Noise filter

Abstract: A noise filter for use in digital electronic equipment is formed on an insulating substrate (1) and comprises a first spiral electrode (3a), a dielectric layer (5), and a second spiral electrode (6a) in at least partial overlapping relationship with the first electrode (3a). The noise filter operates in a normal mode with one end of one of the electrodes (3a,6a) grounded and both ends of the other electrode connected in series with a signal transmission circuit. The noise filter operates in a common mode with both ends of each electrode (3a,6a) connected in series with a signal transmission circuit.

Description: The present invention relates to a noise filter to be used in a signal transmission circuit, power supply circuit and the like in various electronics equipment for suppressing noise passing therethrough. In digital electronic equipment such as computers,noise filters are widely used in signal transmission circuits, power supply circuits, etc. One example of conventional noise filters comprises a pair of spiral electrodes 21,22 disposed in opposed relationship to each other by way of a dielectric member 20. Both terminals 23,24 of one spiral electrode 21 are inserted in series in a signal transmission circuit while either one electrode 25 or the other spiral electrode 22 is grounded as illustrated in Fig.7. An equivalent circuit of the noise filter is illustrated in Fig.8. Distributed capacitors formed between the pair of spiral electrodes 21,22 and the inductance of the one spiral electrode 21 forming the signal transmission circuit provide a low pass filter having a sharp cut-off frequency. In such conventional filters, ceramic, plastics, glass, mica, etc. are used as the dielectric member 20. Spiral electrodes are formed on the opposite faces of the dielectric member 20 by the printing process. However, as portable electronics equipment such as portable telephone, portable personal computer, etc. handling digital signals become popular, more compact noise filters are required. Unfortunately, in the conventional noise filters, there is certain limit in increasing distributed capacitance by reducing the thickness of the dielectric member and forming longer spiral electrodes by the printing technique, thereby making it difficult to achieve significantly compact noise filters. It is therefore an object of the present invention to provide noise filters suited for quantity production, significantly compact in design and having excellent noise suppressing performance. The present invention consists in a noise filter characterised in that a first spiral electrode is formed on an insulating substrate and a second spiral electrode is formed over said first spiral electrode, with a dielectric layer between the two spiral electrodes. The second spiral electrode is preferably formed in such a manner that it at least partly overlaps with the first spiral electrode. Preferably, a high purity semiconductor substrate having a large bulk resistance is used as the insulating substrate. The invention will now be described in more detail, and by way of example only, with reference to the accompanying drawings, in which : Fig.1 illustrates in cross section of one embodiment of the noise filter according to the present invention, at various stages of its manufacture; Fig.2 illustrates a plan view (a) and a cross sectional view (b) of the noise filter made by the process steps of Fig.1. Fig.3 illustrates in cross section a second embodiment of the noise filter according to the present invention; Fig.4 illustrates in cross section a third embodiment of the noise filter according to the present invention during the manufacture thereof; Fig.5 illustrates in cross section a packaged noise filter according to the invention; Fig.6 illustrates in cross section a fourth embodiment of the noise filter according to the present invention. Fig.7 illustrates a front view (a), a side view (b) and a rear view (c) of one example of the conventional noise filters. Fig.8 illustrates an equivalent circuit of the conventional noise filter. A first embodiment of the noise filter according to the present invention will be described in accordance with the process of making such a noise filter illustrated in cross sectional views in Fig.1. (1) Firstly, an insulating substrate 1 is prepared such as glass, plastics, pure silicon wafer, etc. A high purity silicon wafer having high bulk resistance is particularly suited for the insulating substrate 1. (2) A first silicon dioxide (SiO 2 ) film 2 is formed on one surface of the silicon wafer 1 by a chemical vapor deposition (CVD) technique. (3) A metal film 3 such as, for example, gold, tungsten,molybdenum, tantalum, niobium, etc. as a resist for the subsequent CVD steps is formed on the silicon dioxide film 2 on the silicon wafer 1 by a vapor deposition technique. (4) A spiral pattern of a photoresist 4 is formed on the metal film 3 by photo lithography. (5) The photoresist 4 is used as a mask to form a first spiral metal electrode 3a before washing off the photoresist 4. (6) A second silicon dioxide film 5 is formed on the first spiral metal electrode 3a and the exposed silicon dioxide film 2 by CVD. (7) An aluminium film 6 is vapor deposited on the second silicon dioxide film 5. (8) A spiral pattern of a photoresist 7 wider than the gap between electrodes is formed on the aluminium film 6 along the center line between adjacent electrodes of the first spiral metal electrode 3a. (9) A spiral aluminium electrode 6a having a terminal 6b at one end is formed using the photoresist 7 as a mask and then the photoresist 7 is washed off. (10) The second silicon dioxide film 5 at one or both end portions of the first spiral metal electrode 3a is etched to expose the first spiral metal electrode 3a and terminals 3b, 3c are formed (as shown in Figure 2(b)). The noise filter element made on the insulating substrate or the silicon wafer 1 by a series of the above steps (1) through (10) is illustrated in Fig.2(a) in a plan view and in Fig.2(b) in a cross section view along the line B-B in Fig.2(a). Preferably, a plurality of spiral metal electrodes 3a, 6a are formed simultaneously at different areas on the silicon wafer 1 which is subsequently divided. The terminals 3b, 3c and 6b are bonded to the two spiral metal electrodes 3a, 6a before packaging. Although the second spiral metal electrode 6a is formed in overlapped relation to the first spiral metal electrode 3a along both side edges in the above described first embodiment, it is to be understood that the second spiral metal electrode 6a overlaps the first spiral metal electrode 3a along one side edge of the second spiral metal electrode 6a. In other words, it is only required that the first and the second spiral metal electrodes 3a, 6a partly overlap along their lengths for electrical coupling therebetween, as shown in Fig. 3. Next, described below are the steps for making a noise filter according to a third embodiment of the present invention. The steps (1) through (7) are identical to the corresponding steps in the first embodiment described hereinbefore. (1) Firstly, an insulating substrate is prepared such as a pure silicon wafer 1. (2) A first silicon dioxide film 2 is formed on the silicon wafer 1 by CVD. (3) A metal film 3 as a resist for the subsequent CVD process is formed on the silicon dioxide film 2 on the silicon wafer 1 by vapor deposition. (4) A spiral pattern of a photoresist 4 is formed on the metal film 3 by photo lithography. (5) A first spiral metal electrode 3a is formed using the photoresist 4 as a mask and then the photo-resist 4 is washed off. (6) A second silicon dioxide film 5 is formed by CVD technique on the first spiral metal electrode 3a and the exposed first silicon dioxide film 2. (7) An aluminium film 6 is vapor deposited on the second silicon dioxide film 5. (8) A spiral pattern of a photoresist 7a is formed on the aluminium film 6 at a location corresponding to the first spiral metal electrode 3a. (9) A second spiral aluminium electrode 6d having a terminal at one end is formed using the photoresist 7a as a mask. Subsequently, the photoresist 7a is washed off. (10) The second silicon dioxide film 5 at one or both ends of the first spiral metal electrode 3a is etched to expose a part of the first spiral metal electrode 3a where a terminal is formed. The silicon wafer 1 having preferably a plurality of spiral metal electrode 3a, 6a is divided and packaged after bonding the terminals to the two spiral metal electrodes 3a, 6a. Now, packaging the noise filter made by the steps in the above first, second or third embodiments is described by reference to Fig.5. The divided silicon wafer chip 12 is coated with a silicon dioxide film 13 on the entire surface by CVD technique. Holes are bored in the silicon dioxide film 13 by etching at the locations where the terminals are formed. The holes are filled with a solder 14 to extend from the outer surface of the silicon dioxide film 13. The solder bumps 14 can be contacted directly to lands or terminal areas on a printed circuit board. This is particularly suited for surface mounting technology (SMT). It is appreciated that plastics or other insulating materials can be used for surface protection of the chip 12 and that a laser beam can be used to bore holes in such protection films. Next, a fourth embodiment of the present invention for fabricating a noise filter in 3-dimensions on the surface of a silicon wafer where integrated circuits have been fabricated. As illustrated in Fig.6(a), (b) and (c) in cross section a silicon dioxide film 2 is formed by CVD on the surface of a silicon wafer 10 on which integrated circuits 11 have been fabricated. A noise filter (or noise filters) is formed on the silicon dioxide film 2 in accordance with the steps as described in one of the above embodiments. It is appreciated that the 3 dimensional fabrication of the noise filter achieves effective use of the limited and expensive space on the silicon wafer. In operating the noise filter made in accordance with any one of the above first through fourth embodiments in a normal mode, the terminal of the first spiral metal electrode of a pair of spiral metal electrodes is connected to a source of reference potential, e.g., ground while both terminals of the second spiral metal electrode are connected in series with a signal transmission circuit. Since the second spiral metal electrode is AC coupled to ground through the first spiral metal electrode, it forms a distributed circuit to prevent transmission of high frequency noise. On the other hand, in case of operating the noise filter in a common mode, the both spiral metal electrodes are inserted in series with the signal transmission circuit. The spiral metal electrodes 3a, 6a, 6d have a certain resistance depending on their film thickness and width. Thus, the quality factor (Q) of the distributed circuit can be adjusted by choosing the film thickness and/or width. If the noise filter has high Q, there is a sharp attenuation at a particular frequency. If Q is low, a relatively flat attenuation curve is obtained over a wide frequency range. As a result, the bulk resistance of the silicon wafer as the insulating substrate, length, thickness and width of the spiral metal electrodes and the thickness of the silicon dioxide film as the dielectric film can be chosen to obtain a desired attenuation characteristic of the noise filter. Also, in a case where capacitance between the two spiral metal electrodes is too large and/or inductance of one spiral metal electrode is too small, one spiral metal electrode can be chosen longer than the other. In the above embodiments, there are formed two spiral metal electrodes 3a, 6a or 3a, 6d on a single layer. However, it is appreciated that the spiral metal electrodes may be formed on a plurality of films and interconnected between the layers in order to increase inductance and/or capacitance. In case of increasing the inductance of spiral metal electrodes, a magnetic insulating material such as ferrite can be used as the insulating substrate. The noise filter according to the present invention can be used as a discrete part by packaging one element in a case. It is also possible that the noise filter can be fabricated as a part of other circuits on a printed circuit board or an IC card to be used by connecting to a power supply circuit and a signal transmission circuit. Also, a plurality of noise filters can be formed on a common insulating substrate and packaged in a single case as an integral discrete part for preventing noise in signal transmission circuits for a plurality of bits. In order to protect electrostatic breakdown of the dielectric layer between the two spiral metal electrodes, a spark gap, a plurality of series connected diodes, or a zenner diode can be connected between the two spiral metal electrodes. The dielectric layer between the two spiral metal electrodes may be a silicon nitride (Si 3 N 4 ) film rather than the silicon dioxide film. A pure semiconductor substrate such as a silicon wafer has a significantly higher bulk resistance as compared with the metal conductor of the spiral metal electrodes 3a, 6a, 6d and is conveniently used as the insulating substrate. If the noise filter of the present invention is fabricated on a silicon wafer as the insulating substrate, the noise filter can be fabricated, inspected and packaged using the existing conventional facilities for fabricating integrated circuits, thereby eliminating the need for new facilities. As apparent from the above description of various embodiments of the present invention, the noise filter according to the present invention is very compact, less expensive to make and suited for mass production using the conventional and established process for fabricating IC devices without the need for a new facility. Also, noise filter elements having various attenuation characteristics can be obtained easily.

Claims: A noise filter characterised in that a first spiral electrode (3a) is formed on an insulating substrate (1) and a second spiral electrode (6a) is formed over said first spiral electrode, with a dielectric layer (5) between the two spiral electrodes. A noise filter according to claim 1 characterised in that the second spiral electrode (6a) is formed on said dielectric layer at least partially overlapping with said first spiral electrode. A noise filter as claimed in claim 1 or 2, characterized in that at least one end of one of said spiral electrodes (3a,6a) is grounded and both ends of the other spiral electrode are connected in series with a signal transmission circuit to operate in a normal mode. A noise filter as claimed in claim 1 or 2, characterized in that both ends of each of said spiral electrodes (3a,6a) are connected in series with a signal transmission circuit to operate in a common mode. A noise filter as claimed in any one of claims 1 to 4, characterized in that a pure semiconductor substrate is used as said insulating substrate (1). A noise filter as claimed in any one of claims 1 to 4, characterized in that a magnetic insulating substrate is used as said insulating substrate (1). A noise filter as claimed in any one of claims 1 to 5, characterized in that a plurality of noise filters are formed as an integral part on an insulating substrate (1). A noise filter as claimed in any one of the preceding claims, fabricated on a substrate having IC circuits fabricated thereon. A method of making a noise filter characterized by preparing an insulating substrate (1) forming a first spiral electrode (3a) on said substrate by photolithography, depositing a dielectric layer (5) on said first spiral electrode (3a) and said substrate (1), and forming a second spiral electrode (6a) on said dielectric layer by photolithography so that said second spiral electrode at least partially overlaps said first spiral electrode.

Inventors: IKEDA TAKESHI, IKEDA, TAKESHI
Applicants: IKEDA TAKESHI, OKAMURA SUSUMU, IKEDA, TAKESHI, OKAMURA, SUSUMU
Application year: 1993
Main classification: H03H7
IPC codes: H05K3, H03H7, H03H1, H05K1