1. Determining the Index of Refraction of AlF3
Zoe Hughes
Mentor: Dr. R Steven Turley
MAIN IDEA: WHAT IS YOUR PROJECT ABOUT?
Good morning! My project is about determining the index of refraction of AlF3. My mentor is Dr. Turley, but I have also had a chance to work with Dr. Allred

2. Motivation
NASA’s Project: LUVOIR (Large UV/Optical/Near-infrared Telescope)
Aluminum oxidizes when exposed to the atmosphere
Why AlF3? The index of refraction of AlF3 in EUV spectrum is still unknown.
My project is find the index of refraction for AlF3 for EUV light
MAIN IDEA: WHY ARE YOU DOING THIS PROJECT?
REMIND PEOPLE WHAT IT IS ABOUT
This mirror will be for a NASA space telescope and needs to have optical properties for EUV light.
Aluminum is commonly used for mirrors but when it is exposed to the atmosphere it oxidizes, which affects the mirror’s ability to reflect EUV extreme ultraviolet light. Dr. Allred’s group is exploring new ways to coat the mirror to protect the aluminum. Dr. Turley’s group then will test the mirror’s optical properties.
One possible coating we are looking into is Aluminum Fluoride. My project this summer is to work with Dr. Turley’s research team and measure the index of refraction for Aluminum Fluoride for EUV light and ultimately determine if it is the right coating for the space telescope.
Also Aluminum Fluoride is interesting to look at because its index of refraction for EUV light is still unknown so this information would be intriguing information to know.

3. Reflectance vs. Angle for 45 nm
Methods
1. Measure reflectance vs Incidence Angle (Θ)
2. Fit Reflectance vs. Θ to find index of refraction for all wavelengths
Thickness
Index of refraction
3. Graph index of refraction vs. wavelength
Θ-2Θ fit:
Reflectance vs. Angle for 45 nm
Reflectance
Angle
We tested three samples:
Sample A:
6 nm
Sample C:
8 nm
Sample D:
3 nm

4. Initial Results: n= 1-δ + i𝛃
Comparison of all samples index of refraction vs. wavelength
(nm)
(nm)
MAIN IDEA: HOW DID YOU GET THESE GRAPHS?
WHAT DO THESE GRAPHS SHOW?
WHAT ARE THE NEXT STEPS?
So I left off telling you last time about the results from the data we took at the Advanced Light Source in Berkeley. We tested three samples and calculated the index of refraction using the model in mathematica. Here are the graphs that compare the index of refraction to the wavelength. All three graphs follow the same shape as you can see.
So after these initial results, the next steps are to:
Compare the results with the known index of refraction
Adjust the model if the analysis did not come out as expected
Check it with data collected at BYU
n= 1-δ + i𝛃

5. Compare with the CXRO data
(nm)
(nm)
MAIN IDEA: WHAT IS CXRO DATA AND HOW DID THEY GET IT?
WHAT IS THE DIFFERENCE BETWEEN THEIRS AND OURS?
HOW ARE THE GRAPHS?
So number one is compare with known data for the index of refraction. And we get this data from the CXRO which stands for The center of X-ray Optics. This website provides an X-ray database where you can look up the index of refraction of different compounds. Now the CXRO data about index of refraction for AlF3 is not done experimentally, like we are doing here.
CXRO calculated the index of refraction semi-empirically by using the properties of each compound separately. They use the relationship that the index of refraction is dependent on the density. They use another fluoride and the independent atom approximation to find AlF3 index of refraction. We expected the values to be similar for the shorter wavelengths and they are here, but for the longer wavelengths we expected to see our data diverge. It seems to diverge around 22nm for delta and for beta it is similar for the most part.

6. 2. Adjust Model: Using Predicted Thicknesses
Sample
Experimental Avg
(Ångstroms)
Dr. Allred's Prediction
Dr. Allred's 2nd Prediction A
57.42
60.9
62.02 C
78.602(Day 1)
88.9
93.54
(Day 2) D
36.7
43.62
MAIN IDEA: WHAT IS THE SECOND STEP AND HOW DID YOU CHANGE IT FROM THE FIRST?
WHAT DOES THE TABLE SHOW?
WHAT IS EXPERIMENTAL AVERAGE?
Next the second step is to adjust the model to see if we can improve the fit. The fit change we made was to replace the experimental thickness with the predicted thickness from Dr. Allred, which he calculated from ellipsometry that Stephanie and Spencer discussed.
As you can see from the table, the experimental thicknesses are not far off from the predictions for the most part. For sample C there are two values because we did measurements of it on two separate days. Surprisingly we got very different values for thickness on each day. This could be because we picked a weird spot on the mirror that day.
It is an experimental average because I averaged the thicknesses predicted from the model for all of the wavelengths in the sample. They are all smaller than the predictions.

7. 2. Adjust Model: Using Predicted Thicknesses
(nm)
(nm)
MAIN IDEA: HOW DO THE GRAPHS COMPARE TO THE CXRO DATA?
EXPLAIN WHY THEY LOOK THIS WAY
IS IT BETTER THAN THE FIRST?
Here are the graphs after the change in the model. Unfortunately, the data does not fit as well with the CXRO. The change made the samples not as similar to each other. Sample C on the second day is extremely off, but that makes sense since it was so far off of the predicted thickness. Sample A has the highest values followed by D and C for delta. For beta, A, C and then D for the longer wavelengths.
So we concluded that the fits with the estimated thickness are more accurate.

8. 2. Adjust model: Add a layer
First Model
Second Model
Vacuum
Vacuum
AlF3
AlF3
AlF2O
SiO2
SiO2
MAIN IDEA: HOW ELSE DID YOU CHANGE THE FIT?
WHAT IS THE REASON FOR THIS CHANGE?
There came up a concern that there was a leak that let more oxygen in while the samples were made. This created the possibility that there might be a second layer. The layer we originally thought was all AlF3, could be split into two layers Aluminum Fluoride and under it Aluminum OxyFluoride.

9. 2. Adjust model: Add a layer
(nm)
(nm)
Sample A Thickness: AlF3- 49 angstroms
Sample C Thickness: AlF3- 51 angstroms
MAIN IDEA: WHY ONLY SAMPLES A AND C?
HOW DO THE THICKNESS COME OUT/HOW DID YOU CALCULATE THEM?
HOW DO THE GRAPHS COMPARE TO PREVIOUS FIT?
This new model would only have made a difference for the thicker samples. Sample A and C were the thickest, A is around 6.202nm and C is 9.354nm. Sample D is the thinnest around nm. So I only refitted samples A and C. The fit with another layer was more complicated. We assumed the Aluminium oxyfluoride was around 30 angstroms and then approximates the value of the aluminum fluoride like the first model. The combined thicknesses of the AlF3 and AlF2O should add up to the total thickness of the first model.
Yet the thickness of the AlF3 for sample A averaged out to be 49 angstroms and sample C estimated at 51 angstrom.
These fits are different from the first model in the way that sample A now has much lower values for both delta and beta. Sample C also has lower values. Before the scale for delta reached 0.2 and for beta it reached 0.5.

10. 3. Check Data with BYU Data
Monarch
GrIM
He gas
High Voltage power supply
Reflectometer
Monochromator
Reflectometer
MAIN IDEA: HOW IS COLLECTING DATA AT BYU DIFFERENT?
Step 3 is to check our data collected at the ALS with measurements we take here at BYU. Ideally they should be the same because we are measuring the same sample. But before we can take data, i spent a lot of the time aligning the system. Dr. Turley recently moved into a new lab this summer, so I got to see how each instrument is setup. I helped with aligning the two monochromators GrIM and Monarch and the sample stage and detector for our reflectance scans. First we did a rough alignment with a HeNE laser and then we used the plasma. Once the alignment is set then we could start taking data.

11. 3. Check Data with BYU Data
Sample D at Wavelength: 30.4 nm
ALS
BYU
MAIN IDEA: WHY ARE THE GRAPHS SHAPED LIKE THIS?
HOW DO THEY COMPARE INDEX OF REFRACTION
So here in the lab we measured the sample D at two wavelengths 30.4nm and 25.6nm. We used the GrIM monochromator. So as the angle increases, the reflectance decreases. The shape of the first graph indicates that the beta to delta ratio is close to 1. This is because the curve goes straight down. We also did a log graph of the data to see the relationship more clearly. The data we took at BYU has higher reflectivity that the ALS data. The index of refraction : the ALS data has a higher n and a lower beta than the BYU.

12. 3. Check Data with BYU Data
Sample D at Wavelength: 25.6 nm
ALS
BYU
The same pattern can be seen for the 25.6nm

13. Our Experimental Data Compared to Bridou’s Data

14. Conclusion and Future work
Collected Data from ALS and BYU
Find why there is a difference in results
Publish a paper on Index of Refraction for wavelengths 49.5nm to 13nm
MAIN IDEA: WHAT HAPPENS NEXT WITH DATA?
WHAT IS LEFT UNFINISHED/MYSTERY?
WHAT WILL HAPPEN TO THE DATA YOU COLLECTED?

15. Acknowledgements NSF Grant PHY-1461219 Dr. Turley Dr. Allred
Margaret Miles

16. Questions
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