Patent Application
20210184652
Dense integration of multi-frequency and multi-band acoustic spectral processor is essential for realization of the emerging ultra-wideband mobile communication systems that operate based on carrier aggregation. These systems require a large set of resonators with frequencies over ultra- and super-high-frequency regimes to enable spread-spectrum data communication with minimum latency. Current radio frequency bulk acoustic wave (BAW) resonator technologies rely on planar architectures, such as film bulk acoustic resonators (FBAR) or solidly mounted resonators (SMR), with large surfaces to accommodate the required electromechanical transduction area for low-loss operation. The frequency of planar BAW resonators is tied to the thickness of the piezoelectric transducer film that is constant across the substrate. This limitation prevents single-chip integration of multi-frequency and multi-band spectral processors needed for carrier aggregation. Furthermore, planar BAW resonators occupy large chip area since their loss is inversely proportional to the electrode surface dimensions of the piezoelectric film. This becomes more pronounced in carrier aggregation schemes that require several spectral processors at various frequencies and impose excessive integration costs and challenges. An alternative architecture that miniaturizes the planar footprint relies on integration of aluminum nitride (AlN) piezoelectric film transducers on the sidewall of silicon fins to realize high-performance fin bulk acoustic resonators (FinBAR). FinBARs enable ultra-dense integration of high Q resonators and filters in a small chip footprint. Furthermore, operating in width-extensional bulk acoustic modes, their frequency can be lithographically tailored over wide spectrums in ultra- and super-high-frequency regimes.
Embodiments are directed to a method of fabricating a fin bulk acoustic wave resonator (FinBAR). In some embodiments, the method comprises forming a micro-fin structure on a substrate, the micro-fin structure comprising one or more sidewalls. In some embodiments, the method further comprises smoothing the one or more sidewalls. In some embodiments, the method further comprises depositing a bottom electrode layer on top of the micro-fin structure. In some embodiments, the method further comprises forming a layer of aluminum nitride (AlN) on the bottom electrode layer, where a c-axis of the aluminum nitride layer is substantially perpendicular to the one or more sidewalls of the micro-fin structure. In some embodiments, the method further comprises forming a top electrode layer on top of the layer of aluminum nitride (AlN). In some embodiments, the method further comprises patterning the top electrode layer and etching the layer of aluminum nitride (AlN) to create access windows to the bottom electrode layer.
In some embodiments, the substrate and micro-fin structure comprise silicon.
In some embodiments, smoothing the one or more sidewalls comprises annealing.
In some embodiments, the annealing comprises hydrogen (H2) at 1100C.
In some embodiments, smoothing the one or more sidewalls comprises treatment in RF plasma discharge at a power of 70W providing argon (Ar) ion bombardment.
In some embodiments, the bottom electrode layer comprises molybdenum (Mo).
In some embodiments, the method further comprises depositing a seed layer of aluminum nitride (AlN) on the micro-fin structure. In some embodiments, the molybdenum (Mo) comprised in the bottom electrode is sputtered on the seed layer.
In some embodiments, the bottom electrode layer comprises platinum (Pt).
In some embodiments, the platinum (Pt) has a thickness of about 30 nanometers.
In some embodiments, the top electrode layer comprises molybdenum (Mo) with a thickness of about 50 nanometers.
In some embodiments, the micro-fin structure is formed using a deep reactive ion etching technique. In some embodiments, the micro-fin structure is formed using a number of cycles with each cycle comprising a nearly isotropic etching step and a step of deposition of a passivation layer.
In some embodiments, the layer of aluminum nitride has a thickness of about 720 nanometers. In some embodiments, the aluminum nitride layer is formed by a reactive sputtering technique at a base pressure of less than 2×10−10 bar and a power of about 5.5 kW.
In some embodiments, the reactive sputtering technique uses Argon (Ar) and nitrogen (N2) gas flows of about 3 and 15 standard cubic centimeters per minute (SCCM) respectively.
In some embodiments, the layer of aluminum nitride is etched using a tetramethylammonium hydroxide (TMAH) solution at about 50° C. as an etchant to create the access windows to the bottom electrode layer.
So that the present disclosure can be understood by those of ordinary skill in the art, a more detailed description can be had by reference to aspects of some illustrative embodiments, some of which are shown in the accompanying drawings.
In accordance with common practice some features illustrated in the drawings cannot be drawn to scale. Accordingly, the dimensions of some features can be arbitrarily expanded or reduced for clarity. In addition, some of the drawings cannot depict all of the components of a given system, method or device. Finally, like reference numerals can be used to denote like features throughout the specification and figures.
Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the some described embodiments. However, it will be apparent to one of ordinary skill in the art that the some described embodiments may be practiced without these specific details. In other instances, well-known methods, procedures, components, circuits, and networks have not been described in detail so as not to unnecessarily obscure aspects of the embodiments.
It will also be understood that, although the terms first, second, etc. are, in some instances, used herein to describe some elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first contact could be termed a second contact, and, similarly, a second contact could be termed a first contact, without departing from the scope of the some described embodiments. The first contact and the second contact are both contacts, but they are not the same contact, unless the context clearly indicates otherwise.
The terminology used in the description of the some described embodiments herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used in the description of the some described embodiments and the appended claims, the singular forms “a,”, “an,” and “the” are intended to comprise the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term “and/or” as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms “comprises,” “comprising,” “comprises,” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
As used herein, the term “if” is, optionally, construed to mean “when” or “upon” or “in response to determining” or “in response to detecting,” depending on the context. Similarly, the phrase “if it is determined” or “if [a stated condition or event] is detected” is, optionally, construed to mean “upon determining” or “in response to determining” or “upon detecting [the stated condition or event]” or “in response to detecting [the stated condition or event],” depending on the context.
It should be appreciated that in the development of any actual embodiment (as in any development project), numerous decisions must be made to achieve the developers' specific goals (e.g., compliance with system and business-related constraints), and that these goals will vary from one embodiment to another. It will also be appreciated that such development efforts might be complex and time consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art of FinBAR having the benefit of this disclosure.
FinBARs are ideally poised to provide a superior keff2 (effective electromechanical coefficient) and Q (quality factor), compared to planar BAW and contour mode resonators, due to the low acoustic dissipation in Si and the large piezoelectric coefficient d33 of AlN (Aluminum Nitride) sidewall transducer film. In practice, however, the performance of FinBAR is limited by the texture and crystalline orientation of sidewall AlN film.
Referring to
In some embodiments, the aluminum nitride layer 140 is positioned above the bottom electrode layer 130. In some embodiments, the aluminum nitride layer 140 covers at least portions of the bottom electrode layer 130. In some embodiments, the aluminum nitride layer 140 comprises textured sidewall AlN films. A c-axis orientation of the textured sidewall AlN films is substantially perpendicular to one or more sidewalls of the micro-fin 120. That is, the crystalline orientation of the textured sidewall AlN films is substantially perpendicular to the one or more sidewalls of the micro-fin 120. In some embodiments, the aluminum nitride layer 140 comprises densely textured AlN films with substantially perpendicular (e.g., 90°±5° in some embodiments) c-axis on the one or more sidewalls of the micro-fin 120. In some embodiments, the top electrode layer 150 is positioned above the aluminum nitride layer 140. In some embodiments, the top electrode layer 150 covers at least portions of the aluminum nitride layer 140.
Described otherwise, a micro-fin structure is formed on a substrate, the micro-fin structure comprising one or more sidewalls. The sidewalls are smoothed. In some embodiments, a seed layer of aluminum nitride (AlN) is deposited on the micro-fin structure. In some embodiments, a bottom electrode layer is deposited on top of the seed layer. A layer of aluminum nitride (AlN) is formed on the bottom electrode layer, where a c-axis orientation of the layer of aluminum nitride is substantially perpendicular to the one or more sidewalls of the micro-fin structure. A top electrode layer is formed on top of the layer of aluminum nitride (AlN). In some embodiments, the top electrode layer is patterned, and the layer of aluminum nitride (AlN) is etched to create access windows to the bottom electrode layer. It should be noted that, in various embodiments, the steps to fabricate different components and layers of the disclosed acoustic wave resonator can take place in different order.
In some embodiments, the bottom electrode layer 130 covers at least portions of: a top surface of the substrate 110, a top surface of the micro-fin 120, and sidewalls of the micro-fin 120. In some embodiments, the substrate 110 comprises silicon. In some embodiments, the micro-fin 120 forms a pin-shape over the substrate 110. The micro-fin may comprise silicon.
The micro-fin may be fabricated by a deep reactive ion etching (DRIE) technique. DRIE is a highly anisotropic etch process used to create deep penetration, steep-sided holes and trenches in wafers/substrates. DRIE typically creates structures with high aspect ratios. In some embodiments, a Bosch DRIE is used to fabricate the micro-fin 120. The Bosch DRIE process can fabricate 90° (truly vertical) sidewalls. The Bosch process (also known as pulsed or time-multiplexed etching), alternates repeatedly between two modes, i.e., an etching mode and a depositing mode, to achieve nearly vertical micro-fins. The etching mode comprises a standard, nearly isotropic plasma etches. The plasma contains some ions, which attack the wafer from a nearly vertical direction. Sulfur hexafluoride is often used for silicon. The depositing mode comprises deposition of a chemically inert passivation layer. Each phase lasts for several seconds. A resulted passivation layer protects the entire substrate from further chemical attack and prevents further etching. However, during the etching phase, directional ions that bombard the substrate attack the passivation layer at the bottom of the trench (but not along the sides). The ions collide with the bottom of the trench and sputter it off, exposing the substrate to the chemical etchant. These etch/deposit steps are repeated many times over resulting in many very small isotropic etch steps taking place only at the bottom of the etched pits.
In some embodiments, the acoustic wave resonator system 100 is fabricated on (110) Si substrate. In some embodiments, and due to a finite number of isotropic etch and passivation cycles, sidewall surface of the micro-fin 120 suffers from roughness and scalloping. In order to reduce adverse effect of surface roughness, in some embodiments, hydrogen (H2) annealing at temperature between 650C.-1300° C. is used to smoothen the sidewalls of the micro-fin 120.
In some embodiments, the bottom electrode layer 130 comprises platinum (Pt). In some embodiments, a crystalline Pt layer is formed over portions of the substrate 110 and the micro-fin 120. The Pt layer may be deposited via an atomic layer deposition (ALD) method. In some embodiments, the ALD deposition takes place at temperature greater than 100° C. For example, the ALD deposition may take place at about 150° C. In some embodiments, the thickness of the crystalline Pt layer may be 30 nanometers.
In some other embodiments, the bottom electrode layer 130 comprises molybdenum (Mo). In some embodiments, the bottom electrode layer 130 comprises sputtered Mo deposited on a aluminum nitride (AlN) seed layer. The aluminum nitride seed layer and the Mo thin film can be formed via a physical vapor deposition (PVD) clustering method. The PVD method may be employed by an AC powered S-gun magnetron method and a DC powered S-gun magnetron method for deposition of the aluminum nitride seed layer and the Mo thin film, respectively. In some embodiments, a 20-nanometer thick AlN seed layer is formed by the PVD method. In some embodiments, the aluminum nitride seed layer is formed by the AC powered S-gun magnetron with a power of greater than 1 kW.
In some embodiments, prior to forming the aluminum nitride layer 140, the acoustic wave resonator system 100 is treated in a radio frequency (RF) plasma discharge. The RF plasma discharge may take place at power between 50W-300W. The RF plasma discharge uses argon (Ar) ion bombardment to atomically smoothen the acoustic wave resonator system 100. In some embodiments, the RF plasma discharge smoothen surfaces of the micro-fin 120. In some embodiments, the RF plasma discharge is followed by forming the aluminum nitride layer 140. The aluminum nitride layer 140 may comprise aluminum nitride (AlN). In some embodiments, the aluminum nitride layer 140 is so formed that a c-axis of the aluminum nitride layer 140 is substantially perpendicular to the sidewalls of the micro-fin 120. In some embodiments, the aluminum nitride layer 140 is formed via a reactive sputtering. The reactive sputtering can take place at a base pressure of less than about 2×1010 bar with a power greater than 3 kW, for example, with a power of 5.5 kW.
In some embodiments, the reactive sputtering uses Ar and nitrogen (N2) gas. In some embodiments, the Ar and N2 flow rates are at the ratio of about 1:3. In some embodiments, the Ar and N2 gas flows of about 5 and 17 standard cubic centimeters per minute (SCCM) are used respectively. In some embodiments, the Ar and N2 gas flows of about 3 and 15 SCCM are used respectively.
In some embodiments, the aluminum nitride layer 140 is formed via a PVD clustering method. The PVD method may be employed by an AC powered S-gun magnetron method. In some embodiments, the aluminum nitride layer 140 is formed by the AC powered S-gun magnetron with a power of greater than 1 kW.
In some embodiments, the top electrode layer 150 may comprise molybdenum (Mo), Ti, Ta, Ag, Au, etc. In some embodiments, the top electrode layer 150 is formed via a PVD clustering method. The PVD method may be employed by a DC powered S-gun magnetron method. In some embodiments, a Mo layer with a thickness of about 150 nanometers is formed by the PVD method. In some embodiments, the top electrode layer 150 is formed by the DC powered S-gun magnetron with a power of greater than 1 kW, for example, with a DC power of about 3 kW.
In some embodiments, portions of the top electrode layer 150 on the sidewalls of the micro-fin 120 are patterned. The patterned portions of the top electrode layer 150 form a first electrode. In some embodiments, portions of the aluminum nitride layer 140 on the sidewalls of the micro-fin 120 are etched. The etching process exposes portions of the bottom electrode layer 130. In some embodiments, the exposed portions of the bottom electrode layer 130 serve as a second electrode.
In some embodiments, portions of the aluminum nitride layer 140 are etched. A tetramethylammonium hydroxide (TMAH) solution can be used to etch the portions of the aluminum nitride layer 140. The etching process can take place at temperature greater than or about 50° C.
A challenge with characterization of the sidewalls of the aluminum nitride layer is the incapability of X-ray diffraction (XRD) for morphological study. This limitation is due to the small sidewall surface of micro-fins, i.e., the micro-fin, compared to the spot size of the optical ray, which prevents from local characterization of crystal content and orientation. In the absence of XRD results for the sidewalls of the aluminum nitride layer, i.e., the AlN films, selected-area diffraction patterns, extracted from Transmission Electron Microscopy (TEM) images, are used. A detailed set of bright-field cross-sectional transmission electron microscopy (BF-XTEM) images, taken across the sidewall film thickness, are used to identify the relative quality of the films over process variations, and also when compared with the films deposited on the planar surfaces in the same deposition run.
Comparing the images for two processes it is evident that change in sputtering gas pressure significantly reduces the tilt angle of the grains. While the tilt angle of sidewall AlN grains in one process, e.g., wafer 1, is about 41°, it is about 53° for the other process, e.g., wafer 2, with different deposition pressure.
In some embodiments, following the optimization of the sputtering process on Si micro-fins, wafers are used to explore the effect of different bottom electrodes on the texture and crystallinity of the sidewall films. Considering the higher quality of sidewall films sputtered at lower pressure, the process 2 is used for AlN deposition, in some embodiments. It is well-known that addition of bottom electrode tremendously affects the quality of sputtered piezoelectric film. The choice of bottom electrode material and deposition methodology is identified to ensure crystalline texture of the metallic film. In some embodiments, a 30 nanometers Pt layer that is deposited on (110) Si shows a dominant (111) texture (0.1° FWHM on the top surface). In some embodiments, a seed AlN layer of about 10-40 nanometers is sputtered on the sidewall, using the process 2, to promote (110)-crystalline growth of the bottom Mo layer.
This patent application claims priority to Provisional Application Ser. No. 62/947,695, filed Dec. 13, 2019, which is incorporated herein by reference in its entirety.
This invention was made with government support under 1752206 awarded by the National Science Foundation. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 62947695 | Dec 2019 | US |
