Research Equipment
Rutherford Backscattering Spectrometer (RBS)
The following is an introduction to the Rutherford Backscattering Spectrometer (RBS), which can perform both measurement and high-energy ion implantation.
RBS measurement system (NT-1700 model, manufactured by NHV Corporation)
Figure 1 RBS measurement system. The ion producing source is shown on the far left, while the large tank in the center is its acceleration power source. The controller is shown on the right.
First of all, what kind of measurement system is the Rutherford Backscattering Spectrometer (RBS)?
This system uses charged particles (He ions for example), accelerated by hundreds of kV* or up to several MV*, to irradiate an object.
The number of irradiated particles that bounce back, or are “backscattered,” are then measured, along with the amount of energy
released. The system can then nondestructively evaluate the distribution and amount of elements that make up the object, along with the
object’s crystalline content. *[1 kV (kilovolt) = 1,000 volts; 1 MV (megavolt) = 1,000,000 volts]
This system can also evaluate the depth direction to several microns, at a depth resolution of 0.02 microns*, in about five minutes.
This measurement system is second to none for those performing thin film and surface layer research. *(1 micron = 1/1,000,000th of a meter)
The RBS method was also used during the US Apollo mission to analyze rocks on the moon's surface.
Why is this method of RBS comparatively nondestructive and more accurate for measurement than other methods?
The higher the voltage is, the more effectively the size of accelerated charged particles is reduced. Even for an object which usually
looks as if its atoms are condensed with no space between them, when using the RBS at a high voltage, many spaces will appear.
Particles appear separate like stars in the night sky. So, irradiated particles will only collide with or scatter the object’s atoms
at a probability of one in several million. This probability is considered nondestructive. Although scattering is rare, when it does occur it’s
referred to as “Rutherford scattering.” Since this phenomenon can be easily analyzed with a high school level grasp of dynamics, analysis
of the measurement data can be performed simply and accurately.
What's so special about the RBS system at JAIST?
The RBS device at JAIST has the following four significant features:
- Charged particles are accelerated by a tandem system.
Originally "tandem" means a pair of horses side by side pulling a wagon. In this case, the “tandem system” refers to a system that performs acceleration two times. First negative ions are pulled in by positive voltage, causing them to accelerate. Then, high voltage is used to cause negative ions to collide with argon gas, changing them to positive ions.
Those positive ions are then repelled by positive voltage, causing them to accelerate, which ultimately causes twice the acceleration to occur with a single source of positive voltage. This system has a maximum positive voltage of 1.7 MV, and if positive ions are singly-ionized, it will produce a voltage equivalent to 3.4 MV. If they are triply-ionized, it will produce a voltage of 6.8MV.
- It uses a Schenkel-type, accelerated power source that produces positive voltage.
There are many ways to obtain a high voltage of several MV, but they are divided basically into “mechanical pressor systems,” which mechanically rotate insulated belts to carry a static charge, and “stationary device systems,” which use electronic components such as condensers and diodes. JAIST uses the latter system, which is called a Schenkel-type power supply.
A stationary system has comparatively more pulsation than a mechanical system, but structure and handling are simple, and it can obtain a large ion beam current.
- It can implant high-energy ions.
The voltage acceleration of a normal ion implantation system (which implants ions into an object) is about 400 kV, but this system is capable of a voltage of several MV. This basically means it is capable of high-energy ion implantation. This system can obtain heavy ion beams from such elements as boron, phosphorus, oxygen, and silicon with an electrical current of more than 0.05 mA. Such a system that performs RBS measurement, and also performs high-energy ion implantation, cannot be found at any national university in Japan other than JAIST.
- It can take measurements with the PIXE and ERDA methods.
Other than taking RBS measurements, this system can detect X-rays produced from a measured object via ion irradiation, and through the PIXE method it can identify elements that are difficult to measure with the RBS method. Using the ERDA method, it can determine how much hydrogen content is in the object being measured. This is achieved by measuring the amount of hydrogen released by irradiated ions.
Finally, how is this RBS measurement system used at JAIST?
It is mainly used to evaluate the composition and crystalline content of thin films, more specifically, the amorphous silicon film used
for solar cells, and the protective films of integrated circuits, such as silicon oxide and silicon nitride films. Materials used in new electric
devices are also evaluated, such as thin films used for metals, insulators, high-temperature superconductors, and organics.
The schedule for use of this system is not completely full yet. There are still opportunities here for other researchers to use this valuable
system.
