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Fundamentals of Microstrip Silicon Detectors

Tianchi ZhaoUniversity of Washington

• Introduction• Semiconductor Basics• Microstrip Detector Structures and Manufacturing• Front End Readout Chips• Silicon Vertex Detector Construction and Testing• Summary

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Brief History

1951 Point contact germanium diode for -particles1955 Diffusion semiconductor p-n junction detector for -particles Surface barrier (Schottky) silicon diode for -particles 1960-64 Surface passivation Guard ring structures Ion implantation techniques1966 Check board and strip silicon detector1980 Microstrip silicon detector : CERN fixed target experiment1990 First microstrip vertex detector for e-e+ collider : MARK II1992 First microstrip vertex detector for hadron collider : CDF

Every major HEP experiment has a silicon vertex detector

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Advantages of Silicon Microstrip Detector

High position resolution ~ 10 m High double track separation ~ 100 m Perform well in strong magnetic fieldHigh radiation tolerance up to 10 Mrad ~4x1014 1 MeV neutron fluenceSignal very fast ~10 ns L-1 trigger!

Sensor production Industrial standard processDetector stability Stable and maintains free

Disadvantages of Silicon Microstrip Detector Expensive for large size detectorsOne dimensional

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Silicon detector R&D Enormous amount of money and physicist man years in the past 20 years

• Understanding and improving detector structure integrated bias resistors, coupling capacitors double sided detector, p-stop, bias gates guard ring structures for extending operating voltage• Understanding and improving manufacturing processes• Detector physics simulations (TOSCA)• Different types of silicon materials n-type vs p-type, <111> vs <100>, doping methods and levels• Detector construction Wire bonding, Silicon sensor bonding Low mass support structures, low mass find pitch signal cables Alignment techniques Cooling techniques • Sensor testing techniques• Readout chips and signal processing• Recent research has been focused on radiation resistance

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Density : 2.33 g/cm3

Radiation length : 94 mm Energy band gap: 1.12 eVWork function 4.6 V

Energy loss by m.i.p. 380 eV/m (mean)Energy loss by m.i.p. ~260 eV/m (most probable)

Energy required for an e-h pair : 3.6 eV

e-h pairs by m.i.p. 110 per m silicon (mean). e-h pairs by m.i.p. 75 per m silicon (most probable)

Detector thickness : 200-300 m 0.2-0.3% r.l.

Signal size 15-22k e (most probable)Signal/noise ratio Typical ~10Main noise source Shot noise (fluctuations of leakage current)

Properties of Silicon

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N-type silicon

Dopants : group-5 elements (P, As)

Majority carriers : (–) electrons in conductive band n : free electron density

Charge on ionized donor atom : ()

P-type silicon Dopants : group-3 elements (B, Al, Ga)

Majority carriers : () holes in valence band p : hole density

Charge on ionized acceptor atom : (–)

Bulk silicon contains many other impurities. Some of the impurities are at much higher levels than the intentionally added dopants. Most of these impurities are electrically inactive (do not form bonds with silicon).

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Bulk Material of Silicon Semiconductor

• Intrinsic (pure) silicon single crystal

Carrier density 1.381010/cm3 at 300 oK Resistivity 200 kcm• Regular silicon single crystals by Czochralski (CZ) pulling

method

Relatively high doping level : Resistivity : ~10 cm

• “Detector grade” bulk silicon by Floating-Zone (FZ) technique

High purity high resistivity silicon 1 kcm-5 kcm Electrically active impurities are mostly boron and phosphorus at ~11012/cm3 level

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Czochralski (CZ) method

Up to 12” diameter silicon single crystal ingot

Used in semiconductor integrated circuit industry

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Floating Zone

A crucible-free crystallizing technique

A rod of high purity polycrystalline silicon is continuously molten by EM induction heating A single-crystal seed at bottom held in vertical position and rotated Surface tension between melting silicon and growing solid silicon retains the molten silicon

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Flooting Zone Method

High purity detector grade silicon single crystals up to 6” diameter

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More on FZ Silicon Bulk Material

• Electrically active impurities are boron and phosphorus at ~ 1012/cm3 level

• Some electrically inactive impurities such as oxygen, nitrogen, carbon and others can be at much higher level level

• FZ-silicon is always n-type because an excess of phosphorus in polycrystalline silicon

• Resistivity of FZ silicon can fluctuate 30-40% within a wafer

• Highly uniform n-type silicon can be produced by neutron transmutation doping method (30Si + n 31Si + 31P + e-)

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• Doping concentration of detector grade silicon

~11012/cm3 1 dopant atom per m3 1 dopant atom per 5x1010 silicon atoms (Silicon has 51022 atoms/cm3)

Resistively ~2 kcm

• Low doping level

--> low depletion voltage --> high charge collection efficiency --> low leakage current and low noise

• New trend --> Oxygenated (with high O2 concentration) bulk silicon for higher radiation resistance --> p-type bulk

Doping level in Detector Grade Silicon Bulk Material

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Fundamentals of Semiconductor Particle Detectors

Bulk material:

• 2” to 6” diameter n-type FZ wafers are used to make silicon sensors (strips can be up to 12 cm long)

• Ingots are cut to thin wafers (200-300 m) by wire saw

• Wafers are lapped and polished (Chemically and mechanically)

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P-N Junction Diode

All most all silicon particle detectors are based on p-n junction diode structure

microstrip, pixel, photodiode, etc.

It is important to understand how a p-n junction diode work

Three types diode structure

• Surface Barrier Diode

• Diffused p-n junction diode

• Ion implanted p-n junction diode

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Surface barrier detectors

• Evaporated thin gold windows• Based on rectifying properties of Schottky barrier Problems: Metal-semiconductor barriers are difficult to control, to reproduce and they are fragile Higher leakage current than ion implanted p-n junction

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Diffused p-n junction semiconductor detectors were the earliest semiconductor detectors use in nuclear physics experiments. Problems of diffused junctions: -> Require high temperature steps (1000 -1200 oC) -> reduced carrier life time -> increased leakage current

Diffused P-N Junction Diode Detectors

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Well developed standard silicon wafer processing techniques for IC’s No high temperature steps Excellent reproducible detector properties

Ion Implanted P-N Junction Diode Detectors

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Common Detector configuration:

• High resistance n-type bulk (doping level 1011 -1012/cm3)

• (p+) strips on one surface (junction side)

• (n+) for ohmic contact on opposite surface

• Depleted region is positively charged

Other configurations:

• (n+) strip on n-type bulk or p-type bulk

These types may offer higher radiation resistance

Single Sided Microstrip Silicon Detector Summary

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• Strip pitch 25 m or larger

• Readout pitch 50 m or larger

• Strips can be DC or AC coupled to amplifiers

• AC coupling can avoid pedestal shifts due to dark current --> Require integrated capacitors • Integrated capacitors --> very thin SiO2 layer (break down at 1000 V/m)

• Integrated bias resistors --> polysilicon lines with appropriate doping to achieve up to ~100 M

Typical Paramters of Microstrip Detectors

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Detector Manufacturing

Planar technology: successively placing patterns of insulators dopants metallisation contactsby using standard silicon IC manufacturing processes

Photomasks: quartz plates coated by thin film of chrome that is opaque to UV laser light

Photomasks are produced by etching out chrome layer on the quartz plate using photolithography Simplest DC coupled single-sided microstrip detectors need 3 photomasks

Up to 12 photomasks are used to make sophisticated double sided detectors

Complicated, slow and expensive especially for double-sided

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Major steps for planar fabricationprocess of a microstrip detector

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Photomask for D0 double-sided silicon sensors

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Double Sided Microstrip Silicon Detectors

(p+) strips on one side

(n+) strips on the other sideRequire AC coupling --> integrated capacitors

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Problems:

SiO2 layer is always positively charged

(n+) strips under SiO2 layer can be shorted due to electron accumulations Solutions: p-stops for (n+) side strips Special MOS bias structure

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P-stops : P+ dope lines completely enclose (n+) strips

Block shorts under SiO2 layer due to electron accumulations

P-stops

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Guard Ring Structures

Guard ring structure are used to prevent breakdown at the sawn edges and to extend the operating voltage in order to achieve full

depletion

Guard rings : P+ or N+ doped ring shaped structures surrounding the active region at ~ 1mm from the sawn edges

As many as 20 rings are used in some high radiation resistant designs Guard rings can be biased or floating

Operating voltage can be extended to > 1000V

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More difficult to manufacture

Performance poorer than single-sided sensors

higher depletion voltage high leakage current lower break down voltage

Tolerate less radiation

Generic problems of double-sided silicon sensors

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Good Detector Performance Requires Complete Depletion

Depletion starts frompositively biased (n+) side

No drift field in undepleted region

Signal loss due to recombination, diffusion

Capacitance of the junction diode proportional todepletion width W

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It is important to know VFD

How to measure VFD ?

The most commonly used method is base on :

C = (siA/d) = A(q siNeff/2V)1/2 A : diode area

C2 1/V before depletion C constant after depletion

VFD can be determined from measured C2-V curve

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Use C-V curve todetermine VFD

Use I-V curve todetermine VFD

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Effective Doping Density Neff

Effective doping density is an important parameter for understanding the silicon detector operation

Effective doping level can change due to radiation

Neff can be derived from the measured

full depletion voltage VFD and full depletion width w (detector thickness)

Neff = (2 si VFD /qw2)

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Effective doping density (Neff) and depletion voltage (VFD)

Neff = (2 si VFD /qw2)

Depth of depletion

d = (2siVFD/q)1/2 (1/ Neff)1/2 si : Dielectric constant of Si

Junction capacitance

C(V) = (siA/d) = A(q siNeff/2V)1/2 A : Diode area

Fully depleted capacitance

CFD = (siA/w) w : width of full depletion

Resistivity

= 1/qNeff : Mobility Neff : Effective doping density

Some Basic Relations

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Probing the Microstrip Detector

Silicon sensors are first tested on a probe stationin specially designed fixtures

• Inter-strip capacitance measured at full voltage• Inter-strip resistance measured at full voltage• Values of coupling capacitors• Values of bias resistors• Capacitance to voltage (C-V) -> full depletion voltage VFD

• Leakage current - voltage (I-V)

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A Manual Probe Station

A Automated Probe Station

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Probe Fixture : provide bias to the strips and guard rings

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• Clean room

• Automatic (PC-controlled) probe station ($80 k)

(Manual probe station)

• Pico-ammeter for I-V measurements

• Capacitor meter for C-V measurements from 30Hz to 300kHz

• High voltage source (up to several 100V)

Equipment for Silicon Sensor Probing

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Readout Electronics

Hybrid Circuits are bond to edges of silicon sensors

• Kapton flexible circuit board backed by beryllium sheets or ceramic (BeO, B2O3) sheets• Front end readout chips• Bias control• Temperature sensor • Signal and power connector• kapton cable to bring digital signals to outside and to bring DC power in (~1000 A for D0 silicon tracker)

Optical Links

Convert electric signals to optical signals by optical linksOptical cables bring signals to readout crates

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D-zera Flexible Kapton Circuit Board

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Hybrid Circuits for Double-Sided D0 Silicon Detector

D0 High Density Interface (HDI)

Flexible kapton circuit board is laminated on to beryllium sheets

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Integrated 128 channel chips

Amplifiers of amplifier/discriminators Pipelines Multiplexor Output driver Control circuits

Typical size : 5-12 mm

Chips usually consume more power than the silicon sensors

Power consumption : ~ 1 to several mW/channel ~200-500 mW/chip

A moderate sized D0 silicon tracker consumes 5 kW or 1000 A at 5 V !

Radiation hardness depends on the process: up to 10 Mrad

Front End Chips for Microstrip Silicon Detector

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(LBL/Fermilab SVX-II)

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Die picture of the front end chip for D0 silicon tracker

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• Architecture closely related to signal processing and triggering for a particular experiment

• Custom designed 2 experienced engineers 2-3 years 2 - 4 prototype submissions ~ $0.5 M R&D cost

• Large and complicated chip

• Difficult to make (low yield, average ~25% at best)

• Expensive: $500-700 per chip for Fermilab experiments

Comments : Front end chips

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Ladders

• Usually two or more pieces of silicon sensors are joined together to form a ladder• Hybrid circuit board are attached to the end of the ladder• Support structure (carbon fiber/beryllium/foam)

Ladder construction

• Ladders assembled in adjustable fixtures

• Microstrips and hybrid circuits are connected by wire bonding

• Sensors and support structure are glued together

• Aligned and measured on a CMM machine

Detector Construction

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Ladder Structure

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CLEO-II Silicon Vertex Octant

54A DELPHI Silicon Ladder

A CDF Silicon Ladder

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A completed D-zero silicon ladder

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Wire bond: sensor strips to hybrid electronics board

Wire bond: silicon trips to silicon strips

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Wire bonder in action

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Ladder Assembled on a Optical CMM

At Fermilab Silicon Detector (Sidet) Factory

Silicon sensors are in a special fixture

Fiducials on sensors are aligned relative to targets on fixture

The precision can reach 1 m

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Probing the Microstrip Detector by Laser

Completed ladders are probed by a infrared laser on an automated stage

1064 nm laser light can penetrate silicon and mimic a m.i.p.

• Signal size and shape• Charge collection plateau• Depletion voltage

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Mount a ladder in probing fixture

Remote controlled micro stage

Well focused 1064 nm laser beam

Laser Probing

Covered laser probing stationand data acquisition rack

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“Burn-in” completed ladders under real operating conditions at Fermilab SiDet Power is applied to ladders

Cooling is provided

A maximum of 16 ladders can be burned-in at a time

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Partially assembled D-zera barrel silicon tracke is being measured on a 3 meter CMM

Full detector assembly is done on a large CMM

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CDF Barrel Construction

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Silicon microstrip detector has become a common tool for high energyexperimental physics

It is specially useful for detecting secondary vertices from short lived exotic particles and, therefore, very crucial for new discoveries

Microstrip silicon detectors have found their applications in x-ray imaging for material science, biology and medicine, etc.

Designing and constructing silicon microstrip detectors for today’s high energy experiments require great engineering efforts by many people from many different fields.

Let us conclude with pictures of some completed detectors

Summary

65Fermilab E691

Forelimb E687

Early Silicon Detector for Fixed Target Experiments

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ALEPH Silicon Vertex Detector

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CDF SVXII Barrel

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CDF Silicon Vertex Detector

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Largest Silicon Tracker

CMS Silicon Tracker

5.4 m long

2.2 m diameter

240 m2 silicon !!!