5. How are e-beam resists exposed, and how can the optimum exposure dose be determined?
With the use of very short-wavelength electrons for resist exposure, an excellent resolution of up to 2 nm can be achieved (single spot exposure). The exposure is performed with standard equipments for electron beam lithography and is based on the principle of direct writing or raster scan shaped beam procedures. For mix&match technologies with resists of the AR 7000 product line, additional exposure procedures can be realised with (i-, g-line) steppers or contact developers in the respective spectral UV
Values for the photosensitivity as given in our product information are guideline values which we determined in our product-specific standard tests. Every user however employs different procedures and thus has to determine the optimum exposure dose in own tests. With respect to sensitivity, already considerably high differences exist between e.g. silicon wafers and mask blanks (PMMA mask: 15 μC/cm2 PMMA wafer 80 μC/cm2).
For positive PMMA resists, the minimum exposure dose (dose to clear) which is required to develop a large area without structures in a suitable development time (depending on the film thickness, for 0.5 μm: 30–60 s) should be increased by 10 – 20 % for structural imaging. For negative resists, the time for complete development of unexposed areas of 0.5 μm is also about 30 – 40 s. The exposure dose which produces a film of > 90 % of initial resist thickness should similarly be increased by 10 – 20 % for patterning. If a shorter dose to clear is chosen (stronger developer), the sensitivity will decrease, since more crosslinking is required (higher dose).