Isotropic etch profiles

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. 

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. 

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...

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.

Plasma etching. (PE) Physical-chemical etching with free radicals, supported by ions. Etching profile anisotropic-isotropic, good selectivity. 

Since doping affects the etch rate of polysilicon, we investigated the etch rate characteristics of each gas used in the etch process (Table 1). We discovered that, the polysilicon etch rate with SF6 was independent of doping effects. To minimize the isotropic etch characteristics of SF6 in the B.T step(3), we used HBr, which is a well known polymer forming gas, with a SF6 HBr ratio of 1 0.75. With this new process, vertical profiles were obtained after the B.T step, in both types of polysilicon. At the end of the B.T step, the remaining polysilicon thickness in the n-type was comparable to the p-type and was less than the amount of polysilicon after the B.T step In the conventional process (Fig. 2-a 2-b). Also, the later the EP in the M.E step, the thinner was the remaining polysilicon in the p-type, thus... 

Dry-etching techniques, in general, are methods by which a solid state surface is etched physically by ion bombardment or chemically by a chemical reaction with a reactive species at the surface or combined physical and chemical mechanisms. Under chemical methods, one distinguishes between wet etching (solvent, vapor, electrochemical) and dry etching in the gas phase. Depending on the mechanism, isotropic or anisotropic (directional) etch profiles are obtained. 

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