tem sample prep

7/29/2019 TEM sample prep

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TEM Sample Preparation for Beginners:

Dimple Grinding

Dimple grinding is a sample preparation method best suited for brittle materials such as

minerals, ceramics and semiconductors. Ductile metals may be dimpled if special techniques areused. See the Gatan circular Dimpling Ductile Materials at www.Gatan.com.

Sectioning the sample

The first step in sample preparation is to section the material into thin wafers, approximately

0.5mm (or less) in thickness. Use the Struers Accutom high speed wafering saw or the BuehlerIsomet low speed saw. Select the proper wafering blade appropriate to your material and for thesaw you will be using.

This step will determine the orientation of the sample in the microscope, so if crystallographic

orientation is important, position the material carefully in the saw mount. The saw cuts will be

perpendicular to the axis of the electron beam in the final sample.

Cutting 3 mm Disc

The next step is to cut 3 mm discs from the thin wafer. Affix your sample material to thealuminum mounting slide. If you will be dimpling one side only, affix the good side down.

Alternately, you may place with the good side up and affix a clear glass cover-slip on top.

These steps insure that the surface of interest is undamaged during cutting.

To affix with low-temperature melting point wax, place the slide on the hot plate set to ~70 C.

Apply a small amount of the crystal bond wax and allow to melt. Remove the slide from the

hot plate and immediately place the sample onto the melted wax.

If your sample is heat sensitive, affix it to the slide with a cyanoacrylate glue (e.g. SuperGlue or Crazy Glue.) This adhesive is easily dissolved in acetone.

Position the sample/Al slide onto the magnetic base

of the ultrasonic cutting tool. Align the area of

interest of the sample under the cutting tool. Place a

small amount of cutting medium onto the sample.Lower the cutting tool to the sample surface. Use

syringe to wet the powder. Begin the cutting.

The image at right shows the result of two cuts withthe ultrasonic tool. The left example is incompletely

cut; the right is cut through to the Al substrate.

Remove the Al slide from the magnetic base andrelease the sample disc by heating on the hot plate.


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Dimpling First Side

Bulk samples should be dimpled from both sides. Samples with the region of interest on one

surface, (e.g. thin film on substrate) should be dimpled only from the back side. Modify the

following steps appropriately if dimpling only one side.

Measure the initial thickness of the sample and plan the next steps before beginning to dimple.

Affix the 3mm disc onto the center of the dimpling stub, as infigure to the left, with melted wax or cyanoacrylate glue. Position

the stub into the positioning ring on the Gatan Dimple Grinder.

Turn on the Table rotation and observe the rotation of the sample

with the small optical microscope. The cross-hairs indicate wherethe dimple center will be located. Move the positioning ring so that

the region of interest is at the cross-hair intersection. It is not

necessary that the dimple be centered at the exact center of the

disc, however getting it close to the center gives the best result.

The cartoon below shows the concept of double-sided dimpling: Remove material from first side

until the dimple bottom is approximately in the center of the original disc. Turn over, and remove

material from second side until final thickness is achieved.

Judging the amount of material removed as you progress is important. Monitor the dimple depth

carefully using the micron scale on the dimpler. Stop dimpling frequently and examine the

sample with the small microscope attachment, or remove the stub and observe the sample under

a good optical microscope. It is possible to accurately measure the thickness remaining by

focusing on the dimple bottom, then on the dimpling stub and noting the change in the

gradations on the focusing knob.

Use the brass dimpling wheel to start. Mix the abrasive paste with a small amount of water ordiamond paste extender. Use progressively smaller paste to remove calculated thicknesses ofmaterial. Be sure to calculate the total thickness you wish to remove before starting, and adjust

the removal amount accordingly. Remember, best results are obtained when a depth equal to

three times the paste grit size is removed by the next smaller paste.

If dimpling only one side, continue instructions atCompleting the Dimple.

remove

First side

remove

Second side


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Finish first-side dimpling by changing to the felt-

covered wheel, and polish the sample with 3m pastefollowed by 1 m paste until it has a scratch-free

mirror finish. The figure to the right shows what thislooks like on a Silicon sample. The surface color will

vary depending on the material.

Dimpling Second Side

Remove the dimpled disc from the dimpling stub by

heating, or by soaking in acetone. Handle the polished

disc with care!

Measure the thickness of the disc, and calculate the exact amount of material that needs to be

removed from the second side.Note that when using the 15 mm diameter dimpling wheel, the

depth of a dimple that is 3mm in diameter will always be approximately 150 m. Be sure thisvalue figures into your calculations.

Clean wax from stub and melt a fresh piece of wax. Position the sample on to the center of the

stub, dimpled side down. Be sure it is seated flat on the stub with a minimum of adhesive.

Dimple the second side with progressively smaller grit paste using the brass dimpling wheel. Be

sure to complete all the steps in thinning the sample, using progressively smaller diamond paste.

Do not skip any paste size. This is essential in order to remove the scratches of the previous paste

size. Be sure to leave enough material so that you can finish with the felt-covered wheel. Ascratch-free, mirror-like surface is necessary!

Completing the Dimple

The final stages of dimpling are critical to producing a successful sample. Work slowly and

carefully! It is essential that the last polishing step, which produces a scratch-free, mirror finish,

be completed when the sample is at the final thickness desired.

Use the felt-covered wheel and 3 m paste, followed by 1 m paste, to remove the last ~10 m

of material (i.e. when the total thickness remaining at dimple bottom is ~20 m.)

The finished disc with highly polished dimpled surfaces on both sides should ideally be

approximately 300 m in thickness at the edge, and less than 10 m thickness in the center.Thefigure below is a rough sketch of the completed disc in cross-section. (A single-side dimpled

sample should be ~150 m thick at the edge.)

300 m

10 m


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Determining the thickness of material remaining while dimpling the second-side requires careful

measurement, a thorough understanding of the tools, lots of skill, and a bit of luck. If the dimple

is too deep and produces a hole through the sample, it is (99% of the time) useless to continue.

Start over with a new disc. If the dimple is not deep enough, ion-milling time will be excessively

long and this will produce undesired artifacts in the sample.

Many materials will transmit light when thin enough. Silicon is most useful in this regard. It

transmits light at specific wavelengths in relation to material thickness. For example, deep red

color indicates a thickness of ~15 m, bright red ~10 m, orange ~ 8 m, yellow ~5 m.

Practice and experience are key. There is more art than science in TEM sample preparation!

When the second-side dimple is competed, remove the sample from the stub by soaking in

acetone (in a covered container!). Do not attempt to remove the sample by heating: The

possibility of damaging the sample is too great! Soak the sample in clean acetone after removal

to thoroughly remove the wax or glue. Handle the dimpled sample very carefully! It can easilybe damaged, ruining many hours of work!

With practice, it is possible to produce samples dimpled to less than 5 m, which require only a

few minutes in the PIPS ion mill.

Ion Milling

The final step in TEM sample preparation is ion-beam milling in the Gatan PIPS (Precision Ion

Polishing System) or the Gatan Duomill. The Duomill uses low flux and high angle for long-term

milling (several hours). It is equipped with a LN2 cooling stage to prevent sample damage from

heating if needed. The PIPS uses higher flux and lower angles, and is designed for short-term

milling. If ion milling takes longer than 1 hour in the PIPS, the sample was not dimpled thinenough.

Follow the User Guide for the ion mill. If the sample is sensitive to heat, use the cooling stage on

the Duomill or low keV on the PIPS.

Take note that the geometry of a dimpled sample limits the milling angle to greaterthan 6, asthe cartoon below indicates. Be aware that even at this minimum angle, material from the

sample edge may be milled and re-deposited onto the center of the dimple.

3 mm

5.70.150 mm


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Monitor the milling progress periodically. On the Duomill, raise the sample and view with a

magnifier or remove the sample and observe under a good quality optical microscope. On thePIPS, use the mounted camera to observe milling progress. This can be done while milling by

retracting the milling chamber shutter.

Use transmitted light to determine when a hole has been formed in opaque samples. Good TEMviewing can often be obtained at the edge of a small hole.

Examine the sample with reflected light to monitor the surface roughness, and to observe

Newtons Rings.

Reflected light on a highly polished, curved surface

will often produce Newton Rings if the material is

thin enough. These are colored concentric rings that

arise from interference of reflected and transmitted

waves. In ideal situations, these rings can give

accurate indication of sample thickness (see example

in figure to left.) They usually appear only when the

sample is

tem sample prep

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