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.
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.
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.
This paper presents recent results on the microstructural evolution and the resulting mechanical property changes as a function of elevated temperature exposure of two types of electroplated nanocrystalline LIGA Ni. Electroplated Ni structures are the main candidates for LIGA-based MicroElectroMechanical System (MEMS). Initial studies have been conducted to correlate microstructure of electroplated Ni and resulting mechanical properties. A major drawback is that upon exposure to elevated temperatures, electroplated Ni MEMS components suffer dramatic reductions in strength mainly due to grain coarsening. This kind of strength deterioration at elevated temperatures can be detrimental to many MEMS applications, especially to micro-engines and molding inserts. Thus, in order to improve the high temperature performance of LIGA Ni, knowledge of the underlying mechanism is needed. At present, there is very limited understanding of processing-microstructure-property relationship for LIGA Ni at both room and elevated temperatures. The current research is focused on temperature effects on the microstructure of LIGA Ni and the resulting mechanical properties. Two types of sulfuric acid-based solutions were used to produce electroplated Ni samples with different microstructural characteristics. The DSC technique was used to study the stability of plated Ni at elevated temperatures. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to examine the microstructural changes of plated Ni samples as a function of annealing temperature. Nanoindentation tests were performed to study the effects of the evolved microstructures on mechanical properties. The underlying mechanism correlating microstructure and mechanical properties of LIGA Ni at elevated temperatures is discussed.
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.
The normal process to fabricate an x-ray mask involves two steps: make an optical mask by using an optical pattern generator (OPG), and form the pattern on the x-ray mask membrane with the optical mask by UV lithography. The sole function of the optical mask is the pattern transfer from source to target. It is always possible that pattern distortion would happen during its transference. In this paper we present a new process to fabricate deep x-ray lithography (DXRL) mask by direct pattern writing on the first layer of resist of an x-ray mask membrane. A thin layer of gold (1 ~ 2 μm) is deposited on the revealed plating base of the membrane and serves as the absorber for a following x- ray exposure of the second (thicker) layer of resist. Finally a thick (5-10 μm) gold layer is plated in the stencil formed by developed second layer of resist. This process has been demonstrated with Kapton membranes (Polyimide foil). The principle of the process can be applied to other x-ray mask membrane materials and to make ultra deep x-ray lithography (UDXRL) mask as well. In this paper the initial results of the new process are presented. The performance of the fabricated mask is evaluated and the alternative approaches will be discussed.
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.
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.
Many MEMS applications require multi-level microstructures in which two or more levels have to be aligned to each other in the processing. In this paper a passive alignment system based on a mechanical registration method utilizing reference posts is described. A detailed analysis of the test results was conducted to reveal main error sources and estimate the accuracy of this alignment method. An alignment accuracy of +/- 5(mu) m between 2 layers has been achieved. The further work on improving the alignment accuracy and expending in this alignment method to graphite masks for multi-level X-ray or combined optical/x-ray lithography is proposed.
SU-8 has great potential in low cost ultra-thick high aspect ratio MEMS applications. Although a broad range of thickness (from micrometer to mm) can be obtained by spin coating, the works about the sidewall profile and dimension control of SU-8 microstructures have not been published in detail. This paper describes the detailed investigations on the effects of processing parameters such as UV wavelength and exposure dose on dimensional change and sidewall profile of SU-8 microstructures. The optimized processing parameters for SU-8 structures with the thickness from 10 to 360 micrometer are presented.
A new processing technique for thin film tin oxide gas sensor has been described in this paper. Oxygen plasma is used as film sensitizing tool, in which the temperature only goes up to 190 degree(s)C for a short time. By using this technique, the tin oxide thin film sensors with smooth, uniform surface have been made with a reasonable sensitivity to the CO gas at room temperature. The oxygen plasma treatment decreases both film resistivity and film grain size. This new technique makes the tin oxide sensor fabrication compatible with the microelectronic processing.
The Center for Advanced Micro structures and Devices (CAMD) at Louisiana State University supports one of the strongest programs in synchrotron radiation micro fabrication in the USA and, in particular, in deep x-ray lithography. Synchrotron radiation emitted form CAMD's bending magnets has photon energies in the range extending from the IR to approximately 20 keV. CAMD operates at 1.3 and 1.5 GeV, providing characteristic energies of 1.66 and 2.55 keV, respectively. CAMD bending magnets provide a relatively soft x-ray spectrum that limits the maximal structure height achievable within a reasonable exposure time to approximately 500 micrometers . In order to extend the x-ray spectrum to higher photon energies, a 5 pole 7T superconducting wiggler was inserted in one of the straight sections. A beam line and exposure station designed for ultra deep x-ray lithography was constructed and connected to the wiggler. First exposures into 1 mm and 2 mm thick PMMA resist using a graphite mask with 40 micrometers thick gold absorber has been completed.
The X-ray lithography and micromachining facility at CAMD hosts the `print-shop' for the development and prototype exposures in LIGA-like processing techniques for the HI-MEMS Alliance. A simple fixture with alignment, tilt, and rotation modules has been developed. It allows for multiple level exposures with registration. More complex shapes can be achieved by rotating and tilting the mask/wafer assembly with respect to the incident X-ray beam. The alignment system is based on optical registration using an X-ray mask with targets on optically transparent windows. The masks were fabricated at MCNC. The alignment tests and off-axis exposures were performed at CAMD. Overlay accuracy of +/- 5 micrometers has been demonstrated.
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