Patent Application
20080266575
Not Applicable
The present invention relates to a hybrid scanning system that utilizes techniques for cantilever movement detection that directly measure the change in disposition of the cantilever including the tip height, tip rotation at one or more points on the cantilever thereby providing a partial three dimensional reconstruction without the need for actuating the cantilever. This invention covers the use of displacement meters such as confocal and triangulation displacement meters to detect the out-of-plane (vertical) and lateral movements of a cantilever or cantilever arrays in order to produce a topographical and friction/surface roughness map of a specimen in contact with a cantilever equipped with a sharp tip. This invention also covers multiple light beams on a cantilever for the reconstruction of the cantilever's disposition. Finally, this invention covers the usage of these techniques for indentation measurements. This system may be used with specialized cantilevers to measure a number of additional properties such as thermal properties.
The object of the present patent is to describe a method for improving the spatial resolution of displacement meters including laser confocal and triangulation displacement meters by using a cantilever to detect topographical variations, without feedback control of the cantilever position. The terms triangulation meter instead of triangulation displacement meter and confocal meter instead of confocal displacement meter may be used interchangeably. Topographical variations are measured by directly determining the change in disposition of the cantilever to provide a three dimensional topographical reconstruction. Triangulation displacement meters, or alternatively, confocal displacement meters are used to measure the vertical (out-of-plane) movement of the cantilever from the light reflected off the cantilever, and correlate it to the horizontal movement of the cantilever on the sample.
Laser confocal and triangulation displacement meters for measuring topographic variations have matured enough to allow nanometer resolution in the vertical (out-of-plane) direction. Laser confocal displacement meters have been described in literature and are commercially available. Both types of instruments find applications in a number of areas including thickness measurements, alignment, topography measurements, step height measurements, flatness measurements, profile measurements etc. However, these techniques provide inadequate spatial resolution for many applications due to the laser beam diameter, which is typically larger than 2 μm and often about 20 μm.
Scanning probe methods developed within the last two decades offer high-resolution images of sample properties. Scanning probe microscopes (SPM) measure properties at localized spots, such as: topography, thermal conductance, temperature, capacitance, optical absorption, or magnetism. They all use a cantilever with a sharp tip at a very close proximity or in contact with the sample. This close proximity allows for very high resolution. The image is formed by scanning a cantilever over the sample while measuring the desired property. Unlike light based microscopes such as laser confocal and laser triangulation displacement meters, scanning probe microscopes are not wavelength limited; hence their resolution is limited only by the size of the tip at the edge of a cantilever and not by the diffraction effects of light.
The atomic force microscope (AFM) is one of many types of SPM. AFM's employ: a cantilever, a light source, an electronic feedback circuit controlling the out-of-plane (vertical) position of the cantilever, an X-Y-Z piezoelectric transducer, and a photodetector. As the cantilever moves horizontally relative to the sample, topographical variations of the sample change the light reflected off the cantilever. A four-quadrant detector measures the reflection. A closed loop piezoelectric feedback control controls the vertical position of the cantilever. The feedback of the AFM counteracts the signal produced by the four quadrants detector. The cantilever or the sample is moved to maintain the cantilever and the light reflected from it at a constant angle. In almost all SPM's, cantilever positioning is achieved with piezoelectric transducers such as cylindrical piezotubes. Applying a voltage between electrodes of the piezotube causes the length of the tube to change with a limited maximum motion along the tube axis depending on the tube length.
A combination of confocal or triangulation displacement meters with a cantilever can operate without the need for closed loop piezoelectric feedback control. This type of arrangement would allow for improved spatial resolution of confocal or triangulation displacement meters without the additional complexity of an AFM. The cantilever can be easily added and separated from the displacement meter. Confocal and triangulation displacement meters have been used with a cantilever to produce topographical surface maps by the inventors.
Triangulation displacement systems have been widely used. Their use has also been reported in the semiconductor industry for a number of applications including: inspection, quality control, and defect detection of integrated circuits during various manufacturing stages, measuring the change in thickness of a wafer and other planarizing parameters in processes such as chemical-mechanical polishing, and inspection of chip packages.
Triangulation displacement meters measure the position of an object by tracking the light reflected from the target surface. A light beam, typically laser (including a superluminescent laser or diode), is projected on an object. Other light sources such as collimated light or room light may be used. The reflected beam is focused through a lens on a light-receiving element (photodetector) such as a position sensitive device (PSD) or charge coupled device (CCD). As the scan of the sample progresses, variations in the sample topography lead to variations in the position of the reflected signal as measured by the photodetector. A number of mathematical algorithms can be used to calculate the topography from the change in the signal on the photodetector and from the geometry of the set-up. When the term triangulation displacement meter or triangulation meter is used herein it refers to a system that uses any type of light source reflected on a photo-detector to track displacement changes.
Confocal meters have been used for a number of applications, including: surface characterization, measuring the position of micro objects, highly reflective surface measurements, MEMS devices evaluation, characterization of biological structures, and measurement of solder, gold, and stud bumps.
In a typical laser confocal displacement meter, a lens attached to a tuning fork focuses a laser beam on the surface of the sample. The tuning fork oscillates a lens rapidly in the vertical (out-of-plane) direction, focusing and defocusing the laser on the sample. The beam returned from sample is reflected by a half mirror and focused on a pinhole. A peak signal is formed on a receiving element when the focal plane coincides with the sample. A detector transforms the light signal to an electrical signal. Changes in surface reflectance do not affect the focal position and, therefore, the topography measurement.
The present invention addresses a hybrid scanning system (HSS). The HSS system consists of a cantilever or a cantilever array, a scanning stage, a light source, and instrumentation to synchronize and control the individual components. Detection of the cantilever or cantilever array movement is achieved by directly measuring the change in disposition of the cantilever including the cantilever height, cantilever rotation at one or more points on the cantilever thereby providing a partial three dimensional reconstruction without the need for actuating the cantilever or optomechanical feedback.
This has been achieved by employing a triangulation displacement meter to measure the out-of-plane (Z axis) movement of the cantilever from the captured reflected light, and correlates it to the in-plane (X-Y axis) movement of the cantilever at each point. Alternatively, the out-of-plane (Z axis) deflection can be measured with a confocal meter from the captured reflected light, and correlating it to the in-plane (X-Y axis) movement of the cantilever at each point. Devices with these improvements have numerous applications, including high throughput inspection, molecular measurements, microscopy and manipulation technology, lithographic manufacturing, nanometer scale surface profiling, and other aspects of nanotechnology.
The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention.
a and 3b show scans of 5 micron×5 micron squares using a triangulation meter to detect the out-of-plane (Z axis) deflection of a cantilever; and
a and 4b show images obtained with a hybrid confocal meter with a cantilever.
a illustrates angular measurement in order to determine twist of the cantilever using a confocal meter;
b illustrates angular measurement in order to determine twist of the cantilever using a triangulation meter;
In the following are detailed descriptions of the invention of exemplary embodiments. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, but other embodiments may be utilized and logical, mechanical, electrical, and other changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.
In the following description, numerous specific details are set forth to provide a thorough understanding of the invention. However, it is understood that the invention may be practiced without these specific details. In other instances, well-known structures and techniques known to one of ordinary skill in the art have not been shown in detail in order not to obscure the invention.
The present invention provides with a method for detecting the out-of-plane (Z axis) movement of a cantilever or of an array of cantilevers without optomechanical feedback, by directly measuring the change in disposition of the cantilever including the cantilever height, cantilever rotation at one or more points on the cantilever thereby providing a partial three dimensional reconstruction without the need for actuating the cantilever. This has been achieved with a triangulation meter used to measure the out-of-plane (vertical Z axis) movement of the cantilever based on changes in the reflected light, and correlate it to the in plane (X-Y axis) movement of the cantilever at each point. The out-of-plane (Z axis) deflection of a cantilever can also be measured with a confocal meter and correlated to the in-plane (X-Y axis) movement of the cantilever at each point.
Experimental results (presented below) demonstrate that cantilevers with sharp tips can be used to improve the spatial resolution of confocal and triangulation systems. In both cases the minimum detectable feature size was smaller than the laser beam diameter. In addition, the hybrid confocal and triangulation meter have no inherent limitations in the scanning area range and any limitations are due to the scanning stage, in contrast with an SPM, which is limited by the piezoelectric tube (a conventional SPM can usually scan an 80 μm×80 μm area).
Confocal and triangulation meters do not provide information about the sample's properties unlike scanning probe techniques. Embedded sensing elements in a cantilever would permit simultaneous piezo-electric, piezo-resistive, thermal, mechanical, electrical or magnetic properties measurement, providing additional information about the sample. The cantilever may also be an electrostatic actuating cantilever, a magnetic actuating cantilever, a near-field scanning optical microscopy (NSOM) cantilever, or four cantilevers with four tips for sheet resistance measurements. These types of cantilevers may form an array of similar cantilevers. The hybrid systems presented here enable the use of such cantilevers allowing for additional characterization.
Hence, along with topographical information, thermal information can be obtained with an interface circuit to bias both the cantilever tip temperature and read the tip temperature via changes in cantilever resistance. A scanning thermal cantilever probe is well suited for measuring many heat-related parameters, including sample temperature, thermal conductance, heat capacity, glass transition temperature, sub-surface mapping etc. Embedded sensing elements in/on a cantilever may be used for thermal imaging.
As the cantilever moves over a surface, changes in the chemical composition of the surface can give rise to torsions of the cantilever on which the cantilever is mounted. The torsion of the cantilever is then proportional to the friction between the cantilever's tip and the surface. The topographical image is derived from observing the normal forces and the lateral image by observing the torsional movements of the cantilever i.e. the twisting of the cantilever. Frictional force microscopy (FFM) has been used for nanometer inspections and industrial applications to optimize the etch conditions, to detect physical and chemical changes, and to characterize mechanical properties of thin films.
Displacement meters may be used for angular measurements in order to determine the twist of the cantilever. Angular changes are related to the lateral moment and therefore represent a map of the lateral forces on the cantilever due to surface roughness. Lateral measurements are enabled using a confocal meter by directing the laser beam at two locations of the cantilever and measuring the angular twist of the cantilever. This angle is related to the moment and therefore a map of the frictional force applied to the cantilever from scanning a specimen can be generated. Using triangulation methods the angle can be calculated by de-convoluting the photodetector's image into the out-of-plane (vertical Z axis) response and the lateral response. The cantilever's out-of-plane (Z axis) movement is represented on the photo-detector by a characteristic response such as a straight line. Any deviation from that characteristic response represents lateral movement, which can be calculated using trigonometry.
The SPM has been a very successful research tool, but emphasis has not been put on high throughput. Scan speeds of current SPM's are limited to about 180-250 μm/sec. Furthermore, piezoelectric transducers are designed to control a single cantilever. A combination of confocal or triangulation techniques with a cantilever (or an array of cantilevers ) does not require closed loop piezoelectric feedback control allowing for higher throughput and large area scanning, enabling the simultaneous use of multiple cantilevers. Higher throughput is very important in many applications from biological to semiconductor failure analysis and production applications, where entire wafers or large areas need to be examined in relatively short time with sub-micron resolution. The results suggest that these unique detection systems can be used for high-resolution large area high throughput topographical imaging.
The need for higher throughput is addressed using an ultra compliant cantilever or cantilever array. In an array multiple cantilevers scan in parallel a sample with minimal contact force and without mechanical feedback. In this approach integrated piezo actuators are not required. An array of cantilevers refers to a collection of identically cantilever placed in series and joined by a common segment. An array may be a one dimensional array referring to an array possessing only one line of cantilevers or a two dimensional array referring to several lines of cantilevers in parallel forming a two dimensional array.
High-throughput contact mode topography along side with other types of imaging like thermal imaging can be performed with ultra compliant cantilevers or cantilever arrays. This method allows for highly scalable cantilever arrays to scan in parallel and it is possible for many of those arrays to operate simultaneously on a sample increasing analysis speeds. This technique requires minimal sample preparation.
It is preferred that the cantilevers be highly compliant, therefore the cantilever's body may comprise of one of many types of materials or a combination of materials such as photoresist, SU-8, and polymers such as poly(dimethylsiloxane) (PDMS), polyimide, parylene, and elastomers such as silicone and rubber. Cantilevers made from these laterials have low spring constants and therefore are highly compliant.
Topography or the measurement of out-of-plane (Z axis) deflection of each cantilever in an array is achieved by using a triangulation meter or by confocal meter. These techniques are fundamentally different than conventional SPM techniques. In the proposed set-up there is no need for Z axis actuation or Z axis feedback while scanning. The cantilever or cantilever array is scanned across a sample making measurements at discrete points. The X-Y stage controls the position of the sample. In the proposed set-up, the sample is moved in the X-Y direction while the cantilever and detection system are kept fixed. As the sample is moved, any height changes in the out-f-plane (vertical Z direction) will cause the light beam, reflected from the cantilever, to strike the detector at different locations.
Using one or two-dimensional array of cantilevers, multiple light sources may be directed at the cantilever array. The light source may be produced by the use of one or more diffractive optical elements, including diffraction gratings to form a plurality of light beams, each with a selectable shape and intensity, from a single light source and directed on each cantilever. Alternatively, a fiber coupled laser diode array and related focusing optics may be used. Instead of a laser, collimated light may be used. This is light whose rays are nearly parallel. In addition, ambient light (room light) may also be used. Finally, any other type of light source may be used. These light sources may be combined with a cantilever or cantilever array having a miniature mirror on the top side of each cantilever so. The reflected light from each single cantilever is detected by a photodector such as a CCD.
Alternatively, a scanning laser may be employed, in which case one light source scans each cantilever in an array of cantilevers.
In an alternative confocal embodiment a laser confocal meter is focused on the cantilever(s). In this invention a cantilever or an array of cantilevers with sharp tip(s) is scanned across a sample or alternatively it is kept fixed while the sample is being scanned. Any surface variations can be measured by focusing the laser beam of the displacement meter onto the cantilever and measuring the cantilever's deflection that corresponds to the changes in surface height. Multiple cantilevers may be monitored by scanning one laser beam on each cantilever or by a multiprobe confocal 3-D detection system.
A digital feedback may be used. With this capability the topography of the sample is imaged in the mode described, then, the cantilever is lifted above the surface (the height may be determined by using the laser scanning device to scan the sample and the cantilever and determine the cantilever's distance from the sample). The cantilever is then scanned across the sample following the exact shape of the topographical information collected during the scan. This type of measurement might be useful for example in magnetic force imaging to detect differences in magnetization, since magnetic (or electrostatic) forces act over longer distances than atomic forces.
In this system the sample is moved in the X-Y plane by the scanner relative to the cantilever and the photodetector, while the cantilever and the photodetector are kept at a fixed position. However, the cantilever and the photodetector may also be moved in the X-Y plane by the scanner relative to a sample, while a sample is kept at a fixed position.
The instrument is suitable for scanning biological samples. The free end portion of the cantilever may be configured to be submerged in a liquid or in another manifestation the flexible cantilever and the sample surface are all submerged in a fluid.
With the HSS technique, piezoscanners are no longer essential. Other types of scanners that do not have travel range compromises such as motorized scanners or voicecoil scanners etc. can be used.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
Although this invention relates specifically to HSS measurements that use cantilever deflection as a measure of height changes, those skilled in the art will recognize that there are other physico-chemical properties that can be measured using substantially similar cantilevers, instrumentation, and algorithms. In addition, those skilled in the art will recognize that many scanning probe microscopy and atomic force microscopy techniques can be easily transferred and used with this system. Finally, those skilled in the art will recognize that cantilever-based instrument according to this invention; can be combined with an atomic force microscope, a scanning probe microscope, a confocal microscope, a scanning electron microscope, a conventional laser microscope or other types of metrology systems and analytical tools to provide additional functionality.
Referring to
Referring to
Referring to
a and 3b show scans of 5 micron×5 micron squares using a triangulation meter to detect the out-of-plane (Z axis) deflection of a cantilever. Using a cantilever with a sharp tip, features smaller than the laser beam 42, which is 20 micron, were clearly detectable. To obtain the scan in
Referring to
a illustrates an angular measurement in order to determine twist of the cantilever 22 using a confocal meter 39. A laser confocal displacement meter (confocal meter) 39 can directly measure the angle of twist of the cantilever 22 by scanning the laser beam 23 at two locations on the cantilever 22. This angle is related to the moment and therefore a map of the frictional force applied to the cantilever 22 from scanning a specimen 7 can be generated.
b illustrates an angular measurement in order to determine twist of the cantilever 22 using a triangulation meter. Using a triangulation meter the angle of twist of a cantilever can be measured by monitoring the signal location 24 on the detector 25 which depends on the out-of-plane (Z axis) movement of the cantilever 22 and the interfacial friction force between tip 26 and sample 7. The cantilever's out-of-plane (Z axis) movement is represented on the photodetector 25 by a characteristic response such as a straight line. Any deviation from that line represents lateral movement, which can be calculated using trigonometry.
Now referring to
An image of the surface topography is generated using the techniques mentioned above. The cantilever tip is situated at a location of interest on the sample 7 and an indentation is made. Following the indentation, a topographical image can be made to verify the indentation's size and location. Furthermore, a cantilever array may be used, in which case, multiple transducers 29 for force actuation can be used on each cantilever 27 of the array, while the displacement is measured by the techniques described previously. Nanoindentation is used to determine the mechanical properties of materials. It may be used to map the spatial distribution of a cell's mechanical properties, reflecting the structure of the cytoskeleton of a cell.
Thus, it is appreciated that the optimum dimensional relationships for the parts of the invention, to include variation in size, materials, shape, form, function, and manner of operation, assembly and use, are deemed readily apparent and obvious to one of ordinary skill in the art, and all equivalent relationships to those illustrated in the drawings and described in the above description are intended to be encompassed by the present invention. Furthermore, other areas of art may benefit from this method and adjustments to the design are anticipated. Thus, the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.
This application claims priority from U.S. Provisional Patent Application Ser. No 60/926,422, entitled “High throughput scanning probe apparatus and method”, filed on Apr. 27, 2007.
| Number | Date | Country | |
|---|---|---|---|
| 60926422 | Apr 2007 | US |
