Inductive structures for semiconductor integrated circuits

Abstract: A transformer integrable with a semiconductor integrated circuit comprises an electrically insulating semiconductor substrate (1) having a first surface, a first electrical conductor (28) disposed on the first surface of the substrate (1) comprising a first winding having a first pair of leads (32), a first electrically insulating film (33) disposed on the first electrical conductor (28), a second electrical conductor (34) disposed on the electrically insulating film (33) comprising a second winding having a second pair of leads (35), the first and second windings and the electrically insulating film including a common central opening, and a magnetic material (36) disposed on the surface of the substrate (1) in the common central opening increasing the inductive coupling between the first and second windings.

Description: The present invention relates to a transformer integrable with a semiconductor integrated circuit as claimed in the preamble of claim 1. Multiple spiral windings that are inductively coupled to each other can be employed as transformers. An example of another prior art transformer employing two spiral windings disposed on the same surface is shown in plan view in Figure 1. There, an electrically conducting spiral 2 is disposed within an electrically conducting spiral 12. Both the external end 3 and the internal end 4 of spiral 2 require air bridge structures 9, insulating them from and crossing over windings of spiral 12 and giving electrical access to the spiral 2. Because spiral 12 lies outside spiral 2, direct access to external end 13 of spiral 12 is available. However, an air bridge structure 9 must still be provided tc give external access to the internal end 14 of spiral 12. Because of the presence of the three air bridges 9, the transformer of Figure 1 suffers from parasitic capacitance problems. In addition, the magnetic coupling between spirals 2 and 12 is limited because only a relatively low permeability material, i.e., the substrate 1, is present in the vicinity of those windings. Another prior art transformer structure is shown in an exploded view in Figure 2. That transformer structure includes an electrically insulating substrate 1 on which insulating films 20, 30, and 40 are successively disposed. Electrically conducting spirals 2 and 12 are respectively disposed on films 20 and 40. Each of the films includes a through hole through the respective insulating films through which conductors 26 and 27 respectively extend to an underlaying layer, film 30 in the case of spiral 2 and substrate 1 in the case of spiral 12. These electrical conductors passing through the insulating films provide an electrical connection to respective leads 6 from the internal ends 4 and 14 of spirals 2 and 12. The two spirals 2 and 12 have the same sense, i.e., the same direction winding, and overlie each other to maximize their mutual inductive coupling. The mutual inductances of the two spirals is controlled by their geometries with respect to each other and the thicknesses of the insulating films. However, the mutual inductance between the two spirals is limited since the permeability of the adjacent materials is relatively small. In Figures 3(a)-3(c), another prior art transformer structure for use in semiconductor integrated circuits is shown in sectional and plan views. This transformer corresponding to the preamble of claim 1 employs a sheet of a relatively high permeability material in order to improve the inductive coupling between two windings. In this prior art structure, described in Patent Abstracts of Japan: Vol 11, N°. 161 (E-509), 23-05-87 and in Japanese Published Patent Application 61-294850, an electrically conductive spiral 2 is disposed on a semiconductor substrate 1. A second winding 28 comprises a single turn loop spaced from spiral 2 by an electrically insulating layer 4. A ferromagnetic sheet 31 is embedded in the insulating layer 29 between spiral 2 and winding 28. Because the ferromagnetic sheet 31 is disposed between spiral 2 and winding 28 rather than within their central openings, it is not effective in significantly increasing the inductive coupling between those two conductors. Thus, it is difficult in the prior art structures to obtain a large mutual inductance and high efficiency in the transformer. It is an object of the present invention to provide a transformer according to the preamble of claim 1 with a larger mutual inductance and higher efficiency. A transformer according to the invention is characterized by the first and second windings and the electrically insulating film including a common central opening; and a magnetic material disposed on the surface of the substrate in the common central opening increasing the inductive coupling between the first and second windings. The presence of a ferrite within the opening of the windings increases the mutual coupling of the winding and transformer performance compared to the prior art transformers. Embodiments of the invention are discussed below in conjunction with respective prior art to illustrate the improvements achieved in the invention. In the drawings: Figure 1 is a plan view of a prior art transformer employing two spiral inductors disposed on the same surface; Figure 2 is a perspective view of a prior art transformer employing two spiral inductors disposed on different surfaces with an intervening insulating film; Figures 3(a), 3(b), and 3(c) are a sectional view, and two plan views, respectively, of a prior art transformer employing a magnetic sheet between coils; Figures 4(a), 4(b), and 4(c) are plan, sectional, and perspective views, respectively, of a transformer according to a first embodiment of the invention; and Figures 5(a), 5(b) , and 5(c) are plan, sectional, and perspective views, respectively, of a transformer according to a second embodiment of the invention. Figures 4(a), 4(b), and 4(c) are plan, sectional, and perspective views, respectively, of a transformer integrable in a semiconductor integrated circuit according to a first embodiment of the invention. In this structure, a single turn winding 28 made of a thin metal film is disposed on the surface of a semiconductor substrate, such as gallium arsenide. Winding 28 includes a pair of leads 32. An electrically insulating film 33, such as SiN or SioN, is disposed on winding 28. Another single turn winding 34 is disposed on insulator 33 directly above and opposite winding 28. Winding 34 includes leads 35. Windings 28 and 34 and insulating film 33 have common central openings generally aligned with each other to provide a common core. A ferromagnetic body 36, such as a ferrite, is disposed within that common central opening to improve the mutual inductance of windings 28 and 34. The magnetic permeability of body 36 is significantly larger than that of substrate 1. Moreover, the disposition of that magnetic body within the common core of the two windings 28 and 34 ensures good magnetic coupling between those windings. As a result, a relatively high mutual inductance, i.e., a highly efficient transformer, is achieved according to this embodiment of-the invention. As well known in the art, the permeability of ferrite materials may be about twenty-five hundred, providing very strong coupling between the two windings 28 and 34. As illustrated in Figures 4(a) and 4(c), the leads 32 and 35 of the respective windings are preferably aligned in different directions to avoid undesired capacitive coupling between them. The structures of Figures 4(a)-4(c) are readily constructed using conventional semiconductor device techniques, such as patterning of metal and insulating layers with photolithography, and can be made part of integrated circuitry on a substrate including interconnected active and passive circuit elements. The ferromagnetic body 36 may be separately formed and disposed in the common central opening of the windings and insulating film 33. Alternatively, the ferromagnetic material may be deposited in the common opening of the windings and insulating layer by silk screening or other deposition techniques, followed by curing and/or other steps necessary to produce the desired ferromagnetic properties. Preferably, the ferromagnetic material is deposited after both windings 28 and 34 and insulating film 33 have been deposited and patterned. In Figures 5(a), 5(b), and 5(c), an extension of the structure shown in Figures 4(a)-4(c) is shown in plan, sectional, and perspective views, respectively. The structure of Figures 5(a)-5(c) is identical to that of 4(a)-4(c) except that a second insulating film 37 is disposed on winding 34 and a third winding 38 is disposed on insulating film 37. This third winding 38 includes leads 39. The leads of each of the three windings are oriented along different directions. The magnetic body 36 is extended in height to fill the common core of the three windings and two intervening insulating layers. Additional windings and insulating films can be added to the stack. This embodiment is produced in the same manner that the embodiment of Figures 4(a)-4(c) is prepared except that additional steps are required for depositing the second insulating film 37 and electrical conductor forming winding 38. The same desirable large mutual inductive coupling is obtained as in the earlier embodiment except that the coupling occurs between three windings rather than two. In these structures, the direction of mutual coupling is generally perpendicular to the surface of substrate. Since monolithic integrated circuit technology makes single turn transformer windings the easiest to fabricate, integrable transformers inherently have low inductances and low mutual inductances between windings. In the invention, the mutual inductance and transformer efficiency of single turn windings are significantly increased, since the ferromagnetic material is disposed very close to the transformer windings.

Claims: A transformer integrable with a semiconductor integrated circuit, comprising: a semiconductor substrate (1) having a first surface; a first electrical conductor (28) disposed on the first surface of the substrate (1) comprising a first winding having a first pair of leads (32); a first electrically insulating film (33) disposed on the first electrical conductor (28); and a second electrical conductor (34) disposed on the electrically insulating film (33) comprising a second winding having a second pair of leads (35), characterized by the first and second windings and the electrically insulating film including a common central opening; and a magnetic material (36) disposed on the surface of the substrate (1) in the common central opening increasing the inductive coupling between the first and second windings. The transformer of claim 1, characterized in that the magnetic material (36) is a ferrite body. The transformer of claim 1 or 2, characterized in that the electrically insulating film (33) is chosen from the group consisting of SiN and SiON. The transformer of claim 1, characterized in that the pairs of leads (32,35) of the first and second windings are oriented in different directions to minimize the coupling between the respective pairs of leads. The transformer of claim 1, characterized in that the substrate (1) is chosen from the group consisting of gallium arsenide and indium phosphide. The transformer of claim 1, further characterized by a second electrically insulating film (37) disposed on the second electrical conductor (34) and a third electrical conductor (38) disposed on the second electrically insulating film (37) opposite the second electrical conductor (34) comprising a third winding and a third pair of leads (39), the second electrically insulating film (37) and the third electrical conductor (38) having respective central openings aligned with the common central opening of the first and second windings.

Inventors: ANDOH NAOTO, INOUE AKIRA, NAKAHARA KAZUHITO, NAKAJIMA YASUHARU, ANDOH, NAOTO, INOUE, AKIRA, NAKAHARA, KAZUHITO, NAKAJIMA, YASUHARU, ANDOH, NAOTO, MITSUBISHI DENKI K.K., INOUE, AKIRA, MITSUBISHI DENKI K.K., NAKAHARA, KAZUHITO, MITSUBISHI DENKI K.K., NAKAJIMA, YASUHARU, MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1990
Main classification: H01F17
IPC codes: H01L23, H01F21, H01L21, H01F17