Precision measurement of microscopic images

The problems of measuring the dimensions of small geometries using an optical microscope are investigated, with particular attention to the measurement of the critical linewidths on semiconductor integrated circuit wafers and photomasks. Conventional scalar diffraction models are used to investigate the imaging process of the optical microscope and these are extended to include image transducers and measurement devices. Particular attention is paid to the use of a video camera as an image transducer and an image shearing module as a measurement device. The performance of different measurement techniques is investigated both theoretically and experimentally and systematic errors in different measurement techniques are identified. The limits of scalar diffraction theory are investigated experimentally. The design of digital array filters is investigated in order to develop algorithms for automating the location of the feature to be measured, focusing and the measurement process itself. A novel method of automating the image shearing measurement technique using array filters is presented. The models for the optical imaging, transducer response and automation algorithms are used to develop an automated image shearing based measurement system for measuring the gaps in magnetic recording heads. The design of the system is described and experimental performance tests demonstrate good agreement with the predictions of the theoretical system models. The problem of modelling the images formed by thick layer objects is considered and a waveguide model is developed. Experimental and theoretical tests of the model show that the image profiles of shaped, multi-layer objects can be successfully predicted. The model is used to investigate the imaging of thick layer objects in order to study the performance of different linewidth measurement techniques. A novel method of improving the repeatability of measurements on thick layers is presented, based correction technique