Scanning single electron box electrometer microscope

Abstract

A method and device for detecting a quantity of electric charge and electric potential. The device includes a scanning probe, which includes a single electron box located at an apex of the scanning probe configured to sense properties of a sample surface at atomic resolution. The single electron box includes a back electrode, a tunnel barrier controlling a tunneling rate of electrons, and an island.

Description

SCANNING SINGLE ELECTRON BOX ELECTROMETER MICROSCOPECROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application Serial No.63 / 749,505 entitled “Scanning Single Electron Box Electrometer Microscope,” filed on January 25, 2025, which is incorporated by reference herein in its entirety.GOVERNMENT INTERESTS

[0002] This invention was made with government support under Grant Number 2117438 awarded by the National Science Foundation. The U.S. government has certain rights in the invention. TECHNICAL FIELD

[0003] The present disclosure relates generally to scanning probe microscopes, and more particularly to a scanning single electron box electrometer microscope that can detect an extremely small quantity of electric charge and electric potential.BACKGROUND

[0004] A scanning probe microscope (SPM) is a type of microscope that uses a very sharp tip to scan the surface of a sample allowing for incredibly high resolution imaging of nanoscale features by measuring interactions between the tip and the sample surface, such as force, current, or magnetic field, to generate a detailed image of the material's topography and properties at the atomic level.

[0005] An example of a scanning probe microscope is the scanning single electron transistor electrometer. A scanning single electron transistor electrometer is a specialized scanning probe microscope that utilizes a single electron transistor (SET) at the tip to sensitively measure local electric fields and charges on a sample surface with the ability to detect even fractions of an electron charge with high spatial resolution, typically on the order of 100 nanometers.

[0006] The operating principle of the scanning single electron transistor electrometer is based on the high charge sensitivity of single electron tunneling in single electron transistors (SET). In SET, the source-drain current changes abruptly to a small change in the electric potential of an island, which is usually controlled by applying a DC (direct current) voltage to a nearby gate electrode.VR39977.P104WO Page 1 of 21 PATENT

[0007] In the scanning SET, an SET is fabricated at the tip of a scanning probe (SSET probe) without the gate electrode. When an SSET probe is approached to the proximity of the sample surface, the sample surface plays the role of the gate electrode. By monitoring the source-drain current in the SET at the tip apex at various locations above the sample, a map of the electric potential on the sample can be obtained.

[0008] Unfortunately, the SSET technique has not yet become a widely used measurement technique due to various shortcomings. For example, the fabrication of the SSET probe is difficult because SETs are three-terminal devices. That is, the SETs are difficult to fabricate due to the difficulty in defining the source and drain electrodes on a very narrow tip without a short circuit. Another shortcoming in the SSET technique is the difficult operation due to the necessity of measuring small source-drain currents. Furthermore, a shortcoming in the SSET technique is the difficult operation due to the lack of an independent distance regulation mechanism, and the resulting risk of damaging the fragile probe.VR39977.P104WO Page 2 of 21 PATENTSUMMARY

[0009] In one embodiment of the present disclosure, a device for detecting a quantity of electric charge and electric potential comprises a scanning probe comprising a single electron box located at an apex of the scanning probe configured to sense properties of a sample surface, where the single electron box comprises: a back electrode, a tunnel barrier controlling a tunneling rate of electrons, and an island.

[0010] In another embodiment of the present disclosure, a method for fabricating a device for detecting a quantity of electric charge and electric potential comprises depositing a tunnel barrier on a tip of a scanning probe using atomic layer deposition. The method further comprises depositing an island on the tunnel barrier using a focused electron beam-induced deposition technique.

[0011] In a further embodiment of the present disclosure, a method for fabricating a device for detecting a quantity of electric charge and electric potential comprises depositing a tunnel barrier on a tip of a scanning probe. The method further comprises annealing the scanning probe to minimize defects in the tunnel barrier. The method additionally comprises depositing a film on the scanning probe. Furthermore, the method comprises isolating a metallic layer on an apex of the tip of the scanning probe to create an island which confines electrons in a resulting island using ion-beam milling.

[0012] In another embodiment of the present disclosure, a method for fabricating a device for detecting a quantity of electric charge and electric potential comprises depositing a tunnel barrier on a metallic tip body of a scanning probe. The method further comprises depositing a conducting film on the scanning probe. The method additionally comprises depositing an island on the conducting film at an apex of a tip of the scanning probe using a focused electron beam-induced deposition (FEBID) technique. Furthermore, the method comprises isolating a portion of the conducting film to create an electron-confining island by etching the conducting film using the FEB ID-deposited island as a mask.

[0013] The foregoing has outlined rather generally the features and technical advantages of one or more embodiments of the present disclosure in order that the detailed description of the present disclosure that follows may be better understood. Additional features and advantages of theVR39977.P104WO Page 3 of 21 PATENTpresent disclosure will be described hereinafter which may form the subject of the claims of the present disclosure.VR39977.P104WO Page 4 of 21 PATENTBRIEF DESCRIPTION OF THE DRAWINGS

[0014] A better understanding of the present disclosure can be obtained when the following detailed description is considered in conjunction with the following drawings, in which:

[0015] Figure 1A illustrates a schematic of the scanning single electron box (SSEB) probe in accordance with an embodiment of the present disclosure;

[0016] Figure IB illustrates a schematic of single electron box device in accordance with an embodiment of the present disclosure;

[0017] Figure 2 is a flowchart of a method for fabricating a device (single electron box (SEB) probe) for detecting a quantity of electric charge and electric potential in accordance with an embodiment of the present disclosure;

[0018] Figure 3 A illustrates a schematic of the SEB probe using the fabrication method of Figure 2 in accordance with an embodiment of the present disclosure;

[0019] Figure 3B illustrates a schematic of a single electron box (SEB) using the fabrication method of Figure 2 in accordance with an embodiment of the present disclosure;

[0020] Figure 3C illustrates an STEM image of the SEB probe fabricated using the method of Figure 2 in accordance with an embodiment of the present disclosure;

[0021] Figure 4 is a flowchart of an alternative method for fabricating a device (SEB probe) for detecting a quantity of electric charge and electric potential in accordance with an embodiment of the present disclosure;

[0022] Figure 5A illustrates the schematic of the SEB probe before the focused ion beam (FIB) cut using the fabrication method of Figure 4 in accordance with embodiment of the present disclosure;

[0023] Figure 5B illustrates the schematic of the SEB probe after the focused ion beam (FIB) cut using the fabrication method of Figure 4 in accordance with embodiment of the present disclosure;

[0024] Figure 6A illustrates the single electron tunneling peaks in the dissipation signal versus the bias voltage curve in accordance with an embodiment of the present disclosure;VR39977.P104WO Page 5 of 21 PATENT

[0025] Figure 6B illustrates the single electron tunneling rings in the dissipation image taken at a constant bias voltage in accordance with an embodiment of the present disclosure;

[0026] Figure 6C illustrates the single electron tunneling peaks in the dissipation signal versus the bias voltage curve in accordance with an embodiment of the present disclosure;

[0027] Figure 6D illustrates the single electron tunneling rings in the dissipation image taken at a constant bias voltage in accordance with an embodiment of the present disclosure;

[0028] Figure 7A illustrates the single electron tunneling peaks in the dissipation signal versus the bias voltage curve in accordance with an embodiment of the present disclosure;

[0029] Figure 7B illustrates the single electron tunneling rings in the dissipation image taken at a constant bias voltage in accordance with an embodiment of the present disclosure;

[0030] Figure 8 is a flowchart of a method for fabricating a device (SSEB probe) for detecting a quantity of electric charge and electric potential in accordance with an embodiment of the present disclosure; and

[0031] Figures 9A-9C depict the cross-sectional views of fabricating the device (SSEB probe) using the steps described in Figure 8 in accordance with an embodiment of the present invention.VR39977.P104WO Page 6 of 21 PATENTDETAILED DESCRIPTION

[0032] As stated above, a scanning probe microscope (SPM) is a type of microscope that uses a very sharp tip to scan the surface of a sample allowing for incredibly high resolution imaging of nanoscale features by measuring interactions between the tip and the sample surface, such as force, current, or magnetic field, to generate a detailed image of the material's topography and properties at the atomic level.

[0033] An example of a scanning probe microscope is the scanning single electron transistor electrometer. A scanning single electron transistor electrometer is a specialized scanning probe microscope that utilizes a single electron transistor (SET) at the tip to sensitively measure local electric fields and charges on a sample surface with the ability to detect even fractions of an electron charge with high spatial resolution, typically on the order of 100 nanometers.

[0034] The operating principle of the scanning single electron transistor electrometer is based on the high charge sensitivity of single electron tunneling in single electron transistors (SET). In SET, the source-drain current changes abruptly to a small change in the electric potential of an island, which is usually controlled by applying a DC (direct current) voltage to a nearby gate electrode.

[0035] In the scanning SET, an SET is fabricated at the tip of a scanning probe (SSET probe) without the gate electrode. When an SSET probe is approached to the proximity of the sample surface, the sample surface plays the role of the gate electrode. By monitoring the source-drain current in the SET at the tip apex at various locations above the sample, a map of the electric potential on the sample can be obtained.

[0036] Unfortunately, the SSET technique has not yet become a widely used measurement technique due to various shortcomings. For example, the fabrication of the SSET probe is difficult because SETs are three-terminal devices. That is, the SETs are difficult to fabricate due to the difficulty in defining the source and drain electrodes on a very narrow tip without a short circuit. Another shortcoming in the SSET technique is the difficult operation due to the necessity of measuring small source-drain currents. Furthermore, a shortcoming in the SSET technique is the difficult operation due to the lack of an independent distance regulation mechanism, and the resulting risk of damaging the fragile probe.VR39977.P104WO Page 7 of 21 PATENT

[0037] The embodiments of the present disclosure provide a means for overcoming such shortcomings by using a single electron box consisting of an island placed on the apex of a scanning probe (e.g., atomic force microscope tip). Instead of measuring the current, the electric force acting on the probe is measured. The technology of the present disclosure offers simplified measurements while the single electron box tips are easier to make than the SET tips. A further discussion regarding methods for fabricating single electron box tips is provided further below.

[0038] The scanning single electron box (SSEB) technique of the present disclosure overcomes the challenges of the SSET technique by 1) measuring the electric force caused by tunneling a single electron by atomic force microscopy instead of measuring the electric current and 2) using a probe which is simpler and easier to fabricate. In the SSEB technique, a single electron box is fabricated at the apex of the scanning probe tip as shown in Figure 1 A, where the single electron box is configured to sense properties of a sample surface. Figure 1A illustrates a schematic of the scanning single electron box (SSEB) probe 100 in accordance with an embodiment of the present disclosure.

[0039] Referring to Figure 1A, SSEB probe 100 includes a tunnel barrier 101 placed at the apex of the tip of an atomic force microscope (AFM) tip 102. In one embodiment, tunnel barrier 101 is configured to control a tunneling rate of electrons. Furthermore, SSEB probe 100 includes an island 103, such as a metallic dot, placed on tunnel barrier 101. SSEB probe 100 is configured to use tip 102 to scan the surface of a sample 104 allowing for incredibly high resolution imaging of nanoscale features by measuring interactions between tip 102 and the surface of sample 104.

[0040] In one embodiment, the single electron box 105 is located at the apex of AFM tip 102, where single electron box 105 is configured to sense properties of a sample surface (e.g., surface of sample 104), and where the single electron box 105 includes a back electrode, tunnel barrier 101 and island 103 as illustrated in Figure IB in accordance with an embodiment of the present disclosure.

[0041] Referring to Figure IB, single electron box 105 includes a back electrode 106, tunnel barrier 101 and island 103. In one embodiment, back electrode 106 corresponds to a metallic tip of the scanning probe (e.g., SSEB probe 100).VR39977.P104WO Page 8 of 21 PATENT

[0042] Referring to Figures 1A and IB, in one embodiment, in the single electron box (SEB), a single electron tunnels back and forth between island 103 and back electrode 106 in response to an applied AC voltage (not shown) that is applied between gate 108 / sample 104 and back electrode 106. Alternatively, the role of the AC voltage can emanate from the mechanical oscillation of probe 100 which causes the modulation of the probe-sample capacitance. The resulting AC current abruptly changes in response to the electric potential of gate 108 / sample 104 that depends on DC voltage 107 between gate 108 / sample 104 and back electrode 106 and the electric potential of gate 108 / sample 104.

[0043] However, this single electron AC current is too small (several fA=10'15A) to be measured by a current amplifier. Instead, in the SSEB electrometer microscopy, this single electron motion is detected by measuring an electric force with an AFM.

[0044] Furthermore, the fabrication of SSEB probes is fundamentally easier than that of SSET probes. While two electrodes (source and drain electrodes) need to be connected to an island via two tunnel barriers on a narrow tip for fabricating SSET probes, the SSEB probe requires only one tunnel barrier (e.g., tunnel barrier 101) and one electrode (e.g., back electrode 106). To fabricate an SSEB probe, the tip side of an AFM cantilever probe (see AFM tip 102) is coated with a metallic film that serves as the back electrode. This electrode is quantum mechanically coupled to a nanometer-scale metallic island 103 at the probe apex via tunnel barrier 101 as shown in Figures 1A and IB. In one embodiment, island 103 corresponds to a quantum dot. In one embodiment, by carefully engineering tunnel barrier 101, the tunnel rate can be optimized to maximize the damping signal, leading to higher sensitivity. For single electron tunneling to occur, the charging energy of island 103 (Ec= e2 / 2C where C is the capacitance of the island) needs to be much higher than thermal energy (feT), which requires a nanometer-scale island 103 at the apex of the probe to be formed. Because of the probe structure which requires only a single electrode, the island size can be made much smaller (< 20 nm) compared to the 100 nm island size typical for SSET probes. This reduced tip size significantly enhances both spatial resolution and charging energy.

[0045] In terms of operation, the SSEB electrometer leverages force detection techniques, such as those used in single-electron sensitive electrostatic force microscopy (e-EFM), to measure the electric force rather than the small source-drain currents as in SSET. In the SSEB technique, electron tunneling occurs between back electrode 106 and island 103 (e.g., metallic dot) inVR39977.P104WO Page 9 of 21 PATENTresponse to the oscillation of the probe when the gate voltage reaches one or more threshold values. Here, the AC voltage can be omitted because the oscillating capacitance caused by the oscillation of the tip plays the role of the AC voltage. In the SSEB, the gate electrode is replaced by the sample (e.g., sample 104), and electron tunneling occurs at a specific sample bias. Due to the finite tunneling rate, the oscillating electrostatic force due to the single-electron tunneling is delayed relative to the oscillation of the probe (e.g., probe 100). Consequently, the in-phase component of the oscillating electrostatic force generates a frequency shift signal, while the quadrature (90° delayed) component generates a damping (dissipation) signal. The frequency shift and damping (dissipation) signals can be detected independently by using the frequency modulation mode AFM (atomic force microscope) technique.

[0046] This operating principle not only simplifies the probe design and fabrication but also makes SSEB microscopy compatible with various cryogenic atomic force microscopes (AFMs) regardless of their deflection measurement principles, such as fiberoptic interferometry, optical beam deflection, or quartz tuning forks. SSEB probes can be produced by modifying commercially available AFM probes, making them cost-effective and compatible with most standard AFMs that use cantilever-type probes. Additionally, these probes can be attached to quartz tuning fork probes, which are widely used in low-temperature AFMs, further extending the versatility of the SSEB technique.

[0047] Furthermore, the SEB probe of the scanning single electron box (SSEB) microscopy can detect extremely small quantities of electric charge (much smaller than an elementary charge) and electric potentials as low as 1 pV with nanometer spatial resolution. Such an instrument can visualize the charge / potential distribution of samples with unprecedented resolution. Additionally, the fabrication of SEB probes is fundamentally easier, their operation is more straightforward, and they are more economical, making this technique accessible to a broader range of research communities and industries. Three fabrication methods for SEB probes are discussed below in connection with Figures 2, 3A-3C, 4, 5A-5B, 8 and 9A-9C.

[0048] Figure 2 is a flowchart of method 200 for fabricating a device (single electron box (SEB) probe) for detecting a quantity of electric charge and electric potential in accordance with an embodiment of the present disclosure.VR39977.P104WO Page 10 of 21 PATENT

[0049] Referring to Figure 2, in conjunction with Figures 1A-1B, in step 201, a tunnel barrier (e.g., tunnel barrier 101) is deposited on a tip of a scanning probe (e.g., back electrode 106) using atomic layer deposition (ALD). In one embodiment, the tunnel barrier includes a layer of AI2O3 and a layer of TiCh. In one embodiment, a 2 nm thick layer of AI2O3 and a 5.7 nm layer of TiCh are deposited on the back electrode (e.g., back electrode 106) as the tunnel barrier using a plasma-enhanced (PE)ALD technique, with trimethylaluminum (TMA) and Tetrakis(dimethylamido)titanium (TDMATi) as precursors.

[0050] In step 202, the scanning probe (e.g., probe 100) is annealed to minimize defects in the tunnel barrier (e.g., tunnel barrier 101).

[0051] In step 203, a platinum island 103 is deposited on tunnel barrier 101 using the focused electron beam-induced deposition (FEB ID) technique.

[0052] A further discussion regarding method 200 is provided below in conjunction with Figures 3 A-3C. Figure 3 A illustrates a schematic of SEB probe 300 using the fabrication method of Figure 2 in accordance with an embodiment of the present disclosure. Figure 3B illustrates a schematic of single electron box (SEB) 301 using the fabrication method of Figure 2 in accordance with an embodiment of the present disclosure. Figure 3C illustrates an STEM (Scanning Transmission Electron Microscopy) image of SEB probe 300 fabricated using the method of Figure 2 in accordance with an embodiment of the present disclosure.

[0053] Referring to Figures 2 and 3A-3C, in conjunction with Figures 1 A-1B, an SEB includes three main components: a back electrode 106, a tunnel barrier 101, and a metallic dot (island) 103.

[0054] Figure 3A illustrates a schematic of SEB probe 300. In one embodiment, SEB probe 300 includes a cantilever 303 that supports metallic tip 302 at its distal end. In one embodiment, cantilever 303 facilitates the scanning and oscillation of the tip over a sample surface.

[0055] As shown in Figures 3A and 3B, in conjunction with Figures 1A-1B, the metallic tip 302 of SEB probe 300 (also referred to as the metallic tip body), such as AFM tip 102, acts as a back electrode 106 that requires high electrical conductivity to ensure electrical conduction. In one embodiment, the scanning probe tip is coated with platinum, palladium, or gold. In one embodiment, a platinum coated scanning probe tip, such as the HQ: NSC16 / Pt (MikroMasch), was used as the electron reservoir. This ensures good electrical conduction from the base of theVR39977.P104WO Page 11 of 21 PATENTcantilever probe to the tip apex and prevents oxidation thereby enhancing the long shelflife of the manufactured SEB probes.

[0056] As previously discussed, tunnel barrier 101 of the SEB (e.g., SEB 301) controls the tunneling rate of electrons and maximizes the signal. In one embodiment, tunnel barrier 101 is deposited on back electrode 106 by atomic layer deposition (ALD) techniques. ALD enables conformal deposition of various materials, including dielectric materials with monolayer precision. In one embodiment, a 2 nm thick layer of AI2O3 and a 5.7 nm layer of TiCE are deposited on back electrode 106 as tunnel barrier 101 using the plasma-enhanced (PE)ALD (Gemstar XT-P Thermal and PEALD) technique, with trimethylaluminum (TMA) and Tetrakis(dimethylamido)titanium (TDMATi) as precursors. This ensures precise control over the barrier thickness and composition enhancing the stability and reliability of the SEB (e.g., SEB 301).

[0057] In one embodiment, after tunnel barrier 101 was deposited, an island 103 (e g., platinum dot) is deposited on tunnel barrier 101 using the focused electron beam-induced deposition (FEB ID) technique. In one embodiment, island 103 has a 10 nm diameter. In one embodiment, in FEB ID, a focused electron beam is irradiated to the surface of sample 104 in the presence of an organic precursor which contains target material (platinum in this case) to be deposited. This technique enables localized deposition of a platinum island 103 without any lithography resulting in precise positioning of island 103 at the apex of the metallic tip, such as AFM tip 102. The as-deposited platinum island 103 is not electrically conducting because it contains a significant amount of carbon. To reduce the carbon content, probe 300 is annealed in the air for four hours at 215° C. This annealing process makes the platinum island 103 electrically conducting. A STEM image of the fabricated SEB probe (e.g., SEB probe 300) is illustrated in Figure 3C in accordance with an embodiment of the present disclosure.

[0058] An alternative fabrication technique for fabricating an SEB probe is discussed below in connection with Figure 4 and 5A-5B.

[0059] Figure 4 is a flowchart of an alternative method 400 for fabricating a device (SEB probe) for detecting a quantity of electric charge and electric potential in accordance with an embodiment of the present disclosure.VR39977.P104WO Page 12 of 21 PATENT

[0060] Referring to Figure 4, in conjunction with Figures 1A-1B, in step 401, a tunnel barrier (e.g., tunnel barrier 101) is deposited on a tip of a scanning probe (e.g., scanning probe 100).

[0061] In step 402, the scanning probe (e.g., scanning probe 100) is annealed to minimize the defects in tunnel barrier 101.

[0062] In step 403, a conducting film is deposited on the scanning probe (e.g., scanning probe 100).

[0063] In step 404, a metallic layer (e.g., metallic layer of platinum) on an apex of the tip of the scanning probe (e.g., scanning probe 100) is isolated to create an island which confines electrons in a resulting island using ion-beam milling.

[0064] A further discussion regarding method 400 is provided below in conjunction with Figures 5A-5B.

[0065] Figure 5A illustrates the schematic of the SEB probe 500 before the focused ion beam (FIB) cut using the fabrication method of Figure 4 in accordance with embodiment of the present disclosure. Figure 5B illustrates the schematic of the SEB probe 500' after the focused ion beam (FIB) cut using the fabrication method of Figure 4 in accordance with embodiment of the present disclosure.

[0066] Referring to Figures 4 and 5A-5B, in conjunction with Figures 1 A-1B, in one embodiment, AI2O3, TiCh, or HfCh is deposited as tunnel barrier 101 on the metallic (e.g., silicon) tip body 501 of probe 500 using commercially available scanning probe tips (e.g., HQ: NSC16 / Pt). Probe 500 is then annealed in forming gas (96% argon and 4% hydrogen) at 450° C for 1 hour to minimize the defects in tunnel barrier 101. Afterwards, a platinum film 502 of approximately 45 nm thick is deposited on the entire probe 500 using AJA magnetron sputtering as shown in Figure 5A. Finally, the fabrication process is completed using an FIB cut 503 (cut performed by scanning electron microscope / focused ion beam) as illustrated in Figure 5B. In one embodiment, using focused ion-beam milling, platinum layer 502 on the tip apex is isolated to create an island (dot) 103 which confines the electrons in the resulting platinum island 504. When the electrons are confined to nanometer scale island 504, the charging energy of island 504 becomes large enough for the single-electron tunneling to occur between island 504 and the metallic (e.g., silicon) tip body 501 of probe 500'.VR39977.P104WO Page 13 of 21 PATENT

[0067] The performance of such techniques has been evaluated by measuring the force due to single electron tunneling at 4 K by the low-temperature AFM. Evidence of single electron charging is shown in Figures 6A-6D (measurements obtained using the probe that was fabricated using the technique discussed above in connection with Figures 2 and 3A-3C) and Figures 7A-7B (measurements obtained using the probe that was fabricated using the technique discussed above in connection with Figures 4 and 5A-5B).

[0068] Figure 6A illustrates the single electron tunneling peaks in the dissipation signal versus the bias voltage curve in accordance with an embodiment of the present disclosure. Figure 6B illustrates the single electron tunneling rings in the dissipation image taken at a constant bias voltage in accordance with an embodiment of the present disclosure. The measurements shown in Figures 6A-6B were taken using a probe with a tunnel barrier (e.g., tunnel barrier 101) consisting of AI2O3 with a thickness of 2 nm. Figure 6C illustrates the single electron tunneling peaks in the dissipation signal versus the bias voltage curve in accordance with an embodiment of the present disclosure. Figure 6D illustrates the single electron tunneling rings in the dissipation image taken at a constant bias voltage in accordance with an embodiment of the present disclosure. The measurements shown in Figures 6C and 6D were taken using a probe with a tunnel barrier (e.g., tunnel barrier 101) consisting of T1O2 with a thickness of 5.7 nm. Such a probe was annealed in forming gas at 450° C for one hour to reduce the oxide charges.

[0069] Figure 7A illustrates the single electron tunneling peaks in the dissipation signal versus the bias voltage curve in accordance with an embodiment of the present disclosure. Figure 7B illustrates the single electron tunneling rings in the dissipation image taken at a constant bias voltage in accordance with an embodiment of the present disclosure.

[0070] Referring to Figures 6A-6D and 7A-7B, in conjunction with Figures 1A-1B, the rings (Figures 6A, 6D, and 7B) and peaks (Figures 6B, 6C, and 7A) appear when the single electron tunneling is happening between island 103 and back electrode 106.

[0071] Rings appear in the damping (dissipation) images when the SEB probe approaches a charge or potential center around which the electric potential quickly changes spatially. If the potential at the charge or potential center is large enough, the potential at the island (e.g., island 103) reaches one or more threshold potentials for single electron tunneling at a certain probe position. In this situation, a single electron tunnels back and forth between back electrode 106 andVR39977.P104WO Page 14 of 21 PATENTisland 103. The oscillatory motion of the tunneling electron causes the modulation of the electric force leading to the additional damping (dissipation) of the probe. These rings represent the equipotential contours thereby providing the electric potential map across the sample surface.

[0069] Referring now to Figure 8, Figure 8 is a flowchart of a method 800 for fabricating a device (SSEB probe) for detecting a quantity of electric charge and electric potential in accordance with an embodiment of the present disclosure. This method utilizes a combination of Focused Electron Beam-Induced Deposition (FEB ID) and etching to precisely isolate the metallic island at the tip apex. Figures 9A-9C depict the cross-sectional views of fabricating the device (SSEB probe) using the steps described in Figure 8 in accordance with an embodiment of the present invention.

[0072] Referring to Figure 8, in conjunction with Figures 9A-9C, in step 801, a tunnel barrier 901 is deposited on a metallic tip body 902 of scanning probe 900 as illustrated in Figure 9A. In one embodiment, metallic tip body 902 is supported by cantilever 907, which functions similarly as cantilever 303 of Figure 3A. In one embodiment, the composition of tunnel barrier 901 corresponds to AI2O3, TiCh, or HfCh In one embodiment, tunnel barrier 901 is deposited on the metallic (e.g., silicon) tip body 902 of scanning probe 900, such as by using commercially available scanning probe tips (e g., HQ: NSC16 / Pt).

[0073] In step 802, scanning probe 900 is then annealed, such as in forming gas (96% argon and 4% hydrogen) at 450° C for 1 hour, to minimize the defects in tunnel barrier 101.

[0074] In step 803, a conducting fdm 903, such as a platinum conducting fdm, is deposited on scanning probe 900 as shown in Figure 9A. In one embodiment, an approximately 45 nm thick conducting film 903 is deposited on scanning probe 900. In one embodiment, such a deposition is performed by using AJA magnetron sputtering.

[0075] In step 804, an island 904 is deposited on conducting film 903 at an apex of the tip of scanning probe 900 using a focused electron beam-induced deposition (FEBID) technique as shown in Figure 9A.

[0076] In step 805, a portion of conducting film 903 is isolated to create an electron-confining island 906 by etching (see element 905) conducting film 903 using the FEBID-deposited island 904 as a mask as shown in Figures 9B and 9C. In one embodiment, the etching corresponds to focused ion beam (FIB) or reactive ion etching (RIE).VR39977.P104WO Page 15 of 21 PATENT

[0077] The hybrid approach of method 800 allows for the creation of a highly precise and consistent 20 nm-scale islands by leveraging the placement accuracy of FEB ID with the clean material removal capabilities of dry etching.

[0078] In the embodiments of the present disclosure discussed herein, properties of a sample surface can be sensed at atomic resolution by scanning or tapping a needle tip across the surface. Measurements of the electrostatic forces between a sample surface and the AFM tip provide atomic-scale information on surface morphology and other properties. As discussed herein, a single electron box (SEB) is fabricated on the end of a scanning probe tip (e.g., AFM tip). The SEB is fabricated in such a way that it is possible to monitor the quantum tunneling of a single electron between a back electrode and an island on the sample when an SSEB probe is oscillated with a DC bias voltage applied. By scanning the SEB over a sample (as an SSEB), a map of its surface potential can be obtained. In one embodiment, these measurements are carried out at cryogenic temperatures.

[0079] Furthermore, as discussed herein, in some embodiments, the SSEB tips are made by modifying standard AFM tips. An AFM tip is first coated with a metallic film (nominally platinum). A very thin insulating or tunneling layer is then laid down over the metallic film, and a tiny metallic island is then affixed to the apex of the AFM tip on top of the tunneling layer. The charging energy of the dot is engineered to be much higher than the thermal energy. Hence, the island is small and the SSEB probe islands have a dimension on the order of 20 nm. Tunneling of an electron in the SSEB occurs when the sample bias relative to the tip reaches one or more threshold values.

[0080] Additionally, as discussed herein, in some embodiments, SEB probes are fabricated based on commercially available AFM tips. The fabrication methods discussed herein use platinum-coated AFM tips with AI2O3 and T1O2 insulating quantum tunneling barriers that are a few nm thick. In one fabrication method, a 2 nm -thick insulating layer (e.g., tunnel barrier 101) is deposited on the scanning probe tip using atomic layer deposition. The island is then placed onto the end of the probe tip using a focused-electron beam instrument. In an alternative fabrication method, the entire tip is coated with platinum after depositing the insulating layer (e.g., tunnel barrier 101) on the tip of the scanning probe. The platinum layer is then carefully etched off using a focused ion beam until there is an isolated platinum island on the tip of the scanning probe.VR39977.P104WO Page 16 of 21 PATENT

[0081] Benefits of the scanning single electron box (SEB) probes of the present disclosure include being easier to fabricate than single electron transistor probes. Furthermore, the SEB probes can be fabricated at a relatively low cost by modifying commercial AFM tips. Additionally, measuring the electric force by the tunneling of a single electron by AFM is easier than measuring electric currents in SSET measurements. Furthermore, the probe-to-sample distance regulation in SSEB is easier than in SSET, which lacks an independent regulation mechanism.

[0082] The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.VR39977.P104WO Page 17 of 21 PATENT

Claims

CLAIMS:

1. A device for detecting a quantity of electric charge and electric potential, the device comprising:a scanning probe comprising a single electron box located at an apex of said scanning probe configured to sense properties of a sample surface, wherein said single electron box comprises: a back electrode, a tunnel barrier controlling a tunneling rate of electrons, and an island.

2. The device as recited in claim 1, wherein said island corresponds to a metallic dot.

3. The device as recited in claim 1, wherein said back electrode corresponds to a metallic tip of said scanning probe.

4. The device as recited in claim 1, wherein, in response to oscillation of said scanning probe, quantum tunneling of a single electron between said back electrode and said island on said sample surface is monitored.

5. The device as recited in claim 4, wherein an action of said single electron in response to potential changes of said sample surface is detected by an electric force on said scanning probe.

6. The device as recited in claim 1, wherein a tip of said scanning probe is coated with a metallic film.

7. The device as recited in claim 6, wherein said metallic film comprises platinum.

8. The device as recited in claim 6, wherein said tunnel barrier is deposited above said metallic film.

9. The device as recited in claim 8, wherein said island is affixed to said apex of said scanning probe on top of said tunnel barrier.

10. The device as recited in claim 9, wherein a charging energy of said island is greater than a thermal energy.

11. The device as recited in claim 1, wherein tunneling of a single electron occurs when a bias relative to a tip of said scanning probe reaches one or more threshold values.VR39977.P104WO Page 18 of 21 PATENT12. The device as recited in claim 1 , wherein said tunnel barrier is deposited above a tip of said scanning probe using atomic layer deposition.

13. The device as recited in claim 1, wherein said island is affixed to said apex of said scanning probe using a focused-electron beam instrument.

14. The device as recited in claim 1, wherein said tunnel barrier is deposited on a tip of said scanning probe, wherein said tip of said scanning probe is coated with a platinum layer after said tunnel barrier has been deposited on said tip of said scanning probe.

15. The device as recited in claim 14, wherein said platinum layer is etched off using a focused ion beam until there is an isolated platinum island on said tip of said scanning probe.

16. A method for fabricating a device for detecting a quantity of electric charge and electric potential, the method comprising:depositing a tunnel barrier on a tip of a scanning probe using atomic layer deposition; and depositing an island on said tunnel barrier using a focused electron beam -induced deposition technique.

17. The method as recited in claim 16, wherein said tip of said scanning probe is coated with platinum, palladium, or gold.

18. The method as recited in claim 16, wherein said tunnel barrier comprises a layer of AI2O3 and a layer of TiCh.

19. The method as recited in claim 16, wherein a focused electron beam is irradiated to a sample surface in a presence of an organic precursor containing a target material to be deposited.

20. The method as recited in claim 19, wherein said target material comprises platinum.

21. A method for fabricating a device for detecting a quantity of electric charge and electric potential, the method comprising:depositing a tunnel barrier on a tip of a scanning probe;annealing said scanning probe to minimize defects in said tunnel barrier;VR39977.P104WO Page 19 of 21 PATENTdepositing a film on said scanning probe; andisolating a metallic layer on an apex of said tip of said scanning probe to create an island which confines electrons in a resulting island using ion-beam milling.

22. The method as recited in claim 21, wherein said tunnel barrier comprises AI2O3, TiO2, or HfO2.

23. The method as recited in claim 21, wherein said tip of said scanning probe is annealed in forming gas at 450° C for one hour.

24. The method as recited in claim 21, wherein said metallic layer on said apex of said tip of said scanning probe comprises platinum.

25. A method for fabricating a device for detecting a quantity of electric charge and electric potential, the method comprising:depositing a tunnel barrier on a metallic tip body of a scanning probe;depositing a conducting film on said scanning probe;depositing an island on said conducting film at an apex of a tip of said scanning probe using a focused electron beam-induced deposition (FEB ID) technique; andisolating a portion of said conducting film to create an electron-confining island by etching said conducting film using said FEB ID-deposited island as a mask.

26. The method as recited in claim 25 further comprising:annealing said scanning probe in forming gas after depositing said tunnel barrier to minimize defects in said tunnel barrier.

27. The method as recited in claim 25, wherein said tunnel barrier comprises one of the following: AI2O3, TiO2, or HfO2.

28. The method as recited in claim 25, wherein said conducting film comprises platinum.

29. The method as recited in claim 25, wherein said etching comprises one of the following: focused ion beam and reactive ion etching.VR39977.P104WO Page 20 of 21 PATENT

Images