Abstract:
A method for forming STI that allows for in-situ moat/trench width electrical measurement is disclosed herein. A conductive layer ( 18 ) is used in the hard mask ( 20 ) for trench etch. After the hard mask ( 20 ) is formed and the trench ( 12 ) is etched, the resistance of the conductive layer ( 18 ) is measured over a predefined length. Since the length is known, the average width of the hard mask ( 20 )/moat ( 11 ) can be determined. Once the width of the moat ( 11 ) is known, the width of the trench ( 12 ) can easily be determined by subtracting the width of the moat ( 12 ) from the pitch, which is a known factor.

Description:
This application claims priority under 35 USC 119(e)(1) of provisional application Ser. No. 60/111,466 filed Dec. 9, 1998. 
    
    
     FIELD OF THE INVENTION 
     The invention is generally related to the field of test structures for semiconductor processing and more specifically to test structures for in-line electrical measurement of moat/trench width during semiconductor processing. 
     BACKGROUND OF THE INVENTION 
     Shallow trench isolation (STI) is being widely used for isolation in large-scale integrated circuits (ICs) to isolate the active areas of transistors and other devices from each other. STI is formed prior to transistor formation. Typically, a pad oxide and pad nitride are deposited over the surface. The pad oxide and nitride are then patterned and etched to form a hard mask for the trench etch. A shallow trench is then etched into the semiconductor surface. A trench liner is then formed on the surface of the trench and the trench is filled with a dielectric material, such as silicon dioxide. This is followed by CMP and removal of nitride to create active areas. 
     As ICs become denser, both the active areas and the trench shrink. This places increasing demands on the lithography used to pattern the hard mask/trench. It also requires tighter control of the trench etch. The STI lithography and etch can be monitored/evaluated using top-view SEM (scanning-electron-microscope)/cross-sectional SEM. Since large amounts of data are required for wafer uniformity, SEM analysis becomes time-consuming. Accordingly, a method for monitoring/developing/evaluating STI lithography and etch that is less time consuming and provides wafer uniformity information is desired. 
     SUMMARY OF THE INVENTION 
     A method for forming STI that allows for in-situ moat/trench width electrical measurement is disclosed herein. A conductive layer is used in either the hard mask or as part of the resist layer for trench etch. After the mask is formed, the resistance of the conductive layer is measured over a predefined length. Since the length is known, the average width of the mask/moat can be determined. Once the width of the moat is known, the width of the trench can easily be determined by subtracting the width of the moat from the pitch, which is a known factor. The conductive layer is removed either during the resist strip or during etch prior to CMP. 
     An advantage of the invention is providing a structure for in-line measurement of moat/trench width using electrical resistance. 
     This and other advantages will be apparent to those of ordinary skill in the art having reference to the specification in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIGS. 1A &amp; 1B are top-view diagrams of a test structure for trench width according to a first and a second embodiment of the invention, respectively; 
     FIG. 2 is a cross-sectional diagram of the test structure of FIG. 1A or  1 B; 
     FIGS. 3A-3F are cross-sectional views of the test structure of FIG. 1A or  1 B at various stages of fabrication; 
     FIG. 4 is a cross-sectional view of test structure according to a third embodiment of the invention; 
     FIG. 5 is a cross-sectional view of test structure according to a fourth embodiment of the invention; and 
     FIG. 6 is a cross-sectional view of test structure according to a fifth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The embodiments of the invention are test structures for evaluating semiconductor structures and processes during fabrication. They are generally placed in the scribe lines of a wafer. Because the fabricated structures vary across a wafer (for example, polysilicon linewidth is known to vary across a wafer), the test structures are places at various locations of the wafer. The test structures are formed using the same process steps as the IC to as closely match the IC structures (i.e., transistors) as possible. The information gained form the test structures is then used to determine such things as whether the fabrication steps have been performed satisfactorily or predict whether the IC transistors will function within their design specifications. 
     A first embodiment of the invention is shown in FIGS. 1A &amp; 2. FIG. 1A is a top view and FIG. 2 is a cross-sectional view. The hard mask  20  covers the moat regions  11  of semiconductor  10  and exposes the portion of semiconductor  10  that is etched to form trench  12 . Transistors and other devices (not shown) are subsequently formed in the moat or active regions  11 . For a test structure  30  (as shown in FIG.  1 A), probe pads  22  are located at the ends of a portion of hard mask  20 . Hard mask  20  extends back and forth over a portion of semiconductor  10  to create a repeated pitch of moat/trench/moat/trench/moat/trench/etc areas. Although the width of the trench portion may vary depending on the lithography and etch conditions, the pitch remains constant. It will be apparent to those of ordinary skill in the art that the number of moat/trench areas may vary. 
     Although FIG. 1A shows the preferred series layout, FIG. 1B shows an alternative parallel layout according to a second embodiment of the invention. In FIG. 1B, strips  21  of a section of hard mask  20  extend across a section of semiconductor  10  and are connected in parallel. The cross-section shown in FIG. 2 is applicable to both the embodiment of FIG.  1 A and the embodiment of FIG.  1 B. 
     The hard mask  20  comprises a dielectric layer  16  over the semiconductor  10  and a conductive layer  18 . Probe pads  22  comprise the same stack. Dielectric layer  16  may have an underlying pad oxide layer  14 . In the preferred embodiment dielectric layer  16  comprises nitride. However, other dielectric layers that may serve as a CMP (chemical-mechanical-polish) stop layer may alternatively be used. 
     In the preferred embodiment, conductive layer  18  comprises doped polysilicon. The polysilicon may be doped either N+ or P+. However, other conductive materials, such as N+ or P+ doped SiC, metals, or combinations thereof may alternatively be used. The thickness of conductive layer  18  may be in the range of 2000-6000 Å. Typically, the thickness is less than the trench width if it is desired to remove conductive layer  18  during the trench etch. 
     In operation, the electrical resistance of conductive layer  18  is measured using probe pads  22 . Probe pads  22  are placed on opposite ends of a predefined length of conductive layer  18 . Since the length and thickness of conductive layer  18  are known, the average width can be determined from the electrical resistance measured. Because the pitch of the moat  11 /trench  12  is known and unvarying, the trench  12  width can be determined by subtracting the width of the conductive layer  18  from the pitch. 
     An advantage of the invention is that the width of the trench can be determined in-line. In addition, the trench width can be measured at various points across a wafer to determine how the trench width varies across the wafer. The trench width can also easily be measured one multiple wafers to determine trench width variation from wafer to wafer. 
     A method for fabricating and using the test structure according to the first or second embodiments of the invention will now be discussed with reference to FIGS. 3A-3F. Referring to FIG. 3A, an optional pad oxide layer  14  is deposited over the semi conductor  10 . A dielectric layer  16  is then deposited over pad oxide layer  14  (if present). The thickness of pad oxide layer  14  is on the order of 100-200 Å. The thickness of dielectric layer  16  is on the order of 100-300 Å. Dielectric layer  16  may comprise, for example, silicon-nitride. A conductive layer  18  is then deposited over dielectric layer  16 . Conductive layer  18  comprises a conductive material such as doped (N+ or P+) polysilicon, doped SiC, or a metal. For doped polysilicon, conductive layer  18  has a thickness on the order of 1500-3500 Å. 
     After deposition , a moat/trench pattern  24  is formed over conductive layer  18 . Moat/trench pattern  24  covers the areas of semiconductor  10  that are to be active or moat regions and exposes areas over semiconductor  10  where trench isolation is desired. Moat/trench pattern  24  would typically comprise a photoresist material. 
     Referring to FIG. 3B, conductive layer  18 , dielectric layer  16 , and pad oxide  14  (if present) are etched using moat/trench pattern  24  to form a hard mask  20 . At this point the pattern  24  may be removed and contact can be made to probe pads  22  to measure the electrical resistance of conductive layer  18 . Given the electrical resistance, the length and thickness of conductive layer  18 , the average width of conductive layer  18  is determined. The width of the trench  12  can then be determined by subtracting the width of conductive layer  18  from the known pitch. Adjustments to the hard mask  20  or even removal and re-formation of hard mask  20  can be performed prior to etching the trench  12  (and possible scrapping the wafer due, for example, to inadequate isolation). 
     If the hard mask is within specs, semiconductor  10  is etched to form the shallow trench  12 , as shown in FIG.  3 C. If the pattern  24  has been removed, the trench etch will also remove conductive layer  18  if it comprises doped polysilicon. If pattern  24  has not been removed, the moat/trench pattern  24  is removed at this point and the electrical resistance/width determinations are alternatively performed at this point. 
     Next, the trench  12  is filled with a dielectric material  26  as is known in the art, as shown in FIG.  3 D. Typically, a trench liner  28  is formed prior to filling the trench to remove etch damage. Then, the dielectric filler material  26  is deposited. Excess filler material  26  is then typically CMP&#39;d back to planar with dielectric layer  16 , as shown in FIG.  3 E. During the CMP, conductive layer  18  is also removed if it has not been removed previously. Dielectric layer  16  is used as the CMP stopping layer. Accordingly, the material of dielectric layer  16  must be chosen such that the CMP process can distinguish between the dielectric layer  16  and dielectric filler material  26  (and the conductive layer  18  if still present) sufficiently to stop on dielectric layer  16 . 
     Finally, dielectric layer  16  and pad oxide layer  14  (if present) are removed, as shown in FIG.  3 F. Processing then continues with the formation of transistors and other devices (not shown), interconnect, and packaging. 
     In another embodiment of the invention, the order of the conductive layer  18  and dielectric layer  16  is switched as shown in FIG.  4 . This allows for a post-etch electrical resistance measurement. In this case, both the conductive layer  18  and dielectric layer  16  are removed after CMP of the dielectric filler material  26 . Dielectric layer  16  is removed prior to the electrical resistance measurement, but after CMP. After the electrical resistance measurement, the conductive layer  18  is removed. 
     In another embodiment, dielectric layer  16  is omitted, as shown in FIG.  5 . In this case, the conductive layer  18  is used as the CMP stop for the trench dielectric filler material  26 . The resist is left in place during the trench etch and then removed prior to the electrical resistance measurement. The electrical resistance measurement is preferably done prior to CMP. Because the conductive layer is the CMP stop layer, CMP can cause variations in the conductive layer  18  that may make the measurements unreliable thereafter. 
     In yet another embodiment, a conductive photoresist  32  is used, as shown in FIG.  6 . The conductive photoresist  32  is used in place of both the pattern  24  and conductive layer  18 . Electrical resistance measurement is then made before trench etch and/or after the trench etch. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.