Specimen Preparation Technique for a Microstructure Analysis
Using the Focused Ion Beam Process
by Kozue Yabusaki * and Hirokazu Sasaki *In recent years the FIB technique has been widely used for specimen preparationin TEM observation and AES analysis, increasing demand for micro-analysis ofsemiconductor devices. Compared to the conventional process consisting of mechanical polishing and Ar ion milling, the FIB technique has several advantages: rapid process, preventing damage caused by mechanical polishing, and cutting out microstructure parts. However, there arevarious problems such as detection of large amounts of Ga from the ion source and formation ofa damaged layer on the surface. We have developed FIB specimen preparation techniques toobtain valuable data for TEM and AES in consideration of these features of FIB processing.

ABSTRACT1.

INTRODUCTION

As electro-optical semiconductor devices become smaller,a microstructure analysis technique is required to analyze
defects and control the production process. Transmission
electron microscopy (TEM) and auger electron spectroscopy (AES) are analytical techniques used to evaluate
microstructures.
Recently, the focused ion beam (FIB) technique has
been used as a method of fabricating a specimen for TEM
observation, besides the conventional ion milling method.
This technique is indispensable for analyzing semiconductor devices, because cross-sectional TEM observation at
any specified location is possible using the microsampling
method 1), which enables specific parts to be cut out from abulk sample. FIB is also an effective tool for fabricating
samples for AES analysis 2). This paper presents a FIB
specimen preparation technique for TEM observation and
AES analysis.

2.

FIB PREPARATION FOR TEM OBSERVATION

2.1

FIB Process Damage and Problem of Sample
Form
The FIB process, in which a convergent gallium ion beam
is irradiated onto a sample and a specific part is madethinner, has the problem that damage caused is greatercompared to the conventional ion milling method. There
are two types of process-damage in compound semiconductors such as GaAs and InP, the first is the formation of
amorphous layers that contain implanted gallium atoms,

the second is the precipitation of particles. In particular, in
TEM observation, precipitation produces a macula contrast, which greatly influences the image observed. The
level of damage varies depending on the material, forexample, for InP it is remarkably high. It also depends on
acceleration voltage, incident angle, and temperaturewhen fabricating. Damaged layers are created on theprocess surface. For these reasons, it seems that the ion
milling method using a low acceleration voltage and a low
incident angle at a low temperature is effective for reducing the damaged layer.
In conventional FIB preparations, a metal plate from
30 µm to 50 µm in thickness for TEM observation is used
to fix samples. A sample is polished to the proper size by
hand or picked up using a microsampling method, then it
is mounted on a cross-section of Cu plate, and only thepart required for observation is thinly fabricated up to afew hundred nanometers in thickness. To remove thedamaged layer produced during the FIB process, it is necessary to irradiate an argon ion beam at a low angle fromboth sides of the sample surface. However, in the generalprocess, the feature of the sample surface is concave,hence the Cu plate and the unprocessed part of the sample become walls and prevent the argon ion beam fromreaching the processed surface (Figure 1). Furthermore,
there is the problem that materials, which were sputteredfrom the walls, adhere again and contaminate theprocessed surface. Consequently, it is impossible toremove the damaged layer by the conventional sample
preparation method.

* Analytical Technology Center, Yokohama R&D Lab.

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2.2

FIB Sample Preparation Method with Little
Damage2.2.1 Cu Half Plate with Ultra-thin Aluminum Foil andMicrosampling Method
We developed a new specimen preparation techniqueusing the microsampling method and a Cu half plate withultra-thin aluminum foil to overcome the problem of specimen form, and we made it possible to fabricate a FIB
sample with little damage using cold ion milling. The FIB
process conditions are shown below.
FIB: Hitachi FB2000A
Acceleration voltage: 30 kV
Ion beam: Ga+
To irradiate an Ar ion beam onto the processed surface
and prevent redeposition, it is necessary to make a thin
Cu half plate and reduce concavity and convexity near theprocessed surface. To fabricate the thin stand, an ultrathin aluminum foil of 2.5 µm in thickness is attached to thehole of the Cu half plate using epoxy resin. The part of thesample of the same thickness as the aluminum foil was
picked up by the microsampling method, and both the left

Ar+
2 kV

SampleAr+
2 kV

SampleAr+
2 kV

2.2.2 Cooling Ion Milling
After sample preparation using the FIB method, the sample was milled by cooling ion milling (Figure 2-4). The
milling conditions are shown below .
Ion milling: GATAN600N
Acceleration voltage: 2 kV
Incident angle: 15 degrees
Ion beam: Ar+
Ion milling time: 5 min.
Specimen temperature: 77 K
2.3

TEM Observation and Evaluation of Damaged
Layers
2.3.1 TEM Observation
To confirm the effects of removing damaged layers bycooling ion milling, an observation was made using a highresolution transmission electron microscope (HRTEM).
The sample before ion milling was observed for purposesof comparison. The observation condition is shown below.
TEM: Hitachi H9000UHR
Acceleration voltage: 300 kV
Sample: InGaAsP/InP MQW

Ar+
2 kV

Half plate with single hole

Figure 1

and right sides of the sample were fixed on the cross-section of the aluminum foil using tungsten deposition. Not
only the observed area but also the aluminum foil were
processed to be thinner using the FIB method. Using thismethod, the sample became flat, and surroundings of thefabricated part were opened, therefore, an Ar ion beamirradiated at a low angle could reach the processed surface from all directions (Figure 2-1, -2, -3).

Half plate

FIB-TEM Sample.

(1) Picking up(2) Joint
Micro probe

Micro probe
W deposition

W deposition

Bulk sample

Epoxy glueThin Al foil
Half plate with single hole(3) Thinning(4) Cleaning (Ion milling)

30 kV
Ga+
SampleDeposited W
2.5 µ mAr+
2 kV

Thin Al foil
Ar+
2 kV

Figure 22

FIB sample preparation method for TEM observation with little damage.

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2002Photo 1

Photo 2

Dark field image of InGaAsP/InP MQW.

Lattice image of InGaAsP/InP MQW.

Photos 1 and 2 show cross-sectional TEM images(dark-field image and lattice image) of InGaAsP/InP
MQW. In sample A before removing the damaged layers,observation was made of contrast and Moiré fringe, whichappear when crystals with a different orientation or spacing of lattice planes overlap. In addition, the lattice cannot
be observed clearly due to the thin damaged layers
caused by gallium implantation. The influence of damage
to the sample caused only by FIB processing is remarkable. In contrast, in sample B after removing the damaged
layers by ion milling, the lattice is observed clearly without
contrast and Moiré fringe. These investigations demonstrate that the damaged layers on the processed surface
are removed to obtain valuable data for TEM observationby cooling ion milling.

2.3.2 Evaluation of Damaged Layers
We evaluated the damaged layers. Two types of sample
were prepared. In the first sample, damaged layers wereformed by FIB fabrication. In the second sample, damaged layers were formed, and then removed by coolingion milling. Figure 3 shows the sample preparation techniques. Part of bulk sample was fabricated by FIB and thedamaged layers were formed. The damaged layer of thesecond sample was then removed by cooling ion milling.
Processed surfaces of both samples were coated with ametal layer to prevent re-damage due to FIB fabrication.
The samples were picked up using the micro-sampling
method, and fabricated using the method shown in Figure
2. Cross-sectional TEM images of these samples areshown in Photo 3. In the FIB fabricated sample (photo-3(1) FIB milling(2) Cleaning
30 kV
Ga+

Bulk sample

Ar+
2 kV

(3) Picking up(4) Thinning and cleaning

Metal coating

Ar+
2 kV

Thin Al foil

Figure 3

Damaged layer

Ar+
2 kV

Damaged layer evaluation sample preparation method.

that the thickness of the amorphous damaged layer isapproximately 1.5 nm using cooling ion milling (photograph below). It is understood that most of the damaged
layers formed by FIB processing are removed by coolingion milling.

Photo 3

Cross-sectional TEM image of damaged layer.

graph above), the amorphous-like nano-crystalline damaged layer of more than 20 nm thick is formed on theprocessed surface. Contrary to this, it should be noted4

2.3.3 Summary
We developed specimen preparation FIB techniques
using cooling ion milling to reduce damaged layers. In theconventional FIB technique, damaged layers could not be
removed because the specimen's shape prevents theargon ion beam from reaching the processed surface.
This problem was solved using a new specimen preparation technique with Cu half plate, ultra-thin aluminum, andmicro-sampling method, which allows most of the damaged layers to be removed. High-resolution TEM images
were obtained. As a result of an evaluation of thickness ofdamaged layers, it is clear that a damaged layer of morethan 20 nm in thickness is formed on the processed surface. This indicates that, if a thin film sample of 100 nm in

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(2) Cleaning (Ar+ Sputter)(1) FIB Process
30 kV
Ga+

Surface
↑

4 kV
Ar+

Electrode↓
GaAs

Sample2 µm

Figure 4

FIB sample preparation for AES analysis.
Photo 4thickness is fabricated using a conventional FIB sample
preparation technique, about half of the thin film is a damaged layer.
Although only an InP system device was studied in thispaper, it is expected that similar effects are achieved withother materials.

3.

FIB PROCESS TECHNIQUE FOR AESANALYSIS

Sputter depth profiling using AES is an effective techniquewhen multilayer films consisting of layers each with athickness of several tens of nanometers are evaluated.
Since sputter depth profiling outputs one-dimensional data
only, however, it is impossible to execute a precise analysis of complicated samples, such as particles in multilayer
films, which require two- or three-dimensional analysis.
Although there have been direct analysis techniques ofa cross-section exposed by mechanical polishing or crater
edge, it is difficult to expose specified sections using thesetechniques. The FIB technique then makes it possible tofabricate and expose the specified cross-section to beobserved. In addition, several cross-sections from one
sample can be fabricated. More information can beobtained from the exposed section by combining qualitative analysis with line analysis and elemental maps using
AES.
3.1 FIB Process
If the thickness of one layer is several tens of nanometers,
it is difficult to analyze clearly the cross-sections exposed
vertically by the FIB process because the AES spatial resolution is about 15 nm. The thickness of layers are
increased by fabricating diagonally with FIB as shown in
Figure 4. For instance, when a Ga ion beam is irradiatedfrom the vertical direction of 80 degrees, the exposed section is increased about six times, and can be analyzed
using AES.
Because the sample stage in the FIB chamber could be
inclined only up to 60 degrees, a sample was fixed to thesample stand with a slope of 45 degrees so that the sample could be inclined by more than 60 degrees.
The surface layer is cut down through the flare of the
Ga ion beam due to current density distribution in the ion
beam. To avoid this problem, we coated Pt-Pd or carbon
layer on the surface of the sample before the FIB process.

Cross-sectional SEM images of electrode after FIB
processing and ion sputtering.

The FIB process conditions are shown below.
FIB: Hitachi FB2000A
Acceleration voltage: 30 kV
Ion beam: Ga+
Diagonal processing: 80 degrees
Coating: Pt-Pd
3.2 Removing Damaged Layers
A damaged layer is formed on the sample surface after
the FIB process as shown by TEM observation. Ga is
detected on the cross-section in the AES analysis after
the FIB process because the damaged layer is implanted
Ga ion. Then the cross-section is sputter-cleaned using an
Ar ion beam in the AES chamber to remove the damaged
layer with Ga. Steps are formed because sputter rates
depend on the material of each layer, thus SEM images
become clear and specification of analysis area is facilitated. The sputtering conditions are shown below.
AES: PHI670
Acceleration voltage: 4 kV
Ion beam: Ar+
Sputtering time: 3 min.
3.3 Results
The Au/Ge/Ni multilayer film for the electrode, which isdeposited on a GaAs substrate, is processed by the procedure mentioned above, and is inclined in the AES
chamber so that the cross-section is vertical for the electron beam. The AES analysis conditions are shown below.
AES: PHI670
Acceleration voltage: 20 kV
Sample current: 10 nA
Sample: AuGeNi electrode.
A SEM image of the electrode deposited on a GaAs
substrate after FIB process is shown in Photo 4. This electrode with an AuGeNi layer is given an ohmic property byannealing. Intermetallic compounds are seen inmetal/semiconductor interface and electrode. The rougharea at the upper part of the SEM image is the sample
surface. Photo 5 shows the SEM image and results of elemental mapping of Au, Ge, and Ni in part of AuGeNi/GaAs
interface. Diffusion of Au and behavior of Ge and Ni in the5

Au
Barrier layer
AuGeNi

GaAs
500 nmA

D

C

Photo 5

BA) SEM image of electrode/semiconductor interface.
B) Au elemental map. C) Ni elemental map. D) Ge elemental map.

metal/semiconductor interface could be observed clearly
from the elemental mapping results. The three layers on
AuGeNi layer are barrier layers to prevent diffusion of Au,
Ge, and Ni, and the film thickness of the entire barrier layers is about 100 nm. These layers are seen to be about
600 nm layers in Photo 5. Therefore, it can be confirmedthat the layers increased in size six times.

4.

CONCLUSION

A method of FIB sample preparation for TEM observation
and AES analysis was established. The method describedin this article is expected to contribute to the developmentof devices and materials, which involve fine structures
such as laser devices.

REFERENCES
1) H.Koike, Y.Kondo: Proc. of the 56th Annual Meeting of theJapanese Society of Electron Microscopy 203, 2000 (in Japanese)2) K.Mogi, T.Iizuka and M.Suzuki: J. Surface Analysis, 7, 2000, 228(in Japanese)

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