Projection micro-stereolithography using digital micro-mirror dynamic mask

https://doi.org/10.1016/j.sna.2004.12.011Get rights and content

Abstract

We present in this paper the development of a high-resolution projection micro-stereolithography (PμSL) process by using the Digital Micromirror Device (DMD™, Texas Instruments) as a dynamic mask. This unique technology provides a parallel fabrication of complex three-dimensional (3D) microstructures used for micro electro-mechanical systems (MEMS). Based on the understanding of underlying mechanisms, a process model has been developed with all critical parameters obtained from the experimental measurement. By coupling the experimental measurement and the process model, the photon-induced curing behavior of the resin has been quantitatively studied. The role of UV doping has been thereafter justified, as it can effectively reduce the curing depth without compromising the chemical property of the resin. The fabrication of complex 3D microstructures, such as matrix, and micro-spring array, with the smallest feature of 0.6 μm, has been demonstrated.

Introduction

In the past 20 years, continuous investment has led to the rapid advancement of micro electro-mechanical systems (MEMS) technology. MEMS technology, known as chip-level integration of mechanical sensors and actuators, provides attractive advantages of low cost manufacturing, high sensitivity, low power consumption, and high function integration. Beyond laying down planar electronic circuits on the semiconductor substrate, MEMS technologies extend the micro-fabrication into the vertical dimension as required in manufacturing micro-mechanical devices for ultimate performance. However, in the past twenty years, MEMS technology has heavily relied on the silicon micromachining as the primary established technology for micro-fabrication.

Typically, silicon micromachining technology, which includes both surface and bulk micromachining technology, is used in fabricating MEMS devices. However, silicon micromachining technology is rather limited in its ability to fabricate complex microstructures, producing only simple geometric shapes from isotropic and anisotropic etching. In addition, silicon micromachining technology can only be applied to materials such as common semiconductors, metals, and dielectric materials [1].

MEMS technology is now demanding that micro-fabrication technology be capable of fabricating complex microstructures from diverse materials, such as ceramic, metal alloy, polymer, and semiconductor materials. MEMS technology will advance tremendously if more complex microstructures can be fabricated.

The LIGA process (German acronym that stands for lithography, electroplating, and molding) builds high-aspect ratio microstructures by incorporating thick resist layers under masked X-ray or laser irradiation [2]. However, the LIGA process is limited in fabricating 212-dimensional microstructures while fabrication of truly three-dimensional (3D) microstructure remains a challenge. Several free-forming techniques have been explored in solving the critical issue of 3D micro-fabrication. Three-dimensional laser chemical vapor deposition (3D-LCVD) technology fabricates the microstructures by laser-induced chemical vapor deposition (LCVD). The shape of the micro parts is defined by the scanning of a focused laser spot [3]. Electrochemical fabrication (EFAB) technology has been developed as an extension to the LIGA process in order to fabricate complex 3D metal microstructures [4]. The electro-chemically deposited metal layers are defined as electrode masks and a planarizing procedure controls the layer thickness. Nevertheless, the 3D-LCVD and EFAB are still suffering from the drawbacks due to the limited materials that can be incorporated with this process.

As a novel micro-fabrication process, micro-stereolithography (μSL) has been developed to produce high precision, 3D MEMS devices [5], [6]. In principle, μSL utilizes focused light spot scanning over the photo curable resin surface and then a light-induced photo-polymerization occurs, constructing solid microstructures. With focused light spot scanning, a tightly focused laser spot permits micron-scale spatial resolution. Furthermore, sub-micron resolution has been achieved through a two-photon polymerization process [7]. Not limited to polymeric microstructure, fabrication of ceramic and metal green bodies with complex geometry shapes has also been demonstrated by μSL. This was accomplished by mixing UV curable resin with fine powders [6], [8].

Although the fabrication of individual devices can be accomplished in a few hours, the serial nature of the direct writing process limits the yield rate for mass production. To overcome the limitation, a parallel process has been proposed whereby each layer is fabricated simultaneously while the mask pattern is projected onto the liquid resin surface [9]. Being fully compatible with the silicon process, this process permits the integrated fabrication of the micro device onto the IC chip. However, this process has a major drawback: it requires a great number of masks, which significantly increases the processing time and cost. To avoid the difficulties involved in the multiple mask process, this process has been revised by replacing the multiple mask sets with the dynamic mask. The dynamic mask is capable of modulating the multiple mask-pattern electronically, without physically replacing the mask for each layer. The commercially available and large format dynamic mask, is the micro display device, which was initially developed for the high-resolution projection display.

Along with the endless effort to develop large area and high-resolution display techniques, recent progress of the high-resolution SLM devices has generated the commercialization of compact SLM chips, which contain as much as 1280 × 1024 pixel elements, with a typical pixel size of 17–30 μm. These commercial SLM products readily provide a convenient dynamic pattern generator to replace the photo masks in Takagi's design [9]. In fact, the idea of using a μSL projection process with a liquid crystal display (LCD) mask was first demonstrated by Bertsch et al. [10]. As the process time, in projection μSL, was reduced dramatically, microstructures containing more than thousand layers could now be fabricated within a few hours.

However, the LCD technique has some intrinsic drawbacks that hamper its performance. Not only do the large pixel sizes and low filling ratio prevent the device from being more compact, but also the low switching speed (∼20 ms) and low optical density of the refractive elements during the OFF mode hinder the contrast of the transmitted pattern. What's more, in general, LCD's UV light absorption is significantly higher during the ON mode. Therefore, the μSL process in Bertch's system cannot take advantage of a variety of commercial UV curable resins that are optimized for stereolithography [11].

All the above-mentioned difficulties limit further improvement of the micro-stereolithography system with the dynamic mask using LCD technology. Meanwhile, a competing technology in the field of digital display is the Digital Micromirror Device (DMD) [12]. This novel reflective type SLM device, invented by Texas Instruments, is drawing increased attention due to its excellent performance.

Using the DMD chip as a dynamic mask generator, we present in this paper the development of the high-resolution projection micro-stereolithography (PμSL) system. For the first time, the process model of the projection micro-stereolithography system has been studied while the characterization experiment has been carried out to analyze the critical parameters. The UV doping technique has been investigated to further enhance the process resolution.

Section snippets

Principle

The high-resolution PμSL apparatus has been developed by using the DMD as the dynamic mask. Similar to the conventional stereolithography process, the PμSL fabricates the complex 3D microstructures in a layer-by-layer fashion [14]. The shapes of these constructed layers are determined by slicing the design CAD model with a series of closely spaced horizontal planes. By taking the sliced layer patterns in the electronic format, the mask patterns are dynamically generated as bitmap images on a

Three-dimensional micro-fabrication

Fabrications of several complex 3D microstructures have been demonstrated (Fig. 2). The micro-matrix is fabricated by 110 layers with the layer thickness of 5 μm (Fig. 2(a)). The matrix is made up of three freestanding mesh layers suspended on an array of vertical posts, separated at identical distances in a vertical direction. The non-uniform line width of suspended wires presents the cross-talk effect, which is induced by light diffraction. On-going experiments are being aimed to investigate

Process characterization

Spatial resolution is a critical issue to the PμSL process. Two fundamental factors limiting the spatial resolution of PμSL systems are optical resolution of projected image, and the physical-chemical characteristics of chemical resin. Based on diffractive optics, a numerical model is developed to reveal the fundamental mechanism underlying the physical process. This numerical model is also associated with the experimental measurement which determines the process parameters.

Conclusion

In this work, the PμSL system has been successfully developed to fabricate truly 3D microstructures. A process model has been established based on the fundamental study of the underlying physical and chemical mechanisms. The results indicate that the numerical model is in good agreement with the experimental results. The UV curable resin is characterized by the developmental process model with the associated experimental measurements. By introducing 0.3% UV doping, the curing depth of the resin

Acknowledgements

This work was supported in part by the Department of Defense Multidisciplinary University Research Initiative (MURI) under Grant No. N00014-01-1-0803 and Office of Naval Research (ONR) Young Investigator Award under Grant No. N00014-02-1-0224.

C. Sun received his PhD in Industrial Engineering from Pennsylvania State University in 2002 and MS/BS in Physics from Nanjing University. He is currently the Chief Operating Officer at the Center for Scalable and Integrated Nanomanufacturing at University of California Los Angeles. His research interest includes the novel 3D micro- and nano-fabrication technologies and the device applications.

References (17)

  • E.W. Becker et al.

    Microelectron. Eng.

    (1986)
  • X. Zhang et al.

    Sens. Actuat. A

    (1999)
  • X.N. Jiang et al.

    Sens. Actuat. A

    (2000)
  • A. Bertsch et al.

    J. Photochem. Photobiol. A

    (1997)
  • S. Huang et al.

    Microprocess. Microsyst.

    (1998)
  • S.M. Sze

    Semiconductor Sensors

    (1994)
  • K. Williams et al.

    Technical digest

  • A. Cohen et al.

    Technical digest

There are more references available in the full text version of this article.

Cited by (735)

View all citing articles on Scopus

C. Sun received his PhD in Industrial Engineering from Pennsylvania State University in 2002 and MS/BS in Physics from Nanjing University. He is currently the Chief Operating Officer at the Center for Scalable and Integrated Nanomanufacturing at University of California Los Angeles. His research interest includes the novel 3D micro- and nano-fabrication technologies and the device applications.

N. Fang received his PhD in Mechanical Engineering from University of California Los Angeles in 2004. He is currently an Assistant Research Engineer in the Department of Mechanical Engineering at UCLA. His research interest includes: the 3D micro and nanolithography, design and manufacturing photonic metamaterials and devices, energy transfer and mass transport phenomena in micro/nano/biosystems.

D.M. Wu received his Master degree in Physics from Nanjing University. He is now a PhD student in the Mechanical and Aerospace Engineering Department at University of California at Los Angeles. His research interests are in 3D micro-fabrication and molecular electronics.

X. Zhang graduated with a PhD in Mechanical Engineering from University of California, Berkeley in 1996 and MS/BS in Physics from Nanjing University. He joined Pennsylvania State University in 1996 as an assistant professor. In 2000, he joined University of California at Los Angles and now he is a Professor at Mechanical and Aerospace Engineering Department and Director of NSF Nanoscale Science and Engineering Center (NSEC). His research interests includes science and technology in novel micro- and nano-scale fabrication and devices, engineering and characterization on nanophotonic and plasmonic materials and structures, sub-wavelength imaging and nanolithography, bio-sensors and MEMS, transport issues in micro and nano-manufacturing.

View full text