SEM Magnification Calibration & Verification: Building Confidence in Your Scale Bar

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1 SEM Magnification Calibration & Verification: Building Confidence in Your Scale Bar Mark A. Koten, Ph.D. Senior Research Scientist Electron Optics Group McCrone Associates

2 Why check your SEM image calibration? Everyone should want to ensure the scale bar on the image is accurate The initial magnification calibration may not have been as rigorous as you think Serious maintenance on your SEM may have altered the magnification calibration Who checks this? Microscope specialists/supervisors/managers Eperienced users It may be required on a routine basis Industrial labs that have ISO or similar accreditation requirements SEM Magnification Calibration 2

3 SEM Magnification Definition The ratio of a length measured on the SEM monitor (L m ) to the same length measured on the sample (L s ). M = L m L s The length measured could be anything as long as they are equivalent. How does the SEM change magnification? Scan coils change the scan length (L s ). SEM Magnification Calibration 3

4 Magnification Eamples Eample 1 HFOV Eample 2 Piel Magnification Image width Scan length Image piel size Scan piel size (M) (L m1 ) (L s1 ) (L m2 ) (L s2 ) cm mm 190 um 19.0 um cm 2.40 mm 190 um 1.9 um 1, cm 0.24 mm 190 um 0.2 um 10, cm um 190 um 19.0 nm 100, cm 2.40 um 190 um 1.9 nm 1,000, cm 0.24 um 190 um 0.2 nm Monitor Image piels m M = L m L s Problem: beam diameters range from 1 nm 1 um SEM Magnification Calibration 4

5 Digital Resolution vs Imaging Resolution How large is your beam s probe size? How large is your beam s interaction volume? What is the beam s shape? -Interaction volume depends on voltage -Gaussian curve with FWHM Probe size Electron Beam Electron Beam How large are the piels in your image? What distance does the beam travel between two piels? -Piel sizes range from 0.2 nm to 20 um! At higher magnifications the digital resolution can vastly eceed the instrument resolution. SEM Magnification Calibration 5

6 SEM Image Formation Where s the ray diagram? Different from image projection cameras found in optical imaging and bright field TEM applications The beam is controlled very precisely by a scan generator, which deflects the beam over the sample in a grid pattern The detector collects electrons (SE or BSE) for each beam position (, y)-space and saves the number of electrons detected (intensity) at that location. The computer then maps this information into a file with a readable format (tiff, png, jpeg, ) that can display the electron intensity measured on a computer screen as a digital image. Beam Locations Saturation Intensity Grayscale SEM image.tiff -Intensity range corresponds to grayscale values -Contrast is amplitude -Brightness is offset Specimen Detector Zero signal SEM Magnification Calibration 6

7 What is actually being calibrated? The distance the beam moves in both the and y direction The beam positions will be mapped to a square grid (piels are squares) Distortions in the image can arise if the travel distance is not equal in both and y directions If the distances between piels is unknown or inaccurate then measurements will be wrong Specimen Specimen SEM Magnification Calibration 7

8 The Nominal Magnification Parado L m3 Magnification changes from screen to screen L m1 L m2 M = L m L s L m1 L m2 L m3 M 1 M 2 M 3 Scale bars solve this problem by placing a measurement of L s inside the image! SEM Magnification Calibration 8

9 Nominal Magnification vs Scale Bar Nominal Magnification Only No value of L s is saved L m changes depending on the screen you are using (can always measure this) The saved magnification is no longer valid unless you are on the microscope PC Features in your image can t be measured at a later date on different PC If you want to measure something, you have to go back to the microscope PC Scale Bar A value of L s travels with the image in the scale bar L m changes depending on the screen you are using (can always measure this) For different screens you can find your actual magnification Can make measurements of sample features at a later date on any PC Another person can more easily interpret the sizes of the features in the image SEM Magnification Calibration 9

10 Making Measurements from Calibrated Images Advance image analysis often done on third party software such as imagej or image-pro can be used to make precise and comple measurements of your samples These measurements often require distance unit dimensions rather than piels This requires a conversion factor of piels to micrometers/nanometers Your scale bar can be used within imagej to obtain this pi/nm ratio Just measure the length of your scale bar in piels and divide that by its length in dimensional units There is usually a set scale function that allows for the software to apply this ratio to the image or image stack SEM Magnification Calibration 10

11 What features make a good calibration tool? Chemically and structurally stable Sharp edges that don t use a lot of piels Something periodic spanning ~4 orders of magnitude (100 nm to 1 mm) Pitch doesn t deviate too much from the mean at any point Pitch is constant over a wide range of length scales Some things to avoid using for calibration or verification: SEM Magnification Calibration 11

12 What Standard Reference Material Should You Use? Best sources for SRMs are National Metrological Institutes (NMI) United States: NIST United Kingdom: NPL (National Physical Laboratory) France: LNE (Laboratoire National d'essais) Germany: PTB (Physikalisch-Technische Bundesanstalt) An SRM that is traceable back to an NMI like NIST Unbroken chain of validation from processing to measurement Sample-to-sample uncertainties are known and acceptable Measurement uncertainties are known and acceptable These are the requirements to satisfy Category I traceability for an ISO accreditation body Some good SRM eamples: MetroBoost s MetroChip (SEM) NIST RM 8820 (SEM) MAG*I*CAL (TEM) SEM Magnification Calibration 12

13 MetroChip manufacturing process Fabricated using advanced semiconductor processing methods 1. A protective layer is applied to heterostructure 2. Protective layer is eposed to light through a mask and developed 3. Poly-crystalline Si surface under mask is etched 4. Protective layer is removed revealing features derived from NIST certified mask Protective layer Polysilicon (150 nm) SiO 2 (5 nm) Si wafer This method can very reliably reproduce periodic structures sample-to-sample SEM Magnification Calibration 13

14 The Nuances of Periodic Structures Pitch = linewidth + spacewidth MetroBoost guarantees pitch, but DOES NOT guarantee linewidth Pitch reproducibility sample-to-sample is 2 parts per million Features such as line and space widths can vary as much as +/- 10 %. SEM Magnification Calibration 14

15 Metro Chip Certification by NIST NIST demonstrated that the pitch reliability of MetroBoost s mask is very high Deviation is only 2 ppm NIST then certified the pitch accuracy for a single metro chip (July 12, 2005) Used line scale interferometer This means that the pitch for ALL metro chips is BOTH accurate and reliable! Height, linewidth, and sidewall angle were not certified These features can vary up to +/- 10 % 120 μm SEM Magnification Calibration 15

16 Calibration Procedure via MetroChip Most vendors perform annual preventive maintenance activities Older SEMs have scan generators that can be adjusted via potentiometers by the service engineer Newer SEMs are corrected digitally during the mapping process within the SEM operation software Some vendors will perform calibrations with traceable SRM to a recognized authority (NMI), but others will not It is a good idea to perform your verification while the service engineer is still on site, since you will need their help if the verification fails SEM Magnification Calibration 16

17 Magnification Calibration Verification with Metro Chip 1. Set up your instrument for SEM imaging with Metro Chip installed 2. Locate the calibration rulings on the Metro Chip for both and y orientations 3. Capture images of both the and y oriented rulings without rotating your scan or stage 4. Calculate the percent error with the formula % error = 100 known value measured value known value 5. Make sure the distances measured in both and y directions meet your acceptance criteria (5 %) 6. This will verify that both and y are accurate and not distorted Pitch # Meas Known % Error Pitch = 250 nm SEM Magnification Calibration 17

18 MetroChip Features and Uses 1. Linear Microscales 2. Scatterometry and Measurement Features 3. Distortion Targets SEM Magnification Calibration 18

19 Cleaning and Maintenance Things to avoid: Wiping the sample with a cloth. This will destroy the features. Spaying the sample with gases that leave residues. Also immersing the sample in liquids that leave residues. Using cleaning solutions that attack silicon dioide (glass). Any HF dip will attack the oide and should be avoided. Avoid plasma treatments that attack silicon dioide (glass). The list includes gas mitures containing CF 4 or CHF 3 or similar gases. Harsher plasma treatments, such as those using chlorinated gases, should be avoided since the polysilicon can be etched. Good cleaning procedures: Large particles can be blown off the sample by nitrogen gas sprayed from a nozzle, i.e. Dust-off. Rinsing the sample in DI water is safe and effective for removing larger particles. For cleaning fingerprints, chemical baths that remove photoresist, if available, are effective. Any resist strip used in semiconductor manufacturing is safe, including sulfuric peroide. One can also use plasma sample cleaners with pure oygen, or mitures of oygen and forming gas (N 2 /H 2 ) for removing fingerprints and also for cleaning hydrocarbon residues formed as a result of inspection in a scanning electron microscope. SEM Magnification Calibration 19

20 MAG*I*CAL for TEM magnification calibration Cross-sectional TEM sample with alternating layers of Si-Ge and Si. Grown via molecular beam epitay (MBE) Four sets of 5 alternating layers make for a wide range of image magnifications. Si-Ge layers are about 10 nm thin Since the heterostructure is epitaial, phase contrast imaging can also be used for HRTEM calibration and diffraction mode camera length calibration. SEM Magnification Calibration 20

21 Thank you for joining us. Mark A. Koten, Ph.D. Senior Research Scientist (630) SEM Magnification Calibration

22 The NIST Line Scale Interferometer A detailed eplanation is available on the NIST website at: SEM Magnification Calibration 22