Zhong-Geng Ling

Fabrication of monolithic multilevel high-aspect-ratio ferromagnetic devices

This paper describes a process to fabricate monolithic multilevel high-aspect-ratio microstructures (HARMs) for ferromagnetic devices built on silicon wafers using aligned X-ray lithography in conjunction with electrodeposition. Two X-ray masks were fabricated, each consisting of gold (Au) absorber structures on a transparent polyimide membrane. One mask was used to print a polymethyl methacrylate (PMMA) resist layer. Then, a second PMMA layer was applied to the same wafer, and the second mask was used to pattern it. Transparent alignment windows in the second mask, combined with a piezoelectrically controlled X-ray aligner, allowed for high alignment accuracy between the two print patterns over large areas (>4 inch in diameter). Au circuits were electroplated into first PMMA layer from a sulfite-based electrolyte, and nickel-iron (NiFe) ferromagnetic HARMs were formed in second PMMA resist from a sulfate-based bath. The deposition resulted in well-defined NiFe structures with aspect-ratios up to 67:1 as well as smooth sidewalls and top surfaces. Chemical composition measurements with energy X-ray dispersive spectroscopy (EDS) and wavelength X-ray dispersive spectroscopy (WDS) showed that Fe content increased during the electrodeposition process. To electrically isolate the NiFe posts and Au circuits, both wet chemical etching and sputter etching were explored to remove the exposed seed layer, and the latter approach completely removed the seed layers without damaging the electroplated features.

Thermal stability of SU-8 fabricated microstructures as a function of photo initiator and exposure doses

SU-8 has been used directly as structural material for MEMS/BioMEMS components as well as optical MEMS components. Although the applications of SU-8 photoresist have widely been presented, the material properties and behavior at elevated temperature have rarely been reported. In this paper, the thermal stability of the SU-8 structures as the function of exposure doses and photo initiator concentration changes has been studied. Differential Scanning Calorimeter (DSC), Thermogravimetric Analyzer (TGA) and Dynamic mechanical analysis (DMA) are employed to study the thermal stabilities of exposed SU-8 microstructures. Mass loss as the function of exposure doses and post-baking time were studied by TGA. The results show that the relative mass loss is inversely proportional to the exposure dose as well as the post-baking time, which also directly affect the thermal stability of SU-8 components. The DSC results reveal that there is a phase change reaction occurs around the temperature of 150°C and is directly related to the photo initiator. The effects of this phase change on the tensile strength and creep behavior of SU-8 fabricated microstructures were also explored using DMA. These results will provide the MEMS researchers and engineers with the usable information in SU-8 applications. At the end, how to optimize SU-8 processing parameters to increase its thermal stability is discussed.

Expansion of SU-8 application scope by PAG concentration modification

In recent year SU-8 has became the most attractive photoresist in both optical and x-ray lithography. In our early work we have optimized its exposure parameters to improve the patterning quality in UV lithography and concluded that the UV absorption in SU-8 is proportional to the concentration of photoacidgenerator (PAG) and limiting the applicable SU-8 thickness in UV lithography. Actually, the PAG concentration plays an important role in all aspects of SU-8 processing in both optical and x-ray lithography. The motivation of this work is to expand the applicable thickness and application scope and improve processing control of SU-8 by optimizing its PAG concentration. In this paper we present the most recent experimental results on lithographic performance of SU-8 with different PAG concentration (varying up to 2 orders of magnitude). It includes determining the minimum bottom dose and minimum effective energy density in x-ray and UV lithography of SU-8, respectively, observing the dimensional change of SU-8 microstructure at different post exposure bake (PEB) temperature and time and measuring UV absorption spectrum of SU-8 as the function of PAG concentration. The modified SU-8 resists have moderate sensitivities and lower absorption coefficients. The application of the modified SU-8 will be addressed and demonstrated.

Interconnected multilevel microfluidic channels fabricated using low-temperature bonding of SU-8 and multilayer lithography

This paper describes a novel fabrication method for the manufacture of multi-level microfluidic structures using SU-8. The fabrication method is based on wafer bonding of SU-8 layers and multilayer lithography in SU-8 to form microchannels and other structures at different levels. In our method, non-UV-exposed SU-8 layers are transferred to SU-8 structured wafers at desirably low temperatures. This technique is particularly useful for building multi-level fluidic structures, because non-UV-exposed SU-8 can be used as cover for microchannels and the cover can then be lithographically structured, i.e., to form interconnects, after which subsequent transferring of non-UV-exposed SU-8 onto the wafer allows for the fabrication of interconnected multi-level channels and other structures. Examples of interconnected multi-level microchannels were realized using this newly developed method. Liquid has been introduced into the microchannels at different levels to reveal the desirable functionality of the interconnected multi-level channels. The method described here is easily implementable using standard photolithography and requires no expensive bonding equipment. More importantly, the fabrication procedure is CMOS compatible, offering the potential to integrate electronic devices and MEMS sensors into microfluidic systems.

An injection micromixer fabricated by improved SU-8 processing for biochemical microfluidic systems

In this research, first a modular polymer-based (PMMA) injection micromixer prototype has been designed, fabricated and tested. This micromixer is easy to be integrated into biochemical microfluidic systems under development for BioMagnetICs DARPA funded project at CAMD. To improve the mixing efficiency, layout of micronozzles of the mixer was optimized according to the simulation results. Also because SU-8, an epoxy-based negative photoresist, has high chemical resistance, an SU-8 injection mixer was designed and fabricated to run some biochemical sample liquids. Internal stress in patterned SU-8 structures has been reduced and multi-layer SU-8 processing has been successfully developed to fabricate SU-8 injection mixer.

High-aspect-ratio microstructures for magnetoelectronic applications

This paper describes a process to fabricate three-dimensional multilevel high-aspect-ratio microstructures (HARMs) for magnetoelectronic devices using aligned x-ray lithography in conjunction with electrodeposition. In this process, x-ray masks were constructed on a seed layer coated polyimide membrane with ultraviolet (UV) patterned and electrodeposited gold absorbers. The optically transparent polyimide allows one to align and print large areas (>4 inch in diameter) with high alignment accuracies. Patterns that contain 5-10 μm diameter posts and 7-10 μm wide lines were printed to 100-120 μm polymethyl methacrylate (PMMA) resist prepared on silicon wafers using x-ray lithography. Nickel-iron was electroplated to form ferromagnetic HARMs, while electroplated gold formed circuits. The composition profile measured with an electron probe x-ray microanalyzer (EPMA) suggested that iron content increases as NiFe plating proceeds inside the recess. The electrodeposition resulted in well-defined NiFe structures with aspect-ratios up to 20:1, smooth sidewalls and top surfaces. To isolate the magnetic structures and circuits, both wet chemical etching and sputter etching were explored to remove seed layer, and the latter yielded complete removal without noticeable damage to the features. A complete aligned x-ray exposure and electrodeposition protocol applicable to universal multilevel microstructures was established.

Microstructure Enhanced Heat Exchanger for Pressurized Water Reactor

The goal of this research is to enhance the heat exchanger efficiency of pressurized water reactor (PWR) by using LIGA or LIGA-like technique made microstructures. The heat transfer inside the boiler is a complex combination of different physical phenomena, which, besides the traditional convection, conduction, and radiation, includes liquid to vapor phase change, vapor nucleation and evolution, surface tension between the liquid and heating element, and so on. This paper presents the updated total heat transfer enhancement results of the boiling process by adding microstructures on the surface of the heating elements. Different types of microstructure configurations were tested. The power input, temperature of the heating element, and boiling phenomena were recorded. The behavior of increase in power versus rise in temperature of testing coupon is used to evaluate the heat transfer efficiency of the heating element. The steam generating efficiency at fixed input power and fixed temperature have been used to assess the performance of heating elements with different microstructure configurations. The preliminary results show that by simply adding micro-sized poles on the surface of the heating element, the power input can be increased almost 100% higher than that without poles on the surface at 360°C. The current results suggest that the main factors leading to the enhanced boiling process are the surface morphology and configuration of the microstructures. These provide enhanced vapor nucleation sites at heating surface, which result in a better vapor evolution processes, and yield a low superheat temperature. It ultimately results in a higher boiling heat transfer efficiency.Copyright

Fabrication of large-area x-rays masks for UDXRL on beryllium using thin film UV lithography and x-ray backside exposure

A method to fabricate a high precision X-ray mask for Ultra Deep X-ray Lithography (UDXRL) is presented in this paper by use of a single substrate. Firstly, an 8-μm layer of positive photoresist is patterned on a 500 μm thick beryllium substrate by use of UV lithography and 5 μm gold is electroplated out of a sulfite based commercial plating solution. Secondly, the photoresist is removed and 15 μm of SU-8 is spincoated and baked. The layer of SU-8 is patterned by use of an exposure from the backside of the substrate with a soft X-ray source, followed by post-exposure bake and development. An additional 5 μm layer of gold is electroplated on top of the first gold pattern thereby increasing the total thickness of the absorber on the X-ray mask to 10 μm. After the removal of the SU-8 resist, the second step of the process is repeated by use of a thicker layer of SU-8 (up to 100 μm) to obtain the high-precision and high-aspect ratio absorber pattern. Using this method, the maximum dimensional error of the fabricated gold pattern remains under 1 μm, while the smallest absorber feature size is 10 μm.

Instrumentation for synchrotron based micromachining at the Center for Advanced Microstructures and Devices (abstract)

The J. Bennett Johnston Sr., Center for Advanced Microstructures and Devices (CAMD) is a synchrotron radiation facility owned by Louisiana State University and operated with financial support from the State of Louisiana (for information how to submit a project proposal go to: http://www.camd.lsu.edu). The centerpiece of CAMD is a 1.3–1.5 GeV electron storage ring. CAMD supports a strong program in x-ray lithography micromachining (XRLM) or LIGA. A total of four beamlines equipped with different scanners is available for exposures. A 2.500 sq. ft class 100 clean room provides basic processing capability for MEMS including optical lithography, thin film deposition, electroplating, and metrology. Three micromachining beamlines are connected to bending magnets. All beamlines are “white light” beamlines, terminated with a beryllium window. The typical source point to scanner distance is 10 m and the horizontal acceptance ranges from 6.5 to 10 mrad. A number of low Z filters can be inserted into the beam adapti...