US20150331029A1 - Charge decay measurement systems and methods - Google Patents

Charge decay measurement systems and methods Download PDF

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US20150331029A1
US20150331029A1 US14/690,256 US201514690256A US2015331029A1 US 20150331029 A1 US20150331029 A1 US 20150331029A1 US 201514690256 A US201514690256 A US 201514690256A US 2015331029 A1 US2015331029 A1 US 2015331029A1
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sample
optical
decay
charging
curve data
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Viktor Koldiaev
Marc Kryger
John Changala
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Femtometrics Inc
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R29/00—Arrangements for measuring or indicating electric quantities not covered by groups G01R19/00 - G01R27/00
    • G01R29/24—Arrangements for measuring quantities of charge
    • 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
    • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
    • G01N21/636—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited using an arrangement of pump beam and probe beam; using the measurement of optical non-linear 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
    • G01N21/8806—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
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/88—Investigating the presence of flaws or contamination
    • G01N21/94—Investigating contamination, e.g. dust
    • 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
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/26—Testing of individual semiconductor devices
    • G01R31/2601—Apparatus or methods therefor
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/26—Testing of individual semiconductor devices
    • G01R31/265—Contactless testing
    • G01R31/2656—Contactless testing using non-ionising electromagnetic radiation, e.g. optical radiation
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/28—Testing of electronic circuits, e.g. by signal tracer
    • G01R31/282—Testing of electronic circuits specially adapted for particular applications not provided for elsewhere
    • G01R31/2831—Testing of materials or semi-finished products, e.g. semiconductor wafers or substrates
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/28—Testing of electronic circuits, e.g. by signal tracer
    • G01R31/302—Contactless testing
    • G01R31/308—Contactless testing using non-ionising electromagnetic radiation, e.g. optical radiation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L22/00—Testing or measuring during manufacture or treatment; Reliability measurements, i.e. testing of parts without further processing to modify the parts as such; Structural arrangements therefor
    • H01L22/10—Measuring as part of the manufacturing process
    • H01L22/12—Measuring as part of the manufacturing process for structural parameters, e.g. thickness, line width, refractive index, temperature, warp, bond strength, defects, optical inspection, electrical measurement of structural dimensions, metallurgic measurement of diffusions
    • 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
    • G01N2201/061—Sources
    • G01N2201/06113—Coherent sources; lasers

Definitions

  • the subject filing relates to systems for based wafer inspection, semiconductor metrology, materials characterization, surface characterization and/or interface analysis.
  • Second Harmonic Generation is a non-linear effect in which light is emitted from a material at a reflected angle with twice the frequency of an incident source light beam.
  • the process may be considered as the combining of two photons of energy E to produce a single photon of energy 2E (i.e., the production of light of twice the frequency (2 ⁇ ) or half the wavelength) of the incident radiation.
  • the subject systems and methods variously enable capturing the quantitative information for making the determinations required for such activity.
  • a wafer sample with optical electro-magnetic radiation (at a specific site with a pulsed laser or with a flash lamp or other electro-magnetic energy source or light source or other means) a plurality of measurements are made to monitor transient electric field decay associated with heterointerfaces controlling the decay period.
  • decay dependent data is collected and used to provide systems by which charge carrier lifetimes, trap energies and/or trapped charge densities may be determined in order that defects and contaminants can be discerned or parsed from one another, for species differentiation if a contaminant is detected and/or for contaminant quantification if detected
  • Such activity is determined on a site-by-site basis with the selected methodology typically repeated to scan an entire wafer or other material sample or region thereof.
  • the computer processing required to enable such determination it may occur in “real time” (i.e., during the scanning without any substantial delay in outputting results) or via post-processing.
  • control software can run without lag in order to provide the precise system timing to obtain the subject data per methodology as described below.
  • sample material charge-up is monitored in connection with SHG signal production.
  • the information gained via this signal may be employed in material analysis and making determinations.
  • system embodiments may include an ultra-short pulse laser with a fast shutter operating in the range of 10 2 seconds to picosecond (10 ⁇ 12 seconds) range.
  • Such systems may be used to monitor SHG signal generation at a sample site from surface and buried interfaces of thin film materials after the introduction of a plurality of short blocking intervals. These intervals may be timed so as to monitor the field decay of interest.
  • the subject systems may also include an optical line delay.
  • the delay line may be a fiber-based device, especially if coupled with dispersion compensation and polarization control optics.
  • the delay line may be mirror-based and resemble the examples in U.S. Pat. No. 6,147,799 to MacDonald, U.S. Pat. No. 6,356,377 to Bishop, et al. or U.S. Pat. No. 6,751,374 to Wu, et al.
  • the delay is used in the system in order to permit laser interrogation of the material in the picosecond (10 ⁇ 12 second) to femtosecond (10 ⁇ 15 second) and, possibly, attosecond (10 ⁇ 18 second) ranges. Such interrogation may be useful in detecting multiple charge decay-dependent data points along a single decay curve.
  • the subject methods include one that involves measuring an SHG signal for decay data points acquired after successive charge-up events.
  • the conditions for obtaining a SHG signal may be different at each charge-up event. Additionally, the time interval between successive charge-up events may be different.
  • the multiple data points (at least two but typically three or more) can be correlated and expressed as a single composite decay curve.
  • Another method employs minimally disruptive (i.e., the radiation used to produce the SHG signal does not significantly recharge the material) SHG signal interrogation events after a single charging event.
  • Yet another method for determining transient charge decay involves measuring discharge current from the sample material (more accurately, its structures that were charged by optical radiation). The time dependence (kinetics) of this signal may then be treated in the same way as if SHG sensing had been employed. Further, as above, such sensing may be done in the span of one decay interval and/or over a plurality of them following charge to a given level. In any case, electrode-specific hardware for such use is detailed below.
  • charge or charging level this may be taken to a point of apparent saturation when charge dynamics are observed in standard linear time or against a log time scale.
  • the subject methodologies optionally observe, record and analyze charging kinetic as this may yield important information.
  • the system may omit further or subsequent characterization.
  • what may be regarded as “not far” may mean about 1% to about 10% of charge increase versus the initial charge state to be determined by learning when the subject tool is used for a given time of sampling.
  • saturation is a relative term. Using a linear time scale, material will appear saturated very quickly. But if an SHG signal intensity associated with charging is observed in log scale from 10-100 seconds, it can be observed that the later part of saturation occurs with a different time constant and is relatively more gradual or time-consuming. Thus, while examples of the methodology provided herein discuss charging to saturation, the delay and other timing may be regarded as occurring with respect to apparent saturation. Rather than waiting the full amount of time for 100% saturation, as this may be unnecessarily time consuming to reach, instead, the instrument may delay until the time it takes to get to apparent saturation or the time in which can extract important parameters, regardless of how long it takes for full saturation.
  • the subject methods and systems may operate with charge and/or re-charging levels at less than saturation (as discussed above) while still yielding meaningful decay curve information. Without such measurement, however, when approximate saturation is a known parameter (e.g., by experience with the subject tool with a given material) charge to saturation is employed as the target level.
  • various interfacial material properties may also be determined using laser beam blocking or delay as further described in the portion of U.S. Provisional Application No. 61/980,860, filed on Apr. 17, 2014, titled “W AFER M ETROLOGY T ECHNOLOGIES ,” referred to as Section III, titled “T EMPERATURE -C ONTROLLED M ETROLOGY ,” which is incorporated herein by reference in its entirety.
  • Introducing a DC bias across the sample being tested can also assist in analysis of the material.
  • Employing a DC bias actively changes the initial charge distribution at the interfaces before photo-induced voltage has any effect.
  • the sample being tested may be mounted atop a conductive chuck which can be used as a ground for DC biasing across the sample using sample top surface probes.
  • Other means of introducing induced voltage biases are possible as well without the use of surface probes as further described in the portion of U.S. Provisional Application No. 61/980,860, filed on Apr. 17, 2014, titled “W AFER M ETROLOGY T ECHNOLOGIES ,” referred to as Section IV entitled, “F IELD -B IASED SHG M ETROLOGY ,” which is incorporated herein by reference in its entirety. See also co-pending U.S. patent application Ser. No. ______, filed Apr. 17, 2015 titled “F IELD -B IASED S ECOND H ARMONIC G ENERATION M ETROLOGY ”, published as U.S. Publication No. ______, which is incorporated herein by reference in its entirety.
  • the subject systems may use a secondary light source in addition to the primary laser involved in blocking-type analysis for charge decay determination.
  • a secondary light source in addition to the primary laser involved in blocking-type analysis for charge decay determination.
  • Such a set of sources may be employed as a radiation pump/probe combination as further described in the portion of U.S. Provisional Application No. 61/980,860, filed on Apr. 17, 2014, titled “W AFER M ETROLOGY T ECHNOLOGIES ,” referred to as Section I entitled, “P UMP AND P ROBE T YPE SHG M ETROLOGY ,” which is incorporated herein by reference in its entirety. See also co-pending U.S. patent application Ser. No. ______, filed Apr. 17, 2015 titled “PUMP AND PROBE TYPE SECOND HARMONIC GENERATION METROLOGY”, published as U.S. Publication No. ______, which is incorporated herein by reference in its entirety.
  • FIGS. 1A-1C are diagrams of systems embodiments
  • FIG. 2 is a chart of system function
  • FIGS. 3A and 3B are charts representative of the manner of delivering such function
  • FIG. 4 represents system function in a graphical output.
  • FIGS. 5 and 6 plot SHG interrogation-related method embodiments
  • FIGS. 7A-7E plot time dynamics associated with the system in FIG. 1C that may be employed in the methods of FIGS. 5 and 6 .
  • FIG. 8 plots a current-based interrogation method for observing transient electric field decay
  • FIGS. 9A and 9B illustrate hardware configurations that may be employed in the method of FIG. 8 .
  • FIG. 1A is a diagram of a first system 2100 as may employed in connection with the subject methodology.
  • Alternative systems 2100 ′ and 2100 ′′ are shown in FIGS. 1B and 1C .
  • Each system includes a primary laser 2010 for directing a primary beam 2012 of electro-magnetic radiation at a sample wafer 2020 , which sample is held by a vacuum chuck 2030 .
  • the chuck 2030 includes or is set on x- and y-stages and optionally also a rotational stage for positioning a sample site 2022 across the wafer relative to where the laser(s) are aimed.
  • a beam 2014 of reflected radiation directed at a detector 2040 will include an SHG signal.
  • the detector may be any of a photomultiplier tube, a CCD camera, a avalanche detector, a photodiode detector, a streak camera and a silicon detector.
  • the sample site 2022 can include one or more layers.
  • the sample site 2022 can comprise a composite substrate including at least two layers.
  • the sample site 2022 can include an interface between two dissimilar materials (e.g., between two different semiconductor materials, between two differently doped semiconductor materials, between a semiconductor and an oxide, between a semiconductor and a dielectric material, between a semiconductor and a metal, between an oxide and a metal, between a metal and a metal or between a metal and a dielectric).
  • shutter-type devices 2050 are employed as described in connection with the methodology below.
  • the type of shutter hardware used will depend on the timeframe over which the laser radiation is to be blocked, dumped or otherwise directed away from the sample site.
  • An electro-optic blocking device such as a Pockel's Cell or Kerr Cell is used to obtain very short blocking periods (i.e., with switching times on the order of 10 ⁇ 9 to 10 ⁇ 12 seconds).
  • electro-optic blocking device such as a Pockel's Cell or Kerr Cell is used to obtain very short blocking periods (i.e., with switching times on the order of 10 ⁇ 9 to 10 ⁇ 12 seconds).
  • mechanical shutters or flywheel chopper type devices may be employed.
  • a photon counting system 2044 capable of discretely gating very small time intervals, typically, on the order of picoseconds to microseconds can be included to resolve the time-dependent signal counts.
  • the system(s) may include delay line hardware 2060 .
  • Beam splitting and switching (or shuttering on/off) between a plurality of set-time delay lines for a corresponding number of time-delayed interrogation events is possible.
  • a variable delay line may be preferred as offering a single solution for multiple transient charge decay interrogation events on a time frame ranging from immediately (although delay of only 10 ⁇ 12 seconds may be required for many methodologies) to tens of nanoseconds after pump pulse.
  • the desired delay time may even go into the microsecond regime if using a slower, kilohertz repetition laser.
  • such hardware is uniquely suited for carrying out the subject methodology (both of which methodology and such hardware is believed heretofore unknown), it might be put to other uses as well.
  • the beam 2012 from the laser 2010 can be split by a beam splitter 2070 between two optical paths.
  • the beam splitter 2070 can split the beam 2012 unequally between the two optical paths. For example, 70% of the energy of the beam 2012 can be directed along a first optical path (e.g., as beam 2016 ) and 30% of the energy of the beam 12 can be directed along a second optical path (e.g., as beam 2018 ). As another example, 60% of the energy of the beam 2012 can be directed along the first optical path and 40% of the energy of the beam 2012 can be directed along the second optical path. As yet another example, 80% of the energy of the beam 2012 can be directed along the first optical path and 20% of the energy of the beam 2012 can be directed along the second optical path.
  • the beam splitter 2070 can comprise a dielectric mirror, a splitter cube, a metal coated mirror, a pellicle mirror or a waveguide splitter.
  • the beam splitter 2070 can include an optical component having negligible dispersion that splits the beam 2012 between two optical paths such that optical pulses are not broadened.
  • the path of an “interrogation” beam 2016 taken off a beam splitter 2070 from primary beam 2012 can be lengthened or shortened to change its arrival timing relative to a “pump” beam 2018 wherein each of the beams are shown directed or aimed by various mirror elements 2072 .
  • Another approach employs fiber optics in the optical delay component and/or other optical pathways (e.g., as presented in U.S. Pat. No. 6,819,844 incorporated herein by reference in its entirety for such description).
  • the output from the detector 2040 and/or the photon counting system 2044 can be input to an electronic device 2048 (see, e.g., FIGS. 1A and 1B ).
  • the electronic device 2048 can be a computing device, a computer, a tablet, a microcontroller or a FPGA.
  • the electronic device 2048 includes a processor or processing electronics that may be configured to execute one or more software modules.
  • the processor may be configured to execute one or more software applications, including a web browser, a telephone application, an email program, or any other software application.
  • the electronic device 2048 can implement the methods discussed herein by executing instructions included in a machine-readable non-transitory storage medium, such as a RAM, ROM, EEPROM, etc.
  • the electronic device 2048 can include a display device and/or a graphic user interface to interact with a user.
  • the electronic device 2048 can communicate with one or more devices over a network interface.
  • the network interface can include transmitters, receivers and/or transceivers that can communicate over wired or wireless connections.
  • the split may be unequal (e.g., 70-30%, 80-20%, 60-40% or any range therebetween, such as between 60-90% in one path and between 40-10% in another path as well as outside these ranges), sending a majority of the power in the pump beam, and a minority in the probe beam.
  • the split may be 60-70% and 40-30%, for the pump and probe, respectivley, 70-80% versus 30-20% for the pump and probe, respectively, 80-90% versus 20-10%, for the pump and probe respectively, or 90-99.999% versus 10-0.001%, for the pump and probe respectively.
  • the probe beam could be between 0.001% to 49.99% while the pump beam could be between 50.001% and 99.999%, for example.
  • the sum of the two beams may be 100% or approximate thereto.
  • the split may be determined by the particular material system being characterized in some cases. The value (at least in part) of doing so may be to help facilitate methods such as shown in FIGS. 5 and 6 in which the power involved in SHG interrogation subsequent to material charging is desirably reduced or minimized as discussed below.
  • the pump and probe beams are brought in at different angles. Such an approach facilitates measuring pump and probe SHG responses separately. In such cases, two detectors may be advantageously employed with one for each reflected beam path.
  • embodiments 2100 and 2100 ′ are shown including a dichroic reflective or refractive filter 2080 for selectively passing the SHG signal coaxial with reflected radiation directly from the laser 2010 .
  • a prism may be employed to differentiate the weaker SHG signal from the many-orders-of-magnitude-stronger reflected primary beam.
  • a dichroic system as referenced above may be preferred.
  • Other options include the use of diffraction grating or a Pellicle beam splitter.
  • an optical bundle 2082 of focusing and collimating/collimation optics may be provided.
  • a filter wheel 2084 , zoom lens 2086 and/or polarizers 2088 may be employed in the system(s).
  • an angular (or arc-type) rotational adjustment (with corresponding adjustment for the detector 2040 and in-line optical components) as shown in system 2100 ′ may be desirable.
  • An additional radiation source 2090 (be it a laser illustrated emitting a directed beam 2092 or a UV flash lamp emitting a diverging or optically collimated or a focused pulse 2094 ) may also be incorporated in the system(s) to provide such features as referenced above in connection with the portion of U.S. Provisional Application No. 61/980,860, filed on Apr.
  • laser 10 may operate in a wavelength range between about 700 nm to about 2000 nm with a peak power between about 10 kW and 1 GW, but delivering power at an average below about 100 mW. In various embodiments, average powers between 10 mW and 10 W should be sufficient.
  • Additional light source 2090 (be it a another laser or a flash lamp) may operate in a wavelength range between about 80 nm and about 800 nm delivering an average power between about 10 mW and 10 W. Values outside these ranges, however, are possible.
  • an SHG signal is weak compared to the reflected beam that produces it, it may be desirable to improve the signal-to-noise ratio of SHG counts.
  • improvement becomes even more useful.
  • One method of reducing noise that may be employed is to actively cool the detector. The cooling can decreases the number of false-positive photon detections that are generated randomly because of thermal noise. This can be done using cryogenic fluids such as liquid nitrogen or helium or solid state cooling through use of a Peltier device.
  • Others areas of improvement may include use of a Marx Bank Circuit (MBC) as relevant to shutter speed.
  • MBC Marx Bank Circuit
  • FIG. 2 illustrates a process map or decision tree 2200 representing such possibilities.
  • a so-called problem 2210 that is detected can be parsed between a defect 2210 (extended defects such as bond voids or dislocations, Crystal Originated Particle (COP) or the like) and a contaminant 2220 (such as copper inclusion or other metals in point defect or clustered forms).
  • a defect 2210 extended defects such as bond voids or dislocations, Crystal Originated Particle (COP) or the like
  • a contaminant 2220 such as copper inclusion or other metals in point defect or clustered forms.
  • the defect type 2222 and/or a defect quantification 2224 determination e.g., in terms of density or degree
  • the contaminant species or type 2232 and/or a contaminant quantification 2234 determination can be made. Such parsing between defect and contaminant and identification of species may be performed in connection with determining charge carrier lifetimes, trap energies, trap capture cross-section and/or trap densities then comparing these to values in look-up tables or databases. Essentially these tables or databases include listings of properties of the material as characterized by the subject methods, and then matching-up the stated properties with entries in a table or database that correspond to particular defects or contaminants.
  • Trap capture cross-section and trap density may be observed in connection with, optionally, detected charging kinetics.
  • charge carrier lifetimes and trap energies the following equation based on work by I. Lundstrom, provides guidance:
  • ⁇ is the tunneling time constant for the tunneling mechanism of the trap discharge
  • ⁇ denotes the trap energy
  • E ON denotes the strength of the electric filed at the interface and the remaining equation variables and context are described at I. Lundstrom, JAP, v. 43, n. 12, p. 5045, 1972 which subject matter is incorporated by reference in its entirety. Further modeling and calculation options may be appreciated in reference to the portion of U.S. Provisional Application No. 61/980,860, filed on Apr. 17, 2014, titled “W AFER M ETROLOGY T ECHNOLOGIES ,” referred to Section III, titled “T EMPERATURE -C ONTROLLED M ETROLOGY ,” which is incorporated herein by reference in its entirety.
  • the decay curve data obtained by the subject sample interrogation can be used to determine the parameters of trap energy and charge carrier lifetime by use of physical models and related mathematics.
  • Representative sets of curves 2300 , 2300 ′ such as those pictured in FIGS. 3A and 3B may be calculated (where FIG. 3B highlights or expands a section of the data from FIG. 3A ) from the equation above.
  • Poisson/Transport solvers can be used to determine trap density in MOS-like structures and more exotic devices using charge carrier lifetimes and known trap energies.
  • the photo-injected current due to femto-second optical pulses induces bursts of charge carriers which reach the dielectric conduction band.
  • the average value of this current can be related to carrier concentration and their lifetimes in the regions.
  • the E-field across the interface is the proxy by which SHG measures these phenomena.
  • the half-life t 1/2 , average lifetime ⁇ , and decay constant ⁇ can be used to characterize the extent of decay for a decay curve (obtained experimentally or by simulation).
  • the parameters A, B, and ⁇ can be obtained from the decay data points that are obtained experimentally as discussed below.
  • An average lifetime ⁇ can then be calculated from the parameters A, B, and ⁇ , using theory of radioactive decay as a way of setting benchmarks for what is qualitatively called partial, or full-decay.
  • ⁇ can be given by the equation (t 1/2 )/(In(2)).
  • the charge state can be considered to have fully decayed after a time span of three average lifetimes ⁇ , which corresponds to ⁇ 95% decay from full saturation. Partial decay can be expressed in terms of signal after a certain number of average lifetimes ⁇ have elapsed.
  • the systems determine parameters (e.g., carrier lifetimes, trap energies, trapping cross-section, charge carrier density, trap charge density, carrier injection threshold energy, charge carrier lifetime, charge accumulation time, etc.) based at least in part on the subject methodology on a point-by-point basis on a portion (e.g., die size portion) of the wafer or an entire wafer.
  • An entire wafer (depending on the material, surface area, and density of scan desired) can often be scanned in less than about 10 minutes, with these parameters determined for each point scanned.
  • a location of the wafer can be scanned in a time interval between about 100 milliseconds and about 3 seconds. For example, a location of the wafer can be scanned in about 950 milliseconds.
  • a matrix of data containing the spatial distributions of the parameters determined can be plotted as individual color-coded heat maps or contour maps for each parameter, as a means for quantitative inspection, feedback and presentation.
  • FIG. 4 illustrates one such map 2400 . It depicts how a defect 2402 may be portrayed. But it is possible to show any of the further refined subject matters in FIG. 2 . Once quantitative data has been obtained, providing such output is merely a matter of changing the code in the plotting program/script.
  • each wafer spatial distribution can be cross-correlated by referencing with ellipsometry data to correct for layer thickness variability and cross-calibrated with independent contamination characterization data obtained, for example, by Total Reflection X-ray Fluorescence (TXRF), Time of Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) and the like.
  • TXRF Total Reflection X-ray Fluorescence
  • TOF-SIMS Time of Flight Secondary Ion Mass Spectroscopy
  • These initial or corrected spatial distributions can then be compared to those from wafers known to be within specification, to determine if the samples in question have any defects or problematic features which warrant further testing.
  • Human decisions may be employed (e.g., in inspecting a generated heat map 2400 ) initially in determining the standard for what is an acceptable or unsatisfactory wafer, until the tool is properly calibrated to be able to flag wafers autonomously. For a well-characterized process in a fab, human decisions would then only need to be made to determine the root cause of any systemic problem with yields, based on the characteristics of flagged wafers.
  • FIG. 5 provides a plot 2500 illustrating a first method embodiment hereof that may be used in making such determinations.
  • This method like the others discussed and illustrated below relies on characterizing SHG response with multiple shutter blocking events in which interrogation laser is gated for periods of time.
  • a section of a sample to be interrogated is charged (typically by a laser) to saturation.
  • a single source is used to generate as pump beam and probe beam, although separate pump and probe sources can be used in other embodiments.
  • the SHG signal may be monitored.
  • the saturation level may be known by virtue of material characterization and/or observing asymptotic behavior of the SHG signal intensity associate with charging (I ch ).
  • the electromagnetic radiation from the laser is blocked from the sample section.
  • the laser (probe beam) is so-gated for a selected period of time (t b11 ).
  • an SHG intensity measurement (I dch1 ) is made with the laser (probe beam) exposing the surface, thus observing the decay of charge at a first discharge point.
  • a second blocking event occurs for a time (t b12 ) different than the first in order to identify another point along what will become a composite decay curve.
  • SHG signal intensity (I dchs2 ) is measured again. This reduced signal indicates charge decay over the second gating event or blocking interval.
  • a third differently-timed blocking event (t b13 ) follows and subsequent SHG interrogation and signal intensity measurement (I mo ) is made for a third measurement of charge decay in relation to SHG intensity.
  • the sample is charged to a saturation level
  • the sample can be charged to a charge level below saturation.
  • the three blocking times t b11 , t b12 and t b13 are different, in other examples, the three blocking times t b11 , t b12 and t b13 can be the same.
  • the sample can be charged to a charging level initially and the SHG intensity measurement (I dch1 ), (I dch2 ) and (I dch3 ) can be obtained at different time intervals after the initial charging event.
  • these three points can be used to construct a composite charge decay curve. It is referred to herein as a “composite” curve in the sense that its components come from a plurality of related events. And while still further repetition (with the possibility of different gating times employed to generate more decay curve data points or the use of same-relation timing to confirm certainty and/or remove error from measurements for selected points) may be employed so that four or more block-then-detect cycles are employed, it should be observed that as few as two such cycles may be employed.
  • the method above can provide parameter vs. time (such as interfacial leakage current or occupied trap density v. time) kinetic curve by obtaining measurements at a few time points.
  • a time constant ( ⁇ ) can be extracted from the parameter vs. time kinetic curve.
  • the time constant can be attributed to a time constant characteristic for a certain type of defect.
  • the decay-dependent data obtained may be preceded (as in the example) by SHG data acquisition while saturating the material with the interrogation (or probe) laser.
  • charging will not necessarily go to saturation (e.g., as noted above).
  • the measurement necessary be made prior to the blocking of a/the charging laser.
  • the charging will not necessarily be performed with the interrogation/probe laser (e.g., see optional pump/probe methodology cited above).
  • the sample material is typically moved or indexed to locate another section for the same (or similar) testing. In this manner, a plurality of sections or even every section of the sample material may be interrogated and quantified in scanning the entire wafer as discussed above.
  • FIG. 6 and plot 2600 illustrate an alternative (or complimentary approach) to acquiring charge decay related data by scanning is shown in plot 2600 .
  • this method after charging to saturation a/the first time, continuous (or at least semi-continuous) discharge over multiple blocking time intervals (t b11 , t b12 , t b13 ) is investigated by laser pulses from an interrogation or a probe laser measuring different SHG intensities (I dch1 , I dch2 , I dch3 ).
  • the intensity and/or frequency of the laser pulses from the interrogation/probe laser are selected such that the average power of the interrogation/probe laser is reduced to avoid recharging the material between blocking intervals while still obtaining a reasonable SHG signal. To do so, as little as one to three laser pulses may be applied. So-reduced (in number and/or power), the material excitation resulting from the interrogation or probe laser pulses may be ignored or taken into account by calibration and or modeling considerations.
  • a separate pump source can be used for charging.
  • the probe beam can be used to charge the sample.
  • the delay between pulses may be identical or tuned to account for the expected transient charge decay profile or for other practical reason.
  • the delay is described in terms of “gating” or “blocking” above, it is to be appreciated that the delay may be produced using one or more optical delay lines as discussed above in connection with FIG. 1C . Still further, the same may hold true for the blocking/gating discussed in association with FIG. 5 .
  • the method in FIG. 6 may be practiced with various modifications to the number of blocking or delay times or events.
  • SHG signal may or may not be measured during charge to saturation.
  • the method in FIG. 6 may be practiced (as illustrated) such that the final gating period takes the SHG signal to null. Confirmation of this may be obtained by repeating the method at the same site in a mode where charging intensity (I ch ) is measured or by only observing the SHG signal in (re)charging to saturation.
  • FIGS. 7A-7E are instructive regarding the manner in which the subject hardware is used to obtain the decay-related data points.
  • FIG. 7A provides a chart 2700 illustrating a series of laser pulses 2702 in which intermediate or alternating pulses are blocked by shutter hardware (e.g., as described above) in a so-called “pulse picking” approach. Over a given time interval, it is possible to let individual pulses through (indicated by solid line) and block others (as indicated by dashed line).
  • FIG. 7B provides a chart 2710 illustrating the manner in which resolution of a blocking technique for SHG investigation can be limited by the repetition (rep) rate of the probe laser.
  • a decay curve like decay curve 2712 it is possible to resolve the time delay profile with blocking of every other pulse using a pulsed laser illustrated to operate at the same time scale as in FIG. 7A .
  • a shorter curve 2714 cannot be resolved or observed under such circumstances.
  • use optical delay stage(s) can offer additional utility.
  • chart 2720 in FIG. 7C illustrates (graphically and with text) how blocking and introducing a delay with respect to a reference time associated with charging the sample can offer overlapping areas of usefulness, in terms of the decay time of the curve relative to the rep rate of the laser. It also shows how there are short time ranges when only delay stages would allow interrogation of the decay curve, and longer time ranges when only blocking the pumping and/or the probing beam would be practical.
  • FIGS. 7D and 7E further illustrate the utility of the combined block/delay apparatus.
  • Chart 2730 illustrates exemplary SHG signals produced by individual laser pulses 2702 .
  • X range between each such pulse may be interrogated by varying optical delay.
  • additional utility over a range (Y) may be achieved with a system combining a delay stage and blocking or shutter means such as a chopper, shutter, or modulator.
  • chart 2740 such a system is able to measure decay curves (and their associated time constants) in the range from one to several pulse times.
  • time constants can range between 0.1 femtosecond and 1 femtosecond, 1 femtosecond and 10 femtoseconds, 10 femtoseconds and 100 femtoseconds, 100 femtoseconds and 1 picosecond, between 1 picosecond and 10 picoseconds, between 10 picoseconds and 100 picoseconds, between 100 picoseconds and 1 nanosecond, between 1 nanosecond and 10 nanoseconds, between 10 nanosecond and 100 nanoseconds, between 100 nanoseconds and 1 microsecond, between 1 nanoseconds and 100 microseconds, between 100 microseconds and 1 millisecond, between 1 microsecond and 100 milliseconds, between 100 microsecond and 1 second, between 1 second and 10 seconds, or between 10 second and 100 seconds or larger or smaller.
  • time delays ( ⁇ ) for example between the probe and pump (or pump and probe) can be, for example, between 0.1 femtosecond and 1 femtosecond, 1 femtosecond and 10 femtoseconds, 10 femtoseconds and 100 femtoseconds, 100 femtoseconds and 1 picosecond, between 1 picosecond and 10 picoseconds, between 10 picoseconds and 100 picoseconds, between 100 picoseconds and 1 nanosecond, between 1 nanosecond and 10 nanoseconds, between 10 nanosecond and 100 nanoseconds, between 100 nanoseconds and 1 microsecond, between 1 nanoseconds and 100 microseconds, between 100 microseconds and 1 millisecond, between 1 microsecond and 100 milliseconds, between 100 microsecond and 1 second, between 1 second and 10 seconds, between 10 second and 100 seconds. Values outside these ranges are also possible.
  • FIGS. 9A and 9B Two such approaches are illustrated in FIGS. 9A and 9B .
  • Systems 2900 and 2900 ′ use gate electrodes 2910 and 2920 , respectively, made of a conductive material that is transparent in the visible light range. Such an electrode may touch a wafer 2020 to be inspected, but need not as they may only be separated by a minimal distance.
  • the electric field in the dielectric can be estimated by extracting the electrode-dielectric-substrate structure parameters using AC measurement of the Capacitance-Voltage curve (CV-curve).
  • CV-curve measurement can be done by using a standard CV-measurement setup available on the market, connected to a material sample in the subject tool (e.g., the applied voltage is to provide the electric field in the dielectric between about 0.1 MV/cm and about 5 MV/cm).
  • the wafer may be held upon a conductive chuck 2030 providing electrical substrate contact.
  • a gate electrode would be an ultra-thin Au film or Al film on a glass of 10-30 A thickness which can reduce the sensitivity due to absorption of some photons by the thin semi-transparent metal layer.
  • electrodes 2910 and 2920 present no appreciable absorption issues (although some refraction-based considerations may arise that can be calibrated out or may be otherwise accounted for in the system).
  • These electrodes may comprise a transparent conductor gate layer 2930 made of a material such as ZnO, SnO or the like connected with an electrical contact 2932 .
  • An anti-reflective top coat 2934 may be included in the construction.
  • Gate layer 2930 may be set upon a transparent carrier made 2936 of dielectric (SiO 2 ) with a thickness (D gc ) as shown.
  • the transparent carrier comprises an insulator that is used as a gate for a noncontact electrode that may employ for example capacitive coupling to perform electrical measurements, similar to those described in the portion of U.S. Provisional Application No. 61/980,860, filed on Apr. 17, 2014, titled “W AFER M ETROLOGY T ECHNOLOGIES ,” referred to as Section IV entitled, “F IELD -B IASED SHG M ETROLOGY ,” which is incorporated herein by reference in its entirety. See also co-pending U.S. patent application Ser. No. ______, filed Apr.
  • D gc would be calibrated by measuring CV curve on the semiconductor substrate with a non-invasive approach and used in electric field (E) calculation when applied voltage is known.
  • a negligible gap distance between the gate and sample can be an air gap.
  • the electrode can be directly in contact with the sample rather than being separated by an air gap or dielectric. Accordingly normal CV or IV measurements may be performed in various embodiments.
  • deionized water may be helpful in reducing boundary-layer reflection without any ill effect (or at least one that cannot be addressed).
  • Deionized (or clean-room grade) water can maintain cleanliness around the electrically sensitive and chemically pure substrate wafers. Deionized water is actually less conductive than regular water.
  • FIG. 9B a related construction is shown with the difference being the architecture of the carrier or gate-holder 2938 .
  • the carrier or gate-holder 2938 is configured as a ring, optimally formed by etched away in the center and leaving material around the electrode perimeter as produced using MEMS techniques. But in any case, because of the large unoccupied zone through with the laser and SHG radiation must pass, it may be especially desirable to fill the same with DI water as described above.
  • each embodiment would typically be stationary with respect to the radiation exciting the material in use.
  • the electrode structure(s) Prior to and after use, the electrode structure(s) may be stowed by a robotic arm or carriage assembly (not shown).
  • the electrode directly contacts the wafer to perform electrical measurements such as measuring current flow.
  • non-contact methods of measuring current such as for example using electrodes that are capacatively coupled with the sample, can also be used.
  • the systems and methods described herein can be used to characterize a sample (e.g., a semiconductor wafer or a portion thereof).
  • a sample e.g., a semiconductor wafer or a portion thereof.
  • the systems and methods described herein can be used to detect defects or contaminants in the sample as discussed above.
  • the systems and methods described herein can be configured to characterize the sample during fabrication or production of the semiconductor wafer.
  • the systems and methods can be used along a semiconductor fabrication line in a semiconductor fabrication facility.
  • the systems and methods described herein can be integrated with the semiconductor fabrication/production line.
  • the systems and methods described herein can be integrated into a semiconductor fab line with automated wafer handling capabilities.
  • the system can be equipped with an attached Equipment Front End Module (EFEM), which accepts wafer cassettes such as a Front Opening Unified Pod (FOUP)
  • EFEM Equipment Front End Module
  • FOUP Front Opening Unified Pod
  • the system can be configured such that once the cassettes are mounted on the EFEM, the FOUP is opened, and a robotic arm selects individual wafers from the FOUP and moves them through an automatically actuated door included in the system, into a light-tight process box, and onto a bias-capable vacuum chuck.
  • the chuck may be designed to fit complementary with the robotic arm so that it may lay the sample on top. At some point in this process, the wafer can be held over a scanner for identification of its unique laser mark.
  • a system configured to be integrated in a semiconductor fabrication/assembly line can have automated wafer handling capability from the FOUP or other type of cassette; integration with an EFEM as discussed above, a chuck designed in a way to be compatible with robotic handling, automated light-tight doors which open and close to allow movement of the robotic wand/arm and software signaling to EFEM for wafer loading/unloading and wafer identification.
  • DSP Digital Signal Processor
  • ASIC Application Specific Integrated Circuit
  • FPGA Field Programmable Gate Array
  • a general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine.
  • the processor can be part of a computer system that also has a user interface port that communicates with a user interface, and which receives commands entered by a user, has at least one memory (e.g., hard drive or other comparable storage, and random access memory) that stores electronic information including a program that operates under control of the processor and with communication via the user interface port, and a video output that produces its output via any kind of video output format, e.g., VGA, DVI, EIDMI, DisplayPort, or any other form.
  • a user interface port that communicates with a user interface, and which receives commands entered by a user
  • has at least one memory e.g., hard drive or other comparable storage, and random access memory
  • stores electronic information including a program that operates under control of the processor and with communication via the user interface port, and a video output that produces its output via any kind of video output format, e.g., VGA, DVI, EIDMI, DisplayPort, or any other form.
  • a processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. These devices may also be used to select values for devices as described herein.
  • a software module may reside in Random Access Memory (RAM), flash memory, Read Only Memory (ROM), Electrically Programmable ROM (EPROM), Electrically Erasable Programmable ROM (EEPROM), registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art.
  • An exemplary storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium.
  • the storage medium may be integral to the processor.
  • the processor and the storage medium may reside in an ASIC.
  • the ASIC may reside in a user terminal.
  • the processor and the storage medium may reside as discrete components in a user terminal.
  • the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on, transmitted over or resulting analysis/calculation data output as one or more instructions, code or other information on a computer-readable medium.
  • Computer-readable media includes both computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another.
  • a storage media may be any available media that can be accessed by a computer.
  • such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer.
  • the memory storage can also be rotating magnetic hard disk drives, optical disk drives, or flash memory based storage drives or other such solid state, magnetic, or optical storage devices.
  • any connection is properly termed a computer-readable medium.
  • the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave
  • the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium.
  • Disk and disc includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media.
  • Operations as described herein can be carried out on or over a website.
  • the website can be operated on a server computer, or operated locally, e.g., by being downloaded to the client computer, or operated via a server farm.
  • the website can be accessed over a mobile phone or a PDA, or on any other client.
  • the website can use HTML code in any form, e.g., MHTML, or XML, and via any form such as cascading style sheets (“CSS”) or other.
  • the computers described herein may be any kind of computer, either general purpose, or some specific purpose computer such as a workstation.
  • the programs may be written in C, or Java, Brew or any other programming language.
  • the programs may be resident on a storage medium, e.g., magnetic or optical, e.g. the computer hard drive, a removable disk or media such as a memory stick or SD media, or other removable medium.
  • the programs may also be run over a network, for example, with a server or other machine sending signals to the local machine, which allows the local machine to carry out the operations described herein.

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