Thierry Corman

Deep wet etching of borosilicate glass using an anodically bonded silicon substrate as mask

Deep wet etching of borosilicate glass using an anodically bonded silicon substrate as mask is presented. Depths of m or more can be achieved very easily. The structured glass wafer can be bonded anodically on the same side to another silicon wafer, after having removed the bonded silicon mask. A lateral underetching 1.5 times larger than the depth was measured. An application using this masking technique is also presented. It consists of using the anodically bonded frame of a resonant silicon structure as a mask for deep glass etching to increase the gap between the glass wall and the resonator, thus yielding a high Q-factor.

Gas Damping of Electrostatically Excited Resonators

Abstract The aim of one present investigation is to determine the influence of both pressure and cavity depth on the mechanical Q-factor for electrostatically excited and capacitively detected encapsulated microresonators. In this paper, we present vibration Q-factor results for bulk-micromachined resonator structures in silicon which have been anodically bonded to glass lids of different recess depths. The parameters investigated are the air pressure, extending from 0.1 to 1000 mbar, and the distance between the resonator structures and the glass-lid wall, ranging from 15 to 45 μm. Another structure without a glass lid has also been tested and is used as a reference. The measurements are performed inside a vacuum chamber. We also present results on low-pressure encapsulated resonators. The structures are excited electrostatically with an external electrode, while the detection is achieved optically by a He/Ne laser combined with a lateral photodetector. The measurements show that the resonator vibration damping is dominated by ‘squeeze-film’ damping for small recess depths (15 μm or less) and that a pressure below 1 mbar is needed to achieved Q-factor of more than 3000. We present a theoretical model for the squeeze-film Q-factor which takes into account both the pressure and the recess-depth parameters. This model matches very well with the measurements. For the first time, a pressure of 1 mbar has been demonstrated inside a low-pressure encapsulated resonator, starting from a bonding pressure of 10−4 mbar, without using any getter material or gas evacuation after the bonding process.

Low-pressure-encapsulated resonant structures with integrated electrodes for electrostatic excitation and capacitive detection

Abstract Low-pressure-encapsulated resonant structures with lateral electrical feedthrough conductors for electrostatic excitation and capacitive detection are presented. The encapsulated device consists of a triple-stack wafer sandwich. The middle wafer is the silicon substrate with the resonant structure. The top and the bottom substrates are micromachined Pyrex 7740 glass wafers with metal electrodes. The resulting pressure inside the hermetically sealed cavity is 1 mbar, obtained by low-pressure anodic bonding, starting from 10−5 mbar, without using any getter material or gas-evacuation procedure after the bonding. A special electrode design is presented, making it possible to have electrodes on both glass lids using only standard fabrication steps. Low power consumption can be achieved and voltages of only 5 to 10 Vr.m.s. are sufficient for the excitation. A long-term stability test for low-pressure-encapsulated structures shows that after storage for one year (without integrated electrodes) and three months (with integrated electrodes) no leakage has been observed. Finally, a new fabrication technique is investigated to improve the quality factor of the resonator. It consists of using the anodically bonded frame of the silicon structure as a mask for deep glass etching to increase the gap between the electrode wall and the resonator, thus yielding a high Q-factor.

Low pressure encapsulated resonant structures excited electrostatically

Low pressure encapsulated resonant structures were built with a cavity pressure of 1 mbar, obtained and measured for several structures bonded anodically to glass lids with different recess depths from 20.5 to 44 /spl mu/m. The same pressure has also been demonstrated for structures with integrated electrodes for electrostatic excitation and capacitive detection. No getter material or gas evacuation procedure was used. No leakage has been observed and measurements indicate 1 mbar and the same Q-values after six months storage. A theoretical model for the squeeze-film Q-factor taking into account both the pressure and the recess depth is also presented.

New CO2 filters fabricated by anodic bonding at overpressure in CO2 atmosphere

Abstract New bulk micromachined CO 2 infrared filters are presented. The filters consist of a chamber in which CO 2 is encapsulated during an anodic bonding procedure. The chamber is formed by three fusion bonded silicon wafers joined to a glass wafer by anodic bonding at overpressure in a CO 2 atmosphere. The two central silicon wafers are etched in KOH to form a cavity. The high temperature anodic bonding process (430 °C) is performed at overpressures up to 2 bar, enabling an optimized gas absorption at a final CO 2 chamber pressure up to 1 bar at room temperature. CO 2 filters of different bonding pressures were fabricated and evaluated. An experimental investigation was conducted to improve the optical performances of the device at the wavelength of interest, i.e., 4.23 μm, where the absorption peak of CO 2 is located. We studied both the influence of the glass thickness and antireflective coatings on optical losses. Theoretical calculations for transmission with antireflective coating materials are presented. From these calculations, the thickness of two antireflective coating layers (silicon nitride and silicon dioxide) was optimized to obtain a maximum transmission at 4.23 μm. Finally, an optimized filter with thinner glass and silicon dioxide as antireflective coating is presented.