CA2017624A1 - Scanning micromechanical probe control system - Google Patents

Abstract

ABSTRACT OF THE INVENTION

The scanning micromechanical probe control system for controlling relative movement between a sensor probe and an adjacent sample surface includes a sensor probe for measuring a parameter which varies relative to the relative positioning of the probe and the adjacent surface adapted to generate an error signal indicating one of at least two discrete position conditions; an up/down counter for integrating the error signal and for generating an up/down count signal; and a position control servo for controlling the relative positioning of the probe and the surface responsive to the up/down count signal. An adaptive feedback control most preferably controls the rate of up/down positioning of the sensor probe and the rate of raster scanning of the probe relative to the target surface.

Description

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SÇANNING MICROMECHANIC L PRC)BE CONTR

BACKGROUNDT OF THE_I_VENTION

Field o~ th~ Invention:
This invention relates generally to control devices for position control servosy~tem~, and more particularly relates to adaptive f~edback control sy~tems for position control of scann~ng micromechanical instrla~ents.
Descriptlon of_ Related Art: Va:rious position control servosystems for machining devices and motor drive system~ hav~ been developed in industry. Systems ~or altering f .ed rate o~ to :;1 speed to acc:omRIodate changes in volume or cross sectiollal area of a work piece adaptively are Xnown; and a f~ontrol system for po~i~iorl-ing an elec:trical discharge machining c:ontxol device util~ zirl~r an up/down digital csunt~r for integrating ser~rosignals for ::alculating a gross position error value ~s known. Diyi~al speed control systems for sewing ma ::hines using a digital rate counter to determine whether to acc~lerate or decelerate the machine are also known. A closed loop con~rol sys~em for measuring and maintain~ng a tunnel effect variable con~tant ~n a scanning tunnelliny microscop~ is known. Such conven-tional in~;truments integrating position signals typically utilize an analog ~ntegrat~ng circuit to determine an actual position; an anals~g to digital converter for deter-mining a digital value corresponding to the position, a logic circuit comparing pres~nt position wi~h a commanded position, and a digital counter ~or determining a po~ition error value, which can then be converted into an ' '''.. ' ., ' analog signal by a digital-l:o-analog converter for correc-tion of the m chine position.
In position control system~ requiring extr~mely fast and precise position corrections, such as for posi-tioning of scanning tunneling microscope probes, it would be desirable to provide suc:h a highly precise and quickly responsive position control s~stem with an adaptive :feed-back circuit for adjust~ng the rates of relative movement along the axis o~ movernent of the pos.ition con~rol devic~. It would also be desirable to utilize a circuit de~ign elim~nating an analog integrating circuit requir ing the time-consuming interm~diate analog to digital converter. The present invention meets these needs.

SUMMARY OF THE INVENTION
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The present invention provides an improved scanning micromechanical probe control system for c:ontrolling relative movement between a sensor probe and an adjac~nt surface, for an instrument such as a scanning tunnelling micrQscope in which a tunnelling current passes across a gap between the sen~or probe and the adjacent target surface, which is significantly more e~fective in ma~taining a desired gap size, for improved consistency of high resolution scanning ns:t availa~le in the prior art. The feedback control mechanism, along with the rapid and efficient upJdown sounter mechanism which is used to adjust the size of the probe gap, insure high accuracy and speed of the sensiti~re instrumentation. A sen~or circuit determines the amount of tunnelling current across the gap, to compare the tunnelling current with a re~er~nce value for generating a position e.rror signal indicating one of at least two position conditions. An up/down counter integrates the error signal to maintain a running up/down error count, : ' , :
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and a position control mechanism adju ts the gap ~n respon~e to the up/down error count.
Briefly, and in general terDls, a scanlling appaxatus according to the invention includes a sensor for m~a~uring a parameter which varies relative to th0 relative position~ng of the p~obe and an adjacent surface, adapted to generate an error signal indicating one of at least two discrete positiOn condikiorls: m~ans for integrating the error signal and for generating an error count signal: and a position control servo for controlling t.he relat~ve positionirlg O:e the probe and the sur~ace re~E~onsive ko the error count signal.
In a preferred embodiment, the probe control ~yst2m ~ncludes an adaptive feedback control mechanism including at least one adjustable clock for contr~lling a rate of raster scanning i~ a plane perpendicular to the axis between the prGbe and the target surface, whe:rein the clock is operati~ely conn~cted to the sensor, and the clock rate of the clock i5 ad~usted in response to the error sig~al. The adapti~e f~edback control mechani~m most preferably controls the rate of vertical positioning of the probe along a vertical axis in line between the probe and the target sur~ace, and the rate of raster scanning in a horizontal plane.
Other aspects and advantages of the invention will beco~e apparent from the following deta~ 1 ed descrip-tion, and the accompanying drawings, illustrating by way o~ example the features of the invention.
BRIEF~ DESCRIPTION_OF THE DRAWINGS

FX&URE 1 is a schematic diagram of the feedb2ck elect:ronics :Eor the position control circuitry ~n a scannlng tunneling microscope;
FIG. 2 is a schematic diagram of the gap sensor ci~cui~ of ~ Fig. l;

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~r~,~ ~.r~ iq - 4 - 6294~-132 ~IG. 3 is a schematic diagram of ~he integrator circuit of Fig. 1:
FIG. 4 is a ~chematic diagram of an alternative embodi-ment includinq adaptive.feedback control circuitry;
FIG. S is a ~chematic diagram of the Yen~or circuitry of Fig. 4;
- FIG. 6 i~ a schematic diagram of the integrator circuit of ~i~. 4;
FIG. 7A i~ a timing diagram of the X ra~ter scanning waveform of the position control apparatus; and FIG. 7B is a timing diagram of the Y ra~ter ~canning waveform of the po~ition control apparatusO

As is shown in the drawing~, which are provided for the purpo~e o~ illustration, the invention lS embodied in a ~canning micromechanical probe control ~yqtem for use in combination with a sensor probe and an adjacent tar~et 3urface. ~uch as in a scan-ning tunneling microscope, in which tunnelling current pas~e3 across a gap between the sensor probe and target ~urface~ A gap sen30r circuit connected between the sensor probe and target sur-face mea~ure~ the tunnellin~ current acro~ the gap, and compares the mea~ured current with a reference current ~alue which can either be preqet or dynamically determined, to generate an error ~ignal indicating one of at lea~t two possible poRition condi-tions. The err~r ~ignal i9 generally a binary ~ignal indicating . . .

- 4a - 62948-132 that the discharqe current is less than a threshold amount, or greater than or equal to the threqhold amount. The error signal is then received by an up/down counter for digitally integrating the error signalO The~digital count i3 then converted to an analog qignal which iq then received by a po~ition control ~ervo mechani.sm controlling the distance between the sensor probe and the tarqet surface~

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Since a scanning tunneling microscope generally scans the sensor probe in an X-Y plane perpendicular to a Z axis between the probe and the target surface, an adap ti~re feedback control system i5 also preferably included for accelerat~ng or decelerating the clock sampling rates of the Z axis up/down counter, and ~or accelerating and decelerating the X axi~ and Y axis cloc:ks which control the rate of ss::anning along the X and Y axes. This adaptive feedback control allows a scanning tllnneling microscope probe to quickly and precisely react to elevational changes in a target surface as raster scannirl~ proceeds, to maintain an optimum gap distance ~`or consistently high resolution. Although the invention will be described in connec:tion with a typical scanning tunneling microscope, the positioning control and adap-tive feedback control of the invention would be equally well sui~ed for us with a scanning capacitanc:e micro-scope/ an atomic force microscope, a scanning magnetic ~ic:roscope, a scann~ng thermal microsc:ope, or other ~canning micromechanical instru:ments in which it i~
useful to maintain either a desired gap, or a desired force of the probe on the taryet: sur:~aoe, in the case o~
an atomic ~orce microsc:pe, to achieve constantly high resolution of the instrument~ Thus, onc:e the reference paramPter is set for opkimum sensor resolutiorl, either by an operator, or dynamically by the system it~elf, the resolution can be ma~ntained during scann~n~ of an entire sample.
In accordance with the inventionl there is therefiore provided a scanning micromechanic:al probe control system for controllillg relative movement between a sensor probe and an adjacerlt target surface, comprising sensor means operatively connected hetween the sensor probe and the target surfac~ for measuring a parameter ,1 35 which varies relative to the relative positioning of the probe and the surface, and adapted to gen~rate an error ;
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signal indicat~ng one of at least l:wo possible discrete position conditions; an up/down counter for integrating the exror signal, and for generating an up/down count signal; and a position control servo .~or controlling the relative positioning of the probe and the sur~ace responsive to the up/down count signal.
The invenl:ion also provides for a scanning micromechanical proh~ control system for controlling relative mo~ement between the sensor probe and an ad~acent target surface, comprising serlsor mean~
operatively connected between tha sensor probe and the tar~et surface for measuring a parameter which varie~
relat~ve to th2 relative positionlng of the probe and the surface and adapt~d to generate an error signal indicat-~ng one of at least two possible discrete position conditions; means for ~ntegrat~ng the error signal and for generatirlg an up/down count signal; adaptive feedback control means ~ncluding at lsast one raster clock means or controlling a rat~ of raster scannirlg in a plane p~rpendicular to an axi~ between the sensor probe and the target surface, wherein the raster cl~ck is operatively ::onnected to the means, and the clock rate o~ the raster clo¢k mean~ is adapted to be ad~usted in response to the .~ error signal: and a position control servo or controlling the relati~e posi~ioning of the probe and the surface responsive to the up/down count signal.
P~ is shown in the draw~ngs, a scann~ng tunnel-ing microscope lO sense~ the passage of a tunnellirly ;.~ current across a gap 12 between a sensor probe and an adjacent target sur~ace 160 A gap voltage source 15 provides a constant voltage between the sensor probe and the target sur~ace, with the amount of current flow varying as the size of th~ gap betwPen the probe and th~
target change~. The current discharged across t:he gap is 3~ carried along line 17 to the gap sensor ci:rcuit 18, which compares the measured current with a reference c:urrent ?

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- 7 - ~2948-132 value and g~nerate~ a digital signal r~presenting the result o~ the comparlson. The ~en~or slectronic:s prefer-ably generates a binary digltal slgnal indic:ating ~ither a ~tate ln which the m~3a~ured curr~nt is le~ than tha r~ferencQ current, indicating thQ gap is too large~ or a qtata irl which th~ a~;ured c:urr~nt 1~ gre~ter than or equal to thQ ref~rence current indicating that the gap ~lz~ is too small. It i~ of course readily apparent that other digltal code~ coul.d bQ ge2lerated by the sQnsor electronls~. Fcsr exampla, a two b.~ code could be u~ed to generatQ signals indicating alternative gap ~ize ~tates of being two small, exa~ctly a~ spe~ifiPd, or too large. The ditigal error signal generated by thl3 gap sen~or circui~ xec:eived by the integrator .5 circult 22, whlch genQrate~ an analog signal over line 24 repr~senting the accumulated error slgnal and com~anding tllQ probe posi~ion contrc~l sarv~ 26 to maks a corrective ad~u~tmerlt in the gap size.
ThQ tunneling microscope sensor probe i s prefer ably moul~tQd ~or linear tran~lational motlon up and down along a Z axis between tha prob~3 and the ~3urfacQ of the ; karget in re~pon3a to the analog count command signal from thQ integrator circ:uitry. For extremely ~ Q
ad~u6tment o~ th~ gap size, the sensor probQ is prefer~
ably m~unted w~h a piezoelectrls ltransduc~r, typlcally a piezoelectric cera~ic stac:k, wh~ch expand~ along the ~
axi~ whs3n sub~actad to voltage. Thus, when the gap is relativ01y large, the c:urrent acrosE~ the gap i~3 rela~-tlv~ly low, causlng th~ count ~ the intégratr clrcuit 3G t:o de~:rQase. A correspondlng vc~ltage increa ~
dlrected to the piezoelacrtric transducer to properly ad~ust thç~ gap c~lze to thQ id~al valu~0 Altçlrnat:iv01y, ~; the sen30r prob~ could be mounted inversely w~th a pie:~oelec:tric tr~nE:ducer so that a!3 tha gap increases the count of the integrator circuit decreases and the voltage ;' .

level of th~ piezoel~c:tric transducer would be correspond-~ngly reduced to reduce the size of the gap. L~ss precise devices for moving the sansor along the Z axis, such a~ a servo motor, could also be used.
Translational motion in an XY plane perpendicu-lax to the Z axis is preferably perform~d in a similar manner. Thus, a digi.tal waveform for controlling voltage to an X piezoele~ric transducer is produced by a raster X 28 in synchronization with the clock pulses provided by the clocX 30, to produce exparlsion and contraction along -~ an X axis of a pi.eæoelectric posil:ion control 5el:'V0 32.
Similarly, the raster Y 34 produces a scanning digital waveform controllirlg voltage for a Y piezoelectric transducer in synchronization with the clos:::k pulses of clock 3 6, for producing Pxpansion and contraction of a piezoelsctric control servo 3 8 along th~3 Y axis.
Referring to Fig. 2, the gap sensor circuitry is connected by l:ine 17 to the scanning tunneling microscope probe tip 40 or directly to the target substrate being .` 20 ~canned. The gap sensor c:ircuit includes a pic:kup amplifier 42, a di~erential amplifier 44 which compares the measured current as amplified by the pickup ampli:eier wi~h a re~erence current either set by an operator or dynamically determined by a current threshold set circuit 460 In the currently pre~`err~d embodi~eIlt of the gap sensor circuit, a logic encoder 48 produces a binary error signal, with one ~tate ~ndicating that the current level is below the threshold reference, and the other bin~ry state indicating that the measured current is greater than or equal to the r~ferenc:e c::urrent. Thi~
binary signal is then output to an up/down counter 52 of the integrator circuit. Alt:ernatively, a two bit or higher l:)it code could be us~d to generate signals indicat~g that gap sizes are too small, too large or exactly as specified, or where multiple thresholds are :

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utilized, a digital code w~th more exact information as to probe po ition could be generated.
The up/down counter 52 operates to sample the b~rlary signal from the hinary encoder at a rate deter~

5 mined by the clock rate of the adjustable clock 54, and accumulates a runn~ng count of the binary error signals ~n a register, between upper and lower counting limits which are determined hy the counting limit set circuit 50, so as to prevent the register, which is typically a 16 bit register, from drastically changing the integer count a~ the count passes through its minimum or maximum value. A digital to analog onverter 56 conve~s the running count from the up/down counter to an analog command signal which may be amplified by the amplifier 5~, for instructing the up/down Z axis servo to make a translational adjustment of the sensor probe gap.
Referring to Figs. 4, 5 and 6, the inventiorl pref~rably includes feedhack control circuitry for adjusting the clock rates of the integrator, and for adjust~ng the clocl~ rate~ controll~ng the X and Y rasker waveform production, so that in the event that a large adjustmerlt in the gap ~ize needs to be made, the sampling rate of the of Z axis up/down counter can be increased, and the clock rates for the X and Y raskar scanning patterns can be reduced to insure high resolution during the ad~ustmsnt o~ the gap size. Conversely, when little or no adjustment of the gap si2e is required, the clock rate~ of the X and Y raster scanning patterns can be increased back to normal rates. In this manner, an optimum gap size can be ef~iciently maintained throughout the entire scanning process.
P.~ is shown in Figs. 4, 5 and 6/ the gap sensor error signal is output ~rom the gap sensor circult 1~' to the clock 54 of the up/down counter 52 to adjust the rate of sampling of the up/down counter. Feedback line 62 similarly is connec:ted to the Y axis raster clock and :
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- 10 - 629~8-132 line 64 i~ connected to X axis ra~ter clock for adjustment of the tran~lational ~canning pattern along the XY plane in re~ponse to the binary gap error ~ignal. By continuous adju~tmen~ of the clock rates in this fashion, the clocks 54, 36 and 30 operate a~
integrating circuitq in themselve~, and adjust their clock rates proportional to the count kept in the up/down counter. In the moct currently preferred embodiment, a separate differential amplifier 66 having a separate current ~et circuit 68 for provi-: ding an independent current level comparison generates a signal for a separate logical encoder 70 for the Z axi~ up/down motion which i~ output independently to the integrator clock, and for a ; logic encoder 72 for both the X and Y raster clocks.
Illu trative of the manner of synchronization of the canning digital waveforms of the X and Y raster scanners i~ ~he timing diagram shown in Fign 7A and Fiq. 7B. As the clock pulses 74 are generated by the X and Y ra~ter clock~, a relatively 810w movement of the X axis piezoelectric position control servo i~
commanded by the X waveform 76, while a relatively ~ore rapid back and forth scanning pattern i~ directed by the Y axis wave-form 78. In co~bination, the X and Y raster scanning waveforms generate a scanning pattern which can cover a generally rectangu-lar area 3everal times to produce high resolution data repre~ent-ing the distance of the probe tip fro~ the target ~urface, which can be u~ed to develop a ~icroscopic topographical ~ap of the target surface.

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In view of the foregoing, it ha~ been dem~nstrated that the ~canning micromecharlical probe control ~ystem oE the inven-tion is advantageous in maintaining a desired parameter for main-ta.ining optimum resolution for an in~trument 3uch as a ~canning tunneling micro~cope. It is al~o significant that the present invention provldes adaptive feedback control of the sampling and "' - ~

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scanning rates of the sensing circuitry and the position-ing circuitry to maintain high resolution during adjust-men$ of the gap size. The feedback control mechanism along with the rapid and efficient up/down ccsunter mechanism which is u~;ed to adjust the size of the probe ;. gap, insure high accuracy and speed o* the sensitive ' instrumentation.
~` Although specific embodiments of the invention have been dascribed and illustrated, it is clear that it 0 i9 suscepti~le to numeroug modifications and adaptakions wi~hin the ability of those skilled in the art and without the exercise of the inventive faculty. Thus, i~
should be understood that various changes in form, detail and use of the present invention may be made without departing from the spirit and scope of this invention.
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Claims (11)

1. A scanning apparatus for controlling relative movement between a probe and an adjacent surface, comprising;
sensor means operatively connected between said probe and said surface for measuring a parameter which varies relative to the relative positioning of said probe and said surface;
means for comparing said parameter with a reference threshold, and generating a two-state error signal in which one state of the error signal indicates that the measured parameter is less than said reference threshold, and the other state indi-cates that the measured parameter is greater than or equal to the reference threshold;
means for integrating said error signal, and for generating an error count signal; and position control servo means for controlling the rela-tive positioning of said probe and said surface responsive to said error count signal.

2. The apparatus of Claim 1, wherein said means for inte-grating said error signal includes an adjustable clock for con-trolling a sampling rate of the error signal for integrating said error signal.

3. A scanning apparatus for controlling relative movement between a probe and an adjacent surface, comprising;
sensor means operatively connected between said probe and said surface for measuring a parameter which varies relative to the relative positioning of said probe and said surface;
means for comparing said parameter with a reference threshold, and generating an error signal indicating one of at least two possible discrete position conditions;
a digital counter for integrating said error signal and for generating an error count signal, including a register for storing an incremented and decremented count, and including means for setting an upper limit and a lower limit o said count; and position control servo means for controlling the rela-tive positioning of said probe and said surface responsive to said error count signal.

4. A scanning apparatus for controlling relative movement between a probe and an adjacent surface, comprising;
sensor means operatively connected between said probe and said surface for measuring a parameter which varies relative to the relative positioning of said probe and said surface;
means for comparing said parameter with a reference threshold, and generating an error signal indicating one of at least two possible discrete position conditions;
means for integrating said error signal, and for generating an error count signal;

position control servo means for controlling the rela-tive positioning of said probe and said surface responsive to said error count signal; and adaptive feedback control means including at least one adjustable raster clock means for controlling a rate of raster scanning of said probe relative to said adjacent surface in a plane perpendicular to a probe axis between said probe and said adjacent surface, wherein the adjustable clock means is opera-tively connected to the sensor means and the adjustable clock means clock rate is adjustable in response to said error signal.

5. The apparatus of Claim 4, wherein said means for inte-grating said error signal includes an adjustable clock control-ling a sampling rate of said means for integrating said error signal responsive to said error signal, said adaptive feedback control means includes a first adjustable raster clock for con-trolling the rate of raster scanning of said probe relative to said adjacent surface along a second axis perpendicular to said probe axis, and a second adjustable raster clock for controlling the rate of raster scanning along a third axis perpendicular to said probe axis and said second axis responsive to the error signal.

6. A scanning micromechanical probe control system for controlling relative movement between a probe and an adjacent surface aligned along a first axis, comprising;

sensor means operatively connected between said probe and said surface for measuring a parameter which varies relative to the relative positioning of said probe and said surface, com-paring said parameter with a reference threshold, and generating an error signal indicating one of at least two possible discrete position conditions:
means for integrating said error signal and for genera-ting an error count signal;
adaptive feedback control means including at least one raster clock means for controlling a rate of raster scanning of relative movement between said probe and said surface in a plane perpendicular to the first axis, wherein said raster clock means is operatively connected to said sensor means, and the clock rate of said raster clock means is adapted to be adjusted in response to said error signal; and position control servo means for controlling the rela-tive positioning of said probe and said surface along said first axis responsive to said error count signal.

7. The control system of Claim 6, wherein said sensor means generates a two-state error signal in which one state of the binary error signal indicates that said measured parameter is less than said reference threshold, and the other state indicates that said measured parameter is greater than or equal to the reference threshold.

8. The control system of Claim 6, wherein said measured parameter is a tunnelling current and said error signal is based upon the difference between said measured tunnelling current and said reference tunnelling current.

9. The control system of Claim 8, wherein said sensor means includes means for setting said reference current.

10. The control system of Claim 6, wherein said means for integrating said error signal comprises a digital counter inclu-ding a register for storing an incremented and decremented count, and includes means for setting an upper limit and a lower limit of said count.

11. The control system of Claim 6, wherein said means for integrating said error signal includes an adjustable clock for controlling a rate of sampling said error signal.