Cooled optical window for semiconductor wafer heating

A vapor deposition system is provided which uses electromagnetic radiation for heating of a semiconductor wafer. The source of the electromagnetic radiation is typically a lamp having a color temperature corresponding to a wavelength in the range of 0.3 to 0.9 micrometers, and generally for a particular semiconductor to an energy greater than the energy required to cause transitions from the valence band to the conduction band of the semiconductor material used to construct the wafer and more preferably to a color temperature corresponding to an energy substantially at or above the energy required for direct (vertical) transitions from the valence band to the conduction band, thereby providing very high absorption of the incident radiation and very efficient direct heating of the wafer. No substrate is required for conducting heat to the wafer. The radiation is directed by a reflector through a window forming one side of the deposition chamber and impinges directly on the surface of the wafer. Although the windown is typically chosen to be substantially transparent at the frequencies desired for heating the wafer, some absorption does occur, thereby heating the window as well. To maintain optimum control over the deposition process, the window is typically constructed with two spaced-apart plates and water is pumped therethrough to actively control window temperature.

TECHNICAL FIELD 
This invention relates to an apparatus for the heating of semiconductor 
wafers in a vapor deposition system with electromagnetic radiation, and 
more particularly to a cooled optical window for transmitting such 
radiation. 
BACKGROUND OF THE INVENTION 
In a typical expitaxial reactor, it is common to use an evacuated quartz 
bell jar to house the wafer on which deposition is desired and a 
supporting substrate adjacent to the wafer. The wafer and substrate are 
then irradiated by an external source, generally infrared radiation, to 
bring the wafer to the required temperature for the desired chemical 
reactions to occur. In the most common configurations, the wafer is 
typically of silicon, the substrate is of graphite or graphite coated with 
silicon carbide, and the wavelength typically used for heating the silicon 
is generally from about 2 to 10 micrometers. 
Such a configuration is, however, very inefficient for the heating of 
silicon wafers, since in this range of infrared wavelengths, silicon is 
substantially transparent, exhibiting an absorption coefficient typically 
less than 100/cm, and more often less than about 10/cm. Considering this 
low coefficient of absorption, it is likely that a significant portion of 
the heating of the wafer occurs by conduction from the graphite substrate 
which acts as a good absorber, rather than by absorption of the infrared 
in the wafer itself. Because of this rather indirect heating process, 
reactor design can often become the fine art of choosing the proper 
substrate materials. 
For technical and economic reasons, it would be very beneficial to be able 
to heat semiconductor wafers directly in a reactive gas stream without the 
use of a supporting substrate for conductively heating the wafer. In 
particular, it would enable chemical reactions to take place directly only 
at the surface of the heated wafer rather than on both the wafer and its 
heated substrate. 
SUMMARY OF THE INVENTION 
In accordance with the preferred embodiments of the invention, a vapor 
deposition system is provided which uses electromagnetic radiation for 
heating of a semiconductor wafer. The radiation is directed by a reflector 
through a window forming one side of the deposition chamber and impinges 
directly on the surface of the wafer. Although the window is typically 
chosen to be substantially transparent at the frequencies desired for 
heating the wafer, some absorption does occur, thereby heating the window 
as well. Therefore, to control deposition stoichiometry, the window is 
typically constructed with two spaced-apart plates for permitting water to 
flow therethrough, in order to maintain the window at a desired 
temperature. 
The source of the electromagnetic radiation is typically a metal halide 
lamp having a color temperature corresponding to a wavelength in the range 
of 0.3 to 0.9 micrometers. As a further improvement, for more efficient 
heating of a particular semiconductor, the color temperature can be chosen 
within this region to correspond to an energy greater than the energy 
required to cause transitions from the valence band to the conduction band 
of the semiconductor material used to construct the wafer, and more 
preferably to a color temperature corresponding to an energy substantially 
at or above the energy required for vertical transitions from the valence 
band to the conduction band, thereby providing very high absorption of the 
incident radiation and very efficient direct heating of the wafer. No 
substrate is required for conducting heat to the wafer.

DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the preferred embodiments of the invention, shown in 
FIG. 1 is a deposition chamber 11, typically constructed of aluminum, 
containing a semiconductor wafer 12, which is minimally supported at its 
periphery by a support stand 14 located near the middle of housing 11. 
Reactant gases are introduced into the deposition chamber through 
diffusion chamber 15. Support stand 14 is generally of the rim-contact 
point-support type to permit direct exposure of nearly 99% of the wafer 
surface and does not involve the use of a substrate placed directly 
against the wafer surface for conducting heat to the wafer. Chamber 11 is 
bounded on one side by a window 16 which is typically made up of two 
parallel quartz plates 17 and 18 that are transparent at the frequencies 
of interest, each welded to a spacer 19 circumscribing the perimeter of 
the plates to form a cavity therebetween. Window 16 also has an inlet 20 
at the bottom and an outlet 21 at the top for passing a temperature 
control fluid, preferably water, from pump 22 through the cavity in order 
to actively control the temperature of window 16 for optimum deposition. 
Typical temperature control fluids are water, air, or the like, but water 
is preferred. The quartz plates are usually about 8 to 15 inches square 
and 1/4 to 1/2 inches thick, with the spacer typically 1/2 to 1 inch wide, 
to provide a cavity of that same width. Typical spacer materials include 
quartz and/or metal. 
On the other side of window 16 is a second chamber 23 which houses a lamp 
24 and a reflector 25. Lamp 24 is used to supply the radiant energy to 
heat wafer 12, and reflector 25 is for directing the energy from lamp 24 
to the wafer. Unlike typical prior art devices, lamp 24 is not an infrared 
device, but instead is of the metal halide variety and, for most 
semiconductor wafers, is typically chosen to produce light substantially 
in the visible region, i.e., in the region of 0.3 to 0.9 micrometers. For 
silicon, operating in this region promotes high efficiency in the heating 
of the wafer, since for light between about 0.3 micrometers and 0.9 
micrometers, the absorption coefficient for silicon ranges between 400/cm 
and 1,000,000/cm, the higher absorption occurring at the shorter 
wavelengths, whereas above about 1.1 micrometers, in the infrared, the 
absorption coefficient decreases sharply and silicon becomes essentially 
transparent. (See FIG. 2.) 
This high absorption is thought to be due to carriers being raised from the 
valence band to the conduction band, the band gap for silicon being about 
1.15 eV and corresponding to a wavelength of about 1.08 micrometers. As 
can be seen from the band structure of silicon shown in FIG. 3, however, 
this minimum band gap does not occur for vertical transitions but instead 
occurs at different wavevectors, so that the two states cannot be 
connected by an allowed optical (vertical) transition. The threshold 
energy, ET, for strong optical absorption then would be expected to lie 
near the minimum energy for vertical transitions, which occurs at about 
2.5 eV or at a corresponding wavelength of about 0.50 micrometers, as 
appears to be verified by noting the sharp increase in the slope of the 
absorption curve occuring at 0.50 micrometers. (See FIG. 2.) At lower 
energies, i.e. below ET, the increased absorption is thought to be due to 
phonon-assisted transitions, that is vertically by absorption of a photon 
and then to the appropriate minimum in the conduction band by emission or 
absorption of a phonon of a large enough wavevector. The probability of 
these phonon-assisted transitions, however, is typically smaller than for 
direct (vertical) transitions, thus leading to a smaller absorption 
coefficient. Also, these transitions have some temperature dependence 
since the transition probability depends on the occupancy of the phonon 
states in the crystal. 
At the same time that an electron is raised to the upper band, a hole is 
created in the lower band, typically leading either to bound electron-hole 
pairs or, at higher energies of incident radiation, to essentially free 
electrons and free holes. At the temperatures of interest, the interaction 
of the electrons and holes with lattice vibrations is thought to be 
sufficiently high to effectively limit the lifetime of the electrons and 
holes as bound states or as free carriers, so that a substantial portion 
of their kinetic energy is quickly converted to thermal energy, thereby 
raising the temperature of the crystal. Hence, it is very beneficial to 
use a lamp having a frequency corresponding to an energy greater than the 
band gap, or more preferably to an energy at or above the energy for 
direct transitions between the valence band and the conduction band, 
rather than an infrared lamp having an energy below the band gap. As a 
practical matter, however, high power lamps usually have a spectrum of 
energies which they emit rather than a single frequency, so that what is 
desired is a lamp having a range of frequencies corresponding to the 
regions of high absorption of the wafer. With this in mind, it is more 
useful to discuss the output of the lamp in terms of a measure related to 
its spectral density, e.g. its Wein color temperature. Hence, a practical 
lamp for the efficient heating of semiconductor wafers can be 
characterized as having a color temperature corresponding to an energy 
above the band gap (which for silicon would correspond to a color 
temperature above about 2700 degrees K.) and preferably substantially at 
or above the energy for direct transitions between the valence band and 
the conduction band (which would correspond to a color temperature of 
about 5800 degrees K.). 
Experimentally it has been found that a particularly efficient system for 
heating silicon wafers is to use a tungsten halide arc lamp having a color 
temperature of about 5600 degrees K. For example, for a 1.5 kW lamp with 
this characteristic temperature, a wafer positioned approximately 2 inches 
from the window can be heated to about 500 degrees C. in about 70 seconds, 
and with a 2.5 kW lamp of the same type, the wafer temperature can be 
raised to about 700 degrees C. in about 90 seconds. Furthermore, even 
higher wafer temperatures appear to be easily attainable. In addition it 
is not surprising that this color temperature of 5600 degrees K., even 
though it is somewhat lower than the optimum range discussed above, is 
quite efficient in heating the wafers since the tails of the energy 
distribution of the lamp overlap sufficiently into the region above the 
energies for direct transition. This is thought to be generally true, so 
that if the color temperature chosen is even as much as 20% below the 
temperature for direct transitions, the heating will still be quite 
efficient. More specifically, if the color temperature corresponding to 
the energy required for direct transitions is T1, then it is beneficial to 
heating efficiency if the lower bound of the color temperature chosen for 
lamp 24 is greater than about 0.8 T1, and it is even more preferable if it 
is chosen to be greater than T1. 
For the same reason that color temperature is a useful parameter for 
description of the desired lamp characteristics, the composition of window 
16 is also a relevant consideration. Since, lamp 24 will typically emit at 
least some radiation in the infrared and since it is desirable that as 
much of the radiation introduced into chamber 11 as possible be absorbed 
by the wafer rather than by the chamber walls, it would be very beneficial 
if the infrared from lamp 24 be absorbed in window 16. Since water is 
known to have strong absorption in the infrared, the infrared will be 
absorbed directly in the water if plate 17 of window 16 adjacent to 
reflector 25 transmits infrared. Similarly, efficient heating of the wafer 
dictates that both plates used for window 16 be very transparent at the 
energies corresponding to optimal wafer heating. For these reasons, and as 
indicated earlier, quartz has been found highly desirable for construction 
of the plates. Other materials can also be used, however, one such example 
being glass. 
A particularly important benefit of directly heating the wafer through the 
temperature controlled window and avoiding the use of a substrate for 
conducting heat to the wafer is that all surfaces within the deposition 
chamber having any substantial area can be easily maintained at whatever 
temperature is desired, the window by means of pumped fluid and the 
balance of deposition chamber 11 by whatever means may be chosen. Such 
control is very beneficial in obtaining the desired stoichiometry of the 
deposited materials. 
Those skilled in the art will realize that the principles described above 
apply to semiconductors other than silicon, e.g., such as gallium 
arsenide. For example, the band gap (and threshold energy for direct 
transitions) for gallium arsenide is about 1.4 eV corresponding to a 
minimum desirable color temperature of about 3280 degrees K. Hence, it 
would be expected that a lamp having a color temperature of 5600 degrees 
K. as in the previous example would still be satisfactory since it is 
higher than the minimum desirable color temperature.