Negative resist contrast

The impact on negative-CA resists of airborne base contamination differs qualitatively from their positive tone counterparts. Suppression of acid-catalyzed chemistry at the surface of a negative resist results in some film erosion at the top of the exposed fields and in some cases an apparent loss of photosensitivity, but in general the reUef images formed exhibit the expected cross-sectional profile. This is in sharp contrast with the typical behavior seen with positive-tone CA resists, where suppression of acid-catalyzed chemistry at the surface causes an insoluble surface skin. 

Negative resists generally exhibit high sensitivity but low contrast. For instp.ncin our laboratory, polymers containing thiirane groups / g v ere found to be extremely sensitive (o = 6 x 10 T C/cm2) to electron beam irradiation at a 20 kV... 

Figure 2 Typical lithographic response (contrast) curves for (a) positive and (b) negative resists.

Contrast curves were obtained for each resist by measuring the thickness after development of a series of 1 mm by 5 mm exposed areas the exposure dose typically varied from approximately 1 mJ/ cm2 to several J/cm2 for the slowest resists. The majority of the resists were developed in ethyl acetate for 30 to 60 sec followed by a 20-sec rinse in 2-propanol. Initially, THF or a THF/2-propanol mixture was used as the developer they were replaced by ethyl acetate because it provided superior contrast. Resist sensitivity was taken to be the incident dose which resulted in 50% exposed thickness remaining after development, Dg 5. This is the standard convention for a negative resist. 

Figure 2. A typical sensitivity curve for a negative resist showing the relationship between the gel dose (D ), the sensitivity (Q), and the contrast (7).

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. 

Sensitivity and contrast are conveniently measured experimentally by exposing areas of resist of known size to varying radiation doses and measuring the film thickness remaining after development for each area. In the case of negative resists, gel is not formed until a critical dose, denoted as the interface gel dose Dp, has been reached. At this dose no lithographi-... 

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... 

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).

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. 

Although mono-dispersed PSt acts as a negative resist with the highest contrast known, it exhibits low sensitivity (30). 

In this paper, optimum design method for poly(chloromethylstyrene-co-2-vinylnaphthalene) is presented, based on theoretical analysis for copolymer sensitivity and on the dry etch rate dependence on polymer structure obtained by a series of experiments. 2-vinylnaphthalene was selected as a main constituent of the polymer, as poly(2-vinylnaphthalene) was found to be a negative resist with high contrast and high dry etch resistance ( 4 ). 

As for quantitative prediction concerning crosslinking negative resist resolution, no theories are available except relationships between polydispersivity (Mw / Mn) and contrast (JT). Charlesby showed... 

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