Susumu Sugiyama

Anisotropic etching of silicon in TMAH solutions

Abstract Detailed characteristics of tetramethyl ammonium hydroxide (TMAH, (CH3)4NOH) as silicon anisotropic etching solutions with various concentrations from 5 to 40 wt.% and temperatures from 60 to 90 °C have been studied. The etch rates of (100) and (110) crystal planes decrease with increasing concentration. The etched (100) planes are covered by pyramidal hillocks below 15 wt.%, but very smooth surfaces are obtained above 22 wt.%. Etch rates of 1.0 μ/min for the (100) plane and 1.4 μ/min for the (110) plane at 90 °C are obtained using a 22 wt.% solution. The etch-rate ratio of (111)/(100) varies from 0.02 to 0.08. The etch rate of thermally oxidized SiO2 is almost four orders of magnitude lower than that for (100) and (110) planes. The etch rates of aluminium are reduced by dissolving silicon in TMAH solution. Etch-stop techniques using a heavily boron-doped layer or p—n junction prove to be applicable to TMAH solutions.

Mechanical property measurements of thin films using load-deflection of composite rectangular membranes

Abstract The internal stress and Youngs modulus of thin films are determined by measuring the deflection versus pressure of rectangular membranes made of them. In order to reduce the measurement error for Youngs modulus due to the unknown Poissons ratio, a 2 mm × 8 mm rectangular membrane is adopted. Measurements are made by using a computerized measurement system. Low-pressure chemical vapor deposition (LPCVD) silicon nitride films are characterized and found to have an internal stress of 1.0 GPa and Youngs moduls of 290 GPa, showing that the rectangular membrane load-deflection technique could be utilized to measure the internal stress and Youngs modulus of films deposited onto LPCVD silicon nitride membranes. By using this composite membrane technique, a LPCVD polysilicon film and a plasma-CVD silicon nitride film are characterized. The internal stress and Youngs modulus are found to be −0.18 GPa and 0.11 GPa for the LPCVD polysilicon film and 0.11 GPa and 210 GPa for the plasma-CVD silicon nitride film, respectively.

Integrated piezoresistive pressure sensor with both voltage and frequency output

Abstract An on-chip integrated piezoresistive pressure sensor with both voltage and frequency outputs has been developed. The sensor chip, having a size of 3 × 3.8 mm 2 , was realized by the use of a standard bipolar IC process. The output voltage span is 1 to 4 V for a pressure range of 0 to 750 mmHg. The pressure sensitivity of the voltage output is 4 mV/mmHg. The non-linearity is less than 0.4 % of the full scale. The sensitivity of the frequency output is about 30 kHz for a 750 mmHg change in pressure. The output is in the T 2 L level. The temperature coefficient of the sensitivity is less than 0.06%/°C in the temperature range −20 to 110 °C.

Tactile image detection using a 1k-element silicon pressure sensor array

Abstract A 32 × 32 (1k)-element silicon pressure sensor array with CMOS processing circuits has been fabricated for the detection of a high-resolution pressure distribution. The sensor array consists of an X – Y matrix organized array of pressure- sensing cells with a spacing of 250 μm; CMOS processing circuits are formed around the array on the same chip. Fabrication of the sensor array is carried out using a 3 μm CMOS process combined with, Si micromachining techniques. The diaphragm size is 50 μm × 50 μm square. The sensor array chip size is 10 mm × 10 mm. The main features of the design for the tactile image detector are the two-line readout system for a full bridge of piezoresistors in each individual cell for noise reduction, and the low power consumption array exciting system with n-MOS power switches selecting the X – Y address in large-scale sensor integration. A tactile image-detecting system is set up for indicating tactile images as visible pictures. The detecting system is provided with functions for signal amplification and offset cancellation. High-resolution tactile images are stably shown as two- or three-dimensional figures on the display through computer processing of the readout data.

Surface micromachined micro-diaphragm pressure sensors

A micro-diaphragm pressure sensor with a silicon nitride diaphragm of 100 mu m diameter has been developed using surface micromachining (type-C). To increase the yield in the diaphragm forming process, the generation of hydrogen bubbles is reduced by eliminating the anisotropic etching of the silicon substrate and adopting a flat vacuum chamber fabricated by undercut etching of a polysilicon sacrificial layer. The sacrificial layer etching is carried out with a concentric circle around a central etch hole to reduce the stress concentration in the etching process. A tetramethyl ammonium hydroxide solution compatible with the LSI process is utilized as an etchant for polysilicon sacrificial etching. The arrangement of the polysilicon piezoresistors is determined according to stress analysis using a finite element method. The yield of the diaphragm forming process is improved to more than 90%. The pressure sensitivity of approximately 10 mu V/V/kPa is five times that of the conventional micro-diaphragm pressure sensor (type V).<<ETX>>

Micro-diaphragm pressure sensor

A micro-diaphragm pressure sensor with silicon nitride diaphragm of 80 µm × 80 µm was fabricated by applying micromachining technique. The main feature is that it is a complete planar type pressure sensor formed by single-side processing solely on the top surface of

Mechanical property measurements of thin films using load-deflection of composite rectangular membrane

The internal stress and Youngs modulus of thin films are determined by measuring the deflection versus pressure of the rectangular membranes of materials. In order to reduce the measurement error for the Youngs modulus due to an unknown Poissons ratio, a 2 mm*8 mm rectangular membrane is adopted. Measurements are made by using a computerized measurement system. Low-pressure chemical-vapor-deposited (LPCVD) silicon nitride films are characterized and found to have an internal stress of 1.0 GPa and a Youngs modulus of 290 GPa. By using this composite membrane technique, an LPCVD polysilicon film and a plasma-CVD silicon nitride film are characterized. The internal stress and Youngs modulus were found to be -0.18 GPa and 160 GPa for the LPCVD polysilicon film and 0.11 GPa and 210 GPa for the plasma-CVD silicon nitride film.<<ETX>>

Control of internal stress and Young’s modulus of Si3N4 and polycrystalline silicon thin films using the ion implantation technique

Boron and phosphorus ions are implanted in order to control the internal stress and Young’s modulus of 200‐nm‐thick silicon nitride and polycrystalline silicon films prepared by low‐pressure chemical vapor deposition. These ions are implanted into the middle layer of the films at doses of 1×1014 and 1×1015/cm2, then the films are annealed at 900u2009°C. The internal stress and Young’s modulus of these films are measured by a rectangular membrane load deflection technique [Tech. Digest, IEEE Micro Electromechanical Systems Workshop, IEEE, Salt Lake City, UT, 1989, p. 152]. Both internal stress and Young’s modulus of the silicon nitride film decrease with dose, while those of the polycrystalline silicon film do not necessarily decrease.

In situ observation and analysis of wet etching process for micro electro-mechanical systems

Details of the etching of polysilicon sacrificial layers using a KOH etching solution have been studied. Etching rates showed strong dependence on KOH concentration and the structure pattern of the layers. For the clarification of the etching process, in situ observation of an etching process for various micro-membrane structures with polysilicon sacrificial layers has been done. Etching observation equipment was created for this purpose. The equipment is capable of monitoring and recording the etching process in situ under various etching conditions such as temperatures, flow rates and concentrations. It is shown from in situ observation that hydrogen bubbles play an important role in supplying fresh etching solution to the surface to be etched away. Repeated accumulation and exhaust of hydrogen bubbles cause strong oscillatory movement of the membrane structures. This movement results in the fracture of the membrane due to the increase in membrane stress. Based on these results, a system for in situ observation and stress analysis of a wet etching process is proposed.<<ETX>>

Digital compensated capacitive pressure sensor using CMOS technology for low-pressure measurements☆

Abstract A capacitive pressure sensor with digital output for low-pressure measurements has been fabricated using CMOS technology. The sensor output is compensated and adjusted by a newly developed method. The sensor has a hybrid configuration of an integrated sensor chip and a digital IC chip. Because of this configuration, the thermal sensitivity shift and thermal zero shift of the output are adjusted at the sensor chip, while the offset and full-scale span of the output are adjusted at the digital IC chip, independently. By using the new compensation and adjustment technique, a thermal sensitivity shift of 0.026% FS/°C and thermal zero shift of 0.013% FS/°C for a pressure range of 0-200 mm H2O (0-20.39 Pa) and a temperature range of 25–75 °C have been obtained. These characteristics using the new compensation method are about 1/7 and 1/4 of those obtained by using a conventional method. Furthermore, a humidity countermeasure is proposed.