Synchronous sampling scanning force microscope

Claims

What is claimed is:

1. In a synchronous sampling scanning force microscope for examining surface contours of a specimen, said specimen surface also having an attractive region and a repulsive region, said microscope having a body, a cantilever arm having a first end secured to said body and a free end, said cantilever arm having a fundamental resonance frequency, probe means secured to said free end of said cantilever arm and including a probe tip adapted to follow surface contours of the specimen, scanning means for mounting said specimen for examination by said microscope and adapted to scan said specimen relative to said body, deflection measuring means mounted to said body for measuring deflection of the cantilever arm and generating a deflection signal indicative of deflection of said cantilever arm, the improvement which comprises:

oscillator drive means connected to said cantilever arm for causing the cantilever arm and probe tip to oscillate toward and away from said specimen surface contours at a desired frequency in cycles of near and far excursions of said probe tip relative to said specimen surface;

sampling means for sampling selected portions of cycles of said deflection signal corresponding to said cycles of near and far excursions of said probe tip and for generating output signal data indicative of elevation of said surface contours of said specimen;

phase control means connected to said deflection measuring means for receiving said deflection signals, said phase control means being connected to said sampling means for controlling the sampling of said selected portions of cycles of said deflection signal; and

means for storing said output signal data.

2. The microscope of claim 1, wherein said desired frequency of oscillation is different from the resonant frequency of the cantilever arm.

3. The microscope of claim 1, wherein said desired frequency of oscillation is a frequency less than the resonant frequency.

4. The microscope of claim 1, wherein said desired frequency of oscillation is a frequency between about 5-500 kHz.

5. The microscope of claim 1, wherein said desired frequency of oscillation is a frequency between about 80-250 kHz.

6. The microscope of claim 1, wherein said oscillator drive means is operative to drive said cantilever arm with an oscillation amplitude between about 2.ANG. and 1000.ANG..

7. The microscope of claim 1, wherein said oscillator drive means is operative to drive said cantilever arm with an oscillation amplitude between about 100.ANG. and 1000.ANG. at about 50-100 kHz.

8. The microscope of claim 1, wherein said oscillator drive means is operative to drive said cantilever arm with an oscillation amplitude between about 2.ANG. and 10.ANG. at about 5-500 kHz.

9. The microscope of claim 1, wherein said sampling means is operative to sample measurements of amplitude of oscillation of said cantilever arm.

10. The microscope of claim 1, wherein said sampling means is operative to sample measurements of phase shift of oscillation of said cantilever arm.

11. The microscope of claim 1, further including probe position control feedback means for maintaining the probe in the attractive region of the specimen surface.

12. The microscope of claim 1, further including probe position control feedback means for maintaining the probe in intermittent contact with a contamination layer on the specimen surface.

13. The microscope of claim 1, wherein said sampling means includes means for sampling at a plurality of times during each cycle.

14. The microscope of claim 1, wherein said sampling means includes means for sampling at intervals of 90.degree. and 270.degree. phases of each cycle.

15. The microscope of claim 1, wherein said sampling means includes means for determining said output signal data as a difference between measured amplitude at 90.degree. and 270.degree..

16. The microscope of claim 1, further including display means connected to said means for storing, for displaying an image of the surface of the specimen based upon said output signal data.

17. The microscope of claim 1, further including means for averaging successive output signal data to generate an image.

18. The microscope of claim 17, wherein said means for averaging comprises means for averaging a root mean square of voltage of said output signal data.

19. The microscope of claim 17, wherein said means for averaging comprises means for averaging output signal data of a plurality of images of the surface contours of a specimen.

20. The microscope of claim 1, wherein said sampling means includes means for generating a position feedback signal for positioning the probe tip a desired distance from the surface of the specimen.

21. A synchronous sampling scanning force microscope for examining surface contours of a specimen, said specimen surface also having an attractive region and a repulsive region, the microscope comprising:

a body, said body including a cantilever arm having a first end secured to said body and a free end, said cantilever arm having a fundamental resonance frequency;

probe means secured to said free end of said cantilever arm, said probe means including a probe tip adapted to follow the surface contours of the specimen with a substantially constant amount of force;

scanning means for mounting said specimen for examination by said microscope and adapted to scan said specimen relative to said body;

deflection measuring means for measuring motion of said cantilever arm and for generating an output signal indicative of an amount of deflection of said cantilever arm means;

oscillator drive means connected to said cantilever arm for causing the cantilever arm and probe tip to oscillate toward and away from said specimen surface contours at a desired frequency in cycles of near and far excursions of said probe tip relative to said specimen surface;

sampling means for sampling selected portions of cycles of said deflection signal corresponding to said cycles of near and far excursions of said probe tip and for generating output signals indicative of elevation of said surface contours of said specimen;

phase control means connected to said deflection measuring means for receiving said deflection signal and connected to said sampling means for controlling the sampling of said selected portions of cycles of said deflection signal; and

memory means for storing said output signals.

22. The microscope of claim 21, wherein said means for measuring motion of said oscillating cantilever arm comprises laser light source means mounted to said body for producing a focused laser beam directed at and deflected by said cantilever arm, and photodetector means mounted to said body for receiving said laser beam deflected by said cantilever arm and generating said output signals.

23. The microscope of claim 21, wherein said desired frequency of oscillation is a frequency different from the resonant frequency of the cantilever arm.

24. The microscope of claim 21, wherein said desired frequency of oscillation is a frequency less than the resonant frequency.

25. The microscope of claim 21, wherein said desired frequency of oscillation is a frequency between about 5-500 kHz.

26. The microscope of claim 21, wherein said desired frequency of oscillation is frequency between about 80-250 kHz.

27. The microscope of claim 21, wherein said oscillator drive means is operative to drive said cantilever arm with an oscillation amplitude between about 2.ANG. and 1000.ANG..

28. The microscope of claim 21, wherein said oscillator drive means is operative to drive said cantilever arm with an oscillation amplitude between about 100.ANG. and 1000.ANG. at about 50-100 kHz.

29. The microscope of claim 21, wherein said oscillator drive means is operative to drive said cantilever arm with an oscillation amplitude between about 2.ANG. and 10.ANG. at about 5-500 kHz.

30. The microscope of claim 21, wherein said sampling means includes means for sampling amplitude of oscillation of said cantilever arm.

31. The microscope of claim 21, wherein said sampling means includes means for sampling phase shift of oscillation of said cantilever arm.

32. The microscope of claim 21, wherein said sampling means includes dual sample and hold amplifiers.

33. The microscope of claim 21, wherein said sampling means includes a plurality of sample and hold amplifiers.

34. The microscope of claim 21, wherein said sampling means includes probe position control feedback means for maintaining the probe in the attractive region of the specimen surface.

35. The microscope of claim 21, wherein said sampling means includes probe position control feedback means for maintaining the probe in intermittent contact with a contamination layer on the specimen surface.

36. The microscope of claim 21, wherein said sampling means includes clock output means.

37. The microscope of claim 21, further including display means for displaying an image of the surface of the specimen based upon said output signals.

38. A method for synchronous sampling detection of an oscillating cantilever of a scanning force microscope for examining surface contours of a specimen, said microscope having a body, a cantilever arm having a first end secured to said body and a free end, said cantilever arm having a fundamental resonance frequency, probe means secured to said free end of said cantilever arm and including a probe tip adapted to follow the surface contours of the specimen with a substantially constant amount of force, scanning means for mounting said specimen for examination by said microscope and adapted to scan said specimen relative to said body, deflection measuring means for measuring deflection of said cantilever arm and for generating a deflection signal indicative of an amount of deflection of said cantilever arm, the method comprising:

oscillating said free end of said cantilever arm to cause the probe tip to oscillate toward and away from said specimen surface contours at a desired frequency in cycles of near and far excursions of said probe tip relative to said specimen surface;

sampling selected portions of cycles of said output signal corresponding to said cycles of near and far excursions of said probe tip and for generating output signal data indicative of relative elevation of said surface contours of said specimen;

controlling the sampling of said selected portions of cycles so that said sampling occurs during predetermined phases of each cycle of said deflection signal;

storing said output signal data; and

displaying an image of said surface contours of said specimen based upon said output signal data.

39. The method of claim 38, wherein said step of oscillating said free end of said cantilever arm comprises oscillating the cantilever arm at a frequency between about 5-500 kHz.

40. The method of claim 38, wherein said step of oscillating said free end of said cantilever arm comprises oscillating the cantilever arm at a frequency between about 80-250 kHz.

41. The method of claim 38, wherein said step of oscillating said free end of said cantilever arm comprises driving oscillations of the cantilever arm at an amplitude between about 2.ANG. and 1000.ANG..

42. The method of claim 38, wherein said step of oscillating said free end of said cantilever arm comprises driving oscillations of the cantilever arm at an amplitude between about 100.ANG. and 1000.ANG. at about 50-100 kHz.

43. The method of claim 38, wherein said step of oscillating said free end of said cantilever arm comprises driving oscillations of the cantilever arm at an amplitude between about 2.ANG. and 10.ANG. at about 5-500 kHz.

44. The method of claim 38, wherein said step of sampling selected portions of cycles of said deflection signal comprises sampling amplitude of said deflection signal.

45. The method of claim 38, wherein said step of sampling selected portions of cycles of said deflection signal comprises sampling phase shift of said deflection signal.

46. The method of claim 38, wherein said step of sampling selected portions of cycles of said deflection signal comprises sampling amplitude of said deflection signal for about 10% of the period of each cycle.

47. The method of claim 38, wherein said step of sampling selected portions of cycles of said deflection signal comprises sampling amplitude of said deflection signal at a plurality of times during each cycle.

48. The method of claim 38, wherein said step of sampling selected portions of cycles of said output signal comprises sampling amplitude of said deflection signal at intervals of 90.degree. and 270.degree. phases of each cycle.

49. The method of claim 38, wherein said output signal data is determined as the difference between measured amplitude at 90.degree. and 270.degree..

50. The method of claim 38, further including the step of averaging successive output signal data to generate topographical data for displaying said image.

51. The method of claim 38, further including the step of averaging the root mean square of said output signal data to generate topographical data for displaying said image.

52. The method of claim 38, further including the step of averaging output signal data of a plurality of images on a pixel by pixel basis to generate topographical data for displaying an averaged image of said surface contours of specimen.

53. The method of claim 38, further including the step of generating a position feedback signal based upon said deflection signal for positioning the probe tip a desired distance from the surface of the specimen.

54. The method of claim 38, further including the step of generating a probe position feedback signal based upon said deflection signal for maintaining the probe in an attractive region of the specimen surface.

55. The method of claim 38, further including the step of generating a probe position feedback signal based upon said deflection signal for maintaining the probe in intermittent contact with a contamination layer on the specimen surface.

56. In a scanning force microscope for examining surface contours of a specimen, said microscope having a body, a deflection arm having a first end secured to said body and a free end, said deflection arm having a fundamental resonance frequency, probe means secured to said free end of said deflection arm and including a probe tip adapted to follow the surface contours of the specimen, scanning means for mounting said specimen for examination by said microscope and adapted to scan said specimen relative to said body, and deflection measuring means mounted to said body for measuring motion of the deflection arm and generating a deflection signal indicative of deflection of said deflection arm, the improvement which comprises:

oscillator drive means connected to said deflection arm for causing deflection arm and the probe tip to oscillate toward and away from said specimen surface contours at a desired frequency in cycles of near and far excursions of said probe tip relative to said specimen surface;

sampling means for sampling selected portions of cycles of said output signal corresponding to said cycles of near and far excursions of said probe tip and for generating output signals representing deflection of said near and far excursions of said probe tip at said selected portions of said cycles;

control means connected to said deflection measuring means for receiving said deflection signal and connected to said sampling means for controlling the sampling of said selected portions of cycles of said deflection signal; and

means for storing said output signals.

57. A method for examining surface contours of a specimen using a scanning force microscope, said microscope having a body, a deflection arm having a first end secured to said body and a free end, said deflection arm having a fundamental resonance frequency, probe means secured to said free end of said deflection arm and including a probe tip adapted to follow surface contours of the specimen, scanning means for mounting said specimen for examination by said microscope and adapted to scan said specimen relative to said body, and deflection measuring means mounted to said body for measuring motion of the deflection arm and generating a deflection signal indicative of an amount of deflection said deflection arm, the steps of the method comprising:

oscillating said free end of said deflection arm to cause the probe tip to oscillate toward and away from said specimen surface contours at a desired frequency in cycles of near and far excursions of said probe tip relative to said specimen surface;

sampling selected portions of cycles of said output signal corresponding to said cycles of near and far excursions of said probe tip and for generating output signal data representing deflection of said near and far excursions of said probe tip at said selected portions of said cycles;

controlling the sampling of said selected portions of cycles so that said sampling occurs during predetermined phases of each cycle of said deflection signal; and

storing said output signal data.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to scanning force microscopes, and more particularly concerns a scanning atomic force microscope with an oscillating cantilever probe, and a synchronous sampling detection method of generating data points for imaging surface contours of a specimen.

2. Description of Related Art

Scanning force microscopes, also known as atomic force microscopes, are useful for imaging objects as small as atoms. Scanning force microscopy is closely related to scanning tunneling microscopy and the technique of stylus profilometry. However, in a scanning force microscope, a laser beam is typically deflected by a reflective lever arm to which the probe is mounted, indicative of vertical movement of the probe as it follows the contours of a specimen. The deflection of the laser beam is typically monitored by a photodetector in the optical path of the deflected laser beam, and the sample is mounted on a stage moveable in minute distances in three dimensions. The sample can be raster scanned while the vertical positioning of the probe relative to the surface of the sample is maintained substantially constant by a feedback loop with the photodetector controlling the vertical positioning of the sample.

The interactive forces between the probe and surface of the specimen change at different distances. As the probe approaches the surface of an uncontaminated specimen, it is initially attracted to the surface by long range attractive forces, such as van der Waals forces. As the probe tip approaches further, repulsive forces from the electron orbitals of the atoms on the probe tip and the specimen surface become more significant. Under normal ambient conditions, the surface of a specimen will also be covered by a thin contamination layer, typically composed of water and other ambient contaminants, and contaminants remaining from production of the specimen. The thickness of the contamination layer can vary due to humidity and specific ambient conditions, but is generally between 25 and 500 .ANG.. This contamination layer can also have an interactive effect on the probe tip. As the probe tip approaches the contamination layer of a specimen, capillary surface forces can strongly attract the probe tip toward the surface of the specimen. When the probe tip is being retracted from the surface of the specimen, the capillary attraction forces can also strongly resist retraction of the probe tip from the surface of the specimen.

In conventional non-modulated modes of operating atomic force microscopes, where the lever arm is not oscillated, output from the detector monitoring the deflection of the reflective probe lever arm is typically used as feedback to adjust the position of the probe tip to maintain the interactive forces and distance between the probe tip and specimen surface substantially constant. In a conventional non-modulated, DC-contact mode of operation, the detected displacement of the probe is used in a feedback loop to adjust the position of the probe so that the force between the probe and the specimen surface remains substantially constant. The "constant force" mode can be used with a slow scan, but if a sufficiently rapid scan is executed, the feedback loop may not be able to keep up with the changes in the features of the surface of the specimen to be able to adequately maintain a constant force. In a non-modulated "non-contact" operating mode, the probe is maintained in the attractive region near the surface of the specimen, being attracted to the surface of the specimen primarily by capillary attractive forces from contaminated specimens, or by van der Waals forces from uncontaminated specimens. When the "non-contact" mode of operation is used without oscillating the lever arm, as long as the probe tip is in contact with the specimen surface, the "non-contact" mode is substantially identical to the DC-contact imaging mode. In such non-modulated "non-contact" modes of operation, the feedback loop may also not be adequate to accurately position the probe tip at high scan rates.

In modulated modes of operating a scanning force microscope, the reflective lever arm is typically mounted to a piezoelectric ceramic material which can be driven by an alternating voltage to cause the lever arm and the probe tip to oscillate at a desired frequency. In modulated "non-contact" and "intermittent contact" scanning modes, as the oscillating probe tip approaches the surface of the specimen, both the amplitude and phase shift of the probe relative to the driving oscillator are perturbed by the surfaces forces. Measurements are typically made of the average cantilever amplitude or the shift in phase of the cantilever relative to the driven oscillation, in order to monitor the interaction of the tip with the attractive and repulsive forces of the surface of the sample, generally due to a contaminant layer on the surface of the sample, in ambient, open air conditions. Either the change in amplitude or the change in phase can also be used in a positioning feedback loop of a scanning force microscope. Conventional methods of averaging measurements of the change in amplitude or phase over several oscillations of the cantilever are satisfactory for obtaining topographical image data at low speed scan rates. However, in order to achieve satisfactory resolution at high speed scanning, it would be desirable to utilize individual, unaveraged measurements of amplitude or phase shifts for generating topographical image data. There is an increasing need for real-time imaging, particularly for applications such as scanning force microscope electrochemistry, and scanning of living biological specimens. At higher scan rates, the reflective lever arm oscillating frequency must approach the physical limits of the lever arm design, and the data sampling times must be minimized.

In a conventional high amplitude resonance modulation mode, in which the probe is oscillated at its resonant frequency, typically at 50-500 kHz, at a high amplitude of from 100 to 1,000 .ANG., the probe has intermittent contact with the surface of the specimen, rapidly moving in and out of the contamination layer. In this mode, the topographical image is not significantly affected by the contamination layer, since the probe rapidly penetrates this layer. Either the probe or the sample can be damaged in this mode, which is more appropriate for imaging soft specimens. In a conventional low amplitude resonance mode, in which the probe tip is also typically oscillated at it resonance frequency at from 50-500 kHz at a low amplitude, the probe remains within the contamination layer, in the attractive region. However, since the contamination layer can change, due to warming of the specimen, changes in humidity or other ambient surface conditions, images made with in this mode of operation can also change.

Resonance modes of operation also present special problems, in that changes in amplitude and phase during oscillation of the lever arm due to long and short range forces occurring between the tip and the surface of the sample are most greatly affected when the frequency is at or near the fundamental resonance frequency. At resonance, the oscillation is quickly damped when the probe tip is at or near the sample surface. The quality factor, Q, of the oscillating lever arm at resonance, further increases the effect of the interacting surface forces on the amplitude and phase shift. For a single optical lever arm made of silicon (100 microns long, 15 microns wide, 6 microns thick), the resonance frequency is about 300 Khz, and the Q factor is well over 100 in air. However, operation of a scanning force microscope with a lever arm having a high Q factor in "non-contact" mode at the resonance frequency can cause "ringing" problems, reducing frequency response. Consequently, conventional resonance modes of operation typically result in low resolution imaging of the surface of a specimen.

Utilization of unaveraged individual measurements of amplitude or phase shift changes of oscillation of the probe in modulated modes of operation can also present special problems of increased noise and differences in interaction of the probe tip with a surface contamination layer during near and far excursions of the probe tip relative to the surface of the specimen. It would be desirable to provide a way of overcoming problems of noise and differences in interaction of the probe tip during near and far excursions to provide for high resolution imaging at high scan rates.

For specimens having a surface in which the compliance changes, compliance measurements can also be useful in providing enhanced contrast in analyzing topographic images. Compliance of the surface is typically measured by successive application of increasing force of an atomic force microscope probe from point to point, starting each measurement with the probe tip off the surface. As such sampling results in a slow scan, it would be desirable to perform multiple force measurement sampling at a rate greater than the probe oscillation frequency, to provide multiple sampling during the period of each oscillation, to monitor compliance of the probe tip on the surface of the specimen. The invention addresses these needs.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention provides for a scanning force microscope with an oscillating cantilever probe, and a synchronous sampling detection method for using the microscope which allows high speed scanning of surface contours of a specimen for producing an image of the specimen surface contours. The method and apparatus of the invention permit monitoring of compliance of the surface of the specimen by multiple sampling at a rate greater than the period of oscillation of the cantilever probe of the microscope. The method of the invention can be used with any method for sensing a signal proportional to the position of the probe tip at the end of the cantilever.

The invention accordingly provides for a synchronous sampling scanning force microscope for examining surface contours of a specimen, the microscope having a body including a reflective cantilever arm having a first end secured to the body and a free end which is oscillated at a desired frequency which is preferably different from the resonance frequency of the cantilever arm. A probe is secured to the free end of the cantilever arm, and includes a probe tip adapted to follow the surface contours of the specimen with a substantially constant amount of force. Scanning means are provided for mounting the specimen for examination and for scanning the specimen relative to the probe tip. Means are provided for measuring the motion of the oscillating cantilever arm, operating to generate a deflection signal indicative of the amplitude of deflection or phase shift of the cantilever arm. The means for measuring the motion of the oscillating cantilever arm preferably comprises laser light source means mounted to the body for producing a focused laser beam directed at and deflected by the reflective cantilever arm, with photodetector means mounted to the body for receiving the laser beam deflected by the cantilever arm. Means are provided for sampling selected portions of cycles of the deflection signal corresponding to cycles of motion the probe tip, for generating output signal data or phase shift data indicative of elevation of the surface contours of the specimen.

The invention also provides a method for synchronous sampling detection of an oscillating cantilever of the scanning force microscope for examining surface contours of a specimen, generally by the steps of oscillating the free end of the cantilever arm to cause the probe tip to oscillate toward and away from the specimen surface contours at a desired frequency preferably different than the resonance frequency of the cantilever arm of the microscope, sampling selected portions of cycles of the deflection signal corresponding to the cycles of near and far excursions of the probe tip, generating output signal data indicative of elevation of the surface contours of the specimen, controlling the phase of sampling of the selected portions of cycles of the output signal, and storing the output signal data for displaying an image of the surface contours of the specimen based upon the output signal data.

These and other aspects and advantages of the invention will become apparent from the following detailed description, and the accompanying drawings, whic