Methods and devices configured to operated scanning tunneling microscopes using out-of-bandwidth frequency components added to bias voltage and related software
View Patent ↗In the system and method disclosed, an ultrahigh vacuum (UHV) scanning tunneling microscope (STM) tip is used to selectively desorb hydrogen atoms from the Si(100)-2X1:H surface by injecting electrons at a negative sample bias voltage. A new lithography method is disclosed that allows the STM to operate under imaging conditions and simultaneously desorb H atoms as required. A high frequency signal is added to the negative sample bias voltage to deliver the required energy for hydrogen removal. The resulted current at this frequency and its harmonics are filtered to minimize their effect on the operation of the STM's feedback loop. This approach offers a significant potential for controlled and precise removal of hydrogen atoms from a hydrogen-terminated silicon surface and thus may be used for the fabrication of practical silicon-based atomic-scale devices.
1. A scanning tunneling device comprising:
a scanning tunneling tip configured to receive a bias voltage with respect to a sample to be processed by the scanning tunneling device, the bias voltage modulated with a dither voltage at a dither frequency and a dither amplitude; and
a control system, connected to a z-actuator and an x-y scanner, the control system configured to:
receive a time varying tip-sample current signal, including components at the dither frequency, tunneling between the tip and the sample; and
provide the time varying tip-sample current signal as a feedback signal in the control system.
2. The scanning tunneling device of claim 1 wherein the control system is further configured to filter the time varying tip-sample current signal to attenuate the components at the dither frequency to provide a filtered tip-sample current signal as the feedback signal to a control feedback loop of the control system.
3. The scanning tunneling device of claim 2 wherein the control feedback loop has an associated operating bandwidth including an upper frequency limit that is less than the dither frequency.
4. The scanning tunneling device of claim 1 wherein the time varying tip-sample current signal comprises a tunneling current signal that is in-phase with the bias voltage modulated with the dither voltage and a capacitive current signal that is 90 degrees out of phase with the bias voltage modulated with the dither voltage.
5. The scanning tunneling device of claim 4 wherein the scanning tunneling device is configured as a scanning tunneling lithography instrument wherein the control system is configured to:
operate the x-y scanner to position the tip opposite a location on a surface of the sample;
monitor for an indication of a change in height of the tip above the surface at the location; and.
(a) increment a present value of the dither amplitude to ramp up the bias voltage modulated with a dither voltage at the location while monitoring for the indication of the change in height.
6. The scanning tunneling device of claim 5 wherein the control system is configured to:
(b) indicate desorption of an atom terminating the surface at the location responsive to detecting the indication of the change in the height being greater than a threshold value.
7. The scanning tunneling device of claim 6 wherein the control system is configured to:
return the dither amplitude to a value of about zero volts; and
actuate the x-y scanner to position the tip opposite a next location on the surface of the sample responsive to determining that additional positions remain to be processed.
8. The scanning tunneling device of claim 6 wherein the control system is configured to:
(c) indicate lack of desorption of an atom terminating the surface at the location responsive to detecting the indication of the change in the height being less than the threshold value; and
perform operations (a)-(c) until the dither amplitude reaches a final value.
9. The scanning tunneling device of claim 4 wherein the scanning tunneling device is configured as a scanning tunneling lithography instrument wherein the control system is configured to:
operate the x-y scanner to position the tip opposite a location on a surface of the sample;
monitor for an indication of a change in the time varying tip-sample current signal; and.
(a) increment a present value of the dither amplitude to ramp up the bias voltage modulated with a dither voltage at the location while monitoring for the indication of the change in the time varying tip-sample current signal.
10. The scanning tunneling device of claim 9 wherein the control system is configured to:
(b) indicate desorption of an atom terminating the surface at the location responsive to detecting the indication of the change in the time varying tip-sample current signal being greater than a threshold value.
11. The scanning tunneling device of claim 10 wherein the control system is configured to:
return the dither amplitude to a value of about zero volts; and
operate the x-y scanner to position the tip opposite a next location on the surface of the sample responsive to determining that additional positions remain to be processed.
12. The scanning tunneling device of claim 10 wherein the control system is configured to:
(c) indicate lack of desorption of an atom terminating the surface at the location responsive to detecting the indication of the change in the time varying tip-sample current signal being less than the threshold value; and
perform operations (a)-(c) until the dither amplitude reaches a final value.
13. The scanning tunneling device of claim 4 wherein the scanning tunneling device is configured as a scanning tunneling microscope instrument wherein the control system is configured to:
(a) operate the x-y scanner to position the tip opposite a location on a surface of the sample;
(b) measure time varying tip-sample current signal;
(c) determine a measured current from the time varying tip-sample current signal;
(d) adjust a position of the tip above the surface by a controller, which sends a command sinal to a z-actuator to maintain the measured current signal at a predefined value within a control frequency bandwidth of a control feedback loop of the control system;
(e) determine a relative height of the tip based on the command signal of the controller.
14. The scanning tunneling device of claim 13 wherein the measured current comprises a measured amplitude of capacitive current, a measured amplitude of time varying AC tunneling current at a fundamental frequency or higher harmonics, or a measured DC tunneling current.
15. The scanning tunneling device of claim 14 wherein the control system is configured to:
operate the x-y scanner to move the tip approximately parallel to the surface at a set of x-y positions on the surface to provide while carrying out operations (a)-(e).
16. The scanning tunneling device of claim 15 wherein the control system is configured to:
measure the time-varying AC tunneling current as a function of voltage to provide I-V curve data for each position included in the set of x-y positions, corresponding to an image pixel in a scanned image.