Ultra-deep X-ray Lithography Research Articles

Development and characterization of ultra high aspect ratio microstructures made by ultra deep X-ray lithography

A technique of casting of thick pre-polymer layers was developed based on the proposed model of diluent evaporation. For the first time, microstructures with 7mm height and up to 18 microns wide (an aspect ratio of up to 389) were manufactured by means of ultra deep X-ray lithography in a cured pre-polymer that can be functionalized. The microstructures were characterized. The difference in the size of patterned microstructures and their initial size on the X-ray mask decreases to 400nm with the lessening lateral dimension of the microstructures from 1500 to 18μm. The sidewall roughness also decreases to 20nm.

A hybrid approach for fabrication of polymeric BIOMEMS devices

Polymeric microfluidic chips are an enabling component for cost-effective, point of care analytical devices for pharmaceutical, agriculture, health, biological and medical applications. The microfluidic structures can be completed with active elements like pumps and valves as well as sensor components for more complex so called total analysis systems. Often, systems are designed as reader and disposable cartridge where the fluidic structures are simple devices that will be inserted into the reader, which executes the analytical protocol and displays the information in digital form, and disposed after completion of the analysis. In this paper, a hybrid fabrication approach was employed to build a polymeric microfluidic device, so-called sweatstick, suitable for collecting small, precise amounts (600 μl) of human sweat, which were further analyzed for the amount of calcium ions indicating bone mass loss. The device was assembled from different parts fabricated by ultra deep X-ray lithography, precision micro-milling, and molding. Surface treatment of liquid exposed surfaces by oxygen plasma ensures hydrophilic behavior and proper capillary action. Preliminary testing of the device was performed by collecting defined amounts of sweat simulant and determining the calcium ion content using a fluorescent technique. The results for low calcium ion concentration typical for human sweat were excellent and repeatable with variation less than 5% demonstrating the ability to perform indirect bone loss measurements.

Ultra-deep x-ray lithography of densely packed SU-8 features: I. An SU-8 casting procedure to obtain uniform solvent content with accompanying experimental results

Although the benefits of SU-8 as a thick photoresist material for MEMS applications have been shown by various researchers, its use for ultra-thick applications (>1 mm) has been hampered by the difficulties associated with the high solvent gradient of the resultant layers. In this paper, the lithographic performance of the traditional methods of thick resist formation based on the casting of high solvent content SU-8 is presented. As an extension of the work, a new photoresist preparation method called ‘dry chip casting’ was developed. The new method involves the reduction of the solvent content of a given photoresist until the desired content is reached, the collection of the ‘dry’ chips and re-constitution on a substrate with the aid of heat and vacuum. The dry chip casting method yielded average solvent contents of about 7% (±0.5%) throughout the thickness of the SU-8 layers, with excellent x-ray lithographic characteristics. Results of ultra-deep x-ray lithography experiments in SU-8 layers as thick as 3 mm are presented in this paper.

Ultra-deep x-ray lithography of densely packed SU-8 features: II. Process performance as a function of dose, feature height and post exposure bake temperature

Ultra-tall (2–4 mm), densely packed arrays of high aspect ratio micro structures (HARMs) are required for a variety of heat transfer and mass transfer devices currently under development. Great success has been reported lithographically defining relatively tall features using SU-8 as an x-ray resist, but excellent results are scarce with respect to patterning ultra-tall features with tight packing and small gaps between features. Diffusion of a cross-linking species from exposed regions to unexposed regions is believed to be the primary mechanism that prevents such features from being defined. In a companion paper, solvent content is shown to be an extremely important parameter that controls the quality of ultra-tall, tightly packed micro structure arrays. In this paper, a simple quasi-two-dimensional model is developed to qualitatively show that the adverse effects of diffusion of a cross-linking species can be significant. Also, experiments were performed to isolate the effects of parameters other than solvent content on performance (post exposure bake temperature, dose and top-to-bottom dose ratio, and feature height). These effects are placed in the context of the general hypothesis that preventing diffusion of cross-linking species is a key to successfully defining ultra-tall, tightly packed SU-8 HARMs. Experimental results are provided that demonstrate the ability to define SU-8 features with heights of 3 mm, and gaps between adjacent features of 125 µm (the gap therefore having an aspect ratio of 24).

Improvements in graphite-based X-ray mask fabrication for ultradeep X-ray lithography

Hard x-ray masks for ultradeep x-ray lithography (UDXRL) at synchrotron radiation sources, such as the Advanced Photon Source, require a gold absorber thickness of 20–100 μm on a low-Z substrate, such as silicon, graphite, or beryllium. Graphite sheets of 0.5 mm were used for the fabrication of x-ray masks by standard optical lithography using a SU-8 photoresist. The conductivity of graphite is sufficient to perform electroplating directly without the need for a metal-plating base layer. Gold electroforming was used to deposit a 85-μm-thick patterned absorber layer. The masks were used for UDXRL using hard x-rays at the Advanced Photon Source.

Oxygen quenching effect in ultra-deep x-ray lithography with SU-8 resist

An ultra-thick (1.5 mm) SU-8 resist was prepared and patterned using synchrotron x-rays. The lithographic sensitivity of the SU-8 resist is over 100 times higher than that of the traditional PMMA resist. However, preliminary results showed that the standard resist process produced poor lithographic quality. The fluorescence induced by high-energy x-ray irradiation was believed to blur the transfer pattern. The lithographic quality improved significantly if the resists were relaxed in an oxygen atmosphere before being soft baked. The FTIR spectrum displayed new absorption peaks, corresponding to the C=O bond, in the oxygen-relaxed resists. The development rates of the low-dosed resists increased if they were held in the oxygen atmosphere, implying that trapped oxygen restrains the cross linking of the SU-8 resist. Based on these observations, this study proposes an oxygen quenching mechanism for interpreting these phenomena. When the SU-8 is oxygen relaxed, the oxygen molecules will gradually diffuse into the resist. Oxygen is a strong free-radical scavenger, and thus kills the free radicals generated by fluorescence, providing a clear pattern defined by x-ray irradiation. After the SU-8 resist was properly oxygen treated, the maximum dimension error was only about 2 µm for a 1.5 mm-thick SU-8 resist. This lithographic quality is good enough to meet the micromachining requirements for mm-wave RF (radio frequency) cavity applications.

Visualization of transport: NMR microscopy experiments with model objects for porous media with pore sizes down to [formula omitted

Hydrodynamic flow and electric currents through model porous media were investigated. The transport rates through the individual pathways of the pore network are determined by the local width of the pore channels and by the driving mechanism. The model objects represent quasi two-dimensional random site percolation clusters. The calculated design was realized by milling the structure in polystyrene sheets. Velocity maps of stationary flow and current density maps of stationary currents through the cluster were acquired by magnetic resonance imaging methods. The findings were compared to the results of numerical simulations based on the same structure. Since the difference in the transport patterns of the different driving mechanisms are expected to be more pronounced in smaller pore spaces, ultra deep X-ray lithography has been used for the fabrication of downsized model objects with a spatial resolution of better than 50 μm and an aspect ratio as large as 20. First results obtained with these objects are reported.

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.

Side wall roughness in ultradeep X-ray lithography

An experimental characterization of the roughness of side walls in ultradeep, hard X-ray lithography was performed. Several sources of roughness were distinguished: the roughness of the absorber layer on the mask, the roughness of the mask substrate, nonhomogeneities in the X-ray beam, incomplete developing, cracks generated in the PMMA, and the thermal treatment of PMMA. The intrinsic roughness of the correctly developed PMMA was estimated to about 4 nm rms. The best results showed an anisotropy of roughness with minimal values of 4 nm rms along the Oz direction (parallel with the X-ray beam) and 25 nm rms in the Ox direction.

Deep X-ray lithography for the fabrication of microstructures at ELSA

Two beamlines at the Electron Stretcher Accelerator (ELSA) of Bonn University are dedicated for the production of microstructures by deep X-ray lithography with synchrotron radiation. They are equipped with state-of-the-art X-ray scanners, maintained and used by Forschungszentrum Karlsruhe. Polymer microstructure heights between 30 and 3000 μm are manufactured regularly for research and industrial projects. This requires different characteristic energies. Therefore, ELSA operates routinely at 1.6, 2.3 and 2.7 GeV, for high-resolution X-ray mask fabrication, deep and ultra-deep X-ray lithography, respectively. The experimental setup, as well as the structure quality of deep and ultra deep X-ray lithographic microstructures are described.

Consumption-related development in microelectroforming

High-aspect ratio microelectroforming is one of the most challenging techniques in MEMS microfabrication. This is particularly true with plating metal into the very tall micropatterned polymer molds made by X-ray lithography for primary or secondary metal structures or metal mold inserts within the framework of the LIGA process. Among various problems are: (1) the time consumption in plating very tall parts or using microelectroplating as a replication technique; (2) the cost of material, in particular in the formation of very high-aspect-ratio absorber structures for X-ray masks in the deep and ultra-deep X-ray lithography step.

Graphite-based x-ray masks for deep and ultradeep x-ray lithography

The cost-effective fabrication of high-aspect-ratio microstructures using x rays largely depends on the availability and quality of x-ray masks. The architecture of a mask is mainly determined by the photon energy of the synchrotron source, the x-ray flux, and the thickness of the resist. Typically, the mask membrane is made from a low-atomic-number material and can either be a frame-supported, several microns thin membrane (carbon, silicon carbide, silicon nitride, or silicon) or a bulk material (beryllium) with a thickness of up to 1 mm. The absorber pattern is formed from high atomic number materials such as gold, tungsten, or tantalum, and the final pattern geometry can be defined either with additive (electroplating) or subtractive (etching, milling) processes. One approach that is designed to reduce cost and turn-around time is the fabrication of x-ray masks using graphite sheet stock for the mask membrane. Rigid graphite offers unique properties, such as moderate x-ray transmission, relatively low cost, and the ability to be used with either subtractive or additive processes. This article will report details on the lithographic fabrication of graphite x-ray masks using intermediate x-ray masks for pattern transfer. First experimental results will be presented for synchrotron x-ray exposures performed at the Center for Advanced Microstructures and Devices synchrotron in Baton Rouge and the Advanced Photon Source at Argonne National Laboratory.

Research, development and manufacturing plans at CAMD

The X-ray micromachining activities at CAMD are described. Through the HI-MEMS Alliance Program, CAMD has developed an extensive technical infrastructure and process experience. The “X-ray Print-Shop” supports prototyping and is prepared for cost effective low-to-medium volume production of primary parts with high aspect ratio, including exposure of multiple level devices with alignment, as well as tilted and rotated exposures. Important lessons learnt during the course of the program will be reported. An up-grade of the facility to install a 7.5 Tesla superconducting wiggler in the ring is in progress. It will produce hard X-rays for ultra-deep X-ray lithography.

X-Ray Microfabrication at the Center for Advanced Microstructures and Devices (CAMD)

The Center for Advanced Microstructures and Devices (CAMD) has established infrastructure and experience for deep X-ray lithography and functions as a print-shop for high aspect ratio microstructures in the US. A major up-grade of the facility will be completed by the end of 1997. It includes the installation of a wavelength shifter as a hard X-ray source for ultra-deep X-ray lithography and for cost effective, batch fabrication of high aspect ratio micro-components. This paper describes the X-ray micromachining facility and programs at CAMD. The scientific and engineering activities exemplified by results of current studies are reported.