NA62 Gigatracker Working Group Meeting 2 February 2010 Massimiliano Fiorini CERN

Outlook IR 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 assemblies Advantages: controllable source repeatable 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) I 0 : 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

Reflections at the boundary between two different optical media, the light is split into a reflected and transmitted part the fraction of light which is reflected back is given by: for the air-silicon interface, then n 1 =1 and k 1 ~=0 therefore: 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 long cut-off < 930 nm M.F.D. 6.2  1064 nm N.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 output mode field diameter 6.2  1064 nm numerical 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 1 cm 2.8 mm8 micron

“Focusing” optics f 1 = 11 mmf 2 = 55 mm ~3  m ~15  m 3  m * rad = 15  m * 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:

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 mrad55 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 angle stable mechanics and table a 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)

Conclusions Laser test setup could be a valuable tool for the characterization of GTK bump-bonded assemblies very precise timing information perform 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)