Patent Number: 
Section: description

The present invention provides a wafer holder assembly that is well-suited for SIMOX wafer processing, which includes the use of relatively high ion beam energies and temperatures. In general, the wafer holder assembly has a structure that maintains its integrity and reduces the likelihood of wafer contamination during extreme conditions associated with SIMOX wafer processing. The wafer holder assembly can be formed from electrically conductive materials to provide an electrical path from the wafer to ground for preventing electrical charging of the wafer, and possible arcing, during the ion implantation process. FIGS. 1-2 show a wafer holder assembly 100 in accordance with the present invention. The assembly includes first and second main structural rail members 102,104 that are substantially parallel to each other and spaced apart at a predetermined distance. In the exemplary embodiment shown, the main structural members 102,104 are generally C-shaped. A first wafer-holding arm 106 is rotatably secured to a distal end 108 of the holder assembly and a second wafer-holding arm 110 is pivotably secured to the assembly at a generally proximal region 112 of the assembly. The first arm 106 includes a transverse member 114 having first and second portions 116,118 each of which terminates in a respective distal end 120,122. Wafer-contacting pins 124,126 are secured to the distal ends 120,122 of the first and second arm portions. The first arm 106 is rotatable about a first axis 128 that is generally parallel to the first and second main structural members 102,104. By allowing the first arm 106 to rotate about the first axis 128, the first and second arm portions apply substantially equal pressure to the wafer edge via the spaced apart wafer-contacting pins 124,126. The second arm 110 is pivotable about a second axis 130 that is generally perpendicular to the main structural members 102,104 to facilitate loading and unloading of the wafers. A wafer-contacting pin 132 is affixed to the distal end 134 of the second arm to provide, in combination with the pins 124,126 coupled to the first arm, three spaced apart contact points to securely hold the wafer in place. Typically, placement of the pins about the circumference of the wafer is limited by a notch or xe2x80x9csignificant flatxe2x80x9d in the wafer that is used for orientating the wafer on the holder assembly. Some processing techniques include rotating the wafer a quarter turn, for example, one or more times during the implantation process to ensure uniform doping levels. The wafer holder assembly can further include a series of retaining members for securing the components of the assembly together without the need for conventional fasteners and/or adhesives. It is understood that adhesives can vaporize or outgas during the ion implantation process and contaminate the wafer. Similarly, conventional fasteners, such as exposed metal screws, nuts, bolts, and rivets can also contaminate the wafer. In addition, such devices may have incompatible thermal coefficients of expansion making the assembly prone to catastrophic failure. In one embodiment, the assembly includes a distal retaining member 136 coupling the first arm 106 to the assembly and an intermediate retaining member 138 affixed to a bottom of the assembly to maintain the spacing of the first and second main structural members 102,104 in a middle region 140 of the assembly. The assembly can further include a proximal retaining member 142 securing the structural members in position at the proximal region 112 of the assembly. FIGS. 3-7 (shown without the wafer-contacting pins), in combination with FIGS. 1 and 2, show further details of the wafer holder assembly structure. The first arm 106 includes a support member 144 extending perpendicularly from the transverse member 114 (FIGS. 3-4). The support member 144 includes an intermediate region 146 and an arcuate coupling member 148. A bearing number 150 extends through a longitudinal bore 152 in the intermediate region 146 of the support member 144 (FIGS. 3-4). A first cross member 154 is matable with the distal ends 156,158 of the main structural members 102,104 and a second cross member 160 is matable to the main structural members at a predetermined distance from the first cross member 154 (FIGS. 5-6). The first and second cross members 154,160 are adapted for mating with opposite edges of the main structural members 102,104. It is understood that notches can be formed in the various components to receive mating components. Each of the first and second cross members 154,160 includes a respective bore 162,164 for receiving an end of the bearing member 150. (FIG. 7). In one embodiment, the bearing member is a rod having each end seated within respective sleeve members 166,168 disposed within an aperture in the cross members 154,160. The sleeve members 166,168 allow the first arm 106 to freely rotate while minimizing particle generation due to graphite on graphite contact during rotation of the first arm. In one embodiment, the sleeves are formed from a hard, insulative material, such as aluminum oxide (sapphire). FIG. 8, in combination with FIGS. 1 and 2, show further details of the distal retaining member 136 having a first end 170 with a first notch 172 for coupling to one of the main structural members 102 and a second notch 174 for engaging the coupling member 148 (FIG. 3) of the first arm. A second end 176 of the distal retaining member 136 is matable to the intermediate region 140 of the assembly. Indents 178 can be formed in the main structural members 102,104 to facilitate engagement of the second end 176 to the assembly (FIG. 1). FIGS. 9-10 show alternative embodiments of the distal retaining member in the form of a helical spring 136xe2x80x2 and a bellows 136xe2x80x3, respectively. It is understood that one of ordinary skill in the art can readily modify the geometry of the retaining members. In one embodiment, the distal retaining member 136 is under tension so as to apply a force having a direction indicated by arrow 180 (FIG. 5) on the coupling member 148 of the support member. The force applied by the distal retaining member 136 pressures a neck 182 (FIG. 3) of the support member against the second cross member 160. The applied force also pressures the first cross member 154, via the bearing member 150, against the main structural members 102,104 as the second cross member 160 functions as a fulcrum for the support member 144. However, the transverse portion 114, as well as the support member 144 of the first arm, freely rotate about the first axis 128, i.e., the bearing member 150, such that the pins 124,126 at the distal ends of the first arm portions 116,118 provide substantially equal pressure on the wafer. FIGS. 11 and 12 (bottom view), in combination with FIGS. 1 and 2, show further details of the second proximal region 112 of the wafer holder assembly 100. FIG. 11 is shown without the second main structural member 104 for clarity. First and second stop members 184 (FIG. 1), 186 extend from the main structural members 102,104. In an exemplary embodiment, the second arm 110 includes wing regions 188 (FIG. 1) that are biased against the ends of the stop members 184,186 by a bias member 190. In one embodiment, the bias member 190 is under compression so as to pressure the second arm 110 against the stop members 184,186, e.g., the wafer-hold position. The bias member 190 includes a U-shaped outer portion 192 having a first end 194 mated to the first structural member 102 and a second end 196 coupled to the second structural member 104 (FIG. 12). A spring portion 198 of the second bias member includes one end abutting the second arm member 110 and the other end extending from a bottom of the U-shaped outer member 192. The second arm 110 pivots at its bottom end about a second bearing member 200 disposed on the second axis 130, which is generally perpendicular to the main structural members 102,104. The second bearing member 200 extends through a bore in the second arm with each end of the bearing member being seated in a sleeve inserted within a respective main structural member 102,104. Rotation of the second arm 110 is limited by respective brace members 202,204 extending from the main structural members 102,104. FIG. 13 (bottom view), in combination with FIGS. 1 and 2, shows further details of the intermediate retaining member 138, which is mated to the main structural members 102,104 in the intermediate region 140 of the assembly. The intermediate retaining member 138 includes first and second opposing U-shaped outer members 206,208 with a spring member 210 extending therebetween. The first outer member 206 has first and second arms 212,214 for mating engagement with corresponding notched protrusions 216,218 formed on the bottom of the main structural members 102,104. Similarly, the second outer member 208 includes arms that are matable with notched protrusions 220,222. In one embodiment, the U-shaped outer members 206,208 are forced apart to facilitate mating to the protrusions. Upon proper positioning, the outer members 206,208 are released such that spring member 210 biases the outer members against the protrusions. The intermediate retaining member 138 is effective to maintain the spacing between the first and second main structural members 102,104 and enhance the overall mechanical strength of the assembly. FIG. 14 shows the proximal retaining member 142, which provides structural rigidity in the proximal region 112 of the wafer holder assembly. In one embodiment, the proximal retaining member 142 includes upper and lower members 224,226 coupled by a spring member 228. The spring member 228 can be engaged to the main structural members such that the spring member is under tension. The proximal retaining member 142 can include a protruding member 230 having a slot 232 formed therein. As shown in FIG. 15, the assembly 100 is matable with a rotatable hub assembly 250 to which a series of wafer holder assemblies can be secured. A shield 252 can be secured to the proximal region 112 of the assembly to protect exposed regions of the assembly from beam strike. The shield 252 prevents sputtering from the assembly components, as well as any metal devices used to affix the assembly to the hub 250, during the ion implantation process. In addition, the assembly components are not heated by direct exposure to the ion beam. In one embodiment, an edge of the shield 252 is captured in the slot 232 (FIG. 14) located in the proximal retaining member 142. It is understood that the shield 252 can have a variety of geometries that are effective to shield the assembly components from beam strike. In one embodiment, the shield 252 is substantially flat with an arcuate edge 254 proximate the second wafer-holding arm 110 to increase the shielded region of the assembly. It is further understood that the shield can be formed from various materials that are suitably rigid and are opaque to the ion beam. One exemplary material is silicon having properties that are similar to a silicon wafer. The wafer-contacting pins 124,126,132 coupled to ends of the wafer-holding arms are adapted for contacting and securing the wafer in the wafer holder assembly 100. In general, the pins should apply sufficient pressure to maintain the wafers in the holder assembly during the load and unload process in which the wafers are manipulated through a range of motion that can include a vertical orientation. However, undue pressure on the wafers should be avoided since damage to the wafer surface and/or edge can result in the formation of a slip line during the subsequent high temperature annealing process. In addition, the wafer-contacting pins should not electrically insulate the wafer from the assembly. Further, the pins should be formed from a material that minimizes contamination of the wafer. FIGS. 16A-B show a wafer-contacting pin 300 adapted for use with a wafer holder assembly in accordance with the present invention. The pin has a distal portion 302 having a geometry adapted for holding the edge of a wafer and a proximal portion 304 having a contour complementing a corresponding channel formed in the ends of the wafer arms 106,110 (FIG. 1). It is understood that a variety of shapes and surface features can be used to securely and releasably mate the pin 300 to the wafer-holding arms. The distal portion 302 of the pin includes a ridge 306 extending from an arcuate wafer-receiving groove 308 in the pin. A tapered surface 310 extends proximally from the groove 308. As shown in FIG. 17, the pin should contact the top 352 and bottom 354 of the wafer 350 to prevent movement and/or vibration of the wafer as the holder assembly is rotated during the implantation process. In addition, the tapered surface 310 provides a ramp on which the wafer edge may first contact and slide upon during the wafer load process until meeting the ridge 306. FIGS. 18-23 show a wafer-contacting pin 400 in accordance with the present invention having a more limited profile. The pin 400 includes a distal portion 402 for holding a wafer and a proximal portion 404 for coupling to the arm ends. The distal portion 402 of the pin is rounded to minimize the amount of pin material proximate the wafer edge for reducing the likelihood of electrical discharge from the wafer to the pin. In addition, the pin geometry is optimized to maximize the distance between the wafer edge and the pin except at the wafer/pin contact interface. Further, the wafer-contacting region of the pin 400 should be smooth to minimize the electric field generated by a potential difference between the wafer and the pin. The pin should also minimize the wafer/pin contact area. The distal portion 402 of the pin includes a wafer-receiving groove or neck 406 disposed between a wedge-shaped upper region 408 and a tapered surface 410. The neck 406 can be arcuate to minimize the contact area between the wafer edge and the pin. The upper region 408, including the neck 406, can taper to a point or edge 412 for reducing the amount of pin material near the wafer edge to inhibit electrical arcing between the wafer and the pin. It is understood that the term wedge-shaped should be construed broadly to include a variety of geometries for the pin upper region. In general, the wedge-shaped upper region broadens from a point nearest a center of a wafer held in the assembly. Exemplary geometries include triangular, arcuate, and polygonal. In a further aspect of the invention, a wafer-contacting pin, such as one of the pins 122,300,400 shown in FIGS. 1, 15, 18, is coated with a relatively hard, electrically conductive film, such as titanium nitride (TiN) or titanium aluminum nitride (TiAlN). The coating provides a relatively hard, abrasion resistant material that enhances the ruggedness of the pin. In the case where the pin is formed from silicon, the TiN coating, for example, is more conductive than the silicon pin such that the likelihood of electrical arcing is reduced in comparison with an uncoated pin. In addition, the coating inhibits so-called wafer bonding in which two silicon surfaces tend to stick together during extreme processing conditions, e.g., relatively high temperatures. It is understood that potentially contaminating particles can be generated when a wafer bond between a wafer and a wafer-contacting pin is broken. The coating can be applied to the pin using a variety of techniques including chemical vapor deposition and reactive sputtering. For chemical vapor deposition to provide a TiN coating, an exemplary precursor gas is titanium chloride. For reactive sputtering a titanium target can be used and nitrogen gas can be added to an argon gas environment. It is understood that the TiN or TiAlN coating can be applied to cover the entire pin, as well as only targeted portions corresponding to the pin/wafer interface. It is further understood that the TiN coating can be applied in discrete portions or as a continuous coating. The thickness of the coating can vary from about 0.1 micrometers to about 10.0 micrometers, and more preferably from about 2 micrometers to about 5 micrometers. A preferred coating thickness is about 5 micrometers. In a further aspect of the invention, the materials for the various components are selected to provide desired features of the assembly, e.g., mechanical durability; electrical conductivity; and minimal particulation. Exemplary materials for the wafer-contacting pin include silicon and graphite. It is understood that silicon is conductive in its intrinsic state at elevated temperatures. Exemplary materials for the main structural members, the retainer members, and the bias member include silicon carbide, graphite and vitreous or vacuum impregnated graphite, which can be coated with titanium carbide. The graphite retainer and bias members can be fabricated from graphite sheets using wire electron discharge machine (xe2x80x9cwire EDMxe2x80x9d), laser machining and conventional cutting techniques. The graphite bias and retaining members maintain a steady, i.e., invariant, spring constant over a wide range of temperatures. This allows the wafer holder assembly to be adjusted at room temperature for operation at temperatures of 600xc2x0 C. and higher, which can occur during the ion implantation process. The graphite components also provide a conductive pathway for grounding the wafer, even where insulative sleeves for the bearing members are used. The wafer holder assembly of the present invention provides a structure that withstands the relatively high temperatures and ion beam energies associated with SIMOX wafer processing. In addition, the likelihood of wafer contamination is reduced since the ion beam strikes only silicon thereby minimizing carbon contamination and particle production. Furthermore, the likelihood of the electrical discharge from the wafer is minimized due to the selection of conductive materials/coatings for the assembly components and/or the geometry of the wafer-contacting pins. One skilled in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.