Silicon wet etch

The device was realized by deep reactive ion etching (DRIE) using the SU-8 technique, producing vertical side walls [72-74]. This fabrication route was chosen to avoid crystallization, which is known to occur at sharp channel edges. Using DRIE smooth, curved corners can be realized, unlike by conventional silicon wet etching. 

The steam reformer is a serpentine channel with a channel width of 1000 fim and depth of 230 fim (Figure 15). Four reformers were fabricated per single 100 mm silicon wafer polished on both sides. In the procedure employed to fabricate the reactors, plasma enhanced chemical vapor deposition (PECVD) was used to deposit silicon nitride, an etch stop for a silicon wet etch later in the process, on both sides of the wafer. Next, the desired pattern was transferred to the back of the wafer using photolithography, and the silicon nitride was plasma etched. Potassium hydroxide was then used to etch the exposed silicon to the desired depth. Copper, approximately 33 nm thick, which was used as the reforming catalyst, was then deposited by sputter deposition. The reactor inlet was made by etching a 1 mm hole into the end... 

Micro fabrication was made by conventional silicon wet etching. Sealing was achieved by anodic bonding to Pyrex glass [67]. 

KOH Potassium hydroxide etch, silicon wet etching method... 

A fonn of anisotropic etching that is of some importance is that of orientation-dependent etching, where one particular crystal face is etched at a faster rate than another crystal face. A connnonly used orientation-dependent wet etch for silicon surfaces is a mixture of KOH in water and isopropanol. At approximately 350 K, this etchant has an etch rate of 0.6 pm min for the Si(lOO) plane, 0.1 pm min for the Si(l 10) plane and 0.006 pm miiG for the Si(l 11) plane [24]. These different etch rates can be exploited to yield anisotropically etched surfaces. 

Despite the fact that dry etching techniques have improved dramatically in recent decades, the manufacture of microelectromechanical systems (MEMS) is still a domain of wet etching and silicon electrochemistry. The multiplicity of structures that can be achieved with silicon, together with its excellent mechanical properties [Pe6], have led to an immense variety of micromechanical applications. 

The distribution of impurities over a flat silicon surface can be measured by autoradiography or by scanning the surface using any of the methods appropriate for trace impurity detection (see Trace and residue analysis). Depth measurements can be made by combining any of the above measurements with the repeated removal of thin layers of silicon, either by wet etching, plasma etching, or sputtering. Care must be taken, however, to ensure that the material removal method does not contaminate the silicon surface. 

If skin is placed in a water bath under controlled conditions [14] the primary barrier to transdermal delivery, the epidermal membrane comprising the stratum corneum and viable epidermis, can be readily removed and used to analyze the penetration and diffusion of materials. Figure 18.3a and Figure 18.3b show the appearance of human breast epidermal membrane, with epidermis facing uppermost, following application of the cylindrical dry-etch and pyramidal wet-etch silicon microneedles, respectively. In each case the microneedles are clearly shown to pierce the stratum corneum and viable epidermis to facilitate controlled access of molecules to the target region of skin. 

FIGURE 18.2 Scanning electron micrographs of silicon microneedles, (a) Silicon microneedles micro-fabricated using a modified form of the BOSCH deep reactive ion etching process. The microfabrication process was accomplished at CCLRC Rutherford Appleton Laboratory (Chilton, Didcot, Oxon, UK). The wafer was prepared at the Cardiff School of Engineering, Cardiff University, UK. Bar = 100 pm (b-d) platinum-coated silicon microneedles prepared using a wet-etch microfabrication process performed at the Tyndall National Institute, Cork, Ireland. Bar = 1 mm (b), 100 pm (c,d). 

FIGURE 18.3 Scanning electron micrographs of epidermal membrane treated with dry-etch and wet-etch silicon microneedles. The epidermal membrane, consisting of stratum corneum and viable epidermis, was obtained by heat separation of full-thickness human breast skin. The tissue was immersed in distilled water preheated to 60°C for 60 s and the upper layers carefully peeled off from the dermal layer using tweezers. Epidermal membranes were treated with microneedles for 30 s at an approximate pressure of 2 kg/cm2. (a) Dry-etch microneedle-treated epidermal membrane. Bar = 200 pm (b) wet-etch microneedle-treated epidermal membrane. Bar = 500 pm. 

Tanaka et al. [73] developed another MEMS system for the catalytic combustion of butane. It is composed of a combustion chamber 8 mm wide, 14 mm long and 150 pm deep which was prepared by anisotropic wet etching of a silicon substrate. The substrate was then covered with Pyrex glass applying anodic bonding. Combustion was performed on a platinum/titania catalyst... 

A negative photoresist, SU-8 (Microchem), was used in the microreactor mold process for preparing the PDSM-E microreactors. When exposed to ultraviolet light, material may be removed via a wet etching process leaving high-definition features in micrometer dimensions. Additionally, a microreactor has been constructed in silicon onto which layer-bylayer self-assembled polyelectrolytes and enzymes are deposited. This system is being used for comparison with the PDMS-E system performance. 

What are the differences between isotropic and anisotropic wet etch of silicon (2 marks)... 

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