Application of Dual Doped TMAH Silicon Etchant in the Fabrication of a Micromachined Aluminium Flexing Beam Actuator

One of the main goals of micromechanical systems (MEMS) fabrication is microdevice integration with standard integrated circuit (IC) technologies, such as bipolar or the more prevalent complementary metal oxide semiconductor (CMOS) processes. To that end, it has been found that the anisotropic silicon etchant tetra-methyl ammonium-hydroxide (TMAH) can be effectively used in a post-processing step with CMOS-based fabrication by doping it with silicic acid to prevent the unwanted etching of exposed alluminum. Furthermore, the addition of ammonium persulfate to the TMAH/silicic acid solution enhances etch rate and surface quality. The final etching solution, called dual-doped TMAH, is a CMOS-compatible, highly selective to silicon over aluminum, and can therefore allow an aluminum layer to be used as an etch mask. In this paper, we utilize dual-doped TMAH towards the fabrication of a microstructure made entirely of aluminum. A flexing beam microactuar suspended over a bulk micromachined silicon cavity is presented for use as a magnetomete

Tile and till

International audienceThis paper presents the study of gold/gold thermocompression bonding at silicon wafer level. The first samples contains sealing rings and electrical pads and are characterized on pull and shear test showing bond strength similar to silicon/glass anodic bonding (10MPa-80MPa). A sealed cavity and a piezoresistor on a 30µm-thick silicon membrane are added in the second samples. Helium test, membrane deflection and piezoresistor signal monitoring after aging 14 days at 250°C confirm the vacuum stability inside the cavity after bonding. Motivation and results Several bonding techniques [1-5] exist to ensure hermeticity and protection of sensor inside micro-cavities. The analysis of gold thermo-compression bonding performed here contains both sealing ring and electrical contacts. To qualify our process, two different structures are realized to test bond strength (Figure 1) and hermeticity (Figure 2). For both structures, two 4 inches 500µm-thick, double-sided polished silicon wafers with 200nm-thick thermal SiO2 are used. On each wafer, a diffusion barrier followed by 50nm/500 nm Ti/Au evaporated seed layer is deposited. Then a 3µm-thick electroplating gold is deposited inside a patterned resin mold in order to define sealing rings and electrical pads. For the second structure, a square membrane (30µm-thick and 2000µm-side) and piezoresistors are added. For both structures, we used a SUSS-SB6 bonder to perform the thermocompression bonding (420°C, 5,7MPa, 50 minutes). We include spacers between the wafers during alignment to obtain vacuum inside the cavity (5.10-3 mbar). After bonding and dicing, some dies are polished to observe the gold structure at the interface (Figure 3). No delamination is observed between the different materials showing a complete atomic diffusion at the gold interface bonding. Batches of 20 dies are then selected from different wafer areas for pull and shear tests. Most of the dies exhibit cohesive fracture in silicon with tensile strength comparable or superior to silicon/glass anodic bonding (figure 4). Even if pull tests are often used for the qualification of bond strength, this technique don't give reproducible results compared to shear test. High rate leakage through bonding interface has been evaluated with structure 2 by monitoring the membrane deflection several hours after bonding with a mechanical profiler. Measurements show deflection between 4µm and 5µm for 75% of the cells, which is consistent with simulation and technological process variations. In order to check more precisely the leakage, the dies (after breaking the thin silicon membrane) are glued on a special tool where one side of the die is exposed to He and the other side is connected to vacuum (1.10-9 mbar) detector equipment (Figure 5). The mean leak measured by He detector was 10-8 mbar.l/s for the best dies structure bonded which correspond to an excellent hermeticity. The hermeticity reliability has been characterized by following the response of a piezoresistor placed on the membrane after aging at 250°C during 14 days for 6 cells (Figure 6). The piezoreristance shift is lower than 250 ppm for the best cell which correspond to a 75mbar variation inside the cavity

Characterization of Silicon Anisotropic TMAH Etch for Bulk Micromachining

In this paper the possibility to passivate the aluminum metalization in properly saturated TMAH solution is demostrated by doping the solution with appropiate amounts of silicic acid. This increases the range of application of this etchants simplifing both the post processing and the etch set-up configuration. We investigate the effect of these additives on the etch rate and the quality of (100) and (111) silicon sufaces obtained for different TMAH concentrations. We therefore also investigate the effect of ammonium persulfate (NH4)2S2O8 on the etch rate under different addition conditions for a 6.25wt.% TMAH doped by silicic acid in order to keep aluminum passivated

Characterization of TMAH Silicon Etchant Using Ammomium Persulfate as an Oxidizing Agent

Among the silicon anisotropic etchants, tetramethyl ammonium hydroxide [TMAH] is of great interest due to the absence of metal ions. Therefore using TMAH solutions at low concentrations has the advantage of being more economical, both in terms of cost and time. Unfortunately the surface quality of the etched silicon is strongly influenced by the concentration of the solution i.e. at low concentrations (less than 15%), the etched surfaces are very often covered with pyramidal-shaped hillocks, thus producing a very rough surface finish. Ammonium Persulfate (NH4)2S2O8 can be added to TMAH to suppress hillock formation. We investigated the effects of this additive under different oxidizer addition conditions. The influence of TMAH concentration and etchant temperature was evaluated

Realization of Thin Film Specimens for Micro Tensile Tests

International audienc

Realization of Thin Film Specimens for Micro Tensile Tests

International audienc

Electrochemical effects during anisotropic bulk-silicon etching with doped and undoped TMAHW solutions

Tetra-methyl ammonium hydroxide/water (TMAHW) solutions are gaining considerable interest as alternative anisotropic silicon etchant to the more usual KOH and EDP etchants because of their compatibility with CMOS processing and relatively low toxicity. TMAH exhibits a high etch rate for lower concentrations of solution, however this rate falls dramatically due to the inevitable formation of hillocks on the surface. For TMAH concentrations above approximately 20 wt.%, hillock formation is markedly reduced, however the intrinsic etch rate at these concentrations is low. Using TMAH at lower concentrations has the advantage of being more economical, both in terms of cost and time. Furthermore, it has been shown that oxidizing agents can be added to TMAH, such as ammonium persulfate (NH4)2S2O8 to eliminate hillock formation at these reduced concentrations. Our investigations have the aim to determine the effects of this additives on the silicon etch rate and anisotropy under different conditions, i.e. TMAH concentration, temperature, oxidizer concentration and frequency of oxidizer additions. In addition the role of the redox potential of the etchant solution is investigated and anomalous high silicon etch rates under anodic polarisation with respect to OCP, are reported and discussed

Low-Power Silicon Microheaters for Gas Sensors

Silicon-based SnO2 gas sensors are commonly operated at relatively high temperatures (up to 400 °C and even beyond for specific applications), thus necessitating suitable heating modules to guarantee high temperature uniformity over the sensitive surface area with minimal power consumption. Silicon micromachining allows low-power microheaters to be fabricated, with a low-cost (for mass production) technology, which is potentially suitable for the integration of the sensor, the heating element, as well as the required electronics into the same battery-operated microsystem. In this paper, we report on the development of a microheater structure consisting of a dielectric stacked membrane micromachined from bulk silicon, with an embedded polysilicon resistor acting as the heating element, Different technological solutions in the fabrication process for the micromachined structures have been investigated. In particular, hoth uniform membranes and suspeded microbeam structures have been realized to characterize the different thermal behavior toward the ambient. The microheaters have been designed to enable temperatures in the excess of 500 °C to be reached on the hotplate with a power consumption lower than 50 mW. Extensive thermoelectric and thermomechanical finite-element numerical simulations have been carried out, to predict microheater temperature vs. electric-power characteristics and mechanical stability, respectively. Simulations have also provided helpful hints in view of the optimization of the proposed structures