MOSFET device with multiple gate contacts offset from gate contact area and over source area

[0002] This invention relates to semiconductor fabrication, and more specifically to MOSFET contact fabrication.

[0003] BCB: benzocyclobutene, an insulating polymer widely used as a photosensitive patterning material and insulator.

[0004] RDSon: the on-state drain-to-source resistance in a MOSFET.

[0005] UBM: under-bump metal, the conductive metal used to connect a source or gate contact to an external circuit via a surface solder bump.

[0006] MOSFET die layouts are designed to maximize the source contact area for optimum RDSon performance. This approach leaves just enough area for the gate pad to establish one wirebonding connection or bump connection. With the increasing trend toward wireless packaging via wafer bumping and flipchip technologies, a single gate bump contact serves the purpose. A disadvantage of single gate bump contact designs is that mechanical and thermal stress experienced by this single gate bump contact can cause disconnection. Inherent in multilayer metal stack technology, as commonly used in MOSFET contact areas, is the frequent problem of delamination of such metallization from its underlying dielectric layer due to differences between the coefficients of thermal expansion of the metallization and the dielectric when the stack is subjected to thermal stress. Such delamination of the metallization renders the parent device inoperable and unusable. Increasing the gate contact area in an attempt to guarantee robustness does not help, since the resulting contact still constitutes only a single connection. Producing more gate contacts through die redesign is a more-costly option, and consumes a larger portion of the source area.

[0007] By carefully repositioning MOSFET gate pads and increasing their number, the invention makes MOSFET gate connections more durable and reliable without requiring changes in either the leadframe design or the assembly process. The invention extends under-bump metal laterally from the gate pad metallization contact out to two or more gate pads not overlying the gate metallization. This approach minimizes the risk of delamination of the metallization due to thermal and mechanical stresses in assembly and operation. The invention's use of more than one gate pad further reduces such failure risks. The gate pads are positioned sufficiently close to the original gate contact to permit use of the same leadframe design and assembly process as would be used for a single gate pad placed directly over the gate contact. The result is a reliable, durable MOSFET gate contact compatible with current assembly methods.

[0008] FIG. 1 shows a schematic layout of the multiple gate pad, with a foreground cross-section.

[0009] FIG. 1ashows a detail of FIG. 1.

[0010] FIG. 2 shows a plan view of the gate pad of FIG. 1.

[0011] FIGS. 3 through 20 show the steps in the fabrication of multiple gate contacts using solder bumps.

[0012] FIGS. 21 through 23 show alternate steps in the fabrication of multiple gate contacts using non-copper metal platings instead of solder bumps.

[0013] FIG. 24 illustrates the invention's dual gate bump on the gate lead of the leadframe. The invention does not require any change on the leadframe design or assembly process with respect to existing designs and processes.

[0014] FIG. 24ais a close-up view of the dual gate pad layout with the gate lead layout of the leadframe.

[0015] FIG. 24bshows a triple gate pad layout in another embodiment of the invention. The die position in this embodiment requires movement on the x-axis, as shown, but no changes are required on the leadframe design and assembly process.

[0016] A legend for FIGS. 1, 1a, and 3-23 is shown on the drawing sheet with FIG. 23. The legend shows conductive layers with diagonal lines upward from left to right, and insulating layers with diagonal lines downward from left to right.

[0017] This invention details a practical approach of creating more gate contacts on a MOSFET die without introducing alteration in either the internal die layout or the subsequent assembly process. The invention's approach resembles the redistribution or routing techniques used in ball grid array (BGA) chips, in which the contact opening of a pad is relocated to another position in substrate layout design. The present invention creates dual or multiple gate pads for a single gate: contact. For bumped gate pads, the stress is therefore distributed among the pads, so that the stress experienced by one connection is equal to the total stress divided by the number of connections (i.e., for a dual connection, stress is divided by 2; for a triple connection, stress is divided by 3). The strength of the entire gate connection is multiplied twice if it uses a dual gate contact, or three times if it uses a-triple gate contact. This approach sharply reduces the mechanical stress introduced during assembly processing, and avoids multi-metal stacking on top of the gate metallization.

[0018] The invention eliminates the multilayer metal from atop the gate contact by laterally offsetting the gate bump contact and multi-metal layers from the gate contact region. See FIG. 1, showing a first solder bump 10 in cross section, and a second solder bump 11 in the background, with under bump metal (UBM) 20 extending from gate contact 21 to solder bump 10. FIG. 1ashows the details of the gate bump contact offset. Under bump metal (UBM) 20 contacts gate pad metallization 24 at gate contact 21 at one horizontal location 35, and runs laterally over a first layer BCB 26 of insulating material to a second horizontal location 37. UBM 20 at location 37 is built up into a bump termination 22, which emerges through a second layer BCB 27 and rests atop first layer BCB 26 and source metallization 25. There is no bump termination above gate pad metallization 24. A second bump termination is likewise created for second solder bump 11.

[0019] On top of gate metallization 24 there is no actual stacking of the under bump metal and no UBM with a solder bump. For this reason the relocation of the bump terminations on top of a BCB layer over the source area minimizes the stress on the gate pad. With the UBM or UBM with solder bump seated on top of a polymer such as BCB 26, torsional stress or thermal stress introduced during assembly processing or board mounting are better tolerated and withstood, sharply reducing the overall probability of connection failure.

[0020] For a plan view of the gate contact region and the solder bumps, see FIG. 2. Gate contact metallization region 24 surrounds passivation opening 29, where UBM 20 makes contact. UBM 20 extends to bump terminations 22, 23 protruding through the second layer BCB. Gate solder bumps 10, 11 are fabricated above bump termination 22, 23 respectively. FIG. 2 shows that the invention positions two gate bump terminations 22, 23 in areas offset from gate contact metallization area 24. This positioning minimizes the thermal and mechanical stresses that threaten the reliability and integrity of electrical connection. Supplying more than one gate bump further enhances the invention's connection reliability and integrity.

[0021] The fabrication of multiple gate contacts according to the invention proceeds as follows. See FIG. 3. A semiconductor wafer 50, typically silicon or other suitable semiconductor material, has gate and source metallizations 52 and 54 (typically aluminum or other suitable contact metal) respectively, silicon oxide or silicon nitride passivations 53 and 55 respectively, and contact passivation openings 56 and 58 respectively already in place. The wafer is cleaned. A first BCB layer 61 is coated and baked on the wafer as shown (FIG. 4). The BCB is exposed to define the exposed gate contact 63 and source contacts 65, and then developed (FIG. 5) to expose contacts 63 and 65. Titanium, copper, NICHROME, or TiCu 67 is then sputtered onto gate contact 63 and source contacts 65 for overall metallization (FIG. 6). A first photoresist coating 69 is added (FIG. 7), UV-exposed and developed to protect the areas 68 where TiCu is to be retained (FIG. 8). The unprotected Cu and Ti are etched in areas 71 (FIG. 9), creating each lateral pathway 73 between the gate contact 63 and each eventual gate bump termination 75 (FIG. 10), and the photoresist is stripped off. A second BCB layer 81 is coated, baked, exposed and developed (FIGS. 11 and 12) on the wafer to define each gate bump termination, or pad opening, 85, and each source bump termination, or pad opening, 86. Cu is sputtered onto the wafer surface to provide a conductive layer 87 (FIG. 13). A second photoresist coating 89 is added onto the sputtered Cu (FIG. 14), and is UV-exposed and developed (FIG. 15) to expose the gate bump areas 91 and the source bump areas 93 of Cu where gate solder bumps and source solder bumps respectively will eventually be placed. Cu 95 is plated onto the exposed Cu areas 91 and 93 (FIG. 16) to ensure retention of bulk copper interconnect material after soldering process; part of it is consumed with the formation of Cu-Sn intermetallics. Gate solder bumps 101 and source solder bumps 103 are plated onto Copper or nickel [Cu] 95 (FIG. 17), with photoresist 89 supporting the edges of the bumps around each Cu area. The photoresist 89 is stripped (FIG. 18), exposing sputtered Cu layer 87, and sputtered Cu layer 87 is etched (FIG. 19) to define the conductive areas and expose the second BCB layer 81. The plated solder is reflowed (FIG. 20) to form the final gate solder bumps 101 and source solder bumps 103. A drain contact (not shown) is formed on the lower second surface of the wafer in a manner well-known in the art. The drain contact is suitably implanted with dopants of the same polarity used to form source regions and the drain contact is covered with a metal layer, typically aluminum.

[0022] In an alternate embodiment, the invention's process does not require the use of solder bumps. Instead of the Cu plating 95 of FIG. 16, another metal such as Ni 109 is plated (FIG. 21) in place of the UBM. The photoresist is stripped (FIG. 22), exposing the sputtered Cu layer 87, and the sputtered Cu is etched (FIG. 23) to define the conductive areas for each gate pad 113 and each source pad 115 and expose the second BCB layer 87. In this alternate embodiment, connections are made directly to the plated metal.

[0023] The invention's repositioning and replication of the gate pads is done so as to have no effect on leadframe design or the assembly process. See FIG. 24, showing a die 131 with source bumps 141 and gate bumps 153, and leadframe 149 in working position superimposed on die 131 and having gate lead 151. FIG. 24ashows an enlargement of the gate bump area. Gate lead 151 of leadframe 149 contacts gate bumps 153 as shown, with UBM extensions 154 from gate contact area 155. The contour of leadframe 149 is exactly the same as it would be for use with a single gate bump situated directly over gate contact area 155.

[0024] In still another embodiment.shown in FIG. 24b, the invention provides for three gate bumps 163 connected with gate contact area 165 as in the two-gate-bump embodiment. The three-gate-bump embodiment of FIG. 24brequires a repositioning of die 131 horizontally, which can be seen by comparing FIGS. 24band 24a, but there is still no change in leadframe design or assembly process. The provision of three gate bumps improves the reliability of connection.

[0025] From the above descriptions, figures and narratives, the invention=s advantages in providing reliable, durable, and economical MOSFET gate contacts without affecting leadframe design or die assembly should be clear.

[0026] Although the description, operation and illustrative material above contain many specificities, these specificities should not be construed as limiting the scope of the invention but as merely providing illustrations and examples of some of the preferred embodiments of this invention.

[0027] Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given above.