Unconventional methods of patterning polymers

Dahl-Young Khang, Kahpyang Suh, Yunsang Kim, Hyunsik Yoon, and Hong H. Lee

School of Chemical Engineering, Seoul National University

비전통적인 방법에 의한 고분자 패터닝

강달영, 서갑양, 김연상, 윤현식, 이홍희

Introduction

Future integrated circuits will require reliable techniques for fabricating nanometer-scale devices. Optical lithography or photolithography, which has been and will be the mainstay of lithography for the near future, is expected to be limited to ~100nm resolution. If the economics is not a major concern, there are technologies already available for sub-100nm features such as electron-beam lithography, X-ray lithography, and scanning probe lithography, etc [1,2].

Other than photolithography, there are emerging unconventional lithographies that can be directly applicable to polymers for large-area patterning such as nano-imprint lithography [3], capillary force lithography [4], micro-contact printing [5], and electrically induced structure formation [6]. Here, we introduce the room-temperature lithography and the capillary force lithography, which is in pursuit for large-area patterning of polymers.

Unconventional methods of patterning polymers

1. Room-temperature imprint lithography

Nano-imprint lithography generates resist relief patterns in a thermoplastic layer by physically compressing the resist that has been thermally softened, rather than by modifying the resist's chemical structure by radiation [3]. In typical imprint lithography (hot-embossing lithography), a mold with nanostructures on its surface is pressed into a thin resist that has been heated above its glass transition temperature (Tg) to endow the resist with fluidity. This fluidity enables the resist to conform to the mold pattern. After the resist is cooled below Tg, the mold is removed. An anisotropic etching process, such as reactive ion etching (RIE), is used to remove the residual resist in the compressed region.

In this high-temperature process, clean release of the resist from the mold is one of the most important characteristics required. Usually, anti-adhesion agents have been used to achieve clean mold release [3]. Distortion of the imprinted structures or mold features due to the thermal cycle of heating followed by cooling is another problem to contend with.

Here, we demonstrate room-temperature imprint lithography based on plastic flow along with free-volume contraction and plastic deformation of the resist layer [7]. This room-temperature process has the unique features of enabling step-and-repeat and multiple imprinting, which is impossible in the conventional high-temperature imprint processes.

In amorphous polymeric materials, a certain number of vacancies, called free volume, must always be present due to the absence of long-range order and chain-like character with finite thickness and persistence [8]. Typically 10~30% of the original thickness can be compressed to by free volume compaction only. If the free volume compaction is completed and the applied stress is in the range between the yield stress of polymer and that of the inorganic mold material, further imprinting of the mold features into the underlying polymer layer can be realized through the plastic deformation of polymer.

In typical experiments, the substrate was a Si wafer ~2cm×2cm in size with a top layer of oxide 1□ thick. The mold was prepared by electron beam lithography or photolithography. Polystyrene (PS), which was prepared by our anionic polymerization system, was used as the thermoplastic resist. It has a molecular weight of 100,000g/mol with a polydispersity index of 1.02~1.05. Prior to spin-coating onto a SiO2/Si substrate with a PS/toluene polymer solution, the substrate was cleaned by nitrogen blowing, ultrasonication for 5min in trichloroethylene (TCE) and acetone, and then in isopropylalcohol (IPA), rinsing with de-ionized water and blow-drying with nitrogen. The thickness of the dried resist layer, as determined by ellipsometry, ranged from 50 to 550nm depending on the coating conditions.

The PS/SiO2/Si substrate was loaded and pressed at room temperature for a period of 5-20s using a conventional hydraulic press, at a pressure between 30 and 150MPa. When the pressure was released, the mold detached freely without being forced and no adhered polymer was found on the mold surface when examined by scanning electron microscopy (SEM). This mold can be re-used directly tens of times for the multiple or step-and-repeat imprinting with manual alignments.

Fig.  (a) Schematic diagram of the room-temperature imprint lithography and (b) an example of the pattern transfer by the two-step etching.

Schematic diagram of the experimental procedure and an example of the pattern transfer by the two-step etching is shown in Fig. 1. For comparison purpose, SEM micrographs are shown of the master with which the pattern is imprinted, the pattern imprinted, and the pattern transferred into the underlying SiO2 substrate (from top to bottom). The first step of etching polystyrene that was used as the polymer layer was carried out with CF4 plasma. After the underlying SiO2/Si layer had been exposed, it was etched with CHF3/CF4 plasma to transfer the pattern into SiO2. The cross-sectional SEM micrographs in Fig. 1 show that a line and space pattern is transferred well into a SiO2 substrate by the two-step RIE process.

2. Capillary force lithography

In the capillary force lithography presented here [4], the essential feature of the imrpint lithography [3] of molding a polymer melt is combined with the prime element of micro-contact printing [5] or soft lithography [2] of using an elastomeric mold. As a result, the advantage of the imprint lithography over the micro-contact printing is retained in meeting the stringent pattern fidelity requirements in fabricating integrated circuits while eliminating the need to use an exteremely high pressure that is needed in the imprint lithography. When an elastomeric mold such as PDMS with a desired pattern is placed on a polymer surface and heated above glass transition temperature, capillary force allows the polymer melt to fill up the void space of the channels formed between the mold and the polymer, thereby generating the negative replica of the mold.

We used polydimethyl siloxane (PDMS, Sylgard 184, Dow Corning) as an elastomeric mold and fabricated a PDMS master that has a planar surface with recessed or protruding patterns by casting PDMS against a complementary relief structure prepared by photolithography or electron beam method. For the polymer, we used commercial polystyrene (PS) (molecular weight = 2.3×105) and styrene-butadiene-styrene (SBS) block copolymer (molecular weight = 3×105, styrene fraction = 0.3). Typically, silicon wafer (100) was used as the substrate, which can readily be expanded to other kinds of organic or inorganic substrate if the surface is planar enough to allow a conformal contact with the PDMS mold. Silicon wafer was cleaned by ultrasonic treatment in trichloroethylene and methanol for 5 minutes each and dried in nitrogen. Polymer films were spin-coated onto the Si substrate to 30nm to 1.5m thickness and the PDMS mold was placed on the polymer surface. In spite of the spontaneous wetting property of the PDMS mold, care should be taken to avoid small gaps or bubbles between the mold and the polymer surface. The film was then annealed at a temperature above the glass transition temperature, Tg.

Fig.  (a) Schematic diagram of the capillary force lithography and (b) examples of the pattern formation:  3□ line and space, 500nm network, and 100nm line pattern, respectively (from up to bottom).

Typically, polymer films of PS (Tg=101℃) were annealed at 130℃ and those of SBS (Tg=-36℃) at 120℃ to make the polymer chain sufficiently mobile. PDMS has a very low reactivity toward the polymers and its elasticity is sufficient to allow its separation from the polymeric structure. After annealing for 30 minutes to 24 hours and cooling to ambient temperature, the mold was removed from the surface and the remaining polymer structure was examined by scanning electron microscopy (SEM).

Schematic diagram of the experimental procedure and examples of the pattern formation is shown in Fig. 2. When the polymer film is thick enough to completely fill the cavity of the mold, residual polymer remains on the substrate as shown in Fig. 2. However, if the polymer film is thin and the interaction between the polymer and the substrate is sufficiently weak, no residual film remains and the substrate can be exposed. Features down to 100nm can be replicated by CFL as shown in Fig. 2 (right bottom). In the figure, the line features on the original mold were poor due to limited electron-beam capability, which is replicated in the lines formed.

Conclusion

The room-temperature imprint lithography, which is based on a mechanism different from that for the conventional imprinting method, can lead to more versatile and practical nano-scale patterning. Capillary force lithography (CFL) presented here is a simple and yet robust method for large-area patterning. With further development, the techniques could become a strong candidate for an alternative to photolithography in fabricating large-scale integrated circuits.

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