Robert Aigner

Miniaturized Thermoelectric Generators Based on Poly-Si and Poly-SiGe Surface Micromachining

We report on miniaturized thermoelectric generators which are being developed to convert waste heat into a few µW of electrical power sufficient to supply microelectronic circuitry. A BiCMOS realization using standard materials is favored to make these generators amenable to low cost applications. In order to optimize our device, the design and the material properties have been studied. The use of micromachining techniques allowed us to improve the thermal efficiency of the generator significantly. Low thermal conductivity of the thermoelectric materials proved to be the most important factor to increase the output power. The materials we have investigated are poly-Si and poly-SiGe. Experimental results of the fabricated devices show good agreement with the predictions of thermal simulations.

Fracture strength and fatigue of polysilicon determined by a novel thermal actuator [MEMS]

A novel thermal actuator for the determination of polysilicon fracture strength and investigation of its fatigue is presented. The actuator consists of two narrow beams, which expand due to electrical heating, and a cold plate to which a short fracture beam is attached. Because of its small dimensions, the actuator can be used for on-wafer testing. This method is suitable for tensile and compressive material. Using different types of fracture beams fracture strengths were compared for uniaxial tension and bending test. Using Weibull statistics, the fracture strength for polysilicon has been found to be (2.9/spl plusmn/0.5) GPa in tensile tests and (3.4/spl plusmn/0.5) GPa in bending tests. In fatigue investigations, we observe that fracture strength decreases slowly with time to 2.2 GPa after 10/sup 6/ cycles.

Spurious resonance free bulk acoustic wave resonators

In this paper a device structure eliminating the spurious modes arising from lateral standing Lamb waves is presented. The operating principle behind the device is described with the use of a simple model. Experimental results of ZnO and AlN based resonators in SMR configuration are given. Laser interferometer measurements are presented that confirm applicability of the idea.

Optimization of acoustic mirrors for solidly mounted BAW resonators

The overall performance of bulk acoustic wave (BAW) filters is dominated by the effective coupling coefficient and the quality factor of the constituting BAW resonators. Whereas the effective coupling coefficient and its dependency on the layer stack is quite accurately modeled with a simple one-dimensional acousto-electric model (e.g. Masonstransmission line model), the prediction and optimization of the resonators quality factor - particularly for solidly mounted resonators (SMR) - completely fails with this model: whereas a calculation of the acoustic reflectance of a standard quarter-wavelength mirror stack leads to theoretical Q-factors well above 10000, experimental SMR devices with this type of mirror show values of typically well below 1000. This discrepancy is commonly explained by either visco-elastic loss in the materials and/or laterally leaking waves leaving the active resonator area. However, we have found a new, far more important loss mechanism relating to shear waves generated in the device. These waves can be created by injection from the resonators border area as well as by reflection/refraction of longitudinal waves at non-perpendicular angle of incidence to a material interface. In this paper, a quantitative methodology for the optimization of the acoustic mirror layer stack will be proposed. The influence of the mirror structure on the trapping of both longitudinal and shear wave energy will be discussed based on this very simple approach. Trade-offs with respect to the other important device parameters, such as effective coupling coefficient, temperature coefficient of frequency (TCF) and purity of the electrical response, are analyzed. The usefulness of this approach for the optimization of resonator Q-values will be proven by experimental results demonstrating Q-factors of 1500 and higher.

Acoustic reflector for a BAW resonator providing specified reflection of both shear waves and longitudinal waves

A BAW resonator includes a piezoelectric layer, a first electrode, a second electrode, a substrate, and an acoustic reflector disposed between the substrate and the second electrode. The acoustic reflector has a plurality of layers. A performance of the acoustic reflector is determined by its reflectivity for a longitudinal wave existing in the BAW resonator at the resonance frequency of the BAW resonator and by its reflectivity for a shear wave existing in the BAW resonator at the resonance frequency of the BAW resonator. The layers of the acoustic reflector and layers disposed between the acoustic reflector and the piezoelectric layer are selected, with reference to their number, material, and thickness, such that the transmissivity for the longitudinal wave and the transmissivity for the shear wave in the area of the resonance frequency is smaller than −10 dB.

Coupled bulk acoustic wave resonator filters: key technology for single-to-balanced RF filters

Coupled Resonator Filters (CRF) are a new type of Bulk Acoustic Wave (BAW) device in which two piezoresonators are stacked on top of each other in a way that a certain degree of acoustic interaction occurs. CRFs feature complete galvanic isolation between input and output and thus enable to offer BAW filters with mode-conversion (single-ended to balanced) as well as impedance transformation. The basic concept behind these devices is reviewed. The size of a CRF BAW filter is excitingly small as compared to SAW filters. First experimental results for CRFs operating at 1.8 GHz which have been manufactured at Infineon Technologies will be presented. Certain manufacturing issues are discussed and measurements of the temperature coefficient TCF are shown.

Single-to-balanced filters for mobile phones using coupled resonator BAW technology

The coupled resonator filter (CRF) is a new type of bulk-acoustic-wave (BAW) device in which two piezoresonators are stacked on top of each other in a way that a certain degree of acoustic interaction occurs. Experimental results of single-ended and mode-converting CRF for GSM applications also featuring impedance conversion, operating at 1.8 GHz and manufactured at Infineon Technologies are presented. For the balanced port, those results demonstrate excellent amplitude- and phase-imbalance, obtained by a very symmetrical design. Also, a method for suppression of spurious resonances is demonstrated. Moreover, the latest results confirm that unwanted passbands can be overcome by modification of the acoustic mirror. Furthermore, the temperature coefficient of frequency (TCF) has been measured. Due to the compensating effect of SiO/sub 2/ in the CRF stack the values obtained are exceptionally small.

Sensitivity Enhancement of MEMS Inertial Sensors Using Negative Springs and Active Control

In this paper we present a device in which the inherent control scheme of force feedback loops in Σ/Δ-archi-tecture [1] is used for increasing the mechanical sensitivity of differential capacitive inertial sensors in combination with “negative springs”. As a consequence, the input-referred electronic noise and quantization noise is significantly reduced in the signal band. Since electronic noise can influence the loop’s stability, Σ/Δ-theory is tailored to micromechanical sense loops. Results from system simulation and measurement of an accelerometer demonstrate a significant improvement in sensitivity.

Energy loss mechanisms in SMR-type BAW devices

Bulk Acoustic Wave (BAW) filters owe their per- formance advantages particularly to the excellent Q-values of the acoustic resonators. Solidly Mounted Resonator (SMR) type BAW devices achieve Q-values greater than 1500 when acoustic mirror layers with a large impedance ratio and, hence, a high reflectivity are used. Tungsten and Silicon Oxide are the materials of choice for low loss BAW resonators. This paper describes the loss mechanism still present in low loss BAW resonators and presents comprehensive methods to acquire quantitative data about energy losses. These methods are based on electrical measurements, 1D transmission line models and higher dimensional FEM simulations, and laser interferometer measurements. The lateral currents across the electrodes and the resulting ohmic power dissipation are derived from surface vibration measurements and an inverse calculation of local charge distribution. It is demonstrated that combining these methods allows to detect acoustic leaks and to analyze resistive losses and, thus, the major energy losses in the respective operating conditions can be identified.

A low-voltage torsional actuator for application in RF-microswitches

This paper reports a new surface micromachined torsional actuator for integrated microswitches. The device requires an actuation voltage below 10 V and is fabricated in a BiCMOS compatible process, allowing for monolithic integration with an actuation circuitry. Insensitivity against mechanical vibrations is achieved by a bistable operation principle. Measured pull-in voltages of the structures are in good agreement with the electromechanical simulations. The dynamic behavior of the actuator is investigated by interferometric determination of the resonance modes. Measurements of the switching times of completed devices result in values below 10 μs.