On the mechanism of anisotropic etching of silicon

A new model is proposed that explains the anisotropy of the etch rate of single crystalline silicon in certain etchants. It is inspired from theories of crystal growth. We assume that the (111)-face is flat on an atomic scale. Then the etch rate should be governed by a nucleation barrier of one atomic layer deep cavities. The origin of the nucleation barrier is that the formation of a too small cavity increases the free energy of the system due to the step-free energy. The step-free energy and the undersaturation governs the activation energy of the etch rate. Having the largest step-free energy, the (111)-face etches the slowest. The model explains qualitatively why the etching is isotropic in certain etchants and anisotropic in others

Influence of applied potentials on anisotropic etching of silicon described using kinematic wave etch model

Anisotropic etch rates of silicon in KOH solutions were studied as a function of an externally applied potential. A combination of three micromachined samples consisting of predry-etched wagon-wheel patterns and masked trench offset patterns was used to measure the etch rates at a large number of crystal orientations simultaneously. The measured data was described in terms of microscopic properties, including step velocities, terrace roughening, and step anisotropy, using the kinematic wave etch model. All parameters show distinct changes due to the applied potential and resulting additional electrochemical reaction path. A decrease in step velocity shows the electrochemical oxidation and subsequent passivation of the Si surface. Trends in terrace roughening show a minimum in roughness and a corresponding change in anisotropic etch-rate ratio at the non-open-circuit potential of −1250 mV vs saturated calomel electrode. The observed decrease in step anisotropy and subsequent step-anisotropy reversal at more positive potentials indicates an anisotropy in not only chemical etching but also electrochemical oxidation of (111) surface steps.\u

Micro mixer with fast diffusion

A concept for micromixing of liquid is introduced, and its feasibility is demonstrated. The mixer allows fast mixing of small amounts of two liquids and is applicable to microliquid handling systems. The mixer has a channel for the liquid, an inlet port for the reagent, a 2.2-mm×2-mm×330-μm mixing area, and 400 micronozzles (15 μm×15 μm) through with a reagent is injected into the sample liquid. The resulting microplumes greatly increase the contact surface between the two liquids and hasten the speed of the mixing by diffusion. The fabrication process is extremely simple. The mixing is complete within a few seconds; a homogeneous state of mixing is reached in 1.2 s when the total volume injected is 0.5 μl and the injection flow rate is 0.75 μl/

Determination of young's modulus of PZT-influence of cantilever orientation

Calculation of the resonance frequency of cantilevers fabricated from an elastically anisotropic material requires the use of an effective Young’s modulus. In this paper a technique to determine the appropriate effective Young’s modulus for arbitrary cantilever geometries is introduced. This technique is validated using a combined analytical and finite element simulations (FEM) approach. In addition, the effective Young’s modulus of PbZr0.52Ti0.48O3 (PZT) thin films deposited on dedicated micromachined cantilevers was investigated experimentally. The PZT films were deposited on the cantilevers by pulsed laser deposition (PLD). The change in flexural resonance frequency of the cantilevers was measured both before and after deposition of the PZT thin film. From this frequency difference we determined the Young’s modulus of PZT deposited by PLD to be 103 ± 2 GPa. Even though the PZT is grown epitaxially, this value is independent of the in-plane orientation

Nonlinearity and hysteresis of resonant strain gauges

Nonlinearity and hysteresis effects of electrostatically activated, voltage driven resonant microbridges have been studied theoretically and experimentally. It is shown, that, in order to avoid vibration instability and hysteresis to occur, the choices of the ax. and d.c. driving voltages and of the quality factor of a resonator, with a given geometry and choice of materials, are limited by a hysteresis criterion. The limiting conditions are also formulated as hysteresis-free design rules. An expression for the maximum attainable figure of merit is also given. Experimental results, as obtained from electrostatically driven vacuum-encapsulated polysilicon microbridges, are presented and show good agreement with the theory

Performance of thermally excited resonators

A study of electrothermal excitation of micromachined silicon beams is reported. The temperature distribution is calculated as a function of the position of the transducer, resulting in stress in the structure which reduces the resonance frequency. Test samples are realized and measurements of resonance frequency, vibration shape and vibration amplitude are carried out. There is a satisfactory agreement between theory and experiment at small thermal stresses. Near the buckling load we find distinct deviations from theory which are ascribed to mechanical imperfections of the beams

A UHF 4th-order band-pass filter based on contour-mode PZT-on-silicon resonators

A UHF 4th-order band-pass filter (BPF) based on the subtraction of two 2nd-order contour-mode resonators with slightly different resonance frequencies is presented. The resonators consists of a 1 μm pulsed-laser deposited (PLD) lead zirconate titanate (PZT) thin-film on top of a 3 μm silicon (PZT-on-Si). The resonators are actuated in-phase and their outputs are subtracted. Utilizing this technique, the outputs of the resonators are added up constructively while the feed-through signals are eliminated. The BPF presented a bandwidth of approximately 28.6 MHz and more than 30 dB stopband rejection at around 700 MHz

Optimisation of a two-wire thermal sensor for flow and sound measurements

The Microflown is an acoustic sensor measuring particle velocity instead of pressure, which is usually measured by conventional microphones. In this paper an analytical model is presented to describe the physical processes that govern the behaviour of the sensor and determine its sensitivity. The Microflown consists of two heaters that act simultaneously as sensors. Forced convection by an acoustic wave leads to a small perturbation of this temperature profile, resulting in a temperature difference between the two sensors. This temperature difference, to which the sensitivity is proportional, is calculated with perturbation theory. Consequently the frequency dependent behaviour of the sensitivity is analysed; it is found that there are two important corner frequencies, the first related to the time constant velocity of heat diffusion between the sensors, the second related to the heat capacity of the heaters. The developed model is verified by experiments. Previously a very good model has been given for the performance of the Microflown in a channel, i.e. with both heaters between fixed walls walls in the positive and negative z-direction. Here, a model is presented that describes the situation of the present used sensors: without walls under and above them. Model predictions are compared to experimental result