This is the peer reviewed version of the following article: Suzurikawa J, Nakao M, Takahashi H, "Surface Passivation of the Thin‐Film LAPS with
Perhydropolysilazane‐ Derived Silica Treated by O2 Plasma.", IEEJ Trans. Electr. Electron. Eng., 6, pp. 392‐393, 2011, which has been published in final
form at http://dx.doi.org/10.1002/tee.20674. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for
Self-Archiving.

Letter
Surface Passivation of the Thin-Film LAPS with
Perhydropolysilazane-Derived Silica Treated by O2 PlasmaJun Suzurikawa*, Member
Masayuki Nakao**, Member
Hirokazu Takahashi***, Member
Here we describe a novel method for surface passivation of the thin-film light addressable potentiometric sensor (LAPS). In
order to form a uniform passivation layer, perhydropolysilazane (PHPS) was utilized as a spin-coatable precursor of silica.
After transformation of PHPS into silica by baking, we introduced an O2 plasma treatment aiming for enhancement of the water
resistance of the PHPS-derived silica (PDS) film by completing the PHPS-to-silica conversion. To confirm the effect of the O2
plasma treatment, the PDS film was deposited on a thin-film LAPS electrode and tested by immersion in a cell culturing medium.
The immersion test demonstrated that the plasma-treated PDS film could keep the electrode stable longer than the untreated could.
With the treated PSD film of 600 nm in thickness, a lifetime of the thin-film LAPS was estimated at over two weeks, which is
sufficient for cell culturing experiments.
Keywords : perhydropolysilazane, silica, LAPS, passivation, O2 plasma treatment

1. Introduction

2. Materials and Methods

In order to improve the spatial resolution of the lightaddressable potentiometric sensor (LAPS), amorphous silicon(a-Si) has been utilized as a thin-film photoconductor instead ofbulk Si(1). Due to the vulnerability of a-Si to alkaline, thethin-film LAPS requires surface passivation for long-term use inbiological electrolytes like cell culturing media, pH of which mostoften ranges between 7.2 and 7.6. Although our previous works
have proposed a conductive passivation layer(2)~(4), few studies
investigated insulating materials for a stable electrolyte-insulatorsemiconductor (EIS) interface on the thin-film LAPS.
Recently, perhydropolysilazane (PHPS), which is an inorganic
polymer with Si-N skeleton, has drawn attention as a liquid
precursor of SiO2(5). Because of the easy fabrication process,spin-coat and bake, the PHPS-derived silica (PDS) coating could
be an attractive option for surface passivation of the thin-film
LAPS. To our knowledge, however, little is known on how
stable the PDS film is in the biological environment.
Here in this study we investigated the water resistance propertyof the PDS film by a long-term immersion test in a cell culturing
medium. Previous studies on PDS have pointed out that theextent of PHPS-to-silica conversion is a critical factor for the film
stability(5),(6). In order to enhance the water resistance of the PDS
film, therefore, we introduced and evaluated an O2 plasma
treatment, which is expected to remove nitride residues and otherimpurities at the PDS surface(7).

The thin-film LAPS electrodes were fabricated with the PDS
passivation coating. During the immersion test, dark impedancesand luminous sensitivities (dark to bright impedance ratios) of theelectrodes were monitored to detect the deterioration of PDS anda-Si(2). The different test conditions are summarized in Table 1.
A PHPS solution in xylene (20 wt% with catalysts for
low-temperature transformation; Clariant, NL110A) wasspin-coated on an SnO2 conductive glass substrate (AGCFabritech, A110U80), on which hydrogenated amorphous silicon(a-Si:H) had been deposited by chemical vapor deposition. Afterbaking at 250 °C for 2 h, the substrate was treated with O2 plasma
at 150 W for 5 min in a vacuumed reactor (Yamato, PR31). TheSnO2 layer was, then, partly exposed by wet etching of the a-Si:Hlayer to be wired to external devices. Lastly, a plastic dish with a
-4mm through hole on bottom was attached as a solutionchamber.
In the immersion test, the electrodes were exposed toDulbecco's Modified Eagle Medium (DMEM) in a humidified
incubator at 37 °C with or without 5 % CO2, which makes pH of
DMEM 7.2 or >9, respectively. The test condition with pH of >9
was used to accelerate electrode deterioration due to alkaline.
During the immersion, dark and bright impedances at 1 kHz wereintermittently measured with the LCR meter (Agilent
Table 1.

Summary of the test conditions.

Film thickness (nm)Electrode #* Department of Assistive Technology, Research Institute of National
Rehabilitation Center for Persons with Disabilities
4-1, Namiki, Tokorozawa-shi, Saitama 359-8555
E-mail address: suzurikawa-jun@rehab.go.jp** Graduate School of Engineering, The University of Tokyo
7-3-1, Bunkyo-ku, Tokyo 112-8656*** Research Center for Advanced Science and Technology, TheUniversity of Tokyo
4-6-1, Komaba, Meguro-ku, Tokyo 153-8904© 200● The Institute of Electrical Engineers of Japan.

a-Si:H

1Plasma treatment

pHPDS

11000-->921000

600

Untreated>931000

600

Treated>941000

600

Treated7.2

5140300

Treated

7.2

PDS film increases as the PHPS-to-silica conversion proceeds(5),(6).
The O2 plasma treatment was reported to effectively purify theskeletal network of sol-gel-derived SiO2(7), and thus is expected tocomplete the PHPS-to-silica conversion. The water resistanceenhancement of the PDS film by the O2 plasma treatment,
therefore, is well consistent with these studies.
Regarding a cell-semiconductor hybrid on the conventional
LAPS, SiO2 was found to be a more effective insulating material
than Si3N4(8), which has been the first choice for the EIS
configuration(9). With the PDS film shown here, the concept ofon-LAPS cell monitoring can be expanded to the thin-film LAPS,
although some effort would be necessary to make the PDS layer
thinner for the more efficient capacitive coupling between a-Siand cells.

Technologies, 4274A) in dark and under illumination, respectively.
For light illumination, a 150-W halogen lamp was utilized.

3.

Results and Discussion

Figure 1 shows time courses of the dark impedances andluminous sensitivities of different substrates. Both the no-coatedelectrode (electrode #1) and the electrode with an untreated PDS
layer (#2) exhibited decreases in the dark impedance and luminous
sensitivity, while those of the electrode with the O2 plasma-treated
PDS layer (#3) reached stable plateau. The decrease of theluminous sensitivity accompanied with the decrease of the dark
impedance means water penetration to the a-Si:H layer and itssubsequent dissolution.
The initial decrease and increase
observed in the dark impedance and luminous sensitivity of theelectrode #3 seem to reflect surface hydration of the PDS film notdeterioration, because the same phenomenon was observed in an
immersion test with a neutral saline solution instead of DMEM
(data not shown). These observations clearly demonstrate that
the O2 plasma treatment can enhance the water resistance of thePDS film. Then, the stable duration of the thin-film LAPS
electrode was estimated in the standard biological condition, i.e. inDMEM of pH 7.2 with 5% CO2. Both the test electrodes withthe O2 plasma-treated PDS layer (#4) and with the thinner a-Si:Hand PDS layers (#5) showed stable time courses of theimpedances over two weeks. This long-term stability will be
sufficient for biological experiments even including cell culturing.
The previous studies on PDS reported that the stability of the4.

Conclusion

The main findings of this study were that the O2 plasma
treatment enhances the water resistance of the PDS film, and thatthe plasma-treated PDS film can protect the thin-film LAPSelectrode from corrosion in the biological electrolyte. The
immersion test in DMEM with impedance monitoring confirmed
an over two-week lifetime of the thin-film LAPS electrode withthe plasma-treated PDS passivation layer, while the electrode withthe untreated immediately deteriorated. With this long-term
durability, this simple and reliable method of surface passivation
will be applicable for the cell-semiconductor hybrid on thethin-film LAPS.
(Manuscript received Jan. 00, 2000, revised May 00, 2000)
References
(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)Time courses of the measured dark impedances (top panel) andluminous sensitivities (bottom). Each line corresponds to the coatingand test condition indicated and numbered in Table 1. *: The luminous
sensitivity value of the electrode #1 at 0 h was about 200 and out of therange shown.

T. Yoshinobu, H. Iwasaki, Y. Ui, K. Furuichi, Y. Ermolenko, Y. Mourzina,
T. Wagner, N. Näther, and M. J. Schöning: "The light-addressable
potentiometric sensor for multi-ion sensing and imaging", Methods, 37, pp.
94-102 (2005)
J. Suzurikawa, H. Takahashi, R. Kanzaki, M. Nakao, Y. Takayama, and Y.
Jimbo: "Light-addressable electrode with hydrogenated amorphous silicon
and low-conductive passivation layer for stimulation of cultured neurons",
Appl. Phys. Lett., 90, 093901 (2007)
J. Suzurikawa, M. Nakao, Y. Jimbo, R. Kanzaki, and H. Takahashi:"Light-addressed stimulation under Ca2+ imaging of cultured neurons",
IEEE Trans. Biomed. Eng., 56, pp. 2660-2665 (2009)
J. Suzurikawa, M. Nakao, R. Kanzaki, and H. Takahashi: "Microscale pH
gradient generation by electrolysis on a light-addressable planar electrode",
Sens. Actuators, B, 149, pp. 205-211 (2010)
T. Kubo, E. Tadaoka, and H. Kozuka: "Preparation of hot water-resistantsilica thin films from polysilazane solution at room temperature", J. Sol-Gel
Sci. Technol., 31, pp. 257-261 (2004)
C. Kato, S. Tanaka, Y. Naganuma, and T. Shindo: "Room temperature
photochemical fabrication of silica glass coatings using vacuum ultraviolet
excimer lamps", J. Photopolym. Sci. Technol., 16, pp. 163-164 (2004)
H. R. Kim, H. H. Park, S. H. Hyun, and G. Y. Yeom: "Effect of O2 plasma
treatment on the properties of SiO2 aerogel film", Thin Solid Films, 332, pp.
444-448 (1998)
A. B. M. Ismail, T. Yoshinobu, H. Iwasaki, H. Sugihara, T. Yukimasa, I.
Hirata, and H. Iwata: "Investigation on light-addressable potentiometricsensor as a possible cell-semiconductor hybrid", Biosens. Bioelectron., 18,
pp. 1509-1514 (2003)
T. Matsuo, and M. Esashi: "Methods of ISFET fabrication", Sens. Actuators,
1, pp. 77-96 (1981)

Fig. 1. Time courses of the electrode impedances during theimmersion test.
2IEEJ Trans. ●●, Vol.●●, No.●, ●●●