1. Technical Field
The present invention relates to a MEMS oscillator including a MEMS vibrator and a method of manufacturing thereof.
2. Related Art
FIG. 13 shows the circuit configuration of a related-art MEMS vibrator. One of electrodes of the MEMS vibrator 201 is connected to a coupling capacitance 203a, and the coupling capacitance 203a is connected to an input terminal. The other electrode of the MEMS vibrator 201 is connected to a coupling capacitance 203b, and the coupling capacitance 203b is connected to an output terminal 220. A bias voltage applying circuit 202 is connected between the electrodes of the MEMS vibrator 201 via bias resistances 204a and 204b. 
When the MEMS vibrator 201 is operated, a DC bias voltage is applied between the electrodes of the MEMS vibrator 201 by the bias voltage applying circuit 202 including a reference voltage generating circuit so that the MEMS vibrator 201 is free from the influence of an external DC voltage. The input terminal 210 and the output terminal 220 which are coupled to an external circuit and the electrodes of the MEMS vibrator 201 are connected via the coupling capacitances 203a and 203b. To obtain AC insulation between the bias voltage applying circuit 202 which applies the DC bias voltage and the electrodes of the MEMS vibrator 201, the bias resistances 204a and 204b are inserted between the electrodes of the MEMS vibrator 201 and the bias voltage applying circuit 202.
FIG. 14 shows an exemplary MEMS oscillator including the related-art MEMS vibrator and an oscillator circuit. One of the electrodes of the MEMS vibrator 201 is connected to the coupling capacitance 203a, while the other electrode of the MEMS vibrator 201 is connected to the coupling capacitance 203b. The bias circuit 202 is connected between the electrodes of the MEMS vibrator 201, and a bias voltage input terminal is connected to the bias circuit 202. The coupling capacitance 203a is connected to an input side of an amplifier circuit 230, while the coupling capacitance 203b is connected to an output side of the amplifier circuit 230. The output side of the amplifier circuit 230 and the coupling capacitance 203b are connected to an input side of a buffer circuit (buffer amplifier) 240, while an output side of the buffer circuit 240 is connected to an output terminal.
For the MEMS vibrator 201, a MEMS vibrator utilizing change in electrostatic capacitance due to mechanical displacement is generally used in many cases. As the simplest structure, a cantilever structure can be cited as a representative example. FIG. 15 shows an exemplary appearance of the MEMS vibrator 201 using this structure; and FIGS. 16A to 16C show the configuration of the MEMS vibrator 201 in detail.
As shown in FIGS. 15 and 16A, the MEMS vibrator 201 is composed of a fixed electrode 101 formed on a substrate 100 and a movable electrode 102 formed apart from the fixed electrode 101 with a constant gap. The movable electrode 102 is composed of a fixed portion 104 formed on the substrate 100, a movable portion (beam) 103 provided to face the fixed electrode 101 and capable of vibrating, and a supporting portion 105 coupling the movable portion 103 with the fixed portion 104 and supporting them (refer to JP-A-2008-153817 (FIG. 2), for example).
A method of manufacturing the MEMS vibrator 201 is as follows.
As shown in FIGS. 16B and 16C, a silicon oxide film 111 is formed on a Si substrate 110, and a silicon nitride film 112 is formed on the silicon oxide film 111. Next, the fixed electrode 101 formed of a first polysilicon film is formed on the silicon nitride film 112, and an insulating film (not shown) covering the fixed electrode 101 is formed. Next, the movable electrode 102 formed of a second polysilicon film is formed on the insulating film. Next, the insulating film around the movable electrode 102 is removed. In this manner, the MEMS vibrator 201 is manufactured.
In the MEMS vibrator 201, the movable portion 103 vibrates in a thickness direction of the substrate 100. Since the gap between the movable portion 103 and the fixed electrode 101 can be set narrow, the capacitance change rate relative to the fluctuation of the movable portion 103 is high, so that the MEMS vibrator 201 can obtain high sensitive resonance characteristics.
The MEMS vibrator 201 is manufactured by a so-called surface MEMS process in which the fixed electrode 101 and the movable electrode 102 are formed by depositing polysilicon or the like by a CVD (chemical vapor deposition) method. Since the MEMS vibrator 201 can be formed by the relatively simple process, it has such an advantage that the MEMS vibrator can be manufactured at low cost.
On the other hand, the MEMS vibrator 201 has the following disadvantage.
As described above, since the fixed electrode 101 and the movable electrode 102 are formed by depositing polysilicon or the like by a CVD method, the film formation by deposition results in great variation in film thickness, and the tolerance usually reaches up to ±10% in some cases. In a vibration mode, since the film thickness of the fixed electrode 101 and the movable electrode 102 serves as a factor in determining the resonant frequency of the vibrator, the variation in film thickness is directly linked to variation in resonant frequency. Therefore, in the fixed electrode 101 and the movable electrode 102 formed by depositing polysilicon or the like, there is a problem that it is difficult to obtain relatively high frequency accuracy compared to a vibrator which vibrates in a horizontal direction of a substrate.
Means for securing resonant frequency accuracy by managing and selecting the film thickness, optimizing the beam strength, or the like is also conceivable. In view of mass production, however, the process control has its limit, and an increase in manufacturing cost due to the addition of processes therefor is of concern.
FIG. 17 shows the relation between variation in resonant frequency caused by variation in the fixed electrode and movable electrode of a MEMS vibrator and the target accuracy (tolerance) of resonant frequency required for the MEMS vibrator as a product. However, the degree of variation in film thickness varies depending on the performance of a film-forming apparatus, film-forming conditions, and the like, and therefore, the variation in resonant frequency shown in FIG. 17 is illustrative only. The target accuracy also varies depending on products, and therefore, the target accuracy shown in FIG. 17 is illustrative only.
For example, when the specification value for the resonant frequency and frequency accuracy of a MEMS vibrator is 20 MHz±100 kHz (±0.5%), the resonant frequency of the vibrator has to fall within a range from 19.9 MHz to 20.1 MHz. For realizing this, the variation in the film thickness of the MEMS vibrator has to be a desired value or less (for example, within ±0.05 μm).
As described above, however, if the variation in the film thickness of the MEMS vibrator results in, for example, four times the desired value (for example, ±0.2 μm) due to the performance of a film-forming apparatus, or film-forming conditions, it is presumed that the resonant frequency of MEMS vibrators which are mass-produced under this condition varies from 19.6 MHz to 20.4 MHz (±400 kHz=±2%).
As described above, when the variation in resonant frequency caused by the variation in film thickness is great compared to the target accuracy required as a product, the related-art manufacturing method cannot obtain a MEMS vibrator having a resonant frequency within the target accuracy with high yield.