Method for forming micropattern of polyimide using imprinting

Abstract

Provided is a method for forming a pattern of polyimide that is simpler and is more excellent in the pattern shape and in the dimensional accuracy in comparison with the conventional techniques of patterning polyimide, such as photolithography and laser processing. In a method for forming a micropattern of polyimide, which includes using as polyimide a solvent-soluble polyimide resin composition that is photosensitive and is moldable at a temperature of less than or equal to a glass-transition temperature; patterning the composition using thermal imprinting; and thermally curing the composition, ultraviolet irradiation is performed after the composition is released from a mold after a molding step.

Claims

1. A method for forming a micropattern of polyimide using thermal imprinting, the method comprising: forming a polyimide film made of a solvent-soluble polyimide resin composition that is photosensitive and is moldable at a temperature of less than or equal to a glass-transition temperature; heating the polyimide film at a temperature of less than or equal to the glass-transition temperature and pressing a mold against the polyimide film to form a micro projection/recess pattern on a surface of the polyimide film; cooling the polyimide film and the mold and releasing the mold from the polyimide film; irradiating the polyimide film with ultraviolet rays to perform light exposure thereon; and heating the polyimide film to thermally cure the polyimide film.

2. The method for forming a micropattern of polyimide according to claim 1, wherein thermally curing the polyimide film comprises a two-stage heating where the polyimide film is held at a post exposure bake temperature for a given period of time while a temperature is increased from a room temperature to a heat treatment temperature.

3. The method for forming a micropattern of polyimide according to claim 1, wherein the micro projection/recess pattern is one of a micrometer or submicrometer projection/recess pattern with a rectangular cross-section.

4. The method for forming a micropattern of polyimide according to claim 1, wherein the solvent-soluble polyimide resin composition is a solvent-soluble block copolymerized polyimide resin composition.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIGS. 1A-1E are diagrams showing a summary of a method for forming a pattern of the present invention.

(2) FIG. 2 is a graph showing the relationship between the heat treatment (curing) temperature and pattern structure deformation (a) when a photosensitive polyimide resin composition is used and (b) when a non-photosensitive polyimide resin composition is used.

(3) FIG. 3 is a graph showing the influence of the temperature conditions of imprint molding ((a) 100 C. and (b) 120 C.) on the stability of pattern dimensions.

(4) FIGS. 4A-4B are graphs showing the influence of the conditions of heat treatment after molding on pattern shrinkage.

(5) FIGS. 5A-5E are photographs showing L & S of microscale imprint patterns obtained in an example.

(6) FIGS. 6A-6C are photographs showing L & S of nanoscale imprint patterns obtained in an example.

(7) FIGS. 7A-7B are photographs of a flexible polyimide substrate obtained in an example, specifically, FIG. 7A is a photograph after thermal imprint molding, and FIG. 7B is a photograph of a polyimide substrate after it is peeled off a Si substrate (and before it is cured).

DESCRIPTION OF EMBODIMENTS

(8) A method of the present invention will be described in detail below.

(9) The present invention is characterized by, in a method for forming a submicrometer pattern of polyimide through thermal imprinting, using a solvent-soluble polyimide resin composition, which is photosensitive and has a glass-transition temperature of less than or equal to 200 C., and performing ultraviolet irradiation after releasing the composition from a mold and before thermally curing the composition.

(10) FIGS. 1A-1E are diagrams showing a summary of the thermal imprinting of the present invention.

(11) As shown in FIG. 1A, a polyimide film is first applied to a substrate, and is then prebaked ((a) film formation step). After that, the polyimide film is heated to a temperature of less than or equal to the glass-transition temperature, and then, as shown in FIG. 1B, projection/recess patterns of a mold are pressed against the polyimide film to form a pattern thereon through thermal imprint molding (thermal imprinting step).

(12) After the imprint molding, as shown in FIG. 1C, the polyimide and the mold are cooled to a temperature of less than the glass-transition temperature so that the polyimide is released from the mold ((c) cooling and releasing step), and then, as shown in FIG. 1D, light exposure is performed ((d) light exposure step). Then, as shown in FIG. 1E, heat treatment is performed as a final step ((e) heat treatment step).

(13) As described above, a printing method that uses the conventional polyimide has a problem in that thermal molding at a high temperature is necessary as the glass-transition temperature of common polyimide is greater than or equal to 300 C. In order to solve such a problem, using a polyimide precursor (i.e., polyamic acid) that can be thermally molded at a low temperature is considered. However, there remains a problem in that although molding at a low temperature is possible, large shrinkage occurs due to an imidization reaction (i.e., dehydration-condensation reaction).

(14) Thus, the present invention allows thermal molding at a low temperature and suppresses shrinkage, which would occur due to an imidization reaction, by using a solvent-soluble polyimide resin composition that has been imidized and can be molded at a temperature of less than or equal to the glass-transition temperature.

(15) Polyimide that has been molded through thermal imprinting is, after being released from a mold, subjected to heat treatment (curing) in order to have improved characteristics in heating resistance, light resistance, and the like. However, in the heat treatment step, a film thickness reduction (i.e., a reduction in the thickness of a polyimide film) and deformation in the height of a pattern structure occur.

(16) The present invention solves such problems by providing photosensitivity and performing ultraviolet exposure before the heat treatment step.

(17) Hereinafter, each step will be described in further detail.

(18) (a) Film Formation Step

(19) A solvent-soluble polyimide resin composition of the present invention is applied to a substrate of Si or the like (see FIG. 1A). Although the coating method and the thickness of the applied film are not particularly limited, it is preferable to form a film with a thickness of about 25 m using spin coating.

(20) Next, the applied film is prebaked and thus dried.

(21) Solvent-soluble polyimide as used in the present invention has already been imidized. Therefore, high-temperature treatment required for imidization is not needed, and molding can be performed at a low temperature of less than or equal to the glass-transition temperature. Further, as the polyimide is soluble in solvents, it can be a perfect polyimide varnish. Thus, it is possible to form a film only by evaporating a solvent.

(22) Such solvent-soluble polyimide is already known. In the present invention, commercially available solvent-soluble polyimide that can be molded at a temperature of less than or equal to the glass-transition temperature can be appropriately selected and used. In particular, solvent-soluble block copolymerized polyimide, which is obtained by allowing polyimide to contain a given amount of a given region that imparts a desired property through block copolymerization, is preferably used.

(23) Examples of such solvent-soluble block copolymerized polyimide include siloxane-modified block copolymerized polyimide with a low warping property that is formed using a diamine(s) having siloxane bonds in molecular skeleton thereof (siloxane bond-containing diamine(s)) as the diamine compounds (see WO2002/023276 A and WO2005/116152 A). As the block copolymerized polyimide can be molded at a temperature of less than or equal to the glass-transition temperature, and has been already imidized, molding at a low temperature and with low shrinkage is possible.

(24) Resin compositions that use solvent-soluble polyimide are generally divided into photosensitive compositions and non-photosensitive compositions. An imprinting technique is a press technique using a mold. Thus, a pattern formation step using light exposure is not needed. Therefore, it is desirable to select a non-photosensitive composition from the perspective of simplifying the step. However, there is a problem in that deformation of a pattern structure occurs during heating for the heat treatment (curing).

(25) Thus, the present invention can provide a method for forming a micropattern with high accuracy by using a photosensitive solvent-soluble polyimide resin composition.

(26) FIG. 2 is a graph showing the relationship between the heat treatment (curing) temperature and pattern structure deformation (a) when a photosensitive polyimide resin composition, which is used in the following example, is used and (b) when a non-photosensitive polyimide resin composition (without a photosensitizing agent) is used.

(27) As is clear from FIG. 2, when a non-photosensitive polyimide resin composition is used, the dimensional accuracy of the surface of a polyimide substrate would undesirably decrease if the composition is heated to a temperature of greater than or equal to 160 C. in a curing step. In contrast, when a photosensitive polyimide resin composition is used, it can be seen that shrinkage due to deformation of a pattern structure can be significantly reduced.

(28) The photosensitive solvent-soluble polyimide resin composition of the present invention contains the aforementioned solvent-soluble polyimide, a photosensitizing agent, and a solvent. As the photosensitizing agent, a photosensitizing agent that imparts negative photosensitivity is used.

(29) (b) Thermal Imprinting Step

(30) After that, a pattern is formed (transferred) through thermal imprint molding using a mold that has projections/recesses formed thereon (see FIG. 1(b)). That is, after a substrate with a polyimide film obtained in the previous step is heated to a predetermined temperature, pressure molding is performed using a mold that has projection/recess patterns formed thereon.

(31) The characteristics that are required of the mold materials are the releasability from polyimide, heat resistance and durability against a thermal imprint molding step, and the like. For example, Si, quartz, or Ni is used, or resin is also used for some of molds.

(32) In particular, a flexible resin mold is suitable for uniformly transferring a pattern to a large area because, in comparison with a mold of Si, quartz, Ni, or the like, it allows molding of a pattern in a large area, has high releasability, allows uniform press on a non-flat plane, and is inexpensive.

(33) The conditions of the main process parameters in the step, which include an imprint temperature, imprint pressure, and imprint time, vary in accordance with an object to be molded.

(34) In particular, the imprint temperature is preferably greater than or equal to 40 C. and less than or equal to the glass-transition temperature of the polyimide resin composition used, or more preferably, 120 C.

(35) FIG. 3 is a graph showing the influence of the imprint molding temperature conditions ((a) 100 C. and (b) 120 C.) on the stability of pattern dimensions.

(36) As shown in FIG. 3, the temperature of the thermal imprint molding is one of the factors that can change the influence on pattern shrinkage after heat treatment described below. Increasing the temperature of the thermal imprint molding before the light exposure from 100 to 120 C. can reduce the amount of pattern shrinkage after curing to less than or equal to 4%.

(37) (c) Cooling and Releasing Step

(38) After the imprint molding, the polyimide and the mold are cooled to a temperature of less than the glass-transition temperature, for example, down to 60 C., and then, the polyimide is released from the mold (See FIG. 1C).

(39) The cooling method is not particularly limited, and natural cooling may be employed. However, forced cooling is preferably performed by, for example, flowing water through a water pipe that is disposed around the molding space from the perspective of molding cycles.

(40) (d) Light Exposure Step

(41) After the polyimide is released from the mold, light exposure is performed (see FIG. 1D).

(42) Light exposure is typically performed through ultraviolet irradiation, though it differs depending on a photosensitizing agent used.

(43) A reason for performing light exposure after the polyimide is released from the mold is that there has been a problem that if light exposure is performed before the polyimide is released from the mold, the flexible resin mold and the polyimide film may stick together after the light exposure. In addition, although optical imprint molding that uses typical photosensitive resin should necessarily use a transparent mold (i.e., quartz or resin), the method of the present embodiment is advantageous in that the range of selectable molds is wide because an opaque mold of Si, Ni, or the like can also be used.

(44) (e) Heat Treatment Step

(45) The final step is heat treatment (FIG. 1E).

(46) The thermal imprinting step that uses the negative photosensitive polyimide resin composition of the present invention does not require a development step unlike photolithography. Therefore, post exposure bake (PEB) can be integrated with a curing step.

(47) FIGS. 4A-4B are graphs showing the influence of the conditions of heat treatment after molding on pattern shrinkage. As shown in FIG. 4A, pattern shrinkage under the following four heating conditions was inspected.

(48) (1) Heated for 20 minutes using a hot plate heated to 200 C.

(49) (2) Heated from the room temperature to 200 C. at a heating rate of 20 C./min.

(50) (3) Heated from the room temperature to 200 C. at a heating rate of 5 C./min.

(51) (4) Held at a post exposure bake temperature (100 C.) of the photosensitive polyimide resin composition used for a given period of time, and was then heated from the room temperature to 200 C. at a heating rate of 5 C./min.

(52) As shown in FIG. 4B, the range of changes in the shrinkage due to changes in the conditions of the heat treatment after molding is 70 to 12%.

(53) As shown in the following examples, it was found possible to improve the shrinkage down to 14% or less by holding the photosensitive polyimide resin composition used at 100 C., which is a post exposure bake temperature of the composition, during two-stage heating performed at a heating rate of 5 C./min, that is, during heating, and then heating the composition again.

EXAMPLES

(54) Although the present invention will be hereinafter described based on examples, the present invention is not limited thereto.

Example 1: Formation of Micrometer Pattern

(55) In the present example, a commercially available organic-solvent-soluble polyimide resin composition (Q-RP-X1149-X, PI R&D Co., Ltd) was used.

(56) The composition contains a solvent-soluble siloxane-modified block copolymerized polyimide (with a glass-transition temperature of 192 C.), gamma butyrolactone (solvent), methyl benzoate (solvent), and a photosensitizing agent. The amount of shrinkage in the film thickness after curing was about 1%, which is quite small in comparison with a shrinkage of 30% in the film thickness of the commonly used non-photosensitive polyimide after curing (see FIG. 2(a)).

(57) For a substrate, a Si substrate (with a thickness of 725 m and a diameter of 8 inches) was used.

(58) A mold was produced through a thermal imprinting process as follows, using a commercially available resin sheet (i.e., an intermediate polymer stamp (IPS) sheet produced by Obducat AB; a glass-transition temperature of 150 C.) that has a hydrophobic property and thus has excellent releasability.

(59) A resin film with a thickness of 200 m was heated to a temperature around the glass-transition temperature until it softens. Then, using a Si master mold, pressure molding was performed with an imprint pressure of 4 MPa for two minutes. Then, the resin film and the mold were cooled down to 60 C. so that the resin film was released from the mold.

(60) In the present example, five projection/recess patterns with depths of 4 m and line widths of 3 m, 5 m, 10 m, 20 m, and 50 m, respectively, were produced.

(61) The aforementioned resin composition was applied to the Si substrate to a thickness of 25 m through spin coating, and then, prebaking was performed at 100 C. for 4 minutes to form a resin film.

(62) Next, with the aforementioned mold placed on the substrate with the resin film, the resin film was heated to 100 to 120 C. until it softens, and was then pressure-molded. After that, the resin film and the mold were cooled down to 60 C. so that the resin film was released from the mold.

(63) A surface of the molded resin film was irradiated with ultraviolet rays (with a wavelength of 365 nm and an illuminance of 30 mW/cm.sup.2) using an ultraviolet light source provided in a nanoimprint system. Then, heat treatment of two-stage heating was performed where the film was first heated to 100 C. at a heating rate of 5 C./min, and was then held at 100 C., and was further heated to 200 C.

(64) The illumination, the exposure time, and the exposure dosage of the ultraviolet irradiation were 30 mW/cm.sup.2, 40 seconds, and 1200 mJ/cm.sup.2, respectively.

(65) FIGS. 5A-5E are photographs showing L & S of imprint patterns produced in the present example.

(66) As shown in FIGS. 5A-5E, L & S pattern structures of 50 to 3 m, which have sharp line edges and rectangular cross-sections, are favorably formed.

Example 2: Formation of Submicrometer Patterns

(67) In the present example, molds having produced thereon three types of projection/recess patterns, which have depths of 188 nm and line widths of 150 nm, 200 nm, and 300 nm, respectively, were used in the same way as in Example 1 except that a Si substrate with a thickness of 500 m and a diameter of 4 inches was used and the pressing time was 3 minutes.

(68) Submicrometer patterns were formed in the same way as in Example 1 except the substrate and the mold.

(69) FIGS. 6A-6C are photographs showing L & S of imprint patterns produced in the present example.

(70) As shown in FIGS. 6A-6C, submicrometer pattern structures with rectangular cross-sections are favorably formed.

(71) As shown in Examples 1 and 2, when an imprinting technique is used, each case has no problems in the patterning accuracy, such as the light diffraction limit of photolithography, and the obtained patterns theoretically conform to the mold patterns.

(72) It should be noted that the molded polyimide can be physically peeled off if the base Si substrate is subjected to water-repellent treatment in advance, for example, either before or after curing so that the adhesion becomes lower. Thus, a flexible polyimide substrate can be produced.

(73) FIGS. 7A-7B are photographs of a flexible polyimide substrate obtained in the example, specifically, FIG. 7A is a photograph after thermal imprint molding, and FIG. 7B is a photograph of a polyimide substrate after it is peeled off a Si substrate (and before it is cured).