The present invention relates to a device for generating a variable magnetic field, and in particular, though not limited to a device for generating a magnetic field of variable direction.
Many manufacturing processes require that the manufacturing process be carried out in a magnetic field. Magnetic fields are also required in the field of medical healthcare, and in particular, in the field of medical diagnostics, for example, in NMR scanners.
In the semiconductor industry semiconductor substrates are formed from single large silicon crystals which are grown from an electrically conductive melt in relatively strong magnetic fields. In general, single crystal silicon from which semiconductor substrates for use in semiconductor chips are formed are in the form of circular wafers cut from boules grown by a method known as the Czochralski method. The current trend in the formation of such wafers is towards wafers of ever increasing diameter. The industry standard has progressed from four inch through six inch to eight inch diameter wafers, and now twelve inch and even greater diameter wafers are envisaged. Semiconductor materials other than silicon are also grown by the Czochralski method, such as germanium, silicon-germanium alloys, gallium arsenide, indium antimonide and other Class III-V or II-VI compounds. Other methods of growing semiconductor crystals from a melt include methods which are referred to as the floating-zone and Bridgeman methods for forming circular boules, as well as the dendritic-web method for forming flat strip material.
During crystal growth it is important to control parameters in the melt such as the crystal orientation, thermal flux, rotation and pull rates, convection in the molten bath and distribution of impurities in the crystal. In order to stabilise the convective flow pattern in the electrically conducting melt and to dampen oscillations, as well as to achieve a uniform, low and radially homogeneous distribution of impurities, such as, oxygen, large diameter pure boules of silicon are grown from the melt in a magnetic field. In general, three magnetic field patterns are used in the commercial growing of semiconductor crystals, these are:
(i) a transverse field,
(ii) an axial field, and
(iii) a cusp field.
The effects of these three field configurations in the growth of semiconductor crystals is discussed in an article entitled xe2x80x9cUse of a Magnetic Field in Melt Growthxe2x80x9d by DTJ Hurle and R W Series in the xe2x80x9cHandbook of Crystal Growth, volume 2Axe2x80x9d, Chapter 5 published by North Holland, Amsterdam in 1994. Suitable magnetic fields have been produced by devices which comprise electromagnets or Helmholz coils. However, the physical size of the device must be such as to accommodate the growing crystal, the melt and an associated furnace. Electromagnets are bulky and require continuous expenditure of energy to drive the current in the excitation coils required to produce the necessary magnetic flux. Helmholz coils suffer from similar disadvantages. Additionally, a cooling system is required in order to remove the heat generated in electrically resistive coils, and in super conducting coils continuous cooling is required in order to maintain the super conductor below its super conducting transition temperature.
Additionally, in practice it is not possible to predict the precise effect of a magnetic field on crystal growth from first principles, and thus, it is necessary to proceed empirically to optimise the magnitude and direction of the field, as well as the field profile. Indeed, in many instances, different field conditions are required during the growth process of the crystal, for example, a magnetic field which is suitable for the early stage of the growth may not be suitable for the middle stage, and similarly, the latter stage of the crystal growth may require a further different magnetic field. While it is possible to vary the magnitude of the field produced by devices comprising electromagnets and Helmholz coils by varying the electric current, there is only limited scope for varying the field profile and direction in such devices.
There is therefore a need for a device for generating a variable magnetic field, and there is also a need for a device for generation a variable magnetic field which in particular is suitable for use in the semiconductor industry for growing large single crystal silicon from a melt. Indeed, there is also a need for a device for generating a variable magnetic field for use in other industrial applications, for example, in the melt growth of metals by techniques such as jet casting, where a magnetic field is used to stabilise a molten jet of metal, and also, in the medical and medical diagnostic field.
The present invention is directed towards providing a device for generating a variable magnetic field.
According to the invention there is provided a device for generating a variable composite magnetic field, the device comprising a ring member defining a main central axis and a central area around the central axis within which the composite magnetic field is generated, a plurality of discrete magnetic field generating means for generating respective magnetic fields mounted at spaced apart intervals around the ring member for generating the composite magnetic field in the central area, wherein the respective magnetic field generating means are orientably mounted on the ring member for varying the orientations of the magnetic fields generated by the magnetic field generating means for varying the composite magnetic field, and a means is provided for varying the orientations of the magnetic field generating means and for retaining the magnetic field generating means in desired orientations.
In one embodiment of the invention each magnetic field generating means defines a corresponding secondary axis.
In another embodiment of the invention the respective magnetic field generating means are mounted on the ring member with their respective secondary axes extending parallel to each other and parallel to the main central axis.
In a further embodiment of the invention the respective magnetic field generating means are mounted on the ring member with their respective secondary axes located on a common pitch circle, the centre of which coincides with the main central axis.
Preferably, the respective magnetic field generating means are mounted with their secondary axes equi-spaced apart circumferentially around the main central axis. Advantageously, each magnetic field generating means generates a magnetic field extending transversely of its corresponding secondary axis. Preferably, each magnetic field generating means generates a magnetic field extending perpendicularly to its corresponding secondary axis. Ideally, each magnetic field generating means is magnetised such that its direction of magnetisation extends transversely of its secondary axis.
In one embodiment of the invention each magnetic field generating means is rotatably mounted about its secondary axis for varying the orientation of the magnetic field generated by the magnetic field generating means (18) relative to the main central axis.
In another embodiment of the invention the means for orienting each magnetic field generating means rotates the respective magnetic field generating means about their respective secondary axes. Preferably, a plurality of means for orienting the magnetic field generating means are provided, one orienting means being provided for each magnetic field generating means. Advantageously, each orienting means comprises an electrically powered motor. Preferably, each electrically powered motor is a stepper motor.
In one embodiment of the invention a monitoring means is provided for determining the angular position of each magnetic field generating means about its secondary axis, and the orienting means is responsive to the monitoring means for orienting the magnetic field generating means.
Preferably, the monitoring means comprises a plurality of angular position determining encoders, one encoder being provided for each magnetic field generating means.
In another embodiment of the invention a retaining means is provided for retaining each magnetic field generating means in any one of a plurality of orientations.
Preferably, a plurality of retaining means are provided, one retaining means being provided associated with each magnetic field generating means. Advantageously, each retaining means comprises the stepper motor corresponding to the magnetic field generating means.
Preferably, each magnetic field generating means is independently orientatable relative to the other magnetic field generating means.
Advantageously, each magnetic field generating means comprises a permanent magnet.
Advantageously, each magnetic field generating means is provided by a rod-shaped permanent magnet.
Preferably, the secondary axis of each rod-shaped permanent magnet is defined by the longitudinal geometrical central axis of the rod-shaped permanent magnet.
Advantageously, each rod-shaped permanent magnet is of circular transverse cross-section.
In one embodiment of the invention each permanent magnet is of a high coercivity grade material.
In another embodiment of the invention each permanent magnet is of a rare-earth material.
In a further embodiment of the invention at least two ring members are provided, the ring members being co-axially arranged with their respective main central axis coinciding with each other.
Preferably, the respective ring members are arranged side by side axially along the main central axis.
Advantageously, the respective ring members are of similar diameter.
In one embodiment of the invention the respective ring members are arranged for axially abutting each other.
Preferably, the pitch circle diameters of the respective ring members along which the corresponding magnetic field generating means are located are of similar diameter.
Advantageously, each ring member is of circular shape.
In one embodiment of the invention each ring member comprises at least eight magnetic field generating means.
Further the invention provides a method for generating a variable composite magnetic field using the device according to the invention, wherein the method comprises orienting the respective magnetic field generating means located on each ring member to an orientation for generating the composite magnetic field of the desired type and direction.
Additionally, the invention provides a method for growing a semiconductor crystal using the device according to the invention wherein the crystal being grown is grown in the composite magnetic field.
The advantages of the invention are many. The device according to the invention allows the composite magnetic field generated by the device to be varied readily and easily. All that is required is to orient the magnetic field generating means in order to vary the composite magnetic field generated by the device. By virtue of the fact that the device generates the composite magnetic field in a central area around the main central axis defined by the ring member, the device according to the invention is particularly suitable for use in industrial and indeed medical fields. In particular, the devices particularly suitable for use in industrial manufacturing processes where a magnetic field is required during the carrying out of the manufacturing process, and in particular, where it is desired to vary parameters of the magnetic field during the manufacturing process, such as, for example, in the growth of large, pure single crystal semiconductors. By virtue of the fact that the orientation of the magnetic fields generated by the respective magnetic field generating means can be independently orientated both the orientation and the profile or type of the composite magnetic field can readily easily be varied. Indeed the strength of the composite magnetic field can also be varied. Needless to say, the advantages which are derived from the device according to the invention are equally beneficial in the application of the device to other industrial processes, medical diagnostic applications, as well as medical therapeutic methods.