Etch factor

EMM can be applied to either Al films, typically deposited on SiCVSi substrates, or Al foils. The dimensions of metallic features are determined by the same rules the etch factor, the depth of porous-type anodization, the mask design, and process conditions. In addition to the fabrication of metallic microstructures, EMM can be used to produce microstructured ceramic substrates composed of porous AI2O3. For the fabrication of both types of 3D microstructures by localized porous-type anodization, the following technological problems have to be addressed the reliability of a mask material, the fidelity of the mask transfer, volumetric expansion of porous AI2O3 during anodization, and the effect of the mask design on the rate of porous-type anodization and on the completion of anodization of the entire thickness of Al without traces of Al islands. 

In etching technology, where a surface resist stencil is produced from photoresist imaging, screen stencil, or maskant cutting, chemical attack can occur into the body of the substrate, and once a sidewall has been formed, the etchant can attack beneath the stencil to form what is known as undercut (U) where U = 0.5 (B-A) (see Fig. 2). Normally the etch factor (DAJ) is required to be as high as possible so that smaller holes can be etched into thicker materials. 

Etch time, S Undercut, U mil Etch factor, F Extent of etch, R/B... 

There is an important conclusion to be drawn from these data. In order to compare or characterize an etch process (etcher to etcher, etchant to etchant, condition to condition, etc.), it is not sufficient to only use the undercut or etch factor alone. The data must be evaluated at a given R/B point for the same foil, resist, artwork, and panel size. If these factors are not held constant, the statement that process A gives less undercut has little meaning— additional etching gives more undercutting and straighter side walk. 

Limitations—Practical Rule of Thumb. It is useful to have a practical limit to express an understanding of where the technologies may limit performance. In the case of etched linewidth, it has been expressed that the total of the resist thickness pins the etched foil thickness wonld limit the gap between the etched features. Therefore, for 1.2-mil dry-film resist over 1-oz copper foil, the total thickness is 2.6 mil. This conld be nsed as a practical limit for both the trace and gap. Further limits can be determined by the nndercnt and etch factor experienced for the same type of etched featnres. Therefore, nsing the R/B = 1 data from Table 34.2 (17= 0.525 mil), a 2.6-mil resist line wonld be 2.6 mil at the trace bottom and would have a 1.5-mil etched top reduction, leaving only a 1.1-mil top snrface. It mnst then be determined if these dimensions (with allowance for variations) are snfficient for the design functionality. 

There are appropriate correction factors for the etching rates of each combination. Between conductors, patterns, thickness, and etching, solutions should be introduced for the photo masks, based on the actual trials, especially for the multiple parallel fine lines very common to high density flexible circuits. Usually, there are remarkable differences in etching factors between middle conductors and edge conductors, as shown in Fig. 63.13. 

A set of dummy conductor patterns helps to minimize the difference of the etching factors. 

FIGURE 63.13 Pattern correction for the etching factors (a) Unacceptable (b) Preferred. 

ETCH factor The ratio of the depth of etch to lateral etch. 

Undercuts always present. The etch factor for a material is the ratio of the etched depth to the size of undercut. 

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 +... 

Figure 29. Dependence of etch factor v,/v on etchant composition at 20°C and on etching temperature for 3HCI + 1H3PO4 etchant. Based on the data of Ref. (111).

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