AFM Imaging with Creep Correction

Abstract

An atomic force microscope (AFM) and method of operating the same includes a separate Z height sensor to measure, simultaneously with AFM system control, probe sample distance, pixel-by-pixel during AFM data acquisition. By mapping the AFM data to low resolution data of the Z height data, a high resolution final data image corrected for creep is generated in real time.

Claims

1. A method of atomic force microscopy (AFM), the method comprising: providing relative scanning motion between a probe and a sample in a region of interest of the sample to perform a data scan having a fast scan axis and a slow scan axis; detecting probe deflection during the providing step, probe deflection being indicative of topography of a surface of the sample; generating surface data based on the detecting step; measuring, simultaneously with the detecting step, Z height between the probe of the AFM and the sample; generating Z height data based on the measuring step; extracting low resolution topography reference data from the Z height data; mapping the surface data to the low resolution topography reference data; and generating a final data image based on the mapping step.

2. The method of claim 1, wherein the generating a final data image step is performed in real time during the providing and detecting steps.

3. The method of claim 1, wherein the extracting step is performed in the slow scan axis of the data scan.

4. The method of claim 1, wherein the measuring step is performed with a sensor coupled to a head of the AFM.

5. The method of claim 4, wherein the sensor is supported by a actuator mounted in the AFM head.

6. The method of claim 4, wherein the sensor is a capacitance sensor.

7. The method of claim 4, wherein the actuator is a piezoelectric tube scanner.

8. The method of claim 1, wherein the low resolution topography reference data defines a reference plane.

9. The method of claim 1, wherein the detecting step is performed in one of tapping mode, peak force tapping (PFT) mode and contact mode.

10. An atomic force microscope (AFM) comprising: a scanner that provides relative scanning motion between a probe of the AFM and a sample in a data scan having a fast scan axis and a slow scan axis; a detector that measures a deflection of the probe in response to probe-sample interaction during AFM operation, the deflection being indicative of sample topography and stored as surface data; a sensor to measure Z height between the probe and the sample simultaneously with measuring probe deflection, Z height being stored as Z height data; and a processor that extracts reference data from the Z height data and maps the surface data to the reference data to generate a final data image of the sample.

11. The AFM of claim 10, wherein the reference data corresponds to the slow scan axis of the data scan and is low resolution topography data.

12. The AFM of claim 10, wherein the processor maps the surface data to the reference data in real time during AFM operation.

13. The AFM of claim 10, wherein the scanner is a piezoelectric tube scanner.

14. The AFM of claim 10, wherein the sensor is a capacitance sensor coupled to the scanner.

15. The AFM of claim 11, wherein the AFM is operated in one of peak force tapping (PFT) mode, contact mode and tapping mode.

16. The method of claim 1, wherein the mapping step maps the surface data to the reference data to compensate for the effects of AFM creep in the surface data.

17. The AFM of claim 10, wherein the processor maps the surface data to the reference data to compensate for the effects of AFM creep in the surface data.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Preferred exemplary embodiments of the invention are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which:

[0031] FIG. 1 is a schematic illustration of a Prior Art atomic force microscope (AFM);

[0032] FIG. 2 is a block diagram of an AFM system for acquiring AFM data and correcting for creep without post-data acquisition processing;

[0033] FIG. 3 is a flow chart illustrating a creep correction method of the preferred embodiments;

[0034] FIG. 4 is a raw AFM topography image, without creep correction;

[0035] FIG. 5 is a plot illustrating the effects of creep on AFM data shown in FIG. 4;

[0036] FIG. 6 is a height image acquired with a separate Z height sensor to measure probe sample separation independent of AFM control;

[0037] FIG. 7 is a plot corresponding to the height image of FIG. 6, similar to that in FIG. 5, illustrating height data immune to creep;

[0038] FIG. 8 is a low resolution height image extracted from the Z sensor height image of

[0039] FIG. 6;

[0040] FIG. 9 is a plot corresponding to FIG. 8, illustrating the low resolution height data;

[0041] FIG. 10 is a composite high resolution topography image resulting from employing the real time creep correction method of the preferred embodiments; and

[0042] FIG. 11 is a plot corresponding to FIG. 10 illustrating creep correction of the AFM data.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] The preferred embodiments are directed to a creep correction method and system for Atomic Force Microscopy (AFM) which allows for minimizing the adverse effects of system creep in “Z” on AFM data, without post-image processing. The methods described herein include generating two sets of height data, one from the AFM control signal (standard AFM image) and one from a separate Z sensor coupled to the Z piezo actuator and measuring the distance from the piezo to the sample. While the Z command signal “creep” is primarily affected by the piezo response, which has both a command duration and temperature dependent sensitivity, to an input control voltage. The height measurement provided by the Z sensor is immune to system creep because the z sensor measures the physical displacement of the scanner to the sample surface which is primarily affected by a mechanical distance change of the scanner to the sample and to only a negligible amount by either command duration or temperature.

[0044] A scanning probe microscope instrument 150 (e.g., AFM) according to a preferred embodiment is shown in FIG. 2. In this embodiment, a probe 152, having a tip 154 extending from a distal end of a cantilever 155 is held by a probe holder (not shown) that supported by an actuator, such as a piezoelectric tube scanner 156. Scanner 156 is a “Z” or vertical scanner responsive to sample properties in the closed loop control system to position the tip 154 relative to a sample 158 during AFM imaging (alternative Z actuators may be provided, including self-actuated probes). In this embodiment, tube scanner 156 is coupled to an XY scanner 160, preferably also a piezoelectric tube, that is used to raster the probe tip 154 relative to the sample surface during AFM operation. A mechanical Z-stage 162 is employed for providing large movement in Z between tip 154 and sample 158, for example, during AFM start-up to engage tip 154 and sample 158 prior to operation and image acquisition. Tube scanners 156, 160 and mechanical Z stage 162 are mounted as part of the AFM head (not separately shown). Sample 158 is mounted on an XY stage 164 that primarily provides coarse XY motion to position probe 152 at a region of interest of sample 158. An XY stage controller 166 controls stage 164 to locate the probe/sample at that region of interest. However, stage 164 may be configured to provide relative scanning motion between tip 154 and sample 158 at a selected scan speed, depending on the application. Controller 174 provides the instructions to position the image scan at a region of interest. Controllers 166, 174 are implemented by a computer 180.

[0045] AFM 150 also includes a Z metrology or position sensor 190 for sensing the distance between scanner 156 (probe coupled thereto) and sample 158, i.e., Z height. Sensor 190 is preferably a capacitive sensor, but other suitable options may be used. Because the height sensor is primarily sensitive to the physical distance change of the scanner to the sample, and not the varying response of the piezo to the incoming control signal, which is responsible for creep, the height measurement is immune to the creep artifact.

[0046] In operation, after tip 154 is engaged with sample 158, a high speed scan of the sample is initiated with XY scanner 160 in an AFM mode of operation (e.g., PFT mode), as discussed previously. As tip 154 interacts with the surface of sample 158, the probe 152 deflects and this deflection is measured by an optical beam-bounce deflection detection apparatus 168. Apparatus 168 includes a laser 170 that directs a beam “L” off the backside of cantilever 155 and toward a photodetector 172 which transmits the deflection signal to, for example, a DSP 176 of AFM controller 174 for high speed processing of the deflection signal.

[0047] AFM controller 174, via DSP 176, continuously determines a control signal according to the AFM operating mode, and transmits that signal to the piezo tube 156 to maintain the Z position of probe 152 relative to sample 158, and more specifically, to maintain deflection of the probe at the feedback setpoint. Controller 174 also implements the real-time processing of the data acquired from both the AFM control and the Z height sensor. This real time processing is further illustrated in the method shown in FIG. 3.

[0048] Turning to FIG. 3, a method 200 of real-time correction of the creep effect in AFM image data is shown. In Block 202, the AFM tip is engaged with the sample surface at a region of interest. Then a scan of the sample is initiated in Block 204 in a selected mode of AFM operation. In addition to the Z command position signal (a voltage, indicative of topography), the sensed height by the Z metrology sensor (190 in FIG. 2, e.g., capacitive sensor) is captured and recorded. This data is acquired simultaneously at each scan location. In Block 206, method 200 extracts low-resolution topography reference data from the Z metrology sensor. This is the low resolution portion of the height data acquired by sensor 190 (i.e., the sensor height data extracted in the slow scan axis of the data scan). This data provides a reference plane of the region of interest for subsequent removal/minimization of time/voltage dependent errors in position that do not relate to the physical sample surface.

[0049] The creep correction method 200 then includes, in Block 208, using the low-resolution topography reference data to achieve creep correction. More particularly, the surface data of corresponding to the AFM control signal, i.e., the fine feature Z command voltage, is superimposed/mapped on the low-resolution topography reference image data, line by line, in real time. By superimposing or mapping (e.g., subtracting) the surface data of the data scan to the low-resolution topography reference data (defining a reference plane), method 200 corrects for creep in the acquired AFM data. In Block 210, the method generates an image of the sample surface using the “creep corrected” data. As a result, AFM resolution is improved in real time. This also allows for real time adjustment of scan parameters to achieve improved AFM data quality. Method 200 and exemplary data are shown and described below in connection with FIGS. 4 and 5, FIGS. 6 and 7 along with a low resolution Z height image in FIG. 8 (showing transitions in FIG. 9). FIGS. 10 and 11 illustrate the creep corrected AFM image.

[0050] More particularly, turning initially to FIG. 4, a conventional AFM image of Z control signal 300 is shown in which a sample is imaged in a selected mode of AFM operation. This image includes the creep artifact in the data, illustrated in the graphs of the data in FIG. 5. As expected, the creep artifact compromises image detail/resolution. FIG. 5 is a graph 302 of a bare silicon wafer, where the plotted data shows the line to line z height variation on the Y axis, and the slow scan axis line position in the image on the X axis. The creep artifact associated non-linear change in Z position is highlighted in a region 304. Turning to FIG. 6, a height image generated from the output of Z sensor 190 (FIG. 2) is shown. In this case, details of the sample surface are not compromised by creep for the reasons noted previously. This is illustrated in FIG. 7. In FIG. 7, a graph of the line-to-line z height variation on the Y axis, and the slow scan axis line position in the image on the X axis does not show the abrupt non-linear change in position as highlighted by region 314. Sensor 190 data is time and voltage independent and therefore does not exhibit the 2.sup.nd order “creep” error.

[0051] Next, as part of method 200 of the preferred embodiments, data corresponding to the low resolution portion of the height data acquired by sensor 190 is extracted. This image 320 shown in FIG. 8, and its data plotted in graph 322, shown in FIG. 9, is used as a topography reference (Block 206 in FIG. 3). As described previously, this topography reference 320 is free of creep effects. The fine feature Z command voltage AFM data (FIG. 4) is then mapped to the topography reference 320 to correct for creep in the AFM image 300. The result is shown as high resolution AFM image 330 in FIG. 10. Similar to the data 312 plotted in FIG. 7 for the height measurement of the sample surface using sensor 190 of system 100 (FIG. 2), creep corrected data 332 similarly no longer includes the 2.sup.nd order creep error. Resolution of surface features is significantly improved, as shown in the referenced figures by removal of approximately 5 nm of creep artifact induced false sample topography. In a typical case, 1-10 nm of creep artifact may be successfully removed using the techniques described herein, significantly enhancing the resolution of surface features over a range of samples.

[0052] In sum, the preferred embodiments are directed to a method and apparatus that provide high resolution AFM images substantially free of 2.sup.nd order creep error without the need for post-processing of the image. The creep correction technique is fully automated, being performed in real time while the image is being acquired—no creep is visible as the image is being acquired. AFM system creep is suppressed and no additional image distortion is introduced.

[0053] Although the best mode contemplated by the inventors of carrying out the present invention is disclosed above, practice of the above invention is not limited thereto. It will be manifest that various additions, modifications, and rearrangements of the features of the present invention may be made without deviating from the spirit and the scope of the underlying inventive concept.