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Q1-1. How is BEPE series compared with products of other companies?
A1-1. Our BEPE series are series of machines measuring the film
thickness and refractive index in which two new revolutionary features,
2-dimension Ellipsometer (PEL) and multiple thin film ellipsometer
(MFE), is unified in one machine. Conventional ellipsometer, a single
ellipsometer (SEL) only measure one point at one time. On the other
hand, PEL is a machine which has put together the features of 100,000
SELs in parallel. MFE can measure and calculate with multiple parameters
for each point of multilayer film. Conventionally, it was believed
that with single wavelength, only 2 parameters measurement was possible.
However, with MFE algorithm, measurement of each layer of triple-layered
film or simultaneous measurement of optical constants of film and
substrate of monolayer film became possible.
Q1-2. Can you explain the method of calculation to measure the thickness
of film?
A1-2. Calculation of film thickness is done by measuring δψ (changes
in the azimuth angle and the ellipcity, after linearly polarized
incident light is reflected on the sample) of each point, then using
Fresnel Formula with the refractive index as a parameter. The refractive
index is optimized so that the error function which is shown as
imaginary part of film thickness is minimized.
Q1-3. How is your product compared with products of other companies?
A1-3. When using an interferometer for film measurement, the refractive
index is put as a parameter. Therefore, an interferometer is limited
to the measurement for the sort of sample that has a fixed refractive
index to obtain an accurate data. 1000Angstrom or more is the reliable
number for film thickness of Silicon-dioxide (SiO2) on silicon.
With SEL, a reliable simultaneous measurement of film thickness
and refractive index can not be made with less than 200Angstrom.
The refractive index has to be a constant, with film thickness less
than this; otherwise, there will be huge repetitive error in the
measured film thickness. However, the density and refractive index
of oxide film in this range could vary according to the process
parameters. Therefore, with less than 200Angstrom, SEL not only
is unable to measure the refractive index, but film thickness can
not be measured correctly. Probably, the reason for this limit in
SEL is because of the beam size and sample*s roughness. In Ellipsometry,
calculation is made with assumption that the incident light is a
plane wave. Therefore, beam size of 50μm which is about 100 times
of the wavelength is the limit of accurate measurement. Normally,
SEL measures with 1mm resolution. On the other hand, finer resolution
is necessary for thin film because of irregularities on the surface
and influence of barriers. The resolution of BEPE series depends
on the relation of measurement area and the amount of pixels of
the CCD Camera. We guarantee that our BEPE series, which are 2-dimension
Ellipsometers (PEL), can measure refractive index and film thickness
of up to 50Angstrom simultaneously.
Q2-1. The refractive index of a oxide film with 50Angstrom is 1.38
at maximum and 1.35 at minimum according to the result of your measurement.
The difference seems quite large.
A2-1. In fact, the difference of this sample is rather small compared
with others. A thin film thinner than 70Angstrom, in which the structure
is not definite, the dispersion of density in thin film is larger
than that of a thick film. Therefore the difference of the refractive
index is usually larger.
Q2-2. Is it possible to measure etched oxide film which are normally
less than 25Angstrom ?
A2-2. Actually, the film thickness and the refractive index of
samples with 10Angstrom have been measured. But the result was somewhat
different from the measurement by the machines of other companies,
since their machines regard the refractive index fixed and measure
only the film thickness.
Q2-3. What is the limit of thickness for BEPE series?
A2-3. In a case of a film that is too thick for the light to permeate
through, our machine would either be unable to measure it or the
accuracy would drop drastically. The depth of penetration of light
is aboutλ/k. Around 20μis the measuring limit for a silicon.
Q2-4. When a film of poly-silicon is formed on the oxide film, is
it possible to measure each of them?
A2-4. Yes. It is possible to measure multilayer films such as Poly-Silicon/Silicon-dioxide/Poly-Silicon/Quartz
substrate, SOI, AlGaAs/GaAs, ONO, OPO, etc. Please contact us, if
you would like to see the data on the above.
Q2-5. Is it possible to measure a photo-resist film?
A2-5. Yes. Any film thickness which are in practical use can be
measured by our products.
Q3-1. Can your PEL measure just the designated 9 points (1μm×1μm)?
How long would be the measurement time?
A3-1. BEPE series have a great freedom in selecting the method
of measurement and measuring area and point. They can be designated
through the software. The measurement time is same, but the calculation
time would change proportionally depending on the number of measuring
points. The resolution of QR120K is about 100μm. HR and FR have
a resolution of μｍ order. HR can measure 400,000 points of each
9 areas with the resolution of 1μm. You can freely designate the
area and points to be measured from 1 to maximum points.
Q3-2. How would the resulted image be after measuring the same point
repetitively for ten times?
A3-2. We have measured the refractive index and the film thickness
of an SiO2 film of 30Angstrom , using a CCD camera with 40,000 pixel.
The variance of film thickness was ±1.5Angstrom . There is a Cold
CCD for option, which improves the accuracy of repetitive measurement
dramatically.
Q3-3. Would the diffused reflection from the edge of the wafer influence
the result?
A3-3. No. Since our PEL is focused on the entire surface of the
wafer, the diffused reflection would not influence the result. But
you can not obtain an accurate measurement around edges, because
there are slants, etc.
Q3-4. What is done against bends on the sample?
A3-4. Our machine smooth it by absorbing the sample stage with
a vacuum. In case of a sample that has surface to be measured on
both sides like a hard disk, it has to rise from the stage; therefore,
the sample itself must be flat.
Q3-5. Would your PEL be accurate as the spectroscopic ellipsometer
even with the use of a lens?
A3-5 . Our company succeeded in the miniaturization of the device by solving the problems such as the spherical aberrations of the
lens, without losing its accuracy (QR400K).
As a result of these developments, we came to introduce two
dimensional spectroscopic ellipsometer.
Our two dimensional spectroscopic ellipsometer is the device which has solved various problems (such as the purity of the source of light, the wavelength dependency of the refractive index that
current spectroscopic ellipsometer had ) through an original
optical design and the algorithm, and gives physically reasonable solutions to the structure of the sample and the wavelength
dependency of the characteristic.
Q3-6.Why is the wavelength of the semiconductor laser a variant
as a Peltier device?
A3-6. The wavelength is a variant to measure multilayer films.
By drifting the wavelength the structure of film is surmised. Although
the variable range is very narrow, sufficient number of measurement
is possible for the structure of multilayer film because of the
purity of wavelength. Later, multilayer calculation is done using
the MFE algorithm. Stripe observation by measurement of Interferometry
became possible, since BEPE measures as an image. Through controlling
the temperature with Peltier device, the wavelength is more stabilized.
Q3-7. How long are measurement and calculation times?
A3-7. The measurement time is 15-30 seconds per one measurement.The
calculation times for 1 point on a standard thin film are: 15 msec
with 5-parameters (n1, k1, d1, ns, ks) calculation, 4 msec with
2-parameters (n1, d1) calculation, and 0.2msec with 1-parameter
(d1) calculation. (See our specification sheet for further details)
Q3-8. What kind of PEL are there in BEPE series?
A3-8. There are:
(1) QR120K
- With a wafer up to 300mm, it can measure the entire surface
in one time, and in case of a wafer up to 300mm in two times. -
Standard number of points it measures: 120,000 points - Resolution:
100μm to 300μm
(2) HR400K
- IT CAN MEASURE 300mm WAFER. - It can measure a scribeline and
parts in a chip. - Area of measurement: 3×10 mm (at 4μm resolution)
- Standard number of points it measures: 400,000 points - Resolution:
more than 1μm
(3) FR1000K (currently under construction)
- It can measure delicate patterns inside a chip. - Standard number
of points it measures: 1,000,000 points - Resolution: less than
0.2μm - XY scan stage is equipped.
Q3-9. Is it possible to widen the measurement area for HR-type?
A3-9. Yes. It is possible when a camera with a high pixels is used,
but the price would rise.
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