Transcript RD50_lowRstrips_progress - Indico

Low Resistance Strip Sensors
– RD50 Common Project –
RD50/2011-05
CNM (Barcelona), SCIPP (Santa Cruz), IFIC (Valencia)
Contact person: Miguel Ullán
Outline
 Motivation
 Technological challenges
 Preliminary experiments
 Designs
 Status and plan
 Summary
Miguel Ullán (CNM-Barcelona)
RD50 meeting (CERN) – Nov 2011
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Motivation
 In the scenario of a beam loss, a large charge deposition in the
sensor bulk can lead to a local field collapse.
 A conducting path to backplane is created and implant strip potential
could reach a significant voltage.
 Coupling capacitors can get damaged by the voltage difference between
the implant and the readout strip
They are typically qualified to 100 V
 Punch-Through Protection (PTP) structures
 used at strip end to develop low impedance to
the bias line
HPK
 Reduce distance from implant to bias ring
 Placement of the resistor between the implant
and bias rail (“transistor effect”).
Micron
Miguel Ullán (CNM-Barcelona)
RD50 meeting (CERN) – Nov 2011
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PTP effectiveness at ‘far’ end
 Measurements with a large charge injected by a
laser pulse showed that the strips can still be
damaged
 The voltages on the opposite end of the strip keep
rising well above the 100V objective
 The large value of the implant resistance
effectively isolates the “far” end of the strip from
the PT structure leading to the large voltages
C. Betancourt, et al.
“Updates on Punchthrough Protection”
ATLAS Upgrade week,
Oxford, March 31, 2011.
Miguel Ullán (CNM-Barcelona)
RD50 meeting (CERN) – Nov 2011
Near end, plateau
for PT structures
Opposite end, no plateau.
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Proposed solution
 To reduce the resistance of the strips on the silicon sensor.
 A desired target value is 1.5 kOhm/cm (~ 1 order of magnitude reduction)
 Not possible to increase implant doping to significantly lower the
resistance. Solid solubility limit of the dopant in silicon + practical
technological limits (~ 1 x 1020 cm-3)
 Alternative: deposition of Aluminum on top of the implant:
 R□(Al) ~ 0.04 W/□  20 W/cm
Top
view
Cross
section
Longitudinal
section
Miguel Ullán (CNM-Barcelona)
RD50 meeting (CERN) – Nov 2011
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Technological challenges
 Metal layer deposition before the coupling capacitance is
defined
 2 metals processing
 A layer of high-quality oxide/nitride with metal strips on top to
implement the AC-coupled sensor readout (MIM cap).
 Deposited (not grown)
 Low temperature processing
 PTP structure
 Not tried before at CNM
 Very dependent on surface effects (difficult to simulate)
Miguel Ullán (CNM-Barcelona)
RD50 meeting (CERN) – Nov 2011
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Metal on strip
 Metal layer deposition on top of the implant before the
coupling capacitance is defined.
 MIM capacitors
 Low temperature deposited isolation
– PECVD (300-400 ºC)
– Risk of pinholes (Yield, Breakdown)
– > 50 pF  ~ 3000 Å
 Alternatives (for high T deposition)
– Polysilicon + TaSi (tantalum silicide)
– Tungsten
 Experiments already performed at CNM on smaller
dimensions. More experiments needed.
Miguel Ullán (CNM-Barcelona)
RD50 meeting (CERN) – Nov 2011
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Preliminary experiments
 6 wafers batch of MIM capacitors
 Different sizes
– C1: 1100 x 1100 mm2 = 1.20 mm2
– C2: 600 x 600 mm2 = 0.36 mm2
– C3: 300 x 300 mm2 = 0.09 mm2
– …
(short strips ~ 0.5 mm2)
 Low-temperature deposited isolation
 PECVD (300-400 ºC)
 Use of a multi-layer to avoid pinholes
 3 technological options:
 Op1: 3000 Å of SiH4-based silicon oxide (SiO2) deposited in 2 steps
 Op2: 3000 Å of TEOS-based oxide deposited in 2 steps (Tetra-Etil Orto-Silicate)
 Op3: 1200 Å + 1200 Å + 1200 Å of TEOS-based ox. + Si3N4 + SiH4-based ox.
Miguel Ullán (CNM-Barcelona)
RD50 meeting (CERN) – Nov 2011
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MIM results
 All 3 options give good MIM capacitors
 Yield is high even for the largest caps (> 1 mm2)
(Not yet large statistics from all the wafers)
 I-V meas: ILEAK < 3 pA @ 20 V for the largest cap (C1)
 Capacitance:
 C(op1) = 122 pF/mm2
 C(op2) = 120
BREAKDOWN
pF/mm2
1.E-05
 C(op3) = 110 pF/mm2
1.E-06
1.E-07
Current (A)
 Breakdown:
 VBD(op1)|C1 = 170 V
1.E-08
1.E-09
1.E-10
 VBD(op2)|C1 = 150 V (low statistics)
 VBD(op3)|C1 = 210 V (low statistics)
1.E-11
1.E-12
100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270
Applied Voltage (V)
Miguel Ullán (CNM-Barcelona)
RD50 meeting (CERN) – Nov 2011
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PTP design
 Reduce implant distance to bias ring to favor punch-through
effect at low voltages
 Placement of the resistor between the implant and bias rail
(“transistor effect”).
s
p
d
Bias rail
Polysilicon
“bridge/gate”
Implant
 Compromise between PT effect and breakdown
 Design of experiments varying d, p, n (2 indep. variables)
 Simulations ?
Miguel Ullán (CNM-Barcelona)
RD50 meeting (CERN) – Nov 2011
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Test structures
 Test structure to measure voltage in the implant under laser
injection at different strip distances
 Laser tests (SCIPP-Santa Cruz)
Miguel Ullán (CNM-Barcelona)
RD50 meeting (CERN) – Nov 2011
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Wafer mask design




DOE for the PT structure
Control structures with standard resistive implant structure
Test structures
Extra structures for “slim edges”
Miguel Ullán (CNM-Barcelona)
RD50 meeting (CERN) – Nov 2011
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Status and Plan
 Final measurements (more statistics) on MIM experiement
wafers
 DOE being planned and test structures designed
 Final technological options being defined
 Finish designs by end of January 2012
 Fabrication (4-5 months): June 2012
 Device electrical characterization, PTP validation, laser tests,
Irradiation, re-characterization, post-irrad laser tests…
Miguel Ullán (CNM-Barcelona)
RD50 meeting (CERN) – Nov 2011
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Summary
 RD50 Common Project: Low Resistance Strip Sensors (June 2011)
 CNM-Barcelona, SCIPP-Santa Cruz, IFIC-Valencia
 Reduced resistivity of the strip:
 Proposed metal layer on top of the strip implant
 MiM coupling capacitor
 PTP structures implemented
 Preliminary experiments on MIM coupling capacitors:
 Good results (yield, large area, ILEAK, C, VBD). More statistics needed
 Options to be chosen
 DOE on PT structures implemented
 Final designs, start fabrication ~January 2012  Samples ~June 2012
 This could be a very innovative solution for new generations of long-strip
detectors and with great applications in future HEP experiments.
Miguel Ullán (CNM-Barcelona)
RD50 meeting (CERN) – Nov 2011
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