How Small Should Pixel Size Be?

Ting Chen1, Peter B. Catrysse1, Abbas El Gamal1 and Brian A. Wandell2

1Information Systems Laboratory

Department of Electrical Engineering, Stanford University, CA 94305, USA

2Department of Psychology, Stanford University, CA 94305, USA

EI 2000, 3965-53 1

Pixel Design

• Pixel design is crucial in image sensor design

– Selection of pixel architecture

– Selection of photodetector type

– Selection of pixel layout and size

• Pixel size tradeoff

– Small pixel size/large pixel count

improves spatial resolution and MTF

– Large pixel size improves DR and SNR

=⇒ an optimal pixel size may exist

EI 2000, 3965-53 2

Pixel Size Selection

• Difficult to determine the optimal pixel size

analytically, depends on

– Sensor parameters

– Imaging optics

– Human perception of image quality

• We describe a methodology using

– Camera simulator (Catrysse et al. 1999)

– Image quality metric S-CIELAB (Zhang et al.

1997)

EI 2000, 3965-53 3

Outline

• APS pixel circuit and layout

• Pixel size tradeoff

– DR and SNR

– Spatial Resolution and System MTF

• Methodology for determining optimal pixel size

• Simulation results

• Conclusion

EI 2000, 3965-53 4

APS Pixel Circuit and Layout Topology

vdd

iph + idc

Cpd

Co

M1

M2

M3

M4

Reset

Word

Bias

IN

Bitline OUTColumn and Chip

Level Circuits

EI 2000, 3965-53 5

DR and SNR versus Pixel Size

5 6 7 8 9 10 11 12 13 14 1535

40

45

50

55

60

65

70

DR

SNR @ 20% Well Capacity

Pixel size (µm)

DR

and

SN

R (

dB)

EI 2000, 3965-53 6

Spatial Resolution and System MTF versus Pixel Size

0 0.2 0.4 0.6 0.8 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

6µm8µm

10µm12µm

Normalized spatial frequency

MT

F

EI 2000, 3965-53 7

Methodology for Determining Optimal Pixel Size

• Given :

– Process information

– Fixed die size

– Imaging optics parameters

– Scene luminance range

– Highest spatial frequency in the scene

– Maximum exposure time

EI 2000, 3965-53 8

Procedure for Determining Optimal Pixel Size

• Vary pixel size (pixel count) for the given die size

• For each pixel size

– Use the camera simulator with a synthetic

Contrast Sensitivity Function (CSF) scene to

estimate the resulting image

– Determine rendered image quality in

S-CIELAB

• Select optimal pixel size to achieve highest image

quality

EI 2000, 3965-53 9

Synthetic Contrast Sensitivity Function Scene

Spatial frequency (lp/mm)

Con

tras

t

5 10 15 20 25 30

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

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Simulation Flow

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Example : 0.35µ CMOS Process

pixel size : 8µm

Lscene : 25 - 1000 cd/m2

tmax : 100 ms

fmax : 33 lp/mm

optics f/# : 1.2

5 10 15 20 25 30

0.2

0.4

0.6

0.8

1

5 10 15 20 25 30

0.2

0.4

0.6

0.8

1

5 10 15 20 25 30

0.2

0.4

0.6

0.8

1

5 10 15 20 25 30

0.2

0.4

0.6

0.8

1

Spatial frequency (lp/mm)Spatial frequency (lp/mm)

Spatial frequency (lp/mm)Spatial frequency (lp/mm)

Contr

ast

Contr

ast

Contr

ast

Contr

ast

Perfect Image Camera Output Image

∆E Error Map Iso-∆E Curve

1

2

3

5

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Determining the Optimal Pixel Size

• 0.35µ process

• fixed die size

• variable pixel size : 5.3 - 15 µm

• scene luminance range : 25 - 1000 cd/m2

• maximum integration time : 100 ms

• highest spatial frequency in the scene : 33 lp/mm

• examine iso-∆E curves for different pixel sizes

• imaging optics f/# : 1.2

EI 2000, 3965-53 13

Iso-∆E = 3 curves for different pixel sizes

5 10 15 20 25 30

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

5.3µm

6µm

8µm

15µm

Spatial frequency (lp/mm)

Contr

ast

EI 2000, 3965-53 14

Average ∆E versus pixel size

5 6 7 8 9 10 11 12 13 14 151.2

1.4

1.6

1.8

2

2.2

2.4

2.6

Pixel size (µm)

Ave

rage

∆E

EI 2000, 3965-53 15

Effect of Technology Scaling on Pixel Size

2 3 4 5 6 7 8 9 10 11 12 13 14 151

1.5

2

2.5

3

3.5

4

4.5

5

5.5

0.35µm

0.25µm

0.18µm

Pixel size (µm)

Ave

rage

∆E

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Optimal pixel size versus technology

0.1 0.15 0.2 0.25 0.3 0.35 0.40

1

2

3

4

5

6

7

8

Linear Scaling

Simulated

Technology (µm)

Opt

imal

pix

el s

ize

(µm

)

EI 2000, 3965-53 17

Conclusion

• Proposed a methodology using a camera simulator,

synthetic CSF scenes, and S-CIELAB for selecting

the optimal pixel size

• Applied the methodology to photodiode APS

implemented in CMOS technologies down to 0.18µ

• For a 0.35µ process, the optimal pixel size is found

to be around 6.5µm with fill factor 30%

• The optimal pixel size scales with technology,

albeit at slower rate than the technology

EI 2000, 3965-53 18