Polymeric materials that are sensitive to electrons, such as poly(methyl methacrylates) (Rai-Choudhury 1997), copolymers of glycidyl methacrylate (Rai-Choudhury 1997) and ethyl methacrylate, and polysulphones (Roberts 1987) have been used as electron beam resists. The first two are referred to as positive tone resists in which the e-beam fragments the polymer backbone leading to an enhanced solubility of the fragmented area relative to the non-fragmented. The polysulphones are referred to as negative tone resists in which the polymer chains are cross-linked by the e-beam, resulting in a lower solubility of the non-irradiated area, relative to the irradiated area. The differential solubilities allows the SiO2 surfaces coated with the polymers to have the e-beam written pattern, to be transferred to the underlying SiO2, after washing off the more soluble component, and plasma or chemically etching away the exposed SiO2. Although PMMA has led to structures of less than 10 nm being created on SiO2 surfaces, there are limitations to its use. These limitations include the low etch durability of PMMA (i.e. the etchants that etch the SiO2 also etch the PMMA at a similar rate) (Fujita 1996). This limitation can be overcome by the use of high etch durability polymeric composite resists such as SAL-601 (Dobisz 1998). However, this polymer composite has poor lateral resolution leading to structures of ~60 nm (Dobisz 1997). In addition polymeric materials by virtue of their large size, relative to molecular materials, will limit their own ultimate resolution to the size of the polymers that are used (Tada 1996). Thus, attention has recently turned to using low molecular organic materials (~1 kDa) as e-beam resists. Properties such as cross-linking or selective cleavage upon e-beam irradiation can be designed, in order to improve the negative/positive tone resist properties and etch durability, as well as improve the final resolution of the structures by virtue of molecular materials having a much lower molecular weight and size.
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