A new technique for generating registered patterns on opposite sides of a substrate has been developed. Rather than introducing the patterns sequentially and registering them with an infrared or dual viewing microscope, the photomasks for the two surfaces are pre-aligned and assembled into a hinged configuration referred to as an "alligator" mask. Substrates can then be inserted into the "alligator" mask and exposed on both sides without on-line alignment. Since a registered set of exposed patterns now exist, subsequent processing can be performed simultaneously on both surfaces. "Alligator" masks have been utilized for silicon wafer diffusions at Western Electric Company, Allentown, Pa. since July, 1976 and produced devices with top-to-bottom registrations better than ±10pm. Device registration is a function of initial mask alignment and hinge fatigue (fatigue occurs only in direction orthogonal to hinge axis). With improved mask fabrication, better top-to-bottom registration should be possible. An average mask lifetime of 766 wafers has been observed with mask wear being the primary failure mechanism. The exposure system, designed and built by Western Electric Company, is capable of exposing two 1.5" wafers on both sides with a beam divergence of <1° and a beam uniformity of 10%.
R A. Heinz,
J T. Chuss,
C. M. Schroeder,
"Double-Sided Photolithography", Proc. SPIE 0135, Developments in Semiconductor Microlithography III, (6 September 1978); doi: 10.1117/12.956124; https://doi.org/10.1117/12.956124