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.
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).
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 potential1,2,3,4,5,6.
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 characteristic redox reaction of the Ag/AgCl electrode in a chloride containing electrolyte. For the oxidation path, the silver at the imperfection sites on the surface of the electrode combines with the chloride ions in the solution to form soluble AgCl complexes, in which they diffuse to the edges of the AgCl deposited on the surface of the electrode for precipitation in the form of AgCl. The reduction path involves the formation of soluble AgCl complexes using the AgCl on the electrode. The complexes diffuse to the silver surface and reduces back to elemental silver7,8.
The morphology of the AgCl layer is a pivotal influence in the physical property of Ag/AgCl electrodes. Various works showed that the large surface area is key to form reference Ag/AgCl electrodes with highly reproducible and stable electrode potentials9,10,11,12. Therefore, researchers have investigated methods to create Ag/AgCl electrodes with a large surface area. Brewer et al. discovered that using constant voltage instead of constant current to fabricate Ag/AgCl electrodes would result in a highly porous AgCl structure, increasing the surface area of the AgCl layer11. Safari et al. took advantage of the mass transport limitation effect during AgCl formation on the surface of silver electrodes to form AgCl nanosheets on top of them, increasing the surface area of the AgCl layer significantly12.
There is a rising trend to design AgCl electrode for sensing applications. A low contact impedance is crucial for sensing electrodes. Thus, it is important to understand how the surface coating of AgCl would affect its impedance property. Our previous research showed that the degree of AgCl coverage on the silver electrode has a pivotal influence on the impedance characteristic of the electrode/electrolyte interface13. However, to correctly estimate the contact impedance of thin film Ag/AgCl electrodes, the AgCl layer formed must be smooth and have well-controlled coverage. Therefore, a method to form smooth AgCl layers with designated degrees of AgCl coverage is needed. Works have been done to address this need partially. Brewer et al. and Pargar et al. discussed that a smooth AgCl can be achieved using a gentle constant current, fabricating the AgCl layer on top of the silver electrode11,14. Katan et al. formed a single layer of AgCl on their silver samples and observed the size of individual AgCl particles8. Their research found that the thickness of a single layer of AgCl is around 350 nm. The aim of this work is to develop a protocol to form fine and well-controlled films of AgCl with predicted impedance properties on top of silver electrodes.
1. Fabrication of a Cr/Au adhesion layer using liftoff
2. Fabrication of thin film Ag electrodes on the adhesion layer using liftoff
3. Passivation of the wafer to expose only the electrodes and contact pads
4. Preparation for the fabrication of thin film Ag/AgCl electrodes (chip)
5. Preparation for the fabrication of thin film Ag/AgCl electrodes (reagents)
6. Preparation for the fabrication of thin film Ag/AgCl electrodes (macro electrodes)
7. Cathodic cleaning of the micro Ag electrodes
NOTE: All of the following processes use the CHI660D electrochemical analyzer/workstation and its accompanying software.
8. Fabrication of single layer AgCl on top of the thin film Ag electrodes
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, the layer of AgCl is only a single layer, which is proved by the absence of stacked AgCl particles and a distinctive Ag/AgCl intersection. Figure 2 shows more successful examples of thin film Ag/AgCl electrodes fabricated using this protocol, which are 80 µm x 80 µm electrodes with a designated AgCl coverage of 70% and 30%, together with 160 µm x 160 µm electrodes with a designated AgCl coverage of 75% and 90%, confirming the robustness of this protocol.
Figure 1: Exemplar SEM image of the thin film Ag/AgCl electrode with a dimension of 80 µm x 80 µm and designated AgCl coverage of 50%. The observed AgCl coverage is 55%, demonstrating the effectiveness of the protocol. This figure has been modified from Tjon et al.13. Please click here to view a larger version of this figure.
Figure 2: Exemplar SEM images of thin film Ag/AgCl electrodes with various electrode areas and AgCl coverages. (A) 80 µm x 80 µm with 70% AgCl coverage. (B) 80 µm x 80 µm with 30% AgCl coverage. (C) 160 µm x 160 µm with 75% AgCl coverage. (D) 160 µm x 160 µm with 90% AgCl coverage. These figures have been modified from Tjon et al.13. Please click here to view a larger version of this figure.
Figure 3 illustrates a negative result where the polishing step is omitted (i.e., step 4.2). Figure 3A shows a polished electrode surface whereas Figure 3B shows an unpolished electrode surface. For the unpolished electrode, finger-like structures can be observed on the surface, which is illustrated in Figure 4, where the polished electrode surface is smooth with minor scratch marks caused by the polishing process. Figure 5 shows an unpolished 80 µm x 80 µm Ag/AgCl electrode with a designed AgCl coverage of 50%. By observation, the area of the sparsely covered AgCl is only around 40 µm x 40 µm, which is 25% of the apparent electrode surface area. Furthermore, compared to Figure 1 where the protocol is properly observed, for the unpolished electrode, the AgCl formed appears to be recessed inwards instead of protruding outwards.
Figure 3: SEM images for bare silver electrodes. (A) Polished 160 µm x 160 µm electrode (B) Unpolished 40 µm x 40 µm electrode. Please click here to view a larger version of this figure.
Figure 4: Zoomed SEM image for unpolished silver electrodes. Finger-like structures can be observed. Please click here to view a larger version of this figure.
Figure 5: Suboptimal fabrication of thin film Ag/AgCl electrodes. Without polishing, the degree of coverage of the formed AgCl on the surface of the electrode is smaller than the predicted value. The designed AgCl coverage for this 80 µm x 80 µm thin film silver electrode is 50% but the actual coverage is only 25%. Please click here to view a larger version of this figure.
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:
Where X is the thickness of a single AgCl layer in cm (350 nm = 3.5 x 10-5 cm); P% is the percentage of AgCl coverage on the surface of the Ag electrode (100% = full coverage); j is the applied current density in A/cm2 (0.5 mA/cm2), M is the molar weight of AgCl (143.5 g/mol), t is the duration of anodization in s (262 s for 100% coverage); F is the Faraday's constant (~96485 C/mol); D is the density of AgCl (5.56 g/cm3). To ensure the success of the protocol, several critical steps in the protocol must be observed. Step 4.2, which is about the polishing of the surface of the thin film silver electrode, is crucial to define the surface area of the electrode prior to the galvanostatic formation of AgCl on the electrode surface. From Figure 3 and Figure 4, the difference in the surface structure and roughness of the thin film silver electrodes fabricated by sputtering can be seen clearly; the unpolished silver surface has finger-like structures, whereas the polished silver surface is mostly smooth with minor scratch marks caused by sandpaper rubbing. This creates a major problem as the finger-like structures effectively increases the surface area of the electrode. This makes the determination of the surface area of the electrode and subsequently the degree of AgCl coverage on the electrode impossible. The effect of this is well-illustrated in Figure 1 and Figure 5. The protocol compliant Ag/AgCl electrode has a smooth, single layer of AgCl with a well-controlled AgCl coverage, whereas the electrode without observing the polishing step has an overestimated coverage of AgCl on the electrode. Step 8.6.2, which is about the use of a constant current density of 0.5 mA/cm2 to form the AgCl layer on the thin film Ag electrode, is crucial to create a smooth AgCl layer with single layer thickness. Newly formed AgCl deposits at the edges of existing AgCl on the surface due to its low energy7,8. This allows the AgCl particles to form a single layer first before growing thicker. However, if a high current density is applied during the galvanostatic formation of the AgCl layer, the newly formed AgCl could have enough energy to form directly on the electrode other than along the existing AgCl edges, creating a rougher AgCl surface14. This makes the degree of AgCl coverage on the electrode unable to be controlled as the AgCl formation sites cannot be predicted under such condition. Also, this makes the determination of the AgCl surface area impossible as its roughness affects the surface area, which was shown to affect the electrode's impedance characteristics in our previous work13.
There are several ways to troubleshoot whether a single layer of AgCl is formed properly. Firstly, to check whether the polishing step is carried out successfully. The sample should be observed under an SEM microscope without gold coating after the polishing step to see whether the finger structure is replaced by a smooth surface. Furthermore, when the surface of the electrode is fully covered with AgCl, further galvanostatic oxidation will cause a sudden increase in the applied potential to the system as the thickening of the AgCl increases the ohmic resistance of the AgCl layer. This can be used to determine whether the surface of the electrode is fully covered with AgCl already.
There is a major limitation concerning the use of this method to fabricate thin film Ag/AgCl electrodes with good control of the AgCl coverage. Electrodes fabricated using this method are not reworkable. During the process of galvanostatic oxidation of the silver electrode to form the AgCl deposit layer, the sites of imperfection on the surface of the electrode will grow in size in an unpredictable way. If the electrode is reduced to revert AgCl back to Ag, it is unable to guarantee that these sites at the electrode surface will be filled back the way it was. Instead, the surface will become rougher. If the surface is re-polished using sandpaper after an attempted rework, some silver will be removed from the surface during polishing. Hence, one can only do this for a few times before the underlying gold layer becomes exposed.
This method, when compared to typical Ag/AgCl electrode fabrication methods, focuses on a fine control of the coverage of AgCl on the surface of the thin film Ag electrode, whereas other methods focus on creating a porous layer of AgCl. From the best of the author's knowledge, this is the first time a protocol is developed to fabricate a single layer of finely controlled AgCl on top of a silver electrode. This is due to different design aims. Most previous works aimed to achieve a reference Ag/AgCl electrode with high electrode potential stability, while our protocol aims to design sensing Ag/AgCl electrode with low contact impedance for impedance sensing systems, such as impedance flow cytometers and interdigitated electrode arrays.
Future experiments can include a more sophisticated polishing step, for example using a polishing system to achieve an even smoother surface. Further investigations can also be carried out to evaluate the quantitative relationship between the thickness of the AgCl layer and the faraday's equation of electrolysis.
The authors have nothing to disclose.
This work was supported by a grant from the RGC-NSFC Joint Fund sponsored by the Research Grants Council of Hong Kong (Project No. N_HKUST615/14). We would like to acknowledge Nanosystem Fabrication Facility (NFF) of HKUST for the device / system fabrication.
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 |