Abstract:
A process for removing both the silicon nitride layer and polysilicon layer in a poly-buffered LOCOS process which utilizes hot phosphoric acid is disclosed.

Description:
TECHNICAL FIELD 
     This invention relates to integrated circuit manufacturing in general, and more particularly to etching processes utilized in silicon semiconductor integrated circuits. 
     BACKGROUND OF THE INVENTION 
     In the initial stages of conventional silicon integrated circuit processing, a silicon substrate is covered by a pad oxide and a silicon nitride layer. The silicon nitride and pad oxide are subsequently patterned to exposed predetermined regions of more or less bare silicon. An oxidation step is performed to create a field oxide, sometimes called an isolation oxide, in the regions where bare silicon had been exposed. The above-described process is often termed the LOCOS process (&#34;local oxidation of silicon&#34;). 
     A variant of the LOCOS process is the poly-buffered LOCOS process in which a layer of polysilicon is formed between the pad oxide and the silicon nitride layer. The polysilicon layer helps to relieve stresses created during the growth of the field oxide. 
     In both the LOCOS and the poly-buffered LOCOS processes, after the field oxide is grown, it is desirable to remove both the silicon nitride and the polysilicon layer. (Frequently, the underlying pad oxide is also removed.) Typically, the silicon nitride layer is removed by etching in hot phosphoric acid. The polysilicon layer is subsequently removed by a dry etch process. Unfortunately, the phosphoric acid wet etch often used to remove the silicon nitride creates small pits in the polysilicon. The dry etch process which is subsequently utilized to remove the polysilicon, being anisotropic, transfers the pits in the polysilicon downward into the pad oxide, or, under over-etch conditions, into the silicon substrate. This pitting of the pad oxide and the underlying substrate is undesirable. 
     SUMMARY OF THE INVENTION 
     Risk of damage to the pad oxide or underlying substrate is alleviated by the present invention which utilizes phosphoric acid to etch both polysilicon and silicon nitride layers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1-3 are cross-sectional views of an integrated circuit, which are helpful in understanding the principles of the invention. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1, reference numeral 11 denotes a substrate which may be silicon, doped silicon, epitaxial silicon, etc. In general, the term substrate is used to refer to any material having a surface upon which other materials may be adhered or deposited. Pad oxide 15, polysilicon layer 17, and silicon nitride layer 19 have been deposited upon substrate 11, and polysilicon layer 17 and silicon nitride layer 19 have been subsequently patterned. (In some processes, polysilicon layer 17 is not patterned, and the polysilicon is consumed in the subsequent field oxide oxidation process.) A wide range of thicknesses for layers 15, 17, and 19 is common in the semiconductor industry. 
     After polysilicon layer 17 has been formed (and before silicon nitride layer 19 is formed), the upper surface of polysilicon layer 17 is cleaned in, for example, 15:1 HF for approximately two minutes. The cleaning step removes any oxide which may have formed on the upper surface of the polysilicon. Removal of the oxide facilitates subsequent wet etching of polysilicon layer 17. If the oxide is not removed, it resists attach by phosphoric acid and considerably impedes removal of polysilicon layer 17. 
     After the cleaning step, the wafer is not exposed for any significant period of time to room ambient (to prevent re-oxidation) and nitride formation is begun without significant delay. 
     Previously-employed removal techniques, which included dry-etching of the polysilicon, do not include a cleaning step prior to nitride formation. 
     In FIG. 2, field oxide 21 has been grown, for example, by thermal oxidation in steam. Other methods of oxidation may also be employed, such as oxidation in dry oxygen (which requires more time) or high pressure oxidation (which is less easily controlled). It is then desirable to remove both silicon nitride layer 19 and polysilicon layer 17 without significant risk of damage to underlying pad oxide 15 or upper substrate surface 23. Before attempting to remove both layers 19 and 17, another cleaning step, utilizing 15:1 HF for approximately two minutes is performed to remove any silicon dioxide or silicon oxynitride which may have formed on the upper surface of layer 19. (The presence of silicon dioxide or silicon oxynitride will inhibit the effectiveness of the subsequent wet etch.) 
     Approximately two minutes is sufficient for the cleaning step if the field oxide has been grown in steam to a thickness of 5000-6000 Å. Shorter time periods (e.g., one and one-half minutes) may be suitable for thinner field oxides or field oxides formed by high pressure oxidation. 
     Our process for removing both layers 19 and 17 employs hot phosphoric acid, H 3  PO 4 . The hot phosphoric acid removes both the silicon nitride and the underlying polysilicon. Typically, the phosphoric acid is at a temperature of 140° C.-200° C. (A temperature of 165° C. seems to provide adequate removal of nitride and polysilicon without causing undue equipment corrosion.) Higher acid temperature promotes quicker nitride/polysilicon removal. Whatever temperature is chosen, the acid concentration may be selected anywhere between fully concentrated (i.e., approximately 95%) and that concentration at which the acid solution bath boils. 
     For example, at 165° C. with at least a partially boiling bath, etch rates of approximately 55 Å/min. have been observed for silicon nitride; 13 Å/min. have been observed for polysilicon; and 0.7 Å/min. have been observed for silicon dioxide. The etch rate of single crystalline silicon is at least two times less than the etch rate of silicon dioxide. Thus, the etch process is highly selective to silicon dioxide (and crystalline silicon). 
     In other words, the etch rates of silicon nitride, polysilicon, and silicon dioxide, respectively, are in the ratio 80:20:1. The goal of the etching process is to completely strip the silicon nitride 19 and polysilicon 17 without significant damage to underlying silicon dioxide 15 or crystalline silicon 11. Consequently, the desired thickness of the underlying silicon dioxide 15, relative to the thicknesses of the polysilicon 17 and silicon nitride 19 is given by the formula (assuming safety factor of 2): ##EQU1## where A=thickness of silicon nitride layer, 
     B=thickness of polysilicon layer, 
     C=thickness of silicon dioxide. 
     Depending on the application, safety factors greater or less than 2 may be acceptable. 
     Applicants have found that the growth process which produces field oxide 21 induces high stresses in the adjacent polysilicon layer 17. The stressed polysilicon layer 17 etches at a rate somewhat greater than the rate of 13 Å/min. mentioned above (which was obtained on an unstressed sample). 
     FIG. 3 illustrates the structure of FIGS. 1 and 2 after layers 17 and 19 have been removed, leaving only pad oxide 15. Subsequent processing may then be performed in accordance with conventional semiconductor practice.