Sven Achenbach

Influence of developer temperature and resist material on the structure quality in deep x-ray lithography

Deep x-ray lithography is a fabrication method for the production of microstructures with aspect ratios of up to 100 in up to several millimeter thick resist layers. Usually, poly(methylmethacrylate) is used as the resist layer. We have measured the molecular weight and developing rates of crosslinked and noncrosslinked PMMA foils at different GG developer temperatures for dose values between 0.1 and 8 kJ/cm3. The determined developing rates cover a region of 7 orders of magnitude: With decreasing temperature the contrast of the resist-developer system and the limit of the developing dose are increased. Crosslinked PMMA has a higher contrast compared to noncrosslinked material. These effects lead to an enhanced quality of microstructures, which is demonstrated by the grating of the LIGA microspectrometer.

High Aspect Ratio Vertical Cantilever RF-MEMS Variable Capacitor

An electrostatically actuated, microwave microelectromechanical system variable capacitor fabricated using deep X-ray lithography is presented. A single exposure has been used to produce the novel high aspect ratio microstructure, which includes a thin, vertically oriented, movable nickel cantilever beam and a 40:1 vertical aspect ratio capacitance gap. The 0.8-pF capacitor operates in the 1-5GHz region and has Q-factors of 36 at 4GHz and 133 at 2 GHz. The variable capacitance ratio is 1.24:1 over a 20-V tuning range at 4GHz

Reducing pull-in voltage by adjusting gap shape in electrostatically actuated cantilever and fixed-fixed beams

A gap with variable geometry is presented for both cantilever beam and fixed-fixed beam actuators as a method to reduce the pull-in voltage while maintaining a required displacement. The method is applicable to beams oriented either in a plane parallel to or perpendicular to a substrate, but is most suitable for vertically oriented (lateral) beams fabricated with a high aspect ratio process where variable gap geometry can be implemented directly in the layout. Finite element simulations are used to determine the pull-in voltages of these modified structures. The simulator is verified against theoretical pull-in voltage equations as well as previously published finite element simulations. By simply varying the gap in a linear fashion the pull-in voltage can be reduced by 37.2% in the cantilever beam case and 29.6% in the fixed-fixed beam case over a structure with a constant gap. This can be reduced a further 4.8% by using a polynomial gap shape (n = 4/3) for the cantilever beam and 1.2% for the fixed-fixed beam by flattening the bottom of the linearly varying gap.

High vertical aspect ratio LIGA microwave 3-dB coupler

This paper focuses on microwave directional couplers fabricated using synchrotron deep X-ray lithography (DXRL), an advanced micro- and nanofabrication technology allowing the design of microwave structures with improved performance by exploring the third dimension - metal height. Demonstrated are the influence of increased metal height on the structure characteristics and performance unattainable through conventional planar fabrication techniques.

Structure quality of high aspect ratio sub-micron polymer structures patterned at the electron storage ring ANKA

We describe the fabrication of polymer structures with lateral dimensions in the sub-micron regime using hard x rays (λ≈0.4nm) from the electron storage ring ANKA. PMMA and Novolak resists have been analyzed with respect to development rates and contrast. Films with a thickness from 2μmto11μm have been patterned using a high resolution x-ray mask consisting of 2μm thick gold absorbers on a suspended silicon nitride membrane. The fabrication of those sub-micron x-ray lithography structures is confined by the mask absorber sizes of down to 400nm and by the process conditions. The yield of resist structures with aspect ratios of 9 and above is limited by bending of the structures. An intermediate buffer layer of polyimide enhances the resist adhesion and reduces cracking in the microstructures. Diffraction at mask absorber edges biases the resist feature size on the order of tens of nanometers. It may also result in surface attack of periodic resist structures if the proximity gap between mask and resist cho...

Application of scanning probe microscopy for the determination of the structural accuracy of high aspect ratio microstructures

Deep X-ray lithography is a well known technique to fabricate microoptical systems. The performance of these systems, especially microspectrometers, strongly depends on the structural accuracy. To characterize the structural details of gratings of differently processed LIGA microspectrometers, we applied a high resolution atomic force microscope (Digital Instruments Dimension 3100). Structural details of the sidewall of the microspectrometer grating were evaluated by AFM measurements as a function of resist/developer parameters and the lateral and vertical location on the microstructure. The measurements revealed considerable differences in step heights and opening angles of the gratings. The results coincide with measurements of the optical performance of the differently processed spectrometers.

Vertical High-

Compact reactive lumped-element circuits fabricated using a single thick metal-layer deep X-ray lithography process are presented. Vertically oriented capacitive features are combined with inductive features in >0.25-mm-thick metal layers to realize lumped-element filter and coupler microstructures operating at up to 12 GHz. Measurements for separate thick metal reactive structures are also presented, including variable capacitors and single-turn square loop inductors. Devices feature impressive vertical structure, including a 77:1 aspect ratio, 1.3-μm-wide cantilever gap structure in 100-μm-thick photoresist. A 0.6-pF capacitor has -factors of 95 at 5.6 GHz and 214 at 3.5 GHz, and a structurally compatible 1.2-nH loop inductor has a -factor of 47 at 6.8 GHz and a self-resonant frequency of 18.8 GHz. Together, these types of devices could form the building blocks for various integrated reactive lumped-element-based circuits.

Q

An ultra-deep polymer cavity structure exposed using deep X-ray lithography is used as a template for metal electroforming to produce a 24-GHz cavity resonator. The metal cavity is 1.8 mm deep and has impressive structure, including extremely vertical and smooth sidewalls, resulting in low conductor loss. The measured resonator has an unloaded quality factor of above 1800 at a resonant frequency of 23.89 GHz.

RF-MEMS Devices for Reactive Lumped-Element Circuits

Periodically structured dielectric media with lattices on a sub-wavelength scale are receiving increased attention as they enable a variety of photonics applications. Fabrication, however, still imposes challenges to the scientific community. This article discusses process modifications in deep X-ray lithography to reduce minimum feature size and eventually allow the fabrication of high aspect ratio photonic crystal slabs with a moderate refractive index. Proximity printing requires an X-ray mask with high contrast and lateral resolution. Electron beam writing exposure doses were optimized to pattern feature sizes down to 400 nm in 3 μm thick resist. The voids were subsequently electroformed with 2 μm gold to generate the absorbers on a suspended silicon nitride membrane. The mask was copied into PMMA films of 5 μm thickness using X-ray lithography at about 0.4 nm. The yield of free standing smallest features is limited by adhesion of the resist to the substrate. Structures with aspect ratios as high as 8 to 12 tend to collapse after dip development. Periodic features are increased on the order of tens of nanometers compared to mask absorbers. Lattice constants need to be slightly reduced at fixed pore diameters before first photonic demonstrators made of PMMA can be fabricated.

An Ultra-Deep High-Q Microwave Cavity Resonator Fabricated Using Deep X-Ray Lithography

A 2-mm deep microwave cavity resonator is proposed for UGA micro-fabrication. LIGA provides highly vertical and optically smooth sidewalls, leading to potentially high quality (Q) factor. Simulation using 3-D finite element method (FEM) shows an unloaded Q (Q/sub u/) very close to the theoretical calculation. Feasibility of obtaining such cavities with hard X-ray lithography (XRL) is demonstrated.