Patent Application
20140158943
1. Field of Invention
This application relates to processes and systems for making particles, and more particularly processes and systems for making particles having a dimension less than about 1 mm.
2. Discussion of Related Art
The contents of all references, including articles, published patent applications and patents referred to anywhere in this specification are hereby incorporated by reference.
An important emerging class of non-spherical colloidal materials are microscopic and nanoscopic particles that have designed shapes and are created by lithographic means (see e.g. Hernandez, C. J.; Mason, T. G. Colloidal alphabet soup: Monodisperse dispersions of shape-designed LithoParticles. J. Phys. Chem. C 2007, 111, 4477-4480). (These will also be referred to as LithoParticles in this specification.) Optical pattern replicating systems, such as high-fidelity lens-based steppers (Madou, M. J. Fundamentals of microfabrication: The science of miniaturization. 2nd ed.; CRC Press: Boca Raton, 2002), typically used to print electronic structures on computer chips, have been used to mass-produce LithoParticles and create Brownian dispersions of an entire particulate alphabet: “Colloidal Alphabet Soup” (Hernandez, C. J.; Mason, T. G. Colloidal alphabet soup: Monodisperse dispersions of shape-designed LithoParticles. J. Phys. Chem. C 2007, 111, 4477-4480). In the basic implementation of this approach, a polymer resist layer can be cross-linked by the optical exposure and, after development, the polymer resist particles can be lifted off of the substrate. This optical approach for making LithoParticles has important and non-obvious differences from earlier approaches (Higurashi, E.; Ukita, H.; Tanaka, H.; Ohguchi, 0. Optically induced rotation of anisotropic micro-objects fabricated by surface micromachining. Appl. Phys. Lett. 1994, 64, 2209-2210; Brown, A. B. D.; Smith, C. G.; Rennie, A. R. Fabricating colloidal particles with photolithography and their interactions at an air-water interface. Phys. Rev. E 2000, 62, 951-960; Sullivan, M.; Zhao, K.; Harrison, C.; Austin, R. H.; Megens, M.; Hollingsworth, A.; Russel, W. B.; Cheng, Z.; Mason, T. G.; Chaikin, P. M. Control of colloids with gravity, temperature gradients, and electric fields. J. Phys. Condens. Matter 2003, 15, S11-S18) that required etching as part of the procedure. Although robotically automated optical exposure can be used to create significant quantities of monodisperse LithoParticles, expensive lithography exposure systems must be continuously used to optically pattern films during the particle production process. Due to the limited availability and expense of these precise optical exposure systems, there would be advantages to other LithoParticle production methods that could rapidly produce shape-designed particles without relying on such optical equipment during the repetitive production process.
Mechanical imprinting, whether thermal or step-and-flash, is a technology that involves bringing two solid plates into contact after depositing a desired material between them (Madou, M. J. Fundamentals of microfabrication: The science of miniaturization. 2nd ed.; CRC Press: Boca Raton, 2002; Chou, S. Y. Nanoimprint lithography and lithographically induced self assembly. MRS Bulletin 2001, 26, 512; Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Nanoimprint lithography. J. Vacuum Sci. Tech. B 1996, 14 (6), 4129-4133; Resnick, D. J.; Mancini, D.; Dauksher, W. J.; Nordquist, K.; Bailey, T. C.; Johnson, S.; Sreenivasan, S. V.; Ekerdt, J. G.; Willson, C. G. Improved step and flash imprint lithography templates for nanofabrication. Microelectronic Engineering 2003, 69, 412-419). Once the surfaces of the two plates touch, the material only fills trenches or wells in one plate that has been prepared with the desired patterns. Imprinting essentially forces a desired material into voids that have been created in one of the surfaces to form a mold. While the two plates are touching (or nearly touching), a process, such as cross-linking in the case of polymers, can be used to rigidify the material in the mold, and then the plates are separated. During the separation, if the release of the desired material from the corrugated surface can be made efficiently, then the result is a set of raised structures of the desired material on the flat surface of the other plate. Imprinting is a subset of the more general process of embossing, in which a mold is pressed into the surface of a material that is not as rigid and then removed to create raised corrugations that reflect the mold. However, by contrast to embossing, mechanical imprinting involves squeezing out material between two solid plates where they touch, so that only the negative relief corrugations in one plate become filled with the desired material.
Performing mechanical imprinting reproducibly in a production setting can be problematic for many reasons. It is often difficult to achieve good mechanical contact between the two plates over large surface areas. To mitigate this, large sections of the plates are often cut away so that only small, disconnected pedestals containing the desired patterns touch the flat plate. Using pedestals decreases the surface area and production rate significantly. Defects in the surfaces of the plates, dust, or enhanced surface roughness due to wear can preclude the exact contact of the plates, especially for larger substrate sizes. For very small shapes, the wetting properties of the material to be imprinted with the plates can play an important role in determining the success and reproducibility of the imprinting procedure. These are some of the primary reasons why mechanical imprinting has not been widely adopted by the electronics industry as a replacement to more reliable optical approaches. Although imprinting is making some inroads into certain specialty electronics applications, it is uncertain if mechanical imprinting technology will advance to a degree of robustness necessary to overtake existing optical methods in the current race to the sub-50 nm level. Although it is possible to create LithoParticles using imprinting methods, as we and others (Rolland, J. P.; Maynor, B. W.; Euliss, L. E.; Exner, A. E.; Denison, G. M.; DeSimone, J. M. Direct fabrication of monodisperse shape-specific nanobiomaterials through imprinting exists (J. Am. Chem. Soc. 2005, 127, 10096-10100), yet developing alternative approaches for rapidly mass-producing LithoParticles that do not involve imprinting or repetitious exposure by an optical lithography system would be highly useful.
A method of producing at least one of microscopic and submicroscopic particles according to some embodiments of the current invention includes providing a template comprising a plurality of discrete surface portions, each discrete surface portion having a surface geometry selected to impart a desired geometrical property to a particle while being produced; depositing a constituent material of the at least one of microscopic and submicroscopic particles being produced onto the plurality of discrete surface portions of the template to form at least portions of the particles; separating the at least one of microscopic and submicroscopic particles comprising the constituent material from the template into a fluid material, the particles being separate from each other at respective discrete surface portions of the template; and processing the template for subsequent use in producing additional at least one of microscopic and submicroscopic particles. The method of producing at least one of microscopic and submicroscopic particles according to an embodiment of the current invention is free of bringing a solid structure, other than the constituent material, into contact with the template proximate the plurality of discrete surface portions during the producing, and is free of bringing the solid structure into contact with the constituent material during the producing.
A multi-component composition according to some embodiments of the current invention includes a first material component in which particles can be dispersed, and a plurality of particles dispersed in the first material component. The plurality of particles is produced by methods according to embodiments of the current invention.
A system for manufacturing at least one of microscopic and submicroscopic particles according to some embodiments of the current invention includes a template cleaning and preparation system; a deposition system arranged proximate the template cleaning and preparation system to be able to receive a template from the template cleaning and preparation system upon which material will be deposited to produce the particles; and a particle removal system arranged proximate the deposition system to be able to receive a template from the deposition system after material has been deposited on the template. The system for manufacturing particles is free of a structural component, other than the constituent material, for contacting with the template proximate a plurality of discrete surface portions of the template, and is free of a structural component, other than the constituent material, for contacting with the constituent material during the producing.
The invention is better understood by reading the following detailed description with reference to the accompanying figures in which:
a) is a schematic illustration of a method of producing a well-template suitable for W-DePT according to an embodiment of the current invention. An SiO2 layer is deposited on a flat solid Si substrate and is then spin-coated with a photoresist layer. This top resist layer is exposed using an optical lithography system. The exposed resist is developed, yielding a continuous resist pattern that contains holes that reflect the desired particle shapes. Reactive ion etching of the exposed SiO2 regions then exposes similarly shaped regions of the Si surface. Subsequent chlorine etching to the desired depth creates impressions of the desired well patterns in the Si substrate, and the residual photoresist and SiO2 are stripped and removed.
b) shows an SEM image of wells that have the desired square-cross shape that have been etched into a silicon wafer using the method of
a)-3(c) show optical micrographs of several stages of the process described in
a)-4(b) show number-weighted size distributions of square crosses produced by W-DePT, as measured using SEM images of fifty particles.
a) is a schematic illustration of a method of producing a pillar template suitable for P-DePT according to an embodiment of the current invention. A flat solid substrate is coated with a resist layer; this resist layer is exposed using a lithography system, the exposed resist is developed and descummed, yielding resist islands that reflect the desired particle shape; the exposed substrate is etched, the residual photoresist is stripped away, and the etched substrate is cleaned.
b) shows a scanning electron micrograph of a pillar-template for making a plurality of plate-like particles that resemble square crosses. This template is made by ion etching a silicon surface according to an embodiment of the current invention.
a)-14(d) show reflection optical micrographs for examples according to an embodiment of the current invention.
In describing embodiments of the present invention illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. It is to be understood that each specific element includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.
Some embodiments of the current invention provide methods for producing microscopic and/or submicroscopic particles. The methods according to some embodiments of the current invention include providing a template that has a plurality of discrete surface portions, each discrete surface portion having a surface geometry selected to impart a desired geometrical property to a particle while being produced. Each of the discrete surface portions can be, but are not limited to, a flat surface, a curved surface, a complex contoured surface, a surface with a plurality of subsurface regions, or any combination thereof. Herein, microscopic refers to the range of length scales equal to and greater than one micrometer, including length scales ranging up to about one millimeter. Herein, submicroscopic refers to the range of length scales below one micrometer, including length scales ranging down to about one nanometer.
The methods according to some embodiments of the current invention also include depositing a constituent material of said at least one of microscopic and submicroscopic particles being produced onto said plurality of discrete surface portions of said template to form at least portions of said particles. The constituent material is a material in the composition of the particles being manufactured. The broad concepts of the current invention are not limited to any specific constituent materials. There is an extremely broad range of materials including organic, inorganic, composite, multi-component and any combination thereof that could be used in various embodiments of the current invention. The depositing can be a directional deposition in some embodiments of the current invention that, for example, leaves at least a fraction of wall portions around the discrete surface portions uncoated by the constituent material. The depositing can include spin-coating, spray-coating, dip-coating, sputtering, chemical vapor deposition, molecular beam epitaxy, electron-beam metal deposition, or any combination thereof in some embodiments of the current invention.
The methods according to some embodiments of the current invention further include separating at least one particle from the template in which the particle separated has the constituent material in its composition. The particle may be separated into a fluid, for example, into a liquid in some embodiments of the current invention. In some embodiments there may be one or a small number of particles separated from the template, but in other embodiments, there can be a very large number of particles separated in the same separation step. For example, in some embodiments there could be hundreds of thousands, millions and even billions or more particles separated from the template in the same step.
The methods according to some embodiments of the current invention further include processing the template for subsequent use in producing additional particles. Once the template is processed for subsequent use, the above-noted depositing and separating steps can be repeated to produce additional particles. The template may be reprocessed many times according to some embodiments of the invention to mass produce, in assembly-line fashion, very large numbers of the particles. The method of producing particles according to such embodiments of the current invention does not include pressing a structural component against the template to control the application of material to the template, such as is done with printing methods.
An embodiment of the current invention is a process which will be referred to as “Well-Deposition Particle Templating” (W-DePT). W-DePT involves only a single patterned solid plate and an appropriate deposition and release scheme. A solid “well-template” is created by permanently etching a solid surface to make one or more wells that reflect the desired shape or shapes. Although optical or electron beam (e-beam) lithography is typically used in combination with etching to first make this “well-template”, the remaining steps that are repeated for mass-producing particles do not require any exposure or etching systems.
In a simple implementation, W-DePT can be achieved by: (1) depositing a thin layer of a release agent, such as a temporary sacrificial release layer (e.g. fluid-soluble polymer) or a permanent molecular coating (e.g. fluorinated siloxane chains) over the corrugated surface of the well-template; (2) depositing the desired particle materials at a desired thickness through various deposition processes, such as sputtering, physical vapor deposition (PVD), chemical vapor deposition (CVD), or spin-coating; and then (3) releasing the particles from the wells into a fluid, usually using some form of agitation (See
Many lithographic methods can be used to create a patterned “well-template” suitable for W-DePT. As an example, we describe one approach that can be used to create a well-template for making cross-shaped particles with W-DePT. This process is shown schematically in
Choosing appropriate etching conditions and rates is important in some embodiments in order to obtain uniform side-walls without undesirable defects, such as pronounced scalloping, that could inhibit release. Furthermore, the etch depth has been made larger than the maximum desired thickness of the particles. Extremely high etch depths of many microns may not be desirable in some embodiments since deeper wells can reduce the rate and efficiency of release of particles that are formed in them. The basic requirement for the template according to this embodiment of the invention is that it is a solid material containing a permanent patterned structure of wells that define desired particle shapes. Usually, polished solid materials, such as silicon or quartz wafers, represent the easiest candidates for patterning at length scales less than ten microns for making colloidal particles. However, materials other than silicon and quartz can be used for the well-template.
A wide variety of lithographic approaches other than the one we have described can be used to produce the patterned “well-template”. These approaches may not involve depositing a silicon oxide layer onto a silicon wafer, performing resist-based optical lithography to print the repeating disconnected patterns of particle shapes, nor etching silicon dioxide, as we have described in our example. The key characteristic of a well-template according to this embodiment of the invention is essentially a solid material that has at least one surface that has been permanently patterned to have one or more wells of a desired shape into which at least the desired particle material can be deposited.
Once the well-template has been made, LithoParticles can be mass-produced by a succession of steps that involve deposition and fluid-assisted release. As an example, using the well-template of square-crosses, we produce an aqueous suspension of cross-shaped gold particles by the process outlined in
As a by-product of the W-DePT process, a large interconnected film of the desired particle material is created. In the example given above, a layer of patterned gold with cross-shaped holes is also created and lifted off into solution at the same time as the particles. In principle, the intact patterned film could be used to create an optical mask by deposition onto a quartz surface or for shape-specific filtration if mounted on an appropriate porous substrate. Because this film is much larger than the particles that are produced, it can be easily separated from the particles during or after the fluid-assisted release process. If the particle material is valuable and a continuous film is not a desired product, then this interconnected layer can be recovered and potentially recycled. In practice, thin continuous films can be very fragile, and more vigorous agitation used to release particles can potentially tear or break them into smaller pieces. As a result, mild agitation that does not lead to release of the particles can be used to recover an intact film after lift-off, and subsequent stronger agitation can be used to release the particles.
Scanning electron microscopy (SEM) images reveal that the number-weighted polydispersity of the arm lengths and thicknesses of the crosses to less than 5%. In
The polydispersity of the edge lengths is essentially set by the precision of the well-template (i.e. through the exposure and etching processes), whereas the polydispersity of the thickness by the uniformity of the deposition process for coating the wells with the desired materials. For the example W-DePT implementation that we have described using a polymer release layer and gold, the surface roughness of the top and bottom flat layers of the particles is determined by the roughness of the deposited polymer layer and the uniformity of the sputtering process. We have performed W-DePT using the same template repeatedly without any noticeable degradation of the well-template or deterioration of the particle uniformity. Occasionally, the surfaces of the silicon well-template can be non-destructively cleaned using piranha and HF solutions to ensure maximum fidelity. For the method we have described to make gold crosses, using optical reflection microscopy, we estimate the efficiency of release to be greater than 99% after agitating for less than two minutes using an ultrasonic bath, with less than one particle in a thousand remaining stuck in a well. Non-directional vapor deposition of the sacrificial layer, rather than spray-coating and spin-coating, would most likely improve this release efficiency. It is obvious that this approach for making particles will not be successful if the sides of the wells become coated with the particle material, thereby connecting the continuous film on top of the template to the particles in the wells. So, directional deposition methods that do not coat the side-walls, such as sputtering deposition or evaporative deposition normal to the surface or using physical vapor deposition (e.g. thermal or e-beam), offer distinct advantages for the simple example of W-DePT that we have shown. Likewise, W-DePT may not yield discrete particles in its simplest form if the particle layer becomes too thick due to over-deposition, such that the material in the wells would form rigid contacts with the top continuous film.
Completely automated W-DePT can be performed in parallel using many templates that are continuously recirculated by a robotic track system. Identical well-templates are circulated into a spray/spin coater, a baker, a sputterer, a fluid agitation bath, a cleaning tank, a drying stage, and then back to the spray/spin coater to complete the loop (see
A simple alternative method for making the LithoParticles using W-DePT involves permanently bonding a low-surface energy release agent to the surfaces of the well-template. This release agent can take the form of a fluorocarbon, fluorohydrocarbon, or fluoro-siloxane with appropriate reactive groups for bonding these molecules to the well-template surfaces. This type of low-surface energy coating can be applied using standard methods of surface treatment. After treating the well-template by coating and bonding a high surface density of such molecules to all of the patterned surfaces, the treated well-template surface will have only a very weak attractive interaction with a desired particle material. Once this particle material has been deposited into the wells, the permanent release coating permits facile fluid-assisted release of particles from the wells without the need for the fluid to dissolve a sacrificial release layer. In this variation of W-DePT, shown in
Another interesting variation of W-DePT, which can employ either a temporary or permanent release layer, involves depositing a desired target particle material in a liquid base into the wells and causing a solidification of that target material by some other process, such as aggregation, gelation, phase changes due to temperature or pressure, or evaporation. This process is shown in
Well-templates that have overhanging side-walls (Madou, M. J. Fundamentals of microfabrication: The science of miniaturization. 2nd ed.; CRC Press: Boca Raton, 2002) can be used for W-DePT, provided directional deposition of the desired target material for the particles is used. For instance, for gold deposition normal to the surface of an overhang well-template, particles can still escape from the wells during the release step, as shown in
Several situations can lead to difficulties with the efficiency of production and release of particles by basic forms of W-DePT. The simplest W-DePT approaches may not produce well-separated and discrete particles if a well-template has side-walls that are “underhanging”, rather than vertical or overhanging. For instance, deposition of the particle material into wells that have beveled underhanging side-walls, created by anisotropically etching silicon (Powell, O.; Harrison, H. B. Anisotropic etching of {100} and {110} planes in (100) silicon. J. Micromech. Microeng. 2001, 11, 217-220), could simply create a continuous layer of the desired particle material over a sacrificial release layer, as shown in
Thus, one of the requirements of the simplest versions of W-DePT is that the deposition onto the well-template should create separate, disconnected regions of the desired particle material in each of the wells. The efficiency and rate of release of the LithoParticles from the wells can depend strongly on the thickness of the sacrificial layer, the side-wall geometry of the wells, and the method of deposition of both the sacrificial and particle layers. If the release layer is very thin on the side-walls, then the convective hydrodynamic penetration of the fluid to dissolve the release layer underneath the particles in the wells can be slow, because the region where it can penetrate is more highly constricted. Ultrasonic agitation can be used to expedite the release process, but even this more extreme form of agitation may fail. The combination of the well-template structure and the deposition steps should be chosen in such a manner as to (1) provide discrete structures of the desired particle material in the wells, (2) ensure that these discrete particle structures can be essentially completely liberated from the wells on the well-template, and (3) preserve the structural fidelity of the well-template so that it can be re-used.
The bottoms of the wells in the well-template need not be flat, and if they are appropriately shaped by either deposition or etching processes (Powell, O.; Harrison, H. B. Anisotropic etching of {100} and {110} planes in (100) silicon. J. Micromech. Microeng. 2001, 11, 217-220), it is possible to create particles that have highly complex three-dimensional geometries. In
W-DePT can be used to make particles that are not slab-like, even with undercut well-templates, if the top continuous layer of material can be removed by a process without also removing material deposited into the wells. This can be achieved by processes such as, for liquid-borne materials, by spinning off the top continuous layer in a whole surface process reminiscent of edge bead removal of resist at the edges of wafers (Madou, M. J. Fundamentals of microfabrication: The science of miniaturization. 2nd ed.; CRC Press: Boca Raton, 2002). For instance, it would be possible to make particles such as solid pyramids by etching a well-template that has indentations in the form of pyramids, depositing a release layer and then particle materials, spinning off the top surface of the deposited particle layer (thereby creating disconnected islands of particle materials in the wells), solidifying the material in the wells, and releasing the particles from the wells, as we show in
Many variations of deposition of the sacrificial layer and for the target material layer are possible once the well-template has been made. These materials include organics (e.g. polymers), natural and synthetic biomolecules, inorganics (e.g. conductors, semi-conductors, insulators, including nitrides and oxides), metal-organic frameworks (MOFs)(Roswell, J.; Yaghi, O. M. Effects of functionalization, catenation, and variation of the metal oxide and organic linking units on the low-pressure hydrogen adsorption properties of metal-organic frameworks. J. Am. Chem. Soc. 2006, 128, 1304-1315), and metals, or combinations of any of these compositions. Particles can be comprised of dense solids, porous solids, flexible solids, or even tenuous gels. LithoParticles made using W-DePT can also contain nanoscopic particulates, such as quantum dots, gold or silver nanoclusters, magnetically-responsive iron oxide, or molecules, such as fluorescent dyes or biologically active drugs. Performing multiple depositions of different desirable target materials prior to the release step can be used to make hybrid bi-layer or multi-layer particles. These deposition methods include, but are not limited to, spin-coating, spray-coating, dip-coating, sputtering, chemical vapor deposition (CVD), molecular beam epitaxy (MBE), and electron-beam metal deposition (EBMD). Release can be made into aqueous or non-aqueous solvents for further chemical surface treatment to increase particle stability against aggregation. Particle release could take place in a wide range of fluids, including supercritical fluids or even gases, not just liquids.
Well-Deposition Particle Templating is considerably different than mechanical imprinting of features including discrete particle shapes. To perform W-DePT, no mechanical lithography device for imprinting, necessary to ensure good mechanical contact between two plates everywhere over the entire surface of the wafer, is needed. Moreover, the performance of W-DePT in reproducibly creating shapes repeatedly from the same template is not nearly as sensitive to dust, wear, and surface imperfections as mechanical imprinting. Instead, to make LithoParticles, only a single patterned substrate, the “well-template”, is required, along with an appropriately chosen deposition and release method. The internal feature sizes and overall dimensions of the particles are not limited to the microscale; direct e-beam writing, x-ray lithography, or deep-UV lithography to a resist-coated surface and subsequent etching could make templates with internal particle features, such as arm widths on the crosses, and overall particle lateral dimensions, smaller than 50 nm.
According to another embodiment of the current invention, LithoParticles can be mass-produced from a solid template that has been permanently etched to make pillars that define their cross-sectional shape in a process called “Pillar-Deposition Particle Templating” (P-DePT). Although making the patterned pillar-template, which may contain billions of replicas of a portion of a desired particle shape or different shapes in positive relief, can rely on optical or electron beam (e-beam) lithography, the remaining steps for particle production do not. A simple implementation of P-DePT consists of the following steps: coating the pillars with a thin layer of a release agent, such as a sacrificial layer of water-soluble polymer; depositing the desired particle materials at a desired thickness through various deposition processes, such as sputtering, chemical vapor deposition (CVD), or spin-coating; and then releasing the particles from the pillars into water by dissolving the sacrificial layer using an aqueous solution, as shown in
The P-DePT method can facilitate the large-scale production of new kinds of soft multi-phase materials, particularly dispersions of particulates in viscous liquids (Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal dispersions. Cambridge Univ. Press: Cambridge, 1989). These particles can be used as interesting probes for applications such as microrheology (Mason, T. G.; Ganesan, K.; van Zanten, J. H.; Wirtz, D.; Kuo, S. C. Particle tracking microrheology of complex fluids. Phys. Rev. Lett. 1997, 79, 3282-3285; Cheng, Z.; Mason, T. G. Rotational diffusion microrheology. Phys. Rev. Lett. 2003, 90, 018304) or bio-microrheology (Weihs, D.; Mason, T. G.; Teitell, M. A. Bio-microrheology: A frontier in microrheology. Biophys. J. 2006, 91, 4296-4305). Concentrated dispersions of solid shape-designed particles could exhibit interesting liquid-crystalline phases and exotic phase transitions as the particle volume fraction is increased quasi-statically. Moreover, by rapidly is concentrating the particles in the liquid, one may quench in glassy disorder (Torquato, S.; Truskett, T. M.; Debenedetti, P. G. Is random close packing of spheres well defined? Phys. Rev. Lett. 2000, 84, 2064-2067). Understanding how the shape of the particles can influence jamming (Donev, A.; Cisse, I.; Sachs, D.; Variano, E. A.; Stillinger, F. H.; Connelly, R.; Torquato, S.; Chaikin, P. M. Improving the density of jammed disordered packings using ellipsoids. Science 2003, 303, 990-993) in concentrated dispersions can provide key insights into the structure and dynamics of disordered soft materials.
To create a solid pillar-template suitable for P-DePT, as an example, we begin by creating a reticle mask containing a plurality of disconnected cross-shapes suitable for optical lithography; this reticle mask can be designed using computer aided design software and stored electronically in a digital file, and the mask can be produced from the digital file using a standard e-beam lithography writing system (e.g. MEBES). Using a mercury i-line stepper exposure system (Ultratech XLS-7500), ultraviolet light passes through the reticle's clear cross shapes to expose a one micron thick resist-coated (Shipley AZ-5214) flat silicon wafer. Following development and de-scumming, which removes the unexposed resist from the wafer's surface, a pattern of raised crosses of cross-linked resist remains on the wafer's surface, and the wafer is permanently etched using a reactive ion etcher to a depth of 8 microns in the regions outside the crosses where the wafer is exposed and unprotected. The residual protective resist is then stripped and the wafer is cleaned using piranha (a mixture of 70% sulfuric acid and 30% hydrogen peroxide). This process is shown schematically in
Using the pillar-template, as an example, we produce an aqueous suspension of cross-shaped gold plate-like particles according to general scheme of
Using scanning electron microscopy, we have characterized the number-weighted polydispersity of the arm lengths and widths of the crosses to be about 2%. The polydispersity of the thickness is more difficult to measure for such a thin layer, and we estimate it to be about 45±5 nm. The polydispersity of the edge lengths is essentially set by the precision of the pillar template (i.e. through the exposure and etching processes), and the polydispersity of the thickness by the uniformity of the deposition process for coating the pillars with the desired materials. In general, for directional deposition of the desired particle material, we do not observe overhangs, burs, or other defects, and the side-walls are flat. Other forms of deposition, such as solution delivery of a desired organic material, to the tops of the release-coated pillars and subsequent baking could lead to rounding of the top corners of the particles by liquid surface tension. This may be a desirable feature in some cases.
The P-DePT process can be repeated many times without degradation of the pillar template. If deposited materials accumulate in the interconnected trenches beneath the pillars, occasionally, it may be necessary to clean off this excess material by dipping the wafer in piranha or HF solutions. If the trenches are also coated with a release agent when the tops of the pillars are coated, then large continuous interconnected regions of the deposited material containing negative images of the desired particles can also be released into solution. These regions can be easily separated from the particles through sedimentation or filtration, since they are typically tens to hundreds of microns in size.
We have characterized the rate of efficiency of the lift-off of the particles from the pillars by using optical reflection microscopy to examine the tops of the posts after the sacrificial layer has been dissolved. When the sacrificial layer is properly coated over all of the tops of the pillars, it is very difficult to find any gold crosses that remain on the posts after the fluid assisted release step, and we estimate the efficiency of release of the particles to be greater than 99%, with less than one particle in ten thousand remaining on the wafer. The few bound particles that do remain are found near the edges of the wafer where the spin-coating of the release agent may have been adversely affected by the high effective contact angle introduced by the pillars. Vapor deposition of the sacrificial layer, rather than spin-coating, would most likely improve the release efficiency. The simplicity of release and the exceptional release efficiency is one of the strengths of the P-DePT approach.
To continuously produce particles at a high rate, an automated system containing the essential non-optical devices for each step in the above process can be set up in a continuous loop. For the example we gave, several identical pillar templates held in a wafer boat can be fed by an automated robotic track system into a hexamethyldisilizane (HMDS) applicator, a spin-coater, a baker, a sputterer, an ultrasonic bath, a cleaning tank, a drying stage, and then back to the HMDS applicator to complete the loop (
Although P-DePT is well suited for making particles that are slab-like and have a uniform thickness, it is also possible to make particles that have more complex three-dimensional shapes by appropriately modifying the surfaces of the pillars. For instance, it is possible to make a pillar-template suitable for creating pyramid-shaped particles by filling the trenches of the well-template with an inert material, leaving the tops of the pillars exposed, and then etching the tops of the pillars at an angle, as can be achieved by angular etching of an appropriately oriented polished silicon wafer surface. After etching, the surfaces of the pillars can be coated with a release agent. As shown in
In addition to gold particles, we have produced plate-like square-cross particles made of aluminum that have thicknesses in excess of one micron, showing that P-DePT can be used to fabricate particle structures that are quite thick and robust. The ultimate limit of the particle thickness is set by the height of the pillars; if the wells outside of the pillars become filled with the particle material, then the particle material will form a continuous interconnected layer, and no particles can be produced. However, if the height of the permanent pillars is larger than the lateral dimensions of the particles, as it is in our example, then the thickness of the particles can actually exceed the lateral dimensions, without a loss in the definition of the lateral shape. So, both thin and thick particles can be made using the P-DePT method.
Residual stress in the layer of deposited particle material can cause the particles to deform into non-planar shapes, especially when the thickness of the deposited layer is much less than a micron. This effect has been reported previously (Brown, A. B. D.; Smith, C. G.; Rennie, A. R. Fabricating colloidal particles with photolithography and their interactions at an air-water interface. Phys. Rev. E 2000, 62, 951-960), but, in our method, the gold particles remain quite planar, even after release, as can be seen in the optical micrographs. Further electron microscopy shows that the gold particles do not exhibit significant distortions away from planar shapes. In principle, by depositing a thin layer of a particle material that is known to have an inherent stress, it could be possible to design continuously curved particle shapes. Indeed, by relying upon stresses created by controlling the composition (e.g. stoichiometry) of multi-elemental particle materials, one can induce a desired curvature after lift-off. One can also create bilayer deposition of two desired particle materials that have different thermal coefficients of expansion, yielding two-faced Janus particles that have continuously variable shapes that can be controlled as a function of temperature. This can be accomplished by simply depositing a layer of one desired material, and then a second layer of a different desired material having a different coefficient of thermal expansion onto the tops of the pillars before releasing these bi-layer particles into a fluid.
Many different deposition scenarios, both for the sacrificial layer and for the target material layer, are possible once the permanent pillar template has been made. These materials include organics (e.g polymers), biomaterials, inorganics (e.g. nitrides and oxides), and metals, or combinations of any of these compositions. Performing multiple depositions of different desirable target materials prior to the release step can be used to make hybrid multi-layer particles. These deposition methods include, but are not limited to, spin-coating, spray-coating, dip-coating, sputtering, physical vapor deposition (PVD), chemical vapor deposition (CVD), molecular beam epitaxy (MBE), and electron-beam metal deposition (EBMD). Directional deposition at other than normal to the pillar's top surface could provide a method of making particles with slanted side-walls. Release can be made into aqueous or non-aqueous solvents for further chemical surface treatment to increase particle stability against aggregation. Particle release could take place in any fluid, including supercritical fluids or gases, not just liquids. Lastly, it may be possible to omit the sacrificial layer if a suitable surface coating can be used to prevent the particles from sticking to the pillars. Such a permanent coating may take the form of fluorinated molecules that are attached in high density to the template surfaces.
In P-DePT, we employ a re-usable patterned substrate with permanent pillars and do not require exposure by any source of radiation, thereby clearly differentiating this approach from earlier optical approaches of Higurashi et al. (Higurashi, E.; Ukita, H.; Tanaka, H.; Ohguchi, O. Optically induced rotation of anisotropic micro-objects fabricated by surface micromachining. Appl. Phys. Lett. 1994, 64, 2209-2210), Brown, et al. (Brown, A. B. D.; Smith, C. G.; Rennie, A. R. Fabricating colloidal particles with photolithography and their interactions at an air-water interface. Phys. Rev. E 2000, 62, 951-960), Harrison, Chaikin, and Mason (Sullivan, M.; Zhao, K.; Harrison, C.; Austin, R. H.; Megens, M.; Hollingsworth, A.; Russel, W. B.; Cheng, Z.; Mason, T. G.; Chaikin, P. M. Control of colloids with gravity, temperature gradients, and electric fields. J. Phys. Condens. Matter 2003, 15, S11-S18), and Hernandez and Mason (Hernandez, C. J.; Mason, T. G. Colloidal alphabet soup: Monodisperse dispersions of shape-designed lithoparticles. J. Phys. Chem. C 2007, 111, 4477-4480). P-DePT can offer a clear advantage of a re-usable permanently patterned template, excellent uniformity, and high-throughput without the complexity of optical exposure at every stage in the process. Because a stamping, or “imprinting” procedure (Chou, S. Y. Nanoimprint lithography and lithographically induced self assembly. MRS Bulletin 2001, 26, 512; Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Nanoimprint lithography. J. Vacuum Sci. Tech. B 1996, 14 (6), 4129-4133; Resnick, D. J.; Mancini, D.; Dauksher, W. J.; Nordquist, K.; Bailey, T. C.; Johnson, S.; Sreenivasan, S. V.; Ekerdt, J. G.; Willson, C. G. Improved step and flash imprint lithography templates for nanofabrication. Microelectronic Engineering 2003, 69, 412-419), in which particles can potentially be stuck in wells with vertical side-walls that can inhibit facile release, is not necessary, we anticipate that P-DePT will be more efficient than other particle methods involving mechanical imprinting that we have also developed. Moreover, no special fluorinated surface coatings or expensive mechanical imprinting stages are required. The internal feature sizes and overall dimensions of the particles are not limited to the microscale; direct e-beam writing to a resist-coated surface or deep-UV lithography and subsequent etching could make templates with internal particle features, such as arm widths on the crosses, and overall particle lateral dimensions, smaller than 50 nm.
Both pillars and wells can be made on the same template surface to yield a mixed template that can produce particles by both processes. For this kind of mixed template, the same deposition step can create discrete disconnected regions in the form of the desired particles on the tops of the pillars and in the bottoms of the wells simultaneously. A single lift-off step can release the particles both from the pillars and from the wells.
More generally, the solid template can be created in such a manner as to provide several different plateau levels at different depths from its topmost surface upon which the desired material can be deposited. The desired material can be deposited in a manner that leaves disconnected regions of this material at different levels in the form of the desired particle shapes. These disconnected regions can be released from the template, yielding particles in solution. In principle, using this approach, all of the deposited material can be used to form desired particles without waste, provided the different shapes can be formed on the template at different levels and completely fill the available surface area. This would be a highly efficient implementation that would make excellent use of the deposited material. The example in
Templates can potentially have many different forms other than being made on a flat wafer surface. The overall template surface does not have to be flat for either the pillar deposition templating process or the well deposition templating process in order to produce useful particles. For instance, a template can be made on a curved surface, such as a cylinder, which could be spun to expose different portions of the cylinder to cleaning, deposition, and release processes. Using such a curved template that has appropriate pillars and/or wells on the surface, one may be able to optimize the processing steps into a continuous particle production device that does not require repeated exposure with radiation. Templates made from flexible solid materials could be adhered to a solid surface. Well templates could potentially be made by making a thin porous film of a flexible solid material that has holes of the desired particle shape and then adhering this film to a non-porous solid support. Indeed, lifting off the top contiguous layer of the simple well deposition templating process could potentially produce a film that could be used, in turn, to make another well template if this film is deposited and bonded to a solid support.
Templates can be made by many different possible procedures. Standard lithography procedures, such as electron beam lithography and optical lithography, can be used in conjunction with etching, to make the templates. However, other methods can be used, too. One method involves coating a wafer surface with diblock polymers that form phases of dots or short stripes that can be etched onto the wafer's surface to provide either pillars or wells in the form of the dots or stripes. Another possible method is to coat the wafer surface with a solution of polymer particles and use these particles as a mask during an etching process. This type of process could be used to make circular pillars or even ring-like pillars. If complex particle shapes, such as those made using lithographic methods, are deposited, templates for reproducing their shapes could potentially be made this way. Yet another method of making a template could be to cover a wafer's surface with a microporous or nanoporous membrane or film. This kind of well template may not be comprised of only one material but may be made instead from two or more materials that have been put together to create the desired pillars and wells. Optionally, the exposed surface of the wafer could be selectively etched using an ion etcher in the regions where the holes appear and the membrane could then be removed from the surface.
Multiple deposition steps using different materials can be used in combination with templates in order to make complex particles that have layers of different kinds of materials, including organics, inorganics, metals, alloys, and biomaterials. By combining sequences of deposition of different desired materials in controlled amounts with complex templates that have multiple levels in different shapes, it is possible to produce very complex particles that have differently shaped substructures of particularly desired materials located in pre-specified regions. In particular, selective spatially patterned deposition can be used in combination with the templates to create local sites for producing pre-specified interactions, whether attractive or repulsive, between different particles. Alternatively, local regions on the surfaces of the particles can be made rough through a selective deposition process that coats only part of the particles' surfaces with a desired material in a manner that produces an enhanced surface roughness in a desired sub-region of the particle. Thus, by controlling the deposition as well as the template, it is possible to design particles that have customized localized surface coatings that can interact with local sites on the surfaces of other particles to form assemblies of particles that have either the same or different shapes.
Before the particle is separated, it typically will be or will become at least partially solid so that it retains a geometrical feature of the surface portion of the template (or coated template) that it was in contact with, after the separation. The forming of a particle could involve depositing a liquid dispersion and then inducing a chemical reaction, thermal polymerization of a polymer component, photo-induced polymerization, plasma-induced polymerization, sintering, a crosslinking reaction, a gelation, an evaporation of the solvent, an aggregation or agglomeration of materials, a jamming, an entanglement, a denaturation, and/or a bonding.
The constituent material as first applied to the template can be a vapor, a liquid, or a solution, for example. The maximum dimension associated with any of the components contained within the constituent material should be smaller than the maximum dimension associated with the portion of the surface for creating the particles. For example, it may not be reasonable to coat the surfaces of the pillars with giant particles that are larger than the pillars themselves.
The structured substrate can be produced from a flat smooth substrate by a lithographic process involving at least one of electron-beam lithography, optical lithography, ultraviolet lithography, dip-pen lithography, x-ray lithography, imprinting, stamping, deposition, patterning, and etching.
Some strategies for directing the assembly of colloidal particles can involve site-specific interactions between a portion of the surface of one particle and the portion of a surface of a second particle. Some surface-surface interactions can be repulsive, some can be attractive, and there can be variations of these interactions depending upon the nature of different stabilizing materials attached to the surfaces of the particles. The processes of attachment, usually through means such as deposition, adsorption, or bonding, can be material-specific and shape-specific. Therefore, it is important to develop method for producing particles that have custom-modified surfaces that can interact in pre-specified and desirable ways. The surface modifications can be designed to create multi-component assemblies of the particles that are based on controlling the attractive and repulsive interactions between different portions of the surfaces of the particles through selective surface treatments that can be localized.
Further embodiments of the invention include methods to modify the surfaces of particles that are produced using relief deposition templating (RDT). The fundamentals of the process for making particles using relief deposition templating (i.e. through processes such as pillar deposition templating (P-DePT) or well deposition templating (W-DePT)) is described above and in references. (Hernandez et al., “Pillar-Deposition Particle Templating: A High-Throughput Synthetic Route for Producing LithoParticles,”Soft Materials, vol. 5, pp. 1-11, 2007; and Hernandez et al., “Well-Deposition Particle Templating: Rapid Mass-Production of LithoParticles Without Mechanical Imprinting,”Soft Materials, vol. 5, pp. 13-31, 2007)
Variations of the process of making particles using RDT couple the steps of the process of forming the particles described previously with the steps of modifying their surfaces. The specific steps to decorate specific portions of the particles are sufficiently complicated and involved so as to be non-trivial and non-obvious. These further steps may be used with RDT, but may also be applicable to methods of making lithographic particles using spatially patterned radiation (Hernandez et al., “Colloidal Alphabet Soup: Monodisperse Dispersions of Shape-Designed LithoParticles,” J. Phys. Chem. C, vol. 111, pp. 4477-4480, 2007) and also to relief radiation templating (U.S. Provisional Application 61/103,677, incorporated by reference in its entirety).
Coating of the entire surfaces of particles after release from the substrate is typical for the purpose of stabilizing the particles against agglomeration by attractive interactions and thermally driven aggregation. However, this simple process of coating cannot create the complex decorations over portions of the surfaces of particles that may be necessary to design and build multi-component colloidal structures.
One method of creating patches (i.e. portions of the surface of a lithographic particle) of different materials on the surfaces of particles is based on lithographic patterning (e.g. through deposition, exposure of a radiation-sensitive resist, development, and etching). In some cases, no further surface treatment is necessary and the patches created through patterning are all that are necessary to achieve the desired surface modification. In other cases, the patches of deposited material over a portion of a particle's surface serve as localized regions that facilitate the selective attachment of molecules in solution or other colloidal species (i.e. objects) dispersed in solution only onto the patches and not onto the other surfaces.
As an example, we consider the modification of patches by molecules that can be designed to interact with different strength and range. Such molecules typically have a region that enables them to attach to the particle material and a region that enables them to interact with the surfaces of other particles or with molecules or other species that are on the surfaces of other particles. Such multi-functional molecules are sometimes referred to as “functionalized” or “derivatized” molecules (Malmsten et al, “Biopolymers at Interfaces,” Vol. 110 of the Surfactant Science Series, 2nd. ed., Taylor and Francis, 2003). In some cases, certain molecules (e.g. derivatized single-stranded oligomeric DNA) can be designed to have a functionality that enables them to bind to complementary molecules (e.g. derivatized single-stranded oligomeric DNA that has a complementary sequence of nucleic acids) that can likewise be attached to patches on other particles. If the attractive binding of one or more molecules (e.g. that have been previously attached to the surfaces of particles) occurs, and if the attractive binding energy exceeds thermal energy, then the particles having patches containing complementary particles can be strongly bound together without coming apart due to thermal fluctuations as they remain in the suspending fluid material. For example, a first particle having a patch of material onto which derivatized oligomeric single-stranded DNA molecules have been attached can be bound to a second particle having a patch of material onto which different and complementary derivatized oligomeric single-stranded DNA molecules (ss-DNA oligos) have been attached. The binding can occur when, through diffusion, flow, or manipulation by external fields, the first particle can be brought into the vicinity of the second particle with an appropriate position and orientation for the binding to occur. In some cases, it may be advantageous to extend the ss-DNA oligos beyond the surfaces of the particles by placing them on a polymer strand (i.e. like a stalk) that holds the oligos away from the particle surface, thereby letting complementary oligos bind more readily as entropic forces enable them to explore more space as they dangle and undergo thermal fluctuations. This polymer strand might be a segment of double-stranded DNA that would not interfere with the interaction of the ss-DNA.
In some embodiments, binding of molecules on a first patch on a first particle to complementary molecules on a second patch on a second particle may lead to several different degrees of relative motion of the particles. For instance, binding of two particles that arises from a single pair of complementary molecules on proximate patches may lead to a bond that prevents the particles from coming apart under thermal excitations, flow, or external fields; however, the two particles may be able to rotate with respect to each other if the bond has little resistance to twisting (illustrated in
The strength of the binding between different types of patches can be selectively controlled through the temperature and the ionic strength of the suspending fluid material. For instance, the strength of bonds between complementary ss-DNA oligos can be greatly reduced by heating the suspension of particles to temperatures that are typically below the boiling point of the suspending material (e.g. up to about 100° C. for aqueous nucleic acid materials). The larger thermal energy effectively overcomes the attractive energy between the complementary molecules, and the molecules dissociate. By controlling the length and complementarity of the sequences of the ss-DNA oligos on the proximate patches, it is possible to adjust the dissociation temperature of molecules on different patches on the same particle surfaces. Thus, by heating the suspension of particles that contain patches of the complementary particles, it is possible to create a stable suspension of individual unbound particles. Then, by lowering the temperature below a first dissociation temperature associated with a first pair of complementary ss-DNA oligos, it is possible to cause only a specific first patch on one particle to associate with a specific second patch. Usually, a higher dissociation temperature is associated with longer oligomeric sequences that have a significant degree of complementarity. Waiting sufficient time enables a first binding of the particles to occur (e.g. as thermal energy drives the particles close enough to each other to enable the complementary molecules on patches of particles to come into proximity so that binding can readily occur). Subsequently, the temperature is then lowered below a second dissociation temperature associated with a second pair of complementary ss-DNA oligos. Waiting a sufficient time enables a second binding of particles to other particles and/or to multi-particle structures that have been previously formed through the first binding to occur. Repeating this process of changing temperature and waiting for that step of the assembly to occur can thereby enable mass production of multi-component assemblies of one or more types of particles by designing the location and appropriate molecular types of a plurality of patches on the surfaces of said one or more types of particles. This step-wise method of causing particles to assemble could even be potentially used to create a self-replicating colloidal system based on templates that are grown through a step-wise hierarchical assembly process.
As another example of this approach, the temperature could be fixed, but the ionic strength of the fluid material in which the surface-treated particles are suspended may be likewise systematically controlled. Certain patches on the surfaces of particles may modified with surface treatments to respond to changes in the ionic strength of the fluid material (e.g. by adding salts or saline solutions to the fluid material in which the particles are suspended). Similar extensions of this approach for changes in pH (e.g. by adding acids or bases) or other physiochemical variables are likewise anticipated.
The use of intermediate linker molecules of ss-DNA that contain sequences of nucleic acids may also be introduced after the surface treatment steps to induce attractive linking. These linker molecules would contain at least complementary portions of the sequences that are on the ss-DNA attached to the surfaces of the particles that have been designed to be bound together. It can be anticipated that intermediate linkers other than ss-DNA that interact with the molecules or other species on the modified surfaces of particles could also be added to bind together specific sites on the surfaces of two or more particles to initiate the creation of multi-component assemblies.
Note that herein, although we use the term oligomeric to refer to bound polymer-like molecules, we intend to mean by this term both shorter oligomeric molecules and also longer polymeric molecules and do not intend to imply any strict restriction on the length of the molecules.
Although the examples of the embodiments of this invention are primarily focused on the design of the molecular or other species bound to specific portions of the surfaces of the particles, it is this design that enables the broader concept outlined of creating multi-component assemblies through control over the surface interactions between one or more particle species that can be caused to bond together in an predetermined and desirable manner. This approach enables the creation of colloidal devices and machines out of lithographic particles in a highly parallel manner that is off-chip and is typically in the suspending fluid material.
An example of a specific type of linkage that can be used to attach derivatized molecules is the thiol linkage resulting when thiolated derivatized molecules are attached to a patch composed of gold. Other types of materials-specific surface chemistries can be used to attach molecules and particles, as are commonly known in the art through references on surface chemistry and molecular derivatization (Malmsten et al, “Biopolymers at Interfaces,” Vol. 110 of the Surfactant Science Series, 2nd. ed., Taylor and Francis, 2003). Complementary natural and synthetic biopolymers that have amino acid sequences that create attractive interactions between the biopolymers (e.g. the streptavidin-biotin pair) can also be used instead of complementary ss-DNA oligos as the linking molecules.
Although we refer to molecules bound to patches, other species can also be attached to patches in order to provide site-specific binding of a patch on a particle with a patch on another particle. These other species include: nanoparticles, molecularly-coated nanoparticles, dendrimers, microgels, nanoemulsions, double nanoemulsions, vesicles, viruses, viral capsid proteins, peptides, oligopeptides, polypeptides, block copolypeptides, graft copolypeptides, block copolymers, proteins, biopolymers, organelles, membranes, lipids, lipoproteins, amphiphilic molecules, derivatized molecules, molecular motors, derivatized molecular motors, structural biomolecules, derivatized structural biomolecules, membrane biomolecules, and derivatized membrane biomolecules.
Attachment of molecularly coated nanoparticulate materials onto a patch can be used to control surface roughness of that patch in addition to the molecular surface chemistry on that patch. Since surface roughness has been shown to strongly influence depletion attractions (Zhao et al., “Roughness-Controlled Depletion Attractions for Directing Colloidal Self-Assembly,” Phys. Rev. Lett., vol. 99. pp. 268301/1-4, 2007), then the attachment of coated nanoparticulate materials on patches on micron and sub-micron particles can provide a particularly versatile method of performing assembly that can be controlled by a combination of binding of complementary molecules and of surface roughness.
In some cases, the molecules or other species can be attached by exposing patches created on certain portions of the surfaces of particles to a fluid surface-treatment material that contains a plurality of molecules that readily bind, attach, or adsorb onto the surfaces of the patch. The density of molecules or other species attached to a patch can be controlled by the concentration of molecules in the fluid surface-treatment material, the time the particles are exposed to the fluid surface-treatment material, and the rates of reaction of the molecules or other species with the patch material on the surface of a particle. If some surfaces of the particle cannot be exposed to the fluid surface-treatment material, then such unexposed surfaces will not have the molecules attached to them. Even different portions of the surfaces of particles that do not have any patches and are comprised of only one material can be treated in this manner. For bi-layer or multi-layer particles that are made using different materials for two or more layers, the specific attractive interactions between particular molecules and the layer materials can be used to provide methods of attaching several different molecular types to the various layers, either before or after the release of the particles from the substrate. In some cases, to obtain a high level of complexity, pre-release treatment of particles using a fluid surface-treatment material can be combined with release and post-release treatment of particles, potentially using different fluid surface-treatment materials.
After use of a fluid material containing molecules or other species for pre-release, release, and post-release treatment to attach molecules or other species onto specific portions of the surfaces of the particles, it is frequently necessary to remove the fluid treatment material in order to proceed to the next step. For pre-release treatments, it is possible to simply immerse the substrate to which the particles are bound into a container that holds a sufficient quantity of pre-release fluid surface-treatment material for a predetermined and sufficient length of time for the desired attachment to occur, and to then simply remove the substrate from the pre-release fluid surface-treatment material. Subsequently, the particles and substrate upon which they are bound can be washed by a fluid material that does not cause unintended release of the particles and also does not cause the molecules or other species to detach from the portions of the surfaces of the particles that remain bound to the substrate. In some cases, molecules or other species can be included in the fluid release material in order to cause attachment of a portion of these molecules or other species
Usually, in a single surface-treatment step, the attachment of molecules or other species creates at most a monolayer of those molecules or other species on a portion of the surfaces of the particles. In some cases, it could be desirable to attach molecules or other species to form two or more layers.
A fluid surface-treatment material consists of a fluid material (e.g. a liquid, liquid mixture, supercritical fluid, or a gas) in which molecules or species of a colloidal material are dissolved or dispersed. These molecules or colloidal species have a preference to attach to at least a portion of the surfaces of particles that may be placed in contact with the fluid surface-treatment material. In some cases, the fluid surface-treatment material acts as a reservoir of a large number of molecules or other species such that attachment of some of those molecules or other species from the reservoir onto the surfaces of the particles does not significantly lower the concentration of those molecules or other species in the fluid-surface treatment material. This can be desirable since the same fluid surface-treatment material can be re-used repeatedly to alter the surfaces of particles in several different steps or re-used to modify the surfaces of other particles.
In some embodiments of the current invention, more than one species of molecule or other colloidal structure can be attached to the same patch or portion of the surface of the particles. This diversification of the species attached to a patch or portion of the surface of the particles can be obtained through just one surface-treatment step, or it can be obtained through two or more successive surface treatment steps. In some cases, it is desirable to have more than one type of molecule or species attached to the same localized area on the surfaces of the particles.
Sample embodiments of the present invention are shown in
In the example embodiments related to the steps for creating the surface-modified particles (
In the four example embodiments related to the steps for assembling the surface-modified particles (
By incorporating surface treatment into the steps of the fabrication of shape-designed particles using RDT in a non-obvious and non-trivial manner, invention provides control for the attachment of molecules or other colloidal species to specific desired portions of the surfaces of the lithographic particles. This invention provides a well-defined method for causing the highly parallel assembly of many particles in solution into multi-component structures (e.g. small-scale devices or machines) through a set of well-defined steps by changing temperature, ionic strength, or other parameters that induces attractive interactions only between sites on the surfaces of the particles that have been specifically designed to bind together for certain ranges of conditions and not for others.
Post-Release Attachment of One Material onto LithoParticles' Surfaces
This exemplary embodiment is shown in
Post-Release Attachment of More than One Material onto LithoParticles' Surfaces
This exemplary embodiment is shown in
Post-Release Attachment of One or More Materials onto LithoParticles' Surfaces: Permanent Release Layer
This exemplary embodiment is shown in
Post-Release Attachment of One or More Materials onto LithoParticles' Surfaces
This exemplary embodiment is shown in
Pre-Release Attachment of One Material onto Portion of LithoParticles' Surfaces
This exemplary embodiment is shown in
Pre-Release and Post-Release Attachment of Two Different Materials onto Specific Portions of LithoParticles' Surfaces
This exemplary embodiment is shown in
Post-Release Attachment of Two Different Materials onto Site-Specific Portions of Bilayer LithoParticles' Surfaces
This exemplary embodiment is shown in
Pre-Release and Post-Release Attachment of Three Different Materials onto Site-Specific Portions of Bilayer LithoParticles' Surfaces
This exemplary embodiment is shown in
This exemplary embodiment is shown in
Pre-Release Attachment of Two Different Materials onto Site-Specific Portions of Bilayer LithoParticles' Surfaces
This exemplary embodiment is shown in
Pre-Release and Post-Release Attachment of Four Different Materials onto Site-Specific Portions of Surfaces of LithoParticles Comprised of Two Particle Materials
This exemplary embodiment is shown in
This exemplary embodiment is shown in
Incorporating an Internal Layer of Microscale or Nanoscale Particulates into LithoParticles
This exemplary embodiment is shown in
These LithoParticles contain an inhomogeneous distribution of particulates on the outside in the form of a layer that imparts a surface roughness. The particulates stick and bind to the surface of the particle material layer after the deposition of the continuous matrix of particle material onto the tops of the pillars. Deposition of particulates onto the surface of the release material can be uniform or non-uniform at low or high surface density. By changing the type and size of the particulates, the surface roughness of the LithoParticles can be accurately controlled. Controlling the surface roughness on different portions of the surfaces of LithoParticles can be used to control site-selective assembly of the LithoParticles. Typically, the particulates are smaller than the size of the LithoParticles, but this is not a necessary condition. Optionally, particulates can also be deposited on the surfaces of the lower trenches, but this will just result in more waste of the particulates since they will not be incorporated into the LithoParticles.
Incorporating an external layer may be performed before release of the LithoParticles, as shown in
This exemplary embodiment is shown in
Size and Shape Modification of LithoParticles: Coating LithoParticles in Fluid with a Second Type of Particle Material
The LithoParticles are coated with a different particle material in a manner that can change the size and shape of the LithoParticles. Coating material can consist, for example, of monomers or oligomers that can be polymerized onto the surfaces of the released LithoParticles using known methods for making polymer spheres. This coating material can be directly applied through mechanical contact of a liquid to the surfaces of the particle material. The coating material can also be contained in a fluid solvent and essentially grown on the exposed surfaces of the particle material. Thin coatings may significantly alter the surface properties of the LithoParticles without altering their sizes and shapes much. Thick coatings may alter the surface properties of the LithoParticles and also their sizes and shapes quite significantly. Size and shape modification may occur after release of the LithoParticles (
This exemplary embodiment is shown in
This exemplary embodiment is shown in
This exemplary embodiment is shown in
This exemplary embodiment is shown in
The current invention is not limited to the specific embodiments of the invention illustrated herein by way of example, but is defined by the claims. One of ordinary skill in the art would recognize that various modifications and alternatives to the examples discussed herein are possible without departing from the scope and general concepts of this invention.
This application claims priority to U.S. Provisional Application No. 61/100,471 filed Sep. 26, 2008. This application is a Continuation-in-Part of PCT/US2008/003679, filed Mar. 20, 2008, which claims priority to U.S. Provisional Application No. 60/918,896, filed Mar. 20, 2007. The entire contents of all the above documents are hereby incorporated by reference.
This invention was made with Government support under Grant No. CHE-0450022 awarded by the National Science Foundation. The Government has certain rights in this invention
| Number | Date | Country | |
|---|---|---|---|
| 61100471 | Sep 2008 | US | |
| 60918896 | Mar 2007 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12563907 | Sep 2009 | US |
| Child | 14025526 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/US2008/003679 | Mar 2008 | US |
| Child | 12563907 | US |
