Radiant energy – Inspection of solids or liquids by charged particles
Reexamination Certificate
2000-04-26
2002-09-24
Anderson, Bruce (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
C250S307000, C438S014000, C423S447200, C204S157470, C073S105000
Reexamination Certificate
active
06455847
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to scanning probe microscopy, and in particular to a system and method for detecting partially open/closed contacts using a scanning probe microscope employing a nanotube as a scanning tip.
BACKGROUND OF THE INVENTION
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down the device dimensions (e.g., at submicron levels) on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller features sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as optical light, x-rays, etc.) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The lithographic coating is generally a radiation-sensitive coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive image of the subject pattern. Exposure of the coating through a photomask causes the image area to become either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Due to the extremely fine pattern dimensions employed in present day integrated circuits, techniques are being generated to help accurately measure such dimensions. One generic class of tools employed for such high accuracy measurements is the scanning probe microscope (SPM). Generally, scanning probe microscopy provide pictures of atoms on or in surfaces, thereby providing atomic level surface imaging. One form of a Scanning Probe Microscope is an Atomic Force Microscope (AFM), which is sometimes alternatively referred to as a Scanning Force Microscope (SFM). AFMs include a sensor with a spring-like cantilever rigidly mounted at one end and having a scanning tip at a free end. AFMs may operate in contacting and non-contacting modes. In the contacting mode, the tip of an AFM is placed in low force contact with a surface of a semiconductor wafer or other workpiece of interest. The workpiece is then displaced relative to the AFM in one or more directions in a plane (e.g., the tip contacts the workpiece in a Z axis while the workpiece is displaced in the X and/or Y directions), to effect a scanning of the workpiece surface. As surface contours or other topographic features are encountered by the tip during scanning, the cantilever deflects. The cantilever deflection is then measured, whereby the topography of the workpiece may be determined.
In non-contacting operation, the tip is held a short distance, typically 5 to 500 Angstroms, from the workpiece surface, and is deflected during scanning by various forces between the workpiece and the tip. Such forces may include magnetic, electrostatic, and van der Waals forces. In both the contacting and non-contacting modes, measurements of a workpiece topography or other characteristic features are obtained through measuring the deflection of the cantilever. Deflection of the cantilever may be measured using precisely aligned optical components coupled to deflection measurement circuitry, although other techniques are sometimes employed.
Another form of SPM is a Scanning Tunneling Microscope (STM). Where a feature of interest is non-topographic, AFMs as well as STMs may be utilized used to measure the workpiece feature. Examples of non-topographic features include the detection of variations in conductivity of a semiconductor workpiece material. An AFM can be used to scan a workpiece in the non-contacting mode during which deflections in the cantilever are caused by variations in the workpiece conductivity or other property of interest. The deflections can be measured to provide a measurement of the feature. STMs include a conductive scanning tip which is held in close proximity (within approximately 5 Angstroms) to the workpiece. At this distance, the probability density function of electrons on the tip spatially overlap the probability density function of atoms on the workpiece. Consequently, a tunneling current flows between the workpiece surface and the tip if a suitable bias voltage is applied between the tip and the workpiece. The workpiece and tip are relatively displaced horizontally (in the X and/or Y directions) while the tip is held a constant vertical distance from the workpiece surface. The variations in the current can be measured to determine the changes in the workpiece surface.
In another mode of operation, an STM can be used to measure topography. The scanner moves the tip up and down while scanning in the X and/or Y directions and sensing the tunneling current. The STM attempts to maintain the distance between the tip and the surface constant (through moving the tip vertically in response to measured current fluctuations). The movements of the tip up and down can be correlated to the surface topography profile of a workpiece.
In both types of SPMs, the dimensions of the scanning tip is important. It is desirable for high resolution imaging that the tip be sharp and hard. A hard tip will not wear quickly and thus will provide high resolution imaging for a longer period of time before needing to be replaced. Because scanning tips are expensive, such a tip characteristic is desirable. Nevertheless, hard tips will still wear over a period of time, which will result in reduced imaging accuracy by the SPM.
It is therefore desirable to have a system and/or method of utilizing an SPM which provides high resolution imaging/measuring capabilities.
SUMMARY OF THE INVENTION
The present invention relates to a system and method of analyzing a feature such as performing linewidth measurements, performing profile analysis, and analyzing contact holes using a scanning probe microscope (SPM) having a nanotube scanning tip.
According to the present invention, an SPM employs a nanotube scanning tip such as a carbon nanotube. The nanotube scanning tip provides a substantial improvement over conventional scanning tips. The nanotube scanning tip has a length and a cross sectional area associated therewith. The cross sectional area of at least a portion of the nanotube tip's length is generally constant, and thus the scanning resolution provided by the tip is generally constant as the nanotube scanning tip experiences wear, which represents a significant improvement over conventional solutions.
According to one aspect of the present invention, a system for analyzing a film or feature on a substrate (e.g., performing linewidth or profile measurements or analyzing topography) comprises an SPM such as an AFM or an STM which includes a scanning tip assembly. The scanning tip assembly also includes a nanotube scanning tip such as a carbon nanotube. The system further comprises a controller which is operably coupled to the SPM and provides control signals to the SPM to control one or more scanning characteristics associated with the scanning assembly. The controller also receives scanning signals from the SPM associated with the detected tip characteristics associated with the film or feature on the substrate, and provides scanning output data to a user in a format that is capable of analysis by a user. The scanning tip assembly having the nanotube scanning tip provides relatively constant scanning resolution as the sca
Choo Bryan K.
Singh Bhanwar
Yedur Sanjay K.
Anderson Bruce
Eschweiler & Associates LLC
Wells Nikita
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