The requirements for non-contact temperature measurement capabilities for electronic materials processing in space are assessed. Non-contact methods are probably incapable of sufficient accuracy for the actual absolute measurement of temperatures in most such applications but would be useful for imaging in some applications

Lehoczky, S. L.

Szofran, F. R.

English

NASA Technical Reports Server

5/88-23898
Non-Contact Temperature Measurement Requirements for
Electronic Materials Processing
S.L. Lehoczky and F.R. Szofran
Space Science Laboratory
NASA/Marshall Space Flight Center
Huntsville, AL 35812
ABSTRACT
The requirements for non-contact temperature measurement
capabilities for electronic materials processing in space are
assessed. Non-contact methods are probably incapable of sufficient
accuracy for the actual absolute measurement of temperatures in most
such applications but would be useful for imaging in some
applications.
67
https://ntrs.nasa.gov/search.jsp?R=19880014514 2020-03-20T07:03:32+00:00Z
I. Introduction
In a terrestrial environment the crystal growth of electronic and
electro-optical materials are significantly effected by gravitational
forces. In particular, free-convection caused by density gradients
resulting from thermal and compositional gradients in the fluid phase
can alter the compositional distribution of components, increase the
probability of incorporation of impurities leached from the container
walls, and cause growth rate fluctuations that lead to an increase in
the densities of intrinsic crystal defects.
It is generally believed that the reduction or elimination in
space of gravity-induced, convective flows and hydrostatic pressure
related phenomena will result in materials with improved electrical
and optical properties due to concomitant improvements in
compositional homogeneity and reductions in crystal defect density.
Additionally, the elimination of the need for confinement for some of
the solidification processes is expected to lead to a reduction in
impurity concentrations and in the densities of stress-induced defects
such as dislocations.
The materials of interest include inorganic semiconductors (Si,
Ge, GeAs, CdTe, HgCdTe, PbSnTe, etc.), organic and inorganic
non-linear materials, various conducting and insulating oxides and
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selected magnetic and piezoelectric materials. The preparation
methods include solidification from molten and vapor phases,
crystallization from saturated solutions, and various solid-state
compaction and crystallization processes. The latter are not expected
to be effected by reduced gravity; thus, only the processes that
involve solidification from a fluid phase will be considered further.
In general, the various solidification methods used do not
depend on a precise knowledge of the material surface temperatures,
and thus, usually do not employ non-contact temperature measurement
data for process control and implementation. Nonetheless, some of the
methods, e.g., Czochralski and float-zone, frequently use similar
optical measurement techniques to assist crystal diameter measurement
and control. In the near term, of the commonly used melt growth
methods (Czochralski, Bridgman, Bridgman-Stockbarger, float-zone,
traveling heater-zone, etc.) only the Czochraiski and float-zone
growth processes are expected to benefit significantly from advances
in non-contact temperature measurement technology. Thus, the
remainder of the paper will primarily address issues associated with
these two melt growth methods.
Section two of the paper gives a brief description of the two
processes. Section three gives an example of the use of an infrared
television system for thermal imaging and diameter control for a
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Czochralski-type crystal growth process. Finally in section four
details of the specific requirements for future non-contact
temperature measurements are discussed.
7o
2. Crystal Growth Methods and Related Non-Contact Temperature
Measurement Problems
The Czochralski and float-zone crystal growth processes are
briefly described below. In the Czochralski growth process the source
material is placed in an inert crucible and melted by using RF or
resistive heating methods. A seed crystal is inserted into the melt
and crystal growth proceeds by controlled withdrawal of the crystal
from the melt. The crystallographic orientation of the growing
crystal is determined by the orientation of the seed and its diameter
is determined by the temperature distribution in the growth system,
which is controlled by the power input into the heaters.
In the classical float zone method a molten zone is established
at the juncture of a mono-crystalline seed and the source material.
The zone width is controlled by the amount of heat input into the zone
region. Similarly to the Czochralski method either RF or resistive
heating techniques are used to establish and maintain the desired
temperature distribution in the zone region. Additional process
control can be derived from provisions for the rotation and relative
axial displacement of the seed and source materials. Usually, for
both growth prDcesses, the particular vapor pressures of the materials
at the growth temperatures are large and thus evaporation from the
melts can result in deposition of the materials on the container walls
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and the "viewing-port" windows. For some of the smaller vapor
pressure electronic materials, e.g. Si, Ge, etc., the evaporation rate
can be significantly reduced by the introduction of an inert gas into
the process chamber. The processing of the higher vapor pressure
materials, e.g., compound semiconductors, usually requires both large
inert gas pressures (up to I00 atm.) and the use of liquid
encapsulants, e.g., B203. Changes in either the window or encapsulant
spectral transmissivities with time are expected to significantly
impact the accuracy of temperature values derived from the measured
intensity of the thermal radiation from the surface region of
interest. Additional errors may result from the lack of knowledge of
the pertinent emissivities as well as of their temperature, wavelength
and composition dependences. A related problem is the potential for
time-dependent changes in the surface compositions caused by either
preferential surface segregation of minority components, e.g.,
impurities and dopants, or by preferential evaporation and thus
surface depletion of some of the major components caused by
differences in component chemical potentials at the phase boundaries.
Finally, as the crystal grows, the relative volumes of the crystal and
melt will change. Related changes in surface geometries will result
in changes in the various geometrical view-factors leading to further
temperature determination errors.
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The extreme sensitivity of device yield and performance to
variations in material properties necessitates a high degree of
reproducibility in the material processing variables, in particular,
thermal conditions. Because of the potential sources of error
described above, we believe that in most cases, contact temperature
measurements cannot provide accurate enough data to assure routine
reproduction of the desired temperature values. Nonetheless, data
provided by such measurements can be very useful for determining
relative variations in surface temperatures and thus thermal
gradients. Such data can be useful in establishing thermal boundary
conditions for process modeling and optimization.
?3
3. System for Thermal Imaqinq and Crystal Diameter Control
Non-contact measurement systems have been in use for diameter
control in Czochralski growth systems for more than 20 years.[l] By
1972 non-contact imaging systems had already reached the fundamental
physical limits discussed in Section 2. Improvements since that time
have, therefore, been limited to faster processing and better
graphical display of the information. The improved graphics
capabilities" are due in part to faster computers and in part to
improvements in detector technology, especially detector arrays. For
example, the earlier use of ordinary photovoltaic diode arrays has
more recently given way to the significant use of charge coupled
arrays. CCDs have higher sensitivity and inherent integration
capability for noise filtering. The 1972 system of Kwap et al.
described below. A current system for non-contact temperature
measurements in a Czochralski system was described by Wargo at this
Workshop.
[2] is
The system of Kwap, et al. used an infrared TV camera to
determine relative temperatures of the melt surface and growing
crystal. The diode array in the camera had a range of 0.55 to 1.15_m
with peak sensitivity near 0.83 _m. By computer analysis of the TV
/
data, crystal diameter measurements were made and the power to the
crystal puller was controlled by computer to regulate the diameter.
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This system was used to grow oxides near 1400°C. The system could
measure diameter to 0.I mm and data were taken from the camera at the
rate of 30 scans per second. By changing filters at 300°C intervals,
the system could measure temperatures from 300°C to 1500°C. The
output voltage sensitivity ranged from 4 to 12 mV/°C so that
temperature resolution for most applications would not be a problem.
The limitations on temperature measurement accuracy were primarily due
to the laws of nature and the lack of knowledge of the emissivities,
as discussed" in Section 2. These limitations remain unchanged.
4. Thermal and Optical Imaqinq Requirements
In summary, requirements exists for thermal and optical imaging
of free or liquid encapsulated melt/crystal surfaces for the
Czoohralski-type and float-zone type crystal growth. The temperature
range of interest is from 200 to 1600°C. Requirements also exist for
thermal mapping of growth interfaces and real-time monitoring of
solidification rates during vapor crystal growth. The
shape/temperature/growth-rate data can be used for feedback control of
thermal fields, crystal growth rates and crystal/ingot diameters.
The crystal growth of electronic materials, in most cases,
requires the precise knowledge of the absolute values of the
temperatures at selected locations on the sample/growth-system
surfaces. Usually the desired absolute temperature measurement
accuracies range from: i)_1.5°C for the 200 to 700°C range,
2) _2.5°C for the 700°C to ll00°C range, 3) _3.5°C for the II00 to
1350°C and _5°C for the highest temperatures. Sampling rates of up to
5 samples per second are required. We believe that these accuracy
requirements very likely cannot be met in most cases using non-contact
temperature measurement techniques because of (i) the errors resulting
from the lack of precise knowledge of some of the required
thermophysical properties, e.g., emissivities, and (2) the various
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inherent process-imposed limitations described in some detail in
Section 2.
The requirements for temperature resolution measurements range
from about 0.2°C to about 4°C. For imaging purposes the required
sampling rates are of the order of 30 samples/second. The spatial
resolution requirements range from about 0.I to I mm. In contrast to
the absolute temperature accuracies, for most cases, especially for
the higher temperature ranges (>700°C), these imaging requirements
probably can be met by using the combination of judiciously designed
imaging optics and commercially available photovoltaic or CCD array
cameras. The most stringent requirements will probably require the
use of special cryogenically cooled detector arrays or scanning
detector systems.
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5. References
lo E.J. Patzner, R.G. Dessauer, and M.R. Poponiak, Solid State
Technology, October 1967, p. 25.
1 T.W. Kwap, D.F. O'Kane, and L. Gulitz, in Temperature. It's
Measurement and Control in Science and Industry, Vol. 4,
Part i, Instrument Soc. of America, Pittsburgh (1972) p. 541,
and D.F. O'Kane, T.W. Kwap, L. Gulitz, and A.L. Bednowitz,
J. Crystal Growth 13/14 (1972) 624.
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Non-contact temperature measurement requirements for electronic materials processing

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