Abstract:
Photoreflectance spectroscopy is used to measure strain at or near the edge of a wafer in a production process. The strain measurement is used to anticipate defects and make prospective corrections in later stages of the production process. Strain measurements are used to associate various production steps with defects to enhance later production processes.

Description:
PRIORITY 
       [0001]    The present application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 61/982,196, filed Apr. 21, 2014, and U.S. Provisional Application Ser. No. 62/057,126, filed Sep. 29, 2014, both of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is directed generally toward photoreflectance (PR), and more particularly to PR for strain measurement in silicon wafers. 
       BACKGROUND 
       [0003]    Photoreflectance (PR) and piezoreflectance (PzR) of substrates such as silicon-germanium (SiGe), gallium arsenide (GaAs), indium gallium sulfate (InGaS) exhibit a typical band gap of approximately 1.5 eV or 3.5 eV. 
         [0004]    Rapid PR (RPR) spectroscopy requires the simultaneous detection of a small modulated reflectance (ΔR) signal and a large time-invariant, unmodulated reflectance signal R, and the relation of the small modulated reflectance and unmodulated reflectance (ΔR/R). Even at parts of the spectrum where it is significant, ΔR is relatively small compared to the unmodulated reflectance R, of the order of several to hundreds parts per million. Various sources of optical and electrical noises will be present and these dominate the ΔR signal. However, the ΔR signal is always present at the known modulation frequency and so methods of frequency-discriminating signal recovery are typically employed to detect a signal at this known frequency. Therefore, phase sensitive lock-in amplification was required for the measurement of the ΔR signal in PR spectroscopy and the measurement was to be made at different times for each wavelength, so that the ΔR/R spectrum was therefore generally recorded in a serial spectral mode. This limits the practical speed of the PR measurement and precludes its widespread industrial application in high-speed production line inspection of semiconductor wafers. 
         [0005]    Strained silicon (sSi) refers to silicon in which strain is engineered locally in a device structure or globally across a wafer by a local or global stress to accelerate electrons, which allows manufacture of faster devices. Faster sSi transistors due to increased electron mobility and velocity have already been proven. As a result, the technology of strain engineering is being widely used to speed carrier mobility in transistor channels in order to increase the drive currents. 
         [0006]    Globally sSi on a wafer comprises a very thin layer of single-crystal silicon strained by pseudomorphic growth up to a critical thickness on a relaxed Si x Ge 1-x  stressing layer of wider lattice constant dependent on the Ge mole fraction x. 
         [0007]    In all global sSi technologies with one exception, the strain in the silicon is described as biaxial, the result of two effects, namely, the expansion of the silicon lattice due to the wider lattice constant of the relaxed Si 1-x Ge x  layer, which is tensile stressing it, and the contraction of the silicon lattice in the vertical direction because of its behavior as a near-perfect Poisson solid. These two strain effects, hydrostatic tension, and uniaxial compression, will be presently shown to be mirrored by two competing effects in the electronic band structure in the vicinity of the E 1  critical point of silicon, whose combined effect is measurable by PR spectroscopy. Such a sSi layer can be transferred onto an SiO 2  buried oxide layer while retaining the strain to form a strained silicon-on-insulator (SSOI) wafer. 
         [0008]    X-ray diffractometry (XRD) and Raman spectroscopy (RS) have been used for strain measurement in silicon. XRD suffers from excessive measurement time, and RS has poor resolution. Other optical inspection techniques look for slip-lines on the surface or cracks in the edge of silicon wafers. Optical inspection methods detect defects rather than measuring strain and are therefore unsuitable for identifying problems before defects occur, controlling strain sources, relating strain defects to specific process parameters, or managing strain levels in wafers. 
         [0009]    Consequently, it would be advantageous if an apparatus existed that is suitable for directly measuring strain in a wafer production process in time to apply the measurements to prevent defects. 
       SUMMARY 
       [0010]    Accordingly, the present invention is directed to a novel method and apparatus for directly measuring strain in a wafer production process in time to apply the measurements to prevent defects. 
         [0011]    In at least one embodiment of the present disclosure, PR spectroscopy is used to measure strain at or near the edge of a wafer in a production process. The strain measurement is used to anticipate defects and make prospective corrections in later stages of the production process. In another embodiment, strain measurements are used to associate various production steps with defects to enhance later production processes. 
         [0012]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The numerous advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying figures in which: 
           [0014]      FIG. 1  shows a block diagram of a system according to one embodiment of the present invention for implementing RPR in a wafer fabrication process for edge strain measurement; 
           [0015]      FIG. 2  shows an environmental representation of one embodiment of the present invention; 
           [0016]      FIG. 3  shows an environmental representation of one embodiment of the present invention; 
           [0017]      FIG. 4  shows a block diagram for a computer system according to one embodiment of the present invention; 
           [0018]      FIG. 5  shows a flowchart of a method for edge strain measurement; 
           [0019]      FIG. 6  shows optical elements in an embodiment of the present invention; 
           [0020]      FIG. 7  shows a block diagram according to another embodiment of the present invention for implementing RPR in a wafer fabrication process. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. The scope of the invention is limited only by the claims; numerous alternatives, modifications and equivalents are encompassed. For the purpose of clarity, technical material that is known in the technical fields related to the embodiments has not been described in detail to avoid unnecessarily obscuring the description. 
         [0022]    An understanding of embodiments of the present disclosure may be further illuminated by U.S. Pat. No. 7,623,228, U.S. Pat. No. 7,659,979, and U.S. Patent App. Pub. No. 2013/0003050, all of which are hereby incorporated by reference. 
         [0023]    PR spectroscopy provides a useful alternative to XRD and RS to meet the requirements for a nondestructive in-line strain metrology tool. Rapid PR (RPR), and specifically micro RPR (μRPR), provides a measurement of strain in sSi with a high degree of accuracy in a sufficiently short time for application as an on-line production quality assurance tool. 
         [0024]    Embodiments of the present invention measure local wafer strain and local/global strain profiles on and near the edge of a silicon wafer based on μRPR methods. Such measurements allow for monitoring and management of the resulting impact of wafer edge strain on process yield and health. 
         [0025]    Rapid thermal processing (RTP) of silicon wafers can lead to various forms of residual strain in the wafers. For example, the silicon wafer and wafer carrier heat-up and cool-down at different rates. As a result, the contact points between the silicon wafer and the wafer carrier can cause a local thermal gradient in the wafer that results in a corresponding thermally induced local residual strain that remains after the wafer cooled. Under some conditions, the residual strain is local and on a microscopic scale can exceed the yield strength of silicon. This may result in lattice defects such as slip dislocations and micro-cracks. These defects are undesirable both in terms of device yield and process health. Slip dislocations may induce slip line defects in surface films that propagate with further processing and compromise yield. Micro-cracks can lead to wafer breakage during further processing that can damage expensive processing equipment and reduce process throughput. 
         [0026]    Measurement of wafer strain at/near the wafer edge provides insight into the status of a wafer&#39;s slip dislocations or susceptibility to developing slip dislocations, strain-related micro-cracking or susceptibility to developing micro-cracks, and susceptibility to strain-related wafer breakage. 
         [0027]    Referring to  FIG. 1 , a block diagram of a system  100  according to one embodiment of the present disclosure for implementing RPR in a wafer fabrication process for edge strain measurement is shown. In one embodiment, a probe light source  101  generates an incident probe beam and directs the incident beam through optical components  102 , including fiber optics, onto an edge of a sample  104 . In one embodiment, the incident light passes through an incident filter  116  to deliver a blue ultraviolet (UV) beam to the edge of the sample  104 . Contemporaneously, a perturbation is optically applied to the edge of the sample  104  by a modulated pump beam. The modulated pump beam is produced by a pump laser  110 , with the pump laser  110  beam passing through a modulator  112 , such as a mechanical chopper, and modulated pump beam optical components  114  to the edge of the sample  104 . In one embodiment, the modulated pump beam may be filtered by a pump beam filter  118  to a desirable frequency range. The reflectance of the edge of the sample  104  is altered by photoinjection of charge carriers via the interaction of the incident beam and modulated pump beam. In another embodiment, the modulated reflected incident beam may pass through a spectrograph filter  106  to filter the modulated reflected incident beam to a bandpass region of the spectrum in the region of the E 1  transition of silicon at approximately 3.4 eV. In another embodiment, the modulated reflected incident beam is transmitted through receiving optical components  120  and dispersed into its constituent wavelengths by a spectrograph  108  and the constituent components are sampled by a detector  122 . In another embodiment, a processor  124  connected to the detector  122  analyzes the spectrographic data to measure wafer strain in-line in sSi wafers including SSOI and silicon-germanium on insulator by measuring the direct bandgap energy of the sSi layer and converting the value to strain using a theoretical model. 
         [0028]    In another embodiment, the processor  124  reads the modulated reflectance ΔR signal as well as the unmodulated reflectance R signal from each pixel of the detector  122  in sufficiently rapid succession such that multiple measurements of the ΔR signal can be made within a single period of the modulation cycle. In this way, the system allows the simultaneous measurement of the PR signal at all wavelengths in parallel by means of multiplexing the readout. The acquisition time for the entire spectrum is approximately 1-2 ms (e.g., 1.03 ms). Depending on the measured structures, the acquisition may be averaged a plurality of times in order to improve the signal to noise ratio. 
         [0029]    By way of example, in the case of a typical 200 Å top strained silicon layer thickness SSOI wafer, measured over the spectral range of the Si direct bandgap energy of 2.8 to 3.7 eV during a 10 s duration, and fitted to a summation of two low field PR line shapes, the RPR spectrum is modeled using an approximation based on the third derivative functional form, in which the critical point dimensionality is 2-d, and index parameter m=3, the RPR spectrum model may be defined by: 
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         [0030]    It is noted that this model is appropriate given the negligible electro-optic effect and absence of Franz-Keldysh oscillations in the silicon PR spectra. Here, A is an amplitude factor, θ is a line shape phase factor, E g  is the bandgap energy, and Γ is the broadening energy parameter. The two transitions may be measured by fitting the RPR spectrum to correspond to the light hole (LH) and heavy hole (HH) direct bandgaps. It is noted that hydrostatic tensile strain may shift the interband transition (L band) energy, while compressive uniaxial strain along an axis normal to the wafer lifts the degeneracy of the valence band, leading to split LH and HH bands, and a splitting of the transition. 
         [0031]    The behavior of the energy shift ΔE of the E 1  and associated transitions under hydrostatic strain and splitting in the energy shift due to uniaxial strain contribution have been described by a relationship based on extensive analysis of the electroreflectance response of silicon under uniaxial stress: 
         [0000]      Δ E =√{square root over (1/3)} D   1   1 (ε ⊥ +2 E   ∥ )±√{square root over (2/3)} D   3   3 (ε ⊥   +E   ∥ )
 
         [0000]    where the ± sign implies addition to obtain the shift in the HH and LH branches, ε ⊥  is strain perpendicular to growth, ε ∥  is strain in-plane, D 1   1  is the hydrostatic deformation potential, measured by Kondo and Moritani as −9.8 eV, and D 3   3  is the intraband strain deformation parameter along the direction perpendicular to growth, having a value of 4.7 eV. The relationship may therefore be rewritten as: 
         [0000]      Δ E=− 5.658(ε ⊥ +2ε ∥ )±3.837(ε ⊥ +ε ∥ )
 
         [0032]    The strain tensor elements perpendicular to growth take the form: 
         [0000]    
       
         
           
             
               
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         [0000]    where x is the germanium concentration, C 11  and C 12  are the elastic constants, and the Bir-Pikus Hamiltonian for a G1 type band having the form: 
         [0000]        a   0 (Si 1-x Ge x )= a   0 (Si)+0.200326 x (1 −x )+[ a   0 (Ge)− a   0 (Si)] x   2  
 
         [0000]    Therefore the lower energy branch of the split transition shifts according to: 
         [0000]      Δ E=E   LH −3.4=−0.1375ε ∥ (eV)
 
         [0000]    where E ∥  is expressed in percentage strain. As a result, the strain value is calculated using the bandgap shift of the E LH  transition compared to the unstrained silicon direct bandgap 3.4 eV. It is conventional to quote the tensile strain in the plane of the wafer. 
         [0033]    RPR results may be compared to RS. Actual calibration against a range of RS values gives a calibration result of 3.399 and −0.1368 eV as unstrained silicon bandgap and conversion factor, respectively. 
         [0034]    PR techniques may be used to measure tensile in-plane edge strain in biaxially strained SSOI wafers. A decrease in the acquisition time by a factor of 100 to 500 may be achieved for a nondestructive technique suitable for in-line sSi metrology. A decrease and split (E LH  and E HH ) in the silicon bandgap have been observed upon tensile strain. Based on theoretical models, the E LH  transition value may be converted to biaxial tensile strain, which is consistent with RS results. 
         [0035]    Referring to  FIG. 2 , an environmental representation of one embodiment of the present disclosure is shown. In one embodiment, strain measurements taken at or near the edge of a wafer  224  are made with an optical scanning system  200  as described herein. In another embodiment, the optical axis  206  of the optical scanning system  200  is maintained substantially normal to the portion of the surface of the wafer  202  where the measurement is made. In addition, the portion of the surface of the wafer  204  where the measurement is made is maintained substantially at the optimal focus position of the optical scanning system  200  throughout the entire trajectory  202  traveled by the optical scanning system  200 . It is noted that maintain optimal (or adequate) focus may be accomplished using the optically measured wafer edge profiles for feed-forward to wafer r and z stages or any other metrics necessary to maintain focus and normality of the optical axis  206  even where the edge is irregular. 
         [0036]    The feed-forward procedure may be combined with a real-time auto-focus mechanism to mitigate errors in the measurement of the wafer  204  edge profile. Alternatively, the feed-forward procedure may be replaced with a combined real-time auto-focus mechanism and default wafer profile. For example, a semi-circle or other pattern may be used for the wafer edge profile. 
         [0037]    Referring to  FIG. 3 , an environmental representation of one embodiment of the present invention is shown. In one embodiment, a wafer  304  in a production process may be inspected by various inspection modules  300 ,  306 . In one embodiment, module  300  includes an edge inspection module. In another embodiment, module  306  includes a μRPR edge strain measurement module  306 . In one embodiment, the μRPR strain measurement module  306  may measure strain at sample spot sizes  302  of several millimeters down to less than 10 micro-meters. In another embodiment, the μRPR strain measurement module  306  may sample wafer  304  edge locations as the wafer  304  moves. Alternatively, the μRPR strain measurement module  306  may take measurements in a continuous fashion, repeatedly, over a track defined by an edge profile. Multiple tracks may be measured serially or in in parallel. The μRPR edge strain measurement module  306  may include a time-delay integration (TDI) sensor for performing the actual image collection. Furthermore, because the μRPR strain measurement module  306  collects both modulated and unmodulated light reflected from the wafer  304 , a modulating pump beam source may be periodically turned on and off to modulate an incident beam at a given sampling rate. 
         [0038]    It is noted that the μRPR optics head may provide for multiple simultaneous measurement points as a means to increase throughput and mitigate laser damage. In one embodiment, the μRPR optics head may be on a dedicated wafer inspection system, or may be integrated into a wafer inspection platform with multiple inspection systems. In another embodiment, the μRPR may be mounted such that a step-wise or continuous measurement is made of a specific zone circumference of the wafer. 
         [0039]    Referring to  FIG. 4 , a block diagram for a computer system according to one embodiment of the present invention is shown. In one embodiment, the computer system  400  includes a processor  422 , memory  404  connected to the processor  422  for storing computer executable code, and a data storage element  406  for storing wafer profile data. In another embodiment, a spectral detector  424 , such as a TDI sensor, is connected to the processor  422  to receive reflected light from the edge of a wafer and modulated reflected light from the edge of the wafer via a spectrograph. 
         [0040]    In a wafer production process, the edge of a spinning wafer may be inspected for strain. In one embodiment, an illumination device may provide an incident beam arranged to illuminate a portion of the wafer. The incident beam is then reflected to a spectrograph for detection by the spectral detector  424 . In another embodiment, the illumination device may also include a pump laser to produce a modulation beam that illuminates the portion of the wafer to produce a modulated incident beam reflected to the spectrograph for detection by the spectral detector  424 . 
         [0041]    In one embodiment, the processor  422  receives spectral data and produces a strain measurement associated with the illuminated portion of the wafer, the illuminated portion being a surface on the or about the edge of the wafer. In another embodiment, the processor  422  may adjust the location and orientation of the spectral detector  424  based on an edge profile stored in the data storage element  406 , or a predetermined spot locations where strain measurements are desirable. Based on the strain measurements, the processor  422  may determine that a defect is likely in the production process or that a strain threshold for a portion of the wafer has been reached. Furthermore, given known production process parameters, the processor  422  may determine that strain related defects in the edge of a wafer may be correlated to such parameters; for example, temperature disparities during the production process. 
         [0042]    Local and global strain profiles on or near a wafer edge may allow the processor  422  to identify problematic stain profiles before defects or micro-cracks appear, or at least identify wafers that may already have developed strain-related defects; control strain-related defects such as slip lines and micro-cracks at or near the edge of the wafer; identify and control strain sources; and manage strain level in wafers. 
         [0043]    Multiple μRPR strain measurements can be made on a wafer in a stepwise or continuous fashion about the circumference of the wafer. For example, measurements may be made of a top edge surface, bottom edge surface, and side surface of an edge of the wafer, and may incorporate care areas and/or exclusion zones for the measurement. Multiple μRPR strain measurements may be post-processed to generate a continuous strain profile at virtually any scale; e.g. globally such as around the circumference of the top edge, side edge, and bottom edge of the wafer down to a local region whose smallest size is limited by the sample spot size. 
         [0044]    Referring to  FIG. 5 , a flowchart depicting a method for edge strain measurement is shown. In a first step  500 , a wafer (e.g., silicon wafer) is loaded into a wafer production apparatus. For example, the production apparatus may include, but is not limited to, an embodiment of the present disclosure. In a second step  502 , the wafer is aligned for edge inspection. In a third step, the wafer production apparatus then measures  504  an edge profile of the wafer. In a fourth step  506 , the wafer production apparatus performs a strain measurement on an edge surface of the wafer. For example, the measurements may include, but are not limited to, spot measurements or continuous measurements. In another embodiment, the wafer production apparatus may track multiple parameters when performing the strain measurements; such parameters may include identifying particular areas of interest for spot measurement, particular areas identified via previous production processes that are prone to strain related defects, monitoring strain values for a pre-determined threshold, or SNR parameters. In a fifth step  508 , the wafer production apparatus may modifies a feature of the wafer production process based on the edge strain measurements. 
         [0045]    Embodiments of the present disclosure may have an edge strain measurement resolution of better than 0.1%, a silicon yield stress of approximately 1 GPa at room temperature and approximately 10 GPa at 1000° C., and a Young&#39;s modulus of approximately 130. 
         [0046]    Referring to  FIG. 6 , optical elements of an embodiment of the present disclosure are shown. In one embodiment, the edge of a spinning wafer  622  in a wafer production process is illuminated. For example, as shown in  FIG. 6 , a white probe light may illuminate the wafer and scan the wafer along a selected spatial scan direction. In another embodiment, the reflected illuminated light may be collimated by a first curved mirror  602  and reflected onto a diffraction grating  604 . In one embodiment, the diffraction grating  604  reflects a diffracted image from the edge of the spinning wafer  622  to a second curved mirror  606  that focuses the diffracted image onto a TDI sensor  608 . As shown in  FIG. 6 , light received by the TDI sensor  608  may be across the sensor as a function of wavelength, with the scanning direction being arranged orthogonal to this direction. In another embodiment, data from the TDI sensor  608  is used to determine a strain measurement associated with the edge of the spinning wafer  622 . 
         [0047]    Referring to  FIG. 7 , a block diagram according to another embodiment of the present disclosure for implementing RPR in a wafer fabrication process is shown. In one embodiment, a reference beam  700 , such as a reference laser beam (e.g., reference laser suitable for AF), passes through optical elements  702 ,  704 ,  706 ,  708 ,  712  to the edge of a wafer  722 . Contemporaneously, a pump beam  710 , such as an S-wave pump beam, passes through the same optical elements  702 ,  704 ,  706 ,  708 ,  712  to the same spot on the edge of the wafer  722 . 
         [0048]    In one embodiment, optical element  708  includes one or more prisms  708  (e.g., beam separation prism) for directing the reference beam  700  and pump beam  710  along a common entrance path. In another embodiment, optical element  712  includes a first beam splitter  712  (e.g., polarized beam splitter or GLAN-type beam splitter) for reflecting the reference beam  700  and pump beam  710  to a first mirror  702 . 
         [0049]    For example, the first mirror  702  may be, but is not required to be, a convex mirror for reflecting the reference beam  700  and pump beam  710  to a second mirror  704  and third mirror  706  respectively. By way of another example, the second mirror  704  and third mirror  706  may be, but are not required to be, concave mirrors for focusing the reference beam  700  and pump beam  710  to a particular portion of the edge of the wafer  722 . In another embodiment, the second mirror  704  and third mirror  706  may be arranged so as to focus the reference beam  700  and pump beam  710  onto the wafer  722  from different angles. 
         [0050]    In at least one embodiment, a P-wave illuminator  714  may introduce a probe beam to a second beam splitter  716  (e.g.,  50 / 50  beam splitter), which directs a portion of the probe beam through one or more optical elements  702 ,  704 ,  706 ,  712  to combine with the reference beam  700  and pump beam  710 . 
         [0051]    In another embodiment, light reflected from the wafer  722  traverses one or more of the optical elements  702 ,  704 ,  706  through the first beam splitter  712  and the second beam splitter  716  to a detector  724 . In at least one embodiment, a slit diffraction element  718  (e.g., slight has long width aligned with TDI) and a band-pass filter  720  may be interposed between the second beam splitter  716  and the TDI detector  724 . 
         [0052]    Embodiments of the present invention provide a means to measure and control wafer edge strain, in-line, before associated defects and adverse consequences occur as described. 
         [0053]    It is believed that the present invention and many of its attendant advantages will be understood by the foregoing description of embodiments of the present invention, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.