IMPROVED PHOTOCHEMICAL VAPOR DEPOSITION METHOD

Abstract: The specification discloses an apparatus and process for use in the photochemical vapor deposition of a selected radiation-opaque material on a target in a photochemical vapor deposition system having a reaction chamber with at least a portion thereof formed of quartz to provide a quartz window therein, and which prevents undesirable deposits on the quartz window to thereby enhance the efficiency and rate of the photochemical vapor deposition process. The appar­ atus comprises, among other things, a film of a predeter­ mined material, such as a perfluorinated polyether, depo­ sited on the internal face of the quartz window within the reaction chamber, with the predetermined material being characterized by being transparent to a selected wavelength of radiation and having a relatively low adhesive affinity for the selected material being deposited.

Description: IMPROVED PHOTOCHEMICAL VAPOR DEPOSITION APPARATUS AND METHOD The Government has rights in this invention pursuant to Contract No. F33615-78-C-5049 awarded by the Department of the Air Force. BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to an apparatus and method for the photochemical vapor deposition of a layer of a selected material, and, more particularly, to such an apparatus and method using a selected polymeric coating on the window through which the initiating radiation for the photochemical reaction must pass. 2. Description of the Prior Art In the fabrication of semiconductor integrated circuits and devices, the deposition of a layer of a selected material, such as a dielectric material, a semiconductive material, or a metal, is frequently required. In some cases, this deposition may be most satisfactorily accomplished by a photochemical reaction, i.e., a chemical reaction which is induced or initated by radiation of a chosen wavelength. For example, one process for depositing a silicon nitride (Si3N4) dielectric layer uses a photosensitized reaction between silane (SiH4) and hvdrazine (N2H4), as disclosed in U.S. Patent No. 3,620,27 to M. C. Collet. In such a process, mercury (Hg) is excited by radiation of a particular wavelength (i.e. 2537 angstroms) to form mercury in an excited state (Hg*). The Hg* then collides with the SiH4 and the N2H4 to cause these molecules to form radicals which then react to form Si3N4- Ill the above-discussed prior art process, the photochemical vapor deposition process is initiated by the transmission of ultraviolet radiation (e.g. 2537 angstroms) through a quartz window which forms the top surface of the vacuum deposition chamber. The photochemical vapor deposition reaction then occurs in the deposition chamber and the selected material, such as silicon nitride, is deposited on the surface of a selected substrate or other target. However, while the selected material is being deposited on the substrate or target, additional deposits of the same selected material are also formed on the internal face of the quartz window of the deposition chamber. If this deposited material is opaque to the ultraviolet radiation, as silicon nitride is, this deposited material diminishes the transparency of the quartz window, which, in turn, diminishes the amount of radiation which can enter the deposition chamber to initiate the desired photochemical reaction. Since the rate of the photochemical reaction is dependent upon the amount of initiating radiation entering the deposition chamber, this unwanted deposit of ultraviolet-opaque material on the quartz window significantly decreases the rate of the deposition reaction, and ultimately the reaction may cease altogether. It is the alleviation of this latter problem of diminished window transparency to the selected radiation required for the initiation or inducement of a photochemical reaction caused by the formation on the window of deposits'which are opaque to the selected radiation, to which'the present invention is directed. One such process and apparatus for eliminating this problem of diminished window transparency is disclosed in our copending allowed application Serial No. 63,213. This copending application discloses a mobile transparent window apparatus and method for use in a photochemical vapor deposition system having a reaction chamber with a quartz window integral with the top surface thereof, in which a film of a predetermined material is drawn across the internal face of the quartz window within the reaction chamber. The predetermined material is transparent to the selected wavelength of radiation required for the selected photochemical reaction. The film placed at the internal face of the quartz window during the deposition process prevents undesirable or extraneous deposits of the selected material on the internal face of the quartz window. In addition, the movement of the film across the internal face of the quartz window removes the selected material deposited on the film from close proximity to the internal face of the window. Thus, the window is maintained in a clean and transparent state and the efficiency and rate of the deposition reaction is enhanced. The process and apparatus of the present invention provide yet another means for alleviating the prior art problem of diminished window transparency in a photochemical vapor deposition process. In a somewhat related area of prior art processes, certain vacuum systems, such as for the thermal evaporation and/or deposition of a metal, have used a window assembly in order to permit the operator to visually monitor the deposition as it occurs -in the chamber. Such a system using a window assembly comprising a clear Teflon film that is manually advanced is described in the catalog of AIRCO Temescal of Berkeley, California, entitled "Viewvac Viewing Ports Model W400." In addition, a vacuum deposition process using a coating of a perfluoropolyether on the inner surface of the viewing window is described in U.S. Patent No. 4,022,928 to B. P. Piwcyzk. However, in both of the vacuum deposition processes just described, the window is provided for convenience only and is not an integral part of the deposition reaction and process, affecting the rate and efficiency thereof. SUMMARY OF THE INVENTION The general purpose of this invention is to provide a new and improved apparatus and method for the photochemical vapor deposition of a layer of a selected radiation-opaque material on the surface of a chosen substrate which possesses most, if not all, of the advantages of the prior art photochemical vapor deposition processes and apparatuses, while overcoming their abovementioned significant disadvantages. The above general purpose of this invention is accomplished by a photochemical vapor deposition apparatus comprising: (a) a reaction chamber having at least a portion thereof formed of quartz to provide a quartz window for the transmission of a selected wavelength of radiation through the quartz window into the reaction chamber; (b) a film of a predetermined material deposited on the internal face of the quartz window to a predetermined thickness, with this predetermined material being transparent to the selected wavelength of radiation and having a relatively low adhesive affinity for the selected material to be deposited; (c) means for introducing a reactant gas mixture into the reaction chamber; (d) means for heating a substrate within the reaction chamber; (e) means for producing radiation of the selected wavelength and introducing this radiation into the reaction chamber through the quartz window; and (f) means for producing a vacuum in the reaction chamber of sufficiently low pressure to enable the deposition reaction to occur. During the deposition process, the film formed on the internal face of the quartz window prevents undesirable or extraneous deposits of the selected radiation-opaque material from forming on or immediately in front of the internal face of the quartz window. Thus, the window is maintained in a clean and transparent state which provides continuous transmission of the reactioninducing radiation into the reaction chamber, and thus enhances the efficiency and rate of the deposition process. Accordingly, it is a purpose of the present invention to provide a new and improved photochemical vapor deposition apparatus which enhances the efficiency of the deposition process for radiation-opaque materials. Another purpose is to provide an apparatus of the type described which significantly increases the rate at which a selected radiation-opaque material may be deposited on a substrate. Still another purpose is to provide an apparatus of the type described which prevents unwanted deposition of the selected material on the internal face of the quartz window of a reaction chamber. A further purpose of the present invention is to provide an apparatus of the type described which maintains the quartz window in a clean and transparent state throughout an extended time period required for the deposition process. Another purpose is to provide a new and improved apparatus of the type described which permits the deposition of a uniform and high quality layer of a desired material on a substrate. Yet another purpose of the present invention is to provide a new and improved photochemical vapor deposition method which minimizes the resistance to the transmission of reaction-inducing radiation caused by extraneous deposition products, and thus maximizes the efficiency and rate of the vapor deposition reaction. The foregoing and other advantages and features of the present invention will be apparent from the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of one photochemical vapor deposition system embodying the present invention. FIG. 2 is a schematic diagram showing the details of the coated quartz window of FIG. 1. DETAILED DESCRIPTION OF THE INVENTION FIG. 1 shows, in simplified form, one photochemical vapor deposition system incorporating the coated quartz window in accordance with the present invention. The reaction chamber 3, which may be formed of stainless steel, aluminum, or other suitable structural material, has a window 4 which is made integral with the top surface of the reaction chamber 3 as described in further detail with reference to FIG. 2. The window 4 is formed of a material which is transmissive to the selected wavelength of radiation used to initiate the desired photochemical reaction, such as quartz. This radiation 20 is produced by the radiation-producing means 22 which may be, for example, an array of low pressure mercury vapor arc lamps. The internal face of the quartz window 4 is coated with a film of a predetermined material as described in greater detail with respect to FIG. 2. Within the reaction chamber 3, there is a substrate holder 24, which holds the substrate 26 onto which a layer of a selected material is to be deposited. The substrate 26 may be formed of any material which is capable of being subjected to a photochemical vapor deposition reaction. External to the reaction chamber 3 at the bottom surface thereof, there are heating elements 28 which are used to heat the substrate 26 to the required temperature so that appropriate film properties, such as density, may be obtained. A tube 30 leads from the reaction chamber 3 and passes through a valve 32 and then to a vacuum-producing means (not shown). External to the reaction chamber 3, there are placed the chambers 34 and 36 which contain the individual reactant gases for the selected photochemical reaction, for example, silane and ammonia. The chambers 34 and 36 are connected to the valves 38 and 40, respectively, which are used to control the amounts of reactants that are introduced into a tube 42. The reactant gases flow through the tube 42 into a chamber 44 which contains a pool of mercury having mercury vapor above it. The reactant gases thus become mixed in the chamber 44 with mercury vapor and this reactant gas mixture then passes through the tube 46 into the reaction chamber 3, where the photochemical reaction may be brought about. The components of the apparatus shown in FIG. 1 may be constructed of stainless steel or aluminum unless otherwise specified. The reaction chamber 3 with the flat quartz window 4 integral with the top surface thereof as shown in FIG. 1 is but one example of a reaction chamber embodying the present invention. Alternatively, the reaction chamber 3 may have any configuration in which at least a portion thereof is formed of quartz or other material which is t'ransmissive to the selected wavelength of radiation, to allow this radiation to enter the reaction chamber. For example, the reaction chamber 3 may comprise a hollow quartz tube with the entire internal surface or a selected portion thereof coated with the predetermined radiation-transparent, low surface energy material in accordance with the present invention, as described in relation to FIG. 2. Furthermore, in addition to being used in a mercurysensitized photochemical vapor deposition process of the type described above with respect to silicon nitride, the present invention may also be used in a photochemical vapor deposition process in which a reactant gas is directly dissociated by photo-excitation, without the use of a mercury sensitizer. In such a direct dissociation process, the apparatus is the same as that described in relation to FIG. 1 with the deletion of the chamber 44 for containing the mercury. Attention is now directed to FIG. 2 for more particular consideration of the quartz window 4. Prior to insertion of the quartz window 4 into the reaction chamber 3 as shown in FIG. 2; a layer or film of a predetermined material is deposited on one surface of the quartz window 4, which will ultimately become the internal face 5 of the quartz window 4 when the quartz window 4 is incorporated into the chamber 3 as shown in FIG. 2. (The term "internal face" of the quartz window is used herein to mean that face of the quartz window which is internal to the reaction chamber.) This predetermined material is selected to be transparent to the selected wavelength of radiation used to initiate the desired photochemical reaction, and further, to have a relatively low adhesive affinity for the material to be deposited on the substrate. The term "adhesive affinity" is used herein to mean the probability for attraction and adhesion of-the photochemical reaction products or fragments thereof on the predetermined material. In addition, this predetermined material must be stable at the elevated temperature required for the deposition reaction and must have sufficiently lo vapor pressure so as not to significantly increase the evacuation time nor to produce contamination of the deposited layer. One such material is :r.s-y be j':.-. is a perfluorinated polyether, i.e. a polymer with repeating ether linkages (-C-O-C-bonds) and carbon atoms saturated with fluorine. One particular perfluoropolyether which may be used is a compound sold under the trademark name Fomblin Y25 by the Montecatini-Edison Company. A layer or film o the predetermined material is deposited on the internal face 5 of the quartz window 4 to a thickness in the range of 0.2 to 2.0 micrometers, for example, by spraying, painting, or rolling a solution of the Fomblin Y25 polymer in trichlorotrifluoroethane or chloroform onto the internal face 5 and then allowing or causing the solvent to evaporate, as is known in the art of forming polymer films. Thus, the relatively thin film 6 of the predetermined material is formed on the internal face 5 -of the quartz window 4 as shown in FIG. 2. After the film 6 has been deposited on the internal face 5 of the quartz window 4, the quartz window 4 is positioned on the top surface of the reaction chamber 3 as illustrated in FIG. 2, with an O-ring 11 being placed at the junction 12 between the quartz window 4 and the top surface 10 of the reaction chamber 3. Then, a vacuum is formed in the reaction chamber 3 by withdrawing ambient air through the tube 30 and the valve 32 by the vacuum-producing means (not shown), and the quartz window 4 thus becomes integral with the reaction chamber 3. (The reference designators 24, 26, and 46 shown in FIG. 2 indicate the same structures as set forth in FIG. 1 and are shown for purposes of correlating FIGS. 1 and 2.) Using the apparatus shown in FIGS. 1 and 2 for a photochemical vapor deposition reaction, the material to be deposited, such as silicon nitride, does not adhere to film 6 of the predetermined material. It is believed that this predetermined material has a molecular structure and surface energy such that this predetermined material has a low adhesive affinity for the material to be deposited on the substrate. The film 6 of the predetermined material prevents reactant gases from coming in contact with the internal face 5 of the quartz window 4 and, further, prevents extraneous and unwanted deposits from forming either on the-internal face 5 of the quartz window or immediately in front of this internal face 5 (i.e. on the film 6 itseif). Consequently, the internal face 5 of the quartz window 4 is maintained in a clean and transparent state by the process of the present invention since the accumulation of deposition products on the internal face 5 which are opaque to reaction-inducing radiation and would attenuate, absorb, or provide a resistance to the radiation entering the deposition chamber through the quartz window 4 has been avoided. (These materials which are opaque to the reaction-inducing radiation are referred to herein as "radiation-opaque" materials). Since the rate of deposition is directly proportional to the intensity of the radiation passing through the quartz window 4, the rate of the deposition reaction is significantly increased by the process of the present invention because the internal face 5 of the quartz window 4 is maintained in a clean and transparent state which provides continuous transmission of the reaction-inducing radiation into the reaction chamber. EXAMPLE 1 Using the apparatus described in FIGS. 1 and 2, a layer of silicon 'nitride was photochemically deposited on a silicon substrate by the reaction of silane, ammonia, and mercury vapor. The source 22 of the ultraviolet radiation 20 shown in FIG. 1 was an array of low pressure mercury vapor arc lamps which produced radiation having a wavelength of 2537 angstroms. The film 6 was formed of Fomblin Y25 obtained from the Montecatini-Edison Company of Italy, and was a perfluorinated polyether. The layer 6 was formed to a thickness of approximately 4,000 to 5,000 angstroms by the spraying procedure previously described herein. A photochemical vapor deposition process was used which is similiar to that described in U.S. Patent No. 4,181,751 to T.C. Hall and J.W. Peters, assigned to the present assignee, with the optional omission of the silane pre-treatment of ammonia. The reaction chamber 3, with the coated quartz window 4 in place as shown in FIG. 2 herein, was evacuated to a pressure of 10 3 Torr. The heating ele- ments 28 were activated to heat the substra-te 26 to a temperature of 150"C. Using the apparatus and procedure just described, a layer of silicon nitride was deposited on a silicon substrate at a rate in excess of 4,000 angstroms per hour. By comparison, the deposition rate of the prior art process discussed herein is typically 50 angstroms per minute initially and decreases rapidly due to the accumulation of ultraviolet radiation-absorbing deposits on the quartz window. Deposition of the silicon nitride in such prior art processes typically ceases after approximately 45 minutes, providing a total maximum film thickness of approximately 1000 to 1400 angstroms. Thus, a significant increase in the deposition rate can be achieved by using the apparatus and process of the present invention. In addition, the process of the present invention is particularly useful for avoiding the prior art leveling-out problem during a continuous photochemical vapor deposition process, in which the amount of material deposited decreases significantly with extended processing time periods. The deposition rate of the present invention can be maintained essentially constant for a period in excess of 3 hours. In actual practice, it has been found that one polymer-coated quartz window may be used for four successive deposition processes in accordance with the process of the present invention. After such time, the polymer coating degrades and must be removed and a new polymer coating formed on the quartz window. While the present invention has been particularly described with respect to the preferred embodiments thereof, it will be recognized by those skilled in the art that certain other modifications in form and detail may be made without departing from the spirit and scope of the invention. In particular, the scope of the invention is not limited to the photochemical vapor deposition of silicon nitride, which was used nerely as an example, but is intended to include the photochemical vapor deposition of any material which is opaque to the radiation used to initiate or induce the photochemical reaction. In addition, it is intended to include within the scope of the present invention any process in which radiation is introduced into a reaction chamber through a transparent window, including both mercury-sensitized photochemical reactions and direct dissociation of a reactant gas or gases by photoexcitation. Moreover, the predetermined material of which the film is made may be selected as required in order to provide a film that is transparent to the wavelength of radiation necessary to initiate the desired photochemical reaction and that has a low adhesive affinity for the material to be deposited. The predetermined material may be deposited as a film onto the quartz window in accordance with known processes for forming such films and is not limited to the particular process described herein. Further, while a quartz reaction chamber window has been discussed, the reaction chamber window may be made of any material which is suitable for the desired photochemical reaction process. Furthermore, it is not intended to limit the process of the present invention to the particular apparatus described herein, but rather it is intended to include any method for passing radiation to a predefined enclosed region or chamber in which a photochemical vapor deposition reaction is produced, and in which the window through which the selected radiation passes into the enclosed region is protected on its inner face by a selected radiation-transmissive, low surface energy material in order to minimize the resistance to the transmission of the radiation caused by unwanted or extraneous deposition of reaction products on or immediately in front of the window. Finally, it is also intended to include within the scope of the present invention, a method for depositing a layer of a selected material on a substrate or target in a predetermined pattern, such as lines or stripes, rather than as a continuous film. Such deposition of a patterned layer would require that the initiating radiation for the photochemical reaction be caused to strike only selected portions of the substrate or target onto which it is desired to form a deposit. The radiation could be focused on selected portions of the target or the radiation could be passed through a mask, such as a stainless steel mask placed over the target within the reaction chamber, to cause the radiation to strike selected portions of the 'target. Optionally, a layer of the perfluoropolyether material which is used to protect the internal face of the quartz window may also be formed on the surface of the substrate in a predetermined pattern to serve as a mask during the photochemical vapor deposition of the selected material on the substrate in a predetermined pattern. As yet another option, laser radiation of the appropriate wavelength could provide the required limited and directed radiation for depositing such a patterned layer.

Claims: CLAIMS What is Claimed is: 1. Apparatus for the photochemical vapor deposition of a layer of a selected radiation-opaque material on the surface of a chosen substrate comprising: a) a reaction chamber having at least a portion thereof formed of quartz, to provide a quartz window for the transmission of a selected wavelength of radiation therethrough into said reaction chambers b) a film of a predetermined material deposited on the internal face of said quartz window to a predetermined thickness, said predetermined material being transparent to said selected wavelength of radiation, and having a relatively low adhesive affinity for said selected material; c) means external to said reaction chamber and associated therewith for introducing a reactant gas mixture into said reaction chamber; d) means located within said reaction chamber for heating said substrate; e) means external to said reaction chamber for producing radiation of said selected wavelength and introducing said radiation into said reaction chamber through said quartz window; and f) means external to said reaction chamber for producing a vacuum in said reaction chamber of sufficiently low pressure to enable said deposition to occur, whereby said film on said internal face of said quartz window during the deposition process prevents undesirable deposits of said selected radiation-opaque material on or immediately in front of said internal face of said quartz window and maintains said window in a clean and transparent state which provides continuous transmission of said radiation into said reaction chamber to thus enhance the efficiency of said deposition process. 2. The apparatus set forth in Claim 1 wherein said predetermined material is a perfluorinated polyether polymer. 3. The apparatus set forth in Claim 1 wherein: a) said reaction chamber has a flat quartz window integral with the top surface thereof; b) said selected material deposited is silicon nitride; c) said selected wavelength of light is 2537 angstroms; and d) said film comprises Fomblin Y25 having a thickness in the range of 0.2 to 2.0 micrometers. 4. In an apparatus for the photocheinical vapor deposition of a layer of a selected material on a substrate which comprises a reaction chamber with a quartz window integral with the top surface thereof, means for introducing a reactant gas mixture into said reaction chamber, means for heating said substrate in said chamber, means for producing radiation of a predetermined wavelength and introducing said radiation into said cham- ber through said quartz window, and means for producing a vacuum in said chamber, the improvement comprising a film of a predetermined material deposited on the internal face of said quartz window to a predetermined thickness, said predetermined material being transparent to said wavelength of said radiation, and having a sufficiently low surface energy to produce a relatively low adhesive affinity for said selected material, whereby said film on said internal face of said quartz window during the deposition process prevents undesirable deposits of said selected material on or immediately in front of said internal face of said quartz window to thereby maintain said internal face of said window in a clean and transmi- sive state and to enhance the efficiency of said deposition process. 5. A method for passing radiation to a predefined enclosed region for producing therein a photochemical vapor deposition reaction while simultaneously minimizing the resistance to the transmission of said radiation caused by extraneous deposits formed by said photochemical vapor deposition reaction, which method comprises the steps of: a) providing a window of a chosen material adjacent to said enclosed region and suitable for passing radiation of a selected wavelength therethrough and into said enclosed region to produce said photochemical vapor deposition reaction within said enclosed region; and b) providing a film of a selected radiationtransmissive, relatively low surface energy material deposited on the internal surface of said window, operative to prevent the deposition of said extraneous deposits formed by said photochemical vapor deposition reaction onto or immediately in front of said window whereby said resistance to said transmission of said radiation caused by said extraneous deposits in the path of said radiation is minimized and thereby in turn maximizes the rate and efficiency of said photochemical vapor deposition reaction within said enclosed region. 6. A method for maximizing the radiation-induced rate of a photochemical vapor deposition reaction within an enclosed chamber which is sustaining said deposition reaction, including: a) providing a radiation-transparent window on one wall of said chamber, said window having deposited on the internal surface thereof a film of a selected radiation-transparent, relatively low surface energy material; and b) passing radiation of a chosen wavelength through both said window and said film and into said chamber to induce said photochemical vapor deposition reaction therein, whereby said film prevents extraneous deposition of reaction products both on and immediately in front of said internal surface of said window to thereby minimize the resistance to the transmission of said radiation therethrough and into said chamber caused by extraneous deposits of said reaction products and to thus maximize the efficiency and rate of said photochemical vapor deposition reaction occuring therein. 7. A method as set forth in Claim 6 wherein said radiation of said chosen wavelength is passed through both said window and said film to strike predetermined portions of a target for said photochenidal vapor deposition, positioned within said chamber, to thereby form a desired patterned layer of said reaction products on said target. 8. A method for reducing residual film buildup on the surface of a quartz window which is adjacent to a controlled photochemical reaction, comprising depositing a relatively thin perfluorinated polyether layer on said surface of said quartz window, whereby the relatively low adhesive affinity of said polyether layer for reaction products produced by said photochemical reaction substantially retards any film deposition of said reaction products on said g ther layer.

Inventors: GEBHART, FRANK L., PETERS, JOHN W.
Applicants: HUGHES AIRCRAFT COMPANY
Application year: 1981
Main classification: C23C14
IPC codes: H01L21, B01J19, C23C16