Fabrication of Thin Film Silver/Silver Chloride Electrodes with Finely Controlled Single Layer Silver Chloride

Summary

This paper aims to present a method to form smooth and well-controlled films of silver chloride (AgCl) with designated coverage on top of thin film silver electrodes.

Abstract

This paper aims to present a protocol to form smooth and well-controlled films of silver/silver chloride (Ag/AgCl) with designated coverage on top of thin film silver electrodes. Thin film silver electrodes sized 80 µm x 80 µm and 160 µm x 160 µm were sputtered on quartz wafers with a chromium/gold (Cr/Au) layer for adhesion. After passivation, polishing and cathodic cleaning processes, the electrodes underwent galvanostatic oxidation with consideration of Faraday's Law of Electrolysis to form smooth layers of AgCl with a designated degree of coverage on top of the silver electrode. This protocol is validated by inspection of scanning electron microscope (SEM) images of the surface of the fabricated Ag/AgCl thin film electrodes, which highlights the functionality and performance of the protocol. Sub-optimally fabricated electrodes are fabricated as well for comparison. This protocol can be widely used to fabricate Ag/AgCl electrodes with specific impedance requirements (e.g., probing electrodes for impedance sensing applications like impedance flow cytometry and interdigitated electrode arrays).

Introduction

The Ag/AgCl electrode is one of the most used electrodes in the field of electrochemistry. It is most commonly used as the reference electrode in electrochemical systems due to its ease of fabrication, non-toxic property and stable electrode potential^1,2,3,4,5,6.

Researchers have attempted to understand the mechanism of Ag/AgCl electrodes. The layer of chloride salt on the electrode has been found to be a fundamental material in the characterist....

Protocol

1. Fabrication of a Cr/Au adhesion layer using liftoff

1. Spincoat HPR504 positive photoresist of 1.2 µm thickness onto a quartz wafer using a spread speed of 1,000 rpm for 5 s and a spin speed of 4,000 rpm for 30 s.
2. Softbake the photoresist on the quartz wafer at 110 °C for 5 min on a hot plate.
3. Using a mask aligner, expose the wafer such that locations for Cr/Au deposition are exposed with ultraviolet (UV) light. The exposure power density and time is 16 mW/cm² and 7.5 s respectively (exposure energy density = 120 mJ/cm²).
4. Develop the wafer by submersing it in positive resist developer FHD-5 for ....

Results

Figure 1 shows an 80 µm x 80 µm Ag/AgCl electrode with a designed AgCl coverage of 50% fabricated following this protocol. By observation, the area of the AgCl patch is around 68 µm x 52 µm, which corresponds to around 55% of AgCl coverage. This shows that the protocol can finely control the amount of AgCl coverage on the thin film Ag electrodes. The AgCl layer fabricated is also very smooth, as evident by the clumping of adjacent AgCl particles. Furthermore,.......

Discussion

The physical properties of an Ag/AgCl electrode is controlled by the morphology and the structure of the AgCl deposited on the electrode. In this paper, we presented a protocol to precisely control the coverage of a single layer of AgCl on the surface of the silver electrode. An integral part of the protocol is a modified form of the Faraday's Law of Electrolysis, which is used to control the degree of AgCl on the thin film silver electrodes. It can be written as:

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
AST Peva-600EI E-Beam Evaporation System	Advanced System Technology		For Cr/Au Deposition
AZ 5214 E Photoresist	MicroChemicals		Photoresist for pad opening
AZ P4620 Photoresist	AZ Electronic Materials		Photoresist for Ag liftoff
Branson/IPC 3000 Plasma Asher	Branson/IPC		Ashing
Branson 5510R-MT Ultrasonic Cleaner	Branson Ultrasonics		Liftoff
CHI660D	CH Instruments, Inc		Electrochemical Analyser
Denton Explorer 14 RF/DC Sputter	Denton Vacuum		For Ag Sputtering
FHD-5	Fujifilm	800768	Photoresist Development
HPR 504 Photoresist	OCG Microelectronic Materials NV		Photoresist for Cr/Au liftoff
Hydrochloric acid fuming 37%	VMR	20252.420	Making diluted HCl for cathodic cleaning
J.A. Woollam M-2000VI Spectroscopic Elipsometer	J.A. Woollam		Measurement of silicon dioxide passivation layer thickness on dummy
Multiplex CVD	Surface Technology Systems		Silicon dioxide passivation
Oxford RIE Etcher	Oxford Instruments		For Pad opening
Potassium Chloride	Sigma-Aldrich	7447-40-7	Making KCl solutions
SOLITEC 5110-C/PD Manual Single-Head Coater	Solitec Wafer Processing, Inc.		For spincoating of photoresist
SUSS MA6	SUSS MicroTec		Mask Aligner
Sylgard 184 Silicone Elastomer Kit	Dow Corning		Adhesive for container on chip