Patent Application
20100304575
The invention relates to a method and an apparatus for a annealing SiC-Wafers.
Silicon carbide wafers (SiC-wafers) as well as silicon disks have to be annealed, which is also called healed or tempered, after implanting impurities therein (for example Al, P, B) in order to incorporate the impurities in the SiC crystal structure, in order to make them electrically conductive. The damage which is generated by implanting impurities into a crystal structure can only be “healed” partially during this annealing process. This process typically occurs in a high-temperature process at temperatures between 1600-2000° C. in the process chamber of an annealing oven for singles SiC-wafers or a plurality of SiC-wafers simultaneously.
At these temperatures, the problem occurs that first atomic layers of silicon may be stripped from the crystal structure of the silicon carbide layer (SIC) thereby damaging the smooth SiC surface, or a “step bunching” occurs, i.e. crystal structures may be shifted within the wafer.
In order to provide relief to these problems, it is possible to introduce silan into the process chamber thereby enabling an increase of the silicon partial pressure. Typically, a mixture of silan and an inert gas is used, wherein argon is used as the inert gas.
Silan (SiH4) is, however, dangerous due to its tendency to spontaneously combust and is thus difficult and cumbersome to handle.
It is therefore an object of the invention to provide a method and an apparatus for annealing of SiC-wafers, which enables generating a sufficient Si partial pressure in the process chamber with reduced operating costs.
This object is solved by a method in which a plurality of SiC-wafers is introduced into a process chamber of an annealing oven and by generating a vacuum in the process chamber and concurrently heating the SiC-wafer to a process temperature of 1600-2000° C. and by increasing the Si partial pressure in the atmosphere of the process chamber to a value above the vapor pressure of the silicon bound in the Si-wafer over a predetermined period of time at a constant process temperature.
In so doing, pure silicon in gas or vapor form or a mixture of a carrier gas and silicon in gas or vapor form may be introduced into the process chamber. The carrier gas may be argon, helium or H2.
In the interest of a constant process temperature, the silicon in gas or vapor form or the mixture of carrier gas and silicon in gas or vapor form is introduced into the process chamber at a temperature above 1600° C.
In a specific embodiment of the invention, the silicon in gas or vapor form is generated by vaporization of silicon from an SiC surface. This can be the surface of a SiC-wafer or fragments thereof or of molten silicon.
Vaporization is carried out at a temperature above 1400° C.
The object of the invention is also achieved by an apparatus for annealing SiC-wafers which is characterized in that a source of at least silicon in vapor or gas form is connected to the process chamber for receiving at least one wafer, for increasing the Si partial pressure.
The source for silicon in vapor or gas form is a vaporizer, to which a carrier gas may be fed to generate a gas flow over molten silicon, wherein the vaporizer is connected to the process chamber via a conduit or is arranged therein.
In an embodiment of the invention the vaporizer is a box made of graphite, silicon carbide or from silicon coated graphite or tantalum carbide, ceramics, sapphire, or molybdenum.
The source for the silicon in vapor or gas form in the vaporizer is a silicon wafer or fragments of silicon or molten silicon in the vaporizer.
The vaporizer is arranged in the annealing oven below the process chamber in an area of the annealing oven which is at a temperature of 1450-1700° C. Thereby, the vaporizer does not need its own heating.
In another variation of the invention, the vaporizer is arranged within the process chamber below the wafers.
As a carrier gas a noble gas such as argon or helium or H2 could be used. It is a requirement here to have an atmosphere free of oxygen.
The temperature in the vaporizer is in the range of 1450-1700° C. and the temperature in the process chamber lies between 1600 and 1900° C.
In accordance with the invention, an Si-wafer or a mixture of Si and a carrier gas is introduced into the process chamber from a vaporizer (bubbler).
The invention will be described herein below in more detail in accordance with an embodiment. In the respective drawings:
The vaporizer 4 consists of a box below the process chamber 2 of an annealing oven 1, below a high-temperature area (up to 2000° C.), in which the SiC-Wafers 3 are tempered. The vaporizer 4 consists either of graphite, silicon graphite or of SiC coated graphite. The vaporizer can be made from materials such as tantalum carbide, sapphire, ceramics or molybdenum. In said vaporizer there is a silicon wafer 3.1 acting as a Si-vapor source or there may be silicon fragments, wherein the latter is preferred.
The required temperature in the vaporizer lies between 1400-1600° C., at least above the molting temperature of silicon (1414° C.). This temperature is achieved below heat insulation 7 and above insulating layers 8 below the process chamber 2 (
When feeding the carrier gas 5.1 from a gas supply via the conduit 5 and during the subsequent conducting of the carrier gas 5.1 through the molten silicon, vaporized silicon is entrained and the gas mixture, for example Ar—Si, can then be conducted via a duct 6 (
It is essential that through the invention it is possible to build up the required Si partial pressure in the process chamber 2 in a completely safe manner simply by the speed and volume of feeding the carrier gas.
The corresponding
Below the process chamber 2 there is a vaporizer 4 in a temperature range of above 1400° C. with a Si-wafer 3.1 which is molten at its surface (
In order to realize different temperature levels, there is provided a heat insulation 7 (C Baffle Layer) between the high-temperature area and the vaporizer 4 and below the vaporizer 4 a further heat insulation is provided consisting of a plurality of insulating layers 8 and a quartz baffle 9. This enables maintenance of a constant temperature of approx. 150° C. at the bottom area 10 of the annealing oven 1.
As can be seen from
In accordance with the inventive method, a stack of SiC-wafers 3.1 is loaded into a process chamber 2 of an annealing oven 1 and is heated within the process chamber to a process temperature of 1600-2000° C. while generating a vacuum, wherein at the same time the Si partial pressure in the atmosphere of the process chamber 2 is increased to a value above the vapor pressure of the silicon bound in the SiC-wafer 3.1 over a predetermined period of time at a constant process temperature.
Pure silicon in gas or vapor form or a mixture of a carrier gas and silicon in gas or vapor form may be introduced into the process chamber 2. Argon, helium or H2 are considered for the carrier gas. The carrier gas is conducted along the molten fragments 11 (over 1414° C.) and thereby takes along silicon in vapor form to the process chamber (as shown by arrows in
By arranging the vaporizer 4 in the process chamber 2 (
In order to ensure a constant process temperature, the silicon in gas or vapor form or the mixture of the carrier gas and the silicon in gas or vapor form are introduced into the process chamber 2 at a temperature of above 1600° C. application:
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 059 814.0 | Dec 2007 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP08/67197 | 12/10/2008 | WO | 00 | 8/16/2010 |
