Abstract:
A new type of indenter is described. This device combines certain sensing and structural elements of atomic force microscopy with a module designed for the use of indentation probes, conventional diamond and otherwise, as well as unconventional designs, to produce high resolution and otherwise superior indentation measurements.

Description:
CROSS-REFERENCE 
     This application is a continuation application of U.S. Ser. No. 12/729,339 filed Mar. 23, 2010, now U.S. Pat. No. 8,196,458 issued Jun. 12, 2012, which is a continuation application of U.S. Ser. No. 11/724,067 filed Mar. 13, 2007, now U.S. Pat. No. 7,685,869 issued Mar. 30, 2010, which claims priority from provisional 60/782,072, filed Mar. 13, 2006, the entire contents of the disclosure of which is herewith incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     An AFM is a device used to produce images of surface topography (and other sample characteristics) based on information obtained from rastering a sharp probe on the end of a cantilever relative to the surface of the sample. Deflections of the cantilever, or changes in its oscillation, which are detected while rastering correspond to topographical (or other) features of the sample. Deflections or changes in oscillation are typically detected by an optical lever arrangement whereby a light beam is directed onto a cantilever in the same reference frame as the optical lever. The beam reflected from the cantilever is made to illuminate a position sensitive detector (PSD). As the deflection or oscillation of the cantilever changes, the position of the reflected spot on the PSD changes, causing a change in the output from the PSD. Changes in the deflection or oscillation of the cantilever are typically made to trigger a change in the vertical position of the cantilever base relative to the sample, in order to maintain the deflection or oscillation at a constant pre-set value. It is this feedback that generates an AFM image. AFMs can be operated in a number of different imaging modes, including contact mode where the tip of the cantilever is in constant contact with the sample surface, and oscillatory modes where the tip makes no contact or only intermittent contact with the surface. 
     Actuators are commonly used in AFMs, for example to raster the probe relative to the sample surface or to change the position of the cantilever base relative to the sample surface. The purpose of actuators is to provide relative movement between the probe and the sample. For different purposes and different results, it may be useful to actuate the sample, or the tip or some combination of both. Sensors are also commonly used in AFMs. They are used to detect movement of various components of the AFM, including movement created by actuators. For the purposes of the specification, unless otherwise specified, the term “actuator” refers to a broad array of devices that convert input signals into physical motion, including piezo activated flexures, piezo tubes, piezo stacks, blocks, bimorphs, unimorphs, linear motors, electrostrictive actuators, electrostatic motors, capacitive motors, voice coil actuators and magnetostrictive actuators, and the term “position sensor” or “sensor” refers to a device that converts a displacement, velocity or acceleration into an electrical signal, including capacitive sensors, inductive sensors (including eddy current sensors), differential transformers (such as described in co-pending applications US20020175677A1 and US20040075428A1, Linear Variable Differential Transformers for High Precision Position Measurements, and US20040056653A1, Linear Variable Differential Transformer with Digital Electronics, which are hereby incorporated by reference in their entirety), variable inductance, optical interferometry, optical deflection detectors (including those referred to above as a PSD and those described in co-pending applications US20030209060A1 and US20040079142A1, Apparatus and Method for Isolating and Measuring Movement in Metrology Apparatus, which are hereby incorporated by reference in their entirety), strain gages, piezo sensors, magnetostrictive and electrostrictive sensors. 
     SUMMARY OF THE INVENTION 
     We have developed a nanoindenter that produces very accurate, quantitative characterization for a wide spectrum of materials. The new nanoindenter may be implemented on an atomic force microscope (AFM) platform, but unlike indentation that might be effected with an AFM cantilever, the invention drives the indenting tip perpendicularly into the sample. Displacement and force are measured with optimized LVDT sensors and an optical lever, respectively, the same devices that eliminate inaccuracies (e.g. non linearity) present in measurements made with AFMs, and this greatly increases sensitivity and resolution in comparison to commercial indenters. This highly quantitative tool, incorporating high end AFM capabilities breaks new ground in characterization of a great diversity of materials including thin films, coatings, polymers, etc. As noted, the nanoindenter may be implemented on an AFM platform and when integrated with the native metrology abilities of the Molecular Force Probe-3D AFM of Asylum Research Corporation, it enables the user to perform quantitative indentation measurements and to make quantitative statements regarding the indenter tip shape and indentation volumes and profiles, all with the same instrument set-up. 
     In addition to an AFM platform, the new nanoindenter may be implemented on other cantilever-based instruments. Cantilever-based instruments include such instruments as AFMs, molecular force probe instruments (1D or 3D), high-resolution profilometers and chemical or biological sensing probes. For the sake of convenience, the specification focuses on AFMs. However, it should be understood that problems addressed and solutions presented by the present invention are also applicable to other instruments with nanoscale motion and metrology applications. 
     The systems and techniques described herein provide a novel device for nanoscale metrology that permits quantitative measurements of indentation and related parameters better than is presently possible with commercially available tools. 
     Specifics of the invention will be described in greater detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is the new indenter module attached to an existing AFM. 
         FIG. 2  is the new indenter module placed in the MFP-3D AFM of Asylum Research Corporation. 
         FIG. 3  is the new indenter module prior to assembly. 
         FIG. 4  is the flexure mounted on the circular plate/retaining ring of the new indenter module. 
         FIG. 5  is the flexure of the new indenter module. 
         FIG. 6  is a cross sectional view of the mechanical converter system of the new indenter module. 
         FIG. 7  is a perspective view from the top of the mechanical converter system of the new indenter module. 
         FIG. 8  is another new indenter module prior to assembly. 
         FIG. 9  is a bottom-side plan view of the indenter module of  FIG. 8  after assembly. 
         FIG. 10  is a plan view of the planar leaf springs of the assembled flexure of indenter module of  FIG. 8 . 
         FIG. 10  through  FIG. 15  are plan views of planar leaf springs of other assembled flexures for other indenter modules. 
         FIG. 16  is another new indenter module prior to assembly. 
         FIG. 17  is a bottom-side plan view of the indenter module of  FIG. 16  after assembly. 
         FIG. 18  is a plan view of the planar leaf springs of the assembled flexure of indenter module of  FIG. 16 . 
         FIG. 19  is a perspective view of an optical lever detection system designed to substitute for the mechanical converter system depicted in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     One preferred embodiment of the current invention is depicted in  FIG. 1 , showing a cross-sectional view of a module embodying the invention which is installed in an AFM in place of the cantilever holder. Panels (A) and (B) of  FIG. 1  each show the module rotated 90 degrees about the vertical central shaft of the module which culminates in the indenter probe  303  positioned over a stage  307  on which a sample could be attached. This embodiment of the invention allows it to take direct advantage of some or all of the existing sensors and structures of an AFM. 
       FIG. 2  shows the module embodying the invention  12  installed in a Molecular Force Probe-3D AFM from Asylum Research Corporation (only the head  13  of the MFP-3D is shown). In the configuration depicted in  FIG. 2 , the indenter module  12  is removable and makes use of the actuators, sensors and optics of the head  13 , In particular, the z-piezo  14  of the MFP-3D is used to actuate the indenter probe  303  and the z-LVDT sensor  17  of the MFP-3D may be used to measure the displacement of the indenter probe  303 . The position sensitive detector  16  of the MFP-3D is used to measure the motion of the flexure controlling the displacement of the indenter probe  303 . Using data from these sensors, it is possible to quantify the displacement and force acting between the indenter probe  303  and a sample (not shown). 
     Another useful feature of the indenter module  12  being installed in the MFP-3D head  13  is that the module can use certain optical features of the head for providing an optical view of the indenter probe  303  and the sample. The top-view objective  18  and steering mirror  19  of the MFP-3D head  13  work with the prism of the indenter module  12  to provide an optical view of both the indenter probe  303  and the sample (not shown), as well as to illuminate both. This is of great utility for aligning the indenter probe  303  with particular structures on the sample. 
     The indenter can similarly be installed in AFMs manufactured by other companies and, with modifications, in yet other AFMs. The MFP-3D of Asylum Research Corporation is particularly amenable to conversion to an indenter because the actuators, sensors and optics located in its head are appropriate and convenient for indenter purposes. The same is true of other AFMs, or could be made to be true with modifications. 
     Except where clearly stated otherwise, the remainder of this detailed discussion of the current invention is directed at an indenter module that might be installed in any AFM, not just the MFP-3D of Asylum Research Corporation, as well as to a stand-alone indenter that is not a component of an AFM. 
       FIG. 3  shows a perspective view of the module embodying the invention prior to assembly. The indenter probe  303  is mounted on a removable chuck  304 . The chuck  304  allows the user to employ a variety of standard and custom indenter probes and to change easily from one probe to another. As is well-known to those versed in the art, these probes are formed of different materials, including diamond, tungsten, silicon nitride and others. The chuck  304  is attached to a monolithic three-dimensional leaf spring flexure  305  which is designed to constrain the motion of the indenter probe  303  to the z-axis only, that is, perpendicular to the sample. Precluding motion in the other axes is a major contributor to the results available with the invention. The flexure  305  is rigidly attached to the bottom of a circular plate with a retaining ring  310  on its top which is designed to mount into an AFM in place of the cantilever holder. This permits the actuator (not shown), which in an AFM would be used to change the position of the cantilever base in response to changes in the deflection or oscillation of the cantilever, to be used to actuate the circular plate/retaining ring  310  and through it to actuate the flexure  305 , the chuck  304  and the indenter probe  303 . Measurement of this actuation may be improved through the use of a sensor (not shown) to measure the displacement. 
     As shown in  FIG. 3 , the module embodying the invention may also include a prism  306  to permit an oblique view of the sample (not shown) on the sample holder  307 , the indenter probe  303  and their interface where the AFM in which the module is mounted includes an optical view system. The module may also include a dust cover  308  to protect the flexure  305 , the circular plate/retaining ring  310  and the prism  306  from external contamination and prevent damage from handling. The dust cover  308  may also provide mechanical hard stops (not shown) to prevent the flexure  305  from being overextended. A shake piezo (not shown) to oscillate the flexure  305 , and thus the chuck  304  and the indenter probe  303 , when the measurement is to be made in an AC mode could be added at the point where collar  320  screws into the flexure  305  or alternatively could be attached to the circular plate/retaining ring  310 . 
       FIG. 4  is a perspective view of the flexure  305  mounted on the circular plate/retaining ring  310 , with the chuck  304 , the indenter probe  303  and the prism  306  shown assembled. The flexure  305  is a monolithic, and three-dimensional leaf spring with three supporting ends  403 , a flexing portion, a central shaft  404 , which is free to move in response to the application of force. The supporting ends  403  are each rigidly attached to the circular plate/retaining ring  310 . The chuck  304  is rigidly attached to one end of the flexing portion, the central shaft  404 , and the indenter probe  303  is attached to the chuck  304 , facing the sample to be indented. 
       FIG. 5  is a second perspective view of the flexure  305 , absent the other items shown in  FIG. 4 . Such flexures are fabricated using metals and machining methods, including electronic discharge machining, well-known to those versed in the art. 
       FIG. 6  shows a cross-sectional view of the central shaft  404  of the flexure  305 , with the chuck  304  attached to one end (and the indenter probe  303  attached to the chuck  304 ) and a collar  320  attached to the other end which is inserted through a hole in the center of the circular plate/retaining ring  310  and gives access to the central shaft  404  from within the AFM to enable a mechanical converter assembly. The purpose of the mechanical converter assembly is to convert the zaxis linear motion of the central shaft  404 , chuck  304  and indenter probe  303  into an angular change that an optical lever detector system, which in an AFM would be used to detect changes in the deflection or oscillation of the cantilever, can measure. When the indenter module is mounted into an AFM in place of the cantilever holder and the circular plate/retaining ring  310  is actuated, thereby displacing flexure  305 , chuck  304  and indenter probe  303  toward the sample, one end of a second flexure, planar flexure  314 , which is rigidly attached to the circular plate/retaining ring  310  is also displaced. At the same time, the other end of planar flexure  314 , which on one side is linked though a ball bearing mechanism  318  to the central shaft  404  and on the other side is attached to a wedge-shaped mirror mount  315 , and is free to flex is also displaced, with the displacement following that of the central shaft  404 . This displacement of the flexing end of the planar flexure  314  tilts the mirror mount  315 , and mirror  309  attached to the mount, and thereby steers the light beam of an optical lever detection system (not shown) from one place on the position sensitive detector of the system to another thereby providing a measure of the displacement of the indenter probe  303 . A small permanent magnet  324  keeps the mirror mount  315  and planar flexure  314  in contact with the ball bearing surface  318 . 
     The design of the mechanical converter assembly allows the motion of the indenter tip  303  to be measured in much the same manner as cantilever tip deflection or oscillation are measured in a conventional AFM. Together with the use of the z-axis actuator previously discussed, this allows the indenter module to be easily swapped with the cantilever holder of an AFM, resulting in a unique, useful and versatile instrument that allows the user to bring the functionality of an AFM to bear on indenting. 
       FIG. 7  shows a perspective view from above of the mechanical converter assembly discussed in the preceding paragraph. The two ends of a planar flexure  314 , the support end  316  rigidly attached to the circular plate/retaining ring  310 , and therefore displaced in tandem with actuation of the circular plate/retaining ring  310 , and a flexing end  317  linked though a ball bearing mechanism  318  to the central shaft  404  and also attached to the mirror mount  315 , and therefore displaced in tandem with the central shaft, are displayed with greater clarity.  FIG. 7  also shows the hard stops  311  built into the mechanical converter assembly to prevent the flexure  305  from being overextended or damaged by normal handling. 
     The planar flexure  314  has a very high compliance when compared to the indenting flexure  305 , that is, the spring constant of the planar flexure  314  is very low compared to the spring constant of the indenting flexure  305 . 
     It will be observed that the indenting flexure  305  plays a major role in the performance of the preferred embodiment of the current invention which has just been discussed. A second preferred embodiment, employing a different indenting flexure but, like the first preferred embodiment, designed to be installed in an AFM in place of the cantilever holder in order to make use of the actuators, sensors and optics of the AFM is depicted in  FIG. 8 , a perspective view of the module embodying the invention prior to assembly. 
     Unlike the monolithic three-dimensional leaf spring flexure  305  of the first preferred embodiment, the flexure of  FIG. 8  is an assembled flexure. Like that flexure, however, the assembled flexure of  FIG. 8  is designed to constrain the motion of the indenter probe  303  to the z-axis only, that is, perpendicular to the sample. The flexing portion of the assembled flexure of  FIG. 8  is provided by the central shaft of two circular, planar leaf springs  506  and  507 , which are firmly constrained at the support end, that is the perimeter, by interleaved clamps,  503 ,  504  and  505 . The assembly of planar leaf springs  506  and  507  and interleaved clamps,  503 ,  504  and  505  is rigidly attached to the circular plate/retaining ring  310 . The central shaft of the planar leaf springs  506  and  507  is composed of spindles  508  and  509 . The upper end of spindle  508  screws into the lower end of spindle  509 , through holes  514 , clamping planar leaf spring  506  against the stop at the lower end of spindle  509 . The stop at the upper end of spindle  509  is fastened to planar leaf spring  507  and the portion of the spindle above the stop extends through a hole in the center of the circular plate/retaining ring  310  and gives access to the central shaft from within the AFM to enable the mechanical converter assembly discussed above in connection with the first preferred embodiment to function. 
     As with the first preferred embodiment, the central shaft of the assembled flexure of  FIG. 8  ends with the chuck  304 , rigidly attached to the lower end of spindle  508 , and the indenter probe  303  attached to the chuck  304 , facing the sample to be indented. The assembled flexure, the chuck  304  and the indenter probe  303  are actuated by actuating the circular plate/retaining ring  310  with the actuator (not shown) which in the AFM would be used to change the position of the cantilever base in response to changes in the deflection or oscillation of the cantilever. 
       FIG. 9  is a bottom-side plan view of the flexure of  FIG. 8  after assembly.  FIG. 10  is a plan view of the planar leaf springs  506  and  507  of the flexure of  FIG. 8 , showing different parts of the springs. Such springs are fabricated using metals, including beryllium copper, and machining methods well-known to those versed in the art. 
     Other preferred embodiments of the current invention, also designed to be installed in an AFM in place of the cantilever holder in order to make use of the actuators, sensors and optics of the AFM, but employing different indenting flexures than indenting flexure  305  of the first preferred embodiment or the assembled flexure of  FIG. 8 , form part of the invention.  FIG. 10  shows springs  604 , holes  605  and central shaft  606 .  FIG. 11  shows other springs  607 , holes  608  and central shaft  609 .  FIG. 12  shows other springs  610 , holes  611  and central shaft  612 .  FIG. 13  shows the springs  805 , central connection  807  and holes  806 .  FIG. 14  shows springs  808 , central receiving shaft  807  and holes  806 .  FIG. 15  shows the shaft  807 , springs  809  and holes  806 . Altogether,  FIG. 11  through  FIG. 15  are plan views of the planar leaf springs of such other preferred embodiments. Each of them consists of a flexing portion in the center of the spring, support ends at the perimeter and connections from the flexing portions to the support ends of variously shaped beams. In addition to the circles and other depicted shapes of these planar leaf springs, any polygon would in general produce similar results when paired with appropriate connections. Holes  806  and the shaft  807  are also shown in  FIGS. 16 and 18  Another embodiment showing springs  810  is also shown in  FIG. 18 . 
     An additional preferred embodiment, again employing a different indenting flexure but, like the other preferred embodiments disclosed above, designed to be installed in an AFM in place of the cantilever holder in order to make use of the actuators, sensors and optics of the AFM is depicted in  FIG. 16 , a perspective view of the module embodying the invention prior to assembly. Like the flexure of  FIG. 8 , the flexure of  FIG. 16  is an assembled flexure and is designed to constrain the motion of the corresponding probe  703  (which is also an optical element) to the zaxis only, that is perpendicular to the sample. 
     The components of the assembled flexure of  FIG. 16  are similar to those of the assembled flexure of  FIG. 8 : two planar leaf springs  707  and  708  separated by clamps  705  and  706 . However, the assembled flexure of  FIG. 16  does not have a third clamp separating the upper planar leaf spring  708  from the plate  709  which facilitates installation of the assembly in an AFM in place of the cantilever holder. Instead, planar leaf springs  708  is rigidly attached directly to the plate  709 . 
     The central shaft of the planar leaf springs  707  and  708  of the assembled flexure of  FIG. 16  also differs from the central shaft of the assembled flexure of  FIG. 8 . The central shaft of the planar leaf springs  707  and  708  consists of just a chuck  704 , and the probe  703  attached to the chuck  704 , facing the sample to be indented. The upper end of the chuck  704  extends through a hole in the center of the plate  709  and gives access to the central shaft from within the AFM to enable the mechanical converter assembly discussed above in connection with the first preferred embodiment to function. 
     The probe  703  of the assembled flexure of  FIG. 16  is a new device not currently known in the art and as such is significantly different from the indenter probe  303  of the assembled flexure of  FIG. 8 . Probe  703  has both a different function than indenter probe  303  and a different construction. Probe  703  has a dual function: it is both an optical element and an indenting probe. As an optical element, probe  703  acts as a lens allowing the optical viewer system of the AFM head in which assembled flexure of  FIG. 16  is installed with a direct optical view of the sample to be indented. As an indenting probe, probe  703  is a rigid element that applies force and indents the sample as with traditional indenter probes, such as  303 . With this dual function comes a requirement for a different construction. A probe like probe  703  must be fabricated from a material which is both very hard and transparent to light. Suitable materials include diamond and sapphire. 
     Probe  703  has many advantages over other indenter transducers or indenting probes. These include making available a true top optical view of the sample to be indented, allowing for accurate positioning of indentations and having a combination of low mass and high resonant frequency, which allows unprecedented resolution in the measurement of mechanical behavior of materials through both static and dynamic material testing methods. 
       FIG. 17  is a bottom-side plan view of the flexure of  FIG. 16  after assembly.  FIG. 18  is a plan view of the planar leaf springs of the flexure of  FIG. 16 . Such springs are fabricated using metals, including beryllium copper, and machining methods well-known to those versed in the art. 
       FIG. 19  shows an optical lever detection system designed to free the modules embodying the current invention from the use of the mechanical converter assembly described above in order to access the optical lever detection system of an AFM. A mirror  1901  is attached to the collar  320  which is rigidly attached to central shaft  404 , chuck  304  and indenter probe  303  of the flexure of the first preferred embodiment of the invention. Mirror  1901  deflects an incident light beam coming into the optical lever detection system and as this mirror moves relative to the indenter probe  303 , a PSD of the optical lever detection system measures the displacement. An additional lens  1902  may be placed above mirror  1901  to increase sensitivity and facilitate calibration of the optical lever detection system. 
     The described embodiments of the invention are only considered to be preferred and illustrative of the inventive concept. The scope of the invention is not to be restricted to such embodiments. Various and numerous other arrangements may be devised by one skilled in the art without departing from the spirit and scope of the invention.