Calibration strategies for scanning probe metrology

Abstract

Within the Dutch standards laboratory (NMi Van Swinden Laboratorium) a traceable atomic force microscope (AFM) is currently being developed where we aim at a measurement uncertainty of 1 nm. As part of this development novel methods have been developed for the calibration of this instrument. The SPM has been constructed using a commercial AFM head that has been embedded in a metrology frame using an accurate 3D translation stage. The position of the AFM probe relative to the sample is determined along three orthogonal measurement axes by three individual laser interferometers. Due to the properties of the 3D translation stage the Abbe offset between the probe position and the measurement axes should remain below 0.1 mm in order to realize an overall measurement uncertainty of 1 nm. Since the laser interferometers use a four-pass optical configuration the measurement axis is defined by the virtual centre of the four positions of the laserspots onto the plane retro mirror. In order to accurately determine this virtual centre and therefore the position of the measurement axis a device has been designed to measure the four laserspot positions of each axis within the SPM. Since the AFM probe position is determined in 3D accurate calibration of the angles between the measurement axes is important if the measurement error is to be minimized to 1 nm. Different calibration setups to accurately determine these angles will be presented. Additionally, a method for the alignment of the optical axes of the laser interferometers with respect to the end mirrors of the interferometers will be discussed. In order to calculate the overall measurement uncertainty of the scanning probe microscope we decided to use the virtual measurement machine concept since a purely analytical approach for these types of measurement instruments is nearly impossible (Schwenke et al 2000 Ann. CIRP 49 395–8). The model or virtual SPM that has been developed for the simulation of the instrumental characteristics will be presented.

Influences of misalignments on circular path measurement based on dual-beam plane mirror interferometer

“…(10) and (11) and the fitting lines illustrate that the lateral error can be reduced effectively by the decrease of the misalignment (angle deviation).…”

“…EX 2 = 0.0005y + 0.0278 (11) EX 1, EX 2 mean the lateral errors of X-measurement of misalignments under the state 1 and after 2 respectively; y means the displacement of the linear Y-motion, also means lateral displacement of X-mirror for X-measurement.…”

“…Besides, the DPMI is widely applied for the nano-metrology in the micro-field, e.g. the positioning of the photomask [9][10][11][12]. In the measurement for the circular path of the NC machine tool, the target plane mirror is not only driven in the longitudinal (measurement beam direction), but also in the lateral direction (Orthogonal with the measurement beam).…”

Metrological scanning probe microscope based on a quartz tuning fork detector

“…1, which defines the coordinate system of the instrument. Experience in characterizing mSPMs at other metrology institutes [4][5][6][7][8][9] has shown that the metrological frame is the dominant source of measurement uncertainties arising from alignment errors, particularly Abbé errors and nonorthogonality of the measurement axes, and environmental vibration. To achieve the target total uncertainty of dimensional measurements, minimizing these contributions is a principal consideration in the design of the metrological frame and the material selected for its construction.…”

“…1,2 The challenge in realizing this mSPM concept is to create a macroscopic mechanical instrument capable of traceable dimensional measurement at the nanoscale with a combined uncertainty of less than 1 nm. Experience in designing ultraprecision mechanical stages and instruments 3 and in operating similar mSPMs at other metrology institutes [4][5][6][7][8][9] has shown that there are many contributions to the uncertainty of the displacement measurements. These include alignment errors (particularly Abbé errors 10 ), deformations of the mechanical structures (for instance, due to thermal expansion), motion errors of the translation stage, form errors of the interferometer mirrors, nonlinearities of the interferometers, and fluctuations in the refractive index of air.…”