Design and Fabrication of an Efficient Extreme Ultraviolet Beam Splitter

EUV Beam Splitter 1 Design and Fabrication of an Efficient Extreme Ultraviolet Beam Splitter First Semester Report Full Report By: Andrew Wiley Maram Alfaraj Prepared to partially fulfill the requirements for ECE401 Department of Electrical and Computer Engineering Colorado State University Fort Collins, Colorado 80523 Project Advisor: Mario Marconi Approved by: Mario Marconi

EUV Beam Splitter 2 Abstract The project is about designing and fabricating an efficient EUV beam splitter. The idea of the beam splitter is not a new idea; it has been used in optical systems for a long time. For example in devices as Mach-Zehnder interferometer which measures the phase shift between two different beams that went different paths. However the beam splitter that we are trying to fabricate in this project is difficult to design and fabricate because of the highly absorbent nature of extreme ultraviolet (EUV) light. This beam splitter is to be integrated into a laser system that has a beam of 46.9nm wavelength because every laser system needs its own design of beam splitter. The beam splitter is being designed using multilayer interference coatings technics. Simply the multilayer interference coatings technics consist of multi-bilayers pairs on top of each other. In all the bilayers we are using two different materials one is minimal absorption (Silicon) the other is highly reflective (Scandium). Both elements are specifically for this EUV wavelength. The thickness each pair of bilayer has to be half the wavelength (46.9nm). The reason is because if the bilayers thicker than half the wavelength then the multilayers become highly absorbent. This is the best technique because we need to get an efficient percentage of reflectance and transmittance to form the beam splitter. This can t be achieved with only one pair of bilayers due to the highly absorbent nature of EUV light. The multilayer that we fabricated for our beam splitter is working in a way but it is not highly efficient due to fabrication error. The testing results we got don t match our design expectations. It can be easily improved by improving the fabrication method. We will soon be looking at other methods of fabrication and test the results and compare them with what we currently have.

EUV Beam Splitter 3 Table of Contents: Title 1 Abstract 2 Table of Contents.3 List of Tables and Figures 4 1. Introduction.5 2. Summary of previous work..7 2.1 : Fabrication Method..7 3. Current Work..7 3.1 : Design of bilayers.7 3.2 : Fabrication of our prototype..11 3.3 : Testing and analyzing the results 11 3.4 : Validation of fabricated design 13 4. Conclusions.15 4.1 Future work.15 References 18 Appendix A- Abbreviations..19 Appendix B - Budget.20 Appendix C- Project Plan Evolution..21

EUV Beam Splitter 4 List of Figures Figure#: Title Page# Figure1: Schematic of Beam Splitter 5 Figure 2: Ion Beam Sputtering 6 Figure 3: Test design 6 Figure 4: Ion beam hitting sputtering target 7 Figure 5: Transmittance of Si at 17nm 8 Figure 6: %transmission, Si=17nm 9 Figure 7: %Reflected, Si=17nm 9 Figure 8: %Transmission, Sc=10nm 10 Figure 9: %Reflected, Sc=10nm 10 Figure 10: Prototype of our bilayers 11 Figure 11: Reflectance at an AOI of 10 12 Figure 12: Reflectance at a AOI of 40 12 Figure 13: Reflectance at a AOI of 45 12 14 Figure 14: Sample Plot from XRR Figure 15: XRR diagram 15 Figure 16: Our Measured data 15 Figure 17: Table of Estimated Thicknesses 16 16 Figure 18: Figure Showing Estimated Thicknesses Figure 19: Ellipsometer Design 17

EUV Beam Splitter 5 Figure 20: AFM design 17 Figure 21: AFM sample 19 Figure 22: Evaporator Deposition 19 Figure 23: Final Test design 19 Figure 24: Mach- Zehnder interferometer device 20

EUV Beam Splitter 6 Introduction: Beam Splitter Every laser system has multiple components beside the laser source. One of those components that is being used in multiple applications in the industry is the beam splitter. Also, every laser system is unique which means we need to design different components for each system depending on what wavelength we are working with. The Engineer Research Center at Colorado State University has an Extreme ultraviolet laser system of 46.9 nm wavelength yet they haven t come up with a beam splitter for it. The project task is to design and fabricate an efficient extreme ultraviolet beam splitter to work along with this laser system. This project is thought to be working best with multilayer design due to the highly absorbent nature of the very short wavelength it is using. The design will be discussed more in chapter 3. This project was started after researching and reading about the lithography and multilayer interference coatings for the team to get better understanding about the topic. This design is depending on the concept of the multilayers interference coating which is represented by the following Equation: 1. mλ=2d*sin(θ) m=1,2,3 λ (wavelength) d (thickness of the bi- layers film) θ (incident angle from normal) After reading and researching the team started using multiple different kinds of software which some of them was installed in the ERC and others that were online to design the beam splitter. After being done with the designing the team received training on the lathe, ellipsometer, sander and the laser system. Later on the team made multiple holders for the beam splitter. Figure1: Schematic of Beam Splitter

EUV Beam Splitter 7 We first fabricated a multilayer mirror that will act like the reflectance of the beam splitter. The difference being that the multilayer mirror is made from a Silicon substrate not a Silicon membrane. The Silicon substrate will absorb most of the transmitted light so this is why we can only look at the reflected light. The reason is because it is cheaper and easier to test the multilayer fabrication on a thick wafer before attempting the fabrication on a thin Si membrane. The method that we are using to grow these layers is called ion beam sputtering. Figure 2: Ion Beam Sputtering This system injects an ion beam (Argon+ for our project) that then collides with the target which is either Si or Sc. The element of the target is then broken off and is attracted to the substrate that we are growing on. This method is a very accurate way to grow our layers for our beam splitter. This will be discussed more in detail in chapter 2.1. Once we have a beam splitter/multilayer mirrors fabricated we have to test the reflectance and the transmittance. The way to do this is to put the beam splitter into the laser system to measure the input and output intensities. Figure 3: Test design The initial beam will be measured using an inline detector that doesn t damage the beam and lets it pass through mostly intact. Then the laser beam will then go through the beam splitter and the output intensities will then be measured by the photodiodes. Once we have this data we can then see the quality of our beam splitter.

EUV Beam Splitter 8 2. Summary of Previous work There has been very little previous work specifically towards our project. On the other hand there has been a lot work done on what kind of laser we are going to use and also the fabrication method. The pulse laser at the specific wavelength of 46.9nm was given to us. 2.1 Fabrication method The Process that we are going to use to fabricate our design is ion beam sputtering. This process uses an ion sputtering gas, in our case Argon+, which shoots the argon ions to the sputtering target (Figure 9). The sputtering target will be either Si or Sc depending on what we want to grow at that time. The Ar+ ions are attracted to the sputtering target because it is negatively charged. When the Ar+ ion hit the target ions of the Sc or Si and breaks away then is attracted to the charged substrate. We know how thick to make the layers by timing it. Figure 4: Ion beam hitting sputtering target This process is the best way we know right now to fabricate the layers. One reason is because since we are growing the layers an atom at a time we can avoid excessive diffusion between the layers. Also this produces a very little amount of surface roughness. 3. Current Work The current work that we have done so far: 1. Design of the bilayers to maximize the transmitted and reflected beam with minimal absorption. 2. Fabrication of our first prototype that should mimic the reflectance of the beam splitter 3. Testing and analyzing the results that we get. 3.1: Design of Bilayers

EUV Beam Splitter 9 For the beam splitter there are conditions that we need to follow. The elements that we are using for our bilayers were given to us based on previous work done at the ERC. We will be using Scandium and Silicon to make our bilayers. The reason we are Using silicon is because it lets light transmit with minimal absorption of the light. This is the same reason why the 100nm membrane will be made out of Si. Sc on the other hand is highly reflective at these small thicknesses. The trouble with using Sc is that if the layer is made to large then most of the light will be absorbed and not reflected. The thicknesses of our bilayers half to follow the equation mλ=2d*sin(θ), which comes out to be that the bilayers have to be half the wavelength of the laser pulse. For our applications the laser will be a 46.9nm laser which makes our thicknesses be approximately equal to 27nm. If we don t follow this equation then there will be destruction of the beam and no light will be reflected. Along with this there is a dependence on the AOI from normal; we are using an AOI of 10 degrees. There are some conditions that we need to worry about when we are designing our layers. One being that these bilayers has to be put onto a silicon membrane that is very fragile. So for our application we are limited to about three pairs of bilayers because any more will have a very good chance of breaking the membrane. To design this we need to use two different programs to look at the theoretical reflectance and transmittance. The program that was used for the transmittance was from the Center for X- ray Optics. This can take in a specification on the thickness and the element of the bilayer and show what amount of light will be transmitted. For example this is the percentage of transmitted light for Si at 17nm. Figure 5: Transmittance of Si at 17nm

EUV Beam Splitter 10 The problem with this program is that it can only calculate one layer at a time and have to do some calculations to get the true transmittance. The equation is: (Trans%Si Trans%Sc)!"#$%&!"!"#$%&'( Tans%Si@100nm The other program that we used was IDL at the ERC to measure what the reflectance would be. With this program we are able to look at all the bilayers at once, take into account surface roughness, diffusion between the layers, and the AOI. With both of these programs we are able to get enough data points to effectively design our beam splitter. 41 % of transmission vs. Sc thickness 40 39 % of transmission 38 37 36 35 34 7 8 9 10 11 12 13 Sc thickness[nm] Figure 6: %transmission, Si=17nm

EUV Beam Splitter 11 Figure 7: %Reflected, Si=17nm 43 % of transmission vs Si thickness 42 41 % of transmission 40 39 38 37 36 35 13 14 15 16 17 18 19 20 Si thickness[nm] Figure 8: %Transmission, Sc=10nm Figure 9: %Reflected, Sc=10nm From the graphs one can easily see that the best thicknesses for our layers is Si=17nm and Sc=10nm for 3 bilayers. So this is the design that we are going to use going forward. We expect the transmitted percentage to be 37.37% and the reflection percentage to be 42.5%. We have a backup plan if for whatever reason this design will not work. Our backup plan is 2 bilayers on top of a 200nm Si membrane with the Si and Sc layers at the same thicknesses.

EUV Beam Splitter 12 3.2 Fabrication and testing of our prototype Currently we are fabricating a prototype of our beam splitter that is using a Si substrate instead of a Si membrane. So this is a multilayer mirror not a beam splitter. This should give us good approximation of the reflectance of the actual beam splitter. There is a drawback to using a Si substrate as opposed to the Si membrane. It will make it unable for us to test transmittance because the substrate will only allow a minimal amount of light to be transmitted. But one of the benefits of using the Si Substrate is that it is a lot easier and cheaper to make. Also the substrate is stronger than the membrane which has a lower risk of breaking. This is what the bilayers on top the substrate looks like: 3.3 Testing and analyzing the results Figure 10: Prototype of our bilayers We are going to test our prototype just like we would for our beam splitter. The only difference being we will only be looking at the reflectivity. The basic idea is that we will be firing the pulse layer about 50 to 60 times to get a good spread of data. The beam will first go through a IDL to measure the input intensity of the beam. The beam would then impact the bilayer mirror and be reflected at an angle depending on what we have the AOI at. Then the light will hit the PD to measure the output intensity. When we plot the input intensities vs. the output intensities and then make a line of best fit. The equation has the form: Intensity of PD = P1 Intesity of ILD + P2 P2 from this equation is usually a very small value and can be neglected. From this equation we can find out what the reflectance would be by putting the values into this equation: Reflectance = Intensity of PD Intensity of ILD P1

EUV Beam Splitter 13 The output of this equation will then be plotted to see what the reflectance will be: Figure 11: Reflectance at an AOI of 10 Figure 12: Reflectance at a AOI of 40 Figure 13: Reflectance at a AOI of 45

EUV Beam Splitter 14 From these results we are getting that the reflectance is about 2% at a AOI at of 10 which is way off from our calculated result of around 42%. So from this we can see that something went way wrong with our fabrication. Some of the possibilities could be that the surface roughness was too high, there was a lot of diffusion, the layers are not at the correct thicknesses, or the Si substrate absorbed more light then we thought. But the plan now is to investigate and figure out what went wrong. 3.4 Validation of Fabricated design After we are done testing our product we will potentially, based on the results, go through some validation methods. The first test is to measure the thickness of one of the layers of the beam splitter; this is done using an X- Ray Reflectance (XRR). The next test is, using an AFM(Atomic Force Microscope), measuring the surface roughness of the layers because if it is too rough then the reflected beam will be at a low quality. XRR: The Process used to verify the bi- layer mirror or the beam splitter is called X- Ray Reflectivity (XRR). The basic idea behind this process is that an X- ray beam is reflected off of the sample to find the thickness and surface roughness of it. The reflected beam goes into a detector where the data is stored. As the AOI (Angle of Incidence) changes the reflected intensity changes and will look like the sample plot below. Figure 14: Sample Plot from XRR

EUV Beam Splitter 15 Figure 15: XRR diagram The ripples that are seen here comes from the fact that as the AOI changes the reflected beam will have constructive and destructive interference according to Braggs law. After the AOI has passed the critical angle then the thickness of the film can be estimated based on the distance between the peaks. Once the data was collected then the program will find the best fit model and estimate the thickness of all the layers in the beam splitter. This is a very accurate way to measure the thickness but for this project the error margin of the thicknesses is only ±1nm. At these low error margins even this process is still not accurate enough but this is the best we have. Using this process we were able to collect some data about our beam splitter. These show the reflected intensity of the XRR and the estimated thickness of the layers. Figure 16: Our Measured data

EUV Beam Splitter 16 Figure 17: Table of Estimated Thicknesses Figure 18: Figure Showing Estimated Thicknesses The optimal thicknesses for silicon and Scandium are 17nm and 10nm respectively. From the figures above, showing the estimated thicknesses, it can be seen that the fabrication process was not the best.

EUV Beam Splitter 17 Atomic Force Microscope (AFM): Figure 19: AFM design The AFM is a device that measures the surface roughness of a sample using resonant frequency s and can be accurate to fractions of a nanometer. The system works by as the tip gets closer to the sample surface a force is applied back onto the tip according to Hooke s law. The AFM can read this force and create a very good picture of what the surface of the sample looks like: Figure 20: AFM sample From this data we evaluate the quality of the surface roughness of the device. For this project the limit of the hills and valleys is no more than 7nm. This limit is another strict limit because if

EUV Beam Splitter 18 there is too much surface roughness then the reflected and transmitted beams will be at a very low quality. 4. Conclusions The beginning of this project included a lot of background research of the theory of the design. For example what is the purpose of the beam splitter or why we need to have our bilayers is half of the wavelength that we are using. The programs that we were able to use really showed some insight into what happens when we play around with the thicknesses of the bilayers and impurities of the device. We have been able to come up with a good design for our project and have learned a lot about optical electronics. We have many accomplishments to be proud about going through this project. One being that we have been able to successfully design a product that will work for the specifications we have. Another successes being the successful fabrication and a test of a prototype. Something that we were not successful at doing was to make a great prototype. The results can show that but we feel optimistic that we will be able to figure what went wrong and how to improve our design. Once we can find out what went wrong with our fabrication we will be able to fix it and make a better product. Some possibilities could be that it was user error when fabricating or there could be something wrong with our design. Once we are able to get a great product we can start making our photo diodes to do the testing. Then when we have those done we can do our final testing and show that our concept works. 4.1 Future Work With what we were able to accomplish this year has given us a good idea what we need to accomplish next year. Our goal is by the time E- days rolls around we will be able to have a final product that has been successfully tested. We are very confident that we will be able to accomplish this because of how far we were able to get this semester. We were unable to get a successful fabrication this semester because of fabrication errors but we still will be able to continue on with our work. We know we have a good design the only problem we have is the fabrication of the layers. One problem that we address is that maybe our fabrication method was not the best process. The alternative method that we will use to grow our thin films is evaporation deposition. This process uses a super- heated source that will heat our element until the point of evaporation. The evaporated Si or Sc will then rise up to our substrate or membrane and by timing it we will be able to grow our layers. This process will work but there are a lot of things that could go wrong. One being that this method has a tendency of creating a high amount of surface roughness which will change the reflectance and the transmittance. Also there is a high risk of diffusion between the layers which will also make a poor product. One benefit of using this method is that this is a cheap and easy way to grow our thin layers.

EUV Beam Splitter 19 Figure 21: Evaporator Deposition Once we able to fabricate a beam splitter that will work then we can do our final test. Our final test design will include the measure of transmission and reflection at the same time. First devise that needs to make is the two identical photo diodes. They need to be identical because the conditions for measuring the two output beams need to be the same or our results will be useless. Once we are able to fabricate both the diodes we will run our final test. Figure 22: Final Test design

EUV Beam Splitter 20 When we are done testing our Extreme Ultraviolet beam splitter and making sure that it meets all the design requirements and tolerances, we will be adding our EUV beam splitter to the laser system at the Engineering Research Center of Colorado State University. This beam splitter we are designing is going to be used in Wei s PhD research. Wei is going to use the Beam splitter that is being designed by our team for the Mach-Zehnder interferometer device that he is working on. The Mach-Zehnder interferometer device measures the phase shift between two different beams that went different paths. Below is a picture of the Mach-Zehnder interferometer device. Figure 23: Mach-Zehnder interferometer device

EUV Beam Splitter 21 References [1] "Atomic force microscopy". RetrievedDecember, 2014 Available: http://en.wikipedia.org/wiki/atomic_force_microscopy [2] Attwood, David, Soft X- ray and Extreme Ultraviolet Radiation principles and applications. Cambridge University PressFirst Edition ed, Vol. ;.1999. [3] "Mach Zehnder interferometer". Retrieved December, 2014 Available: http://en.wikipedia.org/wiki/mach%e2%80%93zehnder_interferometer [4] "Sputter deposition". Retrieved December, 2014 Available: http://en.wikipedia.org/wiki/sputter_deposition

EUV Beam Splitter 22 Appendix A Abbreviations: EUV: Extreme Ultra Violet Si: Silicon Sc: Scandium Laser: Light Amplification by Stimulated Emission of Radiation PD: photodiode ILD: Inline detector ERC: Engineering Research Center AOI : Angle of Incidence AFM: Atomic Force Microscope XRR: X- Ray Reflectance

EUV Beam Splitter 23 Appendix B Budget: Silicon Membranes: 280$ for a pack of ten XRR Lab: 35$ an hour

EUV Beam Splitter 24 Appendix C 9/17/14 Timelines: 4 Weeks Design and training on elipsometer 4-6 Weeks Fabrication of bi- layers 2-4 Weeks Testing 4 Weeks Redesign of bi- layers based on tests 4-6 Weeks Prefabrication of new designs 2-4 Weeks Testing (All times and projections are tentative and are subject to change) 9/29/14 Sept 26, 2014 Design and training on elipsometer Nov 7, 2014 Fabrication of bi- layers Dec 5, 2014 Testing Feb 13, 2015 Redesign of bi- layers based on tests Mar 27, 2014 Prefabrication of new designs Apr 24, 2014 Testing (All times and projections are tentative and are subject to change) 11/26/2014 Sept 26, 2014 Design and training on elipsometer Completed Nov 7, 2014 Fabrication of bi- layers Completed Dec 5, 2014 Testing In Progress Feb 13, 2015 Redesign of bi- layers based on tests NA Mar 27, 2014 Prefabrication of new designs NA Apr 24, 2014 Testing NA (All times and projections are tentative and are subject to change) 4/13/2015 Sept 26, 2014 Design and training on elipsometer Completed Nov 7, 2014 Fabrication of bi- layers Completed Dec 5, 2014 Testing Completed April 13, 2015 XRR and other testing Completed Feb 13, 2015 Redesign of bi- layers based on tests Unable to accomplish Mar 27, 2015 Prefabrication of new designs Unable to accomplish Apr 24, 2015 Testing Unable to accomplish (All times and projections are tentative and are subject to change)