Different concepts for the manufacture of masks for deep X-ray LIGA are presented. The variation in the accuracy of the final mask as a function of the mask manufacture concept (low, medium or high resolution) influences the sidewall roughness of the irradiated PMMA structures considerably. The mask membrane also influences structure quality. Graphite masks, which yield PMMA resist structures with straight sidewalls, have increased surface roughnesses (>2 μm Rz) comopared to beryllium.A design of experiments approach to the optimisation of PMMA for LIGA applications with deep X-ray lithography has been undertaken. For the optimisation, 18 different resist mixtures were produced on the basis of the significant parameters, holding the parameters within values that were experimentally shown to be conducive for PMMA resist production. High ratios of crosslinker in the resist mixture was shown to lead to significant amounts of resist residue between the microstructures; resist development was significantly hindered. Other resists with higher ratios of adhesion promotor in the resist also had significant problems with development. PMMA resist mixtures containing between 2% and 5% adhesion promotor led to microstructures with sufficient adhesion to the substrate surface without any substantial problems with the development step.A variant of the LIGA-process, which involves deep silicon etching for mastering, has successfully been applied for the fabrication of disposable plastic chips for 2-dimensional electrophoresis. The high precision and structure accuracy of the replicated plastic devices as well as the possibility to pattern complex multilevel structures demonstrate the huge potential of LIGA based on silicon micromachining for the realization of microfluidic and biomedical devices

Manufacturing Aspects of LIGA Technologies

A method for the fabrication of three-dimensional microstructures by deep X-ray lithography (DXRL) comprises a masking process that uses a patterned mask with inclined mask holes and off-normal exposures with a DXRL beam aligned with the inclined mask holes. Microstructural features that are oriented in different directions can be obtained by using multiple off-normal exposures through additional mask holes having different orientations. Various methods can be used to block the non-aligned mask holes from the beam when using multiple exposures. A method for fabricating a precision 3D X-ray mask comprises forming an intermediate mask and a master mask on a common support membrane.

Method for the fabrication of three-dimensional microstructures by deep X-ray lithography

The production of X-ray masks is one of the key techniques for X-ray lithography and the LIGA process. Different ways for the fabrication of X-ray masks has been established. Very sophisticated, difficult and expensive procedures are required to produce high precision and high quality X-ray masks. In order to minimize the cost of an X-ray mask, the mask blank must be inexpensive and readily available. The steps involved in the fabrication process must also be minimal. In the past, thin membranes made of titanium, silicon carbide, silicon nitride (2-5μm) or thick beryllium substrates (500μm) have been used as mask blanks. Thin titanium and silicon compounds have very high transparency for X-rays; therefore, these materials are predestined for use as mask membrane material. However, the handling and fabrication of thin membranes is very difficult, thus expensive. Beryllium is highly transparent to X-rays, but the processing and use of beryllium is risky due to potential toxicity. During the past few years graphite based X-ray masks have been in use at various research centers, but the sidewall quality of the generated resist patterns is in the range of 200-300 nm Ra. We used polished graphite to improve the sidewall roughness, but polished graphite causes other problems in the fabrication of X-ray masks. This paper describes the advantages associated with the use of polished graphite as mask blank as well as the fabrication process for this low cost X-ray mask. Alternative membrane materials will also be discussed.

Cost-effective masks for deep x-ray lithography

Lithographie Galvanoformung Abformung (LIGA) is a promising approach for fabrication of high aspect ratio 3D microactuator for dual-stage slider in hard disk drive. However, this approach involves practically challenging X-ray lithography and structural transfer processes. In this work, electrostatic MEMS actuator is developed based on a LIGA approach with cost-effective X-ray lithography and dry-film-transfer-to-PCB process. X-ray lithography is performed with X-ray mask based on lift-off sputtered Pb film on mylar substrate and photoresist application using casting-polishing method. High quality and high aspect ratio SU8 microstructures with inverted microactuator pattern have been achieved with the interdigit spacing of ~5 μm, vertical sidewall and a high aspect ratio of 29 by X-ray lithography using the low-cost Pb based X-ray mask. A new dry-film-transfer-to-PCB is employed by using low-cost dry film photoresist to transfer electroplated nickel from surface-treated chromium-coated glass substrate to printed circuit board (PCB) substrate. The dry film is subsequently released everywhere except anchor contacts of the electrostatic actuator structure. The fabricated actuator exhibits good actuation performance with high displacement at moderate operating voltage and suitably high resonance frequency. Therefore, the proposed fabrication process is a promising alternative to realize low-cost MEMS microactuator for industrial applications.

Fabrication of electrostatic MEMS microactuator based on X-ray lithography with Pb-based X-ray mask and dry-film-transfer-to-PCB process

In this work, a low-cost and high-resolution x-ray micromask is developed by sputtered lead film on a Mylar sheet substrate with the lift-off process and the x-ray mask is experimented for patterning SU-8 negative photoresist on a glass substrate. Sputtering is selected for Pb thick film deposition due to its high sputtering yield. The Pb mask is used for x-ray lithography of SU-8 photoresist with 5 ?m closely spaced square array patterns, designed for electrowetting electrodes on a microfluidic chip. For 140 ?m thick SU-8 photoresist, a Pb film thickness of around 10 ?m was used to block x-rays with 95% x-ray image contrast at a critical dose of 4200 mJ cm?3. A high aspect ratio of 26.5 of SU8 microstructure with 5 ?m lateral resolution has been demonstrated by the developed low-cost Pb-based x-ray mask. In addition, a steep sidewall angle of nearly 90? for SU-8 structure is confirmed. The results demonstrate that the Pb-based x-ray mask offers high-resolution x-ray lithography at a very low cost. Therefore, it is highly promising for commercial applications.

https://iopscience.iop.org/article/10.1088/0960-1317/20/7/075026/pdf

Low-cost and high-resolution x-ray lithography utilizing a lift-off sputtered lead film mask on a Mylar substrate

High precision mask fabrication for deep X-ray lithography using 40-kV shaped electron beam lithography

Deep X-ray lithography with synchrotron radiation represents the primary process step of the LIGA technique, by means of which high volume production of micro-mechanical, micro-optical and micro-fluidic components becomes possible. We report on a new approach where the direct patterning of an intermediate mask has been performed by an upgraded Leica ZBA23 shaped beam electron writer with an acceleration voltage of 40 kV. Optimised development and exposure processes as well as the use of particularly performed proximity correction methods allowed to produce feature sizes down to 0.4 μm. Taking CD-values of the final gold absorber structure as a target, an optimised parameter set has been found to manufacture periodic lines-and-spaces structures of 1.5 μm width with an accuracy of 0.18 μm per edge which were written into 2 μm thick PMMA resist. Additionally, new intermediate mask membrane materials have been investigated. Special PECVD deposition processes have been developed in order to produce thin mask membranes made from silicon nitride, silicon carbide, and diamond. We produced a fully operational LIGA working mask with a thickness of 4.5 μm on the basis of a diamond membrane with a 30% thickness uniformity over a process diameter of 60 mm. Finally, thorough investigations have been performed in order to determine the accuracy of structure transfer in the X-ray lithography process to the final LIGA mask. Taking all the structure accuracy limiting aspects into account, we found a runout per edge of 0.15 μm using the BESSY I radiation spectrum as a reference. These data fit very precisely into experimentally obtained results.

Gerhard Himmelsbach

Papers

High precision mask fabrication for deep X-ray lithography using 40-kV shaped electron beam lithography

High precision mask fabrication for deep X-ray lithography