Cort N. Johnson and Dr. David D. Allred, Physics and Astronomy
The extreme ultraviolet (EUV) optics group at Brigham Young University uses thin film technology to develop multiplayer mirrors and designed to reflect well at certain EUV wavelengths. We need methods of measuring layer thickness on the order of a few angstroms. One method often used in thin film sample analysis is ellipsometry.
Ellipsometric measurements analyze how the polarization of light changes upon reflection from a sample. The plane of incidence is defined as the plane traced out by the incident and reflected beam. Incident light can be polarized perpendicular to this plane, s-polarized, parallel to this plane, p-polarized, or a combination of both. An ellipsometer has a light source which passes through a polarizer before it reaches the sample. Thus the incident polarization is known. The polarization changes upon reflection from the sample. The ellipsometer has an analyzer which measures the intensity ratio and phase difference of p-polarized light to s-polarized light in the reflected beam. Both layer thickness and optical constants can be calculated from such measurements.
Our group currently uses an ellipsometer which makes measurements at 44 wavelengths simultaneously. These wavelengths are either in the visible of the UV. Since we design EUV optics, an EUV ellipsometer would be much more useful. We could directly measure optical constants in the EUV while we tested the reflectivity of our mirrors. While visible and UV ellipsometers are common, EUV ellipsometers are not sold. There are two reasons. First, development of a multiple wavelength EUV source is very difficult. Second, air absorbs EUV light, so the ellipsometer must be under vacuum.I proposed to use an existing femtosecond Ti:Sapphire laser system as an EUV source for an ellipsometer and to begin construction on a vacuum chamber for the ellipsometer.
The laser system was being revamped when I first started working on the project. A new pump laser had been purchased, so I helped prepare for its arrival and stallation by moving lab furniture and purchasing needed supplies. After its arrival I installed the needed plumbing for a nitrogen purge system for the pump laser cavity.
The existing laser system, which is maintained by Dr. Justin Peatross and Dr. Brett Hess, was already a reliable EUV source when I began this project. Infrared femtosecond pulses are generated by a Ti:Sapphire laser system and sent into a vacuum chamber. High harmonics up to the 100th order are generated by focusing the laser pules in an argon gas jet within the chamber. A low intensity counter propagating pulse boosts high harmonic production. Thus the fundamental pulse and many EUV pulses with the same polarization as the initial pulse exit the gas jet. A reflection grating spatially separates the various wavelengths and projects them onto a CCD camera to measure the intensity of the different harmonics.
I spent many hours reading literature on ellipsometric design. One common method uses a rotating polarizer and stationary analyzer while another rotates the analyzer and keeps the polarizer still. After consultation with Dr. Justin Peatross, we decided that the rotating polarizer was the most feasible way to build an ellipsometer from the existing laser system. A half waveplate in a rotating mount served as our rotating analyzer. Just before the femtosecond pulses enter the vacuum chamber we pass them through the waveplate. As the half waveplate is rotated, the polarization axis of the pulses continually changes between p-polarization, spolarization, and a combination of both. We hoped the reflection grating would serve as an analyzer. We measured the intensity of the harmonics as we rotated the half waveplate. Unfortunately, there was no noticeable change in intensity as we changed pulse polarization from p to s polarized. Thus the reflection grating cannot serve as an analyzer and with the current system we cannot make ellipsometric measurements.
However, it is still very useful to use the laser system EUV source to make pure reflectance measurements. The EUV optics group can currently only measure reflectance of their mirrors one wavelength at a time. The laser system EUV source contains pulses of many wavelengths which can be separated by the reflection grating. Thus reflectance at multiple wavelengths can be made simultaneously, cutting down substantially on data acquisition time. Dr. Justin Peatross received additional National Science Foundation this summer to hire two students to proceed with this project. They designed and built a new vacuum chamber which will make reflectance measurements at 45 degrees. This fall the first measurements should be taken. When successfully completed, further NSF funds will be sought to more fully develop and automate the system. Although ellipsometric measurements cannot be made now, the reflectance measurements will still be very useful to test the effectiveness of mirrors designed by the EUV optics group.
Further thought will be devoted to developing an effective analyzer for the system so ellipsometric measurements will one day be possible.