simulation of new p-type strip detectors with trench to enhance...

Simulation of new P-Type strip detectors RESMDD’10, Florence – 12-15.October.2010 1/15

Centro Nacional de Microelectrónica Instituto de Microelectrónica de Barcelona

Simulation of new P-type strip detectors with trench to enhance the charge multiplication effect in the n-

type electrodes

Simulation of new P-type strip detectors with trench to enhance the charge multiplication effect in the n-

type electrodes

P. Fernández-Martínez, G. Pellegrini, J. P. Balbuena, D. Quirion, S. Hidalgo, D. Flores, M. Lozano

Centro Nacional de Microelectrónica (IMB-CNM-CSIC)

G. CasseLiverpool University

Work partially supported by RD50 collaboration



Introduction

a. Irradiated detectors suffer displacement damage that change their charge

collection dynamics

• Defects induced by radiation increase the electric field at the junction

of the N+ diffusion

• The electric field increase leads to a multiplication of the collected

charge in irradiated devices

b. Project: fabricate a p-type strip detector with small gain �� Similar signal

before and after irradiation

• Multiplication occurs at low bias voltage

• Gain should be limited between 2 and 10:

- Avoid Crosstalk

- Avoid exceeding the dynamic range of readout electronics

• Capacitance should not increase

- Higher capacitance �� Higher noise



Technological proposals

I. Trench filled with doped polysilicon along the centre of the strip pitch

− A N+ contact is created into the silicon bulk that modifies the

electric field in the collection region �� multiplication

285µm

80µm

n+

p-

p+

32µm

5µm

5µm

20µm

8µm

p-stop

Poly trench



Technological proposals

II. P-type diffusion along the centre of the strip pitch

− Under reverse bias conditions, a high electric field region is created

at the N+– P junction �� multiplication

p-

p+

285µm

32µm

80µm

20µm

8µm

n+

5µmp-stop

P-type diffusion



Simulation of the Electric Field

Strip Detector Poly Trench

P diffusionHigh Electric Field

region driven deep in the bulk

High Electric Field peak at the centre of the strip



0 10 20 30 40 50

102

103

104

105

Ele

ctr

ic F

ield

(V

/cm

)

Depth (µµµµm)

No Irradiated

ΦΦΦΦeq

= 1 x 1016

n/cm2

Strip Detector: Section @ Strip Center

High Electric Field peak at the junction

Simulation of the Electric Field

0 10 20 30 40 50

102

103

104

105

Ele

ctr

ic F

ield

(V

/cm

)

Depth (µµµµm)

Strip

Poly Trench

P Diffusion

Section @ Strip Center

High Electric Field region driven deep in

the bulk

Achieved Electric Field values are comparable with the irradiated devices



Simulation of charge collection: MIP

A/cm2

Strip Detector Poly Trench P Diffusion

� We are developing a simulation procedure to obtain the gain value

� We are developing a simulation procedure to obtain the gain value



Simulation of the Capacitance

p-

p+

n+

CBulk

CInterStrip

0 50 100 150

4

8

12

16

Bu

lk C

ap

ac

ita

nc

e (

pF

)

Applied Voltage (V)

Strip

Poly Trench

P Diffusion

0 50 100 1500

20

40

60

80

100

CIn

ter_

Str

ip (

pF

)

Applied Voltage (V)

Strip

Poly Trench

P Diffusion

Once Fully Depleted, the bulk capacitance is the same for all the

structures

Increment related with the depletion of the P

diffusion

� We need experimental results

to extract any conclusion

� We need experimental results

to extract any conclusion



Variation over the basic proposals

I. P – type diffusion, implanted through a trench filled with oxide, along the

centre of the strip pitch

− N+/ P-type diffusion junction creates a high electric field region ��

multiplication

n+

p-

p+

285µm

32µm

80µm

5µm

5µm

20µm

8µm

p-stop

Oxide trench

P-type diffusion




• The deeper the trench, the

larger the volume of

multiplication

- We are considering trenches

of different depth

Electric FieldCharge Collection

0 50 100 1500

20

40

60

80

100

CB

ulk (

pF

)

Anode Voltage (V)

Strip

Oxide Trench




II. Large P-Type covering all the strip width


multiplication

p-

p+

285µm

32µm

80µm

20µm

8µm

n+

30µmp-stop

P-type diffusion




DRAWBACK: If N+ and P diffusions are performed with

the same mask, a premature breakdown is expected

due to the curvature of the junction

- N+ diffusion should overlap P diffusion

�� One extra level of Mask

�� Optimisation of the overlap size

p-

p+

n+

Premature Cylindrical Breakdown



Status of the Work: Fabrication

The fabrication run includes:

•Conventional Strip Detectors

•Poly Trench structures with

different trench depths:

− 5 µm− 10 µm− 50 µm

•Structures with small P layer

along the center of the strip

•Oxide filled trench structures

with a P layer implanted through

the trench:

− 5 µm− 10 µm− 50 µm

●Devices with large P layer along

the center of the strip



Status of the Work: PAD Diodes

We have fabricated PAD diodes with a P layer diffused under the N+ diffusion


multiplication

285 µµµµm

N+

P

High Electric Field region leading to multiplication

5000 µm

5000 µm

� First Measurements: Gain ~2

� First Measurements: Gain ~2



Conclusions and Applications

11 We have presented several new designs to enhance the

multiplication process.

22 Presence of trenches or diffused P layers in the strips can

provide small gain values.

33 Capacitance value can be deteriorated. It will be better

established with the experimental results.

44 A run containing the discussed designs is being fabricated for

subsequent characterisation.

55 A PAD detector with small gain has been fabricated following

some of the procedures described in this work.

6 Applications:

- Radiation Hard Detectors

- Tracking Detectors- Charge multiplication permits the fabrication of thinner detectors

simulation of new p-type strip detectors with trench to enhance...

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