Material selection

In addition to improvements in the effective bandwidth, sensitivity, and radiated output pressure, a proper selection of fabrication materials can lead to improvements in the fabrication process and, in turn, lower the cost. We specifically selected the photopolymer SU-8 2000 series32 as a structural material for our applications, given its unique dielectric and thermal properties, as well as its low density, photopatternability, optical transparency, and mechanical flexibility.

A crucial component in the fabrication of the cavities required for CMUTs is the sacrificial layer, where a high selectivity of the etchant is required to properly dissolve the sacrificial layer without damaging the membranes.

Different materials have been used as sacrificial layers when working with SU-8. Song et al.33 used overbaked positive photoresists to withstand the attack of solvents present in SU-8 to form electroplating molds, and the sacrificial layer was later removed using plasma etching. Moser et al.34 altered the chemical composition of SU-8 to make it more sensitive to UV light, and a standard SU-8 composition was used as a sacrificial layer to create microchannels. Foulds et al.35 patterned polydimethylglutarimide (PMGI) to be used as a sacrificial layer, using a combination of positive photoresists and repetitive sequences of near UV and deep UV light exposures. Chiriaco et al.36 created microchannels by patterning PMGI without using deep UV light; the masking material for PMGI was removed in acetone, leaving the sacrificial layer prone to damage since acetone also attacks the PMGI. Finally, different metals have also been used as sacrificial layers37,38, where they are either evaporated or sputtered, requiring long deposition times and leaving rough surfaces. Metals also require extra fabrication steps to be patterned for use as sacrificial layers.

One of the main novelties of this work is the use of a well-known material in a novel way to create the CMUT cavities. Omnicoat32 has been widely used as a lift-off material to release structures from silicon wafers. It has an excellent selectivity during etching and enhances the adhesion of photoresists to different substrates.

Despite these properties, Omnicoat is not photosensitive and the typical thickness during spin coating ranges from 5 to 15 nm. Since a sacrificial layer in the order of hundreds of nanometers is needed, we evaporated 85% of the solvents present in Omnicoat. Thus, a much denser version of the same chemical was used for spin coating. Using this approach, we were able to increase the coating thickness from 15 to 300 nm in a single step at 1000 RPM. Multiple coatings could be stacked, or a higher percentage of the solvents could be evaporated if an even thicker sacrificial layer is desired. This might be useful in microfluidic applications where channels with different heights are typically used39. In our approach for CMUTs, the thickness of Omnicoat can be accurately controlled by spin coating, and it can be patterned at the same time as a masking material deposited on top. Omnicoat also has an excellent adhesion to different substrates, possesses excellent selectivity against solvents, and allows the substrates to be re-worked if necessary.

Transducer fabrication

Fig. 1 depicts the fabrication steps for these devices. A maskless lithographic system SF-100 (St. Petersburg, USA) was used for UV exposure given its rapid prototyping capabilities; however, if needed, a standard mask aligner with photomasks can be used to replicate the results presented in this article.

Fig. 1 Overview of the six fabrication steps used to create polymer-based CMUTs Full size image

The fabrication started with a clean and low electrical resistance silicon wafer that acts as the bottom electrode. We spin coated the concentrated version of Omnicoat (15% of its original volume) onto the wafer and baked it at 150 °C for 3 min to obtain a 320 nm sacrificial layer. A layer of positive photoresist S1813 (ref. 32) was then deposited and patterned using UV light to create a masking layer to selectively remove the Omnicoat underneath. The sample was then immersed in alkaline-based MF319 developer32 for 60 s, simultaneously etching the exposed areas of both S1813 and the Omnicoat; the etching was stopped by rinsing the sample in de-ionized water. The S1813 masking layer was then removed in acetone. Because of its excellent resistance to solvents, a patterned Omnicoat sacrificial layer was left intact without damage. A plasma etching step in O 2 for 1 min is recommended at this time to completely eliminate all possible residues of Omnicoat left after etching and to increase adhesion of SU-8 to the silicon wafer. After plasma etching, the effective thickness of the sacrificial layer was 300 nm.

A layer of SU-8 2000.5 was spun at 2000 RPM onto the sample to obtain a thickness of 0.67 μm. The thickness facilitated conformal coating of the Omnicoat areas that became the cavities and etch channels of our device. A short pre-baking step at 95 °C was performed for 3 min prior UV exposure. The membranes, etching via holes, and clamping areas of the CMUTs were patterned in UV light, and the sample was post-exposure baked and developed. The SU-8 layer acts as a mechanical support for the top electrode and as dielectric material to avoid any short circuit since the dielectric strength of SU-8 reaches as much as 50% of that of SiO 2 (ref. 40).

A layer of AZP 4110 photoresist was coated and patterned to act as a lift-off layer before evaporating on 570 nm of chromium. Chromium was selected because of its excellent adhesion properties to SU-8 films and low electrical resistivity41. A metal evaporation was preferred over sputtering because of its directional deposition, which simplifies the lift-off process. Once the electrodes are properly patterned, a second layer of SU-8 2002 was spin coated at 1000 RPM to obtain a thickness of 2.40 μm, thereby covering the previous stack of Omnicoat, SU-8, and chromium layers. After pre-baking, the sample was exposed to UV light to pattern the top part of the CMUT membranes, leaving open areas for the etching holes. The sample was cured at 150 °C for 5 min and then gradually cooled down to room temperature. This annealing step removes any possible cracks created and increases the Young’s modulus of the SU-8 films42.

The sample was then immersed in the developer MF319 (ref. 32). This etchant penetrates through the etch channels and gradually removes the Omnicoat material underneath the cavities. The circular cavities have a radius of 50 μm, and the etch channels are 5 μm wide and 15 μm long. After 3 h, the Omnicoat sacrificial layer was completely removed. The sample was then immersed in de-ionized water for 2 h to displace the MF319 developer trapped inside the cavities. Finally, the sample was immersed in isopropanol to replace the water.

A Tousimis (Rockville, MD, USA) critical point dryer was used to completely release the membranes, avoiding any stiction problems43. In order to make our membranes watertight, a 3.67 μm layer of Parylene-C was deposited in a low-pressure chamber, creating a sealed vacuum cavity inside the CMUT cell. In addition to providing excellent electrical insulation, Parylene-C is also biocompatible, optically transparent, has a low Young’s modulus, and has a coefficient of water absorption close to zero44. The dimensions of the fabricated CMUTs are specified in Table 1. The materials used allow for a visual inspection of the fabricated device, facilitating the identification of any possible defect.

Table 1 Parameters of fabricated polymer CMUTs Full size table

A slight modification in the process can lead to the fabrication of polyCMUTs on flexible substrates. Polyimide is a good candidate for this purpose given its elevated thermal stability, strong chemical resistance, and potential biocompatibility45,46; moreover, it has an excellent adhesion to metals and SU-8 films41,47,48. This polyimide film needs to be temporarily fixed to a rigid carrying substrate prior implementing the fabrication steps outlined in this article. This fabrication approach is still under investigation.

By using SU-8 and maskless lithography, a fully functional prototype was created in 17 h. This compares favorable to multi-user wafer fabrication services, such as MEMSCAP49 or MicraLyne50, where multiple designs are combined and processed with fixed fabrication protocols and long turnaround times. It is important to mention that the described fabrication process employs non-hazardous materials, that is, only organic solvents are used during manufacturing (acetone, isopropanol, SU-8 developer, and positive photoresist developer). The health risks associated with an accidental prolonged exposure to these materials do not go beyond drowsiness or minor skin irritation. This contrasts with the hazardous gas silane required to deposit silicon nitride and silicon dioxide in traditional CMUT fabrication51.

The final performance of the device is affected by several design parameters such as cavity height, membrane radius and thickness, electrode size, etc. The inherent resolution of the UV exposure system used allowed us to have release channels measuring 5 μm in width and 15 μm in length. Based on experimentation, a membrane radius of 50 μm gave us an acceptable fill factor while maintaining a low risk of stiction during releasing.

The CMUT cavity was designed to account for the natural deflection of the membrane caused by the atmospheric pressure after vacuum sealing. The remaining gap should be large enough to allow the membrane to vibrate during normal operation given the maximum allowed voltage.

The thickness of the first layer of SU-8 depends on the maximum operating voltage of the CMUT; it should be as thin as possible to maintain a small effective gap between electrodes while providing a good electrical insulation to prevent any voltage breakdown in case the membrane is brought in contact with the bottom electrode. A thermal annealing of this layer removes any possible microcracks and enhances the adherence of SU-8 to the substrate and to the subsequent chromium layer. A gradual cooling procedure is recommended as it reduces the risks of delamination.

Chromium was chosen as the top electrode material based on the excellent adhesion strength to SU-8 (ref. 41) and to the fact that it requires one single masking layer during lift-off. The metallization radius was set to 46% of the membrane radius in order to increase the bandwidth of the final device25. This metal layer should be as thin as possible to minimize its effects on the mechanical properties of the membrane. A 500 nm layer was sufficient to maintain a good electrical conductivity between electrodes. Having a layer too thin can result in resistive interconnection paths that, combined with the capacitance of the CMUT cells, would behave as a natural low-pass RC filter that degrades the driving signals during excitation.

The thickness of the second SU-8 layer is tailored to the desired operational frequency of the device. Since the effective gap between the top and bottom electrodes (and therefore the operational voltage) is dictated by the cavity height and thickness of the first SU-8 layer, this design allows the fabrication of low-frequency and high-frequency CMUTs with constant operational voltages. This contrasts with the design in traditional CMUTs where the operational voltage increases with membrane thickness. The thickness of this second SU-8 layer should also take into account the effects from the Parylene-C layer during vacuum sealing as it has similar mechanical properties as SU-8.