Michael Pedersen

A silicon condenser microphone with polyimide diaphragm and backplate

A new technology for the fabrication of silicon condenser microphones is presented. The technology, which is based on the use of polyimide, can be performed entirely as a post process on substrates already containing integrated circuits. Microphones with an open-circuit sensitivity of 8.1 mV Pa?1, and a flat frequency response (±2 dB) between 100 Hz and 15 kHz have been fabricated with this technology. The bias voltage used in these measurements is 15 V, and the measured noise level with zero bias is 24 dB SPL, which is sufficient for most acoustic applications, including hearing aids.

On the mechanical behaviour of thin perforated plates and their application in silicon condenser microphones

In this paper an alternative approach to the modelling of plates with a large number of holes is presented. By means of plate theory, it is shown that perforated plates can be modelled by conventional orthotropic plates with modified elastic properties. The modification of the elastic constants is derived by equalizing the strain-energy of the perforated and the orthotropic plate. The model obtained is then compared with previous methods and applied in the electrochemical simulation of a silicon micromachined microphone structure. The microphone structures are simulated numerically, using an algorithm based on finite differences.

An integrated silicon capacitive microphone with frequency-modulated digital output

In this paper a new integrated capacitive silicon microphone with frequency-modulated digital output is described. The FM output can be processed directly by a digital system without the need of analog-to-digital conversion, which is normally required with capacitive microphones in digital systems. The integrated microphone comprises a polyimide structure fabricated directly on a silicon substrate, on which the oscillator has already been completed using a standard CMOS process. This is possible since the polyimide microphone process is fully IC-compatible. The capacitance of the microphone modulates the output frequency of the oscillator, and from experiments a sensitivity of 234 Hz/Pa is measured at a carrier frequency of 263 kHz. The frequency response is flat (±1 dB) within the measured range of 100 Hz to 15 kHz. The A-weighted noise is 4.5 Hz for a bandwidth of 15 kHz, yielding an equivalent noise level of the microphone of 60 dB(A) re. 20 μPa. The measurements have shown to be in good agreement with the theoretical models.

A capacitive differential pressure sensor with polyimide diaphragm

A capacitive differential pressure sensor has been developed. The process used for the fabrication of the sensor is IC-compatible, meaning that the device can potentially be monolithically integrated on one chip with a suitable signal conditioning circuit. A sensor for a differential pressure range of was fabricated and tested with a frequency modulated detection circuit, and good agreement was found with the theoretical model of the sensor. A nominal sensitivity of 18% has been measured for a positive differential pressure of 1 bar.

An IC-compatible polyimide pressure sensor with capacitive readout

A capacitive differential pressure sensor has been developed. The process used for the fabrication of the sensor is IC-compatible, meaning that the device potentially can be integrated on one chip with a suitable signal-conditioning circuit. A sensor for a differential pressure of ±1 bar has been fabricated and tested with a frequency-modulated detection circuit, and good agreement is found with the theoretical model of the sensor. A nominal sensitivity ?C/C of 17% has been measured for a positive differential pressure of 1 bar. The resolution of the complete detection system is 2.5 mbar (250 Pa).

On The Electromechanical Behaviour Of Thin Perforated Backplates In Silicon Condenser Microphones

In this work an alternative approach to the modelling of plates with a large number of holes, is presented. By means of plate theory, it is shown that perforated plates can be modelled by conventional orthotropic plates with modified elastic properties. The modification of the elastic constants is derived by equalising the strain-energy of the perforated and the orthotropic plate. The model obtained is then compared with previous methods and applied in the electromechanical simulation of a silicon micromachined microphone structure. The microphone structures are simulated numerically, using an algorithm based on finite differences.

A polymer condenser microphone on silicon with on-chip CMOS amplifier

In this paper the development of a capacitive microphone with integrated preamplifier is described. The condenser microphone is made by micromachining of polyimide on silicon, and is compatible with CMOS technology. Therefore, the structure can be realised by post processing on substrates containing integrated circuits, independently of the IC process. Microphones with a required DC bias voltage of 4 V have been realised on a CMOS substrate containing PMOS buffer preamplifiers. From the measurements on these structures, it is illustrated how an immediate improvement of 4.8 dB of the microphone sensitivity and noise level can be obtained by using the integrated preamplifier. The measured sensitivity of the integrated condenser microphone was 2.5 mV/Pa and the equivalent noise level (ENL) was 29.5 dB(A) SPL.

Fabrication of IC-compatible capacitive sensors by polymer processing

A low-temperature (< 300 degree(s)C) polymer micromachining process has been developed, whereby the sensor can be fabricated directly on substrates containing complete electronic circuits. This approach is strong since any IC process can be selected with no regard to the sensor process. Condenser microphones have been fabricated with a sensitivity of 8.1 mV/Pa, flat frequency response between 100 Hz and 15 kHz, and an equivalent noise level of 24 dBA SPL. Differential pressure sensors have been made with a nominal sensitivity (Delta) C/C of 17%/bar for a pressure range 1 bar. Furthermore, uni-axial accelerometers with a nominal sensitivity of 0.43%/g have been implemented. From these results it may be concluded that IC-compatible capacitive sensors with good performances can be achieved with this technology, and it is suggested that the use of polymer processing on silicon therefore may become an important issue in smart sensors of the future.

Thermal IR imager utilizing a thermal-to-visible transducer

We have been developing a novel thermal-to-visible transducer (TVT), an uncooled thermal-IR imager that is based on a Fabry-Perot Interferometer (FPI). The FPI-based IR imager can convert a thermal-IR image to a video electronic image. IR radiation that is emitted by an object in the scene is imaged onto an IR-absorbing material that is located within an FPI. Temperature variations generated by the spatial variations in the IR image intensity cause variations in optical thickness, modulating the reflectivity seen by a probe laser beam. The reflected probe is imaged onto a visible array, producing a visible image of the IR scene. This technology can provide low0cost IR cameras with excellent sensitivity, low power consumption, and the potential for self-registered fusion of thermal-IR and visible images. We describe development of high-reflectivity coatings deposited on both surfaces of the IR-absorbing material.