Megabit chip builders can't resist this.

These are not poker chips on a gaming table. Magnified 6,300 times, they are details from a photoresist, used in the making of microfine-featured integrated-circuit chips. A new photoresist developed by researchers at IBM's San Jose (Calif.) Research Laboratory allows the etching of these microstructures--some only one-hundredth the width of a human hair. In contrast to the current generation of computer memory chips that typically have minimum features between 2 and 2.5 micrometers wide, the new photoresist permits etching of structures a single micrometer wide.

The key to IBM's new photoresist is its sensitivity to shorter wavelengths of light--those around 300 to 400 nanometers (in the middle ultraviolet). Shorter wavelengths allow finer resolution of chip structures, and hence denser circuitry. This new photoresist was instrumental in IBM's ability to fabricate the experimental 1-megabit (1-million-bit) computer memory chip announced earlier this year (SN: 3/2/85, p. 135).

A photoresist is a light-sensitive chemical used to etch a three-dimensional mask onto the silicon wafers from which computer chips are made. When light is shone onto a chip coated with this new material, the photosensitive molecule (a member of the diazonaphthoquinone family) in those regions struck by light is converted into carboxylic acid. Later, when the coated chip is immersed in a weak-alkaline developer bath, the acid regions dissolve, leaving behind bas-relief structures like those shown here.

If those structures covered part of a metal plating on the chip blank, they would selectively mask the metal destined to become conducting circuitry on the finished chip. Metal uncovered by the etched-away portions could be removed in a subsequent etching process. To finally unmask the metal cirtuis, the photoresist structures are chemically stripped away. Chip designers can sculpt complicated layers of metal and semiconductor circuitry through repeated maskings with etched photoresists.