Patent Grant
10291203
The present invention relates to a MEMS resonator, and, more particularly, to a MEMS resonator with a high quality factor.
Microelectromechanical system (“MEMS”) resonators are small electromechanical structures that vibrate at high frequencies and are often used for timing references, signal filtering, mass sensing, biological sensing, motion sensing, and other applications. MEMS resonators are considered a common alternative to quartz timing devices. In general, quartz resonators have a high quality factor and piezoelectric coupling. High quality factor indicates a low rate of energy loss relative to the stored energy of the resonator, i.e., the oscillations die out more slowly. However, one limitation for quartz resonators is that they are difficult to design in smaller sizes.
Typically, MEMS resonators are made of silicon using lithography based manufacturing processes and wafer level processing techniques. Designers and manufacturers have found that pure silicon resonators often demonstrate very high quality factors comparable to quartz crystals, for example, as described in Non-patent document 1, identified below. However, bare silicon is not piezoelectric and pure silicon resonators have high motional impedance making them unsuitable to replace quartz resonators in many applications.
In order to lower the motional impedance of MEMS resonator, some designs have added piezoelectric material, such as a layer of thin film of aluminum nitride (AlN), as described in Non-patent document 2, for example, identified below. In a typical piezoelectric MEMS resonator, a thin film of molybdenum may be sputtered onto the silicon followed by a layer of AlN and an additional layer of molybdenum. After thin film deposition, the metal layers, the AlN layer and the silicon are etched to form the resonator shape. With the resulting design, the lower and upper layers of molybdenum serve as electrodes to excite and detect the mechanical vibrations of the resonator.
One limitation with the design of the film bulk acoustic resonator 10 shown in
The asymmetrical design causes vibrations in the thickness direction of the resonator that result in energy leakage out of the resonator. Typically, the piezoelectric MEMS resonators have quality factors that are about an order of magnitude lower than bare silicon resonators at the same frequency. This low quality factor increases the noise in oscillator applications and increases the motional impedance.
One design that attempts to overcome the low quality factor of piezoelectric MEMS resonators is to increase the size of the resonator by using a higher order overtone design, for example, as described in Patent document 1, identified below. While a higher order overtone design directly decreases the motional resistance, it also increases the size of the resonator. Moreover, since the manufacturing cost of the resonator is proportional to the size, the larger resonator size is not preferred. In addition, even for larger resonators, the low motional impedance is still not sufficient for low noise oscillator applications and a higher quality factor is required.
Accordingly, the MEMS resonator disclosed herein increases the quality factor of the resonator that results in lower motional impedance without increasing the resonator size.
In one embodiment, the MEMS resonator includes a silicon layer having opposing surfaces, a piezoelectric layer above one of the surfaces of the silicon layer, and a pair of electrode (e.g., metal layers) disposed on opposing surfaces of the piezoelectric layer, respectively. Moreover, the piezoelectric layer has a crystallographic axis that extends at an angle relative to the vertical axis of the MEMS resonator.
In a refinement of the embodiment, the MEMS resonator is a face shear mode resonator that primarily vibrates in an x,y plane of the piezoelectric layer when the pair of electrodes are excited with an electrical current. In yet a further aspect, the MEMS resonator can be a Lamé-mode resonator.
Preferably, the crystallographic axis of the piezoelectric layer extends in the direction at an angle that is equal to or greater than 10°. Preferably, the angle is between 40° and 75° (and ideally at 56°) relative to the direction orthogonal to the first surface of the silicon layer.
In one embodiment, the piezoelectric layer comprises aluminum nitride (AlN) and the pair of metal layers comprise molybdenum (Mo).
In another embodiment, adjacent edges of the piezoelectric layer are oriented along the [100] and [010] directions relative a crystal axis of the silicon layer.
In another embodiment, adjacent edges of the piezoelectric layer are oriented along the [110] directions relative a crystal axis of the silicon layer.
Preferably, the silicon layer, the piezoelectric layer, and the pair of metal layers collectively form a square shape in the x,y plane. Moreover, the square shape of the MEMS resonator has sides, preferably, each with a length between 20 μm and 300 μm. Moreover, the thickness of the MEMS resonator can be between 5 μm and 30 μm.
In another embodiment, the silicon layer forms one of the pair of electrodes.
In another embodiment, a MEMS resonator array comprising a plurality of MEMS resonators is provided. In this embodiment, MEMS resonator a silicon layer having a first surface and a second surface opposite the first surface; a piezoelectric layer disposed above the first surface of the silicon layer and having a crystallographic axis extending in a direction at an angle greater than 0° relative to a direction orthogonal to the first surface of the silicon layer; and a pair of metal layers disposed on opposing surfaces of the piezoelectric layer, respectively. Moreover, each of the plurality of MEMS resonators is arranged laterally with respect to each other as an array.
The above simplified summary of example embodiments serves to provide a basic understanding of the present disclosure. This summary is not an extensive overview of all contemplated aspects, and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects of the present disclosure. Its sole purpose is to present one or more aspects in a simplified form as a prelude to the more detailed description of the disclosure that follows. To the accomplishment of the foregoing, the one or more aspects of the present disclosure include the features described and particularly pointed out in the claims.
The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more example embodiments of the present disclosure and, together with the detailed description, serve to explain their principles and implementations. The drawings provided are for illustrative purposes only and are therefore not drawn to scale.
Example aspects are described herein in the context of a MEMS resonator or MEMS resonator array, and, more particularly, Lamé-mode resonators and other face shear mode resonators where the vibrational motion is in the x,y plane of the resonator and does not vary significant in the thickness direction, i.e., z plane of the resonator. Preferably, the MEMS device described herein can be used in any MEMS based devices that benefit from a high quality factor, such as sensor including gyroscopes and bolometers, for example.
Those of ordinary skill in the art will realize that the following description is illustrative only and is not intended to be in any way limiting. Other aspects will readily suggest themselves to those skilled in the art having the benefit of this disclosure. Reference will now be made in detail to implementations of the example aspects as illustrated in the accompanying drawings. The same reference indicators will be used to the extent possible throughout the drawings and the following description to refer to the same or like items.
According to the exemplary embodiment, the resonator 200 is formed from a square shaped plate with a length of each side between 20 μm to 300 μm. Moreover, the resonator 200 preferably has a thickness of the plate between 5 μm to 30 μm.
Conventionally, pure Lamé-mode resonators that have been made of single crystal silicon have demonstrated very high quality factors. For example, this type of design for a Lamé-mode resonator can achieve a quality factor of over 1 million at 5 MHz for electrostatically actuated MEMS resonators, for example. Unfortunately, using a single crystal silicon for Lamé-mode resonators does not result in an effective electrostatic coupling. For example, the coupling coefficient may be less than 0.1%, which is certainly too low in many applications and results in too large of a motional impedance.
In contrast, piezoelectric MEMS resonators provide improved coupling coefficient. As described above with respect to
In addition, the thickness of the metal electrodes is typically 50 nm to 400 nm and the thickness of the piezoelectric film is typically 400 nm to 2 μm. Additional thin film layers may also be provided on such resonators. For example, silicon dioxide thin film can be used to change the temperature coefficient of frequency of the resonator.
However, conventional Lamé-mode resonators are not effectively actuated with the piezoelectric thin film. This is because Lamé-mode operate pure shear resonance mode, meaning that the motion is pure shear deformation and the volume of the resonator is unchanged, as noted above.
According to an exemplary embodiment of the present disclosure, a MEMS resonator is provided with a resonance mode being a Lamé mode. In particular, the c-axis (i.e., the vertical crystallographic axis) of the piezoelectric layer is formed to effectively actuate the resonator 200.
Specifically,
According to the current embodiment, the resonator 200 is effectively coupled by orienting the c-axis of the piezoelectric layer 320 at an angle relative to the vertical axis (z-axis). In other words, the piezoelectric layer is disposed above the first surface of the silicon layer and has a c-axis that extends in a direction at an angle relative to a direction orthogonal to the first surface of the silicon layer.
As described in more detail below, it is known that for lower quality polycrystalline piezoelectric films, and even for high quality piezoelectric film, there may be some slight variation for individual crystal alignment during the manufacturing process. In other words, although the majority of crystals are closely aligned with the vertical axis, which is a direct effect of the manufacturing process (discussed below), there will be some unintended spread/variation on an individual crystal basis even though the average for the crystal alignment will be equal to the vertical axis or within a single degree. In contrast and as will be discussed in detail below, the piezoelectric layer 320 according to the exemplary embodiment is intentionally manufactured such that the average angle for crystals of the layer relative to the vertical axis is not insignificant. Thus, to obtain an effective coupling coefficient, the average angle for crystal alignment of the piezoelectric layer 320 must be at least 5°, although the preferred angles will be discussed below with respect to
In particular, as shown
In operation, when an electrical signal is applied between the pair of metal electrodes, the piezoelectric thin film deforms due to the piezoelectric effect. The deformation in the thickness direction (z-axis) is commonly used for actuation of thin film bulk acoustic mode resonators that vibrate in thickness mode. In thickness mode, these resonator vibrate mainly in the thickness direction and the mode can be visualized as two opposing surfaces (top and bottom surfaces) moving out of phase up and down. In addition to the thickness deformation, the piezoelectric film deforms in lateral directions (x and y axis). In conventional resonator devices, there is no preferential lateral direction for the preferential lateral deformations and the lateral stress generated by the piezoelectric effect is equal in all directions in x,y plane. Moreover, the lateral piezoelectric effect has been successfully applied for extensional mode resonators, for example, longitudinal mode beam resonators and extensional plate resonators that vibrate in x,y plane.
In contrast, shear mode resonances, such as face shear modes or Lamé-mode resonances, cannot be excited in conventional structures as there is no preferential direction for the lateral piezoelectric effect. Referring to
Due to the vertical c-axis of the piezoelectric layer 420 of such MEMS designs, when a voltage is applied between the top and bottom electrodes 430a and 430b, the electric field causes stress deformation in the piezoelectric thin film 420. This stress deformation effectively actuates extensional modes such as length extension, width extension, or square extension modes. However, due to this design, pure face shear modes, such as the Lamé mode, cannot be actuated.
Typically, during piezoelectric thin film deposition, the c-axis of the piezoelectric film is aligned along z-axis. This alignment follows from the symmetry properties of the deposition substrate and the physical arrangement of the deposition process. Conventionally, neither of them has any preferential in-plane component and the piezoelectric material grows preferentially in crystal structure where the average crystal is aligned to the z-direction.
Conventionally, in a high quality piezoelectric film, the majority of the crystals are closely aligned to the z-axis direction with the spread of the individual crystal alignment generally less than 0.5° from the vertical axis. In lower quality polycrystalline piezoelectric films, the spread of individual crystal alignments can be up to 15 degrees depending on film growth conditions, but the average alignment is still typically in z-direction. By changing the physical arrangement of the deposition, for example, by depositing the piezoelectric material in an oblique angle relative to sample surface, it is possible to grow piezoelectric material where the average c-axis is in angle relative to the z-axis as described herein according to the exemplary embodiment.
In contrast, referring back to
According to an exemplary embodiment, the c-axis of the piezoelectric layer 320 is preferably between 40° and 75° relative to the z or vertical axis. As shown in the graph of
According to the exemplary embodiment, the preferred c-axis for piezoelectric crystal is between 40 to 75 degrees. However, it is appreciated that growing crystals with such a large angle relative to the vertical axis may not be economical due to manufacturing constraints or the like. As shown in
For MEMS resonators implemented in timing device applications, it is important that the resonator has good temperature stability, as would be understood to one skilled in the art. Moreover, uncompensated Lamé-mode resonators show a negative temperature coefficient of frequency (TCF). This TCF can be compensated by doping the silicon substrate to make the TCF more positive.
In this case, L is the sides length of the resonator 600a and ρ is the density of the silicon. In general, for silicon resonators, the TCF is largely determined by the elastic constant. According to this orientation, the temperature dependency (i.e., the TCF) is determined by the elastic constants C11 and C12 of silicon. It is known that C11=16.60*1011 dyn/cm2 and that C12=6.40*1011 dyn/cm2 and these constant have negative temperature coefficient of temperature. Thus, a resonator made of silicon has negative TCF. The TCF can be made more positive by doping the silicon heavily with phosphorus or other n-type dopant. In a preferred embodiment of the resonator 600a, the phosphorus doping density is over 2*1019 1/cm3.
Moreover,
Again, L is the sides length of the resonator and ρ the density of the silicon. According to this orientation, the temperature coefficient of the shear modulus C44 can be made more positive by doping the silicon heavily with boron or other p-type dopants. This results in more positive TCF. In a preferred embodiment of the resonator 600b, the boron doping density is over 1020 1/cm3.
It should be appreciated that for both resonators 600a and 600b, as described above, the resonance frequency is mainly determined by the lateral resonator dimension, i.e., the length L, of the sides of the resonators 600a and 600b and not by the resonator thickness. This is significantly different than a thickness shear mode resonator, such as the bulk film acoustic resonator 10 shown in
Moreover, thickness shear mode resonators typically do not have single crystal silicon as a structural layer. Metals and most piezoelectric thin-films, such as aluminum nitride, has positive TCF that makes them unsuitable for timing reference applications without additional compensation. As already mentioned, silicon TCF can be made positive by doping the silicon. By adjusting the silicon thickness and doping level relative to the thin-film properties, it is possible to make resonator with nearly zero TCF. In typical application, the silicon thickness ranges from 3 μm to 30 μm. In addition, thin film of silicon dioxide can be incorporated in the resonator to make the TCF even more positive.
Moreover, it should be appreciated that each resonator 700a, 700b, 700c, and 700d, can correspond to resonator 200 illustrated in
It should be appreciated that according to alternative embodiments, the metal sample can be positioned at an angle during the ion beam irradiation or the ions can be redirected with another ion beam. As long as the angle of the incidence of sputtering is changed to be at an angle, the c-axis of the resulting piezoelectric layer will be tilted according to the exemplary embodiment.
Finally, it should be appreciated that a benefit of the Lamé-mode resonator according to the exemplary embodiment is that the device is a face shear mode resonator. As described above, the resonator volume does not change during the vibration cycle, and, thus, the quality factor of the resonator can be much larger than for the typical extensional mode resonators. Moreover, the TCF of the face shear mode is more easily compensated than the extensional mode as the dopant density does not need to be as high to achieve zero first order TCF, as described above with respect to
In the interest of clarity, not all of the routine features of the embodiments are disclosed herein. It should be appreciated that in the development of any actual implementation of the present disclosure, numerous implementation-specific decisions must be made in order to achieve the designer's specific goals, and these specific goals will vary for different implementations and different designers. It is understood that such a design effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the art, having the benefit of this disclosure.
Furthermore, it is to be understood that the phraseology or terminology used herein is for the purpose of description and not of restriction, such that the terminology or phraseology of the present specification is to be interpreted by those skilled in the art in light of the teachings and guidance presented herein, in combination with the knowledge of the skilled in the relevant art(s). Moreover, it is not intended for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such.
While the foregoing has been described in conjunction with exemplary embodiments, it is understood that the term “exemplary” is merely meant as an example. Accordingly, the application is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the MEMS resonator disclosed herein.
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