The present invention relates to the field of oxidation treatment for semiconductor wafers and more particularly to the oxidation furnace used for this semiconductor wafer oxidation process.

In microfabrication, thermal oxidation is a way to produce a thin layer of oxide, usually of silicon dioxide Si02, on the surface of a semiconductor wafer, generally on a silicon substrate. But thermal oxidation can be applied to various materials.

The thermal oxidation of silicon is usually carried out in a furnace at a temperature between 800°C and 1200°C. It may use either water vapour (usually UHP steam), or molecular oxygen as oxidant; it is consequently referred to as wet or dry oxidation. Wet oxidation is preferred to dry oxidation for growing thick oxides, because of the higher growth rate, for example for insulating and/or passivating layers.

In the case of wet oxidation, oxide formation is due to a reaction between the silicon and the water molecules originating from the water vapour H2O. The water molecules may be supplied to the furnace in several different ways. In the case of a pyrogenic steam distribution system, the hydrogen and the oxygen are supplied to a torch where they are burnt to produce water molecules. A pyrogenic steam distribution system may comprise an internal torch, which is located at the opening of the furnace, or else it may comprise an external torch, in this case the torch is located far from the inlet of the furnace, thus the hydrogen and oxygen are burnt outside the furnace and the water vapour is introduced into the furnace. The advantage of the external torch system is that the torch ignition process does not disturb the temperature of the furnace.

A single oxidation furnace accepts many wafers at the same time, either in a horizontal boat, where the wafers are held vertically, beside each other, or in a vertical boat, where the wafers fit horizontally, above and below each other. However, the semiconductor wafers introduced into the oxidation furnace have problems of non- uniformity of the oxide layer fabricated.

These problems of non-uniformity of the oxide fabricated are especially due to a temperature difference that exists between the gas flow originating from the torch and the temperature of the furnace tube for the oxidation process. As a general rule, the gas flow leaving the torch is at a lower temperature than the temperature of the furnace tube. Thus, the insertion of the gas flow into the furnace tube creates a temperature variation which affects the oxidation process and consequently the oxide layer fabricated. This temperature variation effect is greater when the gas flow entering the furnace tube is high. This problem of non-uniformity of the oxide layer fabricated limits the wafer load capacity for one batch in the furnace, for example to grow a 400 nm thick oxide layer, the load is limited to 50 wafers for one batch. At most, a load batch of the furnace may contain up to 100 wafers. In order to improve the uniformity of the oxide layer fabricated, it has been proposed to reduce the gas flow entering the furnace tube, in order to reduce the effect of the temperature difference on the fabrication of the oxide layer. However, the possibility of reducing the gas flow originating from the torch is limited by the design of the torch, given that the standard torches do not exhibit stable operation for obtaining a low gas/steam flow. There are, for this purpose, adjustable torches, i.e. a torch with a tube that varies in size, in order to adjust the gas/steam flow leaving the torch. However, these adjustable torches are expensive and make the oxidation process expensive. Furthermore, the adjustment of the parameters of the oxidation process with the use of an adjustable torch remains insufficient due to an imperfect stability. The objective of the invention is therefore to overcome the drawbacks described above by providing an oxidation furnace that can improve the uniformity of the oxide layer fabricated from one wafer to the next, making it possible to increase the number of wafers per load for one oxidation batch.

The objective of the invention is achieved by a semiconductor wafer oxidation furnace, comprising a torch connected to a furnace tube by a fluidic connection, and a means for adjusting a flow of gas or steam downstream of the torch in the direction of the furnace tube. Thus, it is possible to adjust the flow of gas or steam entering the furnace tube, in particular it is possible to reduce the flow of gas entering the furnace tube, without having to intervene at the torch. Specifically, it is thus possible to use a torch under stable and safe operation, i.e. for a high gas flow originating from the torch, while reducing the gas flow entering the furnace tube. In this way, the number of semiconductor wafers introduced per load may be increased for an oxidation batch. According to one variant of the invention, the means for adjusting the flow of gas or steam in the furnace tube comprises an exhaust pipe connected to the fluidic connection, between the torch and the furnace tube, in order to be able to adjust the gas flow entering the furnace tube. Thus, with simple technical means, the gas flow produced by the torch may be high and a portion of this gas flow passes into the exhaust pipe, which results in a reduction of the gas flow entering the furnace tube.

According to another variant of the invention, the gas exhaust pipe comprises a means for adjusting the flow rate of the gas flow passing through the gas exhaust pipe. For example, the flow rate of the gas flow passing through the gas exhaust pipe may be adjusted in a simple and inexpensive manner, by using a pipe sleeve. According to another variant of the invention, a mass flow regulator may also be used to adjust the flow rate of the gas flow passing through the exhaust pipe.

According to one alternative of the invention, the adjustment means is placed at the branch connection between the fluidic connection and the gas exhaust pipe. Thus, the pipe sleeve adjusts the flow rate of gas in the gas exhaust pipe, at the inlet of the gas exhaust pipe.

According to another alternative of the invention, the adjustment means is placed inside the gas exhaust pipe. Thus, the adjustment means may be easily introduced and exchanged in order to modify the flow rate of the gas flow passing through the gas exhaust pipe.

According to another variant of the invention, the adjustment means may comprise a central orifice of fixed size. When the desired gas flow in the furnace tube is known, this makes it possible to choose the appropriate adjustment means for the desired gas flow. In fact, the adjustment means may be a simple pipe sleeve for example, the size of the central orifice of which has been defined relative to the desired oxidation parameters. This makes it possible to use an inexpensive adjustment means and allows a versatile and inexpensive use of the oxidation furnace.

According to another variant of the invention, the adjustment means may comprise a central orifice of adjustable size, in particular a mass flow regulator. Thus, the flow rate of the gas flow passing through the gas exhaust pipe may be changed without having to manually exchange parts in the system.

According to one variant, the flow rate of the gas flow may be adjusted so that the flow rate of the gas flow passing through the gas exhaust pipe is between 50% and 95% of the gas flow leaving the torch, in particular between 60% and 80%. Thus, it becomes possible to reduce the gas flow in the furnace tube so that, with a stable torch, it is nevertheless possible to obtain an improved stability of the oxide thickness.

According to one variant of the invention, the semiconductor wafer oxidation furnace comprises a fixed-flow torch. Thus, the oxidation furnace enables an adjustment of the flow rate of the gas flow while using a standard torch, which makes it possible to avoid the use of a torch with an adjustable flow and therefore to avoid increasing the cost of the oxidation process. Furthermore, this makes it possible to use a standard torch under stable operating conditions, i.e. at high flow rate of the gas flow, and to reduce the flow rate of the gas flow after the torch and before the inlet into the furnace tube. According to one variant ^' of the invention, the gas exhaust pipe may be, downstream, connected to a gas discharge pipe, the gas discharge pipe being in direct connection with the furnace tube. Thus, the gas flow passing through the gas exhaust pipe located between the torch and the furnace tube reaches the gas discharge pipe of the oxidation furnace. This enables a safe operation of the furnace at the gas exhaust, and also a simple design of the oxidation furnace.

The invention may be understood by referring to the following description taken together with the appended figure, in which numerical references identify the elements of the invention.

Figure 1 represents a semiconductor wafer oxidation furnace according to one alternative of the invention.

The oxidation furnace 10 makes it possible to fabricate, by a thermal oxidation treatment, a thin layer of oxide, usually of silicon dioxide S1O2, on the surface of a semiconductor wafer, for example on a silicon substrate. The semiconductor wafer oxidation furnace 10 may be a furnace of horizontal type where the wafers are held vertically in a boat, beside each other, or a furnace of vertical type, where the wafers fit horizontally, above and below each other, in a vertical boat. The oxidation furnace 10 comprises a furnace tube 11 , a torch 12 and a gas exhaust pipe 13. The furnace tube 11 is connected to the torch 12 by a fluidic connection 14. Placed on this fluidic connection 14 is the gas exhaust pipe 13 located downstream of the torch 12, between the torch 12 and the furnace tube 11. This gas exhaust pipe 13 may comprise an adjustment means 15 placed inside the gas exhaust pipe 13, or close to the opening 16 with the fluidic connection 14.

The torch 12 is fed with gas by several gas feed lines, here for example three lines 17a, 17b and 17c, each connected to a different gas container 18, 19 and 20. Positioned on each feed line 17a, 17b and 17c is, in addition, a gas mass flow regulator 21 a, 21 b and 21c, in order to be able to control the flow rate of the gas flow leaving each container 18, 19 and 20 and feeding the torch 12. The gases used and stored in the containers may for example be gases H2, O2 and N2.

The oxidation furnace 10 comprises heating elements 22 placed inside the furnace tube 11 , which make it possible to attain a high temperature in the furnace tube 11 in order to carry out the thermal oxidation treatment. A gas discharge pipe 23 is directly connected to the furnace tube 11 and comprises a firebox 24. The firebox 24 may be installed on the gas discharge pipe 23 of the furnace tube 11 , to ensure the combustion of the excess hydrogen H2 if present.

The furnace tube 11 comprises an inlet air lock 25 in order to be able to introduce the semiconductor wafers 26, for example in a batch of 100 wafers, into the furnace tube 11.

The fluidic connection 14 joins the torch 12 to the furnace tube 11 and a pipe flange 27 connects the fluidic connection 14 with the furnace tube 11 and a pipe flange 28 connects the fluidic connection 14 with the torch 12. Furthermore, the gas exhaust pipe 13 is connected to the fluidic connection 14 by a pipe flange 29. The pipes flanges 27, 28 and 29 are standard pipe flanges for a vacuum system that enable access to various locations within the vacuum system. Thus, the adjustment means 15 may be introduced into the gas exhaust pipe 13 and may be placed at the opening 16 of the gas exhaust pipe 13 with the fluidic connection 14 by means of the connection 29.

The fluidic connection 14 joining the torch 12 to the furnace tube 1 1 makes it possible to space out the torch 12 and the furnace tube 1 1. Thus, the operation of the torch 12 does not disturb, or barely disturbs, the temperature of the furnace tube 1 1. Furthermore, the fluidic connection 14, owing to its length, makes it possible to choose and/or vary the distance between the torch 12 and the furnace tube 1 1.

For an oxidation process, a gas O2, contained in the gas container 18, and a gas H ₂ , contained in the gas container 19, are supplied to the torch 12, which, once ignited, burns the gases O2 and H ₂ to form water vapour H ₂ 0. To prevent an explosion, the 0 ₂ :H ₂ ratio is always greater than 1 :2. This water vapour H ₂ 0 and also the remaining 0 ₂ then pass into the fluidic connection 14 in order to arrive in the furnace tube 1 1 where the semiconductor wafers 26 are, which were previously introduced into the furnace tube 11 through the inlet air lock 25 of the furnace tube 11.

The gas mass flow regulators 21a and 21 b respectively make it possible to regulate the flow rate of 0 ₂ gas and the flow rate of H ₂ gas introduced into the torch 12, and therefore make it possible to regulate the production of steam H2O. The gas container 20 may for example contain N ₂ gas, needed for purging the furnace tube 1 1 before and after the oxidation process. The discharge pipe 23 makes it possible to purge the furnace tube 1 1 of the gas supplied by the fluidic connection 14 for the oxidation process, when the oxidation process is finished.

The presence of the exhaust pipe 13 downstream of the torch 12, between the torch 12 and the furnace tube 1 1 , connected to the fluidic connection 14 makes it possible to adjust the flow of gas, in particular of water vapour H ₂ 0 and of O2 entering the furnace tube 11. Specifically, a portion of the gas flow leaving the torch 12, present in the fluidic connection 14, enters the exhaust pipe 13 controlled by the adjustment means 15.

According to one variant of the invention, the adjustment means 15 for adjusting the flow rate of the gas flow entering the gas exhaust pipe 13 may be a pipe sleeve. According to another variant of the invention, the adjustment means 15 for adjusting the flow rate of the gas flow entering the gas exhaust pipe 13 may be a mass flow regulator.

The shape of the adjustment means 15 is such that it can be installed inside the exhaust pipe 13 in a fixed and secured manner. For this, the adjustment means 15 may, for example, comprise two grooves positioned on the outer lateral side of the adjustment means, opposite one another. The two grooves may fit over protrusions in the gas exhaust pipe 13 in such a way that the adjustment means 15 is inserted inside the gas exhaust pipe 13 at the branch connection 16 between the fluidic connection 14 and the gas exhaust pipe 13 according to Figure 1.

The adjustment means 15 may also be positioned inside the gas exhaust pipe 13 at the branch connection 16 between the fluidic connection 14 and the gas exhaust pipe 13 according to another variant of fixation, such as for example with the use of clamping screws or magnetic positioning means. In the case of a sleeve, the adjustment means 15 may have a shape that is concentric with a central orifice. The size of the central orifice is fixed for a given sleeve, and in order to adjust the flow rate of the gas flow passing through the gas exhaust pipe 13 a sleeve having a first opening is exchanged for another sleeve with an orifice of a different size. In this case, provision is made for the possibility of interchanging the sleeves in a simple manner.

It is sufficient to choose the adjustment means 15 with the size appropriate for the flow rate of the gas flow desired for the semiconductor wafer oxidation process and to install it into the exhaust pipe 13, as represented in Figure 1.

According to one alternative of the invention, the adjustment means 15 may comprise a central orifice of variable size, that can be adjusted by the user. Thus, the flow rate of the gas flow passing through the gas exhaust pipe 13 can be adjusted, for example, by the use of a mass flow regulator, placed at the inlet of the gas exhaust pipe 13.

Thus, the same adjustment means 15 may enable a variation and an adjustment of the flow rate of the gas flow in the gas exhaust pipe 13, simply by varying the size of the central orifice. In this variant, there is no need to change the adjustment means 15 manually in order to change the flow rate of the gas flow as in the case of the sleeves.

The size of the central orifice may, in both variants, be such that the flow rate of the gas flow passing through the gas exhaust pipe 13 is between 50% and 95% of the gas flow leaving the torch 12, in particular between 60% and 80%. The remaining gas flow will enter the furnace tube 1 1.

The gas flow originating from the torch 12 and entering the furnace tube 11 is at the temperature set by the torch ignition process in order to create water vapour H ₂ O.

As a general rule, this temperature is lower than the temperature of the oxidation process. Indeed, the H ₂ ignition temperature is around 600°C and the oxidation process is normally carried out at a temperature between 800°C and 1200°C, in particular around 1000°C. This temperature difference creates a temperature variation for the wafers subjected to the oxidation process. This temperature variation is large at the opening 30 of the furnace tube 1 1 with the fluidic connection 14, and also varies with the flow rate of the incoming gas flow. The higher the flow rate, the larger the temperature variation and the further the temperature variation spreads into the furnace tube 1 1. The result of this is a non-uniformity of the oxide layer fabricated in a batch of wafers 26 and consequently this leads to a wafer load limitation for each oxidation process. Specifically, in the furnaces of the prior art, only the region in the furnace tube 1 1 far from the opening 30 through which the gas flow originating from the torch 12 enters is occupied by the semiconductor wafers 26 in order to ensure sufficient uniformity of the oxide layers fabricated on the semiconductor wafers 26 is obtained.

According to the invention, the gas flow originating from the torch 12 is reduced since a portion of this gas flow passes through the gas exhaust pipe 13 located downstream of the torch 12, between the torch 12 and the furnace tube 1 1. Thus, this temperature variation effect is therefore reduced and consequently the non-uniformity of the oxide can be reduced in a batch of semiconductor wafers 26 subjected to the oxidation process. This thus makes it possible to be able to increase the load of semiconductor wafers 26 for each oxidation process. In this way, it would be possible to use the oxidation furnace 10 with a maximum load of semiconductor wafers 26 for each oxidation process. A certain number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications and improvements may be made without departing from the subject of the invention.