Frank J. Bachner

Transparent heat mirrors for solar-energy applications

Transparent heat-mirror films, which transmit solar radiation but reflect ir thermal radiation, have potentially important applications in solar/thermal/electric conversion, solar heating, solar photovoltaic conversion, and window insulation. We have used rf sputtering to prepare two types of films: TiO(2)/Ag/TiO(2) and Sn-doped In(2)O(3). To characterize the properties of heat-mirror films for solar-energy collection, we define the parameters alpha(eff), the effective solar absorptivity, and epsilon(eff), the effective ir emissivity. For our Sn-doped In(2)O(3) films, alpha(eff)/epsilon(eff) is comparable to the values of alpha/epsilon reported for the leading selective absorbers. Even higher values of alpha(eff)/epsilon(eff) are obtained for the TiO(2)/Ag/TiO(2) films.

Properties of Sn‐Doped In2 O 3 Films Prepared by RF Sputtering

An rf sputtering process was used without postdeposition annealing to prepare Sn-doped In/sub 2/O/sub 3/ films with low electrical resistivity (down to 2 x 10/sup -4/ ohm-cm), high visible transmission, and high infrared reflectivity (up to 93 percent at 10 ..mu..m) for applications as transparent conductors and heat mirrors. Substrate heating is accomplished entirely by the electron bombardment intrinsic to rf sputtering, rather than by using an auxiliary resistance heater. The film properties improve with increasing substrate temperature up to 650/sup 0/C, the maximum employed, and are relatively independent of other sputtering parameters. The electrical and optical properties of the films do not depend significantly on the crystallographic orientation, degree of texture, or substrate material.

Transparent heat‐mirror films of TiO2/Ag/TiO2 for solar energy collection and radiation insulation

Transparent heat‐mirror films of TiO2/Ag/TiO2 on glass with a visible transmission of 84% (at 0.5 μm) and an infrared reflectivity of 98–99% (at 10 μm) have been fabricated by rf sputtering. Initial tests indicate that the films are thermally stable in air at 200°C and inert to water attack. Because of their excellent optical properties and apparent stability, these transparent heat‐mirror films offer great promise for use in solar‐thermal power conversion and as transparent thermal insulators.

Effect of O2 pressure during deposition on properties of rf‐sputtered Sn‐doped In2O3 films

The electrical and optical properties of rf‐sputtered Sn‐doped In2O3 (ITO) films have been found to depend strongly on the O2 partial pressure during deposition. For the sputtering conditions used, films with both low electrical resistivity (ρ ∼ 3 × 10−4 Ω cm) and high visible transmission (∼ 90%) were obtained only over a narrow range of O2 pressures, from 3 × 10−5 to 4 × 10−5 Torr. Our results appear to explain the difficulties that have previously been encountered in obtaining high‐quality ITO films, and indicate that control of the O2 pressure during deposition is essential for reproducible preparation of such films.

ac electron tunneling at infrared frequencies: Thin‐film M‐O‐M diode structure with broad‐band characteristics

A high‐speed diode element consisting of a metal‐metal‐oxide‐metal electron‐tunneling junction is formed by thin films deposited on a substrate. These junctions are integrated with deposited narrow resonant antenna structures which couple the junction to incident radiation. Broad‐band characteristics from radio and microwave frequencies to the infrared region are shown. Frequency mixing and the possibility of utilizing large numbers of elements simultaneously are also demonstrated.

Thin‐film conducting microgrids as transparent heat mirrors

A new type of transparent heat mirror for solar‐energy applications has been fabricated by chemically etching a Sn‐doped In2O3 film to form a transparent conducting microgrid. For square openings 2.5 μm on a side, separated by lines 0.6 μm wide, the solar transmission increases from 0.8 for the original continuous film to 0.9 for the microgrid. Although 65% of the area of the film is removed by etching, the infrared reflectivity decreases by only 9%, from 0.91 to 0.83. A smaller decrease in the infrared reflectivity may be possible if materials with higher optical conductivity are used.

Monolithic silicon bolometers

A new type of bolometer detector for the millimeter and submillimeter spectral range is described. The bolometer is constructed of silicon using integrated circuit fabrication techniques. Ion implantation is used to give controlled resistance vs temperature properties as well as extremely low 1/f noise contacts. The devices have been tested between 4.2 and 0.3 K. The best electrical NEP measured is 4 × 10 - 1 6 W/Hz at 0.35 K between 1- and 10-Hz modulation frequency. This device had a detecting area of 0.25 cm2 and a time constant of 20 msec at a bath temperature of 0.35 K.

Thin‐film VO2 submillimeter‐wave modulators and polarizers

Submillimeter‐wave modulators and switchable polarizers have been fabricated from VO2 thin films deposited on sapphire substrates. By passing electric current pulses through elements made from these films, the films can be thermally cycled through the insulator‐to‐metal transition that occurs in VO2 at about 65 °C. In the insulating state, the films are found to have negligible effect on the transmission at submillimeter wavelengths, while above the phase transition the transmission is strongly reduced by the free‐electron effects characteristic of a metal. Other possible applications of such switchable VO2 elements include variable bandpass filters and diffraction grating beam‐steering devices.

Aluminum beam leaded chips, substrates and crossovers: A single metal system

A single metal system for beam-leaded chips, substrates and crossovers has been shown to be feasible using aluminum. Beams were formed on both ceramic substrates with holes tented with Riston (a sheet photoresist) and on silicon wafers by vacuum evaporation of aluminum from multiple tungsten filaments, and subsequent delineation and etching using photolithographic techniques. The chips with beams were separated from the wafer by anisotropic etching, and the tenting material removed from the ceramic by dissolution in appropriate solvents or an oxygen plasma. Beams were formed on polyimide sheet by photolithographically etching an adhesiveless laminate of aluminum on polyimide. Actual working samples have been fabricated of aluminum beam leads on ceramic and plastic substrates, aluminum beam leaded crossovers on ceramic, aluminum crossovers on plastic using multilayers and aluminum beam leaded silicon chips. Standard integrated circuit chips have been ultrasonically bonded into these substrates and the beam leaded chips have been ultrasonically bonded to an aluminum coated substrate. Environmental physical tests have shown the beam leads and crossovers to be rugged.

A beam-lead substrate package for a six-stage TTL shift register☆

Abstract Commercial integrated circuit chips have been incorporated into a new type of final package which is called a beam-lead substrate. The technique utilizes glass or ceramic substrates with interconnexion metallization and the beams cantilevered over holes in the substrate. The commercial dice are inserted into the holes and bonded to the overhanging beams. TTL chips when incorporated into a six-stage shift register utilizing this type of package show a temperature rise of approximately 0·5°C/W when the package is attached to a heat sink, and 14·3°C/W when the circuit is operated suspended in air. The performance characteristics of the chips are unchanged from the manufacturers specifications after incorporation into the package. The beam-lead substrate package is made by depositing the interconnexion and beam-lead metallization on the substrate, and etching suitable holes under the beams. Chromium/platinum/gold and molybdenum/gold metallizations were evaluated. Sputtered molybdenum/gold was chosen as the best metallization system for this application because of the excellent bondability of the beams after final processing.