Sputter Coating 101

4/8/97

Two questions:

1. When a gold coated specimen is viewed in SEM at ~10k or higher, a

fine structure of gold coating becomes visible. Are there optimal

conditions of sputter coating (sputter current, time, distance

target-specimen, argon pressure, other gas than argon) to minimise the

artefact?

2. What is Au-Pd target? I thought that it was just an alloy one, but

a supplier of the coater says that using the alloy target is not

enough, that a coater with simultaneous sputtering Au and Pd from two

different targets is required to produce continuous coating.

Of course, it's better to get a FEG and use low voltage, but I want

to do it with a magnetron sputter coater and conventional JSM 6400.

Alexander Titkov

Millennium Inorganic Chemicals

PO Box 245

Bunbury WA 6231

Australia

Ph (097) 808 505

FAX: (097) 808 444

E-mail: scm!atitkov@scmaust.attmail.com

Dear Alex,

1. Assuming that you are seeing gold, this indicates

that your coating is too thick. When a sample is coated it

should take on a bluish color. This is most easistly seen on

flat areas where there is no sample. Try using a cover slip

to set up the conditions.

AS to conditions. Don't rush the coating. It should take

about a minute to coat your sample. The manuel that came

with the sputter coater will give you a starting point.

2. You are right, it is an alloy target. This target should give you a

finer grain size, but the secondary electron return is not as good.

Gregory Rudomen

Greg@UMIC.SUNYSB.EDU

516-444-3126

University Microscopy Imaging Center

S.U.N.Y. Stony Brook

You will probably need to experiment, since the optimum coating will depend

to some extent on your specimen - rough specimens will need a coarser,

larger grained coating to prevent charging. In addition, you will get a

huge amount of advice from all directions, much of it contradictory! so

you'll only find out what is right for you by experiment. Anyway, MY advice

is:

Argon is generally the best option and you also want to make sure that the

argon isn't contaminated with N or O - this will substantially reduce the

sputtering rates. If you're Ar supply is good, then this only requires that

you flush the system for a short period before sputtering.

Argon pressure/flow rate should be adjusted so that the plasma is just

steady - turn up the Ar flow rate until you get a plasma, and then slowly

reduce the flow rate (be slow because there will be a significant lag

between changing the flow rate and the system stabilising). At some point,

the plasma will start to flicker, and then go out. Increase the flow rate

to just higher than the point at which the plasma flickers (obviously, this

all needs to be setup either without the specimen in the coater, or with a

shutter over the specimen).

Lower voltages and shorter times will tend to produce finer and thinner

coatings, respectively. Most suppliers will advise approx 1.5 to 1.8 kV,

but you can produce some very nice coatings at 600 to 800 V. You should

coat for a period just long enough to stop specimen charging.

> 2. What is Au-Pd target? I thought that it was just an alloy one, but

> a supplier of the coater says that using the alloy target is not

> enough, that a coater with simultaneous sputtering Au and Pd from two

> different targets is required to produce continuous coating.

Au/Pd target is an alloy - I'm not aware of commercial coaters that

simultaneously work from a pair of targets. Such an arrangement might be

more effective but I want some evidence that it was, and I expect it would

be more expensive:) Au/Pd alloy targets are effective at producing finer

grained coatings. It seems that the Pd provides nuclei for the Au, leading

to more, and smaller Au grains rather than the Au grains growing larger as

with a pure Au target.

Larry Stoter

LPS@teknesis.demon.co.uk

Dear Alex,

You should not see the structure of a Au-Pd film at 10K. In the 1980 SEM

Inc. Conference Proceedings there is an extensive study of the various

conditions for making films, with the films studied by TEM. It is worth

reading if you have access to it. The result was a recommendation to use Ar

gas, 60%Au-40%Pd instead of Au and a lower voltage, 600 to 700 volts. The

specimen-surface distance is about 2 cm. I have been using a 1 to 3 minute

coating in Ar at 700v. ever since and can only see the film at 50,000 times.

Coatings using W or Ni were even finer. I have never heard of using two

targets instead one alloyed one. I'm sure the ionized particles don't care.

The very fine coatings required by FEG microscopes are best made by an

ion-beam coater (much more expensive).

Mary Mager

mager@unixg.ubc.ca

I bought a gold/palladium coater recently and found that the default

time listed in the manual (> 1 minute) resulted in a coating about the

thickness of the chrome on a 1950's Buick. I now coat even the most

problematic samples (Mo oxide crystals, glass fractures, rare-earth

phosphor particles, etc.) for 15 seconds or less. I also dropped the

current to about 70% of the spec value. The samples still generate Au

and/or Pd x-rays occasionaly, but I still detect light elements. It's a

balancing act.

Remember, it is easier to recoat than de-coat.

my 2 cents.

Harold J. Crossman

OSRAM SYLVANIA INC.

Lighting Research Center

71 Cherry Hill Dr.

Beverly, MA 01915

Phone: (508) 750-1717

E-mail: crossman@osi.sylvania.com

Alex;

In my experience, it would be very unusal to be able to see any details of

a Au sputtered coating of normal thickness (5-20 nm.) at a magnification of

only 10,000x. Coating artifacts usually become a problem at much higher

magnification such as 50,000x and above.

If your sputtered coating is very thick this could be a problem and you

should review the operation of your specific coater to determine the best

parameters for the coating thickness that you desire. The sputter current,

gas pressure, gas quality, distance to the target, and sputtering time are

all factors that must be considered.

While Au provides an efficient emitter of secondary electrons in the SEM,

the grain size is quite large and does become a problem at higher mags.

Au/Pd is usually an alloy target that combines the better secondary

electron emission characteristics of Au along with the smaller grain size

of Pd. I've never seen a sputter coater for SEM sample prep that had two

targets mounted and ready to go in the same pump down cycle, this sounds

like a device not designed for SEM sample prep but for industrial sputter

applications.

Pt and Cr are other common target materials that you might also consider,

however I suspect your problem is not the coating but more likely the

sample. You did not state what the sample was so I am guessing that there

may be some sample deformation of the sample related to vacuum

incompatibility or heat from the coater. This could be shrinkage from the

evacuation of the chamber or heat from the magnetron sputter head.

If there is a possibility that the sample may be the problem, try sputter

coating your sample and a piece of metal or carbon disc at the same time.

If the coating is the source of the artifact the artifact should be

observable on all of the samples.

John Humenansky

Braun Intertec Corp.

6875 Washington Ave. So.

Minneapolis, MN 55439

(612) 942-4822

jhumenansky@brauncorp.com

Thank you very much All answered the questions about optimal

conditions for sputter coating. Reducing Argon pressure and voltage

works great even with a gold target!

Thanks again,

Alex

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