This invention relates to a high frequency thin film electrical circuit element comprising an elongate conductor coupled magnetically with at least one layer of magnetic material extending along at least a part of the conductor above and below the conductor.
Embedding or sandwiching the conductor of an inductive element in a magnetic material can significantly increase its inductance at a given size or reduce its size while maintaining a given inductance. Similarly, embedding or sandwiching a conductor in a magnetic material can improve containment of the magnetic field generated by current flowing along the conductor: this may be especially valuable if the conductor is formed as part of a semiconductor device such as an integrated circuit, since it can improve signal isolation from other elements of the device.
A reduction in circuit element size is especially valuable for microscopic circuit elements made using semiconductor-type manufacturing techniques such as mask-controlled deposition and etching of materials on a support layer, since it leads to a reduction in occupied chip area which enables more devices to be produced for a given sequence of manufacturing operations and a given overall support layer (‘wafer’) size.
However, ferromagnetic resonance (FMR) losses have restricted the applicability of such devices to below 1 GHz, even using high resistivity ferromagnetic materials.
A report entitled “Soft ferromagnetic thin films for high frequency applications” by Fergen, I. et al. in the Journal of Magnetism and Magnetic Materials vol. 242-245 p. 146-51 April 2002 describes a study of the properties of sputtered thin films of magnetic material at high frequencies.
A report entitled “Ferromagnetic RF inductors and transformers for standard CMOS/BiCMOS” by Zhuang Y et al. in the International Electron Devices Meeting 2002 Technical Digest, IEEE 8 Dec. 2002 p. 475-478 describes an RF inductor comprising an elongate electrical conductor coupled magnetically with a thin layer of magnetic material extending along at least part of the conductor above and below the conductor, the layer having a thickness of 0.5 μm and a lateral dimension of 100, 200, 400 or 800 μm.
A need exists for a practical high frequency thin film electrical circuit element for high frequency applications that has a small occupied chip area.
The present invention provides an inductive element incorporating carefully chosen layers of magnetic material and a method of making an inductive element as described in the accompanying claims.
The embodiment of the invention shown in the drawings comprises an elongated conductor 1 formed in a layer of conductive material on an electrically insulating support layer 2. The electrical conductor 1 may consist of a single straight element, or a series of parallel straight elements connected at alternate ends to the adjacent elements so as to form a meander, or could be part of a planar or non-planar spiral inductor. In the embodiment of the invention shown in
The conductor 1 may be used as a self-inductance, or as part of a transformer. In order to increase the inductance of the conductor 1, it is embedded in a layer of thin film magnetic material of permeability greater than 1, preferably ferromagnetic material. (Note: the magnetic material is not shown in
In one embodiment of the invention, the magnetic material 5 is a sputtered film of highly resistant ferromagnetic material of suitable thickness. Suitable ferromagnetic materials are alloys such as FeCoSiB and FeTaN.
In another embodiment of the invention, the material of the magnetic layer 5 is a composite material that comprises particles of ferromagnetic material densely packed in a substantially non-magnetic, electrically resistive matrix material. Such composite materials present reduced eddy current losses and the inductor presents reduced series resistance and reduced parasitic capacitance leading to high quality factor (“QS”) at high RF frequencies. The magnetic particles may be magnetic nanoparticles of iron (Fe) or iron cobalt (FeCo) alloys. The matrix material may be an organic resin or ligant.
Typical permeability characteristics of the layer 5 are shown in
The permeability of the magnetic layer 5 depends upon its saturation magnetisation, Ms, which is an element for property of the magnetic material, and the anisotropy, Hk, which depends on the crystal structure and morphology of the layer. In both bulk and thin film configurations, the permeability of the material is as follows:
μ=1 +Ms/Hk Equation 1
As shown in
The demagnetization factors are in general a diagonal tensor function of the sample shape. Their impact on the ferromagnetic resonance can be expressed as follow:
FMR=γ√{square root over ([Hk+(Ny−Nz)Ms][Hk+(Nx−Nz)Ms])}{square root over ([Hk+(Ny−Nz)Ms][Hk+(Nx−Nz)Ms])}
where γ is the gyromagnetic ratio, Nx, Ny, Nz are the demagnetization factors of the particle and Ms the saturation magnetization, Hk is the crystal anisotropy field.
The demagnetization factors are calculated as: Nx+Ny+Nz=1 with their individual expressions for rods and ellipsoid widely calculated and tabulated (see for instance Modern Magnetic Materials, Principles and Application, R. C. O'Handley Wiley Interscience p. 41)
For thin films, Ny=Nz=0; Nx=1 and FMR=γ√{square root over (H2k+HkMs)}≈γ√{square root over (HkMs)} if Ms>>Hk
For spheres: Nx=Ny=Nz=⅓ and FMR=γHk
For intermediates configurations the Nz and FMR are dependent on the sample shape (aspect ratio) as depicted in
As shown in
Nonetheless, there is a lower limit to the useful aspect ratio of the layer 5. The smaller the aspect ratio for a given thickness of the layer, the wider are its lateral dimensions. For an example of an inductance of the order of 1 to 5nH at frequencies above 1 GHz and a practical example of the layer 5 with permeability μ of the order of 10, the thickness of the layer 5 of
In fact, the dimensions of the inductor will depend not only on the aspect ratio of the magnetic material but also on its permeability: magnetic materials may be used exhibiting permeability substantially greater than the value of 10 given for a typical material that is currently readily available.
The inductance of the conductor 1 embedded in the layer 5 relative to the same conductor surrounded by air (“LO”) is shown in
It will be appreciated that current flowing along the conductor 1, that is to say perpendicular to the plane of the drawing, will generate magnetic flux circularly around the conductor and accordingly contained in the transverse extent of the layers 6 and 7 and in the interconnections 8 and 9.
It will be appreciated that the embodiment shown in the lower part of
As shown in
It will be appreciated that the electrical circuit device as shown in the drawings may be used in electrical circuit apparatus together with devices that are responsive to the inductance the electric circuit device presents to a periodic current flowing along the conductor.
Number | Date | Country | Kind |
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03292965 | Nov 2003 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/EP2004/013645 | 11/29/2004 | WO | 00 | 8/25/2006 |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO2005/052961 | 6/9/2005 | WO | A |
Number | Name | Date | Kind |
---|---|---|---|
6114937 | Burghartz et al. | Sep 2000 | A |
6593841 | Mizoguchi et al. | Jul 2003 | B1 |
20020005565 | Forbes et al. | Jan 2002 | A1 |
20020008605 | Gardner | Jan 2002 | A1 |
20020048668 | Inoue | Apr 2002 | A1 |
Number | Date | Country |
---|---|---|
59144105 | Aug 1984 | JP |
Number | Date | Country | |
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20070159285 A1 | Jul 2007 | US |