Patent Publication Number: US-2003235930-A1

Title: Multi-impression nanofeature production

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001] The present invention is directed, in general, to nanotechnology and, more specifically, to methods of producing nanofeatures or nanocircuits on a substrate.  
       BACKGROUND OF THE INVENTION  
       [0002] Techniques for fabricating and patterning integrated circuits include nanocontact printing to construct gate and source/drain electrodes and appropriate interconnections for functional circuits. Nanocontact printing forms, for example, a patterned self-assembled monolayer (SAM) of “ink” on a uniform layer of metal deposited on a substrate. The SAM may comprise a material resistant to etching, such that subsequent etching of the unprinted regions of the metal layer, followed by removal of the SAM with mild heating, forms electrodes or interconnects. In another conventional nanocontact printing process, the SAM may comprise a catalyst material, the catalyst subsequently being activated to initiate growth of the underlying layer to form electrodes or interconnects. Accordingly, those skilled in the art understand that nanocontact printing may be employed to create a mask resistant to subsequent etching, as well as to create a “seed layer” to be subsequently activated or grown.  
       [0003] One conventional nanocontact printing process employs a stamp comprising polydimethylsiloxane (PDMS) and having an electrode pattern molded or otherwise formed in or on a transfer surface of the stamp. The “ink” may be painted or otherwise deposited on the transfer surface, often including portions of the transfer surface beyond the borders of the pattern to be transferred. Typically, the PDMS stamp is merely dipped into the ink solution for a few seconds to coat the transfer surface with the ink. Conventional “inks” may comprise phosphonic acid, thiol, and/or silane. The stamp provides the means for transferring the ink in a predetermined pattern to the substrate. For instance, the inked surface of the stamp may be brought in contact with the substrate with enough pressure to transfer the ink on the transfer surface onto the substrate.  
       [0004] Conventional nanocontact printing methods exhibit numerous processing disadvantages. For example, the PDMS stamp is dipped into the ink solution only momentarily, such that only a thin layer of ink remains on the transfer surface of the stamp. Once the stamp is brought in contact with the substrate, most of the ink is transferred onto the substrate, such that the ink on the transfer surface is substantially consumed. Even if a substantial portion of the ink on the transfer surface is not transferred to the substrate, the ink remaining on the transfer surface will not form a uniform pattern, because some portions of the transfer pattern will be bare after transferring ink to the substrate. Accordingly, to transfer another instance of the pattern onto the substrate (or another substrate), the pattern on the transfer surface must be re-inked in order to provide a uniform layer of ink on the transfer surface. Such an inconvenience increases the complexity, labor hours and production time of each device being fabricated.  
       [0005] In addition, the material from which the stamp was fabricated also poses a threat to complete transfer of the ink on the pattern on the transfer surface. Specifically, the stamp material typically includes contaminants in the form of residual molecules dissolved in the stamp material. For instance, a stamp comprising PDMS will typically include uncrosslinked siloxane dissolved in the PDMS elastomer. These contaminants can dissolve on or migrate to the transfer surface, such that the contaminants compete with the ink molecules during pattern transfer. When the pattern is transferred to the substrate, the contaminants may be transferred instead of the desired ink molecules. The contaminants will not bind to the substrate as well as the ink, if at all, such that the pattern transferred to the substrate will not be uniform and complete.  
       [0006] Since the contaminants inadvertently transferred to the substrate will not bind well to the metal layer on the substrate, they will dislodge, especially during the subsequent etching process. Accordingly, the substrate portions underlying the contaminants, or the gaps left thereby after the contaminants dislodge, will not be protected during the subsequent etching. The unprotected portions will, therefore, be etched away and prevent uniform formation of the intended metal feature.  
       [0007] The inadvertently transferred contaminants also cause problems in nanocontact printing processes in which the ink transferred is a catalyst to subsequently encourage growth of the underlying metal layer. Specifically, because a complete pattern of catalyst ink is not transferred to the substrate, the subsequent metallization will not occur at the sites of the transferred contaminants. Accordingly, the electrodes or interconnects intended to be grown from the metal layer will not adequately develop, again leaving an electrode pattern that is not uniform or complete.  
       [0008] Accordingly, what is needed in the art is a nanocontact printing process that overcomes the above-described disadvantages of conventional nanocontact printing processes.  
       SUMMARY OF THE INVENTION  
       [0009] To address the above-discussed deficiencies of the prior art, the present invention provides a method of producing a nanofeature on a substrate. The method includes soaking a portion of a stamp having a nanopattern thereon in an ink to allow the ink to absorb into the stamp and provide an inked surface. The method also includes applying the inked surface against a substrate to transfer an ink pattern onto the substrate. The ink within the inked stamp replenishes the pattern, in response to the transfer of the ink pattern.  
       [0010] In another embodiment of the present invention, the method of producing a nanofeature on a substrate includes extracting contaminants from the stamp.  
       [0011] The foregoing has outlined preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012] The invention is best understood from the following detailed description when read with the accompanying FIGUREs. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be increased or reduced for clarity of discussion. Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
     [0013]FIG. 1 illustrates a three-dimensional view of a stamp that may be employed in one embodiment of a method of producing a nanofeature on a substrate according to the principles of the present invention;  
     [0014]FIG. 2 illustrates a three-dimensional view of the stamp shown in FIG. 3 being soaked according to the principles of the present invention;  
     [0015]FIG. 3 illustrates a side view of the inked stamp shown in FIG. 2 as the stamp is brought in contact with a substrate;  
     [0016]FIG. 4 illustrates a plan view of the substrate shown in FIG. 3 after the inked stamp has been brought in contact with the substrate and removed;  
     [0017]FIG. 5 illustrates a side view of the inked stamp shown in FIG. 3 during another pattern transfer process;  
     [0018]FIG. 6 illustrates a plan view of the substrate shown in FIG. 5 after the inked stamp has been brought in contact with the substrate and removed a second time;  
     [0019]FIG. 7 illustrates a side view of the inked stamp shown in FIG. 5 during another pattern transfer process;  
     [0020]FIG. 8 illustrates a plan view of the substrate shown in FIG. 7 after the inked stamp has been brought in contact with the substrate and removed a number of times;  
     [0021]FIG. 9 illustrates a side view of the substrate shown in FIG. 8 during an electrode formation step according to the principles of the present invention; and  
     [0022]FIGS. 10 and 11 illustrate respective plan and side views of the substrate shown in FIG. 9 after an etching or metallization process is performed according to the principles of the present invention.  
    
    
     DETAILED DESCRIPTION  
     [0023] Referring initially to FIG. 1, illustrated is a three-dimensional view of a stamp  100  that may be employed in one embodiment of a method of producing a nanofeature on a substrate (see FIGS.  3 - 11 ) according to the principles of the present invention. The stamp  100  may include a transfer pattern  110 . The transfer pattern  100  may be a nanopattern of the electrode or circuit pattern to be formed on the substrate, and may represent nanofeatures, which are raised and/or depressed features having lateral dimensions along the surface of less than about 20 microns. The transfer pattern  110  may represent only a portion of a circuit to be formed on the substrate, or may represent the entire circuit. The stamp  100  may be manufactured by conventional methods. The stamp  100  may include a material  200  that comprises a poly(dimethylsiloxane), a copolymer of dimethyl-siloxane, a copolymer of diphenylsiloxane, or a polymer having a glass transition temperature less than about 10 degrees Celsius.  
     [0024] In an advantageous embodiment, the stamp is made of Sylgard 184, a product of Dow Chemical, Corp., in Michigan, U.S. Sylgard 184 comprises two materials: a cross-linkable silicone polymer and a curing agent. The two materials are mixed together to form a viscous liquid that is poured over a mold (not shown) and heated to about 70-90 Celsius for about 3 hours. At this temperature, the silicone polymer will cross-link and solidify into an elastomer, such that the cured stamp  100  may then be peeled away from the mold.  
     [0025] The stamp  100  may have formed thereon patterns other than that shown in FIG. 1, including patterns formed in a transfer surface  120  rather than on the transfer surface  120 . In addition, the stamp  100  may be formed by other processes and/or with other materials than those just described. However, in advantageous embodiments, the stamp  100  may comprise materials in which capillary action causes significant absorption of ink, e.g., the ink to be used in transferring the nanopattern with the stamp  100 . For instance, the stamp  100  may comprise materials having solubility parameters and diffusion coefficients compatible to those of the ink.  
     [0026] Turning to FIG. 2, illustrated is the stamp  100  shown in FIG. 1, wherein the stamp  100  is being soaked in ink according to the principles of the present invention. In the illustrated embodiment, the stamp  100  is completely immersed in a liquid solution  200  within a container  210 . In other embodiments, however, the stamp  100  may be only partially immersed in the liquid solution  200 .  
     [0027] During the soaking, the stamp  100  may remain in contact with the liquid ink solution  200  for a significantly longer period than in the processes of the prior art. For example, the stamp  100  may remain in contact with the liquid solution for a period ranging from about 5 minutes to about 10 hours. However, other soaking periods are within the scope of the present invention.  
     [0028] The liquid solution  200  may comprise solution that includes a mixture of surface reactive inks and solvents. For example, the ink solution may comprise thiol, phosphonic acid, silane, or mixtures thereof. Of course, the ink may comprise other materials that are capable of leaving a print on a substrate. By soaking the stamp  100  in the liquid solution  200  comprising the ink solution, the ink diffuses into the material of the stamp  100 , effectively creating an ink reservoir within the stamp  100 . Accordingly, the stamp  100  absorbs more ink than is required for a single pattern transfer. The additional ink absorbed in the stamp  100  replenishes the surface of the transfer pattern  110  with ink after each pattern transfer. By replenishing the surface of the transfer pattern  110 , it is intended that the ink absorbed into the interior of the stamp  100  diffuse or otherwise migrate to the surface of the transfer pattern  110  after each pattern transfer. As discussed below, the stamp  100  may therefore transfer ink patterns two or more times before re-inking, thereby increasing manufacturing efficiency.  
     [0029] In an advantageous embodiment, the stamp  100  may undergo an extraction or cleaning step prior to soaking the stamp  100  in the liquid solution  200 . The extraction step removes low molecular weight components which might cause contamination of the ink on the transfer pattern  110 . However, an exemplary extraction step includes soaking the stamp  100  in a solvent comprising hexane, thiol, acetone or methylenechloride. The soaking may be for a period ranging between about 5 minutes and about 10 hours, although longer periods are within the scope of the present application. In an advantageous embodiment, the extraction solution comprises solvents having solubility parameters and diffusion coefficients compatible to those of the stamp  100 . After soaking, the stamp  100  may be dried in an ambient environment, or at an elevated temperature in a vacuum oven.  
     [0030] In performing the above-described extraction process, the stamp  100  may swell in volume as much as  100  percent or more. During this swelling, the contaminants within the matrix of the material of the stamp  100  may chemically bond with the molecules of the soaking solution, or simply diffuse into the soaking solution. Once the stamp  100  is removed from the soaking solution and allowed to dry, the swelling subsides, and more contaminants within the stamp  100  may diffuse to the surface of the stamp  100 , which may then be washed or wiped clean. Alternatively, a continuous extraction may be performed in a Soxhlet extractor. In this manner, contaminants, such as uncrosslinked siloxane polymer, silicon, or residual cyclics from the synthesis of the silicone polymer, are partially or completely removed from the stamp  100  prior to its inking. In an advantageous embodiment, the extracting decreases the concentration of contaminants within the stamp  100  to less than about 1% by weight. The extraction process may be performed in a manner similar to the ink soaking process shown in FIG. 2.  
     [0031] Turning to FIG. 3, illustrated is a side view of the inked stamp  100  after being soaked as discussed above. In the embodiment shown, the interstitial spaces within the matrix of the material of the stamp  100  are substantially saturated with ink  300  from the liquid solution  200 , such that the soaked material of the stamp functions as an ink reservoir. In other embodiments, a block  310  integrally forms a single unit with the transfer pattern  110 . The block  310  may be comprised of the same material. In such embodiments, the block  310  and the transfer pattern  110  both serve as the ink reservoir.  
     [0032] In the embodiment shown, the inked surface of the stamp  100  can be brought in contact with a substrate  320 , such that the transfer pattern  110  makes substantially complete contact with the substrate  320 . In an advantageous embodiment, the substrate  320  may include a metal layer  330  against which the inked stamp  100  is pressed. The metal layer  330  may comprise gold, copper, silver, palladium and/or nickel.  
     [0033] Turning to FIG. 4, illustrated is a plan view of the substrate  320  shown in FIG. 3 after the inked stamp  100  has been brought in contact with the substrate  320  and removed. The transfer pattern  110  of the stamp  100  has transferred to the substrate  320  an ink pattern  400  corresponding to the shape of the transfer pattern  110 . In one embodiment, the ink pattern  400  may correspond to a nanopattern of gate electrodes, interconnects or other nanofeatures to be formed on the substrate  320 . In an advantageous embodiment, the ink pattern  400  may correspond to features of a partial or complete nanocircuit to be formed on the substrate  320 .  
     [0034] Turning to FIG. 5, illustrated is a side view of the inked stamp  100  after the transfer process described above with reference to FIGS. 3 and 4. The ink reservoir has replenished the transfer pattern  110  with additional ink  300 , such that the reservoir contains less ink  300  than it did prior to the transfer. Since the reservoir has replenished the transfer pattern  110 , a second pattern transfer may be performed without re-inking the stamp  100 . Accordingly, the stamp  100  may be brought in contact with or pressed against the substrate  320  again, as described above.  
     [0035] Turning to FIG. 6, illustrated is a plan view of the substrate  320  shown in FIG. 5 after the inked stamp  100  has been brought in contact with or pressed against the substrate  320  and removed a second time. The transfer pattern  110  has transferred to the substrate  320  a second ink pattern  600  corresponding to the shape of the transfer pattern  110 . The second ink pattern  600  is substantially similar or identical to the first ink pattern  400 .  
     [0036] Turning to FIG. 7, illustrated is a side view of the inked stamp  100  after the transfer process described above. The reservoir has again replenished the transfer pattern  110  with additional ink  300  a second time, such that the reservoir contains less ink  300  than it did prior to the second transfer process. Since the reservoir has replenished the transfer pattern  110 , additional pattern transfers may be performed without re-inking the stamp  100 .  
     [0037] Turning to FIG. 8, illustrated is a plan view of the substrate  320  after the inked stamp  100  has been brought in contact with or pressed against the substrate  320  and removed a number of times. The transfer pattern  110  of the inked stamp  100  has transferred to the substrate  320  a plurality of ink patterns  800  corresponding to the shape of the transfer pattern  110 . The ink patterns  800  are identical to the ink pattern  400 .  
     [0038] Turning to FIG. 9, illustrated is a side view of the substrate  320  shown in FIG. 8 during an electrode formation step according to the principles of the present invention. In the embodiment shown, the substrate  320  may be subjected to etching, such as wet etching, as indicated by the arrows  900 . The ink patterns  800  protect underlying portions of the substrate  320  or the metal layer  330  from the effects of the etching process. In such instances, the ink patterns  800  mask or protect portions of the substrate  320 , such that the etching removes unprotected portions of the substrate  320  to thereby substantially duplicate the transfer pattern  110  onto the substrate  320  or metal layer  330 , as the case may be.  
     [0039] However, in other embodiments, the substrate  320  may be subjected to a metallization process, such as electroless plating, wherein metallization may be catalyzed or otherwise initiated on the portions of the substrate  320  or metal layer  330  underlying the ink patterns  800 . In such embodiments, the ink patterns  800  may comprise a catalyst or a complexing agent for a metal ion, which can bind a metal ion and become a catalyst for electroless metallization. For instance, the ink patterns  800  may comprise one or more materials from amino groups or other nitrogen containing functional groups. The ink therein may then bind to palladium catalysts in the substrate  320 , metal layer  330  or ink patterns  800 , and initialize plating of nickel, gold, palladium or other metals at areas corresponding to the ink patterns  800 . Accordingly, the ink patterns  800  may be seed layers, and the ink transferred by the transfer pattern  110  may include a catalyst, a metal or both. In alternative embodiments, the metallization process may include anodizing. Those having skill in the art understand how such etching and metallization processes may be accomplished.  
     [0040] Turning to FIGS. 10 and 11, illustrated are respective plan and side views of the substrate  320  after the etching or metallization process is performed as described with reference to FIG. 9. As a result of the etching or metallization process, nanofeatures  1000  are formed on the substrate  320  in a pattern corresponding to multiple instances of the transfer pattern  110  (see FIG. 3). The nanofeatures  1000  may comprise gate electrodes, interconnects and/or other features, including those of a thin-film transistor. In an advantageous embodiment, the nanofeatures  1000  may form a partial or complete pattern for a nanocircuit. The nanofeatures  1000  may comprise metallic or semiconductor material, depending on the materials selected for the ink, the substrate  320  and the metal layer  330 .  
     [0041] The present invention thus provides a process for transferring several instances of electrode or interconnect nanopatterns or nanocircuits to substrates without requiring the re-inking of the transfer medium between each transfer, because the transfer medium may absorb the transfer ink. In addition, the present invention also provides a process for transferring a more uniform pattern free of contaminants, because the transfer medium may be soaked in a decontamination solution or extracted prior to soaking the medium in the transfer ink.  
     [0042] Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.