A path: From electroplating through lithographic masks in electronics to LIGA in MEMS

Abstract

This paper reviews the historic development of electroplating through lithographic mask technology in IBM starting from the 1960s to the present day, the influence this technology has had on electronics, and the influence it may have in the future on HI-MEMS. The technology of electroplating through lithographic masks onto thin seed layers was born in electronics in the late 1960 s primarily as a result of a need to produce, using optical lithography, very fine conductors and to pack them into a very small space inside the magnetic thin film head yoke. Shortly thereafter, electroplating through lithographic masks was demonstrated to have application in X-ray mask fabrication, bubble memory and in many electronic devices used in computers. The e-beam lithography and X-ray lithography were in their early stages of invention and laboratory demonstration in the late 1960 s and early 1970 s. Soon after first patterns were generated in PMMA using e-beam and X-rays we have demonstrated that the approach of electroplating through optically produced lithographic masks, when combined with these new lithographic techniques, had an excellent potential for patterns into molds formed by these new lithographic techniques produced extremely high fidelity replicas, not only down to submicron, but all the way into nano dimensions. Because of the excellent collimation and large depth of focus, X-ray lithography particularly produced by synchrotron sources, permits fabrication of extremely high aspect ratio structures in the micron and submicron dimensions. By 1974, electroplating through mask was demonstrated to be the only viable technique which assured sharp vertical edges in X-ray masks, a requirement essential for very high resolution. In this paper we give examples of the early work and show the structures produced using optical, e-beam and X-ray lithography in connection with the use of the electroplating-through-mask-technique of pattern replication. The electroplating through masks not only had a pronounced effect in electronics but it is beginning to have a similar effect in MEMS. Today, X-ray lithography in combination with electroplating is becoming an extremely powerful technique in the fabrication of HI-MEMS devices. Under the name of LIGA, the plating-through-mask process was developed in the 1980 s in KfK, in Germany, in which PMMA is exposed through thick gold masks using synchrotron radiation with an objective to produce 3D molds for subsequent injection molding of low cost copies. © 1997 Published by Elsevier Science Ltd.