We have designed and constructed the calibration system of line standards such as tape and rule for the secondary
calibration laboratories. The system consists of the main body with linear stage and linear encoder, the optical
microscope with digital camera, and the computer.
The base of the system is a aluminum profile with 2.9 m length, 0.09 m height and 0.18 m width. The linear stage and
the linear encoder are fixed on the aluminum profile. The micro-stage driven by micrometer is fixed on the carriage of
the long linear stage, and the optical microscope with digital camera and the tablet PC are on the this stage. The linear
encoder counts the moving distance of the linear stage with resolution of 1 μm and its counting value is transferred to the
tablet PC. The image of the scale mark of the tape is captured by the CCD camera of optical microscope and transferred
to the PC through USB interface. The computer automatically determines the center of the scale mark by image
processing technique and at the same time reads the moving distance of the linear stage. As a result, the computer can
calculate the interval between the scale marks of the tape. In order to achieve the high accuracy, the linear encoder
should be calibrated using the laser interferometer or the rigid steel rule. This calibration data of the linear encoder is
stored at the computer and the computer corrects the reading value of the linear encoder.
In order to determine the center of the scale mark, we use three different algorithms. First, the image profile over
specified threshold level is fitted in even order polynomial and the axis of the polynomial is used as the center of the line.
Second, the left side and right side areas at the center of the image profile are calculated so that two areas are same.
Third, the left and right edges of the image profile are determined at every intensity level of the image and the center of
the graduation is calculated as an average of the centers of the left and right edges at all intensity levels.
The system can measure the line standards up to 2.5 m. The expanded uncertainty for the tape calibration is U = [(0.04)2+ (0.015•L)2]1/2 mm, where L is measured length of the tape or rule in meters. At this system, the long distance
measuring instruments such as ultrasonic distance meter or laser displacement sensor can be also calibrated.
We developed a simple and accurate method for measuring the refractive indices of transparent plates by analyzing the
transmitted fringe pattern as a function of angle of incidence. By using two different wavelengths, we resolved the 2π-
ambiguity inherent to the phase measurement involving a thick medium, leading to independent determination of the
absolute index of refraction and the thickness with a relative uncertainty smaller than 10-5 for a 1 mm-thick fused silica plate. The accuracy of our method was confirmed with a standard reference material.
We propose a microscopic system which could be applied to three-dimensional surface profile measurement. In the
system, a two-dimensional pinhole array is imaged onto the surface under measurement by an objective lens. These spots
act as discrete object points which are then imaged to the CCD chip by the microscope which contains two orthogonal
cylindrical lenses. Due to the astigmatism of the two cylindrical lenses, the shape of the image of object points on the
CCD camera becomes oval unless the object point is located at a position which satisfies the best imaging condition. By
calculating the focus error signal using the intensities measured at a group of CCD cells, the information on the distance
of the corresponding object point could be found out.
The basic concept of the system was checked by computer simulation on the point spread function of various object
points. A preliminary measurement system which consists of the same optical components used in the computer
simulation has been set up for verification of the idea. Since this system requires only one image to analyze the surface
profile, it is a one-shot measurement system, and is insensitive to environmental noises such as mechanical vibration.
A total integrated scattering (TIS) system consisting of an integrating sphere has been developed in KRISS for the
purpose of measuring the effective roughness amplitude of gauge blocks and platens, which are necessary for the
correction of phase shift due to roughness difference between gauge block and platen, in the calibration of gauge blocks
by optical interferometry. Details on the TIS system and its calibration by using two different methods are described. The
uncertainty of the effective roughness amplitude measurement by using the TIS system is evaluated to be 2 nm (k=1).
This paper presents the design and fabrication of a precision dual level stage composing a dimensional metrological
system for large range surface topography, such as mask patterns for lithography, fine artifacts on a semi-conductor
wafer and micro roughness on a large specular surface. The stage was configured as dual level, a fine stage on a coarse
stage, to obtain large moving range and high resolution simultaneously. In the design of the coarse stage, we focused on
a simple structure with low profile to achieve insensitivity to vibration and high accuracy. Therefore, a high quality flat
surface plate was used as the reference plane of the coarse stage's movement, instead of a conventional simple stacking
of two long stroke one-axis stages. The surface plate also has a role of metrological frame for very low thermal
expansion coefficient and its size is 800 mm × 800 mm. The coarse stage is guided horizontally by a cross structure with
two precision straight bars perpendicularly linked and vertically by the surface plate. The sliding pads made of PTFE are
used to guarantee the smooth motion of the coarse stage for both horizontal and vertical directions. The fine stage fixed
on the coarse stage generates five-axis fine motion, such as two-axis in-plane translation, one-axis in-plane and two-axis
out-of-plane rotation. The fine stage is composed of flexure guided structures and actuated by five PZTs. The developed
dual level stage can achieve a large range of 200 mm × 200 mm and a nanometric resolution simultaneously. Its
movement is monitored and controlled using a five-axis laser interferometer system to be applied to a dimensional
metrology having direct meter-traceability.
A compact linear and angular displacement measurement device was developed by combining a Michelson interferometer in Twyman-Green configuration and an autocollimator to characterize the movement of a precision stage. A precision stage usually has 6 degrees of freedom of motion (3 linear and 3 angular displacements) due to the parasitic motions, thus linear and angular displacement should be measured simultaneously for the complete evaluation of precision stage. A Michelson interferometer and an autocollimator are typical devices for measuring linear and angular displacement respectively. By controlling the polarization of reflected beam from the moving mirror of the interferometer, some parts of light are retro-reflected to the light source and the reflected beam can be used for angle measurement. Because the interferometer and the autocollimator have the same optic axis, the linear and angular displacements are measured at the same position of the moving mirror, and the moving mirror can be easily and precisely aligned to be orthogonal to the optic axis by monitoring the autocollimator's signal. A single mode polarization maintaining optical fiber is used to deliver the laser beam to the device, and all components except the moving mirror are fixed with bonding to achieve high thermal and mechanical stability. The autocollimator part was designed to have the angular resolution of 0.1" and the measurement range of 60". The nonlinearity error of interferometer was minimized by trimming the gain and offset of the photodiode signals.
The pitch and orthogonality of two-dimensional (2D) gratings have been calibrated by using an optical diffractometer (OD) and a metrological atomic force microscope (MAFM). Gratings are commonly used as a magnification standard for a scanning probe microscope (SPM) and a scanning electron microscope (SEM). Thus, to establish the meter-traceability in nano-metrology using SPM/SEM, it is important to certify the pitch and orthogonality of 2D gratings accurately. ODs and MAFMs are generally used as effective metrological instruments for the calibration of gratings in nanometer range. Since two methods have different metrological characteristics, they give complementary information for each other. ODs can measure only mean pitch value of grating with very low uncertainty, but MAFMs can obtain individual pitch value and local profile as well as mean pitch value, although they have higher uncertainty. Two kinds of 2D gratings, each with the nominal pitch of 700 nm and 1000 nm, were measured, and the uncertainties of calibrated values were evaluated. We also investigated the contribution of each uncertainty source to the combined standard uncertainty, and discussed the causes of main ones. The expanded uncertainties (k = 2) of calibrated pitch values were less than 0.05 nm and 0.5 nm for the OD and the MAFM, and the calibration results were coincident with each other within the expanded uncertainty of the MAFM.
To calibrate a squareness standard and a height micrometer, the Korea Research Institute of Standards and Science (KRISS) has built a new linear measuring machine moving vertically. The main requirement on design of the machine is to achieve the flexibility to calibrate several kinds of standards such as square master, cylindrical square, height micrometer and linear height gauge which are positioned vertically on the surface plate. The system consists of a precision granite column with an air bearing state, a laser interferometer and two electronic probes. In order to calibrate the squareness standards, the granite beam is used as a reference of squareness and a guide of vertical movement. The instrument incorporates a frequency stabilized He-Ne laser. The vertical movement is measured by a laser interferometer whose operation is based on the heterodyne measurement technique. Positioning for calibrating the height micrometer is undertaken by using a vertical stage and a horizontal stage, and its measurement is performed by combining the laser interferometer reading and the electronic probe readings. The paper gives a description of the system and a preliminary measurement results.
The characteristics of the carbon nanotube AFM tip was investigated as it is used to measure the critical dimensions in the high aspect ratio structures. The research has been done to demonstrate the limitations of the CNT probe in imaging steep or vertical sidewall. Two kinds of samples, silicon dot and the lines in the ArF resist pattern were profiled by using carbon nanotube tip in the tapping mode AFM. There is a large oscillation at the steep sidewall, which cannot be controlled by merely changing scan variables, except by slowing down the scan up to the impractical level. The interaction between the long, slim CNT probe and the vertical sidewall severely limits the usefulness of AFM as a CD metrology tool. To achieve hi-resolution and high aspect ratio imaging simultaneously, a stiffer and/or modifed probe under clever non-contact 2D feedback is needed.
We proposed a novel interferometer, which can measure the 3-dimensional coordinates of freely moving objects in space. The interferometer is composed of two fiber ends and three photo detectors. Frequency stabilized laser is incident into an optical fiber, and then split by the fiber beam splitter into two fiber ends. One fiber is used as the fixed reference arm, and the other as the movable test arm of the interferometer. By using servo control on one arm, the phase difference of the laser beams being emitted from the two fiber ends is forced to be kept constant. Beams from each fiber end are combined at the photo detectors on which interference fringes are formed. The displacement of the distances of the moving fiber end (MFE) from the three detectors can be measured by analyzing the interference fringe. From the measured distances of the MFE from each detector, the 3-dimensional coordinates of the MFE are obtained in real time by using the triangulation. Since the wavefront of the beam from the MFE is spherical, the displacement of the MFE is not restricted to a specific direction, which requires no guiding rail for the moving object. This interferometer could be applied to situations where 3-dimensional coordinates of freely moving objects are being measured with high precision.
A phase encoding electronics capable of compensating the nonlinearity in a heterodyne laser interferometer is described. The system consists of the phase demodulating electronics and the nonlinearity compensating electronics. For phase demodulation, the device uses the phase-quadrature mixing technique. And, for nonlinearity compensation, the offsets, the amplitudes and the phase of two output signals from demodulator are adjusted electrically so that their Lissajous figure can be a circle. As a result, the correct phase can be obtained. The brief analysis of nonlinearity in heterodyne interferometer and the design of the phase encoding electronics are presented. The test was performed in a Michelson-type interferometer using a transverse Zeeman stabilized He-Ne laser. The experiment demonstrates that this method can encode the phase of the heterodyne interferometer with sub-nanometer accuracy.
Since the carbon nanotube (CNT) tip was first reported in 1996, its application to CD metrology has been an interest because of its unique properties. It is needle-like, which is the ideal shape as the scanning probe. In addition it elastically buckles, and therefore is robust and does not produce damage to the surface. It is also hydrophobic. The conventional AFM tip has the pyramidal or conical shape, so that either the steep side wall or high aspect ratio pattern like deep and narrow trench is difficult to measure, although the obtained image tends to be more stable than with the other kinds of tips. Recently, FIB(focused ion beam) tip or HDC(high dense carbon) tip with high aspect ratio structure became commercially available, improving the measurement capability of AFM. They are, however, weaker compared with the conventional tip, and still subject to the wear and contamination. In spite of the theoretical advantages, more researches seem to be needed either to make the CNT tip practicable for the precision CD measurement or even to examine its usefulness as a routine metrology tool. At KRISS, the geometry of the CNT tip is being improved by using the 3D manipulator in SEM for better performance, while commercial CNT tips are already available. Two kinds of patterns, groove and dot, have been measured with the three kinds of tips(conventional, HDC, and CNT as home-made). The AFM measurement results are compared, and the characteristics of CNT tips at the current stage, are discussed from the practical point of view.
We have developed a linewidth/pitch measurement system of wafers used in semiconductor manufacturing process. The system was modified from a conventional scanning electron microscope and can be used to measure. In our system we can measure by two methods. The first method is digital scanning method in which the electron beam is digitally scanned by two D/A converters and the signal of secondary electron obtained by an A/D converter is analyzed. And then we can determine linewidth/pitch using edge defining algorithm. The second method is absolute method using laser interferometer in which the electron beam is fixed and the specimen is set on a precise scanning stage driven by a piezo electric transducer. The linewidth/pitch of the specimen has been determined from the signal of edge and the distance between signal of left and right edges is measured by laser interferometer. In this paper, we describe our overall measuring system and measurement method and show the uncertainty estimation of digital scanning method according to a guide published by the International Organization for Standardization (ISO). In this estimation, the expanded uncertainty from instrument and sample was 0.040 micrometer and that only from instrument was 0.033 micrometer.
KEYWORDS: Scanning electron microscopy, Electron beams, Interferometers, Electron microscopes, Monte Carlo methods, Image resolution, Calibration, Integrated circuits, Sensors, Distortion
Two methods of measuring the linewidth of IC using a scanning electron microscope (SEM) have been studied. In the first method, the electron beam was digitally scanned by D/A converters and the signal intensity of secondary electrons obtained by an A/D converter was analyzed by image processing technique to determine the linewidth. This method was found to be very simple and fast, but it was necessary to have a standard specimen to calibrate the magnification of the SEM. Moreover, the distortion of the electron optics induced additional errors in the linewidth measurement. In the second method, the electron beam was fixed and the specimen was set on a precise scanning stage driven by a piezoelectric transducer. The linewidth of the specimen has been determined from the signal intensity of the secondary electron and the displacement of the stage measured by a laser interferometer. This method was used to calibrate the linewidth of the standard specimen. For this study, a system which can be used to measure the linewidth by either of the two methods has been developed. The Monte Carlo simulation was also carried out to obtain the intensities of secondary and backscattered electrons. The results of the measurements and the simulation are discussed.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.