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BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
cantilever, and more particularly to a cantilever of an atomic force microscope.
2. Description of the Related Art An atomic force microscope (AFM), which is a type of scanning
microscope, forms a secondary observation image of a surface by the force acting between
substances. AFM is expected to have a wide range of applications because it can observe material
surfaces and organic molecules without electrical conductivity on a nanometer scale (Yamada,
Applied Physics, Vol. 59, No. 2, P191-192).
FIG. 2 shows a conceptual view of the principle of the conventional AFM. The AFM consists of a
cantilever consisting of a needle tip with a small tip radius of curvature and a flexible plate, and a
displacement detection system that measures the deflection (flexure) of the flexible plate. When
the needle tip at the tip of the cantilever is brought close to the sample (about 10 nm), the
electrostatic plate, magnetism and van der Waals force act between the sample atoms and the
needle tip, and the flexible plate is bent. The measurement is performed by detecting the
displacement amount of this deflection by a displacement detection system.
And, by scanning the sample, two-dimensional information of the force on the sample surface can
be obtained. In addition, the microscopic shape of the surface can be known by scanning the
sample while controlling the position of the sample so as to make the deflection of the flexible
plate constant. For example, Japanese Patent Application Laid-Open No. 3-218998 describes a
cantilever consisting of a silicon substrate and a sharp silicon chip integrated therewith, or a
cantilever consisting of a silicon nitride substrate and a sharp silicon chip.
In addition to the above-described cantilever, Japanese Patent Laid-Open No. 1-262403
describes a cantilever having a structure in which a plate made of silicon is rotated. As a
displacement detection system for detecting the displacement caused by the force received from
the sample atoms of these cantilevers, a tunnel detection system, an optical wave interference
system, and an optical lever system have been used.
In either method, since the movement of the needle tip is measured by the relative displacement
between the cantilever and the displacement detection system, the displacement reading error
becomes large if the displacement detection system is not fixed to the cantilever. Therefore,
conventionally, the cantilever and the displacement detection system are fixed, and the sample is
scanned. However, in this method, it is necessary to process the sample thinly and smallly
because the mechanical properties deteriorate as the sample becomes large. Therefore, when
observing a large sample, it is desirable to scan the cantilever, but in the conventional AFM, as
described above, it is necessary to scan the cantilever and the heavy displacement detection
system integrally, which escapes deterioration of the characteristics. It was not.
Also, AFM has a field of view of ˜100 μm, and in that range takes one frame several minutes,
and does not have a smooth finder function like an optical microscope or an electron microscope.
Also, the inability to align the cantilever and the sample with a large optical microscope from the
top with a large optical detection microscope from the top because of the large displacement
detection system also worsened the operability of the AFM.
Therefore, attempts have been made to integrate the displacement detection system and the
cantilever. For example, a piezoresistive method in which a piezoresistive portion is formed in a
silicon cantilever and displacement is sensed by the resistance change of this (International
Conference on Solid-State Sensors and Actuators 1991, preliminary report, PP 448 to 451), a
micro-optical system is fabricated on the cantilever There is an optical interference method
(Japanese Patent Application Laid-Open No. 2-196209) which measures light wave interference
due to a change in optical path length and senses displacement.
However, in the method of integrating the displacement detection system and the cantilever as
described above, the material of the cantilever itself is used as the material constituting the
displacement detection system or a part of the material of the cantilever There is a problem in
that the material for the cantilever is limited by the detection method, because For example, in
the piezoresistive method, silicon can only be used as a material to obtain piezoresistance, and in
the light interference method, an optical material such as silicon nitride or silicate glass must be
used. Furthermore, when such a material is used, the cantilever often has a double or triple
complex structure in which it is combined with other materials.
In this way, the limitation of the material of the cantilever leads to the complexity of the device
structure and many limitations. Furthermore, it is considered that production of a cantilever
involves a complicated manufacturing process and is difficult, and even if it can be actually
manufactured, problems may be left in production cost and the like. In view of the problems of
the prior art, the present invention aims to provide a cantilever made of various materials such as
silicon dioxide and metals.
SUMMARY OF THE INVENTION The present inventors have conceived that miniaturization can
be achieved by using a thin film displacement sensor as a displacement detection system. And, by
providing and integrating this thin film type displacement sensor on a cantilever, it is found that
even a large sample can be measured by scanning the cantilever and that the material of the
cantilever is not limited and the element configuration is not complicated, the present invention
Came to
Therefore, the present invention provides a cantilever comprising a flexible plate and a needlelike tip fixed near its tip, wherein a thin film displacement comprising a lower electrode, an
intermediate layer having piezoelectric or electrostrictive properties and an upper electrode on
the plate. It provides a cantilever characterized in that a sensor is attached.
In the present invention, a thin film displacement sensor comprising a lower electrode, an
intermediate layer having piezoelectric or electrostrictive properties, and an upper electrode is
formed on a flexible plate, and is used as a cantilever for sensing polarization charge due to
strain. There is a feature.
In the present invention, a thin film displacement sensor is provided on a cantilever used for AFM
or scanning tunneling microscope (STM). However, the thin film displacement sensor of the
present invention is not limited to other minute sensors such as micromachines. The system can
also be used as a displacement detection system such as a displacement sensor or a strain sensor.
FIG. 1 shows one aspect of the configuration of the cantilever of the present invention, and the
operation of the present invention will be described. The cantilever bends when the probe
receives repulsion or attraction from the sample atom. A thin film made of a material with
piezoelectric or electrostrictive properties is formed with the upper and lower electrodes on the
surface of the base of the plate that generates the maximum stress in the flexible plate, and
receives the maximum displacement as the probe moves. It will be. In FIG. 1, the thin film
displacement sensor made of a piezoelectric / electrostrictive material is formed on the upper
surface of the flexible plate, but the same effect can be obtained by forming it on the lower
surface. When a piezoelectric material is used as the intermediate layer, since it is in the form of
a thin film, it can be easily poled using the upper and lower electrodes, and the polarization
charge becomes intermediate layer along with the bending of the flexible plate in the direction
perpendicular to the poling direction. (The phenomenon indicated by the piezoelectric constant
d31). Polarization occurs in the same manner when an electrostrictive material is used as the
intermediate layer. The amount of displacement of the probe (needle tip) can be known by
measuring this polarization charge through the electrode.
In order to make the most of the ability of such a displacement sensor, it is desirable to use a
material having a high piezoelectric constant or electrostriction constant, and a titanate
zirconate-lanthanum oxide solid solution, lead niobate magnesium titanate-titanic acid Lead solid
solution is a representative material of each. Furthermore, barium titanate, which is a
piezoelectric material, is also excellent as a material of the middle layer of the displacement
sensor. In addition, since the lower electrode is required to withstand the heat treatment at the
time of thin film formation, and to be firmly bonded to the cantilever material, etc., platinum is
used as a main material and titanium or It is desirable to use tantalum with platinum. Such a
small thin film displacement sensor can be formed on the plate regardless of the material of the
flexible plate.
EXAMPLE 1 A flexible plate is produced by using a silicon dioxide film and an intermediate layer
of a thin film displacement sensor with lead zirconate titanate as shown in FIG. [Manufacture of
Cantilever] A cantilever made of silicon dioxide can be produced by a conventional method using
microfabrication and anisotropic etching of Si (for example, T. R. Albrecht and C. F. Quate: J. Appl.
Phys 62, 2599). [Fabrication of thin film displacement sensor] On the above-mentioned
cantilever, in a vacuum chamber, at a normal temperature and an Ar-oxygen mixed gas (9: 1)
pressure of [email protected] Torr, first, about 50 .ANG. A tantalum buffer layer is formed, and a
platinum electrode layer of about 5000 Å is formed under the same conditions using platinum as
a target. Next, an intermediate piezoelectric layer of 1 μm is formed on the platinum electrode
by a sputtering method using a sintered body of lead zirconate titanate as a target at an oxygen
gas pressure of 10 -4 Torr or less in a vacuum chamber. Finally, a gold thin film to be an upper
electrode is formed by vapor deposition under an oxygen gas pressure of 10 −5 Torr or less.
Thereafter, a resist film is formed on the gold thin film, and exposure and development are
performed by an exposure device to remove the resist film other than the sensor formation
portion. Subsequently, etching is performed with argon gas under conditions of an acceleration
voltage of 500 V, an incident angle of 0 °, and an ion current density of 1 mA / cm 2 to remove
the upper and lower electrodes and the intermediate piezoelectric layer. Finally, the remaining
resist film is removed to obtain a cantilever provided with the thin film displacement sensor
shown in FIG. The lead wire is not shown in FIG. 1, but is made in the usual manner using a metal
such as aluminum.
The shape of the cantilever is 100 μm in length (l 1), 20 μm in width (w 1) and 2 μm in
thickness (t 1). Assuming that the shape of the piezoelectric thin film is 10 μm in length (l 2), 20
μm in width (w 2), 1 μm in thickness (t 2) and the displacement (d) of the probe tip is 1 nm, the
stress (σ) generated in the ferroelectric thin film portion As an approximation,
Given by
The polarization charge shown by
Here, E is the Young's modulus of the silicon dioxide film formed on silicon, and d31 is the
piezoelectric constant in the direction of transverse vibration indicated by 31 of lead zirconate
titanate. Although this polarization charge changes depending on the material constants E and
d31, a potential difference of about 1 mV is applied between both electrodes of the ferroelectric
thin film. The amount of displacement can be obtained by performing normal signal processing
used for sonar or the like for sound wave diagnosis with respect to this induced voltage.
As shown in FIG. 1, the thin film displacement sensor of the present invention has a very simple
structure, and the sensor portion itself does not have to be a cantilever or a part of a cantilever,
so the oxidation shown in this embodiment is Not only a flexible plate made of silicon thin film
but also a flexible plate made of various materials can be used as a displacement sensor.
As described above, according to the present invention, a displacement sensor integrated with a
cantilever made of any material can be provided.
In particular, when the present invention is applied to an atomic force microscope, since the
space above the force detection cantilever is released from the light source and the light
detection unit, the alignment between the force detection cantilever and the object to be
measured becomes extremely easy. Improves the observation of the object under measurement
by an optical microscope.