The present invention relates to creating patterns in phase change solids using a nanoscale heated probe.
Phase change materials (PCMs) are solids which can be reversibly transitioned between multiple solid phases (for example, between amorphous and crystalline or between two different crystalline phases), typically through application of heat. Examples of PCMs include many chalcogenides (compounds containing elements from column VI of the Periodic Table), as well as some oxides, including VO2. For many applications, different phases of these materials often exhibit very different physical properties. For instance, in many Te-containing chalcogenides, the amorphous phase is electrically insulating, optically absorptive, and less dense, while the crystalline phase is electrically conductive, optically reflective, and more dense. Chalcogenide-based PCMs have widely been utilized as the recording media in rewritable optical data storage devices (e.g. rewritable CDs and DVDs), where a laser is used to induce the amorphous-to-crystalline phase transition. Additionally, PCMs have more recently been examined as candidates for non-volatile electrical memory (i.e. Phase Change Memory), and are currently being researched for use in RF devices, hybrid metamaterial devices, and other applications.
Some applications benefit from local, nanoscale lithography of materials to fabricate minute patterns with distinct contrast from the surrounding field. The contrast required varies by application, but can include electrical (e.g. metal features within an insulating matrix), optical (e.g. reflective features within a transparent or absorptive field), volumetric (e.g. depressed features within a raised field), and chemical (e.g. etchable vs. etch-resistant) properties. The dramatic difference in property between the phases of PCMs makes them a promising material for applications that require such patterning. Previous methods utilized to induce localized crystallographic phase change include the use of electrically biased atomic force microscopy (AFM) tips (i.e. conductive AFM), AFM-based pressure-induced phase change (i.e. use of nanomechanical force microscopy—NFM), laser-based lithography or utilizing embedded heaters. In each of these cases, different drawbacks exist which would prevent the PCM patterning in certain applications or on certain substrates. Electrically biased tips use an electrical current to induce a transformation, requiring an electrically conducting buried layer or substrate, preventing patterning on insulating dielectric substrates. NFM requires hundreds of micronewtons of force to induce a phase change, which can cause abrasion to tips and erode pattern fidelity over time. Laser lithography often requires complex optical components, which are difficult to integrate. Embedded heaters require prior definition of the patterned area, preventing arbitrary in-situ changes in device design.
The aforementioned problems are overcome in the present invention which provides a method for creating nanometer patterns, with features down to the nanometer scale, in phase change solids—materials that demonstrate a thermally-induced phase transition—by use of a nanoscale heated probe. The method of the present invention uses a heated probe, such as an AFM tip, to create arbitrary nanoscale patterns on a PCM surface with significant electrical, optical, or topographic contrast, using simple piezo-electric control. The heated probe locally induces the phase change material to transform from its amorphous phase to its crystalline phase, resulting in a dramatic contrast in property—including electrical resistance, optical reflectance, and volume—relative to the unheated regions. In some cases, the material can be converted back to its amorphous phase (i.e. the patterns can be erased) by appropriate thermal cycling. This approach to patterning can be utilized for a variety of applications, including, but not limited to, creating adaptive circuits and devices, defining nanoscale lithographic patterns, defining nanoscale devices, developing rewritable circuitry.
Patterning PCMs via heated probe offers several advantages relative to other techniques. The nanoscale dimension of the probe allows extremely small feature sizes, and the pattern width and depth can be varied easily using a single probe through controlling the probe temperature and write-speed. Furthermore, the technique limits modification to the surface of the PCM, allowing materials of varying thickness and on arbitrary substrates to be patterned. Established techniques for multi-AFM-tip arrays can be exploited to increase scalability, simultaneously generating patterns across a large area in parallel.
Though nanoscale patterning of chalcogenide-based PCMS has previously been demonstrated, the reported means and modalities for patterning differ significantly from the present invention. Conductive AFM and NFM both use similar AFM probes, but have significant disadvantages in terms of limiting the material stacks that can be used and inducing probe damage. Neither makes use of direct heating of the nanoscale probe to transfer heat to the PCM surface. Laser lithography utilizes optical energy to locally heat the PCM surface and often requires complex optics for control and variation of pattern dimensions. Buried heaters prevent arbitrary patterning in situ.
These and other features and advantages of the invention, as well as the invention itself, will become better understood by reference to the following detailed description, appended claims, and accompanying drawings.
A heated nanoscale probe, such as a metallic AFM tip, is brought into contact with the PCM surface. During contact, heat is transferred from the probe to the surface, increasing the temperature of the PCM material in the immediate vicinity of the tip. If the surface temperature exceeds the PCM crystallization temperature, the tip will induce a local phase change from amorphous to crystalline state, resulting in dramatic changes in electrical, optical, and volumetric properties of the crystallized region, while leaving the properties of the surrounding amorphous PCM material unchanged. The width and depth of the patterned region is dependent upon the dimension and temperature of the heated probe, the speed at which the probe is moved across the surface, and the effectiveness of thermal transport within the film and at the surface-probe junction. Smaller, cooler probes and faster writing result in narrower patterned linewidths, with linewidths of ˜200 nm and topographic features <30 nm in depth achieved without optimization. The pattern can be effectively erased by re-amorphizing the crystallized region by heating the region above its melting temperature and quenching, thereby allowing for re-writability. The writing process works on an amorphous PCM surface regardless of substrate material or film thickness. Potential applications include, but are not limited to, rewritable arrays of resonators, with dimension (and thus resonance frequency) which can be changed on demand, rewritable spiral inductors, rewritable conductive wires to optimize circuits and other nanoscale devices.
The PCM can be a chalcogenide material. Commonly used chalcogenide PCMs include GeTe and GeTe-based alloys, including GeSbTe.
Described herein is the nanopatterning of GeTe thin films by inducing localized crystallization via a fast-scanning heated-tip AFM. The binary chalcogenide GeTe, which has a low crystalline-phase electrical resistivity and a relatively high crystallization temperature (˜180° C.), was used; although, the approach will work for many PCM chalcogenide alloys. Either tip power or tip temperature may be used as a variable. Tip power can be actually measured, while tip temperature is estimated.
The GeTe thin films were prepared by pulsed DC sputtering from compound stoichiometric targets at 100 W in a 5 mTorr Argon atmosphere at room temperature, resulting in the deposition of an amorphous GeTe layer. Film thickness and substrate could be varied, but most work in this example was performed on films ˜620 nm thick and deposited onto fused silica substrates, unless otherwise noted. Following deposition, GeTe composition and structure were probed using a Thermoscientific K-alpha x-ray photoelectron spectroscopy (XPS) system and a Rigaku Smartlab X-ray diffraction (XRD) instrument, respectively. Variable temperature XRD was performed by pairing the Rigaku instrument with an Anton Paar DHS 1100 domed heated stage to evaluate film structure during a stepped anneal, allowing precise determination of the GeTe crystallization temperature. Four-point probe measurements characterized the conductivity of the amorphous and crystalline films. Nanopatterning of the GeTe was performed using an Asylum Cypher AFM with a heatable tip in contact mode, with tip temperature and write speed varied to tune the resultant pattern properties. Patterned regions were subsequently imaged via tapping-mode AFM, taking advantage of the volumetric contraction associated with the amorphous-to-crystalline phase transition to identify regions of crystallized GeTe, which appear as surface depressions. Cross-sections of the patterned regions were milled using a focused ion beam (FIB) instrument and observed via a transmission electron microscope (TEM), while the electrical properties of the patterned regions were probed.
While blanket anneals crystallize the entire amorphous GeTe film, by using a heatable AFM probe as a confined source of thermal energy, the phase transition and pattern arbitrary crystalline regions can be localized with nanometer-scale precision. Crystalline square patterns 1 μm on each side were generated using an AFM probe whose temperature was nominally held from 200° C. to 800° C., with the tip scanned at a speed of 500 nm/s and rastered over the area for ˜20 minutes. AFM images of the patterned squares depicted in the inset of
To evaluate the impact of tip speed on pattern depth and width, multiple lines were generated at a fixed probe temperature of 700° C. (approximate surface temperature of 230° C., as estimated from thermal diffusion properties of the film and tip) while the heated probe scanned a single line at rates ranging from 0.2 μm/s to 1.0 μm/s, with the resulting AFM topograph and line profile shown in
To probe the extent of GeTe crystallization induced by the heated tip, cross-sectional lamella of a series of lines patterned at a probe power of 6.83 mW and write speeds varying from 100 to 1000 nm/s were investigated using transmission electron microscopy (TEM).
As crystalline GeTe is considerably more conductive than amorphous GeTe, significant electrical contrast between the patterned regions and the surrounding amorphous field are expected. Electrical measurements of crystallized and amorphous regions were obtained using a Nanoprobe instrument with two independently controlled scanning tunneling microscopy (STM) tips, where an in situ high-resolution scanning electron microscope (SEM) enables positioning of the tips with sub-micron precision.
In addition to contrasting electronic behavior, the two phases of GeTe possess different optical responses, with amorphous GeTe transparent and crystalline GeTe absorptive across much of the visible spectrum. To optically characterize the patterned regions, near-field scanning optical microscopy (NSOM) was used to measure transmission at 532 nm through GeTe films treated with a heated tip. To limit loss through the sample, all optical measurements were performed on thinner (˜62 nm) films of GeTe deposited onto fused silica substrates. The patterns studied were ˜1 μm×1 μm squares written by a rastered probe at different powers (4.61 mW-9.06 mW), as well as single-pass lines written at tip speeds ranging from 200 to 1000 nm/s (7.95 mW). Regions treated with different dissipated probe powers, and thus different surface temperatures, exhibited a linear reduction in transmission with increased treatment temperature (
T=e
−4πv[k
L(1−p)+k
L(p)] (1)
where v is the light frequency (18797 cm−1 corresponding to the 532 nm laser), L is the total film thickness, and P is the fraction of film crystallized. This crystallization fraction was extracted and plotted in
GeTe PCM films were locally patterned with nanometer-scale precision through heated-tip AFM lithography. Conductive channels of crystalline GeTe were written in amorphous thin films, with the width, depth, and volume of crystalline material varied by controlling the tip temperature and write-speed. Cross-sectional TEM imaging verified the crystallinity of the transformed volume, while KPFM provided evidence of the local enhancement of conductivity in the patterned regions. This approach to nanopatterning is extensible to a wide range of other chalcogenide-based PCM alloys and, when coupled with an anneal-quench process for re-amorphization, enables the realization of non-volatile, nanoscale rewritable conductive pathways without the need for special substrates or laser optics.
The above descriptions are those of the preferred embodiments of the invention. Various modifications and variations are possible in light of the above teachings without departing from the spirit and broader aspects of the invention. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any references to claim elements in the singular, for example, using the articles “a,” “an,” “the,” or “said,” is not to be construed as limiting the element to the singular.
The present application is a divisional application of U.S. application Ser. No. 15/430,725, filed on Feb. 13, 2017 by Laura Ruppalt et al., entitled “NANOPATTERNING OF PHASE CHANGE MATERIALS VIA HEATED PROBE,” which was a non-provisional application claiming the benefit of U.S. Provisional Application No. 62/298,069, filed on Feb. 22, 2016 by Laura Ruppalt et al., entitled “NANOPATTERNING OF PHASE CHANGE MATERIALS VIA HEATED PROBE,” the entire contents of each are incorporated herein by reference.
Number | Date | Country | |
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62298069 | Feb 2016 | US |
Number | Date | Country | |
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Parent | 15430725 | Feb 2017 | US |
Child | 16419273 | US |