Backside metallization scheme for semiconductor devices

Abstract: A backside metallization scheme for semiconductor devices includes a vanadium layer (16) disposed on the backside (14) of a wafer (10) and a silver layer (18) disposed on the vanadium layer (16). An optional intermediate layer (20) comprising either a mixture of vanadium and silver or nickel may be disposed between the vanadium layer (16) and the silver layer (18). The vanadium layer (16) exhibits excellent adhesion characteristics on the backside (14) of wafers (10) having a finish at least as fine as a 300 grit equivalency while the silver layer (18) exhibits excellent solderability characteristics.

Description: Background of the Invention This invention relates, in general, to semiconductor devices and more particularly to a backside metallization scheme for semiconductor devices. Backside metallization schemes of this type generally must have three main properties. First, the layer disposed directly on a wafer must adhere to that wafer. Second, the outer surface of the metallization scheme must be solderable so that the semiconductor device may be bonded to lead frames and the like. Third, the metallization scheme itself must not come apart. Existing metallization schemes encounter various problems in regards to the above properties. One existing metallization scheme includes a chromium layer disposed directly on the backside of a wafer, an intermediate layer of silver and chromium disposed on the chromium layer and a silver layer disposed on the intermediate layer. The silver layer exhibits excellent solderability and the scheme, in general, does not come apart. However, while the chromium layer adheres well to the backside of silicon wafers having a relatively rough surface (approximately 300 grit equivalency), the adhesion of the chromium layer to the silicon wafer backside decreases as the surface becomes finer. When a chromium layer is disposed directly on the backside of a silicon wafer having a 2000 grit equivalency, the adhesion of the chromium to the silicon is extremely poor. Another metallization scheme of this type includes forming a vanadium layer directly on a wafer, a nickel layer on the vanadium layer and then, an optional gold-germanium layer may be formed on the nickel layer. Vanadium has excellent adhesion properties in conjunction with silicon. However, the nickel layer does not have good solderability characteristics in this context. Further, the optional gold-germanium layer does not have good solderability characteristics in this context and is very expensive to employ. Therefore, it would be highly desirable to have a metallization scheme that has good adherence to smooth wafers, excellent solderability characteristics and that will not come apart during the lifetime of the semiconductor device. Summary of the Invention Accordingly, it is an object of the present invention to provide an improved backside metallization scheme for semiconductor devices. Another object of the present invention is to provide an improved backside metallization scheme for semiconductor devices that is relatively low in cost. In accordance with the present invention there is provided a metallization scheme for semiconductor devices, the invention comprising and being characterized by: a vanadium layer disposed on the backside of a wafer; and a silver layer disposed on said vanadium layer. A method of fabricating a backside metallization scheme for semiconductor devices in accordance with the present invention is also disclosed and claimed in claim 4 of the accompanying claims. The foregoing and other objects and advantages are achieved in the present invention by one embodiment in which, as a part thereof, includes a vanadium layer disposed directly on the backside of a wafer, and a silver layer disposed on the vanadium layer. It had previously been believed that the combination of vanadium and silver could not be employed in backside metallization schemes because the vanadium/silver system exhibits immiscibility in both solid and liquid states ( See Smith , Bulletin of Alloy Phase Diagrams 1981, Volume 2, No. 3, pages 343-44). Brief Description of the Drawings FIG. 1 is a highly enlarged cross-sectional view of an embodiment of a backside metallization scheme for semiconductor devices; and FIG. 2 is a highly enlarged cross-sectional view of another embodiment of a backside metallization scheme for semiconductor devices. Detailed Description of the Invention FIG. 1 is a highly enlarged cross-sectional view of an embodiment of a backside metallization scheme for semiconductor devices. FIG. 1 depicts a wafer 10 having a frontside 12 and a backside 14. In this embodiment, wafer 10 is comprised of silicon although one skilled in the art will understand that it is possible to employ wafers comprised of many other well-known materials. Generally, frontside 12 of wafer 10 will have a semiconductor structure or a series of semiconductor structures formed thereon or therein. Although a backside metallization scheme is disclosed herein, it is also possible to employ the scheme on the frontside of a wafer. Backside 14 of silicon wafer 10 has been ground so that it has a surface finish at least as fine as a 300 grit equivalency. Many metals will not adhere to silicon having this smooth a surface. However, vanadium layer 16 shown disposed on backside 14 of silicon wafer 10 has been found to adhere well to silicon wafers having finishes well in excess of a 2000 grit equivalency. Vanadium layer 16 has a thickness in the range of 500 to 3,000 angstroms and may be either sputtered or evaporated onto backside 14 of wafer 10. A silver layer 18 is shown disposed on vanadium layer 16. Silver layer 18 has a thickness in the range of 10,000 to 20,000 angstroms and may be formed by either sputtering or evaporation. Silver has excellent solderability characteristics to the various die bond materials used to attach semiconductor chips to leadframes and the like and therefore, is highly desirable as the final layer of a backside metallization scheme. Since vanadium and silver exhibit immiscibility in both solid and liquid states (see Summary of the Invention) those skilled in the art have been led to believe that these two materials could not be used in direct contact with each other. However, silver layer 18 and vanadium layer 16 have exhibited more than adequate adhesion to each other in this application. FIG. 2 is a highly enlarged cross-sectional view of an embodiment of a backside metallization scheme for semiconductor devices which does not form part of the invention. Wafer 10 having frontside 12 and backside 14 along with vanadium layer 16 disposed thereon are virtually the same as those described in FIG. 1 above. However, an intermediate layer 20 is shown disposed on vanadium layer 16. Intermediate layer 20 may be comprised of a mixture of vanadium and silver that has a thickness in the range of 3,000 to 7,000 angstroms. Intermediate layer 20 may be formed either by evaporation or sputtering. It should be understood that both materials are evaporated or sputtered simultaneously. Intermediate layer 20, has a vanadium:silver ratio in the range of 10:90 to 50:50. Again, although the silver/vanadium system exhibits immiscibility in both solid and liquid states, intermediate layer 20, comprised of a mixture of vanadium and silver, adheres adequately. Intermediate layer 20 may also be comprised of nickel. Nickel intermediate layer 20 may be either evaporated or sputtered on vanadium layer 16 and has a thickness in the range of 500 to 3,000 angstroms. Nickel adheres well to both vanadium layer 16 and silver layer 18 which is later disposed on intermediate layer 20.

Claims: A metallization scheme for semiconductor devices, the invention comprising: a vanadium layer (16) disposed on the backside (14) of a wafer (10); and a silver layer (18) disposed on said vanadium layer (16). The metallization scheme of claim 1 wherein the vanadium layer (16) has a thickness in the range of 50 to 300 nm (500 to 3,000 angstroms) and the silver layer (18) has a thickness in the range of 1 to 2 µm (10,000 to 20,000 angstroms). The metallization scheme of claim 2 wherein the vanadium layer (16) is disposed on the backside (14) of a silicon wafer (10), said backside (14) of said silicon wafer (10) having a backside finish at least as fine as a 300 grit equivalency. A method of fabricating a backside metallization scheme for semiconductor devices comprising the steps of: providing a wafer (10) having a backside (14); forming a vanadium layer (16) on said backside (14) of said wafer (10); and forming a silver layer (18) on said vanadium layer (16).

Inventors: HILEMAN BENNETT L, LYTLE WILLIAM H, RAGONA ANGELA, SHARMA RAVINDER K, HILEMAN, BENNETT L., LYTLE, WILLIAM H, RAGONA, ANGELA, SHARMA, RAVINDER K.
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1990
Main classification: H01L21
IPC codes: C23C28, H01L21, H01L23