Abstract:
The invention provides a transformer suitable for use in a high frequency semiconductor device, and the transformer can be formed without the use of a conventional core and coils by forming at least two spiral inductors selected from a plurality of spiral inductors on a semiconductor substrate so as to overlap each other substantially in the vertical direction with an interlayer insulator interposed in between.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a transformer suitable for use in a semiconductor device, and particularly, to a transformer for use in a high frequency semiconductor device inside a monolithic microwave integrated circuit (MMIC), and the like, that is used in a fast radio transmission system, and so forth. 
     2. Description of the Related Art 
     There has been described a conventional transformer in a literature, the college text, “Electromagnetics”, by Koichi Shimoda and Soshin Ckikazumi, pp. 214, 216. FIG. 1 is a view broadly showing a conventional transformer described in the literature mentioned above. Referring to the figure, the conventional transformer is described hereinafter. Reference numeral  21  denotes a primary coil,  22  a secondary coil, and  23  a core. The primary coil  21  and secondary coil  22  are wound round the core  23 . 
     Now, the operation is described hereinafter. Flow of alternating current in the primary coil  21  causes magnetic fluxes to be induced in the core  23 , whereupon an electromotive force is generated in the secondary coil  22  by the agency of the magnetic fluxes induced. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a transformer that can be applied to a high frequency semiconductor device, and the like. The transformer can be formed without the use of a conventional core and coils by forming at least two spiral inductors selected from a plurality of spiral inductors on a semiconductor substrate so as to overlap each other substantially in the vertical direction with an interlayer insulator interposed therebetween. 
     According to the invention, a transformer can be implemented without the use of a core and coils by forming a plurality of spiral inductors on a semiconductor substrate, and by forming at least two spiral inductors selected from the plurality of the spiral inductors so as to overlap each other substantially in the vertical direction to the substrate with an interlayer insulator interposed therebetween such that at least the two spiral inductors are insulated from each other in terms of d.c., but continuous with each other in terms of a high frequency wave. 
     Further, the transformer with the features described above may be formed wherein at least the two spiral inductors selected from the plurality of the spiral inductors are formed in the shape of a rectangle such that the rectangles overlap each other along the longer sides thereof, so that advantageous effects of mutual inductance can be enhanced by enlarging an area of overlapping portions without enlarging areas of elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view broadly showing a conventional transformer; 
     FIG. 2 is a schematic representation illustrating a first embodiment of a transformer according to the invention; and 
     FIG. 3 is a schematic representation illustrating a second embodiment of a transformer according to the invention 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 2 and 3 are schematic representations illustrating embodiments of a transformer according to the invention. The embodiments of the invention will be described hereinafter with reference to FIGS. 2 and 3. 
     In FIG. 2 as well as FIG. 3, respective spiral inductors are insulated from each other in terms of d.c., but continuous with each other in terms of a high frequency wave. Magnetic fluxes are induced by the flow of electric current in one of the spiral inductors, whereupon an electromotive force having any suitable current and voltage can be caused to occur in the other of the spiral inductors through mutual induction. Accordingly, any suitable current and voltage can be generated by a single power supply source without the use of a plurality of power supply sources, and the like. 
     Not less than two of the spiral inductors are sufficient. Optional current values and voltage values can be obtained by varying the number of turns and an overlapping manner with respect to the spiral inductor in which electric current flows, and the spiral inductor in which the electromotive force is induced. 
     The embodiments wherein two of the spiral inductors are used are described hereinafter, however, it is evident that three or more of the spiral inductors may be used instead. 
     FIG. 2 is a view showing a first embodiment of a transformer according to the invention. The first embodiment is described hereinafter with reference to FIG.  2 . 
     Reference numeral  1  is a primary spiral inductor, and is formed of a first layer wiring over a semiconductor substrate  8 . Reference numeral  2  is a secondary spiral inductor, and is formed of a second layer wiring. Reference numeral  3  is a connection terminal for the primary spiral inductor, and is connected to the second layer wiring (not shown). Reference numeral  5  is a connection terminal for the secondary spiral inductor, and is connected to the first layer wiring (not shown). Reference numeral  7  is an interlayer insulator for insulating the first layer wiring from the second layer wiring. 
     Now, the operation is described hereinafter. Both the primary spiral inductor and the secondary spiral inductor have inductance at a value, respectively. When electric current flows in the primary spiral inductor, magnetic fluxes are induced. An electromotive force is generated in portions of the secondary spiral inductor, where the primary spiral inductor and the secondary spiral inductor overlap each other, by the agency of the magnetic fluxes, and thereby electric current is caused to flow in the secondary spiral inductor, thereby enabling such a constitution as described to function as a transformer. 
     Thus, according to the first embodiment of the invention, it becomes possible to form a transformer on top of a high frequency semiconductor device of a MMIC, or the like by installing a plurality of the spiral inductors formed so as to overlap each other on a semiconductor device. 
     FIG. 3 is a view showing a second embodiment of a transformer according to the invention. The second embodiment is described hereinafter with reference to FIG.  3 . 
     Reference numeral  11  is a primary spiral inductor, and is formed of a first layer wiring over a semiconductor substrate  18 . Reference numeral  12  is a secondary spiral inductor, and is formed of a second layer wiring. Reference numeral  13  is a connection terminal for the primary spiral inductor, and is connected to the second layer wiring (not shown). Reference numeral  15  is a connection terminal for the secondary spiral inductor, and is connected to the first layer wiring (not shown). Reference numeral  17  is an interlayer insulator for insulating the first layer wiring from the second layer wiring. 
     Normally, the spiral inductors are often formed substantially in the shape of a square but, in this case, are formed in the shape of a rectangle on purpose. 
     Now, the operation is described hereinafter. Both the primary spiral inductor and the secondary spiral inductor have inductance at a value, respectively. When electric current flows in the primary spiral inductor, magnetic fluxes are induced. An electromotive force is generated in portions of the secondary spiral inductor, where the primary spiral inductor and the secondary spiral inductor overlap each other, by the agency of the magnetic fluxes induced, and thereby electric current is caused to flow in the secondary spiral inductor, thereby enabling such a constitution as described to function as a transformer. With this embodiment, since both of the spiral inductors are rectangular in shape, an area of the portions of the secondary spiral inductor, where the primary spiral inductor and the secondary spiral inductor overlap each other, becomes greater than that in the case of the first embodiment, and thereby a transfer efficiency from the primary spiral inductor to the secondary spiral inductor is improved on that for the first embodiment. 
     Thus, according to the second embodiment of the invention, it is possible to further improve the transfer efficiency without enlarging an area of elements by forming the spiral inductors in the shape of a rectangle in addition to advantageous effects of the first embodiment gained by installing the plurality of the spiral inductors formed so as to overlap each other on a semiconductor device. 
     With reference to the first and second embodiments described above, a specific case where two of the spiral inductors are used is described, however, it is to be pointed out that there are no limitations whatsoever as to the number of turns of the respective spiral inductors, width thereof, and so forth. 
     Further, output from the connection terminals may be taken out via wiring in any suitable layer through contact holes, or the like, and a manner in which the output from the connection terminals are taken out is not limited to that according to the first or second embodiment.