Yoonyoung Jin

SU-8-based deep x-ray lithography/LIGA

Poly-methylmethacrylate (PMMA), a positive resist, is the most commonly used resist for deep X-ray lithography (DXRL)/LIGA technology. Although PMMA offers superior quality with respect to accuracy and sidewall roughness but it is also extremely insensitive. In this paper, we present our research results on SU-8 as negative resist for deep X-ray lithography. The results show that SU-8 is over two order of magnitude more sensitive to X-ray radiation than PMMA and the accuracy of the SU-8 microstructures fabricated by deep X-ray lithography is superior to UV-lithography and comparable to PMMA structures. The good pattern quality together with the high sensitivity offers rapid prototyping and direct LIGA capability. Moreover, the combinational use of UV and X-ray lithography as well as the use of positive and negative resists made it possible to fabricate complex multi-level 3D microstructures. The new process can be used to fabricate complex multi-level 3D structures for MEMS, MOEMS, Bio-MEMS or other micro-devices.

LiGA Research and Service at CAMD

Since 1995 CAMD has been offering exposure services, so called print shop for a variety of users interested in making precision High-Aspect-Ratio Microstructures (HARMST) for various application. Services have been expanded beyond only the print shop service in recent years and now include x-ray mask fabrication, substrate preparation for PMMA and SU- 8 resists, electroplating, finishing and molding. Metallic and polymeric parts are now routinely fabricated for precision engineering, micro-fluidic and micro-optic applications. This paper presents a brief overview of the actual status of LiGA services provided at CAMD including ongoing research efforts and examples of LiGA components for different applications.

Surface modification of silicon-containing fluorocarbon films prepared by plasma-enhanced chemical vapor deposition

Surface modification of silicon-containing fluorocarbon (SiCF) films achieved by wet chemical treatments and through x-ray irradiation is examined. The SiCF films were prepared by plasma-enhanced chemical vapor deposition, using gas precursors of tetrafluoromethane and disilane. As-deposited SiCF film composition was analyzed by x-ray photoelectron spectroscopy. Surface modification of SiCF films utilizing n-lithiodiaminoethane wet chemical treatment is discussed. Sessile water-drop contact angle changed from 95°±2° before treatment to 32°±2° after treatment, indicating a change in the film surface characteristics from hydrophobic to hydrophilic. For x-ray irradiation on the SiCF film with a dose of 27.4kJ∕cm3, the contact angle of the sessile water drop changed from 95°±2° before radiation to 39°±3° after x-ray exposure. The effect of x-ray exposure on chemical bond structure of SiCF films is studied using Fourier transform infrared measurements. Electroless Cu deposition was performed to test the applic...

Borosilicate-glass-based x-ray masks for LIGA microfabrication

During the past few years, graphite based X-ray masks have been in use at CAMD and BESSY to build a variety of high aspect ratio microstructures and devices where low side wall surface roughness is not needed In order to obtain lower sidewall surface roughness while maintaining the ease of fabrication of the graphite based X-ray masks, the use of borosilicate glass was explored. A borosilicate glass manufactured by Schott Glas (Mainz, Germany) was selected due to its high purity and availability in ultra-thin sheets (30 μm). The fabrication process of the X-ray masks involves the mounting of a 30 μm glass sheet to either a stainless steel ring at room temperature or an invar ring at an elevated temperature followed by resist application, lithography, and gold electroplating. A stress free membrane is obtained by mounting the thin glass sheet to a stainless steel ring, while mounting on an invar ring at an elevated temperature produces a pre-stressed membrane ensuring that the membrane will remain taut during X-ray exposure. X-ray masks have been produced by using both thick negative- and positive-tone photoresists. The membrane mounting, resist application, lithography, and gold electroplating processes have been optimized to yield X-ray masks with absorber thicknesses ranging from 10 μm to 25 μm. Poly(methyl methacrylate) layers of 100 μm to 400 μm have been successfully patterned using the glass membrane masks.

High-resolution x-ray masks for high-aspect-ratio microelectromechanical systems (HARMS)

X-ray lithography is commonly used to build high aspect ratio microstructures (HARMS) in a 1:1 proximity printing process. HARMS fabrication requires high energy X-rays to pattern thick resist layers; therefore the absorber thickness of the working X-ray mask needs to be 10-50 μm in order to provide high contrast. To realize high resolution working X ray masks, it is necessary to use intermediate X-ray masks which have been fabricated using e beam or laser lithographic techniques. The intermediate masks are characterized by submicron resolution critical dimensions (CD) but comparatively lower structural heights (~2 μm). This paper mainly focuses on the fabrication of high resolution X-ray intermediate masks. A three-step approach is used to build the high resolution X-ray masks. First, a so called initial mask with sub-micron absorber thickness is fabricated on a 1 μm thick silicon nitride membrane using a 50KeV e beam writer and gold electroplating. The initial X-ray mask has a gold thickness of 0.56 μm and a maximum aspect ratio of 4:1. Soft X-ray lithography and gold electroplating processes are used to copy the initial mask to form an intermediate mask with 1 μm of gold. The intermediate mask can be used to fabricate a working X-ray mask by following a similar set of procedures outlined above.

Low temperature deposition of carbon nanotubes

We introduce a novel method for low substrate temperature carbon nanotube (CNT) deposition utilizing photo-chemical vapor deposition (PCVD). Aluminum and nickel catalyst layers are deposited on thermally oxidized silicon substrates for CNT growth. The catalyst layers of varying thicknesses are deposited by electron beam evaporation. Different catalyst annealing temperatures and pressures are investigated. The CNT deposition is carried out immediately following the annealing process. The presence of light source during CNT deposition assists in fragmentation of the CCl4 precursor molecules used, thereby permitting a lower substrate temperature during growth. We have successfully deposited CNTs at substrate temperatures as low as 400 °C by this technique.

LIGA-like process for high-aspect ratio PZT microstructures

Processing technique for fabrication of high-aspect ratio structures in PZT is developed in this work using deep X-ray exposures at the CAMD synchrotron storage ring. Arrays of posts were successfully fabricated in PMMA mold as thick as 2700 micrometers and with aspect ratio as high as 15. This LIGA- like technique allows fabrication of structures of shapes and sizes not possible with standard techniques using bulk PZT material or by the existing thin film techniques for PZT deposition. The process shows promise for various applications including high resolution medical imaging and optical projection.