Masks for LIGA applications requiring deep X-ray lithography have different specifications compared to those masks used in microelectronic applications. Generally, deep X-ray LIGA applications require resist heights greater than 100 ?m, whereas accurate pattern transfer in this depth of resist is obtained by using highly parallel synchrotron radiation beam from storage rings in the lithography. Given the unusual nature of the lithography, the requirements for the mask blank and the absorber structures are quite different from those used in optical lithography. This paper discusses different approaches to mask making, and weighs up the advantages and disadvantages of the different approaches. In particular, emphasis will be placed on the changes in critical dimensions and sidewall roughnesses in the resist structures produced by the different mask technologies.
Moulding of plastics enables fluidic and optical features to be integrated into a single element. This is particularly an advantage for product designs that impose space and weight constraints. Therefore, the use of plastic for biomedical and non telecommunications orientated optical applications continues to grow as design engineers take advantage of the ease of fabrication and the material flexibility.
LIGA presents itself as a method ideally suited for the production of moulds for the manufacture of plastic microcomponents. Although LIGA is synonymous for lithography using synchrotron radiation x-rays, many other lithography and non-lithography methods for master production have been developed in the last few years, offering cost effective solutions to template production. These include UV LIGA methods, where deep resists such as SU-8 and AZ 4562, are employed for the master production. In addition, excimer laser micromachining offers a cost effective and efficient method for master fabrication, which later forms a template for electroforming. Furthermore, the use of Advanced Silicon Etching methods to prestructure silicon templates for electroforming, allows to the production of stepped mould inserts, which are particularly useful for microfluidic applications.
This paper provides an overview of the different technologies, emphasizing the strengths and application areas of the different master structuring technologies. Considerations for the electroforming of microstructured mould inserts are also presented.
Moulding of plastics enables optical features to be integrated into a single unit. This is particularly an advantage for product designs that impose space and weight constraints. Therefore, the use of plastic for biomedical and non telecommunications orientated optical applications continues to grow as design engineers take advantage of the ease of fabrication and the material flexibility.
Deep X-ray LIGA presents itself as a method ideally suited for the production of moulds for the manufacture of plastic microcomponents. LIGA is synonymous for the lithography preferably carried out with synchrotron radiation X-rays, although many other lithography and non-lithography methods for master production have been developed in the last few years. Nevertheless, the exceptional resist heights, the enormous accuracy and low runout as well as the low sidewall roughnesses cannot be copied by these other methods of master production. In particular, the low sidewall roughnesses achieved through deep X-ray LIGA is essential for the manufacture of waveguide coupling systems based on polymers. The design and conceptualisation of such waveguides systems is presented here. In addition however, the exceptional resist heights and low runout can be employed to produce passive structures for the packaging of optical components.
This paper provides an overview of the deep X-ray LIGA technology, emphasizing its strengths and application areas. Considerations for the design and manufacture of the plastic structures are also elucidated.
The most common resist used for LIGA applications with deep X-ray lithography is PMMA. It achieves very good resolution of the mask structures with a typical resist thickness from 200 micrometers to 2 mm. The resist system employed here is crosslinked on the substrate after preparation, and this can lead to problems with internal stresses and resist cracking. To optimize the resist performance, a design of experiments procedure was implemented. Design of experiments assists the determination of optimized process parameters through the use of statistical procedures. The principal components of the PMMA resist employed in the optimization procedure were PMMA, adhesion promoter, crosslinker, initiator and accelerator. The PMMA type employed was kept constant and the concentrations of adhesion promoter, crosslinker, initiator and accelerator were altered following a design of experiments methodology. At first 18 resists compositions were prepared and irradiated to determine preliminary exposure behavior. A selection was made from this composition on the basis of the performance, and then these resist composition were irradiated with standard test structures. This determined the performance of the resist to stress, its inherent stability and the inherent resolution that can be achieved. A smaller parameter set was then implemented in the final optimization of the resist. A design of experiments procedure has assisted the optimization of the PMMA resist for LIGA application with deep X-ray lithography. The results obtained demonstrate the effectiveness of the procedure in quickly attaining resist compositions suitable for deep X-ray lithography.
Although the process of deep x-ray lithography with PMMA achieves good resolution, it requires significant exposure times because of the low sensitivity of PMMA to x-rays. Therefore resist materials, which can achieve high resolution, but which are inherently more sensitive than PMMA, are desirable. Here it is shown, that x-ray exposures of the SU-8 resist can achieve high resolution with substantially reduced exposure times. Irradiation at the synchrotron source of DCI at Lure (Paris) and MAXLAB (Lund, Sweden) demonstrated a reduced exposure time for a 600 micrometers thick SU-8 relative to PMMA. The does needed to obtain standing structures was 30 J/Cm3 for DCI and 52 J/CM3 for MAXLAB. A 600 micrometers thick PMMA resist requires a typical bottom does of 4 kJ/cm3, so Su-8 is considerably more sensitive to x-rays than PMMA. Preliminary critical dimension measurements (CD) of the 600 micrometers SU-8 resist structures have been obtained for the entire height of the structure, which was exposed at DCI. The CD measurements were made in a Scanning Electron Microscope (SEM) using 10 micrometers wide structures, which have a 20micrometers pitch, this being used to calibrate the measurements. These measurements show that the gain in the critical dimension per structure edge is dependent on the bottom dose. Doses of 30 J/cm3 achieved a CD gain per edge of +0.5 micrometers , while doses of 40 J/cm3 Yielded a CD gain per edge of 0.9 micrometers . However, the gain in the CD per edge is critically dependent on the solvent content in the resist. Doses of 40 J/cm3 into a resist with a 2% residual solvent content yielded CD gains per edge of 0.3micrometers . In addition, the dose profile in the resist does not change the CD values significantly. It has been shown that the resolution of the x-ray exposed SU-8 structures compare quite favorable with PMMA, but the exposure time for SU-8 is approximately 100 times less than that for PMMA. This significantly improves throughput for deep x-ray lithography processes.
Photoemission electron microscopy (PEEM) has turned out to be one of the most promising methods for surface analysis in the recent years. It is a full field imaging technique based on the emission of secondary electrons by far ultraviolet light or X-rays. The emission intensity of secondary electrons is critically dependent upon the acceptance angle of the incident radiation. However, the size of the microscope restricts this angle substantially. Miniaturizing the objective lens of the microscope reduces the restriction of the acceptance angle and improves the performance of the PEEM considerably. We report on the fabrication of a miniaturized objective lens containing the extraction electrode, the electron column, the contrast aperture and the electron optical correction system for a PEEM. The extraction electrode as well as the electron column have been manufactured using precision milling techniques and electron discharge micromachining. For the fabrication of the correction system (stigmator / bending unit), a process combining aligned photolithography into a thick SU-8 resist and electroforming has been used. All electrodes were made in gold with a height of 150 (mu) m. After attaching a FOTURAN substrate to the electrode and etching under the electrodes, free standing apertures in an octupole and quadruple arrangement were obtained. The outer diameter of the electrodes is 5 mm and the inner diameter is 1 mm, respectively. Each electrode is connect individually to the external power supply which controls their operation. The overall size of the miniaturized objective lens is 23 mm, which has reduced the size of the lens by one order of magnitude when compared to commercially available instruments.