1ｓｔ International Symposium on Thermal Design and Thermophysical Property for Electronics, June 18-20, 2008, Tsukuba

Development of thin film reference material for thermal diffusivity○Takashi YAGI1, Naoyuki TAKETOSHI1, Tetsuya BABA2
1. Thermophysical Properties Section, NMIJ, AIST
Tsukuba Central 3, 1-1-1, Umezono, Tsukuba, Ibaraki 305-8563, JAPAN
Corresponding author: Takashi YAGI, E-mail: t-yagi@aist.go.jp
2. Material Properties and Metrological Statistics Division, NMIJ, AIST
Tsukuba Central 3, 1-1-1, Umezono, Tsukuba, Ibaraki 305-8563, JAPANThere has came into existence some practical instruments to measure the thermophysical properties of thin films, and those
instruments have been recognized to be of benefit to R&D in the thin film technology. However, there are no systems to calibratesuch instruments by standards that are guaranteed to trace up to national standards or reference materials. Accordingly, NMIJ/AIST
has established a high speed laser flash apparatus as a national standard in Japan. The apparatus can measure the thermal diffusivityof thin films normal to the surface with small uncertainty using a thermoreflectance technique. Based on this apparatus, a calibration
service of the thermal diffusion time for user’s specimen has started at the end of 2007 FY.
We have moved to develop a thin film reference material for thermal diffusivity, since the reference material is more useful tocalibrate the practical instrument. The reference material is made of a titanium nitride (TiN) thin film with 700 nm in thickness. TheTiN film was deposited on a synthesized silica wafer (76.2 mm in diameter, 0.525 mm in thickness, Shin-Etsu Chemical Co., Ltd.)
using a reactive dc magnetron sputtering. The target metal is pure Ti and the sputtering gas is pure N2 gas. The TiN film on the wafer
was patterned with each individual chips using a photo-resist and metal-resist coatings by lithography technique. The chip is 10mm x10 mm, and an etched line of 1 mm x 100 µm is positioned at the center of the film. This etched line is for a surface profiler in order
to measure the film thickness. The thermal diffusion time of a typical film is 145.9 × 10-9 s, and its expanded uncertainty is 3.6 %. We
are planning to establish the thin film reference materials ｂｙ end of 2008 FY in Japan.

1. Introduction
Thermophysical properties for thin films become

2. Development of Reference thin film
2.1 Standard apparatus for thermal diffusion time

important in order to manage heat transport in high

NMIJ/AIST has established a standard apparatus totechnology devices. For example, heat management between

measure the thermal diffusion time, τf of the reference film.

interconnects and interlayer dielectric materials for aThe thermal diffusivity, κf is represented as follows equation,

microprocessor will be a critical problem in very near future.
In additions, rewritable optical disks, hard disks using heatassisted magnetic recording technology, phase change

κf =d2

τf(1)memories and thermo-electric devices also require reliable

where d is a characteristic length, i.e. film thickness. Thus,

thermophysical properties values of the thin film materials
for the optimum thermal design. There has came into

the value of κf can be derived from the data of τf and d. The
standard apparatus is shown in Figure 1. Twoexistence some practical instruments [1-3] and methods [4] to

sub-nanosecond pulsed lasers with wavelength of 1064 nm

measure the thermophysical properties of thin films, andthose instruments have been recognized to be of benefit to
R&D in the thin film technology. Accordingly, NMIJ/AIST
has established a high speed laser flash apparatus as anational standard in Japan. The apparatus can measure thethermal diffusion time of thin films normal to the surface
with small uncertainty using a thermoreflectance technique.
We have moved to develop a thin film reference material
for thermal diffusivity. This reference film is of use toguarantee the practical instruments for suppliers, andinstrument users also have the benefit with regard to
calibration for their own instrument. In this paper,
preparation of the reference material and measurement
procedure are reported.

Figure 1 Block diagram of a standard apparatus for the thermal
diffusion time measurement.

for a pump beam and 782 nm for a probe beam are used. TheThe synthesized TiN film

pulse width of both lasers is 500 ps. The pump laser pulse ison the wafer was patterned

irradiated at a rear surface of the film through the substrate,with individual chips using

which induces heat flux inside the film. This thermal

adiffusion is one-dimensional normal to the film surface, sincemetal-resist coatings by athe spot diameter of the pump laser is 300 µm, which is muchlithography technique as

Figurelarger than the typical film thickness such as 1 µm. In order
to measure the thermal diffusion time of the film thickness,shown in Figure 2. The chippositioned at the center ofsize is 10 mm x 10 mm, andthe reference film.

the transient temperature change at the bare surface is

32 pieces of chip was mademeasured using a thermoreflectance technique as follows.

from a wafer. There is an etched line of 1 mm x 100 µm
positioned at the center of the chip. This etched line is for a

The probe beam is focused on the bare surface opposite to thephoto-resistand4

Etchedline

heated area with a spot diameter of 100 µm. The power of theprobe beam is 0.2 mW. Intensity change of the probe beam issurface profiler in order to measure the film thickness. The

detected by a Si photo detector. The pulse repetition

was deposited over the TiN film. Then a photo-resist wasfrequencies of the both lasers are locked by signals with 500

coated over the Cr/Au layer. The chip pattern as shown in

kHz from a function generator. Then, the phase of the signal

Figure 2 was copied on the photo-resist. The photo-resist wasfor pump beam to that for probe beam is controlled to record

developed and the Cr/Au layer was etched as the pattern.

a temperature history curve.

Subsequently, the exposed part of TiN layer was etched.

detail of the patterning procedure is as follows; a Cr/Au layer

Finally, the Cr/Au layer was perfectly removed. After
patterning, the wafer was cut into each chip.

3. Results and Discussion
3.1 Physical properties of reference material
Figure 3 is appearance of the reference thin films.
Figure 4 shows a microscope photograph of the typicaletched line positioned at the center of the film. Users can use
this etched line for a film thickness measurement by asurface profiler. Figure 5 is a cross sectional photograph forthe film by a transmitted electron microscope (TEM).
Figure 2 Illustration of thin film reference material for

Columnar structure is clearly seen in the photograph. The

thermal diffusivity.

microscopic surface roughness originates in these columns.
However, this roughness is relatively smaller than the2.2 Fabrication of Reference material

uncertainty of the surface profiler (several percent).

The reference thin film

Typical physical properties of the reference film are

made of TiN was prepared

given in Table 1. The resistivity data shows that the film is ausing a dc reactive magnetron

good electrical conductor. Thus, the film exhibits a metal likesputtering. Ohtsuka et al[5]has reported that the thermalreflectivity. At the measurement of thermal diffusion time, τffor the film, almost optical energy of the pump laser pulse isdiffusivity

absorbed within the skin depth and converted to heat. TheofTiN

films

changes from 0.2 to 1.4 × 10-5

m2/s according to the nitrogenconcentrations. The films ofunder- or over-stoichiometricFigure 3 Appearance ofthe reference thin films
made of TiN.

values of the skin depth are 21 nm at 1065.3 nm, and theeffect of this penetration on the thermal diffusion isconsidered when the temperature history curve of the film isanalyzed.

composition show smaller thermal diffusivity than that ofstoichiometric composition. Consequently, we chose theover-stoichiometric composition, since a small thermal

Table 1 Physical properties of the reference film (TiN).

diffusivity is suitable for reducing the film thickness. A

Thicksynthesized silica wafer (76.2 mm in diameter, 0.525 mm inthickness, Shin-Etsu Chemical Co., Ltd.) is used for thesubstrate. The surface roughness of the wafer is smaller than
0.2 nm. The target metal is pure Ti and the sputtering gas is
pure N2 gas. The nominal thickness of the film was 700 nm.

Resistivit

Refractive

Extinction

Reflecti

Skin

ness

y,index,coefficient

vity,depth,

d/nm

ρ/Ωmn,kR

l/nm

1.530*

2.584*

0.53*

24*

640

1.4×10-61.800**

3.952**
0.69**
21***Values at 781.9 nm in wavelength**Values at 1065.3 nm in wavelengthThermoreflectance signal/a.u.

Figure 5 Cross sectional TEM image of the reference film.
3.2 Calibration result

TiN 640 nm
analysis curve

050Time/ns100150

Figure 5 Temperature history curve for the reference thin

Figure 5 shows the typical temperature history curve offilm. Thickness of the film is 640 nm.

the reference film obtained by the standard apparatus forthermal diffusion time measurement. The shape of the curveis very similar to that obtained by the conventional laser flash
method, because the configuration of the apparatus is alsosame as that of the laser flash. Thus, from this temperature
history curve, the thermal diffusion time can be determinedreliably with uncertainty evaluation based on Guide to theExpression of Uncertainty in Measurement (GUM).
The uncertainty evaluation was carried out with respect
to the time scale, the origin of the pulse heating for theapparatus, analysis of the value of τf based on the analyticalsolution[6] and the effect of steady temperature rise of thefilm due to the laser heating. The result of the calibration isNOMENCLATURE
κf:thermal diffusivity, m2/s
τf:thermal diffusion time, sd:thickness, mρf:resistivity, Ωmn:refractive index
k:extinction coefficient
R:reflectivity
l:skin depth, mgiven in Table 2 along with the expanded uncertainty. Thecalibrated value of τf is 145.9×10-9 s with the expanded
uncertainty of 3.6 %. The value of the thermal diffusivity forthe film calculated from the value of τf and the film thickness
d using eq. (1) is 2.8×10-6 m2/s, which coincides with thevalue of that for over-stoichiometric TiN film[5] reported byOhtsuka et al.
Table 2 Calibration results of the reference thin film.
Thermal diffusion time, τf for
the reference film

Expanded uncertainty,
U(k = 2)

(TiN 640 nm)
145.9×10-9 s

5.2×10-9 s (3.6 %)4. Conclusions
The development of the reference thin film for thermal
diffusivity is under way. This reference thin film is expected
to be of use to guarantee and calibrate the practical
instruments widely. We are planning to establish thereference material by end of 2008FY.

5. Acknowledgement
This work was supported by the Nanotechnology
Material Metrology Project of the New Energy and Industrial
Technology Development Organization (NEDO).

REFERENCES[1] T. Baba, N. Taketoshi, K. Hatori, K. Shinzato, T.
Yagi, Y. Sato, Y. Shigesato, Proc. 25th Jpn. Symp.
Thermophys. Prop., (2004), 240.
[2] R. Kato, I. Hatta, Int. J. Thermophys. 26, (2005),
179.
[3] I. Hatta, Y. Sasuga, R. Kato, A. Maezono, Rev. Sci.
Instrum., vol. 56, (1985), 1643.
[4] ISO/TTA4:2002(E), “Measreument of thermal
conductivity of thin films on silicon substrates.”[5] T. Ohtsuka,, A. Miyamura, Y. Sato, Y. Shigesato, T.
Yagi, N. Taketoshi and T. Baba, Proc. of 26th Jpn.
Symp. on Thermophys. Prop. (2005) A113
[6] N. Taketoshi, T. Baba, A. Ono, Meas. Sci. Technol.,12, (2001), 2064.