What is a SIMS?
A Secondary Ion Mass Spectrometer is a tool used by scientists and researchers to analyze the surface of a material. A single ion beam is shot at the surface of the material and in the collision of the ion beam and surface, secondary ions are released [1]. The mass spectrometer then collects the secondary ions using different electrical potentials to attract the ions. The mass spectrometer then analyzes the ions using their mass to charge ratios to determine different properties and characteristics of the material. Secondary Ion Mass Spectrometers, also known as SIMS, are expensive and complex and therefore most SIMS devices are built in-house. The objectives of this project are to design an ultra-high vacuum chamber (UHV), align an ion beam and ion extractor to the same spot on a sample material and develop a spectrum using a TOF spectrometry instrument. Dr. Michael V. Lee, our sponsor for this project, is an assistant professor and analytical chemist at Northern Arizona University researching organic electronics and photovoltaic cells. A SIMS device will give Dr. Lee the ability to precisely analyze the organic electronic material in hopes to slow the decay rate. The organic electronic would then lower the cost of photovoltaic cells making renewable solar energy more cost effective and available to the public. The SIMS device, outside of Dr. Lee's research, would also be available to other programs at Northern Arizona University to analyze a material to better understand the materials potential applications. The engineering program could use the mass spectrometer for environmental, petroleum and biological analysis along with some applications into genetic engineering.
Background
Secondary ion mass spectrometry (SIMS) uses one ion beam to bombard a surface. The ions are accelerated with high potential, usually on the order of kV, toward a sample surface. When the ions strike the surface, they penetrate a few nanometers into the sample and eject or sputter off material from the sample. All of the material is small molecules or single atoms and some of them carry a charge. A separate pole piece uses a potential to collect the ions (either positive or negative ions) and accelerate them into a mass spectrometer. The mass spectrometer identifies the ions by mass (technically mass-to-charge ratio). The spectrum of the different ions that were sputtered gives a chemical fingerprint of the sample surface. Rastering the beam can then provide a chemical map of the surface. SIMS acts as a chemical microscope imaging the chemical nature of a surface with spatial resolution that can reach even 50 nm.
SIMS Diagram and Focus of Ion Gun and TOF
The SIMS system is composed primarily of a primary ion gun and an ion extractor that connects to the mass spectrometer, sealed in an ultra-high vacuum (UHV) system (~
). For this project, a Cs+ ion gun and a refurbished triple quadrupole mass spectrometer will be integrated onto a custom vacuum chamber pumped with a turbo pump.
The tasks required for the overall design project include:
- Design a custom UHV system with proper structure, flanges, and angles to align an ion beam, and ion extractor to the same spot on a sample. The distance from the gun to the sample and the extractor to the sample also need to be appropriately defined. The design should account for baking, as well as required vacuum gauges, venting, and later addition of sputter gun for the depth profiling. The design should also account for the weight of the additional parts of the system.
Estimate student involvement: 2-3 ME capstone students.
Budget for fabrication of customized chamber: TBD
- Design a custom UHV enclosure for the triple-quadrupole mass spectrometer that integrates with the main UHV system. Design should minimize the required vacuum pumping required and determine appropriate pumping needs, accounting for needs of both the viscous flow and molecular flow regimes. The group should also design a testing mechanism for the mass spectrometer under UHV to ensure proper operation.