Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
1999-05-21
2001-05-29
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C338S018000
Reexamination Certificate
active
06239432
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the use of SiC for IR radiation sensing and resistance control.
2. Description of the Related Art
IR radiation is presently sensed, for applications such as measuring the power or energy output from IR lasers, by pyroelectric, bolometer and calorimeter detectors. Generally, pyroelectric and bolometer detectors employ materials that directly absorb IR; pyroelectric materials include lithium niobate and tantalum niobate; bolometer materials include silicon, germanium, gallium arsenide, metal-oxide ceramic thermistors and various glasses. Generally, calorimeter detectors employ materials that must be coated with an IR-sensitive absorption coating. The performance of IR sensing detectors is constrained by the IR-sensor material's capacity to absorb energy without damage; which limits the maximum energy/power intensity, maximum exposure time and minimum volume and area for the sensor. All pyroelectric, bolometer and calorimeter materials have limited thermal shock tolerances.
Materials currently used for the direct absorption of IR radiation are easily damaged if they get too hot (above about 400° C.), or if the sensor temperature increases too rapidly. To sense the power or energy output of medium and high power IR lasers, present materials are exposed to only a fraction of the output IR energy by interposing a beam splitter or a disbursing medium between the laser and the sensor. However, this results in reducing the power or energy measurement to an estimate. Furthermore, presently available sensors for medium and high power IR sources require fan or water cooling, and are subject to calibration drift.
A related application for IR radiation sensitive materials is in sensing the temperature of other materials that are heated by IR radiation. For example, the rapid thermal annealing (RTA) process, used extensively in the semiconductor industry, uses high intensity IR lamps to ramp the temperature of semiconductor wafers (principally silicon) by several hundred degrees centigrade per second. Wafer temperatures are presently monitored by remote sensing using either optical or IR pyrometers, or by direct contact thermocouples.
Pyrometers measure the wafer temperature by absorbing radiation emitted from the wafer surface through a transparent view port in the RTA process chamber wall. This type of temperature sensing is limited by a need to know the precise emissivity of the observed wafer surface, a need to prevent particulates or dispersive gas between the wafer surface and view port or any deposits on the view port or wafer surface, and a requirement to avoid any changes in the wafer surface such as contamination or chemical reaction or texture changes.
Thermocouples measure the wafer temperature by touching its surface. Key problems with this approach are that the thermocouple must be enclosed to prevent reactions between it and the wafer, the thermocouple-wafer contact is very difficult to ensure because the wafer is spun at about 1200 rpm to ensure uniform processing, and contacting the wafer with the thermocouple actually changes the local temperature.
Materials presently used for IR radiation power or energy sensing could theoretically also be used as temperature sensors, but they would not survive the environments or temperatures often required, particularly RTA processing in which temperatures can reach 1300° C.
SUMMARY OF THE INVENTION
The invention provides a unique approach to IR radiation sensing that is applicable to IR power and energy sensing, heat sensing, and IR controlled varistors. The new IR radiation sensor is formed from SiC, preferably in a single crystal structure. A SiC body receives IR radiation while an electrical circuit applies an electrical signal to the SiC body, which responds to the radiation by changing its response to the electrical signal. When employed as an IR energy and/or power sensor, either a constant current or a constant voltage is applied to the SiC body which receives the IR radiation, and an output circuit provides an indication of the IR energy and/or power incident on the SiC body as a function of the body's resistance.
The invention can also be employed to sense the temperature of a test body, such as a wafer in an RTA process chamber. In this application the SiC body is positioned such that the IR-radiation intensity received by it, relative to the test body is known. As in the IR energy/power sensor, a constant current or voltage is applied to the SiC body, while an output circuit is calibrated to produce an indication of the test body's temperature as a function of the SiC resistance.
The invention can also be employed as a varistor, in which a laser directs a controlled IR beam onto an SiC body that is incorporated into a larger circuit; the laser cooperates with the SiC body to function as a controlled varistor.
The electrical resistance of pure SiC normally has a positive temperature coefficient, resulting from direct absorption of IR energy by the lattice. Its temperature coefficient (TC) can be ‘tuned’ by incorporating impurity atoms. Impurity atoms can be used to add lattice IR energy absorption bands. IR energy can be absorbed by electrically active impurity atoms (dopants), which are incompletely ionized at room temperature (in the absence of the IR radiation) Direct IR energy absorption by the lattice or impurity atoms produces a positive TC, and absorption by dopant ionization produces a negative TC. Since the different absorption mechanisms operate at different IR wavelengths, the SiC body can be tuned to produce a desired response as a function of IR radiation wavelength.
In some applications, such as temperature sensing and certain varistors, the SiC body is secured in a mounting structure having an AlN substrate. An electrically conductive mounting layer that includes W, WC or W
2
C electrically and mechanically connects the SiC body to the substrate via electrodes on the body. The mounting layer itself can have a W, WC or W
2
C adhesion layer adhered to the substrate, and one or more metallization layers adhered to the adhesive layer and bonded to electrodes for the SiC body, with the metallization layers having a coefficient of thermal expansion not greater than 3.5 times that of the substrate over the temperature range of interest. The mounting layer can be discontinuous, consisting of a plurality of mutually separated mounting elements that are connected to respective mutually separated electrodes on the SiC body.
The invention produces a very stable and reproducible resistance vs temperature characteristic, can withstand both absolute temperatures of at least 1,400° C. and very rapid temperature ramping without the need for fan or water cooling, is robust and not easily damaged, maintains calibration well, and offers improvements in the ability to withstand high IR energy/power intensities, exposures times and thermal shock. Smaller SiC devices can be used because of their favorable high power density handling capabilities, without being subject to large piezoelectric signals that can be induced by focused laser pulses on other materials.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.
REFERENCES:
patent: 4695733 (1987-09-01), Pesavento
patent: 5025243 (1991-06-01), Ichikawa
patent: 5122668 (1992-06-01), Kajiwara et al.
patent: 5868497 (1999-02-01), Jung
Choyke, W.J., “Optical and Electronic Properties of SiC”, NATO ASI Series vol. “The Physics and Chemistry of Carbides, Nitrides and Borides”, Manchester, England, pp 1-25, (Sep. 18-22, 1989).
Spitzer et al., “Infrared Properties of Hexagonal Silicon Carbide”, Physical Review, vol. 113, No. 1, pp. 127-132, (Jan. 1, 1959).
Hannaher Constantine
Hetron
Koppel & Jacobs
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