SU-8 as resist material for deep X-ray lithography

 A new negative tone resist for deep X-ray lithography is presented. This resist is a nine parts to one mixture of the EPON SU-8 resin with 2,2-bis-(3,5-dichloro-4-hydroxyphenyl)propane (Tetrachlorobisphenol A, TCBA), the latter acting as the photoinitiator. The resist was irradiated at the synchrotron source of DCI at LURE. It was dried for 7 to 20 days beforehand over silica gel while under a light vacuum (20 mbar). Best results for a 150 μm high resist were obtained with a X-ray bottom dose of 3 kJ cm⁻³ and a post exposure bake at 33 °C. Differential Scanning Calorimetry measurements (DSC) determined the glass transition temperature of the resist. The glass transition for the undried, loose resist was 34.7 °C, and it was 28.7 °C when the resist was pressed on a silicon substrate. For a sample of the dried resist, the glass transition was 33.4 °C for the loose resist and 29.8 °C when it was pressed on a Silicon substrate. CD measurements were made on top surface of a set of 100 μm long columns structures, which were produced in 150 μm of this resist. These structures have a constant 100 μm pitch, and the structures themselves varied in width from 20 to 17 μm. For these structures, the CD was calculated to be 0.15 ± 0.03 μm. Received: 8 February 2000/Accepted: 3 March 2000     

Numerical simulation of thermal distortions in deep and ultra deep X-ray lithography

 One major process step in deep X-ray lithography is the exposure of the resist with synchrotron radiation. High energy photons are absorbed in mask, resist and substrate. About 95% of this energy is deposited as thermal heat [Schweizer (1997)]. This may lead to a temperature rise in the system and result in thermal distortions during the patterning process. A sample layout is used to determine the distortions during irradiation. Typical radiation parameters of the ELSA storage ring at Bonn University (2.7 GeV, 35 mA) and material properties are applied to simulate the heat effects. Mask membranes made of titanium or beryllium are modeled to irradiate PMMA layers of 200 and 2500 μm thickness. Copper is used as substrate material. Mask support and the bottom of the substrate are cooled to 21 °C as the system is scanned through the synchrotron beam. In the case of 200 μm PMMA and titanium mask membranes, mask temperatures increase to 40.1 °C, whereas only 22.3 °C are reached if beryllium masks are simulated. Maximum distortions are 0.74 μm for Ti-masks and 0.03 μm for Be-masks. With increasing resist thickness, the incident synchrotron radiation power as well as the temperature rise are reduced. In the case of 2500 μm thick PMMA, temperatures of 21.45 °C are simulated. Received: 10 August 2001/Accepted: 24 September 2001 This paper was presented at the Fourth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.     

Microfabrication of thick tungsten films for use as absorbers of deep X-ray lithography masks

 We fabricated thick (5 μm) tungsten (W) film patterns by sputtering and dry etching, and realized a new deep X-ray lithography mask. The X-ray mask with 5-μm-thick W absorbers could expose about 1-mm-thick resist structures. In the deposition process of W films, the column structure of about 0.2 μm grain size, from which pattern edge roughness originates, disappeared by adding nitrogen into the sputtering gas. W film etching was carried out by reducing gas pressure and cooling the substrate (−40 °C), and a side etch width of below 0.2 μm was obtained. From the results of the pattern edge roughness and the side etch width, a pattern fabrication accuracy below ±0.5 μm was achieved. Furthermore, film stress, which induces pattern distortion, was reduced to below 50 MPa by controlling the sputtering gas pressure. The obtained mask achieved a pattern distortion below ±0.3 μm. Received: 7 July 1999/Accepted: 29 May 2000     

Optical DUV-lithography for high microstructures

 A new technology called 3D UV-Microforming consisting of an advanced resist preparation process, a UV lithographic step, resist development, a moulding procedure by electrodeposition, and finally stripping and cleaning for finishing the structures was developed for application in microsystem technology. It enables the low-cost fabrication of a wide variety of micro components for many different users. During resist preparation, layers up to two hundred μm thickness were obtained until now. By using a standard UV mask aligner as an exposure tool followed by immersion development, thick resist layers up to 100  μm could be patterned in a single step on pre-processed silicon wafers. Repeated exposure and development were successfully used for structuring resist layers of up to 200  μm thickness. High aspect ratios of more than 10 as well as steep edges of more than 88° could be fabricated. The resist patterns were moulded by using pulse or DC electroplating. For microsystem applications some metals and alloys were deposited, Three-dimensional micro components were fabricated as demonstrators for the new technique. It allows the use of materials with interesting properties which could not be provided by standard processes. Received: 30 October 1995 / Accepted: 9 December 1995     

Swelling of PMMA-structures in aqueous solutions and room temperature Ni-electroforming

 LIGA-structures are known to have a lateral dimensional precision of 1 μm and below. These specifications strongly depend on the process parameters. Fabrication of 400 μm tall nickel LIGA-structures by electroplating of X-ray patterned (DXRL) resist structures revealed much higher distortions. These are caused on the one hand by thermal expansion of the structured resist since the electroforming is usually performed at 52 °C, but on the other hand to a higher degree by swelling of the resist in the aqueous electroplating bath. Reducing the electrolyte temperature not only eliminates the thermal expansion but also particularly reduces the swelling. At room temperature the latter is reduced by a factor of 3 in comparison to the usual 52 °C-bath leading to an overall reduction of the deformation of up to 80%. In this paper the experimental data are presented and the results are explained. Furthermore first microstructures electroplated at room temperature with optimized precision are shown.     

Cyanate ester based resin systems for snap-cure applications

 Resin compositions comprising cyanate ester have been demonstrated to be useful as die attach adhesives, underfills and encapsulants, where the characteristics of the resins can be varied in a wide range by copolymerization with functionalized comonomers such as epoxies, phenols, rubbers, thermoplastics and others. To reach a combination of properties such as long pot life, short cure time and high glass transition temperature, we encapsulated small particles of effective hardeners to make them insoluble and non-reactive when mixed with the resin at room temperature. Pot lifes of more than 3 months could be reached, whereas the same cyanate ester gels and becomes solid within 30 min at room temperature, if the neat hardener is used instead of the capsules. At a certain elevated temperature, which mainly depends on the structure of the hardener, the capsules open and the curing reaction starts immediately. Low-temperature systems with cure times less than 5 min at 80 °C reach glass temperatures of about 140 °C, and a glass transition temperature of 220 °C after 10 s cure can be achieved with another combination. The developed snap cure resin systems can be easily mixed with a lot of common additives such as minerals, tougheners, metallic powders and others to cover a wide range of performance characteristics for use as adhesives, underfills, encapsulants and the like. Received: 15 May 2001/Accepted: 15 October 2001     

A flip-chip LIGA assembly technique via electroplating

 A novel flip-chip assembly technique by means of electroplating at a low processing temperature has been developed. Mesoscopic gears, made of nickel with 1500 μm in diameter and 250 μm in height, have been bonded to glass substrate. The bonding process is conducted by nickel electroplating at 50 °C with a current density at 53.3 A/m². The nickel electrolyte is found to penetrate about 100 μm from the perimeter to the center of the gear and form a good bond. This process can be applied to the integration of high-aspect-ratio microstructures with substrates that have pre-fabricated microelectronics massively and in parallel. As such, it provides a powerful way to achieve the integration of meso- and microscopic electromechanical systems. Received: 7 July 1999/Accepted: 30 June 2000     

Structural characterisation of ionising-radiation detectors based on CVD diamond films

 A detailed structural characterisation of synthetic diamond films, previously investigated as UV photodetectors, has been carried out by SEM, X-ray diffraction, catholuminescence (CL), micro-Raman spectroscopy and micro-photoluminescence. The films were deposited by microwave plasma enhanced chemical vapour deposition using a CH₄–CO₂ gas mixture. The effect of a systematic change of the methane concentration on film morphology, preferential orientation and crystal quality has been investigated at two different substrate temperatures, 750°C and 850°C. A strong decrease of both band-A CL and width of the diamond Raman line at 1332 cm^-1 has been observed, at lower substrate temperature, going towards (1 0 0) texturing, consistent with the attribution of band-A luminescence to the presence of structural defects such as dislocations. A strong correlation between methane-induced texturing and UV detector performance has been evidenced: poorly oriented films exhibit a better UV photoresponse than highly textured films. Raman and luminescence measurements suggest that the limiting factor for the detector performance is related, rather than to structural defects to centres of different nature, whose density strongly depends on the sample preferential orientation. Received: 30 March 1999/Accepted: 12 April 1999     

High aspect ratio micromachining by synchrotron radiation direct photo-etching

 We have carried out micromachining of Teflon-polymers such as PTFE, PFA and FEP as well as optical crystal such as NaCl and LiF by synchrotron radiation direct (without any chemicals) photo-etching and succeeded in creating microstructures with very high aspect ratios. The maximum aspect-ratio achieved was 50 and the maximum processing depth was 1500 microns. Dependence of the etching rate on the synchrotron beam current and on the substrate temperature was studied. Based on the study, we could use only x-rays from the synchrotron radiation so as to apply x-ray lithography technology (such as using an x-ray mask and processing in He atmosphere) to our process. A rise in the sample temperature results in significant enhancement of the etching rate. The etching rate measured was on the order of a few 100 μm/min. So that this process is much faster than hard x-ray deep lithography for the processing of more than 100 μm deep microstructures. Received: 25 August 1997/Accepted: 23 October 1997     

Polydimethylsiloxane, a photocurable rubberelastic polymer used as spring material in micromechanical sensors

 Polydimethylsiloxane (PDMS) is a commercially available physically and chemically stable photocurable silicone rubber which has a unique flexibility (G≈250 kPa) at room temperature. Further properties of PDMS are a low elasticity change versus temperature (1.1 kPa/°C), no elasticity change versus frequency and a high compressibility. PDMS is an interesting polymer to be used as spring material in micromechanical sensors such as accelerometers. The spring constant of the PDMS structures was theoretically calculated and measurements were done on accelerometers with PDMS springs to validate the theory. The measured and calculated spring constants showed a good correspondence, so the measurement results showed that the PDMS structures can successfully be used as mechanical springs. Received: 29 October 1996 / Accepted: 13 November 1996     

High aspect ratio electrostatic micro actuators using LIGA process

High-power electrostatic microactuators using LIGA process have been fabricated. Comb drive type actuators and a wobble motor were designed and fabricated. A basic structure of the microactuators was composed of movable and fixed electrodes of Ni, a sacrificial layer of SiO₂ and a Si substrate, and carried out by one mask process. As design rules, a minimum resist width of 2 μm, resist height of 120 μm, maximum width of movable parts of 10 μm, minimum width of fixed parts of 40 μm and driving voltage of about 100 V, were decided. A 120 μm-thick PMMA resist was formed on a Si substrate by a casting method. The PMMA was exposed using a compact SR source “AURORA”, using an X-ray mask with 7 μm-thick Au absorber on a 2 μm-thick poly-Si membrane. The exposed PMMA was developed by a developer. Ni microstructures with 100 μm-height, 2 μm-minimum width, 2 μm-minimum gap, and then maximum aspect ratio of 50, were made by electroforming. Ni microstructures used for movable electrodes were separated from the substrate by lateral etching of SiO₂. After lateral etching of SiO₂, Au wires were bonded to electrodes. Actuation of the comb drive type actuator and rotation of the wobble motor were confirmed. The applied voltage to the comb drive actuator and the wobble motor were 65 and 125 V.     

Optical loss of metal coated optical fibers at temperatures up to 800°C

Temperature band of ordinary telecommunication optical fibers is −60...85°C. The developing fiber optic sensors which can work at higher temperatures, required to develop metal coated optical fibers. The Purpose of the work is a researching additional optical loss of copper alloy coated optical fibers which were drawn from low hydroxyl group contamination preforms at temperatures 20...800°C. It is reached that metal coated optical fiber worked at temperature 700°C for 7 hours, while the optical losses changed from 2 to 3 dB/km at the wavelength of λ = 1300 nm. It is not observed intensive growth of optical losses on hydroxyl groups at 800°C, which was observed in metal coated optical fiber when it was heated at 700°C.     

Nickel and copper electroplating of proton beam micromachined SU-8 resist

 Proton beam micromachining (PBM) has been shown to be a powerful technique to produce three-dimensional (3D) high-aspect-ratio microstructures (Watt et al., 2000). Potential commercial applications of PBM, which is a fast direct write technique, will become feasible if the fabrication of metallic molds or stamps is realised. Metallic components can be produced by electroplating a master from a microstructure produced in resist. The production of high-aspect-ratio metallic stamps and molds requires a lithographic technique capable of producing smooth and near 90° sidewalls and a one to one conversion of a resist structure to a metallic microstructure. PBM is the only technique capable of producing high-aspect-ratio microstructures with sub-micron details via a direct write process. In PBM, SU-8 (Lorenz et al., 1997) resist structures are produced by exposing the SU-8 resist with a focused MeV proton beam followed by chemical development and a subsequent electroplating step using Ni or Cu. The data presented shows that PBM can successfully produce high-aspect-ratio, sub-micron sized smooth metallic structures with near 90° sidewall profiles. Received: 10 August 2001/Accepted: 24 September 2001     

Assembly of hybrid integrated micro-optical modules using passive alignment with LIGA mounting elements and adhesive bonding techniques

 Assembling hybrid integrated micro-optical modules for mono-mode applications requires alignment tolerances better than ±1 μm which, so far, were achieved by complex active alignment of the micro-optical components. This article describes a passive assembly concept employing high-precision LIGA structures to simplify assembly and reduce manufacturing costs. The advantages of this passive assembly concept are demonstrated in the micro-optical assembly of a heterodyne receiver. The micro-optical components (ball microlenses, prisms, glass fibers, photodiodes) were aligned passively by means of alignment structures made of PMMA, and were subsequently fixed by UV-curing adhesive. Photodiodes were additionally contacted with electrically conducting adhesive cured at 70 °C. The mono-mode glass fibers were mounted in fiber mounts into the fiber grooves of which the glass fibers were inserted and immobilized with UV-curing adhesive. Measurements of the optical quality of the heterodyne receiver indicated an accuracy of assembly better than 1 μm. Received: 3 January 2000/Accepted: 9 February 2000     

Characterisation of silicon carbide JFETs with respect to microsystems for high temperature applications

 In this work first commercially available SiC-transistor prototypes were tested with regard to their applicability in high temperature electronic circuits for sensor signal conditioning. The influence of the temperature on the device behaviour (drain-saturation current, gate leakage current, I–V-characteristics, long-term effects) was investigated. The devices showed reliable operation up to 450 °C. The maximum forward transconductance g _m and the short circuit drain source current I _DSS decreased to approximately 30% of the room temperature values. Also, a slight increase of the pinch-off voltage V _p was observed. The gate leakage current I _GSS rose with temperature, staying below 1 μA at 450 °C. A pre-ageing study was carried out to verify changes in the device characteristics with time. The devices were exposed to a 270 °C environment and it was observed that the DC parameters tend to stabilise after about 100 h. From the I–V-characteristics the SPICE parameters were extracted for a series of temperatures, allowing the design and optimisation of amplifier gain stages. The SPICE device simulation results are in good agreement with the measured characteristics. Received: 28 November 1996/Accepted: 2 December 1996     

A microvalve matrix using piezoelectric actuators

 A microvalve matrix is proposed for controlling gas flow. Mircovalves in the matrix are controlled independently and each one handles a very small gas flow. The microvalve matrix can thus control gas flows precisely in very small steps, over the range from zero flow to fully-open flow, by digitally opening and closing the appropriate number of microvalves. The microvalve proposed for this matrix has a compact and simple design for a high degree of integration. This valve is normally closed and is opened by using the deformation of the port caused by the piezoelectric effect. Calculations show that a microvalve smaller than 1×1 mm can handle a maximum gas flow rate of the order of 10^-4 Pa m³/s (Air, 20 °C). It is easy to reduce the flow rate. Therefore these results indicate that a microvalve matrix can achieve wide dynamic-range flow-control in small flow steps. Received: 1 November 1996/Accepted: 14 November 1996     

Low-temperature anodic bonding of silicon to silicon wafers by means of intermediate glass layers

Silicon wafers have been anodically bonded to sputtered lithium borosilicate glass layers (Itb 1060) at temperatures as low as 150–180 °C and to sputtered Corning 7740 glass layers at 400 °C. Dependent on the thickness of the glass layer and the sputtering rate, the sputtered glass layers incorporate compressive stresses which cause the wafer to bow. As a result of this bowing, no anodic bond can be established especially along the edges of the silicon wafer. Successful anodic bonding not only requires plane surfaces, but also is determined very much by the alkali concentration in the glass layer. The concentration of alkali ions as measured by EDX and SNMS depends on both the sputtering rate and the oxygen fraction in the argon process gas. In Itb 1060 layers produced at a sputtering rate of 0.2 nm/s, and in Corning 7740 layers produced at sputtering rates of 0.03 and 0.5 nm/s, respectively, the concentration of alkali ions in the glass layers was sufficiently high, at oxygen partial pressures below 10^-4 Pa, to achieve anodic bonding. High-frequency ultrasonic microanalysis allowed the bonding area to be examined non-destructively. Tensile strengths between 4 and 14 MPa were measured in subsequent destructive tensile tests of single-bonded specimens.     

Deep X-ray lithography at ELETTRA using a central beam-stop to enhance adhesion

 In the deep X-ray lithography process adhesion of resist structures on metallic coated substrate is strongly correlated with the spectral distribution of the radiation. At ELETTRA the bending magnet radiation spectrum extends up to 20 keV which can severely increase the secondary radiation process from the metal coated resist substrate, especially when the deep lithography process is used for mask replication involving lower resist and absorber thicknesses. To reduce the proportion of high energy photons in the radiation spectrum the central part of the beam is blocked by a beam-stop acting as a low energy band-pass filter. The results of the first expositions at ELETTRA using different size beam-stops are presented and discussed. Received: 10 August 2001/Accepted: 24 September 2001     

Studies of polymer deformation and recovery in micro hot embossing

In large areas of micro hot embossing, process temperature plays a critical role to both the local-area fidelity and global uniformity of microstructure formation. Higher embossing temperature could improve structure fidelity, however, at the expense of demoulding easiness. Micro embossing at the lowest possible temperature with acceptable fidelity can improve global flatness after demoulding. This study focuses on polymer deformation and recovery in micro embossing when the process temperature is below the polymer glass transition temperature (Tg). PMMA (Polymethyl Methacrylate) substrates (Tg = 105°C) were employed with the process temperature ranging from 25°C to its Tg. At temperature below Tg −55°C, significant recovery occurred after processing, but permanent structures could still be formed with sufficiently high applied stress. With an increase in temperature, plastic deformation increased and was the dominant polymer deformation mode for permanent cavities formation. However, the formation of protrusive structures was not complete since there was little polymer flow. The polymer will lose its storage modulus at an even higher temperature and microstructures could be formed with high fidelity. A compromise between local fidelity of embossed patterns and global flatness of substrate has to be reached in micro hot embossing.     

Silicon hotplates for metal oxide gas sensor elements

 Low-power-consumption metal oxide gas sensors and gas sensor arrays can be produced by combining micromachining and thin-film technologies. In the present paper the state-of-the art in this field is reviewed. In the first part the problem of thermal losses from a heated metal oxide film is addressed and the necessity of miniaturisation of gas sensing devices is pointed out. The thermal properties of realized silicon hotplates are compared and analysed with respect to thermal equivalent circuit models. In addition, the results of accelerated thermal ageing tests are presented. These latter results demonstrate that micromachined heater elements are likely to exhibit device lifetimes of the order of 30 years when operated at membrane temperatures around 400 °C. In the second part of the paper attention is drawn to novel methods of gas detection which are enabled by employing stacks of different thin film materials on micromachined hotplates. In this context, recent results on temperature-and field-effect modulated gas detection experiments are discussed. Received: 16 May 1997 / Accepted: 22 May 1997