The temperature control and calibration issues encountered in the growth, processing, and characterization of electronic materials are summarized. The primary problem area is identified as temperature control during epitaxial materials growth. While qualitative thermal measurements are feasible and reproducibility is often achievable within a given system, absolute calibration is essentially impossible in many cases, precluding the possibility of portability from one system to another. The procedures utilized for thermal measurements during epitaxial growth are described, and their limitations discussed

Wilson, B. A.

English

NASA Technical Reports Server

N90-17914
Temperature Control and Calibration Issues in the Growth,
Processing and Characterization of Electronic Materials
B.A.Wilson,
Jet Propulsion Laboratory, California Institute of Technology
This paper summarizes the temperature control and calibration issues
encountered in the growth, processing and characterization of electronic
materials in the Microdevices Laboratory of JPL's Center for Space
Microelectronics Technology. The primary problem area is identified as
temperature control during epitaxial materials growth. While qualitative
thermal measurements are feasible and reproducibility is often achievable
within a given system, absolute calibration is essentially impossible in
many cases, precluding the possibility of portability from one system to
another. The procedures utilized in the MDL for thermal measurements
during epitaxial growth are described, and their limitations discussed.
INTRODUCTION
The charter of JPL's Microdevices Laboratory (MDL) is the
development of electronic materials and devices for space applications.
The MDL, a 38,000 square foot facility opened in late 1988, contains class
100,000 area for materials growth and characterization, class 100 and 1,000
areas for deposition and device processing, and class 10 areas for e-beam
and photolithography. The facilities also include a variety of deposition,
processing and characterization capabilities, as detailed in Table I. In the
area of semiconductor materials and device development, the main
program thrusts include strained-layer superlattices, silicides and
amorphous silicon, medium to far infrared silicon-compatible detectors,
photonic devices, and neural network systems. There are also strong
programs in conventional and high-temperature superconducting
materials, SIS mixers, superconducting electronics, and miniature electron-
tunneling based sensors. Low-temperature STM capabilities, and a new
technique developed in the MDL, ballistic electron emission microscopy,
which enables microscopic imaging of the electronic properties of
subsurface interfaces, enhance these programs.
Among the wide variety of deposition, processing and
characterization methods utilized in the MDL, the primary area where
temperature control remains an unsolved problem is in the epitaxial
growth of electronic materials. Three important epitaxial growth
techniques, molecular beam epitaxy (MBE), metal-organic chemical vapor
deposition (MOCVD), and laser-assisted chemical vapor deposition (LACVD)
have been selected for the focus of this paper. While thermal control is
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critical for the reproducible fabrication of high-quality films by any of
these techniques, in each case the growth environments interfere with
accurate temperature measurements of the growth surface. For each
deposition technique, I discuss the thermal control requirements, the
growth environment, the methods currently utilized for temperature
measurement and control, and the limitations of these methods.
MOLECULAR BEAM EPITAXY (MBE)
MBE places the most demanding requirements on thermal control of
any of the epitaxial growth techniques. Reproducible growth of high-
quality films requires reproducible sample temperatures controlled to
within 1 K. Sample homogeneity requires even better uniformity across
the growth surface, which is typically 1 to 3 inches in diameter. This is
because the mobility of the deposited atoms is thermally activated, and
thus depends exponentially on the local ambient temperature. Thus small
variations in temperature across the growth surface can result in large
changes in surface atom kinetics, causing significant sample
inhomogeneity. MBE growth is carried out in ultra-high vacuum, typically
~ 10 -11 Torr, and temperatures of 400 - 600 °C for the growth of III-V
materials, and room temperature and up for group IV samples. Growth
occurs when a beam of the desired atoms or molecules impinges on a
substrate. For growth homogeneity the sample mount is often rotated
during deposition, making good thermal contact difficult or impossible. In
addition, the thermal capacity, and surface emissivity evolve as deposition
occurs, so that the sample temperature can vary greatly even when a fixed
heater current is provided. In addition, the deposition of material within
the chamber is not totally limited to the substrate surface, and any
surfaces in the vicinity of the growth surface will eventually become
coated with sample material.
An approximate measurement of the temperature in the growth
chamber is normally obtained using a thermocouple. In order to escape
contamination during sample growth, the thermocouple must be placed
under the sample mount where it is shielded from the beams of
atoms/molecules which are impinging on the growth surface. If the
sample mount is stationary during growth, the thermocouple can be
inserted in a well carved out at the back of the mount for better thermal
contact. However, if the sample mount is being rotated, which is
frequently done for better deposition uniformity, then the thermocouple
must be placed below the mount, and cannot be effectively heat sunk to
provide an accurate measure of the sample mount temperature. Even
when an accurate measure of the sample mount is obtained, there can also
be large thermal gradients between the back of the sample mount and the
sample surface, so an estimate of the temperature at the back surface may
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not provide a realistic measurement of the growth surface temperature.
Since the discrepancy depends on the instantaneous sample emissivity, it
can be strongly dependent on the evolving (and usually unknown) sample
characteristics during growth, and consequently cannot be effectively
calibrated.
Another method of calibrating the growth temperature takes
advantage of an easily observable eutectic critical point. For example, the
sharp transition at N 570 °C for the interdiffusion of aluminum on silicon,
which can be identified by a large change in reflectivity, is often used to
determine the control settings required to achieve this temperature in a
given growth chamber. Such calibration measurements are normally
carried out in separate runs with no gas flow, rather than during an actual
sample deposition, and are especially useful in establishing consistent
growth temperatures within a given system over long periods of time.
This method offers the advantage of measuring the temperature at the
front surface of the substrate where growth actually occurs, rather than at
the back of the substrate holder, and thus avoids problems associated with
thermal gradients in the sample mount and substrate. Inherently, it also
provides a solid thermal contact with the substrate. However, since the
surface temperature of a growing sample depends on the instantaneous
surface emissivity, this calibration cannot provide an absolute measure of
the growth surface temperature for an arbitrary sample, and as mentioned
earlier, fixed heater current may not be equivalent to a fixed sample
temperature if the surface emissivity changes during growth.
Standard IR pyrometry, in which one measures the difference in
brightness of the sample emission at two IR wavelengths in order to
determine the equivalent black-body temperature, offers an alternative
approach. Since it is inherently a contactless measurement, it bypasses the
problems associated with poor heat sinking. In principle it can also be
used during the deposition process, offering the possibility of active
feedback for temperature control during growth. However, this method
suffers from its own set of problems. First, the emission flux from the
sample is often rather low, because the deposition is carried out at
relatively low temperatures. Coupled with the small solid angle for optical
access available in typical MBE systems, this weak emission can be difficult
to observe with enough sensitivity for an accurate determination of the
temperature. In addition, since the samples are usually rather imperfect
black-body emitters, a two-point determination may not provide enough
information for an accurate determination of the temperature. However,
the measurement of the entire spectrum is not usually practical given the
low flux and the changing emissivity of the sample surface. Finally, in
most cases the sample and substrate materials are transparent to the IR
emission, so that the desired signal from the sample surface may be
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completely overwhelmed by the emission from the sample mount and
heater elements.
Finally, an additional contactless optical method, photoreflectance,
offers many advantages. In this technique, the optical gap of the deposited
material is determined from the phororeflectivity spectrum, which in turn
is used to determine the temperature from previously determined data on
the band gap of the material as a function of temperature. This technique
samples only the material within the optical absorption length of the
sample surface, and for heteroepitaxy can even be used to determine the
temperature of the growing layer without interfering signals from the
substrate. The measurement can be made using light of relatively high
intensity travelling in an optical fiber within the deposition chamber,
offering more readily observable optical signals than available with IR
emission pyrometry. However, a wide enough range of frequencies must
be scanned to permit a fit to the complex derivative spectra. Since the
temperature dependence of semiconductors is typically on the order of 0.1
to 1.0 meV per degree Kelvin, this method will not provide accurate
temperature readings if the spectrum is greatly broadened by crystal
imperfections or by thermal effects at high growth temperatures. In
addition, this method is not useful for the deposition of alloys, because the
band gap depends not only on the temperature, but also on the precise
alloy composition, which cannot be determined independently. The effects
of confinement on the electron states will also tend to cause calibration
difficulties for thin-layer structures. Of course this method also fails for
the deposition of gapless materials such as metal overlayers. Nevertheless,
it offers distinct advantages over the other methods described, and is
likely to become more widespread in the future.
METAL-ORGANIC CHEMICAL VAPOR DEPOSITION (MOCVD)
As with MBE deposition, temperature control is critical for the
growth of high-quality films by MOCVD. The most stringent requirements
are actually for the source temperatures (+ 0.1 K), but fortunately these
demands are relatively easily met, since the sources can be immersed in
constant-temperature baths. In reality, control of the substrate
temperature to a desired uncertainty of + 2 K is considerably harder to
achieve. The vacuum used for MOCVD deposition is not as extreme as for
MBE, typically only in the range 0.1 to 1 atmosphere, and the growth
temperatures range from 600 - 750 °C for III-V materials, and 350 450
°C for II-VI growth. The primary additional constraint on techniques for
thermal control in MOCVD growth is the presence of toxic, corrosive and
almost opaque vapors in the growth chamber during deposition. Their
presence requires that any in situ sensor be encapsulated for protection
213
from the environment, and precludes any optical measurement during
growth. In addition, the relatively high density of these gases in the
vicinity of the sample surface results in significant convective cooling of
the growing surface, as well as continuous deposition on all nearby
exposed surfaces.
The two primary techniques used in the MDL for temperature
measurements in MOCVD growth are the thermocouple and IR pyrometry
approaches described in the previous section on MBE growth. The
corrosive gas environment requires that the thermocouple be
encapsulated, normally in a glass tube, which further degrades the thermal
link between the sensor and sample. The thermocouple must also be
placed behind or inside the sample mount to keep the device away from
the area of heavy materials deposition. IR pyrometry capabilities are also
limited by the MOCVD growth environment. The relatively opaque gases
present in the chamber during growth preclude the use of this technique
during actual growth cycles. IR pyrometry can only be used to
precalibrate the control settings before growth is commenced, and the
additional convective cooling which occurs under growt h conditions limits
the accuracy of such pregrowth calibrations.
LASER-ASSISTED CHEMICAL VAPOR DEPOSITION (LACVD)
Since LACVD growth is not a heater-driven process, temperature
control of the sample environment is less critical than for the previously-
described growth techniques, and + 25 K is considered acceptable. The
growth environment is basically the same as for MOCVD, with the
exception of the ambient vacuum, which is typically held at ~ 10 -6 Torr for
LACVD growth, intermediate to that of MOCVD and MBE. LACVD is an
ultra-low temperature growth technique, typically utilizing temperatures
close to room temperature. Growth is stimulated at these low ambient
temperatures by pulsed laser excitation of the surface.
The only technique currently employed for monitoring the
temperature during LACVD growth within the MDL is thermocouple based.
The IR emission signals are too small for the IR pyrometry approach due to
the low growth temperatures. As with MBE growth, the thermocouple is
placed inside the sample mount, rather than at the growth surface where
material deposition is occurring. Unfortunately, this does not provide a
good measure of the temperature at the laser-stimulated growth surface.
The discrepancy has been measured directly using a second thermocouple
at the surface with all conditions identical to those during growth, but
without initiating the gas flow, and temperature gradients as large as 100
K have been observed between the back of the sample mount and the
substrate surface. In addition this discrepancy depends on the ambient
temperature and sample parameters, and thus cannot easily be calibrated
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and corrected for. Finally, the laser pulses may cause transient local
excursions in the temperature which are not reflected by the thermocouple
measurement which has a rather slow response. The ability to measure
these transient surface effects would not only allow growth temperature
specifications which would be portable from one system to another, but
would also be useful in elucidating the underlying mechanisms responsible
for growth by this technique. Unfortunately, there is currently no method
capable of obtaining such measurements.
SUMMARY
This manuscript has summarized the temperature measurement and
control issues important in the growth, processing and characterization of
electronic materias in JPL's MDL. Although accurate thermal control is
required for a variety of processes, the major obstacles remain in the area
of materials growth. In this paper these issues have been examined in
detail for MBE, MOCVD and LACVD growth. In all three cases, the existing
temperature measurement technologies are marginally adequate. In
general it is possible to reproduce consistent growth conditions for the
growth of a particular type of sample in a given growth chamber.
However, sample-to-sample differences due to effects such as surface
emissivity (MBE), convective cooling (MOCVD) and laser heating effects
(LACVD) cause unknown variations in the temperature at the growth
surface during the deposition of an arbitrary new material or structure.
The inability to measure the temperature of the growth surface itself
precludes the ability to compare the actual growth temperature among
different growth chambers. As a result, portability of precise growth
conditions is unfortunately still beyond existing capabilities, and the
optimal growth parameters must be determined empirically for each
growth chamber and for each new material or structure.
The author acknowledges helpful discussions with F. Grunthaner, C. Lewis,
P. Grunthaner, G. Radhakrishnan, P. Maker and B. Lewis. The work
described in this paper was carried out by the Jet Propulsion Laboratory
(JPL), California Institute of Technology, and was sponsored by the
Strategic Defense Initiative Organization, Innovative Science and
Technology Office, the National Aeronautics and Space Administration, and
the Defense Advanced Research Projects Agency. The work was performed
as part of JPL's Center for Space Microelectronics Technology.
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TABLE I. CAPABILITIES OF THE MICRODEVICES LABORATORY
MATERIAL DEPOSITION
• evaporation
• molecular beam epitaxy (Si and III-V)
• liquid phase epitaxy (LPE)
• metal organic chemical vapor deposition (MOCVD)
• laser assisted chemical vapor deposition (LACVD)
SURFACE / INTERFACE
CHARACTERIZATION
• STM / BEEM
• ESCA / SAM
• TEM
• SEM
LITHOGRAPHY / DEVICE FABRICATION
• electron-beam and optical lithographies
• diffusion and oxidation furnaces
• wet and dry etching
• reactive ion etching
BULK MATERIALS
AND DEVICE
CHARACTERIZATION
• transport
• optoelectronic
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