In January of 2023 NIST released SRM 3461, Standard Reference Cantilevers for Atomic Force Microscopy (AFM) Spring Constant Calibration. Each device consists of small chip of single crystal silicon carefully microfabricated with an array of seven (7) uniform rectangular cantilevers, each with a well-defined stiffness at the end. Each cantilever is nominally 1.45 µm thick, 50 µm wide (about the width of a human hair), and vary in length from 300 µm to 600 µm in steps of 50 µm.
Each cantilever has been certified by NIST for stiffness at the free end using a laser Doppler vibrometer (LDV) instrument housed in the Nanomechanical Properties Group's Advanced Measurement Laboratory (AML) clean-room suite. This instrument has been carefully augmented and refined for optimization as a means of very accurately calibrating cantilever stiffness using the LDV Thermal Calibration Method. The accuracy of the method has been verified with comparison measurements made using the NIST Electrostatic Force Balance (EFB) which is SI traceable. Uncertainties in SRM 3461 cantilever stiffness with a 95% confidence limit are better than ±3.0%. A full description of the certification process including a rigorous statistical analysis has been published as NIST SP 260-227 "Certification of Standard Reference Material 3461 Reference Cantilevers for AFM Spring Constant Calibration," available free online at https://doi.org/10.6028/NIST.SP.260-227 . It should be noted that even though the stiffness units for SRM 3461 are listed as near 0.1 N/m using standard Système International (SI) base units of force (N) and length (m), the actual force and displacement regimes measured by LDV on these cantilevers are closer to a pico-scale ( i.e., pN and pm), a trillionth (10-12) smaller.
SRM 3461 is especially useful for AFM manufacturers, AFM cantilever manufacturers, LDV Instrument makers and AFM and LDV researchers in the field interested in verifying accurate force measurements at the nanoscale. AFM manufacturer's use an AFM Thermal calibration method to estimate the stiffness of test cantilevers in the field and SRM 3461 can help validate the AFM Thermal calibration algorithms used. Cantilever manufacturer's often use LDV Thermal calibration measurements to estimate commercial test cantilever spring constants for their customers. Utilizing this NIST standard will verify the accuracy of the actual calibration algorithm and methodology used. Even AFM researchers in the field can utilize SRM 3461 along with the AFM Reference Cantilever method as an independent test cantilever stiffness calibration. SRM 3461 is available for purchase at the NIST SRM store.