Abstract:
A CMP heterodyne in-situ sensor (C-HIS) system utilizes optical heterodyne interferometry. A wafer undergoing CMP is illuminated through the wafer thickness using an infrared laser beam at a wavelength of 1.1 μm or greater. The beam is transmitted through the wafer and is reflected from the front wafer surface. As the wafer is polished, the optical beam path through the wafer is shortened, causing the reflected optical frequency to undergo a Doppler shift. By measuring this shift, the change in wafer thickness is determined. The frequency shift generates a signal, which enables dynamic process control. In embodiments where the wafer includes a planarization film, the frequency shift provides a measurement of changing film thickness. Embodiments of the invention utilize phase detection independent of intensity, and hence do not suffer from intensity fluctuations. Some embodiments detect thickness changes less than 2.5 nm. C-HIS sensors operate in both polished-to-thickness and polished-to-stop scenarios.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to Coon et al., U.S. application Ser. No. 09/021,767, filed Feb. 11, 1998; Aiyer al., U.S. application Ser. No. 09/021,740, filed Feb. 11, 1998; and Aiyer et al., U.S. application Ser. No. 09/047,322 filed Mar. 24, 1998, which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to an apparatus and method for in-situ process monitoring and more specifically, to an apparatus and method for in-situ monitoring of chemical-mechanical planarization of semiconductor wafers. 
     2. Background 
     Planarization of the active or device surface of a substrate has become an important step in the fabrication of modern integrated circuits (ICs). Of the several methods of planarization that have been developed, Chemical Mechanical Polishing (CMP) is perhaps the most commonly used method. This popularity is due, in part, to its broad range of applicability with acceptably uniform results, relative ease of use, and low cost. However, the move to larger diameter wafers and device technologies that require constant improvement in process uniformity requires that an improved planarization system become available. 
     A typical CMP system uses a flat, rotating disk or platen with a pliable monolithic polishing pad mounted on its upper surface. As the disk is rotated, a slurry is deposited near the center of the polishing pad and spread outward using, at least in part, centrifugal force caused by the rotation. A wafer or substrate is then pressed, typically face down, against the working surface of the polishing pad such that the rotating polishing pad moves the slurry over the wafer&#39;s surface. In this manner, surface high spots are removed from the wafer and an essentially planar surface is achieved. 
     The planarization of an interlayer dielectric is one common use for CMP. As the topography of the underlying surface is not uniform, coating that surface with a dielectric film replicates or even magnifies those non-uniformities. As the surface is planarized, the high spots are removed and then the total thickness of the dielectric film is reduced to a predetermined value. Thus, the planarized dielectric film will be thinner over high points of the underlying surface than over low points of that surface. Typically, it is important to maintain a minimum dielectric thickness over each of the highest points of the underlying layer, both locally (within a die) and globally (across the wafer). Thus, uniform removal of the dielectric layer at all points of the wafer is required. 
     A problem with most existing CMP systems is their inability to perform in-situ thickness monitoring. As the surface of the wafer is pressed against the polishing pad during removal, typically no measurements as to the progress of the polishing can be made. Thus, wafers are either polished for fixed times, and/or periodically removed for off-line measurement. Recently, Lustig et al., U.S. Pat. No. 5,433,651 (Lustig) proposed placement of at least one viewing window in the working surface through the thickness of the polishing pad to provide access for in-situ measurement. However, a window placed in a polishing pad creates a mechanical discontinuity in the working surface each time the window passes across the surface of the wafer. A more conventional approach is to use a monolithic polishing pad. 
     Thus there is a need for a CMP apparatus, and method thereof, that provides optical access to the wafer front surface for continuous in-situ process monitoring, without undue process complexity or expense. 
     SUMMARY OF THE INVENTION 
     A Chemical Mechanical Polishing heterodyne in-situ sensor (C-HIS) apparatus and method for enhanced optical access to a wafer surface is provided. The C-HIS system is based on conventional optical heterodyne interferometry. In some embodiments, a front surface of the wafer is illuminated through the wafer using an infrared laser source emitting light at a wavelength of 1.1 μm or greater. In some embodiments, the wafer also comprises a planarization film. For such embodiments the front wafer surface will be understood to encompass the planarization film. Light at such wavelengths is transmitted through the wafer and planarization film to the front wafer surface, where it is at least in part reflected back to the C-HIS apparatus. As the planarization film is polished, the optical path length of the beam propagating through the film is reduced. This causes the optical frequency of the reflected beam to undergo a Doppler frequency shift. By measuring this Doppler shift, the instantaneous change in planarization film thickness can be determined. In some embodiments of the invention, the measured Doppler shift generates an input signal to enable dynamic process control. 
     Existing optical in-situ sensors are intensity-dependent devices and hence are subject to noise due to source intensity fluctuations and variable transmittance in the optical path. Unlike those existing in-situ sensors, the embodiments of the present invention provide for measurement based on phase detection independent of intensity, and hence do not suffer from problems related to intensity fluctuations. Some embodiments are capable of detecting thickness changes of about 2.5 nm. In accordance with embodiments of the present invention, C-HIS sensors operate in both polished-to-thickness and polished-to-stop scenarios. Thus, these embodiments provide a system and method for optically accessing a wafer surface to enable enhanced and versatile in-situ monitoring of a CMP process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art, by referencing the accompanying drawings. 
     FIG. 1 is a schematic cross-sectional view showing a portion of a CMP apparatus including a wafer and a C-HIS optical assembly, in accordance with the invention; and 
     FIG. 2 is a plot of a simulation of measurement and reference signals, in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     As embodiments of the present invention are described with reference to the aforementioned drawings, various modifications or adaptations of the specific structures and or methods may become apparent to those skilled in the art. All such modifications, adaptations, or variations that rely upon the teachings of the present invention, and through which these teachings have advanced the art, are considered to be within the spirit and scope of the present invention. 
     FIG. 1 is a schematic cross-sectional view showing a portion of a Chemical Mechanical Polishing (CMP) apparatus  100  including a wafer  110  and a CMP heterodyne in-situ sensor (C-HIS) optical assembly  120 , in accordance with the present invention. Wafer  110  has a front surface  108  and a back surface  106 . In some embodiments, wafer  110  includes a substrate  148  and a planarization film  112 . In such embodiments, film  112  is separated from substrate  148  by an interface surface (hereinafter interface  104 ), and front surface  108  is understood to encompass film  112  as depicted in FIG.  1 . Planarization film  112  has a thickness  102  measured from interface  104  to front surface  108 . Likewise a substrate thickness  114  is defined between back surface  106  and interface  104 , and a wafer thickness  116  is defined between back surface  106  and front surface  108 . Back surface  106  of wafer  110  is attached to a rotatable carrier  150  using conventional methods (see for example, Aiyer et al., U.S. application Ser. No. 09/047,322, filed Mar. 24, 1998). Front surface  108  is pressed downward against the working surface of a rotatable platen-mounted polishing pad (not shown). Typically a polishing slurry is applied between front surface  108  and the working surface of the polishing pad. 
     Optical assembly  120  comprises a laser source  118 , a conventional reference beam splitter (BS)  122 , a polarization beam splitter (PBS)  124 , a reference beam quarter-wave plate  126 , a reference beam reflector  128 , a measurement beam quarter-wave plate  130 , a measurement mixing polarizer  132 , a measurement photodetector  134 , a reference mixing polarizer  136 , a reference photodetector  138 , and a signal-processing assembly  140  electrically connected to the outputs of measurement photodetector  134  and reference photodetector  138  using signal leads  182  and  184  respectively. The optical arrangement is similar, for example, to that described by Sommargren et al. U.S. Pat. No. 4,688,940 issued Aug. 25, 1987 (hereinafter Sommargren). The components of optical assembly  120  are mounted above and proximate to rotatable carrier  150  to which wafer  110  is attached. In some embodiments optical assembly  120  is physically attached relative to the rotation axis of rotatable carrier  150 . 
     Laser source  118  is configured to produce two substantially superimposed collinear beams B 1  and B 2  of optical frequencies ω 1  and ω 2  respectively. Frequency ω 1  is offset from frequency ω 2  by a heterodyne offset frequency Δω, such that ω 1 =ω 2 +Δω. Additionally beams B 1  and B 2  are orthogonally polarized; illustratively beam B 1  is initially plane polarized perpendicular to the plane of FIG. 1 (shown by solid circles) and beam B 2  is initially plane polarized parallel to the plane of FIG. 1 (shown by arrows). Generation of beams B 1  and B 2  is typically accomplished by placing an acousto-optic device (not shown) in the output beam B 1  of a well-stabilized laser having a linearly polarized output of single-frequency ω1. By driving the acousto-optic device at an acoustic frequency equal to heterodyne offset frequency Δω, a portion of the output is shifted into an orthogonally polarized beam B 2  of frequency ω2 (for example, see Sommargren). 
     Beams B 1  and B 2  propagate collinearly from laser source  118  to reference beam splitter  122 , where a fraction of both beams  142  and  144 , respectively, is deflected through reference mixing polarizer  136  into reference detector  138 . The transmitted portions of beam B 1  and beam B 2  continue to propagate as beam  152  and beam  154  respectively. At polarization beam splitter (PBS)  124 , beam  154  is transmitted without deflection and passes through quarter-wave plate  126 , where the polarization of beam  154  is converted from plane to circular. Beam  154  is then reflected from reflector  128  back through quarter-wave plate  126  as beam  164 ; the circular polarization of beam  154  is converted to plane polarization for beam  164 . The polarization plane of beam  164  is oriented perpendicular to the original polarization plane of beam B 2 . Beam  164  is then reflected from PBS  124 . As can be seen, all optical elements encountered by beams B 2 ,  154 , and  164  collectively are fixed in their positions. Thus beams B 2 ,  154 , and  164  collectively traverse an optical path of fixed length. 
     Beam  152  is reflected from PBS  124  through quarter-wave plate  130 , whereupon the polarization of beam  152  is converted from plane polarization to circular polarization. To provide optical access of beam  152  to back surface  106 , it is necessary for rotatable carrier  150  to be optically transparent in whole or in part. This is accomplished, for example, by forming rotatable carrier  150  in whole or in part of transparent materials, e.g. acrylic plastic or fused silica; or alternatively by forming open slots through rotatable carrier  150  to provide an optical transmission path. 
     Beam  152  undergoes partial reflections at back surface  106 , interface  104 , and front surface  108  respectively, producing partially reflected beams  162 S,  162 T, and  162 B respectively, which propagate back through quarter-wave plate  130 . Upon transmission back through quarter-wave plate  130 , the polarization of each of beams  162 S,  162 T, and  162 B is converted from circular polarization to plane polarization; each of the respective polarization planes is perpendicular to the original polarization plane of beam B 1 . Reflected beams  162 S,  162 T, and  162 B are then transmitted through PBS  124 , whereupon each of beams  162 S,  162 T, and  162 B propagates collinearly with reference beam  164 . During a polishing process, planarization film thickness  102  is reduced, thereby shortening the optical path traversed by beams B 1 ,  152 , and  162 B collectively. Thus beams B 1 ,  152 , and  162 B collectively traverse an optical path of variable length. 
     The polarization planes of reference beam  164  and reflected beams  162 S,  162 T,  162 B are each rotated  90  degrees relative to the polarization planes of their respective original beams B 2  and B 1 . Thus reflected beams  162 S,  162 T,  162 B are still polarized orthogonally relative to reference beam  164 . Reference beam  164  and reflected beams  162 S,  162 T,  162 B now propagate collinearly from PBS  124  onto mixing polarizer  132 . Mixing polarizer  132  provides respective output beams  172 S,  172 T,  172 B and  174  all having the same polarization (for example, see Sommargren). These similarly polarized beams  172 S,  172 T,  172 B and  174  are then mixed on the face of the measurement photodetector  134  to produce an electrical measurement signal S 1 . Likewise the orthogonally polarized fractions  142  and  144 , respectively, of initial beams B 1  and B 2  are combined by reference mixing polarizer  136  (for example, see Sommargren) and then are detected by reference photodetector  138  to produce an electrical reference signal S 2 . 
     Measurement signal S 1  and reference signal S 2  are applied using electrical leads  182  and  184  respectively, or by wireless means to signal processing assembly  140 . Signal processing assembly  140  comprises analog and/or digital circuitry to amplify, condition, compare, and process measurement and reference signals S 1 , S 2 , respectively. Signal processing assembly  140  thereupon generates output signals representing the status of the CMP process. Optionally, signal processing assembly  140  generates output signals to provide dynamic process control, as described in detail below. 
     In accordance with principles of optical heterodyne interferometry familiar in the art, reference beam  174  at measurement photodetector  134  is represented by 
     
       
           E   1 = E   01   e   i(ω     2     *t) ; 
       
     
     wherein 
     E 01  is amplitude; 
     t is time; and 
     the exponential factor represents the frequency and phase dependence of reference beam  174  having frequency ω 2 . 
     Likewise measurement beams  172 S,  172 T, and  172 B at measurement photodetector  134  are represented respectively by 
     
       
           E   2 = E   02   e   i(ω     1     *t+φ     S     ) ; 
       
     
     
       
           E   3 = E   03   e   i(ω     1     *t+φ     T     ) ; 
       
     
     and 
     
       
           E   4 = E   04   e   i(ω     1     *t+φ     B     ) ; 
       
     
     wherein 
     E 02 , E 03 , and E 04  are amplitudes of beams  172 S,  172 T and  172 B, respectively; t is time; and 
     the exponential factors represent the frequency and phase dependencies of the above measurement beams, respectively, having frequency ω 1  and undergoing optical phase shifts of φ S , φ T , and φ B  upon reflection from back surface  106 , interface  104 , and front surface  108  respectively. 
     In accordance with the principles of square-law mixing, familiar in the art, measurement signal S 1  of measurement photodetector  134  arising from combined beams  174 ,  172 S,  172 T, and  172 B is given by: 
       S   1 ∞ E   2 +2 E   02   E   03  cos(Δφ TS )+2 E   02   E   04  cos(Δφ SB ) 
     
       
         +2 E   03   E   04  cos(Δφ TB )+2 E   01   E   02  cos(Δω* t−φ   S )+2 E   01   E   03  cos(Δω* t−φ   T )+2 E   01   E   04  cos(Δω* t−φ   B ); 
       
     
     wherein 
     
       
         E 2 =E 01   2 +E 02   2 +E 03   2 +E 04   2 ; 
       
     
     
       
         Δφ TS =φ T −φ S ; 
       
     
     
       
         Δφ SB =φ S −φ B ; 
       
     
     and 
     
       
         Δφ TB =φ T −φ B . 
       
     
     The first two terms of measurement signal S 1  are time invariant terms. The third and fourth terms will change in magnitude as the film is polished. The consequence of this magnitude change is a change in signal contrast. The fifth and sixth terms are heterodyne terms, but do not undergo any Doppler shift, since phases φ S  and φ T  do not change during a polishing operation. The last term of measurement signal S 1  is the only heterodyne term that undergoes a Doppler frequency shift during the polishing process. 
     Reference signal S 2 , generated by reference photodetector  138 , is represented by 
     
       
           S   2 ∞4+4 cos(Δω* t ), 
       
     
     which depends on heterodyne offset frequency Δω, but does not undergo a phase shift. 
     Illustratively, FIG. 2 is a plot of a simulation of measurement and reference signals S 1  and S 2  respectively, normalized to an arbitrary vertical scale. The horizontal axis represents time in fractional microseconds. A lower plot  212  represents reference signal S 2 ; an upper plot  214  represents measurement signal S 1 . For convenience of simulation, a 2-MHz heterodyne frequency Δω is assumed. The shift between plots  212  and  214  is equivalent to a change in planarization film thickness  102  of about  10  nm. 
     Thus, in accordance with the invention, signals are generated by an in-situ method, that provide sensitive, accurate, and fluctuation-free measurement of film thickness change during a polishing process. In this manner, the thickness  102  of planarization film  112 , as it approaches a predetermined value, is determined without the need to stop the polishing process. 
     Optionally, the apparatus depicted in FIG. 1 also incorporates a dynamic feedback system  52  for routing a signal  52   a  from signal processing assembly  140  to a computing device  53 . In embodiments of the present invention, signal  52   a  is a signal derived from C-HIS optical assembly  120  that represents the thickness  102  of planarization film  112  on wafer  110 . Typically, signal  52   a  is routed from signal processing assembly  140  through dynamic feedback system  52  which includes computing device  53 . 
     In some embodiments of the present invention, computing device  53  is a general purpose computing device having software routines encoded within its memory for receiving, and evaluating input signals such as signal  52   a.  In some embodiments, computing device  53  is an application specific computing device, essentially hardwired for a specific purpose. In some embodiments, device  53  is a combination of general purpose and specific purpose computing devices. Regardless of form, device  53  receives one or more input signals  52   a  and, using encoded routines, generates a result as one or more output signals  52   b,    52   c,    52   d,  and  52   e.  Each output signal  52   b,    52   c,    52   d,  and  52   e  can be a control signal for providing dynamic process control of one or more of the various sub-systems of CMP apparatus  100 . 
     Illustratively, an input signal  52   a  from C-HIS optical assembly  120  enables computing device  53  to continuously calculate a rate of removal of planarization film  112 . In turn, process variables, for example platen drive speed, platen pressure, slurry supply, and/or rotatable carrier motion are each dynamically controlled based upon the input signal  52   a  and rate calculated by computer device  53 . In some embodiments, one or more of output signals  52   b-   52   e  are informational display or alert signals intended to call the attention of a human operator rather than dynamic control signals. For example, in some embodiments of the invention, computing device  53  produces an output signal  52   b-   52   e  that planarization film thickness  102  is approaching or reaching a predetermined value. 
     In addition to receiving and evaluating input signals  52   a  from C-HIS optical assembly  120 , computing device  53  is also capable of receiving process programming inputs from human operators or from other computing devices (not shown). In this manner, computing device  53  is used to control essentially all functions of CMP apparatus  100 . 
     In view of the foregoing, it will be realized that embodiments of the present invention have been described, wherein an improved planarization system has been enabled. Embodiments of the present invention allow improved optical access to the active surface being polished, as compared to prior art systems, thus allowing continuous in-situ monitoring of the process, for example thickness and end point detection, as well as dynamic process control. 
     Although the invention has been described in terms of a certain preferred embodiment, other embodiments apparent to those skilled in the art are also within the scope of this invention. Accordingly, the scope of the invention is intended to be defined only by the claims which follow.