Patent Publication Number: US-2019176376-A1

Title: Pattern forming method and device

Description:
TECHNICAL FIELD 
     The technical field relates to a method and a device for forming patterns using an imprint technique. 
     BACKGROUND 
     In optical parts used for products such as displays and lighting, there is a demand for the development of a device equipped with new functions that control reflection and diffraction of light. There is also a demand for miniaturization of wires for high integration of semiconductors such as system LSIs. These demands are met by, for example, forming nanometer (nm)- to micrometer (μm)-scale fine patterns that exhibit special optical characteristics. Imprinting has attracted interest as a technique for forming such fine structures. In this technique, a mold with predefined surface patterns is used to form patterns by pressing the mold against a resin applied to a substrate surface. 
     With reference to  FIGS. 4 and 5 , the following describes typical pattern-forming procedures of UV (ultraviolet) imprinting, a type of imprinting technique.  FIG. 4  is a schematic view representing steps of a common plate imprinting process. First, in step (a) of  FIG. 4 , a mold  42  with a predefined pattern (irregularities) is placed on a flat stage  41 , and a transfer material  43  is applied to the mold  42  with, for example, a dispenser or an inkjet device. Alternatively, the transfer material  43  is applied to the mold  42  in advance, and the mold  42  is placed on the stage  41 . In step (b) of  FIG. 4 , a cylindrical roller  45  is conveyed over the top surface of a film  44  to linearly press the film  44 , filling the pattern of the mold  42  with the transfer material  43 . In step (c) of  FIG. 4 , the transfer material  43  is cured by applying UV light from above the film  44 , using a UV irradiator  49 . Finally, in step (d) of  FIG. 4 , the film  44  is moved diagonally or vertically above the mold  42  to release the transfer material  43  from the mold  42 . After these steps, the irregularities of the pattern of the mold  42  are formed on the film (a transfer target) in inverted form. 
       FIG. 5 , (b- 1 ) is a schematic view magnifying the region filled with the transfer material in step (b) of  FIG. 4  in the above transfer method.  FIG. 5 , (b- 2 ) is a schematic view magnifying the shape of the transferred material after curing in step (c) of  FIG. 4 . As shown in (b- 1 ) of  FIG. 5 , fine bubbles  52  present in the transfer material  53  remain even after filling the transfer material  53 . The fine bubbles  52  create a defective portion in the pattern after curing, as shown in (b- 2 ) of  FIG. 5 . Bubbles  51  also remain in the pattern, as shown in (b- 1 ) of  FIG. 5 . These phenomena occur when, for example, the pressure application by the rolling cylindrical roller  45  on the top surface of the film  44  is finished before the transfer material  43  completely fills the pattern of the mold  42  in step (b) of  FIG. 4 . This creates defective portions due to the bubbles  51  in the shape of the cured pattern, as shown in (b- 2 ) of  FIG. 5 . These defective portions result in poor transfer accuracy. 
     SUMMARY 
     A method is known that is intended to solve this problem by performing an imprinting process in a condensable gas, as described in JP-A-2004-103817. The method can reduce the pattern shape defects due to the remaining bubbles  51  in the pattern. 
     However, it is not possible with the method of the related art to reduce defective portions created in the pattern by the fine bubbles remaining in the transfer material. Another drawback is that the transfer material absorbs the condensable gas, and disrupts the shape of the transferred pattern. This leads to poor transfer accuracy. 
     It is accordingly an object of the present disclosure to provide a method and a device for forming a pattern with improved transfer accuracy by reducing the number of fine bubbles remaining in the transfer material, and the number of bubbles that remain in the pattern when the pressure application on a film is finished before the transfer material completely fills the mold pattern. 
     A method for forming a pattern according to an aspect of the present disclosure includes: 
     a step A of filling a pattern of a mold with a transfer material; and 
     a step B of transferring the transfer material filling the pattern to a transfer target, 
     wherein an ultrasonic vibration is applied to the transfer material in the step A. 
     A device for forming a pattern according to an aspect of the present disclosure includes: 
     a pressing tool that presses a mold or a transfer target against a transfer material; 
     a curing unit that cures the transfer material filled in a pattern of the mold by the pressing tool; 
     an ultrasonic generator; and 
     a controller, 
     wherein the controller causes the ultrasonic generator to apply an ultrasonic vibration to the transfer material being filled into the pattern by the pressing tool. 
     The method and the device for forming a pattern according to the aspects of the present disclosure can improve transfer accuracy by reducing the number of fine bubbles remaining in the transfer material, and the number of bubbles that remain in the pattern when the pressure application on a film is finished before the transfer material completely fills the mold pattern. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view schematically representing a pattern forming device of an embodiment of the present disclosure. 
         FIG. 2  is a cross sectional view schematically representing an imprinting process of an embodiment of the present disclosure. 
         FIG. 3  is a cross sectional view schematically representing an imprinting process of another embodiment of the present disclosure. 
         FIG. 4  is a cross sectional view schematically representing a related art plate imprinting process. 
         FIG. 5  is a cross sectional view magnifying the region filled with a transfer material, and the shape of the transferred material in the related art plate imprinting process. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present disclosure are described below, with reference to the accompanying drawings.  FIG. 1  shows a pattern forming device of an embodiment of the present disclosure. The pattern forming device includes a pressing tool that presses a transfer material; a curing unit that cures the transfer material; an ultrasonic generator; and a controller. The pattern forming device of the embodiment of the present disclosure is configured from a stage  11 , a mold  12  with a predefined pattern (irregularities), a transfer material  13 , a film  14  (a transfer target), a roller  15  (a pressing tool), an ultrasonic transducer  16  (an ultrasonic generator), an oscillator  17  installed in the ultrasonic transducer  16 , a channel  18  (a temperature adjuster), a UV irradiator  19  (a curing unit), and a controller  20  that controls operations of these members. 
     A pattern forming method using the pattern forming device is described below.  FIG. 2  represents steps of the pattern forming method of an embodiment of the present disclosure. First, in step (a) of  FIG. 2 , a mold  22  is placed on a flat stage  21 , and a transfer material  23  is applied to at least a part of the mold  22 . Alternatively, the transfer material  23  may be applied to the mold  22  in advance, and the mold  22  may be placed on the stage  21 . Inside the stage  21  are provided an ultrasonic generator, and a temperature adjuster that cools the transfer material  23  via the mold  22  to attain a controlled constant temperature. 
     The material of the mold  22  is not particularly limited, as long as it has the properties required as a mold, including rigidity and hardness. For example, the mold  22  may be made of a metallic material, or a resin material. The metallic material is desirably a material, for example, nickel, that is easily releaseable from the transfer material  23 . The resin material may be, for example, a material having a fine UV curable resin pattern formed on a PET film by imprinting. 
     The transfer material  23  may be selected from various UV curable resins, for example, such as a urethane acrylate resin, an epoxy acrylate resin, a polyester acrylate resin, and an acryl acrylate resin. The transfer material  23  may be appropriately selected, taking into consideration factors such as the shape of a film  24 , and the quantity of UV light needed for curing. 
     The method used to apply the transfer material  23  to the whole surface of the mold  22  may be appropriately selected according to the properties of the transfer material  23 , and the shape of the film  24 . Examples of usable methods include dispenser coating, roll coating, gravure coating, and screen coating. 
     On the patterned surface of the mold  22 , a release layer may be formed over the pattern to improve releasability from the transfer material  23 . The release layer is formed by bonding a coupling agent to the top surface of the pattern. By forming the release layer using a coupling agent, an extremely thin film as thin as, for example, a monomolecular film, can be formed, and the influence of the release layer on the transferred shape can be greatly reduced. The coupling agent may use various metal alkoxides of elements, for example, such as Ti, Li, Si, Na, K, Mg, Ca, St, Ba, Al, In, Ge, Bi, Fe, Cu, Y, Zr, and Ta. Particularly preferred are Si-containing metal alkoxides, specifically, a silane coupling agent. 
     In step (b) of  FIG. 2 , an ultrasonic vibration is generated by applying an AC voltage to the oscillator  27  installed in the ultrasonic transducer  26 , and the ultrasonic vibration is applied to the transfer material  23  through the bottom surface of the mold  22 . Under the ultrasonic vibration, a cylindrical roller  25  is conveyed on the top surface of the film  24  to linearly press the film  24 , and fill the transfer material  23  in the pattern of the mold  22 . 
     The roller  25 , which has a cylindrical shape in this embodiment, is not limited to such a shape, provided that it can press the film  24  in a linear fashion. For example, the film  24  can be linearly pressed with an end portion of a polyhedral object, or with a surface of a thin plate by conveying these objects. When using objects of such shapes, damage to the film  24  can be prevented by chamfering the object before use. 
     Inside the ultrasonic transducer  26 , a magnetostrictive material or a piezoelectric material is installed as the oscillator  27  that generates ultrasonic waves. Examples of the magnetostrictive material include iron, and an iron-gallium alloy. Examples of the piezoelectric material include piezoelectric ceramics such as barium titanate, and lead titanate; piezoelectric thin films such as zinc oxide, and lead zirconate titanate; vinylidene fluoride; and trifluoroethylene copolymers. Under applied AC voltage, the magnetostrictive material or the piezoelectric material provided as the oscillator  27  of the ultrasonic transducer  26  expands and contracts in a repeated fashion, and the resulting vibrations generate ultrasonic waves. 
     When ultrasonic vibrations are applied to a liquid, a liquid is alternately subjected to positive and negative pressure due to the acoustic pressure cycle of ultrasonic waves, and bubbles in the liquid are compressed under positive pressure, and decompressed under negative pressure. Under negative pressure, the bubbles expand into vacuum bubbles, and crush when subjected to positive pressure again. This phenomenon is called cavitation, and can be used to crush bubbles in a liquid, or eliminate bubbles in a liquid with the impact of crushing vacuum bubbles. Applying ultrasonic vibrations to a liquid also causes the liquid to flow in a straight line under the acoustic pressure. Such a straight flow of liquid causes convection, agitating and dispersing the bubbles in the liquid, and releasing the bubbles into the atmosphere from the liquid. That is, fine bubbles remaining in the transfer material  23 , and bubbles that remain in the pattern when the pressure application on the film  24  is finished before the transfer material  23  completely fills the pattern can be eliminated by cavitation, or can be released into the atmosphere by the effect of a straight flow by filling the pattern of the mold  22  with the transfer material  23  under applied ultrasonic vibrations to the transfer material  23 . 
     The ultrasonic waves (frequencies greater than 20 kHz) used in the ultrasonic generator of the present disclosure are low-frequency ultrasonic waves with a frequency of no greater than 1 MHz. Frequencies greater than 500 kHz raise the threshold for inducing cavitation, and it becomes necessary to apply ultrasonic waves for a long time period to eliminate bubbles and fine bubbles. This reduces the efficiency of pattern formation. In order to generate a straight flow, it is desirable to apply a frequency of at least 100 kHz, and provide a sufficient vibrational energy. It is accordingly desirable that the frequency be 100 kHz to 500 kHz to efficiently eliminate or release the fine bubbles or bubbles. 
     Desirably, ultrasonic waves are applied at controlled frequencies that vary with the size of the bubbles remaining in the pattern. Cavitation can be enhanced, and larger bubbles can be more effectively eliminated by applying ultrasonic waves of lower frequencies and larger wavelengths. Conversely, smaller bubbles can be more efficiently eliminated by allowing cavitation to occur more frequently with high-frequency vibrations produced by applying higher frequencies. The size of remaining bubbles in the pattern depends on two factors: the aspect ratio of the pattern, and the wettability of the transfer material  23 . 
     The aspect ratio is a normalized value of a groove depth of the pattern with respect to a groove width. Contrary to the simplified shape of the pattern schematically represented in  FIG. 2 , the pattern in the surface of the mold  22  includes patterns of various aspect ratios in actual practice. The bubble size increases as the aspect ratio increases, and decreases as the aspect ratio decreases. It is accordingly desirable to increase frequency when the aspect ratio is large. 
     In order to more effectively eliminate bubbles, it is desirable to store an aspect ratio of the pattern in advance along with a corresponding frequency value, and control the frequency according to the aspect ratio of the pattern to be filled with the transfer material  23 , using the pre-stored corresponding frequency value. Alternatively, detecting means that detects the aspect ratio of the pattern to be filled with the transfer material  23  may be provided, and the frequency may be controlled by using pre-stored corresponding frequency values for different aspect ratios after collating the detected aspect ratio by the detecting means with the pre-stored aspect ratios. For example, a frequency a 1  corresponding to an aspect ratio A 1  of the pattern is stored beforehand, and the frequency is changed to a 1  when the pattern to be filled with the transfer material  23  has aspect ratio A 1 . 
     For simpler control of the ultrasonic generator, it is desirable to control frequency as follows. A mean value of aspect ratios of the pattern is found, and a reference frequency value is set for the mean value. The frequency is then controlled according to the result of a comparison of the mean value with the aspect ratio of the pattern to be filled with the transfer material  23 , using the reference frequency value. For example, when the aspect ratio of the pattern is larger than the mean value, a frequency higher than the reference value is set because higher frequencies are desirable for larger aspect ratios, as stated above. 
     The wettability of the transfer material  23  can be considered as two separate properties: one for the film  24 , and one for the mold  22 . The transfer material  23  show different wettability for the film  24  when the surface state of the film  24  is different, for example, when the film  24  uses a different material in a part of the film, or when another transfer material has already been formed on a part of the film  24 . The bubble size increases as the wettability of the transfer material  23  for the film  24  improves, and decreases as the wettability of the transfer material  23  for the film  24  becomes poor. It is accordingly desirable to increase frequency when the film  24  has a larger contact angle for the transfer material  23 . 
     In order to more effectively eliminate bubbles, it is desirable to store a contact angle of the film  24  for the transfer material  23  in advance along with a corresponding frequency value, and control the frequency according to the contact angle between the film  24  and the transfer material  23  to be pressed with the film  24 , using the pre-stored corresponding frequency value. Alternatively, detecting means that detects the surface state of the film  24  used to press the transfer material  23 , or that detects the contact angle between the film  24  and the transfer material  23  may be provided, and the frequency may be controlled by using pre-stored corresponding frequency values for different surface states or contact angles after collating the detected surface state or contact angle by the detecting means with the pre-stored surface states or contact angles. For example, a frequency b 1  is stored that corresponds to a contact angle B 1  of the film  24  for the transfer material  23 , and the frequency is changed to b 1  when the film  24  used to press the transfer material  23  has contact angle B 1 . 
     For simpler control of the ultrasonic generator, it is desirable to control frequency as follows. A mean value of contact angles of the film  24  for the transfer material  23  is found, and a reference frequency value is set for the mean value. The frequency is then controlled according to the result of a comparison of the mean value with the contact angle between the film  24  and the transfer material  23  to be pressed with the film  24 , using the reference frequency value. For example, when the contact angle between the film  24  and the transfer material  23  is larger than the mean value, a frequency higher than the reference value is set because higher frequencies are desirable for larger contact angles, as stated above. 
     When controlling frequency in the manner described above, bubbles can be effectively eliminated by preferably applying an ultrasonic vibration to a position corresponding to the pattern being filled or to the film  24  being pressed. It is accordingly desirable to locate the position of the pattern or the film  24  being pressed with the roller  25 , and control the frequency of ultrasonic waves according to the located position of the pattern or the film  24 . For example, a position can be located by storing the conveyance direction and speed of the roller  25 , or by using a sensor that detects pressure and movement, or by capturing an image of the mold  22  and the film  24  with an image capturing device equipped with an imaging device. 
     In-plane vibration of the mold  22  at different frequencies can be achieved with the use of the ultrasonic transducer  26  that includes a plurality of magnetostrictive materials or piezoelectric materials as the oscillator  27 . 
     Unlike bubbles that remain in the pattern, the fine bubbles remaining in the transfer material  23  are not introduced in the step of filling the transfer material  23  into the pattern. Accordingly, the fine bubbles may be eliminated by cavitation, or may be released into the atmosphere with a straight flow by applying ultrasonic vibrations to the transfer material  23  before pressing. 
     When applying ultrasonic vibrations, it is preferable to cool the transfer material  23  via the mold  22  by sending coolant water or a coolant gas to a channel  28  provided in the stage  21 . This is to prevent a temperature increase in the mold  22  and in the transfer material  23  via the mold, and viscosity fluctuations in the transfer material  23  due to application of ultrasonic vibrations. 
     When the temperature of the transfer material  23  becomes higher than 2.5° C. above room temperature, the viscosity of the transfer material  23  decreases, and this results in changes in the thickness of the transferred object, and thus poor accuracy. Conversely, the viscosity of the transfer material  23  increases when the temperature of the transfer material  23  becomes lower than 2.5° C. below room temperature. This encourages generation of remaining bubbles in the pattern, or influences the bubble size. It is accordingly desirable to control the temperature within ±2.5° C. of room temperature. Optimally, the temperature is controlled within ±0.5° C. of room temperature. 
     In order to more reliably control temperature, it is preferable to provide a temperature detector, and control temperature by using the detected information. The temperature detector may be, for example, a non-contact sensor that makes use of infrared rays or luminance to measure temperature, or a contact sensor that takes advantage of magnetism or electrical resistance changes. 
     Temperature control of the transfer material  23  is not limited to the channel  28  that cools the transfer material  23  via the mold  22  with a flow of coolant water or coolant gas, provided that a temperature change of the transfer material  23  can be reduced. For example, it is possible to use a temperature adjuster using a cooling fan, or a temperature adjuster using a refrigerant. 
     In step (c) of  FIG. 2 , the transfer material  23  is cured by applying UV light from a UV irradiator  29  from above the film  24 , in the absence of ultrasonic vibration application. 
     Here, the film  24 , which is a UV transmissive translucent material, may be, for example, a PET film. However, the film  24  is not limited to a PET film. The UV irradiator  29  may be, for example, an LED. However, the UV irradiator  29  is not limited to an LED, provided that it is a device that cures the transfer material  23 . 
     The above embodiment of the present disclosure is based on pattern formation by UV imprinting. However, the disclosure also can be implemented by a similar process using thermal imprinting or photo imprinting. In this case, the transfer material  23  is cured by applying heat or light. 
     In step (d) of  FIG. 2 , the film  24  is moved diagonally or vertically above the mold  22  to release the transfer material  23  from the mold  22 . Here, the film  24  can be released from the mold  22  with a smaller resistance, and the transferred shape becomes more desirable when the film  24  is slowly released from the mold  22  with a smaller release angle. 
     In the steps of the pattern forming method described above, the pattern forming device is controlled by the controller  20  connected to the device with wires or without wires, as shown in  FIG. 1 . The above embodiment of the present disclosure is based on an example in which the pattern forming device is controlled as a whole by a single controller  20 . The controller  20  can be a general purpose CPU executing instructions in an associated memory or an application specific integrated circuit (ASIC) including the memory. Moreover, more than one controller may be present. 
     The mold  22  and the film  24  of the embodiment may be disposed in opposite orientation from that shown in  FIG. 2 . Specifically, as shown in  FIG. 3 , a film  34  may be placed on a stage  31  equipped with an ultrasonic generator and a temperature adjuster, and a transfer material  33  may be pressed with a mold  32  after applying the transfer material  33  to at least a part of the film  34 . In this case, an ultrasonic vibration is generated by applying AC voltage to an oscillator  37  installed in an ultrasonic transducer  36 , and the ultrasonic vibration is applied to the transfer material  33  through the bottom surface of the film  34 . The transfer material  33  is cooled via the film  34  with a flow of coolant water or coolant gas in a channel  38 . 
     The material of the mold  32  needs to be transmissive to UV light applied from above by the UV irradiator  39 , and be bendable to follow the shape of the cylindrical roller  35 , in addition to having the properties required as a mold, including rigidity and hardness. For example, the mold  32  may be one having a fine UV curable resin pattern formed on a PET film by imprinting. 
     With the foregoing configuration, the transfer accuracy can improve with reduced numbers of fine bubbles remaining in the transfer material, and with reduced numbers of bubbles that remain in the pattern when the pressure application on a film is finished before the transfer material completely fills the mold pattern. 
     The present disclosure enables accurate pattern formation, and is useful, for example, as an imprinting method and an imprinting device that transfers a pattern to a transfer target.