utilized for pattern definition. For high throughput a spooled, wire-fed evaporator unit could be employed [53], The electrical bond at the metallized pad of a solar cell is the interface between the solar cell and the transmission line. This interconnect is often a major problem in microelectronic systems. For space utilization where the solar cells are being periodically annealed solder wire bonding will not be considered. This still leaves a variety of techniques, which include: thermocompression bonding, ultrasonic wire bonding and welding utilizing lasers and electron beams [54], Thermocompression bonding is further divided into three types: Ball bonding, stitch bonding and wedge bonding. Thermocompression bonding is a process in which both heat and pressure are used to make a diffusion-bonded connection. Ball bonding can most probably be eliminated from consideration since a hydrogen flame is used to sever the wire and to form a metal ball at the end of the severed wire. In wedge bonding the wire is placed under a wedge-shaped tungsten carbide tool prior to the application of force and heat. The wedge tool is internally heated and special scissors are used to cut the wire so this technique is space compatible. Stitch bonding is a series of thermocompression bonds made with the same wire without breaking the wire. The wire is fed through a heated capillary. This technique could also be space compatible. Ultrasonic die bonding is employed to break down the oxide layer grown on the metal pad. Such an oxide layer may not be present in vacuum processed solar cells. Both thermocompression and ultrasonic bonding may be difficult to use with iron and titanium metallized solar cells. Laser or electron-beam welding may be more suitable for such relatively hard metals. If the pads are not annealed with the rest of the solar cell, aluminum may be employed as a wire material. The use of aluminum wire for bonding is a mature technology [55]. All wire-bonding techniques which use coiled wire are very easily automated. Fabrication of Point Contact Solar Cells in Space The fabrication of a very high efficiency silicon solar cell using point-contact technology has recently been announced by Swanson [7], Efficiencies as high as 27.5% have been obtained. Three main design features are responsible for the cell's high efficiencies. First, the front surface through which light enters is textured to trap light in the cell. This texturing is accomplished by chemical etching. Second, all electrical contacts are on the back of the cell so that no metal blocks the incoming light. Third, the current is drawn off through a series of tiny holes instead of from an entire
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