Development of 3D Trenched-Electrode Pixel Sensors with improved - - PowerPoint PPT Presentation

Development of 3D Trenched-Electrode Pixel Sensors with improved Timing performance

• G. Forcolin, R. Mendicino, G.F. Dalla Betta

University of Trento and TIFPA-INFN

• M. Boscardin, S. Ronchin

TIFPA-INFN and FBK

• A. Lai

INFN Cagliari

• A. Loi

INFN Cagliari and University of Cagliari

• S. Vecchi

INFN Ferrara

PIXEL 2018 10 December 2018

• G. Forcolin

Outline

• Introduction to 3D sensors
• Fabrication process
• 3D trench sensors

– Design and TCAD simulation – Fabrication tests

• Outlook

PIXEL 2018 10 December 2018

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3D Sensors

• Advantages:

– Low Depletion Voltage – Lateral Drift

• Fast Response
• Low Sensitivity to mag. fields

– Short inter-electrode distance

• Fast Response
• Reduced trapping probability => more rad hard
• Disadvantages

– Non-Uniform Electric Field – High Capacitance – Complicated + expensive

manufacturing process

~100s μm ~10s μm PIXEL 2018 10 December 2018

• G. Forcolin

3D sensors at HL-LHC

• Requirements:

– higher hit-rate – increased granularity – higher radiation tolerance – lighter detectors

• To meet Requirements:

– Produce thinner sensors (~100μm) – Reduce electrode spacing (~30μm) – Narrower electrodes (5μm) – Small/Active edges (<100μm) PIXEL 2018 10 December 2018

• G. Forcolin

Single sided production method

• Use single sided process with support wafer

– Can reduce active thickness without compromising

mechanical properties

– Active edges – Post processing required to thin support layer and deposit

metal

– Front side layout => processing can be complicated

P- high Wcm wafer Handle wafer to be thinned down p++ low Wcm wafer P- high Wcm wafer p++ low Wcm wafer SiO2

p-spray

Metal to be deposited after thinning Metal to be deposited after thinning Handle wafer to be thinned down

PIXEL 2018 10 December 2018

• G. Forcolin

Fabrication Process

• Electrodes produced using Deep Reactive Ion Etching (DRIE) by the

Bosch process

• Alternating etch cycles (SF6) and passivation cycles (C4F8)
• Can achieve high aspect ration (~30:1 or better) and good uniformity

5.3 mm! 5.3 m m! 117 mm!

155 mm ! 4.7mm ! 3.4 mm!

G.-F. Dalla Betta et al., NIMA 824 (2016) 386 and 388 PIXEL 2018 10 December 2018

• G. Forcolin

FBK Fabrication Process

• Production steps:

– Etch ohmic electrodes > active thickness – Fill with Poly-Si (at least partially) – Etch junction electrodes < active thickness

PIXEL 2018 10 December 2018

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FBK Fabrication Process

PIXEL 2018 10 December 2018

• G. Forcolin

PIXEL 2018 10 December 2018

• G. Forcolin

Timing in 3D Trench Sensors

• 3D trench sensors being

investigated for timing applications

• Some sensors produced

at CNM in 2013, sensors worked but with high leakage current

Advantages:

• High average field
• Uniform weighting field
• Initial pulse (largely)

independent of position

• Very Radiation Hard

Drawbacks:

• Possible fabrication problems
• High electrode capacitance
• A. Montalbano et. al.

NIMA 765 (2014), 23

PIXEL 2018 10 December 2018

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Trenched Sensors

• Investigating TIMEPIX compatible trench sensors
• Trenches are dead area, so minimize thickness (~4μm)
• Tests of fabrication procedure underway to optimize fabrication

parameters

• Test mask produced with wide range of possible geometries and

spacings for tests

• Design optimized with TCAD simulations
• New mask designed for first lot of sensors

p+ trench n+ dashed trenches pitch-y pitch-x width gap PIXEL 2018 10 December 2018

• G. Forcolin

Thin metal Trench Length: 41 mm 43 mm 45 mm 47 mm 49 mm Thin metal poly Trench Length: 41 mm 43 mm 45 mm 47 mm 49 mm Wide metal Trench Length: 41 mm 43 mm 45 mm 47 mm Bump Trench Length: 45 mm

Trenched Sensors

PIXEL 2018 10 December 2018

• G. Forcolin

Trenched Sensors Fabrication

• Test masks produced to test fabrication procedure of devices
• Try to use stepper

–

Minimum feature size 350nm

–

Alignment accuracy 80nm

–

Projection => low defect level

–

Max exposure area ~2x2cm2

PIXEL 2018 10 December 2018

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Trenched Sensors Fabrication

• Tests concluded
• Finalized fabrication process
• Ready to move on to real devices

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Weighting Field Simulations

PIXEL 2018 10 December 2018

• G. Forcolin

High dependence with geometry Low dependence with geometry

gap=16 mm gap=14 mm gap=12 mm gap=10 mm gap=8 mm gap=6 mm

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Inter-pixel capacitance (1 neighbour) Opposite electrode capacitance (half- pixel)

• Strong dependance of trench dimension on inter-pixel capacitance
• Small change in capacitance between opposite electrodes due to

trench dimensions

Capacitance Simulations

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Capacitance Simulations

• Find relationship between interpixel capacitance and trench length
• Approximate inter-pixel capacitance using parallell plate capacitor equation
• Find effective area ~1790μm2

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Capacitance Simulations

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• At full depletion, negligible difference in capacitance between

different geometries

Capacitance Simulations

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MIP Simulations

PIXEL 2018 10 December 2018

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• MIP simulations carried out to understand charge collection properties of

devices

• Looked at 3 hit positions and 3 trench dimensions:

– Trench = 41μm (Gap = 14μm) – Trench = 45μm (Gap = 10μm) – Trench = 49μm (Gap = 6μm)

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MIP Simulations

PIXEL 2018 10 December 2018

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• Strong dependence of pulses on hit position
• Dependance on gap significant only in low field

region

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MIP Simulations

θ

2Y

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• Simulate hits at an angle going through low field

region

• High angle => more charge generated outside of

low field region

22

MIP Simulations

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• Increased charge at higher angles due to increased length of track
• Improvement in rise time for increased angles

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3D Test Structures TIMEPIX sensor Temp Metal

First Sensor Batch

Technological test devices PIXEL 2018 10 December 2018

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Outlook + Conclusions

PIXEL 2018 10 December 2018

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• 3D trench devices being investigated to provide good

position+timing resolution

• Fabrication tests have been completed, verifying that

such devices can be produced

• TCAD simulations have been carried out to

understand and optimize properties of devices

• First batch of devices has been designed

=> beginning production

Backup Slides

PIXEL 2018 10 December 2018

• G. Forcolin

ATLAS IBL Sensors

• Used double sided 3D sensors
• Advantages:

– Reduced process

complexity

– Backside accessible for

bias

– Allows slim edge

• Downsides:

– Active edge not possible – Mechanically more fragile – Wafer bowing ATLAS IBL, JINST 7 (2012) P11010

• G. Giacomini, et al., IEEE TNS 60(3) (2013) 2357

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3D Test Structures

Single Pixel Multi-Pixel Strips Test Pixel Devices PIXEL 2018 10 December 2018

• G. Forcolin