Single-crystal silicon is being increasingly employed in a variety of new commercial products not because of its well-established electronic properties, but rather because of its excellent mechanical properties. In addition, recent trends in the engineering literature indicate a growing interest in the use of silicon as a mechanical material with the ultimate goal of developing a broad range of inexpensive, batch-fabricated, high-performance sensors and transducers which are easily interfaced with the rapidly proliferating microprocessor. This review describes the advantages of employing silicon as a mechanical material, the relevant mechanical characteristics of silicon, and the processing techniques which are specific to micromechanical structures. Finally, the potentials of this new technology are illustrated by numerous detailed examples from the literature. It is clear that silicon will continue to be aggressively exploited in a wide variety of mechanical applications complementary to its traditional role as an electronic material. Furthermore, these multidisciplinary uses of silicon will significantly alter the way we think about all types of miniature mechanical devices and components.

Silicon as a mechanical material

A high efficiency back side contact solar cell is fabricated using a self-aligning process that reduces the number of masks and alignments as compared with prior processes. The back surface of the cell is patterned by etching into an array of bilevel, interdigitated mesas and trenches, separated by inclined surfaces. Doping of the back surface region produces laterally alternating and overlapping P and N regions associated with the mesas and trenches. A metalization layer is deposited over the entire back side of the cell. The portions of the metalization on the inclined surfaces are readily removed by etching. Removal of the metalization on the inclines surfaces separates the mesa conductors from the trench conductors, leaving a well defined interdigitated array of positive electrodes and negative electrodes.

Bilevel contact solar cells

The principles governing the performance of silicon solar cells are reviewed with emphasis on clarifying the essential concepts. Principal attention is devoted to the planar p−n junction cell and recent developments that have contributed to increased efficiency. Other solar cell structures are also reviewed and their relative advantages and shortcomings are discussed. Important areas where our present understanding is inadequate are pointed out and some common misconceptions are corrected.

Silicon photovoltaic cells

In order to evaluate the field-assisted bonding process, reactions during bonding are studied by measurements of current temperature surge and bonding front displacement. The current characteristics are simulated by Wallis's equivalent circuit. From an emissivity calibrated temperature thermography, it is concluded that Joule heating at the interface is not the most important part of displacement of the bonding front. It is also shown that the displacement of the bonding front is a function of the square root of time. This suggests that the bonding is controlled by diffusion. A bonding mechanism is proposed.

The mechanism of field-assisted silicon-glass Bonding

A new type of silicon photovoltaic converter has been developed called the V-Groove Multijunction (VGMJ) solar cell. The VGMJ solar cell consists of an array of many individual diode elements connected in series to produce a high-voltage low-current output. All the elements of the cell are formed simultaneously from a single silicon wafer by V-groove etching. The results of detailed computer simulations predict a conversion efficiency in excess of 24 percent for this cell when it is operated in sunlight concentrated 100 or more times. The advantages of this cell over other silicon cells include the capability for greater than 20-percent conversion efficiency with only modest bulk carrier lifetimes, a higher open-circuit voltage, a very low series resistance, a simple one-mask fabrication procedure, and excellent environmental protection provided by a glass front surface.

The V-groove multijunction solar cell

The conversion efficiency limit of p+-i-n+silicon solar cells in concentrated sunlight is explored with numerical simulations of an idealized p+-i-n+cell having field-induced junctions. Conversion efficiencies greater than 30 percent are calculated for this cell operating in sunlight concentrated 1000 times. The relative importance of bulk and surface recombination in limiting the cell conversion efficiency is illustrated for operation in 1 to 1000 suns. For surface recombination velocities below 100 cm/s, it is shown that bulk recombination losses limit the cell performance rather than recombination losses occurring in the p+or n+regions. The results show that Auger recombination in the bulk region will limit ultimately the cell conversion efficiency.

T.I. Chappell

Papers

The V-groove multijunction solar cell

A study of the conversion efficiency limit of p + -i-n + silicon solar cells in concentrated sunlight

The V-Groove Multijunction solar cell

TP-A5 a simple contactless microwave method for measuring carrier lifetime during solar-cell fabrication