Patent Grant
7405162
The present invention relates to an etching method; and more particularly, to an etching method for performing an etching on a silicon substrate by using a plasma of a reactive gas.
A device of a silicon-on-insulator (SOI) structure is expected to be applicable to micro electro mechanical systems (MEMS) as well as various semiconductor devices such as ultra large scale integrated circuits (LSI), for the device is capable of reducing power consumption, while realizing a high speed operation. In a manufacturing process of a semiconductor device having an SOI structure, a hole or a trench is formed while a silicon (Si) layer formed on a buried oxide (BOX) layer being etched. At this time, a gaseous mixture of SF6+O2, SF6+O2+SiF4, SF6+C4F8 or the like is used as an etching gas, and the Si layer is etched until the BOX layer with the SOI structure is exposed.
However, when the etching is progressed down to near the BOX layer, there occurs a notching phenomenon, i.e., the etching of the silicon layer progresses in a horizontal direction at an interface between the Si layer and the hard BOX layer. It is believed that this notching phenomenon is caused because a balance in inflow of positive ions and electrons is broken at a bottom portion of a hole or a trench with a high aspect ratio, thereby resulting in an excessive presence of the ions. The excessive inflow of the positive ions makes the BOX layer be positively charged, which in turn causes the path of the positive ions among the incident plasma to be bent, resulting in the etching of the sidewall of the silicon layer or a protective film therefor.
Moreover, in general, etching rate at a central portion of a substrate to be processed is different from that at an edge portion thereof. Thus, when etching a silicon layer of a substrate having an SOI structure by using the above mentioned gas system, if the etching is performed until the etching on a lower etching rate portion of the silicon layer is completed, a higher etching rate portion thereof will be kept being etched even after the BOX layer is exposed, resulting in an overetching on that portion. As a result, notching is further likely to occur.
As a technique to prevent notching, there is proposed an etching method for performing an etching by mixing an etching gas with an additive gas such as silicon tetrachloride containing an element identical to that constituting a thin film to be etched (see, for example, Japanese Patent Laid-open Publication No. H8-213368, e.g., claim 1: Reference 1).
Further, there is also proposed an etching method which employs SF6+HBr+O2 as a first etchant and HBr+O2 as a second etchant to prevent notching (see, for example, U.S. Pat. No. 6,391,788, e.g., claim 1: Reference 2).
In general, in the aspect of improving a throughput, a higher etching rate is preferable and, in case of performing an etching on a substrate to be processed with an SOI structure, it is necessary to conduct the processing at a high etching rate. However, both the above-described conventional etching methods aim at avoiding notching at the expense of the etching rate. For example, in Reference 1, though prevention of notching is attempted by forming a protective film through an addition of silicon tetrachloride, the etching rate decreases inevitably due to the presence of the protective film. Further, in the method disclosed in Reference 2, since a gas containing HBr, which forms deposits readily, is used as a first etchant, its etching rate may be in a range between 2 and 4 μm/min at the most, and an etching rate of a second etchant is also as low as 6000 to 8000 Å/min. Accordingly, although the conventional methods for avoiding notching succeed in preventing notching, they have difficulty in finding practical applications, because their etching rates are excessively low.
It is, therefore, an object of the present invention, to provide an etching method capable of performing an etching at a high etching rate, while avoiding notching.
In accordance with a first aspect of the present invention, there is provided an etching method for forming an opening with a substantially vertical profile extending to a stopper layer by performing an etching with a plasma of an etching gas acting on an object to be processed loaded in an evacuable processing vessel, wherein the object has a mask layer of a predetermined pattern, a silicon layer to be etched formed below the mask layer and the stopper layer formed below the silicon layer, the method including: a first etching process for forming an opening with a tapered wall surface in the silicon layer by using a first etching gas including a fluorine-containing gas and O2 but not HBr; and a second etching process for etching the opening by using a second etching gas including a fluorine-containing gas, O2 and HBr.
Preferably, the fluorine-containing gas is SF6. Further, in the first etching process, the etching may be performed until a part of the stopper layer is exposed or overetching may be performed for a preset time period after a part of the stopper layer is exposed.
Preferably, during the first and the second etching process, an internal pressure of the processing vessel is maintained within a range between 13.3 and 66.7 Pa (100 and 500 mTorr) and a high frequency power applied for generating the plasma is set to be within a range between 1.6 and 7.6 W/cm2 on the object to the processed.
Preferably, a ratio of HBr to the sum of the fluorine-containing gas and O2 (HBr/(fluorine-containing gas+O2)) contained in the second etching gas is set to be in a range between 0.25 and 1 exclusive.
Preferably, the stopper layer is a silicon oxide film and the object to be processed is a silicon wafer having an SOI structure.
In accordance with a second aspect of the present invention, there is provided an etching method for performing an etching with a plasma of an etching gas acting on an object to be processed loaded in an evacuable processing vessel, wherein the object has a mask layer of a predetermined pattern, a layer to be etched formed below the mask layer and a stopper layer formed below the layer to be etched, the method including: a first etching process for forming an opening with a tapered wall surface in the layer to be etched; and a second etching process for expanding the tapered wall surface of the opening in horizontal direction and concurrently elongating the opening vertically such that the opening has a substantially vertical profile extending to the stopper layer.
Preferably, in the first etching process, etching is performed until a part of the stopper layer is exposed.
Preferably, an etching rate of the second etching process is smaller than that of the first etching process.
In accordance with a third aspect of the present invention, there is provided a computer readable storage medium for storing therein a control program executable in a computer, wherein, when executed, the control program controls a plasma etching apparatus for use in performing an etching method for etching an object to be processed for forming an opening with a substantially vertical profile extending to a stopper layer with a plasma of an etching gas acting on the object to be processed, wherein the object to be processed has a mask layer of a predetermined pattern, a silicon layer to be etched formed below the mask layer and the stopper layer formed below the silicon layer, the method including: a first etching process for forming an opening with a tapered wall surface in the silicon layer by using a first etching gas including a fluorine-containing gas and O2 but not HBr; and a second etching process for etching the opening by using a second etching gas including a fluorine-containing gas, O2 and HBr.
Preferably, the fluorine-containing gas is SF6. Further, in the first etching process, the etching may be performed until a part of the stopper layer is exposed or overetching may be performed for a preset time period after a part of the stopper layer is exposed.
Preferably, during the first and the second etching process, an internal pressure of a processing vessel is maintained in a range between 13.3 and 66.7 Pa (100 and 500 mTorr) and a high frequency power for generating the plasma is set to be within a range between 1.6 and 7.6 W/cm2 on the object to the processed.
Preferably, a ratio of HBr to the sum of the fluorine-containing gas and O2 (HBr/(fluorine-containing gas+O2)) contained in the second etching gas is determined between 0.25 and 1 exclusive.
Preferably, the stopper layer is a silicon oxide film and the object to be processed is a silicon wafer having an SOI structure.
In accordance with a fourth aspect of the present invention, there is provided a computer readable storage medium for storing therein a control program executable in a computer, wherein, when executed, the control program controls a plasma etching apparatus for use in performing an etching method for etching an object to be processed loaded in an evacuable processing vessel with a plasma of an etching gas acting on the object to be processed, wherein the object to be processed has a mask layer of a predetermined pattern, a silicon layer to be etched formed below the mask layer and a stopper layer formed below the silicon layer, the method including: a first etching process for forming an opening with a tapered wall surface in the silicon layer; and a second etching process for expanding the tapered wall surface of the opening in horizontal direction and concurrently elongating the opening vertically such that the opening has a substantially vertical profile extending to the stopper layer.
Preferably, in the first etching process, etching is performed until a part of the stopper layer is exposed. Further, Preferably, an etching rate of the second etching process is smaller than that of the first etching process.
In accordance with the etching method of the present invention, which includes at least two steps of a first etching process for forming a hole or trench with a tapered wall surface in the silicon layer by using a first etching gas including a fluorine-containing gas and O2 and a second etching process for etching the hole or the trench to have a substantially vertical wall surface by using a second etching gas including a fluorine-containing gas, O2 and HBr, it is possible to maintain the etching processes at a high etching rate, while avoiding notching effectively.
The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which:
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.
Installed in the chamber 1 is a supporting table 2 for horizontally supporting a wafer W that is a silicon (Si) substrate with an SOI structure. The supporting table 2 is formed of, for example, aluminum and is supported by a conductive support 4 via an insulator 3. Furthermore, a focus ring 5 formed of a material other than Si, for example, quartz, is mounted on the periphery of the top surface of the supporting table 2. The supporting table 2 and the support 4 are moved upward and downward by a ball screw mechanism including ball screws 7, and a driving portion below the support 4 is covered with a bellows 8 formed of stainless steel (SUS). Further, a bellows cover 9 is installed to surround the bellows 8. Also, a baffle plate 10 is installed outside the focus ring 5, and the focus ring 5 is electrically connected to the chamber 1 via the baffle plate 10, the support 4 and the bellows 8. The chamber 1 is grounded.
A gas outlet port 11 is formed in a sidewall of the lower portion 1b of the chamber 1, and a gas exhaust system 12 is connected to the gas outlet port 11. By operating a vacuum pump of the gas exhaust system 12, the chamber 1 can be depressurized to a predetermined vacuum level. Further, a gate valve 13 for opening and closing a loading/unloading port for the wafer w is installed at an upper sidewall of the lower portion 1b of the chamber 1.
A first high frequency power supply 15 for generating a plasma is connected to the supporting table 2 via a matching unit 14 such that a high frequency power of a predetermined frequency is applied to the supporting table 2 from the first high frequency power supply 15. Meanwhile, disposed above the supporting table 2 to face it in parallel is a shower head 20 to be described later in detail. The shower head 20 is grounded. Thus, the supporting table 2 and the shower head 20 function as a pair of electrodes.
A second high frequency power supply 26 is connected to a power feed line of the first high frequency power supply 15 via a matching unit 25. The second high frequency power supply 26 applies a high frequency power of a frequency lower than that from the first high frequency power supply 15, so that it is superposed upon the high frequency power for the plasma generation.
An electrostatic chuck 6 for electrostatically attracting and holding a wafer W thereon is mounted on a top surface of the supporting table 2. The electrostatic chuck 6 has an electrode 6a embedded in an insulator 6b, and the electrode 6a is connected to a DC power supply 16. By applying a voltage to the electrode 6a from the DC power supply 16, the wafer W is attracted and held by the electrostatic chuck 6 by the help of the electrostatic force, i.e., a Coulomb force.
A coolant path 17 is formed within the supporting table 2. A coolant is introduced into the coolant path 17 via a coolant introducing line 17a and is discharged via a coolant discharge line 17b. By the circulation of the coolant, a heat transfer between the wafer W and the coolant is carried out via the supporting table 2, whereby a to-be-processed surface of the wafer W is maintained at a desired temperature level.
Further, even while the chamber 1 is evacuated by the gas exhaust system 12 and is maintained in a vacuum state, a heat transfer gas is introduced between the top surface of the electrostatic chuck 6 and the rear surface of the wafer W at a preset back pressure from a gas introduction mechanism 18 via a gas supply line 19, to facilitate the cooling of the wafer W by the coolant which is circulated through the coolant path 17. By introducing the heat transfer gas, the heat transfer between the wafer W and the coolant can be efficiently carried out, so that the cooling efficiency for the wafer W can be improved.
The shower head 20 is disposed at a ceiling portion of the chamber 2 to face the supporting table 2. The shower head 20 is provided with a plurality of gas discharge openings 22 on its lower surface and has a gas inlet 20a in an upper portion thereof. Further, the shower head 20 has a hollow space 21 formed therein. A gas supply line 23a is connected at one end to the gas inlet 20a, and the other end of the gas supply line 23a is coupled to a processing gas supply system 23 for supplying a processing gas containing an etching gas and a dilution gas.
The processing gas is introduced into the space 21 of the shower head 20 from the processing gas supply system 23 via the gas supply line 23a and the gas inlet 20a, and is sprayed toward the wafer W through the gas discharge openings 22.
Further, a dipole ring magnet 24 is concentrically disposed around the upper portion 1a of the chamber 1. As shown in a horizontal cross sectional view of
Accordingly, in a space between the supporting table 2 and the shower head 20, a vertical electric field EL is formed due to an electric power supplied from the first high frequency power supply 15 and the horizontal magnetic field B is formed by the dipole ring magnet 24, as shown in
Each component of the plasma etching apparatus 100 is connected to and controlled by a process controller 50 with a CPU. A process manager can operate the plasma etching apparatus 100 by using a user interface 51 connected to the process controller 50, wherein the user interface 51 includes a keyboard for inputting a command, a display for showing an operational status of the plasma etching apparatus 100, and the like.
Moreover, also connected to the processing controller 50 is a memory unit 52 for storing therein a recipe including a control program, processing condition data and the like to be used in realizing various processings performed in the plasma etching apparatus 100 under the control of the process controller 50.
Further, when a command is received from the user interface 51, the process controller 50 retrieves a necessary recipe from the memory unit 52 to execute it, so that a desired processing is performed in the plasma processing apparatus 100 under the control of the process controller 50. Moreover, the necessary recipe can be retrieved from a readable storage medium such as a CD-ROM, a hard disk, a flexible disk or the like, or retrieved on-line through, for example, a dedicated line to another apparatus available all the time.
Hereinafter, an etching method in accordance with a preferred embodiment of the present invention, which is performed by the plasma etching apparatus 100 configured as described above, will be described with reference to
In the first etching process, the gate valve 13 shown in
A processing gas including an etching gas and a dilution gas is supplied into the chamber 1 from the processing gas supply system 23 at a preset flow rate. Then, while the internal pressure of the chamber 1 is controlled to be kept within a range between 13.3 and 66.7 Pa (between 100 and 500 mTorr), a predetermined high frequency power is applied to the supporting table 2 from the first high frequency power supply 15. It is preferred that the high frequency power for generating the plasma is set to be within a range between, e.g., 1.6 and 7.6 W/cm2 on the wafer W. At this time, by a preset voltage applied to the electrode 6a of the electrostatic chuck 6 from the DC power supply 16, the wafer W is attracted and held by the electrostatic chuck 6 due to, for example, a Coulomb force, and, at the same time, a high frequency electric field is formed between the shower head 20 serving as an upper electrode and a supporting table 2 serving as a lower electrode. Since the horizontal magnetic field B is also formed between the shower head 20 and the supporting table 2 due to the presence of the dipole ring magnet 24, an orthogonal electromagnetic field is formed in the processing space between the upper and the lower electrode where the wafer W is located. The orthogonal electromagnetic field makes electrons produced drift, which in turn generates a magnetron discharge. Then, the wafer W is etched by the plasma of the etching gas generated due to the magnetron discharge. Here, by setting the gas pressure within the chamber 1 high, a sufficient amount of radicals can be generated as well as charged particles including ions and electrons, so that a high-rate etching of 20 μm/min or greater can be realized by the help of the effective action of the radicals.
Moreover, to increase the number of radicals present above the wafer W, it is preferable to apply a high frequency power to the electrode on which the wafer W is mounted. In the preferred embodiment of the present invention, since a high frequency power is applied to the supporting table 2 serving as the lower electrode on which the wafer W is mounted by using an RIE type plasma generating mechanism, the plasma can be generated right above the object to be processed. Accordingly, the extinguishment rate of the radicals decreases, thus resulting in an increase in the number of radicals right above the wafer W to facilitate the contribution of the radicals to the etching of the wafer W. Furthermore, the etching is performed while the magnetic field orthogonal to the electric field between the electrodes is formed, so that an E×B drift occurs immediately above the object to be processed. As a result, a high plasma density can be realized right above the object to be processed. Accordingly, the etching can be performed at a higher rate due to the synergy effects with the high gas pressure.
To increase the etching rate of the wafer W, the etching gas for use in the first etching process preferably includes a gas containing fluorine that features high reactivity and a gas containing oxygen. As for the fluorine-containing gas, its reactivity improves as the number of fluorine atoms (F) per a single molecule increases. If the molecule of the fluorine-containing gas is written as AxFy (A represents an arbitrary element, and x and y represent valence), it is preferable that y is a value of 4 or larger and, more preferably, a value of 6 or larger for achieving high reactivity. Such fluorine-containing gas includes CF4, C3F8, SF6, S2F10 and the like. Further, by using oxygen gas together with the fluorine-containing gas, etching anisotrophy, i.e., etching profile can be improved. Specifically, it is preferable to use an etching gas containing SF6 and O2 whose ratio of SF6/O2 ranges from 1 to 10, more preferably from 3 to 4, for achieving a high etching rate and improving the etching profile.
In improving the etching profile, it is also effective to keep the temperature of the wafer W low. For the reason, there is provided the coolant path 17 through which a coolant is circulated. With this configuration, heat can be transferred between the coolant and the wafer W via the supporting table, whereby the to-be-processed surface of the wafer W can be controlled to be kept at a desired temperature. To improve the etching profile, i.e., etching anisotrophy, a coolant of, e.g., about −30° C. is circulated.
Moreover, a heat transfer gas for effectively transferring the heat between the coolant and the wafer W is introduced between the top surface of the electrostatic chuck 6 and the rear surface of the wafer W at a preset pressure (back pressure) from the gas introduction mechanism 18 via the gas supply line 19. As the heat transfer gas, it is preferable to employ a gas used as an etching gas such as SF6 or C4F8 in lieu of a typical He gas. Since respective cooling efficiencies of those gases are better than that of the He gas, the efficiency of cooing the wafer W can be further improved and the etching profile can be improved as well.
The frequency and the output of the first high frequency power supply 15 for generating plasma is properly adjusted to be set to optimally generate a desired plasma. To increase the plasma density immediately above the wafer W, its frequency is preferably set to be not smaller than 27 MHz.
The second high frequency power supply 26 is for supplying a high frequency power to control the ion energy of the plasma. Preferably, the frequency of the second high frequency power supply 26 is set to be smaller than the frequency of the first high frequency power supply 15 but not smaller than 2 MHz.
The dipole ring magnet 24 provides the magnetic field in the processing space between the supporting table 2 and the shower head 20 serving as the oppositely facing electrodes, respectively, in order to increase the plasma density right above the wafer W. To be effective in increasing the plasma density, the dipole ring magnet 24 needs to have a strength capable of providing a magnetic field of 10000 μT (100 G) or greater. Though it is believed that a stronger magnetic field generates a higher plasma density, it is preferred that the magnetic field is not greater than 100000 μT (1 kG) for safety.
Referring to
The taper shape can be adjusted by controlling the internal pressure of the chamber 1 and the high frequency power for generating the plasma in the first etching process. For the purpose, in the first etching process, the internal pressure of the chamber 1 is set to range from 13.3 to 66.7 Pa (100 to 500 mTorr) and the high frequency power for plasma generation is set to range from 1.6 to 7.6 W/cm2 on the wafer W.
There will now be described a simulation conducted to investigate the influences of the pressure and the high frequency power upon the taper shape of the hole 110.
Conditions for the Plasma Etching:
In
The above experiment proves that etching can be performed to form a hole 110 of a tapered profile by way of controlling the internal pressure of the chamber and the high frequency power while using the processing gas including SF6 and O2.
As shown in
A second etching process is performed by using the plasma etching apparatus 100 after changing a processing gas. As can be seen from
A gas including, for example, a fluorine-containing gas such as SF6, O2 gas and HBr gas is used as a processing gas for the second etching process. The same fluorine-containing gas as employed in the first etching process may be used.
Here,
In this experiment, the flow rate of SF6/O2 was fixed at 180/60 mL/min, while the flow rate of HBr was varied. In cases (a) to (e) of
Further, other conditions for the plasma etching are as follows.
Conditions for the Plasma Etching:
From the cases (a) to (e) of
From the above experiment, one can conclude that it is preferable to perform the second etching process by setting the ratio of HBr to the sum of the fluorine-containing gas and O2 (HBr/(fluorine-containing gas+O2)) in the range between 0.25 and 1 exclusive, which covers the cases (c) and (d) of
Further, to prevent notching and control etching profiles, the internal pressure of the chamber 1 is preferably set to range from 26.7 to 66.7 Pa (200˜500 mTorr) and the high frequency power for generating plasma is preferably set to range from 1.6 to 3.3 W/cm2 on the wafer W in the second etching process.
Moreover, in the second etching process, taper angles are detected by means of an etching profile monitoring device using an optical mechanism, and the etching may be terminated at a point when a desired etching profile is obtained.
Based on the above-described experimental examinations, etching was performed by using the plasma etching apparatus 100, and the results are provided in
(1) Conditions for the Etching:
For the samples in
Referring to
(1) Conditions for the First Etching Process:
In the case of the sample in
In the case of the sample in
In the case of the sample in
In the case of the sample in
Referring to
Considering the results in
(1) Conditions for the First Etching Process:
Here, as for a central portion in
As for an edge portion in
After the completion of the first etching process, the taper angle of the sidewall 111 of the hole 110 in the central portion was measured to be 83.7° (not shown) and after the second etching process, the taper angle of the sidewall 112 of the hole 110 was found to be 87.0°, so that the sidewall 112 of the hole 110 became almost vertical. Furthermore, as illustrated in
Moreover, it was confirmed that in the second preferred embodiment, high etching rates were realized: 20 μm/min or greater in the first etching process and 5 to 10 μm/min in the second etching process.
As described above, it was proved that, by employing the etching method in accordance with the present invention, notching can be prevented and a wafer having, for instance, an SOI structure can be etched at a high etching rate. It was also proved that the etching profile can be improved.
Here, it is to be noted that the present invention is not limited to the preferred embodiments described above.
For example, though a dipole ring magnet is employed as a magnetic field forming device in the magnetron RIE plasma etching apparatus 100, other device may be employed instead of the dipole ring magnet, and the formation of the magnetic field is not essential. In addition, besides the magnetron RIE plasma processing apparatus, various plasma etching apparatuses including a capacitively coupled or an inductively coupled plasma etching apparatus can be used as long as they can generate plasma at a gas pressure in the range in accordance with the present invention. Here, to generate a plasma under a high pressure level, the capacitively coupled plasma etching apparatus is preferred to the inductively coupled plasma etching apparatus. Moreover, in order to have an object to be processed to be positioned closer to a plasma generation region, an RIE type is more preferable. Also, though the preferred embodiments have been described for the case of etching a wafer, the present invention may be applied any process as long as it etches Si of an object to be processed.
While the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-274637 | Sep 2004 | JP | national |
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| 4560436 | Bukhman et al. | Dec 1985 | A |
| 4980316 | Huebner | Dec 1990 | A |
| 6461934 | Nishida et al. | Oct 2002 | B2 |
| 20050029221 | Chang et al. | Feb 2005 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20060063385 A1 | Mar 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60620718 | Oct 2004 | US |
