The geometry of the Schottky contact electrode is important in the design of Schottky power diodes. This work focuses on the optimum shape of the Schottky contact geometry and uses finite element modeling to determine the effects of the shape on electrical characteristics of a diode. The investigation considers the typical situation where the contact is smaller than the substrate area. Simulations were run with different shapes ranging from perfect square to perfect circle with the size of the diode substrate (die) and the distance between the edge of the diode and edge of the Schottky contact as a constant. The different models were examined and compared with magnitude the occurrence of the maximum current density (for a particular output current) and hence the breakdown regions at current density approaching the critical value for breakdown (most likely destruction of a diode) due to high current density. There as an optimum geometry determined for the highest current that the given diode substrate could deliver. The results clearly show that the optimum geometry for the Schottky contact should be neither perfect square nor perfect circle, but an exact geometry in between. This optimum geometry gives the optimum distribution of current density around the edge of the Schottky contact. Investigation is done using Synopsys TCAD. The forward and reverse bias situations were investigated to optimize the electrode geometry.
The four-point probe technique is well known for its use in determining sheet resistance and resistivity (or effective resistivity) of thin films. Using a standard four-point probe setup, relatively large area samples are required. The convention is that the distance from any probe in the probe arrangement should be at least ten times the probe spacing from the sample boundary in order to use the fixed correction factor. In this paper we show, using computer modelling, how accurate measurements can be made using appropriate correction factors for samples that are either small or of any thickness. For the significant extent of variations used, the correction factor does not vary significantly.
Crystalline germanium substrates were amorphised to a depth of one micron by ion implantation of germanium ions at a series of relatively high energies and dose. Using the ion implantation modeling software TRIM, this paper compares the amorphisation results from the ion implantation simulations and experimental results from transmission electron microscopy (TEM) analysis of cross-sections of implanted samples. TEM cross-section micrographs show a clear boundary between amorphous and crystalline germanium. The effect of amorphisation of Ge on the subsequent formation of Nickel germanide is demonstrated and one significant issue is the increased depth of NiGe grains formed on a-Ge compared with c-Ge.
The specific contact resistivity of a metal-semiconductor ohmic contact can be determined in various ways and several of these use the transmission line model approach. Concentric circular contacts have circular equipotential and using this and the transmission line model equations for such contacts, a new technique for determining specific contact resistivity is presented. An analytical technique is used to determine the error of this structure and the developed analytical equations are presented. Finite-element modeling results for Al-SiC ohmic contacts are presented to validate the analytical equations. The scaling behavior of this structure is also discussed.
Low resistance contracts to highly doped silicon carbide (SiC) are investigated. Using a novel test structure that is easy to fabricate and easy to use, this paper demonstrates how it is used to reliably determine relatively low specific contact resistivities which vary with heat treatment. The test structure requires no error correction and is not affected by parasitic resistances. Using the test structure, small changes in specific contact resistivity are determined for small temperature changes. Results will be presented and discussed on the application of this novel test structure for nickel to highly doped SiC.
This paper proposes a method to determine the design of the Circular Transmission Line Model (CTLM) in order to ensure accurate results are obtained. The CTLM is used to measure the specific contact resistance of a metalsemiconductor barrier. Through analytical modelling it has been shown that the accuracy of the measurements obtained using a particular CTLM pattern, depends on the geometry chosen. By determining which geometries will yield the most sensitive measurement will ensure an accurate result when compared to the sheet resistance and specific contact resistance of an ohmic contact sample. Analysis of the equations reveals that for any given sample a smaller geometry is preferable. This is determined by comparing the differential of specific contact resistance of the sample with the contact end resistance of the test structure. It has been found that for confident results to be obtained then the annular (centre) ring of the structure should be as narrow as is possible within testing constraints.
Nickel germanide is used as a contact material in germanium devices for making low resistance electrical contacts. It forms at relatively low temperatures compared to other germanides. Metal thickness, reaction temperature and duration of temperature are critical parameters. Here we report on the minimum temperature of formation of nickel germanide and on the effect of duration of temperature. Nickel germanide forms rapidly at higher temperatures and more slowly at lower temperatures and below a critical temperature it does not form, for any duration.
We present a novel method of etching lithium niobate during the Ti diffusion process. A hypothesis for this etching process is explained by defining the kinetics of the Ti diffusion process as an electrochemical reaction. The Ti ions diffuse into the X cut LiNbO3 crystal by swapping with Nb ions generating an electric field. Investigations were carried out by placing a bare LiNbO3 wafer on top of the Ti patterned LiNbO3 substrate during the diffusion process in a wet oxygen atmosphere. The built-in electric field during the Ti diffusion process is neutralised with the bare LiNbO3 placed on top and is evident from the material removal that takes place from the top bare substrate and deposited on the bottom substrate were Ti is diffused. Hence the bare substrate is etched in the regions where Ti is present on the bottom substrate. Features can be arbitrarily defined (using Ti etching) and can have dimensions of 1 micron or smaller. Etch depths of the order of 1 micron have been demonstrated while maintaining smooth surfaces. The crystalline nature of the etched surface is analysed using X-ray diffraction techniques. The refractive index measurement and the surface roughness of the etched surface are also presented.
Silicides have been used in CMOS technology for some years mainly to reduce sheet resistance in the source and drain
regions. This paper discusses in detail the formation of nickel silicide (NiSi) and titanium silicide (TiSi2). The
composition of silicides formed using sputtered and evaporated metals are compared. Metal films (titanium or nickel) on
silicon deposited by DC magnetron sputtering or electron beam evaporation were vacuum annealed to form
corresponding metal silicide thin films. The problem of oxygen contamination during silicidation is also discussed.
Analyses of the silicide thin films formed were carried out using Auger Electron Spectroscopy (AES) depth profiles,
Atomic Force Microscopy (AFM) surface scans, and surface profilometry for measurement of feature heights. The
average surface roughness of the silicide thin films is also compared, and it was observed that nickel silicide thin films
were much smoother than titanium silicide thin films.
Strontium-doped lead zirconate titanate (PSZT) is a piezoelectric ceramic with relatively high values of piezoelectric
coefficients. Perovskite oriented PSZT thin films are also reported to exhibit a variety of other properties including
ferroelectricity and pyroelectricity. This paper reports on a study of the surface morphology and resulting stress of PSZT
thin films, deposited under a variety of RF magnetron sputtering conditions. The study compares PSZT thin films
deposited on metal (gold and platinum) coated silicon wafers. The surface morphology of the deposited PSZT thin films
was studied using Atomic Force Microscopy (AFM). Grain size and average surface roughness measurements were
used to study the quality of the films. The thin film stress was determined using the changes in the radius of curvature of
the sample due to an added layer of thin film, and by applying Stoney's equation to relate the stress to the radius of
curvature. The variations in the level of stress for different thermodynamic conditions during RF magnetron sputter deposition are also reported.
Electrowetting, the phenomena of changing interfacial energy of an interface, has been demonstrated to be an excellent actuation and pumping mechanism for microfluidics and lab-on-a-chip applications. Individual droplets can be moved and deformed on microchips using voltages as low as 15V. In electrowetting, application of a voltage across the electrodes of a micro-droplet causes it to change the interfacial energy of solid-liquid interface which in turn changes the contact angle of the liquid on the solid. The contact angle is a measure of the extent of wetting of the liquid on the surface. In conventional electrowetting, it has been found that the polarity of the applied potential does not affect the contact angle change. However, our experimental results show that the change of polarity across the electrodes of a micro-droplet can reverse the contact angle change. We call this phenomenon 'dewetting'. The actual physics behind this still remains unexplored. In our experiments we used 100 nm of aluminium on a silicon substrate to form the bottom electrode. A 60 nm silicon dioxide or a 1.4 μm thickness strontium doped lead zirconium titanate (PSZT) layer was used as the dielectric and 380 nm of Teflon was used to make a hydrophobic surface. A platinum wire, which was inserted into the micro-droplet, formed the top electrode. The highest dewetting contact angle change was found to be 9o for a 5μl droplet at 60 V. This compared to a maximum of 41o which we obtained for conventional electrowetting.
Silicide contacts are used in semiconductor devices because of their relatively low sheet resistance as thin films and because they form contacts with relatively low values of specific contact resistivity leading overall to low values of contact resistance. Determining the true values of the specific contact resistivity of metal-to-silicide interfaces is a challenge that requires suitable test structures. The Cross Kelvin Resistor (CKR) structure is a commonly used test structure for the extraction of the specific contact resistance of ohmic contacts. Analysis using this structure has errors associated with it and the challenge is often in determining this error. This paper demonstrates a technique that uses several Cross Kelvin Resistor structures connected in a chain and determines the specific contact resistance of aluminium to nickel silicide contacts using extrapolation rather than determining the error. The formation of the nickel silicide films and the fabrication and testing results for the Cross Kelvin Resistor structures are presented.
Layered Surface Acoustic Wave (SAW) based sensors with: InOx / SiNx / 36° YX LiTaO3 structure were developed for sensing different hydrogen (H2) concentrations between 0.06% (600ppm) and 1% H2 in synthetic air. This paper presents a comparative study of the sensors performances in terms of response time, recovery time and response magnitude as a function of operational temperature. The SAW devices consist of metal interdigitated electrodes fabricated on lithium tantalate (LiTaO3) piezoelectric substrate forming the input and output Interdigital Transducers (IDTs). A 1 μm thick silicon nitride (SiNx) intermediate layer was deposited over these finger pairs, either by Plasma Enhanced Chemical Vapour Deposition (PECVD) or by r.f. magnetron sputtering. A 100 nm thin film of indium oxide (InOx) deposited by r.f. magnetron sputtering provides the selectivity towards hydrogen. The highest sensitivity for the sensor with r.f. magnetron sputtered SiNx intermediate layer was recorded at 190° C, when the frequency shift of 361 KHz for 1% H2 in synthetic air was recorded. However larger responses were obtained for the sensor with the PECVD SiNx intermediate layer at 290° C, when the large frequency shift of 516 KHz was recorded for the same H2 concentration. Microstructural characterization of the InOx and SiNx films by Atomic Force Microscopy (AFM) and X-Ray Photoelectron Spectroscopy (XPS) is also presented.
The paper investigates conditions for depositing perovskite-oriented strontium-doped lead zirconate titanate (PSZT) thin films using RF magnetron sputtering. PSZT is a material that can exhibit high piezoelectric and ferroelectric properties. The deposition was conducted using an 8/65/35 PSZT sputtering target. The effects of sputtering conditions and the deposition rates for films sputtered onto several surfaces (including gold and platinum coated substrates) were studied. Combinations of in-situ heating during sputtering and post-deposition Rapid Thermal Annealing (RTA) were performed and resulting phases determined. RTA was carried out in argon to observe their effects. The sputtered films were analyzed by Scanning Electron Microscopy (SEM), X-ray Diffractometry (XRD), and X-Ray Photoelectron
Spectroscopy (XPS). Results show dramatic differences in the grain structure of the deposited films on the different surfaces. The stoichiometry of the sputtered films is demonstrated using XPS. In the case of gold and platinum coated substrates, sputtering was also carried out for different durations, to establish the growth rate of the film, and to observe the variation in grain size with sputtering duration. The deposited thin films were resistant to most chemical wet etchants and were Ion Beam Etched (IBE) at 19 nm/min.
We investigated linear optical and second-order nonlinear optical (NLO) properties of films of urethane-urea copolymer (UU2) functionalised with a high concentration of an azobenzene chromophore. The polymer films on ITO-coated substrate were corona poled to induce a noncentrosymmetric organization of chromophore dipoles and data on the second harmonic generated with the laser beam (the fundamental wavelength 1053 nm, 6 ps/pulse, 20 Hz repetition rate) was acquired as a function of time and temperature. Second harmonic generation (SHG) was used to monitor in situ the polar alignment and relaxation of orientation of the side-chain Disperse Red-like chromophore molecules in the films poled at room temperature and high above the glass transition temperature (Tg 140-150oC). The deff coefficient was determined from the Maker-fringe method and corrected for absorption. A strong second harmonic effect with a fast relaxation was observed in "cold" (room temperature) poling experiments. A large second-order resonantly enhanced optical nonlinearity (d33 of the order of 200 pm/V) was obtained in high temperature poling. A strong and stable nonlinearity has persisted for years after the films were high-temperature poled.
This paper reports the thermoelectric properties of intrinsic N-type bismuth telluride (Bi2Te3) thin films (2.5-10 μm thickness). These films were deposited using radio frequency (R.F.) magnetron sputtering. These properties include; Seebeck coefficient and electrical resistivity at different temperatures. It has been observed that the Seebeck coefficient and electrical resistivity of thin films are approximately -150 μV/°C and 4 x 10-5 ohm-m at room temperature, respectively. The maximum value of Seebeck coefficient of approximately -287 μV/°C was observed at 54 °C for a film thickness of 9.8 μm. The microstructural characteristics of the thin films were investigated using Scanning Electron Microscopy and X-Ray Diffraction analysis. It was observed that the thicker the Bi2Te3 film, the larger the grain size. The observed grain sizes were approximately 900 nm and 1500 nm for Bi2Te3 film of 2.6 μm and 9.8 μm thicknesses, respectively. The XRD analysis indicated the presence of rhombohedral (Bi2Te3) crystal structures.
The spin-on photoresist SU8 from MicroChem has a relatively high refractive index (n=1.57 at 1550nm) compared with other polymers. It is stable and has high optical transmission at optical communication wavelengths. In this paper we study rib waveguides fabricated using SU8 as the core layer and thermoset polymers UV15 (n=1.50 at 1550nm) from Master Bond and NOA61 (n=1.54 at 1550nm) from Gentec as the cladding layers. The rib height is varied from 0.3 to 1.7μm high. This is part of the SU8 layer sandwiched between the cladding layers. The waveguides are tested to determine the effects of varying this geometry for single mode optical transmission. The lengths of the waveguides were 1.5 cm to 5 cm.
In this paper, the design of a thin film thermoelectric microcooler module is examined. The module consists of n-type bismuth telluride and p-type antimony telluride thermoelectric materials. The commercial software CFD-ACE+ is used to implement and analyse the model. A two-dimensional coupled electrical and thermal synthesis was performed. The influence of the thickness of the thermoelectric materials on the change in temperature has been investigated. The thickness of the thermoelements was varied between 0.5 and 20 μm. The device performance in terms of change in temperature with and without a load has been studied. The optimal thickness for the thermoelements was found to be 2μm. At 30mA, a temperature difference of 3K below ambient was obtained.
A technique is presented for the fabrication of an integrated microfluidic surface acoustic wave (SAW) sensor suitable for a wide range of fluidic analyses. The device was fabricated in Microchem SU8-25, on a PMMA substrate, employing a modified SU8 process. Standard SU8 procedures are sufficiently close to the glass transition temperature for PMMA to cause significant warping. PMMA is advantageous as a substrate material as it is cheap, easily machined, hydrophobic, and exhibits superior adhesion with SU8. A novel die bonding technique was explored to join the SU-8 fluidic structures to a lithium niobate SAW sensor. This involved additional fluidic structures, and the optical adhesive, NOA-73. A device with a 1cm by 1.5cm by 25um chamber was fabricated with these techniques, and was used to explore fundamental properties of flow at the microscale, in particular at the surface of the SAW device.
High quality thermoset polymer solutions are available from several commercial suppliers. These are suitable for forming thin films for optical waveguides because of their high transmission, suitable refractive indices and thickness uniformity of films obtained by spinning solutions on a substrate such as silicon. The solution's viscosity and the spinning speed determine the thickness of resulting films. The plasma etch rate was examined for trenches (of the order of 1 μm depth, suitable for photonic waveguide fabrication) formed in such films from a relatively high viscose polymer solution (UV15 from Master Bond). The cross-link density of the polymer is dependent on its curing process, that is, the exposure to ultra-violet light radiation and heat. The curing process can have a profound affect on the etch rate. Different paths were taken in the curing and etching process of the spun polymer film in order to examine the relation between the cross-link density and the polymer etching process. We examine the polymer films using FTIR to qualitatively measure the cross-link density and DSC for changes in the glass transition temperature (Tg). FTIR is used to show if chemical changes occur for different levels of curing. Determining Tg is important because this will show how it changes with the baking step. We have observed that for films with low Tg, the plasma etch process can cause the polymer surface to flow and hence wrinkle. Post-curing at higher temperatures increases Tg. By choosing appropriate curing steps, Tg and the etch rate can be optimized to obtain an optimum etch rate and preserve the smooth polymer surface during etching.
A layered Surface Acoustic Wave (SAW) hydrogen gas sensor, based on a delay line structure with 64 finger pairs on input and output port, is fabricated on 64° Y-cut, X-propagating LiNbO3 substrate. A guiding layer of ZnO is used to increase the sensitivity of the structure. A WO3 selective layer is employed to H2 gas sensing applications at different operating temperatures between room temperature and 300°C. In this paper, the fabrication process of WO3/ZnO/64° YX LiNbO3 sensor is described and the sensor’s response features are analyzed. The improvement of the response with the addition of a gold catalytic layer on the sensor surface is also investigated.
A layered Surface Acoustic Wave (SAW) device based on an InOx/Si3N4/36° YX LiTaO3 structure is investigated for sensing ozone in air at different operating temperatures and concentrations. These concentrations are between 25 ppb and 150 ppb. Layered SAW devices are of a great interest as they show a remarkable performance for liquid and gas sensing applications. This structure is a single delay line SAW device with 64 input and output finger pairs, having periodicity of 24 μm. They were fabricated on a 36° Y-cut X-propagating lithium tantalate (LiTaO3) piezoelectric substrate. A 1 μm thick silicon nitride (Si3N4) layer was deposited over the finger pairs and a 100 nm indium oxide (InOx) sensing layer was deposited over the Si3N4 layer. Both layers were deposited by RF magnetron sputtering. InOx was chosen as it has a remarkable sensitivity towards ozone. Si3N4 was chosen as it is inert and has stable characteristics at high temperature. The sensor performance is analysed in terms of response time, recovery time and response magnitude as a function of operational temperature. The operational temperature ranges between 185°C and 205°C. The sensor shows repeatability, reversibility, fast response and recovery time. At approximately 190°C the highest sensitivity was observed. A frequency shift of 5.0 kHz at 25 ppb, 6.5 kHz at 50 ppb ozone was recorded. The presented results show this structure is promising for gas sensing applications.
Polymer films with high optical transmission have been investigated for making optical devices for several years. SU8 photoresist and optical adhesives have been investigated for use as thin films for optical devices, not what they were originally designed for. Optical adhesives are typically a one component thermoset polymer and are convenient to use for making thin film optical devices such as waveguides. They are prepared in minutes as thin films unlike SU8, which has to be carefully thermally cured over several hours for optimum results. However SU8 can be accurately patterned to form the geometry of structures required for single mode optical waveguides. SU8 in combination with the lower refractive index optical adhesive films such as UV15 from Master Bond are used to form single and multi mode waveguides. SU8 is photopatternable but we have also used dry etching of the SU8 layer or the other polymer layers e.g. UV15 to form the ribs, ridges or trenches required to guide single modes of light. Optical waveguides were also fabricated using only optical adhesives of different refractive indices. The resolution obtainable is poorer than with SU8 and hence multi mode waveguides are obtained. Loss measurements have been obtained for waveguides of different geometries and material combinations. The process for making polymer waveguides is demonstrated for making large multi mode waveguides and microfluidic channels by scaling the process up in size.
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°).
A comparison between the performance of conductometric and layered surface acoustic wave (SAW) hydrogen sensors is presented. Both sensor structures employ an R.F. magnetron sputtered tungsten trioxide (WO3) thin film as a selective layer for hydrogen (H2) sensing applications. The conductometric device is based on an alumina substrate, while the layered SAW device structure is fabricated on a 36° Y-cut, X-propagating LiTaO3 substrate with a zinc oxide (ZnO) guiding layer. The sensors were investigated for different operational temperatures and various concentrations of H2 in synthetic air.
A technique is presented for fabricating microchannels for flow investigation with fluorescent particles. The channels were fabricated using plasma etching of a thermoset polymer film, UV15 from Master Bond. The UV15 was spun on a silicon wafer to give a depth of 100μm. A 100nm thick patterned aluminium film was sputtered and patterned on the polymer surface for the etch pattern mask. Sputtering conditions were optimised to prevent damage to the polymer layer. Etch depths to 100μm were obtained. Curing conditions were optimised to prevent wrinkling of the Al/polymer surface during etching. There is a wide variation in the polymer etch rate which can be attributed to many factors. However, one of the most significant is the energy dose (mW/cm2) to cure the polymer. For etch depths greater than 20μm the channels varied from the rectangular cross section shape by undercutting on the walls and deeper etching at the bottom of the channel walls. Conditions for obtaining uniform microchannels for 100μm wide and 50μm deep channels, 5cm long are presented.
Layered Surface Acoustic Wave (SAW) devices that allow the propagation of Love mode acoustic waves will be studied in this paper. In these devices, the substrate allows the propagation of Surface Skimming Bulks Waves (SSBWs). By depositing layers, that the speed of Shear Horizontal (SH) acoustic wave propagation is less than that of the substrate, the propagation mode transforms to Love mode. Love mode devices which will be studied in this paper, have SiO2 and ZnO acoustic guiding layers. As Love mode of propagation has no movement of particles component normal to the active sensor surface, they can be employed for the sensing applications in the liquid media.
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