Systematic Study of Micro-Discharge Characteristics of ATLAS SCT Modules

K. Hara, “Micro-discharge characteristics of ATLAS SCT modules

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Systematic Study of Micro-Discharge Systematic Study of Micro-Discharge Characteristics of ATLAS SCT Modules Characteristics of ATLAS SCT Modules

K. Hara, T. Kuwano, S. Shinma (Univ. of Tsukuba)

Y. Ikegami, T. Kohriki, S. Terada, Y. Unno (KEK)

ATLAS Japan Group has constructed 981 ATLAS SCT barrel modules, using 3924 microstrip Si sensors fabricated by HPK. Among these, 111 modules showed a rapid increase in IV curve (micro-discharge) below 500V at least once in the series of quality assurance. The characteristics of micro-discharge modules have been investigated through:

• Hot spot identification by IR imaging• Any visual trace associated to the hot spot• Noise levels• Leakage current decay for 24 hrs


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ATLAS SCT Barrel ModuleATLAS SCT Barrel Module• 4 pcs of 64x64mm Si microstrip s

ensors (HPK 4” process) glued back-to-back onto a baseboard

• flexible hybrid bridges over the sensors (This prevents thorough IR inspection)

• 16um wide p-implants at 80um pitch• 22um wide Al electrodes • SiO2 and Si3N4 insulators

80um

Si3N4

Al

p-implant

design guideline: no touching/gluing on the strip faceexceptions: vacuum sacking via clean-room paper & wirebonding

HV tested up to 500V, maximum HV for innermost barrel.


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IV curves of Modules 1-200IV curves of Modules 1-200

Out of 981 modules B: micro-discharge modules 111 including 6 C: failure


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History of 71* MD Modules History of 71* MD Modules MD in sensors but never laterAl overhang suppresses MD

*71 modules were tested in the specified sequence (e.g. no 500V test in some sensors)

(module LT) more MD in N⇒ 2

damaged at module production?damaged at testing?

IR imaging

: temperature: humidity

IR hot spot imaging to understand the problem

tiny MD,VMD~500V,mostly


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IR hot spot imagingIR hot spot imagingHPK 512x512 pixel CCD with C4880 controller (16-bit imaging),cooled to -55oC by the Peltier

procedure:1. wide-angle lens covers the area ½ of the module (4 shots/module). under the hybrid is out of sense2. microscope lens to localize3. conventional image to visualize

2 3 requires a visible mark⇒

Ileak>1uA for IR imaging

1&2: hot spot image overlaid on image with zero bias and some external light

1 2

3


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Visual images of the hot spot 1/5Visual images of the hot spot 1/5A) 7 modules with irregularly shaped p-implants problem of lithography

Final, 450V

module, 470V

module, 450V

VMD: +,390~470V3/3/1 modules since “sensor”/module/final

Images shown may be spectacular, but most of HPK sensors areclean, as we know…

MD first seen at


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Visual images of the hot spot 2/5Visual images of the hot spot 2/5B) 7 modules with scratch-like trace

module, 10VIsensor~2×nominal

module, 490V

module, 480V

VMD: 10, 390, >460V1/6 modules since “sensor”/“module”

problem since sensor processing


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Visual images of the hot spot 3/5Visual images of the hot spot 3/5C) 5 modules with dots along Al

module, 425Vmodule, 440V

VMD: 410~490V4 modules since “module”1 module since “LT”

module, 490Vproblem since sensor processingIncomplete cleaning?


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Visual images of the hot spot 4/5Visual images of the hot spot 4/5D) 7 modules with red spots

LT, 380Vsensor, 420V

VMD: +,>460V (3/1/3 since “sensor”/”module”/”LT”)

sensor, 440V

E) 15 modules with black spots

module, 260V module, 380V

sensor, 350V


droplets of, or impurities in photoresist


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Visual images of the hot spot 5/5Visual images of the hot spot 5/5F) 8 modules with no visible/clear flaw

module, 425V


G) 12 modules not categorized*

VMD: >400V (2/7/2/1 since “sensor”/”module”/”LT”/”final”)

sensor, 430V module, 475V

module, 475V

*No visible mark to localize the position (9) No time to take photo for scheduled shipping…(3)

No visible damages, but clear IR hot spots :(problem underneath the Al structure?) problem since sensor processing


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Summary of hot spot classification Summary of hot spot classification

O) 2 modules scratched/damaged by miss-handling (damages are recorded) V⇒ MD~0VA) 7 modules with irregularly shaped p-implants B) 7 modules with scratch-like trace C) 5 modules with dots along AlD) 7 modules with red spotsE) 15 modules with black spotsF) 8 modules with no visible/clear flawG) 12 modules not categorized

61 modules: IR spot identifiedmodule, 25V

50 modules: IR spot not identified

4 modules: recorded damage (noise is large at damaged strips) 19 modules: too tiny leakage for hot spot detection (I< 1uA)~10 modules: hidden under the hybrid (16% in area x 61 modules identified/0.84)~17 modules: large enough leakage but IR not detected

These, except O), are problems since sensor processing.

Q: what are they?

*2 modules are in 2 different categories

“Dark modules”


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Micro-discharge onset voltage: 111 modulesMicro-discharge onset voltage: 111 modules

1 IR: Type B)5 damaged ～ Type O) since sensor

?

tiny MD @500Vsince LTdamaged

“Dark modules”

Investigate using the noise distributions


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Noise of “Dark” modules (1/2)Noise of “Dark” modules (1/2)

Vb=410V I=2.5ATdecay = 0.6h

no hot spot / no flaw associated⇒　 flaw under hybrid ? flaw not visible ?

typical MD noise

Vb=260V I=2.5ATdecay = 8.6h Not a typical MD noise

Typical module with flaw at bias ring ? (since sensor)


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Noise of “Dark” modules (2/2)Noise of “Dark” modules (2/2)5 modules

Al short

Vb=330V I=1.5ATdecay = 7.2h

typical MD noiseTdecay=7.2h

4 modules: typical MD noise

Tdecay2.1h0.14h0.03h0.83h

no flaw associated

Noise distribution consistent with luminous MD

no inspection made


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History of 71 MD modules (II)History of 71 MD modules (II)all MD modules

IR failed

IR failed & I>1uA“Dark modules”

Nu

mb

er

of M

D m

odu

les

Tiny MD

Clear flaw identified:MD not detected at sensor probing⇔MD has developed…

history consistent with luminous MD


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Leakage Current Decay at 500VLeakage Current Decay at 500V

ATLAS group agreed to evaluate leakage decay time for MD modules

measured for >24 h3 damaged modules showed instable behavior: no decay assigned

MD is tendered with time

Decay time :typically < 1 h


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Leakage Current Decay at 500VLeakage Current Decay at 500V

Most of the modules decrease the leakage to the level of genuine modules within 24hrs.But not all…

The decay time is typically less than 1hr for most modules. But not all…

“Dark modules” tend to be irregular⇔ modules with longer decay time is not luminous

hot spot not identifiedhot spot identified

YES


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Flaws of modules with longer leakage decayFlaws of modules with longer leakage decay

Td=10h

Td=1.3hTd=4.7h

Td=6.4h Td=1.7h

Td=7.2h

NoiseWhat are the characteristics of Modules with longer decay?tend to be “no flaw”, or “mild” defects…

Hot spot seen

VMD=25V

Td=1.1h

Td=4.6h


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SummarySummary

Among 981 ATLAS SCT barrel modules constructed by Japan, 111 modules showed micro-discharge below 500V.

6 modules are damaged at assembling or testing

　 59/105 modules are IR hot spots identified:

2/3 incomplete photoresist processing, cleaning,*

1/3 no clear flaw associated

46/105 modules are IR failed:

2/5 (I<1A) tiny micro-discharge

3/5 (I>1A) noise distribution consistent with luminous MD no anomaly in the MD history in test sequence leakage decay time tends to be longer (Td>1h) (“mild” defects for modules with longer decay time?) 　 MD is humidity dependent (sensor probing in air vs module operation in N2) MD develops (some sensors with clear flaws were OK at sensor probing)

*HPK 6” process is better


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Installation to the ATLAS BarrelsInstallation to the ATLAS Barrels

4 barrel layers

Japan cluster (981 modules) 83 MD with VMD>350 & Tdecay<1h outer 3⇒ rd/4th barrels 14 MD with 350>VMD>150 &Tdecay<6h spares⇒ 14 MD with (VMD<150 or Tdecay>6h) FAIL⇒ other reasons (23 3rd/4th , 7 spares, 7 FAIL, 10 SQ)823 modules ⇒ any barrel

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Systematic Study of Micro-Discharge Characteristics of ATLAS SCT Modules