Abstract:
A method for detecting semiconductor-manufacturing conditions includes providing a photomask with a plurality of pattern areas each having a plurality of test lines with different pitches, exposing a plurality of wafer with the photomask in different manufacturing conditions, measuring the critical dimensions of the plurality of pattern areas, generating a library of relationships between the pitches and the critical dimension of the pattern areas, exposing a test wafer in an unknown manufacturing condition, finding out a relationships between the pitches and the critical dimension of the pattern areas of the test wafer, searching for a most similar relationship in the library, and detecting a set of manufacturing parameters used to expose the test wafer.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is related to a method for detecting a condition of a semiconductor wafer, and more particularly, to a method for detecting the condition by using scatterometry critical dimension metrology. 
   2. Description of the Prior Art 
   With the advancements of wafer manufacturing processing, advanced lithography makes it possible to mass manufacture semiconductor elements that have line widths of less than 100 nano-meters. However, while the scale of semiconductor is reduced, the critical dimensions (CD) of channel length, junction depth, and gate thickness of a field effect transistor (FET), for example, are reduced, as well. These critical dimensions should be accurately controlled because a little change in the critical dimensions may cause a lot of change in the characteristics of the semiconductor element. Therefore, the measurements of critical dimensions with advanced lithography will become more and more important in the future. Rapid and repeatable measurements will be used to detect the characteristics of semiconductor products on a production line. The accuracy of the measurements influences the yield and the reliability of the semiconductor products. 
   Some favorable metrologies for semiconductor process include advanced bright-field microscopy, scanning electron microscopy (SEM), scatterometry, and atomic force microscopy (AFM). Among them, the scatterometry metrology provides much progress in critical dimension measurement and profile analysis. Scatterometry can provide more information than other metrologies, such as pitch, sidewall angles, thickness, and the thickness of the underlying thin film. Therefore, the scatterometry CD metrology has been widely used in the photolithography process to provide the critical dimension measurement and profile analysis. The advantages of using scatterometry in the photolithography process lie in its non-destructive measurement, the ability to measure the critical dimension as small as 40 nm, and installation without expensive equipment. 
   The major principle of scatterometry is that the intensity of incident light of the periodic gratings varies with the incident angle or with the wavelength of the incident light and the relationship between reflectance and angle/wavelength can be recorded as different signatures. Scatterometry can be divided into forward metrology and reverse metrology. Forward metrology is to measure the spectra of the laser beam reflected by the periodic grating. Reverse metrology is to compare the measured spectra with the theoretical spectra model to provide the structure information of the grating. The reverse metrology can be further divided into library matching method and direct regression method. The library matching method compares the measured data with the data in a library to find the closest grating structure. The direct regression method compares the measured data with a theoretical model, and modifies the parameter to gradually reduce the difference between measured and theoretical data. 
   In the prior art, only the impact on the critical dimensions from limited process parameters is analyzed, such as the relationship between the exposure energy and the critical dimension. Most of the other process parameters, such as post-bake temperature and illumination conditions, are seldom discussed and it is difficult to know their impact on the critical dimension. 
   SUMMARY OF THE INVENTION 
   It is therefore an objective of the invention to provide a method for detecting the impact of a plurality of semiconductor manufacturing parameters. 
   A method for detecting a condition for manufacturing a semiconductor wafer comprises providing a photomask having a plurality of pattern areas, each of the pattern areas having a plurality test lines, pitches between the test lines of different pattern areas being different; exposing a plurality of wafers via the photomask with different sets of process parameters so as to form pattern areas with test lines of different pitches on each of the wafers; measuring a critical dimension of each of the pattern areas of each of the wafers; establishing a relationship library between the pitches of the pattern areas of each of the wafers and the measured critical dimensions of the pattern areas of the wafer; exposing a test wafer via the photomask so as to form pattern areas with test lines of different pitches on the test wafer; measuring a critical dimension of each of pattern areas of the test wafer; establishing a relationship between the pitches of the pattern areas of the test wafer and the measured critical dimensions of the pattern areas of the test wafer; finding a relationship from the relationships library which is most similar to the relationship of the test wafer; and detecting a set of process parameters used to expose the test wafer according to the relationship found in the relationships library. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a wafer used in critical dimension metrology of the present invention. 
       FIG. 2  shows a setting of the scatterometry critical dimension metrology of the present invention. 
       FIG. 3  shows a grating structure on the wafer for critical dimension measurement of the present invention. 
       FIG. 4  shows proximity profile curves measured according to the metrology in  FIG. 2 . 
       FIG. 5  shows a proximity profile error signature curve according to a difference of the proximity profile curves. 
       FIG. 6  shows how an under-test proximity profile error signature is compared with the proximity profile error signatures in the library. 
       FIG. 7  shows a way a wafer is exposed. 
       FIG. 8  shows a way an under-test wafer is exposed. 
       FIG. 9  is a flow chart describing how to detect process drift by using a proximity profile error signature. 
       FIG. 10  shows a way an under-test spectra proximity error signature is compared with spectra proximity error signatures in the library. 
       FIG. 11  is a flow chart describing how to detect process drift by using a spectra proximity error signature. 
   

   DETAILED DESCRIPTION 
   As progress continues in the semiconductor industry, the reduction in the size of semiconductor devices makes proximity effects more significant. Proximity effects can be related to certain photolithographic process parameters, such as illumination setting, defocus, post exposure bake conditions, properties of photoresist, and so forth. These process factors have different impacts on proximity behavior. The present invention proposes methods to analyze the respective impact of these photolithographic process parameters on the proximity behavior and therefore to detect and modify the problem in semiconductor manufacturing. 
   Please refer to  FIG. 1 .  FIG. 1  shows a wafer  31  for sactterometric critical dimension measurement of the present invention. A plurality of production dies  42  are formed above the wafer  31  and the production dies  42  are separated from each other by scribe lines  42 A. After the fabrication processes are completed, the wafer  31  will be cut along the scribe lines  42 A to separate the production dies  42 . There is an alignment notch  37  that is used to provide relatively rough alignment of the wafer  31  in the manufacturing process. An illustrative array  61  of a plurality of grating structures  60 A- 60 I is formed above the wafer  31 . The size, shape, number, location and orientation of the grating structures  60 A- 60 I may be varied as a matter of design choice and not limited to what is shown in  FIG. 1 . Please refer to  FIG. 2  and  FIG. 3 .  FIG. 2  shows a illustrative block diagram of a sactterometric critical dimension measurement of the present invention. An illustrative scatterometry tool  74  comprises a representative light source  73  and a detector  75 . The grating structure  60 A- 60 I in  FIG. 1  may be referred to individually and/or collectively by the grating structure  60  in  FIG. 2 . The layer of photoresist is formed above a layer of thin film  36 . The photoresist features  38 A comprising the grating structure  60  have a thickness  65  and sidewalls  62  disposed at a sidewall angle  63 . The photoresist features  38 A are separated by a space  52  that defines the critical dimension of the illustrative grating structure  60 . The thickness  65 , the sidewall angle  63 , the pitch  64  of the photoresist features  38 A, and the spacing  52  between the photoresist features  38 A may be varied as a matter of design choice. The principle of scatterometry is well-known to those skilled in this field and will not be explained further. The scatterometry critical dimension measurement shows very low measurement noise level whose 3-sigma noise level is as low as 1-2 nm, and therefore it is very suitable for the present invention. However, any critical dimension measurement method which can give an equivalent noise level as the one in the scatterometry critical dimension measurement is also a good substitute in the present invention. 
   In order to separately analyze the impacts from the manufacturing parameters in the photolithography process, a proximity profile error signature (PPES) is used to quantitatively classify these impacts in an embodiment of the present invention. PPES is defined as the difference between the baseline proximity profile and the shifted proximity profile. The proximity profile can be calculated from the measurement of the scatterometry CD metrology in  FIG. 2 . Please refer to  FIG. 4 .  FIG. 4  shows a relationship between critical dimension and pitches when varying one of the process parameters. There are a baseline top CD proximity curve  134 , a baseline bottom CD proximity curve  132 , a shifted top CD proximity curve  138 , and a shifted bottom CD proximity curve  136  in  FIG. 4 . Please refer to  FIG. 5 . The top PPES  144  is generated by calculating the difference between the baseline top CD proximity curve  134  and the shifted top CD proximity curve  138 , and the bottom PPES is generated by calculating the difference between the baseline bottom CD proximity curve  132  and the shifted bottom CD proximity curve  136 . Then, the respective PPES is generated according to each process parameter and a PPES library is established. Please refer to  FIG. 6 . When there is a manufacturing condition to be verified, the PPES  110  is generated and compared with the PPES  120  in the PPES library, and the parameters that make the process drift are found. In addition, the proximity profile can be represented by a formula with two variables. Therefore, the PPES can also be represented by the difference of the baseline formula and the shifted formula, and both are two-variable formulas. Please refer to  FIG. 7 ,  FIG. 8 , and  FIG. 9 . The operating steps of the present invention are as follows: 
   Step  901 : Start; 
   Step  902 : Provide a photomask  500  with a plurality of pattern areas  560 A- 560 I, wherein each of the pattern areas has a plurality test lines and pitches between the test lines of different pattern areas are different; Expose a wafer  31  via the photomask  500  to form a plurality of pattern areas  60 A- 60 I with test lines of different pitches on the wafer  31 ; 
   Step  903 : Measure the critical dimension of each of the pattern areas  60 A- 60 I on the wafer  31  by scatterometry CD metrology and generate a proximity profile curve according to the relationship between the pitches of pattern areas  560 A- 560 I and the critical dimension of the pattern areas  60 A- 60 I; 
   Step  904 : If the process parameters are changed, go to step  902 , else continue to step  905 ; 
   Step  905 : Generate a PPES library by calculating the difference of the baseline proximity profile and the shifted proximity profile of different process parameters; 
   Step  906 : If there is a unknown manufacturing process to be verified, go to step  907 , else continue to step  911 ; 
   Step  907 : Expose a wafer  631  via the photomask  500  to form a plurality of pattern areas  660 A- 660 I with test lines of different pitches on the wafer  631 ; 
   Step  908 : Measure the critical dimension of each of the pattern areas  660 A- 660 I on the wafer  631  by scatterometry CD metrology and generate a proximity profile curve according to the relationship between the pitches of pattern areas  560 A- 560 I and the critical dimension of the pattern areas  660 A- 660 I; 
   Step  909 : Generate the PPES of the wafer  631  by calculating the difference of the baseline proximity profile and the shifted proximity profile; 
   Step  910 : Compare the PPES of wafer  631  with the PPES library and determine the process parameters when exposing the wafer  631 ; 
   Step  911 : End. 
   In the embodiment of the present invention, the PPES library can be generated and saved in advance and is not necessarily generated repeatedly. Therefore it is also feasible for real-time analysis on the production line. The exposing processes in  FIG. 7  and  FIG. 8  are only illustrative. Actual exposing process may comprise exposing the wafers  31  and  631  with a plurality of photomasks and the exposing system  580  by using a light source  570  and generating the pattern areas on the wafers  31  and  631 . 
   In other embodiments of the present invention, the spectra curve measured in the scatterometry CD metrology is used to quantitatively classify these impacts. A spectra proximity error signature (SPES) is defined as the difference between the baseline spectra and the shifted spectra. The spectra can be measured in the scatterometry CD metrology in  FIG. 2 . The respective SPES is generated according to each process parameter and a SPES library is established. When there is a manufacturing condition to be verified, the SPES  210  is generated and compared with the SPES  220  in the PPES library, and the parameters that make the process drift are determined. Please refer to  FIG. 7 ,  FIG. 8 , and  FIG. 11 . The operating steps of the present invention are as follows: 
   Step  1101 : Start; 
   Step  1102 : Provide a photomask  500  with a plurality of pattern areas  560 A- 560 I, wherein each of the pattern areas has a plurality test lines and pitches between the test lines of different pattern areas are different; Expose a wafer  31  via the photomask  500  to form a plurality of pattern areas  60 A- 60 I with test lines of different pitches on the wafer  31 ; 
   Step  1103 : Measure the critical dimension of each of the pattern areas  60 A- 60 I on the wafer  31  by scatterometry CD metrology and generate a spectra curve according to the relationship between the pitches of pattern areas  560 A- 560 I and the critical dimension of the pattern areas  60 A- 60 I; 
   Step  1104 : If the process parameters are changed, go to step  1102 , else continue to step  1105 ; 
   Step  1105 : Generate a SPES library by calculating the difference of the baseline spectra and the shifted spectra of different process parameters; 
   Step  1106 : If there is a unknown manufacturing process to be verified, go to step  1107 , else continue to step  1111 ; 
   Step  1107 : Expose a wafer  631  via the photomask  500  to form a plurality of pattern areas  660 A- 660 I with test lines of different pitches on the wafer  631 ; 
   Step  1108 : Measure the critical dimension of each of the pattern areas  660 A- 660 I on the wafer  631  by scatterometry CD metrology and generate a spectra curve according to the relationship between the pitches of pattern areas  560 A- 560 I and the critical dimension of the pattern areas  660 A- 660 I; 
   Step  1109 : Generate the SPES of the wafer  631  by calculating the difference of the baseline spectra and the shifted spectra; 
   Step  1110 : Compare the SPES of wafer  631  with the SPES library and determine the process parameters when exposing the wafer  631 ; 
   Step  1111 : End. 
   In the embodiment of the present invention, the SPES library can be generated and saved in advance and is not necessarily generated repeatedly. Therefore, it is also feasible for real-time analysis on the production line. In addition, in this embodiment the comparisons are based on the directly measured spectra data, alleviating the need to convert the spectra data to the profile data. 
   The present invention analyzes the respective impact of these photolithographic process parameters on the proximity behavior and detects drift in semiconductor process parameters. By providing both the profile analysis and spectra analysis methods, these methods are feasible for more critical dimension measurement metrologies. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.