Abstract
(Englisch)
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Histack aims at developing and applying a technology for tight integration of electronics and transducers to build multifunctional and smart microsystems. This technology is based on stacking wafers and chips directly using electrical and mechanical wafer-to-wafer feedthroughs. The concept is demonstrated and tested through two devices which are defined by the industrial partners, MICROTRONIC (DK) and BALTEADISK (I). The technologies developed in collaboration with the partners MIC/DTU (DK), DELTA (DK), and LETI/CEA (F) are transferred to foundry process lines, a.o. at CSEM (CH), within the scope of the project.
The two products, an integrated microphone for hearing instruments and an integrated print head for ink jet printers, are addressing new markets for MICROTRONIC and BALTEADISK, respectively. The silicon microphone with integrated buffer amplifier and A/D converter is designed for use in future digital completely-in-the-channel hearing instruments, which will allow for a fully digital design of the signal processor. The integrated print head will be applied for post franking and ticket stamping, requiring the assembly of several print and driving chips to achieve a larger print area. A further goal is to develop encapsulation and shielding for the stacked systems and to acquire testing methods for these.
The project has been successful at fabricating the two products, even though the format of each of them has been modified. The integrated microphone stack has been manufactured in the research facility of MIC/DTU, with the key component (the actual microphone die) fabricated at CSEM. The two consecutive fabrication runs at CSEM resulted both in working microphones and the issues of fabrication yield and process simplifications are now the only ones remaining. The entire microphone stack proved to be difficult to manufacture and simplifications of the stacking process are now foreseen to simplify the stacking process. The integrated printing head approach has been modified to reduce the production cost. The original electrical feedthroughs required too many process steps to be economically feasible. The concept of electrical feedthroughs has therefore been abandoned, focusing the efforts on the flip-chip soldering technique for the precise alignment of different printing heads on a PCB and for the creation of a closed fluid path for the ink supply to printing head. Using this flip-chip technique, printing head assemblies of two precisely aligned printing head have been fabricated and tested.
As an example of the results obtained by CSEM within the project, a picture is shown of a microphone wafer. The picture is taken prior to the final etch, since after this etch and prior to the stacking the microphones are very sensitive to dust. The final etch removes a sacrificial oxide that is under compressive stress. This stress actually causes the slight bending of the membranes that can be observed on the picture. This bending disappears during the final etch. Besides the picture, a measured response curve of a microphone measured at CSEM is depicted. The microphone demonstrates a high sensitivity of 4mV/Pa and a flat frequency response until 12.5 kHz. The oscillations above this frequency are mainly due to measurement artefacts.
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