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In-Situ 4000 Process Monitor (Frequently Asked Questions)
What is the smallest substrate size I can use?
What substrates may I use with the In-Situ 4000
Process Monitor?
Can I use the In-Situ 4000 Process Monitor
on transparent substrates?
What is the index of refraction of my film?
What is the reflectance of my substrate?
How should I calibrate the reflectometer to
my chamber?
Should I use one-color or two-color pyrometry?
What is the difference between one-color and
two-color pyrometry?
Will the In-Situ 4000 Process Monitor
work with a heated viewport?
How accurate is the deposition rate measurement?
Can I use the In-Situ 4000 Process Monitor during
substrate rotation?
Will the In-Situ 4000 Process Monitor automatically
control the substrate temperature?
Q: What
is the smallest substrate size I can use?
A: The standard In-Situ 4000 Process Monitor optics will
measure a circular area of the substrate with diameter D/25 where
D is the substrate to pyrometer distance. For example, if your substrate
to pyrometer distance is 38 inches, then the area measured is a
circle of 1.5 inch diameter. SVT Associates also offers an alternate
version of the In-Situ 4000 Process Monitor which measures a smaller
area corresponding to a diameter of D/90, with the consequence of
an upward shift in the temperature measurement range to 500-1200°C.
It is important that the substrate surface completely fill the measured
area so that the measured radiation corresponds solely to the substrate
and not filament heaters, or wafer mounting surfaces.
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Q:What substrates may
I use with the In-Situ 4000 Process Monitor?
A:The In-Situ 4000 Process Monitor performs
single channel pyrometry at 950 nm wavelength, so any substrate
which is opaque at 950 nm will work. We have tested the system with
GaAs, InP and silicon substrates. Silicon is semitransparent at
temperatures below 550°C so some heater radiation may penetrate
the substrate and provide a slightly higher than accurate reading.
Transparent substrates such as sapphire may be used if an opaque
coating is applied to the back side of the substrate, in which case
the temperature measurement is being made on the backside surface
of the wafer. See the related answer to the question of transparent
substrates in this FAQ.
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Q:Can
I use the In-Situ 4000 Process Monitor on transparent substrates?
A:It is possible under limited circumstances to use substrates
which are transparent at 950 nm. The conditions which permit proper
measurement are:
- The back surface of the substrate must be coated
with an opaque material. In this case it is the backside surface coating
which is being measured for radiation rather than the front surface.
- The back surface of the substrate must be left rough or unpolished
(i.e. single sided polish only). This requirement ensures only the
front surface of the substrate contributes to a reflectance measurement.
In theory, a polished backside might work, but the reflectance of
a metal coated backside is often much higher than the front surface
and makes a proper front surface reflectance impractical. In our application
laboratory, we have successfully used the In-Situ 4000 Process Monitor
to measure the temperature and growth rate of GaN films on transparent
sapphire substrates. We used a thin film of a high temperature refractory
metal on the unpolished back side to form an opaque surface for pyrometry.
It is important to note however that use of the backside coating technique
deviates from the factory temperature calibration and that a substrate
temperature re-calibration must be performed by the customer. Fortunately
this is easily performed with the In-Situ 4000 Process Monitor software
provided some sort of in-situ melting point experiment is used. Also,
the rough backside of the substrate can serve as a scattering surface
for stray light in the chamber so that radiation from an effusion
cell or an ion gauge filament may enter the pyrometer and cause an
error in the reading. This is most significant at lower substrate
temperatures where the radiated signal is weak.
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Q:What is
the index of refraction of my film?
A:Here are some published values of index
of refraction for several semiconductor materials:
| Material |
950 nm Index |
470 nm Index |
| Silicon |
3.67 +i0.005 |
4.32 + i0.07 |
| GaAs |
3.54 + 0.08 |
4.33 + i0.44 |
| Ge |
4.65 + i0.3 |
4.3 + i2.4 |
| AlAs |
2.974 + i0.00 |
3.51 + i0.00 |
| GaSb |
4.3 + i0.34 |
4.3 + i2.3 |
| InAs |
3.7 + i0.43 |
4.3 + i1.8 |
| InP |
3.37 + i0.2 |
3.818 + i0.5 |
| InSb |
4.42 + i0.64 |
3.57 + i2.2 |
| GaP |
3.17 +i0.00 |
3.6 + i0.01 |
| GaN |
2.37 + i0.00 |
2.47 + i0.00 |
These values are obtained from the Palik database (Handbook of Optical
Constants of Solids, Edward D. Palik, Academic Press, First edition 1985,
2nd Edition 1991) and the Landolt-Bornstein tables (Semiconductors: Group
IV Elements and III-V compounds, O. Madelung ed., Springer-Verlag, 1991).
The data for GaAs is based on ellipsometry measurements by Aspnes et. al.
(Phys. Rev. B 27, p. 985 (1983).
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Q:What is the
reflectance of my substrate?
A:The In-Situ 4000 Process Monitor reflectometer
needs to be calibrated against a substrate of known reflectance.
Listed below are the relfectances of an assortment of substrate
materials (at normal incidence and at room temperature) from published
values.
| Substrate |
950 nm Reflectance |
470 nm Reflectance |
| Silicon |
0.326 |
0.419 |
| Ge |
0.419 |
0.492 |
| GaAs |
0.313 |
0.438 |
| GaSb |
0.39 |
0.48 |
| InAs |
0.33 |
0.454 |
| InP |
0.305 |
0.349 |
| InSb |
0.41 |
0.45 |
| sapphire |
0.075 |
0.078 |
These values are obtained from the Palik database (Handbook of
Optical Constants of Solids, Edward D. Palik, Academic Press, First
edition 1985, 2nd Edition 1991) and the Landolt-Bornstein tables
(Semiconductors: Group IV Elements and III-V compounds, O. Madelung
ed., Springer-Verlag, 1991). The data for GaAs is based on
ellipsometry measurements by Aspnes et. al. (Phys. Rev. B 27, p.
985 (1983).
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Q:How should I calibrate
the reflectometer to my chamber?
A:Here is a step-by-step reflectance calibration procedure
which should work for most users.
1) Adjust the position and tilt of the optical head and the tilt
of the wafer to achieve an image of the LEDs in the video monitor.
2) Rotate the substrate to change the wafer tilt, or tilt the
instrument slightly such that the LEDs are not visible in the video
monitor. This case represents zero reflectance.
3) Calibrate the reflectometer instrument by right-clicking in
the Reflectance window and select "calibrate low" from the menu.
Enter 0 for both the 470 and 950 nm reflectance values in the "Low"
row. Click the "OK" button to dismiss the dialog box.
4) Restore the LED reflection again by returning the tilt back
to its previous value. If you use substrate rotation, ensure the
substrate is rotating and the "wobble filter" is on when performing
this step. With the LED reflections centered in the video image,
right-click in the Reflectance window and select "calibrate high"
from the pop-up menu. Enter the reflectance values for your substrate
in the "High" row. Click "OK" to dismiss the dialog box. At this
point, the reflectometer is calibrated to your wafer and instrument
position. Do not change the mechanical position after this point
to maintain the calibration you have just performed. Also, if your
substrate holder has a large amount of tilt as it rotates, you may
need to perform this calibration for each new substrate.
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Q:Should I use one-color
or two-color pyrometry?
A:The In-Situ 4000 Process Monitor is designed
to operate using the one-color, or single channel, pyrometry. The
radiated intensity at 950 nm is converted to temperature and is accurate
even when depositing thin films. Some users may have some difficulty
measuring temperature accurately if the viewport window itself becomes
coated with materials during deposition. When this happens, the coating
generally attenuates the radiation and the observed temperature drops.
To address the problem of viewport window coating we have implemented
a technique to re-calibrate the pyrometer using measurements at two
wavelengths (950 nm and 850 nm). For those cases where the viewport
coating is "metallic" as is the case of arsenic in GaAs epitaxy, the
attenuation of light at 950 nm is largely the same as that at 850
nm and so the ratio of the two intensities is unaffected. We have
provided a means of measuring the intensity ratio to easily update
the single color temperature calibration in software so that drifts
in temperature due to viewport coating can be easily accommodated.
The two-color, or ratio, measurement in fact becomes invalid once
thin films are deposited since the thin film interference affects
the two wavelengths differently.
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Q:What is the difference
between one-color and two-color pyrometry?
A:One of the main features of the In-Situ 4000
Process Monitor is the ability to properly measure the temperature
of wafers with growing dielectric films. Other pyrometers cannot do
this.
There are two basic types of pyrometers available: single color and
two color. Single Color Pyrometry:
Single color simply measures the intensity of emitted radiation in
a given wavelength band. That intensity is mathematically converted
to temperature based on Planck's law. The law however requires 2 unknown
parameters: the "geometry factor" and the surface emissivity at that
wavelength. The "geometry factor" is a measure of how much light is
collected by the optics, how big an area on the wafer is measured,
and how sensitive the sensing electronics is in converting the radiation
to a voltage. Typical pyrometer manufacturers will calibrate the "geometry
factor" against a "black body" which is at a known temperature. Black
bodies have an emissivity of 1.0. A cavity or oven will approach an
ideal black body, so black bodies are often constructed as small ovens
with a hole the pyrometer can peer into. The emissivity of any surface
is the fraction of light emitted from that surface with respect to
an ideal black surface. Real wafers will have an emissivity less than
a black body (<1.0), so the user must know what the sample emissivity
is to translate radiation intensity to temperature. All commercial
pyrometers have dials on them which allows the user to set the emissivity
of any real surface, which must be known from some other means.
There are a number of problems in using a standard pyrometer in a
MBE system, some of these are:
1) The wafer emissivity may not be known,
2) The viewport window may be coated, or coat with time so that the
geometry factor will be different than the factory calibration, or
will change over time.
3) The wafer (Si, GaAs, InP, etc) may be partially transparent due
to the fact it has a band gap. It is improper to use a pyrometer where
the wafer is transparent. In your case, if you had no metal on the
backside of the sapphire wafer, the pyrometer would not work. It would
"see" the heat from the filament and measure the filament temperature
instead. Since the Mo on your wafer is opaque, the pyrometer is technically
measuring the back side of the wafer. One then assumes that the temperature
difference between the back side and front side is small.
4) Even if the wafer is opaque, the emissivity may change with temperature
as the wafer band gap shifts (not in your case).
5) Depositing films on the wafer with a different index of refraction
will alter the emissivity due to interference effects. The emissivity
will vary with time as well as the films get thicker. This produces
an apparent temperature change which is false. This effect can be
quite strong when depositing very small gap materials like InSb or
InAs on GaAs.
6) In looking through a viewport, if the user mis-positions the lens
and cuts off some of the light, or the viewport shutter obstructs
some of the light to the pyrometer, then the factory calibration will
be wrong.
7) Pickup of stray light from incandescent heaters will cause a false
reading, especially at lower wafer temperatures.
Given these problems with standard pyrometry for MBE. SVT Associates
intended to try to fix them.
The In-Situ 4000 has two special features which can fix all of the
above problems.
A) Problems 1, 4, and 5 can be compensated if one can MEASURE the
emissivity rather than assume an emissivity. This is what the In-Situ
4000 Process Monitor does, it measures the emissivity in real time
and uses that measurement to calculate the temperature properly. If
one makes 2 assumptions: the wafer is opaque, and the front surface
is optically smooth (i.e. no scattering of light) our reflectance
measurement at 950 nm allows us to calculate the emissivity at 950
nm only. Most MBE systems use very smooth surface wafers so this is
usually ok. Wafer opacity is assured by properly choosing the pyrometry
wavelength, so for example 950 nm was chosen because Si, GaAs and
InP are reasonably opaque at 950 nm at operating temperature. The
calculation is e=1-R where e is the emissivity and R is the specular
reflectance. So in our temperature conversion using Planck's law,
we substitute 1-R for the emissivity. This is why you must align and
calibrate the reflectometer part of the instrument to measure temperature.
This is also why one of the reflectometer wavelengths is 950 nm, to
match the 950 nm pyrometer channel.
B) Problem 3 is again fixed by ensuring the wafer (or the backside
metal) is opaque at the pyrometer wavelength. Problem 7 is fixed if
the wafer is also opaque so that IR light from other sources will
not be reflected into the pyrometer. NOTE: This is not the case in
your system as your wafer is transparent and the rough back side metal
may scatter interference IR light into the pyrometer!!
C) Problem 2 is dealt with by performing a calibration which is insensitive
to window coating effects. The two-color technique provides this (see
below). Thus, the In-Situ 4000 Process Monitor has the capability
to compensate for these listed problems encountered in MBE systems
making a single color pyrometry measurement useful.
Two Color Pyrometry
There are other more conventional pyrometers which
employ two wavelengths to measure temperature rather than one. If
you use a pyrometer which measures emitted intensity at two different
wavelengths and make the assumption that the emissivity is THE SAME
AT BOTH WAVELENGTHS then you may perform a slightly different calculation
than that performed in single color pyrometry. Here, you compute the
ratio of the two intensities. If the emissivity of the wafer is the
same at both wavelengths then the ratio of the intensities is INDEPENDENT
of absolute emissivity. That is an ideal black surface with emissivity
of 1.0 will produce the same RATIO as a surface with emissivity of
0.2. This frees the user from needing to know precisely what the emissivity
is. The resulting ratio is used then to compute temperature. This
technique may be useful when the vacuum viewport window becomes coated
over time. The window transmission will drop as it becomes coated
and the single color measurement will then read lower than it should.
If one assumes that the coating has the SAME transmission at BOTH
measurement wavelengths (good assumption for many metallic coatings),
then the ratio technique is still accurate. Thus the two-color system
is designed to compensate for problems #2 and #6 above.
However, the two color system in the In-Situ 4000 breaks down when
films are deposited on the wafer due to problem #5 above. Since there
is no reflectance measurement at both wavelengths, you cannot use
the two color technique once film deposition has begun. Only the single
color system is valid after film deposition has started.
In our experience, we have found that the ratio measurement is very
dependent on the substrate material. We therefore have provided the
software with the ability to alter the ratio calculation to suit different
substrates. This is the function of the "Temperature LUT" or "look
up table". We have provided a LUT which converts measured ratios to
temperature for a silicon substrate. We measure this for each instrument
on a special, calibrated, silicon wafer.
Combining The Techniques
Thus the complete solution is:
1) Use the two-color system which solves problem 2 and 6 to calibrate
the geometry and window coating effects of the single color system
BEFORE coating has begun. The two-color measurement is only needed
for an instant to provide a reference temperature for the single color
system to follow from that point in time. After calibration of the
single-color system, the ratio system is not needed anymore.
2) After calibration of the single-color system, use the reflectometer
to follow the emissivity changes in the film and compute temperature
based solely on the single color system. As films are deposited, the
single color system will still be correct.
This is the design philosophy of the In-Situ 4000 Process Monitor
system. The software is intended to be flexible enough for users to
use the information however they see fit.
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Q:Will the
In-Situ 4000 work with a heated viewport?
A:In principle, yes the In-Situ 4000 Process
Monitor may be used with a heated viewport. Two problems which may
occur are:
- Often
the heated elements in a heated viewport are smaller than the 1.4"
diameter viewport clearance needed to receive the radiated light from
the substrate. Thus, there may be some light loss in passing through
the viewport and the temperature calibration will be affected. The
user may easily re-calibrate the instrument if the substrate temperature
is known through some other independent means such as a melting point
measurement.
- Some heated viewports run so hot that they emit radiation themselves.
This radiation may enter the In-Situ 4000 Process Monitor optical
head and affect the proper reading of the temperature.
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Q:How
accurate is the deposition rate measurement?
A:The In-Situ 4000 Process Monitor performs
deposition rate measurements by fitting the oscillating reflectance
as a function of time to a mathematical model.
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Q:Can
I use the In-Situ 4000 Process Monitor during substrate rotation?
A:Yes. Most substrate holders which rotate during
deposition have some mechanical errors which cause the reflection
angle to vary as the substrate rotates. This so-called "wobble tilt"
causes a periodic change in reflection angle resulting in an error
in the reflectance measurement and produces a variation in reflectance
synchronized to the substrate rotation motion.
The In-Situ 4000 Process Monitor system contains a special software
sample and hold filter which accommodates even large amounts of wobble
tilt. The reflectance is sampled once per rotation so that the measurement
is correct, even when the wobble tilt is very large.
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Q:Will the In-Situ 4000
Process Monitor automatically control the substrate temperature?
A: The answer is yes.
It can be integrated into your temperature control system through
a software interface. The pyrometer is not a "stand-alone" unit and
must be run under the control of a Windows 9x/2000/XP computer system running
the provided RoboMBE software program. The RoboMBE program will calculate
the substrate temperature based on the substrate radiant intensity
and the measured reflectance. The computed temperature is displayed
on the computer screen, saved to a log file, and optionally available
to another computer through the "Remote Control Interface". Please
refer to our In-Situ 4000 Process
Product Data sheet for additional information.
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Contact SVTA for more
information.
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