JP5028646B2 - Small oscillator - Google Patents

Description

  The present invention relates to a small oscillator used for a communication device, a portable device, a timepiece and the like and a method for manufacturing the same.

Many communication devices and portable devices use a crystal resonator as an oscillator. MEMS (Micro Electro Mechanical Systems) is a device in which component parts are integrated on a single silicon substrate. In portable devices, silicon-based oscillators using MEMS technology are attracting attention in order to reduce the volume. MEMS resonators have stability and accuracy comparable to quartz and are miniaturized. An example is Sitime.
Further, as a main element of a phase-locked loop (PLL) used in a communication device or the like, there is a voltage-controlled oscillator (VCO, Voltage-Controlled Oscillator) capable of changing an oscillation frequency with a voltage. With the widespread use of mobile phones and wireless LANs, PLLs are frequently used, and the demand for VCOs is increasing.
If the entire VCO can be reduced rather than merely reducing the oscillator, the oscillator can be reduced in size.

Since neither a crystal oscillator nor an oscillator using MEMS is self-oscillating, it functions as a filter in the feedback circuit. Therefore, an external feedback circuit is necessary to maintain oscillation.
Some VCOs use MEMS technology but require an external coil or capacitor. Even in a conventional VCO that does not use MEMS technology, a capacitance change diode or the like is required.
However, since it is difficult to reduce the size of the coil, the use of the coil results in a large device as a whole. If a VCO that does not require an external capacitor or coil can be realized, a dramatically smaller VCO can be realized.

  Patent Document 1 discloses a passive element that can be used for an oscillator. The passive element of Patent Document 1 is electrostatically driven but does not self-oscillate. Further, the oscillation frequency does not change even when the applied voltage is changed.

  Therefore, it is desired to develop a further small oscillator. Also, a method for manufacturing such a small oscillator at low cost is desired. There is also a need for a small oscillator that can change the oscillation frequency.

JP 2006-253493 A

An object of the present invention is to provide an extremely small and small oscillator that does not require an external capacitor or coil.
Another object of the present invention is to provide a method for manufacturing such a small oscillator using micromachining technology.
Another object of the present invention is to provide a small oscillator capable of changing an oscillation frequency.

The present inventors have developed an oscillator that self-oscillates by simply applying a DC voltage without requiring an external capacitor or coil. This oscillator can change the oscillation frequency by changing the applied voltage.
In the small oscillator according to the present invention, a vibrating member and a drive electrode are formed on a substrate using a semiconductor manufacturing technique using a silicon substrate on an insulator in which two silicon substrates are bonded via a silicon oxide film. Is. When a DC voltage is applied to the drive electrode, the vibrating member is attracted to the drive electrode by electrostatic attraction, and when the oscillator contacts the drive electrode, it discharges and returns to its original position. When this is repeated and a DC voltage is applied, self-oscillation occurs.
In the present invention, an extremely small small oscillator can be obtained.

One embodiment of the present invention is a small oscillator. This small oscillator has a substrate,
An anchor formed on the substrate;
An elongated vibration member having one end fixed to the anchor, extending from the anchor, and floating from the substrate;
A drive electrode provided along the vibration member for attracting the vibration member by electrostatic attraction;
A resistance between the power source and the drive electrode,
When a voltage is applied from the power source to the drive electrode,
The vibration member is attracted to the drive electrode by electrostatic attraction, and when the vibration member comes into contact with the drive electrode, the electric charge is discharged, and the vibration member repeatedly leaves the drive electrode, and the cantilever self-oscillates. .

  Since the resonator according to the present invention does not require an external capacitor or coil, the entire resonator can be made small.

It is preferable that a voltage equal to or higher than a voltage at which the vibrating member is attracted to and contacts the driving electrode is applied to the driving electrode.
The charging time for the drive electrode is preferably longer than the resonance period of the vibrating member.
If it carries out like this, a vibrating member will contact a drive electrode and will discharge, and a vibrating member will return to an original rest position with a restoring force.
It is preferable that a capacitance is formed between the vibration member and the drive electrode, and when a voltage is applied to the drive electrode, the voltage between the vibration member and the drive electrode increases with time.
As a result, when the voltage applied to the drive electrode is changed, the time until the vibrating member comes into contact with the drive electrode changes, and the oscillation frequency of the vibrating member changes.
Preferably, the anchor is fixed on the substrate via an insulating film, and a capacitance is formed between the substrate and the anchor.

The small oscillator is preferably made using a silicon substrate on an insulator.
Thereby, an extremely small and small oscillator can be obtained.
The vibration member is preferably a cantilever having one end fixed to the anchor.
In order to eliminate the influence of charges other than the drive electrode on the vibration member, it is preferable to provide a shield along the vibration member.

According to another aspect of the present invention, a small oscillator including an anchor, a vibration member, and a drive electrode is integrally formed from a silicon substrate on an insulator in which two silicon substrates are bonded via a silicon oxide film. Is the method. This manufacturing method is
(a) depositing and patterning a mask layer on the upper surface of the silicon substrate on the insulator;
(b) Etching from the upper surface, etching other than the portion masked by the mask layer, forming the drive electrode, the vibration member, and the drive electrode on the upper layer,
(c) a step of removing the silicon oxide film of the silicon substrate on the insulator by sacrificial layer etching to form a structure in which the vibrating member is lifted from a lower layer.

The mask layer may be an aluminum layer or a silicon oxide film layer.
The etching step (b) is preferably performed by deep reactive ion etching.

  According to the method of the present invention, since a component of a small oscillator can be simultaneously formed on a single substrate using a silicon substrate on an insulator and using micromachining technology, a small small resonator can be manufactured. It can be obtained in a simple way.

According to the present invention, a small and lightweight small oscillator can be obtained. In particular, since an external capacitor or coil is not required, an extremely small oscillator can be obtained.
Further, when the applied voltage is changed, the oscillation frequency can be changed.
Moreover, since the manufacturing method is simple, such a small oscillator can be manufactured at low cost.

(Construction)
Embodiments of the present invention will be described below. FIG. 1 is a diagram showing the structure of a small oscillator 10 according to an embodiment of the present invention. The small oscillator 10 is made from an SOI (Silicon-on-insulator) substrate. The small oscillator 10 includes a substrate 20, and includes an anchor 13, a cantilever 11 as a vibration member, and a drive electrode 12 on the substrate 20. An anchor 13 for fixing the cantilever 11 is provided. An elongated cantilever 11 extends from one end of the anchor 13. One end of the cantilever 11 is fixed to the anchor 13 and is lifted from the substrate 20 along its length. Since the cantilever 11 is fixed to the anchor 13 at one end thereof, the tip end portion 11a is movable in the vertical direction in FIG. A drive electrode 12 for attracting the cantilever 11 is provided on the substrate 20 along the cantilever 11. A portion of the drive electrode 12 facing the tip 11a of the cantilever 11 is a convex contact point 12a. When a voltage is applied to the drive electrode 12 to attract the cantilever 11 to the drive electrode 12, the tip end portion 11a of the cantilever 11 comes into contact with the contact point 12a of the drive electrode 12 and stops.
In the present embodiment, the cantilever 11 is used. However, the vibrating member is not cantilevered, but can be a both-end supported in which both ends are fixed and the center vibrates.

A power supply 15 for applying a voltage to the drive electrode 12 is provided between the substrate 20 and the drive electrode 12. A resistor 14 is provided between the drive electrode 12 and the power source 15. The resistor 14 can be provided on the same substrate 20. The resistance value can be adjusted by changing the length of the resistor 14. Therefore, it is not necessary to provide a resistor outside. The resistor 14 controls the current flowing through the drive electrode 12. The time constant for charging the drive electrode 12 is determined by the magnitude of the resistance value of the resistor 14.
Since the anchor 13 is formed on the silicon oxide film, a capacitance Ce is formed between the anchor 13 and the substrate 20.
In order to prevent the surrounding electric field other than the drive electrode 12 from affecting the cantilever 11, a shield 16 is provided along the cantilever 11 on the side opposite to the drive electrode 12. The shield 16 is connected to the anchor 13 and is provided on the same substrate 20.

  FIG. 2 is a cross-sectional view of the small resonator 10 taken along the line 2-2 in FIG. 1 and shows that the small resonator 10 has a capacitance. In FIG. 2, the shield 16 is not shown. Since the cantilever 11 (anchor 13) is formed on the substrate 20 via an insulating film, a capacitance Ce is formed between the cantilever 11 (anchor 13) and the substrate 20. That is, it is not necessary to provide a capacitor outside. Since there is a space between the cantilever 11 and the drive electrode 12, a capacitance Ca is generated.

(Operating principle)
FIG. 3 is a diagram showing the principle that the cantilever 11 performs self-excited oscillation. In FIG. 3, the shield 16 is not shown. (a) When a DC voltage is applied between the drive electrode 12 and the substrate 20 by the power supply 15, the drive electrode 12 and the cantilever 11 are charged, and a potential difference is generated between the drive electrode 12 and the cantilever 11. Due to this potential difference, an electrostatic attractive force is generated, and the cantilever 11 in the stationary position is attracted to the drive electrode 12.
(b) When the potential difference between the cantilever 11 and the drive electrode 12 becomes higher than the voltage at which the tip 11a of the cantilever 11 is attracted to the contact point 12a of the drive electrode 12, that is, the pull-in voltage, the tip 11a of the cantilever 11 Contact 12 contact points 12a. When the tip 11a of the cantilever 11 comes into contact with the contact point 12a of the drive electrode 12, the charge moves between the cantilever 11 and the drive electrode 12, and the potential difference between the cantilever 11 and the drive electrode 12 disappears.

(c) At this time, if the charging time for the cantilever 11 and the drive electrode 12 is longer than the resonance period of the cantilever 11, electrostatic attraction does not work between the cantilever 11 and the drive electrode 12, and the cantilever 11 is restored. The force leaves the drive electrode 12 to return to the original rest position.
Then return to (a). A DC voltage is applied between the drive electrode 12 and the substrate 20 by the power supply 15. The drive electrode 12 is charged again, and the cantilever 11 is attracted to the drive electrode 12. When the potential difference between the cantilever 11 and the drive electrode 12 becomes higher than the pull-in voltage, the cantilever 11 comes into contact with the drive electrode 12 again.
As described above, when a DC voltage is applied to the drive electrode 12, the small oscillator 10 repeats (a) to (c) and self-oscillates.

(Principle of oscillation frequency change)
FIG. 4 is a diagram showing the principle that the oscillation frequency changes when the voltage applied to the drive electrode is changed. FIG. 4A is a diagram showing an equivalent circuit of the small oscillator 10.
In FIG. 4A, the position of the anchor is indicated by 13 and the position of the drive electrode is indicated by 12. Ca represents the capacitance between the cantilever 11 and the drive electrode 12. Va is a voltage applied between the cantilever 11 and the drive electrode 12. Ce is a capacitance generated between the anchor 13 and the substrate 20. Vc is a voltage applied between the anchor 13 and the substrate 20.

Reference is made to FIG. When a stepped voltage Vn is applied to the drive electrode 12, the potential difference Va (t) between the drive electrode 12 and the cantilever 11 responds transiently according to the following equation.
(1)
When Va (t) gradually increases, the cantilever 11 is attracted to the drive electrode 12, and when the pull-in voltage Vp of Va (t) cantilever 11 is exceeded, the cantilever 11 contacts the drive electrode 12.

If the resonance period of the cantilever 11 is smaller than the time constant CaR of this transient response, the potential difference Va (t) between the drive electrode 12 and the cantilever 11 becomes the pull-in voltage Vp for the time Tn until the cantilever 11 contacts the drive electrode 12. Equals the time to reach.
The time Tn until the cantilever 11 contacts the drive electrode 12 depends on the applied voltage Vn and can be expressed by the following equation.
(2)

  FIG. 4B shows that when various applied voltages Vn (V1, V2, V3) are applied to the drive electrode 12, the potential difference Va (t) between the drive electrode 12 and the cantilever 11 depends on time according to the equation (1). Indicates that it will change. As shown in the figure, when the drive voltage V1 is applied to the drive electrode 12, Va (t) shows the transient response shown by the solid line in the figure according to the equation (1), and the arrival time until the pull-in voltage Vp is reached is t1. It becomes. When the drive voltage V2 is applied, a transient response indicated by a dotted line in the figure is shown, and the arrival time is t2. When the drive voltage V3 is applied, a transient response indicated by a one-dot chain line in the figure is shown, and the arrival time is t3. In this way, when the voltage Vn applied to the drive electrode 12 is changed, the arrival time until the pull-in voltage Vp is reached changes, and accordingly, the time Tn until the cantilever 11 contacts the drive electrode 12 changes. Therefore, the oscillation frequency of the cantilever 11 of the oscillator 10 changes.

(Manufacturing process)
FIG. 5 shows a method of creating the small oscillator 10 according to the embodiment of the present invention. FIG. 5 is a cross-sectional view of the small oscillator 10. First, a silicon on insulator (SOI (Silicon on Insulator)) substrate is prepared. This SOI substrate has a silicon layer 22 formed on the upper surface of a silicon oxide film 21 and a silicon layer 23 formed on the lower surface.
(a) An aluminum layer 26 is deposited on the upper surface of the SOI substrate, and the structure of the cantilever 11, the drive electrode 12 and the shield 16 is formed on the upper layer of the small resonator 10 by using a photolithography technique. Pattern.

  (b) Deep RIE is performed from the upper surface, and portions other than the portion masked by the aluminum layer 26 are etched to form the cantilever 11, the drive electrode 12, the anchor 13 (not shown in FIG. 5), and the shield 16. At this time, since the silicon oxide film 21 becomes an etching stop layer, the underlying silicon layer 23 is not etched.

  (c) The silicon oxide film 21 on the SOI substrate is removed with hydrofluoric acid. Since the cantilever 11 is elongated, the silicon oxide film 21 under the cantilever 11 is removed, and the cantilever 11 is lifted from the silicon layer 23 on the lower surface. The silicon oxide film 21 under the drive electrode 12, the anchor 13, and the shield 16 is not removed. Therefore, one end of the cantilever 11 is fixed to the anchor 13 and the other end is movable.

  According to the method of the present invention, the small oscillator 10 including the cantilever 11, the drive electrode 12, the anchor 13, and the shield 16 can be formed by a very simple process.

(Example)
FIG. 6 shows an electron micrograph of a small oscillator according to an embodiment of the present invention. FIG. 6A is a photograph of the main part of the small oscillator. (b) is an enlarged photograph of the vicinity of the contact point between the cantilever 11 and the drive electrode 12 in the lower right part of (a). The cantilever 11 is in a stationary position where no voltage is applied. The width of the cantilever was 5 μm, the length was 300 μm, and the height was 20 μm. The gap between the tip 11a of the cantilever 11 and the contact point 12a of the drive electrode 12 was 10 μm at a stationary position where no voltage was applied to the drive electrode 12. The pull-in voltage was 40V. This pull-in voltage can also be set lower.

(Operation result)
With respect to the small oscillator according to the embodiment of the present invention, an experiment was performed in which voltage is applied to the drive electrode 12 to oscillate the cantilever 11. FIG. 7 shows the result of measuring the displacement of the cantilever 11 using a laser displacement meter. (a) shows the relationship between the time when the drive electrode 12 is applied with 39.0 V as the drive voltage Vn and the displacement of the cantilever 11. The voltage Va (t) applied to the drive electrode 12 changes according to the equation (1). In the figure, point A indicates that the cantilever 11 is attracted to and contacted the drive electrode 12. Point B indicates that the cantilever 11 has discharged and returned to a rest position where no force is applied. As time further elapses from point B, the cantilever 11 is gradually attracted to the drive electrode 12 and moves. When the pull-in voltage is reached again, the cantilever 11 contacts the drive electrode 12. The oscillation period was 620 ms.

(b) shows the relationship between the time when 40.7 V is applied as the drive voltage Vn to the drive electrode 12 and the displacement of the cantilever 11. The oscillation period was 100 ms. When the voltage applied to the drive electrode 12 is changed from 39.0 V to 40.7 V, it can be seen that the oscillation period has changed from 620 ms to 100 ms. As described above, when the voltage applied to the drive electrode is changed, the oscillation frequency changes.
In the embodiment of the present invention, the oscillation period was relatively slow. This is probably because the value of the externally attached resistor was large and the time constant during charging was large.

In the embodiment of the present invention, a small oscillator is formed using an SOI substrate, but a semiconductor such as GaAs other than silicon, or a metal material can be used. If a metal material is used, the contact resistance can be lowered.
In addition, since the small resonator according to the present invention can be produced by the same process as the integrated circuit production process, it can be produced on an integrated circuit such as an IC. By creating the small oscillator on the same chip as the IC, the small oscillator can be greatly reduced in size.

The small resonator of the present invention is suitable for use in portable devices such as a mobile phone, a game machine, and a wireless LAN.
Since the small oscillator of the present invention can change the frequency by changing the voltage, the VCO used in the PLL can be greatly reduced in size, and has many uses for communication devices such as a communication device. is there.

The figure showing the principle of the small oscillator of this invention. Sectional drawing of the small oscillator by embodiment of this invention. The figure which shows the principle which the small oscillator of this invention vibrates. (a) is a figure which shows the equivalent circuit of the small resonator of this invention, (b) is a figure which shows the transient response of a drive voltage. Sectional drawing which shows the manufacturing process of the small oscillator of this invention. The electron micrograph of the upper surface of the small oscillator by the Example of this invention. The figure which shows the result of having measured the displacement by vibrating a cantilever.

Explanation of symbols

10 Small oscillator
11 Cantilever
12 Driving electrode
13 Anchor
14 Resistance
15 Power supply
16 Shield
20 Support substrate
21 Silicon oxide layer
22 Silicon layer
23 Silicon layer
26 Aluminum layer

Claims (8)

A substrate,
An anchor formed on the substrate;
An elongated vibration member having an end fixed to the anchor, extending from the anchor, and floating from the substrate;
A drive electrode provided along the vibration member for attracting the vibration member by electrostatic attraction;
A resistor connected to the drive electrode,
When a voltage is applied to the driving electrode from a power source, the vibrating member is attracted to the driving electrode by electrostatic attraction, and when the vibrating member comes into contact with the driving electrode, the electric charge is discharged and the driving member is repeatedly separated from the driving electrode. A small oscillator in which the vibration member self-oscillates. A voltage equal to or higher than a voltage at which the vibrating member is attracted to and contacts the drive electrode;
The small oscillator according to claim 1, wherein a charging time for the driving electrode is longer than a resonance period of the vibrating member. A capacitance is formed between the vibration member and the drive electrode, and when a voltage is applied to the drive electrode, the voltage between the vibration member and the drive electrode increases with time,
The small oscillator according to claim 2, wherein when the voltage applied to the drive electrode is changed, the time until the vibrating member comes into contact with the drive electrode changes, and the oscillation frequency of the vibrating member changes.   The small oscillator according to claim 1, wherein the anchor is fixed on the substrate via an insulating film, and a capacitance is formed between the substrate and the anchor.   2. The small oscillator according to claim 1, wherein the oscillator is made of a silicon substrate on an insulator, and the vibration member is a cantilever having one end fixed to the anchor.   The small oscillator according to claim 1, further comprising a shield along the vibration member in order to eliminate the influence of charges other than the drive electrode on the vibration member. A manufacturing method for integrally forming the small oscillator according to claim 1 ,
(a) depositing and patterning a mask layer on the upper surface of the silicon substrate on the insulator;
(b) Etching from the upper surface, etching other than the portion masked by the mask layer, forming the drive electrode, the vibration member, and the drive electrode on the upper layer,
(c) The silicon oxide film of the silicon substrate on the insulator is removed by sacrificial layer etching, and the vibration member is lifted from the lower layer.
A manufacturing method comprising a process.   The manufacturing method according to claim 7, wherein the etching step (b) is performed by deep reactive ion etching.