CHAPTER 1 Introduction Typically, when we think of microscopes, we think of optical or electron microscopes. Such microscopes create a magni ed image of an object by focusing electromagnetic radiation, such as photons or electrons, on its surface. Optical and electron microscopes can easily generate twodimensional magni ed images of an object s surface, with a magni cation as great as 1000X for an optical microscope, and as large as 100,000X for an electron microscope. Although these are powerful tools, the images obtained are typically in the plane horizontal to the surface of the object. Such microscopes do not readily supply the vertical dimensions of an object s surface, the height and depth of the surface features. Unlike traditional microscopes, the AFM does not rely on electromagnetic radiation, such as photon or electron beams, to create an image. An AFM is a mechanical imaging instrument that measures the three dimensional topography as well as physical properties of a surface with a sharpened probe, (see Figure 1-1). FIGURE 1-1 In the AFM, a sharp probe is scanned across a surface, left, and by monitoring the motion of the probe from each pass across the surface, a 2-D line pro le is generated. Then the line pro les are combined to create a three dimensional image of the surface, right. 1 Chapter 1 INTRODUCTION The sharpened probe is positioned close enough to the surface such that it can interact with the force elds associated with the surface. Then the probe is scanned across the surface such that the forces between the probe remain constant. An image of the surface is then reconstructed by monitoring the precise motion of the probe as it is scanned over the surface. Typically the probe is scanned in a raster-like pattern. In an AFM the probe is very sharp, typically less than 50 nanometers in diameter and the areas scanned by the probe are less than 100 um. In practice the heights of surface features scanned with an AFM are less than 20 um. Scan times can range from a fraction of a second to many 10 s of minutes depending on the size of the scan and the height of the topographic features on a surface. Magni cations of the AFM may be between 100 X and 100,000,000 X in the horizontal (x-y) and vertical axis. Figure 1-2 illustrates the block diagram of an <a href="/keyword/atomic-force-microscope/" >atomic force microscope</a> . In the microscope, the force between a nanoscopic needle and the surface is measured with a force sensor, the output of the force sensor is then sent to a feedback controller that then drives a Z motion generator. The feedback controller uses the force sensor output to maintain a xed distance between the probe and the sample. X-Y motion generators then move the probe over the surface in the X and Y axis. The motion of the probe is monitored and used to create an image of the surface. Image Out Imag e Out X-Y Motion X-Y Motion Gener ator Generator Z Motion Z m otion gener ator Generator Force Force Sensor Sensor Feedb ack Feedback Contro ller Controller FIGURE 1-2 Basic block diagram of an AFM. Figure 1.2. Basic block diagram of an AFM 2 Chapter 1 INTRODUCTION The force sensor in an <a href="/keyword/atomic-force-microscope/" >atomic force microscope</a> is typically constructed from a light lever, see (Figure 1-3). In the light lever, the output from a laser is focused on the backside of a cantilever and re ected into a photodetector with two sections. The output of each of the photo-detector sections is compared in a di erential ampli er. When the probe at the end of the cantilever interacts with the surface, the cantilever bends, and the light path changes causing the amount of light in the two photo-detector sections to change. Thus the electronic output of the light lever force sensor, So, is proportional to the force between the probe and sample. FIGURE 1-3 Illustration of the light lever force sensor (left). An AFM SEM image of the cantilever/probe used in an AFM force sensor (right). Although the AFM is capable of extreme magni cation, it is not a large instrument. An AFM that is capable of resolving features as small as a few nanometers can be easily installed in a laboratory on a desk top. The greatest deterrent to high magni cation with the AFM is often environmental vibrations that cause the probe to have unwanted vibrations. Although the AFM is an amazing instrument for visualizing and measuring nanometer scale features, it has several characteristics that make it unique. They are: a) Built-in, Atomic-scale Sensitivity Most measuring instruments become larger when greater sensitivity is required. With an AFM the sensitivity is built-in at the nanometer or atomic scale. Thus to make the instrument more sensitive, there is no need to make it larger. b) Fabrication Technology An AFM may be used for rapidly making changes in surface structures at the nanometer scale. Such changes can be made for a fraction of the amount it would cost with traditional technologies such as e-beam or photolithography. 3 Chapter 1 INTRODUCTION c) Motion Control Precise motion control technology is required to accurately scan and position the probe in an AFM at the nanometer scale. Such accurate motion control technology allows cost e ective motion control at a level not achievable with other methods. These three unique characteristics may be applied to other technological and scienti c areas such as data storage, genetic engineering and nanorobotics. 1.1 History of AFM Magni cation of the vertical surface features of an object, those features leaving the horizontal plane and extending in the vertical direction, have historically been measured by a stylus pro ler. An example of an early pro ler is shown in Figure 1-4. This pro ler, invented by Schmalz1 in 1929, utilized an optical lever arm to monitor the motion of a sharp probe mounted at the end of a cantilever. A magni ed pro le of the surface was generated by recording the motion of the stylus on photographic paper. This type of microscope generated pro le images with a magni cation of greater than 1000X. FIGURE 1- 4 Light Lever design used for one of the early designs of a surface pro ler in the 1920 s. This pro ler had a vertical resolution of approximately 25nm. A common problem with stylus pro lers was the possible bending of the probe from collisions with surface features. Such probe bending 4 Chapter 1 INTRODUCTION was a result of horizontal forces on the probe caused when the probe encountered large features on the surface. This problem was rst addressed by Becker2 in 1950 and later by Lee3. Both Becker and Lee suggested oscillating the probe from a null position above the surface to contact with the surface. Becker remarked that when using this vibrating pro le method for measuring images, the detail of the images would depend on the sharpness of the probe. In 1971 Russell Young4 demonstrated a non-contact type of stylus pro ler. In his pro ler, called the topographiner, Young used the fact that the electron eld emission current between a sharp metal probe and a surface is very dependent on the probe sample distance for electrically conductive samples. In the topographiner (see Figure 1-5), the probe was mounted directly on a piezoelectric ceramic used to move the probe in a vertical direction above the surface. An electronic feedback circuit FIGURE 1-5 Schematic of the rst non-contact mechanical pro ler developed by Russel Young (Top). Light lever design developed by IBM for use in the AFM (Bottom). 5 Chapter 1 INTRODUCTION monitoring the electron emission was then used to drive the piezoceramic and thus keep the probe sample spacing xed. Then, with piezoelectric ceramics, the probe was used to scan the surface in the horizontal (XY) dimensions. By monitoring the X-Y and Z position of the probe, a 3-D image of the surface was constructed. The resolution of Young s instrument was controlled by the instrument s vibrations. In 1981 researchers at IBM were able to utilize the methods rst demonstrated by Young to create the scanning tunneling microscope (STM). Binnig and Rohrer5 demonstrated that by controlling the vibrations of an instrument very similar to Young s topographiner, it was possible to monitor the electron tunneling current between a sharp probe and a sample. Since electron tunneling is much more sensitive than eld emissions, the probe could be used to scan very close to the surface. The results were astounding; Binnig and Rohrer6 were able to see individual silicon atoms on a surface. Although the STM was considered a fundamental advancement for scienti c research, it had limited applications, because it worked only on electrically conductive samples. A major advancement in pro lers occurred in 1986 when Binnig and Quate7 demonstrated the <a href="/keyword/atomic-force-microscope/" >atomic force microscope</a> . Using an ultra-small probe tip at the end of a cantilever, the <a href="/keyword/atomic-force-microscope/" >atomic force microscope</a> could achieve extremely high resolutions. Initially, the motion of the cantilever was monitored with an STM tip. However, it was soon realized that a light-lever, see Figure 1-5, similar to the technique rst used by Schmalz, could be used for measuring the motion of the cantilever. In their paper, Binnig and Quate proposed that the AFM could be improved by vibrating the cantilever above the surface. The rst practical demonstration of the vibrating cantilever technique in an <a href="/keyword/atomic-force-microscope/" >atomic force microscope</a> was made by Wickramsinghe8 in 1987 with an optical interferometer to measure the amplitude of a cantilever s vibration. Using this optical technique, oscillation amplitudes of between .3 nm and 100 nm were achieved. Because the probe comes into close contact with the surface upon each oscillation, Wickramsinghe was able to sense the materials on a surface. The di erences between photo-resist and silicon were readily observed. 6 Chapter 1 INTRODUCTION 1.2 Comparison of AFM to Other Microscopes &amp; Instruments The AFM can be compared to traditional microscopes such as the optical or scanning electron microscopes for measuring dimensions in the horizontal axis. However, it can also be compared to pro lers for making measurements in the vertical axis to a surface. One of the great advantages of the AFM is the ability to magnify in the X, Y and Z axes. Figure 1-6 shows a comparison between several types of microscopes and pro lometers. One of the limiting characteristics of the AFM is that it is not practical to make measurements on areas greater than 100 um. This is because the AFM requires mechanically scanning the probe over a surface: (1000X for 100um to 10 cm) ( 1,000,000X 100nm to 10 cm) FIGURE 1-6 Comparison of the imaging length scale of many types 2-D and 3-D pro ling and imaging instruments. An AFM is capable of resolving features in the dimensions of a few nanometers with scan ranges up to a hundred microns. When compared to a pro ler, the AFM has a greater X-Y resolution because in the AFM the probe is sharper. Pro lers can have high vertical resolutions, as low as .05 nm. However, the bandwidth of the pro ler measurements is much lower than an AFM. To achieve a resolution of .05 nm a pro le has a bandwidth of approximately .1 Hz. The AFM bandwidth for the equivalent measurement is between 5 kHz and 10 Khz. 7 Chapter 1 INTRODUCTION The length scale of an optical microscope overlaps nicely with an AFM. Thus, an AFM is typically combined with an optical microscope and with this combination it is possible to have a eld of view dynamic range from mm to nm. In practice, an optical microscope is typically used for selecting the location for AFM scanning. The AFM is most often compared with the electron beam techniques such as the SEM or TEM. In general, it is easier to learn to use an AFM than an SEM because there is minimal sample preparation required with an AFM. With an AFM, if the probe is good, a good image is measured. A comparison of the some of the major factors follows: SEM / TEM Samples Magni cation Environment Time for image Horizontal Resolution Vertical Resolution Field of View Depth of Field Contrast on Flat Samples Must be conductive 2 Dimensional Vacuum 0.1 - 1 minute 0.2 nm (TEM) 5 nm (FE-SEM) n/a 100 nm (TEM) 1 mm (SEM) Good Poor AFM Insulating / Conductive 3 Dimensional Vacuum / Air / Liquid 1 - 5 minute 0.2 nm .05 nm 100 um Poor Good FIGURE 1-7 Comparison of an AFM and SEM. FIGURE 1-8 Both the AFM and SEM measure topography. However, both types of microscopes can measure other surface physical properties. The SEM is good for measuring chemical composition and the AFM is good for measuring mechanical properties of surfaces. 8 Chapter 1 INTRODUCTION SEM/TEM instruments are capable of doing much more than topography measurements. For example, electron beam instrumentation can do EDX measurements or even electron beam initiated lithography. Likewise, the AFM can make many types of measurements other than AFM topographical measurements. For example, AFM instruments can make thermal, magnetic and electric eld maps of a surface. Like the SEM/ TEM, an AFM can also initiate lithographic changes on a surface. Although the time required for making a measurement with the SEM image is typically less than an AFM, the amount of time required to get meaningful images is similar. This is because the SEM/TEM often requires substantial time to prepare a sample. With the AFM, little or no sample preparation is required. Figure 1-9 shows the comparison between a TEM image of nano-particles and the AFM image of the same nanoparticles. FIGURE 1-9 AFM image, color, and TEM, grayscale, of 500 nm diameter particles. A line pro le from the AFM image shows the height of the particles. In comparison with an optical microscope and the SEM/TEM an AFM is more di cult to use than the optical microscope and easier to use than the SEM/TEM. Also, the AFM is typically more expensive than the optical microscope and less costly than an SEM/TEM. Figure 1-10 compares the relative time and cost for optical, AFM, and SEM/TEM microscopes. FIGURE 1-10 Comparison of the time for measurements and instrumentation cost of optical, AFM, and SEM/ TEM microscopes. 9 Chapter 1 INTRODUCTION Lastly, an optical microscope requires the least amount of laboratory space, while the SEM/TEM requires the most amount of laboratory space. An AFM is in the middle of these two. Finally, in comparison to an optical pro ler, the AFM is more di cult to use. This is because the optical pro ler does not need any adjustments. However, the AFM requires adjustments of the scan speed and the feedback control parameters. 1.3 Enabling Nanotechnology Approximately 20 years ago scientists and engineers began discussing a technological revolution that would be as dramatic and far-reaching to society as the industrial revolution - the nanotechnology revolution. At rst the primary promoters of the nanotechnology revolution were considered eccentric at best, and a little crazy at worst. However, their ideas and visions are becoming accepted by the mainstream intellectual, scienti c and engineering communities. Recently, governments and major corporations around the world have committed several billion dollars per year for the advancement of nanotechnology and nanoscience research and development. Atoms and Molecules The systematic study, manipulation, and modi cation of atoms and molecules having nanometer-sized dimensions began several hundred years ago. Society has bene ted greatly because chemists can use chemical reactions to combine several types of atoms to create new types of molecules. With the advent of quantum physics, physicists, chemists and biologists routinely studied the spectra of atoms and molecules. Biochemists discovered the usefulness of all types of molecules from proteins to enzymes to DNA several decades ago. Until recently however, working with and controlling atoms and molecules was limited to large quantities of these nanometer-sized objects. Realistically, chemists would modify hundreds of trillions of molecules in a typical chemical reaction. When chemists synthesize new molecules, they make them in large quantities by using macroscopic methods such 10 Chapter 1 INTRODUCTION as heat to initiate chemical reactions. Biologists can identify and create new types of genetic material, but only on a large number of molecules. So what s new? The nanotechnology revolution is being driven by a number of developments, ideas, and technical advancements. The primary driving forces behind the nanotechnology revolution are instruments that measure and manipulate atoms and molecules. The invention of the Scanning Tunneling Microscope permitted us for the rst time to see single atoms on a surface. Before this, using techniques based on electromagnetic radiation, it was possible to view and create images of lattices of many molecules. For example, with x-ray techniques it is possible to recreate the positions of atoms in a complex matrix or lattice. With tunneling electronic microscopes (TEM) it is possible to directly image atoms in a lattice. However, these techniques rely on the scattering of electromagnetic radiation from a collection of atoms, and thus cannot see single atoms. Another important innovation is the laser tweezer . By using the momentum of photons it is possible to isolate in a single location collections of several hundred molecules or atoms. The possibility of isolating a few molecules, or even a few hundred molecules, was not considered possible before this invention . The drive to make smaller computer chips &amp; higher density information storage Moore s law, popularized in the late 20th century, dictates that there is a relationship between the size of electronic devices such as transistors and time. This relationship has been very e ective in predicting advances in the world of microelectronics for almost thirty years. However, physicists are predicting that Moore s law will begin breaking down when the size of electronic devices becomes less than 100 nanometers. There is a great e ort to discover new methodologies for creating electronic devices with dimensions that are less than 100 nanometers. 11 Chapter 1 INTRODUCTION The storage of information is considered an essential advancement of modern civilization. At rst, recording information and ideas on written paper was a great achievement; books and newspapers allowed the ow of knowledge and information throughout the world. Today information is stored digitally and transmitted electronically. Digital bits with dimensions of less than a micron are stored on magnetic disks and compact discs. There is an ever-increasing need to store information in smaller spaces and transmit information with faster methodologies. Emerging belief that it is possible to mimic the mechanisms of biology Researchers in the elds of life sciences discovered over the past few decades that there are many fundamental mechanisms that facilitate the recreation and support of all life forms. At a distance these mechanisms can be characterized as machines or engines. They absorb energy and in a very e cient way cause events to occur. For example, a virus will permeate a cell and then integrate with the genetic material of the cell. Presently, we can observe these activities on a macroscopic scale. In many cases we do not understand how they work or why they work. But there is a belief that we can understand, emulate, and even use these fundamental activities or machines that occur in biological systems. Creation of mechanical devices having nanometer tolerances and motions (MEMS) To a great extent, the industrial revolution occurred because it became possible to shape mechanical objects, and thus create new types of machines. Before the industrial revolution, it was possible to routinely make objects that had dimension on the order of a few hundredths of an inch. An artist could paint pictures; a potter could make dishes and pots. After the industrial revolution, it was possible to routinely make machines that had tolerances of a few thousands of an inch (25 to 100 microns). Of course this led the way to the invention of the steam engine, railroads, the car, and airplane transportation. With MEMS technology it is now possible to use machining technologies to create machines that are less than the size of a human hair. This ability 12 Chapter 1 INTRODUCTION is presently used in the sensors that activate air bags in cars, set the frequency of computers, and allow digital projection. Nanoscience Applying the scienti c method to further understand the behavior of atoms and molecules at the nanometer scale will push forward the frontiers of human knowledge. Currently our vision of the nano-world is based only on evidence that we collect from the macroscopic world that we live in. Presently biologists, chemists, physicists and engineers only have a mental picture of what is occurring at the nanometer scale. In fact, only very recently have they actually seen or directly observed nano-events. As an analogy, suppose you were presented with a gift that was in a box and wrapped with paper. In an e ort to guess what is in the package, you could shake it or maybe drop it. Based on how the package behaves under this interrogation , you may get an idea of what is in it (i.e. is it heavy, does it make a noise?). With the Nano-R evolution, scientists will be able to open the package... and really see what is inside. With new ideas and methods, scientists are beginning to further understand how a single atom or molecule behaves. Even more interesting is the direct understanding of how collections of two or three or even a dozen atoms or molecules behave. Nanotechnology The fundamental knowledge gained through nanoscience and developments in nanotechnology will certainly accelerate over the next several decades. With the control of materials at the nanometer dimension, engineers are already able to create new types of products and services. The smallest transistors we make today in a factory are about 130 nanometers wide. With future nanotechnology advancements engineers will be able to make chips that have transistors that are two or three nanometers wide. Today, cosmetic manufacturers use liposomes with diameters of a few tens of nanometer to reduce the dehydration of skin. We expect the nanotechnology revolution will result in the creation of new types of products and services that will greatly bene t our lives. 13 Chapter 1 INTRODUCTION What is possible? When the ideas and concepts that are discussed as part of the nanotechnology revolution are fully implemented, what is possible? Many of the possible advancements that are discussed today seem like science ction. We can only imagine what is possible. Imagine . . . 1. All of recorded history will t in a package that will t in our pockets. This includes all written documents, all music and all movies. 2. Our world will be safer because computers and sensing systems that t in a package the size of a pill will be able to warn us of dangers. 3. Life will be extended because we can create systems and modules that replicate the functions and systems in our bodies. 4. New types of quantum computers will make calculations billions of times faster than today s digital computers. 5. We can create new types of molecules with the mechanical assembly of chemical systems instead of today s assembly by thermodynamic chemical reactions. What is the <a href="/keyword/atomic-force-microscope/" >atomic force microscope</a> s Contribution to Nanotechnology? Measurement An <a href="/keyword/atomic-force-microscope/" >atomic force microscope</a> (AFM) creates a highly magni ed three dimensional image of a surface. The magni ed image is generated by monitoring the motion of an atomically sharp probe as it is scanned across a surface. With the AFM it is possible to directly view features on a surface having a few nanometer-sized dimensions including single atoms and molecules on a surface. This gives scientists and engineers an ability to directly visualize nanometer-sized objects and to measure the dimensions of the surface features. With an <a href="/keyword/atomic-force-microscope/" >atomic force microscope</a> it is possible to measure more than the physical dimensions of a surface. This is because there is a physical 14 Chapter 1 INTRODUCTION interaction of the probe with a surface. By lightly pushing against a surface with the probe, it is possible to measure how hard the surface is. Also the ease by which the probe glides across a surface is a measure of the surface friction . Modi cation Just as a pen is used for writing on a paper s surface, it is possible to write on a surface with an <a href="/keyword/atomic-force-microscope/" >atomic force microscope</a> . This new type of lithography results in a completely new method for making surface modi cations at the nanometer scale. It is already possible to modify surfaces by physically scratching the surface, by directly depositing molecules on a surface, and by using electric elds to modify surfaces. Presently this use of the AFM is in a very exploratory phase, but showing tremendous promise. One of the important technological issues that must be solved is the writing speed of the AFM lithography systems. Manipulation With an AFM probe it is possible to directly move objects across a surface. The objects may be pushed, rolled around, or even picked up by the probe. With such methods it is possible to create nanometer sized objects. One of the important aspects of using an AFM for direct manipulation is the user interface that is used for generating the motions of the probe. There are interfaces that measure the locations of particles, such as microspheres on a surface, and then automatically move the spheres into a pre-established location. In another type of interface, called the nanomanipulator, the motion of the probe follows the motion of your hand. When you move your hand up and down, the probe moves up and down. Such an interface also allows users to feel and touch a surface. The Nanotechnology Timeline Nanotechnology has had impact on our lives for 20 to 30 years. As early as 1970 there were many products that relied on nanometer sized components to operate, such as in the semiconductor industry. However, the time line for radically new, social structure changing advancements is still many years away. Although it impossible to predict the future, Figure 1-11 illustrates the time frames for the development of new and advanced products from nanotechnology innovations. 15 Chapter 1 INTRODUCTION FIGURE 1-11 Timeline for the integration of nanotechnology and nanoscience into products. In the short term efforts are underway to enhance existing products with nanotechnology concepts. Far in the future new types of products not presently imagined will be created from nanotechnology References 1. Uber Glatte und Ebenheit als physikalisches und physiologishes Problem, Gustev Shmalz, Verein Deutscher Ingenieure, Oct 12, 1929, pp. 1461-1467 2. U.S. Patent 2,728,222 3. UK Patent 2,009,409 4. R. Young, J. Ward, F. Scire, The Topogra ner: An Instrument for Measuring Surface Microtopography, Rev. Sci. Inst., Vol 43, No 7, p 999 5. G. Binnig, H. Rohrer, Ch. Gerber, E. Weibel, Surface Studies by Scanning Tunneling Microscopy, Vol. 49, No 1, 1982, p 57 6. G. Binnig, C.F. Quate, Ch. Geber, <a href="/keyword/atomic-force-microscope/" >atomic force microscope</a> , Phys. Rev. Letters, Vol. 56, No 9, 1986 p 930 7. Y. Martin, C.C. Williams, H.K. Wickramasinghe, <a href="/keyword/atomic-force-microscope/" >atomic force microscope</a> -Force Mapping and Pro ling on a sub 100-E scale. J. Appl. Phys. Vol 61, No 10, 1987, p 4723 16