JPH0436602A - Scanning probe controller - Google Patents

Abstract

PURPOSE: To control relative positioning between a probe and the surface by comparing a parameter with a reference threshold and generating an error signal indicative of one of at least two possible discrete positional conditions. CONSTITUTION: In operation, the sensor electronic unit (sensor circuit) 18 in a controller is connected between a probe and an adjacent plane in order to measure parameters variable depending on the relative position between the probe and the adjacent plane. The sensor circuit 18 compares the magnitude of a tunneling current traversing a gap 12 with a reference value and generates a positional error signal indicative of one of at least two positional conditions. An up/down counter 52 integrates 22 the error signal and sustains the up/down error count during execution and a probe position control servo 26 regulates the gap 12 in response to the up/down error count. According to the arrangement, relative positioning between the probe and the surface can be controlled in response to an error count signal.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates generally to controllers for position control servo systems, and more particularly to adaptive feedback control systems for position control of scanning micromechanical instruments.

[Conventional technology]

Traditionally, it has been used industrially for machining equipment and motor drive equipment.
Various position control servo systems have been developed. Systems are known for adaptively adjusting for changes in the volume or cross-sectional area of a workpiece by changing the feed rate of a tool.

That is, a control system is known that positions an electrical discharge machining control device using an up/down digital computer that integrates a servo signal to calculate a large position error value. Digital speed control systems for sewing machines are also known that use digital speed counters to determine whether to accelerate or decelerate the machine. Closed loop control systems for measuring and keeping constant the tunneling power in tunnel scanning microscopes are also known. Conventional instruments that integrate or 71 position signals usually include an analog integration circuit to determine the actual position, an analog-to-digital converter to determine the digital value corresponding to the same position, and an analog-to-digital converter to determine the digital value corresponding to the same position, and convert this position to the command position. and a digital counter that determines the position error value and then converts it to an analog converter with a digital-to-analog converter to correct the machine position.

For position control systems that require extremely fast and precise position corrections, such as the positioning of tunnel scanning microscope probes, such high precision can be achieved with an adaptive feedback circuit to adjust the relative speed of movement along the axis of movement of the position controller. It would be desirable to provide a highly responsive position control system. It would also be desirable to use a circuit design that eliminates analog integrated circuits that require time-consuming intermediate analog-to-digital converters.

[Problem to be solved by the invention]

The present invention provides an improved scanning method for controlling scanning motion between a sensor probe and an adjacent target surface in an instrument such as a scanning tunneling microscope, where the tunneling current flows across the gap between the sensor probe and the adjacent target surface. A micromechanical probe for use in the present invention is significantly more effective than conventional ones in maintaining a desired gear knob size and provides more consistent high resolution scanning than is available in the prior art.

A feedback control mechanism provides efficient up/down of the high speed force used to adjust the size of the probe gear.
Together with the down counter mechanism, high precision and speed of the sensitive instrument can be ensured. A sensor circuit determines the magnitude of the tunneling current across the gap, compares the tunneling current to a reference value, and generates a position error signal indicative of one of at least two position conditions. The up/down counter integrates the error signal to maintain a running up/down error count, and the position control mechanism adjusts its gap in response to the up/down error count.

Briefly and - expressed in casual terms - a scanning device according to the invention is a sensor that measures a parameter that varies as a function of the relative positioning of a probe and an adjacent surface, in at least two discrete positional conditions. means for integrating the error signal and generating an error count signal; and a position control for controlling relative positioning of the probe and the surface in response to the error count signal. Equipped with a servo.

The probe control system comprises at least one adjustable clock and an adaptive feedback control mechanism for controlling the raster scan rate in a plane perpendicular to the axis between the probe and the target surface, the clock being operationally connected to the sensor. And black.

This includes one in which the clock speed of the clock is adjusted in response to an error signal. Most preferably, the adaptive feedback control mechanism controls the vertical positioning rate of the probe along the vertical axis in the line between the probe and the target surface, and the raster scanning rate in the horizontal plane.

Other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate features of the invention.

〔Example〕

As shown in the drawings, the present invention is a scanning micromechanical probe control system for use in conjunction with a sensor probe and an adjacent target surface, such as in a scanning tunneling microscope, in which a tunneling current flows across the gap between the sensor probe and the target surface. It is embodied in what has become. A gap detection circuit connected between the sensor probe and the target surface measures the tunneling current across the gap and compares the measured current to a preset or dynamically determinable reference current value, at least 2.
An error signal is generated indicating one of two possible position conditions. The error signal is generally a binary signal indicating whether the discharge current is less than, greater than, or equal to a threshold value. The error signal is then received by an up/down counter which digitally integrates the error signal. The digital count is then converted to an analog signal, which is then received by a position control servomechanism that controls the distance between the sensor probe and the target surface.

Scanning tunnel microscope: The sensor probe as a whole is moved perpendicular to the Z axis between the probe and the target surface in the
To scan within the plane, the clock sampling speed of the Z-axis up/down counter is accelerated or decelerated, and the X and Y
It is also desirable to include an adaptive feedback control system for accelerating and decelerating the X, Y axis locks to control the scan rate along the axes. This adaptive feedbank control optimizes the gap for consistently high resolution by allowing the scanning tunneling microscope probe to react quickly and accurately to elevation changes in the target plane as the raster scan progresses. Able to maintain distance.

Although the present invention will be described with respect to a typical scanning tunneling microscope, the positioning control and adaptive feedback control of the present invention may be applied to a scanning capacitance microscope, an atomic force microscope, a scanning magnetic microscope, a scanning thermal microscope, or a scanning tunneling microscope. It would be equally suitable for use with scanning micromechanical instruments that serve to maintain the desired force of the probe on the target surface, as in the case of atomic force microscopy, to achieve consistently high resolution of the instrument. Thus, once a reference value for optimal sensor resolution is dynamically set by the operator or the system itself, that resolution can be maintained during scanning of the entire sample.

In accordance with the present invention, there is therefore provided a scanning micromechanical probe control system for controlling the relative movement between a sensor probe and an adjacent target surface, which is operationally connected between the sensor probe and the target surface, and which controls the relative positioning of the probe and the surface. sensor means adapted to measure parameters that vary accordingly and generate an error signal indicative of one of two possible discrete position conditions; /down counter and a position control servo for controlling relative positioning of the probe and surface in response to up and down count signals.

The present invention also provides a scanning micromechanical probe control system for relative motion control between a sensor probe and an adjacent target surface, the probe control system being connected between the sensor probe and the target surface and changing in response to the relative positioning of the probe and the surface. sensor means for measuring para, l - height, adapted to generate an error signal indicative of one of at least two possible discrete position conditions, and integrating the error signal to produce an upper and lower count signal; and at least one raster clock means, the raster clock being operationally controlled by an adaptive feedback control means for controlling the rate of the raster scanning in a plane perpendicular to the axis between the sensor probe and the target surface. connected such that the clock speed of the raster clock means is adjusted in response to the error signal; and a position control servo for controlling the relative position of the probe and the surface in response to the up and down count signals. provide.

As shown in the drawings, tunneling scanning microscope 10 detects the passage of tunneling current across a gap 12 between a sensor probe and an adjacent target surface 16.

Gap voltage source 15 provides a constant voltage between the sensor probe and the target. At that time, the value of the current flowing is the gear 1. between the probe and the target. changes as the size of the drop changes. The current discharged across the gear is carried along line 17 to a gap detection circuit 18 where the measured current is compared with a reference current value and a digital signal is generated representing the result of the comparison. The detection electronics detect either a condition in which the measured current is less than the reference current, indicating that the gap is too large, or a condition in which the measured current is greater than or equal to the reference current, indicating that the gear size is too small. It is desirable to generate a binary digital signal that indicates. It goes without saying that other digital codes can easily be generated by the sensor device. For example, a two-bit code could be used to generate a signal that selectively indicates gap size conditions that are too small or too large, as described above. The digital gap detection error signal generated by the gap detection circuit is received by an integrator circuit 22 that generates an analog signal on line 24 to display and transmit the accumulated error signal to the probe position control servo 26. It is ordered to make a correction adjustment of the gear knob size.

The microscope detection probe is preferably mounted for linear up-and-down translation along the Z-axis between the plane of the probe and the target in response to analog counting command signals from the integrated circuit. For extremely precise adjustment of the gap size, it is desirable to mount the sensor probe with a piezoelectric transducer, typically a piezoceramic stack that extends along the Z-axis when a voltage is applied. Thus, if the gear is relatively large, the current across the gear will be relatively low and the count in the integrator circuit will drop. A corresponding voltage increase is directed against the piezoelectric transducer to suitably adjust the gap size to the ideal value. Instead, the sensor probe is mounted with a piezoelectric transducer in reverse so that as the gear increases, the count of the integrator circuit decreases, and the voltage level of the piezoelectric transducer decreases with increasing gear, reducing the size of the gap. You could do that too. Like a servo motor, Z
Less precise devices for moving the sensor along the axis could also be used.

It is desirable to perform parallel movement in the XY plane perpendicular to Z@22 in the same manner. Thus, a digital waveform for controlling the voltage to the X piezoelectric transducer is created by raster issued. Similarly, raster Y 34 controls the voltage to the Y piezoelectric transducer in synchronization with the clock pulses of clock 36, creating a scanning digital waveform that creates the expansion and contraction of piezoelectric control servo 38 along the Y axis.

Referring to FIG. 2, the gap detection circuit is connected by line 17 to the scanning tunneling microscope probe tip 40 or directly to the target substrate being scanned. The gap sensor circuit includes a pickup amplifier 42 and a differential amplifier 44 that compares the measured current amplified by the pickup amplifier with a base current dynamically determined by an operator or a current threshold constant circuit 46. In the presently preferred gap detection circuit, the logic encoder 48 has a one state indicating that the current level is below a threshold reference value and another binary state indicating that the measured value is greater than or equal to the reference current value. Create a binary error signal. This binary signal is then used as an up/down signal for the integrator circuit.
It is output to the down counter 52. Instead, two or more bit codes are used to signal whether the gap size is too small, too large, or as specified, or where multiple thresholds are used. could be used to generate a digital code with more accurate information regarding probe position.

The up/down counter 52 outputs two signals from a binary encoder at a rate determined by the clock rate of an adjustable clock 54.
The value signal is sampled and the two in the Lujistor are set between the upper and lower count limits determined by the count limit setting circuit 50.
A running count of the value error signal is accumulated and a register (usually a 16-bit register) prevents the integer value from changing dramatically as the count passes from its minimum value to its maximum value. Digital to analog converter 56 converts the elapsed count from the up/down counter to an analog command signal that is amplified by amplifier 58 to command the up/down Z-axis servo to make parallel adjustments to the sensor probe gap.

Referring to Figure 4.5.6, the present invention increases the gap size by adjusting the integrator clock rate and including a feedback control circuit that adjusts the clock rate to control the generation of the X, Y raster waveform. If adjustment is needed, increase the sampling rate of the Z-axis up/down counter and decrease the clock rate of the X and Y raster scan patterns to ensure high resolution during gap size adjustment. Conversely, when there is little or no need to adjust the gear size, the clock rate for the X and Y (75-7 star scan pattern can be increased back to the normal ratio. Optimum gap size can be efficiently maintained throughout the process.

As shown in Figure 4.5.6, the gap detection error signal is output from the gap detection circuit 18' to the up/down counter 5.
2 clock 54 to adjust the sampling rate of the up/down counter. Similarly, feedback line 62 is connected to the Y-axis star clock and line 6
4 is connected to the X-axis star clock, and the parallel scanning pattern is adjusted along the XY plane in accordance with the binary gap error signal. By continuously adjusting the clock rates in this manner, the clocks 54, 36, 30 themselves act as integrator circuits, adjusting their clock rates proportional to the counts maintained in the up/down counters. In most presently preferred embodiments, a separate differential amplifier 66 with a separate current setting circuit 68 providing independent current level comparisons provides separate logic for the Z-axis movement that is independently output to an integration clock. Signals are generated for the encoder 70 and the logic encoder 72 for both the X and Y axes.

A method of synchronizing the scanning digital waveforms of an X;Y raster scanner is illustrated in the timing diagram shown in FIG. When clock pulse 74 is generated by the X and Y raster crosses, relatively slow movement of the X-axis force position control servo is commanded by X waveform 76, while
A relatively fast back and forth scan pattern is dictated by the Y-axis waveform 78. The X and Y raster scan waveforms are combined to generate a scan pattern that collectively covers a rectangular area several times, producing high resolution data representing the distance of the probe loss to the target plane, which data is It can be used to create a microscopic phase map of the target surface.

In view of the above, the scanning micromechanical probe control system of the present invention has proven useful in maintaining desirable parameters to maintain optimal resolution for instruments such as tunneling scanning microscopes. It is also important that the present invention provides adaptive feedback control of the sampling and scanning rates of the detection and positioning circuits to maintain high resolution during gap size adjustment. The feedback control mechanism, together with the fast and efficient up/down counter mechanism used to adjust the size of the probe gap, can ensure high accuracy and speed of the sensitive instrument.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of the feedback electronics of the position control circuit of a tunnel scanning microscope. Figure 2 is a schematic diagram of the gap detection circuit (Figure 1). Figure 3 is a schematic diagram of the integration circuit of Figure 1. , FIG. 4 is a schematic diagram of an alternative example including an adaptive feedback control circuit, FIG. 5 is a schematic diagram of the sensor circuit of FIG. 4, and FIG.
FIG. 7 is a schematic diagram of the integrating circuit, and FIG. 7 is a timing diagram of the raster scanning waveform of the position control device.・Tunnel scanning microscope, ・Adjacent target plane, ・Gap, ・Line 1, gap detection circuit, ・Integrator circuit, ・Probe position control servo, ・Clock, ・Raster X, ・Raster Y, ・Logic encoder, ・Adjustable clock , ・Amplifier/down counter. Procedure for amendment (method) 1. Indication of the case 1990 Permit No. N144260 2. Name of the invention Scanning probe control device 3. Person making the amendment Relationship with the case

Claims (16)

[Claims] (1) A scanning device for controlling relative movement between a probe and an adjacent surface, comprising: sensor means operatively connected between the probe and the adjacent surface for measuring a parameter that varies depending on the relative position of the probe and the adjacent surface; , means for comparing said parameter with a reference threshold and generating an error signal indicative of one of at least two possible discrete position conditions; and integrating said error signal;
Said apparatus comprising: means for generating an error count signal; and position control servo means for controlling the relative position of said probe and adjacent surface with respect to said error count signal. 2. The apparatus of claim 1, wherein said means for integrating said error signal comprises an analog integrator. 3. The apparatus of claim 1, wherein said means for integrating said error signal comprises a digital integrator. (4) the sensor means generates a two-state error signal, one state of the error signal in which the measured parameter is less than the reference threshold, and another state in which the measured parameter is greater than or equal to the reference threshold; 2. The apparatus of claim 1, wherein the apparatus is adapted to exhibit: (5) the parameter is a tunneling current, the sensor means includes means for comparing a reference tunneling current with a measured tunneling current, and generating a binary error signal based on the difference between the measured tunneling current and the reference tunneling current; 2. The apparatus of claim 1, comprising means for generating. 6. The apparatus of claim 5, wherein the means for comparing the reference tunneling current with the measured current comprises means for setting the reference tunneling current. 7. The apparatus of claim 1, wherein said means for integrating said error signal is a digital counter including a register for storing incremented and decremented counts, and further comprising means for setting upper and lower limits of said count. 8. The apparatus of claim 1, wherein said error signal integrating means includes an adjustable clock for controlling the sampling rate at which said error signal is integrated. (9) adaptive feedback control means comprising at least one adjustable raster clock means for controlling a raster scan rate of the probe relative to the adjacent surface in a plane perpendicular to the probe axis between the probe and the adjacent surface; 2. The apparatus of claim 1, wherein said adjustable clock means is operatively connected to sensor means, and wherein the clock speed of said adjustable clock means is adjustable in response to said error signal. (10) said means for integrating said error signal comprises an adjustable clock for controlling a sampling rate of said means for integrating said error signal in response to said error signal, said adaptive feedback control means being adapted to a probe axis of said probe; a first adjustable raster clock that controls a raster scan rate along a second axis perpendicular to the probe axis relative to the adjacent surface; and a first adjustable raster clock that controls a raster scan rate along a third axis relative to the probe axis and the second axis in response to an error signal. and controlling a second adjustable raster clock. (11) A scanning micromechanical probe controller for controlling relative motion between a probe and an adjacent surface aligned along a first axis, the scanning micromechanical probe controller being operatively connected between the probe and the adjacent surface; sensor means for measuring a parameter that varies with relative position, comparing said parameter to a reference threshold, and generating an error signal indicative of one of at least two possible discrete position conditions; means for integrating and generating an error count signal; and at least one raster clock means for controlling the speed of raster scanning the relative motion between the probe and the adjacent surface in a plane perpendicular to the first axis; raster clock means operatively connected to said sensor means, said adaptive feedback control means adapted to adjust the clock speed of said raster clock means in response to said error signal; and said adaptive feedback control means in response to said error count signal. Position control servo means for controlling the relative position of the probe and the adjacent surface along a first axis. (12) the sensor means generates a two-state error signal, one state of the binary error signal in which the measured parameter is less than the threshold reference value, and another state in which the measured parameter is greater than the reference threshold; 12. The control device of claim 11, wherein the control device is adapted to indicate that: or equal to the above. (13) The control device of claim 11, wherein the measured parameter is a tunnel current, and the error signal is based on a difference between the measured tunnel current and a reference tunnel current. (14) The control device according to claim 13, wherein said sensor means includes means for setting said reference current. (15) the means for integrating the error signal comprises a digital counter including a register for storing an increment/decrement count;
12. The control device according to claim 11, further comprising means for setting upper and lower limits of said count. 16. The apparatus of claim 11, wherein said means for integrating said error signal comprises an adjustable clock for controlling the rate at which said error signal is sampled.