Patent Application
20230042586
The present disclosure generally relates to a method to mitigate the effect of surface reflections during holographic lithography of a photoresist.
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it may be described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present technology.
The formation of a three-dimensional photonic crystal yields a structure where, due to a favorable geometry of voids and mass of high refractive index, photonic band gaps can be prepared where light does not propagate in any direction. These crystals have been developed with the goal of forming enabling devices for optical switching, sensors, filters, wave guides, or any other structure that is functional due to a universally connected structure with percolating pores throughout. These materials have features with dimensions of the nanometer and micrometer scales. A method to produce such materials is by multibeam interference lithography, also called holographic lithography. Multibeam interference lithography does not provide a flexibility and resolution that is exhibited by other techniques, such as electron or ion beam lithography (EBL or IBL), however, superior patterning speed and pattern coherence are observed at scales larger than about 10 nanometers.
In multibeam interference lithography, four laser beams can be used to generate three-dimensional periodicity by the interference pattern of light activated volumes and void volumes. Three-dimensional structures are fabricated in the light activated medium by a photoinitiated polymerization of a photoresist. The polymerization is carried out in a post-patterning baking process, as initiation occurs without significant propagation during the lithographic process at normal ambient room temperatures. Light reflection from a substrate supporting the photoresist can reduce the resolution and quality of the interference pattern that is generated, causing photoinitiation in volumes where direct light is designed to result in destructive interference to ultimately generate a volume void of condensed mass.
Typically, reduction or removal of the reflectance is carried out by coating a substrate with an anti-reflective coating. The coating can be deleterious to a subsequent process required to fabricate a device where the three-dimensional photonic crystal is employed as a template. For example, an electroplating metallization process requires an electrically conductive substrate. To this end, an improved holographic lithography process that gives a high-resolution pattern formed on a reflective substrate is desirable.
This section provides a general summary of the disclosure and is not a comprehensive disclosure of its full scope or all features of embodiments.
In various aspects, the present teachings provide a method to form a three-dimensional periodic structure having features of about 0.1 to about 10 microns prepared by a multibeam interference lithography process with a negative photoresist on a reflective substrate, which, after development of the exposed photoresist, forms a periodic pattern of solid constructive volumes and void destructive volumes. This method employs four or more laser beams of non-equal intensity, and the beams' interference allow the periodic structures to be formed on the reflective substrate. The reflective substrate has sufficient electrical conductivity for use of the resulting periodic structure as a three-dimensional photonic crystal template for subsequent processing. The three-dimensional photonic crystal template can be subsequently modified to generate active devices that are supported by the solid template or formed within its voids. Electrodeposition of metals can be performed using the three-dimensional photonic crystal template where the conductive reflective substrate is a portion of an electrode.
In another aspects, the present teachings provide a method to perform multibeam interference lithography with at least four laser beams, where a negative photoresist is patterned on a reflective substrate without anomalies due to undesired mass in the destructive volumes. In one embodiment, a central beam can be delivered perpendicularly to the reflective substrate and a multiplicity of equally spaced beams relative to the central beam can be disposed at an angle of less than about 40° to the central beam. By having a central beam of greater intensity than the other beams, and by using a light dose that is small, reflected light is insufficient to activate sufficient photoinitiator within the direct destructive volumes of a negative photoresist composition to generate defects in a generated three-dimensional photonic crystal.
Further areas of applicability and various methods of enhancing the above coupling technology will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
The present teachings will become more fully understood from the detailed description and the accompanying drawings, wherein:
It should be noted that the figures set forth herein are intended to exemplify the general characteristics of the methods, algorithms, and devices among those of the present technology, for the purpose of the description of certain aspects. These figures may not precisely reflect the characteristics of any given aspect and are not necessarily intended to define or limit specific embodiments within the scope of this technology. Further, certain aspects may incorporate features from a combination of figures.
The present teachings provide a three-dimensional photonic crystal template on a reflective conductive substrate. For purposes of embodiments, a conductive substrate is a non-insulating material with a conductivity greater than deionized water. In various embodiments, the three-dimensional photonic crystal template has a periodic structure without defect features due to an unintended cross-linking initiated by reflected light rays in destructive volumes, which are regions where destructive interference occurs and are intended to be removed upon dissolving a photoresist composition from constructive volumes, which are regions with no destructive interference of light waves during multibeam interference lithography. No antireflective coating is situated between the substrate and the patterned cured photoresist to minimize reflectance, as sufficient conductivity is intended for use of the three-dimensional photonic crystal template for subsequent processing. The three-dimensional photonic crystal template is suitable for preparing periodic porous supported catalysts, metallic heat transfer devices, or other metallic devices, where the fabrication of these devices is facilitated by having an electrically conductive substrate that allows electroplating and other electrolysis processes to be carried out.
The presently disclosed three-dimensional photonic crystal templates on a reflective substrate are generated by an interference lithographic method using four or more laser beams where the beams are of unequal intensity. The periodicity of the three-dimensional photonic crystal template is affected by the angle of incidence of the laser beams relative to each other and the substrate in the lithographic process. Although differing orientations and alignments of beams are possible, in one embodiment, one central beam is situated perpendicular to the substrate supporting the patternable photoresist. The three other peripheral beams are situated about 120° from each other in a periphery to the central beam and directed at an acute angle, generally, but not necessarily, less than about 40° to the central beam, and intersecting with the central beam at an interface of air or inert gas and a planer photoresist surface. The central perpendicular beam can be of greater intensity, for example, about four times the intensity, of the other three beams, either individually or collectively. By using this geometry and a reflective substrate, the dose required for pattern formation is achieved with a very short irradiation period, for example, less than about 0.3 seconds for a 5-watt laser. The laser interference provides a sufficient dose with the short irradiation for initiator generation in a pattern defined by the interference of the laser beams throughout the photoresist. The oriented laser beams define a periodic pattern of constructive volumes without destructive interference of the light waves where photoactivation of the photoinitiator occurs and destructive regions where the irradiation geometry insures destructive light interference to avoid photoinitiation. By employing a limited light dose and the inequal intensities of the beams, the reflected light is of insufficient intensity to allow an insoluble mass to be formed in the destructive volumes where voids are to be formed after development of the photoresist composition. By fixing the laser intensity, modification of the process requires manipulation of only the exposure time.
As detailed herein, the present teachings not only include the method for making periodic photonic crystal templates with few aberrations due to reflected light, but by using a fixed, though not equal, laser intensity for all beams, the fraction of the final three-dimensional photonic crystal templates that are voids is controllable and is inversely proportional to the dose period. As the exposure period is controlled by effectively switching the light on and off, the void portion of the template can be altered as needed to preserve the integrity of the void volume, even during a production run. By appropriate choice of the angle of the laser beams to the substrate, at least a portion of reflected light can be complementary to the incident light for generating a desirable three-dimensional photonic crystal template and unintended photoactivation designed destructive regions can be minimized to the point of no significant anomalies in the pattern.
The three-dimensional photonic crystal templates of the present disclosure can be used as a template for processes that require a sufficiently conductive substrate, such as a silicon substrate, to allow electrodeposition or other processes requiring the substrate to be used as an electrode. In this manner, a relatively large area template with nanometer to micrometer features can be formed with a simple short burst of laser light. The three-dimensional photonic crystal template can be of various negative photoresists, when the initiation of polymerization can be generated photolytically by the laser wavelengths employed. In general, but not necessarily, the polymerization is a living or controlled diffusion polymerization, such as a photo-cationic, photo-anionic, or a photo-generated reversibly stabilized radical polymerization. For example, the negative photoresist can be SU-8, where a multi-epoxy monomer is polymerized via a cationic photo-initiator that is activated by the light projected across the surface and the interference pattern of initiating cations is formed within the glassy photoresist.
In one aspect of the present technology, the laser beams may be oriented in the manner illustrated in
In one aspect, the method does not require any modification of an established development protocol for initiation, polymerization, crosslinking, and developing to form the cured photoresist resin. A traditional process for forming and developing a photoresist is outlined in the flow chart provided in
Portions of the process of
The exemplary structure of
As shown in
The post irradiation processing is represented by the transition of
After baking, development continues with the dissolving of the monomer and non-crosslinked polymer in the destructive volumes that will generate the voids of the three-dimensional photonic crystal template. As shown in
As shown in
In the development of the polymer, which further effects the dissolving of the unpolymerized or insufficiently polymerized regions, diffusion rates of the generated active species and propagation rates of the photoresist polymer must be considered. As the degrees of initiation and propagation affect the diffusing species structures, controlling the three-dimensional photonic crystal template structure becomes a complicated process where a viable window to generate the desired structure is easily missed and where small difference in any of time, temperature, and concentration can promote a loss of control required to avoid defect structures. The control of the dosing rate has little error associated with it, as control is exclusively of the amount of initiator generated, particularly with photoinitiators with a high quantum yield, such as, but not limited to, iron-arene salts, such as, cyclopentadienyl(fluorene) iron (III) hexafluorophosphate as a cationic initiator. Photoinitiators that do not act as thermal initiators at temperatures employed for deposition on the substrate or baking during development are advantageous. Other iron-arene salts that can be used include Irgacure 261, [Cp-Fe-Naph]PF6, CFC, and CFA. Other photoinitiators include diaryliodonium salts and triarylsulfonium salts. The anion coupled to the cation of these photoinitiators can be, but are not limited to, triflate, tosylate, PF6−, SbF6−, and BF4−. A photosensitizer can be included with the photoinitiator. Useful photosensitizers include DIEF, RBAX, TIHF, Acridine Orange, and Erythrosin B. In this manner, the method is implemented by fine control of only one variable, the photonic dose, which can be controlled by the irradiation period, and does not require modification of the development steps where complicated multiple processes occur to define the resin volumes and void volumes.
To avoid structural collapse of the voids during a final drying of the three-dimensional photonic crystal template due to surface tension effects, drying can be carried out using supercritical CO2 where the liquid-vapor interface is beyond the critical point. The wet three-dimensional photonic crystal template is placed in liquid CO2 followed by heating past the critical point and releasing the pressure until ambient pressure and temperature is achieved. The resulting three-dimensional photonic crystal template is isolated with distinct constructive volumes of cross-linked photoresist polymer and destructive volumes void of condensed matter.
Modification of the irradiation profile during the formation of the three-dimensional photonic crystal template or by decorating the deposited photoresist surface with reflective or absorbing particles allows the formation of uniform, patterned or gradient periodic structures along any axis and bias the proportion of void volume in the template. This permits the formation of a metal matrix device that displays the negative of the template, where the void volumes result in the metal features and the resin features of the template generate the voids of the metal matrix upon degradation.
The resulting polymeric three-dimensional photonic crystal template is one that, due to a substrate with sufficient conductivity, can be used to fill the voids with a metal or other material that can be deposited by an electrodeposition process. In this manner, the avoidance of defects is critical to assure that the necessary percolation of voids through the template allows the growth throughout the entire template formed from the photoresist layer. Removal of the template allows development of a metallic device, where the positive features of the device are formed in the voids in the template. Other devices, such as a high surface area catalytic supports, can employ the three-dimensional photonic crystal template as a scaffold where a catalyst layer is deposited on the template.
Methods
A diode pump solid state laser, Verdi 5W (Coherent Inc.), a frequency doubled Nd:YVO4 laser, was used at 532 nm for multi-beam interference lithograph. The Verdi 5W laser has a maximum power of 5.5 W and a beam diameter of 2.25 mm with the beam expanded to as much as 9 mm using a spatial filter and sub-second exposure times. The optical platform employed a rigid table-top (Newport RS-4000) and pneumatic isolators (Newport I-2000) for vibration dampening. The beam was split into four beams and arranged by the mirrors into the umbrella geometry shown in
The photoresist composition was SU-8, on average an octafunctional monomer, with cyclopentadienyl(fluorene) iron (III) hexafluorophosphate as photoinitiator and deposited as a solution in cyclopentanone and spin-coated on a silicon wafer. Solvent evaporation from the photoresist composition was carried out on a hot plate to a temperature of about 65° C. for ten minutes and about 95° C. for twenty minutes. After cooling to room temperature, exposure was restricted to 0.15 seconds using an electric shutter. After exposure, development was started with a bake at about 85° C. for twenty minutes. After cooling to room temperature, PGMEA was used with gentle stirring to dissolve the destructive volumes and form the periodic structure of the three-dimensional photonic crystal template. Immersion of the template into isopropanol resulted in the extraction of residual PGMEA from the cured SU-8 resin. Supercritical drying was carried out by immersing the isopropanol wet SU-8 resin on the Si wafer in liquid CO2 using a critical point dryer (Samdri 790) at about 0° C. and slowly heating through the critical point followed by slowly releasing the pressure to ambient and cooling to ambient temperature, which yielded the three-dimensional photonic crystal template on the Si wafer, as shown in
The preceding description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical “or.” It should be understood that the various steps within a method may be executed in different order without altering the principles of the present disclosure. Disclosure of ranges includes disclosure of all ranges and subdivided ranges within the entire range.
The headings (such as “Background” and “Summary”) and sub-headings used herein are intended only for general organization of topics within the present disclosure, and are not intended to limit the disclosure of the technology or any aspect thereof. The recitation of multiple embodiments having stated features is not intended to exclude other embodiments having additional features, or other embodiments incorporating different combinations of the stated features.
As used herein, the terms “comprise” and “include” and their variants are intended to be non-limiting, such that recitation of items in succession or a list is not to the exclusion of other like items that may also be useful in the devices and methods of this technology. Similarly, the terms “can” and “may” and their variants are intended to be non-limiting, such that recitation that an embodiment can or may comprise certain elements or features does not exclude other embodiments of the present technology that do not contain those elements or features.
As used herein, the term “about” includes exactly the term or number that it modifies and slight variations therefrom. “Slight variations therefrom” can include within 15 degrees/percent/units or less, within 14 degrees/percent/units or less, within 13 degrees/percent/units or less, within 12 degrees/percent/units or less, within 11 degrees/percent/units or less, within 10 degrees/percent/units or less, within 9 degrees/percent/units or less, within 8 degrees/percent/units or less, within 7 degrees/percent/units or less, within 6 degrees/percent/units or less, within 5 degrees/percent/units or less, within 4 degrees/percent/units or less, within 3 degrees/percent/units or less, within 2 degrees/percent/units or less, or within 1 degree/percent/unit or less. In some instances, “about” can include being within normal manufacturing tolerances.
The broad teachings of the present disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the specification and the following claims. Reference herein to one aspect, or various aspects means that a particular feature, structure, or characteristic described in connection with an embodiment or particular system is included in at least one embodiment or aspect. The appearances of the phrase “in one aspect” (or variations thereof) are not necessarily referring to the same aspect or embodiment. It should be also understood that the various method steps discussed herein do not have to be carried out in the same order as depicted, and not each method step is required in each aspect or embodiment.
The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations should not be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.
