Undercutting, mask

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.

Waiting for the natural etch stop means that L , T or U -shaped V-troughs necessary for microreactors cannot be produced. To produce these troughs, therefore, etching is interrupted when the desired depth has been reached. These V-cross-sectional troughs have convex 90° comers at any bends, and at these points there is considerable undercutting of the mask. The remedy is the corner compensation. Sacrificial surfaces are created at the convex corners, calculated in such a way that the etch time in the direction of the convex 90° comer will produce the correct shape. 

Whether or not a chemical process step has been successful is difficult to measure, since there are few on-line measurable electrical properties. For example, film thickness and grain structure of polycrystalline silicon can be measured after a deposition step. However, their effect on device performance might not show up until subsequent doping or patterning steps fail. Similarly, it is possible to measure etch rates on-line by laser interferometry, but the etch profiles must be checked by electron microscopy. Unexpected mask undercutting or undiscovered etch residues can result in subsequent contact and device lifetime problems. 

West et al. [51] simulated shape changes in through-mask electrochemical micromachining. The degree of undercut was predicted for vertictilly-walled cavities of different aspect ratios, assuming primary current distribution. The boundary-element method was used. 

In the first approach to the ECMM, the required localization is achieved by the use of inert photoresist masks on the WP areas, which should not be dissolved. This method has some specific features (for instance, the problem of mask undercutting and island formation) and was considered, for example, in Ref. 91. 

To avoid undercutting, compensating structures must be used, generally simple polygonal extensions of the mask at the convex corners. A large variety of such convex corner-compensating structures have been proposed [22, 23]. 

When using more aggressive acidic etchants, preferential etching does not occur, and rounded isotropic patterns are created. An example is the isotropic etching of silicon under a patterned mask. The etchant moves down and out from an opening in the oxide mask, undercutting the mask and enlarging the etched... 

Neutral species do not have any directionality. They can etch equally well in all directions undercutting the mask (e.g., etching of silicon by fluorine atoms). When ion bombardment is necessary for etching to occur (e.g., etching of undoped silicon in chlorine plasma), anisotropic (vertical) wall profiles can result (Fig. 2, left). The mask pattern can then be faithfully reproduced into the film. There is a host of gases [9-12] that have been used for plasma etching materials encountered in microelectronics. Table 2 provides only a small sample. 

---