Ultrafast laser annealing of thin films

Abstract

A method for locally annealing and crystallizing a thin film by directing ultrashort optical pulses from an ultrafast laser into the film. The ultrashort pulses can selectively produce an annealed pattern and/or activate dopants on the surface or within the film.

Claims

1. A method for selectively activating dopants in an amorphous thin film, comprising the steps of: directing one or more laser pulses having a sub-picosecond pulse width at a predetermined location or plurality of locations on a surface of an amorphous thin film situated on a substrate, each of the laser pulses having a predetermined laser wavelength, pulse width, pulse energy, pulse repetition frequency, excitation spot size, beam profile, and scanning rate; wherein the wavelength, pulse width, pulse energy, pulse repetition frequency, excitation spot size, beam profile, and scanning rate of the laser pulses produces a predetermined absorption of energy from the laser pulse into the amorphous thin film without absorption of the energy into the substrate; and wherein energy from the laser pulses is deposited into the amorphous thin film and locally thermalizes the amorphous thin film so as to activate dopants within the amorphous thin film in at least one predetermined area in the amorphous thin film without thermal diffusion to other areas of the film.

2. The method according to claim 1, wherein the material is amorphous VO.sub.2 with dopants dispersed in the amorphous phase; wherein the laser pulses produce a first doped region in the material and a second doped region in the material, the first region being spatially separated from the second region and having a phase transition temperature different from a phase transition temperature of the second region.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A-1D illustrate aspects of thermal annealing of VO.sub.2 films in accordance with the prior art.

(2) FIGS. 2A-2C are atomic force microscopic (AFM) images showing the results obtained for thermal annealing VO.sub.2 at different O.sub.2 partial pressures.

(3) FIGS. 3A, 3C, and 3D are images illustrating the results of millisecond-pulse laser annealing of amorphous VO.sub.2 films in accordance with the prior art. FIG. 3B is Raman spectroscopy of regions in FIG. 3A showing crystalline regions of VO.sub.2 or V.sub.2O.sub.5 produced in this annealing process.

(4) FIG. 4 is a block schematic illustrating the time evolution of “hot” electron thermalization produced by a sub-picosecond laser pulse used to achieve ultrafast laser annealing of a thin film in accordance with the present invention.

(5) FIG. 5 is a block schematic illustrating the time scale of ultrafast laser annealing of a thin film in accordance with the present invention versus other processes.

(6) FIG. 6 is a block schematic illustrating how the ultrafast laser annealing of a thin film, in accordance with the present invention, can be used to pattern the surface of the film.

(7) FIGS. 7A-7C further illustrate aspects of patterning of VO.sub.2 thin films using ultrafast annealing in accordance with the present invention.

DETAILED DESCRIPTION

(8) The aspects and features of the present invention summarized above can be embodied in various forms. The following description shows, by way of illustration, combinations and configurations in which the aspects and features can be put into practice. It is understood that the described aspects, features, and/or embodiments are merely examples, and that one skilled in the art may utilize other aspects, features, and/or embodiments or make structural and functional modifications without departing from the scope of the present disclosure.

(9) The present invention provides a method for using an ultrafast laser that emits sub-picosecond optical pulses to locally anneal and crystallize thin films within nanoseconds or faster. Since this laser technique performs localized annealing and crystallization, this method enables patterning of thin films with the ability to achieve sub-micrometer resolution. In addition, this method also enables annealing thin films on substrates that cannot handle traditional growth and annealing temperatures. Finally, this ultrafast annealing technique can be conducted in atmosphere without chambers or ambient gas control, which is required many thermal annealing processes.

(10) Unlike other laser annealing techniques which use Excimer and ultraviolet lasers, the method of the present invention can use near-infrared lasers, which can avoid absorption of the laser energy in the substrate material, and thereby enable efficient transfer of laser energy to the thin film to be crystallized. In addition, instead of the longer pulse durations used by Excimer and other lasers, the sub-picosecond pulses used in the method of the present invention produce rapid energy excitation of the sample.

(11) In the present disclosure, annealing in accordance with the present invention is described in the exemplary context of annealing an amorphous VO.sub.2 film so as to produce a crystalline VO.sub.2 thin film. However, one skilled in the art will readily recognize that the annealing principles and techniques described herein can be applied to annealing of other amorphous and non-amorphous materials within the scope and spirit of the present disclosure.

(12) In accordance with the present invention, a series of sub-picosecond laser pulses are applied to the surface of the thin film to be annealed.

(13) Typical laser parameters depend on optical and thermal properties of the materials and substrates used.

(14) For example, to avoid linear absorption in a silicon substrate a user ideally would want to use an excitation energy less than the bandgap energy of silicon (˜1.1 eV, or 1100-nm wavelength).

(15) In addition, ideally the user would choose an excitation energy where the cross-section for multi-photon absorption was greater in the film material than in the substrate.

(16) Along with the materials' absorption cross section, the incident laser pulse fluence will also impact how much energy is deposited in the materials, and this energy must not exceed material damage thresholds.

(17) Parameters such as the laser's pulse repetition rate will depend on the deposited energy density as well as the thermal properties of the film and substrate. A pulse repetition rate that is too high may produce a local temperature increase which does not decay between pulses, and make this annealing process similar to that of the current long-pulse annealing techniques. However, in some instances, depending on the material, a nominal rise in the local temperature may also be a catalyst for ultrafast annealing through modification of materials' physical properties.

(18) The ultrafast sub-picosecond laser pulses are absorbed into the electronic system of the sample through a process of linear and nonlinear multiphoton processes which take place in less than a picosecond. This is unlike other laser annealing processes which use longer (mostly ns- and ms- time-scale) laser pulses and interact with the samples via linear absorption. In addition, other laser annealing techniques use multiple laser beams and also target absorption by the substrate as an indirect thermal heating mechanism. In contrast, use of sub-picosecond laser pulses in accordance with the present invention enables rapid athermal and thermal processes to occur and further enables nonequilibrium interactions between the laser pulse and the sample to crystallize and anneal the amorphous material.

(19) The annealing process resulting from nonlinear absorption interactions can cause the annealed area to be smaller than the laser beam area. For example, an optical pulse with a Gaussian spatial profile will have the nonlinear absorption concentrated near the peak of the spatial distribution, and thereby reduce the absorption area compared with the area of the optical pulse.

(20) Due to its combination of athermal and thermal responses, the sub-picosecond laser pulse used in the annealing process in accordance with the present invention also produces a smaller heat-affected zone, and therefore the interaction (heating/annealing) with surrounding regions is dramatically reduced. As a result, by focusing the laser on a specific location on the film surface, sub-micron-scale annealed features can be produced.

(21) For thicker films, nonlinear optical absorption is highly dependent on the optical intensity, so that focusing the laser below the surface enables large laser absorption in sub-micron volumes below the surface of the material, enabling the formation of “channels” of crystalline material within a bulk object. Laser pulse parameters for achieving this depend on the linear and nonlinear optical properties of the thicker film materials, such that there is minimal linear absorption thereby locally restricting the absorption to the neighborhood of the focused laser.

(22) These ultrafast laser pulses interact with the sample and deposit their energy at sub-picosecond time scales. This enables nonequilibrium electronic states which produce “hot” carriers that have a larger kinetic energy than they would in equilibrium. These hot carriers can be rapidly ionized, and their rapid ionization produces energetic interactions with atoms and their bonds, which at higher laser excitation intensity can rapidly produce spatially-complex highly-ionized regions that lead to Coulombic explosions. Even for energies far below that needed for Coulombic explosion, the rapidly-ionized nonequilibrium electronic states can modify surface states with enough energy to break bonds with adsorbed species, enable dielectric screening, or polarize the material all within picoseconds, and long before thermal processes begin.

(23) The ultrafast laser pulses used in the annealing process in accordance with the present invention have a high spatio-temporal photon density, which can create strong nonlinear photon effects. In an exemplary case, a continuous wave (cw) laser having an average power of 1 mW and a spot size of 1 μm will generate approximately 200 photons per each 50 fs period. The same 1-mW average power from a pulsed laser emitting 4 nJ pulses having a pulse duration of 50 fs at a 250 kHz repetition rate will generate approximately 2.sup.10 photons for each 50 fs period. As these photons are absorbed by the thin film, a large nonequilibrium distribution of carriers forms in the thin film. Because large numbers of these carriers thermalize via electron-electron scattering before they relax, the laser pulse produces billions of energized electrons having electron kinetic energies of 0.2-2 eV and electron temperatures of 1,000 to 24,000 K, while the lattice remains at near ambient temperature during the duration of the 50 fs pulse, as illustrated by the block schematic shown in FIG. 4.

(24) These energetic hot carriers transfer their energy to the phonons/lattice, as shown in FIG. 4, where the time scale (x-axis in FIG. 4) is material dependent, but for many materials is much less than one nanosecond. This energy provides the mechanism to anneal the thin film on this sub-nanosecond time scale. This excitation and relaxation of the thin film occurs in a matter of 10 s of picoseconds, with the electrons and phonons coming into local equilibrium at a temperature a little above ambient in much less than a nanosecond.

(25) As illustrated by the block schematic in FIG. 5, all of this occurs in much less time than that needed for ablation, thermal diffusion and melting, and/or re-solidification of the film to occur. In many cases, ablation takes place at a ns time scale, thermal diffusion and melting takes place at a μs time scale, and re-solidification takes place at a >μs time scale. However, in other cases, one or more of these processes in accordance with the present invention can occur on other time scales, e.g., some ablation can occur in 10+ps, while thermal diffusion, melting and resolidification can occur on time scales as short as 1-10 ns.

(26) For some materials, like VO.sub.2, thermal annealing often requires a procedure to ensure that its constituent elements or other materials do not diffuse into or out of the film, which would leave non-stoichiometric films and inhibit the formation of a film with the required phase. Since diffusion lengths are proportional to the square root of the diffusion time, an ultrafast optical pulse of a sub-picosecond time scale produces sub-nanometer diffusion lengths, thereby ensuring elements would not diffuse through the thin films. One advantage of this method is that these ultrafast laser processes enable annealing to be conducted in atmosphere without chambers or environmental gas control.

(27) Due to its potential for annealing sub-micron regions without influencing the surrounding regions, this ultrafast laser annealing technique can be applied to laser patterning a portion of the sample, rather than, for example, annealing the entire sample and then removing the unwanted material.

(28) Thus, as illustrated by the block schematic in FIG. 6, a sub-picosecond pulsed laser 601 can be directed onto a thin film 602 on a substrate 603, with an annealed area 604 of crystalline material being formed as a result of the thermal and athermal processes described above. As the laser is moved relative to the surface of the film, either by moving the laser or by moving the substrate/film sample, the film can be selectively annealed and a desired pattern of crystalline areas of the film can be produced. This laser-produced patterning provides a planar, patterned surface, whereas an etched sample produces a non-planar patterned surface.

EXAMPLE

(29) We demonstrated this technique on amorphous vanadium dioxide films by using 50-fs optical pulses at 800-nm to locally anneal a pattern of crystalline VO.sub.2 into an amorphous VO.sub.2 film in a manner as described above with respect to FIG. 6 The laser annealing was conducted in an ambient environment, i.e., at room temperature and standard atmospheric pressure.

(30) The resulting annealed patterning is shown in the optical microscope image in FIG. 7A, where the dark background is the amorphous VO.sub.2 and the lighter line that outlines the perimeter of a square is a pattern comprising crystalline VO.sub.2 written with a 5-mW, 50-fs, 800-nm laser while the sample was scanned. In this case, the laser diameter was 8-μm yet the annealed width is much smaller, owing to the nonlinear process which is selective to only the peak spatial amplitude of the optical pulse.

(31) Raman spectroscopy was performed on the sample, and an image of the Raman spectra for the lower part of this patterned laser anneal is shown in FIG. 7B. This Raman image shows that the laser-exposed area exhibits the 195-cm' Raman mode of crystalline VO.sub.2 while the surrounding area does not.

(32) The plot in FIG. 7C further demonstrates that the portions of the film exposed to the sub-picosecond laser pulses are crystallized VO.sub.2 by comparing the Raman spectra of the amorphous area with that of the laser annealed regions. The laser annealed region shows a clear indication the Raman modes of crystalline VO.sub.2 while the amorphous regions reflect the background Raman contributions from the substrate. Taking the difference between the Raman spectra of amorphous and laser patterned regions removes the influence of the substrate and clearly reveals five of the well-known A.sub.g Raman modes of VO.sub.2 (195, 224, 312, 394, and 614 cm.sup.−1). In addition, these spectra also show the absence of the Raman modes for V.sub.2O.sub.5, which often forms when amorphous VO.sub.2 is annealed in ambient, thus, demonstrating that our ultrafast pulsed laser crystallizes the VO.sub.2 without adding or removing oxygen from the sample.

(33) Finally, using a continuous wave laser with over 10× the average power of the pulsed laser produced no discernable annealing or crystallization of the amorphous VO.sub.2, further establishing the multiphoton nonlinear absorption process of the sub-picosecond pulsed laser.

(34) By utilizing athermal and thermal processes, this sub-picosecond laser anneal can crystallize the sample on time-scales shorter than nanoseconds. Using this technique enables rapid patterning of the thin film by scanning the laser and/or sample.

(35) The laser pulses will have a predetermined laser wavelength, pulse width, pulse energy, pulse repetition frequency, excitation spot size, laser beam profile, laser scanning rate, substrate, film thickness, and environment needed to obtain a predetermined rate and extent of annealing, depending on the optical and thermal properties on the materials and substrates used. To reduce and/or eliminate linear absorption the laser wavelength is chosen for the film and substrate used. Another aspect to choosing the laser wavelength, pulse energy, and pulse width, is that they can be selected in order to increase the multi-photon absorption cross-section of the film material compared to the substrate. The laser spot size will influence the rate at which the nonlinear process occurs and enable spatial control by producing maximum absorption at the minimum size (i.e., maximizing the optical intensity/fluence). The laser's pulse repetition rate and scanning rate will depend on the thermal properties of the film and substrate, the thickness of the film and substrate, and the environment.

(36) Advantages and new features.

(37) The crystallization is due to ultrafast laser absorption in film via linear and nonlinear optical multiphoton process setting up nonequilibrium hot carriers, whose rapid energy relaxation produces an ultrafast annealing process. This is contrasted with current laser annealing techniques that use only linear absorption in substrate and/or UV lasers that are linearly absorbed in both the film and substrate, and whose fluence is insufficient to produce a nonequilibrium carrier distribution in the material.

(38) This technique has the advantage of not requiring substrate absorption nor additional absorption layers, and using a low average power. This is due to the sub-picosecond excitation time-scale that enable nonequilibrium interactions. This allows annealing of films on substrates that cannot handle traditional growth and annealing temperatures without damage to the substrate. Hence, an example would be a low-temperature growth of amorphous VO.sub.2 on a temperature sensitive polymer, and then ultrafast laser anneal, all occurring at temperatures of less than 150° C., thereby crystallizing the VO.sub.2 film without damaging the polymer substrate.

(39) Continuous wave (cw) lasers with the same or greater optical power do not perform annealing and crystallization of VO.sub.2 from amorphous VO.sub.2.

(40) The sub-picosecond laser pulse provides rapid energy excitation through a combination of thermal and athermal crystal changes and produces annealing and crystallization on time-scales shorter than nanoseconds thereby enabling rapid patterning.

(41) A single laser beam can be used, unlike other laser annealing techniques that require multiple laser beams.

(42) Ultrafast athermal and thermal processes crystallize materials (such as VO.sub.2) so rapidly that the diffusion lengths are greatly reduced as well as surface interactions. For example, this can prevent oxygen from entering or leaving the sample during the anneal time allowing for crystallization without environmental control or in atmospheric ambients. As another example, this could anneal regions without dopant diffusion, which for example is critical to create regions of different phase transition temperatures, thereby creating devices with multiple phase transition temperatures spatially separated from one another on the same film.

(43) For polymorphic crystals, this laser annealing technique could be used to change one crystal type to another, e.g., rutile to anatase.

(44) As an extension to modifying thin films, by focusing the laser within a bulk material complex 3D crystallized patterns can be created.

(45) Focused laser annealing enables detailed patterning with sub-micron-scale feature size. The nonlinear processes resulting from sub-picosecond lasers produce a smaller heat-affected zone, and therefore does not interact (heating/annealing) with surrounding regions.

(46) Although preheating the substrate is not required as in other laser annealing techniques, in some embodiments, preheating can be used to optimize annealing using the ultrafast annealing method in accordance with the present invention.

(47) Alternatives

(48) As described above, in many embodiments, the ultrafast laser annealing technique in accordance with the present invention will employ one or more sub-picosecond laser pulses applied to one or more predetermined locations on a thin film surface to crystallize and anneal the film. However, in other embodiments, the laser pulses can be directed at the surface of a bulk material to form annealed areas either on the surface or within the bulk material.

(49) The ultrafast laser annealing technique can be accomplished using appropriate wavelengths and at appropriate pulse speed best suited for the ultrafast material annealing given the material system and surrounding environment. This could include using near-infrared lasers and/or UV ultrafast lasers.

(50) As described above, in many embodiments, the ultrafast laser pulses used in the method for laser annealing in accordance with the present invention will have a sub-picosecond pulse duration. However in other embodiments, other ultrafast laser pulses having a pulse duration greater than a picosecond but less than about 10 ns can be used to produce nonlinear processes for the annealing of the present invention.

(51) The ultrafast laser annealing technique in accordance with the present invention is not limited to annealing of amorphous VO.sub.2 films but can be used with many other ALD-grown films that require crystalline structures to provide functionality such as oxide and nitride based films, III-Nitride films, films formed from ferroelectric oxides, or piezoelectric materials. In addition, the ultrafast laser annealing technique of the present invention is not limited to ALD-grown films and can be used with any thin-film fabrication technology.

(52) In some embodiments, the ultrafast laser annealing technique in accordance with the present invention can be used to achieve heterogeneous integration of dissimilar materials that prevent crystallization or prevent the formation of desired properties in the film. For example, it can be used with metal contacts that are generally annealed but then diffuse to active areas, oxides on nitrides, nanolaminates (e.g., Josephson junctions, gate dielectrics, high electron mobility transistors, or neuromorphic films), etc.

(53) The ultrafast laser annealing technique of the present invention can also be applied to different layers in a layer-by-layer fashion, thereby enabling its use in additive, or “3-D,” manufacturing. This lends itself especially well to industrial processing of ALD-deposited films. In such layer-by-layer fabrication, the ultrafast laser annealing technique can be used as each layer is being created, after each layer is created, or after all layers have been created by adjusting laser parameters, such as laser focus and wavelength for transparency through the layers.

(54) Other films that can benefit from the ultrafast laser annealing technique of the present invention include films requiring dopant activation, especially in films such as wide-bandgap semiconductors that require high temperatures that can degrade the films. The ultrafast laser annealing technique of the present invention could also be used to control dopants within a material without thermal diffusion. For example, in VO.sub.2 the deposition of dopants in the amorphous phase combined with ultrafast laser annealing could produce regions with different phase-transition temperatures, and these regions could be spatially separated from one another. In one example, on a single microelectronic chip different circuits could be integrated each with different phase-transition thresholds.

(55) This method of local annealing without influencing the surrounding areas enables ultrafast laser annealing and patterning within already fabricated systems. For example, tuning the wavelength to produce nonlinear optical absorption in the target region, yet remaining non-absorbing (or transparent) in other regions.

(56) Since crystallization can influence etch rates, local ultrafast laser annealing can provide opportunities for enhancing localized etching and patterning due to the etch rate variation for different crystallized regions.

(57) Using this ultrafast laser annealing technique for creating arrays of small features in a larger background enables creation of electromagnetic metamaterials and metasurfaces.

(58) Thus, the ultrafast laser annealing technique described herein can be used with many different materials to achieve many different ends. Although particular embodiments, aspects, and features have been described and illustrated, one skilled in the art would readily appreciate that the invention described herein is not limited to only those embodiments, aspects, and features but also contemplates any and all modifications and alternative embodiments that are within the spirit and scope of the underlying invention described and claimed herein. The present application contemplates any and all modifications within the spirit and scope of the underlying invention described and claimed herein, and all such modifications and alternative embodiments are deemed to be within the scope and spirit of the present disclosure.