Patent Application
20100196661
The present invention relates to the field of probe-based methods for patterning a surface of a material, such as scanning probe lithography (herein after SPL). In particular, embodiments are directed to high resolution patterning of molecules on a surface of a material, such as nano-scale patterns with feature sizes of less than 30 nanometers (nm).
Lithography is a process for producing patterns of two dimensional shapes, including drawing primitives such as lines and pixels within a layer of material, such as, for example, a resist coated on a semiconductor device. Conventional photolithography (also called optical lithography) is running into problems as the feature size is reduced, e.g. below 65 nm. These problems arise from fundamental issues such as sources for the low wavelength of light, photoacid migration, photoresist collapse, lens system quality for low wavelength light and masks cost. To overcome these issues, alternative approaches are required.
Examples of such alternative approaches are known in the field of the so-called nanolithography, which can be seen as high resolution patterning of molecules. Nanolithography refers to fabrication techniques of nanometer-scale structures, including patterns having one dimension sizing up to about 100 nm (hence partly overlapping with photolithography). Beyond the conventional photolithography, they further include such techniques as charged-particle lithography (ion- or electron-beams), nanoimprint lithography and its variants, and SPL (for patterning at the deep nanometer-scale). SPL is for instance described in detail in Chemical Reviews, 1997, Volume 97 pages 1195 to 1230, “Nanometer-scale Surface Modification Using the Scanning Probe Microscope: Progress since 1991”, Nyffenegger et al. and the references cited therein.
In general, SPL is used to describe lithographic methods wherein a probe tip is moved across a surface to form a pattern. In other words, scanning probe lithography makes use of scanning probe microscopy (SPM) techniques, which relies on the availability of the scanning tunneling microscope. In short, it aims at forming images of sample surfaces using a physical probe. SPM techniques rely on scanning such a probe, e.g. a sharp tip, just above a sample surface whilst monitoring interactions between the probe and the surface. An image of the sample surface can thereby be obtained. Typically, a raster scan of the sample is carried out and the probe-surface interaction is recorded as a function of position. Data are thus typically obtained as a two-dimensional grid of data points.
The resolution achieved varies with the underlying technique; atomic resolution can be achieved in some cases. Use can be made of piezoelectric actuators to execute motions with precision and accuracy, at any desired length scale up to better than the atomic scale. The two main types of SPM are the scanning tunneling microscopy (STM) and the atomic force microscopy (AFM).
In particular, the AFM is a device in which the topography of a sample is modified or sensed by a probe or probe mounted on the end of a cantilever. As the sample is scanned, interactions between the probe and the sample surface cause pivotal deflection of the cantilever. The topography of the sample may thus be determined by detecting this deflection of the probe. Yet, by controlling the deflection of the cantilever and the physical properties of the probe, the surface topography may be modified to produce a pattern on the sample.
Following this idea, in a SPL device, a probe is raster scanned across a resist surface and brought to locally interact with the resist material. By this interaction, resist material is removed or changed. In this respect, one may distinguish amongst: constructive probe lithography, where patterning is carried out by transferring chemical species to the surface; and destructive probe lithography, which includes physically and/or chemically deforming a substrate's surface by providing energy (mechanical, thermal, photonic, ionic, electronic, or X-rays energy, etc.). SPL is accordingly a suitable technique for nanolithography.
High resolution patterning of molecules is relevant to several areas of technology, such as alternatives to optical lithography, patterning for rapid prototyping, direct functionalization of surfaces, mask production for optical and imprint lithography, and data storage.
In particular, lithography can be used for the fabrication of microelectronic devices. In this case, electron-beam (or e-beam) and probe-based lithography are mostly in use.
For high resolution optical mask and nano-imprint master fabrication, e-beam lithography is nowadays a standard technology. However, when approaching high resolutions, writing times for e-beam mask/master fabrication increase unfavorably.
As a possible alternative, the use of probes for such patterning is still under development. At high resolution (<30 nm), the speed of single e-beam and single probe structuring converges.
In the case of data storage, various approaches have been proposed to make use of probes for storage in the archival regime. However, a main challenge that remains is to achieve long bit-retention. Using thermomechanical indentation allows for instance to achieve satisfactory endurance and retention of data. A thermomechanical approach, however, produces indentations that are inherently under mechanical stress. Therefore it is difficult to obtain retention times in excess of ten years, as is usually desired in the archival domain.
In one embodiment, a probe-based method for patterning a surface of a material is described. The method includes providing a material having a polymer film with a network of molecules cross-linked via intermolecular, non essentially covalent bonds. The method also includes patterning the polymer film by desorbing molecules from the network with a heated, nano-scale dimensioned probe.
In another embodiment, the method includes providing a material having a polymer film with a network of molecular glass molecules. The molecules are cross-linked essentially via hydrogen bonds. An average desorption energy of the molecules in the film is approximately between 2 eV and 3 eV. The method also includes patterning the polymer film by desorbing molecules from the network with a heated, nano-scale dimensioned probe. The temperature of the probe is approximately between 300° C. and 600° C. The time of exposure of the probe to the surface is approximately between 0.3 microsecond and 10 microseconds. Other embodiments of the patterning method are also described.
Also, embodiments of a material are described. The material includes a polymer film. The polymer film includes a network of molecules. The molecules are cross-linked via intermolecular, non essentially covalent bonds. The polymer film also includes nano-scale dimensioned patterns of molecules in the network. Other embodiments of the material are also described.
In another embodiment, a method of accessing (e.g., writing and/or reading) patterns of molecules is described. The method includes providing a material such as the material described above. The method also includes accessing the patterns of molecules of the material. Accessing the patterns of molecules of material includes writing and/or reading the patterns of molecules. Other embodiments of the accessing method are also described.
Examples of methods and materials embodying aspects of the present invention are described below, by way of non-limiting example, and in reference to the accompanying drawings.
a-c show embodiments of schematized sections of nano-scale dimensioned patterns in a patterned material;
As an introduction to the following description, embodiments described herein relate to a method for patterning a surface of a material. In one embodiment, a material having a polymer film thereon is provided. A probe is then used to create patterns on the film, by desorbing molecules at the surface thereof.
The film includes a network of molecules which are cross-linked via intermolecular, noncovalent bonds, such as van der Waals forces, or hydrogen bonds. More specifically, such bonds are not of a covalent bonding nature (at least not essentially), that is, there is no clear electron pairing between radicals that characterizes the ordinary Heitler-London covalent bond. Rather, the interaction energy of the intermolecular bonds at stake could be divided into various physically meaningful components such as electrostatic, exchange, dispersion, relaxation, etc. Yet, none of the above components could be clearly called “covalent,” inasmuch as anti-bonding mixing of atomic orbitals is likely to be involved, rather than bonding mixing. Should substantial charge-transfer be involved and be regarded as a coordinate-covalent interaction, the occurrence of a substantial overlap repulsion (i.e., the exchange component) would not make the molecules be viewed as covalently bonded.
Rather, the intermolecular bonds provide a better comprise than the usual chemical bonds, inasmuch as the film can remain stable under normal conditions, less energy being yet required at the probe to create the patterns.
In this regards, patterning the film is carried out by means of a nano-scale dimensioned probe, which is further heated, such as to desorb molecules when interacting with (e.g., urged against) the film. In other words, molecules evaporate upon interaction with the probe. The probe thereby directly engraves patterns into the film.
Both the temperature of the probe and the time of exposure of the probe to the surface can be adjusted according to a characteristic of the cross-linked molecules, in order to achieve desired desorption performances. The average desorption energy of the molecules can be seen as such a characteristic, which is impacted by the intermolecular bonds.
Since the binding energy caused by the intermolecular links is small (at least compared to covalent links), the process can work at moderate temperatures and short probe-sample interaction times. This, in turns, allows for scaling to fast writing times.
In reference to
The probe 10 is an AFM probe mounted on the end of a cantilever, as schematically represented in
Engineering solutions may further be provided such that it is possible to accurately control the relative position of the probe and surface, and possibly to ensure good vibrational isolation of the AFM. This can, for instance, be achieved using sensitive piezoelectric positioning devices, as known in the art. Both vertical 50 and horizontal 60 controls of the probe are thus typically provided together with the AFM.
In a usual AFM device, the probe 10 is likely to be raster scanned above the sample surface, such that imaging of the surface topology can be carried out. Here, the probe 10 will rather be used to engrave patterns on the surface 111, as to be explained in reference to
How the surface is patterned can be decomposed into several substeps. First, the probe 10 is maintained in a “non-patterning position,” that is, close to the surface 111 of the film 110 (step S100,
In some embodiments, the probe temperature (Tp) and the time (te) of exposure of the probe to the surface are suitably adjusted, such as to change or optimize desorption of molecules. For example, the parameters may be adjusted so as to obtain regular patterns while minimizing the exposure time. Accordingly, high rates of primitive patterning can be achieved. In some embodiments, these are close to or even greater than one megahertz (MHz).
Incidentally, a person skilled in the art may appreciate in light of the subject matter described herein that, for a given load force, the above parameters (Tp, te) determine the desorption process, inasmuch as its rate constant roughly obeys, in a simple physical picture, the law r=Ae−E
In addition, the skilled person may appreciate in light of the subject matter described herein that the force applied to the probe while patterning may suitably be adapted in respect of the sample. Basically, in one embodiment, both temperature and force pulses are applied to the probe, at positions where the molecules are to be removed. Quantitative details are provided below.
The desorption of a molecule 75 is depicted in
The resulting material has nano-scale patterns of molecules on its surface. By heating this material above a certain temperature, it is locally evaporated, as sketched in
Now, the patterns obtained so far are merely bidimensional inasmuch as no information can be exploited from the depth of the pattern. Rather, a gradient of depth would be required to encode information. In this regards, the present invention can be embodied such as to create three-dimensional (3D) patterns of molecules in the film, as illustrated in
To this aim, one may first engrave a first pattern of molecules at a given location on the film, as represented in
Alternatively, direct 3D-patterning in a single exposure step can be attained by adjusting an evaporation volume at each location on the surface that is exposed. For example, one may contemplate modulating the force applied to the probe during an exposure, e.g., using electrostatic actuations. Varying the force applied during an exposure results in a pattern with modulated depth. Similarly, direct 3D-patterning can further be controlled by varying the temperature of exposure (using e.g. an integrated heater in the probe tip), or the exposure time.
Accordingly, a continuous change of topography can be carried out. This way, 3D patterns can be obtained within a single exposure, i.e., a single indent step. Depth modulated patterns are likely to allow for a dramatic improvement of a writing density. Incidentally, varying any of the above parameters (force, temperature, exposure time) or a combination thereof can already be contemplated for creating 2D patterns.
In addition, before the proper patterning steps, the depth of indents may be calibrated as a function of applied load and temperature, so as to set specific and/or optimal working conditions. For example, for a patterning depth of around 4 nm, a temperature of 300° C. and a load force of 80 nN may result to be optimal (especially for films as described below). Within such conditions, writing indents with a pitch of 23 nm typically yields uniform removal of material over large areas. This results in patterned areas with distinct patterning depths, as discussed below with reference to
Furthermore, the ability to image the surface prior to patterning enables very accurate positioning. This becomes important notably when it comes to patterning very fine features at high resolution over a pre-patterned surface with features that do not require such a high resolution (and which can be realized using more conventional patterning mechanisms with much higher throughput). Once the pattern is written, it is possible to image it before further processing steps. A post-imaging allows for quality control of the written pattern and its eventual correction.
At present, variants as to the types of suitable polymers are discussed.
In one embodiment, the average molecular mass of molecules within the film is less than about 4000 dalton (Da), in order to enable a desorption process. Yet, tests have shown that molecular masses in the approximate range from 100 Da to 2000 Da may make the process easier. More specifically, masses in the approximate range from 150 Da to 1000 Da may allow for increased and/or optimal desorption, at least for specific samples.
As described above, the molecules are cross-linked via e.g., hydrogen bonds. A hydrogen bond is typically defined as the attractive force occurring between a hydrogen atom attached to an electronegative atom of a first molecule and an electronegative atom of a second molecule. While its energy can be compared to that of the weakest covalent bonds, the underlying physics cannot. Incidentally, a typical covalent (bonding) bond is about twenty times stronger than a typical hydrogen bond. Accordingly, relatively low desorption temperature and short interaction times can be contemplated in practice.
In one embodiment, the average desorption energy of the molecules is approximately in the range from 2 eV to 3 eV, as the result of various intermolecular links in the media (including long-distance interactions). More generally, desorption energies approximately between 1 and 4 eV may be convenient.
Closely related, the temperature of the heated probe is approximately between 300° C. and 600° C. In some embodiments, it is approximately between 300° C. and 500° C., which may be optimal in some cases. As a side note, the temperature of the probe is believed to be about twice as much as the temperature of the desorbing molecules.
Meanwhile exposure times are typically in the range of 1 microsecond (μs) and 10 μs. Yet, in some embodiments, it is possible to set the exposure time to about 0.3 μs, with acceptable results. Roughly, exposure times of less than about 1 μs allows for indent rates of 1 MHz.
Next, the network of molecules forming the film may include molecular glasses. An example of a molecular glass molecule is represented in
The deposition of a thin film of this material onto a substrate (e.g., Si-wafer) is simply done by spin-coating a solution of molecular glass, followed by a brief annealing step (e.g., about 1 minute at 130° C.) to drive out the solvent. No further cross-linking reaction is required.
Due to the high number of hydrogen bonding interactions, the polymer exhibits a relatively high glass transition temperature, Tg. In short, below the temperature Tg, the structure of the polymer can be termed glassy, as it has a merely random arrangement of chains, similar to molecular arrangements seen in glasses. In some embodiments, Tg is of about 120° C., which is suited for use in patterning. More generally, the glass transition temperature is approximately between 80° C. to 160° C., and in some embodiments between 100° C. and 130° C. (e.g. 120° C.), which may be more suited for patterning in practice. Furthermore, hydrogen bonds make the material stable against repeated scanning with the probe. It is accordingly suitable for use in the contexts of storage and lithography, for mask-repair and inspection.
A proof of concept has been successfully performed by using a substrate prepared as already mentioned. With typical writing conditions, patterns have been structured onto the polymer media. Specifically, the patterned surface at stake is made of a polymer of molecular glass molecules, cross-linked via hydrogen bonds. A load force of about 80 nN was used to indent the patterns, together with a probe temperature of nearly 400° C. The surface image was then obtained with the same AFM probe as used to pattern.
In
Next, it is pointed at the fact embodiments discussed above present significant advantages in terms of pattern transfer. In this regards, pattern transfer into silicon can be performed by using standard dry-etching technique directly on a patterned molecular glass that serves as the resist.
In an experimental test, the etch conditions used were 20 seconds in a deep-reactive-ion etching tool, using a standard process gas mixture of 50% SF6 and 50% C4F8. The resulting pattern in silicon reflected the topography of the pattern in the molecular glass. Yet, amplification of the pattern in the vertical axis could be controlled via the processing conditions. In particular, the pattern was amplified five times.
The results are illustrated in
Lastly, a final experiment is briefly discussed. In this experiment, a molecular organic glass is patterned. A thin film of 10-100 nm thickness is prepared by spin-coating or evaporation. By fine-tuning the inter-molecular interaction, the material can be desorbed by applying a thermomechanical trigger, i.e., a probe, leaving behind a well defined void. The probe tip temperature and the mechanical force are on the order of 300-500° C. and 50-100 nN, respectively. By laterally displacing the probe and repeating the process, any arbitrary pattern can be written, the resolution of the process being determined by the apex dimensions of the probe. The patterns are written with a pitch of 29 nm, corresponding to 5×104 written marks, resulting in uniformly recessed structures of 8±1 nm depth. The volume of material contained in the box amounts to 0.2 μm3, yet no traces of material displacement or material redeposition are found. Similarly, no material pick-up by the probe tip could be detected by SEM after writing. Next, the structured glass could be used without any development step as a selective etch mask. Using a three layer technique and exploiting etch rate selectivities between organic materials and silicon/silicon oxide, it is possible to transfer the structure into silicon with excellent shape conformity.
In addition, material removal could be accumulated, thereby enabling the fabrication of three-dimensional structures. As a test, a replica of the Matterhorn was accomplished by consecutive removal of molecular glass layers with defined thickness. An almost perfect conformal reproduction of the original was obtained, proving that the final structure is a linear superposition of well defined single patterning steps. Moreover, results made it clear that the organic material is neither densified nor chemically altered during patterning.
The unique capabilities of embodiments of the technology recited above open up new perspectives, notably for the fabrication of complex textured substrates for guided and directed assembly of shape-matching objects. The technique further offers a competitive alternative in terms of resolution and speed to known techniques, such as high-resolution electron beam lithography.
While the present invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiment disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims. For example, the present invention may be contemplated for various applications. While embodiments described above merely focus on uses for lithography and data storage, the skilled person may appreciate potential applications to pattern transfer of patterned regions into silicon.
