Spatially variable dielectric layer for digital microfluidics

Abstract

A digital microfluidics device includes an active matrix of actuation electrodes controlled by thin film transistors. The device includes at least two regions of different actuation electrode density. One region can be driven by directly driving the actuation electrodes from a power supply or function generator. In a first, higher density region, a first dielectric layer covers the actuation electrodes. The first dielectric layer has a first dielectric constant and a first thickness. In a second, lower density region, a second dielectric layer has a second dielectric constant and a second thickness that covers the actuation electrodes.

Description

[0001] Related applications

[0002] This application claims priority to U.S. Provisional Application No. 62 / 962,238, filed January 17, 2020. All references, patents, and patent applications disclosed herein are incorporated herein by reference in their entirety. Background Technology

[0003] Digital microfluidic (DMF) devices use individual electrodes to propel, split, and bind droplets in confined environments, thus providing a “lab-on-a-chip”. DMF devices are also known as electrowetting on dielectrics or “EWoD” to further differentiate this approach from competing microfluidic systems that rely on electrophoretic flow and / or micropumps. Figure 1 A typical EWoD device is shown, including propulsion and sensing on the same active matrix. Wheeler, in "Digital Microfluidics," Annu. Rev. Anal. Chem. A 2012 review of electrowetting techniques is provided in 2012, 5:413-40. This technique allows for sample preparation, assays, and synthetic chemistry using small amounts of sample and reagents. In recent years, the use of electrowetting to control droplet manipulation in microfluidic units has become commercially viable, and products are now available from large life science companies such as Oxford Nanopore.

[0004] Typically, EWoD devices consist of a conductor stack, an insulating dielectric layer, and a hydrophobic layer. A droplet is placed on the hydrophobic layer, and once the stack is activated, the droplet deforms and wets or dewets from the surface depending on the applied voltage. Most literature on EWoDs reports concern so-called “passive matrix” devices (also known as “segmented” devices), where 10 to 20 electrodes are directly driven by a controller. While segmented devices are easy to fabricate, the number of electrodes is limited by space and driving constraints. Therefore, large-scale parallel measurements, reactions, etc., are not possible in passive matrix devices. In contrast, “active matrix” devices (also known as active matrix EWoD, or AM-EWoD) can have thousands, hundreds of thousands, or even millions of addressable electrodes. The electrodes are typically switched by thin-film transistors (TFTs), and the droplet motion is programmable, allowing AM-EWoD arrays to be used as general-purpose devices, providing great freedom for controlling multiple droplets and performing simultaneous analysis processes.

[0005] Electrodes are typically switched by thin-film transistors (TFTs), and droplet motion is programmable, allowing AM-EWoD arrays to be used as general-purpose devices, thus providing great freedom for controlling multiple droplets and performing simultaneous analysis processes. TFT arrays are well-suited for this application because they have thousands of addressable pixels, thus allowing for massive parallelization of droplet procedures. In some cases, the pixel electrode sizes of the array can vary; for example, high-density small pixel electrode regions are adjacent to low-density large pixel electrode regions. Regions with different pixel sizes facilitate rapid droplet distribution from memory and subsequent droplet segmentation.

[0006] Traditionally, a single dielectric layer is used across the entire active surface of an EWoD array, including regions with different functions or pixel densities. Because the maximum operating voltage of an electrode depends heavily on the properties of its dielectric, a single dielectric layer produces a relatively uniform maximum operating voltage across the entire device. However, in most analytical applications, different regions of the EWoD array serve different purposes, thus requiring some regions to withstand greater electrical strain, which can lead to voltage leakage and eventually substrate breakdown. These failure modes are particularly severe in the memory regions, which perform repetitive high-voltage processes such as droplet splitting, and because the memory is immovable relative to the array, there is no flexibility to cycle different spatial regions for these processes. Summary of the Invention

[0007] This application addresses a problem typically associated with providing different voltages and / or waveforms to different regions of a digital microfluidic device by introducing a novel structure with a spatially variable dielectric, which is well-suited for enabling different electrodes to operate at different potentials and frequencies. This architecture helps maintain functionality in high-strain regions, such as near memory. Consequently, the digital microfluidic device of this invention has a longer lifespan than digital microfluidic devices without this architecture.

[0008] In one aspect, this application provides a digital microfluidic device comprising a first plurality of electrodes having a first density coupled to a set of switches, a controller operatively coupled to the set of switches and configured to provide a drive voltage to at least a portion of the first plurality of electrodes, and a second plurality of electrodes having a second density and configured to operate at a higher voltage than the first plurality of electrodes. A first dielectric layer having a first dielectric constant and a first thickness covers the first plurality of electrodes, and a second dielectric layer having a second dielectric constant and a second thickness covers the second plurality of electrodes. In one embodiment, the density of the first electrodes is greater than the density of the second electrodes: therefore, the first electrodes form a high-resolution region, while the second electrodes form a low-resolution region. In another embodiment, the dielectric constant of the first dielectric layer is greater than the dielectric constant of the second layer. In another embodiment, the thickness of the first dielectric layer is less than the thickness of the second dielectric layer. The first and second dielectric layers may be continuous or partially overlapping. The device may also include a third plurality of memory electrodes configured to operate at a higher voltage than the first electrodes. In some cases, the device may include only the first and third memory electrodes and not have the second electrodes. In one embodiment, the first electrode is configured to operate at a potential between about 10 V and 20 V. In another non-exclusive embodiment, the second electrode is configured to operate at a potential between about 100 V and about 300 V. In yet another embodiment, the third electrode is configured to operate at a potential between about 100 V and about 300 V. In an exemplary embodiment, the first dielectric layer has a thickness between about 50 nm and about 250 nm. In a further non-exclusive embodiment, the second dielectric layer has a thickness between about 500 nm and about 5 μm. The first electrode may be configured to operate at a first frequency and the electrode may be configured to operate at a second frequency. In one embodiment, the operating frequency of the first electrode is less than the operating frequency of the second electrode. Example types of switches include thin-film transistors (TFTs) and electromechanical switches. Attached Figure Description

[0009] Figure 1 The basic structure of an exemplary EWoD device is shown.

[0010] Figure 2 This is a schematic diagram of a propulsion electrode controlled by a thin-film transistor, as is common in EWoD devices.

[0011] Figure 3A An architecture of an exemplary spatially variable dielectric structure embodiment is shown in the context of an electrowetting on dielectric (EWoD) array. Figure 3B This is a cross-sectional view of two overlapping exemplary dielectrics. Figure 3C This is a cross-sectional view of another example of two partially overlapping dielectrics.

[0012] Figure 4A This is a schematic diagram of an EWoD memory using the standard AM-TFT architecture. Figure 4B This is a schematic diagram of an alternative memory architecture that uses dedicated electrodes that can be directly driven at higher voltages.

[0013] Figure 5 The architecture of a spatially variable dielectric structure is shown in the context of an EWoD array with dedicated memory electrodes. Detailed Implementation

[0014] As disclosed herein, the present invention provides an active matrix dielectric-on-electrowetting (AM-EWoD) device comprising a spatially variable dielectric structure. Therefore, a much larger voltage can be applied in higher dielectric breakdown regions (e.g., memory devices covered with a thicker dielectric) compared to the main array region (e.g., TFT pixels). This architecture allows for the use of different driving schemes in different regions of the EWoD device depending on their dielectric properties. In some cases, the thicker, robust dielectric can be removed and reapplied to the memory or adjacent regions. This design allows for recycling after these regions have undergone complete fatigue, thereby extending the device's lifespan.

[0015] The use of spatially variable dielectrics over a wide area of ​​the AM-EWoD device allows for the independent application of different voltages and / or waveforms to specific regions of the device. Fatigue and breakdown issues are also addressed by allowing higher-stress areas to operate with thicker dielectrics at higher voltages, while preventing catastrophic device failure. Furthermore, the variable dielectric structure increases the actuation strength in the reservoir region, making it easier to overcome capillary forces from the fluid input system. Because the actuation strength can be increased by applying higher voltages, droplets from the reservoir exhibit more predictable breakage, which helps to regulate the volume of each droplet of fluid in the reservoir. Additionally, the higher actuation strength expands the range of materials that can be introduced into the device from the reservoir.

[0016] Generally, thicker dielectrics operating at higher voltages are more resistant to fatigue, while thinner dielectrics, which are inherently more complex and fragile, are more prone to failure under electrical loads. Furthermore, the minimum voltage required for actuation is inversely proportional to the square root of the capacitance, or proportional to the square root of the thickness. Therefore, achieving operation at lower voltages (ideal for high-density TFT arrays) solely through variations in dielectric thickness is challenging. Similarly, using materials with increased dielectric constants requires complex deposition processes and inherent problems related to leakage due to interstitial electronic states, structural deformation, and other factors.

[0017] The basic structure of an exemplary EWoD device is as follows: Figure 1The cross-sectional view is shown. The EWoD 200 includes oil-filled cells 202 and at least one aqueous droplet 204. Cell spacers are typically in the range of 50 to 200 μm, but spacers can be larger. In a basic configuration, such as... Figure 1 As shown, multiple propulsion electrodes 205 are disposed on the substrate, and a single top electrode 206 is disposed on the opposing surface. The unit also includes a top hydrophobic layer 207 on the surface in contact with the oil layer, and a dielectric layer 208 between the propulsion electrodes 205 and the bottom hydrophobic layer 210. (The upper substrate may also include a dielectric layer, but...) Figure 1 (Not shown in the image). The hydrophobic layer is typically 20 to 60 nm thick and prevents droplets from wetting the surface. When no voltage difference is applied between adjacent electrodes, the droplets will remain spherical to minimize contact with the hydrophobic surface (oil and hydrophobic layer).

[0018] When a voltage difference is applied between adjacent electrodes, the voltage on one electrode attracts the opposite charge in the droplet at the dielectric-droplet interface, and the droplet moves toward that electrode, similarly... Figure 1 As shown. As mentioned above, the voltage required for acceptable droplet propulsion depends largely on the properties of the dielectric. AC drive is used to reduce the degradation of the droplet, dielectric, and electrodes by various electrochemical processes. The operating frequency of EWoD can range from 100 Hz to 1 MHz, but for TFTs with limited operating speeds, a lower frequency of 1 kHz or lower is preferred.

[0019] Return to Figure 1 The top electrode 206 is a single conductive layer, typically set to zero volts or a common voltage value (VCOM) to account for the offset voltage on the drive electrode 205 due to capacitive backlash from the TFT used to switch the voltage on the electrode (see [link]). Figure 2 A square wave can also be applied to the top electrode to increase the voltage across the liquid. This arrangement allows for a lower propulsion voltage to be used for the propulsion electrode 205 of the TFT connection, since the top plate voltage is supplemented by the voltage provided by the TFT.

[0020] like Figure 2 As shown, the active matrix of the propulsion electrodes can be arranged to be driven by data lines and gate (select) lines, much like the active matrix in a liquid crystal display. The gate (select) lines are scanned for addressing one row at a time, while the data lines carry the voltage to be transmitted to the propulsion electrodes for electrowetting operations. If no movement is required, or if the droplet is to move away from the propulsion electrode, 0 V is applied to the (non-target) propulsion electrode. If the droplet is to move towards the propulsion electrode, an AC voltage is applied to the (target) propulsion electrode.

[0021] Figure 3AThe architecture of an exemplary spatially variable dielectric structure embodiment is shown in the context of an EWoD array 100. A first dielectric 102, characterized by a dielectric constant ε1 and a thickness t1, is deposited on a high-density region of the array. A second dielectric 104, having a dielectric constant ε2 and a thickness t2, is deposited on a second low-density region of the array, which has driving electronics separate from the high-density region. Figure 3B and 3C As illustrated in the cross-section, the first and second dielectrics can at least partially overlap each other and be formed according to various methods with different deposition sequences. Return to Figure 3A The third dielectric 106 may be formed of either the first or second dielectric material. Alternatively, the dielectric 106 may be made of a third material with a dielectric constant ε3 different from ε1 and ε2. The number of dielectrics may be further extended to four, five, or more, depending on the number of regions present on the EWoD, each requiring its own specific combination of dielectric constant and thickness. In some embodiments, one or more dielectrics may be formed of two or more materials, mixed together, or stacked on top of each other to form a material with the desired effective thickness.

[0022] Equation (1) establishes the relationship between the actuating contact angle θ, the stationary contact angle θ0, the capacitance per unit area C, the voltage V, and the liquid / environment surface tension γ:

[0023] (1)

[0024] EWoD performance is highly dependent on the difference (θ-θ0) between the stationary contact angle and the actuated contact angle. According to equation (2), the capacitance C per unit area is a function of the dielectric constant ε and the dielectric thickness d.

[0025] (2)

[0027] It can be seen that, in order to increase the degree of actuation, it is desirable to have one or more of the following: high dielectric constant, low thickness, and high voltage.

[0028] It is conceivable to adjust the parameter space so that the EWoD device can withstand a breakdown voltage V B Operating at 75% of the capacity, thus making V = 0.75·V B Therefore, the relationship with the breakdown voltage can be seen in equation (3), where F represents the actuation efficiency proportional to the difference in contact angle, and V B It is expressed as dielectric thickness d multiplied by dielectric strength D S V B = D s ·d:

[0029] (3)

[0031] It can be seen that, assuming the operating voltage is close to V B The actuation efficiency will increase with higher thickness and voltage, and this benefit will not be completely offset by the decrease in dielectric constant of a thicker dielectric.

[0032] Equation (4) reflects the minimum voltage V min Proportional to the square root of the dielectric thickness d in equation (2), α is the hysteresis of wetting and dewetting:

[0033] (4)

[0035] This explains why operating at low voltages is quite challenging, as it requires aggressively reducing dielectric thickness or increasing the dielectric constant. The dielectric thickness required for operation in relatively low voltage ranges (e.g., around 10 V) makes devices more susceptible to fatigue and failure. It has also been found that thicker dielectrics operating in high voltage ranges tend to be more robust and provide a larger actuation contact angle compared to conventional low-voltage platforms on thin-film transistors (TFTs).

[0036] Exemplary high-stress EWoD operation includes a reservoir region with a special electrode pattern and a designated medium-density electrode region for low-resolution operation. An example of a reservoir region with special electrodes is shown in... Figure 4A and 4B Examples are provided. For instance... Figure 4A and 4B As shown, gray represents liquid droplets and grid lines represent electrodes.

[0037] Figure 4A This is a schematic top view of a reservoir defined by a relatively high electrode density grid, and the resulting droplets 420 can have different sizes and different aspect ratios. However, in Figure 4A In this configuration, if the electrodes are controlled by TFT switches, the total voltage amplitude is typically limited to between 10 and 20 volts, such as -15V, 0V, and 15V. To reliably generate droplets 420 of the desired size from the storage region 450, the small electrodes must be driven at high frequencies with the maximum voltage difference, increasing the likelihood of failure in this region.

[0038] As an alternative, such as Figure 4B As shown, dedicated electrodes 470 and 475 that can be driven by higher voltages can be implemented. Furthermore, since the memory region 450 occupies a large area, fewer electrodes (e.g., lower density) can be used to address this region, thereby simplifying manufacturing and reducing costs. Figure 4BAs shown, direct-drive (i.e., segmented) electrodes of various sizes can be used to facilitate rapid and consistent segmentation into the desired sample droplets 420. Furthermore, the reservoir region 450 typically requires more frequent actuation (constant or periodic) to form and distribute droplets to prevent fluid escape from the reservoir region 450. This results in increased voltage strain in the reservoir region. The present invention allows for greater electrowetting forces to be generated in more reservoir regions and enables operation of the reservoir and adjacent regions independently of the rest of the EWoD array in terms of voltage and frequency. By coupling dedicated electrodes 470, 475 to low-voltage TFT electrodes, such as... Figure 4B As shown, identical droplets 420 can be formed, and then directly addressed, thus allowing, for example... Figure 4A It features variable frequency operation and advanced waveform modes, but with higher reliability.

[0039] Figure 5 The architecture of a spatially variable dielectric structure is illustrated in the context of an EWoD array 500 with regions having different electrode densities. This embodiment includes a substrate 502, a low-voltage TFT array 504 operating in the range of approximately 10V to 20V, and high-voltage electrodes directly driven by an external source at a variable frequency and operating in the range of approximately 100V to approximately 300V. The high-voltage electrodes comprise a regular grid of custom-designed memory electrodes 506 and adjacent low-resolution mobile electrodes 508. A thicker, more robust dielectric covers the high-voltage electrode regions. The thicker dielectric is typically in the range of approximately 500 nanometers (nm) to approximately 5 micrometers (μm) and may comprise materials with low or medium dielectric constants. Example materials suitable for the thick dielectric include polymers such as parylene, fluorinated polymers such as ethylene tetrafluoroethylene (ETFE) and polytetrafluoroethylene (PTFE), or ceramic materials such as titanium dioxide and alumina. The low-voltage regions are covered by a thin dielectric with a high dielectric constant. Typically, the thinner dielectrics range from approximately 50 nm to 250 nm and include ceramic materials such as silicon dioxide, silicon nitride, hafnium oxide, aluminum oxide, tantalum oxide, and barium strontium titanate. In one example, the dielectric covering the TFT array 504 is a hybrid ceramic stack with a high dielectric constant and a thickness of approximately 50 nm to 250 nm, while the dielectric covering the low-resolution moving electrode 508 is a parylene C layer with a thickness of approximately 1 µm.

[0040] Dielectric layers can be fabricated using deposition methods commonly used in the art, such as sputtering, atomic layer deposition (ALD), spin coating, chemical vapor deposition (CVD), and other vacuum deposition techniques. Spatial profiles of dielectrics with two or more different materials and thicknesses can be created using techniques such as shadow masks, photolithography, and dry or wet etching. If desired, regions with high dielectric thicknesses can be stripped for reuse, as their robustness allows them to better withstand repeated actuation.

[0041] It will be apparent to those skilled in the art that various changes and modifications can be made to the specific embodiments of the present invention described above without departing from the scope of the invention. Therefore, the entire foregoing description is to be interpreted as illustrative rather than restrictive.

Claims

1. A digital microfluidic device, comprising: A first plurality of electrodes having a first density and operablely coupled to a set of switches; A controller is operatively coupled to the set of switches and configured to provide a push voltage to at least a portion of the first plurality of electrodes; The second plurality of electrodes, having a second density, are configured to operate at a higher voltage than the propulsion voltage of the first plurality of electrodes; A first dielectric layer having a first dielectric constant and a first thickness, the first dielectric layer covering the first plurality of electrodes, and The second dielectric layer has a second dielectric constant and a second thickness, and the second dielectric layer covers the second plurality of electrodes; Wherein, the first density of the first plurality of electrodes is greater than the second density of the second plurality of electrodes, and wherein the dielectric constant of the first dielectric layer is greater than the dielectric constant of the second dielectric layer, or wherein the thickness of the first dielectric layer is less than the thickness of the second dielectric layer.

2. The digital microfluidic device according to claim 1, wherein, The first dielectric constant of the first dielectric layer is greater than the second dielectric constant of the second dielectric layer.

3. The digital microfluidic device according to claim 1, wherein, The thickness of the first dielectric layer is less than the thickness of the second dielectric layer.

4. The digital microfluidic device according to claim 1, wherein, The first dielectric layer and the second dielectric layer partially overlap each other.

5. The digital microfluidic device of claim 1 further includes a third plurality of storage electrodes configured to operate at a higher voltage than the drive voltage of the first plurality of electrodes.

6. The digital microfluidic device according to claim 1, wherein, The first plurality of electrodes are configured to operate at a potential between 10 V and 20 V.

7. The digital microfluidic device according to claim 1, wherein, The second plurality of electrodes are configured to operate at a potential between 100 V and 300 V.

8. The digital microfluidic device according to claim 1, wherein, The first dielectric layer has a thickness between 50 nm and 250 nm.

9. The digital microfluidic device according to claim 1, wherein, The second dielectric layer has a thickness between 500 nm and 5 μm.

10. The digital microfluidic device according to claim 1, wherein, The first plurality of electrodes are configured to operate at a first frequency, and the second plurality of electrodes are configured to operate at a second frequency.

11. The digital microfluidic device according to claim 10, wherein, The first frequency of operation of the first plurality of electrodes is less than the second frequency of operation of the second plurality of electrodes.

12. The digital microfluidic device according to claim 1, wherein, The switch is a thin-film transistor.

13. The digital microfluidic device according to claim 1, wherein, The switch is an electromechanical switch.

14. The digital microfluidic device according to claim 1, wherein, The first dielectric layer includes silicon dioxide, silicon nitride, hafnium oxide, aluminum oxide, tantalum oxide, or barium strontium titanate.

15. The digital microfluidic device according to claim 1, wherein, The second dielectric layer includes parylene, ethylene tetrafluoroethylene (ETFE), polytetrafluoroethylene (PTFE), titanium dioxide, or aluminum oxide.

16. The digital microfluidic device according to claim 1, wherein, The second dielectric layer comprises a combination of layered materials selected from the group consisting of silicon dioxide, silicon nitride, hafnium oxide, aluminum oxide, tantalum oxide, barium strontium titanate, parylene, ethylene tetrafluoroethylene (ETFE), polytetrafluoroethylene (PTFE), titanium dioxide, and aluminum oxide.

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