Abstract:
An interferometer system is disclosed. The interferometer system includes two spaced apart reference flats having corresponding reference surfaces forming a cavity therebetween for placement of a polished opaque plate. The surfaces of the plate are approximately 2.5 millimeters or less from the corresponding reference surfaces when the plate is placed in the cavity. The interferometer system also includes two interferometer devices located on diametrically opposite sides of the cavity to map the surfaces of the plate. A light source is optically coupled to the interferometer devices. The light source includes an illuminator configured for producing light of multiple wavelengths and an optical amplitude modulator configured for stabilizing power of the light produced by the illuminator. The interferometer system further includes two interferogram detectors, and at least one computer coupled to receive the outputs of the interferogram detectors for determining thickness variations of the plate.

Description:
TECHNICAL FIELD 
       [0001]    The disclosure generally relates to the field of measuring technology, particularly to a method and apparatus for measuring the shape and thickness variation of a wafer. 
       BACKGROUND 
       [0002]    Thin polished plates such as silicon wafers and the like are a very important part of modern technology. A wafer, for instance, refers to a thin slice of semiconductor material used in the fabrication of integrated circuits and other devices. Other examples of thin polished plates may include magnetic disc substrates, gauge blocks and the like. While the technique described here refers mainly to wafers, it is to be understood that the technique also is applicable to other types of polished plates as well. 
         [0003]    Generally, certain requirements may be established for the flatness and thickness uniformity of the wafers. There exist a variety of techniques to address the measurement of shape and thickness variation of wafers. One such technique is disclosed in U.S. Pat. No. 6,847,458, which is capable of measuring the surface height on both sides and thickness variation of a wafer. It combines two phase-shifting Fizeau interferometers to simultaneously obtain two single-sided distance map between each side of a wafer and corresponding reference flats, and computes thickness variation and shape of the wafer from the data and calibrated distance map between two reference flats. However, it is noted that sensitivity to wafer vibration in such a measurement system need to be further improved. Other shortcomings of this technique may include the lack of the ability to provide absolute wafer thickness information, and that the system is physically large for measuring larger wafers. 
         [0004]    Another technique is disclosed in U.S. Pat. No. 7,009,696, which is also able to measure the surface height on both sides and thickness variation of a wafer. It combines two grazing incidence interferometers, simultaneously obtains front and backside topography data and stitches multiple measurements of portions of the wafer together to form full wafer topography data maps. The thickness variation and shape of the wafer from may then be computed based on the topography data maps. However, this measurement system has a long, non-common optical path length between object and reference which makes it susceptible to air temperature gradients (air turbulences). It also lacks the ability to provide absolute wafer thickness information. In addition, the damping arrangement utilized in this system does not cover the entire surface area of the wafer and is applied only on one side of the wafer. 
         [0005]    A further technique is disclosed in U.S. patent application Ser. No. 12/388,487, the disclosure of which is incorporated herein by reference in its entirety, improves the vibration damping of the system disclosed in the U.S. Pat. No. 6,847,458 (described above). However, the measurement accuracy and precision may still be further improved. In addition, the optical design of the system results in a larger physical profile, which may not be desirable in various locations such as manufacturing facilities, labs or the like. Furthermore, the system does not combine an active and passive vibration isolation with the small gap air damping design so that floor vibration will affect the system performance. 
         [0006]    Therein lies a need for a method and apparatus for measuring the shape and thickness variation of a wafer, without the aforementioned shortcomings. 
       SUMMARY 
       [0007]    The present disclosure is directed to an interferometer system. The interferometer system includes first and second spaced apart reference flats having corresponding first and second parallel reference surfaces forming a cavity therebetween for placement of a polished opaque plate. The first and second surfaces of the polished opaque plate are approximately 2.5 millimeters or less from the corresponding first and second reference surfaces of the first and second reference flats when the polished opaque plate is placed in the cavity. The interferometer system also includes first and second interferometer devices located on diametrically opposite sides of the cavity to map the opposite first and second surfaces of the polished opaque plate. A light source is optically coupled to the first and second interferometer devices. The light source includes an illuminator configured for producing light of multiple wavelengths and an optical amplitude modulator configured for stabilizing power of the light produced by the illuminator. The interferometer system further includes first and second interferogram detectors, and at least one computer coupled to receive the outputs of the first and second interferogram detectors for determining thickness variations of the plate. 
         [0008]    Furthermore, the interferometer system may include a vibration control unit configured for providing active vibration isolation for at least the first and second reference flats, the first and second interferometer devices and the first and second interferogram detectors. The interferometer system may also include a temperature controlled enclosure configured for providing a thermally stable environment for at least the first and second reference flats, the first and second interferometer devices and the first and second interferogram detectors. The interferometer system may further include a wafer handling mechanism configured for handling and transferring the polished opaque plate in and out of the cavity. In addition, the optical paths in the interferometer devices may be folded to reduce the physical size of the interferometer devices. 
         [0009]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the present disclosure. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate subject matter of the disclosure. Together, the descriptions and the drawings serve to explain the principles of the disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The numerous advantages of the disclosure may be better understood by those skilled in the art by reference to the accompanying figures in which: 
           [0011]      FIG. 1  is a diagrammatic representation of an interferometer system for measuring shape and thickness variation of a wafer according to an embodiment of the present invention; and 
           [0012]      FIG. 2  is a flow diagram illustrating a method for measuring the shape and thickness variation of a wafer utilizing the interferometer system shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. 
         [0014]    Silicon wafers are available in a variety of sizes. Semiconductor fabrication plants (also known as fabs) are defined by the size of wafers that they are tooled to produce. The size has gradually increased to improve throughput and reduce cost with the current standard considered to be 300 mm diameter. The next standard is projected to be 450 mm or even greater. It is noted that addressing the measurement of shape and thickness variation of such larger wafers is not a simple scale up tool of the existing measurement systems. As the size of the wafer increases, simply scaling up an existing tool makes the resulting tool more expensive (to produce and/or operate), physically larger, harder to transport and more sensitive to vibrations. 
         [0015]    The present disclosure is directed to a method and apparatus for measuring a large size (e.g., greater than 300 mm diameter), thin opaque plate (e.g., a silicon wafer) for its shape, surface height on both sides and thickness variations. The apparatus in accordance with the present disclosure also utilizes an improved interferometric system for profiling both sides of a wafer simultaneously and computing the wafer thickness variation that is independent of the shape of a reference plate. Various other features of the method and apparatus in accordance with the present disclosure will be described in details. 
         [0016]    Referring to  FIG. 1 , a block diagram depicting the measurement system  100  in accordance with the present disclosure is shown. The measurement system  100  in accordance with the present disclosure utilizes two Fizeau interferometers similar to that disclosed in U.S. Pat. No. 6,847,458 (the disclosure of which is incorporated herein by reference in its entirety). However, the measurement system  100  in accordance with the present disclosure differs from that disclosed in U.S. Pat. No. 6,847,458 in various ways in order to achieve high precision and high accuracy measurements for larger wafers (e.g., 450 mm or greater). 
         [0017]    As depicted in  FIG. 1 , the measurement system  100  is configured for measuring the shape and thickness of a wafer  60 . The wafer  60  may be placed in a cavity in the center between two Fizeau interferometers  20  and  40 . The reference flats  32  and  52  of the interferometers are placed very close to the wafer  60  in accordance with the present disclosure. In one embodiment, the distance between the reference flat surfaces  33  and  53  and the wafer surfaces  61  and  62  is less than 2.5 mm, respectively. 
         [0018]    The measurement system  100  provides two light sources for Channel A and Channel B through fiber  22  and fiber  42  from a single illuminator  8  that generates a constant power output during its wavelength tuning. In one embodiment, the light source  24 , 44  provides light that passes through a quarter-wave plate  28 , 48  aligned at 45° to the polarization direction of light after it is reflected from the polarizing beam splitter  26 , 46 . This beam then propagates to the lens  30 , 50 , where it is collimated with a beam diameter larger than the wafer diameter. 
         [0019]    The beam then goes through transmission flat  32 , 52 , where the central part of the transmitted beam is reflected at the test surface  61 , 62  that forms an interferogram with the light beam reflected from the reference surface  33 , 53 . The outer part of the transmitted beam travels on to the opposite reference flat  52 , 32 , where it is reflected at the reference surface  53 , 33  that forms an annular shape interferogram with the light beam reflected from the reference surface  33 , 53 . An interferogram detectors (e.g., an imaging device such as a camera or the like)  36 , 56  is utilized to record the interferograms and send the interferograms to a computer  38 , 58  for processing to produce the desired information such as the shape and the thickness variation of a wafer. 
         [0020]    In one embodiment, the distance between the reference surfaces  33  and  53  and the measuring wafer surfaces  61  and  62  is approximately 1.8 mm, respectively; the wavelength of the light provided by the light sources  24  and  44  is approximately 639 nm; the diameter of each reference flat  32  and  52  is approximately 480 mm; and each imaging device  36  and  56  has a resolution of about 4 k by 4 k pixels. However, it is understood the configuration described above is merely exemplary. Various components of the measurement system  100  may be configured differently without depart from the spirit and scope of the present disclosure. 
         [0021]    Reducing the distance between the reference flat surface  33 , 53  and the wafer surface  61 , 62  to less than 2.5 mm provides several advantages. For instance, the reduced distance damps down vibration that becomes much more serious due to the increased size of the wafer  60 . In addition, the reduced distance enables the measurement system  100  to compute the absolute wafer thickness based on optical images obtained by the imaging system  36 , 56 . 
         [0022]    In addition to the reduced distance between the flat and wafer surfaces, the measurement system  100  in accordance with the present disclosure also folds the optical paths in a different way to reduce the physical dimensions of the measurement system  100 . For instance, in order to minimize the ray-tracing error in Fizeau interferometer design, a predetermined optical distance (e.g., about 1 meter) may be required between the quarter-wave plate  28 , 48  and the lens  30 , 50 . In one embodiment, in order to reduce the overall physical size of the measurement system  100 , a plurality of mirrors may be positioned between the quarter-wave plate  28 , 48  and the lens  30 , 50  and may be utilized to fold the optical path between the quarter-wave plate  28 , 48  and the lens  30 , 50 . Utilizing such a folded optical path reduces the physical dimension of the measurement system  100  while still maintaining the optical distance between the quarter-wave plate  28 , 48  and the lens  30 , 50 . 
         [0023]    While placing the reference flat surfaces close to the measurement wafer passively damp down wafer vibrations, it is contemplated that the measurement system  100  may be further equipped with an active vibration isolation mechanism to minimize the system sensitivity to noises and vibration. For instance, the metrology unit that encloses both Fizeau interferometers  20  and  40  may be placed on a vibration control unit that have both active and passive damping capability to isolate the entire metrology unit from seismic and acoustic vibration. In this manner, the vibration isolation for the measurement system  100  is enhanced by integrating active isolations with passive isolations. Such a configuration may be appreciated since it does not require the user to provide an external isolated platform to support the measurement system  100 . 
         [0024]    It is also contemplated that the measurement system  100  may be placed in a temperature controlled enclosure. Such an enclosure is capable of providing a thermally stable environment for the measurement system  100  to operate within, enhancing the accuracy of the measurements obtained thereof. For instance, in a particular embodiment, the temperature in the thermally stable environment for the measurement system  100  may range between ±0.1° C. 
         [0025]    It is further contemplated that an automated wafer handling mechanism  11  may be employed for handling and transferring the wafer  60  in and out of the cavity between the transmission flats  32  and  52 . Such an automated wafer handling mechanism  11  may be realized in the form of a single or multi-deck mechanical arm or the like, which may be fully automated for rapidly and precisely handling wafers without human interaction and/or exposures to potential contaminants. 
         [0026]    Furthermore, it is understood that as the cavity between the transmission flats  32  and  52  becomes smaller (i.e., the distances between the reference flat surface  33 , 53  and the wafer surface  61 , 62  become smaller), the wavelength tuning range of the illuminator  8  (i.e., a tunable laser source) need to be increased accordingly in order to achieve the desired phase-shifting between adjacent frames during data acquisition. However, increasing the tuning range of the illuminator  8  inherently results in a large light power variation that may deteriorate the measurement result. In one embodiment, an external Optical Amplitude Modulator (OAM)  9  is positioned at the output of the illuminator  8  to modify the light power as the light goes through. In this manner, the OAM  9  stabilizes the output of the illuminator  8 , and therefore minimizes the light power variation due to the increased wavelength tuning range. 
         [0027]    Referring now to  FIG. 2 , a method  200  for measuring the shape and thickness variation of a wafer utilizing the measurement system  100  described above is shown. Step  202  may first calibrate the measurement system. For instance, the phase shifting speed of the interferograms in the two interferometer channels may be calibrated. The phase shifting speed may be calibrated by placing a polished opaque plate in the cavity between the reference flats  32  and  52 . Alternatively, this calibration may be conducted by the cavity itself (without the polished opaque plate). Upon completion of the phase shift calibration, or when the phase shift between any adjacent frames is within ±1 degree or less of its expected value such as 90 degrees for the phase shift between any adjacent frames, the cavity characteristics of the reference flats  32  and  52  may then be calibrated with the cavity itself. 
         [0028]    Once the measurement system is calibrated, the wafer  60  that is to be measured may be placed in the cavity in step  204 . The wafer  60  may be placed in between the two Fizeau interferometers  20  and  40  (more specifically, between the reference flats  32  and  52 ). A holding container may be utilized to removably secure the wafer  60  when the wafer  60  is placed in the cavity. The holding container may be configured in a manner such that both wafer sides  61  and  62  are minimally obscured by the holding container. While it may be beneficial to place the wafer  60  in the center of the cavity (i.e., the distance between the reference surface  33  and  61  is substantially equal to the distance between the reference surface  53  and  62 ), such a placement is not required. It is contemplated that if the wafer  60  is placed in an off-center position and/or rotated from its expected position inside the cavity, image processing algorithms associated with the imaging systems  36  and  56  may be utilized to compensate for such an off-center placement and/or rotation. 
         [0029]    Step  206  may then acquire two sets of intensity frames that record interferograms in Channel A and Channel B with different phase shifts by varying the wavelength of the light source  8 . Step  208  may extract phases and phase shifts of interferograms from these intensity frames, and step  210  may compute the shape and thickness information based on the phases and phase shifts of interferograms extracted in step  208 . In one embodiment, the shape and thickness information may be computed in a manner similar to that disclosed in U.S. Pat. No. 6,847,458. For instance, let A denote the phase of interferogram formed by reference flat  32  and wafer surface  61 , let B denote the phase of interferogram formed by the reference flat  53  and wafer surface  62 , and let C denote the phase of interferogram formed by the cavity of two reference flats  32  and  53 . Thus A provides information regarding the height of the wafer surface  61 , B provides information regarding the height of the wafer surface  62 , and C−(A+B) provides information regarding the thickness variation of the wafer  60 . 
         [0030]    It is contemplated that steps  206  through  210  may be carried out multiple times in order to increase the precision and accuracy of the measurement result. The number of iterations to be performed may be customized to meet requirements demanded by different users and/or for different types of wafers. 
         [0031]    The phase shifts of interferograms extracted in step  208  also allows the absolute wafer thickness to be calculated in step  212 . In one embodiment, the absolute thickness of a particular location of the wafer  60  is computed from the amount of phase shift per known wavelength change. For instance, let A denote the phase shift of interferogram formed by reference flat  32  and wafer surface  61  during the data acquisition, let B denote the phase shift of interferogram formed by the reference flat  53  and wafer surface  62  during the data acquisition, and let C denote the phase shift of interferogram formed by the cavity of two reference flats  32  and  53  during the data acquisition. Thus A provides the absolute distance between reference flat  32  and wafer surface  61  at that particular location, B provides the absolute distance between reference flat  52  and wafer surface  62  at that particular location, C provides the absolute distance between reference flat  33  and  53 , and C−(A+B) gives the absolute wafer thickness for that particular location. 
         [0032]    It is understood, however, that step  212  is optional. The absolute thickness of a particular location of the wafer  60  may be computed utilizing other techniques not particularly based on phase shift per known wavelength change as described above. 
         [0033]    It is contemplated that the method and apparatus in accordance with the present disclosure may be utilized for measuring large wafers (e.g., 450 mm or larger) without merely scale up an existing system. An improved interferometric system is provided for profiling both sides of a wafer simultaneously and computing the wafer thickness variation that is independent of the shape of interferometric reference plates. The apparatus in accordance with the present disclosure also employs a light source that keeps a constant power output during its long-range, without the need for mode-hop wavelength tuning. The external Optical Amplitude Modulator utilized to stabilize the output of the light source minimizes the light power variation and increases the measurement precision and accuracy. 
         [0034]    The method and apparatus in accordance with the present disclosure provides several advantages over existing measurement techniques. For instance, folding the optical path and minimizing the non-common paths of interferometers reduces the physical size of the apparatus. In addition, reduced cavity distance with a constant power wavelength tuning minimizes measurement errors and improves wafer vibration damping. The reduced cavity distance also enables high precision phase shift extraction, making the measurement of the absolute thickness based on optical information possible as described above. Furthermore, the measurement system in accordance with the present disclosure may be enclosed in a thermally stable enclosure and/or equipped with an active vibration isolation mechanism to further improve its measurement repeatability. A fully automated wafer handling mechanism may also be employed for precisely handling wafers without human interaction and/or exposures to potential contaminants. 
         [0035]    It is to be understood that the present disclosure may be implemented in forms of a software/firmware package. Such a package may be a computer program product which employs a computer-readable storage medium/device including stored computer code which is used to program a computer to perform the disclosed function and process of the present disclosure. The computer-readable medium may include, but is not limited to, any type of conventional floppy disk, optical disk, CD-ROM, magnetic disk, hard disk drive, magneto-optical disk, ROM, RAM, EPROM, EEPROM, magnetic or optical card, or any other suitable media for storing electronic instructions. 
         [0036]    The methods disclosed may be implemented as sets of instructions, through a single production device, and/or through multiple production devices. Further, it is understood that the specific order or hierarchy of steps in the methods disclosed are examples of exemplary approaches. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the method can be rearranged while remaining within the scope and spirit of the disclosure. The accompanying method claims present elements of the various steps in a sample order, and are not necessarily meant to be limited to the specific order or hierarchy presented. 
         [0037]    It is believed that the system and method of the present disclosure and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components without departing from the disclosed subject matter or without sacrificing all of its material advantages. The form described is merely explanatory.