Projected Capacitive Touch ITO Layout evolution

May 2012

© 2012 PIXCIR 2

Self and Mutual capacitance concepts

Requirements for proper multi-touch functionality

Introduction

© 2012 PIXCIR 3

Self capacitance

Self capacitance is

modified by the

direct coupling

between a finger

and one electrode

Finger

air

glass

© 2012 PIXCIR 4

Mutual capacitance

Mutual

capacitance is the

coupling between

two electrodes. Finger

Note the presence of

the finger reduces the

mutual coupling

air

glass

© 2012 PIXCIR 5

The return path

The return path is the capacitance closing the circuit between the device and the user.

The return path depends on: the casing of the device

the size of the device

the connection to AC supply or battery

Position and holding of the user

Return path

© 2012 PIXCIR 6

6

Ghost stories

A good “return path” and low “self capacitance” are needed to avoid crosstalk between fingers

self

capacitance

self

capacitance

© 2012 PIXCIR 7

The importance of selecting an appropriate layout

PIXCIR recommendations based on

3D simulations

Measurements

Case studies

How to improve the multi-touch performance

© 2012 PIXCIR 8

How to read charts

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0.0

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0.4

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

M1 Signal, er = 2.3

10pF return (floating)

200pF return (grounded)

Signal variation range with return path

Mutual capacitance signal,

in Farads, versus cover

lens thickness.

This signal car vary

strongly with the handling

and AC connection of the

portable device, ranging

between grounded and

floating cases.

Return

path

Signal (pF)

Cover thickness (mm)

© 2012 PIXCIR 9

How to read charts: M1 inversion

Recognize operation

where mutual

capacitance signal is

near inversion or

already inverted.

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0.0

0.1

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0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

M1 Signal, er = 2.3

Multitouch is

impossible

Cover thickness (mm)

Signal (pF)

© 2012 PIXCIR 10

How to read charts: comparing multiple layouts

Comparing different

layout styles at once

with transparent

overlap

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0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

M1 Signal, er = 2.3

Cover thickness (mm)

Signal (pF)

© 2012 PIXCIR 11

The SITO case

Layout candidates and comparison

© 2012 PIXCIR 12

Some of the tested layouts

Electrode pitch: 6mm; finger : 8mm

Diamonds (D) Hollow (H) Empty (E)

Radiator (R) Islands (I) Matrix (M)

© 2012 PIXCIR 13

-1.5

-1

-0.5

0

0.5

1

1.5

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Tan

go M

eth

od

1 s

ign

al (

pF)

Cover thickness (mm)

Signal M1, glass cover

Mutual capacitance signal strength

Multitouch is

impossible

© 2012 PIXCIR 14

Merit comparison

Layout Name Linearity

( : stylus)

Floating (Portable device)

SNR Thin

cover < 0.5mm

Thick cover

> 1.1 mm

Large screen > 12"

Radiator

Hollow

Matrix

Diamond

© 2012 PIXCIR 15

Radiator type

Layout Name Linearity

( : stylus)

Floating (Portable device)

SNR Thin

cover < 0.5mm

Thick cover

> 1.1 mm

Large screen > 12"

Radiator

© 2012 PIXCIR 16

Radiator layout

Radiator fins

The actual number of fins must

be adjusted according to the

rules of next page. This drawing

is only an example.

© 2012 PIXCIR 17

Pitch < Cover Pitch = Cover Pitch > Cover

Mutual signal

Good Good Weak

ITO loading High Good Good

Radiator layout: optimize the fin pitch

© 2012 PIXCIR 18

Matrix type

Layout Name Linearity

( : stylus)

Floating (Portable device)

SNR Thin

cover < 0.5mm

Thick cover

> 1.1 mm

Large screen > 12"

Matrix

© 2012 PIXCIR 19

Matrix layout

Item Value Min / Max

Extension 1.5 mm Min

Pitch1) 8.0 mm Max

5.0 mm Typical

Gap2) Cover Max

Width3) 3 mm Max

1.5 mm Typical

Square4) 2 mm Max

Cbridge5) 0.5 pF Max

1) Caution: finger separation > 2.5 x pitch 2) Gap should consider optical result 3) Width should consider ohmic resistance 4) Please adapt number of squares to pitch 5) Cbridge accounts exclusively for the crossing

node plate to plate capacitance, and does not

apply for the fringing.

© 2012 PIXCIR 20

Matrix: other layout examples

© 2012 PIXCIR 21

Matrix type: linearity and border accuracy

x1 x2 x3 x4 x1 x2 x3 x4

y1

y2

y3

y1

y2

y3

Region with

reduced linearity

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Linearity and border accuracy (2)

x1 x2 x3 x4 x1 x2 x3 x4

y1

y2

y3

y1

y2

y3

Keep 1.5 mm minimum

© 2012 PIXCIR 23

Pixcir Pen -- linearity test using Matrix layout

mm

mm

Wireless and Tethered versions

Palm rejection Yes

Pen tip size 1 mm

Pen hover height (Z) 10 mm

Resolution 400 dpi

Report rate 200 Hz

Linearity +/- 0.4 mm

© 2012 PIXCIR 24

Hollow type

Layout Name Linearity

( : stylus)

Floating (Portable device)

SNR Thin

cover < 0.5mm

Thick cover

> 1.1 mm

Large screen > 12"

Hollow

© 2012 PIXCIR 25

Hollow optimum ring width

Hollow layout offers, for a small mask modification, a valuable improvement over diamond style.

Ring width must match the cover lens thickness.

The ITO to ITO gap should match the process rule, and should follow the optical requirement (typically 40 to 100um)

width

© 2012 PIXCIR 26

Hollow: effect of the ring width

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-1

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0

0.2

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0.6

0.8

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Ta

ngo

Me

tho

d 1

sig

na

l (p

F)

Cover thickness (mm)

Signal M1, glass cover

Ring: 0.4 mm

Ring: 0.7 mm

© 2012 PIXCIR 27

The DITO case

Layout candidates and comparison

© 2012 PIXCIR 28

DITO case study (low cost version / large pitch)

Simulated DITO

Finger size: 8 mm diameter

DITO glass thickness: 0.5 mm

X pitch 7.8 mm

Y pitch 8.0 mm

Cover lens: glass

7” – 28 electrodes

© 2012 PIXCIR 29

DITO simulation result : Mutual capacitance

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-1

-0.8

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0

0.2

0.4

0.6

0.8

0.0 0.5 1.0 1.5 2.0

pF

Cover thickness (mm)

Method 1 signal, glass cover

Diamonds 6mm

DITO top

DITO bottom

© 2012 PIXCIR 30

Further optimization with floating elements

© 2012 PIXCIR 31

Umbrella effect of DITO with floating elements

Top layer with empty spaces Top layer filled with dummy elements

A water drop modifies the fringing fields

between Lift and Sense, causing offsets

A water drop cannot directly modify

the fringing fields, causing less offsets

(simplified explanation)

© 2012 PIXCIR 32

DITO, effect of floating elements

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0

0.1

0.2

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0.4

0.5

0.6

0.7

0.0 0.2 0.4 0.6 0.8 1.0 1.2

pF

Cover thickness (mm)

Method 1 signal, glass cover, misc. return path

None

Top

Floating element:

200pF

7pF

3pF

1pF

Top layer must have floating elements

© 2012 PIXCIR 33

Electrodes pitch

© 2012 PIXCIR 34

All layouts : electrode pitch

For all layouts

Electrode pitch influence FINGER SEPARATION

RULE:

separation > 2.5 electrode pitch

Electrodes pitch

(total electrodes count)

RECOMMENDATION: 4mm < electrode pitch < 6mm

Pitch

© 2012 PIXCIR 35

Summary

The floating issues are related to a ratio of capacitances The Mutual capacitance

The Self capacitance

The Return path capacitance depending on

conductive area of device

the “connection” to the body

The floating does not depend on Driving voltage

Sensing technology

Optimization of multi-touch performance can be achieved through Electrodes layout selection

Cover lens thickness

Casing design

qdc

www.pixcir.com info@pixcir.com

Thank You for your attention

PIXCIR Microelectronics Co., Ltd.

3F, Building M1, No.2 Peiyuan Road,

Microsystem Park

Suzhou Science & Technology Town PIXCIR AG - Switzerland

Siewerdtstrasse 105 CH-8050 Zurich

Rue Galilée 15 CH-1400 Yverdon

PIXCIR - Taiwan

17F-7, No. 75, Xin-Tai 5 Road, Sec 1, Xizhi,

New Taipei City