Abstract:
A method and apparatus for measuring wafers, thin films, or other planar layers are disclosed. This invention utilizes a tunable, monochromatic light source reflected from or transmitted through the layer to be measured. The wavelengths of light are selected such that the light is partially transmitted through the material to be measured so as optical interference is seen among the interfaces of the layer(s). The wavelengths are also controlled to sufficiently small increments to resolve these interference features. This apparatus relates to the need to monitor wafer thinning, film deposition, and other semiconductor device related processes.

Description:
CLAIMS THE BENEFIT TO PROVISIONAL APPLICATION 
   Pursuant to title 35, United States Code, Section 119(e) of the United States, this application claims benefit to the prior provisional patent application 60/486,371, filed on Jul. 11, 2003, the entire disclosure of which is hereby incorporated-by-reference for all purposes. 

   FIELD OF THE INVENTION 
   The present invention relates to an optical spectroscopic measurement system used to precisely determine the thickness of a layer of material typically of a semiconductor wafer. 
   BACKGROUND OF THE INVENTION 
   Considerable interest exists in determining the thickness of thin films used in the manufacture of micro-electronic, micro-optic, and micro-mechanical devices. Such measurement devices typically utilize visible light and are used to measure films of thickness on the order of the wavelength of light or less. Measurement of thicker materials such as semiconductor wafers is generally done with mechanical means. Thicknesses of semiconductors which are thinner than the typical wafer thickness of 200 microns or more, however, currently are difficult to measure. Their thinness makes them too fragile to touch mechanically. One technique utilizes single wavelength optical interference to measure a differential thickness change either from one portion of the wafer relative to the thinned part, or from a known value prior to thinning. Using this technique, optical interference occurs outside of the measured material rather than within it. Another technique measures the capacitance between electrodes above and below the wafer. This invention solves the problem of direct measurement of the thickness of such a layer, done with a non-contact, optical technique, with a high degree of accuracy. 
   SUMMARY OF THE INVENTION 
   The present invention provides an optical spectroscopic measurement system used to precisely determine the thickness of a layer of material and a method of measuring material thickness using the system. 
   In the spectroscopic system aspect of the invention, a system is provided that includes the following elements: a material; a tunable light source, wherein the source provides a beam of light incident to the material; a first detector that detects light from the light source either reflected or transmitted through the material; and, a computing device that computes the material thickness based on data received from the detector. 
   In the method aspect of the invention, a method of measuring material thickness is provided that includes the following steps: loading the material to be measured into a holder of a spectroscopic system, wherein the system comprises a tunable light source which provides a beam of light incident to the material, a detector which detects light from the light source either reflected from or transmitted through the material, and a computing device that computes the material thickness based on data received from the detector; measuring the light reflected from or transmitted through the material at least two different wavelengths using the detector; and, computing material thickness using a computing device based on data received from the detector. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts the reflectivity of a 200 μm thick layer of silicon. 
       FIG. 2  depicts a preferred embodiment of a spectroscopy system of the present invention. 
       FIG. 3  depicts a flow chart of a method for measuring material thickness using the spectroscopy system of the present invention. 
       FIG. 4  depicts a flow chart of a method for computing material thickness using the spectroscopy system of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An objective of the invention is the determination of the thickness of a material quickly, accurately, and without contact, by means of measuring the material&#39;s reflectivity or transmissivity at multiple wavelengths at which the material is at least partially transparent. 
   The typical material is a silicon wafer which has been thinned, yet whose top and bottom surfaces remain co-planar enough to provide some optical interference between the surfaces. Preferably, the top and bottom surfaces vary less than 1 wavelength of light over an area having a 100 μm diameter. More preferably, the top and bottom surfaces vary less than 1 wavelength of light over the same area. Currently available measurement instruments which operate using visible light are unable to determine the thickness of the silicon, which is opaque to such light. This invention utilizes infrared light which is able to partially transmit through the material. Semiconductors become transparent at energies less than their bandgap energy; the bandgap of silicon is 1.1 eV, corresponding to a 1 micron wavelength. This technology can be used to measure semiconductors transparent to the operating wavelength such as GaAs, SiC, SiGe, GaN, and InP. It may also be used to measure relatively thick organic films, such as dielectric films (e.g., photoresists). 
   The material may have coatings of other materials on either side, or may be immersed in liquid, gas, or vacuum. Any coatings on the side of the incident light must be partially transparent to the light; coatings on the side opposite the incident light may be non-transparent, such as metals. Typical coating materials used in semiconductor process are photoresists, insulators such as silicon dioxide, and conductors. 
   A beam of light composed of or tunable over multiple, distinct wavelengths is incident upon the layer of material whose complex refractive index is known. By measuring the incident and reflected light intensity, the material&#39;s reflectivity at each wavelength may be determined by utilizing the Fresnel formulae. See, “Principles of Optics,” Born &amp; Wolf, 6 th  ed., Pergamon Press, Oxford 1980, p. 38–42. These relationships associate reflectivity and transmissivity as a function of polarization of the light, wavelength, angle of incidence, and complex refractive indexes of the materials. Similarly by measuring the incident and transmitted light intensity, the material&#39;s transmissivity at each wavelength may be determined. 
   By knowing the polarization of the incident light, the material&#39;s complex refractive index and either its reflectivity or transmissivity at the multitude of wavelengths, the thickness of the material may be determined. The first step is to calculate the power reflectivity of the sample under test. This is calculated using Fresnel&#39;s equations to calculate the amplitude reflectivity at each material boundary, coupled with a computation of the phase change induced by transmission through each layer. At normal incidence the amplitude reflectivity simplifies to:
 
 R   TM =( n− 1)/( n+ 1) R   TE =−( n− 1)/( n+ 1)
 
where “TM” signifies “transverse magnetic field polarization, “TE” signifies “transverse electric field polarization,” and “n” is the ratio of complex refractive indexes of the two materials on either side of the boundary. The amplitude reflectivity within the layer is computed as:
 
 R =exp(− j 4 πnd /λ)
 
where “n” is the complex refractive index of the material, “d” is the thickness of the material, and λ is the wavelength. The total power reflectivity is obtained by squaring the complex amplitude reflectivity. For each wavelength, a periodic array of possible thickness solutions exist. By utilizing multiple wavelengths, a single thickness solution is possible. Increasing the number of wavelengths improves the accuracy of the measurement.
 
   A typical reflectivity as a function of wavelength for a silicon layer is shown in  FIG. 1 . This is the reflectivity of a 200 μm thick silicon layer. The separation of the measurement wavelengths must be less than half of the period of the oscillations which occur in the reflectivity and transmissivity in order to resolve such oscillations. 
   To facilitate measurements and optical alignment, the incident light angle should be as close to normal to the surface as possible. Preferably, the incident light angle is less than ±10° from normal. More preferably, the incident light angle is less than ±7.5° from normal. More preferably, the incident light angle is less than ±5° from normal. The region of the wafer illuminated should also be as small as possible to reduce the effects of thickness non-uniformity. Preferably, the illuminated region is a roughly spherical area less than 200 μm in diameter. More preferably, the illuminated region is a roughly spherical area less than 150 μm in diameter. Most preferably, the illuminated region is a roughly spherical area less than 100 μm in diameter. Measurement of patterned areas of the wafer are possible providing the desired measurement location on the wafer may be positioned within the light&#39;s area. 
   The typical thickness range of a material measured using the apparatus and method of the instant invention is 1 μm to 1000 μm. Oftentimes, the thickness range is 50 μm to 500 μm or 1 μm to 150 μm. Repeatability of the thickness measurement—in other words, the repeated measurements&#39; standard deviation—is less than 2%. Preferably, the standard deviation is less than 1%. More preferably, the standard deviation, expressed in a unit of measurement, is less than 20 nm or even less than 15 nm. 
   The first embodiment uses a quasi-monochromatic light source (laser) capable of varying its wavelength. The detection is done by means of a photodiode sensor. The laser is tuned to a specific wavelength, a portion of the light is reflected upon or transmitted through the wafer under test, and a portion of the light is detected by the sensor. The wavelength of the light is varied, with the reflection or transmission measured at each wavelength. Typically, the wavelength of light is varied in increments less than 1 nm. Preferably, the wavelength is varied in increments less than 0.5 nm. More preferably, the wavelength is varied in increments of about 0.1 nm. The resulting reflectivity or transmission versus wavelength allows computation of the optical thickness of the sample, as summarized below. 
   A second embodiment uses a broadband light source with a spectrometer detection system to separate the wavelengths of light. Light from this source is reflected upon or transmitted through the wafer under test, and a portion of the light is detected by the spectrometer. The spectrometer separates the light into discrete wavelengths, producing the reflectivity or transmission version wavelength information. The computation of thickness is as in the first embodiment. 
   A curve-fitting technique is used to match the best theoretical model of reflectivity or transmission as a function of optical thickness with the measured value. Optical thickness is the product of the true thickness with the optical refractive index. By prior knowledge of the refractive index of the materials measured, the actual sample thickness is thus determined. This technique may be extended to multiple layers of different materials, so that the thickness of both the wafer and a layer such as photoresist on the wafer may be simultaneously measured. 
   The spectroscopy system in the preferred embodiment is depicted in  FIG. 2  and is described in reference to the figure. The system includes a narrow-band tunable wavelength source  1 , which is typically a laser with light output that may be precisely controlled and varied in wavelength. Such lasers are commercially available, for example, from New Focus of San Jose, Calif. Wavelength source  1  is connected to a fiber optic splitter  9  through fiber optic cable  8 . Fiber optic splitter  9  is used to split the laser output into a measurement light path  10 , which is directed to the sample under test  4 , and a reference light path  11 , which is used to calibrate wavelength. 
   Measurement light path  10  passes through a collimating/focusing lens  5 , which is typically pigtailed. A portion of the light is directed to the sample under measurement  4 —which is typically positioned on a chuck ( 40 ) for reflectivity measurements or on a metal ring that is open in the middle (not shown) for transmissivity measurements—by means of a partially transmitting beamsplitter  2 . The light reflects from the sample  4 , passes (partially) through the beamsplitter  2  again, and is directed toward photosensitive detector  3 . Such detectors are commercially available, for example, from Fujitsu Corporation. A defocusing lens  6  may be used to reduce area of the light reflected from the sample which reaches the detector (known as the measurement spot size). Additionally, this lens helps to correct for curvature in the sample surface by allowing some light reflected at non-normal incidence to reach the detector. The detector  3  is typically a photodiode sensitive to the wavelengths of light of the laser source with an integrated fiber optic “pigtail.” Its sensitive area is very small in order to further limit the measurement spot size. 
   The thickness of the sample to be measured generally needs to be uniform to less than a fraction of the light&#39;s wavelength across the measurement spot. Minimizing this measurement spot increases the non-uniformity or surface variations allowed in the sample. The current output of the detector  3  is converted to a voltage by means of electrical amplifier  12 , and an analog to digital converter  13  provides output to a data acquisition system within computing device  7 . Typically, computing device  7  is a general purpose computer or any processor, where operation of the processor is directed by firmware or software. Computing device  7  also acts to control the output of wavelength source  1  by providing a wavelength control signal  18 —after conversion of a digital signal to an analog signal by means of digital to analog converter  19 —which is a voltage used to tune the laser wavelength. 
   Reference light path  11  passes through a collimating lens  14  and onto a reference etalon  15 . Etalon  15  is a layer of material, such as silicon, whose thickness is precisely known. The reflectivity of this etalon produces a curve of known shape. As the light wavelength is tuned, the transmitted (or reflected) signal amplitude follows a theoretically known curve. Light transmitted through etalon  15  is detected by photosensitive detector  16 , which produces a current output. The current output is converted to a voltage by means of electrical amplifier  17 , and analog to digital converter provides output to computing device  7 . 
   All the elements of the spectroscopic system, or a portion thereof, may be included in a single housing (not shown in  FIG. 2 ). Typically, elements  7 ,  18 , and  19  are not included in a housing with the other elements. 
   A method of operating the spectroscopic system is described in reference to the flow chart depicted in  FIG. 3 . In step  20 , a material (e.g., silicon wafer) is loaded into the apparatus. A reference measurement  21  is taken using a sample of known reflectivity, such as gold. A desired location on the material for measuring thickness is chosen in step  22 , sample reflectivity is measured and sample thickness is computed (Step  23 ). Steps  22  and  23  are repeated over a band of discrete wavelengths until the resulting optical detected intensity as a function of wavelength is obtained. The wavelength increment is chosen to be less than half of the period of oscillation of sample&#39;s reflectivity to fully determine the thickness. For a 100 um thick sample of silicon, this increment must be less than about 400 pm. When it is determined that enough measurements to obtain optical detected density have been taken (Step  24 ), the material is then unloaded from the apparatus (Step  25 ). 
   A method of computing material thickness is described in reference to the flow chart depicted in  FIG. 4 . In step  26 , the system gain, which is proportional to reflectivity/(A/D counts) at each wavelength, is set by measuring and saving values for a reference (e.g., gold). A/D counts with no sample present are taken to measure and save “zero” (Step  27 ). Sample reflectivity and etalon transmission at a fixed wavelength point are measured at step  28 . When it is determined that enough points (measurements) have been taken (Step  30 ) to accurately measure the material thickness, etalon transmission is fitted to theoretical values to determine the actual wavelength at each point (Step  31 ). If enough points have not been taken, then the wavelength is varied by changing the tuning voltage to the laser (Step  29 ), and steps  28  and  29  are repeated over a number of points until enough points have been taken to accurately measure the material thickness. 
   At step  32 , sample signal is converted to reflectivity using the reference data. This step provides reflectivity as a function of wavelength. A theoretical model of sample reflectivity is computed for a guess thickness (Step  33 ). The value of sample thickness is then varied until a best fit occurs between measured and modeled thickness values (least square fit of R(λ)) at step  34 . The thickness value is then displayed (Step  35 ). 
   The primary utility of the described optical spectroscopic measurement system is the measurement of material thickness (e.g., thickness of silicon wafers). Within this use, there are areas of particular relevance for the technology. Such areas include, without limitation, the following: measuring wafer thickness during spin etching; measuring the thickness of bumped wafers; and, measuring the thickness of gallium arsenide containing materials (e.g., gallium arsenide wafers). This method also is useful for the production of wafers for inclusion in smart cards. 
   Other embodiments of the invention include, but are not limited to, transmission mode and continuous scanning. For transmission mode, in reference to  FIG. 2 , beamsplitter  2  is replaced with a mirror, and detector  3 , collimating lens  5 , and optical element  6  are placed on the opposite side of sample  4 .