E-beam sensitivity

While electron beams can produce cations, they are not effective at producing cationic cure in the absence of suitable photoinitiators. The same cationic photoinitiators used for UV cure are often also e-beam sensitive. Examples are triaryl sulfonium or diaryl iodonium salts [41]. 

Another class of "chain scission" positive resists is the poly(olefin sulfones). These polymers are alternating copolymers of an olefin and sulfur dioxide. The relatively weak C-S bond is readily cleaved upon irradiation and several sensitive resists have been developed based on this chemistry (49,50). One of these materials, poly(butene-l sulfone) (PBS) has been made commercially available for mask making. PBS exhibits an e-beam sensitivity of 1.6 pC cm-2 at 20 kV and 0.25 pm resolution. 

While "conventional positive photoresists" are sensitive, high-resolution materials, they are essentially opaque to radiation below 300 nm. This has led researchers to examine alternate chemistry for deep-UV applications. Examples of deep-UV sensitive dissolution inhibitors include aliphatic diazoketones (61-64) and nitrobenzyl esters (65). Certain onium salts have also recently been shown to be effective inhibitors for phenolic resins (66). A novel e-beam sensitive dissolution inhibition resist was designed by Bowden, et al a (67) based on the use of a novolac resin with a poly(olefin sulfone) dissolution inhibitor. The aqueous, base-soluble novolac is rendered less soluble via addition of -10 wt % poly(2-methyl pentene-1 sulfone)(PMPS). Irradiation causes main chain scission of PMPS followed by depolymerization to volatile monomers (68). The dissolution inhibitor is thus effectively "vaporized", restoring solubility in aqueous base to the irradiated portions of the resist. Alternate resist systems based on this chemistry have also been reported (69,70). 

Lithographic Characteristics. Based on the potential of crosslinker 3 to show high sensitivity and contrast and wide process latitude, it was of interest to evaluate its lithographic capability, using crosslinker 1 as the standard for comparison. Crosslinkers 1 and 3 (equal weight) were each incorporated into otherwise identical experimental AHR resist formulations. E-beam exposures were performed so that differences in DUV absorbance characteristics of the crosslinkers could be ignored. The e-beam sensitivities of the resists containing crosslinkers 1 and 3 were 6.2 and 4.2 (lC/cm2,... 

Table III. The inherent E-beam sensitivity of PMOTSS and its copolymers...

Q = E-beam sensitivity (1) Q = proton beam sensitivity. Qp = Qh x (9.65xl010 /iC/mole) / (6.02xl023 ions/mole). 

Figure 7. The E-beam sensitivity (Qe) of MOTSS copolymer resists as a function of the proton beam sensitivity (Qp) with 90 and 125 keV protons.

Figure 29. The 15 kV e-beam sensitivity and the y-ray G(s) of a variety of PMMA analogs. Note that increasing G (s) is inversely related to the lithographic sensitivity.

Figure 36. Effect of change in molecular weight at constant dispersity on the e-beam sensitivity of poly (chloromethylstyrene), PCMS. (Reproduced with permission from Ref 47 J...

The etch rate measurements for positive and negative-behaving e-beam resists are found in Table V. It is apparent that the etch resistance is lower the more sensitive the positive resist. The exception would be PMCN, which exhibits better dry-etch resistance than that which would be predicted based on e-beam sensitivity alone. Where e-beam sensitivity and etch resistance are needed, copolymerization becomes very important. This has been demonstrated for the MCN/MMA and MCA/MCN model copolymer systems in references 9 and 10, respectively. 

DNQ-novolac positive resists have been used also with e-beam exposure. The 2,1,4 DNQ isomers give superior performance in these applications (101). The e-beam sensitivity of these materials is 40 xC/cm2. 

The e-beam sensitivity is not particularly high being about 300 fiC/crn for an accelerating voltage of 20 to 30 kV which is comparable to PMMA. However, the contrast is extremely high with a 7-value equal to 8 (8). The resist has been reported to be sensitive to ion beams (9) and to soft x-ray irradiation as well. Thus, the inorganic resists are applicable to all non-optical lithographies. 

Poly (2-methyl-1-pentene sulfone) may be used as a dissolution inhibitor to effect e-beam sensitivity (38). Trimethylsilylalkoxyphenol is another monomer that has been used in the preparation of oxygen-etching-resistant no-volacs for resist applications (39). For all of the novolac-based systems studied to date, the hydrophobic nature of the silicon moiety limits the incorporation of silicon to —10 wt %. However, this level is sufficient to allow use of these resins as oxygen RIE masks. 

The depolymerization mechanism from the polymer end has been recently revisited in the design of positive electron beam resists. 2-Phenylallyl-termi-nated poly(a-methylstyrene) was prepared by living anionic polymerization, which exhibited a significantly lower depolymerization temperature on TGA than the H-terminated counterpart [340]. The 2-phenylallyl-terminated polymer depolymerized completely when treated with n-BuLi in THF at room temperature. A single-component resist (without PAG) formulated with the 2-phenylallyl-terminated poly(a-methylstyrene) demonstrated a higher e-beam sensitivity (500 pC/cm2 at 20 keV) than the one based on the H-terminated polymer when developed with methanol/methyl isobutyl ketone (2/3 vol/vol) [340]. However, the sensitivity of the non-catalyzed single-component system... 

Babich ED, Paraszcak J, Hatzakis M et al. (1989) A comparison of the E-beam sensitivities and relative 02-plasma stabilities of otganosilicon polymers. Part 111. Lithographic characteristics of poly-1,1,3-trimethyl-l-sila- and poly-1,1,3,3-tetramethyl-l,3-disilacyclobutenes and related silmethylene polymers. Microelectronic Eng 9 537-542... 

A similar phenomenon occurs in the TBS copolymer system described above ((26, 27), cf. Eq. 3) in which a dramatic e-beam sensitivity decrease is observed whereas the TBS system is a very sensitive x-ray resist, its e-beam speed of 90 mC/cm 30 keV is at best moderate. The ratio of e-beam to x-ray sensitivity is thus very different for this resist (cf. Figure 2). One may speculate that lack of water and evaporation of SOj result in reduced acid formation, and hence lower sensitivity. 

A clever variation of the monomeric dissolution inhibitor concept (3-compo-nent system) makes use of the base-catalyzed opening of the lactone ring in cresolphthalein which becomes possible after catalytic deprotection (17b). In the t-BOC protected inhibitor, the quinomethane system cannot form, and the lacton ring is not hydrolysed. The additional acidic functionality improves the dissolution rate in the e q>osed resist (see Eq. 6). The e-beam sensitivity of a 3-component tystem based on this inhibitor in combination with a novolak matrix and a triphenylsulfonium triflate photoacid generator was reported to be 2-3 fiC/cm (17b). 

The above examples of e-beam sensitive polymer blends were concerned with compatible polymer mixtures. Intuitively, compatibility would seem to be necessary for lithographic performance. However, a recent paper describes e-beam resists based on apparently incompatible PMMA and poly(a-methylstyrene) blends. Although considerable information concerning contrast, sensitivity and plasma etch resistance are given, imaging and resolution results are not presented. 

Figure 43 E-beam sensitivity at 20 keV and y-ray G(s) vaiues for a variety of polymers analogous to PMMA which were studied for use as e-beam resists. In general, higher G(s) values correlate with lower required e-beam exposure doses (i.e., higher sensitivity resists).

Electron beam lithography is possible with allyl-containing arylaminophosphazene polymers (40), Work in the group of V. T. Stannett has demonstrated that these species have an e-beam sensitivity and sufficiently high Tg s that they are appropriate for microlithography. 

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