Etching isotropic

Figure C2.13.5. Schematic illustrations of isotropic etching by a neutral gas and anisotropic plasma etching.

Eig. 8. Schematic of etching directionaHty showing (a) isotropic etch, and (b) anisotropic etch. 

Fig. 2.72a-c Construction of buried micro-channels by combined anisotropic and isotropic etching. Reprinted from Kandlikar and Grande (2002) with permission... 

Microfabrication is achieved by photolithography and isotropic etching of glass using HF [4—13]. Thermal bonding serves for interconnection. Holes are drilled in the top plate for connection to the fluidic peripherals. 

Isotropic etching (i.e. etching of bulk material with equal etch rates in all material/crystal directions). 

Fig. 5.72 Underetching of etch mask during isotropic etching [2].

The high selectivity of wet etchants for different materials, e.g. Al, Si, SiOz and Si3N4, is indispensable in semiconductor manufacturing today. The combination of photolithographic patterning and anisotropic as well as isotropic etching of silicon led to a multitude of applications in the fabrication of microelectromechanical systems (MEMS). 

Acidic silicon etchants are mainly used for two purposes for the delineation of crystal defects, as discussed in Section 2.5, or to remove silicon in an isotropic manner. Isotropic etching adds another degree of freedom to the design of micromechanical structures, because all alkaline etches are anisotropic. Most isotropic etchants for silicon were developed in the early days of silicon crystal technology and exhaustive reviews on this topic are available [Tu3, Rul]. A brief summary is given below. 

Once the resist has been patterned, the selected regions of material not protected by photoresist are removed by specially designed etchants, creating the resonator pattern in the wafer. Several isotropic (etching occurs in all directions at the same etch rate) and anisotropic (directional) etching processes are available. Wet chemical etching is an isotropic process that... 

C.I. Silicon and Poly silicon. The isotropic etching of silicon (Si) and polycrystalline silicon (Poly-Si) by atomic fluorine (F) is probably the most completely understood of all etch processes, particularly for the cases in which F atoms are produced in discharges of F2 (36) and CF4/O2 (42). Fluorine atoms etch (100) Si at a rate (A/min) given by (36) ... 

Figure 15. Isotropic etching a film (thickness = h) showing undercutting 6c) of the mask. Overetching from 0% (x/h ) to 300% (x/h 4) results in profiles which appear more anisotropic.

Figure 7. Metal lift-off process using a trilevel-resist scheme, (a and b) The image created in the top-layer resist is transferred via the isolation layer to the bottom planarizing layer by an isotropic etch, (c) The sloped side wall of the planarizing layer has an overhanging transfer layer that breaks up the continuity of the metal film sputter deposited onto the system. (d) Subsequent dissolution of the bottom layer carries off parts of the metal film adhering to the resist layers, and well-defined metal lines are left.

Heavily doped (>1018/cm3) n-type Si and poly-Si etch faster in Cl- and F-containing plasmas than do their boron-doped or undoped counterparts (103a, 105, 111, 112). Because ion bombardment is apparently not required in these cases, isotropic etch profiles (undercutting) in n + poly-Si etching often occur. Although the exact mechanisms behind these observations are not completely understood, enhanced chemisorption (103b, 111) and space charge effects on reactant diffusion (112) have been proposed. 

The simplest and perhaps the most useful measure of anisotropy is the ratio of lateral or horizontal undercut distance (dh) to the vertical etch distance (dy). This ratio is inversely related to the quality of replication. In anisotropic etching, djdv = 0, and exact dimensional transfer is achieved. A low-quality transfer is obtained with isotropic etching, in which djdv = 1. Anisotropic etching is imperative for high-density-device fabrication. 

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