Diana Nyyssonen

Optical Microscope Imaging of Lines Patterned in Thick Layers with Variable Edge Geometry: Theory

A monochromatic waveguide model that can predict the optical microscope images of line objects with arbitrary edge geometry is presented. The lines may be patterned in thick layers, including multilayer structures with sloping, curved, and undercut edges; granular structures such as lines patterned in polysilicon; and asymmetric objects. The model is used to illustrate the effects of line edge structure on the optical image. Qualitative agreement with experimentally obtained optical image profiles is demonstrated. Application of the model to studying the effects of variations in layer thickness and edge geometry on linewidth measurements made at different stages of manufacturing integrated-circuit devices is discussed.

Modeling The Optical Microscope Images Of Thick Layers For The Purpose Of Linewidth Measurement

A monochromatic, waveguide model is presented which can predict the optical microscope images of thick-layer objects including multilayer structures with sloping, curved, and undercut edges, granular structures such as polysilicon, and asymmetric objects. The model is used to illustrate the effects of line structure on the optical image. Qualitative agreement with experimentally obtained optical image profiles is demonstrated. Application of the model to study the effects of variations in layer thickness and edge geometry on linewidth measurements made at different stages of manufacturing an MOS device is discussed.

Calibration of optical systems for linewidth measurements on wafers

In contrast to earlier work with nearly opaque photomasks, optical linewidth measurements on wafers encompass materials with a much wider variation in optical parameters and material profiles. Accurate optical edge detection requires corrections for both the relative reflectance and phase at the line edge because of the partial coherence present in optical microscopes. However, measurement systems which cannot provide the appropriate corrections and cannot detect edge location accurately can be calibrated. Since the correction curve is material dependent, calibrated standards are theoretically required for each step in the wafer fabrication process where linewidths are measured. In the proposed approach for thin layers (less than 200 nm), a small number of etched silicon-dioxide-on-silicon wafers can be used for calibration of a large class of wafer materials. Examples of wafer calibration data for filar, image-splitting, and image-scanning systems are given. The prob-lems associated with accurate linewidth measurement and calibration for thick layers are also discussed.

Comparison Of Linewidth Measurements On An Sem/Interferometer System And An Optical Linewidth-Measuring Microscope

In the current linewidth-measurement program at the National Bureau of Standards, the primary measurement of micrometer-wide lines on black-chromium artifacts is made with an interferometer located in a scanning electron microscope (SEM). The data output consists of a line-image profile from the electron detector and a fringe pattern from the interferometer. A correlation between edge location and fringe location is made for both line edges to give the linewidth in units of the wavelength of a He-Ne laser. A model has been developed to describe the interaction of the electrons with the material line and thereby relate a threshold value on the SEM image profile to a selected point on the material line. An optical linewidth-measuring microscope is used to transfer the primary measurements to secondary measurement artifacts; these artifacts will be used to transfer the linewidth measurements to the integrated-circuit industry. Linewidth measurements from the SEM/interferometer system and the optical linewidth-measuring microscope are compared, and the level of measurement uncertainty for each system is discussed.

Design Of An Optical Linewidth Standard Reference Material For Wafers

Optical linewidth measurements on patterned wafers are complicated by the wide variety of materials and correspondingly wide variation in optical parameters, complex refractive index and thickness, used in the manufacture of integrated circuits. It has been shown that in addition to linewidth, two key parameters, the normalized local reflectance R and the optical phase difference Φ at the line edge, determine the characteristics of the optical image and, therefore, affect the accuracy and precision of linewidth measurements. Both of these parameters, R and Φ, are dependent upon the illuminating wavelength or spectral bandpass and the coherence parameter of the optical system. To achieve the measurement precision and accuracy required for VLSI dimensions (e.g., 10% tolerance for 1-μm linewidths), it is necessary to control coherence, spectral bandpass, and image integrity as well as to achieve reproducible edge detection and focus criteria. When a system can be operated without further operator intervention despite changes in the materials being mea-sured, it is possible to calibrate the linewidth measurement system using a standard fabri-cated from only a few materials representing a range of image characteristics. The desirable characteristics of such a standard are discussed with respect to durability, edge definition, and equivalence of the image characteristics to materials used in the manufacture of ICs. A prototype design consisting of combinations of SiO2 and chromium layers on a silicon substrate is presented.

Partial Coherence In Imaging Systems

An improved method of measuring spatial coherence is described and some sources of measurement errors are discussed. Partial coherence in the image plane of an optical system is discussed and results of coherence measurements are given that demonstrate the scaling of the coherence function for coherence intervals large compared to the diameter of the Airy disk and the limiting value for the coherence interval equal to the diameter of the Airy disk. The application of these results to microdensitometry is discussed and results of coherence measurements in the source plane of currently-used classical microdensitometers are given.

The Effects Of Operating Parameters On Line-Width Measurements With An Optical Microscope

A current micrometrology program at the National Bureau of Standards is concerned with the development of calibrated artifacts and accurate methods for line-width measurements in the 1 to 10 μm region with application to measurements of critical dimensions on IC photomasks. A major program objective is to develop improved theory and experimental verification for line-width measurements made with commonly used optical-microscope systems. Most line-width measurement techniques utilizing an optical microscope apply some edge detection criterion to the image profile. The measurement is therefore affected by system parameters which affect the image profile, such as defocus, spherical aberration and coherence of the illumination. The effects of some of these parameters have been treated previously in the literature. As examples, image profiles for incoherent, coherent, and partially coherent illumination are presented. Image profiles are also given for oaring amounts of defocus with and without spherical aberration. However, these curves only indicate trends and do not represent image profiles that would result with realistic values of the system parameters.

Lens Testing with a Wavefront Shearing Interferometer

A wavefront shearing interferometer for testing lenses has been developed at the National Bureau of Standards. In contrast to most interferometric test systems, the wavefront shearing interferometer is inexpensive, portable, relatively insensitive to vibration, does not need laser illumination, and requires only a minimum of experimental time and operational expertise. Reading of the interferograms and subsequent data reduction require the major effort in testing with the wavefront shearing interferometer. However, with automatic scanning of the interferograms and a high-speed electronic computer to perform the analysis, the data reduction may be completely automated.