Patent Application
20070285191
The following description of the embodiment below is merely an example and is in no way intended to limit the invention or its application or uses. The present invention discloses an SMR resonator device and a method for manufacturing an SMR resonator with a highly efficient all-dielectric acoustic reflector constructed with alternating layers of low acoustic impedance material such as SiO2 and high acoustic impedance material such as SiC, Si-DLC or DLC.
Referring to
If DLC or Si-DLC is used instead of SiC then a modified deposition chamber is employed. DLC is deposited, for example, in a 150 mm parallel plate reactor RFCVD chamber with the upper plate being the gas distribution source and the lower plate being the cathode on which the substrate lies. In this configuration, the upper plate is positive and the cathode is negative (grounded). An RF source, typically 13.56 MHz, is coupled directly to the substrate through the cathode. After the chamber is evacuated, any hydrocarbon gas, typically but not limited to CH4, and/or a Si containing hydrocarbon forming gas if Si-doping is required, typically but not limited to tetramethyidisiloxane (4MS), is introduced into the chamber until the desired pressure is achieved and flow is stable. Other gases such as Ar and H2 can be used in addition to the hydrocarbon forming gases to control the chemical composition of the final DLC film. At this point power is delivered to the cathode to strike a plasma and DLC is deposited for a fixed amount of time until the desired thickness is achieved. Next the power is shut off and the chamber is vented using an inert gas, typically Ar or N2, until ambient pressure is reached and the DLC deposited substrate is removed. Variables that affect DLC physical properties include: RF power, pressure, total gas flow, different gas ratios and cathode to upper plate spacing. Prior to DLC deposition, an Ar plasma is used to pre-condition the substrate surface for the deposition for 1-2 minutes. DLC deposition is typically done at ambient temperatures. DLC thickness and index of refraction can be measured directly using, for example, a pre-calibrated ellipsometer.
Next, the acoustic Bragg reflector is completed by depositing alternate layers of low and high acoustic impedance material 295-291. In one example, the thickness of the individual layers of low and high acoustic impedance materials 297-291 are chosen to be equal to one quarter wavelength of the resonant frequency of the device. Once the acoustic Bragg reflector 290 is complete the next step is to deposit the first resonator electrode 280. In one embodiment, the resonator electrode is sputter deposited. In one example, the material for the first electrode is molybdenum (Mo) although other materials will be apparent to those skilled in the art such as, but not limited to, Ti, W, Au, Pt or Al. In one example, the material for the resonator electrode, can be, a low thermoelastic loss and can have a thickness of less than 1000 A in order to maintain a smooth surface.
After the first resonator electrode 280 has been deposited the next step is to deposit the piezoelectric material 270. One material for the piezoelectric layer, can be, aluminum nitride (AlN) although other materials will be apparent to those skilled in the art such as, but not limited to, zinc oxide (ZnO) or lead zirconate titanate (PZT). In one example the AIN layer is reactively sputter deposited with nitrogen gas using an optimized process yielding a low stress, dense layer with a c-axis orientation. The thickness of the piezoelectric layer 270, can be, in the range from 0.1 to 10 microns.
Finally a top electrode 280 is deposited to complete the resonator 260. Again the top electrode, can be, a sputter deposited layer of Mo. The above embodiment is given as an illustrative example only, and is not intended to impose any limitations on the invention.
Turning to
Although the methodology 400 is illustrated and described hereinafter as a series of acts or events, it will be appreciated that the present invention is not limited by the illustrated ordering of such acts or events. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein. In addition, not all illustrated steps may be required to implement a methodology in accordance with one or more aspects of the present invention. Further, one or more of the acts may be carried out in one or more separate acts or phases.
It will be appreciated that a methodology carried out according to one or more aspects of the present invention may be implemented in association with the formation and/or processing of structures illustrated and described herein as well as in association with other structures not illustrated or described herein. By way of example, the method or variants thereof may be used to fabricate a SMR as illustrated and described below with respect to
The Solidly Mounted Resonator (SMR) is formed upon a semiconductor substrate 300 that is provided at process step 402. The substrate 300 can be, for example, formed out of silicon such as a single crystal silicon wafer, GaAs, AlAs, and the like. At 404 a first layer of low acoustic impedance material 297 in
Other suitable novel low acoustic impedance materials belong to the range of materials used in advanced semiconductor devices to provide a low dielectric constant. These so-called low-k materials are usually based on spin-on siloxane monomers with additives to promote the formation of pores. The degree of porosity is generally related to the k or dielectric constant of the film. In the case of low acoustic impedance materials high porosity also means low density and low sound velocity, both of which translate into a low acoustic impedance. Suitable specific materials are nano-porous methyl silsesquioxane (MSQ), nano-porous hydrogensilsesquioxane (HSQ), nano-porous mixtures of MSQ and HSQ, nanoglass, aerogel, or a variety of other spin-on-glasses (SOG) with the deliberate addition of pores in the final film.
In
Other gases such as Argon (Ar) and Hydrogen (H2) can be used additionally with the hydrocarbon forming gases to direct the chemical composition of the final diamond-like carbon film. At this point power is delivered to the cathode to cause plasma formation and the DLC is deposited for a pre-determined time or until the preferred thickness is obtained, for example. The power is then shut off and inert gas is used to vent the chamber, typically Ar or N2. Once ambient pressure is attained, the high acoustic impedance material layered substrate is removed. Variables that affect DLC physical properties include, for example, RF power, pressure, total gas flow, different gas ratios, cathode to upper plate spacing, and the like. Prior to DLC deposition, Argon plasma is used to pre-condition the substrate surface for the deposition for 1-2 minutes. DLC deposition is typically done at ambient temperatures. DLC thickness and index of refraction can be measured directly using a pre-calibrated ellipsometer. An ellipsometer is the most common way of measuring the thickness of thin films and is based on the detection of a phase shift of a polarized incident light beam during incidence and reflection from the surface.
At 408 of the illustrated method 400 alternate layers of low acoustic impedance material (297, 295, 293, and 291) and high acoustic impedance material 296, 294, and 292) are deposited on the layered substrate. The thickness of the various low and high impedance layers can be, for example chosen to be equal to one quarter wavelength of the resonant frequency of the device. Once the layer criteria at 408 have been met the acoustic Bragg reflector is complete and the method 400 continues at step 410. At step 410 the first layer of a resonant electrode 280 is deposited on the acoustic Bragg reflector that was fabricated at the end of step 408. The resonant electrode 280 can be, for example, sputter deposited by methods well known by those skilled in the art. Molybdenum is the favored material in one example; however other materials will be apparent to those skilled in the art, for example, Titanium (Ti), Tungsten (W), Gold (Au), Platinum (Pt), Aluminum (Al), and the like. In one example the material selected for the resonator electrode 280 preferably has a low thermoelastic loss and preferably has a thickness of less than 1000 Å in order to maintain a smooth resonator electrode 280 surface, for example.
After the first resonator electrode 280 at step 410 has been deposited the next step 412 is the deposit of the piezoelectric material 270, for example. Piezoelectric materials create an electric field when subjected to a change in dimension caused by an imposed mechanical force (known as piezoelectric or generator effect). On the other hand, an applied electric field to the piezoelectric material results in a mechanical stress (known as electrostrictive or motor effect). In one example, the material for the piezoelectric layer 270 can be Aluminum Nitride (Al—N) although other materials will be apparent to those skilled in the art such as Zinc Oxide (ZnO), Lead Zirconate Titanate (PZT), quartz (SiO2) or Barium titanate (BaTiO3) and the like. The Al—N layer can be reactively sputter deposited with nitrogen gas using an optimized process yielding a low stress, dense layer employing a c-axis orientation. The thickness of the piezoelectric layer can be in the range from about 0.1 to 10 microns.
Finally a top electrode may be deposited to complete the resonator at step 414. The top electrode 280 can again be a sputter deposited layer of Mo. The above method is given as an illustrative example only, and is not intended to impose any limitations on the invention.
Although the invention has been shown and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art based upon a reading and understanding of this specification and the annexed drawings. The invention includes all such modifications and alterations and is limited only by the scope of the following claims. In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.”
