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Hobbyist finds special sapphire chip in old HP floppy disk drive

A hobbyist in restoring old computers and reverse engineering integrated chips, Ken Shirriff, came across a very special find while repairing an HP 8-inch floppy drive. He shows the chip on his blog; with a substrate of sapphire, with silicon and metal parts attached to it.

This ‘silicon-on-sapphire’ chip is partly transparent and had to serve as an interface between the interface bus and the Z80 processor in the floppy drive controller. The use of sapphire gives the chip some special properties. The transistors are well insulated from electrical interference because the material is a good insulator. This also means there is less capacitance between the transistors, which provides protection against radiation and certain forms of short circuit.


The chip is clearly somewhat transparent as the gold-colored cross is clearly visible behind the chip.

Thanks to these useful properties, especially regarding radiation, these types of chips have been produced since 1963. Including for space applications where radiation can be a major problem. HP’s MC2 16-bit processor from 1977 also used a sapphire substrate. This resulted in an impressive chip for that time with 10,000 transistors at 8 MHz and a consumption of only 0.35 Watt. The well-known Intel 8086 that was released a year later had almost three times as many transistors, but could only run at 5 MHz and consumed more than seven times more energy. Silicon-on-sapphire chips did provide advantages in terms of clock frequency and efficiency, but the transistors could be placed less closely together because silicon and sapphire do not bond very easily. This also resulted in a low yield of only 9% for HP. These reasons were ultimately decisive for choosing full silicon substrates for chip production.

Source: Techspot

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