Microwave impedance microscopy using a tuning fork

• US 20170299525A1
• Filed: 04/05/2017
• Published: 10/19/2017
• Est. Priority Date: 04/18/2016
• Status: Active Grant

• Current Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford Management Co.)
• Original Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford Management Co.)
• Inventors: Cui, Yongtao, Ma, Yue, Shen, Zhixun
• Granted Patent: US 10,060,862 B2
• Time in Patent Office: Days
• CPC Class Codes: G01N 22/00 Investigating or analysing materials by the use of microwaves or radio waves, i.e. electromagnetic waves with a wavelength of one millimetre or more G01N3/00 - G01N17/00, G01N24/00 take precedence
• CPC Class Codes: G01N 27/02 by investigating impedance
• CPC Class Codes: G01Q 10/045 Self-actuating probes, i.e. wherein the actuating means for driving are part of the probe itself, e.g. piezoelectric means on a cantilever probe
• CPC Class Codes: G01Q 20/04 Self-detecting probes, i.e. wherein the probe itself generates a signal representative of its position, e.g. piezoelectric gauge
• CPC Class Codes: G01Q 60/30 Scanning potential microscopy

Abstract

A microwave impedance microscope including a tuning fork having a high-aspect ratio etched metal tip electrode extending transversely to one tine of the fork and having a high aspect ratio to thereby reduce parasitic capacitance. The metal tip may be electrochemically etched from a wire, then bonded to the tine. The fork is slightly inclined from the surface of the sample and the tip electrode projects transversely to the fork. A microwave signal is impressed on the tip. Microwave circuitry receives microwave signals reflected from the sample back into the tip and demodulates the reflected signal according to the impressed signal. Further circuitry further demodulates the reflected signal according to the lower-frequency signal causing the fork to oscillate at its mechanically resonant frequency. A multi-wavelength matching circuit interposed between the microwave circuitry and the probe includes a coaxial cable of length half a fundamental microwave wavelength.

Claims

1. A microwave impedance microscope, comprising:

a support having a support surface for supporting a sample to be tested;

a tuning fork which has two tines extending parallel to a fork axis with a gap therebetween and being caused to vibrate with respect to the fork axis in response to an oscillatory signal applied thereto;

a probe electrode formed from a metal member having a tapered probe tip extending parallel to a probe axis generally perpendicular to the support surface for a height H between a rim and an apex of the probe tip and having an aspect ratio AR of at least 3 between the height H and a diameter D of the rim, wherein the metal member including the probe tip is fixed to one of the tines and projects to the sample along the probe axis; and

microwave circuitry for impressing an incident microwave signal to the probe tip, for receiving a reflected microwave signal from an interaction of the incident microwave signal with the sample, and for producing at least one output signal representing an electrical characteristic of the sample.

2. The microscope of claim 1, wherein the microwave circuitry demodulates the reflected microwave signal according to the incident microwave signal.

3. The microscope of claim 2, wherein the microwave circuitry includes a mixer demodulating the reflected microwave signal according to the incident microwave signal.

4. The microscope of claim 1, further comprising a demodulating circuit demodulating the at least one output signal according to the oscillatory signal.

5. The microscope of claim 1, further comprising:

a microwave source capable of producing the incident microwave signal at a fundamental microwave frequency and at at least one harmonic frequency thereof; and

a matching circuit disposed between the microwave circuitry and the probe electrode that is resonant at the fundamental microwave frequency and at the at least one harmonic frequency.

6. The microscope of claim 5, wherein the matching circuit includes a microwave transmission line having a length equal to one-half a wavelength of the fundamental microwave frequency on the transmission line and a capacitor connected in series between the microwave circuitry and the transmission line.

7. The microscope of claim 6, wherein the capacitor has an impedance at the fundamental microwave frequency greater than a characteristic impedance of the transmission line.

8. The microscope of claim 1, wherein the tuning fork is aligned with the fork axis extending from its distal end upwardly at no more than 25°

from a plane parallel to the support surface.

9. The microscope of claim 1, wherein the probe tip is aligned with the probe axis extending at no more than 25°

from a normal to a plane parallel to the support surface.

10. The microscope of claim 1, wherein the probe tip has a concave conical shape and is formed from a distal end of a wire forming the metal member and having a diameter D away from the probe tip of no more than 40 μ

m, wherein the rim forms a boundary between the wire and the probe tip and wherein AR=H/D is greater than 3 and wherein the wire is bonded to the one tine.

11. The microscope of claim 10, wherein a middle portion of the wire is bonded to the one tine and a proximal end of the wire is fixed to a support member supporting a transmission line to the microwave circuitry and wherein the wire is free standing between the middle portion and the proximal end.

12. The microscope of claim 10, wherein the diameter D is no more than 25 μ

m.

13. A microwave impedance microscope, comprising:

a probe having an electrode probe tip extending towards a sample to be tested;

microwave circuitry for impressing an incident microwave signal to the probe tip, for receiving a reflected microwave signal from an interaction of the incident microwave signal with the sample, and for producing at least one output signal representing an electrical characteristic of the sample, the microwave circuitry including a microwave source capable of producing the incident microwave signal at a fundamental microwave frequency and at at least one harmonic frequency thereof; and

a matching circuit disposed between the microwave circuitry and the probe electrode that is resonant at the fundamental microwave frequency and at the at least one harmonic frequency.

14. The microscope of claim 13, wherein the matching circuit includes a microwave transmission line having a length equal to one-half a wavelength of the fundamental microwave frequency and a capacitor connected in series between the microwave circuitry and the transmission line having an impedance at least ten times greater than a characteristic impedance of the transmission line.

15. The microscope of claim 13, further comprising a tuning fork which has two tines extending parallel to a fork axis with a gap therebetween and being caused to vibrate with respect to the fork axis in response to an oscillatory signal applied thereto and which is aligned with the fork axis extending from its distal end upwardly at no more than 25°

from a plane parallel to a support surface of a support supporting the sample.

16. The microscope of claim 13, further comprising a tuning fork which has two tines extending parallel to a fork axis with a gap therebetween and being caused to vibrate with respect to the fork axis in response to a oscillatory signal applied thereto and which is aligned with the fork axis and wherein the probe axis extends at no more than 25°

from a perpendicular to the support surface.

17. A microwave impedance microscope, comprising:

a support having a support surface for supporting a sample to be tested;

a tuning fork which has two tines extending parallel to a fork axis with a gap therebetween and being caused to vibrate with respect to the fork axis in response to a oscillatory signal applied thereto, wherein the fork axis extends from distal ends of the tines upwardly at no more than 25°

from a plane parallel to the support surface;

a probe electrode having a probe tip extending parallel to a probe axis generally perpendicular to the support surface and fixed to one of the tines and projects along the probe axis to the sample; and

microwave circuitry for impressing an incident microwave signal to the probe tip, for receiving a reflected microwave signal from an interaction of the incident microwave signal with the sample, and for producing at least one output signal representing an electrical characteristic of the sample.

18. The microscope of claim 17, wherein the probe tip is formed from a metal member and is conically shaped for a height H between a rim and an apex of the probe tip and having an aspect ratio AR of at least 3 between the height and a diameter D of the rim.

19. The microscope of claim 17, wherein the probe tip is formed from a metal member comprising a metal wire having a diameter of no more than 40 μ

m.

20. The microwave impedance microscope of claim 17, further comprising a demodulating circuit demodulating the at least one output signal according to the oscillatory signal.