jaap velthuis (university of bristol)1 radiation damage in silicon sensors overview radiation damage...
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Jaap Velthuis (University of Bristol) 1
Radiation damage in silicon sensors
• Overview radiation damage due to heavy particles
• State-of-the-Art sensors
In case of any questions:
Jaap Velthuis (University of Bristol) 2
sLHC radiation dose
• 5 year radiation dose close to beam pipe ~1016 neq/cm2
– too high for state-of-the-art standard silicon sensors
Jaap Velthuis (University of Bristol) 3
Radiation with protons/neutrons
• Energetic radiation knocks atoms out of lattice: similar to doping
• Energy needed to displace atom from lattice=15eV
• This damage is called Non-Ionizing Energy Loss (NIEL)
• Displacement changes band structure– Donor removal– Acceptor generation
Jaap Velthuis (University of Bristol) 4
Radiation damage: Leakage current
• I = Volume• Material
independent– linked to defect
clusters
• Scales with NIEL• Temp dependence
– Thermal runaway1011 1012 1013 1014 1015
eq [cm-2]
10-6
10-5
10-4
10-3
10-2
10-1
I /
V
[A/c
m3 ]
n-type FZ - 7 to 25 Kcmn-type FZ - 7 to 25 Kcmn-type FZ - 7 Kcmn-type FZ - 7 Kcmn-type FZ - 4 Kcmn-type FZ - 4 Kcmn-type FZ - 3 Kcmn-type FZ - 3 Kcm
n-type FZ - 780 cmn-type FZ - 780 cmn-type FZ - 410 cmn-type FZ - 410 cmn-type FZ - 130 cmn-type FZ - 130 cmn-type FZ - 110 cmn-type FZ - 110 cmn-type CZ - 140 cmn-type CZ - 140 cm
p-type EPI - 2 and 4 Kcmp-type EPI - 2 and 4 Kcm
p-type EPI - 380 cmp-type EPI - 380 cm
kT
ETTI g
2exp2 = 3.99 0.03 x 10-17Acm-1
after 80minutes annealing at 60C
Jaap Velthuis (University of Bristol) 5
Type inversion• Dopants may be
captured into defect complexes.
• Donor removal and acceptor generation– type inversion: n p– depletion width
grows from n+ contact
• Increase in full depletion voltage
biasbi
DA
DAd VV
NqN
NNx
2
0 0.5 1 1.5 2eq [1014cm-2]
1
2
3
4
5
|Nef
f| [1
012 c
m-3
]
50
100
150
200
250
300
Vde
p [V
] (
300
m)
1.8 Kcm Wacker 1.8 Kcm Wacker 2.6 Kcm Polovodice2.6 Kcm Polovodice3.1 Kcm Wacker 3.1 Kcm Wacker 4.2 Kcm Topsil 4.2 Kcm Topsil
Neutron irradiationNeutron irradiation
cNN effeff exp0
= 0.025cm-1 measured afterbeneficial anneal
P-strips in p-bulk
Jaap Velthuis (University of Bristol) 6
Partially depleted detectors
• Depletion zone grows from p-n-junction. Need depleted area around strips for isolation– Signal proportional depleted
area• Undepleted region like high-
ohmic resistor• If detector partially depleted
– from strip side only charge in depleted region
contributes smaller signal, similar spatial resolution
– from backplane carriers travel towards strips, but
don’t reach it signal spread over many strips poor spatial resolution
undepleted
undepleted
Jaap Velthuis (University of Bristol) 7
Solutions radiation damage
• N+-on-n detectors (LHCb)– Need full depletion before type
inversion– After radiation p-type bulk
• P-type bulk sensors– Just becomes more p-type
• Cool to cryogenic temperatures – No trapping, no leakage current
• Diamond sensors– Very high bandgap: no background– Very though lattice
Jaap Velthuis (University of Bristol) 8
Solutions radiation damage
• Oxygenation– Oxygen binds and neutralises
vacancies• Czochralski silicon
– “cheap” Si, contains loads of oxygen– no type inversion donor generation
overcompensates acceptor generation• 3D sensors
– Spacing electrodes so small: full depletion at very low voltages
– “Edgeless”
Jaap Velthuis (University of Bristol) 9
State-of-the-Art devices
• DEPFETs– Very thin p-n sensor with in-pixel
amplification• ISIS
– Pixel sensor with short CCD each pixel• MAPS• 3d integrated devices
• Trend is towards thin, fast and integration
Jaap Velthuis (University of Bristol) 10
DEPFET Principle
• Was developed towards ILC, so needs to be very thin and fast• Will be used at SuperBelle • A p-FET transistor (=amplification!) integrated in every pixel.• By sidewards depletion potential minimum created below
internal gate.• Electrons, collected at internal gate, modulate transistor
current
~1µm
p+
p+ n+
rear contact
drain bulksource
p
sym
met
ry a
xis
n+
ninternal gate
top gate clear
n -
n+p+
--
++
++
- 50
µm
------
MIP
Jaap Velthuis (University of Bristol) 11
DEPFET Principle (II)• Advantages:
– Fast signal collection due to fully depleted bulk
– Low noise due to small capacitance and amplification in pixel
– Transistor can be switched off by external gate – charge collection is then still active !
– Non-destructive readout
• Disadvantages:– Need to clear internal gate.
Jaap Velthuis (University of Bristol) 12
Ladder proposal
• Detectors 50µm thick, with 300µm thick frame yields 0.11% X0
• SWITCHER & CURO chips connected by bump bonding
• Radiation hardness not an issue: can change operating voltage to correct Vthres shift
SWITCHER
CURO
Jaap Velthuis (University of Bristol) 13
Thinning
sensor wafer
handle wafer
1. implant backside on sensor wafer
2. bond wafers with SiO2 in between
3. thin sensor side to desired thick.
4. process DEPFETs on top side
5. etch backside up to oxide/implant
first ‘dummy’ samples:50µm silicon with 350µm frame
thinned diode structures:leakage current: <1nA /cm2
Jaap Velthuis (University of Bristol) 14
Testbeam results• Placed 450µm thick
DEPFET in testbeam • Cluster signal (5σ seed,
2σ neighbour cut) • S/N=112.0±0.3 for 450
μm S/N≈12 for 50 μm
• Position resolution 1.82µm (incl telescope error) for 22µm pitch• Intrinsic resolution 1.25μm• Second best ever
measured!
Jaap Velthuis (University of Bristol) 15
ISIS• Operational Principles:– Every pixel has mini CCD to store charge: burst camera with
multiframes– Charge collected at photogate– Transferred to storage pixel during bunch train– 20 transfers per 1ms bunch train– Readout during 200ms quiet period after bunch train
Jaap Velthuis (University of Bristol) 16
ISIS• It works! • Here you see Fe55
spectrum• Results of a laser
scan• And position
resolution in a beam test
• ISIS2 is currently available
preliminarypreliminaryUsing η
Jaap Velthuis (University of Bristol) 17
MAPS operation principle
• Epitaxial layer forms sensitive volume (2-20m)
• Charge collection by diffusion (no field!)
• Charge collected by N-well• Build amplifiers in P-well
(Intrinsic amplification)– Only NMOS possible
• Small signals (~800e-), but small noise (~15e-)
• Developed for:– SuperBelle, STAR vertex
detector, replacing CCDs in camera’s & satellites
Vreset Vdd
Out
Select
Reset
Jaap Velthuis (University of Bristol) 18
MAPS (dis)advantages• Advantages:
– Integrated detector and electronics• High S/N (first amplification in pixel)• Possible in-pixel or on-chip intelligence (System on chip)
– Low power consumption– Radiation hardness (w.r.t. CCDs)– Small pixel size (10-20 m)– Thin
• can be less than 20 m• 50µm in industry
– Standard CMOS “cheap”– Room temperature operation– Excellent position resolution
• Disadvantages:– Thin active volume low signals (80 eh pairs /µm)– Smaller CMOS sizes usually yield thinner epilayer thickness
Jaap Velthuis (University of Bristol) 19
On chip data processing:MIMOSA VIII
• On chip data processing:– TSMC 0.25 µm (8 µm
epitaxial layer)– 32//columns of 128
pixels– 25x25 µm2 pixels– On-pixel CDS– Discriminator on each
column
55Fe
Jaap Velthuis (University of Bristol) 20
MAPS with storage: FAPS• Active pixel with memory
cells: sample and store charge during bunchtrain can be read out in 200 ms in between trains no high speed readout required!
• FAPS:– 10 memory cells in each pixel– First step of incorporating in-
pixel intelligence– S/Ncell between 14.7±0.4 and
17.0±0.3
• Issue: need 20 C’s per pixel. Small pixels small C’s. Then spread in actual C-values large. Bad for S/N.
FAPS
Column Output
Write amplifier
RST_W
SELA
1
Memory Cell #0
Memory Cell #1
Memory Cell #9
Ibias
Seed 3x3 5x5
Jaap Velthuis (University of Bristol) 21
MAPS• Problem with MAPS: charge
collection by n-well. So can only make p-mos transistors and electronics.
• Now trying to do proper CMOS in-pixel using deep p-well
• Plan to incorporate signal processing logic inside the pixels!– Store X and Y location (14μm
res. in X and Y)– Digitize seed signal with 5bit
ADC– Get 13 bit time stamp– Sum lower signals for total
cluster charge– Use higher threshold for hit flag– Per strixel only one 32 bit
output word/train
Jaap Velthuis (University of Bristol) 22
3D integrated devices• New development in electronics
industry• Put memory directly on processor
– Reduces R, L, C– Improves speed– Can optimize technology for each layer
• Problems:– Dies must be same size– Precise alignment is essential
Jaap Velthuis (University of Bristol) 23
Mechanical bonding techniques
• Direct silicon fusion bonding– Mechanical bond only– High temperature and pressure– Need very flat surface
• Adhesive bonding– Glue– Low temperature
Jaap Velthuis (University of Bristol) 24
Electrical+mechanical
• Copper to copper fusion bonding– Press copper surfaces
together– Need 400oC
• Copper-tin eutectic bonding– Soldering– Need 250oC
Jaap Velthuis (University of Bristol) 25
Processing
• Thinning– Wafers can be
easily thinned to 50μm, much thinner (6μm) done
• Making contact– Drill hole– Fill with Cu
Jaap Velthuis (University of Bristol) 26
3D sensors (example I)
• 3 Layer Infrared camera– HgCdTe sensor– 0.25μm CMOS (analog)– 0.18μm CMOS (digital)
Jaap Velthuis (University of Bristol) 27
3D (example II)• 3D Laser rader imager
– 64x64 array, 30µm pixels– 3 tiers
• 0.18µm SOI• 0.35 µm SOI• High resistivity substrate
diodes
• Oxide to oxide wafer bonding
• 1.5µm vias• dry etch• 6 3D vias/pixel
Jaap Velthuis (University of Bristol) 28
Summary radiation hardness
• Radiation damage in sensors mainly bulk damage– Atoms knocked out of their lattice
position extra levels in band gap • Effectively donor removal (type inversion)• High leakage currents
– High noise– Thermal runaway
• Problems to get full depletion
Jaap Velthuis (University of Bristol) 29
Summary radiation hardness (II)
• Solutions:– n+-on-n or even better n-on-p detectors– Material engineering (oxygenated
Si/Cz)– Cool to cryogenic temperatures
(Lazarus effect)– Use different materials like diamond– Use different detector type like 3D
Jaap Velthuis (University of Bristol) 30
Summary
• Trend towards more integration– Sensor and electronics in same device– In-device, or in-pixel signal processing– Faster, smaller feature sizes– Less material
Jaap Velthuis (University of Bristol) 31
Summary• Tried to show that Particle Physics is more
than hunting for Higgs and CP violation• Forefront of
– Engineering (stiff light weight support structures, cooling, tunnel building)
– High speed and radiation hard electronics– Computing (web, grid, online)– Accelerators (e.g. cancer therapy, diffraction)– Imaging sensors (e.g. nth generation light
source, medical imaging)
• If you find these things interesting, why don’t you join us?
Particle Physics
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