PRESENTATION ON RUTHERFORD BACKSCATTRING
SPECTROMETRY
BY
PARVEZ S. MALDAR
UNDER THE GUIDANCE OF
DR. A.V. MOHOLKAR
M.Sc. Ph.D.
BOYSCAST FELLOW
DEPARTMENT OF PHYSICS
SHIVAJI UNIVERSITY, KOLHAPUR- 416 004
1
Outline of presentation
❑Introduction
❑What is Rutherford backscattering spectrometry(RBS)?
❑RBS terminologies
❑Kinematics considerations
❑Depth profiling
❑Detection of backscattered particles
❑Channeling
❑Applications
2
What should be the nature of perfect surface analysis
technique?
➢It should Provide as maximum information as regarding
the properties of the surface.
➢Be non destructive
➢Be sensitive to all elements in the periodic table
3
Why do we want to analyze just the surface?
Surfaces determine properties such as
➢Chemical reactivity (catalysis),Corrosion, Adhesion
➢Electrical properties of interfaces (semiconductors
structure)
➢Optical properties (photovoltaic cells)
We can modify these by-
➢Surface treatments
➢Surface coatings
➢Nanotechnology structures
4
Spectroscopy or spectrometry?
➢ Prefix SPECTRO, meaning energy measurement.
➢ If the incident species is the same as the emitted species, the
technique is a SPECTROMETRY
❑Rutherford backscattering spectrometry and X-ray diffractometry.
➢ If the incident species is different from the emitted species, then the
term SPECTROSCOPY is used
e.g.-Auger electron spectroscopy and X-ray photoelectron
spectroscopy.
5
Method
Destructive
Incident
particle
Outgoing Resolution
particle
RBS
No
Ion
Ion
10 nm
XPS
Yes
X-Ray photon
Electron
1µm
EDX
Yes
Electron
X-Ray
photon
1.5 nm
Auger
Yes
Electron
Electron
1.5nm
6
Why is RBS Useful?
➢ Non-destructive technique
➢Measures the composition and thickness of thin films.
➢Provides information regarding surface contaminations.
➢Mass and depth of the target sample
➢Crystalline quality of thin films
7
Rutherford Backscattering ProcessNear surface regions are bombarded with a beam of 1-3 MeV alpha
particles.
➢Particle beam – 4-He++ from accelerator (1-3 MeV)
➢Detector - silicon surface-barrier
➢Entire system is under a vacuum.
8
RBS Terminologies➢Kinematic Factor- The ratio of the energy of the projectile
before and after collision is called the kinematic factor.
➢E1=K(m2)²E0, where E0= energy of incident particles,
E1=Energy of backscattered particles. It is given by
 m2


2
2
E1  ( M 2 − M 1 sin ) + M 1 cos  
=
=

E0 
M 2 + M1


1
2
2
Where ф=angle of scattering.
9
➢If the mass of the projectile and the scattering angle are
known, the mass of the target atoms can be determined
Variation in kinematic factor for different scattering angles.
10
e.g. for scattering angle Ф=1200 ,Km2=ratio of initial and
final energy of projectile=0.44,M1 = 4,
M2 can be find out using equation
 m2


2
2
E1  ( M 2 − M 1 sin ) + M 1 cos  
=
=

E0 
M 2 + M1


1
2
2
M2 = corresponds to oxygen(16)
11
➢Scattering Cross Section- The probability that a material
will cause a collision is called its scattering cross section.
12
➢Differential scattering cross section -The relative number of
particles backscattered from a target atom into a given solid angle
for a given number of incident particles is related to the differential
scattering cross section.
➢Stopping cross section- The ratio of energy loss to two-
dimensional atom density for a given material is known as its
stopping cross section .
13
Kinematics
➢1)when He++ strikes light elements such as C, N, or O, a
significant fraction of the projectile's energy is transferred to the
target atom and the energy recorded for that backscattering event is
much lower than the energy of the beam.
➢2) It is usually possible to resolve C from N or P from Si, even
though these elements differ in mass by only about 1 amu.
➢3)If the mass of the atom being struck increases, a smaller and
smaller portion of the projectile energy is transferred to the target
during collision, and the energy of the backscattered atom
asymptotically approaches the energy of the beam.
14
➢4)An important related issue is that He will not scatter backwards
from H or He atoms in a sample.
➢ 5)Elements as light as or lighter than the projectile element will
instead scatter at forward trajectories with significant energy. Thus,
these elements cannot be detected using classical RBS.
➢6)However, by placing a detector so that these forward scattering
events can be recorded, these elements can be quantitatively measured
using the same principles as RBS.
15
Depth Profiling
➢When a particle which backscatters from an element at some
depth in a sample will have measurably less energy
than a
particle which backscatters from the same element on the sample
surface.
➢The amount of energy a projectile loses per distance traversed
in a sample depends on the projectile, its velocity, the elements in
the sample, and the density of the sample material.
➢Typical energy losses for 2 MeV He range between 100 and
800 eV/nm..
16
substrate
Projectile -ion beam of He++ at 2.2 MeV.
➢ The high energy edge of the tantalum peaks near 2.1 MeV
corresponds to backscattering from Ta at the surface.
➢The high energy edge of the silicon peaks near 1.3 MeV corresponds
to backscattering from Si at the surface.
17
substrate
➢The illustration shows that particles scattered from tantalum at the
TaSi/Si interface of the 230 nm film have a final energy of about 1.9 MeV.
➢ while particles scattered from the same interface of the 590 nm film
have less final energy (about 1.7 MeV) because they have passed through
more TaSi.
➢ The entire Ta peak spans a greater energy range, because of the
18
increased thickness of the layer it represents.
Detection of scattered particles-
➢The high energy charged particles produce electron-hole
pairs in the semiconducting material.
➢ The detector is operated with an electrical potential
(typically 4 kV) between the front and back surfaces.
➢ In the resulting electric field, the electron-hole pairs
produce a current proportional to the energy of the charged
particle.
19
➢Channeling
➢RBS can also be used to study the structure of single crystal samples.
➢When a sample is channeled, the rows of atoms in the lattice are
aligned parallel to the incident He ion beam.
➢ For example, the backscattering signal from a single crystal Si
sample which is in channeling alignment along the (100) axis will be
approximately 3% of the backscattering signal from a non-aligned
crystal, or amorphous or polycrystalline Si.
➢By measuring the reduction in backscattering when a sample is
channeled, it is possible to quantitatively measure and profile the
crystal perfection of a sample, and to determine its crystal orientation.
20
Applications
➢Semiconductor-quantitative depth analysis of metal silicide
films, barrier metals, insulating layers, multilayer stacks and
crystal damage vs. depth.
➢High Tc superconductors- quantitative depth profiling
➢Thin surface structure(composition and depth profile.
21
22
References1) Fundamentals of Nanoscale Film Analysis-Terry L. Alford,
2)Backscattering spectroscopy - W. Chu, J. Mayer, M. Nicolet
3) Rutherford Backscattering Spectrometry- F. Ernst
23
24