Chapter 1
Lithography with a Pattern of Block Copolymer Microdomains as a Positive or Negative Resist 1
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Christopher Harrison , Miri Park , Paul M . Chaikin , Richard A. Register , and Douglas H. Adamson Downloaded by ST JOSEPHS UNIV on July 24, 2013 | http://pubs.acs.org Publication Date: September 1, 1998 | doi: 10.1021/bk-1998-0706.ch001
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Department of Physics, Department of Chemical Engineering, and Princeton Materials Institute, Princeton University, Princeton, NJ 08544 Dense, periodic arrays of holes and troughs have been fabricated in silicon, silicon nitride, and germanium, at a length scale inaccessible by conventional lithographic techniques. The holes are approximately 20 nanometers (nm) wide, 20 nm deep, spaced 40 nm apart, and uniformly patterned with 3X10 holes on a three inch silicon wafer. To access this length scale, self-assembling resists were synthesized to produce either a layer of hexagonally ordered polyisoprene (PI) spheres or polybutadiene (PB) cylinders in a polystyrene (PS) matrix. The PI spheres or PB cylinders were then chemically modified by either degradation or stained with metal compounds to produce a useful mask for pattern transfer by fluorine -based reactive ion etching (RIE). A mask of spherical microdomains was used to fabricate a lattice of holes or posts and a mask of cylindrical voids was used to produce parallel troughs. This technique accesses a length scale difficult to produce by conventional lithography and opens a route for the patterning of surfaces via self-assembly. 12
Recent advances in photolithography have pushed the feature size down to 150 nm in production processes(7), and even smaller feature sizes have been reported in experimental research(2,5,4). However, dramatic improvements in the circuit density with photolithographic processes are not anticipated because the minimum feature size is limited by the wavelength of light, typically 193 or 248 nm in current processes. A s an alternative to photolithography, self-assembled structures, such as monolayers of spheres or cylinders(5,6), have been advanced by researchers due to their nanoscopic feature sizes and the control demonstrated in uniformly coating substrates. Though these morphologies do not allow one
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© 1 9 9 8 American Chemical Society In Micro- and Nanopatterning Polymers; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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the high degree of pattern control necessary for microelectronic circuits, there are a variety of applications in which regular patterning or texturing of a surface at the 10 nm lengthscale is ideal: for example, the periodic patterning of an electric potential on a two-dimensional electron gas system(7,#,9,70,77), fabrication of quantum dots or anti-dots(72), synthesis of D N A electrophoresis media(73), filters with nanometer pore sizes, and creation of quantum confinements for light emission. For these applications, the self-assembled structures observed in ordered diblock copolymer thin films would seem ideal as lithography templates because of their self-assembly and the ability to parallel-process wafer-sized areas. B l o c k copolymers with a narrow polydispersity (the chains are of near uniform length) and with %N>10 microphase separate above their glass transition temperature, where % is the Flory-Huggins interaction parameter and N is the degree of polymerization(74). Macroscopic phase separation of the components of the block copolymer is prevented by a covalent bond which connects the unlike blocks. The resulting morphology depends largely on the relative volume fraction of the components. Some of the more commonly seen microdomain morphologies are lamellae, cylinders, and spheres (Figure 1). The periodicity of these structures is determined by the length of the polymer chains and is typically on the order of 20-100 nm. (a) A
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Figure 1: (a) A block copolymer consists of two or more homogeneous blocks, drawn here as blocks A and B . (b) Microphase separation typically produces a lamellar morphology for equal lengths of blocks A (light) and B (dark). A s block B is shortened with respect to block A , hexagonally packed cylinders are typically observed. For an even shorter block B , packed spheres on a body centered-cubic lattice are observed.
In Micro- and Nanopatterning Polymers; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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Researchers have recently studied thin films of block copolymers because of both the rich set of phenomena that have been observed(75,76,77) and because of their application to lithography(7#,79,20). Earlier work with poly(styrene-fcbutadiene), P S - P B , and subsequent work with poly(styrene-£-isoprene), PS-PI, block copolymers showed that by choosing the appropriate film thickness, a single layer of spherical or cylindrical microdomains (a monolayer of spheres or cylinders, see Figure 2) could be produced as a lithographic template. Brush like wetting layers of P B or PI (collectively referred to as the diene component) on the free and confined surfaces were shown to sufficiently decouple the microdomains from the substrate to allow the microdomains to order with a grain size of up to 25 by 25 unit cells, or about 1 square micron. Subsequent dynamic secondary ion mass spectrometry analysis confirmed the existence of wetting layers of P B on the free and confined surface of films spin coated onto silicon wafers(27). The diene component preferentially wets the free surface due to its lower surface tension(22) while the confined surface is wet by the diene component due to a combination of a lower interfacial tension and a possible silica-poly diene chemical bonding(23,24).
Figure 2: (a) A monolayer of PI spheres in a PS matrix, with accompanying free and confined surface PI wetting layers, (drawn for SI 68/12) (b) A monolayer of P B cylinders in a PS matrix, with accompanying free and confined surface P B wetting layers, (drawn for S B 36/11) The microdomain template as shown above (Figure 2) is not an effective mask for R I E because the P S , PI, and P B blocks etch approximately at the same rate under either C F or CF4/O2 R I E , which we find to be the most effective etching process for pattern transfer. Therefore, further modification of the microdomains is necessary to make a useful mask. B y allowing the monolayer to act as a template and taking advantage of the different chemical properties of the component blocks, we found that the template could function as either a positive or negative resist for pattern transfer. 4
In Micro- and Nanopatterning Polymers; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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Sample Preparation Asymmetric P S - P B and PS-PI diblock copolymers were synthesized (designated S B 36/11 and SI 68/12 respectively, with the molecular weights of the blocks in kilograms per mole) by standard high-vacuum anionic techniques(25). In bulk, S B 36/11 microphase separates into a cylindrical morphology and produces hexagonally ordered P B cylinders in a PS matrix. SI 68/12 adopts a spherical morphology and produces PI spheres in a PS matrix with body-centered-cubic order, as shown in Figure l b . Thin polymer films were produced by spin-coating solutions of polymer dissolved in toluene onto various substrates, and the film thickness was controlled by varying the spinning speed and polymer concentration. The films were annealed in vacuum between 130°C and 170°C, a temperature above the glass transition temperatures of both blocks, for 24 hours to obtain well-ordered morphologies. Pattern Transfer as a Positive Resist For pattern transfer, thin films of spherical or cylindrical microdomain monolayers were directly spin coated on silicon wafers. Previous work on imaging the microdomain pattern on silicon wafers with a combination of lowvoltage, high resolution S E M and a REE allowed us to determine the optimum film thickness deposited by spin coating to form a monolayer of spheres or cylinders(27). T o make a monolayer of spheres with SI 68/12, a 70 nm thick-film was required, and for a monolayer of cylinders using S B 36/11, a 50 nm film was required (see Figure 2). For silicon nitride patterning, - 6 0 nm of silicon nitride was deposited on silicon wafers at 250°C by plasma enhanced chemical vapor deposition ( P E C V D ) , followed by spin coating polymer films. T o pattern germanium, we first evaporated germanium on silicon wafers, then deposited a - 1 0 nm isolation layer of P E C V D silicon nitride, and subsequently spin coated polymer solutions. W e found it necessary to protect the germanium during the ozonation (discussed in the following paragraph) process with silicon nitride to prevent the formation of germanium oxides which damage the sample. During pattern transfer, the microdomain pattern was etched through the silicon nitride and into the germanium underneath. B y selectively degrading and removing the PI or P B microdomains, the template functioned as a positive resist on the substrate, making a mask of microdomain voids. The positive resist was created by placing the coated wafers in an aqueous environment through which ozone was bubbled for .four minutes(26). The ozone cleaved the carbon-carbon double bonds of the diene component and the degradation fragments dispersed in the water(27,2
