Old-Style Design Process

Before personal computers, EDA, and automated tools, engineers originally used rubylith, a red plastic film sold by art-supply houses that is still used today by sign makers and graphic artists. Early chip designers would cut strips of rubylith with X-acto knives and tape them to large transparent sheets hanging on their wall. Each layer of silicon or aluminum in the final chip required its own separate sheet, covered with a criss-crossing pattern of taped-on stripes.

By laying two or more of these transparent sheets atop one another and lining them up carefully, you could check to make sure that the rubylith from one touched the rubylith from another at exactly the right points. Or, you'd make sure the tape strips didn't touch, creating an unwanted electrical short in the actual chip. This was painstaking work, to be sure, but such are the tribulations of the pioneers. The whole process was a bit like designing a tall building by drawing each floor on a separate sheet and stacking the sheets to be sure the walls, wiring, stairs, and plumbing all match up precisely.

This task was called taping out, for reasons that are fairly obvious. Tape out was (and still is) a big milestone in every chip-design project. Once you'd taped out, you were nearly done. All the preliminary design work was complete, all the calculations were checked and double-checked, and all the planning was finished. About the only thing left to do was to wait for the chip to be made.

One last step remained, however, before you could get excited about waiting for silicon. You had to make film from your oversized rubylith layers. This was a simple photographic reduction process. Each rubylith-covered layer was used as a mask, projecting criss-crossed shadows onto a small film negative. It works just like a slide projector showing vacation snapshots on a big screen, but instead of making the images bigger, the reduction process makes them smaller. Each separate rubylith layer is projected onto a different film negative that is exactly the size of the chip itself, less than one inch on a side. Now you have a film set that you can send for fabrication.