Single layer thin photoresist soft etch mask for MEMS applications

Microsystem Technologies - Substrate masking plays an important role in wet chemical etching process, however; coating a cost effective masking material with higher stability in the harsh chemical...     

Fabrication of intermediate mask for deep x-ray lithography

 This paper presents the fabrication of intermediate x-ray mask for deep x-ray lithography. In order to have working mask with absorbers thickness larger than 10 μm, the intermediate mask should have absorbers of 0.7 μm in thickness. To demonstrate intermediate mask fabrication, x-ray zone plates are fabricated on the 1.2 μm low-stress silicon-rich silicon nitride (SiN_x) membrane with the tri-layer Chromium-Tungsten-Chromium (Cr–W–Cr) as the x-ray absorbers. The chromium layers both 200 angstroms are used as adhesion and for stress relief. The SiN_x film is deposited with low pressure chemical vapor deposition (LPCVD) and the free standing membrane are formed by KOH silicon backside etching. With the e-beam lithography and reactive ion etching, width of 0.8 μm of outmost zone of the x-ray zone plates has been achieved on the membrane. The scanning electron microscopy (SEM) images of the x-ray zone plates and pictures of intermediate masks are demonstrated. Received: 25 August 1997/Accepted: 3 September 1997     

Novel positive-tone thick photoresist for high aspect ratio microsystem technology

 A methacrylate copolymer combining chemically amplified concept and casting technique was developed as a novel thick photoresist for the UV-LIGA process. Photoresist layers up to 500 μm in thickness can be fabricated easily. Microstructures fabricated by the novel thick photoresist were demonstrated. At present, the ring-shape microstructures with 150 μm tall and 15 μm wide have been realized and the calculated aspect ratio is 10. Received: 10 August 2001/Accepted: 24 September 2001     

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.     

Regularized level-set-based inverse lithography algorithm for IC mask synthesis

Inverse lithography technology(ILT)is one of the promising resolution enhancement techniques,as the advanced IC technology nodes still use the 193 nm light source.In ILT,optical proximity correction(OPC)is treated as an inverse imaging problem to find the optimal solution using a set of mathematical approaches.Among all the algorithms for ILT,the level-set-based ILT(LSB-ILT)is a feasible choice with good production in practice.However,the manufacturability of the optimized mask is one of the critical issues in ILT;that is,the topology of its result is usually too complicated to manufacture.We put forward a new algorithm with high pattern fidelity called regularized LSB-ILT implemented in partially coherent illumination(PCI),which has the advantage of reducing mask complexity by suppressing the isolated irregular holes and protrusions in the edges generated in the optimization process.A new regularization term named the Laplacian term is also proposed in the regularized LSB-ILT optimization process to further reduce mask complexity in contrast with the total variation(TV)term.Experimental results show that the new algorithm with the Laplacian term can reduce the complexity of mask by over 40%compared with the ordinary LSB-ILT.     

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.     

High precision, low cost mask for deep x-ray lithography

 The precision of transferred patterns are highly dependent on the quality of the mask in deep x-ray lithography. Many parameters, such as the critical energy of the synchrotron light, beamline optics and even the microstructure to be exposed should be considered in mask design. In this paper, the design rules and the boundary conditions for deep x-ray mask are discussed in general. The method of making a precision, multilayer mask using UV lithography technique is also described. Received: 25 August 1997/Accepted: 3 September 1997     

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 m 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-m-thick patterned absorber layer. The masks were used for UDXRL using hard x-rays at the Advanced Photon Source.This work has been supported by the U.S. Department of Energy, Office of Science, under Contract No. W-31–109-ENG-38, by AFOSR Grant 49620–00–1-0088, and by DUSD (S&T) under the Innovative Microwave Vacuum Electronics MURI program managed by the AFOSR under grant F49620–99–1-0297.     

Wall profile of thick photoresist generated via contact printing

High-aspect-ratio patterns generated by direct contact or proximity printing in LIGA and other similar processes have recently gained great interest in the field of MEMS. One key issue for a successful thick-film lithography Is the control of wall profile. This paper deals with this issue based on an approximation including the effects of Fresnel diffraction and exposure kinetics for various types of photoresist. This approach leads to simple but practical formulas for estimating the wall profile and resolution for the near-field lithography of thick photoresist     

Micromachining of electroformed nickel mold using thick photoresist microstructure for imprint technology

In order to fabricate polymer-based microstructures with feature sizes on the order of micrometers, we have been developing a microimprint technology with a fine nickel (Ni) mold instead of a conventional photolithography technique. The Ni mold was successfully fabricated by electroforming using a positive thick photoresist microstructure patterned on a silicon substrate as a replication master. The photoresist microstructure with excellent edge quality can be obtained under irradiation with single wavelength (g line) selected from a high-pressure mercury lamp. In addition, its sidewall angle in the range of 65° to 84° can be controlled precisely by varying the distance between a photomask and a photoresist surface. On the structured photoresist master, Ni was electroplated up to a thickness of about 110 μm, and then removed from the master. In this process, two-step electroplating at different current densities was carried out in order to prevent deformation of the photoresist master due to stress generated in a Ni electrodeposit. With the Ni mold, fine patterns with a width of 10 or 30 μm and a depth of 24 μm were almost completely transferred to polymetric materials (PMMA). The geometrical dimensions of the fabricated PMMA microstructures were found to be only about 10% reduction against the Ni mold.     

Low cost transparent SU-8 membrane mask for deep X-ray lithography

Deep X-ray lithography masks require good transparency and mechanical resistance to the intense synchrotron X-ray beam, large active areas (cm)² and compatibility with the standard fabrication processes (optical lithography and gold electroforming). Moreover higher resolution can be achieved with low roughness flat membrane. Furthermore multiple aligned exposures require an optically transparent material. Diamond like Carbon membranes fulfill those requirements but have a prohibitive cost. Our approach consists in using an SU-8 epoxy resin layer as membrane material. In this communication the different steps of the fabrication process will be presented, as well as the results obtained using the mask for particular applications.     

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.     

Deep X-ray lithography using mask with integrated electrothermal actuator

We present a deep X-ray mask with integrated bent-beam electrothermal actuator for the fabrication of 3D microstructures with curved surface. The mask absorber is electroplated on the shuttle mass, which is supported by a pair of 20-m-thick single crystal silicon bent-beam electrothermal actuators and oscillated in a rectilinear direction due to the thermal expansion of the bent-beams. The width of each bent-beam is 10 m or 20 m and the length and bending angle are 1 mm and 0.1 rad, respectively, and the shuttle mass size is 1 mm × 1 mm. For 10-m-wide bent-beams, the shuttle mass displacement is around 15 m at 180 mW (3.6 V) dc input power. For 20-m-wide bent-beams, the shuttle mass displacement is around 19 m at 336 mW (4.2 V) dc input power. Sinusoidal cross-sectional PMMA microstructures with a pitch of 40 m and a height of 20 m are fabricated by 0.5 Hz, 20-m-amplitude sinusoidal shuttle mass oscillation.This research, under the contract project code MS-02-338-01, has been supported by the Intelligent Microsystem Center, which carries out one of the 21st centurys Frontier R & D Projects sponsored by the Korea Ministry of Science & Technology. Experiments at PLS were supported in part by MOST and POSCO.     

Optimization of laser writer-based UV lithography with high magnification optics to pattern X-ray lithography mask templates

Deep X-ray lithography is a preferred fabrication approach for those micro devices that depend on smooth and vertical sidewalls of comparatively deep structures rather than extreme lateral resolution. The structure quality obtained depends on, and is limited by, the quality of the X-ray mask applied. A critical component of the mask is its absorber patterns. They get fabricated by electroplating into voids of a polymer template. These templates must usually be at least 3 μm deep and exhibit smooth and vertical sidewalls with a lateral resolution of micrometers and possibly below. Primary patterning of the templates is very demanding. Best results are obtained when dedicated electron beam writers with acceleration voltages of 100 kV and above are applied. This, however, limits access to patterning infrastructure and substantially drives delivery timeline and cost, making mask absorber template patterning a bottleneck of the entire process sequence. We propose, evaluate and optimize an alternative absorber patterning approach based on direct laser writing. An ultraviolet laser with 355 nm wavelength and 250 mW beam power by Heidelberg Instruments is applied to expose 2.9 μm thick, chemically amplified, high contrast, negative tone resist mrx-5. Exposure parameters analyzed include the dose and focal settings. Experiments are carried out on bare silicon wafers as well as on chrome-gold and on titanium oxide plating bases. For all cases, results with and without an additional antireflective coating of 200 nm AZ BAR-Li are studied. Aspects of the resist template structure quality analyzed include the sidewall verticality and its smoothness and defects, resist adhesion to the substrate, minimum feature size and structure accuracy, as well as irregularities due to stitching of partial layouts. In an optimized process, a dose of 14 mW on oxidized titanium and BAR-Li was used. We were able to demonstrate 1.5 μm minimum feature size of isolated structures and structural details of about 1 μm. The sidewalls are vertical and exhibit a roughness of dozens of nanometers. When an antireflective coating is used, chamfers are observed at the resist bottom. The structure accuracy occasionally deviates from the original layout by 200–300 nm, particularly at stitching singularities or towards the end of resist walls. The described absorber template patterning process delivers a resolution that much extends beyond previous UV patterning approaches. The structure accuracy, however, is inferior to electron beam written samples. Given the cost and timeline benefit, results of the study will allow users to identify which primary patterning approach is best suited for their micro devices.

     

Moving mask deep X-ray lithography system with multi stage for 3-D microfabrication

 Based on a moving mask deep X-ray lithography concept, a new deep X-ray exposure system with multi stage has been built up, which can fabricate 3 dimensional microstructures with controllable free shaped wall such as inclined, curved and vertical wall. The system has 6 stages, an X-stage and a Y-stage for substrate scanning, a substrate tilt stage and a substrate rotation stage for controlling an incident X-ray angle to a substrate, an X–Y stage for mask movement and X–Y stage for substrate and mask alignment. The system performance has been confirmed by fabricating microstructures such as gratings, micro grid and micro prism. Received: 10 August 2001/Accepted: 24 September 2001     

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.     

Microfabrication of thick tungsten films for use as absorbers of deep X-ray lithography masks

 We fabricated thick (5 μm) tungsten (W) film patterns by sputtering and dry etching, and realized a new deep X-ray lithography mask. The X-ray mask with 5-μm-thick W absorbers could expose about 1-mm-thick resist structures. In the deposition process of W films, the column structure of about 0.2 μm grain size, from which pattern edge roughness originates, disappeared by adding nitrogen into the sputtering gas. W film etching was carried out by reducing gas pressure and cooling the substrate (−40 °C), and a side etch width of below 0.2 μm was obtained. From the results of the pattern edge roughness and the side etch width, a pattern fabrication accuracy below ±0.5 μm was achieved. Furthermore, film stress, which induces pattern distortion, was reduced to below 50 MPa by controlling the sputtering gas pressure. The obtained mask achieved a pattern distortion below ±0.3 μm. Received: 7 July 1999/Accepted: 29 May 2000     

High throughput projection UV lithography of high-aspect-ratio thick SU-8 microstructures

This paper presents a method and an ultra-violet (UV) lithography system to fabricate high-aspect-ratio microstructures (HARMS) with good sidewall quality and nice dimension control to meet the requirement for industrial high throughput and high yield production of micro devices. The advantages, equipment, working principle of UV projection scanning exposure, and scanning exposure strategies are introduced first. Following the numerical simulation for the UV projection scanning exposure of thick SU-8 photoresist, experiment results are demonstrated for different exposure strategies. With Continually Changing Focus Projection Scanning (CCFPS), SU-8 microstructures with 860 μm high and 15 μm feature size are demonstrated. For microstructure with 866 μm height, 20 μm width, from the top layer to the bottom layer, the dimension can be controlled in the range of +0.7 to ?1.7 μm; also, the vertical sidewall angle can be controlled inside 90 ± 0.16°. It approves that the CCFPS exposure for HARMS can achieve much straighter and more vertical sidewall compared with UV contact print or UV projection exposure with focusing image on the resist surface or an optimized depth.     

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.