Method of manufacturing semiconductor device

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and, more particularly, to a method of manufacturing a semiconductor device having an improved dry etching step.
2. Description of the Related Art
In recent years, micropatterning of an element has advanced in accordance with the progress of the technique of a semiconductor integrated circuit, pattern dimensions having high accuracy has been demanded. A semiconductor integrated circuit can be generally obtained by stacking an insulating thin film such as a silicon oxide film having a predetermined pattern and a conductive thin film such as a polysilicon, aluminum, copper, tungsten, or silicide film on a semiconductor substrate such as a silicon substrate.
As techniques for processing the thin film into a predetermined pattern, lithographic, dry etching, and removing techniques are used. In the lithographic technique, after a photosensitive resist is coated on the thin film, the resist is exposed to a light beam or an ultraviolet beam in accordance with the predetermined pattern, and an exposed portion or non-exposed portion is selectively removed by development to form a resist pattern. In the dry etching technique, the underlying thin film is etched using the resist pattern as a mask. In the removing technique, the resist is removed.
However, as a degree of integration of semiconductor elements is increased, the required minimum size becomes small and dimensional precision of a pattern becomes more strict. Recently, a micropattern having a size of 0.5 .mu.m or less has been required. In order to cope with the above pattern in a small region, since the above techniques for forming a pattern have various problems, the techniques must be largely improved.
These problems are described below in detail.
As one method of processing an underlying thin film using a small resist pattern, an RIE method using a plasma is popularly used. According to this method, a substrate on which a target film is deposited is loaded in a vacuum vessel having a pair of parallel plate electrodes, and after the vessel is evacuated, a reactive gas having a halogen element or the like is supplied into the vessel. A plasma is produced by the gas using discharging caused by application of an RF power, and the target film is etched by the produced plasma.
According to this etching method, ions of various particles in the plasma are accelerated by a DC electric field generated at an ion sheath on the surfaces of the electrodes, and the ions having high energy are collided with the target film, thereby performing an ion-assisted chemical reaction. For this reason, etching is performed in the direction of the incident ions, and directional etching having no undercut can be performed.
Since all materials are excited or activated by this ion collision, differences between reactivities unique to materials cannot be easily obtained in this etching compared with etching using only radicals, and a ratio of etching rates of different materials, i.e., a selection ratio is generally low. For example, since an etching rate of a resist is high with respect to Al, it is difficult to form a pattern with high accuracy due to a large pattern conversion difference. In addition, since the thickness of a resist is small at a stepped portion, a wiring portion is disadvantageously etched to disconnect wiring lines.
In etching of a silicon oxide film, it has a low selection ratio of the silicon oxide film to an underlying material. That is, since underlying silicon (Si) or aluminum (Al) has a high etching rate, etching cannot be immediately stopped with high precision when the surface of the underlying material is exposed. For this reason, when contact holes having different depths are to be formed by etching, silicon or aluminum serving as the underlying layer at the bottom of a shallow hole is undesirably etched in a considerable amount, thereby degrading device characteristics.
In such dry etching, since the moving directions of radicals are not aligned with each other, an increase in etching rate ratio (selectivity) of a film to be etched to a mask at a desired constant etching rate of the film to be etched makes it impossible to form a highly precise pattern due to undesirable side-etching or deposition of the resultant pattern.
Anisotropic etching free from side-etching, an increase in etching rate ratio (selectivity) of the film to be etched to the mask, and a high etching rate of the film to be etched have a trade-off relationship. That is, it is difficult to simultaneously satisfy these three conditions.
In recent years, a mechanism for maintaining and controlling a wafer temperature to a low temperature of 0.degree. C. or less during etching is employed. Etching can be performed at a high etching rate by an ion-assisted reaction in a direction of a depth, and anisotropic etching can be performed in a lateral direction while the reaction is "frozen". Low-temperature wafer control allows control of a reaction on side walls of a pattern, so that the pattern shape can be controlled. For example, in etching of a silicon oxide film (SiO.sub.2), Oiwa (Dry Process Symposium P. 105, 1990) proposed etching of a tapered silicon oxide film (SiO.sub.2) within an appropriate range between the pressure and the substrate temperature.
Technical specifications required for a contact hole along with an increase in integration of semiconductor elements are a decrease in hole diameter and an increase in hole depth. When the hole diameter is decreased and the hole depth is increased, the diameter of the bottom of the hole can be made smaller than that defined in the device specifications because the side wall of the contact hole is tapered. A contact hole is a hole for electrically connecting underlying silicon and a wiring layer formed on the silicon oxide film. For this purpose, a metal (e.g., aluminum or tungsten) or polysilicon is buried in the contact hole. It is thus known that a more perfect electrical connection can be achieved when a contact area between the metal or polysilicon to be buried and the underlying silicon is increased. In view of improvement of electrical characteristics and an increase in integration density, the side wall of the contact hole must be vertical. That is, the specifications required for forming a contact hole used in a highly integrated device are a high selection ratio (at least 20) to silicon and a vertical side wall.
In a silicon oxide film, although it is possible to form a vertical side wall of a contact hole at a high selection ratio to silicon and a high substrate temperature, a resist pattern is thermally deformed at a substrate temperature of 160.degree. C. or more. Therefore, the upper limit of the taper angle of the side wall of the pattern is 83.degree.. It is therefore impossible to obtain a desired pattern with high precision. In ion milling of Al, Au, or Pt, since high-energy particles bombard against the substrate, the temperature of the substrate is increased during etching. The resist pattern is thermally deformed to disable high-precision etching.
Use of a silicon oxide or nitride film as a heat-resistant mask to etch copper or the like at high temperatures is reported. In this case, since copper tends to be oxidized at high temperatures, a residue may be produced, the shape is deformed, or diffusion of copper into the mask material occurs. Therefore, the electrical characteristics are degraded, and it is impossible to form a good wiring layer.
In etching of tungsten or the like, the etching rate of a peripheral wafer portion is different from that of a central wafer portion. When an area having a low etching rate is completely etched, overetching occurs in an area having a high etching rate. An underlying material is etched in a considerable amount, and the pattern shape is undesirably changed. When the size of the wafer is increased, it is impossible to form a desired pattern on the entire surface of the wafer.
In addition, when an insulating thin film such as a silicon oxide film is used as a mask, in an etching method using a plasma, ions and electrons in the plasma are incident on a thin film to be etched. The ions and electrons incident on the thin film cause charges to be accumulated in the thin film (charge-up). For example, when electrons are incident on a mask pattern from a diagonal direction, since the electrons are collided on any one of the right and left walls, amounts of charges to be accumulated in the right and left walls of the mask pattern are different from each other. An electric field newly generated due to asymmetry of the charge amounts in the right and left directions of the walls acts on ions to curve the movement direction of the ions, thereby degrading the anisotropy of the shape of the mask pattern. The micropattern cannot be easily etched with high accuracy.
When a metal material, especially AlSiCu or the like, is to be etched, after a resist film serving as an etching mask is removed and left to stand in the air, corrosion occurs in the metal material. Device characteristics are degraded, and a highly reliable device cannot be easily formed.
The following problems are posed by anisotropic etching of target substrates in accordance with conventional reactive ion etching techniques.
(1) It is impossible to etch a silicon oxide film to obtain a vertical side wall at a high selection ratio to the silicon oxide film.
(2) The etching rate of the central wafer portion is different from that of the peripheral wafer portion in a refractory metal film as of tungsten, a refractory metal silicide film, or a refractory metal oxide film when the size of the wafer is increased, thereby disabling uniformity on the entire wafer.
(3) Since a dry etching selection ratio of the etching mask to a material to be etched is low in reactive ion etching, the thickness of the etching mask material is largely reduced during etching. In addition, when the temperature of the target substrate is increased, the mask pattern is degraded due to a low heat resistance of the mask material. Therefore, high-precision etching cannot be performed.
(4) Assume that copper or the like is etched at a high temperature. Since copper tends to be oxidized at high temperatures, a residue is produced, the shape is degraded, and diffusion of copper to the mask material occurs. As a result, the electrical characteristics are degraded, and excellent wiring layers cannot be obtained.
(5) When an organic thin film is used as a mask material, since the thin film contains an impurity such as fluorine (F), the impurity is mixed in a plasma during the reactive ion etching to maintain the Al, Al alloy, or Si thin film. Especially, corrosion caused by this contamination occurs, and a highly reliable device cannot be obtained.
(6) When a mask material is an organic material or comprises an insulating thin film such as a silicon oxide thin film, the mask pattern is charged up by an amount of charge stored in the mask due to a balance between electrons and ions incident on the thin film in a plasma. The incident direction of the ions are bent, and a micropattern cannot be formed with high precision.
(7) It is often impossible to remove a mask material without damaging a material to be etched and materials adjacent to the material to be etched due to a combination of the mask material and the material to be etched and a combination of the material to be etched and adjacent materials.
(8) While the required minimum size of a pattern is decreased and the required dimensional precision becomes more strict along with an increase in integration degree of semiconductor elements, a micropattern having a size of 0.5 .mu.m or less is recently required. When an underlying thin film having a high reflectance, such as a polysilicon film and an aluminum film is to be patterned, light or an ultraviolet ray passing through the resist is reflected by the surface of the thin film during exposure, and a resist portion except other than the predetermined pattern is undesirably exposed to degrade the dimensional precision.
In order to solve the above problem, a method of forming a micropattern using a carbon film mask is proposed in, e.g., Published Unexamined Japanese Patent Application No. 58-212136. According to this method, a carbon film excellent in etching resistance is formed on the film to be etched. A resist is applied to the carbon film, and a resist pattern is formed by a conventional lithographic means. The carbon thin film is etched by reactive ion etching using this resist pattern as a mask. The resist is selectively removed from the carbon film by using an organic solvent to form a mask consisting of only the carbon thin film pattern. Reactive ion etching is performed using the carbon film pattern as a mask, thereby forming a thin film to be etched. Etching having a high selection ratio can be performed.
The carbon film formed on the film to be etched serves as an anti-reflection film, as described above, and an etching mask having resistance to dry etching.
In the step of forming the carbon film mask pattern, the organic resist is removed in an organic solvent, a solution mixture of H.sub.2 SO.sub.4 and H.sub.2 O.sub.2, or a solution obtained by adding H.sub.2 O thereto. When the material to be etched consists of Al as a major constituent, and the resist is removed by the solution mixture of H.sub.2 SO.sub.4 and H.sub.2 O.sub.2, the material to be etched itself is also etched.
Even if an organic solvent is used, the photo-cured resist or the like cannot be perfectly removed. An alkaline organic solvent or the like has a limited number of types of thin films to be etched because a metal material such as Al may be etched or corroded.
In the process using a solution, a lot of problems are posed in view of solution management and safety measures in operations. Therefore, this process is not suitable for the process for manufacturing semiconductor elements in a dry state.
On the other hand, dry ashing is available to cause an oxygen plasma to remove an organic resist. According to this method, a sample having an organic resist film is placed in a barrel or flat parallel palate type discharge reaction chamber, and oxygen gas is discharged to remove the organic resist film. According to this method, as compared with the method using the solution, the resist can be easily removed, and an underlying material may be a metal. The type of underlying material is not limited to a specific one. However, according this dry ashing method, since the sample is placed in a discharge to obtain a predetermined removal or etching rate required in practical use, both the organic resist and the carbon film are undesirably etched. It is therefore impossible to remove the organic resist with high selectivity to the carbon film.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a highly reliable method of manufacturing a semiconductor element, capable of forming a pattern with high precision, wherein various problems (e.g., an etching selection ratio to a target film, a selection ratio to an underlying material, charge-up, damage during mask removal, heat resistance, contamination of a target body, and a taper shape) caused by mask materials and dry etching can be eliminated at the time of anisotropic etching of the target substrate in accordance with dry etching techniques.
It is another object of the present invention to provide a method of manufacturing a semiconductor device capable of appropriately dry-etching an organic film pattern at high speed with a high selection ratio to the carbon film.
It is still another object of the present invention to provide a method of manufacturing a semiconductor device capable of appropriately dry-etching an organic film pattern at high speed with a high selection ratio to the carbon film, and the target substrate can be etched at a high selection ratio by using a carbon film pattern as a mask.
It is still another object of the present invention to provide an apparatus for manufacturing a semiconductor device, capable of appropriately dry-etching an organic film pattern at high speed with a high selection ratio to the carbon film, and the target substrate can be etched at a high selection ratio by using a carbon film pattern as a mask.
According to the present invention, there is provided a method of manufacturing a semiconductor device, comprising the steps of: forming a carbon film on a silicon oxide film formed on a substrate; forming an organic film pattern on the carbon film; etching the carbon film using the organic film pattern as a mask to obtain a carbon film pattern; removing the organic film pattern; and heating the substrate to a temperature of not less than 160.degree. C., supplying an etching gas containing a gas having fluorine and carbon atoms, and anisotropically etching the silicon oxide film using the carbon film pattern as a mask.
According to the present invention, there is also provided a method of manufacturing a semiconductor device, comprising the steps of: forming a carbon film on a copper film formed on a substrate; forming an organic film pattern on the carbon film; etching the carbon film using the organic film pattern as a mask to form a carbon film pattern; removing the organic film pattern; and heating the substrate to a temperature of not less than 150.degree., supplying an etching gas, and anisotropically etching the copper thin film using the carbon film pattern as a mask.
According to the present invention, there is also provided a method of manufacturing a semiconductor device, comprising the steps of: forming a carbon film on a target film formed on a substrate, the target film being selected from the group consisting of a tungsten film, a nickel film, a titanium film, a tantalum oxide film, a strontium titanate film, an aluminum oxide film, and an aluminum nitride film; forming an organic film pattern on the carbon film; etching the carbon film using the organic film pattern as a mask to form a carbon film pattern; removing the organic film pattern; and heating the substrate to a temperature of not less than 130.degree. C., supplying an etching gas, and anisotropically etching the target film using the carbon film pattern as a mask.
According to the present invention, there is also provided a method of manufacturing a semiconductor device, comprising the steps of: forming a carbon film on a target film formed on a substrate, the target film consisting of a metal or alloy containing aluminum as a major constituent; forming an organic film pattern on the carbon film; etching the carbon film using the organic film pattern as a mask to form a carbon film pattern; removing the organic film pattern; supplying an etching gas containing chlorine and/or bromine atoms, and anisotropically etching the target film using the carbon film pattern as a mask; and heating the substrate to a temperature of 250.degree. C. or more, preferably, falling within a range of 250.degree. to 450.degree. C.
According to the present invention, there is also provided a method of manufacturing a semiconductor device, comprising the steps of: forming a metal wiring layer containing aluminum as a major constituent and formed on a substrate; forming an insulating film on the metal wiring layer; forming a carbon film on the insulating film; forming an organic film pattern on the carbon film; etching the carbon film using the organic film pattern to form a carbon film pattern; removing the organic film pattern; supplying an etching gas having fluorine atoms, and anisotropically etching the insulating film using the carbon film pattern as a mask; and heating the substrate to a temperature of 250.degree. C. or more, preferably, falling within a range of 250.degree. to 450.degree. C.
According to the present invention, there is also provided a method of manufacturing a semiconductor device, comprising the steps of: forming an insulating film on a substrate; forming a carbon film on the insulating film; forming an organic film pattern on the carbon film; etching the carbon film using the organic film pattern as a mask to form a carbon film pattern; removing the organic film pattern; supplying an etching gas, and anisotropically etching the insulating film using the carbon film pattern as a mask; and heating the substrate to a temperature of 250.degree. C. or more, preferably, falling within a range of 250.degree. to 800.degree. C.
Preferred embodiments of the present invention are exemplified as follows.
(1) In the etching step of the carbon film, oxygen, nitrogen, halogen gas, an inert gas (e.g., argon, krypton, or xenon), hydrogen, or fluorocarbon gas is used as an etching gas.
(2) The carbon film is formed by sputtering, vacuum deposition, or CVD.
(3) As a means for removing a resist pattern, a target substrate is placed in a vacuum vessel, a gas mixture consisting of a gas having at least fluorine atoms and oxygen gas is excited in an area other than the vacuum vessel, and an active species produced by this excitation is supplied to the vacuum vessel, thereby achieving down-flow etching.
According to the present invention, there is also provided a method of manufacturing a semiconductor device, comprising the steps of: forming a carbon film on a target substrate; forming an organic film pattern on the carbon film; etching the carbon film along the organic film pattern to form a carbon film pattern; and etching to remove the organic film pattern by an etching gas containing an active species having halogen atoms and an active species having oxygen atoms, or by an etching gas containing an active species having oxygen atoms or an etching gas containing an active species having halogen atoms and an active species having oxygen atoms during heating of the target substrate.
According to the present invention, there is also provided a method of manufacturing a semiconductor device, comprising the steps of: forming a carbon film on a target substrate, forming an organic film pattern on the carbon film; etching the carbon film along the organic film pattern to form a carbon film pattern; etching to remove the organic film pattern by a first etching gas containing an active species having halogen atoms and an active species having oxygen atoms, or by a first etching gas containing an active species of oxygen atoms or a first etching gas containing an active species of halogen atoms and an active species of oxygen atoms during heating of the target substrate; and anisotropically etching the target substrate using the carbon film pattern as a mask and a second etching gas having halogen atoms.
According to the present invention, there is further provided an apparatus for manufacturing a semiconductor device, comprising: a first process chamber for storing a target substrate having a surface on which a carbon film and an organic film pattern are formed, and for etching to remove the organic film pattern by a first etching gas containing an active species having halogen atoms and an active species having oxygen atoms, or by a first etching gas containing an active species of oxygen atoms or a first etching gas containing an active species of halogen atoms and an active species of oxygen atoms during heating of the target substrate; and a second process chamber, connected to the first process chamber, for storing the target substrate which is conveyed from the first process chamber and from which the organic film pattern is removed, and for anisotropically etching the target substrate using the carbon film pattern as a mask and a second etching gas having halogen atoms.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.
FIGS. 1A to 1D ar sectional views showing the steps in forming a pattern according to Example 1 of the present invention;
FIG. 2 is a view showing a schematic arrangement of an etching apparatus used in Example 1 of the present invention;
FIG. 3 is a graph showing a relationship between a substrate temperature, a taper angle of a side wall of a pattern, an etching rate, and a selection ratio;
FIGS. 4A to 4H are sectional views showing the steps in forming a pattern according to Example 2 of the present invention;
FIG. 5 is a graph showing a relationship between a substrate temperature and an etching rate;
FIG. 6 is a graph showing a relationship between a gas flow rate and an etching rate;
FIGS. 7A to 7C are sectional views showing different shapes of side walls of patterns in accordance with different gas compositions;
FIG. 8 is a graph showing a relationship between a substrate temperature and an etching rate;
FIGS. 9A to 9F are sectional views showing the steps in forming a pattern according to Example 4 of the present invention;
FIG. 10 is a graph showing a relationship between a substrate temperature and a degassed component;
FIGS. 11A to 11E are sectional views showing the steps in forming a pattern according to Example 4 of the present invention;
FIGS. 12A to 12C are sectional views showing the steps in forming a pattern according to Example 5 of the present invention;
FIG. 13 is a graph showing an impurity amount obtained in use of a carbon mask in comparison with an impurity amount obtained in use of a resist mask;
FIGS. 14A to 14D are sectional views showing the steps in forming a pattern according to Example 6 of the present invention;
FIG. 15 is a schematic diagram showing an etching apparatus for embodying Example 7 of the present invention;
FIG. 16 is a graph showing a relationship between a total flow rate of CF.sub.4 and O.sub.2 gases and etching rates of a carbon film and a resist film;
FIG. 17 is a graph showing a relationship between a target substrate temperature and etching rates of a carbon film and a resist film; and
FIG. 18 is a schematic diagram showing an etching apparatus for embodying Example 8 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to the present invention, in order to examine dry etching characteristics in use of a carbon film as an etching mask, a substrate temperature was changed from room temperature to 600.degree. C., and an etching apparatus capable of performing reactive ion etching was prepared. Various gases were used as etching gases and the substrate temperature was changed to etch a silicon oxide film, a copper film, a tungsten film, or a tantalum oxide film. Etching rates, shapes of etched patterns, and their uniformity were examined.
A resist pattern was used as a mask a reactive gas containing at least C, F, and H, such as CHF.sub.3 gas, and CO gas were used, and a substrate temperature was changed from 50.degree. C. to 300.degree. C. at a predetermined pressure upon application of an RF power, thereby etching a silicon oxide film. When the substrate temperature was changed from 50.degree. to 160.degree., a taper angle of a side wall of the resultant silicon oxide film pattern could be changed from 80.degree. to 83.degree., so that the side wall could be close to a vertical side wall. When the substrate temperature was increased to 160.degree. C. or more, the resist pattern is thermally deformed, and a pattern having desired pattern dimensions could not be obtained with high precision.
Etching of a silicon oxide film was performed under the same conditions as described above except that a carbon film was used as an etching mask. It was found that a taper angle of a side wall of the resultant silicon oxide film pattern etched at a substrate temperature of 50.degree. C. was 80.degree. and that at a substrate temperature of 170.degree. C. was 83.degree.. At a substrate temperature of 260.degree., a selection ratio for silicon was 20, and a taper angle was 90.degree., thereby satisfying both a high selection ratio to silicon and a highly precise etched shape. Although a substrate temperature was increased to 300.degree. C. or more, neither degradation nor deformation of the carbon film mask itself were observed, and degassing was found to rarely occur. In addition, it was also found that the etching rate of the carbon film was very low.
Reactive ion etching had never been performed such that the substrate temperature was increased to a temperature which caused thermal degradation of the resist and the substrate temperature was controlled. However, it was found that high-precision etching of a silicon oxide film could be performed by using an appropriate etching gas and a carbon film mask at a high substrate temperature.
When a copper thin film was etched using chlorine gas as in the above method, it was found that the copper thin film was vertically etched at a substrate temperature of 250.degree. C. or more and that the etching rate of the carbon thin film was low. That is, as can be apparent from the above fact, no residue is produced from a copper thin film which produces only an etching product having a very low vapor pressure in conventional dry etching, thereby achieving high-precision anisotropic etching.
When a mixing ratio of a gas mixture consisting of fluorine gas and chloride gas was appropriately changed to etch a tungsten film and a tantalum oxide film, uniformity of the etching rates of the tungsten film and the tantalum oxide film was improved with an increase in substrate temperature. Therefore, high-precision etching could be performed on the entire surface of the wafer.
The present inventors used a carbon thin film mask to etch a silicon oxide film having an underlying AlSiCu thin film or a silicon oxide film having an underlying Si substrate. The present inventors then performed annealing of the etched silicon oxide film in a vacuum or in a predetermined gaseous atmosphere upon an increase in substrate temperature. The present inventors found that a contaminant or residue produced by etching could be removed. By this annealing, corrosion or degradation of electrical characteristics of the device could not be observed. That is, a highly reliable device can be formed in formation of a wiring structure in a semiconductor integrated circuit and formation of a capacitor.
The present invention will be described in detail by way of its examples with reference to the accompanying drawings.
EXAMPLE 1
Example 1 of the present invention will be described with reference to FIG. 1.
As shown in FIG. 1A, a silicon oxide film 2 having a thickness of 1.4 .mu.m was deposited on a silicon substrate 1 by thermal oxidation. After a carbon film 3 was formed to have a thickness of 250 .mu.m by sputtering, made of novolak resin (TSMR-CRBl: manufactured by Tokyo Oka Ltd.) a resist pattern 4 having a desired pattern was formed on the carbon film 3 in accordance with conventional photolithographic techniques. The carbon film 3 was vertically etched using the resist pattern 4 (thickness: 1.5 .mu.m) as a mask in accordance with a dry etching technique, thereby forming a carbon film pattern 3.
The remaining resist 4 was removed by down-flow ashing using CF.sub.4 /O.sub.2 gas. As shown in FIG. 1B, the carbon film pattern 3 was left on the silicon oxide film 2.
As shown in FIG. 1C, the silicon oxide film 2 was etched using a dry etching apparatus, thereby forming a silicon oxide film pattern 2a.
The step in FIG. 1C will be described in detail below.
The dry etching apparatus used in Example 1 will be described with reference to FIG. 2.
In the etching apparatus shown in FIG. 2, an etching chamber 20 comprises a vacuum vessel 20a, a first electrode 22 for supporting a target substrate 21 placed in the vacuum vessel 20a, an RF power source 24 connected to a blocking capacitor 29 to apply a 13.56-MHz RF power to the first electrode 22, and a heater 25 for heating the first electrode 22 and controlling the temperature of the target substrate 21 to a desired temperature. Alternatively, the substrate may be heated by a heated silicon oil circulating through the first electrode 22.
The etching chamber 20 is also connected to a CHF.sub.3 gas supply line 28a and a carbon monoxide gas supply line 28b. CHF.sub.3 and CO are supplied from the lines 28a and 28b to the vacuum vessel 20a, and an RF voltage is applied across the first electrode 22 and the inner wall (upper wall) of the vacuum vessel 20a serving as the second electrode.
The vacuum vessel 20a is grounded. The gas supply lines 28a and 28b have valves 28a and 29b and flow adjustors 30a and 30b, respectively. Therefore, the flow rates and gas pressures can be adjusted to desired values.
A permanent magnet 26 made of Sm-Co is arranged above the upper wall serving as the second electrode of the vacuum vessel 20a. The permanent magnet 26 is eccentrically rotated about a rotating shaft 27 by a motor. A high-density plasma can be generated and maintained even at a high vacuum of 10.sup.-3 Torr or less by a magnetic field of 50 to 500 Gauss generated by the permanent magnet 26. A large amount of ions are extracted from the high-density plasma and are radiated on the target substrate 21, thereby performing etching. In this case, a magnetic field strength on the surface of the substrate 21 is set to 120 gauss.
As shown in FIG. 1C, the silicon oxide film 2 was dry-etched using the dry etching apparatus shown in FIG. 2. A gas mixture of CHF.sub.3 and CO was used as an etching gas. As etching conditions, a CHF.sub.3 gas flow rate was 45 SCCM, a CO gas flow rate was 155 SCCM, a power was 2.6 W/cm.sup.2, and a pressure was 40 mTorr. The substrate temperature was changed from 50.degree. C. to 300.degree. C. to perform etching.
FIG. 3 is a graph showing an etching rate of the silicon oxide film 2 etched with a change in substrate temperature, a selection ratio to silicon, and a taper angle of an etched section of the silicon oxide film pattern 2a upon observation with a scanning electron microscope (SEM).
As is apparent from FIG. 3, the taper angle of the wide wall of the pattern at a substrate temperature of 50.degree. C. was 80.degree., the taper angle at a substrate temperature of 175.degree. C. was 84.degree., and the taper angle at a substrate temperature of 260.degree. C. was 90.degree., i.e., a vertical side wall. When the substrate temperature exceeded 260.degree. C., etching of the silicon oxide film was accelerated by radicals in the plasma, and under-cut occurred immediately under the carbon film mask. The etching rate of the silicon oxide film was linearly decreased with an increase in substrate temperature. Since the etching rate of silicon was also linearly decreased in the range of 50.degree. C. to 170.degree. C. with an increase in substrate temperature, so that the selection ratio was not largely changed. However, in the temperature range of 170.degree. C. or more, it was found that diffusion components from the carbon film pattern was increased, and then the etching rate of silicon was further decreased, thereby increasing the selection ratio in this temperature range.
As shown in FIG. 1D, only the carbon film pattern 3 was removed by O.sub.2 plasma etching to obtain the highly precise silicon oxide film pattern 2a having a side wall ranging from a tapered side wall to a vertical side wall.
When the silicon oxide 2 was etched under the same conditions described above by using a resist pattern mask as a comparative example, the taper angle of the side wall could fall within the range of 80.degree. to 83.degree. in the temperature range of 50.degree. C. to 160.degree. C. When the substrate temperature exceeded 160.degree. C., the resist pattern was thermally deformed, and it was difficult to obtain a pattern having desired dimensional precision.
The silicon oxide film was dry-etched using an etching gas containing only CHF.sub.3 gas (i.e., without containing CO gas). Etching was performed under the same conditions (i.e., power: 2.6 W/cm.sup.2 ; pressure: 40 mTorr; and substrate temperature: 50.degree. C. to 300.degree. C.) except that CO gas was not used and the flow rate of CHF.sub.3 gas was 200 SCCM.
The taper angles were respectively 74.degree., 84.degree., and 90.degree. at substrate temperatures of 50.degree. C., 125.degree. C., and 170.degree. C. using the etching gas containing only CHF.sub.3. It was apparent that a vertical side wall could be obtained at lower temperatures than those using the etching gas containing CHF.sub.3 and CO gases. At either temperature, a selection ratio to silicon was less than 10. When the content of CO gas was increased at the constant total flow rate of 200 SCCM, the selection ratio to silicon was 20 at a CHF.sub.3 gas flow rate of 70 SCCM, a CO gas flow rate of 130 SCCM, and a substrate temperature of 170.degree. C. At this time, the shape of the pattern was observed with an SEM, and the taper angle was 84.degree..
The pressure and power were changed in addition to the changes in substrate temperature described above. In either case, the taper angle and the selection ratio to silicon were slightly influenced.
It is found that the substrate temperature must be controlled to fall within the range of 160.degree. C. (inclusive) to 260.degree. C. (exclusive) to satisfy the taper angle of 83.degree. C. and the selection ratio for silicon of 20 in Example 1. Though CHF.sub.3 gas is used in Example 1, CF.sub.4 gas or the like may be used. Further, H.sub.2 gas may be used instead of CO gas or in addition to Co gas.
EXAMPLE 2
Example 2 using a carbon film as an etching mask in dry etching of copper (Cu) according to the present invention will be described below. Example 2 used a carbon film pattern in place of an organic material (e.g., a resist) as an etching mask because a halogen compound as of Cu had a low vapor pressure and a high ion energy impact and a high substrate temperature were required.
As shown in FIG. 4A, an SiO.sub.2 film was formed on an Si substrate 41, and a Cu film (400 nm) 43 was formed on the SiO.sub.2 film 42 by sputtering. As shown in FIG. 4B, a carbon film 44 having a thickness of 200 nm was formed by the same sputtering as described above. In addition, as a comparative example, a sample having a silicon oxide film (SiO.sub.2) 44' having a thickness of 200 nm was also formed by sputtering.
As shown in FIG. 4C, resist films 45 each having a thickness of 16 .mu.m were deposited on the carbon film 44 and the SiO.sub.2 film 44', respectively. The resist films 45 were exposed and developed using the conventional lithographic techniques, as shown in FIG. 4D, thereby forming resist patterns 45a. In the step shown in FIG. 4D, elusion and peeling of the carbon film 44 or the SiO.sub.2 film 44' did not occur.
As shown in FIG. 4E, the carbon film 44 was patterned by reactive ion etching using the corresponding resist pattern 45a as a mask. A dry etching apparatus used in this case was a reactive ion etching apparatus having the magnetron. The etching conditions were set such that an H.sub.2 gas flow rate was 100 SCCM, a pressure was 1.5 Pa, an RF power was 1.7 W/cm.sup.2, and a substrate temperature was 25.degree. C.
As shown in FIG. 4E, a carbon film pattern 44a was formed. The resist pattern 45a was left on the carbon film pattern 44a. The SiO.sub.2 film 44' was patterned by the same etching apparatus as described above, using the corresponding resist pattern 45a as a mask. The SiO.sub.2 film 44' was patterned by etching under the same conditions as described above, using H.sub.2 gas, thereby obtaining the SiO.sub.2 film pattern 44a'. The resist pattern 45a was left on the SiO.sub.2 film pattern 44a'.
As shown in FIG. 4F, in order to remove the resist patterns 45a from the corresponding thin films, the resist patterns 45a were removed using an organic solvent. The resist patterns 45a respectively formed on the carbon film pattern 44a and the SiO.sub.2 film pattern 44a' were perfectly etched and removed, thereby obtaining an etching mask pattern consisting of the carbon film pattern 44a or the SiO.sub.2 film pattern 44a'.
As shown in FIG. 4G, the Cu film 43 was etched using the carbon film pattern 44a or the SiO.sub.2 film pattern 44a' as an etching mask. An etching apparatus was a reactive ion etching apparatus having a magnetron used as in Example 1. The etching gas was Cl.sub.2 (total flow rate: 100 SCCM), a pressure was 0.5 Pa, an RF power was 1.7 W/cm.sup.2, and a substrate temperature was changed from 200.degree. C. to 400.degree. C.
The etching rate of the Cu film was measured while an RF power was not applied and the pressure and substrate temperature were changed. As shown in FIG. 5, Cu was found not to be etched at a substrate temperature of less than 250.degree. C. even if the pressure was changed. When the pressure was increased, the etching rate of the Cu film was found to be increased. In addition, the carbon film pattern was not etched at all under the above conditions and wa free from thermal deformation.
The RF power wa applied to etch the Cu film. The etching rate of the Cu film was found to be low at a substrate temperature of less than 250.degree. C.
Etching was performed after the substrate temperature was increased to 300.degree. C. or more. As shown in FIG. 4G, Cu was etched to have an almost vertical side wall. In this case, no residue was observed.
The Cu etching rate was 400 nm/min, the carbon film etching rate was 100 nm/min, and the selection ratio of Cu to the carbon film was 4. Since the carbon film has heat resistance so as to serve as an excellent etching mask for reactive ion etching using a halogen gas even at high temperatures of 300.degree. C. or more, high-precision etching for a halogen compound as of Cu having a low vapor pressure can be performed.
As a comparative example, etching using the mask pattern having the SiO.sub.2 film formed on the Cu film was performed under the same conditions as described above. A residue was observed at a substrate temperature of 300.degree. C. due to the following estimation. Oxygen is produced from the SiO.sub.2 film during etching, so that a Cu etching product is produced to locally oxidize the Cu etched surface and produce a Cu oxide having a very low vapor pressure. As a result, Cu tends not to be etched.
When a carbon film mask is used, no oxygen is produced from the mask. Carbon or a carbon chloride produced from the mask is reacted with moisture and oxygen present in the atmosphere of the residue to eliminate the moisture and oxygen. The Cu surface tends not to be oxidized, and no residue is produced.
A conventional resist pattern was formed on a Cu film, and the substrate temperature was increase to perform etching. Even if this resist pattern is a photocured resist pattern, the resist pattern was found to be thermally degraded at a substrate temperature of 160.degree. C. or more. On the other hand, when a carbon film pattern was used as a mask, no pattern degradation occurred even at a substrate temperature of 400.degree. C. or more.
As shown in FIG. 4H, an etching apparatus using conventional parallel plate electrodes was used to etch and remove the carbon film pattern 44a from the Cu film 43. An etching gas was a gas (e.g., SF.sub.6 or NF.sub.3) containing at least fluorine atoms but not containing oxygen atoms, or H.sub.2 gas. A pressure was 50 mTorr, an RF power was 150 W, and a substrate temperature was 50.degree. C.
After the carbon film pattern 44a was removed, the shape of the Cu film pattern 43a was observed with an SEM. No degradation of the pattern was observed, and high-precision 0.4-.mu.m line-and-space etching can be performed.
Where trialkyl phosphine is used as an etching gas, a Cu film can be etched at a temperature of 150.degree. C. or higher.
Though Cl.sub.2 gas is used in Example 2, HBr gas or a mixture containing HBr may be used.
EXAMPLE 3
Example 3 for applying the method of the present invention to formation of a tungsten pattern will be described below.
As shown in FIG. 4A, as in Example 2, a 10-nm thick silicon oxide (SiO.sub.2) film 42 was formed on a silicon substrate 41 by thermal oxidation, and a 200-nm thick tungsten (W) film 43 was formed by CVD. As shown in FIG. 4B, a carbon film (thickness: 100 nm) 44 was formed on the tungsten film 43.
As shown in FIG. 4D, a photoresist was applied to the carbon film 44 and is patterned by the conventional lithographic techniques to obtain a resist pattern 45a. As shown in FIG. 4E, the carbon film 44 was etched by reactive ion etching using the resist pattern 45a as a mask and H.sub.2 gas to obtain the carbon film 44 having a vertical side wall. As shown in FIG. 4F, the resist pattern 45a was removed by down-flow etching using CF.sub.4 /O.sub.2 gas to form a carbon film pattern 44a.
As shown in FIG. 4G, the W film 43 was etched using the carbon film pattern 44a as an etching mask. The dry etching apparatus described above was used to etch the W film 43.
The etching conditions were set as follows. A gas pressure within the reaction vessel was 50 mTorr, an etching gas was Cl.sub.2 gas or SF.sub.6 gas, or a mixture thereof, and an RF power was 150 W. In this case, the mixing ratio of the gas mixture was changed to perform etching at room temperature. As shown in FIG. 6, when SF.sub.6 gas (100%) was used as an etching gas, the etching rate of the W film was 350 nm/min. To the contrary, when Cl.sub.2 gas (100%) was used as an etching gas, the etching rate of the W film was found to be reduced to 20 nm/min. At this time, the etching rate of the carbon film was found to be 10 nm/min for the SF.sub.6 gas (100%) and to be as small as 5 nm/min for the Cl.sub.2 gas (100%). The distribution of the etching rates within the wafer was measured. It was found that the peripheral portion of the wafer had the highest etching rate for the SF.sub.6 gas and that uniformity was as low as 75% (3.sigma./x: average etching rate). The sectional shape of the etched pattern was observed with an SEM. Under-cut of the W film 43 occurred in the presence of SF.sub.6 gas (100%), as shown in FIG. 7A. The side wall of the pattern was tapered in the presence of Cl.sub.2 gas (100%), as shown in FIG. 7C. Therefore, the side wall was tapered, and high-precision patterned could not be performed.
Etching was then performed while the mixing ratio of the gas mixture of SF.sub.6 and Cl.sub.2 was appropriately changed and the substrate temperature was increased. As a result, a vertical side wall shown in FIG. 7B could be obtained at a partial pressure ratio of Cl.sub.2 to SF.sub.6 being 7:3 at a substrate temperature of 130.degree. C.
As shown in FIG. 8, the etching rate of the W film was increased with an increase in substrate temperature. More specifically, an etching rate was 450 nm/min at 130.degree. C., while the etching rate of the carbon film was as small as 50 nm/min. Therefore, the W film was found to be etched at a high selection ratio. A measured distribution of etching rates within the wafer is shown in FIG. 8. As is apparent from FIG. 8, etching uniformity was improved with an increase in substrate temperature. Uniformity (3.sigma./x) at a substrate temperature of 160.degree. C. was found to be 10% due to the following reason. The concentration of tungsten chloride (WCl.sub.6) as an etching product is higher at the central portion of the wafer than at the peripheral portion thereof. When the product is deposited again, the etching rate is decreased. This may derive the following estimation. Although the etching rate at the central portion of the wafer is lower than that at the peripheral portion thereof, the vapor pressure of the etching product (WCl.sub.6) or the like is increased with an increase in substrate temperature. Therefore, redeposition tends not to occur. When the substrate temperature was further increased, the etching rate was increased and etching uniformity was improved. However, under-cut occurred.
Etching was performed by adding CO gas in this region. The etching rates of the W film and the carbon film were decreased with an increase in CO gas, but under-cut was suppressed. The shape of the pattern could be controlled by adding CO gas. In addition, etching uniformity was found not be largely changed by adding CO gas.
In order to highly precisely etch the W film and the like at a high etching rate with high uniformity within the wafer, it was very effective to increase the substrate temperature and perform etching using the carbon film mask at a high selection ratio. The shape of the etched pattern was changed with an increase in substrate temperature. In this case, the mixing ratio of the etching gas was appropriately changed, or, CO gas, for example, was added to the etching gas, thereby performing high-precision etching.
Finally, as shown in FIG. 4H, the carbon film pattern 44a was etched in a barrel type plasma etching apparatus using O.sub.2 gas. After the carbon film pattern 44a was removed, the W film pattern 43a was evaluated with an SEM. It was found that a vertical 0.4-.mu.m wide line pattern was found to be formed on the entire surface of the wafer.
Though Cl.sub.2 gas and SF.sub.6 gas are used in Example 3, HBr gas or a mixture containing HBr may be used.
EXAMPLE 4
Example 4 for applying the present invention to formation of an Al alloy film pattern will be described below.
FIGS. 9A to 9F are sectional views showing the steps in forming an Al alloy film pattern. As shown in FIG. 9A, an SiO.sub.2 film 52 was formed on an Si substrate 51. A Ti film and a TiN film (constituting a TiN/Ti film 53), and an AlSiCu thin film 54 (Si: 1 wt %; Cu: 0.5 wt %) were sequentially deposited on the SiO.sub.2 film 52. The surface of the AlSiCu thin film 54 was exposed in a plasma using oxygen gas to modify the surface of the AlSiCu thin film 54. As shown in FIG. 9B, a carbon film 55 (thickness: 200 nm) was formed on the thin film 54. Note that the carbon film 55 was deposited by a magnetron sputtering apparatus.
As shown in FIG. 9C, a photoresist 56 (thickness: 1.6 .mu.m) was applied to the carbon film 55, and was exposed and developed using the conventional lithographic techniques to form a resist pattern 56. In the step of FIG. 9C, an alkaline organic solvent was used as a developing solution. However, elusion and the like of the AlSiCu film 54 did not occur because its underlying layer was the carbon film 55.
The carbon film 54 was patterned by reactive ion etching using the resist pattern 56 as a mask. A dry etching apparatus used in this etching process was a reactive ion etching apparatus having the magnetron described above. Etching conditions were set as follows. An H.sub.2 gas flow rate was 100 SCCM, a pressure was 1.5 Pa, an RF power was 1.7 W/cm.sup.2, and a substrate temperature was 25.degree. C. A carbon film pattern 55a was formed. Only the resist pattern 56 was removed from the carbon film 55 by a down-flow ashing apparatus using CF.sub.4 /O.sub.2 gas. As shown in FIG. 9D, the resist pattern 56 was perfectly etched and removed, and an etching mask pattern consisting of the carbon film 55 wa formed.
As shown in FIG. 9E, the AlSiCu film 54 and the TiN/Ti film 53 were etched using the carbon film 55a as an etching mask. This etching was performed using the dry etching apparatus having the magnetron described above. The etching conditions were set as follows. A substrate temperature was maintained at 25.degree. C., an etching gas was a gas mixture of Cl.sub.2 and BCl.sub.3, an etching pressure was 2.0 Pa, and an RF power was 0.8 W/cm.sup.2.
At this time, the etching rate of the AlSiCu film 54 was about 350 nm/min, the etching rate of the TiN/Ti film 53 was about 150 nm/min, the etching rate of the carbon film was 20 nm/min, and a selection ratio of the AlSiCu film to the carbon film was about 13. An amount of residue produced on the wafer under the above conditions was measured, but no residue was observed. The shape of the etched AlSiCu film 54 was observed with an SEM, and a pattern having an almost vertical side wall was obtained. After etching, the above sample was annealed in an N.sub.2 atmosphere such that the substrate was heated at 200.degree. C. for 2 min. The annealed sample was left to stand in air, and corrosion was evaluated with an optical microscope. Even if the sample was left to stand in air for a week, no corrosion was observed. When corrosion was evaluated while the sample was left to stand and moistened, corrosion was observed with a lapse of 6 hours.
The substrate was heated to 350.degree. C. upon etching, and was annealed for 2 min. No corrosion was observed while the sample was left to stand and moistened for 6 hours.
In order to examine a corrosion mechanism, a gas produced from the substrate was examined using a TDS method (Thermal Desorption Spectra), i.e., heating the substrate, in accordance with a mass analysis method. As shown in FIG. 10, it was found that Cl as an etching gas component and AlCl as an etching product were produced from the substrate with an increase in substrate temperature. It is thus estimated that corrosion during exposure at a high humidity may be caused by the residual Cl and AlCl upon etching. When the substrate was heated to 450.degree. C. by the TDS method, the Cl and AlCl components were found to be perfectly eliminated. In the prior art, when a resist film mask is used, the resist is thermally deformed near 160.degree. C., and decomposed components from the resist are attached to the AlSiCu film pattern, so that corrosion heavily occurs.
To the contrary, by using the heat-resistant carbon film mask almost free from degassing, it is possible to increase the substrate temperature to about 500.degree. C. prior to removal of the carbon film pattern. In practice, even if the substrate temperature was increased to 1,100.degree. C., degassing was not observed from the carbon film mask 55, and degradation of the pattern by thermal deformation was not observed, either. However, when annealing was performed at a temperature exceeding 450.degree. C., the AlSiCu film 54 was thermally deformed. Therefore, annealing is preferably performed at a temperature of 250.degree. C. to 450.degree. C.
EXAMPLE 5
Example 5 of the present invention will be described as an example for applying a carbon film as an etching mask in the process for forming a connecting portion (via contact) between upper and lower metal wiring layers with reference to FIGS. 11A to 11E.
As shown in FIG. 11A, a first metal wiring layer 63 such as an Al alloy wiring film (Si: 1 wt %; Cu: 0.5 wt %) having a thickness of about 800 nm was deposited on a first insulating interlayer 62 deposited on a semiconductor substrate 61 having an element thereon in accordance with sputtering. An SiO.sub.2 film 64 was deposited by a low-temperature CVD (Chemical Vapor Deposition) apparatus using TEOS (tetraethoxysilane).
As described in Examples 1 to 4, a carbon film 60 having a thickness of 200 nm was deposited on the SiO.sub.2 film 64 by sputtering. A photoresist 65 was deposited on the carbon film 60, and the photoresist 65 was removed from only a prospective connection hole forming portion of the second insulating interlayer 64 in accordance with the conventional photolithographic process.
As shown in FIG. 11B, as described in Example 2 in detail, a carbon film 60 was anisotropically etched by a dry etching technique using H.sub.2 gas and the photoresist 65 as a mask, thereby forming a carbon film pattern 60a. As shown in FIG. 11C, the resist film 65 was etched following the same procedures as in Example 2 in accordance with down-flow ashing using the CF.sub.4 /O.sub.2 gas.
As shown in FIG. 11D, the SiO.sub.2 film 64 was anisotropically etched by reactive ion etching using the dry etching apparatus as in Examples 1 to 4 until the first metal wiring layer 63 was exposed, thereby forming a connection hole 66. At this time, the etching conditions were set as follows. An etching gas was CHF.sub.3, a power was 1.4 W/cm.sup.2, a pressure was 40 mTorr, a gas flow rate was 20 SCCM, and a substrate temperature was 150.degree. C. The etching rate of the SiO.sub.2 was measured to be 200 nm/min, and the shape of the etched film was observed with an SEM. The side wall of the connection hole was almost vertical. However, when the substrate temperature during etching was decreased to 100.degree. C. or less, the side wall of the SiO.sub.2 film 64 defining the connection hole could be tapered.
When the connection hole had a vertical side wall, a small amount of deposited substance 68 was formed on the side wall. However, when the substrate temperature was decreased during etching and a tapered side wall was obtained, no deposited substance 68 was observed.
As described with reference to Example 2, the carbon film 60 was removed by O.sub.2 gas plasma etching. A second metal wiring layer 67 (e.g., an AlSiCu film) was deposited on the entire surface by sputtering and was patterned to form the second wiring layer.
As a comparative example, a resist used in the conventional fabrication method was used as an etching mask to form a via contact.
As shown in FIG. 12A, an AlSiCu film 73 (Si: 1 wt %; Cu: 0.5 wt %) having a thickness of about 800 nm was deposited as an Al alloy wiring layer on a first insulating interlayer 72 deposited on a semiconductor substrate 71 having an element formed thereon. An SiO.sub.2 film 74 was deposited on a second insulating interlayer in accordance with low-temperature plasma CVD. These steps are substantially the same as those in FIGS. 11A to 11E.
A photoresist (thickness: 1.6 .mu.m) was deposited on the SiO.sub.2 film 74, and the resist 75 was removed from only a prospective connection hole forming portion of the second insulating interlayer 74 in accordance with a conventional photolithographic process. As shown in FIG. 12B, the SiO.sub.2 film 74 was anisotropically etched using CHF.sub.3 gas and the resist 75 as a mask under the same conditions as described above until the first metal wiring layer 73 was exposed, thereby forming a connection hole 76.
At this time, the side wall of the connection hole defined by the SiO.sub.2 film 74 was slightly tapered at a substrate temperature of 130.degree. C. When the resist 75 was removed by conventional O.sub.2 plasma ashing, a deposited substance 78 called a fence was formed on the side wall of the connection hole defining the SiO.sub.2 film 74, as shown in FIG. 12, in an amount larger than that obtained in the process using the carbon film 60 in FIGS. 11A to 11E. When the substrate temperature was decreased during etching and the side wall of the connection hole defined by the SiO.sub.2 film 74 was tapered, no deposited substance 78 was observed.
As shown in FIG. 12C, an AlSiCu film 77 serving as the second metal wiring layer was deposited on the entire surface by sputtering and was patterned to form a second metal layer 77.
Corrosion properties of via contact holes formed in the process (FIGS. 11A to 11E) using the carbon film as a mask and in the process (FIGS. 12A to 12C) using the resist as a mask were evaluated. This evaluation was performed such that each sample was left to stand in air for a long period of time and corrosion amount within the chip was examined with a microscope. As a result, in the process using the resist as the mask, corrosion was observed in a large amount. However, in the process using carbon as a mask, no corrosion was observed even if the sample was left to stand for a week.
In order to clarify the cause of corrosion, a wafer having a via hole was dipped in distilled water, and ion chromatographic analysis was performed. Cl and F were detected as impurities, as shown in FIG. 13. In particular, an amount of Cl and F measured using the carbon film mask was found to be larger than that measured using the resist mask.
When the wafer is left to stand in air upon formation of a connection hole, the first metal wiring layer 73, an impurity containing Cl and F on the surface of the layer 73, and moisture in air are reacted with each other to form HCl and HF. When HCl and HF were contained in water, water becomes an electrolyte, thus causing the following reactions:
Al+3Cl.sup.- .fwdarw.AlCl.sub.3 +3e.sup.-
2AlCl.sub.3 +6H.sub.2 O.fwdarw.2Al(OH).sub.3 +6H.sup.+ +6Cl.sup.-
Once these reactions occur, AlSiCu constituting the first metal wiring layer 73 may be corroded by the produced Cl.
As shown in FIGS. 11D and 12B, an impurity contained in the photoresist 75 is emitted in the plasma, and the impurity is attached to the side wall of the via hole. The impurity is not attached to the carbon film because it is highly pure.
When etching reaches the upper surface of the underlying first metal wiring layer 73, metals contained in the first metal wiring layer and atoms contained in the mask material (75 or 60) and the SiO.sub.2 film 74 are sputtered, and these substances are attached to the side and bottom surfaces of the connection hole 76. Since these substances cannot be eliminated by O.sub.2 ashing, a deposited substance (fence) 78 is formed on, e.g., the side surface of the connection hole. In a post process, overhanging occurs during sputtering of the upper second metal wiring layer 77, thereby causing poor step coverage or disconnection of the wiring layer.
To the contrary, when a carbon film is used as an etching mask, an amount of decomposed substance is small during plasma etching, and the amount of substance deposited on the side wall of the via hole can be reduced. Wiring disconnections do not occur. Since an impurity such as Cl and S is produced from a resist mask, the impurity is reacted with moisture in air upon removal of the resist, thereby forming Cl.sup.- and F.sup.- ions. Corrosion occurs in a large amount. To the contrary, since the resist is removed in advance in the carbon film mask, an amount of the impurity in the via hole is reduced. Therefore, no corrosion occurs.
Corrosion was evaluated when the sample was left to stand in air and moistened. Corrosion occurred with a lapse of 6 hours as in Example 4. Various post processes were taken into consideration to suppress corrosion.
More specifically, after a contact hole was formed, the substrate temperature was increased to 250.degree. C. or more or the sample was exposed to Si.sub.2 H.sub.6 or CO gas. F or S left on the surface of AlSiCu was directly reacted with Si, CO, or H and SiH.sub.x decomposed from SiH.sub.4, thereby producing SiH.sub.x F, COF, HF, HS, and COS as a result of mass analysis. The residues left on the SiO.sub.2 or AlSiCu surface upon etching could be removed by a post process by substrate heating and proper selection of a reactive gas.
Corrosion of the sample was evaluated while the treated sample was moistened. No corrosion was observed. In this process, the substrate must be heated to a temperature of 250.degree. C. or more. When the resist mask was used, components degassed from the resist were attached to the SiO.sub.2 or AlSiCu surface and could not be perfectly removed. It was found that the above process could be performed using a heat-resistant carbon film mask having a very small amount of degassed components. As shown in Example 4, since the underlying AlSiCu film was thermally deformed, post annealing was found to be appropriately performed in the temperature range of 250.degree. C. to 450.degree. C.
Finally, as shown in FIG. 11E, the carbon film 60 was removed by conventional O.sub.2 plasma ashing, electrical characteristics of the resultant wiring structure were evaluated, and a highly reliable device having good characteristics can be obtained.
EXAMPLE 6
Example 6 according to the present invention exemplifies use of a carbon film as an etching mask in a step of forming a metal wiring contact hole in a semiconductor device will be described with reference to FIGS. 14A to 14D.
As shown in FIG. 14A, an impurity was doped in an Si substrate 81 having a plane direction of (100) to form a diffusion layer 82. As shown in FIG. 14B, an SiO.sub.2 film was deposited to have a thickness of about 300 nm by CVD, and BPSG (BoroPhosphoSilicate Glass) was deposited on the SiO.sub.2 film to a thickness of about 600 nm. The surface was flattened through a low-temperature reflow step, and an insulating interlayer 83 was formed.
A hole will be formed in the insulating interlayer 83 as follows. In this step, as shown in FIG. 14C, a photoresist as deposited on the insulating interlayer 83 and a hole was formed in the insulating interlayer 83 in accordance with conventional dry etching techniques following the same procedures as in Example 5. The hole was formed in the insulating interlayer 83 using a carbon film as a mask in the steps of FIGS. 11A to 11D.
After a mask 84 consisting of a resist or carbon film was removed, an AlSiCu thin film 85 serving as a wiring metal film was deposited on the entire surface so as to bury a contact hole 86 which exposes the diffusion layer 82.
The properties of the contact holes thus formed were evaluated. An increase in contact resistance, junction breakdown, variations in contact resistances, and corrosion of the AlSiCu wiring layer were found for the contact hole formed using the resist as the mask. An increase in corrosion amount of the contact portion was observed with a lapse of time, as shown in FIG. 13. To the contrary, degradation such as an increase in contact resistance, and corrosion were not observed.
In the contact hole formed using the resist mask, impurities such as S, F, and Cl as components decomposed from the resist during reactive ion etching may be attached to the contact hole to cause degradation of electrical characteristics. On the other hand, in the contact hole formed using the carbon film mask, since an etching selection ratio is high, the carbon film is rarely degraded during etching. In addition, when carbon is attached in the contact hole, it does not cause corrosion of the wall surface of the contact hole. Therefore, it is assumed that electrical characteristics are not degraded.
When a carbon-containing halogen gas such as CHF.sub.3 is used as an etching gas, the electrical characteristics are not degraded because the etching gas itself contains carbon.
A corrosion acceleration test while the same was left to stand and moistened was performed following the same procedures as in Example 5. Corrosion was observed in the same formed using the carbon film mask. As in Example 5, the substrate temperature was increased to 250.degree. C. or more, and the sample was exposed to a gas such as Si.sub.2 H.sub.6, CO, or B.sub.2 H.sub.6. Residual substances on the Si or SiO.sub.2 surface could be removed. In this process, no thermal deformation of the Si or SiO.sub.2 did not occur even if the substrate temperature was increased to about 1,000.degree. C. However, an impurity distribution in a diffusion layer formed in Si substrate changes at 800.degree. C. It is, therefore, desirable that heating temperature of the substrate is 800.degree. C. or lower so that the impurity distribution does not change.
The characteristics of the contact hole upon completion of the above treatments were evaluated. Even in the corrosion acceleration test performed while the sample was left to stand and moistened, no corrosion was observed. In addition, degradation of electrical characteristics such as an increase in contact resistance was not observed. Therefore, a highly reliable device can be manufactured.
In Examples 1 to 6, the magnetron reactive ion etching apparatus having parallel plates was used as an etching apparatus. However, a reactive ion etching apparatus using an ECR electric discharge applied with a microwave, a reactive ion etching apparatus for applying a target substrate to be etched in the presence of a discharge plasma produced upon application of a microwave or electron beam, or a conventional etching apparatus using parallel plates may be used.
In Example 4, etching of a tungsten film was exemplified. However, a carbon film can be used as an etching mask for nickel, titanium, tantalum, a tantalum oxide, strontium titanate, an aluminum oxide, an aluminum nitride, a refractory metal, a refractory metal silicide, a metal oxide, and a metal nitride, and substrate temperatures are then increased to etch these materials.
In order to examine dry etching properties of carbon and organic resist films, a down-flow etching apparatus, a cylindrical etching apparatus, and parallel plate type etching apparatus were used, various etching gases were used, and etching gas pressures, RF or microwave powers, and substrate temperatures were changed to measure etching rates.
In the cylindrical and parallel plate type etching apparatuses, when organic resist films were to be etched and removed at high etching rates, carbon and organic resist films were dry-etched. Selective etching of the organic resist film with respect to the carbon film was found to be impossible. Voltage dependency of the carbon film in the oxygen plasma was examined. It was found that etching was abruptly progressed from a given voltage.
On the other hand, the etching rate of the resist was monotonously increased as a function of voltage applied thereto. A voltage applied across the resist and the plasma is preferably minimized.
In etching using a gas mixture of carbon tetrafluoride and oxygen in the down-flow etching apparatus for performing etching by means of radicals without causing ion bombardment, when the mixing ratio is set to be a given value, the organic resist film was etched at a high etching rate, and the carbon film was rarely etched.
When ozone or a gas mixture of carbon tetrafluoride and steam was used under the control of substrate temperatures, the organic resist film was etched at a high etching rate, and the carbon film was rarely etched.
In the above cases, Al, silicon, a silicon oxide film, or a metal film was found not to be etched at all.
When the etching conditions are appropriately selected in the down-flow etching apparatus, the resist pattern can be etched at high speed. In addition, the carbon film, and a target material to be etched (e.g., Al, silicon, a silicon oxide film, or a metal film) can be removed at a high selection ratio.
As described above, according to the method of the present invention, since a carbon film is used as a dry etching mask pattern, it is possible to increase the temperature of the target substrate and perform etching at high temperatures. In addition, after etching, the target substrate is annealed at a high temperature before the mask pattern is removed. The side wall of the silicon oxide can be vertically etched, and a high selection ratio to the underlying Si film can be obtained. Furthermore, a pattern having a vertical side wall can be formed without producing residual substances for a material (e.g., copper) having a very low vapor pressure of an etching product.
A material such as tungsten, nickel, titanium, tantalum, a tantalum oxide, strontium titanate, an aluminum oxide, or an aluminum nitride can be accurately etched at a high etching rate with good planar uniformity.
Since high-temperature annealing is performed after etching using a halogen gas is performed but before the mask pattern is removed, degradation of electrical characteristics such as an increase in contact resistance rarely occurs, and a highly reliable device can be manufactured.
EXAMPLE 7
FIG. 15 is a schematic view showing an apparatus used in Example 7. Reference numeral 101 denotes a reaction chamber. A heater 102 is arranged inside the reaction chamber 101. A target object 103 is placed on the heater 102. A supply pipe 104 is connected to the reaction chamber 101 to supply an active spices containing oxygen atoms. A gas containing oxygen atoms or a gas mixture consisting of a gas containing oxygen atoms and a gas containing halogen atoms is supplied from the supply pipe 104, and a discharge tube 106 is connected to a microwave power source 105 and the supply pipe 104 to generate the active species. The reaction chamber 104 is evacuated to a vacuum through an exhaust port 107. A gas mixture of oxygen and CF.sub.4 is supplied from the supply pipe 104 in the apparatus (FIG. 15) having the above arrangement to perform etching. The introduced gas is discharged to form neutral radicals, that is 0* in the case of oxygen gas, and 0*+F* in the case of a gas mixture of O.sub.2 and CF.sub.4. These neutral radicals are conveyed to the reaction chamber isolated from plasma, where the substrate is etched. Numeral 8 denotes a wave guide for microwave.
The target object 103 to be etched was an object obtained such that a carbon film (thickness: 20 .mu.m) was formed on an Si substrate or an AlSiCu film formed on an SiO.sub.2 formed on an Si substrate, a photoresist (thickness: 1.6 .mu.m) was applied to or coated on the carbon film, and a photoresist pattern was formed using conventional lithographic techniques.
In order to check the etched shape, the carbon film was anisotropically etched by reactive ion etching using oxygen gas as an etching gas and the above photoresist pattern as a mask.
FIG. 16 shows a relationship between etching rates of a resist and a carbon film at a substrate temperature of 25.degree. C. when a mixing ratio of oxygen gas and CF.sub.4 gas is changed.
When only oxygen gas was used as an etching gas, neither the resist nor the carbon film were etched. When a small amount of CF.sub.4 gas was mixed in the oxygen gas, the etching rate of the resist was increased. When the flow rates of CF.sub.4 and O.sub.2 were 20 SCCM and 480 SCCM, respectively, the etching rate of the resist was 1,000 .ANG./min, and the etching rate of the carbon film was 16 .ANG./min, so that a high selection ratio of 600 was obtained. At this time, the shape of the carbon film was evaluated. The carbon film pattern was not etched at all, and the resist pattern on the carbon film could be perfectly removed.
FIG. 17 shows a relationship between a target object temperature and etching rates of a carbon film and a resist when oxygen gas is used as an etching gas. As is apparent from FIG. 17, when only oxygen gas or a gas mixture of oxygen and CF.sub.4 was used as an etching gas, etching of the carbon film was started from about 100.degree. C., and the etching rate of the carbon film was increased with an increase in temperature of the target object. On the other hand, when only O.sub.2 gas is used, etching of the resist was started from about 50.degree. C. Further, when a gas mixture of O.sub.2 and CF.sub.4 is used, etching of the resist was started from a room temperature (20.degree. C.). Since the carbon film was not heated to 100.degree. C. or more, it was not etched. In down-flow etching using O.sub.2 gas, or a gas mixture of a gas containing oxygen atoms and a gas containing halogen atoms such as a gas mixture of O.sub.2 and CF.sub.4 at a constant substrate temperature of 100.degree. C. or less, only the resist on the carbon film could be removed.
The sectional shape of the target object upon removal of the resist pattern by using the gas mixture of oxygen and CF.sub.4 at a constant substrate temperature of 100.degree. C. was evaluated with an SEM. A carbon film pattern was rarely etched, and the resist pattern on the carbon film was perfectly removed.
EXAMPLE 8
FIG. 18 is a schematic view of an apparatus used for etching a target object by reactive ion etching, i.e., down-flow etching of Example 8, using a carbon film mask pattern obtained such that a resist is selectively removed from a carbon film to form the carbon film mask pattern.
The apparatus shown in FIG. 18 is obtained by connecting a magnetron build-in reactive ion etching apparatus to the apparatus of FIG. 15.
A dry etching apparatus of this example will be described with reference to FIG. 18.
Reference numeral 111 denotes a reaction chamber. A heater 112 is arranged in the reaction chamber 111 to heat a target object, and a target object 113 is placed on the heater 112. A gas containing oxygen atoms or a gas mixture consisting of the gas containing oxygen atoms and a gas containing halogen atoms is supplied from a pipe 114 for supplying an active species containing oxygen atoms to the reaction chamber, and a discharge tube 116 connected to a microwave power source 115 and the supply pipe 114 generates the active species. The reaction chamber 111 can be evacuated to a vacuum from an exhaust port 117.
This apparatus is constituted by an etching chamber 120 and an unloading preliminary chamber 140, and the etching chamber 120, the reaction chamber 111, and the unloading preliminary chamber 140 are partitioned by gate valves 131 and 141, respectively. The etching chamber 120 is kept evacuated, a target substrate is loaded from a gate valve 132 arranged in a loading preliminary chamber 130, and the target substrate is unloaded from a gate valve 142 arranged in the unloading preliminary chamber 140, thereby preventing the substrate from being adversely affected by, i.e., humidity or oxygen in the air. In addition, substrate mounting tables 133 and 143 are arranged in the preliminary chambers 130 and 140, respectively.
The etching chamber 120 comprises a first electrode 122 for mounting a target substrate 121 arranged in a vacuum vessel 120a, a high-frequency power source 124 connected through a blocking capacitor 129 for applying a 13.56-MHz high-frequency power to the first electrode 122, and a cooling pipe 125 for cooling the first electrode 122 and controlling the substrate temperature of the target substrate 121 at a predetermined temperature.
While Cl.sub.2, BCl.sub.3, HBr, O.sub.2, H.sub.2, and He (Ar or Kr) are supplied in the vacuum vessel 120a from a chlorine gas (Cl.sub.2) supply line 128a, a boron trichloride (BCl.sub.3) supply line 128b, a hydrogen bromide (HBr) supply line 128c, an oxygen gas (O.sub.2) supply line 128d, an inert gas (He, Ar, or Kr) supply line 128e, a hydrogen gas (H.sub.2) supply line 128f, and a carbon monoxide gas (CO) supply line 128g, respectively, a RF voltage is applied across the first electrode 122 and the inner wall (upper wall) of the vacuum vessel 120a serving as a second electrode.
At this time, the vacuum vessel 120a is grounded. The gas supply lines 128a to 128g have valves and flow rate adjustors 129a to 129g, respectively, so as to control their flow rates and gas pressures at predetermined values, respectively.
A permanent magnet 126 is arranged above the second electrode portion of the vacuum vessel 120a and eccentrically rotated about a rotating shaft 127 by a motor. A high-density plasma can be produced and maintained by a magnetic field of 50 to 500 gauss generated from the permanent magnet 126 even in a high degree of vacuum of 10.sup.-3 Torr or more. A large amount of ions are extracted from the high-density plasma produced as described above to be radiated on the target substrate 121, thereby etching the target substrate 121.
Etching processes of samples having the following structures were performed using the above dry etching apparatus. The steps in this process are the same as those in FIGS. 4A to 4H and will be described with reference to FIGS. 4A to 4H below.
As shown in FIG. 4A, an SiO.sub.2 film was formed on an Si substrate 41, and an AlSiCu (Si concentration: 1 wt %; Cu concentration: 0.5 wt %) thin film 43 was formed on the SiO.sub.2 film 42. A carbon film 44 (thickness: 200 nm) was formed on the thin film 43, as shown in FIG. 4B.
As shown in FIG. 4C, a photoresist (thickness: 1.6 .mu.m) was applied to the photoresist 45 on the carbon film 44. The resist 45 was exposed in accordance with the conventional lithographic techniques. As shown in FIG. 4D, the resist 45 was developed to form a resist pattern 45a. In the step of FIG. 4D, an alkaline organic solvent was used as a developing agent. No problem such as elusion or peeling did not occur in the carbon film 44a during development.
Each etching sample was placed on the same table 121 in the dry etching reaction chamber 120 through the gate valves 132 and 131 in the apparatus shown in FIG. 18.
As shown in FIG. 4E, the carbon film 44 was etched using O.sub.2 gas at a constant substrate temperature of -75.degree. C. As etching conditions, the flow rate of O.sub.2 gas was set to be 100 SCCM, the pressure was set to be 40 mTorr, and an RF power of 1.7 W/cm.sup.2 was applied. The shape of the etched carbon film 44a was observed with an SEM. It was found that the carbon film 44 had almost vertical wall surfaces.
As shown in FIG. 4F, the resist pattern 45a was removed. That is, after the carbon film 44 was etched, the sample was loaded in the down-flow etching reaction chamber 111 in a vacuum through the gate valve 131 and was placed on the sample table 112. A CF.sub.4 O.sub.2 gas mixture was used as the etching gas. The flow rate ratio of the CF.sub.4 /O.sub.2 gas mixture was kept at 20/480 SCCM, and the pressure was kept at 0.3 Torr. The temperature was kept at room temperature, and a microwave was applied to generate an electric discharge, thereby performing etching. As shown in FIG. 4F, the resist pattern was perfectly removed without any residue.
As shown in FIG. 4F, the AlSiCu film 43 was selectively etched using the carbon film 44a as an etching mask. The above dry etching apparatus was used to etch the AlSiCu film 43. That is, the sample was loaded in the dry etching reaction chamber 120 in a vacuum through the gate valve 131 (FIG. 18) again and was placed on the sample table 121.
As etching conditions, the substrate temperature was kept at 50.degree. C., a Cl.sub.2 /BCl.sub.3 gas mixture (flow rate: 100 SCCM) was used as an etching gas, and CO (flow rate: 20 SCCM) was used as a deposition gas.
An etching pressure was 2.0 Pa, and an RF power density was 0.8 W/cm.sup.2 to etch the AlSiCu film 43.
After etching, the sample was loaded in or unloaded from the unloading preliminary chamber 140 through the gate valve 142.
The etched AlSiCu film pattern 43 was observed with an SEM. As shown in FIG. 4G, a 0.4-.mu.m L/S (line and space) pattern was accurately etched.
Finally, as shown in FIG. 4H, the carbon film pattern 44a was etched. A conventional barrel type plasma etching apparatus was used as an etching apparatus, O.sub.2 was used as an etching gas, and plasma ashing was performed. It was confirmed that the carbon film pattern 44a was easily removed.
After removal, the AlSiCu pattern 43a was evaluated with an SEM. Even if a gas mixture consisting of Cl.sub.2 and BCl.sub.3 or a gas mixture consisting of Cl.sub.2 and HBr was used, an AlSiCu film having an accurate 0.4-.mu.m/0.4-.mu.m L/S pattern having a tapered shape was found to be formed.
No residue was found on the surface of the AlSiCu film pattern 43a.
The carbon film was dry-etched to obtain a carbon film pattern by using a resist mask and a gas such as H.sub.2, an inert gas, or a mixture thereof. Even if any one of the gases was used, the carbon films could be accurately patterned. AlSiCu film patterns 53a obtained by these processes have good shapes without any residue.
An AlSiCu/TiN/Ti/SiO.sub.2 thin structure was used as an etching sample and wa etched by adding CO. After etching, the sample from which the carbon film pattern was removed was left to stand in air, and the state of corrosion was evaluated with an optical microscope. No corrosion was observed even if the sample was kept left to stand in air for a week.
In this case, a gas used to remove the resist may be a gas mixture consisting of a ga containing oxygen atoms and a gas containing halogen atoms. This etching gas is not limited to CF.sub.4, but can be replaced with SF.sub.6, FCl, NF.sub.3, C.sub.2 H.sub.6, C.sub.3 F.sub.8, BF.sub.3, X.sub.2 F.sub.6, or F.sub.2.
A means for etching a resist is a method of preventing etching of the carbon film or a method of controlling a substrate temperature. The former method uses a gas mixture consisting of a gas containing steam or at least hydrogen atoms in an active species having halogen atoms.
In Example 8 described above, when the carbon film pattern 44a was removed using the barrel type ashing apparatus upon etching of the AlSiCu film 43. However, when a function of removing the carbon film pattern 44a is provided in the dry etching apparatus shown in FIG. 18, the carbon film pattern 44a can be removed in a vacuum apparatus without exposing it in air.
The method of the present invention described above is not limited to the examples described above. Various changes and modifications may be made without departing the spirit and scope of the invention.
According to the present invention, as has been described above, a resist film can be removed at high speed and a high selection ratio to the carbon film without damaging the target object. In addition, the target object can be patterned at a high selection ratio by using a carbon film pattern mask from which the resist film is removed.
Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices, and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.

Number	Date	Country
3-021569	Jan 1991	JPX
3-211302	Jul 1991	JPX
4-004197	Jan 1992	JPX
4-004198	Jan 1992	JPX

Number	Name	Date
4620898	Banks et al.	Nov 1986
4904338	Kozicki	Feb 1990
5022959	Itoh et al.	Jun 1991
5102498	Itoh et al.	Apr 1992

Number	Date	Country
58-212136	Dec 1983	JPX
62-136025	Jun 1987	JPX
62-136820	Jun 1987	JPX
63-76438	Apr 1988	JPX

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