Optical measurement system and method for measuring critical dimension of nanostructure

An optical measurement system for measuring a critical dimension having a nanostructured surface including a nanostructure formed on a plane. The optical measurement system includes an image recording module including a microscope optical system which records a defocused image having an nonuniform degree of defocusing with respect to the nanostructured surface, an optical scheme parameter control module which sets and outputs to the microscope optical system optical scheme parameters for the microscope optical system, an image calculation module which receives receiving the optical scheme parameters set by the optical scheme parameter control module and calculates an image of the nanostructured surface, and a comparison module which compares the defocused image recorded by the image recording module and the image calculated by the image calculation module.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from Russian Patent Application No. 2011142372, filed on Oct. 20, 2011, in the Russian Patent Office, and from Korean Patent Application No. 10-2012-0085388, filed on Aug. 3, 2012, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entirety by reference.

BACKGROUND

Methods and apparatuses consistent with exemplary embodiments relate to measurement technology, and more particularly, to measuring a geometric variable with respect to a nanostructure by means of scattered light investigation while scanning an object having a nanostructure.

2. Description of the Related Art

In micro-lithography technology developed for manufacturing semiconductor chips, there is a recent trend to reduce a critical dimension (CD) of a manufactured structure. CD signifies a size of a nanostructure to be examined and has a value of about tens of nanometers. Currently, a CD that may be embodied is about 30 nm and will be 20 nm soon.

For mass production of a semiconductor structure having a very small CD, a demand for accuracy and reliability of measurement equipment, speed, and economical efficiency of a measurement process have been increased. Also, a conventional measurement method using a scanning electron microscope (SEM) and an atomic force microscope (AFM) is very slow and expensive particularly when examining a chip having a well-known topology in which critical dimensions of structures are slightly different from required values. For the measurement of the type above, optical methods based on technologies such as ellipsometry [see reference 1] and scatterometry [see reference 2] have been developed. In particular, a well-known optical critical dimension (OCD) method [see reference 3] is used to identify a semiconductor structure having a CD smaller than a Rayleigh limit.

These optical analysis methods have their merits and demerits. In the OCD method, dependence of sub-wavelength structure reflectivity from critical dimension value, wavelength of incident irradiation, and angle of incidence are used. There are two types of methods: one is to fix a wavelength of incident light and measure a degree of dependence on reflectivity with respect to an incident angle of light irradiated onto an object by a through-angle scanning method and the other is to fix an incident angle and measure a degree of dependence on reflectivity with respect to a wavelength by a through-wavelength scanning method. In the second method, spectrums of irradiation of incident and reflected light are measured and a degree of dependence on reflectivity with respect to a wavelength is calculated based on the spectrum of the incident and reflected light. The measured dependence is compared with a dependence calculated with respect to various CD values and the best coincidence of measured curve and calculated curve gives a required CD value.

Although the OCD technology is widely used in a semiconductor manufacturing process, it is impossible to perform an analysis of a non-periodic structure, a low periodic structure, and a structure formed of one or more isolated objects by the technology.

The non-periodic and isolated objects may be analyzed by a through-focus scanning optical microscopy (TSOM) [see reference 4] based on analysis of a non-contrast defocused image of an object recorded using an optical microscope while scanning the object in an optical axis direction.

In the TSOM technology, a mechanical scanning system for moving an object through-focus at an accuracy of tens of nanometers is a basic one and simultaneously is the weakest in view of reliability of a module for measuring a TSOM-plant. As the size of an object decreases and scanning steps decreases, a demand for reduction of scanning steps and the accuracy of positioning of an object increases. In this situation, when vibrations exist, the accuracy and reliability in measurement are generally lowered.

SUMMARY

One or more exemplary embodiments may provide an optical measurement system and method for measuring a CD of a nanostructure based on defocus image processing without using mechanical through focus scanning with respect to an object to be examined.

According to an aspect of an exemplary embodiment, an optical measurement system for measuring a CD having a nanostructured surface including a nanostructure formed on a plane includes an image recording module including a microscope optical system which records a defocused image having an nonuniform degree of defocusing with respect to the nanostructured surface, an optical scheme parameter control module which sets and outputs to the microscope optical system optical scheme parameters for the microscope optical system, an image calculation module which receives the optical scheme parameters set by the optical scheme parameter control module and calculates an image of the nanostructured surface, and a comparison module which compares the defocused image recorded by the image recording module and the image calculated by the image calculation module.

A normal of the nanostructured surface of the sample may be inclined by a predetermined angle θ with respect to an optical axis of an objective lens included in the microscope optical system.

The predetermined angle θ may satisfy a condition that α<θ<2 α, wherein α≈d2/λD, λ is a center wavelength of an illumination spectrum of the microscope optical system, d is a spatial resolution of the objective lens included in the microscope optical system, and D is a maximum character size of the nanostructure.

The optical scheme parameter control module may be configured to measure and/or modify the optical scheme parameters.

The microscope optical system may include a light source, a polarizer, an amplitude mask, a beam splitter, an objective lens, and an image sensor.

The optical scheme parameters may include at least one of a frequency of an illumination spectrum irradiated by the light source, a polarization axis direction of the polarizer, a size and/or a shape and/or a location of an opening of the amplitude mask, a numerical number of the objective lens, and an angle of inclination of the nanostructured surface to an optical axis of the objective lens.

A bandwidth of the illumination spectrum may be 100 nm or less and a wavelength range may be 350 nm˜700 nm.

The amplitude mask may be disposed on a surface that is optically conjugated with a back focal plane of the objective lens.

The size of an opening formed in the amplitude mask may satisfy a condition that 0.1<(NAill/NA)<0.8, wherein NAill is a numerical number of illumination and NA is a numerical number of the objective lens.

A numerical number of the objective lens may have a value between 0.4 and 0.9.

The optical scheme parameter control module may include a spectrometer, an amplitude mask positioning system setting a position of the amplitude mask, a charge-coupled device (CCD) camera which measures a size and a shape of the opening of the amplitude mask, and a nanostructured surface positioning system which determines an angle of inclination of a normal of the nanostructured surface with respect to a normal of the objective lens.

The microscope optical system may use a Kohler illumination method.

The image calculation module may calculate an image by using a rigorous coupled wave analysis (RCWA) method and a finite-difference time-domain (FDTD) method.

According to an aspect of another exemplary embodiment, a method of measuring a critical dimension (CD) having a nanostructured surface including a nanostructure formed on a plane includes selecting an optical scheme parameter of a microscope optical system which records an image of the nanostructured surface, recording a defocused image having an nonuniform degree of defocusing with respect to the nanostructured surface by using the microscope optical system, calculating an image of the nanostructured surface within a predetermined CD range according to a selected optical scheme parameter, and determining an estimated value of a CD by comparing the calculated image with the recorded defocused image.

The T normal of the nanostructured surface of the sample may be inclined by a predetermined angle θ with respect to an optical axis of an objective lens included in the microscope optical system.

The predetermined angle θ may satisfy a condition α<θ<2 α, wherein α≈d2/λD, wherein λ is a center wavelength of an illumination spectrum of the microscope optical system, d is a spatial resolution of the objective lens included in the microscope optical system, and D is a maximum character size of the nanostructure.

The optical scheme parameter may include at least one of a frequency of an illumination spectrum irradiated by a light source, a polarization axis direction of a polarizer, a size and/or a shape and/or a location of an opening of an amplitude mask, a numerical number of the objective lens, and an angle of inclination of the nanostructured surface with respect to an optical axis of the objective lens.

A bandwidth of the illumination spectrum may be 100 nm or less and a wavelength range may be 350 nm˜700 nm.

The size of the opening formed in the amplitude mask may satisfy a condition that 0.1<(NAill/NA)<0.8, wherein NAill is a numerical number of illumination and NA is a numerical number of the objective lens.

A numerical number of the objective lens may have a value between 0.4 and 0.9.

A bright field technology may be used to record the defocused image.

The calculating of the image of the nanostructured surface may include using a rigorous coupled wave analysis (RCWA) method and a finite-difference time-domain (FDTD) method.

The determining of the estimated value of the CD includes using an optimization technique for calculating a CD value when an absolute value between a measured image and a calculated image is minimum.

The determining of the estimated value of the CD may include comparing calculated images from a library of calculated images with the recorded defocused image step-by-step.

The determining of the estimated value of the CD may include extracting, for each of the calculated image and the measured image, a focus metric curve depending on a degree of defocusing and a topology of the nanostructure and comparing the extracted focus metric curves with each other.

DETAILED DESCRIPTION

FIG. 1is a view schematically illustrating an arrangement structure in which a sample2to be examined is arranged at an incline with respect to an objective lens1to detect a defocused image of a nanostructured surface NS in an optical measurement system and method according to an exemplary embodiment.

A nanostructure is a structure in which at least one character has a size less than a Rayleigh resolution limit, that is, about tens of nanometers. In the optical measurement system and method according to the present embodiment, only one defocused image is registered with respect to the sample2to be examined having a nanostructured surface NS and then a CD of the nanostructured surface NS is measured by using the registered defocused image.

Referring toFIG. 1, the sample2is disposed under the objective lens1and has the nanostructured surface NS formed on a plane surface thereof. The sample2is disposed under the objective lens1such that an optical axis6of the objective lens1is inclined by an angle θ with respect to a normal7of the nanostructured surface NS. According to this arrangement, an area4where an image is clearly formed on an image sensor of a microscope optical system, that is, an optimal focus position, exists on the nanostructured surface NS to be examined. Also, areas3and5, where an image is not clearly formed, that is, a non-focus area of the objective lens1, exists on the nanostructured surface NS. A degree of defocusing appears to be nonuniform in a direction in which the sample2is inclined, that is, in a direction x. For example, the degree of defocusing changes linearly depending on the viewing angle. Thus, mechanical through-focus scanning, in which the sample2is moved in a direction penetrating the focus of the objective lens1, which is generally used to obtain an image having a different defocus degree, is not required.

FIG. 2is a block diagram schematically illustrating a structure of an optical measurement system100according to an exemplary embodiment.FIG. 3is a view schematically illustrating an optical arrangement of an image recording module120that can be employed in the optical measurement system100, according to an exemplary embodiment.

The optical measurement system100may be a combination of equipment and software. The optical measurement system100includes an optical scheme parameter setting module110, an image recording module120, an image calculation module130, and a comparison module140.

The image recording module120is an optical module recording an image of the nanostructured surface NS, based on a microscope optical system. The recorded image is an image including complex superposition of diffractive patterns. An analysis to extract an unknown CD may be performed from the recorded image.

Referring toFIG. 3, the image recording module120forms the microscope optical system to obtain a defocused image of the nanostructured surface NS of the sample2to be examined. The image recording module120may employ, for example, a microscope optical system configuration using a Kohler illumination method performed in a bright field mode. The image recording module120may include a light source8, a condenser lens9, a polarizer11, an amplitude mask12, a beam splitter14, the objective lens1, and an image sensor17.

The light source8provides illumination to obtain an image of the sample2. The sample2may be provided with an illumination spectrum of light in a predetermined wavelength range. The illumination spectrum may be determined by the light source8or the light source8and a color filter10.

The beam splitter14branches an optical path to allow light irradiated from the light source8to proceed toward the sample2and also allow light reflected or scattered by the sample2to proceed toward the image sensor17. The beam splitter14may be a polarized beam splitter.

The amplitude mask12includes an opening having a predetermined shape and size and adjusts the size of a cross section of a beam. The amplitude mask12may be arranged on a plane that is optically conjugated with a back focal plane15of the objective lens1.

A tube lens16may be provided between the beam splitter14and the image sensor17. A relay lens13may be provided between the amplitude mask12and the beam splitter14. The image sensor17is a device that converts an optical image to an electric signal and a charge-coupled device (CCD), for example, may be used therefor.

The image recording module120records an image of the nanostructured surface NS according to the optical scheme parameters set by the optical scheme parameter setting module. Also, the optical scheme parameters are transferred to the image calculation module130so that the image calculation module130may calculate an image of the nanostructured surface NS.

The optical scheme parameters include parameters related to the arrangement of optical elements or illumination conditions. For example, the optical scheme parameters may include at least one of a frequency of an illumination spectrum irradiated by the light source8, a direction of a polarization axis of the polarizer11, a size and/or shape and/or position of the opening of the amplitude mask12, a numerical aperture of the objective lens1, and an angle of inclination of the NS to the optical axis6of the objective lens1.

Optimal optical scheme parameters for measurement of the nanostructured surface NS may depend on a topology and a CD value of the nanostructured surface NS and may be determined within a range, as discussed below.

The illumination spectrum may be a spectrum having a bandwidth of about 100 nm or less in a wavelength range of 350˜700 nm.

The amplitude mask12may be disposed on a surface that is optically conjugated with the back focal plane15of the objective lens1. The size of the opening of the amplitude mask12may be set to satisfy a condition that 0.1<(NAill/NA)<0.8, where NAill is a numerical aperture of illumination and NA is a numerical aperture of the objective lens1. The NA of the objective lens1may satisfy a condition that 0.4<NA<0.9.

The angle θ of inclination of the normal7of the nanostructured surface NS to the optical axis6of the objective lens1may satisfy a condition that α<θ<2α. In the condition, α≈d2/λD, where λ is a center wavelength of the illumination spectrum, d is a spatial resolution of the objective lens1, and D is the maximum character size of the nanostructure.

To this end, the optical scheme parameter setting module110may include a spectrometer, an amplitude mask positioning system setting a position of the amplitude mask12, a CCD camera measuring a size and shape of the opening of the amplitude mask12, and a nanostructured surface positioning system setting an angle of inclination of the normal7of the nanostructured surface NS to the optical axis6of the objective lens1.

The image recording module120, assuming that an unknown CD varies within a known range that CD1<CD<CD2, calculates an image with inputs of a predetermined CD value in the range and the parameters set by the image calculation module130. The image calculation module130may calculate an image by using a rigorous coupled wave analysis (RCWA) method [see reference 5] and a finite-difference time-domain (FDTD) method [see reference 6].

The comparison module140compares a calculated image and a measured image. The comparison module140for comparing the measured image and the calculated image of the nanostructured surface NS is an important module in a measurement system. For comparison, the measure image of the image recording module120and the calculated image of the image calculation module130calculated in consideration of a set CD value change range are transferred to the comparison module140as inputs. As a result of comparison, the best estimation of a measured CD value and/or a narrower adjusted range of CD value variation are produced.

The calculation by the image calculation module130and the comparison by the comparison module140continue by changing a CD value until the calculated image and the measured image match. A CD value when the calculated image and the measured image match is the best estimated value of the measured CD value of the nanostructured surface NS.

FIG. 4is a flowchart for explaining an optical measurement method according to an exemplary embodiment.

First, optical scheme parameters for setting a microscope optical system to record an image of a nanostructured surface NS are set (S201). The optical scheme parameters may be the parameters described with reference toFIGS. 2 and 3.

A defocused image of the nanostructured surface NS is recorded by using a microscope optical system formed according to the optical scheme parameters (S202). To record a defocused image, as illustrated inFIG. 1, the sample2having the nanostructured surface NS is arranged at an incline with respect to the objective lens1.

The image of the nanostructured surface NS is calculated in consideration of the set optical scheme parameters and the predetermined CD range (S203). A software module for calculation is realized owing to a combination of methods of electromagnetic field calculation RCWA [see reference 5] and FDTD [see reference 6], which are based on an accurate solution of the Maxwell's equation. Input parameters for calculation are optical scheme parameters set to define conditions for illumination and nanostructured surface recording, and a CD value variation range. In an actual semiconductor manufacturing process, an approximate CD value variation range is well known and may be determined according to the opinions of experts.

Next, the calculated image and the measured image are compared with each other (S204). As a result of comparison, the best estimation of the measured CD value and/or a CD value variation range that is more narrowly adjusted are produced. Some exemplary methods for comparing digital images to be applied to the software module for comparison are known. For example, a result of the comparison may be a number indicating a degree of matching between the measured image and the calculated image. A focus metric parameter [see reference 7] which is defined to analyze images, each having a different degree of defocus may be used and is described below with reference toFIGS. 5A, 5B, and 6.

A degree of matching between a calculated image and a measured image is determined within a given range of accuracy (S205). A CD value at which the two images are matched is output as a measured CD value. If the two images do not match, a CD variation range is changed (S206), an image of the nanostructured surface NS is calculated (S203), and a newly calculated image is compared with the measured image (S204). A more narrowly adjusted CD value variation range may be determined according to a result of the comparison between the measured image and the calculated image.

There are many methods of selecting a CD value by comparing the measured image and the calculated image. For example, an optimization technique and an image library calculation technology may be used. When the optimization technique is used, the optimal estimation of a CD value is obtained when an absolute value between the measured image and the calculated image is minimum. When the library calculation technology is used, the measured image is compared with previously calculated images step-by-step on an assumption that a CD value changes within a known range.

FIG. 5Ais an example of a measurement image of a nanostructured surface of a sample, in which a periodic grating is formed on a glass surface with a period of about 3 μm and a grating line having a height of about 100 nm.FIG. 5Bshows a focus metric curve extracted from the image ofFIG. 5A.

A focus metric parameter denotes a characteristic of contrast of an image in a given degree of defocusing and may be a standard deviation calculated from a digital image indicating a data array like the equation below. Parameters M(Ii,j) depending on a topology of an object and the degree of defocusing are given corresponding to defocused images Ii,j. The parameter characterizes image contrast at the given degree of defocusing.

In the equation, M(Ii,j) denotes a focus metric, Ii,jdenotes a pixel intensity in coordination (i,j) in an image, Ī denotes an average intensity of all pixels of an image, and N denotes the total number of image pixels.

A through-focus focus-metric curve M(h) may be obtained by calculating a focus metric of an image at each defocus position. In the M(h), h is the position of an object in a focus direction.

In the current embodiment in which the nanostructured surface NS is arranged at an incline with respect to the objective lens1, since the degree of defocusing varies according to the direction of inclination of the nanostructured surface NS, the focus metric curve may be calculated along the investigated inclined surface. To this end, a predetermined window is defined. The size of a window is small so that a degree of defocusing may be regarded as a constant in the window. Next, the window is scanned along the nanostructured surface NS and the focus metric is calculated at each window position. A focus metric M(x) is obtained as a result of the image processing, where x denotes a coordinate indicating the position of the window, that is, a coordinate according to a direction in which the nanostructured surface NS is inclined.

InFIG. 5A, an arrow indicates a surface slope direction. Moire strips are easily identified by an arrow direction on the image and scanning windows are seen at some locations on the image.

FIG. 5Billustrates a focus metric curve extracted from the image ofFIG. 5A, that is, an M(x) graph. The shape of the curve and all essential features are dependent on the optical scheme parameters defining conditions for illumination and image recording, and a CD of the grating (having a height of 100 nm).

FIG. 6shows three focus metric curves calculated within the range of grating lines height variation CD−10 nm≦CD≦CD+10 nm, as an example of a library construction of a calculated image for comparison with a measured image.

InFIG. 6, it is easily observed that the focus metric curves M(x) are normalized to 1 and the shapes of curves calculated with respect to grating having a height difference of 10 nm are similar to one another. The curves have three maximum points that are clearly shown. The measured and calculated focus metric curves may be compared with a predetermined parameter related to the measured CD. In this case, the parameter for comparing the focus metric curves extracted from the measurement and the calculation corresponds to the left maximum value of the curves and is a focus metric value satisfying the following condition.
MCD−10nm<MCD<MCD+10nm

When a focus metric curve (MCD*) extracted from the measured image and a calculated focus metric curve (MCD) are determined, with a required accuracy, that MCD*=MCD, a CD value may be assumed to be the best estimation of a measured CD* value.

According to the above-mentioned optical measurement system and method, a single defocused image is measured with respect to a nanostructured surface of a sample to be examined and a CD value is estimated by comparing the estimated defocused image with a calculated image. The single defocused image is an image having a different degree of defocusing. Thus, a process of mechanically scanning a sample in a focus direction in order to obtain an image having a different degree of defocusing is not needed so that reliability, stability, and accuracy of the measurement are high.

According to the above-mentioned optical measurement system and method, a CD of a nanostructure having not only a periodic structure but also a non-periodic structure may be measured.

Although an optical measurement system and method that measure a CD of a nanostructure according to exemplary embodiments are described with reference to the accompanying drawings, it should be understood that the exemplary embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments.

The following is a list of reference documents quoted in the above description.