The synchrotron experiments
4.1 The National Synchrotron Research Center
Located in Hsinchu Science Park, Taiwan, the synchrotron accelerator of the Na-tional Synchrotron Radiation Research Center (NSRRC) was opened in October 1993. It was the first synchrotron light source of the third generation built in Asia.
By the end of 2004, twenty-seven beamlines had been completed at the NSRRC, each providing a varied flux, brightness and spectral range. In total, the NSRRC possesses more than fifty experimental stations located at the beamlines, covering experimental technology of all types such as chemical dynamics, a photoemission electron microscope, an inelastic X-ray scattering end station and a X-ray lithogra-phy beamline.
Figure 4.1 represents a schematic of the synchrotron in Taiwan:
Insertion device (wiggler/undulator)
Sextupol magnet
Bending magnet Quadrupol magnet
RF cavity
Storage ring
Transport line Beamline
LINAC
Booster ring Experimental
station
Figure 4.1: Layout of the NSRRC. Reproduced from Ref. [9].
High energy electrons produced in a booster ring (1) enter the storage ring (3) via a transport line (2); in the storage ring, the electrons produce synchrotron light after
28 4. The synchrotron experiments being deflected by bending magnets or insertion devices, and the emitted radiation is collected and channeled through beamlines (4) to experimental units (5) at which the experiments are eventually performed.
1. Booster ring
A linear accelerator (LINAC) accelerates an electron beam produced with an electron gun to an energy of 50 MeV. The electron beam then enters a booster ring of 72 m diameter and increases its energy to 1.5 GeV, reaching 99.999995 % of the speed of light.
2. Transport line
From the injector, the electron beam passes the transport line into the storage ring. The transport line has a length 70 m.
3. Storage ring
The electron beam then enters the hexagonally shaped storage ring, which has a diameter 120 m. Bending magnets located around the ring steer the motion of the electrons along an orbital path, the electron beam thus continuously emits synchrotron light tangentially to the direction of their movement. To ensure the stability of the electron beam, an absolute vacuum condition of less than 1×10−10 Torr is maintained within the storage ring. The electrical current of the beam is 200 mA, while approximately 50 billion electrons are orbiting inside.
4. Beamlines
The beamlines serve as links between the synchrotron light source and the experimental stations. In principle, to direct the synchrotron light to an ex-perimental unit, a port might be opened at each location at which electrons are deflected, or directly downstream from each insertion device.
5. Experimental unit
On arriving at the experimental unit, the synchrotron light is applied to an experimental sample. Depending on the type of measurement a great variety of experiments can be set up.
The insertion devices used in the NSRRC comprise magnets in rows with alternating polarity, which deflect the passing electron beam multiple times. If the intensity of the magnetic fields is increased, the frequency of emitted light can increase to the level of soft X-rays, or even hard X-rays. Such a device that increases the energy of the light is known as a wiggler. If the spatial frequency of the alternating magnetic fields is shrunk, the magnitude of the electron motion is tapered. Thereby the synchrotron light gains constructive interference at a particular wavelength, at which the brightness of light is greatly enhanced. A device increasing the brightness of the light is known as an undulator.
4.1. The National Synchrotron Research Center 29
4.1.1 The beamline
The beamline used for the synchrotron experiments was the micro-machining/LIGA beamlineBL18B. The acronym LIGA is derived from the German words Lithogra-phie, Galvanoformung and Abformung. LIGA signifies the use of photolithography to produce precision moulds, which are then usable to produce microstructures in large quantities. LIGA is especially suitable for mass production of highly precise microstructures with large aspect ratios. This beamline provides photon beams with energies above 500 eV from the bending magnet and is designed for LIGA related studies. It is applicable for high-speed exposure processes with an exposure area big enough for a six-inch wafer.
Re-focussing mirror
Exit slit and gate valve
To experimental station
Monochromator
Entrance slit and gate valve
Horizontal focussing mirror
From light source Vertical focussing mirror
Figure 4.2: Schematic of the micro-machining/LIGA beamline.
The layout of the LIGA beamline is illustrated in Figure 4.21. Focusing mirrors focus the photon beam horizontally and vertically. The entrance and exit slits allow for control of intensity and resolution of the focused beam. A monochromator (grating or crystal) can be used to select light of a particular wavelength. The re-focusing mirror focuses the beam of light on a sample in the experimental station.
Gate valves and metal bellows split the beamline into three major sections. Each is equipped with its own independent pumping system. The first section splits the light and guides a beam with a beam width of 10 mrad to the second section. A photon shutter and diamond window are located at the end of the second section.
The third section is a low-conductance delay line. A sensor detects any possible vacuum failure in the exposure chamber and protects the upstream UHV condition.
During maintenance of the wiggler beamline, this delay line can be removed.
4.1.2 The experimental unit
The experimental unit is manufactured byJENOPTIK Technologie GmbH.
4.1.2.1 The filter chamber
The filter chamber is located directly after the beam pipe. It operates under high vacuum and serves to accommodate aluminium filter foils which can be inserted into the beam. In this manner the X-ray radiation spectrum can be adjusted or limited.
4.1.2.2 The work chamber
The work chamber is sealed off from the filter chamber and represents the next section of the experimental unit. It is separated from the radiation-source vacuum
30 4. The synchrotron experiments by a 125µm beryllium window, which leaves a beam-passage aperture of 100 mm× 10 mm. The chamber serves as a vacuum enclosure capable of creating a vacuum as low as 10−1 mbar. The required pump-down level can be selected arbitrarily. When the final vacuum has been achieved helium may be allowed to enter the chamber at controlled levels.
4.1.2.3 The scanner stage
The scanner stage inside the work chamber is designed to provide a uniform vertical motion of mask and resist carrier relative to an impinging synchrotron beam. The stage scans in a horizontal direction with an adjustable speed and work length.
The scanning speed can be set from 1 mm/s to 50 mm/s. Non-uniformities of the exposure dose are supposed to be less than 1 % across the entire exposure area. The maximum work length is 120 mm and has been selected to enable exposure at a uniform radiation dose within an area of 100 mm2 referred to both the mask and resist plane. By respectively adjusting the exposure time, work length and scanning speed, the actual dose applied to the surface of the sample can be controlled.
4.1.2.4 Radiation Spectrum and deposited dose
The energy spectrum of the photon beam provided by the 1.5 GeV LIGA beamline is presented in Figure 4.3. A first change in the spectrum is caused when the beam passes the beryllium window. By inserting up to three filters into the beam the spectrum can be further modified to select a range of higher energy photons.
0 5 10 15 20 25 30
Flux (1010 photons/(mA mrad mrad2 bw)
Photon Energy (keV) Beamline spectrum
unfiltered
after 125µm Be window after 50µm Al filter
Figure 4.3: The radiation spectrum of the LIGA/micro-machining beamline of the NSRRC, calculated with DoseSim [32]. Absorption of photons with a low energy in the beryllium window and aluminium filer shift the intensity peak towards higher energies.
To calculate the radiation dose actually deposited at different depths in silica the DoseSim software [32] was used. The results shown in Figure 4.4 on the facing page