Parylene-based stretchable neural electrodes with serpentine interconnects

Parylene C has rapidly gained popularity as a flexible and biocompatible material for next generation chronic probes. However, the mechanical strains attributed to the intracranial pressure and micromotion may compromise the longevity and biostability of implanted neural devices. To obtain conformable bioelectronic interfaces, it is essential to incorporate serpentine metal interconnects in flexible neural electrodes to achieve stretchability. In this paper, the influence of straight segments on the mechanical behavior of serpentine-shaped Parylene C probes has been investigated by finite element analysis. The etching performance of Parylene C with different masks are compared and the optimum masking material is the SiO2 film grown at a low temperature by inductively coupled plasma chemical vapor deposition (ICP-CVD). In vitro electrochemical measurements verify excellent electrode function with a low impedance of 13±0.4 kΩ at 1 kHz, which is beneficial for high-resolution neural recording.


Introduction
Flexible neural interfaces have been popularly used as a neurophysiological tool for recording the electrical brain activity.High-quality inorganic electronic materials such as metals have been utilized for fabricating high-performance electrocorticogram (ECoG) microelectrode arrays.However, the mechanical mismatch between the inorganic electrode materials and brain tissues may trigger an immune response which would impact the long-term stability [1].Although in-plane serpentine structures have been applied as the stretchable configuration to accommodate the mechanical deformation, the serpentine shape remains to be optimized under practical constraints.Parylene C has been widely used in implantable devices due to its excellent biocompatibility and processability.It remains challenging to find an appropriate mask for the Parylene C etching.As standard photoresists cannot resist the strong oxygen plasma, the etching selectivity is poor.Oxygen plasma etching with nickel or titanium metal masks may create residues due to the micro-masking.In this work, the Parylene C etching performance has been carefully compared to propose an optimized etching strategy.The optimal serpentine shape of a unit cell has been determined by the mechanical finite-element analysis to achieve higher stretchability.The characteristics of Parylene C based stretchable neural electrodes have been assessed by the electrochemical impedance spectroscopy (EIS).

Design parameters of serpentine interconnect
The shape of the serpentine interconnect is closely correlated with the stretchability and compliance.The serpentine unit cell can be analytically described by four independent geometric parameters as illustrated in Fig. 1(a): the ribbon width w, the arm length L, the arc radius R, and the breadth Y [2].Considering the limited size of the target brain region, the geometric parameters are optimized to construct more stretchable serpentine interconnects, provided the same ribbon width and breadth.The commercial software ABAQUS has been used to perform plane strain modelling of the serpentine cell.The stretchability can be enhanced by more than three folds when the arm length L increases from 0 to 120 μm as shown in Fig. 1(b).Meanwhile, the increase of L leads to the gradual reduction of the maximum principal strain and the decreasing tendency slows down.Thus, L=90 μm has been determined as the optimized parameter of the serpentine unit cell.Parylene C etching Firstly, a layer of 200 nm aluminum was deposited using electron beam evaporation and subsequently patterned by photolithography on the Parylene C film as the etching mask.Parylene C etching was performed by a reactive ion etching (RIE) system under the plasma power of 200 W and oxygen flow rate of 30 sccm.After oxygen plasma etching, the nanoforest structures appeared on the etched surface of Parylene C possibly due to metal sputtering during etching as shown in Fig. 2(a).The strong plasma induced micro-masking may attack the Parylene layer underneath and compromise the device functionality.The Parylene residues were also reported to be observed in the openings with nickel or titanium metal masks [3].
Standard photoresists cannot withstand strong oxygen plasma and deteriorate rapidly with the thermal gradient, thus reducing the etching selectivity.A 14 μm-thick AZ 4620 photoresist was used as the photoresist mask for Parylene C etching.Based on the etching depth measurement using a profilometer as shown in Fig. 2(b), It can be noted that the sidewall for the thick photoresist was not steep, which would inevitably reduce the pattern accuracy.
The SiO2 film exhibits high etching selectivity and plasma-resistance, rendering it an ideal choice as a hard mask for Parylene C etching.Hence, the SiO2 etching mask with a thickness of 200 nm was deposited using inductively coupled plasma chemical vapor deposition (ICP-CVD) at a low temperature of 75°C and patterned by lift-off as shown in Fig. 2(c).The conformal coverage of the SiO2 etching mask is able to withstand high power oxygen plasma, with no observable residues and sharp profile cut as illustrated in Fig. 2(d).Fabrication of neural electrode Based on the study of serpentine interconnects and etching masks, an optimized fabrication method has been proposed.An adhesion promoter (A-174 silane) was added to the deposition chamber immediately before the deposition of Parylene C. Firstly, a 3-inch silicon wafer was cleaned with acetone and isopropyl alcohol (IPA), and then 5 μm-thick Parylene C was deposited by chemical vapor deposition on the silicon wafer in MQP-3001 system.Next, the AZ 5214 photoresist was spin-coated and patterned followed by electron-beam evaporation of a 10/100 nm Ti/Pt metal layer to form microelectrode sites, traces, and contact pads as shown in Fig. 3(a).Subsequently, the neural device was again encapsulated with Parylene C and A-174 silane for electrical insulation.Then, a 200 nm-thick SiO2 layer was deposited by ICP-CVD at 75 °C and patterned by photolithography process to form a hard mask for dry etching of Parylene C under an oxygen atmosphere.After removing the SiO2 mask in a diluted HF solution, the device was released from the silicon substrate by UV laser cutting followed by soaking in the deionized water as shown in Fig. 3(b).A laser-cut polyimide film was attached on the backside of contact pad sites to facilitate the insertion into the zero-insertion force (ZIF) connector, as demonstrated in Fig. 3(c

Electrochemical characterization
The electrochemical impedance spectroscopy (EIS) has been performed in phosphate buffered saline (PBS) with a potentiostat (PP 211, Zahner, Germany).Figure 4 shows the EIS results of the impedance and phase across the frequencies of interest (1 Hz-100 kHz).The average impedance at 1 kHz is 13±0.4kΩ in PBS.It can be inferred from the corresponding phase plot of the impedance that the microelectrode exhibits the capacitive behavior in the low frequency range and becomes more resistive at higher frequencies.Similar trends have been observed in each channel, indicating an excellent reliability within the neural electrode array [4].

Figure 1 .
Figure 1.(a) Schematics of a serpentine unit cell where w, L, R, and Y represent the ribbon width, the arm length, the arc radius, and the breadth, respectively.(b) Finite element analysis results of the elastic stretchability as a function of L.

Figure 2 .
Figure 2. (a) Optical images of Parylene C after O2 plasma etching using the aluminum mask.(b) The step profile of the AZ4620 mask after O2 plasma etching.(c) Surface morphology and (d) step profile of the ICP-CVD grown SiO2 mask after O2 plasma etching. ).

Figure 3 .
Figure 3. (a) Optical image of the recording sites and serpentine interconnects of nine-channel arrayed neural electrode.(b) Photograph of the neural electrode released from wafer.(c) Attachment of a polyimide film onto the back of electrode pad sites.

Figure 4 .
Figure 4. Representative Bode magnitude and phase plots for the parylene C-based neural electrodes in PBS at room temperature.