Patent Application
20090262335
1. Field of the Invention
This invention generally relates to a scatterometry system and, more particularly, to a holographic scatterometry system
2. Background of the Invention
Over the past several years, there has been considerable interest in using optical scatterometry (i.e., optical diffraction) to perform measurements associated with semiconductor fabrication. One area of great interest has been the critical dimension (CD) measurements of two-dimensional structures (e.g., line gratings) and three-dimensional structures (e.g., patterns of vias or mesas) included in integrated circuits. Scatterometry measurements have also been proposed for monitoring etching, planarity of a polished layer, control of gate electrode profiles, film stack fault detection, stepper control, deposition process control and resist thickness control.
Various optical techniques have been used to perform optical scatterometry. These techniques include spectral ellipsometry (i.e., measuring phase and amplitude of the scattered light for fixed glanced incident and azymuthal angle), normal incidence spectral reflectometry (i.e., measuring amplitude of the scattered light for spectrum of wavelengths) and angular reflectometry (i.e., measuring amplitude of the scattered light for spectrum of incident angles). These conventional scatterometry techniques have been used for the 45 nm semiconductor technology node. However, with the downsizing of scatterometry target (e.g., total area or number of features used to scatter light) and feature dimensions (e.g., height and lateral dimensions), the signal-to-noise ratio for scatterometry measurements significantly reduces with each technology node.
Attempts have also been made to combine various optical scatterometry techniques in order to obtain both phase and amplitude information of the scattered light from the measurement target for multiple incident and azymuthal angles. This requires combining different scatterometry techniques and different data libraries together as well as requires an additional challenging analysis for finding the best match from multiple (at least two) data libraries In addition, due to the difference between optical schemes used for the scatterometry, it is a challenge to combine them in a single tool.
Thus, there is a need to overcome these and other problems of the prior art and to provide a scatterometry system that can be used in a single tool to measure both phase and amplitude information for spectrum of incident and azymuthal angles.
According to various embodiments, the present teachings include a method for holographic scatterometry. The holographic scatterometry can be performed by first providing a test light that is coherent with a reference light and is directed to emerge from a test object, followed by bringing the emerged test light and the reference light together on an image sensor to record the holographic information. Such holographic information can include the amplitude information and the phase information of the emerged test light from the test object.
According to various embodiments, the present teachings also include a method for holographic scatterometry. In this method, the amplitude and the phase of a test light that is caused to emerge from a test object can be simultaneously recorded as a holographic pattern using a CCD camera. On the CCD camera, the test light can interfere with a reference light that is derived from a common source with the test light. The recorded holographic pattern can then be compared with a data library to determine a topographic feature of the test object.
According to various embodiments, the present teachings further include a holographic scatterometry that includes a beam splitter. The beam splitter can split an incident light into a reference light and a test light, which is focused on and scattered from a test object. The holographic scatterometry can further include a CCD camera that is placed on a focal plane of where the scattered test light interferes with the reference light to simultaneously and instantaneously record the amplitude and the phase of the scattered test light from the test object.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.
Reference will now be made in detail to the present embodiments (exemplary embodiments) of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. In the following description, reference is made to the accompanying drawings that form a part thereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the invention. The following description is, therefore, merely exemplary.
While the invention has been illustrated with respect to one or more implementations, alterations and/or modifications can be made to the illustrated examples without departing from the spirit and scope of the appended claims. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular function. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.” As used herein, the term “one or more of” with respect to a listing of items such as, for example, A and B, means A alone, B alone, or A and B. The term “at least one of” is used to mean one or more of the listed items can be selected.
Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Moreover, all ranges disclosed herein are to be understood to encompass any and all sub-ranges subsumed therein. For example, a range of “less than 10” can include any and all sub-ranges between (and including) the minimum value of zero and the maximum value of 10, that is, any and all sub-ranges having a minimum value of equal to or greater than zero and a maximum value of equal to or less than 10, e.g., 1 to 5.
Exemplary embodiments provide a system and method for holographic scatterometry by using holography in scatterometry to record amplitude and phase of scattered light from a featured object in order to measure the surface topography and/or feature dimensions of the object. The amplitude and phase information can be obtained simultaneously and instantaneously in a single tool with incident and azymuthal angular resolution.
Specifically, the disclosed holographic scatterometry can include a beam splitter to produce two coherent beams including a test beam and a reference beam. The test beam can be focused on and emerged (e.g., scattered, diffracted and/or reflected) from the featured object and interfered with the reference beam on an image sensor (e.g., a charge-coupled device (CCD) camera). When the reference light and the scattered light are coherent, due to the superposition of the light waves, optical interference between the lights can produce a series of intensity fringes that can be recorded as hologram (or interferogram) on the exemplary CCD camera, which is in focal plane of light collection optical system from the sample object. The resulting holographic information on the camera plane can include all angular (including azymuthal) amplitude and phase information of the scattered light from the measured object.
The holographic scatterometry can thus provide many advantages as compared with conventional optical scatterometry techniques, for example, ellipsometry. As compared, in one aspect, ellipsometry techniques provide dependent or limited amplitude and phase information. This is because ellipsometry measures the change of polarization (which is in turn determined by the sample properties, e.g., thickness, complex refractive or dielectric function tensor) by measuring the ratio or difference of amplitude and phase rather than the absolution value of either. In another aspect, ellipsometry techniques provide restricted or no angular information for phase nor for amplitude. In general, ellipsometry is restricted on one set of amplitude ratio and phase shift per measurement, which covers a fixed spectral range. In addition, conventional optical scatterometry techniques often measure only one of the phase and amplitude in one single tool. For example, angular reflectometry is used to measure amplitude of the scattered/reflected light from the test sample for spectrum of incident angles.
The disclosed holographic scatterometry can be used to measure both amplitude and phase of scattered test light simultaneously and instantaneously for all angles of incidence and all azymuthal angles recorded in the form of hologram (or interferogram) in focal plane of light collection optical system. That is, the holographic information can present scattered light characteristic averaged over scatterometry target of repeated periodic measured objects. In addition, instant amplitude and phase angular (including azymuthal) distribution of scattered light can be provided by the hologram, which can avoid analyzing time-components of the scattered test light (e.g., in order to extract angular dependences) and avoid varying incident angles.
Analyzing time-components of the signal is often used in conventional optical scatterometry, such as, in ellipsometry or interferometry. For example, in general, ellipsometry includes an incident light that is polarized by a polarizer After the incident light is scattered/ reflected from the sample, it passes a second polarizer as an analyzer and then falls into a detector. In ellipsometry, one needs to rotate incident light polarization or characterizers to analyze polarization and to read phase of the scattered light. In another example for interferometry, typically, an incident light is split into two (or more) coherent beams, which travel different paths, and the beams are then combined to create interference and the differences between the beams are detected. When the two beams have the same frequency that have the same phase, they add to each other and interfere constructively to increase the amplitude of the output wave. When the two beams have opposite phase, they subtract to decrease the amplitude of the output. Thus anything that changes the phase of one of the beams by 180°, shifts the interference from a maximum to a minimum. This makes interferometers sensitive measuring instruments for anything that changes the phase of a beam, such as path length or refractive index. For example, time components need to be analyzed from varying path of the two coherent beams.
The holographic scatterometry can thus include a combined power of, e.g., angular reflectometry and ellipsometry, to measure both amplitude and phase of scattered test light simultaneously and instantaneously for all incident and azymuthal angles without analyzing time components as used for other optical scatterometry techniques in the art. Further, by a simple comparison with a single scatterometric data library, the topographic feature information of the measured object can be obtained and there is no need to generate two libraries, e.g., for both angular reflectometry and ellipsometry, as that used in the prior art.
The exemplary scheme shown in
The incident light 110 can be illuminated from an optical source (not shown), such as, for example, lasers, mercury-arc lamps or other optical sources. The incident light 110 can also be a white light or any monochromatic light, for example, blue, green, yellow, red, etc. The incident light 110 can have various incident angles and/or various wavelengths.
The incident light 110 can be partially reflected by the beam splitter 104 to define the reference light 120 and partially transmitted by the beam splitter 104 to define the measurement light 130. The measurement light 130 can be focused by the optical system 135 onto the test sample 140. The optical system 135 can be an objective to process the measurement light 130 that is transmitted from the beam splitter 104. The test sample 140 can be any featured object to be examined in-line or off-line from, e.g., a semiconductor manufacturing. For example, the disclosed holographic scatterometry can be used for, such as linewidth measurements, resist thickness measurements, overlay measurements, or side wall spacer analysis for transistor devices.
The measurement light 130 can be scattered, diffracted, and/or reflected from the test sample 140 at various angles, propagated back through the measurement optical system 135 resulting in an emerged light 130′, and reflected by the beam splitter 104 whereby resulting in a test light 130″. Similarly, reference light 120 can be reflected from the mirror 125, and transmitted through the beam splitter 104 as a reference light 120″ that is interfered with the test light 130″, whereby imaged and recorded by the image sensor 150. In various embodiments, the reference light 120 can be focused by a second optical system, e.g., a reference objective (not shown) having common properties (e.g., matched numerical apertures) with the exemplary objective 135, onto the mirror 125.
The image sensor 150 can be a charge-coupled device (CCD) camera and can be placed in the focal plane of a light collection optical system (not shown) of the interfered coherent lights 130″ and 120″ as shown in
Angular resolution is important for scatterometry applications. This is because the scattered test light in scatterometry often can have a strong signal in a direction that is perpendicular to the incident plane, i.e., out of phase of the incident plane, instead of in phase of the incident plane. In addition, with rapid development of technologies, more complex features and/or geometries can be introduced and used in, for example, semiconductor industries. The complexity of the test structure may scatter the test light in unexpected directions when using scatterometry to measure, e.g., feature sizes and/or critical dimensions. Holographic scatterometry, however, can provide an ability to collect scattered information not only in the incident plane but also in all azymuthal directions. In this manner, holographic scatterometry can differ from conventional scatterometry, where the detector screen is placed in an image plane (e.g., as opposed to the focal plane for holographic scatterometry); sample images (e.g., as opposed to the holographic information) are therefore obtained on the detector plane; and azymuthal angles are not identified to provide angular resolution.
Additionally, incident and azymuthal angular resolution can improve the quality of the scatterometry. For example, angular resolution can improve the signal-to-noise ratio of the disclosed scatterometry. In general, scatterometry utilizes a large target to collect sufficient information from similar structures and average the collected information in order to improve the signal-to-noise properties. Averaged information about structures from the test sample can then be extracted. Due to angular resolution of the holographic scatterometry, the averaged information (e.g., each spot) on the exemplary CCD camera can respond to specific incident angle and specific azymuthal angle without any conversion, which can provide significant improvement on the signal-to-noise ratio and to improve the quality of the holographic scatterometry.
Referring back to
Geometries and/or feature dimensions of the test sample 140 can then be evaluated based on the holographic results obtained on the screen of the CCD camera. The holographic results (i.e., hologram) can include, for example, an information pattern including both amplitude and phase information for all incident angles and all azymuthal angles from the scattered test light from the measured sample. For example, the pattern-type results of the hologram can be a two-dimensional mixture of dots as opposed to an image illustration of structures/shapes of the test sample in a conventional scatterometry. The holographic results can further be analyzed using standard scatterometric data library to transform scatterometry measurements into geometric measurements.
For example, the holographic scatterometric data library can be established in a standard way based on an analysis of a theoretical model that is defined for various physical structures. The theoretical model can predict the empirical measurements that scatterometry systems would record for the structure. The theoretical results of this calculation can then be compared to the measured data from scatterometric signals. To the extent the results do not match, the theoretical model can be modified, calculated once again and compared to the empirical measurements. This process can be repeated iteratively until the characteristics of the theoretical model and the physical structure are very similar. The scatterometric data library, e.g., a data library including 2-dimensional data patterns, can be established.
By comparing the sample pattern from the holographic information on the exemplary CCD camera with the patterns calculated from the established scatterometric data library, the sample character can be obtained from the best match between the sample hologram and the library hologram. The best fit of the library signature can be “automatically” selected based on amplitude and phase information of the scattered light from the test sample. There is no need to generate two types of libraries for both reflectometer (as for amplitude information) and ellipsometer (as for phase information).
In this manner, the holographic scatterometry system (as shown in
In various embodiments, the disclosed holographic scatterometric analysis of a featured object can be used on a real-time basis during, e.g., semiconductor manufacturing, so that manufacturers can immediately determine when a process is not operating correctly. Such need is becoming more acute as the industry moves towards integrated metrology solutions wherein the metrology hardware is integrated directly with the process hardware.
Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.
