Hybrid control system for scanning probe microscopes

• US 5,805,448 A
• Filed: 02/21/1997
• Issued: 09/08/1998
• Est. Priority Date: 03/10/1995
• Status: Expired due to Term

• Current Assignee: Keysight Technologies, Inc.
• Original Assignee: Molecular Imaging Corp (Agilent Technologies, Inc.)
• Inventors: Jing, Tianwei, Lindsay, Stuart M.
• Primary Examiner(s): Ruggiero, Joseph
• Application Number:
  US08/805,030
• Time in Patent Office: 564 Days
• Field of Search: 364/148, 364/161, 364/180, 364/481, 364/483, 364/487, 364/550, 364/551.01, 364/571.01, 364/571.02, 364/176, 250/306, 250/307, 250/234, 310/311-317
• US Class Current: 700/71
• CPC Class Codes: B82Y 35/00 Methods or apparatus for measurement or analysis of nanostructures
• CPC Class Codes: G01Q 10/065 Feedback mechanisms, i.e. wherein the signal for driving the probe is modified by a signal coming from the probe itself
• CPC Class Codes: Y10S 977/851 Particular movement or positioning of scanning tip

Abstract

A scanning probe microscope controller includes a digital signal processor (DSP) and an analog feedback control loop. The DSP serves to process the output of the scanning probe in the digital realm after conversion of the signal to digital form. After processing, the signal is restored to analog form. The height correction signal to be applied to a transducer controlling the distance between the scanning probe and a sample surface is then generated by an analog feedback control circuit, at least one parameter of which is under computer control. At the end of each scan-line, a variance may be calculated for the data and the inverse of this quantity is used to adjust the gain with which digitization of the data is carried out. Linearization of the data to correct for non-linearities in the scanning transducers may be carried out by the DSP after the data is acquired. This permits the scanning ramps applied to the transducers to be linear but the final displayed data do not show the effects of non-linearities. Adjustment of the feedback control loop gain may be carried out by monitoring the signal level from the scanning probe as each line is scanned. A computer determines if slow variations in this signal level have occurred across a line-scan. If so, the gain is increased. This process is continued until signals characteristic of excess gain (oscillation) are detected. At that point, the gain is reduced to a level slightly below the onset of the oscillation.

Claims

1. A controller for a scanning probe microscope having:

(1) a sensor probe tip which is used to generate an analog height signal indicative of the distance between said sensor probe tip and a sample surface under investigation, and (2) a height transducer coupled to said sensor probe for controlling said distance between said sensor probe tip and a sample surface under investigation, said controller comprising;

a computer for controlling programmable elements of the controller;

an analog to digital converter having as its input the analog height signal and as its output a digital height signal;

a programmable digital signal processor under control of said computer for carrying out selected signal processing operations on said digital height signal to form a processed digital height signal;

a digital to analog converter having as its input said processed digital height signal and as its output a processed analog height signal;

means for inputting an analog set point signal indicating a desired distance between the sensor probe tip and the sample surface under investigation;

means for generating an error signal proportional to a difference between said analog set point signal and said processed analog height signal;

means for integrating said error signal to form an integrated error signal;

means for adding said integrated error signal to a signal proportional to said error signal to form a transducer control signal;

means for applying said transducer control signal to the height transducer to control the distance between said sensor probe tip and a sample surface under investigation;

means for applying a gain and an offset to said transducer signal to form an adjusted transducer signal, said means for applying operating under control of said programmable digital signal processor;

means for digitizing said adjusted transducer signal to form a digitized adjusted transducer signal; and

means for applying said digitized adjusted transducer signal to said programmable digital signal processor.

2. A controller for a scanning probe microscope having:

(1) a sensor probe tip which is used to generate an analog height signal indicative of the distance between said sensor probe tip and a sample surface under investigation, and (2) a height transducer coupled to said sensor probe for controlling said distance between said sensor probe tip and a sample surface under investigation, said controller comprising;

a computer for controlling programmable elements of the controller;

an analog to digital converter having as its input the analog height signal and as its output a digital height signal;

a programmable digital signal processor under control of said computer for carrying out selected signal processing operations on said digital height signal to form a processed digital height signal;

a digital to analog converter having as its input said processed digital height signal and as its output a processed analog height signal;

means for inputting an analog set point signal indicating a desired distance between the sensor probe tip and the sample surface under investigation;

means for generating an error signal proportional to a difference between said analog set point signal and said processed analog height signal;

means for integrating said error signal to form an integrated error signal;

means for adding said integrated error signal to a signal proportional to said error signal to form a transducer control signal;

means for applying said transducer control signal to the height transducer to control the distance between said sensor probe tip and a sample surface under investigation,

means for applying a gain and an offset to said transducer signal to form an adjusted transducer signal, said means for applying operating under control of said programmable digital signal processor;

means for digitizing said adjusted transducer signal to form a digitized adjusted transducer signal;

means for applying said digitized adjusted transducer signal to said programmable digital signal processor,

a mean line value determined by averaging a plurality of values for said digitized adjusted transducer signal taken at a plurality of positions along a line under scan;

a variance line value determined by comparing a plurality of values for said digitized adjusted transducer signal taken at a plurality of positions along a line under scan;

an inverse of said variance line value is determined;

said gain is set so as to be proportional to said inverse of said variance; and

said offset is set so that the mean line value is digitized at the zero of said digitizing means.

3. In a method of operating a scanning probe microscope having a scanning probe, a sample stage for supporting a sample surface to be scanned and a nonlinear piezoelectric scanner having an attached end and a free end to produce a relative cyclic lateral scanning motion between the scanning probe and the sample stage in response to the application of a cyclically changing scan voltage to electrodes carried by the piezoelectric scanner, the improvement by which data recovered by the scanning probe in scanning the sample surface is made linear with space, said method comprising the steps of:

causing the cyclically changing scan voltage applied to the piezoelectric scanner electrodes to change in voltage linearly with time over a substantial portion of each voltage cycle;

scanning a test object having markings of known spacings located thereon and detectable by the scanning probe microscope;

obtaining information which relates the operation of the nonlinear piezoelectric scanner to a linear space; and

converting spatially non-linear data obtained by scanning the sample surface to spatially linear data by converting said spatially non-linear data as a function of the distance from the center of the scan.

4. A method according to claim 3 further comprising:

displaying said spatially linear data.

5. A method according to claim 3 further comprising:

displaying said spatially linear data in line-by line fashion as it is converted.

6. A method for correcting non-linearities in a scanning transducer of a scanning probe microscope driven by application of a cyclically changing scan voltage applied to electrodes of said scanning transducer wherein raw data, obtained at points that are not equally separated in space, are converted into a data set in which elements of said data set correspond to points equally separated in space, said method comprising the steps of:

causing the cyclically changing scan voltage applied to the electrodes of the scanning transducer to change in voltage linearly with time over a substantial portion of each voltage cycle;

scanning a test object having markings of known spacings located thereon and detectable by the scanning probe microscope;

obtaining polynomial coefficients for a set of equations which translate points in the raw data of the scan of the test object to equally separated points in a linear space;

based upon said polynomial coefficients, determining, for each direction of the scan to be mapped into the data set, an integer factor by which each point in the raw data is to be mapped into the data set; and

for each point in the data set, calculating the value of the point in the data set based upon inputs including a plurality of elements of the raw data and a plurality of said integer factors.

7. A method according to claim 6 further comprising:

displaying said spatially linear data.

8. A method according to claim 6 further comprising:

displaying said spatially linear data in line-by line fashion as it is converted.

10. A method of operating a scanning probe microscope as claimed in claim 7 further including as a first step calculating the parameters required to transform the points in said spatially non-linear data to spatially linear form prior to the start of a scan over a chosen area.

9. A method of automatic adjustment of the overall servo gain of a scanning probe microscope having:

(1) a sensor probe tip which is used to generate an analog height signal indicative of the distance between said sensor probe tip and a sample surface under investigation, and (2) a height transducer coupled to said sensor probe tip for controlling said distance between said sensor probe tip and a sample surface under investigation, said method comprising;

operating the microscope to obtain a line of data corresponding to a path scanned across the sample surface by the sensor probe tip;

analyzing the line of data to determine the existence of one of at least three conditions in said line of data, said three conditions including;

Condition 1;

relatively constant signal across the line scan;

Condition 2;

oscillations present in signal across the line scan; and

Condition 3;

signal varies with topography across the line scan; and

employing a digital signal processor to automatically alter the servo gain of said scanning probe microscope such that on Condition 1, the gain is kept constant, on Condition 2, the overall servo gain is reduced, and on Condition 3, the overall servo gain is increased.

11. In a method of operating a scanning probe microscope having a scanning probe, a sample stage for supporting a sample surface to be scanned and a nonlinear piezoelectric scanner having an attached end and a free end to produce a relative cyclic lateral scanning motion between the scanning probe and the sample stage in response to the application of a cyclically changing scan voltage to electrodes carried by the piezoelectric scanner, the improvement by which data recovered by the scanning probe in scanning the sample surface is made linear with space, said method comprising the steps of:

calculating the parameters required to transform the points in said spatially non-linear data to spatially linear form prior to the start of a scan over a chosen area;

causing the cyclically changing scan voltage applied to the piezoelectric scanner electrodes to change in voltage linearly with time over a substantial portion of each voltage cycle;

scanning a test object having markings of known spacings located thereon and detectable by the scanning probe microscope;

obtaining information which relates the operation of the nonlinear piezoelectric scanner to a linear space; and

converting spatially non-linear data obtained by scanning the sample surface to spatially linear data.