Ultraflat Templated Polymer Surfaces - American Chemical Society

The roughness of spin-cast polymer films arises from thermally activated capillary waves during preparation and typically amounts to about 0.5 nmrms m...
6 downloads 0 Views 4MB Size
Download PDF
pubs.acs.org/Langmuir © 2009 American Chemical Society

Ultraflat Templated Polymer Surfaces David Pires,† Bernd Gotsmann,† Fabrizio Porro,‡ Dorothea Wiesmann,† Urs Duerig,† and Armin Knoll*,† †

:: :: IBM Zurich Research Laboratory, Saumerstrass 4, 8803 Ruschlikon, Switzerland and ‡STMicroelectronics, P. le Enrico Fermi 1, Porto del Granatello, 80055 Portici, Italy Received December 19, 2008. Revised Manuscript Received February 9, 2009

The roughness of spin-cast polymer films arises from thermally activated capillary waves during preparation and typically amounts to about 0.5 nmrms measured on a micrometer-sized surface area. Templating from atomically flat mica substrates allows the creation of polymer films with a surface roughness approaching the molecular scale. Three regimes of spatial frequencies are identified in which the roughness is controlled by different physical mechanisms. We find that frozen-in elastic pressure waves ultimately limit the flatness of polymer films.

Introduction The roughness of amorphous polymer films is governed by thermally activated capillary waves, which are excited on the surface of a film in the liquid or molten state1,2 and frozen in during the transition to the glassy state.3,4 Because typical amorphous polymers have similar surface tension values (i.e., in the range of 20 to 40 mJ/m2)5 the rms roughness of a spin-cast film is a rather universal number of approximately 0.3-0.6 nmrms 4 measured on micrometer-sized areas. In silicon manufacturing, the roughness of spin-cast resist films may become an important issue as dimensions decrease toward the nanometer range. For other applications, the roughness value is already critical: its influence on contact mechanics can be substantial;4 the quality of molecular selfassembled monolayers improves with decreasing surface roughness,6 and in thermomechanical-probe data storage,7 the depth of the indents encoding the information is ultimately limited by the polymer roughness as densities extend to the multi-Tbit/in2 range.8 In this article, we describe an efficient way to reduce the polymer roughness by templating smoother surfaces and thereby suppressing the formation of capillary waves. In our study, we use atomically flat muscovite mica as a templating substrate. Mica has been employed as a template to create ultrasmooth metal layers9 and to assemble stacks of thin * Corresponding author. E-mail: [email protected]. (1) Buff, F. P.; Lovett, R. A.; Stillinger, F. H. Phys. Rev. Lett. 1965, 15, 621. (2) Meunier, J. J. Phys. (Paris) 1987, 48, 1819. :: (3) Jackle, J.; Kawaski, K. J. Phys.: Condens. Matter 1995, 7, 4351. (4) Persson, B. N. J. Wear 2008, 264, 746. (5) Wu, S. In Polymer Handbook, 4th ed.; Brandrup, J., Immergut, E. H., Grulke, E. A., Eds; Wiley: New York, 1999; pp VI/521-VI/523. (6) More, S. D.; Graaf, H.; Baune, M.; Wang, C.; Urisu, T. Jpn. J. Appl. Phys. 2002, 41, 4390. :: (7) Pantazi, A.; Sebastian, A.; Antonakopoulos, T. A.; Bachtold, P.; Bonaccio, A. R.; Bonan, J.; Cherubini, G.; Despont, M.; DiPietro, R. A.; Drechsler, U.; Duerig, U.; Gotsmann, B.; Haeberle, W.; Hagleitner, C.; Hedrick, J. L.; Jubin, D.; Knoll, A.; Lantz, M. A.; Pentarakis, J.; Pozidis, H.; Pratt, R. C.; Rothuizen, H.; Stutz, R.; Varsamou, M.; Wiesmann, D.; Eleftheriou, E. IBM J. Res. Develop 2008, 52, 493. :: (8) Wiesmann, D.; Durig, U.; Gotsmann, B.; Knoll, A.; Pozidis, H.; Porro, F.; Vecchione, R. Proceedings of Innovative Mass Storage Technologies 2007 “IMST 2007”; Enschede: The Netherlands, 2007; http://imst2007.ewi.utwente.nl/imst_program.pdf, p 19. :: (9) Diebel, J.; Lowe, H.; Samorı, P.; Rabe, J. P. Appl. Phys. A 2001, 73, 273.

Langmuir 2009, 25(9), 5141–5145

polymer films for polymer interdiffusion measurements.10 However, the surface roughness quality of polymeric films obtained by a replica technique has not yet been studied. We find that the roughness can be reduced by approximately one order of magnitude in the spatial frequency range, >1/150 nm-1, which is relevant for probe-storage applications.

Experimental Section Polymer. The polymer used is a polyaryletherketone (PAEK), shown in Figure 1, incorporating diresorcinol units in the backbone for control of the glass-transition temperature (Tg) and phenyl ethynyl groups in the backbone and as endgroups for cross-linking functionality. These polymers exhibit excellent thermal stability up to 450 °C. The polymer used here was specifically tailored for the probe-storage application investigated and has 18% phenyl ethynyl groups in the backbone to enhance the overall cross-linking. The molecular weight is approximately 5.5 kDa, and a Tg of 150 °C is measured by dynamic scanning calorimetry. Unlike homopolymer films, such chemically cross-linked polymers are stable against repetitive imaging in contact-mode scanning probe microscopy as described below.11 Thin films are prepared by spin-casting from anisole solution. To induce the cross-linking reaction, the films are cured at 400 °C for 1 h in a nitrogen atmosphere. Homogeneous films are obtained, with average thicknesses of 160 and 430 nm as measured by ellipsometry, by using solutions of 5 and 15 wt %, respectively. Templating. A freshly cleaved mica sheet (muscovite mica grade V1 obtained from SPI Supplies) is used as a template.12 The square mica samples, 2.5  2.5 cm2 in size, are mechanically cleaved. Typically, fewer than three large steps are observed by optical inspection throughout the sample area, resulting in atomically flat surfaces in between the steps, which are in the square centimeter range. The templating of the polymer surface follows the procedure illustrated in Figure 2. In the first step, a (10) Scheffold, F.; Eiser, E.; Budkowski, A.; Steiner, U.; Klein, J. J. Chem. Phys. 1996, 104, 8786. (11) Gotsmann, B.; Duerig, U. T.; Sills, S.; Frommer, J.; Hawker, C. J. Nano Lett. 2006, 6, 296. (12) By analogy, the method can be applied to other template substrates. Successful transfer has been achieved from silicon oxide surfaces whereby the silicon oxide serves as a sacrifical layer. In that case, the roughness of the templated film is a mere replica of the silicon oxide roughness, and no intrinsic roughening effects can be observed.

Published on Web 3/10/2009

DOI: 10.1021/la804191m 5141

Article

Pires et al.

Figure 1. Chemical structure of the polyaryletherketone (PAEK) used in the experiments. The functional cross-link unit fraction (1 - x) is 0.18.

Figure 2. Process flow used in the templating of a polymer. (a) The polymer is depositied onto the mica template via spin casting. (b) The target is put into contact with the free surface of the polymer. (c) The polymer is released from the substrate by immersion in H2O and thus transferred onto the target. (d) The templated surface of the polymer medium is exposed. thin film of polymer is spin cast and cured on a freshly cleaved mica substrate, which serves as the template (Figure 2a). In the second step, a silicon wafer, referred to as target, is put into contact with the free surface of the polymer (Figure 2b). The sandwich is clamped using tweezers and applying moderate pressure and is submerged in ultrapure water (Figure 2c). Because mica is very hydrophilic, water penetrates between the mica and the polymer surfaces, thereby releasing the polymer film. The transfer to the target substrate is assisted if hydrophobic hydrogen-terminated silicon wafers or polymercoated wafers are used as targets. In this case, the water does not penetrate between the polymer and target surface, and the transfer is completed over the full area in less than 1 min. After the mica substrate is lifted off, the templated polymer surface is exposed (Figure 2d). Atomic Force Microscopy. Atomic force microscope (AFM) images are obtained using home-built equipment. All three axes of the piezo scanner (Physical Instruments P-733-2DD and P-753) are linearized to ensure reliable metrology. We use cantilever-style force sensors with a lowest resonance frequency of >60 kHz, a stiffness of 0.15 ( 0.05 N/m, tip radii of 5-10 nm, and integrated thermal sensors13 for topographic sensing. The instrument is operated in contact mode using contact forces on the order of 10 nN. For reliable roughness imaging, no feedback loop is used to control the height of the cantilever base over the surface. The soft cantilever spring compensates for the corrugation of the substrate, and the vertical displacement of the tip is directly measured via the integrated thermal sensor. The peakto-peak corrugation of the films investigated here is below 5 nm, and the tilt of the surface plane can be corrected to contribute less than 10 nm of modulation of the out-of-plane component. Therefore, the load force on the tip varies by less than 3 nN, and the nominal load force, 10 nN, ensures stable tip-sample contact. Because of the direct imaging method, the setup is able to capture high roughness frequencies reliably. The resolution of (13) Vettiger, P.; Cross, G.; Despont, M.; Drechsler, U.; Duerig, U.; Gotsmann, B.; Haeberle, W.; Lantz, M. A.; Rothuizen, H. E.; Stutz, R.; Binnig, G. K. IEEE Trans. Nanotechnol. 2002, 1, 39.

5142

DOI: 10.1021/la804191m

Figure 3. (a) Topographical image of a spin-cast polymer of size 1.6  1.6 μm2. The gray scale is 3 nm. (b) 2D-PSD image of panel a using a logarithmic color scale from 10-3 to 103 nm4. (c) Azimuthally averaged PSD spectra of two polymeric surfaces (solid dots) and the respective electronic noise spectra (solid lines) of the thermal sensor. The scan sizes are 19.6 (red dots, orange line) and 1.6 μm (blue dots, light blue line). The spectra were averaged azimuthally in an angular range of -45 to 45°, as indicated by the dashed lines in panel b. the topographic image is limited by the fundamental mechanical response time of the lever of ∼17 μs, the thermal time constant of the sensor of ∼20 μs,14 and the sharpness of the tip. We adjusted the imaging speed to 60 μs/pixel to ensure that the imaging resolution is limited solely by the tip sharpness. Power Spectral Density. The power spectral density is defined as the Fourier transform of the autocorrelation function of the real-space topography and is a measure of the mean square amplitude of the sinusoidal roughness components for a given wave vector q. The roughness power spectral density15 is obtained from real-space images consisting of 1800 pixels  1800 pixels. An example of a topographic image of size 1.6  1.6 μm2 is shown in Figure 3a. To avoid artifacts and edge effects from the nonperiodic nature of the images, we use a cosine apodization function.16 The resulting 2D power spectral density (2D-PSD) image is shown in Figure 3b. Because of the line-byline AFM imaging, low-frequency electronic and mechanical noise accumulates along the y axis of the 2D-PSD, appearing as a yellow line emanating from the center along the y axis in Figure 3b. To eliminate these noise artifacts, the 2D spectrum is azimuthally averaged in an angular range of -45 to 45° with respect to the x axis as indicated by the dashed lines in Figure 3b. The resulting 1D spectrum, henceforth termed PSD, is displayed in Figure 3c as solid blue circles, along with a spectrum of the electronic noise of the lever shown as the light-blue line. The electronic noise is measured by recording the lever signal far from the surface. Note that the electronic noise density in q space is proportional to the real-space sampling interval. As a consequence, the noise is reduced by 2 orders of magnitude if the scan size is reduced by a factor of 10 using the same number of (14) Gotsmann, B.; Lantz, M. A.; Knoll, A.; Duerig, U. In Nanotechnology; Fuchs, H., Ed.; Wiley-VCH: Weinhein, Germany; Vol. 2, in press. (15) Persson, B. N. J.; Albohr, O.; Tartaglino, U.; Volokitin, A. I.; Tosatti, E. J. Phys.: Condens. Matter 2005, 17, R1. (16) Weisstein, E. W. Concise Encyclopedia of Mathematics; CRC Press: Boca Raton, FL, 2003, p 95ff.

Langmuir 2009, 25(9), 5141–5145

Pires et al.

Article

pixels. To demonstrate the effect, a second PSD obtained from a topographic image of size 19.6  19.6 μm2 (red solid circles) and the corresponding noise signal (orange line) are shown in Figure 3c. In the following, to avoid electronic noise artifacts, the 1D spectra shown are cropped at a level where the PSD signal drops below 3 times the value of the electronic noise.

Results We use the free surface of a 160-nm-thick spin-cast, cured sample on a silicon wafer as a reference for the roughness of a spin-cast film, henceforth referred to as a reference surface. We first compare it with a 160-nm-thick film templated from mica and transferred onto a silicon wafer. Both surfaces are imaged using scan sizes ranging from 20  20 μm2 to 160  160 nm2 as shown in Figure 4a. The data for the reference and the mica-templated surface are shown in the top and bottom rows, respectively. The PSD spectra for each surface, displayed in Figure 4b, are obtained by stitching the PSDs of the respective images. Blue and red symbols denote the data for the reference and the templated surface, respectively. The perfect superposition of all data subsets for a given sample confirms the ergodic character of these surfaces. For q < 2  10-1 nm-1, the PSD amplitudes of the reference surface decay proportionally to the inverse square of the wavenumber, as shown by the power law fit (solid blue line) in Figure 4b. Owing to the isotropic nature and the q-2 dependence, amorphous polymeric surfaces belong to the class of self-affine fractal surfaces having a Hurst exponent of H = 0. This corresponds to a fractal dimension of Ds = 3 - H = 3.4 In other words, laterally magnified images (i.e., without rescaling the z axis) with a small scan size cannot be distinguished from images obtained from large scan sizes. This can be seen in the first two images of the reference surface (marked with an asterisk). Although the size of the second image is more than 10 times smaller, it is remarkably similar to the large-scale image. Interestingly, the largest scan image of the templated surface is indistinguishable from the reference surface. Accordingly, at large wavelengths (i.e., for small wavenumbers q < 10-2 nm-1), the spectrum is identical to that of the reference surface. We will return to this point later in the article. For higher q > 10-2 nm-1, the roughness of the templated film is significantly reduced (Figure 4a,b). For 10-2 < q < 4  10-2 nm-1, the PSD drops exponentially as indicated by the solid red line in Figure 4b. In an intermediate region of 4  10-2 < q < 3  10-1 nm-1, q-1 scaling is observed, as shown by the dashed red line. Finally, at wavenumbers higher than 3  10-1 nm-1, a second cutoff is observed, which is common to all spectra. The cutoff arises from the low pass filtering due to the finite sharpness of the tip, which limits the imaging resolution. A direct measure of the roughness improvement is provided by the rms roughness calculated in the wavenumber interval of 4  10-2 to 3  10-1 nm-1, where the roughness is significantly suppressed. For the reference and the templated surface, values of 0.26 and 0.036 nmrms, respectively, are obtained (i.e., the roughness is reduced by almost an order of magnitude). As mentioned above, a motivation for this work is to improve the data density in thermomechanical probe data storage. In this application, data is stored in form of thermomechanically written indents into the polymer film using heated tips.7,13,17 The presence of an indent represents a logical 1, and the absence of an indent represents a logical 0. :: (17) Pozidis, H.; Haberle, W.; Wiesmann, D. W.; Drechsler, U.; Despont, M.; Albrecht, T.; Eleftheriou, E. IEEE Trans. Magn. 2004, 40, 2531.

Langmuir 2009, 25(9), 5141–5145

Figure 4. (a) Topographic images of the reference surface (top row) and a templated mica surface (bottom row, film thickness 160 nm) at different length scales. Note the self-similarity between the images marked with an asterisk. (b) PSD spectra of the reference sample (blue symbols) and two templated mica surfaces with film thicknesses of t = 160 (red symbols) and 430 nm (green symbols). The scan size is marked by the symbol type: circles = 19.6 μm, squares = 1.6 μm, down triangles = 0.48 μm, and up triangles = 0.16 μm. The solid lines and the dashed line are fits to the respective models, as mentioned in the text. Typical images of a data bit field written at a storage density of 1 Tbit/in2 on the as-cast surface and of 3 Tbit/in2 on a micatemplated surface are shown in Figure 5a,b, respectively. Both images are shown at the same scale using the same dynamic range on the gray scale of 3 nm. The data pattern consists of a preamble of 11 consecutive 1’s followed by random data. The data is (d, k) encoded with d = 1 and k = 7, which means that two 1’s are separated by at least one and not more than seven 0’s.17 As a consequence, the symbol pitch is half of the minimum distance between indents (Figure 5a). The pitch between tracks (i.e., lines of data) corresponds to the minimal indent pitch. The corresponding PSD spectra of both topographies are shown in Figure 5c together with the PSD spectra of the virgin surfaces. The sharp peaks at 0.42 and 0.74 nm-1 observed in the PSD spectra and marked in Figure 5c by the vertical lines correspond to the carrier frequency at the symbol pitch for 1 and 3 Tbit/in2, respectively. The information itself is encoded in the modulation side bands. The lower side band is clearly visible in the PSD spectra as broad peaks centered at approximately half the carrier frequency, whereas the upper side band is suppressed by the finite imaging resolution. The average bit depth in the fields is 7 nm at 1 Tbit/in2 and 2 nm at 3 Tbit/in2. Although the bit depth is decreased by more than a factor of 3, the measured signal-to-noise ratio in both images is similar, as indicated by the arrows in Figure 5c, enabling error-free read back of the data.

Discussion It is well known4 that thermally excited capillary waves dominate the surface roughness of spin-cast and annealed polymer films. The surface can be modeled as a membrane DOI: 10.1021/la804191m 5143

Article

Pires et al.

Figure 5. Topographic images of data patterns written at data densities of (a) 1 Tbit/in2 on the reference surface and (b) 3 Tbit/in2 on a mica-templated surface. (c) PSD spectra of the data storage fields for 1 Tbit/in2 (black squares) and 3 Tbit/in2 (green stars) added to the roughness PSDs. The vertical lines and the arrows mark the wavenumber corresponding to the symbol pitch and the estimated signal-to-noise ratio, respectively. having a surface tension γ. A quasi-continuous spectrum of thermally excited modes exists at the interface. In general, the energy E of a surface mode is proportional to the square of its amplitude Aq: E ¼ jAq j2 f ðqÞ

ð1Þ

where f (q) is a characteristic function for the physical origin of the excitation. In the following text, we consider capillary waves f (q) = fc(q) and elastic waves f (q) = fe(q). In the case of thermally excited and overdamped capillary waves, the function fc(q) is proportional to the square of the wave vector q and is given by2,4,16 1 ð2Þ fc ðqÞ ¼ γq2 2 Equating the mode energy given by eqs 11 and 22 to the thermal energy 1/2kBT per mode yields the average thermally excited power P(q) = |Aq|2 of the mode as kB T Pc ðqÞ ¼ ð3Þ γq2 The lower wavenumber limit of this equation is given 19 by qmin = (A/2πγt4)1/2 ≈ 10-5 nm-1, where A (∼10-20 J) is the Hamaker constant of the system and t (>160 nm) denotes the film thickness. The upper limit is given by the molecular dimensions,19 approximately qmax = 10 nm-1. Both values are outside the range of wave numbers studied, hence deviations from eq 3 are not expected to be observed for the reference surface. The surface tension γ also depends on q,2 but the effect is small for our system and the range of q vectors investigated and thus can be ignored. (18) (a) Harden, J. L.; Pleiner, H.; Pincus, P. A. J. Chem. Phys. 1991, 94, 5208. (b) Cao, B. H.; Kim, M. W.; Cummins, H. Z. J. Chem. Phys. 1995, 102, 9375. (19) Doerr, A. K.; Tolan, M.; Prange, W.; Schlomka, J.-P.; Seydel, T.; Press, W.; Smilgies, D.; Struth, B. Phys. Rev. Lett. 1999, 83, 3470.

5144

DOI: 10.1021/la804191m

Indeed, the roughness PSD of the reference polymer film follows the q-2 law up to a wave vector of 2  10-1 nm-1, as expected for capillary waves. The surface waves are excited during the curing of the polymer at 400 °C and frozen in upon cooling through the glass transition (i.e., close to the glass-transition temperature (Tg)). In other words, we observe a topography that represents a snapshot of the polymer surface at a temperature close to Tg. Using eqs 1 and 2 and Tg = 150 °C, we obtain a surface tension of 30 ( 10 mN/m, which is in agreement with literature values for similar polymeric systems.20 At frequencies above 2  10-1 nm-1, a significant drop in the power amplitude is observed for all data sets, which can be attributed to a tip-convolution effect. The position of the drop depends on the sharpness and the load force of the tip (data not shown). Because of the high compliance of the sample, the tip penetrates the surface on the order of 1 nm, thereby effectively averaging over the contact area and limiting the imaging resolution. A typical apex radius of the tips used in this study is 10 nm, thus we estimate a contact area with a diameter of ∼10 nm, in good agreement with the observed drop for q > 0.3 nm-1. Let us now consider the origin of the exponential drop in amplitude for q > 10-2 nm-1 in the PSD of the templated surface. To clarify its origin, we prepared a second micatemplated sample with a significantly higher film thickness of 430 nm. The PSD of a 20  20 μm2 scan on this sample is shown as green data points (circles) in Figure 4b and exhibits the same exponential drop, albeit at significantly smaller wave numbers. This dependence on the film thickness can be explained by an evanescent wave model: the surface in touch with the target substrate was exposed to free space during preparation and prior to contact with the target and therefore exhibits the full roughness spectrum of capillary waves (Figure 6a). The target substrate is a silicon wafer with roughness smaller than that of the free polymer surface. Once the film is in close proximity to the target, adhesive forces will pull the film into full contact and thereby induce (elastic) deformations of the polymer matrix (Figure 6b). These deformations are propagated into the polymer film as evanescent elastic waves with a decay length of 1/q; therefore, only long-wavelength modulations appear on the templated surface. As is characteristic of evanescent waves, the cutoff frequency is proportional to the product of the wavenumber q and the film thickness t. In fact, the red and green solid lines in Figure 4b are obtained by assuming the PSD to be directly proportional to exp(-qt) using the corresponding film thicknesses of 160 and 430 nm, as measured by ellipsometry. The drop in amplitude is followed by a straight line in the PSD plot having a slope of -1 for 4  10-2 nm-1 < q < 3  10-1 nm-1. This dispersion is characteristic of bulk elastic waves.18 We propose the following scenario: after the curing process at 400 °C, during the cool down of the melt, elastic waves are thermally excited when the rubbery plateau of the polymer shifts to longer time scales and the elastic modulus is building up. Upon further cooling, the polymer relaxation time scale diverges, and the waves are frozen in during the transition to the glassy regime at Tg ≈ 150 °C (Figure 5a). After the release of the polymer, the surface of the glassy polymer is free to relax elastically on a local scale in response to the stress distribution associated with the elastic waves. The :: (20) (a) Janocha, B. Ph.D. Thesis, University of Tubingen, Germany, 1998, summarized in: (b) http://www.igb.fraunhofer.de/www/gf/grenzflmem/gf-physik/en/GFphys-PolymOberfl.en.html.

Langmuir 2009, 25(9), 5141–5145

Pires et al.

Article

Conclusions

Figure 6. (a) During the curing of the polymer in contact with the template, bulk pressure waves are excited and frozen in upon cooling the sample through the glass transition. (b) Long-wavelength deformation of the surface due to evanescent elastic waves induced by the conformal contact of the rippled polymer surface and the target substrate. (c) Once the polymer has been detached from the template, the pressure waves relax and are manifested as long-wavelength corrugations of the surface. restoring force is due to the (shear) deformation of the material and is therefore proportional to q ð4Þ fe ðqÞ ¼ Gq with G being the shear modulus of the material. Accordingly, the power spectral density is given by 1 kB T Pe ðqÞ ¼ ð5Þ 2 Gq This results in a static corrugation of the surface (Figure 6c). Using eq 55, we find that the dashed red line in Figure 4b corresponds to a modulus of about 0.3 GPa, which is the right order of magnitude for highly cross-linked polymers close to Tg and corroborates our elastic wave picture.

Langmuir 2009, 25(9), 5141–5145

We studied the surface roughness properties of polymeric films using cleaved mica substrates as templates. Because of the atomically flat templating surface, short-wavelength capillary wave excitations are effectively suppressed, resulting in ultrasmooth surfaces with a residual roughness on the order of 0.04 nmrms. The roughness is controlled by elastic pressure waves with a characteristic q-1 dependence in the power spectrum. These pressure waves cannot be avoided on the templated surface and therefore represent a lower bound for the surface roughness that can be achieved. The long-wavelength roughness, however, displays characteristics of capillary wave excitations. We show that these longwavelength corrugations are caused by the elastic response of the templated surface to corrugations on the back side of the polymer film. This corrugation transfer proceeds via evanescent waves and is therefore manifest only for long wavelengths with a cutoff given by the inverse film thickness. The long-wavelength roughness is not critical, however, as long as the size of the features of interest is smaller than the cutoff wavelength. Ultraflat amorphous polymer surfaces have been used to achieve extremely high storage densities of up to 4 Tbit/in2 without losing the ability to read back the information.8 In fact, the surface roughness reaches a level at which it would be possible to study individual molecules, such as DNA, or the assembly of small molecules on a polymeric substrate without significant interference of the substrate. Acknowledgment. We gratefully acknowledge the invaluable support of the probe storage team at the IBM Zurich :: Research Laboratory in Ruschlikon, in particular, Peter :: Bachtold for the design of the electronics hardware; Michel Despont, Richard Stutz, and Ute Drechsler for the fabrication of the thermomechanical probe sensors; Marilyne Sousa, Meinrad Tschudy, and Walter Haeberle for technical support; and Evangelos Eleftheriou for stimulating discussions. We thank Russell Pratt, James Hedrick, Jane Frommer, Charles Wade, and Robert Miller from the IBM Almaden Research Center for the synthesis and characterization of the polymer materials. This work is supported by the European Research Council under the project ProTeM.

DOI: 10.1021/la804191m 5145