Patent Application
20180224384
Test methods measure some property of a sample. That property may be used to make inferences about the sample. For example, measuring the conductivity of salt water may allow inference of the ion concentration in the salt water. However, in measurement of sample properties, the measurement is also a function of the piece of equipment and the environment. Accordingly, when assessing measurements, it is relevant to consider the impact of equipment and the environment. It also follows that, depending on the environment, some equipment designs may be more or less effective at producing accurate measurements of sample properties. Accordingly, it is desirable for measurement systems to minimize environmental noise and produce accurate, repeatable measurements.
The accompanying drawings illustrate various examples of the principles described herein and are a part of the specification. The illustrated examples are intended to describe possible implementations and do not limit the scope of the claims. Like numerals denote similar but not necessarily identical elements.
One property relevant to the characterization of a fluid is the impedance of the fluid. Impedance is the relationship between voltage and current through the fluid. Impedance may be defined as the effective resistance to alternating current.
Measuring impedance of a solution may be performed by measuring the voltage and current between two electrodes in contact with the solution. However, current may be distributed non-uniformly across the surface of the electrodes. It can be conceptually helpful to think of the current as distributing through a number of parallel paths traveling between the two electrodes. The current distributes so that the voltage drop is equal for all the paths. Thus, if one path is less resistive and causes a smaller voltage drop, the amount of current traveling through that path increases until the voltage drop is the same. Similarly, if a given path has a higher voltage drop, then the amount of current traveling that path drops until the voltage drop is equal. One contributor to the variation in the paths is the geometries and relative positions of the electrodes.
With a pair of flat, infinite electrodes separated by a fixed distance, the field between the two electrodes is the same everywhere, and any measurement is similarly uniform. However, creating large electrodes may be challenging in many designs due to cost and space constraints. The introduction of real world geometries can produce non-uniformity. For example, consider two square electrodes separated by a fixed distance. Assuming the separation is small compared with the size of the electrodes, the centers of the electrodes act very much like the pair of infinite electrodes. However, the edges and corners act differently. The edge has a lower resistance to current flow because the current can flow not just straight between the plates but also out beyond the edge of the plate. This lower resistance, in turn, produces greater current flow through the edge of the plate than through an equivalent area in the center of the plate. This non-uniform distribution of current is used in electro-polishing and other electrochemical techniques. Broadly speaking, the larger the amount of fluid through which the current from a given area of an electrode can flow, the more current will flow through that portion of the electrode. Thus, peaks, edges, dendrites, points, etc. all show increased current flow per unit of surface area while valleys, holes, hollows, etc. show decreased current flow per unit of surface area.
Returning to the two plate electrodes, if the edge is a small fraction of the surface area of the plate, then its contribution to the measurement may be small compared to the overall relatively uniform behavior of the two plates. However, as the area of the non-uniform areas increases, the impact on the measurement increases and may eventually come to dominate the measurement. This may present a challenge for small electrode systems. In such systems, the edge effects play a significant role in the measurements. This is because, as the electrodes become smaller, the ratio of edge to area of the electrode increases, similar to the way that small particles have very high surface area to mass ratios. Accordingly, as electrode areas become small, the impact of edge effects become larger. Thus, for microelectrodes, achieving uniformity of the field between the two electrodes allows robust measurement.
Another challenge with microelectrodes is that they tend to be placed in close proximity to each other and to other parts of a micro-electromechanical system (MEMS). Close proximity can result in crosstalk and interference to the microelectrode which may increase the noise and/or reduce signal and result in a decreased signal to noise ratio (S/N ratio). Accordingly, there is a need for microelectrode designs that facilitate accurate, repeatable measurement of solution properties.
Accordingly, the present specification describes, among other examples, an electrode system, An electrode system, the system comprising: a substrate defining a plane; a first, inner electrode, on the substrate; a second, outer electrode with a cutout, on the substrate, wherein the first electrode is inside the cutout of the second electrode; and an insulating layer separating the first electrode and the second electrode.
The present specification also describes a method of making an impedance measurement, where the method comprises: measuring an impedance of a solution between a first electrode and a second electrode, wherein the first electrode has a circular perimeter, the second electrode has a round interior cutout, and the first electrode is centered in the cutout of the second electrode.
The present specification also describes a system for making electrical measurements, the system comprising: a first electrode and second electrode on a surface. The first electrode has a smooth outer perimeter. The second electrode completely surrounds the first electrode on the surface. The minimum separation between the first electrode and the second electrode on the surface is uniform at all points of a circumference of the first electrode.
Turning now to the figures:
The first electrode (110) may be formed from any suitable conductive material. In one example, the electrode is formed from gold due to its inertness and conductivity. Other potentially used materials include platinum, platinum-group metals, silver, copper and/or alloys thereof. Alternately, metals which form resistive surface oxides can be used, for example, tantalum, titanium, aluminum and similar metals. Conductive polymers, fibers, and carbon black loaded organics are also options.
The second electrode (120) may be formed from the same material as the first electrode (110) or a different material. The use of the same material may reduce the number of operations to manufacture the electrodes (110, 120). It may also avoid generating a galvanic potential between the first electrode (110) and the second electrode (120). Similarly, other features of the may be formed in the same layer, for instance, a portion of a firing electrode may be formed simultaneously. Alternately, other sensor components or conductive traces may be formed simultaneously. With microelectrodes, a number of electrodes may be formed in close proximity, resulting in increased potential for interference. The presence of the surrounding second electrode (120) reduces the noise and crosstalk from other sources, including other electrodes. The second electrode (120) can be connected to a ground plane.
The area between the first (110) and second electrodes (120) includes a non-conductive portion of the substrate (130). This material does not have to be strongly insulating, just sufficiently resistive so as not to affect the measurement of the impedance through the fluid. If the fluid is moderately conductive, e.g., includes water and an ionic species, then as long as the substrate layer (130) is not a conductor, accurate measurements of the fluid properties can be obtained although some calibration may be performed in order to baseline the measurements. A wide variety of suitable materials are used in semiconductor fabrication. Further, it may be helpful to use a material that is being deposited or formed as part of another manufacturing operation to avoid adding additional processing operations. Some example materials include silicon, doped silicon, silicon-oxide, silicon-nitride, epoxies (such as SU-8), polymers (such as polyimide), and various other metal oxides, nitrides, carbides, and mixtures thereof.
In
In one example, the surface area of the first (110) and second electrodes (120) are the same. In one example, the outer electrode (120) is connected to ground. The second, outer electrode (120) serves to shield the first, inner electrode (110) and reduce the impact of other electrical operations near the electrode system (100).
The first electrode (110) is surrounded by the second electrode (120). The first (110) and second electrodes (120) are separated on a substrate (130). The substrate (130) acts as an insulating layer between the first electrode (110) and the second electrode (120). If the first electrode (110) and second electrode (120) were in direct electrical contact then the short would prevent measurement of a fluid in contact with the first and second electrodes (110, 120). In one example, the first electrode (110) is centered relative to an opening in the second electrode (120). The first electrode (110) may have a circular cross section and/or exposed area. The second electrode (120) may have a circular opening. The second electrode (120) may be a ring. The first and second electrodes (110, 120) may have equivalent surface areas. The surface area of an electrode is the area of the electrode exposed (or that will be exposed) to the solution being tested.
The first electrode (110) may have an outer radius of approximately 47 micrometers and the second electrode (120) may have an inner radius of approximately 54 micrometers and an outer radius of approximately 72 micrometers. The outer electrode (120) may have an outer diameter between approximately 20 micrometers and 300 micrometers. The outer electrode (120) may have an outer diameter between approximately 40 and 150 micrometers. The outer electrode (120) may have an outer diameter between approximately 40 and 200 micrometers. As used within this specification, the term approximately when applied to a dimension indicates to within +/−10% of the listed value of the dimension.
An electrode system with a flush surface, as shown in
In one example, the first electrode (110) is an oval. The first electrode (110) may be oblong. The first electrode (110) may be a circle centered in an opening and/or cutout of the second electrode (120), the cutout also being a circle. This configuration provides uniform minimum distance but also makes the volumes of fluid associated with each point on the perimeter of the first electrode (120) symmetrical. Accordingly, this symmetry further improves the signal to noise ratio by reducing variation.
The outer perimeter of the second electrode (120) may be smooth. In some example, the use of a smooth outer perimeter of the second electrode (120) enhances the field uniformity between the first electrode (110) and the second electrode (120). A smooth outer perimeter of the second electrode (120) may facilitate uniform fluid behavior and/or flow over the electrode system (100). The use of a smooth outer perimeter of the second electrode (120) may help avoid dead zones
In one example, the first electrode (110) and second electrode (120) comprises gold. The conductive layer (690) may be formed of, or may comprise, tantalum. The conductive trace (150) may be formed of aluminum. The substrate (130) may be silicon, including doped silicon. The surface layer (630) may be silicon oxide (SiO2). The insulating layer (360) may be formed of silicon carbide. Any of these materials may be substituted with the alternatives described previously.
The insulating layer (360) may include a rounded top between the first electrode (110) and the second electrode (120). The conductive layer (690) may be exposed to a solution being tested. The tested solution is the solution being evaluated by the electrode. In one example, the tested solution is a control solution, such as, phosphate buffered 0.9 wt. % saline (PBS). In one example, the tested solution is an environmental sample, for example, a water sample. In one example, the tested solution is a biological sample, for example, blood or plasma. In one example, The exposed surface of the conductive layer (690) can be oxidized to minimize current transfer through the exposed portions of the conductive layer (690).
By placing the first electrode (110) in the cutout of the second electrode (120), the second electrode serves to shield the first electrode (110). Further, the circular perimeter of the first electrode (110) combined with the circular cutout of the second electrode (120) provides a highly symmetrical relationship between the first and second electrodes (110, 120) which increases the signal to noise ratio compared with other electrode geometries.
It will be understood that, within the principles described by this specification, a vast number of variations exist. It should also be understood that the examples described are just examples, and are not intended to limit the scope, applicability, or construction of the claims.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2016/015568 | 1/29/2016 | WO | 00 |
