Symposia on VLSI Technology and Circuits
RTN Induced Frequency Shift Measurements Using a Ring Oscillator
Based Circuit
Qianying Tang1, Xiaofei Wang1, John Keane2, and Chris H. Kim1
1 University of Minnesota, Minneapolis, MN2 Intel Corporation, Hillsboro, OR
Outline• Background• Proposed ROSC Based RTN
Characterization Circuit• 65nm Test Chip RTN Data• Transistor Vt Shift Estimation• Summary
Slide 1
Slide 2
Random Telegraph Noise (RTN) Basics
• Random trapping and de-trapping of carriers from channel
• Manifests as a fluctuation in Vt resembling a random telegraph signal
Slide 3
RTN Characterization Techniques
SRAM Logic circuit or ring oscillatorIndividual transistor
Parameter of interest SRAM VminDC current Frequency shift
Schematic
VDD
WLWL
BL BLB
Pros
1. Realistic RTN impact on SRAM Vmin
2. Short test time3. Small test area
1. Simple2. High resolution
1. Realistic RTN impact on circuit frequency
2. Short test time3. Small test area
Cons1. Limited resolution2. Rare occurrences3. Averaging effect
1. Long test time2. Large test area
3. Limited insight on circuit level
1. Limited resolution2. Long meas. time3. Averaging effect
D Q
Circuit based methodsProbing
Simple ROSC Based Meas. Circuit
Slide 4
• Measure the divided ROSC frequency using off-chip instrument
• A simple & popular structure for characterizing process variation and reliability issues
• No reported RTN data using this technique– Single ended sensing, common mode noise, long
measurement time poor resolution
Beat Frequency Detection Scheme
Slide 5
• D flip-flop captures the beat frequency of two free running ROSCs
• Benefits of beat frequency detection scheme– Achieves <0.01% frequency measurement resolution
at sub-microsecond sampling times– Insensitive to common mode noise such as V, T drifts– Fully-digital scan-based interface
T. Kim, et al., JSSC, 2008
C A B
Slide 6
RTN Measurements Using BFD SchemeR
TN
Slide 7
RTN Measurements Using BFD SchemeR
TN
Slide 8
RTN Measurements Using BFD SchemeR
TN
Slide 9
RTN Measurements Using BFD SchemeR
TN
• Small frequency shifts induced by RTN amplified • Sub-ps resolution + sub-µs measurement time
Comparison with Prior Art
Slide 10
[2] K. Ito, et al., IRPS, 2011
Cou
nter
Cou
nter
Cou
nter
ROSC Based Test Chip Diagram
Slide 11
• An 8×10 DUT ROSC array, a shared reference ROSC and a beat frequency detection block
• Measure beat frequency between DUT ROSC and shared reference ROSC
Row
Sel
ect
DUT ROSC
DUT ROSC
DUT ROSC
DUT ROSC
Column Select
DUT ROSC
DFF
8bit Counter
Edge Detector
Reset
D Q
Beat Frequency Detection Unit
Ref. ROSC
Scan out
A
BC
row<n>col<m>
Parallel/serial shift register
A
BDUT ROSC
DUT ROSC
DUT ROSC
DUT ROSC
DUT ROSC
N:1
Mux
65nm Test Chip Die Photo
Slide 12
ROSC Frequency Shift Measurements
Slide 13
• Single trap induced ROSC frequency shift is roughly 0.4% at 0.8V
• Capture/emission time constants range from 1 ms to 100’s of milliseconds
Slide 14
Single and Multi Trap RTN
• Time Lag Plot (TLP) used to identify the number of RTN traps [3] [3] T. Nagumo, et al., IEDM, 2009
Frequency Shift versus Voltage
Slide 15
• Frequency shift due to RTN trap decreases at higher voltages– Frequency shift sensitivity decreases– Fermi level shifts with supply voltage
∆f/f
(%)
Peak
-to-p
eak ∆
f/f (
%)
Time Constant Distribution
Slide 16
• Capture and emission time fit exponential distribution under AC condition
Pr = A·exp(-t/τ)
τc=3.68 msτe=14.0 ms
τc=24.9 msτe=8.12 ms
τc=26.7 msτe=8.70 ms
Time Constant versus Voltage
Slide 17
• τc and τe show opposite dependences• τc has negative dependence on voltage for NMOS
and positive dependence for PMOS [3][3] T. Nagumo, et al., IEDM, 2009
Power Spectral Density of RTN
Slide 18
• PSD of RTN induced frequency shift suggests a Lorentzian spectrum
Number of Traps Distribution
Slide 19
• Most ROSCs do not show any RTN behavior• No more than two RTN inducing traps observed
Slide 20
Frequency to Vt Mapping
• Frequency shift is proportional to Vt shift• Vt shift due to single RTN trap is 1.9% for NMOS
and 1.6% for PMOS
Slide 21
Vt Shift versus Supply Voltage
• Freq. shift reduces at higher supply voltages• However, trap induced Vt shift has a weak
dependency on supply voltage
Summary• Beat frequency detection system utilized for
high precision RTN measurement with short measurement time
• ROSC frequency shift due to RTN measured for the first time from a 65nm test chip– Single trap RTN causes 0.4% frequency shift at 0.8V – Wide range of time constants measured– Frequency shift and time constant measured at
different supply voltages• Measured ROSC frequency data can be readily
mapped to transistor Vt shift
Slide 22
