oct. 2004intro to pixel detectors1 introduction to silicon pixel detectors m. garcia-sciveres...

Oct. 2004 Intro to pixel detectors 1

Introduction to silicon pixel detectorsM. Garcia-Sciveres

October 2004


Charged Particle Tracking

Charged particle shooting through space

detector

Negligible change to energy and direction

Record of path taken by particle (:trajectory”). Not the same thing as a “picture” of the particle.


Flying back and forth to CERN

• Think of jet airplanes. • Easy to see where they have been by streaks left in the sky• An insignificant bit of the energy and mass of the airplane

(of the fuel) are used to produce the streak • Atmospheric conditions affect sharpness, persistence, and

amplification (natural cloud formation) of the streak.

• The earliest charged particle tracking detectors were cloud chambers


Cloud and Bubble Chambers

• A gas-liquid (cloud chamber) or liquid-gas (bubble chamber) is exploited to greatly amplify microscopic perturbations due to a charged particle passing through gas or liquid.

• Both require taking a picture of the cloud streak or bubble trail left behind by the particle. See 5th floor hallway “art”.

• The detection is a 3 step process:– Particle goes by, interacting with a few gas or liquid atoms- losing a small amount of energy in

each interaction– A phase change is triggered by the interaction, which eventually involves huge numbers of near-

by atoms in a chain reaction (domino effect). Cloud streaks or bubbles form. – A picture is taken (this is the “raw” data that is recorded and must then be analyzed).

• Bubble chambers are faster and more precise than cloud chambers, but both are painfully slow by today’s standards. The gas or liquid must be brought to its critical point for every event, it takes time for bubbles/clouds to form, and taking pictures is also slow.

(Gargamel bubble chamber on display at CERN)


Drift Chambers: much faster

• Cut out the middleman. Forget bubbles, clouds, and pictures.

• Wire chambers, time projection chambers.• Measure the ionization left behind by a passing particle

(through a gas).– Two step process: use avalanche in gas to amplify the primary

signal (obviously faster than bubbles + pictures)

Wire at –ve voltage

Wire at +ve voltage

+

++

+ --

-

-

Ions created by passing charged particle. Too small for electronic detection- just a few atoms

When ions get close to HV wire they trigger an avalanche- each ion leads to ~10,000 new ions. Now you have a charge you can measure with an electronic circuit.


Why Does This Work?

• (1) An avalanche is a natural phenomenon in gasses (think lightning)• (2) Gasses consist 100% of neutral molecules- there is no natural

contamination of ions an any level. – The ions needed to start an avalanche must be externally introduced (drift

chambers) or are created when the electric field is high enough to rip atoms apart.

– Until ions are introduced the gas will happily ignore the electric field.

• What’s wrong with drift chambers?– Drift: it takes time for ions to move towards the HV wires– Rate limit: must wait for all ions to clear away before device is sensitive

to new particles. – Resolution: The diffusion of ions in the gas and the ionization statistics

(randomness in location of primary ions) limit the ultimate resolution (~100um).


Silicon Detectors: still faster and more accurate

• Cut out the middleman again!• Detect the primary ionization directly• Silicon (a solid) is much denser than gases => more primary ions are

produced (this also means that the charged particle loses more energy in order to be tracked). – Just enough charge for direct measurement with electronic circuits.

• Silicon (a solid) has less diffusion than a gas => higher resolution (~10um).

• The catch: solids in general are not 100% made or neutral atoms, free of ions like gasses. – Need a solid that one can prepare to be 100% ion-free– Has to conduct electricity (so ions can flow to an electronic circuit)


Silicon Detector Basics

• Metals: Atoms arranged in lattice that shares valence electrons. Number of free charge carriers ~1022/cc (~1,000 Coulomb of charge per cc). Impossible to remove this free charge.

• Pure silicon: atoms arranged in metal-like lattice, but number of “charge carriers” is ~1010/cc (~1nC/cc).

– A modest electric field can remove this free charge. • Impure silicon can have varying number and kind (+ or -) of charge carriers,

depending on impurity type and concentration. – N-type has –ve carriers– P-type has +ve carriers– Typical concentration in range 1012 – 1018/cc

• After free carriers are removed by electric field, silicon looks electrically like a gas- 100% neutral. This state is called “depleted”.

• A passing charged particle creates 22,000 (avg.) charge carrier pairs per 300m. (~4fC).

– If my laptop ran on 4fC/s the battery would last ~1010 years (the age of the universe).


Actual ATLAS Pixel Sensor

• A diode junction forms wherever p-doped and n-doped regions touch. Depletion always begins at the diode junction as reverse external voltage is applied.

• Hadron irradiation introduced p-type defects. Eventually this will cause the bulk to “type invert” and become p-type. At this point the diode junction shifts to the top. This was chosen on purpose because it allows to operate without fully depleting the bulk.

Lightly n-doped bulk

Heavily n-doped pixel implants (doping too heavy to deplete)

Heavily p-doped back side contact Guard rings

P-spray doping to isolate individual pixels

Diode junction

Bumps connect to implants


Basic Charge Amplifier

Q=CV

Gain = 1/C(fF) V/fC

Pixel chip input stage gain is ~300mV/fCC = 3.5fF

Q1/C1 = Q2/C2

Pixel capacitance + parasitics ~ 100x inverse gain!

Active amplifier. Increases effective capacitance by factor of “open loop gain” at the expense of addinga rise-time (active takes time)

With Leakage current CompensationDC “current source” compensates for detector leakage


Pixel Chip Front End

comparator

Input from Pixel sensor (bump goes here)

preamp


Front End Output

Threshold (adjustable up and down). Comparator output is on when red line is above threshold, off if below.


Front End Features

• Programmable threshold = Global Threshold + Pixel Threshold

• Calibration charge injection

• Ability to measure leakage current

• Time over Threshold (TOT) charge measurement– How long the red curve says above threshold depends on the size of the

input charge

Can easily change threshold for whole chip

Can fine tune each pixel to compensate for response differences (Tuning)

V1

V2switch

Input from detector

Good old charge amplifierInjection capacitor

(must be small)


CMOS integrated circuits

• CMOS = combination metal oxide silicon

• Combination = p-type and n-type implants on the same wafer.

silicon

photoresist

+ ion implantation

p-implant p-implant

Gate oxide

n-implant n-implant

source

gate

drain source

gate

drain


The MOS transistor

• Transistor

• Diode

• ~Resistor

• Capacitor

S D

G

S D

S D


Complexity in Numbers

• Complex circuits are possible using a large number of MOS transistors (~3M in FE-I3).

• In practice there are some special circuit elements also used in small numbers, such as metal-insulator-metal (MIM) capacitors and polysilicon resistors.


The MOS transistor schematic of the FE-I3 charge amplifier


Pixel Chip Readout Architecture

Data driven

Trigger Trigger driven

Time stamp

oct. 2004intro to pixel detectors1 introduction to silicon pixel detectors m. garcia-sciveres...

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