Abstract:
A method of fabrication of a device having laterally isolated semiconductor regions. In a preferred embodiment, laterally isolated polysilicon features are created with vertical, nitride-sealed sidewalls. The nitride-sealed sidewalls formed using sidewall spacer technology eliminate oxide encroachment while further preventing the loss of dopant laterally during thermal processing. The final structure comprises polysilicon features flanked by either oxide isolation or additional polysilicon features and is planar without requiring a planarization etchback. The process is applicable to polysilicon electrodes over active areas as well as polysilicon resistors over isolation oxide.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates, in general, to semiconductor devices, and more particularly to the fabrication of semiconductor devices having laterally isolated semiconductor regions. 
     Presently, a number of lateral isolation schemes of semiconductor regions are known in the art. However, there are typically problems with the existing schemes. For example, one such scheme employs a nitride mask having a window therein through which the semiconductor regions, commonly polysilicon, are oxidized. Major problems with using this method include a resulting non-planar device structure, a non-vertical polysilicon sidewall and the polysilicon dimensions vary considerably due to the variability in oxide encroachment. 
     In their paper entitled &#34;Planarized Self-Aligned Double-Polysilicon Bipolar Technology&#34;, IEEE 1988 Bipolar Circuits and Technology Meeting, Appendix III, Paper 6.1, Drobny et al. teach a SWAMI modification of polysilicon oxidation through a nitride window. A nitride sidewall spacer reduces oxide encroachment. However, the final planarity of the resulting structure after oxidation is dependent upon etching through half of the polysilicon prior to nitride sidewall deposition and polysilicon oxidation. This is extremely difficult because the problem of controlling the etch so that it stops halfway through the deposited polysilicon thickness hinders reproducibility and manufacturability. Additionally, the sidewalls taught by this method are not sealed. 
     U.S. Pat. No. 4,659,428 entitled &#34;Method of Manufacturing a Semiconductor Device and Semiconductor Device Manufactured by Means of the Method&#34; issued to Maas et al. on Apr. 21, 1987 discloses another approach. In this patent, silicon feature separation is accomplished by a groove having a dimension which is determined by the differential oxidation rates of undoped and heavily doped polysilicon or silicon layers under low temperature, steam oxidation conditions. There is an absolute requirement for heavy doping of the polysilicon or monosilicon layer of interest. Differential oxidation (approximately a tenfold difference) between the heavily doped layer of interest and the undoped sacrificial polysilicon layer is the heart of the process concept. 
     The present invention discloses a method for the fabrication of a device having sealed, laterally isolated semiconductor regions which reduces or eliminates the problems disclosed above. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide for the fabrication of a device having laterally isolated semiconductor regions wherein there is no encroachment into the semiconductor regions. 
     Another object of this invention is to provide for the fabrication of a device having laterally isolated semiconductor regions that may be scaled beyond photolithographically defined parameters. 
     It is an additional object of the present invention to provide for the fabrication of a device having laterally isolated semiconductor regions wherein the device has a planar finished structure. 
     Yet a further object of the present invention is to provide for the fabrication of a device having laterally isolated semiconductor regions wherein vertically insulating sidewalls prevent the loss of dopant laterally during thermal processing to flanking dielectric features. 
     The foregoing and other objects and advantages are achieved in the present invention by one embodiment in which, as a part thereof, provides a semiconductor substrate, forms semiconductor regions on the semiconductor substrate and forms insulating sidewalls that are impervious to impurities and oxidation to laterally insulate the semiconductor regions. 
     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 
     FIGS. 1-11 illustrate highly enlarged cross-sectional views of a portion of a semiconductor structure during processing; 
     FIGS. 12-13 illustrate highly enlarged cross-sectional views of a portion of a semiconductor structure; 
     FIGS. 14-18 illustrate highly enlarged cross-sectional views of a portion of a semiconductor structure during processing; and 
     FIGS. 19-20 illustrate highly enlarged cross-sectional views of a portion of a semiconductor structure. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1-3 illustrate highly enlarged cross-sectional views of a portion of a semiconductor structure during processing. Initially, a semiconductor substrate 10 is provided. In this embodiment, semiconductor substrate 10 is comprised of monocrystalline silicon although one skilled in the art will understand that semiconductor substrate 10 may comprise one of many semiconductor materials well known in the art. Of course, it must be understood that the material comprising various other layers of the semiconductor structure must electrically and physically correspond with the material of which semiconductor substrate 10 is comprised. 
     An isolation oxide layer 12 is then formed on semiconductor substrate 10. Isolation oxide layer 12 is comprised of silicon dioxide in this embodiment. Following the formation of isolation oxide layer 12, an insulating layer 14 is grown thereon. In this embodiment, insulating layer 14 comprises silicon nitride and is deposited on isolation oxide layer 12 by low pressure or plasma enhanced chemical vapor deposition. It is important that insulating layer 14 be impervious to both impurities and oxidation. One skilled in the art will understand that although the preferred embodiment defines a process for forming resistors over isolation oxide, the present invention is equally applicable to the formation of electrodes over active areas. This may be accomplished by eliminating isolation oxide layer 12 and possibly insulating layer 14. 
     A semiconductor layer 16 is formed following the formation of insulating layer 14. Semiconductor layer 16 comprises polysilicon in this embodiment and is formed by low pressure or plasma enhanced chemical vapor deposition. One skilled in the art will understand that amorphous silicon may also be employed to form semiconductor layer 16. Following the formation of semiconductor layer 16, an insulating layer 18 is formed thereon. Insulating layer 18 is comprised of silicon nitride and is formed by low pressure or plasma enhanced chemical vapor deposition. Again, it is important that insulating layer 18 be impervious to both impurities and oxidation. A dielectric layer 20 is then formed on insulating layer 18. Dielectric layer 20 is comprised of silicon dioxide in this embodiment and may be formed by any suitable process such as low pressure chemical vapor deposition or plasma enhanced chemical vapor deposition. 
     Openings 22 are formed through dielectric layer 20 and insulating layer 18. Openings 22 extend to semiconductor layer 16 and may be formed by many methods well known in the art although in this embodiment, reactive ion etching is used following the implementation of a mask. After openings 22 have been etched, a sidewall spacer layer 24 is formed on the surface of the device structure. This includes sidewall spacer layer 24 being disposed in openings 22 and on dielectric layer 20. In this embodiment, sidewall spacer layer 24 comprises silicon nitride which is formed by low pressure or plasma enhanced chemical vapor deposition. After its formation, sidewall spacer layer 24 is etched to expose dielectric layer 20 and semiconductor layer 16 in openings 22 except for sidewall spacers 26 which remain in openings 22. In this embodiment, the etch of sidewall spacer layer 24 is performed by reactive ion etching. It should be understood by one skilled in the art that a highly anisotropic etch is required to minimize reduction in the lateral dimension of sidewall spacer 26. 
     FIGS. 4-11 are highly enlarged cross-sectional views of a portion of a semiconductor structure during processing, the figures depicting one embodiment. Following the formation of sidewall spacers 26 in openings 22, portions of semiconductor layer 16 disposed between sidewall spacers 26 are partially oxidized to form oxidized regions 28 which are relatively thick in this embodiment. The minimum thickness of oxidized regions 28 is defined by the requirement to act as an etch mask for formation of sidewall openings 30 which will be explained presently. This oxidation is generally performed by thermally oxidizing those exposed regions of semiconductor layer 16 between sidewall spacers 26. Once oxidized regions 28 have been formed, sidewall spacers 26 are removed to expose unoxidized portions of semiconductor layer 16 in openings 22. Sidewall spacers 26 are removed in this embodiment by a wet nitride etch. 
     The exposed regions of semiconductor layer 16 in openings 22 are etched away to expose insulating layer 14 thereby forming sidewall openings 30. Sidewall openings 30 are formed by an anisotropic reactive ion etch of semiconductor layer 16. The formation of sidewall openings in the disclosed manner allows for sidewall openings 30 to be narrower than if formed using photolithography techniques. 
     Once sidewall openings 30 have been formed, a conformal layer 32 is formed in sidewall openings 30, as well as on the exposed portions of dielectric layer 20 and oxidized regions 28. In this embodiment, conformal layer 32 comprises silicon nitride and is formed by low pressure or plasma enhanced chemical vapor deposition. It is important that the material used to form conformal layer 32 be impervious to both impurities and oxidation. Conformal layer 32 is then etched away leaving only sidewalls 34 which extend above oxidized regions 28. The etch of conformal layer 32 is performed by reactive ion etching. This is followed by completely oxidizing the portions of semiconductor layer 16 which remain beneath oxidized regions 28. In addition to oxidizing these portions, this step also creates a relatively planar structure. It can be seen that semiconductor layer 16 comprises a series of laterally sealed semiconductor regions 36 that are isolated by relatively small isolation regions. 
     One skilled in the art will understand that it is possible to dope various regions of semiconductor layer 16 at various stages of the disclosed process. It is also possible to dope the various semiconductor regions 36 of semiconductor layer 16 at this time. To do this, the entire device structure is etched back to insulating layer 18. In this embodiment, an unmasked reactive ion etch is used. Once insulating layer 18 has been exposed, dopants may be implanted through insulating layer 18 or insulating layer 18 may be etched away. The latter may be done with a wet etch. 
     Now that semiconductor regions 36 of semiconductor layer 16 have been exposed, the exposed surfaces are thermally oxidized and a thin screen oxide layer 38 is formed. Screen oxide layer 38 protects the surfaces of semiconductor layer 16 during photolithography and the implantation of impurities. If it is desired that semiconductor regions 36 be completely sealed so that they are impervious to oxidation and impurities, a thin layer of silicon nitride may be deposited over the entire structure. This thin layer may be formed on screen oxide layer 38 or directly on semiconductor layer 16 in place of screen oxide layer 38. The silicon nitride layer will eliminate the loss of dopants during post doping thermal cycles. One skilled in the art will understand that an optional mask may now be employed if more than one impurity level is required. Next, impurity doping and annealing are performed by methods well known in the art. 
     FIGS. 12 and 13 illustrate highly enlarged cross-sectional views of a portion of semiconductor structure. As shown, FIG. 12 includes an oxide layer 40 formed over the entire device structure. Oxide layer 40 serves to insulate semiconductor regions 36 from metallization. As shown in FIG. 13, an encapsulating layer 42, comprised of silicon nitride in this embodiment is disposed on each semiconductor region 36 thereby completely insulating each semiconductor region 36 from the surrounding oxide layers. One skilled in the art will understand that encapsulating layer 42 may be formed by leaving insulating layer 18 (see FIG. 10) in place or by forming a completely new layer on the structure of FIG. 11. 
     FIGS. 14-18 are highly enlarged cross-sectional views of a portion of a semiconductor structure during processing, the figures depicting another embodiment. Initially, the processing steps are the same as those disclosed earlier and represented by FIGS. 1-3. After sidewall spacers 26 have been formed in openings 22, the portions of semiconductor layer 16 exposed in openings 22 are partially oxidized to form oxidized regions 28&#39;. Oxidized regions 28&#39; are extremely thin as depicted herein. Next, sidewall spacers 26 are etched away as described in the previous embodiment and sidewall openings 30 are formed as previously described. 
     Once sidewall openings 30 have been formed, dielectric layer 20 and oxidized regions 28&#39; are etched away by methods well known in the art. This is followed by a wet etch of insulating layer 18. Following the wet etch of insulating layer 18, a thin conformal layer 32&#39; is formed in sidewall openings 30 as well as on the exposed portions of semiconductor layer 16. The resulting structure includes a plurality of semiconductor regions 36 separated from each other only by sidewalls 34. 
     Again, dopants may be implanted into the various semiconductor regions 36 through conformal layer 32&#39; or the regions may have been doped at various stages of the disclosed process. Semiconductor regions 36 may also be doped by removing conformal layer 32&#39; to expose semiconductor regions 36 and thermally oxidizing the exposed surfaces to form a thin screen oxide layer 38. Screen oxide layer 38 will protect semiconductor layer 16 during photolithography and the implantation of impurities. If it is desired that semiconductor regions 36 be completely sealed so that they are impervious to oxidation and impurities, a thin layer of silicon nitride may be deposited over the entire structure as disclosed previously. 
     FIGS. 19 and 20 illustrate highly enlarged cross-sectional views of a portion of a semiconductor structure. Both figures include an oxide layer 40 formed over the entire device structure to insulate semiconductor regions 36 from metallization. FIG. 20 includes encapsulating layer 42 that completely isolates and seals each semiconductor region 36 from the surrounding oxide layers. Encapsulating layer 42 may be formed by leaving conformal layer 32&#39; in place or by forming a completely new layer. 
     Thus it is apparent that there has been provided, in accordance with the invention, an improved semiconductor device having laterally isolated semiconductor regions and a method of fabrication for the device which meet the objects and advantages set forth above. While specific embodiments of the present 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.