Positive resists, contrast

For a positive resist, the film thickness of the irradiated region after development decreases until eventually a critical dose Dp is reached which results in complete removal of the film 8,9). The sensitivity and contrast (7p) are evaluated in a manner similar to that for a negative resist. After they have been spin-coated and prebaked, a series of pads of known area are exposed to varying doses. The substrate is developed in a solvent that does not attack the unexposed film and the thickness of the film remaining in the exposed areas measured. The film thickness is normalized to the original thickness, and this value is plotted as function of log dose, as shown in Figure 5 where Dp represents the sensitivity of the positive resist. Contrast (7p) is determined from the extrapolated slope of the linear portion of the response curve as... 

Numerous studies have probed how novolac microstmcture influences resist hthographic properties. In one example, a series of resists were formulated from novolacs prepared with varying feed ratios ofpara-jmeta-cmso. These researchers found that the dissolution rate decreased, and the resist contrast increased, as thepara-jmeta-cmso feed ratio increased (33). Condensation can only occur at the ortho position ofpara-cmso but can occur at both the ortho- and i ra-positions of meta-cmso. It is beheved that increased steric factors and chain rigidity that accompany increasedpara-cmso content modify the polymer solubihty. 

Figure 3 Representative contrast curves for a) positive resists and b)...

Figure 6. Exposure curves of positive resist Hitachi Chemical R1-7000P with and without the Dl-CEL. The contrast (r-value) of the Dl-CEL resist is three times of that of the non-CEL resist.

The resolution capability of a resist is directly related to resist contrast (7) which, for a negative resist, is related to the rate of crosslinked network formation at a constant input dose. It is somewhat more complicated for a positive resist being related to the rate of chain scission and the rate of change of solubility with molecular weight with the latter being markedly solvent dependent. Contrast, like sensitivity, is governed by the type of chemical reactions that occur in the polymeric resist and is affected by molecular parameters such as molecular weight distribution and chemical composition. 

Figure 3. Typical lithographic response or contrast plots for (a) positive resists and (b) negative resists in terms of the developed thickness normalized to initial resist thickness (p) as a function of log (dose).

Poly(p-formyloxystyrene) was also subjected to quantitative sensitivity analysis using a calibrated multidensity resolution mask. A plot of the normalized thickness remaining as a function of dose is provided in Figure 11. The resist exhibits a sensitivity of approximately 70 mJ/cm2 in the deep UV and has a contrast ( ) comparable to that of the classical diazonaphtoquinone-novolac positive resists that are commonly employed in semiconductor manufacturing. 

Electron Beam Microliihography Good sensitivity and contrast, acceptable plasma etching resistance less scattering of electrons aiul therefore better resolution negative and positive resists possible. 

In the PCM systems just described, both top and bottom resists are positive working. Residual exposure of a bottom positive resist during UV or electron beam imaging of a positive top resist is acceptable. However, when a negative resist is used as the top layer, the residual exposure may reduce the contrast of a bottom positive resist. The several PCM systems involving the use of a negative resist as a top layer listed in Table 3.8 (imaging layers 3-6) indicate that if there is a sufiScient sensitivity difference between the top and the bottom resists, the residual exposure can be tolerated. 

M. Kakuchi, S. Sugawara, K. Murase, and K. Matsuyama, Poly (fluoro methacrylate) as highly sensitive, high contrast positive resist, ]. Electrochem. Soc. 124, 1648 (1977). i66j Crosshnked poly(2,2,2 trichloroethyl methacrylate) as a highly sensitive positive elec... 

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