Etch-Resistant Block Copolymers

By: Prof. Padma Gopalan Prof. Shu Yang , Material Matters 2006, 1.1, 6.

Material Matters 2006, 1.1, 6.

Prof. Padma Gopalan Department of Materials Science and Engineering, University of Wisconsin, Madison, WI
Prof. Shu Yang Department of Materials Science and Engineering University of Pennsylvania, Philadelphia, PA


Introduction

Block copolymers offer a means of combining the desirable characteristics of different polymers in a new hybrid material. Polymers consisting of hydrophobic and hydrophilic blocks, for example, can be used to encapsulate organic molecules and deliver them into aqueous media. There has been tremendous interest in the self-assembly of block copolymers in nanoscale dimensions, especially in thin-film configuration.

Conventional lithography has its limitations when features of less than 30 nm are desired. Accessibility to a wide range of periodic structures with feature sizes less than 30 nm make block copolymers attractive as templates for nanopatterning.1–3 Most of the literature approaches use selective ozonolysis or preferential staining of one block with heavy metals to increase etch selectivity between the blocks. Often, an intermediate silicon nitride (SiN) layer and selective etching of one block over another is required for successful pattern transfer. In general, the use of organic block copolymers is limited at high temperatures because of low thermal/mechanical stabilities. Thus, direct patterning of semiconductors that requires high growth temperature (>500 °C) using organic block copolymers as templates is nearly impossible.

It has been well established that incorporation of silicon (at least 10 wt %) in resist polymers provides improved oxygen–RIE (reactive ion etching) etch resistance. When exposed to oxygen plasma, the silicon-containing polymers are oxidized to silicon oxide that is stable in an O2 environment. The high etch resistance to oxygen plasma compared to organic polymers makes silicon-containing polymers favorable as bilayer resists to pattern high-aspect ratio structures and to create nanoporous ceramic thin films in a variety of morphologies.4–8 In addition, silicon oxide has high thermal and mechanical stability at a temperature greater than 500 °C, making it a long-time dielectric in microchip fabrication. Thus, the possibility of combining acid labile groups and silicon-containing groups in block copolymers offers a new route to directly pattern nanostructured semiconductors.

As the synthesis of silicon-containing block copolymers is quite challenging using traditional living anionic polymerization, post functionalization of polymers is often used to incorporate silicon. Recent advances in controlled living free-radical polymerization (LFRP),9–11 including nitroxide-mediated radical polymerization (NMRP), atom transfer radical polymerization (ATRP), and reversible addition fragmentation chain transfer (RAFT), make it possible to design and synthesize a variety of block copolymers with novel functionalities. The LFRP procedures in general are easier to carry out as they are tolerant to a variety of functionalities and do not require stringent purification of the starting materials, unlike living anionic or cationic polymerization. We had recently applied NMRP towards, (i) the synthesis of narrow dispersed silicon-containing homopolymers from three kinds of silicon-containing styrenic monomers, including 4-(pentamethyldisilyl)-styrene (Si2St), 4-(bis(trimethylsilyl)methyl) styrene (Si2-CSt), and 4-(pentamethyldisiloxymethyl) styrene (OSi2-St) (Scheme 1), each containing two silicon atoms to enhance the etch selectivity, and (ii) the synthesis of block copolymers from silicon-containing styrenic monomers with styrene and acid labile acetoxystyrene by sequential monomer addition using an nitroxide unimer initiator.

Scheme 1.Transmission electron micrograph (TEM) images of PAcOSt-PSi2St (21/79 v/v) before and after O2 plasma for 10 minutes, showing intact cylindrical morphology.

Scheme 1.Transmission electron micrograph (TEM) images of PAcOSt-PSi2St (21/79 v/v) before and after O2 plasma for 10 minutes, showing intact cylindrical morphology.


By optimizing conditions such as solvent polarity, temperature of polymerization, and the monomer addition sequence, welldefined narrow dispersed silicon-containing block copolymers were synthesized from the above monomers. Both TEM (transmission electron microscopy) and SAXS (small angle X-ray scattering) data showed that these polymers formed cylindrical, lamellae, or disordered structures depending on the volume ratio between the blocks and their molecular weights. When the silicon-containing block was the major phase and silicon content was greater than 12 wt %, block copolymer morphology and its domain size were well maintained under exposure to oxygen plasma12 Figure 1).

Figure 1.Transmission electron micrograph (TEM) images of PAcOSt-PSi2St (21/79 v/v) before and after O2 plasma for 10 minutes, showing intact cylindrical morphology.

Figure 1.Transmission electron micrograph (TEM) images of PAcOSt-PSi2St (21/79 v/v) before and after O2 plasma for 10 minutes, showing intact cylindrical morphology.


Synthetic access to novel silicon-containing block copolymers via LFRP enables potential applications such as (1) growth of nanostructured semiconductor crystals at high temperatures, (2) formation of nanoporous ceramic films, or (3) creation of hierarchical hybrid nanostructures by combining photolithography and self-assembly of photosensitive siliconcontaining block copolymers.

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References

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