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Tip Shape Issues

Rev. F

Scanning Probe Microscope Training Notebook

19.0 Tip Shape Issues

The SPM image is a result of the interaction of the tip shape with the surface topography.

There are two primary features of the tip which affect the SPM image: the radius of curvature and
the tip sidewall angles.

The smaller the radius of curvature, the smaller the feature that can be resolved. A sharper tip will
be able to laterally resolve smaller features than a dull tip with a larger radius of curvature. This can
be seen in the example below.

Figure 19.0a

Schematics and Image Profiles of Spheres Scanned with a Sharp (left) and Dull (right) Probe

The accumulation of debris on the end of the tip can also dull the tip and result in image distortion,
as shown below.

Figure 19.0b

Schematic and Image Profile of Trenches Scanned with a Dirty Tip

Note:

A dull or dirty tip may not affect the measurement of the vertical dimensions of
these samples.

Summary of Contents for 004-130-000

Page 1: ...t and Format Update N A C Kowalski Rev D 08 05 2003 Overall Content and Format Update N A L Burrows Rev C 07 30 2003 Content Update N A L Burrows Rev B 08 01 1998 Format Update N A C Fitzgerald Rev A 05 09 1997 Initial Release N A J Thornton Copyright 2003 2011 Bruker Corporation All rights reserved SPM Training Notebook 004 130 000 standard 004 130 100 cleanroom ...

Page 2: ...ied reproduced or translated into another language without prior written consent Copyright Copyright 2003 2011 Bruker Corporation All rights reserved Trademark Acknowledgments The following are registered trademarks of Bruker Corporation All other trademarks are the property of their respective owners Product Names NanoScope MultiMode Dimension BioScope Atomic Force Profiler AFP Dektak Software Mo...

Page 3: ... AFM and Non contact Mode AFM 12 6 1 Contact Mode AFM 12 6 2 TappingMode AFM 12 6 3 Non contact Mode AFM 13 7 0 Piezoelectric Scanners How They Work 14 8 0 Piezoelectric Scanners Hysteresis and Aging 16 8 1 Hysteresis 16 8 2 Aging 18 9 0 Piezoelectric Scanners Creep and Bow 19 9 1 Creep 19 9 2 Bow 20 10 0 Probes 21 10 1 Silicon Nitride 21 10 2 Silicon 22 11 0 Types of SPM Probes 23 11 1 Contact Mo...

Page 4: ... 15 2 Mount Probe 32 15 3 Select Scanner 32 15 4 Mount Sample 32 15 5 Place Optical Head on Scanner 32 15 6 Align Laser and Tip Sample Approach 2 Methods 33 15 7 Adjust Photodiode Signal 34 15 8 Cantilever Tune TappingMode Only 34 15 9 Set Initial Scan Parameters 34 15 10 Engage 34 15 11 Adjust Scan Parameters 35 15 12 Set Desired Scan Size Scan Angle and Offsets 35 16 0 Realtime Operation 36 16 1...

Page 5: ...roscope Page 5 Contact Mode AFM Page 8 TappingMode AFM Page 10 Non contact Mode AFM Page 11 Advantages and Disadvantages of Contact Mode AFM TappingMode AFM and Non contact Mode AFM Page 12 Piezoelectric Scanners How They Work Page 14 Piezoelectric Scanners Hysteresis and Aging Page 16 Piezoelectric Scanners Creep and Bow Page 19 Probes Page 21 Types of SPM Probes Page 23 Atomic Force Microscopy B...

Page 6: ...2 Scanning Probe Microscope Training Notebook Rev F Offline Operation Page 44 Tip Shape Issues Page 53 Typical Image Artifacts Page 58 Calibration Page 62 ...

Page 7: ...tomic Force Microscope AFM Developed in 1986 by Binning Quate and Gerber as a collaboration between IBM and Stanford University 1 2 Definitions Scanning Probe Microscopy SPM Consists of a family of microscopy forms where a sharp probe is scanned across a surface and probe sample interaction is monitored The two primary forms of SPM 1 Scanning Tunneling Microscopy STM 2 Atomic Force Microscopy AFM ...

Page 8: ...Electric Force Microscopy EFM Surface Potential Microscopy Phase Imaging Force Volume Electrochemical STM AFM ECM Scanning Capacitance Microscopy SCM Scanning Thermal Microscopy SThM Near field Scanning Optical Microscopy NSOM or SNOM Scanning Spreading Resistance SSRM Tunneling AFM TUNA Conductive AFM CAFM NanoIndentation Torsional Resonance Mode TR Mode NanoLithography NanoManipulation SECPM ...

Page 9: ...Scanning Tunneling Microscope Rev F Scanning Probe Microscope Training Notebook 5 2 0 Scanning Tunneling Microscope Basic SPM Components ...

Page 10: ...ple c constant d tip sample separation distance As the tip scans the sample surface it encounters sample features of different heights resulting in an exponential change in the tunneling current A feedback loop is used to maintain a constant tunneling current during scanning by vertically moving the scanner at each x y data point until a setpoint current is reached The vertical position of the sca...

Page 11: ...ng Probe Microscope Training Notebook 7 2 1 Applications Atomic resolution imaging STM is the only technique which detects atomic scale defects Electrochemical STM Scanning Tunneling Spectroscopy Low current imaging of poorly conductive samples ...

Page 12: ...urface while monitoring the change in cantilever deflection with a split photodiode detector The tip contacts the surface through the adsorbed fluid layer on the sample surface A feedback loop maintains a constant deflection between the cantilever and the sample by vertically moving the scanner at each x y data point to maintain a setpoint deflection By maintaining a constant cantilever deflection...

Page 13: ...antilever deflection Force constants usually range from 0 01 to 1 0 N m resulting in forces ranging from nN to µN in an ambient atmosphere The distance the scanner moves vertically at each x y data point is stored by the computer to form the topographic image of the sample surface Operation can take place in ambient and liquid environments F kx ...

Page 14: ... a constant oscillation amplitude by maintaining a constant RMS of the oscillation signal acquired by the split photodiode detector The vertical position of the scanner at each x y data point in order to maintain a constant setpoint amplitude is stored by the computer to form the topographic image of the sample surface By maintaining a constant oscillation amplitude a constant tip sample interacti...

Page 15: ... adsorbed fluid layer on the surface during scanning The cantilever s resonant frequency is decreased by the van der Waals forces which extend from 1nm to 10nm above the adsorbed fluid layer and by other long range forces which extend above the surface The decrease in resonant frequency causes the amplitude of oscillation to decrease The feedback loop maintains a constant oscillation amplitude or ...

Page 16: ...e Disadvantages Lateral shear forces can distort features in the image The forces normal to the tip sample interaction can be high in air due to capillary forces from the adsorbed fluid layer on the sample surface The combination of lateral forces and high normal forces can result in reduced spatial resolution and may damage soft samples i e biological samples polymers silicon due to scraping betw...

Page 17: ...aration Slower scan speed than TappingMode and Contact Mode to avoid contacting the adsorbed fluid layer which results in the tip getting stuck Non contact usually only works on extremely hydrophobic samples where the adsorbed fluid layer is at a minimum If the fluid layer is too thick the tip becomes trapped in the adsorbed fluid layer causing unstable feedback and scraping of the sample Note Due...

Page 18: ...ming a scanner which can manipulate samples and probes with extreme precision in 3 dimensions In some models e g MultiMode SPM the scanner tube moves the sample relative to the stationary tip In other models e g STM Dimension Series and BioScope SPM s the sample is stationary while the scanner moves the tip Figure 7 0b Typical scanner piezo tube and X Y Z configurations AC Signals applied to condu...

Page 19: ...7 0c Waveforms applied to the piezo electrodes during a raster scan with the X axis designated as the fast axis Scan Angle 0 V V 0 X V V 0 X V V 0 Y t t V V 0 Y t t z X AXIS YAXIS 1416 410 Schematic of piezo movement during a raster scan Voltage applied to the X and Y axes produce the scan pattern ...

Page 20: ...f movement vs applied voltage is nonlinear This causes the forward and reverse scan directions to behave differently and display hysteresis between the two scan directions Figure 8 1a Effect of Nonlinearity and Hystersis The effect of nonlinearity and hysteresis can be seen from the curve above see Figure 8 1a As the piezo extends and retracts throughout its full range it moves less per applied vo...

Page 21: ...h scan direction is demonstrated This nonlinear relationship is corrected during the calibration routine by applying a nonlinear voltage in Realtime to produce a linear scan in X and Y in both trace and retrace scan directions U S Pat 5 051 646 Figure 8 1c Nonlinear waveform solid line applied to the piezo electrodes to produce linear scanner movement The unaltered triangular waveform dashed line ...

Page 22: ...sensitivity to occur at the beginning of a scanner s life as shown in the graph to the left Scanners are run approximately 48 hours before they are shipped from the factory to get the scanner past the point where the sensitivity changes dramatically over short periods of time As the scanner ages the sensitivity will change less with time and will eventually get to the point where it very seldom ne...

Page 23: ...lowly moving in the direction of the offset Creep is the result of this slow movement of the piezo over the remainder of the offset distance once scanning has resumed Creep appears in the image as an elongation and stretching of features in the direction of the offset for a short period of time after the offset Figure 9 1a Calibration Grating An example of creep is shown in the image of a calibrat...

Page 24: ...ple or tip on the other the free end does not move in a level plane The mechanical properties of the piezo as well as the kinematics of motion often result in 2nd order or 3rd order curvatures from an ideal plane This is commonly called Bow which increases with scan size The bows can be removed from a captured image by using Modify Planefit Manual or Auto ...

Page 25: ... to provide a small force constant For the silicon nitride tips there are 4 cantilevers with different geometries attached to each substrate resulting in 4 different spring constants Characteristics of the standard silicon nitride probes Models NP DNP are listed below Table 10 1a Silicon Nitride Probe Characteristics Spring Constant k 0 58 0 32 0 12 0 06 N m a a Calculated spring constant values a...

Page 26: ...er than the silicon nitride probes resulting in larger force constants and resonant frequencies Characteristics of the TappingMode Etched Silicon Probes Model TESP are listed below Table 10 2a TappingMode Etched Silicon Probe TESP Characteristics Figure 10 2a Scanned Electron Microscope SEM Images of Silicon Cantilever and Tip Spring Constant k 20 100 N m Resonant Frequency 200 400 kHz Nominal Tip...

Page 27: ...gMode Etched Silicon Probes described previously Hardened TappingMode Etched Silicon Probes TappingMode Focused Ion Beam FIB Machined Silicon Probes Olympus Tapping Tips 11 3 Probes for Other SPM Techniques Force Modulation Probes Force Modulation Etched Silicon Probes Magnetic Force Microscopy MFM and Electric Force Microscopy EFM Probes Magnetic Force Etched Silicon standard Probes Low Moment Ma...

Page 28: ... Iridium STM Tips Tungsten STM Tips Probes for Electrical Techniques PIT PtIr coated Si probes for SCM and TappingMode PIC PtIr coated Si probes for contact mode SCM DDESP Doped diamond coated Si probes with medium to high spring constant For a listing and description of SPM probes available from Bruker Corporation please go to www bruker com Products AFM SPMs NSOMS Buy SPM Probes Probes ...

Page 29: ...on sensitive detector PSD consisting of two closely spaced photodiodes whose output signal is collected by a differential amplifier Angular displacement of cantilever results in one photodiode collecting more light than the other photodiode producing an output signal the difference between the photodiode signals normalized by their sum which is proportional to the deflection of the cantilever It d...

Page 30: ...rations 26 Scanning Probe Microscope Training Notebook Rev F 13 0 SPM Configurations Figure 13 0a Scanned Tip SPM Figure 13 0b Scanned Sample SPM Labels 1 Laser 2 Mirror 3 Cantilever 4 Tilt mirror 5 Photodetector ...

Page 31: ...SPM Configurations Rev F Scanning Probe Microscope Training Notebook 27 Figure 13 0c Feedback Loop Contact Mode 62 5 KHz ...

Page 32: ...SPM Configurations 28 Scanning Probe Microscope Training Notebook Rev F Figure 13 0d Feedback Loop Tapping Mode 62 5 KHz ...

Page 33: ...on the cantilever using the two screws on the top of the scanner Tapping The Sum signal should be about 2V Contact The Sum signal should be 4 6V 14 3 Adjust the Photodetector Adjust the photodetector so that the red dot moves towards the center of the square using the two screws on the side of the scanner Tapping Center the red dot and set the vertical deflection Vert Defl to a value close to 0 0 ...

Page 34: ...quency is at 100 kHz and the End Frequency is at 500 kHz Target Amplitude should be 2 3V 2 Click Auto Tune A Tuning sign should appear and then disappear once Auto Tune is done When done quit the Cantilever Tune menu 3 Set the Peak Offset to 3 10 percent The Peak Offset is the percentage of the cantilever s free air resonant frequency to be automatically offset Peak offset is used to compensate fo...

Page 35: ... try scanning at 2Hz for 5 10µm try 1Hz and for large scans try 1 0 0 5Hz Next try increasing the Integral Gain using the right arrow key As you increase the Integral Gain increase the Proportional Gain as well Proportional Gain can usually be 30 100 more than Integral Gain The tracking should improve as the gains increase although you will reach a value beyond which the noise will increase as the...

Page 36: ...er A E or J Mount and plug the scanner into the base Attach the corresponding springs to the microscope base In the software choose the appropriate scanner parameters with Microscope Select 15 4 Mount Sample Mount a sample on a metal disk with a sticky tab sample width should be limited to the disk s 15mm diameter Mount the disk and sample on top of the scanner 15 5 Place Optical Head on Scanner 1...

Page 37: ...ng optical microscope equipped with zoom Use the base screws on the stage of the optical microscope to center the cantilever in the field of view Lower the focus of the optical microscope to the sample surface You will still see an out of focus shadow of the cantilever in the field of view Keep the optical head level while lowering the cantilever until it is almost in the same plane of focus keep ...

Page 38: ...value bottom LCD to zero using the screw on the back left side of the optical head 15 8 Cantilever Tune TappingMode Only Select View Cantilever Tune or select the Cantilever Tune icon For Auto Tune Controls make sure Start Frequency is at 100 kHz and End Frequency is at 500 kHz Target Amplitude should be 2 3 volts Click on Auto Tune A Tuning sign should appear and then disappear once Auto Tune is ...

Page 39: ...e For scan sizes of 1 3µm try scanning at 2Hz for 5 10µm try 1Hz and for large scans try 1 0 0 5Hz Next try increasing the Integral Gain using the right arrow key As you increase the Integral Gain increase the Proportional Gain as well Proportional Gain can usually be 30 100 more than Integral Gain The tracking should improve as the gains increase although you will reach a value beyond which the n...

Page 40: ...ace right to left scan lines being acquired in Realtime Scope Mode may be used to diagnose how well the tip is tracking the surface If the feedback parameters are adjusted optimally the trace and retrace scan lines should look very similar but may not look completely identical An optimized scope trace of a 40µm TappingMode scan across the pits of the grating is shown below Figure 16 0a Scope Trace...

Page 41: ...medied by decreasing the Setpoint in TappingMode or increasing the Setpoint in Contact Mode 16 2 Integral Gain The Integral Gain controls the amount of the integrated error signal used in the feedback calculation The higher this parameter is set the better the tip will track the sample topography However if the integral gain is set too high noise due to feedback oscillation will be introduced into...

Page 42: ...hat the feedback loop has time to respond to changes in the sample topography Setting the scan rate too high will result in poor tracking of the surface as shown below Figure 16 3a Scope Trace Demonstrating the Effect of Setting the Scan Rate Too High The actual tip speed or of the tip depends on two parameters Scan Rate and Scan Size Since the Scan Rate units are lines per second Hz increasing th...

Page 43: ...ation Typically set 30 to 100 higher than the Integral Gain Data Type Selects the signal viewed on the display monitor this parameter is set to Height for topographical measurements Line Direction Selects the direction of the fast scan during data collection Trace left to right or Retrace right to left Microscope Mode Sets mode of operation to Tapping Contact or Force Mod Data Scale Controls the v...

Page 44: ...set in Realtime and Offline by the Realtime Planefit and Offline Planefit parameters respectively Realtime Planefit The setting of this parameter only effects how the image is viewed in Realtime It has no effect on how the data is saved Line Removes tilt by applying a first order planefit and removes the Z offset from every line scan individually in the fast scan direction X direction at 0 during ...

Page 45: ...ight of the step then setting the Offline Planefit to Full will result in the tilting of the data However if the Offline Planefit is set to Offset then a planefit is not applied and the step orientation is not altered If the Full setting were applied a first order planefit could be applied in the Offline software to reoriented the features without negatively affecting the measurement Figure 16 4a ...

Page 46: ...is moved with respect to a stationary sample A force curve plots the deflection of the cantilever as it contacts and separates from the sample during the extension and retraction of the scanner Figure 17 0a Typical Force Curve with Schematic Labeling Corresponding Tip Sample Interaction Points On the force curve graph the distance of the scanner movement is represented by the horizontal axis and t...

Page 47: ...ntal Setpoint line in the center of the graph The force may be calculated by Hooke s Law F kx where F force k spring constant and x deflection See your microscope s manual for further instruction Force curves may also be acquired in TappingMode to view the change in amplitude and phase with tip sample separation Care should be taken not to completely dampen the oscillation since this may result in...

Page 48: ...nd Create Directory commands for transferring data between directories The Browse command also under the File menu places up to twenty four images on the screen with a variety of commands available at the top of the Browse screen There is also a File menu within the Browse commands which provides the ability to specify in any order which images to Move Copy Delete or Rename Browse also provides th...

Page 49: ... of the screen Cursors may be added or deleted with the Marker menu located at the top of the Display monitor The measurements are displayed in the same colors as the corresponding markers The power spectrum fast Fourier transform FFT along the cross section is displayed in the lower center window The cursor may be used to determine the predominate periodicities along the cross section by placing ...

Page 50: ...n area Zi is the current Z value and N is the number of points within a given area Ra The Mean Roughness Ra represents the arithmetic average of the deviations from the center plane Zcp is the Z value of the center plane and Zi is the current Z value and N is the number of points within a given area Note The above equations for RMS and Ra are presented for ease in understanding The formulas used b...

Page 51: ...heights The area depth measurement can be made by determining the relative depth between the two cursors Planefit and or Flatten may need to be performed before making this measurement This will produce sharper peaks in the depth histogram The Depth feature may be used to make automated depth histogram measurements over many images See manual for details Figure 18 2c Area Depth Measurement of GaAs...

Page 52: ...ge Flattening can be performed by applying a 0th 1st 2nd or 3rd order polynomial fit to each scan line Figure 18 3a Schematic Showing the Effects of Flattening Order on Two Scan Lines in X Acquired at Different Locations in Y in the Image Flattening is a powerful filter which you may or may not use often However it is important to understand how flattening can effect subsequent measurements First ...

Page 53: ...ements It can be seen from the above example that in order to make accurate cross sectional measurements on the grating after flattening the structures should be oriented so that they are scanned in the fast scan direction at a 0 scan angle this is the X scan direction If the features of interested are not oriented perpendicular to the fast axis this may require rotating the scan with the Scan Ang...

Page 54: ...ass Substrate Using the Boxes to Exclude the Cells from the Flattening Calculation Planefit Auto Planefit is commonly used to remove tilt or bow from images Planefit calculates a single polynomial fit for the entire image and then subtracts the polynomial fit from the image Planefit may be applied by calculating a 1st 2nd or 3rd order polynomial fit to the image in the X or Y directions 1st order ...

Page 55: ...calculated from a manually positioned line drawn across the image in X and or Y Erase Scan Lines Removes scan lines from image due to skips noise etc This routine replaces the selected scan line with the average of the two scan lines adjacent to it Zoom Expands a selected portion of the captured image to fill the display screen Note This does not increase the resolution or fine detail of the captu...

Page 56: ... This may also be accomplished at any time in Realtime and Offline by hitting the Print Screen button at the top of the keyboard Data may also be exported and imported as TIFF and ASCII files for use with other software programs Automated Offline analysis may be performed with the Auto Program File feature This provides the capability to perform a series of modification and analysis routines to ma...

Page 57: ...at can be resolved A sharper tip will be able to laterally resolve smaller features than a dull tip with a larger radius of curvature This can be seen in the example below Figure 19 0a Schematics and Image Profiles of Spheres Scanned with a Sharp left and Dull right Probe The accumulation of debris on the end of the tip can also dull the tip and result in image distortion as shown below Figure 19 ...

Page 58: ...Tip Shape Issues 54 Scanning Probe Microscope Training Notebook Rev F 35 35 35 35 Tip Cantilever Tip Sidewall Angles of Silicon Nitride Probes Tip Sidewall Angles of Etched Silicon Probes ...

Page 59: ...w the resulting angles and image profiles of silicon nitride and silicon tips scanned over trenches with 90 sidewall angles The tip is mounted in the cantilever at 10 so the angles measured with the front and back sides of the tip will be skewed by 10 Figure 19 0c Sidewall Angle Measurements of Trench with Vertical Sidewalls Acquired with Silicon Nitride Probe Figure 19 0d Sidewall Angle Measureme...

Page 60: ... the image in the X and Y scan direction These data points are commonly called pixels Available choices are 512 256 and 128 Pixelization affects resolution in that you cannot resolve features smaller than the pixel size of the image For example if you are acquiring a 50µm x 50µm image with the Samples line parameter set to 512 then the pixel size is 98nm 50µm 512 0 098µm or 98nm Thus you will not ...

Page 61: ...of data points in the vertical dimension is determined by the conversion of 16 bits over the full vertical range of the scanner 440V The Z limit parameter may be used to increase the sampling resolution in the vertical dimension to image sub angstrom height changes with large Z range scanners Noise The overall system noise will generally be the primary limiting factor in the highest obtainable ver...

Page 62: ...Dirty Tip Double or multiple tip images are formed with a tip with two or more end points which contact the sample while imaging The above images are examples see Figure 20 0b Figure 20 0b Double or Multiple Tips Loose debris on the sample surface can cause loss of image resolution and can produce streaking in the image The image on the left is an example of the loss of resolution due to the build...

Page 63: ...is artifact is most often seen in contact mode on highly reflective surfaces in force calibration plots and occasionally in TappingMode images This artifact can usually be reduced or eliminated by adjusting the laser alignment so that more light reflects off the back of the cantilever and less light reflects off the sample surface or by using a cantilever with a more reflective coating MESP TESPA ...

Page 64: ...e seen in the image of the Ti grains on the left Decreasing the Drive Frequency during imaging can eliminate this artifact as shown in the image on the right when reducing the Drive Frequency the Setpoint voltage may need to be reduced as well see Figure 20 0f Figure 20 0f Rings During High Frequency Operation The arch shaped bow in the scanner becomes visible at large scan sizes left The bow in t...

Page 65: ...ining Notebook 61 Figure 20 0g 2nd Order Bow At large scan sizes the bow in the scanner may take on an S shaped appearance left This may be removed by performing a 3rd order planefit in X and Y right Data Scale 324nm see Figure 20 0h Figure 20 0h 3rd Order Bow ...

Page 66: ...the dimensions in X Y at scan sizes of 440V and 150V to see if the pitch measurement falls within 2 10 0 2µm If this needs correcting run the automated calibration routine found under Capture Capture Calibration This routine captures twelve images at various scan sizes and settings to calibrate the scanner sensitivity sensitivity change with scan size and scanner rotation The operator should watch...

Page 67: ...f the measurement is not 200nm 1nm calculate the new sensitivity by the following formula Input the new sensitivity into the Z sensitivity parameter in the Microscope Calibrate Z menu then repeat this procedure until the scanner is calibrated New Sensitivity Old Sensitivity x Actual Depth 200nm Measured Depth ...

Page 68: ...Calibration 64 Scanning Probe Microscope Training Notebook Rev F ...

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