Jarmo Hietanen

Capacitive microphone with low-stress polysilicon membrane and high-stress polysilicon backplate

Abstract A capacitive single-chip silicon microphone with very low-stress polysilicon membrane was fabricated. A mechanism for stress-releasing due to the high stress of the perforated membrane was introduced. With the achieved stress level of 2 MPa, a microphone with the membrane area of 1 mm 2 can be optimally designed, although the measured components did not show the optimal resolution due to excessive acoustic resistance. With a membrane area of 1 mm 2 , the acoustical sensitivity was 4 mV/Pa (at 1 kHz) and the noise equivalent sound level was 33.5 dB (A), which are adequate values for many applications. The packaged components were tested with a thermal cycle between −40°C and +60°C, and due to low packaging-related stresses, no buckling of the membranes was observed.

Quantitative theory for V-groove capacitive transmitting transducers

The applied voltage to radiated far field, on-axis sound pressure relationship of V-groove capacitive air ultrasonic transducers was predicted using a drumhead model for the V-groove resonator. The electrostatic applied voltage to force relationship was obtained analytically based on a known solution for the electrostatic problem of the V-groove membrane system. The result was further used in the numerical calculation of the transfer function from harmonic applied voltage to radiated, on axis sound pressure. Fair agreement was found between the measured and predicted sound pressures versus frequency and Q-values for representative transducer design operating between 40 to 90 kHz.

An integrated printed circuit board (PCB) microphone

An electret microphone has been implemented into a printed circuit board. Due to novel construction the printed circuit board serves as the microphone backplate. Furthermore, other features of printed circuit board manufacturing technology are applied to minimize the number of components of the microphone. This results in a microphone that can be manufactured with the assembly of other electronic components during the manufacturing process. As a result a complete manufacturing process has been developed. To analyze the manufacturing process and the acoustical performance of the so-called PCB-microphone, the acoustics have been modeled, and backplate surface roughness, air gap thickness, electret charge, and overall sensitivity have been measured from prototypes.

Internal coupling of multiple-sized cavities in a capacitive ultrasonic transducer

The internal leakage between resonator elements in capacitive ultrasonic transducers with V-grooved backplates has been studied. The phenomenon is modelled and simulated using the lumped parameter method. The model can be used to explain signal vanishing at certain frequencies in capacitive ultrasonic transducers using multiple-sized resonators, i.e., cavities in the transducer backplate. The internal leakage together with multiple-sized cavities can be used to increase the frequency band of the capacitive ultrasonic transducer.

Closed-form formulation for sensitivity of capacitive ultrasonic transducers using V-grooved backplates

Abstract Capacitive air ultrasonic transducers with uniformly V-grooved backplates have been studied. It was found out that the variation of 18% in the electrostatic excitation force profile is converted down into 1 % to the side of the mechanical bending profile of the membrane over the V-groove. Therefore a modified electric field can be used to analyse the membrane-V-groove resonator. The treatment results in a relatively simple analytical equation which can be used to guide the design process of the transducer.

Integration of small transducers in commercial products

Miniaturization of electronic devices requires smaller components as well as a higher level of integration. Whenever audio acoustics is involved, as in mobile phones, it is exactly the acoustics components that offer the largest challenge in this respect. There are physical limitations to how small those components can be made, and with present methods of integration, even the smallest acceptable size is still too large. Progress in material research is expected to offer new possibilities for solving problems in further integration of components, e.g., microphones. In the development of silicon microphones, new materials and processes are utilized already. Decreasing the size of components and systems makes the problem of vibration coupling through structures more severe. Furthermore, in smaller components there are a number of physical effects to be accounted for, not only in practice but also in numerical simulation models of these systems. An example of this is the effect of viscosity when predicting a...