DISCHARGE SYSTEM FOR PLASMA PROCESSING

Abstract: An apparatus (10) for the downstream etching of semiconductor devices (31) which utilizes internal electrodes (12, 18) while avoiding the formation of loosely bonded particles in the discharge chamber (26).

Description: BACKGROUND OF THE INVENTION Plasma or dry etching is a well known technique used in the fabrication of integrated circuits wherein gas which has been dissociated into positive and negative ions and radicals reacts with unprotected areas of a material disposed on a substrate to remove elected portions of the material from the substrate. The etching generally takes place in a chamber containing the etching gas which is formed into a plasma by the application of R.F. Frequency power in a relatively low pressure environment. Another etching technique often used in conjunction with the above technique is known as downstream etching. In this technique the plasma is created outside the main etching chamber with the reactive species being transported to the main chamber or auxilliary chamber where the etching takes place. Downstream etching is a specialized form of etching and is used, e.g., to strip remaining photoresist from the substrate as well as to remove materials such as polycrystalline silicon and silicon nitride. The foregoing ethcing is often used as a clean-up operation after the main etching has been accomplished. A typical method for creating the plasma is to pass the etching gas through a tube with a portion thereof subjected to R.F. (or) radio frequency power, microwaves or the line external to the tube. These sources must necessarily be of high voltage and frequency to overcome the high impedance of the tube through which the etching gas must pass. Obviously it would be a great advantage of plasma in the tube could be generated by means of low frequency power. This advantage can be realized by the use of internal electrodes. While internal electrodes can generate the required plasma, these electrodes sputter creating loosely bonded particles or dust which are transported downstream along with the etching plasma which deposit on the wafer in quantity which often effectively destroys the usefulness of the wafers which must then be discarded. The present invention overcomes this problem. BRIEF SUMMMARY OF THE INVENTION The present invention relates to an apparatus for downstream etching in which internal electrodes are used to permit plasma generation by a low frequency power source without the attendant disa vantage of loose particles formation. In the present invention electrodes are used which are of a material that reacts chemically to produce a volatile species advoid of particles which may then be pumped out of the etching chamber leaving the wafer unharmed. BRIEF DESCRIPTION OF THE DRAWING The Figure illustrates a preferred embodiment of the present invention. DESCRIPTION Referring now more particularly to the Figure there is shown the downstream etching apparatus 10 of the present invention. It comprises a tube 11 made of alumina or other suitable material. Disposed at one end of tube 11 is an electrode 12. The electrode 12 is made of a reactive material such as graphite, silicon or silicon carbide for reasons that will become apparent as this description proceeds. An electrically conductive casing 13 such as, aluminum has an opening 14 in which electrode 12 is secured, e.g., by force fit. The end of tube 11 abuts electrode 12. The casing 13 has an extension which forms an opening for the end of tube 11 which is secured therein, e.g., by force fit holding the end of tube 11 in gas tight abutting relationship with electrode 12. An Oring is secured on tube 11 in abutting relationship with the extension 15 of casing 13 to further ensure a gas tight fit at that end of tube 11. Casing 13 and electrode 12 have an opening or port 17 which is connected to a source of etching gas (not shown). In a similar manner an electrode 18 is secured within opening 20 of aluminum casing 19, e.g., by force fit. Extension 21 holds the other end of tube 11 in abutting gas tight relationship with electrode 18. 0ring 22 disposed around tube 11 in abutting relationship with extension 21 of casing 19 further provides a gas tight seal between tube 11 and electrode 18. Electrode 18 and casing 19 have an opening 23 therethrough which is connected by any suitable means to conduit 27. The inner volume 24 of etching chamber 25 communicates with the inner volume 26 of tube 11 via conduit 27. The inner volume 24 of etching chamber is connected to a pump 29 via port 28 for evacuating gases within volume 24 and maintaining pressure therein at a predetermined amount suitable for etching. In evacuating gases from etching chamber 25 the gases input into volume 26 of tube 11 are transported into chamber 25 with the products of etching leaving chamber 25 via pump 29 and port 30. Within chamber 25 a wafer 31 to be etched is disposed on platform 32. An A.C. source 33 of low frequency, e.g., ordinary 60 Hz house current is electrically connected to aluminum casings 13 and 19 via switch 34, as shown. Alternately, a d.c. source providing of power may be substituted for the A.C. source. This use of a low frequency power source is possible inasmuch as electrodes 12 and 18 are located within the plasma chamber or volume 26. If the electrodes were external to volume 26, the frequency would have to be very high, i.e., the order of an R.F. frequency in the mega-hertz range. The electrodes 12 and 13 are made of carbon (graphite), silicon or silicon carbide. The materials react chemically with the gas products created by the plasma and in so doing the formation of loosely bonded particles is avoided. In operation when the source of etching gases is connected to tube 11 and pump 29 is on, the etching gases flow into volume of 26 of tube 11. When switch 34 is closed, the 60 cycle source is connected between electrodes 12 and 18 which creates a plasma of the etching gas which in the excited state flow into volume 24 of etching chamber 25 where they attack and remove the desired material. As aforesaid, since the electrodes are internal of the volume 26 low power frequency, e.g., 60 hertz may be used. Furthermore since the electrodes are made of graphite, silicon or silicon carbide which chemically react with the plasma products, the formation of particles or dust is avoided and therefore, do not deposit on-the wafer 31 in etching chamber 25 thus eliminating damage to the wafer orthe circuit patterns already built-up thereon. When stripping photoresist from a wafer, the etching gas is composed of 1 part of CF4 to 3 or 4 parts of 02 When silicon is etched three parts of CF4 to 1 part of 2 are used for the etching gas. Also, for both of these processes, the pressure in volumes 24 and 26 should be about 0.3 Torr (higher or lower pressures can be used) when gas is flowing. and the discharge is on. Temperature of the walls of tube 11 is approximately 40C and the temperature of the chamber 25 is approximately 230C. Other modifications of the present invention are possible in light of the above description which should not be construed as placing limitations on the invention other than those expressly set forth in the claims which follow:

Claims: WHAT IS CLAIMED IS: 1.. An apparatus for the downstream etching of semiconductor devices, comprising in combination; an etching chamber containing the semiconductor device to be etched, a tube having a discharge chamber connected between a source of etching gas and said etching chamber, said tube including a pair of electrodes disposed therein made of a material which chemically reacts with the plasma products created when the etching gases in said tube are excited to form an etching plasma whereby the etching plasma entering said etching chamber is substantially particle free. 2. An apparatus according to-claim 1 wherein said electrodes are disposed at opposite ends of said tube. 3. An apparatus according to claim 2 wherein said electrodes are made of graphite. 4. An apparatus according to claim 2 wherein said electrodes are made of silicon. 5. An apparatus according to claim 4 wherein said electrodes are made of silicon carbide. 6. An apparatus according to claim 2 further including, a low frequency power source connected to said electrodes for creating a plasma of said etching gas in said tube. 7. An apparatus according to claim 6 wherein said low frequency power source is a 60 hertz, 1,000 volt source. 8. An apparatus according to claim 2 further including, a d.c. power source connected to said electrodes for creating a plasma of said etching gas in said tube. 9. An apparatus according to claim 8 wherein said d.c. power source provides approximately 300 watts of power. 10. An apparatus according to claims 1, 2, 3, 4, 5, 6, 7, 8 or 9 further including pump means connected to said etching chamber for evacuating said etching chamber at a predetermined rate. 11. A apparatus according to claim 10 wherein the etching gas is 1 part CF4 to 3 or 4 parts 0 2 when photoresist is being stripped from the semiconductor device. 12. An apparatus according to claim 10 wherein the etching gases are 3 parts CF4 and one part 0 2 when silicon is being etched.

Inventors: REINBERG, ALAN R., STEINBERG, GEORGE N.
Applicants: THE PERKIN-ELMER CORPORATION
Application year: 1982
Main classification: H01J37
IPC codes: H01L21, H01J37