Patent Description:
The present disclosure is directed at methods, systems, and techniques for fabricating a layered structure, such as a capacitive micromachined ultrasound transducer, and the structure itself.

Ultrasound imaging is the most widely used medical imaging modality in the world in terms of images created annually. Ultrasound is useful for generating images of a variety of different targets within the human body. It is important that images are acquired with high quality and in an accessible, cost-effective manner since ultrasonic imaging has many medical uses. The ultrasound transducer is the key hardware involved sending and receiving ultrasonic waves to and from the body. Consequently, there exists a continued need to improve the capabilities of the transducer.

<CIT> discloses a method of fabricating a polymer-based capacitive ultrasonic transducer, which comprises the steps of: (a) providing a substrate; (b) forming a first conductor on the substrate; (c) coating a sacrificial layer on the substrate while covering the first conductor by the same; (d) etching the sacrificial layer for forming an island while maintaining the island to contact with the first conductor; (e) coating a first polymer-based material on the substrate while covering the island by the same; (f) forming a second conductor on the first polymer-based material; (g) forming a via hole on the first polymer-based material while enabling the via hole to be channeled to the island; and (h) utilizing the via hole to etch and remove the island for forming a cavity.

<CIT> discloses a capacitive ultrasonic transducer including a flexible layer, a first conductive layer on the flexible layer, a support frame on the first conductive layer, the support frame including a flexible material, a membrane over the support frame being spaced apart from the first conductive layer by the support frame, the membrane including the flexible material, a cavity defined by the first conductive layer, the support frame and the membrane, and a second conductive layer on the membrane.

According to an aspect, there is provided a method for fabricating a layered structure as claimed in claim <NUM>. Optional features are claimed in the dependent claims.

According to another aspect, there is provided a layered structure as claimed in claim <NUM>.

This summary does not necessarily describe the entire scope of all aspects. Other aspects, features and advantages will be apparent to those of ordinary skill in the art upon review of the following description of specific embodiments.

In the accompanying drawings, which illustrate one or more example embodiments:.

In an ultrasound imaging system, ultrasonic waves emitted by a transducer travel along soft tissues, creating wave reflections (echoes) at the interfaces between tissues with different densities (e.g., fat and muscle); these echoes travel back to the transducer and are collected and processed to form an ultrasound image. The collection and manipulation of multiple echo signals along different directions is the basis of ultrasound image formation. Ultrasound transducers are a key component in an ultrasound imaging system, which transform electrical voltage into acoustic waves and vice versa.

Medical ultrasound systems have traditionally used piezoelectric materials for their transducers since the <NUM>. Materials such as piezoelectric crystals (e.g., quartz), ceramics (e.g., lead zirconate titanate (PZT)), and thermoplastic fluoropolymers (e.g., polyvinylidene fluoride (PVDF)) have been used as the transducer materials. Despite the fact that piezoelectric transducers technology is mature, it suffers from many drawbacks such as the technical challenges in fabricating large two-dimensional arrays due to interconnect technologies and integration with electronics at the die-level.

Acoustic impedance (i.e., speed of sound in a material multiplied by its density, units: Rayls) is a measure of the opposition that a system presents to the acoustic flow resulting from an acoustic pressure applied to the system. It is an important figure in piezoelectric-based ultrasound systems since it determines their acoustic efficiency, which represents how much of the acoustic power is effectively transferred to tissues. An acoustic matching layer is typically used in biomedical piezoelectric-based systems to reduce the impedance mismatch between the crystals and tissues (<NUM> MRayls to <NUM> MRayls); otherwise just a fraction of the acoustic power could be used. These matching layers are typically made of high-density rubber combined with liquid gel and are located between the crystals and the body.

Capacitive micromachined ultrasound transducers (CMUTs) are an alternative technology to conventional piezoelectric-based transducers. A CMUT may be modeled as a parallel-plate capacitor with a fixed electrode at bottom, a suspended membrane over a closed cavity, and another electrode patterned on top of the cavity. Ultrasound waves are generated when an AC signal superimposed on a DC voltage is applied between both electrodes; conversely ultrasound waves can be detected by measuring the variation in capacitance of the device while a DC voltage is applied in the presence of incoming ultrasound waves. The effective distance (i.e., thickness of the cavity and membrane) is preferably as small as possible for two reasons: <NUM>) in order to maintain a low (e.g., less than <NUM> V) operating voltage during transmission (i.e., ultrasound waves generated from CMUT) and <NUM>) to maintain a good sensitivity during reception (i.e., ultrasound waves arriving to the CMUT), since the capacitance variation is greater for large capacitance devices (i.e., those devices with comparatively thin dielectrics).

Silicon nitride and polysilicon are the most popular materials for membranes in conventional CMUTs fabrication, while metals such as aluminum or chromium are patterned on top of the membranes to become the top electrode. The membrane materials are chosen mainly because of their mechanical properties so the membranes can be as thin as possible in order to minimize the effective gap between the bottom and top (or "hot") electrodes.

By decreasing the effective gap between electrodes, the electric-field share of the gap and the capacitance increase, and the impedance matching to the electronics improves. Starting with a desired operational frequency and a specific limit for the biasing voltage, the CMUT membrane should be designed to be as thick as possible given the fact that its bandwidth linearly increases with its thickness.

In contrast to the above materials, photopolymers are inexpensive and can be patterned using UV light; their low density and high mechanical strength make the application of these polymers interesting in the ultrasound field mainly because acoustic impedance matching with the medium into which ultrasonic waves are sent and received can be greatly improved. Nonetheless the challenge in fabricating CMUTs using polymers is that a thick membrane with a metal electrode on top is needed to reach the MHz region, contravening the required short gap between electrodes for low operational voltages and maximum sensitivity. There has been some research in fabricating CMUTs using polymer materials; however, given their large membrane thickness the operational voltages were in the order of hundreds of volts, which is incompatible with biomedical ultrasound applications. Moreover, the mentioned devices are suitable operating in air only, and not for operating in conjunction with human tissues.

The embodiments described herein are directed at methods for fabricating a layered structure, such as a CMUT, and at that structure itself. In at least some example embodiments, surface micromachining may be used to fabricate the layered structure. When surface micromachining is used, a sacrificial layer is deposited on a substrate; the sacrificial layer is patterned into a first shape; a first polymer-based layer is deposited on the sacrificial layer; an electrode is deposited, on the first polymer-based layer, above the sacrificial layer; a second polymer-based layer is deposited on the electrode such that the electrode is between, and in some embodiments embedded, within the first and second polymer-based layers; and the sacrificial layer is then etched away to form a cavity under the electrode.

In at least some example embodiments in which surface micromachining is used to manufacture a CMUT, the sacrificial layer is deposited on to a substrate assembly that functions as a bottom electrode; the sacrificial layer is patterned to be shaped as a cavity of the CMUT; the first polymer-based layer is deposited on the sacrificial layer; a via hole is patterned through the first polymer-based layer to the sacrificial layer; a top electrode is patterned, above the sacrificial layer, on the first polymer-based layer; a second polymer-based layer is deposited on the top electrode such that the top electrode is embedded within the first and second polymer-based layers; the sacrificial layer is etched away, using the via hole, to form the cavity of the CMUT; and the cavity is closed.

In at least some different embodiments, wafer bonding may be used to fabricate the layered structure. When wafer bonding is used, a first polymer-based layer is deposited on a first substrate; the first polymer-based layer is patterned to be a cavity; a sacrificial layer is deposited on a second substrate; a second polymer-based layer is deposited over the sacrificial layer; an electrode is deposited on the second polymer-based layer; a third polymer-based layer is deposited on the electrode such that the electrode is between, and in some embodiments embedded, within the second and third polymer-based layers; the second and third polymer-based layers are cross-linked; the first and third polymer-based layers are adhered together such that the cavity is sealed by those layers; and the sacrificial layer is etched away such that the second polymer-based layer is released from the second substrate.

In at least some example embodiments in which wafer bonding is used to manufacture a CMUT, the first polymer-based layer is deposited on the substrate assembly, which functions as the bottom electrode; the first polymer-based layer is patterned to be a cavity of the CMUT; the sacrificial layer is deposited on a separate substrate; the second polymer-based layer is deposited over the sacrificial layer; the top electrode is deposited on the second polymer-based layer; the third polymer-based layer is deposited on the top electrode such that the top electrode is embedded within the second and third polymer-based layers; the first and third polymer-based layers are adhered together such that the cavity is closed; and the sacrificial layer is etched away such that the second polymer-based layer is released from the separate substrate.

As used herein, "embedding" an electrode with a polymer means completely covering the electrode with the polymer, except for any electrical connections made with that electrode.

Also as used herein, "patterning" a material means to selectively remove that material either directly (e.g., if it is photosensitive) or by using a masking layer (e.g., in the case of the OmniCoat™ composition, as discussed further below).

In at least some of the embodiments in which a polymer-based CMUT is fabricated, the polymer material may be inexpensive, easy to process, and be capable of being made in large arrays. Additionally, in contrast to conventional CMUTs, the top electrode is embedded within two polymer layers, with the bottom layer being thinner than the top layer; this, combined with forming a sufficiently thin CMUT cavity by etching away a sacrificial layer, permits the CMUT to reach the MHz operative region without requiring unacceptably high operating voltages.

A detailed description of the fabrication operations and relevant information about the materials used follows. <FIG> are schematic diagrams sequentially arranged for illustrating operations comprising a method for fabricating a polymer-based CMUT according to a surface micromachining embodiment. <FIG> depict an analogous method according to a wafer bonding embodiment.

Referring now to <FIG>, there is shown a cross-sectional view of a substrate assembly in the form of an electrically conductive substrate <NUM> (an electrically conductive silicon wafer in this case). A low electrical resistance of this substrate <NUM> facilitates the substrate <NUM> acting as a bottom electrode in the finished CMUT. In some different embodiments (not depicted), a dedicated bottom electrode can be patterned over an insulating substrate <NUM> as an alternative. CMUTs are typically fabricated using silicon wafers as substrates, but any version of the rigid to semi-rigid surface with a smooth hydrophilic surface is sufficient to be used with this methodology. The surface of the substrate <NUM> is hydrophilic in at least some example embodiments in order to achieve a wet release of the final membrane described in subsequent operations.

A sacrificial layer <NUM> is deposited on the substrate <NUM> by spin coating. This sacrificial layer <NUM> will become the evacuated cavity <NUM> in the finished CMUT. The required thickness of the sacrificial layer <NUM> for a CMUT can range from a few hundreds of nanometers (nm) (e.g., <NUM>) to a couple of micrometers (µm) (e.g., <NUM>). A highly selective etchant is used to etch away the sacrificial layer <NUM> without damaging the CMUT's membranes, which are formed in subsequent operations as described below.

The OmniCoat™ composition by MicroChem Corp. has an excellent selectivity during etching and it enhances the adhesion of photoresists to different substrates. The two main chemical components in OmniCoat™ composition are cyclopentanone (a solvent that gets evaporated) and Propylene Glycol Monomethyl Ether (PGME). The OmniCoat™ composition also comprises a polymer (less than <NUM>% of total volume) and a surfactant (also less than <NUM>% of total volume). The OmniCoat™ composition is not photosensitive and its typical thickness during spin coating ranges from <NUM> to <NUM>, which limits its conventional use to releasing large structures by immersing them in developer for a few hours until the structures get released and float away from the carrying substrate. Still, this thickness range (<NUM>-<NUM>) is well below the typical thickness used for sacrificial layers when conventional CMUTs are fabricated (<NUM> - <NUM>,<NUM>). In the depicted example embodiments, the OmniCoat™ composition is used for the sacrificial layer <NUM>. In at least some example embodiments, the OmniCoat™ composition is evaporated prior to depositing it as the sacrificial layer <NUM>. For example, evaporating a certain percentage of the solvents of the off-the-shelf OmniCoat™ composition (e.g., <NUM>%) prior to its deposition allows a relatively thick sacrificial layer <NUM> (e.g., <NUM>), to be deposited in a single step. This may help to increase efficiency and to allow for greater precision in laying a sacrificial layer <NUM> of a desired thickness. In at least some other embodiments (not depicted), the OmniCoat™ composition may not be evaporated at all prior to its deposition as the sacrificial layer <NUM>; for example, without any pre-deposition evaporation, multiple layers of the OmniCoat™ composition may be deposited in order to reach a desired thickness. In further additional embodiments (not depicted), while a certain proportion of its solvents may be evaporated, that proportion may be more or less than <NUM>%. For example, if a thinner sacrificial layer <NUM> is desired, then a smaller percentage (e.g., <NUM>%) of the solvents in the OmniCoat™ composition may be evaporated; alternatively, more (e.g., <NUM>%) of the solvents may be evaporated. More generally, this pre-deposition evaporation may be performed on whatever composition is used as the sacrificial layer <NUM>.

The sacrificial layer <NUM> is patterned to create the areas that will become the cavity <NUM> (shown in <FIG>) in the final device as well as releasing channels to permit access to this cavity <NUM>. Given the fact that the OmniCoat™ composition is not photosensitive it cannot be directly patterned and needs to be removed indirectly.

Referring now to <FIG>, a layer of positive photoresist (PR) <NUM> is deposited on top of the sacrificial layer <NUM>. This photoresist (S <NUM>) is selected so that its developer dissolves the cross-linked photoresist <NUM> (shown in <FIG>), which results after ultraviolet light (UV) exposure as well as the sacrificial material <NUM>.

Referring now to <FIG>, the photoresist layer <NUM> is exposed to UV using a photomask and a mask aligner. The areas exposed to UV become cross-linked photoresist <NUM> and the areas not exposed to UV are left intact (uncross-linked).

Referring now to <FIG>, the cross-linked photoresist <NUM> is etched away (removed) by placing the sample in an aqueous solution containing an alkaline-based photoresist developer (MF319). The photoresist <NUM> that is uncross-linked remains intact.

Referring now to <FIG>, while the sample is still in the photoresist developer (MF319) from <FIG>, the etching continues and starts dissolving the sacrificial layer <NUM>. The patterned photoresist layer <NUM> acts as a masking layer to protect the sacrificial layer <NUM> underneath. The etching is stopped as soon as the sacrificial layer <NUM> under the cross-linked photoresist <NUM> is removed, leaving the substrate <NUM> exposed for subsequent operations.

Referring now to <FIG>, the masking layer of positive photoresist <NUM> is removed by immersing the sample in acetone or any other solvent suitable to dissolve the positive PR <NUM> without damaging the sacrificial layer <NUM>. The sacrificial layer <NUM> offers an excellent selectivity (chemical resistance) to the solvent used (acetone). What is left behind is a patterned sacrificial layer <NUM> containing the areas that will become the cavity <NUM> in the final device as well as the etch channels.

Referring now to <FIG>, a first polymer-based layer <NUM> comprising a negative photosensitive polymer-based material (SU8 photoresist, hereinafter interchangeably referred to simply as "SU8") <NUM> is deposited, conformally covering the sacrificial layer <NUM>. The thickness of the layer <NUM> is designed to be as thin as possible to conformally coat the sacrificial layer <NUM> and to be able to maintain good electrical insulation between the conductive substrate <NUM>, which acts at the finished CMUT's bottom electrode, and a top electrode <NUM> (shown in <FIG>). The SU8 comprises Bisphenol A Novolac epoxy dissolved in an organic solvent, and comprises up to <NUM> wt% Triarylsulfonium/hexafluoroantimonate salt; in different example embodiments (not depicted), the polymer-based material may have a different composition. The SU8 is also optically transparent, which facilitates inspection of the finished device. In at least some different embodiments, a material may be used in place of the SU8, and that replacement material may be non-opaque (i.e., partially or entirely transparent).

The layer <NUM> in at least the depicted example embodiment comprises a photopolymer. Photopolymers are inexpensive and can be patterned using UV; their low density and high mechanical strength make the application of these polymers interesting in the ultrasound field mainly because the impedance matching with the medium into which ultrasonic waves are transmitted and from which reflected waves are received can be greatly improved. Nonetheless the challenge in fabricating CMUTs using polymers is that, conventionally, a thick membrane with a metal electrode on top is needed to reach the MHz operational region, contravening the required short gap between electrodes that facilitate low operational voltages and maximum sensitivity.

Referring now to <FIG>, the layer <NUM> is exposed to UV using a photomask and a mask aligner. The areas exposed to UV light become cross-linked areas <NUM> of the layer <NUM> and the areas not exposed to UV are left as uncross-linked areas.

Referring now to <FIG>, the uncross-linked areas of the first polymer-based layer <NUM> are etched away (removed) by placing the sample in an aqueous solution containing a negative photoresist developer (SU8 developer). The cross-linked areas <NUM> remain intact.

Referring now to <FIG>, an electrically conductive top electrode (chromium) <NUM> is patterned on top of the cross-linked areas <NUM> of the first polymer-based layer <NUM> using lift-off methods. This electrode <NUM> is in at least some example embodiments made as thin as possible without sacrificing electrical conductivity in order to not greatly modify the structural properties of the cross-linked areas <NUM>, which will later become the membrane of the finished device.

The material for this electrode <NUM> in at least the depicted example embodiment is typically metallic; nevertheless any other material capable of fulfilling the functions of the top electrode <NUM> can be used (e.g., conductive polymers, optically transparent materials, etc.). A good adhesion between this top electrode <NUM> and the cross-linked areas <NUM> is present in order to avoid any potential delamination during normal operation of the finished device. Using chromium as the top electrode <NUM> when the cross-linked areas <NUM> comprise SU8 may help to facilitate adhesion.

At this point the overall thickness of the membrane (i.e., the cross-linked areas <NUM> and the top electrode <NUM>) is thin compared to its diameter so that its resonant frequency would be just a fraction of the desired operational frequency in the finished device. A much thicker membrane is required in order to reach the desired operational frequency in, for example, the MHz range.

Referring now to <FIG>, a second polymer-based layer <NUM> is deposited over the membrane, conformally coating the sacrificial layer <NUM>, the first polymer-based layer <NUM>, and the top electrode <NUM>. The second polymer-based layer <NUM> is of the same photosensitive polymer (SU8) as the first polymer-based layer <NUM> in at least the depicted example embodiment; however, in different embodiments (not depicted) the layers <NUM>,<NUM> may comprise different polymers. The thickness of this second polymer-based layer <NUM> is designed to be a few times (~<NUM>) thicker than that of the cross-linked areas <NUM> of the first polymer-based layer <NUM>.

Referring now to <FIG>, following the same process as described in respect of <FIG>, the second polymer-based layer <NUM> is exposed to UV using a photomask and a mask aligner. The areas exposed to UV light become cross-linked areas <NUM> and the areas not exposed to UV are left intact (uncross-linked).

Referring now to <FIG>, the uncross-linked areas of the second polymer-based layer <NUM> are etched away (removed) by placing the sample in an aqueous solution containing a negative photoresist developer (SU8 developer). The cross-linked areas <NUM> remain intact.

At this point, the top electrode <NUM> becomes embedded between the cross-linked areas <NUM>,<NUM> of the two polymer-based layers <NUM>,<NUM>. The advantage of this approach is that the membrane is still able to operate in the MHz region because of the added thickness from the second polymer-based layer <NUM>, which increases the effective stiffness while still maintaining a low operational voltage thanks to the small effective distance between the bottom substrate <NUM> and the embedded top electrode <NUM>.

This fabrication process is not limited to the operation in the MHz range for biomedical ultrasound imaging. If desired, the same or an analogous fabrication process can be used to obtain membranes that operate in the Hz and kHz region for air-coupled operation applications, for example. The final operational frequency of the membrane depends on the geometry of the cell. This means that membranes that resonate at different frequencies can be operated with very similar voltages. For example, two membranes with the same diameter can operate with the same voltage (same effective distance between electrodes), but one can be thinner for lower frequencies and the other ticker for high-frequency operation.

Referring now to <FIG>, the sample is then immersed in an aqueous alkaline-based solution containing the etchant (MF319) of the sacrificial layer <NUM>. The patterned sacrificial layer <NUM> is gradually removed though via holes and etch channels until it is fully dissolved. At this point the etchant is replaced by water and then by isopropanol (IPA). A critical point dryer system is used to release the membrane, avoiding stiction problems and remaining with a membrane suspended above an un-sealed cavity <NUM>.

Referring now to <FIG>, the sample is encapsulated by a bio-compatible material <NUM> (a poly(p-xylylene) polymer, such as parylene) inside a low-pressure chamber; while various pressures may be used, in the depicted example embodiment the pressure within the chamber is <NUM> × <NUM>-<NUM> Torr. The encapsulating material <NUM> conformally coats the entire sample, sealing the via holes and etch channels to form a closed cavity <NUM>. In at least the depicted example embodiment, the cavity <NUM> is vacuum sealed and is watertight and airtight, which helps to avoid squeeze-film effects and to reduce the risk of voltage breakdown. In different example embodiments, the cavity <NUM> may not be vacuum sealed, or may be watertight and not airtight.

At this point the fabrication process is complete and the finished device (i.e., the CMUT) results. Any electrical interconnection is made before this step as the biocompatible material (parylene) is an excellent electrical insulator and is safe for use on humans.

This fabrication process may use optically transparent or semi-transparent materials for any one or more of the substrate (e.g. glass or quartz), for the electrodes (e.g. Indium oxide, which is semi-transparent) and for the sealing layer (parylene). This leads to an optically transparent or semi-transparent transducer.

In the CMUT depicted in <FIG>, the cross-linked areas <NUM> of the first polymer-based layer <NUM> is significantly thinner than the cross-linked areas <NUM> of the second polymer-based layer <NUM>. <FIG> depicts an example CMUT in which the electrode <NUM> is located above the second polymer-based layer <NUM> and then encapsulated by the encapsulating material <NUM>.

The operating voltage of the CMUT of <FIG> is much lower than that of <FIG>. For instance, the resonant frequency of the membranes in <FIG> and <FIG> is the same since all the materials and thickness remain the same except for the location of the top electrode. The operational voltage of the CMUT shown in <FIG> is <NUM> Volts, whereas the operational voltage of the CMUT shown in <FIG> is <NUM> Volts, which is prohibitive in medical ultrasound systems.

The described materials and fabrication process of <FIG> may be used to fabricate the CMUT on flexible substrates. This is an advantage for conformal imaging systems in which the ultrasound elements are to be curved around different parts of the human body as depicted in <FIG>. The polymer materials used for fabrication are sufficiently flexible, allowing the CMUT to bend around small radii of curvature without sacrificing performance or mechanical stability.

Traditional CMUTs fabricated with polysilicon and silicon nitride are generally inflexible and employ hazardous chemicals (potassium hydroxide and hydrochloric acid) during etching. The chemicals may present a risk for people working with these materials as they are corrosive and the vapours can cause internal organ damage, resulting in severe and in some cases fatal consequences. The fabrication operations according to at least some of the example embodiments herein can be performed in simple low-cost and safe fabrication facility.

The aforementioned fabrication process in at least some example embodiments employ non-hazardous materials, i.e. only organic solvents are used during fabrication (acetone, isopropanol, SU8 developer, and positive photoresist developer). The health risks associated with an accidental prolonged exposure to these materials are generally limited to drowsiness and minor skin irritation. The etchant used to remove the OmniCoat™ composition (MF319 or Tetramethylammonium hydroxide diluted in water) can be safely disposed of in ordinary laboratory drain systems when diluted in water as it is considered a mild base.

The fabrication costs associated with the fabrication process depicted in <FIG> is significantly less than the cost required to manufacture conventional designs. As of December, <NUM>, the estimated material costs to fabricate an array of ultrasound transducers is less than US$<NUM> inside a university laboratory, with a potential cost reduction if mass produced; meaning that the fabricated devices can be considered at some point disposable.

The maximum temperature required to manufacture CMUTs using the described process in <FIG> for this process is <NUM>, consequently requiring minimal thermal protection systems and using minimal thermal budget compared to conventional fabrication processes using polysilicon.

Additionally, using polymers as structural material for CMUTs means that if an acoustic matching layer is required it can be manufactured using the same kind of polymer materials with embedded fillers.

Referring now to <FIG>, there are depicted perspective views sequentially arranged for illustrating operations comprising a method for fabricating a polymer-based CMUT, according to another example embodiment.

Referring now to <FIG>, the electrically-conductive substrate <NUM> (e.g., silicon wafer) is uniformly coated with a sacrificial material (e.g., the OmniCoat™ composition) and baked to form the sacrificial layer <NUM>. A layer of positive photoresist (S1813) is deposited on top of the OmniCoat™ composition and baked. The sample is selectively exposed to UV to pattern the sacrificial layer <NUM>'s design. The sample is immersed in positive photoresist developer (MF319). The developer dissolves both unexposed areas in S1813 and the OmniCoat™ composition underneath, thereby leaving a patterned design of the sacrificial layer <NUM>. The sacrificial layer <NUM> comprises an area to eventually form the CMUT cavity <NUM> as well as etch channels <NUM> and etch via holes <NUM>.

Referring now to <FIG>, the sample coated with the first polymer-based layer <NUM> comprising a polymer-based material (SU8), conformally covering the sacrificial layer <NUM>. The thickness of the layer <NUM> is by design selected to be as thin as possible as long as the sacrificial layer <NUM> is covered and the breakdown voltage of the polymer exceeds the desired operational voltage. The sample is exposed to UV to pattern the anchor points of the sample as well as the first layer of the membrane. The sample is baked and developed in SU8 developer, leaving open windows for the etch channels <NUM>.

Referring now to <FIG>, the electrically conductive electrode <NUM> (chromium) is patterned on top of the first polymer-based layer <NUM> using lift-off micromachining methods; electrical connections <NUM> to the electrode <NUM> are concurrently patterned. The thickness of the electrode <NUM> is as thin as possible as long as a low resistance path is maintained. In at least some different embodiments (not depicted), the electrode <NUM> may comprise non-metallic materials, such as one or more conductive polymers.

Referring now to <FIG>, the second polymer-based layer <NUM> is conformally coated on top of the electrode <NUM>, covering the stack comprising the sacrificial layer <NUM>, first polymer-based layer <NUM>, and metal electrode <NUM>. The second polymer-based layer <NUM> also comprises the SU8. The sample is exposed to UV to pattern the CMUT membrane and leave open areas for the via holes <NUM> on the first polymer-based layer <NUM>. The purpose of this second polymer-based layer <NUM> is to increase the effective thickness of the membrane and therefore increase its resonant frequency. Electrical contacts are exposed to air. In the depicted example embodiment, only the areas corresponding to where the cavity <NUM> will be located in the finished CMUT is patterned with the second polymer-based layer <NUM>; in different embodiments (not depicted), more than these areas may be patterned with the second polymer-based layer <NUM>.

Referring now to <FIG>, the sample is immersed in positive photoresist developer (MF319, same etching chemical as for the OmniCoat™ composition). Developer removes the sacrificial material through the via holes <NUM> and etch channels <NUM>. The developer (MF319) is replaced by water and then by isopropyl alcohol (IPA) in a wet environment. The sample is immersed in IPA inside a critical point dryer system to release the membrane. Liquid CO<NUM> replaces IPA in a highpressure environment and then the liquid CO<NUM> is converted to gaseous CO<NUM>. At this point the membrane is suspended on the cavity <NUM>. While a critical point system is used in the fabrication depicted in <FIG>, in different embodiments (not depicted) it may be omitted, particularly if the CMUT membrane is not prone to stiction given its dimensions.

Referring now to <FIG>, the sample is placed in a low-pressure chamber (e.g., operated at <NUM> × <NUM>-<NUM> Torr) and conformally coated with the encapsulating material <NUM> comprising polymer materials (parylene) so that the cavity <NUM> is vacuum-sealed (i.e., airtight) and watertight. The parylene's thickness is selected so the mechanical properties of the encapsulating material <NUM> in the finished CMUT are very similar (and in some embodiments identical) to the SU8 (e.g., in terms of density and Young's modulus); accordingly, in at least some embodiments the collective thickness of the encapsulating material <NUM> and the second polymer-based layer <NUM> are considered when comparing that thickness to that of the first polymer-based layer <NUM>, the ratio of which influences the finished CMUT's operational frequency. The encapsulating material <NUM> seals the via holes <NUM> and etch channels <NUM>, leaving a vacuum-sealed and watertight cavity once the sample is removed from the low-pressure chamber. Areas for electrical interconnections are protected prior this sealing step.

The resulting finished CMUT is a sealed CMUT element with a low pull-in voltage given its small effective separation between electrodes. The carrying substrate need not be limited to rigid materials; flexible material temporarily attached to a rigid carrier may be used as well. The sample may be electrically interconnected to an interface circuit prior to sealing (<FIG>). An acceptable adhesion between polymer material and electrode is used to avoid mechanical failure during operation.

In at least some example embodiments, wafer bonding technology can be used to manufacture a similar version of the CMUT depicted in <FIG>. In this approach, the materials are deposited and processed in two separate substrates, such as silicon wafers. The materials deposited on the separate substrates are then adhered together and further processed to obtain CMUTs. The detailed fabrication description follows.

Referring now to <FIG>, the first polymer-based layer <NUM> comprising a polymer-based material (SU8) is deposited on top of a substrate assembly comprising the bottom substrate <NUM>, which acts as the bottom electrode in the finished CMUT and which in the depicted example embodiment is electrically conductive.

Referring now to <FIG>, the first polymer-based layer <NUM> is exposed to UV using a photomask. The areas exposed to UV become the cross-linked areas <NUM> and the areas not exposed to UV are left uncross-linked.

Referring now to <FIG>, the uncross-linked areas of the first polymer-based layer <NUM> are etched away (removed) using photoresist developer (SU8 developer). The cross-linked areas <NUM> remain intact. The cross-linked areas <NUM> that remain following etching will act as pillars supporting the CMUT membranes.

Referring now to <FIG>, in a separate substrate <NUM> (silicon wafer or any other rigid and smooth substrate) a sacrificial layer <NUM> is deposited on top by spin coating. This sacrificial layer <NUM> will be used to release the separate substrate <NUM> following adhering, as discussed further below.

Referring now to <FIG>, the second polymer-based layer <NUM>, which in the depicted example embodiment comprises the same photosensitive polymer (SU8) as used for the first polymer-based layer <NUM>, is deposited on top of the sacrificial layer <NUM>; this layer <NUM> will become the top part of the finished CMUT.

Referring now to <FIG>, the second polymer-based layer <NUM> is exposed to UV using a photomask and a mask aligner. The areas exposed to UV become the cross-linked areas <NUM>.

Referring now to <FIG>, the electrically conductive top electrode <NUM> (chromium) is patterned on top of the cross-linked areas <NUM> using lift-off methods.

Referring now to <FIG>, a third polymer-based layer <NUM> of the same photosensitive polymer (SU8) comprising the first and second polymer-based layers <NUM>,<NUM> is deposited on the top electrodes <NUM>, conformally coating the metal electrodes <NUM> and the cross-linked areas <NUM> of the second polymer-based layer <NUM>. At this point, the top electrode <NUM> becomes encapsulated between the cross-linked areas <NUM> of the second polymer-based layer <NUM> and the third polymer-based layer <NUM>.

Referring now to <FIG>, the third polymer-based layer <NUM> is exposed to UV using a photomask and a mask aligner. The areas exposed to UV become cross-linked areas <NUM>. The purpose of the cross-linked areas <NUM> is to act as dielectric layer between the two electrodes (the bottom substrate <NUM> and the top electrode <NUM>) in the finished CMUT. Cross-linking the first and third polymer-based layers <NUM>,<NUM> serves to promote adhesion between those layers; in at least some different example embodiments (not depicted), the cross-linking may be skipped if the layers <NUM>,<NUM> can be suitably adhered to each other without cross-linking.

Referring now to <FIG>, the surfaces of the separate samples as shown in <FIG> and <FIG> are treated with oxygen plasma, which allows the surfaces of both samples to be permanently adhered. The samples are aligned and placed face to face in a vacuum environment and pressed against each other. The vacuum may be any suitable pressure, such as <NUM> × <NUM>-<NUM> Torr as described above in the surface micromachining embodiment. In different embodiments (not depicted), the samples may not be treated with oxygen plasma.

Referring now to <FIG>, after releasing the pressure both samples are now permanently attached, creating an array of vacuum-sealed cavities <NUM>.

Referring now to <FIG>, the sample is immersed in an aqueous alkaline-based solution containing the etchant (MF319) of the sacrificial layer <NUM>. The sacrificial layer <NUM> is gradually removed until the separate substrate <NUM> is released. This etchant does not attack the polymer not the metallic materials used. At this point the fabrication process is complete and the device becomes watertight. As with the surface micromachining embodiment of <FIG>, in certain embodiments the cavities <NUM>, while closed, may not be airtight or watertight; in other embodiments, the cavities <NUM> may be watertight and not airtight.

The CMUT fabricated using wafer bonding does not comprise the encapsulating material <NUM> in the depicted example embodiments as the cavities <NUM> are vacuum-sealed following adhesion. This simplifies fabrication.

The fill factor (number of CMUTs per unit area) may be improved using wafer bonding vs. surface micromachining, as the CMUTs can be placed closer to each another since the releasing holes (vias) and channels do not exist. By using hexagonal or square membranes the fill factor can be increased, relative to circular membranes.

In at least some example embodiments, Roll-to-Roll (R2R) technology may be applied to fabricate the cavity <NUM>.

In the wafer bonding embodiment, one or both of the substrates <NUM>,<NUM> may be flexible if bonded to a rigid carrier.

Charge trapping effects in CMUTs may be observed, for example, when a zero-bias resonator is fabricated by purposely trapping electrical charges in a dielectric layer by applying a large bias voltage beyond pull-in. More generally, charge trappings effects may be observed for any resonator (including a CMUT) or layered device fabricated according to the embodiments described herein, including those that are not zero-bias. In the examples described herein, the trapped charging effect contributes positive to the normal operation of the resonator (e.g., a materially lower operational voltage may be used when trapped charges are present).

Referring now to <FIG>, electrical charges get trapped in the SU8 membrane underlying the top electrode <NUM> of the CMUT of, for example, <FIG> or <FIG>, when a DC voltage larger than pull in (VPI = 65V) is applied between the top electrode <NUM> and the bottom substrate <NUM>, which acts as the bottom electrode. This causes the membrane to collapse (e.g., to be pulled into contact with the substrate <NUM>), resulting in the electrical field acting on the membrane to increase. In at least some different example embodiments (not depicted) the cavity <NUM> is sufficiently tall that the membrane does not contact the bottom substrate <NUM> when the DC voltage is applied. After removing the DC voltage, the membrane returns to its initial position having electrical charges <NUM> trapped in the dielectric film (SU8).

The electrical charges <NUM> trapped in the membrane contribute to the electrostatic force during operation (acting like a built-in voltage), meaning that a lower DC bias voltage may be used to bring the membrane closer to the bottom substrate <NUM>.

It has been experimentally shown that the electrical charges <NUM> get trapped in the volume of the SU8 film (in theory, by the molecules' dipole alignment) and not on the metal electrode <NUM> (as an ordinary capacitor). This prevents the CMUT from getting "discharged" even if its terminals are shorted.

Using the surface micromachining embodiment described in respect of <FIG>, a set of linear arrays containing <NUM> and <NUM> CMUT elements, with each element comprising an interconnected matrix of CMUT cells sharing a common bottom electrode in the form of a conductive substrate <NUM> (a silicon wafer), were fabricated. These CMUT elements are shown in <FIG> (<NUM> elements) and <FIG> (<NUM> elements). The total fabrication time was <NUM> Hrs.

Detailed views of the fabricated arrays are shown in <FIG> for a <NUM> and <NUM> element array, respectively. The diameter of the CMUT cells is <NUM> and <NUM> for the <NUM> and <NUM> element arrays, respectively. The thickness of the CMUT membranes is <NUM>, which includes the top electrode <NUM> and the cross-linked areas <NUM> of the first polymer-based layer <NUM> and the vacuum-filled cavity <NUM> has a height of <NUM>. Electrical connections for external interface are located at each end on the elements.

Acoustic measurements were performed in an oil bath using a piezoelectric transducer to validate the operation of the fabricated polymer CMUTs. The measured response is shown in <FIG>, showing a short pulse characteristic of ultrasound transducers. The frequency spectrum (FFT) of the measured pulse is shown in <FIG>, having a fractional bandwidth of <NUM>°%.

Preliminary results show that it is possible to measure ultrasound pulses using a polymer CMUT element as a passive receiver (i.e., no DC bias voltage applied). <FIG> shows the measurement of an ultrasound pulse generated by a piezoelectric crystal located above a polymer CMUT element in a liquid medium operating at an acoustic pressure typical for medical ultrasound imaging. The terminals of the CMUT (top and bottom electrode) were directly connected to an oscilloscope.

The amplitude of the received signal when the CMUTs are operated as a passive device (no DC bias voltage) was <NUM> mVpp; this represents much more than the expected voltage obtained from typical piezoelectric-based transducers, in which the expected generated voltage across the terminals ranges between a few microvolts and <NUM> mV. The amplitude of the received signals was increased even further to almost <NUM> mVpp when a bias voltage of 15V was applied.

This implies that ultrasound signals can be directly processed without the need of low-noise and high-gain amplifiers used in commercial piezoelectric-based ultrasound systems, potentially reducing the physical volume and weight in ultrasound probes and marking a step forward towards a lightweight, low-power conformal ultrasound system.

In at least some embodiments, no acoustic matching layer is required to couple the fabricated CMUTs (regardless of whether fabricated using surface micromachining or wafer bonding) to an aqueous medium. This contrasts with the mandatory acoustic matching layer in conventional piezoelectric-based ultrasound imaging systems.

Additionally, in at least some example embodiments, one or both of the surface micromachining and wafer bonding embodiments may further comprise an annealing operation. When SU8 is used during fabrication, annealing may be done at, for example, <NUM> for five minutes to anneal any cracks that may have formed during development.

Accordingly, as used herein, the singular forms "a", "an", and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and "comprising", when used in this specification, specify the presence of one or more stated features, integers, steps, operations, elements, and components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and groups. Directional terms such as "top", "bottom", "upwards", "downwards", "vertically", and "laterally" are used in the following description for the purpose of providing relative reference only, and are not intended to suggest any limitations on how any article is to be positioned during use, or to be mounted in an assembly or relative to an environment. Additionally, the term "couple" and variants of it such as "coupled", "couples", and "coupling" as used in this description are intended to include indirect and direct connections unless otherwise indicated. For example, if a first device is coupled to a second device, that coupling may be through a direct connection or through an indirect connection via other devices and connections. Similarly, if the first device is communicatively coupled to the second device, communication may be through a direct connection or through an indirect connection via other devices and connections.

It is contemplated that any part of any aspect or embodiment discussed in this specification can be implemented or combined with any part of any other aspect or embodiment discussed in this specification.

Claim 1:
A method for fabricating a layered structure, the method comprising:
(a) depositing a sacrificial layer (<NUM>) on a substrate assembly (<NUM>) that functions as a bottom electrode;
(b) patterning the sacrificial layer into a first shape;
(c) depositing a first polymer-based layer (<NUM>) on the patterned sacrificial layer;
(d) patterning a top electrode (<NUM>) on the first polymer-based layer above the patterned sacrificial layer;
(e) depositing a second polymer-based layer (<NUM>) on the top electrode such that the top electrode is between the first and second polymer-based layers, wherein the second polymer-based layer is at least five times thicker than the first polymer-based layer; and
(f) after the second polymer-based layer has been deposited on the top electrode, etching away the patterned sacrificial layer to form a cavity (<NUM>) under the top electrode.