THIN METAL-FILMS AS ABSORBERS FOR INFRARED-SENSORS

In order to achieve the maximal sensitivity of a thermal IR sensor, the incident light must be efficiently absorbed. This may be done either by the sensor material with its own electrodes or by an additional absorber structure. Freely suspended thin metal films are known to act as wide-band absorbers for IR, radiation. Prepared on the solid surface of the sensor, the absorbing properties of the metal layer are, however, strongly influenced by the dielectric function of the sensor material. Results are presented for thermoelectric and pyroelectric sensors. For the thermoelectric sensor, a 400 nm thick silicon layer is used as a support for the absorbing silver film with a sheet resistance of 150 OMEGA/square, evaporated onto the rear side of the silicon layer. This absorber has an absorbance of nearly 50% independent of the wavelength over the whole IR range. For pyroelectric sensors two examples of absorbers on PVDF are presented. The first is a broad-band absorber and has an absorbance of about 50% within the wavelength region 3-200 mum. The other is a selective absorber and has an absorbance of about 90% within the region 7-15 mum. The selective absorber is completely opaque and can be realized by means of a PVDF film with, a thickness of about 2 mum, covered on both sides by metal films. This structure may serve as an excellent absorber for a crystalline pyroelectric sensor.