Session W24: Focus Session: Lithography

Nanoimprint Lithography: Process Induced Stresses and Pattern Stability

Nanoimprint lithography is emerging as an economical technique for fabricating polymeric nanostructures. Features as small as 10 nm in a hard master or mold can be faithfully replicated by imprinting this master into a polymer film. At elevated temperatures and pressures, the molten polymer fills the nanoscale cavities of the mold. When the film is cooled to the vitreous state and the mold removed, freestanding polymeric nanostructures remain. In this presentation we illustrate that the NIL process induces large degrees of residual stress into these structures. Upon heating imprinted nanostructures to just above the glass transition temperature of the polymer, a physical relaxation of the nanostructure shape occurs. The features shrink in height and broaden in width with increased annealing time. However, this decay or slumping of the imprinted pattern is not driven by a simple viscous flow. High molecular mass polymer patterns slump faster than their low molecular mass analogs, contrary to the viscosity changes. Rather, the high viscosity resins generate greater shear stresses along the mold interfaces that lead to extensional flow of the polymer in the fill directions of the patterns. This traps residual stresses in the nanostructures when they are cooled into the glassy state. We quantify this slumping process using X-ray scattering and reflectivity techniques for a range of polymers and pattern sizes and explore potential relations with the glass transition of the polymer within the nanostructure.   

Nanometer-scale control of the crystallization of oligomers and polymers.

The ability to control the position and local orientation of organic crystals at the nanometer scale paves the way to the fabrication of hybrid nano-devices displaying better properties. Here, we present two ways to control the assembly of organic chain compounds into nanometric crystals of defined location or orientation. We first show how the location of crystals of model oligomers can be directed by chemical nano-templates [1]. The templates are obtained by combining electron-beam lithography with the deposition of self-assembled monolayers [2]. These surfaces can then be used to control a variety of assembly processes [3], such as the crystallization of model alkane-1-ol oligomers in solution. By using directing maps with the appropriate chemical inks, nano-squares, nano-corrals and nano-lines of organic crystals are rapidly and massively grown at pre-defined locations, at least down to 60 nm. At this scale, confinement effects mediated by van der Waals forces become prominent, providing a unique handle to design crystal growth. Then, we show how the nucleation and orientation of polymer crystals can be controlled by nano-imprint lithography [4]. The combination of confinement, and of preferential nucleation at the vertical walls of the nano-molds probably arising from partial chain orientation due to the polymer flow during embossing, results in local control over the 3D orientation of the crystals. We demonstrate that crystals may be guided through complex geometries, and investigate the case of systems where conflicting instructions are delivered to the crystallizing chains. \newline \newline References: \newline [1] J. Plain et al., submitted. \newline [2] A. Pallandre et al., Nano Letters 2004, 4, 365. \newline [3] A. Pallandre et al., J. Am. Chem. Soc. 2005, 127, 4320; F.A. Denis et al., Small 2005, 1, 984; A. Pallandre et al., Adv. Mater., in press. \newline [4] Zhijun Hu et al., Nano Letters 2005, 5, 1738.   

UV Polarizer Fabricated by Diblock Copolymer Lithography

Transmission UV polarizers are desired for next-generation semiconductor device fabrication using ArF or F$_{2}$ excimer laser lithography. Controlling polarization is essential especially for high numerical aperture (NA) immersion lithography processes. The polarizer requirements are thickness less than 1 mm and low absorption of the light used for the exposure. A wire grid polarizer is ideal for this purpose but it requires wires with a pitch less than quarter of wavelength of the light. A cylinder-forming polystyrene-polyhexylmethacrylate diblock copolymer (PS-PHMA, 21-64 kg/mol) was used as a mask for fabrication because its cylinders macroscopically align by simple application of shear stress, and the PHMA domains etch faster than PS by reactive-ion etching (RIE), providing sufficient contrast for pattern transfer. The diblock was spin-coated on a UV transparent fused silica substrate and shear-aligned. The stripe pattern was transferred by RIE onto the substrate by a multilayer technique to enhance the pattern height, then a metal was deposited by evaporation. Finally, the remaining polymer was lifted off to complete the wire grid, having a 33nm pitch (16.5nm line and space). The UV light polarization characteristics of these grids will be presented.   

Simple Analytic Model for Nanowire Array Polarizers

Cylinder-forming diblock copolymers can be used to pattern nanowire arrays on a transparent substrate to be used as a polarizer, as described by Koji Asakawa in a complementary talk at this meeting. With a 33nm period, these wire arrays can polarize UV radiation, which is of great interest in lithography, astronomy and other areas. One can gain an analytical understanding of such an array made of non-perfectly conducting material of finite thickness using a model with an appropriately averaged complex dielectric function in an infinite wavelength approximation. This analysis predicts that the grid can go from an E-polarizer to an H-polarizer as the wavelength decreases below a cross-over wavelength, and experimental data confirm this cross-over. The overall response of the polarizing grid depends primarily on the plasma frequency of the metal used and the volume fraction of the nanowires, as well as the grid thickness. A numerical approach is also used to confirm the analytical model and assess departure from infinite wavelength effects. For our array dimensions, the polarization is only slightly different from this approximation for wavelengths down to 150nm.   

Robust Nanopatterns from Self-Assembly of a Diblock Copolymer and an Inorganic Precursor

Nanoscopic patterns from self-assembled block copolymer thin films have been recognized as a promising route to sub-lithographic patterns on substrates. Line patterns from lamellar phase of block copolymers are particularly attractive as they can be used as an etch mask for transferring patterns into substrates. A few organic block copolymers have been studied for generating line patterns by controlling the orientation of lamellar microdomains. The organic nature of the block copolymers, however, often gives poor thermal stability and etching contrast, which limits potential applications. Indeed robust nanostructures of sub-lithographic length scales are highly desirable to comply with common nanofabrication processes. Here we report a simple method to create robust nanoscopic line patterns on surfaces from self-assembly of mixtures of a diblock copolymer and an inorganic precursor. The organic diblock copolymer directs the structure of the inorganic precursor and can be removed by thermal treatment. By tuning the interfacial energy at two interfaces, normally oriented lamellar patterns of approximately 20nm half-pitch and 40nm thick were obtained. Results on transferring patterns to substrate will be reported as well.   

Fabrication of inorganic photonic crystals from interference lithography

We have fabricated 3D FCC-like microstructure using multi-beam interference pattern. This polymeric structure was used as a sacrificial template. Silica was deposited into the pores by alternating exposure to water and silicon tetrachloride vapors under atmospheric pressure and at room temperature. This inorganic structure can provide a platform for the deposition of high refractive index materials such as silicon, germanium, and titania. We investigate the photonic bandgap property of this structure as a function of refractive index as well as filling ratio. Using a two-parameter level-set approach, we find that the FCC-like structure has multiple complete photonic bandgaps at 2-3 and 7-8 bands, respectively, while the bandgap width is sensitive to the morphology of coated-structure. Our calculation results suggest that the complete-filled structure possessed a wider photonic bandgap between 2 and 3 bands than the incompletely-coated core-shell structure.   

Effects on Low Voltage Electron Beam Lithography

To examine the practical limits and problems of low voltage operation we have studied e-beam lithography in the low (few keV) to ultra-low ($<$500eV) energy range, employing commonly used resists such as PMMA and compared the results to those from conventional high voltage processing. We have directly imaged, exposed and developed resist profiles as well as deposited metal after liftoff, using scanning electron microscopy, and compared to our advanced Monte Carlo simulations which incorporate elastic, inelastic, fast secondary electrons, and plasmon contributions. The results show that the exposed profiles and resolutions experimentally achieved at low energy can only be matched by simulations which include a significant FSE and plasmon contributions to the energy PSF. With an optimized resist thickness proximity effects are greatly reduced and process latitude is improved.   

Hierarchical Organization of Nanoparticle Composites through Nano-Imprinting

Formation of highly ordered and morphologically controlled nanoparticle/polymeric assemblies is highly desirable in a variety of applications including optoelectronics, sensing, photonics and catalysis. Structure-guiding polymer matrices, such as block copolymers, have demonstrated to be an effective means for controlling the composite morphology as well as localizing particles in nanoscale domains. Yet, fabrication of polymer/nanoparticle composites with precise control over final morphology and particle location is still a major challenge. In this paper, we will show the use of nano-imprint lithography to pattern polystyrene/gold composites at different length scales (20 nm to 1micron). Imprint lithography (the technique of physically pressing and deforming a polymer layer for patterning purposes) is a low cost and high throughput alternative to conventional patterning. PS coated gold nanoparticles were synthesized following Brust's method and subsequently dispersed in PS matrices of varying molecular weights. Nano- imprinting was conducted under different temperature and pressure conditions. TEM, SEM, AFM and UV-Vis techniques were used to characterize these composites.   

Mesoscale Simulation of the Lithography Process

Process simulation is an important tool for the semiconductor industry. Optimization of the microlithography process is extremely expensive since exposure tools that cost in excess of 20 million dollars, which could be dedicated to manufacturing, must be used in the optimization experiments. There exist several commercial simulation packages that work efficiently and accurately all of which are based on parameterized continuum models. Continuing improvements in materials and equipment for microlithography have now provided the ability to print circuit elements with minimum dimensions approaching the size of the molecules that comprise the photoresist. As a result, stochastic and molecular scale effects such as line edge roughness have become an increasing concern and may limit continued progress in this industry. These effects can not be captured by continuum models. Hence, we have worked to develop a mesoscale simulation of the process. The simulator is based upon with discretization of the photoresist film into cells on a three dimensional lattice and a Monte Carlo approach. The entire process has now been simulated in this way. New models for reactive transport, polymer dissolution, etc. have been incorporated in this simulator. Progress will be reported.   

New Directions in 3-D Multiphoton Lithography.

Multiphoton absorption polymerization (MAP) is a promising technique for the lithographic fabrication of 3-D microdevices. However, this technique also has two major shortcomings that have so far precluded its use in the mass production of devices. First, MAP is an inherently serial technique, and structures must be created on a voxel-by-voxel basis. Second, the fabrication of many desirable 3-D devices requires incorporation of materials other than polymers. We will discuss our recent progress in attacking both of these problems. We have developed soft-lithographic techniques that allow for the creation of complex 3-D structures that can include closed loops based on master structures created using MAP. We have also developed a technique that allows for the selective deposition of materials that include metals, metal oxides, and biomolecules on desired regions of 3-D structures fabricated with MAP. We demonstrate the use of the latter technique in the creation of functional microinductors.   

High efficient LEDs having columnar structure surface fabricated by block copolymer lithography

Recently, the internal quantum efficiency of LEDs has improved, but the external efficiency remains low due to the high refractive index of semiconductors. To extract more light, a columnar structure with sub-micron period was fabricated on the LED surface by block copolymer lithography. Since the desired pattern is relatively larger than the block copolymer microdomains, a super-high molecular weight block copolymer was used. The polystyrene (PS) - polymethyl methacrylate (PMMA) diblock copolymer was used in this study since the PMMA has a much faster etch rate than the PS by reactive-ion etching (RIE). The PMMA was removed by RIE, and the gallium phosphide (GaP) substrate was etched by chlorine-based inductively coupled plasma RIE using the remaining PS dots as a mask. The optical extraction efficiency of the patterned substrates improved 2.6 times compared to unprocessed flat GaP substrates; the pillars' height was 450 nm, diameter was 100 nm, and pitch was 150 nm. We also prototyped a real LED and increased light emission volume 1.8 times compared with conventional LED at the same energy consumption.