A method of this kind is known from American patent 4,925,388, which describes a furnace in which, for cooling purposes, a gas is guided through the furnace either from above or from below. The direction of flow of the gas alternates, and this alternating movement is controlled by a number of valves. The gas moves in the space between the heating element and the core tube and in the space between the heating element and the insulation arranged at a distance therefrom. The air or other gas which is used as cooling medium is discharged via a fan.

It has been found that the discharge of such gases into a discharge system of a relatively large installation is accompanied by major drawbacks, since by definition these gases are hot and consequently can damage a discharge system of this kind, in particular if it is composed of plastic components. Moreover, harmful substances may be present in the gases.

The object of the present invention is to avoid this drawback and to provide a method which, on the one hand, allows rapid and uniform cooling and, on the other hand, does not subject the gas-discharge system to further loading.

This object is achieved by means of the characterizing features of Claim 1.

In principle, it would be possible to discharge the gas solely via the heat exchanger and to guide it into the gas-discharge system, i. e., in contrast to the measures described in Claim 1, without using a closed circuit. However, the drawback of this is that it is constantly necessary to introduce additional gas and, in the case of air, this will generally emanate from the clean room, i. e. it will have been purified extensively. As a result, a particularly large installation for cleaning air is required, with all the associated costs. Moreover, it is undesirable to periodically withdraw air from or introduce air into a clean room.

If, in accordance with the invention, the gas is guided in a closed circuit with a heat exchanger, it is not necessary to constantly supply fresh gas, such as air. Moreover, it is possible as a result to make use of a gas having more specific properties, such as nitrogen. This is because basically there is no loss of gas, with the result that the extra costs of nitrogen or some other gas can be justified. The method

according to the invention makes it possible to treat a greater number of wafers per hour, and the thermal loading to which each wafer is subjected will be reduced. Moreover, the treatment of the various wafers is more uniform.

It has been found that alternating the direction of flow of the cooling gas at a certain frequency enables the temperature gradient across the furnace to be limited as far as possible. It will be understood that the hottest part will now be situated in the centre of the furnace. Moreover, it has been found that the cooling rate can be increased considerably by comparison with designs in accordance with the prior art.

According to an advantageous embodiment of the invention, the reversal frequency of the flow of gas lies between 2 and 600 seconds, and more particularly between 5 and 60 seconds. Advantageously, the reversal frequency lies between 10 and 20 seconds. The above-described design makes it possible to achieve a cooling rate in the region of the wafers of approximately 50°C per minute.

The invention also relates to a furnace assembly, comprising a furnace which is provided with a cooling device for using gas to cool the boundary wall of said furnace, a cooling chamber for gas being present in the region of said boundary wall, which chamber opens out on both sides into a line which is provided with valves, which valves comprise diverter valves, in order to move the gas alternately in one direction along said boundary wall and then in the opposite direction along said boundary wall, in which said diverter valves are connected to a closed circuit in which a liquid-gas heat exchanger is arranged.

According to an advantageous embodiment of the invention, the supply/discharge of gas is arranged in the outer casing of the furnace assembly, i. e. is effected via the wall and not via the end closures, such as the base or cover. This allows a more uniform distribution of flow to be achieved. This is because the movement of the gas in the gap between heating element and core tube is particularly difficult to control. However, this control is important if sufficient uniformity is to be ensured. This distribution of flow can be promoted further by introducing and discharging the gas via a number of openings which are arranged in a ring or other regular curve along the circumference of the furnace wall. These openings form a restriction to the gas flow and in fact locally determine the metering of the gas flow. By distributing these openings regularly along the wall, it is possible to ensure an even

distribution of the gas flow, and hence an even dissipation of heat.

It is possible to design each diverter valve as a single or double valve, provided that the natural convection is eliminated.

The invention will be explained in more detail below with reference to an exemplary embodiment depicted in the drawings, in which: Fig. 1 diagrammatically illustrates the principle of the method and device according to the invention; Fig. 2 shows temperature curves of a number of wafers in a conventional furnace during cooling; and Fig. 3 shows temperature curves at various levels in a furnace according to the invention during rapid cooling.

The treatment furnace according to the invention is denoted overall by 1, and is disposed in a treatment chamber which is otherwise depicted diagrammatically. This furnace comprises an outer casing 7 made, for example, of insulating material, against which a heating element 9 bears. A core tube 5 made, for example, of quartz material or silicon carbide is arranged at a distance from this diagrammatically depicted element 9, in which tube it is possible to accommodate a boat 20 provided with wafers 21-25.18 denotes the top cover of the core tube. Wafer rack 20 is supported by cover 15, which can be moved up and down in a diagrammatically depicted manner in order to remove the boat 20 from the core tube 5. Between heating element 9 and core tube 5, there is arranged a gap 6, through which cooling gas, such as nitrogen, can be moved. Two series of approximately sixteen openings 16 and 17 each are arranged in outer casing 7. Lines 3 and 8, which are respectively connected to valves 12,13 and 10,11, are connected to these openings. These valves are in turn connected to a gas-liquid heat exchanger 4 and a fan 2.

The structure described above operates as follows. During cooling of treated wafers, in a first operating condition cool gas emanating from line 8 is introduced through openings 17 into the gap 6 between core tube 5 and heating element 9. This gas moves downwards in the direction of the illustrated arrow, and leaves the furnace in a heated state at 16, after which it is guided via pipe 3 and valve 12 to heat exchanger 4. After having been cooled, it is guided through fan 2 towards valve 11, and is then fed back into line 8 and then to the treatment furnace.

After a certain time, such as typically 20 seconds, the direction of flow is reversed by switching over the valves, i. e. closing the valves 11 and 12 and opening the valves 10 and 13. This means that

cooling gas moves from line 3 and openings 16 towards openings 17 and line 8. From line 8, the gas passes through valve 10 towards heat exchanger 4 and, via fan 2 and valve 13, back into line 3.

If the valves 10,11,12 and 13 are closed, the design in accordance with the prior art would be the result. Fig. 2 shows a typical cooling rate for the wafers 21-25 in such a case.

The exit gas will be at a higher or lower temperature depending on the temperature of the furnace. A value of the exit gas (air) of 300- 400°C is mentioned by way of example. If this gas is discharged via a conventional outlet system, this results in two problems. Firstly, it is possible for low levels of pollutants, such as asbestos-like particles, or leakage gas to be present. Secondly, the increase in temperature frequently subjects the discharge system to impermissible loading. This applies in particular if the discharge system comprises components which are unable to withstand such high temperatures, such as plastic components. In such a case, a discharge device of this kind is designed to carry out cooling, with the result that extra cold air is used. This means that the discharge system has to be enlarged further in order to have sufficient capacity for such rapid cooling of a furnace.

In order to avoid these problems, it is now proposed to use the heat exchanger 4, which is connected to the liquid-cooling system of the production installation. In general, the heat which is gained as a result can be put to good use. In addition, an expensive gas can be used in the cooling circuit, since there is no consumption of this gas.

By arranging the openings 16 and 17 in series in the wall and introducing the cooling gas or air through these openings, it is possible to obtain an optimum distribution of flow. It is assumed here that the diameter of the openings 16,17 determines the volume which flows through, i. e. the flow resistance in gap 6 is much lower than the flow resistance through these openings 16,17. In designs in accordance with the prior art, the gas is introduced simply from above or below, and consequently it is impossible to ensure correct distribution of the cooling medium throughout gap 6, with the result that the temperature distribution is no longer uniform.

Fig. 3 shows the cooling rate of the same wafers when the gas flow is reversed every 20 seconds in accordance with the invention.

A comparison between Figs. 2 and 3 will firstly show that the cooling rate has increased by a number of times, so that the residence time in the furnace and the time for which the wafers are subject to

elevated temperature are reduced considerably. Moreover, it is clear that- the temperature spread between the wafers 21 and 25 has been limited to a considerable extent, so that more constant process conditions can be ensured.

Although the invention has been described above with reference to a preferred embodiment, it should be understood that it is possible for numerous modifications to be made to this preferred embodiment. For example, the gas inlet openings and gas outlet openings may be arranged at different positions over the height of the furnace, or may even open out at the top cover.

These and further variants, which will be obvious to the person skilled in the art after the above description has been read, lie within the scope of the appended claims.