Monday, February 8, 2016

Semiconductor manufacturing - part 4, complex parts

Continuing the series, here is a look at more complex devices like flash memory and microprocessors.

 The takeaway is that with minimal additional hardware and an ongoing focus on recycling all reagents, we can advance from LEDs to full microprocessors and other products of high complexity. Realistically this would require a team of engineers to develop and maintain in addition to a human presence for operation and maintenance.

Details after the break.
(part 1) (part 2) (part 3)

 For the most part these devices will use the same equipment and techniques that were required for LEDs. Feature size becomes a significant challenge, so precision in masking and the use of high-UV lasers becomes essential. Part failure becomes significant; it is thought that on Earth the yield of cutting-edge microprocessors might be as low as 30%.

 Nickel would make a reasonable mask material. It can be 3d printed in a thermal process where nickel carbonyl gas is decomposed using several infrared lasers to heat a spot to about 200 °C. That means a mask file from Earth or the local system can be printed in an immediately useful (and recyclable) form.

 In most cases the added complexity is simply the number of masking steps to be performed; each requires a very precise alignment of all components. In the case of microprocessors with copper interconnects, a different technique is used that requires repeated surface polishing. Basically the metallic layers are deposited on top of etched trenches without masking, then polished away so that only the trenches are filled with metal.
 Applying masking to the thin-film process allows us to manufacture thin-film transistor arrays. A TFT array is an essential component of flat-panel visual displays, whether they be LCD or LED.

 Consumables are much the same as for earlier products, but in larger quantities; acid etch solution (hydrochloric and / or hydrofluoric acids), wafer cleaning solution (nitric acid or ammonium nitrate with peroxide), photoresist, polishing slurry, wirecutting slurry and diamond sawblades. For this to be economical, the materials must be recycled or locally produced. The acids are not difficult, but photoresist is a mixture of complex organic molecules and often polymers. (See for example PMMA and SU-8.) Cutting slurry and wafer saw blades both require precisely-sized particles of diamond or rare-earth oxides, which can be produced with the CVD equipment already present. Polishing pads are generally polymer or fiber based, which means shipping from Earth or manufacturing them alongside other plastic goods using hydroponics outputs. Sawblades are often nickel, so it may be possible to thermoform them in the printer with abrasive particles embedded throughout (with electroforming as an alternative). If the printer cannot achieve the strict tolerance required then the blades could be polished or etched to size.

 Making the transition to fully generic patterns on wafers allows the facility to produce anything down to a certain limit on feature size. By this point each individual step has been demonstrated on less-demanding products, so the development work is focused on optimization and reliability. A facility that has reached this point can manufacture microprocessors, power conversion circuitry (such as solid-state RF rectenna panels), touchscreen displays, solid-state storage devices and most of the other bits of technological wizardry that define our culture.

 This capability would allow for orbital construction (and maintenance) of satellites without parts from Earth. Avionics and communication gear could be printed to order along with structural members, PV panels and propulsion units. At this level of ability an orbital facility would be competing with launch service providers on Earth for customers.

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