Thermal detectors expansion

Thermal expansion detectors that use a heat-sensitive metal link that melts at a predetermined temperature to make contact and sound an alarm. 

Before any slit operation check, write down, or save the old motor positions Operation of slits can be useful to change the beam intensity (instead of operating absorbers). Imperfect thermal stabilization of mirrors and monochromators can be compensated by proper slit operation. Before such operation is undertaken, it should be made sure that the instrument is close to thermal equilibrium. In particular after opening the main beam shutter for the first time, it may be indicated to wait for several hours. Otherwise the operator will have to follow the thermal expansion continuously. This bears the risk to destroy the adjustment or even the detector. 

Figure 12.4 Bending response of a cantilever (as measured by the voltage output from a position-sensitive detector) to applied voltage pulse with and without TNT adsorbed on the surfaces. The bending of the uncoated cantilever follows the time profile of the applied voltage pulse (except the lengthening of the rise and fall times) and is presumably due to the difference between the thermal expansion coefficients of silicon and the doping material. The exothermic nature of the TNT deflagration event is clear due to the enhancement in bending of the cantilever.

The traditional source in IR absorption spectroscopy is a glowing rod or wire heated by the passage of an electric current the hot body emits radiation over a continuous frequency range. The radiation is dispersed using a prism NaCl, which is transparent over much of the IR region, is commonly used for IR prisms and windows. The sample may be a solid, liquid, or gas. Various detectors are used the most common are thermocouples, photoconductive materials such as PbS, bolometers (which are temperature-dependent resistors), and the Golay cell (which uses the thermal expansion of a gas contained in a chamber). 

The problem of cross-talk between adjacent photodiodes and the problem of different thermal expansion coefficients between a silicon substrate and an HgCdTe substrate are approached in JP-A-63281460. A detector is made up of HgCdTe wells, comprising photodiodes, formed in a CdTe substrate. The detector is bonded to a silicon substrate by a flip-chip process, and finally the CdTe substrate is etched away. 

The problem of difference in thermal expansion coefficients between a silicon read-out substrate and an HgCdTe detector substrate is approached in US-A-4783594 by filling the space between the two substrates with a resilient electrically insulating polymeric material and thereafter separating the detector elements from each other by removing a layer of the HgCdTe detector substrate. 

To reduce the effect of difference in thermal expansion coefficients between a silicon readout substrate and an HgCdTe detector layer, which is bump bonded to the silicon substrate, it is proposed in JP-A-1061056 to affix a second silicon substrate to the HgCdTe detector layer with an intermediate layer of GaAs. 

Mechanical stress in connection bumps formed between a read-out circuit in a silicon substrate and detectors in an HgCdTe substrate due to a difference in thermal expansion coefficients of the two substrates is reduced in US-A-4943491 by bonding a layer of a material, having a greater thermal expansion coefficient than the coefficient of the two substrates, to the silicon substrate. 

A detector layer of an imager presented in US-A-S264699 is thinned to allow the detector to act like a flexible membrane and to elastically respond to thermal mismatch resulting from different coefficients of thermal expansion between the detector and a semiconductor read-out circuit. 

In WO-A-9417557 an integrated circuit assembly is presented which includes a silicon thin film circuit bonded to a substrate of a material selected to provide the assembly with an effective thermal expansion characteristic that approximately matches that of an HgCdTe detector array. 

An imager with insulator walls formed between connection bumps is shown in JP-A-6236981. The structure reduces the detrimental effects due to a difference in the coefficient of thermal expansion between a detector substrate and a read-out substrate. 

When the flip-chip technique is used to connect two chips of different materials having different thermal expansion coefficients, the connection will be subjected to mechanical stress to a degree dependent on the the thermal history of the array. In JP-A-55150279 (Fujitsu Ltd, Japan, 22.11.80) the connection between an HgCdTe detector chip and a silicon read-out chip comprises bent buffers which absorb such mechanical stress and which prevent the fragile detector chip being damaged during the connection. 

The hybrid circuit 10 comprises a buffer structure 16 which is comprised of a material which accommodates the difference in thermal expansion coefficients of the HgCdTe detector array 12 and the silicon read-out chip 14. The buffer layer is made of sapphire which also has good thermal conductivity properties. The buffer structure has laser drilled vias 18 which are formed in registration with unit cells of the detector array and the read-out circuit. Each of the vias is provided with indium bumps 20 at opposing ends thereof. The buffer structure is interposed between the detector array and the read-out chip. Cold weld indium bump technology is employed to couple the bumps 20 to the buffer structure. The buffer structure is further... 

The structure is bonded to a substrate 24 which is chosen to have a coefficient of thermal expansion that is selected for providing the resultant read-out chip assembly with an effective coefficient of thermal expansion that is approximately the same as an HgCdTe detector array 36. The substrate material may be GaAs (4.5-5.9 x 10"6 m/mK), CdTe, Ge (5.5-6.4 x 10"6 m/mK), and a-plane sapphire (3.5-7 5 x 10" m/mK) where the coefficients of thermal expansion are given in parentheses. The coefficients of thermal expansion for silicon, HgCdTe and epoxy are 1.2 x 10"6 m/mK, 3.8-4.5 x 1 O 6m/mK and 30-50 x 10"6 m/mK, respectively. Next, the substrate 16 is removed and aluminium pads 34a are formed. Indium bumps 34b are cold welded to corresponding indium bumps 36b. 

One problem which arises when a detector array is attached to the face of a multi-layer module is the inability of the detector material to absorb forces generated by a mismatch of coefficient of thermal expansion between the detector array material and the module. Furthermore, it is difficult to isolate a fault that may be attributable to either the detector elements, module wiring or processing elements. 

The difference of thermal expansion coefficient between an HgCdTe detector material and a silicon read-out material is dealt with in FR-A-2494910 where detector elements in the shape of islands are connected on one side to a silicon read-out chip and on the other side to a substrate having a similar thermal expansion coefficient as the silicon read-out chip. 

A problem which arises when a read-out chip of for example silicon is attached to a detector chip of mercury cadmium telluride is the mechanical damage which may occur when the array is cooled to cryogenic temperatures for operation. The stress is due to a mismatch in the coefficients of thermal expansion between the two materials. 

Individual detector elements 4 of HgCdTe are formed into islands which are connected on a first side via indium connectors 3 to input regions 11 of a read-out chip 1. A second chip 2, which is transparent to infrared radiation L and which comprises common electrodes 22, is connected to an opposite side of the detector elements via indium connectors 21. It is important that the thermal expansion coefficient of the chip 1 and 2 are closely matched. Both chips may, for example, be made of silicon. 

The Golay cell uses the distortion of a reflecting Sb-coated collodion membrane, closing one of the ends of a so-called pneumatic chamber. This distortion is caused by the thermal expansion of a gas heated by the radiation incident in the cell, and produces the deflection of a beam of visible light, which is detected by a photocell. The Golay cell was used, fitted with a diamond window, with the first far IR FTS and its responsivity and response time were comparable to those of the radiation thermocouple. For more details on these detectors, see [15]. 

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