Technology Report Cite This: J. Chem. Educ. 2019, 96, 1268−1272
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Design, Fabrication, and Optical Characterization of a Low-Cost and Open-Source Spin Coater Mohammad Sadegh-cheri* Department of Laser, Institute of Science and High Technology and Environmental Sciences, Graduate University of Advanced Technology, Kerman, Iran
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S Supporting Information *
ABSTRACT: A spin coater is widely used for thin film coating in nano/microtechnology. In this paper, a spin coater with inexpensive mechanical and electronic components was fabricated based on an open-source Arduino microcontroller. To measure and control the spin speed of the spin coater, two sensor types including two infrared (IR) light-emitting diodes (LEDs) or a Hall Effect (HE) integrated circuit (IC) were used. The spin coater was tested for coating polydimethylsiloxane (PDMS) polymer in the spin speed range of 1000−9000 rpm (revolutions per minute). An optical interferometric method was used to determine the thickness and optical transmittance of the spin-coated PDMS films. The results show that the performance of this spin coater is similar to that of a commercial model and therefore it can be used in the laboratory and for student education. The total cost and power consumption of the spin coater are less than $30 and 5 W, respectively. KEYWORDS: Second-Year Undergraduate, Upper-Division Undergraduate, Chemical Engineering, Interdisciplinary/Multidisciplinary, Computer-Based Learning, Hands-On Learning/Manipulatives, Laboratory Equipment/Apparatus, Spectroscopy
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PDMS films were determined by an optical spectroscopy method. To validate the performance of the fabricated spin coater, the thicknesses of the obtained films were compared with those of a commercial spin coater (published in the literature).
INTRODUCTION A uniform thin film (TF) coating of both organic and inorganic materials by a spin coater is widely used in laboratories that work in the field of nano/microtechnology especially for the fabrication of microfluidic and lab-on-a-chip devices,1 organic light-emitting diodes (OLED),2 solar cells,3 transistors,4 and sensors.5 Commercial spin coaters can cost between $400 and $10k, depending on the number of their features. In addition to having a high price, commercial spin coaters need an expert for maintenance and repair. Therefore, using a low-cost and opensource spin coater with a similar performance to that of a commercial model in the laboratory for academic work and student education is desirable. An available, low-cost, and user-friendly electronic board like the Arduino microcontroller6 is a good candidate for the fabrication of laboratory equipment/apparatus. The Arduino microcontroller has recently come into wide use for data acquisition of sensors7−9 and fabrication of laboratory tools like syringe pumps,10 photometers,11,12 portable capillary electrophoresis,13 tensile testers,14 automatic titrators,15 electronic burets,16 3D printed four-point probe stations,17 visible spectrophotometers,18 magnetic stirrers,19 and potentiostats.20 In this work, an open-source and low-cost spin coater based on the Arduino microcontroller was fabricated for less than $30. The spin coater was used for coating polydimethylsiloxane (PDMS) polymer at spin speeds of 1000−9000 rpm. The thickness and optical transmittance of spin-coated © 2019 American Chemical Society and Division of Chemical Education, Inc.
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MATERIALS AND METHODS The wiring configuration of the spin coater for speed measurement by IR sensor is shown in Figure 1. The spin coater consists of an ATmega328 microcontroller (Arduino Nano v3), a brushless direct current (BLDC) motor, a servo tester, a liquid crystal (LCD 16×2) display, an electronic speed controller (ESC 12A), infrared (IR)/ Hall Effect (HE) sensors, and a power supply (12 V, 0.5 A). The speed is controlled by the knob of the servo tester by generating pulse width modulation (PWM) for the ESC. A potentiometer is used to control the backlight of the LCD. The schematic wiring and list of the materials along with their costs and suppliers are available in the Supporting Information (Table S1). In the spin coater, the brushless DC motor is a hard drive disk (HDD) (Maxtor brand). The HDD motor was selected due to its high efficiency, long life, high reliability, compact size, and low heat dissipation. For a given material, one of the most important parameters for the thickness of the thin film is Received: January 6, 2019 Revised: May 4, 2019 Published: May 14, 2019 1268
DOI: 10.1021/acs.jchemed.9b00013 J. Chem. Educ. 2019, 96, 1268−1272
Journal of Chemical Education
Technology Report
Figure 1. Electronic component setup of the spin coater for speed measurement by IR sensor.
syringe. PDMS films were coated on the substrates at spin speeds of 1000−9000 rpm with 1000 rpm steps. The coated PDMS films were baked in the oven at 80 °C for 30 min. The thicknesses of the spin-coated PDMS films were measured by using an interferometer (Avantes, Avaspec-2048-TEC).
the speed of the spin coater. The spin speed is measured by two IR LEDs or a Hall Effect (HE) integrated circuit (IC). The details of speed measurement are available in the Supporting Information. A chuck (rotor) was designed for sample holding. The chuck and the body of the spin coater were designed by Corel software. The detailed instructions for building the spin coater along with CAD files have been provided in the Supporting Information. An Arduino code was developed for displaying the speed and time on the LCD of the spin coater. The Arduino code used to run the spin coater has been provided in the Supporting Information. The final package of our spin coater is shown in Figure 2. To test the fabricated spin coater,
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RESULTS AND DISCUSSION An optical interferometric method was used to determine the thicknesses of the spin-coated PDMS films. The thicknesses were calculated by using the optical interferometric method as follows:21 d=
λ1λ 2 2(λ1n2 − λ 2n1)
(1)
where λ1 and λ2 are the wavelengths of two adjacent minima (maxima) in the interferometric spectrum and the constant values n1 and n2 are the refractive indexes of PDMS and glass, respectively. Figure 3a shows the optical interferometric spectrum of the PDMS film at 9000 rpm that consists of minima and maxima. Figure 3b shows the optical transmittance of PDMS films at five thicknesses (or five spin speeds). At the thickness of 3.67 μm (or the spin speed of 9000 rpm), the PDMS film has a transmittance of about 95%, and with an increase of the thickness the transmittance decreases. The PDMS polymer is spread on the substrate as a thin film due to the centrifugal force during the spinning of the substrate. The relation of the PDMS films and the spin speed can be determined as follows:22 d = kω−α
(2)
where ω is the spin speed and k and α are experimentally derived values. More precisely, the value of k depends on the concentration and viscosity of the material. In this work, the concentration and viscosity of the PDMS are constant in the spin coating process. Figure 3c shows the thicknesses of the PDMS films at spin speeds of 1000−9000 rpm which are compared with those of a commercial spin coater. Each sample is spun for 5 min. According to eq 2, by increasing the spin speed, the thicknesses of the PDMS films decrease.22 A graph is fitted to the experimental data to derive k and α values in eq 2. The derived values are compared with those of a similar work22 in Table 1.
Figure 2. Fabricated spin coater with time and speed controls.
PDMS polymer (SYLGARD 184) was used for coating on a microscopic slide (soda lime glass). After cleaning the substrate, using a double-sided adhesive (DSA) tape, the substrate was fixed on the chuck (rotor). The PDMS polymer was mixed with a ratio of 10:1 (base:curing) and after degassing was dropped on the substrate by using a 10 mL 1269
DOI: 10.1021/acs.jchemed.9b00013 J. Chem. Educ. 2019, 96, 1268−1272
Journal of Chemical Education
Technology Report
Table 1. Comparison of k and α Values Obtained from the Experimental Data in This Work and That of the Literature
a
Source
k
α
this work similar worka
19,897 ± 25 × 102 22,000
0.95 ± 0.02 0.98
See ref 22.
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CLASSROOM SETTING TEST The students in a classroom setting were asked to test the spin coater and investigate the results. Ten clean lamels (2 mm × 2 mm), a DSA tape, some concentrated grape juice, and a 10 mL syringe were given to the students. The students fixed each lamel on the rotor of the spin coater with the DSA tape. Using the 10 mL syringe, some concentrated grape juice was dropped on the surface of the lamel. The spin coater was run for 3 min at 1000 rpm. As was mentioned, due to the centrifugal force, the liquid concentrated juice is spread on the lamel. This experiment was repeated from 2000 to 9000 rpm with 1000 rpm steps. Figure 4a shows the photographs of the concentrated grape juice coated on lamels (the samples were placed on a white piece of paper, and the students took the photographs of the samples with their cell phones). The thickness or color of the coated samples decreases by increasing the spin speed. The difference of color (or thickness) at 1000 and 9000 rpm is clear. However, there is little color difference between the samples with 1000 rpm steps (for example, the samples with 6000 and 7000 rpm). For further investigation, using a free image process software such as ImageJ, the contrast of each pixel in each photograph was obtained. Each pixel has a contrast between 0 for black color and 255 for white color in grayscale mode. For example, the contrast of the photograph of a sample at 1000 rpm is less than that of one at 9000 rpm. The students were asked to open each image and investigate its gray mode. The details of the software are given in the Supporting Information. Figure 4b shows the grayscale contrast for 10 samples versus the number of pixels. As shown in Figure 4b, the thicknesses of all the samples are distinguished by the values of the grayscale contrast. This project enhances the hardware and software abilities of the students. In the hardware section, the students learn the working mechanism and driving of a BLDC motor like a HDD motor and the construction of an Arduino-based spin coater. The students can spin coat solutions and liquid polymers in laboratories for fabrication devices that need spin coating processes such as microfluidic devices, OLEDs, solar cells, transistors, and sensors. In the software section, the students enhance their computer programming skills by speed measurement of BLDC motor. Also by investigating the gray value of the images of thin film layers by the ImageJ software, they learn to distinguish the thickness of thin film layers by the naked eye and see the decrease of thickness of thin film layers with the spin speed (according to eq 2).
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Figure 3. (a) Optical interferometric spectrum of the PDMS film at 9000 rpm. (b) Optical transmittance for PDMS films versus wavelength for the sets (speed (rpm): thickness (μm)) including (1000 rpm: 27.6 μm), (3000 rpm: 9.34μm), (5000 rpm: 5.95μm), (7000 rpm: 4.52 μm), and (9000 rpm: 3.67 μm) (the highest optical transmittance (9000 rpm) is for the lowest thickness (3.67 μm) and the lowest optical transmittance (1000 rpm) is for the highest thickness (27.6 μm)). (c) Thicknesses of the PDMS films versus spin speed (the table shows the curve-fit parameters).
CONCLUSION A low-cost and low-power consuming spin coater based on the open-source platform Arduino microcontroller was fabricated. The total fabrication cost and power consumption of the fabricated spin coater are less than $30 and 5 W, respectively. The spin coater was tested for coating PDMS polymer at spin speeds of 1000−9000 rpm, and using an optical spectroscopy method the thicknesses and optical transmittance spectra of 1270
DOI: 10.1021/acs.jchemed.9b00013 J. Chem. Educ. 2019, 96, 1268−1272
Journal of Chemical Education
Technology Report
construction of Arduino-based devices and learn computer programming and optical spectroscopy.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.9b00013.
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Notes for instructors, including: materials list; wiring circuit; pin and wire connections; principles of spin speed measurement; fabrication of the spin coater; assembly instructions; testing; Arduino code (PDF, DOCX) CAD files and the Arduino code for the spin coater (ZIP)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Mohammad Sadegh-cheri: 0000-0002-9762-6629 Notes
The author declares no competing financial interest.
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ACKNOWLEDGMENTS This work (with Project No. 97.15) was supported financially by the Department of Laser, Institute of Science, High Technology and Environmental Sciences, Graduate University of Advanced Technology, Kerman, Iran.
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Figure 4. (a) Concentrated grape juice coated on a lamel at 1000 to 9000 rpm for 3 min. (b) Grayscale values of 10 samples from (a).
the PDMS films were determined. The results show that the performance of the spin coater is similar to a that of commercial one. Due to its light weight, compact size, lowcost, and good performance, this device is suitable for use in laboratory and hands-on-learning in a classroom setting. Students can easily fabricate and test the spin coater and enhance their hardware and software abilities in the 1271
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DOI: 10.1021/acs.jchemed.9b00013 J. Chem. Educ. 2019, 96, 1268−1272
