A Modified Micromolding Method for Sub‐100‐nm Direct Patterning of Pd Nanowires (2009)

An unconventional modification of a poly(dimethylsiloxane) stamp hosting microchannels leads to the formation of nanochannels entrapping a precursor, which metallizes into sub-100-nm Pd nanowires under the molding conditions (see image). This is a single-step direct-patterning technique termed nanoentrapment molding. The urge to miniaturize electronic circuitry and memory devices originates from demands such as reduced cost of ingredient materials, compactness, low weight and high reliability, and low power requirements, among others.1 As the size of the active device components as well as of the metallic interconnects is projected to decrease well below 100 nm, one of the pertinent questions being addressed is how to cast materials at such small length scales and derive acceptable device characteristics.2 It has been a dream of researchers to learn to cast materials at the nanoscale with the ease and versatility of everyday writing, stamping, or molding where the desired “material ink” is directly deposited. While working with materials in general, it is not always easy and, more often, the desired impression of a material is obtained after several arduous process steps. Thus, any attempt to save the number of process steps would obviously increase the throughput as well as the reliability. In this context, there is increasing interest in developing molecular precursors that not only convert themselves into the desired material in a single step–known as direct patterning–but, importantly, leave behind minimal impurity. Patterning a material feature below 100 nm is generally achieved using focused electron3 or ion beams4, 5 in lithography processes or by writing with a scanning probe.6, 7 These methods, besides being serial and hence time consuming, also demand sophistication in instrumentation. This explains the importance attached to relatively inexpensive, simpler and fast lithography for large areas such as stamping, which emerged over decade ago.8 However, the scope of stamping methods for truly nanoscale patterning is rather limited9 due to the assumed softness and roughness of the elastomeric stamp, which is commonly poly(dimethylsiloxane) (PDMS).10 Nevertheless, there are few examples in the literature. The fabrication of nanostructures with lateral resolution down to 50 nm has been achieved by microtoming11 or by phase-shift lithography employing composite masks.12 Other variations of stamping include nanocontact printing,13 nanoimprint lithography,14 edge-spreading lithography,15 capillary-force lithography,16, 17 as well as transfer printing.18 In a recent report,19 ordered arrays of sub-100-nm metal nanoparticles produced using a block-copolymer template have been transferred on to Si substrates employing PDMS stamps. Dewetting of a polymer from the stamp surface has been applied to create nanotrenches (≈40 nm) for nanomaterial deposition.20 Stamp collapse due to overpressure has also been exploited in contact printing of features with reduced size21 as well as in creating new shapes.22 Yet another way to miniaturize the feature size is to shrink hydrogels used for patterning.23 However, the above stamping-based methods consist of multiple steps. Direct patterning by stamping-based methods, is not common in the literature. In one such study,24 Pt nanowires (≈700 nm) were fabricated by filling the microchannels of a stamp with a carbonyl precursor. Here we report how the deformation of a PDMS stamp induced by on-top pressure in combination with a high-temperature ramp can create fine capillaries to entrap a metal precursor, which in this case is Pd octanethiolate. Trapped precursor thus undergoes thermolysis to give rise to the metallic patterns at the nanometer scale in a single step. Initially, we investigated the metallization behavior itself (Figure 1 and Supporting Information Figures S1 and S2). Pd octanethiolate, Pd(SC8H17)2, coated as a film on a glass substrate and subjected to thermolysis in air, produced a film that contained closely linked Pd nanoparticle domains, as revealed by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images shown in Figure 1a and b, respectively. During thermolysis, the thiol molecules seem to desorb, leaving behind the metallic Pd (see thermogravimetric analysis (TGA) data in Figure S1). The carbon content was found to be less than 10% based on energy-dispersive X-ray spectroscopy (EDS) quantification (see spectrum in Figure 1a), in contrast to 70% from the pristine film. The high-resolution (HR) TEM image in Figure 1b shows the lattice fringes from a nanoparticle with the spacing (2.245 Å) closely matching the Pd(111) value (2.2457 Å). The overall crystallinity of the film is evident from the X-ray diffraction (XRD) and electron diffraction (ED) data in Figures 1c and S2, respectively. The four-probe resistivity measurements (Figure 1d) revealed the room-temperature resistivity of the film to be 0.235 µΩm, nearly twice that of bulk Pd (0.105 µΩm). The temperature dependence was linear, typical of a metallic species and the temperature coefficient of resistance was found to be 0.002713 K−1 (bulk value, 0.00572018 K−1). a) SEM image of the film obtained by thermolysis of Pd octanethiolate in air at 250 °C for 30 min; the EDS spectrum is also shown. b) TEM image. The inset shows a HRTEM image of a nanoparticle. The lattice fringes correspond to the d(111) planes (2.245 Å) of Pd. c) XRD data from the thermolyzed film. d) Temperature-dependent four-probe resistivity for the thermolyzed film (dashed line, this work), along with that of the bulk Pd (solid line, Handbook of Chemistry and Physics CRC Press 88th edition, 2007-08). Under the stamp's weight with no external pressure, we obtained broad and continuous stripes of width ≈1060 nm and spacing 520 nm (Figure 2) in close compliance with the stamp geometry. The PDMS stamp used here hosts relief features of width 505 nm with intermediate channels of 950-nm width. The features seen in Figure 2b and c are micromolded Pd metal stripes. Micromolding was possible this way due to the unique nature of Pd alkanethiolates, in that they are highly soluble in organic solvents unlike other metal thiolates. Previously in this laboratory, the electron-beam resist behavior of a Pd alkanethiolate has been successfully demonstrated.25 While direct molding of Pd, as shown in Figure 2, promises widespread applications of the method to produce circuit interconnects by stamping, it also prompts one to exploit the flow conditions of the precursor inside the microchannels, which is the subject matter of this Communication. Direct patterning of Pd metal by conventional micromolding. a) Schematic image showing injection of the precursor ink (Pd octanethiolate) at the interface of the PDMS stamp and the Si substrate. The Pd stripes are formed by trapping the precursor in the microchannels followed by thermolysis at 250 °C for 30 min. b) SEM image showing the Pd stripes on Si, in conformation with the stamp geometry. c) AFM image of the Pd pattern along with a typical z profile. The process described below may be termed as “nanoentrapment molding (NEM)” and consists of three steps (Figure 3a); i) a PDMS stamp is kept on the substrate, ii) pressure is applied by placing a weight on top of the stamp and the molecular precursor is injected from one edge while the whole setup is heated to 250 °C at a given temperature ramp, and iii) after annealing the stamp is removed, leaving behind metallic nanowires on the substrate. A) Schematic image showing the formation of nanochannels due to the near-complete microchannel roof collapse. B) Large-area SEM micrograph of patterned Pd nanowires formed on Si substrate by NEM. C) Magnified view with the inset showing an individual nanowire of width 50 nm. D) AFM micrograph of the Pd nanowires. The observed width in AFM may have a contribution from tip convolution. For the given PDMS stamp, the best results were obtained when a toluene solution of Pd octanethiolate (1 mM, 60 µL) was injected while applying an external pressure of 448 Pa on the stamp, the whole setup being heated at a temperature ramp of 30 °C min−1 to reach the annealing temperature of 250 °C. Figure 3 shows the scanning electron microcopy (SEM) and atomic force microscopy (AFM) images of the patterned area. Pairs of parallel lines, straight and continuous, each of width 50 nm, can be seen over a large area (Figure 3b). The variation in the width is rather small (standard deviation 4 nm), as can be seen from the magnified image in Figure 3c. The AFM image in Figure 3d provides the height of the patterned wires to be approximately 25 nm. It is amazing that starting with a stamp with nearly micrometer-sized features the present recipe directly imprints truly nanometric features. It is also unique in that it combines the advantages of direct patterning with the versatility of soft lithography, namely simplicity, speed, and throughput, as well as scalability. This is the first report on direct molding of sub-100-nm metal nanowires. From Figure 3c, we observe that the line spacing within a pair (425 nm) is much less than the separation between the pairs (980 nm), the two distances being close to the dimensions of the relief features (505 nm) and the microchannel roof (950 nm), respectively, from an undistorted stamp (Figure 2b). It is therefore tempting to consider that under the pressure and heat treatment given, the precursor gets deposited only at the edges of the relief features, leaving the channels free of any deposit (Figure 3a). However, this process is quite different from edge-lithographic techniques,26 where the molecular ink is constrained to the intersection of the roof and relief features of the stamp by employing a solvent that de-wets the stamp surface, which is not the case here. We identify five important parameters that are responsible for the NEM process described in this article, namely, the applied pressure, the solvent and the metal precursor, the heat treatment given in terms of the temperature ramp and the annealing temperature, and, finally, the stamp geometry. When the same process was performed using different applied pressures, nanowires of different thicknesses were obtained (Figure 4). While broad and continuous stripes corresponding to the stamp microchannels are seen in the absence of external pressure (Figure 2), with the application of a mild pressure (≈110 Pa) thick lines are seen to differentiate in place of broad stripes reminiscent of their boundaries, albeit with some deposits left in between. The differentiation into well-defined lines becomes evident at 127 Pa such that at any higher pressure, only lines were visible with the intervening regions completely clear. The width (W) of Pd nanowires thus obtained varies with the applied pressure, as shown in Figure 4. In pressure regime I, the precursor ink is not strictly restricted and, therefore, the deposits are seen all over the microchannel (marked D in Figure 4a). In regime II, well-defined features form with increasing pressure with no residual deposit on the substrate (Figure 4a), whereas in regime III, due to excess pressure, broken nanowires are seen (Figure 4a). Best results (shown in Figure 3) are obtained at the intersection of regimes II and III, shown by the arrow in Figure 4a. a) Variation of the Pd nanowire width with the external pressure applied on top of the PDMS stamp. In each case, the setup was heated at a ramp of 30 °C min−1 to 250 °C and held for 30 min. Here, zero pressure refers to the pressure due to the weight of the stamp itself (≈54 Pa), without any external pressure. Error bars represent standard deviation in line width. SEM images corresponding to the pressures 110 Pa and 1010 Pa are also shown. The middle arrow refers to the pressure applied to obtain the pattern shown in Figure 3. SEM images of the molded patterns obtained b) using ethanol as the solvent, c) with a temperature ramp of 50 °C min−1, and d) with a 40 mM solution (other conditions as in Figure 3). Large-area images are shown in the Supporting Information (Figures S3–S5). In contrast to edge-patterning methods such as edge-spreading lithography15 and edge-transfer lithography,26 which employ a de-wetting solvent, here the solvent toluene wets the PDMS surface.27 With solvents such as ethanol, which de-wet the PDMS surface and cause negligible swelling, similar experimental conditions produced wide patterns (≈1 µm) almost equal to the microchannel width (Figure 4b). Thus, wetting by the solvent, in some way, seems to be important in this process. The line width is sensitive to the temperature ramp as well. When the temperature was increased and held just above the boiling temperature of toluene (≈110 °C) for nearly 10 min prior to annealing at 250 °C, we obtained broad features (≈1 µm), as shown in Figure 4c. Higher ramp rates, 50 °C min−1 instead of 30 °C min−1 also failed to yield narrower lines (Figure 4c), indicating that the temperature profile influences the process. Similarly, the precursor concentration was found to influence the line width. When the concentration was much higher (40 mM instead of 1 mM), the wires were correspondingly thicker and non-uniform (Figure 4d), while a lesser concentration (0.1 mM) yielded much fainter lines. The discussion below provides an insight into the process by which the precursor is deposited selectively at the edges of the relief features to produce the fine wires shown in Figure 3. It is unlikely that a lateral diffusion of the solvent into the relief features forcing the formation of a “sedimentation cake”28 can entirely be responsible, given that the wire formation is so neat. The line contours may instead be taken to represent the nature of the cavities that might have formed due to stamp roof collapse under the optimal conditions. Inducing roof collapse by applying external pressure on the stamp is not new but the pressures used in such cases are usually high, ≈30 000 Pa. Under the nominal pressures used in this study (≈1000 Pa), a cross-sectional SEM image of the stamp showed no evidence of roof collapse but instead a mild deformation (Figure S6). Accordingly, the footprint of the relief features is seen reduced from 505 nm from an undistorted stamp to 433 nm, as marked in Figure S6. Interestingly, the reduced footprint is close to the spacing observed within pairs of wires (≈425 nm, see Figure 3c). However, the point under discussion is the roof collapse, which is far from complete. For the given stamp geometry with an aspect ratio (microchannel height/relief width) of 0.3, the minimum amount of pressure needed for the stamp to collapse is ≈30 000 Pa29 but the pressure used in the present experiment is at least 30 times less. Obviously, the desired roof collapse cannot take place because of pressure alone (Figure S7). A steady-state swelling due to the solvent can not induce an excessive roof collapse either as maximum swelling of PDMS in toluene is only 106% on weight basis30 and, moreover, such a swelling is expected to be isotropic. Here, we call upon another parameter, namely the temperature ramp as it controls the extent of solvent diffusion into the PDMS surface. While the diffusion rate increases with increasing temperature, the amount of solvent available itself depletes with time. Thus, the diffusion can only be skin-deep, causing differential swelling of the roof surface. Perhaps the roof surface becomes so fragile that it may not be able to withstand even mild pressures (≈100 Pa, see Figure 4a) and collapses, leading to the confinement (or pinning) of the precursor ink at geometrical obstacles, that is, feature edges (see schematic in Figure S8). It is amazing that the confinement is so effective, leaving no trace of precursor in the intervening regions (between the pairs of lines, see Figure 3c). One way to test this proposition is to induce some roughness of the roof surface so that, in spite of collapse, the confinement of the precursor to the edges would not be effective. In such a case (Figure S9), instead of narrow pairs of lines, we observed broad Pd stripes (≈1.25 µm) reminiscent of the topography of the roof surface. The optimal conditions are also linked to the initial stamp geometry. When a DVD master was used instead of a CD to produce the stamp, the above conditions produced Pd wires of width ≈270 nm (Figure S10). As the aspect ratio is 0.4, finer lines may be expected at much higher pressures. In summary, a new method of patterning nanometric metallic Pd wires has been developed using micromolding by a PDMS microstamp, the precursor ink being Pd octanethiolate, which enables direct patterning. It is important that a stamp hosting micrometer-sized channels is used to fill in the precursor solution and confine it, rather than use a stamp with prefabricated nanochannels for nanomolding the precursor. The latter may not serve well as the flow of the precursor solution will be highly constrained. Besides, the stamp fabrication will be expensive. The NEM process described here is essentially a consortium of parameters, which require fine tuning to produce fine lines. These are the applied pressure, solvent, temperature ramp, annealing temperature, and stamp geometry. In the present study using a PDMS stamp made from a CD as the master, a 60 µL of 1 mM Pd octanethiolate precursor solution in toluene yielded Pd nanowires of width ≈50 nm. This value is ideal for circuit interconnects and patterned growth of semiconductor nanowires employing Pd as catalyst. We have proposed that the confinement of the precursor at the relief feature edges arises due to microchannel roof collapse, the latter being the result of a combined effect of external pressure and differential swelling of PDMS due to skin-deep diffusion of the solvent under rising temperature. The process is therefore general and should be applicable in other situations as well, provided the parameters are optimized for a given stamp and an appropriate precursor is chosen. Printing such patterns on flexible substrates is a good possibility. The process may be extendable to other solvated metal precursors as well.31 Elastomeric stamps were fabricated by replica molding of PDMS on a commercially available compact disk (Sony CD-R). PDMS was prepared by mixing Sylgard 184 curing agent (Dow Corning) and its elastomer in the ratio of 1:10 by weight. This ratio was preferred as it results in a flexible stamp with a Young's modulus of 0.75 MPa and Poisson's ratio of The was under for 30 PDMS was the of the master and in the at °C The stamp from the master contained line features of 505 nm width by 950-nm The stamp was thick and The for the of the precursor is as of in toluene was of in toluene and the was the the solution and the to The obtained was with and to excess thiol and in toluene to obtain a 1 mM PDMS stamps were using and in ethanol to any substrates were by in and and under Figure shows the schematic of the In to pattern Pd nanowires the PDMS stamp is kept on and pressure Pa to 1010 Pa) is applied on top of the PDMS by placing an appropriate a 60 µL of the Pd octanethiolate precursor solution is injected at the interface of the stamp and the substrate using a The whole setup is heated to 250 °C at a ramp of 30 °C min−1 and held for 30 min on a annealing the stamp is removed, leaving behind the Pd nanowires. The patterned substrates were using a The EDS analysis was performed with an attached to the SEM AFM measurements were performed using with in contact were used for the topography HRTEM and analysis were with a at XRD measurements were performed using a of importance to are as are but not or are made available as by the The is not responsible for the content or of any by the (other than should be to the corresponding for the