Calibration of AFM with virtual standards; robust, versatile and accurate. Richard Koops VSL Dutch Metrology Institute Delft

Transcription

1 Calibration of AFM with virtual standards; robust, versatile and accurate Richard Koops VSL Dutch Metrology Institute Delft

2 VSL Dutch Metrology Institute VSL is the national metrology institute of the Netherlands Located in Delft Private company with public task About 100 FTE ISO accredited Maintenance of national standards, calibration services and reference materials, uncertainty analysis, training, consultancy 2

3 aim4np automated in-line metrology for nanoscale production Goal Enable measurement of nanomechanical properties of work pieces in a production environment. Industrial carrier A robotically controlled metrology platform with a white light interferometer and an atomic force microscope (AFM) Products Injection moulds and molded products Thin film organic solar cells Role of VSL Ensuring the traceability of measurement results, standards, methods, uncertainty analysis 3

4 aim4np hardware concept Need for highly accurate nanoscale surface measurements in the production environment 4

5 Atomic force microscopy (AFM) trace Source: retrace Lateral range X, Y: 0.1 mm x 0.1 mm Height range Z: 20 µm Probe radius: 2 nm Resolution: sub-nm 5

6 AFM piezo scanners Non-linear response to actuation voltage Distortions of the scan field Inaccurate measurements Height and lateral calibration required Linear scanner response Non-linear scanner response 6

7 Conventional AFM z-axis calibration Top view Physical step height or groove standards Traceable calibration by interference methods Requires wide lateral scanning field crosstalk Limited to heights larger than approx. 5 nm Delicate structures, susceptible to contamination and damage ISO 5436 step height AFM result 7

8 Conventional lateral (x,y) AFM calibration Physical standards with periodic structures, i.e. 1D or 2D gratings Traceable calibration by diffraction (angle) measurements Average pitch used to calibrate the scanner sensitivity Measured pitch deviations used to establish scanner non-linearity Limited to pitch of several 100 nm Delicate structures, susceptible to contamination and damage 8

9 Concept of the virtual height standard Create a programmable surface to mimic surface texture AFM probe Arb. surface Arbitrary waveform generator Nanometer displacement generator Calibration standard library Texture library 9

10 Virtual step height standard implementation for the calibration of the AFM z-axis AFM scanner VSL Veeco Dimension AFM Virtual standard

11 Virtual standard calibration method VSL Picodrift interferometer top view VSL Picodrift interferometer beam paths dut 11

12 Calibration result virtual standard piezo Nearly perfect linearity confirmed by calibration result 12

13 AFM z-axis calibration: histogram method AFM: 10.2 lines/s Virtual standard: Hz square wave trace retrace difference 1/f noise in slow scan direction Histogram Reference value: nm Histogram result: nm Calibration factor:

14 AFM z-axis calibration: ISO 5436 method AFM: 10.2 lines/s Virtual standard: Hz square wave trace retrace difference ISO 5436 analysis Reference value: nm ISO 5436 result: nm Calibration factor:

15 Virtual step height standard benefits Specifically useful for small height ranges No lateral scanning no cross talk Stationary probe no influence from surface texture Insensitive to drift, damage and contamination suitable for harsh environments ISO 5436 virtual step height profile of 197 pm 15

16 Virtual lateral standard concept Create a virtual pitch by shifting the position of an arbitrary surface over an accurately known distance during the calibration Calculate the average scanner sensitivity from the correlation between the two measurements and the known translation Calculate the non-linearity from the sub-image correlations between the two measurements Arb. texture Shear piezo 16

17 Concept simulation Random texture sampled at two positions 5 pixels (1%) apart Added sinusoidal scanner non-linearity of 5 pixels (1%) 17

18 Virtual lateral standard implementation for the calibration of the AFM x and y axes Shear piezo of same material as virtual height standard Sample with arbitrary texture No sample alignment and calibration required Calibration of piezo sensitivity by VSL picodrift interferometer Arb. texture Shear piezo 18

19 AFM calibration procedure Arb. texture Shear piezo Trace line n, sample position 1 Trace line n+1, sample position 2 Interlacing minimizes effects from: Tip and sample instabilities Noise Drift 19

20 AFM calibration measurement result Nominal scan area: 450 nm x 450 nm trace retrace Calibration coefficient = ( ± ) 20

21 Non-linearity analysis and correction Calibration coefficient = ( ± ) 21

22 Virtual lateral standard benefits Specifically useful for small scan ranges No specific texture, sample calibration and sample alignment required Enables lateral calibration with non-linearity correction Insensitive to drift, damage and contamination suitable for harsh environments Arb. texture Shear piezo 22

23 Summary Virtual standards provide robust methods for AFM calibration Simple to implement and very versatile Accuracy optimized for small ranges (< 20 nm height, < 1 µm lateral) Developed for AFM but applicable to other high resolution microscopes The project consortium aim4np acknowledges funding from the EU commission under its FP7 NMP Programme 23

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25 VSL PO Box AR Delft The Netherlands T F E I info@vsl.nl