Bilayer resist system

The conventional bilayer resist systems in which the top imaging layer (typically organosilicon polymer) also serves as an etch mask was first proposed by Hatzakis et al. in 1981, ostensibly for electron-beam lithography. Since then, a number of organosilicon resists for bilayer resist systems have been reported for use in near-UV, DUV, mid-UV, electron-beam, and x-ray applications, a good review of which has been provided by Ohnishi et al. In recent times, negative-tone resist systems and processes based on silicon-backbone polymers such as polysilanes,polysilynes, and plasma-deposited polymers have been developed for 193-nm lithography. 

It should be mentioned that these silicon-backbone polymers are highly absorptive, and must therefore be used in very thin films ( 30 nm), which can be prone to defects. Also, the principal electronic transition in these polysilane (with two-dimensional silicon backbone) and polysilane (three-dimensional silicon backbone) polymers when irradiated at 193 nm is most likely a (t-ct transition involving the all-silicon backbone. Due to this excitation, reaction pathways leading to scission of the Si-Si bonds lead to photodegradation of the polymer. 

Hatzakis, J. Paraszczak, and J. Shaw, Double layer resist systems for high resolution lithogra phy, in Proc. Microelectronics Engineering, p. 386 (1981). 

Ohnishi, M. Suzuki, K. Saigo, Y. Saotome, and H. Gokan, Postirradiation polymerization of e beam negative resists Theoretical analysis and method of inhibition, Proc. SPIE 539, 62 (1985). R.R. Kunz, M.W. Horn, R.B. Goodman, P.A. Bianconi, D.A. Smith, J.R. Eshelman, G.M. Wallraff, R.D. Miller, and E.J. Ginsberg, Surface imaged silicon polymers for 193 nm excimer laser lithogra phy, Proc. SPIE 1672, 385 (1992). 

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Fig. 36. Representative bilayer resist systems. Both CA and non-CA approaches are illustrated (116—119). (a) Cross-linking E-beam resist, 193-nm thin-film imaging resist (b) acid-cataly2ed negative-tone cross-linking system (c) positive-tone CA resist designed for 193-nm appHcations and (d) positive-tone...

Figure 3.52. Patterns formed in a bilayer resist system consisting of a 1.2-p.m-thick planarizing layer of PMMA and a 0.4-[im-thick RD2000N imaging resist. Reproduced with permission from reference 19. Copyright 1984 Technical...

It behaves as a negative photoresist in bilayer resist systems and is highly sensitive to electron beam, X-ray or UV radiation whilst being resistant to 02 reactive ion etching243. 

Bilayer resists are formulated mostly from organosilicon polymers as the imaging layer and novolacs polymers as the underlayer. Many organosilicon resists for bilayer resist systems have been reported for use in near-UV, DUV, mid-UV, electron-beam, and x-ray applications, a good review of which has been provided... 

The inorganic/organic bilayer resist technology has been successfully applied to the A1 interconnection process in bipolar LSI fabrication, including the 1 kbit RAM and several repeater ICs for optical communication systems (6). The minimum feature size of the patterns was 1.5 fim for lines and 1.0 fim for spaces. In order to further evaluate the lithographic... 

Table 1.1. Organosilicon Resists for Bilayer Multilevel Resist Systems...

Mass spectrometry is an analytical method of choice for identification of volatile compounds and has been employed in investigation of thermolysis and acidolysis mechanisms of chemical amplification resists [96, 121, 122]. This technique has been also utilized in screening of resists systems, especially Si-containing 193 nm bilayer resists,for outgassing [438,439]. MALDI-TOF mass spectroscopy has been applied to characterization of dendritic resist polymers. 

The three main approaches to multilayer resist imaging systems (see Chapter 16 for details) include (i) hard mask (HM) processes, (ii) top surface imaging (TSI) processes requiring latent image formation only near the surface of the resist, thus circumventing any transparency requirements, and (iii) bilayer resist (BLR)... 

For further extension, a TSI NOVOBOC resist tem was demonstrated with gas phase sil tion followed by RIE transfer. A bilayer Si system was also formed and processed by simultaneous liquid phase development, liquid phase sifylation and RIE pattern transfer. 

Studies of the reaction of ozone with simplified lipid systems have shown that malonaldehyde can be produced by direct ozonolysis. The use of malonaldehyde assay as an index of lipid peroxidation is therefore invalid in ozone studies. Liposomes formed from egg lecithin and prepared in aqueous media were quite resistant to ozone, but the contribution of polyconcentric spheres to this resistance has not been fully assessed. However, the bilayer configuration, with the susceptible unsaturated fatty acids shielded from ozone by the hydrophilic areas of the molecule, may be resistant. In hexane, where the fatty acid moieties are exposed, ozone reacts stoichiometrically with the double bonds. The experiments with aqueous suspensions of phosphatidylcholine gave no evidence of the formation of lipid peroxides,nor did experiments with films of fatty acids exposed to ozone. ... 

Although the majority of the lipids in M. laidlawii membranes appear to be in a liquid-crystalline state, the system possesses the same physical properties that many other membranes possess. The ORD is that of a red-shifted a-helix high resolution NMR does not show obvious absorption by hydrocarbon protons, and infrared spectroscopy shows no ft structure. Like erythrocyte ghosts, treatment with pronase leaves an enzyme-resistant core containing about 20% of the protein of the intact membrane (56). This residual core retains the membrane lipid and appears membranous in the electron microscope (56). Like many others, M. laidlawii membranes are solubilized by detergents and can be reconstituted by removal of detergent. Apparently all of these properties can be consistent with a structure in which the lipids are predominantly in the bilayer conformation. The spectroscopic data are therefore insufficient to reject the concept of a phospholipid bilayer structure or to... 

Out of a variety of polymerizable lipids tested for possible use of bilayer formation, only three systems exhibited BLM lifetimes of more than a few minutes (Table 2 26)). These BLMs were characterized by measuring their resistance and capacitance (Table 2., see26> for details). The data obtained were comparable with values obtained with egg lecithin the most frequently used material for preparing BLMs. 

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