Determination of local contact potential difference by noncontact atomic force microscopy

Patent metadata

Field	Value
Publication number	US-9316668-B2
Application number	US-201414524261-A
Country	US
Kind code	B2
Filing date	Oct 27, 2014
Priority date	Oct 31, 2013
Publication date	Apr 19, 2016
Grant date	Apr 19, 2016

Abstract

A method for determining a value of a local contact potential difference by noncontact atomic force microscopy. For one or more cantilever positions above a surface of a sample: i) performing two atomic force microscopy measurements, using an oscillating cantilever, ii) thereby determining two distinct voltage values of DC voltage applied between the cantilever and the sample, and iii) obtaining a value of a local contact potential difference based, at least in part, on the two distinct voltage values determined. Wherein substantially similar distinct values indicate a substantially similar value of frequency shifts of cantilever oscillation, as measured for each of said distinct values.

First claim

The invention claimed is:

1. A method for determining a value of a local contact potential difference by noncontact atomic force microscopy, the method comprising:
for one or more cantilever positions above a surface of a sample:
performing two atomic force microscopy measurements, using an oscillating cantilever, thereby determining two distinct voltage values of DC voltage applied between the cantilever and the sample, wherein substantially similar distinct values indicate substantially similar values of frequency shifts of cantilever oscillation, as measured for each of said distinct values; and
obtaining a value of a local contact potential difference based, at least in part, on the two distinct voltage values that were determined.

2. The method of claim 1 , wherein determining the two distinct voltage values further comprises:
during a first AFM measurement:
selecting and storing a first voltage value of DC voltage applied between the cantilever and the sample;
measuring a first frequency shift corresponding to said first voltage value; and
storing a corresponding first voltage value of DC voltage applied between the cantilever and the sample; and

during a second AFM measurement:
measuring a second frequency shift;
adjusting a DC voltage applied between the cantilever and the sample, to obtain a second voltage value distinct from the first voltage value, such that the second frequency shift measured is the same as said first measured frequency shift; and
storing the second voltage value.

3. The method of claim 2 , wherein a constant value of DC voltage is used during the first AFM measurement, for each of said one or more positions.

4. The method of claim 3 , further comprising, prior to using said constant value of DC voltage for the first AFM measurement, selecting said constant value such that a corresponding frequency shift of cantilever oscillation shall be on a same branch of the curve representing the frequency shift vs. an applied DC voltage, where said branch terminates at a principal vertex of the curve, for each of said one or more positions.

5. The method of claim 2 , wherein the first AFM measurement further comprises adjusting a DC voltage applied between the cantilever and the sample to obtain the first voltage value, the latter such that the first measured frequency shift is the same as a predefined frequency shift.

6. The method of claim 1 , wherein determining said two distinct voltage values of DC voltage comprises adjusting a DC voltage during the second measurement or each of the first and second measurements, via a feedback circuit using a frequency shift signal as a feedback signal, for determining a value of DC voltage corresponding to a given value of frequency shift, preferably in constant height mode.

7. The method of claim 1 , wherein the value of a local contact potential difference is obtained as an average of the two distinct voltage values stored.

8. The method of claim 1 , wherein said one or more cantilever positions above the surface of the sample span a subset of a volume above the sample surface, said subset being preferably one of: a 3D, a 2D, or a 1D portion of said volume.

9. The method of claim 1 , the method further comprising:
acquiring a frequency shift map by performing a third AFM measurement for each of said one or more cantilever positions above the surface of the sample, using the local contact potential difference as determined for said each of several cantilever positions, wherein, the first AFM measurement is performed for each of several cantilever positions above the surface of the sample, and then the second AFM measurement is performed for each of the same several cantilever positions, and then the third AFM measurement is performed for each of the same several cantilever positions.

10. An atomic force microscopy apparatus, configured for performing atomic force microscopy measurements using an oscillating cantilever, the apparatus comprising:
a frequency shift measurement unit configured for measuring a frequency shift of cantilever oscillation;
a circuit connected to the frequency shift measurement unit, the circuit being configured to:
determine a DC voltage to be applied by the apparatus between a cantilever and a sample for an AFM measurement; and
adjust such a DC voltage such that a value of a frequency shift, as measured by the frequency shift measurement unit, matches a reference frequency shift;

a memory that is configured to store one or more pairs of distinct values of DC voltages that correspond to a same value of frequency shift of cantilever oscillation; and
a processor connected to the memory to access said one or more pairs of voltage values, the processor being configured to determine values of local contact potential differences based, at least in part, on said one or more pairs.

11. The apparatus of claim 10 , wherein the circuit is i) configured as a feedback circuit; ii) connected to the frequency shift measurement unit; iii) configured to use a frequency shift signal as a feedback signal; and iv) configured to adjust a DC voltage such that a value of frequency shift measured by the frequency shift measurement unit is the same as a reference frequency shift.

12. The apparatus of claim 10 , wherein the circuit has two selectable modes, wherein:
in a first one of the selectable modes, the circuit does not adjust the DC voltage to be applied; and
in a second one of the selectable modes, the circuit adjusts the DC voltage to be applied, such that a value of frequency shift measured by the frequency shift measurement unit matches a reference frequency shift.

13. The apparatus of claim 10 , wherein the processor is further configured to determine a value of the local contact potential difference as an average of two distinct DC voltage values in a given one of said one or more pairs.

14. The apparatus of claim 10 , wherein the apparatus is at least one of a Kelvin probe force microscopy apparatus or an electrostatic force microscopy apparatus.

15. The apparatus of claim 10 , wherein the apparatus is further configured to acquire a frequency shift map by performing one or both of AFM measurement and constant height AFM measurements, using local contact potential differences as determined by the processing means.

Assignees

• IBM

Inventors

• Gross Leo
• Meyer Gerhard
• Schuler Bruno
• Steurer Wolfram

Classifications

• G01Q30/06

  for error compensation · CPC title

• G01R19/0084

  Measuring voltage only · CPC title

• G01Q60/32

  AC mode · CPC title

• G01Q60/30 Primary

  Scanning potential microscopy · CPC title