GB2409581A

GB2409581A - Electromagnet arrangement for generating a uniform planar magnetic field

Info

Publication number: GB2409581A
Application number: GB0426889A
Authority: GB
Inventors: Gordon Green; Robert Trowell
Original assignee: Trikon Technologies Ltd
Current assignee: Trikon Technologies Ltd
Priority date: 2003-12-23
Filing date: 2004-12-08
Publication date: 2005-06-29
Anticipated expiration: 2024-12-08
Also published as: JP2005179781A; GB2409581B; GB0426889D0

Abstract

An electromagnet comprises a winding or windings 13, 15 arranged such that each effective turn of the winding or windings follows the perimeter of a U-shaped core 11, 16. The said electromagnet generates a uniform magnetic field about a planar object 20 which is parallel to the internal base section of the U-shaped perimeter. The U-shaped electromagnet may be formed by a planar electromagnet 13 with an auxiliary coil 15 and core 16 positioned adjacent respective edges of the windings of the planar electromagnet 13 such that they project beyond the face of the planar electromagnet 13. One or more independent power supplies may be used to activate one or more coils. The auxiliary coils may include various tapping points such that the number of windings of respective coils can be adjusted. Current level and number of windings may be adjusted to tune the coils. There is also disclosed a uniform magnetic field generating apparatus comprising a planar electromagnet with auxiliary coils disposed adjacent respective edges of the windings of the planar electromagnet. The electromagnet may provide a compact arrangement for use in generating a uniform planar magnetic field within a vacuum processing environment.

Description

Mannet Assemblies This invention relates to magnet assemblies and

particularly assemblies which are useful for inducing magnetic fields within vacuum chambers.

In the fabrication of thin film magnetic devices - sensors, magnet heads, MRAM magnetic materials are often deposited by sputtering. During such processes, it is also, frequently, a requirement that a magnetic field is applied to the substrate so that the resulting films are magnetically aligned. It will be appreciated, therefore, that if the magnetic field across the substrate is not completely aligned, problems of nonuniformity will arise.

Considerable thought therefore has been given to the design of magnetic field generators for use in connection with vacuum deposition systems.

Although either permanent magnets or eiectromagnetics can be used, the latter are preferred, because they can be switched on or off or otherwise modulated.

Typical field strengths are in the range of 4 to 8 kA.m'. For thin film applications, the component of the magnetic field perpendicular to the growing film is generally of no concern. However alignment of the in plane film needs to be typically no more than +/- 2 over the area of the substrate as in plane non uniformity is often required to be less than +/5%.

Whilst large coils placed externally on opposite sides of the vacuum chamber, might well provide the necessary configuration, they are very bulky and require high power and only provide a magnetic field in a single plane or direction.

It is generally preferred to mount the field generator in close proximity to the substrate support. The approaches generally fall into two types. The first has two opposing pole pieces, connected by a low reluctance magnetic circuit incorporating one or more coils, which can be located beneath or adjacent to the substrate support. The disadvantage of this approach is that the pole pieces must lie in, or very close, to the plane of the substrate. They must also extend laterally well beyond the substrate in order to project an acceptably uniaxial field across the substrate. They thus encompass a relatively large rectangular space surrounding the substrate and this can interfere with other hardware within the process environment. Such a configuration is shown in Figure 1.

A development of this approach is shown in US-A-6545580. It will be noted that in order to achieve acceptable field alignment, additional L-shaped "pole pieces" are required. Since these must be located at or close to the plane of the substrate, much of the advantage of the upward projection is lost.

The second approach is illustrated in US-A-5630916 and US-A-6042707, which aim to project a field from below the substrate support. They utilise a generally planar coil - i.e. a coil wound around a soft magnetic core in the form of a flat plate. The weakness of the planar coil approach, as described in US 5630916, is illustrated in Figure 2, which is that the coil must extend significantly beyond the edge of the substrate, if uniformity over the substrate is to be maintained. In US-A-5630916 it is stated that the coil should be at least 50% larger than the substrate in order to achieve acceptable field uniformity.

From one aspect the applicants invention consists in a magnetic assembly comprising, or for providing the magnetic equivalent of, an electromagnet having U-shaped windings to generate a uniform magnetic field extending between the arms of the U and parallel to the base thereof.

The applicants have realised that by having such a configuration a uniform field can be generated, but without the electromagnet needing to be dimensionally much greater than the dimension of the planar coil, so that the electromagnet can sit readily within conventional support structures. Using current technology, it is probably impractical to provide an electromagnet with U shaped windings on a commercial basis and the applicants have further appreciated that the equivalent can be constituted by a generally planar electromagnet and a pair of auxiliary coils disposed adjacent respective edges of the windings of the planar electromagnet to project beyond one face of the planar electromagnet.

The auxiliary coils would preferably have respective magnetically soft cores and, in use, should be arranged such that the current flowing in adjacent sections of the windings of the electromagnet and the respective auxiliary coils are in the opposite direction. This not only puts the current flow in the correct direction in respect of the projecting portions of the auxiliary coils, it also means that the fields generated by the edge of the planar electromagnetic windings are substantially cancelled or compensated for.

The auxiliary coils and/or the electromagnet may be independently powered and means may be provided for tuning the auxiliary coils, for example by adjusting the number of active coils in the auxiliary windings, e.g. by using appropriate tapping points, and/or by varying the current through the respective coils.

From another aspect the invention consists in apparatus for generating a substantially uniform magnetic field across a planar location for a planar workplace including a generally planar electromagnet locatable beneath the location and parallel thereto and a pair of auxiliary coils disposed adjacent respective edges of the windings of the generally planar magnet for creating a magnetic field in opposition to the magnetic field generated in use by the associated edges of the windings of the electromagnet.

Preferably the auxiliary coils project beyond the face of the generally planar electromagnet. The electromagnet and respective auxiliary coils may be independently powered and additionally or alternatively the auxiliary coils may include tapping points or other arrangements for varying the active number of windings in them.

From a still further aspect the invention consists in a planar workplace apparatus including a vacuum chamber, a support for a workplace located in the chamber to support the workplace in a generally planar workplace location and an assembly as claimed in claims 1 to 7 or a magnetic field generating apparatus as claimed in claims 8 to 10 disposed external to the chamber adjacent the support to generate a uniform magnetic field through the location in the plane of the location.

The invention may be performed in various ways and specific embodiments will now be described by way of example, with reference to the accompany drawings, in which: Figure 1 is a schematic side view of a prior art configuration; Figure 2 is a schematic plan view of the configuration of Figure 2; Figure 3 is a schematic view of an alternative prior art arrangement; Figure 4 is a cutaway plan view of a magnet assembly of a specific embodiment of the invention; Figure 5 is a cutaway view showing the assembly mounted in a wafer support; Figure 6 is the equivalent diagram to Figure 3 for the construction of Figure 4; Figure 7 is a printout from a mathematical model modelling the in plane

field generated by the Figure 3 arrangement; and

Figure 8 is the equivalent print-out to Figure 7 for the Figure 4 arrangement.

Returning to the diagram in Figure 2, it is evident from the Biot-Savart Law that current flowing upwards in the plane of the page, as indicated, cannot produce components of magnetic field in a parallel direction. Thus the looping field lines found in proximity to the upper and lower edges of the coil cannot have their origin in the current plane formed by the upper windings of the coil.

Stated another way, if the current plane could terminate abruptly just beyond the edge of the substrate, then the resulting field would be perfectly aligned at the plane of the page. However, this is not practicably possible. The current must pass into and out of the plane of the page in order to make a closed circuit with the parallel returning path beneath. This arrangement is illustrated in Figure 3 and the vertical return paths, which enable the current to flow through the planar parts of the windings, are the source of the unwanted looping field which creates the poor alignment. One approach to overcoming this would to be to make these vertical paths very small, but practically this would result in a loss of mechanical rigidity; saturation of the soft magnetic core and, if the two current sheets are brought too close together their magnetic fields will tend to cancel.

Turning to Figure 4, the applicants accordingly propose a magnetic assembly, generally indicated at 10, which includes a rectangular magnetically soft core 11 carrying windings 12, which together form the main planar electromagnet, generally indicated at 13. Immediately adjacent the edges of the magnet 13 are placed auxiliary coils 14. These are each formed from windings wound about ablate cores 16 and it will be noted that they project upwardly above the upper surface of the planar magnet 13. Pole pieces 17 are attached to the ends of the planar magnet 13 and these too project above the upper surface of the planar magnet 13. Finally the assembly is completed by mounting brackets 18.

Turning now to Figure 6, it will be seen that the inner vertical limbs of auxiliary coils 15 have oppositely flowing current to the current flowing in the respective adjacent edge 19 of the planar magnet 13. Thus one way of considering the arrangement of Figure 4 is that the magnetic fields in the auxiliary windings serve to compensate for the edge effects. It may be possible to achieve this by auxiliary coils which do not project. However, another way of viewing the Figure 4 construction is that it is the magnetic equivalent of an electromagnet having U-shaped windings and therefore produces a uniform magnetic field parallel to the upper surface of the electromagnet 13 and between the arms formed by the projecting portions of the auxiliary coils 15. Accordingly if the substrate 20 can sit in the vicinity of these upwardly projecting arms, it will

lie in an aligned field.

This notable improvement is well illustrated by comparing Figures 7 and 8. As has been mentioned above the Figure 7 field is generated from the arrangement illustrated in Figure 3. The outside edge of a wafer or substrate 20 is indicated at 21 and it will be seen that there is an area of poor alignment. (As only a quadrant is shown, corresponding patches will also appear on the corresponding portions in the other quadrant). In contrast, in Figure 8, the field is well aligned over all of the surface of the substrate 20.

Figure 5 illustrates how the assembly 10 can be located into a practical support arrangement. Thus an enclosure is formed by upper and lower plates 22, 23 which seal around the periphery, at 22a, to form a vacuum tight enclosure. The enclosure is intended to be mounted within a vacuum process chamber, by means of the stem 24 on the lower enclosure plate 23 such that a vacuum exists outside of the enclosure and normal atmosphere exists on the inside. The atmospheric region connects to the outside of the process chamber by means of penetrations 25 (not shown) in the stem as is well known in the art.

The central region of the top surface of the upper enclosure plate forms the support surface 26 for the wafer or substrate 20. This is the region where the magnetic field needs to be projected. Above the upper plate 20 is the edge shield 27. This protect the exposed part of the upper plate 22 from the deposition of flux during the process. The edge shield 27 has a central aperture 28 which is sized slightly smaller than the substrate 20 in order to protect the edges of the substrate from deposition. The edge ring is support on three actuators 29 which can cause the edge ring to move vertically during wafer loading and unloading operations. Four wafer lift fingers 30 are mounted to the lower face of the edge shield 27. During processing the edge shield 27 is in the fully down position and the fingers 30 are accommodated within pockets 31 in the enclosure upper plate. After processing the edge shield is raised, thereby picking the substrate 20 off the support surface 26 by means of the lift fingers. A substrate handling robot (not shown) then removes the substrate from the support fingers and replaces it by a fresh wafer. The edge shield 27is then lowered so as to place the new substrate down onto the support surface 26.

The assembly 10 is mounted on the lower enclosure plate 23, as shown.

Electrical connections are brought out via penetrations in the stem 24 of the lower plate 23. The upper and lower plates 22, 23 are both water cooled in order to remove heat generated by the coils and also heat transferred from the wafer during processing. The cooling water likewise passes through the penetrations within the stem.

The planar coil 12 typically processes approximately a thousand turns of copper wire and requires a current of approximately 2A to produce a magnetic field at the wafer support of 5,000A.m'. The auxiliary coils 14 also possess a thousand turns and also typically run at 2A in order to provide good field alignment. Field and non-uniformity over the wafer support surface is maintained with in +/-5% and alignment is within +/-2.

One advantage of the Applicants' system is that it achieves this high level performance without being significantly larger than the substrate. Thus useful results have been achieved with an assembly measuring 350mm x 326mm for a 300mm circular substrate.

Claims

Claims 1. A magnetic assembly comprising, or for providing, the magnetic

equivalent of an electromagnet having a U-shaped winding to generate a uniform magnetic field extending between the arms of the U and parallel to the base thereof.
2. An assembly as claimed in claim 1 wherein the equivalent is constituted by a generally planar electromagnet and a pair of auxiliary coils disposed adjacent respective edges of the windings of the planar electromagnet to project beyond one face of the planar electromagnet.
3. An assembly as claimed in claim 2 further including one or more power supplies for supplying current to electromagnet and coils so that the current flows in opposite directions in the adjacent sections of the windings of the electromagnet and the respective auxiliary coils.
4. An assembly as claimed in claim 3 wherein the coils and electromagnet are independently powered.
5. An assembly as claimed in any one of claims 2 to 4 including means for tuning the auxiliary coils.
6. An assembly as claimed in claim 5 wherein the tuning means includes one or more tapping points for altering the active number of windings in the respective coils.
7. An assembly as claimed in claim 5 or claim 6 wherein the tuning means includes means for varying the current through the respective coils.
8. Apparatus for generating a substantially uniform magnetic field across a planar location for a planar workplace including a generally planar electromagnet locatable beneath the location and parallel thereto and a pair of auxiliary coils disposed adjacent respective edges of the windings of the generally planar electromagnet for creating a magnetic field in opposition to the magnetic field generated in use by the associated edges of the windings of the electromagnet.
9. Apparatus as claimed in claim 8 wherein the auxiliary coils project beyond a face of the generally planar electromagnet.
10. Apparatus as claimed in claim 8 or claim 9 wherein the electromagnet and the respective auxiliary coils are independently powered.
11. Planar workplace processing apparatus including a vacuum chamber, a support for a workplace located in the chamber to support the workplace in a generally planar workplace location, and an assembly as claimed in claims 1 to 7 or magnetic field generating apparatus as claimed in claims 8 to 10 disposed external to the chamber adjacent the support to generate a uniform magnetic field through the location in the plane of the location.