Handling Technology of Plastic Substrates in Flexible Display Manufacturing

Min-Feng Chiang, Chun-Cheng Cheng, Chun-Hao Tu, Chu-Yu Liu, Tai-Hsiang Huang, Jen-Kuei Lu, Norio Sugiura, Yu-Chieh Lin

Advanced Technology Research Center, AU Optronics Corporation, No. 1, Li-Shin Rd. 2, Hsinchu Science Park, Hsinchu, Taiwan

Abstract How to handle the flexible substrates is an important topic by using cluster facilities, such as PECVD (Plasma-Enhanced Chemical Vapor Deposition), sputtering and photo-lithography systems. Laminating and de-bonding methods of flexible substrates have been the key factors in flexible display manufacturing. Comparison of handling methods has been discussed in this paper.

Author Keywords Plastic substrate, Flexible display, Laminating, De-bonding.

1. Introduction Flexible substrates have become key components for the flexible display in the near future. An ultra thin glass has advantages of high Tg (glass transition temperature), small CTE (coefficient of thermal expansion), smooth surface, high transparency, good WVTR (water vapor transmission rate) and good chemical resistance. But brittle feature brings inconvenience in display manufacturing and end user experience. A metal foil has weakness of poor surface, and opaque feature to limit the display application. Plastic substrates seem to be the good candidates for the flexible application. But issues of low Tg, large CTE, serious birefringence, poor WVTR and poor chemical resistance still need to be improved. However, some excellent plastic substrates have been already invented and fabricated successfully so far. At the same time, modified panel processes on plastic substrates are helpful and powerful in the display manufacturing. The Roll-to-Roll (R2R) manufacturing is a good method to handle the flexible substrates. But it can not be used to fabricate high-end products due to the poor quality of display processes, such as overlay accuracy, tension control, dielectric leakage and etching uniformity. Cluster systems are still the reliable and mainstream facilities, so we need to put flexible substrates on the rigid glass to be compatible with the sheet-to-sheet (S2S) process. Since 2001, many companies published many plastic prototypes in international technical conference. Sharp and Sumitomo Bakelite co-published the 4” plastic reflective AMLCD (Active Matrix Liquid Crystal Display) on plastic substrates in 2002 [1]. Seiko Epson published the 2.1” flexible AMOLED (Active Matrix Organic Light Emitting Diode Display) and 7.1” flexible AMEPD

(Active Matrix Electrophoretic Display) on plastic substrates by the SUFTLA (Surface Free Technology by Laser Annealing/Ablation) transfer process in 2003 and 2006, respectively [2-3]. Samsung published the 5” and 7” plastic AMLCDs on PES (Polyethersulfone) substrates in 2005 and 2006, respectively [4-5]. Samsung published the 5.6” flexible AMOLED on stainless steel foils in 2006 [6]. ITRI (Industrial Technology Research Institute) published the 4.1” and 7” plastic AMLCDs on colorless Polyimide substrates in 2006 and 2007, respectively [7-8]. UDC (Universal Display Company) and LG co-published the 4.3” flexible AMOLED on metal foils in 2009 [9]. Samsung published the 6.5” flexible AMOLED on Polyimide substrates in 2010 [10]. ITRI published the 4” flexible AMOLED on Polyimide substrates in 2010 [11]. SEL (Semiconductor Energy Laboratory) published the 3.4” flexible AMOLED on plastic substrates by transfer process in 2011 and 2013 [12-13]. Three kinds of handling methods were well known and summarized, including (1) flexible substrates laminated on the glass and de-bonded from the glass, (2) Polyimide coated, formed on the glass and de-bonded from the glass, and (3) devices transferred from the glass onto plastic substrates. In this paper, we would like to discuss that plastic substrates laminated on the glass and de-bonded from the glass in advance.

2. Experiment and Method Table 1 shows the comparison on material properties of several flexible substrates. Thermal properties, such as Tg, CTE and dimension stability, would be important factors to limit the highest temperature of display manufacturing. Serious warpage of glass carrier is not allowed during several thermal cycles. Poor adhesion and film cracks often appear if the thermal stress of multilayer stacking does not balance. Low dielectric leakage and good thin film quality with low temperature annealing require adjusting the deposition parameter. Optical properties, such as transmittance and birefringence, are relative to display feature, especially in LCD applications. In order to laminate the flexible substrate on the glass carrier, several kinds of adhesives have been tested and the peeling force of each adhesive has been measured after thermal cycles shown in Figure 1.

Table 1. Comparison on material properties of flexible substrates

*PI: Polyimide, *FRP: Fiber Reinforced Polymer, *PEN: Polyethylene naphthalate, *COC/COP: Cyclic Olefin Co-polymer/Cyclic Olefin Polymer

Plastic substrate Tg (Tm) ( ) CTE (ppm/ ) Thickness (um) Transmittance (%) Birefringence Dimension stability Stainless steel foil 1400 14-16 20-100 - - Excellent

Ultra thin glass 600 3-8 50-100 93 Excellent Excellent Yellowish *PI 400 10-20 20-125 - - Good Colorless *PI 350 10-20 25-55 90 Good Good

*FRP 250 8-16 50-100 91 Excellent Excellent *PEN 180 18-20 50-125 87 Poor Good

*COC/COP 150 60-65 75-125 92 Excellent Poor

6.1 / M.-F. Chiang Invited Paper

46 • SID 2014 DIGEST ISSN 0097-966X/14/4501-0046-$1.00 © 2014 SID

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

4th3rd2nd

Thermal cycle

Adhesive1 Adhesive2 Adhesive3 Adhesive4

Peel

ing

forc

e (N

@2c

m)

1st

Figure 1. Peeling force of adhesive after thermal cycles High peeling force of the adhesive could make the flexible substrate, which is laminated on the glass carrier, complete the display processes but the flexible substrate could not be de-bonded safely from the glass carrier. But the flexible substrate could be peeled easily from the glass carrier during the display processes, due to low peeling force of the adhesive. It’ very difficult to define the correct peeling force to achieve the high peeling force during the display process and keep the low peeling force during the de-bonding process. Figure 2 shows the adhesive pattern and cross section coated by two kinds of adhesive area with two kinds of peeling force. The main TFT array was formed and patterned on the weak adhesive area for de-bonding. The strong adhesive area was coated around the weak adhesive area for fixing the flexible substrate. Other methods could also have the same concept so far. For example, DBL (De-Bonding Layer) could be used as the weak adhesive area coated on a high peeling force of the adhesive to form two kinds of adhesive area. The peeling force of some special adhesives could be controlled by temperature or UV dosage. More and more novel methods have been discovered by panel makers and material suppliers.

Weak adhesive area

Strong adhesive area

Weak adhesive area

Strong adhesive area

Glass carrier

Low peeling forceFlexible substrate

Glass carrier

Low peeling forceFlexible substrate

High peeling force

Gla

ss c

arrie

r

Low

pe

elin

g fo

rce

Fle

xibl

e su

bstr

ate

Gla

ss c

arrie

r

Low

pe

elin

g fo

rce

Fle

xibl

e su

bstr

ate

Figure 2. Adhesive pattern and cross section coated by two kinds of adhesive area with two kinds of peeling force

Flexible substrateThin film stacking

Glass carrier

Low peeling force

Glass carrier

Low peeling forceFlexible substrateThin film stacking

Laser Cutting

Flexible substrateThin film stacking

Glass carrier

Low peeling force

Flexible substrateThin film stacking

Glass carrier

Low peeling force

Glass carrier

Low peeling force

Glass carrier

Low peeling force

Glass carrier

Low peeling forceFlexible substrateThin film stacking

Laser Cutting

Glass carrier

Low peeling forceFlexible substrateThin film stacking

Glass carrier

Low peeling force

Glass carrier

Low peeling forceFlexible substrateThin film stackingFlexible substrateThin film stacking

Laser Cutting

Figure 3. Mechanical de-bonding process The hybrid substrates (Figure 2) were prepared and compatible with the S2S process in the cluster systems. The process temperature of display manufacturing needs to be controlled to

make the adhesives keep the ideal peeling force without serious warpage of glass carrier. After the TFT process and cell assembling finished, the flexible display could be released safely and easily by mechanical de-bonding. Very low peeling force of the weak adhesive area was kept after all processes, and this area was cut by the laser cutting technique finally (Figure 3).

3. Result and Discussion In the development of plastic display, a suitable plastic substrate plays an important role. The highest process temperature and the thermal budget of total thermal cycles were limited by the suitable plastic substrate and the suitable adhesive. In this paper, three display applications have been demonstrated and published, including (1) Unbreakable AMLCD, (2) Flexible E-Paper and (3) Three-Dimension Switching Cell. For the plastic AMLCD applications, a FRP (Fiber Reinforced Polymer) substrate was laminated on glass carrier with the adhesive before the TFT fabrication. Island-Stop (IS) amorphous-InGaZnO (a-IGZO) TFTs were employed for array side of plastic display. First of all, a barrier layer was deposited on the plastic substrate to prevent the water and oxygen passing through from the back side of plastic substrate and improve the interface adhesion between the plastic substrate and inorganic layers. The electrical characteristics of a-IGZO TFTs on plastic substrates at the highest process temperature of 220°C are shown in Figure 4. The saturation mobility is about 13.7 cm2/Vs, the threshold voltage is about -0.21 V, and the sub-threshold swing is about 0.3 V/decade. The channel width and length of a-IGZO TFTs were 10 m and 10 m, respectively. Finally, 12.1-inch plastic AMLCDs were developed to verify the low temperature a-IGZO TFTs for display applications [14].

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 301E-14

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

VDS=0.1V

Dra

in C

urre

nt (A

)

Gate Voltage (V)

LT a-IGZO TFTsW/L=10um / 10umMobility=13.7cm2/V-sSS=0.3V/dec.VTH=-0.21V

Ion/Ioff > 108 @ VDS=10V

VDS=10V

Figure 4. Transfer characteristics of a-IGZO TFTs on plastic substrates

Figure 5. (a) photography and (b) display image of 12.1-inch Plastic AMLCD Figure 5 shows (a) the photography and (b) display image of 12.1-inch plastic AMLCD. The plastic panel has the resolution of 125 ppi (pixel per inch), the aperture ratio of 50%, and the cell mode is TN (Twisted Nematic) Liquid Crystal. The module thickness and weight without BLUs (Backlight Units) are 0.37 mm and 24.9 g,

(a) (b)

Invited Paper 6.1 / M.-F. Chiang

SID 2014 DIGEST • 47

respectively [15]. In general, the mechanical stress has great concern in flexible display applications. Hence, in this work, not only electrical stress but also mechanical stress was discussed. Meanwhile, in order to clarify the device performance under mechanical stress, a-IGZO TFTs were measured with concave bending (compressive state). Figure 6 shows the transfer curve of a-IGZO TFTs on plastic substrates laminated on glass and de-bonded from glass with bending radius 20 mm, 10 mm, and 5 mm. In the beginning, plastic substrates with a-IGZO TFTs were laminated on glass. The electrical characteristics of a-IGZO TFTs exhibit the threshold voltage of -0.21 V and the sub-threshold swing of 0.3 V/decade. After plastic substrate de-bonded from glass, the threshold voltage and sub-threshold swing of a-IGZO TFTs were changed to -0.73 V and 0.37 V/decade, respectively. Furthermore, the bending ability of a-IGZO TFTs was proved with concave bending test. The threshold voltage showed a slight negative shift and the sub-threshold swing increased slightly with the radius decreasing [15].

-25 -20 -15 -10 -5 0 5 10 15 20 25 301E-14

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

LT a-IGZO TFTs

Laminated on Glass De-bonded from Glass Bending Radius = 20mm Bending Radius = 10mm Bending Radius = 5mm

Dra

in C

urre

nt (A

)

Gate Voltage (V) Figure 6. Transfer characteristics of a-IGZO TFTs on plastic substrates under different conditions For the E-paper applications, the transfer characteristics of Organic TFTs (OTFTs) and the cross-section view of OTFTs fabricated on polyethylene naphthalate (PEN) substrates with bottom gate and bottom contact configuration are shown in Figure 7 [16]. Process temperature is kept below 120 °C in all steps. The mobility is around 0.6 cm2/Vs with W/L = 125 um / 20 um. Such high mobility is sufficient to drive EPD (Electrophoretic Display), even to drive LCD. A very low off-current leads to high On/Off ratio exceeding 106 and extends the holding time of the storage capacitance. The threshold voltage and subthreshold swing of Organic TFTs are 0 V and 0.5 V/decade, respectively.

Figure 7. Transfer characteristics of OTFT devices

Furthermore, the OTFTs give nearly no hysteresis phenomenon as well. A SiPix film is laminated onto the OTFT backplane to complete a rollable AMEPD. This prototype has a size of 6-inch in diagonal with SVGA resolution (800x600). The pixel size is designed as 150um x 150 um. The total thickness of AMEPD, including OTFT backplane and SiPix film, is around 125 um. Such a thin feature gives this display rolling ability. The minimum rolling diameter can reach 25 mm. (Figure 8)

Figure 8. 6-inch of OTFT e-Paper prototype For the 3D display applications, the comparison of two types of switchable auto stereoscopic 3D displays is shown in Figure 9 [17]. Figure 10 shows the flexible LC cells were fabricated by sheet-to-sheet process consist of four steps: i. Bonding: attach the plastic substrate to rigid carrier substrate; ii. Array process: metal, transparent conductive electrode sputtering and photo spacer formation; iii. Cell process: alignment layer coating, one drop fill (ODF) for LC filling; iv. De-bonding: remove the plastic LC cell from rigid carrier substrate [18].

Figure 9. Comparison of viewing distance in different switchable 3D displays

Figure 10. Manufactory process of plastic LC cell

6.1 / M.-F. Chiang Invited Paper

48 • SID 2014 DIGEST

The plastic substrate is made by transparent cyclic olefin copolymer (COC) with 100 um thickness. All fabrication temperature is well controlled under 100°C. The appearance of 10.1-inch and 4.3-inch flexible LC cells were shown in Figure 11. Figure 12 shows the display module of switching and parallax barrier LCDs are each 10.1” with 224 ppi LCD base panels and 4.3” with 256 ppi OLED base panels which perform good image quality [17].

Figure 11. (a) 10.1-inch flexible switching LC cell (b) 4.3-inch flexible parallax barrier LC cell

Figure 12. 3D demonstration (a) 10.1-inch flexible switching cell with 224 ppi LCD base panel (b) 4.3-inch flexible parallax barrier with 256 ppi OLED base panel

4. Summary The flexible display has many benefits over than the currently display. Factors such as a reduction in thickness and weight, easy to carry, unbreakable and a non-linear form are all positive aspects of the flexible display. The success of low temperature process development on plastic substrates is an important milestone to achieve the flexible display. The electrical and mechanical reliability for these TFTs were also studied and these TFTs will have highly potential for flexible electronics and flexible displays in the future.

5. References [1] Yoshihiro Okada et al, “A 4-inch Reflective Color TFT-

LCD Using a Plastic Substrate”, SID’ 02 Digest, pp 1204-1207

[2] Sumio Utsunomiya et al, “Flexible Color AM-OLED Display Fabricated Using Surface Free Technology by Laser Ablation/Annealing (SUFTLAR) and Ink-jet Printing Technology”, SID’ 03 Digest, pp 864-867

[3] Yuko Komatsu et al, “A Flexible 7.1-in. Active-Matrix Electrophoretic Display”, SID’ 06 Digest, pp 1830-1833.

[4] MunPyo Hong et al, “Large Area Full Color Transmissive a-Si TFT-LCD Using Low Temperature Processes on Plastic Substrate”, SID’ 05 Digest, pp 14-17.

[5] Woojae Lee et al, “Transmissive 7” VGA a-Si TFT Plastic LCD Using Low Temperature Process and Holding Spacer”, SID’ 06 Digest, pp 1362-1364.

[6] Dong Un Jin et al, “5.6-inch Flexible Full Color Top Emission AMOLED Display on Stainless Steel Foil”, SID’ 06 Digest, pp 1855-1857.

[7] Chun-Cheng Cheng et al, “4.1-inch Color QVGA TFT LCD with a-Si:H TFTs on Plastic Substrates”, AMFPD’ 06, pp 7-10

[8] Yung-Hui Yeh et al, “7-inch Color VGA flexible TFT LCD on Colorless Polyimide Substrate with 200 oC a-Si:H TFTs”, SID’ 07 Digest, pp 1677-1679.

[9] Ruiqing Ma et al, “Wearable 4-inch QVGA Full Color Video Flexible AMOLEDs for Rugged Applications”, SID’ 09 Digest, pp 96-99.

[10] Dong-Un Jin et al, “Highly Robust Flexible AMOLED Display on Plastic Substrate with New Structure”, SID’ 10 Digest, pp 703-705.

[11] Cheng-Chung Lee et al, “A Novel Approach to Make Flexible Active Matrix Displays”, SID’ 10 Digest, pp 810-813.

[12] Kaoru Hatano et al, “3.4-inch Full-Color QHD AMOLED Display using Large-Size Flexible Substrate with Highly Reliable OS-FETs”, SID’ 11 Digest, pp 498-500.

[13] Akihiro Chida et al, “3.4-inch Flexible High-Resolution Full-Color Top-Emitting AMOLED Display”, SID’ 13 Digest, pp 196-198.

[14] Wei-Ting Lin et al, “Low Temperature Oxide TFTs on Plastic Films for Flexible Display Applications”, SID’ 13 Digest, pp 1232-1235.

[15] Shuo-Yang Sun et al, “12.1-inch WXGA Plastic AMLCDs Driven by Low Temperature Amorphous IGZO TFTs”, IDW’ 13 Digest, pp 272-275.

[16] Ching-Yang Liu et al, “6-inch Rollable Active-Matrix Electrophoretic Display Driven by Organic Thin-Film Transistor”, SID’ 12 Digest, pp 506-507.

[17] Shiuan-Iou Lin et al, “The Application of Flexible Liquid-Crystal Display in High Resolution Switchable Autostereoscopic 3D Display”, SID’ 13 Digest, pp 5-6.

[18] Pin-Hsiang Chiu et al, “Reliability and Barrier Layer Dependency of Flexible 2D/3D Switchable LC Cell”, SID’ 13 Digest, pp 810-811.

Invited Paper 6.1 / M.-F. Chiang

SID 2014 DIGEST • 49