PRODUCTION OF HARD MAGNETIC MATERIAL FILMS

Abstract: Magnetic films for very small magnetic devices are produced by applying a paste to a substrate and curing it. The paste has magnetic particles in a binder which has binding particles dissolved in a solvent. The solvent evolves during curing and thus the cured film has a very high magnetic density. The method of curing ensures that porosity does not arise to a significant extent.

Description: "Production of Hard Magnetic Material Films The invention relates to the production of hard magnetic material films on a substrate such as a ceramic substrate, Al 2 0 3 , or a silicon wafer. More particularly, the invention relates to production of such films for applications such as micro-motors or very small medical instrument circuits. Various approaches to production of such films have been adopted in the past. One such approach is to employ the sputtering technique, however, the film thickness is very small - of the order of 1 micron. As described in US 5100604 (Matsushita), high pressure compression can be used to achieve high magnetic density magnets, which are then applied to the relevant substrate. This method appears to be effective for some applications, however, it involves several production steps, and application of the magnets may prove difficult in some devices . An alternative approach is to distribute magnetic particles in a binder and apply a paste to the substrate. Thus, the steps of producing the magnet itself and applying to the substrate are integrated. One such approach is described in EP 0650635 (Magnet Applications Limited) . This involves application to two backing sheets and subsequently mating the sheets together after curing. These steps appear to be unsuitable for some very small applications in which space limitations would make handling of the backing sheets very difficult. According to the invention, there is provided a method of producing a magnetic film, the method comprising the steps o t- distributing magnetic particles in a binder comprising solid binding particles dissolved in a solvent, applying the paste to a substrate, curing the paste at temperatures and a time duration sufficient to cause binding operation of the binding particles and evolution of the solvent in the binder, and magnetising the cured paste. In one embodiment, the paste is evacuated during curing, and the evacuation pressure is preferably less than 1.5 mbar, and most preferably less than 1.0 mbar. In one embodiment, the paste is cured with a ramping gradient of slower than 1.2°C per minute, and preferably the full curing temperature is in the range of 145°C to 150°C, and preferably the duration of curing at the full curing temperature is approximately 60 mins . In another embodiment, the binder is of polyester material . Preferably, the magnetic particles are in excess of 75% by volume, and preferably the magnetic particles have a size distribution of 95% being smaller than 23 microns. The paste is preferably applied to a thickness of 50 to 70 microns . In one embodiment, the magnetic particles are prepared by grinding in a ceramic rollermill in an inert atmosphere, and preferably the milling time is in excess of 9 hours. In another embodiment, high purity cyclohexane is added as a lubricant for milling, and preferably the milled magnetic particles are dried in a vacuum after removal from the mill . In one embodiment, the paste is aligned before curing to improve magnetisation of the film if the magnetic particles are anisotropic, and preferably the paste is aligned by exposure to a DC magnetic field of 0.2 to 0.3 T for up to 10 seconds. In a further embodiment, the films are magnetised in a magnetic field of approximately 8 T. The invention also provides a magnetic film produced by the method described above and a magnetic device incorporating such a film. The invention will be more clearly understood from the following description of some embodiment thereof, given by way of example only with reference to the accompanying drawings :- Figs. 1(a), 1(b) and 1(c) are diagrams showing the manner in which MQP-B magnetic particles are ground to a desired particle size; Figs. 2(a)-2(d) inclusive are photographs showing printed films; Fig. 3, 4(a) and 4(b) show substrate details; Figs. 5 and 6 are characteristic curves of samples; and Figs. 7(a) and 7(b) are photographs showing patterned magnetic films. In the drawings, a method of the invention for applying a thick film of hard magnetic material to a substrate is described. The method involves producing a paste of magnetic particles and a binder. The paste is viscous enough to be applied as a thick film to a substrate by a method such as screen-printing. The paste is then aligned, cured to form a thick film, and is then magnetised. If desired, the film may then be patterned using a suitable technique. Production of Magnetic Paste The magnetic particles used are Magnequench MQA-T or MQP- B Nd 2 Fe, Λ B material. These materials have the following characteristics . Magnetic Characteristics 1 : MQP-B MQA-T Remnant Induction Br 0.80-0.84 (0.80-0.87) T Coercive Force He 2 460 (557-597) KA/m Intrinsic Coercivity Hci 3 640-800 (995-1154) KA/m Energy Product (BH) max 96 (112-136) KJ/m 3 Temperature Coefficient of Br to 100°C -0.105 (-0.09) %/°C Temperature Coefficient of Hci to 100°C -0.40 (-0.55 %/ C Required Magnetising Force (open circuit) Hs 2785 (1910) KA/m In the above table, the data is uncorrected for demagnetisation effects. He is defined as the reverse field required to reduce the magnetic induction B to zero. Hci is defined as the reverse field required to reduce the magnetisation M to zero, and is always bigger than He. The following are the physical properties. Physical Properties: MQP-B MQA- •T Density (Solid Nd 2 Fe l4 B) 7 . 55 6 . 1 g/cm 3 Density (Crushed Ribbon) 3 . 02 g/cm 3 Curie Temperature 360 420 °C Hardness 60 Rockwell-C Particle Size 0.1%>420μm, 15%<44μm, typically-200μm-250μm To produce a film having a thickness in the range of 50- 100 microns, the target particle size is 10-20 microns. The particles are ground to the desired size by milling in a ceramic rollermill in the inert argon atmosphere of a glovebox. High purity cyclohexane is added as a lubricant and samples are removed at various time periods for particle sizing. The graphs of Figs. 1(a), 1(b) and 1(c) illustrate the distribution of particle size for the MQP- B powder sample after times 5 hrs 25 mins., 9 hrs, and 21 hrs respectively. A very important aspect of the milling procedure is that it is carried out over a lengthy period of time to ensure that the temperature of the particles does not rise to an extent where magnetisation can be adversely affected. As is clear from Fig. 1(c), 50% of sample 6 has a particle size of 8.21 microns or less and 95% of the particles are smaller than 23 microns. The particle sizes for the sample 6 are therefore suitable for production of a magnetic thick film having a thickness of 50 to 70 microns as this is more than double the diameter of the larger particles . After removal from the mill, the particles are dried by being placed under vacuum, and all of the cyclohexane is evaporated. To produce the paste, the particles are then distributed within a binder. Mechanical mixing takes place until a stage is reached where the paste comprising the magnetic particles and binder still has sufficient viscosity for screen-printing. An important aspect of the binder is that it comprises solid binding particles dissolved in a solvent. There is preferably a high solvent proportion - ideally above 60% by weight and in this embodiment 70% by weight. The reason for this is that during subsequent curing, the solvent evolves, thus leaving a very high magnetic particle proportion. In this embodiment, the binder used is polyester and more particularly a linear, high molecular weight copolyester based on aromatic dicarboxylic acids and aliphatic diols. In more detail, the polyester has the following characteristics: Molecular Weight: 18,000 to 20,000 Density (at 20°C) : 1.25 Glass Transition Temperature: 67-75°C Softening Point: 145 Alternative binders which have solvents which evolve to a significant extent during curing could also be used. The mixing ratios of magnetic particles to binder is 80.11% magnetic particles to 19.9% binder, by volume. However, other values in excess of 75% magnetic particles would be suitable. Printing The paste is then screen printed using a 325 mesh, 25 μm thick emulsion screen, onto 96% alumina substrates. The photographs of Figs. 2(a) to 2(d) inclusive show SEM micrographs of the cross-sections of the films in which it is clear that the magnetic particles occupy a large volume of the paste. Printing onto pre-etched silicon wafers Pre-scribed silicon wafers are used as substrates. The patterns etched into these wafers have dimensions typical of those one might expect in some microsystem/microactuator applications . Sample wafers A and B (shown in Figs. 3 and 4(a) respectively) are cut into approximately 1 cm 2 pieces using a wafer scribing system. Each of the B samples contained two individual circular wells, approximately 3.5 mm in diameter and 200 μm deep. Plan views of the wafers are shown in Figs. 3 and 4(a), while a cross sectional profile of a B wafer well is shown in Fig. 4(b). The wells are 6 mm long etched trenches in silicon. The width of the six individual trenches are as follows: 580, 600, 640, 720, 880, 1200 μm respectively, while the depth of the trenches is 133 μ . The pastes are printed into the wells of the silicon samples through patterned screens. The screens used to print the samples are 31 lines cm "1 stainless steel screens, with an average pore size of about 0.25mm 2 . The emulsion thickness is 13μm. The wells of the samples are filled with magnetic paste by means of a screenprinter. In another example, 5x5cm x lOOμm prints are printed onto standard (non-etched) silicon wafers using a screen with a 150μm thick emulsion. These samples are suitable for adhesion testing. Alignment If the magnetic particles are of the MQA-T type, the magnetic material in the paste is aligned before the paste is cured. This involves exposing the wet samples to a DC magnetic field of 0.2 to 0.3 T for up to 10 seconds. This step helps to increase the magnetic energy product upon subsequent magnetisation if the magnetic particles are anisotropic . Curing The curing step of the method is particularly important as it governs how the paste transforms to a hard magnetic film with a high magnetic density. The wet print is slowly evacuated in a vacuum below 1.5 mbar and preferably below 1.0 mbar before full curing temperature is reached. The evacuation stage is carried out at a gradually increasing temperature at a gradient of less than 1.2°C per min., and in this embodiment 1°C per min. Full curing then takes place at the same vacuum level. The temperature is 145°C to 155°C and in this embodiment 150°C. The duration is 60 mins . This brings the binding particles above the glass transition temperature for sufficiently long to bind the magnetic particles and bind the paste to the substrate. The length of the curing stage and the fact that it is carried out in a vacuum helps to ensure that the solids in the paste fill up the pores which remain as the solvent in the binder evolves. In this way, a magnetic density of approximately 80% is achieved. Thus, a magnetic loading content compatible with that achieved from compression loading techniques is achieved in a very simple manner. Magnetising The films are magnetised five at a time in an 8 T pulsed magnet and immediately examined using a vibrating sample magnetometer (VSM). This instrument is capable of producing a magnetisation (M in J\τ kg) vs. applied field (μoHa in T) curve up to a maximum field of 1.1 T. Based on mass measurements of the films, a typical M- oHa curve, is shown in Fig. 5. From this curve, it will be understood that the intrinsic coercivity Hci of the sample is 844 kA/m. The B-μoHi curve is shown in Fig. 6. The remnant induction Br is the induction at zero field and can be read from Fig. 6. To calculate BH(max) , the 2nd quadrant absol u te values of B and Hi were multiplied and plotted against Hi in Fig. 6. The maximum point of this curve is BH( ax) in units of J\m 3 . For optimum performance, this material should be used as close to the BH(max) point as possible in its eventual application. The results are as follows: D ier D mav I 5 17 84-1 0 37 7 8J Patterning The eventual physical requirements of the magnetic films depend entirely on the nature of the application. For example, if 50 micron thick films or lines with 50 micron spacings are required, screen-printing alone is not sufficient. In this situation, one method which could be used is application of photoresist as a blanket thick film layer and exposing the pattern onto the resist. A 200 x 60 μm patterned track produced by this method is shown in Figs. 7(a) and 7(b) at two resolutions. Alternatively, a blanket photo resist layer could be deposited onto the substrate and exposed to a suitable pattern and etched. The magnetic paste is then printed on top, filling the lines etched out of the original photo resist. After curing, the photo resist could be removed together with any magnetic paste which was on top of it, leaving the well defined lines of magnetic material. Further, recent developments in thick film technology have produced photo-imageable pastes whereby the paste itself can be patterned without requiring photo resist. It is envisaged that such a paste or binder could be used, however, the exposure of the binder may be blocked by the solid particles, leading to incomplete etching. Successive exposures may overcome this problem. In order to improve the characteristics of the film the percentage magnetic powder in the paste should be increased. It is envisaged that a combination of large particles and small particles could be used so that the particles fit within the binder for a higher density population. For the production of sintered Nd ; Fe H B magnets, coarse powders from cast Nd 2 Fe,iB ingots may be used. The average particle size may range from 15-20μm. The coercivity of such alloy powders amounts to <160 kA\m and hence is not appropriate for the manufacture of high energy product pastes. The low coercivity of the coarse alloy powder depends on the microstructure of the cast Nd 2 Fe 14 B ingots which consists of grains with dimensions ranging from 10- lOOμ . At this size, each powder particle contains only a few domains. A surface defect such as a sharp corner can nucleate a reverse domain which will turn the neighbouring domains out of alignment, resulting in a " strong decrease in the coercivity to <160 kA\m, whereas the coercivity of the sintered magnets with an average grain size of lOμm ranges from 1.0 to 3.3 depending on the grade of material. However, grinding this sintered material back onto powder will not mean that the coercivity is retained due to the surface effects mentioned above. The Magnaquench™ MQP-B powder has sub- micron sized grains with nanometer sized domains. With more domains per grain, surface effects are less influential . Finally, consideration must be given to the Curie temperature, Tc, of the material. Although the Curie temperature for Nd 2 Fe u B is about 420°C, its magnetisation decreases quite quickly with temperature. So although NdFeB might be the best choice at room temperature, it may not be as temperature increases. Other materials such as Sm 2 Feι 7 N 3 have lower magnetisation and coercivity but they do retain these properties well with increasing temperature. Therefore to make a final choice about which material to proceed with, an estimate of the working temperature of the device must be made. It will be appreciated that the invention provides for production of magnetic films at a very small scale for "micro" applications. A very high magnetic loading has been achieved without the need for high pressure and temperature stages. For example, the temperature levels are compatible with silicon processing generally. Binders of the type required are readily available and act in a very effective manner to both achieve the necessary binding and to ensure a very high magnetic density. The invention is not limited to the embodiments hereinbefore described, but may be varied in construction and detail within the scope of the claims.

Claims: CLAI MS 1. A method of producing a magnetic film, the method comprising the steps of:- distributing magnetic particles in a binder comprising solid binding particles dissolved in a solvent, applying the paste to a substrate, curing the paste at temperatures and a time duration sufficient to cause binding operation of the binding particles and evolution of the solvent in the binder, and magnetising the cured paste. 2. A method of producing a magnetic film as claimed in claim 1, wherein the paste is evacuated during curing. 3. A method of producing a magnetic film as claimed in claim 2, wherein the evacuation pressure is less than 1.5 mbar. 4. A method of producing a magnetic film as claimed in claim 2 or 3, wherein the paste is cured with a ramping gradient of slower than 1.2°C per minute. 5. A method of producing a magnetic film as claimed in claim 4, wherein the full curing temperature is in the range of 145°C to 150 C C. 6. A method of producing a magnetic film as claimed in claim 5, wherein the duration of curing at the full curing temperature is approximately 60 ins . 7. A method of producing a magnetic film as claimed in any preceding claim, wherein the binder is of polyester material. 8. A method of producing a magnetic film as claimed in any preceding claim, wherein the relative proportion of magnetic particles is in excess of 75% by volume. 9. A method of producing a magnetic film as claimed in any preceding claim, wherein the magnetic particles have a size distribution of 95% being smaller than 23 microns . 10. A method of producing a magnetic film as claimed in claim 9, wherein the paste is applied to a thickness of 50 to 70 microns. 11. A method of producing a magnetic film as claimed in any preceding claim, wherein the magnetic particles are prepared by grinding in a ceramic rollermill in an inert atmosphere. 12. A method of producing a magnetic film as claimed in claim 11, wherein the milling time is in excess of 9 hours . 13. A method of producing a magnetic film as claimed in claim 11 or 12, wherein high purity cyclohexane is added as a lubricant for milling. 14. A method of producing a magnetic film as claimed in any of claims 11 to 13, wherein the milled magnetic particles are dried in a vacuum after removal from the mill . 15. A method of producing a magnetic film as claimed in any preceding claim, wherein the paste is aligned before curing to improve magnetisation of the film if the magnetic particles are anisotropic . 16. A method of producing a magnetic film as claimed in — claim 15, wherein the paste is aligned by exposure to a DC magnetic field of 0.2 to 0.3 T for up to 10 seconds. 17. A method of producing a magnetic film as claimed in any preceding claim, wherein the films are magnetised in a magnetic field of approximately 8 T. 18. A method substantially as hereinbefore described with reference to the drawings . 19. A magnetic film whenever produced by a method as claimed in any preceding claim. 20. A magnetic device comprising a film as claimed in claim 19.

Inventors: MURRAY CHRISTOPHER PATRICK, O'MATHUNA SEAN CIAN, MURRAY, CHRISTOPHER, PATRICK, O'MATHUNA, SEAN, CIAN
Applicants: FORFAS TRADING AS PEI TECHNOLO, MATHUNA SEAN CIAN O, MURRAY CHRISTOPHER PATRICK, FORFAS TRADING AS PEI TECHNOLOGIES, MURRAY, CHRISTOPHER, PATRICK, O'MATHUNA, SEAN, CIAN
Application year: 1997
Main classification: H01F41
IPC codes: H01F41