Performance evaluation of a curved electrode actuator fabricated without gold/chromium conductive layers

Abstract

Curved electrode actuators displace on the order of a biological cell diameter at low actuation voltages, making them a potential candidate for probing and micromanipulation of cells in an underwater, closed microenvironment. Here, a closed microenvironment is formed by bonding a Polydimethylsiloxane-based cap-like structure to the actuator; however, the presence of Au/Cr conductive layers on the actuator reduce the available silicon surface area of the actuator—resulting in poor adhesion. Therefore, we propose to fabricate the actuators without these conductive layers. In this paper, we demonstrate the absence of these layers has a minimal effect on the actuator’s peak-to-peak displacement and force. The actuator achieves the following performance metrics: large displacement (1–10 μm) and force (1–21.5 μN) range; broad frequency range (1–500 Hz); and low actuation voltages (0–8 V) in viscous media that are necessary for cell probing and micromanipulation applications.

Appendix

The curved electrode electrostatic actuator shown in Fig. 3 is fabricated on a silicon-on-glass (SOG) wafer with the physical parameters given in Table 1. The SOG wafers are fabricated (purchased from PlanOptik, Elsoff, Germany) by anodic bonding of a silicon wafer (<100> p-type boron doped to 100 ppm) to a 500 μm thick Borofloat33 glass wafer and then thinning and polishing the silicon wafer to a thickness of 45 μm. Components are micromachined from the silicon layer, while the glass layer acts as a handling substrate and provides optical access for viewing with an inverted microscope. The typical fabrication workflow is illustrated in steps (a) → (h) in Fig. 9, which results in the fabrication of the actuator with Au/Cr conductive layers that form electrode pads and tracks.

First, on the SOG wafer, a composite metal film comprising of 200 nm Au/30 nm Cr is deposited on the silicon side by e-beam evaporation and patterned by photolithography to make electrode pads and tracks (a, b). Next, an etch mask to be used for bulk micromachining of actuator components is defined by depositing a 1000 nm thick SiO₂ layer using a plasma enhanced chemical vapor deposition (PECVD) process (processed at 250 °C using SiH₄/N₂O gases), patterned using a photoresist mask, and then etched using inductively coupled plasma assisted reactive ion etching process (RIE-ICP; using CHF₃/Ar gases) (c, d). Then, the wafer is diced into 11 mm × 18 mm dies, each containing an actuator; dicing is an aggressive process and performing at this early stage of fabrication increases the yield, as the silicon layer is still monolithic. After dicing, the high-aspect ratio actuator features (curved electrodes, beam electrode, reinforcing beam) are machined by DRIE using a standard Bosch process in 5s:2s step cycles; the glass substrate acts as an etch stop (e). After DRIE, the movable structures of the actuator are released by timed HF (49% HF:H₂O::1:1 volume ratio) wet etch process for 3 min at standard room temperature, triple rinsed in water, then in methanol, and then dried at room temperature (f). Next, the actuator is coated with 10 nm thick Al₂O₃ insulator film using atomic layer deposition (ALD) process (processed at 120 °C using TMA/O₂ precursors) (g). Finally, the 10 nm thick Al₂O₃ insulator film is anisotropically etched using inductively coupled plasma assisted reactive ion etching process (RIE-ICP; using BCl₃/Cl₂ gases) to unmask the Au/Cr electrode pads that were coated with Al₂O₃ during the ALD process (h); the anisotropic etch does not appreciably effect Al₂O₃ on the sidewalls. This completes the typical fabrication approach.

To study the effect of not having the Au/Cr conductive layers, we wet etch the Au/Cr layers using corresponding etchants (i). This completes the fabrication process of the actuator (without conductive layers) used in the study.

Fig. 9

Fabrication scheme of the actuator. Steps (a) → (h) are the traditional steps in the fabrication. Actuators from step (h) have conductive layers and are used for benchmarking purposes. To study the effect of not having the Au/Cr conductive layers, we fabricate the actuators by wet etching the Au/Cr layers using respective wet etchants [step (i)].