Patent Publication Number: US-7589843-B2

Title: Self referencing heterodyne reflectometer and method for implementing

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a Continuation-In-Part of U.S. patent application Ser. No. 11/237,225 entitled “Self Referencing Heterodyne Reflectometer and Method for Implementing,” filed Sep. 27, 2005 now U.S. Pat. No. 7,545,503. The present application is related to co-pending U.S. patent application Ser. No. 11/178,856 entitled “Method for Monitoring Film Thickness Using Heterodyne Reflectometry and Grating Interferometry,” filed Jul. 10, 2005, and co-pending U.S. patent application Ser. No. 11/066,933 entitled “Heterodyne Reflectometer for Film Thickness Monitoring and Method for Implementing,” filed Feb. 25, 2005, both assigned to the assignee of the present application. The above identified applications are incorporated by reference herein in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to reflectometry. More particularly, the present invention relates to a reflectometer system and method for obtaining thickness information by measuring phase shift in reflected split frequency signals via heterodyne interferometry. Furthermore, the present invention relates to a method and system for using the heterodyned signals from a heterodyne reflectometer for measuring the thicknesses of thin and ultra thin films formed over substrates. Still more particularly, the present invention relates to a self referencing heterodyne reflectometer for monitoring of film thickness which compensates for detector drift. Additionally, the present invention relates to a heterodyne reflectometer which compensates for spurious noise generated in the optical measurement components. The present invention also relates to a heterodyne reflectometer for in situ monitoring of film thickness. 
     Due to the increasing demand for ultra precise tolerances in chip fabrication, the physical characteristics of the subsequent layers must be very carefully controlled during processing to achieve satisfactory results for most applications. Broadly defined, interferometry relates to the measurement of the interaction of waves, such as optical waves. An interferometer works on the principle that two coherent waves that coincide with the same phase will enhance each other while two waves that have opposite phases will cancel each other out. One prior art monitoring system utilizes interferometry for measuring variations in surface profiles, from which feature height information can be inferred. Hongzhi Zhao, et al., in “A Practical Heterodyne Surface Interferometer with Automatic Focusing,” SPIE Proceedings, Vol. 4231, 2000, p. 301, which is incorporated herein by reference in its entirety, discloses an interferometer for detecting a phase difference between reference heterodyne signal, and a measurement signal. Height information related to the sharp illumination point on the surface can be inferred from the measurement. Although the reference and measurement signals are generated by beams that are propagated over different paths, this is a common path interferometer. This approach is sometimes referred to as the common-axis approach or the normal-axis approach because the incident and reflected beams occupy a common path or axis to a target location, which is normal to the surface being examined. 
     One shortcoming of the common-path heterodyne interferometers known in the prior art is that the height information is calculated from an average height of the large illumination area of the reference signal. Thus, the accuracy of the results is adversely affected by surface roughness. Another limitation of the prior art common axis method is that it does not measure or calculate an actual thickness parameter for a film layer. 
     Other attempts in monitoring film thicknesses achieve heterodyning by frequency modulating the light source. U.S. Pat. No. 5,657,124 to Zhang, entitled “Method of Measuring the Thickness of a Transparent Material,” and U.S. Pat. No. 6,215,556 to Zhang, et al., entitled “Process and Device for Measuring the Thickness of a Transparent Material Using a Modulated Frequency Light Source,” disclose such devices, and are incorporated herein by reference in their entireties. With regard to these devices, a polarized light beam having a modulated frequency is directed to the target surface and heterodyne interference signals are detected from two rays, one reflected off the top surface of a target and a second from a bottom surface of a target. A thickness is determined from the number of beats per modulation period by comparing the heterodyned interference signals with the linearly modulated intensity of the light source. The principle drawback of these types of devices is that since the heterodyning is achieved by frequency modulating, the source and thinnest film measurable is limited by its bandwidth. 
     Other heterodyne interferometers obtained a heterodyned signal from two separate beams, a first beam at a first frequency and polarization, and a second beam at a second frequency and polarization. U.S. Pat. No. 6,172,752 to Haruna, et al., entitled “Method and Apparatus for Simultaneously Interferometrically Measuring Optical Characteristics in a Noncontact Manner,” and U.S. Pat. No. 6,261,152 to Aiyer, entitled “Heterodyne Thickness Monitoring System,” which are incorporated herein by reference in their entireties, disclose this type of interferometer. 
       FIG. 1  is a diagram of a heterodyne thickness monitoring apparatus in which a pair of split frequency, orthogonally polarized beams are propagated in separate optical paths prior to being mixed and heterodyned, as is generally known in the prior art, for use with a Chemical Mechanical Polishing (CMP) apparatus. Accordingly, heterodyne thickness monitoring system  100  generally comprises a CMP apparatus, a wafer  110  and a measurement optical assembly. Wafer  110  includes substrate  112  and film  114 . 
     The measurement optical assembly generally comprises various components for detecting and measuring a Doppler shift in the optical frequency of the reflected beam, including laser source  140 , beam splitter (BS)  144 , polarization beam splitter (PBS)  146 , beam quarter-wave plate  148 , beam reflector  152 , beam quarter-wave plate  150 , mixing polarizer  143 , photodetector  147 , mixing polarizer  145 , photodetector  149 , and signal-processing assembly  154  electrically connected to the outputs of photodetectors  147  and  149 , which is in turn connected to thickness processor  160 . 
     In operation, laser diode  140  emits a beam having first linear polarized light component  102  at a first wavelength and second linear polarized component  103  at a second wavelength, but orthogonally polarized to the first polarization component. The first and second polarization components  102  and  103  propagate collinearly to BS  144  where a portion of both components are reflected to mixing polarizer  145  as beams  134  and  135  and then to detector  149  as beams  116  and  117 , where signal I 2  is produced. 
     The transmitted portions of polarization components  102  and  103  propagate to PBS  146  as beams  104  and  105 . At PBS  146  component  104  follows a first transmission path as beam  106  and passes through reference quarter-wave plate  148  to reflector  152  and is reflected back through quarter-wave plate  148  as beam  122  (orthogonally polarized to beam  106 ), where it reflects at PBS  146  to mixing polarizer  143  and on to detector  147  as beam  124 . 
     The second polarization component, from component  105 , follows a separate transmission path, from the first path, as beam  107  and is orthogonally oriented to first polarization component  104  and, therefore, reflects off PBS  146 , passes through quarter-wave plate  150  as beam  109  and propagates to optically transparent rotatable carrier  115 . Beam  109  experiences partial reflection at the back surface of rotatable carrier  115 , the interface between substrate  112  and the top surface of film  114 , thereby producing partially reflected beams  111 S,  111 T and  111 B, respectively. Each of reflected beams  111 S,  111 T and  111 B propagate back through quarter-wave plate  150 , are transmitted through PBS  146  as beams  113 S,  113 T and  113 B and propagate collinearly with beam  122  to mixing polarizer  145  as beams  124 ,  135 S,  135 T and  135 B and then detected at photodetector  147  as signal I 1 . Importantly, I 1  is produced from both beam  107 , which oscillates at one optical frequency and interacts the film, and beam  122 , which oscillates at another optical frequency and that propagates in a second optical path that does not interact with the film. Signals I 1  and I 2  are compared for finding a thickness measurement. 
     When the measurement beam undergoes an optical path length change, the beat signal will experience corresponding phase shift. The amount of phase shift can be determined by comparing the phase of the measurement beam with the phase of the beam without the optical path length change. The phase shift between the beams can be extrapolated to a distance, from which a thickness may be inferred (or change in thickness) for the target sample. 
     As might be apparent, because signal I 1  is detected from two beams having different optical paths, only one of which interacts with the sample, any change in the optical path of either beam will be inferred as a change in the distance to the surface of the film. Furthermore, because only the distance to a single point on the surface of the film is measured; extraneous factors that interfere with that measurement can be interpreted as a change in thickness, such as wafer tilt. Therefore, this reflectometer is largely relegated to profile measurements. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a self referencing heterodyne reflectometer system and method for obtaining highly accurate phase shift information from heterodyned optical signals, without the availability of a reference wafer for calibrations. The heterodyne reflectometer is generally comprised of an optical light source with split optical frequencies, a pair of optical mixers to generate the optical beat signal, a pair of optical detectors for detecting and converting the optical beat signal to electrical heterodyne beat signals, and a phase shift detector for detecting a phase shift between the two electrical signals. 
     The self referencing heterodyne reflectometer operates in two modes: a heterodyne reflectometry (HR) mode in which an HR beam comprised of s- and p-polarized beam components at split angular frequencies of ω and ω+Δω is employed; and a self referencing (SR) mode in which an SR beam comprised of p-polarized beam components at split angular frequencies of ω and ω+Δω is employed. A measured phase shift δ Ref/film  is derived from the I ref  and I het  signals detected from HR beam and a reference phase shift δ Ref/Sub  is derived from the I ref  and I het  signals detected from SR beam. The measured phase shift δ Ref/film  generated from the beat signals of the HR beam is used for film thickness measurements. The SR beam is p-polarized and no significant reflection will result from a film surface. The reflection returning from the film-substrate interface will not carry any phase information pertaining to the film. Therefore, the reference phase shift δ Ref/Sub  generated from the beat signals of the SR beam is equivalent to that obtained using a reference sample. 
     By alternating between the HR and SR modes in rapid succession, temperature induced noise and phase drift in the detector can be assumed to be the same as for both measurements. A film phase shift Δφ film  can then be calculated from the measured phase shift δ Ref/film  and the reference phase shift δ Ref/Sub . In so doing, the temperature induced detector noise and phase drift on both detectors is effectively canceled out, yielding a temperature independent Δφ film . 
     Since the reference phase shift δ Ref/Sub  is not affected by changes in the film, and the substrate does not change, any variation between successive reference phase shift values is attributable to detector noise or temperature related phase drift. Unacceptable noise levels can be detected by monitoring sequential reference phase shift values for change. The magnitude of phase change between the measurements can then be compared to a noise threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a diagram of a heterodyne interferometer as is generally known in the prior art; 
         FIG. 2  is a diagram of a heterodyne reflectometer for measuring thin film thicknesses; 
         FIGS. 3A and 3B  are diagrams showing the interaction of a linearly polarized incident beam, comprised of s-polarization component having an optical angular frequency of ω, and a p-polarization component having a split optical angular frequency of ω+Δω, with a thin film; 
         FIGS. 4A-4C  are diagrams of operating states of a self referencing heterodyne reflectometer for measuring thin film thicknesses without the availability of a reference wafer in accordance with an exemplary embodiment of the present invention; 
         FIG. 5  is a flowchart depicting the method for finding a film thickness using a self-referencing heterodyne reflectometry in accordance with an exemplary embodiment of the present invention; 
         FIGS. 6A and 6B  diagrammatically illustrate the interactions between the HR beam and/or SR beam with the film and substrate; 
         FIG. 7  is a flowchart depicting the method for identifying detector noise that may be resistive to the noise canceling in accordance with an exemplary embodiment of the present invention; 
         FIGS. 8A and 8B  are diagrams of a self referencing heterodyne reflectometer configured without moving optical components in accordance with an exemplary embodiment of the present invention; 
         FIGS. 9A and 9B  are diagrams of a self referencing heterodyne reflectometer with separate SR beam and HR beam paths in accordance with an exemplary embodiment of the present invention; 
         FIGS. 10A and 10B  are diagrams of a self referencing heterodyne reflectometer with counter rotating SR and HR beam paths in accordance with an exemplary embodiment of the present invention; 
         FIGS. 11A and 11B  are diagrams of a self referencing heterodyne reflectometer employing a liquid crystal variable retarder (LCVR) for electronically switching between HR and SR operating modes in accordance with an exemplary embodiment of the present invention; 
         FIGS. 12A and 12B  are diagrams of a self referencing heterodyne reflectometer in which the SR beam bypasses the sample for addressing the sole issue of detector phase drift in accordance with an exemplary embodiment of the present invention; 
         FIG. 13A  and  FIG. 13B  depicts a self referencing heterodyne reflectometer which employ a mechanical or electrical-electromagnetic device for switching between the HR and SR beams, or vice versa, at a faster rate than the temperature drift in a detector; 
         FIGS. 14A and 14B  are diagrams of a self referencing heterodyne reflectometer with counter rotating SR and HR beam paths which utilize a high frequency optical switch to minimize error in the phase measurement resulting from changes in the temperature of the detectors in accordance with an exemplary embodiment of the present invention; 
         FIGS. 15A and 15B  are diagrams of a self referencing heterodyne reflectometer, utilizing a chopper, in which the SR beam bypasses the sample for addressing the sole issue of detector phase drift in accordance with an exemplary embodiment of the present invention; and 
         FIGS. 16A and 16B  are diagrams of a self referencing heterodyne reflectometer in which the SR beam path is replaced by an amplitude modulated (AM) beam in accordance with an exemplary embodiment of the present invention. 
     
    
    
     Other features of the present invention will be apparent from the accompanying drawings and from the following detailed description. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Element Reference Number Designations 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 100: 
                 Heterodyne monitoring system 
               
               
                 102: 
                 First linear polarized light component 
               
               
                 102: 
                 First linear polarized light component 
               
               
                 103: 
                 Second linear polarized component 
               
               
                 104: 
                 First polarization component 
               
               
                 105: 
                 Second polarization component 
               
               
                 106: 
                 Orthogonally polarized to beam 
               
               
                 107: 
                 Beam 
               
               
                 109: 
                 Beam 
               
               
                 110: 
                 Wafer 
               
               
                 111s: 
                 Reflected beam 
               
               
                 111b: 
                 Reflected beam 
               
               
                 111t: 
                 Reflected beam 
               
               
                 112: 
                 Substrate 
               
               
                 113s: 
                 Beam 
               
               
                 113b: 
                 Beam 
               
               
                 113t: 
                 Beam 
               
               
                 114: 
                 Film 
               
               
                 115: 
                 Rotatable carrier 
               
               
                 116: 
                 Beam 
               
               
                 117: 
                 Beam 
               
               
                 122: 
                 Beam 
               
               
                 124: 
                 Beam 
               
               
                 134: 
                 Beam 
               
               
                 135: 
                 Beam 
               
               
                 135S: 
                 Beam 
               
               
                 135B: 
                 Beam 
               
               
                 135T: 
                 Beam 
               
               
                 140: 
                 Laser diode 
               
               
                 143: 
                 Mixing polarizer 
               
               
                 144: 
                 Beam splitter 
               
               
                 145: 
                 Mixing polarizer 
               
               
                 146: 
                 Polarization beam splitter 
               
               
                 147: 
                 Photo detector 
               
               
                 148: 
                 Quarter-wave plate 
               
               
                 149: 
                 Photo detector 
               
               
                 150: 
                 Quarter-wave plate 
               
               
                 152: 
                 Beam reflector 
               
               
                 154: 
                 Signal-processing assembly 
               
               
                 160: 
                 Data processing system 
               
               
                 200: 
                 Heterodyne reflectometer 
               
               
                 202: 
                 Split freq. beam with orthogonal, linearly polarized components 
               
               
                 203: 
                 Incident beam 
               
               
                 204: 
                 Split beam 
               
               
                 205: 
                 Reflected beam 
               
               
                 210: 
                 Table system 
               
               
                 212: 
                 Substrate 
               
               
                 214: 
                 Film 
               
               
                 215: 
                 Table 
               
               
                 220: 
                 Light source 
               
               
                 222: 
                 Optics 
               
               
                 224: 
                 Beam splitter 
               
               
                 232: 
                 Incident prism 
               
               
                 234: 
                 Reflection prism 
               
               
                 240: 
                 Signal detector 
               
               
                 242: 
                 Reference signal i ref , 
               
               
                 250: 
                 Signal detector 
               
               
                 252: 
                 Heterodyne signal i ref , 
               
               
                 254: 
                 Mixing polarizer 
               
               
                 255: 
                 Mixing polarizer 
               
               
                 256: 
                 Reflected beam 
               
               
                 260: 
                 Data processing system processor 
               
               
                 262: 
                 Δφ m  measured phase shift detector 
               
               
                 264: 
                 Memory 
               
               
                 266: 
                 Δφ corrector 
               
               
                 268: 
                 D f  calculator 
               
               
                 269: 
                 Film thickness d f   
               
               
                 303: 
                 Incident s-polarization 
               
               
                 303s: 
                 S-polarization freq. ω 
               
               
                 303p: 
                 P-polarization freq. ω and ω + δω 
               
               
                 305-1s: 
                 Reflected s-ray 
               
               
                 305-2s: 
                 Refracted s-ray 
               
               
                 305-1p: 
                 Reflected p-ray 
               
               
                 305-2p: 
                 Refracted p-ray 
               
               
                 400: 
                 Light source generates 
               
               
                 402: 
                 Split freq. beam with orthogonal, linearly polarized components 
               
               
                 403: 
                 Split freq. beam with orthogonal, linearly polarized components 
               
               
                 404: 
                 Reflected hr beam 
               
               
                 405-1: 
                 Reflected beam component 
               
               
                 405-2: 
                 Reflected beam component 
               
               
                 410: 
                 Polarizer 
               
               
                 411: 
                 λ/2 plate 
               
               
                 412: 
                 Beam splitter 
               
               
                 413: 
                 Aperture 
               
               
                 414: 
                 Polarizer 
               
               
                 416: 
                 Reference detector 
               
               
                 418: 
                 Reflective optical component 
               
               
                 420: 
                 Reflective optical component 
               
               
                 422: 
                 Polarizer 
               
               
                 424: 
                 Focusing optics 
               
               
                 426: 
                 Optics 
               
               
                 426: 
                 Measurement detector 
               
               
                 433: 
                 P-polarized heterodyne beam 
               
               
                 434: 
                 Self-referencing (SR) beam 
               
               
                 435: 
                 Reflected SR beam 
               
               
                 442: 
                 Reference signal i ref   
               
               
                 452: 
                 Heterodyne signal i het   
               
               
                 460: 
                 Data processing system 
               
               
                 461: 
                 Slider controller 
               
               
                 462: 
                 δ ref/sub  detector 
               
               
                 463: 
                 δ ref/film  detector 
               
               
                 465: 
                 Threshold noise detector 
               
               
                 466: 
                 Δφ film  calculator 
               
               
                 467: 
                 Δφ film  averager 
               
               
                 468: 
                 D f  calculator 
               
               
                 469: 
                 D f  film thickness 
               
               
                 470: 
                 Slider 
               
               
                 800: 
                 Light source 
               
               
                 801: 
                 Beam splitter 
               
               
                 802: 
                 HR beam 
               
               
                 803: 
                 HR beam 
               
               
                 804: 
                 Reference beam 
               
               
                 805-1: 
                 Ray 
               
               
                 805-2: 
                 Ray 
               
               
                 807: 
                 Beam splitter 
               
               
                 809: 
                 Sliding shutter 
               
               
                 810: 
                 Stationary polarizer 
               
               
                 811: 
                 λ/2 plate 
               
               
                 812: 
                 Beam splitter 
               
               
                 815: 
                 Polarizer 
               
               
                 816: 
                 Reference detector 
               
               
                 818: 
                 Reflective optical component 
               
               
                 820: 
                 Reflective optical component 
               
               
                 822: 
                 Polarizer 
               
               
                 824: 
                 Focusing optics 
               
               
                 826: 
                 Measurement detector 
               
               
                 828: 
                 Optical component 
               
               
                 829: 
                 Optical component 
               
               
                 833: 
                 Sr beam 
               
               
                 834: 
                 Beam 
               
               
                 835: 
                 Ray 
               
               
                 900: 
                 Light source 
               
               
                 901: 
                 Beam splitter 
               
               
                 902: 
                 HR beam 
               
               
                 903: 
                 HR beam 
               
               
                 904: 
                 Reference beam 
               
               
                 905-1: 
                 Ray 
               
               
                 905-2: 
                 Ray 
               
               
                 909: 
                 Sliding shutter 
               
               
                 910: 
                 Stationary polarizer 
               
               
                 911: 
                 λ/2 plate 
               
               
                 912: 
                 Beam splitter 
               
               
                 915: 
                 Polarizer 
               
               
                 916: 
                 Reference detector 
               
               
                 917: 
                 Reflection optics 
               
               
                 918: 
                 Reflective optical component 
               
               
                 920: 
                 Reflective optical component 
               
               
                 922: 
                 Polarizer 
               
               
                 924: 
                 Focusing optics 
               
               
                 926: 
                 Measurement detector 
               
               
                 928: 
                 Optical component 
               
               
                 929: 
                 Reflective optical component 
               
               
                 933: 
                 SR beam 
               
               
                 934: 
                 Beam 
               
               
                 935: 
                 Ray 
               
               
                 1000: 
                 Light source 
               
               
                 1002: 
                 HR beam 
               
               
                 1003: 
                 HR beam 
               
               
                 1004: 
                 HR beam 
               
               
                 1005: 
                 Reflected HR beam 
               
               
                 1010: 
                 Stationary polarizer 
               
               
                 1011: 
                 λ/2 plate 
               
               
                 1015: 
                 Optics 
               
               
                 1016: 
                 Detector 
               
               
                 1018: 
                 Beam splitter 
               
               
                 1020: 
                 Reflective optical component 
               
               
                 1021: 
                 Reflective optical component 
               
               
                 1022: 
                 Polarizer 
               
               
                 1023: 
                 Beam splitter 
               
               
                 1024: 
                 Focusing optics 
               
               
                 1026: 
                 Detector 
               
               
                 1033: 
                 SR beam 
               
               
                 1034: 
                 SR beam 
               
               
                 1035: 
                 Reflected sr beam 
               
               
                 1041: 
                 Beam splitter 
               
               
                 1042 
                 Beam splitter 
               
               
                 1050: 
                 Corner cube 
               
               
                 1051: 
                 Shutter 
               
               
                 1052: 
                 Shutter 
               
               
                 1100: 
                 Light source 
               
               
                 1102: 
                 HR beam 
               
               
                 1103: 
                 HR beam 
               
               
                 1104: 
                 Reference beam 
               
               
                 1105: 
                 Rays 
               
               
                 1107: 
                 Beam splitter 
               
               
                 1111: 
                 Liquid crystal variable retarder 
               
               
                 1112: 
                 Beam splitter 
               
               
                 1114: 
                 Polarizer 
               
               
                 1116: 
                 Reference detector 
               
               
                 1118: 
                 Beam splitter 
               
               
                 1119: 
                 Polarizing beam splitter 
               
               
                 1120: 
                 Reflective optical component 
               
               
                 1122: 
                 Polarizer 
               
               
                 1124: 
                 Focusing optics 
               
               
                 1126: 
                 Measurement detector 
               
               
                 1128: 
                 Optical component 
               
               
                 1129: 
                 Optical component 
               
               
                 1133: 
                 Beam 
               
               
                 1134: 
                 Beam 
               
               
                 1135: 
                 SR ray 
               
               
                 1135-1: 
                 HR ray 
               
               
                 1135-2: 
                 HR ray 
               
               
                 1200: 
                 Light source 
               
               
                 1202: 
                 HR beam 
               
               
                 1203: 
                 HR beam 
               
               
                 1204: 
                 Beam 
               
               
                 1205: 
                 Reflected HR beam 
               
               
                 1209: 
                 Shutter 
               
               
                 1212: 
                 Optics 
               
               
                 1214: 
                 Polarizer 
               
               
                 1216: 
                 Detector 
               
               
                 1217: 
                 Shutter 
               
               
                 1218: 
                 Beam splitter 
               
               
                 1220: 
                 Reflective optical component 
               
               
                 1221: 
                 Reflective optical component 
               
               
                 1222: 
                 Polarizer 
               
               
                 1226: 
                 Detector 
               
               
                 1233: 
                 Beam 
               
               
                 1300: 
                 Light source 
               
               
                 1301: 
                 Beam splitter 
               
               
                 1302: 
                 HR beam 
               
               
                 1303: 
                 HR beam 
               
               
                 1304: 
                 Reference beam 
               
               
                 1305-1: 
                 Ray 
               
               
                 1305-2: 
                 Ray 
               
               
                 1309: 
                 Chopper shutter 
               
               
                 1310: 
                 Stationary polarizer 
               
               
                 1311: 
                 λ/2 plate 
               
               
                 1312: 
                 Beam splitter 
               
               
                 1315: 
                 Polarizer 
               
               
                 1316: 
                 Reference detector 
               
               
                 1318: 
                 Reflective optical component 
               
               
                 1320: 
                 Reflective optical component 
               
               
                 1322: 
                 Polarizer 
               
               
                 1324: 
                 Focusing optics 
               
               
                 1326: 
                 Measurement detector 
               
               
                 1328: 
                 Optical component 
               
               
                 1329: 
                 Optical component 
               
               
                 1333: 
                 SR beam 
               
               
                 1334: 
                 Beam 
               
               
                 1335: 
                 Ray 
               
               
                 1361: 
                 Chopper controller 
               
               
                 1400: 
                 Light source 
               
               
                 1402: 
                 HR beam 
               
               
                 1403: 
                 HR beam 
               
               
                 1404: 
                 HR beam 
               
               
                 1405: 
                 Reflected HR beam 
               
               
                 1410: 
                 Stationary polarizer 
               
               
                 1411: 
                 λ/2 plate 
               
               
                 1415: 
                 Optics 
               
               
                 1416: 
                 Detector 
               
               
                 1418: 
                 Beam splitter 
               
               
                 1420: 
                 Reflective optical component 
               
               
                 1421: 
                 Reflective optical component 
               
               
                 1422: 
                 Polarizer 
               
               
                 1424: 
                 Focusing optics 
               
               
                 1426: 
                 Detector 
               
               
                 1433: 
                 SR beam 
               
               
                 1434: 
                 SR beam 
               
               
                 1435: 
                 Reflected sr beam 
               
               
                 1441: 
                 Beam splitter 
               
               
                 1442 
                 Beam splitter 
               
               
                 1450: 
                 Corner cube 
               
               
                 1451: 
                 Chopper shutter 
               
               
                 1452: 
                 Chopper shutter 
               
               
                 1500: 
                 Light source 
               
               
                 1502: 
                 HR beam 
               
               
                 1503: 
                 HR beam 
               
               
                 1504: 
                 Beam 
               
               
                 1505: 
                 Reflected HR beam 
               
               
                 1509: 
                 Chopper shutter 
               
               
                 1512: 
                 Optics 
               
               
                 1514: 
                 Polarizer 
               
               
                 1516: 
                 Detector 
               
               
                 1517: 
                 Chopper shutter 
               
               
                 1518: 
                 Beam splitter 
               
               
                 1520: 
                 Reflective optical component 
               
               
                 1521: 
                 Reflective optical component 
               
               
                 1522: 
                 Polarizer 
               
               
                 1526: 
                 Detector 
               
               
                 1533: 
                 Beam 
               
               
                 1600: 
                 HR light source 
               
               
                 1601: 
                 AM split freq., amplitude mod. beam 
               
               
                 1602: 
                 HR split freq. beam 
               
               
                 1603: 
                 HR split freq. beam 
               
               
                 1604: 
                 HR split freq. reference beam 
               
               
                 1605-1: 
                 AM ray 
               
               
                 1605-2: 
                 AM ray 
               
               
                 1609: 
                 Chopper shutter 
               
               
                 1611: 
                 Lght source 
               
               
                 1612: 
                 Beam splitter 
               
               
                 1613: 
                 Aplitude modulator 
               
               
                 1614: 
                 Polarizer 
               
               
                 1616: 
                 Reference detector 
               
               
                 1618: 
                 Reflective optical component 
               
               
                 1620: 
                 Reflective optical component 
               
               
                 1622: 
                 Polarizer 
               
               
                 1624: 
                 Focusing optics 
               
               
                 1626: 
                 Measurement detector 
               
               
                 1633: 
                 AM amplitude mod. incident beam 
               
               
                 1634: 
                 AM amplitude mod. reference beam 
               
               
                 1635: 
                 AM amplitude mod. reflected beam 
               
               
                 D f : 
                 Film thickness 
               
               
                 I 1 : 
                 Signal 
               
               
                 I 2 : 
                 Signal 
               
               
                 I het : 
                 Heterodyne signal 
               
               
                 I ref : 
                 Reference signal 
               
               
                   
               
            
           
         
       
     
     In a Michelson heterodyne interferometer, the interfering reference beam and measurement beam have slight optical frequency difference, typically ˜KHz to MHz. The interference between the two is represented by the equation:
 
 I=A+B  cos(Δω t +φ)  (1)
         A is a direct current component;   B is the signal component that represents fringe visibility;   φ is the phase difference between reference beam and measurement beam; and   Δω is the angular frequency difference between the two signals. The interference between the two can be observed as a beat signal with an angular frequency equal to the difference angular frequency, Δω.       

     When the measurement beam undergoes an optical path length change (Δd), the beat signal will experience corresponding phase shift Δφ=(4π×Δd)/λ. 
     The present inventor has disclosed an uncomplicated heterodyne reflectometer approach to thin film measurements in co-pending U.S. patent application Ser. No. 11/178,856 entitled “Method for Monitoring Film Thickness Using Heterodyne Reflectometry and Grating Interferometry,” filed Jul. 10, 2005, and also in co-pending U.S. patent application Ser. No. 11/066,933 entitled “Heterodyne Reflectometer for Film Thickness Monitoring and Method for Implementing,” filed Feb. 25, 2005. In accordance with this approach, the measurement signal is heterodyned from two beam components that each interact with the sample. One of the beam components is almost totally refracted into the film and reflected off the bottom of the film and the other is reflected off the surface. Thus, the phase of the heterodyned measurement signal is due to the difference in the optical paths of the two beam components, which, in turn, is related to the thickness of the sample. This concept will be understood with the discussion of the heterodyne reflectometer in  FIG. 2 . 
       FIG. 2  is a diagram of a heterodyne reflectometer for measuring thin film thicknesses. Heterodyne reflectometer  200  generally comprises optics for directed incident beam  203  incident on film  214  and substrate  212  at incidence angle α. Light source  220  generates beam  202  having two linearly polarized components, operating at split optical frequencies, that are orthogonal with respect to each other for illuminating the target. For instance, the beam may have an s-polarized beam component at frequency ω and a p-polarized beam component at frequency ω+Δω. 
     Beam  202  is split by beam splitter  224  into beam  204  and beam  203 . Beam  203  comprises two linearly polarized components that are orthogonal to each other, with split optical frequencies, i.e., s- and p-polarized beam components at split frequencies of ω and ω+Δω, respectively. As used herein, Δω is approximately 20 MHz, but is merely exemplary and other frequency splits may be used without departing from the scope of the present invention. Light source  220  for generating this beam may be, for example, a Zeeman split He—Ne laser. Alternatively, the beam from a single mode laser source can be split into two separate beams with one or both of the separate beams being frequency shifted to a predetermined frequency using, for example, an acousto-optic modulator. The split-frequency beams can then be recombined prior to incidence with film  214 . The light beam is directed into the plane of incidence, and toward film  214 , using any suitable optical component for redirecting the path of the aforementioned light beam. As depicted in the figure, a pair of triangular prisms (incident prism  232  and reflection prism  234 ) direct incident beam  203  to film  214  and receive reflected beam  205  from film  214 , but optionally may be any suitable optical component for directing the light path while retaining the beam&#39;s polarization. For example, light source  220  may be directed in the plane of incidence (at incidence angle α from normal), using a mirror or other reflecting optical component, or, alternatively, coupled into polarization preserving fibers which are then positioned to launch the beam at the predetermined incidence angle. 
     Notice that the paths of both optical frequencies interact with the film along a single path, i.e., the s-polarization component and the p-polarization component of the measurement beam are substantially collinear beams and approximately coaxial. Furthermore, the illuminated areas on film  214  from s-polarization and p-polarization components are approximately coextensive at the target location. 
     A primary function of a heterodyne reflectometer of the present invention is to determine the actual phase shift, Δφ, from a measured phase shift, Δφ m . Measured phase shift Δφ m  is the phase difference between the phase of reference signal I ref  and the phase of measurement signal I het , i.e., the beat of a signal obtained from a non-reflected path (the reference signal) and the beat signal obtained from a reflected path. The true (or actual) phase shift Δφ is necessary for determining an error-free and accurate thickness of a film layer, d f . Therefore, finding measured phase shift Δφ m  necessitates employing two signal detectors, one for detecting/generating reference signal I ref  and a second for detecting/generating the measurement signal I het . 
     Signal detector  240  senses the split beam (reference beam)  204  from mixing polarizer  254 , which mixes the s- and p-polarization components of beam  204 , prior to reflecting off of film  214 , and produces reference signal I ref ,  242 , which is indicative of the phase of beam  204 , phase φ. Detector  240  may be, for example, a PIN (Positive-Intrinsic-Negative) detector, or any photo detector that responds to the beat frequency, and produces reference signal I ref  with a beat frequency of |ω−(ω+Δω)|. Reference signal I ref    242  is transmitted to Δφ m  measured phase shift detector  262 , where it is used as the reference phase for determining measured phase shift Δφ m  induced by film  214 . 
     Signal detector  250 , on the other hand, senses reflected beam  256  from mixing polarizer  255 , which mixes the s- and p-polarization components of beam  205 , propagated from prism  234 , and after interacting with film  214 . Signal detector  250  produces measurement signal I het ,  252 , which is indicative of the phase of beam  256 , phase φ+Δφ, and is phase shifted from the phase of reference signal I ref  by Δφ. Detector  250  may be, as an example, a PIN detector, which monitors the reflected optical beam  256  and produces heterodyne measurement signal I het , also with a heterodyne angular frequency of Δω. 
     Signal  252  is received at Δφ m  measured phase shift detector  262 , which compares measured heterodyne measurement signal I het    252  with reference signal I ref    242  and determines measured phase shift Δφ m . Phase shift Δφ is induced by film  214 , and the amount of the phase shift depends on several factors, including the thickness of film  214 , the refractive index n f  for the particular film being monitored, and in higher phase shifts, a correction factor. The interrelationship between the factors will be discussed in greater specificity further below. In any case, an accurate film thickness d f    269  can then be determined by processor  260  from corrected phase shift Δφ, which is obtained from measured phase shift Δφ m . However, since measured phase shift Δφ m  has an inherent error, at least at higher phase shifts, accurate thickness measurements are possible only after the measured phase shift is corrected. 
     Data processed system  260  may take a variety of forms depending on the particular application. Often data from inline wafer processing is processed in real time on a computer or PC that is electrically coupled to reflectometer detectors  240  and  250  or Δφ m  measured phase shift detector  262 . However, the reflectometer systems may be pre-configured with internal data processors and/or discrete firmware components for storing and processing monitored data in real time. Also, the raw measured data from the reflectometer may be handled by a data processing system resident on the wafer process equipment. In that case, the wafer processing firmware performs all data processing for the reflectometer, including thickness computations. Accordingly, heterodyne reflectometer system  200  is depicted with generic data processing system  260 , which may include discrete firmware and hardware components. These components generally include measured phase shift corrector  266  and thickness calculator  268 . Optionally, system  260  may include error correction data memory  264 , the operation of which will be discussed below. 
     More particularly, Δφ m  phase shift detector  262  receives reference signal I ref    242  and heterodyne measurement signal I het    252  from the respective detectors and measures phase shift Δφ m  between the two. Phase shift detector  262  may use any appropriate mechanism for detecting corresponding points on reference signal I ref  and measurement signal I het  for phase detection. 
     Although not depicted in the figure, phase shift detector  262  may also be equipped with an I/O interface for entering wavelength and/or oscillator frequency information for facilitating signal detection. 
     Once measured phase shift Δφ m  has been detected, it is passed to Δφ m  measured phase shift corrector  266  for error correction. The error in measured phase shift Δφ m  may be appreciable at higher phase shifts, but the error can be corrected by applying a polynomial function to Δφ m , with an appropriate set of correction coefficients. Furthermore, Δφ m  corrector  266  requires certain parametric data for performing the error correction computations. These data include the source wavelength, λ, the top film layer refractive index, n f , and the incidence angle, α. α will be typically set at a default, α=60°, rather than precisely at the Brewster&#39;s angle for the source wavelength and film refractive index n f , the reasons for which are discussed in U.S. patent application Ser. No. 11/066,933 “Method for Monitoring Film Thickness Using Heterodyne Reflectometry and Grating Interferometry,” and also in co-pending U.S. patent application Ser. No. 11/066,933 entitled “Heterodyne Reflectometer for Film Thickness Monitoring and Method for Implementing.” 
     Finally, d f  thickness calculator  268  receives the corrected phase shift, Δφ, from Δφ m  corrector  266  and computes a corrected film thickness d f  for the film being examined, i.e., film  214 . Alternatively, d f  thickness calculator  268  may receive measured phase shift Δφ m  directly from Δφ m  phase shift detector  262  and then algebraically correct the measured thickness with film thickness correction data it fetches from memory  264 . The thickness error correction data, or a look-up table (LUT), are loaded into memory  264  beforehand based on the refractive index n f  for film  214 . 
     Still another option is to store a table of corrected thickness values, d f , in memory  264  which are indexed to discrete measured phase shift values. In that case, on receiving Δφ m  from phase shift detector  262 , d f  thickness calculator  268  retrieves a corrected thickness value from memory  264  and outputs the value. 
     This method relies on the anisotropic reflection of the radiation from the top surface of the film. Therefore, the heterodyne reflectometer set-up is optimally configured with incidence angle α near Brewster&#39;s angle. The maximum sensitivity to phase shift for a film is achieved at the Brewster&#39;s angle for the refractive index of a particular film under examination. At the Brewster&#39;s angle, the amount of reflected p-polarized light from the top surface of the film is nil or minimal. Thus, signal, I het ,  252  from detector  250  is rich with film-thickness information. 
     However, as a practical matter, the optical components in a monitoring system may be semi-permanently configured for cooperating with a particular processing apparatus (e.g., at a preset 60° angle of incidence, α=60°). In those systems, adjusting the incidence to a precise angle may be difficult or impossible. Nevertheless, as will be shown in the following discussions, one benefit of the presently described invention is that the thickness measurements are highly accurate over a wide range of angles around the Brewster&#39;s angle for a particular film&#39;s refractive index. 
     Furthermore, in addition to the anisotropic reflection from the film surface, reflective anisotropy may also be present in the film itself and the bottom film surface or the substrate. It has been assumed that the film material and the lower interface are isotropic for the s- and p-polarizations. However, this assumption may not always be correct for every film type, see T. Yasuda, et al., “Optical Anisotropy of Singular and Vicinal Si—SiO 2  Interfaces and H-Terminated Si Surfaces,” J. Vac. Sci. Technol. A 12(4), July/August 1994, p. 1152 and D. E. Aspnes, “Above-Bandgap Optical Anisotropies in Cubic Semiconductors: A Visible-Near Ultraviolet Probe of Surfaces,” J. Vac. Sci. Technol. B 3(5), September/October 1985, p. 1498. Accordingly, in those situations where the top film and/or the substrate exhibit significant reflectance anisotropy, the optimized incidence angle can be between normal incidence and Brewster incidence. 
     The heterodyne reflectometer set-up incidence angle α for configuring system  200  is related to, and could change with, the refractive index, n f , of the film under inspection and the wavelength, λ, of the illumination source. Since different films have different refractive indexes, the angle α could be adjusted corresponding to changes in the index. If this is desired, a means should be provided for adjusting the incident angle of heterodyne reflectometer system  200  based on the refractive index of the various films to be examined. This may be accomplished by enabling table system  210  and/or prisms  232  and  234  to move. For example, mirrors  232  and  234  may be configured with two degrees of movement, one in a rotational direction about an axis that is perpendicular to the plane of incidence formed by beams  203  and  205 , and the normal of film  214 , and a translation movement direction that is parallel to the surface normal. Alternatively, mirrors  232  and  234  may have one degree of rotational movement about a direction perpendicular to the plane of incidence and table assembly  210  will then have one degree of translational movement in the normal direction. The latter exemplary embodiment is depicted herein with mirrors  232  and  234  and table assembly  210  (depicted herein as table  215 , film  214  and substrate  212 ) shown with phantom lines indicting movement. The phantom components show incident beam  203  and receiving reflected beam  205  redirected to a different incident angle α, in response to a change in the value of refractive index n f . However, as emphasized above and below, using a default incidence angle, α=60°, is advantageous over setting the incidence angle precisely at the Brewster&#39;s angle for the film and light source. 
     Turning to  FIGS. 3A and 3B , the source of phase shift Δφ attributable to film  214  is depicted. The s-polarization component of the HR beam is depicted as being separated ( FIG. 3A ) from the p-polarization component of the HR beam ( FIG. 3B ) for clarity. Turning to the s-polarization component of the HR beam is depicted in  FIG. 3A , incident beam  303  is comprised of s-polarization component  303   s  (having an optical angular frequency of ω) and p-polarization component  303   p  (having an optical angular frequency of ω+Δω), which are orthogonal to each other. Both component  303   s  and component  303   p  are incident to the normal of film  214  at angle α. At the surface of film  214 , a portion of beam component  303   s  is reflected as reflected ray  305 - 1   s , while another portion of beam component  303   s  refracts into film  314  at a refraction angle, ρ, then reflects off substrate  212  and refracts out of film  214  as refracted ray  305 - 2   s . Turning to the p-polarization component of the HR beam as depicted in  FIG. 3B , incident beam component  303   p  is split into a reflected ray  305 - 1   p  and refracted ray  305 - 2   p.    
     Basic to calculating accurate film thicknesses is optimizing the light interaction with the film to be more sensitive to film thickness, which in turn enhances the heterodyne phase shift, Δφ m . The aim is to increase the phase shift of the heterodyned signal as much as possible from the reference signal, i.e., increase Δφ m . This is done by optimizing the incidence angle. Since the reflected beam is composed of s- and p-component rays that are both reflected and refracted, it is advantageous for one polarization component to have a greater portion of reflected rays from the film surface than the other. Because s- and p-polarized light with split frequencies is used for the measurement, it is possible to adjust the incident angle, α, to achieve this result. As is well understood in the art, linear polarized light will exhibit this result by setting the incident angle to the Brewster&#39;s angle for the source wavelength. At Brewster&#39;s angle, virtually the entire p-polarization component of incident beam  303   p  is refracted into the film as  305 - 2   p  with very little, if any, reflected as ray  305 - 1   p . Conversely, operating at Brewster&#39;s angle, the s-polarization component of incident beam  303   s , sees significant reflection as ray  305 - 1   s  with the rest penetrating the film as refracted ray  305 - 2   s . Therefore, angle α may be adjusted such that more of one polarized light component is not reflected, but almost totally refracted in the film. Hence, after the rays are mixed, the resulting beam is sensitized for phase shift due to a disproportionate contribution of the s-polarization component reflected from the film&#39;s surface. Therefore, it can be appreciated that a phase shift results from the time necessary for refracted components to travel over the increased path distance, Δd=2d f √{square root over (n f   2 −sin 2  α)}, where 
     
       
         
           
             
               δ 
               f 
             
             = 
             
               
                 
                   2 
                   ⁢ 
                   π 
                 
                 λ 
               
               ⁢ 
               
                 
                   
                     n 
                     f 
                     2 
                   
                   - 
                   
                     
                       sin 
                       2 
                     
                     ⁢ 
                     α 
                   
                 
               
               × 
               
                 d 
                 f 
               
             
           
         
       
     
     δ is the phase shift attributable to the film thickness; 
     α is the angle of incidence; 
     n is the refractive index of the film; and 
     d is the film thickness. 
     With the heterodyne reflectometer configured toward being more sensitive to thickness, a calculation for determining thickness from phase shift Δφ can be established. In the classical heterodyne interferometer, the phase shift is measured and a corresponding change in the beam path difference, Δd, can be calculated using the following expression:
 
Δφ=4 π×Δd/λ   (2)
 
     Δφ is the phase shift of the measured signal, I het , 
     Δd is the corresponding beam path difference; and 
     λ is wavelength of the heterodyne illumination source. 
     Thus:
 
 Δd=Δφλ/ 4π  (3)
 
     In heterodyne reflectometry, since Δφ=2δ, and 
               δ   =         2   ⁢   π     λ     ⁢         n   2     -       sin   2     ⁢   α         ×   d       ,         
the thickness of the film can then be found by the following equation:
 
     
       
         
           
             
               
                 
                   d 
                   = 
                   
                     ( 
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         ϕ 
                         × 
                         λ 
                       
                       
                         4 
                         ⁢ 
                         π 
                         × 
                         
                           
                             
                               n 
                               2 
                             
                             - 
                             
                               
                                 sin 
                                 2 
                               
                               ⁢ 
                               α 
                             
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     The proofs of Equations (2)-(4) can be found in U.S. patent application Ser. Nos. 11/178,856 and 11/066,933 discussed above. 
     Heterodyne reflectometry by nature is a differential measurement technique. In accordance with the prior art, phase shift corresponding to a film is measured with respect to a reference substrate that has a film of known thickness. Ideally, the operator has access to the reference sample in order to take a reference measurement each time before measuring the product/monitor wafer. In the absence of that, one would require the heterodyne reflectometry sensor to be robust enough not to have (systematic) phase drift before the next reference sample measurement is made. Highly precise measurements (˜0.001 deg.), are influenced by a drift in the heterodyne frequency, phase shift induced by optical components, presence of surface contaminants, and detector response to temperature change. Some obstacles can be overcome. Because of the common mode nature of heterodyne reflectometry, long-term frequency drift will not influence measurement. Optical component induced phase shift can be eliminated by using appropriate coatings and angles of incidence. Taking data in a controlled environment will prevent surface contaminants from influencing measurement. Studies done with heterodyne reflectometry detectors have shown that phase drift as much as 0.01 deg/° C. can occur in a heterodyne reflectometry system if the detector temperature is not controlled. 
     Therefore, in accordance with one aspect of the present invention, a self referencing heterodyne reflectometer and method for implementing is disclosed. In accordance with another aspect of the present invention, a heterodyne reflectometer and method for implementing is disclosed which does not rely on the availability of reference wafer sample for accuracy. These aspects of the present invention, as well as other aspects, will be better understood through a discussion of  FIGS. 4A-4C  below. 
       FIGS. 4A-4C  are diagrams of a self referencing heterodyne reflectometer for measuring thin film thicknesses without the availability of a reference wafer in accordance with an exemplary embodiment of the present invention.  FIG. 4A  depicts a self referencing heterodyne reflectometer showing a composite operational state for obtaining δ Ref/film  phase measurements and δ Ref/Sub  reference phase measurements.  FIG. 4B  shows the operational state for obtaining δ Ref/film  measurements and  FIG. 4C  shows the operational state for obtaining δ Ref/Sub  measurements. 
     Similar to heterodyne reflectometer  200  discussed in  FIG. 2 , the self referencing heterodyne reflectometer of the present invention generally comprises optics for directing a beam incident to film  214  and substrate  212  at incidence angle α. Light source  400  generates two collinear beams (beam  402 ) having two linearly polarized components, operating at split optical angular frequencies, that are orthogonal with respect to each other for illuminating the target; an s-polarized beam component at frequency ω and a p-polarized beam component at frequency ω+Δω. This beam will be referred to hereinafter as a HR (heterodyne reflectometer) beam and will be designated as a soling line in each of  FIGS. 4A ,  4 B,  4 C,  8 A,  8 B,  9 A,  9 B,  10 A,  10 B,  13 A,  13 B,  14 A and  14 B. As used herein, Δω is approximately 20 MHz, but is merely exemplary and other frequency splits may be used without departing from the scope of the present invention. Light source  400  for generating this beam may be, for example, a Zeeman split He—Ne laser. Alternatively, the beam from a single mode laser source can be split into two separate beams with one or both of the separate beams being frequency shifted to a predetermined frequency, using for example, an acousto-optic modulator. The split-frequency beams can then be recombined prior to incidence with film  214 . The light beam is directed into the plane of incidence, and toward film  214 , using any suitable optical component for redirecting the path of the aforementioned light beam. 
     HR beam  402  propagates as HR beam  403  and split by BS (beam splitter)  412  into reflected HR beam  404 , through polarizer  414  (@45°) where a reference signal, I ref , is detected by detector  416 . It should be appreciated that the use of cubes gives rise to certain disadvantages, principally associated with the generation of thermal stress-induced birefringence that degrades the polarization performance of the component. Therefore, the beam splitters employed for use with the present invention should have low thermal stress-induced birefringence, such as, for example, low linear birefringence SF57 glass. Fused silica components exhibit more thermal birefringence and those with BK7 substrate appear to be even less desirable, as their thermal birefringence properties appear to be on the order of two magnitudes worse than SF57. Thermal stress-induced birefringence should be considered in the selection of other optical components, such as mirrors and the like. Reference signal I ref  provides phase information for the beam before the beam interacts with the sample. The portion of beam  403  transmitted through BS  412  propagates off of reflective optical components  418  and  420  (mirrors or the like) and incident on film  214  and substrate  212  (typically a wafer). As discussed above, the angle of incidence, α, (not shown) is typically set near the Brewster angle for the source wavelength, λ, of light source  400  and the film&#39;s refractive index n f , the reasons for which are discussed in U.S. patent application Ser. Nos. 11/178,856 and 11/066,933 (or at a default, e.g., α=60°, rather than precisely at the Brewster angle). 
     HR beam  403  interacts with film  214  and substrate  212  resulting in reflected beam components  405 - 1  and  405 - 2 , which pass through polarizer  422  (@45°) where a heterodyne measurement signal, I het , is detected by detector  426 , from focusing optics  424 , for the HR beam. As mentioned above, because this method relies on the anisotropic reflection of the radiation from the top surface of film  214 , beam component  405 - 1  is almost exclusively s-polarization reflected from the surface of film  214 , while beam component  405 - 2  results from interactions below the surface of film  214 . Therefore, beam component  405 - 2  comprises virtually the entire p-polarization component from the incident beam, in addition to some s-polarization component. Film thickness information can be obtained from heterodyne measurement signal I het  and reference signal I ref  as discussed above with respect to  FIG. 2 . 
     When polarizer  410  and λ/2 plate  411  combination (referred to hereinafter as polarizer/λ/2 combination  410 / 411  or component  410 / 411 ) is introduced into the path of beam  402 , p-polarized heterodyne beam  433  results which is a composite beam made up of both ω and ω+Δω frequencies. This beam will be referred to hereinafter as a SR (self-referencing) beam. 
     SR beam  433  is split by BS (beam splitter)  412  into reflected SR beam  434 , through polarizer  414  (@45°) where reference signal I ref  is detected by detector  416  for the SR beam. The portion of beam  433  transmitted through BS  412  propagates off of reflective optical components  418  and  420  and incident on film  214  and substrate  212  following the same path as incident HR beam  403 . Incident interacts with film  214  and substrate  212  resulting in reflected beam  435 , which pass through polarizer  422  (@45°) where heterodyne measurement signal I het  is detected by detector  426  for the SR beam. 
     When the SR beam is incident on a dielectric film, there is no or insignificant reflection (˜10 −3 ) from the dielectric film surface. The reflection returning from the film-substrate interface will not carry any phase information pertaining to the film. Therefore, the beat signals generated by the SR beams can be used to obtain a reference phase value, which is equivalent to that obtained using a reference sample. Thus, because incident SR beam  433  is p-polarized, virtually none is reflected from the surface of film  214 , but instead interacts with, and is reflected from the interface between film  214  and substrates  212 . Both the ω and ω+Δω frequency components of the reflected p-polarized SR beam are reflected as an SR beam, beam  435 . Consequently, measurement signal I het  detected by detector  426  from SR beam  435  provides a reference phase value that is not affected by changes in film thickness. 
       FIG. 4A  depicts a composite operational state for generating SR and HR beams, but as a practical matter the SR and HR beams are propagated sequentially, with δ ref/Sub  and δ ref/film  also generated sequentially.  FIG. 4B  shows the operational state of the self referencing heterodyne reflectometer in the HR beam generation mode for detecting measurement phase δ ref/film . In accordance with this exemplary embodiment of the present invention, polarizer/λ/2 combination  410 / 411 , rather than being stationary, is a sliding optical component further including aperture  413 . Sliding polarizer/λ/2/aperture combination  410 / 411 / 413  provides a mechanism for rapidly alternating between an HR beam and an SR beam. In HR beam mode, sliding polarizer/λ/2/aperture combination  410 / 411 / 413  is positioned such that aperture  413  aligns in the path of beam  402 , thereby allowing the HR beam generated by light source  400  to pass. Conversely, in SR beam mode, sliding polarizer/λ/2 aperture combination  410 / 411 / 413  is positioned such that polarizer/λ/2 combination  410 / 411  align in the path of beam  402 , thereby converting HR beam  402  into SR beam  433 . The movement force necessary is provided by slider actuator  470 , which is controlled by slider controller  461 . 
     Continuing, in the HR beam generation mode, slider controller  461  instructs slider actuator  470  to move in the HR beam position with aperture  413  aligned directly in the path of beam  402 . Incident HR beam  403  propagates to detectors  416  and  426  as described above resulting in reference signal I ref  and measurement heterodyne signal I het . Signals I ref  and I het  are routed to slider controller  461  which, in turn, switches the path of the signals to δ Ref/Sub  detector  462  or δ Ref/film  detector  463  depending on the propagation mode; in HR mode the signals I ref  and I het  are routed to detector  463  and in SR mode the signals I ref  and I het  are routed to δ Ref/Sub  detector  462 . δ Ref/film  is the phase difference between the signals I ref  and I het  operating in HR mode. Using Equation (5) below, δ Ref/film  detector  463  detects δ Ref/film  from signals I ref  and I het .
 
δ Ref/film =(φ Ref +φ noise1 )−(φ het +φ Sub +φ film +φ noise2 )  (5)
         Where, δ Ref/film  is the phase shift due to the film,   φ Ref  is the phase shift associated with the reference detector from BS  412 ,   φ noise1  is the phase shift associated with the detector noise,   φ het  is the phase shift associated with the heterodyne measurement detector from BS  412 ,   φ noise2  is the phase shift associated with the detector noise,   φ Sub  is the phase shift associated with the substrate, and   φ film  is the phase shift associated with the film.       

       FIG. 4C  shows the operational state for the self referencing heterodyne reflectometer in the SR beam generation mode. Here, slider controller  461  instructs slider actuator  470  to move in the SR beam position with polarizer/λ/2 combination  410 / 411  directly in the path of beam  402 , thereby converting HR beam  402  into a p-polarized SR beam with split optical frequencies of ω and ω+Δω (SR beam  433 ). Incident SR beam  433  propagates to detectors  416  and  426  as described above resulting in reference signal I ref  and measurement heterodyne signal I het . Signals I ref  and I het  are routed to slider controller  461  which now switches the path of the signals to δ Ref/Sub  detector  462 . Reference phase δ Ref/Sub  is the phase difference between the signals I ref  and I het  operating in SR mode. Using Equation (6) below, δ Ref/Sub  detector  462  detects δ Ref/Sub  from signals I ref  and I het .
 δ Ref/Sub =(φ Ref +φ noise1 )−(φ het +φ Sub +φ noise2 )  (6)         Where, δ Ref/Sub  is a reference phase shift due to the substrate,   φ Ref  is the phase shift associated with the reference detector from BS  412 ,   φ noise1  is the phase shift associated with the detector noise,   φ het  is the phase shift associated with the heterodyne measurement detector from BS  412 ,   φ noise2  is the phase shift associated with the detector noise, and   φ Sub  is the phase shift associated with the substrate.       
     Notice that unlike Equation (5), Equation (6), for finding δ Ref/Sub , does not contain any terms that depend on the film phase shift, and therefore, the value of δ Ref/Sub  is unaffected by changes in the film phase (i.e., changes in the thickness of the film). 
       FIG. 5  is a flowchart depicting the method for finding a temperature independent film thickness using a self-referencing heterodyne reflectometry in accordance with an exemplary embodiment of the present invention. The process begins by propagating HR beam with s-polarization at frequency ω and p-polarization at frequency ω+Δω to a target sample at α angle of incidence (step  502 ). The reflected HR beam is detected at a reference detector and a heterodyne measurement detector (step  504 ) and δ Ref/film  determined from I ref  and I het  signals from the respective detectors (step  506 ). The self-referencing mode processing is similar. An SR beam with p-polarization at frequency ω and p-polarization at frequency ω+Δω is propagated to a target sample at a angle of incidence (step  508 ). The reflected SR beam is detected at a reference detector and a heterodyne measurement detector (step  510 ) and δ Ref/Sub  determined from I ref  and I het  signals from the respective detectors (step  512 ). Next, temperature independent phase shift, Δφ film , attributable to the film is calculated from difference of δ Ref/Sub  and δ Ref/film  (step  514 ). Finally, the film thickness, d f , can be calculated from Δφ, n f , α and λ using, for example, Equation (4) (step  516 ). The refractive index, n f , for the particular film should be known beforehand. Alternatively, the measurement detector of the self-referencing heterodyne reflectometer can be augmented with a grating interferometer as disclosed in U.S. patent application Ser. Nos. 11/066,933 and 11/178,856, from which the refractive index, n f , for the film and be dynamically measured.
 2Δφ film =δ Ref/Sub −δ Ref/film   (7) 
     Where, Δφ film  is the phase shift due to the film layer. 
     Using Equation (7), the phase shift due to the film layer, Δφ film , is calculated by Δφ film  calculator  466  subsequent to each δ Ref/Sub  and δ Ref/film  measurement. Assuming the noise levels between successive measurements are the same (or sufficiently small), the thickness, d f , of film  214  can then be determined directly by d f  calculator  468  using Equation (4) above, with the refractive index for the particular film, n f , the wavelength, λ, of light source  400 , and the angle of incidence, α. 
     The level of detector noise may be monitored by comparing successive δ Ref/Sub  measurements for changes. Recall that δ Ref/Sub  is calculated from a self referencing beam that is unaffected by changes in film thickness, hence δ Ref/Sub  is also unaffected by changes in film thickness. From Equation (6) above, it is apparent that the value of δ Ref/Sub  will not change between successive δ Ref/Sub  measurements unless the level of detector noise changes. Therefore, the severity of the detector noise can be determined by comparing the change in successive δ Ref/Sub  measurements to a noise threshold. 
     Therefore, in accordance with another aspect of the present invention, detector noise is monitored and when the noise level is unacceptable, the Δφ film  is averaged over several measurement cycles. Returning to  FIGS. 4A-4C , threshold noise detector  465  monitors successive δ Ref/Sub  measurements from δ Ref/Sub  detector  462  and compares changes in the noise level to a threshold. If the level is below the threshold level, threshold noise detector  465  takes no action, but if the noise level is found to be higher than the acceptable noise threshold, δ Ref/Sub  detector  462  instructs Δφ film  averager  467  to average several or more cycles of Δφ film  data from Δφ film  calculator  466 , and output an averaged Δφ film(AVG)  to d f  calculator  468 . 
       FIG. 5  is a flowchart depicting the method for finding a temperature independent film thickness using a self-referencing heterodyne reflectometry in accordance with an exemplary embodiment of the present invention. The process begins by propagating HR beam with s-polarization at frequency ω and p-polarization at frequency ω+Δω to a target sample at a angle of incidence (step  502 ). The reflected HR beam is detected at a reference detector and a heterodyne measurement detector (step  504 ) and δ Ref/film  determined from I ref  and I het  signals from the respective detectors (step  506 ). The self-referencing mode processing is similar. An SR beam with p-polarization at frequency ω and p-polarization at frequency ω+Δω is propagated to a target sample at α angle of incidence (step  508 ). The reflected SR beam is detected at a reference detector and a heterodyne measurement detector (step  510 ) and δ Ref/Sub  determined from I ref  and I het  signals from the respective detectors (step  512 ). Next, temperature independent phase shift, Δφ film , attributable to the film is calculated from difference of δ Ref/Sub  and δ Ref/film  (step  514 ). Finally, the film thickness, d f , can be calculated from Δφ, n f , α and λ using, for example, Equation (4) (step  516 ). The refractive index, n f , for the particular film should be known beforehand. Alternatively, the measurement detector of the self-referencing heterodyne reflectometer can be augmented with a grating interferometer as disclosed in U.S. patent application Ser. Nos. 11/066,933 and 11/066,933, from which the refractive index, n f , for the film and be dynamically measured. 
       FIGS. 6A and 6B  diagrammatically illustrate the interactions between the HR beam and/or SR beam with the film and substrate.  FIG. 6A  shows the HR beam interactions and  FIG. 6B  depicts the SR beam interactions.  FIG. 6A  is essentially a composite of  FIGS. 3A and 3B  discussed above, and shows incident HR beam  403  comprised of two linearly polarized s- and p-polarized beam components that are orthogonal to each other, with split optical frequencies of ω and ω+Δω. The s-polarization component interacts with the surface of film  214  and is partially reflected as ray  405 - 1 . Ray  405 - 1  is almost completely s-polarization. Incident angle α is at the Brewster of film  214  for optimizing the reflected s-polarization component of ray  405 - 1 . On the other hand, the p-polarization component does not interact with the surface of film  214  and refracts from the interface between film  214  and substrate  212  at angle ρ as ray  405 - 2 . However, because some of the s-polarization component is also refracted, ray  405 - 2  comprises both s- and p-polarization components. Clearly, as the thickness of film  214  changes, the distance traversed by the HR beam (beam  403  and ray  405 - 1  will change and consequently the phase will also experience a corresponding change at the detector. 
       FIG. 6B  shows incident SR beam  433  comprised of two linearly polarized p- and p-polarized beam components, with split optical frequencies of ω and ω+Δω. With incident angle α approximating the Brewster of film  214 , only a minimal reflection of the p-polarized SR beam occurs at the surface of film  214 . Instead, incident SR beam  433  refracts into film  214  and reflects off the interface between film  214  and substrate  212  at angle ρ as ray  435 . Unlike the HR beam, the SR beam is unaffected by changes in the thickness of film  214  because the beam does not interact with the surface of the film. The beat signals generated by the SR beams can be used to obtain a reference phase value, which is equivalent to that obtained using a reference sample. This reduces the need to have periodic access to reference wafer. The availability of a reference phase, which is unaffected by changes in film thickness, but drifts with temperature by a corresponding amount as the measured film phase, allows for real-time phase drift corrections to the measured film phase. 
     In addition to compensating for temperature related phase drift and eliminating the necessity for calibration wafers, the availability of a reference phase also provides a mechanism for assessing detector noise. As mentioned above, temperature induced phase drift (or noise) from the detector can be assumed to be the same as for successive measurements and therefore can be canceled out. However, it is possible for the level of spurious noise in the detector to reach a level where this may not hold true. In that case, merely canceling the noise may provide an inferior result. In accordance with another exemplary embodiment of the present invention, the level of detector noise can be monitored in real-time, thereby providing a basis for implementing more rigorous noise reduction measures. 
       FIG. 7  is a flowchart depicting the method for identifying detector noise that may be resistive to the noise canceling in accordance with an exemplary embodiment of the present invention. The process begins by finding successive values of δ Ref/Sub  from successive SR beam measurements, i.e., δ Ref/Sub1  and δ Ref/Sub2  (step  702 ). Recall that reference phase δ Ref/Sub  is unaffected by changes in film thickness, but is affected by detector noise and drift with temperature. Phase drift, and other noise affecting the phase, is assumed to be negligible between successive measurements made in rapid succession. However, some noise may exist. If δ Ref/Sub1 −δ Ref/Sub2 =0, the detector noise and/or drift are negligible. However, where the phase difference between successive measurements is greater than 0, i.e., δ Ref/Sub1 −δ Ref/Sub2 &gt;0, some noise is present and, depending on the amount, should be suppressed. A noise threshold level can be adopted for a particular application, below which the results are acceptable and additional suppression is not necessary. Therefore, threshold noise detector  465  compares the phase difference between successive reference phase measurements to a noise threshold, i.e., |δ Ref/Sub1 −δ Ref/Sub2 |&gt;THRESHOLD (step  704 ). If the noise increment is below the threshold level, d f  calculations proceed by finding the film induced phase shift, Δφ film , from the measurement phase δ Ref/film  and the reference δ Ref/Sub  (step  706 ) and then the film thickness d f  from Δφ film , n f , α and λ (step  710 ). If, at step  704 , |δ Ref/Sub1 −δ Ref/Sub2 | is greater than the noise threshold, additional noise suppression procedures should be implemented. One exemplary procedure is to smooth the noise profile by averaging the result over several successive measurement cycles (step  708 ). Any of Δφ film , δ Ref/Sub  and δ Ref/film  or d f  can be averaged, but averaging Δφ film  or δ Ref/Sub  and δ Ref/film  can be accomplished in earlier stages of the process. In any case, film thickness d f  from Δφ film , n f , α and λ, albeit and averaged thickness (step  710 ). 
     The present invention, as depicted in  FIGS. 4A-4C , rapidly alternates between HR mode and SR modes for detecting δ Ref/film  phase measurement and δ Ref/Sub  reference phase by sliding polarizer/λ/2—aperture component in the path of the HR beam.  FIGS. 8A and 8B  are diagrams of a self referencing heterodyne reflectometer configured without moving optical components in accordance with an exemplary embodiment of the present invention.  FIG. 8A  shows the self referencing heterodyne reflectometer in HR mode for detecting a δ Ref/film  phase measurement and  FIG. 8B  depicts the reflectometer in SR mode for detecting δ Ref/Sub  reference phase. Much of the structure is similar to that discussed above with regard to  FIGS. 4A-4C , and therefore only the distinctions will be discussed in greater specificity. 
     In accordance with this exemplary embodiment, HR beam  802  is selectively propagated in an HR path and an SR path. Sliding shutter  809  selectively opens one path, while simultaneously closing the other. Slider controller  461  provides the operational control signals for repositioning sliding shutter  809 . In the HR mode the HR path is open with sliding shutter  809  blocking the SR path. HR beam  802  reflects off BS  812  as beam  804 , through polarizer  815 , to detector  816 , resulting in reference signal I ref . Incident HR beam  803 , the portion of HR beam  802  transmitted through BS  812 , and across optional reflective optical components  818  and  820 , interacts with film  214 , and on to detector  826 , via polarizer  822  and focusing optics  824 , as rays  805 - 1  and  805 - 2 . Slider controller  461  receives signals I ref  and I het  as described above, which are passed to δ Ref/film  detector  463  for detection of δ Ref/film  measurement phase. 
     In the SR mode, sliding shutter  809  blocks the HR path and opens the SR path. HR beam  802  from light source  800 , is deflected at BS  801  and reflected at optical component  828  to stationary polarizer/λ/2 combination  810 / 811  where SR beam  833  is formed. Recall that HR beam is a split frequency, linearly polarized where one polarization component at frequency ω is orthogonal with respect to the other polarization component at frequency ω+Δω. The SR beam is a split frequency, p-polarized beam. Incident SR beam  833  converges back to the path of the incident HR beam  803  at BS  807 . SR beam  833  reflects off BS  812  as beam  834  to detector  816 , resulting in reference signal I ref . The portion of SR beam  833  transmitted through BS  812 , interacts with film  214 , and on to detector  826  as ray  835 . Slider controller  461  receives signals I ref  and I het  as described above, which are passed to δ Ref/Sub  detector  462  for detection of δ Ref/Sub  measurement phase. 
     In this exemplary embodiment, a significant amount of light is lost and hence light source  800  should be selected to accommodate the loss of light. As a sidebar, the combination of beam splitters  801  and  807  with reflection components  828  and  829  suggest the look of a Mach Zehnder interferometer, but because self referencing beam  833  and the HR beam  803  are not used simultaneously, there is no optical interference between them and hence no finite fringe issue. Also, the different paths traveled by the beams before reaching BS  812  will have no effect on phase measurement since the phase differencing, between I Ref  and I het  signals, for each beam is accomplished after BS  812 . 
     The δ Ref/Sub  reference phase provides a reference from which accurate temperature independent film phase shifts, Δφ film , may be derived without the use of a reference wafer. It can be assumed that φ Sub1 ≈φ Sub2 ≈φ Subn  across a wafer, and therefore the HR and SR beam spots on a film need not be coextensive. Thus, the self referencing heterodyne reflectometer depicted in  FIGS. 8A and 8B  can be made significantly less lossy by separating the SR beam and HR beam paths. 
       FIGS. 9A and 9B  are diagrams of a self referencing heterodyne reflectometer with separate SR beam and HR beam paths in accordance with an exemplary embodiment of the present invention. The self referencing heterodyne reflectometer shown in  FIGS. 9A and 9B  is identical to that depicted in  FIGS. 8A and 8B  with the exception of the paths of the incident and reflected SR beams. Rather than utilizing a pair of beam splitters for diverging and collimating the separate beam paths, SR beam  933  propagates in an essentially parallel path to that of HR beam  903  via reflection optics  917  which replaces BS  807 , is out of line with BS  901 . HR beam  902  is deflected at BS  901  into the SR path to stationary polarizer/λ/2 combination  910 / 911  where SR beam  933  is formed, and on to reflection optics  917 . 
       FIGS. 10A and 10B  are diagrams of a self referencing heterodyne reflectometer with counter rotating SR and HR beam paths in accordance with an exemplary embodiment of the present invention. Two synchronous shutters are necessary for switching modes, one at either detector. As should be appreciated, because the HR and SR beams propagate in directions counter to each other, detector  1016  detects signal I ref  for HR beam  1004  and I het  for SR beam  1035 . Conversely, detector  1026  detects signal I ref  for SR beam  1034  and I het  for HR beam  1005 . In HR mode, light source  1000  generates HR beam  1002  beam is reflected at BS  1018  as incident HR beam  1003  and off BS  1041 , passed open shutter  1051 , through focusing optics  1015 , to detector  1016 . At BS  1041 , the transmitted portion of beam  1003  propagates in a counter clockwise direction (with respect to  FIGS. 10A and 10B ) off optical reflector  1020  to film  214  and reflected HR beam  1005  continues off optical reflector  1021 , through BS  1023  and passed open shutter  1052 , through polarizer  1022  and focusing optics  1024 , to detector  1026 . In SR mode, HR beam  1002  beam is transmitted through BS  1018  to polarizer/λ/2  1010 / 1011  and converted to p-polarized, split frequency SR beam  1033 . At BS  1023  the transmitted portion of SR beam  1033  is turned at corner cube  1050  to BS  1023  and passed open shutter  1052  to detector  1026 . At BS  1023  the reflected portion of SR beam  1033  propagates in a direction counter to HR beam  1003  off optical reflector  1021  to film  214  and reflected SR beam  1035  continues off optical reflector  1020 , reflecting off through BS  1042  and passed open shutter  1051  to detector  1016 . 
       FIGS. 11A and 11B  are diagrams of a self referencing heterodyne reflectometer employing a liquid crystal variable retarder (LCVR) for electronically switching between HR and SR operating modes in accordance with an exemplary embodiment of the present invention. The configuration of self referencing heterodyne reflectometer shown in  FIGS. 11A and 11B  is similar to that depicted in  FIGS. 8A and 8B  with the exception of polarizing BS  1119  in the HR beam path and LCVR  1111  in the SR beam path but operationally is quite different. LCVR  1111  is a device, which causes the polarization of a light beam to be rotated by an angle, which is dependent upon the voltage applied to it. When the retarder is set so that the polarization is not rotated, the device operates as a heterodyne interferometer as previously described. When the retarder is set to rotate the polarization by 90°, then the beams at both frequencies are p-polarized, and the SR function is obtained. In this embodiment, the amount of light lost is reduced. On the other hand, the path between PBS  1119  and BS  1107  acts as a Mach Zehnder interferometer. 
     In the HR mode, HR beam  1102  is separated into p- and s-polarization components at polarizing beam splitter PBS  1119 , the p-polarization component (at frequency ω+Δω) propagates as beam  1103  and the s-polarization component (at frequency ω) propagates as beam  1133 . Beam  1103  reflects off BS  1112  as beam  1104  to detector  1116 . Incident HR beam  1103 , the portion of HR beam  1102  transmitted through BS  1112 , interacts with film  214 , and on to detector  1126  as rays  1105 . The s-polarization component of HR beam  1102  passes through LCVR  1111 , which is switched OFF in HR mode, as beam  1133 . Beam  1133  reflects off BS  1107  and also reflects off BS  1112  as beam  1134  to detector  1116 . HR beams  1134  and  1104  result in reference signal I ref . The portion of beam  1133  transmitted through BS  1112  interacts with film  214 , and on to detector  1126  as rays  1135 - 1  and  1135 - 2 . Taken together, beams  1103  and  1133  are HR. Reflected HR beam components  1105 ,  1135 - 1  and  1135 - 2  result in heterodyne measurement signal I het . 
     In the SR mode, HR beam  1102  is separated into p- and s-polarization components at polarizing beam splitter PBS  1119 , with the p-polarization component (at frequency ω+Δω) propagating as beam  1103  as described above. The s-polarization component of HR beam  1102  is transformed into p-polarized beam  1133  by LCVR  1111 , which is ON in SR mode. Beam  1133  reflects off BS  1107  and again off BS  1112  as beam  1134  to detector  1116 . Beams  1134  and  1104  result in reference signal I ref  for the SR mode. The portion of beam  1133  transmitted through BS  1112  interacts with film  214 , and on to detector  1126  as ray  1135 . Reflected SR beam components  1105  and  1135  result in heterodyne measurement signal I het . 
       FIGS. 12A and 12B  are diagrams of a self referencing heterodyne reflectometer in which the SR beam bypasses the sample for addressing the sole issue of detector phase drift in accordance with an exemplary embodiment of the present invention. To remove the effects of phase drift from the system, the split frequency light is measured in an alternating fashion by the detector  1226 . In the HR mode, HR beam  1202  is reflected as HR beam  1203  by beam splitter  1218 . Beam  1203  reflects off BS  1212  as beam  1204  to detector  1216 , resulting in reference signal I ref  for the HR mode. Incident HR beam  1203  interacts with film  214 , passing open shutter  1217  and on to detector  1226  as rays  1205  (actually  1205 - 1  and  1205 - 2 ), resulting in heterodyne measurement signal I het  for the HR mode. The portion of beam  1202  transmitted through BS  1218  is blocked by shutter  1209 . In wafer measurement mode (SR mode) shutters  1209  and  1217  reverse their positions with shutter  1217  blocking reflected HR beam components  1205 , and shutter  1209  in the open position. Beam  1205  transmitted through BS  1218  is measured at detector  1226 , while reflected beam  1204  is measured at detector  1216 . This measurement allows the ability to determine the phase offset between each detector just prior to or after each measurement of the wafer. 
     The present invention, is directed to a self-referencing heterodyne reflectometer which rapidly alternates between HR mode and SR modes for detecting a δ Ref/film  phase measurement and a δ Ref/Sub  reference phase. The exemplary embodiment depicted in  FIGS. 4A-4C  utilizes a sliding polarizer/λ/2—aperture component in the path of the HR beam to create the SR beam. In accordance with another embodiment of the present invention, a self-referencing heterodyne reflectometer is disclosed that alternates between HR mode and SR modes by propagating the HR beam in a separate path from the SR beam, thereby allowing the HR and SR beams to be separately controlled. In accordance with the exemplary embodiment disclosed in  FIGS. 8A and 8B , a sliding shutter, under the control of a slider controller, is employed to alternate the beams on the target, between measuring δ Ref/film  phase measurement with the HR beam and measuring δ Ref/Sub  with the SR beam (of course controller  461  also routes the I het  and I ref  signals to either of δ Ref/film  phase detector  463  or δ Ref/Sub  phase detector  462  depending on which beam is incident on the target). As a practical matter, however, sliding shutters are somewhat slow which results in a relatively long cycle time for calculating of Δφ film  data. As mentioned elsewhere above, error in the Δφ m  measurement is the detector error due to the effect of temperature on the detectors. Thus, the detector temperature error is greater with a longer measurement cycle and, consequently, can be reduced by shortening the measurement cycle. 
     Therefore, in accordance with another exemplary embodiment of the present invention, a high frequency optical switch is employed for rapidly alternating between the HR beam and the SR beam in the measurement cycle. One such optical switch is a rotating chopper. Rotating optical choppers are well known in the related art as a metal disc with slots etched into it and is mounted on a drive axle and rotated. The disc is placed in the beam path which causes the beam to be periodically interrupted by the blocking part of the disc. Thus, the measurement beam can rapidly switch from HR mode to SR mode, and vice versa, thereby greatly reducing the time period for the detector temperature to drift and thereby prevent unwanted temperature induced error in the phase measurements. It should be understood that although the embodiments of the present invention below are described with reference to a rotating optical chopper, the chopper is merely an exemplary device for switching the heterodyne reflectometer between HR mode and SR modes at a higher rate than temperature change in the detector. In so doing, any error in the phase measurement due to detector temperature will be comparable in consecutive HR and SR measurements and effectively cancel out in the phase calculations. Those of ordinary skill in the art will readily understand that other optical switching devices may exist and/or will exist in the future that are equivalent to a mechanical chopper for the purposes discussed hereinabove. 
     In this regard,  FIG. 13A  and  FIG. 13B  show an exemplary self referencing heterodyne reflectometer which employ a mechanical or electrical-electromagnetic device for switching between the HR and SR beams, or vice versa, at a faster rate than the temperature drift in a detector. The figures illustrate the operational modes of an exemplary self referencing heterodyne reflectometer which is similar to that discussed above with respect to  FIGS. 8A and 8B .  FIG. 13A  shows the self referencing heterodyne reflectometer in HR mode for detecting a δ Ref/film  phase measurement and  FIG. 13B  depicts the reflectometer in SR mode for detecting δ Ref/Sub  reference phase. Much of the structure is similar to that discussed above with regard to  FIGS. 4A-4C , as well as  FIGS. 8A and 8B  and therefore only the distinctions will be discussed in greater specificity. 
     In accordance with this exemplary embodiment, HR beam  1302  is selectively propagated in an HR path and an SR path. High frequency optical switch  1309  selectively opens one path, while simultaneously closing the other. Optical switch  1309  is depicted in the figure as a pair of rotating optical choppers, one positioned in the path of HR beam  1303  and the other positioned in the path of SR beam  1333 . Optical switches  1309  are out of phase such that as the path closes, completing the first part of the measurement cycle, the opposite paths opens for completion of the entire measurement cycle. Alternatively, a single optical switch, such as a rotating chopper, could be positioned across both beam paths, with the opening slots out of phase with respect to the beams. As discussed with regard to  FIGS. 8A and 8B  above, chopper controller  1361  provides the operational control signals for repositioning (rotating) optical choppers  1309  and synchronizes them to the measurement cycle if the self referencing heterodyne reflectometer is configured with a pair of choppers rather than a single chopper. In the HR mode, depicted in  FIG. 13A , the HR path is open with chopper  1309  blocking the SR path. HR beam  1303  reflects off BS  1312  as beam  1304  to detector  1316 , resulting in reference signal I ref . Incident HR beam  1303 , the portion of HR beam  1303  transmitted through BS  1312 , interacts with film  214 , and on to detector  1326  as rays  1305 - 1  and  1305 - 2 . Chopper controller  1361  receives signals I ref  and I het  as described above, which are passed to δ Ref/film  detector  463  for detection of δ Ref/film  measurement phase. 
     In the SR mode, depicted in  FIG. 13B , chopper  1309  blocks the HR path and in so doing aligns an opening slit with the SR path, thereby opening the SR path. HR beam  1302  from light source  1300 , is deflected at BS  1301  and reflected at optical component  1328  to stationary polarizer/λ/2 combination  1310 / 1311  where SR beam  1333  is formed. Recall that HR beam is a split frequency, linearly polarized where one polarization component at frequency ω is orthogonal with respect to the other polarization component at frequency ω+Δω. The SR beam is a split frequency, p-polarized beam. Incident SR beam  1333  converges back to the path of the incident HR beam  1303  at BS  1307 . SR beam  1333  reflects off BS  1312  as beam  1334  to detector  1316 , resulting in reference signal I ref . The portion of SR beam  1333  transmitted through BS  1312 , interacts with film  214 , and on to detector  1326  as ray  1335 . Chopper controller  1361  receives signals I ref  and I het  as described above, which are passed to δ Ref/Sub  detector  462  for detection of δ Ref/Sub  measurement phase. 
     In this exemplary embodiment, a significant amount of light is lost and hence light source  1300  should be selected to accommodate the loss of light. As a sidebar, the combination of beam splitters  1301  and  1307  with reflection components  1328  and  1329  suggest the look of a Mach Zehnder interferometer, but because self referencing beam  1333  and the HR beam  1303  are not used simultaneously, there is no optical interference between them and hence no finite fringe issue. Also, the different paths traveled by the beams before reaching BS  1312  will have no effect on phase measurement since the phase differencing, between I Ref  and I het  signals, for each beam is accomplished after BS  1312 . 
     The δ Ref/Sub  reference phase provides a reference from which accurate temperature independent film phase shifts, Δφ film , may be derived without the use of a reference wafer. It can be assumed that φ Sub1 ≈φ Sub2 ≈φ Subn  across a wafer, and therefore the HR and SR beam spots on a film need not be coextensive. Thus, the self referencing heterodyne reflectometer depicted in  FIGS. 13A and 13B  can be made significantly less lossy by separating the SR beam and HR beam paths. 
     Although not depicted in a figure, the self referencing heterodyne reflectometer shown in  FIGS. 9A and 9B  may also employ a high frequency optical switch, such as a rotating chopper, for generating parallel path HR and SR beams. 
       FIGS. 14A and 14B  are diagrams of a self referencing heterodyne reflectometer with counter rotating SR and HR beam paths which utilize a high frequency optical switch to minimize error in the phase measurement resulting from changes in the temperature of the detectors in accordance with an exemplary embodiment of the present invention. The self referencing heterodyne reflectometer depicted in  FIGS. 14A and 14B  is identical to that shown in  FIGS. 10A and 10B , with the exception of the optical switches. Two synchronous choppers are necessary for rapidly switching modes, one at either detector, as shown both choppers have two beam channels, one for regulating the HR path and the other for the SR path (in so doing the beams can be incident the respective chopper at an identical azimuth but in different optical channels). As discussed above with regard to  FIGS. 10A and 10B , the HR and SR beams propagate in directions counter to each other and, therefore, detector  1416  detects signal I ref  for HR beam  1404  and I het  for SR beam  1435 . Detector  1426  detects signal I ref  for SR beam  1434  and I het  for HR beam  1405 . In HR mode, choppers  1451  and  1452  rotate open with respect to HR beam  1404  and HR beam  1405 , respectively, thereby allowing the beams to propagate incident to detector  1416  and detector  1426 , respectively. Choppers  1451  and  1452  simultaneously rotate closed with respect to SR beam  1435  and SR beam  1434 . At the close to the HR mode, the HR portion of the measurement cycle, choppers  1451  and  1452  rotate closed with respect to HR beam  1404  and HR beam  1405  and open to SR beam  1435  and SR beam  1434 . SR beam  1435  and SR beam  1434  propagate incident to detector  1416  and detector  1426 , which produce I het  and I ref  signals, but at the opposite detectors from the HR mode. 
       FIGS. 15A and 15B  are diagrams of a self referencing heterodyne reflectometer in which the SR beam bypasses the sample for addressing the sole issue of detector phase drift in accordance with an exemplary embodiment of the present invention. Although the use of high frequency optical switches reduce the amount of error resulting from temperature related detector drift, some phase drift is inevitable, the effects of which can be measured as discussed above with regard to  FIGS. 12A and 12B . 
     Although the self referencing heterodyne reflectometer embodiments discussed above are highly accurate and stable, they suffer from two shortcomings. First, in an effort to reduce the disparity in the measurement between the SR beam and the HR beam, each of the previous embodiments use a single light source for generating the SR and HR beams. Thus, the strength of the beam is reduced by half for either operation mode (with the exception of those using a shutter which suffer from other shortcomings discussed immediately above). Furthermore, and secondly, the use of parallel SR and HR beam paths increases the complexity of the setup and alignment. These and other shortcoming are overcome by the use of a second light source in conjunction with an amplitude modulator, for generating an amplitude modulated (AM) reference beam. Modulating the amplitude of the independently generated light beam between the two modulated amplitudes of α and α+Δα results in reference and heterodyne signals that allow for accurate phase detections regardless of the path distances to the separate detector, similar to that discussed above for using HR beam components at frequencies ω and ω+Δω. 
       FIGS. 16A and 16B  are diagrams of a self referencing heterodyne reflectometer which utilizes an amplitude modulated beam as a reference in conjunction with the HR beam in accordance with an exemplary embodiment of the present invention. The self referencing heterodyne reflectometer described herein employs a chopper  1609  for switching between HR and amplitude modulated (AM) operating modes in accordance with an exemplary embodiment of the present invention. The configuration of the HR path in the self referencing heterodyne reflectometer shown in  FIGS. 16A and 16B  is similar to that depicted in  FIGS. 13A and 13B , however the SR beam path is replaced by an amplitude modulated (AM) beam. In accordance with this exemplary embodiment, light source  1600  generates HR beam  1602  having two linearly polarized components, operating at split optical angular frequencies, that are orthogonal with respect to each other for illuminating the target; an s-polarized beam component at frequency ω and a p-polarized beam component at frequency ω+Δω. HR beam  1602  is propagated in only an HR path. In further accordance with this exemplary embodiment, light source  1611  generates a beam that&#39;s amplitude is modulated by amplitude modulator  1613 , resulting is AM beam  1601  having a single frequency, ω′, at two modulated amplitudes of α and α+Δα. The frequency, ω′ may be different from the frequency of the HR beam, ω, however, amplitude modulator  1613  oscillates the amplitudes operates at approximately the frequency of the HR beam, ω. Furthermore, although the AM beam is depicted as a p-polarized beam, it need only have a p-component. 
     Chopper  1609  selectively opens one path, while simultaneously closing the other. A chopper controller (not shown) provides the operational control signals for the chopper  1609  and detector signal paths (see discussion of  FIGS. 4 and 8 ). In the HR mode (depicted in  FIG. 16A ), the HR path is open with chopper  1609  blocking the AM beam path. HR beam  1602  passes through BS  1612  as beam  1604  to detector  1616 , resulting in reference signal I ref . HR beam  1603 , the portion of HR beam  1602  reflecting off BS  1612 , interacts with film  214 , and on to detector  1626  as rays  1605 - 1  and  1605 - 2  resulting in reference signal I ref . δ Ref/film  phase measurement is detected from signals I ref  and I het  of the HR beam in the same manner as described above. 
     In the AM mode (depicted in  FIG. 16B ), chopper  1609  blocks the HR path and opens the AM path. The amplitude of the beam from light source  1601 , is modulated at amplitude modulator  1613  and a portion deflected at BS  1612  as beam  1634  to detector  1616 , resulting in reference signal I ref . The portion of AM beam  1603  that passes through BS  1612 , interacts with film  214 , and on to detector  1626  as ray  1635 , resulting in signal I het . δ Ref/Sub  phase measurement is detected from signals I ref  and I het  of the AM beam in the same manner as described above with respect to the SR beam. Phase φ film  and the film thickness, d f , are derived from δ Ref/film  phase and δ Ref/Sub  phase as also discussed above. 
     The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.