Abstract:
A method and apparatus for reducing the parachuting of a probe used in an atomic force microscope. The apparatus includes an oscillating probe, a phase detection circuit coupled to the oscillating probe, and a probe drive boosting circuit coupled to the phase detection circuit and the probe, wherein the phase detection circuit detects a reduction of a variation of a phase signal from the probe and the probe drive boosting circuit boosts a signal to the probe based on the phase signal detected by the phase detection circuit to produce a boosted probe drive signal. The phase detection circuit includes a precision full wave rectifier, and an envelope detector coupled to the precision full wave rectifier, wherein the precision full wave rectifier rectifies a phase signal of the probe to produce a rectified phase signal and the envelope detector detects the rectified phase signal to produce an envelope detected signal. The phase detection circuit further includes a comparator coupled to the envelope detector, and an event detector and hold off circuit coupled to the comparator, wherein the comparator and the event detector and hold off circuit generate an event signal from the envelope detected signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention is directed to scanning probe microscopy. More particularly, the present invention is directed to a method and apparatus for reducing the parachuting of a probe used in an atomic force microscope. 
     2. Description of Related Art 
     Scanning probe microscopes are used to make extremely high resolution measurements. An oscillating cantilever or probe is used to scan a sample surface and obtain information representative of the surface. Activity of the probe responsive to surface variations is monitored by a detection system. The detection system is typically an optical beam system. The height of the probe relative to the sample surface is then adjusted to maintain constant one or more probe operational parameters based on the monitored activity. Piezoelectric positioners are often used to adjust the height of the probe relative to the sample surface. Correlation of the adjustment amount in the Z direction versus the position of the probe along the surface in the X and Y directions is used to create a map of the surface. 
     Unfortunately, as the probe traverses the surface of the sample, the probe may not accurately follow the surface, particularly at abrupt drop-offs or ledges in the surface where the probe will tend to depart from and “parachute” over some portion of the surface. For example, when the probe reaches a drop off in the surface, the probe will only gradually lower itself again to the surface (the bottom of the drop-off) as the probe continues its scan, instead of immediately dropping to the surface below the ledge. This parachuting effect causes abrupt vertical drops that actually exist in the surface to be erroneously represented as gradual surface changes. 
     In particular, in current configurations of Tapping™ AFM scanning probe control, a probe is set to tap the surface at a constant oscillating amplitude. The change in the probe tapping amplitude, or other probe operational parameter, is used as a feedback error signal. As the cantilever probe traverses off an abrupt ledge or edge of a plateau, the cantilever leaves the surface, i.e., parachutes. Accordingly, the probe&#39;s oscillation amplitude will grow until the error is sufficiently large to cause a vertical (“Z”) motion piezo to respond and lower the cantilever towards the surface, reestablishing the pre-set amplitude, or other operational parameter. The duration of amplitude growth, and therefore error generation, can be quite long such that while the probe remains off the surface, surface features are passed by and not measured. To compensate for prolonged and slow error generation, and in an attempt to lower the probe to the surface more quickly so that surface features are not missed, the gain of the feedback error signal can be increased but this causes the tip to tap hard on the surface. Such hard tapping reduces error growth time and shortens the free parachuting state of the cantilever but causes other problems. Hard tapping causes the tip to impact the surface with great force which quickly results in damage to the tip and/or sample. Hard tapping is particularly damaging if the tip runs into an upward sloping feature where the probe slams into the surface and damages the tip and/or sample. Finally, these problems are exacerbated as scan rates increase. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for reducing the parachuting of a probe in an atomic force microscope. The apparatus includes an oscillating probe, a phase detection circuit coupled to the oscillating probe, and a probe drive boosting circuit coupled to the phase detection circuit and the probe. The phase detection circuit detects the phase signal corresponding to parachuting of the probe, and the probe drive boosting circuit boosts a signal to the probe based on the phase signal detected by the phase detection circuit to produce a boosted probe drive signal. 
     The phase detection circuit includes a precision full wave rectifier and an envelope detector coupled to the precision full wave rectifier, wherein the precision full wave rectifier rectifies a phase signal of the probe to produce a rectified phase signal and the envelope detector detects the rectified phase signal to produce an envelope detected signal. The phase detection circuit further includes a comparator coupled to the envelope detector, and an event detector and hold off circuit coupled to the comparator, wherein the comparator and the event detector and hold off circuit generate an event signal from the envelope detected signal. The phase detection circuit farther includes a multiplier coupled to the event detector and hold off circuit, wherein the multiplier combines the event signal with a probe drive signal to produce the boosted probe drive signal. 
     The apparatus further includes an event level setting circuit coupled between the event detector and hold off circuit and the multiplier, wherein the event level setting circuit sets an event level of the event signal. The boosted probe drive signal is boosted to a level 20 to 30 percent higher than the probe drive signal. This higher level is adjustable. The event detector and hold off circuit delays generation of the event signal for a predetermined time. 
     The method includes scanning the surface of the sample with an oscillating probe, detecting a reduction of a variation of a phase signal of the probe indicative of free oscillation of the probe, and increasing a rate of the probe response to steep variations of sample surface features. 
     The detecting step further includes rectifying the phase signal of the probe to produce a rectified phase signal, and envelope detecting the rectified phase signal of the probe to produce an envelope of the detected phase signal of the probe. The increasing step further includes boosting a drive signal of the probe to produce a boosted drive signal of the probe in the event of parachuting. The detecting step also further includes triggering an event signal based on the detected phase signal, and the boosting step further comprises boosting the drive signal of the probe by combining the event signal with the drive signal of the probe to produce a boosted drive amplitude signal. The boosted drive signal is 20 to 30 percent of the drive signal above the drive signal. 
     The present invention locates the parachuting (free oscillation) event by using phase criteria and boosts the probe drive signal to expedite error generation. Since the probe drive signal is only boosted during the parachuting event, the tip is able to continue to tap lightly without sacrificing scanning speed. Consequently, probe parachuting is reduced or eliminated without causing hard tapping and thus without damaging the tip and/or sample. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The preferred embodiments of the present invention will be described with reference to the following figures, wherein like numerals designate like elements, and wherein: 
         FIG. 1  is an exemplary block diagram of a paraboost system for reducing the parachuting of a probe; 
         FIG. 2  is an exemplary block diagram of a paraboost module according a preferred embodiment; 
         FIGS. 3  is an exemplary illustration of probe operation without a paraboost module; 
         FIG. 4  is an exemplary illustration of probe operation with a paraboost module; 
         FIG. 5  is an exemplary block diagram of the paraboost module according to a preferred embodiment; 
         FIGS. 6-10  are exemplary illustrations of a phase signal at stages a-g of the paraboost module; 
         FIG. 11  is an exemplary illustration of resulting signals of a probe system without a paraboost module; 
         FIG. 12  is an exemplary illustration of resulting signals of a probe system when the paraboost module is used; and 
         FIG. 13  is an exemplary flowchart of the operation of the paraboost system. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     When a scanning probe of a Tapping™ AFM taps the surface of a sample there are several operational parameters of the probe that can be detected and used to measure surface properties. One such operational parameter is the oscillation amplitude of the tapping cantilever. Another operational parameter is the phase difference between a sinusoidal drive signal and a cantilever response signal. When the cantilever is oscillating freely in air (“free oscillation”), the phase is approximately 90 degrees. When the probe is tapping the surface, the phase varies significantly. The deviation of the tapping phase from the free oscillation phase is normally a few tens of degrees with the sign depending on repulsive or attractive tip/surface interaction. As a result, the free oscillating state of the cantilever is uniquely defined as the state which holds a 90 degree phase. In the figures provided, the phase is plotted with a 90 degree offset so that free oscillation appears at zero degrees. 
       FIG. 1  is an exemplary block diagram of a paraboost system  100  for reducing the parachuting of a probe. The paraboost system  100  includes a paraboost module  110 , a probe  120  including a cantilever and a tip, an oscillator  130 , a Z-module  140 , a control module  150 , a sample  160 , and a surface  170 . The Z-module  140  is a mechanism which causes the probe to approach and separate from the surface  170 , such as a Z-axis piezoelectric element that changes a distance between the probe  120  and the surface  170 . The Z-module adjusts either the distance of the probe relative to the surface or the surface relative to the probe. The oscillator  130  preferably oscillates the probe  120  so as to intermittently contact the surface  170  in accordance with this assignee&#39;s Tapping™ AFM to map the surface  170 . 
     In overview, the paraboost module  110  detects operation of the probe  120 , including parachuting of the probe  120  and, indirectly, boosts an error signal sent to the control module  150 . The control module  150  controls movement and location of the probe  120  with respect to the surface  170 . In particular, the control module  150  controls the distance between the probe  120  and the surface  170  based on detected operational parameters of the probe. The control module  150  may include an additional signal processor which operates in response to error signals from the paraboost module  110 . For example, the control module  150  may integrate an error signal from the paraboost module  110  caused by under or over oscillation of the probe  120 . The control module  150  then adjusts the distance between the probe  120  and the surface  170  to compensate for the error signals. For example, the control module  150  lowers the probe  120  if the paraboost module  110  or another sensor detects that the probe  120  is too far from the surface.  170 . 
     In a preferred embodiment, the probe  120  oscillates and taps the surface  170  at a predetermined set point amplitude. The probe also has a free amplitude oscillation when it is away from the surface  170  (i.e., oscillating in air). The set point amplitude of oscillation of the probe  120  when it taps the surface  170  is usually 70%-90% of the free oscillation amplitude. To obtain an accurate map of the surface  170 , it is desired to keep the tapping amplitude of oscillation constant as the probe scans the surface, i.e., to return it to the set point oscillation amplitude if it strays. Accordingly, when an operational parameter of the probe  120 , such as amplitude of oscillation changes (e.g., decreases) indicating that the probe is too close to the surface  170 , the Z-module  140  retracts the probe  120  from the surface  170  until the probe again oscillates at its set point. Likewise, when the amplitude of oscillation of the probe  120  increases above the set point, indicating that the probe  120  is too far from the surface, the Z-module  140  lowers the probe  120  towards the surface  170  to again reestablish the amplitude of oscillation at the set point. 
     In controlling the Z-module, the paraboost module  110  processes oscillation amplitude, phase, and or other properties of the probe  120 . These properties are detected, for example, by processing the signals produced by any number of standard detector schemes  105 , such as a laser reflecting off the back side of the cantilever and onto a bi-cell or quad detector. An error signal from the detector is sent to the control module  150 . If the oscillation amplitude of the probe  120  is too high, for example, the non-zero error signal is integrated and accumulated by the control module  150 . When enough error signal is integrated, the control module  150  commands the Z-module  140  to lower the probe  120  towards the surface  170 . The error is integrated so that abrupt changes are not detected too quickly, which may cause the Z-module  140  to engage in unwanted oscillation. If the probe  120  encounters a deep recess  175  in the surface  170 , it is desirable to lower the probe  120  to the bottom of the recess  175  as quickly as possible. In conventional devices, this is accomplished by increasing the gain of the error signal. Unfortunately, high gain makes the system susceptible to instability of the Z-module. In the present invention, however, no such Z-module instability occurs when the probe is lowered quickly because instead of increasing gain of the error signal, probe oscillation is increased by boosting the probe drive signal. This causes the error signal to accumulate more rapidly in the control module  150  which causes the Z-module  140  to lower the probe  120  more rapidly. Therefore, the Z-module  140  responds to and reduces parachuting of the probe  120  without causing the probe to oscillate or become unstable. 
     According to a preferred embodiment, the paraboost module  110  detects parachuting of the probe  120  by detecting properties of the phase of the probe  120 . In particular, the present invention takes advantage of the fact that the phase of the probe oscillation is distinctly different when the probe  120  is tapping surface  170  than when it begins to freely oscillate. During tapping the phase is “noisy,” while during free oscillation the phase quiets. Therefore, parachuting can be detected when the phase of the probe  120  becomes quiet. 
       FIG. 2  is an exemplary block diagram of a paraboost module  110  according to a preferred embodiment. The paraboost module  110  includes a detector module  210  and a boost module  220 . In operation, the detector module  210  detects the phase of the oscillating probe  120  with a phase detection circuit  212 . When the detector module  210  detects the reduction of a vibration of the phase signal from the probe  120 , the detector module  210  instructs the boost module  220  to increase the oscillator  130  drive signal supplied to the control module  150  to increase the amplitude of the oscillating probe  120 . By boosting the drive to the oscillating probe  120 , the vibration amplitude of the cantilever  120  is increased, the error signal is increased, and the control module  150  integrates the error more quickly. Accordingly, the Z-module  140  is instructed to lower the probe  120  towards the surface  170  faster. 
       FIGS. 3 and 4  are exemplary illustrations of the desirable effects of the paraboost module  110 .  FIG. 3  illustrates a probe  120  scanning a surface  170  from left to right without using the paraboost module  110 . As illustrated, when the probe  120  reaches an edge of the surface  170 , the probe  120  gradually lowers along the dotted line to the bottom of the surface  170 . Therefore, the edge of the surface  170  is not properly detected and mapped.  FIG. 4  illustrates the scanning of a surface  170  with the probe  120  from left to right using the paraboost module  110 . As illustrated, the probe  120  quickly reacts to the edge of the surface  170 . 
       FIG. 5  is an exemplary block diagram of the paraboost module  110  according to a preferred embodiment. The paraboost module  110  includes a detector module  210  having a phase detection circuit  212 , a differential amplifier  510 , a precision full-wave rectifier  520 , a clamp and gain circuit  530 , an envelope detector  540 , a comparator with hysteresis circuit  550 , an event detector and hold off circuit  560 , a correction period and reset event detector circuit  570 , and a boost module  220  having an event level setting circuit  580 , and an analog multiplier  590 .  FIGS. 6-10  are exemplary illustrations of a phase signal at stages a-g of the paraboost module  110 . 
     In operation, the paraboost module  110  detects parachuting of the probe  120  based on the probe phase signal when the phase signal quiets as illustrated in  FIG. 6 , waveform (a). In particular, the phase signal waveform (a) enters the paraboost module  110  through the differential amplifier  510  at location (a). The differential amplifier  510  is useful to reduce noise of the signal when the paraboost module  110  is relatively far from the sensors detecting the phase of the probe  120 . The amplified phase signal then enters a precision full-wave rectifier  520  and then is rectified to produce full-wave rectified waveform (b). Waveform (b) then enters a clamp and gain circuit  530  and proceeds through an envelope detector  540  which converts the ragged edges into perimeter edges to produce the envelope detected waveform (c) illustrated in FIG.  7 . 
     The event detector and hold off circuitry  560  prevents false triggering. In other words, the event detector and hold off circuitry  560  waits a set amount of time before signaling a no-phase condition. For example, the event detector and hold-off circuitry  560  can ignore events less than 1 ms in duration for cantilevers with 100 kHz resonance frequency and Q 100. The holding time can be shorter if the cantilever resonance frequency is higher. The waveform (c) then enters the comparator with hysteresis  550  and proceeds through the event detector and hold off circuitry  560  to produce waveform (d). 
     Waveform (d) then enters correction period with reset event detector circuitry  570  to produce waveform (e). As illustrated in  FIG. 8 , and discussed above, the event detector and hold off circuitry  560  and the correction period and reset event detector circuitry  570  ignores false events, such s those that are less than one millisecond. Accordingly, on events that are greater than one millisecond are detected and output as a dashed line waveform (e). Waveform (c) then enters event level setting circuitry  580  to adjust the pulse of the waveform (e) to the desired level by using a level setting input and to produce waveform (f) as illustrated in FIG.  9 . Waveform (f) then proceeds through an analog multiplier  590  where it is combined with the cantilever drive signal to boost the drive amplitude resulting in waveform (g) as illustrated in FIG.  10 . Boosted drive amplitude waveform (g) is then used to drive the probe  120 . Note, if a parachuting event is not detected, the cantilever drive is applied directly to oscillator  130  (FIG.  1 ). 
       FIG. 11  is an exemplary illustration of resulting signals of a probe system without a paraboost module. The signals include a detected map signal  1110  of the surface  170  using a probe  120  scanning from right to left, a phase signal  1120  of the probe  120 , and an amplitude of a drive signal  1130  to the probe  120 .  FIG. 11  shows how the probe begins parachuting in map signal  1110  at point  1115  when the phase signal  1120  is level (quiet) at point  1125 , without using the paraboost module  110 . Thus, without the use of paraboost module  110  the amplitude of the drive signal  1130  is not boosted to compensate for the parachuting. 
       FIG. 12  is an exemplary illustration of resulting signals when the paraboost module  110  is used while other experimental conditions are identical to those associated with FIG.  11 . The signals include the mapped surface signal  1210 , the phase signal  1220 , and the amplitude of the drive signal  1230 . As illustrated, when the paraboost module  110  detects a leveling of the phase signal  1220  at point  1225  indicating an abrupt drop in the surface  170  at point  1215 , the amplitude of the drive signal  1230  is adjusted at point  1235 . In particular, the leveling of the phase signal is a reduction of the variation of a phase signal from the probe  120 . Thus, the leveling is a quieting of the phase signal from the probe  120 . Therefore, the paraboost module boosts the cantilever drive signal, the control module  150  integrates the error more rapidly and the Z-module  140  lowers the probe  120  faster. Accordingly, as shown in signal  1210 , an abrupt variation in the surface  170  is more accurately detected. 
       FIG. 13  is an exemplary flowchart of the operation of the paraboost system  100 . The flowchart begins at step  1310 . In step  1320 , the probe  120  scans across the surface  170 . In step  1330 , the paraboost system  100  detects the phase signal of the probe  120 . In step  1340 , the paraboost system  100  boosts the drive signal of the probe  120 . In step  1350 , the paraboost system  100  adjusts the distance between the probe  120  and the surface  170 . For example, the paraboost system  100  reduces the distance between the probe  120  and the surface  170  when the paraboost system  100  detects free oscillation of the probe  120 . In particular, when free oscillation is detected, the paraboost system increases an error signal which in turn increases a drive signal of the Z-module  140  to reduce the distance between the probe  120  and the surface  170 . For example, the error signal or the drive signal are boosted 20-30 percent. 
     While this invention has been described with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth herein are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention.