Manufacturing Server Sky

This is pretty old, a holdover from the glass substrate days. Needs updating.

Devices and materials

A lot can be done on a very thin planar surface. Other things cannot be done easily. Here are some common electronic devices:

Here are some relevant properties of silicon, SiO2 glass, gallium arsenide, copper, aluminum, silver, gold, tantalum, indium tin oxide, kovar, invar, low expansion borosilicate glass, indium phosphide, and pyrolytic carbon, which will make up 99.9% of the weight of a thinsat:

Data mostly from wikipedia and various places online. See also Matweb the material properties website, B.Y.U. CTE table.

Tensile strength untrustworthy, and many parameters are anisotropic. Silicon often has small crystallographically aligned and very sharp surface pits (intentionally on solar cells) that can cause 33x stress concentrations, nucleating cracks and causing fractures at 300MPa instead of the bulk 7000MPa. Use these tensile strengths only for rough estimates.

The vast bulk of the material , and the largest pieces of of the thinsat, will be laminated engineering glass and metal. Since the thinsat undergoes wide temperature changes when it passes in and out of shadow, or undegoes thermal annealing, it will be more survivable if the glass can match silicon's 2.6E-6/Kelvin coefficient of thermal expansion (CTE). Metals have very high CTEs, while SiO2 has a very low CTE, so slotted metal wires with SiO2 in the gaps is one way to make a "material" that is both conductive and has the same CTE as silicon.

Stack compression during launch Booster systems vibrate during launch. If there are regions of higher and lower compressibility and mass density, there will be standing waves and resonances in a stack of thinsats. Ideally, the design would match the mechanical properties of all the materials used, but that is unlikely given other constraints. So the stacks may need to be resonance isolated from the boosters, reducing the payload fraction. A somewhat simpler constraint is to match the mechanical compression of the stack caused by acceleration forces. The materials (plus spacers if necessary) should have the same ratio of compressibility (modulus) to mass density, that is, the same speed of sound. That will minimize shear forces on the connections from the solar cells to the electronics and thruster ring. Mismatches will restrict the number of thinsats that can be stacked between spacers. Of course, all these problems must be designed out with mechanical CAD, and tested with centrifuges and shock tables on the ground.

Composite Silicon and Pyrolytic Carbon matched to Borosilicate Glass One possibility is to match mechanically to the glass rather than the silicon. From the table above, borosilicate glass has a speed of sound of 5.3km/s, silicon has a speed of sound of 8.0km/s, and pyrolytic carbon has the very low speed of sound of 1.5km/s. If the silicon is thinned from 50 microns to 48 microns, and the remaining 2 microns is replaced with pyrolytic carbon, then the average speed of sound vertically through the two layers is the same 5.3km/s as the glass. The "elastic impedance" is proportional to the energy stored by propagating sound, and sound waves will reflect at impedance discontinuities. The impedance of borosilicate glass is 11.9 Kg/mm²s, silicon is 18.7 Kg/mm²s. The composite material is 12.4 Kg/mm²s, a much better match to glass than pure silicon, so standing waves at the interface are less likely. Finally, pyrolytic carbon is an excellent thermal conductor. The composite structure has 50% better thermal conductivity than silicon alone.

There may still be issues with the Si/polyC composite material; it may help or hurt adhesion between thinsats. black body radiation, trapped charge, etc. The stacked material is acoustically dispersive for high frequencies, and this may help with shatter resistance and handling. The carbon help may damp out vibrations faster. The final thinsat stack will consist of many layers of materials, and they will be evaluated empirically to find the best mix.

Curling can occur if the front and back sides of a thinsat (especially the solar cell) have different CTEs, and the thinsat undergoes repeated thermal cyclings. There is nothing inherent in a thinsat that establishes "flat" - it will flex until tensions and compressions are minimized. A slightly curved thinsat is not a severe operational problem. If the edges are turned up a few degrees, that will reduce collected solar energy very little. The main problem is the effect on the phasing of the radios. Significant curling will change the spacing of the radios at opposite sides of the curl, and lift them above the plane of the radios at the center of the curl. Without some means of determining precisely how much curl is there, the radios may be in incorrect phases.

Here are some curls for various temperature and CTE changes and sizes:

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Factories

Much of the manufacturing for Server Sky can happen in Washington County, Oregon, around Hillsboro.

The main component by weight is the glass substrate, the size of a large solar cell wafer. The Solar World plant in Hillsboro is the largest solar cell manufacturer in the US - certainly the most highly automated. They recently upgraded their line to 200 mm solar cells.

Glass layers in flat screen displays are very similar to thinsats. Sharp makes flat screens, and I expect Sharp Labs America in Camas, Washington may become a world leader in thinsat design and a prime contractor. While the components can be attached with pick-and-place, the fluidic self-assembly techniques pioneered by Alien Technologies may provide further cost reductions. Eventually, we will be making thousands of square kilometers of thinsats - hyperautomated planar manufacturing will be essential.

The most complicated components are the microprocessors. Some version of the Intel Atom may be suitable. For a thinsat, it is preferable to use a fast, deep submicron, 1V processor with heavy doping (less sensitive to radiation damage) and an epi substrate, though a trench isolated SOI process is preferred. AMD processors are all trench isolated SOI. Intel's Penryn process, with thick Hafnium oxide gates and work-function controlled gate metalization, will also be more radiation resistant. Over time, most process improvements desirable for high performance processors will also be desirable for radiation hardness.

The most radiation sensitive components are likely to be the flash memory. These incorporate error correction, but software error correction and frequent rewrites may be necessary to correct for radiation-induced charges. Some errors may need to be restored from caches on other thinsats partway around the orbit.

The gallium arsenide radios will probably be made by Triquint.

The electrochromic light shutter material will be pretty simple. It must survive freezing. The light shutters should be divided into separately-addressed cm-sized cells so that meteorite punctures will not disable a whole shutter. They shutters only need to switch very slowly, and consequently have wide flexibility in operating voltage. They will probably be constructed from a 1 micron layer of electrochromic material with a thin layer of indium tin oxide. Electrocromic light shutters are used for light control in tall buildings, and are used as electronically controllable passenger window shades on Boeing's 787 Dreamliner.

Washington county companies such as D.W. Fritz build wafer handling equipment .

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