Singapore Synchrotron Light Source

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LiMiNT - Lithography for Micro- and NanoTechnology

Contact person: Dr. Jian Linke (slsjlnus.edu.sg)
Contact person: Userdesk (slsuserdesknus.edu.sg)


The micro- and nano-manufacturing facility at SSLS offers (deep) x-ray lithography to the scientific and commercial high-tech community in Singapore and the region. SSLS is providing all_process_steps for prototyping of micro- and nanodevices, starting from pattern design, over mask fabrication, lithography, electroforming to plastic moulding. The LiMiNT_beamline thereby plays a vital role as its x-ray scanner makes SSLS a unique micro- and nanotechnology lab in South East Asia. The deep, unscattered penetration of x-rays through resist materials allows the fabrication of microstructures with very high aspect ratios (LIGA technology) on the one hand while, on the other, the short wavelength of the x-rays enables batch fabrication of nanostructures on wafer level (e.g. super-resolution_process). SSLS is working with groups from local and overseas universities and institutes as well as industry on several projects in the fields of biotechnology, polymer processing, X-ray and microoptics, and microfluidics.

The LiMiNT beamline comprises 4 groups. Group 1 with Power Shutter and Filter Unit is an UHV group directly connected to Dipole 2 of Helios 2. A collimator blocks radiation that would not be able to reach the Scanner and thus reduces heat load on all downstream components. The Power Shutter in closed position absorbs all synchrotron radiation in order to protect uncooled beamline components which have to be moved out of the beam before the control system allows operation of the Power Shutter. A Filter Unit in Group 1 of the LiMiNT beamline offers the opportunity to condition the spectrum of the transmitted synchrotron radiation. The 1st Beryllium Window of 200-µm thickness separates the UHV in Group 1 from the HV in Group 2. The g-Shutter in Group 2 absorbs - in closed position - the g-radiation from Helios 2 so that LiMiNT staff can work on the Scanner without any radiation hazard. The operation of the g-Shutter is linked to the operation of the Power Shutter, i.e. both shutters can only be actuated simultaneously. Group 2 is connected with a beamline tube through the radiation shield wall to Group 3 which is a mere Beam Position Monitor. The beamline tube then penetrates the cleanroom wall. Right inside the cleanroom is a 2nd Beryllium Window of 200 µm thickness which separates this time the HV in Group 3 from the Scanner section which is Group 4 of the beamline. Between Scanner and the 2nd Beryllium window is a gate valve which is interlocked by the control system in a way that the 2nd Beryllium window cannot be set under atmospheric pressure even when venting the Scanner. The drawing below illustrates the set-up of the beamline, the pictures show the respective beamline components as they are installed at SSLS.




X-ray Lithography at the SSLS:
The SSLS LiMiNT beamline provides reasonable photon flux for (deep) x-ray lithography. The useful spectral flux at the sample covers a bandwidth from 2 keV to 10 keV delivering a power of 0.9 W to a 4" wafer at an electron current of 300 mA. The relatively soft x-ray spectrum reduces the requirements on absorber thickness for x-ray masks. Lower absorber thickness in turn facilitates smaller lateral dimensions, since the aspect ratio on the mask restricts the minimum feature size. In case of SU-8 as a resist for deep x-ray lithography, exposure time is of minor significance since the overall process time will be dominated by mainly resist preparation and evacuating and venting the scanner. In case of PMMA, long exposures have to be accepted for a 4" wafer (17 h for 500 µm, 9 h for 300 µm, 4 h for 100 µm).




Micrographs of SU-8 structures:

Left: Microstructures in 250-µm SU-8. Right: Microstructures in 1000-µm SU-8. The bridges intersecting the cylindrical structures are 10 µm wide yielding an aspect ratio of 100. The inside lamellae are only 5 µm wide yielding an aspect ratio of 200. The mask was provided by SSLS' strategic partner CAMD, Baton Rouge, Louisiana.

Process infrastructure
The equipment for micro- and nanofabrication at the LiMiNT beamline is located in a class 1000 cleanroom of 76 m2.

Pattern generation for mask fabrication and rapid prototyping
Left: Heidelberg Instruments DWL 66 direct-write laser system for DXRL-mask fabrication with 4-µm resolution for direct mask writing and 0.8-µm resolution for intermediate masks.

Right: The NPGS tool from JC Nabity Lithography Systems for nano-mask fabrication by electron beam lithography. Patterns with CD 40 nm are achieved.



In-house fabrication of x-ray masks

Left: A mask for deep x-ray lithography with 10 µm gold as absorber pattern on a 90-µm graphite membrane. The usable area is 75-mm in diameter.

Right: A Si3N4-membrane (5 x 5 mm2 usable area) serves as mask blank for the super-resolution process. Here with nickel microstructures plated on top.




Facilities for chemistry
Left: Fume hoods with hot plate and spin coater for resist application, development and wet chemistry.

Right: Chemistry lab for electrolyte analysis and preparation of process chemicals.



Plasma processing

Left: The NSP 12-1 (Nanofilm Technologies International) magnetron sputtering system with one DC and one RF magnetron sputtering gun.

Right: The RIE 2321 (Nanofilm Technologies International) for plasma cleaning and descum.

 

 



X-ray lithography
  
Left: 4" wafer with 250-µm high SU-8 microstructures after x-ray lithography and SEM micrograph of the structures

Right: The Oxford Danfysik scanner in the LiMiNT cleanroom (LiMiNT beamline)



Electroforming

Left: The µGalv plant (M-O-T) with one process circuit dedicated to gold electroplating for x-ray masks, one Cu and one Ni (Ni-alloy) process circuit.

Right: Ni-microstructures on a 4" wafer.

 

 



Inspection and metrology

Left: The Wyko NT 1100 profiler for measurement of height and surface roughness of microstructures, the optical microscope DMLM (Leica) and the stereomicroscope SZ40 (Olympus)

Middle: The Sirion scanning electron microscope (FEI Company) which also serves as platform for electron beam lithography with NPGS and the imaging set-up (Right) of SSLS’ PCI beamline which is used for the analysis of hollow high aspect-ratio microstructures by phase contrast imaging.



Hot embossing, mechanical machining and infrastructure

Left: The HEX 01 hot embossing systems from Jenoptik-Mikrotechnik. The fabrication of moulds is currently in progress.

Middle:The Engis 380-J lapping and polishing machine and the Buehler Isomet 4000 precision saw.

Far right: Service corridor of the LiMiNT cleanroom with DI water supply and storage of process chemicals.


LIGA process
LIGA is the German acronym for LIthographie (lithography), Galvanoformung (electroforming) and Abformung (moulding). It is an established technology for the fabrication of MEMS/MOEMS, particularly in combination with deep x-ray lithography (DXRL) for the fabrication of high aspect ratio microstructures. The basic process steps are depicted below.

LIGA process flow at SSLS:


Mask fabrication

SSLS is doing in-house mask fabrication as there are no x-ray masks commercially available and specifications on masks for deep x-ray lithography need to be adapted to the respective synchrotron radiation source and beamline. Different requirements for individual processes and applications have also led to a diversity of mask materials and mask fabrication processes in the LIGA community.

SSLS is using either direct laser writing or electron beam lithography for the pattern transfer of a user design into a resist applied on a suitable mask substrate. The substrate is an x-ray transparent membrane with a seed layer for subsequent electroforming of an x-ray absorbing metal.

     

Substrate preparation

The substrate for the microstructures should be flat with a low surface roughness. A seed layer for the electroforming is deposited by magnetron sputtering on which the resist is applied and soft-baked.

     

Irradiation

The substrate is then loaded into the scanner. The mask is aligned relative to the substrate and a proximity gap between mask and substrate is set by shims. The scanner will then be evacuated and back-filled with Helium at low ambient pressure. The irradiation transfers the absorber pattern on the mask into a latent image in the resist.

     

Development

After the irradiation, the exposed resist will be developed. Negative resists require a post-exposure bake prior to development.

     

Electroforming

The resist structure on the substrates serves as a mask for conformal electroplating.

     

Resist removal

After electroplating the resist must be removed. The metal structures can either serve as the final product or as mould for subsequent hot embossing. Polishing or lapping might be required in order to achieve a metal structure of uniform height and low surface roughness.

     

Hot embossing

The mould produced in the precedent process can be used as a stamp for hot embossing, thus enabling quick reproduction at lower costs.



Super-resolution process
Following earlier work of Guo and Cerrina [1] the super-resolution process has been developed at SSLS in order to achieve a size reduction for clear mask features [2][3]. It demands an appropriate choice of exposure and development parameters together with a larger proximity gap. The principle of the super-resolution process is illustrated below together with first results of a feature size reduction of 380-nm holes on a mask to 180-nm holes in resist. The experiments were carried out by SSLS staff Kong Jong Ren at CAMD, Louisiana State University.

1. Guo J. Z. Y. and Cerrina F., IBM J. Res. Develop., 37 (1993) 331-349
2. Kong Jong Ren, Leonard Q. J., Vladimirsky Y., and Bourdillon A. J., Proc. SPIE 3997 (2000) 721-728
3. Vladimirsky Y. and Bourdillon A., US Patent # 6383697


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