阅读说明：本技术 具有用于分子识别的纳米孔的隧道结的制造 (Fabrication of tunnel junctions with nanopores for molecular recognition ) 是由 J·托普兰奇克 Z·马吉克 F·米切尔 于 2019-04-09 设计创作，主要内容包括：本技术的实施例可允许改进的和更可靠的隧道结以及制造隧道结的方法。电气短路问题可通过沉积不具有尖锐侧壁和拐角而是相反地具有倾斜或弯曲侧壁的电极来减少。沉积在电极层顶部上的层可然后能够充分覆盖下面的电极层,并且从而减少或防止短路。另外,两个绝缘材料可用作介电层,可减少不完全的覆盖的可能性和剥落的可能性。此外,电极可从接触区域到结区域逐渐减小,以提供薄的电极,在该薄的电极处,孔被图案化,而较厚的接触区域减少薄层电阻。电极还可被图案化为在接触区域处较宽并且在结区域处较窄。(Embodiments of the present technology may allow for improved and more reliable tunnel junctions and methods of fabricating tunnel junctions. Electrical shorting problems can be reduced by depositing electrodes that do not have sharp sidewalls and corners, but instead have sloped or curved sidewalls. The layer deposited on top of the electrode layer may then be able to sufficiently cover the underlying electrode layer and thereby reduce or prevent short circuits. In addition, two insulating materials may be used as dielectric layers, reducing the likelihood of incomplete coverage and the likelihood of flaking. Further, the electrodes may taper from the contact area to the junction area to provide a thin electrode where the holes are patterned, while a thicker contact area reduces sheet resistance. The electrodes may also be patterned to be wider at the contact regions and narrower at the junction regions.)

1. A method of making a system for analyzing molecules, the method comprising:

depositing a first conductive material on a surface of a substrate to form a first electrode having non-vertical sidewalls, the first electrode having a first longitudinal axis, wherein a first plane includes the first longitudinal axis and is orthogonal to the surface of the substrate;

forming an insulating layer on the first electrode;

depositing a second conductive material on the insulating layer to form a second electrode having non-vertical sidewalls, the second electrode having a second longitudinal axis, wherein a second plane includes the second longitudinal axis and is orthogonal to the surface of the substrate, and wherein the first plane and the second plane intersect at a non-zero angle; and

an aperture is defined in the substrate, the first electrode, the insulating layer, and the second electrode.

2. The method of claim 1, wherein forming the insulating layer comprises:

depositing a first insulating material to a first thickness over the first electrode,

a first via is defined in the first insulating material to expose a portion of a top surface of the first electrode,

depositing a second insulating material to a second thickness over the first insulating material, the second thickness being less than the first thickness,

depositing the second insulating material on the portion of the top surface of the first electrode to define a second via,

wherein:

defining the aperture includes removing material defining a portion of a bottom surface of the second via, an

The aperture is defined in the second insulating material and not in the first insulating material.

3. The method of claim 2, wherein:

depositing the first conductive material by biased target deposition, and

defining the first via includes patterning using e-beam lithography and wet etching the first insulating material.

4. The method of claim 1, wherein the non-zero degree angle is from 85 degrees to 95 degrees.

5. The method of claim 1, further comprising:

forming a first resist layer prior to depositing the first conductive material,

defining a first trench in the first resist layer, the first trench having a first width at a bottom of the first trench and a second width at a top of the first trench, the first width being greater than the second width,

removing the first resist layer after depositing the first conductive material,

wherein:

depositing the first conductive material includes depositing the first conductive material at a non-perpendicular angle while rotating the substrate.

6. The method of claim 5, wherein the non-perpendicular angle is from 40 to 50 degrees.

7. The method of claim 5, wherein:

the first width decreases from an end of the first groove along the first longitudinal axis, and

the second width decreases from an end of the first groove along the first longitudinal axis.

8. The method of claim 1, wherein the first electrode tapers in width and thickness along the first longitudinal axis from an end of the first electrode.

9. The method of claim 1, further comprising:

forming a first resist layer prior to depositing the second conductive material,

defining a first trench in the first resist layer, the first trench having a first width at a bottom of the first trench and a second width at a top of the first trench, the first width being greater than the second width,

removing the first resist layer after depositing the second conductive material, wherein:

depositing the second conductive material includes depositing the second conductive material at a non-perpendicular angle while rotating the substrate.

10. The method of claim 1, further comprising:

connecting at least one of the first electrode or the second electrode to a power source, an

Connecting at least one of the first electrode or the second electrode to an electrical meter.

11. A system for analyzing molecules, the system comprising a device comprising:

a first electrode comprising a first conductive material, the first electrode having a non-vertical sidewall, the first electrode contacting a surface of a substrate, the first electrode having a first longitudinal axis, wherein a first plane comprises the first longitudinal axis and is orthogonal to the surface of the substrate;

a second electrode comprising a second conductive material, the second electrode having a non-vertical sidewall, the second electrode having a second longitudinal axis, wherein a second plane comprises the second longitudinal axis and is orthogonal to the surface of the substrate, and wherein the first plane and the second plane intersect at a non-zero angle;

an insulating layer disposed between the first electrode and the second electrode;

wherein:

an aperture passes through the first electrode, the second electrode, and the insulating layer.

12. The system of claim 11, further comprising:

a power source in electrical communication with at least one of the first electrode or the second electrode, and an electrical meter in electrical communication with at least one of the first electrode or the second electrode.

13. The system of claim 12, wherein the power source is a first power source, the system further comprising:

a second power source configured to apply an electric field through the aperture, the second power source not in electrical communication with the first electrode and not in electrical communication with the second electrode.

14. The system of claim 11, the orifice being cylindrical.

15. The system of claim 11, wherein:

the insulating layer comprises a first insulating material and a second insulating material,

a first portion of the second insulating material is disposed between the first insulating material and the second electrode,

a second portion of the second insulating material is disposed between the first electrode and the second electrode,

the first insulating material is characterized by a first thickness,

the second insulating material is characterized by a second thickness,

the first thickness is greater than the first thickness, and

the aperture is defined in the second insulating material but not in the first insulating material.

16. The system of claim 11, wherein:

the first electrode is formed by:

after forming the first resist layer with the overhang, the first conductive material is deposited at a non-perpendicular angle while rotating the substrate.

17. The system of claim 11, wherein:

the first electrode is characterized by a thickness and a width,

the first electrode has an end portion that is,

the thickness of the first electrode decreases gradually from the end portion to a portion of the first electrode defining a portion of the aperture, and

the width of the first electrode decreases gradually from the end to a portion of the first electrode that defines the portion of the aperture.

18. The system of claim 17, wherein:

the second electrode is characterized by a thickness and a width,

the second electrode has an end portion that is,

the thickness of the second electrode gradually decreases from the end portion to a portion of the first electrode defining a portion of the aperture, and

the width of the second electrode decreases gradually from the end to a portion of the second electrode that defines the portion of the aperture.

19. The system of claim 13, further comprising:

a third electrode in electrical communication with the second power source,

a fourth electrode which is provided on the substrate,

wherein:

the aperture is centered on the longitudinal axis, and

the longitudinal axis intersects the third electrode and the fourth electrode.

20. The system of claim 11, wherein the orifice is characterized by a diameter in the range of 2 nm to 30 nm.

21. The system of claim 11, wherein the insulating layer is characterized by a thickness in a range of 1nm to 2 nm.

22. The system of claim 11, wherein:

the device is a first device, and

the system includes a plurality of devices identical to the first device.

23. A method of analyzing a molecule, the method comprising:

applying a voltage across a first electrode and a second electrode separated by an insulating layer, the first electrode having a non-vertical sidewall, the first electrode having a first longitudinal axis, the second electrode having a non-vertical sidewall, the second electrode having a second longitudinal axis, wherein a first plane comprises the first longitudinal axis and is orthogonal to a surface of a substrate, wherein a second plane comprises the second longitudinal axis and is orthogonal to the surface of the substrate, and wherein the first plane and the second plane intersect at a non-zero angle;

contacting molecules to the first electrode and the second electrode across the insulating layer in the aperture;

measuring an electrical characteristic through the first electrode and the second electrode; and

identifying a portion of the molecule based on the electrical characteristic, wherein:

the aperture passes through the first electrode, the second electrode, and the insulating layer.

24. The method of claim 23, wherein:

the insulating layer comprises a first insulating material and a second insulating material,

a first portion of the second insulating material is disposed between the first insulating material and the second electrode,

a second portion of the second insulating material is disposed between the first electrode and the second electrode,

the first insulating material is characterized by a first thickness,

the second insulating material is characterized by a second thickness,

the first thickness is greater than the first thickness, an

A portion of the aperture is defined by the second insulating material.

25. The method of claim 23, wherein the electrical characteristic is current, voltage, or resistance.

26. The method of claim 23, wherein identifying the portion of the molecule based on the electrical characteristic comprises comparing the electrical characteristic to a calibrated electrical characteristic measured from a known molecule or a known portion of a molecule.

27. The method of claim 23, wherein the molecule is a nucleic acid molecule.

28. The method of claim 23, wherein the portion of the molecule comprises a nucleotide.

29. The method of claim 23, wherein identifying the portion of the molecule comprises identifying the portion of the molecule from a predetermined set of portions of the molecule.

Technical Field

The present application relates to systems for analyzing molecules using tunnel junctions, methods of making such systems, and methods of using such systems. Such analysis of the molecules may include sequencing of biopolymers such as nucleic acids.

Background

Nanopores have the ability to detect single molecules, which is a promising technology in the fields of chemical and biological detection. For example, nanopores may be used for nucleic acid sequencing. Solid state nanopores are one type of molecular sensing technology used for rapid biosensing. In some cases, the solid-state nanopore forms a channel in the ionic liquid between two electrodes. The two electrodes may not be part of the nanopore itself, but may be positioned in the ionic liquid. As a molecule passes through a nanopore channel, the current and other electrical properties through the channel change. These electrical properties can provide information about the molecule, but manufacturing issues can make it difficult to identify individual nucleotides in a nucleic acid molecule.

Nanopore devices use tunneling recognition. Tunneling recognition is based on placing the nucleotides of a nucleic acid between electrodes, which may be in the nanopore device itself. The orbital path of the nucleotide will allow electrons to be transferred from one electrode to the other, thereby creating a tunneling current. The size and other characteristics of solid state nanopores can be difficult to adapt to large scale production processes. To sequence nucleic acid molecules with ionic currents, nanopore sizes may need to be on the order of nanometers, e.g., less than 2 nm. Creating channels of such size may require precise and expensive techniques. However, reducing the size of the nanopore may result in incomplete or poor wetting of the nanopore required for use as a sensing device. There remains a need for improvements in the design and manufacturability of nanopore-containing devices for chemical and biological detection, as well as processes involving the devices. Design and manufacturability improvements should not be at the expense of accurate and precise analysis. These and other problems are solved by the techniques described in this document.

Disclosure of Invention

For a tunnel junction, a thin dielectric between two metal electrodes is desired. The tunnel junction may include a hole through the electrode and the dielectric. It is difficult to fabricate these tunnel junctions with dimensions on the order of nanometers. The electrodes may be patterned perpendicular to each other for alignment purposes. However, the vertical alignment of the electrodes can result in short circuits caused by the sharp sidewalls of the electrodes and the thin dielectric covering the sharp sidewalls. In addition, the thin dielectric itself may be a poor barrier to shorting. The metal from the electrodes may be embedded in the dielectric. The metal and dielectric materials may flake off, creating the possibility of voids and shorts. In addition, electrode thickness can also present challenges. Because the holes can be patterned in the electrode, a thin electrode can make patterning easier. However, thin electrodes also result in increased sheet resistance.

Embodiments of the present technology may allow for improved and more reliable tunnel junctions and methods of fabricating tunnel junctions. By depositing electrodes that do not have sharp sidewalls and corners, but instead have sloped or curved sidewalls, the electrical shorting problem can be reduced. The layer deposited on top of the electrode layer may then be able to sufficiently cover the underlying electrode layer and thus reduce or prevent short circuits. In addition, two insulating materials may be used as the dielectric layer, thereby reducing the possibility of incomplete coverage and the possibility of flaking. Furthermore, the electrodes may taper from contact area to junction area to provide a thin electrode where the holes are to be patterned, while thicker contact areas reduce sheet resistance. The electrodes may also be patterned to be wider at the contact regions and narrower at the junction regions.

A better understanding of the nature and advantages of embodiments of the present invention may be obtained with reference to the following detailed description and the accompanying drawings.

Drawings

Figure 1A illustrates a metal-insulator-metal junction according to an embodiment of the present invention.

Fig. 1B and 1C show views of a solid-state nanopore device according to an embodiment of the invention.

Fig. 1D illustrates a region of a solid-state nanopore device according to an embodiment of the invention.

Fig. 2 shows a schematic view of a system 200 according to an embodiment of the invention, the system 200 having an apparatus 201 with electrodes without vertical side walls.

Figure 3A shows a process flow for depositing an insulator layer according to an embodiment of the present invention.

Fig. 3B, 3C, and 3D illustrate cross-sections during a process for forming a nanopore according to an embodiment of the invention.

Figures 4A and 4B illustrate views of junction and contact regions in accordance with embodiments of the present invention.

Figure 5 illustrates a process flow for depositing an electrode having non-vertical sidewalls in accordance with an embodiment of the present invention.

FIG. 6A illustrates the deposition of a metal layer using different sized resist openings according to an embodiment of the invention.

FIG. 6B shows a top view of a resist opening according to an embodiment of the invention.

FIG. 7A illustrates a method of manufacturing a system for analyzing molecules according to an embodiment of the invention.

Fig. 7B and 7C show cross-sections of trenches in a resist layer according to an embodiment of the invention.

FIG. 8 illustrates a method of analyzing molecules according to an embodiment of the present invention.

Fig. 9A shows a configuration of a device under test according to an embodiment of the present invention.

Fig. 9B shows current-voltage curves for different diameters according to an embodiment of the invention.

Fig. 9C shows current at constant voltage for different diameters according to an embodiment of the invention.

Fig. 10A shows a configuration of a device under test according to an embodiment of the present invention.

Fig. 10B shows current-voltage characteristics for different thicknesses of the insulator according to an embodiment of the present invention.

Fig. 11 shows an SEM image of a tunnel junction device according to an embodiment of the present invention.

FIG. 12 illustrates a computer system according to an embodiment of the invention.

FIG. 13 shows an analysis system according to an embodiment of the invention.

FIG. 14 illustrates a computer system according to an embodiment of the invention.

Detailed Description

Term(s) for

The term "contact" may refer to bringing one object into close proximity to another object so that electrons can tunnel from one object through the other. At the sub-atomic level, two objects may never physically contact each other because the repulsive forces from the electron cloud in the objects may prevent the objects from coming closer together.

"nucleic acid" may refer to deoxyribonucleotides or ribonucleotides and polymers thereof in either single-or double-stranded form. The term can encompass nucleic acids comprising synthetic, naturally occurring and non-naturally occurring known nucleotide analogs or modified backbone residues or linkages that have similar binding properties as the reference nucleic acid and are metabolized in a manner similar to the reference nucleotide. Examples of such analogs include, but are not limited to, phosphorothioate, phosphoramidite, methylphosphonate, chiral methylphosphonate, 2-oxymethylribonucleotide, Peptide Nucleic Acid (PNA).

Unless otherwise indicated, a particular nucleic acid sequence also implicitly encompasses conservatively modified variants thereof (e.g., degenerate codon substitutions) and complementary sequences, as well as the sequence explicitly indicated. In particular, degenerate codon substitutions mayBy generating sequences in which the third position of one or more selected (or all) codons is replaced by mixed base and/or deoxyinosine residues (Batzer et al,Nucleic Acid Res.19：5081 (1991); Ohtsuka et al.，J. Bio l. Chem.260：2605-2608 (1985); Rossolini et al.，Mol. Cell. Probes8: 91-98(1994)). The term nucleic acid is used interchangeably with gene, cDNA, mRNA, oligonucleotide, and polynucleotide.

The term "nucleotide" in addition to referring to naturally occurring ribonucleotide or deoxyribonucleotide monomers, can also be understood to refer to related structural variants thereof, including derivatives and analogs, which are functionally equivalent with respect to the particular environment in which the nucleotide is used (e.g., hybridization to a complementary base), unless the context clearly indicates otherwise.

The term "oscillation" may refer to the movement of an object in a fluid due to brownian motion or other forces. The object may oscillate without human or machine active intervention. In some cases, the object may oscillate due to an applied electric field or pressure driven flow.

Directional terms for the semiconductor processing layers and steps (such as "over" or "on top of … …") may use the reference coordinate system, where these terms refer to locations further away from the plane defined by the substrate surface. The "bottom" may be the underside of the substrate or the underside facing the substrate. Those skilled in the art will appreciate that even if the substrate is processed upside down, the "bottom" of the layer may still refer to the side of the layer closest to the underside, or non-processing side, of the substrate.

The term "electrical characteristic" may be understood to refer to any property associated with an electrical circuit. The electrical characteristic may refer to voltage, current, resistance, impedance, inductance, or capacitance, and temporal changes thereof (e.g., current frequency).

Detailed Description

Conventional nanopore-based devices currently on the market may comprise protein nanopores inserted into metastable lipid bilayers. Lipid bilayers can be fragile and can compromise the stability of the device. Solid-state atom-scale nanopore layers may be less fragile than protein nanopores and have the potential for improved manufacturability. Possible methods involving these devices include confining the nucleic acid molecules in a gap of 2 nm or less between the electrodes and using electrons that tunnel through the electrodes and nucleic acid molecules to recognize nucleotides and nucleotide sequences. Conventional solid state methods can be difficult to accommodate for reliable mass production of nanopores, devices containing nanopores, and analytical instruments. Devices fabricated by conventional methods may be electrically shorted even before the nanopores are patterned. After patterning, the remaining devices may degrade rapidly during operation.

Embodiments of the present technology may allow for improved and more reliable tunnel junctions and methods of fabricating tunnel junctions. By depositing electrodes that do not have sharp sidewalls and corners, but instead have sloped or curved sidewalls, the electrical shorting problem can be reduced. The layer deposited on top of the electrode layer may then be able to sufficiently cover the underlying electrode layer and thus reduce or prevent short circuits. In addition, two insulating materials may be used as the dielectric layer, thereby reducing the possibility of incomplete coverage and the possibility of flaking. Further, the electrodes may taper from contact area to junction area to provide a thin electrode where the holes are to be patterned, while thicker contact areas reduce sheet resistance. The electrodes may also be patterned to be wider at the contact regions and narrower at the junction regions.

Nanopores using tunneling

Fig. 1A shows a simple apparatus 100 that can be used for tunneling recognition of molecules. Insulating layer 102 separates metal 104 and metal 106. The metal 104 and the metal 106 may be electrodes. A voltage may be applied to the metal 104 and the metal 106 from a power supply 108. When a molecule contacts both metals 104 and 106, electrons can tunnel from one electrode to the other through the nucleic acid molecule, thereby generating an electrical current. The current may be measured by meter 110. The molecules may oscillate and the measured current may have an amplitude and a frequency. The amplitude and frequency may be variable. The nature of the current may assist in identifying a particular molecule or portion of a molecule. The electrical property can be used almost as a fingerprint to identify a molecule or a part of a molecule.

Fig. 1B shows a solid state nanopore device 120 fabricated by conventional techniques. The device 120 has an orifice 122. The orifice 122 may allow only one molecule to pass at a time, which may simplify the identification of the molecule. The apertures 122 may be formed in the top metal electrode 124, the insulating layer 126, and the bottom metal electrode 128. The bottom metal electrode 128 may be on a substrate 130. The top metal electrode 124 may be connected to a power supply 132. The bottom metal electrode 128 may be connected to an electrical meter 134. Top electrode 136 and bottom electrode 138 may generate an electric field to help drive molecules into aperture 122. The bottom electrode 138 may be connected to a power source 140. The top electrode 136 may be connected to an electrical meter 142.

Fig. 1C shows a cross-sectional view of device 120. The effect of the sharp sidewalls of the metal electrodes is more pronounced in region 150. The layer on top of the step formed by the bottom metal electrode 128 can be seen. On the top surface of the bottom metal electrode 128, the insulating layer 126 may have a thickness on the order of nanometers (e.g., 1-2 nm). The insulating layer 126 in the schematic shows a conformal coverage of the bottom metal electrode 128. However, the insulator may in practice conformally cover the step. The sidewall coverage of the bottom metal electrode 128 may be thinner than the coverage on the top surface of the bottom metal electrode 128. The thin overlay may be an area between the top metal electrode 124 and the bottom metal electrode 128 where a short circuit is more likely to occur.

Fig. 1D shows a two-dimensional view of the device 120. The figure also shows that the junction region is the region where the top metal electrode 124 and the bottom metal electrode 128 overlap. The contact area is an area where the electrodes do not overlap.

Techniques for improving reliability and manufacturability

Three techniques are described to improve the reliability and manufacturability of nanopore devices. As described above, a potential weakness in the reliability and manufacturability of the nanopore device in fig. 1B and 1C may be the sharp sidewalls of the electrodes and the thin insulating layer covering between the electrodes. First, an electrode with sloped sidewalls may be deposited such that the insulating layer coverage on the electrode is thicker, thereby reducing the likelihood of shorting. Second, the insulating layer may include a plurality of insulator layers, which may prevent void formation and formation of short circuits. Third, the electrode layer may be tapered in two dimensions, making the electrode easier to manufacture and will not have too high a sheet resistance for normal operation.

Electrode with inclined side wall

By forming the electrodes without vertical or substantially vertical sidewalls, the likelihood of shorting can be reduced. The electrodes may have sloped sidewalls or may be curved or rounded. The structure of the electrodes is described herein. The process for depositing the electrodes will be discussed later.

Fig. 2 shows a schematic of a system 200 with an apparatus 201, the apparatus 201 having electrodes without vertical side walls. Top metal electrode 202 is located above bottom metal electrode 204. A first insulator layer 206 and a second insulator layer 208 separate the top metal electrode 202 and the bottom metal electrode 204. The bottom metal electrode 204 is on top of the substrate 210. An aperture 212 may pass through the top metal electrode 202, the second insulator layer 208, the first insulator layer 206, the bottom metal electrode 204, and the substrate 210. As can be seen in region 214, no vertical sidewalls of bottom metal electrode 204 are present covered by first insulator layer 206. In contrast, the bottom metal electrode 204 has a gradual curve or slope that can be conformally covered by the insulator layer more easily than the vertical sidewalls.

The top metal electrode 202 may also have sloped sidewalls. The inclined side walls ensure a proper coverage for the insulator layer covering the electrode. With the sloped sidewalls, the insulator layer deposited over the top metal electrode 202 may avoid the formation of voids and seams. During operation of the nanopore device, the molecules to be analyzed may be in a liquid medium. The voids and seams may allow liquid to penetrate to the electrode, which may allow for unwanted electrical paths from the ionic liquid to the top metal electrode 202. In other words, having sloped sidewalls of the top metal electrode 202 may help isolate the top metal electrode from the liquid.

Multiple insulator layers

A thin insulating layer is desired between the electrodes so that when a molecule contacts both electrodes, a tunneling current through the molecule can pass through a sufficiently small portion of the molecule to facilitate analysis. For example, if the molecule to be analyzed is a DNA molecule, the insulating layer should be thin so that when the DNA molecule contacts both electrodes, current can only pass through one nucleotide. However, a thin insulating layer provides less barrier between the electrodes than a thick insulating layer. At the edge of the bottom electrode, metal particles may be present. During or shortly after deposition of the electrode, the metal particles may have flaked off the electrode edges. If a thin insulating layer is used to cover these metal particles at the edge of the bottom electrode, the thin insulating layer can be removed or etched away before depositing the other electrode. These metal particles may then no longer be covered by the insulating layer when the other electrode is deposited onto the bottom electrode. The bottom and top electrodes may contact and form a short circuit. Metal particles present at the edges of the electrodes may also be more prevalent in geometries with vertical sidewalls.

Fig. 3A illustrates a process flow that can maintain the integrity of a thin insulating layer between electrodes. At step 300, a substrate 302 begins with a metal electrode 304 on top. At step 320, a first insulator material is deposited to form a first insulator layer 306.

At step 340, contact vias 308 may be etched in the first insulator layer 306. The bottom of the contact via 308 may be an exposed portion of the metal electrode 304. Fig. 3B shows a cross-section through the contact via 308 at step 340.

At step 360, a second insulator material may be deposited to form the second insulator layer 310. The second insulator layer 310 may comprise a material that functions as a junction insulator. A second insulator material may be deposited on the surface defining the contact via 308, including on the exposed portion of the metal electrode 304 and on the portion of the first insulator layer 306. The contact via 308 is also covered by a second insulator layer 310 to form a second via 312. The second via 312 may have a smaller diameter than the contact via 308 because the thickness of the second insulator that the contact via 308 is deposited is reduced. The bottom of the second via 312 is part of the second insulator layer 310. Fig. 3C shows a cross-section through the second via 312 at step 360.

At step 380, a metallic material may be deposited to form the top electrode 314. Also at step 380, a hole 316 may be etched through all layers, including the top electrode 314, the second insulator layer 310, the first insulator layer 306, the metal electrode 304, and the substrate 302. The size of the hole 316 may be smaller than both the contact via 308 and the second via 312. Fig. 3D shows a cross-section of the through hole 316. The second insulator layer 310 separates the metal electrode 304 from the top electrode 314 and forms the width of the tunnel junction.

The two insulator layers in fig. 3 may allow for better coverage of the metal electrode 304 and may reduce the likelihood of shorting. The first insulator layer 306 covers the edges of the metal electrode 304, which acts as a thick insulator layer that can provide adequate coverage of any metal particles that flake off the sides of the metal electrode 304. Additionally, etching contact vias through the first insulator layer 306 may facilitate subsequent etching of the nanopores themselves and reduce alignment and processing issues.

Gradually decreasing from the contact region to the junction region

It is desirable for the electrodes for the nanopores to be thinner to make fabrication easier. It is easier to etch or otherwise form holes through thin electrodes than thick electrodes. Thinner electrodes also increase sheet resistance. To some extent, a greater sheet resistance is desired so that a change in current or voltage when a molecule contacts an electrode can be more easily detected. However, increasing sheet resistance can increase heat and can lead to device failure. To address these problems, the electrodes may be tapered in at least two directions from the contact region to the junction region. The structure of the electrode is described below. The process flow of tapering the electrodes will be described later.

FIG. 4A shows a side-by-side view of a junction region and a contact region. The junction region may have a height H₁And has a width W₁The electrode 402 of (a). The contact regions may have the same electrode 402, but with a height H₂And width W₂. Width W₂Can be larger than the width W₁. Height H₂Can be greater than the height H₁。

Figure 4B shows how both the height and width may taper from the contact region to the junction region. The taper may be from the width W₂Continuously or monotonically decreasing to widthW₁. May be gradually decreased from H₂Continuously or monotonically decreasing to H₁. In some embodiments, the taper may be linear. In other embodiments, the gradual decrease may follow a curve. In some embodiments, the gradual decrease in width may be symmetrical on both sides, as shown in fig. 4B. In other embodiments, the gradual decrease in width may be asymmetric. For example, one side may be flat while the other side is tapered. For height, the tapering may only occur on one side, as the bottom of the electrode is fixed by the underlying layer.

Tapering the thickness may allow nanopores to be formed through thin portions of the electrodes. Tapering the width will allow the sheet resistance to be higher near the junction region and lower near the contact region.

Embodiments of the present technology may result in near 100% process yield even when the insulating layer is approximately 2 nm thick. The tunneling current in the device may be proportional to the cross-sectional area of the junction, which is consistent with standard metal-insulator-metal junction models. The method of tapering in at least two directions may be independent of forming electrodes with sloped sidewalls or depositing multiple insulator layers.

System III