Pure Tetramethylammonium Hydroxide Research Articles

Combining Isotope Dilution and Standard Addition-Elemental Analysis in Complex Samples.

A new method combining isotope dilution mass spectrometry (IDMS) and standard addition has been developed to determine the mass fractions w of different elements in complex matrices: (a) silicon in aqueous tetramethylammonium hydroxide (TMAH), (b) sulfur in biodiesel fuel, and (c) iron bound to transferrin in human serum. All measurements were carried out using inductively coupled plasma mass spectrometry (ICP–MS). The method requires the gravimetric preparation of several blends (bi)—each consisting of roughly the same masses (mx,i) of the sample solution (x) and my,i of a spike solution (y) plus different masses (mz,i) of a reference solution (z). Only these masses and the isotope ratios (Rb,i) in the blends and reference and spike solutions have to be measured. The derivation of the underlying equations based on linear regression is presented and compared to a related concept reported by Pagliano and Meija. The uncertainties achievable, e.g., in the case of the Si blank in extremely pure TMAH of urel (w(Si)) = 90% (linear regression method, this work) and urel (w(Si)) = 150% (the method reported by Pagliano and Meija) seem to suggest better applicability of the new method in practical use due to the higher robustness of regression analysis.

Effective improvement in the etching characteristics of Si{110} in low concentration TMAH solution

An aqueous tetramethylammonium hydroxide (TMAH) solution is widely used for silicon wet anisotropic etching to perform bulk micromachining for the fabrication of microstructures on a silicon wafer. To reduce the etching time to increase the productivity, etchant must provide high etch rate. In the present work, the etching characteristics of Si{110} in low concentration TMAH (5 wt%) with the addition of various concentrations (5–20%) of reducing agent hydroxylamine (NH2OH) have been studied to increase the etch rate of Si{110} to reduce the etch time for the fabrication of microstructures. Moreover, it is aimed to enhance the undercutting at convex corner for the fast release of the structure. The etch rate of Si{110} and the undercutting at convex corners with the addition of NH2OH increases by more than three times that in pure TMAH. In addition to the etch rate and undercutting, the effect of NH2OH on etched surface morphology is investigated systematically. The present study is focused to enhance the application of wet etching in silicon micromachining for the fabrication of various kinds of microstructures for applications in microelectromechanical systems.

Silicon Anisotropic Etching in Ternary Solution Composed of TΜΑΗ+Triton+NH2ΟΗ

Present work reports the anisotropic etching characteristics of silicon in various concentrations of tetramethylammonium hydroxide (TMAH) without and with addition of 10% hydroxylamine (NH2OH) and Triton X-100 ranging from ppb to ppm. The etch rate, undercutting at convex corners, and etched surface morphology are investigated in pure TMAH, TMAH+NH2OH, and different compositions of TMAH+NH2OH+Triton (i.e. ternary solution). High etch rate and undercutting are achieved in NH2OH-added 15 wt% TMAH, while 25 wt% TMAH + 10% NH2OH + 1 ppm Triton provided significantly less corner undercutting with reasonably good etch rate of Si{100}. This work is very useful for engineering applications for the fabrication of microstructures using silicon wet bulk micromachining for MEMS-based devices.

Sharp silicon tips with different aspect ratios in wet etching/DRIE and surfactant-modified TMAH etching

A simple, high yield method of surfactant-modified wet anisotropic etching for the fabrication of sharp silicon tips is proposed in the applications of scanning probe microscope (SPM) and vacuum microelectronics. The formation of this new tip has been explained as the change in the local etch rate brought about by the strong adsorption of surfactant molecules in the apex of a silicon tip. In order to understand this point, the different etch rates of the same orientations tested on the hemispherical silicon (with curvature) and flat samples (no curvature) are investigated, and the adsorption thicknesses (or densities) of surfactants on various silicon surfaces measured by an ellipsometer are obtained. Tips with different aspect ratios (0.8:1 and 6:1) have been successfully fabricated on silicon (100) and (111) by the combination of etching in pure tetramethylammonium hydroxide (TMAH) or deep reactive ion etching (DRIE) and etching in surfactant-modified TMAH solutions, having the advantages of good uniformities, low temperatures, low costs and complementary metal oxide semiconductor (CMOS)-compatible. The apex of each resultant tip after the treatment in surfactant-added wet etchants typically can have a radius of curvature of 4–5nm on silicon (100) or even ≤2nm on silicon (111) without any subsequent oxide sharpening process.

Complex three-dimensional structures in Si{1 0 0} using wet bulk micromachining

Complex three-dimensional structures for microelectromechanical systems (MEMS) are fabricated in Si{1 0 0} wafers using wet bulk micromachining. The structures are divided into two categories: fixed and freestanding. The fabrication processes for both types utilize single wafers with sequentially deposited nitride and oxide layers, local oxidation of silicon (LOCOS) and two steps of wet anisotropic etching. The fixed structures contain perfectly sharp edges. Thermally deposited oxide is used as the material for the freestanding structures. Wet etching is performed in tetramethyl ammonium hydroxide (TMAH) with and without Triton X-100 (C14H22O(C2H4O)n, n = 9–10). For the fixed structures, both etching steps are performed either in 25 wt% TMAH + Triton or pure TMAH or both, depending upon the type of the structures. In the case of freestanding systems, TMAH + Triton is utilized first, followed by pure TMAH. The fabrication methods enable densely arrayed structures, allowing the manufacture of corrugated diaphragms, compact size liquid (or gas) flow delivery systems, newly shaped mold for soft MEMS structures (e.g. PDMS (polydimethylsiloxane)) and other applications. The present research is an approach to fabricate advanced MEMS structures, extending the range of 3D structures fabricated by silicon anisotropic etching.

Various shapes of silicon freestanding microfluidic channels and microstructures in one-step lithography

In this research, we have developed and demonstrated a fabrication method for the formation of various shapes of silicon freestanding microfluidic channels and microstructures in one-step photolithography. The fabrication process utilizes the silicon direct wafer bonding with silicon nitride as an intermediate layer, local oxidation of the silicon (LOCOS) process and wet anisotropic etching. Two different types of etchants (non-ionic surfactant (Triton-X-100) added and pure 25 wt% TMAH solutions) are used in series to perform silicon anisotropic etching. Surfactant-added tetramethyl ammonium hydroxide (TMAH) is employed to define the shapes of the structures, while pure TMAH is used to get high undercutting for their fast releasing. The non-ionic surfactant is preferred considering the complementary metal-oxide semiconductor (CMOS) post process issue of wet anisotropic etching. The undercutting at sharp and rounded concave corners, edges aligned along ⟨1 0 0⟩ directions, is measured and analyzed in both pure and surfactant-added TMAH solutions. Mask design issues that must be taken into consideration for the fabrication of desired shape and size structures are also presented.

Study of corner compensating structures and fabrication of various shapes of MEMS structures in pure and surfactant added TMAH

In this research, we have studied the various shapes of corner compensation structures such as triangles, squares as well as 〈1 1 0〉 and <1 0 0> oriented beams for use with pure and surfactant added tetramethyl ammonium hydroxide (TMAH) solutions. The etchant concentrations of 10%, 20% and 25% were used with and without a small amount of surfactant NC-200 (0.1% of the total volume of the etchant). This surfactant contains 100% polyoxyethylene-alkyl-phenyl-ether with a hydrophobic head ‘CH 3(CH 2) 8–C 6H 4’ and a hydrophilic tail ‘O(CH 2CH 2O) 20H’. As the etching characteristics (such as etch rate, convex corner undercutting and etching anisotropy) of pure TMAH change dramatically when a small amount of surfactant is added, all the designs proposed for pure TMAH (or KOH) cannot be adopted for surfactant added TMAH. Among all the compensating designs, the triangular shaped geometry is found to be the optimal choice for the surfactant added 25% TMAH solution. The optimized design is successfully used for demonstrating the fabrication of grooves for chip isolation and mesa structures surrounded by bent V-grooves where limited space is available for the compensating pattern. Furthermore, the NC-200 added 25% TMAH was also used to fabricate microfluidic channels, microstructures with rounded concave and sharp convex corners, and a 45° mirror with smooth sidewalls.

Study of rounded concave and sharp edge convex corners undercutting in CMOS compatible anisotropic etchants

In this paper, we have studied the undercutting at rounded concave and sharp convex corners in (1 0 0)-silicon wafers using a complementary metal-oxide semiconductor (CMOS) compatible tetramethyl ammonium hydroxide (TMAH) solution with and without surfactant. In order to minimize the undercutting at both corner types while keeping reasonable etch rates, smooth etched-surfaces and CMOS compatibility, the non-ionic surfactant NC-200 that contains 100% polyoxyethylene-alkyl-phenyl-ether is considered. The effect of concentration and etching temperature is studied using 10, 20 and 25 wt% TMAH solutions at 60, 70 and 80 °C. When NC-200 at 0.1% of the total volume of the etchant is used, the undercutting ratio at both rounded concave and sharp convex corners is beneficially reduced as the etchant concentration is increased while, simultaneously, the etch rate increases. This is the opposite trend to the etch characteristics of pure TMAH. In addition, the rough etched surface morphology at low concentration is also improved by using NC-200.

Anisotropic etching and electrochemical etch-stop properties of silicon in TMAH:IPA:pyrazine solutions

Anisotropic etching and electrochemical etch-stop properties of single-crystal silicon in tetramethylammonium hydroxide (TMAH): isopropyl alcohol (IPA) solutions containing pyrazine were investigated. With the addition of IPA to TMAH solution, an improvement in flatness on the etching front and a reduction in undercutting were observed, but the etch rate on (100) silicon decreased. However, the (100) silicon etch rate is improved by the addition of pyrazine. An etch rate of 0.8 μm/min on (100) silicon, which is faster by 13% than a 20 wt.% solution of pure TMAH, is obtained using 20 wt.% TMAH:0.005g/ml pyrazine solutions, but the etch rate on (100) silicon is decreased if there is an increase in the amount of pyrazine in the solution. With the addition of pyrazine to a 25 wt.% TMAH solution, no significant variations in flatness on the etching front were observed and the undercutting ratio was reduced by 30∼50%. The addition of pyrazine to TMAH:IPA solutions increased the etch rate of (100) silicon, thus the time required by the etch-stop process was shortened. The current-voltage (I–V) characteristics of n- and p-type silicon in TMAH:IPA:pyrazine solutions were obtained. The passivation potential (PP) of n- and p-type silicon was obtained and the applied potential was selected between n- and p-type silicon PPs. The electrochemical etch-stop method was applied to the fabrication of 801 silicon microdiaphragms of 20 μm thickness on a 5-inch silicon wafer. The average thickness of fabricated 801 microdiaphragms on one wafer was 20.03 μm and the standard deviation was ±0.26 μm. The silicon surface of the etch-stopped microdiaphragms was extremely flat with no noticeable taper or non-uniformity. The benefits of the electrochemical etch-stop method for the anisotropic etching of silicon in TMAH:IPA:pyrazine solutions become apparent when reproducibility of silicon microdiaphragm thickness for mass production is realized. The results indicate that use of the electrochemical etch-stop method for the anisotropic etching of silicon in TMAH:IPA:pyrazine solutions provides a powerful and versatile alternative process for fabricating high-yield silicon microdiaphragms.

TMAH/IPA anisotropic etching characteristics

Abstract The main advantages of tetramethyl ammonium hydroxide (TMAH)-based solutions is their full compatibility with IC technologies. In this work a new etching system of TMAH/IPA (isopropyl alcohol) is suggested. The influence of the addition of IPA to TMAH solutions on their etching characteristics is presented. The etch rates of (100) oriented silicon crystal planes decreases linearly with decreasing the IPA concentration for all experimental conditions. Etch rates for TMAH/IPA solutions (10–45 μm/h) are lower than those for KOH/IPA solutions (30–100 μm/h) but they are still applicable for micromachining purposes. The etch rates of most commonly used masking layers in IC technologies has been investigated. Low-pressure chemical vapour deposited (LPCVD) Si 3 N 4 and thermally-grown SiO 2 have excellent stability in TMAH/IPA solutions. Low-temperature deposited silicon oxide (LTO) etch rates are low enough to be used as masking layers in anisotropic etching processes. Quality of etched surfaces is mainly dependent on TMAH wt.% concentration. For pure TMAH solutions the observed undercutting ratio (5–7) is much larger than in KOH case. The addition of IPA to TMAH solutions reduce the undercutting by a factor of more than 2 and leads to smoother surfaces of sidewalls etched planes. We have studied briefly the p ++ etch-stop characteristics by means of heavily boron-implanted layers. The etching selectivity with respect to high boron-doped silicon is improved in TMAH/IPA solutions. Implant doses used in our experiments (2 × 10 16 ion/cm 2 ) stands the etching during more than 90 min.

水酸化テトラメチルアンモニウム

Tetramethylammonium hydroxide (TMAH) is a relatively strong base among organic compounds, and decomposes at 130-140°C into gaseous substances (Me3N, Me2O, MeOH, and H2O). TMAH has been prepared by alkali treatment of tetramethylammonium chloride (TMAC) obtainable from trimethylamine and methyl chloride, but use is made recently of developer electrolysis of TMAC in the presence of ion exchange membrane. The latter method gives rise to a highly pure TMAH aqueous solution free from chloride anions. TMAH currently finds some utilities in the production of highly functional materials such as developer for positive-working photoresist and the surface treatment of fibers, glasses, and plastics, and also as reagent for the methyl esterification of fatty acids and phospholipids, reagent for organic synthesis and preparation of catalysts, etc.