上海交通大学：《材料导论》教学资源（英文）SEM and AFM：Complementary Techniques for High Resolution Surface Investigations

Veeco The Digital World Instruments Metrology Group Leader In 3D Surface Metrology SEM and AFM:Complementary Techniques for High Resolution Surface Investigations formation mechanisms are quite By There are a wide range of analyti- History cal techniques which may be used different,resulting in different P.Russell, for materials characterization types of information about the The first SEM was constructed in D.Batchelor, surface structure.The occurrence 1938 by von Ardenne by rastering J.Thornton depending on the type of infor- mation needed.For high resolu- of the SEM and AFM side-by-side the electron beam of a Transmis- tion surface investigations,two is becoming more common in sion Electron Microscope(TEM) commonly used techniques are today's analytical laboratories. to essentially form a Scanning Atomic Force Microscopy(AFM) This article will compare and Transmission Electron Microscope and Scanning Electron Micros- contrast the two techniques with (STEM)(1,2).In 1942,Zworkin copy(SEM)-Figures 1 and 2, respect to specific types of surface et.al.developed the first SEM for respectively.Each of these measurements,and demonstrate bulk samples.This configuration techniques resolves surface how these analytical techniques contains many of the basic structure down to the nanometer provide information which is principles of today's SEMs(2,3). scale.However,the image complementary in nature. Cambridge Scientific Instruments Electron gun Feedback Loop Maintains NanoScope llla Controller Gun alignment ooll Electronics Laser 1st condenser lens 2nd condenser lens Objective lens Defiection coll ×,y Scanner Detector Main piping Electronics Split Cantilever Tip Photodiode Detector Sample Oil ditusion pump (DP)Oi ditusion pump in high vacaum mode oil rotary pump for low vacuum mode (RP) Figure 1.Schematic of the major components of an AFM Figure 2.Schematic of the primary components of a showing the feedback loop for TappingModeTM operation. typical SEM

SEM and AFM: Complementary Techniques for High Resolution Surface Investigations The World Leader In 3D Surface Metrology There are a wide range of analyti￾cal techniques which may be used for materials characterization depending on the type of infor￾mation needed. For high resolu￾tion surface investigations, two commonly used techniques are Atomic Force Microscopy (AFM) and Scanning Electron Micros￾copy (SEM)–Figures 1 and 2, respectively. Each of these techniques resolves surface structure down to the nanometer scale. However, the image formation mechanisms are quite different, resulting in different types of information about the surface structure. The occurrence of the SEM and AFM side-by-side is becoming more common in today’s analytical laboratories. This article will compare and contrast the two techniques with respect to specific types of surface measurements, and demonstrate how these analytical techniques provide information which is complementary in nature. History The first SEM was constructed in 1938 by von Ardenne by rastering the electron beam of a Transmis￾sion Electron Microscope (TEM) to essentially form a Scanning Transmission Electron Microscope (STEM) (1, 2). In 1942, Zworkin et. al. developed the first SEM for bulk samples. This configuration contains many of the basic principles of today’s SEMs (2, 3). Cambridge Scientific Instruments Figure 2. Schematic of the primary components of a typical SEM By P. Russell, D. Batchelor, J. Thornton Figure 1. Schematic of the major components of an AFM showing the feedback loop for TappingModeTM operation

produced the first commercial used form of SPM,many other instrument in 1965.A number of SPM techniques have been improvements have occurred since developed which provide informa- this time,resulting in an increase tion on differences in friction, in resolution from 50nm in 1942 adhesion,elasticity,hardness, to-0.7nm today.Besides the electric fields,magnetic fields, development of morphological carrier concentration,temperature imaging,the SEM has been distribution,spreading resistance, developed to detect signals which and conductivity. are used to determine composi- tional information,such as X-rays, backscattered electrons, Imaging Mechanisms cathodoluminesce,Auger elec- trons,and specimen current. Scanning Electron Microscopy ×300 25KV 100UE The operation of the SEM The development of the AFM was consists of applying a voltage preceded by the development of between a conductive sample and Figure 3.SEM image of an integrated single crystal the Scanning Tunneling Micro- filament,resulting in electron silicon cantilever and tip which has an end radius of 2 scope(STM)in 1981 at IBM emission from the filament to the to 10nm.Tips for AFM are typically made of silicon or Zurich Research Laboratory by silicon nitride.Bar=100um. sample.This occurs in a vacuum Binnig and Rohrer(4).Its ability environment ranging from 10to to view the atomic lattice of a 10-10 Torr.The electrons are sample surface earned the inven- guided to the sample by a series of tors the Nobel Prize in Physics in electromagnetic lenses in the 1986.Although the STM pro- electron column.A schematic of a vides subangstrom resolution in all typical SEM is shown in Figure 2. three dimensions,it is limited to The resolution and depth of field conductive and semiconductive of the image are determined by samples.To image insulators as the beam current and the final well as conductors,the Atomic spot size,which are adjusted with Force Microscope(AFM)was one or more condenser lenses and developed in 1986(5),and the the final,probe-forming objective first commercial AFMs were lenses.The lenses are also used to produced in 1989 by Digital shape the beam to minimize the Instruments effects of spherical aberration, chromatic aberration,diffraction, AFM provides three-dimensional and astigmatism. surface topography at nanometer lateral and subangstrom vertical The electrons interact with the Figure 4:TappingMode AFM image of 1.4A resolution on insulators and sample within a few nanometers monoatomic steps on epitaxial silicon deposited on (100)Si.1um scan. conductors.From this beginning, to several microns of the surface, the field of Scanning Probe depending on beam parameters Microscopy(SPM)was born and sample type.Electrons are which consists of a family of emitted from the sample primarily techniques that involves scanning a as either backscattered electrons or sharp tip across the sample surface secondary electrons.Secondary while monitoring the tip-sample electrons are the most common interaction to form a high resolu- signal used for investigations of tion image.Although the AFM surface morphology.They are has become the most commonly produced as a result of interac- 2

2 produced the first commercial instrument in 1965. A number of improvements have occurred since this time, resulting in an increase in resolution from 50nm in 1942 to ~0.7nm today. Besides the development of morphological imaging, the SEM has been developed to detect signals which are used to determine composi￾tional information, such as X-rays, backscattered electrons, cathodoluminesce, Auger elec￾trons, and specimen current. The development of the AFM was preceded by the development of the Scanning Tunneling Micro￾scope (STM) in 1981 at IBM Zurich Research Laboratory by Binnig and Rohrer (4). Its ability to view the atomic lattice of a sample surface earned the inven￾tors the Nobel Prize in Physics in 1986. Although the STM pro￾vides subangstrom resolution in all three dimensions, it is limited to conductive and semiconductive samples. To image insulators as well as conductors, the Atomic Force Microscope (AFM) was developed in 1986 (5), and the first commercial AFMs were produced in 1989 by Digital Instruments. AFM provides three-dimensional surface topography at nanometer lateral and subangstrom vertical resolution on insulators and conductors. From this beginning, the field of Scanning Probe Microscopy (SPM) was born which consists of a family of techniques that involves scanning a sharp tip across the sample surface while monitoring the tip-sample interaction to form a high resolu￾tion image. Although the AFM has become the most commonly used form of SPM, many other SPM techniques have been developed which provide informa￾tion on differences in friction, adhesion, elasticity, hardness, electric fields, magnetic fields, carrier concentration, temperature distribution, spreading resistance, and conductivity. Imaging Mechanisms Scanning Electron Microscopy The operation of the SEM consists of applying a voltage between a conductive sample and filament, resulting in electron emission from the filament to the sample. This occurs in a vacuum environment ranging from 10-4 to 10-10 Torr. The electrons are guided to the sample by a series of electromagnetic lenses in the electron column. A schematic of a typical SEM is shown in Figure 2. The resolution and depth of field of the image are determined by the beam current and the final spot size, which are adjusted with one or more condenser lenses and the final, probe-forming objective lenses. The lenses are also used to shape the beam to minimize the effects of spherical aberration, chromatic aberration, diffraction, and astigmatism. The electrons interact with the sample within a few nanometers to several microns of the surface, depending on beam parameters and sample type. Electrons are emitted from the sample primarily as either backscattered electrons or secondary electrons. Secondary electrons are the most common signal used for investigations of surface morphology. They are produced as a result of interac￾Figure 3. SEM image of an integrated single crystal silicon cantilever and tip which has an end radius of 2 to 10nm. Tips for AFM are typically made of silicon or silicon nitride. Bar=100µm. Figure 4: TappingMode AFM image of 1.4Å monoatomic steps on epitaxial silicon deposited on (100) Si. 1µm scan

tions between the beam electrons conducted by a piezoelectric and weakly bound electrons in the tube scanner which scans the tip conduction band of the sample. in a raster pattern with respect Some energy from the beam to the sample (or scans to the electrons is transferred to the sample with respect to the tip). conduction band electrons in the The tip-sample interaction is sample,providing enough energy monitored by reflecting a laser for their escape from the sample off the back of the cantilever surface as secondary electrons. into a split photodiode detector. Secondary electrons are low By detecting the difference in energy electrons (<50eV),so only the photodetector output those formed within the first few voltages,changes in the cantile- nanometers of the sample surface ver deflection or oscillation NCSU have enough energy to escape and amplitude are determined.A Hit Zero Crossing Stopband Eneest be detected.High energy beam schematic of this can be seen in b electrons which are scattered back Figure 1. out of the sample(backscattered electrons)can also form secondary The two most commonly used electrons when they leave the modes of operation are contact surface.Since these electrons mode AFM and travel farther into the sample than TappingModeTM AFM,which the secondary electrons,they can are conducted in air or liquid emerge from the sample at a much environments.Contact mode larger distance away from the AFM consists of scanning the impact of the incident beam probe across a sample surface which makes their spatial distribu- while monitoring the change in tion larger.Once these electrons cantilever deflection with the escape from the sample surface, split photodiode detector.A Figure 5:(a)SEM image of rugged polysilicon thin they are typically detected by an feedback loop maintains a film.100,000x,Bar=0.1um;(b)TappingMode AFM Everhart-Thornley scintillator- constant cantilever deflection by image of the same with roughness measurement. photomultiplier detector.The vertically moving the scanner to 1um scan. SEM image formed is the result of maintain a constant photodetec- the intensity of the secondary tor difference signal.The electron emission from the sample distance the scanner moves at each x,y data point during the vertically at each x,y data point rastering of the electron beam is stored by the computer to across the surface. form the topographic image of the sample surface.This Atomic Force Microscopy feedback loop maintains a constant force during imaging, AFM consists of scanning a sharp which typically ranges between tip on the end of a flexible 0.1to100nN. cantilever across a sample surface while maintaining a small, TappingMode AFM consists of constant force.An integrated oscillating the cantilever at its silicon tip and cantilever can be resonance frequency(typically seen in Figure 3.The tips -30okHz)and lightly“tapping” typically have an end radius of on the surface during scanning. 2nm to 20nm,depending on tip The laser deflection method is type.The scanning motion is used to detect the root-mean- 3

3 tions between the beam electrons and weakly bound electrons in the conduction band of the sample. Some energy from the beam electrons is transferred to the conduction band electrons in the sample, providing enough energy for their escape from the sample surface as secondary electrons. Secondary electrons are low energy electrons (<50eV), so only those formed within the first few nanometers of the sample surface have enough energy to escape and be detected. High energy beam electrons which are scattered back out of the sample (backscattered electrons) can also form secondary electrons when they leave the surface. Since these electrons travel farther into the sample than the secondary electrons, they can emerge from the sample at a much larger distance away from the impact of the incident beam which makes their spatial distribu￾tion larger. Once these electrons escape from the sample surface, they are typically detected by an Everhart-Thornley scintillator￾photomultiplier detector. The SEM image formed is the result of the intensity of the secondary electron emission from the sample at each x,y data point during the rastering of the electron beam across the surface. Atomic Force Microscopy AFM consists of scanning a sharp tip on the end of a flexible cantilever across a sample surface while maintaining a small, constant force. An integrated silicon tip and cantilever can be seen in Figure 3. The tips typically have an end radius of 2nm to 20nm, depending on tip type. The scanning motion is conducted by a piezoelectric tube scanner which scans the tip in a raster pattern with respect to the sample (or scans to the sample with respect to the tip). The tip-sample interaction is monitored by reflecting a laser off the back of the cantilever into a split photodiode detector. By detecting the difference in the photodetector output voltages, changes in the cantile￾ver deflection or oscillation amplitude are determined. A schematic of this can be seen in Figure 1. The two most commonly used modes of operation are contact mode AFM and TappingModeTM AFM, which are conducted in air or liquid environments. Contact mode AFM consists of scanning the probe across a sample surface while monitoring the change in cantilever deflection with the split photodiode detector. A feedback loop maintains a constant cantilever deflection by vertically moving the scanner to maintain a constant photodetec￾tor difference signal. The distance the scanner moves vertically at each x,y data point is stored by the computer to form the topographic image of the sample surface. This feedback loop maintains a constant force during imaging, which typically ranges between 0.1 to 100nN. TappingMode AFM consists of oscillating the cantilever at its resonance frequency (typically ~300kHz) and lightly “tapping” on the surface during scanning. The laser deflection method is used to detect the root-mean￾Figure 5: (a) SEM image of rugged polysilicon thin film. 100,000x, Bar=0.1µm; (b) TappingMode AFM image of the same with roughness measurement. 1µm scan. (a) (b)

square (RMS)amplitude of will discuss measurements of cantilever oscillation.A feedback different vertical scales of topogra- loop maintains a constant oscilla- phy,beginning with very smooth tion amplitude by moving the surfaces and working up to very scanner vertically at every x,y data rough surfaces to determine how point.Recording this movement the surface topography affects the forms the topographical image. ability of each technique to The advantage of TappingMode perform the measurement. with respect to contact mode is that it eliminates the lateral,shear Atomically Smooth Surfaces forces present in contact mode. This enables TappingMode to Atomically smooth surfaces can image soft,fragile,and adhesive occur either naturally,such as on surfaces without damaging them, mineral surfaces,or by processing, which can be a drawback of such as polishing and epitaxial contact mode AFM. growth on semiconductor,data storage,and optical surfaces.A Comparison of TappingMode AFM image of an Techniques epitaxial silicon surface is shown in Figure 4.Note that,unlike There are a number of different SEM,the AFM can measure in all ways to compare and contrast three dimensions(x,y,and z)with these two techniques with respect a single scan.Since the AFM has to each other.Although investi- a vertical resolution of <0.5A,it gations that use both SEM and can resolve the 1.4A monoatomic AFM to characterize a material are silicon steps on the surface as well (b) common,there are just a few as calculate an RMS roughness of studies that directly discuss the 0.7A (14).On a sample this complementary nature of the smooth,the SEM has difficulty techniques(6-13).A comparison resolving these features due to the of these techniques will be subtle variations in height. conducted with respect to 3 factors:(1)Surface Structure,(2) Thin Films Composition,and(3)Environ- ment.The comparisons are On most thin films,the SEM and presented for typical equipment AFM produce a similar represen- configurations and operating tation of the sample surface.A (c) 5.0 7.5 10.0 procedures. common application of surface Length CUN] investigations of thin films Surface Structure consists of determining changes in Figure 6.(a)SEM image of partially GaP-covered Si morphology with variations of after chemical beam epitaxy deposition for 10 minutes Although both SEM and AFM are deposition parameters,such as 30,000x,Bar=1um;(b)AFM image of the same similar in lateral resolution,there temperature,pressure,time,etc. sample as in figure 6a showing the presence of nodules during the growth of GaP by chemical beam are situations in which one Figure 5 shows SEM and AFM epitaxy.10um scan;(c)Cross-sectional measurement technique can provide a more images of a polysilicon thin film at with AFM across the image in Figure 6b showing 3 complete representation of the approximately the same lateral nodules which have a height of approximately 70nm. sample surface,depending on the magnification.The two images (16) information desired.One show similar surface structure, principle difference is in how the however,they differ in the other two techniques process vertical types of information that can be changes in topography.Below we

4 square (RMS) amplitude of cantilever oscillation. A feedback loop maintains a constant oscilla￾tion amplitude by moving the scanner vertically at every x,y data point. Recording this movement forms the topographical image. The advantage of TappingMode with respect to contact mode is that it eliminates the lateral, shear forces present in contact mode. This enables TappingMode to image soft, fragile, and adhesive surfaces without damaging them, which can be a drawback of contact mode AFM. Comparison of Techniques There are a number of different ways to compare and contrast these two techniques with respect to each other. Although investi￾gations that use both SEM and AFM to characterize a material are common, there are just a few studies that directly discuss the complementary nature of the techniques (6-13). A comparison of these techniques will be conducted with respect to 3 factors: (1) Surface Structure, (2) Composition, and (3) Environ￾ment. The comparisons are presented for typical equipment configurations and operating procedures. Surface Structure Although both SEM and AFM are similar in lateral resolution, there are situations in which one technique can provide a more complete representation of the sample surface, depending on the information desired. One principle difference is in how the two techniques process vertical changes in topography. Below we will discuss measurements of different vertical scales of topogra￾phy, beginning with very smooth surfaces and working up to very rough surfaces to determine how the surface topography affects the ability of each technique to perform the measurement. Atomically Smooth Surfaces Atomically smooth surfaces can occur either naturally, such as on mineral surfaces, or by processing, such as polishing and epitaxial growth on semiconductor, data storage, and optical surfaces. A TappingMode AFM image of an epitaxial silicon surface is shown in Figure 4. Note that, unlike SEM, the AFM can measure in all three dimensions (x, y, and z) with a single scan. Since the AFM has a vertical resolution of <0.5Å, it can resolve the 1.4Å monoatomic silicon steps on the surface as well as calculate an RMS roughness of 0.7Å (14). On a sample this smooth, the SEM has difficulty resolving these features due to the subtle variations in height. Thin Films On most thin films, the SEM and AFM produce a similar represen￾tation of the sample surface. A common application of surface investigations of thin films consists of determining changes in morphology with variations of deposition parameters, such as temperature, pressure, time, etc. Figure 5 shows SEM and AFM images of a polysilicon thin film at approximately the same lateral magnification. The two images show similar surface structure, however, they differ in the other types of information that can be (a) (b) (c) Figure 6. (a) SEM image of partially GaP-covered Si after chemical beam epitaxy deposition for 10 minutes. 30,000x, Bar=1µm; (b) AFM image of the same sample as in figure 6a showing the presence of nodules during the growth of GaP by chemical beam epitaxy. 10µm scan; (c) Cross-sectional measurement with AFM across the image in Figure 6b showing 3 nodules which have a height of approximately 70nm. (16)

acquired on this sample.The tedious and time consuming. etched three-dimensional nature of the Since the aFM data contains the poly AFM can be used to calculate height information,determining changes in roughness and surface whether a feature is a bump or pit area variations due to differences is straightforward.As can be seen in deposition parameters.For the in Figures 6b and 6c,the features SEM,a large area view of the on this sample are bumps.This under- variations in surface structure can information was used in the study cutting oxide be acquired all at once (such as of the growth mechanisms of GaP several mm's),whereas a 100um x on Si during chemical beam 100um area is typically the largest epitaxy deposition(16).Determi- area viewed by an AFM.These nation of whether these features 100nm images are an example of“rugged” were small bumps or depressions (a) polysilicon films which are used as would have changed how the sor Marker Spectrum Zoom Center Line offset Cipar Section Analysis capacitors in memory devices.By deposition process was altered to making these films rough,the produce an epitaxial GaP film. surface area is increased which makes it possible to hold more High Aspect Ratio Structures charge without increasing the lateral dimensions of the capaci- Semiconductor processing tors on the chips.By adjusting commonly requires measurements the deposition parameters and of high aspect ratio structures such using the AFM to analyze the as trenches and via holes.In a surface area of the films,the SEM,these structures are typically deposition parameters needed to measured in cross section by produce a film with the maximum cleaving the wafer and imaging surface area were determined(15). the sample on end to obtain the dimensions of the structure.A Another example of the difference common example of this is seen in Figure 7.(a)Cross-sectional SEM image of polysilicon lines which shows undercutting due to between the two techniques is in Figure 7a.In contrast,the AFM reactive ion etching.Scale bar=100nm;(b)Cross- interpreting subtle differences in image of a trench or via is made sectional measurement of developed and incompletely height.In the SEM image, by scanning over the sample developed vias in photoresist acquired by changes in slope can result in an surface.The ability of the AFM TappingMode AFM.In order to image the high aspect ratio structures on the sample,a silicon tip machined increase in electron emission from to measure these structures with a focused ion beam was used to scan the vias. the sample surface,producing a nondestructively makes it possible 6.2um scan. higher intensity in the image. for the wafer to be returned to the However,it can sometimes be production line after the measure- difficult to determine whether the ment is acquired.An AFM image feature is sloping up or down.For of vias in photoresist is shown in instance,in the SEM image in Figure 7b.To image some higher Figure 6a it is very difficult to aspect ratio structures,the proper determine whether the small tip shape is needed for the AFM round structures are bumps or to scan narrow openings and steep pits,even when tilting the sample sidewalls.Although the SEM stage in the SEM.The only other measurement is destructive to the option would be to cleave the sample,the ability to image the sample through one of these undercuts of these lines is a useful features and look at the sample in application that AFMs are not cross-section,which would be typically designed to perform. 5

5 acquired on this sample. The three-dimensional nature of the AFM can be used to calculate changes in roughness and surface area variations due to differences in deposition parameters. For the SEM, a large area view of the variations in surface structure can be acquired all at once (such as several mm’s), whereas a 100µm x 100µm area is typically the largest area viewed by an AFM. These images are an example of “rugged” polysilicon films which are used as capacitors in memory devices. By making these films rough, the surface area is increased which makes it possible to hold more charge without increasing the lateral dimensions of the capaci￾tors on the chips. By adjusting the deposition parameters and using the AFM to analyze the surface area of the films, the deposition parameters needed to produce a film with the maximum surface area were determined (15). Another example of the difference between the two techniques is in interpreting subtle differences in height. In the SEM image, changes in slope can result in an increase in electron emission from the sample surface, producing a higher intensity in the image. However, it can sometimes be difficult to determine whether the feature is sloping up or down. For instance, in the SEM image in Figure 6a it is very difficult to determine whether the small round structures are bumps or pits, even when tilting the sample stage in the SEM. The only other option would be to cleave the sample through one of these features and look at the sample in cross-section, which would be tedious and time consuming. Since the AFM data contains the height information, determining whether a feature is a bump or pit is straightforward. As can be seen in Figures 6b and 6c, the features on this sample are bumps. This information was used in the study of the growth mechanisms of GaP on Si during chemical beam epitaxy deposition (16). Determi￾nation of whether these features were small bumps or depressions would have changed how the deposition process was altered to produce an epitaxial GaP film. High Aspect Ratio Structures Semiconductor processing commonly requires measurements of high aspect ratio structures such as trenches and via holes. In a SEM, these structures are typically measured in cross section by cleaving the wafer and imaging the sample on end to obtain the dimensions of the structure. A common example of this is seen in Figure 7a. In contrast, the AFM image of a trench or via is made by scanning over the sample surface. The ability of the AFM to measure these structures nondestructively makes it possible for the wafer to be returned to the production line after the measure￾ment is acquired. An AFM image of vias in photoresist is shown in Figure 7b. To image some higher aspect ratio structures, the proper tip shape is needed for the AFM to scan narrow openings and steep sidewalls. Although the SEM measurement is destructive to the sample, the ability to image the undercuts of these lines is a useful application that AFMs are not typically designed to perform. Figure 7. (a) Cross-sectional SEM image of polysilicon lines which shows undercutting due to reactive ion etching. Scale bar=100nm; (b) Cross￾sectional measurement of developed and incompletely developed vias in photoresist acquired by TappingMode AFM. In order to image the high aspect ratio structures on the sample, a silicon tip machined with a focused ion beam was used to scan the vias. 6.2µm scan. (a) (b)

Rough Surfaces give a more complete "picture"of the sample. One of the key advantages of the SEM with respect to other types of microscopy is its large depth of Composition field.This ability makes it Both SEM and SPM provide possible to image very rough compositional information surfaces with millimeters of through a variety of techniques. vertical information within a SEM is the only one of the two single image.A SEM image of techniques which provides non-woven polyethylene oxide elemental analysis,however,both fibers can be seen in Figure 8a. SEM and AFM are associated with The depth of field and small beam techniques which can provide size makes it possible to image the compositional information fibers far below the top layer.This through analyzing materials and ability also makes it possible to physical properties of the sample. measure very rough surfaces over Some of the most common of larger lateral areas as well.Al- these methods are described though the AFM can measure below. vertical surface variations below 0.5A,its ability to measure a tall SEM structure comes from how far the scanner can move vertically. Along with the secondary electron Standard scanners typically have 5 emission which is used to form a (b) NCSU 5ku×18.66815mL81 to 6um of vertical range,however, morphological image of the in some configurations the vertical surface in the SEM,a number of range approaches 10um or larger. other signals are emitted as a result Figure 8.(a)SEM image of a non-woven textile For scanning areas that have of the electron beam impinging sample of polyethylene oxide fibers.The large depth heights of greater than 5 to 10um's on the surface,as shown in Figure of field of the SEM makes it possible to image fibers of variation,the SEM would be 9.Each of these signals carries which are 10's of um's below the upper layer of fibers. better suited for the analysis. Bar=10um;(b)SEM image of Y,O crystal.Bar=1um. information about the sample which provide clues to its compo- Another example of a complex sition three-dimensional surface struc- ture which shows how the SEM Two of the most commonly used and AFM can complement each signals for investigating composi- other can be seen in Figure 8b. tion are x-rays and backscattered The convoluted three dimensional electrons.X-ray signals are Y,O,oxide crystal shown growing commonly used to provide out of a relatively flat Y,O,thin elemental analysis by the attach- film on a Si substrate is easily ment of an Energy-Dispersive imaged in the SEM(Figure 8b). Spectrometer (EDS)or Wave- Although the AFM would have length-Dispersive Spectrometer probelms imaging the obtuse (WDS)to the SEM system.X- angels and enclosed areas of this ray emission results from inelastic surface,the roughness of the Y,O scattering between the beam film can be measured whereas in electrons and the electrons of the the SEM image the surface sample atoms.This interaction roughness is not evident.There- results in the ejection of an inner fore,the two techniques together shell electron from the atom, 6

6 Rough Surfaces One of the key advantages of the SEM with respect to other types of microscopy is its large depth of field. This ability makes it possible to image very rough surfaces with millimeters of vertical information within a single image. A SEM image of non-woven polyethylene oxide fibers can be seen in Figure 8a. The depth of field and small beam size makes it possible to image the fibers far below the top layer. This ability also makes it possible to measure very rough surfaces over larger lateral areas as well. Al￾though the AFM can measure vertical surface variations below 0.5Å, its ability to measure a tall structure comes from how far the scanner can move vertically. Standard scanners typically have 5 to 6µm of vertical range, however, in some configurations the vertical range approaches 10µm or larger. For scanning areas that have heights of greater than 5 to 10µm’s of variation, the SEM would be better suited for the analysis. Another example of a complex three-dimensional surface struc￾ture which shows how the SEM and AFM can complement each other can be seen in Figure 8b. The convoluted three dimensional Y2 O3 oxide crystal shown growing out of a relatively flat Y2 O3 thin film on a Si substrate is easily imaged in the SEM (Figure 8b). Although the AFM would have probelms imaging the obtuse angels and enclosed areas of this surface, the roughness of the Y2 O3 film can be measured whereas in the SEM image the surface roughness is not evident. There￾fore, the two techniques together give a more complete "picture" of the sample. Composition Both SEM and SPM provide compositional information through a variety of techniques. SEM is the only one of the two techniques which provides elemental analysis, however, both SEM and AFM are associated with techniques which can provide compositional information through analyzing materials and physical properties of the sample. Some of the most common of these methods are described below. SEM Along with the secondary electron emission which is used to form a morphological image of the surface in the SEM, a number of other signals are emitted as a result of the electron beam impinging on the surface, as shown in Figure 9. Each of these signals carries information about the sample which provide clues to its compo￾sition. Two of the most commonly used signals for investigating composi￾tion are x-rays and backscattered electrons. X-ray signals are commonly used to provide elemental analysis by the attach￾ment of an Energy-Dispersive Spectrometer (EDS) or Wave￾length-Dispersive Spectrometer (WDS) to the SEM system. X￾ray emission results from inelastic scattering between the beam electrons and the electrons of the sample atoms. This interaction results in the ejection of an inner shell electron from the atom, Figure 8. (a) SEM image of a non-woven textile sample of polyethylene oxide fibers. The large depth of field of the SEM makes it possible to image fibers which are 10’s of µm’s below the upper layer of fibers. Bar=10µm; (b) SEM image of Y2 O3 crystal. Bar=1µm. (a) (b)

creating a vacancy that is filled by electrostatic fields,carrier concen- an outer shell electron.This jump tration,temperature distribution, from an outer to inner shell results spreading resistance,and conduc- e beam in a change in energy that pro- tivity.Many of these techniques duces either a x-ray or Auger consist of looking simultaneously electron.The emitted x-ray has at another signal while performing energy equal to this change.The standard AFM imaging.One of x-rays are then detected by either a the most common techniques for lithium-drifted silicon detector for mapping differences in materials an EDS system,or a gas propor- properties is PhaselmagingTM tional counter detector for a WDS Phaselmaging is conducted during system.A typical x-ray spectrum TappingMode AFM operation by collected with an EDS system is monitoring the phase lag between shown in Figure 10. the oscillating drive signal used to drive the cantilever and the Figure 9.Signals emitted from a sample surface after interaction with an electron beam. Backscattered electrons are the oscillating detection signal from result of beam electrons being the photodiode detector.This scattered back out of the sample. signal will indicate differences in In this case,the incident beam viscoelasticity and/or adhesion electrons undergo a number of across the imaged area.This scattering events within the technique is commonly applied to specimen in which very little mapping the distribution of energy is lost,allowing these polymers in a heterogeneous electrons to go much deeper into system,or mapping the distribu- the sample than secondary tion of filler,such as silica or electrons and still emerge from the carbon black,in a polymer matrix. sample surface to be detected. An example of Phaselmaging on a The percentage of beam electrons polyethylene film is shown in that become backscattered Figure 12.Other ways to get electrons has been found to be similar information are by Force dependent on the atomic number Modulation AFM,which maps Figure 10.EDS X-ray spectrum of an AlGaN thin of the material,which makes it a differences in elasticity across the film on SiC substrate showing the presence of N, useful signal for analyzing the Ga,and Al. sample surface,and Lateral Force material composition.Once these Microscopy(LFM),which maps electrons escape from the surface differences in friction across the they are detected by either the sample surface. Everhart-Thornley detector or a solid state detector.An example There are also techniques that can of a backscattered image of a PbSn be used to investigate long range alloy is shown in Figure 11. forces across the imaged area. Magnetic Force Microscopy AFM/SPM (MFM)and Electric Force Microscopy(EFM)map the Although an AFM does not magnetic and electrostatic field provide elemental analysis,it can gradients,respectively,which supply compositional information extend from the sample surface. by differentiating materials based These techniques are performed Figure 11.Backscattered SEM image of an PbSn on physical properties,such as by using either a magnetic or alloy showing contrast based on the atomic number stiffness,elasticity,compliance, conductive probe to map the of the two components.The brighter areas are Pb- friction,adhesion,magnetic and attractive and repulsive forces rich.5.000x.Scale bar=1um

7 creating a vacancy that is filled by an outer shell electron. This jump from an outer to inner shell results in a change in energy that pro￾duces either a x-ray or Auger electron. The emitted x-ray has energy equal to this change. The x-rays are then detected by either a lithium-drifted silicon detector for an EDS system, or a gas propor￾tional counter detector for a WDS system. A typical x-ray spectrum collected with an EDS system is shown in Figure 10. Backscattered electrons are the result of beam electrons being scattered back out of the sample. In this case, the incident beam electrons undergo a number of scattering events within the specimen in which very little energy is lost, allowing these electrons to go much deeper into the sample than secondary electrons and still emerge from the sample surface to be detected. The percentage of beam electrons that become backscattered electrons has been found to be dependent on the atomic number of the material, which makes it a useful signal for analyzing the material composition. Once these electrons escape from the surface they are detected by either the Everhart-Thornley detector or a solid state detector. An example of a backscattered image of a PbSn alloy is shown in Figure 11. AFM/SPM Although an AFM does not provide elemental analysis, it can supply compositional information by differentiating materials based on physical properties, such as stiffness, elasticity, compliance, friction, adhesion, magnetic and electrostatic fields, carrier concen￾tration, temperature distribution, spreading resistance, and conduc￾tivity. Many of these techniques consist of looking simultaneously at another signal while performing standard AFM imaging. One of the most common techniques for mapping differences in materials properties is PhaseImagingTM. PhaseImaging is conducted during TappingMode AFM operation by monitoring the phase lag between the oscillating drive signal used to drive the cantilever and the oscillating detection signal from the photodiode detector. This signal will indicate differences in viscoelasticity and/or adhesion across the imaged area. This technique is commonly applied to mapping the distribution of polymers in a heterogeneous system, or mapping the distribu￾tion of filler, such as silica or carbon black, in a polymer matrix. An example of PhaseImaging on a polyethylene film is shown in Figure 12. Other ways to get similar information are by Force Modulation AFM, which maps differences in elasticity across the sample surface, and Lateral Force Microscopy (LFM), which maps differences in friction across the sample surface. There are also techniques that can be used to investigate long range forces across the imaged area. Magnetic Force Microscopy (MFM) and Electric Force Microscopy (EFM) map the magnetic and electrostatic field gradients, respectively, which extend from the sample surface. These techniques are performed by using either a magnetic or conductive probe to map the attractive and repulsive forces Figure 9. Signals emitted from a sample surface after interaction with an electron beam. Figure 10. EDS X-ray spectrum of an AlGaN thin film on SiC substrate showing the presence of N, Ga, and Al. Figure 11. Backscattered SEM image of an PbSn alloy showing contrast based on the atomic number of the two components. The brighter areas are Pb￾rich. 5,000x, Scale bar=1µm

between the tip and the sample. hardness from indentations made MFM is commonly used to detect at the same forces,producing the domain structure of magnetic different sized indents.Scratching bits written on magnetic media,to and wear testing may also be evaluate the performance of conducted with this configuration magnetic heads,and to investigate to investigate adhesion and the magnetic structure of experi- delamination of films under a mental materials.This is con- small applied force. ducted by a routine called LiftModeTM in which a Tapping- Figure 12.Phase image of two components which are Environment used to form a polyethelene (PE)film.The phase Mode topographic image and a image(right)clearly shows the distribution of the two magnetic image are acquired over One of the primary differences polymers due to differences in stiffness which is not the same area.LiftMode consists between these two types of evident from the topographic image (left).2um scan. of first collecting a line scan in microscopy is the environment in TappingMode of the surface which they are performed,i.e., morphology.The tip is then lifted SEM is conducted in a vacuum above the surface and a second environment,and AFM is scan is made over the same line conducted in an ambient or fluid using the saved topographic scan environment.There are several to maintain a constant tip-sample issues which make environment separation.The long-range an important issue.First,there is magnetic forces shift the reso- a frequent need in fields such as nance frequency of the oscillating biology and biomaterials to study cantilever,which is detected to hydrated samples.These two produce the magnetic image.An techniques compensate for this example of bits written on a need by different means:an textured hard disk is shown in environmental chamber for a Figure 13.Magnetic Force Microscopy(MFM)image of overwritten tracks on a textured hard disk.The Figure 13. SEM,and a fluid cell for the topography(left)was imaged using TappingMode;the AFM.Second,the SEM is magnetic force image of the same area(right)was Although the AFM is applied so required to work in a vacuum captured with LiftMode(lift height 35 nm)by mapping that it is nondestructive to the environment due to the nature of shifts in cantilever resonant frequency.25um scan. sample surface,it can be used to the technique which brings up the (17) study differences in mechanical issues of vacuum compatibility of properties by performing the sample,the conductivity of nanoindention to investigate the surface,and vacuum mainte- hardness differences between nance.To image poorly conduc- materials.This technique uses a tive surfaces without sample diamond tip mounted on a stiff, charging may require conductive stainless steel cantilever.A coatings or staining,which may TappingMode AFM image is alter or obscure the features of collected with the probe to interest;or it may require low determine the area of interest for voltage operation,or an environ- indentation,the nanoindention is mental chamber,which may then made at a specified force,and sacrifice resolution. an image is then collected of the indented area.An example of For SEM,hydrated samples are comparing the difference between addressed by placing a specimen diamond like carbon films on a in an environmental chamber with hard disk is shown in Figure 14. either an electron transparent In this example,the two films window or a small aperture for the demonstrate a difference in beam to enter the chamber.The 8

8 between the tip and the sample. MFM is commonly used to detect the domain structure of magnetic bits written on magnetic media, to evaluate the performance of magnetic heads, and to investigate the magnetic structure of experi￾mental materials. This is con￾ducted by a routine called LiftModeTM in which a Tapping￾Mode topographic image and a magnetic image are acquired over the same area. LiftMode consists of first collecting a line scan in TappingMode of the surface morphology. The tip is then lifted above the surface and a second scan is made over the same line using the saved topographic scan to maintain a constant tip-sample separation. The long-range magnetic forces shift the reso￾nance frequency of the oscillating cantilever, which is detected to produce the magnetic image. An example of bits written on a textured hard disk is shown in Figure 13. Although the AFM is applied so that it is nondestructive to the sample surface, it can be used to study differences in mechanical properties by performing nanoindention to investigate hardness differences between materials. This technique uses a diamond tip mounted on a stiff, stainless steel cantilever. A TappingMode AFM image is collected with the probe to determine the area of interest for indentation, the nanoindention is then made at a specified force, and an image is then collected of the indented area. An example of comparing the difference between diamond like carbon films on a hard disk is shown in Figure 14. In this example, the two films demonstrate a difference in Figure 12. Phase image of two components which are used to form a polyethelene (PE) film. The phase image (right) clearly shows the distribution of the two polymers due to differences in stiffness which is not evident from the topographic image (left). 2µm scan. Figure 13. Magnetic Force Microscopy (MFM) image of overwritten tracks on a textured hard disk. The topography (left) was imaged using TappingMode; the magnetic force image of the same area (right) was captured with LiftMode (lift height 35 nm) by mapping shifts in cantilever resonant frequency. 25µm scan. (17) hardness from indentations made at the same forces, producing different sized indents. Scratching and wear testing may also be conducted with this configuration to investigate adhesion and delamination of films under a small applied force. Environment One of the primary differences between these two types of microscopy is the environment in which they are performed, i.e., SEM is conducted in a vacuum environment, and AFM is conducted in an ambient or fluid environment. There are several issues which make environment an important issue. First, there is a frequent need in fields such as biology and biomaterials to study hydrated samples. These two techniques compensate for this need by different means: an environmental chamber for a SEM, and a fluid cell for the AFM. Second, the SEM is required to work in a vacuum environment due to the nature of the technique which brings up the issues of vacuum compatibility of the sample, the conductivity of the surface, and vacuum mainte￾nance. To image poorly conduc￾tive surfaces without sample charging may require conductive coatings or staining, which may alter or obscure the features of interest; or it may require low voltage operation, or an environ￾mental chamber, which may sacrifice resolution. For SEM, hydrated samples are addressed by placing a specimen in an environmental chamber with either an electron transparent window or a small aperture for the beam to enter the chamber. The

chamber is typically flushed with accessories,AFM can also be used an inert gas saturated with water in varied gaseous environments vapor.Common applications are and at elevated temperature.The to either investigate hydrated latter is particularly important for surfaces to preserve their surface research and development of structure when hydrated,or to polymers. reduce charging on insulating samples.An example of imaging of a pesticide film on skin can be Further Discussion seen in Figure 15.For the One thing to keep in mind when Figure 14:Indentations on two different diamond-like electron beam to interact with the comparing these two techniques is carbon thin films using three different forces(23,34, surface in this configuration,it that although SEM and AFM and 45uN)with four indents made at each force to must go though an environment appear very different,they actually compare differences in hardness.500nm scans. of gas and water vapor.One share a number of similarities. drawback of this configuration is Both techniques raster a probe that it will result in an increase in across the surface to detect some scattering of the electron beam on interaction with the surface to the way to and from the surface, form an image.Both have a which may result in the sacrifice lateral resolution which is similar of image quality and resolution. in scale(although under certain conditions AFM is superior).And One of the primary attractions to both techniques have image the AFM is its ability to image artifacts that the operator is insulating surfaces at high resolu- trained to identify.The SEM has tion in fluid.Imaging samples in had a much longer time to mature a hydrated state with an AFM is as a technique and to develop an commonly performed by enclos- understanding of how to identify ing the sample and probe in a and avoid artifacts,but the rapid fluid environment,as shown in adoption and implementation of Figure 16.Since AFM is not AFM has resulted in a similar based on conductivity,the image understanding of artifacts.This and scanning mechanism is not article has avoided discussing such Figure 15.Environmental SEM image of a pesticide film on skin.A hydrated environment was needed in disturbed by the presence of the artifacts unless they are relevant to order to maintain the integrity of the pesticide layer fluid.Common applications for the comparison.Furthermore,by and to reduce charging.Bar=1mm. AFM investigations in fluid are in using two techniques which are the biological sciences,biomateri- complementary,one technique als,crystal growth,force interac- will often compensate for the tion studies,and for investigating imaging artifact of the other processes in situ (Figures 17,18). technique. The resolution of the image will be determined by the radius of the However,one should be wary of tip,the applied force,and the combined systems in which an noise floor of the instrument. AFM is placed inside the SEM Because of these factors,this chamber.One of the true configuration allows the study of advantages of the AFM is its hydrated specimens at a lateral ability to perform high resolution resolution of I to 5nm and a measurements outside of a vertical resolution down to 0.5A vacuum environment.Placing it without sample damage,as seen in inside a vacuum environment the image of the GroES chaperon reduces its flexibility and increases (Figure 17).With the appropriate its operating time.Combined 9

9 chamber is typically flushed with an inert gas saturated with water vapor. Common applications are to either investigate hydrated surfaces to preserve their surface structure when hydrated, or to reduce charging on insulating samples. An example of imaging of a pesticide film on skin can be seen in Figure 15. For the electron beam to interact with the surface in this configuration, it must go though an environment of gas and water vapor. One drawback of this configuration is that it will result in an increase in scattering of the electron beam on the way to and from the surface, which may result in the sacrifice of image quality and resolution. One of the primary attractions to the AFM is its ability to image insulating surfaces at high resolu￾tion in fluid. Imaging samples in a hydrated state with an AFM is commonly performed by enclos￾ing the sample and probe in a fluid environment, as shown in Figure 16. Since AFM is not based on conductivity, the image and scanning mechanism is not disturbed by the presence of the fluid. Common applications for AFM investigations in fluid are in the biological sciences, biomateri￾als, crystal growth, force interac￾tion studies, and for investigating processes in situ (Figures 17, 18). The resolution of the image will be determined by the radius of the tip, the applied force, and the noise floor of the instrument. Because of these factors, this configuration allows the study of hydrated specimens at a lateral resolution of 1 to 5nm and a vertical resolution down to 0.5Å without sample damage, as seen in the image of the GroES chaperon (Figure 17). With the appropriate Figure 14: Indentations on two different diamond-like carbon thin films using three different forces (23, 34, and 45µN) with four indents made at each force to compare differences in hardness. 500nm scans. accessories, AFM can also be used in varied gaseous environments and at elevated temperature. The latter is particularly important for research and development of polymers. Further Discussion One thing to keep in mind when comparing these two techniques is that although SEM and AFM appear very different, they actually share a number of similarities. Both techniques raster a probe across the surface to detect some interaction with the surface to form an image. Both have a lateral resolution which is similar in scale (although under certain conditions AFM is superior). And both techniques have image artifacts that the operator is trained to identify. The SEM has had a much longer time to mature as a technique and to develop an understanding of how to identify and avoid artifacts, but the rapid adoption and implementation of AFM has resulted in a similar understanding of artifacts. This article has avoided discussing such artifacts unless they are relevant to the comparison. Furthermore, by using two techniques which are complementary, one technique will often compensate for the imaging artifact of the other technique. However, one should be wary of combined systems in which an AFM is placed inside the SEM chamber. One of the true advantages of the AFM is its ability to perform high resolution measurements outside of a vacuum environment. Placing it inside a vacuum environment reduces its flexibility and increases its operating time. Combined Figure 15. Environmental SEM image of a pesticide film on skin. A hydrated environment was needed in order to maintain the integrity of the pesticide layer and to reduce charging. Bar=1mm

Laser Diode AFM/SEM systems often have References Position Sensitive reduced capabilities and typically Detectoz 1.Van Ardenne,M.(1938)Z. compromise the performance of Phys.109,407 Glass Cell both instruments. Cantilever/Tip 2.Goldstein,J.I.,Newbury, Summary D.E.,Echlin,P.,Joy,D.C.,Fiori, Flow-thrt SEM and AFM are complemen- C.,Lifshin,E.,Scanning Electron Fluid- Chamber tary techniques that provide a Microscopy and X-ray Mi- O-ring Seal more complete representation of a croanalysis,1981,Plenum surface when used together than if Publishing Corp.,New York,p.3. Cells or Molecules each were the only technique available.These techniques 3.Zworykin,V.K.,Hillier,J., Specimen overlap in their capabilities to Snyder,R.L.,(1942),ASTM provide nanometer scale lateral Bulletin 117,p.15. information.However,they Figure 16.Fluid cell for an AFM which allows imaging deviate in the fact that the afm 4.Binning G,Roher H,Gerber in an enclosed.liguid environment. can provide measurements in all C,Weibel E,"Surface Studies by three dimensions,including Scanning Tunneling Microscopy, height information with a vertical Phs.Reu.Let.49(1982)57. resolution of <0.5A,whereas the SEM has the ability to image very 5.Binnig G B,Quate C F,and rough samples due to its large Gerber Ch.,"Atomic Force depth of field and large lateral Microscope,"Phys,Rev.Lett.,12 field of view. (1986930 The SEM can provide elemental 6.Neves,B.R.A.,Salmon,M.E., analysis using X-ray detection, Russell,P.E.,Troughton,E.B. whereas the AFM can provide "Comparitive Study of Field compositional information based Emission-Scanning Electron on physical properties.The fact Microscopy and Atomic Force that the two techniques operate in Microscopy to Access Self- different environments can be a Assembled Monolayer Coverage strength when used together since on Any Type of Substrate," the AFM does not encounter Microscopy and Microanalysis 5 (1999)413. Figure 17.Image of two GroES molecules positioned vacuum issues(difficult sample side-by-side in physiologic fluid,demonstrating 10A preparation,sample modification, etc.)and may image samples in an 7.Castle,I.E.,Zhdan,P.A., lateral resolution and 1A vertical resolution.The entire molecule measures 84A across,and a distinct enclosed fluid or other environ- "Characterization of Surface 45A heptameric "crown"structure protrudes 8A above ment.The vacuum environment Topography by SEM and SFM: the remaining GroES surface and surrounds a central of the SEM makes it possible to Problems and Solutions,"J.Phys. depression.18nm scan.Image courtesy of Z.Shao. D:App Phys,.30(1997）722. University of Virginia.(18) conduct a number of techniques that require vacuum,such as X-ray analysis.By having both tech- 8.Lemoine,P.,Lamberton,R.W., niques side-by-side in an analytical Ogwu,A.A.,"Complementary facility,the overall scope of Analysis Techniques for the analytical capabilities is broad- Morphological Study of Ultrathin ened,adding to the flexibility of Amorphous Carbon Films,". the facility. App.Phs,86(1999)6564. 10

10 Figure 16. Fluid cell for an AFM which allows imaging in an enclosed, liquid environment. Figure 17. Image of two GroES molecules positioned side-by-side in physiologic fluid, demonstrating 10Å lateral resolution and 1Å vertical resolution. The entire molecule measures 84Å across, and a distinct 45Å heptameric “crown” structure protrudes 8Å above the remaining GroES surface and surrounds a central depression. 18nm scan. Image courtesy of Z. Shao, University of Virginia. (18) AFM/SEM systems often have reduced capabilities and typically compromise the performance of both instruments. Summary SEM and AFM are complemen￾tary techniques that provide a more complete representation of a surface when used together than if each were the only technique available. These techniques overlap in their capabilities to provide nanometer scale lateral information. However, they deviate in the fact that the AFM can provide measurements in all three dimensions, including height information with a vertical resolution of <0.5Å, whereas the SEM has the ability to image very rough samples due to its large depth of field and large lateral field of view. The SEM can provide elemental analysis using X-ray detection, whereas the AFM can provide compositional information based on physical properties. The fact that the two techniques operate in different environments can be a strength when used together since the AFM does not encounter vacuum issues (difficult sample preparation, sample modification, etc.) and may image samples in an enclosed fluid or other environ￾ment. The vacuum environment of the SEM makes it possible to conduct a number of techniques that require vacuum, such as X-ray analysis. By having both tech￾niques side-by-side in an analytical facility, the overall scope of analytical capabilities is broad￾ened, adding to the flexibility of the facility. References 1. Van Ardenne, M. (1938) Z. Phys. 109, 407 2. Goldstein, J. I., Newbury, D.E., Echlin, P., Joy, D.C., Fiori, C., Lifshin, E., Scanning Electron Microscopy and X-ray Mi￾croanalysis, 1981, Plenum Publishing Corp., New York, p. 3. 3. Zworykin, V.K., Hillier, J., Snyder, R.L., (1942), ASTM Bulletin 117, p. 15. 4. Binning G, Roher H, Gerber C, Weibel E, “Surface Studies by Scanning Tunneling Microscopy,” Phys. Rev. Lett. 49 (1982) 57. 5. Binnig G B, Quate C F, and Gerber Ch., “Atomic Force Microscope,” Phys, Rev. Lett., 12 (1986) 930 6. Neves, B.R.A., Salmon, M.E., Russell, P.E., Troughton, E.B. “Comparitive Study of Field Emission-Scanning Electron Microscopy and Atomic Force Microscopy to Access Self￾Assembled Monolayer Coverage on Any Type of Substrate,” Microscopy and Microanalysis 5 (1999) 413. 7. Castle, J.E., Zhdan, P.A., “Characterization of Surface Topography by SEM and SFM: Problems and Solutions,” J. Phys. D: App Phys, 30 (1997) 722. 8. Lemoine, P., Lamberton, R.W., Ogwu, A.A., “Complementary Analysis Techniques for the Morphological Study of Ultrathin Amorphous Carbon Films,” J. App. Phys., 86 (1999) 6564. Laser Diode Position Sensitive Detector Oscillating Cantilever/Tip Flow-thru Fluid Chamber Cells or Molecules Glass Cell Sample or Specimen Support O-ring Seal