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)