S8-2
On-wafer Monitoring Sensor for Precise Control of Plasma Processes
高精度プラズマプロセス用オンウエハーモニタリングセンサー
Seiji Samukawa
寒川誠二
1Intelligent Nano-Process laboratory, Institute of Fluid Science, Tohoku University
Katahira 2-1-1 Aoba-ku, Sendai, 980-8577, Japan
東北大学・流体科学研究所・知的ナノプロセス研究分野
仙台市青葉区片平２−１−１

We developed “on-wafer monitoring sensor”, such as, charge build-up sensor, sidewall conductivity
sensor, photon radiation sensor and ion energy analyzer, for in-situ monitoring in plasma etchingprocesses. By using these techniques, precise control of plasma processes and prediction of plasma
radiation damages in processes could be realized.
1.

Introduction
Gate insulator for metal-insulator silicon (MIS) devices
needs high breakdown voltage, low leakage current and low
interface states for robust semiconductor devices. During
the fabrication of the MIS devices using plasma process,the amounts of the ultraviolet (UV) photon are irradiated to
the gate insulators. In particular, the UV with wavelengths
more than the band gap energy of the dielectric film cangenerate electron-hole pairs in the UV-irradiated dielectricfilms. This generation increases the charge densities
trapped in dielectric film and in the interface [1], thus
affecting the conductivity of the dielectric film layer [2,3].
Especially, the image of the charge-coupled devices(CCDs) was degradated by the UV photon radiations.
Because the SiO2 / Si interface state were generated by theUV photon radiation. The generation of the interface state
density might caused by the generation of the electron-holepairs in dielectric films [4]. Therefore, it is necessary toevaluate the generation of the electron-hole pairs indielectric films during the plasma etching process toimprove the reliability of the MIS devices.
However, it spends high costs a lot of time to evaluate
the UV radiation damage by using the actual MSI devices.
Thus, we proposed the “on-wafer monitoring” system forthe evaluation of UV radiation damages by simple structuredevices [5]. The on-wafer monitoring devices consist of
only the insulator of the MIS devices, and it is easily toevaluate the UV radiation damage by measuring the“plasma-induced current” in insulator films during theplasma etching processes. We had already reported that thegeneration of the plasma-induced current depended on theUV wavelength that depends on the gas chemistry [5].
In this paper, we investigated the UV radiation damagesby measuring the plasma-induced current in on-wafer
monitoring devices with several insulator materials orstructures. In addition, we evaluate the effect of the gas
chemistry and compared with the damages in CCDs.
2.

Experimental

2.1 Experimental setup
Figure 1 shows the experimental setup for the plasmairradiation. Applying 13.56 MHz RF power to one turnantenna generates CF4/O2, C2F4/O2 and C4F8/O2 inductively
coupled plasma. We used ultraviolet (UV) spectrometer tomeasure UV light intensity. The flow ratio of thefluorocarbon gas verses oxygen were fixed at 50 sccm / 15
sccm under the 2.6 Pa gas pressure. The electron density
were fixed at 5.5×1010 cm-3 by varying the RF power.

Fig. 1 Experimental setup. Plasma was irradiated uponon-wafer monitoring devices
2.2 On-wafer monitoring devices
To evaluate the plasma-induced electron-hole pairs indielectric films, we used the on-wafer monitoring device.
We made four kinds of devices. Every device has thepolycrystalline- silicon (poly-Si) electrode (100 nm) on thethermal oxidized silicon substrate (SiO2: 500 nm). Figure 2a) shows the on-wafer monitoring devices with SiO2 film.
The SiO2 thickness was 150 nm on the poly-Si electrode.
SiO2 film was deposited on the poly-Si electrode by low
pressure CVD (LP-CVD) process. In Figure 2 b) and c) ,the dielectric film consist of the SiO2 (75 nm)/Si3N4 (75nm) and Si3N4 (75 nm) SiO2 (75 nm) films, respectively.
We irradiated the each gas plasma to these on-wafer
monitoring devices and evaluated the generation ofelectron-hole pairs as “plasma-induced current” during theplasma etching process. Plasma-induced current is expected
to increase when the UV lights were higher energy than theband gap energy of the dielectric films. The band gap
energy of the SiO2 and Si3N4 film are 8.8 eV and 5.0 eV,respectively. Therefore the plasma-induced current mustdepend on the kind and structure of the dielectric films.
These plasma-induced currents were measured at the bias
voltage of –30 V. The chamber had ground potential. The
background current when turning off the plasma was lessthan 20pA.

electron-hole pairs in MIS devices by using a simple
on-wafer monitoring device. The generation of theelectron-hole pair depends on not only the gas chemistry
but also the structure of the insulators. This results werecaused by the differences of band gap energy between
dielectric films. This on-wafer monitoring devices is very
effective for controlling the UV radiation damages in MIS
devices such as CCD image sensors.
a)4.

Conclusion
We evaluate the generation of the UV inducedb)
SiO2

SiO2

Si3N4

SiO2

SiO2
Si substrate

Si substratec)
Si3N4
SiO2
SiO2
Si substrate

Plamsa-Induced Current (µA)

Fig.2 On-wafer monitoring device. Structure of the dielectricfilms consist of a) SiO2, b) SiO2 on Si3N4, c) Si3N4 on SiO2.
100

CF4/O2
C2F4/O2
C4F8/O2806040
200

SiO2

SiO2/Si3N4

Si3N4/SiO2Structure of Insulator Films

Fig.3 Plasma induced currents in SiO2, SiO2/Si3N4, andSi3N4/SiO2 film.
7

Increased in CCD dark cuurent(arb. unit)

3. Results and Discussion
Figure 3 shows the plasma-induced current in the dielectric
film of SiO2 , SiO2/Si3N4 and Si3N4/SiO2 . In the case of the
Si3N4/SiO2, the plasma induced current drastically reduced
comparing with the film of SiO2 and SiO2/Si3N4. This result
indicates that the generation of the electron-hole pairs ininsulator depends on the substance and structure of thedielectric films.
In the case of the SiO2 / Si3N4 structure, most ultraviolet
photons of less than 140 nm were absorbed in SiO2 film.
However the UV lights of longer than 140 nm can betransmitted through the SiO2 film because of the lower
energy photon of less than the energy of SiO2 band gap.
Then the transmitted UV lights of shorter than 250 nm (5eV, the band gap energy of the Si3N4 film) induce theelectron-hole pairs in the Si3N4 film. As a result theplasma-induced current were generated.
On the other hand, in the case of the Si3N4 / SiO2, most
UV lights of less than 250 nm (higher than 5 eV) wereabsorbed at the Si3N4 film on the SiO2. Because there arelittle UV light absorbed in the SiO2 film can transmit theSi3N4 film, the electron-hole pairs in the SiO2 film werehardly generated.
From this result, we found that the damage creation
mechanism strongly depend on the structure of the insulator
film.
In addition, we compared the plasma-induced damage ofCCD image sensors. We investigated the dependence ofgas-chemistry with the plasma-induced current in theon-wafer monitor devices. The insulator of the CCD image
sensor consists of the Si3N4 on SiO2 film [6]. Therefore, theplasma-induced current in the Si3N4/SiO2 wasinvestigated ,as shown in fig. 4. In the CCD image sensor,
the dark current was increased with an increase in gas
molecular weight [6]. The trends of dark current correspondto the plasma-induced current in the Si3N4/SiO2 film. Thismeans that the plasma-induced current and the CCD darkcurrent in case of the C4F8 were larger than those in othergases.
This result indicates that the damages in CCD image
sensor can be predicted by measuring the plasma-inducedcurrent. Moreover, we can control the damages in actual
MIS devices with evaluating the plasma-induced current
using a simple on-wafer monitoring device.

C4F8/O2

6
54
32
1CF4/O2

0
0.8

1.0

C2F4/O2

1.2

1.4

1.6

1.8

2.0

Plasma-induced current (µA)

Fig. 4 Comparison between CCD dark current andplasma-induced current.

References[1] T. Yunogami, T. Mizutani, K. Suzuki, and S. Nishimatsu : Jpn.
J. Appl. Phys. 28 (1989) 2172.
[2] C. Cismaru, J. L. Shohet, and J. P. Mcvittie : Appl. Phys. Lett.
76 (2000) 2191.
[3] C. Cismaru and J. L. Shohet : J. Appl. Phys. 88 (2000) 1742.
[4] M. Okigawa, Y. Ishikawa, and S. Samukawa: Jpn. J. Appl.
Phys. 28 (1989) 2172. Part 1 42 (2003) 2444.
[5] S. Samukawa, Y. Ishikawa, S. Kumagai, and M. Okigawa : Jpn.
J. Appl. Phys. 40 (2001) L1346.
[6] Y. Ishikawa, M. Okigawa, Y. Ichihashi and S. Samukawa:IEDM Technical Digest (2003) 16.3.