This invention relates to pulsed laser deposition of thin films to tune their properties.
Pulsed laser deposition (PLD) has been used as a fabrication technique to grow nano-particles, nano-rods, nano-wires, and thin films. Ablation mechanisms in PLD are complicated and not yet fully understood. Film quality can be controlled by changing the laser fluence and/or repetition rate, growth substrate temperature, and/or background gas pressure in a vacuum chamber. Tunability of the thin films is limited by at least those parameters.
Tunability of thin film properties is desirable for many applications of PLD. Various embodiments provide tunability of thin film properties by setting one or more laser temporal pulse parameters for thin film growth, for example a pulse duration.
In at least one embodiment a distinct and controlled change in a thin film property occurs as a function of pulse duration, for example by setting a pulse duration to a value in the range from about 20 ps to 200 ps.
In some embodiments various thin film properties can be tuned by setting a pulse duration to less than about 20 ps.
In some embodiments a combination of burst-mode operation and laser pulse duration control may be utilized for tuning the thin film properties. Burst-mode operation and examples are disclosed in U.S. patent application Ser. No. 12/401,967, entitled “A Method for Fabricating Thin Films”.
In some embodiments other laser parameters may be further utilized to tune thin film properties, for example laser pulse energy.
At least one embodiment provides a method to obtain desired magnetic properties with control of particle size, crystallinity, and/or thin film morphology.
In at least one embodiment the conductivity and/or resistivity of a film may be tuned with control of particle size, crystallinity, and/or thin film morphology.
In at least one embodiment a material property of a thin film electrode may be tuned to improve performance of an electrochemical device, for example to improve the reaction speed of a Li ion battery.
A material property may include one of more of a physical property and chemical property, and may comprise an optical and/or electrical property.
At least one embodiment provides a method to grow desired thin films. Thin film properties are tuned by controlling particle size and thin film morphology.
At least one embodiment provides for tuning crystallinity of nanoparticles and the thin films.
In some embodiments a laser pulse duration may be set in the range of about 100 fs to 50 ns to tune a thin film property.
In some embodiments pulse duration may be the range from about 10 fs to 200 ns, 100 fs-1 ns.
In some embodiments a pulse duration may be set in the range from about 1 ps to 200 ps, or from about 1 ps to 50 ps.
In some embodiments a pulse duration may be set to a value in the range from about 20 ps to 200 ps.
Various embodiments may be utilized for growth of thin films, wherein the film includes one or more properties which can be tuned as a function of laser parameter(s).
Various embodiments can be applied for control of thin film properties for magnetic materials design.
In at least one embodiment a PLD system for tuning a material property is provided. The system includes a pulsed laser and components for adjusting and/or selecting output pulse widths in at least a portion of the picosecond-nanosecond regime, and/or for providing pulse repetition rates in at least a portion of the range from about 1 MHz to about 1 GHz.
The quality of thin films produced with PLD varies with lasers used and their associated parameters. For example, improved crystalline GaN phase using femtosecond pulsed laser deposition was disclosed in X. L. Tong et al., “Comparison between GaN thin film grown by femtosecond and nanosecond pulsed laser depositions” J. Vac. Sci. Technol. B, Vol. 26, (2008) 1398-1403. PLD configurations producing pulses in the range from femtoseconds to picoseconds are known, for example as disclosed in U.S. Pat. Nos. 5,432,151, 6,372,103 and U.S. Patent Application Pub. No. 20080187684. Tunability of magnetic thin films, among other things, was disclosed in Reilly et al., “Pulsed laser deposition with a high average power free electron laser: Benefits of subpicosecond pulses with high repetition rate”, Journal of Applied Physics, Vol. 93, 3098, 2003. In Reilly et al. results obtained with a free electron and Ti:sapphire laser were compared. The sensitivity of coercivity to laser parameters was identified as a point of interest. Effects of pulse energy and repetition rate of laser pulses, and in particular a high repetition rate, was studied. In Reilly et al. a combination of laser parameters, including sub-picosecond pulses, were investigated.
Applicants discovered that thin film properties may be tuned by setting pulse widths in the picosecond regime. It was also found that films properties may be further controlled with a combination of burst-mode operation and laser pulse duration control. In least one embodiment a method to tune thin films properties by controlling laser parameters is provided, particularly with control of at least the pulse duration. In various embodiments the number of pulses provides additional control. Surprising results disclosed herein show that pulse duration and other temporal parameters may be utilized to tune magnetic properties.
In various embodiments the tunability of the thin film properties is achieved by setting laser parameters such as pulse duration, the number of burst laser pulses, and/or the pulse energy. Such embodiments are particularly desirable for PLD on heat sensitive substrates, for example an organic film.
In accordance with an embodiment,
In some embodiments other suitable techniques for varying a pulse duration may be utilized. For example, a combination of a laser diode and pulse modulator may be used to as an input source to generate pulses of different width, under computer control. By way of example, published U.S. Patent Application Pub. No. 2007/0053391, entitled “Laser Pulse Generator”, illustrates operation with a laser diode and modulator. The pulses may then be amplified with a fiber amplifier as illustrated in
A PLD system was used to tune thin film properties: magnetic properties (coercivity), and for control of thin film morphology of metal and metal oxide thin films.
At least one embodiment provides a method to obtain desired magnetic properties, and to tune magnetic properties with control of particle size, crystallinity, and thin film morphology. In previous work magnetic metal thin films have been grown using thermal and e-beam evaporation, and magneto-sputtering. The magnetic properties can be controlled by deposition rate, power of the source etc, but most effectively by changing growth temperature. Tuning magnetic properties of the thin films without temperature control has been limited. Providing tunability of the thin film properties is a desirable advancement. The following results demonstrated new capability for tuning magnetic properties, and other physical properties.
As shown in
In a further experiment, magnetic properties of samples were measured.
In the example of
The results show that magnetic coercivity is increased by increasing the number of burst pulses as shown in
The dependency of coercivity of magnetic metals upon ablation pulse width, as discovered with the examples of Ni3Fe and Co thin films, is not expected to change beyond about 200 ps, and particularly in the ns regime. Therefore, the tunability in such large pulse width ranges (ns and greater) is insubstantial.
The results obtained with picosecond operation for tunability of magnetic properties were surprising. It is known that picosecond pulsed laser processing of several types of materials can exhibit thermal behavior (e.g.: melt formation), even in the range of a few tens of picoseconds and higher. Ultrashort pulse widths, for example below about 10 ps, are therefore preferred for various precision micro-machining applications, and are generally desirable for many PLD applications also. Pulsed laser deposition using nanosecond lasers has not been regarded as a suitable method to grow magnetic metal thin films because of the known droplet generation problems caused by thermal ablation processes.
The tunability of magnetic properties is desirable for the design of device applications based on magnetic materials. If the magnetic properties are precisely controlled the design of the devices may be simplified and potentially provide for new devices based on these magnetic materials.
Smooth TiO2 thin films were also obtained using burst-mode pulses with 200 ps pulse duration (not shown). The results were comparable to those illustrated in
At least one embodiment provides for tuning the crystallinity of nanoparticles and the thin films. The crystallinity can be tuned by setting the pulse duration, the number of burst-mode pulses, and the pulse energy. In some embodiments, growth may be carried out at room temperature. Further crystallinity tuning can be achieved by controlling growth temperature, as an optional parameter.
In some embodiments conductivity may be tuned. The conductivity depends on several parameters, such as number of defects in the materials, and the crystallinity of the materials. Conductivity control of the thin films has been achieved by optimizing growth conditions such as substrate temperature and processing gas pressure. Conductivity control of thin films with control of laser parameters can be beneficial when the films must be grown under heat sensitive conditions, such as on a polymer substrate.
Experiments were carried out to investigate tuning of conductive properties. Table 1 below illustrates measurements of thickness (Å), resistance by using a 4 probe multi meter (kΩ), resistivity (Ωcm), electron mobility, and carrier density (cm−3) of the corresponding thin films shown in
The results of
In various embodiments properties of electro-chemical devices, or portions thereof, may be tuned by varying temporal parameters of one or more pulses. To advance thin film battery technology, embodiments providing for growth of electrode thin films with tunable size and morphology in a vacuum chamber can produce several benefits. For instance, during fabrication of the thin film batteries, exposure to air can create undesired structure or material created by such as oxidation by air or moisture during the exposure. Moreover, performance control of the electrode materials for the electrode by adjusting laser parameters can be useful when heat sensitive materials are used for the battery structure (such as in the solid electrolyte and the substrate).
At least one embodiment provides a method to grow desired thin films and tune their properties by controlling particle size and thin film morphology. When the particle size and density (particles/area) change, the surface morphology of the thin films is also changed. As a result, by controlling particle size and thin film morphology, the thin film properties are modified.
In the PLD experiments discussed above processing was carried out with a spot size of about 30 μm. Thus, a pulse may provide a minimum fluence of about 10−4 J/cm2 with pulse energy of about 1 nJ. If pulse energy of 1 μJ is applied to the target material the fluence will be increased to about 0.1 J/cm2. In various embodiments a laser spot size may be in the range from about a few microns to a few hundred microns. In various embodiments a PLD system includes optical elements for delivering the laser beam such that the beam is focused onto the target surface with an appropriate average energy density and an appropriate energy density distribution.
In summary, materials used for the above examples were Ni3Fe (permalloy), cobalt, and manganese for magnetic applications. TiO2 samples were grown with both pulse duration and burst-mode operation to demonstrate tuning of film morphology. Additionally Nb:TiO2 samples were grown with conductivity, crystallinity, and morphology control. LiMn2O4 samples were processed with burst-mode, and the results show such processing may be utilized for lithium ion battery fabrication. Substrates used for above examples were Si, glass, SrTiO3, and Pt foil. In various embodiments other targets and substrates may be utilized for thin film growth.
The above description of the preferred embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the structures and methods disclosed. It is sought, therefore, to cover all such changes and modifications as fall within the spirit and scope of the invention, as defined by the appended claims, and equivalents thereof.
This application claims priority to U.S. Patent Application Ser. No. 61/267,153, filed Dec. 7, 2009, entitled “A method of tuning properties of thin films”, which is hereby incorporated by reference in its entirety. This application is related to U.S. patent application Ser. No. 12/401,967, entitled “A Method for Fabricating Thin Films”, which claims priority to Application No. 61/039,883, filed Mar. 27, 2008. This application is also related to U.S. patent application Ser. No. 12/254,076, entitled “A Method for Fabricating Thin Films”, filed Oct. 20, 2008, which claims priority to Application No. 61/039,883, filed Mar. 27, 2008. The '967 application is published as U.S. Patent Application Pub. No. 2009/0246530. The '076 application is published as U.S. Patent Application Pub. No. 2009/0246413. This application is also related to U.S. patent application Ser. No. 11/798,114, entitled “Method for Depositing Crystalline Titania Nanoparticles and Films”, filed May 10, 2007, now published as U.S. Patent Application Pub. No. 2008/0187684. The disclosures of application Nos. 61/039,883, 11/798,114, 12/254,076 and 12/401,967 are hereby incorporated by reference in their entirety.
Number | Date | Country | |
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61267153 | Dec 2009 | US |