Sensitivity resist

The response of crosslinkers 1-4 to pTSA catalysis as a function of acid concentration is shown in Figure 2 for a 75°C/1 minute softbake and 105°C/1 minute hardbake cycle. The concentration of acid required to crosslink these films to a given LP is an indication of the relative resist sensitivities, while the steepness of the LP curves reflects resist contrast. Crosslinkers 3 and 4 are about twice as sensitive to pTSA catalysis as 1, while 2 requires a higher concentration of pTSA for crosslinking. Furthermore, the steepness of the curve for 3 suggests that it would show higher contrast in a resist formulation. 

Figure 8 shows the effect of the alkaline concentration in TMAH solutions on the contrast and sensitivity of the new resist. Sensitivity of the resist increases as the alkaline concentration increases, however, the contrast is maxima (4.72) at 0.83% TMAH solution. This means that the higher concentration over 0.83% cannot distinguish the difference of the dissolution rate between the unexposed and exposed resist film. For instance, the higher concentrated developer also attacks the exposed areas and the loss of resist thickness occurs. The alkaline concentration in TMAH solution, therefore, is optimized at 0.83%. This developer concentration was subjected to the following lithographic evaluation. 

Optimization of both resist sensitivity and contrast requires a fundamental appreciation of the radiation chemistry in addition to appreciation how polymer molecular parameters affect the lithographic behavior of the resist. The intent of this chapter is to further the readers understanding of the polymer and radiation chemistry that is associated with a large part of the microelectronics industry, and provide some of the necessary background to effect future developments. 

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. 

The substrate must be transparent to the source radiation in the range of maximum resist sensitivity ... 

The measurement of intrinsic radiation sensitivity of various materials (defined by 0, Gis) or G(x)), in one laboratory correlates well with meas-urenients made in other laboratories. Measurement of lithographic sensitivity, on the other hand, is not nearly as precise. The literature is pervaded by papers describing resist sensitivity simply in terms of dose per unit area without the relevant experimental details. Interpretation of such results and their utility in comparing one resist with another demands extreme caution. 

Resist sensitivity expressed in terms of dose per unit area is like an EPA mileage rating and at best should be used for comparison only. 

When reporting the sensitivity of a resist, all of the parameters used in the processing should be stated and an exposure curve given. Unfortunately such detail is often omitted in the literature, making comparison of resist sensitivities difficult if not impossible. 

MLR systems offer many advantages in optical, e-beam, x-ray, and ion-beam lithography. An advantage common to all imaging methods is in enhancement of resist sensitivity. As the resolution and the aspect ratio requirements are separated in an MLR system, faster resists that are usable only for low aspect ratio images can now be candidates for the top layer. Other advantages of MLR systems differ from one imaging method to the other. They will be discussed separately. 

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