INTERNALLY TRANSDUCED PN-DIODE-BASED ULTRA HIGH FREQUENCY MICROMECHANICAL RESONATOR
专利汇可以提供INTERNALLY TRANSDUCED PN-DIODE-BASED ULTRA HIGH FREQUENCY MICROMECHANICAL RESONATOR专利检索，专利查询，专利分析的服务。并且A radio frequency microelectromechanical (RF MEMS) device can comprise an actuation p-n junction and a sensing p-n junction formed within a semiconductor substrate. The RF MEMS device can be configured to operate in a mode in which an excitation voltage is applied across the actuation p-n junction varying a non-mobile charge within the actuation p-n junction to modulate an electric field acting upon dopant ions and creating electrostatic forces. The electrostatic forces can create a mechanical motion within the actuation p-n junction. The mechanical motion can modulate a depletion capacitance of the sensing p-n junction, thereby creating a motional current. At least one of the p-n junctions can be located at an optimal location to maximize the efficiency of the RF MEMS device at high resonant frequencies.，下面是INTERNALLY TRANSDUCED PN-DIODE-BASED ULTRA HIGH FREQUENCY MICROMECHANICAL RESONATOR专利的具体信息内容。
This application claims priority benefit of U.S. Provisional Patent Application Ser. No. 61/369,124, filed Jul. 30, 2010, the disclosure of which in its entirety is hereby incorporated herein by reference.
This invention was made with Government support under grant number W911QX-07-C-0086 awarded by the Army Research Laboratories. The Government has certain rights in the invention.
This invention relates generally to microelectromechanical devices and, more specifically, to radio frequency microelectromechanical resonators.
The potential of micromechanical devices as high quality factor resonators has been recognized since the seminal paper by Nathanson et al. describing the resonant gate transistor [1]. Continued research in micromechanical resonators has since pushed resonant frequencies into the multi-gigahertz range while improving transduction efficiency and reducing motional impedance.
In one aspect, there is provided a radio frequency microelectromechanical (RF MEMS) device comprising an actuation p-n junction and a sensing p-n junction formed within a semiconductor substrate. The RF MEMS device can be configured to operate in a mode in which an excitation voltage is applied across the actuation p-n junction varying a non-mobile charge within the actuation p-n junction to modulate an electric field acting upon dopant ions and creating electrostatic forces. The electrostatic forces can create a mechanical motion within the actuation p-n junction. The mechanical motion can modulate a depletion capacitance of the sensing p-n junction, thereby creating a motional current. At least one of the p-n junctions can be located at an optimal location to maximize the efficiency of the RF MEMS device at high resonant frequencies.
In one embodiment, the mechanical motion within the actuating p-n junction can occur in a direction orthogonal to the substrate plane.
In one embodiment, the mechanical motion within the actuating p-n junction can occurs in a direction parallel to the substrate plane.
In one embodiment, the semiconductor substrate can be provided by a monocrystalline silicon wafer.
In one embodiment, at least one of the p-n junctions can be located at a minimum displacement location.
In one embodiment, the RF MEMS device can further comprise two conductive pads formed on the surface of the semiconductor substrate. One conductive pad can be electrically coupled to the sensing p-n junction, and the other conductive pad can be electrically coupled to the actuation p-n junction.
In one embodiment, the RF MEMS device can be configured to operate as a high-Q resonator.
In one embodiment, the actuation p-n junction can be reverse-biased.
In one embodiment, the thickness of said semiconductor substrate can be chosen to achieve a desired resonant frequency.
In another aspect, there is provided an RF MEMS device comprising an actuation p-n junction and a sensing p-n junction formed within a semiconductor substrate. An equivalent circuit model of the RF MEMS device can comprise a first shunt capacitance equivalent to a depletion capacitance of the sensing junction and a second shunt capacitance equivalent to a depletion capacitance of the actuation junction. The equivalent circuit model can further comprise a motional capacitance, a motional impedance, and a motional inductance connected in series with the first shunt capacitance and the second shunt capacitance. The equivalent circuit model can further comprise a third shunt capacitance connected in series with the first shunt capacitance and the second shunt capacitance.
In another aspect, there is provided a method of operating an RF MEMS device comprising an actuation p-n junction and a sensing p-n junction. The method can comprise the step of applying an excitation voltage across the actuation p-n junction causing a non-mobile charge within the actuation p-n junction to produce an electric field acting upon dopant ions and creating electrostatic forces. The electrostatic forces can create a mechanical motion within the actuation p-n junction. The mechanical motion can modulate a depletion capacitance of the sensing p-n junction, thereby creating a motional current.
In one embodiment, the RF MEMS device can be configured to operate as a high-Q resonator.
In another aspect, there is provided a method of manufacturing an RF MEMS device. The method can comprise the steps of selectively doping a semiconductor-on-insulator (SOI) wafer using boron implantation to create two p-n junctions, annealing the SOI wafer to achieve a desired junction depth, etching a device layer of the SOI wafer to define a resonator and two pads, depositing and etching a conductive layer to define interconnect and pad structures, and releasing the device layer.
In one embodiment, the device can be etched by a reactive ion etch of a monocrystalline device layer of an SOI wafer.
In one embodiment, the desired junction depth can be equal to 0.65 μm.
In one embodiment, the thickness of the device layer can be equal to 1.3 μm.
In one embodiment, the resistivity of the device layer is less than 0.006 Ω·cm.
In one embodiment, the conductive layer can be provided by a 50-nm chromium layer and a 100-nm aluminum layer.
In one embodiment, the step of releasing said device layer can be performed by hydrogen fluoride (HF) etching.
In one embodiment, the thickness of said device layer of SOI wafer can be chosen to achieve a desired resonant frequency.
In one embodiment, the RF MEMS device can be configured to operate as a high-Q resonator.
In one embodiment, at least one of the p-n junctions can be located at a minimum displacement location.
The features described herein can be better understood with reference to the drawings described below. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the drawings, like numerals are used to indicate like parts throughout the various views.
In one embodiment, there is provided a radio frequency (RF) microelectromechanical (MEMS) resonator actuated by electrostatic forces acting on the immobile charge within the depletion region of a reverse-bias pn-diode. Due to the internal nature of depletion-layer transduction (i.e., the force is applied within the resonator), such resonators can be efficiently actuated at high frequencies when the junction is placed at optimal locations within the resonator [2].
The electrostatic force distribution schematically shown in
where e is the elementary charge, N is the symmetric doping concentration, εsi is the relative permittivity of silicon, ε0 is the permittivity of free space, Aj is the junction area, and 2zd is the junction depletion width given by the expression
where Φbi is the junction built-in voltage.
Equations (1)-(4) indicate that this function is strongly nonlinear with the input AC voltage described by vin=v0e−jω0t. If v0<<Φbi−VDC, then (3) can be linearized around the bias point, yielding the distributed force at the excitation frequency
This force distribution yields the following equation of motion:
Here, ρ is the mass density, E is the Young's Modulus, b is the loss factor, da is the location of the actuation junction, A is the resonator cross-sectional area, and u(z,t) is the displacement field within the resonator. This field can be described by
From this point forward, the subscripts a and s are used herein to differentiate between parameters of the actuation and sensing junctions, respectively. The spatial shift of the force'distribution is necessary since the expressions for the force distribution assume the junction is located at z=0. This shift does not change the analytical form of any of these expression's, only the ranges in which they apply. Multiplying equation (6) by the mode shape and integrating over the thickness d of the resonator allows one to find the electromechanical transduction efficiency of the actuation junction
where zd0,a=zd,a(vin=0).
Using a two-port configuration, a similar pn-junction 134 can be used to sense the motion at the output (see
As noted herein supra, RF MEMS device 110 can be efficiently actuated at high frequencies when the actuation and sensing junctions are placed at optimal locations within the resonator. To achieve direct actuation of a longitudinal bulk mode resonator using pn-diodes without Poisson ratio penalty (caused by converting perpendicular stress into stress in the direction of motion), the p-n diode should be oriented in the direction of the main resonant mode. In such a configuration, depending on how symmetrically doped the diode is, the optimal locations of the actuation and sensing junctions with respect to the frequency setting dimension will change. For a completely symmetric p-n junction, the optimal locations of the actuation and sensing junctions are at displacement nodes. When the junctions are placed at optimal locations, the transduction efficiency increases with increasing frequency.
An equivalent Butterworth-Van Dyke (BVD) circuit model of the RF MEMS device 100 is now being described with references to
Assuming small displacement (Uo<<Wd,s), the capacitive sensing efficiency can be expressed as follows:
Defining the motional impedance to be RX≡vo/im, this can be expressed as
Equations (7)-(10) show the strong dependence of the motional impedance on the junction locations and the depletion layer widths, similar to what is shown in [2].
Thus, to maximize the efficiency of RF MEMS device 100 at high resonant frequency, the junctions should be placed at minimum displacement locations (maximum strain locations). It is also true that the motional impedance will exhibit a minimum near frequencies where the depletion width approaches half of the acoustic wavelength.
The rest of the Butterworth-Van Dyke model is obtained by using the expressions for LX and CX given by
As is the case with other electrostatically actuated resonators, there is a static capacitance associated with the transducer. For this resonator, the static capacitance is equal to the depletion capacitance given by
This capacitance usually sets the sensitivity of the resonance in electrostatically transduced resonators, but the dual p-n junction configuration used in the resonator may make possible some self-gain mechanism (e.g., BJT configuration) to boost the resonant signal, as demonstrated in air-gap FET sensing [3].
Note that the preceding analysis assumes quasi-static electrical behavior, i.e., the charge carriers are able to respond within a time much shorter than the period of the input stimulus. This requires both junctions to be in reverse bias where electrical dynamic behavior is governed by majority carriers. Dynamics in forward bias are governed by minority carriers, which respond much more slowly.
Thus, an equivalent Butterworth-Van Dyke (BVD) circuit model of the RF MEMS device 100 can comprise a motional capacitance 310, a motional impedance 320, and a motional inductance 330, all connected in series, as illustrated by
Using the above described BVD model allows for an easy simulation of the RF MEMS device in conventional circuit simulators, greatly simplifying the design of oscillators and filters using these devices.
As noted herein supra, in one embodiment, RF MEMS device 100 can be designed to operate in the vertical motion mode, as schematically shown in
In another embodiment, RF MEMS device 100 can be designed to operate in the lateral motion mode, as schematically shown in
In another aspect, a single chip can comprise many different lateral motion mode resonators having different resonant frequencies.
Experimental results obtained by the inventors are now being described with references to
One embodiment of a method of manufacturing an RF MEMS device is now being described with references to
Then, at step (b), the SOI wafer can then be annealed to achieve a desired junction depth. In an exemplary embodiment, a rapid thermal annealing at 1100° C. for 45 seconds can be used for dopant activation and to create the junction at the desired depth of 0.65 μm.
Next, at step (c), the device layer can be etched to define a resonator and two pads. Then, interconnect and pad structures can be defined by depositing an etching a conductive layer. In an exemplary embodiment, the interconnect and pad structures can be defined by sputter deposition and wet etching of 50 nm chromium (for adhesion and to prevent spiking) and 100 nm aluminum.
Finally, at step (d), the device layer can be released. In an exemplary embodiment, the device layer can be released using hydrogen fluoride (HF) etching.
While the present invention has been described with reference to a number of specific embodiments, it will be understood that the true spirit and scope of the invention should be determined only with respect to claims that can be supported by the present specification. Further, while in numerous cases herein wherein systems and apparatuses and methods are described as having a certain number of elements it will be understood that such systems, apparatuses and methods can be practiced with fewer than the mentioned certain number of elements. Also, while a number of particular embodiments have been described, it will be understood that features and aspects that have been described with reference to each particular embodiment can be used with each remaining particularly described embodiment.
Citation of a reference herein shall not be construed as an admission that such reference is prior art to the present invention. The following references, to the extent that they provide exemplary procedural or other details supplementary to those set forth herein, are specifically incorporated herein by reference:
[1] H. C. Nathanson, W. E. Newell, R. A. Wickstrom, J. R. Davis, Jr., “The resonant gate transistor,” IEEE Trans. Elec. Dev., 14 (3), pp. 117-133 (1967).
[2] D. Weinstein, S. A. Bhave, “Internal dielectric transduction of a 4.5 GHz silicon bar resonator,” IEDM 2007, pp. 415-418.
[3] D. Grogg, M. Mazza, D. Tsamados, A. M. Ionescu, “Multi-gate vibrating-body field effect transistor (VB-FETs),” IEDM 2008, pp. 663-666.
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