Advances in the Chemistry of Resists for Ionizing Radiation - ACS

Chapter 17

Advances in the Chemistry of Resists for Ionizing Radiation Ralph Dammel

Downloaded by FUDAN UNIV on December 16, 2016 | http://pubs.acs.org Publication Date: November 23, 1993 | doi: 10.1021/bk-1994-0537.ch017

Hoechst Celanese Corporation, 500 Washington Street, Coventry, RI 02816

The lithographic techniques using high-energy radiation may be distinguished according to their use of electromagnetic (x-ray) or particle radiation (e-beam or ion beam). In x-ray lithography, a further distinction may be made depending on whether 1:1 proximity printing or reducing optics are employed; one may further differentiate between systems using synchrotron radiation, and those using e.g. laser focus sources or cathode ray tubes (Figure 1). Similarly, in e-beam lithography, one may distinguish mask-using (projection) or serial writing techniques; the latter are most conveniently further subdivided.according to the market segment they serve. Each of these subfields of high-energy lithography has inherent constraints, arising from its physics, that define what resist properties and processes will be required for a successful practical implementation (1-3). Most organic materials have first ionization energies ranging from 8.5 to 10 eV. Disregarding multi-photon ionization processes, ionizing radiation could therefore be defined as electron beams with > 10 eV kinetic energy, or electromagnetic radiation of wavelength shorter than 124 nm. However, this formalistic definition is somewhat misleading since the energies used in lithographic techniques are typically several orders of magnitude higher: electron beam energies range from 10 to 100 keV, and the radiation used in x-ray lithography lies in the wavelength range from 1.4 nm to 0.4 nm. At these energies, the molecular and electronic structure of the resist material has no influence on the absorption behavior; instead, absorption events occur primarily in the inner non-valence shells of atoms (4). Consequently, the energy deposition within a resist depends solely on its constituent atoms, and may be calculated easily from the atomic composition using atomic absorption cross sections, irrespective of the molecular structure of its components. Cascades of secondary and tertiary electrons, initiated by the primary event, dissipate the energy of the incoming photon or electron over a more extended volume. While minor deviations caused by different cross sections towards

0097-6156/94/0537-0252$08.50/0 © 1994 American Chemical Society Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

non-synchr. \ lithography |

reducing projection

reducing T| projection J

Figure 1. Family tree of lithographic methods employing ionizing radiation. The physics of each method leads to specific demands on resist chemistry.

I synchrotron | lithography

1:1 proximity^ printing J

Ionizing h Radiation J


254

POLYMERS FOR MICROELECTRONICS

electrons and x-rays have been reported (5), x-ray and e-beam sensitivities show an approximately linear correlation (see Figure 2). This has traditionally been interpreted to mean that the basic mechanism of energy transfer proceeds via the secondary electrons and is thus very similar for the x-ray and e-beam experiments. The absorbed energy is thus distributed unspecifically over all atoms in the entire irradiated photoresist volume according to a stochastic physical process. In contrast, in near-UV materials, the photon energy is directly deposited in a specific absorption band of a molecule, leading to an excited state from which a definite chemical reaction occurs, such as e.g. N extrusion from diazonaphthoquinones (DNQs): in other words, the energy is directly delivered to the chemical bond that is to be broken. It is therefore not surprising that the sensitivity of DNQ resists in x-ray and e-beam resists (e.g. AZ1450J in Figure 2) is much lower than for near-UV irradiation. The challenge facing photoresist chemists has therefore been to design a resist material for high energy radiation that provides a chemical mechanism for focusing the indiscriminately deposited energy into specific chemical reaction channels.


2

TAXONOMY OF PHOTORESIST MATERIALS FOR IONIZING RADIATION The multitude of materials described for high-energy applications may be classified in a "taxonomy" of resists for ionizing radiation: the two main sub-kingdoms of resists, positive and negative tone, each may be subdivided into three phyla according to the mechanism of the photoinduced solubility change (Figures 3 and 4). Each phylum may be further classified according to whether the solubility change is effected directly by the incoming radiation, or whether it proceeds by the intermediacy of a photogenerated cationic agent. The distinction of chemically amplified and non-amplified resists (6) yields a subdivision of these classes into different orders; for each order, one specific resist genus (e.g. PBS) is given as an example. Actually, to keep the analogy to zoological taxonomy, an additional taxonomic level, the family, will have to be recognized; e.g. tertiary polyalkenesulfones (such as PMPS) and polyaldehyde resists both belong to die family of low-T unzipping polymers. In this paper, resist taxonomy has not been carried to this level. The high sensitivity required of resists for ionizing radiation is usually achieved by some kind of chemical amplification scheme (7). In the case of polybutenesulfone (PBS), the bonds between monomer units are so weak that the chain scission process, with its concomitant decrease in molecular weight and increase in dissolution rate, occurs with great efficiency. In PBS, the polymer is, however, not so unstable that a single chain break event would lead to a large number of consecutive monomer split-offs. In the closely related poly(methylpentenesulfone) (PMPS), the ceiling temperature of the polymer is so low that one chain break can lead to an "unzipping reaction" in which the polymer thread unravels outward in both directions from the original scission site, reverting to monomers and leading to substantial volatilization of the resist. c

Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


17.

DAMMEL

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1 5

P 0.2

Advances in the Chemistry of Resists for Ionizing Radiation 255

I

I 0.6

I

I 1.0

I

I 1.4

I

I 1.8

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I 2.2

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log (x-ray sensitivity [mJ/cm ]) 2

Figure 2. Double logarithmic plot of e-beam vs. x-ray sensitivity. Circles denote negative, squares positive-tone resists. For the off-diagonal point (TBS/S0 ) see section 4, for further references see [1]. A Z PF514: acetal-based 3-component system, A Z PN114: 3-component system based on electrophilic novolak crosslinking, AZ1450J: diazonaphthoquinone/novolak resist, CMPS, CMPS-X: chlorinated polymethylstyrene, COP: poly(glycidylmethacrylate-co-ethylmethacrylate), EPB: epoxidized polybutadiene, F B M types: polj

Advances in the Chemistry of Resists for Ionizing Radiation - ACS

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