1063-7397/02/3106- $27.00 © 2002 MAIK “Nauka /Interperiodica”
Russian Microelectronics, Vol. 31, No. 6, 2002, pp. 346–350. Translated from Mikroelektronika, Vol. 31, No. 6, 2002, pp. 408–413.
Original Russian Text Copyright © 2002 by Levin, Gitlin, Tatarintsev, Ostrouhov, Kadmensky.
The radiation-assisted process technologies of mod-
ern microelectronics consist in the controlled alteration
of the properties of semiconductor materials and struc-
tures by different forms of irradiation [1, 2]. They are
easy to implement and monitor, provide good unifor-
mity, and are compatible with established ﬂow routes.
Initially, the effect of ionizing radiation on MOS cir-
cuits was addressed from the radiation-hardness stand-
point . It was found that radiation-induced charge in
gate oxide is unstable, and this view, indeed true of alu-
minum-gate p-channel MOSFETs, remained a wide-
spread belief after the industry changed to the n-chan-
nel local-oxidation-of-silicon (LOCOS) technology
with self-aligned polycrystalline-silicon gates. This ste-
reotype has long been a hindrance to the use of ionizing
radiation for adjusting MOS-circuit parameters.
On the other hand, efforts to employ high-energy
particles and gamma rays in IC manufacture were ham-
pered by their poor absorption in MOS layers. Using
higher incident ﬂuxes impaired over-the-wafer unifor-
mity, placed much more stringent safety requirements
on the equipment, and, therefore, made the approach
incompatible with existing ﬂow routes in MOS-circuit
This paper presents some techniques for the radia-
tion ﬁne adjustment of threshold voltage in MOS cir-
cuits. Our approach is based on x-rays and near-UV
radiation. It has been implemented as a mass-produc-
tion technology and has led to a signiﬁcant improve-
ment in circuit yield and reliability.
OPTIMIZING IRRADIATION CONDITIONS
Our research on radiation adjustment began with
comparing the effects of different forms of radiation on
the parameters of mass-produced MOS circuits . The
investigations covered 1.2-MeV gamma-ray photons,
15-keV and 120-keV x-ray photons, 10–35-keV and
3.5-MeV electrons, and 5-MeV alpha particles.
The gamma irradiation was performed with an Issle-
dovatel’ apparatus, using a
Co source. The x-irradia-
tion was carried out with a RUM-17 apparatus, using a
4VTM-250 x-ray tube (tungsten anode, anode voltage
90–250 kV, current about 15 mA), and a modiﬁed
SRM-20 apparatus, using a BKhV-9 x-ray tube (palla-
dium anode, anode voltage 15–40 kV, current 50 mA).
At least 70% of x-ray photons were emitted with an
corresponding to 2/3
anode voltage. The x-ray exposure dose was measured
with an RFT-VA 7 ionization dosimeter accurate to
within 5%. The ﬂux was uniform to within 10% over a
100-mm wafer. The electron irradiation was performed
with a REM-200 scanning electron microscope and an
ELU-4 electron accelerator. The alpha-particle treat-
ment was effected with an RTU OPI-AM apparatus,
Po sources. An individual source had an
activity of (3.7–11.1)
Each form of irradiation was carried out just before
the dicing of the wafer into chips, with exposure doses
The radiation-induced change in MOSFET thresh-
, was measured as a function of the
observed for all forms of irradiation employed [5, 6].
Figure 1 displays the curves for gamma rays and high-
and low-energy x-ray photons. Notice that the effect
gains in magnitude as the forms of radiation are
changed in the stated order. The reasons for this are as
follows. The effect of radiation on matter grows with
capture cross section, which in turn varies as .
However, optimal energies range between 10 and
20 keV. The radiation of energy below 10 keV is
strongly absorbed by the passivating layers of MOS
X-ray and UV Adjustment of Threshold Voltage
in MOS-Circuit Manufacture
M. N. Levin, V. R. Gitlin, A. V. Tatarintsev, S. S. Ostrouhov, and S. G. Kadmensky
Voronezh State University, Voronezh, Russia
Received March 14, 2002
—Techniques are presented for the radiation ﬁne adjustment of threshold voltage in MOS circuits,
employing x-rays (photon energies 10–20 keV) and near-UV radiation. The adjustment consists in the con-
trolled generation of heat-resistant charge in gate oxide under irradiation, due to the presence of phosphorus in
the oxide. The experience gained by using the techniques in the mass production of a wide variety of MOS cir-
cuits is brieﬂy reviewed.