Optical characterization of Black 2.0 and its application in the optical projection microlithography device
Keywords: microfabrication, Optical Lithography, black paints, optical characterization, resolution enhancement
|OPML||Optical Projection Micro Lithography|
|LED||Light Emitting Diode|
|DOF||Depth of Focus|
|ABS||Acrylonitrile Butadiene Styrene|
|D.I water||Deionized water|
|CMOS||Complementary Metal Oxide Semiconductor|
|FZP||Fresnel Zone Plates|
Optical Lithography refers to the Lithography technique that uses UV or visible light to form a specific pattern on the wafer, covered with a photo reactive material called photoresist for fabricating or printing the circuits on the substrate by depositing or removing the pattern from the photoresist. Printing the image of the mask at the micro and nanometer scale can be made using the Micro and Nano Optical Lithography Techniques [1-3].
Types of Optical Lithography
Optical Lithography can be divided into three major types namely a) Contact aligner b) Proximity aligner and c) Projection aligner. Each of the types has its applications and limitations. In Contact aligner, there is no gap between the mask and the photoresist film and they both are in contact. In the Proximity aligner, there is a very small gap between the mask the photoresist film. These two types are used in researches and they are simple and cheap. The projection aligner is another type of Optical Lithography in which the image of the mask is projected on the substrate using the projection (convex) lens. In this method, the image size can be reduced by four times and it is very much useful in IC production and provides a longer mask life [4-5].
Resolution of the Image
The resolution of the image produced by the contact and proximity method is given by the equation,
where ‘g’ is the gap between the mask and the photoresist and ‘t’ is the thickness of the photoresist, ‘λ’ is the wavelength of the light used.
The Rayleigh resolution criteria for the circular aperture is given by the equation,
where, ‘λ’ is the wavelength of the light source and ‘NA’ is the Numerical Aperture, which is the ability of the lens to collect the light and is given by, Where, ‘n’ is the refractive index of the medium where the lens is placed and ‘α’ is the maximal half-angle of the cone of light that can enter or exit the lens. The generalized image resolution is given by,
where the ‘k’ represents the ability to approach the physical limits depending on the contrast of the resist, aberration in the lens, etc. To get higher resolution the value of ‘k’ and ‘λ’ needs to be decreased and the value of ‘NA’ should be increased. But this will reduce the Depth of Focus (DOF) which is one of the major issues in the optical lithography. The DOF is given by,
As the Resolution and the DOF are depending on the Numerical Aperture value, they can be increased by introduction a medium with the index of refraction greater than one and has a low optical absorption. By using the theoretical knowledge of Optical Lithography, a Customizable device was designed and fabricated using the 3D printer to print micro circuits in the laboratory.
The Black paints with very low Reflectivity have been used in Optics, Spacecrafts, Solar heaters, Telescopes, Coatings and Blackbody radiation[7-12],etc. Due to their high absorptivity, they absorb most of the light hitting them in the visible region. They are been used in the places where complete darkness or absence of light is needed. Black 2.0 by Stuart Semple, is one of the blackest paints commercially available in the market which was reported to absorb most of the light and has very low reflectivity. The Black 2.0 was characterized and compared with a few other black paints and also some experiments were carried out to verify the practical uses of Black 2.0 in some optical applications. Finally, the Black 2.0 paint was used to coat the inner walls of the OPML device to reduce the internal scattering of light inside the device, so that the image quality got enhanced and a high clarity image was produced.
To understand the optical properties of Black paints, the initial optical characterization was made. Acrylic black paint (Pedilite Fevicryl), Black Enamel oil paint (Asian paints), Black Acrylonitrile Butadiene Styrene (ABS) and Black 2.0 acrylic paint (Stuart Semple) were taken and a comparative study on their optical properties was done.
Absorbance study was carried out to check whether the black paint is completely black and also verified whether they absorb all the wavelengths of visible light. A very small quantity of black paint was taken and dissolved in suitable solvents like D.I. Water or Acetone and shaken well. Absorbance spectroscopy was done for all the Black paints by using the Perkin Elmer Lambda 750 UV/Visible/Near IR Spectrometer in the range 350 nm to 800nm in JNCASR Measurement Lab.
From the graph (Fig1), it was observed that all the black paints absorb all the wavelengths of the visible light and the value of Absorbance decreases with an increase in wavelength. Every Black paint except Black 2.0 absorbs the different wavelengths of light at different quantities. But the Black 2.0 absorbs every wavelength of the visible light almost at the same quantity as reflected by the absorbance plots and so, the Black 2.0 is perfectly black than other black paints.
The Reflectance of the black paints were also taken, to measure how much of the incoming EMR (visible light) is reflected by them. As the total intensity of light (I0) after striking a surface is equal to the sum of reflected (Ir), absorbed (Ia) and transmitted (It) intensities,
where, ‘I’ is the number of photons impinging on a surface per unit area per unit time. By knowing the value of the Reflectance, the value of Absorbance can be determined (consider the Transmission is negligible) as the complementary value of the Reflectance gives the value of Absorbance. To measure the Reflectance of the Black paints they are needed to be coated uniformly to avoid diffused reflection which may affect the values of reflection considerably. So, the following coating process was adopted so that the coating is quite flat and smooth.
A 2x2 cm2 glass slide was taken and soaked in soapy water, distilled water, Acetone, and IPA consecutively and kept in the ultrasonic cleaner for 3 minutes with each of them to remove unwanted dirt and dust particles from the glass slide. Then the glass slide was kept in a Plasma cleaner for about 5 minutes to remove the impurities and contaminants from the surfaces. After the plasma cleaning process, the glass slide was coated with the black paints with the spin coating method. The glass slide was placed in the spin coater and few drops of the black paint were dropped in the center of the glass surface. The spin coater was driven at about 500 rpm for the first 30 seconds and then the rpm was gradually increased to about 2500 and maintained in the same for about 3 minutes. After the spin coating process, the glass slide was placed inside the oven at 550 for 10 minutes to dry the paint in the glass slide. Then the glass slide is kept under a light source and checked for the transmission of light through the glass slide. To avoid the transmittance of light the paint was coated for the second time using the same procedure and dried. After the coating process, the Reflectance of the black paints are analyzed using Perkin Elmer Lambda 750 UV/Visible/Near IR Spectrometer and the Wavelength vs Reflectance graph was obtained.
Observations and Inference
From the graphical data (Fig4) it was observed that each painting reflects the incident radiation of light at a different amount. Acrylic Black paint reflects about 13% of the light and hence it is absorbing nearly 87% of the incident light. Likewise, the Black Enamel Oil Paint Reflects about 8% of the light and absorbs 92% of incoming light. The Black 2.0 Reflects only about 3 to 4 % of the light and so it is absorbing 96-97% of the light and hence we can conclude that the Black 2.0 is the Blackest paint among these three.
EXPERIMENTS USING BLACK 2.0
From the obtained results of Spectroscopic studies, it is clear that the Black 2.0 absorbs most of the light and reflects a very small amount of light, it can be used in many optical applications where a surrounding complete Darkness and complete absorption of light is needed. So, some basic experiments were done using Black 2.0 to find its industrial, laboratory and research applications.
Sphere for Detecting Reflected Light
A hollow sphere with the radius of 30mm and thickness of 3 mm is designed and 3D printed with Black ABS plastic, in which a power LED is placed in one end of the sphere and a photodiode, which acts as a detector is placed in another end at 900 from the LED as shown in the (Fig4). A solid light Blocker with a 10mm height and 3mm thickness is placed near the Detector which ensures that none of the direct light hits the detector. The LED is connected to a Source meter and the detector is connected to a multimeter that was set at the 2V range (DC) to measure the photovoltage. The LED connected to the source meter is driven at a constant voltage of 3.2V throughout the experiment and the value of input current is changed from 1 mA to 100 mA at 1 mA increment in each step. First, the detector is checked for the zero reading when the source meter is OFF. After ensuring the zero reading from the detector the input current given to the LED was increased from 1 mA-100mA and the corresponding photovoltage produced by the photodetector is noted. Then the inner wall of the sphere is coated with Black 2.0 and the same experiment was repeated. The Input current vs Output photovoltage graph is plotted and analyzed for both the experiments (Fig6).
From the graph, it was observed that without any of the coating, the ABS plastic reflects more light and so we get higher output photo voltage from the detector. But after the coating of Black 2.0 in the inner wall of the sphere, the reflection of light by the inner wall reduces considerably as the Black 2.0 absorbs about 96% of the light that hits it. From the graphical data, we can conclude that for the same amount input current the output photo voltage is very less for the sphere coated with Black 2.0, as it absorbs most of the light.
Sphere and Cylinder for Detecting both Direct and Reflected light
The same experiment (Exp 3.1) was done with the detector positioned directly opposite to the power LED so that it can capture both the direct and reflected light. This experiment was done to verify the previous experimental results and also to check how the reflected light is affected by the shape. So, the experiment was done with a Hollow sphere (Fig7.) and a Hollow cylinder (Fig8.) and obtained results were plotted in a graph (Fig9.) and (Fig10.).
Observation and Inference
From this graphical result, it was very clear that the Black 2.0 absorbs the light hitting its surface and reflects only a small quantity of the light, which agrees with the previous experiment. The experiment using the hollow cylinder with and without the Black 2.0 coating provides the data, from which it can be observed that the value of photo voltage is decreasing considerably with Black 2.0 coating in a cylinder on comparing with the sphere. So, we can use Black 2.0 to coat the materials to decrease the scattering of light.
DESIGNING AND DEVELOPMENT OF DEVICE FOR OPML
After knowing the theories and Resolution enhancement techniques of Projection Optical Lithography, a device for OPML was designed using Autodesk Inventor Professional Software and printed using the PP3DP 3D printer. The components of the device were designed and printed in such a way that it could be integrated to make a complete device and also customizable, and easy to use.
A movable LED holder rod (Fig11.) was designed to hold the LED which will act as a source and can slide inside the Lens Holder freely and can be fixed at the desired position. It was designed in a way so that a bulb of power LED could exactly fit inside the hole. And also, we can attach a mask in front of the LED by which the shape of the effective light source could be changed which will enhance the resolution of the mask (image) in the photoresist.
The Lens holder (Fig12.) was designed in which the LED holder can slide through it from one end and also it can hold the convex lens at the other end using screws. After fixing the lens in the lens holder, the position of the LED holder can be adjusted front and back, so that the light source will be at the focal point of the convex lens by which the diverging light from the LED will be collimated. After keeping all the components in the desired position, they are fixed tightly and the degrees of freedom are controlled by screwing it with screws.
A mask (printed pattern in transparent sheet) can be placed inside the mask holder (Fig13.) and after that, the Lens holder along with the lens can be fitted inside the mask holder and tightened using the screws. Now the collimated light from the convex lens strikes the mask and the opaque regions in the mask block the light, while the transparent region of the mask will allow the light to pass through. This light can be converged using another converging lens.
Converging Lens Holder
For converging the collimated light from the mask, COMPUTAR Bio imaging systems H6Z0812 camera lens was used and a Lens holder (Fig14.) was designed to hold the lens perfectly as well as the holder can be fitted inside the mask holder. This lens provides manual adjustment of focal length (8 to 48mm) and also has an adjustable aperture so that the amount of the light from the mask can be adjusted.
After 3D printing the device, the inner walls of the device were coated with Black 2.0 to reduce the light scattering inside it. And after assembling the components of the device, a mask of 2mm width and 30mm length printed with 1200 dpi resolution was placed inside the device and the white LED source was illuminated and the projected image of the mask was obtained. The focal length and the Iris of the camera lens were adjusted so that the projected image of the mask was well focused on the CMOS camera. After focusing the image in the camera, the image pattern was checked to find any of the deformities and distortions. After getting the clear image on the camera the distance between the projection lens and the camera was determined and now the camera can be replaced by the substrate with photoresist so that the clear miniature image of the mask can be patterned in the photoresist (Fig17)
Fresnel Zone Plates (FZP) with the outer most circle’s diameter of about 20mm was designed and printed in a transparent PET sheet and was used as a mask. By using the OPML the image of FZP (mask) was projected on the glass substrate with the negative photoresist using a UV LED for about 10 minutes. After the UV exposure, the photoresist is cured by using the Developer Solution (Sodium Carbonate). The patterned image formed on the photoresist was about 6mm which is 4x demagnification (Fig18.). So, a smaller image can be obtained using the OPML.
RESULTS AND DISCUSSIONS
After capturing the projected micro-image of the mask using the CMOS camera, it is seen that there were no considerable deformities and the image obtained from the mask is crisp and clear. The experimental results, the size of the mask can be reduced four times its original size without any distortion using this method. This experiment could be done using a UV LED source of the wavelength 365nm, which can be projected in the photoresist and can be cured using a developer solution. As the resolution of the pattern will increases for source with higher frequency, better resolution can be obtained by using the UV or Extreme UV sources. The modified light source with lower wavelength, Phase shifting mask, Medium with higher Refractive index and Lens with greater Numerical Aperture value (NA) can be used in this device to get an enhanced image with higher resolution.
In this project, characterization of Black 2.0 and few other black paints were done and from the results, it is concluded that the Black 2.0 is one of the Blackest paints which has high absorptivity, as it absorbs almost 97% of the incident light and has very less reflectivity. Then, Black 2.0 was used in some experiments by which the practical application of the Black 2.0 was seen and observed. Finally, A customizable device was designed and developed with various components along with a Black 2.0 coating, which can be very useful for the Projection Micro-Optical Lithography and hence very small electronic circuits and patterns can be easily printed in the laboratory in a cost-efficient manner.
In the future, the efficiency and customizability of the device can be improved by using some Resolution Enhancement Techniques. Usage of many lens systems inside the device can increase the image quality of the mask and hence the image with a higher resolution is obtained. Multiple condenser lenses along with spatial filters can be used to double the resolution. An attachment can be designed, inside which some transparent materials with a higher refractive index like water, glass, oil, etc. can be filled and attached to the device which will increase the Resolution. Coating the walls of the device with material Blacker materials than Black 2.0 like VANTA black (absorbs almost 99.96% of the light) may completely block the light scattering from the walls of the device. The pattern in the mask can be printed using completely opaque black ink so that the transmittance of the light through the ink can be prevented which will produce a sharper image of the mask.
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It is my immense pleasure to convey my gratitude to my research supervisor Prof. K. S. Narayan for his invaluable guidance, continuous support and great encouragement in various ways and for allowing me to use his Laboratory throughout my Summer Internship.
I would like to express my hearty gratitude to Mr. Anaranya Ghorai for his motivational words in hard times and for the help and guidance in all my experiments.
I would like to thank Mr. Sinay Simanta Behera for introducing me to the chemical room and for giving me an overview of every machine in the Molecular Electronics Lab. I extend my gratitude to Mr. Ganesh for introducing me to the optics lab and for teaching me about the usage of optical components.
I would like to thank Mr. Sumukh Anil Purohit and Mr. Manish Tiwari for teaching me the usage of Adobe Illustrator and Autodesk Inventor software, and for helping me with the 3D printing for teaching me about the spectroscopy.
I would like to thank Mr. Manvendra Singh for his great help and for teaching me about the cleaning process, chemical usages, and chemical equipment.
I would like to thank Mr. Abdul Azeez for teaching me about solar cells and for creating an interest in organic solar cells.
I would like to thank Mr. Gaurav Dhopeshwarkar for teaching me the programming languages.
I would like to thank my colleague and best friend Mr. Abhilash for helping me with my experiments.
I would like to convey my gratitude to my Lab seniors Mr. Anil Krishna, Mr. Deepak, Mr. Raaghesh, Ms. Sukanya Das, Dr. Prashanth Kumar, Dr. Suman Banerjee and Dr. Ravi for their support, pieces of advice and for spending their great time with me.
I would like to convey my sincere thanks to Mr. Rajakumar, Mr. Sunoj and Mr. Vasu for helping me in the workshop and in the Measurement Lab.
I am very grateful to Jawaharlal Nehru Centre for Advanced Scientific Research for its beautiful infrastructure and facilities which gave me a fruitful experience.
I am forever indebted to my College Management, Secretary, Director, Principal, Vice Principal and all my Professors for permitting me to attend this Summer Internship.
I would like to express my hearty thanks to our Dean Dr. S. Madhu, HoD Dr. M. Karnan and my professors Mr. R. Nirmal Kumar and Mr. R. Vijayakumar for their motivation and guidance.
I would like to thank my colleagues in Bengaluru, my classmates and all my friends for making this summer internship as a pleasant experience and I would like to convey my special thanks to my friend Mr. S. B. Raman for his Moral support and helps in my hard times.
Especially, I would like to thank the Indian Academy of Sciences for providing me this golden opportunity which helped me to gain a lot of experience and knowledge.
Finally, I would like to thank Almighty for blessing me with this wonderful opportunity and above all, I would like to thank my parents, my brother for their continuous support and encouragement.