PROCESS FOR FORMING A CMOS STRUCTURE

Abstract: A process for forming a CMOS structure in a silicon substrate, polysilicon gates (27N, 27P) for n-channel and p-channel devices. Then, p-channel device regions are protected by a photomask while arsenic is implanted into the unmasked polysilicon (27N) and n-type source/drain regions (29S, 29D). During a subsequent drive-in step the arsenic-doped silicon acquires a relatively thick oxide layer (30N) which is used as a mask for the n-channel device regions during a subsequent p-type implant for the p-type source/drain regions (31S, 31D). An interlayer oxide (33) is then provided having openings to contact areas in the substrate, and a polysilicon layer (35) is provided thereover. While masking the p-channel device regions with a photomask, arsenic is implanted into the unmasked polysilicon layer (35). During heating in an oxidizing atmosphere a further oxide masking layer (37N) is formed which masks the n-channel device regions during a subsequent boron implant into p-type contact regions. In a subsequent drive-in step, both n-type and p-type contact regions (39S, 39D; 41S, 41D) acquire an increased doping level. Finally, aluminum contact conductors (42) are provided on the polysilicon (35) above the contact regions (39S, 39D; 41S, 41D). The process provides reliable contacts for reduced dimension CMOS devices in a simple manner.

Description: PROCESS FOR FORMING A CMOS STRUCTURE Technical Field This invention relates to processes of the kind for forming, in a semiconductor substrate, integrated circuit structures having complementary n-channel and p-channel insulated gate devices. Such structures may be in the form of CMOS structures utilizing silicon gate technology. The invention has a particular application to the formation of electrical contacts to CMOS structures. Background Art A process of the kind specified is known from published international patent application No. WO 82/01380. In the known process, the formation of complementary field effect transistors is followed by the provision of a thick oxide layer over the substrate, the opening of contact apertures therethrough to the source/drain re¬ gions, and the provision of metal contacts to such re¬ gions. The topographical and material properties of contact formation in MOSFET integrated circuits such as CMOS structures make this operation one of the most critical in the fabrication sequence. Contacts between two conductive materials, such as metal and an underlying diffusion or polycrystalline silicon layer (also termed "poly" or "polysilicon") involve forming deep, steep- walled openings in an interlayer dielectric separating the two conductive layers . Aluminum metal contacts to diffusions tend to suffer from the problem of metal spiking through the diffused region, causing a short. A consideration common to all contacts is the necessity for controlling the size and location with respect to the feature being contacted. Misalignment or oversizing causes short circuits with adjacent features. This possibility causes design rules to be expanded in order to avoid the nearby feature. Consequently, as -$ff *t O PI overall circuit design rules shrink, minimum contact geometries and spacings can dominate other factors and determine the lower limit on device/cell sizes. Additional problems arise when attempts are made to provide devices having channel lengths of 3 microns or less and in which the source and drain junc¬ tion depths are dimensionally very shallow (perhaps 0.3 micron) and the concentration of dopant such as arsenic, phosphorus or boron is low (approximately 1 x 10 /cm or less) . Nonetheless, this concentration in short channel devices may result in high field effects such as punch-through and hot electron injection. Lowering the concentration to reduce or minimize such high field ef¬ fects increases the contact resistance and may increase the tendency for metal spiking. Disclosure of the Invention It is the object of the present invention to provide a process of the kind specified, suitable for use in the formation of electrical contacts to comple- mentary integrated circuit structures, which is simple yet has the capability of providing reliable contacts even when design dimensions are reduced. Therefore, according to the present invention, there is provided a process of the kind specified, characterized by the steps of: forming a silicon layer over the structure including first contact regions contacting n-type substrate contact regions and second contact regions contacting p-type substrate contact regions, doping selected regions of said silicon layer with an n-type dopant, the selected regions including said first contact regions; heating the resulting struc¬ ture in an oxidizing atmosphere to form an oxide masking layer over the doped selected regions of said silicon layer; utilizing said oxide masking layer as a mask, doping unmasked regions of the structure with a p-type dopant? and heating the resulting structure to increase the n-type dopant concentration in said n-type substrate contact regions and the p-type dopant concentration in said p-type substrate contact regions. It will be appreciated that a process according to the invention has the advantage that the increased dopant concentration in both the n-type and p-type sub¬ strate contact regions resulting from the heating steps and the utilization of the silicon layer provided over the substrate contact regions permits the use of rela¬ tively low concentrations of source/drain dopant to minimize high field effects, reduces the incidence of metal spiking through shallow junctions in the insulated gate devices, and decreases the contact resistance. It will be appreciated that the conversion of part of the silicon layer to oxide over the n-type substrate contact regions results in an increase in n-type dopant concen¬ tration in the remaining part of the silicon layer above such substrate contact regions thereby enabling acceler¬ ated diffusion of the n-type dopant into the n-type substrate contact regions during a subsequent diffusion step. Furthermore, these advantages are achieved without added process complexity in the form of photomasking steps, since the utilization of an oxide masking layer formed from the silicon layer is advantageous as com¬ pared with a process wherein an additional photomasking step is utilized to provide the requisite mask. Thus a considerable advantage is achieved as compared with a process wherein two separate photomasking operations are utilized to provide the increased dopant concen¬ trations in the respective n-type and p-type substrate contact regions. A further advantage is that the pro¬ vision of a silicon layer over the substrate contact regions alleviates the problem of thinning at struc¬ tural steps of the substrate surface which tends to occur with metal contacts, such as aluminum. According to a further feature of the inven¬ tion, prior to said step of forming a silicon layer, there are effected the steps of selectively implanting the structure with n-type dopant excluding selected MPI regions for p-channel type devices; heating the substrate to drive in the n-type dopant thereby forming n-doped regions for the n-channel type devices and to form an initial oxide masking layer at the n-type dopant im- planted surfaces; using said initial oxide masking layer as a mask, doping the substrate with a p-type dopant to form p-doped regions for the p-channel type devices; and forming on the structure an interlayer dielectric layer having openings corresponding to said n-type and p-type substrate contact regions. It will be appreciated that this further feature has the advantage that the utilization of the initial oxide masking layer as a mask during the form¬ ation of p-type doped regions for the p-channel devices avoids the provision of a separate photomask for this purpose. In brief summary of a preferred embodiment of the invention, conventional processes are followed up to the formation of a first layer polysilicon gate. Then all p-type regions, such as the p-type active region, are masked with photoresist during the n-type source/ drain doping step. The n-type areas are then preferen¬ tially oxidized during the anneal/drive-in step and the resulting oxide layer is used as an implant mask over the n-type regions during implantation of the p source and drain regions. It is to be understood that by "preferential oxidation" is meant the faster oxidation rate which is experienced with silicon which has been suitably doped. Thus, silicon which is heavily doped with arsenic oxidizes at a much faster rate than undoped or p-type (boron-doped) silicon. For example, the preferential oxidation rate of arsenic-doped (and/or phosphorus-doped) silicon may be 5-10 times higher than undoped or boron- doped silicon at a temperature of 700-800 β C. This relatively high oxidation rate of arsenic-doped silicon may be used to selectively form oxide layers which serve as implantation masks. OMPI After forming an interlayer dielectric, cutting contact windows to the doped substrate regions, and forming a second polysilicon layer, the second prefer¬ ential oxidation step is used to implement an enhanced contact diffusion (ECD) process. In the second prefer¬ ential oxidation process, the contact windows to p-type regions are masked with photoresist, and the wafer, including all contact openings to n-type contacts, is implanted with an n-type dopant such as arsenic or phosphorus. The n-type implanted surface is then pre¬ ferentially oxidized and the resulting oxide layer is used as a mask during the implant of the p-type dopant into the polysilicon over the contact windows to p-type substrate regions. Subsequently, a short anneal removes implant damage and activates the implanted dopants with 4-. -A- minimal diffusion of the underlying p and n source/ drain junction depths. In addition to the advantageous complementary use of preferential oxidation and enhanced metal-to- diffusion contact technology, the preferential oxide mask eliminates the use of photoresist during the boron implant. This eliminates any problem of boron ion- induced polymer cross-linking, which can cause resist strip problems and limit implant current as well as process throughput. Brief Description of the Drawings One embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figures 1-9 are cross-sectional representa¬ tions of a complementary metal oxide semiconductor (CMOS) wafer showing sequentially the major fabrication steps employed in implementing a preferred embodiment of the present invention. Best Mode for Carrying Out the Invention The present invention will be presented in the OMPI context of a p-well CMOS process employing a polysilicon gate, a locally oxidized isolation oxide and atmospheric pressure oxidation. There is, however, no restriction in its applicability to other approaches for forming silicon gate CMOS structures having arsenic N regions or to the use of other refractory gate materials such as molybdenum, polysilicon-silicide alloys, etc. Also, the selective, arsenic-enhanced oxidation described herein is part of a normal thermal activation/anneal cycle, but could be performed using a low temperature- high pressure oxidation approach. This latter approach substantially preserves the arsenic implant doping profile, except for an enhancement of arsenic concen¬ tration at the oxide/silicon interface. In considering the invention in detail, refer¬ ence is made to the cross-sectional representations of Figs. 1-9. These non-scale drawings illustrate the se¬ quence of major fabrication steps involved in construc¬ ting a CMOS structure using the invention. In particular, referring to Fig. 1, there is shown the starting point for application of the present process. The illustrated structure is intermediate to the overall process for forming a CMOS circuit. The partially-completed CMOS device is based upon a lightly-doped, n-type silicon layer 11 of approximately 10 ohm-cm resistivity. Typi¬ cally the layer 11 may have 1-20 ohm-cm resistivity and be either "epi" or bulk material. An "epi" (epitaxial) layer 11 would be formed on a heavily doped n-type substrate 12 such as 0.01 ohm-cm antimony- doped or arsenic- doped silicon. (To preserve drawing space, - substrate 12 is not shown in Figs. 2-9.) An epitaxial layer gives added SCR latch up protection in CMOS, but is not essential for the present process. The intermediate Fig. 1 structure has surface- adjacent p-well 17 and n-well 18. Actually, although "n-well" is used for simplicity of nomenclature here, 18 may be either an n-type surface-adjacent region in which O PI the p-well 17 is formed or a separately-masked n-doped region. Both p-well 17 and n-well 18 are formed within any existing epitaxial region 11. Thick field isolation oxide regions 23-23 define active areas 10P and ION for the subsequently formed PMOS and NMOS complementary devices, respectively. The various approaches for fabricating the intermediate structure, Fig. 1, will readily be apparent to those skilled in the art. However, to describe one approach, the upper, surface-adjacent region of the epi layer 11 is given a phosphorus implant for ultimately forming the n-well 18. An oxide layer is deposited/grown. Using a photomask, the oxide is then formed into a mask which delineates the p-well region, and boron is implanted (mask 1; see TABLE). The wafer is subsequently given a thermal drive-in to diffuse the p-well 17 and n-well 18 to a depth of 3-6 microns. After stripping oxide mask 1, a dual-layer mask of oxide and nitride is provided, typically by growing an oxide layer, depositing a nitride layer and using a second photomask to form the nitride into a field oxidation mask (mask 2). Mask 2 covers the PMOS and NMOS active regions 10P and ION, respectively. TABLE Photomask Level Conventional CMOS Process Present Embodiment (No ECD Structure) (ECD Structure) 1. P-well 1. P-well 2. Active Areas 2. Active Areas 3. Field Implant 3. Field Implant 4. Gates 4. Gates 5. N-channel S&D 5. N-channel S&D 6. P-channel S&D 7. Contact Cuts 6. Contact Cuts 7. N + Contacts 8. Metal 8. Metal 9. Silox Passivation 9. Silox Passivation Prior to oxidation of the field regions, a third photo¬ masking step is used to delineate the p-well field regions 22-22, then these regions are implanted using boron, to improve the field inversion threshold. Next, the isolation oxide 23 is grown in the field regions in the presence of the oxide-nitride mask. Subsequently, the oxide-nitride mask is itself removed and gate oxide layer 24 is grown in the active device regions 10P and ION to a thickness of 650 Angstroms. The wafer is then given a blanket implant to adjust the ultimate n-channel and p-channel threshold voltages to their desired values. To complete the Fig. 1 structure, a layer 27 of polycrystalline silicon 0.5 microns thick is deposi¬ ted on the wafer and doped using n-type dopant such as phosphorus to provide the desired conductivity. The gate regions are masked, as by photoresist (mask 4), and the polysilicon is etched by plasma or a wet chemical etchant to form the PMOS and NMOS gates 27P and 27N and interconnect 27C. Mask 4 is then removed and the gate oxide is etched back to a thickness of about 400 Ang¬ stroms to allow subsequent ion implant penetration. The crucial steps of my invention are illus¬ trated commencing with Figure 2. As shown there, a photomask is applied to cover the PMOS device areas (mask 5), then the wafer is implanted with arsenic to form source 29S and drain 29D which are self-aligned with the gate 27N of NMOS device ION. After mask 5 is stripped, the first arsenic-enhanced oxide mask 30N is formed during the furnace operation which diffuses the arsenic to the desired junction depth. See Figure 3. To achieve a junction depth of 0.4 microns and to form the differential oxide mask 30N, there is used a masked arsenic implant of 5 x 10 15 cm-2, 80 keV and a furnace cycle which consists of inserting the wafer into a fur- nace at relatively low temperature (e.g. 600°C) and a ramp up to approximately 750°C for implant annealing; a 750°C, 120 minute preferential oxidation in steam; and then a diffusion at 900°C for 60 minutes in nitrogen to drive-in the arsenic to the desired junction depth. The resulting oxide mask 30N has a thickness of about 1500-2000 Angstroms in the arsenic source/drain regions 29S and 29D and the arsenic-implanted gate 27N (Fig. 3). The exact time of oxidation and oxide growth depends on the arsenic dose as well as the selected drive-in temperature. For the exemplary 5 x 10 15 cm—9 dose and 750°C preferential oxidation temperature, the 120 minute oxidation operation suffices to form the 1500-2000 Angstrom oxide thickness. Elsewhere, layer 30P comprises about 100-200 Angstroms of additional oxide over the unimplanted areas which will later become the P+ source and drain regions and approximately 650 Ang- stroms of oxide on the phosphorus-doped polysilicon gate 27P. After oxidation, the wafer is subjected to a short dip etch to etch back the oxide 3OP to the orig¬ inal implant oxide thickness of approximately 400 Ang- stroms. After this etch, oxide mask 30N is still ap¬ proximately 1300-1700 Angstroms thick over the N+ regions such as ION. The P+ implant (boron or BF 2 ) is now carried out as a "blanket" implant, as illustrated in Fig. 3. A boron implant of 5 x 10 15cm-2, 30 keV provides PMOS source and drain 31S and 31D of 0.65 microns junction depth after a subsequent 950°C, fifteen minute, furnace diffusion cycle in oxygen. In addition to forming the PMOS source/drain regions, this sequence is also used to simultaneously form other p diffusions, such as p-well contacts. Please note, it is not necessary to strip the oxide mask 30. The foregoing sequence allows relatively un¬ constrained selection of the P+ and N+ doses, since only an implant "tail" from the boron will penetrate the mask 30N to the N+ regions. It should be explained that the word "tail" refers to a low boron doping con¬ centration profile which consists of those few boron atoms which penetrate all the way through the oxide 30N. Such tail penetration is effectively confined by the presence of the arsenic. These boron atoms do not dif¬ fuse out of the arsenic-doped source/drain regions be- cause the presence of arsenic greatly reduces the dif¬ fusion coefficient of boron. Consequently the boron has no effect on any of the NMOS device parameters. Still another advantage arises from the absence of a photoresist mask on the wafers during the boron implant. That is, the elimination of the photoresist mask (mask 6 of conventional process; see TABLE) elim¬ inates any problem of boron ion-induced polymer cross- linking which can cause resist strip problems and limit the implant current as well as process throughput. The above-mentioned brief, 15 minute oxygen anneal cycle also forms an oxide layer on the poly¬ silicon gates. This oxide layer provides better die¬ lectric integrity between the polysilicon gates and the subsequently deposited metal, in conjunction with interlayer dielectric 33. As shown in Fig. 4, interlayer dielectric 33 (0.7 microns thick silicon dioxide) is then deposited on the wafer. Various techniques may be used, including chemical vapor deposition at atmospheric pressure or low pressure, or plasma enhanced deposition. A photomask (mask 6; not shown) is used to delineate the contact locations in the deposited oxide, and contact cuts or windows are etched, as by plasma etching or by wet chemical etching in a buffered solution of ammonium fluoride and hydrogen fluoride. The photoresist mask is now stripped. It should be noted that Fig. 4 illus¬ trates five contact cuts. There are two each of cuts 34N and 34P to the PMOS and NMOS sources and drains, while cut 34C is for contact to the polysilicon 27C. It is unlikely that these five contact cuts would appear in the same vertical cross-section in an actual fabri¬ cation sequence. Rather, they are shown in this manner to facilitate illustration. The next phase of the fabrication sequence implements ECD contacts. Referring to Fig. 5, a layer 35 of undoped polysilicon is deposited on the wafer to a thickness of 0.2-0.5 microns. Photoresist mask 7 is applied to cover the PMOS active areas 10P and all p + diffusions. All regions not covered by mask 7 are im¬ planted using arsenic (5 x 10 to 2 x 10 cm dose, 80 keV) . Referring to Fig. 6, after stripping mask 7, a thermal cycle in an oxidizing atmosphere (750°C, four hours in steam) forms a preferentially-grown, 0.4 micron , _J. thick oxide mask 37N over the arsenic-doped N poly¬ silicon regions and forms oxide 37P of about only 800 Angstroms thickness in the undoped regions. This thermal cycle also provides oxidation-enhanced drive-in of the arsenic implant. Next, and referring to Figure 7, a "blanket" boron implant and an anneal/drive-in are undertaken, to form highly doped regions 41S and 41D which produce en- hanced contacts to the p source and drain regions 31S and 31D. The boron is effectively masked from the n- type contact areas by oxide 37N. The P anneal/drive-in is at 900°C for 30 minutes. This anneal/drive-in also enhances the poly- to-n contacts. That is, the high degree of segregation of arsenic from the oxide grown on the poly 35 combined with the slow diffusion of arsenic in silicon dioxide increases the arsenic concentration at the oxide-poly interface above the poly-to-n contacts 39S and 39D and accelerates diffusion of the arsenic dopant into the N + region below. This establi.shes hi•ghly doped N+ re¬ gions 39S and 39D and thereby provides ECD source and drain contacts to the n-channel device ION. It also forms highly doped N+ region 44 and thereby provides improved contact between the polysilicon 27C and the subsequently-deposited metallization 42. Referring to Fig. 8, after an oxide strip to remove the oxide 37 from the poly II layer 35, conductor layer 42 such as aluminum (or an aluminum alloy) is deposited on the wafer and is etched in the presence of photomask 8 to define conductor interconnections. The wafer is then dip-etched approximately 30 seconds in polysilicon etchant or plasma etched to everywhere remove the polysilicon 35 except under the aluminum interconnections, then mask 8 is removed. Referring to Figure 9, a passivation layer 43 is now deposited. As will be appreciated by those skilled in the art, it may be the more usual process to form additional interlayer dielectrics and additional conductor/metal interconnection layers prior to the final passivation layer. Finally, mask 9 (not shown) is used to define openings in the passivation layer over metal bonding pads, the mask is removed and an alloying heat treatment is applied. The final CMOS structure 10 shown in Figure 9 exhibits the advantages previously referred to, yet requires no additional photomask steps as compared to a standard CMOS process. Referring to the TABLE, it is seen that the present process with its ECD formation sequence utilizes nine photolithographic mask formation steps. Thus, the present process provides enhanced contacts using the same number of mask steps as the exemplary conven- tional CMOS process, which does not provide enhanced contacts. The N contact mask (mask 7 in the present process) is added while the conventional p-channel mask (mask 6) is deleted. If ECD contacts were not to be formed in the present process, N contact mask 7 would be unnecessary, thereby resulting in the elimination of one photolithographic masking step as compared to the conventional process. If a conventional enhanced con¬ tact diffusion were to be applied to the conventional process, that process would require two additional photolithographic mask steps as compared to the present process, to separately mask the N + and P + ECD implant steps. Thus, the process effectively saves two photo- lithographic masking steps if ECD structures are fabri¬ cated. While the present exemplary process is com¬ pared to and is an improvement of a nine-mask process that is believed representative of current state-of-the- art CMOS processing, the benefits are by no means limi¬ ted thereto. If the present invention were applied to a different CMOS process, for example, to one having a greater number of mask steps, the same relative savings in the number of mask steps and the other benefits would nonetheless result. These savings and benefits are the result of the use of the phenomena of preferential oxidation and retarded boron diffusivity in heavily doped arsenic regions. _ OMPI _

Claims: CLAIMS : 1. A process for forming, in a semiconduc¬ tor substrate, integrated circuit structures having complementary n-channel and p-channel insulated gate devices, characterized by the steps of: forming a silicon layer (35) over the structure including first contact regions contacting n-type substrate contact regions (39S, 3 D) and second contact regions contacting p-type substrate contact regions (41S, 41D) ; doping selected regions of said silicon layer (35) with an n- type dopant, the selected regions including said first contact regions; heating the resulting structure in an oxidizing atmosphere to form an oxide masking layer (37N) over the doped selected regions of said silicon layer (35); utilizing said oxide masking layer (37N) as a mask, doping unmasked regions of the structure with a p-type dopant; and heating the resulting struc¬ ture to increase the n-type dopant concentration in said n-type substrate contact regions (39S, 39D) and the p-type dopant concentration in said p-type substrate contact regions (41S, 41D) . 2. A process according to claim 1, charac¬ terized by the steps of: removing the oxide (37N, 37P) formed on said silicon layer (35); providing a metallic layer (42) over the resulting structure; and delineating said metallic layer (42) to form interconnection con¬ ductors contacting said first and second contact regions of said silicon layer (35). 3. A process according to claim 2, charac¬ terized by the step of forming a passivation layer (43) over the structure after said step of delineating. 4. A process according to claim 1, charac¬ terized in that, prior to said step of forming a silicon layer (35), there are effected the steps of: selectively OMPI implanting the structure with n-type dopant excluding selected regions for p-channel type devices; heating the substrate to drive in the n-type dopant thereby forming n-doped regions (29S, 29D) for the n-channel type devices and to form an initial oxide masking layer (30N) at the n-type dopant implanted surfaces; using said initial oxide masking layer (30N) as a mask, doping the substrate with a p-type dopant to form p-doped regions (31S, 31D) for the p-channel type devices; and forming on the structure an interlayer dielectric layer (33) having openings corresponding to said n-type and p-type substrate contact regions (39S, 39D; 41S, 41D). 5. A process according to claim 4, charac¬ terized in that said n-type substrate contact regions (39S, 39D) are included in said n-doped regions (29S, 29D) and said p-type substrate contact regions (41S, 41D) are included in said p-doped regions (31S, 31D) . 6. A process according to claim 4, charac¬ terized in that, prior to said step of selectively implanting the structure with n-type dopant, there are effected the steps of providing polysilicon gate elec- trodes (27N, 27P) for n-channel type and p-channel type devices, and polysilicon interconnects (27C), and in that said initial oxide masking layer (30N, 30C) covers said n-channel type gate electrodes (27N) and said polysilicon interconnects (27C). 7. A process according to claim 6, charac¬ terized in that said silicon layer (35) includes third contact regions in contact with regions of said poly¬ silicon interconnects (27C). 8. A process according to claim 1, charac¬ terized in that said silicon layer (35) is deposited in the form of undoped polycrystalline silicon. 9. A process according to any one of the preceding claims, characterized in that said n-type dopant is arsenic and in that said p-type dopant is boron. OMPI

Inventors: ELLSWORTH DANIEL LLOYD, ELLSWORTH, DANIEL, LLOYD
Applicants: NCR CO, NCR CORPORATION
Application year: 1983
Main classification: H01L21
IPC codes: H01L21, H01L27, H01L29