Active solid-state devices (e.g. – transistors – solid-state diode – Integrated circuit structure with electrically isolated... – Passive components in ics
Reexamination Certificate
2000-04-24
2001-11-27
Flynn, Nathan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Integrated circuit structure with electrically isolated...
Passive components in ics
C257S275000, C257S295000, C257S379000, C257S664000
Reexamination Certificate
active
06323533
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a semiconductor device with an operating frequency above 50 MHz comprising a body composed of a soft ferrite material, said body having a surface to which a semiconductor element, a pattern of conductors and a passive element are fastened, which passive element is shaped like an inductor.
Such a semiconductor device can be used as a receiver for application in radio apparatus (about 100 MHz), television apparatus (about 450 to 860 MHz) and mobile telephone apparatus (about 900 MHz). In practice, the semiconductor device may comprise many active and passive elements, which active elements may be arranged in an integrated circuit comprising a very large number of transistors, and which passive elements may comprise besides one or several inductors, for example, capacitors and resistors.
A semiconductor device of the kind mentioned in the opening paragraph is known from published patent application WO 96/13858. The known semiconductor device is suitable for processing microwave signals and comprises a body composed of ferrite material, which body of ferrite material advantageously influences the performance of the inductor compared to a body composed of glass.
The performance of an inductor is often measured in terms of the quality factor Q, which is defined as the inductive reactance &ohgr;L, with &ohgr; being the angular frequency and L the inductance, divided by the total resistance R. From a theoretical article entitled “Planar inductors on magnetic substrates”, written by Roshen and published in IEEE Transactions on Magnetics, Vol. 26, No. 1 (1990), pp. 270-275, it is known that the use of a magnetic carrier body might result in a 100 percent enhancement in inductance and, hence, in the quality factor of an inductor compared to a non-magnetic carrier body, provided that the magnetic carrier body has a magnetic permeability much larger than one.
As the quality factor Q of an inductor is proportional to the typical width (e.g. diameter) of the inductor, which may be, for instance, a substantially circular, square or rectangular spiral, a doubling of the quality factor can be turned into a saving in inductor surface area of approximately 75%. Considering a typical surface area of an inductor of the order of 1 mm
2
, it is evident that a saving of 75% in inductor surface area strongly influences the total number of semiconductor devices that can be manufactured per unit of surface area.
In order to attain the above-mentioned doubling of the quality factor of an inductor to be integrated in a semiconductor device with an operating frequency above 50 MHz, generally a soft ferrite material is selected which is capable of following the alternations of the magnetic field at the operating frequency of the semiconductor device. This is supported by the statement given in a standard textbook entitled “Soft ferrites: properties and applications”, second edition, written by E. C. Snelling and published by Butterworths, London (1988), p. 90, that the useful frequency range of a ferrite is limited by the onset of ferromagnetic resonance, either because the permeability begins to fall or, at a somewhat lower frequency, the losses rise steeply.
A disadvantage of a semiconductor device of the type mentioned above, in which a soft ferrite material is applied which is capable of following the alternations of the magnetic field at the operating frequency of the semiconductor device, is that its manufacturing costs are high owing to the high initial costs of the soft ferrite material applied.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a semiconductor device of the kind mentioned in the opening paragraph, in which a soft ferrite material is applied which brings about an enhancement in inductance and, hence, in the quality factor of the integrated inductor compared to glass, and allows manufacture of the semiconductor device at moderate costs.
According to the invention, this object is achieved in that the soft ferrite material has a ferromagnetic resonance frequency smaller than the operating frequency of the semiconductor device. Surprisingly, it has been found that by using a soft ferrite material, which is not capable of following the alternations of the magnetic field at the operating frequency of the semiconductor device, the inductance and, hence, the quality factor of an inductor are almost doubled. The initial costs of soft ferrite materials in general decrease with decreasing ferromagnetic resonance frequency. Soft ferrite materials with a lower ferromagnetic resonance frequency are generally less fine-grained, and need less processing during preparation, and/or are generally poorer in expensive metals such as, for instance, nickel, magnesium and manganese. This means that a less expensive soft ferrite material can be applied in a semiconductor device with an operating frequency above 50 MHz, thereby giving rise to a significant reduction of the manufacturing costs of the semiconductor device without reducing the semiconductor device's performance.
The selection of the soft ferrite material employed in the semiconductor device in accordance with the invention is contrary to the teaching of the above-mentioned statement by E. C. Snelling in the standard textbook “Soft ferrites: properties and applications”, second edition, Butterworths, London (1988), p. 90, which statement teaches that a person skilled in the art would select a soft ferrite material having a ferromagnetic resonance frequency lying above the operating frequency of the semiconductor device, that is to say a soft ferrite material which is capable of following the alternations of the magnetic field at the operating frequency of the semiconductor device. Such a soft ferrite material has unnecessarily high initial costs and, hence, gives rise to unnecessarily high manufacturing costs of the semiconductor device.
In order to further improve the inductance and, hence, the quality factor of the inductor, it is advantageous to select a soft ferrite material having a magnetic permeability larger than about 5 at the operating frequency of the semiconductor device.
If the inductor is disposed on a relatively highly conductive body of soft ferrite material, electric currents are induced in the body of soft ferrite material by the magnetic field of the inductor. These electric currents, which are free to flow in the above-mentioned conductive body, cause extra resistance losses and a decrease in inductance and, hence, in the quality factor of the inductor. In order to counteract these resistance losses, it is therefore advantageous to apply a soft ferrite material with an electric resistivity larger than about 10
3
&OHgr;m.
The category of soft ferrite materials with an electric resistivity larger than about 10
3
&OHgr;m comprises, for instance, some MnZn-ferrites, NiZn-ferrites and MgZn-ferrites, which ferrites are of the spinel-type and have the general chemical formula Me
x
Zn
(1−x)
Fe
2
O
4±&dgr;
, with Me representing either Mn, Ni or Mg, x representing the fraction of Me of the total of Me and Zn, which lies between 0 and 1, and &dgr; representing the off-stoichiometry of the ferrite, which is typically smaller than or equal to 0.05. The ferrites advantageously possess a slight Fe-deficiency, as this suppresses the presence of Fe
2+
ions, which ions in general lead to increased eddy-current losses. From the above-mentioned category of soft ferrite materials, a NiZn-ferrite or a MgZn-ferrite is advantageously selected, as these types of soft ferrite materials can be comparatively easily manufactured with a resistivity larger than about 10
3
&OHgr;m.
In order to facilitate the manufacture of a semiconductor device in accordance with the invention, the inductor is advantageously produced in substantially planar form.
The inductor may be shaped like, for instance, a substantially square or rectangular spiral. However, the inductor preferably has the shape of a substantially circular spiral, since the use of a circular inductor l
Dekker Ronald
Dolmans Wilhelmus Mathias Clemens
Van Der Zaag Pieter Jan
Biren Steven R.
Flynn Nathan
Forde Remmon R.
U.S. Philips Corporation
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