Plasma ablation by femtosecond lasers creates thin films with tailored structure through laser MBE, and isotopically enriched films through electromagnetic fields formed within the plasma itself.
Peter Pronko and Xiaoqing Pan
Ultrafast lasers have become valuable tools in applications relating to precision micromachining and other highly controlled material removal and cutting processes. Such worth is largely a consequence of their ability to remove material by plasma ablation in a way that can be decoupled from the thermal response properties of a material.1 It is now becoming recognized that these same attributes are of value when using such ablation plasmas as pulsed sources for thin-film growth by laser-based molecular-beam epitaxy (MBE).
Ultrafast pulse parameters affect plumesSeveral interesting and important characteristics of ultrafast-laser ablation plumes come to the fore when applied to atomic-layer growth processes. Electron temperatures, neutral atom densities, ion energetics, and particulate-free plumes are among some of these characteristics.2, 3 The common use of chirped-pulse amplification in the processing of ultrafast laser pulses further enhances the case for laser MBE by providing a means to independently adjust pulse duration. A range of pulse durations from 200 ps to 100 fs is commonly available for such work. Frequency-doubling and tripling techniques provide an opportunity to work with ultrafast-laser wavelengths in the range from near infrared (1 µm) to the near ultraviolet (276 nm).FIGURE 1. An epitaxial film of stannic oxide (SnO2) is deposited on R-cut sapphire using 100-fs, 780-nm laser pulses. A cross-sectional high-resolution transmission electron microscope (HRTEM) image of the film shows the material's single-crystal quality, as well as the atomically sharp interface between the substrate and the film (top). X-ray diffraction patterns confirm the film's single-crystal nature. An HRTEM image of another SnO2 film deposited onto (0001) sapphire using ultrafast pulses reveals the film's nanocrystalline structure (bottom). (Photo courtesy of CUOS)
The availability of a wide range of pulse repetition rates (10 Hz to 100 MHz) allows for investigation of growth properties at extreme ranges of pulse rate and associated maximum or minimum peak power densities. The growing variety of commercial femtosecond lasers has resulted in a greatly increased interest in this subject. Such access and versatility make it possible to investigate atomic-layer-controlled thin-film deposition for a host of applications and to compare these results with more conventional pulsed laser deposition (PLD) techniques. In the latter case, the normally used nanosecond pulsed excimer lasers result in thermal loading of the ablation target (causing particulates to form) and unavoidable optical pumping of the ablation plasma (acting to increase the mean thermal energy of the plasma).4 Further experimental work is needed to determine how important such differences between the effects of conventional and ultrafast lasers may become; however, early experiments are proving interesting and suggest an important role for ultrafast PLD.
Ultrafast-laser MBE system facilitates researchIn order to perform this type of ongoing work, an ultrahigh-vacuum laser MBE system has been set up at the Center for Ultrafast Optical Science (CUOS) at the University of Michigan wherein such opportunities are being explored. In one result, a stannic oxide (SnO2) film deposited on sapphire using ultrafast laser pulses has single-crystal quality (see Fig. 1). Growth rates of the films depend on factors such as oxygen gas pressure during deposition and whether the background gas is in an activated electrical discharge state. Microstructures ranging from high-quality epitaxial single crystal, textured film, nanocrystalline film, to nearly amorphous material can be formed depending on the choice of substrate and the deposition temperature. The SnO2 films grown on the (0001) sapphire (C-cut) substrate consist of nanosized [100]-oriented columnar grains (approximately 5 to 10 nm, depending on deposition temperature) with six different in-plane orientations. These studies indicate that ultrafast-pulsed-laser deposition is an appropriate method to synthesize high-quality SnO2 thin films with tailored microstructures and compositions.FIGURE 2. Isotopic enrichment on the central axis of copper plasmas occurs with use of 100-fs laser pulses for ablation (natural abundance is normalized to unity in the graph). The isotope ratio varies with the amount of charge carried by the copper ions in the plasma. The overall quantitative extent of this enrichment can vary by factors of two or more depending on circumstances.
A variety of characteristics in ultrafast-laser ablation plumes lend themselves to the successful application of this technique to atomic layer growth. For example, the electron temperatures in ultrafast-laser ablation plumes can be kept low in comparison to longer-pulse lasers. This effect again is associated with the fact that femtosecond laser ablation removes very thin layers of material with each shot and is capable of launching a very clean plasma plume that is essentially free of particulates. The nature of ionization processes and fundamental avalanche breakdown within the ablating material is largely responsible for these effects.5 Such advantages have been greatly exploited in ultrafast-laser micromachining and are valuable attributes for thin-film growth.
Isotope enrichmentIn another consequence of thin-film deposition, the spatial distribution of elemental isotopes can be systematically influenced within an ultrafast-laser ablation plume. The central cores of such ablative plasmas are observed to be highly enriched in the ionic component of the lighter isotope of an ablating element.6 This differentiating effect presents the intriguing possibility of growing films that are directly enriched in a specific isotope. It is well known in materials and solid-state science that control of the isotopic content of a material can significantly affect such properties as thermal conductivity, superconductivity, and other transport related phenomena resulting in important physical design consequences. In one example using copper as the ablated material, the ionic component of light isotopes on the central axis of ablation plasma is enriched (see Fig. 2).The phenomenon has been observed to work for elements up to mass 73 in the periodic table. It is hypothesized that a plasma centrifuge process is driven by the internal electromagnetic fields of the plasma itself. This interpretation is reasonably successful in accounting for the on-axis enrichment of the lighter isotope and also for the observed variation in angular distribution of the isotopic ratios about the central axis. For example, the angular distribution of such isotopic ratios varies accordingly for boron ablation plumes generated by 100-fs laser pulses on boron nitride (see Fig. 3). The increase of the heavier fraction with angle is consistent with a plasma centrifuge model if the assumption is made that the internal magnetic fields in the ablation plasmawhich are on the order of a megagauss near the ablation target surfaceare acting to form such an ionic rotational motion.7, 8
FIGURE 3. Isotope ratio varies as a function of angle for boron ablation plasmas generated by 100-fs laser pulses incident on a boron nitride target (vertical coordinate is normalized to the on-axis observed value in the center of the plane).
This model and the physics associated with it continue to be a subject of investigation at CUOS. Regardless of the specific mechanism by which the enrichment process occurs, the phenomenon appears to be a potentially interesting way to grow isotopically enriched thin films. Such films are of special interest to the semiconductor community, as thin-film circuits can dissipate larger amounts of operating power when thermal conductivities in the materials from which they are grown are enhanced through changes in isotope ratio.
Since the plasma centrifuge process would operate on the ionic component in the ablation plasma, it is necessary to generate and maintain such plasmas in an ionically dense form. Once again, the nature of ultrafast laser-pulse interactions in materials suggest that they will produce such ionically dense plumes as a result of the nonthermal electric-field-driven ionization processes involved. Special attention needs to be directed to keeping the plasmas ionized during the expansion process while they pass through the magnetic-field centrifuge zone. These details are an active part of ongoing research.
REFERENCES
- P. P. Pronko et al., J. Appl. Phys. 78, 6233 (1995); Optics-Comm. 114, 106 (1995).
- E. G. Gamaly et al., J. Appl. Phys. 85, 4213 (1999); 85, 4222 (1999).
- Z. Zhang et al., Proc. SPIE, 3935, 86 (2000).
- Pulsed Laser Deposition of Thin Films, D. B. Chrisey and G. K. Hubler, John Wiley & Sons, N.Y. (1994).
- P. P. Pronko et al., Phys. Rev. B, 58, 2387 (1998).
- P. A. VanRompay et al., Phys. Rev. Lett. 83, 2596 (1999); Proc. SPIE, 3934, 43 (2000).
- M. Krishnan et al., Phys. Rev. Lett. 46, 36 (1981).
- J. Briand et al., Phys. Rev. Lett. 54, 38 (1985); Phys. Fluids 30, 2893 (1987).
ACKNOWLEDGMENTS
The research presented here was performed at the Center for Ultrafast Optical Science (University of Michigan) with support and encouragement from its director Gerard Mourou. The following graduate students contributed research results: P. R. VanRompay, Z. Zhang, J. E. Dominquez, and W. Tien. We acknowledge John Nees for important technical contributions. This work has been supported in part by the National Science Foundation under Grant STC PHY 8920108.
PETER PRONKO is a research scientist and the associate director for industry liaison at the Center for Ultrafast Optical Science and XIAOQING PAN is an associate professor at the Department of Materials Science and Engineering, both at the University of Michigan, Ann Arbor, MI 48109; e-mails: [email protected] and [email protected].