1. Technical Field
This invention relates to a process for forming a pattern of conductive material on a substrate, and more particularly, to an improved lift-off process for forming a pattern of metallization under high temperature conditions.
2. Description of the Prior Art
Lift-off techniques are commonly employed in forming a pattern of metallization on a processed semiconductor substrate. One approach involves coating a photoresist layer on a substrate, and then patterning the layer through a photomask. A metal layer is then formed on the patterned photoresist, which is subsequently lifted-off, taking with it the undesired portions of the metal layer. See, for example, U.S. Pat. No. 3,934,057, issued to Moreau, et al., on Jan. 20, 1976. One drawback to this type of process, however, is that the metal layer must be deposited at a relatively low temperature, due to the relatively poor heat resistance of the photoresist materials typically employed. This is undesirable as it degrades the electrical characteristics of the devices and lowers the yield of the metallization pattern.
To overcome this problem, processes have been developed, in which polyimide films are employed as the lift-off layer. See, in general, Frary, J. M., et al., "Lift-Off Techniques For Fine Line Metal Patterning", Semiconductor International (Dec. 1981), pp. 72-85. See also, Homma, Y., et al., "Polyimide Lift-Off Technology For High-Density LSI Metallization", IEEE Transactions On Electron Devices, Vol. Ed.-28, No. 5 (May 1981), pp. 552-556, which uses a heat-resistant polyimide (available from Hitachi Chemical Co., Ltd., under the trademark "PIQ") in a high temperature metal deposition process.
At high temperatures, however, the polyimide materials become generally insoluble, and thus not completely and readily removable with commonly used solvents. As a result, rather cumbersome and complex removal processes have been devised. See, for example, U.S. Pat. No. 4,428,796, issued to Milgram on Jan. 31, 1984, which discloses a process for removing a polyimide from a semiconductor substrate by heating it to a temperature of 450.degree.-490.degree. C. and immersing the substrate in a solution of methylene chloride or ethylene diamine/hydrazine, followed by ultrasoneration.
Further, a barrier layer is generally required in order to protect the polyimide during the photolithographic process. In U.S. Pat. No. 4,451,971, issued to Milgram on Jun. 5, 1984, for example, a select polyimide layer is applied to a semiconductor substrate. A barrier layer is deposited on the polyimide, and these layers are patterned using a photoresist mask. A metal layer is deposited, and the polyimide is then stripped, lifting off the undesired portions of metal.
A process which seeks to eliminate the need for a barrier layer is disclosed in U.S. Pat. No. 4,606,998, issued to Clodgo, et al. on Aug. 19, 1986. In that patent, a first polyimide layer is applied to a substrate, followed by application of a high temperature polyimide layer. After the two polyimide layers are etched through a photoresist mask, a metal layer is applied; and then, the high temperature polyimide layer is lifted off, the first polyimide layer remaining as a passivation layer. While satisfactory in some respects, this process still requires the use of a photoresist and involves a number of photolithographic steps, which, for cost reduction, would be desirable to reduce.
Thus, for a variety of reasons, none of the techniques described above has proven to be entirely satisfactory.