Abstract:
A semiconductor device, including a semiconductor substrate having an element region on a surface thereof, an electrical element being formed in the element region; an insulating layer formed on the semiconductor substrate and covering the electrical element; and an inductor formed on the insulating layer and overlapping with the element region. In an exemplary embodiment, the element region is free from being overlapped by a center axis of the inductor. In another exemplary embodiment, the inductor includes a wiring region, a center region, and a wiring pattern formed in the wiring region and winding spirally to surround the center region, the element region being free from being overlapped by the center region. In a further exemplary embodiment, the inductor includes a voltage controlled oscillator, and the electrical element is electrically connected to the inductor and includes at least one of a varactor and a MOSFET.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 10/863,389 filed Jun. 9, 2004, now U.S. Pat. No. 7,088,195. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a voltage controlled oscillator which utilizes the resonance of parallel-LC tank circuits and more particularly to a voltage controlled oscillator that is preferably used as a local oscillator in a phase locked loop circuit. 
     2. Description of the Related Art 
     Conventionally, ring-voltage controlled oscillators (R-VCO) have been used as a local oscillator (LO) in a phase locked loop (PLL) circuit that is employed for frequency multiplication and phase synchronization. The R-VCO has an odd number of CMOS (Complementary Metal Oxide Semiconductor) inverters that are annually coupled to each other. This configuration provides an advantage that the R-VCO can be incorporated into a MOS integrated circuit. However, the R-VCO has disadvantages of high jitter and high phase noise. 
     On the other hand, the voltage controlled oscillator utilizing the resonance of a parallel-LC tank circuit (LC-VCO) has been recently used as a local oscillator. The LC-VCO includes inductors and variable capacitors which are coupled in parallel to each other to form a parallel-LC tank circuit. The resonance of the parallel-LC tank circuit causes an AC signal to be delivered at a resonant frequency. A resonant frequency is a frequency at which the reactance of a parallel-LC tank circuit is infinite, and the resonance refers to a phenomenon in which a current flows alternately through the inductors and variable capacitors in a parallel-LC tank circuit. The variable capacitor is a varactor element or the like, the capacitance of which varies with a control voltage applied thereto. The capacitance of the variable capacitor is adjusted to thereby control the frequency of the oscillating AC signal. For example, such an LC-VCO is disclosed in the literature, Ali Hajimira and Thomas H. Lee, “Design Issues in CMOS Differential LC Oscillators”, IEEE JOURNAL OF SOLID-STATE CIRCUIT, Vol. 34, No. 5 (MAY 1999). 
     The LC-VCO has the following advantages over the R-VCO. First, the LC-VCO has a lower noise level as compared with the R-VCO. This feature derives from the fact that the LC-VCO includes a less number of transistors which may cause noise because it is primarily based on the resonance of a parallel-LC tank circuit. This feature thus allows the LC-VCO to be preferably incorporated into high-speed optical telecommunications devices, cellular phones, wireless LANs or the like. 
     Secondly, the LC-VCO can easily provide higher oscillation frequencies than the R-VCO. This is because the LC-VCO is primarily based on the resonance of a parallel-LC tank circuit, whereas the R-VCO is made up of transistors and utilizes their logic gate delays. 
     Thirdly, the LC-VCO has a smaller range of variations in oscillation frequency for a control voltage as compared with the R-VCO. This feature allows for a lower tuning sensitivity and less variations in oscillation frequency caused by variations in control voltage, resulting in low noise. 
       FIG. 1  is a circuit diagram showing a conventional LC-VCO, and  FIG. 2  is a plan view showing the conventional LC-VCO. As shown in  FIG. 1 , the conventional LC-VCO  101  is connected to a supply potential line VCC and a ground potential line GND. An inductor section  2 , a variable capacitor section  3 , a negative resistance section  4 , and a current regulation section  5  are connected with one another in that order from the supply potential line VCC towards the ground potential line GND in the LC-VCO  101 . 
     The inductor section  2  is provided with two spiral inductors  6   a  and  6   b . Ends of the spiral inductor  6   a  and  6   b  are connected to the supply potential line VCC, with the other ends being connected to output terminals  7   a  and  7   b , respectively. 
     The variable capacitor section  3  is provided with two varactor elements  8   a  and  8   b . One end of the varactor element  8   a , e.g., a well electrode is connected to the output terminal  7   a , while one end of the varactor element  8   b , e.g., a well electrode is connected to the output terminal  7   b . The varactor elements  8   a  and  8   b  are connected to each other at the other ends thereof, e.g., the gate electrodes, to which a control voltage is applied. 
     The negative resistance section  4  is provided with N-channel transistors  9   a  and  9   b . The N-channel transistor  9   a  has the drain connected to the output terminal  7   a  and the gate connected to the output terminal  7   b . On the other hand, the N-channel transistor  9   b  has the drain connected to the output terminal  7   b  and the gate connected to the output terminal  7   a.    
     The current regulation section  5  is provided with an N-channel transistor  10 , with the drain of the N-channel transistor  10  connected to the sources of the N-channel transistor  9   a  and the N-channel transistor  9   b . Additionally, the N-channel transistor  10  has the source connected to the ground potential line GND and the gate to which a bias voltage is applied. 
     Now, a layout of the conventional LC-VCO  101  will be described below. As shown in  FIG. 2 , the LC-VCO  101  is provided in a semiconductor integrated circuit device in which multilayer interconnection layer  12  are deposited on a semiconductor substrate (not shown). The spiral inductors  6   a  and  6   b  are deposited in the topmost layers of the multilayer interconnection layer  12 , respectively. A varactor element formed region in which the varactor elements  8   a  and  8   b  are to be formed and a transistor formed region in which the N-channel transistors  9   a ,  9   b , and  10  are to be formed are located out of the underlying region of the spiral inductors  6   a  and  6   b  on the surface of the semiconductor substrate. On the other hand, no other elements nor conductors such as wirings are formed in the underlying region of the spiral inductors  6   a  and  6   b.    
     In this manner, the conventional LC-VCO layout is designed such that the spiral inductors are deposited in the topmost layers of the multilayer interconnection layer, with no elements such as varactor elements or transistors nor conductors being deposited in the underlying region of the spiral inductors. This design is intended to prevent magnetic fields created in the spiral inductors from having an adverse effect on the operation of active elements or from inducing a current flowing through conductors to cause power loss. Such an LC-VCO having a layout of this type is disclosed in the aforementioned literature and Japanese Patent Laid-Open Publication No. 2002-9299. 
     However, the aforementioned prior art technique has the following problems. That is, the conventional LC-VCO has a larger layout area than the R-VCO. For example, the R-VCO may have a layout of a rectangular area with a 75 μm vertical side and a 150 μm horizontal side, whereas the conventional LC-VCO  101  has a layout of a rectangular area with a 250 μm vertical side and a 300 μm horizontal side, being about seven times the R-VCO in layout area. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a voltage controlled oscillator which has good characteristics with a reduced layout area. 
     The present invention provides a voltage controlled oscillator which is incorporated into a semiconductor integrated circuit device that includes a substrate and multilayer interconnection layer deposited on the substrate. The voltage controlled oscillator includes output terminals, spiral inductors deposited in the multilayer interconnection layer and connected to the output terminals, variable capacitors, and a negative resistance section. In this configuration, the variable capacitors are formed in a region which includes the underlying region of the spiral inductors but does not include the center axes of the spiral inductors, and connected in parallel to the spiral inductors to form a resonant circuit in conjunction therewith. The negative resistance section is formed in the region. And, the negative resistance section is connected between the resonant circuit and a power supply to supply currents to the resonant circuit in sync with a resonant signal produced by the resonant circuit. 
     According to the present invention, the resonant circuit made up of the spiral inductors and the variable capacitors is supplied with a current from the negative resistance section connected to the power supply, thereby allowing an AC signal to be delivered from the output terminals at a frequency equal to the resonant frequency of the resonant circuit. The variable capacitors and the negative resistance section are formed in a region including the underlying region of the spiral inductors, thereby providing a reduced layout area for the voltage controlled oscillator. Additionally, the variable capacitors and the negative resistance section are formed in the region which does not include the center axes of the spiral inductors, thereby preventing magnetic fields created by the spiral inductors from having effects on the operation of the variable capacitors and the negative resistance section. It is also possible to prevent the efficiency of the spiral inductors from being reduced due to a current being induced. This allows for providing a voltage controlled oscillator which has a reduced layout area with good characteristics. 
     On the other hand, the spiral inductors may be a full-symmetric inductor. This makes it possible to provide a further reduced layout area for the voltage controlled oscillator. 
     According to the present invention, the variable capacitors and the negative resistance section are formed in the region which includes the underlying region of the spiral inductors but does not include their center axes, thereby providing a reduced layout area without degradation in the characteristics of the voltage controlled oscillator. This allows a semiconductor integrated circuit device incorporating this voltage controlled oscillator to be reduced in size and more densely populated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing a conventional LC-VCO; 
         FIG. 2  is a plan view showing the conventional LC-VCO; 
         FIG. 3  is a plan view showing a voltage controlled oscillator according to a first embodiment of the present invention; 
         FIG. 4  is a sectional view taken along the line A-A′ of  FIG. 3 ; and 
         FIG. 5  is a plan view showing a voltage controlled oscillator according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, the present invention will be described below more specifically with reference to the accompanying drawings in accordance with the embodiments. First, a first embodiment of the present invention will be described below.  FIG. 3  is a plan view showing a voltage controlled oscillator (LC-VCO) according to this embodiment,  FIG. 4  is a sectional view taken along the line A-A′ of  FIG. 3 . 
     As shown in  FIGS. 3 and 4 , the LC-VCO  1  according to this embodiment has a layout different from that of the aforementioned conventional LC-VCO  101  (see  FIG. 2 ). The circuit configuration of the LC-VCO  1  of this embodiment is identical to that of the conventional LC-VCO  101  shown in  FIG. 1 . The LC-VCO  1  according to this embodiment is a local oscillator for use with a phase locked loop circuit. 
     As shown in  FIGS. 3 and 4 , the LC-VCO  1  is incorporated into a semiconductor integrated circuit device in which the multilayer interconnection layer  12  is formed on a semiconductor substrate  11 . The spiral inductors  6   a  and  6   b  are deposited in the topmost layers of the multilayer interconnection layer  12 , respectively. For example, each of the spiral inductors  6   a  and  6   b  is an inductor having five turns of a conductor trace formed in rectangular shapes. 
     The varactor element formed region in which the varactor element  8   a  is deposited is positioned in the underlying region of the spiral inductor  6   a  on the surface of the semiconductor substrate, the region excluding the center axis O of the spiral inductor  6   a . As described above, the varactor element  8   a  is connected to the spiral inductor  6   a . Additionally, the transistor formed region in which the N-channel transistor  9   a  is formed is positioned adjacent to the varactor element formed region. The transistor formed region is also positioned in the underlying region of the spiral inductor  6   a , the region excluding the center axis O of the spiral inductor  6   a.    
     The center of the spiral inductor  6   a  refers to the point of intersection of two diagonal lines that each connects two diagonal corners of the rectangles formed by the turns of the conductor trace. In the case of a spiral inductor of turns of a conductor trace formed in annular shapes, the center of the annular rings is the center of the spiral inductor. 
     Likewise, the varactor element formed region in which the varactor element  8   b  is formed and the transistor formed region in which the N-channel transistor  9   b  is formed are positioned in a region which includes the underlying region of the spiral inductor  6   b  on the surface of the semiconductor substrate but does not include the center axis of the spiral inductor  6   b.    
     Furthermore, the transistor formed region in which the N-channel transistor  10  is formed is also positioned in a region which includes the underlying region of the spiral inductors  6   a  and  6   b  but does not include the center axes of the spiral inductors  6   a  and  6   b . Still furthermore, the wirings for connecting between the aforementioned elements and those for connecting between other elements in the semiconductor integrated circuit device are also positioned in a region which does not include the center axes O of the spiral inductors  6   a  and  6   b . Accordingly, in this semiconductor integrated circuit device, the center axes O of the spiral inductors  6   a  and  6   b  are designed to go through only interlayer insulating layers of the multilayer interconnection layer and the semiconductor substrate  11 . 
     Now, referring to  FIGS. 3 and 4 , the operation of the LC-VCO  1  will be described below. First, a bias voltage is applied to the gate of the N-channel transistor  10 . This causes the N-channel transistor  10  to reach a level of saturation of its I-V property, in the case of which a drain current that is defined by a gate voltage and does not depend on a drain voltage flows between the source and the drain. As a result, with a bias voltage being fixed to a certain value, a constant current is allowed to flow through the LC-VCO  1  irrespective of the potential difference between the source and drain of the N-channel transistor  10 . 
     Under this condition, suppose that the LC-VCO  1  is connected to the supply potential line VCC and the ground potential line GND. In this case, with some electric stimulus being applied to the LC resonant circuit having the inductor section  2  and the variable capacitor section  3 , the LC resonant circuit delivers a complementary resonant signal from the output terminals  7   a  and  7   b.    
     However, using only the LC resonant circuit would cause loss due to parasitic resistance, resulting in the oscillation being terminated sooner or later. To address this situation, a positive supply potential is applied to the supply potential line VCC and a ground potential is applied to the ground potential line GND to supply a current to the LC-VCO  1 , while the negative resistance section  4  is provided. This allows the LC resonant circuit to deliver a resonant signal permanently. 
     That is, for example, with the output terminal  7   a  being at a low level and the output terminal  7   b  at a high level, the N-channel transistor  9   a  is turned on. As a result, both the supply potential and the ground potential are applied to the output terminal  7   a , resulting in the output terminal  7   a  being at a potential intermediate therebetween. On the other hand, since the N-channel transistor  9   b  is turned off, the supply potential is applied to the output terminal  7   b . Likewise, with the output terminal  7   a  being at a high level and the output terminal  7   b  being at a low level, the supply potential is applied to the output terminal  7   a  while both the supply potential and the ground potential are applied to the output terminal  7   b , resulting in the output terminal  7   b  being at a potential intermediate therebetween. This allows the oscillation delivered from the output terminals  7   a  and  7   b  to be maintained without attenuation. 
     Then, changes of the control voltage to be applied to the gate electrodes of the varactor elements  8   a  and  8   b  cause the capacitance of the varactor elements  8   a  and  8   b  to vary. As a result, the LC resonant circuit changes its resonant frequency in response to the control voltage, thereby allowing the frequency of the AC signal delivered by the LC-VCO  1  to vary. 
     At this time, as shown in  FIG. 4 , the spiral inductors  6   a  and  6   b  produce a magnetic field. However, magnetic force lines  14  concentrate in the vicinity of the center axes O of the spiral inductors  6   a  and  6   b . The LC-VCO  1  according to this embodiment has the varactor elements  8   a  and  8   b  and the N-channel transistors  9   a ,  9   b , and  10  formed in a region that does not include the center axes O. This prevents the magnetic field produced by the spiral inductors  6   a  and  6   b  from having substantial adverse effects on the operation of the varactor elements  8   a  and  8   b , and the N-channel transistors  9   a ,  9   b , and  10 . Furthermore, no conductors such as wirings are also located in the region that intersects the center axes O, thereby causing almost no loss due to induced currents. 
     As described above, this embodiment allows the magnetic fields produced by the spiral inductors to have no adverse effects on the varactor elements and the transistors. Furthermore, in this embodiment, since the varactor elements and transistors are formed in the underlying region of the spiral inductors, the LC-VCO  1  is approximately equal in layout area to the spiral inductors and thus provided with a reduced layout area as compared with the conventional LC-VCO. For example, the LC-VCO  1  according to this embodiment has a rectangular layout area of a 150 μm vertical side and a 300 μm horizontal side, which is thus 0.6 times the layout area of the conventional LC-VCO  101  (see  FIG. 2 ). As a result, the entire semiconductor integrated circuit device incorporating the LC-VCO  1  according to this embodiment is reduced in size and more densely populated. 
     In this embodiment, only the varactor elements  8   a  and  8   b  and the N-channel transistors  9   a ,  9   b , and  10  are shown as the elements other than the spiral inductors in the LC-VCO  1 . However, the LC-VCO according to the present invention may include other elements than those mentioned above. For example, invariable capacitors having a fixed capacitance can also be connected in parallel to the varactor elements  8   a  and  8   b  between the output terminals  7   a  and  7   b . Furthermore, for example, resistors may also be connected between the supply potential line VCC and the output terminals  7   a  and  7   b . In this case, the elements other than the spiral inductors are located in a region which includes the underlying region of the spiral inductors but does not include the center axes of the spiral inductors. 
     Furthermore, if the layout allows, the elements other than the spiral inductors may be preferably formed in the underlying region of the spiral inductors which does not include the underlying regions of the inner areas of the innermost conductor trace turns of the spiral inductors. 
     Now, a second embodiment of the present invention will be described below.  FIG. 5  is a plan view showing a voltage controlled oscillator according to this embodiment. As shown in  FIG. 5 , an LC-VCO  21  according to this embodiment has a layout different from that of the aforementioned LC-VCO  1  according to the first embodiment (see  FIG. 3 ). The circuit configuration of the LC-VCO  21  according to this embodiment is identical to that of the conventional LC-VCO  101  shown in  FIG. 1 . 
     As shown in  FIG. 5 , the LC-VCO  21  is provided with one full-symmetric inductor  16  in the topmost layer of a multilayer conductor trace stack  12  instead of the two spiral inductors  6   a  and  6   b  provided in the LC-VCO  1  according to the first embodiment. The full-symmetric inductor  16  has two spiral inductors, which are nested alternately and interconnected at the innermost turn, and is formed with a total of five turns of wirings in rectangular shapes, for example. 
     A varactor element formed region in which the varactor elements  8   a  and  8   b  are formed is located in the underlying region of the full-symmetric inductor  16  on the surface of the semiconductor substrate, the region excluding the center axis O of the full-symmetric inductor  16 . A transistor formed region in which the N-channel transistors  9   a ,  9   b , and  10  are formed is also positioned adjacent to the varactor element formed region. This transistor formed region is also positioned in the underlying region which underlies the full-symmetric inductor  16  and does not include the center axis O thereof. The configuration and operation of the LC-VCO  21  according to this embodiment other than those mentioned above are identical to those of the LC-VCO  1  according to the aforementioned first embodiment. 
     This embodiment allows for providing a less layout area for the LC-VCO as compared with the aforementioned first embodiment. For example, the LC-VCO  21  according to this embodiment has a rectangular layout of a 150 μm vertical side and a 150 μm horizontal side. The effects other than those described above according to this embodiment are identical to those of the aforementioned first embodiment.