Pirmin Rombach

The first low voltage, low noise differential silicon microphone, technology development and measurement results

The first differential silicon microphone is presented. This capacitive working device consists of two backplates with a membrane in between. Due to the balanced arrangement the air gap can be minimized. Thus, a higher electrical field and sensitivity can be achieved for low voltages. A dedicated process sequence has been developed in order to get the optimum mechanical and electrical properties for all structural layers. Furthermore, a sandwich structure has been developed to achieve a reproducible, very sensitive microphone membrane with a thickness of only 0.5 /spl mu/m and a stress of 45 MPa. The total sensitivity for a bias of 1.5 V was measured to be 13 mV/Pa and the A-weighted equivalent input noise was measured to be 22.5 dB SPLA. The upper limit of the dynamic range has been determined to be 118 dB SPL and the total harmonic distortion at 80 dB SPL is below 0.26%.

Vapor-Phase Self-Assembled Monolayers for Anti-Stiction Applications in MEMS

We have investigated the anti-stiction performance of self-assembled monolayers (SAMs) that were grown in vapor phase from six different organosilane precursors: CF₃(CF₂)₅(CH₂)₂SiCl₃ (FOTS), CF₃(CF₂)₅(CH₂)₂Si(OC₂H₅)₃ (FOTES), CF₃(CF₂)₅(CH₂)₂Si(CH₃)Cl₂ (FOMDS), CF₃(CF₂)₅(CH₂)₂Si(CH₃)₂Cl (FOMMS), CF₃(CF₂)₇(CH₂)₂SiCl₃ (FDTS), and CH₃(CH₂)₁₇(CH₂)₂SiCl₃ (OTS). The SAM coatings that were grown on silicon substrates were characterized with respect to static contact angle, surface energy, roughness, nanoscale adhesive force, nanoscale friction force, and thermal stability. The best overall anti-stiction performance was achieved using FDTS as precursor for the SAM growth, but all coatings show good potential for solving in-use stiction problems in microelectromechanical systems devices.

Thermal stability of vapor phase deposited self-assembled monolayers for MEMS anti-stiction

Six different source chemicals (organosilanes) were successfully used for deposition of self-assembled monolayers (SAMs) onto silicon substrates by a vapor phase process. Five different fluorocarbon coatings and one hydrocarbon coating were deposited. The thermal stability of the coatings was studied in detail with respect to degradation as a function of temperature, and for the fluorocarbon coatings also the degradation rate at 400 °C. For fluorocarbon coatings deposited from FDTS a useful lifetime of approximately 90 min at 400 °C was found allowing the coating to survive high temperature MEMS packaging operations, while fluorocarbon coatings deposited from FOTS, FOMDS, FOTES and FOMMS were less stable. The hydrocarbon coating deposited from OTS degrades already at approximately 200 °C. The thermal stability of the SAM coatings was found to be significantly reduced if aggregations from the deposition process are present on the coatings.

Silicon microphones - a Danish perspective

Two application areas of microphones are discussed, those for precision measurement and those for hearing instruments. Silicon microphones are under investigation for both areas, and Danish industry plays a key role in both. The opportunities of silicon, as well as the challenges and expectations, are discussed. For precision measurement the challenge for silicon is large, while for hearing instruments silicon seems to be very promising.

Balanced membrane micromachined loudspeaker for hearing instrument application

Different actuator principles are rated with respect to an application as a loudspeaker in a hearing instrument. Fundamental rules are presented that have to be considered when different actuation principles are compared. A loudspeaker design using the most promising actuator principle, namely the balanced change in reluctance principle, is presented and first results of the production of planar coils are discussed.

A compact CMOS MEMS microphone with 66dB SNR

Silicon MEMS microphones that offer small size, ease of integration with CMOS electronics, and the ability to withstand lead-free solder reflow cycles, are becoming increasingly popular for high-volume consumer electronic products, and are competing in price and performance with traditional electret condenser microphones [1]. The design of a MEMS microphone, consisting of a compliant membrane and a stiff back-plate forming a variable capacitor, is a challenging task with a number of design trade-offs [2]. For cost reasons, a small-area membrane is desired; however, lower acoustical noise is obtained with a larger membrane. In this work, we demonstrate a method to increase the SNR of a microphone system without the need for a complicated and risky MEMS die redesign. An SNR of 66dB is achieved using two microphones (instead of a single one) in a differential configuration, thus doubling the total membrane area.

Magnetic flux generator for balanced membrane loudspeaker

This paper reports the development of a magnetic flux generator with an application in a hearing aid loudspeaker produced in microsystem technology (MST). The technology plans for two different designs for the magnetic flux generator utilizing a softmagnetic substrate or electroplated NiCoFe as core material are presented and the production and characterization of four different mono- and double-layer planar coil types are reported.

An integrated sensor head in silicon for contactless detection of torque and force

Abstract An integrated micromachined torque sensor head in silicon for contactless detection of torque and force is described. It is based on a magnetic yoke with an exciting and a receiving coil that detects the change of permeability of a rotating shaft and an amorphous ribbon with strong magnetostrictive properties, respectively. The designed sensor head determines directly the change of permeability μ by measuring the magnetic flux density in the air gap between the yoke and the amorphous ribbon (shaft). The problem arising with this kind of torque measurement is the variation of the air gap when the shaft is rotating, and especially when a torque is applied. The sensor presented here allows the influence of the air-gap variation to be eliminated by differential measurement of the magnetic flux density. 3-D simulations of the magnetic circuit show the magnetic properties of the sensor head. The development of the complete integrated sensor head starts with components like an NiFe yoke shaped by KOH etched membranes, coils with a large cross-sectional area and CMOS MAGFETs.

Chip-size-packaged silicon microphones

The first results of silicon microphones that are completely batch-packaged and integrated with signal conditioning circuitry in a chip stack are discussed. The chip stack is designed to be directly mounted into a system, such as a hearing instrument, without further single-chip handling or wire bonding. The devices are fully encapsulated and provided with a well-determined interface to the environment. The integrated microphones operate at a bias of 1.5 V and are expected to reach a sensitivity of 5 mV/Pa, an A-weighted equivalent input noise of 24 dB sound pressure level, and a power consumption of about 50 μW in the near future, thereby living up to the tight specifications of microphones for hearing instruments. Other potential applications include mobile phones, headsets, and wearable computers, in which space is constrained.

A low-voltage silicon condenser microphone for hearing instrument applications

Silicon microphones have been the subject of investigations since the early 1980s. Due to their poor performance, no silicon microphone for hearing instrument applications has been commercially available until today. Usually the sensitivity of the electromechanical transducer is too low. Thus the input‐related noise of the following preamplifier stage becomes dominant and results in a high equivalent input‐related noise. Here a silicon condenser microphone with the potential for hearing instrument applications will be presented. To get the best properties for the different mechanical parts, e.g., membrane and back plate, a dedicated process sequence has been developed. Therefore, circuitry and mechanical parts have to be produced separately and mounted later in a stacking process. The microphone has a 2×2 mm2, 0.4 μm‐thick membrane and an air gap of 1.0 μm. Wafers with different membrane stress have been produced. The microphones have been acoustically, mechanically, and electrically characterized, partly...