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

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

Vertically structured thin membranes by a lost mold technique

For MEMS applications it will be more and more important to fabricate thin membranes with low intrinsic stress while maintaining high mechanical and structural performance in terms of rigidity and resilience. In this paper a new approach to obtain stiff thin membranes is presented as an alternative to conventional membranes obtained by bulk micromaching methods (like chemical or electro-chemical etch stop technique). The reinforcement of the membranes is achieved by vertical structures obtained by filling a mold of deeply etched silicon trenches with different materials (SiO2, Si3N4, polysilicon) and subsequent removal of the bulk silicon. The theoretical estimation of the rigidity of a plate with vertical enforcement strips has been studied by the comparison of the inertial modulus of beams with T-shaped section. The mechanical behavior of membranes stiffened by vertical structures with different dimensions (in terms of depth, thickness and pitch) has been investigated through simulation using ANSYS as finite elements analysis software. The silicon trenches have been fabricated by deep reactive ion etching (DRIE) based on SF6/O2 plasma, using a 500 nm thick TEOS as masking layer. These structures have been subsequently sealed by deposition of a silicon oxide layer and filled by poly-silicon. The silicon oxide has been used both to calibrate the trench dimensions and as etch-stop layer for the TMAH anisotropic etch that is required to release the three dimensional structure. For this reason, some experiments have been carried out to define the best conformality condition in terms of thickness uniformity of the layer between different type of silicon oxide (thermal oxide, TEOS, LTO). In order to minimise the intrinsic stress distribution due to the deposition process and to optimise the polysilicon conformality, the effects of the annealing temperature and the boron doping conditions have been characterised. The mechanical behaviour of these structures has been measured under different load conditions

Optimization of TMAH etching for MEMS

Tetra-methyl ammonium hydroxide (TMAH) is an anisotropic silicon etchant that is gaining considerable use in silicon sensor micromachining due to its excellent compatibility with CMOS processing, selectivity, anisotropy and relatively low toxicity, as compared to the more used KOH and EDP etchants. In this paper, the influence of temperature and concentration of the TMAH solution together with oxidizer additions is studied in order to optimize the anisotropic silicon etching for MEMS fabrication. In particular this optimized etchant formulation has been employed ad ITC-Irst in the development of a basic fabrication process for piezoresistive pressure sensors based on a silicon membrane and four resistors connected in a Weathstone bridge configuration. The active element of the sensor, i.e. the thin silicon membrane, is formed by etching anisotropically from the backside of the wafer. Both process and etchin have to be tuned and matched in order to obtain an optimum fabrication sequence. Some improvements such as higher etch rate and better surface finish have been obtained by the addition of ammonium peroxidisulfate as oxidizing agent under different conditions. This simplifies both the post processing and the etch set-up. The process parameters and the thermo-electro-mechanincal characteristics of the pressure sensors (as piezoresistors resistivity, device sensibility, temperature coefficients. membrane thickness) were tested and are compared with the analytical and numerical simulations (ANSYS, ISE-TCAD)

Feasibility study on fabrication of piezoresistive pressure sensors using silicon micromachining technology

This paper describes a feasibility study on design and fabrication of piezoresistive pressure sensors for the pressure range 0.5 - 350 bar, using silicon micromachining technology. Different technological steps are studied in order to optimize the fabrication process and the electro-mechanical parameters of the device. The sensing membrane is etched in (100)-oriented silicon by anisotropic etching using different concentration of TMAH (tetramethyl anmmnium hydroxide) in water solution. The software package ISE-TCAD. based on the finite element method (FEM), has been used to calculate the stress distribution on the membrane in order to provide information for the proper location of the piezoresistors.Devices with different membrane thickness (between 120 and 40 um) have been investigated. All devices show a good linearity (better than 1%) and their sensibility ranges from 3 to 58 mV/Vbar depending on the membrane thickness