Thermal Stress Preventing Measures

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. 

A detector chip 10 of HgCdTe comprising detector elements 11 is connected via bumps 13 to bent buffers 17 of gold. The buffers 17 are in turn connected to input regions 15 of a CCD formed in a silicon substrate 14. A method to form the buffers is also provided. 

When a silicon read-out chip and an HgCdTe detector chip are bonded together by a flip-chip bonding process, the chips are exposed to a mechanical stress which may lead to damages, especially of the fragile detector chip. Photo-resist films are used in JP-A-61059771 (Fujitsu Ltd, Japan, 27.03.86) to form a spacer. The spacer reduces the mechancial stress of the semiconductor chips during the bonding process. 

A read-out device, comprising an input region 23, is formed in a silicon substrate 21. A positive photo-resist film 29A, a negative photo-resist film 29B and a second positive photoresist film 29C are patterned into a spacer. Connecting bumps 34 are applied and a detector chip 31, comprising photodiodes 33, is bonded to the silicon chip by pressing the two chips together. Finally, the photo-resist films 29A, 29B and 29C are removed. 

In JP-A-63268271 (Fujitsu Ltd, Japan, 04.11.88) metal rods are used to provide spacers between a detector substrate and a read-out substrate. 

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The research activity here presented has been carried out at the N.D.T. laboratory of l.S.P.E.S.L. (National Institute for Occupational Safety and Prevention) and it is aimed at the set up of the Stress Pattern Analysis by Measuring Thermal Emission technique [I] applied to pressure vessels. Basically, the SPATE system detects the infrared flux emitted from points resulting from the minute temperature changes in a cyclically stressed structure or component. 

The stabilised austenitic stainless steels for cladding contain an alloying element (niobium), which forms stable grain boundary carbides. This prevents chromium depletion along the grain boundaries and makes the material immune to stress corrosion cracking. Non-stabilised material was used for the first layer because the thermal expansion coefficient of the material is closer to that of the low-alloy pressure vessel material. The presence of niobium in the second layer allows performance of so-called retrospective dosimetry in the RPV inner surface by machining out some scraps for further chemical separation and activity measurement to determine real neutron fluence on the RPV inner surface. 

Direct molding begins with the preheating of the PP-textiles in an infrared station, a convection oven or directly in the shaping tool. As previously discussed, suitable measures must be taken (utilization of a tenter or blank holder) to prevent the thermally activated, orientation-induced shrinking stresses [57,68]. Nevertheless, a good thermal contact should be ensured, so that an even temperature distribution is possible between the single layers. 

The sample preparation for a bulk pyroelectric measurement is very similar to what is required for a bulk piezoelectric measurement, namely a well-sintered ceramic disc that has been electrically poled. Determining the pyroelectric coefficient may be divided into two groups - the measurement of the pyroelectric current and the measurement of the charge. We will describe measurement techniques for both groups. In addition, the pyroelectric effect can be subdivided into primary and secondary effects. The primary effect is observed when the material is rigidly clamped under a constant strain to prevent any thermal expansion or contraction. Secondary effects occur when the material is permitted to deform, i.e. the material is under constant stress. Thermal expansion results in a strain that changes the spontaneous polarisation, attributable to the piezoelectric effect. Thus the secondary pyroelectric effect includes contributions caused by piezoelectricity. Exclusively measuring the pyroelectric coefficient under constant strain is experimentally very difficult. What is usually experimentally measured is the total pyroelectric effect exhibited by the material - the sum of the primary and secondary effects. 

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