INFLUENCE OF GAMMA IRRADIATION
ON SILICON NITRIDE MIS CAPACITORS
AND RADIATION HARDNESS
Dr. Ercan YILMAZ
Abant Izzet Baysal University
Bolu-Turkey
Contents
• Basic Properties of Dielectrics
• Radiation Hardness
• Silicon Nitride Thin Films
• Radiation Interactions with Silicon Nitride Thin
Films
• Results
Basic Properties of Dielectrics
In electromagnetics, we classify materials generally
into three broad categories:
1. Conductor - free charge moves easily
2. Semi-conductor - free charge moves somewhat
3. Dielectric (insulators)-no free charge, but alters
electric field
When a dielectric material is placed in an exernal electric field, the dielectric
alters this electric field due to bound or polarization charges that are
formed in the dielectric. A capacitor is an example of this.
A simplistic model of the atomic conditions that produce this bound charge is
the displacement of the electron cloud around a nucleus. In an electric field
, the negative charged electron cloud becomes displaced very slightly from
the positively charge nucleus:
The electrostatic effects of this displacement (potntial and electris field)
are model by an electric dipole moment p.
Electric dipole moment model is only an approximation, but for charge natural
molecules and relatively “small” electric fields, it is a very good model.
Using this model, a polarized material can be visualized as
These bound charges cannot move. Unlike free charge, bound charge is
induced by an external Ē field and vanishes when the external Ē field is
removed.
Dielectric materials, interfaces, metal gate, work
function
Radiation Hardness/Hardening
Radiation hardening is a method of designing and testing electronic
components and systems to make them resistant to damage or malfunctions
caused by ionizing radiation (particle radiation and high-energy electromagnetic
radiation).
CCDs were generally expected to exhibit radiation tolerance comparable with
other CMOS circuits.
Some experiments with Co-60 irradiation shows loss of charge transfer
performance at doses of 10krads [Lumb and Holland 1988b].
These studies also shows that the charge transfer degradation is a more
severe problem than conventional integrated circuit damage mechanisms.
Detailed experiments with proton irradiation show that displacement damage
creates measurable loss of charge transfer efficiency at doses as low as 10's
rads, and hence radiation hardening will require optimized shielding and CCD
architectures to minimize loss of data.
Normal commercial electronic components are not qualified to work in
environments with radiation. Special designed and/or qualified components are
required
above
a
radiation
dose
of
a few hundred
Rad.
Radiation effects on electronics are normally divided into 3 different categories
according to their effect on the electronic components.
•
Total ionizing dose (TID): Total Ionizing Dose effects on modern integrated
circuits cause the threshold voltage of MOS transistors to change because of
trapped charges in the silicon dioxide gate insulator.
•
Displacement damage: Hadrons may displace atoms (therefore called
displacement effect) in the silicon lattice of active devices and thereby affect
their function.
•
Single event effects (SEE): Single Event Effects refer to the fact that it is
not a cumulative effect but an effect related to single individual interactions in
the silicon. Highly ionizing particles can directly deposit enough charge
locally in the silicon to disturb the function of electronic circuits.
Silicon Nitride Thin Films
Al
SiNx
Al
 SiNx thin films with thickness of 100 nm were deposited on p-type (100) silicon
substrate by using (PECVD) method.
 After deposition, samples annealed at 500 and 700 °C for 1h at N2 ambient.
 The chemical bonds and their densities inside the films were investigated by
Fourier transform infrared (FTIR) spectroscopy.
0.08
Absorbance (a.u.)
AS
Si-N
2
0.04
3
0.02
Peak Analysis
WR
Si-H
0.02
1
B
N-H
Data Set:% ([Book2]FitPeaks2,@WL,Input.IDTC2)
SS
Si-N
B
N-H
WR
Si-H
SS
Si-H
0.00
Chi^2=--
0.00
600
SS=--
700
Adj. R-Square=--
800
900
Date:21/11/2011
1000
1100
# of Data Points=% ([B
1200
1300
Degree of Freedom=% ([Book2]FitPeaks2,@WL,RegStats.C1.D
-1
Wavenumber (cm )
Fitting Results
400
600
800
1000
1200
2200
-1
Wavenumber (cm )
Peak Type
0.08
(b)
Peak #
1
2
3
4
5
Gaussian
Gaussian
Gaussian
Gaussian
Gaussian
Area Intg
0,42306
0,90447
5,45
4,39997
0,35636
AS
Si-N
FWHM
44,53727
59,91479
110
175,01493
96,74341
Max Height
0,00893
0,01418
0,04654
0,02362
0,00347
Center
Annealed
at Grvty
700C
664,29743
790,49003
835
922,23928
1185
Area In
3,668
7,8418
47,252
38,148
3,0897
0.06
Annealing temperature evolution of the FTIR spectra of
the SiNx films.
The following peaks are identified:
Absorbance (a.u.)
Absorbance (a.u.)
As-deposited
AS
Si-N
0.06
0.04
(a)
0.06
As-Deposited
Annealed at 500C
Annealed at 700C
2
0.04
1
0.02
N-H (~3340
cm-1)
and Si-H (~2150
cm-1)
Si-H (~660
cm-1)
wag-rocking mode, and
symmetric Si-N (~475 cm-1) stretching mode
WR
Si-H
0.00
N-H (~1200 cm-1) bending mode,
asymmetric Si-N (~830 cm-1) stretching mode,
B
N-H
3
stretching modes,
600
700
800
900
1000
1100
1200
1300
-1
Wavenumber (cm )
Fitting Results
Peak #
1
2
3
4
5
Peak Type
Gaussian
Gaussian
Gaussian
Gaussian
Gaussian
Area Intg
0,03846
5,04999
8,99993
3,56236
0,74605
FWHM
44
102
154,28233
179,68801
85,02112
Max Height
8,5E-4
0,04651
0,0548
0,01863
0,00825
Center Grvty
633
803,55
882
1056,84391
1237,03954
Deconvoluted FTIR spectra of the as-deposited
sample (a) and the sample annealed at 700°C (b).
Area IntgP
0,20908
27,4504
48,92116
19,36404
4,05533
Radiation Interactions with Silicon Nitride Thin
Films
The as-deposited and annealed (MIS) capacitors were exposed to 60-Co gamma irradiation with a
dose rate of 0.015 Gy/s. Capacitance-voltage (CV) measurements were performed at 1000 kHz
before and after radiation exposure with doses of up to 40 Gray.
10
8
6
-24
-22
-20
-18
-16
Gate Voltage (V)
C-V curves of as-deposited Al/Si3N4/Si MIS capacitor before and after
gamma radiation at different doses.
10
)
No Radiation
1 Gray
5 Gray
10 Gray
20 Gray
40 Gray
-10
No Radiation
1 Gray
5 Gray
10 Gray
20 Gray
40 Gray
Capacitance (Farad x 10
Capacitance (Farad x 10
-10
)
12
8
6
-20
-15
-10
-5
Gate Voltage (V)
C-V curves of Al/Si3N4/Si MIS capacitor annealed at 700 0C before and
after gamma radiation at different doses.
Conclusions
 Before exposure to gamma irradiation, as-deposited sample shows
relatively large Vfb shift towards negative voltages indicating a
reduction in positive charge buildup due to annealing process.
 On the other hand, after gamma irradiation at different doses, all
samples show a Vfb shift toward more negative values being more
pronounced in the sample annealed at higher annealing
temperature of 700 °C.
 We attribute this to the structural characteristics of the films, in which
the as-deposited sample, having an amorphous network and
containing H atoms is metastable, compared with annealed samples
making it less affected by irradiation.
 The more streaking and common feature is observation of radiation
hardening after first irradiation in as-deposited and annealed
samples which can be used in space and nuclear applications as a
radiation hardened device.
“If we knew what we were doing
it wouldn't be research.”
Albert Einstein