25-07-2012, 10:20 AM
Using Oxide-Trap Charge-Pumping Method in Radiation-Reliability Analysis
Using Oxide-Trap Charge.pdf (Size: 525.63 KB / Downloads: 42)
Abstract
In this paper, a thorough investigation of the application
possibilities of the oxide-trap charge-pumping (OTCP)
extraction method to evaluate the radiation-induced traps in short
lightly doped drain (LDD) transistors is conducted. We have
successfully demonstrated that the OTCP is able to determine all
kind of traps induced by radiation in short LDD transistors. First,
we have presented a methodical approach to take out the LDD
effect from CP curves, leaving only the effective channel-length
CP. Second, we have extracted the radiation-induced interface, oxide,
and border traps for LDD-NMOSFET and LDD-PMOSFET
with varied gate length and fixed gate width. Finally, we have
performed a comparison between OTCP and subthreshold slop
(STS), midgap (MG), dual-transistor CP (DTCP), and DT border
trap (DTBT). OTCP method shows perfect agreement with all
methods regarding oxide-trap.
INTRODUCTION
THE WELL-KNOWN charge-pumping (CP) technique [1],
[2], as well as CP-based extraction methods [3]–[7], is
widely used in reliability MOSFET transistor characterization.
Several authors [8], [9] have presented different ways to apply
this technique for the determination of trap density, trap energetic
distribution, and trap location distribution. Regarding
radiation effect, dual-transistor CP (DTCP) [10], DT border
trap (DTBT) [11], and oxide-trap CP (OTCP) [6], [7] methods
are used for the extraction of radiation-induced trap. However,
the accuracy of the CP-current (ICP) measurements, in all CP
technique variants (such as constant-amplitude (VG), fixed
low-level voltage (VL), fixed high-level voltage (VH), and
multifrequencies), is affected by lightly doped drain (LDD)
region, particularly short LDD devices.
EXPERIMENTAL SETUP
We characterized LDD-NMOS and LDD-PMOS transistors
fabricated on the same chip at the Institute for Silicon Technology
of Fraunhofer, Germany. The process is a dual-layer-metal
1-μm LDD-CMOS twin-well technology on P-type 12-μm
epilayer on silicon 100 substrate with 20-nm-thick gate oxide
layer grown in dry O2. The gate capacitance per unit area Cox is
about 2.12 × 10−7 F · cm−2. The nonpackaged transistors have
a gate length ranging from 1 to 10 μm with fixed gate width
at 10 μm.
The irradiation was performed at the Division for Nuclear
Applications (DAN), Algeria. The devices were irradiated up
to 500 krad with a dose rate of 200 rad/min, using 1.25-MeV
rays at room temperature and zero bias voltage. Therefore,
the devices were submitted to internal field (0.55 MV/cm)
during irradiation, allowing a fraction of holes to escape the
initial recombination [19].
CP CHARACTERISTICS OF LDD TRANSISTORS
Before using OTCP method to extract the radiation-induced
traps in the LDD device, it is important to understand the
ICP–VL curves of these devices in order to remove the LDD
region effect.
Fig. 2 shows the CP curves in logarithmic scale of the LDD
transistors with varied gate length from 1 to 10 μm and fixed
width at 10 μm. This clearly illustrates the hump caused by
the LDD region. Obviously, all transistors show the same hump
even for 10-μm-gate-length transistor. However, its relative
effect on Vth and ICPmax is negligible in 10-μm-gate-length
transistors, since it is three orders of magnitude smaller .
CONCLUSION
We have established that OTCP extraction method is valid
to evaluate radiation-effect issues in short LDD transistors. We
have presented a method to separate the LDD region effect
from the effective channel CP. We extracted the radiationinduced
traps in both LDD-NMOSFET and LDD-PMOSFET
using OTCP, STS,MG, DTCP, and DTBT methods. The results
of OTCP have been compared to those given by the other methods.