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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Laser stabilization in atom lithography based on LIF signal from the chromium beam

Tong Zhang, Yuejin Zhao, Cong Yin

Published Sep 29, 2016• 0 citations

Abstract

Atom lithography has now become an important means for manufacturing Nano-scale gratings, which are indispensable instruments for Nano-scale dimensional metrology. However, the presence of laser frequency drift makes the results unsatisfactory. Using the atoms’ laser induced fluorescence (LIF) to stabilize the laser frequency provides an effective method of overcoming the drift. This paper gives an analysis with regard to our own experimentation conditions in some aspects, such as fluorescence of Cr beam, difference signal, the correlation between laser frequency and the central of fluorescence. Based on the analysis, the laser induced fluorescence frequency stabilizing equipment is built in our selfdesigned system of chromium atom lithography. The laser is detuned near the wavelength of the 7S3→7P40 transition of 52Cr, so that the atom beam could be illuminated. The power of the laser we used is 15mW. The position of the fluorescent spot depends on detune of the laser. This spot is imaged onto a split- photodiode. Whenever the laser frequency exhibits some deviations from the desired value, the difference signal between the right and the left area of the detector is non-zero. The measured signal is used to servo-lock the laser. It is acquired by the servo-lock port of the Ti:sapphire laser. As a result, the laser frequency is stabilized in the wavelength corresponding to 7S3→7P40 transition of 52Cr. The stability is superior

DOI: 10.1117/12.2237099

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Atom lithography has now become an important means for manufacturing Nano-scale gratings, which are indispensable instruments for Nano-scale dimensional metrology. However, the presence of laser frequency drift makes the results unsatisfactory. Using the atoms’ laser induced fluorescence (LIF) to stabilize the laser frequency provides an effective method of overcoming the drift. This paper gives an analysis with regard to our own experimentation conditions in some aspects, such as fluorescence of Cr beam, difference signal, the correlation between laser frequency and the central of fluorescence. Based on the analysis, the laser induced fluorescence frequency stabilizing equipment is built in our selfdesigned system of chromium atom lithography. The laser is detuned near the wavelength of the 7S3→7P40 transition of 52Cr, so that the atom beam could be illuminated. The power of the laser we used is 15mW. The position of the fluorescent spot depends on detune of the laser. This spot is imaged onto a split- photodiode. Whenever the laser frequency exhibits some deviations from the desired value, the difference signal between the right and the left area of the detector is non-zero. The measured signal is used to servo-lock the laser. It is acquired by the servo-lock port of the Ti:sapphire laser. As a result, the laser frequency is stabilized in the wavelength corresponding to 7S3→7P40 transition of 52Cr. The stability is superior

To meet the requirement of nanoscale dimensional metrology, lengthy standards with features below 100 nanometers are indispensable instruments. Our group has successfully fabricated length standards through atom lithography. For further improvement of the quality of these standards, laser cooling of Chromium atom beam was studied through a transverse Doppler cooling scheme. Moreover, utilizing the software developing kit (SDK) of CCD camera, we developed an image collecting software, which had functions of real-time display for gray-scale image and image measure. In our experimental setup, with the software, we could detect the laser induced fluorescence spots from marginal beams to monitor and analyze the effect of laser cooling.

Chromium atom photolithography gratings are a promising technology for the development of nanoscale length standard substances due to their high accuracy, uniformity, and consistency. However, the inherent difference between the interaction of positive and negative frequency detuning standing wave field and the atoms can cause a difference in the adjacent peak-to-valley heights of the grating in positive and negative frequency detuning chromium atom lithography, which greatly reduces its accuracy. In this study, we performed a controlled variable growth simulation using the semi-classical theoretical model and Monte Carlo method with trajectory tracking and ballistic deposition methods to investigate the influence of key experimental parameters on the surface growth process of positive and negative frequency detuning atomic lithography gratings. We established a theoretical model based on simulation results and summarized empirical equations to guide the selection of experimental parameters. Our simulations achieved uniform positive and negative frequency detuning atomic lithography gratings with a period of 1/4 of the wavelength corresponding to the atomic transition frequency, and adjacent peak-to-valley heights differing by no more than 2 nm, providing an important theoretical reference for the controllable fabrication of these gratings.

By using of polarization spectrum, a laser frequency stabilization system for Cr atom lithography was designed. In order to eliminate the noise and improve the signal-to-noise ratio, a Lock-in amplifier was used. Calculation shows that the frequency discrimination signal is a purely dispersive signal. The energy sublevel distributions of Cr atom, which are important for frequency discrimination signal as the designed system would be used for Cr atom lithography, were also discussed. The nuclear magnetic torque of Cr atom is zero, which in turn has no influence on the energy level of Cr atom. Calculation shows that several other factors, such as the isotopes and the earth magnetic field, had little influence on the energy level of Cr atom, indicating that they could be ignored in the current experimental system. Some factors which will influence the line width and the linearity of the discrimination signal are also discussed.

In this paper,a size detecting system based on Laser induced fluorescence(LIF)is designed to measure the Cr atomic beam divergence of laser collimation by using one-dimensional transverse Doppler cooling scheme.Under the parameters of cooling laser power of 45mW,interaction distance of 24mm and detuning of-0.5Γ,we observe the angular distribution of Cr atoms(50%quantile) on the beam axis is from 1.07±0.10mrad to 0.46±0.10mrad by laser collimation.we also present detailed measurements of collimation angles over different laser powers and compare and analysis our results with numerical calculation based on the semiclassical model.

Background: As progress on the nanofabrication has made semiconductor developed rapidly, there is an increasing need in precise pitch standards to calibrate the structure of devices at nanoscale. Nano-gratings fabricated by atom lithography are unique and suitable to act as precise pitch standard because its pitch distance is directly traceable to a natural constant. As the scaling down of nano-devices, it is very challenging to double the spatial frequency of nano-grating while keeping the self-traceability in atom lithography. Methods: In this study, the switching-detuning light mask is utilized for Cr atom lithography. During a single deposition process, the standing wave frequency is switching from positive detuning to negative detuning alternatively. Results: Nano-gratings fabricated using switching-detuning light mask is successfully replicated with double spatial frequency and self-traceability. Non-uniformity between neighboring Cr lines shows up with a corrected pitch of 107.15 Conclusion: Non-uniformity is mainly caused by the dipole force discrepancy between positive and negative detuning light mask. Therefore, to increase the high uniformity of nano-gratings, the deposition time of negative detuning should be at least twice as the positive detuning. On the other hand, to reduce the pitch uncertainty, it is necessary to reduce the distance between the atom beam and reflection mirror as close as possible. These two significant optimization designs are promising to increase the spatial frequency doubling performance with high uniformity and accuracy.

A quantum mechanical Monte Carlo simulation is carried out for one-dimensional laser cooling of Chromium atoms. Chromium atom beam collimation via transverse laser cooling is essential for the fabrication of periodic nanometer scale structures by laser-focused atomic deposition. Such structures can be used as precision pitch standards for nanometer scale measurement and engineering. We explore the roles played by several factors, such as laser intensity and detuning, in the cooling process. Calculation results show different dependencies for cooling rate and ultimate temperature to be reached with given sets of parameters. Also, for atoms with different initial velocities in the transverse direction, the conditions under which optimal collimation effect is achieved can differ. Consequently, the current experimental setup, which utilizes LIF spots from marginal beams to monitor the laser collimation, can produce cooling effects which are not optimal for the end product.