Patent Publication Number: US-6660643-B1

Title: Etching of semiconductor wafer edges

Description:
FIELD OF THE INVENTION 
     This invention pertains to plasma etching of semiconductor wafers and more particularly to the etching of the edges of semiconductor wafers. 
     BACKGROUND OF THE INVENTION 
     It is a common practice to etch semiconductor wafers to remove selected material from their front and back surfaces and their edges. Thus etching of silicon has been accomplished in various ways, e.g., by chemical (wet acid) etching, plasma etching, reactive ion etching, ion beam etching and chemically assisted mechanical polishing (“CMP”). It also is common to remove material from wafers by grinding. 
     Silicon solar cells are commonly made from silicon wafers. These wafers are subsequently subjected to various processing steps to produce solar cells, one of those steps involving formation of a photovoltaic p-n junction adjacent to one side of each wafer. The wafers are formed in different ways. They may be sliced from large single crystal Czochralski-grown boules or polycrystalline blocks, e.g., typically by means of a diamond blade saw or a wire saw. Another common method of producing silicon solar cell wafers is to grow hollow polycrystalline bodies by the Edge-Defined Film-Fed Growth (“EFG”) method, and then laser cut those bodies into wafers. Wafers cut from large single crystal boules or polycrystalline blocks are characterized by opposite surfaces that are flat and smooth (sufficiently so that adjacent wafers tend to adhere to one another), while the corresponding surfaces of EFG silicon wafers are uneven. Additionally EFG wafers generally are rectangular in shape, in contrast to those produced from large single crystal boules which commonly have a generally circular configuration (the wafers cut from relatively large cast polycrystalline blocks also are generally rectangular). 
     EFG semiconductor silicon wafers received from the laser cutting station have edges that have micro-cracks and are tend to be rough due to resolidified material. It has been determined that EFG wafers with micro-cracks at their edges tend to break during subsequent solar cell processing. However, it has been determined that the resistance to fracture of EFG wafers is improved if their edge portions (margins) are etched to give smooth edges that are free of micro-cracks. It should be noted that silicon wafers cut from boules or cast blocks also have edge damage that needs to be removed. 
     EFG wafers are typically etched using a wet acid etching process, since the other etching techniques mentioned above have not been satisfactory because the etching process throughput rates are too slow and costly. The wet acid etching of EFG wafers serves two purposes: (1) remove edge damage (microcracks) resulting from the laser cutting of the wafers and (2) isotropically remove a thin layer of a film of SiO/SiC that is formed on the back surface of the wafers during the EFG growth process. The etching solution typically is a mixture of nitric and hydrofluoric acids. Using the wet chemical etching process, the EFG-grown wafers are submerged in a bath of the mixed acid solution, with the wafers being kept in the bath long enough to achieve the desired etching. 
     Although it is effective in improving the resistance to fracture of the EFG wafers, the wet etching process poses other immediate problems. Much floor space is required for the etching equipment and the chemical waste disposal facility. Also the use of the highly corrosive acid mixture poses a continuous hazard. The wet acid etching process also has the disadvantage of producing copious quantities of spent acid which must be neutralized before disposal, and legally disposing of the chemical waste involves substantial cost. Moreover, even with neutralization, an environmental problem remains due to limits on the disposal of the resulting chemical compounds imposed by government authorities. 
     Prior to this invention it has been deemed desirable to use a plasma etch process to etch the edges of silicon solar cells, after they have been provided with front and back metal contacts, for the purpose of removing sufficient material to assure junction isolation. However, use of a plasma etch process for such purpose is feasible economically only if a plurality of wafers can be etched at the same time and the amount of material required to be removed at each face is relatively small. For effecting junction isolation, the amount of material required to be removed at each edge face typically is in the order of 1 micron. Prior to the present invention it was recognized also that the etch production rate could be increased by stacking a plurality of solar cell wafers together face-to-face in what is commonly called a “coin stack”, and then subjecting that stack to a plasma etch process that is conducted in a plasma etching chamber under a partial vacuum, i.e., at sub-atmospheric pressures. Such a process typically produces what is commonly characterized as a relatively low density plasma which envelops the stack. Edge etching a coin stack of solar cells for junction isolation purposes is disclosed by U.S. Pat. No. 4,158,591, issued Jun. 19, 1979 to J. E. Avery et al. and Charles F. Gay For “Solar Cell Manufacture”. In the Avery et al. process of making solar cells, the stack of solar cells are enveloped by the reactive plasma for a selected period of time sufficient to achieve the desired junction isolation. The Avery et al patent teaches that the plasma etching may be carried out using plasma etching apparatus as described in U.S. Pat. No. 3,795,557. 
     Using a reactive plasma in the manner taught by Avery et al. to etch the edges of a coin stack of EFG wafers is not satisfactory for several reasons. For one thing, since a plasma produced under a pressure that is less than atmospheric pressure tends to have a relatively low density, the rate of removal of wafer material is comparatively low. That limitation is especially severe in the case of edge etching EFG wafers since the amount of material required to be removed at each edge face to eliminate the micro-cracks is substantially in excess of one (1) micron. EFG silicon wafers currently being used to make solar cells frequently have micro-cracks or roughness due to resolidified material that require as much as 100 microns to be removed from each edge. 
     A second reason is due to the nature of the EFG wafers. As noted above, the opposite faces of EFG wafers are uneven. More specifically, the surfaces of EFG wafers have undulations that are clearly visible and result in surface flatness deviations of as much as 500 microns. Because of those undulations, and also because of the roughness at the edges caused by resolidified material, when EFG wafers are arranged in a coin stack, they do not adhere to one another and instead there may exist small gaps therebetween into which the reactive plasma can intrude. In this connection it is to be noted that the mean free path of the reactive ions (e.g., fluorine ions) in an etching plasma depends on the density and pressure of the plasma. Under the partial pressures created in the etching chamber, the mean free path of the reactive ions in the plasma surrounding a stack of wafers is such that the ions can travel in the order of millimeters between adjacent EFG wafers, causing uneven etching of the opposites surfaces of each wafer as well as uneven etching of the edge faces of the same wafers. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The primary object of this invention is to provide a new method of etching the edges of semiconductor blanks for the purpose of removing damaged edge portions. 
     Another specific object of the invention is to provide a method of edge-etching semiconductor wafers that are stacked together, with the coin stack being exposed to a reactive plasma in a manner that removes edge damage from the wafers, while limiting the etching substantially to the edges of the wafers. 
     Another object is to provide a novel method which involves supporting a plurality of semiconductor wafers in a stack and moving that stack so that successive edge portions of the wafers are etched to within predetermined tolerances. 
     Still another object is to provide a novel method of edge etching semiconductor wafers for use in making solar cells or other semiconductor devices. 
     A more specific object is to provide an improved method of making solar cells that is characterized by a novel technique for edge etching the wafers to remove microcracks. 
     A further specific object is to provide an improved method of edge etching EFG wafers so as to removed damaged portions from said wafers. 
     These and other objects are achieved by a process which comprises subjecting semiconductor wafers to dry etching using a relatively high density plasma which is produced at atmospheric or near-atmospheric pressure. More particularly the invention comprises etching the edges of a stack of wafers using apparatus like or similar to the plasma apparatus disclosed in U.S. Pat. No. 5,767,627, issued Jun. 16, 1998 to O. Siniagnine for Plasma Generation And Plasma Processing Of Materials. 
    
    
     Other features and advantages of the invention are presented by the following detailed description which is to be considered together with the attached drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view illustrating a stack of rectangular semiconductor wafers; 
     FIG. 2 is a plan view showing the same stack of wafers supported by a clamping type holder; 
     FIG. 3 is a rear elevation of the wafer holder shown in FIG. 2; 
     FIG. 4 is a schematic plan view of a carousel for transporting a plurality of stacks of wafers into and out of an etching plasma; 
     FIG. 5 is a schematic side elevation of the same carousel; and 
     FIG. 6 is a schematic view illustrating a plasma generating system adapted to produce a high density plasma in combination with the carousel of FIGS. 4 and 5, and showing how a stack of wafers is exposed to the etching plasma. 
    
    
     SPECIFIC DESCRIPTION OF THE INVENTION 
     The present invention is based on the concept of using a plasma generator of the type described in U.S. Pat. No. 5,767,627 which operates at atmospheric or near atmospheric pressure and produces a plasma which has a density that is high in relation to the density of the plasma produced by the apparatus described in U.S. Pat. No. 3,795,557. However a plasma generator of the type described in said U.S. Pat. No. 5,767,627 produces a magnetically confined plasma stream. Accordingly the plasma generator needs to be positioned so that its magnetically confined plasma stream is directed at the edges of the wafers in the stack of wafers to be etched. The invention also recognizes that the magnetically controlled plasma cannot be directed simultaneously at all edges of the wafers in a given stack of wafers. Therefore, the method of this invention also involves rotating the coin stack so as to expose different edges of the wafers (or different edge portions in the case of circular wafers) to the plasma stream. The time of exposure of the wafer edges to the plasma stream is controlled so as to avoid overheating the wafers, since overheating produces undesirable results. 
     Referring first to FIGS. 1-3, a plurality of rectangular wafers  10  are mounted face-to-face in a stack  11 , and that stack is disposed in and supported by a holder  12 . It should be noted that the wafer thickness is exaggerated in FIGS. 1,  2  and  4 - 6  for convenience of illustration. The holder  12  preferably comprises a main strap portion  14 , a supporting arm portion  16  which is joined to the center of strap portion  14  and has a hole  18  at its upper end which is used to attach it to a hanger (not shown), and a pair of arms  20  at opposite ends of strap portion  14 , with arms  20  terminating in right-angle flanges  22 . As a result, as seen in FIG. 2, the members  14 ,  20  and  22  cooperate to define a space for receiving the stack of wafers. The two arms  20  engage the adjacent edges of the wafers. In this particular embodiment of the invention, the arms  20  are made of a spring metal and are formed so that together they exert a clamping force on the wafers that is just sufficient to hold the wafers in place without incurring any wafer damage. The wafers are held only at two opposite edges (the vertically-extending edges as viewed in FIGS. 3,  5  and  6 ), leaving the other two edges (the horizontally-extending edges as viewed in FIGS. 3,  5  and  6 ) fully exposed so that they can be etched as hereinafter described. 
     Turning now to FIGS. 4-6, there is shown one form of apparatus for etching stacks of wafers according to this invention. The apparatus comprises a housing  30  that defines an etching chamber containing a carousel  32  and a plasma generator identified generally by the numeral  34 . Carousel  32  is carried by a rotatable shaft  36  which is mounted to a bearing support  37  that is affixed to the housing. Mounted on shaft  36  is a pulley  38  that is coupled by a belt  40  to a second pulley  42  that is mounted on the output shaft of an electric motor  44 . The latter is mounted to a motor support  45  affixed to housing  30 . Operation of motor  44  will cause pulleys  42 , belt  40  and pulley  38  to rotate the carousel in a direction determined by operation of motor  44 . 
     Carousel  32  has an a polygonal configuration in plan view, comprising a plurality of flat facets  46  (FIG. 4) that are provided with hangars in the form of pins  48  (FIGS.  5 , 6 ) that are sized to fit in holes  18  of holders  12 . The facets and pins  48  serve to permit a plurality of holders  12 , each supporting a stack of wafers to be etched, to be attached to the carousel, one at each facet of the carousel. 
     Referring now to FIG. 6, the plasma generator identified generally by numeral  34  comprises two hollow electrode units  52  and  54  which also serve as discharge nozzles, their internal passageways (not shown) being connected by lines  56  and  58  to a supply of at least an inert gas such as argon. The two electrode units are disposed so that their axes are disposed in converging relation with one another, with the result that the gas streams emitted by them converge on one another in the manner shown in FIG. 1 of said U.S. Pat. No. 5,767,627. The plasma generator also comprises at least a pair of electromagnets represented schematically at  60  and  62 , and electrical leads  64  and  66  for connecting the electrodes and electromagnets to power supplies (not shown), plus other associated components (not shown) that complete and operate electrical circuits for (a) energizing the electrode units so as to pass an electrical current through and between the two inert gas streams and thereby form a plasma stream, and (b) energizing the electromagnets so as to create magnetic fields for shaping the plasma. Other details of the plasma generator are not shown herein and instead the teachings of said U.S. Pat. No. 5,767,627 are incorporated herein by reference thereto. In this connection it is to be appreciated that other forms of plasma generators that operate at or near atmospheric pressure, e.g., the plasma generators disclosed by the several patents and PCT patent applications listed in said U.S. Pat. No. 5,767,627, may be used in practicing the present invention. By way of example but not limitation, the power supply for the electrode units may provide a d.c. voltage in the range of 100 to 200 volts when the distance between the centers of the discharge orifices of the two adjacent electrode units is in the range of 20 to 100 mm and the angle between the electrode units is in the range of 30° to 50°. 
     Thus, further by way of example but not limitation, the power supply for the two electrode units may be such as to generate a plasma current in the order of 90 amperes. The plasma has the appearance of a flame, as represented at  70  in FIG. 6, and reaches a maximum temperature in the order of thousands of Centigrade degrees in the region where the current arcs between the two gas streams emitted by the two electrode units. 
     The plasma generator is disposed below the carousel and is positioned so as to direct the gas streams upwardly at the periphery of the carousel. Consequently as the carousel rotates each stack of wafers in turn will pass over the plasma flame produced by generator  34 . The carousel and plasma generator are disposed so that the bottom edges of the wafers in each of the stacks that are mounted to the carousel are spaced approximately 45 cm. above the level of the gas discharge ends of the two electrode units. This gap between the stack of wafers in etching position and the discharge ends of the two electrodes assures that the wafers will be in a downstream plasma zone that is characterized by a relatively low temperature (in the order of 500° C.) rather than in the much higher temperature zone where the current arcs between the two gas streams emitted by the electrodes. This spacing between the plasma generator and the wafers carried by the carousel helps assure that the wafers will not be overheated. 
     A selected gas that serves as the source of a desired reactive species, i.e., the ions that react with the wafers so as to chemically etch their edges, may be introduced with the inert gases through the internal passageways of the electrode units, so that the gas streams emitted from the discharge nozzles of the electrode units contain both an inert gas and the desired reactive species, e.g., fluorine ions. Alternatively or additionally, the gas that serves as the source of the desired reactive species may be introduced directly into the plasma via an injection tube  74  which corresponds to the injection tube  21  shown in said U.S. Pat. No. 5,767,627. As suggested by said U.S. Pat. No. 5,767,627 (col.  4 , lines  10 - 15 ), the injection tube is located below and is spaced at a distance from the point of intersection of the axes of the two electrode units where it is not likely to experience thermal damage by the plasma heat during operation of the plasma generator. U.S. Pat. No. 5,767,627 suggests that in some embodiments the distance between the discharge end of the injection tube and the point of intersection of the axes of the two electrodes may be as much as 10-50 mm. 
     In practicing this invention it is preferred that the gases introduced via the electrode units consist of only inert gases and that the reactive species be introduced solely via injection tube  64 . The reason for this preferred way of introducing the reactive species is that it is easier to control the concentration of reactive species in the downstream plasma zone where the plasma contacts the wafers. By way of example but not limitation, the inert gas may be argon flowing through each of the two electrodes at a rate in the order of one (1) liter per minute, while the reactive species introduced via injection tube  12  may be CF 4  flowing at a rate in the order of 2.5 liters per minute. 
     Various compounds may be used as the source of the reactive species that etches the wafers. In the case of silicon wafers, the source is a halogen-containing compound, preferably one that contains fluorine. By way of example but not limitation, for etching silicon the source of the reactive species comprise one or more of the following in gaseous form: carbon tetrafluoride (CF 4 ), trifluoromethane (CHF 3 ), difluoromethane (CH 2 F 2 ), fluorine, fluoroform, hexafluoroethane, hydrogen bromide, hydrogen fluoride, iodine pentafluoride, tetrafluoroethylene, trichlorofluoromethane, bromine pentafluoride, bromine trifluoride, bromotrifluoroethylene, bromotrifluoromethane, chlorodifluoromethane, chloropentafluoroethane, chlorotrifluoroethylene, chlorotrifluoromethane, dibromodifluoromethane, 1,2-dibromotetrafluoroethane, dichlorodifluoromethane, dichlorofluoromethane, 1,2-dichlorotetrafluoroethane, 1,1-difluoro-1-chloroethane, 1,1-difluoroethane, 1,1-difluoroethylene, tetrafluoroethylene, and trichlorofluoromethane. 
     The gas introduced to the electrode units for creating the plasma preferably is an inert gas such as argon or helium, or a mixture thereof, although any other gas capable of sustaining an electrical current may be used to create the plasma, provided, of course, that such gas does not have a deleterious effect on the wafers or the components, operation or operating results of the plasma generator. 
     Following is a specific example of how to practice the invention using apparatus as shown in FIGS. 2-6. A plurality of rectangular EFG wafers, each measuring 4 inch×4 inch and having a thickness of approximately 300 microns (300 μm), are arranged in face-to-face contact in a plurality of stacks, with each stack being placed in a holder  12 . Each holder supports a stack of 85 wafers. The stacks are mounted on the facets  46  of the carousel, with the arm portions  16  of the holders  12  being suspended from hanger pins  48 . The carousel has  12  facets, so that in excess of 1000 wafers are supported at its periphery. The housing  30  is not air-tight and thus the pressure in the chamber defined by the housing is atmospheric. The housing is provided with an exhaust line  31  that may be coupled to an exhaust fan (not shown) to assure removal of spent gases. 
     Argon gas is introduced to the two electrode units at a rate of approximately 0.8 liter/minute and CF 4  gas is introduced via injection tube  64  at a rate of 2.5 liters/minute. The plasma generator is energized with a d.c. voltage so as to provide an electrical current of approximately 90 amperes to arc between the two gas streams emanating from the two electrodes, resulting in the formation of a plasma containing ions of fluorine. The flow velocity in the downstream plasma region, i.e., the relatively cold plasma zone through which the bottom edges of the wafers are transported, is less than 30 meters/second. 
     Rotation of the carousel is initiated before or substantially at the same time that the plasma generator is energized to create the plasma. The carousel is rotated at a linear speed of about 120 inches/second, so that each stack of wafers is exposed to the plasma for a period on the order of milliseconds, thereby preventing overheating of the wafers. Under the foregoing conditions, the etch rate is in the range of 50-75 cm 2 /sec./μm. The carousel operates continuously until the bottom edges of all of the wafers on the carousel have been etched a desired amount, e.g., their bottom edges are etched back by about 35 μm. Then the carousel is stopped, the flow of gases is terminated and the flow of electrical power to the plasma generator is shut off. Thereafter the wafers are allowed to cool and then removed from the chamber. The wafers are then reversed 180° in holders  12 , so that what were previously the wafer top edges are now the wafer bottom edges, and the wafer-containing holders are then mounted again to the carousel. The etching process is then repeated to etch the new bottom edges of the wafers. Subsequently the process is terminated again, the wafer holders are removed from the chamber and the wafers rotated 90° in their holders. Then the holders are again mounted in the carousel and the etching process is repeated. Subsequently, the process is terminated a third time, the wafer holders removed, and the wafers rotated 180° in the holders. Then the holders are again mounted on the carousel and the process repeated to etch the fourth edges of the wafers. 
     Wafers etched in the manner described by the foregoing example have been found to have improved resistance to fracture during subsequent solar cell manufacturing steps. Additionally the process of this invention offers the advantage that the throughput of wafers is as high or higher than it is using wet acid etching. Also waste disposal problems are substantially reduced and the overall cost of etching is substantially less than wet acid etching. 
     It is contemplated that the process may be modified by providing the carousel with wafer holders that are capable of rotating on axes that are perpendicular to the facets of the carousel and have a first set of grippers for gripping a stack of rectangular wafers at first and second opposite edges of the wafers and a second set of grippers for gripping same wafers at their other two opposite edges, with the carousel also being provided with means operable on command for selectively and alternately operating the two sets of grippers and for rotating the wafer holders in 90° or 180° increments so that each of the edges of the wafers is exposed in turn to the etching plasma. 
     The invention is not limited to etching EFG silicon wafers. Instead the invention may be used to etch the edges of wafers cut from single crystal boules or from polycrystalline blocks made by casting or some other process. Also the invention is not limited to etching wafers that have a rectangular shape. By way of example, the invention may be used to etch the edges of a stack of circular wafers, in which case the carousel is modified to include (a) wafer holding means that are adapted to grip the circular wafers at their centers rather than their edges, and (b) means for rotating the wafer holding means so that successive portions of the edges of the circular wafers are exposed to the plasma flame. 
     Another contemplated modification is to modify the wafer holders  12  and the carousel by providing modified wafer stack holders that have a center axis for rotation and providing the carousel with (a) means on each of its facets for rotatably and removably supporting the modified holders and (b) detent means for releasably holding each wafer stack holder in a selected rotational position. With such apparatus, rotation of the holders carrying the wafers in 90° increments may be effected by means of a manually manipulated tool (not shown) or a manually operated mechanism (also not shown) that is carried by the turntable. Another possible modification is to use more than two electrodes to shape the plasma stream, as suggested by said U.S. Pat. No. 5,767,627. 
     It also is recognized that the reactive gas, e.g., CF 4  gas injected via tube  64 , may be admixed with an inert carrier gas such as argon. Also, if desired, a mixture of reactive gases may be introduced to the plasma for the etching purposes. It also is appreciated that the method of this invention may be used to edge etch wafers may of a material other than silicon and that in such case, the same or other gaseous etchant materials may be used in practicing the invention. 
     It is contemplated that the edge etching method herein described and claimed forms an integral part of a solar cell manufacturing process which comprises the following conventional steps: (1) growing a hollow polycrystalline body of silicon by the EFG method, e.g., a so-called “octagon”, (2) severing the hollow body with a laser so as to form a plurality of rectangular wafers, (3) edge etching the wafers as herein described, (4) subjecting the wafers to diffusion doping so as to form photovoltaic junctions therein, and (5) forming metal contacts on the front and back surfaces of the wafers so as to form operative solar cells. Except for the edge etching operation, the foregoing process for making solar cells comprises steps that are old in the art and hence need not be described in greater detail herein. In this connection see the following U.S. Patents which are incorporated herein by reference: U.S. Pat. No. 5,476,553, issued to J. I. Hanoka et al.; U.S. Pat. No. 5,270,248, issued to M. D. Rosenblum et al.; U.S. Pat. No. 5,102,494, issued to D. S. Harvey et al; and U.S. Pat. No. 5,156,978, issued to B. R. Bathey et al. 
     Still other modifications and applications of the invention will be obvious to persons skilled in the art from the foregoing specific description.