The Moving Finger Writes: Carbon Nanotubes As AFM Probe Tips

Commercially available carbon nanotube (CNT) probe tips

The Moving Finger Writes: Carbon Downloaded from for atomic force microscopes (AFMs) typically consist of multi- Nanotubes as AFM Probe Tips wall nanotubes (MWNT) mounted onto standard AFM cantilever Katerina Moloni probes, usually tapping mode etched silicon probes. The tips can be mounted on either standard silicon probes or silicon probes coated nPoint, Inc.

with platinum/tridium (Pt/Ir). The metallic coating enhances electri- https://www.cambridge.org/core [email protected] cal conductivity and the reflectivity of the laser beam that is used to After carbon nanotubes (CNT) were discovered in 1991 [1], detect the probe deflection during scanning. As previously indicated, many applications have been proposed that utilize their extraordi- CNT tips offer a much greater scanning life than standard probes and nary electrical and mechanical properties. One application is as tips have the ability to achieve high resolution and measure high-aspect for scanning probe microscopy where CNTs offer several advantages ratio features. CNT probes enhance performance in intermittent and including high resolution and the capability to image deep, narrow non-contact modes but are not recommended for contact mode. Two structures [2, 3], A recent study oi'CNT scanning probes for atomic applications where CNT AFM tips have unique advantages are ad- force microscopy (AFM) in semiconductor surface science concluded vanced semiconductor manufacturing and biological research. that an AFM with CNT tips has immense potential as a surface char- Advanced Semiconductor Manufacturing . IP address: acterization tool in integrated circuit manufacture [4]. Previously re- Surface Characterization searchers had to construct their own CNT probes, but recently CNT The AFM offers a simple, nondestructive method to characterize AFM probes have become commercially available. advanced semiconductor structures in the nanoscale realm, hi contrast 170.106.202.8 Carbon nanotubes (sometimes called buckytubes) are closed to techniques such as scanning electron microscopy or transmission seamless shells ot graphitic carbon typically one to tens of nanome- electron microscopy, AFMs do not require a special (usually destruc- ters in diameter and several micrometers long. The structure of a , on CNT probe Conventional | CNT probe closed-dome single-walled nanotube is illustrated in Figure 1. Carbon 29 Sep 2021 at 17:02:21 nanotube probe tips offer several advantages. CNTs can be as small as 1-2 nm in diameter and tens of nm long, provide high image resolution, and can accurately measure extremely high- aspect-ratio topography. CNTs are 100

times stronger than steel yet are quite , subject to the Cambridge Core terms of use, available at flexible and do not deform biological stir- Zftm x Zfzm ljj,m x lfim lfj,m. x', faces. In addition, the extreme hardness Figure 2, Resolution comparison: CNT probe vs. conventional probe of the CNT tip provides wear resistance tip. Thesame location on a photoresist surface was scanned. The maximum and greater scanning life and their flex- height difference measured by the CNT was 114 nm vs. 91 tint measured ibility makes them resistant to damage by the conventional tip. \7}. trom probe tip crashes. tive) sample preparation or a vacuum environment. AFM has been CNT probe tips for atomic force mi- deemed the best solution for examining surface morphology and croscopy (AFM) consist of single-walled uniformity of coverage of non-conducting ultra-thin films. CNT tips or multi-wall nanotubes (MWNTs) extend the capability of conventional AFMs, providing wear resistance, mounted onto AFM probes, typically high resolution, and the ability to image very small, high-aspect ratio attached to the ends of etched silicon features. Nguyen, et al. used singie-walled nanotube (SWNT) probes in tips on silicon cantilevers. MWNT tips imaging silicon nitride, indium,.and gold films on mica substrates and are now commercially available, CNTs MWNT probes for metrology of 300-nm-thick photoresist patterns olfer unique advantages over standard on a silicon substrate [4].They found that although a single-wall CNT AFM tips in applications ranging from gave better resolution than the larger-diameter MWNT, the MWNT https://www.cambridge.org/core/terms probing the interiors of biological cells Figure 1. Diagram of is much stiffer and can be longer (up to several urn) without exhibit- to advanced integrated circuit manu- the structure of a single- ing thermal vibration. The MWNT coulci trace deep features, which walled carbon nanotube facture. This article presents examples is important for accurate measurement of the critical dimensions of applications of CNT probes and pointers on matching the probe (both widths and thicknesses) of hues and spaces in semiconductor type to the application. masking layers and patterned structures. To be useful, the tip shape CNT Tip Properties must remain stable during extended use. They found that after con- Carbon nanotubes have been used as tips for A.FM and scanning 10 jim tunneling microscopy (STM) to take advantage of their nanoscale size, electrical conductivity, wear resistance, and mechanical strength. An

. example of the enhanced resolution offered by a CNT tip is shown by https://doi.org/10.1017/S1551929500051804 the images of a photoresist surface in Figure 2. The large length/width aspect ratio plus the small diameter permits imaging of nanometer- scale, high-aspect ratio features, as found on integrated circuits and micro electromechanical systems (MEMS) and structures (Pigure 3). Most conventional AFM tips are silicon nitride or silicon. The radius and shape of these tips vary from one to the other and must be characterized, i.e., their shape accurately determined, to ensure Mini I >.» VI pi accurate imaging. These tips are subject to wear after repeated scans. Figure 3. Anarrow CNJ'probe tip improves theineq$ure.meta accuracy In general, the sharper the tip, the better the image resolution, and a of 'high-aspect ratio features such as deep, narrow trenches. [8]. worn tip will degrade the image.

34 • miCROICOPYTODflY January/February 2004 Downloaded from

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https://doi.org/10.1017/S1551929500051804

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'Dimension 31OQ and Veeco are registered marks of Veeco. Downloaded from tinuous scanning for over 15 h,a ~10-nm-diameter MWNT showed nanotubes can bind reversibly with chemically recictive functional no detectable degradation in lateral resolution of 8-10 nm silicon groups that can sense target molecule [13,14]. For example a car- nitride grains. The probe length also did not change with time. This boxylic acid (-COOH) group, when attached to a CNT tip, can "see"

was in contrast with the rapid wear of a silicon probe tip. The thinner amide (-NH2) groups on protein molecules. https://www.cambridge.org/core SWNT probe tips were capable of lateral resolution as small as 2 nm. Commercial Nanotube AFM Tips Similar results were found in the degredation studies performed by In much of the work described above the researchers had to Larsenetal. [5]. prepare their own CNT-tipped AFM probes. Today, CNT AFM probes Advanced Lithography are available in configurations suitable for a broad range of applica- Semiconductor structures start out as a photoresist pattern on tions. Table 1 lists the characteristics of typical probe configurations. a thin film that will be subsequently patterned by a plasma etch- CNT AFM probes provide extremely long life and high resolution for ing process. The pattern is formed by first exposing the photoresist imaging high aspect-ratio samples, and samples that require gentle layer to a deep ultraviolet, X-ray, or electron-beam pattern and then probe-sample interaction.

. IP address: chemically developing the latent image. As the widths of the resist pattern shrink below 100 nm, the roughness of the de- Table I. Characteristics of Carbon Nanotube Tips A. High Aspect Ratio/ B. High Aspect veloped pattern edge becomes an increasingly large fraction of Characteristic C, High Resolution the allowed critical dimension tolerance budget. Many factors High Specification Ratio 170.106.202.8 may contribute to line edge roughness (LER), including aerial Tube Angle ±5° ±15* image fluctuations, resist material, exposure parameters, and the Diameter 10-40 nftii .10-40 nm 5-20 titfi resist development: process. AFMs with CNT probe lips offer the , on Length 500-2000 nm 500-2000 nm <500 nm

resolution needed to non-destructively and quantitatively study 29 Sep 2021 at 17:02:21 the effects of photomask imaging parameters on LER [6,7]. Pig- Typical CNTs are produced by attaching multi-wall carbon nano- ure 4 shows a 3-D image of a tubes onto commercial silicon (Si) tips. They function best scanning in 0.5-urn line/space photoresist intermittent contact mode. They are available in three types with each pattern that resolve line edge configuration available on standard Si and platinum/Indium (Pt/ir) roughness. Figure 5 shows the coated Si probes. The combinations of configurations and coatings

results of an investigation of allow optimization across a broad array of applications depending , subject to the Cambridge Core terms of use, available at the effect of aerial image con- on the dimensions and alignment of the nanotube tip. The silicon % and space patterns. trast on LER using CNT AFM cantilever on which the nanotubes are mounted has a typical spring Figure 4. Right: 3-D CNT measurements of LEIL constant of 50 N/m and resonant frequency of 300 kHz. A Pt/Ir coated AFM image of 0.5-fim line/space Biological Research Si tip has a typical cantilever spring constant of 5 N/m and a resonant photoresist pattern thai resolves line edge roughness. Left: Transmission One of the advantages of frequency of 80 kHz. electron microscope images, of the an AFM is that it can operate Figure 6 shows SEM images of typical examples of each of the tip nanutube probe tip, [9j. in a liquid medium, making it types. SEM images for each specific tip are provided with the tip. possible to examine biological Type A. High Aspect Ratio/High Specification Tips specimens such as proteins, ceEular structures, and DNA in a "natural" These tips are inspected in the scanning electron microscope environment [10]. Carbon nanotube tips extend the resolution of from two directions to ensure that they are within ±5° of perpen- AFMs used in this manner [11]. An extension of this technique is an dicularity to the sample. Lengths and diameters vary with typical operating mode where the AFM cantilever is driven with an oscillat- lengths being 500-2000 nm and typical diameters 10-40 nm. A typical ing magnetic field, minimizing the effect of acoustic resonance from application is the imaging of high aspect ratio features such as narrow, the cantilever holder and the body, increasing the signal/noise, and

deep trenches that have sharp (nearly perpendicular) edges. https://www.cambridge.org/core/terms reducing the required vibration amplitude [12]. Type B. High Aspect Ratio Tips CNTs have other advantages for biological studies. They are flex- These tips are inspected in the scanning electron microscope ible and are less likely to damage fragile entities. In addition, carbon from one direction. Their alignment is within +15° from perpendicular to the sample in one direction. Typical lengths are 500-2000 nm and typical diameters 10-40 nm. These tips are used in similar applications like the ones mentioned above, but their alignment with respect to the sample is not guaranteed to better than 15 degrees. Type C. High Resolution Tips These tips have a multi-wall nanotube with diameter of 5-20 mn

and a length generally shorter than 500 nm. Such dimensions make https://doi.org/10.1017/S1551929500051804 these tips well suited for applications where high resolution, more than high aspect ratio, is desirable. Due to die short length of tliese tips, tip angle is not a critical. Typical applications include inspection of biological samples and topography produced by advanced lithog- Figure 5. Left: Typical CNT AFM image of photoresist pattern. Right: raphy that has high aspect ratio features with critical dimensions on The relation between line edge roughness (LER) and image contrast (% the order of a few tens of nanometers. modulation) for various exposure conditions. LER decreases as image The Future contrast increases up to a certain threshold. EBL, EUVL, and XRL refer The potential benefits of carbon nanotube tips are yet to be to electron beam, extreme ultraviolet, and x-ray lithography respectively. fully explored. As the advantages of CNT AFM tips become widely CMTF (critical modulation transfer function) ti the minimum intensity recognized and experience is gained with their properties they swing (contrast) required to prim lines and spaces [6], will become an even more valuable addition to the AFM arsenal

36 • miOROSOOPYTODftY January/February 2004 Downloaded from

of techniques. Very recently the synthesis of tubular graphite cones using chemical MRS-4 https://www.cambridge.org/core vapor deposition has A ISO-9000 ruidISO Guide-SSStaiidard'forMicroscopy been reported [15]. Calibrate fivm 1'OXto200,000X They are composed of cylindrical graphite sheets coiled into a This is our third generation, cone shape and have traceable, magnification nanometer-si zed tips reference standard for a types of microscopy (SEM, . IP address: and micrometer-sized Optical, STM, AFM, etc.). The roots. They also have MRS-4 has multiple X and Y potential for use as pitch patterns that range

170.106.202.8 scanning probe tips, from Vs |jm l± 0.045 pm) to but with greater rigid- 500 urn (± 0.1 \im) and a 6 ity and easier mount- mm ruler with 1 pm increments. ing than carbon nano- , on

tubes. However, their 29 Sep 2021 at 17:02:21 pyramidal shape may Visit our website and send for our free resources guide. limit their ability to accurately resolve very narrow high-aspect ratio features. • MICROANALYTICAL LABORATORY , subject to the Cambridge Core terms of use, available at References 1. S. [ijima, "Helical micro- 436E BOSTON STREET (ET. 1) * TOPSFTED, MA. 01983 Uibnlcs of graphitic carbon," 97&887-7000 • 97&«S7-6e7I * [email protected] Nature 354, p. 56(1991). http; //www.geUennicro. com 2. H. Dai, J. H. Hafner, A. G, Rinzler, D. T. Colbert, and R. E, Smalley, "Nanolubes as nanoprobes in scanning probe micros copy," Nature Director Figure 6. Scanning electron 384, pp.147-150 (1996). Cell Imaging Core Facility 3. K. Moloni, A. Lai, and micrograph of typical CNT probe tips on Fox Chase Cancer Center has an opening to lead M. G. Lagally, "Sharpened silicon pyramids on silicon cantilevers. See carbon narujritube probes," its facility which contains state-of-the-art computer and Table I for tip characteristics. Proc. SP1E 4098, pp. 76-83 light microscopy equipment, including a Bio-Rad Radi- (2000). ance 2000 Laser Scanning Confocal Microscope, an 4. C.V. Nguyen, et al,,"Carbon nanotiibe tip probes- Stability and lateral resolution in optical imaging system with upright and inverted Nikon

scanning probe microscopy and application to surface science in semiconduc- https://www.cambridge.org/core/terms tors," Nanoiechnology 12. pp. 363-367 (2001). microscopes, and an automated fluorescent dissecting 5. T. Larsen, K. Moloni, F. Flack, MA. Eriksson, M.G, Lagally, and C.T. Black, microscope. Day to day responsibilities include the training "Comparison of" wear characteristics of etched-silicon and carbon nanoiube of new users, maintenance and operation of the imag- atomic-force microscopy probes," APL S0( 1 l),pp 1996-1998 (2002). ing systems, archiving of images on Unix and PC based 6.J. Shin, G.'Han, Y. Ma, K. Moloni, and F. Cerrina, "Resist edge line roughness and computer systems, as well as the independent design aerial image contrast," f. Vac. Sq. Techno], B 19(5), pp. 2890-2895 (Nov/Dec and implementation of imaging-based experiments. This 2001), 7. Y. Ma, J. Shin, and P. Cerrina, "Line edge roughness and photoresist percolation facility supports up to 70 investigators and is supported development model," I Vac. Sci. Technol. B 21(1), pp. 112-117 (Jan/Feb 2003). by an NCI core grant. The ideal candidate will possess a 8. J, Shin and R Cerrina, "Carbon Nanotubes Application to Sub-100-nm Pattern B.Sc, MS or Ph.D. degree and possess experience with Metrology," Semiconductor Research Corp. (unpublished), 17 July 2001.

confocal and wide field microscopy as well as state-of- .

https://doi.org/10.1017/S1551929500051804 9,1. Shin, G. Han, F. Cerrina, and K. Moloni,"Resist line-edge roughness and aerial the-art imaging software. Salary will be commensurate image contrast," Semiconductor Research Corp. (unpublished), 17 July 2001. with training and experience. Send CV and at least two 10. R. l;ewis,"From Ruckyballs to Nanotubes: Carbon nanotube atomic force microscopy probes Alzheimer's disease," The Scientist 15(4}, p. 8 (19 [:eb 2001). letters of recommendation to: TR-Director, c/o Human Re- 11. S.S, Wong, et al., Carbon nanotube tips: High-re solution probes for imaging sources, Fox Chase Cancer Center, 333 Cottman Ave, biological systems," J. Am. Chem. Soc, 120, pp. 603-604 (1998). Philadelphia, PA19111-2497.www.fccc.edu 1 I2.J, Li and A. Cassell, "Carbon Nan (tlubt-Tips for MAC Mode" AFM Measurements Equal Opportunity Employer in Liquids," Application Note, Molecular Imaging, Inc., Phoenix. AZ. 13.S.S. Wong, E.Ioselevich,A.T.Woolley,C. L.Cheung, and C. M, Lieber/'Covalenlly fu fictionalized nanntubes as nanometre-sized probes in chemistry and biology," Nature 394,pp. 52-55 (1998), FOX CHASE 14. E" J. Boul, el al, "Reversible Sidevrall Functionalization of Buckytubes," Chem. Phys. Lett. 310, pp. 367-372(1999). CANCER CENTER 15,G,2riang,XJian;g,an-dE, Wang,Science 300,pp.472-474 (18 April 2003).

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