Atomic force microscope

In a method of operating an atomic force microscope having a sample holder and a probe with a sensing tip, wherein a relative scanning movement between a sample in the holder and the sensing tip is created in X, Y, and Z coordinate directions and the position of the sensing tip is sensed, and a feedback operation is performed to maintain the tip in a pre-established relationship with respect to a scanned surface of the sample in the Z direction based on the sensed position of the tip, thereby to obtain height information about the scanned surface of the sample, the improvement comprising the steps of:

• performing digital feedback comprising,
• obtaining an analog signal position from the sensing means,
• converting the analog signal with analog-to-digital conversion means into a digital signal,
• receiving the digital signal from the analog-to-digital conversion means,
• calculating and outputting with digital computation means first digital vertical control signals which create desired relative movement between a sample in the holder and the sensing tip in the Z coordinate direction which maintain the tip in the pre-established relationship with respect to the scanned surface of the sample in the Z direction,
• receiving the first digital vertical control signals from the digital computation means, and
• outputting analog control signals to the scanning means to affect movement thereof in the Z direction;
• additionally comprising:
• discontinuing outputting of said first digital vertical control signals which maintain the tip in the said pre-established relationship with respect to the scanned surface in the Z direction and utilizing said digital computation means to produce second digital control signals which vary the vertical position of the sample with respect to the tip so as to obtain a force position curve as a function of sample position and tip deflection.

An atomic force microscope comprising:

• a sample holder and a probe with a sensing tip;
• scanning means for creating relative movement between a sample in the holder and the sensing tip in X, Y and Z coordinate directions;
• sensing means for sensing the vertical position of the sensing tip;
• and
• feedback means connected between the sensing means and the scanning means for maintaining the tip in a pre-established relationship with respect to a scanned surface of the sample in the Z direction, thereby to obtain height information about the scanned surface of the sample, wherein said feedback means comprises,
• analog-to-digital conversion means for obtaining an analog signal from the sensing means and for converting said analog signal into a digital signal,
• digital computation means including program means for receiving said digital signal from said analog-to-digital conversion means and for calculating and outputting first digital vertical control signals which create desired relative movement between the sample in the holder and the sensing tip in the Z coordinate direction which maintain the tip in the pre-established relationship with respect to the scanned surface of the sample in the Z direction,
• first digital-to-analog conversion means for receiving said first digital vertical control signals from said digital computation means and for outputting analog control signals to the scanning means to affect movement thereof in the Z direction, and
• said digital computation means comprising means for modulating the relative movement between the tip and scanned surface of the sample in the Z direction so as to vary the force on the sample and for calculating a measure of stiffness of the sample from the changes in the force and changes in the position of the tip;
• wherein modulation done by the modulating means of said digital computation means is done during scanning in the X and Y directions to obtain the measure of stiffness of the sample as a function of X and Y.

In an atomic force microscope having a sample holder and a probe with a sensing tip, scanning means for creating relative movement between a sample in the holder and the sensing tip in X, Y, and Z coordinate directions, sensing means for sensing the position of the sensing tip, and feedback means connected between the sensing means and the scanning means for maintaining the tip in a pre-established relationship with respect to a scanned surface of the sample in the Z direction to obtain height information about the scanned surface of the sample, the improvement to also allow material characteristics information about the scanned surface of the sample to be obtained simultaneously wherein the feed-back means comprises:

• a) analog-to-digital conversion means for obtaining an analog signal from the sensing means and for converting said analog signal into a digital signal;
• b) digital computation means including program means for receiving said digital signal from said analog-to-digital conversion means and for calculating and outputting first digital vertical control signals which create desired relative movement between a sample in the holder and the sensing tip in the Z coordinate direction which maintain the tip in the pre-established relationship with respect to the scanned surface of the sample in the Z direction;
• and
• c) first digital-to-analog conversion means for receiving said first digital vertical control signals from said digital computation means and for outputting analog control signals to the scanning means to affect movement thereof in the Z direction;
• wherein said digital computation means includes means for determining a tip null point.

In an atomic force microscope having a sample holder and a probe with a sensing tip, scanning means for creating relative movement between a sample in the holder and the sensing tip in X, Y, and Z coordinate directions, sensing means for sensing the position of the sensing tip, and feedback means connected between the sensing means and the scanning means for maintaining the tip in a pre-established relationship with respect to a scanned surface of the sample in the Z direction to obtain height information about the scanned surface of the sample, the improvement to also allow material characteristics information about the scanned surface of the sample to be obtained simultaneously wherein the feed-back means comprises:

• a) analog-to-digital conversion means for obtaining an analog signal from the sensing means and for converting said analog signal into a digital signal;
• b) digital computation means including program means for receiving said digital signal from said analog-to-digital conversion means and for calculating and outputting first digital vertical control signals which create desired relative movement between a sample in the holder and the sensing tip in the Z coordinate direction which maintain the tip in the pre-established relationship with respect to the scanned surface of the sample in the Z direction;
• and,
• c) first digital-to-analog conversion means for receiving said first digital vertical control signals from said digital computation means and for outputting analog control signals to the scanning means to affect movement thereof in the Z direction;
• wherein said digital computation means includes means for varying the vertical position of the sample with respect to the tip so as to obtain a force position curve at the end of a scan line.

In an atomic force microscope having a sample holder and a probe with a sensing tip, scanning means for creating relative movement between a sample in the holder and the sensing tip in X, Y, and Z coordinate directions, sensing means for sensing the position of the sensing tip, and feedback means connected between the sensing means and the scanning means for maintaining the tip in a pre-established relationship with respect to a scanned surface of the sample in the Z direction to obtain height information about the scanned surface of the sample, the improvement to also allow material characteristics information about the scanned surface of the sample to be obtained simultaneously wherein the feed-back means comprises:

• a) analog-to-digital conversion means for obtaining an analog signal from the sensing means and for converting said analog signal into a digital signal;
• b) digital computation means including program means for receiving said digital signal from said analog-to-digital conversion means and for calculating and outputting first digital vertical control signals which create desired relative movement between a sample in the holder and the sensing tip in the Z coordinate direction which maintain the tip in the pre-established relationship with respect to the scanned surface of the sample in the Z direction;
• and,
• c) first digital-to-analog conversion means for receiving said first digital vertical control signals from said digital computation means and for outputting analog control signals to the scanning means to affect movement thereof in the Z direction;
• wherein said digital computation means includes means for first determining a force-position curve and for then setting the force of the tip on the sample near a minimum value as determined from said curve.

In a method of operating an atomic force microscope having a sample holder and a probe with a sensing tip, wherein a relative scanning movement is created between a sample in the holder and the sensing tip in X, Y and Z coordinate directions and the vertical position of the sensing tip is sensed, and a feedback operation is performed to maintain the tip in a pre-established relationship with respect to a scanned surface of the sample in the Z direction based on the sensed position of the tip, thereby to obtain height information about the scanned surface of the sample, the improvement comprising the steps of:

• obtaining an analog signal indicative of the sensed tip position;
• converting the analog signal with analog-to-digital conversion means into a digital signal;
• receiving the digital signal from the analog-to-digital conversion means;
• calculating and outputting, with digital computation means, first digital vertical control signals which create desired relative movement between the sample in the holder and the sensing tip in the Z coordinate direction which maintain the tip in the pre-established relationship with respect to the scanned surface of the sample in the Z direction;
• receiving the first digital vertical control signals from the digital computation means;
• outputting analog control signals to the scanning means to affect movement thereof in the Z direction;
• modulating with the digital computation means the relative movement between the tip and scanned surface of the sample in the Z direction so as to vary the force on the sample;
• and
• calculating a measure of stiffness of the sample from the changes in the force and changes in the position of the tip;
• wherein said step of modulating the relative movement between the tip and scanned surface of the sample in the Z direction is done during scanning in the X and Y directions to obtain a measure of stiffness of the sample as a function of X and Y.

In a method of operating an atomic force microscope having a sample holder and a probe with a sensing tip, wherein a relative movement is created between a sample in the holder and the sensing tip in X, Y, and Z coordinate directions and the vertical position of the sensing tip is sensed, and a feedback operation is performed to maintain the tip in a pre-established relationship with respect to a scanned surface of the sample in the Z direction based on the sensed tip position, thereby to obtain height information about the scanned surface of the sample, the improvement comprising the steps of:

• performing digital feedback comprising,
• obtaining an analog signal from the sensing means,
• converting the analog signal with analog-to-digital conversion means into a digital signal,
• receiving the digital signal from the analog-to-digital conversion means,
• calculating and outputting with digital computation means first digital vertical control signals which create desired relative movement between the sample in the holder and the sensing tip in the Z coordinate direction which maintain the tip in the pre-established relationship with respect to the scanned surface of the sample in the Z direction,
• receiving the first digital vertical control signals from the digital computation means, and
• outputting analog control signals to the scanning means to affect movement thereof in the Z direction;
• and
• further comprising,
• discontinuing outputting of said first digital vertical control signals which maintain the tip in said pre-established relationship with respect to the scanned surface of the sample in the Z direction and utilizing said digital computation means to produce second digital control signals which create the following movements,
• bringing the tip and the sample into contact,
• varying the vertical position of one of the sample and the tip until the attractive force of the tip to the sample is overcome by an external force applied to the tip and the tip therefore breaks free of the sample, and
• determining a tip null point as the vertical position of the one of the tip and the sample whose position was varied at the time the tip breaks free of the sample.