Radiant energy – Inspection of solids or liquids by charged particles
Reexamination Certificate
2000-06-30
2003-06-03
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
C250S252100
Reexamination Certificate
active
06573497
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to semiconductor processing and, more particularly, to a system and method for measuring a sample using a scanning electron microscope and calibration thereof.
BACKGROUND OF THE INVENTION
In the semiconductor industry there is a continuing trend toward higher device densities. To achieve these high densities there have been, and continue to be, efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such a high device packing density, smaller features sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as the corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photo lithographic processes as well as high resolution inspection and measurement instruments. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which, for example, a silicon wafer is coated uniformly with a radiation-sensitive film (e.g., a photoresist), and an exposing source (such as ultraviolet light, x-rays, or an electron beam) illuminates selected areas of the film surface through an intervening master template (e.g., a mask or reticle) to generate a particular pattern. The exposed pattern on the photoresist film is then developed with a solvent called a developer which makes the exposed pattern either soluble or insoluble depending on the type of photoresist (i.e., positive or negative resist). The soluble portions of the resist are then removed, thus leaving a photoresist mask corresponding to the desired pattern on the silicon wafer for further processing.
In order to control quality in the design and manufacture of high density semiconductor devices, it is necessary to measure critical dimensions (CDs) associated therewith. Semiconductor device features having CDs of interest include, for example, the width of a patterned line, the distance between two lines or devices, and the size of a contact. CDs related to these and other features may be monitored during production and development in order to maintain proper device performance. As device density increases and device sizes decrease, the ability to carry out quick, inexpensive, reliable, accurate, high-resolution, non-destructive measurements of CDs in the semiconductor industry is crucial. The ability to accurately measure particular features of a semiconductor workpiece allows for adjustment of manufacturing processes and design modifications in order to produce better products, reduce defects, etc.
CDs are usually measured during or after lithography. Various operations performed during the lithography process may affect the critical dimensions of a semiconductor device. For example, variations in the thickness of the applied photoresist, lamp intensity during the exposure process, and developer concentration all result in variations of semiconductor line widths. In addition, line width variations may occur whenever a line is in the vicinity of a step (a sudden increase in topography). Such topography-related line width variations may be caused by various factors including differences in the energy transferred to the photoresist at different photoresist thicknesses, light scattering at the edges of the steps, and standing wave effects. Since these factors can greatly affect CDs, fast and reliable monitoring of semiconductor device features is important in order to guarantee acceptable device performance.
Different technologies are currently available to measure CDs associated with semiconductor devices. These include: optical microscopy, stylus profilometry, atomic force microscopy, scanning tunneling microscopy, and scanning electron microscopy. Scanning electron microscopes (SEMs) are commonly used for inspection and metrology in semiconductor manufacturing. The short wavelengths of scanning electron microscopes have several advantages over conventionally used optical microscopes. For example, scanning electron microscopes may achieve resolutions from about 100 to 200 Angstroms, while the resolution of optical microscopes is typically about 2,500 Angstroms. In addition, scanning electron microscopes provide depths of field several orders of magnitude greater than optical microscopes.
In a typical SEM wafer inspection system, a focused electron beam is scanned from point to point on a specimen surface in a rectangular raster pattern. Accelerating voltage, beam current and spot diameter may be optimized according to specific applications and specimen compositions. As the scanning electron beam contacts the surface of a specimen, backscattered and/or secondary electrons are emitted from the specimen surface. Semiconductor inspection, analysis and metrology is performed by detecting these backscattered and/or secondary electrons. A point by point visual representation of the specimen may be obtained on a CRT screen or other display device as the electron beam controllably scans a specimen.
Scanning electron microscopes (SEMs) operate by creating a beam of electrons accelerated to energies up to several thousand electron volts. The electron beam is focused to a small diameter and scanned across a CD or feature of interest in the scanned specimen. When the electron beam strikes the surface of the specimen, low energy secondary electrons are emitted. The yield of secondary electrons depends on various factors including the work function of the material, the topography of the sample, the curvature of the surface, and the like. These secondary electrons can be employed to distinguish between different materials on a specimen surface since different materials may have significantly different work functions.
Topographic features also affect the yield of secondary electrons. Consequently, changes in height along a specimen surface may be measured using an SEM. Electron current resulting from the surface-emitted secondary electrons is detected and used to control the intensity of pixels on a monitor or other display device connected to the SEM. An image of the specimen may be created by synchronously scanning the electron beam and the display device.
Although SEMs can achieve resolution in the range of angstroms, calibration is difficult. For example, the magnification of an SEM may be calibrated by placing a sample of known dimensions, such as a chip or wafer having a conductor line of known width, in the instrument and measuring the dimension of the sample. The magnification of the SEM is determined by dividing the SEM measurement of the image of the sample by the known dimension of the sample. The magnification calibration information may then be used to construct a calibration curve, or the SEM's magnification controls may be trimmed accordingly.
Calibration according to these prior methods requires samples of known dimensions. The actual dimensions of a sample, however, may not be precisely known, or may change. In particular, repeated usage of a single reference sample as a calibration standard results in degradation of the reference sample. Charge buildup on a reference sample caused by repeated measurement in a SEM affects the secondary electron emission. Contaminant deposition or buildup also has deleterious effects on measurement of a calibration standard reference sample over time. Conventional SEM calibration methods and systems do not account for the errors in estimating the actual size of a reference feature, and also fail to account for degradation in reference features.
The measurement of a calibration standard reference sample typically involves determining where an edge of the sample is. At the sub-micron range, an edge of a sample may be a complex waveform, as opposed to a flat line. Therefore, in measuring the sample there is uncertainty as to edge location. Where the calibration involves determining the length of a sample, two edges must be located, and thus the edge determination uncertainty i
Phan Khoi
Rangarajan Bharath
Singh Bhanwar
Templeton Michael K.
Advanced Micro Devices , Inc.
Amin & Turocy LLP
Kalivoda Christopher M.
Lee John R.
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