CMOS structure fabrication
This application is a continuation of prior application Ser. No. 07/588,867, filed Sep. 27, 1990, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates, in general, to semiconductor devices, and more particularly to the fabrication of CMOS devices which may be integrated into a BICMOS process. 
     A major trend in the semiconductor art is towards the fabrication of circuits integrating devices of multiple technologies. For example, a BICMOS integrated circuit which contains both bipolar and CMOS devices is highly desirable because the best characteristics of both technologies may be obtained and superior high performance integrated circuits may be fabricated. 
     There have been many problems associated with the integration of bipolar and CMOS devices into a single circuit. It is generally impractical to fabricate CMOS devices in a traditionally bipolar structure and vice versa. Many attempts at doing so have resulted in circuits having poor performance and requiring large amounts of real estate. Accordingly, the practical fabrication of BICMOS integrated circuits must have process integration flexibility as well as enhanced scalability characteristics. 
     In view of the above, it would be highly desirable to have a method of fabricating a CMOS structure that could be easily integrated into a BICMOS process. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a method of fabricating a CMOS structure that may be integrated into a BICMOS process. 
     Another object of this invention is to provide a method of fabricating a CMOS structure having process integration flexibility. 
     It is an additional object of the present invention to provide a method of fabricating a CMOS structure having excellent scalability characteristics. 
     The foregoing and other objects and advantages are achieved in the present invention by one embodiment in which, as a part thereof, includes providing an isolation module having a plurality of active device regions in which at least one N doped well and at least one P doped well are formed. A first conformal nitride layer is formed over the surface of the structure and portions thereof disposed over the doped wells are removed. Gate oxide is formed above the exposed doped wells prior to forming a conformal polycrystalline semiconductor layer on the surface of the structure. The conformal polycrystalline semiconductor layer is then doped so that it comprises a P conductivity type where disposed above an N doped well and an N conductivity type were disposed above a P doped well. Gate electrodes are then formed from the conformal polycrystalline semiconductor layer and are employed in the self-aligned formation of first portions of source and drain regions in the doped well. Once these first portions have been formed, dielectric spacers abutting the edges of the gate electrodes are formed and used in the self-aligned formation of second portions of the source and drain regions. A second conformal nitride layer is then formed on the surface of the structure and a conformal oxide layer is formed on the second conformal nitride layer. Source and drain contacts are then fabricated. 
     A more complete understanding of the present invention can be attained by considering the following detailed description in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a highly enlarged cross-sectional view of a portion of an isolation module of the type to be used in conjunction with the present invention; and 
     FIGS. 2-4 are highly enlarged cross-sectional views of a portion of a CMOS structure during processing. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a highly enlarged cross-sectional view of an isolation module 10 of the type to be used in conjunction with the present invention. Although isolation module 10 as depicted herein comprises specific materials, conductivity types and dopant concentrations, it should be understood that these may vary. The fabrication of isolation module 10 may be in accordance with the method disclosed and taught by copending U.S. patent application Ser. No. 07/431,420, now U.S. Pat. No. 4,994,406 entitled &#34;Method of Fabricating Semiconductor Structures&#34; filed on Nov. 3, 1989 by B. Vasquez and P. Zdebel. It should be understood that the invention may also be employed with other type isolation modules or structures. 
     Isolation module 10 comprises a monocrystalline silicon substrate 12 having a P+ conductivity type and a dopant concentration of approximately 2×10 16  to 2×10 17  atoms/cm 3 . An epitaxial silicon layer 14 is formed on substrate 12. Epitaxial layer 14 may be either N type or P type and in this embodiment, has a relatively low dopant concentration on the order of 1×10 14  atoms/cm 3 . A maskless buried layer 16 is formed on epitaxial silicon layer 14. Buried layer 16 comprises epitaxial silicon, has an N conductivity type and a peak dopant concentration on the order of 3×10 19  to 4×10 19  atoms/cm 3 . An epitaxial silicon layer 18 is formed on buried layer 16. Epitaxial layer 18 is approximately one micron thick, may be either N type or P type and has a dopant concentration on the order of 5×10 15  atoms/cm 3 . Generally, active devices will be formed in epitaxial layer 18 of isolation module 10. 
     A plurality of isolation trenches 20 are formed through epitaxial layer 18, buried layer 16, epitaxial layer 14 and extend into substrate 12. Trenches 20 are lined with a trench liner oxide 22 and filled with polysilicon 24. Field oxide regions 26 are formed on the surface of isolation module 10 and with isolation tenches 20, generally separate and insulate active device regions 28 where the semiconductor devices will be formed. 
     FIGS. 2-4 are highly enlarged cross-sectional views of a portion of a CMOS transistor structure 30 during fabrication in accordance with the present invention. Although the fabrication of basically only a single transistor is depicted by these FIGS., it should be understood that the present invention is intended to fabricate CMOS structures and more specifically, integrating a CMOS process into a BICMOS process flow. 
     Now referring specifically to FIG. 2. After forming a screen oxide layer (not shown) on active device regions 28, epitaxial silicon layer 18 of active device regions 28 is doped to form doped wells 32. A P doped well 32A is formed by implanting a P type dopant such as boron into epitaxial silicon layer 18 of active device region 28. This will allow for the formation of an N channel device. An N doped well 32B is then formed by implanting an N type dopant such as phosphorous into epitaxial silicon layer 18 of other active device regions 28 where P channel devices are desired. Doped wells 32 may be formed by either a single implant or by multiple implants with staged energies and implant doses. Doped wells 32 may either be driven simultaneously or separately between implanting P type and N type dopants. Optimally, the surface concentration of P doped well 32A will be on the Order of 5×10 16  atoms/cm 3  and the surface concentration of N doped well 32B will be on the order of 8×10 16  atoms/cm 3 . 
     Following the formation of doped wells 32, a first conformal nitride layer 34 is deposited over the entire surface of structure 30. It should be understood that first conformal nitride layer 34 is deposited over the entire surface of isolation module 10 (see FIG. 1) in a BICMOS process flow. In a preferred embodiment, first conformal nitride layer 34 will have a thickness of approximately 500 angstroms. After its formation, first conformal nitride layer 34 is masked and its portions disposed above doped wells 32 are etched from the surface of structure 30. The mask should slightly overlap onto field oxide regions 26 so that they become partially exposed after the portions of first conformal nitride layer 34 are etched away. 
     Once first conformal nitride layer 34 has been etched as desired, the screen oxide layer (not shown) is removed from above doped wells 32. This oxide etch may be performed by many methods well known in the art although a wet etch employing hydrofluoric acid is preferably employed. Following the removal of the screen oxide layer (not shown), a gate oxide layer 36 having a thickness in the range of 100 to 150 angstroms is grown over doped wells 32. Gate oxide layer 36 is thermally grown in this embodiment although it may be formed by other methods. 
     After the formation of gate oxide layer 36, a conformal polysilicon layer 38 is formed on the surface of structure 30. It should be understood that in the entire BICMOS process, conformal polysilicon layer 38 would be formed over the entire surface of isolation module 10 (See FIG. 1). Initially, a first portion of conformal polysilicon layer 38 comprising approximately 500 angstroms is formed immediately after the formation of gate oxide layer 36. This first portion of conformal polysilicon layer 38 satisfies boundary states. Threshold implants are then performed through the first portion of conformal polysilicon layer 38 and gate oxide layer 36 into doped wells 32. Once these threshold implants have been performed, additional polysilicon is formed on conformal polysilicon layer 38 so that its total thickness is approximately 3000 angstroms. It should be understood that conformal polysilicon layer 38 will form the resistors and base electrodes of bipolar devices fabricated by the BICMOS process as well as the CMOS gate electrodes which will be explained presently. 
     Now referring specifically to FIG. 3. Conformal polysilicon layer 38 is doped following its formation. In MOS structure 30, the portions of conformal polysilicon layer 38 which will become gate electrodes are appropriately doped. In the entire BICMOS process flow, the portions of conformal polysilicon layer 38 which will become the resistors and base electrodes in the bipolar devices are also appropriately doped at this time. Conformal polysilicon layer 38 is masked and etched following its doping. On MOS structure 30, a doped gate electrode 40 is formed from conformal polysilicon layer 38. Doped resistors and base electrodes are formed by etching conformal polysilicon layer 38 in the bipolar regions of isolation module 10 (see FIG. 1). The etching of conformal polysilicon layer 38 is preferably performed by reactive ion etching although other types of etching may be employed. 
     Once gate electrode 40 has been etched from conformal polysilicon layer 38, source and drain regions 42 which are interchangeable in this structure are formed. Initially, first portions of source and drain regions 42 are implanted. The implant is self-aligned to gate electrode 40. Following the implantation of the first portions of source and drain regions 42, sidewall spacers 44 abutting gate electrode 40 are formed. The formation of spacers 44 is consistent with methods well known in the art including the formation of a conformal oxide or nitride layer in a low temperature deposition process such as PECVD from which spacers 44 are etched. The formation of spacers 44 may be done simultaneously with the formation of spacers abutting the resistors and base electrodes of the bipolar devices in a BICMOS process flow. Once sidewall spacers 44 have been formed abutting gate electrode 40, second portions of source and drain regions 42 are implanted. This implant is self-aligned to sidewall spacers 44. 
     The device depicted in structure 30 will comprise N type source and drain regions 42 because doped well 32A is P type. N type source and drain regions 42 are formed by implanting arsenic or another N dopant into doped well 32A as described above. In an MOS device where doped well 32 is of an N conductivity type, source and drain regions 42 would be of a P conductivity type and formed by implanting boron or another P type dopant. 
     Following the formation of source and drain regions 42, a conformal nitride layer 46 is formed on the surface of structure 30. Conformal nitride layer 46 is approximately 500 angstroms thick and forms an ion contamination barrier over structure 30. A low temperature oxide layer 48 is then deposited on nitride layer 46 over the entire surface of structure 30. Once deposited, oxide layer 48 is then planarized over gate regions 40. A maskless planarizing etch is employed in this embodiment. 
     After the deposition and planarization of oxide layer 48, the fabrication of the emitter-base structure of the bipolar device is the next step to occur in the BICMOS process. The fabrication of the emitter-base structure is set forth in copending application Ser. No. 07/382,879, now U.S. Pat. No. 5,026,663 entitled &#34;Method of Fabricating a Structure Having Self-Aligned Diffused Junctions&#34; filed on Jul. 21, 1989 by P. Zdebel and B. Vasquez. 
     Now refering specifically to FIG. 4. Once the bipolar emitter-base structure has been fabricated, source and drain contacts are formed for the CMOS devices. Initially, source and drain contact openings 50 are formed through oxide layer 48, nitride layers 46 and 34 and gate oxide layer 36. The openings extend to epitaxial silicon layer 18. Source and drain contact openings 50 are formed simultaneously with collector contact openings for the bipolar devices in the BICMOS process. Once openings 50 have been etched, they are filled with contact polysilicon 52 which is planarized and recessed so that the top surface of polysilicon 52 is below the top surface of oxide layer 48. Where contact polysilicon 52 is formed to contact source and drain regions 42 of a device having a P type doped well 32A, it will be doped with an N type dopant such as arsenic. Where contact polysilicon 52 contacts the source and drain regions of a device having an N type doped well 32B it will be doped with a P type dopant such as boron. If contact polysilicon 52 is desired to contact doped wells 32A and 32B, its conductivity type must be the same as the well it contacts. It should be understood that the formation of contact polysilicon 52 is performed simultaneously with the formation of polysilicon to contact the collectors and emitters of the bipolar devices in the BICMOS process. 
     Once contact polysilicon 52 has been formed and doped, a gate electrode contact opening 54 is formed by etching through oxide layer 48 and nitride layer 46 to expose polysilicon gate electrode 40. In the BICMOS process, contact openings to the polysilicon resistors and base electrodes of the bipolar devices are also formed at this time. Silicide 56 is formed on all exposed polysilicon including gate electrode 40 and contact polysilicon 52 of structure 30 as well as the polysilicon exposed on the bipolar structure. Once silicide 56 has been formed, contact metal 58 is formed on silicide 56. The formation of silicide 56 and contact metal 58 is by methods well known in the art. 
     Thus is is apparent that there has been provided, in accordance with the invention, an improved method of fabricating a CMOS structure that may be integrated into a BICMOS process flow. While specific embodiments of the invention have been shown and described, further modifications and improvements will occur to those skilled in the art. It is desired that it be understood, therefore, that this invention is not limited to the particular form shown and it is intended in the appended claims to cover all modifications which do not depart from the spirit and scope of this invention