PROCESS FOR FORMING CONTACTS AND INTERCONNECTS FOR INTEGRATED CIRCUITS

Abstract: In a process for forming ohmic contacts and interconnects for an integrated circuit structure such as a CMOS device structure, a thin layer of intrinsic polycrystalline or amorphous silicon (22) is formed over the structure and patterned to leave portions of the polysilicon layer (22) at contact and interconnect locations. The structure is then heated in an inert atmosphere to updiffuse dopant from doped substrate regions into regions (24A, 25A) of the polysilicon in contact with the substrate surface. A layer of tungsten (26, 27) is selectively deposited on the polysilicon by a CVD process. The tungsten (26, 27) acts to shunt p-n junctions in the polysilicon and any undoped polysilicon. In a second embodiment a tungsten layer (42, 43, 44) is provided to bridge a polysilicon layer (32) to a substrate region (38) and to shunt surface-adjacent p-n junctions (37, 39).

Description: PROCESS FOR FORMING CONTACTS AND INTERCONNECTS FOR INTEGRATED CIRCUITS Technical Field This invention relates to processes for forming contacts to substrate regions in a semiconductor substrate. The invention has a particular application to the provision of contacts and interconnects for CMOS devices formed in a semiconductor substrate. Background Art Metallization technology presently includes three broad areas: aluminum metallization processes; refractory metal processes, including those using tungsten; and composite layer processes, in which two or more separate conductors are deposited sequentially and then etched coextensively to the desired pattern. Each of these three approaches has advantages and disadvantages which are, typically, unique to the selected technology area. Consider first the prevalent aluminum metallization technology. Aluminum, or aluminum alloyed with a small percentage of silicon and/or copper, has been the standard microelectronics interconnect metallization for a number of years. Accordingly, aluminum has the advantage of being very well characterized. However, as device component thicknesses and lateral dimensions are made increasingly smaller in progressing from LSI to VLSI technologies, and beyond, the disadvantages of aluminum — the difficulty in etching small geometry aluminum lines, electromigration susceptability at small geometries, and aluminum spiking of shallow junctions — have become major impediments to its continued use. An article by Miller et al., "CVD Tungsten Interconnect and Contact Barrier Technology for VLSI", Solid State Technology, December 1982, pages 85-90 discloses a process wherein CVD tungsten is deposited selectively on exposed contact areas of a silicon substrate to provide low resistance contact barriers between aluminum metallization and the silicon substrate contact areas. This article also discloses the use of CVD tungsten deposited on polysilicon lines such as polysilicon gates to act as sheet resistance reducing shunt material. U.S. Patent No. 4,441,247 discloses the selective deposition of tungsten on the upper surface of polysilicon gates and on source and drain regions. The U.S. Patent also discloses the provision of a tungsten region in contact with a polysilicon gate and with a doped surface region of the substrate to form an electrical coupling member. U.S. Patent No. 4,042,953 relates to refractory metal contact/interconnect fabrication techniques. Representative completed structures are comprised of a layer of high temperature inert refractory metal, such as molybdenum, tungsten, platinum, nickel or palladium, sandwiched between upper and lower silicon layers. In one application, the refractory metal sandwich composite is used to provide ohmic contact between a semiconductor substrate and a metal interconnect. In a second application, the composite forms MOSFET gate electrodes and interconnects. For the contact application, the two silicon layers provide ohmic contact between the substrate and the refractory metal and between the refractory metal and the metal interconnect. A typical fabrication sequence involves forming a thick isolation oxide layer on the substrate and defining contact cuts therein, then sputter depositing the silicon and refractory metal, etching the metal to an interconnect pattern and thereafter sputter depositing the second silicon layer. Disclosure of the Invention It is an object of the present invention to provide a simple process for forming contacts to doped semiconductor, without regard to the impurity type of the semiconductor regions being contacted. According to one aspect of the present invention, there is provided a process for forming contacts to substrate regions in a semiconductor substrate, including the step of providing a silicon substrate having doped surface regions, characterized by the steps of: forming a layer of silicon over at least said doped surface regions, heating the structure to diffuse dopant from the silicon substrate through the surface-adjacent regions of the silicon layer; and selectively depositing a layer of metal on the silicon layer to provide a low resistance ohmic contact between the doped surface regions and the metal. According to another aspect of the present invention, there is provided a process for forming contacts to substrate regions in a semiconductor substrate, characterized by the steps of: providing a silicon substrate having first exposed regions for making buried contacts and second exposed regions for making contacts between the silicon substrate and a silicon interconnect layer; forming a silicon interconnect layer on the silicon substrate to cover part of the second exposed regions; and selectively depositing a layer of tungsten on exposed surfaces of the silicon substrate and the silicon interconnect layer to shunt any surface-adjacent p-n junctions. A process according to the invention has the advantage of suitability for use in a process wherein contacts and interconnects can be electrically connected to semiconductor regions without regard to the impurity type of the semiconductor regions being connected. Thus, a process according to the invention is particularly suitable for use in ' the manufacture of CMOS devices. Moreover, the process of the present invention is useful for fabricating low resistance, self-aligned, ohmic contacts to substrate diffusion regions as part of a unitary process for forming integrated circuit metallization using a common composite structure and with a minimum number of additional processing operations. Brief Description of the Drawings Embodiments of the present invention will now be described by way of example, with reference to the accompanying drawings, in which:- FIGS. 1 through 4 are schematic cross- sections of an integrated circuit taken sequentially during the formation of lower level metallization and self-aligned contacts in accordance with the present invention. FIG. 5 is a schematic of the CMOS circuit formed by the structure depicted in FIG. 4. FIG. 6A-- 6D depict various stages in the fabrication of an alternate embodiment for making polysilicon to diffusion ohmic contacts, following a procedure in which the polysilicon layer is formed prior to the creation of the S/D or diffused interconnect regions in the substrate. Best Mode for Carrying Out the Invention FIG. 1 is a cross-sectional representation of a CMOS type integrated circuit at an intermediate stage of fabrication, configured as an inverter. FIG. 1 depicts the CMOS integrated circuit structure 10 immediately after contact cuts have been made to the device source/drain regions. The structure 10 is the starting point for the application of the present invention. At this point, the integrated circuit structure 10 comprises an NMOS FET structure 11 and a PMOS FET structure 12 which are formed, respectively, in p-well 13 and n-well 14 of a <100> silicon substrate 15, with the active regions separated by a field isolation oxide 16. The NMOS FET 11 and PMOS FET 12 have source/drain regions 18 and 17, respectively, which are self-aligned with the polysilicon FET gate electrodes 19. Heavily doped well contact regions 8 and 9 immediately adjacent source and drain regions 17 and 18 provide ohmic connections to respective wells 14 and 13. As shown in the figure, the contact cuts 20-20 have been formed in the silicon dioxide interlevel dielectric layer 21. This can be done by any one of many conventional photolithographic masking and etching techniques. FIG. 1 quite obviously is only- one, exemplary, application among the various NMOS, PMOS, CMOS, bipolar and other applications of the present contact and interconnect formation process. Next, referring to FIG. 2, a layer of intrinsic polycrystalline or amorphous silicon 22, preferably about 50 nanometers or less in thickness, is conformally deposited on the IC structure of FIG. 1, typically using low pressure chemical vapor deposition (LPCVD) process. For instance, to deposit polysilicon one might use silane at 625-675°C temperature and 240 mTorr pressure. Optionally, silicon layer 22 is doped to a relatively low concentration with n-type impurity either during or after the deposition process using any of a number of standard technologies, e.g. by diffusion or ion implantation, to provide a concentration of about lxl0 iu atoms/cc. The deposited silicon layer provides excellent adhesion to the monocrystalline silicon of substrate 15 and field oxide 16, and being conformal also provides excellent step coverage. Referring next to FIG. 3, where layer 22 is polysilicon, the layer is delineated by photoresist (PR) etch mask 23 and patterned by etching to define fine geometry contacts 24 and contact/interconnect 25. Polysilicon layer 22 (Fig. 2) is then patterned in the presence of mask 23 using, for example, an anisotropic reactive ion etch (RIE) to form very fine geometries of < 2 micrometers. A representative etch process is reactive ion etching using a fluorine based etching gas such as nitrogen trifluoride (NF3). In reactive ion etching, the mechanical component or ion bombardment component dominates the chemical reaction component and provides very well defined, vertical geometries which are desirable in order to minimize lateral conductor dimensional variations. A sinter operation, to updiffuse dopant from regions 8, 9, 17, and 18 through surface adjacent regions 24A and 25A, Fig. 4, of polysilicon layers 24 and 25, follows a strip of photoresist mask 23. A representative sinter operation would include an 2 ambient conditioning at 800-1000°C for 10-60 minutes at atmospheric pressure. In updiffusing, p-type and n-type dopants from the source/drain regions 17 and 18, and well contact regions 8 and 9, intrinsic polysilicon layers 24 and 25 are made conductive in surface adjacent regions 24A and 25A with corresponding dopant type. Note that, if the thin silicon layer 22 is lightly doped, in accordance with the option mentioned hereinabove, the updiffusing dopant can offset a light opposite dopant and thereby prevent the function of a p-n junction at the source/drain region. Following the sinter or anneal operation used to updiffuse, a suitable preclean of the polysilicon outer surface, using, for example, HNO3 at 85°C for 10 minutes followed by 30°1 HF at 27°C for 30 seconds, tungsten is selectively deposited, for example, to a thickness in the range of 250 to 500 nanometers. For instance, one may use a low pressure chemical vapor deposition (LPCVD) process, with representative conditions including the use of WFg + H2 at a temperature of 300°C and a pressure of 100 mTorr. This process deposits tungsten 26 and 27 over all the exposed silicon areas, in particular patterned polysilicon regions 24, 24A, 25 and 25A. This provides, as is evidenced in Fig. 4, self-aligned, ohmic, low resistance connections to the shunting layer of tungsten 26 and 27. The use of undoped silicon, together with dopant updiffusion during the sinter, permits the simultaneous formation of contacts to n-type and p-type regions such as 8, 9, 17 and 18. Any p-n junctions created within the updiffusion doped polysilicon or amorphous silicon layer region 24 and 25 are shorted by the immediately overlying tungsten layer. During this selective deposition step, no tungsten is formed over the oxide regions, such as the exposed interlevel dielectric 21. Because the tungsten deposits selectively on monocrystalline, polycrystalline or amorphous silicon, the tungsten pattern is defined by the shape of the exposed silicon, thereby providing automatically patterned, low resistance shunting regions 26 and 27 over the intrinsic and updiffusion doped regions of polysilicon, i.e., 24, 24A, 25 and 25A. Recall that such silicon exhibits excellent adhesion to the underlying oxide. Furthermore, tungsten is known to be highly resistant to electromigration, which feature becomes critical at small geometries. Note further, as shown in FIG. 4, that the selectively deposited tungsten occurs on the sidewall surfaces of the polysilicon as well as on the upper surface thereof, providing excellent confor al coverage of the polysilicon. Preferably the sintering or annealing performed to updiffuse dopant through the intrinsic polycrystalline or amorphous silicon is performed prior to the selective deposition of tungsten. This avoids the temperature induced conversion of such silicon into tungsten suicide, which conversion is known to produce stresses as a result of volumetric changes associated with such suicide formations. The structural embodiment depicted in FIG. 4 is shown by way of electrical schematic in FIG. 5. Note that the use of selectively deposited tungsten has allowed a direct, buried contact type connection of the supply voltage VQD both to n-well 14 via n+ diffusion 8 and to p+ doped S/D diffusion 17 of p- channel FET 12, and further provides for a similar buried contact arrangement for the connection of electrical ground both to p+ diffusion 9 of p-well 13 and to S/D diffusion 18 of FET 11. As shown, the . interconnect pattern for the two FET CMOS inverter 10 is formed by the composite of silicon layer 25 and tungsten layer 27, which interconnect has self-aligned contacts at S/D diffusions 17 and 18. Note that this interconnect corresponds to the output of the inverter depicted in Fig. 5. In addition, if sufficiently thin layers of polysilicon or amorphous silicon 24 and 25 are used in the contact regions, the tungsten will, by complete reduction of the silicon, thereby make direct, self- aligned, ohmic, low resistance contacts, to the p-type and n-type source/drain and diffusion regions in the substrate. Furthermore, it is also possible to reduce the silicon to tungsten alone, whereby 2WF5 + 3Si is reacted to form 2W + 3SiF4, with the latter being removed as a gas. Using the process parameters described above, the desired complete polysilicon reduction to tungsten and the resulting direct contact results from using a polysilicon thickness of 40 nanometers or less for the silicon layer regions 24 and 25 and a tungsten thickness of at least about 50 nanometers. It will be appreciated that the retention of the polycrystalline or amorphous silicon layer 22 over the doped substrate regions such as 8, 17, and its subsequent reduction to tungsten has an advantage as compared with an alternative possibility of etching away the polycrystalline or amorphous silicon layer 22 over such substrate regions and directly depositing tungsten thereat, in that the precise removal of the polycrystalline or amorphous silicon without undesired etching of the underlying substrate surface is difficult to control. Following the above delineated contact formation, the integrated circuit structure 10 is completed conventionally by subjecting the substrate to passivation. Thus, there has been described a process • which uses selective tungsten deposition in a metallization scheme for VLSI devices. Either polycrystalline or amorphous silicon can be used as the base conductor layer for the selective tungsten deposition/patterning. When undoped silicon is used for the base layer, self-aligned, ohmic contacts can be formed simultaneously both in n-type and p-type silicon regions (such as source and drain diffusion regions) using a low temperature sinter operation. Alternatively, the sinter can be used to form self- aligned direct ohmic contacts to the source/drain regions or to other regions such as polysilicon gates by reduction of the very thin silicon layer to tungsten alone. The combined contact and contact/interconnect process forms contacts and metallization as a common structure. A variation of the process in the present invention is depicted schematically in FIGS. 6A - 6D, by cross-section at different stages of fabrication. The process differs in that the selectively deposited tungsten forms polysilicon to substrate diffusion contacts by tungsten bridging. It is particularly noteworthy that according to this practice of the invention neither the thickness nor the impurity type of the polysilicon or amorphous silicon patterned according to the interconnect layer affects the shunting value of the selectively deposited tungsten. Beginning with FIG. 6A, there is shown a partially completed FET, having a gate electrode 28, an overlying silicon nitride layer 35, and relatively thick sidewall silicon dioxide regions 29. The active regions beyond sidewall oxides 29 are covered by thinner oxide layer 30. Preferably, according to the present practice of the invention, oxide layer 29 is thermally grown to extend approximately 100-150 nanometers laterally, while layer 30 is grown during the same operation, to be in the range of 25-50 nanometers thick. The FET structure depicted in Fig. 6A includes the presence of patterned photoresist 31 preparatory to an etch of unmasked oxide 30, which etch selectively removes oxide 30 over the surface of the monocrystalline substrate, such as depicted p-well 13 in Fig 6A. As shown in FIG. 6B, following the etch of exposed oxide 30, photoresist 31 is stripped, the wafer is conformally covered by an LPCVD layer of polysilicon 32, preferably to a thickness of approximately 10-50 nanometers, and in succession, masked by patterned photoresist 33. Note that at location 34 the pattern of photoresist 33 is spaced apart from the residuals of oxide layer 30. Because the impurity type of polysilicon layer 32 is of no consequence according to this practice of the invention, it may be in situ doped upon deposition, later doped by blanket implant, or doped in conjunction with the subsequent doping of the S/D regions. FIG. 6C depicts the shape of the structure after processing the configuration in FIG. 6B firstly with a silicon etchant, for which silicon dioxide and silicon nitride are etchant barriers, and then with a silicon nitride strip operation, both operations being commonly known and practiced. As is schematically depicted, the silicon etchant also removes a thin layer of p-well substrate 13 in exposed surface regions 34 and 36 by virtue of conventionally implemented overetching procedures. To reach the state of fabrication depicted in FIG. 6D, the structure in FIG. 6C is subjected to either diffusion or preferably implant operations to create source/drain regions 37 and 38, and by separate operation well contact diffusion 39. . The need for individual masking and doping operations to create regions of different impurity type, such as 37 and 39, should be self-evident to those of skill in the art. The electrical shunts by which such doped silicon regions of different impurity type are interconnected are again provided by selective deposited tungsten, layers 42, 43 and 44. As depicted in FIG. 6D, the FET at 45 now has S/D diffusion 37 connected to p-well contact diffusion 39, by way of a buried, low resistance, tungsten shunting layer 42, has polysilicon gate electrode 28 shunted by low resistance tungsten layer 43, and has interconnect polysilicon layer 32 making contact to S/D diffusion 38 by way of shunting tungsten layer 44. Tungsten layer 44 electrically bridges any p-n junction between diffusion 38 and polysilicon 32. Therefore, differences in the impurity type of diffusion 38 and polysilicon layer 32 have no consequence on the making of the contact. Furthermore, because updiffusion is not utilized according to this practice of the invention to make ohmic contact, the thickness of polysilicon 32 is also of no consequence. Though the embodiment depicted in the fabrication sequence of FIGS 6A - 6 ' D employs a capping nitride 35 to mask gate polysilicon 28 during the patterning of polysilicon contact/interconnect layer 32, it is equally feasible to use oxide alone, which oxide can be grown from polysilicon gate 28 or LPCVD deposited over the patterned polysilicon gate. In that case, an anisotropic etch of the oxide enclosing the FET gate electrode, so as to selectively retain sidewall oxide adjacent gate polysilicon 28, would supplant the nitride strip operation performed between Fig. 6B and 6C of the drawings.

Claims: CLAIMS 1. A process for forming contacts to substrate regions in a semiconductor substrate, including the step of providing a silicon substrate (15) having doped surface regions (8, 9, 17, 18), characterized by the steps of: forming a layer.of silicon (22) over at least said doped surface regions (8, 9, 17, 18); heating the structure to diffuse dopant from the silicon substrate (15) through the surface-adjacent regions of the silicon layer (22); and selectively depositing a layer ' of metal (26) on the silicon layer (22) to provide a low resistance ohmic contact between the doped surface regions (8, 9, 17, 18) and the metal (26). 2. A process according to claim 1, characterized by the step of patterning said layer of silicon (22) to define contact regions (24, 25) over said doped surface regions (8, 9, 17, 18), and to define interconnect regions (24A, 25A) . 3. A process according to claim 1, characterized in that said layer of silicon (22) is formed to a thickness in the range of about 50 nm. 4. A process according to claim 1, characterized in that said layer of silicon (22) is formed of intrinsic silicon. 5. A process according to claim 1, characterized in that said step of forming a layer of silicon (22) is effected by conformally depositing polycrystalline or amorphous silicon. 6. A process according to claim 1, characterized in that said heating step is effected by sintering at a temperature in the range of 800-1000 degrees Celsius and an inert atmosphere of nitrogen at atmospheric pressure. 7. A process according to claim 1, characterized in that said step of selectively depositing a layer of metal (26) includes selectively depositing tungsten by low pressure chemical vapor deposition. 8. A process according to claim 7, characterized in that the step of selectively depositing tungsten by chemical vapor deposition causes at least a portion of said layer of silicon (22) to be reduced to tungsten silicide. 9. A process for forming contacts to substrate regions in a semiconductor substrate, characterized by the steps of: providing a silicon substrate having first exposed regions (36) for making buried contacts and second exposed regions (34) for making contacts between the silicon substrate and a silicon interconnect layer (32); forming a silicon interconnect layer (32) on the silicon substrate to cover part of the second exposed regions (34); and selectively depositing a layer of tungsten (42, 43, 44) on exposed surfaces of the silicon substrate and the silicon interconnect layer (32) to shunt any surface-adjacent p-n junctions. 10. A process according to claim 9, characterized by including, after the step of forming a silicon interconnect layer (32), the step of doping source/drain regions (38) and diffused interconnect regions (37, 39) of the silicon substrate. 11. A process according to claim 9, characterized in that said step of forming a silicon interconnect layer includes: forming a conformally deposited interconnect layer (32) o the substrate; and patterning the conformally deposited interconnect layer (32) in the presence of a mask (33) to expose silicon substrate regions forming at least a part of the regions for making contacts (34, 36). 12. A process according to claim 11, characterized in that the silicon interconnect layer (32) is of polycrystalline form and is conductively doped in conjunction with the doping of the source/drain and diffused interconnect regions (37, 38, 39).

Inventors: DRURY MICHAEL JOSEPH, METZ WERNER ADAM JR, MILLER GAYLE WILBURN, SULLIVAN PAUL ANDREW, SZLUK NICHOLAS JOHN, DRURY, MICHAEL, JOSEPH, METZ, WERNER, ADAM, JR., MILLER, GAYLE, WILBURN, SULLIVAN, PAUL, ANDREW, SZLUK, NICHOLAS, JOHN
Applicants: NCR CO, NCR CORPORATION
Application year: 1986
Main classification: H01L21
IPC codes: H01L21, H01L27, H01L29