KEYWORDS: Ions, Polymers, Diffusion, Hydrogen, Finite element methods, Microelectromechanical systems, Mechanics, Biological and chemical sensing, Computer simulations, Chemical elements
Hydrogels are 3-D network polymeric materials that exhibit a large volume phase-transition due to a of change
in their environment so that the response causes the hydrogel to swell or shrink. Since hydrogels have been
found to be useful for chemical sensing and delivery, there is a growing interest in their use for medicine. This
,requires a thorough understanding of the hydrogels characteristic response to pH. The hydrogel response can be
explained by various physical equations which are often challenging to solve. We discuss the simulation of such
phase-transitions in steady-state conditions emphasizing the response to solvent pH and other environmental
stimuli. We demonstrate a method for simulating pH response of hydrogels and describe numerical model
and its implementation in detail. Though a few models have been developed for simulation of these hydrogel
characteristics, these have been based on custom programs implemented in individual laboratories and often
not generally accessible. Hence, our modeling effort is implemented using the generic finite element software
COMSOL and the method can be used with any software having similar capabilities. The effect of buffer
solution concentration, fixed charge density, the solution pH on the swelling characteristics are studied. Results
are compared with published experimental data.
In this study, we compare the characteristics of ferrogels prepared using γ-Fe2O3 and Fe3O4 nanoparticles. The
magnetic nanoparticles with ~ 20 nm diameter were distributed in N-isopropylacrylamide (NIPAM) gel prepared using
N,N'-methylenebisacrylamide (BIS), ammonium persulfate (APS) and N,N,N',N'-tetramethylethylenediamine
(TEMED). Particle distribution and agglomeration characteristics of the prepared ferrogels were investigated using ultra
small angle x-ray scattering (USAXS) and transmission electron microscopy (TEM). The ferrogel samples prepared
using Fe3O4 and γ-Fe2O3 particles have similar particle distribution. The ferrogels, prepared with γ-Fe2O3 nanoparticles,
however, demonstrate significantly different agglomeration characteristics compared to the ferrogels prepared using
Fe3O4. In both systems, the agglomerated particles appear to be spherical, with few of those indicating chain like
structures. Based on the particle concentration and sizes, the DC SQUID magnetometry data of these samples showed
the magnetic moments range between 0.9 to 2.5 emu/g. Details of our results and analysis are presented.
KEYWORDS: Ions, Finite element methods, Sodium, Chlorine, Actuators, Chemical elements, Systems modeling, Temperature metrology, Diffusion, Focus stacking software
This paper presents the results of our work in building a finite element based numerical model, to study and understand
the actuation characteristics of gels especially in response to the changes in pH, temperature and electrical potential.
Steady state behavior of the gel is considered initially within an overall model based on 'equilibrium mechanical
condition' with buffer and fixed ion concentrations as the most affecting parametric variables. The finite element
analysis was carried out using commercially available multi-physics software, 'COMSOL'. This simulation used the
variables fixed charge density, pH (2-12), buffer solution ionic concentration (2- 10 mM) and electric potential (0 to 2
V). The variation in gel deformation characteristics with respect to temperature and applied electrostatic potential are
presented. The typical dimensions of the actuator considered are 1 mm × 3 mm. The gel deformation or displacement
with varying length to width ratio and applied potential is also described. A detailed analysis of these results is
discussed in this paper.
We present the details of our study on the internal stresses and adhesion strengths of SU-8 structures to different substrate seed layers. The effect of adhesion promoter-methacryloxy [propl] trimethoxysilane (MPTS), and OmniCoat-and different seed layer combinations (Ti/Cu/Ti, Ti/Cu, Cr/Au, and Cr/Au/Cr) was examined for internal stress and adhesion strength in 650-µm-thick SU-8 films. Increased stress and poor adhesion have led to the delamination of SU-8 in some cases. Adhesion and stress have proven to be the function of process parameters such as soft bake (time and temperature), exposure dose, post-exposure bake (time and temperature), and development time. We have found that a 100 silicon wafer containing a titanium-copper-titanium (Ti/Cu/Ti) seed layer with MPTS as the adhesion promoter yielded a thick SU-8 film with a lower value of stress and consequently better adhesion for processing in deep x-ray lithography (DXRL). A detailed correlation of the effects of adhesion and internal stress on the SU-8 film is discussed. An analysis of the possible chemical bonding interactions occurring between SU-8, and its adhesion promoter and/or substrate is also presented.
Magnetic actuation has been very successful in both macro and micro scales and magnetic microactuators with different microstructures have also been extensively studied. Almost all of these microactuators are based on the electromagnetic effect and the force is generated by the electromagnetic coil, between the soft magnetic core and the (magnetic) cantilever. To integrate all of these magnetic components on the same chip, microstructures with different aspect ratios were fabricated in a single layer by using a photolithography process using the negative photoresist SU-8 2000. A copper microcoil, a permalloy core and a nickel metal post for suspending a cantilever beam were electroplated on the same seed layer with aspect ratios varying from 1:1, 5:1 to 10:1, and the dimensions of the features ranging from 25 μm to 500 μm. The three different features were fabricated separately in three layers. Different viscosity of SU-8 2000 was used in three layers. 50 μm, 80 μm and 200 μm thick SU-8 are used to pattern the molds for electroplating three different features. 25 μm thick copper microcoils with an aspect ratio 1:1 was first electroplated on the Ti/Cu/Ti seed layer. This is followed by electroplating of 50 μm thick permalloy layer with an aspect ratio 10:1 on the same seed layer. A 100 μm thick copper post with an aspect ratio 5:1 was electroplated on to the same seed layer in the next step. SU-8 resist molds were removed between each of these layers. Finally, the residual SU-8 was cleaned by CF4 plasma etching. The coils have 19 turns with a footprint area of 10 mm2. The microcoils were tested to ascertain their maximum current densities before burnout. Similarly, the magnetic properties of the Permalloy core were also tested. The profiles of different layers and the coil and permalloy core test results were discussed in detail in this paper.
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.
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.
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°).
Titanium Nitride (TiN) is a wear resistant and complementary metal oxide silicon (CMOS) compatible material that is increasingly being investigated for MEMS applications. Incorporating any new material into a MEMS device requires the development of a processing strategy. This paper discusses a wet-etching strategy for patterning and releasing TiN features on Cr sacrificial layers. Filtered arc TiN films were deposited onto Cr coated Si (100) substrate. A Cr contact mask was sputtered over the TiN and patterned using UV photolithography. Patterned TiN features were examined using scanning electron microscopy (SEM). Rutherford Backscattering Spectroscopy (RBS) was carried out to investigate the selective etching of TiN and Cr in their respective etchants, which consisted of SC-1 for etching the TiN and a commercial chromic acid solution for etching the Cr. The results showed that Cr was not etched by SC-1 and that TiN was not etched by the Cr etchant.
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.
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.
This paper presents our design and experimental results of nickel microcantilevers, which were fabricated using a laser-LIGA process, based on KrF (248 nm) excimer laser micromachining. A chrome-on-quartz mask, containing the desired mask patterns was prepared for this work. The substrate of copper (30 μm thick) clad printed circuit board (PCB) was laminated with Laminar 5038 photopolymer to be laser patterned. Following laser patterning and laser cleaning, all the samples were electroformed with nickel on top of the copper layer. To release the Ni microcantilevers, the excimer laser was employed again to remove the polymer in the localised area to facilitate Cu selective etching. Here, copper acted as the sacrificial layer as well. The Cu selective etching was carried out with ~ 20 % (wt) aqueous solution of ammonium persulfate. Because the Cu selective etching is isotropic, some undercuts happened next to the anchor area. The samples were characterised using optical microscope, confocal laser scanning microscope and SEM, and some of Ni cantilevers were tested electro-thermally. Their performance was analyzed with respect to the simulation results.
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.
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.
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.
A pulsed excimer laser (248 nm) based LIGA-like process is presented for the fabrication of Ni serpentine microstructures, such as those that might be used for micro-heaters. The structures were produced on both Cu (60 micrometers ) clad PCB and on Cu/Ti (up to 4 micrometers /15 nm) sputtered Si (100) substrates. The substrates were coated with a Laminar dry film (35 micrometers ) photoresist, which was then patterned by laser ablation to produce the mould for Ni electroforming. The optimal ablation conditions were identified for laser patterning to prepare the micro polymer mould. Beam fluence (~ 1 J/cm2) and number of shots (~ 60 pulses) for 50 micrometers wide features on this photoresist were established, and it was observed that an increased number of shots and increased fluence were needed for features less than 20 micrometers wide. Additionally, the Cu layer surface was cleaned by the use of 5 -10 laser pulses at the same fluence. Ni electroforming has been carried out using standard Ni sulfamate bath at a current density of ~ 10 mA/cm2. After Ni electroforming, both the Laminar dry film and the Cu layers around the electroformed Ni patterns were removed using a combination of acetone, laser and Cu selective etching. Finally, a series of Ni microstructures were fabricated consisting of up to 50 micrometers wide and 35 micrometers thick serpentine tracks. The devices were measured using a scanning confocal microscope and it was found that using the excimer laser to remove the remaining dry film laminate also smoothed the electroplated Ni surfaces from a pre-laser treated Ra of 1.20 micrometers to 0.19 micrometers . Laser ablation also released the finer features from the substrate.
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.
In this work, the relative performance of patterning TiN film from metal sacrificial layers using a 248nm excimer laser is presented. Patterning performance was determined by investigating etching behavior in terms of edge quality, film delamination and layer selectivity. Using <100> silicon as a substrate, TiN was arc deposited onto sputtered Cr and Cu sacrificial layers and silicon in a partially Filtered Arc Deposition (FAD) system at 150 degree(s)C. The TiN films were directly patterned into matrixes of fluence verses number of shots. The results show excellent patterning of TiN from Cr sacrificial layers in terms of pattern quality and film selectivity. The TiN ablated from a Cu sacrificial layer produced poor patterning and no layer selectivity. The experimental results are presented and discussed in relation to the explosion mechanism of ablation.
In this paper we investigate excimer laser micromachining of TiNi shape memory alloy using an image projection system as an alternative to photolithographic patterning. We report on the characteristics of material removal by KrF excimer laser induced ablation at 248 nm and the dependence of material removal rates on laser parameters such as fluence and pulse frequency. Fluences at the workpiece using a 10x projection lens were up to 2.5 J cm-2 with pulse repetition rates up to 100Hz. Conventional chrome-on-quartz masks were used for pattern transfer. Material removal mechanisms and rates of material removal are compared with those observed during excimer laser micromachining of polymers and ceramics and limitations on achievable lateral and depth resolution explored. Data obtained by a variety of characterization methods are correlated to assess the effects of laser induced damage.
MoO3-WO3 thin films have been fabricated via the sol-gel method. FESEM, TEM, RBS and SIMS analysis techniques have been employed to analyse the films and material properties for use as gas sensors to detect CO and NO2. FESEM shows the film made up of segregated molybdenum crystals. TEM highlights the nano-sized grains sructure and crystallinity. RBS analysis confirmed the films are stoichimetric and that the Mo component of the system decreases as the annealing temperature is increased. SIMS illustrates the interesting elemental depth profiles of the films. The films were exposed to CO and NO2. MoO3-WO3 shows better NO2 sensitivity and selectivity compared to its single metal oxide constituents.
This paper presents the results of our investigations on the laser micromachining of structures in a dry film photoresist polymer (Dynachem, Laminar AX dry film) laminated on a copper clad Printed Circuit Board (PCB) and (100) Silicon wafer coated with Ti (15nm)/Cu (100 to 4000 nm) and copper seed layers. This study concentrated on investigating and comparing the effect of laser fluence (0.01 to 2 J/cm2) and number of shots (1 to 1000) on the etch characteristics of the Laminar AX dry film on both substrates. The other important aspects that were studied include the minimum required seed layer thickness for electroplating. The removal of the residual polymer layer at the end of the laser micromachining process and its effect on plating characteristics has been studied. The surface quality and roughness of the laser micromachined sites and their effect on the plated Nickel structures were studied in detail. The laser fluence and the number of shots used at this stage affected the conditions of the seed layer, which in turn influenced the plated film growth kinetics. The seed layers with thicknesses less than or equal to 0.8 mm were completely removed when high fluence (> around 1 J/cm2) was used. The seed layer surface after micromachining was characterised using Scanning Electron Microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and profilometer. High Aspect Ratio Structures (HARS) of Nickel were fabricated using the Laminar AX moulds. The highest aspect ratio achieved so far in this work is 6. The characteristics of these structures are discussed in detail.
A simple technique is described for the formation of microfluidic channels and reservoirs in polymer substrates. The technique is based upon excimer laser mask projection and includes a simple, low cost lift-off procedure, using readily available adhesively backed tapes, for the production of a metallisation layer. An additional advantage of the lift-off technique described is the removal of the debris layer usually associated with excimer laser ablation. The cross sectional profile and texture of the channels have also been controlled.
For the electrokinetic flow device presented here, up to six reservoir structures, including electrode pads, were ablated into polycarbonate, through a sacrificial layer. The entire substrate was then coated with Cu or Au by sputter deposition. Channels, 135μm wide and 135μm deep were engraved into the samples, joining the reservoirs with a selection of "T", "Y" and "X" junctions. The sacrificial layer was then peeled off of the substrate removing both the unwanted metallisation and debris deposited during the ablation process. The channels of the devices were then sealed using a dry film photoresist material which was applied by a simple lamination process. Water was flowed through devices fabricated in this manner by pumping with a syringe and no leakage was observed.
Microengineering has evolved in the last decade as a subject of its own with the current research encompassing every possible area of devices from electromagnetic to optical and bio-micro electromechanical systems (MEMS). The primary advantage of the micro system technology is its small size, potential to produce high volume and low cost devices. However, the major impediments in the successful realization of many micro devices in practice are the reliability, packaging and integration with the existing microelectronics technology. Microengineering of actuators has recently grown tremendously due to its possible applicability to a wide range of devices of practical importance and the availability of a choice of materials. Selection of materials has been one of the important aspects of the design and fabrication of many micro system and actuators. This paper discusses the issues related to the selection of materials and subsequently their effect on the performance of the actuator. These will be discussed taking micro magnetic actuators and bearings, in particular, as examples. Fabrication and processing strategies and performance evaluation methods adopted will be described. Current status of the technology and projected futuristic applications in this area will be reviewed.
Binary metal oxide MoO3-TiO2 thin films have been prepared by the sol-gel process. These films were deposited on sapphire substrates with interdigital electrodes and single crystal silicon substrates. The films were annealed at different temperatures of 400 degrees C, 500 degrees C, and 600 degrees C for 1 hour. The morphology, crystalline structure and chemical composition of the films have been analyzed using SEM, XRD, RBS and XPS techniques. The SEM analysis showed that the films annealed at 500 degrees C are smooth and uniform with nanosized grains and probes. RBS and XPS characterizations have revealed that the films are nearly stoichiometric. In this work, we have investigated the sensitivity of this material for oxygen and ozone gases. The MoO3-based gas sensor is capable of detecting O2 down to 50 ppm with a very fast response time. Adding TiO2 to MoO3 thin films tremendously reduced the resistance, which assisted the measurement of ozone gas sensing.
KEYWORDS: Magnetism, Plating, Photoresist materials, Control systems, Copper, Position sensors, Semiconducting wafers, Actuators, Electroplating, Control systems design
A micro magnetic bearing actuator has been designed using elector-magnets. In this design, the rotor position is actively controlled in the radial direction and passively supported in the axial directions. A micro position sensor along with a Proportional Derivative control system constitutes the feed-back network, which ensures the rotor actively suspended in radial directions. The circular stator has four control coils which are sandwiched between two stator end plates. The diameter of the stator and rotor are 2.1 and 2.6 mm respectively, while the thickness is fixed at 250(mu) . The air gap between the stator and rotor has been fixed at 10(mu) . Stator and rotor plates were fabricated using permalloy electroplating, while the control coils of the stator were hand wound using conventional wire winding techniques. An enamel coated copper wire of 20(mu) diameter was used for this purpose. These parts are assembled using normal micro assembly techniques. The details of the fabrication and assembly techniques employed for the micro bearing are presented along with the preliminary test results.
The ablation characteristics of the photoresist SU-8 under 248 KrF excimer pulsed laser radiation have been studied. The variation of SU-8 etch rate with fluence has been investigated in the range 0.05 J/cm2 to 3.01 J/cm2. Threshold fluence for ablation of SU-8 is measured to be about 0.05 J/cm2. The etch rate of SU-8 is found to be higher than that of polyimide under identical conditions. The curves for etch rate dependence on fluence are in agreement with those reported in the literature for polyimides and also obey a theoretical model. We have investigated the effect of different prebake temperatures on the ablation characteristics which are found to be similar for all temperatures. The effect of increasing the number of laser shots has been examined at different fluences to understand the etch rate variation near the 'end of film' stage of ablation. The results of our analyses of these samples using SEM, profilometery and optical microscopy reveal - very smooth morphology of the etched surfaces without significant debris, no noticeable damage to underlying silicon, gradual build up of a carbonaceous film outside and around the etch pits. We find SU-8 very suitable for rapid excimer ablation lithography which has been demonstrated by patterning a gear structure in an SU-8 resist layer with aspect ration of 4.5.
This paper presents the results of our investigation on deposition and characterization of sol-gel prepared TiO2 thin films for oxygen sensing applications. The properties of pure TiO2 thin films are compared with those doped with niobium oxide and 1%pt. These films are characterized using Rutherford backscattering spectroscopy (RBS), x-ray diffraction (XRD) and scanning electron microscopy (SEM) to study their chemical composition, structure and surface morphology respectively. Both kinds of the films are stoichiometric. Pure TiO2 as well as doped films were amorphous as deposited. Pure TiO2 films after annealing to 450 degrees Celsius and above showed the formation of anatase phase, while the doped films still predominantly amorphous, barely showing the onset of crystallinity. Pure films after annealing to 600 degrees Celsius appear to have become granular and porous. Doping with niobium oxide and Pt resulted in modification of film microstructure also. As a result of doping, the gas sensitivity of the films is increased from 8 to 24 and operating temperature decreased from 320 degrees Celsius to 190 degrees Celsius.
In this paper we present a novel method for selective electroplating on silicon without the need of using a continuous plating base film. Ion implantation on the polished front face of a silicon wafer is employed to induce selective seeding of electroplating. Silicon substrates have been implanted at RT with 19 keV Pd+ ions at several doses 7e14, 2e15, 1e16 and 1e17 ions/c,2 using a metal vapor vacuum arc ion implanter. These are then electroplated under similar bath conditions using a Ni-Fe plating solution to produce permalloy films. The unimplanted regions do not get plated showing the selective nature of this seeding process. Adhesion is better for films plated on substrates implanted at higher dose. To study the effect of implanted species, a silicon wafer was implanted with self ions and plated under similar conditions. This sample showed deposition of an extremely thin film. The potential of the technique has been demonstrated by patterning 50 and 100 (mu) wide metallic structures on silicon substrate.
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