SEMICONDUCTOR CONTACT METAL PROTECTION

Abstract: In a photoresist lift-off process for depositing metal on a semiconductor substrate, a protective coating (40) of silicon is applied to an etched via hole (32) prior to the contact metal (42) deposition step. This prevents the formation of contami­ nant trapping voids and contaminated chemical residues which would otherwise occur at the base (42) of the metal line subsequently deposited in the bottom of the via hole. The protective silicon layer (40), which has a thickness of from 10 to 30 nanometers, remains intact as a permanent part of the structure.

Description: Field of the Invention The invention disclosed broadly relates to semiconductor processing and more particularly relates to metal deposition techniques in semiconductor processing. Background of the Invention Alkaline developers and chemical etchants, used to develop the photoresist layer for first and second metal layer depositions in existing FET processes, severely attack the contact metal, thereby producing poor interlevel contacts and possibly electrical failures. Also, contaminant residues, or voids created by the reaction of the etchants with existing contact metals or semiconductor substrates, will impact reliability and life of the device structures. These residues and voids increase resistance which is a significant cause of device failure. It is known to protect ion implanted and diffused contacts by depositing a barrier metal over the implanted area so that it will not be depleted by subsequent etching and/or other processes. This provides protection for the ion implanted diffused and/or buried contacts. However, it does not protect against chemical attack on the sides of the metal contact in via holes. Objects of the Invention It is therefore an object of the invention to provide an improved metallization process for semiconductor devices. It is another object of the invention to provide an improved metallization process which diminishes the problem of alkaline developers attacking contact metal structures. It is still another object of the invention to diminish void formations and eliminate chemical residues in the metal contact area in order to maintain contact integrity. Summary of the Invention These and other objects, features and advantages of the invention are accomplished by the process disclosed herein. In a photoresist lift-off process for depositing metal on a semiconductor substrate, a protective layer is applied to the etched via hole prior to the metal deposition step. This prevents the formation of contaminant trapping voids and chemical residues which would otherwise occur at the base and/or sides of the metal subsequently deposited in the contact via hole. The protective layer, which has a thickness of from 10 to 30 nanometers, remains intact as a permanent part of the structure. Description of the Figures These and other objects, features and advantages of the invention will be more fully appreciated with reference to the accompanying figures. FIGS. 1, 2 and 3 are a sequence of steps in existing photolithographic development processes, to produce a typical photoresist structure prior to metallization over contact metallurgy. FIG. 4 illustrates the first step of the invention relating to developing the resist of the contacts. FIG. 5 illustrates the steps of etching the insulator to produce contacts in the substrate and/or over the ion implanted diffused areas. FIG. 6 illustrates the step of flash evaporating a thin protective layer over the etched contacts and insulator sidewalls. FIG. 7 illustrates the step of blanket depositing contact metal. FIG. 8 illustrates the step of lifting off unwanted metal. FIG. 9 illustrates the step of developing the photoresist structure for interconnect metal prior to metal deposition. Discussion of the Preferred Embodiment In the preferred embodiment of the invention, a photoresist lift-off process is performed for depositing metal on a semiconductor substrate or ion implanted contacts. Then, a protective film is formed over the etched via holes, prior to the metal deposition step. This prevents the formation of contaminant trapping voids and chemical residues which would otherwise occur at the base and/or sides of the metal subsequently deposited in the etched contact via hole. The protective film, which has a thickness of from 10 to 30 nanometers, remains intact as a permanent part of the structure. In prior art methods, alkaline developers attack the contact metal. This is shown in FIGS. 1 through 3 which illustrate the prior art problem. In FIG. 1, the developer 10 has progressed almost all the way through photoresist layer 12, providing a structure for later to be made deposition of interconnect metal but short of contacting the previously deposited contact metal 14 over the substrate or ion implanted doped area 16. In FIG. 2, the developer 10 is shown to have completely removed the photoresist 12 for subsequently depositing interconnect metal over the contact 14. The developer 10 is now in contact with contact metal 14 at the bottom as well as the sides of via hole 18. In FIG. 3, it is seen that the developer 10 is actually attacking the physical structure of the previously deposited contact metal 14 and corroding it thereby leaving a void 19. The corroded contact metal is shown to have a reduced cross-sectional area 20 which increases the resistance per unit length of the metal contact. This also presents the problem of entrapping some of the alkaline developer and/or other chemical reaction residues 22 which may present contamination problems, thereby impacting the reliability and life of the product. The invention is shown in the sequence of steps illustrated in FIGS. 4 through 9. In FIG. 4, a contact photoresist layer 30 is shown with a via hole 32 made by a photoresist developer 34 over an insulator 36. The appropriate portion of the insulator is then wet or dry chemical etched to the semiconductor substrate 38 as shown in FIG. 5. An ion implant to produce a doped area 37 in the semiconductor substrate can be done at this time if desired. Referring to FIG. 6, in accordance with the invention, a flash evaporation step is then performed. From 10 to 30 nanometers of a protective film 40, such as silicon, is flash evaporated onto the etched contact sidewalls and the exposed semiconductor surface. This step protects the sides of the subsequently to be deposited contact metal from attack by alkaline resist developer or other chemical etchants and thereby protects it from corrosion. Thereafter, contact metal 42, 44 is blanket deposited by sputtering onto all exposed surfaces, as is shown in FIG. 7. Referring to FIG. 8, unwanted contact metal 44 shown in FIG. 7 is lifted off and the desired remaining contact metal 42 is intact on the silicon substrate 38 or ion implant doped region 37 in the etched region of the insulator layer 36. As shown in FIG. 9, interconnect metal photoresist layer 44 is applied, exposed and developed so as to expose the contact metal 42 which was previously deposited. For carrying out the process steps for depositing interconnect metal, reference is made to U.S. Patents 4,377,633; 4,076,575; 3,985,597; and 3,873,361. Photolithography and metal deposition procedures can be applied to complete the device and are described in detail in these patents. Referring to FIG. 2, by preventing the subsequently applied developer from etching along the interface between the insulator layer 17 and the lateral sides of the metal contact 14, the cross-sectional dimensions of the contact can be preserved as shown in FIG. 9, thereby maintaining a low electrical contact resistance. This becomes especially important for submicron geometries where this phenomenon of photoresist developer attack of the metal structures produces significantly high contact resistance. For example, although silicon has been described as the preferred protective film 40, other materials such as titanium, germanium, molybdenum silicide, and tungsten silicide can also be used. It is this class of materials which is preferred because they are resistant to corrosion effects caused by chemicals, and can be deposited with state-of-the-art equipment. Also, although the semiconductor material 38 is disclosed herein as preferably silicon, other substrate materials such as gallium arsenide and indium phosphide can also be used. Further, although the preferred contact metal 42 used in this invention is aluminum, other metals such as molybdenum, tungsten, titanium, and combinations of these materials can be utilized. The preferred photoresist 30 utilized with this invention is any positive photoresist. However, other photoresist such as optical materials, electron beam or negative photoresist, wet or dry developable, can also be used.

Claims: In a semiconductor fabrication process a method for protecting the sides of a contact metal (42) in a metal structure from lateral attack by chemicals used in subsequent process steps comprising the steps of: applying a layer of photoresist (30) to an insulator layer (36); forming a via through said photoresist on top of said insulating layer (36) to expose the material arranged below said insulating layer; depositing a layer (40) of protective material on the surface (32) of the sidewalls and the bottom of said window, said protective material being resistant to attack by said chemicals; depositing contact metal (42) into said window producing a continuous coverage of the exposed bottom and sidewall surface of the window with no gap at the interface between said metal (42) and said protective layer (40) on the window sidewalls; applying etching chemicals for photoresist removal, whereby said protective layer (40) prevents the sides of said contact metal (42) to be in contact with said chemicals. The method of claim 1, wherein said protective material (40) is selected from the group consisting of silicon, germanium, molybdenum silicide, and tungsten silicide. The method of claims 1 or 2, wherein said insulator layer is formed on a substrate (38) selected from the group consisting of silicon and gallium arsenide; said contact metal (42) is selected from the group consisting of aluminum, molybdenum, tungsten and titanium; and said photoresist (30) is selected from the group consisting of optical photoresist, electron beam photoresist, positive photoresist and negative photoresist, wet or dry developable. The method of any one of the preceding claims, wherein said subsequently applied chemical is an alkali photoresist developer. The method of any one of the preceding claims, wherein the thickness of said protective layer (40) is in the range 10-30 nanometers. An electrical contact structure for an integrated circuit, formed through a window in an insulator layer (36) on a silicon substrate (38) comprising a thin protective layer (40) of silicon coating the sidewalls (32) and bottom region of said window in said insulating layer; a contact metal (42) deposited on said silicon coating layer (40), filling said window, having no gaps at the interface between said contact metal (42) and said protective Layer (40).

Inventors: FREDERICKS, EDWARD C., NANDA, MADAN M.
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1985
Main classification: H01L21
IPC codes: H01L21, H01L23