Have you ever wondered what it would be like to look down at a surface and see

down to the level of an atom? Our entire existence is influenced by how the atoms

around us are arranged. From the transparency of glass in windows to the processor

speeds in servers, it is important to precisely measure the placement and

composition of atoms.

Why engineers need RBS for materials analysis?

Many engineers (process, materials, integration, and failure analysis) struggle to

know what is the structure and composition of the material that they deposited. Are

the layers pure? How thick are the layers? Is there intermixing? A host of analytical

tools exist to answer these questions but most of the tools only yield surface

information 1 – 10 nm. There are depth profiling tools such as XPS, AES and SIMS

that can give depth profiles but they require drilling into the samples with high energy

gas through a process known as sputtering. This sputtering can cause fine atomic

structure to mix resulting in process induced artifacts. This brings us to our focus for

the month, Rutherford Back Scattering (RBS).

What information does RBS give?

This will serve as a high-level overview of RBS to convey basic theory, system setup

and basic data interpretation. The actual data is modeled using detailed physics

programs that can handle complex structures. For a simple case where the heaviest

elements are nearest the surface, the analysis can be more easily explained.

RBS is a distinctive technique that can probe from the surface (1nm) to ~ 1 micron

deep and provide compositional analysis (to about 1 atm %) along with atomic

packing density and even crystallographic defect detection. RBS does not require a

reference samples for compositional analysis because the technology is based on

collision physics. RBS works by directing high energy ions, usually Helium (He), at the

sample, such as a gold (Au) foil on a silicon substrate. The helium deflects off the

gold. Angle and energy of the backscattered Helium are used in the physics modeling

to determine which type of atom (M2) matches the collision equation. This process

is repeated by measuring the position and energy of all the backscattered helium

ions resulting in accurate location and atomic composition throughout the sample.

Gold Foil Experiment

Starting in 1909, Ernest Rutherford, born in New

Zealand in 1871, initiated some experiments at the

Cavendish Lab (University of Cambridge) showing

the back scattering of α particles on gold. This

research lead to a 1911 publication establishing

the nuclear model of the atom.

(see below for details on the gold foil experiment)

Linear Particle Accelerator

In order to support their research on transmutation of atomic

nuclei, John Douglas Cockcroft (pictured) and Ernest T.S.

Walton from the University of Cambridge developed the first

linear particle accelerator allowing high intensity controllable

beams. This work was recognized by the Nobel Prize in 1951.

Surface Barrier Detector

Various labs including CEA (France), Chalk River

(Canada) and Oak Ridge (USA) worked on the

development of Surface Barrier Detectors also

known as Solid State Detectors (SSD). By replacing

Magnetic Spectrometers, SSD simplified the

acquisition of data in RBS experiments.

Surveyor 5

By performing a back-scattering

experiment from the moon, NASA

Surveyor 5 mission brought mass

media attention to RBS. The aim

was to quantify the major elements

on the surface of the moon.

1932

1955-

1965

1967

1911

1973

HOW DID WE GET HERE?

HOW DOES IT WORK?

WHY CHOOSE OUTERMOST TECHNOLOGY FOR RBS?

Due to the development of the

technique, the Electrochemical

Society asked several labs including

Caltech to prepare a joint review on

back scattering techniques. RBS

was no longer a specialized nuclear

technique but was becoming a

standard analytical method.

ECS Chicago

The Gold Foil Experiment:

Starting in 1909, Hans Geiger (that would later invent and give his name to the

radioactive counter) and Ernest Marsden from the Rutherford lab put together an

experiment to study the behavior of α−particles ( He

) when interacting with gold.

The theory predicted that all the alpha particles would go through the gold with some

possible deflection. The experiment showed that 1/20,000 α−particles bounced back.

For Rutherford, “it was almost as incredible as if you fired a 15 inches shell at a piece

of tissue paper and it came back and hit you”.

From this experiment, in 1911, Rutherford proposed a new atomic model, the nuclear

model, that would contradict the current plum pudding model of JJ. Thomson. In the

center of the atom, there has to be a something very small, very heavy and positively

charged: the nucleus.

α-particles

Gold Foil

Detector

Gold Atom

Nucleus

Electrons

α-particles

Atomic Level View (Nuclear Model)

1/2M

=1/2M

+1/2M

Incident Ion

Scattered Particle

Detector

Scattering Angle Φ

Target

Au Layer

Mo Layer

Fe Layer

Si Substrate with

Trace C and O

Sample Surface

Au Layer, Au: 197

Mo Layer, Mo: 96

Fe Layer, Fe: 56

Si Substrate, Si: 28

C: 12

O: 16

Reading RBS Spectra

PIXE

RBS

Incident

Particle

/He

Sample

Additional Analysis

NRA

Analyze non-destructively thin film composition up to 1 µm

Measure hydrogen profile in thin films and bulk materials

APPLICATIONS

RBS

Measure specific isotopes for high accuracy composition

of elements such as B, C, N and O

Study the X-rays released by the sample, good for trace

contamination detection

Study photons emitted by electron-hole pairs recombining,

useful for locating and identifying types of crystal dislocations

HFS

NRA

PIXE

Rutherford Backscattering

(GC)

Typical RBS spectra are presented as Counts (total counts detected by the detector)

and Channels. Channels are bins for the histogram of energies and they have a

correlation to the energy of the backscattered helium. Higher energy helium (higher

channel number) will tend to be from heavier elements near the surface. The higher

the count and wider the channel number the denser and thicker the layer is

respectively in the sample. If the structure has small pockets of other elements the

signals will superimpose such as C and O signals on top of the Si signal as shown in

the example RBS spectra.

Fast Turnaround Time, Data Report Within 4 days of Sample Receipt

• Samples can be pre-diced or diced onsite further saving your time

Competitive Pricing - RBS/HFS

• Commonly used for analyzing hydrogen profiles for samples on silicon

• Can distinguish hydrogen in the various layers of a film stack

• Also used to verify deposition quality and monitor carbon content

Great Combination Deal - RBS/HFS/NRA

• NRA is often used to distinguish B and C in depositions

• Combining NRA and HFS further improves the accuracy of the RBS modeling

• Additional NRA options available, please inquire for more details

*Reference: Rutherford backscattering spectrometry: reminiscences and progresses _W.K. Chu,J.R. Liu _Materials Chemistry and Physics_1996

Rutherford Backscattering Spectrometry