Solid StateDetectors-2

T. Bowcock

2

Schedule

1 Position Sensors

2 Principles of Operation of Solid State Detectors

3 Techniques for High Performance Operation

4 Environmental Design

5 Measurement of time6 New Detector Technologies

3

Why use a Solid StateDetector?

• Physics requires– high rate capability

• rare processes imply huge event rates

– high efficiency and low dead time

– good signal./noise ratio– good resolution

– electronics r/o– high speed

4

B-physics

• Detecting vertices ...

5

Silicon Properties

• Electron-hole production at few eV– compare with 30eV in gas

• Density reduces deltas– remember bubble chamber photos(!)

• 100 e-h pairs/micron• solid

– easy to install close to interaction point

6

Silicon

• Properties of Si– Crystal structure

– Group IV– 4 electrons in

valence shell

• 2D representation

7

Ionisation and holes

• 1.1eV• Holes

– F=mhdv/dt

• In semiconductorselectrical conductiontakes place via twomodes of electron motion.Can be viewed as motionof e-’s with charge -q andeffective mass m*e andholes, +q, m*h

• Intrinsic semiconductors

+

e-

8

Valence and ConductionBands

• In intrinsic semiconductors (noimpurities)

E

conduction band

valence bandE=Ev

E=Ec

Ef

9

Thermal Properties

• In intrinsic semiconductor

• Note that in intrinsic semiconductorsn=p=intrinsic carrier density

• For Si (at room temp) ni=1.5*1010

)exp(

)exp(

kT

EENp

kT

EENn

vfv

fcc

−−=

−−= Nc=3*1019

Nv=1*1019

−==

kT

ENNnnp g

vci exp2

10

Carriers properties

• Carrier mobility µ=v/E– 1300 cm2/Vs for e’s

– 500 cm2/Vs for holes

• Resistivity– reflects the doping level )(

1

pnq hn µµρ

+=

11

Intrinsic Silicon cont’d

• At room temp in a 300 micron thickdetector with an area of 1cm2 totalfree carriers would be about 109

– cool (lower T)• can be done but cryogenics are bulky and

expensive

– reverse biased diode operation

12

Impurities

• Group III (e.g. B)– acceptor type atoms

– majority carriers=holes– p-type

• Group V (e.g. P)– donor type atoms

– majority carrier=e’s– n-type

III

h

V

e

13

Impurities

• Do not lead to net charge!– Donor concentration Nd

– Acceptor concentration Na

• simplifies if impurities >> ni

−++−=

−++−=

=−+−

22

22

)(42

1

)(42

1

0

adida

adiad

ad

NNnNNp

NNnNNn

NNnp

14

Band Structure

• Doped silicon

E

conduction band

valence bandE=Ev

E=Ec

Ea

Ed

(p-type)

(n-type)

15

pn-Junction

• Abrupt junction– diffusion

- + + -+ + + + + + + + + + + +

- - -

- - -- - -- - -

+ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + +

- - - - - - -- - - - - - -- - - - - - -- - - - - - -

+ - - +

- - - - - - -

+ - - ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + +

- - - - - - -- - - - - - -- - - - - - -- - - - - - -

- + + -+ + + + + + + + + + + +

- - -

- - -- - -- - -

+ - - +

16

pn-junction

- - - - - - -

+ - - ++ + + + + + ++ + + + + + ++ + + + + + ++ + + + + + +

- - - - - - -- - - - - - -- - - - - - -- - - - - - -

- + + -+ + + + + + + + + + + +

- - -

- - -- - -- - -

+ - - +

Space charge

Carrrier density

Field and potential

p-type n-type

acceptor

donor

17

Diode Behaviour

• Built in potential

• Calculate depletion width (neutral)

• Use 1D Poisson Eq.

=

2ln

i

da

n

NNq

kTV

21 WNWN da =

εερ qN

dx

Vd=−=

2

2

18

Depletion Depth

• Problem: Prove

• Conversely applying a potential increasesdepletion width

• Reverse biased diode• Note dependence on doping

( )

( )add

Bs

daa

B

NNqN

VW

NNqN

VW

/1

2

/1

21

+=

+=

ε

ε

19

Depletion Region as aDetector

• Build a p+n diode– Na=1015

– Nd=1013

• At 50-100V bias voltage get 300micron depletion in the n-part (bulk)and < 1 micron in the heavily dopedpart.

20

Depletion region

• In the depletion region continualthermal generation of eh pairs– leakage current

– depends on volume

• Ionisation will also create pairs thatwill also drift and be collected– signal/noise

21

IV and CV of diodes

22

Fabrication

• Key to use of Si is the processing• Photolithography

Organic Photoresist usually “spun” on

Photoresist forms a layer a fewmicrons thick in 30s

23

Patterning

• Photoresist exposedusing a “mask”

• Mask contains thedesign and isproduced with e-beam lithography– feature size down to

0.25microns

• Negative or Positive

24

Etching

• Negative Process• Chemical etch

– exposed partunaffected byetch 1

• Exposed patternmay be removedlater– second etch

25

Example:

• Aluminium line

26

Simple Strip Detector

• Oxide passivation

• Windows• Doping

– B– As

• Al Metallization• Al patterning

• Rear Contact

SiO2Al

n+

p+

27

2D strips

Al

Si

28

Wafer

Main detector

Test structures

29

Pulse Height

Landau Distribution

30

Signal Shape

• Simulation– charges follow e-

field– Ramo’s

Theorem– Finite element

• Matches data

0

100

200

300

400

500

600

700

800

900

0 8

16 24 32 40 48 56 64 72 80 88 96

250V Irrad

250v Unirrad

60V Unirrad

0

0.2

0.4

0.6

0.8

1

1.2

1 9 17 25 33 41 49 57 65 73 81 89 97

250V Irrad

250v Unirrad

60V Unirrad

31

Strip Pitch

• Strip pitch is the dominant factor indetermining resolution

Typically 40-100 microns(why not smaller)

Resolution better than about pitch/4(why?)

32

Charge Sharing

• Charged shared (see Ramo’sTheorem!) between strips

c

Pulse height

x

33

Pulse on p-strip detector

Al

Si

-V

+-

34

Electronics

35

Electronics

• VLSI• ASIC

bonds

36

Electronic Noise

• Noise sources– coupling of strips to each other an back

plane (extra load and signal loss)– intrinsic to amplifier

• ENC = a +b×C– d may vary from 100-1000 depending on speed– b varies from 20 to 100 depending on speed

– Sources from• leakage current• load resisitor

R

kTt

q

eENC

qIt

q

eENC

p

p

2

4

=

=

37

Signal/Noise

• Electronics– speed

– intrinsic characteristics

• Thickness of detector and Vbias• MIPS• Capacitance of Strips

– resistors

• Desire about 10/1 S/N

38

AC Coupled Devices

• Avoid drainingbulk current intoelectronics

• Usual detectorbuilt

• Higher cost

39

Biasing Techniques

• FOXFET

• Reachthrough• Polysilicon resisitor

– 1-10MΩ

poly Al bias strip

40

Double Sided Detectors

• Using the Ohmic side– divide up the “plane”

+++++++++- - - - - - - -

+ ++ +- -- -

p-stops

41

Pixel Detectors

• Pixel detectors are identical inprincipal to strip detectors– shape of pads smaller

• few microns or 10’s of microns comparedwith strips of 6cm or so,

– more diffiucult to route out• expensive bump bonded electronics

– low capacitance(noise)– intrinsically 2D rather than 1D

42

Charge Coupled Devices

• Very high precision (0.2microns)

• Moves charge in a potential well• 2D device• Slow

43

CCD

• Use small low capacity elements andexchange information <10e noise

• Matrix of potential wells created

p-type

n-doping

“cell”

44

Summary

• Have seen the basics of how astrip/pixel detector works– capable of adequate S/N

– only cost effective for last 10 years withadvent of second generation fabrication

– easily modifiable geometry

• Next: high performance operation