Muralidhar K. Ghantasala

An investigation of SU-8 resist adhesion in deep x-ray lithography of high-aspect-ratio structures

The SU-8 negative photo resist has been recognised as an unique resist, equally useful for UV lithography and deep x-ray lithography (DXRL) applications; but it is in DXRL where SU-8 has shown a significant advantage over other resists. When compared with the common DXRL resist poly-methyl methacrylate (PMMA), SU-8 has been found to significantly reduce x-ray exposure time, processing time and cost, thus making SU-8 a strong candidate for commercial DXRL applications. Despite these advantages, several factors associated with SU-8 processing are not well understood. Resist-substrate adhesion, which is the key for successful lithography, is one such example. This paper examines the effect of substrate (silicon and graphite), seed layer (Ti/Cu/Ti, Ti/Cu, Cr/Au and Cr/Au/Cr), and the use of adhesion promoters (OmniCoat and MPTS) on the adhesion of SU-8 structures. In addition, parameters such as SU-8 thickness (450 μm, 650 μm, 900 μm) and substrate roughness values (silicon, Ra < 10 nm and Ra = 0.5 μm) have also been investigated. The results of our work highlight the importance of material selection for a given process and the relationship between the different parameters investigated. Increased stress for thicker films (> 850 μm) has lead to the delamination of SU-8 on some substrates. The adhesion has also proven to be a function of process parameters such as pre-bake (time and temperature), exposure dose, development time and post exposure bake (time and temperature). We have found that a <100> silicon wafer (Ra = 0.5 μm) containing a titanium-copper-titanium (Ti/Cu/Ti) seed layer, provided an adequately adhered resist for DXRL, while a chromium-gold (Cr/Au) seed layer on silicon (Ra = 0.5 μm) showed poor adhesion. A detailed correlation of the effect of these parameters on SU-8 adhesion will be discussed in this paper.

Fabrication, measurement and modeling of electroosmotic flow in micromachined polymer microchannels

Electroosmotic pumping in the microchannels fabricated in polycarbonate (PC), polyethyleneterephthalate (PET) and SU-8 polymer substrates was investigated and species transportation was modeled, in an attempt to show the suitability of low cost polymer materials for the development of disposable microfluidic devices. Microchannels and the fluid reservoirs were fabricated using excimer laser ablation and hot embossing techniques. Typical dimensions of the microchannels were 60μm (width) x 50μm (depth) x 45mm (length). Species transportation in the microchannels under electroosmosis was modeled by finite element method (FEM) with the help of NetFlow module of the CoventorWareTM computational fluid dynamics (CFD) package. In particular, electroosmosis and electrophoresis in a crossed microfluidic channel was modeled to calculate the percentage species mass transportation when the concentration shape of the Gaussian input species plug and the location of the injection point are varied. Change in the concentration shape of the initial species plug while it is electroosmotically transported along the crossed fluidic channel was visualized. Results indicated that Excimer laser ablated PC and PET devices have electroosmotic mobility in the range 2 to 5 x10-4 cm2/V.s, zeta potential 30 to 70 mV and flow rates of the order of 1 to 3 nL/s under an electric field of 200 V/cm. With the electroosmotic mobility value of PC the simulation results show that a crossed fluidic channel is electroosmotically pumping about 91% of the species mass injected along one of its straight channels.

Single-shot selective laser micromachining of filtered arc deposited TiN films from chromium underlayer

This paper presents the results on single-shot laser micromachining of filtered arc deposited TiN films and compares the machining characteristics of the films deposited under partially and fully filtered conditions. Machining performance was evaluated in terms of patterning quality and the ability to perform selective removal of top TiN film with minimal interference to an underlying layer. TiN was arc-deposited onto silicon substrate with a chromium layer on the top. These films were analysed for their composition and microstructure using Rutherford Backscattering Spectroscopy (RBS) and Scanning Electron Microscopy (SEM) before and after laser machining. Under single shot conditions the effect of fluence on the machined features has been investigated. The results showed selective removal of TiN films with a single shot from the underlying Cr layer. Further, this work clearly shows a distinction between the laser machining characteristics of the films deposited under different filtering conditions and substrate temperatures.

Patterning of SU-8 resist structures using CF4

Carbon Tetraflouride (CF4) plasma etching condition for SU-8 negative photoresist is characterized for its potential applications in photonics and bioMEMS. The effects of main plasma etching parameters such as rf power, gas flow rate, chamber pressure and time were systematically studied and the parameters were optimized by a three-level, L9 orthogonal array of the Taguchi method. By optimization, the optimal parameter range and the weighted percent of each parameter on the final results i.e. depth, surface roughness and wall angle were determined. Photoresist & metal were used and compared as masks for plasma etching. The minimum feature size was 1µm in both cases. Results indicated that with the increase of rf power, etch rate and roughness increases almost linearly. With increase in gas flow rate, etch rate increases while roughness decreases non-linearly. Etch rate is linear with time but roughness is significantly dependent on time initially. The side-wall angle of the samples with metal mask was found to be nearly 90° whereas samples with photoresist as the mask showed poor side-wall angle and surface roughness mainly due to poor mask-resist selectivity. Optimized values of rf power, gas flow rate, time and pressure were found to be 200W, 240sccm, 20minutes and 1Torr respectively, which yielded high etch rate (80nm/min), low surface roughness (5nm) and nearly vertical side-walls (89°).

Integration of microfluidic channels and optical waveguides using low-cost polymer microfabrication techniques

Recent progress on the realization of a silicon integrated biophotonic chip using plasma etching and laser ablation is presented. The chip utilizes films of SU-8 and UV-15 polymer material. An intersecting optical waveguide and microfluidic channel exhibiting good optical transmission across the channel is demonstrated.

Fabrication of 3D high-aspect-ratio microfluidic components using laser machining and LIGA

This paper mainly reviews the current status of the LIGA fabrication technology for use in microfluidics applications. It also presents the work that the group is involved in the fabrication of 3-D high aspect ratio structures. This work explored the possibility of using LIGA and excimer laser micromachining as complimentary processes in two sequential steps for making some of the components required for fluidic applications. A microflask is designed and fabricated using this combination of processing technology. The details of the design and fabrication technology are presented in this paper.

Effect of argon gas pressure and substrate temperature on magnetic properties of magnetron sputtered SmCo thin films

Preparation of hard magnetic SmCo thin films onto silicon substrate has been considered as one of the important steps in the realisation of magnetic MEMS devices. In this paper, we report the results of our investigations on the deposition and characterisation of these films. In particular, this paper deals with the study of the effect of argon gas pressure (10 ~ 40 mTorr) and substrate temperature (R.T. ~ 600 °C) on the composition, structure, and magnetic properties of SmCo thin films. These films were characterised using RBS, XRD and SQUID. The results indicate that the Co/Sm ratio of the films decreases with increasing argon gas pressure, but increases with increasing substrate temperature. As substrate temperature rises, both the degree of crystallinity and in-plane texturing increase, resulting in an increased in-plane intrinsic coercivity. Films prepared at lower substrate temperatures exhibit lower coercivity values due to the amorphous or partially crystallised phases in the films.

Design, modeling and fabrication of piezoelectric polymer actuators

Piezoelectric polymers are a class of materials with great potential and promise for many applications. Because of their ideally suitable characteristics, they make good candidates for actuators. However, the difficulty of forming structures and shapes has limited the range of mechanical design. In this work, the design and fabrication of a unimorph piezoelectric cantilever actuator using piezoelectric polymer PVDF with an electroplated layer of nickel alloy has been described. The modeling and simulation of the composite cantilever with planar and microstructured surfaces has been performed by CoventorWare to optimize the design parameters in order to achieve large tip deflections. These simulation results indicated that a microstructured cantilever could produce 25 percent higher deflection compared to a simple planar cantilever surface. The tip deflection of the composite cantilever with a length of 6mm and a width of 1mm can reach up to 100μm. A PVDF polymer with a specifically designed shape was punched out along the elongation direction on the embossing machine at room temperature. The nickel alloy layer was electroplated on one side of the PVDF to form a composite cantilever. The tip deflection of the cantilever was observed and measured under an optical microscope. The experimental result is in agreement with the theoretical analysis.

Patterning of magnetron sputtered SmCo thin films by excimer laser ablation

This paper presents the results of our investigations on excimer laser micromachining of SmCo thin films. These films were prepared onto silicon substrate with a SiO2 layer and chromium film on the top. The ablation characteristics of the films were analysed by examining the surface topography using optical microscopy and Scanning Electron Spectroscopy (SEM). The quality of the etched patterns was evaluated in terms of sharpness of the edges, side wall profile and ratio of pattern mask size to feature size. The etch characteristics were studied over a wide range of laser parameters with the fluence in the range 0.25 J/cm2 and 1.6 J/cm2 and the number of shots varied from 1 to 50. The results showed that the precise nature of the resulting structure is a strong function of fluence and number of shots. These studies provide insight into the ablation behaviour of SmCo films and their potential applications in MEMS.

Influence of patterning geometry on the electrodeposition of microstructures fabricated by laser LIGA

This paper describes recent experimental studies on the effect of patterning geometry on the laser machining parameters and electrodeposition rate of nickel microstructures fabricated using laser LIGA. The effect of shape, size and spacing of features has been studied for structures plated into Laminar AX moulds. In contrast to previous work, which has concentrated on low aspect ratio (< 0.1) geometries or on large (> 1 mm) structures, we specifically address here problems relating to aspect ratios in the range 0.14 - 8.75 and feature sizes of ~ 4 micrometers to 200 micrometers . Mould structures and plated features have been examined using optical, scanning electron and laser scanning confocal microscopy. Results show that for features >50 micrometers , the thickness profile of plated shapes varies by approximately +/- 1 micrometers m over most of the surface area with the edges demonstrating corner rounding with a radius ~ 5 micrometers . Below 20 micrometers in size, thickness profiles become peaked towards the center of a feature. A surface roughness (Ra) of ~ 1.0 micrometers is also observed. The reduction in deposition rate over the 3 hour electroforming process has also been explained in terms of an increase in plating area due to the profile of the laser ablated moulds.