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Development of the technology and applications of the scanning probe microscope H. Kumar Wickramasinghe, Departments of Electrical Engineering and Computer Science, Biomedical Engineering, and Chemical Engineering and Materials Science, University of California, Irvine, USA

IntroductionScanning probe microscopes that can image nanoscale properties of surfaces are considered essential tools in most nanotechnology laborato-ries. It was not at all obvious, however, at the time of the development of the various techniques described in this article that these tools would gain such widespread acceptance. This widespread use was clearly accelerated by the fact that the AFM was rapidly developed from a scientific tool into a user-friendly instrument for important applications in industry. This paper provides a brief account of the research conducted in the laboratory of the author at the IBM Research Division during the 1984-1994 time-frame that contributed to the the scanning probe microscopy (SPM) technology and its applications.

The need for the resolution of SPMsMoving from a faculty position at University College London to IBM Research in New York in 1984, I was assigned leadership of a group named “Physical Meas-urements” within the newly formed department of Measurement Science and Technology. At IBM, my manager Tom DiStefano gave me an amazing amount of freedom to work on practically anything within the broad area of measurement science with just one proviso – that I did “world-class science” ! Not knowing exactly what that meant, I assembled a team of four staff members from IBM Research – two from Yorktown Heights, New York, and two from IBM Almaden Research Center, San Jose, and spent the next six months visiting various IBM manufac-turing and development labs requesting their management to present their current and future metrology needs to us. What was striking from all those visits was that practically all the metrology we saw was being performed using optical instru-ments and feature sizes were getting close to the optical resolution limit.

Figure 1 shows a chart we presented to manage-ment in 1984 showing some of the metrology trends in lithography and storage showing a Moore’s law-type decrease in feature sizes with time. Projecting ten years into 1994, it was clear that some new form of metrology tools were needed. However, none of us was clear at the time what those tools were actually going to look like.

Table 1 shows in more detail the metrology Figure 1 (left)Semiconductor and storage feature size as a function of time

needs that we identified following our visits to the various IBM development and manufactur-ing sites. I got down to work on the top item on that list - semiconductor surface profiling - with Clayton Williams, the first recruit into my group. The requirements were clear: we needed a non-contact, non-destructive technique. The projected semiconductor minimum critical dimension was approximately 250 nm in the mid 1990s. The required resolution needed to be about 1/5 the feature size and the precision needed to be about 1/50 of that value or just 5 nm. The scanning tun-neling microscope (STM) [1] introduced in 1982 more than satisfied the resolution requirement, but it did not work on insulators and most of the samples of interest were insulators - so a new scheme was needed.

Scanning thermal microscopy The first approach we looked at was inspired

by two papers, one on photothermal surface profiling by Fournier and Boccara [2] and the other on the STM by Binning et al [1]. Fournier and Boccara showed that one could exploit the thermal gradient of the air in contact with a heated solid surface to map out its surface relief pattern by locally measuring the refractive index gradient variation close to its surface; they used a technique called the “mirage effect”. The thought that came to me at the time was whether one could follow a similar approach by constructing

a heater and sensor at the end of a sharp probe tip. If such a probe could be fabricated, it would satisfy all the requirements for the surface pro-filer that we were trying to develop: it would be non-contact and non-destructive, it would work on conductors, semiconductors and insulators and, in principle, it could achieve the high spatial resolution requirement of 5 nm – stipulated by our customers - that is if we were able to make a tiny 5 nm sensor at the end of the probe.

Clayton Williams took on this challenge and in a very short time built such a device and got it to work– the scanning thermal microscope (SThM) [3] was born and it was based on a heated probe with a thermocouple sensor built at its very end, as shown in Figure 2. Our thermal probe worked in much the same way as an STM except that the tunneling current was replaced by heat flow and the tunneling voltage was replaced by tempera-ture. The SThM earned a place as one of the earli-est members of the SPM family [4].

The spatial resolution reached by our SThM was 100 nm and we were able to further miniatur-ize the probes and drive the resolution down to almost 30 nm but improvements beyond that resolution proved very difficult – and we were still a factor of 6 away from our goal of 5 nm for surface profiling. During the decades that followed, the lateral resolution of SThM has in fact not improved significantly; nevertheless, the SThM has found many important uses includ-

Table 1 (above)Some key metrology needs at IBM in 1984.

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ing the local modification of surfaces using the very high (400oC) temperatures attainable at the tip end, the temperature profiling of hot spots in semiconductors, imaging of sub-surface features [5] and the recording of differential calorimetric spectra for detecting local phase transitions.

Figure 3 from Anasys Instruments (courtesy of Roshan Shetty) shows some recent results. Figure 3a - a comparison of an AFM image (left) with a SThM (right) of a copper trace and solder joint in a cross-sectioned ball grid array shows thermal contrast not visible in the AFM topography image. Figure 3b shows a SThM image (left) and corresponding thermal analysis data (right) of a partially miscible polymer blend demonstrating that local thermal expansion traces can provide useful thermal spectroscopy information.

Development of atomic force microscopy technology As for the pursuit of our goal of semiconductor surface metrology, fortunately for us – just as we were becoming increasingly discouraged by the lack of resolution of the SThM – we came across an article by Binnig et. al. in 1986 on the atomic force microscope (AFM) [6]. Their AFM was a modified stylus profilometer (a diamond shard glued to the end of a gold foil with an STM tunneling into the gold foil acting as a highly sensitive deflection sensor; the lateral resolution reached was 3 nm. This AFM was very promising and had most of the stipulated requirements for semiconductor profiling except for one serious drawback – Binnig’s demonstrations were based on scanning a stylus in contact with the sample and detecting a force response for feedback control and no semiconductor manufacturing line wanted a metrology tool that contacted their wafers. I shifted the focus of my group to further explore the AFM concept.

Together with a recently recruited postdoctoral student Yves Martin and Clayton Williams, we started exploring ways that might allow us to use the AFM as a surface profiler in semiconductor metrology. How could we achieve a non-contact, non-destructive probe with the AFM just as we did with the SThM? My first thought was that one might vibrate the tip/cantilever system at its resonance frequency f and try to detect its interac-tion with a surface by looking for a response at 2f. I thought that this might be a way to mechani-cally sense the presence of the surface in a very sensitive manner. To perform this experiment, we had to make several important changes to the original AFM instrument of Binnig et. al. The first change was to replace the STM sensor with a laser sensor. All of us were quite familiar with var-ious forms of laser sensors for vibration detection from our previous research. Although simpler laser detection schemes existed, we chose the laser heterodyne sensor for our experiments as it pro-vided the highest signal-to-noise (S/N) ratio and allowed us to precisely quantify the cantilever/tip vibration amplitude signal against the laser wavelength. Secondly, we replaced the diamond shard/gold foil with an electro-etched tungsten wire cantilever that Yves Martin carefully bent into an L-shaped hook to serve as the tip. Thirdly, we vibrated the cantilever at its resonant fre-quency. When we focused the laser probe on the wire, vibrated the wire at its resonance frequency f and looked for a signal at 2f as we approached

the surface, disappointingly, we only saw a very weak response. However, we were very excited when we noticed that the effect on the vibration signal at f was quite substantial– and it decreased as the tip approached the sample! From that point onward, progress was rapid. We learnt that we were detecting the weak attractive van der Waals forces between the tip and sample – forces that were 10,000 times weaker than those reported in the original contract mode demonstration of the AFM, allowing us to control the tip over a sample without contacting it. We developed a simple theory to show that the decrease in cantilever os-cillation amplitude as we approached the sample was due to the resonant frequency shift in the cantilever caused by the force gradients between tip and sample and not the force as exploited in the original Binnig paper. We stabilized the AFM probe over the surface by exciting the cantilever close to its resonant frequency with constant energy and detecting the dec-rease in cantilever oscillation as the tip approached the surface.

We published our first paper showing AC AFM imaging and profiling with 5 nm resolution in 1987 [7]. This paper demonstrated force gradient sensitivity down to 10-4 N/m and a force sensitiv-ity down to 10-13 N. It showed non-contact opera-tion and demonstrated a form of peak force detec-tion which was based on an intermittent contact or tapping mode of the probe with the sample. We also predicted that with stiffer cantilevers in the range of 100 N/m, the AC AFM technique could be used in the repulsive regime to achieve atomic

Figure 2(a) Schematic of scanning thermal microscope. (b) Surface profile of aluminum line on silicon.

Figure 3(a) AFM image (left) and SThM im-age (right) of a copper trace and a solder joint in a cross-sectioned ball grid array. Scan field 75x75 µm (b) SThM image (left) and local thermal expansion versus temp-erature (right) showing local ther-mal analysis curves of a partially miscible polymer blend. Courtesy of Roshan Shetty, Anasys Instruments.

resolution. Force gradient imaging, non-contact and tapping mode AFM imaging are commonly used modes today and atomic resolution in ultrahigh vacuum has already been achieved with the AC AFM [8]

New scanning probe techniques: EFM, MFM, KPFM, SNOMShortly after our first publication, we introduced many new scanning probe techniques. Our group demonstrated that electrostatic forces and charge could be detected by applying electric fields between tip and sample (EFM) [9] and that both static and dynamic magnetic domains can be imaged using magnetized tips (MFM) [10, 11]. We showed that both amplitude and phase images could be recorded with the AC AFM and that phase imaging produced superior resolution [12]. Together with my postdoctoral student Martin Nonnenmacher we introduced the Kelvin probe force microscope (KPFM) which was capable of measuring the local work function differences between tip and sample by detecting the weak electric fields that are created between them as a result [13]. Finally, in 1994, together with my post doctoral student Frederic Zenhausern, we introduced AC techniques to scanning near field optical microscopy (NSOM) by combining the AC AFM technique with light scattering from the end of a sharp tip (named apertureless near field optical microscopy or scattering scanning near field optical microscopy – sSNOM). We were able to improve the resolving power of near-field opti-

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Figure 4(a) Schematic of AC atomic force microscope showing scheme for detecting force gradients between tip and sample. Cantilever frequency shifts are detected through a change in the vibration amplitude measured using a laser probe.(b) Photograph of first fully automated and manufacturing hardened AFM.(c) Surface profile of a small region of a 16 MBit DRAM chip showing sidewall features and information not accessible before.

cal microscopes by almost 20x [14,15] compared with previous instruments.

All these techniques have been significantly improved today; the KPFM recently demonstrat-ed charge distribution imaging within a single molecule [16]. Follow-on work has shown that the cantilever frequency shift due to the force gradi-ents between tip and sample can be monitored by making the cantilever a part of an electrical self oscillator circuit and then dir-ectly measuring the self-oscillation frequency change of the cantilever (FM AFM) [17]; this implementation achieves faster mechanical response of the feedback loop and is not limited by the quality factor Q of the cantilever as it is for AM AFM.

In subsequent work on AFMs the heterodyne sensor was replaced by a much simpler laser deflection sensor [18]. Several other laser sensors were developed as well for use with AFMs during the same time period. A sensor that has been employed in low-temperature or high-vacuum AFMs is based on the one developed by G. Mc-Clelland at IBM Almaden who was working on an AFM scheme that vibrated the sample instead of the tip and used a Michaelson interferometer to detect the induced cantilever vibrations [19].

AFM for manufacturing and developmentIn our laboratory, progress toward building a hardened AFM for development and manufac-turing was rapid. By December 1988, with the help of another postdoctoral student Phil Hobbs and an engineer, we designed and built the first bench top prototype – transitioning from a research in-strument capable of looking at 1-mm samples to an instrument that could handle 1-inch samples and where a 100 µm x 100 µm scan field could be positioned any where within a 1 square inch area using a reflection optical visualization system. These were major steps in the right direction for user friendly operation and we installed six proto-types of this instrument in strategic development laboratories within IBM. With these instruments we were soon able to identify at least 50 metrology application areas within IBM ranging from semi-conductors, liquid crystal displays, X-ray masks, to magnetic disk drives.

At this point, my manager and I went out with a proposal to IBM corporate headquarters seeking funds to develop an 8-inch full-wafer metrology tool. We managed to secure a substantial amount of funding in spite of negative comments from a few respected senior scientists. Their com-ments were based on an assessment that such a tool would never work because of issues such as

mechanical stability and large thermal drifts. In retrospect, their concerns were probably under-standable. To put it in perspective, if one scaled up our proposed AFM system 60,000 times, we were proposing to build a profiling device where we would stabilize a tip 1 foot long and 0.01 inch in diameter at its apex, 0.01 inch away from a sample, then be able to scan a field of 1 square foot without crashing it – and do this anywhere on the sample surface over an 8 mile radius!

With our funding approved, we assembled a team consisting of a group led by Johann Gresch-ner at IBM Sindelfingen, Germany, charged with replacing our tungsten cantilever/tips with single-crystal silicon micromechanical tips, a group led by Mike Servedio at IBM Boca Raton

charged with engineering development, and my group at IBM Research to provide overall leader-ship and guidance. The project started in Sep-tember 1989. In parallel with this internal effort, we contracted with an external vendor (Digital Instruments) to develop a similar instrument just to hedge our bets. It turned out that both efforts were successful (Digital Instruments delivering to us an 8-inch wafer system – which became their Dimension 3100) and was used primarily in IBM disk development labs.

Our internal effort was completed in Jan- uary 1991 – much earlier than scheduled and six months before Digital Instruments delivered theirs. Our AFM, dubbed the SXM Workstation, provided more functionality for manufactur-

Figure 5(a) AC AFM image of the atomic arrangement of a calcite crystal surface in water showing point defects; scan size 20 nm, Z scale 3.2 Å, cantilever amplitude 4 Å, cantilever frequency 454 KHz. (b) Comparison of MFM images of a 2TB hard disk using two different MFM cantilevers with cobalt tips: image with a conventional cantilever (240um x 30 um) (left) and image with an ultra-small cantilever (10 µm x 5 µm) (right) demonstrating the higher spatial resolution resulting from reduced thermal noise. The images (courtesy of Terry Mehr) were taken using an Asylum Research Cypher AFM.

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biography H. Kumar Wickramasinghe, PhD, is a member of the National Academy of Engineering and a respected pioneer in nanotechnology. Prior to joining UC Irvine, he managed nanoscience and technology research at IBM’s Almaden Research Center in San Jose. Holding 70 patents, some of his most significant inventions and contributions to the nano field include the development of the vibrating mode atomic force microscope, the magnetic force microscope, the electrostatic force microscope, the Kelvin probe force microscope, the scanning thermal microscope, and the apertureless near-field optical microscope.

abstractThis paper chronicles the research in the laboratory of the author at the IBM Research Division during the 1984-1994 time frame that significantly contributed to the development of the scanning probe microscopy technology and its applications. Several key technologies that are now considered commonplace in SPM were developed during that time. These include the vibrating probe AFM (also called AC AFM, non-contact AFM, or tapping-mode AFM), introduction of single crystal silicon cantilevers/tips, laser sensing of the cantilever vibration, full wafer positioning and closed-loop scanning systems. Our application-driven approach led to several new microscopes for nanoscale probing of the magnetic, electrostatic, thermal and optical properties of surfaces that are widely used today. They are among the early members of the family that is now classified as scanning probe microscopes.

acknowledgements H. K. W. would like to thank all members of his group, especially Yves Martin, Phil Hobbs, Clayton Williams and the SXM development team, whose support was essential for the successful development of the SXM workstation. I would also like to thank my management at IBM, Tom DiStefano, Russ Lange, Carol Kovac and Jim Yeh, for their unwavering and enthusiastic support of our efforts that led to the development of the first fully hardened SPMs for industrial deployment.

corresponding author details Professor H. Kumar Wickramasinghe,Departments of Electrical Engineering and Computer Science, Biomedical Engineering,and Chemical Engineering and Materials Science, University of California, Irvine, CA, 92697, USATel: +1 949 824 0378 Email: hkwick@uci.edu ang

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ing applications such as full-wafer handling and metrology capability and was transferred almost immediately to several manufacturing sites within IBM. The team effort introduced several new technologies. We developed the first silicon micromechanical tips, and cantilevers were fabricated in October 1988. We introduced capaci-tive sensors with feedback control to linearize the piezo scanners, We developed a system to rapidly position the probe anywhere on the wafer surface within seconds, followed by magnetic engage-ment of the wafer carrier to the tip scanner body prior to scanning to minimize vibrations. We developed calibration edge standards using ani-sotropically etched silicon to calibrate the probe tips before and after use and introduced etched silicon mounting pieces to accurately position the cantilever chips auto-loaded from a cassette. We introduced a three-stage auto app-roach system; an optical autofocus to approach within several microns, followed by a slow piezo inch-worm mechanism to detect cantilever air damping fol-lowed by a fine piezo for final approach. We also introduced a dual Z-piezo actuator system, one for coarse tracking and another for fast tracking of the topography. Figure 4 shows a picture of the SXM Workstation and one of the early scans of a line profile from a 16 Mbit DRAM chip showing sidewall detail not attainable before.

In the meantime the various SPM techniques and technology continue to advance. Figures 5 and 6 show some recent images from an Asylum Cyber AFM. Figure 5a shows that atomic resolu-tion has been achieved in water on a calcite crystal surface with an AC AFM and Figure 5b shows the latest MFM images showing bit patterns on a 2 TB hard drive; Cobalt magnetic tips were grown at the end of silicon cantilever tips using a focused ion beam to record these images. Figure 6 shows an AFM scan of the surface of an atomic terrace on silicon showing 0.43 Å surface roughness.

ConclusionsWe have primarily focused on the key technol-ogy developments that emerged from my group that eventually resulted in the introduction of hardened SPM technologies into manufacturing and development due to a highly focused team effort within IBM. The technology found its way to the external world through sale of the SXM

Figure 6AFM image of a silicon wafer annealed in argon; the single atomic terrace shows a roughness of 0.43 Å. The images (courtesy of Terry Mehr) were taken using an Asylum AFM. Sample courtesy of T. Suwa Ohmi, Tohoku University.

technology to Veeco and spin-off of the AC AFM technology to Digital Instruments. Many of the advances highlighted in this article are incorpo-rated into most AFMs used today.

We have left out several important SPM tech-nologies such as scanning capacitance microscopy that has been used for 2D and 3D semiconductor dopant profiling [20] and other SPM techniques that have emerged since 1994 for lack of space. SPM techniques will continue to evolve with new techniques being introduced and the technology will continue to develop with ever increasing automation. The next few decades should spawn instruments with increasing ease of use and real-time operation.

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