Process for making compound films

BACKGROUND OF THE INVENTION
1. Technical Field
The present invention is directed to a process and apparatus for depositing thin films made of a compound material formed from at least two gaseous reactants, wherein the interaction of the reactants must be controlled in order to obtain a film of the desired quality.
2. Art Background
Compound films, i.e. films of materials formed from at least two different elements such as silicon nitride (Si.sub.3 N.sub.4), silicon dioxide (SiO.sub.2), aluminum oxide (Al.sub.2 O.sub.3), aluminum nitride (AlN) and titanium oxide (TiO.sub.2) are formed by introducing reactive gases into a chamber. The reactive gases are precursor gases for films of the desired material. Since the gases react to form the desired compound material, the reactions must be controlled in order to obtain a film of the desired material.
Compound films are formed using techniques such as Chemical Vapor Deposition (CVD). In CVD, a non-volatile solid film is formed on a substrate by a surface-pyrolized reaction of the gaseous reagents. A typical CVD reaction process comprises the following steps, (1) gaseous reagent and inert carrier gas are introduced into the reaction chamber, (2) gaseous reagent is transported by convection and diffusion to the surface of the substrate, (3) reagent species are absorbed onto the substrate where they undergo migration and film forming reactions and (4) gaseous byproducts of the reaction and unused reagents are removed from the chamber. The pressure in the deposition chamber may be atmospheric or reduced as low as a fraction of 1 Torr, as in the respective cases of Atmospheric Pressure CVD (APCVD) or Low Pressure CVD (LPCVD). The energy required to drive the reaction is supplied as heat to the substrate. For practical reaction rates, substrates are typically heated to temperatures ranging from about 500.degree. C. to as high as 1600.degree. C.
If the energy for the reaction is supplied by an RF electric field which powers a gas plasma discharge in the deposition chamber near the substrate surface, then the substrate temperature need not be as high. In such processes (referred to as Plasma Enhanced CVD (PECVD), the substrate temperature is 300.degree. C. or less. However, in PECVD, the substrate and the film formed thereon are immersed in the plasma discharge, which will potentially damage the substrate and the film during growth. Other disadvantages of CVD processes include the reaction and nucleation of the reactants in the gas phase. When growing compound films, the reaction between the reagent gases must occur in the film to obtain a film with the desired uniformity. If the reaction occurs in the gas phase, the reaction products precipitate onto the substrate surface and contaminate the growing film.
Downstream plasma CVD processing has been employed to avoid the problems associated with permitting the plasma to contact the substrate. As its name implies, in downstream plasma processing, the substrate is downstream of the plasma (including the CVD reagent gas). Physical Vapor Deposition (PVD) has also been utilized. PVD includes methods of evaporation (metallizing), sputtering, molecular beam epitaxy, and vapor phase epitaxy. PVD processes typically occur in a chamber evacuated to a pressure of less than 10.sup.-6 Torr. The material from which the film is formed is present in the chamber in bulk solid form. The material is converted from the solid, condensed phase to the vapor phase using thermal energy (i.e. evaporation) or momentum transfer (i.e. sputtering). The atoms or molecules of the material condense on the substrate (and the chamber walls) as a thin film. If the pressure becomes too high, the molecules or atoms start to collide with a frequency that reduces the deposition rate.
Reactive evaporation and sputtering processes involve the intentional introduction of oxygen, nitrogen, or other reactive gas to form a thin film of an oxide, nitride or other compound material. In such processes, the pressure must be carefully controlled to maintain an effective environment for deposition. If the pressure is too high, the atoms or molecules will react in the gas phase. Furthermore, the source of the reactive atoms or molecules is subject to contamination by the reactant gases if it is allowed to come into contact with these gases.
U.S. Pat. No. 5,356,672 to Schmitt, III et al. describes a method for forming a thin film of a compound material on a substrate. The compound film is formed by the interaction between a first reagent gas and a second reagent gas. The interaction is controlled by translating a substrate from a first position where it is subjected to the discharge from a first gas jet apparatus to a second position where it is subjected to the discharge from a second gas jet apparatus. In one embodiment, the discharge from the first gas jet apparatus contains a mixture of one reactive species and a carrier gas. The discharge from the second gas jet apparatus contains a mixture of a second reactive species and a carrier gas. This method requires the substrate to be moved from the first position to the second position in a time less than the time required to complete film formation and it also requires that the wafer be moved frequently from the first position to the second position. In a second embodiment, the discharge from the gas jet apparatus contains a mixture of the first and second reactive species and the substrate is scanned or otherwise moved to subject all portions of the substrate to the discharge.
Such methods require that the substrate be subjected to background gases in the chamber as it is moved. The background gases typically contain molecular or atomic species that compete with the desired interaction between the first and the second gases. These competing interactions, if they occur to any significant extent, will have an undesired effect on the composition of the film. Furthermore, the requirement for fast and frequent translation is mechanically rigorous. Therefore, a simpler and more effective process solution is required.
SUMMARY OF THE INVENTION
In the process of the present invention, a compound film is formed on a substrate from gases that are discharged from at least one source. The compound film is formed from a combination of reactive species provided from the source and directed onto the substrate. In one embodiment of the invention, the reactive species are provided by reactant gases, which are subjected to plasma discharge mechanism that generates the reactive species from the reactant gases. Other embodiments in which the reactant gases are sufficiently reactive without being subjected to a plasma discharge are also contemplated. The composition of the film is controlled by using inert carrier gas to control the relative amounts of reactive species in the discharge. The substrate is not removed from the purview of either the inert carrier gas, the reactant gas, or combinations thereof while the film is being formed on the substrate.
In the process of the present invention, the source is a chamber that is adapted to receive gas streams through at least two separate ports. A first reactant gas flows into the cavity through the first port and a second reactant gas flows into the cavity through the second port. The first port and the second port are oriented so that the reactant gases combine in the cavity. An illustrative arrangement is one wherein a supply line is in fluid communication with the cavity. This is referred to as an outer nozzle. Inside the supply line is a second supply line with a nozzle at the end thereof. This is referred to as the inner nozzle. The inner nozzle is oriented such that the discharge of the nozzle is placed in the cavity and directed toward the substrate on which the desired compound film is formed. One of the reactant gas streams enters the cavity through the outer nozzle and the other reactant gas stream enters the cavity through the inner nozzle. The cavity is equipped with a plasma generator such as a microwave cavity. A microwave cavity suitable for this purpose is described in U.S. Pat. No. 5,356,672 which is hereby incorporated by reference. The microwave cavity is situated on the nozzle cavity downstream of the points where the individual gas streams enter the nozzle cavity.
In one embodiment of the present invention, the gas stream that flows through the second port is either a mixture of a second reactant gas and an inert carrier gas or pure inert carrier gas. The gas stream that flows through the second port alternates between the second reactant gas/carrier gas mixture and the carrier gas to control the composition of film formed from the reaction of the first reactant gas and the second reactant gas. The two gas streams intermix in the nozzle cavity.
In an alternate embodiment multiple sources are used. The combined discharge of these multiple sources is designed to continuously shower the entire surface of the substrate on which the film is grown during film growth. This embodiment contemplates at least three sources. The discharge from the first source is a mixture of the first reactant and a carrier gas. The discharge from the second source is a mixture of a second reactant and a carrier gas. Carrier gas is discharged from at least one other source. The discharge from these sources is directed at the surface of the substrate so that the surface of the substrate is continuously bathed in the combined discharge during film growth. The combined discharge does not have a uniform composition however. The combined discharge is controlled by positioning the sources and moving the surface of the substrate so that, while the film is being grown on a targeted area of the surface of the substrate, the untargeted portions of the substrate are bathed in a stream of carrier gas which acts as a barrier to the background gases in the chamber. The substrate surface is moved relative to the sources so that every portion of the surface on which the film is grown is within the targeted area at some point in the process.
In the embodiment of the present invention wherein the two reactant gases and an inert carrier gas are all introduced into a single cavity, the flow rates of the three gases (i.e., the first reactant gas, the second reactant gas, and the inert carrier gas) into the cavity are controlled to obtain a film of the desired composition and uniformity. This is based upon the recognition that a reaction does not occur, and a film of the desired composition does not form, when the first reactant gas and the non-reactive carrier gas are the only gases present in the cavity and, consequently, are the only gases in the cavity. This is true even when the plasma generator is operating. This reaction is "controlled" by controlling the flow of the second reactant gas. When film formation is desired, a mixture of all three gases flows from the cavity into the chamber.
The relative flow rates of the first reactant gas, the second reactant gas, and the carrier gas depends upon the kinetics of the reaction between the first and second reactant gases and the kinetics of the formation of the film. For example, if the desired compound film is Si.sub.3 N.sub.4, the flow rate of nitrogen and the silicon precursor gas, SiH.sub.4, are determined relative to the stoichiometry of the desired film. The desired compound has three atoms of silicon for every four atoms of nitrogen. However, in a given volume of N.sub.2 gas there are approximately two atoms of nitrogen for every one atom of silicon in the same volume of SiH.sub.4 at the same pressure. Therefore, to form a film of Si.sub.3 N.sub.4, it is advantageous if the volumetric flow rate of the silane gas relative to the flow rate of the nitrogen is about 3 to about 2.
However, the stoichiometry is not the only factor that determines the relative flow rates of the gases into the inner nozzle. The kinetics of the reaction that forms the compound determine the concentration of the reactant gases in the gas streams flowing into the cavity. Since one reactant typically reacts at a different rate than the second reactant to form the desired compound, the flow of one reactant needs to be suspended periodically to allow the slower reactant to react with any excess of the faster reactant to avoid the formation of undesired compounds through competing reactions. One skilled in the art will appreciate that the relative flow rates of the first and second reactant gases must be determined empirically, because the flow rates depend on the kinetics of the reaction and the stoichiometry of the particular compound film that is desired. The process of the present invention allows further flexibility because the flow of one of the reactant gases is capable of periodic suspension during film formation. Such control enables the formation of a film with the desired stoichiometry (the desired stoichiometry is either a uniform stoichiometry throughout the film thickness or a stoichiometry that varies with the depth in the film) while maintaining the substrate on which the film is being formed in a continuous flow of gas (i.e. a gas stream containing only one of the reactant gases and the non-reactive carrier gas). This ensures that the film being formed is constantly subjected to a protective shower of gas. The flux of gas in this shower directed toward the substrate surface serves to sweep away molecules or atoms in the background gas so that they do not reach the film being formed. The background gas contains water vapor and other molecular and gaseous species that adhere to the wall of the chamber from previous processing, cleaning and cycling to normal room air.
In another embodiment of the invention, gas streams are directed onto the substrate from a plurality of sources. One or more of these sources are contemplated as being equipped with a plasma discharge source, as required by the process for film formation. One skilled in the art will appreciate the flexibility provided by this embodiment, which permits the substrate on which the film is formed to be continuously showered by the discharge from these sources in either a combination of reactant gases and carrier gases or pure inert carrier gas during film formation. In this embodiment, portions of the substrate are restored or otherwise rotated in and out of the purview of the discharge streams that contain the reactant gases. The portions of the substrate that are not directly within the purview of the gas from the source or sources from which the reactant streams emanate are still showered by streams of inert carrier gas during film formation.
The process of the present invention is used to form a compound film that contains at least three elements. In these compound films, the element is an element such as hydrogen (H), deuterium (D) or oxygen (O) which is added to the film to provide the film with additional properties. Since elements such as H and O are also typically found in the background gas and since the amount of these elements in the films must be carefully controlled in order to obtain the desired properties, the present process is advantageous because the compound film is completely isolated from the background gases when being formed. The advantages of introducing hydrogen into a silicon nitride film are described in Lucovsky, G., et al., "Defect Reduction in Remote Plasma Deposited Silicon Nitride By Post-Deposition Rapid Thermal Annealing," Mat. Res. Symp. Proc. Vol. 525, pp. 187-192 (1988).

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic of an apparatus used to practice the process of the present invention.
FIG. 2 is a detail of a portion of the apparatus illustrated in FIG. 2.
FIG. 3 is a schematic of an alternate embodiment of an apparatus used to practice the process of the present invention.

DETAILED DESCRIPTION
The present invention is directed to a process for forming compound films. Compound films are films that are composed of more than one element. The compound films formed by the process of the present invention contain at least three elements.
Compound films are used in a variety of applications. For example, since compound films have useful dielectric properties, they are used as insulating layers in semiconductor devices. Examples of compound films used for this purpose include silicon nitride (Si.sub.3 N.sub.4), silicon dioxide (SiO.sub.2), silicon oxynitride, aluminum oxide (Al.sub.2 O.sub.3), aluminum nitride (AlN), and titanium oxide (TiO.sub.2). The controlled addition of at least one additional element to these compound materials during film formation has been observed to improve the dielectric properties of the resulting films. The additional element is either hydrogen, deuterium, O (for the non-oxide films only), or some combination thereof.
The process of the present invention is also useful for forming compound metal films such as metal silicides (e.g., TiSi.sub.2, MoSi.sub.2, TaSi.sub.2, WSi.sub.2, CoSi.sub.2, NiSi.sub.2, PtSi, and Pt.sub.2 Si), metal nitrides (e.g. TiN, TaN, NbN, ZrN), metal carbides (TiC, TaC), metal oxides (e.g., Ta.sub.2 O.sub.5, Al.sub.2 O.sub.3, SiO.sub.2), and metal borides (TiB.sub.2) for use as conductive layers in semiconductor devices. The controlled addition of an additional element such as H, D, or O to these compounds during film formation has also been observed to improve the electrical properties of these films. Because the properties of such films are highly composition-dependent, processes are required which ensure that the resulting films are not contaminated with species that adversely affect film properties.
Compound films are formed by providing an environment that is favorable for forming the desired compound. In the process of the present invention, the compound film is formed from the interaction of at least two reactants. In one embodiment of the present invention, the reactants are reactive species that are generated by introducing the reactant gas into a plasma generator. Each gas stream contains a reactant gas from which a reactant molecular or atomic species is generated when the gas is subjected to the energy from the plasma-generator. Thus the plasma contains a mixture of compound, molecular, atomic and ionic species. The species then combine to form the desired compound. The reactive environment in which the compound is formed is controlled to favor the kinetics of compound formation over the kinetics for incorporating impurities. Other embodiments are contemplated in which one or more of the reactant gases need not be subjected to a plasma-generator in order to be reactive. In these embodiments a plasma-generator is optionally used to generate additional reactive species.
One source of impurities that compete with the formation of the desired compound is the background gas in the chamber in which the compound film is formed. In the process of the present invention, the substrate is isolated from the chamber background during compound film formation by maintaining the substrate within the purview of the plasma discharge during film formation. The flow of gas from the discharge source to the substrate provides a high flux of directed gas flow which sweeps away background gases that would otherwise diffuse to the surface of the substrate. This reduces the amount of interaction that would otherwise occur between the background gas and the reactive species that form the compound film during film formation.
The process of the present invention is especially advantageous for use when forming compound films in which one of the elements in the compound is also found in the background gas. Specifically, since the film composition must be carefully controlled in order to obtain a film with the desired properties, it is necessary to control the relative amounts of the reactants that form the film. The composition of the background gases cannot be carefully controlled. Since the background gas typically contains elements such as hydrogen and oxygen, it is particularly advantageous to isolate the compound film from background gases during film formation when a controlled amount of an element that is found in the background gas is being incorporated into the compound films.
In the embodiment of the present invention in which the reactant gases are subjected to a plasma-generator that generates the desired reactive species, a plasma is generated in a cavity that communicates with the chamber in which deposition occurs via an opening. The reactants are introduced into the chamber through the cavity so that the reactants are properly directed to the substrate on which the compound film forms. In one embodiment of the present invention, the reactant gases are introduced through separate ports and combine in a single cavity equipped with a plasma generator. For ease of explanation, the present invention will be described in terms of an embodiment in which two reactant gas streams are combined to form the desired compound. One skilled in the art will readily appreciate how to apply the principles of the present invention to those instances where three or more reactant gas streams are combined to form the desired compound.
In order to accommodate the kinetics of film formation, and to maintain gas flow rates and pressures that are high enough to sustain the plasma discharge in the cavity and to suppress plasma formation in the inner nozzle, the reactant gas is combined with an inert carrier gas. The higher flow rates permitted by the presence of the inert carrier gas also keep the walls of the nozzle cooler, which suppresses the dissociation of reagent species thereon. The reactant gases are introduced into the cavity in a manner that provides adequate intermixing of the two gases. In the embodiment wherein the two reactant gases are introduced into a single cavity equipped with a plasma generator, adequate intermixing is achieved by the introduction of one reactant gas into the cavity via a first port and the introduction of the second reactant gas into the cavity via a second port in the form of an inner nozzle which directs the second gas stream directly into the cavity. This arrangement prevents significant intermixing of the reactant gases upstream of the cavity. The gas flow through both the first port and the inner nozzle is commenced before the plasma is struck, and is continuous throughout film formation. The composition of the gas flowing through the inner nozzle is controlled through the use of a manifold which is connected to two separate gas supplies. One of the supplies is pure, inert carrier gas with no reactant gas mixed therein. The second supply is inert carrier gas with a small fraction of reactant gas therein. The flow rates and pressures of the gases in the two manifold supply lines are approximately equal. During film formation, the manifold is switched from the first line to the second line to provide a constant flow rate of gas into the plasma generator. However, film formation only occurs when the plasma contains species from both reactant gases.
Since the kinetics of film formation varies from compound to compound, the present invention is described in term of the embodiment in which the desired compound is Si.sub.3 N.sub.4. In this embodiment, the first reactant feed stream is a mixture of nitrogen and a first non-reactive carrier gas and is introduced into the cavity via the outer nozzle. Through the inner nozzle is directed either a stream of the pure inert carrier gas or a stream of the inert carrier gas mixed with silane, SiH.sub.4, which is the second reactant gas. Both streams are provided to the inner nozzle via a manifold. A first supply line to the manifold connects the manifold with a supply of the pure inert carrier gas. A second supply line connects the manifold with a source for the pure inert carrier gas combined with the second reactant gas. Each supply line is equipped with a valve that controls the flow of the gas therein to the manifold. The backing pressure and conductances of the first and second supply lines are approximately equal and the valves are interconnected so that one shuts off simultaneously as the other turns on. Switching in this manner ensures that there is no significant change in pressure or flow rate in the gas stream exiting the inner nozzle.
Since a particular component of the compound film is only added when the reactant gas providing this component is supplied to the plasma generator, formation of the desired compound film is controlled by controlling the rate and duration of the addition of each component to the reactive gas mixture. As previously described, this enables the film to be formed in a manner that is tailored to the kinetics of film formation for the particular film.
The following example is provided as an illustration of the general concepts outlined above.
EXAMPLE 1
Silicon nitride (Si.sub.3 N.sub.4) compound films were formed on silicon wafers according to the process of the present invention. The films were prepared in the apparatus depicted in FIG. 1. A silicon wafer 10 was placed onto a holder 12 and loaded into a plasma deposition chamber 14 using standard equipment and procedures well known to those skilled in the art. The silicon wafer was positioned so that the surface on which the compound film is formed directly faced a source 16 equipped with a dual gas nozzle. The silicon wafer 10 was positioned about 3.5 cm from the source 16. The dual gas nozzle source 16 is illustrated in detail in FIG. 2. The dual gas nozzle source 16 had a first outer nozzle 28 that was standard quartz tubing (10 mm ID and 12 mm OD) affixed to a quartz disc 30 with a 10 mm hole 32 drilled therein. The outer nozzle 28 was attached to the disc so that it was centered on the hole 32. The inner nozzle 34 was standard quartz tubing (3 mm ID and 5 mm OD) centered in the outer gas hole. The tubing had an exit orifice 36 (0.4 mm diameter) which directed the gas flow toward the wafer surface 10 (in FIG. 1). Both the outer nozzle 28 and the inner nozzle 34 had ports (38 and 40, respectively) through which reagent gases enter the nozzles. The outer nozzle 28 was equipped with another port 42 for measuring the pressure in the nozzle.
The chamber 14 (FIG. 1) was evacuated to a pressure in the range of about 10 mTorr to about 100 mTorr using a 140 l/sec roots blower 18. Once the chamber pressure was in this range, gas flow was commenced through the dual nozzle source 16. The reactant gases from which the Si.sub.3 N.sub.4 films were formed were nitrogen (N.sub.2) and silane (SiH.sub.4). Both reactant gases were diluted with a non-reactive carrier gas. Helium gas (He) was used for this purpose.
The dual nozzle source 16 contained an inner chamber 20 into which the two gas streams flowed. The dual nozzle source 16 was equipped with a quartz flange 30 which was used to fix the dual nozzle source 16 to the chamber 14 in which the wafer 10 was placed. The silane gas, diluted with helium, flowed through the inner nozzle 34 at a pressure of about 600 Torr. The nitrogen gas, also diluted with helium, flowed into the nozzle chamber through outer nozzle 28. The pressure in the outer nozzle was about 2 Torr. A pressure gauge interfaced to a pressure controller 26 was used to regulate the pressure of the gas to the outer nozzle 28. A similar pressure gauge interfaced to a pressure controller (not shown) was used to regulate the pressure of the silane gas/carrier gas mixture and pure carrier gas in the inner nozzle 34.
The gas stream flowing through the outer nozzle 28 was about 10 volume percent nitrogen diluted with helium. The flow rate of the dilute nitrogen gas was about 400 sccm at a pressure of 2 Torr in the outer nozzle. The gas stream flowing through the inner nozzle 34 was initially pure helium. The flow rate of the helium was about 2300 sccm at a pressure of 600 Torr in the inner nozzle.
The dual nozzle source 16 was equipped with a microwave cavity 24. The cavity 24 surrounded a one-inch length of the dual nozzle source 16. The microwave cavity 24 was positioned about 2.8 cm downstream from the inner nozzle 34. While the nitrogen/helium mixture and the pure helium were flowing through the outer and inner nozzles, respectively, the microwave discharge was commenced. The microwave cavity used was an Evenson style cavity such as the one described in U.S. Pat. No. 5,356,672. Absorbed microwave powers ranging from about 200 W to about 400 using a Micro-Now Model #420B1 Microwave Power oscillator to generate 2.455 GHz microwaves. The oscillator was connected to the microwave cavity via a coaxial cable. The microwave discharge was stabilized for about five minutes before the silane flow was commenced.
The effect of the microwave power used on the refractive index of the resulting film is reported in Table 1 below. The refractive index of the resulting films was measured using ellipsometry. For silicon nitride films, the refractive index indicates the stoichiometry of the film. Stoichiometric films of Si.sub.3 N.sub.4 have a refractive index of 2.01. If the measured refractive index varies substantially from this value, that fact indicates that the film does not have the desired stoichiometry. If the refractive index is lower than 2.01, that fact indicates that the film also contains silicon dioxide (SiO.sub.2). The presence of SiO.sub.2 in the film will result if there are oxygen-containing species present in the chamber during film formation. In the system used in the present example, the species appears to be molecular oxygen (O.sub.2) or water (H.sub.2 O). The presence of non-silicon/oxygen/nitrogen/hydrogen species was monitored and, due to their very low concentration, determined to be unlikely sources for the decreased refractive index.
If the refractive index is significantly higher than 2.01, this indicates that the film contains unreacted silicon. Since unreacted silicon will react with oxygen to form SiO.sub.2 when the film is exposed to air, the presence of unreacted silicon in the film is highly undesirable.
TABLE 1______________________________________Microwave Film ThicknessPower T.sub.L (O) Refractive IndexAbsorbed (Watts) (Angstroms) N.sub.L______________________________________400 668 1.982330 674 1.957255 617 1.938185 576 1.932______________________________________
The data in Table 1 illustrates how the index of refraction (N.sub.L) changed as the microwave power that was absorbed by the plasma gas source was varied. The 10% N.sub.2 /He and 23.4 ppm SiH.sub.4 /He flows were maintained at 377 sccm and 2090 sccm, respectively, for each film. Also, each film was grown for 30 minutes using an alternating gas duty cycle of 25% (i.e. the inner nozzle alternated between having SiH.sub.4 /He flowing for 1.2 seconds and helium flowing for 3.6 seconds). The refractive index increased with increasing microwave power. The relationship between an increase in absorbed microwave power and an increase in refractive index is not linear, however. For example, the 70 Watt increase in the absorbed power from 185 Watts to 255 Watts provided a 0.3% in N.sub.L whereas the 70 Watt increase from 330 Watts to 400 Watts provided a 1.3% increase in N.sub.L.
A gas manifold 46 was provided through which the silane gas/inert carrier gas mixture and the pure inert carrier gas are supplied to the inner nozzle 34. The pressure gauge interfaced to a pressure controller 26 regulated the flow of gas from the manifold 46 to the inner nozzle 34. The silane gas mixture flowed into the manifold via line 48 and the pure inert carrier gas flows into the manifold via line 40. Lines 48 and 40 were equipped with valves 50 and 52, respectively, which regulate the flow of the individual gas streams into the manifold 46. Although bellows-sealed electropneumatic valves which have a nominal opening or closing time of less than one second were found to be suitable for valves 52 and 50, one skilled in the art will recognize that a variety of control valves are suitable for this purpose. The valves 52 and 50 are operated such that one valve is closed while the other valve is opened. As a result of this simultaneous operation, little or no change in pressure or flow rate occurred when switching between the two gases. Another embodiment of the present invention is contemplated in which the pure inert carrier gas flows continuously into the manifold 46 and the reactive gas is pulsed into the manifold to obtain the desired the amount of silane gas in the feed to the nozzle 16. However, it is more difficult to maintain a constant pressure and flow rate in this embodiment.
The effect of flow rate of the 10% N.sub.2 /He reagent on film quality was also observed. Films were grown under conditions of constant microwave power (400 Watts absorbed), constant 23.4 ppm SiH.sub.4 /He flow (2250 sccm), and a 25% alternating gas duty cycle. However, for this set of films, the total deposition time was adjusted in order to obtain films of approximately equal thickness. The data in Table 2 summarizes the effect that changing the 10% N.sub.2 /He flow rate has on film quality. The results are reported in Table 2 below.
TABLE 2______________________________________10% N.sub.2 /He Deposition ThicknessFlow Time (.ANG.) N.sub.L______________________________________821 sccm 15 minutes 787 2.008582 sccm 17 minutes 743 1.990382 sccm 20 minutes 758 1.981228 sccm 25 minutes 808 1.974______________________________________
Table 2 illustrates that the refractive index of the films increased from 1.974 to 2.008 as the flow rate of the 10% N.sub.2 /He gas stream increased from 228 sccm to 821 sccm.
The effect of the SiH.sub.4 /He flow rate on the refractive index of the film was also examined. A 25% alternating gas duty cycle was used to control the SiH.sub.4 /He flow rate as described above. The silane concentration was 22.7 ppm, the 10% N.sub.2 /He flow rate was maintained at 325 sccm, and the absorbed microwave power was maintained at 400 Watts. Table 3 below illustrates that, when the SiH.sub.4 /He flow was reduced from 2330 sccm to 2030 sccm, the refractive index remained relatively constant (1.932 at 2330 sccm and 1.934 at 2030 sccm). However, when the flow was reduced to 1720 sccm and 1370 sccm, small decreases in the refractive index (1.924 at 1720 sccm and 1.918 at 1370 sccm) were observed. A film with a refractive index of 1.924 has an oxygen content which is about eight percent of the atomic fraction of nitrogen (atomic ratio of O/N=0.08) and a film with a refractive index of 1.918 has an oxygen content of about 10% of the atomic fraction of nitrogen. Thus, a decrease in the flow rate of the reactant gas permits the incorporation of greater amounts of oxygen (and, presumably, other elements in the background gases in the film).
TABLE 3______________________________________ Deposition T.sub.L (O)SiH.sub.4 /He Flow Time (.ANG.) N.sub.L______________________________________2330 sccm 15 minutes 362 1.9322030 sccm 18 minutes 385 1.9341720 sccm 21 minutes 343 1.9241370 sccm 25 minutes 282 1.918______________________________________
Based on the pressures maintained in the inner nozzle (600 Torr) and outer nozzle (2 Torr) specified above, flow rates of 2300 sccm to the inner nozzle and 400 sccm to the outer nozzle, the following objectives were met when the pressure in the chamber was maintained in the range of 300 mTorr to 400 mTorr:
1. the plasma was sustained in the outer nozzle 28;
2. the pressure in the inner nozzle suppressed the formation of a plasma in the inner nozzle 34;
3. the chamber pressure was sufficiently low to provide an environment that did not favor the recombination or the reaction of the dissociated molecular and atomic species in the plasma;
4. the pressure and flow rates of the gases flowing through both the inner and outer nozzles were adequate to suppress the dissociation of the reagent gases on the walls of the nozzles;
5. the flow rates of the reagent gases are adequate to provide a reasonable rate of film formation; and
6. the flow rates did not exceed the available pumping so that the desired background pressures are maintained relative to the dimensions of the inner and outer nozzles.
Although the conditions used in this example were adequate to provide an environment in which these objectives are met, it will be readily apparent to one skilled in the art that there are numerous conditions that are useful to achieve these objectives.
The gas stream introduced through outer nozzle 28 contained 10 percent by volume nitrogen diluted with helium. The gas stream from the manifold 46 through the inner nozzle 34 was either pure helium, or helium mixed with silane gas. Several films were prepared and the concentration of the silane gas in the feed stream was varied from film to film. The duty cycle was also varied. The duty cycle is defined as the percent of time that the silane/helium gas mixture flowed to the inner nozzle as a function of total time that gas flowed to the inner nozzle.
Before film growth commenced, the dual nozzle source was stabilized. To accomplish this, gas flow was maintained to the cavity through both the inner and outer nozzles while sustaining the microwave plasma discharge. Reaction was avoided by flowing helium through the inner nozzle. This condition was maintained for about five minutes to stabilize the plasma.
Table 4 below summarizes the properties of the films that were made according to the above-described procedure. In addition, two additional films were prepared wherein the flow of both reagent gases into the microwave cavity was continuous (i.e. the duty cycle for the SiH.sub.4 /He mixture was 100 percent. The substrate on which these films were formed were Si(100) wafers which were nominally at room temperature during film growth. The composition (atomic ratio of oxygen/nitrogen) of the film was measured by Rutherford Backscattering, which is a film diagnostic technique well known to those skilled in the art.
TABLE 4______________________________________Silane Film Film Atomiccontent in Deposition Thick- Ratio offeed gas SiH.sub.4 /He Time ness Refractive Oxygen/(ppm) Duty Cycle (min) (.ANG.) Index* Nitrogen**______________________________________15 100% 10 1060 1.933 0.094 .+-. 0.00215 50% 20 1150 1.967 0.05 .+-. 0.0115 25% 40 850 2.002 0.007 .+-. 0.00215 10% 100 1024 2.041 0.011 .+-. 0.0077.6 100% 22 1100 1.949 0.10 .+-. 0.02______________________________________ *Measurement Made Using Ellipsometry **Measurement Made Using Rutherford Backscattering
Table 4 demonstrates that the duty cycle affected the amount of oxygen in the film. For a silane concentration of 15 percent in the feed gas, the refractive index of the resulting film increased from a low of 1.93 when a duty cycle of 100 percent was used to the desired index when a duty cycle of 10 to 25 percent was used. The refractive index of the film formed using a duty cycle of 50 percent, while a significant improvement over the index of the film formed using a duty cycle of 100 percent, was still less than preferred.
Another effect observed from the data in Table 4 was that cycling the flow of the silane reagent from on to off provides a film with a more desirable refractive index than simply reducing the concentration of silane in a feed stream that flows continuously to the cavity during film formation. This is observed by noting that the refractive index of the film formed from a feed stream with a silane concentration of 15 ppm and a duty cycle of 100 percent was 1.933. When the silane concentration in the feed stream was reduced to 7.6 ppm, but the duty cycle remained 100 percent; the refractive index of the resulting film was 1.949. However, when the concentration of the feed stream was maintained at 15 ppm and the duty cycle changed to 50 percent, the refractive index of the resulting film was 1.967. Consequently, in the context of the data in Table 4, halving the SiH.sub.4 /He duty cycle from 100 percent to 50 percent provided a larger increase in the refractive index of the film than halving the concentration of the SiH.sub.4 /He in the feedstream from 15 ppm to 7.3 ppm.
FIG. 3 illustrates an alternate embodiment of the present invention in which a plurality of sources are arranged such that the surface of the substrate on which the compound film is grown is continuously bathed in the discharge from these sources during film growth. In the embodiment depicted in FIG. 3, three sources, 310, 315, and 320, are affixed to a flange 330. Nozzles 310 and 315 are equipped with microwave cavities 335 and 340 respectively. Nozzle 320 is not equipped with a microwave cavity. An inert carrier gas is introduced through nozzle 320. The inert carrier gas serves to isolate the growing film from the background gases during film formation.
One skilled in the art will appreciate that the embodiment depicted in FIG. 3 can be modified in many different ways and still remain within the scope of the present invention. For example, the nozzles can be the dual nozzle type depicted in FIG. 2. Also, there can be more than the three nozzles depicted in FIG. 3. Furthermore, if certain reactants are used, the source for the reactant gas will not have to be equipped with a microwave source to generate the reactant species.
EXAMPLE 2
Silicon nitride films formed from the process of the present invention is contrasted with silicon nitride films formed by a process in which the growing film is contacted by background gas. This comparison demonstrates that, unless the growing film is isolated from the background gases in the manner prescribed by the present process (i.e. by maintaining the growing film within the purview of the reactant and carrier gases during film formation) the properties of the resulting film are adversely affected.
An apparatus was constructed that permitted a wafer to be moved from a first position to a second position. The apparatus had a wafer holder and, when a wafer was placed in the holder, the wafer was positioned at the source gas outlet in position one and was positioned in the background gas in position two. The center of the wafer was offset from the source outlets by 4.6 cm. The apparatus was also equipped with a stepper motor and a Geneva mechanism. By rotating this mechanism by 180.degree., the center of the growing film was 3 inches removed from the center of the source outlet. A barrier was employed to further isolate the growing film from the source. The barrier was placed so that it was separated from the wafer by about two to three millimeters and divided the wafer in half.
The following experiments were performed to determine if exposing the films to background gases, by rotating the film out of the direct purview of the source, affects the film quality. A spot on the wafer was exposed to the nitrogen/helium flow plus the silane/helium mix (through the inner nozzle) for 1.2 seconds, after which the wafer was continuously rotated 360.degree. (time required=7.0 seconds). When the film returned to the output of the source, it was then nitrided (i.e. only pure helium flows through inner nozzle) for 6.9 seconds. This experiment was done with and without the barrier in place. The control for these experiments was exposing a spot on a wafer to the nitrogen/helium flow plus the silane/helium mix for 1.2 seconds, immediately followed by the nitriding step for 6.9 seconds. Table 5 indicates that the resulting films all had approximately the same refractive index and, therefore, roughly the same composition. The refractive index of these films was measured using ellipsometry.
TABLE 5______________________________________ Fraction of Time Film Thickness Out of Purview ofFilm (.ANG.).sup.1 Refractive Index.sup.2 Source Gases______________________________________A 762 1.995 0.5.sup.3B 775 2.008 0.5.sup.4C 809 1.998 0.sup.5______________________________________ .sup.1 Measured by ellipsometry and Rutherford Backscattering .sup.2 Measured using Rutherford Backscattering ellipsometry .sup.3 Without barrier, portions of wafer were rotated from purview of gases; time measured as a fraction of total seconds some portion of wafer was exposed to source gas. .sup.4 With barrier, portion of wafer were rotated from purview of gases; time measured as a fraction of total seconds some portion of wafer was exposed to source gas. .sup.5 Without barrier; entire wafer was maintained in purview of gases during entire time the wafer was exposed to source gas.
The timing was then altered so that a spot on the wafer was again exposed to the nitrogen/helium flow plus the silane/helium mix for 1.5 seconds. In the next step, the wafer was rotated away from the source to the other side of the barrier for 38 seconds. Then the film was rotated back to its original position in front of the source where it was nitrided for 11 seconds and the cycle was then repeated. Table 6 describes the refractive index and the film thickness for this film, compared to the film thickness and the refractive index of a film that was formed without rotating the wafer from the purview of the incoming gases. The difference (2.4%) in the average refractive index between the film grown during rotation and the control film (grown under the previously described conditions) illustrates that exposure to background gases during film formation adversely affects the resulting film. Although applicants do not wish to be held to a particular theory, applicants believe that the decrease in refractive index is the result of increased exposure to the background gases, which permits increased amounts of oxygen to be incorporated into the film during formation.
TABLE 6______________________________________ Fraction of Time Film Thickness Out of Purview ofFilm (.ANG.).sup.1 Refractive Index.sup.2 Source Gases______________________________________D 330 1.940 0.25.sup.3E 425 1.990 0.sup.4______________________________________ .sup.1 Measured by ellipsometry and Rutherford Backscattering .sup.2 Measured using Rutherford Backscattering ellipsometry .sup.3 With barrier, portion of wafer were rotated from purview of gases; time measured as a fraction of total seconds some portion of wafer was exposed to source gas. .sup.4 Without barrier; entire wafer was maintained in purview of gases during entire time the wafer was exposed to source gas.
It should be noted that the partition does not completely isolate one portion of the wafer from the source gases because of the space between the partition and the substrate. Therefore, one would expect the film to be more adversely affected by exposure to background gas if it were rotated into a chamber entirely isolated from the source gases, rather than only partially isolated as in the above example.
In the embodiments of the present invention wherein the compound film contains additional elements, the source of the additional element is either one of the two reactant gases or a third reactant gas. As previously noted, it is advantageous to incorporate H, D, or O into compound films that contain two other elements. The H, D, or O is used to enhance the properties of the compound film.
Hydrogen, deuterium, or both, are incorporated into the film in a number of different ways. In one example, the hydrogen gas (or a source for hydrogen such as NH.sub.3 or H.sub.2 O) is introduced into the chamber that contains the substrate on which the compound film is formed from a separate source via a separate nozzle. That is, in addition to the sources for the first and second reactant gases and the carrier gas, a separate source for hydrogen is also provided. Suitable sources for deuterium are the deuterated analogs (either partially or completely perdeuterated analogs of hydrogen and hydrogen containing compounds) of the sources for hydrogen described herein.
In other examples, the hydrogen gas is mixed with either the carrier gas or one of the reactant gases. In these examples, the mixture of hydrogen and carrier gas is introduced into the chamber in the same manner as previously described for the carrier gas alone. That is, the hydrogen is either introduced into the chamber directly through a separate nozzle from a separate source or into a cavity in fluid communication with the chamber, into which other reactant gas/carrier gas combinations are introduced. In another example, hydrogen is the carrier gas for one of the reactant gases. For example, in the above-described examples for forming silicon nitride films, hydrogen gas is selected as the carrier gas for the SiH.sub.4 reactant gas. In this example, the silane acts as an additional source of hydrogen.
Also, in other embodiments one reactant gas is the single source for two elements in the compound film. For example, in the case of hydrogen-containing silicon nitride films, the SiH.sub.4 reactant gas is used as the sole source for hydrogen (i.e., without H.sub.2 gas) as well as the source for silicon in the film. Other silanes such as SiCl.sub.2 H.sub.2, SiF.sub.2 H.sub.2 and higher silanes are further examples of sources of both silicon and hydrogen.
In the examples wherein hydrogen gas or a gaseous source of hydrogen is used to incorporate hydrogen into silicon nitride films, the placement of the energy source for dissociating the gases into reactant species and the combination of the various gases is controlled to prevent substantially complete dissociation of the SiH.sub.4 reactant species. In one example, the SiH.sub.4 gas is introduced into the plasma discharge cavity at a place where the SiH.sub.4 gas is not subjected to the full force of the plasma. Controlling the dissociation of the SiH.sub.4 into reactant species in the plasma discharge cavity improves the likelihood that the desired film will be formed on the surface of the substrate.
Other examples of arrangements that provide such control include the use of energy sources that do not fully dissociate SiH.sub.4 at the discharge nozzles for the other gases that are introduced into the cavity or the chamber. These energy sources include radiation such as light and electron beam radiation. Previously described methods for controlling the dissociation of the SiH.sub.4 into reactant species (i.e. using a series of nozzles to direct different gases onto the surface of the substrate) is also contemplated as suitable.
The placement of the nozzle relative to the plasma discharge is also used to control the composition of the compound film. In one example, a hydrogen-containing silicon nitride film is formed using SiH.sub.4 gas as both a source of silicon and a source of hydrogen. In this example SiH.sub.4 is introduced into gaseous environment through two different nozzles. A first nozzle introduces a dilute stream of SiH.sub.4 into the cavity in which the primary plasma discharge is generated. A second nozzle is also employed to introduce SiH.sub.4 directly onto the substrate surface. In this second nozzle the dissociation of the SiH.sub.4 is controlled by either having no plasma discharge occur therein, or, if plasma discharge does occur, maintaining the plasma discharge power in the second nozzle significantly lower than the plasma discharge in the first nozzle. As previously noted gases other than silane (e.g. NH.sub.3, H.sub.2, and H.sub.2 O) are also used as sources for hydrogen. In the two-nozzle arrangement described above, the source for hydrogen is introduced through the second nozzle.
After the hydrogen-containing compound film is formed, the compound film is annealed at a temperature and for a duration selected to redistribute the third element in the film and to substantially eliminate the dangling bonds that result from such redistribution. Suitable annealing conditions that will accomplish this objective are well known to one skilled in the art provided. The annealing conditions that are selected must be within the temperature tolerances of the particular film. Annealing temperatures in the range of about 300.degree. C. to about 1000.degree. C. are generally contemplated as suitable.

Number	Name	Date
5356672	Schmitt, III et al.	Oct 1994
5508067	Sato et al.	Apr 1996
5525379	Takada et al.	Jun 1996
5593741	Ikeda	Jan 1997

Process for making compound films

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

Field of Search

US

International Classifications

Abstract

Description

Claims

STATEMENT OF RELATED INVENTION

US Referenced Citations (4)

Non-Patent Literature Citations (1)

Continuation in Parts (1)