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  • LiMiNT - Lithography for Micro- and NanoTechnology

  • LiMiNT - Lithography for Micro- and NanoTechnology

    Contact: Prof Mark B H Breese (slshead@nus.edu.sgnus.edu.sg)

  • What we do?

    Limint beamline is dedicated to micro-and nano-manufacturing through Deep X-Ray Lithography (DXRL) for the scientific, commercial, and high-tech community in Singapore and overseas. DXRL is enabled via the LIGA technique, a German acronym for Lithographie, Galvanoformung, Abformung (Lithography, Electroplating, and Molding), a fabrication technology used to create high-aspect-ratio microstructures. Limint is working with groups from local and overseas universities, institutes, and industries on several projects in the fields of biotechnology, polymer processing, X-ray and micro-optics, and microfluidics.

    What we offer?

    LiMiNT beamline of SSLS provides all process steps of LIGA technique for prototyping micro-and nano-devices, starting from pattern design and mask fabrication. It plays a vital role as its x-ray scanner makes SSLS a unique micro-and nanotechnology lab in South-east Asia.

    Why X-rays?

    The deep and unscattered penetration of x-rays through resist materials enables the fabrication of microstructures with very high aspect ratios. On the other hand, the shorter wavelength of the x-rays with rectangular beam architecture enables batch fabrication of nanostructures on wafer level.

    What’s new?

    Conventional lithography processes done with usage of single mask results in 2 dimensional components. Over the years, Limint beamline acquired new capabilities of manufacturing 3-dimensional micro-components of optical quality and at wafer scale through simultaneous usage of multiple masks. The wafer can either be glass, Si, or even a bulk polymer. These manufactured components can directly be applied to commercial devices for various functionalities such as micro-mirror arrays, filter arrays, or gratings. Precise control of dose deposited is also enabled through the usage of stages with micro-controllers that offer determined chain scission leading to 1800 discrete gray levels in a chip of area 20 mm2.

  • Beamline architecture

    • The LiMiNT beamline comprises four groups. Group 1 with Power Shutter and Filter Unit is a UHV group directly connected to Dipole 2 of Helios 2.
    • A collimator blocks radiation that would not reach the Scanner and thus reduces the heat load on all downstream components.
    • The Power Shutter in a closed position absorbs all synchrotron radiation to protect uncooled beamline components, which have to be moved out of the beam before the control system allows the 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 function 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 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 that is interlocked by the control system. The 2nd Beryllium window cannot be set under atmospheric pressure even when venting the Scanner.
    • The drawing below illustrates the described set-up of the beamline, the pictures show the respective beamline components as they are installed at SSLS.

  • Key features of X-ray lithography process at SSLS

  • Key features of X-ray scanner
    • 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 facilitates smaller lateral dimensions since the aspect ratio on the mask restricts the minimum feature size.
    • In the case of chemically amplified photoresist specifically designed for DXRL such as SU-8, exposure time is of minor significance since the overall process time will be dominated by mainly resist preparation and evacuating and venting the Scanner.
    • While for less sensitive photoresist such as PMMA, a longer exposure time can be expected for a 4" wafer, typically 17 h for 500 µm, 9 h for 300 µm and 4 h for 100 µm.

     

  • Micro-components fabricated using x-ray lithography at SSLS

  • SU-8 Microstructures

      250 µm – 1 mm tall structures (Left) Split ring resonators – 75 µm tall structures

     
    Masks manufactured for X-ray lithography

      Mask for deep x-ray lithography – gold absorber on graphite wafer (Left) Nano x-ray mask: 1 µm Au on Si3N4 membrane

     
    Electromagnetic Metamaterials

      Metamaterial structures made using multi-level aligned lithography with minimum structure feature of 5 µm – 3D archi-tecture (a) foils – made of Gold (Au) (b) Optical microscope image of mesh of 3D metamaterials (c, d, f) SEM image of two layer aligned S-shaped resonators (e) Photograph of electromagnetic metamaterial chip (g)

     
    Stencil masks manufactured to fabricate micro-components of optical quality

      Mask fabricated on graphite membrane (Left) Stencil mask without membrane (Right)

     
    Optical phase shifting module

      100 nm steps inside SSLS with sub-micrometer depth resolution and 500 nm steps for optical phase shifting module (Left) Optical metrology of manufactured structures acquired using vertical scanning interferometer (Right)

     
    3-Dimensional optical components through usage of multiple masks and moveable stage

      Schematic of multiple masks aligned exposure (Top left) Chip with micromirror array (Top right) 3D representation of multiple masks aligned exposure (Bottom left) 3D representation of micromirror array with lamellae manufactured using multiple masks aligned exposure

      Nanolithography

      Nano scale trenches fabricated using moving mask technology

     
    Slanted micro-structures

      Slanted cylindrical structure array fabricated using slanted x-ray exposure through tilting of sample stage to an optimized angle with respect to the incoming x-ray beam

     
    Micro-mirror / filter array

      Micro-chip with 1800 unique phase arrays or optical filters in a chip of size 5x4 mm

  • Facilities available at Limint beamline for micro-fabrication

  •  
    Class 1000 cleanroom

     
    Oxford Danfysik X-ray scanner

     
    Equipment for X-ray masks alignment Stereomicroscope (Left) Mask aligner (Right)

     
    Metrology – Optical profiler (Left) Scanning electron microscope (Middle) Optical microscope (Right)

     
    Thin film processing – Hotplates and spin coater (Left) Wet bench (Middle) Convection ovens and ultrasonic bath (Right)

     
    Gold electroplating bath with DC and pulsed plating option (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)

     
    The RIE 2321 (Nanofilm Technologies International) for plasma cleaning and descum
    The NSP 12-1 (Nanofilm Technologies International) magnetron sputtering system with one DC and one RF magnetron sputtering gun

     
    The HEX 01 hot embossing systems from Jenoptik-Mikrotechnik (Left) The fabrication of moulds Service corridor of the LiMiNT cleanroom with DI water supply and storage of process chemicals (Right)

  • LIGA lithography process flow

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  • Novel feature of LIGA process

  • Super-resolution
     
    Following 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

     

  • Selected publications

    • Wafer scale manufacturing of high precision micro-optical components through X-ray lithography yielding 1800 Gray Levels in a fingertip sized chip, S. M. P. Kalaiselvi, E. X. Tang, H. O. Moser, M. B. H. Breese, S. P. Turaga, H. Kasi & S. P. Heussler, Nature Scientific Reports volume 12, Article number: 2730 (2022)
    • S.M.P, Kalaiselvi., Xiaosong, E. T., Oskar, H. M., B.H., M. B., Turaga, S. P., Kasi, H., & Heussler, S. P. (2021). Wafer Scale Manufacturing of High Precision Micro-Optical Components through X-Ray Lithography yielding 1800 Gray Levels in a Fingertip Sized Chip. Nature Scientific Reports (Under consideration).
    • Ze, X., Sippanat, A., Sophie, L., Leigh, E. M., Ong, Z., Chua, W., Kalaiselvi, S. M. P. (2021). A wireless and battery-free wound infection sensor based on DNA hydrogel. Science Advances (Under consideration).
    • HEUSSLER*, S P, H O Moser, S M P Kalaiselvi, C Quan and C J Tay, "Multichannel Fourier-transform interferometry for fast signals". Optics Express, 19, no. 13 (2011): 12628-12633. (United States).
    • H.O. Moser, L.K. Jian, H.S. Chen, M. Bahou, S.M.P. Kalaiselvi, S. Virasawmy, X.X. Cheng, A. Banas, K. Banas, S.P. Heussler, B.-I. Wu, W.B. Zhang, S.M. Maniam and Wei Hua, “THz meta-foil – a platform for practical applications of metamaterials”, Journal of Modern Optics, Vol. 57, No. 19, 10 November 2010, 1936–1943.
    • Moser, H O, L.K. JIAN, H S Chen, M Bahou, S M P Kalaiselvi, S Virasawmy, S M Sivakumar, X X Cheng, S P Heussler, S B Mahmood And B I Wu, "All-Metal Self-Supported Thz Metamaterial - The Meta-Foil". Optics Express, 17, No. 26 (2009): 23914-23919. (United States).
    • Moser, H O, X.X Cheng, J.A Kong, L.K. JIAN, H.S Chen, G LIU, B Mohammed, S M P Kalaiselvi, S M Sivakumar, B.I Wu, P Gu, A Chen, S P Heussler, B M Shahrain and L WEN, "Free-Standing Thz Electromagnetic Metamaterials". Optics Express, 16, No. 18 (2008): 13773. (United States).
    • Moser, H. O., Heussler, S. P., Kalaiselvi, S. M. P. Single-shot capable fast FTIR based on microfabricated 3D multimirror array. Proceedings of SPIE 19, 13 84280Y-1 - 84280Y-10 (2012).
    • Heussler, S. P., Moser, H. O., Kalaiselvi, S. M. P. 3D deep X-ray lithography for producing a multi-channel Fourier transform interferometer. Microsystem Technologies 19, 3 335-341 (2013).
    • Heussler, S. P., Moser, H. O., Kalaiselvi, S. M., Quan, C. G., Tay, C. J. Multichannel Fourier-transform interferometry for fast signals. Optics Express 19, 13 12628-12633 (2011).