Annular apodizer for small target overlay metrology.

Abstract

Metrology is performed on a semiconductor wafer using a system with an apodizer. Spots with diameters between 2 nm and 5 nm are formed on the semiconductor wafer. The wavelength of the associated light beam is between 400 nm and 800 nm. Small target measurements can be performed over a range of optical wavelengths.

Description

[Technical field]

[0001] This disclosure relates to semiconductor metrology. [Background technology]

[0002] As the semiconductor manufacturing industry evolves, the demands on yield management, especially for metrology and inspection systems, are intensifying. As critical dimensions continue to shrink, the industry still needs to reduce the time to achieve high yields and high value production. Reducing the total time from detecting a yield problem to correcting it maximizes the return on investment for semiconductor manufacturers.

[0003] In fabricating semiconductor devices, such as logic and memory devices, semiconductor wafers are typically processed using a number of manufacturing processes to form the various features and multiple tiers of the semiconductor devices. For example, in a semiconductor manufacturing process known as lithography, a pattern is transferred from a reticle onto a photoresist disposed on the semiconductor wafer. Further examples of semiconductor manufacturing processes include, but are not limited to, chemical mechanical polishing (CMP), etching, deposition, and ion implantation. An array of multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.

[0004] Metrology processes are used to monitor and control various steps during semiconductor manufacturing. Metrology processes are distinct from inspection processes, which detect defects on wafers, and are used to measure one or more wafer characteristics that cannot be determined using existing inspection tools. Metrology processes can be used to measure one or more wafer characteristics, which in turn can be used to determine process performance. For example, metrology processes can measure the dimensions (e.g., line widths, thicknesses, etc.) of features formed on the wafers during the process. Additionally, if the one or more wafer characteristics are unacceptable (e.g., outside a predetermined range for the one or more characteristics), the measurements of the one or more wafer characteristics could be used to modify one or more parameters of the process so that additional wafers produced by the process exhibit acceptable characteristic(s).

[0005] Scatterometry overlay metrology uses the asymmetry between the intensity of +1 and -1 diffraction orders reflected from a grating-over-grating structure. The gratings are placed in the layers for which the overlay is to be measured. Light diffracted by the target is collected by a CCD camera in the pupil plane of the metrology tool. As an example, a scatterometry overlay (SCOL) target consists of four cells, two cells per direction, with a known additive overlay shift ±f0. A relative shift between the current and previous layer due to lithographic misalignment leads to a symmetry break, which results in an intensity difference between +1 and -1 diffraction orders. Such a difference can be related to an overlay value.

[0006] Apodizers in scatterometry can be used with on-axis or off-axis illumination. Both on-axis and off-axis apodizers present challenges when measuring small targets.

[0007] On-axis illumination has a limited spectral range because the diffraction direction is along λ / pitch, where λ is the wavelength. For small targets, the pitch must be small to have enough bars in the target, so objectives lacking a pupil cut can only collect short wavelengths. To allow enough intra-pupil space for multi-order diffraction collection, a small pupil opening (illumination numerical aperture (NA)) creates a large spot on the target. A large spot creates target noise as the incoming light diffracts as it hits the edge of the target.

[0008] Off-axis illumination is more tolerant of diffraction angles and can accommodate a wider wavelength range, but requires at least two illumination spots to collect the two orders needed for overlay calculations, and the interference on the target creates a large spot, making it more sensitive to target position. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Patent No. 10006865 [Patent Document 2] US Patent Application Publication No. 2018 / 0180406 Summary of the Invention [Problem to be solved by the invention]

[0010] Using the red spectrum (approximately 600nm to 800nm) with conventional metrology tools has been difficult because the red spectrum requires an off-axis apodizer. There are certain applications for which an off-axis apodizer is not suitable. Interference can occur, which can affect pupil-to-field differences. For small targets this can cause the beam to strike the edge of the specimen, and possibly even strike outside the specimen.

[0011] Improved systems and techniques are needed. [Means for solving the problem]

[0012] In a first embodiment, a method is provided. The method includes generating a light beam having a wavelength between 400 nm and 800 nm. Directing the light beam through an apodizer. Performing metrology on a semiconductor wafer using the light beam. Using the apodizer, causing the light beam to form a spot on the semiconductor wafer having a diameter between 2 nm and 5 nm.

[0013] The semiconductor wafer may have a diffraction-based overlay target.

[0014] The apodizer may define an aperture with a numerical aperture of about 0.2.

[0015] The wavelength can be 400 nm to 600 nm or 600 nm to 800 nm.

[0016] The metrology can be performed in the field of view.

[0017] The light beam can be used in the form of on-axis illumination or off-axis illumination.

[0018] In a second embodiment, a system is provided. The system includes an illumination source generating a light beam, an apodizer disposed on a path of the light beam, and a stage configured to hold a semiconductor wafer on the path of the light beam. The illumination source generates the light beam to have a wavelength between 400 nm and 800 nm. Using the apodizer, the light beam is configured to form a spot on the semiconductor wafer having a diameter between 2 nm and 5 nm.

[0019] The wavelength can be 400 nm to 600 nm or 600 nm to 800 nm.

[0020] The metrology can be performed in the field of view.

[0021] The apodizer may define an aperture with a numerical aperture of about 0.2.

[0022] The apodizer can be located near the pupil plane.

[0023] For a more complete understanding of the nature and objects of the present disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: [Brief description of the drawings]

[0024] [Figure 1] FIG. 1 illustrates one embodiment of an annular apodizer disk according to the present disclosure. [Diagram 2] FIG. 2 is a diagram illustrating an example of an illumination pupil. [Diagram 3] FIG. 2 shows the resulting spot on the target. [Figure 4] FIG. 13 illustrates illumination pupil cuts for annular, off-axis, and on-axis. [Diagram 5] FIG. 1 shows pupil images for 400 nm for three different wavelengths, with outline markings of areas from which overlay is determined. [Figure 6] FIG. 1 shows pupil images for 600 nm for three different wavelengths, with outline markings of areas from which overlay is determined. [Figure 7] FIG. 1 shows pupil images for 800 nm for three different wavelengths, with outline markings of areas from which overlay is determined. [Figure 8] 1 is a flow chart of one embodiment of a method according to the present disclosure. [Figure 9] FIG. 1 is a block diagram of a system according to the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Although the claimed subject matter is described in terms of certain embodiments, other embodiments are within the scope of this disclosure, including embodiments that do not provide all of the benefits and features described herein. Various structural, logical, process step, and electronic changes may be made without departing from the scope of this disclosure. Thus, the scope of this disclosure is defined solely by reference to the appended claims.

[0026] Annular apodization can be used to measure small grating targets in SCOL metrology tools. The disclosed embodiments allow for small target metrology in an optical wavelength range. In particular, the small target can have a diffraction-based overlay (DBO).

[0027] In the method 100 of Figure 8, a light beam having a wavelength between 400 nm and 800 nm is generated at 101. For example, the wavelengths used in the method 100 can be between 400 nm and 600 nm or between 600 nm and 800 nm. Specifically, the wavelengths can be or include red wavelengths between about 600 nm and 800 nm (e.g., 620 nm and 750 nm). Wavelengths below 400 nm or above 800 nm may be difficult to use effectively because light sources may be low intensity in those wavelength ranges and anti-reflective coatings in the system may affect the results.

[0028] At 102, the light beam is directed through an apodizer. For example, one embodiment of an annular apodizer disk is depicted in Figure 1. The apodizer defines an aperture with an NA of approximately 0.2, although the NA could be ±5%, ±10%, or ±25% of that value.

[0029] The apodizer can have a large aperture resulting in a tight light beam spot. A clear area in the center of the collection pupil aperture allows a wide range of working wavelengths for the light beam. A wider wavelength range or pitch range is also possible.

[0030] The apodizer edge can be refined to reduce the Airy rings. For example, cos 2 A function can be used to reduce spot wings and avoid cell edges.

[0031] The ring parameters can be selected to best fit and / or be optimized for a recipe, layer or target. A wider ring would allow more light to be admitted to produce a tighter spot for dark layer measurements. A tighter ring would allow for a wider spectral range. With that additional spectral range, wavelengths up to 800 nm could be achieved with sufficient light.

[0032] Returning to FIG. 8, metrology is performed on the wafer using the light beam at 103. The diameter of the spot formed on the semiconductor wafer by the light beam is approximately 2 nm to 5 nm using an apodizer, although other diameters are possible. The spot diameter range can also be ±5% or ±10% of the range. Spot sizes within this range are difficult to obtain with conventional metrology tools because they have small NAs. In general, larger NAs result in smaller spot sizes. If a smaller spot size is used, metrology can be performed on smaller targets since the spot does not strike the edge or outside of the target.

[0033] Method 100 may be performed, at least in part, by at least one computer processor (e.g., in a weighing module). In certain embodiments, a computer readable program configured to perform method 100 is embodied by a computer readable storage medium in a computer program product.

[0034] In one example method 100, a stationary diffractive target is illuminated by a stationary illumination source. Overlay information is determined based on the intensity difference between the +1st and -1st orders.

[0035] The resulting spot on the wafer is presented in Figures 2-3. The spot is well localized within a 3 x 3 micron (micrometer) area, even with the inevitable array ring resulting from the annular assembly, e.g., the full width at half maximum (FWHM) of the spot is about 200 nm to 250 nm in Figure 3.

[0036] The semiconductor wafer can have a diffraction-based overlay target having a dimension of about 16 μm to about 16 μm, and the target can be made larger than the spot size to prevent the spot from interacting with the edge of the cell.

[0037] The metrology in 103 can be performed in the field plane. The light beam in method 100 can be used with on-axis illumination or off-axis illumination. An apodizer can determine whether the illumination is on-axis or off-axis.

[0038] To compare the spot size of the novel annular apodizer with that of a conventional apodizer, a cut through the middle lobe of the generated spot is presented in Figure 4. Intensity is shown on the vertical axis in Figure 4. The spot with the novel apodizer disclosed herein ("annular") is better for small targets than conventional on-axis or off-axis systems.

[0039] In the disclosed embodiments, the light at the edge of the pupil plane can be collected at a wider angle than with an on-axis apodizer, allowing overlay to be measured over a wider wavelength range. In Figures 5-7, the collection pupil is shown at three separate wavelengths across the entire usable spectral range. Overlay can be measured over the entire wavelength range. In Figures 5-7, the area from which overlay is determined is marked by a contour in the inner ring.

[0040] 9 is a block diagram of an example metrology tool system 200. System 200 may be used to perform method 100 or other embodiments of the present disclosure.

[0041] 9 depicts several metrology arrangements that may be used alternatively or complementary. The system 100 comprises a stationary illumination source 201 configured to illuminate a stationary diffraction target 202 on a stage 211, a metrology unit 203 configured to repeatedly measure a signal composed of diffraction signals (e.g., a zeroth order diffraction signal and a first order diffraction signal) while the diffraction target 202 and illumination source 201 are stationary, and a processing unit 204 configured to derive the diffraction signals. The diffraction target 202 may be a diffraction-based overlay target, although other designs may be used. The diffraction target 202 may be a portion of a semiconductor wafer. The metrology unit 203 may be configured to perform measurements in a pupil plane 205 and / or in a field plane 206.

[0042] An example of the illumination source 201 generates a light beam having a wavelength of 400 nm to 800 nm (e.g., 400 nm to 600 nm or 600 nm to 800 nm). The stage 211 is configured to hold a semiconductor wafer with a diffraction target 202 on the path of the light beam. The light beam from the illumination source 201 forms a spot on the semiconductor wafer with the diffraction target 202 with a diameter of 2 nm to 5 nm.

[0043] The illumination may be coherent and measurements may be performed in a pupil plane 205 (shown diagrammatically) relative to a diffractive target 202. Appropriate selection of the illumination wavelength and pitch of the diffractive target 202 can result in partial overlap of the zeroth and first diffraction orders at the pupil plane 205.

[0044] The illumination may be incoherent and measurements may be performed in a field plane 206 (shown diagrammatically) relative to the diffractive target 202. The system 200 may further include a mask 207 at a pupil plane 205 of the diffractive target 202. The mask 207 may be configured to block first order diffraction signals that are not of interest for measurement. An apodizer may be located at or near the pupil plane 205.

[0045] The metrology unit 203 may be configured to perform one or more measurements of the phase relationship between the diffraction signals (e.g., between the zeroth and first order diffraction signals). The metrology unit 203 may further be configured to perform iterative measurements for multiple angles and / or phases and / or wavelengths of the illumination to modify the relationship between the diffraction signals. The system 200 may optionally include an optical phase scanner 212 (e.g., digital light processing elements, tiltable mirrors, focus aberrating elements, etc.) configured to perform iterative phase measurements. A reference beam 208 may be used instead of or in addition to the zeroth order diffraction signal to enhance the measurement of the first order diffraction signal as described herein. The system 200 may include a beam displacer 208 configured to modify the illumination beam incidence angle between the iterative measurements. Physical movement of the diffractive target 202, the illumination source 201, and / or the measurement plane may replace or augment the phase scan.

[0046] The system 100 may include order separation optics 209 configured to separate field of view signals associated with different diffraction orders, which are measured by at least two respective detectors 210 associated with a metrology unit 203 which measures the separated field of view signals. The detectors 126 may, for example, be part of the metrology unit 203. Due to the order separation using optics, the combined 0th and ±1st diffraction orders may be detected simultaneously on separate detectors 210. Such field plane detection has the advantage of simultaneous measurement of all target cells.

[0047] The metrology unit 203 and / or processing unit 204 and the subsystems therein may comprise a personal computer system, an image computer, a mainframe computer system, a workstation, a network appliance, an internet appliance, or other device. The subsystem or system may comprise any suitable processor known in the art, such as a parallel processor. Additionally, the subsystem or system may comprise a platform with high speed processing and software, whether a stand-alone tool or a networked tool. Although depicted as separate units, the metrology unit 203 and / or processing unit 204 may be components of a single system.

[0048] In certain embodiments, the metrology unit 203 and / or processing unit 204 and subsystems therein and various steps, functions and / or operations of the methods disclosed herein may be performed by one or more of electronic circuits, logic gates, multiplexers, programmable logic devices, ASICs, analog or digital controllers / switches, microcontrollers and information processing systems. Program instructions implementing the methods, such as those described herein, may be transmitted or stored on a carrier medium. The carrier medium may include storage media, such as read-only memory, random access memory, magnetic or optical disks, non-volatile memory, solid-state memory, magnetic tape, etc. The carrier medium may include transmission media, such as wires, cables or wireless transmission links. For example, the various steps described throughout this disclosure may be performed by a single processor (or computer system) or, alternatively, by multiple processes (or multiple computer systems). Additionally, the various subsystems in the metrology unit 203 and / or processing unit 204 may include one or more information processing or logic systems. As such, the above description should be construed as merely illustrative and not limiting on the present disclosure.

[0049] Although the present disclosure has been described in connection with one or more specific embodiments, it will be understood that other embodiments of the present disclosure may be made without departing from the scope of the present disclosure, that is, the present disclosure is to be considered limited only by the appended claims and the reasonable interpretation thereof.

Claims

1. 1. A method comprising: generating a light beam having a wavelength between 400 nm and 800 nm; directing the light beam through an apodizer; and a method of performing metrology on a semiconductor wafer using the light beam, comprising: The method uses the apodizer to form a spot on the semiconductor wafer with the light beam having a diameter of 2 nm to 5 nm.

2. 10. The method of claim 1, wherein the semiconductor wafer comprises a diffraction-based overlay target.

3. 2. The method of claim 1, wherein the apodizer defines an aperture having a numerical aperture of about 0.

2.

4. The method of claim 1, wherein the wavelength is between 400 nm and 600 nm.

5. The method of claim 1, wherein the wavelength is between 600 nm and 800 nm.

6. The method of claim 1 , wherein the metrology is performed in a field plane.

7. 2. The method of claim 1, wherein the light beam is used in an on-axis illumination manner.

8. 2. The method of claim 1, wherein the light beam is used in an off-axis illumination manner.

9. 1. A system comprising: an illumination source generating a light beam, the illumination source generating the light beam having a wavelength between 400 nm and 800 nm; an apodizer disposed on a path of the light beam; a stage configured to hold a semiconductor wafer in the path of the light beam; wherein the light beam is configured to form a spot having a diameter of 2 nm to 5 nm on the semiconductor wafer using the apodizer.

10. 10. The system of claim 9, wherein the wavelength is between 400 nm and 600 nm.

11. 10. The system of claim 9, wherein the wavelength is between 600 nm and 800 nm.

12. 10. The system of claim 9, wherein the metrology is performed in a field plane.

13. 10. The system of claim 9, wherein the apodizer defines an aperture having a numerical aperture of about 0.

2.

14. 10. The system of claim 9, wherein the apodizer is disposed near a pupil plane.

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