I. Introduction: Context and motivations.
0.1. Why Silicon-on-Insulator technology ? 0.2. Why a thin-film membrane ? 0.3. Why co-integration and CMOS compatibility ? 0.4. Contents of the work
II. Techniques and materials.
1. Silicon bulk micromachining with TMAH. 1.1. Introduction. 1.2. Generalities about silicon micromachining. 1.3. TMAH silicon etching. 1.4. Selectivity versus dielectrics. 1.5. Selectivity versus aluminum. 1.6. Selectivity versus other metals. 1.7. Etch-Stop. 1.8. Undercutting. 1.9. Summary.
2. Thin dielectric films stress extraction. 2.1. Introduction - Definitions, 2.2. Stress measurements by substrate curvature method. 2.3. Strain measurements using micromachined structures. 2.4. Final conclusions.
1. Low power microhotplate as basic cell. 1.1. Introduction. 1.2. Motivations. 1.3. Materials selection. 1.4. Thermal design. 1.5. Device fabrication. 1.6. Microheater characterization and results. 1.7. Conclusions.
2. Microheater based flow sensor. 2.1. Introduction. 2.2. Design and fabrication. 2.3. Measurements results. 2.4. Discussions and comparison with the state-of-the-art. 2.5. Conclusions.
3. Gas Sensors on microhotplate. 3.1. Introduction. 3.2. Interdigitated electrodes: from design to deposition. 3.3. Sensitive layer deposition. 3.4. Summary of the fabrication steps. 3.5. Measurements results without gas. 3.6. Measurement results under gas and discussions. 3.7. Conclusions.
4. SOI-CMOS compatibility validation. 4.1. Introduction. 4.2. Basics of SOI technology. 4.3. Post-processing steps. 4.4. Measurements. 4.5. Transistors on membrane as final demonstrator. 4.6. Conclusions.
IV. Conclusions and outlook
Appendixes. A. (100) Silicon crystallography. B. About Interferometry. C. About Reflectometry.
Publications originated from this work.
Co-integration of sensors with their associated electronics on a single silicon chip may provide many significant benefits regarding performance, reliability, miniaturization and process simplicity without significantly increasing the total cost.
Micromachined Thin-Film Sensors for SOI-CMOS Co-integration covers the challenges and interests and demonstrates the successful co-integration of gas-flow sensors on dielectric membrane, with their associated electronics, in CMOS-SOI technology.
We firstly investigate the extraction of residual stress in thin layers and in their stacking and the release, in post-processing, of a 1 µm-thick robust and flat dielectric multilayered membrane using Tetramethyl Ammonium Hydroxide (TMAH) silicon micromachining solution. The optimization of its selectivity towards aluminum is largely demonstrated.
The second part focuses on sensors design and characteristics. A novel loop-shape polysilicon microheater is designed and built in a CMOS-SOI standard process. High thermal uniformity, low power consumption and high working temperature are confirmed by extensive measurements. The additional gas flow sensing layers are judiciously chosen and implemented. Measurements in the presence of a nitrogen flow and gas reveal fair sensitivity on a large flow velocity range as well as good response to many gases. Finally, MOS transistors suspended on released dielectric membranes are presented and fully characterized as a concluding demonstrator of the co-integration in SOI technology.
Co-integration of MEMS and MOS in SOI technology is promising and well demonstrated here
The impact of Micromachining on SOI devices is deeply analyzed for the first time
Include extensive TMAH etching, residual stress, microheaters, gas-flow sensors review
Residual stresses in thin films need to be more and more monitored in MEMS designs
TMAH micromachining is an attractive alternative to KOH