Modified Spin Coating Method for Coating and Fabricating Ferroelectric Thin Films as Sensors and Solar Cells Modified Spin Coating Method for Coating and Fabricating Ferroelectric Thin Films as Sensors and Solar Cells

Spin coating process with a modified spin coater is performed well, especially the second generation of modified spin coater, which has a maximum value of 18,000 rpm, is able for manufacturing/coating photonic crystal‐based ferroelectric thin films that require a high angular velocity (rpm). Ferroelectric thin films that use both 3000 and 6000 rpm have given good results in energy gap, electrical conductivity, etc. In addition, the modified spin coater has also produced several applications such as sensors in the device of blood sugar level noninvasively, sensors in the automatic drying system, sensors in the robotic system, and photovoltaic cells in the system of solar cells/panels which are being developed at present. These applications used ferroelectric material such as barium strontium titanate (BST), lithium niobate (LiNbO 3 ), cuprous oxide (CuO), and lithium tantalate (LiTaO 3 ).

Spin coating is a method for coating and fabricating uniform thin films by rotating substrate and solution of thin films with a certain angular velocity. Purwanto and Prajitno [29] stated that spin coating is a method to deposit a thin film by spreading the solution onto a substrate by utilizing the centripetal force, the substrate is rotated at a constant velocity and then thin film precipitate is obtained on the substrate. Spin coating process has several advantages: namely, it is a simple method that can be done at room temperature, and low cost, yet effective enough for manufacturing thin films [30]. Coating technique with spin coating method is the best technique used to produce thin films with uniform thickness ranging from 0.3 to 5.0 μm on the substrate surfaces that are relatively small [31,32]. The film thickness is determined by the flow rate and plating time [33]. A simple process of spin coating can be seen in Figure 1.

First generation of modified spin coater
Proper design of the modified spin coating can reduce cost in the international market because its components can be purchased in Indonesia and adequate for our laboratory-scale research. A schematic design of portable spin coating type 2004 can be seen in Figure 2. This device can be carried easily because it is very light and small as well as better than the traditional one in operational and production costs as well as in efficiency. For generating or rotating the disk in a spin coating, the step-down transformer of 1 A is required with the output voltages based on the digital system. The output voltages of 7, 9, and 12 V are connected to the potentiometer, diode, and capacitor which resulting in the spin coating rotation of 3480, 4380, and 5840 rpms, respectively. The accuration test of the rotational velocity can be conducted by using a stroboscope.
The work mechanism of the device is as follows: the solution of thin films is dripped on a substrate which has been placed on the spin coating device. Then, the attached solution on a substrate is rotated at the desired rpm velocity. The voltage source used in the device is 220 V AC with current of 1 A, which is obtained through the step-down transformer. The output voltage of 7, 9 or 12 V can be selected via potentiometer setting. A diode serves to rectify the AC current into DC and then the electrical charges stored in a capacitor. The rotations are about 3480, 4380, and 5840 rpms.
The modified spin coating device (Figure 2), which its patent has been registered in Indonesia with a number of P00201201122 in 2013, has a structural design that consists of four components.

Current source
A current source of 220 V AC is connected to the switch and fuse safety systems.

Step-down transformer and potentiometer
A step-down transformer of 1A with input voltages of 7, 9 and 12 V. These are a set based on the resistance that changes in the potentiometer.

Diode and capacitor
A rectifier diode of standard currents is used to deliver a DC current of 50 V into a capacitor of 2200 μF and 50 V. The capacitor serves to store the charges and acts as a charge source for powering the disk spinner.

Disk of spin coating
The aluminum spinner device with a radius of 4 cm and thickness of 0.2 cm is used to rotate the substrate that has been dripped by solution of thin films. To generate rotation, a rotator machine is used to achieve the desired rpm velocity. Figure 3 shows the exterior of the modified spin coating device. A disk spinner installed on a plastic container of PVC with a radius of 6 cm and thickness of 0.3 cm and a steel cantilever with a thickness of 2 cm and an area of 4 cm × 4 cm to stabilize the disk spinner. The electrical circuits are arranged in a container of 6.2 cm × 19 cm.

Second generation of the modified spin coater
Currently, the development of the spin coating device has started in which the angular velocity of the device is increased up to 18,000 rpm. This is done because the manufacture of ferroelectric-based photonic crystals thin films requires a high-angular velocity. There are five stages in the development of a high-velocity spin coating device, namely designing, manufacturing, testing, analyzing, and developing the device.

Designing the device
In this stage, designing the workflow is conducted which is described in the block diagram and flowchart. The block diagram in Figure 4 illustrates the Arduino Uno as a data processing center that received input from infrared sensors, controlled by the buttons, and the output as a signal to adjust the angular velocity of a brushless motor as the driving component via the ESC motor driver and the LCD will show the magnitude of the angular velocity in rotation per minute (RPM). Figure 5 shows the program flowchart for a device that further will be integrated with the Arduino Uno.

Manufacturing the device
The schematic circuit diagram in Figure 6 describes the use of Arduino Uno, buttons, infrared sensor, ESC, brushless motor, and LCD. There are eight Arduino pins, six digital, and two analog pins, which are used for the data path. Pin 4-7 on Arduino connected to four buttons. In connecting the ESC to Arduino, pins that support the pulse width modulation (PWM) are needed, so pin 3 was used on the Arduino. Infrared sensors require attach interrupt function on the program of pin 2. Then, to connect Arduino to LCD, I2C interface used to conserve the use of pins on the Arduino. From the circuit scheme, A4 pin on the Arduino is connected to the SLC in the I2C interface and A5 pin on the Arduino is connected to the SDA in I2C interface. The brushless motor has only three wires and those are connected to the ESC.
The used voltage source is an adapter of 12 V and 5 A. Most of the voltage source used to drive a brushless motor and ESC by using a voltage of 12 V and current of 5 A. Arduino gets voltage from the same source, but also need a resistor of 20 Ω on V in pin in order not to damage Arduino. While other components such as infrared sensors, I2C interface, and LCD can be run at a voltage of 5 V. After making a circuit scheme, the program code is conducted in accordance with the device workflow diagram that has been made at the designing stage. When the device circuit and the program code are completely made, it is necessary to merge the process of the device circuit with the program code so that the device can work in accordance with the flowchart. The incorporation using an application that matches to the Arduino device is Arduino IDE 1.6.8.
Interface of Arduino IDE 1.6.8 application is shown in Figure 7.

Testing the device
LCD displays the measurement of angular velocity and value rpm. As can be seen from Figure 8, the first line of LCD includes the word "Speed" that shows the measurement of the angular velocity in units of rpm. The second line of LCD is"Set' that shows the rpm setting value that users want in units of rpm as well. Table 1 shows the measured voltages and currents between circuit blocks of the spin coater which explains that an infrared sensor will produce an output voltage of 4.92 V if detects black color and will produce an output voltage of 0.11 Vif detects another colors. Arduino circuit blocks will produce different output voltage according to the type of the use of the IC.
The next is the accuracy of angular velocity of the spin coater using tachometer type DT-2234C + as an angular velocity comparator (Figure 9). Angular velocities of the brushless motor are determined on the value from 1000 to 20,000 rpm.   From the experimental data, the brushless motor is only capable of producing the minimum angular velocity of 4000 rpm and the maximum of 18,000 rpm with no load. In addition, angular velocity on the spin coater has an average difference of 82.67 rpm obtained from the tachometer. The data can be seen in Table 2 and Figure 10.

Development analysis
A function menu is added in the program of development. In this function, users can choose the mode of use, i.e., manual and automatic modes. In automatic mode, users need to set Thin Film Processes -Artifacts on Surface Phenomena and Technological Facets the number of repetitions (step), the length of timer (timer), and the angular velocity (rpm set). The number of repetitions is only for a substrate. When the timer is up it is necessary to notify such as a sound, hence the buzzer circuit is added as a component to produce a sound as shown in Figure 11(a). In addition, a button is also added, as shown in Figure 11(b), as a supporter of the functions that will be created. The added button is useful for stop the button when the spin coater is started. The development of block diagram shown in Figure 12 is not much different from the block diagram of before device development.

Development
The development of schematic circuit designing and flow of program are conducted in this stage. The developed schematic circuit shown in Figure 13 is not much different from previous schematic, only different in add buzzer and button components. A buzzer and a button are connected directly to the Arduino via pins 9 and 8, respectively. Overall device development and its power supply can be seen in Figure 14.
A flow diagram in Figure 15 shows the flow of the program code. First, the user is prompted to select the mode to be used. The available modes are manual and automatic modes are shown in Figure 16. If users select the manual mode, it will display a condition before the device was developed. When choosing the automatic mode, the user is prompted to set the number of repetitions as shown in Figure 17, then adjust the angular velocity in rpm as shown in Figure 18, and set the length of time in seconds as shown in Figure 19. Once the setup process is complete, it will display a summary which is shown in Figure 20 and then confirm to save the setting on Electrical Erasable Programmable Read-only Memory (EEPROM) on Arduino      semipermanently which is not lost when the power supply is not connected as in Figure 21.

Set value Spin coater (infrared sensor)
The save settings will display words of"Saving Data" as shown in Figure 22, after the save process is completed it will display the words of"Data saved" as shown in Figure 23 and a buzzer will sound twice. If not saved then it will return to the setup process.
Once the data has been stored, the spin coater will begin the process of spin coating which displays some information such as repetition, duration, time remaining on the first line of the LCD, and the rpm setting value and measurement results of angular velocity on the second line of the LCD as in Figure 24. At each repetition process, the buzzer will sound once. If the spin coating process has been completed, it will display word of"Done" as shown in Figure 25 and a buzzer will sound three times, after that confirmation of the spin coating reprocess will be also displayed as in Figure 26.
The aim of this stage is to improve the device accuracy by conducting device reconfiguration. The device reconfiguration will change the measurement of angular velocity, then retest must be done for the accuracy of device. The testing method as well as the testing method before development is compared to tachometer-type DT-2234C + .
In the measurement of angular velocity of the brushless motor, the rpm setting value on a spin coater is determined which starts from 4000 to 18,000 rpm. The brushless motor only produces the minimum angular velocity of 4000 rpm and the maximum angular velocity of 16,637 rpm under no-load conditions. From data, the created spin coater has an accuracy rate of 98.9%. The results can be seen in Figure 27 and Table 3.             Table 3. Comparison data of angular velocity after development.

Set value
Thin Film Processes -Artifacts on Surface Phenomena and Technological Facets

Results and applications
This method has been developed in our laboratory to fabricate thin films, especially ferroelectric thin films such as barium strontium titanate (BST), lithium tantalate (LiTaO 3 ), lithium niobate (LiNbO 3 ), and copper oxide (CuO) thin films. Specifically for BST ferroelectric thin films, its patent has been registered in Indonesia with a number of P00201201119 in 2013.
BST thin films of the modified spin coating method resulting in a dielectric constant of 2-18 and conductivity values of 1.6 × 10 -6 to 2.4 × 10 -9 S/cm at the voltages of 1-4 V [34]. In addition, the particle distribution size of BST 0.45 is 134.93 nm which is smaller than BST 0.25 which gives 186.26 nm, BST 0.35 gives a value of 467.86 nm, and BST 0.55 is 407.49 nm [34].
BST thin films have been developed in a number of applications such as temperature and light sensors that are applied into the automatic drying system [35], light sensors into the luxmeter system [36] and sensors in the blood sugar level system, its patent has been registered in Indonesia with a number of P00201508327 in 2015.
In the automatic drying system [35], light sensors have 0.176 mV/lux, which is the best sensitivity. While the best temperature sensor has a value range of 30-109°C, a sensitivity of 0.862 mV/°C, a film resolution of 1°C, an accuracy level of 92.2%, and small hysteresis [35]. These sensors, which are then integrated into microcontroller, have been successfully conducted on an ATMega8535 microcontroller based on our automatized drying system model.
In the luxmeter system [36], Ba 0,25 Sr 0,75 TiO 3 (BST) thin films have been deposited on a Si (100) p-type substrate by a chemical solution deposition (CSD) method followed by the spin coating technique (at 3000 rpm rotational speed for 30 seconds). This BST has an electrical conductivity of 2.79 × 10 -7 to 5.3 × 10 -7 S/cm, which are in the range of semiconductor materials. The I-V measurement on the films which was carried out under dark and bright conditions results in convincing conclusion that the films are photodiodes. The maximum optical absorbance found for green light with a wavelength of around 550 nm.
The blood sugar level system also uses the modified spin coating method. The test results showed that the light intensity that was received by photodiode sensor, represented by the output voltage, will change in line with changes in the value of blood sugar levels, follows the equation y = 0.0014x 2 − 0.6652x + 139.34. The coefficient of R 2 = 0.9544 indicates that x has a major effect on y, so that it can be concluded that the photodiode sensor can work well as a sensor of blood sugar level instrument. Accuracy and precision of the instrument are 98.92 and 97.41%, respectively.
BST thin films as solar cells enhanced by photonic crystals are currently being developed [37]. This enhancement needs higher angular velocity (>6000 rpm) than previous works in BST (3000 rpm). The second generation of modified spin coater is used for coating the thin films to fulfill the requirement. The optical characterization of BST (Ba x Sr 1-x TiO 3 ) using a photonic crystal resulted in the average absorption percentages for mole fraction x = 0. 25 Furthermore, BST thin films are worked both as sensors and solar cells can be applied to the more sophisticated fields such as temperature and light sensors in the satellite technology [38,39], photodiode in satellite technology [40], as well as solar cells for substituting conventional battery in satellite technology [41].
LiTaO 3 thin films have also been developed by using the modified spin coating method as infrared sensors that are expected to be developed as an automatic switch in satellite technology [42]. These thin films have electrical conductivities of 10 -6 -10 -5 S/cm and the diffusion coefficient values of 57-391 nm 2 /s [43] as well as the energy gaps of 3.41-4.56 eV [42] while the LiTaO 3 thin films have energy gaps of 2.43-2.80 eV [44]. The LiNbO 3 thin films that were enhanced by a lanthanum dopant also use a velocity of 3000 rpm and have the energy gaps of 2.43-2.80 eV [45].
In addition to BST, LiTaO 3 , and LiNbO 3 , we are also develop other thin films, i.e., CuO thin films. This film is also developed as solar cells. CuO thin films were enhanced by photonic crystals that have absorbance in the visible region and energy gaps of 1.89-2.05 eV [46].

Conclusion
The modified spin coaters both first and second generation have given very good results on ferroelectric films, which are BST, LiTaO 3 , LiNbO 3 , and CuO, as sensors and solar cells. In BST, the modified spin coater enhances the photon absorption percentages by using photonic crystals which need higher angular velocity (6000 rpm). Another ferroelectric thin film that use 3000 rpm also have given good results in energy gap, electrical conductivity, etc. From these data, they were succeed to be applied in an instrument or a device such as a blood sugar level, automatic drying, and luxmeter systems. In addition, these thin films also have potential to be applied in more sophisticated fields such as photodiode in satellite technology, temperature, and light sensors in the satellite technology, as well as solar cells for substituting conventional battery in satellite technology.