Si Based Pillar Structured Thermal Neutron Detectors · Lawrence Livermore National Laboratory...

Lawrence Livermore National Laboratory

Rebecca Nikolic

A.Conway1, C. Britton3, C. Cheung2, M. Dar2, M.N. Ericson3, L. Fabris3, R. Radev1, Q. Shao1, L. Voss1, T.F. Wang1

LLNL1, Univ. of Neb.2,ORNL3

This work performed under the auspices of the U.S. Department of Energy by

Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, LLNL-PRES-474261

Si Based Pillar Structured Thermal Neutron Detectors

Funded by

DHS/DNDONebraskaUniversity of

Lincoln

[email protected] IAEA March 23, 2011

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2Lawrence Livermore National Laboratory

0 500 1000 1500 200010

-1

100

101

102

103

104

105

Co

un

ts

Energy (keV)

Neutron 3V

Gamma 3V

No source 3V

We are developing high-performance neutron

detectors

252Cf

DUT

252Cf

DUT

252Cf

DUT

137Cs

20 % recent result (26 µm pillar)

105 neutron/gamma discrimination

Applied Physics Letters, Vol. 93, Issue 13, Sept. 2008, p. 133502

50µm Pillar, > 90% fill

Detector noise

Neutron events

200 keV discriminator

3


Thermal neutron detectors are needed for the detection of SNM: 3He tube is current standard

10Boron is interspersed between silicon pillars to detect thermal neutrons

10Boron chosen for high thermal neutron cross-section 3837 barns and process compatibility

4Lawrence Livermore National Laboratory

Comparison: Thermal Neutron Converter Materials

10B+n → 7Li +

10 100 1000

0.0

0.2

0.4

0.6

0.8

1.0 10B

10B

5C

B

B5C

6LiF

Pro

ba

bili

ty o

f N

eu

tron

In

tera

ctio

n

Thickness ( m)

ηint = 1-exp(-t/mfp)

1st Excited State Ground State

Energy (MeV) 0.84 Li 1.47 α 1.02 Li 1.78 α

Distance in 10B (µm) 1.85 3.6 2.0 4.4

Distance in Si (µm) 2.4 5.2 2.8 6.4

Pillar height determined by

probability of n interaction

Pillar spacing determined by

& Li travel ranges

Mean free path = 1/N [ m]

N = number density, nuclei/volume

= microscopic cross-section

5


Pillar Detector enables

high efficiency neutron detection

Standard 2D Design LLNL 3D Design

Thermal

neutrons

Si

10B

Electron-hole pairs

Charged

particles

pitch

+ -

+ -

+ -

( , 7Li)

4 m

50 m

10B efficiently produces particles

Most /7Li do not reach the detector

Limited efficiency: 2-5 % (@ thermal)

Simulations show 3-D structure will

increase efficiency towards 50+%

Device geometry:

etch depth (50 m) no capture

pitch (4 m) alpha particle range

+ - Electron-hole pairs

De

tec

tor

C

on

ve

rte

r

Thermal neutrons

10Boron

Charged particles( , 7Li)

( , 7Li)

3 m

6


Pillar Detector Fabrication

1. Pattern pr mask 2. Deep RIE etching 3. Conformal Boron

Deposition

boronMulti-step Process

Si

photoresist

4. Plasma etching

of boron5. Deposit contacts

electrodes

7


OUO

OUO

Inductive-Coupled Plasma (ICP)

Control ion density and RF power independently

1. Less damages to etched materials; 2. Flexibility in mask materials;

3. High etch rate; 4. Low substrate temperature

RF biased wafer

chuck

RF input

1000 W

N S

Magnet Magnet

RF waveguide

Plasma

H2, Ar2, CH4, Cl2,SF 6 Gas

Electron Cyclotron Resonance (ECR)

SF6 & C4F8 Gas

High-density plasma processing for

high aspect ratio structures

8


OUO

OUO

“Pillar” Chips for 10B R&D and Detector Fabrication

ICP Etch

Pillar Litho well-developed

Large area pillar litho developed

Developing negative pr process for large area

50 µm pillar process developed

Compatible with 2’’ and 4’’ wafers

50µm Pillar Chips, 2µm Width

nLOF 2020

Vertical

sidewalls

Vertical CVD system provides improved film reproducibility and safety.

Vertical vs. Horizontal CVD System for 10B Deposition

Horizontal CVD System

Features

Vertical CVD System

Pump

Substrate

heater

• Better control of precursor and sampleheating

• Quick sample loading• Large conductance → Effective pump

rate & lower operation pressure• Tailor for scale up with larger wafers• Low chemical exposure

• Inexpensive set up• Easy cleaning of quartz tube chamber• Limited control of precursor and

heating profiles for samples• Long sample loading time• Potential for chemical exposure

Pump

Mass flow

Controller:

B10H14

B10H14

source

CVD

Furnace

Features

Heat shielding improves heat profile and boron filling.

Activating New Vertical CVD System

Vertical CVD system50- m silicon pillar samples

before boron coatingSubstrate

heater

Side wall

shielding

Top side shielding

Sample location

Shower head

High fill factor achieved

First device runs had low

efficiency

Optimizing vertical process in

parallel with 1’’ CVD tube

11


We have developed specialized processing and characterization

tools for 10B

Raman Spectroscopy

Stress induced on silicon pillars

calculated from Raman shift

1E+14

1E+15

1E+16

1E+17

1E+18

1E+19

1E+20

1E+21

1E+22

1E+23

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Depth (microns)

B C

on

ce

ntr

atio

n

1E+02

1E+03

1E+04

1E+05

1E+06

1E+07

1E+08

1E+09

Co

un

ts P

er

Se

co

nd

Si (sec. ion counts)->

B

C06M1250B05.SWF

LLNL, Sample 228

9/12/2006

Cs Evans Analytical Group

Boron CVD 600 C – 900 C

SIMS:No B diffusion Si/B interface

Boron CVD and Etch Back

A) Pillar Platform 1:25 aspect ratio

B) Conformal boron filling

C) Rapid removal of boron with

high B:Si selectivity

Stress in 10B Coating

4 µm

12


0%

10%

20%

30%

40%

50%

60%

70%

80%

0 20 40 60 80

Pillar Height ( m)

Ne

utr

on

Eff

icie

ncy (

%)

1.0E+04

1.0E+05

1.0E+06

Ne

utr

on

to

Gam

ma

Dis

cri

min

ati

on

Neutron Effeciency

200 keV

662 keV

1 MeV

104

105

106

Simulation Tool-box

• MCNP Neutron history

•TRIM alpha, lithium trajectory

• Silvaco semiconductor device physics

Efficiency by simulations and measurement

Simulations Measurements

Applied Physics Letters, vol. 93, issue 13, Sept. 2008, p. 133502

0 500 1000 1500 200010

-1

100

101

102

103

104

105

Co

un

ts

Energy (keV)

Neutron 3V

Gamma 3V

No source 3V

20 % recent result (26 µm pillar)

105 neutron/gamma discrimination

13


We are increasing detector area and channel number

Interposer board 4 mm x 4 mm detectors

Scaling to Larger Area

Excellent uniformity

across 9, 4 mm x 4 mm detectors

Jleak: 10-3 A/cm2

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3

J (A

/cm

2 )

V (V)

4mm x 4mm

5mm x 5mm

10mm x 10mm

14

Thermal neutron detection efficiencies measured at LLNL

Part # Efficiency

EPI 26-1-1* 22

EPI 26-1-4 23

EPI 26-7-1 ~20

EPI 26-6-3 16.4

EPI 26-7-2 16.1

EPI 26-7-3 19.2

EPI 26-7-4 17.8

EPI 26-1-2 ~20

EPI 26-2-3 ~20

EPI 26-2-4 ~20

Process for measuring thermal

neutron efficiency:

MCNP of source and lab for # of incident

thermal neutrons

Eff = Measured counts / simulated incident

counts

Measure “calibration” detector using poly

moderated 252Cf source

Other detectors:

• Compare to calibration detector to

determine relative efficiency

All detectors from “EPI-26” family

1’’ CVD system

*Calibration detector

Packaged LLNL Pillar Detector

TO can

2 mm x 2 mm detectors

15


Characterization of 9 Element Detector Array

Detector size 2 mm x 2 mm

Moderated 252Cf sources

1.5 m distance

Detector unbiased

Measurement time 3000 sec.

Total counts Channels 1-9 = 16k

14 counts/sec/cm2

1 2 3

3

54 6

7 8 9

Element 1

Element 4

Element 7

Measurements with

rack-mount electronics

16

9 channel custom circuit with discrete components

Custom design and layout with

discrete COTS components

Integrates 9 detectors with individual

readout chain

• Packaged individually

• Or, on interposer board

Readout chain includes:

• Charge sensitive preamp

• Shaping amplifier, 250 ns

shaping time

• Low level discriminator

• Driver circuit for TTL output

• LED display and driver circuits

• Can readout either electron or

hole signal

CSP Shaper LLD

Output to

LED display

Individually

packaged

detectors

17

Video: 3 detector channelsdetector size 2 mm x 2 mm

18


Example for single channel, 8 channel was implemented

Read Out Design: ASIC

May be a full detector

or a sub-area depending

on the size of the pixel

capacitance and leakage

To Counter/Multiplicity

Logic

Pillar

DetectorShaper

Leading Edge

Discriminator

Charge Integrating

Front-End

t0 t0n

LVDS

Logic Stage

Single ASIC Channel

ASIC

2.8 mm x 4.0 mm

8 Channels per ASIC

Mosis Foundry

20 ASICs received Dec. 2010

Differential signaling

19

Silicon Detector for HLC: High radiation tolerance possible

NIEL - nonionizing energy loss

IEL – ionizing energy loss

1013 5 1014 5 1015 5 1016

eq [cm-2]

5000

10000

15000

20000

25000

sig

nal

[el

ectr

on

s]

n-in-n (FZ), 285 m, 600V, 23 GeV p

p-in-n (FZ), 300 m, 500V, 23GeV p

p-in-n (FZ), 300 m, 500V, neutrons

p-in-n-FZ (500V)n-in-n FZ (600V)

M.Moll - 08/2008

References:

[1] p/n-FZ, 300 m, (-30oC, 25ns), strip [Casse 2008][2] n/n-FZ, 285 m, (-10oC, 40ns), pixel [Rohe et al. 2005]

FZ Silicon Strip and Pixel Sensors

strip sensorspixel sensors

*Silicon FZ Strip Detectors for High Luminosity Colliders

Radiation Damage to Sensors:

Bulk damage due to NIEL

Change of effective doping concentration

Increase of leakage current

Increase of charge carrier trapping

Surface damage due to IEL

(accumulation of positive charge in oxide

& interface charges)

Determine if Pillar Detector has elevated damage due to:

large surface area

composite material structure

20

High fluence measurements: gamma

15 cm

Pillar Detector

137Cs

Test setup Electrical measurements done before and after

irradiation

Time, 60ksec (16 hours)

Gamma flux: 2.64×109 photons/(cm2·sec)

Total gamma fluence delivered: 2.0×1014 photons/cm2

Electrical properties of I-V, C-V and transient voltage

decay remain same:

I-V remains the same

No trapping centers introduced

No damage in the depletion

region

Transient voltage decay remains the same

No change in effect lifetime: ~ 1.0 µs

No damage in bulk or pillar sidewalls

C-V remains the same

No change in voltage pulse for a

certain amount of induced charges

Neutron detection performance not

degraded

1.E-07

1.E-05

1.E-03

1.E-01

1.E+01

-10 -8 -6 -4 -2 0 2

J (A

/cm

2)

V (V)

before 60k sec irradiation

after 60k sec irradiation

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.E+00 5.E-05 1.E-04

V (V

)

Time (sec)

before irradiation

60k sec irradiation

1.E-11

1.E-10

1.E-09

-10 -5 0

Cap

acit

ance

(F)

V (V)

before 60k sec irradiation


21

High fluence measurements: fast neutron

Electrical measurements done before and after

irradiation

Time, 60ksec (16 hours)

Neutron flux: 3.2×103 n/(cm2·sec)

Total neutron fluence delivered: 1.9×108 n/cm2

Electrical properties of I-V, C-V and transient voltage

decay remain same

1.5 m

Pillar Detector

252Cf

Test setup

I-V remains the same

No trapping centers introduced

No damage in the depletion

region

Transient voltage decay remains the same

No change in effect lifetime: ~ 1.0 µs

No damage in bulk or pillar sidewalls

C-V remains the same

No change in voltage pulse for a

certain amount of induced charges

Neutron detection performance not

degraded

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

-10 -8 -6 -4 -2 0 2 4

J (A

/cm

2)

V (V)

before irradiation


0

0.1

0.2

0.3

0.4

0.5

0.6

0.E+00 5.E-05 1.E-04

V (V

)

Time (sec)

before irradiation


1.E-11

1.E-10

1.E-09

-10 -8 -6 -4 -2 0

Cap

acit

ance

(F)

V (V)

before irradiation


22


High fluence measurements summary

Thermal neutron counts monitored before & after irradiation testing

Thermal Neutron Counts Set-up

• Moderated 252Cf source 4x108 n/sec

• 30 cm diameter D2O covered with 1 mm Cd

•Time 60k sec (16 hours)

•0.5 micro-sec shaping time

• Detector bias = 0V

Gamma (#/cm2s)

109

Gamma and Neutron Radiation

Neutron (#/cm2s)

103

No reduction in n counts

23


3He detector can be used for efficiency

calibration

He-3 detector used:

• Cylindrical 3He from LND, Inc.

• Active volume

1.55 cm diameter, 2.39 cm long

• 4 Atm 3He, 850V

• Data sheet efficiency ~30% thermal

neutrons

Initial measurements performed

Efficiency calculations underway

Measurement of He-3 detector for comparison with Pillars

Conditions Counts

No Source 1393

Bare 252Cf 622413

Moderated* 252Cf 887504

2.0 m

3He Tube252Cf

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 50 100 150 200 250 300 350

LOG

(nu

mb

er o

f co

un

ts)

Channel number

*15cm D2O sphere

Background

Bare 252Cf

Moderated 252Cf

24


3He vs Pillar Detector

Detector 3He tube*

(LND-25177)

Pillar detector

(EPI26-1-1)

Active dimensions

(cm)

Diameter:1.55

Length:2.390.2 x 0.2

Operation voltage (V) 850 0

Total counts (1 hour) 886,111 3067

CPS/area

(#/cm2·s)66.44 21.30

*Gas pressure: 3040 torr

3He

Pillar detector

2m

2m 252Cf

D2O moderator

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120 140

Nu

mb

er o

f co

un

ts

Channel Number

0 DEG

45 DEG

90 DEG

180 DEG

Isotropic source

No angular dependence (as shown in measurements)

Non-Isotropic source

Little streaming 4.6 ° solid angle to deflect out Si

Isotropic source

Pillar Detector

25


OUO

OUO

Mapping Pillar Detector Over 3He Tube Form:

Efficiency Comparison

Configurations Pillar EfficiencyNumber of

detectors

Total counts

per second

Sensitivity

normalized to 3He

1Single side 22 % 7 x 11 (77) 65.45 0.27

Single side 44 % 7 x 11 (77) 130.9 0.53

2

Single side 22 % 12 x 11 x 2 (264) 224.4 0.91

Single side 44 %12 x 11 x 2 (264)

449 1.82

Comparison of 3He and pillar detector(246.14 cps for 3He)

L

D

Config. 1 Config. 2

D

LD=1.55 cm

L=2.39 cm

D=1.55 cm

L=2.39 cm

Pillar efficiency can increase by lining multiple layers within the form factor

26


Summary

Pillar based neutron detectors can be a 3He replacement

(small form factor applications)

Ideal material system is Si (electron/hole collection) and 10B

(neutron conversion)

High aspect ratio structures of 2 m x 2 m pillars, with a 4 m

pitch and scaled to 50 m can achieve 50 % thermal neutron

detection efficiency, currently at 20 %

8-channel ASIC read-out integration underway

Scaling for efficiency and area

Output from

ASIC

ASICDetector

Interface

Discriminator

Threshold Control

ASIC Power

27


Acknowledgement

DHS DNDO Program Manager:

» Dr. Alan Janos

LLNL Radiation Detector Materials PEL:

» Dr. Steve Payne

LLNL Technician Staff:

» Cathy Reinhardt

» Tim Graff

» Marianne Ammondolia

Si Based Pillar Structured Thermal Neutron Detectors · Lawrence Livermore National Laboratory...

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