PROCESS FOR FORMING A POLYSILICON GATE INTEGRATED CIRCUIT DEVICE

Abstract: In a process for forming a CMOS integrated circuit structure having polysilicon gates (18, 24) and interconnections (19) which are all of the same conductivity type, preferably n<s+>s-type, polys ilicon is formed into the gate (18) for the n-FET, a barrier layer (20) for the p-FET region (15) and the interconnection pattern (19). Then a layer of arsenosilicate glass (ASG) (23) is formed over the n-FET active region (14), the interconnections (19) and in an area to define the p-FET gate (24) which is etched using the ASG layer (23) as a mask. The device is heated to drive in impurities from the ASG layer (23) to n<s+>s dope the polysilicon and form the n-FET source and drain (27, 28). Boron is then implanted into the p-FET source and drain (25, 26), the ASG layer serving to mask the polysilicon from p-type doping. Since the polysilicon which is etched is undoped, highly accurate self alignment is obtained.

Description: PROCESS FOR FORMING A POLYSILICON" GATE INTEGRATED CIRCUIT DEVICE Technical Field This invention relates to processes of the kind for forming an integrated circuit including the steps of providing a body of semiconductor material having first and second active regions of respective first and second conductivity types therein, providing a silicon dioxide gate insulator layer over said active regions and providing first and second doped polysilicon gates having the same conductivity type doping over said gate insulator layer. Background Art A process of the kind specified is known from U.S. Patent Specification No. 4,209,797 which discloses a process for making a CMOS integrated circuit device having polysilicon gates. According to this known pro¬ cess a semiconductor substrate is provided with p-type and n-type wells. A doped n-type (or p-type) poly- silicon layer is formed over the gate insulator layer. An oxide layer is formed over the polysilicon layer and is removed except in areas corresponding to the gate regions. The polysilicon layer except for those portions under the remaining oxide layer is then removed by plasma etching. Then the gate insulator layer is removed except for those portions under the remaining polycrystalline layer. Next a phosphosilicate glass (PSG) layer and oxide film are formed on the whole sur¬ face of the resultant semiconductor structure. The PSG and oxide film except for those portions over the p-type well are removed. Boron is then diffused into the n-type well to form p-type source and drains and at the same time phosphorus is diffused from the PSG layer into the n-type well to form n-type source and drains. It will be appreciated that the known process has the advantage that the polysilicon gates are all -2- of the same conductivity type. However, it will further be appreciated that the known process utilizes predoped polysilicon to form the gates. Such predoped poly¬ silicon is not amenable to controlled etching and pre- cise gate definition. The known process therefore has the disadvantage of difficulty in controlling the etch¬ ing for precise gate definition of the polysilicon gates. Disclosure of the Invention It is an object of the present invention to provide a process of the kind specified wherein the aforementioned disadvantage is alleviated. Therefore, according to the present invention there is provided a process of the kind specified, characterized by the steps of forming said first poly¬ silicon gate over said first active region and a poly¬ silicon layer over said second active region; forming a mask containing impurities of the second conductivity type over the structure and laterally encompassing said first active region and defining a gate region in the second active region; forming said polysilicon layer in the second active region to said second polysilicon gate defined by the mask; implanting first conductivity type impurities in source and drain regions defined in said second active region by said mask; and diffusing second conductivity type impurities from said mask into source and drain regions defined in said first active region by said first polysilicon gate and into said polysilicon gates, the combination of implantation and diffusion forming a self-aligned source and drain in each of said first and second active regions and said doped polysilicon gates. It will be appreciated that in a process accor¬ ding to the invention since the polysilicon is etched prior to doping, such etching is easily controllable thereby alleviating the aforementioned disadvantage. Furthermore only a small number of masking steps are performed and the gates are accurately self-aligned with the sources and drains. According to a further feature of the inven- tion the first conductivity type is p-type and the second conductivity type is n-type. Thus the mask contains n-type impurities. This is advantageous since if the polysilicon gate electrode is doped with a p-type im¬ purity, typically boron, a phenomenon called boron pene- tration occurs. Thin gate insulators allow boron from p-doped polysilicon to diffuse through the insulator to the channel region of the substrate. This uncontrolled p-type channel doping lowers the p-channel transistor threshold voltages and raises n-channel threshold volt- ages. Consequently, boron p - gates require relatively thick gate insulators to prevent penetration, but the thick insulator, in turn, decreases the operating speed of the device. In contrast, n-type doped polysilicon not only allows use of thin gate insulators but makes possible FETs having a higher gain and lower threshold voltage than is possible with ρ + - polysilicon gates. Description of the Drawings Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 illustrates in a cross-sectional view the oxide isolation and the CMOS p-well process used in the present invention; Figs. 2-7 are cross-sectional views illus- trating various steps involved in forming a CMOS device using the present invention; * Fig. 3A is a plan-view of Fig. 3; Fig. 4A illustrates in a cross-sectional view an alternative embodiment; Fig. 5A is a plan view of Fig. 5; and Fig. 8 illustrates in cross-sectional view the finished device. Detailed Description of the Preferred Embodiments Reference is now made to Figs. 1-8 wherein the successive steps of the fabrication process of the present invention are illustrated in detail. The steps to be described hereinafter are illustrative of one technique for implementing the process of the subject invention. In addition, it should be noted that the specific processes for implementing the various steps of the fabrication method are well-known in the art and may be carried out in a number of different ways which are readily apparent to those of ordinary skill in the art. Fig. 1 shows a portion of a semiconductor sub¬ strate 10 having a well region 11 formed therein adjacent to its surface. Electrical isolation of adjacent de- vices is achieved by thick silicon dioxide regions 12 grown using a conventional localized oxidation of silicon (LOCOS) process at an elevated temperature (typically in the range 90G-1200°C.) for 1-20 hours. The thickness range of the region 12 so formed is typi- cally 10,000-20,000 Angstroms (1-2 microns).- After implantation of the well with the desired type of im¬ purities followed by diffusion of the impurities to the proper depth, a relatively thin layer of silicon dioxide 13 is grown over the active regions 14 and 15. The oxide layer 13 is grown in the conventional manner in a dry oxygen atmosphere at a temperature of 700-1000°C. for a period of 1-4 hours. The thickness of this thin oxide layer grown in this manner is in the range of 300-1200 Angstroms (0.03-0.12 microns). In the embodi- ment shown, the well 11 is p-type and is formed in an n- type substrate 10. This is only exemplary of the fabri¬ cation of a class of devices which embody the principles of the present invention, for it is readily possible to fabricate a CMOS device having a n-well in a p-type substrate. In addition, it should be noted that the thicknesses and other dimensions shown herein are selec¬ ted for clarity of illustration and are not to be interpreted in a limiting sense. The dimensions can be smaller or larger depending upon the operating environ¬ ment in which the device is going to be used. In this regard, it should be further pointed out at this time that the goal of microelectronics technology is toward smaller dimensions to achieve greater device density, better performance, etc. Next, as shown in Fig. 2, the polysilicon gates and interconnects (that is, conductors) are formed by depositing a layer of polycrystalline silicon 16 to the desired thickness over the entire structure using conventional techniques such as low pressure chemical vapor deposition (LPCVD) , or pyrolysis of silane (SiH.). The thickness of the polysilicon layer 16 is typically in the range 3,000-8,000 Angstroms (0.3-0.8 microns), the preferred thickness being about 5,000 Angstroms (0.5 microns). Beyond 8,000 Angstroms (0.8 microns), the well-known step-coverage problems during metallization will begin to appear. Below about 3000 Angstroms (0.3 microns), there will be a performance limitation of the device due to long signal propagation times caused by increased resistance of the thin polysilicon inter¬ connections. A silicon oxide masking layer 17 having a thickness typically in the range 500-3,000 Angstroms (0.05-0.3 microns) is then grown on the polysilicon 16 (Fig. 2) using one of the conventional techniques such as steam growth, growth in dry oxygen, pyrolytic decom¬ position, or plasma deposition. A photoresist mask (not shown) is then applied to the oxide layer 17. As shown in Figs. 3 and 3A, conventional photoengraving tech¬ niques are then used to pattern the polysilicon 16 into the gate 18 for the n-channel active region 14, the interconnect 19, and a mask 20 for the p-channel active region 15. Referring to Fig. 3, the portions 21-21 designate the portions of the oxide layer 17 that remain over the patterned polysilicon at the completion of this step. The oxide masking layer 21 over the poly¬ silicon regions 18, 19, 20 is now removed using a con¬ ventional etchant such as buffered hydrofluoric acid. During this step the thin oxide layer 13 in the n- channel active area 14 is also etched away except under the polysilicon mask 20 for the p-channel active region and under the polysilicon gate 18 where the oxide remains intact as the n-channel gate insulator 22 (Fig. 4). Next, referring to Fig. 4, a layer 23 of a material containing impurities of a chosen conductivity type is formed over the gate area of the p-channel active region 15, over the entire n-channel active region 14 and over the polysilicon interconnect 19. Preferably, the material 23 is arsenosilicate glass (ASG) . An example of ASG consists of a solution of an arsenic-doped polymer in alcohol and is sold by Allied Chemical Corporation under the trade name Accuspin and by Emulsitone Company under the trade name Arseno- silicafilm. The process of application of ASG is as follows. A small quantity of the doped polymer in alcohol solution is applied to the structure of Fig. 4. The structure is spun at a rate of 3,000-5,000 rotations per minute to obtain uniform coating. The structure is placed in an oven at a temperature of 150-200°C. for about 10-30 minutes to drive off the solvents from the ASG. The thickness of the ASG layer following the com¬ plete evaporation of the solvents is approximately 1,500 Angstroms (0.15 microns). Then, using conventional photolithographic techniques, the layer 23 of arsenosilicate glass is patterned to provide the configuration shown in Fig. 4. ASG layer 23 now completely masks the entire n-channel active region 14 and the polysilicon interconnections 19, but masks only the gate area of the p-channel active region 15. Referring to Fig. 5, the next step in the present process is to form p-channel gate electrode 24 ■y . * O--.PI ' by etching away the polysilicon in the active region 15 outside the arsenosilicate glass. A suitable etchant is a mixture of hydrofluoric acid, acetic acid, and nitric acid. Plasma etching can also be used. It may be desirable at this stage of the present process to perform a short step of dry thermal oxidation of the polysilicon gate walls 24A (Fig. 5) to prevent penetration of p-type impurities (to be implant¬ ed subsequently as explained below) into the polysilicon gate side walls. This step, however, is not essential to this invention. Referring to Fig. 5, p-type impurities (typi¬ cally boron) are then implanted by conventional, ion implantation into the active region 15 to form the p source 25 and drain 26 therein. A typical boron im¬ plantation utilizes an energy of 30-100kev and a dose of 10 ions per square centimeter. The resulting source and drain junction depth can be quite shallow, of the order of 0.3 microns. The junction depth, increases after the annealing step described below-, to the final, desired dimension. From the foregoing sequence of steps it is clear that the p source 25 and drain 26 are self- aligned to the overlying polysilicon gate 24. This is a much desired feature, because it eliminates stray capacitances such as gate overlap capacitance and in¬ creases the speed of the device. Next, the n drive-in diffusion step is accom¬ plished by introducing the structure of Fig. 5 into a furnace having an inert atmosphere such as dry gaseous nitrogen atmosphere at a high temperature in the range of 900-1,100°C. for a period of about 0.5-4 hours. During this step arsenic ions from the arsenosilicate glass form the n source 27 and drain 28 of the n-channel active region 14 and, at the same time, diffuse into the polysilicon areas to establish the high conductivity of the now n polysilicon gates 18 and 24 and the n poly¬ silicon interconnections 19 (Fig. 6). During this step the previously implanted boron ions in source 25 and drain 26 diffuse deeper into the silicon substrate to the desired concentration profile. The junction depth at the completion of this step is typically 0.3-0.8 microns for the n junction and 0.8-1.5 microns for the p junction. During this step gates 18 and 24 mask the channel regions from the arsenic ions. The determining factor in selecting the order in which the preceding two steps are carried out is the controllability of the boron penetration depth. At drive-in temperatures of greater than about 1,000°C, for example, it is preferable to carry out the n drive- in first as indicated by the dashed source and drain in Fig. 5, otherwise the implanted boron ions, due to their generally higher mobility at these higher temperatures, penetrate too deep in the p-channel source 25 and drain 26, thereby adversely affecting the p-FET threshold voltage. At drive-in temperatures below about 1,00Q°C., however, the boron implantation may be accomplished first, followed by the arsenic drive-in. Another possibility, regardless of the drive- in temperature, is simultaneous drive-in of boron during the arsenic drive-in of Fig. 6. It will be appreciated that an anneal step may be used, as desired, to repair any structural damage caused by ion implantation and to activate the implanted species. Prior to the drive-in diffusion step an oxi¬ dation step may be used to form a thin insulating layer 29 (Fig. 6) of thermally grown oxide over the arseno¬ silicate glass layer. The layer 29 will prevent unde¬ sirable outgassing of arsenic from the upper surface of the arseno-silicate glass and will also protect the device from possible autodoping. This capping oxide step, while not essential to the present invention per se, is recommended in the interests of safety and better yields. Next, the arsenosilicate glass layer 23 (and the capping oxide 29, if it was formed) is (are) etched away using a conventional etchant such as hydrofluoric acid. During this step the gate thickness oxide 13 in the active region 15 (Fig. 6) is also etched away except under the polysilicon gate 24 where the oxide remains intact as the p-channel gate insulator 30 as shown in Fig. 7. The processing at this point is substantially complete as far as the novelty of the present invention is concerned. The balance of the processing is generally conventionally performed. Referring to Fig. 8, this includes forming a thick oxide 31-31, opening contact holes, defining metal conductors 32-32, and forming a passivation layer 33-33 over the circuit. Although the CMOS device of the present inven¬ tion has been described in detail by selecting an n-type substrate having a p-well, ' the invention is fully ap¬ plicable to a p-type substrate having an n-well as shown . in Fig; 4A. In the latter case, the polysilicon is patterned such that a poly gate electrode 34 is formed in the n-channel region (i.e., the p-substrate) and the p-channel region (i.e., the n-well) is completely covered with the polysilicon layer 35. Layer 35 is then etched using arsenosilicate glass layer 36 as a mask. Layer 36 completely masks the n-channel active region. The remaining steps are the same as those described in connection with Figs. 5-8. The invention described above is a process tailored to permit n polysilicon gate doping for both p-channel and n-channel CMOS FETs and n + doping of the poly . interconnections. This process may also be advan¬ tageously adopted to form p + doped poly gates and inter¬ connections by substituting borosilicate glass (3SG) in place of arsenosilicate glass. It will be appreciated that devices having all p -doped poly gates and inter¬ connections are not suitable for CMOS VLSI circuits unlike the devices having all n poly gates and inter¬ connections since the former must necessarily have thick gate oxide layers (of the order of at least 1,000 Ang¬ stroms (0.1 microns) in order to overcome the boron penetration problems discussed above. By carrying out the above process, four differ¬ ent types of CMOS FETs may be obtained by means of only two masking steps, i.e., the steps illustrated in Figs. 4 and 5. These are: (1) n-substrate device with all n + poly gates and interconnects, (2) n-substrate device with all p poly gates and interconnects, (3) p-substrate device with all n poly gates and interconnects, and (4) p-substrate device with all p poly gates and intercon¬ nects. The advantages of this process include self- aligned gates, all-n -doped or all-p^-doped poly gates and interconnections, and a small number of masking steps. This leads to better yields and a reduction in cost. An advantage of the described embodiment is obtained since the ASG- layer 23 is formed in the p-FET active region 15 over a dual layer of gate oxide 13 and polysilicon 20. This arrangement avoids the problem of counterdoping which would occur if the impurity-con¬ taining layer 23 were formed in contact with the p-FET active region of the substrate 10. Although the layer 23 is removed from the p-FET active region 15 prior to the diffusion and implanting steps it will be appreci¬ ated that there would be a risk of leaving an impurity residue after such removal which could produce counter- doping of the p-FΞT source and drain if the layer had been in contact with the substrate. In summary it will be appreciated that the ASG layer 23 serves (1) as an etch mask for the p-FET gate; (2) as a source of n-type dopant for the poly- silicon gates and interconnects; (3) for self-aligned formation of n-FET source and drain regions; and (4) as an implantation mask during self-aligned formation of the p-FΞT source and drain.

Claims: CLAIMS : 1. A process for forming an integrated cir¬ cuit including the steps of providing a body of semi¬ conductor material (10) having first (14) and second (15) active regions of respective first and second con- ductivity types therein, providing a silicon dioxide gate insulator layer (13) over said active regions and providing first (18) and second (24) doped polysilico ' n gates having the same conductivity type over said gate insulator layer (13), characterized by the steps of form- ing said first polysilicon gate (18) over said first active region (14) and a polysilicon layer (20) over said second active region (15); forming a mask (23) con¬ taining impurities of the second conductivity type over the structure and laterally encompassing said first active region (14) and defining a gate region in the second active region (15) ; forming said polysilicon layer (23) in the second active region (15) to said second polysilicon gate (24) defined by the mask (23); implanting first conductivity type impurities in source and drain regions (25, 26) defined in said second active region (15) by said mask (23); and diffusing second conductivity type impurities from said mask (23) into source and drain regions (27, 28) defined in said first active region (14) by said first polysilicon gate (18) and into said polysilicon gates (18, 24) the combination of implantation and diffusion forming a self- aligned source and drain in each of said first and second active regions (14, 15) and said doped polysili¬ con gates (18, 24) . 2. A process according to claim 1 charac¬ terized in that said first conductivity is p-type and said second conductivity type is n-type. 3. A process according to claim 1, charac¬ terized in that the thickness of the gate insulator layer 3. ( concluded ) (13) is 300-1200 Angstroms (0.03-0.12 microns) and the thickness of the polysilicon layer (20) is 3,000-8,000 Angstroms (0.3-0.8 microns). 4. A process according to claim 1, charac¬ terized by the step of forming a layer of silicon diox¬ ide (29) over said mask (23) prior to said diffusion step, whereby surface outgassing of impurities from the mask (23) is prevented. 5. A process according to claim 1 charac¬ terized by the steps of forming polysilicon intercon¬ nects (19) for said integrated circuit and forming said mask (23) over said interconnects (19). 6. A process according to claim 2, wherein said integrated circuit is a CMOS integrated circuit characterized in that said body of " semiconductor material (10) is n-type semiconductor material and in that said first active region (14) includes a p-well (11) provided in said semi-conductor body (10). 7. A process according to claim 2 charac¬ terized in that said mask (23) is formed of arseno¬ silicate glass. 8. A process according to any one of the preceding claims characterized in that said step of diffusing is carried out prior to said step of implant¬ ing.

Inventors: BROWER RONALD W
Applicants: NCR CO, NCR CORP
Application year: 1981
Main classification: C22B21
IPC codes: H01L21, H01L27, H01L29