Abstract:
A process for making a semiconductor structure, includes forming a second dielectric layer on exposed regions of an intermediate structure. The intermediate structure includes: a semiconductor substrate having the regions, a first dielectric layer on at least a first portion of the semiconductor substrate, an etch-stop layer on at least a second portion of the first dielectric layer, and spacers on at least a third portion of said semiconductor substrate. The spacers are adjacent edges of the etch-stop layer and adjacent the exposed regions.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present relates to an isolation technology for semiconductor devices.  
           [0002]    A variety of methods and structures have been used to isolate areas on semiconductor devices. One conventional technology is based on localized oxidation of silicon (LOCOS), shown in FIG. 1. In FIG. 1, on the silicon substrate  102  is a field oxide  104  that acts to isolate an active area of the semiconductor device; the field oxide is contiguous with the gate oxide  110  on the active region of the silicon substrate. The active area in this illustration includes a layer of silicon nitride  106 . The field oxide encroaches on the active area in the shape of a bird&#39;s beak  108 , before narrowing into the gate oxide. This large encroachment distorts the active region, and interferes with the submicron lithography necessary to construct elements in the active region.  
           [0003]    Another widely used isolation technique is silicon trench isolation (STI), shown in FIG. 2. The field oxide  104  does not have the shape of a bird&#39;s beak where it narrows into the gate oxide  110  on the silicon substrate  102 . FIGS.  3 ( a )- 3 ( e ) illustrate the step used to prepare the structure shown in FIG. 2. In FIG. 3( a ) thermal oxidizing forms a thermal oxide layer  111  on the silicon substrate  102 , followed by depositing a silicon nitride layer  106  using low pressure chemical vapor deposition (LPCVD). In FIG. 3( b ) a photoresist layer  112  is applied, and patterned using a mask. Etching of those portions of the silicon nitride, thermal oxide and silicon substrate not covered by the photoresist layer, in a single operation, opens a trench  114 .  
           [0004]    In FIG. 3( c ) the photoresist layer is first stripped, and the substrate is cleaned. A thin oxide layer  118  is then grown by dry oxidation of the exposed portions of the silicon substrate. An oxide layer  116  is then deposited into the trench and across the surface of the structure by chemical vapor deposition (CVD). In FIG. 3( d ) chemical-mechanical polishing (CMP) planarizes the surface, leaving the oxide layer  116  only in the trench. In FIG. 3( e ) the silicon nitride and thermal oxide layers are removed, and a gate oxide layer  110  is grown on the silicon substrate.  
           [0005]    In the STI process described above, silicon etching is usually performed by reactive ion etching (RIE), which may result in damage to the etched area. The damage may create defects, contamination and stress that may result in electrical degradation of the device, such as high leakage and low efficiency isolation performance.  
           [0006]    Often the STI process will include thermal treatments such as thermal oxidation and annealing or densification to remove the defects. However, these treatments are frequently insufficient to obtain a damage-free silicon surface in the trench, and may lead to even greater damage at the lower corners of the trench oxide structures (e.g., dislocations in the surrounding silicon) due to greater thermal expansion of oxide relative to silicon. The upper corners of the trench (and the adjacent silicon) also experience problems resulting from a similar stress concentration, as well as an increased risk of electrical “leakage” from local electric field effects.  
           [0007]    CVD of oxide into the trench may also exacerbate the defect problems, possibly by aggregating contamination from the oxide at the defects in the silicon surface during annealing or densification, and/or by bearing the stress from the silicon corners.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    In one aspect, the present invention concerns a process for making a semiconductor structure, including forming a dielectric layer on exposed regions of an intermediate structure. The intermediate structure includes (a) a semiconductor substrate having the regions, (b) a first dielectric layer, on at least a second region of the semiconductor substrate, (c) an etch-stop layer, on at least a portion of the dielectric layer, and (d) spacers on at least a third region of the semiconductor substrate. The spacers are adjacent edges of the etch-stop layer and adjacent the exposed regions.  
           [0009]    In a second aspect, the present invention concerns a semiconductor structure, including: (i) a semiconductor substrate, (ii) field oxide regions on a first portion of the semiconductor substrate, and (iii) gate oxide regions on a second portion of the semiconductor substrate. The surface of the first portion of the semiconductor substrate does not contain reactive ion etch damage.  
         Definitions  
         [0010]    A thick dielectric layer is a dielectric layer (for example, a silicon oxide layer or silicon nitride layer) that has a thickness greater than the thickness of a thin dielectric layer, and preferably has a thickness of 1000 to 50,000 Å, more preferably a thickness of 5000 to 10000 Å.  
           [0011]    A thin dielectric layer is a dielectric layer (for example, a silicon oxide layer or silicon nitride layer) that has a thickness less than the thickness of a thick dielectric layer, and preferably has a thickness of 10 to 999 Å, more preferably a thickness of 100 to 250 Å.  
           [0012]    The term “adjacent” means that there are no functional structures between the specified structures. A functional structure is a structure intentionally placed on the semiconductor device that affects the function of the device. For example, a functional structure may mean a structure having a predetermined set of dimensional and/or compositional parameter values, that has an electrical, mechanical and/or optical function (e.g., conductive, insulative, masking, photolithographic, antireflective).  
           [0013]    The term “exposed regions” means regions of a structure that are not covered by another structure.  
           [0014]    The term “planarizing” means to flatten, level or remove material, preferentially in the vertical direction, to enhance the planarity of the structure.  
           [0015]    The term “reactive ion etch damage” means damage associated with reactive ion etching that can affect the performance of a semiconductor device containing such damaged areas.  
           [0016]    The term “oxide” refers to a metal oxide conventionally used to isolate electrically active structures in an integrated circuit from each other, typically an oxide of silicon and/or aluminum (e.g., SiO 2  or Al 2 O 3 , which may be conventionally doped with fluorine, boron, phosphorous or a mixture thereof; preferably SiO 2  or SiO 2  conventionally doped with 1-12 wt. % of phosphorous and 0-8 wt. % of boron). 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    Various other objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood from the following detailed description when considered in connection with the accompanying drawings in which like reference characters designate like or corresponding parts throughout the several views and wherein:  
         [0018]    [0018]FIG. 1 shows an edge-on view of a portion of a semiconductor device containing field oxide formed by LOCOS;  
         [0019]    [0019]FIG. 2 shows an edge-on view of a portion of a semiconductor device containing field oxide formed by STI;  
         [0020]    FIGS.  3 ( a )- 3 ( e ) illustrate a series of successive edge-on views for forming the structure of FIG. 2; and  
         [0021]    FIGS.  4 ( a )- 4 ( d ) and FIGS.  5 ( a )- 5 ( e ) illustrate a series of successive edge-on views for an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    With the present invention it is possible to form a damage-free semiconductor substrate in a trench, using a sacrificial layer. Wet chemical removal of the sacrificial layer prevents damage to the semiconductor substrate surface, in order to for a trench. The trench may then be fill to form a dielectric isolation structure. This eliminates the field oxide portion having the shape of a bird&#39;s beak, typical of conventional LOCOS.  
         [0023]    FIGS.  4 ( a )- 4 ( d ) illustrate a first portion of an embodiment of the invention. In FIG. 4( a ), an etch-stop layer  6  covers a thin dielectric layer  11 , which in turn covers a semiconductor substrate  2 . Preferably, the semiconductor substrate is single crystal silicon, the dielectric layer is a thermally grown oxide layer, and the etch-stop layer is silicon nitride. More preferably, etch stop layer  6  comprises a silicon-rich silicon nitride having a refractive index of at least 2.1 and a thickness of from 500 to 3000 Å, even more preferably from 1500 to 2000 Å. In FIG. 4( b ), a photoresist layer  12  is applied to the surface of the structure, and patterned.  
         [0024]    In FIG. 4( c ), the exposed portions of the etch-stop layer (and optionally the thin dielectric layer) are removed. The photoresist layer is stripped and the surface cleaned. If the exposed portion of dielectric layer  11  was removed, a thin dielectric layer may be reformed or grown (preferably by dry thermal oxidation) on the exposed portion(s) of the semiconductor substrate  2 , and may be contiguous with the remaining thin dielectric layer  11 . Then a thick dielectric layer  20  is deposited over the surface. The thick dielectric layer is preferably an oxide deposited by CVD, more preferably by decomposition of TEOS, having a thickness of from 500 to 5000 Å, even more preferably from 1000 to 2500 Å.  
         [0025]    In FIG. 4( d ) the thick dielectric layer  20  has been etched to make spacers  22  adjacent the edges of the etch-stop layer, and adjacent the newly exposed region  28  of the semiconductor substrate, which in one embodiment, has a width of from ½ to ¾ of the distance between the sidewall interface between the corresponding adjacent spacers  22  and etch stop layer structure(s)  6 . In a more preferred embodiment, the width of region  28  is about ⅔ of the distance between the corresponding spacer-etch stop layer interfaces. Preferably, the spacers are in contact with the etch-stop layer and the exposed regions of the semiconductor substrate.  
         [0026]    The spacers may inhibit or prevent defect introduction in subsequent steps of the process (e.g., a trench etch step, an oxide growth step, etc.). For example, the spacers enable one to etch a relatively narrow trench (compared to the method illustrated in FIGS.  3 ( a )-( e ) for an isolation structure of approximately the same width) and grow the oxide for the isolation structure, rather than deposit it (e.g., by CVD). In addition, oxide grown in a subsequently formed trench may exhibit increased corner rounding relative to the method illustrated in FIGS.  3 ( a )-( e ), thus reducing stress-induced defects and corner-induced transistor “leakage.” 
         [0027]    Optionally, and preferably, the exposed semiconductor substrate surface between the spacers may be etched to from a trench. Such a trench may have a depth the same as or less than a predetermined depth of the isolation structure  16  to be formed (see FIGS.  5 ( d ) and  5 ( e )), preferably from about 5 to about 90% of the depth of the isolation structure, more preferably from about 25 to about 75% of the depth of the isolation structure.  
         [0028]    FIGS.  5 ( a )- 5 ( e ) illustrate a second portion of an embodiment of the invention. In FIG. 5( a ), after cleaning, a (sacrificial) oxide structure  24 / 26  is grown on the exposed semiconductor substrate regions, preferably by conventional dry and/or wet thermal oxidation (more preferably a dry thermal oxidation step to form lower portion  26 , followed by a wet thermal oxidation step to form upper portion  24 ). In its entirety, this oxide structure may comprise the spacers  22 , and newly formed materials from oxidation of the substrate: a section below the spacers  26 , and a section between and (optionally) above the spacers  24 . In certain embodiments, section  26  has a depth of from 1500 to 8000 Å relative to the upper surface of the substrate  2  (i.e., that surface closest to etch stop layer  6 ), more preferably from 2000 to 5000 Å, and even more preferably from 3000 to 3500 Å; and a section  24  has a height of from 1000 to 5000 Å relative to the upper surface of the substrate  2 , more preferably from 1500 to 3500 Å, and even more preferably from 2000 to 2500 Å.  
         [0029]    Optionally, as illustrated in FIGS.  5 ( b ) and  5 ( c ), the sacrificial oxide structure  24 / 26  may be removed and replaced. In FIG. 5( b ), the sacrificial oxide structure  24 / 26  may be etched (for example with a conventional wet chemical etching process) to create trench  30 . For example, the sacrificial layer could be etch with fluorine-based wet chemicals. When the etch-stop layer comprises silicon nitride and the sacrificial oxide structure comprises silicon dioxide, then buffered HF is preferred. FIG. 5( c ) shows (an optional) thermal oxide layer  18  grown at the bottom of the trench, and a thick dielectric layer  16  deposited on the surface. Preferably, dielectric layer  16  comprises an oxide deposited by CVD.  
         [0030]    In FIG. 5( d ), the surface of the semiconductor structure is planarized, preferably by CMP. Therefore, in one embodiment, the etch stop layer may also comprise a polish stop layer (which may be the same as or different from the etch stop layer; e.g., under conditions known to those of ordinary skill in the art, silicon nitride may provide both etch stop and chemical-mechanical polish stop functions). In FIG. 5( e ), the etch-stop layer  6  (and optionally dielectric layer  11 ) is removed, and a gate dielectric layer  10  may be grown on the semiconductor substrate. Preferably, the gate dielectric layer is a gate oxide, grown thermally.  
         [0031]    As noted above, the operations illustrated in FIGS.  5 ( b ,) and  5 ( c ) are optional. Other variations are also possible. For example, the process may be stopped at the point illustrated in FIG. 5( b ), in order to provide a substrate with a trench that may be used to construct other semiconductor devices. Alternatively, if the compositional difference(s) between the grown oxide structure  22 / 24 / 26  and the desired trench oxide  16  are functionally insignificant, the process may simply comprise chemical-mechanical polishing a structure such as that shown in FIG. 5( a ), in order to provide a dielectric for isolating regions of the semiconductor substrate.  
         [0032]    The invention provides a semiconductor devices that may have higher reliability, and that may reduce manufacturing costs per device.  
         [0033]    The individual processing steps for use in the present invention are well known to those of ordinary skill in the art, and are also described in Encyclopedia of Chemical Technology, Kirk-Othmer, Volume 14, pp. 677-709 (1995); Semiconductor Device Fundamentals, Robert F. Pierret, Addison-Wesley, 1996; and Microchip Fabrication 3rd. edition, Peter Van Zant, McGraw-Hill, 1997. Conventional chemical-mechanical polishing is described in U.S. Pat. Nos. 4,789,648, 4,954,142 and 5,262,354.  
         [0034]    The substrate may typically be a semiconductor material conventionally known by those of ordinary skill in the art. Examples include silicon, gallium arsenide, germanium, gallium nitride, aluminum phosphide, and alloys such as Si 1−x Ge x  and Al x Ga 1−x As, where 0≦x≦1. Many others are known, such as those listed in Semiconductor Device Fundamentals, on page 4, Table 1.1 (Robert F. Pierret, Addison-Wesley, 1996). Preferably, the semiconductor substrate is silicon, which may be doped or undoped.  
         [0035]    Any of the dielectric layers, and the etch-stop layer, may be made from a dielectric material conventionally known to those of ordinary skill in the art. Examples include conventional oxides, nitrides, oxynitrides, and other dielectrics, such as borophosphosilicate glass (BPSG), borosilicate glass (BSG), phosphosilicate glass, spin-on glass (SOG), silicon oxide, P-doped silicon oxide (P-glass), and silicon nitride. The dielectric layer must be made from a material that is different from the etch-stop layer.  
         [0036]    The dielectric layer may be deposited by conventional methods known to those of ordinary skill in the art, such as by spin-on methods, sintering (which may further include sol-gel oxide formation), chemical vapor deposition, etc. A glass layer deposited by a chemical vapor deposition technique may be subject to a glass reflow step (e.g., by heating) to smooth, densify and further improve the contact between the protection layer and the substrate.  
         [0037]    When a dielectric layer is to be etched, it is made from a material that can be etched more quickly than the etch-stop layer. If the dielectric layer is made from silicon oxide, or a glass, such as borophosphosilicate glass (BPSG), borosilicate glass (BSG), phosphosilicate glass, spin-on glass (SOG), or P-doped silicon oxide (P-glass), it may be doped in order to enhance the selectivity etching the glass or oxide instead of the etch-stop layer. Preferably, the etch selectivity (i.e., the ratio of (a) the rate of dielectric etching to (b) the rate of etch stop material etching) is at least 2:1, preferably at least 3:1, more preferably at least 5:1 and even more preferably at least 10:1. In the case of silicon oxide using silicon nitride as the etch-stop layer, an etching solution of one part HF (49%) in one part deionized water will give a selectivity of greater than 1:300.  
         [0038]    The isolation structures of the present invention may be incorporated into a semiconductor device such as an integrated circuit, for example a memory cell such as an SRAM, a DRAM, an EPROM, an EEPROM etc.; a programmable logic device; a data communications device; a clock generation device; etc.  
         [0039]    Etching of deposited films may be conducted by conventional methods known to those of ordinary skill in the art. The specific etching method and materials depend on the material being removed, the resist material and the compatibility of the etching material with the existing structure. Selection of suitable etching materials, resist materials and etching conditions is within the level of ordinary skill in the art.  
         [0040]    Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.