Method and apparatus for the ultrasonic actuation of the cantilever of a probe-based instrument

• US 6,779,387 B2
• Filed: 03/12/2002
• Issued: 08/24/2004
• Est. Priority Date: 08/21/2001
• Status: Expired due to Fees

• Current Assignee: Georgia Tech Research Corporation (University System of Georgia)
• Original Assignee: Georgia Tech Research Corporation (University System of Georgia)
• Inventors: Degertekin, F. Levent
• Primary Examiner(s): RAEVIS, ROBERT R
• Application Number:
  US10/095,850
• Publication Number:
  US 20030041657A1
• Time in Patent Office: 896 Days
• Field of Search: 731/05, 73/81, 73/82, 250/306, 250/307
• US Class Current: 73/105
• CPC Class Codes: B82Y 35/00 Methods or apparatus for measurement or analysis of nanostructures
• CPC Class Codes: G01N 2291/02827 Elastic parameters, strength or force
• CPC Class Codes: G01N 2291/0427 Flexural waves, plate waves, e.g. Lamb waves, tuning fork, cantilever
• CPC Class Codes: G01N 29/0681 by acoustic microscopy, e.g. scanning acoustic microscopy
• CPC Class Codes: G01N 29/2456 Focusing probes focusing arrangements G01N29/221
• CPC Class Codes: G01N 29/346 with amplitude characteristics, e.g. modulated signal
• CPC Class Codes: G01Q 30/14 Liquid environment
• CPC Class Codes: G01Q 60/32 AC mode
• CPC Class Codes: G01S 17/08 for measuring distance only indirect measurement G01S17/46; active triangulation systems G01S17/48
• CPC Class Codes: Y10S 977/872 Positioner

Abstract

The cantilever of a probe-based metrology instrument such as an AFM is deflected by directing a beam of ultrasonic energy at the cantilever to apply ultrasonically generated acoustic radiation pressure to the cantilever. The energy is generated by an ultrasonic actuator such as a ZnO transducer driven by a power source such an RF signal generator. The transmitted beam preferably is shaped by focusing, collimation, or the like so that it impinges at least primarily on a region of interest of the cantilever such as the free end. The ultrasonic actuator produces a much better controlled force on the cantilever than can be achieved through the use of a traditional piezoelectric actuator and, accordingly, produces a response free of spurious effects (at least when the cantilever is operating in liquid). It also has a frequency bandwidth in the MHz range.

Claims

1. A metrology instrument comprising:

(A) a probe including a cantilever; and

(B) an ultrasonic actuator configured to direct a beam of ultrasonic energy at the cantilever that imposes a force on the cantilever.

2. The instrument as recited in claim 1, wherein the ultrasonic actuator is configured to deflect the cantilever.

3. The instrument as recited in claim 2, wherein the instrument is configured to transmit an RF oscillation signal to the ultrasonic actuator.

4. The instrument as recited in claim 3, wherein the instrument is configured to modulate the amplitude of the RF oscillation signal using a modulation signal having a modulation frequency that is lower than the frequency of the RF oscillation signal.

5. The instrument as recited in claim 4, wherein the modulation signal has a time-varying modulation characteristic.

6. The instrument as recited in claim 3, wherein the amplitude of the RF oscillation signal is adjustable to provide an adjustable force to the cantilever.

7. The instrument as recited in claim 6, wherein the instrument is configured to alter the amplitude of the RF oscillation signal at a rate so as to permit a quasistatic measurement to be performed.

8. The instrument as recited in claim 3, wherein the amplitude of the RF oscillation signal is adjustable to provide an adjustable deflection of the free end of the cantilever.

9. The instrument as recited in claim 7, wherein the instrument is configured to use acquired data indicative of the deflection of the cantilever versus amplitude of RF oscillation to generate a measurement of the spring constant of the cantilever.

10. The instrument as recited in claim 3, wherein the RF oscillation signal has a frequency of between 10 MHz and 1 GHz.

11. The instrument as recited in claim 1, wherein the cantilever has a free end supporting a probe tip and a base supported on a holder, and wherein the instrument is configured to shape the beam of ultrasonic energy so that the beam substantially strikes the cantilever.

12. The instrument as recited in claim 11, wherein the instrument is configured to shape the beam such that the beam is sufficiently larger than the cantilever to accommodate limited mispositioning of the cantilever arising from mounting to mounting tolerance.

13. The instrument as recited in claim 11, wherein the instrument is configured to shape the beam such that the beam is sufficiently smaller than the cantilever to assure that all non-deflected components of the beam strike the cantilever.

14. The instrument as recited in claim 11, further comprising a focusing device positioned between the actuator and the cantilever and configured to focus the ultrasonic beam at least substantially onto a designated region of the cantilever.

15. The instrument as recited in claim 14, wherein the focusing device comprises a Fresnel lens.

16. The instrument as recited in claim 14, wherein the focusing device includes a hemispheric cutout in the surface of a substrate supporting the acoustic actuator.

17. The instrument as recited in claim 14, wherein the focusing device focuses the beam to a spot diameter of no more than about 10 Mm.

18. The instrument as recited in claim 17, wherein the focusing device focuses the beam to a spot diameter of no more than about 5 μ

m.

19. The instrument as recited in claim 11, wherein the ultrasonic actuator is configured to generate a collimated or minimally divergent beam having a divergence of less than 10 degrees.

20. The instrument as recited in claim 19, wherein the ultrasonic actuator is configured to generate a collimated or minimally divergent beam having a divergence of less than 30 degrees.

21. The instrument as recited in claim 20, wherein said ultrasonic actuator has a dimension of no more than about 500 microns.

22. The instrument as recited in claim 20, wherein said ultrasonic actuator has a dimension of no more than about 50 microns.

23. The instrument as recited in claim 1, wherein the ultrasonic actuator comprises a zinc oxide transducer.

24. The instrument as recited in claim 1, further comprising a detector that is configured to detect cantilever deflection.

25. The instrument as recited in claim 24, wherein the cantilever and the detector are positioned on a common side of the cantilever disposed opposite a sample holder.

26. An atomic force microscope comprising:

(A) a fluid cell;

(B) a probe including

     1) a cantilever having a base and having a free end portion extending into the fluid cell and

     2) a tip located on the free end portion of the cantilever;

(C) an ultrasonic actuator positioned on a side of the cantilever opposite the fluid cell and configured to direct a shaped beam of ultrasonic energy onto the cantilever that drives the cantilever to oscillate;

(D) a detector positioned on the side of the cantilever opposite the fluid cell and configured to detect cantilever deflection.

27. A method comprising:

(A) generating a beam of ultrasonic energy using an ultrasonic actuator;

(B) directing the beam onto a cantilever of a probe of a metrology instrument to impose a force on the cantilever.

28. The method as recited in claim 27, wherein the cantilever has a free end and has a base attached to a holder, and wherein the directing step includes shaping the beam so that it impinges substantially on the surface of the cantilever.

29. The method as recited in claim 28, wherein the shaping step comprising constraining the beam such that all undeflected components of the beam impinge on the cantilever.

30. The method as recited in claim 28, wherein the shaping step comprising constraining the beam such that the beam is sufficiently larger than the cantilever to accommodate limited mispositioning of the cantilever arising from mounting tolerance.

31. The method as recited in claim 28, wherein the shaping step comprises focusing the ultrasonic beam onto a designated region of the cantilever.

32. The method as recited in claim 31, wherein the designated region comprises a free end portion of the cantilever.

33. The method as recited in claim 31, wherein the designated region has a diameter of no more than about 10 μ

m.

34. The method a recited in claim 32, wherein the designated region has a diameter of no more than about 5 μ

m.

35. The method as recited in claim 31, wherein the focusing step is performed by one of a Fresnel lens and a hemispheric cutout in the surface of a substrate supporting the acoustic actuator.

36. The method as recited in claim 28, wherein the shaping step comprises collimating the beam sufficiently to produce a beam divergence of no more than about 30°

.

37. The method as recited in claim 36, wherein the shaping step comprises collimating the beam sufficiently to produce a beam divergence of no more than about 10°

.

38. The method as recited in claim 27, wherein the directing step imposes a sufficient force on the cantilever to deflect the cantilever.

39. The method as recited in claim 38, further comprising altering a power supply to the ultrasonic actuator to alter the magnitude of the force applied to the cantilever.

40. The method as recited in claim 39, wherein the magnitude of the force imposed on the cantilever and the magnitude of cantilever deflection are proportional to the power supplied to the ultrasonic actuator.

41. The method as recited in claim 38, further comprising measuring cantilever deflection and determining a spring constant of the cantilever by comparing the deflection of the cantilever at a specified drive voltage to the ultrasonic actuator to a measured deflection of a cantilever of known spring constant at the specified drive voltage.

42. The method as recited in claim 27, wherein at least a free end of the cantilever is immersed in a fluid.

43. The method a recited in claim 42, wherein the fluid is a liquid.

44. The method as recited in claim 42, wherein the fluid is a gas.

45. The method as recited in claim 27, further comprising detecting cantilever deflection.

46. The method as recited in claim 45, further comprising generating a force curve using data collected as a result of the detecting step.

47. The method as recited in claim 27, further comprising transmitting an RF oscillation signal to the ultrasonic actuator.

48. The method as recited in claim 47, further comprising modulating the RF oscillation signal via a modulation signal having a modulation frequency that is lower than the frequency of the RF oscillation signal.

49. The method as recited in claim 48, wherein the modulation signal has a time-varying modulation characteristic.

50. The method as recited in claim 47, further comprising adjusting the amplitude of the RF oscillation signal to provide an adjustable force to the cantilever.

51. The method as recited in claim 50, wherein the amplitude of the RF oscillation signal is altered at a rate so as to permit a quasistatic measurement to be performed.

52. The method as recited in claim 47, further comprising adjusting the amplitude of the RF oscillation signal to provide an adjustable deflection of the free end of the cantilever.

53. The method as recited in claim 47, further comprising determining a spring constant of the cantilever using acquired data concerning the deflection of the cantilever versus amplitude of RF oscillation.

54. The method as recited in claim 47, wherein the RF signal has a frequency of between 10 MHz and 1 GHz.

55. The method as recited in claim 54, wherein the RF signal has a frequency of between 50 MHz and 500 MHz.

56. A method comprising:

(A) generating a beam of ultrasonic energy using an ultrasonic actuator of an AFM, the AFM including a probe that includes

     1) a cantilever having a base supported on a holder and having a free end portion and

     2) a tip located on the free end portion, at least the free end portion of the cantilever being immersed in a liquid;

(B) transmitting the beam onto a cantilever to impose a force on the cantilever of sufficient magnitude and frequency to drive the cantilever to oscillate while shaping the beam sufficiently to impinge primarily on the cantilever; and

(C) detecting cantilever deflection.