23G07
Non-contact monitoring of wafer temperature in plasma processing
by using optical interferometer
光干渉計を用いたプラズマプロセス中の非接触ウエハ温度モニタリング
Takayuki Ohta1, Takayoshi Tsutsumi2, Masafumi Ito1 and Masaru Hori3
太田貴之, 堤 隆嘉，伊藤昌文，堀 勝
1

Faculty of Science and Technology, Meijo University
1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan
名城大学 理工学部 〒468-8502 名古屋市天白区塩釜口1-501
2Graduate School of Systems Engineering, Wakayama University930, Sakaedani, Wakayama 640-8510, Japan
和歌山大学 システム工学部 〒640-8510 和歌山市栄谷930
3Graduate School of Electrical Engineering, Nagoya University, Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
名古屋大学大学院 電子工学専攻 〒464-8603 名古屋市千種区不老町
In order to control the processing such as plasma etching and plasma enhanced chemical vapor deposition
precisely, the temperature of Si substrate was monitored with non-contact by optical low coherence
interferometry. We demonstrated the real-time monitoring of the temperatures of a Si wafer in capacitively
coupled plasmas. The response of the low-coherence interferometry was faster than that of a fluorescencetemperature sensor, and the low-coherence interferometry enabled the monitoring of the actual temperatureduring plasma processing.

1. Introduction
Various plasma processing technologies suchas the plasma etching and plasma enhanced
chemical vapor deposition (PECVD) have beenused for the fabrication of semiconductor devices.
Control of the temperature of the wafer in various
plasma processing technologies is important toprocess with high-preciseness [1, 2]. However,contact measurements such as thermo-couples
and fluorescent temperature sensor cannot
measure the accurate temperature in low pressure
due to poor thermal conductance. To solve thisproblem, we have performed non-contacttemperature and thickness measurement of
substrate by using time domain low coherence
interferometer [3, 4]. This system has been
experimentally performed to measure an accurate
wafer temperature in plasma processing [5].

However, the measurement time depends the scantime of the reference mirror. Moreover thevibration noise of the scanning reference mirror
reduces the accuracy of temperature measurement.
In order to achieve the real-time monitoring of
the temperature, it is required that themeasurement speed improve to be about 1 msec.
We focused on frequency domain low coherence
interferometer to improve more accuracy andsimplify the interferometer of optical sysytem [6].
The Cross-Correlation Type (CCT) using
frequency domain low coherence interferometermeasures the optical path length of wafer fromcross-correlation terms which shows interferences
between the reference arm and the sample arm by
the inverse Fourier transform of spectralinterferogram. On the other hand, theAuto-Correlation Type (ACT) without a reference

Optical fiber Coupler
50:50

L1

SiAr gas

SLD:1570nm
Isolator

PC

Optical spectrum
analyzer

Sample arm

L2M
Reference armFig. 1. Optical system for temperature measurement.
APP:Atmospheric Pressure Plasma. M:Mirror. L:Lens

APP

High voltage

Intensity [nW]

100002000 4000 6000 8000
Wavelength [nm]10000

Fig. 2. the spectral interferograms(CCT).

(b)

1.5
1(a)

0.5
002000
4000
6000
Optical path length [m]Fig. 3. the inverse fourier transforms of the spectralinterferograms. Optical path length of Si for CCT(a)and ACT(b).

408Thermocouples
ACT35

630

425

2200100200300 400
Time [s]500

Voltage [V]

3. Results
Figure 2 shows the spectral interferogram ofCCT at room temperature. Figure 3 shows theinverse Fourier transform of the spectralinterferogram of Figure 2. The correlation signalsof ACT appeared at optical path length of 0 and5593 μm and those of the CCT appeared at 484 μm
and 6077 μm, which are interferences at the top andthe bottom of Si wafer, respectively. The opticalpath length of Si wafer is equivalent to be theinterval of the peaks of CCT [Fig.3. (a)] or ACT[Fig.3. (b)]. The relation between the temperature ofSi (780μm) and the optical path length was derivedfrom the experiment using furnace and described asfollows
T = - 1.6 × 10-2 L2 + 9.4 × 10L - 1.4 × 105 (1)were T (°C) and L (μm) is the temperature andthe optical path length, respectively. The standard
deviations of the optical path length of CCT andACT were 92 nm and 7 nm at room temperature,
which were equivalent to be 0.52 °C and 0.04 °C intemperature. It is found that ACT improved themeasurement accuracy in addition to simplify theoptical system.
The temperature measurement of Si wafer by
ACT was performed during the plasma treatment byusing atmospheric pressure plasma. Argon gas wasflowed with 3 L/min between the two electrodes

200

Intensity [a.u.]

2. Experimental
The setup for the measurement system offrequency domain low coherence interferometer isshown in Figure 1. Temperature measurement using
ACT was performed without the reference arm.
SLD (Super luminescent diode; center wavelength:1570 nm, spectral width: 38 nm) was used as thelow coherence light source. This system canmonitor the temperature by measuring optical path
length of Si wafer that depends on temperature. The
optical path length of Si wafer was deduced fromthe position of interferences which were obtainedby the inverse Fourier transform of the spectralinterferogram. The spectral interferogram wasmeasured by the optical spectrum analyzer which
spectral range and wavelength resolution were1520~1620 nm and 0.065 nm, respectively.

and the plasma was generated by applying a highvoltage. Figure 4 shows time evolution of Si wafer
temperature measured by ACT and thermocouple.
The temperature was successfully measured byusing ACT during plasma processing.

Temperature [℃]mirror can be used for Si wafer which has highreflectivity of the surface and is expected toimprove the measurement accuracy and time. We
report the temperature measurement of Si wafer
using ACT. We have demonstrated thetemperature measurement of Si wafer during thetreatment of atmospheric pressure plasma by
ACT.

0600Fig. 4. time evolution of Si wafer temperature measured by
ACT and thermocouple

References[1] T. Ohiwa et al., Jpn. Appl. Phys., 31, 405 (1992).
[2]H. Oshino et al., J. Electrochem. Soc. 147 (2000).
[3] K. Takeda et al., Jpn. J. Appl. Phys., 43, 7737 (2004).
[4] T. Ohta et al., J. Appl. Phys., 105, 013110 (2009).
[5] C. Koshimizu et al., Appl. Phys., 3, 056201 (2010).
[6] A. F. Fercher et al., Opt. Commun. 117, 43-48,(1995).