Fabrication of microgratings on PMMA plate and curved surface by using copper mesh as X-ray lithography mask

We demonstrate experimentally the X-ray lithography technique to fabricate microgratings on a PMMA plate and on curved surfaces such as PMMA cylinder lens surfaces with X-ray lithography by copper mesh as mask. Some gratings with 12.7 μm pitches on the plate and on PMMA curved surface with large area (10 mm × 10 mm) by vertically moving or rotating the resist stage exposure are realized.     

Proton micromachining: a new technique for the production of three-dimensional microstructures

A novel technique for the fabrication of high aspect ratio three-dimensional (3D) microstructures is presented. A suitable resist (e.g. PMMA or SU-8) is exposed using focused MeV (million electron volt) protons in a direct write process to produce 3D microstructures with sub-micrometer feature sizes. By adjusting the energy of the proton beam, the depth of the microstructures can be controlled very accurately (e.g. between 5 and 160 μm). Single layer SU-8, a newly developed, chemically accelerated, negative tone, near UV, photo-resist, has been used in multiple exposures using different proton energies to produce intricate 3D microstructures. The combination of a well controlled exposure depth coupled with the ability to tilt the sample with respect to the beam increases the manufacturing capability, and allows the production of complex microstructures with well defined edges in single layers of resist. Received: 15 July 1999/Accepted: 30 July 1999     

Validation of X-ray lithography and development simulation system for moving mask deep X-ray lithography

This paper presents a newly developed 3-Dimensional (3-D) simulation system for Moving Mask Deep X-ray Lithography (M/sup 2/DXL) technique, and its validation. The simulation system named X-ray Lithography Simulation System for 3-Dimensional Fabrication (X3D) is tailored to simulate a fabrication process of 3-D microstructures by M/sup 2/DXL. X3D consists of three modules: mask generation, exposure and resist development (hereafter development). The exposure module calculates a dose distribution in resist using an X-ray mask pattern and its movement trajectory. The dose is then converted to a resist dissolution rate. The development module adopted the "Fast Marching Method" technique to calculate the 3-D dissolution process and resultant 3-D microstructures. This technique takes into account resist dissolution direction that is required by 3-D X-ray lithography simulation. The comparison between simulation results and measurements of "stairs-like" dose deposition pattern by M/sup 2/DXL showed that X3D correctly predicts the 3-D dissolution process of exposed PMMA.     

Proton beam writing: a tool for high-aspect ratio mask production

P-beam writing (proton beam writing), a direct write 3D nano lithographic technique has been employed for the production of X-ray masks in a single step fabrication process, with high aspect ratios and extremely smooth absorber edges. P-beam writing employs a focused MeV proton beam scanned in a predetermined pattern over a resist (e.g. PMMA or SU-8), which is subsequently chemically developed. P-beam writing in combination with electroplating appears ideally suited to directly write X-ray masks with nano sized features, high aspect ratios, small lateral feature sizes, and smooth and vertical sidewalls. Sub 100 nm resist structures with aspect ratios up 160 have been produced, as well as metallic (nickel) structures down to the 100 nm level. Preliminary tests on p-beam written X-ray test masks show that Ni stencils can be fabricated with a thickness of 2–20 μm, smooth side walls, feature details down to 1 μm, and aspect ratios up to 20.     

Direct, high throughput LIGA for commercial applications: a progress report

Direct LIGA; LIGA without injection molding; has the potential to become a cost effective, high throughput form of LIGA. The process requires high energy photons; near 20,000 eV; which are best produced in facilities such as the X-ray ring at Brookhaven National Laboratory. The increased absorption lengths over lower energy photons eliminates the need for a membrane type X-ray mask. This in turn facilitates very large area X-ray masks fabricated from standard silicon wafers with 20 μm gold absorbers. The absorption length increase in PMMA to 2 cm is used to implement stacked PMMA exposures in which 1 mm thick PMMA layers are used to produce exposed PMMA sheets. These sheets are eventually solvent bonded to working substrates with plating bases. New high energy X-ray masks have been developed. Two exposure stations at Brookhaven are operational. The recently commissioned manufacturing exposure station which uses a 22 inch scanner which can expose four separate PMMA-mask combination is in the testing phase. Received: 7 July 1999/Accepted: 30 July 1999     

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.     

Comparison of PMMA and SU-8 resist moulds for embossing of PZT to produce high-aspect-ratio microstructures using LIGA process

 In this paper results are presented from a range of experiments to explore the feasibility of inserting a ceramic material PZT (lead zirconium titanate) into different kinds of high-aspect-ratio resist moulds. Polymethylmethacrylate (PMMA) and SU-8 on silicon substrates and free-standing SU-8 membranes with micro-cavities or through-holes (defined by X-ray lithography) have been used as moulding medium. Processing conditions for the resist materials including pre-bake, exposure, post-bake, development and stripping have been compared. The advantages of different types of resist mould for the LIGA process has been evaluated. Additionally a comparison of photosensitivity of PMMA and SU-8 has been carried out. Using a range of load pressures (5 to 60 MPa), appropriate conditions for PZT embossing into resist moulds have been determined (ensuring minimum void formation in the final PZT structures). In the final form, SU-8 moulds have been removed by laser ablation. This is the first reporting of high-aspect-ratio ceramic microstructures fabricated using a combination of SU-8 moulds and PZT embossing. Received: 10 August 2001/Accepted: 24 September 2001     

Characterization of exposure and processing of thick PMMA for deep x-ray lithography using hard x-rays

 The first step for the fabrication of microstructures using deep x-ray lithography (DXRL) is the radiation of a sensitive polymer like poly(methyl methacrylate) (PMMA) by hard x-ray. At the Advanced Photon Source a dedicated beamline¹ allowed the proper exposure of very thick resist (several mm). In this work we give a characterization of the PMMA/development system. As a result the resist dissolution rate (μm/min) of the exposed PMMA in different developer is calculated. We also analyze the conditions that produce bubbles and cracks in the thick exposed resist and investigate the scanning parameters that reduce them. Received: 25 August 1997/Accepted: 3 September 1997     

X-ray lathe: An X-ray lithographic exposure tool for nonplanarobjects

An X-ray lithography lathe has been developed that can pattern cylindrical, ellipsoidal, and other nonplanar objects. This lathe is capable of patterning on a micron scale a wide variety of shapes including shapes impossible to achieve with a conventional lathe. A cylindrical core covered with a suitable resist is rotated while being exposed with a collimated X-ray source through a mask. The mask absorbs X rays up to a particular radius from the center of the core and the resist beyond that radius is removed in a developer. Several cylindrical cores were coated with poly(methylmethacrylate) (PMMA) 5 to 125 μm thick and patterned with X-rays down to a 250-μm horizontal scale (along the lathe axis). The exposure time for a cylindrical PMMA layer is ~three-four times longer than a planar layer with the same thickness. The capabilities of this technology, lathe apparatus, dose calculations, and initial exposure results are described     

Equations of exposure time and X-ray mask absorber thickness in the LIGA process

 The LIGA X-ray exposure step was modeled into three inequalities from exposure requirements. From these inequalities, equations for the minimum and maximum exposure times required for a good quality microstructure were obtained. An equation for the thickness of an X-ray mask absorber was also obtained from the exposure requirement of threshold dose deposition. A power function of photon energy, approximating the attenuation length of the representative LIGA resist, PMMA, and the mean photon energy of the X-rays incident upon an X-ray mask absorber were applied to the above mentioned equations. Consequently, the trends of the minimum and maximum exposure times with respect to mean photon energy of X-rays and thickness of PMMA were examined and an equation for the maximum exposable thickness of PMMA was obtained. The trends of the necessary thickness of a gold X-ray mask absorber with respect to photon energy of the X-rays and PMMA thickness ratio were also examined. The simplicity of the derived equations has clarified the X-ray exposure phenomenon and the interplay of exposure times, the attenuation coefficient and the thickness of an X-ray mask absorber, the attenuation coefficient and the thickness of a resist, and synchrotron radiation power density. Received: 22 November 1999/Accepted: 27 January 2000     

Process conditions in X-ray lithography for the fabrication of devices with sub-micron feature sizes

This article describes the fabrication of polymer structures with lateral dimensions in the sub-micron regime using hard X-rays (λ_c ≈ 0.4 nm) from the electron storage ring ANKA. Spincoated polymethylmethacrylate (PMMA) grades have been analyzed with respect to development rates and contrast. The contrast has been determined to be constant over a wide dose regime but rapidly decreases for dose values below 1 kJ/cm³. Films with a thickness from 2 to 11 μm have been patterned using a high resolution X-ray mask consisting of 2 μm thick gold absorbers on a suspended 1 μm thick silicon nitride membrane. The fabrication of sub-micron X-ray lithography structures with feature sizes down to 400 nm is confined by the mechanical parameters of the resist material and the process conditions. Surface tension after development limits the achievable aspect ratio of isolated pillars and walls, depending on the actual resist height. PMMA structures have been successfully used as template for electroplating of 1 μm thick gold to demonstrate the fabrication capability of sub-micron scale metal parts.     

The influence of X-ray fluorescence on LIGA sidewall tolerances

 Fluorescence radiation during LIGA X-ray exposure and the impact of the resulting secondary dose on feature sidewall tolerances are examined using numerical methods. The study addresses fluorescence emitted by the substrate of the PMMA resist and focuses on the tolerances obtained for resists patterned with a range of feature sizes. New models of the secondary dose and subsequent feature development are presented and discussed. These models are coupled such that the evolving feature geometry during development depends on the local total dose, as well as the development temperature and the transport of PMMA fragments away from the dissolution front. We find that sidewall tolerances for typical exposure and development conditions are less than 0.1 micron for a metallized silicon substrate, but exceed several microns for thick titanium, copper and nickel substrates. We also find that these tolerances are very sensitive to the feature geometry, development temperature and the magnitude of the primary dose. Received: 10 August 2001/Accepted: 24 September 2001     

Study on fabrication of high aspect ratio MEMS microparts using a compact SR beamline

 A compact beamline dedicated to hard x-ray deep lithography for fabrication of high aspect ratio MEMS microparts has been developed. The exposure stage was only 3 meters away from the synchrotron radiation (SR) source so that a relatively high photon flux could be achieved with a compact super-conducting SR source. The deep lithography using PMMA resist could be as deep as 1000 μm and the maximum aspect ratio achieved was about 50. The throughout for the 200 μm-deep lithography was found to be on the order of 5 cm²/h using the membrane-free mask under the routine SR conditions. Templates with the high aspect ratio microstructures have been made of the PMMA resist based on conducting substrates and applied further to electroforming to create metallic microstructures. In order to fabricate microparts for the MEMS applications, we have concentrated on development of masks for the hard x-ray deep lithography. The masks now can be made to have the 8 μm-thick gold absorber on the 2 μm-thick SiC membrane. Received: 25 August 1997/Accepted: 3 September 1997     

Structure quality in deep X-ray lithography applying commercial polyimide-based masks

The structure quality of deep X-ray lithography components strongly depends on the quality of the applied X-ray mask. In this article we compare the results obtained with two different mask types. Sophisticated working masks generated by e-beam lithography, soft X-ray lithography and electroplating of gold absorbers on a titanium mask membrane have been fabricated at the Institute for Microstructure Technology, Research Center, Karlsruhe (FZK/IMT), Germany. Prototype masks generated by e-beam lithography, optical lithography and electroplating of gold absorbers on a polyimide mask membrane have been fabricated by Optnics Precision, Japan, with the aim to offer commercially available low cost masks. Both mask types were applied to pattern PMMA resist layers of 300–750 μm thickness at the 2.5 GeV electron storage ring ANKA, Germany, using comparable process parameters. FZK/IMT masks provide microstructures with significantly better structure quality. The layout area, however, is currently limited to 12 cm², and the Ti mask membrane tends to lead to a slight resist surface attack, such as rounding of the resist edges. Optnics masks provide microstructures with reduced structure quality due to sidewall striations (sidewall roughness up to 2 μm) and thermal distortions (of up to 3–5 μm) which limit the potential scope of applications. They could nevertheless potentially be applied as low quality, low cost X-ray masks. High resolution and high accuracy applications, however, require more sophisticated but also more expensive masks, like the Ti-masks from FZK/IMT.     

A hard X-ray prototype production exposure station at NSLS

 Exposures conducted at the NSLS R&D beamline (X-27B) for High Aspect Ratio Precision Manufacture have proven sufficiently successful that we are constructing a dedicated hard X-ray exposure beamline. The new beamline (X-14B) provides an exposure field ∼120 mm wide, three times larger than that of X-27B. The scanner is based on the hydraulic system from the X-27B program. It is optimized for planar exposures and takes advantage of the full 525 mm stroke available. Exposures of multiple substrates and masks will be possible, with the fixturing supporting mounting of substrate holders from other groups (ALS, APS, CAMD, and UW). The function of this beamline is to establish a hard X-ray exposure station where manufacturing scale protocols can be developed and ultimately exploited for production runs. Received: 25 August 1997/Accepted: 3 September 1997     

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     

Characterisation of defects in very high deep-etch X-ray lithography microstructures

 In deep X-ray lithography synchrotron radiation is applied to pattern several hundred micrometer thick resist layers. This technique has been used to obtain micro structures with an aspect ratio up to 100 and dimensions in the micrometer range. The structures are characterised by straight walls and a typical sidewall roughness of approximately 50 nm. To be able to fabricate n-coherent structures with any lateral shape and to have the possibility to use these resist microstructures in an additional electroforming process the resist is usually mounted on a ceramic or metallic substrate. Due to the different thermal expansion coefficients of the resist material and the substrate a developing temperature of 37 °C produces cracks in the resist structures depending on the microstructure design. These defects are not observed if the developing temperature is reduced to 20 °C. Better structure quality is obtained using the GG-developer instead of MIBK/IPA, but the developing rate is decreased. Measurements of the developing rate of PMMA in GG-developer at different temperatures show that the contrast of the developer-resist system is increased at 20 °C compared to 37 °C. Received: 25 August 1997/Accepted: 3 September 1997     

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. Received: 10 August 2001/Accepted: 24 September 2001 This work is supported by the U.S. Department of Energy, Office of Science, under Contract No. W-31-109-ENG-38. This paper was presented at the Fourth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.     

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     

The influence of mask substrate thickness on exposure and development times for the LIGA process

Optimizing mask substrate thickness is an important practical concern in the X-ray exposure of PMMA resists for LIGA. An overly thick substrate necessitates long exposure times due to excessive beam filtering, while a substrate too thin leads to long development times due to low absorbed doses at the PMMA bottom surface. To assist in this optimization, we have developed numerical models describing both the exposure and development of a PMMA resist. These exposure and development models are coupled in a single interactive code, permitting automated adjustment of mask substrate thickness to yield the minimum of a prescribed cost object function that depends on both the exposure and development times. Results are presented for several synchrotron sources and over a wide range of the PMMA thickness. Received: 15 July 1999/Accepted: 23: August 1999