Proceedings of 7th Thin Film Materials & Devices Meeting
November 5-6, 2010, Nara, Japan
http://www.tfmd.jp/Oxygen Plasma Followed by High-Pressure H2O Vapor Heat Treatment
Used for Passivation of Silicon Surfaces
S. Yoshidomi, M. Hasumi and T. Sameshima
Tokyo University of Agriculture and Technology, 2-24-16, Naka-cho, Koganei, Tokyo,
184-8588
Keywords: passivation, oxygen plasma, silicon, lifetime, recombination velocity

Abstract. We report passivation of silicon surfaces by combination of oxygen plasma withhigh-pressure H2O vapor heat treatment. 150 m-thick n-type silicon substrates were treatedwith radio frequency (RF) oxygen plasma at 100 W, 133 Pa and room temperature for 1 min.
The samples were subsequently heated with 8x105 Pa H2O vapor at 240oC. 3.5 nm thick SiO2layers were formed on the surfaces. The effective minority carrier lifetime and recombination
velocity were estimated to be 440 s and 17 cm/s. Those results indicate a possibility ofpassivation of silicon surfaces with thin SiO2 layers at 240oC.

Introduction
We want to passivate silicon surfaces well by forming thin oxide layers, which allow tunneling
effect current in order to fabricate solar cells [1]. When silicon passivated with thin oxide layers
is sandwiched by two different kinds of metals, the difference in their work functions can cause
built in potential in silicon. Holes and electrons excited by light illumination in silicon are
separated each other. Holes move to the metal with a high work function according to theinternal built in potential distribution. On the other hand, electrons move to the metal with a low
work function. Photo induced current are consequently produced through tunneling current via
thin oxide layers.
In this paper, we discuss combination of oxygen plasma treatment with high-pressure H2Ovapor heat treatment to achieve passivation of silicon surfaces by forming thin oxide layers on
silicon surfaces at low temperature [2,3]. We report analyses of the effective minority carrier
lifetime, recombination velocity, and oxide thickness to discuss passivation of silicon surface at
low temperatures.

Experimental procedure150-m thick n-type silicon wafers were prepared. Native oxide was removed by 5% diluted
HF solution. Top surfaces of the silicon substrates were then irradiated with 100-W RF plasma
at an oxygen gas pressure of 133 Pa and room temperature for 1 and 3 min. The other rearsurfaces of the silicon substrates were subsequently irradiated with oxygen plasma in the samemanner. Some substrates were kept with no plasma treatment for making high pressure H2Ovapor heat treatment alone. The samples were then annealed with 8x105-Pa H2O vapor attemperature ranging from 220 to 260oC for 3 h. The minority carrier effective lifetime eff was110107077-1

precisely
analyzedby
thephoto-induced carrier microwave
absorption method, as shown by aschematic apparatus in Fig. 1 [4,5].
The9.35
GHzmicrowave
transmittance measurement system
was constructed using waveguidetubes. It had a narrow gap for
placing a sample wafer. A small hole
was opened on a wall of theFig. 1: Schematic apparatus of microwave
waveguide to place an optical fibertransmittance measurement system.
for introducing light from a 620 nm
light-emitting diode (LED). The
light was switched by switching the operation voltage applied to the diode with pulse widths
ranging from 1x10-5 to 1x10-2 s. A Teflon plate was placed aslant in the waveguide tube toreflect and diffuse LED light. Consequently, the sample was uniformly illuminated at 1.55
mW/cm2 with LED light pulses. Microwave was coincidentally switched with the light pulse
turned ON or OFF, respectively, using a coincident switching circuit to obtain changes in
microwave transmittance during light ON or OFF. The microwave, which transmitted thesample, was rectified using a high-speed diode and integrated with time with a time constant of
5 s. The integrated voltages with different pulse widths was analyzed to determine eff. In thecase of eff lower than 1x10-5 s, continuous wave (CW) light illumination was used. eff was
obtained as, eff NG ,(1)where N is photo induced minority carrier density per unit area and G is a carrier generation
rate of 2.81x1015 s-1cm-2, which was previously determined by comparison of periodicallypulsed light illumination measurement with CW light illumination measurement [5]. Therecombination velocity of silicon surfaces S was estimated from the minority carrier lifetime.
We supposed that the bulk lifetime was long enough, and that eff was governed by S. S wasgiven as,Sd2 eff,(2)

where d is the thickness of silicon wafer [6,7].
SiO2 formation and its thickness were investigated by analysis of optical reflectivity spectra.
Silicon wafer coated with 12.6 nm-thick thermally grown SiO2 layers was used as a reference.
Reflectivity spectra of samples R(), where  is wavelength, and the reference Rr() weremeasured in ultraviolet range between 250 and 340 nm. The ratio of R()/Rr() was precisely
analyzed by numerical calculation program constructed with the multiple layered Fresnel
coefficient model and refractive indexes of SiO2 and crystalline silicon which depended on
wavelength [8]. Fitting calculated R()/Rr() to experimental R()/Rr() gave the optical judgeof SiO2 formation and its thickness.

Results and discussion

110107077-2

eff of initial bare silicon wafers was4.6 s in average. Oxygen plasma
treatment for 1 and 3 min. decreasedeff to 0.9 and 1.6 s, respectively, asshown by an arrow A on the verticalaxis in Fig. 2. Those eff resulted inthat oxygen plasma treatment
increased S to 8550 and 4350 cm/s,respectively, as shown by arrows on
the vertical axis in Fig. 3. 8x105 Pa
H2O vapor heat treatment for 3 hincreased eff, as shown in Fig. 2. In
the case of oxygen plasma treatment
Fig. 2: Effective minority carrier lifetime of n-type
for 3 min, eff increased to 210 s as
silicon wafers as a function of temperature for 8x105temperature of H2O vapor heating
Pa H2O vapor heat treatment for 3 h. The arrow A
increased to 240oC and it almost
presents lifetime as-oxygen plasma treated for 1 andleveled off for H2O vapor heating
3 min.
oabove 240 C. On the other hand, effmarkedly increased to 440 s as
temperature of H2O vapor heating
increased to 240oC in the case ofoxygen plasma treatment for 1 min. It
decreased to 240 s as temperature ofH2O vapor heating further increased
to 260oC. In contrast, eff, was low on
the order of 10 s when 8x105 PaH2O vapor heat treatment alone wasapplied. Those results of Fig. 2 showthat oxygen plasma had an importantrole for silicon surface passivation.
We believe that oxygen plasma
Fig. 3: Recombination velocity of n-type siliconincorporated oxygen atoms into thewafers as a function of temperature for 8x105 Pa
silicon surface regions, although it
H2O vapor heat treatment for 3 h.
alone did not complete surface
passivation. High pressure H2O vapor
heat treatment clearly passivated the silicon surfaces. Our previous study has revealed that highpressure H2O vapor heat treatment makes Si-O bonding networks stable [2]. We believe thatthin and stable SiO2 layers were formed on the silicon surfaces by bonding silicon and oxygen
atoms incorporated by oxygen plasma during 8x105 Pa H2O vapor heat treatment at 240oC for 3h. It is interesting that oxygen plasma only for 1 min. was effective for surface passivation withhigh eff. It suggests a possibility of surface passivation by formation of thin SiO 2 layer on thesilicon surfaces. S markedly decreased to 17 and 35 cm/s for oxygen plasma treatment for 1 and3 min, respectively, followed by 8x105 Pa H2O vapor heat treatment at 240oC for 3 h, as shownin Fig. 3. The very low recombination probability of hole minority carriers was achieved by thelow temperature processes.
Figure 4 shows experimental and calculated normalized reflectivity ratio. The ratio ofR()/Rr() defined in Experimental procedure was normalized by that at 250 nm R(250)/Rr(250).
An experimental spectrum was presented for oxygen plasma treatment for 1 min. followed by
8x105 Pa H2O vapor heat treatment at 240oC for 3 h in Fig. 4. Calculated spectra were also

110107077-3

presented for the SiO2 thickness
ranging from 0 to 4 nm with a 0.5 nm
step. The normalized reflectivity ratio
increased as the SiO2 thickness
increased for every wavelength
becausethereflectivity
ratio
increases 1 as the SiO2 thickness
increased 12.6 nm. Anti-reflection
effect is high in low wave length
region because of high refractive
index of SiO2 in low wave length
region. This effect resulted in
low normalized reflectivity ratio forlong wavelength region, as shown inFig. 4. There was a good coincidence
between the experimental spectrum
and the calculated spectrum withFig. 4: Experimental and calculated normalizedreflectivity ratio. The ratio of R()/Rr() defined inExperimental procedure was normalized by that at
250 nm R(250)/Rr(250).

3.5-nm thick SiO2 layer.
Figure 5 shows the thickness ofSiO2 layer analyzed by the method
described above as a function oftemperature of 8x105 Pa H2O vapor
heat treatment for 3 h. Similar valuesof the SiO2 thickness were obtained
among temperatures of H2O vapor
heating between 220 and 260oC foreach oxygen plasma condition. Thethickness of SiO2 layer was estimatedto be 3.5 nm in average for oxygen
plasma treatment for 1 min, which
was lower than 4.5 nm in average
formed by oxygen plasma for 3 min.
Fig. 5: Thickness of SiO2 layer as a function ofThe long eff of 440 s was achieved
temperature for 8x105 Pa H2O vapor heat treatment
by formation of 3.5-nm thick SiO2for 3 h.
layer of the silicon surfaces. On theother hand, 2-nm thick SiO2 layer
was also formed when bear silicon was heated with 8x105 Pa H2O vapor for 3 h alone.
Optimum conditions of oxygen plasma and H2O vapor heat treatment should be further
investigated to achieve passivation by formation of SiO2 layers thinner than 2 nm.

Summary
The passivation of the silicon surfaces was investigated using combination of oxygen plasma
treatment with high pressure H2O vapor heat treatment. 150-m thick n-type silicon substrates
were irradiated with 100-W RF plasma at an oxygen gas pressure of 133 Pa and room
temperature for 1 and 3 min. Samples were subsequently annealed in 8x105 Pa H2O vapor from
220 to 260oC for 3 h. eff was obtained by analysis of the photo-induced carrier microwave
absorption. The thickness of SiO2 layer was estimated by analysis of optical reflectivity spectra.

110107077-4

eff increased to 440 s as temperature of H2O vapor heating increased to 240oC for oxygen

plasma treatment for 1 min, while it increased to 210 s at most for oxygen plasma treatment
for 3 min. S was estimated to be 17 and 35 cm/s for oxygen plasma treatment for 1 and 3 min,respectively. Formation of SiO2 layer on the silicon surfaces was observed. The thickness ofSiO2 layer was estimated to be 3.5 and 4.5 nm for oxygen plasma treatment for 1 and 3 min,respectively. These results indicate a possibility of effective passivation of silicon surfaces bycombination of oxygen plasma treatment with high pressure H2O vapor heat treatment at 240oC.

Acknowledgments
This work was partly supported by a Grant-in-Aid for Science Research C (No. 22560292)
from the Ministry of Education, Culture, Sports, Science, and Technology, Japan

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