Chapter 1 Introduction
1.6 IJP in color filter fabrication
At present, color filters are manufactured by the pigment dispersion method which includes three lithography processes for the red (R) green (G), and blue (B) color resists [1].
The color resist is spin coated onto a substrate, exposed to UV light, and then developed to prepare the colored pixels. The color resist includes curable resins and dispersed color pigments. It became reactive when irradiated by light, and was further cured by heating. The procedure of coating, light exposure, and development steps must be performed three times in order to obtain pixels of the three RGB colors. Therefore, the equipment cost is increased and the yield decreased due to the complexity of the processes. To solve these problems, the ink-jet printing systems and processes were developed for the manufacture of color filters [2][3].
Compared with the conventional pigment dispersion method, the ink-jet method is a
simplified process that is more environmentally friendly, and requires fewer raw materials.
The RGB inks are ejected onto the micro color areas of the color filter, and are fixed by a UV curing reaction. The colored layers of R, G and B can be formed in a single step. This method reduces the process steps and the equipment investment cost. Moreover, the inks can be deposited selectively on pixel areas without waste [5]. Additionally, productivity can be improved and production costs can be reduced. Table. 1.1 shows the ink-jet print and conventional color filter process [9].
（
Generation 3.5 RGB process）
Ink-jet Printing Spin coating
Equipment Only one printing system No mask
Three mask process equipment Mask (1500~2000 K NT/per) Process Few process step
Printing ink in pixel using only one step
Many process step and complex Using three mask process
Material >95%
Almost no waste
<10%
About 90% material wasted in spin coating process
70% material removed in mask process
Next generation
Suitable for large area and flexible substrate in next generation
Coating for large area substrate is hard
Table. 1.1 Compare with ink-jet print and conventional color filter process
Chapter 2
Surface Treatment
2.1 Surface treatment in IJP color filter application
An emerging process for manufacturing color filter has been technically developed by an inkjet printing system. The piezoelectric printing head has been innovated as the emerging tool for fabricating large-sized color filter, organic thin-film transistor devices, flexible display, and organic light emitting diode in mass production. However, literature on the manufacturing of color filter by inkjet printing technique is very few [3]. From the economical viewpoints of competition in production, inkjet printing is a high efficiency technology for resist-saving and low-cost productivity in the processing of large size panels [5][6].
Generally speaking, there is an issue for color filter printing process that color ink overflows when printing. To solve this problem, many methods of preventing ink overflowing are published and used for ink-jet printing color filter process. The common method is to use rib building on BM, but it needs mask process. Here, we use surface treatment as a method to block ink overflowing and the surface treatment uses only one step process.
Different surface treatments of the substrate resulted in different performances of hydrophobic and hydrophilic effect [13]. Use chemical coating and plasma surface treating as
two kinds of surface treatment for hydrophobic treatment. Color ink would not be wetting on hydrophobic treatment thus the color ink would be blocked in subpixels without overflowing when printing [14][15].
2.2 Chemical surface treatment
The black matrices were coated with a monolayer and the surface became hydrophobic and the surface energy was changed due to the monolayer chemicals. The monolayer chemical contains the chemical compound of fluorine as shown. Chemical 1:
R(X)SiOMe3
Where R is alkyl containing fluorides X is spacer with the alkyl group
Fig. 2.1 shows the monolayer chemical formed on the substrate. The process includes coating chemical on substrate and baking. After monolayer chemical is formed, the surface of substrate will show a hydrophobic property because of the R alkyl containing fluorides. A simple process is provided to form a hydrophobic surface for preventing color ink overflowing.
R(X)Si(OMe)
3Fig. 2.1 Monolayer chemical formed on the substrate
2.3 Plasma surface treatment
Plasma treatment is a common surface treatment to form a hydrophobic or hydrophilic surface. Different plasma has different effect. Plasma is named by Irvin Langmuir in 1929;
and plasma is partial ionized gases which can be define as a quasi-neutral gas of charged &
neutral particles characterized by a collective behavior. Fig. 2.2 shows the principle of plasma generation using Ar as an example.
Fig. 2.2 Principle of plasma generation using Ar as an example
The Kinetic Energy is the energy for plasma generation as shown in 2.2.1.
(Kinetic Energy) gained = F．d = q．ε．λ = q．(V/de)．λ. (2.2.1) where λ：mean free path ; de：electrode distance
ε：electric field ; V：electrode voltage
Different plasma has different effect. CFx plasma is a common method for chemical stability, low surface energy, low refractive index, good electrical and thermal insulation applications. For the purpose to prevent color ink overflowing, here we use CFx plasma as a surface treatment to decrease surface energy and to increase contact angle.
Fig. 2.3 shows the effect of plasma in the chamber. When we treat the BM substrate, other effect would influence the results. The etching effect of CFx plasma would be an issue when we using a CFx plasma surface treatment.
Fig. 2.3 Effect of plasma in the chamber
Chapter 3 Experiment
3.1 Experiment design
The ink-jet printer Litrex 70 as shown in Fig. 9 is used for printing R,G,B color film.
Fig. 3.1 Litrex 70 ink-jet printer
The system of inkjet printing equipment includes the glass substrate laying on the X-Y
ink storage syringe, ink supply system, controller and printing head driver for ink injection, shown in Fig.3.2. The ink droplets are injected through the orifice of printing head and dropped in the substrate. The compatibility between color photoresists and printing head should be the main bottleneck issue due to different ingredients and properties of color ink and piezoelectric material of printing head [16][19].
Fig. 3.2 Basic structure of ink jet printing system
The ink-jet printing equipment is a Drop-On-Demand type printer. Print head is Spectrum piezoelectric head [17][18]. Distance between nozzle and nozzle is about 507 um and nozzle diameter is 38 um. Drops size are about 25 to 35 pl which depend on ink property and applied printing voltage. The specification of print head is shown in table 3.1.
The relationship of drop size and drop diameter is shown in Fig. 3.3. Drop diameter of 25 to 35 pL droplet are about 58 to 66 um.
24 26 28 30 32 34 36
Fig. 3.3 The relationship of drop size and drop diameter for a spherical shape drop
Table. 3.1 Specification of print head Print Head type Spectra
SE-128 Addressable Jets 128
Drop size (pL) 25-35 Fluid viscosity range (cP) 8-20
Nozzle diameter (um) 38
3.2 Ink characterizations of IJP
For insuring the inks we used are in the specification of printing system and testing the color quality of color film, ink and color film properties are measured [28~32].
3.2.1 Ink property
blue inks are 11.07, 7.23, and 12.99 cp. Surface tensions are 28.2, 28.0, and 28.1. The optimal viscosity for SE-128 print head is within 8 to 20 cp that the color inks are usable.
Table. 3.2 Ink and color film properties Ink Viscosity
The optimal viscosity for SE-128 print head is within 8-20 Cp.
The viscosities are measured by BROOKFIELD/U.S.A DV-III+ viscosity measurement instrument. Basic theory of the viscosity measurement instrument are shown in Fig. 3.4
(a) (b)
Fig. 3.4 (a) shear for fluid by Newton (b) BROOKFIELD/U.S.A DV-III+ viscosity measurement instrument
BROOKFIELD/U.S.A DV-III+ viscosity measurement instrument uses spindle spinning in the measured material and measures the torque when spindle is spinning as shown in Fig. 3.4(b).
Fig. 3.4(a) shows the shear for fluid proposed by Newton. Tow parallel fluid surfaces
in different plane has the same area of A, and the distance between the two different parallel fluid surfaces is dx. The two different parallel fluid surfaces flow in the same direction with different flowing speed. We will have an equation of:
F/A = ηdv/dx
η is viscosity and dv/dx is shear rate which means the shear of fluid. The unit of shear rate is sec-1. F/A term which called shear stress means the shearing force of fluid in per unit area.
The unit of shear stress is dyne/cm2. We will have an equation of:
η = (F/A) / (dv/dx) = shear stress / shear rate The unit of viscosity is cp or mPa․s.
1 cp = 1 mPa․s
3.2.2 Color film property
Color films are formed by spin coating method. After color films are formed, they were soft baked by 90oC for 10 minutes, cured by UV light for 250 mj/cm2, and hard baked by 230oC for 40 minutes.
Fig. 3.5(a) shows the color film thickness and roughness which is measured by AFM (Atomic Force Microscope). The thickness of red, green, and blue color films are 1.81, 1.80, and 2.00 um and the roughness are 9.628, 9.294, and 8.071 nm.
Fig. 3.5(b) shows UV-visible spectrum of RGB color films of thickness shown in Fig.
3.5(a). The CIE1931 color coordinate of RGB color films are x = 0.663, y = 0330, for red color film and x = 0313, y =0610, for green color film, and x = 0.130, y =0.126, for blue color film after turning UV-visible spectrum of RGB color film to CIE1931 color coordinate [25][26].
(a)
(b)
Fig. 3.5 (a) RGB Color film thickness and roughness (b) UV-visible spectrum of RGB color film
3.2.3 Color film reliability
The reliability of color film is used for insuring that chemical resistance of color film.
Color filter is combined with other components to form a display. The reliability of color film relates to the quality of color filter.
Chemical resistance tests are using different solvent for testing as shown in Fig. 3.6.
Color films are dipped into chemicals for 24 hrs.
Blue (2.00 um)
(a)
(b)
Fig. 3.6 UV-visible spectrum of RGB color film and CIE1931 color coordinate of chemical 1931 CIEx,y Coordinations
Red Green Blue
Original (0.580, 0.318) (0.352, 0.491) (0.161, 0.242) Methanol (0.580, 0.317) (0.351, 0.498) (0.167, 0.245) Ethanol (0.566, 0.316) (0.352, 0.470) (0.161, 0.239)
1931 CIEx,y Coordinations
Chemicals Red Green Blue NTSC
Original (0.655, 0.330) (0.300, 0.621) (0.133, 0.101) 74 % IPA (0.659, 0.330) (0.303, 0.617) (0.134, 0.108) 73 % PGMEA (0.660, 0.329) (0.310, 0.611) (0.132, 0.104) 72 % r-butyrolatane (0.664, 0.329) (0.307, 0.613) (0.133, 0.113) 72 %
T (%)
3.3 BM glass treatment
BM glass surface treatment is used for prevent ink overflow from the pixels. Here are two BM glass surface treatment process.
3.3.1 BM glass chemical treatment
Fig. 3.7 shows the chemical treatment process. A BM glass is dipped in the monolayer chemical (series No. HFC-128) and baked by 100oC for half an hour. This method will coat the chemical on both black matrices and glass. Two chemical is used in this experiment. Difference between these chemicals is the hydrophobic effect to different surface.
Fig. 3.7 Chemical treatment process
3.3.2 BM glass plasma treatment
A BM glass is put in a vacuum chamber for plasma treatment as shown in Fig. 3.8. Gas handling system is mass flow controller. Reactor includes chamber, electrode, gas distribution, gauge, and heat/cool. Pumping unit includes throttle valve, turbo-pump, and rough pump.
Power supply is the source (DC, RF, and MW)/bias for plasma, and pulsed/continuous
controller. CFx gas flows into the chamber to produce CFx plasma. Control the gas flowing rate, plasma power, and treating time to have different effect of plasma treatment. This method will treat both black matrices and glass.
Fig. 3.8 Plasma treatment process
3.3.3 Ink contact angle measurement
Use contact angle measurement system to observe the effect of surface treatment. The contact angles are measured by contact angle system (KRŰSS GmbH). Fig. 3.9 shows the method of contact angle system measurement. The needle will push 3 ~ 4 uL drop at the tip and touch the substrate. After drop contact to the substrate the beadle leave the substrate and the droplet is left on the substrate.
(a) (b)
Fig. 3.9 Method of contact angle system measurement (a) wetting surface (b) not wetting surface
3.4 Printing process
Printing design is assisted by using LSL program which is also the program used to control the Litrex 70 ink-jet printing system. Using Litrex ink-jet printing system print red, green, and blue single color films before printing three color films on same BM [21][22].
3.4.1 Printing pattern design
Red, green, and blue color films are independently printed step by step. Every printed pattern are apart from another by an applicable distance to form red, green, and blue subpixel color filter. Fig. 3.10 shows the design for one of the three primary color print pattern.
Fig. 3.10 Design for one of the three primary color print pattern using Litrex inside program
Every subpixel are designed to print 7 drops as shown in Fig. 3.11. the program will generate a mechanical langrage after completing the patter. The mechanical langrage which is called swath and the printing recipe is shown in Fig. 3.12.
Fig. 3.11 Every subpixel are designed to print 7 drops
Fig. 3.12 The mechanical langrage which is called swath and the printing recipe
3.4.2 Printing process & maintain
[23]: Step1. Pump out the maintenance solvent out of the ink supply syringe.
Step2. Inject the color ink into the ink supply syringe.
Step3. Prime the head to fill the color ink into the inside pipe and printing head.
Step4. Set printing recipe and maintain the printing head to check the head condition.
Step5. Go printing process.
Fig. 3.13 shows the drops jetting out of the nozzle taking picture by camera. Using the system, we could know the condition of nozzle and print head. Fig. 3.13(a) shows the earlier time of drops jetting out of the nozzle and Fig. 3.13(b) shows the later time.
(a) (b)
Fig. 3.13 Drops jetting out of the nozzle (a) earlier time of drops jetting out of the nozzle (b) later time of drops jetting out of the nozzle
Chapter 4
Result & Discussion
4.1 Contact angle of BM glass surface treatment
Contact angle of droplet on substrate relates to the wetting situation between ink and substrate surface. High contact angle means the ink would not be so wetting on the surface.
In order to prevent ink from overflowing the bank when printing, two surface treatments is used to increase the contact angle of droplet on BM substrate as shown in chapter 2.
Use water as a reference to show the contact angle changed after surface treatment.
Fig. 4.1 shows the water on the blank BM, and the contact angle is 65o.
Fig. 4.1 Contact angle of water on blank BM
The contact angle increases from 65o to more than 85 o after surface treatment for BM side as shown in Fig 4.2. The chemical treatment is more effective than plasma treatment as shown in Fig 4.2
WaWatteerr oonn bbllaannkk BBMM
C C /A / A 6 6 5 5
ooFig. 4.2 Contact angle of water on different treatment BM and glass
4.1.1 Contact angle of BM glass chemical surface treatment
Use chemical surface treatment to increase contact angle of color ink droplet. Fig. 4.3 shows the contact angle of red ink drop on blank BM and glass
(a) (b)
Fig. 4.3 Contact angle of red ink drop on blank BM and glass (a) blank BM (b)blank glass
Fig. 4.4 shows the contact angles of red ink drops on chemical treatment BM and glass. Contact angles increase after BM and glass chemical treatment as shown in Fig. 4.3 and Fig. 4.4. The contact angle increases obviously for red ink in chemical 1 treatment case.
In order to prevent overflowing, chemical 1 treatment for large contact angle increasing would better than chemical 2 treatment.
C/C/AA 1122oo C/C/AA 1177oo
Fig. 4.4 Contact angles of red ink drops on (a) chemical1 treated BM (b) chemical1 treated glass(c) chemical2 treated BM (d) chemical2 treated glass
The contact angles of red ink drops on chemical treated surface are increased. The effect of surface treatment to other inks will be discussed next. Fig. 4.5 shows the contact angle of green ink drop on blank BM and glass.
(a) (b)
Fig. 4.5 Contact angle of green ink drop on blank BM and glass (a) blank BM (b)blank glass
Fig. 4.6 shows the contact angles of green ink drops on chemical treatment BM and glass. Contact angles increase after BM and glass chemical treatment as shown in Fig. 4.5
C
C//AA 5522oo CC//AA 4411oo
C/A 30o C/C/AA 2244oo
(a) (b)
(c) (d)
CC//AA 1144oo CC//AA 1166oo
case. In order to prevent overflowing, chemical 1 treatment for large contact angle increasing would better than chemical 2 treatment.
Fig. 4.6 Contact angles of green ink drops on (a) chemical1 treated BM (b) chemical1 treated glass(c) chemical2 treated BM (d) chemical2 treated glass
The contact angles of red ink drops on chemical treated surface are increased. The effect of surface treatment to blue inks will be discussed next. Fig. 4.7 shows the contact angle of blue ink drop on blank BM and glass.
(a) (b)
Fig. 4.7 Contact angle of blue ink drop on blank BM and glass (a) blank BM (b)blank glass
Fig. 4.8 shows the contact angles of blue ink drops on chemical treatment BM and C
C//AA 5544oo
C
C//AA 3344oo
CC//AA 2288oo
C/C/AA 2299o o
(a) (b)
(c) (d)
CC//AA 77oo C/C/AA 1166oo
glass. Contact angles increase after BM and glass chemical treatment as shown in Fig. 4.7 and Fig. 4.8. The contact angle increases obviously for blue ink in chemical 1 treatment case. In order to prevent overflowing, chemical 1 treatment for large contact angle increasing would better than chemical 2 treatment.
Fig. 4.8 Contact angles of blue ink drops on (a) chemical1 treated BM (b) chemical1 treated glass(c) chemical2 treated BM (d) chemical2 treated glass
Contact angles increase after BM and glass chemical treatment. The contact angle increases obviously in chemical 1 treatment case for red, green, and blue ink. Chemical 1 treatment for large contact angle increasing would better than chemical 2 treatment. Contact angle increases both BM and glass side after chemical treatment. In ink-jet printing color filter application, increasing contact angle of ink on BM without on glass would be a best situation to prevent overflowing but not to influence color film uniformity and color properties.
C/A 39o
C/C/AA 4400oo
C/C/AA 3355oo
C/A 32o
(a) (b)
(c) (d)
Use CFx plasma surface treatment to increase contact angle of color ink droplet. The power of plasma is 400w, gas flowing rate is 300 sccm, and treatment time is 5 seconds. Fig.
4.9 shows the contact angles of red ink drops on plasma treatment BM and glass. For red ink, the contact angle of plasma treatment BM side increases as shown in Fig. 4.3 and Fig.
4.9. Plasma treatment is not so effective to glass side that contact angle of glass side is not increasing obviously.
(a) (b)
Fig. 4.9 Red ink drops on CFx plasma treatment (a) BM side (b) and glass side.
Fig. 4.10 shows the contact angles of green ink drops on plasma treatment BM and glass. For green ink, the contact angle of plasma treatment BM side increases as shown in Fig. 4.5 and Fig. 4.10. Plasma treatment is not so effective to glass side that contact angle of glass side is not increasing obviously.
(a) (b)
Fig. 4.10 Green ink drops on plasma treatment (a) BM side (b) and glass side.
Fig. 4.11 shows the contact angles of blue ink drops on plasma treatment BM and CC//AA 2277oo C/C/AA 1155oo
CC//AA 3300o o CC//AA 1155o o
glass. For blue ink, the contact angle of plasma treatment BM side increases as shown in Fig.
4.7 and Fig. 4.11. Plasma treatment is not so effective to glass side that contact angle of glass side is not increasing obviously.
(a) (b)
Fig. 4.11 Blue ink drops on plasma treatment (a) BM side (b) and glass side.
Table. 4.1 Contact angle of color inks on BM and glass without
treatment
Chemical I Chemical II CFx Plasma
BM (o)
Contact angles increase after plasma treatment for BM side. Contrast to chemical treatment, plasma treatment is not so effective for BM. Different treatment time will cause different hydrophobic effect.
C/C/AA 2255o o C/C/AA 1155oo
side effect of over etching was inevitable. To obtain an optimized condition, the plasma treatment was done experimentally between 10 to 50 seconds. Fig. 12 shows that the height of BM decreases with the treatment time increasing.
Fig. 4.12 Height of BM with the treatment time increasing from 0 to 50 seconds.
Height of BM decreases to 80% with treatment time increasing to 30 seconds. To prevent further damage on BM while keeping good hydrophobic condition, CF4 plasma treatment time should be limited within 30 seconds.
Fig. 4.13 shows the contact angles of red ink drops on plasma treatment BM and glass by different time. The contact angle increases with treatment time increasing. Plasma treatment is not so effective to glass side that contact angle of glass side is not increasing obviously even increasing treatment time.
Fig. 4.13 Contact angles of red ink drops on different time plasma treatment BM (a1) 10s (b1) 20s (c1) 30s and glass (a2) 10s (b2) 20s (c2) 30s.
4.1.3 Discussion of surface energy
The adhesion energy, Wa, is the energy dissociating from the original surface of two
The adhesion energy, Wa, is the energy dissociating from the original surface of two