Technical
Brief
SPUTTER
COATING INCORPORATING
EMITECH K500X, K550X, K575X, K650X and K675X
Introduction
When
a target is bombarded with fast heavy particles, erosion of the target
material occurs, this is termed sputtering.
The arrangements of the systems are such that some of the sputtered
atoms will condense on the surface of the specimen to be coated.
The
above process occurring in the conditions of a gaseous glow discharge
between an anode and cathode is termed sputtering and can be enhanced by
the choice of a suitable gas and target material, which together with
other developments of the technique, allows the deposition of a suitable
coating to increase the electrical conductivity of a specimen, probably
the single most common requirement for Scanning Electron Microscopy.
The
development of Sputter Coater systems embodies significant empirical
design, however, an understanding in classical terms of glow discharge
characteristics enhance such designs, and may assist in the comparison of
differing systems.
Gaseous
Conduction
If
an inert gas such as argon is included in a cathode gas tube, the free
ions and electrons are attracted to opposite electrodes and a small
current is produced.
As
the voltage is increased some ionisation is produced by collision of
electrons with gas atoms, the 'Townsend' discharge.
When the voltage across the tube exceeds the breakdown potential, a
self-sustaining glow discharge occurs, characterised by a luminous glow.
The
current density and voltage drop remains relatively constant, the increase
in total current being satisfied by the area of the glow increasing.
Increasing the supply voltage increases current density and voltage
drop; this is the abnormal glow region.
Further
increase in supply voltage concentrate the glow into a cathode spot and
arc discharge is apparent. The
operating parameters of Sputter Coaters are within the glow discharge
regions of the characteristic described.
Glow
Discharge
Once
the condition for a sustained discharge is met, the tube exhibits the
characteristic glow discharge, so called because of the associated
luminous glow. It has been
established that free ions and electrons are attracted to opposite
electrodes producing a discharge; however, for a discharge to be
self-sustaining requires regeneration of the electrons by the positive ion
bombardment of the cathode. This
produces secondary electrons and enhances ionisation.
The resulting positive ion excess creates a positive space charge
near the cathode. The voltage
drop experienced is termed the cathode fall.
If the discharge is established in a long narrow tube it exhibits
the characteristics indicated.
Figure
2
The
positive ion density in the Crookes dark space is very high; as a result a
significant voltage drop is experienced between it and the cathode.
The resulting electric field accelerates the positive ions, which
produce secondary electron emission from the cathode.
These electrons are accelerated in the direction of the anode and
cause ionisation, generating positive ions to sustain discharge.
Subsequently, excitation of the gas results in intense illumination
in the negative glow region. From
this stage the electrons have insufficient exciting or ionising energy,
resulting in the Faraday dark space. Towards
the anode, a small accelerating field can produce ionisation and
excitation, the gas again becoming luminous.
Sputter
Coating
It
has been indicated that under conditions of glow discharge, ion
bombardment of the cathode will occur, this results in the erosion of the
cathode material and is termed plasma sputtering, the subsequent omni-directional deposition of the sputtered atoms forming coatings of the
original cathode material.
This
process is enhanced in Sputter Coaters for use in Scanning Electron
Microscopy where one objective is to provide an electrically conductive
thin film representative of the surface topography of the specimen to be
viewed, such films inhibit 'charging', reduce thermal damage, and enhance
secondary electron emission.
The
most common arrangement for a D.C. (Direct Current) Sputter Coater is to
make the negative cathode the target material to be sputtered (typically
Gold), and to locate the specimens to be coated on the anode (which is
usually 'earthed' to the system and the specimens are effectively at 'ground'
potential). The desired
operating pressure (relative vacuum) is obtained by using a suitable
applied vacuum, usually a two stage rotary pump.
An inert gas, such as argon, is admitted to the chamber by a fine
control valve.
Operating
Characteristics
The
glow discharge in sputtering is significantly dependent on the work
function of the target material and pressure of the environmental gas.
A range of target materials are used including Gold,
Gold-Palladium, Platinum and Silver, although Gold is the most common
having the most effective electrical conductibility characteristics.
The sputter head and sputter power supply should be effective over
a range of anticipated target materials.
The deposition rate is current dependant, and if we operate in the
correct glow region of the characteristic previously described, several
fold changes in current should be available for a relatively small change
in sputtering voltage. The
deposition rate should not be sensitive to small changes in pressure,
which may be experienced in the system.
If
an efficient sputter head design, operating on low voltage and as a result
low energy input, is achieve, then radiant heating from the target and
high energy electrons, (potentially the most significant sources of damage
to delicate specimens) should be considerably reduced.
There is also evidence to suggest that such a sputter head system
may also produce finer grain size for a given target material.
The presence of an inert gas, which will not decompose in the glow
discharge, is obviously desirable. Argon,
having a relatively high atomic weight, provides a suitable source of ions
for effective bombardment of the target material.
The effectiveness is also dependent on the mean free path (m.f.p.)
that is inversely proportional to pressure.
If the m.f.p. is too short, insufficient energy will be gained for
effective bombardment and will inhibit movement of sputtered material from
the target. If the m.f.p. is
too long, insufficient collisions occur and, in addition, the flow of
sputtered material may change from diffusion in the gas to free molecular
flow with a reduction in the effectiveness of omni-directional deposition.
If
these characteristic of sputter heads are achieved it should not be
necessary to cool the specimen stage for the majority of applications.
If not, however, such cooling will only serve to reduce the
baseline temperature, the thermal conductivity of most specimens we are
considering being relatively poor. For
sensitive specimens pre-cooling and subsequent reduction of the baseline
may still be desirable and there is also evidence to suggest a reduction
in grain size of the coating. It
may be apparent that Scanning Electron Microscopy requires a versatile
system without compromising performance.
Specifically, fine grain size, uniform coating and low heat input.
Consideration of these characteristics in design and development
should enable a suitable coating system to be realised.
It was indicated previously that while empirical design may be in
evidence, it should now be apparent that efficient production of positive
ions for glow discharge is required. The
sputter head and its associated power supply should be a primary objective
of design and development.
Certain
sputter heads can employ an annular magnet and shroud assembly, with disc
target. The magnetic lines of
force form enclosed loops at the target surface; deflection and
retardation of electrons resulting in increased ion yield sputtering
efficiency. The power supply
employing solid stage switching for applied voltage control.
Specification
The
overall result is a low voltage head with low energy input.
The possibility of thermal damage due to radiant heating and
electron bombardment is considered negligible.
Vacuum
0.1
to
0.05 Torr
Sputtering Voltage
100
to 150 Volts
Current
10 to
50mA
Deposition
3 to
50nm/min
Grain Size
Less than
5nm
Temperature Rise
Less than
10oC
K550X Sputter Coater |
The
Micrograph is 3-day old concrete, freshly fractured.
This is a typically difficult sample as the surface is
highly granular and uneven and therefore susceptible to
charging during SEM. However,
after coating in the K550X such problems were not encountered.
(Coating conditions: Gold, 20mA, 2 minutes, 0.1Torr,
coating thickness 11nm). |
K575X Turbo pumped high resolution Chromium Sputter Coater
It
is, of course, possible to satisfy very precise parameters by the
selection of target material, voltage deposition current and vacuum.
Under these conditions, it is possible to achieve thin films to
10nm with grain sizes better than 2nm and temperature rises of less than 1oC.
The application of sputter coating has been well established.
However, the improved performance of Scanning Electron Microscopy realizes
the capabilities of this series of Sputter Coaters.
The
Cathode target material is commonly Gold. However, to achieve finer grain
size, and thinner continuous coatings, it is advantageous to use cathode
target materials such as Chromium. To
achieve sputtering with this target material requires vacuums somewhat
better than those achievable with a Rotary Vacuum Pump.
The K575X uses a ‘Turbo’ pump, backed up by a Rotary
Vacuum pump, the complete pumping sequence being under automatic control,
the vacuum of the order of 1 x 10-3mbar.
The
twin head version of the above, the K575XT has two sputter heads.
These are arranged such that for special coating applications two
sequential layers of a target material can be deposited without breaking
the vacuum seal in this automatic unit.
The
K675X system employs a magnetron target assembly, this enhances the
efficiency of the process using low voltages and giving a fine grain, cool
sputtering. There are three
such target assemblies in the K675X, positioned to give coating over a
large diameter which, together with a rotating sample table, ensures even
depositions. This method
allows standard targets to be utilised, and avoids the necessity of
special large profile targets. The
triple-target system is particularly useful in the semi-conductor wafer
industry. It has a
turbo-molecular pump backed by a rotary vacuum pump.
The
integrated instrument panel and plug-in electronics maximise
‘up-time’ and, with user-friendly designs, ensures
satisfactory multi-user discipline. The
sputtering parameters can be pre-set, including the gas bleed needle valve,
which has electromagnetic valve back-up.
The independent vacuum pump is controlled by the instrument
throughout the fully automatic coating cycle.
It can be used to sputter coat targets such as gold, and also
targets that may need pre-cleaning, or the removal of oxide layers such as
chromium. A shutter assembly
is fitted as standard, which allows a sputter cleaning and the sputter
cycle to be carried out while maintaining the vacuum.
The K695X, launched at the start of the new millennium, is
specifically designed for the 12-inch wafer market.
Selection of Sputter Coaters
K550X
K575X
K650X
K675X
SC3000
SC7620
SC7640
SC7680 sputtering plasma
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