Pelletron Laboratory

The areas of research within the Pelletron Laboratory are ion–matter interactions and their exploitation in IBA, ion beam modification and ion beam lithography. For IBA, the research focuses on the development of new high performance detectors, digitizing data acquisition systems and the development of IBA software. The ion beam modification focuses mainly on the fabrication of nano- and microsized ion tracks by means of energetic heavy ion beams from a cyclotron. Central to ion beam lithography is the fabrication of nano- and microfluidic high aspect ratio structures to polymers, quartz and glasses.

The experimental facilities of the laboratory (Fig. 17) include:

— A 1.7 MV Pelletron tandem accelerator with three ion sources (radiofrequency ion source for helium, sputtering ion source for heavy elements and multicusp ion source for high hydrogen currents), which is mainly used for:

● Heavy ion ERDA;

● Ion beam lithography using a programmable proximity aperture lithography set-up;

● RBS;

● PIXE;

● Ion beam irradiation within the energy range of 0.2–15 MeV.

— A sputtering device for serial sectioning of radiotracer implanted diffusion samples (used for samples implanted at the IGISOL (Ion Guide Isotope Separation On-Line) facility).

— A scanning electron microscope.

— A 3D profilometer.

FIG. 17. Layout of the Jyväskylä Pelletron Laboratory. (Reproduced courtesy of the University

of Jyväskylä.) FIG. 18. Jyväskylä time of flight–E–elastic recoil detection analysis measurement set-up

with a gas ionization detector installed for energy measurement. (Reproduced courtesy of the

The key instrument constructed during this project is the TOF–ERDA set-up (Fig. 18). It can be used for depth profiling thin films down to a thickness of 5 nm or less and, with its gas ionization detector for energy measurement, even heavy masses at low energies down to 3–4 MeV can be separated from each other.

3.4.2. Experimental conditions

The 1.7 MV Pelletron was used. The end station was designed around a six axis goniometer. The detection system was a TOF–energy (TOF–E) telescope with good detection efficiency for hydrogen, higher than 90% for helium and higher than 99.5% for carbon, as shown in Fig. 19.

The timing resolution was measured for helium and hydrogen ions having energies of 4.8 MeV and 0.6 MeV, respectively, and scattered from a thin gold layer onto a silicon dioxide (SiO₂)–silicon wafer, as shown in Fig. 20. The timing resolution reached was 155 ps for helium (FWHM).

A gas ionization detector was built at the University of Jyväskylä and tested in measurements with low energy incident ions. The measured performance of this detector was superior in comparison with a silicon detector, and a good mass resolution could be obtained for a ³⁵Cl beam for energies as low as 3 MeV.

The data acquisition was realized in list mode, and a data stamp with an accuracy of 25 ns was given for each event. Coincident events were determined off-line.

FIG. 17. Layout of the Jyväskylä Pelletron Laboratory. (Reproduced courtesy of the University

of Jyväskylä.) FIG. 18. Jyväskylä time of flight–E–elastic recoil detection analysis measurement set-up

with a gas ionization detector installed for energy measurement. (Reproduced courtesy of the

Stopping forces were measured by the transmission technique (see Section 2.4.1): the incident beam was scattered from a 1 nm gold layer onto a silicon substrate, and this scattered beam either went through the stopping medium under study or was scattered directly to the TOF–E telescope. A schematic view of the set-up is shown in Fig. 21 together with a photo of the sample holder containing both the gold scatterer and the Si₃N₄ membrane sample (see Section 4.1).

The energy spectra for scattered ions were calculated from the TOF spectra, FIG. 20. Time of flight spectra for helium and hydrogen scattered from a thin gold layer.

(Reproduced courtesy of the University of Jyväskylä.)

FIG. 21. Experimental set-up used at the University of Jyväskylä for stopping measurements (left) and photograph (right) of the sample holder containing both a silicon wafer sample with a thin gold layer at the surface for scattering the incident beam and a Si₃N₄ window. The window can be moved to the path of scattered incident ions by rotating the sample holder in a vacuum. (Reproduced courtesy of the University of Jyväskylä.)

FIG. 22. Energy spectra for scattered 0.253 MeV ¹²C (left) and 10.215 MeV ³⁵Cl (right) incident ions with (red line) and without (black line) 100 nm thick Si₃N₄ . (Reproduced courtesy of the University of Jyväskylä.)

FIG. 19. Detection efficiency of the University of Jyväskylä time of flight detector with respect to the energy detector as a function of energy for hydrogen and carbon. The typical hydrogen energy region is marked for hydrogen. (Reproduced courtesy of the University of Jyväskylä.)

of leading to a much better energy resolution than if the signal of the energy detector were used. Examples of low energy ¹²C and higher energy ³⁵Cl spectra, from which the energy loss was determined, are shown in Fig. 22. The systems studied and the beam energies used are given in Table 1.

FIG. 20. Time of flight spectra for helium and hydrogen scattered from a thin gold layer.

(Reproduced courtesy of the University of Jyväskylä.)

FIG. 21. Experimental set-up used at the University of Jyväskylä for stopping measurements (left) and photograph (right) of the sample holder containing both a silicon wafer sample with a thin gold layer at the surface for scattering the incident beam and a Si₃N₄ window. The window can be moved to the path of scattered incident ions by rotating the sample holder in a vacuum. (Reproduced courtesy of the University of Jyväskylä.)

FIG. 22. Energy spectra for scattered 0.253 MeV ¹²C (left) and 10.215 MeV ³⁵Cl (right) incident ions with (red line) and without (black line) 100 nm thick Si₃N₄ . (Reproduced courtesy of the University of Jyväskylä.)

FIG. 19. Detection efficiency of the University of Jyväskylä time of flight detector with respect to the energy detector as a function of energy for hydrogen and carbon. The typical hydrogen energy region is marked for hydrogen. (Reproduced courtesy of the University of Jyväskylä.)

3.5. iTHEMBA LABORATORY FOR ACCELERATOR BASED