Abstract:
Shallow trench isolation is combined with optional deep trenches that are self-aligned with the shallow trenches, at the corners of the shallow trenches, and have a deep trench width that is controlled by the thickness of a temporary sidewall deposited in the interior of the shallow trench and is limited by the sidewall deposition thickness of the deep trench fill.

Description:
FIELD OF THE INVENTION 
     The field of the invention is integrated circuit processing using shallow trench isolation (STI) to separate circuit elements. 
     BACKGROUND OF THE INVENTION 
     STI has come into widespread use in the semiconductor industry because of its space savings as compared with the old LOCOS isolation. 
     In an active well circuit, where the wells are actively biased to improve the device performance, a solid and constant isolation is needed between the wells. The isolation must be deeper (˜2μm) than the shallow isolation (˜0.25 μm) between devices. 
     It is not enough, however, to have only deep trenches because device isolation using deep trenches would require body contacts, as in silicon on insulator technology, and would consume a great deal of space. 
     In addition, it is not practical to have varying widths of deep trenches because of the nature of the filling mechanism. Ordinarily, a deep trench is filled by growth of a layer adhering to the walls of the trench, so that the thickness of the fill layer can be much less than the width of the trench. This mechanism does not work for wide deep trenches. When the width of the trench is greater than twice the depth, the thickness of the fill layer must be at least the depth of the trench; i.e. a wide deep trench would require a 2 μm thick fill layer. 
     The combination of shallow and deep trenches has been suggested in the past, but the previous approaches have suffered from various practical problems. 
     If the deep trenches are not self-aligned to the shallow trenches, the deep trench can overlap the devices that are isolated by the trenches, so that the device size is not correct for that process. 
     SUMMARY OF THE INVENTION 
     The invention relates to a shallow trench isolation that is combined with optional deep trenches and the process for forming the isolation. 
     A feature of the invention is that the deep trenches are self-aligned with the shallow trenches, at the edges of the shallow trenches. 
     Another feature of the invention is that deep trench width is controlled by the thickness of a temporary sidewall spacer deposited in the interior of the shallow trench. 
     Another feature of the invention is that the deep trench width is limited by the sidewall deposition thickness of the deep trench fill in the case in which the two deep trenches entirely fill the shallow trench. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 through 10 show in cross section shallow/deep trenches in various stages of fabrication according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIGS. 1 through 10, there is shown a portion of an integrated circuit at various stages in the process. Each figure represents the results of a step in the process. 
     Initially as shown in FIG. 1, silicon substrate  10  has a pad oxide (SiO 2 ) layer  20  of initial thickness 8 μnm, pad nitride (Si 3 N 4 ) layer  22  of initial thickness 80 μnm, second oxide (first polish stop layer)  24  of initial thickness 150 nm, and second nitride (second polish stop layer)  26  of initial thickness 80 nm. These thicknesses are relevant to a particular process. The choice of thicknesses will be explained below. 
     An aperture 100 sized for the shallow trench and having a nominal width 101 of 1 μnm is etched a nominal 300 nm into the silicon substrate, using a conventional reactive ion etch (RIE) with conventional Ar:NF 3  or HBr:HeO 2  chemistry. The result is shown in FIG. 2, with vertical interior sidewalls on the left and right of the trench. 
     A composite interior spacer is deposited within the shallow trench and directionally etched using conventional chemistry to expose the silicon at the bottom of the shallow trench and also to remove the spacer films outside the trench. The first spacer layer  32  is oxide, having a nominal thickness of 4 nm. The second spacer layer  34  is nitride, having a nominal thickness of 130 nm. The result is shown in FIG.  3 . 
     Second nitride  26  should be thick enough to protect oxide  24  during this step and also to act as a polish stop in the next CMP step of polishing the oxide trench fill, i.e. the residue of layer  26  is thick enough to be an effective polish stop. 
     The shallow trench aperture  100  is filled with oxide  40  that is subject to a chemical-mechanical polish (CMP) operation to remove the oxide outside the shallow trench. The result is a shallow trench filled with oxide, shown in FIG.  4 . 
     The oxide fill  40  is recessed to a nominal remaining thickness of 350 nm using a conventional wet or dry etch selective to nitride, leaving an etch-resistant plug  42  below an aperture  102 . This sequence is preferred because it is insensitive to the various trench widths that may be involved. The result is shown in FIG.  5 . 
     Nitride spacer  34  and the residue of second nitride  24  are stripped in a conventional wet process with hot phosphoric acid, leaving oxide plug  42 , with apertures  104  on either side. The result is shown in FIG.  6 . Oxide plug  42  and the oxide spacer layer  32  serve as a hard mask for the deep trench etch that follows in the next step. The width of the deep trench will be set by thickness  35  of spacer  34  and the offset distance of the deep trench will be determined by the thickness  33  of oxide spacer  32 . The result is shown in FIG.  6 . 
     FIG.  7 ( a ) is the same as FIG.  6 . FIG.  7 ( b ) shows the result of depositing photoresist with a block mask to prevent deep trench etching in selected areas of the chip. 
     Next, a deep trench etch is performed, using HBR: O 2 : NF 3  chemistry to give a high aspect ratio aperture  106 . The nominal depth is 2 μm and the nominal width is 0.15 μm, the thickness of spacer layer  34 . The result is shown in FIG.  8 ( a ). FIG.  8 ( b ) shows the protected shallow trench after a resist strip. 
     FIGS.  9 ( a ) and  9 ( b ) show the deep and shallow trenches after an oxide strip step using vapor phase HF or similar chemistry and a conventional cleaning step to remove metallics, organics and polymeric residuals. 
     Lastly, a layer of 12 nm of thermal oxide is grown on the sides and bottoms of the deep and shallow trenches; the trenches are filled with HDP oxide and planarized with conventional CMP. The result is shown in FIG.  10 . 
     Preferably, an LPCVD oxide is used for both shallow and deep trenches. In the event that geometry does not allow this in a particular case, an alternative is to fill the deep trenches with polysilicon then recess the deep trenches to about the depth of the shallow trenches and fill both trenches with conventional oxide fill for shallow trenches. 
     In the limiting case of large spacer width  35  and a narrow shallow trench width  101 , the two deep trenches will meet and a single deep trench will be etched. In order to fill this limiting case, the thickness of film  35  should be at least half the width  101  of the trench and less than the sidewall deposition thickness—the maximum thickness that can be deposited with a particular fill material and filling technique. 
     The use of the invention is not confined to active well circuits and it may be used with conventional CMOS circuits as well. 
     The materials listed are illustrative only and other materials having appropriate etching and etch resisting properties may be used. 
     While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced in various versions within the spirit and scope of the following claims.