Patent Publication Number: US-2023152714-A1

Title: Method for correcting critical dimension measurements of lithographic tool

Description:
BACKGROUND 
     Technical Field 
     The disclosure relates to a measurements monitoring and correcting method, and more particularly to a method for monitoring and correcting the critical dimension (CD) measurements of a lithographic tool. 
     Description of Background 
     In the manufacture of integrated circuits (ICs), at least one lithographic process is generally required for the formation of any active or electrical passive element in order to define the location and the basic dimensions of the element. As the level of ICs integration continues to increase, the size of semiconductor devices shrinks correspondingly. The manufacture of ICs has relied more heavily on the accuracy of the lithographic process. 
     In order to confirm line widths of the pattern have been precisely transferred to the photo-sensitive layer, quality inspections including a CD measurement may be required. However, the diffraction of the focused electron beam provided by a Scanning Electron Microscope (SEM) used to perform the CD measurement may cause measuring bias, which can deteriorate the reliability of the quality inspections. 
     Therefore, there is a need of providing a method for correcting CD measurements of a lithographic tool to obviate the drawbacks encountered in the prior art. 
     SUMMARY 
     One aspect of the present disclosure is to provide a method for correcting CD measurements of a lithographic tool, wherein the method includes steps as follows. A correction pattern having a first sub-pattern parallel to a first direction and a second sub-pattern parallel to a second direction is provided on a lithographic mask; wherein the first sub-pattern and the second sub-pattern come cross with each other. A first After-Develop-Inspection critical dimension (ADI CD) of a developed pattern transferred from the correction pattern and formed on a photo-sensitive layer is measured using the lithographic tool along a first scanning direction. A second ADI CD of the developed pattern is measured using the lithographic tool along a second scanning direction. The first ADI CD is subtracted from the second ADI CD to obtain a measurement bias value. Exposure conditions of the lithographic tool are adjusted according to the measurement bias value. 
     In accordance with the aforementioned embodiments of the present disclosure, a method for correcting CD measurements of a lithographic tool is provided. A correction pattern having a first sub-pattern and a second sub-pattern intersect with each other is formed on a lithographic mask. After exposure/development, at least two ADI CDs of a developed pattern transferred from the correction pattern and formed on a photo-sensitive layer are measured using the lithographic tool along different scanning directions. A measurement bias value can be obtained by accountings the difference between these two ADI CDs measured along difference scanning directions. Therefore, the exposure conditions and/or measuring parameters of the lithographic tool can be adjusted according to the measurement bias value to alleviate the measurement bias resulted either from the diffraction of the focused electron beam used for measuring the ADI CD or from the developing light used for forming the developed pattern, so as to enhance the processing quality of the lithographic process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objects and advantages of the present disclosure will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
         FIG.  1    is a flowchart illustrating a method for correcting CD measurements of a lithographic tool, according to one embodiment of the present disclosure; 
         FIG.  2 A  is a top view illustrating a correction pattern formed on a lithographic mask and used for correcting the CD measurements of a lithographic tool; 
         FIG.  2 B  is a partial enlarged view illustrating a vertical stepped structure formed on a first sub-pattern of the correction pattern as depicted in  FIG.  2 A ; and 
         FIG.  3    is a top view illustrating a developed pattern transferred from the correction pattern as depicted in  FIG.  1    and formed on a photo-sensitive layer according to one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments as illustrated below provide a method for correcting CD measurements of a lithographic tool, to enable the lithographic tool to alleviate the measurement bias and enhance the processing quality of the lithographic process. The present disclosure will now be described more specifically with reference to the following embodiments illustrating the structure and arrangements thereof. 
     It is to be noted that the following descriptions of preferred embodiments of this disclosure are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed. Also, it is important to point out that there may be other features, elements, steps, and parameters for implementing the embodiments of the present disclosure which are not specifically illustrated. Thus, the descriptions and the drawings are to be regard as an illustrative sense rather than a restrictive sense. Various modifications and similar arrangements may be provided by the persons skilled in the art within the spirit and scope of the present disclosure. In addition, the illustrations may not be necessarily drawn to scale, and the identical elements of the embodiments are designated with the same reference numerals. 
     A lithographic process generally includes exposure of a photo-sensitive layer, such as a photoresist layer, using a lithographic tool to accurately produce an image on the photo-sensitive layer by some form of energy, such as light bean, charged particles or x-rays, so as to transfer a pattern of a lithographic mask or reticle to the photo-sensitive layer. The photo-sensitive layer is subsequently developed to remove some selected (or unselected) areas, thereby leaving portions of the underlying layer, such as a semiconductor substrate, exposed to allow subsequent processes, such as an etching process or an ion implantation process, performed thereon. 
     In order to confirm all line widths of the pattern have been precisely transferred to the photo-sensitive layer, quality inspections may be required, including using a Scanning Electron Microscope (SEM) after development to measure an After-Develop-Inspection critical dimension (ADI CD) of the pattern transferred to the photo-sensitive layer. During the measurements of the ADI CD, a highly focused electron beam is used to scan the surface of the developed photo-sensitive layer, the electronic signal can be obtained by measuring the electron beam scattered by the remaining portion of the developed photo-sensitive layer and the underlying layer not covered by the developed photo-sensitive layer with a detector, and the ADI CD can be obtained by figuring the correlation between the scanning distance and the electronic signal of the developed photo-sensitive layer and the underlying layer. 
     However, the diffraction of the focused electron beam may cause measuring bias of the ADI CD measurements, which can deteriorate the reliability of the quality inspections. Typically, the measuring bias can be observed by the factors that when an optical proximity correction (OPC) is performed along the vertical direction of the pattern, the Scatterometry critical dimension (SCD) obtained may away off from the SCD obtained by the OPC performed along the horizontal direction. 
     In addition, during the exposure process, the light that is projected onto the lithographic mask may diverge and spread due to diffraction (which may be refereed as to a proximity effect). Meanwhile, a portion of the light passing through the lithographic mask may penetrate the photo-sensitive layer and be reflected back by the underlying layer causing some light interference. Hence, multiple exposure of the photo-sensitive layer may make the pattern transferred to the photo-sensitive layer deformed and further contribute the measuring bias of the ADI CD measurements. 
     How to alleviate the measuring bias due to either the developing light or the focused electron beam should be an important issue in the pertinent technical field. 
       FIG.  1    is a flowchart illustrating a method for correcting CD measurements of a lithographic tool, according to one embodiment of the present disclosure. The method for correcting CD measurements of the lithographic tool includes steps as follows. Firstly, referring to step S 11 , a correction pattern  101  is provided on a lithographic mask  100 . 
       FIG.  2 A  is a top view illustrating a correction pattern  101  formed on a lithographic mask  100  and used for correcting the CD measurements of the lithographic tool. In some embodiments of the present disclosure, the correction pattern  101  includes a first sub-pattern  102  extending along a first direction L 1  and a second sub-pattern  103  extending along a second direction L 2 , wherein the first sub-pattern  102  and the second sub-pattern  103  intersect with each other; and the first direction and the second direction make a non-180° angle θ. 
     For example, in some embodiments of the present disclosure, the first direction L 1  is perpendicular to the second direction L 2 . In other words, the non-180° angle θ is a 90° angle. However, the non-180° angle θ is not limited thereto. In some other embodiments, the non-180° angle θ can be a non-90° angle. 
       FIG.  3    is a top view illustrating a developed pattern  301  transferred from the correction pattern  101  as depicted in  FIG.  1    and formed on a photo-sensitive layer  300  according to one embodiment of the present disclosure. In the present embodiment, the developed pattern  301  corresponds to the correction pattern  101 , the correction pattern  101  is formed on a peripheral area  100   a  of the lithographic mask  100 , the developed pattern  301  is formed on a scribe lane  300   a  of a photo-sensitive layer  300 , and the peripheral area  100   a  on which the correction pattern  101  is formed corresponds to the scribe lane  300   a  of the photo-sensitive layer  300 . In other words, the developed pattern  301  that is transferred from the correction pattern  101  can be formed on the scribe lane  300   a  of the photo-sensitive substrate  300  aligned to the peripheral area  100   a  of the lithographic mask  100 , after a develop process. The first direction L 1  and a reference horizontal direction R 1  of the lithographic mask  100  form a substantially 45° angle, the second direction L 2  and a reference vertical direction R 2  of the lithographic mask form a substantially 45° angle. 
     In detailed, the first sub-pattern  102  of the correction pattern  101  includes a plurality of parallel strips, such as strips  102   a - 102   d , wherein the strip  102   a  is separated from the strip  102   b  with a pitch P 1 , the strip  102   b  is separates from the strip  102   c  with a pitch P 2  not equal to the pitch P 1 , the strip  102   c  is separated from the strip  102   d  with a pitch P 3  not equal to the pitch P 1  and pitch P 2 . The second sub-pattern  103  includes a plurality of parallel strips, such as strips  103   a - 103   d , wherein the strip  103   a  is separated from the strip  103   b  with a pitch P 4  substantially equal to the pitch P 1 , the strip  103   b  is separated from the strip  103   c  with a pitch P 5  substantially equal to the pitch P 2 , the strip  103   c  separates from the strip  103   d  with a pitch P 6  substantially equal to the pitch P 3 . 
     In some embodiments, the at least one of the strips  102   a - 102   d  of the first sub-pattern  102  and the strips  103   a - 103   d  of the second sub-pattern  103  includes a vertical stepped structure  205  including a plurality of sections. For example,  FIG.  2 B  is a partial enlarged view illustrating a vertical stepped structure  105  formed in the first sub-pattern  102  of the correction pattern  101  as depicted in  FIG.  2 A . In the present embodiment, the strip  102   d  of the first sub-pattern  102  has a vertical stepped structure  205  including a first section  205   a , a second section  205   b  and a third section  205   c  connected in series; wherein the first section  205   a  is connected to and perpendicular to the second section  205   b ; and the second section  205   b  is connected to and perpendicular to the third section  205   c . The first section  205   a  extends along a direction parallel to the reference horizontal direction R 1  of the lithographic mask  100 . 
     In these embodiments, the presence of the vertical stepped structure  205  of the strip  102   d  can alleviate the diffraction of the light passing through the lithographic mask  100  during the exposure/development process, to avoid the developed pattern  301  formed on the photo-sensitive layer  300  being deformed. 
     In some embodiments of the present disclosure, the lithographic mask  100  may include other correction patterns. For example, in the present embodiment, the lithographic mask  100  may further include three correction pattern  101 ′,  101 ″ and  101 ″. The correction pattern  101 ′ is mirrored and spliced with the correction pattern  101  along a symmetry axis K 1  parallel to the first direction L 1 ; the correction pattern  101 ″ is mirrored and spliced with the correction pattern  101  along a symmetry axis K 2  parallel to the second direction L 2 ; and the correction pattern  101 ′″ is mirrored and spliced with the correction pattern  101  along the reference vertical direction R 2 . 
     Next, referring to step S 12  in  FIG.  1   , at least one exposure/development process is performed to transfer the correction pattern  101  from the lithographic mask  100  to the photo-sensitive layer  300  and then to form a developed pattern  301  on the photo-sensitive layer  300 . In the present embodiment, a lithographic tool is used to project energy, such as light bean, charged particles or x-rays, penetrating through the correction pattern  101  from the lithographic mask  100  to form an image onto the photo-sensitive layer  300 ; and the photo-sensitive layer  300  exposed by the image is then developed. Therefore, the correction pattern  101  of the lithographic mask  100  can be transferred onto the photo-sensitive layer  300  by the image, and the developed pattern  301  can be then formed on the photo-sensitive layer  300  by the development. In other words, the developed pattern  301  corresponds to the correction pattern  101 . 
       FIG.  3    is a top view illustrating a developed pattern  301  transferred from the correction pattern  100  as depicted in  FIG.  1    and formed on the photo-sensitive layer  300  according to one embodiment of the present disclosure. In the present embodiment, the developed pattern  301  is formed on the scribe lane  300   a  and includes a first developed sub-pattern  302  and a second developed sub-pattern  303 . The first developed sub-pattern  302  includes a plurality of patterned lines  302   a - 302   d  corresponding to the strips  102   a - 102   d  of the first sub-pattern  102  respectively. The patterned line  302   a  is separated from the patterned line  302   b  for a distance W 1 ; the patterned line  302   b  is separated from the patterned line  302   c  for a distance W 2 ; and the patterned line  302   c  is separated from the patterned line  302   d  for a distance W 3 . 
     The second developed sub-pattern  303  includes a plurality of patterned lines  303   a - 303   d  corresponding to the strip  103   a - 103   d  of the second sub-pattern  103  respectively. The patterned line  303   a  is separated from the patterned line  303   b  for a distance W 4 ; the patterned line  303   b  is separated from the patterned line  303   c  for a distance W 5 ; and the patterned line  303   c  is separated from the patterned line  303   d  for a distance W 6 . 
     After the exposure/development process, the correction pattern  101 ′,  101 ″ and  101 ′″ of the lithographic mask  100  are also transferred onto the photo-sensitive layer  300  to respectively form a developed pattern  301 ′ spliced with the developed pattern  301  along a symmetry axis KD 1 , a developed pattern  301 ″ spliced with the developed pattern  301  along a symmetry axis KD 2 , and a developed pattern  301 ″ spliced with the developed pattern  301  along a symmetry axis KD 3 . 
     Thereafter, referring to step S 13  in  FIG.  1   , a first After-Develop-Inspection critical dimension (ADI CD) of the developed pattern  301  is measured using the lithographic tool  31  along a first scanning direction LS 1 . In some embodiments of the present, the measurement of the first ADI CD of the developed pattern  301  includes using the lithographic tool  31  to project a highly focused electron beam  31   a  to scan the developed pattern  301  along the first scanning direction LS 1 ; using a detector (not shown) to measure the electron beam scattered by the different materials of the developed pattern  301  and the exposed portion of the photo-sensitive layer  300  to obtain electronic signal; and the first ADI CD of the developed pattern  301  can be obtained by figuring the correlation between the scanning distance and the electronic signal received along the first scanning direction LS 1 . 
     In the present embodiment, the step of measuring the first ADI CD of the developed pattern  301  includes steps as follows. The lithographic tool  31  is used to project a highly focused electron beam  31   a  scanning the first developed sub-pattern  302  along the first scanning direction LS 1  to measure the distance W 1  separating the patterned lines  302   a  and  302   b , the distance W 2  separating the patterned lines  302   b  and  302   c  and the distance W 3  separating the patterned lines  302   c  and  302   d . Wherein, the first scanning direction LS 1  can be (but not limited to) parallel to the extending direction of the distance W 1 . 
     Referring to step S 14  in  FIG.  1   , a second ADI CD of the developed pattern  301  is measured using the lithographic tool  31  along a second scanning direction LS 2 . In the present embodiment, the measurement of the second ADI CD of the developed pattern  301  includes using the lithographic tool  31  project a highly focused electron beam  31   a  to scan the second developed sub-pattern  303  along the second scanning direction LS 2  to measure the distance W 4  separating the patterned lines  303   a  and  303   b , the distance W 5  separating the patterned lines  303   b  and  303   c  and the distance W 6  separating the patterned lines  303   c  and  303   d . Wherein the second scanning direction LS 2  can be (but not limited to) parallel to the extending direction of the distance W 4 . 
     However, the first scanning direction LS 1  and the second scanning direction LS 2  are not limited thereto. In some embodiments, the first scanning direction LS 1  and the second scanning direction LS 2  may not parallel to the extending direction of the distance W 1  and the extending direction of the distance W 4  respectively. 
     Then, referring to step S 15  in  FIG.  1   , the first ADI CD is subtracted from the second ADI CD to obtain a measurement bias value. In some embodiments of the present disclosure, the step of obtaining the measurement bias value includes subtracting the distance W 1  from the distance W 4 , subtracting the distance W 2  from the distance W 5 , or subtracting the distance W 3  from the distance W 6 . In some other embodiments of the present disclosure, the step of obtaining the measurement bias value includes calculating a first average of the distance W 1 , the distance W 2  and the distance W 3 ; calculating a second average of the distance W 4 , the distance W 5  and the distance W 6 ; and subtracting the first average from the second average. 
     Subsequently, the exposure conditions or measuring parameters of the lithographic tool  31  are adjusted according to the measurement bias value (referring to step S 16  in  FIG.  1   ) for improving the reliability of the ADI CD measurements. 
     For example, in some embodiment of the present disclosure, the lithographic tool  31  may include an optical measuring tool, a metrology tool having a SEM for performing the ADI CD measurements and/or an optical stepper having an objective lens for projecting images (light) onto the photo-sensitive layer  300 . The measuring parameters of the SEM, such as the energy of the electron beams, as well as the exposure conditions of the optical stepper, such as the focal length of the objective lens, the exposure time and the thickness of the photo-sensitive layer  300  can be adjusted according to the measurement bias value. The adjusted conditions and parameters can be input to an OPC and simulation software tool, and the correction results of the ADI CD measurements can be thus verified by the SCD obtained from the OPC. 
     In accordance with the aforementioned embodiments of the present disclosure, a method for correcting CD measurements of a lithographic tool is provided. A correction pattern having a first sub-pattern and a second sub-pattern intersect with each other is formed on a lithographic mask. After exposure/development, at least two ADI CDs of a developed pattern transferred from the correction pattern and formed on a photo-sensitive layer are measured using the lithographic tool along different scanning directions. A measurement bias value can be obtained by accountings the difference between these two ADI CDs measured along difference scanning directions. Therefore, the exposure conditions and/or measuring parameters of the lithographic tool can be adjusted according to the measurement bias value to alleviate the measurement bias resulted either from the diffraction of the focused electron beam used for measuring the ADI CD or from the developing light used for forming the developed pattern, so as to enhance the processing quality of the lithographic process. 
     While the disclosure has been described by way of example and in terms of the exemplary embodiment(s), it is to be understood that the disclosure is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.