Heat treatment system using ring-shaped radiation heater elements

In a heat treatment system for heating a semiconductor wafers, around a processing chamber is defined an enclosed heating chamber that can be either filled with gases or evacuated. The heater elements consist of halogen infrared lamps received in concentric grooves having a mirror surface. It is therefore possible to achieve a favorable heat insulation which contributes to a high heat efficiency. When the heating chamber is filled with gases or air, it is possible to achieve a controlled rapid cooling of the processing chamber, and this contributes to a short turnaround time of the heat treatment system.

TECHNICAL FIELD
 The present invention relates to a heat treatment system for heating
 disk-shaped workpieces such as semiconductor wafers, and in particular to
 a heat treatment system which is suitable for conducting various heat
 treatments for manufacturing semiconductor devices such as chemical vapor
 deposition (CVD), impurity diffusion, annealing, and epitaxial growth.
 BACKGROUND OF THE INVENTION
 Manufacture of semiconductor devices requires various heat treatment steps
 under controlled environments. Typically, a large number of semiconductor
 wafers carried by a wafer boat made of heat resistant material such as
 quartz glass is received in a processing chamber (such as a diffusion
 chamber and a reaction chamber) defined inside a pressure vessel typically
 made of heat resistant material such as quartz glass, and process gases
 are introduced into the processing chamber. A heating arrangement is
 placed around the processing chamber, and a desired process is conducted
 in the processing chamber with the semiconductor wafers heated to a
 desired temperature.
 The heating arrangement for such a heat treatment system typically uses a
 coil-shaped electroresistive heater element, and the heater element is
 supported by spacers made of alumina or other refractory material.
 Additionally, insulating material typically consisting of ceramic fibers
 is filled in the gap defined between the exterior of the heating
 arrangement and the outer casing surrounding the processing chamber.
 However, such a conventional heat treatment system using a electroresistive
 heater element has a relatively large heat capacity, and it is not
 possible to rapidly heat or cool the processing chamber. In other words,
 because of the time lag in heating or cooling the processing chamber, it
 is difficult to achieve an accurate and responsive temperature control. In
 particular, because the insulating material prevents rapid cooling, it is
 not possible to lower the temperature of the processing chamber as rapidly
 as desired. This is detrimental in reducing the turnaround time of the
 heat treatment system, and achieving a high quality heat treatment.
 Also, according to a conventional heat treatment system using an
 electroresistive heater element, it is difficult to achieve an even
 temperature distribution in the processing chamber. This is particularly
 significant in view of the increasing demand for semiconductor wafers
 having large diameters. Handing such large semiconductor wafers requires a
 correspondingly large processing chamber, and an electroresistive heater
 element is often unable to heat the central part of the processing chamber
 as much as the peripheral part of the processing chamber.
 Furthermore, an electroresistive heater element tends to produce particles
 during use, and this may seriously contaminate the clean room environment
 which is required for semiconductor manufacturing processes.
 BRIEF SUMMARY OF THE INVENTION
 In view of such problems of the prior art, a primary object of the present
 invention is to provide a heat treatment system which is capable of
 rapidly heating and cooling the processing chamber.
 A second object of the present invention is to provide a heat treatment
 system which is capable of accurately and responsively controlling the
 temperature of the processing chamber.
 A third object of the present invention is to provide a heat treatment
 system which has a short turnaround time, and is therefore economical to
 operate.
 A fourth object of the present invention is to provide a heat treatment
 system which is compact in design and economical to build.
 A fifth object of the present invention is to provide a heat treatment
 system which is relatively free from contamination.
 According to the present invention, these and other objects can be
 accomplished by providing a heat treatment system for heating a
 disk-shaped workpiece, comprising: an outer casing; an inner shell
 received in the outer casing and defining an enclosed cylindrical
 processing chamber therein, the inner shell being made of radiation heat
 transmitting material and provided with an inlet and an outlet for
 admitting and removing processing gases into and out of the processing
 chamber; an enclosed heating chamber defined between the outer casing and
 the inner shell, the heating chamber being provided with a port for
 controlling an inner pressure thereof; and a plurality of ring-shaped
 heater elements disposed concentrically on an inner axial end surface of
 the outer casing, the heater elements being provided with individual power
 feed segments.
 Because the enclosed heating chamber defined around the processing chamber
 can be either filled with gases or evacuated, it is possible to achieve a
 favorable heat insulation which contributes to a high heat efficiency.
 When the heating chamber is filled with gases or air, it is possible to
 achieve a controlled rapid cooling of the processing chamber, and this
 contributes to a short turnaround time of the heat treatment system.
 In particular, when the heater elements consists of halogen infrared lamps,
 the heater elements can be controlled in a highly responsive manner.
 Because the heater elements can be individually controlled it is possible
 to control the heat distribution in the processing chamber at will, and
 can achieve either a uniform temperature distribution or a temperature
 gradient as desired. Preferably, the heater elements are received in
 corresponding ring-shaped grooves formed in the inner wall of the outer
 casing, each groove being provided with a reflective inner surface. Thus,
 the emitted heat radiation may be directed inward, and the requirement for
 insulating the outer casing is minimized so that the system can be
 designed in a both economical and compact manner.
 According to a preferred embodiment suitable for treating a large number of
 semiconductor wafers at the same time, the inner shell comprises a
 cylindrical container coaxially received inside the outer casing so as to
 define the processing chamber inside the container and the heating chamber
 outside the container, and the outer casing comprises a cylindrical part
 and a pair of end plates, inner surfaces of the cylindrical part and the
 end plates being provided with the ring-shaped grooves for receiving the
 heater elements.
 When the cylindrical part of the outer casing consists of at least two
 pieces which are joined by a plane passing through an axial center of the
 outer casing, the heater elements may be attached to the two casing halves
 in an alternating fashion so that the power feed segments which do not
 emit heat may be evenly distributed along the circumference of the casing,
 and a uniform heating result can be achieved. This arrangement also
 simplifies the assembling of the casing for the heat treatment system.
 In such an arrangement using a cylindrical casing, the inner face of at
 least one of the end plates may directly face the processing chamber. This
 end plate is typically adapted to be detachable from the remaining part of
 the outer housing to permit charging and discharging of workpieces into
 and out of the processing chamber. If a cover plate made of radiation heat
 transmitting material is placed over the inner surface of the one end
 plate so as to define the heating chamber in the grooves of the one end
 plate, the interior of these grooves may define an auxiliary heating
 chamber which serves a similar purpose as the heating chamber defined in
 the cylindrical part of the casing. For the ease of evacuating or
 pressurizing the auxiliary heating chamber, the grooves may be
 communicated with each other via communication holes formed in walls
 separating the adjacent grooves.
 According to an embodiment which is suitable for treating a single
 semiconductor wafer at a time, the outer casing consists of a pair of end
 plates having inner surfaces provided with ring-shaped grooves for
 receiving the heater elements, and a cover plate made of radiation heat
 transmitting material is placed over the inner surface of each of the end
 plates so as to define the processing chamber between the cover plates,
 and the heating chamber in the grooves which are separated from the
 processing chamber by the cover plates.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 generally illustrates a heat treatment system 1 embodying the
 present invention. This heat treatment system 1 comprises a processing
 chamber 2 defined inside an inner shell 13 made of quartz glass, an
 annular heating chamber 5 defined coaxially around the processing chamber
 2 by a cylindrical casing 6. The processing chamber 2 accommodates therein
 semiconductor wafers 3 that are going to be processed, and is adapted to
 receive process gases as described hereinafter. The heating chamber 5
 accommodates therein a number of radiation heater elements 4 for heating
 the semiconductor wafers 3 in the processing chamber 2.
 In this embodiment, the heater elements 4 each consist of a ring-shaped
 infrared halogen lamp which includes a ring-shaped infrared radiating
 element 8 having a pair of electric power feed segments 7 formed in one
 part thereof, as best shown in FIGS. 4 and 5. These heater elements 4 are
 arranged coaxially and at a regular interval along the axial length of the
 cylindrical casing 6. These heater elements can be controlled either
 individually or collectively as desired. The inner wall of the cylindrical
 casing 6 is provided with a plurality of annular grooves 9 at a regular
 interval each receiving an associated one of the heater elements 4, and
 the inner surface of each annular groove 9 consists of a mirror surface so
 that the infrared radiation from the heater element 4 is effectively
 concentrated upon the semiconductor wafers 3.
 The cylindrical casing 6 may be made of a heat resistant metallic material
 such as aluminum, and defines the heating chamber 5 jointly with an upper
 end plate 10 and a lower end plate 11 made of similar material and
 attached to either axial end thereof. The cylindrical casing 6 or the end
 plates 10 and 11 may not be provided with any massive insulating material,
 and can be therefore designed both compact and light weight, as opposed to
 the conventional heat treatment systems using electroresistive heater
 elements. The main part of the cylindrical casing 6 consists of two
 semicylindrical casing halves 6A and 6B as illustrated in FIGS. 2 and 3,
 and the heater elements 4 are attached to the two casing halves 6A and 6B
 in an alternating manner so that the power feed segments 7 which do not
 emit infrared radiation may be arranged uniformly along the circumference.
 In the illustrated embodiment, there are four possible circumferential
 positions for the power feed segments 7 to be located. Thus, the infrared
 radiation from the heater elements 4 is distributed evenly in both axial
 and circumferential directions.
 Each of the heater elements 4 is received in the associated groove 9, and
 the two radially extending power feed segments 7 are passed out of the
 cylindrical casing 6 via a hole formed in the bottom of the corresponding
 annular groove 9. The hole for the power feed segments 7 is appropriately
 sealed so as to keep the heating chamber 5 sealed off from the outside,
 and to thermally insulate the heater element 4 with respect to the
 cylindrical casing 6. More specifically, over the outer end of the hole is
 placed a mounting plate 45 having a pair of holes for individually passing
 through the power feed segments 7. Each power feed segment 7 is fitted
 with an O-ring for sealing the annular gap between the power feed segment
 7 and the corresponding hole in the mounting plate 45.
 The power feed segments 7 are electrically connected to a PID temperature
 control unit (not shown in the drawings) via connecting pins so that the
 heater elements 4 may be controlled either individually or collectively.
 Parts of each heater element adjacent to the power feed segments 7 are
 appropriately cooled by a known air or water cooling arrangement so that
 the excessive thermal expansion of various parts of the heater element may
 not destroy the heater element or damage the seal for the heater element.
 The power feed segments 7 are evenly distributed along the circumferential
 direction so as to avoid any uneven heating results. This arrangement also
 simplifies the wiring of the heater elements because an excessive
 concentration of power feed cables can be avoided.
 The heating chamber 5 is sealed also in other respects so that its interior
 may be filled with suitable incombustible gases via a port 12 provided in
 a lower part of the casing for adjusting its internal pressure. For this
 purpose, the heating chamber 5 may be controlled by a PID pressure
 adjusting unit not shown in the drawing. By properly controlling the
 internal pressure of the heating chamber 5, it is possible to protect the
 inner shell 13 of the processing chamber 2 from damage, and prevent
 leakage of process gases from the processing chamber 2. When a thermal
 insulation is desired, the heating chamber 5 may be evacuated to a desired
 vacuum level. Normally, the higher the vacuum in the heating chamber 5 is,
 the better the insulating performance is. When the temperature of the
 processing chamber 2 is desired to be lowered, the heating chamber 5 may
 be brought to the atmospheric condition.
 The processing chamber 2 is defined by the heat resistant and transparent
 cylindrical inner shell 13 consisting of a container having a spherical
 top and an open bottom, and made of quartz glass which permits
 transmission of near infrared light, and is provided with a suitable
 mechanical property. The processing chamber 2 accommodates therein a wafer
 boat 14 carrying a large number of semiconductor wafers 3. The wafer boat
 14 is attached to a free end of a rotary shaft 15 of a boat loader (not
 shown in the drawing) so as to be turned during the course of a heat
 treatment and moved into and out of the processing chamber 2 as required.
 For this purpose, the lower end plate 11 is adapted to be detachable from
 the rest of the casing.
 The inner shell 13 is provided with a process gas inlet 16 at an upper end
 thereof, and a process gas outlet 17 at a lower end thereof. The upper and
 lower end plates 10 and 11 are each provided with an auxiliary heater
 unit. Each auxiliary heater unit comprises a number of heater elements 20
 each including a ring-shaped infrared radiating element 19 and a pair of
 power feed segments 18. In this case, the power feed segments 18 extend
 axially from a plane defined by the ring-shaped infrared radiating element
 19 as best illustrated in FIGS. 6 and 7.
 Each heater element 20 is received in one of a number of concentric grooves
 21 and 22 provided on the inner faces of the upper and lower end plates 10
 and 11. The heater elements 20 are provided with varying diameters which
 correspond to the diameters of the corresponding annular grooves 21 and
 22. The inner surface of each groove consists of a mirror surface. The
 power feed segments 18 of each infrared heater element are each fitted
 with an O-ring, and are passed out of the heating chamber via a hole in
 the end plate 10 or 11 and a mounting plate 24 placed on the outer end of
 the hole. The power feed segments 18 are appropriately cooled by suitable
 means.
 The grooves 20 formed in the lower end plate 11 are communicated with each
 other via communication holes 26 provided in the walls separating the
 adjacent grooves. A cover plate 27 made of heat resistant and light
 transmitting material such as quartz glass is placed over the inner face
 of the end plate 11 to seal off the interior of the grooves 22 from the
 processing chamber 2. Therefore, the interior of the grooves is separated
 from the processing chamber 2, and may be filled with gases for pressure
 control or evacuated as desired. The interior of these grooves may be
 called as an auxiliary heating chamber because it is similar in function
 to the heating chamber 5 defined outside the inner shell 13. Because the
 cover plate 28 is supported by the upper ends of the walls separating the
 adjacent grooves 22 at a regular interval, the cover plate 28 is able to
 withstand a significant pressure difference between the interior of the
 grooves and the processing chamber without requiring the thickness of the
 cover plate to be significant. These infrared heater elements 20 may also
 be controlled either individually or collectively by using a PID
 controller or the like which is not shown in the drawing.
 Because the inner surface of the upper end plate 10 is directly exposed to
 the heating chamber 5, the grooves 21 may not be closed by a cover plate,
 and the heater elements may be simply placed inside the grooves in the
 same way as the heater elements received in the grooves 9 of the
 cylindrical casing 6. However, if desired, for instance, to provide an
 enhanced thermal insulation in the region of the upper end plate, the
 grooves 21 may be covered by a cover plate 27 in the same way as with the
 lower end plate 11 as illustrated in FIG. 1.
 A heat equalizer tube 29 or 30 may be placed between the infrared heater
 elements and the workpieces to evenly distribute the radiant heat applied
 to the workpieces. It may be made of suitable material such as carbon,
 tantalum or silicon carbide, and may be placed either inside or outside
 the inner shell 13. The heat equalizer tube 30 receives radiant energy
 from the infrared heater elements, and radiates the resulting heat evenly
 upon the workpieces in the processing chamber 2. If the heat equalizer
 tube is required to be protected from contaminating gases in the
 processing chamber, it may be placed in the heating chamber as indicated
 by the imaginary lines 29. If such contamination is not a problem, it may
 be placed inside the processing chamber for better thermal efficiency as
 indicated by the solid lines 30.
 The operation of this embodiment is described in the following. A number of
 semiconductor wafers 3 are horizontally supported on the wafer boat 14,
 and are charged into the processing chamber 2 by using a boat loader. At
 the same time, the lower end plate 11 is fitted into the lower end of the
 cylindrical casing 6. The interior of the heating chamber 5 is then
 evacuated to a vacuum level of about 10.sup.-3 Torr. The interior of the
 grooves of the upper and lower end plates is likewise evacuated. Thus, the
 processing chamber is entirely surrounded by a vacuum heat insulation
 layer.
 The interior of the processing chamber 2 is also evacuated by a PID
 pressure control unit connected to the outlet 17, typically to a level of
 50 to 10.sup.-7 Torr. Then, a process gas is introduced into the
 processing chamber 2 from the inlet 16. When oxide film is desired to be
 formed on the surface of the wafers, the process gas may consist of
 N.sub.2 and O.sub.2, and the inner pressure of the processing chamber 2 is
 controlled at a desired level which may range from the pressurized
 condition in the order of 2 kg/cm.sup.2 to the evacuated condition in the
 order of 10.sup.-7 Torr.
 Thereafter, power is supplied to the heater elements 4 and 20, and the
 temperature in the heating chamber 5 and the interior of the grooves
 (auxiliary heating chambers) is controlled to a desired level (such as 700
 to 1,200.degree. C. in the case of forming an oxide layer) by the PID
 temperature control unit. If desired, a number of thermocouple temperature
 sensors may be arranged in various places of the heating chamber and the
 auxiliary heating chambers so that the heater elements may be individually
 controlled and a desired temperature distribution may be achieved.
 Therefore, it is possible to achieve a uniform temperature over the entire
 heating chambers. Alternatively, a desired temperature gradient may be
 achieved in the entire heating chambers.
 When the temperature of the heating chamber is required to be lowered,
 typically following the completion of a desired heat treatment, the vacuum
 in the heating chamber and the auxiliary heating chambers may be removed
 until the interior of these chambers reaches the atmospheric pressure or
 higher. This causes the temperature of the heating chambers to drop
 rapidly so that the subsequent heat treatment can be started without any
 significant time loss. This leads to an improvement in the work efficiency
 and the reduction in power consumption. Also, the temperature may be
 finely and quickly adjusted during the course of a single heat treatment.
 Therefore, undesirable thermal shocks to the semiconductor wafers can be
 avoided.
 The performance of the heat treatment system of the present invention is
 summarized in the following in comparison with that of a prior art heat
 treatment system when 50 semiconductor wafers having a diameter of 300 mm
 were treated.
 TABLE 1
 Invention Prior art
 temperature rise (.degree. C./min) 1,000 15-20
 temperature equalization (.degree. C.) .+-.0.5.degree. C.
 .+-.0.5.degree. C.
 temperature drop (.degree. C./min) 300 5-15
 pressure range (Torr) 1,520-10.sup.-7 760-10.sup.-3
 The present invention may also be applied to a sheet-fed system as
 illustrated in FIGS. 8 and 9. This heat treatment system 31 comprises an
 outer casing 34 consisting of a disk-shaped upper case half 32 and a
 similarly shaped lower case half 33. The opposing faces of these case
 halves are each provided with a number of concentric grooves 40 and 41. A
 circular quartz glass plate 35 or 36 is placed over each of these opposing
 faces via an O-ring or the like so that the interior of the grooves 40 and
 41 is sealed off from a processing chamber 37 defined between the opposing
 faces of the upper and lower casing halves 32 and 33. The adjacent grooves
 40 and 41 are communicated with each other via communication holes 46 and
 47 formed in the walls separating them from each other. The grooves 40 and
 41 which are closed by the cover plates 35 and 36 thus define a pair of
 heating chambers 37.
 A ring-shaped infrared heater elements 38 are received in each of the
 grooves 40 and 41. These heater elements 38 are similar to those used in
 the upper and lower end plates 10 and 11 of the previous embodiment, and
 each include a ring-shaped infrared radiating element 18 having a pair of
 axially extending electric power feed segments formed in one part thereof
 as illustrated in FIGS. 6 and 7. In this case also, the cover plates 35
 and 36 are supported by the upper ends of the walls separating the
 grooves, and are able to withstand the pressure difference between the
 processing chamber 37 and the heating chambers.
 This system is also provided with an inlet 50 for admitting process gases
 into the processing chamber 37, and an outlet 51 for removing gases from
 the processing chamber 37. The heating chambers 39 are provided with ports
 48 for adjusting the inner pressure of the heating chambers 39.
 The performance of this heat treatment system is summarized in the
 following when a semiconductor wafer having a diameter of 200 mm was
 treated.
 TABLE 2
 Invention
 temperature rise (.degree. C./sec) 500
 temperature equalization (.degree. C.) .+-.1
 temperature drop (sec) from 1,000 to 300.degree. C.
 (1) 2.5 sec (when the internal pressure
 is increased from 10.sup.-3 Torr to 50
 Torr)
 (2) 4 sec (when the internal pressure
 is maintained at 10.sup.-3 Torr)
 Although the present invention has been described in terms of preferred
 embodiments thereof, it is obvious to a person skilled in the art that
 various alterations and modifications are possible without departing from
 the scope of the present invention which is set forth in the appended
 claims.