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发信人: hfl (凤凰·凤秘·蜂蜜), 信区: NanoST
标  题: 【范文】原子力显微镜(转载)
发信站: 哈工大紫丁香 (2003年12月04日18:15:35 星期四), 站内信件

【以下文字转载自 NewBoard 讨论区】
【原文由 fangzhe 所发表】

The Atomic Force Microscope (AFM ) is being used to solve processing and ma

terials problems in a wide range of technologies affecting the electronics,

telecommunications, biological, chemical, automotive, aerospace, and energy

industries. The materials being investigating include thin and thick film co

atings, ceramics, composites, glasses, synthetic and biological membranes, m

etals, polymers, and semiconductors. The AFM is being applied to studies of

phenomena such as abrasion, adhesion, cleaning, corrosion, etching, friction

, lubrication, plating, and polishing. By using AFM one can not only image t

he surface in atomic resolution but also measure the force at nano-newton sc

ale. The publications related to the AFM are growing speedily since its birt

h.

The first AFM was made by meticulously gluing a tiny shard of diamond onto o

ne end of a tiny strip of gold foil. In the fall of 1985 Gerd Binnig and Chr

istoph Gerber used the cantilever to examine insulating surfaces. A small ho

ok at the end of the cantilever was pressed against the surface while the sa

mple was scanned beneath the tip. The force between tip and sample was measu

red by tracking the deflection of the cantilever. This was done by monitorin

g the tunneling current tot a second tip positioned above the cantilever. Th

ey could delineate lateral features as small as 300 ?. The force microscope

emerged in this way. In fact, without the breakthrough in tip manufacture, t

he AFM probably would have remained a curiosity in many research groups. It

was Albrecht, a fresh graduate student, who fabricated the first silicon mic

rocantilever and measured the atomic structure of boron nitride. Today the t

ip-cantilever assembly typically is microfabricated from Si or Si3N4. The er

a of AFM came finally when the Zurich group released the image of a silicon

(111) 7X7 pattern. The world of surface science knew that a new tool for sur

face microscope was at hand. After several years the microcantilevers have b

een perfected, and the instrument has been embraced by scientists and techno

logists.
The force between the tip and the sample surface is very small, usually less

than 10-9 N. How to monitor such small forces is another story. The detecti

on system does not measure force directly. It senses the deflection of the m

icrocantilever. The detecting systems for monitoring the deflection fall int

o several categories. The first device introduced by Binnig was a tunneling

tip placed above the metallized surface of the cantilever. This is a sensiti

ve system where a change in spacing of 1 ? between tip and cantilever change

s the tunneling current by an order of magnitude. It is straightforward to m

easure deflections smaller than 0.01 ?. Subsequent systems were based on the

optical techniques. The interferometer is the most sensitive of the optical

methods, but it is somewhat more complicated than the beam-bounce method wh

ich was introduced by Meyer and Amer. The beam-bounce method is now widely u

sed as a result of the excellent work by Alexander and colleagues. In this s

ystem an optical beam is reflected from the mirrored surface on the back sid

e of the cantilever onto a position-sensitive photodetector. In this arrange

ment a small deflection of the cantilever will tilt the reflected beam and c

hange the position of beam on the photodetector. A third optical system intr

oduced by Sarid uses the cantilever as one of the mirrors in the cavity of a

diode laser. Motion of the cantilever has a strong effect on the laser outp

ut, and this is exploited as a motion detector. An expanded view of the imag

e at left, and a legend describing its parts is found here.
According to the interaction of the tip and the sample surface, the AFM can

be classified as repulsive or Contact mode and attractive or Noncontact mode

. Now the Tappingmode shows a prosperous future to image the micro-world.
The principles on how the AFM works are very simple. An atomically sharp ti

p is scanned over a surface with feedback mechanisms that enable the piezo-e

lectric scanners to maintain the tip at a constant force (to obtain height i

nformation), or height (to obtain force information) above the sample surfac

e. Tips are typically made from Si3N4 or Si, and extended down from the end

of a cantilever. The nanoscope AFM head employs an optical detection system

in which the tip is attached to the underside of a reflective cantilever. A

diode laser is focused onto the back of a reflective cantilever. As the tip

scans the surface of the sample, moving up and down with the contour of the

surface, the laser beam is deflected off the attached cantilever into a dual

element photodiode. The photodetector measures the difference in light inte

nsities between the upper and lower photodetectors, and then converts to vol

tage. Feedback from the photodiode difference signal, through software contr

ol from the computer, enables the tip to maintain either a constant force or

constant height above the sample. In the constant force mode the piezo-elec

tric transducer monitors real time height deviation. In the constant height

mode the deflection force on the sample is recorded. The latter mode of oper

ation requires calibration parameters of the scanning tip to be inserted in

the sensitivity of the AFM head during force calibration of the microscope.

Some AFM's can accept full 200 mm wafers. The primary purpose of these instr

uments is to quantitatively measure surface roughness with a nominal 5 nm la

teral and 0.01nm vertical resolution on all types of samples. Depending on t

he AFM design, scanners are used to translate either the sample under the ca

ntilever or the cantilever over the sample. By scanning in either way, the l

ocal height of the sample is measured. Three dimensional topographical maps

of the surface are then constructed by plotting the local sample height vers

us horizontal probe tip position.

The concept of resolution in AFM is different from radiation based microsco

pies because AFM imaging is a three dimensional imaging technique. The abili

ty to distinguish two separate points on an image is the standard by which l

ateral resolution is usually defined. There is clearly an important distinct

ion between images resolved by wave optics and scanning probe techniques. Th

e former is limited by diffraction, and later primarily by apical probe geom

etry and sample geometry. Usually the width of a DNA molecule is loosely use

d as a measure of resolution, because it has a known diameter of 2.0 nm in t

he B form. Some of the best values for AFM imaging are 3.0 nm quoted form DN

A in propanol. Unfortunately, this definition of resolution can be misleadin

g because the sample height clearly effects this value.
Indeed, many authors have seen that it is the radius of curvature that signi

ficantly influences the resolving ability of the AFM. Images of DNA made by

the sharper tip have shown dramatic improvements in resolution widths (see t

he two pictures below). Even greater improvements in resolution have been at

tained with Tappingmode but contact imaging still is capable of high resolut

ion imaging. For brief discussion on resolution see the article by Keller in

Physics Today (October, 1995).

In order to obtain good AFM results, the vibration isolation platform is ne

eded. The vibration isolation consists of a large mass attached to bungy cor

ds firmly anchored to the building. (Notice that the resonance frequency onl

y depends on the stretch of the bungy cord). Damping of the oscillation is b

elieved to result from rubbing of the rubber fibres inside of the bungy cord

against the outside lining material. Between the low resonance frequency of

the bungy cord system and the high resonance frequency of the microscope ha

rdware itself (> 10 kHz ), the AFM effectively comprises a band pass filter.

This allows the microscopists to safely image their samples in the intermed

iate range of about 1 - 100 Hz and obtain atomic resolution.

Comparison of AFM and other imaging techniques
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1. AFM versus STM:
It's interesting to compare AFM and its precursor -- Scanning Tunneling Micr

oscope. In some cases, the resolution of STM is better than AFM because of t

he exponential dependence of the tunneling current on distance. The force-di

stance dependence in AFM is much more complex when characteristics such as t

ip shape and contact force are considered. STM is generally applicable only

to conducting samples while AFM is applied to both conductors and insulators

. In terms of versatility, needless to say, the AFM wins. Furthermore, the A

FM offers the advantage that the writing voltage and tip-to-substrate spacin

g can be controlled independently, whereas with STM the two parameters are i

ntegrally linked.
2. AFM versus SEM:
Compared with Scanning Electron Microscope, AFM provides extraordinary topog

raphic contrast direct height measurements and unobscured views of surface f

eatures (no coating is necessary).
3. AFM versus TEM:
Compared with Transmission Electron Microscopes, three dimensional AFM image

s are obtained without expensive sample preparation and yield far more compl

ete information than the two dimensional profiles available from cross-secti

oned samples.
4. AFM versus Optical Microscope:
Compared with Optical Interferometric Microscope (optical profiles), the AFM

provides unambiguous measurement of step heights, independent of reflectivi

ty differences between materials.

Contact Mode
The contact mode where the tip scans the sample in close contact with the su

rface is the common mode used in the force microscope. The force on the tip

is repulsive with a mean value of 10 -9 N. This force is set by pushing the

cantilever against the sample surface with a piezoelectric positioning eleme

nt. In contact mode AFM the deflection of the cantilever is sensed and compa

red in a DC feedback amplifier to some desired value of deflection. If the m

easured deflection is different from the desired value the feedback amplifie

r applies a voltage to the piezo to raise or lower the sample relative to th

e cantilever to restore the desired value of deflection. The voltage that th

e feedback amplifier applies to the piezo is a measure of the height of feat

ures on the sample surface. It is displayed as a function of the lateral pos

ition of the sample. A few instruments operate in UHV but the majority opera

te in ambient atmosphere, or in liquids. Problems with contact mode are caus

ed by excessive tracking forces applied by the probe to the sample. The effe

cts can be reduced by minimizing tracking force of the probe on the sample,

but there are practical limits to the magnitude of the force that can be con

trolled by the user during operation in ambient environments. Under ambient

conditions, sample surfaces are covered by a layer of adsorbed gases consist

ing primarily of water vapor and nitrogen which is 10-30 monolayers thick .

When the probe touches this contaminant layer, a meniscus forms and the cant

ilever is pulled by surface tension toward the sample surface. The magnitude

of the force depends on the details of the probe geometry, but is typically

on the order of 100 nanoNewtons. This meniscus force and other attractive f

orces may be neutralized by operating with the probe and part or all of the

sample totally immersed in liquid. There are many advantages to operate AFM

with the sample and cantilever immersed in a fluid. These advantages include

the elimination of capillary forces, the reduction of Van der Waals' forces

and the ability to study technologically or biologically important processe

s at liquid solid interfaces. However there are also some disadvantages invo

lved in working in liquids. These range from nuisances such as leaks to more

fundamental problems such as sample damage on hydrated and vulnerable biolo

gical samples.
In addition, a large class of samples, including semiconductors and insulato

rs, can trap electrostatic charge(partially dissipated and screened in liqui

d). This charge can contribute to additional substantial attractive forces b

etween the probe and sample. All of these forces combine to define a minimum

normal force that can be controllably applied by the probe to the sample. T

his normal force creates a substantial frictional force as the probe scans o

ver the sample. In practice, it appears that these frictional forces are far

more destructive than the normal force and can damage the sample, dull the

cantilever probe and distort the resulting data. Also many samples such as s

emiconductor wafers can not practically be immersed in liquid. An attempt to

avoid these problem is the Non-contact Mode.
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Non-contact Mode
A new era in imaging was opened when microscopists introduced a system for i

mplementing the non-contact mode which is used in situations where tip conta

ct might alter the sample in subtle ways. In this mode the tip hovers 50 - 1

50 Angstrom above the sample surface. Attractive Van der Waals forces acting

between the tip and the sample are detected, and topographic images are con

structed by scanning the tip above the surface. Unfortunately the attractive

forces from the sample are substantially weaker than the forces used by con

tact mode. Therefore the tip must be given a small oscillation so that AC de

tection methods can be used to detect the small forces between the tip and t

he sample by measuring the change in amplitude, phase, or frequency of the o

scillating cantilever in response to force gradients from the sample. For hi

ghest resolution, it is necessary to measure force gradients from Van der Wa

als forces which may extend only a nanometer from the sample surface. In gen

eral, the fluid contaminant layer is substantially thicker than the range of

the Van der Waals force gradient and therefore, attempts to image the true

surface with non-contact AFM fail as the oscillating probe becomes trapped i

n the fluid layer or hovers beyond the effective range of the forces it atte

mpts to measure.
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Tapping Mode
Tapping mode is a key advance in AFM. This potent technique allows high reso

lution topographic imaging of sample surfaces that are easily damaged, loose

ly hold to their substrate, or difficult to image by other AFM techniques. T

apping mode overcomes problems associated with friction, adhesion, electrost

atic forces, and other difficulties that an plague conventional AFM scanning

methods by alternately placing the tip in contact with the surface to provi

de high resolution and then lifting the tip off the surface to avoid draggin

g the tip across the surface. Tapping mode imaging is implemented in ambient

air by oscillating the cantilever assembly at or near the cantilever's reso

nant frequency using a piezoelectric crystal. The piezo motion causes the ca

ntilever to oscillate with a high amplitude( typically greater than 20nm) wh

en the tip is not in contact with the surface. The oscillating tip is then m

oved toward the surface until it begins to lightly touch, or tap the surface

. During scanning, the vertically oscillating tip alternately contacts the s

urface and lifts off, generally at a frequency of 50,000 to 500,000 cycles p

er second. As the oscillating cantilever begins to intermittently contact th

e surface, the cantilever oscillation is necessarily reduced due to energy l

oss caused by the tip contacting the surface. The reduction in oscillation a

mplitude is used to identify and measure surface features.
During tapping mode operation, the cantilever oscillation amplitude is maint

ained constant by a feedback loop. Selection of the optimal oscillation freq

uency is software-assisted and the force on the sample is automatically set

and maintained at the lowest possible level. When the tip passes over a bump

in the surface, the cantilever has less room to oscillate and the amplitude

of oscillation decreases. Conversely, when the tip passes over a depression

, the cantilever has more room to oscillate and the amplitude increases (app

roaching the maximum free air amplitude). The oscillation amplitude of the t

ip is measured by the detector and input to the NanoScope III controller ele

ctronics. The digital feedback loop then adjusts the tip-sample separation t

o maintain a constant amplitude and force on the sample.
When the tip contacts the surface, the high frequency (50k - 500k Hz) makes

the surfaces stiff (viscoelastic), and the tip-sample adhesion forces is gre

atly reduced. TappingMode inherently prevents the tip from sticking to the s

urface and causing damage during scanning. Unlike contact and non-contact mo

des, when the tip contacts the surface, it has sufficient oscillation amplit

ude to overcome the tip-sample adhesion forces. Also, the surface material i

s not pulled sideways by shear forces since the applied force is always vert

ical. Another advantage of the TappingMode technique is its large, linear op

erating range. This makes the vertical feedback system highly stable, allowi

ng routine reproducible sample measurements.
Tapping mode operation in fluid has the same advantages as in the air or vac

uum. However imaging in a fluid medium tends to damp the cantilever's normal

resonant frequency. In this case, the entire fluid cell can be oscillated t

o drive the cantilever into oscillation. This is different from the tapping

or non-contact operation in air or vacuum where the cantilever itself is osc

illating. When an appropriate frequency is selected (usually in the range of

5,000 to 40,000 cycles per second), the amplitude of the cantilever will de

crease when the tip begins to tap the sample, similar to TappingMode operati

on in air. Alternatively, the very soft cantilevers can be used to get the g

ood results in fluid. The spring constant is typically 0.1 N/m compared to t

he tapping mode in air where the cantilever may be in the range of 1-100 N/m

.
Summary
In contact AFM electrostatic and/or surface tension forces from the adsorbed

gas layer pull the scanning tip toward the surface. It can damage samples a

nd distort image data. Therefore, contact mode imaging is heavily influenced

by frictional and adhesive forces compared to non-contact or tapping mode.

Non-contact imaging generally provides low resolution and can also be hamper

ed by the contaminant layer which can interfere with oscillation.
TappingMode AFM was developed as a method to achieve high resolution without

inducing destructive frictional forces both in air and fluid. With the Tapp

ingMode technique, the very soft and fragile samples can be imaged successfu

lly. Also, incorporated with Phase Imaging, the tapping mode AFM can be used

to analyze the components of the membrane.

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