Etching profiles

The most commonly used etching solutions for producing etching profiles on the (001) face of GaAs and InP are mixtures of various 

Like the rate of etching, v , of the wafer in a direction perpendicular to the etching plane, the ratio of the rate of undercutting, v, in the lateral direction and the etch factor v, /v depend on the composition of etching solution as well as on the temperature of etching. The dependence of these rates for the etching of (001) wafer of InP in a mixture of 3HCI + 

Etching kinetics of semiconductors may be diffusion-controlled in two ways (24). In the first case, the reduction reaction is diffusion limited and controls the etching kinetics. Under these conditions a well defined crystallographic facet is obtained. This behavior is observed at low pH. In the second case of high pH, the rate of anodic dissolution of the semiconductor wafer depends on the mass transport of OH ions to the electrode. Electroless etching based on this limitation shows rounded profiles typical of diffusion-controlled dissolution. 

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Etch Profiles. The final profile of a wet etch can be strongly influenced by the crystalline orientation of the semiconductor sample. Many wet etches have different etch rates for various exposed crystal planes. In contrast, several etches are available for specific materials which show Httle dependence on the crystal plane, resulting in a nearly perfect isotropic profile. The different profiles that can be achieved in GaAs etching, as well as InP-based materials, have been discussed (130—132). Similar behavior can be expected for other crystalline semiconductors. It can be important to control the etch profile if a subsequent metallisation step has to pass over the etched step. For reflable metal step coverage it is desirable to have a sloped etched step or at worst a vertical profile. If the profile is re-entrant (concave) then it is possible to have a break in the metal film, causing an open defect. 

Fig. 2.71a-c Bulk micromachining etch profiles. Reprinted from Kandlikar and Grande (2002) with permission... 

The etch rates were measured by a surface profiler and field emission scatming electron microscopy (FESEM), and the etch profile were observed by FESEM. In this study, a C /Ar gas chemistry was chosen to obtain high etch selectivity of Si film to niobium oxide mask since CI2 gas was known to be a good etch gas for Si films. The etch rate, etch selectivity and etch profile of niobium oxide nanopillars and Si films were explored by varying the CI2 concentration, coil RF power and dc bias voltage to substrate. 

Figure 17. The influence of gaseous XeF on the ion beam etching profiles of silicon.

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 important point to recognize is that the etch rate of surfaces subjected to energetic particle bombardment (bottom surfaces) will be larger than the etch rate of surfaces not subjected to this bombardment (sidewalls) because of the ion-assisted (or electron-assisted) gas-surface chemistry. The relationship between the shape of an etched profile and the dependence of the etch rate on ion bombardment is shown in... 

For systems in which is not zero, the shape of the etched profile can be controlled to a certain extent by adjusting the stoichiometry of the discharge. This is illustrated in Fig. 3-7 using an idealized Si sample. As hydrogen is added to a CF. discharge the etch rate of Si will decrease as we have seen earlier in Fig. 3.2. However, the etch rate will stop on surfaces not subjected to ion bombardment (point A in Fig. 3.7) before etching stops on surfaces which are exposed to energetic ion bombardment. This means that the lateral etch rate has been eliminated and features with vertical sidewalls can be etched if an etch gas mixture of CF. — 10% is used in this example. 

Planar or Parallel-Plate Reactor. Because very-large-scale integration (VLSI) demands nearly vertical etch profiles, planar or parallel-plate... 

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. 

Figure 11. Schematic diagram comparing the control of etch profiles by the use of erodible and nonerodible mask materials. (Reproduced with permission from reference 199. Copyright 1980 John Wiley.)...

Glow discharges or plasmas have been used extensively to promote chemical reactions for thin-film etching and deposition in a variety of technologically important areas. The reactive chemical atmosphere and complex discharge-surface interactions in these systems permit the attainment of unique etch profiles and film properties. 

The principal impediment to effective process design and analysis is the limited understanding of synergistic effects due to ion, photon, and electron bombardment of solid surfaces during etching and deposition. Fundamental relationships must be established between the gas-phase chemistry the surface chemistry as modified by radiation and etch profiles, rates, selec-tivities, and film properties. 

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