Influence of development parameters on electron resist contrast

A 3D micro/nano structuring is finding more and more applications in different fields of since at last time. One of the 3D micro/nano structuring method is e-beam lithography. Measurement of electron resist characteristics especially contrast is very important for 3D structure fabrication. For this purposes the new express method for contrast determination was developed. This work is devoted to application of the method to investigation of contrast dependence on development conditions. It is based on an expression for resist development velocity: nu = nu₀ (D/D₀)^gamma where v is development rate of the electron resist exposed with dose D, nu₀ is the development rate of the electron resist exposed with dose D₀ (resist sensitivity), and gamma is a contrast. The main feature of the method is the specially designed 3D test structure "dose wedge" in detail described. Three different profiles can, generally, be revealed after the development of the exposed "dose wedge". As shown, when gamma is higher than the real resist contrast gamma_exp the profile is concave, when gamma < gamma_exp the profile is convex, and when gamma and gamma_exp are equal the profile is plane. Therefore, when gamma = gamma_exp the change in the residual resist thickness is linear. The real test structures consist of several dose wedges. The article shows the results of the test structure development of a PMMA 950 K. resist exposed by a JEOL 840 A under NanoMaker hardware/software system control at 25 kV. The exposure data were designed using the software part of the NanoMaker. When the residual resist thickness grows linearly, the distance between neighboring intensity minima or maxima is constant. In the case gamma > gamma_exp, the distance decreases with a thickness increment and, vice versa, if gamma < gamma_exp the distance between minima or maxima increases with a thic- kness increment. For the test structure, the value of the resist contrast is 7. The considered method is similar to approach developed for measurements of proximity function parameters. The main advantage of the method in comparison with others is it speed (design, exposure, development and observation take typically 30 minutes). So this method gives possibility to investigate resist contrast as function of different lithography parameters for acceptable time. The results of the investigations can help to find ways for resist contrast manipulating. As example of the method application the contrast of the PMMA 950 K. for two development parameters were investigated. First, the method was used for the investigation of the PMMA 950 K. temperature contrast dependence. The test structures were exposed at 25 kV in the PMMA 950 K film and then they were developed in IPA-H₂O 8:1 for different temperatures. A significant influence of the developer temperature was observed, and as shown in fig 3a, contrast decreases in two times when temperature changes from 21.5degC to 37.5degC. Second, the contrast for four volume ratio of the developer components IPA and H₂O was found and graphed in fig. 3 b. So developer temperature and the developer staff have material effect on resist contrast value and these factors could be now quantitatively used for development control and optimization.