Electronic Design

Silicon Wafers Used In Fixed Digital Memory

Thin silicon wafers imprinted with ion-deposited conducting paths form the basis of a digital memory with a storage density of 70,000 bits per cu in. Development of the Silicon Permanent Array Memory is being pushed by the Librascope Div. of General Precision Instruments, Glendale, Calif., for use in space-vehicle computers.

In its present experimental configuration, the memory consists of a number of 1-in. wafers, 0.008-in. thick, which are stacked to provide the required memory capacity. With present techniques, each wafer can be imprinted with up to 1000 conducting paths—each path representing a stored bit. The paths are formed by fabrication of diodes on the surface of the wafer. The contents of the memory are thus written in during the manufacturing operation and cannot be altered thereafter. Permanent storage can absorb up to 95 percent of a memory's capacity, according to Librascope's Robert Williamson. Storage of programs and constants in guidance computers is a likely application of the memory.

Each conduction path includes a diode, formed from the surface of the silicon wafer. Diodes are separated from each other by an etched-grid pattern on the wafer. Conduction paths are then deposited from the diodes to the perimeter. The wafers are sandwiched between thin layers of insulating material, stacked, and interconnected.

Typically a "1" bit would be a conducting path across the silicon wafer, and a "0" would be non-conducting. Power per bit is approximately 1 milliwatt and access time is claimed to be in the microsecond range. (Electronic Design, Jan. 18, 1961, p. 5)

Apparently the term read-only memory, or ROM, wasn't current at that time. The use of 1-in. wafers, however, was common in the manufacturing of semiconductor devices.

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