Two systems of linewidth/pitch measurement have been developed. One is precision diffractometer using laser and goniometer for measuring average pitch of standard specimen. The other is absolute measurement system using scanning electron microscope and laser interferometer.
Precision laser diffractometer of the conventional-type has been developed for the grating pitch measurement. The accuracy of the measurement depends on the accuracy of the laser wavelength, goniometer and the determination of diffraction angles. The main source of error in the pitch size measurement by the conventional diffractometry has been the low resolution in the measurement of the diffraction angles. The rotary arm is extended and reliable high-resolution goniometer is chosen, while a focusing lens is inserted between the grating sample and the detection plane to prevent the error due to the parallel shift of the diffracted beam, which will take place when the rotational axis of the arm does not coincide with the grating plane. Obtained values for lamella structure grating samples show quite a good repeatability, which is the order of magnitude of angstrom. The values for the grating, whose pitch is certified as 288 nm by the manufacturer, is 287.595 with the standard deviation of 0.017 nm at worst. That of the 700 nm pitch grating is 700.782 with the standard deviation of 0.054 nm at worst. The expanded uncertainty with the coverage factor of 2 is estimated to be 0.038 nm and 0.10 nm respectively.
For absolute linewidth/pitch measurement system using SEM, The system was modified from a conventional scanning electron microscope and can be used to measure. In the system, absolute method using laser interferometer are used in which the electron beam is fixed and the specimen is set on a precise scanning stage driven by a piezo electric transducer. The linewidth/pitch of the specimen has been determined from the signal of edge and the distance between signal of left and right edges is measured by laser interferometer. In this paper, overall measuring system and measurement method are described and showed the uncertainty estimation of absolute method according to guide published by ISO. In this estimation, the expanded uncertainty from instrument and sample was 6 nm with the confidence level of 95.45%.
And also a Monte Carlo simulation is performed to investigate charging effects of scanning electron microscopes on the linewidth measurement of PMMA insulator patterns in the secondary electron detection mode with the accelerating voltage around 1 kV. The simulation results reveal in detail how the non-unity yield of electron generation in the PMMA target leads to local positive and negative charge accumulation and affects the image profile of secondary electrons. Thus measurement offsets are quantified in consideration of the edge detection criteria of 50% threshold and linear regression. Two types of patterns, isolated and line array, of PMMA formed on a Si wafer are considered with different operating conditions and geometric dimensions. In addition, influences of the charging time elapse on the linewidth measurement are tested and discussed.
