Thomas Glinsner

High resolution lithography with PDMS molds

The resolution, dimension stability, and reproducibility of the Soft UV-Nanoimprint is investigated. The potential for imprinting nanostructures with flexible molds in UV-curable resists in the 100nm regime are explored and the limitations analyzed. The dimensional stability of imprinted patterns is determined by the deformation of the mold that in term depends on the geometry of the structures and the imprint pressure applied.

Nanoimprint lithography with a commercial 4-in. bond system for hot embossing

In order to examine the suitability of nanoimprinting for wafer scale pattern definition, a commercially available hot embossing system, the EV520HE of EVGroup, Austria, has been used to imprint 4 inch substrates. The EV520HE is based on a production-proven wafer bonding system which guarantees compatibility with semiconductor fabrication conditions. A 4 inch silicon wafer fully patterned with structures from 400 nm to 100 micrometers size was used as a stamp. The patterns, having a nominal height of 260 nm were defined in poly-Si over SiO2 by reactive ion etching. Different anti- sticking layers were applied to the stamps by monolayer self-assembling, among them (1,1,2,2 H perfluoroctyl)- trichlorosilane. Two different polymers, polymethylmethacrylate (PMMA) and a commercially available nanoimprint resist were used to spin-coat the substrates. Imprints were performed with temperatures of up to 225 degree(s)C, forces between 10 bar and 55 bar and holding times of 5 and 15 minutes. After separation of stamp and sample the imprints were characterized by a surface profiler and inspected by an optical microscope as well as a scanning electron microscope. Different qualities of pattern transfer according to the used process parameters were achieved, but patterning of the whole sample surface was always observed. In contrast to radiation-based lithography, the difficulties are based in imprinting of larger features whereas structures of 400 nm size were reproduced with high quality. Therefore the largest patterns of the stamp, 100 micrometers square bond pads, were used for imprint quality assessment, judged by the degree of stamp cavity filling around the pads. High quality was achieved by embossing at 225 degree(s)C with a hold time of 5 minutes at a pressure of 55 bar. For full wafer imprint only a small degradation of imprint quality from the center towards the periphery was observed. Further optimization of the process is required to minimize residual layer thickness for the hot embossing lithography step, taking into account the visco-elastic properties of the polymer material.

Cyclo-olefin polymer direct bonding using low temperature plasma activation bonding

Low temperature direct bonding method of Cyclo-Olefin Polymer (COP) plates (20mm /spl times/ 40mm /spl times/ 2.0mm) has been developed employing surface plasma treatment with various gases such as N/sub 2/, O/sub 2/, and 10%-H/sub 2//Ar. Surface energy of the bonded interface has been measured by razor blade method. Reasonable bonding strength for flow devices was achieved even at room temperature. The contact angle measurement on the sample surfaces after plasma exposure indicated that the plasma activated surfaces became hydrophilic and this activated state lasted for longer than 2 months. This method is useful to fabricate micro-flow devices for single-molecule level optical bio-detection systems that requires less residual stress and deformation after bonding.

New Results on Plasma Activated Bonding of Imprinted Polymer Features for Bio MEMS Applications

Nanoimprint Lithography is a well-acknowledged low cost, high resolution, large area 3D patterning process for polymers. It includes the most promising methods: high pressure hot embossing (HE) and UV-Nanoimprint Lithography (UV-NIL). Curing of the imprinted structures is either done by cooling down below the glass transition temperature of the thermoplastic polymer in case of HE or by subsequent UV-light exposure and cross-linking in case of UV-NIL. Both techniques allow rapid prototyping for high volume production of fully patterned substrates for a wide range of materials. The advantages of using polymer substrates over common Micro-Electro-Mechanical Systems (MEMS) processing materials like glass, silicon or quartz are: bio-compatible surfaces, easy manufacturability, low cost for high volume production, suitable for use in micro- and nano-fabrication, low conductivity, wide range of optical properties just to name a few. We will present experimental results on HE processes with PMMA as well as UV-NIL imprints in selected UV-curable resists. In the second part of the work we will describe the bonding techniques for packaging of the micro or nano structures. Packaging of the imprinted features is a key technology for a wide variety of field of applications: µ-TAS, biochemistry, micro-mixers, micro-reactors, electrophoresis cells, life science, micro-optical and nano-optical applications (switches) nanofluidics, data storage, etc. for features down to sub-100 nm range. Most bonding techniques for polymer use adhesives as intermediate layers. We will demonstrate a promising technique for dense and very strong bonds using plasma activation of polymers and glass. This bonding technology allows for bonding at low temperatures well below the glass transition temperature of the polymers, which will ensure that the structures are not deformed.

3D interconnect through aligned wafer level bonding

Wafer level packaging and 3D interconnect technologies are driven by increasing device density and functionality as well as reduction of total packaging cost. Key enabling technologies for 3D interconnect are high precision alignment and bonding systems and thick resist processing. A unique processing equipment has been developed to meet high volume production requirements. This paper reviews advances in equipment processing capabilities and provides a guideline for new processing techniques available. Wafer to wafer alignment can be carried out in various ways. Traditionally aligned wafer bonding is a well-established technology in the MEMS industry. Special requirements for 3D interconnects are high accuracy, the use of single side processed wafers and 8 inch capability. A comparison of wafer alignment technologies is presented. Also, a novel face-to-face alignment method is described and analyzed in terms of alignment accuracy before and after bonding. Wafer bonding is carried out subsequent to the alignment step in a separate process module. A summary of different bonding methods is given. Intermediate layers that act as bonding agent can be spun on to the wafer. Such coating processes for wafer level packaging applications vary greatly from the requirements for VLSI processing. VLSI photoresist processes use thin layers to transfer small features with sub-micron tolerances. Wafer level bumping typically is performed with 5-150 /spl mu/m films, to transfer large (20-250 /spl mu/m) features, with tolerances approaching micron scale. HDI applications require thicker adhesive layers. The paper concludes with application examples with different intermediate layers such as BCB.

Development of nanoimprint processes for photovoltaic applications

Abstract. Due to its high resolution and applicability for large area patterning, nanoimprint lithography (NIL) is a promising technology for photovoltaic (PV) applications. However, a successful industrial application of NIL processes is only possible if large-area processing on thin, brittle, and potentially rough substrates can be achieved in a high-throughput process. The development of NIL processes using the SmartNIL technology from EV Group with a focus on PV applications is described. The authors applied this tooling to realize a honeycomb texture (8  μm period) on the front side of multicrystalline silicon solar cells, leading to an improvement in optical efficiency of 7% relative and a total efficiency gain of 0.5% absolute compared to the industrial standard texture (isotexture). On the rear side of monocrystalline silicon solar cells, the authors realized diffraction gratings to make use of light trapping effects. An absorption enhancement of up to 35% absolute at a wavelength of 1100 nm is demonstrated. Furthermore, photolithography was combined with NIL processes to introduce features for metal contacts into honeycomb master structures, which were initially realized using interference lithography. As a final application, the authors investigated the realization of very fine contact fingers with prismatic shape in order to minimize reflection losses.

Impact of vacuum environment on the hot embossing process

One of the key questions concerning the concept of a system for hot embossing lithography is whether or not it should provide for imprinting under vacuum. We have performed experiments comparing the embossing in vacuum and in atmospheric pressure in a semi-automated imprint system. The stamps used were fully patterned, 10 cm diameter with pattern sizes ranging from 400 nm to 100 μm. It turned out that vacuum enhances the large area uniformity of the imprint by avoiding an air cushion remaining between stamp and sample during automated contact after a non-contact assembly and alignment step. Lower molecular weight polymers turned out to be more sensitive to uniformity deviation than higher molecular weight materials. Detailed analysis showed that defects typically found for relatively high processing temperatures, caused by overheated compressed air, remaining solvent in the polymer layer or even beginning polymer decomposition could be reduced substantially under vacuum embossing conditions, where the excess volume of the polymer is evacuated and free to accommodate gaseous constituents. The best result with complete cavity filling and negligible defects was obtained for imprint of a 99 kg/mol polymer at 200°C and 50 bar under vacuum. Residual layers measured across the diameter of the sample were 44.5 nm ± 9.8 nm. The non-uniformity of the residual layer is a result of the locally different pattern sizes and pattern densities of the stamp, typical for all mechanical patterning processes.

Fabrication of 3D-photonic crystals via UV-nanoimprint lithography

Optical lithography will reach its limits due to the diffraction effects encountered and the necessity for using complex resolution enhancement techniques like optical proximity correction, phase shift masks, and off-axis illumination [L. W. Liebmann et al., in Advanced Semiconductor Lithography (2001), Vol. 45]. The restrictions on wavelength, in combination with high process and equipment costs, make low-cost, simple imprinting techniques competitive with next-generation lithography methods. There are several nanoimprint lithography (NIL) techniques which can be categorized depending on the process parameters and the imprinting method—either step and repeat or full wafer single-step imprinting. A variety of potential applications has been demonstrated using NIL (e.g., surface acoustic wave devices, vias and contact layers with dual damascene imprinting process, Bragg structures, patterned media) [M. D. Stewart et al., Proc. SPIE 5751, 210 (2005); P. Dorsey et al., in Discrete Track Recording (DTR) Media...

Fabrications of Micro-Channel Device by Hot Emboss and Direct Bonding of PMMA

We have fabricated and evaluated the mechanical, optical and fluidic characteristics a 50µm wide and a 30µm deep micro-channel device produced by hot emboss and direct bonding of PMMA plate with dimensions of 20mm × 20mm × 1mm. The fabricated micro-channel device was evaluated the bond strength, which was confirmed to be high enough for practical use as well as for quite severe cleaning conditions as ultrasonic cleaning in pure water. The optical loss around bonded interface was also evaluated and no increase in the light absorption was observed. The above results confirmed that the hot emboss and direct bonding technologies for micro-channel manufacturing using the PMMA plates realizes high performance micro channel devices.

Soft UV-based nanoimprint lithography for large-area imprinting applications

The International Technology Roadmap for Semiconductors (ITRS) lays out a quite challenging path for the further development of the patterning techniques needed to create the ever-smaller feature sizes. In recent years the standard lithography reached its limits due to the diffraction effects encountered and the necessary complexity of compatible masks and projection optics. The restrictions on wavelength, in combination with high process and equipment costs, make low cost, simple imprinting techniques competitive with next generation lithography methods. Nanoimprint Lithography (NIL) is predicted as one candidate for the 32 nm and 22 nm technological nodes according to the ITRS. There are several NIL techniques which can be categorized depending on the process parameters and the imprinting method - either step & repeat or full wafer imprinting. A variety of potential applications has been demonstrated by using Nanoimprint Lithography (e.g. SAW devices, vias and contact layers with dual damascene imprinting process, bragg structures, patterned media) [1,2]. In Soft UV-NIL processes the overlay alignment accuracy was not demonstrated to be prepared for nanoelectronic devices; however other applications are already in high volume manufacturing such as the production of optical components (e.g. micro lenses).