ISSN 1063-7397, Russian Microelectronics, 2006, Vol. 35, No. 5, pp. 329–336. © Pleiades Publishing, Inc., 2006.
Original Russian Text © M.N. Levin, A.V. Tatarintsev, V.A. Makarenko, V.R. Gitlin, 2006, published in Mikroelektronika, 2006, Vol. 35, No. 5, pp. 382–391.
Ionizing-radiation effects on metal–oxide–semicon-
ductor (MOS) structures continue to attract active
research interest [1, 2]. New models of radiation-
induced charge buildup have recently been developed
One possibility under consideration is the use of
irradiation in MOS-circuit manufacture. Levin et al.
[5, 6] carried out a series of investigations into the pre-
cision adjustment of transistor threshold voltage by
x-ray or near-ultraviolet treatment of ﬁnished wafers.
Two components of radiation-induced positive charge
differing in bias–temperature stability have been exper-
imentally identiﬁed in phosphorus-doped silicon diox-
ide layers, the charge being responsible for radiation-
induced threshold-voltage shift. A thermally stable
component is determined by the manufacturing process
employed; the resultant amount of threshold-voltage
shift is typically 30 to 70% of the initial value.
The aim of this study is to formulate a model that
would yield an accurate quantitative description of
radiation effects relevant to MOS-circuit manufacture
in order to evaluate the limits of x-ray or ultraviolet
(UV) threshold-voltage adjustment.
2. PHYSICAL MODEL
Films of thermally grown silicon dioxide are known
to occur as a disordered network of
together by Si–O–Si bonds . There is also a transi-
tion layer between the
and Si regions that consists
of a strained part of thickness 3–10 nm and a
about one monolayer thick with
varying from 2 down
to 0 [7, 8].
The drift of radiation-generated holes toward the
interface breaks strained Si–Si bonds, disinte-
grates the network of SiO
tetrahedra, and creates
centers; these constitute the main type of hole trap
and are responsible for positive-charge buildup in the
oxide. Having reached the interface, holes alter its
defect structure by producing surface states whose
energy levels lie in the forbidden band of the silicon.
The surface states can exchange charge with the con-
duction or valence band of the silicon, and their charge
state varies with the Fermi level of the silicon surface.
The radiation-induced surface states have been
centers—trivalent silicon atoms having
one unsatisﬁed valence bond (
ters are of an amphoteric nature: they act as donors in
the lower half of the silicon forbidden band and as
acceptors in the upper half.
The charge on radiation-induced
centers and sur-
face states is susceptible to thermal annealing at 450–
600 K; therefore, it is unsuitable for threshold-voltage
Hydrogen constitutes another source of positive
charge in thermal-oxide layers, as supported by the
detection of a large amount of Si–H and Si–OH bond-
ing . Fixed positive charge is produced by the inter-
action between these bonds and mobile
ions in the
oxide near the Si/
X-ray or UV Adjustment of MOS Threshold Voltage:
Analytical and Numerical Modeling
M. N. Levin, A. V. Tatarintsev, V. A. Makarenko, and V. R. Gitlin
Voronezh State University, Voronezh, Russia
Received July 27, 2005
—A model is formulated that describes how radiation-induced charge accumulates in the gate oxide
of a MOS structure and how it decays through tunneling and thermal emission. The model is used in a numerical
analysis of the x-ray or UV adjustment of threshold voltage in MOS-circuit manufacture. The limits of this pro-
cess technique are evaluated.
PACS numbers: 85.40.Bh