Spatially resolved electromagnetic property measurement
View Patent ↗A scanning probe detects phase changes of a cantilevered tip proximate to a sample, the oscillations of the cantilevered tip are induced by a lateral bias applied to the sample to quantify the local impedance of the interface normal to the surface of the sample. An ac voltage having a frequency is applied to the sample. The sample is placed at a fixed distance from the cantilevered tip and a phase angle of the cantilevered tip is measured. The position of the cantilevered tip is changed relative to the sample and another phase angle is measured. A phase shift of the deflection of the cantilevered tip is determined based on the phase angles. The impedance of the grain boundary, specifically interface capacitance and resistance, is calculated based on the phase shift and the frequency of the ac voltage. Magnetic properties are measured by applying a dc bias to the tip that cancels electrostatic forces, thereby providing direct measurement of magnetic forces.
1. A method for determining impedance information of an interface in a sample, the method comprising the steps of:
(a) applying an ac voltage to the sample, laterally across the interface, the ac voltage having a predetermined frequency;
(b) disposing a cantilevered tip in a first position proximate to a surface of the sample;
(c) measuring a first response of the cantilevered tip with the cantilevered tip in the first position;
(d) placing the cantilevered tip in a second position proximate to the surface of the sample, the interface being between the first position and the second position;
(e) measuring a second response of the cantilevered tip with the cantilevered tip in the second position; and
(f) determining impedance information of the interface based upon the measured first response and the measured second response.
2. The method as recited in claim 1 , wherein the step of:
measuring a first response comprises measuring a first phase angle of deflection of the cantilevered tip;
measuring a second response comprises measuring a second phase angle of deflection of the cantilevered tip; and
determining impedance information comprises:
determining a phase shift based upon the first phase angle and the second phase angle; and
determining impedance information of the interface based upon the phase shift and the frequency of the ac voltage.
3. The method as recited in claim 2 , wherein the step of determining impedance information further comprises determining an impedance product of the interface according to:
tan
(
φ
gb
)
=
ω
C
gb
R
gb
2
(
R
+
R
gb
)
+
R
ω
2
C
gb
2
R
gb
2
where C gb is the capacitance of the interface;
R gb is the resistance of the interface;
ω is the frequency of the ac voltage;
φ gb is the phase shift; and
R is a resistance of a current limiting resistor in series with the sample.
4. The method as recited in claim 2 , further comprising the step of:
selecting the frequency ω of the ac voltage such that tan(ω gb ) is proportional to ω −1 ; and
wherein the step of determining impedance information further comprises determining the capacitance of the interface according to:
tan
(
φ
gb
)
=
1
ω
R
C
gb
where C gb is the capacitance of the interface;
ω is the frequency of the ac voltage;
φ gb is the phase shift; and
R is a resistance of a current limiting resistor in series with the sample.
5. The method as recited in claim 2 , further comprising the step of:
selecting the frequency ω of the ac voltage such that tan(φ gb ) proportional to ω; and
wherein the step of determining impedance information further comprises determining the resistance of the interface according to:
tan
(
φ
gb
)
=
ω
C
gb
R
gb
2
(
R
+
R
gb
)
where R gb is the resistance of the interface;
C gb is the capacitance of the interface;
ω is the frequency of the ac voltage;
φ gh is the phase shift; and
R is a resistance of a current limiting resistor in series with the sample.
6. The method as recited in claim 1 , wherein the step of:
measuring a first response comprises measuring a first amplitude of deflection of the cantilevered tip;
measuring a second response comprises measuring a second amplitude of deflection of the cantilevered tip; and
determining impedance information comprises:
determining an amplitude ratio based upon the measured first amplitude and the measured second amplitude; and
determining an impedance of the interface based upon the amplitude ratio and the frequency of the ac voltage.
7. The method of claim 6 , wherein the step of determining the impedance further comprises determining the impedance according to:
β
-
2
=
{
(
R
+
R
gb
)
+
R
ω
2
C
gb
2
R
gb
2
}
2
+
ω
2
C
gb
2
R
gb
4
R
2
(
1
+
ω
2
C
gb
2
R
gb
2
)
2
where β is amplitude ratio across the interface,
C gb is the capacitance of the interface,
R gb is the resistance of the interface,
ω is the frequency of the ac voltage, and
R is a resistance of a current limiting resistor in series with the sample.
8. The method as recited in claim 1 , wherein the step of:
measuring a first response comprises the steps of:
measuring a first phase angle of deflection of the cantilevered tip; and
measuring a first amplitude of deflection of the cantilevered tip;
measuring a second response comprises:
measuring a second phase angle of deflection of the cantilevered tip; and
measuring a second amplitude of deflection of the cantilevered tip; and
determining impedance information comprises:
determining a phase shift based upon the measured first phase angle and the measured second phase angle;
determining an amplitude ratio based upon the measured first amplitude and the measured second amplitude; and
determining an impedance of the interface based upon the phase shift and the amplitude ratio.
9. The method as recited in claim 8 , wherein the step of determining an impedance of the interface comprises solving the following equations:
β
-
2
=
{
(
R
+
R
gb
)
+
R
ω
2
C
gb
2
R
gb
2
}
2
+
ω
2
C
gb
2
R
gb
4
R
2
(
1
+
ω
2
C
gb
2
R
gb
2
)
2
tan
(
φ
gb
)
=
ω
C
gb
R
gb
2
(
R
+
R
gb
)
+
R
ω
2
C
gb
2
R
gb
2
where β is amplitude ratio across the interface,
C gb is the capacitance of the interface,
R gb is the resistance of the interface,
φ gb is the phase shift;
ω is the frequency of the ac voltage, and
R is a resistance of a current limiting resistor in series with the sample.
10. The method as recited in claim 8 , further comprising the steps of:
repeating each step of claim 8 , for each of a predefined plurality of ac voltage frequencies; and
determining an impedance of the interface as a function of frequency based upon the determined phase shifts and amplitude ratios.
11. The method as recited in claim 8 wherein the step of determining an impedance further comprises determining the impedance of the interface based upon the phase shift and amplitude ratio and the frequency of the ac voltage by using the circuit termination comprised of average resistive and capacitive components determined by conventional impedance spectroscopy by measuring the frequency dependence of phase angle or measuring phase angle and amplitude ratio at a single frequency with a least square fit procedure.
12. The method as recited claim 8 further comprising the steps of:
repeating each step of claim 8 and applying a first dc signal laterally across the interface while performing each measuring step;
repeating each step of claim 8 and applying a second dc signal laterally across the interface while performing each measuring step; and
determining dc signal bias dependence of the interface resistance and capacitance based upon the first and second dc signal.