Abstract:
A method and process for obtaining a metal stamp from an intermediate polymer stamp comprising the steps of providing a first print layer on top of a first polymer layer, imprinting structures to obtain an intermediate stamp. A conductive layer is provided on top of the structures to obtain a seed layer if the imprinted polymer is a non-conductive, plating metal on top of the intermediate polymer stamp to obtain a metal stamp then separating the intermediate stamp from the metal stamp. This invention demonstrates stamp replication in high throughput and at low cost.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to replication of metal stamps assisted by imprinting lithography technology using an intermediate polymer stamp (IPS) consisting of micro- and nano-structures. 
       BACKGROUND OF PRIOR ART 
       [0002]    The invention of the imprint lithography by Kondo at NTT, a low-cost and high throughput manufacturing process, (Kondo, M., Patent no. JP 22389, 1979) has been widely adopted in many applications, such as photonics, magnetic data storage, display, nano-micro-electromechanical system (NEMSs, MEMSs), nano-micro-electronics, biotech and chemical synthesis. However, one of the key issues in the invented technique is to fabricate the imprint stamp with Nanopatterns in high resolution, large area, at a low cost, and it is simultaneously capable of pattering arbitrary nanostructures over a large area with long-range order at a low cost (J. J. Wang, et al. J. lightwave technol., vol. 23, pp. 474-485, 2005; B. Heidari, et al, J. Vac. Sci. Technol., B 17(6), pp 2961-2964, November/December 1999. However, stamp replication is still a crucial issue in Nano Imprint Lithography (NIL). It is known that the conventional silicon stamp manufacturing with nanoscale patterns is based on high-cost e-beam lithography, and then followed by either dry etching or metal lift-off. However, the silicon stamp is too brittle to be used for compression or injection template for mass production and therefore is not suitable for high volume production in industrial application fields. For instance, Hirai et al. found that the Si-stamp could rarely be used more than 20 times in the imprint process, where the high-pressure shock applied to the master stamp captivity repeatedly (Y. Hirai, et al., Jpn. J. Appl. Phys., vol, 41, pp. 4186-4189, 2002). A nickel stamp on the other hand, not only provides high mechanical strength and durability but also enables cost-effective manufacturing via electroforming process (J. K. Luo, et al., Mater. Lett., vol. 58, pp. 2306-2309, 2004; T. Haatainen, et al., Microelectron. Eng., vol. 83, pp. 948-950, 2006; S. H. Hong, et al., Microelectron. Eng., vol. 84, pp. 977-979, 2007). However, the suitable molds for electroforming should be conductive as well as anti-adhesive to the electroformed nickel stamp in order to facilitate multiple copies of nickel stamps from one original mold. Although some attempts were made using a silicon stamp (J. Kouba, et al., J. Phys. Conference Series, Vol. 34, p. 897 (2006)) or a quartz stamp (Y. Hirai, et al., Jpn. J. Appl. Phys. Vol. 41, p. 4186 (2002)) as electroforming templates in order to facilitate separation, a concentrated alkaline solvent has to be used to dissolve the templates. In this case, the original mold was destroyed and only capable of providing one-nickel stamp. Another well-known method is to use a structured and developed electron beam resist served directly as a galvanic form for nickel electroforming. 
       SUMMARY OF THE INVENTION 
       [0003]    It has been found by the inventors that replicating a maximum amount of nickel stamps based on one Electron Beam Lithography (EBL)-resist master is the key issue to reduce the manufacturing cost of Nano-Imprint Lithography (NIL) and thereby to further promote application of NIL technology in the industry. Nowadays, electroforming via a familiar process facilitated more copies of the Ni-stamp, which have identical structures to that of the original EBL-master. This means that one EBL-master only provides one “father” Ni-stamp with inverse features via electroforming. On the other hand, by performing additional electroforming based on a “father” stamp, a “mother” stamp with identical structures to the EBL-master was obtained. Incidentally, we found that only 10 mother stamps could be produced by this manner due to the more and more severe demoulding damages between father and mother stamps at duration of separation, especially as densely spaced small features are patterned with large a area and high aspect ratio. According to this invention we introduced a novel method to copy large quantities of nickel stamps from the original master via combined NIL and electroforming process. If patterns identical to those of the original stamp were required, then a one-step NIL was performed, otherwise a two-step NIL process will provide the reversed nanostructures on a nickel stamp after electroforming, which is probably more promising considering the fact that patterns inverse to those of the original mold are not readily obtained. 
         [0004]    One aspect of the present invention is provided by a method for obtaining a metal stamp having the same structure as a master stamp from at least one intermediate stamp, comprising the steps of providing a first imprint layer on top of a first carrier substrate, imprinting structures in the first imprint layer using a master stamp to obtain a first intermediate stamp, providing a conductive layer on top of the structured first intermediate stamp to obtain a seed layer, plating metal on top of the seed layer to obtain a metal stamp, and separating the first intermediate stamp from the metal stamp. 
         [0005]    Another aspect of the present invention is related to method for obtaining a metal stump with a structure inverse to that of the master stamp, where the metal stamp is obtained from at least two intermediate stamps according to the following steps: providing a second imprint layer on top of a second carrier layer, using the said first intermediate stamp to imprint structures in the second imprint layer in order to obtain a second intermediate stamp, providing a conductive layer on top of the second intermediate stamp to obtain a seed layer, plating metal on top of the seed layer to obtain a metal stamp, and separating the second intermediate stamp from the metal stamp. 
         [0006]    The first and second carrier substrates may comprise a polymer material, while the first carrier substrate may comprises a transparent material. 
         [0007]    The second carrier substrate may comprise transparent or nontransparent material, wherein the carrier substrates may comprises glass, a semiconductor material or metals. 
         [0008]    Moreover, the first and second imprint layers may be coated on top of the carrier substrates. 
         [0009]    Anti-sticking molecules may be provided in the resist before obtaining the seed layer and therefore the seed layer thickness of at least one atomic conductive layer may be sputtered on top of the structures in the first intermediate stamp. 
         [0010]    For obtaining the metal stamp with inverse structure to that of the master stamp, the seed layer may be sputtered on top of the structures of the second intermediate stamp. 
         [0011]    The conductive material may be a metal composed of at least one of the metals nickel, gold, silver, titanium, copper and aluminum. The metal stamp may be electroplated on top of the conductive layer. 
         [0012]    Also, the structures imprinted in the imprint layer may comprise micro and nano-structures sizes greater than 5 nm. 
         [0013]    If the first imprint layer and the second imprint layer material is conductive polymer, a seed layer is not necessary. In this case the sputtering step is not performed, and the metal stamp is electroplated directly on top of the conductive polymer directly. 
         [0014]    Additionally, the separation step between the intermediate stamps and the metal stamp may be achieved by mechanical demolding. 
         [0015]    It may also be mentioned that manufacturing of any of the metal stamps above may be performed at constant temperature in the range of 15-100° C., preferably 20-70° C. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0016]      FIG. 1  shows a schematic diagram of stamp replication process via electroforming using IPS as the galvanic-template. 
           [0017]      FIG. 2  shows a schematic diagram of stamp replication process via a two-step imprint process using the IPS imprinted substrate as galvanic-template. 
           [0018]      FIG. 3  displays Scan Electron Microscope (SEM) and Atomic Force Microscope (AFM) images of the produced nickel stamp with photonic crystal structures. 
           [0019]      FIG. 4  displays SEM and AFM images of the produced nickel stamp with magnetic storage media structures. 
           [0020]      FIG. 5  displays SEM and AFM images of the imprinted Si-substrate. 
           [0021]      FIG. 6  displays SEM images acquired on the replicated nickel stamp. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    A method for quantitative and cost-effective replication of metal stamps via combined nanoimprint lithography and electroforming has been proposed. The method includes pattern transfer to an intermediate polymer stamp (IPS) where an IPS can either be directly used as a galvanic-master stamp to replicate the metal stamp (one-step imprinting), which has nanostructures identical to those of the original master stamp-The IPS may also be used further to imprint a resist, such as, but not limited to, a thermoplastic/UV-curable resist on substrates (two-step imprinting). In this fashion the nanostructures on electroformed metal stamp will be inversed to that of the original master stamp. The invention offers a significant extension of the life-time of the original master stamp since the imprinting and demoulding only happens between the soft polymer material and the master stamp, thereby avoiding fracturing of the hard material and contaminants such as dust particles present at the interface between the master stamp and the IPS will be enclosed by the IPS. The direct electroforming from the IPS-based master stamp will result in easy separation between the master stamp and the metal stamp after electroforming. It has been shown that using IPS based nanoimprinting about 1000 IPS using one master stamp can be produced without contaminating or damaging the master stamp, which means that 1000 metal stamps could be replicated via electroforming based on one master stamp. Furthermore, since the selected IPS material is UV-transparent, even if the original master stamp is opaque or UV-nontransparent, the UV-imprints can still be performed between the master stamp and IPS and also between the IPS and other opaque/UV-nontransparent substrates. 
         [0023]    The advantages of using an IPS to transfer the nanostructures from the original imprint stamp onto nickel stamps are:
       1) Conformability of the IPS makes it capable of adapting to the non-planar master stamp or substrate;   2) Since the IPS is UV-transparent, even though the original stamp is opaque, it still enables UV processing;   3) Using an IPS avoids fractures on the hard material. For example, if some dust/particles are present between the IPS and the substrate, then they will be enclosed into the polymer without causing cracks on the stamp;   4) The demoulding only occurs between the polymer and the hard material thereby avoiding demolding damage.       
 
         [0028]    The nickel-stamp with structures identical to those of the original stamp is readily obtained via electroforming from an IPS according to  FIG. 1 . 
         [0029]      FIG. 1  shows a schematic diagram of the stamp replication process via electroforming using IPS as the galvanic-template. The nickel stamp contains nanostructures identical to those of the original master stamp. It should be emphasized that either an additional fluorocarbon film via plasma-enhanced CVD, (such as shown by U.S. Pat. No. 5,244,730, “Plasma deposition of fluorocarbon”; U.S. Pat. No. 6,184,572—“Interlevel dielectric stack containing plasma deposited fluorinated amorphous carbon films for semiconductor devices” and U.S. Pat. No. 5,698,901—“Semiconductor device with amorphous carbon layer for reducing wiring delay” for semiconductor device applications) or other releasing layers should preferably adhere onto the IPS prior to the metalized seed layer, since otherwise the separation between the IPS and the electroformed nickel stamp will be hard. This means that the stamp will experience a strong shearing force during the separation. As an example, Hong, S. et al (Microelectronics, Eng. Vol. 84, p. 977, 2007) has to separate the hot embossed thermoplastic PVC film from electroformed nickel stamp in an organic solvent to soften the polymer film. It is believed that as the patterned area is enlarged (e.g., 3-4 inch) the surface area coated by metal seed layer is relatively increased, which in turn leads to the increased shearing force at the separation. Thus there is a high risk that the increased shearing force is sufficient to fracture the nanostructures of the polymer material thereby making the fractured material will fill into the cavities of nanostructures on the nickel stamp. In this case, an expensive and advanced cleaning method is required (e.g., downstream plasma treatment) to clean the stamp. 
         [0030]    If a stamp with features inverse to that of the original master stamp is desired, two-step imprint processes were performed. The first imprint produces an IPS and then the second imprint was performed on the substrate with pre-coated imprint resist. Then the imprinted substrate was used as a template for electroforming and the nickel stamp with reversed features compared to that on the original stamp was obtained. 
         [0031]    A schematic diagram of the stamp replication process via a two-step imprint process is shown in  FIG. 2 . Finally, by using the technology of the applicant, such as the IPS method for yield improvement and combined thermal and UV nanoimprint disclosed in the European patent. EP 1731962 to imprint the UV curable or thermosetting materials, precise pattern transfer with long range order across a large area was facilitated on the nickel stamps via electroforming. However, one point the applicant wished to emphasize is that a release layer deposited onto the imprinted substrate prior to the metallization of a seed layer is very crucial, since without the release layer the substrate resist was peeled off from the substrate and adhered strongly onto the electroformed Ni-stamps. In contrast by adopting a releasing film, e.g. plasma enhanced Chemical VaporDeposition (CVD) of a fluorocarbon film onto the imprinted substrate prior to metallization of the seed layer, this problem has been solved. Thereby, after the final electroformed nickel stamps were formed, only simple cleaning methods, such as electrochemical cathode cleaning and plasma etching were required. 
       EXPERIMENTAL EXAMPLES 
     Fabrication Process for Ni-Stamp Replication from the IPS 
     Example 1 
     Nickel Stamp with Photonic Crystal Structures (PCS) 
       [0032]    The original master stamp was obtained, for instance, by a combined e-beam recording (EBR) and electroforming process. A nickel stamp obtained in this fashion consists of an array of width 230 nm PCS with a pitch of 450 nm and a depth of 130 nm across 4-inch patterned area. An acrylate imprint resist was coated onto a polycarbonate polymer sheet and then used as the substrate for nanoimprinting. After demoulding the IPS, it was inspected by AFM, SEM and an optical microscope. The surface of the IPS was further modified by depositing a thin (˜6 nm) fluorocarbon film via plasma enhanced chemical vapor deposition. Then, a nickel seed layer was sputtered onto the IPS prior to electroforming. The thickness of the sputtered Ni-seed layer was 10 nm. Since we adopted nickel as the seed layer, the definition of the nanostructures should be maintained well. It could be seen that the replicated nickel-stamp has identical structures to that of the original master stamp, and the replicated features showed long-range order as well as high fidelity ( FIG. 3 ). 
       Example 2 
     Nickel Stamp with Magnetic Storage Media Nanostructures 
       [0033]    An original nickel imprint was produced by a combined e-beam recording technique and an electroforming process. In a data track area the patterns have dimensions of 40 nm in width and 120 nm in pitch. The imprinted IPS was inspected with SEM, using an acrylate imprint resist on a polycarbonate polymer sheet. The inverse nanofeatures were transferred with good fidelity. After sputtering of a thin film (˜10 nm) of nickel, the electroforming was performed. The big advantage of sputtering a thin metal layer instead of thick layer is to avoid hole-inclusion due to narrow nanochannels with high aspect ratio and with high pattern density. The electroformed nickel stamp with nanostructures identical to those of the original master stamp was obtained ( FIG. 4 ). 
       Fabrication Process for Ni-Stamp Replication from the Two-Step Imprint 
     Example 3 
     Nickel Stamp with Photonic Crystal Structures 
       [0034]    The stamp consists of an array of dots with 200 nm in diameter with a pitch of 460 nm by 3-inch area. The IPS, which comprises an acrylate imprint resist on a polycarbonate carrier polymer sheet, was imprinted onto master stamp. The IPS was further used to transfer the patterns onto the Si-wafer, which was pre-coated with an epoxy imprint resist ( FIG. 5 ). The imprinted Si-wafer was finally used as a mold for electroforming to obtain the nickel stamp replica, which comprises structures inverse to those on the original stamp. 
       Example 4 
     Nickel Stamp with Magnetic Storage Media Nanostructures 
       [0035]    In order to obtain the inverse nickel replica with magnetic storage media structures, we follow the same procedure as in example 3. The final nickel stamp with complementary structures to that on the original stamp was illustrated by  FIG. 6 .