Abstract:
This invention provides a micromechanical resonator oscillator structure and a driving method thereof. As power handling ability of a resonator is proportional to its equivalent stiffness, a better power handling capability is obtained by driving a micromechanical resonator oscillator at its high equivalent stiffness area. One of the embodiments of this invention is demonstrated by using a beam resonator. A 9.7-MHZ beam resonator via the high-equivalent stiffness area driven method shows better power handling capability and having lower phase noise.

Description:
RELATED APPLICATIONS 
     The application claims priority to Taiwan Application Serial Number 101123301, filed Jun. 28, 2012, which is herein incorporated by reference. 
     BACKGROUND 
     1. Field of Invention 
     The present invention relates to a micromechanical resonator oscillator structure and its driving method. More particularly, the present invention relates to a micromechanical resonator oscillator and its high equivalent stiffness driving method. 
     2. Description of Related Art 
     A quartz crystal oscillator is used for generating clock pluses and widely used in electronic products such as mobile phones, personal computers, digital cameras, electronic clocks and motherboards. However, a conventional quartz crystal oscillator has the disadvantages of being bulky, costly and difficult to be integrated with IC (Integrated Circuit). Recently, the developments of the micromechanical resonator oscillator get more focused. The advantages of the micromechanical resonator oscillator are low cost, small volume and high integration capability to the LSI (Large-Scale-Integrated Circuits). The anti-shock capability of the micromechanical resonator oscillator is also better than that of the Quartz crystal oscillator. 
     U.S. Pat. No. 6,249,073 discloses a micromechanical oscillator structure. The micromechanical oscillator structure comprises a beam oscillator and a supporting structure. The supporting structure supports the beam oscillator in order to form a gap, thus forming an oscillation. The micromechanical oscillator can obtain a high Q-value in a high frequency range. Besides, the working frequency can be expanded by a differential signal technique. 
     U.S. Pat. No. 6,958,566 discloses another mechanical oscillator related to phenomena of dependent electronic stiffness. The mechanical oscillator o comprises a substrate, a mechanical oscillator and a supporting structure. A gap is formed between the nearby electrode and the mechanical oscillator. By means of a controllable voltage between an electrode and the mechanical oscillator, improvement on the instability of the oscillating frequency caused by temperature and acceleration can be made. 
     Although the prior art discloses a micromechanical oscillator structure and a driving method thereof, yet an issued of high phase noise still remains unsolved. 
     SUMMARY 
     A micromechanical resonator oscillator structure and a driving method thereof are provided. The micromechanical resonator oscillator structure comprises a substrate, an insulating layer, a conductive layer, an oscillation unit and a plurality of anchor points. An insulating layer is deposited on the substrate. A conductive layer deposited on the insulating layer, and the conductive layer comprises an electrode set and an input contact. An oscillation unit has a high equivalent stiffness area and a low equivalent stiffness area. At least one of the anchor points connects the high equivalent stiffness area of the oscillation unit to the input contact, and the anchor points support the oscillation unit on the substrate. 
     A driving method applicable to the micro mechanical resonator oscillator structure comprises: 
     inputting an input electronic signal from a driving electrode of the micro mechanical resonator oscillator structure, and outputting an output electronic signal from a sensing electrode of the micro mechanical resonator oscillator structure, and passing the output electronic signal through a number of electronic components, 
     It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the following detailed description of the embodiment, with reference made to the accompanying drawings as follow 
         FIG. 1A  is schematic view showing a first process step of the micromechanical resonator oscillator according to an embodiment of the present invention; 
         FIG. 1B  is schematic view showing a second process step of the micromechanical resonator oscillator according to the embodiment of the present invention; 
         FIG. 1C  is schematic view showing a third process step of the micromechanical resonator oscillator according to the embodiment of the present invention; 
         FIG. 2A  is schematic view showing an oscillation unit being oscillated; 
         FIG. 2B  is a schematic diagram showing the distribution of an equivalent stiffness area of the oscillation unit of the micromechanical resonator oscillator; 
         FIG. 3A  is a schematic diagrams showing the layouts of an electrode set for a driving method of a high equivalent stiffness area of the oscillation unit of the micromechanical resonator oscillator; 
         FIG. 3B  is a schematic diagrams showing the layouts of an electrode set for a driving method of a low equivalent stiffness ea of the oscillation unit of the micromechanical resonator oscillator; 
         FIG. 4  shows a method for measuring an oscillation frequency spectrum of the micromechanical resonator oscillator; 
         FIG. 5  shows a comparison of frequency spectra between the driving method of the high equivalent stiffness area and the driving method of the low equivalent stiffness area of the oscillation unit; 
         FIG. 6  shows a method for measuring a phase noise of the micromechanical resonator oscillator; 
         FIG. 7  shows a comparison of phase noises between the driving method of the high equivalent stiffness area and the driving method of the low equivalent stiffness area of the oscillation unit; 
         FIG. 8  is a schematic diagram showing one structure of the micromechanical resonator oscillator; 
         FIG. 9  is a schematic diagram showing another structure of the micromechanical resonator oscillator; 
         FIG. 10  is a schematic diagram showing another structure of the micromechanical resonator oscillator; and 
         FIG. 11  is a schematic diagram showing another structure of the micromechanical resonator oscillator. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     Referring to  FIG. 1A , an insulating layer  111  is deposited on a substrate  110  for electric isolation. A conductive layer  112  is deposited on the insulating layer  111 . Then, an electrode set  113  and an input point  114  are formed on the insulating layer  111  by a photolithography process. The electrode set  113  comprises one electrode  115  and the other electrode  116 . Then, a sacrifice layer  120  is deposited on the conductive layer  112 , A reactive ion etching method is applied for etching the sacrifice layer  120  to form an anchor point  117  in  FIG. 1B . 
     Referring to  FIG. 1B , a structure layer  118  is deposited on the sacrifice layer  120 . A main structure of an oscillation unit  119  is formed from a structure layer  118  by the photolithography process. 
     Referring to  FIG. 1C , a complete micromechanical resonator oscillator is formed after the sacrifice layer  120  is removed by a chemical wet etching method. 
     Referring to  FIG. 2A , the oscillation unit  119  and the input point  114  are connected by the anchor point  117 . The anchor point  117  supports the oscillation unit  119  and a gap is formed between the oscillation unit  119  and the electrode set  113 , and thus an equivalent capacitance structure is formed. 
     Referring to  FIG. 2B , the distribution of the equivalent stiffness areas of the oscillation unit is formed by a theoretical calculation. The physical phenomena corresponding to the theoretically calculation can be explained by the following description. When the oscillation begins, both side areas of the oscillation unit  119  are connected to and supported at input points  114 , and thus have lower oscillation speeds and higher equivalent stiffness. A central area of the oscillation unit  119  has a larger oscillation speed owing to the nature of the vibrating mode shape. When the oscillation speed is lower, the equivalent stiffness is higher. Therefore, the equivalent stiffness is lower in the central area of the oscillation unit  119  than that in the side areas of the oscillation unit  119 . By applying a theoretical calculation, the high equivalent stiffness area  600   a  and the low equivalent stiffness area  600   b  can be defined. 
     Referring to  FIG. 3A . an electrode  115  is placed under the low equivalent stiffness area, and an electrode  116  is placed under the high equivalent stiffness area. An electrical signal is inputted from the electrode  116  and outputted from the electrode  115 . It is defined that the electrical signal is inputted from a driving electrode and outputted from a sensing electrode,  3 A shows a driving method for the high equivalent stiffness area of the oscillation unit. 
     Referring to  FIG. 3B , an electrode  115  is placed under the low equivalent stiffness area and an electrode  116  is placed under the high equivalent stiffness area. An electrical signal is inputted from the electrode  115  and is outputted from the electrode  116 . It is defined that the electrical signal is inputted from a driving electrode and outputted from a sensing electrode.  FIG. 3B  shows a driving method for the low equivalent stiffness area of the oscillation unit. 
     Referring to  FIG. 4 ,  FIG. 4  shows a method for measuring an oscillation to frequency spectrum of the micromechanical resonator oscillator. A micromechanical resonator oscillator is placed in a vacuum chamber  200  for preventing air damping and noise interference. The vacuum chamber  200  has a connecting point. A power supply  201  is connected to the input point  114  of the micromechanical resonator oscillator  100  through the vacuum chamber  200 . The power supply  201  provides an electrical signal in order to enlarge the oscillation and to tune the frequency of the oscillation unit  119 . An output end of a network analyzer  202  is connected to an electrode of the micromechanical resonator oscillator  100 , and an input end of the network analyzer  202  is connected to the other electrode of the micromechanical resonator oscillator  100 . 
     Referring to  FIG. 5 ,  FIG. 5  shows a comparison of frequency spectra between the driving method of the high equivalent stiffness area and the driving method of the low equivalent stiffness area of the oscillation unit  119 . The measurement described in  FIG. 4  is applied to the electrode layouts of signals are compared. It is found that the frequency spectrum  500   a  outputted by the driving method of the high equivalent stiffness area of the oscillation unit  119  is more stable than the frequency spectrum  500   b  outputted by the driving method of the low equivalent stiffness area of the oscillation unit  119 . 
     Referring to  FIG. 6 ,  FIG. 6  shows a method for measuring a phase noise of the micromechanical resonator oscillator. An electrical circuit (not labeled) is connected in series with the micromechanical resonator oscillator. The electrical circuit comprises a transimpedance amplifier  301 , a variable-gain amplifier  302 , a loop buffer  303  and an output buffer  304 . A micromechanical resonator oscillator  100  is placed in a vacuum chamber  400  for preventing air damping and noise interference. The vacuum chamber  400  has a connecting point. A power supply  403  is connected to the input point  114  of the micromechanical resonator oscillator  100  through the vacuum chamber  400 . The power supply  403  provides an electrical signal in order to enlarge the oscillation and to tune the frequency of the oscillation unit  119 . An oscillation analyzer  401  and a frequency spectrum analyzer  402  are independently connected to the output buffer  304  in order to measure the output signal. 
     Referring to  FIG. 7 ,  FIG. 7  shows a comparison of phase noises between the driving method of the high equivalent stiffness area and the driving method of the low equivalent stiffness area of the oscillation unit  119  The measurement method described in  FIG. 6  is applied to the electrode layouts of a micromechanical resonator oscillator in  FIG. 3A  and  FIG. 3B , and the phase noises are compared. It is found that the phase noise  700   a  of the driving method of the high equivalent stiffness of the oscillation unit  119  is 26.3 dB lower than the phase noise  700   b  of the driving method of the low equivalent stiffness area of the oscillation unit  119 . 
     Referring to  FIG. 8 , one structure of the micromechanical resonator oscillator  100  is a beam type structure, and the electrode set  113  has one electrode set. 
     Referring to  FIG. 9 , another structure of the micromechanical resonator oscillator  100  is a square type structure, and the electrode set  113  has four sets of electrodes, 
     Referring to  FIG. 10 , another structure of the micromechanical resonator oscillator  100  is a disc type structure, and the electrode set  113  has four sets of electrodes. 
     Referring to  FIG. 11 , another structure of the micromechanical resonator oscillator  100  is a disc type structure, and the electrode set  113  has one electrode set. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims.