Post on 27-Dec-2015
Non-destructive testing using magneto-resistive sensors
David P. Pappas
Quantum Devices Group
National Institute of Standards and Technology
Boulder, CO, 80305
A. Nazarov, Fabio da Silva NIST
Ken Marr, Jim Ryan FBI Audio Lab
Erin Gormley, Jim Cash NTSB
Dave Krefft NSA
Objectives• Develop state-of-the art magnetic imaging metrology for NIST
– Forensics
– Bio-magnetic tag detection
– Component failure analysis
– Non-destructive testing
• Probe components non-destructively to determine:
– Power usage
• Defects
• Design flaws
• Unapproved process
– Investigate low level signal monitoring
• Wafer level
• De-processed devices
Procedure• Methods:
– Map out magnetic field above device under test (DUT).
• Single, scanned magneto-resistive (MR) element.
• Array of elements (faster).
– DUT
• Test artifacts – striplines, etc.
• Sample chips – processed, deprocessed & flip-chip
– Invert the magnetic field to find current distribution.
• Metrics
– Low frequency current resolution:
• Limited by noise floor of sensor - field falls off as 1/d from DUT.
– Spatial resolution:
• Deconvolution requires close proximity to DUT, low noise.
– Temporal resolution – High frequency
• Johnson & shot noise of resistors increases with bandwidth.
Magnetic imaging system
x-y-z translation stage Stationary sample Lock-in amplifier Unshielded magnetic sensor – 1 m resolution
– AMR, GMR sensor mounted on flexure– Self aligning - slides on back of Si wafer– Sensors readily available
Scalable, arrayable, fast
Magneto-resistive (MR) sensors• AMR - Anisotropic MR
– Single ferromagnetic film NiFe
– 2% change in resistance
• Spintronic:
– GMR trilayer w/NM spacer
• 60% R/Rmin Co/Cu/Co
• “Spin Valve”
– TMR – Insulator spacer
• 500% R/Rmin at R.T.
• CoFeB/MgO/CoFeB
• Hayakawa, APL (2006)
IM
FM
NM
FM
**
*
*
Large arrays of MR sensors
• NDE Imaging applications
– 256 element AMR linear array
– Thermally balanced bridges
– High speed magnetic tape imaging –
forensics, archival applications
Cassette Tape – forensic analysis4 mm
45 mm
erase headstop event
write headstop event
4 mm
V+V-
I-I-
16 m x 256
I+
BARC.
Magnetic field measurement of current
MR element -
LocalizedCurrent, I
B-field
RR
IB
15320 15340 15360 15380 15400 15420-15
-10
-5
0
5
10
15
Vo
ltag
e (V
)
Position (m)
x
Z (up)
y
BZ
Coordinate system
Magnetic field probe above meander line test structure
•10 m meander line•5 mA current•Spacing = 1000 m•Sensor 100 m above sample
8 x 8 mm, Bz –field image
0 500 1000 1500 2000 2500 3000 3500-3
-2
-1
0
1
2
Sig
nal (
mic
rovo
lts)
Distance (micrometer)
BZ
1 mm
Image magnetic field of test stripline sample
Lockin Amplifier data acquisition +-10 mA in line @ 15 kHz, 1 ms 10 m wide meander
-500 0 500 1000 1500 2000 2500 3000 3500
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Sig
na
l fro
m 2
0 m
A in
10
mic
ron
str
iplin
e (
mV
)
Distance (micro-meter)
Probe height above sample950 m 150 m 50 m 3 m
Cross section scan
Deconvolution of planar currents from magnetic fields
Localized current in y direction Measure Bz Height = z0
z
x
z0
Bz 20
2)(
zx
xKxBZ
X
1/x
x=+z0
-z0
)(B)( z xk
keCxJ xkz
y FF 1-
Transform method Chatraphorn, et. al (2000):
Calculate currents from Bz
Calculated currentsMeasured magnetic field image of test stripline
x
y
-dB/dy
+dB/dy
+dB/dx -dB/dx
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 110000
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
Non-destructive VLSI current measurement
Intel flip-chip RAM
• Wafer thinned
• Had been probed with FIB from back
• Short circuit induced in center
Q – Could we locate short circuit?
2000.00 4000.00 6000.00 8000.00 10000.00 12000.00 14000.00 16000.00
2000.00
4000.00
6000.00
8000.00
10000.00
12000.00
14000.00
Intel flip-chip RAM with shortheight = wafer thickness (~500 m)
Measured magnetic field - Bz Calculated currents
x
y
-Jy
+Jx
-Jx
+Jy
Spatial resolution
“From what height can we resolve two currents that are flowing in the same direction?”
gap
Z
z < g
z >> gsensor
B
Spatial resolution
• In principle, deconvolution is perfect functions in, functions out Works for any current distribution Multiple sources, gnd planes
• In real life: Smaller signal for large z Noise – electronic & mechanical
)(B)( z xk
keCxJ xkz
y FF 1-
z >> g
Test structures for spatial resolutionSplit meander line:• I = 65 mA
• g = 200,100, 50,20,10, 5 m
• Z = 500 and 100 m
0 2 4 6 8-40
-20
0
20200
10050
20105
Distance (mm)
Cur
rent
den
sity
J x
(kA
/m)
current
g (m)
z=100 m
z=500 m
Resolves g ~ z/10
7 mA pulse
Expected signal from stripline I = 7 mA, d = 1 m
mV/mT 8.0
mT/ .070mA 10
RIV
0.07
%125.0 30R
B 0.25 ~
T 1012
B
S
3-0
d
IB
Gain = 1000Single sweep
MR elements as real-time, non-contact probes
V = 1 mV
Signal from stripline with z~0
mV/mT 1VMeas.
High frequency operation of probes - f >100 kHzSample & Average
•Intrinsic sensor response ~GHz•Filtering slows response
•Random noise – can be averaged out:
Johnson -
Shot -
1/f -neglible-
=> noise
TRBkV BRMS 4
BqIV BRMS 2
VV 02.0)kHz 1(
HznV 2
100 101 102 103 104 105 106 107 108 109 10101E-3
0.01
0.1
1
10
100
AMR cutoff @ 50 kHz (with no averaging)
1 mA signal, TMR
TMR cutoff @ 50 MHz
1 mA signal, AMR
Noise level
Vol
tage
(m
V)
Frequency (Hz)
6 s pulse
Average 1000 sweeps
Present & Future Applications• Probes
– Non-destructive, localized current mapping– Monitoring of individual current lines– Spatial, temporal resolution determined by
• probe height• Sensitivity• signal strength• Sampling (real-time vs. averaging)
• Linear & two-dimensional arrays – Field mapping
• VLSI failure analysis• Listen in on high frequency chip emissions
– Localization, analysis– Want gold standards for chip emissions to compare
For non-localized, planar source
w
Current in plane
z-sensor
h
•Ground planes, power, …
•Width of trace greater than height, size of sensor
w >> z0
•Bx = constant over current ~ 0 outside
•Bz = 0 over current ~1/x outside current
Bz
B
x
Bx
x-sensor
B field
Asymmetric stripline
50 m wide line with ground plane, 100 mA, 1 kHz
50
0 5000 10000 15000 20000 25000-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
Asymmetric stripline, z=700 m
Mag
netic
fiel
d B
z (O
e)
Position (m)0 5000 10000 15000 20000 25000
-200
0
200
400
600
800
1000
Asymmetric stripline, z=700 m
Cur
rent
den
sity
Jy
(A/m
)
Position (m)
Field distribution – Bz Deconvoluted current - Iy
Use artifact with both types of currents
0 5000 10000 15000 20000 25000
-200
0
200
400
Cur
rent
den
sity
Jy
(A/m
)
Asymmetric stripline, z=1200 m
Position (m)0 5000 10000 15000 20000 25000
-0.15
-0.10
-0.05
0.00
0.05
0.10
Asymmetric stripline, z=1200 m
Mag
netic
fiel
d B
x (O
e)
Position (m)
Ideal geometry for magneto-resistive arrays
Use either BX & BZ sensor arrays
2 – dimensional image
Field distribution - BxDeconvoluted current -Iy
In-plane field current measurements
Sensitivity of MR vs. SQUID
mV/mT 1VMeas. Signal:
Resistive noise : (50 ohm, Johnson) HznV/ 1
HzpT/ 35 - fT 50 - SQUID
HzpT/ 001 - GMR
HznT/ 1 - AMR
Bz
Small area1 x .05 mTiny flux
Large area>20 x 20 mBig flux
Roth, et. alChatraphorn, et. al
Flux = MR elements comparable to SQUIDS in flux measurement
AB
Test structure with features
Asymmetric meander lineWith holes in return plane
BZ at z = 1 mm
2 mm hole
1 mm
0.5 mm
I
(a) (b)
Current distributions calculated for asymmetric stripline
-0.02
-0.01
0.00
0.01
0.02
0.03
(a)
1 mm
2 mm
Mag
netic
indu
ctio
n Bz (
mT
)
10 8 6 4 2 0 -2 -4
-20
0
20
40
60
80
1 mm
2 mm
(b)
Cur
rent
den
sity
J (
A/m
)Position (mm)
Feature resolution optimal for z ~ d/2