NANO LETTERS
Electrochemical Fabrication of Nanodimensional Multilayer Films
2005 Vol. 5, No. 10 1899-1904
Peter Mardilovich* and Pavel Kornilovitch Hewlett-Packard Company, Imaging and Printing Group, 1000 N.E. Circle BouleVard, CorVallis, Oregon 97330 Received June 23, 2005; Revised Manuscript Received August 17, 2005
ABSTRACT A novel method of fabricating nanodimensional multilayer films using electrochemistry is described. A thin layer of tantalum (Ta) is sputtered on a smooth insulating substrate. Ta is partially electrochemically oxidized (anodized) forming a Ta2O5 layer. The rate of Ta consumption, the rate of Ta2O5 expansion, and the dependence of Ta2O5 thickness on anodization conditions have been carefully characterized to enable accurate predictions of the resulting thicknesses of both layers. Due to strong planarization action of the anodization process, the resulting interfaces Ta/Ta2O5 and Ta2O5/electrolyte are remarkably smooth. The next layer of Ta is deposited on top of Ta2O5, and the process is repeated as many times as needed. The Ta2O5 layers are amorphous and pinhole free. We report fabrication of 10-layer structures with pitches ranging from 200 nm down to 12 nm and with excellent uniformity between the layers. The smallest achieved thickness of Ta layers is only 2.8 ± 0.1 nm. The edges of such films, after proper polishing and etching, could serve as templates in nanoimprint lithography and in other applications.
Progress in nanotechnology critically depends on the availability of methods to fabricate features at deep nanoscale about and below 20 nm. Alongside the well-established electron beam lithography and ion-beam lithography, several novel methods have been developed in the past decade, which include the dip-pen lithography,1 block copolymers,2-4 nanoimprint lithography,5 and microcontact printing.6,7 The latter two techniques use one-to-one templates to either define a nanoscale pattern in a resist layer or directly transfer material on a substrate. Once the template is obtained, fabrication of individual devices has been shown to be feasible.8,9 It is expected that in the future, high throughput processes will be developed because the templates will be used multiple times. At the same time, creation of templates remains a challenging task, which still requires highresolution methods that are usually slow and expensive. One attractive approach to template creation is based on nanodimensional multilayer films. In this method, a multilayer stack of two dissimilar materials is grown on a rigid substrate. Then the edge is polished and selectively etched resulting in a corrugated profile that can be used for imprinting into a resist layer.10 Thus the layer thickness, which can be controlled very accurately, is translated into the width of resist features. This method has already produced spectacular demonstrations. Melosh et al.11 combined epitaxially grown GaAs/AlGaAs superlattices with physical vapor deposition to form 8 nm platinum wires on the superlattice edge, which were then transferred on a * Corresponding author. E-mail:
[email protected]. Phone: +1-541-715-1826. Fax: +1-541-715-7992. 10.1021/nl0511925 CCC: $30.25 Published on Web 09/09/2005
© 2005 American Chemical Society
substrate. Austin et al.12 used the edge of a GaAs/AlGaAs superlattice to define 7 nm lines in a resist layer on a 14 nm pitch. To be suitable for such applications, the multilayer films must possess some unique properties. First, the layer thickness must be reproducible and controllable with subnanometer precision. Second, the interface roughness should be of the order of a nanometer or less. Otherwise the features will not be straight resulting in an unwanted increase of electrical resistance. Third, the layers should have distinctly different etch rates. Other desirable properties are the mechanical hardness of the protrusions (to withstand the pressure of printing) and the uniformity of surface chemistry (for release properties). So far, epitaxially grown GaAs/ AlGaAs superlattices have been the system of choice for this application. In this paper, we report fabrication of a novel metal-metal oxide multilayer system that can be used for the same purposes. Specifically, we have developed Ta/Ta2O5 (tantalum/tantalum pentoxide) films, in which the Ta2O5 layers are formed by electrochemical oxidation (anodization) of the Ta layers, as illustrated in Figure 1a. Ta is one of few metals that form a dense amorphous oxide, with the oxide growing independently of the crystallographic orientation of the underlying metal film.13 Other advantages of such films are the relative simplicity of the Ta deposition process, small natural roughness of the deposited Ta, high chemical stability, and high dielectric constant of Ta2O5, which have resulted in many applications.14-16 In addition, etch selectivity between Ta and Ta2O5 can be very high (for example 100 to 1 in a 30:5:1 mixture of acetic acid, HF, and HNO3), which is required for template fabrication. After the Ta layers are
Figure 1. (a) Fabrication sequence of a multilayer film: a, thickness of Ta as deposited; b, thickness of remaining after anodization Ta layer; c, thickness of Ta2O5 layer; the pitch of a multilayer film is given by b + c. (b) Typical time dependencies of anodization voltage and current density, with galvanostatic and potentiostatic portions indicated.
etched from the side, their exposed edges can be further oxidized creating an all-Ta2O5 corrugated template surface. Such a uniformity of the surface chemistry is beneficial for the release step of the nanoimprinting process. We have found that anodization of tantalum produces highly uniform interfaces with sub-nanometer roughness. We have been able to reproducibly grow five-bilayer films at pitches between 200 and 12 nm. The smallest achieved thickness of the Ta layers is only 2.8 ( 0.1 nm. The structures were fabricated on Si substrates with a 1.7 µm layer of thermally grown SiO2 on top. The tantalum layers were dc sputtered using a 17-in. Ta target in 5 mTorr Ar at a power 500 W for deposition thicknesses less than 20 nm and 1350 W for thicknesses between 20 and 160 nm. The linear translation was between 40 and 110 cm/min in the former case and between 10 and 80 cm/min in the latter case. The thickness of deposited Ta was chosen based on the target pitch of the multilayer film. The roughness of the top Ta surface after deposition depended on the layer thickness. The root mean square (rms) roughness was between 0.22 and 1.1 nm, and Z-range was between 2.1 and 10 nm for thicknesses between 7 and 160 nm, respectively. The roughness of very thin films was limited by the roughness of the substrate, which was 0.19 nm rms and 1.8 nm Z-range in our case. Subsequent anodization of Ta was the central step of our method, which is now described in more detail. The structure is removed from the deposition chamber and placed in an electrochemical cell with a platinum cathode. The anode is the Ta layer to be oxidized. The electrolyte is a 0.01% solution of citric acid C6H8O7. Anodization is performed at room temperature with constant stirring of electrolyte. In terms of the time dependence of anodization voltage and current density, several different regimes have been described in the literature.17 We use a two-step procedure that consists of a galvanostatic step (constant current density) followed by a potentiostatic step 1900
(constant voltage). The sequence is shown in Figure 1b. During the galvanostatic step the applied voltage increases linearly with time reflecting the changing resistance of the growing Ta2O5 layer. After the voltage reaches a predetermined value, Vm, it is kept fixed at this level for the rest of the process. The final oxide thickness depends on Vm and the duration of the potentiostatic step. During the last step, the oxide layer continues to grow and the current density sharply decreases by approximately 2 orders of magnitude. By comparison of focused ion beam scanning electron microscopy (SEM-FIB) cross sections of samples anodized at equal Vm but different potentiostatic times, it was found that oxide growth was not accompanied by consumption of extra Ta. The oxygen ions do not reach the bulk metal layer but instead fill vacancies in the oxide thereby pushing its stoichiometry toward Ta2O5. Figure 2 summarizes experimentally determined dependencies of oxide thickness on the anodization voltage and the duration of the potentiostatic step. The data points were obtained by analyzing SEM and transmission electron microscopy (TEM) cross-sectional images of the films. In general, the oxide thickness h is a linear function of the maximum anodization voltage Vm, the slope being the anodization coefficient. The latter was found to be approximately 1.8 nm/V. The best fit of transmission electron microscopy data on very thin layers (corresponding to Vm < 15 V, see inset of Figure 2a) yielded the dependence h ) 1.80Vm + 3.17 nm. If all thicknesses are included in the fit, the coefficients change slightly to h ) 1.76Vm + 3.20 nm. The offset thickness (i.e., at Vm ) 0) of 3.2 nm is most likely a consequence of the native oxide layer on the Ta surface. Our value is slightly larger than the value of 1.8-2.0 nm reported in the literature.13 As long as Vm and time interval are kept constant, the Ta2O5 thickness reproduces extremely well, with a rms error below 1 nm. An important parameter of anodization is the expansion coefficient kexp, which is defined as the ratio of Ta2O5 volume Nano Lett., Vol. 5, No. 10, 2005
Figure 2. (a) Tantalum pentoxide thickness as a function of the maximum anodization voltage. The dependence is close to linear with the slope of 1.76 nm/V. Inset: transmission electron microscopy data on layers thinner than 15 nm. (b) The same quantity as a function of the duration of the potentiostatic step. Inset: magnified initial part of the main plot.
Figure 3. A 1.04 µm thick Ta layer before (left) and after (right) anodization shown on the same scale. Ta thickness is reduced by 157 nm generating 364 nm of oxide.
to the Ta volume consumed in the process. (kexp is sometimes called the Pilling-Bedworth ratio.) We have found kexp ≈ 2.3 which is in agreement with the values from 2.3318 to 2.4719 reported for Ta oxidation in air. Figure 3 shows the expansion of 157 nm of Ta into 364 nm of Ta2O5. Knowledge of the expansion coefficient is needed to determine how much Ta should be deposited initially to obtain a desired multilayer structure. If c is the oxide thickness produced by anodization (see Figure 1a), and n ) c/b is the target oxideto-metal ratio, then the required deposition thickness is given by a ) c(1/n + 1/kexp) nm. We have discovered that anodization of Ta possesses unique planarization properties. As the oxide grows both interfaces are planarized: the top surface of Ta2O5 and the Ta/Ta2O5 interface. This can be seen in Figure 3. After deposition, the Ta layer is relatively rough (the left image), but after anodization (the right image) both interfaces are much smoother. Figures 4 and 5 present quantitative information on the roughness of the top oxide surface. Threedimensional atomic force microscopy (AFM) images (Figure 4, bottom) show consistent reduction of roughness as the film is anodized at progressively higher voltages. The rms roughness drops from the original 6 nm after sputtering to Nano Lett., Vol. 5, No. 10, 2005
∼0.4 nm at Vm ) 200 V; see Figure 4, top left. Correspondingly, the AFM Z-range drops from 48 to 5 nm; see Figure 4, top right. The AFM images suggest the existence of two surface roughness scales: one around 20 nm and another around 200 nm. The two scales are planarized at different oxide thicknesses. The 20 nm roughness goes away after approximately 80-100 nm of grown oxide; see sample 3 in Figure 4, bottom. The remaining roughness disappears at 200-300 nm of oxide. A similar correlation between the oxide thickness and reducing roughness has been observed for 200 nm Ta films (with initial roughness of 1.1 nm rms and 10.2 nm Z-range) at different stages of anodization. Thick films enable visualization of the planarization effect on the internal Ta/Ta2O5 interface. Figure 5 shows how this roughness progressively reduces as anodization proceeds. Thin films (
