In the Laboratory
Hands-On Classroom Photolithography Laboratory Module To Explore Nanotechnology
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Scott J. Stelick* and William H. Alger Alliance for Nanomedical Technologies, Cornell University, Ithaca, NY 14850; *
[email protected] Jesse S. Laufer, Anna M. Waldron, and Carl A. Batt Nanobiotechnology Center, Cornell University, Ithaca, NY 14850
Photolithography, literally meaning light–stone–writing in Greek, is the process by which patterns on a semiconductor material can be defined using light (1). All nanometersized electronic components are made using this process. Alois Senefelder of Munich invented the basic principle of lithography, printing on stone, around 1798. It is based upon the notion that oil and water do not mix. So patterns can be made on a surface by drawing with an oily substance (like a crayon), and only where the oily substance was not present would a water-based ink adhere. You could also cover the entire surface scribbling with a crayon and then scratch away to “draw” your pattern. Craft people call this material scratch boards. The key is to draw with very fine resolution. Photolithography involves using energy (e.g., light or electrons) to change the solubility of a chemical, which is then “developed” to create a pattern. An image can be produced on a surface by drawing with light or electrons much the same way that you might scratch away the crayon on a scratch board. On the surface we apply a special chemical that is sensitive to light, a photoresist. The mask is a stencil that allows the energy to pass through only certain regions. So a pattern on a mask can be transferred to a surface by passing light or electrons through the mask. When the light or the electrons reach the photoresist on the surface it changes the solubility of the photoresist making it easier or harder to wash away. What is left is the pattern that was originally on the mask and is now transferred to the photoresist. The limitation in resolution is how small a beam of light or electrons can be produced. Lenses are used to reduce the image in a manner analogous to magnification except in reverse. During the exposure process, the resist undergoes a chemical reaction. Depending on the chemical composition of resist, it can react in two ways when light strikes the surface. The action of light on a “positive” resist causes it to become polymerized where it has been exposed to the light. A “negative” resist is the opposite and remains where it has been exposed. A positive resist, PRP 200, is used here so where the pattern is exposed, the resist is removed.
In commercial instruments, the substrate is exposed, then is moved or “stepped” to a different region. In this way a larger substrate can be patterned with multiple patterns. The layout of a basic 5X image reducer using a projection-based system in shown in Figure 2. The 5X reducer that was designed in this article uses the projection design type for transferring the image onto the substrate. Photolithography has been the workhorse of the nanotechnology industry for the past 20 years, and it continues to be a core technology used to fabricate microstructures and devices. Optics and chemistry are taught at the elementary, middle, and high school level, so there is a unique opportunity to introduce the optics and chemistry of photolithography to stimulate students’ interest in nanotechnology.
Experiment The 5X reducer in this article “projects” the mask image onto the photoresist-coated substrate. It uses a set of lenses and a light-emitting diode (LED) to transfer the reduced image (Figure 1). The 5X reducer was custom built from aluminum tubing and mill stock. Details of the construction are presented in the Supplemental Material.W The 5X reducer system uses one ultraviolet LED (UV LED) (Roithner Lasertechnik, Germany) at 380 nm as the excitation source and two lenses to project the image onto the slide. www.JCE.DivCHED.org
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Figure 1. Completed 5X reducer prototype.
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The learning goals of this activity are: • Students will gain an understanding of size and the nanoscale. • Students will gain an understanding of the role of optics in the shrinking of images. • Students will gain an understanding of the photochemistry involved in making nanoscale structures. • Students will gain an understanding of the basic concepts of photolithography.
These goals are supported by the objective of the activity, which is to create microscale circuits using photolithography and test the circuits using LEDs. Hazards The 5X reducer system uses low voltage so a shock hazard is minimal. Safety features have been installed to prevent accidental exposure of the UV light by the student. The vol-
Figure 2. Schematic of projection based 5X image reducer. Top LED exposes the mask and the second lens images it onto the substrate and reduces it by 5 times.
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ume of NaOH used in the development is small (around 200 mL) but care should be taken with the handling. The NaOH is caustic to the skin, so gloves and safety goggles should be worn at all times. Acetone is flammable and care should be taken when handling. PRP 200, the photoresist, is a vapor hazard and should be sprayed on in a fume hood. Gloves should be worn when using the acetone as well. We chose a spray-on photoresist, PRP 200 (2). It is commercially used for coating circuit boards and can be exposed to ambient room light with no effect. Slides coated in PRP 200 can be stored for several months without degrading. Results and Discussion The instrument was characterized to determine the resolution and image reduction. The minimum feature resolution for the 5X reducer system was measured to be 55 µm using the patterned photoresist. The droplet sizes of the photoresist emitted from the spray nozzle and formed on the surface of the aluminum layer limit the creation of any smaller features. The numerical aperture of the 5X reducer was calculated to be 0.052 (301.8-mm mask to lens and 28.6-mm lens clear aperture). The reduction ratio was calculated by measuring the dimensions of the features in the mask (transparency) and the developed photoresist. The width of the line in the photoresist was measured using an ALPHA STEP profiler that drags a needle across the surface of the resist. The reduction ratio was measured to be approximately 4.7 fold. Figure 3 shows the patterned PRP resist with an exposure of 90 s and a development time of 2 min. The edges of the resist are rough, attributed to the droplet size of the PRP photoresist on the surface and resolution of the pattern features on the transparency. For teaching applications, the resolution of the system is adequate to show the overall concept of photolithography. Features of approximately 100 µm are visible with the naked eye and a closer examination can be accomplished using a standard optical microscope.
Figure 3. Image of a wire in patterned PRP resist. Image of exposed line measured to be 492 µm wide.
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In the Laboratory
Aluminum can be etched using dilute sodium hydroxide and the remaining photoresist removed with acetone. Examples of the resultant patterned metal are shown in Figures 4A–C. The electrical properties of the patterned metal are measured with the LED and resistor circuit shown in Figure 4D. The resistance of the patterned metal is inversely proportional to the width of the lines. For line width of the lines ranging from 2.2 mm down to 0.36 mm the resistance of the lines were on average 30 Ω and 100 Ω, respectively. The 5X reducer system was tested as part of a learning activity “Microcircuits” with a group of 12 females in grades 7 through 9 attending an interscholastic event held at Cornell University. A schematic of the circuit is shown in Figure 5. The students were introduced to the topic using a PowerPoint presentation that is included in the Supplemental Material.W After this 15 minute introduction the students selected a “circuit” design and participated in the exposure, development, and testing activities. The entire process was completed by the 12 females in approximately 45 min using three 5X reducer systems. The students were given a brief post-activity survey to determine their comprehension of the topic area. Each question was multiple choice except for the
A
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1.5 VDC
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1 kΩ
Figure 5. Schematic of the LED and resistor circuit used to test the electrical properties of the patterned metal.
last question, which was fill in the blank. The results of that survey are presented in Table 1. Overall the students gained a good understanding of the core concepts of the lesson. Almost all of the students mastered the key concepts of nanotechnology that were reinforced in both the lecture and hands-on activity. These key concepts form the basis for understanding nanotechnology and more specifically the photolithographic process.
B
D C
Figure 4. Results of aluminum etching through patterned photoresist: (A) part of an etched pattern, dark is aluminum, (B) a simple electric circuit that was patterned, (C) a close-up of the same pattern, and (D) the completed electric circuit with LED and resistor attached with silver epoxy (note that reflection in the aluminum surface results in duplication of the circuit components).
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Table 1. Student Postsurvey Responses Question A nanometer is a ______________of a meter.
billionth
11/12
Photoresist is a chemical that is sensitive to ____________.
light
11/12
We use lenses to __________ the image five times.
shrink
11/12
Photolithography is like _________________.
photography
11/12
Conclusion The 5X reducer system and associated lesson plan was used to provide students with hands-on exposure to the basic principles of photolithography and microscale circuit fabrication. The instrument and the protocols have been developed to be used in a standard classroom. The students can design, create, and test their own microscale circuits with minimal supervision. Nanotechnology is an area of significant interest and can be used as a motivator for students in subject areas including physics, chemistry, and the life sciences. The activities would also be suitable for technology subject areas including electronics. With the 5X reducer system, it is now possible to demonstrate these concepts in instructional settings. Acknowledgments The work was supported in part by NYSTAR grant from New York State and by the STC Program of the National
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Science Foundation under Agreement No. ECS-9876771. We would like to acknowledge the use of the Cornell Nanoscale Facility (CNF), a National Science Foundation supported NNIN facility. The authors also thank Dan Woodie at the CNF for initially suggesting the use of the PRP 200 photoresist. W
Supplemental Material
Detailed information about the instrument design, notes for the instructor, experimental procedure for the students, postlab questions, and PowerPoint slides are available in this issue of JCE Online. Literature Cited 1. Madau, M. T. Lithography. In Fundamental of Microfabrication, 2nd ed.; CRC Press: Boca Raton, FL, 2002. 2. Information of the materials used can be found at http:// msds.ehs.cornell.edu/msdssrch.asp (accessed Jun 2005).
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