Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

21.5 Near-Field Raman Imaging Using Optically Trapped Dielectric Microsphere 689

(25 μ m pinhole) with Olympus microscope objective (60 × NA = 1.2 water immer-

sion). A double - frequency Nd:YAG laser (532 nm, 100 mW; CNI Laser) is used as

the excitation laser, and coupled into a 3.5 μ m - core diameter single - mode ﬁ ber. The

linearly polarized Gaussian beam (TEM00) that is used to excite the Raman signal

is also used to trap the microsphere. The laser beam is incident on the sample

through the microsphere. The sample is placed in a sample cell with diluted poly-

styrene microspheres in deionized water; the sample cell is then placed on a transla-

tion stage, which can be moved coarsely along the x - and y - axes (it can also be ﬁ nely

moved by using a piezostage). The Raman scattered light was directed to a 1800

grooves mm

− 1

grating, and detected using a TE - cooled CCD. Scanning electron

microscopy images were recorded using ﬁ eld emission SEM (JEOL JSM - 6700F).

This technique has many advantages over the previous near - ﬁ eld techniques.

The Raman signal collected with microsphere using the present technique is

always much stronger than that without microsphere, typically two - to ﬁ vefold

depending on the objective lens used. This is a critical advantage over the aperture

near - ﬁ eld technique, which is orders of magnitude weaker than the corresponding

far - ﬁ eld signal. As the laser light is focused on the sample through the micro-

sphere, there is no background far - ﬁ eld signal in this experimental conﬁ guration,

which has been one of the major problems in TERS. There is also no requirement

to use a metal or metal - coated probe (e.g., a metal - coated AFM tip) to perform the

experiment. The strong near - ﬁ eld Raman signal, and the simplicity in performing

the experiment, make this technique attractive, easy, and fast. The reproducibility

is also excellent, at near - 100% level. Moreover, the technique can also be used on

different types of sample.

The laser spot size of the near - ﬁ eld Raman technique was determined using

a scanning - edge method [38] . For this, the trapped microsphere was scanned

across a Si/SiO

2

structure and the scanning spectrum (Figure 21.3 c) ﬁ tted with

Equation 21.1 :

Ix

P

erf

xx

w

o

()

=+

−

()

⎛

⎝

⎜

⎞

⎠

⎟

{}

2

1

2

(21.1)

where P is the total power contained in the laser beam, x is the position of

the scanning edge, x

0

is the center of the beam, and w is the 1/ e

2

half - width. The

spot size of the beam, the full width at half maximum ( FWHM ), was calculated

to be ∼ 80 nm, from the following relationship:

FWHM = 22ln w.

The system was used to study the SiGe/Si device sample with poly - Si gate and

SiGe stressors, as shown in the electron micrograph in Figure 21.8 . The patterned

wafers used in this study were prepared using 65 nm device technology. After

spacer formation and Si recess etching, the wafers were cleaned and the epitaxial

SiGe growth was performed using a commercially available low - pressure chemical

vapor deposition ( LPCVD ) system. The capability of the technique was also dem-

onstrated by studying the strain on the channel below the poly - Si gates, which is

compressively strained by the SiGe stressors.

690 21 Near-Field Raman Imaging of Nanostructures and Devices

Straining the silicon can suppress the inter - valley scattering and reduce the

effective carrier mass; the result is an improvement of the effective carrier mobility

in the Si channel. In the past, the semiconductor industry has used mechanical

strain as an alternative to physical scaling to improve transistor performance [39] .

An appropriate strain, when applied to the channel region, can lead to a signiﬁ cant

improvement in transistor performance. However, in the complementary metal -

oxide - semiconductor ( CMOS ) transistor, the negative (n) - MOS and positive (p) -

MOS must be strained differently. A compressive strain is known to be beneﬁ cial

for p - MOS, while a tensile strain is known to improve n - MOS performance [40,

41] . It is for this reason that a technique to characterize strain with sub - 100 nm

resolution, on a reliable basis, is in such high demand.

Micro - Raman spectroscopy has long been a popular tool for strain measure-

ments, because it is both nondestructive and quantitative [42 – 44] . Compressive

strain shifts the Raman peak to higher frequency, while the tensile strain results

in a red shift [45] . However, the spatial resolution of micro - Raman makes it impos-

sible to be used for strain characterization in sub - 100 nm semiconductor devices.

Converging beam electron diffraction ( CBED ) in TEM can be used to characterize

the strain with a nanometer - scale resolution [46, 47] . However, destructive

and complicated sample preparations (which may alter the original strain ﬁ eld)

have made this technique undesirable for large - scale strain characterization. Con-

sequently, whilst the development of a reliable, nondestructive, quantitative

assessment of strain on the nanometer - scale is critical, there is at present no

Figure 21.8 Scanning electron microscopy image with a

cross - sectional diagram of the sample. The line scan of

Raman Si intensity is shown in yellow, and the peak position

in purple. The scanning step size was 20 nm with a 1 s

integration time. The line width measured from the Si

intensity proﬁ le was between 80 and 100 nm.

21.5 Near-Field Raman Imaging Using Optically Trapped Dielectric Microsphere 691

such characterization technique available on the market. Here, we show the

strain measurement on the 65 nm device lines with a much improved repeatability

and SNR.

Figure 21.8 shows the SEM image as the background and the schematic cross -

section diagram of the sample. This ﬁ gure also shows the Raman line - scan Si

intensity (yellow) and peak position (purple) proﬁ les across the sample, with a

scanning step size of 20 nm and a 1 s integration time. Both lines show an excel-

lent correlation with the structure. Higher values of the intensity line scan cor-

respond to positions of the poly - Si lines. It is important to note that the poly - Si

lines of 45 nm width and 400 nm separations can easily be distinguished. The

line - width measured from the FWHM of the Si - peak intensity is between 80 and

100 nm. The Si

–

Si peak position in regions below the poly - Si lines (purple line)

is at a higher frequency, reﬂ ecting the fact that Si is under compressive strain

exerted by the SiGe stressors. In other regions with SiGe lines (yellow line), the

Si

–

Si peak position is at a lower frequency than that of the Si substrate, as would

be expected for unstrained poly - Si lines. The difference between the center poly - Si

line (marked 0) and its neighboring lines is also apparent, showing the high resolv-

ing power of the technique.

Figure 21.9 a and b show the Raman spectra recorded in the SiGe line region

and on top of the poly - Si line, respectively. Each spectrum was ﬁ tted with three

Figure 21.9 Raman spectra from SiGe and

poly - Si lines with ﬁ tted peaks using Lorentzian

function. (a) The Raman peaks correspond to

Si

–

Si phonon vibrations from the SiGe

( ∼ 510 cm

− 1

), tensile - strained Si just below the

SiGe ( ∼ 519 cm

− 1

), and the Si substrate below

( ∼ 520 cm

− 1

), respectively; (b) The Raman

peaks correspond to Si

–

Si phonon vibrations

poly - Si and bulk Si below ( ∼ 516 cm

− 1

and

∼ 520 cm

− 1

), and compressively strained Si in

the channel region ( ∼ 522 cm

− 1

).

692 21 Near-Field Raman Imaging of Nanostructures and Devices

Lorentzian peaks. In Figure 21.9 a, the Raman peaks from SiGe line correspond

to Si

–

Si phonon vibrations from the SiGe ( ∼ 510 cm

− 1

), tensile - strained Si just

below the SiGe ( ∼ 519 cm

− 1

), and the Si substrate below ( ∼ 520 cm

− 1

), respectively.

Similarly, in Figure 21.9 b, the Raman peaks correspond to the Si

–

Si phonon

vibrations of poly - Si and bulk Si below ( ∼ 516

− 1

and ∼ 520 cm

− 1

), and another one is

from the compressively strained Si in the channel region ( ∼ 522 cm

− 1

).

Figure 21.10 shows the SEM image of the device sample, together with the

detailed illustration diagram. From this ﬁ gure, the line proﬁ le of the intensity of

Si

–

Si phonon vibrations from the SiGe (yellow), and the Si

–

Si peak position

(purple), can be seen. The results show an excellent correspondence to the device

structure, with a good SNR. The line proﬁ le data (intensity and peak positions)

are extracted from an area of Raman mapping of 4.0 × 1.3 μ m

2

(100 × 32 pixels),

as shown in Figure 21.11 , which took about 6 min to complete.

Figure 21.11 a shows the Raman image from the intensity of Si

–

Si phonon

vibrations from the SiGe of the structure shown in Figure 21.10 . For comparison,

Figure 21.11 b shows the image from confocal Raman set - up (far - ﬁ eld imaging).

It is clear that the present near - ﬁ eld technique can resolve the periodic lines

with excellent repeatabiliy that far - ﬁ eld technique cannot resolve. The three -

dimensional ( 3 - D ) Raman image in Figure 21.11 c is based on the peak position

of Si

–

Si phonon vibration from Si at ∼ 520 cm

− 1

. From this image, it is possible to

study the higher compressive strain regions, which are under the poly - Si lines and

Figure 21.10 Scanning electron microscopy image with a

cross - sectional diagram of periodic poly - Si lines and SiGe

stressors. The line scan of Raman Si

–

Si intensity from SiGe is

shown in yellow, and the Si

–

Si peak position in purple. The

line scans show excellent correspondence with the structure,

and a good SNR.

21.5 Near-Field Raman Imaging Using Optically Trapped Dielectric Microsphere 693

are compressively strained by the SiGe stressors. The fact that it took only about

6 min to carry out the mapping means that the high - resolution Raman imaging

using this technique can be performed within a reasonable time.

Apart from the device sample, in order to show the capability of the technique

for other types of sample, Raman mapping was also performed on CNTs and gold

nanopatterns. The puriﬁ ed HiPCO single - walled carbon nanotube s ( SWNT s) were

purchased from Carbon Nanotechnologies. The HiPCO SWNTs (10 mg) were

dispersed in 100 ml D

2

O solution with 1 wt% sodium dodecyl sulfate ( SDS ). The

dispersion was then treated by probe sonication (Sonics & Materials Inc.; Model

VCX 130) at a power level of 250 W for 30 min, followed by ultracentrifugation at

140 000 × g for 4 hours. The supernatant was drop - cast on a Si substrate and dried

in air. Individual (or slightly bundled) tubes on Si were then obtained after rinsing

the Si substrate with pure water. A gold nanopattern was also fabricated on a

silicon substrate. For this, polystyrene microspheres (diameter 0.5 μ m) in solution

were dispersed on a silicon substrate, using the drop - coating method. The poly-

styrene microspheres self - assembled to form a monolayer, and a thin ﬁ lm of gold,

of ∼ 100 nm thickness, was deposited onto the Si substrate, using DC magnetron

sputtering. The sample was sonicated for 1 min in chloroform to remove the

Figure 21.11 Raman images (4.0 × 1.3 μ m

2

) of the periodic

poly - Si lines and SiGe stressors obtained in approximately

6 min. (a) Near - ﬁ eld Raman image from the Si

–

Si peak

intensity from SiGe; (b) Confocal Raman image from the

Si

–

Si peak intensity from SiGe; (c) 3 - D near - ﬁ eld Raman

image from the Si

–

Si peak position, showing the relative

strain at different regions.

694 21 Near-Field Raman Imaging of Nanostructures and Devices

polystyrene microspheres, and then annealed in argon ambient at 400 ° C for

30 min. The size of the gold nanopattern was ∼ 100 nm.

Figure 21.12 a – c show the 5.0 × 5.0 μ m

2

(100 × 100 pixels) Raman images of the

CNTs and gold nanopattern. Figure 21.12 a shows the Raman image from the G

band ( ∼ 1590 cm

− 1

) of CNTs; the inset shows the Raman spectrum from the CNTs

on a silicon substrate. Figure 21.12 b shows the Raman image from the Si

–

Si peak

intensity, where the darker regions correspond to the gold nanopatterns. It is clear

that the ∼ 100 nm gold nanopattern can be clearly resolved using the new tech-

nique. The near - ﬁ eld Raman image corresponds well to the electron micrograph,

as shown in the inset of Figure 21.12 b. Figure 21.12 c shows the confocal Raman

image of the gold nanopatterns, which show no pattern details. Hence, the capabil-

ity of this technique for various samples is proven.

Figure 21.12 Raman images (5.0 × 5.0 μ m

2

of

the carbon nanotubes (CNTs) and gold

nanopatterns. (a) Near - ﬁ eld Raman image

from G band of CNTs; (b) Near - ﬁ eld Raman

image from the gold nanopatterns;

(c) Confocal Raman image from the gold

nanopatterns. The inset in (a) is the Raman

spectrum of the CNTs; the inset in (b) shows

the SEM image of the gold nanopatterns,

where the nanopattern size is ∼ 100 nm.

References 695

21.6

Conclusions

Characterization tools in the nanometer regime are very important to reveal and

utilize the unique properties of nanomaterials, and Raman imaging is indeed a

versatile tool for studying both materials and devices. In this chapter, we have

illustrated the value of Raman imaging by using three techniques:

• Micro - Raman imaging for the detection of Si C covalent bonding of CNTs grown

on a Si substrate. The technique is simple and easy to perform, but is limited in

its application to nanomaterials and devices by its limited spatial resolution.

• Apertureless near - ﬁ eld Raman imaging , where laser light is focused to the apex of

a metal or metal - coated tip. This method is much superior to the aperture

technique in terms of spatial resolution and SNR, but faces problems of wear

and tear, oxidation, and contamination of the tip apex (e.g., by carbon). These

may lead to severe adverse effects on the near - ﬁ eld enhancement. Far - ﬁ eld

signals may also be problematic.

• Microsphere near-ﬁ eld Raman imaging [48] , where laser is focused to a spot size

smaller than the diffraction limit through a dielectric microsphere that is trapped

using the optical tweezers principle. This method has many advantages over

previous near - ﬁ eld techniques: the Raman signal is stronger; there is no

background far - ﬁ eld signal; and the use of a metal or metal - coated tip is not

required. The strong near - ﬁ eld Raman signal and simplicity in carrying out the

experiments make this technique attractive, easy, and fast. The method ’ s

reproducibility is also excellent (close to 100%), and it can also be used on

different types of sample.

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Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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