Photon Factory Activity Report 2004 #22 Part B (2005)
Materials Science20A/2004G116

High Insulating SiO2 Thin Film Growth under VUV Irradiation
Atsuyuki FUKANO*1, Hideki HASHIMOTO2, Hiroyuki OYANAGI1
1AIST, Umezono, Tsukuba, Ibaraki 305-8568, Japan
2
TRC, Sonoyama, Otsu, Shiga 520-8567, Japan
Introduction
Silicon industry is now paying attention to high-k and
SiON dielectric materials as a candidate gate insulator
beyond ultra-thin (<1nm) SiO2 film. The presentlimitations in ultra-thin SiO2 film are, however, defects inthe Si-O interface which is a source of leakage current. Inorder to improve insulating performance, sub-oxide layer
in the interface must be minimized in thickness.
Our approach is to densify the SiO2 film to minimize
defect density. There are several polymorphous for SiO2
with different packing (density); i.e. stishovite(4.35g/cm3), coesite (2.93g/cm3), quartz (2.6 ~ 2.65g/cm3),
tridymite (2.28 ~ 2.33g/cm3) and cristobalite (2.32g/cm3).
High density SiO2 in an amorphous form is also known;i.e. lechatelierite (2.5 ~ 2.65g/cm3). Densification ofglassy silica is achieved by high pressure. Density ofthermally oxidized SiO2 film on the silicon wafer is 2.14
~ 2.23g/cm3.[1] High density forms are expected to haveless defects; e.g oxygen imperfection. Recently, we
reported that ultra-thin SiO2 film prepared by VUVoxidation has superior insulating performance and here
we report the study using synchrotron radiation as VUVsource.
We focused attention on VUV (vacuum ultraviolet)
assisted oxidation which is one of low damage method fordepositing highly density of SiO2 film. The firstexperimentation of highly density of SiO2 film wasdeposited by excimer lamps whose wavelength was 126,
172 and 222nm, in atmosphere oxygen gas. The growthof SiO2 using these wavelength values formed ultra-thin
SiO2 films with density higher than those of conventional
thermal oxidation technique. Chemical analysis by XPS(x-ray photoelectron spectroscopy) confirmed that theSiO2 films have only small amount of sub-oxide layer.
The insulating performance of SiO2 prepared by VUVoxidation is summarized in the following; criticalbreakdown voltage at 126nm oxidation is higher while
leakage current is lower than those of thermally oxidized
films.[2] Highly insulating SiO2 film can be grown by
VUV assisted oxidation up to 5 nm thickness.
Mechanism of VUV oxidation is still unclear but thepossible mechanism through radical oxygen is proposed.
However, it is difficult to describe the observation that
SiO2 is grown by 126nm radiation by such a mechanism
considering strong absorption and collisions with oxygen
molecules in atmospheric pressure.

The samples tidied were deposited on Si(001) standard
wafer after chemical treatment. Density profiles of theSiO2 films were obtained by grazing incidence x-rayreflectometry (GIXR). Figure 1 shows depth profile ofSiO2 using 126nm excitation at BL-20A. For comparison,
the results of SiO2 grown by thermal oxidation (Nagai etal) is referred. The VUV oxidation was performed for 10hours and substrate was heated to 350°C. The SiO2
3density and thickness is ~2.3g/cm and 1.4nm,respectively. When substrate temperature is lower orhigher, the density decreases. The deposition rate seemsto depend on photon flux and we are now trying tocalibrate the photon flux.
It is found that the high density SiO2 can be grown by
126nm irradiation using synchrotron radiation. Highly
monochromatic beam will allow us to study thewavelength dependence.
Fig. 1, The depth alternation of the density from GIXR

for 126nm at VUV-SR. The growth condition of substratetemperature is 300, 350, and 450°C and 10 hour. The
condition of growth chamber is kept atmosphere andflowed fresh O2 gas. The wet and dry thermally oxidation
data are referred by Nagai and Hashimoto.
References[1] N.Nagai and H.Hashimoto, Appl. Surf. Sci. 172, 307(2001).
[2] A.Fukano and H.Oyanagi, J. App. Phys. 94, 3345
(2003).
* a.fukano@aist.go.jp

Result and Discussion

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