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Preparation of Electron Transparent Samples for Transmission

Electron Imaging

Michael Berkson, Katayun Barmak, Xiwen Chen

Columbia University

The size effect in Cu has prompted interest in alternate metals for sub-30nm linewidths.

The criteria for alternate metals should include: (a) low resistivity, (b) minimal resistivity

size effect, (c) elimination of diffusion barrier, and (d) high melting temperature.

Electroless-deposited Ni, Co, Ru, and Pt have been identified as candidates for further

investigation. Exploratory characterization of electroless-deposited metal films using

transmission electron imaging and diffraction methods will be conducted. Comparing these

results with results for physically deposited films will help guide interconnect metal

process development. In order to image and analyzing materials using TEM, samples must

be thin enough to be electron transparent. In this project, a method for preparing electron-

transparent solid crystalline TEM samples is developed and practiced.

Introduction

The electron diffraction based metrology me-

thods developed by Profs. Barmak and Coffey

make use of crystal orientation maps obtained by

precession electron diffraction (PED) in the

transmission electron microscope (TEM) using

the ASTAR™ system. The orientation maps can

be used to obtain:

1. Average grain size and grain size

distribution.

2. Orientation distribution (OD), including

fiber fractions, i.e., fraction of grains

orientated with a give crystal axis

parallel to the film normal).

3. Misorientation distribution (MD), i.e.,

fraction of boundaries with a given

misorientation across the boundary.

4. Grain boundary character distribution

(GBCD), i.e., length fraction (in 2D) or

area fraction (in 3D) of boundaries with

a given misorientation and boundary

normal.

The size effect in Cu has prompted interest in

alternate metals for sub-30nm linewidths. The

criteria for alternate metals should include:

1. Low resistivity,

2. Minimal resistivity size effect,

3. Elimination of diffusion barrier, and

4. High melting temperature.

Electroless-deposited Ni, Co, Ru, and Pt have

been identified as candidates for further

investigation. Exploratory characterization of

electroless-deposited metal films using trans-

mission electron imaging and diffraction methods

will be conducted. Comparing these results with

results for physically deposited films will help

guide interconnect metal process development.

The anticipated results are:

Microstructural characterization of

electroless deposited metal films

provided by Lam Corporation using

transmission electron imaging and

diffraction methods.

Comparison of the results with results

for physically vapor deposited (PVD)

films

In order for us to use TEM to characterize these

films, which are deposited on Si wafers hundreds

of microns thick, we must thin our samples

enough to make them electron transparent. This

is a necessary step to obtain ASTAR™ images

from TEM for analysis with ASTAR™.

However, usually only a paragraph of such a

publication is devoted to the TEM sample

preparation procedure. In this report, we will

describe in more detail the process we used to

prepare the deposited thin-film samples for

TEM.

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Detailed TEM sample preparation procedures

have been published by Pakzad [1], Yao [2], and

others. Our approach will closely follow [1],

which consists of several mechanical thinning

methods and no chemical etching.

Experimental Methods

This TEM sample preparation procedure was

developed and practiced on 3-inch Si wafers with

Fe80Ni20 film deposited via sputtering.

Cutting

Si wafers with deposited thin films are cut face-

down with a diamond scribe using a glass slide

as a straightedge. The wafer is braced against a

ruler and cut into 1.5mm-square pieces.

Mounting

A square sample is mounted to a circular Cu grid

of 3mm in diameter and 1mm diameter central

circular opening using M-bond epoxy, with the

thin film facing the grid. This sample-grid

assembly is then fixed with wax to a Pyrex glass

stub whose size is compatible with the grinding

tools described below.

Grinding and Polishing

Mounted samples were ground using a South

Bay Technology Model 900 Grinder/Polisher lap

wheel (Figure 1) and Buehler SiC grinding

papers with particle sizes denoted 800 grit

(8.4µm) and 1200 grit (6.5µm). Samples were

ground to a target silicon thickness of 90µm.

Figure 1. South Bay Technology lap wheel, used for

grinding and polishing.

Samples were then polished with circular cloth

compatible with the lap wheel and 1µm diamond

paste manufactured by Buehler.

During the grinding and polishing process, stubs

containing the samples were held in place by a

tripod (Figure 2).

Figure 2. Tripod used with lap wheel.

Dimple Grinding

Samples were then thinned to a target silicon

thickness of 7µm using a Gatan Model 656

Dimple Grinder (Figure 3). This device grinds a

crater into the sample to thin it further while

preserving its structural integrity to prevent

fracture. The dimple grinder uses a bronze

grinding wheel with 1µm diamond paste as an

abrasive, followed by a felt wheel with colloidal

silica suspension.

Figure 3. Dimple grinder.

Ion Milling

Finally, samples are thinned to under 100nm in

the center using the Gatan PIPS II ion milling

machine, which mills the sample by bombarding

it with ionized argon gas. Since we have no way

of measuring sample thickness directly during

this step, our criterion for sufficient thinness is

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the appearance of an optical interference pattern,

shown in Figure 4.

Figure 4. Optical micrograph showing diffraction pattern

on ion-milled sample.

Results and Discussion

Originally, mounting the grid on the sample was

done between dimple grinding and ion milling.

Mounting at that point in the process proved

difficult because epoxy was too often smeared

onto the middle of the sample, interfering with

electron transparency, or mounting wax got in

the way and prevented the epoxy from bonding

the grid and the sample together.

By mounting the sample on the grid first, a

change borrowed from the doctoral thesis of G.

Lucadamo [3], the sample could maintain its

structural integrity throughout the process with

the additional support from the grid, and the

difficult mounting step later in the procedure was

eliminated.

During the time when the original procedure was

conducted, we attempted to prepare 8 plan-view

samples, none of which made it past the post-

dimple grinding grid-mounting step. With the

procedure described in this report, we prepared 5

plan-view samples in 6 attempts, and the one we

examined using TEM was sufficiently electron

transparent to produce an image (Figure 5).

The Moire patterns seem in the image of Fig. 5

suggest that the film has more than one layer of

grains through the thickness. We attribute this

grain structure to the sample itself and not to the

preparation procedure.

Figure 5. Transmission electron micrograph (top) and

electron diffraction pattern (bottom) for plan-view

permalloy thin film sample.

Conclusions

The sample preparation procedure described in

this report, which involves mounting the sample

on a Cu grid before grinding and polishing, rather

than after dimple grinding, is an effective method

for preparing thin film samples for TEM. The

two-layer grain structure in the sample examined

is more likely to be a product of the sputtering

process than of the TEM preparation process.

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References

1. Pakzad, A.; Granz, S.; Wise, A. Ready, set,

go! Ensuring an identical TEM sample

preparation route again and again. Gatan.

http://www.gatan.com/ready-set-go

-ensuring-identical-tem-specimen

-preparation-route-again-and-again

(accessed June 2, 2016).

2. Yao, B.; Petrova, R.; Vanfleet, R.; Coffey, K.

J. Electron Microsc. 2006, 55(4), 209–214.

3. Lucadamo, G. Ph.D. Dissertation, Lehigh

University, 2000.

Acknowledgments

Funds for this research were provided by the

Materials Research Science and Engineering

Center (MRSEC) and the NSF Engineering

Research Center grant DMR-1420634. Thank

you to Prof. Katayun Barmak for your guidance

throughout the project and for the language in the

introduction, and to Xiwen Chen for your help

with the sample preparation procedure as well as

several of the figures. And thank you to Jiaxing

Liu for the thin film samples.

Michael Berkson is a senior at

Columbia University studying

Materials Science. He is

continuing his studies next year

in the MS program in Materials

Science at Columbia, and he hopes to pursue a

multidisciplinary engineering career.