High temperature ZrN and HfN IR scene projector pixels

Details High temperature ZrN and HfN IR scene projector pixels High temperature ZrN and HfN IR scene projector pixels

1998-06-29

2001-04-03

Wyszomierski, George (Department: 1742)

Metal treatment

Process of modifying or maintaining internal physical...

Carburizing or nitriding using externally supplied carbon or...

C148S537000, C204S192210

Reexamination Certificate

active

06210494

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to ohmic heating elements, and more particularly to emission of thermal energy from refractive metal compounds resistive members. The present invention finds particular application for the production of infrared (IR) or thermal images from tightly packed pixel elements formed from these materials.
A major challenge in resistive IR emitter array technology is to produce a high emittance structure that requires relatively little electrical current during operation. The key factors which contribute to high emittance are the density of the pixels which form the array, and the maximum operating temperature of the pixels. High pixel density has been achieved in the prior art using a multi-level pixel structure. The multi-level pixel structure maximizes the radiating area by placing the pixel drive and addressing electronics directly under the a resistive emitting member. High radiance is achieved by fabricating the resistive emitting member of the pixel using a thin, absorbing film, and placing a reflector below this film to direct radiation outward.
The electrical current used by a thermal emitting pixel is strongly linked to the material used to form its resistive emitting member. In prior art systems the designer traded off low current operation for high temperature operation or compromised on other pixel characteristics. For example, metal films used for the resistive member such as platinum, although potentially having good high temperature properties, do not have high resistivities. Thus, platinum resistive members must be patterned into an extremely thin serpentine film to maximize their resistance. Unfortunately, the adhesion of these platinum films is poor, making the pixel structurally weak.
Titanium nitride is another material which has been used to form resistive members in thermal emitters. Titanium nitride has good temperature properties, satisfactory resistance and structural properties, but unfortunately involves sensitive pixel fabrication steps. Specifically, an annealing is typically done during processing of the thermal emitter to stabilize the device for high temperature operation. The resistance of titanium nitride varies considerably in the range of temperatures used for this anneal. This sensitivity can lead to large variations in pixel resistance from array to array and possibly large variations in emmisivity from pixel to pixel in the same array. In fact, the range of resistance of the titanium nitride resistor can in some cases cause pixels to become inefficient or completely inoperative. Titanium nitride resistive members also suffer from some difficulty with lifetime high temperature stability.
SUMMARY OF THE INVENTION
The present invention solves these and other needs by providing materials to fabricate resistive emitting members which exhibit high resistivity while at the same time providing high temperature operation significantly above that known in the art. Specifically, the use of nitrides of Group IVB transition metals from the periodic table, exclusive of titanium, and having a resistance greater than 50 ohms per square, is described.
The chosen resistive member materials are capable of operating at temperatures in excess of 1000K, and also result in other desirable properties for ohmic heating elements or arrays of such elements—high dynamic range of resistivities, controllable annealing properties at high temperatures, high temperature stability, improved control of resistive properties, improved optical properties, and a low positive thermal coefficient of resistance. The chosen resistive member materials are suitable for use in current high pixel density structures, and thus the applicants' invention may be applied to current thermal emitter fabrication techniques.
While there is literature which describes very high temperature materials for use as IR pixel arrays, (i.e., in the 1000K range), typically these devices exhibit low pixel density. One such device is described in: “Dynamic Infrared Scene Projector for Missile Seeker Simulation”, W. S. Chan, et al., Proceedings of the 1991 International Simulation Technology Conference, Oct. 21-23 1991, pp. 250-255. The low pixel density leads to an array of very hot pixels with cooler inter-pixel regions. The ideal resistive member material should allow for a high pixel density without sacrificing the desired high temperature operation—that is, operation at temperatures in excess of 1000K.
While the chosen materials find particular use for thermal emitting elements (i.e. to produce thermal images), they also find use as heating elements either singularly or as arrays. Typically, when used in this manner, the resistive member is placed in physical contact with the material be heated.


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Article entitled“A High Fill Factor Suspended Resistor IR Scene Generator: Design, Fabrication and Preliminary Performance” by A. P. Pritchard, M. C. Hebbron, S. P. Lake and I. M. Sturland; British Aerospace plc Sowerby Research Centre, FPC 267 Filton Bristol BS12 7 QW UK; pp. 15-26 from SPIE vol. 1967.
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Article entitled “Recent Progress in Large Dynamic Resistor Arrays” by B. Cole, R. Higashi, J. Ridley, J. Holmen, and E. Benser; Honeywell Technology Center, 12001 State Highway 55, Plymouth, MN 55441; R. Stockbridge and L. Murrer; Wright Laboratory/MNGI, 101 West Eglin Boulevard, STE 309, Elgin AFB, FL 32542; L. Jones; Science Applications International Corp., 1247-B North Elgin Parkway, Shalimar, FL 32579 and E. Burroughs; RTTC, STERT-TE-E-SA Bldg. 4500, Redstone Arsenal, AL 35898; pp. 1-13 from SPIE 1997.
Article entitled “Dynamic IR Scene Generation: Basic Requirements and Comparative Display Device Design” by A. P. Pritchard; Sowerby Research Centre, British Aerospace plc, FPC 267, Filton, Bristol BS12 7QW, UK; pp. 14-149 from SPIE vol. 940 Infrared Scene Simulation: Systems, Requirements, Calibration, Devices and Modeling (1988).

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