Abstract:
A method of making and using a reference wafer and a metrology system to calibrate tools in a photolithographic system. The reference wafer includes a silicon substrate, a dielectric or insulating layer disposed above the silicon substrate and a pattern disposed above the insulating layer. The pattern is coupled to the silicon substrate and the silicon substrate acts as a ground for the pattern. As a result, charge buildup on the pattern is mitigated since excess charge is dissipated into the silicon substrate.

Description:
TECHNICAL FIELD 
     The present invention generally relates to semiconductor processing, and in particular to a method for improving a process for calibrating a photolithographic tool. 
     BACKGROUND OF THE INVENTION 
     In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down device dimensions at submicron levels on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as corners and edges of various features. 
     The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as optical light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The lithographic coating is generally a radiation-sensitive coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Exposure of the coating through a photomask causes the image area to become either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer. 
     FIGS. 1 a  and  1   b  illustrate problems associated with conventional polysilicon gate layer test wafers. FIG. 1 a  illustrates a reference wafer  10  having a pattern  15  formed thereon. The pattern  15  could include a plurality of fields, each field having an array of sub-fields and each sub-field having a number of component images formed thereon. FIG. 1 b  illustrates a portion of the reference wafer  10  including a silicon substrate layer  12 , a dielectric or insulating layer  14  on the silicon substrate layer  12 , and a first contact  16  and a second contact  18  on the insulating layer  14 . The first contact  16  and the second contact  18  are made of a conductive material, such as polysilicon or metal. In a metrology measurement system, electron beams  20  are directed toward the reference wafer  10  and secondary electron emissions from the surface are detected by detectors (not shown). However, a charge  22  begins forming on the first contact  16  and the second contact  18  from the electron beams  20  due to the conductivity of the contacts. The charge build up  22  may cause deflection in the incident beam in addition to deflection and/or suppression of the secondary electron emissions. 
     In view of the above, an improvement of the calibration process is needed. In addition an improvement is needed in the structure and formation of the reference wafer. 
     SUMMARY OF THE INVENTION 
     The present invention provides for an improved method of making and using a reference wafer to calibrate metrology tools. Reference wafers are used in SEM to make precision and line width measurements repeatedly in order to maintain the tools in accordance with desired specifications. An electron beam may cause a charge buildup on an area of inspection pattern on the reference wafer if the elements and materials are not connected to a ground, as opposed to actual production wafers. This charge buildup may cause deflection of an incident electron beam and deflection and/or suppression of secondary electron emission resulting in erroneous secondary electron signals from the reference wafer. 
     The reference wafer of the present invention includes a silicon substrate, a dielectric or insulating layer disposed above the silicon substrate and a material (polysilicon, silicon nitride, metal, amorphous silicon) disposed above the insulating layer. The features (e.g., lines) are formed so that they extend through the insulating layer to the silicon substrate. The silicon substrate acts as a ground for the elements or materials forming the pattern (it is to be appreciated that an ion implantation may be performed to modify the electron dissipation properties of the base silicon to be more electron conductive). As a result, charge that is formed on the patterned layer due to charges induced by the electron beam dissipate into the silicon substrate thereby mitigating deleterious charge formation on the patterned layer. 
     In one aspect of the invention a reference wafer for calibrating a metrology tool set is provided. The reference wafer includes a substrate layer, an insulating layer formed over the substrate layer, a pattern formed over the insulating layer and at least one conductive path coupling at least a portion of the pattern to the substrate layer. The at least one conductive path provides a path to dissipate charge from the at least a portion of the pattern to the substrate layer. 
     In yet another aspect of the invention a method for calibrating a line width measurement metrology tool set over time is provided. The method includes the steps of using a reference wafer to calibrate the tool at a first time period, the reference wafer, including a substrate layer, an insulating layer formed over the substrate layer, a pattern formed over the insulating layer, and at least one conductive path coupling at least a portion of the pattern to the substrate layer, the at least one conductive path providing a path to dissipate charge from the at least a portion of the pattern to the substrate layer, and using the reference wafer to calibrate the tool at a second time period. 
     In another aspect of the invention a method for calibrating first and second metrology tool sets is provided. The method includes the steps of using a reference wafer to calibrate the first tool, the reference wafer, including a substrate layer, an insulating layer formed over the substrate layer, a pattern formed over the insulating layer, and at least one conductive path coupling at least a portion of the pattern to the substrate layer, the at least one conductive path providing a path to dissipate charge from the at least a portion of the pattern to the substrate layer and using the reference wafer to calibrate the second tool at a second time period. 
     One aspect of the invention relates to a SEM system. The system includes a line width measurement metrology tool set and a reference wafer adapted to be used to calibrate the tool. The reference wafer includes a silicon layer, an insulating layer above the silicon layer and at least one contact. The contact extends from the top of the silicon layer to above the top of the insulating layer. The system also includes a metrology system adapted to transmit an electron beam to the reference wafer and detect electron emissions based on characteristics of the reference wafer. The electron beam from the metrology system making contact with the at least one contact forms a charge on the at least one contact that dissipates through the silicon layer. 
     Another aspect of the present invention relates to a reference wafer for calibrating a line width measurement metrology tool set. The reference wafer includes a silicon layer, an insulating layer above the silicon layer and at least one contact. The contact extends from the top of the silicon layer to above the top of the insulating layer wherein an electron beam transmitted from a metrology system and making contact with the at least one contact forms a charge on the at least one contact that dissipates through the silicon layer. 
     Yet another aspect of the present invention provides for a method of fabricating a reference wafer. The method includes the steps of providing a substrate having an insulating layer, providing a photoresist layer over the insulating layer, developing the photoresist layer exposing portions of the insulating layer, etching the exposed portions of the insulating layer to form at least one via, stripping off the photoresist layer, filling the via with a conductive layer, the conductive layer covering the insulating layer, providing a second photoresist layer over the conductive layer, developing the second photoresist layer exposing portions of the conductive layer, etching the exposed portions of the insulating layer to form at least one contact and stripping off the second photoresist layer. 
     In yet another embodiment of the invention, a method for calibrating a line width measurement metrology tool set is provided. The method includes the steps of providing a reference wafer adapted to be used to calibrate the tool, the reference wafer having a silicon layer, an insulating layer above the silicon layer and at least one contact, the contact extending from the top of the silicon layer to above the top of the insulating layer, providing a metrology system adapted to transmit an electron beam to the reference wafer and detect secondary electron emissions based on characteristics of the reference wafer, wherein the electron beam transmitted from the metrology system and making contact with the at least one contact forms a charge on the at least one contact that dissipates through the silicon layer, providing a line width measurement metrology tool set coupled to the metrology system, transmitting an electron beam to the reference wafer from the metrology system and measuring the secondary electron emissions from the reference wafer and generating calibration data based on the characteristics of the reference wafer detected from the secondary electron emissions and adjusting the line width measurement metrology tool set based on the calibration data. 
     To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a perspective view of a reference wafer including a pattern in accordance with the prior art; 
     FIG. 1 b  is a schematic illustration of a portion of the reference wafer of FIG. 1 a  in accordance with a conventional calibration process; 
     FIG. 2 a  is a schematic illustration of a calibration system for calibrating a microscope and a tool using a reference wafer in accordance with the present invention; 
     FIG. 2 b  is a schematic illustration of a calibration system for calibrating a microscope and two tools using a reference wafer in accordance with the present invention; 
     FIG. 3 is a schematic illustration of a semiconductor substrate covered with an insulating layer and a photoresist layer in accordance with the present invention; 
     FIG. 4 is a schematic illustration of the structure of FIG. 3 after the photoresist layer has been patterned in accordance with the present invention; 
     FIG. 5 is a schematic illustration of the structure of FIG. 4 undergoing an etching step in accordance with the present invention; 
     FIG. 6 is a schematic illustration of the structure of FIG. 5 after the etching step is substantially complete in accordance with the present invention; 
     FIG. 7 is a schematic illustration of the structure of FIG. 6 undergoing a stripping step to remove excess photoresist in accordance with the present invention; 
     FIG. 8 is a schematic illustration of the structure of FIG. 7 undergoing a contact fill step to form a conductive layer in accordance with the present invention; 
     FIG. 9 is a schematic illustration of the structure of FIG. 8 after a second photoresist layer is applied onto the conductive layer in accordance with the present invention; 
     FIG. 10 is a schematic illustration of the structure of FIG. 9 after the second photoresist layer has been patterned in accordance with the present invention; 
     FIG. 11 is a schematic illustration of the structure of FIG. 10 undergoing a second etching step in accordance with the present invention; 
     FIG. 12 is a schematic illustration of the structure of FIG. 11 after the second etching step is substantially complete in accordance with the present invention; and 
     FIG. 13 is a schematic illustration of the structure of FIG. 12 undergoing a stripping step to remove excess photoresist in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. The present invention involves making and using a reference wafer having a variety of attributes (e.g. reference marks, conductive components) in a metrology process to calibrate various tools used in the lithographic process. The present invention more specifically involves making and using a reference wafer to calibrate a line width measurement metrology tool set. 
     It is also appreciated that the present invention can be employed to calibrate a single system or tool or several systems or tools in a metrology system. In addition, data obtained for one tool or system in a multiple tool system can be used for calibrating other tools or systems in a metrology system. Calibration using the present invention can also be employed for calibrating a single tool or system over different periods of time. 
     Referring now to FIG. 2 a,  a system for calibrating a tool is illustrated including a calibration system  44 , a pair of electron detectors  42 , a line width measurement metrology tool set  46  and a reference wafer  30 . Electron beams  40  are directed at the reference wafer  30  from the calibration system  44 . The electron beams  40  will induce secondary electron emission which will deflect off the reference wafer  30  to the electron detectors  42  coupled to the calibration system  44 . During scanning by the incident electron scan. a secondary electron emission takes place which is used by the detector  42  to form an image. In the case where the number of incident electrons are greater than that of the secondary emission, charge may start to build on the features. The build-up charge may suppress and/or deflect a secondary emission path for the secondary electrons to reach the detector  42 . Broadening or suppression of the signal may lead to erroneous readings. The incident electron beams  40  will begin forming a charge on the first contact  36  and the second contact  38 . However, the contacts are electrically coupled to the semiconductor substrate  32 , which acts as a ground for the contacts. Therefore, the charge formed on the contacts dissipates through the semiconductor substrate  32  and the secondary electron emission will not be disturbed or suppressed due to the charge. It is to be appreciated that the electron detectors  42  can be replaced by detectors used in calibrating the line width measurement metrology tool set  46  or the detectors could be used to calibrate the electron detectors  42 . A variety of calibration setups is contemplated by the present invention which would be apparent to those skilled in the art. 
     Referring now to FIG. 2 b,  a system for calibrating a tool  46  (it is to be appreciated that a second tool may be employed) is illustrated including the calibration system  44 , the pair of electron detectors  42 , the reference wafer  30  and a pair of detectors  50  for receiving emissions from the electron beams  40 . Electron beams  40  are directed at the reference wafer  30  from the calibration system  44 . The electron beams  40  will induce a secondary electron emission that is deflected off the reference wafer  30  to the detectors  50  coupled to the calibration system  44 . The emissions will be evaluated into calibration data used in calibrating the electron detectors  42 , and the tool  46 . The electron beams  40  will begin forming a charge on the first contact  36  and the second contact  38 . However, the contacts are electrically coupled to the semiconductor substrate  32 , which acts as a ground for the contacts. Therefore, the charge formed on the contacts dissipates through the semiconductor substrate  32  and the secondary electron emission will not disturbed or suppressed due to the charge. It is to be appreciated that the calibration data can include calibration data with respect to the electron detector(s) that is different with respect to each tool. The separate calibration data can be used to cross reference each tool with respect to the other. 
     FIGS. 3-13 illustrate an embodiment of the present invention. With regard to the description in connection with the embodiment of FIGS. 3-13, the term substrate includes not only a semiconductor substrate, but also any and all layers and structures fabricated over the semiconductor substrate up to 
     FIG. 2 illustrates a non-patterned portion of a reference wafer  60  including an insulating layer (e.g., an oxide layer)  64  which is formed on a semiconductor substrate  62 . Semiconductor substrate  62  may be any suitable semiconductor material, for example, a monocrystalline silicon substrate. Any suitable technique (e.g., thermal oxidation, plasma enhanced chemical vapor deposition (CVD), thermal enhanced CVD and spin on techniques) may be employed in forming the insulating layer  64 . 
     A photoresist layer is formed on the insulating layer  64 . The photoresist layer  66  has a thickness suitable for functioning as a mask for etching the underlying insulating layer  64  and for forming patterns or openings in the developed photoresist layer  66 . The photoresist layer  66  is patterned using conventional techniques to form a first opening  68  and a second opening  70  (FIG.  4 ). The size of the first opening  68  and the size of the second opening  70  is about the size of the ultimate vias to be formed in the insulating layer  66 . The patterned photoresist  66  serves as an etch mask layer for processing or etching the underlying insulating layer  64 . 
     Turning now to FIG. 5, the insulating layer  64  is shown undergoing an etching process  90  wherein the patterned photoresist layer  66  serves as a mask. For example, the etching process  90  may include a reactive ion etch (RIE), that is highly selective to the insulating layer  64  with respect to the patterned resist layer  66 . It is to be appreciated that any suitable etch methodology for selectively etching the insulating layer  64  over the patterned photoresist layer  66  may be employed and is intended to fall within the scope of the hereto appended claims. For example, the insulating layer  64  at the first opening  68  and the second opening  70  is anisotropically etched with a plasma gas(es), herein carbon tetrafloride (CF 4 ) containing fluorine ions, in a commercially available etcher, such as a parallel plate RIE apparatus or, alternatively, an electron cyclotron resonance (ECR) plasma reactor to replicate the mask pattern of the patterned photoresist layer  66  to thereby create a first via  72  and a second via  74  in the insulating layer  64  (FIG.  6 ). 
     FIG. 6 also illustrates a stripping step  100  (e.g., ashing in an O 2 plasma) to remove remaining portions of the photoresist layer  66 . FIG. 7 illustrates a partially complete reference wafer  60 ′ after the stripping step  100  is substantially complete. Next, a deposition step is performed on the structure  60 ′ (FIG. 8) to form a conductive layer  76  over the structure  60 ′. Preferably, the conductive layer  76  is comprised of polysilicon or metal. 
     FIG. 9 illustrates a second photoresist layer  78  formed on the conductive layer  76 . The second photoresist layer  78  has a thickness of about 500 Å-5000 Å, however, it is to be appreciated that the thickness thereof may be of any dimension suitable for carrying out the present invention. Accordingly, the thickness of the second photoresist layer  78  can vary in correspondence with the wavelength of radiation used to pattern the second photoresist layer  78 . The second photoresist layer  78  may be formed over the conductive, layer  76  via conventional spin-coating or spin casting deposition techniques. The second photoresist layer  78  has a thickness suitable for functioning as a mask for etching the underlying conductive layer  76  and for forming patterns or openings in the developed second photoresist layer  78 . 
     The second photoresist layer  78  is patterned using conventional techniques to eliminate photoresist material around the vias  72  and  74  to form a first contact area  80  and a second contact area  82  (FIG.  10 ). The size of the first contact area  80  and the second contact area  82  is larger than the size of the vias  72  and  74  formed in the oxide layer  64 . The second patterned photoresist  78  serves as an etch mask layer for processing or etching the underlying conductive layer  76 . 
     An etch step  110  (e.g., anisotropic reactive ion etching (RIE)) (FIG. 11) is performed to form a first contact  84  and a second conduct  86  in the material layer  76 . The resultant structure is illustrated in FIG.  12 . The second patterned photoresist  78  is used as a mask for selectively etching the material layer  76  to provide a patterned material layer  76 . Any suitable etch technique may be used to etch the material layer  76 . Preferably, a selective etch technique may be used to etch the material layer  76  at a relatively greater rate as compared to the rate that the material of the second patterned photoresist  78  is etched. The etch step  110  is also highly selective to the material layer  76  over the underlying insulating layer  64 , so as to mitigate damage to the insulating layer  64 . 
     FIG. 12 also illustrates a stripping step  120  (e.g., ashing in an O 2  plasma) to remove remaining portions of the photoresist layer  78 . FIG. 13 illustrates a complete partial reference wafer  60 ″ after the stripping step  120  is substantially complete. The reference wafer  60 ″ includes the first contact  84  and the second contact  86  electrically coupled to the semiconductor substrate  62 . 
     Although the present invention has been described primarily within the context of lines, it is to be appreciated that the present invention is intended to apply to any feature suitable for carrying out the present invention. 
     What has been described above are preferred embodiments of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims.