| Focused Ion Beam
Principles
Focused ion beam devices scan sample surfaces
with finely focused ion beams, and by detecting
generated secondary ions, enable observation of
microscope images and process sample surfaces.
Focused ion beam devices has similar structure and functions as scanning
electron microscopes. We will first introduce the principles of scanning
microscopes. Then, we will introduce the features of focused ion beam
devices, focusing on the major distinctions between scanning electron
microscopes and focused ion beam devices.
Scanning Microscope Overview
Light generated at a light source is converted
to a beam by condensing it through aperture and
condenser lens (CL). Then, the focus is set on
the sample surface using the objective lens (OL).
Figure 1 - Conceptual Diagram
The beam that is condensed and focused on the
sample surface scans the sample surface using a
deflector. Then, the detector detects the secondary
signal generated from the sample surface through
beam irradiation, and the data corresponding to
the secondary signal is saved on image data memory
corresponding to the beam irradiation location
coordinate. By displaying the data saved on the
image data memory on the computer monitor, we can
observe the microscope image of the area irradiated
with the bam.
Figure 2 - Scanning Microscope
Scanning Microscope and Focused Ion Beam Device
Light Source
On a focused ion beam device, the generating
source of the beam is known as the light source,
as with an optical microscope. It is referred to
as an electron gun on a scanning electron microscope
and an ion gun or ion source on a focused ion beam
device.
An electric field is applied between the sharp, metallic tip and the
pulling electrode and the charged particles are pulled out. The charged
particle is accelerated using voltage from acceleration power supply
and collides with the sample.
With the case of ions, the sharp metallic tip is supplied with liquid
metallic gallium and irradiates using power from the electric field.
Generally, about +5kV to +30kV acceleration voltage is used.
The higher the acceleration voltage, the finer we can focus the beam.
However, that also increases the damage to the sample and based on experience,
30kV is used generally for processing observation.
Figure 3 Ion Source Conceptual Diagram
Optics System
The charged particles generated from the light
source are condensed using the magnetic field or
the electric field, then focused and scanned on
the sample surface.
Electron microscopes are generally controlled using the magnetic field
generated by the electromagnet. The current circuit controlling the electromagnet
is not easily affected by external noise and since there is no need to
create an electromagnet in vacuum, high performance can be achieved at
a relatively low cost.
On the other hand, focused ion beams are not controlled using a magnetic
field. In a magnetic field, the path is altered by the mass of the charged
particle. Electric field is used with ion beams, as ions of different
weight are present because of isotopes so they cannot be condensed using
a magnetic field.
When using an electric field, high voltage must impressed on the electrode
placed in the vacuum, and we must make sure that they do not discharge
when the high voltage is impressed. Also, a voltage circuit is more easily
affected by noise than a current circuit so noise solution is necessary.
And performance is determined by the electrode's mechanical positioning.
Seiko Instruments has addressed these issues in order to bring focused
ion beam devices to the market.
Electron Beam & Focused Ion Beam
Samples are irradiated with electron beams in
scanning electron microscopes and ion beams from
gallium ions are irradiated in focused ion beam
devices.
When electron beams are irradiated on sample surface, secondary ions
are generated. This occurs when the electrons excited by the energy of
the irradiated electrons fall from its trajectory. In addition, as the
electron beams' acceleration voltage increases, different characteristic
x-rays are generated depending on the material.
Generated amounts of secondary ions differ depending on the surface shape
and material. By finding the two-dimensional distribution of the generated
amount, the microscope image of the sample surface can be observed.
By analyzing the wavelength distribution (spectrum) of the characteristic
x-ray, the atoms that form the sample surface can be identified. Also,
by finding the two-dimensional distribution, the material distribution
can be observed.
Figure 4 - Electron Beam Irradiation
When ion beam is irradiated on the sample surface,
as with electron beam, secondary electrons are
generated. Also, because gallium ions are significantly
heavier than electrons, a sputtering phenomenon
occurs, where atoms that form the sample are forced
out. The atoms that are forced out exit the sample
as secondary ions.
As with electron beams, by finding the two-dimensional distribution of
the secondary electrons, microscope image of the sample surface can be
observed.
Also, by detecting the secondary ions with the detector and finding the
two-dimensional distribution, microscope image of the sample surface
can be observed.
Figure 5 - Ion Beam Irradiation
Figure 6 - Secondary Electron Image Comparison
The relationship between the spatial resolution
and material dependency of the secondary signal
is generally as shown below.
Figure 7 - Characteristics Comparison
Processing With Focused Ion Beam
As a feature distinct from electron beams, samples
can be processed with ion beam irradiation.
Etching
Previously, we introduced the sputtering phenomenon
that occurs due to ion beam irradiation. By increasing
the ion beam amount which would increase the sputtered
atom amount, etching can be performed on the sample
surface.
Maskless processing, which selectively etches only parts where ion beam
is irradiated, is made possible.
Using this technique, etching can be performed on pre-determined spots
of the sample, enabling cross-section processing observation and TEM
sample creation processing, which removes set points of the sample as
slices.
Figure 8 - 64M-DRAM Cross-Section Processing Observation
Figure 9 - TEM sample Fabrication
Also, by spraying compound gas on the sample
surface near the ion beam irradiation area, deposition
can be performed locally. Conceptual diagram of
beam-assisted deposition is shown in Figure 10.
Secondary electrons are generated when primary ions are irradiated. Secondary
ions contribute to the resolution of compound gas and compound gas splits
into gas and solid parts. The gas part is evacuated in vacuum, but the
solid part builds up on the sample surface. From this, maskless deposition
can be performed selectively on ion beam irradiation areas.
At Seiko Instruments, in order to efficiently supply the compound gas,
the deposition ingredient, efficiently, we have developed and produced
a compound gas supply device. iJP1,866,266j
In the SIM series, following deposition products
are available:
- Carbon C
- Tungsten W
- Platinum Pt
- Silicon Oxide SiOx
Deposition can also be similarly performed with
electron beam irradiation. Seiko Instruments offers
electron beam-assisted deposition as an optional
attachment for SEM. However, while electron beam
deposition can regulate the damage to the sample
because there is no sputtering effect with beam
irradiation, the formation speed of the deposition
film, or the deposition rate, appears to be low.
We hope that you will perform deposition with these
characteristics in mind.
Figure 10 - Beam-Assisted Deposition Conceptual Diagram
Figure 11 shows examples letters and a three-dimensional
wall created with carbon deposition.
Figure 11 - Carbon Deposition Examples
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