Micromachined thin-film sensors for SOI-CMOS co-integration
(2004)
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- Authors
- Supervisors
- Flandre, Denis
- Abstract
- While in some applications such as gas-flow sensors, the co-integration of a sensor with its surrounding electronics on a single chip is an asset, it is unavoidable in other cases such as transistors-based pressure sensors or from the substrate isolated microwave circuits, on a membrane. Moreover, the co-integration performed in the most advanced of the Complementary-Metal-Oxide-Semiconductor (CMOS) technologies to date -namely Silicon-on-Insulator (SOI) technology- provides many significant benefits regarding the performance, reliability, miniaturization and process easiness without significantly increasing the final cost. Thin dielectric membranes constitute the starting material for a large number of sensors thanks to their ability to act as a mechanical support or an electrical and thermal insulator. We particularly focused on the thermal insulation feature to build fully CMOS-SOI co-integrated gas-flow sensors. A one-µm-thick robust and flat dielectric multilayered membrane has been built taking into account the residual stresses in each constitutive layer. A complete review and summary of the main concepts of thin film mechanics is detailed for an in-depth understanding. A new measurement methodology based on substrate curvature and deflected microstructures has been developed in order to quantify accurately the residual stresses in each layer and in their stacking. To release the membrane in post-processing, the Tetramethyl Ammonium Hydroxide (TMAH) silicon micromachining solution has been used and optimized in order to increase its selectivity towards aluminum. This particular wet etching technique was extensively reviewed and enhanced by our own experiments, in particular for assuring CMOS compatibility. A novel loop-shape polysilicon microheater implementing the basic heating cell of new simple gas-flow sensors has been designed and built in a CMOS-SOI standard process. High thermal uniformity, low power consumption and high working temperature have been targeted and confirmed by various measurements. In particular, the electrical properties of polysilicon versus temperature and annealing time have been analyzed in depth. The gas-flow sensor has been optimized to be integrated in intermediate- and post-processing of a standard CMOS-SOI fabrication. The thermopiles of the flow sensor as well as the interdigitated electrodes of the gas sensor have judiciously been chosen in this purpose. The sensing films of the gas sensors consisted in sputtered and drop coated metal oxide layers such as SnO2 and WO3. Measurements in the presence of a nitrogen flow and gas revealed a fair sensitivity on a large flow velocity range for the flow transducer, as well as a good sensitivity to gases such as ethanol, ammonia and nitrogen dioxide for the gas transducer. The whole process has confirmed its full CMOS compatibility by measuring transistors, capacitors and gated diodes on the same chip than the sensors after each post-processing step. Finally, transistors which are integrated in small silicon islands located in the middle of our dielectric membrane have been studied and presented as a concluding demonstrator of the co-integration in SOI technology. Such devices have opened the door to a lot of new applications where integrated circuits and sensors are merged in order to meet higher performance in harsh environments.
- Affiliations
Citations
Laconte, J. (2004). Micromachined thin-film sensors for SOI-CMOS co-integration. https://hdl.handle.net/2078.5/98000