Sensitivity negative resists

Figure 34. The Polystyrenes. Polystyrene is a low sensitivity negative resist, poly (a-methylstyrene) is a low sensitivity positive resist and the para-substituted analogs listed are all sensitive negative resists.

Theoretical sensitivity calculations were carried out for two copolymer systems, poly (chloromethylstyrene-co-2-vinylnaphthalene) (P(CMS-2VN)) and poly(glycidyl methacrylate-co-2-vinyinaphthalene) (P(GMA-2VN)). Polychloromethlstyrene is known as a high sensitivity negative resist ( 7 8 ) (D mw=0.07 C-g /cm2- mole), which shows no postirradiation polymerization. The crosslinking reaction in P(CMS-2VN)... 

Negative resists generally exhibit high sensitivity but low contrast. For instp.ncin our laboratory, polymers containing thiirane groups / g v ere found to be extremely sensitive (o = 6 x 10 T C/cm2) to electron beam irradiation at a 20 kV... 

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. 

Radiation-sensitive polymers are used to define pattern images for the fabrication of microelectronic devices and circuits. These polymers, called resists, respond to radiation by either chain scission (positive resists) or by crosslinking (negative resists). In positive resists, the exposed areas dissolve selectively by chemical developers in negative resists, the exposed areas are insoluble and remain after development. 

High resolution negative resists are needed for masked ion beam lithography (MIBL) and for the fabrication of MIBL masks by E-beam lithography (EBL). The MOTSS copolymer resists were developed to obtain the resolution of fine features that a bilevel resist can best provide. The flexibility afforded by choosing the structure of the HS, the copolymer composition, and the molecular weight allows a resist to be tailored by simple synthesis adjustments to have the particular sensitivity and etch protection which best suits the application. 

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).

Figure 16. GMC, an example of copolymer selected by Bowden and Thompson for optimal sensitivity in pattern generation, through negative resist formation.

COP, the familiar negative e-beam resist developed at Bell Laboratories, is an example of a one-component negative resist system. COP is a copolymer which has excellent film-forming characteristics, resistance to etchants, and intrinsic radiation sensitivity. 

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... 

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