The basic fabrication process is as shown in Figure 3-1. In the beginning of process, the ITO glass substrate needs to be cut into adequate size. Second, the ITO glass substrates should be cleaned since any small particle will cause fatal defect.
Third is the lithography processes including multi-layer structure which contain photomask designing, spin coating of positive photoresist, UV light exposing and wet etching. Fourth, a high resistance material should be spin coated above the patterned ITO. Finally, the glass substrates, spacer, LC (E7) will be assembled as a complete sGD-LC sample.
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Figure 3-1 Flow chart of fabrication process
3.1.1 ITO Glass Substrates Cleaning
A sGD-LC lens sample must be fabricated in clean room. While the first step that is troublesome, so cleaning ITO glass substrates should be an important thing. The purpose of cleaning is to eliminate small particles and chemical compositions above the surface. Figure 3-2 shows the flow chart of glass cleaning. A method of industrial was adopted. First of all, KG, a detergent from Yokohama Chemical Co. was diluted to de-ionized (DI) water, the ratio of KG and DI water was 1:100. Dipping the glass substrate into KG solution and oscillate 30 minutes. The purpose of this step is to remove greasy dirt and chemical compounds. Second, flushing glass substrates 5minutes with DI water, the step is for washout KG since KG is hard to clean. Third, dipping glass substrates into Acetone and oscillate 30minutes for organic compounds and KG cleaning. Fourth, pouring DI water and oscillate another 30miuntes for Acetone eliminating. Finally, the heater is used to evaporate the remaining water.
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Figure 3-2 Flow chart of cleaning process
3.1.2 Lithography
We have already obtained a totally clean ITO glass substrate. Lithography, a process of fabricate ITO pattern what we want will be introduced. Since GD-LC lens needs jump structure in the center, the multi-layer structure must be utilize, this part will be discussed later. The flow chart of lithography is show in Figure 3-3 First, the spin coater was used to generate the uniform and thin photoresist layer on the surface of the cleaned glass substrate, while the spin speed is 1000rpm/15seconds for pre-spin and 2500rpm/25seconds for adequate thick of photoresist. A photoresist acts similarly to the sensitizer of a film in a camera since it could optically transfer the patterns of the layout from the photomask. A 90seconds soft baked should be taken, this step is for strengthen coated photoresist. Second, the UV light beam strokes the photomask which has been well-designed patterns and reacts with the chemical component of the photoresist. Third, the chemical reacted photoresist can be stripped off and remains
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the no chemical reacted parts. Therefore, our desired patterns are generated on the photoresist after the exposures photoresist is developed. Further, the left photoresist is like a protective layer to its cover ITO part. Therefore, HCl is used to etch the ITO layer which has no photoresist covered part. Finally, stripping the left photoresist and our designed ITO pattern is generated.
Figure 3-3 Flow chart of lithography process
A central electrode is needed for concave mode of GD-LC lens, so the jump structure should be used for central electrode. Silicon Nitride (SiNx) was chosen for the insulation layer. The fabrication process of insulation layer and 2nd ITO layer are same as 1st ITO etching, so we skip the detail processes of insulation layer and 2nd ITO layer. The flow chart of these two additional layers is shown in Figure 3-4.
Finally, the process of lithography was ended after etching these three layers.
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Figure 3-4 Flow chart of digging structure
3.1.3 Thin Film Coating
Three different materials were needed to spin above the ITO glass substrate. First is photoresist which has been mention in Chapter 3.1.2. The second one is high-resistance (Hi-R) material which is the key element of GD-LC lens. The last one is polyimide which is used for alignment layer of LC.
Spin coating is a procedure used to apply uniform thin films to flat substrates. In short, an excess amount of a solution is placed on the substrate, which is then rotated at high speed in order to spread the fluid by centrifugal force.
Rotation is continued while the fluid spins off the edges of the substrate, until the desired thickness of the film is achieved. The higher angular speed of spinning obtains the thinner the film. The thickness of the film depends on the concentration of the solution and the solvent.
Hi-R layer is the most important part of sGD-LC lens. In this thesis, Clevious-P from Baytron was chosen to be the Hi-R layer. The spin speed of Hi-R layer is
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4000rpm/30seconds to generate a 30nm thickness of Hi-R layer, the thickness was already measured by AFM. If the resistance is too low, Hi-R layer is like a conducting layer. Oppositely, if the resistance is too high, the driving voltage will increase. The resistance and driving voltage is a tradeoff, in our experiment, the appropriate value of surface resistance was chosen as ~1MΩ/□. Polyimide as an alignment layer was coated after Hi-R layer coated. Both of these two films need heating. For Hi-R layer, the function of heating is to dry it, the hot plate was set as 120℃ for 1hour. The requirement of heating for alignment layer is to harden the thin film. While the Hi-R layer is an organic material, high heating temperature should not be used. The hot plate was set as 150℃ 1.5 hour to prevent the damage of Hi-R layer. The section diagram of all thin films and substrates was shown in Figure 3-5.
Figure 3-5 Section diagram of GD-LC lens
3.1.4 Sample Assembling
The well-spin glass substrates could be assembled to GD-LC lens samples. The flow chart of the assembly process is shown in Figure 3-6. First, the roller made by woolen is used to rub the coated PI and give an arrange direction of LC. An
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anti-parallel rubbing direction was taken for two glass substrates. Second, the spacers are placed on the substrate to support an appropriate cell gap. Furthermore, the glue is used to fix and seal one side of the LC cell for LC injecting. Third, drip the LC (E7) onto the chink of two glass substrate, the LC will drill into the gap automatically since the capillarity. Then, the UV glue is used to seal all the seams of the LC cell completely in order to prevent the LC material contact with the air. We suggest spread additional glue for ensure separation of LC and air. Finally, soldering the wires with the pattern on substrate, the LC lens is accomplished.
Figure 3-6 Flow chart of assembling sample