Patent Publication Number: US-8114751-B2

Title: Multi-angle rotation for ion implantation of trenches in superjunction devices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 12/371,025, filed on Feb. 13, 2009, entitled “Multi-Angle Rotation for Ion Implantation of Trenches in Superjunction Devices,” which claims the benefit of U.S. Provisional Patent Application No. 61/028,215, filed on Feb. 13, 2008, entitled “Multi-Angle Rotation for Ion Implantation of Trenches in Superjunction Devices,” and U.S. Provisional Patent Application No. 61/088,744, filed on Aug. 14, 2008, entitled “Multi-Angle Rotation for Ion Implantation of Trenches in Superjunction Devices,” the entire contents of all of which are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     An embodiment of the present invention relates generally to a method of manufacturing a semiconductor device, and more particularly, to a method of manufacturing a superjunction device by ion implanting trenches using four different implantation directions that are generally non-orthogonal to the orientations of the trenches. 
     Since the invention of superjunction devices by Dr. Xingbi Chen, as disclosed in U.S. Pat. No. 5,216,275, the contents of which are incorporated by reference herein, there have been many attempts to expand and improve on the superjunction effect of his invention. U.S. Pat. Nos. 6,410,958, 6,300,171 and 6,307,246 are examples of such efforts and are incorporated by reference herein. 
     Trench type superjunction devices are expected to replace multi-epi superjunction devices because of the potential lower processing cost.  FIG. 1A  illustrates a top plan view of a wafer  10  used in manufacturing of a plurality of trench-type superjunction devices. A plurality of horizontally oriented trenches  12  are formed therein.  FIGS. 1B and 1C  are enlarged partial cross-sectional views of the wafer  10  both taken along the line A-A′. In general, a superjunction device is produced through formation of n or p columns (not shown) using adjacent sidewalls  14 ,  16  of the trenches  12 . Doping with a dopant of n-type or p-type conductivity is performed by ion implantation at an appropriate tilt angle Φ (or Φ′ in  FIG. 1C ). The dopant impurities are thereby generally uniformly implanted at the tilt angles Φ, Φ′ throughout the sidewalls  14 ,  16  of each of the trenches  12 . 
     Implantation is typically performed at a direction having a rotation angle θ that is generally orthogonal to the orientation of the trench  12 . For example, in  FIG. 1A , the trenches  12  have a horizontal orientation, and a first implantation direction  21  is shown in a vertical orientation, in the plane of the wafer  10 , for implanting the first sidewalls  14  of the trenches  12  (see  FIG. 1B ). A second implantation direction  22  is also shown in a vertical orientation, opposite to the first implantation direction  21 , for implanting the second sidewalls  16  of the trenches  12  (see  FIG. 1C ). 
     In certain applications, a semiconductor wafer will include two or more sets of trenches having different orientations. For example,  FIG. 2  shows a wafer  11  including one set of trenches  12  having a horizontal orientation (as in  FIG. 1A ) and a second set of trenches  13  having a vertical orientation. For wafer  11 , four implantation directions are required in order to properly dope all of the sidewalls (not shown) of all of the trenches  12 ,  13 . For example, implantation directions  21 ,  22 , orthogonal to the trenches  12 , are used for implanting the dopant into the sidewalls of the trenches  12 . Likewise, implantation directions  23 ,  24 , orthogonal to the trenches  13 , are used for implanting the sidewalls of the trenches  13 . 
     The four angle implantation method described above is satisfactory for wafers having trenches or trench sets orthogonally aligned with one another, and the trenches each having identical doping concentrations. However, difficulties arise when the trenches are not orthogonal to one another or in situations where differing doping concentrations are required. The ion dose and/or the acceleration energy of the ion beam must be changed during processing or multiple implantation steps may be required, which increases the complexity, cost, and time for manufacturing the superjunction devices. 
     It is desirable to provide a method of manufacturing trench-type superjunction devices having trenches non-orthogonally aligned to one another without having to change the orientation or the ion beam strength mid-step or perform additional implantation steps. It is further desirable to provide a method of manufacturing superjunction devices wherein varying doping concentrations of different trenches may be achieved simultaneously. 
     BRIEF SUMMARY OF THE INVENTION 
     Briefly stated, various embodiments of the present invention comprise a method of manufacturing a semiconductor device. The method includes providing a semiconductor wafer and forming at least one first trench in the wafer having first and second sidewalls and a first orientation on the wafer. A dopant of a first conductivity is implanted into the first sidewall of the at least one first trench at a first implantation direction. The dopant of the first conductivity is implanted into the first sidewall of the at least one first trench at a second implantation direction. The second implantation direction is orthogonal to the first implantation direction. The first and second implantation directions are non-orthogonal to the first sidewall. 
     In a preferred embodiment, the method further includes implanting, with the dopant of the first conductivity, the second sidewall of the at least one first trench at a third implantation direction. The third implantation direction is orthogonal to one of the first implantation direction and the second implantation direction. The method also includes implanting, with the dopant of the first conductivity, the second sidewall of the at least one first trench at a fourth implantation direction. The fourth implantation direction is orthogonal to the third implantation direction. The third and fourth implantation directions are non-orthogonal to the second sidewall. 
     Embodiments of the present invention also comprise a method of manufacturing a semiconductor device. The method includes providing a semiconductor wafer and forming at least one first trench in the wafer having sidewalls and a first orientation on the wafer. At least one second trench is formed in the wafer having sidewalls and a second orientation on the wafer. The second orientation is different from the first orientation. The method further includes directing an ion implantation beam from a first implantation direction at the at least one first trench and the at least one second trench. An ion implantation beam is directed from a second implantation direction, different than the first implantation direction by 90°, at the at least one first trench and the at least one second trench. An ion implantation beam is directed from a third implantation direction, different than the first implantation direction by 180°, at the at least one first trench and the at least one second trench. An ion implantation beam is directed from a fourth implantation direction, different than the first implantation direction by 270°, at the at least one first trench and the at least one second trench. The first, second, third, and fourth implantation directions are non-orthogonal to at least one of the first orientation and the second orientation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustration, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
       In the drawings: 
         FIG. 1A  is a top plan view of a prior art semiconductor wafer having a plurality of trenches formed thereon, the trenches being implanted from two implantation directions; 
         FIG. 1B  is an enlarged partial cross-sectional elevational view of a portion of the wafer of  FIG. 1A  taken along line A-A′, the trenches being implanted from a first implantation direction; 
         FIG. 1C  is an enlarged partial cross-sectional elevational view of a portion of the wafer of  FIG. 1A  taken along line A-A′, the trenches being implanted from a second implantation direction; 
         FIG. 2  is a top plan view of a prior art semiconductor wafer having a plurality of trenches formed thereon, the trenches being implanted from four implantation directions; 
         FIG. 3  is a top plan view of a semiconductor wafer having a plurality of trenches formed thereon, the trenches being implanted in accordance with a preferred embodiment of the present invention; 
         FIG. 4A  is a greatly enlarged top plan view of a trench and associated sidewalls being implanted in accordance with the prior art; 
         FIG. 4B  is a greatly enlarged top plan view of a trench and associated sidewalls being implanted in accordance with a preferred embodiment of the present invention; 
         FIGS. 5A and 5B  are greatly enlarged top plan views of two trenches of differing orientation and associated sidewalls being implanted in accordance with a preferred embodiment of the present invention; 
         FIG. 6A  is a diagram illustrating the relationship of implant direction and trench angle, in accordance with preferred embodiments of the present invention; 
         FIG. 6B  is a graph illustrating the relationship between effective implantation dose and trench angle; 
         FIG. 7A  is an enlarged cross-sectional side elevational view of a wafer with a trench, the trench being implanted in accordance with a prior art method; 
         FIG. 7B  is an enlarged cross-sectional side elevational view of a wafer with a relatively narrow trench, the trench being implanted in accordance with a prior art method; 
         FIG. 7C  is an enlarged cross-sectional side elevational view of a wafer with the trench of  FIG. 7B , the trench being implanted in accordance with a prior art method; 
         FIG. 8  is a graph illustrating the relationship between possible sidewall implantation depth and trench width for various implantation rotation angles in accordance with a preferred embodiment of the present invention; 
         FIG. 9A  is a schematic diagram illustrating the relationship between trench width, substantial trench width, and rotation angle in accordance with a preferred embodiment of the present invention; 
         FIG. 9B  is a graph illustrating the relationship between substantial trench width and implantation rotation angle; 
         FIG. 10  is a greatly enlarged top plan view of a wafer having a corner trench being implanted in accordance with a prior art method; and 
         FIG. 11  is a greatly enlarged top plan view of a wafer having a corner trench being implanted in accordance with a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Certain terminology is used in the following description for convenience only and is not limiting. The words “right”, “left”, “lower”, and “upper” designate directions in the drawings to which reference is made. The words “inwardly” and “outwardly” refer to directions toward and away from, respectively, the geometric center of the device and designated parts thereof. The terminology includes the above-listed words, derivatives thereof, and words of similar import. Additionally, the words “a” and “an”, as used in the claims and in the corresponding portions of the specification, mean “at least one.” 
     As used herein, reference to conductivity will be limited to the embodiment described. However, those skilled in the art know that p-type conductivity can be switched with n-type conductivity and the device would still be functionally correct (i.e., a first or a second conductivity type). Therefore, where used herein, reference to n or p can also mean either n or p or p and n can be substituted therefor. 
     Furthermore, n +  and p +  refer to heavily doped n and p regions, respectively; n ++  and p ++  refer to very heavily doped n and p regions, respectively; n −  and p −  refer to lightly doped n and p regions, respectively; and n −−  and p −−  refer to very lightly doped n and p regions, respectively. However, such relative doping terms should not be construed as limiting. 
     Referring to the drawings in detail, wherein like reference numerals indicate like elements throughout, there is shown in  FIG. 3  a top plan view of a wafer  110  including a first set of trenches  112  having a first orientation direction and a second set of trenches  113  having a second orientation formed thereon. Each of the trenches  112 ,  113  has first and second sidewalls (not shown in  FIG. 3 ). The first orientation shown in  FIG. 3  is horizontal on the wafer  110  and the second orientation is vertical on the wafer  110 . These orientations, however, are shown for illustrative purposes only, and each set of trenches  112 ,  113  may have any orientation on the wafer as desired. 
     Preferably, the trenches  112 ,  113  are formed by utilizing known techniques such as plasma etching, reactive ion etching (RIE), sputter etching, vapor phase etching, chemical etching, deep RIE, or the like. Utilizing deep RIE, trenches  112 ,  113  can be formed having depths of about 40 micrometers (μm) to about 300 μm or even deeper. Deep RIE technology permits deeper trenches  112 ,  113  with straighter sidewalls. Furthermore, forming deeper trenches  112 ,  113  that have straighter sidewalls than conventionally etched or formed trenches, in addition to other steps in the process, results in a final superjunction device with enhanced avalanche breakdown voltage (V b ) characteristics as compared to conventional semiconductor-transistor devices (i.e., the avalanche breakdown voltage (V b ) can be increased to about 200 to 1200 Volts or more). 
     The sidewalls of each of the trenches  112 ,  113  can be smoothed, if needed, using, for example, one or more of the following process steps: (i) an isotropic plasma etch may be used to remove a thin layer of silicon (typically 100-1000 Angstroms) from the trench surfaces or (ii) a sacrificial silicon dioxide layer may be grown on the surfaces of the trench and then removed using an etch such as a buffered oxide etch or a diluted hydrofluoric (HF) acid etch. The use of smoothing techniques can produce smooth trench surfaces with rounded corners while removing residual stress and unwanted contaminates. 
     The sidewalls of the trenches  112 ,  113  are doped with a dopant of a first conductivity. Typically, the wafer  110  is comprised of an n-type semiconductor material, such as silicon or the like, and the dopant would therefore be of the p-type. For example, the sidewalls may be implanted or doped with a p-dopant such as boron (B) using any techniques known in the art. However, in some cases n-type doping may be required prior to the p-type doping of the trench sidewalls. Preferably, the implants are performed without benefits of a masking step, e.g., at a tilt angle Φ (see  FIGS. 1B ,  1 C) determined by the width and the depth of the trenches  112 ,  113 , at a high energy level in the range of about 40 kilo-electron-Volts (keV) to several Mega-eV (MeV). Preferably, the energy level is in the range of, for example, about 80 keV for boron and about 180 keV for phosphorous. The use of the predetermined tilt angle Φ ensures that only the sidewalls and not the bottoms of the trenches  112 ,  113  are implanted. 
     The sidewalls of the trenches  112 ,  113  are preferably implanted from four different directions, first implantation direction  121 , second implantation direction  122 , third implantation direction  123 , and fourth implantation direction  124 . The first, second, third, and fourth implantation directions, in the present embodiment,  121 ,  122 ,  123 ,  124  are generally orthogonal to one another and are non-orthogonal to at least one of and preferably both of the first and second orientations of the respective trenches  112 ,  113 . As shown in  FIG. 3 , the first, second, third, and fourth implantation directions  121 ,  122 ,  123 ,  124  are non-orthogonal to both the first trenches  112  and the second trenches  113 . 
     Following implanting the p-type implant on the sidewalls of the trenches  112 ,  113 , a drive-in step (i.e., a diffusion) is performed using any known techniques to create p-type doped regions (not shown) proximate the sidewalls of the trenches  112 ,  113 . Preferably, a temperature and a time period for the drive-in step are selected to sufficiently drive in the implanted dopant. For example, for p-type doping, the drive-in step (i.e., a diffusion) may be performed at a temperature of about 1150-1200° Celsius for about 1-2 hours. Alternatively, for n-type doping, the drive in step may be performed at a temperature of up to about 1200° C. for up to about 24 hours. 
       FIG. 4A  shows a trench implanted in accordance with prior art techniques. The trench  12  is illustrated as having a horizontal orientation. The first sidewall  14  of the trench  12  is implanted with a dopant of the first conductivity and the implantation direction  21  is orthogonal to the trench  12  orientation. Similarly, the second sidewall  16  is implanted by an orthogonally oriented implantation direction  22 . Each sidewall  14 ,  16  thereby receives an implant dose n d  during implantation. 
     When ion implantation is performed at a non-orthogonal rotation angle θ, however, the implant dose is reduced because as the ion beam strikes the sidewall surfaces  114  or  116 , the projection area is larger, thus decreasing the concentration. For example, in  FIG. 4B , the ion beam A having the first implantation direction  121  strikes the first sidewall  114  of the trench  112  at an angle θ from the first sidewall  114  (and necessarily the trench orientation). The effective dose N eff  received by the first sidewall  114  is calculated by:
 
N eff =n d  sin [θ]
 
where n d  is, as above, the implant dose of the ion beam. It is seen that for the embodiment where the implantation direction  121  is orthogonal to the first sidewall  114  (as in  FIG. 4A ), θ=90°, and therefore N eff =n d .
 
     In  FIG. 4B , however, two ion beams (A and B) strike the first sidewall  114  from first and second implantation directions  121 ,  122  respectively. The effective dose N eff  is thus found by summing N eff  for beam A and N eff  for beam B. Since the second implantation direction  122  is orthogonal to the first implantation direction  121 , the angle for beam B may written as θ+90°, and therefore N eff  is found simply by:
 
 N   eff   =n   d (sin [θ]+cos [θ])
 
For example, where θ=60°, the effective dose for beam A is 0.87 n d  and the effective dose of B is 0.5 n d . Thus, N eff  for the first sidewall  114  is 1.37 n d . To achieve the desired N eff , the required implant dose n d  for both beam A and beam B is N eff /1.37. Since each implantation direction  121 ,  122 ,  123 ,  124  is separated by 90°, the second sidewall  116  of the trench  112  in  FIG. 4B  will be doped identically to the first sidewall  114 .
 
       FIG. 5A  shows the trench  112  of  FIG. 4B  adjacent to a second trench  113 , shown in  FIG. 5B , formed on the same wafer  110  and having first and second sidewalls  115 ,  117 . The first, second, third, and fourth implantation directions  121 ,  122 ,  123 ,  124  remain the same, but the orientation of the second trench  113  is different than and non-orthogonal to the first trench  112 . Although the ion beam A will be applied simultaneously to the first and second trenches  112 ,  113 , the first ion beam A intersects the first sidewall  115  of the second trench  113  at an angle of θ′ rather than θ. As a result, the effective dose N eff  for the first and second sidewalls  115 ,  117  of the second trench  113  must be calculated using the new θ′. 
     Under this principle, sidewall doping of the trenches  112 ,  113  may be tuned by altering the angle of the trench with respect to the implantation direction. For example,  FIG. 6A  sets parameters for a trench angle by fixing the first implantation direction  121  vertically and fixing the second implantation direction  122  horizontally. The trench angle is thus defined in relation to the second implantation direction  122 , i.e., a trench collinear with the second implantation direction  122  (and orthogonal to the first implantation direction  121 ) is defined as having a trench angle of 0°. Similarly, a trench orthogonal to the second implantation direction  122  is defined as having a trench angle of 90°. 
       FIG. 6B  plots the effective dose N eff  for a trench sidewall against the trench angle (used in the above-described equation in place of θ), wherein the implant dose n d  is set to 1. The minimum N eff  according to the curve is 1, found when the trench angle is 0° or 90°. The maximum N eff  is approximately 1.41, found when the trench angle is 45°. The plot may be used for determining the trench pattern. For example, one wishing to manufacture a wafer having trenches with orientations differing by 50° and having the same doping concentration can examine the plot and find that the trench angles must be 20° and 70° to accomplish the stated objective using the method described above in accordance with preferred embodiments of the present invention. Similarly, one may use the plot to determine trench angles when it is desired to form one trench with a higher doping concentration relative to the second trench. Both results may be accomplished using a constant ion dose and acceleration energy with only four applications of the beam to the wafer  110 . 
     Additional benefits of angled trench implantation are described below. For example,  FIG. 7A  shows that a sidewall  14  of a trench  12  implanted at tilt angle Φ is doped to a depth D when the implantation rotation angle θ is normal to the sidewall  14 . In the case of a deep, narrow trench  12   n  ( FIG. 7B ), a sidewall  14   n  doped at the same tilt angle Φ results in a shallower doping depth D relative to the sidewall  14   n . The prior art solution, shown in  FIG. 7C , was to utilize a smaller tilt angle Φ n . However, tilt tolerance for doping variations becomes smaller as the tilt angle is decreased, and surface ion scattering can become more severe. 
     The possible implantation depth D is given by the equation:
 
 D=W /tan [Φ]
 
 FIG. 8  plots the possible implant depth D for a trench  112  against a width W of the trench  112  ( FIG. 9A ) for varying implantation rotation angles θ and with Φ=4°. It is seen that for smaller implantation rotation angles θ, the possible implantation depth D increases in accordance with preferred embodiments of the present invention. Thus, as the trench width W decreases, a small rotation angle θ can achieve the same implantation depth D without varying the tilt angle Φ. Additionally, with a smaller tilt angle Φ, very narrow and deep trenches  112  can be adequately implanted.
 
     As shown in  FIG. 9A , doping the sidewall  114  at an implantation rotation angle θ appears to provide a “substantial trench width” X to the ion beam, which is greater than or equal to the actual trench width W. The substantial trench width is found by the equation:
 
 X=W /sin [θ]
 
Thus, when the implantation rotation angle θ is 90°, the substantial trench width X is equal to the actual trench width W, and as the implantation rotation angle θ approaches 0°, the substantial trench width X increases exponentially. For example,  FIG. 9B  plots the substantial trench width X against the rotation angle θ for W=3 μm. It is seen that for θ=45°, the substantial trench width X is about 4.24 μm.
 
     An implantation rotation angle of 45° is therefore optimal when doping perpendicular trenches  112 ,  113  (e.g., as shown in  FIG. 3 ). It is also possible to create asymmetric doping profiles, such as, for example, when the desired implant depth D for one sidewall  114  is different than the desired implant depth D for the second sidewall  116 . Accordingly, one may implant the first sidewall  114  at 30° and the second sidewall  116  at 60°, for example. 
     Further, as shown in  FIG. 10 , semiconductor devices often have corners  32 . The corners  32  will have different doping levels than the surrounding trenches  12 ,  13  because of the different trench angle with respect to the ion beam. Because the electric field is circular rather than planar, the potential exists for electrical breakdowns at the corners  32 . It is therefore preferred that the doping levels of the corners  32  be lower than the doping levels of the trenches  12 ,  13 .  FIG. 10  shows that the doping level of the corners  32  will actually be higher because it receives doping from all four implantation directions  21 ,  22 ,  23 ,  24 , while the trenches  12 ,  13  only receive doping from two. Using the calculations described above, the corner  32  will be doped 1.4 times higher than the trenches  12 ,  13 , which creates a lower breakdown voltage. 
       FIG. 11  shows doping in accordance with preferred embodiments of the present invention. The trenches  112 ,  113  are doped from non-normal implantation directions  121 ,  122 ,  123 ,  124 , preferably θ=45°, as shown in  FIG. 11 . The corner  132  therefore receives the lowest implant dose, which is about 0.7 times the implant dose of the trenches  112 ,  113 . A higher breakdown with a wider doping process window is thus achieved. 
     It will be appreciated by those skilled in the art that the above-described multi-angle trench implantation embodiments are not confined to formation of deep trenches. The multi-angle implantation may be used in any trench fabrication requiring sidewall implant, independent of the width or depth of the trenches. 
     It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that the invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.