Abstract:
A method of fabricating a semiconductor processing device includes providing a susceptor including a substantially cylindrical body portion having opposing upper and lower surfaces. The body portion has a diameter larger than a wafer diameter. The method also includes providing a set of holes circumferentially disposed at a first susceptor diameter, the set of holes being evenly spaced with respect to adjacent holes and extending through the upper and lower surfaces in an area. The first susceptor diameter is larger than the wafer diameter, and holes are omitted along the first diameter in a set of predetermined orientations.

Description:
FIELD 
     The field of the disclosure generally relates to semiconductor wafer processing, and more particularly to susceptors and related methods for epitaxial processing. 
     BACKGROUND 
     Epitaxial chemical vapor deposition is a process for growing a thin layer of material on a semiconductor wafer so that the lattice structure is identical to that of the wafer. Using this process, a layer having different conductivity type, dopant species, or dopant concentration may be applied to the semiconductor wafer to achieve the necessary electrical properties. Epitaxial chemical vapor deposition is widely used in semiconductor wafer production to build up epitaxial layers such that devices can be fabricated directly on the epitaxial layer. For example, a lightly doped epitaxial layer deposited over a heavily doped substrate permits a CMOS device to be optimized for latch up immunity as a result of the low resistivity of the substrate. Other advantages, such as precise control of the dopant concentration profile and freedom from oxygen are also achieved. 
     Prior to epitaxial deposition, the semiconductor wafer is typically mounted on a susceptor in a deposition chamber. The epitaxial deposition process begins by introducing a cleaning gas, such as hydrogen or a hydrogen and hydrogen chloride mixture, to a front surface of the wafer (i.e., a surface facing away from the susceptor) to pre-heat and clean the front surface of the wafer. The cleaning gas removes native oxide from the front surface, permitting the epitaxial silicon layer to grow continuously and evenly on the surface during a subsequent step of the deposition process. The epitaxial deposition process continues by introducing a vaporous silicon source gas, such as silane or a chlorinated silane, to the front surface of the wafer to deposit and grow an epitaxial layer of silicon on the front surface. A back surface opposite the front surface of the susceptor may be simultaneously subjected to hydrogen gas. The susceptor, which supports the semiconductor wafer in the deposition chamber during the epitaxial deposition, is rotated during the process to ensure the epitaxial layer grows evenly. 
     Epitaxial delta edge roll-off (DERO) is generally an undesirable effect of epitaxial deposition in that it may negatively affect flatness. DERO varies azimuthally according to the crystal lattice directions in conventional, monocrystalline silicon wafers. Flatness of the wafer may commonly be measured by quantities known as SFQR, SBIR, ROA, ERO, ESFQR, ESFQD and the like. In a conventional (100)-oriented silicon wafer, there are four equidistant points around the circumference of the wafer that correspond to &lt;110&gt; equivalent directions. In conventional wafers, DERO may be largest near certain directions, specifically the &lt;110&gt; directions. On the edge profile, including the edge bevel and a rounded interface between the edge bevel and the lateral surface of the wafer, there are typically exposed surfaces near the (311) orientations. Epitaxial growth on the (311) surfaces of the wafer is hindered by large densities of surface atoms on the (311) planes of the wafer. Accordingly, the gas stream is depleted of silicon precursors to a lesser extent when passing from near the (311) surfaces of the wafer onto the near-edge front and back surfaces of the wafer during processing. The result is enhanced growth rate in the vicinity near the (311) surfaces, which may lead to a large DERO in such areas. Epitaxial DERO on silicon wafers undesirably affects the flatness of the wafer, especially near the edge of the wafer. Thus, there remains a need for a system and method for processing a silicon wafer to reduce variation in DERO. 
     BRIEF SUMMARY 
     One aspect is directed to a susceptor for supporting a semiconductor wafer during an epitaxial chemical vapor deposition process. The susceptor defines a wafer diameter. The susceptor includes a substantially cylindrical body portion having opposing upper and lower surfaces, the body portion has a diameter larger than the wafer diameter. A set of holes in the body portion are circumferentially disposed at a first diameter. The set of holes are evenly spaced with respect to adjacent holes and extend through the upper and lower surfaces. The first diameter is larger than the wafer diameter, and there are no holes along the first diameter in a set of predetermined orientations. 
     Another aspect is directed to a susceptor defining a wafer diameter. The susceptor includes a substantially cylindrical body portion having opposing upper and lower surfaces, the body portion having a diameter larger than the wafer diameter. A set of holes extends through the upper and lower surfaces at a given diameter of the susceptor radially outward of the wafer diameter. A density of the set of holes varies circumferentially around the given diameter. 
     In still another aspect, a method of fabricating a semiconductor processing device includes providing a susceptor including a substantially cylindrical body portion having opposing upper and lower surfaces, the body portion having a diameter larger than a wafer diameter. The method also includes providing a set of holes circumferentially disposed at a first susceptor diameter, the set of holes being evenly spaced with respect to adjacent holes and extending through the upper and lower surfaces in an area. The first susceptor diameter is larger than the wafer diameter, and holes are omitted along the first diameter in a set of predetermined orientations. 
     In yet another aspect, a method of treating a wafer in an epitaxial chemical vapor deposition process includes providing a susceptor having a plurality of holes circumferentially disposed at a diameter larger than a diameter of a wafer to be treated. The method also includes placing the untreated wafer on the susceptor in a predetermined orientation such that &lt;110&gt; directions of the wafer aligns with portions of the susceptor that are free of holes outward from the diameter of the untreated wafer and chemically treating the wafer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of a test susceptor according to an embodiment of the present disclosure. 
         FIGS. 1A-1D  are detail views of the susceptor of  FIG. 1 . 
         FIG. 2  is a plot of azimuthal variation in DERO of a wafer processed on the susceptor of  FIG. 1 . 
         FIG. 3  is a top view of an embodiment of a susceptor according to the present disclosure. 
         FIGS. 3A and 3B  are detail views of the susceptor of  FIG. 3 . 
         FIG. 4  is a top view of another embodiment of a susceptor. 
         FIGS. 4A and 4B  are detail views of the susceptor of  FIG. 4 . 
         FIG. 5  is a cross section of the susceptor of  FIG. 4 . 
         FIG. 5A  is a detail view of the susceptor of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, and in particular to  FIG. 1 , a test susceptor is generally indicated at  10 . Susceptor  10  of this embodiment is substantially circular in shape, though other shapes are contemplated. The susceptor is suitable to support a semiconductor wafer (not shown) in a deposition chamber, such as a chemical vapor deposition (CVD) chamber, during a CVD process. In this embodiment, the semiconductor wafer has a wafer radius RW that is smaller than the susceptor radius RS of susceptor  20 . In this embodiment, the wafer radius is approximately 150 millimeters, but may be other radii between about 25 mm and about 300 mm, such as approximately 25.5 mm, 50 mm, 75 mm, 100 mm, 150 mm, 200 mm, 225 mm, 300 mm and the like. However, the wafer radius RW and susceptor radius RS of susceptor  20  may be any radius that allows the susceptor to operate as described herein. 
     In this embodiment, susceptor  10  has a disk-shaped body  20  with a center  40 . Body  20  is substantially planar and includes a set of through-holes  30 . Through-holes  30  are arranged in a pattern, such as a grid pattern or the like, and may include a through-hole located at center  40 . In this embodiment, each of the through-holes is located at a predetermined distance and angle from center  40 . Angle measurements are taken with reference to horizontal line H, with positive angles increasing in a counter-clockwise direction. 
     Without being bound to a particular theory, a CVD process tends to deposit a small amount of silicon on the back face of the wafer and may thicken the near-edge region of the wafer (within a few millimeters, e.g., within 5-6 mm, within 3-4 mm or within 1-2 mm of the wafer edge) relative to regions that are inward of the edge. Such thickening may increase DERO. 
     In this embodiment, certain through-holes  35  in the susceptor are disposed at a radial distance RH just outward of the wafer radius, to reduce the azimuthal DERO variation. Holes outside the wafer radius  35  may tend to increase the DERO nearby the holes. In one embodiment, to reduce the variation of DERO by angle, holes  35  are added where the DERO is smallest. For example, points near &lt;110&gt; directions have higher DERO than is typical of other points on the wafer, and points near outside holes have higher DERO than is typical of other points on the wafer. Thus, in this embodiment, holes  35  are added outside of the wafer radius RW at hole radius RH to make the DERO between the &lt;110&gt; directions substantially match the DERO at the &lt;110&gt; directions, thereby reducing the DERO variation. In this embodiment, the total DERO averaged over the whole wafer edge is increased compared to a wafer made on a susceptor without the added holes outside the wafer radius  35 . Having a reduced variation of DERO around the wafer edge enables better matching of epi DERO with the incoming wafer ERO, resulting in good flatness. By including the holes just outside the wafer radius  35 , except near the &lt;110&gt; directions, the azimuthal DERO variation is reduced. 
     In order to test the effect of changing the angular position of the holes  35 , a different number of holes were added at locations  1 A,  1 B,  1 C and  1 D shown on  FIG. 1 , at an outside hole radius RH. When referring to angle measurements herein, the convention of 0 degrees being on the right side of the horizontal axis H and angles increase going counterclockwise is used. 
     In the embodiment of  FIG. 1 , susceptor  20  has DERO holes  35  (added outside the wafer radius) disposed at 150.6 mm, and at approximately 90 degree intervals aligned with the &lt;100&gt; directions of a &lt;110&gt; notched wafer with its notch located at reference C (i.e., 270 degrees). The holes  35  are added to increase the DERO locally to the holes. At the &lt;110&gt; locations the DERO is the largest. Detail views of each location  1 A- 1 D are shown in  FIG. 1  as  FIG. 1A ,  FIG. 1B ,  FIG. 1C  and  FIG. 1D . At location  1 B, corresponding to an angle of 45 degrees, measured according to the angle convention used by the KLA-Tencor WaferSight (WS) tool, five holes were added over a 6 degree span. Each of the holes has a diameter of 0.9 mm, with a variance of plus-or-minus 0.05 mm. At location  1 A, corresponding to an angle of 135 degrees, eleven holes were added over a span of 15 degrees. Each of the holes has a diameter of 0.9 mm, with a variance of plus-or-minus 0.05 mm. At location  1 C, corresponding to an angle of 225 degrees, seven holes were added over a span of 9 degrees. Each of the holes has a diameter of 0.9 mm, with a variance of plus-or-minus 0.05 mm. At location  1 D, corresponding to an angle of 315 degrees, fifteen holes were added over a span of 21 degrees. Each of the holes in this embodiment has a diameter of 0.9 mm, with a variance of plus-or-minus 0.05 mm. 
       FIG. 2  shows a plot of the DERO values DV as a function of azimuthal angle AA from a wafer processed on the  FIG. 1  test susceptor having the above described holes added at locations  1 A- 1 D. Angles shown in  FIG. 2  correspond to the angles measured by the WS tool. In  FIG. 2 , DERO values DV are measured in nanometers. The peaks in DERO values DV at 40 degrees, 130 degrees, 220 degrees and 310 degrees may be a result of the holes  35  added outside the wafer radius at locations  1 A- 1 D. Such peaks are absent for wafers processed on conventional susceptors. The peaks in DERO values DV located at 0 degrees, 90 degrees, 180 degrees, and 270 degrees result from the &lt;110&gt; effect. The DERO values DV are measured at a 148 mm wafer radius. Such results may suggest that DERO increases by approximately 15 nm with holes spaced approximately 1.5 degrees apart. Accordingly, to make the DERO away from the &lt;110&gt; positions of the wafer substantially match the DERO at the &lt;110&gt; positions, a response coefficient of 22.5 nm·degrees is applied to calculate a hole density for angles between the &lt;110&gt; directions. The results of the calculations are shown in Table 1: 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                   
                 Hole Density 
                 Hole Spacing 
               
               
                 Angle 
                 (deg{circumflex over ( )}−1) 
                 (deg) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0 
                 0 
                 no holes 
               
               
                 5 
                 0.26 
                 3.83 
               
               
                 10 
                 0.38 
                 2.63 
               
               
                 15 
                 0.63 
                 1.60 
               
               
                 20 
                 0.72 
                 1.40 
               
               
                 25 
                 0.83 
                 1.20 
               
               
                 30 
                 0.86 
                 1.17 
               
               
                 35 
                 0.91 
                 1.10 
               
               
                 40 
                 0.93 
                 1.07 
               
               
                 45 
                 0.95 
                 1.06 
               
               
                   
               
             
          
         
       
     
       FIG. 3  shows another exemplary embodiment of a susceptor  20  having the Table 1 hole spacing. In this embodiment, for a wafer radius RW of 150 mm, holes  35  are disposed at a radius RH of approximately 150.6 mm. In this embodiment, the holes  35  outside the wafer radius are omitted at locations of approximately plus-or-minus 9 degrees of the &lt;110&gt; directions of the wafer. Holes  35  have a diameter of approximately 0.9 mm, but other diameters and radii may be used.  FIG. 3A  shows a detail view of area  3 A of susceptor  10 .  FIG. 3B  shows a detail view of area  3 B of susceptor  10 . 
     In the embodiment of  FIG. 4 , for a wafer radius of 150 mm, holes  35  are disposed at a radius RH of approximately 150.6 mm. In this embodiment, the holes outside the wafer radius  35  are omitted at orientation locations of approximately plus-or-minus 10 degrees of the &lt;110&gt; directions of the wafer. In contrast, in the  FIG. 1  test susceptor, the angular range over which the holes are omitted is different for each of the four &lt;110&gt; directions. In this embodiment, as shown in Detail  4 A ( FIG. 4A ), the holes are located at angular positions of 45.0, 43.9, 42.8, 41.7, 40.6, 39.5, 38.4, 37.3, 36.2, 35.1, 34.0, 32.9, 31.8, 30.7, 29.6, 28.4, 27.2, 26.0, 24.8, 23.6, 22.4, 21.1, 19.8, 18.4, 17.0, 15.5, 14.0, 12.3 and 10 degrees. The hole pattern may be reflected (i.e. symmetrical) about a 45 degree line and repeated up to four times (i.e., may be identical in each quadrant). Detail  4 B ( FIG. 4B ) shows that holes are omitted within 10 degrees of &lt;110&gt; location  50 . 
     In an epitaxial CVD reactor of one embodiment, there are two process chambers referred to as Chamber A and Chamber B. In one mode of operation, wafers processed in Chamber A are rotated such that the wafer notch is 7 degrees counterclockwise of the reference C position. Wafers processed in Chamber B have the wafer notch rotated 7 degrees clockwise of the reference C position. In one embodiment, to accommodate the difference in alignment between the wafers and susceptors, wafers are prealigned in a cassette with the notch in a direction corresponding to the chamber in which it is to be processed (i.e., Chamber A or Chamber B). In another embodiment, the pattern of through-holes  30  may be rotated, corresponding to Chamber A or Chamber B. In yet another embodiment, the plus or minus 7 degree misalignment between the wafer crystal directions and the pattern of holes added outside the wafer diameter may be neglected. 
       FIG. 5  shows a cross section of susceptor  10 . Susceptor  10  has a thickness T. The holes  35  outside the wafer radius RW extend entirely through thickness T of body  20  of susceptor  10 . 
     In other embodiments, wafers may have a notch located at a direction other than the &lt;110&gt; direction, such as the &lt;100&gt; direction. For wafers with &lt;100&gt; direction notches, the wafer may be loaded on susceptor  20  such that the notch is approximately 45 degrees, 135 degrees, 225 degrees, or 315 degrees to the reference C position, shown in  FIG. 3 . However, it is contemplated that other wafer notch positions may be used in accordance with the present disclosure. 
     When introducing elements of the present invention or the embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     As various changes could be made in the above apparatus and methods without departing from the scope of the disclosure, it is intended that all matter contained in the above description and shown in the accompanying figures shall be interpreted as illustrative and not in a limiting sense.