Silicon’s Limitations

Chris Kenney

April 28, 2011

Page 2

Key Parameters

•  Material properties: mobilities, band gap, dielectric constant is 12!

•  Geometry of device

•  Operation: fields, temperature

GLAST/FERMI prior to launch

Page 3

Optical Light Absorption

•  Very sensitive to wavelength

0.01

0.1

1

10

100

0 300 600 900 1200 1500

Abs

optin

Dep

th (m

icro

ns)

Wavelength (nm)

Optical Absorption

Page 4

X-ray Absorption

0.01

0.1

1

10

100

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 Abs

orpt

ion

Dep

th (m

icro

ns)

Energy (eV)

X-Ray Absorption

Page 5

Electron Absorption 3 keV electrons from Casino Almost all energy deposited in a sphere with a 100 nm diameter

Page 6

Electron Absorption

Page 7

Direct Sensing

•  Primary, science particle is absorbed in silicon

•  Match

Page 8

Current pulse

3D sensor Field null point

Page 9

Avalanche Mulitplication

•  Intrinsically limited to thin regions •  Electron multiplication factor much

higher than for holes •  Requires careful device design

and fab •  Can have slower recovery time •  Can add extra noise

Page 10

Mobility - Electrons

Colder is faster Electrons

Page 11

Electrons go the distance

•  Time for complete signal charge higher for holes

Page 12

Electrons go the distance

Collect electrons on the far face, if there is one. Minimize hole drift distance and maximize electron drift distance for a given thickness

p+

n+

i

3D Sensor

Use 200 microns thick sensor

100 micron pitch

50 micron n-to-p electrode spacing

Trench electrodes = uniform electric field

p+

n+

p+

n+

i i i

Delta Rays

•  Changes total charge

•  3D scales waveform

•  Planar introduces waveform distortion

•  Large energy deltas will always degrade the timing

p+

n+

p+

n+

i i i

p+

n+

i

Page 15

Entrance Face

•  Dielectrics – bad for electrons •  Metals – bad for photons and

electrons •  Heavily doped silicon – bad for

both – inefficient and slower •  Anti-reflection coatings critical for

optical photons

Page 16

Diamond

•  Higher mobilities •  Lower dielectric constant •  Higher bandgap •  Optical transparency •  Easy of fabrication?

30 nanometers

Platinum (3000A) on Mo (30 A) 18 Microns Thick Diamond

Collaboration with D. Pickard Nat. Univ. Singapore

ESRF Test

Placed in synchrotron beam

Attached to fast, discrete amplifier

Recorded bunch spacing period

Scope Trigger Count

I n t e r v a l (ns)

0 5

176.01

176.02

176.03

176.04

176.05

176.06

176.07

10 20 30 0

measured: mean=176.036ns σ = 16ps

σ = 16 ps

Collaboration with J. Morse of ESRF

Page 18

Signal to Noise

•  Noise often limits the achievable resolution

•  Encourages indirect sensing after multiplicative amplification of the primary

•  Capacitance can be critical •  Entrance face loss of electron energy

Page 19

Ideal

•  Secondary electron accelerated to 3 keV – decent signal

•  300 nm sensor thickness •  All singal charge collected within 5

picoseconds •  Current pulse will have a rise time

several times faster

Page 20

Summary

•  Match absorption thickness to particle

•  Maximize electric fields

•  Run cold

•  Choose between direct and indirect sensing

•  Entrance-face dead layer must be minimized

•  Have electrons transit the long way

•  Beware of capacitance

Page 21

Personnel

Sherwood Parker, Gary Varner, John Morse, Ed Westbrook, Al Thompson, Jasmine Hasi, Cinzia Da Via, Angela Kok, Giovanni Anelli, Dan Pickard, Niels van Bakel