Apparatus and method for rapid thermal processing

A closable enclosure for rapid thermal processing of semiconductor wafers is presented, wherein the closable enclosure has an enclosed volume less than 10 times the volume of the wafer, and wherein the closable enclosure may closed about the wafer while the closable enclosure is surrounded by the process gas.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and an apparatus for the rapid 
thermal processing (RTP) of sensitive electronic materials. The present 
invention allows high throughput of wafers which must be rapidly processed 
inside a small enclosure to reduce the loss of material from the wafer 
surface. 
2. Description of the Prior Art 
Rapid Thermal Processing (RTP) is a versatile optical heating method which 
can be used for semiconductor processing as well as a general, well 
controlled, method for heating objects or wafers which are in the form of 
thin sheets, slabs, or disks. The objects are generally inserted into a 
chamber which has at least some portions of the chamber walls transparent 
to transmit radiation from powerful heating lamps. The transparent portion 
of the walls is generally quartz, which will transmit radiation up to a 
wavelength of 3 to 4 microns. These lamps are generally tungsten-halogen 
lamps, but arc lamps or any other source of visible and/or near infra-red 
radiation may be used The radiation from the lamps is directed through the 
transparent portions of the walls on to the surface of the object to be 
heated. As long as the objects absorb light in the near infrared or 
visible spectral region transmitted by the transparent portion of the 
walls, RTP techniques allow fast changes in the temperature and process 
gas for the different material processes and conditions. RTP allows the 
"thermal budgets" of the various semiconductor processing to be reduced, 
as well as allows the production of various metastable states which can be 
"frozen in" when the material is cooled rapidly. 
RTP systems are relatively new. In the last 10 or 15 years, such systems 
were used only in research and development. The thrust of the work was 
increasing the temperature uniformity, and developing heating cycles and 
processes which decreased the thermal budget. Prior art RTP machines can 
heat unstructured, homogeneous materials in the form of a flat plate or 
disk, and produce temperature uniformities across the plate adequate for 
semiconductor processing processes. 
The temperature control in current RTP systems is mostly performed by 
monochromatic (or narrow wavelength band) pyrometry measuring temperature 
of the relatively unstructured and featureless backside of semiconductor 
wafers. The results of the temperature measurement are generally used in a 
feedback control to control the heating lamp power. Backside coated wafers 
with varying emissivity can not be used in this way, however, and the 
backside layers are normally etched away or the temperature is measured 
using contact thermocouples. 
A newer method of temperature control is the power controlled open loop 
heating described in U.S. Pat. No. 5,359,693, which patent is hereby 
incorporated by reference. 
German patent DE42 23 133 C2, hereby incorporated by reference, discloses a 
method of producing relatively defect free material in RTP machines. 
Apparatus induced thermal inhomogeneities have been reduced in the last 
few years because of the demand for more uniform processing. Among the 
techniques used have been control of the individual lamp power, use of 
circular lamps, and rotation of the semiconductor wafers with independent 
power control. 
Most RTP machines have a thin rectangular quartz reaction chamber having 
one end open. Chambers meant for vacuum use often have a flattened oval 
cross section. Chambers could even be made in the form of a flat 
cylindrical pancake. In general, the chambers are used so that the thin 
objects to be heated are held horizontally, but they could also be held 
vertical or in any convenient orientation. The reactor chamber is usually 
thin to bring the lamps close to the object to be heated. The reactor 
chamber is opened and closed at one end with a pneumatically operated door 
when the wafer handling system is in operation. The door is usually made 
of stainless steel, and may have a quartz plate attached to the inside. 
The process gas is introduced into the chamber on the side opposite the 
door and exhausted on the door side. The process gas flow is controlled by 
computer controlled valves connected to various manifolds in a manner well 
known in the art. 
Reactors based on this principle often have the entire cross section of one 
end of the reactor chamber open during the wafer handling process. This 
construction has been established because the various wafer holders, guard 
rings, and gas distribution plates, which have significantly greater 
dimensions and may be thicker than the wafers, must also be introduced 
into the chamber and must be easily and quickly changed when the process 
is changed or when different wafer sizes, for example, are used. The 
reaction chamber dimensions are designed with these ancillary pieces in 
mind. Copending patent application Ser. No. 08/387,220 now U.S. Pat. No. 
5,580,830, assigned to the assignee of the present invention, hereby 
incorporated by reference, teaches the importance of the gas flow and the 
use of an aperture in the door to regulate gas flow and control impurities 
in the process chamber. 
The wafer to be heated in a conventional RTP system typically rests on a 
plurality of quartz pins which hold the wafer accurately parallel to the 
reflector walls of the system. Prior art systems have rested the wafer on 
an instrumented susceptor, typically a uniform silicon wafer. Copending 
patent application Ser. No. 08/537,409, assigned to the assignee of the 
present invention, hereby incorporated by reference, teaches the 
importance succeptor plates separated from the wafer. 
Rapid thermal processing of II-VI and III-IV compound semiconductors has 
not been as successful as RTP of silicon. One reason for this is that the 
surface has a relatively high vapor pressure of for example, arsenic (As) 
in the case of gallium arsenide (GaAs). The surface region becomes 
depleted of As, and the material quality suffers. 
Kanack et. al., Appl. Phys. Lett. 55, 2325, (1989) disclose a method of 
annealing contacts in GaInAsP by placing the InP substrate wafer between 
two silicon susceptor wafers. 
Katz et. al., J. of Vac. Science and Tech. B 8, 1285, (1990); Pearton et. 
al. in SPIE 1393, 150, (1991), and Kazior et. al. IEEE transactions on 
Semiconductor Manufacturing 4, 21 (1991) teach a method of enclosing a 
compound semiconductor wafer in an enclosed susceptor having an enclosed 
volume only slightly greater than the wafer volume, then placing the 
susceptor in the RTP system for processing. A sacrificial wafer is 
processed first to charge the walls of the interior of the enclosed 
susceptor with the volatile component of the wafer of interest, and 
thereafter a number of product wafers can be treated. Presumably, the 
partial pressure of the volatile component inside the susceptor is high 
enough that the rate of evaporation from the wafer surface is equal to the 
rate at which the volatile component redeposits on the wafer surface. 
Such a process does not lead to high throughput, however. The wafer and the 
enclosing susceptor must be flushed for a relatively long time to expel 
all the oxygen which was introduced into the enclosure when the wafer was 
introduced. 
The enclosed susceptor could be flushed relatively rapidly if the vacuum 
RTP system is used. However, these systems have much greater cost and 
complexity. 
OBJECTS OF THE INVENTION 
It is an object of the invention to provide an improved rapid thermal 
processing (RTP) apparatus, method and system which increases the 
throughput of objects which must be processed in a small volume enclosure. 
It is an object of the invention to provide a method of heating an object 
in a RTP system so that temperature inhomogeneities due to differences in 
material or absorption coefficient or emission coefficient are reduced. 
It is an object of the invention to provide a rapid thermal processing 
system which is capable of rapid turnaround of semiconductor wafers and 
high throughput of processed wafers with good repeatability even in the 
case where backside emissivity of the wafers varies. 
SUMMARY OF THE INVENTION 
The object to be processed is placed into a closable enclosure having a 
very small enclosed volume. The interior of the closable enclosure is 
filled with a process gas. When the object to be processed is surrounded 
by the process gas, the closable enclosure is closed, and the closed 
enclosure with the enclosed object is then processed in an RTP system

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a wafer 10 held in an RTP apparatus. A guard ring 40 is shown. 
Susceptor plates 20 and 30 are shown. The wafer 10, the susceptor plates 
20 and 30, and the guard ring 40 are all supported by quartz pins 52. The 
gas inlet 60 to the quartz chamber 62 releases process gas which is 
baffled by baffle plates 54. Heating lamps 70 supply the radiation for 
heating wafer. A pyrometer 84 measures the temperature of the susceptor 
plate 20. A door 90 seals the quartz chamber 62. 
FIGS. 2a-b shows a sketch of a closable enclosure which can be closed in 
the process gas of an RTP system. FIGS. 2a-b gives a sketch of the most 
preferred embodiment of the invention. A base 100 and a lid 110 are shown 
in elevation in FIG. 2a. When the closable enclosure is closed, the lid 
110 sits on the upper surface 190 of the base. A semiconductor wafer 
(shown later) to be heated sits on the surface 170 of the base 100. The 
thickness of the semiconductor wafer is slightly less than the depth 180 
of the well 200 in the base 100. The diameter of the semiconductor wafer 
is slightly less than the diameter of the well 200. The volume of the 
semiconductor wafer is less than the enclosed volume formed by the well 
200 and the lid 110 when the lid 110 is resting on the surface 190 of the 
base 100. The well 100 could equivalently be in the lid 110, or there 
could equivalently be a well in both the base 100 and the lid 110. Since a 
small amount of the volatile component of the wafer coats the walls of the 
interior of the enclosure, and this volatile component is driven off when 
the wafer is processed, the interior volume of the closable enclosure must 
be as small as possible to give the largest vapor pressure of the volatile 
component. The enclosed volume of the closable container should be less 
than about 10 times the volume of the enclosed wafer, more preferably less 
than 2 times the volume of the enclosed wafer, and most preferably less 
than 1.5 times the volume of the enclosed wafer. A plurality of holes 120 
are located around the periphery of the base 100 to allow at least 3 
quartz pins (shown later) to slidably pass through the base 100 to support 
the lid 110 above the base 100. A further plurality of holes 130 in the 
base 100 allow at least 3 quartz pins (shown later) to slidably pass 
through the base 100 to support a wafer (shown later) above the base 100 
and below the lid 110. An optional detent 150 ensures that the quartz pins 
(shown later) hold the lid 110 in a repeatable position with respect to 
the base 100. An optional detent 160 (shown later) allows the base 100 to 
be held in a repeatable position with respect to quartz pins (shown 
later). An optional film 102 to control the reflectivity of a portion of 
the exterior surface of the closable enclosure is shown covering a portion 
of the bottom of the base 100. The optional film 102 may be placed in any 
convenient place on the exterior surface of the base or the lid depending 
on the convenience of viewing the film 102 with a pyrometer. The optional 
film may cover the entire surface of the closable enclosure, or just a 
very small part of the surface which is in the field of view of an optical 
temperature measuring instrument. The lower surface 112 of the lid 110 and 
the surface 170 may be specially treated to control contamination of the 
surfaces 112 and 170. FIG. 2b shows a plan view of base 100 where A-A' 
denotes the section taken for FIG. 2a. 
FIG. 3a-f shows a sketch of the operational steps of loading wafers into 
the apparatus of FIGS. 2a-b in a preferred method of the invention. The 
apparatus of FIG. 2 is preferably located inside the reaction chamber of a 
rapid thermal processing system in an atmosphere of process gas. The 
process gas can be an inert gas such as helium, neon, or argon, or it may 
be a gas such as nitrogen or a forming gas such as nitrogen admixed with 
hydrogen or argon admixed with hydrogen. The process gas is not critical 
to the invention, as long as it contains very little admixture of gas 
which would be injurious to the RTP processing of the wafer. The chamber 
of the RTP system is flushed with a laminar flow of flushing or process 
gas in a preferred embodiment of the invention, and little outside 
atmosphere penetrates the reaction chamber. FIG. 3a shows the lid 110 and 
the base 100 of the enclosure supported by a plurality of at least 3 
quartz pins 210 (one pin shown) which pass through the holes 120 in the 
base 100. The base 100 is shown resting on a shoulder 212 of pin 210. The 
end of pin 210 rests in the optional detent 150 of the lid 110. A 
plurality of at least 3 quartz pins 220 (one pin shown) also pass through 
a plurality of holes 130 in the base. The quartz pins are free to slide 
through the holes 120 and 130 in the base 100. The holes 120 may be 
located equidistant from the center of the base 100, and the holes 130 may 
be located equidistant from the center of the base 100 at a smaller radius 
than the holes 120. The number and placement of the holes 130 and 120 is 
not critical to the invention. The holes 120 must be placed so that the 
plurality of pins 210 support the base 100 and the lid 110, and so that a 
wafer may pass between the quartz pins 210 and below the lid 110 to be 
located above the center of the base 100. The number and placement of the 
holes 130 is not critical to the invention. At least 3 holes 120 and pins 
210 are necessary to support the base 100 and the lid 110. The pins 220 
must be close enough together to support a wafer as shown later. The 
quartz pins 210 and 220 are supported on a mounting plate 230. FIG. 3b 
shows a robot pan 240 carrying a wafer 10 entering the space between the 
base 100 and the lid 110 of FIG. 1a. FIG. 3c shows that the robot pan 240 
has lowered the wafer 10 so that the wafer 10 sits on the quartz pins 220. 
The robot pan 240 is withdrawing from the space between the lid 110 and 
the base 100. FIG. 3d shows the robot pan reentering under the base 100. 
FIG. 3e shows the robot pan 240 raising until it contacts the base 100 and 
lifts the base 100 from the mounting pins 210. The base 100 lifts the 
wafer 10 from the pins 220. The base 100 lifts the lid 110 from the pins 
210. FIG. 3f shows that the robot pan 240 has lifted the base 100 free of 
the pins 210, (thereby closing the closable enclosure), moved the base 
100, the wafer 10, and the lid 110 a short distance horizontally until the 
detent 160 in base 100 is above the pin 210, and lowered the base 100 
until the base 100 rests on the end of pin 210. FIG. 3f shows the robot 
pan 240 withdrawing. After the robot pan 240 has withdrawn, the door to 
the RTP chamber is closed, and the wafer 10 within the closed enclosure 
formed by base 100 and lid 110 is ready for rapid thermal processing. 
A preferred embodiment of the invention is shown in FIG. 4. The first steps 
of the method of the invention are identical to the steps of FIG. 3a-c. 
The steps of lining the base and lid of the apparatus of the invention are 
carried out by an elevator mechanism 250 that passes through the walls of 
the RTP chamber 62. FIG. 4 shows a pan 260 connected to the elevator 250. 
The pan 260 has lifted the base 100, the wafer 10 and lid 110 away from 
the supporting pins. In this position, the elevator mechanism 250 has been 
rotated around its axis 252 to bring further detents 270 above the quartz 
pins 210. 
The detents 270 are at the same radius as the holes 120 with respect to the 
elevator 250. FIG. 4 shows the base 100 lowered so that it rests on the 
pins 210. The elevator pan 260 can now be withdrawn so that the closable 
enclosure and the wafer are now ready for RTP. This preferred embodiment 
of the invention is preferred in the case that the process gas inside the 
closable enclosure be as clean as possible. The RTP chamber door may be 
closed after the step represented by FIG. 3c, and the process chamber 
flushed until there is sufficiently little impurity in the process gas 
surrounding the wafer 10. At this point, the closable enclosure is closed, 
and the process continued. 
The most preferred method of closing the closable enclosure when the door 
of the reactor chamber is closed is shown in FIG. 5a-b. In this figure, 
the base 100 is supported by a plurality of quartz pins 300 and does not 
move, and the lid 110 and the wafer 10 are lifted free of the base 100 by 
quartz pins 330 and 320 respectively. The quartz pins 330 and 320 are 
fastened to a quartz carrier frame 340, which may be raised and lowered by 
an elevator mechanism through the wall of the RTP chamber as sketched in 
FIG. 4, or more preferably are raised and lowered by extensions of The 
carrier frame 340 passing through the door 90 of the RTP chamber as 
sketched in FIG. 5a and 5b. The door 90 has an aperture 350 to control the 
flow of process gas, and the aperture 350 is closed by a door 360. The 
frame 340 passes through slots (shown later) cut in the door 90. FIG. 5a 
shows the system after the wafer 10 has been brought into the space 
between the base 100 and the lid 110 when the robot pan 240 has been 
lowered and is withdrawing. FIG. 5a shows the system after the system has 
been flushed, the carrier frame 340 has lowered the wafer 10 and the lid 
110 into contact with the base 100, and the system is ready for RTP 
FIG. 6 shows a sketch of a simple carrier frame 340 arrangement for 
supporting the pins 320 and 330, (with the position of the base 100 
indicated by a dotted line) and for passing the frame 340 through slots 
370 in door 90 of the chamber. The aperture 350 need only be wide enough 
to admit the wafer 10, and high enough to provide clearance on the top for 
the wafer entering the RTP chamber, and low enough to provide clearance 
for the robot pan 240 to withdraw after the robot pan 240 has deposited 
the wafer 10 on the pins 320. The slots 370 allow the frame 340 to be 
raised and lowered, and may be sealed by methods known in the art to 
prevent all or most leakage of outside atmosphere into the reaction 
chamber. Typical sealing methods use bellows to effect a gas tight seal, 
or a sliding plate to reduce process gas flushing flow to a very small 
amount. The frame 340 may be equivalently swivelled at the position of the 
slots 370 in order to raise and lower the pins 330 and 320. 
The invention is not limited to loading the wafer into the enclosable 
enclosure located inside the chamber of a conventional RTP processing 
system. It is an embodiment of the invention that the wafer can be placed 
inside the closable enclosure anywhere, as long as the closable enclosure 
is surrounded with a process gas or a suitably clean environment such as a 
vacuum or low pressure gas environment when the closable enclosure is 
closed. The steps 3a-e may be carried out in such an environment and the 
robot pan 240 used to transport the closed enclosure into the chamber of 
the RTP system for further steps. As long as the closable enclosure 
remains in a clean process gas, or as long as little contaminating 
atmosphere leaks into the closable enclosure, the method of the invention 
may be practiced. It is a further embodiment of the invention that the RTP 
system need not have the conventional reaction chamber in such a 
situation. As long as the closable enclosure retains the process gas or 
the closable enclosure remains surrounded by process gas, the radiation 
from the radiation source may be used to heat the enclosed object without 
passing through an intervening transparent window. 
The techniques and controllers for controlling the radiation source and for 
controlling the process gas are well known in the art. The wafer handling 
systems for translating, picking up, and putting down wafers are also very 
well known in the art of RTP. The invention is not limited to the 
particular methods of controlling process gas, radiation sources, and 
material handling cited above. 
The closable enclosure can be made of any material which prevents the 
escape of the volatile component of the wafer material. Transparent 
materials such as quartz pass the heating radiation from the lamps 
directly to the wafer. The preferred embodiment is a "hot box" enclosure 
which absorbs and is heated by the visible and/or near infra-red radiation 
from the heating lamps. The object to be processed is then heated by 
conduction and/or convection and/or radiation of heat from the walls of 
the closable enclosure. Materials such as heavily doped silicon, silicon 
carbide, boron nitride, graphite, silicon carbide coated graphite, diamond 
coated graphite, or other such materials are suitable. The invention is 
not limited to the choice of materials of the enclosure. 
The interior of the closable enclosure is optionally comprised of a porous 
or etched material which may retain more of the volatile component of the 
wafer per unit area than a dense material. Suitable materials are porous 
tungsten, porous silicon or silicon with pore diameter in the range of 0.1 
to 10 microns. A material which has surface relief features which increase 
the effective surface area or specific area is also be used to good 
effect. Such materials as etched polysilicon have effective surface areas 
greater than 1.5. Anisotropically etched &lt;11O&gt; silicon have effective 
surface areas greater than 10. Such surfaces are of value when long runs 
of processing must be carried out without recharging the surface with the 
volatile material, and when the surface does not have contact with 
contaminating gas when the processed wafer is withdrawn and a new wafer is 
inserted. In the case that the closable enclosure is exposed to a 
contaminating atmosphere, it is more preferable to have an effective 
surface or specific area of the interior of the closable enclosure which 
is as small as possible. In this case, highly polished material which is 
very smooth is preferable. Such surfaces are provided by polished graphite 
coated with titanium carbide, for example. The surface roughness of the 
surface should be less than 1 micron, more preferably less than 0.1 
microns, and most preferably less than 0.025 microns over a substantial 
portion of the interior surface of the closable enclosure. 
Use of the closable enclosure solves a temperature control problem when 
back side pyrometry is used to measure and control the wafer temperature 
where a backside film on the wafer may change the emissivity of the wafer 
and interfere with the temperature measurement and control. The exterior 
walls of the closable enclosure presumably will have constant emissivity 
and reflectivity from wafer to wafer, and the temperature of the wafer can 
be inferred from the measured temperature of a wall of the closable 
enclosure. Use of a pyrometer which measures a narrow band width of 
wavelengths is also made easier, because the closable enclosure may 
reflect less scattered lamp light to the pyrometer than a semiconductor 
surface. For example, a bare silicon surface reflects 32% of the visible 
light incident on the surface. This reflection may markedly vary as the 
silicon has various thin film coatings, and may markedly vary as the 
thickness of the coating varies. The reflectivity for visible and near IR 
light of the surfaces of the closable enclosure should be preferably less 
than 32%, more preferably less than 25%, and even more preferably less 
than 10%. 
It is a further embodiment of the invention to control the reflectivity and 
emissivity of the closable enclosure by coating an outside surface of the 
enclosure with one or more layers of a thin film 102. A single thin film 
thickness is conveniently set so that the reflectivity of the closable 
enclosure has a minimum of less than 3% at the wavelength which the 
pyrometer measures. A multilayer film is used for a pyrometer or a 
pyrometer plus filter combination which measures a very narrow bandwidth 
of infra-red radiation, so that the reflection coefficient is less than 1% 
at the wavelength region which the pyrometer measures. In this manner, the 
light reflected from the surface gives less background in the pyrometer 
measurement. 
It is a further embodiment of the invention to coat an exterior surface of 
the enclosure with a material that has a very high emissivity and low 
reflectivity at the wavelength measured by the pyrometer. Such a coating 
as known in the art of absorbing coatings is formed by dendritic tungsten. 
The absorption coefficient of the material of the closable enclosure for 
radiation should preferably high enough that the closable enclosure 
transmits less than 5% of the visible and/or the near IR radiation light 
incident on to its surfaces. The closable enclosure is most preferably 
opaque to visible and/or to near infra-red light from the heating lamps. 
It is an additional embodiment of the invention to increase throughput by 
holding the temperature of the closable enclosure at an elevated 
temperature while the already processed wafer is removed and a new wafer 
to be processed is introduced. The lamps are run at lower power to keep 
the temperature of the closable enclosure at minimum temperature while the 
wafer is being changed. 
It is an additional embodiment of the invention to control contamination of 
the surfaces of the closable enclosure by heating the closable enclosure 
and the wafer before the closeable enclosure is closed. In this way, the 
contaminant gases adsorbed on the wafer and on the interior surfaces of 
the closable enclosure may be driven off and flushed away by the process 
gas. The enclosure and the wafer should be heated to a temperature 
sufficient to drive off adsorbed contaminant gases, but not high enough to 
drive off the adsorbed component of the semiconductor wafer which is to be 
processed. The preferred temperature is greater than 100.degree. C., a 
more preferred temperature is 200.degree. C., an even more preferred 
temperature is greater than 300.degree. C., and the most preferred 
temperature is greater than 400.degree. C.