Dual gate formation

Abstract: A structure and method for fabrication of an integrated circuit having adjoining n + and p + gates (25,31) is disclosed. The adjoining n + and p + gates are covered by a titanium nitride layer (27). The titanium nitride layer is covered by a protective layer (29), illustratively made from PETEOS and by flanking spacers (not shown in FIG. 2). The protective layers help to prevent oxidation of the titanium nitride layer. (Oxidation of the titanium nitride layer would destroy its shunting capability.) Also disclosed is a heating step which simultaneously activates source/drain implants as well as flowing an overlying dielectric.

Description: Technical Field This invention relates to a method of manufacturing a semiconductor integrated circuits and their manufacture. Background of the Invention Those who manufacture MOS integrated circuits are utilizing designs in which both n + and p + gates are contained within the same integrated circuit. Unfortunately, when an n + gate contacts a p + gate, counterdoping may occur and a region of poor conductivity may occur where the n + and the p + gate contact each other. An attempt at solving the problem is depicted in FIG. 1. In FIG. 1, reference numeral 11 denotes a substrate which may, typically, be silicon. Reference numeral 13 denotes a gate, made from patterned, n + doped polysilicon. Reference numeral 15 denotes a p + doped patterned polysilicon. Typically, gates 13 and 15 are patterned together from the same blanket layer of polysilicon, although each has been doped separately while the other has been masked. There is an opportunity for counterdoping at boundary 19 between gates 13 and 15. Layer 17 is made from titanium nitride. Titanium nitride layer 17 acts as a shunt over gates 13 and 15 and preserves conductivity across boundary 19 where an n + - p + diode exists. Unfortunately, titanium nitride layer 17 is easily oxidized, and its conductivity thereby substantially reduced if thermal processing temperatures go to 800°C or above. A structure as described above is disclosed in US-A-4745079, except that the material of layer 42, which corresponds to layer 17 of FIG. 1 is not specified. US-A-4890141 (corresponding to the preamble of claim 1) discloses a CMOS device including a TiN local interconnect layer for electrically connecting an n + doped polysilicon region to a p + doped polysilicon region, similar to the FIG. 1 prior art depiction illustrates in this application. JP-A-59124161 discloses an MIS-type FET including an n + poly region 11 and a p + poly region 12 disposed in a contiguous relationship across the top surface of a gate oxide 13. Research Disclosure (1991) no. 326 discloses a low resistivity stack gate electrode structure where a layer of TiN is interposed between a lower layer of polysilicon and an upper layer of titanium disilicide. The TiN layer is used as a conductor and also provides a dopant barrier between the layers. Those concerned with the development of integrated circuits have consistently sought for methods to preserve conductivity across the n + / p + gate boundary despite the problems posed by subsequent thermal processing. Furthermore, those concerned with the development of integrated circuits have continually sought methods of reducing thermal budgets. Summary of the Invention The above problems are solved by the method as defined in claim 1. Brief Description of the Drawings FIG. 1 is a cross-sectional view of the typical prior art device. FIGS. 2-5 are partially cross-sectional, partially perspective drawings showing an illustrative embodiment of the present invention. Detailed Description In FIG. 2, reference numeral 21 denotes a substrate which, typically, may be silicon, doped silicon, epitaxial silicon, etc. In general, the term "substrate" is used to refer to a body of material upon which other bodies or layers of material may be subsequently formed. Formed atop substrate 21 is gate oxide layer 23; n + polysilicon gate layer 25 is formed atop gate oxide 23. p + doped polysilicon gate 31 is also formed atop gate oxide 23. p + gate 31 contacts n + gate 25. Titanium nitride layer 27 is located on top of gate 25 and 31. Titanium nitride layer 27 contacts both gates 25 and 31 and electrically shunts them. Layer 29 is a dielectric, desirably formed from plasma enhanced tetraethoxysilane (PETEOS). Applicant has determined that dielectrics formed from low pressure CVD TEOS are unsatisfactory because the dielectric deposition process causes undue oxidation of titanium nitride layer 25 during deposition. Silicon nitride has also been found to be unsuitable for layer 29 because subsequent etching of the silicon nitride/polysilicon/oxide stack is very difficult. Other acceptable dielectrics for layer 29 may be spin on glass (SOG) or Low Temperature Oxides (desirably formed below 400°C). The process for creating the structure of FIG. 2, typically, involves depositing a blanket layer of oxide over substrate 21. Next, a blanket layer of polysilicon is deposited over the oxide layer. The polysilicon is then doped utilizing p + and n + dopants in the appropriate places. Next, blanket layers of titanium nitride and PETEOS are deposited. The PETEOS layer may be patterned using a gate mask. The patterned PETEOS is utilized as a hard mask to pattern the titanium nitride, polysilicon, and oxide layers, thereby producing the structure of FIG. 2. It will be noted that titanium nitride layer 27 is now covered by protective PETEOS layer 29 and that the plasma process utilized to form PETEOS layer 29 has not caused any appreciable oxidation of titanium nitride shunt layer 27. Turning to FIG. 3, LDD (lightly doped drain) source and drain implants 33 and 35 are performed. Next, a blanket layer of PETEOS or silicon nitride or TEOS denoted by reference numeral 37 is formed. Next, an (optional) implant thermal activation of the LDD implant step may be performed. Next turning to Fig. 4, blanket layer 37 is an isotropically etched to produce spacers 39 and 41. Implants 43 and 45, designed to complete the source-drain regions may also be performed. It will be thus noted that the titanium nitride layer 27 is effectively encapsulated and protected by layer 29 and spacers 39 and 41. Turning to FIG. 5, lightly-doped source structures formed by implantations 33 and 43 have been designated by a single reference numeral 47. Similarly, lightly-doped drain structure designated by reference numerals 35 and 45 have been designated by a separate reference numeral 49. Next, a blanket dielectric layer 51, which may be a bilayer of, for example, TEOS beneath a material chosen from the group of: BPTEOS or LPCVD TEOS or PETEOS, is formed. After dielectric 51 has been formed, it is smoothed by flowing it at a temperature of between 800°C and 900°C. The thermal smoothing of dielectric 51 also simultaneously accomplishes an annealing of source/drain regions 47 and 49. The thermal smoothing step is performed in an atmosphere with no more than 2% oxygen. Because of the minimal quantity of oxygen present temperatures as high as 900°C may be used without risk of damage to layer 27. Thus, the same thermal processing step at this point of the process simultaneously smooths dielectric 51 while accomplishing a source/drain activation at the same time. Thus, thermal budget is reduced. Furthermore, the thermal heating step at this point of the process also prevents out-diffusion of boron dopant from those portions of substrates 21 which are doped with boron because no source/drain contact windows have been opened. (If source/drain contact windows had been opened, a thermal, processing step would induce some boron present in substrate 21 to out-diffuse.) From this point further processing may continue, such as the opening of windows to source and drain regions, filling of the windows with conductive material, deposition of further interlevel dielectrics and conductors, etc.

Claims: A method of integrated circuit fabrication comprising: forming an oxide layer (23) upon a substrate (21); forming a polysilicon gate layer upon said oxide layer; doping a first portion of said polysilicon gate layer with n-type dopant (25); doping a second portion of said polysilicon gate layer with p-type dopant (31); forming a layer of titanium nitride (27) upon said polysilicon gate layer; forming a layer of first dielectric material (29) upon said layer of titanium nitride; CHARACTERISED IN THAT said first dielectric material is chosen from the group consisting of PETEOS, SOG and LTO; and in that said method further includes: patterning said first dielectric layer (29) by at least partially etching said first dielectric layer (29) in the presence of a patterned resist; patterning said titanium nitride layer (27), and said polysilicon gate layer (25) and said oxide layer (23) using the patterned first dielectric layer as a mask (29), thereby exposing a portion of said substrate and whereby an n-doped polysilicon gate (25) adjacent a p-doped polysilicon gate (31) is defined, these gates being shunted by the patterned titanium nitride layer (27); forming a blanket layer (37) of a second dielectric material over said gates and over the exposed portion of said substrate; and an isotropically etching said blanket layer to form spacers (41, 39) adjacent said n-doped gate and said p-doped gate. The method of claim 1 further including the steps of: forming source and drain regions (47, 49) by embedding dopants in said source and drain regions; forming a blanket layer of undoped oxide (51) over said gate and said source and drain regions; forming a blanket layer of doped oxide over said layer of undoped oxide; performing a thermal heating step at a temperature between 800°C and 900°C to thereby simultaneously activate said dopants and smooth said doped dielectric; and opening windows through both said dielectrics, thereby exposing said source and drain regions. The method of claim 1 wherein said spacers (41, 39) are formed from a material chosen from the group consisting of PETEOS, silicon nitride and TEOS.

Inventors: LEE KUO-HUA, YU CHEN-HUA DOUGLAS, LEE, KUO-HUA, YU, CHEN-HUA DOUGLAS
Applicants: AT & T CORP, AT&T CORP.
Application year: 1995
Main classification: H01L29
IPC codes: H01L21, H01L29, H01L27