Infrared Laser Test Setup for

Characterization of GTK Assemblies

NA62 Gigatracker Working Group Meeting2 February 2010

Massimiliano FioriniCERN

OutlookIR laser light: generate e-h pairs to mimic (as much as

possible) charged ionizing particles traversing silicon bulk

Objectives:perform measurements and diagnostic tests on bump

bonded assembliesAdvantages:

controllable sourcerepeatable measurements on laboratory bench

Disadvantages:reflection, refraction, attenuation (plus

interference/diffraction) effects have to be carefully taken into account

stability of the whole setup (optics + mechanics) must be accurately monitored

Silicon optical properties

silicon absorption coefficient for 1060 nm light is α=11.1 cm-1 (1/α=900 microns)

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I = I0 ⋅e−α ⋅d

I0: initial light intensity

d: silicon thickness

Sensor back side metallization

opened back side metallization for one single chip size sensor and for 50% of prototype sensors

15 m distance from frame’s edge to last active pixel edge

100 m diameter holes (only for a few pixels) are present in all the structures

Reflectionsat the boundary between two different optical media,

the light is split into a reflected and transmitted partthe fraction of light which is reflected back is given by:

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R =N1 −N2( )

2

N1 + N2( )2

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N i = ni + i ⋅ki

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k =α ⋅λ

4πfor the air-silicon interface, then n1=1 and k1~=0

therefore:

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R =1− n2( )

2+ k2

2

1+ n2( )2

+ k22≅ 0.3

in the sensor many interfaces are present (different layers)

care must be taken when considering all these effects (interference if coherence length is sufficiently large)

Laser system: an example

Laser driver

Laser head

laser head with integrated collimator (no cooling)single mode fiber

5 m longcut-off < 930 nmM.F.D. 6.2 m @ 1064 nmN.A. ~14%

Spatial filtering (1)the

amplified spontaneous emission (A.S.E.) is completely removed by the coupling with the single mode fiber (“spatial filtering”)

IRF = 30 ps

example plots

Spatial filtering (2)the

amplified spontaneous emission (A.S.E.) is completely removed by the coupling with the single mode fiber (“spatial filtering”)

IRF = 30 ps

example plots

Laser output (1)

I = 3.0

Laser output (2)

I = 3.6

Laser output (3)

I = 5.6

Laser output (4)

I = 10.0

Spectral response

“Focusing/collimating” optics

Light output from laser diode

due to diffraction, the beam diverges rapidly after leaving the laser diode

this leads to an elliptical beam shape after the collimating optics with typical dimensions of 1.5 mm × 3.5 mm

the beam shape can be modified using optical fibers

Single mode fiber outputmode field diameter 6.2 m @ 1064 nmnumerical aperture ~14% 8˚ “divergence” (140 mrad)

the beam shape after the fiber is gaussian (only one mode is transmitted within the fiber)

No optics used

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n1 ⋅sin θ1( ) = n2 ⋅sin θ2( )

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n1 =1

n2 = 3.6

θ1 = 8o

=140mrad

⎧

⎨ ⎪ ⎪

⎩ ⎪ ⎪

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θ2 = 2.2o

= 39mrad

1 cm

2.8 mm 8 micron

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n1

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n2

“Focusing” optics

f1 = 11 mm f2 = 55 mm

~3 m

~15 m

3 m * 0.140 rad = 15 m * 0.028 rad

8 mrad * 200 m = ~1.6 m

for gaussian beams the divergence is limited by this relation, where d is the beam spot diameter:

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θ =λ

π ⋅d2

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θ1 = 28mrad =1.6o

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θ2 = 8mrad = 0.4o

Dependence on height

~30 m (at the focus)

1 mm

~86 m

for 100 m vertical displacement, the spot size increases by ~6 m (from to ~30 m to ~36 m)

height should be measured, controlled and kept constant when scanning the GTK assembly (within 100 m or less)

Dependence on tilt angle

~6 m

0.2 rad ≈ 12°

28 mrad 55 mrad

~11 m

0.1 rad ≈ 6°20 mrad ≈ 1°

6 mrad

~1 m

Other components needed

ultra-fast photodiode (precise trigger signal)X-Y motion control system (micrometric precision)variable gain attenuator + power meter (calibration and

power stability control)various optical components (single mode fibers,

connectors, splitters, fixed attenuators, etc…)control of optics-sensor Z distance and tilt anglestable mechanics and tablea stable temperature is needed for repeatable

measurements (change in optical properties of laser diode + attenuation coefficients + silicon behavior etc…)

a complete list of material to be purchased is being prepared (G. Aglieri-Rinella, M. Fiorini, A. Kluge, M. Morel)

ConclusionsLaser test setup could be a valuable tool for the

characterization of GTK bump-bonded assembliesvery precise timing informationperform tests on laboratory bench (accessibility)

Careful calibration is needed and stability of the system must be ensured (repeatability)take into account systematic effects

The project for a test setup at CERN and the corresponding list of materials are being finalized

SPARES

Energy release GTK per hit

mean energy: 72.4 keV (~20.1 k e-h ~3.2 fC)most probable energy: 53.7 keV (~14.9 k e-h ~2.4 fC)FWHM: ~25 keV (~6.9 k e-h ~1.1 fC)minimum energy: ~29 keV (~8.1 k e-h ~1.3 fC)