CHIP SCALE SURFACE MOUNTED DEVICE AND PROCESS OF MANUFACTURE

Abstract: A chip scale package has a MOSFET die (30) which has a top electrode surface (37) covered with a layer of a photosensitive liquid epoxy (111) photolithographically patterned to expose portions of the electrode (37), and to act as a passivation layer or solder mask. A solderable contact layer (40) is formed over the remaining portions of the layer of liquid epoxy (111). Each individual die (30) is mounted drain side down in a metal clip (100) or is mounted drain side down in a can with a drain electrode (34) disposed coplanar with a flange (105) extending from the can bottom (101).

Description: TITLE: CHIP SCALE SURFACE MOUNTED DEVICE AND PROCESS OF MANUFACTURE BACKGROUND OF THE INVENTION This invention relates to semiconductor devices and more specifically relates to a process for the low cost manufacture of a novel semiconductor device. Semiconductor devices and housings are well known. In prior art devices, the housing area is frequently a large multiple of the area of the semiconductor device. Further, in many known semiconductor device packages, heat is taken out only from one side of the die, usually the bottom surface. Further, in present packages the manufacturing process is costly, using single device handling techniques. More specifically, in present semiconductor devices, particularly power MOSgated devices, the top contact (the source) is generally an aluminum contact containing about 1.0% silicon (hereafter an aluminum contact). The aluminum contact is used because it is well adapted to the wafer manufacturing process. However, it is difficult to form electrical connections to such aluminum contacts so a wire bond process is usually used in which a wire is ultrasonically bonded to the underlying aluminum contact. These wire-bond connections have a limited area and are thus a source of electrical resistance (R DS0N ) and of heat generation during operation. However, the bottom drain contact is frequently a trimetal which is easily solderable or otherwise electrically connectable to a wide area contact surface without wire bonding as shown, for example, in U.S. Patent No. 5,451,544. Heat is primarily removed from the silicon die at the back contact surface, even though most heat is generated at the junction in the top surface and at the wire bonds. It is further known that solderable top contacts can be made to the top surface of a die, as shown in U.S. Patent No. 5,047,833. However, the packages used for such solderable top contact structures have had very large "footprints" in comparison to the die area. It would be desirable to produce a package design and process for its manufacture which would use a smaller package for the same die, while improving electrical characteristics such as R DSON of a MOSgated semiconductor type device. It would be further desirable to produce such devices in a process which permits batch handling with reduced equipment on the production line and lower costs. BRIEF SUMMARY OF THE INVENTION In accordance with one aspect of the invention, the source side of a MOSgated device wafer is covered with a passivation layer, preferably a photosensitive liquid epoxy, or a silicon nitride layer, or the like. The wafer is coated by a spinning, screening, or otherwise depositing the liquid epoxy onto the wafer surface. The material is then dried and the coated wafer is exposed using standard photolithographic techniques to image the wafer and openings are formed in the passivation layer to produce a plurality of spaced exposed surface areas of the underlying source metal and a similar opening to expose the underlying gate electrode of each die on the wafer. . Thus, the novel passivation layer acts as a conventional passivation layer, but further acts as a plating resist (if required) and as a solder mask, designating and shaping the solder areas. The openings in the novel passivation layer can be made through to a conventional underlying solderable top metal such as a titanium/tungsten/nickel/silver metal. Alternatively, if the underlying metal is the more conventional aluminum metal the exposed aluminum can be plated with nickel and gold flash or other series of metals, resulting in a solderable surface, using the passivation as a plating resist. The tops of the plated metal segments are easily solderable, or otherwise contacted with low resistance, as compared to the high resistance connection of the usual wire bond to an aluminum electrode. The source contact areas may have various geometries and can even constitute a single large area region. The wafer is then sawn or otherwise singulated into individual die. The individual die are then placed source-side down and a U-shaped or cup shaped, partially plated drain clip is connected to the solderable drain side of the die, using a conductive epoxy or solder, or the like to bond the drain clip to the bottom drain electrode of the die. The bottoms of the legs of the drain clip are coplanar with the source-side surface (that is the tops of the contact projections) of the die. The outer surface of the die is then over molded in a mold tray. A large number of die with such drain clips can be simultaneously molded in the mold tray. The bonding material may be protected with a fillet of passive material or by overmoldmg all, or a part of the assembly. The parts can be made in production by using a lead frame, a continuous strip, or by molding devices in a single block and singulating devices from that block. After molding, the devices are tested and laser marked and are again sawn into individual devices. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a top view of a singulated power MOSFET die which can be housed in accordance with the invention. Figure 2 is a cross-section of Figure 1 taken across section line 2-2 in Figure 1. Figure 3 is a top view of the die of Figure 1 after it has been processed in accordance with the invention to define a plurality of separate "solderable" source contact areas and a "solderable" gate area. Figure 4 is a cross-section of Figure 3 taken across section line 4-4 in Figure 3. Figure 5 is a view like that of Figure 3 of a die with a modified source contact pattern.. Figure 6 is a view like that of Figures 3 and 5 of a still further and large area "solderable" source contact pattern. Figure 7 is a top view of a still further contact topology (with a corner gate) formed using the process of the invention. Figure 8 is a cross-section of Figure 7 taken across section lines 8- 8 in Figure 7. Figure 9 is a perspective view of a first form of a drain clip of the invention. Figure 10 is a top view of the drain clip of Figure 9, with mold lock openings formed in the clip. Figure 11 is a bottom view of the subassembly of the die of Figures 3 and 4 and the clip of Figure 9. Figure 12 is a cross-section of Figure 11 taken across section line 12-12 in Figure 11. Figure 13 shows the subassembly of Figures 11 and 12 after overmolding in a molding tray. Figure 14 is a cross-section of Figure 13, taken across section lines 14-14 in Figure 13. Figure 15 is a cross-section of Figure 13 taken across section line 15-15 in Figure 13. Figure 16 is a perspective view of a further embodiment of a drain clip. Figure 17 is a top view of the clip of Figure 16. Figure 18 is a bottom view of assembly of the clip of Figures 16 and 17 with a die of the general kind of that of Figures 3 and 4 after overmolding. Figure 19 is a cross-section of Figure 18 taken across section line 19-19 in Figure 18. Figure 20 is a bottom view of a cup shaped drain clip with a die of the topology of Figures 7 and 8. Figure 21 is a cross-section of Figure 20 taken across section lines 21-21 in Figure 20. Figure 22 shows a wafer of MOSFET die before singulation. Figure 23 shows process steps for the formation and patterning of a passivation layer on the source surface of the wafer of Figure 22. Figure 24 shows the metalizing atop the passivation layer of Figure 23. DETAILED DESCRIPTION OF THE DRAWINGS The present invention provides a novel package for semiconductor die of the kind having power or other electrodes on opposite surfaces of the die and makes it possible, with low cost manufacturing techniques, to make both electrodes available for surface mounting on a common support surface, for example the metallized pattern on a printed circuit board. While the invention is described with reference to a vertical conduction power MOSFET having the gate and source electrode on one surface and a drain electrode on the opposite surface, the invention is equally applicable to IGBTs, thyristors, diodes and the like of various topologies. Thus, as will be seen, a novel die clip surrounds and contacts at least a portion of the back side electrode (a drain electrode in a MOSFET) and at least one leg of the clip extends over an edge of the die and terminates in a plane which is coplanar with, but insulated from the front surface contacts (gate and source in a MOSFET). The device may then be overmolded around the back and sides of the die and clip to present flat, coplanar solderble contact surfaces for all die electrodes to a mounting surface. All top contact surfaces are formed, using a novel solder mask to form easily solderable contact surfaces on the die top surface, while the die are in the wafer stage. Drain clips are then attached to the die after die singulation and are overmolded in a batch molding process. Figure 1 shows a typical power MOSFET 30 to which the invention can apply. The die 30 may be of the type shown in U.S. Patent 5,795,793 but can be any kind of die having a junction containing silicon body 31, a top aluminum (that is, aluminum with 1.0% silicon) source electrode 32, an aluminum gate electrode 33 and a bottom drain electrode 34, which may be a conventional easily solderable trimetal. The top aluminum layer may be any other suitable metallic material. Connections are normally made to aluminum electrodes 32 and 33 by wire bonding. In accordance with the invention and as will be later described, a plurality of easily solderable contact posts 36 are secured to (formed on) the source electrode 32 and a contact post 37 is secured to the gate electrode 33 as shown in Figures 3 and 4. Contacts 36 and 37 are sub-flush by the thickness of the passivation in the case of a silver top metal die; and by about one-half the passivation thickness in the case of a plated aluminum top metal die. The flat contact tops are coplanar. Contact to these contact surfaces is made by a solder paste, which at minimum printable solder thickness is about 4 to 5 times as thick as layer 38. The pattern of contacts 36 can take different forms such as those shown in Figures 5, 11 and 18. Further, it is also possible to use a large area solderable contact such as source contacts 40 or 41, for the die of Figure 6 and Figures 7 and 8. A metallizing process for forming contacts 36, 37 and 40 shall be later described. In forming the novel package with die prepared as shown in Figures 3 to 8, a novel conductive plated (or partly plated) metal clip 45 of Figure 9 is employed. Clip 45 may be a copper alloy with at least partially plated silver surfaces where contact to other surfaces is to be made. Clip 45 has a general "U-shape" with shallow legs 46 of a length slightly greater than the thickness of die 31 as measured from the surface 47 to the free surfaces of columns 36, 37, plus the thickness of an adhesive used to connect the drain to the plated interior surface 47 of the flat thin web 48 of the clip. For example, the clip may have a total thickness along the full length of legs 45 of 0.7 mm and a length from surface 47 to the free end of legs 46 of about 0.39 mm. the distance between the legs 46 depends on the size of the die, and a distance of 5.6 mm has been used for a size 4.6 die of International Rectifier Corporation, with a total width of about 1.5 mm for each of legs 46. Mold lock openings 48 and 49 may also be formed in the clip 45 as shown in Figure 10. In accordance with a feature of the invention, the solderable bottom drain electrode 34 of the die 30 is electrically connected to and secured to the plated interior of drain clip 45 as by a conductive adhesive 60 as shown in Figure 12. The adhesive can, for example, be a silver loaded epoxy material which is suitably cured. Gaps 61 and 62 are left between the side edges of die 30 and the opposite sides of legs 46 of clip 45. The structure is dimensioned so that the free surfaces of legs 46 (the drain connector) and posts 36 and 37 are coplanar. Thereafter and as shown in Figures 13, 14 and 15, the device of Figures 11 and 12 is overmolded with mold compound 70 in a mold tray. Mold compound 70 lies over the full exposed outer surface of clip 45, except for the outer free surfaces of legs 46. Mold compound fills into the gaps 61 and 62 as shown in Figures 13 and 15. The device is now ready for surface mounting to conductive traces on a printed circuit board, which are aligned with contacts 36, 37 and 46. Figures 16 to 19 show a further embodiment of the invention, using a different clip geometry. Thus, the clip 80 of Figures 16 and 17 has a web 81 and three segmented projecting legs 82, 83 and 84. A die 30, which has projecting contacts 36 and 37 is first adhered, at its drain contact (not shown) to web 81 as shown in Figures 18 and 19 so that contacts 36, 37 and the free surfaces of drain clip projections 82, 83 and 84 lie in a common plane. The device is then overmolded with molded compound 70 in a suitable mold fray. Figures 20 and 21 show a still further embodiment of the invention in which the die of Figures 7 and 8 is mounted in a cup-shaped clip 100 which is a silver plated copper alloy. Clip 100 has an internal area greater in length and width than the die 30, and, the bottom drain electrode of die 30 is connected to the interior web surface 101 (Figure 21) by silver loaded (conductive) epoxy 102. The epoxy is cured. Optimally, a ring of low stress high adhesion epoxy 103 may be applied around the die edge, sealing the package and adding structural strength to the package. The top surface of solderable contact 40 is coplanar with drain clip projection surfaces 105. Thus, all of contacts 105, 40 and 37 will align with contact traces on a printed circuit board. The drain contacts may take any suitable form and could comprise a single contact er side, if desired. Figures 22 to 24 show a novel process for forming conductive posts on the aluminum electrodes of conventional die. Thus, a plurality of identical die, each having a gate electrode 37 and separate source electrodes (not numbered) are shown within wafer 110 prior to die singulation. While still in wafer form, the top surface of the wafer 110 is coated with a photoimagable solder mask 111. Mask 111 is a photosensitive liquid epoxy which will act as a passivation layer, a plating resist (if required) and a solder mask designating and shaping the solder areas. However, other mask materials, for example, silicon nitride, can be used. Using a conventional reticule, multiple openings 11 la to 11 ldare formed through the mask to the underlying source and gate contacts on the die top metal. A laser etch process can also be used to form these openings. As shown in Figure 24, a series of metals 112 are then plated atop the surface of the wafer and the plating adheres to the metal of source 32 (and other electrodes) which are exposed through openings 11 la to 11 lb, forming contacts 112a to 112d with the source and a similar contact to the gate. Metals 112a to 112d can consist of a first layer of nickel which makes good contact to the aluminum, followed by a gold flash. Alternatively, the nickel can be followed by layers of copper or tin, and the like, ending with an easily solderable metal top surface such a silver. The wafer is then sawn to separate the die at lines 112 and 113 for example, and the die are singulated. The typical die 30 has the appearance shown in Figures 3 to 8 and has a plurality of solderable source contacts and gate contacts which project above insulation surface 50. The singulated die are then drain source-side down, into conductive clips which are plated on their interior as with silver or some other conductive coating. The die is bonded to the clip, using conventional bond material such as a conductive epoxy as previously described. The clips/cans can be presented in the form of a lead frame and the devices can be later singulated from the lead frame. Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A semiconductor device package comprising a semiconductor device die having parallel top and bottom surfaces; said top surface having a planar metallic electrode, said bottom surface having a solderable planar metal electrode; at least one solderable conductive layer formed on at least a first portion of said planar metallic layer, said at least one solderable conductive layer having an upper planar surface; a metal clip having a flat web portion and at least one peripheral rim portion extending from an edge of said flat web portion; said bottom surface of said web being electrically connected in surface to surface contact to said solderable planar metal electrode on the bottom surface of said die; said peripheral rim portion of said clip being extended over and spaced from an edge of said die and terminating in a clip rim surface which is in plane parallel to the plane of said upper surface of said at least one solderable planar metal electrode and being insulated therefrom, whereby said clip rim surface and said at least one solderable planar metal electrode are mountable to metallized patterns on a support surface. 2. The package of claim 1 wherein a plurality of spaced solderable planar metal post-shaped electrodes are connected to said metallic power electrode and all terminate in the plane of said upper planar surface. 3. The package of claims 1 or 2 which further includes a second planar metallic electrode on said top surface of said die, said second metallic electrode comprising a control electrode; and a second solderable planar metal electrode having an upper surface which is coplanar with said upper surface of said at least one solderable planar metal electrode. 4. The device of claims 1 to 3 wherein said at least one solderable planar metal electrode includes a nickel layer connected to said metallic electrode and an easily solderable metal connected atop said nickel layer. 5. The device of claims 1 to 3 wherein all of said solderable planar electrodes include a nickel layer connected to the underlying metallic electrode and an easily solderable metal connected atop said nickel layer. 6. The device of claims 1 to 5 which further includes a conductive epoxy connecting said bottom surface of said web to said bottom surface of said die. 7. The device of claims 1 to 6 wherein said clip has a second peripheral rim portion on the opposite side of said clip from said at least one peripheral rim portion which extends over and is spaced from an opposite edge of said die and terminating in said plane parallel to the plane in which said at least one peripheral rim portion terminates. 8. The device of claims 1 to 6 wherein said clip is a cup- shaped structure and wherein said peripheral rim is a continuous rim surrounding and spaced from the exterior of said die. 9. The device of claim 8 wherein said space between said die and said peripheral rim is filled with an insulation bead. 10. The process of forming a solderable top surface for a semiconductor die having a metallic top electrode structure; said process comprising the deposition of a mask layer over the surface of a wafer containing a plurality of identical die having said metallic top electrode structure; photolithographically opening at least first and second openings in said mask layer over each of said plurality of die; depositing a first metal layer which adheres to metallic electrode over said mask layer and onto the metallic electrode surface exposed by said at least first and second openings; depositing a second metal layer which is solderable over said first metal layer; removing said mask layer and the first and second metal layers overlying said mask layer and leaving in place the metals formed in said at least first and second openings, forming a passivation insulation layer over the silicon surface exposed by the removal of said mask layer with said metals remaining in said at least first and second openings and protruding beyond the surface of said passivation insulation layer and terminating in a common plane above said passivation layer; and thereafter singulating said die from said wafer. 11. The process of claim 10, wherein said wafer has a common solderable electrode formed over its full bottom surface. 12. The process of claims 10 and 11 , wherein said aluminum top electrode for each die is segregated into first and second insulated segments; said at least first and second openings formed over said first and second segments respectively. 13. The process of claim 12, wherein said at least first and second openings include a plurality of openings atop said first segment to define a plurality of conductive posts connected to said first segment. 14. The process of claims 12 and 13, wherein said first and second segments are source and gate electrodes respectively of a power - MOSFET. 15. The process of claims 10 to 14, wherein said mask layer is a photosensitive polyimide.

Inventors: SCHOFIELD HAZEL DEBORAH, STANDING MARTIN, SCHOFIELD, HAZEL, DEBORAH, STANDING, MARTIN
Applicants: INT RECTIFIER CORP, INTERNATIONAL RECTIFIER CORPORATION
Application year: 2001
Main classification: H01L21
IPC codes: H01L21, H01L23