Yao N. Focused Ion Beam Systems: Basics and Applications

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ﬁeld, or to create free standing structures for nanotechnology. To assist with

these processes, a reactive neutral gas is introduced to the sample through a

ﬁne capillary tube at the milling site. The gas is able to enhance the etching

rate compared to straight sputtering of the material and also reduce the

amount of redeposited material from the sputtering of the sample. Other

beneﬁts from the use of gas are the different etch rate enhancements obtained

for different materials that can allow for preferential etching of one material

in the presence of another. The use of gas can also result in structures

that cannot be formed by the use of the ion beam alone, such as high aspect

ratio vias.

3.2.1 Gas assisted focused ion beam etching – principles

Gas assistance of the FIB induced etching process relies on gases that react

with the sample to produce a volatile compound that is removed by the

vacuum system. Overall, there are many similarities in the steps used to

describe the deposition processes and those used to describe etching. For

example, deposition and etching gases can adsorb and desorb without

reacting (chemisorption) with the surface they are on. However, some

etching gases behave differently in that instead of just physisorbing on the

surface, the etching gas is able to chemisorb. For example, the use of

chlorine on a clean silicon surface results in a chlorosilyl layer being formed

on the silicon.

The steps in the gas assisted focused ion beam etching process are as

follows:

(1) A chemically neutral reactive gas is introd uced through a ﬁne gas nozzle and is

adsorbed (or chemisorbed) on the sample surface.

(2) The gas reacts with the sample either spontaneously, as in silicon and ﬂuorine, or

in the presence of the ion beam the gas decomposes and reacts with the sample,

as with water and carbon based polymers.

(3) The volatile products leave the surface (desorption).

Winters and Coburn [1] have investigated the spontaneous etching and

effect of ion enhanced spontaneous etching of silicon with ﬂuorine-based

gases. These studies have shown that in the silicon-XeF

system the incoming

XeF

gas is able to spontaneously react with the silicon surface without the

interaction of the ion beam according to the following reaction:

XeF

(g) þSi(s) !‘‘SiF

’’(s) þXe(g), (3.1)

SiF

(s) þXeF

(g) !SiF

(g) þXe(g). (3.2)

Focused ion beam systems68

The symbols (g) and (s) indicate gas phase and solid phase respectively. The

SiF

, while stoichiometrically correct, is probably a mixture of SiF, SiF

, and

SiF

. The means by which the incoming ion from the FIB is able to enhance

the etch rate above the rate for the spontaneous process is by removing the

‘‘SiF

’’ layer through recombination to form SiF

(g):

2SiF

(s) þion ! SiF

(g) þSi(s), (3.3)

SiF(s) + SiF

(s) þion ! SiF

(g) þSi(s), (3.4)

through sputtering of SiF

SiF

(s) þion ! SiF

(g), (3.5)

or by increasing the desorption rate of SiF

. The main gaseous species

detected from the ion enhanced etching of silicon is SiF

and some SiF

.It

should be kept in mind that most of the material sputtered during ion beam

exposure originates from the ﬁrst atomic layer and that the yield is inversely

proportional to the binding energy. Equations (3.1)–(3.5) result in the for-

mation of a volatile species with a low binding energy and hence can increase

the etching rate.

Equations (3.3), (3.4), and (3.5) will result in fresh silicon surfaces being

exposed to the reactive XeF

gas to form a fresh layer of SiF

as in (3.1). This

way more XeF

is able to react with silicon in the presence of the ion beam

than without, increasing the rate of removal of silicon.

The interaction of molecular chlorine on silicon is known to behave in a

similar fashion as the XeF

case with the exception of (3.2) which does not occur

with chlorine at room temperature. However, chlorine is still able to give a

signiﬁcant boost to the etch rate of silicon in the presence of the ion beam as

shown in Table 3.1. The lack of spontaneous etching is more the preferred

situation with gas enhanced ion etching. Here the addition of the gas causes no

etching, even though in some cases the gas will chemisorb to the sample and the

Table 3.1 Selection of halide gases and their etch rate enhancements for semiconductor

materials using a 30 kV Ga

ion beam.

Gas/sample Si SiO

Al W GaAs InP

XeF

[13] 7–12 7–10 1 7–10 NA NA

[13,14] 11 1 5–10 1 50 4

[15,16,17] 5–10 1–1.5 5–15 NA 10–15 11–13

Gas assisted ion beam etching and deposition 69

ion beam is required to drive the etching process. This provides greater control

of the etching process than is possible with the presence of spontaneous etching.

Gas assisted etching of metals

The use of halide etching gases on metals to increase the etching rate is widely

used in the semiconductor industry. In this case the halide helps to remove

the metal lines as a step to rewiring the circuit for failure analysis or para-

meter testing. Here the halide not only aids in the etching rate but is also able

to create an electrically isolated cut since any sputtered metal is oxidized by

the halide and rendered nonconducting. For applications requiring the

removal of aluminum metal the use of either chlorine or iodine is preferred

due to their ease of use, formation of volatile products, and excellent

enhancement rates. XeF

is not used here since the reaction product, AlF

,is

not volatile and there is no improvement in the etching rate.

The case with tungsten is the opposite of aluminum. In this case XeF

is the

preferred gas since the product of the reaction WF

is a gas and is thermo-

dynamically stable. Chlorine does not provide any enhancement over sput-

tering since the WCl

product is a solid with a low vapor pressure.

Gas assisted etching of SiO

is achieved by the use of ﬂuorine based gases.

XeF

is widely used because of the ease of handling this ﬂuorine source.

There is no spontaneous component in the etching of SiO

. XeF

does not

chemisorb to the SiO

surface but only physisorbs in a low concentration

initially. This is due to the paucity of adsorption sites on the oxide surface

prior to milling with the ion beam. However, after a few passes of the ion

beam the damaged surface will readily adsorb up to ten times more ﬂuorine,

which is about the same coverage as one monolayer [2 ,3]. Even when

damaged, the oxide surface does not spontaneously etch which is advanta-

geous and provides excellent control of the etching based solely on the ion

beam current. Studies using argon ions and XeF

have shown that the

majority of the desorbed product from the etching of SiO

is SiF

. This has

been shown to be independent of the energy and type of ion used to excite the

etching [4,5].

Etching of carbon based materials

The use of halide materials as etch enhancing agents provides solutions to

many materials issues. However, they are by no means universal etching

agents and are known to have limits with organic, carbon based materials.

Water has been found to be a very effective etch enhancing material for

Focused ion beam systems70

polymers and biological samples. The effect of water has been noted in the

SEM community for many years in papers and books [6]. This work was then

applied by Stark et al. [7] to focused ion beam systems with great success.

Table 3.2 shows a comparison of the etching enhancement of various gases

on poly(methyl methacrylate) (PMMA) and clearly shows the efﬁciency of

water over other gases including XeF

The products from the reaction of water on polymers have not been

determined but Stark et al. did speculate that CO

is formed and pumped

away. However, this would seem to be unlikely in light of the results reported

in the Si/XeF

system where the XeF

is able to chemisorb and attack the Si

surface prior to the ion beam arriving. In this system, the rate limiting step is

desorption since the gas reacts with the Si to form a saturated layer of

ﬂuorosilanes. The ion interaction with the ﬂuoridated surface results in the

maximum removal rate of Si of 25 atoms/ion [8]. These studies have also

shown the products of the etching reaction to be SiF

and SiF

. Comparing

this to the etching data of water, where no reaction occurs prior to the arrival

of the ion beam and the water is simply adsorbed to the surface, the authors

observed an etching rate of 100 atoms/ion striking the surface. The rate is

four times higher than that observed in the XeF

/Si system. It would seem

likely that the role of the water in the PMMA/water reaction is to cleave the

polymer chains and the volatile products are chunks of monomer or methyl

methacrylate fragments.

3.2.2 Inhibition of the etching process

The addition of gas is also able to inhibit ion beam sputtering of the sample.

In the previous discussion the focus was on gases that produced acceleration

in the etching rate of materials by the production of volatile products.

Table 3.2 Yield and etch rate enhancement for various gases on PMMA

with a 30 kV Ga

ion beam [7].

Gas Yield ( mm

/nC) Enhancement

No gas 0.4 –

0.4 1

Iodine 0.8 2

XeF

1.7 4.2

Oxygen 1.8 4.5

OH 4.2 10

Water 7.5 18

Gas assisted ion beam etching and deposition 71

In some cases there is no acceleration or even inhibition of the etching

process. The situations where the gas does not produce any enhancement can

be due to the gas not reacting with the substrate or the sputter rate of the

product is the same as for the substrate. For a reduction of the etching rate to

occur the gas and the sample must react to form nonvolatile products or

products with a high binding energy.

Water is a good example of a gas that is able to produce all three results;

enhancement, no change, and inhibition of etching. The chart below shows

the log of the enhancement factor versus different materials and clearly shows

enhancement, no affect, and inhibition. The enhancement factor is calculated

by dividing the yield (mm

/nC) with gas by the yield without gas:

enhancement factor ¼

yield with gas

yield without gas

· ð3:6Þ

It can be seen from Figure 3.1 that the etching rate of gold is unaffected by

the addition of water because it is inert and does not react. The etching rate

of aluminum or silicon is greatly reduced by the addition of water and is one-

sixth the normal sputtering rate of the material under a 30 kV Ga

ion beam.

This is very advantageous and allows the user to preferentially etch polymers

from around aluminum lines in integrated circuits. The result of this etching

is seen in Figure 3.2, where the surface layer of resist with contact holes has

been removed to expose the proﬁle of the contact. Note that the silicon

exposed to the ion beam at the bottom of the hole does not show any

apparent damage from the ion beam.

Water etch enhancements

–1 –0.5 0 0.5 1 1.5

Aluminum

Silicon

Silicon oxide

Silicon nitride

Gold

Teflon

Diamond

PMMA

Polyimide

Material

log (Enhancement factor)

Figure 3.1 Water etch enhancement and inhibition for various materials [7].

Focused ion beam systems72

The presence of a chemically active gas is able to inﬂuence the rate of

physical sputtering of a material by three factors:

(1) A modiﬁed surface is formed by the reaction of the gas with the material. For

silicon and XeF

this will be a saturated SiF

layer where x ¼1–3 and for water

the formation of SiO

2y

, y < 1.

(2) The binding energy of the surface atoms may increa se, as in the case of silicon

and water leading to inhibition of etchi ng, or decrease as with silicon and XeF

leading to enhancement of the etch rate.

(3) The number of atoms of the original element or material in the top surface layers

decreases.

For example, using water in the presence of aluminum and silicon creates a

surface where most of the atoms in the top layer are oxygen which originate

from the gas passed over the surface. As mentioned above since most of the

sputtered material originates from the surface layer the ion beam is mostly

removing the oxygen from the top oxide layer. These oxygen vacancies are

rapidly replaced by the gas leading to a net decrease in the rate of sputtering

of the aluminum or silicon in the sample.

The variation in the etching rate with different materials is very useful if

many materials are present in a cross section and the user needs to see where

they are. This is a common practice in the semiconductor industry where

engineers need to see the layers of a device during manufacturing to monitor

the FAB (fabrication plant for semiconductor materials) processes. This is

called delineation etching and utilizes the varying etch rates of materials

Figure 3.2 Water enhanced etching of Phot oresist. (Courtesy of FEI

Company.)

Gas assisted ion beam etching and deposition 73

when exposed to a constant ion beam current and gas ﬂow. Some gases are

designed to have a great variation in etching rate and an example is deli-

neation etch from FEI which is used to delineate the layers in semi-

conductor devices. The use of these gases can change a cross section from a

blank and uninformative view into a series of topographic features based on

the materials present. Figure 3.3 is an example of the end result of a deli-

neated semiconductor device.

3.2.3 Etching with other ion sources

Other ion sources apart from gallium will also produce similar results. Noble

gas ion sources have been used successfully to etch samples by sputtering and

with the addition of reactive gases. There are differences in the etching rates

achieved with the use of other ion sources due to the size and mass of the ion

produced. The interaction of an ion passing through a solid is explained in

depth in the book by Utlaut et al. [9]. The energy delivered to the sample is

via the nuclear losses of the collision cascade as the ion passes into the

substrate. The smaller and lighter the ion is the less energy it is able to deliver

to the sample and hence a lower sputter yield results.

Coburn et al. [8] were able to demonstrate this clearly with various 1 kV

noble gas ions in a set of experiments with silicon and chlorine etching gas.

The heaviest gas used was argon and when used in conjunction with

chlorine a maximum yield of four silicon atoms ejected per argon ion was

observed. When helium was used instead of argon the yield dropped to less

than one silicon atom per helium ion. Figure 3.4 is a summary of these

results.

Figure 3.3 Results of delineation etching of a semiconductor device.

(Courtesy of FEI Company.)

Focused ion beam systems74

3.2.4 Sample damage

As with any physical process the ion beam will change the characteristics of

the top layers of a sample. With a 30 kV gallium ion beam the top 30 nm of

material is implanted with gallium and any atomic structure that was present

will be altered or destroyed depending on the dose the sample received. This

damage layer is very apparent in TEM samples obtained by FIB unless the

operator is careful to follow known damage minimizing procedures, which

can greatly reduced the thickness of the damaged layers.

Another effect of ion beam exposure is the movement of species within the

top most layers of the sample. Material from the surface can move into the

bulk and bulk material can move to the surface. This can be caused by recoil

implantation, cascade mixing, or radiation enhanced diffusion. Thus, etching

gases can be found implanted into the sample and not only on the top surface

of the material [1].

3.3 Gas assisted focused ion beam deposition

In terms of ion beam assisted deposition, FIB induced deposition and con-

ventional ion beam assisted deposition (IBAD) are similar. However, FIB

induced deposition with gas assistance signiﬁcantly differs from the IBAD. In

general, ion beam assisted deposition is used in conjunction with other

sources such as evaporation, sputtering, pulsed laser, and molecular beam

epitaxy. The gas assisted FIB deposition is a direct deposition by using only

the focused ion beam itself along with a gas. In addition, FIB induced

deposition is also a deposition in a very local area only where the controlled

ion beam is scanning.

0 102030

Chlorine flow rate

Si atoms/ion

He etch rate

Ne etch rate

Ar etch rate

Figure 3.4 Yield of silicon atoms sputtered per 1 kV He

,Ne

, and Ar

The chlorine ﬂow rate is 10

molecules/s [8].

Gas assisted ion beam etching and deposition 75

The gas assisted FIB induced deposition process can be described by the

following steps:

(1) A gaseous compound precursor is introduced through ﬁne gas nozzles inserted

close to the surface and adsorbs on the surface of the sample.

(2) The gas molecules adsorbed on the surface are decomposed into nonvolatile

products and volatile products by the incident energetic ion beam where the

focused ion beam is scanning. Simultaneously, the ion beam’s energy results in

sputtering of the sample surface.

(3) The nonvolatile products remain on the surface, producing deposition layers,

while the volatile components, such as oxygen and hydrogen, leave the surface.

Deposition conditions can be optimized by controllable system parameters

such as the properties of precursor gases, gas ﬂux (needle location, the

heating temperature of gas), ion beam current, dwell time, beam overlap,

beam scanning area, and raster loop time.

Once the gas adsorbs to the surface of the sample, the precursor is required to

have sufﬁcient sticking probability to remain on the surface long enough before

it is decomposed. Furthermore, the precursor on the surface must be easily

decomposed by the incident energetic ion beam for any deposition to occur.

The incident ion beam decomposes the precursor gas on the surface as well

as sputters or mills the materials on the surface. So the total deposition yield

dep

), the number of atoms deposited per incident ion, can be expressed as a

function of the decomposition yield (Y

decomp

) of the precursor gas and the

sputtering yield (Y

sputt

) on the surface:

dep

¼ Y

decomp

 Y

sputt

: ð3:7Þ

The decomposition yield is proportional to the number of adsorbed mole-

cules on the surface and the cross section for molecular dissociation. The

number of adsorbed molecules on the surface is known as a function of the

gas ﬂux onto the substrate, the substrate temperature, and the gas/substrate

interaction on the deposited area [10].

For the success of deposition many beam parameters need to be adjusted

properly. In general the deposition rate is a combination of local gas ﬂux or

local pressure, ion beam current, pattern size, dwell time per pixel, beam

overlap, and raster refresh time. The following sections introduce the type of

deposition precursor gases, and parameters to obtain good deposition.

3.3.1 Deposition precursor gases

In 1984, the ﬁrst FIB induced deposition of Al was reported by Gamo et al. from

metal organic material, trimethyl aluminum (TMA) Al

(CH

)

precursor [11].

Focused ion beam systems76

So far there are dozens of different precursors that have been successfully

reported for FIB induced deposition of metal (aluminum, tungsten, tantalum,

platinum, gold, copper, palladium, and iron), insulator (silicon dioxide), and

carbon. The detail of precursor gases for the successful deposition is shown in

Table 3.3.

As one condition of a successful deposition in the focused ion beam system

the proper amount of gas precursor in a local area should be introduced by

the focused ion beam system. If the gas ﬂux is too high the deposition rate

becomes slow at the same beam conditions. If the gas ﬂux is too low all the

gases quickly get consumed resulting in the dominant sputtering of

the sample surface by the incident ion beam. The gas ﬂux can be adjusted by

the nozzle location, e.g. the height from the surface and the distance from the

scanning ion beam, and the temperature of gas crucible. Figure 3.5 shows the

schematic diagram of the gas needle location in a focused ion beam system.

The gas ﬂux or local pressure decreases as the height of the nozzle in the

system increases. However, the nozzle must be high enough to be able to

Table 3.3 Precursor gases for FIB induced deposition.

Deposition Material Precursor gas Reference

Aluminum Trimethyl aluminum (TMA) Al

(CH

)

[11]

Aluminum Trimethylamine alane (TMAA) [18]

Aluminum Triethylamine alane (TEAA) [18]

Aluminum Tri-isobutyl aluminum (TIBA), Al(C

)

[19]

Tungsten Tungsten hexaﬂuoride WF

[20]

Tungsten Tungsten hexacarbonyl, W(CO)

[21]

Tungsten Tungsten hexaﬂuoride, WF

[22]

Gold Dimethyl gold hexaﬂuoro acetylacetonate

[23, 24]

Platinum (methylcyclopentadienyl) trimethyl

platinum C

[25, 26]

Copper Cu(hfac)TMVS [27]

Tantalum PMTA, Ta(OC

)

[20]

Tantalum Pentaetoxy tantalum, Ta(OC

)

[20]

Iron Iron pentacarbonyl, Fe(CO)

[19]

Palladium Palladium acetate [Pd(O

CCH

)

]

[28]

Insulator (TEOS): (C

)

Si [29]

Silicon dioxide Si(OCH3)4 [30]

Silicon dioxide A combination of siloxane and

oxygen gases

[31]

Carbon – [32]

Carbon Naphthalene (C

)[33]

PMMA – [34]

Gas assisted ion beam etching and deposition 77