Thermal processing furnace and liner for the same

A thermal processing furnace, comprising:

FIELD OF THE INVENTION

The present invention generally relates to equipment for processing semiconductor substrates, and more in particular to a vertical thermal processing furnace and a liner for use therein.

BACKGROUND

Thermal processing furnaces or reactors are commonly used for batch processing semiconductor wafers during several fabrication stages of silicon integrated circuits. Processing steps for which a furnace may be used include oxidation, diffusion, annealing and chemical vapor deposition (CVD) and pulsed atomic layer deposition (ALD).

A conventional vertical thermal processing furnace may include a thermally resistive heating coil, powered by an electrical power supply. Within the heating coil there may be provided a bell jar-shaped outer reaction tube and an inner reaction tube that is coaxially disposed within the outer reaction tube. The inner reaction tube is commonly referred to as the liner. The lower end of the outer reaction tube may be open, while the upper end thereof may be closed, typically by a dome-shaped structure. The liner may be open at both its lower and upper end. The lower ends of both the outer reaction tube and the liner may be supported on a flange, which may define a central furnace opening via which a wafer boat holding a plurality of wafers may enter and exit the reaction chamber that is formed by the interior of the liner. The wafer boat may be supported on a thermally insulating pedestal, which in turn may be supported on a door plate that may serve to close off the central furnace opening in the flange. The flange may further be provided with a gas feed conduit that connects to a gas injector disposed inside the liner, and a gas exhaust conduit via which a vacuum pump may be connected to a lower end of the gas passage that exists between an outer wall of the liner and an inner wall of the outer reaction tube.

In operation, a wafer boat may be introduced into the reaction chamber, which may then be evacuated. Subsequently, a process gas may be fed to the reaction chamber via the gas feed conduit and the gas injector. The process gas may flow upwardly within the inner reaction tube while contacting the wafers provided therein. As the process gas exits the upper end of the inner reaction tube and reaches the closed upper end of the outer reaction tube, it may reverse its direction and flow downwardly through the gas passage between the inner and outer reaction tubes, so as to be exhausted from the reaction chamber via the gas exhaust conduit by the vacuum pump.

A general problem associated with thermal processing furnaces is contamination of the reaction chamber atmosphere with small particles. A particle that ends up on a wafer being processed may render a die to be manufactured therefrom inoperable. Contamination of the reaction chamber atmosphere may have different causes.

U.S. Pat. No. 7,736,437 expresses the belief that the dome of a quartz, bell jar-shaped outer reaction tube generates a significant number of micro-particles when it is heated. In operation, these particles may fall into the upper, open end of the inner reaction tube and thence onto the wafer boat and the wafers supported therein. To prevent this, U.S. '437 teaches the use of a cover that is to be disposed on top of the inner reaction tube. The cover may include apertures, such as convolute passageways, in order to allow a substantial upward flow of process gas therethrough while blocking most particles from the dome from falling to within the inner reaction tube.

U.S. Pat. No. 6,503,079 identifies the furnace opening portion of a thermal processing furnace, and more in particular outgassing O-rings, leaking seals and wafer boat rotation mechanisms in the furnace opening portion, as possible sources of contamination. Since the furnace opening portion is located upstream of a flow of process gas within the reaction chamber, the process gas introduced into the reaction chamber may serve as a carrier for the contaminants generated by the aforementioned sources in the furnace opening portion. The contaminants may thus be deposited on and/or adhere to any wafers present in the reaction chamber. Obviously, this may hinder film growth and inhibit process reactions, and so form a cause of poor film quality. To prevent contamination of the reaction chamber, U.S. '079 teaches the use of a reverse-diffusion preventing body, disposed between the reaction chamber and a furnace opening portion space at a side of the furnace opening, within the furnace. In addition, two independently operable gas exhaust systems are provided: a process gas exhaust system for exhausting process gas from the reaction chamber, and a purge gas exhaust system for exhausting purge gas from the furnace opening portion space. The contaminating furnace opening portion is thus being isolated from the reaction chamber by the reverse-diffusion preventing body, while both the reaction chamber and the furnace opening portion space are both provided with their own gas flow management. This combination of features makes it possible to prevent diffusion of a contaminant from the furnace opening portion space to the reaction chamber, and hence to the wafers.

SUMMARY OF THE INVENTION

Research by applicant to further improve the performance of her thermal processing furnaces has revealed that another, yet unrecognized cause of contamination of the reaction chamber atmosphere exists.

The source of the contamination appears to lie partly in the fact that process gases tend to form a deposit when they are being exhausted via the relatively cold lower portion of the furnace, which includes the flange and the gas exhaust conduit. For instance, when TEOS (tetraethyl orthosilicate, Si(OC2H5)), which may be employed as a precursor in the low pressure chemical vapor deposition of a silicon dioxide (SiO2) film, is exhausted from the reaction chamber, together with reaction by-products, it is observed to form solid and/or viscous-liquid by-products. These by-products are the result of complex chemical (surface) reactions at the lower temperatures in the downstream part of the furnace, and deposit on the flange and in the gas exhaust conduit. Another process in which accumulation of deposit in the downstream part of the furnace is reported is the low pressure chemical vapor deposition of silicon nitride.

In itself the deposition of by-products adjacent the downstream end of the gas exhaust path, i.e. on the flange and in the gas exhaust conduit, does not cause contamination of the reaction chamber. It seems, however, that material deposited at the downstream end of the gas exhaust path may be whirled up and transported back, through the gas passage, into the reaction chamber by recirculating gas flows. During normal operation of a thermal processing furnace such recirculating gas flows are unlikely to occur, both because of the low pressure at which processes are typically carried out and the pressure gradient along the gas exhaust path that is imposed and maintained by the vacuum pump. There are situations, however, during which these factors do not prevent the backflow of deposit. For instance, when, after discharging one wafer boat holding processed wafers from the reaction chamber another wafer boat with a fresh batch of wafers is being loaded into the reaction chamber, the reaction chamber may be at atmospheric pressure and the vacuum pumps may be temporarily switched off. The introduction of the new, relatively cold wafer boat with the likewise cold unprocessed wafers into the relatively warm reaction chamber may cause significant temperature gradients within the reaction chamber, in particular between the outer reaction tube, the liner and the wafer boat. These temperature gradients may induce pressure gradients, which may in turn drive convective flows over the liner. These flows may facilitate particle transport from the downstream end of the exhaust path, via the gas passage, back into the reactor chamber. This way, particles may end up on the wafers, in particular those near the top of the wafer boat.

It an object of the present invention to provide for a thermal processing furnace in which the above-mentioned problem of backflow of material that has been deposited at the downstream end of the gas exhaust path is overcome or mitigated.

It is another object of the present invention to provide for a liner that may be installed in a conventional thermal processing furnace (possibly as a replacement for the originally installed liner) so as to overcome or mitigate the problem of backflow of material deposited near the downstream end of the gas exhaust path.

One aspect of the present invention is directed to a thermal processing furnace. The thermal processing furnace may include a generally bell jar-shaped outer reaction tube having a central axis. It may further include an open-ended inner reaction tube for accommodating a wafer boat holding a plurality of substrates. The inner reaction tube is substantially coaxially disposed within the outer reaction tube, thereby defining a gas passage between an outer wall of the inner reaction tube and an inner wall of the outer reaction tube. At least one of the outer wall of the inner reaction tube and the inner wall of the outer reaction tube may be provided with a flow deflector that protrudes radially from the respective wall into the gas passage.

Another aspect of the present invention is directed to an inner reaction tube for installation and use in a thermal processing furnace of the double-tube type. The inner reaction tube may comprise a generally tubular wall having a central axis, which tubular wall may be provided with a flow deflector that protrudes radially outwardly therefrom.

In the thermal processing furnace according to the present invention, the inner wall of the outer reaction tube and/or the outer wall of the liner may be provided with a flow deflector. The flow deflector, which may take any suitable form (e.g. one or more protrusions, ridges, (cantilever-like) baffles, etc.), may protrude from the respective wall into the gas passage in a substantially radial direction with respect to the central axis of the furnace. The flow deflector's primary purpose is to obstruct, e.g. deflect or break down, somewhat turbulent or erratic upwardly directed gas flows carrying contaminating particles through the gas passage towards the reaction chamber.

In one embodiment the flow deflector may—seen in the direction of the central axis—encircle the inner reaction tube, such that a gas flow through the gas passage in the upward direction of the central axis is obstructed at least once by said flow deflector, irrespective of the angular position of the gas flow relative to the central axis,

In another embodiment, the flow deflector may protrude radially from the respective wall by a distance of at least 75% of a local width of the gas passage, so as to warrant sufficient gas flow obstruction. In an embodiment wherein the flow deflector encircles the inner reaction tube, it may preferably protrude from the respective wall by said distance of 75% of the local width of the gas passage over at least one full encirclement.

In yet another embodiment, the flow deflector may extend along, or be distributed over, the axial length of the inner reaction tube, such that the flow deflector extends (i.e. is present) in all of three equally long axially extending portions of the gas passage that together cover a total length thereof. Such distribution or spreading of the flow deflector over the axial length of the gas passage helps to minimize the size of axial gas passage portions in which strong, upwardly directed gas flows may develop in the absence of by the flow deflector.

These and other features and advantages of the invention will be more fully understood from the following detailed description of certain embodiments of the invention, taken together with the accompanying drawings, which are meant to illustrate and not to limit the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1schematically illustrates in a cross-sectional side view an exemplary vertical thermal processing furnace or reactor1according to the present invention. The furnace1is of a double tube type, and includes a generally bell jar-shaped outer reaction tube30and an open-ended inner reaction tube40. The inner reaction tube40may alternatively be referred to as the liner. The outer reaction tube30may be surrounded by heating means, such as a thermally resistive heating coil22that is powered by an electrical power supply (not shown). The heating means may further be secured to a thermally insulating sleeve (not shown) that surrounds the outer reaction tube30. Both reaction tubes30,40may have a generally tubular, for example circular or polygonal, cross-sectional shape. An outer diameter of the inner tube40may be smaller than an inner diameter of the outer reaction tube30. Accordingly, the inner reaction tube40may be at least partially disposed within the outer reaction tube30, and extend substantially coaxially therewith around a common central axis L. A gas passage20may be defined between an inner wall32of the outer reaction tube30and an outer wall41of the inner reaction tube40. In case the reaction tubes30,40have a similar cross-sectional shape, the gas passage20may have a uniform width along its axial length. The (average) width of the gas passage may typically be on the order of several centimeters, e.g. in the range of 1-5 centimeters. Both tubes30,40may be made of quartz, silicon carbide, silicon or another suitable heat resistant material.

In the configuration shown inFIG. 1the inner reaction tube40may delimit a reaction chamber2in which a wafer boat26is receivable. At their lower ends, both the outer and inner reaction tube30,40may be supported on a flange8, which may be made of stainless steel. The wafer boat26may enter and/or exit the reaction chamber2via a central furnace opening10provided in the flange8. The wafer boat26, which may include a plurality, e.g. between 50 and 150, of slots for holding equally many semiconductor wafers27, may be mounted on a (sleeveless) pedestal28that itself may be mounted on a seal cap or door plate12. The pedestal28may act as a heat shield for both the door plate12and the flange8, and reduce heat loss via the lower portion of the furnace1. In some embodiments, the wafer boat26and the pedestal28may be rotatable by motor means (not shown).

To ensure that the reaction chamber2is sealed in a gas-tight manner, several elastomeric O-rings14may be employed in the lower part of the furnace1, in particular between the outer reaction tube30and the flange8, and between the flange8and the door plate12. Since the reliability of elastomeric O-rings and other seals may diminish when subjected frequently or continuously to high temperatures, the lower part of the furnace1may preferably be kept at a lower temperature than that present in the central and upper parts of the reaction chamber2.

The furnace1may further include a gas injector4. The gas injector4may be disposed within reaction chamber2and include a plurality of gas injection holes6provided over the height or axial length of the wafer boat26. A gas feed conduit18may connect to the gas injector4, possibly via the flange8, so as to enable the introduction of process gases, e.g. precursor and/or purge gases, into the reaction chamber2from the gas injection holes6.

To discharge or exhaust gas from the reaction chamber2, a gas exhaust conduit16may be fluidly connected to a lower or downstream end of the gas passage20, possibly via the flange8(as schematically shown inFIG. 1). A downstream end of the gas exhaust conduit16may be connected to a suction side of a vacuum pump24.

During normal operation, gas introduced into the reaction chamber2from the holes6of the gas injector4flows generally upwardly inside the reaction chamber. It turns around at the open upper end of the inner tube40, and then flows downwardly through the gas passage20between the outer and inner tubes30,40, towards the gas exhaust conduit16connected to the vacuum pump24. InFIG. 1this gas exhaust path is indicated with reference numeral21. While being exhausted, reactive gases may form a deposit as they flow through the relatively cold lower portion of the furnace1, which includes the flange8and the gas exhaust conduit16(in the embodiment ofFIG. 1a part of the flange8).

In itself the deposition of by-products adjacent the downstream end of the gas exhaust path21does not cause contamination of the reaction chamber2. Under certain conditions, however, material deposited at the downstream end of the gas exhaust path21may be whirled up and be transported back, via the gas passage20, into the reaction chamber2by recirculating gas flows. For instance, when after discharging one wafer boat26holding processed wafers27from the reaction chamber2another wafer boat with a fresh batch of wafers27is being loaded into the reaction chamber2, the reaction chamber2may be at atmospheric pressure and the vacuum pump24may be temporarily switched off. The introduction of the new, relatively cold wafer boat26with the likewise cold unprocessed wafers27into the relatively warm reaction chamber2may cause significant temperature gradients within the reaction chamber, in particular between the outer reaction tube40, the inner reaction tube30and the wafer boat26. These temperature gradients may induce pressure gradients and/or gas density gradients, which may in turn drive convective flows over the inner reaction tube30. These flows may facilitate particle transport from the downstream end of the exhaust path21, via the gas passage20, back into the reactor chamber.2This way, particles may end up on the wafers27, in particular those disposed near the top of the newly introduced wafer boat26.

To prevent such back flow of deposit, the outer wall41of the inner reaction tube40and/or the inner wall32of the outer reaction tube30may be provided with a flow deflector50. The flow deflector50may protrude from the respective wall into the gas passage20, in a generally radial direction with respect to the central axis L of the reaction tubes30,40.

In the thermal furnace1ofFIG. 1both the outer reaction tube30and the inner reaction tube40are provided with a flow deflector50in the form of a single annular baffle52that protrudes radially into the gas passage20. The flow deflectors50are provided at a point about halfway the axial length of the gas passage20, and sufficiently close to each other to define a narrow Z-shaped gap between themselves and the walls32,41through which gas may pass. The baffles52of the flow deflectors50encircle or surround the inner reaction tube40, such that they necessarily obstruct the flow of gas through the gas passage20in the direction of the central axis L, irrespective of the angular position of the gas flow relative to the central axis.

It will be clear that the annular baffles52in the embodiment ofFIG. 1encircle the inner reaction tube40. With an eye to some more elaborate flow deflector embodiments, however, the following is noted. Whether or not a flow deflector encircles the inner reaction tube40(at least once) may be best judged by viewing the double tube structure30,40in the direction of the central axis L. In such an axial view, a flow deflector that encircles the inner tube40will normally be visible, and be seen to extend through an angle of 360° around the axis L. Hence, to encircle the inner tube40it is no requirement that the flow deflector extends around that tube at a single axial position, such as the baffles52inFIG. 1. Neither is it necessary for the flow deflector to consist of a single part. As will be illustrated with reference toFIGS. 2 and 3, a flow deflector may be composed of multiple parts, e.g. baffles, that may be provided at different axial positions, which parts together encircle the inner tube40in the sense just defined.

In order to warrant an efficient obstruction of a backflow, a flow deflector50may preferably protrude sufficiently far into the gas passage20. Precisely what is ‘sufficiently far’ may depend in particular on the (local) width of the gas passage20, i.e. on the (local) distance between the inner wall32of the outer reaction tube30and the outer wall41of the inner reaction tube40. In general, the flow deflector50may preferably protrude radially from the wall on which it is provided over a radial distance of at least 75% of a local width of the gas passage20. For example, in case the outer and inner reaction tubes30,40define a cylinder jacket-shaped gas passage20with a uniform width of 25 millimeters along the central axis L, the flow deflector50may preferably extend a radial distance of at least 19 millimeters (i.e. 0.75*25 mm) into the gas passage20. In case the inner reaction tube40is disposed slightly off-axis, e.g. by 5 mm, such that the width of the gas passage20varies in the tangential direction between 20 and 30 mm, the distance over which the flow deflector50protrudes into the gas passage20may vary correspondingly, e.g. between 15 and 23 mm.

The outer and inner reaction tubes30,40are normally manufactured individually, and assembled at a later stage to form the double tube structure of the furnace1. To enable such assembly, during which the inner reaction tube40is carefully moved into the outer reaction tube30, at least a few millimeters of clearance between the two components is desirable. The clearance may preferably be at least 2 millimeters, and more preferably be in the range of 2-8 millimeters. Accordingly, a flow deflector may preferably protrude radially from the wall on which it is provided over a radial distance of no more than a local width of the gas passage20minus at least 2 millimeters, or preferably 2-8 millimeters, depending on the desired clearance.

As mentioned, a flow deflector may comprise multiple baffles, which may be provided on the walls32,41of the outer and/or inner reaction tubes30,40. Several embodiments of a such a flow deflector50will now be elucidated with reference toFIGS. 2 and 3. It is noted in advance that in the embodiments depicted inFIGS. 2-3, the baffles52of the flow deflector50are provided on the outer wall41of the liner40, which liner is shown in isolation. One skilled in the art will appreciate, however, that similar patterns of baffles may alternatively, or in addition, also be provided on the inner wall32of the outer reaction tube30.

FIG. 2schematically illustrates two embodiments of a liner40. Each embodiment features a flow deflector50comprising a plurality of identical baffles52that protrude radially from, and extend substantially tangentially along, the outer wall41of the liner at different axial positions. The two embodiments have a number of characteristics in common.

In both embodiments each of the baffles52extends tangentially along the outer wall41of the liner40through an angle α of approximately 35 degrees relative to the central axis L. It is contemplated, however, that in other embodiments the angle α of subtended by at least some of the baffles52may be smaller or larger than 35 degrees, e.g. be in the range of 10-90 degrees. Furthermore, the baffles52in both embodiments extend substantially tangentially, meaning that they do not, or at least not significantly, extend in the axial direction L. In other embodiments, however, one or more baffles52may extend along the outer wall41in a direction having both a tangential and an axial component (cf. the embodiments depicted inFIG. 3).

In either of the embodiments ofFIG. 2, the baffles52are disposed at a discrete number (six) of equidistantly spaced apart axial positions, spread across the axial length or height of the liner40. Consequently, when the liner40is incorporated in a thermal furnace1similar to that shown inFIG. 1, the flow deflector50will be approximately uniformly distributed over the length of the gas passage20, at least such that it extends in all of three equally long axially extending portions of the gas passage20that together cover the total length thereof (e.g. in the depicted orientation: a bottom portion, a middle portion and a top portion of the gas passage20).

Each of the axial positions in the embodiments ofFIG. 2features a series of a same number of equidistantly tangentially spaced apart baffles52; six and three for the embodiment ofFIG. 2AandFIG. 2B, respectively. The configuration of baffles52at the different axial positions is thus the same, beit that a series of baffles at a certain axial position has each time been rotationally offset relative to a series of baffles at an adjacent axial position. The series of baffles52at different axial positions have been rotationally offset relative to each other, and may partially overlap which each other (seen in the axial direction L), in such a way that, seen in the axial direction L, the flow deflector50—i.e. all the baffles52considered in conjunction—completely encircle the liner40. In fact, they may be considered to encircle the liner40more than once. In the embodiment ofFIG. 2each two (axially) adjacent series of baffles52taken together encircle the liner40, while in the embodiment ofFIG. 3the baffles52at each set of four adjacent axial positions account for one full encirclement.

Due to the fact that the flow deflector50is configured such that it encircles the liner40more than once, a gas flow traveling along the outer wall41of the liner40in the axial direction L may be obstructed several times by different baffles52of the flow deflector50. Furthermore, because the flow deflector50is approximately uniformly distributed over the axial length of the liner40, there is no particular axially extending portion of the outer wall41that is devoid of baffles52and that may for that reason facilitate the development of relatively strong back flows. Instead, the flow deflector50may be considered as somewhat of a maze made up of flow breaking/deflecting baffles52that scatter developing, axially directed flows that might be capable of transporting deposit.

FIG. 3schematically illustrates another two exemplary embodiments of a liner40according to the present invention. Both depicted liners40feature a flow deflector50including a number of baffles52that extend helically along the outer wall41of the liner, around the central axis L. Each of the baffles52extends along substantially the entire axial length of the liner40, thereby ensuring that, when the liner40is incorporated into a thermal furnace as shown inFIG. 1, the flow deflector50extends in all of three equally long axially extending portions of the gas passage20that together cover a total length thereof.

The liner40in the embodiment ofFIG. 3Aincludes four baffles52, while the liner in the embodiment ofFIG. 3Bincludes only two baffles. The baffles52in either embodiment are arranged such that the flow deflector50, seen in the direction of the central axis L, encircles the liner40. In addition, the number of baffles52is in both embodiments related to the angle α subtended by a single baffle52with respect to the central axis L as seen in the direction of the central axis. For instance, in the four-baffle-embodiment ofFIG. 3Aeach baffle52subtends an angle α of (360/4=) 90 degrees, while in the two-baffle-embodiment ofFIG. 3Beach baffle subtends an angle α of (360/2=) 180 degrees. In general, flow deflectors50including a plurality of n helically extending baffles52, wherein each baffle subtends an angle of at least 360/n degrees with respect to the central axis L, appear quite effective in back flow prevention. They may also be manufactured economically, in particular because the number of baffles n that needs to be provided may be relatively small, e.g. four or less.

From the perspective of back flow prevention, it may be tempting to construct and employ a flow deflector50with a relatively large number of baffles52. However, a larger number of baffles52may mean an increase in flow resistance along the exhaust path21, which in turn may increase the demands placed on the vacuum pump24of a thermal processing furnace1. Numerical simulations have shown that the increase in flow resistance caused by the presence of a modest number of helically extending baffles52, as shown inFIG. 3(i.e. up to and including four baffles), is relatively small and practically of no concern to most applications.

Although illustrative embodiments of the present invention have been described above, in part with reference to the accompanying drawings, it is to be understood that the invention is not limited to these embodiments. Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, it is noted that particular features, structures, or characteristics of one or more embodiments may be combined in any suitable manner to form new, not explicitly described embodiments.

LIST OF ELEMENTS

1thermal processing furnace/reactor2reaction chamber4gas injector6gas injection holes8flange10central furnace opening12door plate14O-ring16gas exhaust conduit18gas feed conduit20gas passage21gas exhaust path22heating coil24vacuum pump26wafer boat27substrate/wafer28pedestal30outer reaction tube31outer wall of outer reaction tube32inner wall of outer reaction tube40inner reaction tube/liner41outer wall of inner reaction tube42inner wall of inner reaction tube50flow deflector52baffle of flow deflectorL central axis of both the inner and outer reaction tuben number of baffles of flow deflectorα angle subtended by baffle of flow deflector