WO2000042632A1

WO2000042632A1 - Method and apparatus for generating and confining a reactive gas for etching substrates

Info

Publication number: WO2000042632A1
Application number: PCT/US2000/000744
Authority: WO
Inventors: William G. America; Edward J. Gratrix; Michael D. Robbins
Original assignee: Ipec Precision Inc
Current assignee: Ipec Precision Inc
Priority date: 1999-01-12
Filing date: 2000-01-12
Publication date: 2000-07-20
Anticipated expiration: 2001-07-12
Also published as: AU2503500A; WO2000042632B1

Abstract

A method for performing localized etching reactions on a substrate by confining a reactive gas with a non-reactive gas curtain is described. In the method, a reactive gas capable of etching the etchable substrate is generated. The reactive gas is applied to the etchable substrate. A non-reactive gas is flowed towards the substrate in the form of a non-reactive gas curtain around the reactive gas so as to substantially confine the reactive gas to a predetermined etching region on the substrate. An apparatus for carrying out the above described method is also disclosed.

Description

METHOD AND APPARATUS FOR GENERATING AND CONFINING A REACTIVE GAS FOR ETCHING SUBSTRATES

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method and apparatus for controlled material removal from the surface of a substrate over a confined region. More particularly, the present invention relates to a method and apparatus for generating and confining a reactive gas which is used for removing material from the surface of a substrate.

Description of the Related Art Conventional processes for thinning and figuring surfaces and films often employ such methods as mechanical polishing, grinding, sputtering, sand blasting, and chemomechanical processes. Each of these processes usually have substantial processing limitations. For example, chemomechanical processes for thinning semiconductor films is a contact method that leaves contaminants on the surface which cause subsurface damage to the substrate. Also, chemomechanical thinning processes do not allow corrections of the spatial variations within the film thickness.

Plasma assisted chemical etching methods to shape surfaces of substrates have been developed to be an improvement over the above described processes such as chemomechamcal thinning because such plasma processes do not contact the substrate surface, and, therefore, reduce the potential for subsurface damage. U.S. Pat. No. 4,668,336 discloses a method and apparatus for figuring a surface by plasma assisted chemical transport where the substrate surface is mounted in close proximity to at least one electrode of an radio frequency (rf) driven reactor having two parallel plate electrodes. Removal of material from the surface of a substrate is controlled by varying the amount of time an electrode with small surface area spends at a given region wherein the entire surface of the substrate is exposed to a reactive gas which in the presence of an rf field between the two electrodes forms a plasma. Thus, material removal is effected by a plasma which is generated at the surface where material removal is desired. A disadvantage to the invention disclosed therein is that the profile of the surface where the material has been removed may not be precisely controlled.

U.S. Patent No. 5,336,355, discloses a plasma assisted chemical etching device which confines the etching plasma to a local surface area of the substrate surface. The local confinement allows control of the material removal footprint and, thus, the control of the profile of the surface where material has been removed. Because the device disclosed therein is of the type where the plasma excitation occurs by rf discharge directly coupled to the substrate (the local substrate surface is effectively an electrode where the reaction is occurring), underlying surface structure may cause variations in the material removal rate which is a primary cause of "print through" problems.

The "print through" problems associated with local coupled plasma generation can be solved by generating a plasma away from the etching surface to carry out the etching reaction "downstream". When plasma assisted chemical etching is done in a "downstream" mode, long-lived chemically reactive species created in the plasma are carried by the flow "downstream" to the etching reaction site. U.S. Pat. No. 4,431,898 describes a method to generate plasma away from the etching surface for "downstream" stripping of photo resist from an entire wafer. The patent discloses an apparatus which has a plasma chamber inductively coupled to a source of A.C. power wherein the semiconductor devices are etched and an alternative embodiment where the stripping or etching of the substrate takes place "downstream."

The downstream method produces a dry chemical removal process over the substrate which eliminates damage to the substrate which could result from energetic species. However, the method disclosed therein fails to provide a means to make precise changes to the profile of the surface where material is removed and, therefore, is not useful for local error correction of a substrate surface. U.S. Patent No. 5,290,382 discloses another method and apparatus for generating a plasma for "downstream" rapid shaping of surfaces of substrates and films useful for local error correction.

A problem with downstream etching disclosed in these patents is that the reactive gas may uncontrollably etch areas of the substrate in which etching is not desired. Etching outside a desired region occurs because such reactive gas is not confined to the desired etching region.

The present invention incorporates the advantages of decoupled plasma assisted chemical etching with a means to perform confined local etching so as to correct spatial errors in a substrate surface or correct the thickness profile of a film on a substrate.

SUMMARY OF THE INVENTION One object of the present invention is to provide a means for performing a plasma assisted chemical etching reaction.

Another object of the present invention is to provide a means for material removal by a chemically reactive species from a plasma generated independent of the electrical and geometric characteristics of the substrate.

Another objective of the present invention is to provide a means for local application of a decoupled plasma for precision optical figuring or film thinning. Another objective of the present invention is to provide a means for preventing unwanted etching reactions outside of the local material removal footprint.

The aforementioned objects are also accomplished, at least in part, by a method for performing etching reactions in a predetermined etching region on the surface of an etchable substrate with a reactive gas. The method comprises the steps of generating a reactive gas capable of etching the etchable substrate, applying the reactive gas to the etchable substrate, and flowing a non-reactive gas towards the substrate in the form of a non-reactive gas curtain around the reactive gas so as to substantially confine the reactive gas to the predetermined etching region on the substrate.

The aforementioned objects are accomplished, at least in part, by an apparatus for performing etching reactions in a predetermined etching region on the surface of an etchable substrate with a reactive gas. The apparatus includes, in part, a means, such as a plasma chamber, for generating a reactive gas capable of etching the etchable substrate. The apparatus also includes a means, such as a reactive gas transfer conduit, for applying the reactive gas to the etchable substrate. The apparatus further includes a means, such as a gas curtain channel, for confining the reactive gas to the predetermined etching region on the etchable substrate.

Other objects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description read in conjunction with the attached drawings and claims appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS The drawings, not drawn to scale, include:

FIG. 1, which is a schematic diagram of an etching apparatus according to the present invention;

FIG. 2, which is a schematic plan view of the etching apparatus illustrated in FIG. 1, taken along the line 2—2;

FIG. 3, which is an enlarged partial schematic diagram of a portion of the etching apparatus shown in FIG. 1 illustrating confinement of the reactive gas to the predetermined etching region on the substrate; and

FIG. 4, which is a schematic plan view of the substrate shown in FIG. 3, taken along the line 3—3 to illustrate the predetermined etching area on the substrate relative to the boundary of a confining gas curtain directed at the substrate.

DETAILED DESCRIPTION OF THE PRESENT INVENTION FIGS. 1 through 3 illustrate an apparatus 10 for performing local etching of a substrate to remove material therefrom. The apparatus comprises a plasma chamber 12 wherein a plasma 14 is generated to subsequently generate a reactive gas RG capable of etching a substrate. In the embodiment illustrated, the plasma chamber 12 is bounded by an upper electrode 16, an insulator member 18 and a lower electrode 20. The upper and lower electrodes are connected to a radio frequency (rf) signal source 22. A reactive gas precursor RGP is introduced into the plasma chamber 12 via an inlet 24. The excitation of the reactive gas precursor into a plasma by exposure to the radio frequency signal converts at least some of the reactive gas precursor RGP into the reactive gas RG containing chemically active species which will react with the substrate to cause etching thereof. Nitrogen trifluoride (NF₃) is one example of a reactive gas precursor. The reactive gas RG, containing active fluorine ions in the case where NF₃ is the reactive gas precursor RGP, is flowed out of the chamber 12 through a reactive gas transfer conduit 26 having opening 28 which is positioned near a substrate 30 mounted on a substrate holder or platen 32 which is grounded. In the above described embodiment, the plasma 14 and reactive gas RG are generated "upstream" independent of the substrate 30. Those skilled in the art will appreciate that the configuration of the plasma and reactive gas RG generating part of the apparatus described above is merely exemplary, other "upstream" configurations, such as those disclosed in U.S. Patent No. 5,290,382, which is incorporated herein by reference in its entirety, may also be used to generate a decoupled plasma and reactive gas "upstream" from the substrate 30.

Still referring to FIGS. 1 through 3, the apparatus 10 includes a gas curtain channel 34 which surrounds the opening 28 of the reactive gas transfer conduit 26 for confining the reactive gas. The gas curtain channel 34 is connected to a non- reactive gas source (not shown). In the context of the present invention, non- reactive gas means a gas that does not react with the substrate 30. Examples of such gasses may include nitrogen, argon or hydrogen. If hydrogen is used, the chemically active species in the reactive gas RG, such as the fluorine ions, may be inactivated by reacting such ions with the hydrogen to form hydrogen fluoride or by releasing fluorine from the fluorine containing reactive species to form non- reactive F₂. The gas curtain channel is 34 configured or adapted to direct the non- reactive gas towards the substrate 30 and around the reactive gas flowing out of the opening 28 of the gas transfer conduit 26 in the form of a non-reactive gas curtain GC. The non-reactive gas curtain GC surrounding the reactive gas prevents the reactive gas from flowing in contact with the substrate 30 beyond the non-reactive gas curtain GC, thereby confining the reactive gas RG to a predetermined etching region PER (FIG. 3). As illustrated in FIG. 2, the gas curtain channel 34 is annularly shaped and surrounds the circularly shaped opening 28 of the reactive gas transfer conduit 26. Of course, those skilled in the art will appreciate that other shapes, such as elliptical, rectangular, triangular, etc. may be used, if desired. While not required, it is preferred that the shape of the gas curtain channel 34 be configured in a manner which cooperates best with the configuration of the opening 28 of the reactive gas transfer conduit 26. For example, if the opening 28 is square shaped, then the gas channel 34 should be similarly shaped.

In addition to the gas curtain channel 34, the apparatus may be provided with an exhaust channel 36 disposed between the gas curtain channel 34 and the opening 28 of the reactive gas transfer conduit 26. The exhaust channel 36 may be connected to a vacuum source (not shown) so as to provide a region of relatively lower pressure at the exhaust channel 36 to draw reactive and non- reactive gases from the region between the apparatus and the substrate 30 at the border of the predetermined etching region PER. Preferably, the exhaust channel 36 surrounds the opening 28 of the reactive gas transfer conduit 26 and like the gas curtain channel 34, the shape of the exhaust channel 36 should be configured in a manner which cooperates best with the configuration of the opening 28 of the reactive gas transfer conduit 26. For example, if the opening 28 is square, then the exhaust channel 36 should be similarly shaped.

According to the present invention, the above described apparatus 10 enables etching reactions to be performed in a predetermined etching region on the surface of an etchable substrate 30 with a reactive gas which is preferably generated from a plasma "upstream" from the substrate 30 and thereby decoupled from the substrate. In the method, a reactive gas capable of etching the etchable substrate is generated. This generation is preferably performed using a decoupled "upstream" plasma converting a reactive gas precursor, such as NF₃, into a reactive gas containing chemically active species suitable for etching the substrate. The reactive gas generated "upstream" is flowed "downstream" and applied to the etchable substrate. To confine the reactive gas to the predetermined etching region PER on the substrate 30, a non-reactive gas is flowed towards the substrate in the form of a non-reactive gas curtain GC around the reactive gas RG. FIG. 4 illustrates the boundary of the predetermined etching region PER and the boundary of the gas curtain GC. The reactive gas and non-reactive gas forming the gas curtain may be exhausted from the substrate, if desired. Those skilled in the art will appreciate that the flow rates of the reactive gas RG, non-reactive gas forming the gas curtain GC and the rate at which such gases are exhausted from the substrate 30 will have an influence on the size of the predetermined etching region PER, which is also affected by the distance between the opening 28 of the reactive gas transfer conduit and the substrate 30. Optimization of the size of the predetermined etch region PER may be determined from routine experimentation adjusting the flow rates of the above mentioned gases to and from the substrate 30 as well as the distance between the opening 28 and the substrate.

Thus, what has been described is a method and apparatus for performing etching reactions in predetermined etching region on the surface of an etchable substrate with a reactive gas. The invention provides a means for making local surface error corrections on a substrate. The invention finds particular usefulness in the thinning of semiconductor films and figuring of optics. The embodiment of the present invention disclosed herein admirably achieves the objects set forth; however, it should be appreciated by those skilled in the art that departures can be made by those skilled in the art without departing from the spirit and scope of the invention which is limited only by the following claims.

Claims

WHAT IS CLAIMED IS:

1. An apparatus for performing etching reactions in a predetermined etching region on the surface of an etchable substrate with a reactive gas, the apparatus comprising: means for generating a reactive gas capable of etching the etchable substrate; means for applying the reactive gas to the etchable substrate; and means for confining the reactive gas to a predetermined etching region on the etchable substrate.

2. The apparatus of claim 1, wherein the means for generating a reactive gas is a plasma chamber having an inlet for receiving a reactive gas precursor and an outlet for flowing reactive gas from the chamber, the outlet being connected to the means for applying the reactive gas; and means for generating a plasma in the plasma chamber to convert at least some of the reactive gas precursor into the reactive gas.

3. The apparatus of claim 1, wherein the means for applying the reactive gas to the substrate is a reactive gas transfer conduit having an opening adapted to direct the reactive gas towards the substrate.

4. The apparatus of claim 1, wherein the means for confining the reactive gas to the predetermined etching region comprises a gas curtain channel surrounding the opening of the reactive gas transfer conduit, the gas curtain channel being adapted to direct a non-reactive gas towards the substrate and around the reactive gas in the form of a non-reactive gas curtain thereby preventing the reactive gas from flowing in contact with the substrate beyond the non-reactive gas curtain.

5. The apparatus of claim 4, wherein the means for confining the reactive gas to the predetermined etching region further comprises an exhaust channel disposed between the gas curtain channel and the opening of the reactive gas transfer conduit, wherein the exhaust channel is adapted to remove reactive and non-reactive gases from the region between the apparatus and the substrate.

6. The apparatus of claim 5, wherein the exhaust channel surrounds the opening of the reactive gas transfer conduit.

7. The apparatus of claim 6, wherein the opening of the gas transfer conduit is substantially circular in shape, and wherein the curtain gas channel and the exhaust channel are substantially annular in shape.

8. A method for performing etching reactions on a predetermined etching region on the surface of an etchable substrate with a reactive gas, the method comprising the steps of: generating a reactive gas capable of etching the etchable substrate; applying the reactive gas to the etchable substrate; and flowing a non-reactive gas towards the substrate in the form of a non- reactive gas curtain around the reactive gas so as to substantially confine the reactive gas to a predetermined etching region on the substrate.

9. The method of claim 8, wherein a plasma is used to generate the reactive gas.

10. The method of claim 8, wherein the non-reactive gas is selected from the group consisting of nitrogen and argon.

11. The method of claim 8, wherein the non-reactive gas is hydrogen.