Progress in measuring and modeling NBTI in
nitrided SiO2 gate MOSFETs
Kenneth E. Kambour
SAIC On-site Research Contractor
Electronics Foundations Research Group
Space Electronics Branch
Space Vehicles Directorate
DISTRIBUTION STATEMENT D. Distribution authorized to DoD and their DOD contractors only.
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Collaborators
EXPERIMENT
Duc Nguyen, 3rd year student UNM/AFRL RVSE
Camron Kouhestani, 3rd year student UNM/AFRL RVSE
Rod Devine, Think Strategically/AFRL RVSE
THEORY
Ken Kambour, SAIC/AFRL RVSE
Harry Hjalmarson, Sandia National Labs.
TECHNOLOGIES
130 nm IBM Bulk – nitrided SiO2
90 nm IBM Bulk – nitrided SiO2
45 nm IBM SOI – nitrided SiO2
32 nm TI – HfSiON Bulk
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NBTI Experiment
Type I Gate dielectrics – SiO2, lightly nitrided SiO2
Room temperature NBTI due to interface state generation DVth = A t a
0.16 ≤ a ≤ 0.25
Type II Gate dielectrics- nitrided SiO2, HfSiON
Room temperature NBTI due to interface state generation and hole
tunneling from the inversion layer into neutral traps in the near interface
region. Oxide traps charge quickly but also relax quickly if bias is
removed/reduced.
Need to measure NBTI dynamically to capture full effect of charging
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Comparative Stress and Recovery
Data for 130 nm at Room Temp.
Example of room temperature NBTI in 130 nm channel length devices with
3.2 nm nitrided SiO2 gate dielectric.
Recovery (Vgs = 0 V)
Stressing (Vgs = -3.3 V)
Sweep Trial 1
Sweep Trial 2
Single Point
Single Point trial #2
Sweep Method of multiple steep slope points
0.01
0.00
-0.01
-0.01
Threshold Voltage Shift (V)
Threshold Voltage Shift (V)
0.00
-0.02
-0.03
-0.04
-0.05
-0.06
-0.02
-0.03
-0.04
-0.05
-0.06
-0.07
-0.08
-6
-5
-4
-3
-2
-1
10 10 10 10 10 10
-0.07
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
0
10
1
10
2
10
3
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Accumulated Recovery Time (seconds)
Accumulated Stress Time (seconds)
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Evolution of NBTI Study (Experiment)
Stress
Short time regime (tstress < 1 sec)
Model DVth(t) assuming full field dependent tunneling
only oxide trapped charge relevant
Long time regime (tstress > 100 sec)
Model DVth(t) assuming oxide trapped charge saturated
only interface state term evolves.
Recovery
Model short time regime as de-trapping via tunneling
Model of long time interfacial trap relaxation (exists)
Develop an NBTI model enabling prediction of frequency and duty
cycle dependence
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Circuit Response - Cao's approach




Uses the Alam model for NBTI to determine the ΔVth as a function of duty
cycle and age .(only interface states)
Determines the effect of changing the PMOS threshold voltage, the
capacitive load, and the input slew rate on the delay time of a CMOS
NAND gate composed of PMOS and NMOS devices.
Once this is done, treats the NAND gates as single devices rather than
combinations of MOSFETs.
Apply to multiple standard digital logic (benchmark)circuits used to test the
timing. (ISCAS ’89)
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RVSE Approach



Develop our own formula, either theoretical or empirical, for ΔVth as a
function of duty cycle and age.
Recreate Cao's basic results.
Currently using Predictive Technology Model (PTM) SPICE device models
for 65 nm MOSFETS.


PTM (ptm.asu.edu) is a standard set of device libraries.
Channel lengths from 180 nm down to 22 nm.
Ultimately implement modeling software capable of treating much
larger scale circuits

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Xyce


Xyce was developed at Sandia National Labs.
Why choose Xyce?

Designed for large scale problems. (23,000,000 devices have been
simulated)

Potential access to source code.

Can model both digital devices (NAND, NOR, AND, & OR gates) and
transistors.

Access to local expertise.

Xyce has radiation modeling developed which we could obtain in the
future if we wish.
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CMOS NOR and NAND Gates
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Effect of DVth on td
As the threshold voltage changes, for example due to
NBTI, the delay time rises for both NOR (green) and
NAND (yellow) gates.
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Effect of ti on Delay and to
As the input slew rate rises, for example if one input for the NAND
gate is the output of a prior NAND gate experiencing a Vth shift,
the delay (blue points) and output slew rate (red points) rises.
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C17 Benchmark Circuit


Working in Xyce
Simulation of
transient switching
of one input takes
3 seconds.
C432 Benchmark Circuit

233 logic gates including other gates made by sets of NANDS

752 PMOS and 752 NMOS devices
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Ring Oscillator
• Implemented a ring oscillator to determine the effect
of ΔVth on frequency
• 11 NAND gates using the 65 nm PTM models
• If ΔVth=0.1 volts, the frequency changed by 15%
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Conclusions
Experiment
The dynamic measurement system works well
We are close to being able to model the complete
short time  long time behavior of NBTI
Need access to a much larger reservoir of devices
ideally with controlled process variations
Theory
Modeling of the effects of NBTI on limited circuit size
examples is operative
Implementing the modeling in Xyce to predict response
of much more complex circuits
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