Fabrication of metallic microstructure on curved substrate by optical soft lithography and copper electroplating

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Abstract

Microfabrication of copper structures with a high aspect ratio was successfully performed on a curved substrate by optical soft lithography and metal electroplating. This fabrication method comprises mainly of three steps. Firstly, a flexible polymer photomask was fabricated by a direct metal transfer technique via two types of self-assembled monolayer treatments. Next, SU-8 micro patterns were fabricated on a curved glass substrate by optical soft lithography. Here, we utilised a mask with a microslit and a flexible polymer photomask. Lastly, copper electroplating was performed on the curved substrate using the SU-8 patterns as molds to form high-aspect-ratio copper structures. We confirmed that SU-8 microline patterns with an aspect ratio of over 2.5 were formed on the curved substrate. Subsequently, copper structures fabricated on the curved substrate with SU-8 micro patterns were acquired. The unconventional microfabrication technique introduced in this work can be used for the fabrication of 3-D high-aspect-ratio metallic microstructures on curved or nonplanar substrates.

Introduction

Optical photolithography is a well-established, essential process in the fabrication of micro devices. Ultraviolet light is passed through a photomask and incident on a light-sensitive photoresist deposited on a substrate. In general, a rigid and flat glass or quartz substrate is used as the photomask. However, the micropatterning process or microfabrication on a curved substrate with conventional photolithography, which is mainly used for flat substrates, is subject to limitations. The reflection of UV light within the gap between a flat photomask and a curved substrate can cause several problems such as the distortion of patterns and unwanted exposure during the fabrication process. To overcome these limitations and problems, various researches have been performed. Most of these researches have employed either laser lithography or a modified lithography process. The laser lithography technique has been used to directly fabricate accurate 3-D microstructures on curved substrates without a photomask [1], [2], [3]. However, this technique has two major disadvantages: it requires expensive laser lithography equipment and it has a low throughput. Modified lithography processes used for the fabrication of microstructures on a curved substrate include X-ray lithography using LIGA process [4], 3-D microlens array projection lithography [5], capillary force lithography using a PDMS mold [6], photolithography using carbon-black-infused-PDMS elastomeric masks [7], a combination of PDMS mold and copper electroplating [8], lithography using a flexible film mask [9], and stepped rotating lithography using a metal roller [10], [11]. Compared to laser lithography, these techniques have a higher throughput and do not impose restrictions in terms of equipment and process complications. However, several limitations still exist, such as the dimensions of final patterns and the material of substrates or molds. In particular, the stepped rotating lithography technique is very similar to our fabrication technique. However, it is difficult to fabricate connected micro patterns in the rolling direction owing to discontinuous exposure. Meanwhile, the fabrication of SU-8 microstructures using inclined or rotated UV exposure was also performed. They fabricated 3-D microstructures successfully by changing the angle of UV light or rotating a substrate. But, those microstructures were not fabricated on a curved substrate but a flat one [12], [13].

Recently, the optical soft lithography technique was successfully introduced to transfer micro patterns onto a curved substrate [14]. Instead of a rigid glass photomask, a flexible PDMS photomask was used, which can be fit uniformly on a curved substrate. Here, flexible photomasks were fabricated using metal transfer technique by two kinds of self-assembled monolayers (SAMs). Several researches have been performed to fabricate metal patterns on PDMS [15], [16], [17]. However, those metal patterns on PDMS are not suitable for flexible masks because the conformal contact between a substrate and a photomask cannot be achieved owing to the thickness of metals. Although compliant contact photomask was demonstrated by embedding metal patterns in PDMS using SAMs [18], the thickness was relatively thick (3 mm) for flexible photomask with curved or nonplanar substrates. Thus, a flexible photomask for curved substrates was fabricated by embedding gold patterns in PDMS using SAMs and PDMS spincoating.

Electrodeposition is widely used in the fabrication of microcomponents and microstructures in electronic devices. The copper electroplating technique has been researched since the 19th century and provides an easy, fast, and cheap method to fabricate microstructures in the present day. Thus, it has mainly been used for the fabrication of interconnects [19] and thick metallic structures such as coils on a substrate [20]. In addition, the fabrication of microstructures by photolithography and electroplating is attracting attention in the development of MEMS devices such as sensors [21], actuators [22], and electric circuits [23].

This paper describes the fabrication of high-aspect-ratio micro patterns on a curved substrate by copper electroplating. Firstly, SU-8 micro patterns were fabricated on a curved substrate using a novel optical soft lithography technique, which involves the fabrication of a flexible PDMS photo mask. In addition, a micro slit mask was introduced to prevent unwanted exposure of SU-8 with optical soft lithography. As a result, SU-8 micro patterns with thicknesses of 25 and 50 μm were fabricated and compared. Next, copper electroplating was performed to build a high-aspect-ratio structure using the SU-8 micro patterns as molds. Each result was analyzed and compared before and after copper electroplating by scanning electron microscopy. We developed and improved the fabrication method for microstructures on a curved substrate by employing optical soft lithography and copper electroplating.

Section snippets

Fabrication of flexible photomask (Fig. 1(I))

Fig. 1(I) shows a schematic diagram of the fabrication of flexible photomask. A 3-inch silicon wafer was cleaned in a mixture of sulfuric acid and hydrogen peroxide for 15 min and was then rinsed thoroughly with distilled water. The wafer was thermally oxidized with an oxygen rate of 1.5 L/min for 30 min, at 1050 °C, under dry oxidation conditions. This resulted in the growth of a 50 nm thick SiO2 layer. Silicon substrate was then treated with 100 mM of dodecyltrichlorosilane (DTS, Shinetsu Chemical

Fabrication of flexible PDMS photomask

In this work, flexible PDMS photomasks with four different patterning dimensions were fabricated in total. First, two types of photomask had micro patterns that consist of lines of 15 μm in width and 30 μm and 60 μm spaces between each pair of lines. In the same manner, the other photomasks had 22 μm line width and 22 μm and 44 μm spaces between each pair of lines. Fig. 2 shows the flexible PDMS photomask with 22 μm lines and spaces (Fig. 2(a)), as well as the negative photomask with 15 μm lines and 30 

Conclusion

In this work, we demonstrated the fabrication of micro patterns on a curved substrate by optical soft lithography and copper electroplating using the micro patterns as molds. This fabrication method utilises hybrid technology comprising photolithography using a flexible polymer photomask and electroplating.

Using a flexible PDMS photomask in combination with optical soft lithography, we could fabricate micro patterns on nonplanar or curved substrates with a high throughput, low cost, and high

Acknowledgement

One of the authors (J.H. Park) was supported through the Global COE Program ‘Global Center of Excellence for Mechanical Systems Innovation’ by the Ministry of Education, Culture, Sports, Science and Technology.

Jongho Park received a BSc degree in mechanical engineering from Yeungnam University, Gyeongsan, South Korea, in 2005 and an MSc degree in mechanical engineering from Yonsei University, Seoul, South Korea, in 2007. Since 2008, He has been researching unconventional microfabrication techniques as a PhD student at University of Tokyo. His research interests include the development of unconventional microfabrication techniques, flexible photomasks, and nanocontact printing for industrial

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    Jongho Park received a BSc degree in mechanical engineering from Yeungnam University, Gyeongsan, South Korea, in 2005 and an MSc degree in mechanical engineering from Yonsei University, Seoul, South Korea, in 2007. Since 2008, He has been researching unconventional microfabrication techniques as a PhD student at University of Tokyo. His research interests include the development of unconventional microfabrication techniques, flexible photomasks, and nanocontact printing for industrial applications.

    Hiroyuki Fujita received BS, MS, and PhD degrees in electrical engineering from University of Tokyo, Tokyo, Japan, in 1975, 1977, and 1980, respectively. Since 2009, he has been working as the Deputy Director of the Institute of Industrial Science, University of Tokyo. Also, since 2000, he has been working as the Director of the Center for International Research on Micronano Mechatronics. He has worked as an Associate Professor (1981–1993) and a Lecturer (1980–1981) and is currently working as a Professor (1993–present) with the Institute of Industrial Science, University of Tokyo. He is currently engaged in the investigation of micro- and nano-electromechanical systems fabricated by IC-based processes and their application to bio- and nano-technology. He is also interested in autonomous distributed microsystems manufactured by MEMS technology.

    Beomjoon Kim received his BE degree from Seoul National University, Department of Mechanical Design and Production Engineering, Seoul, Korea, in 1993, and his ME and PhD from the Department of Precision Engineering, University of Tokyo, Tokyo, Japan, in 1995 and 1998, respectively. He is currently working as an Associate Professor at the Institute of Industrial Science, University of Tokyo, Japan (since 2000). From 1998 to 1999, he was a CNRS (Centre National de la Recherché Scientifique) Associate Researcher for Microsensors, Nano-instruments for Nanotechnology, at LPMO, Besancon, France. He also worked in the research orientation group of NanoLink, MESA+ Research Institute, University of Twente, up to September 2000. He has been a Co-Director at the CIRMM/CNRS Paris office since 2001 for 3 years, coordinating collaborations with other European countries in MEMS/NEMS research.

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