Patent Application
20090117339
The present invention relates broadly to a method of component assembly on a substrate, and to an assembly of a bound component on a substrate.
The creation of integrated optical devices from separate micro-components has, in the past, required time-consuming and often manually intensive methods. Attempts to alleviate these difficulties have seen the emergence of more mechanized technologies that focus on assembly either via fluidic self-assembly or methods that are based on wafer-to-wafer transfer. Key to all these technologies is the substrate which is either a specifically prepared ‘receptor’ with precisely etched holes that are complementary to the optical components, or substrates that require equally stringent photolithographic alignment and/or masking. The current technologies used for the integration of optical components are restricted by the limited number of compatible substrates (e.g. silicon, silicon oxide, gallium arsenide).
Ideally, the optical designer should not be limited by the fabrication technology. For example, one should be able to integrate III-V light sources and detectors with Si based photonic crystals, modulators and/or micro-mirrors, with SiO2 waveguides, and non-linear optical devices on any substrate. The function and/or complexity of an integrated optical circuit should not be restricted by the substrate.
“Strained layer epitaxy” is used to integrate semiconductors with dissimilar lattice structures, such as growing GaAs on Si, or SiGe alloys on Si, etc. However, this technique is only possible if the respective layer thicknesses are thinner than a critical thickness which is typically extremely thin. In addition, this technique is only useful for crystalline materials, and is not useful for integrating non-crystalline materials such as plastics and glasses. The use of MEMS (Micro-Electro-Mechanical Systems) for integrating mechanical components, sensors, etc with electronics on a silicon substrate using microelectronic technology is also made use of. This technology relies on devices, such as micro-mirrors, waveguides, cantilevers, etc that are Si (and SiO2) based and are micromachined into Si. Again, this method is limited to Si and SiO2 and is not useful to integrate other materials, such as GaAs, electro-optic materials, etc
There are a number of other techniques that are grouped into ‘top-down’ and ‘bottom-up’ approaches. The top-down approach involves a block of material being processed into the desired shape and working unit. In bottom-up fabrication, small building blocks (usually nanoscale as the term originates from nanotechnology) are connected together to fabricate a functioning unit.
Current top-down approaches for integrating optical structures on a substrate typically involve fluidic assembly into defined ‘holes’ in a substrate, lithographic patterning followed by etching or wafer-to-wafer transfer. These are very complicated procedures that lack the ability to be easily scaled up and typically suffer from low fabrication success rates.
On the other hand, while there are many potential bottom-up strategies for fabricating optical structures on different materials, no current method for assembling high quality optical devices (prefabricated) on any substrate has been demonstrated. A sufficient understanding of how to assemble molecular building blocks with sufficient control to produce high quality materials (that is, comparable to microelectronics state of the art) has not been reached.
Recently, methods for electric field assisted self-assembly of functionalized DNA strands as building blocks for assembly and fabrication of devices have been proposed in U.S. Pat. No. 6,652,808. However, the methods disclosed in that document focus primarily on the control and chemical nature of the DNA based building blocks for bonding of components to a substrate, rather than providing any teaching with respect to the properties or functionality of the devices bound to the substrate. Furthermore, an approach for building a photonic band-gap structure is disclosed, where a photonic band-gap structure is built-up from metal beads exhibiting magnetic properties. The photonic band-gap structure is formed on the substrate through a process in which the metal beads are interconnected via DNA bonds. No optical characterization of such grown photonic band-gap structures is provided in that document.
Furthermore, there is no teaching provided in that document that verifies whether the alignment accuracy between the metal beads is actually sufficient to achieve a photonic crystal effect, and on which substrate or type of substrates. A technique for alignment of “larger” structures of the order of 10 to 100 microns is also discussed in that document, using selective derivatisation with different DNA sequences of a device to be positioned and oriented on a substrate. However, no teaching is provided with respect to handling of larger devices, thus limiting the proposed method to techniques in which the devices to be attached are smaller than about 100 microns, and with a need to apply individual devices in that size range to the substrate for assembly. The preparation of free-standing devices in that range of small sizes can constitute a major challenge in the overall assembly process, in particular with a view to mass-production of assemblies of devices on various substrates.
A need therefore exists to provide a method of component assembly on a substrate that seeks to address at least one of the above-mentioned problems.
In accordance with a first aspect of the present invention there is provided a method of component assembly on a substrate, the method comprising the steps of forming a free-standing component having an optical characteristic; providing a pattern of a first binding species on the substrate or the free standing component; and forming a bound component on the substrate through a binding interaction via the first binding species; wherein the bound component exhibits substantially the same optical characteristic compared to the free-standing component.
The forming of the bound component may comprise applying the free-standing component to the substrate for establishing the binding interaction via the first binding species, and removing portions of the free-standing component unbound via the first binding species such that the pattern of the first binding species is transferred to the formed bound component.
The method may further comprise providing a second binding species on the free-standing component or the substrate, and the binding interaction between the free-standing component and substrate is via the first binding species binding with the second binding species.
The substrate may comprise a further component formed thereon, and the at least a portion of the free-standing component is bound to a surface of the further component, and wherein the further component and the bound component from at least part of an integrated component.
The integrated component may comprise an optical component.
The method may further comprise forming a material layer on the further component, the free-standing component, or both, such that the material layer is sandwiched between the further component and the bound component in the integrated component.
The material layer may be chosen such that the integrated component exhibits a desired optical characteristic.
The material layer may comprise at least the first binding species.
The material layer may comprise an organic material, and the further component and the bound component may comprise inorganic materials.
The substrate and the free-standing component may be lattice mismatched.
The substrate may be flexible.
A lateral dimension of the bound component may be in the range of nm to mm.
The method may further comprise the step of providing a blocking species in areas not covered by the pattern of the first binding species, prior to forming the bound component on the substrate through the binding interaction via the first binding species, for enhancing the selectivity of the binding interaction.
The first binding species may be chosen such that the binding interaction comprises one or more of a group consisting of a biomolecular interaction, van der Waals forces, hydrogen bonding, hydrophobic/hydrophilic, metal coordination, electrostatics, and covalent bonding.
The method may comprise forming two or more different free-standing components, each free-standing component having an optical characteristic; providing respective patterns of two or more different first binding species on the substrate; and providing different second binding species on the respective different free-standing components corresponding to the respective different first binding species, forming respective bound components on the substrate through binding interactions between the different free-standing components and the different first binding species via the different corresponding second binding species; wherein the bound components exhibit substantially the same respective optical characteristics compared to the corresponding free-standing components.
In accordance with a second aspect of the present invention there is provided a assembly comprising a substrate; and a bound component assembled on the substrate through a binding interaction via a first binding species provided on the substrate or on a free-standing pre-form of the bound component; wherein the bound component exhibits substantially a same optical characteristic compared to the free-standing pre-form.
The bound component may be a portion of the free-standing pre-form with other portions of the free-standing pre-form unbound via the first binding species removed.
The assembly may further comprise a second binding species on the bound component or the substrate, and the binding interaction between the bound component and substrate is via the first binding species binding with the second binding species.
The substrate may comprise a further component formed thereon and the bound component is bound to a surface of the further component, and wherein the further component and the bound component from at least part of an integrated component.
The integrated component may comprise an optical component.
The assembly may further comprise a material layer on the further component, the bound component, or both, such that the material layer is sandwiched between the further component and the bound component in the integrated component.
The material layer may be chosen such that the integrated component exhibits a desired optical characteristic.
The material layer may comprise at least the first binding species.
The material layer may comprise an organic material, and the further component and the bound component comprise inorganic materials.
The substrate and the bound component may be lattice mismatched.
The substrate may be flexible.
A lateral dimension of the bound component may be in the range of nm to mm.
The assembly may comprise respective patterns of two or more different first binding species on the substrate; and different second binding species on respective different bound components corresponding to the respective different first binding species, the bound components being bound through binding interactions between the bound components and the different first binding species via the different corresponding second binding species; wherein the bound components exhibit substantially the same respective optical characteristics compared to respective corresponding free-standing pre-forms of the different bound components.
Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:
a to d show the characteristic optical reflectivity spectra of a PSi microcavity as prepared, and assembled on GaAs, silicon dioxide and poly carbonate respectively, using the method of
a shows reflectivity spectra of two different microcavities assembled on the same polycarbonate substrate using the method of
b shows a schematic representation of attachment of two different microcavities onto different locations of the same substrate according to an example embodiment.
a and b show reflectivity spectra of structures fabricated using the method of
a to c show reflectivity spectra of a Bragg mirror and different assembled microcavity structures fabricated using the method of
a to d show the optical properties of a substrate reflector and formed microcavities with different spacer layers respectively fabricated using the method of
a shows reflectivity spectra of a PSi Bragg mirror before and after deposition of a PMMA layer by spin coating, according to another example embodiment.
b shows reflectivity spectra of microcavities fabricated using a PMMA spacer layer in the method of
The integration of different optical components on the same substrate, as well as optical components with electronic devices, has been hindered by different components typically being made of different materials. Hence a problem has existed where either optical components are all made from the same material, hence compromising the performance of some or all of the components, or the problem has been how to integrate components made from the different materials onto the same substrate. Thus the problem is one of material incompatibility. The described example embodiments provide methods that can overcome this problem by harnessing the recognition properties of biological molecules to enable the assembly of optical materials on any substrate. Porous silicon (PSi) microcavities and Bragg mirrors are fabricated and assembled on silicon, gallium arsenide and plastic. The substrate material is modified by application of a biological molecule to define the location for assembly. Optical components modified with the complementary biomolecule self-assemble only onto the correct location without compromising their optical integrity. In another embodiment optical components can be deposited onto and adhered to a substrate via patterns of an adhesive ultrathin coating. Furthermore, the technique in the example embodiments allows assembly of new devices from components of different composition as demonstrated by incorporating different spacer layers between porous silicon Bragg mirrors to create a resonant microcavity.
Described embodiments use biomolecule directed or adhesive coating directed assembly of prefabricated high quality optical structures on the micro and macroscale without micromachining requirements. In contrast to biomolecule directed assembly of photonic crystals from colloidal building blocks (described e.g. in U.S. Pat. No. 6,752,868 B2), which cannot produce the high quality optical structures required for the fabrication of optical circuits, in example embodiments high quality Bragg mirrors and resonant microcavities were formed by anodization of silicon. In one embodiment, the macroscale assembly of optical films occurs on substrates patterned with complementary biological molecules. The high affinity of biorecognition causes assembly at the applied pattern only, while the remainder of the film fractures upon rinsing and drying steps leaving a macroscale pattern of optical structures (>1 mm). In another embodiment, a macroscopic free-standing optical structure was fractured by sonication in ethanol to produce microparticles (<100 μm). Utilizing biorecognition, the optical microparticles are assembled in the correct orientation when applied to the biomolecule labelled substrate. Example embodiments of the present invention can create optically flat materials on a macroscale such that high quality optical characteristics are maintained. In contrast to building an optical structure using the bottom up approach, example embodiments can allow assembly of prefabricated high quality optical components over multiple length scales.
Example embodiments assemble optical materials on any substrate that allows biorecognition or deposition of thin coatings to mate the materials together. In one embodiment, resonant microcavities fabricated with porous silicon were removed from silicon and coated with biorecognition molecules. A number of substrates including: silicon, silicon dioxide, galium arsenide and polycarbonate, were patterned with aqueous solutions of complementary biomolecules. Application of the labelled microcavities to the patterned substrates yielded assembly at the biomolecular pattern only, while the remaining microcavity was rinsed away with ethanol.
Example embodiments provide a combination of high quality top-down optical structure fabrication techniques with a bottom-up assembly method (a hybrid approach) exploiting biorecognition or an adhesive coating to form new devices. Previous work on assembling optical structures has involved either 1) the top-down fabrication of optical materials (e.g. PSi microcavity formation) or 2) bottom-up assembly of new optical materials (e.g. colloidal crystal fabrication). By first forming high quality optical materials using top-down fabrication followed by e.g. biomolecule directed assembly of multiple components, a high quality optical structure can be created in example embodiments. Other materials (e.g. responsive polymers and small molecules, metals, nanoparticles and objects, redox and photosynthetic proteins, molecular wires, carbon nanotubes, ionic liquids/liquid crystals, lipid layers, cells, diatoms, silica and polymer beads and many other functional molecules and materials) can be incorporated with the high quality optical structures such that novel properties and new emergent functions may be harnessed.
For the PSi film 100a photonic crystal formation, the Si(100) wafer 104 (p++, B-doped, 0.005 Ωohm cm, single side polished) was cleaned by sonication in ethanol and acetone and blown dry under a stream of nitrogen. The cleaned wafer 104 was etched in an electrochemical cell with a polished stainless steel electrode as back-contact and a Pt ring counter electrode using 25% ethanolic HF (mixture of 50% aqueous HF and 100% ethanol, 1:1, v/v) as electrolyte. The power supply was controlled using custom written software to modulate the current density and etching times during the etching process. Etch stops were incorporated into the etching program to allow recovery of the HF concentration at the etching front. The current densities and etch times required to obtain the PSi layer 100a of desired porosity and thickness were calculated from calibration curves obtained for each batch of Si wafers and etching solutions.
At the end of the electrochemical etching that creates the cavity, a high current pulse is applied (
Subsequently, the modified device 100c is released from the Si wafer 104 (
It is noted that other blocking species may be used in different embodiment, including, but not limited to, thin films of or self assembled monolayers (SAMs) terminated with
ethers and derivatives of poly-/oligo-(ethylene glycol)
amines/ammonium salts
amides, amino acids, peptides
Crown ethers
sugars, polyols (eg mannitol)
surfactants (eg Triton X-100)
zwitterionic groups (eg phosphrylcholine)
perfluorinated groups
protein
synthetic polymers
natural polymers or combinations thereof. It is noted that, depending on the nature of the binding species in different embodiments, the use of a blocking species is optional.
As seen in
It is important to note that the optical properties of the devices advantageously remain the same independent of the substrate in different example embodiments.
n
PSi
1/3=(1−p)nSi1/3+pnair1/3
The starting parameters of the simulation (layer thickness and porosity) were taken from the etching program which calculates current density and etch times for a desired layer thickness and porosity from calibration curves. The values were then refined to achieve good agreement between the measured spectrum and the simulation. For a number of samples the total thickness of the PSi sample was determined by profilometry to validate the layer thickness values used in the simulations. In
The reflection spectra 200-203 of the optical cavity are characterized by sharp ‘dips’ 208-211 in the reflectivity at the resonant frequency in the Bragg plateaus 212-215 (the regions of high reflectivity). The position and spectral width of the resonance is a sensitive measure of the structure and quality of the cavity. As can be seen in
As a self-assembly approach, an advantage of the described embodiments is the possibility of depositing several components simultaneously without the need to individually align them at the desired locations on the substrate, as this task is performed by the biorecognition. Another benefit of using biorecognition to assemble optical structures in the example embodiments is the possibility to self-assemble different optical components onto the same substrate by using different biorecognition pairs. This concept is demonstrated in
In this example, at location B the substrate 304 is modified with avidin 314, whilst at location A the substrate 304 is modified with biotinylated BSA 316. The two separate free standing microcavities, B′ 300 and A′ 302, are modified with biotinylated BSA 318 and avidin 320, respectively. Biorecognition therefore dictates that cavity A′ 300 assembles at position A, and similarly, the avidin modified cavity B′ 302 binds to the biotinylated substrate 304 at location B. It was found that cavity B′ 302 did not assemble over spot A or vice versa. Also, there is no need to align each optical cavity 300, 302 precisely with its respective receptor spot(s) 314, 316 on the substrate 304. Unbound regions of the deposited free-standing structure simply break away during the washing step (compare
In other embodiments, biorecognition is also capable of self-assembling optical devices from separate components. In one example, PSi microcavities were assembled from two independent Bragg mirrors using biorecognition to create the desired resonant cavities. The steps used are shown in
To test the formation of a cavity resonance, the reflectivity spectra 500, 502 of the structures were measured before and after assembly of the mirrors, shown in
To further test this capability, several cavities with spacer layers of different optical thicknesses (which can be achieved either by varying the thickness or the porosity of the layer) were fabricated via deposition of a Bragg mirror with integral spacer layer, and the cavity resonance was always in agreement with theoretical predictions.
Further evidence for the uniformity of the assembly of optical structures is obtained from SEM and profilometry measurements. The SEM image 700 in
Apart from being able to assemble or form high quality optical structures, the usefulness of the biomolecular self-assembly technique in the example embodiments is determined by the success rate of forming the correct device in the correct location.
Using separate components to assemble optical structures has additional benefits. In the case of optical microcavities, the method of example embodiments can allow complete flexibility in choosing the mirrors and the spacer layer.
In a further embodiment, poly(methyl methacrylate) (PMMA), a common laser gain medium and lithographic material, was spin-coated onto a substrate mirror followed by assembling a free-standing mirror to define the microcavity. It was found that by spin-coating different thickness polymer layers, the frequency (wavelength) of the final cavity resonance can be easily tuned. This embodiment enables the integration of organic materials with (inorganic) high quality optical components.
a shows reflectivity spectra 1200, 1202 of a PSi Bragg mirror before and after deposition of an approximately 500 nm thick layer of PMMA by spin coating respectively. The positions of the Bragg plateau and the interference fringes do not shift after deposition of PMMA, which demonstrates that the polymer did not enter the pores of the PSi structure, i.e. the properties of the cavity layer can be adjusted without altering the composition and optical properties of the Bragg mirror.
The high degree of strength and uniformity imparted with biorecognition or with the use of adhesive coatings and the prospect of removing unbound material makes the approach in the example embodiments amenable to lithographic patterning. For instance, inkjet printing or soft lithographic stamping of proteins could define the circuit geography and deposition of silicon photonic material accomplished by the methods of the example embodiments. Furthermore, the approach can be extended for any optical material such that patterning different biomolecules for mixing different components could provide unprecedented ease and flexibility in optoelectronic circuit construction especially when taking into account the wide range of surface functionality that can be introduced on semiconductors (e.g. via hydrosilylation chemistry for Si and PSi), metals and polymers. Incorporating the cavity layer separately was demonstrated using thin PSi layers and PMMA in example embodiments. Different doping schemes can allow material to be confined exclusively to the cavity layer, a major advantage to using PSi for lasing applications. Incorporating alternative polymeric materials into the resultant photonic assembly is also possible and can open the door for new composite materials for diverse applications (e.g. laser gain medium, optical switches, biosensing at the cavity layer etc.).
The described embodiments provide methods that utilize biological recognition as a driving force for assembling photonic components into more complex architectures on a larger range of substrates. With the continued need to develop robust and flexible strategies to incorporate photonic components into complex devices, this advance expands current capabilities into composite materials. In conjunction with the evolving landscape of lithographic techniques and nanofabrication, harnessing the power of nature's complexity with self-assembling systems in the example embodiments can become a powerful synergistic tool for technological advancement in e.g. the photonic industries.
Current strategies for integrating optical components on a substrate require wafer-to-wafer transfer or photolithographic masking and etching to define a precise pattern that physically holds the optical components. In contrast, in the described embodiments, registration of optical components can be performed by spotting a biomolecule solution in a defined location. Importantly, the biomolecule pattern on the substrate dictates the patterning such that rinsing removes any non-specifically bound optical material. Thus the example embodiments allow a simple and flexible method to spatially array optical components which is amenable to existing liquid handling techniques, such as inkjet printing or soft lithographic stamping.
The described embodiments can provide a platform technology that allows, inter alia,
By integrating different components on any substrate and simplifying the registration of optical components on the substrate, the example embodiments can lead to new and novel materials and even multiple different materials to be incorporated into optical devices by using the described biological assembly approach. This described methods in example embodiments have the potential to revolutionize the way optical devices and integrated optical circuits are fabricated and thus can lead to improvements in current technologies and many novel devices.
The example embodiments can allow virtually unlimited resources for fabrication diversity. For instance, different combinations of the four bases of DNA or RNA for hybridization assembly, using DNA ligands that bind proteins, called aptamers, can be fabricated and screened using a process called SELEX, monoclonal/polyclonal antibody production for many different antigens, phage display library screening to optimize recognition, use of combinatorial peptide libraries for the selection of peptides binding to inorgranic substrates, protein:protein recognition. Thus the choice of assembly pairs can be very large including interactions such as van der Waals forces, hydrogen bonding, hydrophobic/hydrophilic, metal coordination, electrostatics, covalent bonding.
Application of the biological species in the example embodiments is predominantly aqueous wet chemistry with mild conditions, thus avoiding any harsh treatment that may damage sensitive optical components (i.e. high temperature). The fabrication can represent a ‘green’ approach. Many techniques can be used and exist to apply biomolecules to a substrate in well-defined patterns, including ink jet printing and soft lithography. In the example embodiments, complementary biorecognition molecules or thin adhesive coatings drive the assembly of optical components onto virtually any substrate without requiring any micromachining. Biorecognition or thin adhesive coatings can allow previously incompatible materials to be integrated seamlessly on the same device. The biorecognition layer or adhesive coating may allow interesting ‘soft’ and ‘hard’ components to be integrated by themselves or as composites with the optical materials (i.e. responsive polymers and small molecules, metals, nanoparticles and objects, redox and photosynthetic proteins, ionic liquids/liquid crystals, lipid layers, cells, diatoms, silica and polymer beads etc.)
Embodiments of the present invention can provide a hybrid top-down/bottom-up strategy for producing optical structures by biomolecular assembly of high quality optical materials. Labelling the optical material with a biological receptor and the substrate with the complementary ligand (or vice versa) can allow the assembly of any optical structure on any substrate in a well defined manner. This can allow previously unrealized components to be assembled together on the same substrate. No micromachining or masking for lithography is necessary on the substrate and simple liquid transfer techniques can define the pattern (circuit geography). Using a biological assembly approach in the example embodiments can allow flexibility in substrate choice such that any planar substrate can be patterned with a biorecognition molecule for assembling optical structures. Thus, any combination of optical structures may be integrated on any material.
Assembling new materials/devices using biomolecule directed assembly or assembly using adhesive thin films of prefabricated high quality optical components was demonstrated in example embodiments. Biomolecule directed assembly of two optical structures can allow formation of a third optical structure, where the joining of the two optical structures produces a new optical characteristic in the resulting structure. Furthermore, incorporating diverse materials into assemblies with high quality optical components is possible in different embodiments towards a range of new optical materials.
It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.
For example, it will be appreciated that other optical characteristics of the free-standing device may be substantially maintained after assembly, other than the transmission/reflectance spectra described for the example embodiments, and including, but not limited to, optically tested characteristics of non-optical devices for substantially maintaining machining tolerances, such as optical interference based characterisation for assembly of micro mechanical or micro electro mechanical systems (MEMS) on a substrate.
