Sample Preparation Procedure for TEM Imaging of 

Semiconductor Materials 

by Wendy L. Sarney 

ARL-TR-3223 June 2004 

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Army Research Laboratory 

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ARL-TR-3223 June 2004 

Sample Preparation Procedure for TEM Imaging of 

Semiconductor Materials 

Wendy L. Sarney 

Sensors and Electron Devices Directorate, ARL 

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Sample Preparation Procedure for TEM Imaging of Semiconductor 

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14. ABSTRACT 

Transmission electron microscopy (TEM) allows detailed materials characterization at high resolutions. Lattice imaging requires

the preparation of electron transparent samples. Preparing such thin samples without changing the characteristics of the material 

is not a trivial process. The Electro-Optics & Photonics Division (EO&P) of the Army Research Laboratory uses TEM for 

structural characterization of MBE-grown semiconductor materials. Major projects that rely on TEM include the following 

structures: type-II superlattices, quantum well and quantum dot superlattices, II-VI films, and GaN-based structures.. TEM 

analysis provides the grower with high and low resolution images of the interfaces, surfaces, and defects. This report discusses 

the typical preparation procedures used to fabricate TEM samples. 

15. SUBJECT TERMS

Transmission electron microscopy, sample preparation. 

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Wendy L. Sarney 

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Contents 

Figure iii

l. Introduction 1

2. Sample Preparation Procedure for Cross-Sectional TEM Samples 2

2.1 Sectioning and Assembling the Sample “Sandwich”......................................................2 

2.2 Polishing the Sample.......................................................................................................4 

2.3 Ion Milling.......................................................................................................................6 

3. Sample Preparation Procedure for Plan-view TEM Samples 8

4. Conclusion 8

5. References 9

Distribution list 10 

Figure 

Figure 1. (a) Wafer before cleaving. (b) Orientation relationships of the two pieces cut from 

the wafer in (a)...........................................................................................................................3 

Figure 2. Schematic of a sample `sandwich.' The films sides of two pieces of the original 

semiconductor wafer are glued together. They are supported by two dummy pieces. The 

dummy pieces are usually scraps remaining from other wafers and are the same material 

as the substrate...........................................................................................................................4 

Figure 3. (a) Sample is mounted onto SEM stub. (b) SEM sub is mounted onto tripod 

polisher. (c) Sample is polished on the polishing wheel............................................................4 

Figure 4. Model 900Tm Grinder/Polisher from South Bay Technology...........................................5 

Figure 5. Orientation relationships of the two pieces in the sample sandwich before (a) and 

after (b) polishing. The dotted line shows the direction that the electron beam will be 

incident on the sample when it is placed in the TEM. (c) The copper grid is mounted onto 

the polished sample....................................................................................................................6 

Figure 6. Gatan pIpSTM ion mill. .....................................................................................................7 

Figure 7. Schematic of sample after ion milling. The area bordering the hole is electron 

transparent. The hole crosses the film/substrate interface twice on each piece (four times 

total| for the sample). .................................................................................................................7 

iii
INTENTIONALLY LEFT BLANK 

iv
l. Introduction 

The quality of TEM images is directly dependent on the sample preparation process, a 

destructive technique that involves cleaving, grinding, polishing, and ion milling the material to 

electron transparency. It is inevitable that the sample preparation process affects the properties of 

the material under investigation. This, in turn, affects and limits our interpretation of TEM 

images. We minimize (but not eliminate) preparation-related artifacts by customizing the process 

for each material. 

The lattice resolution of ARL’s JEOL 2010F microscope operated at 200 keV is approximately 

0.1 nm. Imaging at this resolution requires the TEM samples to be very thin in the region of 

interest. As a minimum requirement, samples must be electron transparent to an extent that the 

electron beam penetrates the specimen and we see intensity on the microscope’s fluorescent 

screen. In practice, TEM specimens should be no thicker than 100 nm for low-resolution 

imaging, and even thinner (~50 nm) for high-resolution imaging. The required sample thinness is 

a function of the electron beam energy and the average atomic number of the sample. Samples 

made of heavy materials need to be thinner than those made of lighter materials. Higher energy 

incident electron beams are able to penetrate though thicker samples than lower energy beams, 

but the likeliness of electron beam damage to the sample increases with increasing beam 

voltages. Some materials commonly used in infrared devices, such as the antimonides and 

HgCdTe, are particularly sensitive to the electron beam. These materials degrade quickly as we 

examine them in the microscope; therefore, we want to minimize the time needed to examine 

them. Having high quality and extremely thin samples minimizes the struggle of obtaining 

meaningful images, and allows us to work more quickly with the TEM. 

The TEM image is a two dimensional projection of a three dimensional sample. This means that 

the image is an average of everything that the electron beam `sees' as it penetrates the sample. 

Averaging leads to difficulties when interpreting TEM images. Therefore, another benefit of 

preparing extremely thin samples is that it minimizes the amount of averaged information in a 

TEM image. 

In this paper, we discuss the preparation of TEM samples from semiconductor wafers grown by 

molecular beam epitaxy (MBE). In the past, we have used this same procedure to prepare 

samples grown by metalorganic chemical vapor deposition (MOCVD) and pulsed laser 

deposition (PLD). The semiconductor samples usually consist of a film grown onto a substrate. 

Typical studies involve examining the substrate/film interface, and identifying, quantifying, and 

characterizing defects. We supplement TEM data with non-destructive characterization obtained 

from x-ray, SEM, AFM, and optical imaging. 

1
2. Sample Preparation Procedure for Cross-Sectional TEM Samples 

2.1 Sectioning and Assembling the Sample “Sandwich” 

We must be mindful of which zone axis we wish to examine in the TEM before we cleave the 

wafer. For Si and III-V substrates the major flat usually indicates the (110) plane. It is worth 

verifying the orientation of the major flat with the grower or the substrate vendor, since wafers 

purchased outside of the United States may not use the same convention. We select the zone axis 

based on which crystal planes we want to image [1]. We typically prefer the [110] zone axis 

because the first order diffraction spots correspond to the relatively easily resolved (002) and 

(111) crystal planes and because the sample readily cleaves along the {110}-type planes. 

Occasionally we examine the [1001 zone axis when we are studying a superlattice structure that 

does not yield sufficient image contrast. Examining this zone axis results in an image of the 

(002) and (020) planes. Dynamical theory suggests that imaging along the [100] zone axis allows 

higher contrast imaging than obtained along the [110] axis [2]. This is because the {002}-type 

diffracted beams are more sensitive to the chemical composition of the sample than the (111) 

beams. One example of an ARL-grown system requiring examination along the [100] zone axis 

is the AlGaAs/GaAs triple quantum well superlattice structures grown for reflection modulator 

applications [3–4]. 

It is important to wear gloves and sometimes a dust mask during the sample preparation process. 

Scribing and cutting the wafers can create dust and some samples leave a chalky residue when 

polished. 

We begin the sample preparation process by cutting two rectangular (approximately 1.5 mm by 4 

mm) pieces of the wafer with a diamond scribe. Figure 1 schematically demonstrates the 

cleaving process for a [110] zone axis wafer having a [001] growth direction. We cleave the 

rectangular pieces so that the longer dimension of one of the pieces is parallel to the major flat 

and the longer dimension of the other piece is perpendicular to the major flat. This results in one 

piece having a zone axis along [110] and the other having its zone axis along [1 10]. This allows 

a more thorough characterization of the specimen, especially in the cases of certain orientationdependent structures such as quantum wires. To examine a sample with the [100] zone axis, we 

would cleave the sample 45° away from the major flat. Occasionally we cut the wafer so that one 

piece has a [110] zone axis and the other piece has a [100] zone axis. 

2
Figure 1. (a) Wafer before cleaving. (b) Orientation relationships of the two pieces cut from the wafer in 

(a). 

After cutting the pieces, we glue the film sides together with epoxy. Usually the film side is 

shiny compared with the substrate side but occasionally a grower will use double-side polished 

substrates. In this case, we lightly scratch the substrate side with our diamond scribe. Currently 

we are gluing the samples with MBond, an epoxy based on bisphenol A. Periodically we must 

change brands of epoxy due to the manufacturer ‘improving’ the strength of the epoxy. Keep in 

mind that epoxy is not designed or marketed for the purposes that we use it, and for most 

consumers, increased strength is a desired feature! Epoxy that is too strong will not ion mill 

properly, and leave an undesired ‘glue line’ between the two pieces after milling. Bits of the 

sample’s surface will tend to stick to the glue line, and these regions are typically too thick along 

the beam direction. Epoxy that is too weak will ion mill quickly and leave the sample’s surface 

unprotected. When we image the film, we usually pick an area that still has some glue visible on 

the surface. This assures us that the surface is ‘intact’ and that the film is true to its original 

thickness. Finding proper epoxy is not a trivial issue. 

After gluing the two film sides together, we attach a dummy piece to the backside of each piece. 

This provides more support and makes handling the sample with tweezers much easier. Figure 2 

is a schematic of the sample ‘sandwich.’ The dummy pieces are usually the same material as the 

substrate, or at least a material of similar hardness. We place the sample into a small vice and 

cure it on a hot plate (at approximately 60 °C) for one hour. We then remove the sample from the 

hot plate (but keep it in the vice) and allow it to cool for another hour. For particularly delicate 

and sensitive samples, such as those containing HgCdTe, we place the vice on the 65 °C hot 

plate but immediately turn the heat off. We leave the vice on the plate until the plate reaches 

room temperature. We then remove the vice from the hot plate and let the epoxy cure for at least 

four hours. 

3
Figure 2. Schematic of a sample ‘sandwich.’ The films sides of two 

pieces of the original semiconductor wafer are glued together. They are 

supported by two dummy pieces. The dummy pieces are usually scraps 

remaining from other wafers and are the same material as the substrate. 

2.2 Polishing the Sample 

We mount the sample sandwich onto an SEM stub with acetone-dissolvable wax as shown in 

Figure 3a. The SEM stub is mounted onto a Model 590 Tripod Polisher© from South Bay 

Technology (Figure 3b). The sample is planarized by adjusting the three micrometers. We hold 

the tripod polisher and polish the sample (Figure 3c) with a Model 900 Grinder/Polisher from 

South Bay Technology (Figure 4). The Model 900 has an 8” polishing wheel upon which we 

place polishing disks and papers. The speed of the wheel can be varied from 0 to 1725 RPM. A 

constant stream of water drips onto the polishing surface whenever the wheel is rotating. Water 

softens the contact between the sample and the polishing medium, it keeps the sample from 

heating during the polishing process, and it traps the small particulates as they polish away from 

the sample (keeping them from becoming airborne). For environmental safety reasons, our water 

supply is part of a custom-built closed pumping system. As shown in Figure 4, the 

Grinder/Polisher machine is inside a custom-built hood, which protects the engineer from any 

gases that may be emitted from the sample as it is polished (such as phosphine gas from InP 

substrates). 

Figure 3. (a) Sample is mounted onto SEM stub. (b) SEM sub is mounted onto tripod polisher. 

(c) Sample is polished on the polishing wheel. 

4
Figure 4. Model 900TM Grinder/Polisher 

from South Bay Technology. 

The hardness and fragility of the sample dictates which polishing medium we use. For hard 

samples, such as GaN, we begin polishing one side of the sample with a 30 ? m metal bonded 

diamond grinding disk. We move through a series of progressively smoother diamond discs and 

then diamond papers until a shiny, scratch-free mirror-like surface appears. We then remove the 

sample from the SEM stub (by melting it off on the hot plate) and re-mount it on its opposite 

side. The sample is polished to a thickness of approximately 20 ? m with a series of 

progressively smoother diamond disks and papers. For softer samples, such as those based on 

GaSb, we use the same procedure, except that we begin polishing with 15 ? m aluminum oxide 

paper and move to progressively smoother papers. Figure 5 shows the orientation relationships 

of the sample sandwich during the thinning process. When the sample is sufficiently thin, we 

mount a copper grid (having an outer diameter of 3 mm) mounted onto it with Ml3ond (Figure 

5c). The sample (still mounted on the SEM stub) is cured on the hot plate for 5 minutes and then 

let sit for another hour at room temperature. We then soak the SEM stub in acetone until the wax 

dissolves and the sample falls off the stub. At this point, we must be handle the sample with 

extreme care since it is very thin and supported only by the Cu grid. 

5
Figure 5. Orientation relationships of the two pieces in the sample sandwich before (a) and after 

(b) polishing. The dotted line shows the direction that the electron beam will be incident on the sample 

when it is placed in the TEM. (c) The copper grid is mounted onto the polished sample. 

2.3 Ion Milling 

We use a Model 691 Gatan PIPSTM (Precision Ion Polishing System) ion mill for final thinning 

to electron transparency. The PIPS (Figure 6) uses two Penning ion guns to produce a stable 

Argon ion beam. The system has a liquid nitrogen trap, which reduces contaminants and water 

vapor. Ion beam damage may affect the sample’s crystalline morphology (making it appear 

amorphous) or introduce surface oxides. Therefore, selecting an appropriate beam voltage is 

critical. Using too low of a beam voltage lengthens the milling time, and increases the likelihood 

of contamination. Using too high of a beam voltage can cause structural damage to the sample. 

Because GaSb and HgCdTe-based samples are very beam sensitive, we tend to mill these 

samples at no higher than 3 keV. At the other extreme, GaN samples are very beam resilient and 

are usually milled at 6eV. We usually position the ion guns so that they are milling on opposing 

sides of the sample. Since the milling rates of different phases or crystal directions can vary 

drastically, it is difficult to obtain a cross-sectional sample that is equally electron-transparent 

across multiple layers. Using low milling angles minimizes differential thinning effects (the slow 

milling regions protect the fast milling regions from the ion beam). The PIPS allows samples to 

be milled at angles ranging from 1 to 10°. Rotating the sample during the milling process further 

minimizes preferential thinning. We align the guns so that the ion beams are incident on the 

center of the specimen. It is important that the thinnest area of the sample after milling be located 

as near to the center of the grid's aperture as possible. This allows the area of interest to stay at 

the same height above the TEM’s objective lens (z-height) during tilting. We monitor the ion 

6
milling process with a CCD zoom camera and video display. Thinning continues until a small 

hole perforates both pieces of the sample in the center. The final TEM sample then has four 

useful thin areas to examine-two on each side of the ion-milled hole (Figure 7). 

Figure 6. Gatan PIPSTM ion mill. 

Figure 7. Schematic of sample after ion milling. The area 

bordering the hole is electron transparent. The hole crosses the 

film/substrate interface twice on each piece (four times total| 

for the sample). 

7
3. Sample Preparation Procedure for Plan-view TEM Samples 

Plan-view samples are prepared when we want the TEM’s electron beam to penetrate the sample 

along the growth direction. We usually examine samples in plan-view when we need to calculate 

the misfit dislocation density of the film. We begin the procedure for preparing plan-view 

samples by cutting one 2 cm by 4 cm rectangle from the wafer. The sample is mounted film side 

down onto an SEM stub. We polish the substrate side of the sample as described in the previous 

section until the total samples thickness is 20 ? m or less. A copper grid is mounted to the 

substrate side of the sample. The sample is ion milled with both guns configured to mill from the 

substrate side only. 

4. Conclusion 

To take advantage of a TEM’s high resolution, we must prepare high quality electron transparent 

samples. Although there is a wide array of known procedures used in research and commercial 

laboratories, sample preparation is really somewhat of an art. The best procedure is the one that 

the individual scientist can perform repeatably and confidently. The reward for the time invested 

in developing reliable sample preparation techniques is the acquisition of beautiful TEM images 

that provide a wealth of information for our MBE growers. 

8
5. References 

[1] Sarney, W. L., Understanding Transmission Electron Microscopy Diffraction Patterns 

Obtained from Infrared Semiconductor Materials, ARL-TR-3128, (December 2003). 

[2] Suzuki, Y. and H. Okamoto, Transmission electron microscope observation of lattice image 

of A1XGa1–X,As-AlYGa1–yAs superlattices with high contrast. J. Appl. Phys, 1985, 58 (9), 

pp 3456-3462. 

[3] Tobin, M. S. and J. D. Bruno, Quantum-confined Stark effect modulator based on multiple 

triple-quantum wells. J. Applied Phys, 2001, 89 (3), pp 1885-9. 

[4] Sarney W. L., L. Salamanca-Riba, M. S. Tobin, and J. D. Bruno, Effect of Interface 

Roughness and Growth Temperature on the Absorption Spectrum of 810 nm Reflection 

Modulator Active Layers. Materials Research Society Fall Meeting, Boston, MA, December 

1998. 

9
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