Research On Dual Tuning-Fork Atomic Force Microscopy For Linewidth Metrology

With the development of micro/nano manufacturing technology and semiconductor industry,the scale of micro/nano structure/device is constantly reduced,the pattern is increasingly complicated,as well as the requirement of the machining accuracy is higher.Micro/nano linewidth is one of the important geometric parameters for evaluating the performance of micro/nanodevices.Typically,AFM is widely used for the characterization of linewidth structure with a few microns,however it does not meet the requirement of accurate measurement of the current micro/nano linewidth due to its intrinsic top-down detection type and the tip’s geometric effect.The measure dilemma seriously hinders the development of the linewidth metrology and the industry quality control.Therefore,the dissertation conducts the research on the key technologies and system of special dual-probe AFM for accurate linewidth measurement,as described below.Dual-probe atomic force microscope based on homemade tuning-fork probe is proposed for linewidth measurement that is implemented by aligning the two probes for building zero reference point,scanning sidewall on both sides separately and stitching the two images.It breaks the limitations from conventional AFM due to unreachable and then undetectable the sidewall and provides a technical solution for the linewidth measurement.The dual-probe alignment is a crucial technique for the dual-probe AFM that is achieved by the tip-scan-tip method based on the interaction between the two probes.However,the typical tip-sample interaction model is invalid since the size of the two tips and the distance between them for the probe-probe interaction case are of the same order magnitude.Therefore,a new interactional model is developed based on the surface element integration method.With the corrected interaction model,the dependence of the probe-probe interaction on the radii and the alignment angle is investigated,following which,the final AFM detection signals is evaluated by establishing a virtual AFM.The simulation describes the dual-probe interaction mechanism and offers a guidance for precise dual-probe alignment.A novel tuning fork force sensor is designed and implemented,which permits compact system design and small-amplitude detection making it is more suitable for dual-probe alignment and measurement than the regular silicon cantilever.First,an automatic tip-preparation device is developed based on the electrochemical etching method for achieving small-radius tip.Next,a balanced method is proposed and achieved for the preparation of the high-Q tuning fork force sensor by theoretical research on the influence of the additional mass on the dynamic properties of tuning fork probe.And also,the optimal parameters of the attached tips are suggested and verified by experiment,which provides optical sensors for dual-probe system.The calibration of the properties of the tuning fork probe is very difficult due to its homemade process,extremely high stiffness and small amplitude features,however,which is required for achieving accurate measurement by optimized imaging control.Therefore,a new calibration method is developed that is based on AFM and the principle of energy equivalence.It is validated by a series of experiments which achieves synchronous calibration for stiffness and amplitude.Furthermore,a tilt scanning method is developed for sidewall measurement that switches automatically between tilt-servo scan and vertical-servo scan depending on the detection region.A dual-probe AFM system is developed for the linewidth metrology based on the above research.It is verified that the tuning fork probe has a low noise feature,high stability and the ability for sidewall characterization based on tilt control method by a series of experiments.Finally,the performance of the dual-probe AFM system is validated by experiments on a CD-standard(linewidth-standard)structure that achieves the true linewidth assessment with good accuracy and repeatability.The research provides an effective method and system for the accurate measurement of critical dimensions such as linewidth of micro-nano structure.