Patent Application
20100119708
Not applicable.
Not applicable.
The present invention relates to compositions, devices and methods of filling structures, particularly structures comprising a plurality of high aspect ratio elements.
Deposition of one or more materials on a structure to form a composition or device with high electrical conductivity, thermal conductivity and mechanical strength have met with some success, but also have many limitations. Such structures, particularly those having a plurality of high aspect ratio elements can be very fragile. An example of such high aspect ratio elements are carbon nanotubes (CNTs). CNTs are often grown as a forest on a substrate using a variety of processes, and while individual CNTs have great strength, forests are found generally to be very fragile because individual CNTs exhibit very weak connections to one another and to the substrate. This can necessitate delicate handling to avoid serious degradation and deformation to the CNT forest. The fragility has prevented the widespread application of CNTs and CNT forests. As such, there remains a need to improve the stability of structures comprising a plurality of high aspect ratio elements, such as CNTs.
Structures having a plurality of high aspect ratio elements, such as CNTs, exhibit many exciting properties, such as high electrical conductivity, high thermal conductivity, and high mechanical strength. Accordingly, composite structures have been developed in which dispersed high aspect ratio elements, such as CNTs, contribute their beneficial properties (e.g., high electrical conductivity, high thermal conductivity, and high mechanical strength) to the composite structure. For example, incorporating CNTs into a brittle and electrically insulating ceramic matrix helps increase fracture toughness and electrical conductivity of the matrix. Unfortunately, incorporation requires uniform dispersion of CNTs in the matrix which, in turn, is possible only by elaborate post-processing. Therefore, there remains a need to prepare and provide a more uniform dispersion of high aspect ratio elements, particularly in a manner that does not deform the original structure.
It is also difficult to deposit typical materials for electronics and micro-electro-mechanical systems (MEMS), such as polysilicon, silicon nitride, metals and polymers, to thicknesses of several tens of micrometers. While many techniques are too slow; others produce films that are too thick. For example: low pressure chemical vapor deposition technologies tend to be slow (typically around 1 μm per hour). Thus, depositing materials as films that are tens of micrometers thick requires tens of hours and quickly becomes infeasible. In addition, the internal stress in such films can become high; leading to a variety of film degradation effects. Therefore, there remains a need to deposit electronic and MEMS materials as films in a manner that is more cost-effective and saving time, reduce internal stress to the film and prevent film degradation.
The present invention solves problems indicated above, including forming stable structures comprising a plurality of high aspect ratio elements and providing high aspect ratio structures that are more uniformly dispersed. In addition, the present invention provides improvements over the current and slow deposition rate of films and alleviates internal stress to the film and film degradation effects.
The present invention overcomes current issues associated with the fragile and delicate nature of high aspect ratio elements synthesized in forests, yarns, strings, fibers, paper, glancing angle deposits and the like. In one form, the present invention provides for mechanically stronger interconnections between a substrate surface and a high aspect ratio element and between high aspect ratio elements. In another form, the present invention provides for mechanically stronger composite structures comprised of high aspect ratio elements. The mechanically stronger composite structures are prepared in accordance with one or more select and defined properties (e.g., more electrically conductive, more electrically insulating, more thermally conductive, chemically functionalized). Accordingly, composite structures of the present invention are prepared as desired and convey such properties to the devices created therefrom.
Compositions of the present invention are prepared by filling one or more structures having a plurality of high aspect ratio elements. Filling includes coating the plurality of high aspect ratio elements with a solidifying material. The solidifying material is provided typically by chemical vapor deposition. The solidifying material may include polysilicon or silicon nitride, as examples. The method fills spaces between high aspect ratio elements to a desired degree thereby providing a composite structure also referred to herein as structured film. The coating process is typically continued in order to coat an exposed surface of individual high aspect ratio elements. The solidifying materials do not deform the original structure. Each structure may include a plurality of high aspect ratio elements provided as a forest, yarn, fiber, paper, strings, or the like. The plurality of high aspect ratio elements may also be arranged as a filter, membrane or as high aspect ratio trenches. High aspect ratio elements may be as long as or longer than 50 μm.
As provided herein, the method of filling one or more structures may occur to any desired endpoint to create a conductive composite structure, insulating composite structure, membranous composite structure or the like having a desired sheet resistance, hardness, mechanical strength and/or density as needed.
In another form, a method of the present invention provides for preparing select and defined compositions. The method includes growing one or more structures having a plurality of high aspect ratio elements on a substrate and coating the plurality of high aspect ratio elements with a solidifying material. The solidifying material may be grown to any desired endpoint to provide a defined thickness of solidifying material around each high aspect ratio element. Coating may be repeated one or more times to prepare a layered composite structure, wherein the solidifying material in each step is the same or different. When desired, electrical connections may be included in order for such compositions to behave like an electrical device. The solidifying material and/or its thickness promotes the formation of a composite structure having any desired sheet resistance, hardness, and/or density as needed. When desired, the step(s) of coating may be followed by a removal of the one or more structures and/or substrate using processes known to one of ordinary skill in the art, such as etching. Similarly, when desired, the composite structure may be further processed by post-processing techniques known to one of ordinary skill in the art, such as polishing, annealing, etching, as examples.
Advantages of the present invention are many, including providing truly stable and long-lasting solid structures having a plurality of high aspect ratio elements and providing a method for depositing typical electronic and MEMS materials to thicknesses greater than a few micrometers. Devices prepared by compositions of the present invention are not limited to the field of electronics but can be applied for use in chemical, optical, biologic and mechanical technologies as well.
Those skilled in the art will further appreciate the above-noted features and advantages of the invention together with other important aspects thereof upon reading the detailed description that follows in conjunction with the drawings.
For more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures, wherein:
Although making and using various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many inventive concepts that may be embodied in a wide variety of contexts. The specific aspects and embodiments discussed herein are merely illustrative of ways to make and use the invention, and do not limit the scope of the invention.
In the description that follows like parts are marked throughout the specification and drawing with the same reference numerals, respectively. The drawing figures are not necessarily to scale and certain features may be shown exaggerated in scale or in a somewhat generalized or schematic form in the interest of clarity and conciseness.
In general, the present invention provides compositions comprising one or more structures having a plurality of high aspect ratio elements and filling such structures thereby preparing composite structures (structured films). The structured films are of any desired thickness with properties that can be controlled, in part, by filling. Such properties include electrical, thermal, chemical, biologic, optical and mechanical properties that are initially dictated by the structure and the plurality of high aspect ratio elements. Filling includes coating the plurality of high aspect ratio elements using a form of chemical vapor deposition. Coating occurs as gas phase molecules dissociate and stick to exposed surfaces of the structure. Accordingly, coating occurs wherever there is a surface that gas phase molecules can stick to, react and form a solid. Such surfaces may include not only exposed surfaces of the high aspect ratio elements as well as any support or substrate on which the structure resides on or is in contact with. A material used for coating is referred to herein as solidifying material.
One feature of the present invention is that each structure of the present invention has a plurality of high aspect ratio elements. High aspect ratio elements are defined as elements with a length-to-width ratio, a height to width ratio, a height-to-diameter ratio or a length-to-diameter ratio that is high aspect; the ratio typically greater than 5. The plurality of high aspect ratio elements may be organized into quasi 1-dimesional (linear), quasi 2-dimensional (areal) or 3-dimensional (volumetric) spaces to form the structure. Examples include: (quasi 1-dimensional) filaments, fibers, strings, yarns; (quasi 2-dimensional) papers, membranes, cylinders, spheres; (3-dimensional) forests, filters, balls, and the like. Structures have an overall dimension (e.g., length, width, height) that is larger than the typical spacing (e.g., distance) between high aspect ratio elements. The ratio for the overall dimension of the structure to the spacing between adjacent elements is typically 5 or greater.
One or more elements of the same or differing material may be combined to form a structure and one or more structures may be combined, as long as each structure comprises a plurality of high aspect ratio elements.
One feature of a structure of the present invention is that it is constructed from a material able to withstand elevated temperatures as further described herein for providing compositions and devices of the present invention.
Referring now to
A schematic of a quasi 2-dimensional structure is shown in
With the present invention, the one or more structures of the present invention may reside on or be in contact with a support or substrate, the support or substrate being any desired shape or in any desired location with reference to the structure(s). A representative example of one or more structures residing on a substrate is shown in
A representative example of a patterned substrate is shown schematically in
In one form, an example of a high aspect ratio element is a CNT, a plurality of which may comprise a forest, as depicted in
Referring now to
A support or substrate as provided herein may be any suitable material that is compatible with structures of the present invention, including their growth or placement, and are able to withstand elevated temperatures as described herein for providing compositions and devices of the present invention. Examples of a support or substrate include a wafer, circuit, filter, membrane, cylinder, sphere, block or template. Suitable support or substrate materials are those that are compatible with the growth or placement of the aforesaid structures, including semiconductor materials (e.g., gallium-arsenide, silicon or other substrates that may be used in integrated circuit fabrication), insulating materials (e.g., quartz slides), single metals (e.g., copper, titanium sheets etc.), metal alloys (e.g., stainless steel, nickel-chromium, titanium-aluminum-vanadium parts), and organic materials (e.g., polymers, diamond, graphite). The materials may further enhance, promote, counteract or provide sense properties to structures of the present invention.
A support or substrate as provided herein may also include surface modifications, such as oxidations, etchings, thin or thick film depositions or patterning. Moreover, a support or substrate may have its own properties, for example electrical, mechanical, optical, biologic or chemical properties. In combination, a structure plus a support or substrate may provide useful properties such as high electrical conductivity, insulation, sensing capabilities, signal processing, filtering functions (e.g., electronic, chemical, optical filtering), and signal integration functions.
As described further, a method of the present invention includes filling one or more structures which includes coating the plurality of high aspect ratio elements of the one or more structures with a solidifying material to form stable, stronger, or thicker structures (typically composite structures) having desirable properties using a form of chemical vapor deposition (CVD). When filled completely, structures of the present invention are solid and absent or nearly free of voids or spaces. Incomplete filling, for example when coating with a thin layer of solidifying material, may promote more voids or spaces. Filled structures (typically composite structures) provide valuable properties to such structures that will be appreciated to those of ordinary skill in the art.
Another feature of the present invention includes providing coating conditions wherein (precursor) molecules of the solidifying material have either a small surface sticking coefficient or a limited surface reaction capability. CVD (e.g., low pressure chemical vapor deposition [LPCVD], plasma enhanced chemical vapor deposition [PECVD], atmospheric pressure CVD, metal organic CVD) and atomic layer deposition (ALD) fit such conditions by providing a substantially uniform thickness of solidifying material to coat individual high aspect ratio elements and preventing or reducing capping or matting down of the structure.
Using LPCVD as an example; thermal energy is used to decompose precursor molecules of the solidifying material, the energy for decomposition being provided through collisions of the precursor molecules with exposed surfaces of the structures that also allow growth of the solidifying material as a film. As is obvious to one of ordinary skill in the art, thermal energy may be provided by a furnace-type apparatus or a planar apparatus, such as a hot plate, heater block, heating pad etc.
With the present invention, coating is either extended in time or repeated to obtain a filled structure, referred to herein as structured film. Suitable solidifying materials of the present invention are solids that are capable of being coated onto and solidifying around a plurality of high aspect ratio elements with substantial uniformity. Examples of solidifying materials include semiconductor materials, insulating materials, metals, polymers and combinations thereof. Semiconductor materials may include polysilicon (doped or undoped), amorphous silicon, gallium arsenide, and germanium, as examples. Insulating materials may include silicon nitride (Si3N4), silicon dioxide, alumina and ceramics, as examples. Metals may include tungsten, aluminum, vanadium, nickel and copper, as examples. Polymers may include fluoropolymers in addition to other forms of carbon such as amorphous carbon, diamond or diamond-like carbon.
With the present invention, the one or more structures are filled by coating individual high aspect ratio elements with at least one solidifying material thereby forming a structured film. Representative schematics of structures having a plurality of high aspect ratio elements, wherein the plurality of high aspect ratio elements are coated by a solidifying material in accordance with various aspects of the present invention are shown in
Additional examples of filling structures by coating with one or more solidifying materials are presented herein in which polysilicon and silicon nitride (Si3N4) are representative solidifying materials. In the examples, polysilicon and Si3N4 were deposited via LPCVD using a standard LPCVD furnace (as used in the semiconductor manufacturing industry). Polysilicon was coated using a feed gas of 75 standard cubic centimeters per minute (sccm) of silane at a furnace temperature of about 630 degrees Centigrade. Si3N4 was coated by flowing 75 sccm of ammonia and 25 sccm of dichlorosilane at a temperature of about 730 degrees Centigrade. Both filling methods were performed at a pressure of about 250 mTorr and for a time period commensurate with a desired final coating thickness (typically less than 1 hour). One of ordinary skill in the art will appreciate that the duration of coating, the furnace temperature, the gases introduced, and the flow rates of those gases factor in to provide the final coating thickness as well as the uniformity of coating thickness on a high aspect ratio element.
Referring now to
Referring now to
Referring now to
Referring now to
Referring now to
Further observations of filling, as performed on structures comprising 50 μm tall CNT forests showed that the solidifying material (e.g., both polysilicon and Si3N4) begins by growing uniformly around an individual CNT. As the solidifying material thickens, it bridges spaces between adjacent CNTs, thereby filling the forest and forming a structured film. Growth continues wherever there is an exposed surface that precursor molecules can contact. A greater number of precursor molecules tend to contact the ends of high aspect ratio elements (or uppermost portions of an element when residing or in contact with a support or substrate). Therefore, growth of the film is often faster in these portions. When the ends (or uppermost portions) become filled by the solidifying material, voids may result, such as those observed in
Methods described herein may be compared to film deposition inside a deep trench, as occurs in semiconductor processing, in which trench width is similar to the space between adjacent high aspect ratio elements and trench depth is similar to the height of the structure. With film deposition in a trench, deposition at any point on the trench sidewall is due to adsorption of an incoming precursor gas molecule and the transformation of the gas molecules to a solid state. This is akin to film deposition on a surface of a high aspect ratio element being due to adsorption of an incoming precursor gas molecule and the transformation of the gas molecules to a solid state. Accordingly, deposition thickness at any point is a product of the reactive sticking coefficient and the total number of impinging molecules.
Precursors to a solidifying material such as polysilicon have a sticking coefficient of approximately 10−3, while precursors to a solidifying material such as Si3N4 have a much smaller sticking coefficient of approximately 5×10−5. As such, for a solidifying material such as polysilicon, it is expected that a larger fraction of precursors stick to high aspect ratio elements at their uppermost portion (an example of an uppermost portion being shown in
With the present invention, a solidifying material is deposited and coats around a high aspect ratio element and, thereby fills the structure. The coating is typically uniform or substantially uniform as shown in
The coating provided by the present invention increases the thickness of each individual high aspect ratio element which offers several advantages. One such advantage is that with the present invention there is an ability to control filling, such as partially filling or complete filling of a structure. Such control is not possible with many other techniques, such as liquid filling procedures and provides structured films of known porosity and/or known stoichiometry. An example of a partially filled structure is shown in
Another advantage is that there is an ability to layer disparate structured films in any desired sequence and at any thickness. Accordingly, the step of coating a plurality of high aspect ratio elements of a structure may be repeated as often as necessary to prepare a desired composition or device having one or more desired mechanical, physical, chemical, optical, biologic and/or electrical properties. The coating process may be repeated with the same solidifying material or with different solidifying materials.
Still another advantage is the present invention as described herein prevents the mere sticking of a solidifying material to the top of individual high aspect ratio elements (thereby, preventing capping) and allows the formation of a continuous structured film. Furthermore, high aspect ratio elements are not matted down with the present invention, which could greatly reduce the vertical height or length and/or alignment of such elements. Instead, the present invention provides structures that have retained their original size and/or shape. It negates the need of post-processing to ensure uniform dispersion of structures comprising high aspect ratio elements as compared with current (conventional) techniques required for making composite structures.
Post-processing techniques known to one of ordinary skill in the art may be provided to structures of the present invention when desired. Techniques may include chemical treatment (e.g., etching), physical treatment (e.g. chemical mechanical polishing), plasma treatment (e.g. surface modification), thermal treatment (e.g. annealing, melting, converting), optical treatment (e.g., irradiation) and combinations thereof. In addition, techniques may involve removing high aspect ratio elements and/or the substrate from the structure. Such techniques are well known to those of ordinary skill in the art.
In some embodiments, deposition of the one or more structures comprising a plurality of high aspect ratio elements and coating the plurality of high aspect ratio elements with one or more solidifying materials may be provided by the same CVD system. For example, as described herein, structures of up to and greater than 50 μm in height may be prepared and then filled to form a structured film in a continuous process.
Compositions and structured films formed by systems and methods of the present invention are typically at least twice as thick (in a given deposition time) as that achieved by conventional processes known in the art to one of ordinary skill. Structured films also exhibit reduced stress as compared to films formed without the initial structure (data not shown).
In accordance with various aspects of the present invention, structured films formed as described herein may be any desired thickness, hardness, density and have any desired electrical property by selection of the structure, its support, and/or solidifying material. In addition, methods as described herein may be varied to provide a desired structured film density, hardness and/or thickness. In another form, the present method provides for fabricating compositions in the absence of a support or substrate (e.g., a stand-alone platform).
The present invention helps overcome the current inability to deposit typical electronic and MEMS materials like polysilicon, silicon nitride, metals and polymers of thicknesses greater than a few micrometers. Current deposition techniques are slow (typically 1 μm per hour). Thus depositing films that are tens of micrometers thick becomes infeasible. In addition, the internal stress in such thick films can become high leading to various film degradation effects. The present invention overcomes such limitation by incorporating and coating around high aspect ratio elements to provide a structured film that alleviates the stress in the film. The reduced stress is, in part, due to the inherent strength of high aspect ratio elements as described herein. In addition, as a result of having structures with high aspect elements, structures now have an increased surface area (and sites) available for adsorption and subsequent coating as compared with a comparative structure (or support or substrate) absent high aspect ratio elements.
While particular embodiments of the invention have been described herein, additional alternatives not specifically disclosed but known in the art are intended to fall within the scope of the invention. Thus, it is understood that other embodiments and applications of the present invention will be apparent to those skilled in the art without departing from the scope and spirit of the appended claims and drawings.
