Review of Polyimides Used in the Manufacturing of Micro Systems

Since their invention, polyimides have found numerous uses in MicroElectroMechanical Systems (MEMS) technology. Polyimides can act as photoresist, sacrificial layers, structural layers, and even as a replacement for silicon as the substrate during MEMS fabrication. They enable fabrication of both low and high aspect ratio devices. Polyimides have been used to fabricate expendable molds and reusable flexible molds. Development of a variety of devices that employ polyimides for sensor applications has occurred. Micro-robotic actuator applications include hinges, thermal actuators and residual stress actuators. Currently, polyimides are being used to create new sensors and devices for aerospace applications. This paper presents a review of some of the many uses of polyimides in the development of MEMS devices, including a new polyimide based MEMS fabrication process

Inkjet printing of polyimide insulators for the 3D printing of dielectric materials for microelectronic applications

In this article, we report the first continuous fabrication of inkjet-printed polyimide films, which were used as insulating layers for the production of capacitors. The polyimide ink was prepared from its precursor poly(amic) acid, and directly printed on to a hot substrate (at around 160 °C) to initialize a rapid thermal imidization. By carefully adjusting the substrate temperature, droplet spacing, droplet velocity, and other printing parameters, polyimide films with good surface morphologies were printed between two conducting layers to fabricate capacitors. In this work, the highest capacitance value, 2.82 ± 0.64 nF, was achieved by capacitors (10 mm × 10 mm) with polyimide insulating layers thinner than 1 μm, suggesting that the polyimide inkjet printing approach is an efficient way for producing dielectric components of microelectronic devices. © 2016 The Authors Journal of Applied Polymer Science Published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43361

Inkjet printing of polyimide insulators for the 3D printing of dielectric materials for microelectronic applications

In this article, we report the first continuous fabrication of inkjet-printed polyimide films, which were used as insulating layers for the production of capacitors. The polyimide ink was prepared from its precursor poly(amic) acid, and directly printed on to a hot substrate (at around 160 °C) to initialize a rapid thermal imidization. By carefully adjusting the substrate temperature, droplet spacing, droplet velocity, and other printing parameters, polyimide films with good surface morphologies were printed between two conducting layers to fabricate capacitors. In this work, the highest capacitance value, 2.82 ± 0.64 nF, was achieved by capacitors (10 mm × 10 mm) with polyimide insulating layers thinner than 1 μm, suggesting that the polyimide inkjet printing approach is an efficient way for producing dielectric components of microelectronic devices. © 2016 The Authors Journal of Applied Polymer Science Published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43361

Modeling and Characterization of Metal/SiC Interface for Power Device Application

Silicon carbide is a wide band-gap semiconductor widely considered to be an excellent material for the fabrication of power devices able to operate in extreme environmental conditions. Its superior properties such as wide energy bandgap, high hardness, chemical inertness, high electrical field breakdown strength and high thermal conductivity enable electronic devices, based on it, to operate at high temperatures, high voltages and high frequencies and make it an attractive semiconducting material for the power electronics industry. Since 1999 a number of electronic devices based on silicon carbide are commercially available such as Schottky barrier diodes with voltage rating of 300 - 1700 V (as of 2011) which often show non-ideal electrical behavior. Non-ideal electrical behavior is manifested in the abnormal current-voltage characteristics and greater than unity ideality factors. Various theories exist as to the origin of these non-idealities some attribute them to different conduction mechanisms such as generation-recombination and edge-related currents and others to the inhomogeneous Schottky barrier. We have considered the approach, taken by Tung, which can explain all the non-ideal behaviors with thermionic emission theory alone by assuming the Schottky barrier height to be inhomogeneous. Inhomogeneous Schottky barrier implies spatially varying isolated low barrier height regions existing alongside a homogeneous high Schottky barrier. These regions are supposed to interact, in case of being situated together, resulting in the region with low barrier height to be pinched-off. If the pinch-off occurs the low barrier height region (or patch depending on the shape) has a Schottky barrier height equal to the 'saddle point potential' in front of that patch or low barrier region. Whole Schottky barrier is assumed to be composed of numerous such low barrier height patches. These patches are considered to be embedded into the high background Schottky barrier and define the overall current transport through the Schottky barrier diode. A similar model is the parallel conduction model presented by D. Defives et al. which instead of considering the Schottky barrier to be composed of various small patches, divides the Schottky barrier into two major parts each with different Schottky barrier height and both existing simultaneously within one Schottky barrier diode. Though accurate to some extent, this model considers the two Schottky barrier heights to be electrically independent of each other; which is not true in all situations. After applying Tung's theoretical model it was possible to extract nearly correct value of Richardson constant for the Schottky diodes with titanium and molybdenum Schottky contacts on 4H silicon carbide. It was also possible to fit the experimental data correctly with Tung's theoretical model. Note: The diodes used in this research work were fabricated during a research project involving Vishay and Politecnico di Torino

FABRICATION AND THERMAL ANALYSIS OF POLYIMIDE-BASED X-RAY MASKS AT THE SYNCHROTRON LABORATORY FOR MICRO AND NANO DEVICES, CANADIAN LIGHT SOURCE

In deep X-ray lithography (DXRL), synchrotron radiation (SR) is applied to transfer absorber patterns on an X-ray mask into the photoresist to fabricate high-aspect-ratio micro and nano scale structures (HARMNST). The Synchrotron Laboratory for Micro and Nano Devices (SyLMAND) at the Canadian Light Source (CLS) is a DXRL laboratory with continuous tuning capabilities for spectrum and power of the synchrotron beam. X-ray mask fabrication is one of the most demanding sequences associated with the entire processing chain and has always constituted a bottleneck in the DXRL technology. In this thesis, X-ray masks based on polyimide membranes are studied, including the development of a fabrication sequence as well as theoretical and experimental analyses of limitations of pattern accuracy. A 30 μm polyimide membrane was obtained by spin-coating photo-sensitive Fujifilm Durimide 7520® polyimide on a stainless steel substrate. Subsequently, a layer of TiO1.9 was sputtered onto the membrane as the plating base for the absorbers. On the plating base, 100 μm of UV-sensitive negative-tone resist Futurrex NR26-25000P® were spin-coated and patterned by UV-lithography. The patterned photoresist served as a template, filling the voids with 80 μm nickel by electrodeposition. These metal structures served as the mask absorbers in the test masks. Two test masks were fabricated, one with complete coverage and one with a center block absorber layout. In the next processing step, the sacrificial steel substrate was locally opened by etching with ferric chloride solution to create an X-ray transparent exposure window. The mask was finally bonded to a mask frame for support and rigidity. Polymer-based mask membranes are often avoided in DXRL because of large thermal distortions expected during X-ray exposure as a result of the low thermal conductivity. The power tuning capabilities at SyLMAND, however, allow the beam power to be adjusted and consequently limit thermal distortions. The heat load of the polyimide masks was analyzed by numerical analysis for the thermal and thermoelastic behavior of the test masks under synchrotron beam exposure. The beam power was calculated by the software LEX-D. ANSYS FLUENT® was used for the thermal analysis by computational fluid dynamics, and ANSYS Mechanical® for thermoelastic analysis using the finite element method. The thermal simulation results indicate that the main heat dissipation mechanism is from the mask absorbers by conduction across the rarefied helium gas in the proximity gap between the mask and its cooled surroundings. In DXRL, masks are vertically scanned through the synchrotron beam. Under the given conditions, this scanning speed of 50 mm/s was faster than the heat dissipation speed, such that a steep temperature gradient is observed between the exposed and unexposed mask areas as the beam scans across the test mask. The low thermal conductivity of the polyimide membrane can cause accumulation of heat in absorber structures such as the isolated center block absorber. At SyLMAND, an intensity chopper can effectively tune the incident beam power, thereby reducing the heat load in the mask during exposure. The temperature rises during exposure scale almost linearly with the incident beam power. Final temperatures of close to 39° C were obtained for both test masks at an incident beam power of about 14.5 W. To verify the numerical analysis, the actual temperature rises in the test masks during exposure were experimentally measured. Five thermocouples were bonded to the surface of the absorber to measure the local temperature. By comparing the recorded temperatures at different beam power settings, the temperature rises in the test masks were found to be proportional to the beam power, which verifies the numerical findings. Furthermore, the shape and size of the absorbers have a significant impact on the physics of the thermoelastic behavior. However, the increased power absorption associated with larger absorbers almost completely compensate the increased heat transfer capability along the higher conductivity mask absorbers in the examined cases. The experiments verified the simulations to a large extent. Deviations typically amount to 5° C at an overall temperature of approximately 40° C, which is mainly attributed to the size of the proximity gap varying in reality, and therefore differing from the model assumption of a constant gap. Finally, the thermal and thermoelastic behavior of the test masks was evaluated by an extended numerical analysis model for different typical exposure scenarios used for the DXRL exposure of 250 μm and 500 μm PMMA resist coated onto a silicon wafer substrate. In these simulations, the resist and substrate were also modeled. For a 25% duty cycle chopper setting, and spectral tuning as required for exposure, about 14 W to 20 W of incident beam power gets absorbed. A maximum mask temperature of 25.4° C is observed for 250 μm PMMA, and 31.0° C for 500 μm PMMA. While the resist deforms on the nanometer level, mask deformations in the lateral plane amount to approximately 2.3 μm for 250 μm PMMA, and to approximately 4 μm for 500 μm PMMA. These are worst-case values without further beam power reduction, and integrated over 6 cm large absorbers. Local deformations would be significantly lower. Such deformations are therefore deemed acceptable. The results prove that polyimide masks can be applied with acceptable thermal deformations under the conditions found at SyLMAND