Abstract:
A method and apparatus is provided to identify material boundaries and assist in the alignment of pattern masks in semiconductor fabrication. The invention probes a layer step or feature edge of an individual wafer using spectroscopic reflectance to detect a change in the reflectance spectral response. In integrated circuit fabrication, a wafer is subjected to wafer fabrication processes to produce a number of individual layers on a semiconductor substrate. During processing a reflectometer, a light emitting and collecting device, emits a specific range of electromagnetic wavelengths which are reflected from the wafer surface. The intensity of the reflected light is monitored for changes which signify the detection of a feature edge. The use of a specific range of electromagnetic wavelengths with the reflectometer allows the apparatus to detect feature edges covered by visibly-opaque material. After a feature edge has been detected, positional information associated with the detected feature edge may be used to accurately align a pattern mask.

Description:
This application is a divisional of application Ser. No. 09/292,396, filed on Apr. 15, 1999, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to semiconductor processing and, in particular, to a method and apparatus for improving the alignment of pattern masks to semiconductor wafers. 
     2. Description of the Related Art 
     Trends toward smaller critical dimensions in semiconductor processing have caused an exponential increase in the precision with which fabrication processes must be performed by the semiconductor device manufacturer. Semiconductor based integrated circuits are typically manufactured through the formation of a set of layers on a wafer containing many integrated circuit areas which will later be separated into individual dies. Very thin layers of material are deposited one on top of the other in patterns to form integrated circuit components. One technique of deposition and patterning is photolithography wherein a material layer is first coated with a light-sensitive photoresist. The photoresist is exposed through a pattern mask of a desired circuit pattern. The exposed photoresist is developed to remove, depending upon the type of photoresist used, either the exposed or unexposed resist. Etching and/or deposition processes are then used to create the desired circuit within the pattern created. 
     It is imperative to the process of photolithography that the pattern mask be precisely aligned on a wafer during processing. The overlay of the mask, the measure of how accurately the pattern mask was aligned, will often determine whether the wafer will be functional or must be discarded. Because each wafer must undergo numerous photolithography processing steps, the alignment of each pattern mask, especially the last ones used, is dependant upon the correct alignment of earlier masks. Poor overlay destroys the intended electrical properties of a circuit device on a wafer. 
     Prior art alignment approaches have used numerous methods for aligning a pattern mask to a wafer. One such method is the formation or use of reflective targets within the material layers deposited on a wafer prior to the alignment of the pattern mask. The targets, such as vertical scores or pronounced feature edges between two material layers, are illuminated by a light source and the resulting contrast created by the target is used to visually align the pattern mask. However, in wafers in which, for example, an oxide layer has been deposited in a silicon substrate such that the surfaces of the oxide and the substrate are even, the system fails because no physically distinct feature edge exists. In addition, the detection of minute feature edges is further complicated after numerous material layers have been deposited on top of the feature edge which must be detected. Visibly opaque materials and variations in colors between material layers will also degrade the performance of such a system. 
     U.S. Pat. No. 5,343,292 (Brueck, et al.), U.S. Pat. No. 4,991,962 (Kantilal Jain), and U.S. Pat. No. 4,631,416 (William Trutna Jr.) use interferometry to establish a phase shift within reflected light to create target patterns for alignment of a mask. The phase shift of a wide light beam as it encounters a feature edge, the boundary between a substrate and a material layer which has been deposited into a substrate, can be detected if that light beam is only reflected by the substrate material. A diffraction grating pattern will emerge in the reflected light and this can be used to align pattern masks. However, the existence of material layers above the edge to be detected dilutes the precision of this measurement technique by weakening the interference pattern. In addition, interferometry systems which rely upon a physically distinct edge are imprecise when two materials have equivalent heights at the material boundary edge. 
     None of the described methods allows for in-line identification of feature edges to allow accurate and repeatable registration of pattern masks within the increasingly reduced critical dimensions made possible by recent advancements in wafer fabrication. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus that is able to overcome some of the problems attendant the alignment of pattern masks in semiconductor fabrication of small critical dimension devices. 
     The above and other features and advantages of the invention are achieved by providing an apparatus for the detection of a layer step or feature edge of a die or wafer using spectroscopic reflectance to detect a change in the reflectance spectral response at the step or edge. The detection of a feature edge may be used, for example, to align a pattern mask for photolithography processing of the wafer. 
     In integrated circuit fabrication, a wafer is subjected to wafer fabrication processes to produce a number of individual layers on a semiconductor substrate. During processing, a reflectometer emits electromagnetic radiation having a predetermined wavelength range. The intensity or reflectivity of the radiation which is reflected from the wafer is monitored for changes which signal the detection of a feature edge within or on the wafer. The use of a specific range of electromagnetic wavelengths with the reflectometer allows the apparatus to detect feature edges covered by material which is visibly opaque, that is the material is opaque or semi-opaque in the visible wavelength range of 400 nm to 700 nm. After a feature edge has been detected, the apparatus may be used to accurately align a pattern mask according to the data collected by the reflectometer. 
     The above and other advantages and features of the present invention will be better understood from the following detailed description of the preferred embodiment which is provided in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a preferred embodiment of the present invention; 
     FIG. 2 is a perspective view of a wafer used to illustrate a preferred embodiment of the present invention; 
     FIG. 3 is another perspective view of a wafer used to illustrate a preferred embodiment of the present invention; 
     FIG. 4 is a flow chart outlining the steps of the present invention; 
     FIG. 5 is a perspective view of a wafer used to illustrate a preferred embodiment of the present invention; 
     FIG. 6 is a graphical representation of reflectivity versus wavelength for the wafer of FIG. 5 measured in accordance with a preferred embodiment of the present invention; 
     FIG. 7 is a graphical representation of reflectivity versus wavelength for the wafer of FIG. 5 measured in accordance with a preferred embodiment of the present invention; 
     FIG. 8 is a perspective view of a wafer used to illustrate a preferred embodiment of the present invention; 
     FIG. 9 is a graphical representation of reflectivity versus wavelength for the wafer of FIG. 8 measured in accordance with a preferred embodiment of the present invention; 
     FIG. 10 is a perspective view of a wafer used to illustrate a preferred embodiment of the present invention; and 
     FIG. 11 is a graphical representation of reflectivity versus wavelength for the wafer of FIG. 10 measured in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. Wherever possible, like numerals are used to refer to like elements and functions in the various figures of the drawings and between the different embodiments of the present invention. 
     A feature edge detection system in accordance with the present invention is illustrated generally in FIG.  1 . The detection system is part of an automated wafer fabrication system for processing a wafer  20 . The wafer  20  is mounted for processing on a coordinate table  36  attached to a base  38  capable of three dimensional movement along the X, Y, and Z axis, preferably at least along the plane of the surface of the wafer  20  which is to be processed. An element  52 , preferably a robotically controlled element, grips pattern mask  50 , and is capable of moving pattern mask  50  in response to control signals from the controller  42 . Reflectometer  32  produces a radiation incident beam  34  which intersects with a surface of the wafer, preferably at an angle approximately perpendicular to that surface. The incident beam  34  is comprised of electromagnetic radiation having a predetermined wavelength range and may have a variable cross-sectional area depending upon the critical dimensions of the feature edge which must be detected on the wafer  20 . The wavelength range of the incident beam  34  is preferably between about 100 nm to about 1000 nm. The reflected beam  60  is the reflection of incident beam  34  which is then collected in the reflectometer  32 . The reflectometer exploits the material property of reflectivity to operate. Reflectivity is the property of illuminated objects, e.g. a wafer, to re-radiate or reflect a portion of the incident electromagnetic radiation energy. The reflectometer  32  measures changes in intensity or reflectivity of the reflected beam  60  relative to the intensity of the incident beam  34 . As illustrated in greater detail in FIG. 2, the intensity of the reflected beam  60  will be different than the intensity of the incident beam  34  depending upon the wavelength range of the beam, the absorption coefficient of the wafer  20  layers  24 ,  26 ,  28 ,  30  and the substrate  22 , and the distance traveled by the incident beam  34  and reflected beam  60  through the material layers  24 ,  26 ,  28 ,  30 . The intensity of the reflected beam  60  may be measured in relative terms of intensity and/or reflectivity to a standard which may chosen by an operator. 
     The reflectometer  32  is attached to a mount  40  which is preferably capable of movement at least along the plane of the wafer  20 . Preferably the reflectometer  32  or the coordinate table  36  is also capable of vertical or Z axis movement to allow focusing of the incident beam  34  on the surface of the wafer  20 . The incident beam  34  is moved along the wafer  20  through relative movement between the coordinate table  36  and the mount  40 . One is generally stationary, while the other is moveable. The movement of the coordinate table  36  or mount  40  is directed by the controller  42  (FIG. 1) responsive to information received from control interface  46 . The coordinate table  36  provides support for the wafer  20  and monitors the position of the wafer  20  in relation to a fixed point or to the reflectometer  32 . Coordinate table  36  is preferably a platform capable of movement along the plane of the wafer  20  and transmitting a position signal to the controller  42 . The position signal is at least the x and y coordinates of the current position of the coordinate table  36  within the plane of the wafer  20  relative to a fixed point. The coordinate table  36  is comprised of motors such as, for example, DC-motors, stepping-motors, pulsed motors, or rotary hydraulic motors and may have a dedicated microprocessor or a general purpose microprocessor programmed with a mathematical coordinate table capable of monitoring the current position of the coordinate table with relation to a fixed point. Operation of the coordinate table  36  may be accomplished, for example, by control signals from the controller  42 . Controller  42  is further connected to a display  44  capable of visually displaying the data from reflectometer  32  in graphical form and a control interface  46 , for example a keypad, which allows an operator to control the use of the FIG. 1 detection system. 
     FIG. 2 shows in an enlarged fashion a portion of the FIG. 1 detection system when in use. The wafer  20  at the illustrated stage of processing includes a plurality of material layers  22 ,  24 ,  26 ,  28 ,  30 . For example, a semiconductor substrate  22  is processed such that an oxide layer  24  is formed therein and visibly-opaque material layers  26 ,  28 ,  30  are formed in series over the substrate  22  containing the oxide layer  24 . Feature edges  62  are formed at the boundary between the substrate  22  and the oxide layer  24 . The substrate  22  of the wafer  20  may be any material suitable for use as a substrate for integrated circuit devices which are reflective in the usable spectrum (150 nm to 1100 nm wavelengths), preferably silicon (Si) or gallium arsenide (GaAs). Visibly-opaque material layers  26 ,  28 ,  30  may be any visibly-opaque material used in the processing of integrated circuits such as, for example, polyimide, polysilicon, Wsix, Nitride, oxide or other resist coatings. Thin layers of metallization, e.g. aluminum, may also be present on the wafer  20  so long as they remain penetrable by the chosen wavelength range of electromagnetic radiation incident beam  34  being used. For example, one or more metallization layers comprised of one or more of a group of metals including titanium, copper, aluminum, platinum, and tungsten having a thickness of less than approximately 500 Angstroms would not significantly degrade the radiation incident/reflected beams  34 ,  60  used in the present invention. 
     Preferably the beam intersects the wafer  20  perpendicular to the plane of the wafer  20 . The incident beam  34 , depending upon the predetermined wavelength range, will pass through visibly-opaque material layers  26 ,  28 ,  30  and oxide layer  24  to the substrate  22 . The beam is reflected by the substrate  22  and reflected beam  60  and is collected in the reflectometer  32 . As shown in FIG. 2, the reflectance path of reflected beam  60  is different on the left and right sides of a feature edge  62 . 
     FIG. 3 shows a pattern mask  50  such as is commonly used in integrated circuit manufacture which is aligned on the wafer  20  such that a feature edge  62  of the mask  50  is parallel to the feature edge  62  separating two layers of wafer  20 , for example the substrate  22  and the material layer  24 . An element  52  moveable in two or, preferably, three dimensions positions the mask  50  responsive to control signals from the controller  42 . 
     The reflectometer  32  may be any device capable of producing an electromagnetic incident beam  34  of a predetermined wavelength range directed at the wafer  20  and collecting reflected beam  60  to measure the intensity or reflectivity of the reflected beam  60  over the predetermined wavelength range. The controller  42  is preferably a general use microprocessor capable of receiving input from the reflectometer  32 , the control table  36 , and the input interface  46 . The controller  42  may be dedicated to the system  5  or may be used to control other wafer fabrication processes as well. 
     The process for implementing the FIG. 1 detection system to align a pattern mask  50  onto the wafer  20  will next be described with reference to FIG.  4 . The wafer  20 , which may ultimately be diced and yield many dies, is subjected to integrated circuit processing such that at least material layer(s)  24  are formed on and/or within the substrate  22  in step S 200 . This may occur through etching, deposition, prior photolithography processing, and other methods known in the art. Often, portions of the substrate  22  are removed and filled with an oxide to form material layers  24  which are substantially flush with the surface of the substrate  22 . For example, a silicon substrate  22  may have silicon oxide (SiO 2 ) layers  24  which are deposited in etched portions of the substrate  22 , as shown in FIGS. 2,  3 . Due to the small size of the oxide layers  24  and the similarity in color between a substrate semiconductor and an oxide of that semiconductor, e.g. Si and SiO 2 , visual or interferometric inspection of the surface of the wafer  20  can be insufficient to detect the feature edge  62  between the substrate  22  and oxide layers  24 . This is especially true in the case in which several visibly-opaque material layers  26 ,  28 ,  30  have been deposited onto the wafer  20  in prior processing. Deposition of the visibly-opaque material layers  26 ,  28 ,  30  may be necessary to the processing of the wafer  20  prior to use of the mask  50 . 
     Returning to FIG. 4, once the wafer  20  has been secured, the mask  50  must be aligned to continue processing, e.g. to complete photolithography processing, of the wafer  20 . The wafer  20  is placed onto and/or secured to the coordinate table  36  in step S 202 . The wafer  20  may already be on the coordinate table  36  if the coordinate table  36  has been used in prior processing steps. In step S 204 , the controller  42  transmits control signals to the reflectometer  32  to begin emission of the incident beam  34 . This step may be part of an automated process or may be initiated by an operator using control interface  46 . The reflectometer  32  emits electromagnetic radiation in the form of an incident beam  34  directed towards at least one surface of the wafer  20  in step S 206 . The incident beam  34  preferably has a minimum spot size, e.g. less than 10 microns in area, preferably less than about 1 micron in area in the form of a rectangle or other right angled shape. The wavelength range of the incident beam  34  is chosen such that it passes through the material layers above the substrate  22  (e.g. material layers  24 ,  26 ,  28   30 ) and is reflected from the surface of the substrate  22  as the reflected beam  60  which is collected by the reflectometer  32  in step S 208 . The reflectometer  32  measures the intensity of the reflected beam  60  over the predetermined wavelength range and outputs the intensity reading to the controller  42  in step S 210 . The controller  42  displays the data from reflectometer  40  onto a display  44  in a form readable by an operator in step S 212 . One such display may be a graph of intensity versus wavelength or, alternatively, a graphical representation of the wafer  20  as reflected by the reflected beam  60 . In step S 214 , the coordinate table  36  and, therefore, the wafer  20  are moved along the plane of the wafer  20  or, alternatively, the reflectometer  32  is moved along the plane of the wafer  20  such that the entire surface or a selected portion of the surface of the wafer  20  may be scanned. 
     A feature edge  62  is detected in step S 216  by monitoring the signal from the reflectometer  32  for a sudden change in the intensity versus wavelength data output from reflectometer  32 . If a sudden change or sharp derivation in intensity is detected, the operator or controller  42  compares the change to known data for the specific chemical composition of the substrate  22  and material layers  24 ,  26 ,  28 , and  30  in step S 218 . If no feature edge  62  is detected of step S 216 , the signal from the reflectometer  32  continues to be monitored in step S 214 . 
     If the change corresponds to known data for a feature edge  62 , the positional data of the feature edge  62  in relation to the wafer  20  is calculated and stored in controller  42  through the use of the coordinate table  36  in step S 218 . In step S 220  the operator or controller  42  determines whether a sufficient number of feature edges  62 , preferably a minimum of two, have been detected to allow for proper alignment of the mask  50 . The feature edge detection process may be repeated in step S 206  if additional feature edges  62  must be detected. 
     To further illustrate detection of a feature edge  62  through analysis of the derivation of intensity patterns, FIG. 5 shows a wafer  20  comprised of a silicon substrate  22  and a variety of material layers including a silicon oxide layer  24  having a thickness of approximately 1000 Angstroms and a Wsix layer  26  having a thickness of approximately 1250 Angstroms. FIGS. 6-7 display graphical representations of reflectivity versus wavelength of the beams  34 ,  60  directed at and reflected from sections  70  and  72  of the wafer  20 , respectively. FIG. 6 corresponds to the reflectivity pattern seen over a wavelength range of about 250 nm to about 750 nm for the sections  70  of the wafer  20  which are comprised of the silicon substrate  22  and the Wsix layer  26 . As the reflectometer  32 , or alternatively, the wafer  20  itself, moves in a direction parallel to the surface of the wafer  20 , the graphical representation of the intensity versus the wavelength of the reflected beam  60  will change to the intensity pattern as shown in FIG. 7, as the beam moves from section  70  to section  72 , the latter of which is comprised of the silicon substrate  22 , silicon oxide layer  24 , and Wsix layer  26 . The obvious significant and sudden change in the graphical display signals the detection of a feature edge  62 . In this example the feature edge corresponds to the boundary between the silicon substrate  22  and the silicon oxide layer  24 . 
     By way of further example, FIG. 8 shows a wafer  20  comprised of a silicon substrate  22  and a variety of material layers including a silicon oxide layer  24  having a thickness of approximately 90 Angstroms, a polysilicon layer  28  having a thickness of approximately 850 Angstroms, and a Wsix layer  26  having a thickness of approximately 1250 Angstroms. FIG. 9 is a graphical display representing reflectivity versus wavelength of the incident beam  34  and  60  directed at and reflected from section  76  of the wafer  20  of FIG.  8 . FIG. 10 shows a wafer  20  comprised of a silicon substrate  22  and a variety of material layers including a silicon oxide layer  24  having a thickness of approximately 1000 Angstroms, a polysilicon layer  28  having a thickness of approximately 4000 Angstroms, and a Wsix layer  26  having a thickness of approximately 1250 Angstroms. FIG. 11 is a graphical display representing reflectivity versus wavelength of the incident beam  34  and  60  directed at and reflected from section  80  of the wafer  20  of FIG.  10 . Known data such as that displayed in FIGS. 6,  7 ,  9 , and  11  may be used to compare reflectivity readings from a wafer  20  being measured using the apparatus and methods of the present invention to determine the exact material boundaries or feature edges of regions  76  (FIG. 8) and  80  (FIG.  10 ). 
     Returning to FIG. 4, once one or more feature edge  62  have been detected, the controller  42  determines the position of the incident beam  34  relative to the wafer  20  using the coordinate table  36  which monitors the position of the wafer  20  on two or three dimensions. Next, a control element  52  positions the mask  50  over the wafer  20  responsive to positional data from controller  42  in step S 222 . The positioning of the mask  50  may be accomplished by a human operator through control interface  46  or by the controller  42  in an automated fabrication production line. The wafer  20  may then be subjected to photolithography or other processing techniques requiring correct alignment of a pattern mask in step S 224 . 
     The feature edge detection system of the present invention may be implemented at various stages within a semiconductor wafer fabrication production line. The detection system may be used prior to deposition of one or more of the material layers  26 ,  28 ,  30  and a plurality of detection systems may be implemented within the same wafer fabrication production line to assist in the identification of feature edges  62  on wafers  20  whenever such identification is required or desired. 
     With the present invention, the identification of material layer feature edges  62  in a wafer  20  by a human or machine at any stage of a wafer fabrication process is simplified and does not suffer from the obfuscation which occurs in current optical systems and, therefore, allows for precise and repeatable registration of masks onto wafers  20 . 
     It should be readily understood that the invention is not limited to the specific embodiments described and illustrated above. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, which are commensurate with the spirit and scope of the invention. Accordingly, the invention is not limited by the foregoing description, but is only limited by the scope of the appended claims.