Patent Publication Number: US-2011053094-A1

Title: Method for fabricating roller mold for nanoimprinting

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority of Taiwan Patent Application No. 98129692, filed on Sep. 3, 2009, the entirety of which is incorporated by reference herein. 
     BACKGROUND 
     1. Technical Field 
     The disclosure relates to fabrication of an imprint mold for a nano-imprinting process, and more particularly, to methods of fabricating a roller imprint mold having nano-patterns formed thereover for a nano-imprinting process. 
     2. Description of the Related Art 
     With the rapid development of 3C science and technology, it is necessary for semiconductor process and information recording media process to reduce line width or the size of recording pit, so as to improve the operation speed and the recording density. Taking optical disc storage as an example, the length of the minimum recording pit of DVD discs is approximately 400 nm, and the length of the minimum recording pit of the next generation optical disc is approximately 170 nm. The line width of semiconductor process is reduced from several hundreds of nanometers to several tens of nanometers. 
     Therefore, in order to fabricate extremely fine line widths or recording pits, nano-processing techniques such as nanoimprint lithography (NIL) have been developed to form nano-dimensional patterns. During fabrication of an imprint mold, nano-dimensional patterns are formed on a plane mold made of organic resist materials by an electron-beam (E-beam) lithography process. However, fabrication of nano-dimensional patterns by E-beam lithography requires high equipment costs and long lithography process times. Thus, it is not feasible to use the method to fabricate large-area nano-dimensional patterns over a plane mold. 
     Nevertheless, a roll-to-roll method has been developed, wherein a nano-dimensional pattern formed by an E-beam lithography process is first imprinted on several flexible metal substrates. The flexible metal substrates are then attached to a roller to form a roller mold. The obtained roller mold could be applied to imprint nano-dimensional patterns on a large area surface by the roll-to-roll method. However, since the flexible metal substrates attached to the roller mold are respectively formed by imprinting method, it is difficult to precisely perform connection and alignment of the nano-dimensional patterns formed on various flexible metal substrates. In addition, an undesired seam may be formed between the flexible metal substrates, such that the nano-dimensional patterns for imprinting may not be completely provided at curved surfaces of the roller mold. Moreover, the flexible metal substrates have hardness issues such that the nano-dimensional patterns formed thereon may be easily damaged due to wear-and-tear, thereby affecting reliability and lifetime of the roller mold. 
     Taiwan Patent No. I305753 discloses a method for fabricating a roller mold by using an imprint roller to compress a plane mold so that patterns formed on the plane mold can be imprinted to a heated thermal imprinting material layer formed on a curved surface of a body structure having a cylinder shape, thereby forming the roller mold. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     An exemplary method for fabricating a roller mold comprises providing a roller substrate, wherein the roller substrate is a cylinder and has a curved surface. An inorganic resist layer is formed over the curved surface of the roller substrate. A laser exposure device is provided for radiating the inorganic resist layer with a focused laser and causing phase change of the inorganic resist layer at exposed regions. The inorganic resist layer in the exposed regions is removed to form a nano-pattern over the roller substrate. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIGS. 1A to 1C  are cross sections showing a method for fabricating a roller mold according to an embodiment of the disclosure. 
         FIGS. 1D to 1E  are cross sections showing a method for fabricating a roller mold according to another embodiment of the disclosure. 
         FIGS. 2A to 2D  are cross sections showing a method for fabricating a roller mold according to yet another embodiment of the disclosure. 
         FIGS. 3A to 3C  are cross sections showing a method for fabricating a roller mold according to another embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     The following description is one of the embodiments for carrying out the disclosure. This description is made for the purpose of illustrating the general principles of the disclosure and should not be taken in a limiting sense. The scope of the disclosure is determined by reference to the appended claims. 
       FIGS. 1A to 1C  are cross sections showing an exemplary method for fabricating a roller mold. 
     In  FIG. 1A , a roller substrate  100  of a cylinder is provided. The roller substrate  100  can be made by, for example, semiconductor materials, glass materials, plastic materials or metal materials. In one embodiment, the semiconductor materials comprise silicon. The roller substrate  100  of a cylinder has a curved surface  102 . A layer of inorganic resist layer  104  is formed on the roller substrate  100  with a thickness of 10˜400 nm, or of 100˜300 nm. The inorganic resist layer  104  may comprise materials such as an incomplete oxide of a phase-change material, an incomplete oxide of a transition metal, metallic glass or ZnS—SiO 2 . The so-called “incomplete oxide” means that the oxygen content of the inorganic resist material is lower than the stoichiometric oxygen content of the complete oxide of the phase-change material or the transition metal. In one embodiment, the inorganic resist layer  104  comprises an incomplete oxide of a phase-change material having a general formula A 1-x O x , wherein A represents a phase-change material, and x is between 5 at. % and  65  at. %. The phase-change material can be, for example, an alloy selected from a group consisting of Se, Te, Sb, As, Sn, Ge, or In, such as, Ge—Sb—Te, Ge—Sb—Sn, or In—Ge—Sb—Te alloy. Taking Ge—Sb—Sn alloy as an example, when the phase-change material is expressed as a general formula of Ge a Sb b Sn 1-a-b , a is approximately 5-15 at. %, b is approximately 10-50 at. %. In another embodiment, the inorganic resist layer  104  comprises an incomplete oxide of a transition metal having a general formula B 1-x O x , wherein A represents transition metal, and x is a non-zero value between 0 at. % and 75 at. %. In this embodiment, the inorganic resist material comprises a transition metal such as Ti, V, Cr, Mn, Fe, Nb, Cu, Ni, Co, Mo, Ta, W, Zr, Ru or Ag. In another embodiment, the inorganic resist layer  104  comprises metallic glass materials such as Mg-based metallic glass materials. The method of forming the inorganic resist layer  104  with an incomplete oxide of a phase-change material, an incomplete oxide of a transition metal, and ZnS—SiO 2  can be, for example, oxygen reactive sputtering. The method for coating the inorganic resist layer  104  with metallic glass materials can be, for example, reactive sputtering. 
     In  FIG. 1B , the inorganic resist layer  104  is subsequently irradiated by a focused laser  106  generated by a set of laser exposure device  108 , so that the exposed regions of the inorganic resist layer transit from an initial state  104   b  to a phase transition state  104   a . The laser exposure device  108  can be an exposure device applied in a lithography process or an optical laser direct writing device used in media drivers such as CD-ROM or DVD-ROM, so that no additional equipment costs are required. If an inorganic resist layer  104  has preferred absorption in the red band, the blue band in the visible band, or the UV-band, lasers of various wavelengths can be used as the exposure sources. Phase change of the inorganic resist layer  104  is performed by adjusting the amount of power and the process time of the focused laser  106  during the exposure. On the other hand, the feature size of the subsequently formed nano-patterns can be controlled by adjusting the amount of power and process time of the focused laser  106 . Furthermore, the oxygen content of the oxide layer of the phase-change material can also be controlled by adjusting the flow ratios of Ar and O 2  during the sputtering process. 
     In  FIG. 1C , portions of the inorganic resist layer  104  transited to the phase transition state (see  104   a  in  FIG. 1B ) are next removed, so as to from a nano-pattern  110  on the inorganic resist layer  104 , in which the nano-pattern  110  can be a line pattern or a recording pit. The portions of the inorganic resist layer  104  transited to the phase transition state  104   a  can be removed by, for example, dissolving the inorganic resist layer of the phase transition state  104   a  with an alkali solution, in which the alkali solution is, for example, a KOH or NaOH solution. 
     Thus, substantially completing fabrication of a roller mold having the nano-pattern  110  as shown in  FIG. 1C . The roller substrate  100  having the nano-pattern  110  can be horizontally mounted to an imprint carrier (not shown) to perform a roll-to-roll imprinting process of the nano-pattern  110 . In this embodiment, since the nano-pattern  110  disposed on the roller mold can be fabricated by a direct-writing exposure source, for example, a conventional lithography process or a laser beam of an optical device in a media driver, the fabrication of the roller mold allows fast fabrication time and low cost. Additionally, due to precise alignment, a highly integrated nano-pattern can be formed on the curved surface of the roller mold. Thus, additional imprinting devices such as a plane mold for fabricating the nano-pattern is not required, as the entire curved surface of the roller mold is fully utilized for forming the required nano-pattern. In addition, since materials of the inorganic resist layer have mechanic strength and are not easily damaged due to wear-and-tear of the roller mold, the roller mold fabricated by the above exemplary method can be widely applied in the nano-imprinting process for fabricating semiconductor devices, recording media, magnetic elements and displays of large areas. 
       FIGS. 1D to 1E  are cross sections showing another exemplary method for fabricating a roller mold. 
     Referring to  FIG. 1D , after the steps in  FIGS. 1A to 1C , a selective deposition process (not shown) such as an electroforming process is performed on the roller mold, which is shown in  FIG. 1C , and thereby form a metal layer  112  on the curved surface  102  of the roller substrate  100  covered by the initial state  104   b  of the a nano-patterned inorganic resist layer. Materials of the metal layer  112  can be, for example, selected from a group consisting of Ni, W, and an alloy thereof. 
     In  FIG. 1E , the initial state  104   b  of the inorganic resist layer  104  is next removed to form a nano-pattern  110 ′ made of the metal layer  112 . Herein, the nano-pattern  110 ′ is a reversed pattern of the nano-pattern  110  illustrated in  FIG. 1C  and the nano-pattern  110 ′ can be a line pattern or a recording pit. The initial state  104   b  of the inorganic resist layer can be removed by, for example, dissolving the initial state  104   a  of the inorganic resist layer with an aqueous solution such as KOH, HNO 3  or HF solution. 
       FIGS. 2A to 2D  are cross sections of yet another exemplary method for fabricating a roller mold, in which the same reference numerals indicate the same components shown in previous exemplary methods. 
     In  FIG. 2A , an intermediate layer  200  is first formed on the curved surface  102  of the roller substrate  100 , and the inorganic resist layer  104  is then formed on the intermediate layer  200 . The intermediate layer  200  can be a thermal barrier layer that can reduce the heat dissipation speed of the film and improve the exposure sensitivity or an etching stop layer. Also, the intermediate layer  200  may comprise materials such as Al 2 O 3 , AlN, SiC, SiO 2 , Si 3 N 4 , ZnS—SiO 2 , or organic polymer materials. A thickness of the intermediate layer  200  can be adjusted according to design requirements for forming the nano-patterns. 
     In  FIG. 2B , the inorganic resist layer  104  is then irradiated with a focused laser  106  by using a laser exposure device  108 , so that exposed regions of the inorganic resist layer transit from an initial state  104   b  to a phase transition state  104   a  (as shown in  FIG. 2B ). 
     In  FIG. 2C , portions of the inorganic resist layer being transited to the phase transition state  104   a  are next removed to the intermediate layer  200  and thereby form a nano-pattern  110  having a plurality of recesses  120  therein. Next, a dry etching process  250  is performed to etch the intermediate layer  200  and the roller substrate  100  exposed by the recesses  120 , using the initial state  104   b  of the inorganic resist layer as an etching mask. The dry etching process  250  can be, for example, a reactive ion etching (RIE) or an inductive coupling plasma (ICP) etching process. The initial state  104   b  of the inorganic resist layer  104  and the underlying patterned intermediate layer  200   a  are next removed and thereby the nano-pattern  110  in  FIG. 2C  is transferred to the roller substrate  100  and form a transferred nano-pattern  110 ″, as shown in  FIG. 2D . Herein, due to formation of the transferred nano-pattern  110 ′, a roller substrate  100  is formed with a curved surface  150  having a concave and convex structure which is different from the curved surface  102  illustrated in  FIG. 2A . 
     Thus, a substantially completed roller mold having the transferred nano-pattern  110 ″ is fabricated as shown in  FIG. 2D . The roller substrate  100  having the transferred nano-pattern  110 ″ can be horizontally mounted to an imprint carrier (not shown) to perform a roll-to-roll imprinting process of the transferred nano-pattern  110 ″. In this embodiment, since the transferred nano-pattern  110 ″ disposed on the roller mold can be fabricated by a direct-writing exposure source, for example, a conventional lithography process or a laser beam of an optical device in a media driver, the fabrication of the roller mold allows fast fabrication time and low cost. Additionally, due to precise alignment, a highly integrated nano-pattern can be formed on the curved surface of the roller mold. Thus, additional imprinting devices such as a plane mold for fabricating the nano-pattern are not required, as the entire curved surface of the roller mold is fully utilized for forming the required nano-pattern. In addition, since materials of the inorganic resist layer have mechanic strength and are not easily damaged due to wear-and-tear of the roller mold, the roller mold fabricated by the above exemplary method can be widely applied in the nano-imprinting process for fabricating semiconductor devices, recording media, magnetic elements and large-area displays. 
       FIGS. 3A to 3D  are cross sections of yet another method for fabricating a roller mold, in which the same reference numerals indicate the same components shown in previous exemplary methods. 
     Referring to  FIG. 3A , an inorganic resist layer  104  is formed over the roller substrate  100 . The inorganic resist layer  104  is next irradiated with a focused laser  106  by using a set of laser exposure device  108 , so that the exposed regions of the inorganic resist layer transit from an initial state  104   b  to a phase transition state  104   a.    
     In  FIG. 3B , the portions of the inorganic resist film transited to the phase transition state  104   a  are next removed, so as to from a nano-pattern  110  having a plurality of recesses  120  in the inorganic resist layer  104 . After completing the nano-pattern  110 , the roller substrate  100  exposed by the recesses  120  is etched by a dry etching process  350  and the initial state  104   b  of the inorganic resist film act as a mask, in which the dry etching process includes an RIE or ICP etching process. The initial state  104   b  of the inorganic resist layer is then removed and the nano-pattern  110  shown in  FIG. 3B  is transformed to the roller substrate  100 , thereby forming a transferred nano-pattern  110 ″ which is the same as the nano-pattern  110  illustrated in  FIG. 3B . Herein, due to formation of the transferred nano-pattern  110 ″, the roller substrate  100  is now formed with a curved surface  150  having a concave and convex structure which is different from the curved surface  102  illustrated in  FIG. 3A . 
     Thus, substantially completing fabrication of roller mold having the transferred nano-pattern  110 ″ as shown in  FIG. 3C . The roller substrate  100  having the transferred nano-pattern  110 ″ can be then horizontally mounted to an imprint carrier (not shown) to perform a roll-to-roll imprinting process of the transferred nano-pattern  110 ″. In this embodiment, since the transferred nano-pattern  110 ″ disposed on the roller mold can be fabricated by a direct-writing exposure source, for example, a conventional lithography process or a laser beam of an optical device in a media driver, the fabrication of the roller mold allows fast fabrication time and low cost. Additionally, due to precise alignment, a highly integrated nano-pattern can be formed on the curved surface of the roller mold. Thus, additional imprinting devices such as a plane mold for fabricating the nano-pattern are not required, as the entire curved surface of the roller mold is fully utilized for forming the required nano-pattern. In addition, since materials of the inorganic resist layer have mechanic strength and are not easily damaged due to wear-and-tear of the roller mold, the roller mold fabricated by the above exemplary method can be widely applied in the nano-imprinting process for fabricating semiconductor devices, recording media, magnetic elements and large-area displays. 
     In the above exemplary methods, a lithographic process can be achieved by performing a thermally direct writing type photolithographic process to an inorganic resist and thereby dramatically reduce laser spot for exposure used therein. Therefore, a nano-pattern can be formed in a cost effective way. In addition, since the methods for fabricating a roller mold having nano-patterns thereon by a direct-writing laser exposure device are disclosed, the inorganic resist layer can be directly used as an imprint layer after direct writing and etching of the inorganic resist layer, or an imprint layer can be further formed after performing an electroforming process, thereby forming a continuous and seamless large area nano-pattern. Moreover, the inorganic resist layer in the above exemplary methods shows a predetermined mechanical strength greater than of a flexible metal substrate and an imprint layer can be further formed by an electroforming process so as to function as a contact surface of the roller mold. Moreover, while conventional organic resist layer can be uniformly formed over a roller mold, process difficulties exist due to diffraction limitations of the photolithography process. Therefore, the inorganic resist layer formed by sputtering methods as disclosed in the above exemplary methods are preferably used to solve the above conventional drawbacks of organic resist layer. 
     While the disclosure has been described by way of example and in terms of the preferred embodiments, it is to be understood that the disclosure is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.