Fabrication of high aspect ratio comb-drive actuator using deep X-ray lithography at Indus-2

We report microfabrication of high aspect ratio comb-drive using deep X-ray lithography at Indus-2 synchrotron radiation source. Analysis shows that the comb-drive actuator of aspect ratio 32 will produce nearly 2.5 μm displacement when 100 V DC is applied. The displacement increases as the gap between the comb finger decreases. For fabrication of comb-drive, polyimide–gold X-ray mask using UV lithography is made for the first time in India. To pattern on an 800 μm thick X-ray photoresist (PMMA) exposures are performed using our deep X-ray lithography beamline (BL-07) at Indus-2. Metallization on the selective regions of the developed X-ray photoresist with comb-drive pattern was carried out by RF sputtering. Following this the comb-drive actuator of PMMA was fabricated by one-step X-ray lithography. The comb-drive can also be used as a sensor, energy harvester, resonator and filter.     

Fabrication of high precision X-ray mask for X-ray grating of X-ray Talbot interferometer

X-ray imaging is used in many applications such as medical diagnosis and non-destructive inspection, and has become an essential technologies in these areas. In one image technique, X-ray phase information is obtained using X-ray Talbot interferometer, for which X-ray diffraction gratings are required; however, the manufacture of fine, highly accurate, and high aspect ratio gratings is very difficult. X-ray lithography could be used to fabricate structures with high precision since it uses highly directive syncrotron radiation. Therefore, we decided to fabricate X-ray gratings using X-ray lithography technique. The accuracy of the fabricated structure depends largely on the accuracy of the X-ray mask used. In our research, we combined deep silicon dry etching technology with ultraviolet lithography in order to fabricate untapered and high precision X-ray masks containing rectangular patterns. We succeeded in fabricating an X-ray mask with a pitch of 5.3 μm. The thickness of the Au absorber was about 5 μm, and the effective area was 60  × 60 mm², which is a sufficient size for phase tomography imaging. We demonstrated the utility of the Si dry etching process for making high precision X-ray masks.     

Fabrication of electrostatic MEMS microactuator based on X-ray lithography with Pb-based X-ray mask and dry-film-transfer-to-PCB process

Lithographie Galvanoformung Abformung (LIGA) is a promising approach for fabrication of high aspect ratio 3D microactuator for dual-stage slider in hard disk drive. However, this approach involves practically challenging X-ray lithography and structural transfer processes. In this work, electrostatic MEMS actuator is developed based on a LIGA approach with cost-effective X-ray lithography and dry-film-transfer-to-PCB process. X-ray lithography is performed with X-ray mask based on lift-off sputtered Pb film on mylar substrate and photoresist application using casting-polishing method. High quality and high aspect ratio SU8 microstructures with inverted microactuator pattern have been achieved with the interdigit spacing of ~5 μm, vertical sidewall and a high aspect ratio of 29 by X-ray lithography using the low-cost Pb based X-ray mask. A new dry-film-transfer-to-PCB is employed by using low-cost dry film photoresist to transfer electroplated nickel from surface-treated chromium-coated glass substrate to printed circuit board (PCB) substrate. The dry film is subsequently released everywhere except anchor contacts of the electrostatic actuator structure. The fabricated actuator exhibits good actuation performance with high displacement at moderate operating voltage and suitably high resonance frequency. Therefore, the proposed fabrication process is a promising alternative to realize low-cost MEMS microactuator for industrial applications.     

Potential of BPN as a new negative photoresist for a very thick layer with high aspect ratio

The BPN is a negative photoresist, sensitive in the UV at 365 nm and was previously dedicated for Wafer Level Packaging applications. This photoresist offers the advantage of forming thick layers; however, it suffers from low aspect ratio (2:1 declared by the supplier). This work reports the BPN’s technological process that allows forming 200–800 μm thick molds for electroplating purposes. Our results revealed an aspect ratio as high as 17:1 while having vertical sidewalls using conventional photolithography.     

BPN a new thick negative photoresist with high aspect ratio for MEMS applications

The BPN is a negative photoresist, sensitive in the UV at 365 nm and was previously dedicated for Wafer Level Packaging (WLP) applications. This photoresist offer the advantage of forming thick layers, however, it suffers from low aspect ratio (2:1 declared by the supplier). This work reports the optimization of BPN’s technological process enabling forming 30–160 μm thick molds for electroplating purposes. Our results revealed an aspect ratio as high as 16:1 while having vertical sidewalls using conventional photolithography.     

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.     

Electroplated compliant metal microactuators with small feature sizes using a removable SU-8 mould

In the work presented in this article a fabrication process for high aspect ratio metal microstructures has been developed using photoresist EPON SU-8. A smallest feature size of 2 μm with near vertical sidewalls has been achieved on a routine basis. The SU-8 photoresist has been used as mould for electroplating high aspect ratio metal micro actuators. A removal process has also been developed. The removal of the SU-8 mould after plating is done in a plasma asher using an oxygen or oxygen/fluorine plasma with plasma power as low as 200 W. To demonstrate the fabrication process a 10 μm thick nickel, electro-thermal micro actuator has been fabricated.     

Dual layer photoresist complimentary lithography applied on sapphire substrate for producing submicron patterns

In this study, the combined technologies of dual-layer photoresist complimentary lithography (DPCL), inductively coupled plasma-reactive ion etching and laser direct-write lithography are applied to produce the submicron patterns on sapphire substrates. The inorganic photoresist has almost no resistance for chlorine containing plasma and aqueous acid etching solution. However, the organic photoresist has high resistance for chlorine containing plasma and aqueous acid etching solution. Moreover, the inorganic photoresist is less etched by oxygen plasma etching process. The organic and inorganic photoresist deposit sequentially into a composite photoresist on a substrate. The DPCL takes advantages of the complementary chemical properties of organic and inorganic photoresist. We fabricated two structures with platform and non-platform structure. The non-platform structure featured structural openings, the top and bottom diameters and the depth are approximately 780, 500 and 233 nm, respectively. The platform structure featured structural openings, the top and bottom diameters and the depth are approximately 487, 288 and 203 nm, respectively. The precision submicron or nanoscale patterns of large etched area and patterns with high aspect ratio can be quickly produced by this technique. This technology features a low cost but high yield production technology. It has the potential applications in fabrication of micro-/nanostructures and devices for the optoelectronic industry, semiconductor industry and energy industry.     

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.     

Influence of mask substrate materials on resist sidewall roughness in deep X-ray lithography

Absorption of X-rays in deep X-ray lithography masks can significantly increase exposure time, harden the X-ray spectrum and lead to heating and distortion of the mask. To reduce the impact of such absorption, we have evaluated low atomic mass (low-z) materials that allow the utilization of thick substrates (>100 μm) for reliable mask fabrication. Various forms of graphite, vitreous carbon (VC), boron nitride and beryllium were chosen for testing. Transmission tests were conducted to evaluate resulting surface roughness in the X-ray resist sidewalls. We found that VC, beryllium and pyrolytic graphite all have minimal effect on the resist sidewall surface roughness; however, graphite and boron nitride both significantly increase the roughness to about 300 nm RMS. We could show that this increase in surface roughness is directly related to the crystal structure of these materials. From the tests conducted, VC proves a promising mask substrate, superior to the more expensive and hazardous beryllium that is commonly used for thick high precision masks. VC has been successfully employed as a mask substrate and corresponding resist structures are introduced.     

Aqueous base developable: easy stripping, high aspect ratio negative photoresist for optical and proton beam lithography

A variety of different photo resists are used for the fabrication of polymer and metal high aspect ratio structures. Among them SU-8, a chemically amplified negative tone photoresist is the mostly used. However, after processing the finished resist pattern (SU-8) is hardly removed from the substrate. In the present work the formulation and process optimization of a negative tone chemically amplified photoresist (TADEP) is presented. TADEP resist owns two advantages: the dissolution of the uncrosslinked areas in IC standard aqueous developers and the easy stripping in acetone by the assistance of ultrasonic bath. The TADEP resist is successfully applied for the fabrication of polymer and metal structures, after electroplating and stripping and also in the case of Proton Beam Writing.     

Mixer slit plates fabricated by direct-LIGA

Direct-LIGA services combining X-ray based micro-fabrication and electroplating are routinely provided to customers worldwide on a “best effort” basis by several LIGA centers. In a publicly founded project in collaboration between BESSY and Ehrfeld Mikrotechnik BTS GmbH the direct-LIGA process was adapted and optimised for fabricating mixer slit plates for micro-reactors. The application of SU-8 as a resist in deep X-ray lithography opens new opportunities for a direct and cost-effective mass fabrication of LIGA metal micro-parts. This joint paper will report on efforts from BESSY, Micro Resist Technology, and Ehrfeld BTS in fabrication of mixer slit plates by direct-LIGA for use in modular micro-reaction technology. The high demands regarding precision and yield of the micro-slit plates need certain improvements in the LIGA processing chain. Our approach consists of X-ray patterned SU-8 resist molds with a layer thickness of up to 350 μm and an aspect ratio of up to 30:1. Furthermore, the pattern-transfer into the SU-8 resist was done by a low-cost X-ray mask based on a graphite membrane. Other processing steps like NiFe electroplating and SU-8 removal will be discussed in this paper.     

Fabrication of X-ray imaging zone plates by e-beam and X-ray lithography

X-ray imaging and microscopy techniques have been developed in worldwide due to their capabilities of large penetration power and high spatial resolution. Fresnel zone plates is considered to be one of the most convenient optic devices for X-ray imaging and microscopy system. The zone plates with aspect ratio of 7 and 13 have been fabricated by e-beam lithography combined with X-ray lithography in this paper. Firstly, the X-ray lithography mask of zone plates with outermost zone width of 100 nm was fabricated by e-beam lithography and gold electroplating techniques. Secondly, the zone plates with gold profile thickness of 700 and 1,300 nm were replicated by X-ray lithography and gold electroplating techniques. X-ray imaging and microscopy techniques were introduced to characterize the high-aspect-ratio zone plates’ inner structures. At the X-ray energy of 7.5 keV, the first-order focusing efficiency of zone plates with gold profile thickness of 700 nm is about 8.63%.     

High aspect ratio micromolds for the electroplating of micro electro discharge machining tools

A new approach using micro systems technology for the fabrication of micro electro discharge machining tool electrodes using a combination of positive and negative photoresist fabricated with near UV-lithography as high aspect ratio micromolds was developed. Micromolds with heights varying from 200 to 650 μm and an aspect ratio of up to 26:1 were fabricated using SU-8?, a negative photoresist, as a micromold for the electrodeposition of the micro tool electrodes. Besides serving as a micromold, SU-8? is also used as an insulator to protect the sidewalls of the tool electrodes from corrosions. The fabrication of the SU-8? micromold was optimized to avoid cracks and delaminations. In the experiments, electrodes made of Cu, WCu, and CoFe were deposited into these High Aspect Ratio Micro Structure Technology (HARMST) micromolds. Furthermore, the process technique for electroplating in deep micromolds is discussed in this paper.     

Inexpensive, quickly producable X-ray mask for LIGA

 The capability to produce X-ray masks inexpensively and rapidly is expected to greatly enhance the commercial appeal of the LIGA process. This paper presents a process to fabricate X-ray masks both inexpensively (under $1000) and rapidly (within a few days). The process involves one UV lithography step and eliminates the need for an intermediate X-ray mask. The X-ray mask produced by this process consists of a 125 μm thick graphite membrane that supports a gold-on-nickel absorber pattern. The thickness of the absorber structures is great enough to supply sufficient contrast even when radiation sources with high characteristic photon energies up to 40 keV are utilized and/or when deep exposures are desired. The mask fabrication process is initiated by spin coating 30–50 μm of SU-8 directly on a graphite membrane. The SU-8 is then patterned using a UV mask. Gold-on-nickel absorber structures are electroplated directly onto the SU-8 covered graphite. Once the remaining SU-8 is removed, attaching the graphite membrane to a frame completes the mask. To test the performance of the mask, a nickel mold insert was fabricated. A sheet of PMMA 500 μm in thickness was bonded to a nickel substrate, then exposed to X-rays through the mask, and developed. Electroplating nickel into the patterned PMMA sheet produced a mold insert. SEM pictures taken of the SU-8, the X-ray mask, and the mold insert are shown. This method of rapidly producing an inexpensive X-ray mask for LIGA resulted in a mold insert with smooth, vertical sidewalls whose dimensions were within two micrometers of the UV mask dimensions. Received: 12 December 1998/Accepted: 2 February 1999     

Magnetic lithography for servowriting applications using flexible magnetic masks nanofabricated on plastic substrates

Magnetic lithography (ML) is a process qualitatively analogous to contact optical lithography which transfers information from a nanopatterned magnetic mask (analog of optical photomask) to magnetic media (analog of photoresist), and is interesting for applications in instantaneous parallel magnetic recording, in particular for servowriting applications. The magnetic mask consists of nanopatterned magnetically soft material (FeNiCo, FeCo) on a thin flexible plastic substrate, typically Polyethylene teraphtalate (PET) or polyimide. When uniformly magnetized media is brought into intimate contact with the magnetic mask, an externally applied magnetic field selectively changes the magnetic orientation in the areas not covered with the soft magnetic material. Flexible substrate of the magnetic mask offers superior compliance to magnetic media which is likely to have imperfect flatness and surface particulate contamination. We discuss nanofabrication challenges of magnetic masks on plastic substrates, including electron beam lithography, electroplating and lift-off processing on the nanometer scale, adhesion of metal thin films on PET and polyimide substrates, and release of plastic films from rigid substrates used during the processing. We present results on fabricated magnetic masks, magnetic force microscopy images of the magnetic transition patterns and disk spinstand tests of servowritten patterns.     

Large area, cost effective X-ray masks for high energy photons

The cost effectiveness of the deep X-ray lithography and electrodeposition process, LIGA, depends directly on the throughput of the process. The use of high energy photons allows the exposure of stacked photoresist and results in high throughput. High energy X-ray exposures require a different mask than low energy X-ray exposures. The high energy mask allows a large area exposure but requires a thicker X-ray absorber. The cost of generating high energy X-ray masks can be drastically reduced by using a thick optical photoresist process rather than an X-ray exposure process. The cost can be further reduced by using alternatives to the typical X-ray absorber, gold. High atomic weight (high Z) materials are ideal absorbers. Lead has been demonstrated as being a useable alternative as an X-ray absorber. Received: 7 July 1999 / Accepted: 1 September 1999     

X-Ray masks for very deep X-Ray lithography

 The high aspect ratio, deep x-ray lithography and electrodeposition process [Becker et al. (1986)] can be expensive unless throughput is high enough. The use of a very high energy synchrotron has allowed the cost of exposure to be significantly reduced through simultaneous exposure of stacked photoresist [Guckel et al (1994)]. Synchrotron radiation at high photon energies has resulted the use of a large area x-ray mask. Both stacked exposures and a large area x-ray masks have significantly increased the throughput of the deep x-ray lithography and electrodeposition process. Received: 25 August 1997/Accepted: 3 September     

Fabrication of HARM structures by deep-X-ray lithography using graphite mask technology

The cost-effective fabrication process for high-aspect-ratio microstructures using X-rays depends largely on the availability and quality of X-ray masks. The fabrication of X-ray masks using commercially available graphite sheet stock, as a mask membrane is one approach that is designed to reduce cost and turnaround time. Rigid graphite offers unique properties, such as moderate X-ray transmission, fairly low cost, electrical conductivity, and the ability to be used with either subtractive or additive processes [1, 2]. This paper will demonstrate the potential of a cost-effective, rapid prototyping of high-aspect-ratio microstructures (HARMs) using graphite masks. The graphite wafer accommodates both the intermediate mask and the working mask. In order to allow a direct comparison of the graphite mask quality with other X-ray masks, the primary pattern was derived from a Ti X-ray mask using soft X-ray lithography (XRL). Received: 7 July 1999 / Accepted: 29 September 1999     

Polyimide-based X-ray masks with advanced performance of pattern accuracy and thermal stability

Polyimide-based X-ray masks which are generated by optical lithography and electroplating of gold absorbers on a polyimide mask membrane gain attention as low cost masks. The organic membranes generally have problem of (1) pattern edge sharpness, (2) miniaturization of pattern and (3) thermal stability. To make the masks commodity in deep and accurate lithography area, Optnics Precision, Japan, has overcome the above difficulties and realized the masks with advanced performance of pattern accuracy and thermal stability by improving the making process and the material. Good results were obtained in the exposure experiment that used this mask. When this X-ray mask is combined with the electroforming technique and the material development technique that Optnics has, the application to various fields like an industrial field and medical field can be expected.