Integrated Silicon Heater for Tip-Based. Nanomanufacturing

Transcription

1 Supporting Information Wear Resistant Diamond Nanoprobe Tips with Integrated Silicon Heater for Tip-Based Nanomanufacturing Patrick C. Fletcher 1, Jonathan R. Felts 1, Zhenting Dai 1, Tevis T.B. Jacobs 2, Hongjun Zeng 3, Woo Lee 4, Paul E. Sheehan 4, John A. Carlisle 3, Robert W. Carpick 2 and William P. King 1 * 1 Department of Mechanical Science and Engineering, University of Illinois at Urbana- Champaign, Urbana, IL 2 Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, PA 3 Advanced Diamond Technologies Inc., Romeoville, IL 4 Chemistry Division, Naval Research Laboratory, Washington DC *wpk@illinois.edu Telephone: ; Fax: Department of Mechanical Science and Engineering, University of Illinois Urbana-Champaign, Urbana, IL

2 Electrical/Thermal Characteristics of Heated Cantilevers. The electrical and thermal properties of the silicon and ultrananocrystalline diamond-coated (UNCD) cantilevers were characterized using Raman spectroscopy. We first characterized device resistance and dissipated power as a function of applied voltage, then characterized the cantilever heater region temperature as a function of applied voltage by monitoring the Stokes peak shift. Figure S1 shows typical cantilever electrical and thermal properties. The cantilever resistance and dissipated power are both non-linear functions of temperature. Figure S1. Temperature and dissipated power as a function of cantilever resistance for a UNCDcoated heated cantilever tip. 2

3 Oxidation of Ultrananocrystalline Diamond. UNCD oxidizes above 600 C in aerobic environments; consequently, we investigated the effect of heating our diamond-coated atomic force microscope (AFM) tips to temperatures exceeding 600 C. For this experiment, the tip temperature was increased incrementally while visually monitoring the color of the UNCD coating, which normally appears reddish in an optical microscope. The tip temperature was monitored concurrently using a Raman spectroscopy measurement system. Figure S2 shows the electrical calibration curve for the cantilever heater. Around room temperature, the cantilever resistance increases with temperature because carrier mobility in doped silicon decreases with temperature. Simultaneously, the intrinsic carriers in the silicon increase with increasing temperature. Over 460 C, the thermally generated intrinsic carriers become the overriding factor affecting cantilever resistance and the resistance decreases. This is also known as thermal runaway. Figure S2. Heater calibration curve for oxidation tests. 3

4 The heater was held at each elevated temperature for a period of 5 minutes. The tip temperature was held at 540 C, 573 C, 578 C, 615 C, 648 C, 674 C, 686 C, 715 C, 735 C, and 760 C. The UNCD coating began to visually fade above 600 C and the coating was completely removed at 760 C, except for some small remaining grains on the tip. Figure S3 shows before and after scanning electron microscope (SEM) micrographs of the oxidation test. The SEM micrographs indicate that the UNCD on the heater region was almost completely removed during oxidation. The surface appears rough which may be caused by UNCD grains or surface damage from ultrasonic seeding. Some diamond is clearly left on the conical probe tip, most likely because the conical tip does not get as hot as the flat heater region around the tip. Figure S3. SEM images shows removal of UNCD from heated cantilever tip through oxidation and self-heating to high temperatures. Top left and right shows before UNCD oxidation and bottom left and right show after UNCD oxidation above 600 C. 4

5 TEM Imaging. The diamond-coated tips were extensively characterized using transmission electron microscopy (TEM). TEM imaging confirmed that the conformal diamond growth process minimized granular voids on the three-dimensional conical tips. Selected area diffraction confirmed the presence of crystalline diamond grains, rather than graphite. Figure 4 shows before and after TEM images of a UNCD-coated tip after wear testing on quartz, and the selected area diffraction patterns of the worn tip area are shown in Figure S4 below. The TEM images show that wear occurs by gradual atom-by-atom attrition of the UNCD surface rather than by delamination or fracture of the diamond coating, and the UNCD does not undergo observable graphitization during wear. We calculated the volume of UNCD removed using a MATLAB script to trace the tip profiles and take the difference of the volume integrals of the tip before and after wear. This method assumes the tip is made of discs defined by the tip profile. It is sensitive to tip orientation, profile tracing, and alignment of profiles so multiple measurements were made for each TEM image. After analysis, it was determined that there is significantly more wear present in probes run at higher temperatures. The diamond-coated probe shown in Figure 4 had more than 170,000 nm 3 of material removed during the wear test, while a probe run under identical conditions without tip heating had no measurable loss of volume. While the overall UNCD tip structure appears broad, protruding supergrains make the effective tip radius smaller. Using adhesive contact mechanics and work of adhesion values determined from a previous study, 1 we estimate that the contact diameter between substrate protrusions and an 8 nm UNCD asperity could be as small as 3.6 nm at 200 nn applied load, and only 0.5 nm at 10 nn applied load, defining an impressively small resolution for imaging and nanomanufacturing. 5

6 Figure S4. Selected area diffraction (SAD) patterns from the tip region of the UNCD-coated AFM tip shown in Figure 4 of the main text (insets), which was rastered on a quartz substrate at 400 C and with 200 nn of force for 1.28 meters. The SAD patterns show the expected diamond cubic diffraction rings for randomly oriented diamond grains. The SAD patterns at the wear site show no change in the diamond lattice parameters, indicating that there is no graphitization during tip wear. 6

7 Ex Situ Tip Wear Measurement. Wear testing of silicon and diamond-coated probes requires assessment of the probe tip sharpness to gage tip wear. Tip radii before and after wear testing were measured ex situ using a high-resolution SEM. Tables S1-S4 show SEM micrographs of the silicon and diamond AFM tips before and after wear testing, and Table S5 shows polymer nanostructures written with a diamond AFM tip. From the resulting images, it is clear that the diamond tips were virtually unaffected by the silicon and polished silicon carbide (SiC) substrates, except for some accumulated debris from rastering 1.28 meters. In contrast, the silicon tip was slightly worn during the silicon substrate test but significantly damaged during the SiC substrate tests. The clumped fragments around the silicon tip after the 200 C and 400 C test on SiC are silicon chips from the tip that were accumulated as the tip deteriorated. The quartz and UNCD substrates were the most abrasive on the AFM probe tips. The silicon tips were universally destroyed on both substrates and the tip fragments are evident in the SEM micrographs. However, the UNCD tips were only either slightly worn or unchanged after testing on the quartz substrate. We anticipated that wear testing on a UNCD substrate with a UNCD probe would cause damage to the tip. The UNCD on UNCD test was conducted three times, with each test resulting in debris blocking our view of the tip. The blunted UNCD tip was imaged using a high accelerating voltage (20 kv) in the SEM to penetrate the debris. The UNCD tip was indeed worn by the UNCD substrate and we believe the gathered debris is the result of wear from the tip and/or substrate. 7

8 Table S1. Wear test results for the polished silicon substrate. SEM micrographs are before and after wear testing of the tips. Polished Silicon Substrate Test Conditions Before Wear Test After Wear Test Test Conditions Before Wear Test After Wear Test 200 nn, 400 C 200 nn, 400 C Radius : Before = 32 nm After = 42 nm Radius : Before = 47 nm After = 49 nm 8

9 Table S2. Wear test results for the polished silicon carbide substrate. SEM micrographs are before and after wear testing of the tips. Polished Silicon Carbide Substrate Test Conditions Before Wear Test After Wear Test Test Conditions 10 nn, 25 C 10 nn, 25 C Before = 25 nm After = 48 nm 200 nn, 25 C Before = 32 nm After = 34 nm 200 nn, 25 C Before = 26 nm After = 65 nm 200 nn, 200 C Before = 27 nm After = 27 nm 200 nn, 200 C Before = 50 nm After = 57 nm 200 nn, 400 C Before = 61 nm After = 61 nm 200 nn, 400 C Before = 31 nm After = N/A Before = 67 nm After = 67 nm Before Wear Test After Wear Test 9

10 Table S3. Wear test results for the amorphous quartz substrate. SEM micrographs are before and after wear testing of the tips. Quartz Substrate Test Conditions Before Wear Test After Wear Test Test Conditions 10 nn, 25 C 10 nn, 25 C Before = 22 nm After = N/A 200 nn, 25 C Before = 46 nm After = 46 nm 200 nn, 25 C Before = 42 nm After = N/A 200 nn, 200 C Before = 44 nm After = 53 nm 200 nn, 200 C Before = 35 nm After = 138 nm 200 nn, 400 C Before = 35 nm After = 43 nm 200 nn, 400 C Before = 67 nm After = 187 nm Before = 89 nm After = 90 nm Before Wear Test After Wear Test 10

11 Table S4. Wear test results for the ultrananocrystalline diamond substrate. SEM micrographs are before and after wear testing of the tips. UNCD Substrate Test Conditions Before Wear Test After Wear Test Test Conditions Before Wear Test After Wear Test 200 nn, 400 C Before = 65 nm After = 245 nm 200 nn, 400 C Before = 26 nm After = 73 nm 11

12 Table S5. Thermal deposition of polymer nanostructures using a heated UNCD cantilever tip polymer nanostructures were written at heater temperature 120 C and consisted of alternating 2.5 µm and 1.5 µm lines. The table shows both the writing distance and the total scan distance for one tip. The table also shows the tip at different times during writing, as well as several nanostructures written at various times. Silicon substrate Total writing distance = 2.16 mm Total scan distance = 3.78 mm Number of Nanostructures = 1080 Total writing distance = 8.64 mm Total scan distance = mm Number of Nanostructures = 4320 Total writing distance = 10.8 mm Total scan distance = 18.9 mm Number of Nanostructures =

13 In Situ Tip Wear Measurement. Tip wear was also monitored in situ during testing by measuring the adhesion of the AFM probe tip to the surface; according to continuum contact mechanics, for a round tip, the pull-off force is directly proportional to tip radius multiplied by the work of adhesion; as the tip gets flat the relation becomes non-linear but pull-off force still increases with tip size. Figure 5 shows typical pull-off forces during wear testing for silicon and UNCD probes. In general, the pull-off force increases with scan distance as the tip is worn. The UNCD tip wear is consistent with atomic wear, or the shedding of individual atoms. 2 This results in a relatively smooth increase in pull-off force with increasing scan distance. In contrast, the silicon tip wear is characterized by progressive fracture of the tip, which results in sudden increases or decreases in tip contact area, depending on the nature of the fracture event. This is shown in Figure 5 as step changes in the pull-off force and a lower signal-to-noise ratio. These changes may also be due to contamination, and variations in the chemical state of the tip such as the exposure of freshly cleaved Si bonds and the resulting oxidation of those bonds in air. Wear tests with no change in tip shape, such as the UNCD tip on quartz at 10 nn and 25 C shown in Figure 5, have relatively little change in pull-off force with increasing scan distance. 13

14 Figure S5. In situ tip wear measurements using pull-off forces on a quartz substrate. The silicon tip was tested at 200 nn, 400 C and the UNCD tips were tested at 200 nn, 400 C and 10 nn, 25 C. REFERENCES 1. Sumant, A. V.; Grierson, D. S.; Gerbi, J. E.; Carlisle, J. A.; Auciello, O.; Carpick, R. W. Surface Chemistry and Bonding Configuration of Ultrananocrystalline Diamond Surfaces and Their Effects on Nanotribological Properties. Phys. Rev. B 2007, Gotsmann, B.; Lantz, M. A. Atomistic Wear in a Single Asperity Sliding Contact. Physical Review Letters 2008,