JP2005123498A - Microwave integrated circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide more simply a microwave integrated circuit having a wide-band and good frequency characteristic. <P>SOLUTION: In the microwave integrated circuit 10, a coplanar line and MIM capacitors, etc. are formed on the top surface of a circuit board 28. A resistor 60 to be an wave absorber is provided extensively on the bottom surface of the circuit board. By virtue of this resistor 60, the electric field generated in the inside of the circuit board is so attenuated as to be able to suppress any resonance phenomenon. Therefore, the microwave integrated circuit having the wide-band and good frequency characteristic can be obtained. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a microwave integrated circuit of a coplanar line type that is mounted face-up on a mounting substrate.

  A conventional microwave integrated circuit mainly uses a microstrip line. On the other hand, in recent years, a microwave integrated circuit using a coplanar line has been proposed. Since the coplanar line type microwave integrated circuit includes a high frequency ground (ground conductor) on the upper surface of the circuit board, a via hole for grounding is not required, and the substrate is not required to be thinned. Further, since the circuit area is small, it is easy to increase the degree of integration. For this reason, the use of the coplanar line type microwave integrated circuit has been expanded in order to reduce the cost of microwave and millimeter wave radio apparatuses. In particular, it is frequently used in a monolithic microwave integrated circuit (hereinafter referred to as “MMIC”) in which active elements and passive elements are integrated on one semiconductor substrate.

  A conventional coplanar line type MMIC will be described with reference to FIG. FIG. 1 is a diagram showing an example when the coplanar line type MMIC 10 is mounted face up. The MMIC 10 and the alumina substrate 12 are attached to a Kovar carrier 14 as a mounting substrate by brazing with a conductive adhesive or gold tin.

  An active element such as a field effect transistor (hereinafter referred to as “FET”) and a passive element such as a coplanar line 20 or a resistor are provided on the upper surface of the MMIC 10. A circuit having desired characteristics is constituted by the plurality of active elements and passive elements. In FIG. 1, only the coplanar line 20 is illustrated for simplification.

  The coplanar line 20 includes a center conductor 24 and surface ground conductors 22 on both sides thereof. The ground conductors on both sides are connected by an air bridge 26 and are kept at an equal potential. This is to prevent the slot line mode that occurs when the potentials are not equal between the two ground conductors.

  On the other hand, a center conductor 42 and a ground pad 44 of the microstrip line 40 are formed on the upper surface of the alumina substrate 12. The ground pads 44 are provided on both sides of the center conductor 42 of the microstrip line 40 and are connected to the carrier 14 (corresponding to a floor ground conductor) by via holes.

  Input / output of a high frequency signal to the MMIC 10 is achieved by connecting the microstrip line 40 formed on the alumina substrate 12 and the coplanar line 20 of the MMIC 10. Specifically, the bonding conductors 50 connect the central conductors 24 and 42 of both the lines 20 and 40 and the surface ground conductor 22 and the ground pad 44.

Japanese Patent No. 3051430

  According to this mounting method, in the case of a chip of several mm square that is normally used, good connection can be made up to a frequency band of 10 and several GHz. Therefore, the characteristics of the MMIC 10 are not deteriorated. However, in the frequency band from quasi-millimeter wave to millimeter wave, the resonance of the microstrip antenna may occur in the coplanar MMIC 10 and the circuit operation becomes unstable.

  The resonance of this microstrip antenna will be described. In the coplanar MMIC 10, the surface ground conductor 22 is connected by an air bridge 26. Therefore, the parallel plate mode can propagate between the carrier 14 (floor ground conductor) and the surface ground conductor 22. At this time, a relationship such as the expression (1) is established between the length a of one side of the circuit board 28 and the resonance frequency fr.

  Here, c is the speed of light, and εr is the relative dielectric constant of the circuit board. In the case of a GaAs substrate, εr = 12.6. When this equation (1) is modified, the following equation (2) is obtained.

  As apparent from the equation (2), when the operating frequency f becomes fr, resonance of the microstrip antenna occurs. For example, in the case of a frequency of 30 GHz, a resonance phenomenon may occur when the length a of one side of the circuit board 28 exceeds about 1.4 mm. The resonance of the microstrip antenna may significantly deteriorate circuit characteristics.

  As a method of reducing this problem, there is a method of connecting the periphery of the surface ground conductor 22 and the carrier 14 with bonding wires 52 as shown in FIG. Thereby, the electric potential of the surface ground conductor 22 on the circuit board and the ground surface of the carrier 14 can be made uniform.

  In this case, however, the bonding wire 52 becomes a pseudo electric wall (dielectric wall). Therefore, a pseudo dielectric loading waveguide is formed in a region surrounded by the bottom surface of the circuit board 28, the surface ground conductor 22, and the bonding wire 52.

  In FIG. 2, when it is assumed that the bonding wire 52 is provided along the side surface of the circuit board 28 without any gap, the bottom surface of the circuit board 28, the surface ground conductor 22, and the bonding wire 52 are inserted into a rectangular dielectric material. It functions as a load waveguide (see FIG. 3). At this time, the shielding frequency fc of the lowest TE01 mode is given by equation (3). Further, FIG. 4 shows the relationship between the shielding frequency fc calculated from the equation (3) and the length a of one side.

  That is, when the use frequency band is higher than the cutoff frequency fc, the waveguide mode propagates in the circuit board 28. For this reason, when the length b of the other side of the circuit board 28 matches the half wavelength of the waveguide mode, a steep resonance phenomenon occurs. This is commonly known as a cavity resonator.

  As is clear from the equations (2) and (3), the resonance frequency fr of the microstrip antenna and the cutoff frequency fc of the waveguide are given by the same relational expression. That is, even if the surface ground conductor 22 is grounded by a large number of bonding wires 52 as shown in FIG. 2, the essential problem cannot be solved. Of course, in reality, a slight change in the resonance frequency fr occurs, so that in a narrow-band circuit, the resonance frequency fr can be shifted out of the band, and deterioration of in-band characteristics can be avoided. However, the problem of the resonance phenomenon is unavoidable in a circuit having a bandwidth including a frequency at which the dimension of the circuit board is about a half wavelength.

  In order to solve this problem, Patent Document 1 discloses a microwave integrated circuit in which a high-frequency Q dump circuit is inserted and connected between the surface ground conductor 22 and the carrier 14. Examples of the Q dump circuit include a circuit in which a resistor is connected from a surface ground conductor to a floor ground. In this microwave integrated circuit, the Q dump circuit operates as an attenuation circuit or an absorption circuit for a microstrip mode electric field. Since the microwave in the microstrip mode that has propagated through the gap between the surface ground conductor and the carrier is terminated by this Q dump circuit, reflection from the surface ground conductor is suppressed, and the standing wave is suppressed and the resonance phenomenon occurs. Can be suppressed.

  However, this method has a problem that a resistor must be newly connected after the microwave circuit is formed, and the manufacturing process of the entire circuit becomes complicated.

  Therefore, an object of the present invention is to provide a microwave integrated circuit that can obtain a good frequency characteristic in a wide band more easily.

  The microwave integrated circuit according to the present invention is a microwave integrated circuit in which at least one passive element including at least a coplanar line is provided on a circuit board, and is face-up mounted on a mounting board having a floor ground conductor. In the microwave integrated circuit, a radio wave absorber is provided on a part of the surface of the circuit board.

  In a preferred embodiment, a radio wave absorber is provided on at least a part of the bottom surface of the circuit board. In another preferred embodiment, the radio wave absorber is provided on the upper surface of the circuit board and below one or more passive elements. At this time, the passive element is preferably at least one of the surface ground conductor and the capacitance of the coplanar line.

  According to the present invention, the radio wave absorber attenuates or absorbs the electric field of the parallel plate mode and the waveguide mode generated inside the circuit board of the microwave integrated circuit. Therefore, the resonance phenomenon is suppressed, and a microwave integrated circuit having a good frequency characteristic even in a wide band can be obtained more simply.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 5 is a top view of the MMIC 10 according to the embodiment of the present invention, and FIG. 6 is a cross-sectional view taken along line AA in FIG. In addition, the same code | symbol is attached | subjected about the structure similar to the conventional MMIC shown in FIG. 1, and description is simplified.

  In the MMIC 10, an MIM (Metal-Insulator-Meta) capacitor 30, an FET 32, and the like are formed on the upper surface of the circuit board 28 in addition to the coplanar line 20.

  The coplanar line 20 includes a center conductor 24 and a surface ground conductor 22 that are electrically connected to an FET 32, an MIM capacitor 30, and the like that are circuit elements. The surface ground conductor 22 is provided on both sides of the center conductor 24. The MIM capacitor 30 has a metal-capacitance insulating film-metal structure, and is formed by a lower layer wiring 34, an insulating film 32, and an upper layer wiring 36 provided on the upper surface of the circuit board.

  As the circuit substrate, for example, a GaAs substrate can be used. At this time, it is not necessary to thin the circuit board. This is because the coplanar line does not depend on the thickness of the circuit board. Therefore, the circuit board can be used with a commercially available wafer thickness, for example, 600 μm. In the present embodiment, the resistor 60 is provided on the entire bottom surface of the circuit board 28. By providing the resistor 60, the resonance phenomenon in the high frequency band can be suppressed. The suppression of this resonance phenomenon will be described.

  As described above, the MMIC 10 can propagate in the parallel plate mode or the waveguide mode, and an electric field is generated inside the circuit board 28. At that time, when the use frequency f and the length a of one side satisfy a predetermined relationship, a resonance phenomenon occurs.

  For example, the case where the waveguide mode can propagate, that is, the case where the side circumference of the surface ground conductor 22 and the carrier (floor ground conductor) 14 are connected by the bonding wire 52 will be described with reference to FIGS. FIG. 7 is a conceptual diagram of the electric field distribution in the circuit board in the waveguide TE01 mode, and FIG. 8 is a diagram showing transmission characteristics.

  When the side periphery of the surface ground conductor 22 and the carrier are connected by a bonding wire, the bonding wire or carrier becomes an electric wall (pseudo-dielectric), and the waveguide mode can propagate (see FIG. 7). At this time, if the resistor 60 is not present, a generally high electric field distribution 66 as shown by a broken line in FIG. 7 is generated inside the waveguide mode circuit board 28. A resonance phenomenon occurs when the operating frequency f and the length of one side of the circuit board 28 satisfy a predetermined relationship. Therefore, when the resistor 60 is not provided, the transmission characteristic 68 has a sharp peak (resonance point) 68a due to a resonance phenomenon at a predetermined frequency, as shown in FIG. 7A. Such a peak 68a significantly deteriorates the frequency characteristic.

  On the other hand, when the resistor 60 is provided on the bottom surface of the circuit board 28, the resistor 60 exhibits the effect of attenuating or absorbing the electric field. That is, the resistor 60 functions as a radio wave absorber and absorbs an electric field generated in the circuit board 28. Therefore, the electric field distribution 64 is attenuated as a whole as shown by a solid line in FIG. In addition, the transmission characteristic 68 is excellent as shown in FIG.

  Of course, the electric field absorption or attenuation effect by the resistor 60 is not limited to the waveguide mode, but is also effective for the electric field by the parallel plate mode. Therefore, by providing a resistor as a radio wave absorber on the bottom surface of the circuit board 28, the resonance phenomenon can be suppressed in a wide band. And a favorable frequency characteristic can be acquired in a wide band.

  In this embodiment, a resistor is used. However, a resistor may be used as long as it absorbs radio waves. The resistor is preferably provided on the entire bottom surface, but may be provided only on a part of the bottom surface.

  Next, a method for manufacturing the MMIC 10 will be described. FIG. 9 shows a flowchart of semiconductor process steps. Here, a manufacturing method using an ion-implanted MESFET semiconductor process step is particularly shown. In the MESFET semiconductor process step, ions are implanted into a substrate (wafer) and a semiconductor is formed by subsequent heat treatment (activation annealing).

  In the ion implantation step, first, n-layer ion implantation is performed (S14). This is ion implantation for forming the active layer of the FET. Next, R layer ion implantation is performed (S16). This is an ion implantation that forms a high resistance (˜800 Ω / sheet) resistive element. Further, n + layer ion implantation is performed (S18). This is an ion implantation that forms an ohmic junction with the electrode and a resistor of low resistance (˜180Ω / sheet).

  In the present embodiment, thereafter, high-concentration ion implantation is further performed on the back surface of the substrate (wafer) (S20). The ion implantation to the back surface is performed on the entire surface of the substrate (wafer). Therefore, unlike ion implantation for element formation, no mask or resist coating process is required. Therefore, it can be carried out in a short time. By performing ion implantation on the entire back surface, a resistor functioning as a radio wave absorber can be formed.

  Thereafter, an annealing cap film that can withstand heat treatment is formed on the front and back of the substrate in the same manner as in a normal semiconductor process (S22). Then, after forming the gate, a heat treatment called activation annealing is performed to form a resistor on the entire back surface of the substrate simultaneously with the FET active layer, high resistance, and low resistance (S24 to S36).

  As is apparent from this description, the MMIC having the resistor 60 on the bottom surface of the circuit board 28 can be manufactured by adding a simple process called back surface ion implantation to a normal semiconductor process process. This backside ion implantation does not require a new mask or the like and can be performed very easily. That is, the resistor 60 can be easily formed. In other words, it is possible to obtain a microwave integrated circuit that can easily suppress the resonance phenomenon without forming a new mask or bonding after manufacturing the circuit.

  As described above, according to the present embodiment, it is possible to form a desired resistor without significantly changing the semiconductor process, and the resistor allows the metal surface on the upper surface of the microwave integrated circuit to be formed. It is possible to attenuate the electric field generated between the (surface ground conductor) and the floor ground conductor of the mounting substrate, and to suppress the resonance phenomenon caused by this. Therefore, the present invention realizes a microwave device having a good frequency characteristic in a wide band without damaging the ultra-small feature associated with high integration when a coplanar line type microwave integrated circuit is mounted on a package or the like. be able to.

  In addition, although embodiment described was MMIC, if it is a coplanar line type | mold microwave integrated circuit in which one or more passive elements were formed, it will not be restricted to MMIC. Further, the capacity is not limited to the MIM structure but may be a capacity having another structure. Furthermore, the resistor may be formed using a HEMT or HBT process in addition to using an ion-implanted MESFET process.

  Next, another embodiment will be described with reference to FIGS. FIG. 10 is a top view of a coplanar line type MMIC 10 according to another embodiment, and FIG. 11 is a cross-sectional view taken along line AA. This embodiment is particularly effective when the circuit board 28 is thinned.

  In the coplanar line type MMIC, unlike the microstrip line type MMIC, the characteristic impedance of the line is not affected by the thickness of the circuit board. Therefore, a substrate thinning process is not required, and generally, a commercially available wafer thickness of 600 μm is often used. However, the substrate thickness may be reduced in order to reduce the height gap with the connection substrate (such as the alumina substrate 12 in FIG. 1) or to reduce the height of the chip package as much as possible. In this case, the wafer back surface grinding is performed after the process out of the semiconductor process, and the substrate thickness is reduced from 200 to 300 μm. When such wafer back surface grinding is performed, the resistor formed on the wafer back surface disappears.

  Therefore, in such a case, the resistors 70 and 72 are provided on the upper surface of the circuit board 28. The first resistor 70 is formed on the upper surface of the circuit board 28 and below the surface ground conductor 22. At this time, the electromagnetic field of the coplanar line 20 wraps around the end face (edge) of the surface ground conductor 22 in contact with the distributed constant line. Therefore, it is not preferable to form the first resistor 70 up to the end face. It is desirable to form the first resistor under the surface ground conductor at least 10 μm inside from the end face.

  The second resistor 72 is formed on the upper surface of the circuit board 28 and below the lower layer wiring 34 that forms the MIM capacitor 30. Similarly, in the case of the second resistor 72, the second resistor 72 is formed at least 10 μm inside from the inner surface of the lower layer wiring 34 of the MIM capacitor 30 in order to prevent circuit loss due to electromagnetic field wraparound. Is desirable. That is, the resistors 70 and 72 are formed on the inner side so as not to straddle the surface ground conductor and the edge of the capacitor when viewed from above.

  As described above, if the resistors 70 and 72 are formed on the upper surface of the circuit board 28 and below the surface ground conductor 22 and the MIM capacitor 30, the same effect as that obtained when the resistors are provided on the circuit board bottom surface can be obtained. it can. That is, the resistors 70 and 72 that are radio wave absorbers absorb or attenuate the electric field generated by the parallel plate mode or the waveguide mode. As a result, the resonance phenomenon can be suppressed and good frequency characteristics can be obtained.

  In particular, in a coplanar line type MMIC of 10 GHz band or more in which the resonance phenomenon is a problem, the surface ground conductor 22 of the distributed constant line and the MIM capacitor 30 occupy most of the surface area of the circuit board 28. Therefore, when the resistors 70 and 72 are formed under the surface ground conductor 22 and the MIM capacitor 30, an electric field absorption effect similar to that when the resistor 60 (see FIG. 6) is formed on the back surface is exhibited and the resonance phenomenon is reduced. Effect is obtained.

  In the present embodiment, the resistor is formed in any lower part of the surface ground conductor 22 and the MIM capacitor 30, but the resistor may be formed only in any one of the lower parts. In addition to the resistors 70 and 72 on the upper surface, a resistor 60 may also be provided on the bottom surface of the circuit board 28.

  Next, a method of forming the resistors 70 and 72 on the upper surface of the circuit board 28 will be described with reference to FIG. FIG. 12 is a flowchart of the manufacturing process of the MMIC 10 in the present embodiment.

  The first resistor 70 and the second resistor 72 can be formed by using a resistance forming mask during the n + layer ion implantation (S18) in the normal process. That is, the n + layer ion implantation is an ion implantation that forms an ohmic junction with the electrode and a low-resistance (up to 180Ω / sheet) resistor. At the time of this ion implantation, a mask provided with an opening at a position where the resistors 70 and 72 are to be formed, that is, a mask provided with an opening at the position of the surface ground conductor and the MIM capacitor is used.

  Then, the first resistor 70 and the second resistor 72 can be formed by manufacturing all except the mask in the n + layer ion implantation by a conventional process. In other words, it is possible to obtain a microwave integrated circuit that can suppress the resonance phenomenon without greatly changing the conventional semiconductor process steps. As a result, a microwave integrated circuit having good frequency characteristics can be easily obtained.

It is a figure which shows an example at the time of carrying out the face-up mounting of the conventional MMIC. It is a figure which shows the other example at the time of carrying out face-up mounting of the conventional MMIC. It is AA sectional drawing of FIG. It is a figure which shows the relationship between the shielding frequency fc and the length a of one side. It is a top view of MMIC which is an embodiment of the invention. It is AA sectional drawing of FIG. It is a conceptual diagram of the electric field distribution in the circuit board in waveguide mode. It is a figure which shows a transmission characteristic. It is a flowchart of the manufacturing process of MMIC. It is a top view of MMIC which is other embodiments. It is AA sectional drawing of FIG. It is a flowchart of the manufacturing process of another MMIC.

Explanation of symbols

  20 Coplanar lines, 22 Surface ground conductor, 24 Center conductor, 26 Air bridge, 28 Circuit board, 30 MIM capacity, 60, 70, 72 Resistor.

Claims (8)

A microwave integrated circuit in which at least one passive element including at least a coplanar line is provided on a circuit board, wherein the microwave integrated circuit is mounted face-up on a mounting board having a floor ground conductor.
A microwave integrated circuit, wherein a radio wave absorber is provided on a part of a surface of a circuit board. The microwave integrated circuit according to claim 1, wherein
A microwave integrated circuit, wherein a radio wave absorber is provided on at least a part of a bottom surface of a circuit board. The microwave integrated circuit according to claim 2, wherein
A microwave integrated circuit, wherein a radio wave absorber is provided on the entire bottom surface of a circuit board. The microwave integrated circuit according to any one of claims 1 to 3,
A microwave integrated circuit, wherein the radio wave absorber is provided on an upper surface of a circuit board and below one or more passive elements. The microwave integrated circuit according to claim 4, wherein
A microwave integrated circuit, wherein the radio wave absorber is provided on an upper surface of a circuit board and below a surface ground conductor constituting a coplanar line. The microwave integrated circuit according to any one of claims 4 and 5,
A microwave integrated circuit, wherein the radio wave absorber is provided on an upper surface of a circuit board and below a capacitor provided on the circuit board. The microwave integrated circuit according to any one of claims 4 to 6,
The microwave integrated circuit, wherein the radio wave absorber has a position and a size that do not straddle the edge of the passive element when viewed from above. The microwave integrated circuit according to any one of claims 1 to 7,
A microwave integrated circuit, wherein the radio wave absorber is a resistor.

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