National Semiconductor Corporation

Anjali Anagol-Subbarao

Eddy Tjhia

Chi Lin

A photolithographic process for forming a lead frame pattern or other pattern is described. In the preferred embodiment, a mask approximately 12 inches by 12 inches contains two nearly identical patterns: a first lead frame pattern is provided on one half of the mask, and a second lead frame pattern, having features reduced by 0.3 mil, is provided on the other half of the mask. The first pattern is positioned over a copper web having a layer of photoresist laminated on it, and ultraviolet light is transmitted through the first pattern. The photoresist is thus exposed by a first image formed by the first pattern. While the copper web remains stationary, the mask is moved perpendicular to the length of the copper web so that the second pattern now forms a second image overlying the photoresist pattern from the first image. The photoresist layer is then exposed by the second image. The two nearly identical patterns on the mask essentially double expose the photoresist except for a 0.3 mil edge around all features, so that particles on either of the mask patterns will not significantly affect the final lead frame pattern formed in the photoresist. By not requiring the copper web to be moved during this process, the overlapping lead frame patterns can be aligned much more precisely by shifting the mask rather than by shifting the copper web.

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Anjali Anagol-Subbarao