Wet-etching process

The critical operation is the etching of the sacrificial layer. With a proper wet etching process, the remaining structural layer shows a high quality surface (Fig. 8). The 10 pm thick sacrificial layer has been etched and the layer stays with a perfect plane shape. Stiffness of the structural layer is also visible in Fig. 8b where the substrate have been cleaved near a structure. Attachment points are 500 pm away and there is no bending of the structure. 

Surface properties can also be changed by removing material from the surface. This is done in various dry and wet etching processes. 

A 5.5 (xm photoresist layer was patterned as the sacrificial layer, followed by the deposition of a second 4.5 p,m parylene layer. The parylene/photoresist/ parylene sandwich structure formed the electrospray nozzle and channel when the photoresist was subsequently dissolved. A 1500 A sputtered aluminum layer was used as a mask for parylene etching to define the shape of the nozzle. Aluminum was removed by a wet etching process. After SU-8 developing, wafers were left inside the SU-8 developer for 2 days to release the photoresist. A serpentine channel (250 pan x 500 pm x 15 mm) extending from the junction of pump channels to the edge of the chip was patterned in the SU-8 layer. Platinum/titanium lines spaced 200 pm apart were patterned under the channel after the electrode deposition step. 

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. 

FIGURE 2.13 Electron micrograph showing the sample injection region of the PDMS device. Channel cross section, 50 pm (width) x 20 pm (height). The geometrical features of the channel junction result from the anisotropy of the wet etching process of the Si master [159]. Reprinted with permission from the American Chemical Society. 

The lift-off process is usually employed to fabricate metal electrodes. This method, as opposed to the wet-etch process, allows the dual-composition electrode to be patterned in a single step [747]. In order to achieve well-defined metal electrodes in a channel recess using the lift-off technique, the metal (Pt/Ta) will not be deposited onto the sidewalls of the photoresist structure (see Figure 2.32). This discontinuity of the deposited metal layer around the sidewalls allows metal on the resist to be removed cleanly from the surface without tearing away from the metal on the surface. Thus negative resists were used because they can be easily processed to produce negatively inclined sidewalls. To achieve this, the photoresist is subjected to underexposure, followed by overdevelopment [141]. 

The detector elements of the imager presented in JP-A-6326342 are separated from each other by providing amorphous material only at regions between the detector elements and removing this amorphous material in a wet etching process. 

The vias are extended down through layers 48, 52 and 53 by using a plasma dry etching process. A wet etching process is then used to smooth the surfaces of the vias. An electrically conductive layer of indium is formed over the photoresist masking layer and within the vias in electrical contact with the region 64 and the input pad 58. The photoresist masking layer and... 

As stated in the introduction, wet etching processes may proceed either with or without external current flow. In the former case, the semiconductor crystal is incorporated as an electrode in an electrochemical cell and polarized anodically under illumination or in darkness for n- and p-type samples, respectively, leading to dissolution of the sample (see Sec. 2). This is referred to as the (photo)anodic etching process. 

Bottle-shaped DT For well controlled engineering of the lateral DT profile a new wet etch process employing diluted NH4OH chemistry was developed... 

Wet etching processes work isotropically, if the material is isotropic or the etching process is... 

Oxidation is also needed in addition to solvation in all etching processes of metals and semiconductors. In most cases, this process is performed without an outer current. This means that the anodic partial process which corresponds to the formation of metal cations is completely compensated by a cathodic partial current, which must be realized by the electrochemical reduction of an oxidizing agent of suitable electrochemical potential. Open-circuit (electroless) wet etching processes lead to the formation of a mixed... 

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