electrical characterization of rollers & belts for high · pdf fileelectrical...

NIP 24 September 2008, Pittsburgh, PA 1

Electrical Characterization of

Rollers & Belts for High

Speed Electrophotography

Ming-Kai Tse and Inan Chen

Quality Engineering Associates (QEA), Inc. Contact information as of 2010:

755 Middlesex Turnpike, Unit 3

Billerica MA 01821 USA Tel.: 1+ 978-528-2034

Fax: 1+ 978-528-2033

www.QEA.com

NIP 24 September 2008, Pittsburgh, PA

Outline

• What characterization method –

measurement of dielectric relaxation

• Why – correlates with process

fundamentals

• Traditional resistance measurement

method – what’s wrong

• ECD method – what’s right

• Analysis & Experimental Results

2


Dielectric Relaxation of Rollers & Belts

is Critical to the Performance of

Several EP Processes

Characterization of dielectric relaxation is the key!

3

Transfer Belt or

Transfer Roller

Media

Charge Roller

Photoconductor

Image

Development

Roller


Why Dielectric Relaxation is Important?

4

Conductive

Elastomer

OPC Drum

Semi-insulating

Coating

Charging

Metering

Blade

Donor

Roll

PR

Development

Need to understand

the process physics Vb

Transfer

Media

PR

Toner

Air Gap

Roller, Belt

or Paper


QEA Research & Publications

5

Year Conference Subject

1999 JHC ECD method for semi-insulators

1999 NIP15 Modeling of electrostatic transfer

2000 NIP16 Transfer media

2001 NIP17 Corona charging current

2002 ICIS Charge mobility measurement

2004 NIP20 Transfer of color images

2005 JHC Semi-insulating devices

2005 NIP21 Roller charging of photoreceptor

2006 ICIS Media non-uniformity issues

2006 NIP22 Counter charge in development rollers

2008 PPIC Aging of donor rolls

2008 NIP24 Characterization for high speed EP


• Roll coating voltage, VR

• Air gap voltage, VA

• VA > Paschen threshold

• PR surface charged, QP

• Counter-charge QR on Roll

• Air-gap voltage, VA

• VA < Paschen threshold

• PR charging stops

• To continue charging,

QR must be neutralized

to VR and VA

• Dielectric relaxation in roll

coating important for high

charging efficiency

Vb

+ + + + + QR

– – – – – QP

Dielectric Relaxation in CR Charging

CR Coating, VR

Air Gap, VA

Photoreceptor, VP

A qualitative description:


Toner Charging in Single Component

Development

7

Counter-charge (+) injection from VB

VRC decays, dielectric relaxation of roller coating

Dev.

Roll

PR

VB

VB

MB

Dev.

Roll

PR

VBVB

VB

MB

Donor Roll = Conductive Core + Semi-ins. Overcoat

Toner Charging (–) at Metering Blade (M-B)

M-B

– – Toner – –

+ + + + + +

Roll Coating

VB


Toner Deposition in Single Component

Development

8

Toner Deposition on Photoreceptor (PR)

Counter-charge (–) injection from VB to Coating

Dielectric Relaxation of Roll Coating layer

Dev.

Roll

PR

VB

VB

MB

Dev.

Roll

PR

VBVB

VB

MBVB

Roller Coating

– + – + – + –

– – – Toner – –

+ + + + + + + PR

Counter-charge injection


Electrostatic Transfer of Developed Toner

Bias voltage applied to multiple layers

VB reverses field direction in toner layer

Receiving media (Paper, Belt),

(semi-insulator) much thicker than

other layers

Dielectric Relaxation in receiver

• Shifts most of VB to toner layer

• Enables efficient transfer

– without very high bias voltages

• occurs by charge injection

9

Charge injection

- - - - Toner - - -

Air-gap

PR

VB

Receiving Media

(100 mm)

+ + + + + +


Common Configuration & Implications

on Characterization Method

VB applied to insulator & semi-insulator

in series

Voltage across semi-insulator decays

with time Dielectric Relaxation

Low intrinsic charge density

Need charge injection

Performance of process closely related

to efficiency of dielectric relaxation,

charge injection and transport

Due to the complexity in the charge

transport processes in dielectric

relaxation – best characterize by a test

method that simulates actual device!

10

Semi-Insulator

VB Insulator (PR, Air)


Process Time

11


Conventional Roller & Belt

Characterization Method

• DC bias voltage is applied between an electrode in contact with

the charge roller and the roller shaft. The current flow through

the roller ISS is measured, typically at “steady state”.

• The roller resistance is the ratio of the applied VB to the

measured ISS.

12

A

VB

ISS

Electrode

Pressure

VB

Iss

R =

A

ISS

VB

A “Close-circuit” Method


Limitations of Conventional

Resistance Method • Most serious is that the underlying physics is not

consistent with the process physics:

– Ohmic relaxation model does not apply

– Semi-insulator to electrode contact is non-ohmic

– Test configuration does not simulate process configuration

- no way to duplicate charge transport physics crucial to

process performance (e.g., electric field dependence)

– Measurement time scale is wrong

• Practical issues:

– Contact pressure dependence

– No mapping capability

• Results do not reliably or consistently predict

performance

13


Ohmic vs Non-Ohmic Contact

“Ohmic” contacts:

Supply charge to maintain q = qi (intrinsic charge

density) in sample

Current density: J = sEo = mqiEo (Eo = applied field)

“Non-Ohmic” contacts:

Supply more or less charge (injection)

Charge density q(x, t) qi ; E(x, t) Eo ; J sE

Semi-insulating devices are typically Non-ohmic.

Injection at interface is key to the relaxation process.

Conductivity, s : not a good figure of merit!

14


An Example of the Role of Injection –

Photo-induced Discharge in Photoreceptor

In the dark: an insulator with high resistivity with long

dielectric relaxation time t Exposed to light: charges photo-generated in CGL; charge

injection into CTL Voltage decreases Photo-induced

Dielectric Relaxation

15

Light on off

V

+ + + + + + + + + + + + +

– – – – – – – – – – – –

CGL

CTL


“Open-Circuit” Method

is Preferred to Simulate Actual Device

16

Semi-Insulator


Semi-Insulator

VB

Semi-Insulator

HV

- - - - - -

Semi-insulator

in Open Circuit

Testing Semi-insulator

In Open Circuit


In Closed Circuit

Traditional Resistance

Measurement Preferred ECD Method Device Configuration


Charge Transport Model of

Dielectric Relaxation (1)

Samples characterized by:

• Charge densities: qp(x,t), qn (x,t)

• Charge mobility: mp, mn

Charge Continuity:

q(y, t)/t = – (mqE)/y

Boundary conditions:

• Injection current at y = 0

J(0, t) = sE(0, t), s = Injection strength

• Interface charge: QL = eIEI – eDED(L)

(Gauss’ theorem)

• Bias VB = VD + VI (constant in time)

Solved by Numerical iterations

17

Semi-Insulator



Voltage VD(t) across semi-

insulator (dielectric) depends

strongly on injection strength

1E-3

1E-2

1E-1

1E+0

0 50 100 150 200Time

Vo

ltag

e :

Die

lectr

ic

Equiv-ckt model

s = 0.01

0.03

1.0

0.3

0.1

Hybrid080213/1

Li = 0.5

ei = 0.5

0.0

0.1

0.2

0.3

0.4

0.5

0.1 1 10 100 1000

Time

VD(0

) -

VD(t

)

s = 1.0

0.01

0.03

0.1

0.3

Hybrid080213/1

Li = 0.5

ei = 0.5

Voltage decay: VD(0) – VD(t):

significant effect of s in time

t 10 to 100 to, (to= L2/mVB)

Charge Transport Model of

Dielectric Relaxation (2)


Photoreceptor surface voltage increases with time

Significant effect of charge injection strength in time t 100 to

0

0.2

0.4

0.6

0.8

1

1 10 100 1000

Time

PR

Su

rface V

olt

ag

e

s = 10.3

0.01

0.03

0.1

ChgR080214/2Vdc = 1.0

qi = 0.1

Conductive

Elastomer

OPC Drum

Semi-insulating

Coating

Roller Charging of Photoreceptor


0

1

2

3

4

5

1 10 100 1000

Time

To

ner

Ch

arg

e D

en

sit

y Vb = 0.2

qi = 0.1

up= un= 1

Lr = 1

Lt = 0.3

e r = 1

e t = 1

SCDC060107/1

s = 1

0.3

0.10.03

0.01

20

Toner Charging (–) at Metering Blade (MB)

Counter-charge (+) injection into Roll Coating

Dependence on injection strength s in t 100 to

Toner Charging in

Single Component Development

Metering

Blade

Donor

Roll

PR


Toner deposition (–) on photoreceptor PR

Counter-charge (-) injection into Roll Coating

Dependence on injection strength s in t 100 to

Toner Deposition in

Single Component Development

Metering

Blade

Donor

Roll

PR 0.5

0.6

0.7

0.8

0.9

1.0

0.1 1 10 100 1000

Time

Dep

osit

ion

Eff

icie

ncy

Vd = -1

qt = 3

qi = 0.1

u =1

s = 1

0.3

0.01

0.03

0.1

SCD060104/2


0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.1 1 10 100 1000

Time

Eff

icie

ncy

Transf81808/2

s = 1.0

0.3

0.1

0.03

0.01

qi=0.1

Qt =-4

Va=1

22

Dielectric Relaxation in receiver

• Enables efficient transfer without very high VB

• increase transfer efficiency; depends significantly

on injection strength s

Electrostatic Transfer of

Developed Toner

Vb

PR

Toner

Air Gap

Roller, Belt

or Paper


Close resemblance in analytical results for voltage decay and

processes provides strong support for the critical role of

dielectric relaxation on EP performance

Significant effects of s in t = 10~100 to, i.e., transient behavior

0.0

0.1

0.2

0.3

0.4

0.5

0.1 1 10 100 1000

Time

VD(0

) -

VD(t

)

s = 1.0

0.01

0.03

0.1

0.3

Hybrid080213/1

Semi-Ins

Insulator

0.5

0.6

0.7

0.8

0.9

1.0

0.1 1 10 100 1000

Time

Dep

osit

ion

Eff

icie

ncy

s = 1

0.3

0.01

0.03

0.1

SCD060104/2

Transient in Dielectric Relaxation is

Most Critical to Performance


• Roller or Belt Relaxation Time:

Charge transit time to = L2/mVB 5 ms; with: L 50 mm, m

10-5 cm2/V-s, VB 500 volts; full relaxation time is tP >

100to 0.5 sec or 500 ms.

Relaxation Time vs Process Time

• Therefore must

consider

transient

behavior in

characterization;

particularly for

high speed

printing.


Implementation – the ECD Method

25

Semi-Insulator

HV

- - - - - -


In Open Circuit


Charge Roller Mapping

• The full-body ECD map shown below for a poor charge roller clearly

demonstrates the correlation between VECD and print quality.

• The non-uniformity in VECD can be mapped directly to a print density

variation map (on a gray page) and a background map (on a white page).

Such results clearly demonstrate the efficacy of the ECD method.

* Charge roller circumference

279.4mm (11”)

37.7mm*

ECD Full-body Map

Print

60 100 140 180 volt


Good Roller

50% Gray Page

ECD Re Map

* The ECD Re map is scaled to physical size.


Bad Roller

50% Gray Page

ECD Re Map

* The ECD Re map is scaled to physical size.


Dot Gain, Optical Density and Tone Reproduction

0.2

0.3

0.4

0.5

0.6

0.7

300 400 500 600 700

Equivalent Resistance, Re (MW-cm2)

Op

ical

Den

sit

y (

OD

)

PD=5

PD=1

Dot Gain, Optical Density & Tone

Reproduction


Background

0

2

4

6

8

10

12

300 400 500 600 700


Back

gro

un

d G

S

PD = 5

PD = 1

Background

where i = 1 to N

a

d

GS i

i-

46 )(1074.4

and a is the ROI area


Ghosting

0.02

0.04

0.06

0.08

0.10

300 400 500 600 700


DO

D

PD = 1

PD = 5

Ghosting


Application Example – Material

Formulation

Use of ECD-DRA in Roller Materials Development

0.1

1

10

100

0 0.4 0.8 1.2 1.6 2

Time (sec)

Ro

lle

r E

CD

Vo

ltag

e (

vo

lt) Old Formulation

New Formulation A

New Formulation B


New

Application Example – Roller Aging

6

14

10

18

V

Used


Development Roller

A cut in the roller


ITB – Failure Analysis

35

0

200

400

600

800

1000

0 2 4 6 8 10

Time (sec)

Vo

lta

ge

(V

) Bad-new

Good-used

Good-new

Bad-used

First data points V(0):

Low for good,

high for bad

same for new and used

intrinsic charge

Voltage decay V(t):

Fast for new

Slow for used

injected charge


ITB - Benchmarking

36


Summary

Performance of EP sub-processes using

rollers and belts is controlled by

dielectric relaxation (DR) of semi-

insulating (SI) layer

Dielectric relaxation induced by charge

injection from bias voltage

Full relaxation of SI often requires time

longer than available in high speed

Electrophotography

37


Conclusions

Electrical characterization of rollers and

belts should emphasize transient values

& spatial variations in DR

Observations of spatial averages, at fully

relaxed states are insufficient

Open-circuit voltage measurements,

efficiently scanning large area of sample

– an extremely valuable tool for R&D, QC

and failure analysis

38


Ming-Kai Tse and Inan Chen Quality Engineering Associates (QEA), Inc.

Burlington, MA USA

Please visit: www.qea.com

See you in the Exhibit Hall

39

Thank you for your attention

http://www.qea.com/

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