of friction we can have F
ps
friction A m
ps ðF a
ps þ F
ps
load Þ where F
ps
load ¼ loading force
and m
ps is the coefficient of friction between the particle and the substrate. The tip
will ‘‘ride’’ smoothly along the surface of the particle when the tangential component of F c on the surface of the particle is larger than the frictional force between
the two, which is determined by F a
tp and m
tp , the coefficient of friction between the
particle and the tip. (The selection of the absorption chemicals is an important
issue because it determines the force of adhesion. It depends on the type of the
substrate and the particle. (For instance, in the manipulation of Au on mica poly-Llysine is used.) To meet condition (1), i.e., to ensure that the particle does not stick
to the tip, we should minimize F a
tp , which for a given g sv happens for a minimum
R t . This can be achieved by a tip of long aspect ratio. When the tip and the particle
have similar size, R eff A R p =2 and F a
tp A F a
ps =2 and the particle is not likely to
adhere to the tip. Condition (2) would imply that there is a minimum contact force
that is needed to move the particle. In order to have a substantial contact force that
can make F
c
k > F
ps
friction . This would imply that one would need a stiff cantilever
(one with a large spring constant typically >10 N m
À1 ) for the manipulation. In
Figure 21.18(a) we show a tentative force diagram for the nanomanipulation.
The scheme for nanomanipulation can be roughly described as below. Generally
a noncontact mode image of the substrate is first taken to mark the particles to be
manipulated and the trajectory of their movement. The noncontact mode ensures
that F
c
k A 0. The cantilever vibrating at an amplitude A is then lowered near the
particle to be moved. At this stage, the feed back loop of the AFM is put to a hold
mode so that the change in vibration amplitude does not make the feedback circuit
adjust the tip–particle distance. Contact with the particle is detected by reduction
in A and an increase in the static deflection. The cantilever carrying the tip is then
moved along the desired trajectory. This movement would require a closed loop
X–Y scanner for the AFM. At the stop position the tip is again pulled out and the
feedback is enabled to start the noncontact mode imaging process again. Sufficient
F a
ps > F a
tp will ensure that the particle is parked at the desired spot as described
before. This can be confirmed by the cantilever oscillating at its previous amplitude
again. A schematic of the manipulation process described above is given in Figure
21.18(b). The description given above of the nanomanipulation process as well as
the important forces is a simplification of the process that occurs, which in reality
is rather involved. It is expected that in the years to come there will be an enhanced
understanding of the basic processes.
21.7
Nano-Electromechanical Systems
In the past 15 years the field of micro-electromechanical systems (MEMS) has progressed tremendously thanks to the innovative utilization of the techniques of microelectronic fabrication. In particular, techniques of lithography using optical and
electron beams and the development of anisotropic etching (both dry and wet) led
to the rapid progress in the field. In recent years there have been new efforts to
21 Nanolithography and Nanomanipulation
718
ps
friction A m
ps ðF a
ps þ F
ps
load Þ where F
ps
load ¼ loading force
and m
ps is the coefficient of friction between the particle and the substrate. The tip
will ‘‘ride’’ smoothly along the surface of the particle when the tangential component of F c on the surface of the particle is larger than the frictional force between
the two, which is determined by F a
tp and m
tp , the coefficient of friction between the
particle and the tip. (The selection of the absorption chemicals is an important
issue because it determines the force of adhesion. It depends on the type of the
substrate and the particle. (For instance, in the manipulation of Au on mica poly-Llysine is used.) To meet condition (1), i.e., to ensure that the particle does not stick
to the tip, we should minimize F a
tp , which for a given g sv happens for a minimum
R t . This can be achieved by a tip of long aspect ratio. When the tip and the particle
have similar size, R eff A R p =2 and F a
tp A F a
ps =2 and the particle is not likely to
adhere to the tip. Condition (2) would imply that there is a minimum contact force
that is needed to move the particle. In order to have a substantial contact force that
can make F
c
k > F
ps
friction . This would imply that one would need a stiff cantilever
(one with a large spring constant typically >10 N m
À1 ) for the manipulation. In
Figure 21.18(a) we show a tentative force diagram for the nanomanipulation.
The scheme for nanomanipulation can be roughly described as below. Generally
a noncontact mode image of the substrate is first taken to mark the particles to be
manipulated and the trajectory of their movement. The noncontact mode ensures
that F
c
k A 0. The cantilever vibrating at an amplitude A is then lowered near the
particle to be moved. At this stage, the feed back loop of the AFM is put to a hold
mode so that the change in vibration amplitude does not make the feedback circuit
adjust the tip–particle distance. Contact with the particle is detected by reduction
in A and an increase in the static deflection. The cantilever carrying the tip is then
moved along the desired trajectory. This movement would require a closed loop
X–Y scanner for the AFM. At the stop position the tip is again pulled out and the
feedback is enabled to start the noncontact mode imaging process again. Sufficient
F a
ps > F a
tp will ensure that the particle is parked at the desired spot as described
before. This can be confirmed by the cantilever oscillating at its previous amplitude
again. A schematic of the manipulation process described above is given in Figure
21.18(b). The description given above of the nanomanipulation process as well as
the important forces is a simplification of the process that occurs, which in reality
is rather involved. It is expected that in the years to come there will be an enhanced
understanding of the basic processes.
21.7
Nano-Electromechanical Systems
In the past 15 years the field of micro-electromechanical systems (MEMS) has progressed tremendously thanks to the innovative utilization of the techniques of microelectronic fabrication. In particular, techniques of lithography using optical and
electron beams and the development of anisotropic etching (both dry and wet) led
to the rapid progress in the field. In recent years there have been new efforts to
21 Nanolithography and Nanomanipulation
718