Presented at The Fourteenth Meeting of the Symposiu m on Polymers for Microelectronics Wilmington, Delaware USA

May 11-13, 2010

Novel OrganosiloxanePolymers with Improved

Device Properties

Edward W. Rutter, Jr., Ahila Krishnamoorthy and Nancy Iwamoto

2

Problem statement

• Liquid crystal displays are made by bonding two substrates: thin film transistor substrate on glass and color filter (CF) on glass with liquid crystal in between.

• Both substrates require planarization layers to dictate uniform liquid crystal thickness.

• Indium tin oxide (ITO) layer is required to electrically orient liquid crystal.

• Planarization layer may contact the LC on the blanket film (TFT side) and influence the electrical behavior

• The bottom planarization layer should meet voltage holding ratio (VHR) requirement (no change in voltage with applied field) in addition to leveling topography

• Planarization of CF is more demanding than for TFT substrates. Requirements for CF planarization are > 90% DOP

• Unmet need: Defining polymers that provide planarization and effective VHR

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Organosiloxane Chemistry: Polymer Formation

• Hydrolysis + condensation reactions: - -OEt groups hydrolyze and form silanol - Silanol groups then condense to form Si-O-Si linkages

• Residual silanol → polarization / enhanced moisture absorption ⇒ poor electrical properties

• Chain extension and crosslinking: - Additives are optionally used as catalysts for the condensation

reaction- Extensive crosslinking at 230 to 300°C (crosslinkin g

temperature is lower in the presence of additives)

RSi(OEt) 3 + H2O RSi-OH + EtOH Hydrolysis

RSi-OH + RSi-OEt RSi-O-Si + EtOH Condensation

CondensationRSi-OH + RSi-OH RSi-O-Si + H 2O

H+

H+

H+

4

General behavior of Organosiloxanes (PTS-E)

-0.02

0.00

0.02

0.04

0.06

0.08

500 1000 1500 2000 2500 3000 3500 4000

0.00

0.02

0.04

0.06

0.08

500 1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm -1)

Silanol peak

100-Ad - 425 °°°°C

0.00

0.02

0.04

0.06

0.08

500 1000 1500 2000 2500 3000 3500 4000

0.00

0.02

0.04

0.06

0.08

Abs

orba

nce

500 1000 1500 2000 2500 3000 3500 4000

Silanol peak

0-Ad - 200 °°°°C

0-Ad - 425 °°°°C

100-Ad - 200 °°°°C

-0.02

0.00

0.02

0.04

0.06

0.08

500 1000 1500 2000 2500 3000 3500 4000

0.00

0.02

0.04

0.06

0.08

500 1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm -1)

Silanol peak

100-Ad - 425 °°°°C

0.00

0.02

0.04

0.06

0.08

500 1000 1500 2000 2500 3000 3500 4000

0.00

0.02

0.04

0.06

0.08

Abs

orba

nce

500 1000 1500 2000 2500 3000 3500 4000

Silanol peak

0-Ad - 200 °°°°C

0-Ad - 425 °°°°C

100-Ad - 200 °°°°C

1.E-11

1.E-09

1.E-07

1.E-05

1.E-03

-4 -3 -2 -1 0Electric field, MV/cm

Cu

rre

nt d

en

sity

, A/c

m2

425°C, 100-Ad200°C, 100-Ad425°C, 0-Ad200°C, 0-Ad

Si/AO-C

• Film formation occurs through crosslinking of silanol groups (monitor by IR)

• Silanol concentration in film is low when high cure temperature (425°C) and crosslinking additives (100 ppm) are used to form films.

• Films cured at low temperature (200°C) show higher weight loss (TGA) due to unreacted silanol functionality.

• Films with high silanol content exhibit higher leakage current (Hg probe).

Cured for1hr at 425°CWt loss from 25 to 450C is 0.88%

Cured for 1hr at 200°CWeight loss from 25 to 450C is 2.78%

96

97

98

99

100

Wei

ght (

%)

0 100 200 300 400 500

Temperature (°C)

Cured for1hr at 425°CWt loss from 25 to 450C is 0.88%

Cured for 1hr at 200°CWeight loss from 25 to 450C is 2.78%

96

97

98

99

100

Wei

ght (

%)

0 100 200 300 400 500

Temperature (°C)

Si wafer

Cured Siloxane film

Hg

Si wafer

Cured Siloxane film

Hg

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Planarizing Thermally Stable- X (PTS-N) • Derived from PTS-R (introduced at SID 2009) in conjunction with a polyfunctional

crosslinker and catalyst• Crosslinker reacts with silanols on base polymer at low temperature (140°C)

producing solvent resistant films• Polymer reflow occurs prior to crosslinking or cure resulting in excellent

planarization – two stage bakes can be used to further enhance planarization• Crosslinking produces volatiles during PAB resulting in low shrinkage during

subsequent cure (11% bake to cure (230°C/40 min))• Silanols are acidic, polar functional groups, not all are equally reactive, thought to

interact with LC materials causing a decrease in VHR• The low processing temperatures are enabling for temperature sensitive device

processing (OTFT, Flexible display, Plastic substrates)

Ladder Examples Cage Example

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Material Properties

Properties PTS-N PTS-R

Thermal Budget Cures at or above 140°C Cures above 300 °C

Film Thickness Single coat → up to 3.5µm Single coat → up to 3.5µm

Transmittance 98% (400 – 700nm) similar to glass 98% (400 – 700nm) – similar to glass

Outgas 0.02% (1hr hold at 200°C) 0.02% (1hr hold at 2 00°C)

Dielectric Constant 3.1 3.1

Field to breakdown 2.1 MV/cm (at 1 µA) 4.2 MV/cm (at 1 µA)

Leakage at 2 MV/cm 8x10-6 A/cm2 2 to 3 x10-8 A/cm2

Chemical ResistanceResistant to PR strippers (PRS 2000/ 70°C/ 3 min)

Resistant to HF and other stripping chemistries

Adhesion (Al, Cr, Mo, SiN, organics) Focused on improving Passes 3M scotch tape test

Film quality No defects. Good No defects. Good.

O2 Plasma resistance Less resistant. Resistant. Surface becomes hydrophilic

ApplicationCF planarization and TFT passivation, thermally sensitive processes and substrates

Color filter planarization and TFT passivation

• PTS-R demonstrates the best balance of properties of the PTS series• PTS-N retains the planarization properties of PTS-R while adding low

temperature processing capability• Blanket film electrical properties of PTS-N appear worse than PTS-R

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Low Temperature Crosslinking

• PTS-N completely (97%) crosslinked at 140°C, 160°C lower than the temperature required for the starting PTS-R (300°C) polymer

• Single stage hotplate bakes are used for the PAB• Solvent resistance tested using a puddle of PGMEA (to mimic exposure to PR)• Lower temperatures may be preferred, but will require longer bake times

0%

20%

40%

60%

80%

100%

100 150 200 250 300

Bake Temperature (°C)

% F

ilm R

eten

tion

PTS-N60 s PAB

PTS-R120 s PAB

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Optical Transmittance – PTS-N

0

20

40

60

80

100

190 390 590 790 990 1190

Wavelength

%T

rans

mitt

ance

After 230°C 40 min cure

After 140°C 60 s bake

Glass reference

60

80

100

380 480 580 680 780

Wavelength

%T

rans

mitt

ance

After 230°C 40 min cure

After 140°C 60 s bake

Glass reference

• Optical transmittance after bake is 100.7% relative to the glass substrate• Optical transmittance after cure is 98.2% relative to the glass substrates• These materials with high transmittance across the entire visible spectrum

are suitable for the display industry

PTS-N After PAB and cure vs. glassPTS-N After PAB and cure vs. glass

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∆t (µm) PTS-R PTS-N

Before OC 0.81

After OC 0.07 0.074

% DOP 91.4 90.9

∆t (µm) PTS-R PTS-N

Before OC 1.67

After OC 0.08 0.14

% DOP 95.2 91.6

Color Filter Planarization

• Film thickness of PTS-N is 1.9 µm and that of PTS-R is 2.2 µm • PTS-N provides outstanding planarization w/o requiring a large overburden• CF planarization is more challenging than TFT planarization

1.9 µm 2.2 µm

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Completely condensed silicate

• Large bandgap (~5.7eV) • Smaller bandgap (~4.4eV)

Effect of Silanol Content on Bandgap

Partially condensed silicate

• Silanols are well known to impart current leakage in films• Bandgap increases as level of silanol decreases in the film

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Blanket Electrical Properties – PTS-N vs. PTS-R

• Bandgap for materials is from molecular modeling studies (DFT Quantum Mechanics)

• Experimentally, PTS-N exhibits lower FBD and higher current leakage relative to PTS-R indicating that it is a poorer dielectric material

• Reduction in the bandgap is consistent with PTS-N being more leaky

Film Gap (eV)(modeled)

Field to breakdown at 1 µA/cm 2

Current density at 2.0 MV/cm (A/cm 2)

Silicate 5.7 3.51-3.57 6.6-7.16E-8

Silicate w/Si-OH 4.4 3.04-3.08 5.14-6.12E-8

PTS-T 3.8 2.62-3.27 8.18-9.52E-7

PTS-E 4.3 4.41-4.42 2.16-2.23E-8

PTS-R 4.03-4.08 2.48-2.90 1.45-3.75E-7

PTS-N 3.71-3.78 1.75-1.76 2.63-3.13E-6

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• VHR test measures how the applied voltage is held by the liquid crystal

when it is in contact with the dielectric.

• Bottom glass substrate has ITO (1000 Å) under 2.5 µm PTS films. A 1 cm

edge exclusion is used permitting external ITO contact.

• Spacer is sprayed on the surface to generate a 4 µm gap between top

and bottom ITO electrodes – to permit LC filling.

• Top glass substrate is patterned ITO on glass

• A seal layer is applied (around the patterned ITO region) leaving a small

gap to fill with LC.

• LC is filled under vacuum requiring almost an hour to fill. LC thickness is

approximately 4 µm (determined by spacer)

VHR Test Cell Fabrication

Glass ITO

ITO

PTS-N

Glass

VacuumLC

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Sealant Sealant

Top ITO

Bottom ITO

Gap to fill LCGap to vacuum

Gap to fill LC

Gap to vacuum

VHR Test CellTop Down ViewBottom Up View

Measure VHR

Time at65%RH/65°C

Measure VHR

Iterate

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PTS-NPTS-R

• A peak pulse potential of 5V was applied and the voltage was measured.• Sequentially, VHR was measured followed by placing test cells in a heated

humidity chamber followed by re-measuring VHR. • Testing is continued for ~ 1000 hrs at 65% RH/65°C • PTS-N shows < 3% voltage drop after 1000 h of testing• The voltage drop for PTS-R is ~ 60% after 100 h of testing

VHR Test Results

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• PTS-N provides a means to minimize voltage drop compared to PTS-R • PTS-N demonstrates less than 3% VHR drop after 1000 h of accelerated

exposure making PTS-N a promising candidate for display devices• PTS-N delivers this improvement in spite of higher blanket electrical leakage

Time dependent VHR: PTS-R and PTS-N

40%

60%

80%

100%

0 200 400 600 800 1000

Time (hours at 65%/65°C)

Vol

tage

(10

0% =

5V

)PTS-N

PTS-R

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60

70

80

90

100

110

120

0 0.5 1 1.5 2 2.5Density

Mea

n P

olar

izab

ility

Material

Mean Polarizability Results

Calculated Literature

PVDF beta (all trans) 9.0 9.05

PVDF alpha 8.1 8.8

PTS-R 84-86 --

PTS-N 103-106 --

PTS-N more cure 105-117 --

Polarizability – Key for Effective Dielectrics

• Modeling predicts that PTS-N with higher polarizability should maintain the electric field better than PTS-R

• Modeling predicts that increasing the extent of cure (reduced silanol) will increase polarizability and produce superior VHR

• Experimentally, PTS-N with higher polarizability holds the electric field in contact with LC much better than PTS-R in spite of showing higher leakage

PTS-N, 250°C cure

PTS-N 230°C cure

PTS-R

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Conclusions and Future Work

• Do not limit film testing to blanket electrical data – can be misleading

• Control of silanol content is critical to both electrical performance, interaction with LC and film formation

• Polarizability is a key criterion for effective VHR stability – modeling can be used to focus R&D efforts on most promising candidates

• VHR results will depend on the specific LC used – modeling results used a traditional LC (ZΙΙΙΙ1132)

• Testing of uncured material (PAB only) is needed to assess performance for processes with limited thermal budgets

• More completely cured material is expected to provide higher polarizability and improved VHR performance

• PTS-N provides improved VHR while retaining the excellent planarization, transmittance and other material properties of PTS-R while delivering low temperature crosslinking

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