September_AMP

A D V A N C E D

M A T E R I A L S

P R O C E S S E S |

S E P T E M B E R

2 0 1 5

2 9

TESTING INORGANIC AND ORGANIC

MATERIALS WITH A NEW ION SOURCE

A new monatomic and gas cluster ion source for XPS instruments uses both Ar +

and Ar

+ (n>1000) gas cluster sputtering to clean surfaces and create depth

profiles for a growing class of advanced materials.

TECHNICAL SPOTLIGHT

common technique called depth

profiling uses x-ray photoelec-

tron spectroscopy (XPS) to

evaluate layered materials using ion

etching. However, before testing can

even begin, samples that arrive at the

laboratory often need to be cleaned of

contamination. Most XPS instruments

include an ion gun that produces mon-

atomic argon ions (Ar

). This gun works

particularly well for most inorganic

materials, keeping the chemical struc-

ture intact as layers are removed by ion

bombardment. However, other classes

of materials, such as polymers, bioma-

terials, and even some metal oxides,

can be damaged by interaction with

the ion beam, changing the material’s

chemistry and distorting test data.

To enable cleaning or depth pro-

filing of these types of materials, an ion

source that sputters the sample surface

using large, singly-charged gas clusters

was developed. The new monatomic

and gas cluster ion source (MAGCIS) for

Thermo Scientific XPS instruments uses

both Ar

and Ar

(n>1000) gas cluster

sputtering, enabling surface cleaning

and depth profiling of a growing class of

advanced materials that include both

hardened inorganic and softer organic

materials.

Complex, multilayered materials

are increasingly used in a wide range of

products and devices. These materials

were traditionally based around me-

tallic or oxide layers. However, with to-

day’s drive toward lightweight and less

expensive components, polymer-based

materials are becoming more common,

particularly in areas such as display

technologies, biomedical devices, and

energy generation. Understanding the

interaction of these layers with each

other and the environment is a crucial

part of the development cycle. Because

these interaction zones

are typically

just a few nm thick, understanding their

chemistry requires techniques that are

especially surface sensitive and can

profile through the material to access

subsurface interfaces.

XPS is one such technique. It is very

surface sensitive, the sampling depth is

generally less than 10 nm, and it pro-

vides quantitative chemical informa-

tion. To probe subsurface layers, an XPS

system is usually fitted with an ion gun

that produces monatomic argon ions

to remove material from the surface.

This works particularly well for inorgan-

ic materials. A range of beam energies

are usually available, typically from a

few hundred eV up to a few keV, giving

operators a choice of etch rates depend-

ing on the thickness of the layers to be

analyzed. Spectroscopy helps with ma-

terial removal by ion milling to generate

a depth profile, typically displayed as

atomic concentration, which shows the

variation in the chemistry with depth

into the surface. Depths up to a few

microns can be investigated using this

approach.

Thismethod has been available for

many years. However, it is particularly

unsuccessful with polymeric systems,

which tend to be chemically modified

by interaction with the ion beam. This

chemistry change affects the spectra

obtained during the depth profile ex-

periment because they no longer rep-

resent the original material. To allow

depth profiling of the increasing num-

ber of polymer-based systems, gas clus-

ter ion sources have been developed.

The key to minimizing damage to

a polymer system during depth profil-

ing is to reduce the energy going into

the surface. With a monatomic beam,

any energy not used to eject material

from the surface generally penetrates

into the surface, breaking bonds and

damaging the remaining material. This

damage is typically just greater than

the XPS information depth, so spectra

become representative of the damaged

surface rather than the real surface.

It is possible to reduce the damage

zone by reducing the beam energy, but

below a threshold of roughly 50 eV, it

no longer has an effect on the surface.

This threshold can be negated by mak-

ing the projectile much heavier: By us-

ing a weakly bound cluster of several

thousand gas atoms, material can still

be removed, and the beam energy can

spread across the whole cluster. Upon

impact, the cluster removes surface

material, but also breaks apart, mini-

mizing penetration of the projectile into

the surface, and instead disperses the

energy in the beam laterally. By having

such a low energy per atom, damage to

the remaining surface is substantially

reduced, so the resultant spectra repre-

sent the sample’s actual chemistry.

PROFILING ORGANIC FIELD

EFFECT TRANSISTORS

The trend toward thin, low cost,

flexible electronic systems has led to

the design of many organic microelec-

tronic components, including field ef-

fect transistors (FETs). Early organic FET

designswere basedon aromatic organic

semiconducting materials, while recent

developments explore organometallic

species, which potentially offer greater