Appl. Phys. A 73, 273–279 (2001) / Digital Object Identifier (DOI) 10.1007/s003390100935
Applied Physics A Materials Science & Processing
Fabrication of large-scale ultra-smooth metal surfaces by a replica technique J. Diebel1 , H. Löwe1,∗ , P. Samor´ı2 , J.P. Rabe2 1 Institut 2 Institut
für Mikrotechnik Mainz GmbH, Carl-Zeiss-Str. 18–20, 55129 Mainz-Hechtsheim, Germany für Physik, Humboldt Universität zu Berlin, Invalidenstaße 110, 10115 Berlin, Germany (Fax: +49-30/20937632, E-mail:
[email protected]) Received: 1 June 2001/Accepted: 5 June 2001/Published online: 25 July 2001 – Springer-Verlag 2001
Abstract. This paper describes the growth and characterization of large-scale ultra-smooth metal surfaces produced by an adapted replica technique. Making use of this method, either amorphous or crystalline masters of different materials with ultra-flat surfaces, e.g. mica, glass or polymer coatings on silicon, were coated by a physical vapor deposition (PVD) process with a thin precious-metal layer. On the top of this layer a thick Ni surface was grown by electroplating. Both the precious-metal layer and the nickel reinforcement can be stripped off from the master, and the free metal surface that is made to appear can be used as a substrate for the self-assembly of molecules, mostly via chemisorption of thiol-functionalized moieties. The use of either gold or silver layers led to films exhibiting different morphologies and roughnesses, which are all strongly influenced by the structure of the master’s surface and by the conditions during the PVD-coating procedure. Utilizing mica as a master it was possible to grow Ag and Au surfaces made of ultra-smooth well-defined 111-oriented crystals. A root mean square roughness down to 0.2 nm was measured over micrometersized areas by scanning tunneling microscopy. Very flat Au and Ag films have been also produced using the amorphous masters. PACS: 68.37.Ef; 68.47.De; 68.55.-a Replica techniques are established methods that have been successfully applied in areas such as microsystem manufacturing, e.g. LiGA technology (deep X-ray lithography, electroforming and molding) [1–3] and sample preparation for scanning probe microscopy (SPM) investigations [4–7]. In the field of deep-space astronomy replica techniques have been shown to be simple and inexpensive procedures to fabricate flat and bright mirrors for X-ray telescopes [8, 9]. Moreover, the replica technique can be adapted and employed for nanotechnological applications [10]. For a few years much effort has been devoted to the construction of microelectronic ∗ Corresponding
author. (Fax: +49-6131/990205, E-mail:
[email protected])
devices using active layers consisting of organic molecules, i.e. electrically conductive polymers [11]. The use of substrates with a flatness in the atomic scale extending over an area of several micrometers represents an essential prerequisite for arranging the molecules into targeted architectures. The arrangement in these self-assembled molecular nanostructures can be characterized with a resolution that reaches the molecular scale using scanning probe microscopies [12], namely scanning force microscopy (SFM) and scanning tunneling microscopy (STM). This latter technique allows us to visualize self-assembled monolayers (SAMs) of properly functionalized molecules chemisorbed on precious-metal surfaces [4–7, 13–15]. The ideal substrate for STM studies possesses a high electrical conductivity and exhibits atomically flat and spatially extended terraces on which SAMs can be chemisorbed or physisorbed. Different kinds of crystalline substrates with native ultra-flat surfaces are routinely employed for SPM investigations; they include cleaved highly oriented pyrolytic graphite (HOPG), dichalcogenides or mica crystals. For the chemisorption of SAMs on metallic substrates the cleaved crystal surfaces must be covered with thin precious-metal films by PVD processes. Several studies have been reported of the growth of gold and silver films on mica, which provide optimized conditions of the PVD deposition leading to crystalline metallic surfaces [16–19]. The surface of those metal films consists of domains with a flatness on the atomic scale, although the overall film is much more rough than the cleaved crystal used as a support for the PVD deposition. Moreover, the sizes of the domains not exhibiting atomic steps are very small. The replication of the original cleaved mica represents a good alternative which allows us to produce better-quality metal surfaces [4]. Such replicas have been prepared by gluing either a silicon wafer [4, 5] or a glass slide [6] onto a gold-coated cleaved sheet of mica or a silicon wafer and stripping it off afterwards. The roughness of those gold replicas has been analyzed by STM and SFM. As expected, the surface roughness was notably decreased [4–6]. One of the major advantages of the replica surfaces is that the metallic film is stored between two layers (master and reinforcement) in a moisture-free and in particular oxygen-
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Fig. 1. a STM image of a 200-nm-thick evaporated gold layer (Rrms = 1.8 nm) on a cleaved mica surface. b STM image of a template-stripped Au surface (Rrms =0.4 nm)
free environment, which ensures chemical purity. This could be of strong interest for the preparation of fresh Ag surfaces, since they are known to strongly oxidize when stored in air. Recently, we have reported the preparation and STM investigation of template-stripped gold surfaces supported on an electroplated Ni thick layer [7]. These Au films were prepared by first evaporating Au onto mica, which leads to crystalline Au layers. Making use of an electroplating process to reinforce the gold film, it is possible to produce replicas with low mechanical malleability and high thermal stability, combined with a smaller substrate thickness and therefore reduced weight if compared to template-stripped gold films glued on Si or glass. Finally, STM measurements (Fig. 1) have shown that the roughness of the replicated surfaces is also improved due to a reduced mechanical stress introduced into the system during each step of the substrate preparation. Moreover the absence of glue in the multilayer sample does not limit the user in the choice of the solvent to be used for, for example, the molecular self-assembly or for any chemical in situ modification of the SAM to be carried out. In this paper we show that the method of templatestripped metallic surfaces supported on Ni can be successfully applied to produce flat Ag and Au films, and also that, making use of both crystalline and amorphous masters, one can fabricate flat crystalline metallic replicated surfaces with different morphologies and roughnesses.
of 1 nm; (b) monocrystalline substances with atomically flat terraces, e.g. layered crystals (mica, highly oriented pyrolytic graphite, MoS2 ) and (c) large-scale ultra-smooth masters with a Rrms of 0.3 nm, e.g. silicon, ZERODUR2 or thin amorphous layers of thermosetting polymers. Various SPM investigations of such masters up to areas of about 10 × 10 µm2 have been performed and they revealed 2 Ultra-smooth polished glass substrate from the Carl-Zeiss Co., Oberkochen, Germany.
1 Experimental procedures 1.1 Replica technique The replica technique makes it possible to produce a negatively shaped copy of a master. In order to obtain flat surfaces that can be used for the chemisorption of SAMs, masters with a flatness on the atomic scale are required. Basically, these masters can be classified into three main groups: (a) masters which were described as flat in general, e.g. commercially polished glass with a root mean square roughness (Rrms )1 1 The surface flatness can be characterized by Rrms = N 2 −1 N×N × ¯ 2 , where N × N is the number of pixels, h mn is the m,n=1 (xmn − x) height value of the pixel mn and x¯ is the average height of the pixel calculated from the N × N values. 1
Fig. 2a–d. Cartoon of the fabrication process: a preparation of the master containing an ultra-smooth surface, b coating of the master surface by a PVD process with a thin precious-metal layer, c reinforcement by electroplating of nickel, d stripping off the replicated surface from the master
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that the smallest roughness can be obtained using cleaved mica, float glass3 and polymer-coated silicon, respectively, as masters [13, 14]. The replication process starts with the preparation of the master (Fig. 2a). Green muscovite mica (size 25 × 25 mm2 ) has been freshly cleaved with a scalpel in order to obtain an adsorbate-free surface [19, 20] with a morphology described elsewhere [21]. Alternatively, float-glass masters have been used after being cleaned with methanol in an ultrasonic bath for several minutes, rinsed again with methanol and dried under a gentle stream of N2 . As a third alternative, thermosetting polymer layers with a thickness of about 1 mm have been spin-coated onto a commercially available polished silicon wafer. These photoresists (Hoechst, AZ series) have been chosen because they are optimized materials for microelectronics applications with respect to adhesion and homogeneity of coverage of the silicon. The roughness of the float glass and the polymer-coated silicon master were measured by SFM and exhibited a Rrms smaller than 0.3 nm. A 200-nm-thick silver or gold layer has been deposited on each of the three types of masters by a thermally assisted PVD method using a vacuum coating system from Bal-Tec (Balzers, BAK 640) (Fig. 2b). The masters have been heated from the rear side through the sample holder and the temperature was monitored using an additionally inserted thermocouple. The metal evaporation on the heated master surfaces has been executed as follows: first, the master has been kept at the chosen temperature for one hour at a pressure lower than 5 × 10−4 Pa, which allows us to degas the surface. Then the evaporation of the metal has been carried out either at 35 ◦ C or at 300 ◦ C with deposition rates ranging between 0.05 nm/s and 0.4 nm/s. Finally, the metal-coated masters have been slowly cooled to room temperature with a speed of approximately 50 ◦ C/h. Finally the vacuum chamber has been purged with N2 . The thin evaporated metal layers have been stabilized by reinforcement with electroplated nickel of approximately 200-µm thickness (Fig. 2c). The electroplating was performed in a sulfamate-type plating bath containing 100 g/l nickel, 40 g/l boric acid and 3 g/l nickel bromide. The pH value of the electrolyte has been adjusted to 3.8 and the bath temperature was kept at 50 ◦ C. A current density of 1 A/dm2 has been set to avoid damaging the evaporated thin metal layer by intrinsic stress of the electrodeposit, which can appear at higher current densities. Finally, the replicated and reinforced surfaces have been carefully stripped off from the masters by lifting them with a scalpel or a pair of tweezers (Fig. 2d). In the case of the float-glass master almost all replicas can be pulled off directly after the electrodeposition of the reinforcement without any difficulties. It is noteworthy that the adhesion between master and evaporated metal layer has been sufficiently strong to perform the electroplating process, but weak enough to permit the stripping of the metallic surface. In comparison, by using mica it has been assumed, first, that thin residues of mica may adhere to the replicated surface after pulling it off. In contrast to this expectation, X-ray diffraction measurements indicated no remaining residues on the replicated surface. This result has been confirmed by STM
measurements, which did not reveal the presence of insulating mica leftovers. On the other hand, stripping off the replicas from masters of polymer-coated silicon resulted in polymer residues remaining on the replica. However, these polymer residues, being soluble in a mixture of peroxodisulfate (K2 S2 O8 ) and sulfuric acid, could be easily washed away. Indeed, after this latter chemical treatment, energy-dispersive X-ray analysis (EDX) showed no remaining carbon on the surface. It is fair to point out that the sensitivity of this method allows the detection of a substance only when present in more than 0.1–1% by weight [22]. Final confirmation of the purity of the surface has been obtained by high-resolution STM studies revealing the absence of electrically insulating regions.
3 This glass is prepared by cooling the molten glass on a liquid-metal surface.
4
1.2 Instrumentation The surface-topography measurements have been performed in air with commercial SFM and STM setups (Park Scientific Instruments). Constant force mode SFM imaging was carried out in contact mode using Si3 N4 cantilevers and tips; the interaction forces between the sample and the tip were ∼ 5 nN. In order to attain higher spatial resolutions, the metal surfaces have been explored by means of STM using Pt/Ir tips, which have been produced by mechanical cutting. Constant current mode STM images have been recorded with an average tunneling current of approximately 5 nA and a tip bias voltage of 500 mV. All images shown in this paper are raw non-filtered data. Two types of piezoelectric scanners have been used, namely 75-µm and 10-µm tube scanners. They were calibrated in x-, y- and z-directions on the micrometer scale using a Park Scientific Instrumentation calibration standard and on the atomic scale utilizing the lattice spacings of freshly cleaved HOPG, mica and gold surfaces. The surface roughness was characterized by peak-to-valley (pv) distances4 and Rrms . All X-ray measurements have been carried out at room temperature with a diffractometer D 500 (Siemens AG) with a Cu K α radiation source. A beam divergence of 0.3◦ has been measured by a NaJ scintillation counter. 2 Results and discussion Masters from glass, mica and polymer-coated silicon have been coated with thin gold or silver films by the PVD process. In order to gain a deeper understanding of the growth of these metallic films, two parameters, namely the substrate temperature and the deposition rate, have been varied systematically. This leads to replicated surfaces with different topographies, roughnesses and crystalline structures. 2.1 Replicas from glass surfaces Figure 3 displays the STM images of replicated Au and Ag surfaces from float glass evaporated at different substrate temperatures. All surfaces are made of grains exhibiting steps with height on the atomic scale. The morphology The pv value gives the distance between the maximal and the minimal height of the surface topography in a given image.
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Fig. 3a–d. STM images of replicated surfaces from float glass. Evaporation of gold at different substrate temperatures: a 35 ◦ C, b 300 ◦ C. Evaporation of silver: c 35 ◦ C, d 300 ◦ C
is uniform within the grain, although the grains exhibit different orientations. We believe that at the beginning stages of the evaporation process only a few crystallization nuclei arise on the amorphous glass surface on which the metallic crystals start to grow. Afterwards, the crystal growth is no longer influenced by the glass-surface topography and, thus, every grain can build up its own individual crystal structure. The topographies of the replicated gold and silver surfaces are similar when evaporated at low substrate temperatures (Fig. 3a,c). Gold films prepared at high evaporation temperatures exhibit grains, which are not separated significantly from each other (Fig. 3b). Note that the average size of the grains increases with higher evaporation temperatures for both metals. Replicated surfaces of Ag fabricated at higher temperatures of 300 ◦ C appear to be more flat (Fig. 3d), although the pv value indicates that the clusters are separated by narrow deep crevices. This is attributed to a higher diffusion of the Ag atoms on the glass surface if compared the Au atoms. In summary, it has been found that the sizes of the grains increase with the substrate temperature during the evaporation. Silver seems to be more suitable in replicating nanometer-size structures on amorphous substrates: this can be observed in Fig. 3d, where a scratch in the glass surface was properly replicated. In addition, the topography of replicated surfaces of float glass is similar to that obtained using polished glass (ZERODUR-glass, DESAG 263-glass; images not shown). Thus, we conclude that by using Au or Ag, i.e. metals with a relatively low melting point, the
topography of the replicated surface is not considerably influenced by that of the glass master in the sub-nanometer scale. 2.2 Replicas from mica surfaces The image in Fig. 4a shows the topography of a gold surface replicated from mica, at a substrate temperature during PVD of 35 ◦ C. The obtained surface is characterized by irregularly shaped three-dimensional crystals having a size of 50–200 nm, which consist of layered gold terraces. These terraces are laterally spaced approximately 15 nm from each other and show different orientations within the gold layer. The calculated roughness of the surface is mainly influenced by deep valleys between the crystals. During the evaporation process at low temperatures the surface diffusion energy of gold atoms is likely to be very low. For this reason, the gold atoms do not have enough energy to overcome the potential barrier which separates this state from the thermodynamic minimum configuration on the mica surface [4]. At higher temperatures of 300 ◦ C epitaxial 111 crystal growth including flat plateaus of several tens of nanometers in diameter has been found on the replicated gold surfaces (Fig. 4b). At this temperature the rapidly diffusing gold atoms can find an energetically favorable position on the mica surface, which leads to the formation of homogeneous layers. Furthermore, Fig. 4b shows that flat regions can have either sharp or round borders. Both types of boundaries appear randomly on the surface. The hexagonal borders (obtuse angle 120◦ ) reflect the symmetry of
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Fig. 4a–e. STM images of replicated surfaces from freshly cleaved mica. Evaporation of gold at different substrate temperatures: a 35 ◦ C, b 300 ◦ C. Evaporation of silver: c 35 ◦ C, d 300 ◦ C, e 400 ◦ C
the crystal structure of mica. Incompletely grown regions are characterized by a round shape. Several experiments have shown that neither the shape nor the size of these regions is influenced by a specific choice of parameters during the evaporation procedure. In all cases, the shape of the terraces is random and their sizes are up to 400 nm in diameter. The topography of areas of 10 × 10 µm2 shows defects with an average diameter of 100 nm and distances of some nm between them. This can be assigned to the different lattice constants of mica (k = 5.2 Å) and gold (k = 4.08 Å). Some theoretical aspects of this effect are described elsewhere [16, 17]. The amount of defects should be minimized by an increase in the evaporation rate. A higher rate leads to an increased vapor pressure and, thus, to a higher concentration of free metal atoms. In this case, the nucleation and the fast grain growth, which are also characterized by the coalescence of neighboring grains, predominates the surface diffusion of the gold atoms. This allows us also to
fill in the defect areas leading to a decrease in their sizes. The roughness of the replicated surfaces can be minimized from a Rrms of 0.5 nm to less than 0.3 nm by choosing an evaporation rate of 0.4 nm/s. This result is in good agreement with the observations on Ag surfaces evaporated on mica [20]. In addition, the quality of the obtained Au surfaces was analyzed by chemisorbing alkanethiol monolayers, which were successfully visualized by STM with molecular resolution [7]. A similar behavior was found for the growth of silver layers on mica. Figure 4c shows the topography of a Ag surface replicated from mica, prepared with a substrate temperature during the evaporation of 35 ◦ C. On the silver layers, holes of 10–100 nm have been detected similarly to the replicated gold surfaces. However, the silver surfaces show larger coherent crystals, due to the higher mobility of silver atoms on mica. Ag films grown at a substrate temperature of 300 ◦ C exhibit surfaces consisting of flat regions (Rrms of approximately 0.3 nm) in areas less than 1 × 1 µm2
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Fig. 5a,b. STM images of replicated surfaces from spin-coated polymer on silicon. Evaporation of gold (a) and silver (b), at a substrate temperature of 35 ◦ C
(Fig. 4d). Within these regions there are large and deep holes filled with nickel by the reinforcing electroplating step, as determined with EDX measurements. Increasing the substrate temperature to 400 ◦ C leads to large-scale ultra-smooth surfaces (Rrms of approximately 0.2 nm). In this case, the topography is characterized by small terraces of 200 nm in diameter (Fig. 4e). In summary, homogeneous surfaces with mainly 111oriented crystals are formed at higher substrate temperatures. The crystal structures and the steps in height of the flat regions (k¯ 111Gold = 2.35 Å; k¯ 111Silver = 2.36 Å) have been determined by X-ray diffraction measurements; these results are in excellent agreement with the ones obtained by STM. 2.3 Replicas from polymer-coated silicon A commercially available polymer (photoresist AZ 1215, Hoechst AG) has been coated onto a polished silicon surface by spin coating that allows us to obtain a smooth, homogeneous surface. Such a polymer has been used because of its optimized adhesion to silicon. SFM analysis of the morphology of a coated resist surface shows a Rrms lower than 0.3 nm across areas of 50 × 50 µm2 . This value can be influenced by the thickness of the resist and the applied temperature. Higher temperatures soften the overall resist surface, but increase the roughness. The topographies of replicated gold and silver surfaces are shown in Fig. 5. Both surfaces are characterized by clusters with a diameter smaller than 100 nm. This might be due to the irregular deposition of the metal atoms during the evaporation procedure on the amorphous polymer surface of the master. In contrast to mica, the polymer surface offers a non-crystalline structure that prevents epitaxial crystal growth. 3 Conclusion The fabrication of ultra-smooth precious-metallic surfaces of Au and Ag can be carried out by the replica technique, using as reinforcement an electroplated nickel thick layer. By stripping it off from the master, a fresh and robust metallic substrate can be made to appear. The gold and silver films replicated from mica masters exhibit a highly reproducible structure characterized by extremely small roughnesses. Table 1
Table 1. Rrms of replicated surfaces, as function of the type of master, evaporation temperature and chosen metal. The Rrms values are expressed in nm Gold
Mica Float glass Polymer-coated silicon
Silver
T = 35 ◦ C T = 300 ◦ C T = 35 ◦ C T = 400 ◦ C 0.6 0.3 0.9 0.2 0.4 0.6 1.1 1.1 0.4 – 0.6 –
summarizes the roughness of replicated surfaces depending on the master used, the temperature during the metal evaporation and the chosen precious metal. It unambiguously reveals that the crystal growth of the precious-metal layer is strongly influenced by the type of master: replicated surfaces from mica evaporated at high temperatures are characterized by epitaxial growth, while replicated surfaces from amorphous glasses or polymers possess a polycrystalline structure. These replicated gold or silver surfaces, in addition to the possibility of being used for large-scale production, could offer a large variety of applications, e.g. in the field of molecular self-assembly that is important both in material sciences and biotechnologies. Acknowledgements. This work was supported by the BMBF project “Muster selbstorganisierender Moleküle”.
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