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