A patterned conductor layer may be formed on a substrate by various methods well known in the field of electronic device fabrication. Subtractive etching is one common method. In subtractive etching, after a blanket conductor layer is deposited on the substrate, the layer is etched through a photoresist mask in order to remove undesired portions thereof. Such processes suffer from a number of disadvantages which restrict their application, particularly in the fabrication of integrated circuit structures having exceptionally small regions. For example, to ensure complete etch removal of deposited material, the pattern must generally be at least slightly over-etched, leading to diminished line widths for particular line spacing. Although this problem may be lessened by using plasma or reactive ion etching, these processes, in turn, create chemical contamination and possible radiation damage problems which must also be solved.
Many of these disadvantages can be circumvented by the use of a lift-off process, another common method for forming a patterned conductor layer. In lift-off, a layer of resist material is deposited on a substrate, patterned and developed so that the resist covers those portions of the substrate which are not to have a coating of a conductor material. Generally, the resist layer is exposed and developed in such a way that the pattern side walls of the resist defining uncovered areas of the substrate have an undercut profile. Next, a metal conductor layer is deposited over the entire surface. If the side walls of the resist have an undercut profile, the portion of the metal layer lying on the surface of the resist will be discontinuous from that lying on the uncovered areas of the substrate, i.e., discontinuous at the resist pattern edges. The unwanted portion of the metal layer, i.e., that lying on the surface of the resist, is then removed and "lifted off" by exposing the resist to a suitable solvent which causes dissolution of the underlying resist material taking with it the unwanted portions of the conductive layer.
Of these two techniques, it has been found that the lift-off process is sometimes more desirable in that the solvents used to remove the resist cause less damage to the underlying substrate than do the various etch processes, e.g., chemical, plasma, or reactive ion etch, used in subtractive etching. Also, because the conductor profile resulting from lift-off processing does not necessarily exhibit undercut features prominent in etched structures, step coverage problems in subsequent dielectric or conductor layers may be minimized.
One example of a lift-off process is disclosed in U.S. Pat. No. 4,662,989 to Casey et al. As known in the art and as acknowledged in that patent, conventional photoresist lift-off processes for metal layers typically require many hours of soaking in a solution before the desired metal layer can be removed. This is because the solution must penetrate the photoresist through the relatively small area of the resist side walls since access to the photoresist's top surface is blocked by the metal covering. According to that patent, the lift-off time may be substantially reduced by applying an additional layer of material on the metal so that the additional material causes microcracks to develop in the metal layer and in the additional layer of material. These microcracks are meant to increase the access of the solvent to the photoresist and thus shorten the amount of time required for the photoresist to be dissolved. U.S. Pat. No. 4,631,250 to Hayashi discloses a process for the removal of a covering film from the surface of a substrate, e.g., a mask used during etching of the substrate. More particularly, it is taught therein that the film is removed by blasting the film with CO.sub.2 particles, which may be mixed with fine ice particles. While that process may be suitable to remove a photoresist mask from a substrate surface, it would be less suitable for use in a situation where the film on the surface included discontinuous portions of metal and metal/resist. Blasting CO.sub. 2 particles against the surface of the substrate would tend to undesirably and indiscriminately remove the metal conductor portions as well as the metal/resist portions.