Abstract:
A method for determining the thickness and other properties of a metal layer comprising producing an acoustic wave at a first frequency in the metal layer with a first laser beam, measuring the angle of diffraction of a second laser beam from the acoustic wave, and calculating the wavelength of the acoustic wave and thickness of the metal layer from the angle of diffraction.

Description:
BACKGROUND  
         [0001]    1. Field of the Invention  
           [0002]    This invention generally relates to nondestructive evaluation of a material, and more specifically to surface photo acoustic wave measurement to determine thickness or other properties of a material used in semiconductor device fabrication.  
           [0003]    2. Related Art  
           [0004]    During fabrication of microelectronic devices, thin films of metals and metal alloys are deposited on silicon wafers and used as electrical conductors, adhesion-promoting layers, and diffusion barriers. Microprocessors, for example, use metal films of copper, tungsten, and aluminum as electrical conductors and interconnects, titanium and tantalum as adhesion-promoting layers, and titanium nitride and tantalum nitride as diffusion barriers. Thickness variations in these films can modify their electrical and mechanical properties, thereby affecting the performance of the microprocessor. The target thickness values of metal films vary depending on their function: Conductors and interconnects are typically 3000-10000 angstroms thick, while adhesion-promoting and diffusion-barrier layers are typically between 100-500 angstroms thick.  
           [0005]    Metal films are typically deposited and patterned in complex geometries in the microprocessor. A geometry currently used in microelectronics fabrication is a “damascene” or “dual damascene” structure. Damascene-type structures, used primarily to form copper conductors and interconnects, are typically formed by a multi-step process: i) an oxide layer on a wafer is first etched to have a series of trenches and then coated with a diffusion-barrier layer of tantalum or tantalum nitride; ii) copper is electrolytically plated onto the wafer to fill the coated trenches; iii) the structure is then mechanically polished to remove excess copper, leaving only trenches filled with the diffusion-barrier layer and copper. The resulting structure is a series of separated copper lines having a thickness of a few thousand angstroms, a width and periodicity of about 0.5 microns, and a length of several millimeters.  
           [0006]    Non-metal thin films also have considerable application in low dielectric constant (k) applications such as optical coatings, sensors, and insulating films for use in ULSI circuit devices. Silica aerogel films are of particular interest. The porosity and density of the insulating film are difficult to measure but are directly related to the dielectric constant (k). Young&#39;s modulus is another important property to be measured that is also correlated with k.  
           [0007]    During typical fabrication processes, films are deposited to have a thickness of within a few percent (e.g., 5-100 angstroms) of their target value. Because of these rigid tolerances, film thickness is often measured as a quality-control parameter during and/or after the microprocessor&#39;s fabrication. Noncontact, nondestructive measurement techniques (e.g., optical techniques) are preferred because they can measure patterned “product” samples, (e.g., damascene samples) rather than “monitor” samples. Measurement of product samples accurately indicates errors in fabrication processes and additionally reduces costs associated with monitor samples.  
           [0008]    One optical technique for film-thickness measurements uses a single, short (100 fs) optical pulse to generate an acoustic pulse that propagates into a multilayer structure. The acoustic pulse reflects off various interfaces (i.e., layer/layer and substrate/layer interfaces) in the structure, thus causing its echo to return to the structure&#39;s surface. The returning echo pulse modulates the surface reflectivity and is measured with a variably delayed optical probe pulse. The thickness of the layers in the structure is determined by analyzing the time dependence of the reflected probe beam and the sound velocities of the film and substrate materials.  
           [0009]    In another prior technique, a measurement system launches a surface acoustic wave with known wavelength λ. A fixed wavelength is imprinted on the wave by illuminating the copper surface with a powerful pulsed laser. The laser beam is divided into multiple beams so that an array of alternating light regions (constructive interference) and dark regions (destructive interference) as above. The period of the array is the imprinted wavelength. The copper is heated and expands in the region of the light stripes, and surface acoustic waves are launched in the two opposite directions perpendicular to the stripes. The surface acoustic wave is a series of ripples on the surface and effectively forms a diffraction grating. A second laser beam is diffracted off the grating and the surface acoustic wave frequency is imprinted on this second beam. The wave frequency is measured by the time dependence of the diffracted beam intensity. If the frequency is measured by the system, then the wave speed c can be calculated. Once c is known, the film thickness can be determined if the material acoustic constants are known.  
         SUMMARY OF THE INVENTION  
         [0010]    A first aspect of the invention is a method for determining the thickness, density or other properties of a material that involves producing an acoustic wave at a first frequency in a material layer with a first laser beam. After the wave with a known frequency is created within the material layer, the angle of diffraction of a second laser beam from the acoustic wave is measured. With the measured angle of diffraction and the known frequency of the wave, the wavelength of the acoustic wave and thickness of the material layer or layers are then determined.  
           [0011]    Another aspect of the invention is a system for measuring the thickness, density or other properties of a material. The system comprises a first laser, the first laser creating a first beam, the first beam creating an acoustic wave at a first frequency in the material, a second laser, the second laser creating a second beam, a portion of which is reflected and a portion of which is diffracted by the material, and a position sensing detector that measures the angle of diffraction of the diffracted portion of the second beam. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0012]    [0012]FIG. 1 is a schematic of a first embodiment of the invention.  
         [0013]    [0013]FIG. 2 is a schematic of a second embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    The fundamental physical basis of surface acoustic wave measurement is that the phase velocity, or sinusoidal wave speed, depends in part on the thickness of the layer in which it propagates. The wave speed also depends on the acoustic constants of both the film and the substrate, as well as the wavelength or frequency of the wave. The speed, wavelength, and frequency for any wave are connected by the following relation: 
         
       c=vλ, 
     
         [0015]    where c is the phase velocity, v is the frequency and λ is the wavelength. For a given film material, substrate material, and film thickness, c is fixed but will be unknown because the film thickness is unknown.  
         [0016]    Referring to FIG. 1, optical system  100  creates an acoustic wave  145  with a definite frequency and measures the wavelength of the acoustic wave  145 .  
         [0017]    Continuous wave (CW) excitation laser  110  creates an excitation beam  115 . The wavelength of CW laser  110  and beam  115  is selected based upon the material to be tested, and will be different for different applications. For instance, to measure a roughly one micron copper layer, a green, frequency doubled YAG or UV laser is preferred. CW laser  110  can be modulated at any frequency ranging from 100 MHz to 2 GHz. Although laser  110  is illustrated as a CW laser, a pulsed laser or any laser that can excite a wave with a known frequency may be used in place of a CW laser. Preferably, if employed, the pulsed laser should have a pulse length of roughly 1 microsecond or longer. Beam  115  travels through acousto-optic modulator  120 , which modulates the beam at a definite frequency, p, of, for instance, 300 MHz. The focused beam  142  is in the form of a single narrow stripe, not an array, with no wavelength or q associated with it. Beam  115  passes through and is focused by cylinder lens combination  130  and  135  in the form of a narrow stripe and repetitively heats a small rectangular region  140  of copper at the same frequency (300 MHz) and the resulting expansion launches surface acoustic waves  145  in opposite directions perpendicular to the rectangular region  140 . The illuminated rectangular region  140  is about 1 micron by about 50-100 microns. These waves  145  have a wavelength λ a  and form a diffraction grating.  
         [0018]    The wavelength λ a  of the acoustic waves  145  is determined by diffracting a probe laser beam  215  from the grating and measuring its angle of diffraction θ. Any order of diffraction can be measured, however, it is preferable to measure the first order to maximize the signal to noise ratio. The angle of diffraction is used to calculate the wavelength of the acoustic wave according to the following formula: 
         λ l =2λ a  sin θ 
         [0019]    where λ l  is the wavelength of the laser light in probe beam  215 , λ a  is the wavelength of the acoustic wave, and θ is the diffraction angle. The wavelength of the laser light λ l  is known or can be measured or determined by any number of ways well known in the art.  
         [0020]    With the calculated acoustic wavelength λ a ,and the known frequency of the acoustic wave  145 , which is the same as the modulation frequency of laser light that induced the wave (300 MHz), the speed of the wave can be calculated with the formula 
         
       c=vλ 
       a 
     
         [0021]    Once c is known, the film thickness can be determined after the material acoustic constants of the material are measured. The material acoustic constants are: the transverse and longitudinal speeds of sound in the substrate material, the transverse and longitudinal speeds of sound in the film material, and the densities of the substrate and film materials. The thickness can be calculated from the measured acoustic wave speed c; the imposed frequency v (300 MHz), and the material constants using acoustic dispersion relations. Acoustic dispersion relations are theoretically calculated data and/or data plots for different layer/substrate combinations relating frequency to velocity or other characteristics. The thickness can then be determined by referencing in real-time the dispersion curves which can be stored in a memory of optical system  100 .  
         [0022]    For further explanation of the dispersion relations please refer to “Real-time optical characterization of surface acoustic modes of polyimide thin-film coatings”, Anil R. Duggal, John A. Rogers, and Keith A. Nelson, Journal of Applied Physics, 72, (1992), pp.2823-2839, which is hereby incorporated by this reference in its entirety.  
         [0023]    The density and Young&#39;s modulus of low-k dielectric films on a substrate can also be determined by exciting a surface acoustic wave within the substrate. This is possible because these films, unlike copper or other metal films, are transparent. The transparency will prevent them from absorbing much laser light, which will then be absorbed by an underlying copper or silicon substrate. The substrate will absorb some of the laser light and create the acoustic wave. If data is taken at more than one frequency, the acoustic dispersion curve can be mapped out and the density as well as the acoustic wave speed within the substrate and films on the substrate can be determined. Four properties of a layer affect the dispersion curve shape, the thickness, density, Young&#39;s Modulus, and Poisson&#39;s ratio. One or more of these variables can be fitted to the dispersion curve, and from the best fit, the desired layer property can be determined. The density is closely related and can be used to determine film properties such as the dielectric constant, stiffness (Young&#39;s modulus), thermal conductivity, and pore size by referencing the previously gathered acoustic dispersion curves. The density can also be used to calculate Young&#39;s modulus, (a measure of the strength of the film material) as density varies with the following relation: 
           E=E   0 (ρ/ρ 0 ) m   
         [0024]    where E is Young&#39;s modulus, E 0  is the modulus at an initial density, ρ is the density, ρ 0  is the initial density, and m is a constant with value of about 3-4. Young&#39;s modulus is also directly related to the dielectric constant of the material which can be determined by referencing experimental data correlating Young&#39;s modulus with the dielectric constant.  
         [0025]    For further explanation please refer to “Characterization of thin-film aerogel porosity and stiffness with laser-generated surface acoustic waves”, C. M. Flannery, C. Murray, I. Streiter, S. E. Schulz, Thin Solid Films, 388, (2001), pp. 1-4, which is hereby incorporated by this reference it its entirety.  
         [0026]    Referring again to FIG. 1, beam  215  produced by probe laser  210  is reflected by mirror  212 , and modulated by modulator  220  at a frequency that differs from the beam  115  by about 100 Hz to 100 kHz. Probe laser  210  can be a CW laser or a pulsed laser as described above regarding laser  110 . In this example, a frequency difference of 1 kHz is illustrated between beam  115  and  215 . Therefore beam  215  is modulated at 300.001 MHz before being focused onto the material by lenses  230  and  235 . A CW pump laser  110  with a large, but reasonable, power of 10 W will produce surface ripples about 0.2 Å high. These small ripples diffract only a small fraction, around 10 −7 , of the probe laser light. With a visible or near-IR probe laser power of 100 mW, there is about 10 nW or 10 10  photons/sec in the diffracted beam  250 . This power is adequate for a precision measurement. With an incident angle of 45° the first order diffracted beam  250  is 10° away from the specularly reflected beam  245 . With a measurement spot  240  of 100 μm length, there are about 20 ripples within the spot and this would make the diffracted beam width about 0.5 degree (the size of the ripples has been exaggerated in the figures for illustrative purposes). In order to make a measurement of film thickness with about 0.5% accuracy, the diffraction angle θ must be measured to about 1%, or 0.1 degree. This involves finding the center of the diffracted spot within about ⅕th of its width. This is possible with about 10 10  photons. Thus, at a position sensing detector  260  located 150 mm away from the surface of the metal, 0.1 degrees corresponds to 250 microns.  
         [0027]    Position sensing detector  260  is shown as a split detector, although any type of position sensing detector, a component well known in the art, can be employed. A split detector is two or more detectors spaced very closely, often arranged as sectors of a circular disk. By combining the separate photocurrents from the detector in various ways, such as subtracting them or computing their ratio, small changes in the beam position can be measured very accurately. One problem with split detectors is that a portion of the light that falls between the two cells is lost. Lateral-effect cells are another type of position sensing device that avoids the problem of split detectors. They are single large photodiodes that use a thin, highly resistive layer for the top electrode of the cell, each end of which has its own lead. The output leads are connected to low-impedance points. The light beam appears as a current source located somewhere on the surface, so that the photocurrent divides itself between the output pins in proportion to the conductance of each path. Because the conductance depends on the distance from the light beam to the output pin, the ratio of the currents in each pin gives the location of the light source. The position sensing detector  260  could also be discrete diodes with a shadow mask or any type of position sensing detector used in the art.  
         [0028]    Lock-in detector/amplifier  270  detects or amplifies only the difference frequency (between excitation laser  110  and probe laser  210 ) at 1 kHz. Optical system  100  has sufficient signal-to-noise ratio and resolution so that an accurate measurement within 0.5% can be made. A microprocessor (not shown) may also be integrated into the circuitry of optical system  100 .  
         [0029]    There is at least one major difficulty in measuring the angle of diffraction of the surface wave that is overcome by optical system  100 . The small amplitude surface ripples (about 0.2 Å) may be approximately as much as a factor of 100 smaller than the inherent surface roughness. This means that the probe beam  215  will be scattered over a wide range of angles, including those of the diffracted beam  250 , with a total intensity about 10 4  times larger than the diffracted beam. Looking for this needle in a haystack might seem impossible, except for the fact that the diffracted light has the frequency of the acoustic ripples imprinted on it (300 MHz), and the scattered light does not. One way to use this fact is to modulate the probe beam  215  at a slightly different frequency, for instance, 300.001 MHz. The probe beam  215  can be modulated at any frequency, but preferably at a frequency such that the difference (from the excitation or pump beam) is about 100 Hz to 100 kHz. In the example above where the difference frequency is 1 kHz, the diffracted probe light would then have several frequency components, including the difference frequency of 1 kHz. The position sensitive detector  260  is used with lock-in amplification/detection set at 1 kHz to monitor only the diffracted beam and reject the stray scattered light. This scheme also would reject the scattered light from the excitation laser beam  115 .  
         [0030]    [0030]FIG. 2 illustrates optical system  200 , another embodiment of the invention. Optical system  200  differs from optical system  100  of FIG. 1 in that it lacks the acousto-optic modulator  220 . Also, position sensing electronics  270  measure the frequency over a range of frequencies, not just a particular frequency (1 kHz for example) as in optical system  100 . Optical system  200  is thus simpler than optical system  100  but also less accurate because the stray scattered light would not be rejected as described above with regard to optical system  100 .  
         [0031]    The system and method of the present invention are advantageous over prior techniques. Although any laser that can create an acoustic wave of known frequency is within the scope of this invention (e.g. pulsed), the preferred CW laser measurement techniques described have better signal-to-noise than pulsed laser techniques, due to the much greater stability of CW lasers. In addition, measuring the difference frequency between the laser beams rejects any inaccuracy from the scattered light resulting in a more accurate and reliable measurement and system.  
         [0032]    While particular embodiments of the present invention and their advantages have been shown and described, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended Claims. For example, different frequencies of modulation can be used and multiple measurements at more than one frequency, even for the same material and thickness can be performed. These multiple measurement frequencies can be used to determine the film thickness, acoustic constants, density, porosity, stiffness, and Poisson&#39;s ratio of the bulk material or of layers upon the bulk material. Furthermore, different types and configurations of the lenses, and position detectors are also within the scope of the invention. Those skilled in the art will realize that other types of lasers and different frequency modulations are within the scope of the invention.