Sensitivity negative resist dose

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... 

Sensitivity. Poly(2-vinylnaphthalene) is a low sensitive negative electron resist, and Dg-M value for the polymer was estimated as 10 Og /cm2-mole ( 9 ). Froduct Dg Mw is. a constant that characterizes the sensitivity for negative resists, where Dgand Mw denote gel dose and weight average molecular weight, respectively ( 6 ). To attain higher sensitivity, modification or copolymerization with other sensitive monomers is needed. 

So far sensitivity calculations were carried out to estimate DgMw, which is a parameter that characterizes the sensitivity for negative resist materials. Theoretically, gel dose (Dj) can be decreased without limit as molecular weight (Mw) increases. Figure 3 shows predicted sensitivity for P(CMS-2VN) with Mw as a parameter. 

Sensitivity is said to increase as the dose required to produce the lithographic image decreases. The sensitivity of a positive resist, Og, is the dose required to achieve complete solubility of the exposed region under conditions where the unexposed region remains completely insoluble. The sensitivity of a negative resist is conventionally defined as the dose at which 70% of the original film thickness has been retained after development, Oq 7. The required sensitivity varies with the type of irradiation and is expressed as energy/surface (e.g., J/m ). 

Figure 3. Size of clear and opaque 2.0 fim features as a function of exposure dose for a negative electron beam resist. The dose Dp that results in the correct feature size is denoted as the "sensitivity .

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