Abstract:
Provided is an oscillator including: a MEMS resonator for mechanically vibrating; an output oscillator circuit for oscillating at a resonance frequency of the MEMS resonator to output an oscillation signal; and a MEMS capacitor for changing a capacitance thereof caused by a change in a distance between an anode electrode and a cathode beam according to an environmental temperature.

Description:
REFERENCE TO THE RELATED APPLICATIONS 
   This application claims priority under 35 U.S.C. §119 to Japanese Patent application No. JP2007-045008 filed Feb. 26, 2007, the entire content of which is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an oscillator using a MEMS technology. 
   2. Description of the Related Art 
   Under the increasing demand for reduction in size and increase in accuracy of electronic devices such as personal computers and wireless mobile devices represented by cellular phones, a small and stable high frequency signal source is inevitable in these electronic devices. A crystal resonator is a representative electronic part that satisfies the demand. It is known that the crystal resonator has an extremely high resonance sharpness (that is, Q value) which is an index of the oscillation device quality, and which exceeds 10,000 owing to the excellent crystal stability. This is a reason that the crystal resonator is widely used as a stable high frequency signal source of the wireless mobile devices, the personal computers, or the like. However, it has been also proved that the crystal resonator cannot sufficiently satisfy a recent demand for further downsizing of the electronic devices. 
   Under the above circumstances, in recent years, there has been reported a MEMS oscillator using, instead of a crystal resonator, a downsized MEMS resonator that is formed with technology for micro electro mechanical system (MEMS) using a silicon substrate (refer to US 2006/0033594 A1). A MEMS oscillator can be made smaller in size than the crystal oscillator, and can also operate in high frequency. Accordingly, MEMS resonators are expected to spread particularly into compact devices such as a cellular phone. Also, it is possible to integrate a MEMS resonator and a peripheral circuit into a single chip since the MEMS resonator can be manufactured from a silicon substrate. 
     FIG. 9  is a principle diagram showing a MEMS resonator. As shown in the figure, the MEMS resonator is disposed opposite to a substrate  10  at a distance of a gap  14  in a floating state to form an oscillation beam  11 . Both ends of the oscillation beam  11  are fixed to the substrate  10  through anchors  16 . A drive electrode  12  and a sense electrode  13  are disposed opposite to each other with capacitive gaps  15  with respect to the oscillation beam  11 , respectively. Since the MEMS resonator is driven with an electrostatic force generated by applied voltage, in which application of a DC bias voltage in addition to an AC signal can draw the same electric characteristics (for example, a Q value) as that of a crystal resonator, it is possible to use a MEMS resonator as a resonator of a oscillator with the same configuration as that of the crystal oscillator. The oscillation frequency f 0  of the MEMS resonator can be represented by using an effective mass Meff and an effective hardness Keff as follows.
   f 0=1/(2 π )√(Keff/Meff) 
   Since material of the oscillation beam  11  expands and contracts according to the environmental temperature, the effective mass Meff changes. And since the Young&#39;s modulus of the material for the oscillation beam  11  changes according to the environmental temperature, the effective hardness Keff also changes. 
   In the MEMS oscillator according to the above conventional art, however, there arises such a problem that the temperature dependency of the oscillation frequency is worse than that of a crystal resonator. This is because the effective mass and the effective hardness change according to the temperature. To correct the temperature dependency a temperature sensor is required though, there arise such problems that the size of the oscillator becomes large, and the cost also increases when a temperature sensor is incorporated separately into the oscillator. Disposition of the temperature sensor at a position apart from the resonator arises another problem that a error in the temperature detection becomes so large that deteriorates accuracy of correction. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above-mentioned circumstances, and it is therefore an object of the present invention to provide an oscillator using a MEMS resonator, which automatically corrects the temperature dependency of the frequency by using the temperature dependency of the MEMS capacitor. 
   An oscillator according to the present invention includes: an output oscillator circuit for oscillating at a resonance frequency of an output resonator to transmit an oscillation signal; and a capacitor changing a capacitance caused by a change in a distance between an anode electrode and a cathode beam according to an environmental temperature. 
   Further, an output resonator according to the present invention is disposed opposite to a support layer which is a semiconductor substrate, and at least a part of the output resonator is fixed to an insulating layer on the support layer. 
   Further, the capacitor according to the present invention includes: a support beam and a cathode beam disposed opposite to a support layer which is the semiconductor substrate with a gap; an anchor for fixing the support beam onto the support layer which is the semiconductor substrate; and an anode electrode fixed onto the support layer which is the semiconductor substrate, in which both ends of the cathode beam are connected to one end of the support beam, respectively, another end of the support beam is fixed to an insulating film on the support layer which is the semiconductor substrate through the anchor, and the anode electrode and the cathode beam are disposed opposite to each other in parallel, and the support beam is connected orthogonally to the cathode beam. 
   Also, according to the present invention, the capacitor includes support beams that are disposed opposite to the support layer which is the semiconductor substrate with a gap, an anode electrode, a cathode electrode, and anchors that fix the support beams on the support layer which is the semiconductor substrate, in which one end of one of the support beams is connected to the cathode electrode, another end of the one support beam is fixed onto an insulating layer on the support layer that is the semiconductor substrate through one anchor, one end of another support beam is connected to the anode electrode, another end of the another support beam is connected to the insulating film on the support layer that is the semiconductor substrate through another anchor, the anode electrode and the cathode electrode are disposed opposite to each other in parallel, the support beam that is connected with the anode electrode and the support beam that is connected with the cathode electrode are disposed in parallel to each other, and the respective anchors thereof are disposed at positions completely opposite to each other. 
   Further, according to the present invention, the oscillator is designed in such a manner that the output resonator, the output oscillator circuit, and the capacitor are formed on the same substrate. 
   According to the present invention, the capacitor that is connected to the output oscillator circuit is designed so as to change the capacitance with respect to the temperature. As a result, it is possible to provide an oscillator which shows little change in the frequency with respect to the temperature and which automatically corrects the change in oscillation frequency of the oscillator caused by the environmental temperature. Also, since the capacitor is located adjacent to the MEMS resonator and manufactured in the same process as that of the MEMS resonator, it is possible to correct the oscillation frequency of the oscillator by precisely detecting the temperature of the MEMS resonator. Further, providing a quadratic characteristic to a change in capacitance of the capacitor with respect to the temperature, it is possible to provide the oscillator that automatically corrects the quadratic component of the frequency temperature change of the oscillator and that shows little change in the frequency with respect to the temperature. Still further, since the MEMS resonator, the capacitor, and the output oscillator circuit can be fabricated on the same substrate through the same process, it is possible to provide the oscillator using the MEMS resonator which is small in size and inexpensive. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing an oscillator according to an embodiment of the present invention; 
       FIG. 2  is a perspective view showing a configuration of a MEMS resonator according to the embodiment of the present invention; 
       FIG. 3  is a perspective view showing a configuration of a MEMS capacitor according to the embodiment of the present invention; 
       FIGS. 4A and 4B  are graphs showing temperature dependency of the capacitance of the MEMS capacitor according to the embodiment of the present invention; 
       FIG. 5  is a perspective view showing a configuration of a MEMS capacitor according to another embodiment of the present invention; 
       FIG. 6  is a graph showing temperature dependency of the capacitance of the MEMS capacitor according to the another embodiment of the present invention; 
       FIG. 7  is a circuit diagram showing an output oscillator circuit according to an embodiment of the present invention; 
       FIG. 8  is a graph showing temperature dependency of the resonance frequency of the MEMS resonator according to the embodiment of the present invention; and 
       FIG. 9  is a perspective view showing a principle configuration of a MEMS resonator. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First Embodiment 
   Hereinafter, a description will be given of an embodiment of the present invention with reference to the accompanying drawings. 
     FIG. 1  is a block diagram showing an oscillator according to an embodiment of the present invention. As shown in the figure, the oscillator includes a MEMS resonator  1 , an output oscillator circuit  2 , and MEMS capacitors  3  and  4 . Also, as shown in  FIG. 2 , the MEMS resonator  1  includes an oscillation beam  11 , a drive electrode  12 , and a sense electrode  13 . The sense electrode  13  of the MEMS resonator  1  is connected to the MEMS capacitor  3  and an input terminal of the output oscillator circuit  2 . The drive electrode  12  of the MEMS resonator  1  is connected to the MEMS capacitor  4 , an output terminal of the output oscillator circuit  2 , and an output terminal  5  of the oscillator. 
   The block diagram of the oscillator shown in  FIG. 1  is constructed based on a Pierce-type oscillator circuit, but is not necessarily limited to this configuration. The feature of the present invention resides in that the MEMS capacitor is used in the output oscillator circuit of the MEMS resonator, which can be applied to other amplifier circuits or oscillator circuits. 
   As shown in  FIG. 2 , the MEMS resonator  1  of  FIG. 1  includes an anchor  16 , the drive electrode  12 , and the sense electrode  13  disposed on a substrate  10  through an insulating layer. No insulating layer exists between the oscillation beam  11  and the substrate  10 , and a gap  14  is formed therebetween. With the above configuration, the oscillation beam  11  can mechanically vibrate. In addition, both ends of the oscillation beam  11  in the longitudinal direction are fixed to the insulating layer on the substrate  10  by means of the anchors  16 . The drive electrode  12  and the sense electrode  13  are  3  and  4  are formed by using a silicon insulator (SOI) substrate by the MEMS technology. The SOI substrate has an active layer, the insulating layer, and the substrate  10 . Among those layers, the active layer (silicon) is used in order to form the oscillation beam  11  of the MEMS oscillator, the support beams  24 ,  25 ,  33 , and  34  of the MEMS capacitor, the cathode electrodes  21  and  31 , and the anode electrode  32 . Those movable portions are formed so as to be disposed opposite to the substrate  10  and adjacent to each other by etching. In order that those movable portions function as electrodes, the active layer is made low in the resistance due to impurity doping in advance. 
   The insulating layer other than the insulating layer under the anchors  16 ,  26 ,  27 ,  35 , and  36  is also removed by etching after the active layer has been processed, to thereby form gaps between the above movable portions and the substrate. The insulating layer under the anchors  16 ,  26 ,  27 ,  35 , and  36  which remains without being etched electrically insulates the substrate  10  from the movable portions, and is also fixed to the substrate  10 . 
   Further, the respective electrodes are electrically connected to the oscillator circuit by vapor deposition of aluminum (Al), or the like. 
   The MEMS resonator  1  and the MEMS capacitors  3  and  4  are formed to be adjacent to each other, so it is possible to detect substantially the same temperature as that of the oscillation beam  11 . Also, because the MEMS resonator  1  and the MEMS capacitor can be formed integrally through the same process, it is possible to remarkably downsize the oscillator and reduce the costs. 
   Next, the operation of the oscillator according to this embodiment will be described in detail. 
   First, a beam bias voltage is applied to the oscillation beam  11  of the MEMS resonator  1  through the anchor  16 . The output oscillator circuit  2  oscillates at the resonance frequency that is determined according to the MEMS resonator  1  and the MEMS capacitors  3  and  4 , and outputs an output oscillation signal. The output signal is transmitted to another circuit through the output terminal  5 , and also transmitted to the drive electrode  12  of the MEMS resonator  1 . An AC voltage as well as a DC bias voltage is applied to the drive electrode  12 , thereby making it possible to provide the same characteristics (for example, a Q value) as that of the crystal resonator. 
   Now, a description will be given of an example of the output oscillator circuit according to the embodiment of the present invention with reference to  FIG. 7 . 
   Both ends of the MEMS resonator  1  are connected to one ends of current suppression resistors  63  and  64 , respectively. Another ends of the current suppression resistors  63  and  64  are connected to both ends of the DC bias voltage  66 , respectively. Also, both ends of the MEMS resonator  1  are connected with one ends of DC component cut capacitors  61  and  62 , respectively. Further, the oscillation beam  11  of the MEMS resonator  1  is connected to a positive pole of a beam bias voltage  68 , and a negative pole thereof is grounded. Another end of the DC component cut capacitor  61  is connected with one end of the MEMS capacitor  3  that is a load capacity as well as an input terminal of an inverter (amplifier)  67  and one end of a load resistor  65 . Also, another end of the DC component cut capacitor  62  is connected with one end of the MEMS capacitor  4  that is the load capacity as well as an output terminal of the inverter  67 , another end of the load resistor  65 , and the output terminal  5  of the output oscillator circuit  2 . Another ends of the MEMS capacitors  3  and  4  that are the load capacitors are grounded, respectively. 
   Next, the operation of the oscillator circuit of this embodiment will be described. 
   As described above, the DC bias voltage  66  and the beam bias voltage  68  are applied to both ends of the MEMS resonator  1  to further increase an electrostatic attraction between the oscillation beam  11  and the drive electrode  12 , and between the oscillation beam  11  and the sense electrode  13 , and also adjust the level of the signal that is input to the input terminal of the inverter (amplifier)  67 . In this case, because the DC bias voltage  66  is applied, the current suppression resistors  63  and  64  are inserted for the purpose of preventing the inflow and outflow of a large current with respect to the MEMS resonator  1 . Also, the DC component cut capacitors  61  and  62  are inserted so that no DC voltage is applied to an input/output terminal of the inverter (amplifier)  67 . 
   The oscillation beam  11  then induces the electrostatic attraction due to the AC voltage that has been added to the drive electrode  12  and the DC bias voltage that has been added to the oscillation beam  11 , and oscillates. Electric charges are induced in the sense electrode  13  due to the oscillation of the oscillation beam  11 , and an increase or decrease in the induced electric charges is transmitted to the input terminal of the inverter (amplifier)  67  as a signal. The signal that has been transmitted to the input terminal is amplified by the inverter (amplifier)  67 , and again added to the drive electrode  12 . 
   Repeating the above operation, the output oscillator circuit  2  finally oscillates at the resonance frequency of the MEMS resonator  1  and outputs an oscillation signal. 
     FIG. 8  shows a temperature dependency of a resonance frequency of the MEMS resonator. As shown in the figure, the resonance frequency of the MEMS resonator is lower when the temperature is higher. Also, a relationship between the temperature and the frequency is not linear but is quadratically lowered. For that reason, a manner that a temperature sensor is disposed to measure the temperature of the MEMS resonator and correct the frequency by the circuit as in the conventional art suffers from severe difficulties. 
   The MEMS capacitors  3  and  4  that are connected to the input terminal and the output terminal of the output oscillator circuit  2  are disposed to provide capacitive gaps  20  between the anode electrodes  22  and  23  and the cathode electrode  21 . The MEMS capacitors  3  and  4  have therefore the capacitance determined by the following expression.
 
 C=εS/d  ( S  is an anode electrode area, and  d  is a capacitive gap)
 
When the environmental temperature changes, the material is expanded or contracted based on the following arithmetic expression.
 
 L′=L   0 (1 +αΔT+βΔT   2 )
 
   L 0  is a length of the material at a reference temperature, L′ is a length of the material at the present environmental temperature, ΔT is a temperature difference between the reference temperature and the present environmental temperature, and α and β are linear expansion temperature coefficients of the material. 
   Thus, when the environmental temperature is, for example, higher than the reference temperature, the lengths of the support beams  24  and  25  expand based on the above arithmetic expression. However, because one ends of the support beams  24  and  25  are fixed to the substrate  10  by means of the anchors  26  and  27 , the support beams  24  and  25  expand to a direction connected by the cathode electrode  21  (to a direction opposite to the ends that are fixed by the anchors  26  and  27 ). And because the anode electrodes  22  and  23  are fixed to the substrate  10 , the capacitive gap  29  between the anode electrode  23  and the cathode electrode  21  reduces, and the capacitive gap  28  between the anode electrode  22  and the cathode electrode  21  expands. Accordingly, when the anode electrode  23  and the cathode electrode  21  are used to constitute the MEMS capacitors  3  and  4 , the capacitance values of the MEMS capacitors  3  and  4  increase when the environmental temperature rises ( FIG.4A ). On the other hand, when the anode electrode  22  and the cathode electrode  21  are used to constitute the MEMS capacitors  3  and  4 , the capacitance values of the MEMS capacitors  3  and  4  decrease when the environmental temperature rises ( FIG. 4B ). When the MEMS capacitor is used in the circuit block of  FIG. 1 , the temperature dependency of the frequency of the MEMS resonator  1  shows that temperature rising causes frequency lowering. As a result, the anode electrode  22  and the cathode electrode  21  are used to constitute the MEMS capacitors  3  and  4  so that the capacitance of the capacitor becomes lower when the temperature rises. The combination of the MEMS resonator  1  and the MEMS capacitor is not necessarily limited to the use of the anode electrode  22  and the cathode electrode  21 . Selection of anode electrode to be connected depends on the circuit configuration of the output oscillator circuit  2 . In the oscillator circuit of the Pierce type described in the present invention, the anode electrode  22  and the cathode electrode  21  are used as described above. 
   When the environmental temperature becomes higher, the capacitance values of the MEMS capacitors  3  and  4  tend to be low and the frequency of the output signal of the output oscillator circuit  2  tends to increase. On the other hand, the resonance frequency of the MEMS resonator  1  tends to be low when the temperature rises. As a result, the resonance frequency of the combined MEMS resonator  1  and MEMS capacitors  3  and  4  can be held constant without changing with respect to the temperature. 
   As described above, the oscillator according to this embodiment compensates the frequency of the MEMS resonator with the temperature by using the temperature dependency of capacitance of the MEMS capacitor. 
   Second Embodiment 
   An oscillator according to another embodiment of the present invention will be described in detail. 
   In the frequency temperature characteristic of the MEMS resonator  1 , as shown in  FIG. 8 , the resonance frequency of the MEMS resonator decreases when the temperature rises. Also, a relationship between the temperature and the frequency is not linear but is quadratic. Up to now, in order to improve the frequency temperature characteristic of the MEMS resonator, there have been used the methods disclosed in UP 2006/0033594 A1 and US 2002/0069701 A1. In those methods, the linear component of the frequency temperature characteristic is reduced, but the effect of reducing the quadratic component of the temperature characteristic is small. Under the circumstances, in another embodiment of the present invention, the capacitance values of the MEMS capacitors  3  and  4  which are connected to the input/output terminals of the output oscillator circuit  2  show quadratic change due to temperature to reduce the quadratic component of the frequency temperature characteristic. 
   As shown in  FIG. 5 , the MEMS capacitors  3  and  4  of  FIG. 1  include the cathode electrode  31 , the anode electrode  32 , the support beams  33  and  34 , and the anchors  35  and  36 . The cathode electrode  31 , the anode electrode  32 , and the support beams  33  and  34  set afloat with a gap  38  with respect to the substrate  10 . The cathode electrode  31  is connected to one end of the support beam  33 . Another end of the support beam  33  is fixed to the insulating layer on the substrate  10  by means of the anchor  35 . The anode electrode  32  is connected to one end of the support beam  34 . Another end of the support beam  34  is fixed to an insulating layer on the substrate  10  by the anchor  36 . The cathode electrode  31  and the anode electrode  32  are disposed with a capacitive gap  37  therebetween in such a manner that the sides of the cathode electrode  31  and the anode electrode  32  which are in parallel to the long sides of the support beams  33  and  34  are opposite to each other. Also, the support beams  33  and  34  are fixed to the insulating layer on the substrate  10  through the anchors  35  and  36 , respectively. In this situation, the anchors  35  and  36  are disposed at positions completely opposite to each other with respect to a face where the cathode electrode  31  and the anode electrode  32  face each other. 
   Next, the operation of the oscillator according to another embodiment of the present invention will be described in detail. 
   First, in order to improve the frequency temperature characteristic of the MEMS resonator  1 , a MEMS resonator using the methods disclosed in US 2006/003594 A1, US 2002/0069701 A1 or the like is selected. Starting point is that the linear component of the frequency temperature characteristic is substantially removed, but that the quadratic component of the temperature characteristic remarkably remains. 
   The MEMS capacitors  3  and  4  that are connected to the input terminal and the output terminal of the output oscillator circuit  2  are disposed to provide the capacitive gap  37  between the anode electrode  32  and the cathode electrode  31 . Therefore, the MEMS capacitors  3  and  4  have the capacitance value that is determined by the following expression.
 
 C=εS/d  ( S  is a capacitor electrode area, and  d  is a capacitive gap)
 
When the environmental temperature changes, the material expands and contracts based on the following arithmetic expression.
 
 L′=L   0 (1 +αΔT+βT   2 )
 
   L 0  is a length of the material at a reference temperature, L′ is a length of the material at the present environmental temperature, ΔT is a temperature difference between the reference temperature and the present environmental temperature, and α and β are linear expansion temperature coefficients of the material. 
   For that reason, when the environmental temperature is, for example, higher than the reference temperature, the lengths of the support beams  33  and  34  expand based on the above arithmetic expression. However, because one ends of the support beams  33  and  34  are fixed to the substrate  10  by means of the anchors  35  and  36 , the support beam  33  expands to a direction connected by the cathode electrode  31  (to a direction opposite to the end that is fixed by the anchor  35 ), and the support beam  34  expands to a direction connected by the anode electrode  32  (to a direction opposite to the end that is fixed by the anchor  36 ). The capacitance of the capacitor is proportional to the area of the electrode as represented by the above expression, so the support beams  33  and  34  expand, to thereby reduce an electrode area where the anode electrode  32  and the cathode electrode  31  face each other, and reduce the capacitance value of the capacitor. 
   On the other hand, in the case where the environmental temperature is lower than the reference temperature, the lengths of the support beams  33  and  34  contract based on the above expression. However, because the support beams  33  and  34  have one ends fixed to the substrate  10  by means of the anchors  35  and  36 , the support beam  33  contracts in a direction of the anchor  35 , and the support beam  34  contracts in a direction of the anchor  36 . The capacitance of the capacitor is proportional to the area of the electrode as represented by the above expression, so the support beams  33  and  34  expend, to thereby reduce an electrode area where the anode electrode  32  and the cathode electrode  31  face each other, and reduce the capacitance value of the capacitor. 
   In other words, the capacitance of the MEMS capacitor has the quadratic characteristic where the capacitance becomes lower even if the temperature is high or low with the capacitance at the reference temperature as the maximum (refer to  FIG. 6 ). The MEMS resonator whose linear temperature characteristic of the frequency has been corrected has a negative quadratic characteristic with a certain temperature as the maximum value. The MEMS capacitor has also the negative capacitance temperature characteristic with the certain temperature as the maximum value. Thus, the resonance frequency of the combined MEMS resonator  1  and MEMS capacitors  3  and  4  can be held constant without changing with respect to the temperature. 
   As described above, the oscillator according to this embodiment compensates the frequency of the MEMS resonator with the temperature by using the temperature dependency of the capacitance of the MEMS capacitor. 
   Further, in the case of the above configuration using an SOI substrate or the like, it is possible to integrate all of the structural elements on the same substrate together in this oscillator. Also, for example, it is possible to form another integrated circuit that processes diverse signals by using an oscillation signal of the oscillator circuit of the present invention on the same substrate. 
   The embodiments of the present invention have been described in detail, but specific configurations are not limited thereto, and various changes may be made without departing from the scope of the invention. 
   For example, the structure of the MEMS resonator is not limited to the structure shown in  FIG. 2 , but may be of a structure using lateral oscillation, or of a structure in which both ends of the resonator are supported. Further, it is needless to say that the configuration of the oscillator circuit is not limited to the structure shown in  FIG. 7 .