Yao N. Focused Ion Beam Systems: Basics and Applications

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predicted by SRIM for two types of target substrates (Si and Au) using two

types of ion sources (Ga and Ar) at normal incidence as a function of the ion

energy. Ga and Ar are the two most popular ion species in FIB milling, while

Si and Au are the two most useful substrates used in nano-fabrication. They

will be used as the example ions and target materials for later discussions. As

shown, the sputter yield grows as the ion energy increases, but its rate of

increase reduces. Although the data are not shown, in most of the cases, the

sputter yield either levels off or decreases for ion energies higher than

100 keV, where the implantation becomes dominant, as ions penetrate into

the substrate and are trapped in the lattices. Since SRIM is based on the

Monte Carlo method, the number of ions used in the simulation can have an

effect on the results. Earlier studies indicate that the simulations become

converged when the ion number surpasses 500. To be conservative, one

thousand ions are used for all of the SRIM simulations presented here.

Figure 7.6 also shows that the sputter yield of the Au substrate is much

higher than that of the Si substrate, and that Ga can produce higher sputter

yields than Ar. In general, heavier ion sources or lower surface binding

energies (or sublimation energy) of target materials can produce higher sputter

yields [1]. Also, a decrease in the target sublimation energy causes higher

sputter yields due to the fact that sputtering is a phase change process: an atom

Sputter yield (atoms/ion)

0 20 40 60 80 100

Ion ener

(keV)

Ion/target

Ga-Au

[23] [24]

[27]

[31]

[34]

[39]

[42]

[26]

[29]

[33]

[36]

[38]

[41]

[25]

[28]

[30]

[63]

[35]

[37]

[40]

Ar-Au

Ga-Si

Ar-Si

SRIM Experiment

[43]

Figure 7.6 Energy dependence of sputter yield of Au and Si substrates by Ga

and Ar ions at normal incidence.

Focused ion beam systems198

in a solid phase being ejected into a gas phase. The atomic (or ion) weights of

Ar and Ga are 39.95 and 69.72, respectively, while the binding energies of Au

and Si are 3.80 and 4.69 eV/atom, respectively.

To gauge their reliability, the SRIM predictions are further compared to a

large group of experimental data. As shown in Figure 7.6, the data for Ga

ions on Au substrates are reported by Blauner et al. [23] and Xu et al. [24],

while the data from Almen and Burce [25], Robinson and Southern [26],

Carter and Colligon [27], Nenadovic et al. [28], EerNisse [29], Mu

ller et al.

[30], and Poate [31] are for the case of Ar ions on Au substrates. For Si

substrates, the results of Yamaguchi [32,63], Pellerin et al. [33], Santamore et

al. [34], Bischoff and Teichert [35], and Frey et al.[36] are shown for the case

of using a Ga FIB, while the data using Ar FIB are obtained from Southern

et al. [37], Sommerfeldt et al. [38], Andersen and Bay [39], Blank and

Wittmaack [40], Kang et al. [41], Morgan et al. [42], and Zalm [ 43]. As shown

in Figure 7.6, the SRIM predictions agree very well with the experimental

data for all the cases considered. Since the data presented by Lehrer et al. [44]

are almost identical to those of Frey et al.[36 ], only the latter is cited.

Figure 7.7 shows the SRIM simulation of the dependence of the sputter

yield on the incident angle at two different ion energies. Again, Ar and Ga, in

the sputtering of Au and Si substrates, are considered. As shown, for all the

cases considered, increasing the incidence angle increases the sputter yield

until it reaches its maximum near 80



then, it decreases very rapidly to zero as

0 102030405060708090

Incident an

le (de

rees)

Correlation: cos

–2

Ion/target

Ga-Au

Ar-Au [28]

[32]

[34]

[44]

[36]

[38]

Ga-Si

Ar-Si

SRIM Experiment

Sputter yield (atoms/ion)

Figure 7.7 Angular dependence of sputter yield of Ar and Ga ions on Au

and Si substrates.

Fabrication of nanoscale structures using ion beams 199

the incident angle approaches 90



. Roughly speaking, as the angle of collision

between the ions and target atoms increases from normal incidence, the

possibility of the target atoms escaping from the surface during the collision

cascades increases and this eventually leads to an increased sputter yield.

After reaching a maximum, the sputter yield decreases again as the ion

approaches glancing incidence because of the increase in reﬂected ions and

the fact that more and more collision cascades terminate at the surface before

they are fully developed.

Figure 7.7 also shows that the sputter yield of the Si substrate by Ar and

Ga ions at 30 keV increases about 12 times from the normal incidence to the

angle at its peak while the corresponding sputtering yield for the Au substrate

increases less than 2.5 times. The incidence angle that is normal to the target

surface is 0



. The experimental results reported by Santamore et al. [34] and

Lehrer et al. [44] for Ga ions/Si substrate at 30 keV and a normalized

correlation of 1/cos

, where  is the incident angle, are also plotted in

Figure 7.7. As shown, a good agreement has been found between the SRIM

predictions and the experimental results, especially for incident angles less

than 60



. In fact, the experimental results fall in between the SRIM predic-

tions and the correlation. For larger incident angles, the SRIM prediction is

much higher than the experimental observation. At glancing angles, surface

channeling plays an important role and causes the sputtering yield to

decrease. The behavior of the angular dependence of sputter yields has been

widely observed by many other researchers, including Sommerfeldt et al.[38],

Yamamura et al.[45], and Vasile et al.[46].

Based on the above discussion, the material removal rate by sputtering, or

the sputter yield, is dependent not only on the substrate material, but also on

many processing parameters, including the ion energy, angle of incidence,

and scanning procedures.

Redeposition and amorphization

It has been found that the majority of the sputtered atoms (neutrals and ions)

can be ejected from the solid surface into a gas phase. Since the ejected atoms

are not in their thermodynamic equilibrium state, they tend to condense back

into the solid phase upon collision with any solid surface nearby. Conse-

quently, a portion of the ejected atoms can bump into the already sputtered

surface and redeposit onto it. For example, in trench milling, the atoms

ejected from the bottom of the milled channel have a certain possibility to

collide with the milled sidewalls and become redeposited.

The milling of different micro- and nanostructures from simpliﬁed 2D

(such as nanoholes and nanochannels) to complicated 3D geometries has

Focused ion beam systems200

been recently reviewed to characterize the basic sputtering, amorphization,

and redeposition phenomena [20]. For instance, Tseng [1] indicated that

redeposition could be greatly reduced if multiple passes instead of a single pass

are used in milling under the same amount of total ions. In multiple

passes, each successive pass removes redeposited material from the previous

pass. In general, redeposition is affected not only by the scanning pattern of

milling, but also by the milled geometry, the dynamics of the ejected atoms,

and the sticking coefﬁcient of the target material. Normally, redeposition

reduces the milling rate and the aspect ratio of the milled structures. It also

makes the amount of the material milled difﬁcult to be controlled and its ﬁnal

geometry hard to predict because a certain portion of the ejected atoms can

be removed by the vacuum pumping system or can be redeposited outside the

milled area.

If the energy or dose level of the incident ions is not high enough for

sputtering, amorphization may occur in the bombarded area of a crystalline

substrate and induce the substrate to swell. For example, in the case of a

crystallized Si substrate bombarded by Ga ions, the dose level to cause

amorphization is in the order of 10

ions/cm

, while the effective sputtering

dose should be at least two orders of magnitude higher than the amorphi-

zation dose [36,47]. In amorphization, the incident ions in most cases are

buried in the target material and may also displace the target atoms from

their lattice sites so that the displaced atoms are relocated on the nearby

region. In the case of silicon, its density at the amorphous state is much lower

than that of crystalline silicon as reported by Custer et al. [48]. Also, since the

volume of swelling is much larger than the volume of the buried or implanted

ions, swelling should be mainly caused by the density changes or the amor-

phization rather than the volume increase caused only by the buried or

implanted ions. The magnitude of the swelling due to amorphization can be

as high as tens of nanometers. Thus, amorphization can diminish the

dimensional accuracy of nanostructures and should be important in nano-

fabrication. An understanding of swelling or amorphization should be the

ﬁrst step to minimizing or controlling it.

Nanostructure milling

Li et al. [49] have used a 30 keV Ga

FIB to mill an array of nanoholes (dots)

with a single pass on highly n-doped single-crystal Si(100) substrates. One of

the dot-array proﬁles milled at 1 pA with a dwell time of 2 s for each dot

using a 10 nm (FWHM) diameter beam is shown in Figure 7.8. As shown,

the hole (dot) has a V-shaped cross section and is approximately 20 nm

in depth and 50 nm in diameter with a pronounced ring-shaped structure

Fabrication of nanoscale structures using ion beams 201

surrounding the crater. In fact, the protruded ring structure is the swelling of

the substrate due to amorphization and is the inherent shape obtained by

single-pass FIB milling. Since most of the FIB roughly resembles a Gaussian

ion distribution, the intensity at the fringe (tail) of the beam is much smaller

than that at the core (center region). Thus it is not strong enough to sputter

materials but is sufﬁcient enough to cause amorphization that induces sub-

strate swelling [ 1]. The ridge or protrusion height shown in Figure 7.8 should

mainly be a result of the swelling of the substrate and can be as high as on the

order of 10 nm. Redeposition can add materials in the ridge, but the amount

of material added should be much less than that of swelling.

Tseng et al. [50, 52] studied the single-pass milling of several nano- and

microchannel patterns on a gold-coated substrate using a 90 keV As

2 þ

FIB

with a beam FWHM diameter of 50 nm at a current of 5 pA with a dwell time

varying from 5 to 50 ms. As shown in Figure 7.9(a), the AFM image depicts

the pattern milled with 5 ms dwell time and indicates ridges being formed

along the channel banks (shoulders). Figure 7.9(b) delineates the corre-

sponding cross-sectional proﬁle, displaying a V-shaped contour similar to

that of the milled hole-array shown in Figure 7.8. Figure 7.9(c) shows the

variations of four proﬁle features with respect to the dwell time indicating

that although all of the features increase with the dwell time, the increase

rates are gradually reduced as the dwell time increases. This may indicate that

Height (nm)

0 1000 2000 3000 4000

Scan len

th (nm)

20.0 n

10.0 n

0.0 nm

500 nm

–10

–20

Figure 7.8 AFM image of hole (dot) array milled at 1 pA on Si substrate.

(Reprinted from [49] with permission from Institute of Physics and IOP

Publishing.)

Focused ion beam systems202

a large amount of sputtered materials are redeposited into the channel since

the mouth width and the milling depth should increase proportionally

with the dwell time without redeposition. The four features are deﬁned in

Figure 7.9 (b): A is the ridge width, which is the distance between the ridge peaks,

B is the mouth width, which is the channel width with respect to the original

surface, C is the depth from the original surface, and D is the ridge height.

Also, the angles to characterize the steepness of the channel walls and the

ridge, which are deﬁned in Figure 7.9 (b), are calculated and reported in

Figure 7.9 (c). The wall slope angle (ﬁ ) approaches 90



if the wall is vertical

or parallel to the incident beam direction, while the ridge angle (ﬂ) is zero

when no swelling occurs. As shown, the effects of the dwell time or

the amount of doses on the angles are insigniﬁcant except in the initial

transient stage. The ridge orﬂ angle varies from 15



to 17



with the dwell

time changing from 10 to 50 ms, while the corresponding wall slope or

ﬁ angle increases from 33



to 39



, which is about twice the ridge angle. It is to

be noted that the swelling angle is highly dependent on the amount of the

density change by amorphization, while the wall angle can be dictated by the

number of the repetitive passes following the pattern that the larger

the number, the higher the wall angle.

Frey et al.[36] and Li et al. [49] have used a 30 keV Ga

FIB to mill

nanoscale channels with a single pass on Si substrates. They used different

–40

–20

0 100 200 300 400 500

Coordinate (nm)

Channel depth (nm)

450

(a)

(c)

(b)

le (de

rees)

400

350

300

250

200

150

100

0 102030405060

Dwell time (ms)

Width between ridges (A)

Channel width at zero level (B)

Channel depth (C)

Ridge height (D)

(channel angle)

(ridge angle)

µm

Figure 7.9 ‘‘ASU’’ nanochannel pattern on gold layer milled by 90 keV As

2þ

FIB: (a) channel cross section proﬁle and feature deﬁnition, (b) AFM image

at 5 ms dwell time, (c) proﬁle measurements for various dwell times.

Fabrication of nanoscale structures using ion beams 203

beam diameters at different beam currents and all learned that the channel

proﬁles milled with a single pass have a ‘‘V’’ shape. Their results further

conﬁrm that the V-shaped channel proﬁle is the inherent shape obtained by

single-pass FIB milling. Also, the measurements shown in Figure 7.9 and in

[49] reveal that the mouth width of the V-shaped channel can be much larger

than the beam diameter by almost one order of magnitude. This may suggest

that at a higher dwell time, the ion intensity outside the core region of the

FIB is sufﬁciently high enough to produce a sizable amount of sputtering.

Kim et al. [ 51] used a 30 keV Ga

FIB to mill an intrinsic Josephson

junction (IJJ) structure used as the basic element for quantum computation.

The IJJ is a Z-shaped stacked structure made by Bi

6þ–

(Bi-2201)

single crystal whiskers. As shown in the Figure 7.10(a), the intrinsic

Josephson junction has a 0.01 mm

(100 nm · 100 nm) tunneling stack area.

The 3D tunneling stacks were patterned on a special rotation stage. The steps

of the FIB milling process are shown in Figure 7.10(b). A width, depending

on the required junction size, was milled in the whisker thickness from the

perpendicular direction. By tilting the sample stage up to 90



, two grooves of

the bridge were then milled completely from the lateral sides in order to

create the required junction size. This technique, FIB in concert with a

Single crystal

whisker

Tunneling stack

(a)

(b)

FIB

Ste

1 Ste

1µm

Figure 7.10 Stacked nanostructure. (a) SIM image of intrinsic Josephson

junction with 0.01 mm

(100 nm · 100 nm) tunneling stack, (b) FIB

fabrication steps. (Reprinted from [51] with permission from American

Institute of Physics.)

Focused ion beam systems204

rotation stage, can fabricate nanoscale stacks from thin ﬁlms and single

crystals.

7.3.4 FIB implantation (FIBI)

The FIB system not only has capabilities of performing subtractive (milling

and etching) processes, but it can also perform additive (implantation and

deposition) processes. In this subsection, the implantation ability of FIB will

be explored by showing the speciﬁc procedures in creating different nano-

structures.

A combination of FIB implantation (FIBI) and wet etching has been used

to fabricate a nanoscale quad-cantilever on a Si (100) substrate having a 10 W

n-type background dope. As shown in Figure 7.11, the quad-cantilever is

ﬁrst patterned by ion implantation using a 30 keV Ga

FIB at a current of

100 pA in the Si substrate at a sufﬁciently high dose, i.e., higher than

ions/cm

. At this level of dose concentration, i.e., higher than 10

ions/cm

in silicon, the etch rate of certain etchants, including KOH

(potassium hydroxide), can dramatically be reduced and the doped silicon

has been used as an etch stop or a mask in IC fabrication. The thickness of

the cantilever structure is dictated by the penetration depth (or the range)

of the implanted ion, which can be controlled by the incident energy of the

ions. Therefore, a selective etching is accomplished as the substrate is dipped

into the 40% KOH solution at 60



C, in which the silicon layer with no

(d)

Mask layer by ion

implantation

(a)

Scanned by FIB

KOH etching

20–30 nm

(c)

<110>

Si (100)

500 nm

5 µm

(b)

Figure 7.11 Nanoquad-cantilever. (a) Implantation by FIB scanning (cross

section), (b) top view of FIB scanned surface with quad-cantilever layout,

of fabricated quad-cantilevers, 30 nm thick, 500 nm wide, and 5 mm long.

(Courtesy of J. Brugger of Swiss Federal Institute of Technology.)

Fabrication of nanoscale structures using ion beams 205

implantation is etched away at a rate of 250 nm per min, while the implanted

region stays relatively unaffected (Figure 7.11(c)). The 3D quad-cantilever is

20–30 nm thick, 500 nm wide, and 5 mm long, as shown in the SEM image

(Figures 7.11(b) and (d)). Using this implantation/wet etching combined

method, other types of nanoscale freestanding structures, including a nano-

cup and a U-shaped nanocantilever, have also been reported [20].

Utilizing a similar procedure, Hiramoto et al.[53] fabricated an array of

grooves on epitaxial AlGaAs layers by implantation combined with etching.

The implantation is performed by line scanning of an Si

FIB with a 100 nm

FWHM diameter at 200 keV. They found that the cross sections of the doped

area could be varied from U to round shapes by reducing scanning speeds or

increasing the doses. Figure 7.12 shows the round-shaped grooves that are

wider in the interior and narrower near the surface. The grooves are formed

by selectively etching the highly doped region that is implanted at a scanning

speed of 0.1 mm/s with a line dose of 1.0 · 10

ions/mm using a hot HCl

etchant at 70



C. The round grooves are about 350 nm in diameter and the

distances between the etched grooves are about 30 nm. Since the groove

diameter is much larger than the beam diameter (100 nm), the implanted ions

are spread into the substrate and the grooves can be implanted very close at a

distance much less than the beam diameter. Hiramoto et al. [ 53] also applied

a similar technique to make very small GaAs conducting wires ranging from

20 to 100 nm in diameter.

FIB implantation has been used to irradiate polymeric resist materials to

increase their resistance to subsequent physical etching. Arshak et al. [11]

used a 30 keV Ga

FIB with 1.17 · 10

 2

C/cm

doses to make implanted

patterns in Shipley SPR660 positive photoresist layers, which can form a

compound against O

reactive ion etching (RIE). This process, also

known as graphitization, can harden the photoresist to be conveniently used as

an etching mask for O

dry etching. As shown in the FIB image in Figure 7.13,

two 100 nm negative resist lines with a 100 nm space (left) and two 30 nm

30 nm

100 nm

Figure 7.12 SEM image of round groove array fabricated by Si-FIB

implantation on AlGaAs substrates. (Modiﬁed from [53] with pe rmission

from American Institute of Physics.)

Focused ion beam systems206

resist lines (right) were patterned after RIE in the SPR660 resist layer. This

demonstrates that anisotropic patterns can be achieved by graphitization and

can be directly incorporated into the standard photolithographic processes.

Ion implantation has also been used to change the electrical, optical, and

magnetic properties of different materials for making nanodevices. In the

case of changing electrical properties, Sumita et al. [54] fabricated a CDW

(charge-density-wave) device with a thickness of 40 nm and a width of

100 nm, and found that the device shows collective sliding and negative

resistance above some threshold electric ﬁeld in the low temperature region.

This suggests that the quantum tunneling phenomena occur in the fabricated

CDW, which can only result from a condition that the structure is a true one-

dimensional conductor or is narrow enough to cause the collective sliding of

electrons without interferences. Shinada et al. [55] have improved FIB optics

for single-ion implantation and this high precision implantation instrument

can perform one-by-one doping of impurity ions into nanoscale semi-

conductor devices for various applications.

7.3.5 FIB induced deposition (FIBID)

The FIB can be used as a deposition tool for fabricating nanostructures. FIB

induced deposition (FIBID) uses ion energy to initiate and localize chemical

vapor deposition (CVD) in a speciﬁc location by a direct writing technique,

also known as FIBCVD. The FIB system should be equipped with a gas

nozzle to inject a variety of organo-metallic precursor gases on the target

surface in the vacuum chamber (Figure 7.14(a)). The high-energy ion beam

can decompose the organo-metallic molecules adsorbed onto the surface of

the substrate. This leads to the release of metal atoms, which are incorpo-

rated onto the surface, while the remainder resulting from the decomposition

300 nm

30 nm

1µm

Figure 7.13 Cross-sectional FIB image of double 100 nm (left) and 30 nm

(right) line patterns in SPR660 photoresist after O

RIE. (Courtesy of Khalil

Arshak of University of Limerick of Irelan d.)

Fabrication of nanoscale structures using ion beams 207