Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
  • G01N29/04—Analysing solids
  • G01N29/06—Visualisation of the interior, e.g. acoustic microscopy
  • G01N29/0654—Imaging
  • G01N29/0681—Imaging by acoustic microscopy, e.g. scanning acoustic microscopy
• • 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
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
  • G01N29/22—Details, e.g. general constructional or apparatus details
  • G01N29/24—Probes
  • G01N29/2418—Probes using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2291/00—Indexing codes associated with group G01N29/00
  • G01N2291/01—Indexing codes associated with the measuring variable
  • G01N2291/011—Velocity or travel time
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2291/00—Indexing codes associated with group G01N29/00
  • G01N2291/02—Indexing codes associated with the analysed material
  • G01N2291/023—Solids
  • G01N2291/0237—Thin materials, e.g. paper, membranes, thin films
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2291/00—Indexing codes associated with group G01N29/00
  • G01N2291/02—Indexing codes associated with the analysed material
  • G01N2291/028—Material parameters
  • G01N2291/02854—Length, thickness
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2291/00—Indexing codes associated with group G01N29/00
  • G01N2291/02—Indexing codes associated with the analysed material
  • G01N2291/028—Material parameters
  • G01N2291/02881—Temperature
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2291/00—Indexing codes associated with group G01N29/00
  • G01N2291/04—Wave modes and trajectories
  • G01N2291/042—Wave modes
  • G01N2291/0427—Flexural waves, plate waves, e.g. Lamb waves, tuning fork, cantilever

Definitions

• This invention relates generally to a method and apparatus for characterizing a sample using electromagnetic radiation.
• a radio frequency pulse is applied to a piezoelectric transducer mounted on a substrate between the transducer and the film to be studied.
• a stress pulse propagates through the substrate toward the film.
• part of the pulse is reflected back to the transducer.
• the remainder enters the film and is partially reflected at the opposite side to return through the substrate to the transducer.
• the pulses are converted into electrical signals, amplified electronically, and displayed on an oscilloscope.
• the time delay between the two pulses indicates the film thickness, if the sound velocity in the film is known, or indicates the sound velocity, if the film thickness is known.
• Relative amplitudes of the pulses provide information on the attenuation in the film or the quality of the bond between the film and the substrate.
• the minimum thickness of films which can be measured and the sensitivity to film interface conditions using conventional ultrasonics is limited by the pulse length.
• the duration of the stress pulse is normally at least 0.1 ⁇ sec corresponding to a spatial length of at least 3 X 10 -2 cm for an acoustic velocity of 3 X 10 5 cm/sec. Unless the film is thicker than the length of the acoustic pulse, the pulses returning to the transducer will overlap in time. Even if pulses as short in duration as 0.001 ⁇ sec are used, the film thickness must be at least a few microns.
• acoustic microscopy projects sound through a rod having a spherical lens at its tip.
• the tip is immersed in a liquid covering the film. Sound propagates through the liquid, reflects off the surface of the sample, and returns through the rod to the transducer.
• the amplitude of the signal returning to the transducer is measured while the sample is moved horizontally.
• the amplitudes are converted to a computer-generated photograph of the sample surface.
• Sample features below the surface are observed by raising the sample to bring the focal point beneath the surface.
• the lateral and vertical resolution of the acoustic microscope are approximately equal. Resolution is greatest for the acoustic microscope when a very short wavelength is passed through the coupling liquid. This requires a liquid with a low sound velocity, such as liquid helium.
• An acoustic microscope using liquid helium can resolve surface features as small as 500 Angstroms, but only when the sample is cooled to 0.1 K.
• Ellipsometers direct elliptically polarized light at a film sample and analyze the polarization state of the reflected light to determine film thickness with an accuracy of 3-10 Angstroms.
• the elliptically polarized light is resolved into two components having separate polarization orientations and a relative phase shift. Changes in polarization state, beam amplitudes, and phase of the two polarization components are observed after reflection.
• the ellipsometer technique employs films which are reasonably transparent. Typically, at least 10% of the polarized radiation must pass through the film. The thickness of metal sample films thus cannot exceed a few hundred Angstroms.
• Another technique uses a small stylus to mechanically measure film thickness.
• the stylus is moved across the surface of a substrate and, upon reaching the edge of a sample film, measures the difference in height between the substrate and the film. Accuracies of 10-100 Angstroms can be obtained. This method cannot be used if the film lacks a sharp, distinct edge, or is too soft in consistency to accurately support the stylus.
• Another non-destructive method based on Rutherford Scattering, measures the energy of backscattered helium ions. The lateral resolution of this method is poor.
• Yet another technique uses resistance measurements to determine film thickness.
• the film thickness is determined by measuring the electrical resistance of the film. For films less than 1000 Angstroms, however, this method is of limited accuracy because the resistivity may be non-uniformly dependent on the film thickness.
• the change in the direction of a reflected light beam off a surface is studied when a stress pulse arrives at the surface.
• stress pulses are generated by a piezoelectric transducer on one side of a film to be studied.
• a laser beam focused onto the other side detects the stress pulses after they traverse the sample. This method is useful for film thicknesses greater than 10 microns.
• a film may also be examined by striking a surface of the film with an intense optical pump beam to disrupt the film's surface. Rather than observe propagation of stress pulses, however, this method observes destructive excitation of the surface.
• the disruption such as thermal melting, is observed by illuminating the site of impingement of the pump beam with an optical probe beam and measuring changes in intensity of the probe beam.
• the probe beam's intensity is altered by such destructive, disruptive effects as boiling of the film's surface, ejection of molten material, and subsequent cooling of the surface. See Downer, M.C.; Fork, R.L.; and Shank, C.V., "Imaging with Femtosecond Optical Pulses", Ultrafast Phenomena IV, Ed. D.H. Auston and K. B. Eisenthal (Spinger-Verlag, N.Y. 1984), pp. 106-110.
• a very high frequency sound pulse is generated and detected by means of an ultrafast laser pulse.
• the sound pulse is used to probo an interface.
• the ultrasonic frequencies used in this technique typically are less than 1 THz, and the corresponding sonic wavelengths in typical materials are greater than several hundred Angstroms. It is equivalent to refer to the high frequency ultrasonic pulses generated in this technique as coherent longitudinal acoustic phonons.
• Tauc et al. teach the use of pump and probe beams having durations of 0.01 to 100 psec. These beams may impinge at the same location on a sample's surface, or the point of impingement of the probe beam may be shifted relative to the point of impingement of the puinp beam.
• the film being measured can be translated in relation to the pump and probe beams.
• the probe beam may be transmitted or reflected by the sample.
• the pump pulse has at least one wavelength for non-destructively generating a stress pul.se in the sample.
• the probe pulse is guided to the sample to intercept the stress pulse, and the method further detects a change in optical constants induced by the stress pulse by measuring an intensity of the probe beam after it intercepts the stress pulse.
• a distance between a mirror and a corner cube is varied to vary the delay between the impingement of the pump beam and the probe beam on the sample.
• an opto-acoustically inactive film is studied by using an overlying film comprised of an opto- acoustically active medium, such as arsenic telluride.
• the quality of the bonding between a film and the substrate can be determined from a measurement of the reflection coefficient of the stress pulse at the boundary, and comparing the measured value to a theoretical value.
• Tauc et al. are not limited to simple films, but can be extended to obtaining information about layer thicknesses and interfaces in superlattices, multilayer thin-film structures, and other inhomogeneous films.
• Tauc et al. also provide for scanning the pump and probe beams over an area of the sample, as small as 1 micron by 1 micron, and plotting the change in intensity of the reflected or transmitted probe beam. While well-suited for use in many measurement applications, it is an object of this invention to extend and enhance the teachings of Tauc et al.
• fitting may be performed using predetermined values for many of the material properties (such as density, sound velocity, optical constants) in order to determine a sample characteristic of interest (such as thickness, or adhesion strength).
• a sample characteristic of interest such as thickness, or adhesion strength
• a method for characterizing a sample comprising the steps of: (a) acquiring data from the sample using at least one probe beam wavelength to measure, for times less than a few nanoseconds, a transient change in an optical response, such as the reflectivity, of the sample induced by a pump beam; (b) analyzing the data to determine at least one material property by comparing a background signal component of the data with data' obtained for a similar delay time range from one or more samples prepared under conditions known to give rise to certain physical and chemical material properties; and (c) analyzing a component of the transient response of the probe beam that is caused by ultrasonic waves generated by the pump beam, using the at least one determined material property.
• the first step of analyzing may include a step of interpolating between reference samples to obtain an intermediate set of material properties.
• the step of acquiring is accomplished with a non- destructive system and method for measuring at least one transient response of the sample to a pump pulse of optical radiation
• the measured transient response or responses can include at least one of a measurement of a modulated change ⁇ R in an intensity of a reflected portion of a probe pulse, a change ⁇ T in an intensity of a transmitted portion of the probe pulse, a change ⁇ P in a polarization of the reflected probe pulse, a change ⁇ in an optical phase of the reflected probe pulse, and a change in an angle of reflection of the probe pulse, each of which may be considered as a change in a characteristic of a reflected or transmitted portion of the probe pulse.
• the measured transient response or responses are then associated with at least one characteristic of interest of the sample. In one embodiment an association is made with a characteristic such as at least one of the structural phase, grain orientation, and stoichiometry.
• This invention also teaches a method for characterizing a sample, comprising the steps of: (a) acquiring data from the sample using at least one probe beam wavelength to measure, for times less than a few nanoseconds, a transient change in the optical response of the sample induced by a pump beam; (b) analyzing the data to determine at least a sample preparation technique by comparing a background signal component of the data with data obtained for a similar delay time range from one or more samples prepared by similar sample preparation techniques; and (c) analyzing a component of the measured time dependent reflectivity caused by ultrasonic waves generated by the pump beam using data corresponding to the determined sample preparation technique.
• a method for characterizing a sample comprising the steps of: (a) acquiring data from the sample using at least one probe beam wavelength to measure, for a range of delay times, a change in at least one transient optical response of the sample induced by a pump beam; (b) assuming a value for a thickness of the film; (c) comparing a background signal, resulting from a non-acoustical component of the acquired data, to data that corresponds to a film of the same general type having the assumed thickness, to determine a most probable composition for the film; and (d) associating determined physical properties of the film as a result of the execution of step (c) with the sample film.
• the method further comprises the steps of (e) deducing an improved value for the thickness from an analysis of the acoustical component of the acquired data; and (f) repeating steps c-e until convergence between film thickness and material properties is achieved.
• This invention also encompasses a method of characterizing a sample that includes the steps of: (a) acquiring data from the sample using at least one probe beam wavelength to measure, for a range of delay times, a change in at Least one transient optical response of the sample induced by a pump beam; (b) assuming a composition of the film; (c) deducing a value for a thickness of the film from an analysis of an acoustical part of the data, the stop of deducing including a step of considering the film's material properties based on a film having the assumed composition; and (d) comparing a background signal corresponding to a film of the same general type, having the deduced thickness, to determine an improved composition for the film.
• the method further includes the steps of (e) associating material properties of the film from step (d) with the sample film; and (f) repeating steps c-e until convergence between the sample film's composition and thickness is achieved.
• FIG. 1A is a block diagram of a first embodiment of a picosecond ultrasonic system that is suitable for use in practicing this invention, specifically, a parallel, oblique beam embodiment;
• Fig. 1B is a block diagram of a second embodiment of a picosecond ultrasonic system that is suitable for ⁇ se in practicing this invention, specifically, a normal pump, oblique probe embodiment;
• Fig. 1C is a block diagram of a third, presently preferred embodiment of a picosecond ultrasonic system that is suitable for use in practicing this invention, specifically, a single wavelength, normal pump, oblique probe, combined ellipsometer embodiment;
• Fig. 1D is a block diagram of a fourth embodiment of a picosecond ultrasonic system that is suitable for use in practicing this invention, specifically, a dual wavelength, normal pump, oblique probe, combined ellipsometer embodiment;
• Fig. 1E is a block diagram of a fifth embodiment of a picosecond ultrasonic system that is suitable for use in practicing this invention, specifically, a dual wavelength, normal incidence pump and probe, combined ellipsometer embodiment;
• Fig. 2 is a graph illustrating representative data showing acoustic echoes superimposed on a background signal
• Fig. 3 is a flow chart that illustrates a method in accordance with this invention.
• Fig. 4 is a graph illustrating picosecond reflectivity versus time for Ti-Si films annealed at temperatures ranging from 350oC to 765oC, wherein a result for an unannealed sample, which overlaps the 350 oC curve, is also shown;
• Fig. 5 is a graph showing curves that correspond to the curves shown in Fig. 4, but from which a slowly varying background signal has been removed in order to emphasize the acoustical vibrations of the Ti-Si film, wherein the curves are offset from one another for the sake of clarity;
• Fig. 6 is a graph illustrating a comparison of a picosecond reflectivity, from an arbitrary point on the curves shown in Fig. 4, versus annealing temperature, and which shows a correlation of this curve with a curve representing sheet resistivity;
• Figs. 7A and 7B are a flow chart illustrating a method of this invention wherein an initial Ti thickness is known a priori;
• Figs. 8A and 8B are a flow chart illustrating a method of this invention wherein an initial Ti thickness is not known a priori;
• a light pulse is directed onto a sample, and is partially absorbed by electronic carriers in the sample, which subsequently transfer their energy to the materials comprising the sample.
• a small, localized transient change in the sample's optical response that is, there is manifested at least one transient and measurable response of the sample to the pump pulse of optical radiation.
• a measured transient response or responses can include at least one of a measurement of a modulated change ⁇ R in an intensity of a reflected portion of a probe pulse, a change ⁇ T in an intensity of a transmitted portion of the probe pulse, a change ⁇ P in a polarization of the reflected probe pulse, a change ⁇ in an optical phase of the reflected probe pulse, and a change in an angle of reflection of the probe pulse, each of which may be considered as a change in a characteristic of a reflected or transmitted portion of the probe pulse.
• the transient response of the sample to the pump pulse decays at a rate which depends mainly on the rates at which the excited electronic carriers transfer their energy to the remainder of the sample, and also on the thermal diffusivities and thicknesses of the materials comprising the sample.
• a second effect of the light pulse may be to generate ultrasonic waves in the sample.
• These ultrasonic waves also give rise to changes in the optical reflectivity of the sample which vary more rapidly in time than the reflectivity changes associated with the sample's return to thermal equilibrium.
• the reflectivity changes associated with the ultrasonic waves and with the change in temperature occur concomitantly.
• all transient changes in the optical response of the sample, such as the reflectivity, and excluding the ultrasonic contribution are referred to as the "background" signal.
• the ultrasonic component of the net reflectivity change may be said to ride on a thermal "background" signal.
• Typical data are illustrated in Fig. 2.
• the reflectivity changes associated with the propagating ultrasonic waves may be analyzed in the manner disclosed by Tauc et al. to determine the mechanical properties of a sample.
• the reflectivity change is measured by means of a second light pulse, the "probe", which is delayed relative to the heating pulse, the "pump", by a time ⁇ .
• the value of ⁇ is typically in the range of 0.01 picosecond to 100 nanoseconds.
• the sign, magnitude, and rate of decay of the "background” reflectivity change depends on the wavelength, angle of incidence and polarization of the probe light pulse, and on the electronic and thermal properties of the sample material.
• the background reflectivity change for samples of the same material may differ substantially due to different preparation conditions.
• the inventors have observed that different background signals arise for samples composed of the same metal, but having a different structural phase, and also for alloys having differing compositions. These observations are indicative of the differences which exist between the thermal and electronic properties of such materials.
• the factors referred to above may affect the elastic properties of a sample material.
• the elastic constants and densities of two structural phases of a particular metal may differ significantly, corresponding to readily observed differences in the sound velocity and volume of samples having identical stoichiometry. Similar differences may be seen for samples such as thin films which have been deposited under conditions leading to different grain orientations and shapes.
• This invention provides a method for obtaining greater accuracy, than that obtained by previous techniques, by removing possible ambiguities.
• data are acquired according to methods and apparatus described below by using at least one probe wavelength to measure, for times generally less than a few nanoseconds, a transient change in an optical property, such as the reflectivity, of the sample induced by the pump laser.
• the data are then analyzed in two steps as follows.
• Step 1 the "background" signal is compared with data obtained for a similar delay time range from samples prepared under conditions known to give rise to certain physical and chemical material properties, such as, by example, structural phase. This may involve interpolation between reference samples to obtain an intermediate set of material properties. Properties including sound velocity, density, optical constants etc. are then associated with the sample of interest.
• the background signal can be compared also, or instead, with results obtained from a modeled diffusion process and the associated physical parameters, such as the thermal reflectance.
• Step 2 one or more sample characteristics of interest, such as sound velocity, film thickness, film adhesion, etc., are determined from the measured time dependent change in the optical property, such as reflectivity, caused by the ultrasonic waves generated by the pump beam. These are analyzed in accordance with the material properties determined in Step 1. As such, the present invention provides a method for obtaining greater accuracy by removing possible ambiguities.
• a correlation can be made with a characteristic of the sample preparation technique, rather than a material property per se.
• the sound velocity appropriate to CVD (Chemical Vapor Deposition) TiN may be different than the sound velocity appropriate to sputtered TiN.
• Figs. 4-6 A sample series consisted of 10 samples which had been subjected to a range of RTP (Rapid Thermal Processing) annealing temperatures. The annealing cycles differed slightly for different annealing temperatures.
• RTP Rapid Thermal Processing
• a reaction between Ti and Si during the anneal cycle produces a layer whose electronic band structure changes as the reaction proceeds.
• the accompanying change in the transient reflectivity of the samples was measured in response to a short laser pulse, as described above.
• One component is a slowly varying "background" signal which is due to a combination of relaxation phenomena which take place after the laser pulse is absorbed by the film.
• the second component varies more rapidly, and is associated with acoustical vibrations of the layer.
• both components may be interpreted and used to characterize a given sample.
• Fig. 4 is a graph which shows transient reflectivity signals measured for all samples. The temperatures at which the samples were annealed is indicated on the graph.
• the data show acoustical oscillations for times less than about 30 picoseconds which are superimposed on the slowly varying background signal.
• the acoustical and background signals are separated, and the acoustical signals are plotted separately in Fig. 5.
• the curves in Fig. 5 have been intentionally offset from one another for the sake of clarity; and the ordering of the curves in Fig. 5 bears no relation to the ordering of the curves in Fig. 4 (to which no offsets have been applied).
• the increase in thickness is apparent from the shift to longer times of the acoustic peaks (which may be thought of as the vibrations of a slab, whose frequency becomes lower as its thickness is increased) . Also observed was an increase in the amplitude of the acoustic signal, indicating that the interface between thn TiSi 2 layer and the underlying Si becomes more distinct with annealing at higher temperatures. There is a final large change in the background signal for the sample annealed at 765 oC, which the Raman data showed to be due to the formation of the C54 phase. The largest acoustical signal of the entire series was observed for this sample. It should also be noted that the rate of decay of the vibrations of the C54 film is smaller than that observed for the C49 samples, indicating that the interface between the C54 layer and Si may be smoother.
• the thickness of the Ti layer was determined to be 231 ⁇ .
• the corresponding theoretical thickness can be shown to be 580 ⁇ .
• a measured thickness of 581 ⁇ 3 ⁇ was obtained, in very close agreement with the expected result. This is important because it shows how the acoustic information itself can be used to identify the suicide phase; i.e., the vibrational period of the suicide layer can be used as a measure of the completeness of the C54 formation if the initial Ti thickness is known.
• the vibrational period of the suicide layer can be used as a measure of the completeness of the C54 formation if the initial Ti thickness is known.
• Figs. 7A and 7B Qualitatively, curves similar to those in Fig. 4 are obtained for other thicknesses of Ti suicide, however, the details depend on the thickness to some degree.
• a first technique illustrated in Figs. 7A and 7B, uses a family of reference curves corresponding to Fig. 4 for a starting target Ti thickness over a wide range of annealing temperatures. For a subsequent "unknown" sample having the same starting Ti thickness, the phase is determined by comparing its picosecond reflectivity with the reference curves.
• a second technique employs a series of curves like those of Fig. 4, but corresponding to several thicknesses of Ti, and then determines the structural phase via Raman scattering (or any other suitable technique) for each curve. From this data the curves are constructed in accordance with the underlying physical parameters which govern the sign, amplitude, and rate of decay of the picosecond reflectivity for each phase.
• the phase is determined by parametrizing its picosecond reflectivity in terms of the same parameters, and these parameters are then compared with their known values for each phase.
• the second technique has the advantage of being potentially more transportable, although the first technique also provides satisfactory results.
• the transient reflectivity change is plotted at an arbitrary point on the Fig. 4 curves versus the annealing temperature. The result is shown in Fig. 6.
• the transient reflectivity change at 45 picoseconds was used, expressed as a percentage change relative to the value obtained for the unannealed sample.
• the qualitative features of the curve correlate well with the graph of resistivity versus annealing temperature.
• the resistivities have been expressed as a percent change relative to the resistivity obtained for the unannealed sample.
• the optical measurement system in accordance with this invention can be packaged as an in-fab optical metrology tool. It is completely nondestructive, and has a small spot size. For silicide monitoring it can be used to make measurements on product wafers within device or scribeline structures which are at least, by example, 5 microns in diameter. This value is a function of the particular focussing optics used. The use of fiber optic focussing optics, such as those having a reduced tip diameter, are especially attractive. Depending on such factors as pattern complexity and film thickness, measurement times range from, by example, 0.1 to 10 seconds per site. This technique can also be applied to small structures, such as regular arrays of lines and dots, using analogous analysis techniques.
• An important aspect of the teaching of this invention is that it uses two independent components of the measured silicide response (i.e. the "background” signal and the "acoustical vibration” signal) to perform a self-consistent analysis.
• Self-consistent analysis allows the technique to not just detect a silicide misprocessing event, Lut to identify its cause.
• the technique in accordance with this invention provides an unambiguous determination of the phase and thickness of an annealed film. It is ideally suited to high-throughput measurements on product wafers, and can produce high resolution film maps comparable to those obtained via four point probe measurements.
• Figs. 1A-1E illustrate various embodiments of sample measurement apparatus that are suitable for practicing this invention. These various embodiments are disclosed in copending U.S. Patent Application S.N. 08/689,287, filed 8/6/96, entitled " Improved Optical Stress Generator and Detector", by H.J. Maris and R.J. Stoner.
• FIG. 1A For illustrating an embodiment of apparatus 100 suitable for practicing this invention.
• the is embodiment is referred to as a parallel, oblique embodiment.
• This embodiment includes an optical/heat source 120, which functions as a variable high density illuminator, and which provides illumination for a video camera 124 and a sample heat source for temperature-dependent measurements under computer control.
• An alternative heating method employs a resistive heater embedded in a sample stage 122. The advantage of the optical heater is that it makes possible rapid sequential measurements at different temperatures.
• the video camera 124 provides a displayed image for an operator, and facilitates the set-up of the measurement system. Appropriate pattern recognition software can also be used for this purpose, thereby minimizing or eliminating operator involvement.
• the sample stage 122 is preferably a multiple-degree of freedom stage that is adjustable in height (z-axis), position (x and y-axes), and tilt ( ⁇ ), and allows motor controlled positioning of a portion of the sample relative to the pump and probe beams.
• the z-axis is used to translate the sample vertically into the focus region of the pump and probe, the x and y-axes translate the sample parallel to the focal plane, and the tilt axes adjust the orientation of the stage 122 to establish a desired angle of incidence for the probe beam. This is achieved via detectors PDS1 and PDS2 and a local processor, as described below.
• the optical head may be moved relative to a stationary, tiltable stage 122' (not shown). This. is particularly important for scanning large objects (such as 300mm diameter wafers, or mechanical structures, etc.)
• the pump beam, probe beam, and video signal can be delivered to or from the translatable head via optical fibers or fiber bundles.
• the BS5 is a broad band beam splitter that directs video and a small amount of laser light to the video camera 124.
• the camera 124 and local processor can be used to automatically position the pump and probe beams on a measurement suite.
• the pump-probe beam splitter 126 splits an incident laser beam pulse (preferably of picosecond or shorter duration) into pump and probe beams, and includes a rotatable half- wave plate (WP1) that rotates the polarization of the unsplit beam.
• WP1 is used in combination with polarizing beam splitter PBS1 to effect a continuously variable split between pump and probe power.
• This split may be controlled by the computer by means of a motor to achieve an optimal signal to noise ratio for a particular sample. The appropriate split depend on factors such as the reflectivity and roughness of the sample. Adjustment is effected by having a motorized mount rotate WP1 under computer control.
• a first acousto-optic modulator (AOM1) chops the pump beam at a frequency of about 1MHz .
• a second acousto-optic modulator (AOM2) chops the probe beam at a frequency that differs by a small amount from that of the pump modulator AOM1.
• the use of AOM2 is optional in the system illustrated in Fig. 1A.
• the AOMs may be synchronized to a common clock source, and may further be synchronized to the pulse repetition rate (PRR) of the laser that generates the pump and probe beams.
• PRR pulse repetition rate
• a spatial filter 128 is used to preserve at its output a substantially invariant probe beam profile, diameter, and propagation direction for an input probe beam which may vary due to the action of the mechanical delay line shown as a retroreflector 129.
• the spatial filter 128 includes a pair of apertures A1 and A2, and a pair of lenses L4 and L5.
• An alternative embodiment of the spatial filter incorporates an optical fiber, as described above.
• WP2 is a second adjustable half wave plate which functions in a similar manner, with PBS2, to the WP1/PBS1 of the beamsplitter 126.
• WP2 the intent is to vary the ratio of the part of the probe beam impinging on the sample to that of the portion of the beam used as a reference (input to D5 of the detector 130.
• WP2 may be motor controlled in order- to achieve a ratio of approximately unity.
• the electrical signals produced by the beams are subtracted, leaving only the modulated part of the probe to be amplified and processed.
• PSD2 is used in conjunction with WP2 to achieve any desired ratio of the intensities of the probe beam and reference beam.
• the processor may adjust this ratio by making a rotation of WP2 prior to a measurement in order to achieve a nulling of the unmodulated part of the probe and reference beam. This allows the difference signal (the modulated part of the probe) alone to be amplified and passed to the electronics.
• the beamsplitter BS2 is used to sample the intensity of the incident probe beam in combination with detector D2.
• the linear polarizer 132 is employed to block scattered pump light polarization, and to pass the probe beam.
• Lenses L2 and L3 are pump and probe beam focusing and collimating objectives respectively.
• the beamsplitter BS1 is used to direct a small part of pump and probe beams onto a first Position Sensitive Detector (PSD1) that is used for autofocusing, in conjunction with the processor and movements of the sample stage 122.
• PSD1 is employed in combination with the processor and the computer-controlled stage 122 (tilt and z-axis) to automatically focus the pump and probe beams onto the sample to achieve a desired focusing condition.
• the detector D1 may be used in common for transient optical, ellipsometry and reflectometry embodiments of this invention. However, the resultant signal processing is different for each application.
• the DC component of the signal is suppressed, such as by subtracting reference beam input D5, or part of it as needed, to cancel the unmodulated part of D1, or by electrically filtering the output of D1 so as to suppress frequencies other than that of the modulation.
• the small modulated part of the signal is then amplified and stored.
• ellipsometry there is no small modulated part, rather the entire signal is sampled many times during each rotation of the rotation compensator (see Fig. 1B), and the resulting waveform is analyzed to yield the ellipsometric parameters.
• the change in the intensity of the entire unmodulated probe beam due to the sample is determined by using the D1 and D2 output signals (D2 measures a signal proportional to the intensity of the incident probe).
• additional reflectometry data can be obtained from the pump beam using detectors D3 and D4.
• the analysis of the reflectometry data from either or both beams may be used to characterize the sample.
• the use of two beams is useful for improving resolution, and for resolving any ambiguities in the solution of the relevant equations.
• a third beamsplitter BS3 is used to direct a small fraction of the pump beam onto detector D4, which measures a signal proportional to the incident pump intensity.
• a fourth beamsplitter BS4 is positioned so as to direct a small fraction of the pump beam onto detector D3, which measures a signal proportional to the reflected pump intensity.
• Fig. 1B illustrates a normal pump beam, oblique probe beam embodiment of apparatus 102.
• Components labelled as in Fig. 1A function in a similar manner, unless indi rated differently below.
• the aoove- mentioned rotation compensator 132 embodied as a quarter wave plate on a motorized rotational mount, and which forms a portion of an ellipsometer mode o ⁇ the system.
• the plate is rotated in the probe beam at a rate of, by example, a few tens of Hz to continuously vary the optical phase of the probe beam incident on the sample.
• the reflected light passes through an analyzer 134 and the intensity is measured and transferred to the processor many times during each rotation.
• the signals are analyzed according to known types of ellipsometry methods to determine the characteristics of the sample (transparent or semitransparent films). This allows the (pulsed) probe beam to be used to carry out ellipsometry measurements.
• the ellipsometry measurements are carried out using a pulsed laser, which is disadvantageous under normal conditions, since the bandwidth of the pulsed laser is much greater than that of a CW laser of a type normally employed for ellipsometry measurements.
• the rotation compensator 132 is oriented such that the probe beam is linearly polarized orthogonal to the pump beam.
• the analyzer 134 may be embodied as a fixed polarizer, and also forms a portion of the ellipsometer mode of the system. When the system is used for transient optical measurements the polarizer 134 is oriented to block the pump beam.
• the analyzer 134 may be embodied as a fixed polarizer, and also forms a portion of the ellipsometer mode of the system. When the system is used for acoustics measurements the polarizer 134 is oriented to block the pump polarization. When used in the ellipsometer mode, the polarizer 134 is oriented so as to block light polarized at 45 degrees relative to the plane of the incident and reflected probe beam.
• Fig. 1B further includes a dichroic mirror (DM2), which is highly reflective for light in a narrow band near the pump wavelength, and is substantially transparent for other wavelengths.
• DM2 dichroic mirror
• PSD2 pump PSD
• PSD1 probe PSD
• a lens L1 is employed as a pump, video, and optical heating focussing objective, while an optional lens L6 is used to focus the sampled light from BS5 onto the video camera 124.
• FIG. IC for illustrating an embodiment of apparatus 104, specifically a single wavelength, normal pump, oblique probe, combined ellipsometer embodiment. As before, only those elements not described previously will be described below.
• Shutter 1 and shutter 2 are computer controlled shutters, and allow the system to use a He-Ne laser 136 in the ellipsometer mode, instead of the pulsed probe beam.
• shutter 1 is open and shutter 2 is closed.
• shutter 1 is closed and shutter 2 is opened.
• the HeNe laser 136 is a low power CW laser, and has been found to yield superior ellipsometer performance for some films.
• Fig. 1D is a dual wavelength embodiment 1D of the system illustrated in Fig. 1C.
• the beamsplitter 126 is replaced by a harmonic splitter, an optical harmonic generator that generates one or more optical harmonics of the incident unsplit incident laser beam. This is accomplished by means of lenses L7, L8 and a nonlinear optical material (DX) that is suitable for generating the second harmonic from the incident laser beam.
• the pump beam is shown transmitted by the dichroic mirror (DM 138a) to the AOM1, while the probe beam is reflected to the retroreflector. The reverse situation is also possible. The shorter wavelength may be transmitted, and the longer wavelength may be reflected, or vice versa.
• the pump beam is the second harmonic of the probe beam (i.e., the pump beam has one half the wavelength of the probe beam).
• F1 is a filter having high transmission for the probe beam and the He-Ne wavelengths, but very low transmission for the pump wavelength.
• Fig. 1E illustrates a normal incidence, dual wavelength, combined ellipsometer embodiment 108.
• the probe beam impinges on PBS2 and is polarized along the direction which is passed by the PBS2.
• WP3 a quarter wave plate, and reflects from the sample, it returns to PBS2 polarized along the direction which is highly reflected, and is then directed to a detector D0 in detector block 130.
• D0 measures the reflected probe beam intensity.
• WP3 causes the incoming plane polarized probe beam to become circularly polarized.
• the handedness of the polarization is reversed on reflection from the sample, and on emerging from WP3 after reflection, the probe beam is linearly polarized orthogonal to its original polarization.
• BS4 reflects a small fraction of the reflected probe onto an Autofocus Detector AFD.
• DM3 a dichroic mirror, combines the probe beam onto a common axis with the illuminator and the pump beam.
• DM3 is highly reflective for the probe wavelength, and is substantially transparent at most other wavelengths.
• D1 a reflected He-Ne laser 136 detector, is used only for ellipsometric measurements.
• a selected one of these presently preferred embodiments of measurement apparatus provide for the characterization of samples in which a short optical pulse (the pump beam) is directed to an area of the surface of the sample, and then a second light pulse (the probe beam) is directed to the same or an adjacent area at a later time.
• the retroreflector 129 shown in all of the illustrated embodiments of Figs. 1A-1E can be employed to provide a desired temporal separation of the pump and probe beams.
• the apparatus 100, 102, 104, 106 and 108 is capable of measuring some or all of the following quantities: (1) the small modulated change ⁇ R in the intensity of the reflected probe beam, (2) the change ⁇ T in the intensity of the transmitted probe beam, (3) the change ⁇ P in the polarization of the reflected probe beam, (4) the change ⁇ in the optical phase of the reflected probe beam, and/or (5) the change in the angle of reflection ⁇ 6 of the probe beam.
• These quantities (1)-(5) may all be considered as transient responses of the sample which are induced by the pump pulse.
• measurements can be made together with one or several of the following: (a) measurements of any or all of the quantities (1)-(5) just listed as a function of the incident angle of the pump or probe light, (b) measurements of any of the quantities (1)-(5) as a function of more than one wavelength for the pump and/or probe light, (c) measurements of the optical reflectivity through measurements of the incident and reflected average intensity of the pump and/or probe beams; (d) measurements of the average phase change of the pump and/or probe beams upon reflection; and/or (e) measurements of the average polarization and optical phase of the incident and reflected pump and/or probe beams.
• the quantities (c), (d) and (e) may be considered to be average or static responses of the sample to the pump beam.
• the simulation may also give a value for the static reflection coefficient of the pump and probe beams.
• the system measures the transient change ⁇ P probe-refl in the power of the reflected probe pulse as determined, for example, by photodiode D1 in Fig. 1C. It also measures the static reflection coefficients of the pump and probe beams from a ratio of the power in the incident and reflected beams.
• the incident probe power is measured by photodiode D2 in Fig. 1C
• the reflected probe power is measured by D1
• the incident pump power is measured by D4
• the reflected pump power is measured by D3.
• f is the repetition rate of the pump pulse train
• R pump is the reflection coefficient for the pump beam
• the change in optical reflectivity of the each probe light pulse will be: TM / P
• I probe-inc and I pump-lnc p are respectively the intensities of the probe and pump beams on the surface of the sample.
• I probe-inc and I pump-lnc p are respectively the intensities of the probe and pump beams on the surface of the sample.
• Analogous expressions can be derived for the change in optical transmission ⁇ T(t), the change in optical phase ⁇ (t), the change in polarization ⁇ P(t), and the change ⁇ (t) in the angle of reflection of the probe light.
• a first method directly measures the intensity variations of the pump and probe beams over the surface of the sample, i.e, I probe-inc and I pump-inc ) as a funttion
• a second method measures the transient response ⁇ P probe- . refl for a sample on a system S for which the area A effective is known. This method then measures the response ⁇ P probe-refl of the same sample on the system S' for which A effective is to be determined.
• the ratio of the responses on the two systems gives the inverse of the ratio of the effective areas for the two systems. This can be an effective method because the system S can be chosen to be a specially constructed system in which the areas illuminated by the pump and probe beams are larger than would be desirable for an instrument with rapid measurement capability.
• a third method measures the transient response ⁇ P probe-refl for a sample in which all of the quantities are known which enter into the calculation of the simulated reflectivity change ⁇ R sim (t) of the sample when it is illuminated with a pump pulse of unit energy per unit area of the sample. Then by comparison of the measured transient response ⁇ P probe-refl with the response predicted from the Eq. 6, the effective area A effective is determined.
• the apparatus shown in Figs. 1A-1E incorporate means for automatically focusing the pump and probe beams onto the surface of the sample so as to achieve a reproducible intensity variation of the two beams during every measurement.
• the automatic focusing system provides a mechanism for maintaining the system in a previously determined state in which the size and relative positions of the beams on the sample surface are appropriate for effective transient response measurements.
• a calibration scheme such as described above is an important feature of the measurement system, wherein calibration includes determining a size and an area of overlap of the pump and probe beams on a surface of the sample.
• V probe- refl C V probe- i nc ⁇ R s im ( t ) V pump-inc (1-R pump ) (6), where ⁇ v probe-refl is the output voltage from the detector used to measure the change in the power of the reflected probe light (D1), V pu mp is the output voltage from the detector used to measure the incident pump light (D4), and V probe-inc is the output voltage of the detector used to measure the incident probe light (D2).
• ⁇ v probe-refl is the output voltage from the detector used to measure the change in the power of the reflected probe light (D1)
• V pu mp is the output voltage from the detector used to measure the incident pump light (D4)
• V probe-inc is the output voltage of the detector used to measure the incident probe light (D2).
• a first method measures the transient response ⁇ V probe- refl for a sample in which all of the quantities are known which enter into the calculation of the simulated reflectivity change ⁇ R sim (t) of the sample, when it is illuminated with a pump pulse of unit energy per unit area of the sample .
• the method measures V probe - inc and V pump-inc' then determines R pump either by measurement or from the computer simulation. The method then finds the value of the constant C such that Eq. 6 is satisfied.
• a second method measures the transient response ⁇ V probe- refl for a reference sample for which the transient optical response ⁇ R(t), when it is illuminated with a pump pulse of unit energy per unit area of the sample, has been measured using a system which has been previously calibrated, for example, by one or more of the methods described above. The method then measure V probe - inc and V pump-inc . , determines R pump by measurement, and then finds the value of the constant C such that the following equation is satisfied:
• V probe- refl C V Probe- inc ⁇ R ( t ) V pump-inc ( 1-R pump ) ( 7 )
• this invention employs an analysis of the background and acoustical data from a sample in order to get a best result for two sample properties, namely the thickness and the composition of the material comprising a thin film, wherein the term "composition" is herein intended to include characteristics such as phase, morphology, crystalline orientation, grain size, etc.
• the analysis may assume a value for one of the properties, and then determine the other by simulation and/or by comparison to data obtained from known reference samples.
• the analysis may also be self-consistent by assuming a value for one property, then determining the other by simulation and/or by comparison to data obtained from known reference samples, then improving the value for the assumed property, and then continuing to iterate until to obtain a best result for both properties. For example, this iteration can be begun by first assuming a thickness of the film, and then determining the film's composition, or by first assuming a composition for the film and then determining the thickness. Both are equally valid approaches.
• (A) Calibrate the measurement system, such as that shown in Fig. 1C, autofocus, and measure the sample.
• (C) Compare the background signal corresponding to a film of the same general type (e.g., TiSi 2 )) having the assumed thickness to determine a most probable composition for the film.
• the material properties for 46-54 TiN differ from the material properties for 47-53 TiN.
• (A) Calibrate the measurement system, autofocus, and measure the sample.
• composition of the film e.g., 50-50 TiN.
• (C) Deduce a value for the thickness from an analysis of the acoustical part of the data.
• (A) Calibrate the measurement system, such as that shown in Fig. 1C, autofocus, and measure the sample.
• (A) Calibrate the measurement system, autofocus, and measure the sample.
• (C) Deduce a value for the thickness from an analysis of the acoustical part of the data.
• any of the forgoing steps of comparing can optionally interpolate between reference data or parameters that are measured or deduced from a plurality of reference samples.
• the methods can include a step of modifying an assumed composition of the sample or film by one or more of the stoichiometry, crystal structure, morphology, structural phase, alloy composition, impurity content, doping level, defect density, isotope content, grain orientation, etc.
• the teaching of this invention can also be employed with compound semiconductors, such as Group III-V and Group II- VI compound semiconductors, having an unknown content of one or more constituent elements.
• the teaching of this invention is especially beneficial for use with samples comprised of a semiconductor material and a metal or silicide.
• the teaching of this invention is also useful with samples comprised of an alloy of at least two elements having an unknown ratio. Examples include Ti-W, Au-Cr, Al-Cu, Al-Cu-Si, Si-Ge, In- Ga-As, Ga-Al-As, and Hg-Cd-Te.
• the methods of this invention are particularly useful with thermally annealed samples for determining annealing temperature, the thickness of an annealed layer, and the phase of an annealed layer.
• This invention can be employed to advantage with a wide range of materials and material systems having a finite (useable) absorption of the pump wavelength that is sufficient to excite strain waves in the sample. As was described above in reference to the embodiment shown in Fig. 1D, the wavelengths of the pump and probe pulses may be different.

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