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FYSZ460 Advanced Laboratory Exercise Electron Beam Lithography Spring 2010 Mikko Palosaari JYFL 1. INTRODUCTION This advanced laboratory exercise is an introduction to operation of Scanning Electron Microscope (SEM) and to Electron Beam Lithography (EBL). In its conventional use as a microscope SEM has many advantages to optical microscopes and has many important applications in different fields. Generally word lithography refers to a printing method and in this context to a method to fabricate very small patterns on a suitable surface. In industry the photo- or UV-lithography is most commonly used method but in basic research the electron beam lithography is widely used as it allows fabrication of very small structures. 2. PRACTICAL INSTRUCTIONS FOR THE LABORATORY ASSIGNEMENT The laboratory exercise will be completed in two parts: Before starting the work in laboratory each student will - Study the chosen literature and instructions and return the homework. The given material should be studied for the different process steps to be familiar already before starting the practical part. Especially the operation and the construction of SEM should be acquainted beforehand. -Design and draw a pattern with Elphy Quantum software. In laboratory the operation of SEM as an electron beam lithography tool and in the conventional use as a microscope is studied. The steps of basic lithography process are gone through as each student will recreate his/her design on a silicone substrate in gold. After the lithography process the created pattern will be inspected using SEM as a microscope. 2.1. The report Each student will write a report which will largely define the grade for the assignment. Good preparation, shown interest and independent working will also be valued. The report should include at least the following things: construction and operation of SEM, the different lithography process steps and their significance for the final outcome. Also answers for the questions given at the end of these instructions should appear as a part of the report. 3. ELECTRON MICROSCOPE The history of the electron microscopes starts already in 1930’s and has many interesting features. As the details of the history and the development can be easily found from internet sources let’s skip it here. An electron microscope is a device in which the light used in traditional optical microscopes is substituted with very thin beam of electrons. The electron microscopes are divided in two main categories: transmission electron microscopes (TEM) and scanning electron microscopes (SEM). According to the name in TEM the image is formed by electrons transmitted through a very thin sample. In SEM the beam of electrons is scanned over the sample and the image is formed by electrons scattered by the inspected area. Apart from method of forming the image the operational principle of both types of electron microscopes is similar. The electron beam is produced by a filament which is heated by electric current (thermionic emission gun). Two types of filaments are used: the hairpin shaped tungsten filament and the LaB6 (lanthanum hexaboride) crystal. (Also other types of electron sources do exist. Which?) The electrons emitted from the source are accelerated by high voltage towards the sample. The electron beam is focused by magnetic lenses and in SEM the scan coils are used to deflect the beam to scan over the sample. More about operation of SEM can be found for example in , , . Explain in the report the advantages of SEM (high resolution, high contrast and depth of field) and possible disadvantages (charging effects, aberrations, astigmatism and vibrations). Explain also the operational principle of the magnetic lenses. To get an impression of the resolution of SEM in comparison to optical microscope calculate the theoretical wave length for electrons (for example with acceleration voltage of 40 kV). Compare the result to the wave length of visible light (600 nm). 4. LITHOGRAPHY The lithography process consists of several steps and each step has importance to the final out come. In research and industry the most commonly used lithography techniques are the photo and electron beam lithography. Although for example X-ray and ion beam lithography are also being studied. In industry the photo lithography is most widely used method as it is fast (parallel exposure) and well controlled. The process often employs DUV light (Deep Ultra Violet). With the electron beam lithography the smaller line widths can be obtained but for industrial applications it is often too slow method (serial exposure). In research the EBL method is widely used because it's flexibility. Figure 1 shows the basic steps of the lithography process. e -beam patterning 2 -layer resist substrate metal coating metal develop lift-off Figure 1. The basic steps of the lithography process with positive resist. 4.1. Resists The resists have two purposes in the process. Firstly the resist has to react to the exposure (to UVlight, electrons,…) in such a way that the desired pattern can be copied to it. Secondly the resist layer has to protect the substrate underneath it. The name resist arises from the material’s capability to resist etching. Usually the resists consist of three components: the film forming polymer, the solvent and the component sensitive to radiation. Resists are also divided in two categories. The resist is called positive if the exposed areas dissolve more readily in the developer (Fig. 1). The negative resists behave in opposite way and after exposure the unexposed parts are more soluble to the developer thus it forms a negative image of the exposed pattern. The quality and characteristics of the resist affect highly to the success of the process. 4.3. The resist deposition The resists are deposited on a suitable substrate often by spin coating. Few droplets of resist is applied on the silicone chip and the chip is rotated around with high speed. This is done to obtain thin and uniform resist layer. The substrate is baked on a hot plate or in an oven to evaporate the solvent and so harden the resist layer. One, two and multilayer resists can be used for different applications The following recipe is used for the resist deposition: - 1. layer (bottom layer): 9% PMMA-MAA, poly(methyl methacrylate-co-methacrylic acid), dissolved in Ethyl Lactate, MicroChem MMA(8.5)MAA EL9 4000 rpm / 45 s, baking at 160 °C / 1 min. - 2. layer (top layer): 4% PMMA, polymethyl methacrylate, dissolved in anisole, MicroChem 950 PMMA A4 4000 rpm / 45 s, baked as previous layer 4.4. Exposure The exposure changes the (positive) resist so that after the exposed areas of the resist dissolve more readily in the developer. The exposure is also the step where different lithography methods differ from each other. The optical lithography is done by exposing the resist through a mask which has the desired pattern. The resolution of photo lithography is restricted by the wave length of light and the diffraction of light in the resist and in the mask and optics. While using the SEM for exposure the deflection coils are controlled by computer according to defined pattern. The patterning is done point by point (serial exposure) and is thus far slower method than photolithography. 4.5. Developing After exposure the silicon chip is immersed into developer which dissolves the exposed positive resist. As two layer resist is used also two developers are used: 1. developer: MIBK (methyl-isobutylketon) in IPA (isopropanol), ratio 1:2 2. developer: Ethyl Glycol Monomethyl Ether in methanol, ratio 1:2 By using two layer resist so called undercut profile can be formed. Undercut helps with the lift-off and is very useful for in fabrication of some more complicated structures. 4.6. Other process steps After developing the metal deposition can be done. The metal deposition is done in vacuum either by evaporating or sputtering chosen metal(s) on the sample. After metal deposition the chip is immersed in (heated) acetone to dissolve the remaining resist and to remove the extra metal on it. After this so called lift-off the drawn pattern is copied on the silicone substrate as a metallic structure (Fig. 1). Instead of the metal deposition also other process steps could be used, for example the drawn pattern could be etched on the surface of the silicon. Questions about (electron beam) lithography to think over for the report: What happens while spin coating and baking? Compare positive and negative resists How exposure is done? What happens during the exposure? What happens during development? Why does undercut –profile form and what are the advantages? How does vacuum evaporator work? How the thickness of metal layer is measured during evaporation? Compare EBL vs. photo lithography, advantages & disadvantages. APPENDIXES: 1. Homework LITERATURE:  JEOL JSM-840A scanning electron microscope (Figures: 3.2 General view of column, 3.3 Cross-section of column)  Invitation to the SEM World  P. Rai-Choudhury (ed.), Handbook of Microlithography, Micromachining and Microfabrication, Vol. 1: Microlithography, SPIE Press, USA / IEE, UK (1997). (pp. 142-163, 202-206) APPENDIX 1 HOMEWORK: 1. Find the data sheets for the resists to be used (MicroChem). Find the thicknesses of the resist layers spun with the parameters given in these instructions (see 4.3 Resist deposition). Describe the resists briefly. 2. What are the main steps of the electron beam lithography process? Describe each process step briefly. (Hint 1: start writing the chapter “Electron beam lithography” for the report, write headers and few sentences for each subject. Hint 2: Figure is very useful here.) 3. What happens during the exposure? 4. What are the main differences of the EBL and photo lithography? (Hint: What does serial/ parallel mean in this context?) 5. In SEM you have e-gun, studied sample and the monitor to observe the image but what happens in between OR how is the image formed in the SEM? Describe briefly. 6. Explain the terms: -depth of focus -astigmatism -positive resist / negative resist The internet sources and the references mentioned in the course website can be used to find information on SEM and lithography processes.