Shrunken Circuits

After the transistor came of age, there was still room for the venerable vacuum tube in the burgeoning world of electronics. But even though that world is getting bigger, its parts are getting smaller. Transistors, diodes, tunnel diodes and their proliferating cousins are getting more versatile as they shrink. And the vacuum tube is slowly dying out like the ancient dinosaur. At the annual exhibition held by the Institute of Electrical and Electronics Engineers in Manhattan's Coliseum last week, there was scarcely a tube anywhere to be seen.

Etching in Patterns. Yesterday's circuitry is being shoved aside by modern micromodules, tiny gadgets that electronic engineers believe will soon replace most of the tangled guts of familiar apparatus. Least radical of the miniature gadgets are the "thin-film circuits." In the Philco version they are glass or ceramic sheets a few hundredths of an inch thick, covered with foil-thin layers of tantalum, chromium and gold. On top of the gold is a photosensitive material that becomes insoluble when exposed to light. The diagram of a desired circuit is printed by strong light on the photosensitive surface. Then the unexposed parts are dissolved, and the bare gold and chromium on top can be etched away. By using a second masking coat in the same manner, the underlying tantalum is also removed in the intricate patterns of an electronic circuit.

Where all the films are removed, the remaining glass is a nonconductor. When a section of tantalum remains, it acts as a resistor. A strip of gold and chromium is a good conductor, and serves as a thin wire. Capacitors can be made by covering tantalum with an oxide that acts as an insulator and then laying on a fresh film of gold. To make a complete circuit, tiny silicon transistors are electrically bonded to the proper sites. Complicated as it is, the process is wholly automatic, and up to one hundred microcircuits can be manufactured on a piece of glass 2 1/2 inches square.

Integrated Silicon. But the stuff gets even smaller. Integrated circuits that include everything, even transistors, are built into a single chip of silicon. Westinghouse starts with a sheet of silicon eight one-thousandths of an inch thick and about the diameter of a quarter. On top of this, an even thinner layer of extra-pure silicon is deposited by evaporation and covered with photosensitive masking material. The mask is removed in patterns, allowing successive parts of the silicon to be exposed to vapors, such as boron, that change its electrical properties. Some of the tiny areas become built-in transistors; others become diodes, capacitors or resistors. Then a pattern of conducting material is plated over the whole circuit, connecting its new-formed parts. As many as 300 microscopic circuits can be formed photographically at the same time.

To test such tiny apparatus, the disk is locked in a machine, and a probe with many electrical contacts in its tip is pressed against each circuit. Currents flowing through the contacts check out every element of the circuit, and if it fails to meet all requirements,' the probe marks it for rejection with a speck of dye. Then another machine makes checkerboard scratches between the circuits, and they are separated into "dice" by breaking the brittle disk along the scratches.

Nearly all the microscopic circuits that are being manufactured at present are designed for military or space equipment, but they will soon revolutionize civilian electronics as well. Although still expensive, they are sure to become much cheaper, and when they are properly protected, they are extremely reliable. They are warming up to shrink today's room-filling computers until tomorrow's mechanical brains can be built into a suitcase.

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