The effects of irradiation and proton implantation on the density of mobile protons in SiO{sub 2} films Page: 3 of 7
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I. INTRODUCTION
In microelectronics technology, annealing in a
H2 containing atmosphere at temperatures < 450
*C is commonly used to passivate electrically
active interface traps. However, the role
hydrogen plays in the Si/Si02 system is far more
complex as reflected in the vast research effort in
this field. Recently, we have begun to explore the
effects of annealing at temperatures > 450 *C, and
have found that hydrogen can generate both fixed
and mobile positive charges in thin film
amorphous SiO2. The fixed positive species show
remarkable similarities to oxidation induced fixed
charge [1] and have recently been attributed to
near-interfacial over-coordinated oxygen sites
induced by interaction with hydrogen [2,3]. The
mobile species are only observed in Si/Si02/Si
structures and have been identified as protons
imprisoned inside the oxide of these layered
structures [4]. These protons are rapidly
rearrangable at room temperature by applying a
bias and immobile in the absence of a bias, which
makes this phenomenon potentially useful as the
basis for a new generation of nonvolatile memory
devices [4,5].
It has been shown that mobile protons can be
generated during a forming-gas anneal (>450 C).
It was furthermore evidenced that this reaction is
induced by hydrogen which diffuses laterally into
the buried oxide (as opposed to through the Si
overlayer). In this study we explore the
technologically interesting alternative of
introducing the mobile protons into the oxide by
proton implantation, instead of the forming-gas
anneal. It is shown that proton implantation, with
or without post-implantation anneal, does not
yield a significant number of mobile protons in
the buried oxide of Si/Si02/Si structures. In a
second part of this work, we investigate the
radiation tolerance of the mobile protons
introduced into thermal oxides capped with poly-
Si. These protons were introduced using standard
forming gas anneal treatments. Proton densities
were analyzed as a function of radiation dose,
temperature and applied oxide field during
irradiation. The irradiation data show that the
mobile protons in the oxide can tolerate doses
over 10 Mrad(Si02) before their density drops to
half of the initial value. No post-irradiation
trapping of mobile protons was observed in these
thermal oxides. This is in contrast with thebehavior of SOI material, where proton trapping
was observed near the substrate interface when
irradiation was performed with a positive bias
applied to the Si overlayer [6]. Our data provide
new insights into the atomic mechanisms
governing the generation and radiation tolerance
of mobile protons in SiO2. This can lead to
improved techniques for production and radiation
hardening of radiation tolerant memory devices
based on this phenomenon [4,5].
II. EXPERIMENTAL DETAILS
Both SOI and thermal oxide material were
investigated. We used Unibond SOI material
formed by implanting hydrogen (- 6 x 1017 cm-2)
into a wafer, below a thermally grown Si02 layer
(380 nm thick), followed by bonding this wafer to
another wafer. Splitting of the first wafer occurs
at the boundary defined by the implant. Finally a
high temperature anneal at 1100 C is used to
strengthen the bonding interface. We also
analyzed 40-nm dry thermal oxides grown at
Sandia's Microelectronics Development
Laboratory. These were capped with a poly-Si
layer and annealed at 1100 C in Ar + 1 % 02.
Finally, titanium silicide was formed on the poli-
Si overlayer by depositing Ti, and reacting it in
nitrogen at 700 C, stripping the unreacted Ti,
and annealing again at 800 C in Ar.
Mobile protons were introduced into the
thermally grown oxide of the Si/Si02/Si
structures (density - 1 x 10" cm-2) by annealing
in forming gas [Ar:H2; 95:5 (by volume)] at
600 C for 5 min. Details of this process are
discussed elsewhere [4]. These samples were
subsequently irradiated using a 10-keV x-ray
source at a dose rate of 4 krad(SiO2)/s. Samples
were irradiated with and without applied bias, and
different biases and substrate temperatures were
used during irradiation.
Protons were implanted into SOI substrates at
room temperature with an energy of 40 keV to
doses of 0.5, 1.0, and 4.0 x 1014 cm-2. TRIM-90
simulation shows that the vast majority of the
protons are stopped inside the buried oxide layer.
For a number of samples, post-implantation
anneals were performed in pure Ar or in forming
gas at 600 C for 5 min.
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Vanheusden, K.; Fleetwood, D. M.; Schwank, J. R.; Shaneyfelt, M. R.; Meisenheimer, T. L. & Draper, B. L. The effects of irradiation and proton implantation on the density of mobile protons in SiO{sub 2} films, report, April 1998; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc691856/m1/3/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.