Anisotropic etching etched feature

Anisotropic etching (i.e. etching of bulk material with etch rates depending on material/crystal orientation, used in single crystal material in order to determine clear features and geometry aspect ratios). 

Figure 14. Etch profiles for isotropic, tapered, and anisotropic etching of a film. Sq, Wq and Sf, Wf represent mask dimensions before etching and feature dimensions after etching, respectively. The degree of undercutting (dfj) and wall taper (6) are indicated for etching to a depth (dy) that exposes just the initial mask dimensions in the substrate. (Reproduced with permission from Ref. 11J...

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. 

Feature Sizes. Although minimum feature sizes in TFML interconnections are large relative to IC feature sizes (i.e., 25- xm versus l- xm line widths), the conductor and dielectric layers are substantially thicker in TFML structures, a fact that results in high aspect ratios. Conductor layers must be several micrometers thick to keep resistive losses low, and dielectric layers must be 10 to 30 xm thick to maintain low interconnection capacitance. Thus a thickness width aspect ratio as large as 1 1 is frequently required. This aspect ratio demands anisotropic-etching processes. 

A four-layer microchip has been constructed to generate total internal reflection (TIR) and an evanescent field (see Figure 7.8). Surface-adhered Nile red-labeled fluorescent microspheres (1 pm) are excited by the evanescent field for fluorescent measurement. An essential feature on the chip was the micromirror that was constructed by depositing Au/Cr on the slanted wall (54.7° due to anisotropic etch of Si). Operation near the critical angle 0C assures strong evanescent intensity [695]. 

FEATURES OF SILICON ANISOTROPIC ETCHING IN AQUEOUS KOH SOLUTIONS... 

Thus, the mechanistic model described above is consistent with the characteristics of the anisotropic etching of silicon in alkaline solutions as well as the isotropic planar etching in HE solutions. It is also a useful simple model for explanation of the etched features and surface roughness as will described in the following sections. 

The surface of the silicon crystal, no matter how it is finished, will have a certain number of lattice defects, which tend to dissolve preferentially resulting in formation of etch pits and other features. Terraces and steps of various sizes are inevitable consequences of anisotropic dissolution of the surfaces misoriented from the (111) surface. Also, a silicon surface, whether initially smooth or not, in HF solutions, has an intrinsic tendency to roughen and form micropores governed by sensitivity of the electrochemical reactions on a semiconductor electrode to surface curvature. Furthermore, the two groups of factors shown in Fig. 7.57 may affect each other. For example, the initial lattice inhomogeneities may provide the sites for deposition whereas localized deposition may enhance the development of etch features such as pits or hillocks. 

Anisotropic etching, that is, different dissolution rates on different crystal planes, is a characteristic feature of silicon etching in alkaline solutions. Strictly speaking, the etch rate of silicon always depends, to a various extent, on crystal orientation in all etching solutions, acidic or alkaline. However, the etch rate difference among different planes is small in acidic HF solutions compared to those in alkaline solutions. Figure 26 shows the etch rate ratios of(100)/(lll) and (110)/(111) planes in various solutions. 

The principle of nanoimprint is quite simple. As shown in Fig. lA, NIL uses a hard mold that contains nanoscale features defined on its surface to emboss into polymer material cast on the wafer substrate under controlled temperature and pressure, thereby creating thickness contrast in the polymer material, which can be further transferred through the resist layer via an O2 plasma-based anisotropic etching process. Nanoimprint has the capability of patterning sub-10 nm features, " yet only entails simple equipment setup and easy processing. This is the key reason that NIL attracted wide attention within only a few years after its inception. 

To create micromachines, films that have been deposited must be patterned and etched to reveal the desired structures. Often, it is important to etch these structures with vertical sidewalls (anisotropic etching). In this case, most pattern transfer operations (lithography and etch) are carried out using plasma etching. Conceptually, this process is the reverse of deposition. The etching process consists of exposure of the patterned and masked substrate to a low-pressure plasma. The reactive species and ions preferentially etch those areas that are not masked, resulting in the definition of features on the surface. The key to plasma etching is that the products of the reaction of the activated gas and the material to be etched must be volatile (see e.g.. Ref ). 

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