Abstract:
A method for depositing a solder layer or solder bump on a sloped surface. The method includes etching a sloped surface on a planar semiconductor substrate, depositing a solder-wettable layer on the sloped surface, masking the wettabler layer with a coating layer to control the position of the solder deposition, and using an organic film to prevent the solder from being deposited at regions not above either the wettable layer or the coating layer. Also, a semiconductor device structure on which a solder layer or solder bump is formed exclusively on a sloped surface.

Description:
BACKGROUND 
     Solder is a material that typically contains tin and lead and that is commonly used during the manufacturing of electronic circuit boards. Solder generally has a lower melting temperature than the metals that may be included, as lines or layers, in the circuit boards. Hence, once two or more metal lines or layers have been formed in a circuit board, solder may be used to form an electrical contact between the layers and/or lines. 
     FIGS. 1A-1E show cross-sections of semiconductor device structures after various steps of a process for depositing solder on a planar semiconductor substrate surface have been performed according to the related art. FIG. 1A is a cross-sectional view of a semiconductor substrate  100 , such as silicon or gallium arsenide, and of an organic film  110 , such as a photoresist film, that has been deposited on the semiconductor substrate  100 . The organic film  110 , according to the related art, is typically spun onto the substrate  100  and is typically in contact with the entire surface of the semiconductor substrate  100 . 
     FIG. 1B is a cross-sectional view of the layers  100 ,  110  discussed above after the organic film  110  has been selectively etched to form a series of holes  120  (or channels, troughs, grooves, or openings) above the substrate  100 . The holes  120  in the organic film  110  may be formed via photo-lithography or by any other process known in the art of semiconductor device manufacturing. 
     FIG. 1C is a cross-sectional view of the substrate  100  and organic film  110  discussed above after the holes  120  in the organic film  110  have been filled, at least partially, with solder paste  130 . Solder paste, in general, typically includes an admixture of flux and solder particles. The solder paste  130  shown in FIG. 1C may be deposited in the holes  120  by any process known in the art. For example, a process similar to the stencil printing process used in the surface mount assembly process can be used. Specifically, a squeegee can be used to “roll” a bead of solder paste  130  across the organic film  110  to deposit the solder paste  130  into the holes  120 . 
     FIG. 1D is a cross-sectional view of the substrate  100  after the solder paste  130  has been heat-treated to form solder bumps  140  on the substrate  100 . In order to form the solder bumps  140 , the temperature of the solder paste  130  that had been in the holes  120  of the organic film  110  was raised. The higher temperature caused the flux portion that had been in the paste  130  to liquefy and activate the metal surfaces and caused the solder particles in the paste to melt. In the molten phase, the solder will wet to a solderable pad on the substrate surface while the surface tension of the liquid solder will cause the molten solder to form the shape of the solder bump. Upon cooling of the melted solder particles, solid solder bumps  140  were formed. Typically, the temperature of the solder paste  130  is raised by the use of an oven or hot plate. 
     FIG. 1E is a cross-sectional view of the substrate  100  and the solder bumps  140  after the organic film  110  has been removed. The organic film  110  may be removed by any process that known in the art. Upon removal of the organic film  110 , the substrate  100  may have additional structures, such as metal layers and metal lines, deposited thereon, and the solder bumps  140  can be used to electrically connect two or more metal layers or lines. 
     SUMMARY 
     A method of depositing solder, the method including the steps of providing a substrate that includes a substantially planar surface and a sloped surface adjacent to the substantially planar surface, forming a wettable layer on a portion of the sloped surface, and forming a solder layer on a first portion of the wettable layer. 
     A semiconductor device including a substrate having a substantially planar surface and an interior sloped surface, a wettable layer adhered to a portion of the interior sloped surface, and a solder layer adhered to a first portion of the wettable layer. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The detailed description will refer to the following drawings, wherein like numerals refer to like elements, and wherein: 
     FIGS. 1A-E show the steps of a process for depositing solder on a planar surface according to the related art; and 
     FIGS. 2A-I illustrate steps of a process for depositing solder on a sloped surface. 
    
    
     DETAILED DESCRIPTION 
     The following detailed description is presented to enable any person skilled in the art to make and use devices that include solder. For purposes of explanation, specific nomenclature is set forth to provide a thorough understanding of making and using such devices. However, it will be apparent to one skilled in the art that these specific details are not required to make and use the devices. Descriptions of specific applications are provided only as representative examples. Various modifications will be readily apparent to one skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the methods and devices described herein. The methods and devices are not intended to be limited to the embodiments shown, but are to be accorded the widest possible scope consistent with the principles and features disclosed herein. 
     Historically, solder bumps have been deposited exclusively on substantially planar surfaces, such as the surface of the substrate  100  shown in FIGS. 1A-E and discussed in detail above. However, solder bumps have generally not been deposited on sloped surfaces. Since semiconductor devices typically include both substantially planar and sloped surfaces, the need exists for methods to deposit solder on sloped (i.e., non-planar) surfaces. FIGS. 2A-I illustrate various embodiments of methods of depositing solder and solder bumps on sloped surfaces. 
     With reference now to FIG. 2A of the Drawings, there is illustrated a cross-sectional view of a substrate  200  that includes two substantially planar surfaces  210  and two sloped surfaces  220 . In some cases, the two sloped surfaces  220  may be on opposite sides of the same square hole or rectangular channel formed in the substrate  200 , as is well understood in the art. As shown, each of the sloped surfaces  220  may be positioned adjacent to a substantially planar surface  210  of the substrate  200 . The slope of the sloped surfaces  220  relative to the planar surface  210  may be at any angle greater than 0° and less than 90°, measured relative to a line extending horizontally from the planar surface  210 . However, the 5°, 10°, 20°, 30°, 45°, 60°, 70°, and 80° angles, plus or minus 2.5°, are preferred in certain embodiments. 
     The sloped surfaces  220  may be formed by etching the substrate  200 . The etching step that forms the sloped surfaces  220  may include anisotropically etching completely through the substrate  200  to form a hole. Alternately, the etching step may form a channel in the substrate  200  and/or may not etch completely through the substrate  200 . 
     FIG. 2B is a cross-sectional view of the substrate  200  shown in FIG.  2 A and of a wettable layer  230  that has been formed on a portion of one of the sloped surfaces  220  of the substrate  200 . The wettable layer  230  may include a metal that is wettable by solder (i.e., a metal on which solder can spread evenly, as opposed to beading up on). The wettable layer  230  may be formed by any process known in the art including, but not limited to, evaporation, sputtering, and plasma deposition. Metals that may be included in the wettable layer  230  include, but are not limited to, gold, silver, and copper. Compounds that are solder-wettable may also be used. 
     As shown in FIG. 2B, the wettable layer  230  may be formed partially on the sloped surface  220  and partially on the planar surface  210  of the substrate  200 . In this case, the portion of the wettable layer  230  that is formed on the planar surface  210  of the substrate  200  may be substantially planar. Alternately, the wettable layer  230  may be deposited exclusively on the sloped surfaces  220 . The benefits of forming a portion of the wettable layer  230  on the planar surface  210  will become apparent from the discussion below, as will the benefits of forming the wettable layer  230  exclusively on the sloped surface  220 . 
     FIG. 2C is a cross-sectional view of the substrate  200  and wettable layer  230  discussed above, after a coating layer  240  has been formed on a portion of the wettable layer  230  and of the substrate  200 . The coating layer  240  may include one or more dielectric materials that are not solder-wettable. Such materials include, but are not limited to, oxides, polyimides and solder masks. The coating layer  240  may be formed by any method known in the art and may be thought of as a mask for the wettable layer  230  during solder deposition, as will be seen below. 
     FIG. 2D is a cross-sectional view of the structure shown in FIG. 2C after an organic film  250  or organic layer has been adhered to a portion of the substantially planar surface  210  of the substrate  200 . The organic film  250  or layer may or may not be adhered to the wettable layer  230  or the coating layer  240 , but may be in contact with both the wettable layer  230  and the coating layer  240 . According to certain embodiments of the methods for solder-deposition discussed herein, the organic film  250  is not in contact with the sloped surfaces  220  of the substrate  200 . Rather, the organic film  250  forms a bridge over the sloped surfaces  220  and over any hole or cavity that has been etched or otherwise formed in the substrate  200 . 
     A convenient method for substantially preventing the organic film  250  from adhering to or contacting the sloped surfaces  220  of the substrate  200  involves using a rigid or semi-rigid and substantially planar sheet of material as the organic film  250 . The sheet may be adhered to the substantially planar surfaces  210  of the substrate  200  after the wettable layer  230  and the coating layer  240  have been formed. Then, because of the inherent rigidity of the substantially planar sheet, the organic film  250  will not dip into the etched portion of the substrate  200  and will therefore not contact the sloped surfaces  220 , as shown in FIG.  2 D. 
     No limitations are made on the materials that may be included in the organic film  250 . However, polymers that can form thin sheets with enough rigidity to bridge the etched portion of the substrate  200  are preferred. The organic film  250  may be fixed or held in place relative to the substrate  200  via electrostatic forces, a chemical adhesive, and/or mechanical forces. For example, the organic film  250  may be rolled out over the substrate  200  or may be wrapped around the substrate  200  like plastic food wrap around a plate. 
     FIG. 2E is a cross-sectional view of the structure shown in FIG. 2D after a section of the organic film  250  has been removed, leaving an empty volume  255  above portions of the wettable layer  230  and coating layer  240 . The removed section of the organic film  250  is divided into two portions to facilitate description. The first portion of the removed section, designated by the reference numeral  256 , is positioned above one of the planar surfaces  210  of the substrate  200  and contacts a planar portion of the wettable layer  230  before removal. In FIG. 2D, the first portion  256  is supported from underneath by the substrate  200 , the wettable layer  230 , and the coating layer  240 . The second portion of the removed section, designated by the reference numeral  257 , is positioned above the etched portion of the substrate  200  before removal and is not supported by the substrate  200 . Instead, the second portion  257  of the removed section is bridging the etched portion of the substrate  200 . 
     Subsequent to the removal of the second portion  257 , as shown in FIG. 2E, the remaining organic film  250  retains its substantially planar shape and continues to bridge across the etched portion of the substrate  200 . In other words, the organic film  250  does not dip or droop into the etched portion of the substrate  200  and does not contact the sloped surface  220 . Hence, a gap  260  or unfilled space is formed between the organic film  250  and the wettable layer  230 . As shown in FIG. 2E, the gap  260  is formed adjacent to one of the sloped surfaces  220  of the substrate  200 . 
     FIG. 2F is a top view of the structure illustrated in FIG.  2 E. In this embodiment, the removed section of the organic film  250 , represented by the empty volume  255 , has a rectangular shape and a width that is slightly larger than the width of the wettable layer  230  and the coating layer  240 . Accordingly, the substrate  200  is exposed on both sides of the wettable layer  230  and the coating layer  240 . 
     FIG. 2G is a cross-sectional view of the structure illustrated in FIGS. 2E and 2F, after the empty volume  255  has been substantially filled with solder paste  270 . The solder paste  270  may be placed in the volume  255  by any means known in the art of semiconductor device manufacturing and does not have to exactly fill the entire volume  255 . Any solder paste that is deposited on the organic film  250  may, optionally, be removed after the volume  255  has been substantially filled. Typically, the solder paste  270  is viscous enough and the gap  260  is small enough such that little or none of the solder paste  270  flows through the gap  260  until the solder paste  270  is heated. 
     FIG. 2H is a cross-sectional view of the structure shown in FIG. 2G after the solder paste  270  has been heated and processed to form a solder layer  280  on at least a portion of the wettable layer  230 . The solder layer  280  may be formed on a portion of or all of the wettable layer  230  that is not covered by the coating layer  240 . The solder layer  280  may be formed by thermally treating the solder paste  270  in such a way that the flux in the paste  270  liquifies and activates the metal surfaces and the solder particles in the paste melt together to form the denser solder layer  280 . According to certain embodiments of methods for depositing solder, the solder layer  280  may be formed by heating the solder paste  270  to about 180° C. or less, plus or minus approximately 5° C. Although a solder layer  280  is shown in FIG. 2G, solder bumps may also be formed if more solder paste  270  is used, as is understood in the art. 
     FIG. 2I is a cross-sectional view of the structure shown on the left-hand side of FIG. 2H after the organic film  250  has been removed. The organic film  250  may be removed by any method known in the art such as, but not limited to, chemical dissolution, heating, and application of mechanical force to cause de-lamination. 
     The structure shown in FIG. 2I includes a substrate  200  having a substantially planar surface  210  and an interior sloped surface  220 . Also included is a wettable layer  230 , which may include a metal, and that is adhered to a portion of the interior sloped surface. According to alternate structures, the entire wettable layer  230  may be adhered to the sloped surface  220 , if desired. 
     Adhered to a portion of the wettable layer  230  is the solder layer  280 . In FIG. 2I, the solder layer  280  is positioned over both a portion of the sloped surface  220  and a portion of the planar surface  210 . However, according to alternate structures, the solder layer  280  may be formed and/or positioned exclusively over all or a portion of the sloped surface  220 . When the wettable layer  230  is formed over a portion of the planar surface  210 , the wettable layer may be used to provide an electrical contact to a line or layer formed on the planar surface  210 . 
     As illustrated in FIG. 2I, a coating layer  240  adheres to a portion of the wettable layer  230 . The coating layer.  240 , during the manufacturing of the structure shown in FIG. 2I, can assist in preventing deposition of the solder layer  280  in undesired locations by effectively masking the wettable layer  230 . Once the solder layer  280  has been formed, the coating layer  240  may, optionally, be removed from the structure to facilitate the formation of electrical connections to lines and/or layers on the adjacent planar surface  210 . 
     The coating layer  240  may include any material that is not wettable by solder (i.e., on which solder does not readily spread). For example, the coating layer  240  may include a dielectric material or, more specifically, an oxide. The solder layer  280  may, among other materials, include a tin-bismuth compound or a eutectic, tin-lead compound. However, no particular restrictions are placed on the materials that may be used to build the structure illustrated in FIG.  2 I. 
     While the aforementioned and illustrated methods for forming a solder on a sloped surface have been described in connection with exemplary embodiments, those skilled in the art will understand that many modifications in light of these teachings are possible, and this application is intended to cover any variation thereof.