Abstract:
A method for fabrication of a III-nitride film over a silicon wafer that includes forming control joints to allow for overall stress relief in the III-nitride film during the growth thereof.

Description:
RELATED APPLICATION 
     This application is based on and claims priority to the U.S. Provisional Application Ser. No. 60/863,427, filed on Oct. 30, 2006, entitled III-Nitride Global Stress Reduction, to which a claim of priority is hereby made and the disclosure of which is incorporated by reference. 
    
    
     DEFINITION 
     III-nitride as set forth herein refers to an alloy from the InAlGaN system including but not limited to AlN, AlGaN, GaN, InGaN, InN, InAlGaN or the like. 
     FIELD OF THE INVENTION 
     The present invention relates to semiconductor device fabrication and particularly to the fabrication of wafers for III-nitride semiconductor devices. 
     BACKGROUND OF THE INVENTION 
     Epitaxial growth or deposition of III-nitride films onto substrates of a different material (e.g. silicon) often results in high epitaxial defect density, high film stresses, high wafer warpage, and cracks in the epitaxial films, which are typically caused by the large lattice constant mismatch between the substrate and the III-nitride film, or by the substrate and the III-nitride film having different lattice structure types. For example, when depositing a 2 μm thick AlGaN/GaN/AlN epitaxial layers onto 100 mm diameter silicon substrates, it is not uncommon to have up to 100 μm total warpage, which occurs despite efforts at strain reduction through careful control and selection of transition layers and compositions. 
     SUMMARY OF THE INVENTION 
     The wafer warpage, epitaxial film cracks, and defects can occur from the cumulative stress buildup over the surface of a whole wafer. 
     To reduce warpage and defects, according to the present invention, an array or grid of “control joints” is fabricated onto a surface of a substrate before epitaxial growth of a III-nitride film, such that the epitaxial film is discontinuous across the control joints. As a result, stresses in the III-nitride film are only able to accumulate inside the boundaries defined by the control joints and cannot propagate past the control joints. The control joints also may force an intentional “crack” in the III-nitride film, localized near the control joint, which can lower the total stress buildup across the wafer, resulting in lower overall wafer warpage. 
     A control joint according to the present invention can take several forms. According to one embodiment a control joint can be a shallow trench etched into one surface of the substrate. A typical trench can be 100 angstrom deep and several micrometer wide. The shallow trenches can be arranged in a rectangular vertical and horizontal array (i.e. a grid) with center-to-center spacings in the range 10-25 mm. 
     According to another aspect of the present invention the shallow trenches can reside inside of the normal saw streets between die and thus do not occupy any additional space on the wafer. 
     According to another aspect of the present invention, if a more planar substrate surface is desired, the shallow trenches could be filled with a dielectric (e.g. oxide) or amorphous silicon or other filling material. 
     Note that the control joints need not be continuous, and can be joints of closely spaced segments. 
     Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a top plan view of a wafer that includes stress reduction trenches formed in one surface thereof to constitute a control joint according to an embodiment of the present invention. 
         FIG. 2  illustrates portion A (circled in  FIG. 1 ) of the wafer of  FIG. 1 . 
         FIG. 3  illustrates a cross-sectional view of the wafer along lines  3 - 3  ( FIG. 1 ) viewed in the direction of the arrows. 
         FIG. 4  illustrates a portion of a wafer that includes stress reduction trenches after growth of a III-nitride body thereon. 
         FIG. 5  illustrates a portion of a wafer that includes stress relief trenches filled with a filler body according to an alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring to  FIGS. 1 ,  2 , and  3 , in a method according to one embodiment of the present invention shallow trenches  10  are etched into one surface of a silicon wafer  12 . Preferably, trenches  10  cross one another to form a grid, as specifically illustrated by  FIGS. 1 and 2 . Trenches  10  function to reduce the overall stress in the III-nitride epitaxial layer that is to be formed over surface  14  of wafer  12 . Each trench  10  may be about 100 angstroms deep, and several microns wide. Furthermore, the center-to-center spacing d of opposing trenches (e.g. trenches  10 ′ and  10 ″) may be in the range of 10-25 mm. 
     According to one aspect of the present invention trenches  10  are formed inside regions  16  of wafer  12  designated as saw street for the dicing of the wafer into individual die. 
     Referring now to  FIG. 4 , according to the present invention, a III-nitride body  18  (e.g. AlN) is epitaxially formed over surface  14  of wafer  12  and filling trenches  10 . Note that film  18  is rendered discontinuous over trenches  10 . As a result, a crack  20  may be formed in III-nitride body  18  over trench  10 , relieving a portion of the stress developed in III-nitride body  18 . 
     Thereafter, any III-nitride device can be formed over III-nitride bodies  18  between trenches  10  followed by a sawing step inside regions  16  (streets) to obtain III-nitride devices. 
     Referring now to  FIG. 5 , in an alternative embodiment, a filler material having the capability of preventing nucleation of III-nitride  18 , is deposited over surface  14  of wafer  12  filling trenches  10 . Thereafter, in a planarization step, excess filler body is removed from surface  14  leaving only filler  22  inside trenches  10  substantially coplanar with the top of surface  14 . Thereafter, a nitride body  18  is epitaxially grown over surface  14 . Note that the filler renders III-nitride body  18  discontinuous over trenches  10 , which may result in a stress relief crack  20  therein. Suitable materials for fillers  22  include dielectrics such as SiO 2 , or amorphous silicon. 
     Thereafter, any III-nitride device can be formed over III-nitride bodies  18  between trenches  10  followed by a sawing step inside regions  16  (streets) to obtain III-nitride devices. 
     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.