LEE, SEOK WOO (388-6, Seoknam-riYugu-eu, Gongju-si Chungchongnam-do 314-896, KR)
LEE, JUNG A (138-13, Shinseong-dongYuseong-gu, Daejeon 305-804, KR)
LEE, SEUNG SEOB (1 Samsung Nareumae Apt, Gajang-dong Seo-gu, Daejeon 302-726, 5-1305, KR)
LEE, SEOK WOO (388-6, Seoknam-riYugu-eu, Gongju-si Chungchongnam-do 314-896, KR)
LEE, JUNG A (138-13, Shinseong-dongYuseong-gu, Daejeon 305-804, KR)
| [CLAIMS] [Claim 1] A method for producing a high-aspect-ratio microstructure, comprising: a photomask attachment step of attaching a photomask with a pattern groove to one surface of a transparent substrate; a photoresist attachment step of attaching a negative photoresist to one surface of the photomask; an exposure step of irradiating light toward the opposite surface of the transparent substrate from the photomask to cure a portion of the negative photoresist with the light irradiated on the negative photoresist through the pattern groove; and a developing step of removing an uncured portion of the negative photoresist while leaving the cured portion of the negative photoresist as a microstructure. [Claim 2] The method as recited in claim 1, wherein the accumulated energy quantity of the light irradiated on the negative photoresist is controlled in the exposure step to specify the shape of the microstructure. [Claim 3] The method as recited in claim 2, wherein the intensity of the light irradiated on the negative photoresist is adjusted to control the accumulated energy quantity of the light. [Claim 4] The method as recited in claim 2, wherein the irradiation time of the light irradiated on the negative photoresist is adjusted to control the accumulated energy quantity of the light. [Claim 5] The method as recited in claim 1, further comprising a pyrolysis step of heating and carbonizing the microstructure to reduce the thickness of the microstructure, the pyrolysis step being performed after the developing step. [Claim 6] The method as recited in claim 2, further comprising a numerical analysis step of, prior to irradiating the light on the negative photoresist, calculating the shape of the portion of the negative photoresist to be cured by exposure using the equations: J(P 0)=^- 1 ^ 0)I2 and where U is the electric fields induced by the propagation of light, λ is the wavelength of light, c is the speed of light, ε is the dielectric constant, Po is the position in the negative photoresist, Pi is the position of the pattern groove, tExp is the exposure time, R1 is the reflection coefficient between the transparent substrate and the negative photoresist, z is the projection distance of light from the transparent substrate, dEXp is the absorption coefficient of the negative photoresist exposed, and αunexp is the absorption coefficient of the negative photoresist unexposed. [Claim 7] The method as recited in claim 5, wherein the pyrolysis step comprises putting the microstructure into a furnace and heating the furnace while feeding a nitrogen gas into the furnace. [Claim 8] The method as recited in claim 7, wherein the interior of the furnace is maintained at a first temperature for a first time period to evaporate a volatile compound from the microstructure and then the interior of the furnace is maintained at a second temperature higher than the first temperature for a second time period to carbonize the microstructure. [Claim 9] The method as recited in claim 8, wherein the first temperature is about 300 °C, the first time period is about three hours, the second temperature is about 700 °C, and the second time period is about thirty minutes. [Claim 10] The method as recited in claim 1, wherein the photomask comprises a chromium film. [Claim 11] The method as recited in claim 1, wherein the negative photoresist comprises an SU-8 photoresist. [Claim 12] A method for producing a high -aspect-ratio microstructure, comprising: a photomask attachment step of attaching a photomask with a pattern groove to one surface of a transparent substrate; a photoresist attachment step of attaching a negative photoresist to the opposite surface of the transparent substrate from the photomask; an exposure step of irradiating light toward the negative photoresist through the pattern groove to cure a portion of the negative photoresist; and a developing step of removing an uncured portion of the negative photoresist while leaving the cured portion of the negative photoresist as a microstructure. [Claim 13] The method as recited in claim 12, wherein the accumulated energy quantity of the light irradiated on the negative photoresist is controlled in the exposure step to specify the shape of the microstructure. [Claim 14] The method as recited in claim 12, further comprising a pyrolysis step of heating and carbonizing the microstructure to reduce the thickness of the microstructure, the pyrolysis step being performed after the developing step. [Claim 15] A high-aspect-ratio microstructure produced by the method of one of claims 1 to 14. [Claim 16] A method for producing a high-aspect-ratio microstructure array, comprising: a photomask attachment step of attaching a photomask with a plurality of pattern grooves arranged at a specified interval to one surface of a transparent substrate! a photoresist attachment step of attaching a negative photoresist to one surface of the photomask; an exposure step of irradiating light toward the opposite surface of the transparent substrate from the photomask to cure portions of the negative photoresist with the light irradiated on the negative photoresist through the pattern grooves; and a developing step of removing an uncured portion of the negative photoresist while leaving the cured portions of the negative photoresist as microstructures. [Claim 17] The method as recited in claim 16, wherein the accumulated energy quantity of the light irradiated on the negative photoresist is controlled in the exposure step to specify the shape of the microstructures. [Claim 18] The method as recited in claim 16, further comprising a pyrolysis step of heating and carbonizing the microstructures to reduce the thickness of the microstructures, the pyrolysis step being performed after the developing step. [Claim 19] A high-aspect-ratio microstructure array produced by the method of one of claims 16 to 18. |
HIGH ASPECT RATIO MICROSTRUCTURE AND METHOD OF FABRICATING THE SAME AND HIGH ASPECT RATIO MICROSTRUCTURE ARRAY AND METHOD OF FABRICATING THE SAME [Technical Field]
The present invention relates to a high-aspect-ratio microstructure and, more particularly, to a high-aspect-ratio microstructure produced by photolithography, a method for producing a high-aspect-ratio microstructure, a high-aspect-ratio microstructure array and a method for producing a high-aspect-ratio microstructure array. [Background Art]
Nanotechnology is directed to the use of new phenomena and characteristics appearing in a material with a size of about 100 nm or less. With the nanotechnology, a material is physically or chemically controlled in terms of atoms and molecules to manifest useful structures and functions. This makes it possible to realize a device whose principle is totally different from that of the conventional devices. The nanotechnology is one of the technical fields usable in many different applications. The nanotechnology is expected to substitute the currently available microtechnology in many different fields such as production, medicine, national defense, energy, transportation, communication and computer.
A nanowire and a nanotube may be said to be the core research fields in the nanotechnology. The nanowire refers to a high-aspect-ratio microstructure of several to several tens micrometers in length and 100 nm or less in diameter. The nanowire shows monocrystalline and one-dimensional characteristics. Use of such characteristics of the nanowire makes it possible to increase the integration degree of electronic elements, which assists in reducing the size of devices. As an example, it has been demonstrated that a vertically arranged nanowire transistor may be realizable. Research has also been made to realize a three-dimensional nano-size electronic element that makes use of nanowires well arranged in multi-layers. In addition, the nanowire may bring about an innovative result in performance when applied to thin film transistors, thin film sensors or transparent flexible electronic elements. Examples of nanowire production methods include a top-down method such as photolithography in which a nanometer- size structure is artificially formed through the processing of nanometer level and a bottom-up method such as chemical vapor deposition in which a desired structure is formed by manipulating atoms or molecules that constitute a minimum unit of a material. [Disclosure]
[Technical Problem]
In the production of microstructures having a micrometer or nanometer size, the bottom-up method suffers from drawbacks of reduced reproducibility, deteriorated size controllability and insufficient versatility, the latter of which makes it difficult to use the method in different applications. It is known that the top-down method has a limit in increasing the aspect ratio of nanowires, which is attributable to the diffraction of light. For example, the I-line ultraviolet rays (with a wavelength of 365 nm) extensively used in the art are scattered by diffraction when passing through a photomask. Thus the aspect ratio of microstructures produced by the top-down method is confined to three or less.
In view of the problems inherent in the prior art microstructures, it is an object of the present invention to provide a method capable of photolithographically producing a high-aspect-ratio microstructure in an easy and cost-effective manner, a method capable of producing an array of high-aspect-ratio microstructures in an easy and cost-effective manner, a microstructure produced by the microstructure production method, and a microstructure array produced by the microstructure array production method. [Technical Solution]
In one aspect of the present invention, there is provided a method for producing a high-aspect-ratio microstructure, comprising: a photomask attachment step of attaching a photomask with a pattern groove to one surface of a transparent substrate; a photoresist attachment step of attaching a negative photoresist to one surface of the photomask; an exposure step of irradiating light toward the opposite surface of the transparent substrate from the photomask to cure a portion of the negative photoresist with the light irradiated on the negative photoresist through the pattern groove; and a developing step of removing an uncured portion of the negative photoresist while leaving the cured portion of the negative photoresist as a microstructure.
In another aspect of the present invention, there is provided a method for producing a high-aspect-ratio microstructure, comprising: a photomask attachment step of attaching a photomask with a pattern groove to one surface of a transparent substrate; a photoresist attachment step of attaching a negative photoresist to the opposite surface of the transparent substrate from the photomask; an exposure step of irradiating light toward the negative photoresist through the pattern groove to cure a portion of the negative photoresist; and a developing step of removing an uncured portion of the negative photoresist while leaving the cured portion of the negative photoresist as a microstructure.
In a further aspect of the present invention, there is provided a high-aspect-ratio microstructure produced by the methods set forth above.
In a still further aspect of the present invention, there is provided a method for producing a high-aspect-ratio microstructure array, comprising: a photomask attachment step of attaching a photomask with a plurality of pattern grooves arranged at a specified interval to one surface of a transparent substrate; a photoresist attachment step of attaching a negative photoresist to one surface of the photomask; an exposure step of irradiating light toward the opposite surface of the transparent substrate from the photomask to cure portions of the negative photoresist with the light irradiated on the negative photoresist through the pattern grooves; and a developing step of removing an uncured portion of the negative photoresist while leaving the cured portions of the negative photoresist as microstructures.
In a yet still further aspect of the present invention, there is provided a high- aspect-ratio microstructure array produced by the method set forth just above.
[Description of Drawings]
Fig. 1 is a perspective view showing a chromium film attached to the surface of a glass substrate in a microstructure production method in accordance with one embodiment of the present invention.
Fig. 2 is a perspective view showing a negative photoresist, SU-8, attached to the chromium film.
Fig. 3 is a perspective view illustrating a step in which the negative photoresist is exposed to ultraviolet rays.
Fig. 4 is a perspective view showing microstructures obtained through a developing step, the microstructures being composed of the exposed and cured portions of the negative photoresist.
Fig. 5 is a scanning electron microscope image showing microstructures produced when the pattern grooves are about 3.0 Um in diameter and the intensity of ultraviolet rays used in
2 exposure is 109.2 mj/cm .
Fig. 6 illustrates a diffraction analysis model of the light irradiated on the negative photoresist through the pattern grooves.
Fig. 7 illustrates the normalized distribution of the energy quantity accumulated in the negative photoresist in case where the pattern grooves are 1.0 μm in diameter and the intensity
2 of ultraviolet rays used in exposure is 100 mj/cm .
Figs. 8 through 10 are scanning electron microscope images showing the change in shape of the microstructures depending on the intensity of ultraviolet rays used in exposure, when the pattern grooves are 1.0 Um in diameter.
Fig. 11 illustrates the process in which the microstructures are pyrolyzed within a quartz tube furnace. Fig. 12 illustrates the temperature change within the quartz tube furnace in the pyrolysis step illustrated in Fig. 11.
Fig. 13 shows carbon microstructures carbonized by the pyrolysis.
[Best Mode]
Hereinafter, a method for producing a high-aspect-ratio microstructure in accordance with one preferred embodiment of the present invention will be described with reference to the accompanying drawings.
The method for producing a high-aspect-ratio microstructure is directed to a top-down method and is largely divided into a photolithography step and a pyrolysis step.
Figs. 1 through 4 show the photolithography step of the present method. As shown in Fig. 1, a chromium film 11 (or a photomask) having a thickness of 1,100 A is attached to one surface of a glass substrate (or a transparent substrate) (Pyrex #7740, a product of Corning, Inc.)
10 having a thickness of 500 μm. The glass substrate 10 may be changed to many other transparent substrates. Similarly, the chromium film 11 may be changed to many other photomasks capable of interrupting light when attached to one surface of a transparent substrate. The chromium film 11 has a plurality of pattern grooves 11a arranged at a specified interval, each of the pattern grooves 11a having a diameter of 1.0 um. The chromium film 11 is attached to the glass substrate 10 by an electron-beam deposition method or other like methods. The pattern grooves 11a allow light to pass therethrough, the diameter and interval of which may be changed in many different ways. The pattern grooves 11a are not limited to the circular shape but may have other shapes. The surface of the glass substrate 10 to which the chromium film
11 is attached serves as a light outgoing surface.
Referring to Fig. 2, after the chromium film 11 has been attached to the glass substrate 10, a negative photoresist 12, e.g., SU-8 (a product of Microchem Inc.), is spin-coated on the surface of the chromium film 11 with a suitable thickness and is then dried. Other kinds of negative photoresists than SU-8 may be attached to the surface of the chromium film 11. Ultraviolet rays (UV) are irradiated on the glass substrate 10 after the negative photoresist 12 has been attached to the chromium film 11.
As shown in Fig. 3, the ultraviolet rays are irradiated on the opposite surface of the glass substrate 10 from the chromium film 11. Then, the ultraviolet rays are incident on the negative photoresist 12 through the pattern grooves 11a of the chromium film 11. Although not shown in the drawings, a bandpass filter of 356 nm in wavelength and 10 nm in band width (079-0550 bandpass filter, a product of Opto Sigma Corp.) is arranged between the light source and the glass substrate 10 to filter the ultraviolet rays emitted from the light source. The light used in exposing the negative photoresist 12 is not limited to the ultraviolet rays but may be deep ultraviolet rays, extreme ultraviolet rays, X-rays or other light capable of curing the negative photoresist 12.
The ultraviolet rays projecting toward the glass substrate 10 and passing through the pattern grooves 11a of the chromium film 11 are concentrated on the central regions of the pattern grooves 11a by diffraction, although some of them are diffused away from the pattern grooves 11a. Among the irradiated portions 12a of the negative photoresist 12, curing occurs only in the portions 12b where the energy quantity of the light is greater than the critical energy value required in curing the negative photoresist 12. Each of the cured portions 12b of the negative photoresist 12 is of a conical shape with an increased aspect ratio. The cured portions 12b of the negative photoresist 12 constitute microstructures 13. The shape of the microstructures 13 can be changed by adjusting the intensity and irradiation time of the ultraviolet rays and eventually controlling the energy quantity of the light irradiated on the negative photoresist 12. If the energy quantity of the light irradiated on the negative photoresist 12 is increased, the cured portions 12b are not thickened but elongated in the light irradiation direction. This is because the light is concentrated on the central regions of the pattern grooves 11a by diffraction.
At the end of the exposure step, the negative photoresist 12 is subjected to a developing step. If the negative photoresist 12 thus exposed is dipped into a developing solution such as a PGMEA solution (a product of Microchem Inc.) for more than one hour, the uncured portions of the negative photoresist 12 are removed and only the cured portions remain as microstructures 13 as shown in Fig. 4. After the developing step, the microstructures 13 are cleansed with a cleaning solution such as isopropyl alcohol, deionized water or the like.
In the photolithography step noted above and unlike the conventional photolithography step, the photomask, i.e., the chromium film 11, is attached to the surface of the transparent substrate, i.e., the glass substrate 10. This ensures that no light diffraction occurs between the photomask and the transparent substrate in the exposure step. This feature is advantageous in concentrating the light irradiated toward the negative photoresist on the central regions of the pattern grooves 11a. Although the negative photoresist is attached to the surface of the photomask in the illustrated embodiment, it may be attached to the opposite surface of the transparent substrate from the photomask. In this case, the light is projected toward the photomask and is irradiated on the negative photoresist through the transparent substrate.
Fig. 5 is a scanning electron microscope image showing the microstructures 13 formed on the glass substrate 10 when the pattern grooves 11a are about 3.0 um (precisely, 2.97 um) in diameter and the intensity of the ultraviolet rays irradiated on the glass substrate 10 is 109.2 mj/cm . It can be seen in Fig. 5 that each of the microstructures 13 has an aspect ratio of 20 or more and a tip end diameter reduced to about 700 nm (precisely, 697 nm). The shape of the microstructures 13 can be predicted by calculating the quantity of the light energy accumulated in the internal region of the negative photoresist 12.
The quantity of the energy (or the exposed dose) accumulated in the internal region of the negative photoresist 12 can be calculated using the diffraction analysis model shown in Fig. 6 and the following equations regarding the Huygens-Fresnel diffraction principle:
In equations (1) and (2), U is the electric fields induced by the propagation of light, λ is the wavelength of light, c is the speed of light, ε is the dielectric constant, Po is the position in
the negative photoresist 12, and P 1 is the position of each of the pattern grooves lla. In
equation (3), tExp is the exposure time, R 1 is the reflection coefficient between the glass substrate 10 and the negative photoresist 12, z is the projection distance of light from the glass substrate 10, ciExp is the absorption coefficient of the exposed negative photoresist 12, and αunexp is the absorption coefficient of the unexposed negative photoresist 12.
The distribution of the accumulated energy quantity can be calculated by numerical analysis based on the above equations. Fig. 7 illustrates the normalized distribution of the accumulated energy quantity (D/Dc), which is the result of numerical analysis in case where the 2 pattern grooves 11a are 1.0 Um in diameter and the irradiated energy D is 100 mj/cm . In the distribution of the accumulated energy quantity, the critical value Dc of curing energy is represented by the contour line linking the points where the energy is great enough to cure the
2 negative photoresist 12. In this regard, the critical value Dc of curing energy is 50 mj/cm .
The portion lying inside the contour line is cured because it has an accumulated energy quantity greater than the critical value. The portion lying outside the contour line is removed in the developing step because it has an accumulated energy quantity smaller than the critical value.
Fig. 8 is a scanning electron microscope image showing the microstructures actually produced when the pattern grooves 11a are 1.0 μm in diameter and the irradiated energy D is
2
109.2 mj/cm . It can be noted in Fig. 8 that the portions of the negative photoresist 12 having
2 an accumulated energy quantity of 63 mj/cm or more were cured. The microstructures 13 thus
obtained are 4.6 μm in height. Figs. 9 and 10 are scanning electron microscope images showing the microstructures 13 actually produced when the pattern grooves lla are 1.0 μm in diameter
2 2 and the irradiated energy D is 218.4 mj/cm and 327.6 mj/cm . It can be seen that the
2 microstructures 13 have a height of 7.0 μm when the irradiated energy D is 218.4 mj/cm and a
2 height of 10.9 μm when the irradiated energy D is 327.6 mj/cm . Therefore, it is possible to produce microstructures 13 of desired shape by properly selecting the size of the pattern grooves lla in view of the critical value of curing energy of the negative photoresist 12 and the wavelength of the irradiated light, calculating the shape of the exposure-cured portions of the negative photoresist 12 by numerical analysis, and suitably controlling the energy of the irradiated light.
The thickness of the microstructures 13 produced through the photolithography step is reduced in a pyrolysis step. Referring to Fig. 11, the microstructures 13 are heated to a high temperature within a quartz tube furnace 20 during the pyrolysis step. At this time, the quartz tube furnace 20 is supplied with heat from the outside. A nitrogen gas (N2) is fed into the
quartz tube furnace 20 to keep the interior of the quartz tube furnace 20 in a high-temperature inert atmosphere.
Fig. 12 illustrates the temperature change within the quartz tube furnace 20 during the pyrolysis step. The internal temperature of the quartz tube furnace 20 is maintained at 300 0 C for a predetermined time period and then increased up to 700 °C, after which the quartz tube furnace 20 is cooled. During the time when the internal temperature of the quartz tube furnace 20 is maintained at 300 °C for three hours, the microstructures 13 are dried so that volatile compounds can be evaporated. Thereafter, the internal temperature of the quartz tube furnace 20 is increased up to 700 "C at a rate of 10°C/min and maintained at that temperature for thirty minutes. During this time, hydrogen and oxygen in the microstructures 13 are decomposed, resulting in reduction in the thickness of the microstructures 13. Subsequently, the quartz tube furnace 20 is naturally cooled in the inert atmosphere. In this way, the microstructures 13 are carbonized through the pyrolysis step and transformed into carbon microstructures 14 with a reduced thickness as shown in Fig. 13. At the end of the pyrolysis step, the microstructures 13 show about 10% reduction in the size and volume thereof. Other furnaces than the quartz tube furnace 20 may be used in heating the microstructures 13 in the pyrolysis step.
With the method for producing a high-aspect-ratio microstructure described above, the photomask is attached to the transparent substrate, and the light is irradiated on the opposite surface of the transparent substrate from the photomask. This enables the light to be concentrated on the central regions of the pattern grooves formed in the photomask. Use of this principle makes it possible to produce microstructures 13 in an easy and cost-effective manner. By controlling the intensity and irradiation time of light, it is also possible to produce microstructures 13 of varying shape and a microstructure array having such microstructures vertically arranged on a transparent substrate.
Although one preferred embodiment of the present invention has been described hereinabove, the present invention shall not be limited thereto. It will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention defined in the claims. [Industrial Applicability]
With the present invention, a microstructure of micrometer or nanometer size can be produced in an easy and cost-effective manner. Therefore, the present invention can be advantageously used in the field of nanotechnology and its application fields such as production, medicine, national defense, energy, transportation, communication, computer and the like.
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