Anisotropic KOH Etching

KOH is one of the most commonly used silicon etch chemistry for micromachining silicon wafers. 

The (110) plane is the fastest etching surface. The (111) plane is an extremely slow etching plane that is tightly packed and is overall atomically flat. 

Etchants Temperature (°C) Direction (plane) Etch rate (pm min ) 

the opening of the etching material depends on the etching angle. Thus, the size of exposed surface L needs to be accordingly decided. Here, 

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A silicon wafer with anisotropically KOH-etched openings was used as shadow mask. The shadow mask is accurately positioned with the help of an optical microscope and fixed using a custom-made wafer holder. A 50-nm-thick TiW-film is deposited by sputtering through the shadow mask. This film serves as adhesion layer and diffusion barrier and covers the rough surface of the CMOS-Al-metallization. A Pt-layer with a thickness of 100 nm was sputtered on top of this TiW-layer. 

In Figure 2.2, anisotropic KOH etch on <100> Si will produce a rectangular cross section, if the channel is 45° relative to the wafer flat (A-A, B-B, and... 

Alkaline etchants are anisotropic. The etch rate for the (111) crystal planes of the Si crystal is smaller by about two orders of magnitude than the etch rate of any other crystal plane. The etch rate ratio between other crystal planes like (100) and (110) depends on etchant concentration and temperature, but doesn t usually exceed a factor of two [Sa6]. Addition of oxidizing agents reduces the anisotropy. The etch rate of (100) Si and Si02 in KOH at different temperatures is shown in Fig. 2.2. 

To create liquid access, holes were punched through the replica. Then it was placed on another thin slab of PDMS for sealing. Because an anisotropic KOH-based etching method was employed to etch the Si master without corner compensation, the channel intersections in the replica were limited in shape by the <111> plane of the Si master [159]. 

Combining DRIE and anisotropic wet etching can produce unique structures. For example, (111) silicon cannot be etched by the standard KOH, TMAH, and EDP etchants, but if an initial trench has been etched by DRIE, fast-etching crystal planes are exposed, and wet etching can proceed. If this is combined with spacer structures to protect sidewalls and another DRIE step, (111) free-standing silicon structures can be made. 

Fins in the silicon are fabricated in a variety of ways. Argonne National Laboratory uses electrical discharge machining (EDM). MESA+ Research Institute, University of Twente, The Netherlands, uses a KOH etch of the silicon to form the channels [33]. Others use an anisotropical etch of KOH... 

Anisotropic wet etching of single crystal Si using KOH. The surface of the wafer is a 100 plane. 

The mass spectrometer is assembled from four wafers, three silicon and one glass wafer, hermetically sealed by anodic bonding The geometries of the orifice and the plasma electrodes are fabricated via anisotropic etching of Si by KOH The plasma chamber for this first approach was drilled into the Pyrex wafer For the electrodes of the mass separator a deep anisotropic plasma etching process is used by which perpendicular trenches more than 200 im deep with micrometer tolerances can be generated The electrodes are metallized for a defined potential within the separator... 

Table 10.7 shows that the KOH etch rate is strongly affected by the crystallography orientation of the silicon (anisotropic). 

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. 

Step 7 Anisotropic etching in 30% KOH solution for 10 minutes to foim 13-20pm deep chamber on backside... 

Step 9 KOH anisotropic etching for 3hr and 40 minutes until only 60pm silicon left... 

Etch rate and homogeneity and anisotropic characteristics are the predominant factors in determining the resulting micro system device properties. Temperature and concentration of the KOH solution as well as the doping concentration of the silicon material have the largest impact on these properties and have to be thoroughly controlled. 

For good aspect ratio and device geometry, anisotropic etching agents have to exhibit a strong difference in etch rate between crystal directions. In a typical KOH solution for single crystal Si, about a two orders of magnitude smaller etch rate in... 

The membranes of the microhotplates were released by anisotropic, wet-chemical etching in KOH. In order to fabricate defined Si-islands that serve as heat spreaders of the microhotplate, an electrochemical etch stop (ECE) technique using a 4-electrode configuration was applied [109]. ECE on fully processed CMOS wafers requires, that aU reticles on the wafers are electrically interconnected to provide distributed biasing to the n-well regions and the substrate from two contact pads [1 lOj. The formation of the contact pads and the reticle interconnection requires a special photolithographic process flow in the CMOS process, but no additional non-standard processes. 

The most characteristic feature of alkaline solution etching is its anisotropic nature [13]. For example, in 20% KOH at room temperature, the etch rate at the OCP of the (100) plane is a hundredfold higher than the (111) plane. In general, because of the anisotropic etching process, the morphology of the surface shows smooth ideal (111) sidewalls thus forming micropyramids on (100) wafers, as shown in Fig. 12. 

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