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In the closing decades of this millennium, our civilization has seen the rise
in scientific and popular interest in tiny entities. We have come to accept
as common place, previously unexpected and seemingly impossible effects that
occur at very short length scales. We have developed new theories that are
rooted in quantum mechanics and built sophisticated tools to model, investigate
and fabricate small, densely-packed devices for faster and more powerful computers,
for storing and retrieving massive amounts of data, for diagnosis and treatment
of diseases, for generation of energy and for providing new insights into
the nature of matter. We have coined the word “nanotechnology”,
which has become a universal term to denote systems that are on the order
of a nanometer - a billionth (10-9) of a meter, comparable to interatomic
spacing in solids.
The
advances in nanotechnology came as a direct result of our ability to ‘observe’
individual atoms. To date, there are only two instruments that possess the
extraordinary resolving power to view structures far smaller than visible
light. These are the transmission electron microscope and the scanning tunneling
microscope. Both use electrons as probes: the former uses electrons that are
accelerated up to several thousand volts that impinge on the specimen, while
the latter relies on low energy electrons (< 10 Volts) that “tunnel”
between the specimen and a very sharp probe. While these instruments earned
their pioneers, Ruska, Binnig and Rohrer, the Nobel prize in physics in 1986
(http://nobelprize.org/nobel_prizes/physics/laureates/1986/),
it is the scanning tunneling microscope (STM) with its elegantly simple principle
that allowed researchers to investigate atoms at relatively lowcost on a tabletop
apparatus. It is a crucial invention that spurred advances in nanotechnology.
The
STM uses an atomically sharp metallic tip and is placed very close to the
surface with about a 1 nanometer gap (Binnig et al. 1982). When a small bias
voltage is applied between the sample and probe, some of the electrons from
the sample manage to cross over to the probe and be detected as current. This
process is known as ‘quantum tunneling’, which does not have an
analog in classical physics. Tunneling is forbidden in the classical theory
of Newtonian mechanics because it implies that an object hitting a wall can
be found on the other side without damage to itself, the wall and without
going above the wall. In quantum theory, this is possible because it offers
the alternative description that physical objects need not be viewed exclusively
as particles with well-defined edges. Instead, objects are described by probability
distributions analogous to clouds that have smeared spatial extent. Thus,
in bringing the probe tip close to the sample, it begins to feel the electron
cloud from the atoms on the sample that extends beyond the gap. In other words,
there is a finite probability that an electron from the sample can be found
across the gap and at the location of the probe. Under this condition, a small
but measurable current flow occurs into the probe tip. The sensitivity to
atomic features arises since the tunneling current depends exponentially on
the separation between the sample and tip. Scanning the tip across the surface
at a fixed height causes the measured current to vary with the concomitant
surface structure. Examples of atomically resolved images are shown in Figure
1 for two different faces of crystalline silicon: Si(100) and Si(111) (Binnig
et al. 1983, Gomez 1990). Notice that the atoms are not in the same configuration
as those in bulk. As happens in most surfaces, the difference in the number
of nearest neighbors between the surface and the bulk causes massive rearrangement
of surface atoms to lower the energy. This is the case with the so-called
‘2x1’ Si(100) and ‘7x7’ Si(111) reconstructions. Si(100)
has a square lattice in the bulk. But due to the truncation of periodicity
at the surface, the atoms in one direction move closer to each other which
form ‘dimer’ rows that appear in the STM images. The 7x7 reconstruction
is even more exotic and it involves the rearrangement of 49 atoms. Metal surfaces
are a little harder to resolve atomically because their electronic probability
clouds are less localized. However, the STM can still provide exquisite details
on how films grow. An example is the atomically registered or epitaxial growth
of Fe on the crystalline surface of MgO (Lee et al. 2006). It forms terraces
resembling ziggurats of ancient Mesopotamia, where each terrace is separated
by a single atomic step. Studies such as these enable VLSI (very large scale
integration) researchers to understand the formation of surface crystallinity,
defects and epitaxy as increasingly smaller devices are fabricated into semiconductor
chips.
From
an implementation point of view, a key feature of the STM is the use of piezoelectric
crystals as highly accurate probe manipulators. Crystals such as lead zirconate
titanate (Pb[ZrxTi1-x]O3 0<x<1) can be formulated to elongate or contract
at a rate of about 0.1 nM/V, so that probe positioners made from these can
be controlled with extraordinary accuracy.
A few years after the STM, it became evident that the same technology can
be used for imaging nonconducting surfaces using a method reminiscent of the
bygone days of vinyl recording. In order to map the topographic features of
the surface, the probe is made out of a cantilever beam with a sharp point
on the free end, which is in contact with the surface. As the probe is scanned
across the surface, the cantilever deflects in accordance with the surface
contour (Binnig et al. 1986). The technique is called atomic force microscopy
(AFM) because the deflection of the cantilever in normal operation is directly
proportional to the force between the surface and the probe tip. The resolution
is typically 10 nm and somewhat less than the STM. However, this is more than
made up for by the versatility of the technique. Apart from the ability of
the AFM to image nonconducting samples, the AFM can be used to detect any
force between the tip and the sample. For example, if the cantilever is electrically
charged then the force is dominated by the electrostatic field from the surface
(Gomez et al. 1998). Similarly if a tiny magnet is placed at the probe tip
and the sample is magnetic, then the image is magnetic in origin. The technique
is called magnetic force microscopy or MFM. Figure 2 shows examples of magnetic
images of nanomagnets comprised of (~100x300x25 nm) cobalt islands (Ganesan
et al. 2000) and various patterns of Ni80Fe20 alloy (Gomez et al. 1999), a
widely used material in magnetic technology. The bright and dark regions on
the cobalt islands are locations of magnetic charges, which we often refer
to as the poles of a magnet, whereas the films of NiFe show interior geometric
shapes known as magnetic domains. When imaging is conducted in the presence
of an external magnetic field, important attributes such as switching and
reversal of domains can be analyzed (Gomez 2001).
Figure
2. A. topography (left) and magnetic structure (right)
of 300x100x15 nm of cobalt grown epitaxy on Si(110) surface. B.
magnetic domain patterns of patterned polycrystalline NiFe alloy, showing the
richness of magnetic phenomena in small structures.
Yet another example of the complex magnetic structure that occur in areas with extreme geometrical confinement such as a nanometer constriction is shown in Figure 3 along with a calculated model of distribution of the magnetic dipoles (Florez et al. 2004). This is a concept device for a new magnetic memory element. The local magnetic structures affect the flow of electrons across the device by virtue of another quantum mechanical entity, the electron spin. In addition to charge, an electron also exhibits angular momentum akin to a spinning top. It could have ‘up’ or ‘down’ spin, which is affected by local magnetic field. Thus, the magnetic images when correlated with electrical measurements provide direct knowledge of the role of the electron spin on electrical properties such as current and resistance. Electrical conduction in polarized materials, i.e. those that have preferentially more spin ‘up’ than spin ‘down’ states, exhibit extraordinary sensitivity to magnetic fields which is further amplified in nanocontacts (Chung et al. 2002). These insights on nanomagnetism allow profound understanding and control of magnetic domains that have enabled today’s high density disk drives, high performance sensors, and magnetic random access memory (Florez et al. 2006).
Figure
3. A test structure showing a micromagnetic model of distribution of local magnetic
dipoles which are consistent with superimposed experimentally derived MFM images.
The long length is 3 microns, the width of the middle segment is 100 nm and
the constrictions are 50. The images show trapped domains on the left constriction
(top image) and on right constriction (bottom image), respectively. This is
a design concept for a magnetic random access memory element.
Finally, the technology for scanned probe microscopy can also be used for surface modification. There are many creative methods that have been successfully implemented, including the patterning of polymer masks in semiconductor fabrication processes, chemical modification and writing of molecular ink using probe technology. One implementation uses the STM probe to produce tiny sparks caused either by the dielectric breakdown of the gap molecules or avalanche field emission. The idea is straightforward. Electrical energy stored in a capacitor is discharged between the probe tip and sample in a highly controlled process. By appropriately tuning the discharge parameters, the energy can be controlled to modify the surface with well defined results. Some examples are shown in Figure 4a. In the left image, 100 nm wide holes are blasted through a 30 nm silicon nitride membrane using high intensity pulses. The right image, on the other hand, was created using low power and longer duration pulses, which formed impressions of the tip shape on the surface. The deliberate pattern (of M and a turtle) is possible because the tip controlled by piezoelectric positioners can be programmed for virtually any configuration. These structures can be useful for applications ranging from DNA sequencing, investigations of ion channels, to nanophotonics.
In conclusion, we are reminded of Richard Feynman’s famous phrase that he uttered about small structures at CalTech in 1959, “There is plenty of room at the bottom.” (http://www.its.caltech.edu/~feynman/plenty.html). Nearly 50 years later and having developed tools to visualize what is at the bottom, we are overwhelmed by its vastness. Indeed, there appears to be a whole gym down there
