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also be used to improve the effectiveness of grease as a thermal contact. Users
often make simple clamping fingers to fit their sample and hold them down with
M3 screws in the tapped holes intended for the lifter tool.
3.
Adhesive tape:
tape over the corners or edges of a sample with vacuum compatible
tape that has a silicon adhesive. Experience has proven that 3M brand Kapton®
tape with silicon adhesive will retain its adhesive properties to as low as 4 K.
4.
Double-sided adhesive tape:
if there is no room on the top surface for Kapton® tape,
double-sided tape can be placed between the sample and sample holder. Some
experimentation may be required to find a tape that does not harden and peel
away at low temperatures.
With all of the adhesive methods, the sample holder top surface can be cleaned with
acetone applied to a soft, clean cloth then rinsed with isopropyl alcohol. Do not use
abrasives or scrub the sample holder, because the gold plating will be removed.
4.3.3.4 Semi-Permanent Mounting
Semi-permanent mounting gives better thermal contact than temporary mounting,
but it requires more time to mount and remove the sample.
1.
VGE 7031 varnish:
you can use VGE-7031 varnish in temperatures ranging from 2 K
to 470 K, and it is compatible with the grounded, isolated, coaxial and triaxial
sample holders. VGE-7031 varnish is available through Lake Shore.
D
To mount: only a small amount of varnish is needed for your sample. For a
25 mm (1 in) sample, place a drop of the varnish on three pads of the sample
holder. Then center and align the wafer on the sample holder. The amount of
varnish used can be increased or decreased for larger or smaller samples,
respectively. VGE-7031 varnish may be air dried or baked according to
manufacturer’s recommendations.
D
To remove: you can remove the sample by soaking in ethanol or toluene. A
solution of equal parts of ethanol and toluene has also been very successful
at removing samples mounted with VGE-7031 varnish. Grooves in the
sample holder permit the remover to flow under the sample.
2.
Photoresist or PMMA material:
the most common semi-permanent mounting tech-
nique is photoresist or PMMA material common to the semiconductor processing
industry. It offers excellent adhesion yet is still removable.
The photoresist, chemical remover, and bake procedures must be compatible with
the wafer and devices. These processes are compatible with grounded and isolated
sample holders. Please note that coaxial and triaxial sample holders contain Kapton®
insulation; any chemicals used must be compatible with this. Also, for the coaxial and
triaxial sample holders, temperatures should not exceed 400 K.
D
To mount: put a drop of the photoresist on three pads of the sample holder.
Center and align the wafer onto the sample holder and bake using the usual
specifications for the resist.
D
To remove: you can remove the sample by soaking in chemical remover.
Grooves in the sample holder permit the remover to flow under the sample.
3.
Silver paint:
if an electrically conductive mounting is required, silver paint can be
used in place of photoresists or VGE-7031 varnish. Please note that the paint
must be dried completely for best electrical conduction.
D
To mount: only a small amount of silver paint is needed for your sample. For a
25 mm (1 in) sample, place a drop of the silver paint on three pads of the
sample holder. Center and align the wafer onto the sample holder. The
amount of silver paint used can be increased or decreased for larger or
