Abstract:
A method of determining overlay layers utilizing advanced lithographic materials utilizes a post-etch overlay metrology. After etching, a relatively opaque layer is removed so that registration markers such as trench isolation structures can be observed. Lithographic parameters associated with the process can be adjusted in accordance with the observations. In a preferred embodiment, an overlay error is determined and adjustments are made to the reduce the overlay error.

Description:
FIELD OF THE INVENTION  
         [0001]    The present disclosure relates generally to lithography, especially optical lithography, such as that used for fabricating semiconductor devices, integrated circuits (ICs) and other devices. More particularly, the present disclosure relates to error measurements when using advanced lithographic materials.  
         BACKGROUND OF THE INVENTION  
         [0002]    Conventional lithographic fabrication systems typically include a semiconductor wafer and one or more layers of materials located on the surface of the wafer. A pattern can be transferred to one or more of the layers of material using a lithographic mask or reticle having a pattern of apertures or object features in the form of an opaque material. Optical radiation is provided to the mask or reticle and is focused by a lens onto a layer of the wafer. In such a fashion, the pattern of the mask is transferred to or printed on the layer of material, typically a photoresist layer.  
           [0003]    Generally, lithographic errors such as overlay errors are measured after the photoresist layer is patterned. For example, an overlay error can be related to distances between a gate conductor and neighboring trench regions. Registration marks or structures are observed on the substrate through the relatively translucent photoresist layers and polysilicon layers with optical equipment, such as a scanner or stepper.  
           [0004]    The scanner or stepper observes the wafer which is situated on a stage. The scanner or stepper can use the actinic (exposure wavelength), white light, or helium neon (HeNe) light (633 nm) to make overlay measurements. Optical equipment such as a KLA5200 overlay measurement optical microscope manufactured by KLA-Tencor can be utilized to measure lithographic errors associated with a patterned photoresist feature with respect to a marking or structure on the substrate.  
           [0005]    According to one conventional process, the STI shallow trench isolation structures or marks can be reasonably observed through the polysilicon layer. After observation of the photoresist feature and the STI marks, an overlay error is determined and the wafer can be reworked if the overlay error is above a threshold. In addition, corrections to alignment and exposure tools can be made in accordance with the overlay error. If the overlay error is below a threshold, the wafer is etched in accordance with the patterned photoresist and the process is continued.  
           [0006]    It is desirable to use advanced lithographic materials such as amorphous carbon materials when patterning integrated circuits (ICs). However, overlay measurements cannot be made through the advanced lithographic materials because applicants have observed that amorphous carbon is relatively absorbing at the wavelengths associated with the optical equipment described above. Therefore, conventional processes cannot be employed to measure registration errors when advanced lithographic materials are used.  
           [0007]    Thus, there is a need for a method of determining overlay errors when advanced lithographic materials are provided. Further, there is a need to observe registration marks and isolation structures when advanced lithographic materials are utilized. Yet further, there is a need for a process flow which enables the determination of overlay errors when amorphous carbon is utilized in the lithographic process.  
         SUMMARY OF THE INVENTION  
         [0008]    An exemplary embodiment relates to the system of processing integrated circuits. The method includes lithographically patterning a photoresist layer above a layer including carbon. The layer including carbon is above an underlying layer or substrate including registration features. The method also includes etching the layer including carbon in accordance with the patterned photoresist layer, etching the underlying layer or substrate, removing the layer including carbon, and observing features to determine at least one error factor. The method also includes adjusting lithographic parameters in accordance with the at least one error factor.  
           [0009]    Another exemplary embodiment relates to a method of patterning a gate stack above a substrate using a patterned carbon layer. The method includes patterning the gate stack in accordance with the patterned carbon layer, removing the patterned carbon layer, and determining a lithographic error using optical equipment. The optical equipment determines a location of features on the substrate through the gate stack to determine the lithographic error.  
           [0010]    Another exemplary embodiment relates to a method of fabricating an integrated circuit above the substrate including trench isolation features. The method includes steps of patterning a photoresist layer above a layer including carbon, etching the layer including carbon in accordance with the patterned photoresist layer, etching the underlying layer, removing the layer including carbon, locating positions of the trench isolation structures to determine an overlay error, and adjusting lithographic parameters in accordance with the overlay error. The layer including carbon is above an underlying layer above the substrate. The underlying layer is translucent. The underlying layer covers at least a portion of the trench isolation structure.  
           [0011]    Other principle features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The exemplary embodiments of the disclosure will hereafter be described with reference to the accompanying drawings, wherein like numerals denote like elements, and:  
         [0013]    [0013]FIG. 1 is a general schematic block diagram of a lithographic inspection system for scanning a wafer;  
         [0014]    [0014]FIG. 2 is a more detailed general schematic block diagram of a portion of the wafer;  
         [0015]    [0015]FIG. 3 is an enlarged schematic block diagram of the portion illustrated in FIG. 2 including a gate structure and four isolation structures;  
         [0016]    [0016]FIG. 4 is a cross-sectional view of the portion illustrated in FIG. 2 about line 4-4, showing a lithographic patterning step;  
         [0017]    [0017]FIG. 5 is a cross-sectional view of the portion illustrated in FIG. 4, showing an ARC etching step;  
         [0018]    [0018]FIG. 6 is a cross-sectional view of the portion illustrated in FIG. 4, showing an amorphous carbon etching step;  
         [0019]    [0019]FIG. 7 is a cross-sectional view of the portion illustrated in FIG. 6, showing a polysilicon etching step;  
         [0020]    [0020]FIG. 8 is a cross-sectional view of the portion illustrated in FIG. 7, showing an amorphous carbon layer removal step;  
         [0021]    [0021]FIG. 9 is a flow diagram showing a process for lithographically patterning an integrated circuit utilizing advanced lithographic materials;  
         [0022]    [0022]FIG. 10 is a flow chart showing a post-etch overlay metrology in accordance with another exemplary embodiment;  
         [0023]    [0023]FIG. 11 is a graph showing the translucency of amorphous carbon deposited at 550° C.; and  
         [0024]    [0024]FIG. 12 is a graph showing the translucency of amorphous carbon including nitrogen deposited at 450° C. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0025]    With reference to FIG. 1, an integrated circuit fabrication system is utilized to provide an image to a substrate, such as, a semiconductor or integrated circuit (IC) wafer  14 . The image is generally provided as light to wafer  14 . The light preferably has a wavelength in the range of 200 nm to 700 nm. The type of semiconductor process, the type of light, the layer being configured on wafer  14 , the type of wafer  14 , and the type of equipment are not described below in a limiting fashion.  
         [0026]    Wafer  14  can be the substrate for a variety of types of integrated circuits including memory units, logic circuits, communications devices, processors, application specific integrated circuits (ASICs), or other devices. Preferably, wafer  14  is a semiconductor (e.g., silicon) wafer upon which insulative, conductive, and semiconductive materials are deposited in an IC fabrication process.  
         [0027]    A system  10  is utilized to inspect wafer  14  for lithographic errors, such as overlay errors. System  10  can be implemented in a variety of semiconductor tools and can be included as part of an ultraviolet (UV) light stepper unit. System  10  includes an inspection tool  12  and a stage  16 .  
         [0028]    With reference to FIG. 1, wafer  14  is provided on a stage  16  and can be viewed (optically analyzed) by inspection tool  12 . Inspection tool  12  or system  10  can be a variety of optical inspection tools, including a KLA 5200 manufactured by KLA-Tencor. Wafer  14  includes a portion  32  including integrated circuit structures  24 . Integrated circuit structures  24  can be any type of integrated circuit structures which are completed or partially completed.  
         [0029]    With reference to FIGS. 2 and 3, portion  32  can correspond to an IC chip or device. Portion  32  includes structures  24  which are shown including at least one transistor including a gate conductor  34  surrounded by isolation structures such as shallow trench isolation structures  46 ,  48 ,  50  and  52 .  
         [0030]    Gate conductor  34  is separated from isolation structure  52  by a distance  42  and is separated from an isolation structure  50  by a second distance  44 . Ideally, according to preferred design, distances  42  and  44  should be equal. However, due to various semiconductor fabrication accuracy and precision issues, distances  42  and  44  can be different. A particular lithographic error, overlay error, is equal to the difference between distance  44  and distance  42  (e.g., overlay error=D 42  minus D 44 ).  
         [0031]    Similar errors can be defined by distances between end points of gate conductor  34  and structures  46  and  48 , widths of conductor  34  and structures  46 ,  48 ,  50  and  52 , distances between structures  46 ,  48 ,  50 , and  52 , etc. Further, errors related to other distances can be measured such as, end cap errors, etc. Preferably, system  10  measures these errors as well as other lithographic errors and deviations optically.  
         [0032]    With references to FIGS.  4 - 9 , a process  100  (FIG. 9) for forming portion  32  (FIG. 1) is described below as follows. In FIG. 4, substrate  62  is etched to form trenches which are filled with insulative material such as silicon dioxide to form isolation structures  50  and  52 . Structures  50  and  52 , as well as structures  46  and  48 , can be formed in a conventional shallow trench isolation process.  
         [0033]    A thin gate oxide layer or gate dielectric layer  64  is provided above substrate  62 . Layer  64  can be thermally grown as a 10-30 Å thick silicon dioxide layer. A gate conductor layer  66 , such as a polysilicon layer, is deposited as a 1,000-2,000 angstrom thick layer by chemical vapor deposition (CVD). Layers  66  and  64  comprise a gate stack for the eventual formation of a transistor.  
         [0034]    A layer  68  of advanced lithographic material is provided above layer  66 . Preferably, layer  68  is a layer containing carbon and can be an amorphous carbon layer. Preferably layer  68  is between approximately 300 and 800 angstroms thick and deposited by plasma-enhanced chemical vapor deposition (PECVD), magnetron sputtering, or a variety of other techniques (e.g., single low-energy beams of carbon ions, dual ion beams of carbon and argon, ion plating, rf sputtering or ion-beam sputtering from carbon/graphite target, vacuum-arc discharges, laser ablation, etc.). Layer  68  can be pure amorphous carbon deposited at approximately 550° C. or can be an amorphous carbon nitrogen layer (e.g., N=0 to 57 atomic percent) deposited at 450° C. Layer  68  is provided in a step  202  of process  100  (FIG. 9).  
         [0035]    After layer  68  is provided, an optional antireflective coating (ARC) layer  70  can be provided above layer  68  in a step  204  (FIG. 9). Layer  70  can be a silicon nitride (Si 3 N 4 ), silicon oxynitride (SiON), or other suitable ARC material. Preferably, layer  70  is between approximately 100 and 400 angstroms thick and deposited by PECVD.  
         [0036]    After layer  70  is provided, a photoresist layer  72  is provided in a step  206  (FIG. 9). Layer  72  is preferably a positive chemically-amplified type photoresist material and provided by spin coating. In alternative embodiments, other types of photoresist or electron beam resist materials can be used for layer  72 . After the provision of layer  72 , layer  72  is lithographically patterned to form a feature  45  associated with the eventual formation of gate conductor  34 . Any lithographic patterning technique can be utilized to form feature  45 .  
         [0037]    With reference to FIG. 5, layer  70  is etched in accordance with feature  45 . Layer  70  can be etched in a dry etching process selective to layer  70  in a step  210  (FIG. 9). At this point in process  100 , layer  72  can be optionally removed.  
         [0038]    After layer  70  is etched, layer  68  can be etched in a reactive ion etch or plasma etch process in a step  210  (FIG. 6). Preferably, the etch process is selective to layer  68  with respect to layer  66 . After step  210 , layer  70  can be removed or both layers  70  and  72  can be removed.  
         [0039]    Conventionally, after layer  72  is provided, it is desirable to inspect wafer  14  for overlay errors. However, wafer  14  cannot be inspected using conventional optical equipment due to the presence of layer  68  which is relatively opaque.  
         [0040]    Applicants have found that when layer  68  is a relatively pure amorphous carbon layer deposited at a temperature of approximately 550° C., layer  68  allows a relatively small percent of transmission through a thickness of 100 nanometers. Applicants have also found that when layer  68  is an amorphous carbon layer including 6 atomic percent nitrogen deposited at approximately 450° C., layer  68  allows a relatively small percent of transmission through a thickness of 100 nm.  
         [0041]    [0041]FIGS. 11 and 12 show dispersion spectra  300  and  350  for layer  68  illustrating the optical properties of layer  68 . FIG. 11 shows the dispersion spectra  300  for an embodiment where layer  68  is pure amorphous carbon deposited at 550° C. FIG. 12 shows the dispersion spectra  350  for an embodiment where layer  68  is amorphous carbon including approximately 6 atomic percent nitrogen deposited at 450° C. FIGS. 11 and 12 are presented in the form of graphs that plot optical property of layer  68  on the Y-axes  302 ,  352  and photon energy of incident light in eV on the X-axes  304 ,  354 , where photon energy is determined by the multiplying Planck&#39;s constant (6.63×10 −34  J-s) by the speed of light and dividing by the wavelength of incident light.  
         [0042]    Values for the optical constant n (shown as curves  310  and  360  in FIGS. 11 and 12, respectively) and k (shown as curves  312  and  362  in FIGS. 11 and 12, respectively) describe how the material of layer  68  interacts with light. The optical constant n is the ratio of the speed of light in a vacuum to the speed of light as it propagates through the material (e.g., layer  68 ). The optical constant k is a quantification of the absorption of light in a material. In an exemplary embodiment, optical properties n and k are measured using a Woolam vacuum ultra violet variable angle spectroscopic ellipsometer. Other masurement systems may also be used in alternative embodiments.  
         [0043]    As shown in FIGS. 11 and 12, layer  68  absorbs light over the entire spectral range typically used by the overlay measurement tool (e.g., even low energy photons between 3 and 5 eV are strongly absorbed by layer  68 , indicated by the large k values). For this reason, overlay measurements are difficult to obtain when layer  68  is present.  
         [0044]    The table below includes data showing shows specific data points as plotted in FIG. 11. Optical constants n and k are described above. The variable a quantifies the absorbance of layer  68  per unit length (e.g., micrometers). The variable T represents the transmittance of layer  68 , and is calculated according to the formula  
           T=exp (− a*d )  
         [0045]    where T is the transmittance, a is the absorbance per unit length, and d is the thickness of layer  68 .  
                                                                             Photon   Wave-               % T through       Energy (eV)   length (nm)   N   k   a (1/μm)   100 nm                                7.9   156.9   1.08   0.13   10.5   35.1       6.4   193.7   1.15   0.31   20.1   13.3       5   248.0   1.40   0.57   29.0   5.5       2   619.9   1.94   0.39   7.9   45.2                  
 
         [0046]    In FIG. 7, in step  210 , layer  66  is dry etched in accordance with feature  45  to form gate conductor  34 . According to one embodiment, layer  64  can also be etched in a dry etching process at this point. According to another embodiment, layer  64  remains intact. If layer  64  is a translucent layer such as silicon dioxide, it is not required to be removed at this point in process  100 .  
         [0047]    After layer  66  is etched to form gate conductor  34 , layer  68  is removed. In one embodiment, layer  68  and  70  can be removed after etching layer  66  in a step  212 . Alternatively, layer  70  can be removed prior to etching layer  66 . Preferably, layer  68  is removed by ashing. In one embodiment, an oxygen plasma based removal process is utilized to remove layer  68 .  
         [0048]    With reference to FIG. 8, a distance  42  and a distance  44  can be used to determine an overlay error for gate conductor  34 . Distances  42  and  44  are measured with respect to a center point of isolation structures  52  and  50  with tool  12 . After removal of layer  68 , alignment can be readily checked in a step  214  because layer  66  is relatively translucent so that alignment marks and/or structures  52  and  50  can be readily observed by system  10 . In a step  216 , the overlay error can be utilized to adjust fabrication parameters.  
         [0049]    Distances  42  and  44  are preferably measured using optical equipment such as a KLA5200 scanner light. A wavelength of between approximately 300 and 600 nanometers can be used to make the measurements. In alternative embodiments, any broad band light wavelengths may be used (e.g., from the mid-visible light spectrum down to the near ultraviolet wavelength ranges).  
         [0050]    With reference to FIG. 10, a flow chart shows a fabrication process utilizing a principle of a preferred embodiment. At a step  230 , wafer  14  is aligned and exposed to pattern photoresist features similar to step  208 . At a step  232 , wafer  14  is etched through an advanced lithographic material and a gate conductor layer to form a gate stack similar to step  210 . Also in step  232 , the advanced lithographic layer (e.g., amorphous carbon layer  68 ) is removed.  
         [0051]    At a step  234 , an overlay measurement is made. Based upon the overlay measurement made in step  234 , a decision to allow wafer  14  to continue processing is made at a step  236 . If the overlay measurement is below a threshold, the wafer can be allowed to pass assuming other criteria are met and process flow continues at a step  240 . However, if the overlay error is above a threshold or other criteria are not met, the wafer  14  does not pass at step  236  and must be scrapped in a step  238 .  
         [0052]    Unlike a conventional process which could send wafer  214  for rework, rework is not possible in the preferred embodiment because gate conductor layer  66  has already been etched. However, overlay corrections can still be made and applied to alignment and exposure tools so that subsequent patterning reduces overlay errors.  
         [0053]    While the exemplary embodiments illustrated in the FIGURES and described above are presently preferred, it should be understood that these embodiments are offered by way of example only. Other embodiments may include, for example, a variety of other errors. The invention is not limited to a particular embodiment, but extended to various modifications, combinations, and permutations that nevertheless fall within the scope and the spirit of the appended claims.