Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/84—Systems specially adapted for particular applications
  • G01N21/88—Investigating the presence of flaws or contamination
  • G01N21/8806—Specially adapted optical and illumination features
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/21—Polarisation-affecting properties
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/84—Systems specially adapted for particular applications
  • G01N21/88—Investigating the presence of flaws or contamination
  • G01N21/95—Investigating the presence of flaws or contamination characterised by the material or shape of the object to be examined
  • G01N21/9501—Semiconductor wafers
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
  • H10P74/00—Testing or measuring during manufacture or treatment of wafers, substrates or devices
  • H10P74/20—Testing or measuring during manufacture or treatment of wafers, substrates or devices characterised by the properties tested or measured, e.g. structural or electrical properties
  • H10P74/203—Structural properties, e.g. testing or measuring thicknesses, line widths, warpage, bond strengths or physical defects
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
  • H10P74/00—Testing or measuring during manufacture or treatment of wafers, substrates or devices
  • H10P74/23—Testing or measuring during manufacture or treatment of wafers, substrates or devices characterised by multiple measurements, corrections, marking or sorting processes
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
  • H10P74/00—Testing or measuring during manufacture or treatment of wafers, substrates or devices
  • H10P74/27—Structural arrangements therefor
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/84—Systems specially adapted for particular applications
  • G01N21/88—Investigating the presence of flaws or contamination
  • G01N21/8806—Specially adapted optical and illumination features
  • G01N2021/8848—Polarisation of light
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2201/00—Features of devices classified in G01N21/00
  • G01N2201/06—Illumination; Optics
  • G01N2201/068—Optics, miscellaneous
  • G01N2201/0683—Brewster plate; polarisation controlling elements

Definitions

• the invention generally relates to the field of wafer inspection systems. More particularly the present invention relates to defect detection,
• an inspection system for inspecting a semiconductor sample comprises an illumination optics subsystem for generating and directing an incident beam towards a defect on the surface of a wafer.
• the illumination optics subsystem includes a light source for generating the incident beam and one or more polarization components for adjusting a ratio and/or phase difference between the two base vectors for the incident beam's electric field components.
• the system further includes a collection optics subsystem for collecting scattered light from the defect and/or surface in response to the incident beam, and the collection optics subsystem comprises an adjustable aperture at the pupil plane, followed by a rotatable waveplate for adjusting the phase difference for the collected scattered light's electric field components, followed by a rotatable analyzer.
• the system also includes a controller that is configured for (i) selecting a polarization of the incident beam, (ii) obtaining a defect scattering map, (iii) obtaining a surface scattering map, and (iv) determining a configuration of the one or more polarization components, aperture mask, and rotatable waveplate, and analyzer based on analysis of the defect and scattering map so as to maximize a defect signal to noise ratio.
• the defect and surface scattering maps are obtained at four or more angles of the waveplate of the collection optics subsystem, and determining a configuration is accomplished by (i) for each pupil position at the pupil plane, determining defect Stokes parameters based on the obtained defect scattering map, (ii) for each pupil position at the pupil plane, determining surface Stokes parameters based on the obtained surface scattering map, (iii) generating a polarization orthogonality map based on the determined defect and surface Stokes parameters, and (iv) comparing relative polarization orthogonality values from the polarization orthogonality map and relative intensity distribution values from the defect scattering map to determine the configuration.
• the one or more polarization components of the illumination subsystem include a rotatable 1 ⁇ 2 waveplate for controlling the incident beam's polarization angle and a rotatable 1 ⁇ 4 waveplate for controlling the incident beam's circular or elliptical polarization.
• the one or more polarization components of the illumination subsystem further comprise another 1 ⁇ 2 waveplate and a linear polarizer for controlling the incident beam's power and increasing a dynamic range.
• the 1 ⁇ 4 waveplate is positioned before the linear polarizer.
• the collection optics subsystem further includes an adjustable field stop for separately obtaining the defect and surface scattering maps.
• the collection optics subsystem further includes a sensor and one or more relay lens for relaying a pupil image to the sensor.
• the illumination optics subsystem includes an aperture that is open to a full size and determining a configuration is accomplished by iteratively mathematically applying different settings for the aperture mask, 1 ⁇ 4 waveplate, and analyzer so as to maximize the defect signal to noise ratio.
• a configuration of the aperture mask is determined so as to block areas of the pupil, except for areas with maximized polarization orthogonality and defect scattering intensity.
• the one or more pol arization components of the illuminati on optics subsystem comprise a linear polarizer, and the rotatable waveplate of the collection optics subsystem is a rotatable 1 ⁇ 4 waveplate.
• the linear polarizer and the rotatable 1 ⁇ 4 waveplate are each positioned at a conjugate plane.
• the light source is a broadband light source
• the illumination optics subsystem is arranged to direct the incident beam through an objective onto the surface of the wafer.
• the invention pertains to a method of inspecting a semiconductor sample.
• the method includes (i) in an illumination optics subsystem of an inspection system, generating and directing an incident beam at a selected polarization state towards a defect on a surface of a wafer, wherein the illumination optics subsystem of the inspection system includes a light source for generating the incident beam and one or more polarization components for adjusting a ratio and/or phase difference for the incident beam's electric field components, (ii) in a collection optics subsystem of an inspection system, collecting scattered light from the defect and/or surface in response to the incident beam, wherein the collection optics subsystem of the inspection system comprises an adjustable aperture at the pupil plane, followed by a rotatable waveplate for adjusting a phase difference of electric field components of the collected scattered light, followed by a rotatable analyzer, (iii) obtaining a defect scattering map based on the collected scattered light, (iv) obtaining a surface scattering map based on the collected
• Figure 1A is a graph of the Total integral Scattering (TIS) under S and P illumination as a function of defect size and different defect materials.
• Figure IB is a graph of normalized TIS as a function of illumination polarization angle.
• Figure 2 is a graph showing how various parameters change as a function of the incidence linear polarization angle and how they can cohesively affect the defect SMI and selection of the illumination polarization angle.
• Figure 3A depicts the alignment of polarization vectors for an 80nm Si02 particle defect for a P polarization illumination.
• Figure 3B depicts the alignment of polarization vectors for a silicon wafer substrate for a P polarization illumination.
• Figure 3C depicts the alignment of polarization vectors for an 80nm Si02 particle defect for an S polarization illumination.
• Figure 3D depicts the alignment of polarization vectors for a silicon wafer substrate for an S polarization illumination
• Figure 4 illustrates the results of a set of numerical simulations based on an 80nm Si0 2 particle defect on a rough seed Cu wafer.
• Figure 5 shows simulation results of 20nm Si02 particle defect scattering on a smooth bare Si wafer.
• Figure 6 is a diagrammatic representation of an inspection system in accordance with one embodiment of the present invention.
• Figure 7 shows detailed schematics of one possible implementation of an inspection apparatus in accordance with a specific embodiment of the present invention.
• Figure 8 is a diagrammatic representation of adjustment of an incident polarization direction in accordance with one embodiment of the present invention.
• Figure 9 A shows a plurality of differently sized field stop configurations.
• Figure 9B shows a plurality of differently sized aperture mask configurations.
• Figure 10 is a flow chart illustrating an optimization process in accordance with a specific implementation of the present invention.
• Figure 1 1 is a flow chart illustrating an optimization process in accordance with an alternative embodiment of the present invention.
• Figure 12 is a diagrammatic representation of an inspection system in accordance with an alternative embodiment of the present invention.
• Figure 13 represents one possible step by step optimization for a 48.5nm Si02 particle deposited on a W film wafer in accordance with a specific application of the present invention.
• Figure 14 is a bar chart of SN values of deposited Si02 particles on five different wafers and under five imaging modes.
• Figure 15 represents an improvement using two sequential scans with different linear incidence polarizations in accordance with one example implementation.
• Figure 6 illustrates images of protrusion defects that are taken under conventional S polarization illumination with unpolarized optimal collection masks and under optimal linear polarized illumination with optimized waveplate, analyzer and collection masks.
• Figure 17 illustrates an alternative embodiment of an inspection apparatus in which a 1 ⁇ 4 waveplate and linear polari zer are sequentially positioned in the collection path at two conjugate field planes.
• Figure 18 illustrates an alternative embodiment of an inspection system that utilizes a broadband source.
• Certain inspection system embodiments are described herein as being configured for inspecting semiconductor structures.
• Other types of structures such as solar panel structures, optical disks, etc., may also be inspected or imaged using the inspection apparatus of the present invention.
• a semiconductor wafer inspection can sometimes use an inspection tool that i s configurable to have a linearly polarized light configuration, such as an S or P polarization setting. Selection of either S or P polarization can be based on the wafer type, defect type (e.g., particle), etc.
• One form of inspection microscopy that utilizes S and P polarization is a laser dark field (DF) mi croscope, which has been widely used in the wafer inspection industry to detect nanoscale anomalies on semiconductor wafers.
• the DF technique often enhances defect detection sensitivity by blocking specular reflection from the wafer while collecting mostly scattered light.
• an illumination polarizer, analyzer, aperture mask, and Fourier filter can be applied to further improve sensitivity.
• illumination polarization is normally considered to have the most direct impact on defect sensitivity.
• Other types of systems such as a polari zed broadband system, may also utilize circularly, S, and, P configurations,
• P polarization illumination is more sensitive for smaller defects on smooth surfaces, such as described in U.S. Patent No. 6, 118,525 by Fossey et al.
• a linear polarizer together with a corresponding mask could be applied to null optical scattering from wafers to thereby enhance defect sensitivity, such as described in U. S. Patent No. 8,891 ,079 by Zhao et al., which patent is incorporated herein by reference.
• FIG. 1 A illustrates that, as particle diameter increases above approximately 80nm, the Total Integral Scattering (TIS) under S illumination exceeds the TIS under P illumination.
• TIS Total Integral Scattering
• This TIS difference results in better detection sensitivity for larger size defects. That is, the S polarization results in a much larger TIS (e.g., 102a) for defects sizes that are about 80nm and greater, as compared to the TIS for P polarization (e.g., 102b).
• the TIS for P polarization e.g., 104b
• Figure IB is a graph of normalized TIS as a function of illumination polarization angle.
• an S illumination results in a wider difference between an 80nm defect and haze.
• P illumination has differently aligned polarization vectors for an 80nm Si02 particle defect and the wafer substrate, resulting in better defect signal to noise ratio (SNR) for an 80nm defect.
• SNR defect signal to noise ratio
• polarization vectors of an 80nm defect and wafer substrate are mostly aligned under S illumination, leaving little room for further improvement of defect signal to noise ratio by excluding wafer surface scattering using a polarizer,
• Circular polarization illumination is a polarization state that is the superposition of P and S polarizations with equal amplitude and a constant phase offset of ⁇ /2. Said in another way, circular polarization can be viewed as an average of P and S illumination with a fixed phase offset. A circular polarization finds limited application, as compared to either P or S illumination, as further described in U.S. Patent No, 4,740,708 by Batchelder.
• polarization orthogonality is a measure of how much defect and wafer surface scatterings may be separated optically based on the difference in their polarization states.
• both particle and wafer scatterings are linearly polarized and, additionally, if one is P polarized while the other is S polarized, their polarization orthogonality can be defined as 1. In this case, wafer scattering could be fully extinguished by a linear polarizer while the other particle signal remains unchanged.
• both scatterings are linearly polarized but parallel to each other, their polarization orthogonality can be defined as 0. In this latter case, there is no means to differentiate the two with a polarizer.
• their polarization orthogonality can be defined as 0.5 in that a linear analyzer can be aligned with the polarization direction of particle scattering while reducing the wafer scattering by 2x.
• a x and a are the amplitudes of the x and y components of the electric field
• the scattered light can be partially polarized.
• the partially polarized scattered light can be separated into two parts of purely polarized light and un-poiarized light and treated separately.
• Partially polarized light can be defined by Stokes vector as given by:
• S 0 1 , 3 are the four elements of the Stokes vector which can be extracted from polarimetry measurements.
• the intensity of the polarized portion is given by: 56] and the intensity of unpolarized light is given by:
• phase difierence between x/y components of the polarized portion of the E field is expressed as: >3] ian ⁇ ⁇ : - : ⁇ (8)
• phase difference between the x/y component of the unpolarized portion of the E field is randomly distributed between 0 to 360 degrees.
• S""' par and Sl u! wafer are the first elements of the Stokes vectors of S 0 ⁇ '. and S° * ' , which are the intensity of particle scattering and wafer scattering after passing through waveplate and analyzer.
• M w is the Mueller matrix of the waveplate and M A is the Mueller matrix of the analyzer.
• Certain embodiments of the present invention pertain to inspection systems that utilize illumination polarization angles that are intermediate between the S and P polarization states to improve defect detection sensitivity. Under different inspection and specimen conditions, a specific intermediate polarization angle can be selected to minimize noise and improve defect SNR and, thereby, improve defect detection sensitivity.
• Figure 2 is a graph showing how various signal parameters change as a function of the incidence linear polarization angle and how they can cohesively affect the defect SNR and selection of the illumination polarization angle.
• the horizontal axis of Figure 2 indicates the linear incidence polarization angle.
• An illumination angle of 0 degrees corresponds to P polarization (E field parallel to plane of incidence) and 90 degrees corresponds to S polarization (E field perpendicular to plane of incidence).
• the defect SNR (206) is proportional to defect signal divided by total wafer haze, and then multiplied by a polarization orthogonality factor.
• the more orthogonality there is between defect and wafer scattering polarization implies a higher probability that defect signal could be separated from wafer signal by an optical analyzer.
• the polarization orthogonality may have an optimum value (210), which is an intermediate polarization state between S and P, and which results in a SNR peak (on curve 206), which also corresponds to maximized defect sensitivity.
• Figure 4 illustrates the results of a set of numerical simulations based on an 80nm Si0 2 particle defect on a rough seed Cu wafer.
• Both the particle scattering and wafer scattering are shown at three linear incidence polarization status, L0(P), L90(S) and L45, where L45 means the incident light is linearly polarized and its polarization angle is 45- degrees relative to L0(P) and L9()(S).
• the corresponding polarization orthogonality maps are plotted at the 3rd column of Figure 4. Increasing brightness levels represent increasing polarization orthogonality values (white being highest). Thus, L45 apparently maximizes polarization orthogonality particularly towards the bottom half of the pupil.
• the particle signal multiplied by the polarization orthogonality factor is plotted since it is desirable to also consider the relative intensity of the particle signal.
• the linear incidence polarization of L45 has a better defect sensitivity compared to either pure S or P illumination polarization.
• the simulation results indicate that a mask applied towards the bottom half of the pupil (encircled by the thick dotted line 402) could further enhance defect sensitivity.
• Figure 5 shows simulations of 20nm Si02 particle defect scattering on a smooth bare Si wafer.
• incidence polarization changes from L0(P) to L90(S)
• both particle signal and wafer haze decreases while particle signal drops at a faster rate.
• polarization orthogonality nionotonically decreases towards L90.
• the best case is to apply an analyzer at L0(P), as disclosed in U.S. Patent No, 8,891,079 B2 by Zhao et al., instead of at an intermediate polarization status.
• the simulation results indicate that a mask applied towards certain portions of the pupil (two areas 502a and 502b encircled by the thick dotted lines) could further enhance defect sensitivity.
• illumination polari zation may be fully optimized at states between P and S states, including states other than P, S, or circular polarization, in addition to optimized collection masks, waveplates, and analyzers. Such combination may additionally offer supplementary improvement of SNR, as compared to strict P and S systems.
• any suitable tool may be utilized, as long as variable polarization states that are between S and P polarization states may be setup on the tool .
• the selectable polarization states include S and P polarization states, as well as states that are not S or P polarization.
• an applicable inspection tool for implementation of techniques of the present invention may include at least one light source for generating an incident light beam at different polarization states.
• Such an inspection may also include illumination optics for directing the incident beam to the area-of-interest, collection optics for directing scattered electromagnetic waveforms (e.g., scattered light, X-rays, etc) from the area-of-interest in response to the incident beam, a sensor for detecting this scattered output and generating an image or signal from the detected scattered output, and a controller or computer subsystem for controlling the components of the inspection tool and facilitating defect detection in various materials and structures as described further herein.
• illumination optics for directing the incident beam to the area-of-interest
• collection optics for directing scattered electromagnetic waveforms (e.g., scattered light, X-rays, etc) from the area-of-interest in response to the incident beam
• a sensor for detecting this scattered output and generating an image or signal from the detected scattered output
• controller or computer subsystem for controlling the components of the inspection tool and facilitating defect detection in various materials and structures as described further herein.
• FIG. 6 is a diagrammatic representation of an inspection system 600 in accordance with one embodiment of the present invention .
• the system includes an illumination subsystem 602 for generating an incident beam (e.g., any suitable electromagnetic waveform) and directing such beam towards an objective system 604 and then towards a sample 606, such as a wafer.
• the illumination subsystem 602 may also be configured to control the incidence polarization state onto the sample 606.
• Examples of light sources for generating the incident beam include a laser-driven light source, a high-power plasma light source, a transillumination light source (e.g., halogen or Xe lamp), a filtered lamp, LED light sources, etc.
• the inspection system may include any suitable number and type of additional light sources, including broadband light sources,
• the incident beam from the light source may generally pass through any number and type of lenses which serve to relay (e.g., shape, focus or adjust focus offset, filter/select wavelengths, filter/select polarization states, resize, magnify, reduce distortion, etc.) the beam towards a sample.
• the illumination module 602 may also include any number of linear pol arizers and waveplates as described further herein,
• each detector may be in the form of a CCD (charge coupled device) or TDI (time delay integration) detector, photornultiplier tube (PMT), or other sensor.
• CCD charge coupled device
• TDI time delay integration
• Analyzer subsystem 6 12 generally includes multiple optical elements for analyzing scattered light and optimizing defect sensitivity, in conj unction with the illumination subsystem 602 being optimized to a selected illumination polarization.
• Malus' law which is named after Etienne-Louis Malus, says that when a perfect polarizer is placed in a polarized beam of light, the intensity, I, of the light that passes through is given by:
• the second polarizer is generally called an analyzer
• the mutual angle between their polarizing axes gives the value of ⁇ in Malus' law. If the two axes are orthogonal, the polarizers are crossed and, in theory, no light is transmitted.
• a computer subsystem is connected to both illumination subsystem and analyzer subsystem for automated control .
• the signals captured by each detector can be processed by computer subsystem 624, which may include a signal processing device having an analog-to-digital converter configured to convert analog signals from each sensor into digital signals for processing.
• the computer subsystem 624 may be configured to analyze intensity, phase, and/or other characteristics of the sensed light beam.
• the computer subsystem 624 may be configured (e.g., with programming instructions) to provide a user interface (e.g., on a computer screen) for displaying resultant images and other inspection characteristics as described further herein.
• the computer subsystem 624 may also include one or more input devices (e.g., a keyboard, mouse, joystick) for providing user input (e.g., as changing wavelength, polarization, mask configuration, aperture configuration, etc.), viewing detection results data or images, setting up an inspection tool recipe, etc.
• input devices e.g., a keyboard, mouse, joystick
• user input e.g., as changing wavelength, polarization, mask configuration, aperture configuration, etc.
• viewing detection results data or images e.g., setting up an inspection tool recipe, etc.
• the computer subsystem 624 may be any suitable combination of software and hardware and is generally configured to control various components or other controllers of the inspection system.
• the computer subsystem 624 may control selective activation of the illumination source, the illumination or output aperture settings, wavelength band, focus offset setting, polarization settings, analyzer settings, etc.
• the computer subsystem 624 may also be configured to receive images or signals generated by each detector and analyze the resulting images or signals to determine whether defects are present on the sample, characterize defects present on the sample, or otherwise characterize the sample.
• the computer subsystem 624 may include a processor, memory, and other computer peripherals that are programmed to implement instructions of the method embodiments of the present invention.
• the computer subsystem 624 may also have one or more processors coupled to input/output ports, and one or more memories via appropriate buses or other communication mechanisms.
• Such information and program instructions may be implemented on a specially configured computer system
• such a system includes program instructions / computer code for performing various operations described herein that can be stored on a computer readable media.
• machine-readable media include, but are not limited to, magnetic media such as hard disks, floppy disks, and magnetic tape; optical media such as CD-ROM disks; magneto-optical media such as optical disks; and hardware devices that are specially configured to store and perform program instructions, such as read-only memory devices (ROM) and random access memory (RAM).
• Examples of program instructions include both machine code, such as produced by a compiler, and files containing higher level code that may be executed by the computer using an interpreter.
• System embodiments of the present invention described herein can be used to characterize optical scattering intensity and polarization states of defects and wafer background while also optimizing the illumination polarization, in conjunction with optimizing the collection aperture mask, waveplate and analyzer. Compared to prior architectures with only P, S, or circular illumination polarization, embodiments of the present invention improve defect sensitivity on bot opaque rough film wafers and pattern wafers.
• system embodiments of the present invention can provide a polarimetry scatterometer capable of capturing high dynamic range (HDR) Stokes vector maps of scattered light at the pupil plane of the microscope objective. Adopting different field stops and aperture masks, both the intensity distribution and polarization status of scattered light from a defect and wafer can be extracted, which later can assist defect sensitivity optimization.
• HDR high dynamic range
• certain embodiments of the present invention are configurable to output an arbitrar illumination polarization state while simultaneously optimizing the collection mask and analyzer, so as to find an optimal combination of configurable variables of the optical system as further described herein.
• Bench test results show 1.6x to 4x improvement of SNR of deposited Si02 particle defects on opaque film wafers and 1.7x SNR improvement of protrusion defects on line pattern wafers, compared to conventionally optimal imaging modes as baselines,
• Figure 7 shows detailed schematics of one possible implementation of an inspection apparatus 700 in accordance with one embodiment of the present invention.
• This inspection system has an S-to-P variable polarization range for illumination and collection.
• a dark field microscope objective 704 may be employed to collect scattered light from the sample.
• the objective lens may have these four features: 1) high numerical aperture (NA) imaging (e.g., greater than about NA 0.9) enabling collection of as much scattered light as possible towards higher spatial frequency, 2) polarization states of scattered light is generally preserved, 3) an accessible pupil plane at which an analyzer and mask are located, and 4) an accessible field plane at which configurable field stops could be inserted.
• NA numerical aperture
• the illumination subsystem 702 shown in Figure 7 may include a light source 702a along with four polarization elements: a first 1 ⁇ 2 waveplate 702b, a linear polarizer 702c, a second 1 ⁇ 2 waveplate 702d, and a 1 ⁇ 4 waveplate 702e.
• the illumination subsystem 702 is generally configurable and freely manipulated so as to produce illumination polarization status other than a P, S, or circular state. More generally, the illumination optics subsystem may include one or more polarization components for adjusting a ratio and/or phase difference for the incident beam's electric field components.
• the illumination subsystem 702 may include any suitable components, optical or otherwise, to control power.
• the linear polarizer 702c has a fixed position to extinguish polarization in every direction, except in one linear direction.
• the first rotatabie 1 ⁇ 2 waveplate 702b shifts the direction of the linearly polarized light that is received from the linear polarizer 702c.
• the combination of the first rotatabie 1 ⁇ 2 waveplate 702b and linear polarizer 702c controls incidence power and increases dynamic range of the system.
• the illumination subsystem may also include one or more polarization components for introducing a polarization state in the incident beam that is between S and P states.
• the 1 ⁇ 2 waveplate 702b serves to rotate the incident light's polarization direction by 0 to 180 degrees without a change in intensity, while the linear polarizer 702c extinguishes polarized light and controls incidence power. Any other suitable components for controlling power and/or increasing the dynamic range may be utilized in place of a 1 ⁇ 2 waveplate and linear polarizer combination,
• the rotatable second 1 ⁇ 2 waveplate 702d generally controls the incidence light' s linear polarization angle
• the rotatable 1 ⁇ 4 waveplate 702e controls the degree of elliptical polarization.
• the addition of the second rotatable 1 ⁇ 2 waveplate 702d in a position after the linear polarizer provides a mechanism to continuously adjust the polarization angle to values between S and P. More specifically, the polarization angle will be 2 ⁇ , which is twice the angle between the incident polarization angle and the fast axis of the 1 ⁇ 2 waveplate 702d.
• the 1 ⁇ 2 waveplate 702d is rotated to a position that is 1 ⁇ 2 the desired rotation for the polarization.
• the incident polarization (Pi) which is oriented in a polarization direction, is rotated by 2 ⁇ to result in vector v when a 1 ⁇ 2 waveplate is positioned at angle ⁇ .
• the 2" d 1 ⁇ 2 waveplate 702d would be rotated to produce a polarization 210 that corresponds to the maximum particle signal 206 (and a significantly lower wafer signal 204).
• the 1 ⁇ 4 waveplate 702e can be used and positioned to change the phase of the incident light.
• the 1 ⁇ 4 waveplate 702e is positioned to generate circular or eliptical polarized incident light. This 1 ⁇ 4 waveplate 702e is optional.
• a collection subsystem includes a full field stop 707, a col limator 708, an aperture mask 710 at a conjugate pupil plane, a rotatable 1 ⁇ 4 waveplate 712a, a rotatable linear polarizer (or analyzer) 712b, relay lens 714, Fourier plane (FP) lens 716, and an image sensor 718
• Both the field stop 707 and the aperture mask 710 may be adjustable so that an area of the pupil plane that is more sensitive to wafer scattering, as compared with background scattering, reaches a sensor 718.
• Both the 1 ⁇ 4 waveplate 712a and linear polarizer 712b, as well as the FP lens may be slidably positioned in and out of the optical path for flexible control of scattered light from the sample 706.
• Figure 9A shows a plurality of differently sized field stop configurations FS 1, FS2, FSB, and FS4, while Figure 9B shows a plurality of differently sized aperture mask configurations AMI , AM2, AM3, and AM4,
• the full size field stop is labelled FS 1
• the full size aperture mask is labeled AMI .
• the light reflected and scattered from a perfectly flat surface tends to not have a phase shift so that the scattered and reflected light has correlated phase, which results in linear or elliptical/circular polarized wafer scattered light.
• different parts of the wafer scattering can be both unpolarized and polarized (e.g., partially polarized). That is, the wafer scattered light can be unpolarized in certain portions of the pupil plane.
• unpolarized light may result from a relative rough surface, such as 10-20 nm or higher relative to a 266 rmi wavelength for incidence light (as compared with less than 0.1 or 1 nm for a smooth surface).
• the wafer scattered light will tend to be linear or elliptical/circular scattered as a function of scattering angle. Said in another way, an incident beam can impinge at different angles with respect to the peaks and valleys in the surface topography and result in the scattered light constructively interfering for certain angles to result in linear or elliptical/circular polarized wafer scattering. Conversely, the scattered wafer light destructively interferes at other angles so that it becomes unpolarized. Accordingly, different portions of unpolarized light, which is difficult to fully extinguish, can be partially blocked by the aperture mask 710.
• the configurable 1 ⁇ 4 waveplate 712a and linear polarizer 712b are positioned relative to one another and together forms the analyzer subsystem 712.
• any waveplate may be utilized and is configured for adjusting a phase difference for the collected scattered light's electric field components.
• Both the 1 ⁇ 4 waveplate 712a and linear polarizer 712b may be insertable into the optical path.
• the 1 ⁇ 4 waveplate 712a serves to produce linear polarized light from elliptical/circular polarized light, such as wafer scattering, while simultaneously modulating the polarization status of particle/defect scattering.
• the linear polarizer 712b extends the output beam's polarization in a particular direction.
• relay lens 714 and FP lens 716 together image the pupil or aperture mask onto the image sensor 718, The sensor 718 only captures the aperture mask image when the FP relay lens 716 is inserted into the optical path.
• the image sensor 718 captures a full size pupil image, instead of the wafer image, when the aperture mask is fully open (AM I), where x' ( Ax) and y' (NAy) are pupil coordinates.
• FIG. 10 is a flow chart illustrating an optimization process 1000 in accordance with a specific implementation of the present invention.
• a wafer with a known defect type and location is provided in operation 1001.
• a wafer may include known defects of various sizes and types, which have been formed at specific locations.
• Each defect may also be, for example, imaged with a high-resolution tool, such as a scanning electron microscope, to locate such defect's location.
• the wafer, along with defect locations may then be loaded onto a stage of an optical tool, and such stage can be moved relative to the optical tool's illumination column so that the defect will be imaged.
• An incidence polarization may then be selected in operation 1002.
• a polarization setting may be initially selected (or adjusted) via a second 1 ⁇ 2 waveplate 702d on the illumination side (e.g., Figure 7).
• Various settings may be tried for the process 1000 below so as to find optimal settings for maximized defect sensitivity.
• a cropped field stop may be employed such as FS2 or FSB illustrated in Figure 9A.
• the size of the field stop may be further reduced to enclose only a single defect of interest, such as FS4.
• this last setting requires the wafer scattering to be much lower compared to defect scattering, such as a relatively- large particle deposited on smooth bare silicon surface, so that the measured scattering map will represent mostly the signal coming from the defect and not the wafer.
• a field stop for imaging the defect location may be selected to obtain a defect scattering map in operation 1004.
• the field stop FS4 (see Figure 9 A) may be selected to position over the defect's known position, excluding the surrounding wafer surface.
• a defect scattering map may then be obtained in operation 1006. That is, the reflected and scattered intensity for each pixel of the imaged pupil or pupil plane is obtained (e.g., as imaged on sensor 718).
• the defect Stokes parameters for each pupil position may then be determined in operation 1008. Assuming the linear polarizer 712b is aligned along x' direction and the 1 ⁇ 4 waveplate rotation angle is ⁇ , the pupil image is related to the four elements of the Stokes vector by the following equation:
• Stokes vectors at each individual pixel may be calculated based on the N pupil images. For example, four different measurements at the four different angles may be obtained to solve for four unknowns: So- S3.
• the output of scatterometiy measurements is a four-elements Stokes vector for each pixel of the pupil image, with each element of Stokes vector in the form of a 2 ⁇ D matrix:
• a similar imaging and Stokes calculation process may be performed for the wafer region (non-defect or bare wafer area) in parallel or sequentially with the defect process.
• the field stop for imaging the wafer region may be selected to obtain a wafer scattering map in operation 101.0.
• a larger field stop such as FS2
• a wafer scattering map may be obtained for the pupil in operation 1012.
• the wafer Stokes parameters for each pupil position may then be determined in operation 1014.
• a polarization orthogonality map may then be generated based on the determined Stokes parameters for the defect and wafer scattering maps in operation 1016. Other polarization or light properties may also be determined in operation 1016. Knowing the four elements of Stokes vector for each wafer and particle position as shown in Equation (12), polarization properties of the combined scattered light may then be extracted, including the polarized light intensity distribution Spol (Equation 4), the phase of polarized light ⁇ (Equation 8), and the degree of polarization p (Equation 6).
• the scatterometry measurements can be used to unveil a full picture of how wafer scattering is distributed, polarized, and depolarized at the pupil of the microscope objective. This full picture can be used to select optimal tool configuration for maximizing defect sensitivity.
• the relative polarization orthogonality and intensity distribution may then be compared in operation 1018.
• the inspection system configuration may then be optimized to maximize defect sensitivity based on relative comparison of polarization orthogonality and intensity distribution in operation 1020, and the process 1000 ends.
• the analyzer setting will be most effective within the pupil space in which polarization orthogonality is as high as possible, while mask optimization makes efficient use of scattering intensity distribution.
• the specifically configured system may then be used to locate unknown defects on a wafer using any suitable inspection process, such as comparing to known defect scattering signatures, a reference image obtained from an identical die or cell, or a rendered reference image.
• the optimization process shall ultimately lead to a preferable combination of configurable variables of the optical system to obtain a maximized defect sensitivity. That is, light from the wafer surface is blocked as much as possible, while blocking only a minimal amount of defect light.
• the wafer signal may be elliptical, while the particle signal is not elliptical. In this example, elliptical light can be blocked.
• a 1 ⁇ 4 waveplate in the collection path (e.g., 712a) can be positioned and rotated to convert circularly or elliptically polarized wafer light into linear polarized wafer light, which can then be blocked with an analyzer (e.g., linear polarizer 712b).
• an analyzer e.g., linear polarizer 712b
• an optimization process 1 100 may be performed offline in an iterative manner as illustrated in Figure 1 1. Similar to the process 1000 of Figure 10, a wafer with a known defect type and location is provided in operation 1001 and an incidence polarization is selected in operation 1002. A defect scattering map (1 104) and wafer scattering map (1 105) may be obtained. The defect and wafer scattering maps may be obtained by imaging with a full-size aperture (AMI ).
• AMI full-size aperture
• an aperture mask, a 1 ⁇ 4 waveplate, and an analyzer could be applied numerically to both defect and wafer scattering maps in operations 1 106, 1 1 10, and 1 1 12, respectively.
• the SNR may then be estimated based on the adjusted defect and wafer scattering maps in operation 1 1 14. It may then be determined whether the estimated S R is optimized in operation 1 1 16.
• the aperture mask, 1 ⁇ 4 waveplate, and analyzer may be configured in various combinations until a maximized SNR. is reached and the process 1 100 ends.
• a 1 ⁇ 4 waveplate is applied ahead of applying the linear analyzer. This configuration can be beneficial in the case in which wafer scattering is elliptically or circularly polarized, which a 1 ⁇ 4 waveplate could convert it into linearly polarized light and, hence, be fully extinguished by the linear analyzer.
• system 1200 may include a translatable small size aperture mask 1210 together with a full size field stop FS 1 1202.
• the FP relay lens 716 may be removed so that image sensor 718 captures the defect image, instead of the pupil image.
• Stokes vectors ( 1206) of the defect pixels may then be calculated at each aperture mask XY position (e.g., 1212a and 1212b). Finally, a full scattering map (1204) of the defect at the pupil plane may be reconstructed by interpolation of Stokes vectors (1206) captured on wafer images. In other words, Stokes vector (1204) at the pupil plane may be discretely sampled by successively translating the aperture mask (1212a, 1212b, . . .) across the entire aperture and then fused from multiple Stokes vectors captured at the image plane (1206).
• any suitable number and type of techniques may be implemented to improve the dynamic range of the scatterometry measurements.
• the i llumination subsystem may be configured to modulate the incident power from 100% to below 0.1%.
• the exposure time of the image sensor may be adjusted from about a microsecond to about a second level.
• neutral density filters may be inserted at the pupil plane to further increase dynamic range. The overall dynamic range, therefore, could well exceed 10 8 , which would be particularly beneficial for characterizing wafers with strong specular reflection or non-zero order diffractions.
• Figure 13 represents one possible step by step optimization for a 48.5nm Si02 particle deposited on a W film wafer in accordance with a specific application of the present invention.
• the starting point is S il lumination with unpolarized full aperture collection. This setting has conventionally been adopted as an optimal mode for detecting medium size particles on rougher films.
• the baseline SN (signal to noise) value is 16.99.
• illumination polarization is changed from pure S (L90) to L56 while leaving the collection optics unchanged.
• An L56 polarization state means that the illumination source is still linearly polarized but the E field is rotated 56-deg off the plane of incidence.
• the SN value has dropped to 9.48.
• a vertically oriented 1 ⁇ 4 waveplate and a 70-deg linear polarizer are applied in sequence in the imaging chain.
• An instant result of this change is that the bottom part of the pupil plane becomes darker indicating wafer scattering is significantly suppressed therein.
• an aperture mask is applied to the bottom half of the pupil plane.
• the resulting optimized SN value is 46.57, which represent about a 2.7x improvement versus the baseline S illumination with unpolarized full aperture collection,
• All SN values are normalized with respect to LQL mode for easier comparison.
• the optimal mode appears to be LQL with an intermediate illumination polarization state between P and S, consistent with simulations shown in Figure 4.
• the SN improvement ratio ranges from 1 .6x (poly with strong depolarization) to 4x (CMP Cu with less depolarization), compared to the best of conventional baseline mode.
• the optimal mode appears to be PL mask (half), in agreement with simulations shown in Figure 5. The reason may be that for small size particles on smooth surface, the particle signal under P illumination is stronger than under S illumination. In addition, the particle scattering polarization is crossed with wafer scattering. Therefore, it may be predicted that as the optimization process keeps being implemented for smaller and smaller particles on smooth film, the LQL mode will eventually converge to the PL mask mode where P polarization is preferred over other polarization states.
• the particle sensitivity may also be further improved by two sequential scans with different linear incidence polarizations.
• the first scan 1502a adopted the same settings of illumination polarization and collection optics as shown in Figure 13.
• illumination polarization is rotated to the opposite direction with respect to the plane of incidence, noted as L-56; collection analyzer is correspondingly changed to 110-deg; and aperture mask is moved to the upper portion of pupil. Since the aperture masks of two scans take two different regions of the pupil space, their speckle noises are uncorrelated.
• the defect images could be added incoherently with an additional ⁇ 1.4x reduction ratio of wafer noise as shown by incoherent summed image 1502c.
• this technique requires defect responsivitv to be consistent with two symmetric linear incidence polarization angles, so that defect signal will not be dropped by the incoherent sum of two images.
• Sensitivity improvement is further shown on pattern wafers with line space structures and protrusion defects.
• Shown in Figure 16 are images 1602a of protrusion DOI (Defect of Interest) that are taken under conventional S polarization illumination with unpolarized optimal collection masks.
• the white arrow in the first image (top-left) indicates the illumination azimuth direction which is also the direction of line patterns.
• Corresponding defect images 1602b with optimized linear incidence polarization, collection aperture, 1 ⁇ 4 waveplate, and analyzer are shown. On average the SN improvement ratio is 1 ,7x compared to baseline S polarization illumination.
• Figure 17 illustrates an alternative embodiment of an inspection apparatus 1700.
• the microscope objective, illumination subsystem, and collection aperture mask are the same as in Figure 7.
• the 1 ⁇ 4 waveplate 1712a and linear polarizer 1712b are sequentially positioned at two conjugate field planes.
• the system also includes relay lens 1714a and 1714b for relaying such image towards the sensor 718. Consequently, either analyzing or modulation of the polarization status of scattered light is realized at an intermediate image plane, instead of at the pupil plane.
• a bright field type microscope objective may be implemented.
• a broadband source 1802a such as a lamp or LED, illuminates the wafer through the microscope objective 1804.
• a linear polarizer 1812a similarly along the illumination path are positioned a linear polarizer 1812a, a 1 ⁇ 2 waveplate 1812b to rotate the linear polarization angle, and a 1 ⁇ 4 waveplate 1812c to modulate the degree of elliptical polarization.
• An illumination aperture 1806 is applied to spatially filter the illumination source so that the collection path may be customized as DF imaging mode.
• the illumination path and collection path are separated by a beamsplitter 808.
• a beamsplitter 808 In the collection path following the beamsplitter 1808, similar optical elements as shown in Figure 7 are arranged.
• This embodiment shows that flexible illumination polarization control and optimization may be carried out on most existing BF wafer inspection tools as on those DF inspection tools,
• Certain embodiments of the present invention provide optimization of illumination polarization at arbitrary polarization states, besides S and P.
• the system improves defect sensitivity by modulating illumination polarization between P and S and balancing between a scattering intensity factor and a polarization orthogonality factor.
• This flexible design inherently provides a new degree of freedom of optimization that rarely has been attempted in prior arts.
• a 1 ⁇ 4 waveplate along with an optimized analyzer maximizes suppression of wafer noise in cases in which it is either ellipticaily or circularly polarized.
• certain embodiments provide a more effective noise suppression solution and, hence, possibly leading to much better sensitivity.
• Certain embodiments of the present invention enable polanmetric scatterometery characterization of scattered light of both defects and wafers.
• the measurements described herein can be used to unveil important characteristics about wafer scattering, including scattering intensity distribution, intensity of polarized scattered light, phase of polarized scattered light, and degree of polarization. This information can then be used either for defect sensitivity optimization or for defect classification, such as using phase and degree of polarization on individual defect pixels.
• each optical element may be optimized for the particular wavelength range of the light in the path of such optical element. Optimization may include minimizing wavelength-dependent aberrations, for example, by selection of glass type, arrangement shapes, and coatings ⁇ e.g., anti -reflective coatings, highly reflective coatings) for minimizing aberrations for the corresponding wavelength range.
• the lenses are arranged to minimize the effects caused by dispersion by shorter or longer wavelength ranges.
• all the optical elements are reflective. Examples of reflective inspection systems and configurations are further described in U.S. Patent No.
• the optical layout of the inspection tool can vary from that described above.
• the system microscope objective lens can be one of many possible layouts, as long as the transmission coatings are optimized for the particular selected wavelength band or sub-band and the aberration over each waveband is minimized.
• Different light sources can be used for each path. For instance, a Xe source may be used for the long wavelength path and an HgXe or Hg lamp may be used for the short wavelength path.
• Multiple LED or speckle buster laser diodes are also possible sources for each path.
• the zoom ratio can be modified to include different magnification ranges either via a lens-only approach, a mostly fixed lens with an optical trombone, or any combination thereof.
• the sample may also be placed on a stage of the inspection system, and the inspection system may also include a positioning mechanism for moving the stage (and sample) relative to the incident beam.
• a positioning mechanism for moving the stage (and sample) relative to the incident beam.
• one or more motor mechanisms may each be formed from a screw drive and stepper motor, linear drive with feedback position, or band actuator and stepper motor.
• the one or more positioning mechanisms may also be configured to move other components of the inspection system, such as illumination or collection mirrors, apertures, FP relay lens, wavelength filters, polarizers, analyzers, waveplates, etc.
• an inspection or measurement tool may have any suitable features from any number of known imaging or metrology tools arranged for detecting defects and/or resolving the critical aspects of features of a reticle or wafer.
• an inspection or measurement tool may be adapted for bright field imaging microscopy, dark field imaging microscopy, full sky imaging microscopy, phase contrast microscopy, polarization contrast microscopy, and coherence probe microscopy. It is also contemplated that single and multiple image methods may be used in order to capture images of the target.
• Non-imaging optical methods such as scatterometry, may also be contemplated as forming part of the inspection or metrology apparatus.
• Any suitable lens arrangement may be used to direct the incident beam towards the sample and direct the output beam emanating from the sample towards a detector.
• the illumination and collection optical elements of the system may be reflective or transmissive.
• the output beam may be reflected or scattered from the sample or transmitted through the sample.
• any suitable detector type or number of detection elements may be used to receive the output beam and provide an image or a signal based on the characteristics (e.g., intensity) of the received output beam.
• the defect detection characteristic data may be obtained from a transmitted, reflected, or a combination output beam. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein.

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