FURNACE AND CONTROL METHOD THEREFOR

Abstract: An "atmospheric" processing furnace (1) is insulated from the naturally-occurring variations in ambient pressure. In one embodiment a gas valve (9) in an exhaust duct (7b) from a sealed scavenger box (7) is controlled by a feedback loop including a control system (10) and a pressure sensor (11).

Description: FURNACE AND CONTROL METHOD THEREFOR Background of the Invention 1. Field of the Invention This invention relates to a furnace for the heat treatment of product and in particular for the heat treatment of semiconductor material used in the fabrication of microelectronic components. The invention also relates to a method of operating a furnace for the heat treatment of semiconductor material. 2. Description of the Related Art It is known to subject wafers of semiconductor material to a variety of different processing operations in a furnace which comprises a tube of highly purified refractory material. A tube of very pure quartz is widely used, the product for treatment being loaded into the tube from one end and heated therein by electrical heating means operating from without the tube. Tubes extending vertically or horizontally are normally used. In the fabrication of microelectronic components it is known to subject wafers of semiconductor material to heat treatment in a variety of different gas environments and these operations are commonly carried out at atmospheric pressure, the appropriate gas being led into the furnace tube at or adjacent to the upstream end thereof and allowed to exhaust from the downstream end of the tube to a scavenger box from which the exhaust gas is drawn away via an exhaust duct. It has long been appreciated that precise control of temperature and gas flow rates is required for reliably consistent semiconductor fabrication techniques but heretofore no attempts have been made, when operating substantially at atmospheric pressure, to compensate, by controlling pressure, for variations in ambient pressure which naturally occur from day to day. Sealed unit low pressure and high pressure systems are available for the heat treatment of product but these systems are custom designed and are relatively expensive. Summary of the Invention Surprisingly, I have now found that improved product (and in particular improved semiconductor fabrication) is possible if in addition to temperature and gas flow rate control in a furnace operating substantially at atmospheric pressure, means is provided to control the pressure so that the daily atmospheric pressure changes that naturally occur do not affect the absolute pressure (or at least the partial pressure of a reactive gas) existing within the furnace. Thus, one aspect of the present invention relates to a method of operating a processing furnace having a gas exhaust which method includes controlling the pressure of the gas exhaust at a constant level which is substantially always above ambient pressure. By choosing a small supra- atmospheric pressure which is somewhat above the maximum ambient pressure likely to occur naturally in the environment in which the furnace is located, and monitoring the exhaust gas pressure to maintain it at this level, I have found it is possible to operate the furnace in an improved manner when an identical operating pressure exists in the furnace at all times. The maximum atmospheric pressure which is likely to occur will, in practice, be related to the altitude of the furnace above sea level and the climatic zone in which the furnace is located. In the United Kingdom, the highest atmospheric pressure recorded is around 1060 millibars and world-wide the maximum figure would be around 1090 millibars (both expressed as sea-level atmospheric pressures) so operating at an effective pressure at sea level of 1100 millibars would be to use a pressure higher than would be expected to be exceeded naturally anywhere in the world. In practice an operating pressure of around 1050 millibars would exceed virtually all naturally occurring atmospheric pressures arising in the United Kingdom and 1040 or even 1030 millibars would be an operating pressure that would be exceeded rarely enough to be used with confidence. If the furnace is to be operated at altitude a considerably lower pressure could be chosen without risk of it being exceeded by the naturally occurring atmospheric pressure so the invention can be thought of as operating at a constant pressure which is between 0.05 and 40% above the naturally occurring ambient pressure and preferably between 0.5 and 10% or even 1 and 6%. The furnace may be located in a "clean room" or other location where some degree of overpressure may be used for dust ingress control. To allow for this possibility the phrase "ambient pressure" is used in this specification to cover the pressure occurring in the environment in which the furnace is located. According to a further aspect of the invention a method of operating a processing furnace, and in particular a semiconductor processing furnace, comprises controlling the partial pressure of the reacting gases at a constant level which is substantially always below the minimum ambient pressure likely to occur naturally in the environment in which the furnace is located. By monitoring the exhaust gas pressure to maintain it at this level, I have found it is possible to operate the furnace in an improved manner when an identical operating partial pressure of the reactant gas exists in the furnace at all times. The minimum atmospheric pressure which is likely to occur will, in practice, be related to the altitude of the furnace above sea level and the climatic zone in which the furnace is located. A deep low pressure is quoted at 980 millibars, while the lowest atmospheric pressure recorded is 870 Millibars during a typhoon in October 1979 (both expressed as sea-level atmospheric pressures) so operating at an effective pressure at sea level of 860 millibars would be to use a pressure lower than would be expected to occur naturally anywhere in the world. In practice an operating pressure of around 950 millibars would be below virtually all naturally occurring atmospheric pressures arising in the United Kingdom and 900 millibars would be an operating pressure that would be exceeded rarely enough to be used with confidence. If the furnace is to be operated at altitude a considerably lower partial pressure must be chosen to avoid risk of it being exceeded by the naturally occurring atmospheric pressure. This pressure dependency is approximately a lOmbar reduction for every 100 meters altitude. So a reduction of 160 mbars on the equivalent sea-level pressure would be adequate for most operations. A further advantage of the invention is that it allows optimum processing conditions determined for an "atmospheric" furnace operating at one geographical location to be reproduced in a furnace operating at a different geographic location having a different altitude above sea level and/or located in a different climatic zone. This would means that, for example, processing conditions for high temperature semiconductor fabrication optimised by research using an "atmospheric" furnace at a location 1000 metres above sea level could be reproduced in an "atmospheric" furnace at one processing station at sea level and at another at 2000 metres elevation by operating all three furnaces at the same pressure. In the case of operating at a slight supra-atmospheric pressure this operating pressure will be much closer to the average ambient pressure occurring at the sea level station than it would at either of the stations at altitude. This feature of the invention is expected to be of particular importance to multinational companies operating processing stations at many different locations around the world but wishing to standardise a quality control. In one embodiment the method of the invention involves feeding the gas exhaust from the furnace into a scavenger box having a door and an exhaust duct, the door being sealed to the box. Suitably the exhaust duct includes a pressure- sensitive motorised valve and control means for maintaining the valve at a degree of opening such that a pre-determined small supra-atmospheric pressure exists in the scavenger box. In one preferred arrangement, a bleed gas (e.g. nitrogen) is fed to the scavenger box to maintain the pressure in the box at the required slight excess above the ambient pressure existing in the furnace environment (e.g. in a "clean room"). According to a further aspect of the present invention a tubular processing furnace comprising a furnace tube, means for heating the furnace tube, means for supplying gas to an upstream end of the furnace tube, a scavenger box at the downstream end of the furnace tube into which exhaust gas from the furnace tube flows, a door to the scavenger box to provide access to the downstream end of the furnace tube and an exhaust duct for removing gas from the scavenger box, is characterised in that the door seals to the box and means is provided to maintain the pressure of gas in the box to a predetermined constant level which exceeds the maximum naturally occurring ambient pressure. Conveniently a loading system is provided for the furnace which comprises an end plate for the furnace tube and a scavenger door for the scavenger box, and is characterised in that the scavenger door is provided with means to seal to the scavenger box and in that means is provided for controlling the gas pressure in the scavenger box to a pre-determined constant level which is at least slightly supra-atmospheric. The invention also extends to a kit of parts to adapt a conventional atmospheric semiconductor processing furnace for operation in accordance with the method herein described. Brief Description of the Drawings The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic of a conventional semiconductor atmospheric furnace shown in cross-section, Figure 2 is a detail of the downstream end of the furnace of Figure 1 showing how it can be modified in accordance with the method of the invention, Figure 3 is a view corresponding to Figure 2 showing a modified means for operating in accordance with the method of the invention, Figure 4 is an enlarged view of the end of the tubular furnace of Figure 3 showing a convenient means for sealing the interior of the scavenger box to the furnace tube, Figure 5 is a view similar to Figure 4 showing an alternative sealing means, Figure 6 is a schematic of a loading arrangement in accordance with a further aspect of this invention, Figure 7 is a schematic end view of the loading arrangement shown in Figure 6, Figure 8 is a schematic view of how the inlet end of a conventional "atmospheric" processing furnace can be modified to ensure the partial pressure of reacting gas in the furnace is at a constant level which is never greater than the existing atmospheric pressure , and Figure 9 is a graphic illustration of the principle behind the present invention. Description of Preferred Embodiments Referring first to Figure 1, the atmospheric furnace shown consists of the following: A furnace tube 1 (normally manufactured of high purity quartz glass, silicon carbide or polysilicon - although other materials providing a low contamination environment for processing within the furnace may be used) with a heating element 2 arranged concentric with the furnace tube 1. A pair of "vestibule blocks" 3 are employed to support opposite ends of the furnace tube 1 and a pair of vestibule housings 4 are used to support the vestibule blocks 3 and provide a snug fit to the heating element 2 to assist in keeping the components aligned. A fume extracted gas supply system is connected to the vestibule housings 4, process gas being fed into the furnace tube 1 by means of a tight connection 5 (normally a ground glass clamp joint - but other systems can be used) at the upstream end. In order to minimise the risk of gas leaks to the environment outside the gas supply system, the gas supply system should be a snug fit to the vestibule housing 4 at the upstream end of the furnace tube 1 and the gas supply system is fume extracted at its downstream end by means of a scavenger box 7. An end plate (or end cap) 6 covers the downstream end of the furnace tube 1 and is used to reduce ingress of atmosphere into the furnace tube. Normally a hole 6a is made in the end plate 6 to provide an exhaust for the process gases fed into the tube 1 from the connection 5. The fume-extracted scavenger box 7 connects to the downstream vestibule housing 4 and reduces the risk of gases from the furnace escaping to the environment outside the scavenger box. The scavenger box has an exhaust duct 7b which is often fitted with a manually adjustable baffle 7a to regulate the exhaust flow. A scavenger box door 8 is provided to prevent any explosion in the system from causing a personal safety hazard. The scavenger box door 8 has a hole 8a in its middle to prevent a seal being made and therefore operates at the pressure of the space communicating through the hole 8a (typically the clean room air pressure which will be typically less than one millibar above atmospheric pressure for dust control purposes) . To ensure that pressures above the naturally occurring ambient pressure do not occur within the scavenger box 7, arrangements are typically made to ensure that the "closed" position of the scavenger box door is spaced away from (typically from 5 to 15 millimetres away from) the scavenger box itself. The end plate 6 and scavenger box door 8 are often mechanically connected when a load carrier for the furnace is used. Figure 1 does not show this arrangement. The process gas flows through the furnace are shown by the double arrows in Figure 1 and the clean room (or ambient) gas flows by triple arrows. It should be noted that the heating element 2, vestibule blocks 3, vestibule housings 4 and scavenger box 7 connections are either sealed or are of a snug-fit construction in order to minimise thermal loss from the furnace and to reduce the risk of exhaust gases from the furnace tube 1 leaking into areas where they should not nass. The known scavenger box extraction system operates on a negative pressure principle, i.e. the exhaust is fume extracted at a rate sufficient to ensure that gas will flow from the outside of the scavenger box to the inside of the scavenger box under all operating conditions and it is for this reason that it is a requirement of the operation of a conventional furnace that the scavenger box is not sealed. The fume extract rate is typically manually adjusted using the baffle 7a, conventional fans being provided to create a differential pressure in the duct 7b which is below the presently-existing ambient pressure. A schematic of the invention is shown in Figure 2 and where applicable, the same reference numbers have been used in Figure 2 as have been used in Figure 1. In Figure 2 only the downstream end of the furnace tube 1 and the scavenger box 7 is shown. However the manual baffle 7a has been replaced with a motorised valve 9 in the exhaust duct 7b. This valve 9 is controlled by a feedback loop from a control system 10 sensing pressure via a sensor 11 taking its input directly from the scavenger pressure. The scavenger door 8 has no vent hole and may be sealed using an O-ring 12. Several implementations of the new system to control the pressure and provide an additional safety seal are possible. These can be classified into two categories, one for manual loading systems and the other for automatic ("paddle" or cantilevered) systems. Some essential features are common to both however. In the first instance, the furnace tube 1 may be sealed against the scavenger box 7 to prevent blow-back down the outside of the furnace tube. The normal condition is to use a quartz wool or similar to minimise thermal loss. This may, in itself, be sufficient to minimise blow-back. However an added safety feature is to provide positive sealing such as shown in Figure 3. Here a sealing block 13 is attached to the scavenger box 7 and to the furnace tube 1. The pressure sensor 11 feeds into a conventional control loop which drives the motorised valve 9 thus regulating the pressure within the scavenger box (and thus within the furnace tube) and thereby over-riding the daily pressure changes which would otherwise modify the operating conditions within the furnace tube 1. Figure 4 shows an enlarged view of the sealing block 13 shown in Figure 3. In Figure 4 mounting blocks 21 fix an 0- ring seal (shown as a double O-ring seal 22 but a single seal on the furnace tube side of the mounting blocks 21 could be used) to the downstream vestibule block housing 4. Extending from this, towards the furnace tube 1, is a pair of threaded flanges 23 and 24, between which is a compression O-ring 25 which completes the seal against blow- back down the outside wall of the furnace tube 1. Figure 5 shows a further arrangement in which the furnace tube 1 is fitted with a flange 26 which locates against the vestibule housing via an O-ring 27. The large diameter of the flange 26 ensures that O-ring 27 is kept cool and thermal damage thereto is reduced to an acceptable level. The size of the O-ring 27 also reduces the possibility of contamination from the seal system diffusing into the interior of the furnace tube 1. Another option for a system according to the invention is to leave the scavenger exhaust duct 7b slightly open to the ambient atmosphere. The control valve 9 can be located in another port in the scavenger box through which a nitrogen (or other bleed gas) is inlet. Since this system need only control to a small fixed supra-atmospheric pressure, increasing the nitrogen flow can regulate the pressure and this has an advantage in that the control baffle primarily passes an inert gas thus reducing corrosion of it to a minimum. In each of these cases, the scavenger door 8 is manually closed against the scavenger box 7 and sealed via an O-ring. This is the main distinguishing feature between the two proposed systems, the manual load and the automatic load. 5 In the case of an automatic load system, a different mechanism is proposed to seal the scavenger box door 8. Figure 6 shows a layout of this different mechanism. A loading "paddle" 30 is moved in and out of the furnace tube 1. The furnace tube may be fitted with the sealing blocks 10 23 as shown in Figure 4. The mouth of the furnace tube 1 is covered by a quartz or similar end plate 6. The plate 6 can be provided with an attached gas outlet port (not shown) directing the exhaust towards the scavenger box extract. The end plate 6 is pressed against the furnace tube 1 by a 15 spring-loaded assembly 31. The scavenger box door 8 is held back from the scavenger box 7 allowing free ingress of clean room ambient. The invention provides a new design of this system which allows both the furnace tube 1 and the scavenger door 20 8 to seal. The added features are included in Figure 6 as items 32 to 34. Item 32 is an O-ring seal on the paddle loader 30. Mechanical bellows, or springs, 33, connect this to the scavenger door 8. Attached to the scavenger door is a similar set of spring-loaded rods shown at 34 which are, 25 however, attached to the paddle loader 30. Thus when the loading system closes, the end plate 6 first meets the furnace tube 1, the compression springs 31 take the load until the scavenger door 8 meets the scavenger box 7, thereupon the compression springs 34 force a seal against 30 the scavenger box 7 itself. This seal may be metal to metal or may use an O-ring. The control system can be as described previously. An end-on view of the system shown in Figure 6 is given in Figure 7. 36 is the paddle port, which though drawn 35 square in Figure 7, may be of any shape (e.g. circular in cross-section) . 37, 38 and 39 are the spring-loaded push rods, attached to which is the quartz or other refractory end plate 6 to close the furnace tube 1. 40, 41 and 42 are push rods, attached to which is the scavenger door 8. The number of push rods can vary. Three suitably provides a balanced system, though one, two or more than two could be used. In setting up the system the amount of compression needs to be known. A convenient way of doing this is to extend the push rods through the scavenger door, within glass, or similar transparent, fittings, maintaining the seal -but allowing a view of the push rod extensions which have calibration markings on them. This has been shown in Figure 6 at 43. A final feature which is recommended is a weakened (e.g. glass) wall at the exhaust port side to act as a safety release in case of a system failure and an excessive over-pressure condition arising in the scavenger box. Figure 8 shows how the partial pressure of a reactant gas in a furnace tube 101 is kept constant at a level below that of the lowest atmospheric pressure ever likely to occur at the geographical location of the furnace. A pressure gauge 111 sensing the pressure in a scavenger box 107 feeds a signal to a control system 110 which drives a valve 109 controlling the supply of one gas (e.g. an inert gas) to the furnace tube 101. The loop 111, 110, 109 acts to ensure the partial pressure of the reactant gas also fed to the furnace tube 101 remains at a constant sub-atmospheric pressure. Although the pressure gauge 111 is shown monitoring pressure in the box 107 it can monitor the current ambient pressure at any convenient location. A further embodiment of the method of the invention involves feeding the gas exhaust from the furnace into a scavenger box having an exhaust duct. Suitably the scavenger box includes a pressure gauge used to monitor the exhaust pressure. Using the absolute pressure value, a control signal may be sent to the gas inlet to the furnace tube such that the partial pressures of the reacting gases may be maintained either by (a) adjusting the respective ratios or flow rates, or (b) varying the proportion of reactive gas(es) in the furnace by the introduction of an inert gas if one is not already present in the gas mix. Thus normal atmospheric pressure fluctuations may be compensated by maintaining constant the partial pressure of the reacting species. Figure 9 shows graphically the principle of the method of the invention, A being the pressure chosen for the exhaust gas in the scavenger box and B showing the diurnal variations in atmospheric pressure which variations are shielded from the processing reaction in the furnace connected to the scavenger box by the pressure A. Although it is clearly preferred that the pressure A chosen for the exhaust gas be always above the ambient pressure, considerable advantage would still be secured by operating at a level for A which is only rarely exceeded by the ambient pressure and the scope of the invention should be understood to cover such a mode of operation. Figure 9 has greatly exaggerated the diurnal variations and the pressures noted are purely exemplary for the purposes of illustration. Figure 9 also shows a pressure C being the pressure chosen for the partial pressure of the reacting gas. The diurnal variations B in atmospheric pressure are shielded from the processing reaction in the furnace by the pressure C. Although it is clearly preferred that the pressure C chosen for the operating partial pressure be below any possible naturally occurring pressure, considerable advantage would still be secured by operating at a level for C which is only rarely attained by the ambient pressure and the scope of the invention should be understood to cover such a mode of operation. In the absence of any action to control the pressure in an atmospheric processing furnace, naturally-occurring changes in ambient pressure are likely to produce variations in the absolute pressure within the furnace of about 50 mbars over a period of a few weeks. By operating in accordance with the method of the invention an improvement of at least one order (i.e. variations around 5mbars) preferably at least two orders (around 0.5 mbar) and more preferably three orders (around 0.05 mbar) or better (around 0.01 mbar) can be achieved. From what has been said, it will be appreciated that the invention allows furnace processes to run at a nominal atmospheric pressure. The actual pressure is less important than the fact that the pressure is controlled to be the same pressure time after time. A slight modification of conventional furnaces as described here will control this pressure. The nature of the control system dictates that the pressure be slightly above ambient or that the partial pressure of the reacting gas be slightly below ambient. The system must be run either at slightly higher than or slightly lower than the maximum atmospheric pressure, allowing for normal weather variations. The oxidation of silicon is one process which is expected to be an immediate benefactor of this invention. However there are other "atmospheric" processes which can also benefit. Some deposition processes use particularly toxic gases, such as phosphene or diborane and could benefit from the safety aspect of the system, as much as the improved pressure control.

Claims: CLAIMS 1. A method of operating a processing furnace substantially at atmospheric pressure, which includes controlling the absolute pressure in the furnace, or at least the partial pressure of a reactive gas in the furnace, to a substantially constant level irrespective of changes in ambient pressure. 2. A method of operating a processing furnace according to claim 1 which has a gas exhaust, which includes controlling the pressure of the gas exhaust to a substantially constant level which lies in the range between 0.05 and 40% above ambient pressure. 3. A method of operating a semiconductor processing furnace according to claim 2, which includes controlling the pressure of the gas exhaust so that variations in absolute pressure within the furnace do not exceed 5 millibars and preferably do not exceed 0.5 millibar. 4. A method according to claim 2, in which the gas exhaust feeds into a scavenger box having a door and an exhaust duct which includes a pressure-sensitive valve, the door being sealed to the box. 5. A method according to claim 4, in which a bleed gas is fed to the scavenger box to always maintain a pressure in the box which is a variable but slight amount above current atmospheric level whereby the operating pressure in the furnace is unaffected by changes in ambient pressure. 6. A method of operating a processing furnace according to claim 1, in which the partial pressure of reacting gas(es) in the furnace is controlled to be at a constant level which is substantially never greater than the existing ambient pressure. 7. A tubular processing furnace comprising a furnace tube, means for heating the furnace tube, means for supplying gas to an upstream end of the tube, a scavenger box at the downstream end of the box into which exhaust gas from the tube flows, a door to the scavenger box to provide access to the downstream end of the tube and an exhaust duct for removing gas from the scavenger box, characterised in that the door seals to the box and means is provided to maintain the pressure of gas in the box at a predetermined constant level which exceeds the maximum naturally occurring ambient pressure. 8. A furnace according to claim 7, in which the pressure maintaining means is set to maintain an overpressure of between 0.05 and 40% of said maximum ambient pressure. 9. A tubular processing furnace according to claim 7 or claim 8, in which a loading system for the furnace comprises an end plate for the tube and a scavenger door for the scavenger box, characterised in that the scavenger door is provided with means to seal to the scavenger box, and in that means is provided for controlling the pressure in the scavenger box to a predetermined constant level which is at least slightly supra-atmospheric. 10. A kit of parts to adapt a conventional semiconductor atmospheric processing furnace for operation in accordance with the method of any one of claims 1 to 6.

Inventors: FALLON MARTIN, FALLON, MARTIN
Applicants: FALLON MARTIN, FALLON, MARTIN
Application year: 1993
Main classification: C30B13
IPC codes: C30B31, F27D19