Ion Solid Interactions Research

 

Activities and research projects in the area of Ion Solid Interactions include:

 

a)   Installation and commissioning of the HIBS system.

 

The HIBS system is truly unique. It has been custom made so there are no other similar systems in the world, and is one of the most sensitive techniques everywhere to detect trace levels of metal impurities on the surface of silicon or similar materials. The HIBS system was disassembled at Sandia National Laboratory, shipped to Orlando, reassembled, and put back in operation by a team consisting of Dr. B. Tonner, Dr. S. Kleckley, myself, and two undergraduate students D. Bateman and T. Bentley. The system became quickly operational with performance similar to that at Sandia. Since then, I have been the sole faculty responsible for the operation and maintenance of the instrument.

 

The basic principles of HIBS are similar to those of Rutherford backscattering spectrometry. A beam of energetic particles impinges on the sample, and the number and energy of backscattered particles are monitored. Analysis of the energy spectrum of the backscattered particles provides information about the composition of the sample as a function of depth. The experimental set-up and parameters, in HIBS, are optimized to obtain maximum sensitivity by:

a)      using a high Z, low energy, ion beam, and

b)      using a large detector solid angle.

 

The beam species is typically 12C+, or 14N+, and the energy about 120 keV. The detection is achieved with a set of six microchannel plate (MCP) detectors, in a time-of-flight configuration. After backscattering, the probing particle traverses a very thin carbon foil (5 mg/cm2 » 25 nm), emitting secondary electrons. Forward emitted, secondary electrons, are accelerated by a grid, biased to about 600 V, and reach an (electron) MCP detector, giving rise to a ‘start’ pulse. Start and stop pulses are routed to respective constant fraction discriminators and, subsequently to a time-to-amplitude converter. The output of the time-to-amplitude converter is fed to a computer based multichannel analyzer, working in pulse height mode. The time it takes for the electrons to reach the electron MCP can be essentially neglected, so the multichannel analyzer provides a display of the time-of-flight of the backscattered probing beam.

There are three pairs of electron-ion MCP detectors to maximize sensitivity. The ion beam is generated by a 200 keV accelerator, that uses a duoplasmatron source, and a mixture of N, and CO, gases. The accelerator has good energy stability (£ 250 eV), and a mass-analyzing magnet with £ 0.5 amu resolution. The scattering chamber has been custom designed to accommodate the MCP detectors, has a computer-controlled goniometer to handle the samples, and is pumped by a cryo-pump (base pressure is usually in the10-8 Torr regime). Samples, up to 8” in diameter, are introduced via a load lock located in a mini-clean enclosure.

 

b)      Current research.

 

Analysis of buried impurities using HIBS. The original design of the system targeted the analysis of impurities on the surface of silicon samples. Dr. Braunstein and his students are exploring the use of HIBS to study impurities buried in a film. They have determined that it is possible to study heavy impurities in silicon or silicon dioxide buried up to a depth of about 50 - 100 nm. They also found that deposition a thin layer of silicon dioxide does not alter significantly the quality of the signal. The students also developed a computer model of the experiment that permits to calculate the time of flight and the energy of the detected backscattered particles. These results have been presented at the International Conference on Ion Beam Analysis, IBA 03, Albuquerque, NM, July 2003, and published.

 

We are presently taking advantage of these results in three exploratory projects:

1) Study of shallow arsenic implants in silicon,

2) Detection of transition metal impurities in carbon nanotubes, and

3) Detection of metal ions in biological samples.

 

c) Future research.

           

Micro-imaging ion beam system. With funds from a University Presidential Equipment Award, we have purchased: I) an ion lens that can focus the beam to about 10 mm, II) a switching magnet, III) beamline components, and IV) a computer controlled sample holder. These parts will be used to start the development of an ion beam imaging system for biological samples based on the HIBS accelerator, and time of flight signal detection. The imaging system will take advantage of the extreme sensitivity of HIBS, to study the role of metals in biological processes as described above.