Yao N. Focused Ion Beam Systems: Basics and Applications

Подождите немного. Документ загружается.

Figure 14.4 shows examples of images of the same cross section. The

secondary electron image (Figure 14.4(a)) shows grains of metal, but the

secondary ion image (Figure 14.4(b)) shows uniform structure. We can

recognize that this portion of the specimen is constructed of one material but

its structure is not uniform.

14.2.2 Sputter etching

An FIB is different to an SEM because it irradiates a much heavier gallium

ion than an electron. Therefore the sample surface is etched by the collision

of heavy gallium ions. Figure 14.5 shows a general diagram of the sputter

etching process. Sputter etching advances by irradiating an ion beam con-

tinuously in the same domain. Figure 14.6 shows a diagram of how the process

area is determined.

At ﬁrst a domain, including the processing area, is observed in the same ion

beam conditions (acceleration voltage, beam current, scan speed, etc.) as used in

processing. An ion beam is then irradiated to each pixel as shown in Figure 14.6.

Topological contrast

Structural contrast

(a) (b)

Figure 14.3 An SEM image (a) and an FIB image (b) of a Cu wire cross

section: SEM V

ACC

¼2 kV; FIB V

ACC

¼30 kV.

Topological Contrast

(a) (b)

Structural Contrast

Figure 14.4 Secondary electron image and secondary ion image. FOV

(ﬁeld of view): 3 mm · 3 mm. (a) Secondary electron image: V

ACC

¼30 kV,

beam

¼1 pA; (b) secondary ion image: V

ACC

¼30 kV, I

beam

¼2 pA.

Focused ion beam systems358

If the setting of the ion beam irradiation position is the same as the condition of

an ion beam, an ion beam is irradiated to the same pixel. Processing happens as

shown in Figure 14.5 if an ion beam is continuously irradiated to the same pixel.

The processing area is observed by an ion beam of the same conditions as

used for processing, and it is decided by assigning pixels of the observation

image.

It is necessary for the ion beam to be focused to a narrow beam. In addition

to the stability of the ion beam, irradiation position must be good.

14.2.3 Gas assisted etching (GAE)

GAE uses the reaction between the gas and the work piece. A reactive gas is

blown in the proximity of the ion beam irradiation. There are two major

types of etching: accelerated etching, in which the etching speed is fast, and

decelerated etching, in which the etching speed is slow.

Accelerated etching

Improve the etching rate Generally, the reactive gas exhibits effects in terms of

its speciﬁc properties. Table 14.1 displays the acceleration ratios for typical

Long time irradiation

= s

utter etchin

Short time irradiation

= observation

Figure 14.5 The sputter etching process.

Pixels

Process are

Observation area

Figure 14.6 Choosing the process area.

Focused ion beam systems as a multifunctional tool for nanotechnology 359

gases. The acceleration rate is expressed as a ratio of the etching rate when

the gas is used and the rate when it is not used. For example, with chlorine

gas (Cl

) the etching speed for aluminum (Al) is 20 times greater than that

without and the rate for silicon (Si) is 10 times greater. It can be seen from

Table 14.1 that no other materials exhibit accelerated etching rates with

chlorine.

Selective etching An example of the selective etching of an insulator on an

integrated circuit is shown in Figure 14.7. XeF

gas was blown onto the

sample surface at the same time as ion beam irradiation was used.

Removal of redeposited material Sputter etching causes processed material to

accumulate as redeposited material around the process area. As a result,

drilling at a high aspect ratio is difﬁcult with only the ion beam. However,

when using a gas that makes evaporation reaction products, the reaction

Table 14.1. Acceleration ratios for typical gases (etching rate with gas/etching rate

without gas).

Al W Si SiO

or SiN

Photo resist

· 20 · 1 · 10 · 1 · 1

· 20 · 1 · 6–10 · 1 · 1

· 8–10 · 2–6 · 4–5 · 1 · 1

XeF

· 1 · 10 · 10–100 · 6–10 · 3–5

O · 0.16 – · 0.19 · 0.28 · 20–30

Figure 14.7 Selective etching using XeF

. The insulator at the surface of the

IC was selectively etched by an ion beam with XeF

. The wires used in this

process can be seen. The etching rate of silicon oxide is large in comparison

with aluminum when we use gas assisted etching with XeF

Focused ion beam systems360

product from the sputter etching in a gas atmosphere is vented by a vacuum

pump. As a result, redeposited material will not form in holes, and holes at a

high aspect ratio can be formed. Figure 14.8 shows an example of using XeF

when etching SiO

ﬁlm or a silicon substrate. Etching without gas has an

aspect ratio of 6 for drilling whereas etching with gas improves the aspect

ratio to 11 in this example.

Decelerated etching

Cu has been used in recent years in semiconductor IC wires. The etching rate

of the ion beam relies largely on the orientation of the crystals. Since the crystal

orientation of the grains is not uniform, it is difﬁcult to uniformly etch Cu.

Figure 14.9 shows an example of Cu etched by FIB. It is clear that grains in

the direction that is difﬁcult to etch will remain when not using the gas

assisted etching. The effect of the grains is remarkably mitigated when water

0.82 µm

9.0 µm

3.32 µm

0.55 µm

(a) (b)

Figure 14.8 High aspect drilling of SiO

ﬁlled by W. (a) Without gas: aspect

ratio ¼6; (b) with gas: aspect ratio ¼11.

Bird's-e

e view Cross sectionTo

view

Without water vapor

With water vapor

Figure 14.9 Cu etching with water.

Focused ion beam systems as a multifunctional tool for nanotechnology 361

vapor is blown into the ion beam irradiation area at the same time that the

ion beam is irradiated, and the processed surface becomes smooth. This

mechanism has not yet been fully veriﬁed, but we know that using water

slows the etching rate, as shown in Figure 14.9.

14.2.4 Deposition

The FIB is able to selectively deposit thin ﬁlms in the ion beam irradiation

area if a compound gas is blown to the specimen surface at the same time.

Since deposition is induced by the ion beam, it is referred to as ion beam

induced CVD.

Secondary electrons are produced when the ion beam irradiates the speci-

men surface. These secondary electrons decompose the compound gas and

gaseous components are vented through the system’s vacuum pump. The

solid components, on the other hand, remain on the surface of the decom-

posed specimen and accumulate to form a thin ﬁlm.

The specimen surface is always being etched due to ion beam irradiation,

but the deposition rate looks for conditions that are faster than the etching

rate thus enabling formation of a thin ﬁlm. Gases used today are shown in

Table 14.2.

Table 14.2. Deposition.

Film Material Properties Uses

Carbon Phenanthrene C

High deposition

rate

3D deposition

Difﬁcult to etch

with ion beam

Protection

mask when

creating

a TEM

specimen

Resistors

Tungsten Hexa-carbonyl-tungsten

W(CO)

Low resistivity

Good step

coverage

Low deposition

rate

Wires

Platinum Methylcyclopentadienyl

trimethylplatinum

(CH

)(CH

)

High deposition

rate

Wires

Silicon oxide

ﬁlm

TEOS Insulation

resistance is high

Insulators

Focused ion beam systems362

Each deposition ﬁlm has its own characteristics which must be applied. For

example, a tungsten ﬁlm is able to ﬁll high aspect ratio holes formed by

accelerated etching from XeF

because the step coverage is excellent. Figure

14.10 shows an example of this process.

14.2.5 System conﬁguration

Figure 14.11 shows a photograph of the SII Nano Technology Inc. SMI3050

FIB tool for small specimens. Its main speciﬁcations are given in Table 14.3.

This tool can perform cross section process observation in any direction on an

entire area of a 50 mm diameter specimen.

The SMI3050 consists of (i) a main console, (ii) an operation table, and

(iii) a control cabinet. Also included are accessories including a roughing

pump and a power transformer capable of being used in all countries and all

regions.

The main console has a specimen chamber and a frame that supports the

specimen chamber. An FIB column, secondary electron detector, and gas

injector are installed in the specimen chamber. A specimen stage is also installed

Table 14.3. SMI3000 series FIB speciﬁcations.

Minimum beam diameter 4 nm @V

ACC

¼30 kV

Beam current 0.15 pA–20 nA

Beam stability 0.1 mm/10 min

Acceleration voltage 30 kV daily use

5–30 kV, 5 kV steps

Maximum current density 30 A/cm

Figure 14.10 W wire formations.

Focused ion beam systems as a multifunctional tool for nanotechnology 363

inside the specimen chamber. The specimen stage has at least ﬁve drive axes

for moving the FIB irradiation angle and position to the specimen. It can

move in three axes, with the x–y plane consisting of an incline that tilts with

the axis of rotation in the x-axis, and the x–y plane rotating with the axis of

rotation of the z-axis.

14.2.6 Scan signal generator

As stated previously, irradiating the ion beam on the specimen surface,

processing, and observation are performed by the FIB tool. This is realized

by applying a scan signal generated from the scan signal generator to the

deﬂection electrodes of the ion beam column, and controlling the irradiation

position of the ion beam. Scan signals are classiﬁed as raster scans and vector

scans. When performing precision processing, these scan signal properties

must be understood and used effectively.

Raster scanning

Figure 14.12 shows a diagram of the raster scanning process. Raster scanning

is a typical scan method using a scanning electron microscope or laser

microscope. A beam is irradiated one by one along the x columns from one

corner of the irradiated area. When the end point of that x column is reached

it advances one y column and at the same time returns to the start of the next

x column. At this time the blanking signal turns on and prevents the beam

Figure 14.11 Photograph of the SMI3050.

Focused ion beam systems364

from reaching the specimen as it moves from the end point of one x column

to the start point of the next x column. A normal rectangular process is a

typical result of this scan method (Figure 14.12).

Dot blanking scanning (or bitmap scanning) is a development of raster

scanning and is shown in Figure 14.13. Here, the x and y scan signals are the

same as for raster scanning, but the blanking signal is controlled at each pixel,

and enables the process to form any pattern. Deﬁnition of the blanking signal

ON/OFF is a general format for displaying picture information on a personal

computer, which is usually stored as a .bmp (bitmap) ﬁle. The .bmp ﬁle can be

created using graphics software such as the software that comes standard with

Microsoft Windows



Figure 14.14 shows two examples of dot blanking scanning. In Figure14.14

(a) Si is etched. In Figure 14.14(b) a carbon ﬁlm is formed on Si substrate.

Vector scanning

Vector scanning is a highly ﬂexible method of scanning in any order at any

pixel, in contrast to raster scanning, which scans periodically in only the x– y

direction. Figure 14.15 shows a diagram of a vector scan drawn in the same

-scan signal

Blanking signal

Waveforms

First pixel

Last

ixel

Figure 14.12 The raster scanning process.

-scan signal

Blanking signal

Waveforms

First pixel

Last pixel

Figure 14.13 The dot blanking scanning process.

Focused ion beam systems as a multifunctional tool for nanotechnology 365

shape as Figure 14.13 but which requires less time to scan than dot blanking

scanning because:

 be am blanking is not requir ed for each line scan;

 be am irra diation can be a t a mini mum exc ept toward the process area.

An operator must deﬁne the beam trace to use vector scanning. There are

two typical methods to deﬁne the beam trace. The ﬁrst is describing using script

language. The second is using automatic generation software from picture data.

The script language has several standard patterns, e.g., a circle, a rectangle, etc.

Operators can describe the detail conditions to draw an ion beam. This

method is suitable for precise processing. An example of script language is

shown on page 367. An array of 5 · 5 circles will be drawn by this script

(Figure 14.16a). The process ﬁeld is 40 000 · 40 000 pixels. Figure 14.16(b)

shows results of etching and deposition. These pictures shows us that the

-scan signal

Blanking signal

Waveforms

First pixel

Last pixel

Figure 14.15 The vector scanning process.

(a) (b)

Figure 14.14 Two examples of dot blanking scanning. (a) Si, (b) carbon ﬁlm

on Si.

Focused ion beam systems366

results of a vector scan are better than those of a raster scan. An arc of a

circle is described at line 5. The direction of the trace is described at line 19.

The method of using graphical software to interpret graphical data is suitable

for complicated shapes that are difﬁcult to describe using script language alone.

Graphical data are expressed by .bmp format, GDSII format, and so on. The

software reads the data and generates a vector for the beam trace. Operators

can decide on the direction of the ion beam trace. By this method, complicated

shapes are described but it is difﬁcult to deﬁne detail.

Scan signals in nanotechnology

Applications in FIB nanotechnology anticipate creating not only ﬂat shapes

but also three-dimensional shapes. Here we introduce an example of scan

signals for creating 3D shapes. Figure 14.17 is an example of creating 3D

shapes with sputter etching. There are several methods for creating scan

signals but, as shown in the ﬁgure, at the time the 3D shapes are created (not

only 2D positions) the ion beam irradiation time for each pixel is controlled

and a speciﬁed 3D shape is realized. Note that the effects of shaping or rede-

position on the processed part must be corrected.

1. FOV=40000 /Size of process ﬁeld

2. p=FOV/800 practical scale on

a specimen surface.

/Conversion from script coordination

3. X0=0 /X coordination of the starting point

4. Y0=0 /Y coordination of the starting point

5. R1=1000 /Size of circle

6. Xp=2000 /X pitch of circles

7. Yp=2000 /Y pitch of circles

8. Xa=5 /X number of repeat

9. Ya=5 /Y number of repeat

10. px0=4*X0/p /Conversion of X starting point

11. px0=4*Y0/p /Conversion of Y starting point

12. pr=4*R1/p /Conversion of size of circle

13. pxp=4*Xp/p /Conversion of X pitch

14. pyp=4*Yp/p /Conversion of Y pitch

15. for i in xrange(0,ya,1): /Repeat order fo Y

16. for j in xrange(0,Xa,1): /Repeat order fo X

17. for k in xrange(1,pr,2): /Repeat order fo circle

18. blanking(0) /BLK Off

19. circle(px0+pxp*i,py0+pyp*j,k,0,360 /Detail of circle

20. blanking(1) /BLK On

21. point(px0+pxp*i+k,py0+pyp*j) /End

Focused ion beam systems as a multifunctional tool for nanotechnology 367