Guenter Hofmann

Process Technologies for High-Resolution Infrared Detectors Based on LiTaO3

IR system designers increasingly demand pyroelectric detectors demonstrating on the one hand a geometry and number of responsive elements, which are highly user- specific, and a very high signal-to-noise ration on the other. This paper describes the principal design of high- resolution single-element detectors and arrays based on the pyroelectric material lithium tentalate together with subtechnologies for manufacturing. It is shown that the production of self-supporting responsive elements with a thickness smaller than 5 micrometers will be feasible by applying a combined chemical and mechanical polishing together with ion beam etching. In the attempt to increase the absorption coefficient of these elements special silver black coatings are deposited, which result in an absorption coefficient (alpha) >= 0.92 in the wave range pitches of 50 micrometers were reached for ultrasonic bonding processes. Selected detector parameters are used to prove the potential advancement of detector characteristics, which will be feasible by applying these technologies.

Pyroelectric IR single-element detectors and arrays based on LiNbO3 and LiTaO3

Starting from a characterization of the used pyroelectric materials LiNbO3 and LiTaO3 the paper describes the hybrid arrangement and essential properties of realized single- element detectors and arrays. It shows that the design and the production technique of pyroelectric chips have a strong influence on the thermal and spatial resolution of the detectors. This technique includes both the thinning process and reticulation of the chips, for which ion beam milling as a universal method was optimized and used. Single-element detectors with extremely thin, self-supporting LiTaO3 chips (dp < 2 micrometers ) were produced. With a responsive element of 2 X 2 mm2 in area, they have a specific detectivity D* (500 K; 10 Hz; 25 degree(s)C) > 1 X 109 cmHz1/2W-1. Linear arrays with 128 responsive elements of 90 X 100 micrometers 2 element size, 100 micrometers pitch, integrated readout circuit, and coated germanium window have a noise equivalent power (NEP) (500 K; 40 Hz; 25 degree(s)C) of 4 nW. The modulation transfer function MTFS (40 Hz; 31 p/mm; 25 degree(s)C) is 0.15 for pyroelectric chips without isolating grooves and was increased up to 0.45 by means of ion- beam milling of 10 micrometers wide isolating grooves between the responsive elements. First results of two-dimensional arrays with 128 X 128 elements, of 50 micrometers pitch and integrated CCD-readout circuit are presented.

High-resolution pyroelectric linear arrays based on LiTaO3

Lithium tantalate (LiTaO3) is a material that is excellently suited for pyroelectric infrared detectors. Its figures of merit are first rate, it has a good long-time stability and it is available at reasonable cost. Researchers at the Institute for Solid State Electronics have been developing a flexible technology to manufacture LiTaO3 detectors for the last twelve years. As a result, devices can be produced that can be optimally adjusted to the planned application. This paper describes the design and basic features of linear arrays with up to 256 responsive elements. The arrays are hybrid devices consisting of the pyroelectric chip and a CMOS multiplexer. It is demonstrated that special patterning technologies (ion beam etching) and optical functional layers on the responsive element considerably increase the thermal and spatial resolution. Thus, NEP values smaller than 0.2 nW were obtained at 40 Hz chopper frequency. Main applications of detectors are the contactless temperature measurement and -- increasingly -- the spectrometry in the wavelength range 0.8 ... 25 μm.

Pyroelectric infrared arrays and their applications

For different applications the use of uncooled thermal infrared detectors is often a low-cost alternative to the cooled semiconductor detectors. By the development of a large number of novel detector technologies the thermal and spatial resolution ofthe thermal detectors could be improved considerably. On the basis of the pyroelectric material lithium tantalate (LiTaO3) a cheap and flexible detector technology was developed for the design and manufacturing of user-specific arrays even at small batch quantities. The possibilities of this technology are described by the example of newly developed linear arrays with 128 responsive elements of different sizes (pitch 100 i.tm) and 256 responsive elements (pitch 50 jtm) as well as a two-dimensional array with 16 x 8 responsive elements. The given detector properties ofthe arrays clearly show that the achieved detector properties meet the demands of many applications. Responsive elements having a thickness of less than 5 jtm can be produced by the use of ion-beam etching for the stmcturing ofthe pyroelectric chips. In connection with a low-noise CMOS read-out circuit, NEP-values lower than 1 .5 nW were obtained by it at a chopping frequency of 128 Hz. The effective capacity of the detectors is illustrated on the basis of two detector applications in devices for the twodimensional temperature measurement and in a spectrometer. Keywords: infrared, uncooled detector, pyroelectric, array, manufacturing, properties, thermal imaging, spectrometer

Influence of nonideal chopper design on nonuniformity in uncooled pyroelectric staring array systems

Choppers are inevitable parts of thermal imager or radiometer systems based upon uncooled pyroelectric focal plane arrays, since pyroelectric detectors require the incident radiation to be modulated. In comparison with other chopper techniques such as oscillator or shutter choppers rotating circular disks are considered superior in both mechanical simplicity and electronic control. However, the use of rotating disks introduces two non-ideal circumstances affecting the systems operation quality. Two-dimensional arrays require a bended chopper edge to approximate an ideal straight edge traveling over the array. Since sampling is performed line- sequentially, a local shift in time occurs between the sampling instant and the change of the optical chopper phase within each line. Furthermore, the chopper can only be placed in front of the entrance window of a detector array and, therefore, modulates the incident radiation in the blurred region of the optical path. This flattens the ideal rectangular modulation shape one would obtain if the chopper were able to run in the focal plane. Both effects attenuate the sensor output signal. The article presents a model for designing optimum chopper disks for 2- D arrays in order to achieve minimal possible signal attenuation. The optimization procedure incorporates all interdependencies between chopper layout, array size, chopper position, and readout velocity. The model also considers the unavoidable distance of the chopper plane from the focal plane and provides a chopper performance prediction method where the varying signal attenuation is discussed as contribution to the arrays nonuniformity. Finally, a sample design is given, and measurements are presented.

Pyroelectric linear array IR detectors with CCD multiplexer

Responsive pyroelectric linear arrays are described. After a short representation of the principal detector function, the pyroelectric materials L-alanine doped triglycine sulfate (DTGS:L-A) and lithium niobate (LiNbO3) are characterized, and the system parts pyroelectric chip, CCD-multiplexer, and hybrid arrangement are described in detail. Finally, the measured properties responsivity, noise equivalent power, and modulation transfer function are summarized.

Infrared line cameras based on linear arrays for industrial temperature measurement

The PYROLINE/ MikroLine cameras provide continuous, non-contact measurement of linear temperature distributions. Operation in conjunction with the IR_LINE software provides data recording, real-time graphical analysis, process integration and camera-control capabilities. One system is based on pyroelectric line sensors with either 128 or 256 elements, operating at frame rates of 128 and 544 Hz respectively. Temperatures between 0 and 1300DGRC are measurable in four distinct spectral ranges; 8-14micrometers for low temperatures, 3-5micrometers for medium temperatures, 4.8-5.2micrometers for glass-temperature applications and 1.4-1.8micrometers for high temperatures. A newly developed IR-line camera (HRP 250) based upon a thermoelectrically cooled, 160-element, PbSe detector array operating in the 3 - 5 micrometers spectral range permits the thermal gradients of fast moving targets to be measured in the range 50 - 180 degree(s)C at a maximum frequency of 18kHz. This special system was used to measure temperature distributions on rotating tires at velocities of more than 300 km/h (190 mph). A modified version of this device was used for real-time measurement of disk-brake rotors under load. Another line camera consisting a 256 element InGaAs array was developed for the spectral range of 1.4 - 1.8 micrometers to detect impurities of polypropylene and polyethylene in raw cotton at frequencies of 2.5 - 5 kHz.

Uncooled pyroelectric arrays for contactless temperature measurements

Uncooled pyroelectric arrays can be advantageously used for contactless measurements of one- and two-dimensional temperature fields. Satisfying values of noise equivalent power NEP, modulation transfer function MTF and long term stability of responsivity, respectively, are necessary for these applications. Linear pyroelectric arrays developed for those purposes are described. The pyroelectric chip based on lithium tantalate contains 128 sensitive elements (element size 90 X 100 micrometers 2 with 100 micrometers pitch). A CMOS read-out circuit (low noise preamplifiers, S&H-stages, analog switching structures with digital components, output-amplifier) is specially designed. Pyroelectric chip, read-out circuit, and a PTAT temperature sensor chip, respectively, are mounted in a metal hermetic package with 8...12 micrometers germanium window. Measured NEP values reach 5 nW at a chopping frequency of 128 Hz. The modulation transfer function MTF (128 Hz, 3 lp/mm) measured is typically 60%. Devices for the measurement of temperature distributions based on linear arrays described contain the uncooled array, infrared optics, chopper, control electronics, analog to digital converter, and a comfortable digital processing unit for multi point pattern correction, accumulation, digital filtering and so on. The measuring range of such PYROLINE systems reaches from 0...80 degree(s)C, 50...300 degree(s)C, 200...700 degree(s)C (1500 degree(s)C), respectively.

Uncooled multispectral detectors

The article describes the basic design and essential properties of developed multispectral detectors based on the pyroelectric material lithium tantalate (LiTaO3). The detector concepts presented here make parallel measurements possible in two to twelve spectral channels in the wavelength range of typically 1 - 25 micrometer. It is shown that these concepts are also useful for other responsive elements (thermopiles, PbS, PbSe). In one case, the detector consists of: (1) an optical unit with a beam splitter and interference filters for the spatial and spectral dispersion of the incident total radiation; and (2) the responsive elements with appropriate low-noise preamplifiers. The detector is a discrete and compact component in a TO 8 transistor casing (0 14 mm X 10 mm) with an aperture of typically 0 2 mm. Essential properties of a four-channel detector for gas analysis (CO; CO2; HC; REF) are described. The influence of the beam splitter geometry on the overlap of the measuring spots in the spectral channels is discussed. In the other case, the paper describes a hybrid arrangement of a six- channel multispectral detector which only uses interference filters for spatial and spectral dispersion of the input radiation. Its dimensions are about (22 X 22 X 33) mm3. It is shown that the radiation efficiency of this arrangement is higher than the first case.

Pyroelectric multispectral detectors and their applications

In many fields of IR technology it is necessary to take a parallel measurement of IR radiation within various defined spectral ranges. And in many cases there is also the demand for an identical measuring dot (moved objects), a high responsivity, a high signal-to-noise ratio, and a minimum cross talk. The paper describes the hybrid arrangement and essential properties of the developed pyroelectric multispectral detectors disposing of up to four channels. The detectors are discrete and handy building bricks fit into a T0 8 case which has an aperture of 2 mm in diameter and an internal spatial and spectral dispersion of the total radiation. It also presents results of these detectors applied especially in a multispectral pyrometer and the possibility of using them in gas analyzers. By using them in a multispectral pyrometer and applying special evaluation algorithms it was possible to reduce the influence of emission rate on the measuring accuracy decisively.