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Probing Surface Microthermal Properties by Scanning Thermal Microscopy V. V. Gorbunov,†,‡ N. Fuchigami,‡ J. L. Hazel,§ and V. V. Tsukruk*,‡ Metal-Polymer Institute, National Academy of Science, Gomel, 246550, Belarus, Department of Materials Science & Engineering, Iowa State University, Ames, Iowa 50011, and Air Force Research Laboratory/MLBP, Wright-Patterson Air Force Base, Ohio 45433 Received June 9, 1999. In Final Form: August 20, 1999 Scanning thermal microscopy (SThM) was used for probing surface microthermal properties of a wide range of materials from polymers to metals. We demonstrated that SThM measurements in contact mode can provide unique capabilities of unambiguous measurements of localized thermal conductivity of a wide variety of surfaces with sensitivity better than 0.05 W m-1 K-1 and lateral resolution in the range from 0.03 µm for hard materials to 1 µm for compliant materials. Variation of surface microthermal conductivity correlates fairly well with known bulk values for hard materials. For compliant materials, significant contribution of local deformation to measured values of thermal response is noted.
Introduction Local probing of surface thermal properties with a submicrometer resolution becomes a reality after the recent introduction of scanning thermal microscopy (SThM) as a new family of scanning probe microscopy (SPM) techniques. 1-3 Two major designs explore either microthermocouple or microthermoresistor as a miniature probe, which scans a surface in a usual SPM mode.2-5 The SThM ability to probe surface thermal conductivity and diffusivity with a submicrometer resolution has been demonstrated for polymer composites as well as for semiconductor and metal surfaces.2-6 Results on buried metal particles in polymer composites,7,8 photothermal FTIR analysis,9 distributed thermal properties of metal alloys,10 thermal conductivity of diamond-like thin films,11 and localized phase transformation on polymer surfaces,12 have been recently published. This approach provides supplementary capabilities and new advantages in comparison with thermal stages and micromechanical analysis at different frequencies.13-15 However, the quantitative characterization of the surface thermal properties is still a challenge for SThM. 8,10 To date, only limited discussions * To whom correspondence should be addressed: phone, 515294-6904; fax, 515-294-5444; e-mail,
[email protected]. † Metal-Polymer Institute. ‡ Iowa State University. § Air Force Research Laboratory/MLBP. (1) Williams, C. C.; Wickramasinghe, H. K. In Photoacoustic and Photothermal Phenomena; Springer-Verlag: Berlin, 1988; p 364. (2) Majumdar, A.; Carrejo, J. P.; Lai, J. Appl. Phys. Lett. 1993, 62, 2501. (3) Oesterschulze, E.; Stopka, M.; Kassing, R. Microelectron. Eng. 1994, 24, 107. (4) Trannoy, N.; Grossel, P.; Troyon, M. Probe Microsc. 1998, 1, 201. (5) Hammiche, A.; Reading, M.; Pollock H. M.; Song, M.; Hourston, D. J. Rev. Sci. Instrum. 1996, 67, 4268. (6) Hammiche, A.; Song, M.; Pollock H. M.; Reading, M.; Hourston, D. J. Polym. Prepr. 1996, 37 (2), 585. (7) Hammiche, A.; Pollock, H. M.; Song, M.; Hourston, D. J. Meas. Sci. Techn. 1996, 7, 142. (8) Hammiche, A.; Hourston, D. J.; Pollock H. M.; Reading, M.; Song, M. J. Vac. Sci. Technol. 1996, 14 (2), 1486. (9) Hammiche, A.; Pollock, H. M.; Reading, M.; Clayborn, M.; Turner, P. H.; Jewkes, K. Appl. Spectrosc., in press. (10) Depasse, F.; Gomes, S.; Trannoy, N.; Grossel, P. J. Phys. D: Appl. Phys. 1997, 30, 3279. Fiege, G. B.; Altes, A.; Heiderhoff, R.; Balk, L. J. J. Phys. D: Appl. Phys. 1999, 32, L13. (11) Ruiz, F.; Sun, W. D.; Pollak, F. H. Appl. Phys. Lett. 1998, 73, 1802. (12) Pollock H. M.; Hammiche, A.; Song, M.; Hourston, D. J.; Reading, M. J. Adhes. 1998, 67, 217.
of resolution, sensitivity, and limits of thermal contrast in SThM images have been attempted. In the present communication, we report the initial results from our studies on quantitative microprobing surface thermal properties. The thermal conductivity of materials studied here varied by 4 orders of magnitude from as low as 0.14 W m-1 K-1 for polystyrene to as high as 317 W m-1 K-1 for gold. Experimental Section The samples for investigation were selected to represent a variety of materials with a wide range of thermal properties (Table 1).16-18 Polymer samples were purchased/received from Bayer, Fluka, Janssen Chimica, and Aldrich. Smooth polymer films of several hundred micrometer thicknesses were prepared on silicon wafers by a solution-casting technique. The glass substrate was a Fisher float slide. The silicon substrate was a highly polished (100) wafer with silicon oxide thickness of 1 nm (PureSilicon). A silicon nitride substrate was taken from a SPM cantilever wafer (Digital Instruments). A planar graphite substrate was obtained from Digital Instruments. Silicon oxidesilicon grids were standard calibration specimens from Thermomicroscopes. Thin films (250 µm) of Pt and Au (both 99.95%) were purchased from Goldsmith. The surfaces of all samples studied were characterized by SPM prior to thermal measurements and were carefully cleaned, dried, and observed to be smooth and homogeneous. The average microroughness within 1 µm × 1 µm area did not exceed 1 nm for all samples. All SThM measurements were done on an Explorer scanning thermal microscope (SThM) (Thermomicroscopes). We used standard thermal probes with Pt-Rh sensors (Thermomicroscopes). The approaching-retracting mode was exploited to collect concurrent force-distance and bridge voltage-distance data beginning a micrometer distance above the sample surface. The probe temperature was kept constant by the electronic (13) Ratner, B., Tsukruk, V. V., Eds. Scanning Probe Microscopy of Polymers; ACS Symposium Series; American Chemical Society: Washington, DC, 1998; Vol. 694. (14) Tsukruk, V. V., Wahl, K., Eds. Microstructure and Microtribology of Polymer Surfaces; ACS Symposium Series; American Chemical Society: Washington, DC, 1999; Vol. 741. (15) Tsukruk, V. V.; Huang, Z.; Chizhik, S. A.; Gorbunov, V. V. J. Mater. Sci. 1998, 33, 4905. Chizhik, S. A.; Huang, Z.; Gorbunov, V. V.; Myshkin, N. K.; Tsukruk, V. V. Langmuir 1998, 14, 2606. Tsukruk, V. V.; Huang, Z. Polymer, submitted. (16) MatWeb, http://www.matls.com/searchindex.html. (17) MEMS Materials Database, http://mems.isi.edu/mems/materials/index.html. (18) CenBASF/Materials, http://www.centor.com/cbmat/visitors/ splast1.html.
10.1021/la990913a CCC: $18.00 © 1999 American Chemical Society Published on Web 09/11/1999
Letters
Langmuir, Vol. 15, No. 24, 1999 8341
Table 1. Thermal Properties of Materials (Data for Thermal Conductivies Are Taken from Refs 7 and 16-18) materials
λ, W m-1 K-1
∆Q/∆T, 10-5, W/K
polystyrene (PS) polyurethane (PU) polypropylene (PP) poly(methyl methacrylate) (PMMA) poly(vinyl chloride) (PVC) polyethylene (PEHD) glass silicon nitride graphite platinum silicon gold air
0.142 0.147 0.18 0.19 0.21 0.37 1.6 19.0 24.0 71.0 156.0 317.0 0.024
1.0 1.04 0.91 0.83 0.96 1.30 1.26 1.26 1.56 1.87 2.0 2.30 0a
a
Taken as a reference level.
feedback during this procedure. We collected sets of thermal data (voltage within the bridge) at three different locations for each sample with four different probe temperatures, namely, ∼40, ∼55, ∼70, and ∼80 °C. For several samples, thermal measurements were repeated with two different tips and gave consistent results ( 1.0 W m-1 K-1.
ments achievable for current setup is better than 0.05 W m-1 K-1. These results are very similar to data reported for a number of inorganic materials in ref 11. However, data for all polymeric materials studied (except polyethylene with the highest thermal conductivity among polymers) are very close to each other and cannot be described by a simple relationship (1). It seems that this relationship cannot be held over the entire range of materials with very different surface properties. A reason for the deviation observed is probable change in effective thermal contact area due to much more compliant nature of polymeric materials. Indeed, further analysis supports this suggestion. The effective radius of the contact area can be estimated using our thermal data from eq 1 as Rc ) π-1∂(∆Q/∆T)/∂λ. Under our experimental conditions, thermal contact diameter is determined to about 30 nm for hard materials. This value correlates with our estimation made from thermal images for silicon oxide-silicon grids (
