Radiant energy – Calibration or standardization methods
Reexamination Certificate
2000-06-15
2004-07-27
Lee, John R. (Department: 2881)
Radiant energy
Calibration or standardization methods
Reexamination Certificate
active
06768102
ABSTRACT:
TECHNICAL FIELD
The invention relates to the field of micro imaging and, in particular, to methods of recalibration to compensate for thermal drift between a micro-imaging system and a sample under investigation.
BACKGROUND OF THE INVENTION
Semiconductor integrated circuits (ICs) are reverse-engineered for the purposes of validation and product quality assurance. Typically a large group of high magnification tile images representative of a sample IC are acquired using a high magnification micro-imaging system in between deconstructive steps. Subsequent processing of the tile images includes their assembly into one or more photo-mosaics. Each photo-mosaic is representative of the IC sample, or portion thereof, at a particular deconstructive step.
Representative dimensions that have to be resolved by the micro-imaging system are related to the width of traces, which are connections between components on the ICs. Trace width reduction is a goal sought in the semiconductor manufacturing industry to provide increased integration, increased switching speed, reduction in drive voltages, etc. Today typical trace widths are in the micron and submicron range.
The time required for micro imaging an IC sample at each deconstructive step is typically on the order of hours or days. It is well known in the art of material sciences that all known materials are subject to temperature induced deformation. The degree of deformation is dependent on a particular material's coefficient of thermal expansion. A high magnification optical microscope is commonly used in micro imaging. The microscope typically has an arm supporting high magnification optic elements above the sample IC. It has been observed that the components of the micro-imaging system are subject to temperature induced deformation. It has also been observed that the temperature-induced deformation is time dependent, typically having a linear variation. This phenomenon is commonly referred to as “thermal drift”. Thermal drift between the optics and the sample IC during micro imaging can cause misalignment between images acquired at different times. It will be understood by those skilled in the art that an imaging system other than optical, such as a beam instrument as used in a Scanning Electron Microscope (SEM) or Focus Ion Beam (FIB), could also be used.
Considering the size of the arm of the optical microscope and coefficient of thermal expansion of the materials used in manufacturing the arm (such as aluminum), thermal drift can sometimes cause a misalignment in excess of 20 microns cumulative between images. Therefore, thermal drift can be a significant impediment to acquiring high magnification images and assembling the images into photo-mosaics representative of a surface of interest of the IC sample.
Misalignments between the optical system and the IC sample during micro-imaging result in either an excess of overlap between the acquired images, or the formation of inter-image gaps. Having excess overlap leads to waste processing time. Having incomplete mosaics due to inter-image gaps results in an inability to validate the IC design. One option for reducing thermal drift is to wait for the temperature of the sample IC and imaging apparatus to stabilize, assuming stable temperature conditions. However, suitable conditions are very rare. Another option is to enforce dynamic temperature control during the micro-imaging process. This is an expensive option that is difficult to achieve in practice, at least partly because the image acquisition process itself generates heat because the sample IC is positioned for each image acquisition by an electromechanical drive mechanism that generates heat when operated. Other factors related to temperature control are heat capacity and heat conductivity coefficients of materials, which impose limits on how quickly, and at what cost dynamic temperature control can be effected.
There therefore exists a need for methods and apparatus for dynamically recalibrating a micro-imaging system to compensate for thermal drift.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method of determining thermal drift between a micro-imaging system and a sample to compensate for temperature-induced deformation.
It is another object of the invention to ensure compensation for misalignment between a micro-imaging system and an imaged sample based on measured thermal drifts determined during a photo-mosaic acquisition period.
It is a further object of the invention to provide a method of determining thermal drift between the micro-imaging system and the sample at predetermined time intervals during a photo-mosaic acquisition period.
It is a further object of the invention to provide a method of determining thermal drift between the micro-imaging system and the sample based on an adaptive recalibration time interval responsive to a rate of thermal drift.
In accordance to one aspect of the invention a method is provided for determining a measure of thermal drift between the micro-imaging system and a sample. The method includes an initial calibration step performed prior to an acquisition of a series of high magnification images of a surface of interest of the sample. The initial calibration includes positioning a pre-selected calibration location on the sample in a field-of-view of the micro-imaging system, determining a reference calibration focus setting by focusing the micro-imaging system on the sample at the pre-selected calibration location and capturing a reference calibration image. The recalibration is triggered during the acquisition of the images to determine the thermal drift. The thermal drift determination includes repositioning the pre-selected calibration location in the field-of-view of the micro-imaging system, determining a recalibration focus setting, capturing a recalibration image, determining a planar shift from a correlation between the reference calibration image and the recalibration image. The determined planar shift and a difference between the reference calibration focus setting and the recalibration focus setting represent the measure of thermal drift between the micro-imaging system and the sample.
A method is provided for acquiring high magnification tile images of an integrated circuit sample using an optical system subject to thermal drifts. A field of view of a high magnification power optical system is positioned over a surface of the sample at a predetermined location. A tile image of the surface is captured and stored. On detecting a trigger event, a thermal drift between the optical system and the sample is determined with respect to a predetermined calibration location on the surface of the sample. The acquisition of tile images is continued in accordance with predefined rules respecting a degree of thermal drift. The rules include aborting tile image acquisition in the event of excessive thermal drift, recapturing all tile images since a last recalibration in the case of a large thermal drift, and otherwise continuing tile image acquisition.
In accordance with another aspect of the invention the trigger event includes the expiration of a recalibration time interval and the recalibration time interval is adaptively varied based on rules respecting the degree of the thermal drift. As such the recalibration time interval is increased when thermal drift is slight and decreased when thermal drift is significant. Other rules ensure that recalibrations are performed during the tile image acquisition period, and that recalibrations occupy only a certain amount of processing time.
The invention also provides a micro-imaging system for acquiring high magnification tile images of a sample while the system is subject to thermal drift, the tile images being used to construct a seamless photo-mosaic of a surface of interest of the sample. The micro-imaging system comprises means for positioning the surface of interest in a field of view of a micro-imaging system at a location on the sample; means for storing a position of the location; means for focusing the field
(Ogilvy Renault)
Chipworks
Gurzo Paul M.
Lee John R.
Wood Max R.
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