This application claims priority of Taiwanese application no. 099104381, filed on Feb. 11, 2010.
1. Field of the Invention
This invention relates to a method for forming a microstructure on a polymeric substrate, more particularly to a method for forming a microstructure on a polymeric substrate at an elevated temperature using a master mold formed with a plurality of nano-sized protrusion portions.
2. Description of the Related Art
A microstructure, especially a nano-scale microstructure having a high aspect ratio is widely used in biological, mechanical, and micro-electro mechanical fields. Nanoimprintlithography techniques, such as hot embossing nanoimprintlithography (HE-NIL), are commonly used to form the microstructure by transferring a microstructure pattern from a master mold onto a polymeric substrate.
However, making a nano-sized microstructure which has a high aspect ratio using a master mold having a nano-sized microfeature is relatively difficult. The reasons reside in that the flowability of the polymeric substrate with high molecule weight in the nano-sized microfeature of the master mold cannot be predicted, i.e., the microstructure pattern might not be successfully transferred from the master mold onto the polymeric substrate in nano-sized scale. Moreover, in general, a master mold is used to form a single pattern of microstructure, e.g., a master mold with protrusion features is used to form an indent microstructure on a substrate. As a consequence, forming different microstructures with different aspect ratios on the polymeric substrate requires the use of different master molds, which results in an increase in production costs and an increase in production time.
Therefore, the object of the present invention is to provide a method for forming a microstructure on a polymeric substrate that can overcome the aforesaid drawbacks of the prior art.
According to this invention, a method for forming a microstructure on a polymeric substrate comprises: providing a master mold formed with a micro-feature thereon, the micro-feature having a base portion and a plurality of protrusion portions protruding from the base portion, each of the protrusion portions having a tapered anisotropic shape, including a free end distal from the base portion, and being spaced apart from an adjacent one of the protrusion portions, a distance between the free ends of two adjacent ones of the protrusion portions being not greater than 40 nm; and impressing the free end of each of the protrusion portions of the micro-feature into the polymeric substrate at an elevated temperature T1, the polymeric substrate having a pyrolysis temperature Tp greater than the elevated temperature T1.
Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments of this invention, with reference to the accompanying drawings, in which:
A method for forming a microstructure on a polymeric substrate according to the present invention includes: providing a master mold 2 formed with a micro-feature thereon (see
Each of the protrusion portions 22 of the micro-feature has a size in nano scale. In the examples of this invention, each protrusion portion 22 has a diameter of about 20 nm at one-half of the height thereof.
Preferably, the polymeric substrate has a glass transition temperature Tg and a heat distortion temperature T2, where T2≦T1<Tp.
Preferably, the method further includes, after the impressing step, removing the master mold 2 from the polymeric substrate under an elevated temperature T3, the elevated temperature T3 being less than the glass transition temperature Tg of the polymeric substrate. More preferably, the elevated temperature T3 is lower than the heat distortion temperature T2 of the polymeric substrate.
Preferably, the method further includes, before the impressing step, forming an anti-sticking layer (not shown), which has a surface energy less than that of the micro-feature, on a surface of each of the protrusion portions 22 of the micro-feature so as to prevent adjacent ones of the protrusion portions 22 from sticking to each other and to facilitate removal of the master mold 2 from the polymeric substrate in the removing step. Preferably, the anti-sticking layer is made from octadecyl-trichlorosilane (OTS) or 1H,1H, 2H,2H-perfluorooctyl trichlorosilane (FOTS).
Preferably, each of the protrusion portions 22 includes a nanopin.
Preferably, each of the protrusion portions 22 has an aspect ratio greater than 1 and less than 14. In the present invention, the aspect ratio of each protrusion portion 22 is defined as H/D, where H and D respectively represent the height of the protrusion portion 22 and the diameter of the protrusion portion 22 at one-half of the height thereof.
During the impressing step, the polymeric substrate is subjected to a pressing pressure and heat (i.e., at the elevated temperature T1). Therefore, mechanical strength and viscosity of the polymeric substrate should be considered. The mechanical strength is determined by Young's modulus and is relevant to the stability of the microstructure formed on the polymeric substrate. Viscosity determines the flowability of the polymeric substrate at a visco-elastic state during the impressing step, and is relevant to the structural integrity and time required for the impressing step. Preferably, the polymeric substrate has a Young's modulus not less than 2 GPa and a melt flow rate (MFR) ranging from 2 g/10 mins to 50 g/10 mins. More preferably, the polymeric substrate is made from polycarbonate (PC), polymethylmethacrylate (PMMA), cyclic olefin copolymer (COC), epoxy resin, polystyrene (PS), or polyvinylchloride(PVC). In the embodiments and the examples of this invention, the polymeric substrate is made from cyclic olefin copolymer (COC) having a glass transition temperature Tg of 158° C. The preferred embodiments of the method for forming microstructures having different aspect ratios and patterns using the aforesaid master mold 2 are illustrated below.
The polymeric substrate 3 under such elevated temperature T1 (i.e., close to Tg+20° C.) has a viscosity lower than that of the polymeric substrate 3 in the second preferred embodiment such that the viscous substrate not only flows into the first space 10 but also into the second space 20, and the depth where the viscous substrate flows into the spaces 10, 20 toward the base portion 21 is greater than that of the first preferred embodiment. However, because of flow resistance, the viscous substrate can not flow into the relatively small spaces among the protrusion portions 22 adjacent to the base portion 21. The microstructure thus formed on the polymeric substrate 3 in the third preferred embodiment includes nanopins 33 having a high aspect ratio greater than 6 (see
In this embodiment, the polymeric substrate 3 at such elevated temperature T1 has a superior flowability, i.e., extremely low viscosity, and the flow resistance for the polymeric substrate 3 is extremely low. As a consequence, the viscous substrate could fill up all of the spaces among the protrusion portions 22, and the microstructure thus formed on the polymeric substrate 3 in the fourth preferred embodiment includes nanopores 34 having a high aspect ratio greater than 10 (see
The applicant surprisingly founds that, by controlling the elevated temperature T1 of the impressing step, the master mold 2 having the nano-sized micro-feature (i.e., the distance Smax between the free ends 221 of two adjacent protrusion portions 22 to be not greater than 40 nm) can be successfully used to form different microstructures on the polymeric substrate 3.
The following examples are provided to illustrate the merits of the preferred embodiments of the invention, and should not be construed as limiting the scope of the invention.
A microstructure formed on a polymeric substrate for Example 1 (E1) was prepared based on the method of the aforesaid first preferred embodiment. A processing chamber was cleaned using CF4 and O2 plasma, followed by controlling the pressure in the cleaned chamber at 5×10−5 torr. A Si substrate was placed in the cleaned chamber, and was then subjected to a hydrogen plasma etching process (see Electrochemical and Solid-State Letters, Vol. 8, No. 10, pp. C131 (2005)) so as to form a master mold formed with nanopins on the Si substrate, each of the nanopins having a height of about 250 nm and a diameter of about 18 nm at one-half of the height thereof (see
A microstructure formed on a polymeric substrate for Example 2 (E2) was prepared based on the method of the second preferred embodiment. The procedures and operating conditions for preparing the microstructure on the polymeric substrate were similar to those of Example 1 (E1), except that the impressing step was conducted at an elevated temperature T1 of 160° C. The microstructure of Example 2 (E2) thus formed on a cyclic olefin copolymer (COC) substrate includes nanopins having a low aspect ratio not greater than 6 (see
A microstructure formed on a polymeric substrate for Example 3 (E3) was prepared based on the method of the third preferred embodiment. The procedures and operating conditions for preparing the microstructure on the polymeric substrate were similar to those of Example 1 (E1), except that the impressing step was conducted at an elevated temperature T1 of 180° C. The microstructure of Example 3 (E3) thus formed on a cyclic olefin copolymer (COC) substrate includes nanopins having a high aspect ratio greater than 6 (see
A microstructure formed on a polymeric substrate for Example 4 (E4) was prepared based on the method of the fourth preferred embodiment. The procedures and operating conditions for preparing the microstructure on the polymeric substrate were similar to those of Example 1 (E1), except that the impressing step was conducted at an elevated temperature T1 of 220° C. The microstructure of Example 4 (E4) thus formed on a cyclic olefin copolymer (COC) substrate includes nanopores having a high aspect ratio greater than 10 (see
In conclusion, by using the master mold in which the distance Smax between the free ends 221 of two adjacent protrusion portions 22 to be not greater than 40 nm, and by controlling the elevated temperature of the impressing step, different microstructures may be formed using such single master mold, thereby reducing the production costs and time.
While the present invention has been described in connection with what are considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation and equivalent arrangements.
Number | Date | Country | Kind |
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099104381 | Feb 2010 | TW | national |