GB2083728A - Annealing furnace - Google Patents

Abstract

An annealing furnace wherein a material which is to be annealed is placed under a flash discharge lamp (3) which radiates light of a short duration for rapidly heating up the material. The radiation area is selected not to exceed twelve times the light emitting cross- sectional area of the flash discharge lamp (3). The table (4) on which the material is placed may be movable away from and towards the flash discharge lamp (3) for conveying the material into and away from the radiation area, or the material may be placed on a conveyor belt passing through the furnace. <IMAGE>

Description

SPECIFICATION Annealing furnace This invention relates to an annealing furnace. It is general practice to anneal metal, glass and other materials by keeping them at a raised temperature for a period of time sufficient to relieve internal strains and various annealing methods and annealing furnaces have been proposed for this purpose.

Where it is required to eliminate the defects in the lattice structure of iron oxide (FeO) or the like due to absence of oxygen atoms therein the material is maintained for an extended period at an elevated temperature in the presence of oxygen having a certain partial pressure. It takes quite a long time before the desired annealing effect is obtained.

However, as in the case of glass rods for generating laser beams and semiconductor materials, the material cannot be heated for an extended period without changing the density and distribution of certain impurities in the materials. Conventional annealing processes which involve a heating over an extended time period are totally unsuitable in cases where a substantial change in the density and the distribution of certain impurity cannot be tolerated.

For example, conventional heating processes and annealing furnaces cannot be used to repair radiation damage occurring during ion implantation of impurities into silicon semiconductor material to produce silicon semiconductor devices.

One of the primary objects of this invention is to avoid the need to heat materials during annealing for an extended period.

According to this invention, we propose an annealing furnace, comprising: a support for materials to be annealed, a flash discharge lamp associated with a reflector the arrangement being such that light is radiated upon the support, in a radiation area not exceeding twelve times the cross-sectional area of the light emitting space of the flash discharge lamp.

Embodiments of this invention will now be described by way of example, with reference to the accompanying drawings, of which: Figure lisa schematic side view of a part of an annealing furnace seen in the longitudinal direction of the flash discharge lamp; Figure 2 is a schematic front view of the annealing furnace shown in Figure 1 seen from the lateral direction of the flash discharge lamp; Figure 3 is a plan view illustrating one possible arrangement of materials to be annealed; and Figures 4 and 5 respectively are schematic side and front views of another annealing furnace.

The furnace shown in Figures 1 and 2, has a box construction with two mutually separable portions, an upper portion 1A and a lower portion 1B. In the upper portion 1A are mounted a downwardly facing reflecting mirror 2 and a flash discharge lamp 3 comprising a sealed glass tube which is so positioned in relation to the reflecting mirror 2 as to cause light to be radiated upon a defined area in the lower portion 1 B. The lower portion 1 B, on the other hand, has a table 4 having an upper surface 4A onto a radiation area of which, impinges light emitted from the discharge lamp 3 and reflected by mirror 2.

The table 4 is positioned at a level such that the radiation area does not exceed twelve times of the cross-sectional area of the light emitting space of the flash discharge lamp 3 defined by its arc length Land the diameter of its glass tube. The flash discharge lamp 3 and the reflecting mirror may be mounted on the upper portion 1A of the Box 1 by a suitable means.

The annealing furnace described above may be used for annealing for example, silicon wafers, in which such case, the flash discharge lamp 3 may be a glass tube of 6mm in diameter d and filled with xenon gas at 0.2 atmospheric pressure and at a temperature of 25 C. The arc length L is preferably 160 mm, so that areas of the light emitting space L x d is 9.6 cm2. The table 4 is positioned such that its upper surface 4A is 16 cm long in the direction of the arc length and 6 cm wide, the upper surface 4A of the table being ten times the cross-sectional area of the light emitting space in area, (i.e. 96 cm2). Further, the level of the table relative to the discharge lamp is set such that the whole of the upper surface 4A is included in the radiation area.

On the table 4 are arranged 12 silicon wafers 5, each having a diameter of 1 inch, in two rows as shown in Figure 3, access to the furnace for arranging the silicon wafers being provided by separating the lower portion 1 B of the box 1 from its upper portion 1A.

When the flash discharge lamp 3 is energized by 1,600 volts of electric voltage, the radiation energy impinging upon the silicon wafers amounts to approximately 800 joules. As a result, the surface layer of the silicon wafers 5 is instantaneously heated up and any defects in the silicon lattice structure are eliminated by being annealed without involving any change in the density and the distribution of impurities. Annealing of silicon wafers can be made in a satisfactory manner only if their surface layer approximately a few thousand A deep where impurities exist is heated up, and the flash discharge lamp 3 offers the radiation of instantaneous light suitable for bringing about such an effect.

When the radiation area is excessively great, we have found that satisfactory annealing can not be obtained over the whole radiation area. To be specific, if the level of the upper surface 4A of the table 4 is lowered away from the flash discharge lamp 3, a greater radiation area will be obtained, but, in this case where the light from a flash discharge lamp 3 is used for instantaneous heating as opposed to the case of conventional annealing furnaces in which heating is conducted over an extended time period, increasing the radiation area will cause the increase in the variation of heating effect due to uneven distribution of light radiation between the middle portion and the peripheral portion of the area.Even if the energy of the light emitted from the flash discharge lamp 3 is increased in any corresponding manner, the middle portion of the radiation area will be excessively heated up causing some change in the distribution of impurities of the materials thereon while the materials on the peripheral portion will not be annealed in any satisfactory manner due to the insufficiency in the radiated light thereon. However, provided the radiation area does not exceed twelve times the crosssectional area of the light emitting space comprising the arc central line of the flash discharge lamp 3, a satisfactory distribution of light radiation is obtained over the whole radiation area irrespective of the level of the upper surface 4A of the table 4, resulting in a satisfactory annealing effect over the same radiation area.

In the furnace, shown in Figures 4 and 5, a slit 12 is formed in a lower side wall of the box 11 and the internal surface 13 of the portion of the box 11 located above the slit 12 is a reflecting surface and in that portion of the box is mounted a flash discharge lamp 3 and a table 4 which is movable as indicated in and out of the box through the slit 12. The crosssectional area defined by the reflecting surface 13 (i.e. the cross-sectional area of the box when viewed in plan) constitutes the radiation area and this area does not exceed twelve times the light emitting cross-sectional area of the flash discharge lamp 3.

It also is possible to provide another slit 14 on the sidewall of the box 11 opposite to the slit 12 so that an elongated table, constructed as a belt conveyor, for example, can convey the workpieces which are to be annealed from the slit 12, across the open end portion of the reflecting mirror box 13, and to the slit 14 along a conveying path P. The arrangement of the table shown in Figures 4 and 5 can be used as an alternative to that shown in Figures 1 and 2.

The above described annealing furnace is of an instantaneous heating type and, by properly selecting the radiation area, a satisfactory annealing effect can always be obtained. The annealing furnace according to this invention is particularly suitable for annealing thin materials such as silicon wafers or for materials only the surface layer of which is required to be annealed.

Claims (5)

1. An annealing furnace comprising: a support for materials to be annealed, a flash discharge lamp associated with a reflector the arrangement being such that light is radiated upon the support in a radiation area not exceeding twelve times the cross-sectional area of the light emitting space of the flash discharge lamp.

2. An annealing furnace according to Claim 1 wherein the flash discharge lamp and the reflecting mirror are disposed in an upper portion of a box-like structure.

3. An annealing furnace according to Claim 1 or 2, wherein the support is movable towards and away from the reflecting mirror.

4. An annealing furnace according to Claim 1, 2 or 3, wherein the support is adapted to convey material to be annealed along a path including a position which lies in the reflected light.

5. An annealing furnace constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.