Patent Publication Number: US-2016246176-A1

Title: Fabrication of Tetherable Patterned Thin Film with 3D Rolled-up Structure

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claim priority to TAIWAN application Numbered 104105887, filed Feb. 24, 2015, which is herein incorporated by reference in its&#39; integrity. 
     TECHNICAL FIELD 
     The present invention generally relates to a fabrication of patterned thin film, and more particularly, to a fabrication of patterned thin film with tetherable 3D rolled-up structure. 
     BACKGROUND OF RELATED ART 
     In microelectromechanical system, micromachining of silicon includes surface micromachining and bulk micromachining. Surface micromachining builds microstructures by depositing and etching different structural layers on top of the substrate. Bulk micromachining builds a silicon substrate selectively etched to produce structures. Each of two micromachining system above has its benefit and disadvantage, respectively. For example, in surface micromachining, due to the fabrication process involved series of 2D thin films stacking, which limit its modification. The control of thickness of the thin film and the techniques involved multiple mask process and sacrificial layers make the process of surface micromachining too complex. Thus, the utility of surface micromachining is restricted to planar configurations due to the difficulty in constructing structures in the direction perpendicular to the substrate. On the contrary, in bulk micromachining, the etched area and un-etched area form specific angle due to the diamond like lattice structure of silicon, which make it difficult to create micro structure with particular shape, in other words, the flexibility of design is limited. 
     In prior art, it has developed three-dimensional (3D) tubular structure with multilayered thin film structures due to strain-induced self-rolled-up, that includes planar layer which is formed by a sacrificial layer and one or more strained layer. 
     It is well-established that mismatch strains between different layers in thin film systems can induce mechanical deformation either in the form of surface waviness formation or in the form of bending and rolling of thin membranes. 
     Deposition method includes plasma-enhanced chemical vapor deposition (PECVD), metal-organic vapor deposition (MOCVD) and molecular beam epitaxy (MBE). 
     The material of thin film layer includes, epitaxial single crystal, amorphous polymers, metal or composite materials. 
     The sacrificial layer includes lattice-matched heterojunction generated from epitaxy, spun-on layers or semiconductor substrate. 
     Defining patterns on the surface can usually be achieved by lithography. Lithographic technique includes extreme ultraviolet lithography (EUV), X-ray lithography, electron projection lithography (EPL), ion projection lithography (IPL), electron-beam lithography (often abbreviated as e-beam lithography) and the like. 
     Recently, scientists have developed microelectromechanical system and/or an apparatus for cell culture in vitro and detection. 
     However, most biomedical devices are formed by 2D system. To integrate 2D material with normal physical property into 3D system, it must construct 3D supportive structure firstly on the substrate, and followed by covering a 2D material onto the 3D substrate, and therefore, it is complicated and difficult. 
     In order to solve the problem of the conventional arts, there is a need to provide simple fabrication process for preparing sensitive device based on 3D system. The present invention can not only maintain the physical property of the 2D patterned thin film material, but also improves detection by integrating 3D structure. 
     In addition, the present invention also provides a tetherable patterned thin film with 3D rolled-up structure to actively trap and detect the target objects. 
     SUMMARY 
     An object of the present invention is to provide a patterned thin film with tube-shaped structure by strain-induced self-rolled-up technique. The present invention can improve biomedical technique and/or apparatus by combining 2D patterned thin film and tetherable 3D structure. 
     The present invention is to provide a method for preparing a patterned thin film with 3D hollow tubular structure. Due to the difference in thermal expansion coefficient between different layer, the 2D thin film can curve and scroll into a 3D tubular structure by etching the substrate to free from adhesion. 
     According to one embodiment, the present invention provides a fabrication of patterned thin film with tethered 3D rolled-up structure. The fabrication includes at least one substrate for allowing steps: covering a supportive layer onto the substrate; defining a pattern portion onto the supportive layer; depositing a thin film layer onto the pattern portion; opening three concavities onto supportive layer; and removing the substrate. The 2D thin film will bend or curl towards the layer (supportive layer or thin film layer) with higher coefficient of thermal expansion and form 3D tube-shaped structure. 
     According to one embodiment, the present invention provides a fabrication of patterned thin film with untethered 3D rolled-up structure. The fabrication includes allowing steps: preparing a substrate, covering a supportive layer onto the substrate; defining a pattern portion onto the supportive layer; depositing a thin film layer onto the pattern portion; opening four concavities onto supportive layer; and removing the substrate. The 2D thin film will bend or curl towards the layer (supportive layer or thin film layer) with higher coefficient of thermal expansion and form 3D tube-shaped structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The components, characteristics and advantages of the present invention may be understood by the detailed description of the preferred embodiments outlined in the specification and the drawings attached. 
         FIG. 1  illustrates a flow chart of preparing the patterned thin film with tethered 3D tube-shaped structure according to an embodiment of the present invention. 
         FIG. 2A  illustrates a sectional view of the substrate, supportive layer and thin film layer according to an embodiment of the present invention. 
         FIG. 2B  illustrates a sectional view of opening a concavity at a side of the supportive layer according to an embodiment of the present invention. 
         FIG. 2C  illustrates a sectional view of rolled-up structure according to an embodiment of the present invention. 
         FIG. 3A  illustrates a diagram of supportive layer without pattern according to an embodiment of the present invention. 
         FIG. 3B  illustrates a diagram of the supportive layer with pattern according to an embodiment of the present invention. 
         FIG. 3C  illustrates a diagram of concavities at three sides of the supportive layer according to an embodiment of the present invention. 
         FIG. 3D  illustrates a diagram of tethered tube-shaped thin film according to an embodiment of the present invention. 
         FIG. 4  illustrates a flow chart of preparing the patterned thin film with untethered 3D tube-shaped structure according to an embodiment of the present invention. 
         FIG. 5A  illustrates a sectional view of the substrate, supportive layer and thin film layer according to an embodiment of the present invention. 
         FIG. 5B  illustrates a sectional view of opening a concavity at a side of the supportive layer according to an embodiment of the present invention. 
         FIG. 5C  illustrates a sectional view of rolled-up structure according to an embodiment of the present invention. 
         FIG. 6A  illustrates a diagram of supportive layer without pattern according to an embodiment of the present invention. 
         FIG. 6B  illustrates a diagram of the supportive layer with pattern according to an embodiment of the present invention. 
         FIG. 6C  illustrates a diagram of concavities at four sides of the supportive layer according to an embodiment of the present invention. 
         FIG. 6D  illustrates a diagram of untethered tube-shaped thin film according to an embodiment of the present invention. 
         FIG. 7A  illustrates a SEM image of the tethered thin film with 3D rolled-up structure. 
         FIG. 7B  illustrates a SEM image of the untethered thin film with 3D rolled-up structure. 
         FIGS. 8A-8B  illustrate SEM images of 3D rolled-up structure. 
         FIG. 9A  illustrates a SEM image of 3D rolled-up structure with single turn. 
         FIG. 9B  illustrates a SEM image of 3D rolled-up structure with double turns. 
         FIG. 9C  illustrates a SEM image of 3D rolled-up structure with triple turns. 
         FIG. 10A  illustrates a SEM image of 3D rolled-up structure with supportive layer. 
         FIG. 10B  illustrates a SEM image of 3D rolled-up structure without supportive layer. 
     
    
    
     DETAILED DESCRIPTION 
     Some preferred embodiments of the present invention will now be described in greater detail. However, it should be recognized that the preferred embodiments of the present invention are provided for illustration rather than limiting the present invention. In addition, the present invention can be practiced in a wide range of other embodiments besides those explicitly described, and the scope of the present invention is not expressly limited except as specified in the accompanying claims. The layout of components may be more complicated in practice. 
     First Preferred Embodiment 
     Tethered 3D Thin Film 
       FIG. 1  illustrate a flow chart of preparing a patterned thin film with tethered rolled-up hollow structure according to an embodiment of the present invention. The method provides at least one substrate, such as silicon material, for following steps: 
     Step  202 : A supportive layer  104  is covered over the substrate  102 .  FIG. 2A  illustrates a sectional view of the substrate  102 , a supportive layer  104  and a thin film layer  106  of the present invention. The supportive layer  104  includes, but is not limited to SiO 2  or Si 3 N 4 , in the preferred embodiment, the supportive layer  104  is SiO 2 . The supportive layer  104  is formed over the substrate  102  by coating, printing or other process. In the preferred embodiment, the supportive layer  104  is covered over the substrate  102  by coating. The thickness of the supportive layer  104  is can be about 10-100 nm, more particularly, about 100 nm. 
     Step  204 : A micro-pattern is defined on the supportive layer  104 .  FIGS. 3A-3B  illustrate processes of patterning portion  110  formed on the supportive layer  104 . It should be noted that the supportive layer  104  and the thin film layer  106  are combined into a single layer in order to simplify references in drawings. It is well understood that the present invention must coat a photoresist agent on the surface of the supportive layer  104  in order to define a pattern. Either positive resist or negative resist can be adapted for defining patterns based on the specific requirements. In the preferred embodiment, a positive resist polymethylmethacrylate (PMMA) is covered on the supportive layer  104  by spin coating, and then continues following steps. 
     Step  206 : A thin film layer  106  is deposited onto the pattern portion  110 . A required material can be coated onto the supportive layer  104 , such as but not limited to magnetic material, conductive/non-conductive material or semiconductive material, after defining the pattern portion  110 . In one embodiment, the thin film layer  106 , coated onto the surface of the supportive layer  104  includes, but is not limited to Nickel-Iron (Ni 80 Fe 20 ) alloy. In the preferred embodiment, the thin film layer  106  is deposited onto the surface of the supportive layer  104  by e-beam evaporation. In the preferred embodiment, the thin film layer  106  includes, but is not limited to Cr and Ni 80 Fe 20  (not shown in drawings). We used the e-beam evaporation system to deposit (1) about 5-20 nm thick Cr as the adhesive layer, preferable 10 nm; (2) a layer of Ni 80 Fe 20  ranges from 30 nm to several micrometers as the sensing layer, preferable 90 nm; and (3) about 5-20 nm thick Cr as the protective layer, preferable 10 nm, in sequence. Accordantly, 2D patterned thin film with will be done through above steps. 
     Step  208 : A concavity  108  is opened on at least one side of the supportive layer  104 . In an embodiment, a concavity  108  is formed at the front (or back), right and left sides, respectively, of the pattern portion  110  of the supportive layer  104 .  FIG. 2B  illustrates a sectional view of the concavity  108  formed at one sides of the supportive layer  104 .  FIG. 3C  illustrates a perspective view of the concavity  108  formed at three sides of the supportive layer  104 . First of all, the shape of required concavities are defined onto the supportive layer  104  and the thin film layer  106  by lithography, and then the concavities are etched by buffered oxide etchant (BOE). As shown in  FIG. 3C , each of left, right and front (or back) side of the supportive layer  104  and the thin film layer  106  has its concavity, respectively, to assist 2D thin film form rolled-up structure  120  (also called tube-shaped or ring-shaped structure or tubular structure). It is well understood that the height and width of concavities  108  can be modified or varied based on requirements by the skilled person in the art. 
       FIG. 7A  illustrates a SEM image of tetherable thin film with tube-shaped structure, that is tethered at a side of the substrate. The width of the concavity  108  is 5 micrometers. The width and diameter of the tube-shaped structure are 8 and 19 micrometers, respectively. 
     Step  210 : The substrate  102  is subsequently etched. The substrate  102 , after step  208 , is immersed into etchant, such as tetramethylammonium hydroxide (TMAH) for removing parts of the substrate  102  to form the tube-shaped structure  120 . Referring to  FIG. 2C , the supportive layer  104  and the thin film layer  106  bend or curl towards a side of the substrate  102  to form the tube-shaped structure  120  in etching process is due to the difference in thermal expansion coefficient between the supportive layer  104  and the depositing material  106 . In the embodiment, the etchant includes, but is not limited to TMAH (N(CH 3 ) 4   + OH − ). 
     Step  212 : The thin film layer  106  and the supportive layer  104  can roll up due to stress induced by the difference in thermal expansion between different layers are released after substrate etching, and a tethered thin film with 3D structure  120  is created. If the thermal expansion coefficient of the supportive layer  104  is greater than that of the thin film layer  106 , they will bent towards a side of the supportive layer  104  (away from a side of the thin film layer  106 ), thereby rolling downward (not shown in FIG.) If the thermal expansion coefficient of the supportive layer  104  is smaller than the thin film layer  106 , they will bent towards a side of the thin film layer  106  (away from a side of the supportive layer  104 ), thereby rolling upward and forming the tube-shaped structure  120 , as shown in  FIGS. 2C and 3D . In the preferred embodiment, the thermal expansion coefficient of Cr, Ni 80 Fe 20  and SiO 2  are 6.2 (10 −6 /mK), 12.8 (10 −6 /mK) and 0.5 (10 −6 /mK), respectively, so the supportive layer  104  will bent towards a side of Ni 80 Fe 20 . The difference of thermal expansion coefficient between the depositing  106  and the supportive layer  104  is about 4.8-12.3 (10 −6 /mK). 
     In another embodiment, various length of the tube-shaped thin film can be formed by modulating the distance between left and right sides, as shown in  FIGS. 8A and 8B , they illustrate SEM images of tube-shaped thin film with lengths of 8 and 140 micrometers. 
     The present invention also provides a fabrication of tube-shaped thin film with single turn and multiple turns by modulating the thickness of the supportive layer  104 , the etching temperature and the distance between the front and the back concavities  108 .  FIGS. 9A-9C , they illustrate tube-shaped thin film with single turn, double turns and triple turns, respectively. In above embodiment, the thickness of the thin film  104  is 100 nanometers, and diameter of tube-shaped thin film with single turn, double turns and triple turns are 15, 17, 19 micrometers, respectively, by modulating the length of front concavity to back concavity. It is well understood that the diameter raises as the number of turns increased. 
     On the other hand, diameter and turns can be modulated by external factors, such as etching time and temperature. In one embodiment, etching rate rises as temperature from 60° C. to 150° C., and thus the number of turns (N) of the tube-shaped structure 120 will be made. In one embodiment, the number of turns (N) is 3 under temperature between 90° C.-110° C.; in contrary, the number of turns (N) is 1 under temperature between 60° C.-80° C. Accordantly, the number of turns (N) are proportional to the temperature. It is well understood that the desired operating temperature is based on the depositing material chosen in thin film layer. 
     In an embodiment, removing the supportive layer  104  of the tube-shaped thin film can reduce the inference problem during sensing/detecting. As shown in  FIGS. 10A and 10B , they illustrate the tube-shaped structure prior to and posterior to removing the supportive layer  104 , respectively. 
     Second Preferred Embodiment 
     Untethered 3D Thin Film 
     Step  302 : A supportive layer  404  is covered over the substrate  402 .  FIG. 5A  illustrates a sectional view of the substrate  402 , a supportive layer  404  and a thin film layer  406  of the present invention. The supportive layer  404  includes, but is not limited to SiO 2  or Si 3 N 4 , in the preferred embodiment, the supportive layer  404  is SiO 2 . The supportive layer  404  is formed over the substrate  402  by coating, printing or other process. In the preferred embodiment, the supportive layer  404  is covered over the substrate  402  by coating. The thickness of the supportive layer  404  is can be about 10-100 nm, more particularly, about 100 nm. 
     Step  304 : A micro-pattern is defined on the supportive layer  404 .  FIGS. 6A-6B  illustrate processes of patterning portion  410  formed on the supportive layer  404 . It should be noted that the supportive layer  404  and the thin film layer  406  are combined into a layer in order to simplify references in drawings. It is well understood that the present invention must coat a photoresist agent on the surface of the supportive layer  404  in order to define a pattern. Either positive resist or negative resist can be adapted for defining patterns based on the specific requirements. In the preferred embodiment, a positive resist polymethylmethacrylate (PMMA) is covered on the supportive layer  404  by spin coating, and then continues following steps. In the preferred embodiment, the pattern portion  410  are created onto the substrate  402  that spin-coated with e-beam resist polymethyl methacrylate (PMMA). Then, the pattern portion  410  will be appeared on the substrate  402  in developer, such as 3:1 mixture of 2-propanol and methyl isobutyl ketone. It is well understood that the lithographic technique is not limited to e-beam lithography, but can be varied or modified by the person in the art in the light of the need in use. Besides, in step  404 , the fabrication further includes dehydration baking, priming, soft baking and hard baking to enhance precision and reliability of the pattern portion  410 . 
     Step  306 : A thin film layer  406  is deposited onto the pattern portion  410 . A required material can be coated onto the supportive layer  404 , such as but not limited to magnetic material, conductive material, non-conductive material or semiconductive material, after defining the pattern portion. In one embodiment, the thin film layer  406 , coated onto the surface of the supportive layer  404 , includes, but is not limited to Fe-Ni alloy. In the preferred embodiment, the thin film layer  406  is depositing onto the surface of the supportive layer  404  by e-beam evaporation. In the preferred embodiment, the thin film layer  406  includes, but is not limited to Cr and Ni 80 Fe 20  (not shown in drawings). We used the e-beam evaporation system to deposit (1) about 5-20 nm thick Cr as the adhesive layer, preferable 10 nm; (2) a layer of Ni 80 Fe 20  ranges from 30 nm to several micrometers as the sensing layer, preferable 90 nm; and (3) about 5-20 nm thick Cr as the protective layer, preferable 10 nm, in sequence. Accordantly, patterned thin film with 2D planar will be done through above steps. 
     Step  308 : A concavity  408  is opened on at least one side of the supportive layer  404 . In an embodiment, a concavity  408  is formed at the front, back, right and left sides, respectively, of the pattern portion  410  of the supportive layer.  FIG. 5C  illustrates a sectional view of the concavity  408  formed at a side of the supportive layer  404 .  FIG. 6C  illustrates a perspective view of the concavity  408  formed at four sides of the supportive layer  404 . First, the shape of required concavities are defined onto the supportive layer  404  and the thin film layer  406  by lithography, then the concavities are etched by buffered oxide etchant (BOE). As shown in  FIG. 6C , each of left, right, front, and back sides of the supportive layer  404  and the thin film layer  106  has its concavity, respectively, for forming rolled-up thin film  420  (also called tube-shaped or ring-shaped structure or tubular structure). It is well understood that the height and width of concavities  408  can be modified or varied based on requirements by the skilled person in the art. 
       FIG. 7B  illustrates a SEM image of untetherable thin film with tube-shaped structure, that is tethered at a side of the substrate. The width of the concavity  408  is 5 micrometers. The width and diameter of the tube-shaped thin film are 8 and 19 micrometers, respectively. 
     Step  310 : Etching the substrate  402 . The substrate  402 , after step  308 , is immersed into etchant, such as tetramethylammonium hydroxide (TMAH) for removing parts of the substrate  402  to form the tube-shaped thin film  420 . Referring to  FIG. 5C , the supportive layer  404  and the thin film layer  406  bend or curl towards a side of the substrate  402  to form the tube-shaped structure  420  in etching process, due to the difference in thermal expansion coefficient between the supportive layer  404  and the thin film layer  406 . In the embodiment, the etchant includes, but is not limited to TMAH (N(CH 3 ) 4   + OH − ). 
     Step  312 : The thin film layer  406  and the supportive layer  404  can roll up owing to stress induced by the difference in thermal expansion between different layers are released after substrate etching, and then an untethered thin film with 3D structure  420  is created. If the thermal expansion coefficient of the supportive layer  404  is greater than that of the thin film layer  406 , they will bent towards a side of the supportive layer  404  (away from a side of the thin film layer  406 ), thereby rolling downward (not shown in FIG.) If the thermal expansion coefficient of the supportive layer  404  is smaller than the thin film layer  406 , they will bent towards a side of the thin film layer  406  (away from a side of the supportive layer  404 ), thereby rolling upward and forming the tube-shaped thin film  420 , as shown in  FIGS. 5C and 6D . In the preferred embodiment, the thermal expansion coefficient of Cr, Ni 80 Fe 20  and SiO 2  are 6.2 (10 −6 /mK), 12.8(10 −6 /mK) and 0.5(10 −6 /mK), respectively, so the supportive layer  404  will bent towards a side of Ni 80 Fe 20 . The difference of thermal expansion coefficient between the thin film layer  106  and the supportive layer  104  is about 4.8-12.3 (10 −6 /mK). 
     On the other hand, diameter and turns can be modulated by external factors, such as etching time and temperature. In one embodiment, etching rate rises as temperature from 60° C. to 150° C., and thus the number of turns (N) of the tube-shaped structure 120 will be made. In one embodiment, the number of turns (N) is 3 under temperature between 90° C.-110° C.; in contrary, the number of turns (N) is 1 under temperature between 60° C.-80° C. Accordantly, the number of turns (N) are proportional to the temperature. It is well understood that the desired operating temperature is based on the depositing material chosen in thin film layer. 
     In the texture, the terms “one end”, “one side”, “two ends” and “two sides” refer to any one side (or end) of the pattern portion. In order to distinguish and clarity, “two ends” and “one end” refer to the front end and/or back end corresponding thereof, for example in  FIGS. 3A-3D , “front end” and “back end” refer to the left side and right side in drawings, respectively. “Two sides” and “one side” refer to the left side and/or right side corresponding thereof, for example, in  FIGS. 3A-3D , “left side” and “right side” refer to the bottom side and top side in drawings, respectively. The term “side” is changeable with “end”, not limited to above embodiment. The difference between tube-shaped and ring-shaped is only length of the specification, theoretically, the length of the tube-shaped is longer than that of the ring-shaped. 
     As description above, the present invention provides fabrication of a patterned thin film with 3D rolled-up structure. The 3D rolled-up thin film can be serve as biosensor to dissolve disadvantage of conventional 2D sensor. In addition, the 3D rolled-up thin film also increase the amount of collected cells and detective direction as a result of its rolled-up structure which can enhance the signal. 
     Various terms used in this disclosure should be construed broadly. For example, if an element “A” is said to be coupled to or with element “B,” element A may be directly coupled to element B or be indirectly coupled through, for example, element C. When the specification states that a component, feature, structure, process, or characteristic A “causes” a component, feature, structure, process, or characteristic B, it means that “A” is at least a partial cause of “B” but that there may also be at least one other component, feature, structure, process, or characteristic that assists in causing “B.” If the specification indicates that a component, feature, structure, process, or characteristic “may”, “might”, or “could” be included, that particular component, feature, structure, process, or characteristic is not required to be included. If the specification refers to “a” or “an” element, this does not mean there is only one of the described elements. 
     The foregoing descriptions are preferred embodiments of the present invention. As is understood by a person skilled in the art, the aforementioned preferred embodiments of the present invention are illustrative of the present invention rather than limiting the present invention. The present invention is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures.