implantation requires a high vacuum in order to prevent collisions between ions and gas molecules, and hence is ideally suited for space applications. An ion implanter consists of an ion source, a magnetic mass analyser that filters unwanted species of ions from the beam, an acceleration, focusing and deflection system, and a target holder. Several solid source systems have been constructed [44, 45]. Pattern definition can be obtained by use of a solid mask as described by Sacher [46] or by controlling the beam to write the desired pattern [47], The implanted ions must be thermally activated, this is usually done at around 900 C [42]. In a continuous space system, a rod- or wire-fed source system could be employed to reduce machine down-time. To keep different areas electrically separated an insulating layer on top of the silicon is needed. Such insulating layers, usually oxides, can also be employed to shield the cells from radiation. Generally a thermally grown oxide layer is used. Thermally grown oxide can be produced from lunar materials but there is no way of etching through to the silicon without the importation photoresists and etchants as well as the aid of gravity, all of which violate the system constraints. Oxide films can be evaporated [48]. Table IV shows the temperatures at which the vapor pressure of common lunar oxides equals 10~2 torr. Silicon monoxide, the most volatile of those shown is produced by heating a mixture of silicon and silicon dioxide under vacuum [49]. Containment of oxides during evaporation is usually done in a refractory metal crucible (Ta, Mo, W) or in an oxide crucible (zirconia). An electron beam or resistance heated crucible is used to obtain such high temperatures. By using solid masks patterning in the dielectric film can be obtained as well. The choice of oxide system to use in solar cells will depend on the electrical, mechanical, and optical properties desired. Electrically the oxides of silicon, aluminum, calcium and magnesium are insulators while those of iron and titanium are semiconductors [50]. Mechanically, a close match between the coefficient of expansion of the oxide and silicon as well as the metal used as a conductor is desirable. The expansion coefficient can be varied by compositional changes in the glass produced [51]. Optically the glass should transmit a high percentage of the incident solar radiation, quartz is perhaps the best at transmitting a high amount of solar radiation. Evaporation of metal can be employed for metallization of the solar cells. Of the possible conductors available (Al, Mg, Fe, Ti, various silicides, and doped polycrystalline silicon) Al and Mg can be eliminated if anneals as high as 750 C must be used to minimize the effect of radiation damage. Titanium and titanium silicides are commonly used in the integrated circuit industry [52] and would be an excellent choice for a single crystal solar cell system. If polycrystalline silicon is used for the photovoltaic system iron and titanium may present a problem with reduced carrier diffusion lengths if repeated high temperature anneals are employed [53]. Solid masking could also be
RkJQdWJsaXNoZXIy MTU5NjU0Mg==