X-ray or gamma ray systems or devices – Specific application – Diffraction – reflection – or scattering analysis
Reexamination Certificate
2000-08-09
2003-04-29
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Diffraction, reflection, or scattering analysis
C378S071000
Reexamination Certificate
active
06556652
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to semiconductor manufacturing and process control, and specifically to measurements of critical dimensions of semiconductor device features.
BACKGROUND OF THE INVENTION
When microelectronic devices are produced on a semiconductor wafer, it is crucial that the critical dimensions of the devices be held within specified tolerances. Critical dimensions, in this context, refer to the widths of features, such as conductors, that are deposited on the wafer and the spacing between adjacent features. Deviations from the specified dimensions lead to performance and yield degradation. The manufacturing process must therefore be carefully monitored and controlled, in order to detect deviations as soon as they occur and to take corrective action to avoid the loss of costly wafers in process. For example, when a critical dimension in photoresist that has been deposited and etched on the wafer is found to be out of specification, it is possible to remove and reapply it.
A variety of systems and methods for measurement of critical dimensions are known in the art. Most microelectronic production facilities currently use optical metrology to monitor critical dimensions. As semiconductor devices become ever denser, however, with design rules of 0.25 &mgr;m and below, it becomes impossible for classical optical metrology systems to provide accurate results. Electron beam (e-beam) metrology has been suggested as an alternative, but e-beam systems also suffer from performance limitations.
A further problem in critical dimension measurements is the high aspect ratio and non-uniform width of features created on the semiconductor wafer. For example, in order to produce vias, a layer of photoresist is deposited on the wafer surface. The photoresist is exposed to ultraviolet radiation, hardened and then etched to form trenches, which are subsequently filled with metal. The photoresist is typically 0.7 to 1.2 &mgr;m thick, while the trenches are only 0.1 to 0.2 &mgr;m wide. (This width is the critical dimension of the vias.) Because of the etching process, the walls of the trenches tend to slope inward. The trenches are therefore wider at the top and narrower at the bottom, where they encounter the underlying wafer surface below the photoresist. It is the smaller width at the bottom of the trench that is most critical. Optical and e-beam metrology are not well suited for measuring this dimension, however, because of the high aspect ratio of the trenches.
X-ray reflectance and fluorescence measurements are commonly used to determine the thickness and composition of thin film layers, including particularly metal layers formed on semiconductor wafers. For example, U.S. Pat. No. 5,740,226, to Komiya et al., and U.S. Pat. No. 5,619,548, to Koppel, whose disclosures are incorporated herein by reference, describe film thickness measurements based on X-ray reflectometry.
Another method for thin film measurement using X-rays is described by Hayashi et al., in an article entitled “Refracted X-rays Propagating near the Surface under Grazing Incidence Condition,” published in
Spectrochimica Acta
, Part B 54 (1999), pages 227-230, which is also incorporated herein by reference. The authors irradiated a silicon wafer having an organic thin film coating with X-rays incident at a grazing angle. They measured the energy of the X-rays that propagated along the surface of the wafer and discovered two peaks: one corresponding to refraction at the upper boundary between the thin film coating and the ambient air, and the other to refraction at the boundary between the coating and the wafer substrate. The energies of these peaks correspond to the critical energies of the respective boundaries. Below the critical energy, the X-rays are totally reflected from the boundary, while above the critical energy the X-rays pass through the boundary and are refracted. The critical energy for any given boundary depends on the angle of incidence of the X-rays and on the refractive indices of the materials separated by the boundary.
The methods of X-ray measurement mentioned above, as well as all other known X-ray methods, are limited to measurement of thickness or depth, i.e., of dimensions measured perpendicular to the plane of the wafer substrate. X-ray techniques known in the art do not generally have sufficient spatial resolution for use in measuring critical dimensions of feature width, i.e., dimensions of the features in a direction parallel to the substrate plane.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide improved methods and apparatus for measurement of critical dimensions of microelectronic devices, particularly during stages of wafer fabrication.
It is a further object of some aspects of the present invention to provide methods and apparatus capable of measuring the dimensions of microscopic features on a semiconductor wafer or other substrate having a high aspect ratio of height to width.
In preferred embodiments of the present invention, X-ray scattering from features on the surface of a substrate is detected in order to determine dimensions of the features, and particularly to measure the width of such features. Typically, the substrate comprises a semiconductor wafer, on which a test pattern is formed for the purpose of measuring critical dimensions of functional features of microelectronic devices in fabrication on the wafer. Preferably, the test pattern comprises a grating structure, made up of a periodic pattern of ridges, having attributes (such as height, width and spacing) similar to those of the functional features in question. When an X-ray beam irradiates the pattern, the resultant scattered radiation has a spatial modulation that is characteristic of the critical dimensions. A detection system senses and analyzes the modulation of the scattered radiation in order to determine the critical dimensions.
In some preferred embodiments of the present invention, the scattered radiation received by the detection system comprises reflected radiation. Preferably, the irradiating beam is well collimated and is incident on the surface at a grazing angle, nearly parallel to the surface of the substrate. Most preferably, the beam is incident in a direction whose projection on the surface is parallel to the ridges of the pattern. As a result, spatial modulation in the form of a “shadow” of the test pattern is imposed on the reflected radiation. The critical dimensions of the pattern are determined based on the characteristics of the modulation.
In one of these preferred embodiments, a collimator, which is used to collimate the irradiating beam, itself comprises a grating made of a pattern of ridges formed on a base. Preferably, the ridges on the collimator comprise metal ridges, having a period substantially equal to the period of the test pattern on the substrate under test. Reflection of the X-ray beam from the collimator at a grazing angle both collimates the beam and imposes on it the periodic pattern of the ridges. When the collimated beam is incident on the test substrate, an interference pattern is formed between the pattern imposed by the collimator and the test pattern. The detection system senses the interference pattern in order to determine the critical dimensions of the test pattern. Most preferably, the test substrate is translated relative to the collimator (or vice versa), causing the interference pattern to vary. The detection system senses this variation, which is then analyzed to determine the critical dimensions.
In other preferred embodiments of the present invention, the detection system captures a portion of the scattered radiation that is refracted at the surface of the substrate at an angle that is roughly parallel to the surface. The detection system analyzes the energy of X-ray photons that it captures and thus determines the critical energies for total external reflection at the surface, in a manner similar to that described in the above-mentioned article by Hayashi et al. Two different
Gvirtzman Amos
Mazor Isaac
Yokhin Boris
Hoffman Wasson & Gitler
Jordan Valley Applied Radiation Ltd.
Song Hoon K.
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