Patent Publication Number: US-7219538-B2

Title: Balanced momentum probe holder

Description:
RELATED APPLICATION 
     The present application is a continuation application of U.S. patent application Ser. No. 10/614,425, filed Jul. 7, 2003, now U.S. Pat. No. 6,861,649,entitled, “BALANCED MOMENTUM PROBE HOLDER,” which is a continuation of U.S. patent application Ser. No. 09/766,555, filed Jan. 19, 2001, now U.S. Pat. No. 6,590,208 entitled, “BALANCED MOMENTUM PROBE HOLDER,” the disclosures of which are incorporated herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention is generally directed to a balanced momentum probe holder for use in metrology systems, especially scanning probe microscopes used to measure sample surfaces down to the nanometer level. Specifically, the invention is directed to such systems employing nested-Z and non-nested parallel feedback loops, to achieve rapid, and highly accurate scanning of a sample surface. The invention also relates to methods of using such a probe holder in such systems. 
     BACKGROUND OF THE INVENTION 
     The ongoing miniaturization of components of a variety of devices makes high-resolution characterization of critical surfaces increasingly important. In the field of metrology, for example, surface-characterization devices such as stylus profilers and scanning probe microscopes (SPM) are routinely used to measure topography and other characteristics of critical samples. Stylus profilers and scanning probe microscopes are in fact frequently used as inspection tools to measure the critical surfaces of industrial devices like semiconductor chips and data storage devices during and after the manufacturing process. To be economically feasible, these profilers and scanning probe microscopes must complete their measurements as quickly, accurately, repeatably and as reliably as possible. The accuracy, precision, reproducibility, and reliability of such metrology instruments are especially critical in view of the ongoing desire that such surface-characterization instruments be capable of quickly and accurately characterizing dimensions smaller than those of the products and devices being fabricated, to assure manufacturing quality, and to provide accurate diagnoses of manufacturing problems. Because critical features continue to shrink in the manufacturing process, it is necessary to improve the accuracy and the speed of scanning probe microscopes and stylus profilers to keep up with the measurement demand. 
     For the sake of convenience, the discussion that follows and throughout this patent specification will focus on Atomic Force Microscopes (AFMs). In this regard, it shall be understood that problems addressed and solutions presented by the present invention shall also be applicable to problems experienced by other measurement instruments including surface-modification instruments and micro-actuated devices. 
     The typical AFM includes a probe which includes a flexible cantilever and a stylus mounted on the free end of the cantilever. The probe is mounted on a scanning stage that is typically mounted on a common support structure with the sample. A typical scanning stage may include an XY actuator assembly and a Z actuator, wherein “X” and “Y” represent what is typically the horizontal XY plane, and “Z” represents the vertical direction. “X” and “Y” and “Z” are mutually orthogonal directions. The XY actuator assembly drives the probe to move in an X-Y plane for scanning. The typical Z actuator mounted on the XY actuator and providing support for the probe, thus drives the probe to move along a Z axis which is disposed orthogonally relative to the X-Y plane. (The definition of the XYZ axes is convenient and typical, but the choice of axis name and orientation is of course arbitrary.) 
     AFMs can be operated in different sample-characterization modes including contact-mode and Tapping™ mode. In contact-mode, the cantilever stylus is placed in contact with the sample surface, cantilever deflection is monitored as the stylus is scanned over the sample surface, and the resulting image is a topographical map of the surface of the sample. In Tapping™ mode (a trademark of Veeco Instruments, Inc.) sample characterization, the cantilever is oscillated mechanically at or near its resonant frequency so the stylus repeatedly taps the sample surface or otherwise interacts with the sample. See, e.g., U.S. Pat. Nos. 5,266,801; 5,412,980; and 5,519,212 to Elings et al., which are illustrative. 
     In either sample-characterization mode, the interaction between the stylus and the sample surface induces a discernable effect on a probe-based operational parameter, such as the cantilever deflection oscillation amplitude, the phase or the frequency, all of which are detectable by a sensor. In this regard, the resultant sensor-generated signal is used as a feedback control signal for the Z actuator to maintain a designated probe operational parameter constant. 
     In contact-mode, the designated parameter may be cantilever deflection. In Tapping™ mode, the designated parameter may be oscillation amplitude, phase or frequency. The feedback signal also provides a measurement of the surface characteristic of interest. For example, in Tapping™ mode, the feedback signal may be used to maintain the amplitude of cantilever oscillation constant to measure the height of the sample surface or other sample characteristics. 
     In analyzing biological samples, polymers, photoresist, metals and insulators, thin films, silicon wafer surfaces, and other surfaces, the ability to accurately characterize a sample surface is often limited by the present ability of an AFM to move the stylus vertically relative to the surface at a rate sufficient to accurately measure the surface while scanning in either the X or Y direction. This ability is inadequate in present day devices for essentially two reasons. 
     In order to accurately measure the height of all features, both large and small, on a sample surface, the Z actuator must have the ability to displace the stylus connected thereto over a large range of heights, i.e., it must have large vertical travel. This necessitates that the Z actuator, whether it is a scanning tube such as is on this assignee&#39;s Dimension series AFM heads or is a flexure such as is on this assignee&#39;s Metrology series AFM heads, must be large enough to move the stylus up and down sufficiently to measure even the largest surface features. 
     Unfortunately, a necessary by-product of a larger Z actuator having greater range is associated greater mass which makes the actuator movement relatively slow. Slow actuators are not able to move the probe rapidly enough in Z while scanning in X or Y at anything more than modest speed without damaging the probe or sample or without sacrificing measurement accuracy. Because it is important while scanning to minimize the force of the stylus on the sample to prevent damage to the stylus and/or sample, the scan rate in X or Y must, of necessity, be reduced to a speed compatible with the Z actuator&#39;s ability to move the stylus up and over surface features without slamming into them, which is obviously undesirable. One present day technique to overcome this limitation and increase responsiveness of the Z-actuator is to increase the gain of its feedback loop. This works only to a limited degree because if the gain is increased more than a modest amount, the Z actuator begins to resonate and that resonance is passed into the AFM, creating parasitic oscillations, which in turn ruin image quality. In essence, a large mass, large displacement Z actuator cannot be made to overcome its inherent physical limitations. 
     In another approach, one does not attempt to wring more performance from the large Z actuator than it is inherently able to deliver. Instead, a separate “fast” Z actuator is used, with its own feedback loop, to move the stylus quickly over small surface variations that the large Z actuator is too slow to react to, which enables one to obtain relatively high quality imaging at even high scan speeds. The fast Z actuator is smaller than and hence of significantly smaller mass than the slow Z actuator. As a result, it is advantageously driven in its own (or shared) fast feedback loop at speeds exceeding that of the slow Z actuator. 
     Unfortunately, at high gain, the high speed of operation and momentum of the fast Z actuator can similarly cause parasitic oscillations which reduce image quality. A device and method which balances these inertial forces created by a fast Z actuator would be of great benefit and commercial interest. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a novel balanced momentum probe holder for scanning probe microscopes and/or stylus profilers that permits the probe to measure the height of small surface features better than is presently possible with commercially available tools. It is specifically an object to provide such a probe holder for an improved atomic force microscope (AFM). 
     Another object of the present invention is to provide a novel AFM that permits more accurate imaging of surface features at high scan rates. 
     Still another object of this invention is to provide an AFM that can measure surface features at high scan rates without inducing parasitic oscillations in the AFM. 
     A further object of this invention is to balance the momentum created by the fast Z actuator in an AFM to allow fast actuation without driving parasitic oscillations. 
     Yet another object of this invention is to provide a fast actuator of sufficiently low mass to allow its use on the lower end of a scanning stylist AFM. 
     Yet a further object of this invention is to provide an AFM with fast actuation optimized for operation in nested or parallel feedback loops. 
     These and other objects are achieved according to the present invention by providing a new and improved AFM having a probe holder that includes a separate, fast Z actuator assembly operated in a fast feedback loop and that balances the momentum of the fast Z actuator assembly. The basic idea is to balance the momentum of the moving probe holder with the momentum of a counterbalance moving in synchronization with the probe holder, but in the opposite direction. In this case, the net momentum of the fast Z-actuator assembly is essentially zero, and thus the motion of the probe does not substantially excite parasitic resonances of the supporting structure and/or XYZ scan assembly. The fast Z actuator assembly is also of low mass and is therefore able to displace the probe in the Z direction more rapidly than a larger, higher mass conventional Z actuator which is part of the piezo tube or the flexure upon which the fast Z actuator assembly is mounted. In order to take advantage of the small size and low mass of the fast Z actuator assembly, it is operated in a fast feedback loop, either nested with the feedback loop of the conventional Z actuator or in a parallel feedback loop. The combination of a low mass fast Z-actuator and the balanced momentum enables extremely accurate scanning of even the smallest surface features and even at high scan speeds where conventional Z actuators perform sluggishly. 
     The present invention, then, is generally directed to an apparatus having a probe for characterizing a surface of a sample. The apparatus may have an X actuator, a Y actuator and a first Z actuator as in an AFM but may also have only a Z actuator such as in a profilometer. The apparatus also has a second Z actuator assembly with the probe mounted on it. The second Z actuator assembly is coupled to the first Z actuator. The second Z actuator assembly is less massive and therefore quicker responding than the first Z actuator. When actuated to move the probe, the momentum of the second Z actuator assembly is balanced so that its motion does not transmit substantial vibration to other actuators or support members. 
     The fast Z actuator assembly comprises first and second fast Z-actuators, sometimes referred to herein as the bottom actuator and the top actuator, respectively. The two actuators are arranged so that the fixed ends are attached to a common central support. Then the top end of the top actuator and the bottom end of the bottom actuator are both free to move. The measurement probe, for example an AFM cantilever probe, is attached directly or through intermediate mounting to the bottom or distal end of the bottom actuator which is proximate the sample. A counterbalance mass is attached to the top or distal end of the top actuator. The top and bottom fast Z-actuators are arranged so that they move in a synchronized manner, but in opposite directions. The probe mount, actuators, and counterbalance mass are arranged to match the momentum carried by the top and bottom actuators. In the simplest case, the mass of the top actuator is the same as the mass of the bottom actuator and the mass of the counterbalance mass matches the mass of the probe mount. Then the two actuators are arranged to move substantially the same distance (in opposite directions) at the same time. Since the motions are the same but opposite and the masses are matched, the net momentum is essentially zero, thus transmitting no vibration to surrounding members. In more complicated arrangements, the momentum can be matched by arranging a top actuator with say half the motion of the bottom actuator, but twice the moving mass, or suitable variations thereof that match combinations of velocity and mass of the top and bottom fast Z-actuators. 
     In one embodiment, the first, bottom actuator includes a first piezo stack disposed between the common central support and the probe mount assembly, and the second, top actuator includes a second piezo stack disposed between the counterbalance and the common central support. 
     In yet another embodiment, the balanced momentum probe holder is incorporated into a nested feedback control system. In still another embodiment, the balanced momentum probe holder is incorporated into a non-nested parallel feedback control system. 
     In both feedback systems, when an error signal to move the probe vertically is sent to the fast Z actuator assembly, the first piezo stack extends or retracts to move the probe to the desired height while, simultaneously, the second piezo stack extends or retracts also. The momentum of the second piezo stack and its associated components balances the momentum of the first piezo stack and associated components including the probe. 
     As a result, the net momentum, and therefore the net force acting upon the larger system is eliminated, thereby eliminating or substantially reducing parasitic oscillations. In a nested feedback control system, the error signal is processed by a control device such as a PID controller and sent to the fast Z actuator assembly to cause it to move the probe. Any residual error signal is sent to the slow Z actuator assembly to cause it to move the probe an additional amount needed. In this way, the probe is able to track, and therefore measure the height of surface features that are quite small, even at high scan speeds, while also being able to measure larger surface features as well. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A clear understanding of the above-summarized features and advantages of the present invention as well as various environments and fields-of-use of the invention, as is presently contemplated by the instant inventor, including the construction and operation of conventional components and mechanisms associated with the present invention, will become more readily apparent to one skilled in the art by referring to the exemplary, and therefore non-limiting, embodiments illustrated in the following drawings which accompany and form a part of this patent specification. 
         FIG. 1  is a schematic illustrating a preferred method and system for controlling the relative distance between a probe and a sample surface. The illustrative system depicted in  FIG. 1  includes an atomic force microscope, a personal computer, and a display device operably coupled to the personal computer for visually displaying information characterizing the surface sample. 
         FIG. 2  is a view depicting select elements from  FIG. 1  on an enlarged scale. 
         FIG. 3  is a schematic depicting an alternate embodiment of the present invention. 
         FIG. 4  is a view depicting select elements from  FIG. 2  on an enlarged scale. 
         FIG. 5  is a perspective view based on  FIG. 4 , and on an enlarged scale relative to  FIG. 2 . 
         FIG. 6  is an exploded perspective view, based on  FIG. 4  and on an enlarged scale, presenting one embodiment of the balanced momentum probe holder of the invention. 
         FIG. 7  is an exploded perspective view, based on  FIG. 4  and on an enlarged scale, presenting another embodiment of the balanced momentum probe holder of the invention. 
         FIG. 8  is a schematic illustrating one preferred embodiment of a method and apparatus for characterizing a sample surface. 
         FIG. 9  is a schematic illustrating another preferred embodiment of a method and apparatus for characterizing a sample surface. 
     
    
    
     Throughout the drawings, like reference numerals refer to like parts. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As suggested above, sample surfaces may be characterized by using probe-based instruments such as scanning probe microscopes, stylus profilers, or any other instrument capable of obtaining, recording, and manipulating sample surface information. While all of these applications are within the scope of the present invention, the preferred embodiment describes the invention as included in an Atomic Force Microscope (AFM) but does not exclude other SPMs. 
     An AFM-based system which incorporates the balanced momentum probe holder of the present invention and which is capable of acquiring sample surface data, recording the surface data, and manipulating the data to perform desired tasks is schematically illustrated in  FIG. 1 . 
     The AFM-based system ( FIG. 1 ) includes an XYZ actuator  100  to which a cantilever arm  102  is operatively connected. A stylus  104  is mounted on the other end of the cantilever  102  for characterizing a surface  106  of a sample  108  releasably affixed to chuck  110 . 
     A displacement sensor  112  detects movement of the stylus  104  above the surface  106  and provides a signal that is related to a measured property of the sample surface, for example the shape of the sample surface. The output of the displacement sensor is sent to an AFM control/computer system  116  as is well known in the art. The control system  116  outputs scan and control signals to the XYZ actuator  100 . AFMs are usually operated in a mode that attempts to maintain (and often minimize) a constant tracking force between the stylus  104  and the sample surface  106 . This is usually accomplished by arranging a feedback loop to keep the output of the displacement sensor constant as the XYZ actuator scans the probe, and therefore the stylus, over an area of interest on the sample surface. To maintain the constant tracking force, the Z portion of the XYZ actuator  100  is raised up and down. In the current invention, this vertical motion is accomplished by either the Z portion of the XYZ actuator or the balanced momentum probe holder  114 , or both, by raising or lowering the stylus  104  relative to the sample surface  106 . 
     The displacement sensor  112  includes a laser and a photodetector, both of which will be discussed in detail below in connection with  FIGS. 8 and 9 . Signals from the displacement sensor  112  may, for example, be used to determine the deflection, oscillation amplitude, frequency, or phase or similar parameter of the cantilever  102  and stylus  104  when they are moving in proximity or in contact with the sample surface  106 . An image display device  118 , operatively connected to the personal computer  116 , is able to display video images in response to a signal from the personal computer  116 . The computer also typically stores the sample images for later viewing and analysis. 
       FIG. 2  is a side view illustrating the balanced momentum probe holder  114  of the present invention mounted on the XYZ actuator  100 , in the form of a scanning tube as is referred to by this assignee as a Dimension tube scanner. The XYZ actuator  100  is a standard piezo tube scanner and includes a conventional XY actuator  120  consisting of cylindrical X and Y piezo elements and a Z actuator  122  consisting of a cylindrical Z piezo element. XY actuator  120  is adapted to move the stylus  104  relative to the sample surface  106  in the “X” and “Y” directions. Z actuator  122  is adapted to move the stylus  104  relative to the sample surface  106  in the “Z” (i.e., height) direction. The balanced momentum probe holder  114  is mounted on the lower end of Z actuator  122 . 
       FIG. 3  is a schematic of another embodiment of an XYZ actuator  100 A, in the form of a flexure, referred to by this assignee as a Dimension Metrology scanner with which the inventive probe holder  114  may be used. Components of a preferred embodiment of XY actuator  120 A include an X actuator  124  and a Y actuator  126 . A Z actuator  122 A is mounted on Y actuator  126 , Y actuator  126  is mounted on X actuator  124 , and X actuator  124  is mounted on a connector  128 , for connecting the XYZ actuators  124 ,  126 , and  122 A to an apparatus for characterizing a surface of a sample. 
     The balanced momentum probe holder  114  is mounted on the lower end of the Z actuator  122 A. Each of the X, Y and Z actuators  124 ,  126 , and  122 A includes a respective piezo element or stack  130 ,  132 ,  134  mounted within respective flexures for moving the probe holder  114  relative to the sample surface as is standard. For this purpose, the X direction actuator piezo stack  130  and the Y direction actuator piezo stack  132 , are each diagonally mounted respectively within their X, Y flexures  124 ,  126  while the Z direction piezo stack  134  is mounted in the Z direction within its flexure  122 A as shown. In operation, a respective piezo element or stack  130 ,  132 ,  134  is energized, causing such piezo stack  130 ,  132  and/or  134  to expand or contract, bending its respective flexures for moving the balanced momentum probe holder  114  relative to the surface  106  of the sample  108 . 
       FIGS. 4 and 5  depict the Z actuator  122  of the piezo tube scanner as an elongated, hollow tube of conventional Z piezo material. A lower end portion  136  of the Z actuator  122  includes a plurality of pins  138  extending away from the end portion  136  of the Z actuator  122 . The novel balanced momentum probe holder  114  includes a base or holder  140  which defines a corresponding plurality of apertures, or sockets,  142  dimensioned for receiving the pins  138  and for operatively connecting the Z actuator  122  and the base  140  together. Further in this regard and referring to  FIG. 5 , the base  140  defines a central, open portion  172  through which other components  146 ,  154  (described in detail below) of the probe holder  114  pass. Still further, the end portion  136  of the Z actuator  122  preferably includes a corresponding central, open portion  174  into which the probe holder  114  is disposed when actuator  100  is joined to the balanced momentum holder  114 , as shown in  FIG. 5 . 
       FIGS. 6 and 7  are exploded perspective views, based on  FIGS. 2 and 4 , and on an enlarged scale relative thereto, presenting a preferred embodiment of the balanced momentum probe holder of the present invention. The illustrated embodiment of the balanced momentum probe holder  114 A of the present invention comprises the holder or base  140  ( FIG. 5 ) connected to the actuator  100 , and a common central support  144  connected to the holder or base  140 . The base or holder  140  is not shown in  FIG. 6  for purposes of clearly presenting the remainder of the components or elements of the first preferred embodiment of the balanced momentum probe holder  114  of the present invention. 
     The balanced momentum probe holder  114 A further comprises a first member  146  which preferably comprises a piezo stack  147 . The first member  146  is carried by the common central support  144 , in a central recessed portion  145  of the support. The first piezo stack  147  has a distal end  148  disposed toward the sample and which is extensible and retractable in the Z axis. The probe holder  114 A further comprises a second member  150  which also preferably comprises a piezo stack  151 . The second member  150  is carried on the opposite side  145 ′ of the common central support  144 . The second piezo stack  151  has a free end  152  disposed away from the sample and which is extensible and retractable in the Z direction. 
     First and second member  146  and  150  may alternatively comprise other piezo actuators such as piezo electric tubes or piezo electric bimorphs. First and second members  146 ,  150  may also comprise voice coil actuators, electrostatic actuators, electrorestrictive actuators or magnetorestrictive actuators, or other suitable actuators. 
     The first and second actuator assemblies  146  and  150  each has a mass that is selected to provide minimal weight to the probe holder  114 A yet achieve the desired sample surface characterization effect. In operation, the free ends  148  and  152  of the first and second actuator assemblies  146  and  150  either both extend or both retract synchronously in response to a signal from a detector, as is described in detail below. Moreover, in the preferred embodiment, the masses of the first and second actuator assemblies  146  and  150  are substantially equal, to balance the momentum of the piezo stacks  147  and  151  during operation of the balanced momentum probe holder  114 A during surface characterization of a sample  108 . In an alternate embodiment, the mass of the actuator assemblies can be different if the range of travel of the two actuators is different. For example, the upper actuator assembly  150  may have twice the mass of the lower actuator assembly  146  if the upper actuator assembly is arranged to move half the distance of the lower actuator assembly. Other effective combinations having matched mass times velocity products may be used as appropriate. 
     Probe holder  114 A further comprises a probe mount assembly  154  carried by the free end  148  of the lower actuator assembly  146 . Probe mount assembly  154  comprises a probe mount  156  and a cantilever probe  158  carried by the mount  156 , consisting of a cantilever substrate  160 , and a cantilever arm  102  carried by the cantilever substrate, and disposed toward the sample. The cantilever probe  158  includes the cantilever arm  102  and stylus  104  (not shown), both of which are depicted in  FIG. 1 . Probe holder  114 A further comprises a counterbalance  162  carried by the distal end  152  of the second member  150 . The mount assembly  154  and counterbalance  162  have substantially equal masses or as indicated above are chosen to ensure that the momentums of the first and second actuator assemblies are balanced. 
     When assembled, alumina insulating layers (not shown) may be placed between common central support  144  and the first piezo stack  147 , between the common central support  144  and the second piezo stack  151 , and between the first piezo stack  147  and the probe mount  156 . The insulating layers are not necessary especially if the common central support is nonconducting. 
     In operation, when activated by a Z actuation signal, each of the first and second piezo stacks  147 ,  151  will extend or retract in the Z direction (up or down by the conventional orientation and as oriented in the figures.) Further, the first and second piezo stacks  147 ,  151  are driven in opposite directions. This can be accomplished by orienting the piezo stacks so that the same control signal will cause them to move in opposite directions or by opposing control signals to two stacks that are oriented with the piezo polarity in the same direction. It may also be desirable to scale the control voltages going to each piezo stack to account for any difference in sensitivity (and therefore response) between the two piezo stacks. Accordingly, the momentum of extending and retracting actuator assemblies  146 ,  150  will be balanced, as will readily be appreciated by those skilled in the art. 
     Additionally, to achieve Tapping™ mode operation, or other A.C. imaging modes such as MFM (magnetic mode microscopy) a signal at a frequency substantially equal to the resonant frequency of the cantilever arm  102  is fed to the first piezo stack  147  in combination with the Z actuation driving signal fed to that stack. In this way, first piezo stack not only causes the stylus  104  to move in the Z direction but to oscillate at resonance and tap the sample surface, or otherwise obtain sample information by various A.C. imaging modes. 
       FIG. 7  is an exploded perspective view, presenting another embodiment of the balanced momentum probe holder of the invention. The base or holder  140  is not shown for purposes of clearly presenting the remainder of the components or elements of the second preferred embodiment of the balanced momentum probe holder  114 B. 
     The balanced momentum probe holder  114 B of  FIG. 7  is similar to the above-discussed embodiment of the balanced momentum probe holder of the present invention with the addition of a separate tapping piezo element. Thus in  FIG. 7 , the first member  146  preferably includes a first piezo stack  147  disposed between the common central support  144  and the probe mount assembly  154 A. Similarly, second member  150  includes a second piezo stack  151  disposed between counterbalance  162 A and the common central support  144 . 
     The mount assembly  154 A illustrated in  FIG. 7  further preferably includes an oscillation piezo element  180  which is disposed between the first piezo stack  147  and the probe mount  156 . The oscillation piezo element  180  is typically used to oscillate the cantilever probe at or near its resonant frequency. Using a separate piezo element  180 , excited with a signal at a frequency substantially equal to the resonant frequency of the cantilever arm  102 , may provide additional robustness as compared to the apparatus of  FIG. 6  which lacks a tapping piezo element and wherein tapping is achieved by combining the tapping signal with the fast Z actuation signal fed to the first piezo stack  147 . 
     Insulators, though not necessary, may be used as before in addition to one on either side of oscillation-piezo element  180 . 
     It should be noted that in the preferred embodiment the mass of the counterbalance  162 A is substantially equal to the mass of the mount assembly  154 A (which includes the tapping piezo element  180 ), to achieve the balanced momentum effect mentioned above. In an alternate embodiment, the masses of the counterbalance  162 A and the probe mount assembly  154 A can be different if the momentum of the assemblies are substantially balanced. For example if the counterbalance  162 A had twice the mass of the probe mount  154 A, the lower actuator would be moved roughly twice the distance of the upper actuator. Other effective combinations of mass and travel may be used as appropriate. 
     A suitable commercially available adhesive material is preferably used to adhesively bond adjacent components of the balanced momentum probe holder  114 A,  114 B together or not. Said components may also be soldered, welded, braised, mechanically constrained, clamped or held together by any other equivalent method. 
       FIG. 8  is a schematic illustrating one preferred embodiment of a method and apparatus in  FIG. 1 . The illustrated apparatus, in operation, includes the XYZ actuator  100  ( FIG. 2 ), wherein the Z actuator portion  122  of the XYZ actuator  100  is extendable both toward and away from sample  108 , alternatively, for characterizing the surface  106 . As mentioned above, one preferred Z actuator is an elongated, tubular, hollow Z actuator  122 . (See  FIGS. 4 and 5 .) The side wall of the hollow Z actuator  122  includes a sufficiently large cut-out portion  188  such that a beam  189  of light from a source (not shown) such as a laser is able to pass longitudinally through the hollow Z actuator  122 , reflect off the cantilever arm  102  of the balanced momentum probe holder  114 , and pass to detector  190 . The detector  190  is adapted to produce a signal which is correlatable to the magnitude of displacement or oscillation of the stylus  104 . 
       FIG. 8  further depicts a nested feedback control system comprising the balanced momentum probe holder  114  discussed above, the detector  190 , amplifier  196 , difference amplifier  197 , first and second control devices  192 ,  194 , and optional amplifier  198 . The first control device  192  is preferably a standard PID controller and is operatively connected to the output of the difference amplifier  197 . Difference amplifier  197  has at its output the difference between the amplified output of the detector  190  and a set point voltage, as is standard. The first control device is connected to the first and second piezo stacks  147 ,  151  of the probe holder  114  through optional amplifier  198  for causing the distal ends  148 ,  152  of the first and the second piezo stacks  147 ,  151  to simultaneously extend or retract in response to the error signal from the amplifier  197  for moving the stylus  104  at a first predetermined rate either toward or away from the surface  106  of the sample  108 . Simultaneous operation of the second piezo stack  151  with first piezo stack  147 , balances the momentum generated by the piezo stack  147  when it extends or retracts. This advantageously eliminates unwanted detrimental parasitic oscillations in the device as a whole. 
     The second control device  194  is also preferably a PID controller, operably connected to the output of first control device  192  and input to the XYZ actuator  100  for causing the extendable Z actuator portion  122  of actuator  100  to move the stylus  104  of probe holder  114  at a second predetermined rate either toward or away from the surface  106  of the sample  108  when the entire error signal is not reduced to zero by operation of the piezo stacks  147  and  151  of the probe holder  114 , as discussed below. 
     The first predetermined rate is greater than the second predetermined rate. In other words, the first piezo stack  147  (the fast Z actuator) of the balanced momentum probe holder  114  moves the stylus  104  toward and away from the sample  108  faster than does the Z actuator  122 . This is possible because the probe holder  114  is significantly less massive than the Z actuator  122  and because the fast feedback loop operates at higher speed relative to the slower feedback loop of the Z actuator  122 . The fast feedback loop comprises, operably coupled: the detector  190 , the amplifier  196 , the difference amplifier  197 , the first controller  192 , the optional amplifier  198  and the first piezo stack  147 . The fast feedback loop operates at higher speed relative to the slower feedback loop of the Z actuator  122 . The slower feedback loop comprises, operably coupled: the detector  190 , the amplifier  196 , the difference amplifier  197 , the first controller  192 , the second controller  194  and the slow Z actuator  122 . The probe holder  114 , by design, thus balances the momentum of its opposing ends, which is of significant interest because it keeps stray oscillations from probe holder  114  from coupling into the actuator  100  and the remainder of the apparatus. 
     By current design, for the apparatus illustrated in  FIGS. 8 and 9 , the maximum range of travel for the Z actuator  122  in the direction toward and away from surface  106  is approximately 15 micrometers, and the maximum range of travel for the first piezo stack  147  of the probe holder  114  in the direction toward and away from the sample surface  106  is approximately 1 micrometer. Because the range of travel of the probe due to the first piezo stack of the probe holder  114  is limited to about 1 micrometer, it may not be able to move the probe the required amount to, for instance, clear a surface feature that is more than 500 nanometers high, which is about one-half of the total travel of the first piezo stack  147 . In this situation, the error signal from difference amplifier  197  is not reduced to zero and a residual error signal will be input to the second control device  194  which will output a signal to the slow Z actuator  122  to move the probe the additional amount required to track or clear the surface feature being scanned. 
     Preferably, the residual error signal input to the second control device  194  will be such as to cause the Z actuator  122  to keep the stylus  104  in the middle of the range of travel (1 micrometer) of the first piezo stack  147  on which the cantilever probe  158  including cantilever arm  102  and stylus  104  are mounted. In this way, probe holder  114 , in particular the first piezo stack  147 , will also have sufficient travel available, toward and away from the sample, to be able to move the probe rapidly in the Z direction to provide the fastest Z actuation possible. This ensures that the stylus will be able to accurately follow even the smallest surface features at high scan rates. Importantly, it will do so without inducing parasitic oscillations into the remainder of the apparatus because the second piezo stack  151  balances the momentum of the first piezo stack  147 . 
       FIG. 9  is a schematic illustrating another embodiment of a method and apparatus for characterizing the surface  106  of the sample  108  shown in  FIG. 1 . The illustrated apparatus ( FIG. 9 ), in operation, includes the XYZ actuator  100  ( FIG. 2 ), wherein the Z actuator portion  122  of the XYZ actuator  100  is extendable both toward and away from the sample  108 , alternatively, for characterizing the surface  106  of the sample  108 . 
     Also, as discussed above in connection with  FIG. 8 , a light beam  189  from a source (not shown) reflects off the cantilever arm  102  of the holder  114 , and passes to the detector  190 , for determining either the displacement or the amount of oscillation of the cantilever arm  102  for controlling the relative force or distance between the sample surface  106  and the stylus  104 . 
     Further in that regard and for that purpose,  FIG. 9  depicts a non-nested parallel feedback system comprising the novel probe holder  114  discussed above, the detector  190 , amplifier  196 A, difference amplifier  197 A, high and low pass filters  200  and  202 , and first and second control devices  192 A,  194 A. The components of the non-nested parallel feedback system ( FIG. 9 ) are substantially as described above in connection with the nested feedback control system ( FIG. 8 ), except as follows. 
     The first control device  192 A has as its input the error signal from difference amplifier  197 A which has been high pass filtered by high pass filter  200 . The output of the first control device  192 A is fed through an amplifier  198 A to the piezo stack  147  to move the stylus  104  at a first predetermined rate either toward or away from the sample surface  106 . 
     The second control device  194 A has as its input the error signal from difference amplifier  197 A which has been low pass filtered through low pass filter  202 . The output of the second control device  194 A is fed to the Z actuator  122  for causing the Z actuator  122  to move the stylus  104  at a second predetermined rate either toward or away from the sample surface  106 . 
     In operation, the first control device  192 A produces a first control signal in response to the higher frequency components of the error signal from the high-pass filter  200 , for causing the distal ends  148 ,  152  of the first and the second members  146 ,  150  ( FIGS. 6 ,  7 ) either to extend or retract, for moving the stylus  104  relative to the sample surface  106  ( FIG. 1 ) within a range of 1 micron and at a rapid rate. The second control device  194 A produces a second control signal in response to the lower frequency components of the error signal from the low-pass filter  202 , for moving the stylus  104  toward or away from the sample surface  106 , within a range of 15 microns at a slower, conventional rate. 
     In this regard, the outputs of the first and second control devices  192 A and  194 A cooperate to move the stylus to the appropriate height above the sample, through their respective Z actuators (piezo stack  147  and Z actuator  122 , respectively), and with sufficient rapidity to ensure accurate measurement even at higher scan rates. 
     As those skilled in the art can well appreciate, the first and second control devices  192 ,  192 A,  194 ,  194 A for the nested feedback control and non-nested parallel feedback control system may be micro computers or microprocessors, as desired. 
     The invention thus allows relatively rapid high-precision sample scanning and characterization, resulting in significantly faster sample tracking than conventional systems can provide, without undesired system resonance and attendant system instability. 
     What has been illustrated and described herein is a balanced momentum probe holder that can be used in a nested feedback control system or in a non-nested parallel feedback control system. However, as the balanced momentum probe holder system has been illustrated and described with reference to several preferred embodiments, it is to be understood that the invention is not to be limited to these embodiments. In particular, and as those skilled in the relevant art can appreciate, functional alternatives will become apparent after reviewing this patent specification. Accordingly, all such functional equivalents, alternatives, and/or modifications are to be considered as forming a part of the present invention insofar as they fall within the spirit and scope of the appended claims.