Abstract:
A photo mask includes an asymmetrical diffraction grating pattern in which diffraction efficiencies of plus primary diffracted light and minus primary diffracted light are different, the asymmetrical diffraction grating pattern including a shielding portion which shields light, a first transmitting portion which transmits light, and a second transmitting portion which transmits light, a ratio of widths of the shielding portion, the first transmitting portion, and the second transmitting portion being n11 where n is a positive real number except 2, the asymmetrical diffraction grating pattern approximately satisfying 163°≦360°/(n+2)+θ≦197° where θ (≠90°) indicates an absolute value of a difference between a phase of the light transmitted through the first transmitting portion and that of the light transmitted through the second transmitting portion, and a reference pattern for obtaining an image as a reference for measuring a shift of an image of the asymmetrical diffraction grating pattern.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2004-217874, filed Jul. 26, 2004, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a photo mask for use in a semiconductor field, a focus measuring method using the mask, and a method of manufacturing a semiconductor device.  
         [0004]     2. Description of the Related Art  
         [0005]     A tolerance of focus permitted in lithography has been narrowed as a design rule of a semiconductor device to be manufactured is miniaturized. When the tolerance of the focus is narrowed, flatness of a wafer and specifications with respect to curvature of field of an exposure apparatus has been strict. Moreover, a high-precision measuring method of the focus, curvature of field and the like using a resist pattern transferred onto the wafer has become important.  
         [0006]     A focus test mask comprising an asymmetrical diffraction grating pattern and a reference pattern, and a focus measuring method using the focus test mask and utilizing a phenomenon in which an image of the asymmetrical diffraction grating pattern shifts in proportion to a focus value have been known (Jpn. Pat. No. 3297423). Since the focus measuring method has a high measurement precision having a measurement error of 5 nm or less, and the measuring is simple, the method can be said to be one of most promising techniques at present.  
         [0007]     The asymmetrical diffraction grating pattern comprises a shielding portion, a transmitting portion, and 90° phase grooved portion. A line width ratio of the shielding portion, transmitting portion, and 90° phase grooved portion is ideally 2:1:1. On the other hand, an alternating type phase shift exposure mask including a pattern (device pattern) for manufacturing an actual semiconductor product comprises a 180° phase grooved portion.  
         [0008]     A method of manufacturing an exposure mask comprising the asymmetrical diffraction grating pattern and the device pattern includes a step of forming the 90° phase grooved portion, and a step of forming the 180° phase grooved portion. When these two steps are performed, a manufacturing process is complicated, and manufacturing costs remarkably rise. This respect will be further described hereinafter.  
         [0009]     The step of forming the 180° phase grooved portion includes a step of forming a trench vertically in the surface of a quartz glass substrate by a dry process (e.g., a vertical etching process such as an RIE process); and a step of expanding the trench by predetermined amounts in a lateral direction and a vertical direction by a wet process (isotropic etching process). A sum of grooved amounts by the dry and wet processes is a grooved amount by which a phase of transmitted light delays by 180° as compared with a case where there is not any grooved portion.  
         [0010]     To obtain a high-precision alternating type phase shift exposure mask, the groove has to be made vertically, and further expanded in the lateral direction. However, an etching process to expand the groove only in the lateral direction does not exist. Therefore, as described above, combined use of the dry and wet processes is required. Since an amount to be expanded in the lateral direction needs to be controlled with a high precision, the grooved amount in the dry process is a depth obtained by subtracting the amount to be expanded in the lateral direction from the amount corresponding to 180°. On the other hand, to obtain a high-precision focus test mask, a grooved portion corresponding to 90° has to be formed only by the dry process.  
         [0011]     Therefore, in a conventional technique, the above-described etching processes have to be separately performed in order to realize the high-precision alternating type phase shift exposure mask and the high-precision focus test mask in one exposure mask, and a mask manufacturing cost rises by at least 30% or more as compared with the conventional alternating type phase shift exposure mask.  
       BRIEF SUMMARY OF THE INVENTION  
       [0012]     According to an aspect of the present invention, there is provided a photo mask comprising: an asymmetrical diffraction grating pattern in which diffraction efficiencies of plus primary diffracted light and minus primary diffracted light are different, the asymmetrical diffraction grating pattern including a shielding portion which shields light; a first transmitting portion which transmits light; and a second transmitting portion which transmits light, a ratio of widths of the shielding portion, the first transmitting portion, and the second transmitting portion being n:1:1 where n is a positive real number except 2, the asymmetrical diffraction grating pattern approximately satisfying 163°≦360°/(n+2)+θ≦197° where θ (≠90°) indicates an absolute value of a difference between a phase of the light transmitted through the first transmitting portion and that of the light transmitted through the second transmitting portion; and a reference pattern configured to obtain an image as a reference for measuring a shift of an image of the asymmetrical diffraction grating pattern.  
         [0013]     According to an aspect of the present invention, there is provided a focus measuring method comprising: preparing a focus test mask, the focus test mask comprising: an asymmetrical diffraction grating pattern in which diffraction efficiencies of plus primary diffracted light and minus primary diffracted light are different, the asymmetrical diffraction grating pattern including a shielding portion which shields light, a first transmitting portion which transmits light, and a second transmitting portion which transmits light, a ratio of widths of the shielding portion, the first transmitting portion, and the second transmitting portion being n:1:1 where n is a positive real number except 2, the asymmetrical diffraction grating pattern approximately satisfying 163°≦360°/(n+2)+θ≦197° where θ (≠90°) indicates an absolute value of a difference between a phase of the light transmitted through the first transmitting portion and that of the light transmitted through the second transmitting portion; and a reference pattern configured to obtain an image as a reference for measuring a shift of an image of the asymmetrical diffraction grating pattern; applying a photosensitive agent on the substrate; exposing images of the asymmetrical diffraction grating pattern and the reference pattern in the photo mask at the same time onto the substrate; developing a pattern transferred on the substrate; and measuring a relative distance between the images of the asymmetrical diffraction grating pattern and the reference pattern formed on the substrate.  
         [0014]     According to an aspect of the present invention, there is provided a method of manufacturing a semiconductor device comprising: preparing an exposure mask, the exposure mask comprising an asymmetrical diffraction grating pattern in which diffraction efficiencies of plus primary diffracted light and minus primary diffracted light are different, the asymmetrical diffraction grating pattern including a shielding portion which shields light, a first transmitting portion which transmits light, and a second transmitting portion which transmits light, a ratio of widths of the shielding portion, the first transmitting portion, and the second transmitting portion being n:1:1 where n is a positive real number except 2, the asymmetrical diffraction grating pattern approximately satisfying 163°≦360°/(n+2)+θ≦197° where θ (≠90°) indicates an absolute value of a difference between a phase of the light transmitted through the first transmitting portion and that of the light transmitted through the second transmitting portion; a reference pattern configured to obtain an image which is a reference in measuring a shift of an image of the asymmetrical diffraction grating pattern; and a device pattern; applying a photosensitive agent on the substrate; exposing images of the asymmetrical diffraction grating pattern, the reference pattern, and the device pattern in the photo mask at the same time onto the substrate; developing a pattern transferred on the substrate; inspecting the device pattern formed on the substrate; and measuring a relative distance between the images of the asymmetrical diffraction grating pattern and the reference pattern in a case where a defect is detected in the device pattern in the inspecting the device pattern formed on the substrate. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0015]      FIG. 1  is a plan view schematically showing a photo mask (focus test mask) of an embodiment;  
         [0016]      FIG. 2  is a plan view schematically showing another photo mask (exposure mask) of the embodiment;  
         [0017]      FIG. 3  is a plan view showing details of a test mask of the embodiment;  
         [0018]      FIG. 4  is a sectional view along arrows A-A′ of  FIG. 3 ;  
         [0019]      FIGS. 5A  to  5 H are sectional views showing a method of manufacturing the exposure mask of the embodiment;  
         [0020]      FIG. 6  is a sectional view showing the exposure mask of the embodiment;  
         [0021]      FIG. 7  is a diagram showing a relation between n and θ;  
         [0022]      FIGS. 8A  to  8 F are sectional views of PSG patterns corresponding to n=0.5, 1.0, 1.5, 2.0, 2.5, 3.0;  
         [0023]      FIGS. 9A  to  9 C are plan views showing test marks including typical reference patterns;  
         [0024]      FIGS. 10A  to  10 C are plan views showing test marks capable of eliminating measurement errors attributed to a measuring apparatus;  
         [0025]      FIGS. 11A  to  11 C are plan views showing another test marks capable of eliminating the measurement errors attributed to the measuring apparatus;  
         [0026]      FIG. 12  is a plan view showing a test mark capable of measuring focus and astigmatism;  
         [0027]      FIGS. 13A  to  13 D are plan views showing test marks corresponding to automatic measurements by an alignment shift inspection apparatus;  
         [0028]      FIGS. 14A and 14B  are sectional views along A-A′ and B-B′ of  FIG. 13D ;  
         [0029]      FIGS. 15A and 15B  are plan views of test marks capable of linearizing a relation between a focus value and a positional shift;  
         [0030]      FIG. 16  is a plan view showing an exposure mask including the test mark of  FIG. 15 ;  
         [0031]      FIG. 17  is an explanatory view of a double exposure method using the exposure mask of  FIG. 16 ; and  
         [0032]      FIG. 18  shows a test mark for measuring a focus based on a profile of reflected light.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.  
         [0034]      FIGS. 1 and 2  are plan views schematically showing a photo mask of the embodiment. Concretely,  FIG. 1  shows an example of a focus test mask PM 1 , and  FIG. 2  shows an example of an exposure mask PM 2 .  
         [0035]     The focus test mask PM 1  comprises a quartz glass substrate  1 , and a test mark TM for focus measurement, provided on the quartz glass substrate  1 .  
         [0036]     The exposure mask PM 2  comprises the quartz glass substrate  1 , the test mark TM provided on the quartz glass substrate  1 , and an alternating type phase shift mask PSM provided on the quartz glass substrate  1  and including a pattern (device pattern) for manufacturing an actual semiconductor product. The phase shift mask PSM comprises a shielding portion, and first and second transmitting portions (phase grooved portions). An absolute value of a difference between phases of lights transmitted through the first and second transmitting portions is 180°.  
         [0037]     Each of the test marks TM in the focus test masks PM 1 , PM 2  comprises an asymmetrical diffraction grating pattern (hereinafter referred to as the PSG pattern)  2  including a periodic pattern which differs in diffraction efficiency with plus and minus primary diffracted lights, and a reference pattern  3 .  
         [0038]      FIG. 3  is a plan view showing details of the PSG pattern  2 .  FIG. 4  is a sectional view along arrows A-A′ of  FIG. 3 .  
         [0039]     The PSG pattern  2  comprises shielding portions  4  which are provided on the glass substrate  1  and shield the light; the first transmitting portions  5  which are provided on the glass substrate  1  and transmit the light; and the second transmitting portions  6  (phase grooved portions) which are provided on the glass substrate  1  and transmit the light.  
         [0040]     The shielding portion  4 , the first transmitting portion  5 , and the second transmitting portion  6  satisfy the following two equations ([1], [2]). 
 
W 1 :W 2 :W 3 =n:1:1  [Equation 1]
 
163°≦360°/( n+ 2)+θ≦197°  [Equation 2]
 
         [0041]     W 1  . . . width of the shielding portion  4   
         [0042]     W 2  . . . width of the first transmitting portion  5   
         [0043]     W 3  . . . width of the second transmitting portion  6   
         [0044]     n . . . positive real number other than 2  
         [0045]     θ . . . absolute value of the difference between the phase of the light transmitted through the first transmitting portion  5  and that of the light transmitted through the second transmitting portion  6 . Additionally, 90° is excluded.  
         [0046]      FIGS. 5A  to  5 H are sectional views showing a method of manufacturing the exposure mask PM 2 . A left side of a one-dot chain line shows a region (PSG region) occupied by the PSG pattern  2  in the mask, and a right side of the one-dot chain line shows a region (alternating PSM region) occupied by the phase shift mask PSM in the mask.  
         [0047]     First, as shown in  FIG. 5A , a chromium film (shielding film)  4  to be processed into the shielding portion is formed on the quartz glass substrate  1 .  
         [0048]     Next, as shown in  FIG. 5B , a resist film is formed on the chromium film  4 , and thereafter exposing and developing are performed with respect to the resist film to form a resist pattern  11 . The resist pattern  11  forms shielding portions in a PSG region and alternating PSM region. The resist film is exposed, for example, by EB drawing.  
         [0049]     Next, as shown in  FIG. 5C , the chromium film  4  is etched using the resist pattern  11  a mask. As a result, the shielding portions  4  comprising chromium are formed in the PSG region and alternating PSM region. Thereafter, the resist pattern  11  is removed.  
         [0050]     In the present embodiment, a step of forming the resist pattern for forming the shielding portion  4  in the PSG region, and a step of forming the resist pattern for forming the shielding portion in the alternating PSM region are simultaneously performed in the step of  FIG. 5B . Furthermore, an etching step for forming the shielding portion  4  in the PSG region, and an etching step for forming the shielding portion in the alternating PSM region are simultaneously performed in the step of  FIG. 5C . Therefore, a manufacturing process of the exposure mask PM 2  of the present embodiment is not changed as compared with the manufacturing process of a phase shift mask of a conventional alternating type.  
         [0051]     It is to be noted that the step of forming the resist pattern for forming the shielding portion  4  in the PSG region, and the step of forming the resist pattern for forming the shielding portion in the alternating PSM region may be performed in separate steps, and the etching step for forming the shielding portion  4  in the PSG region and the etching step for forming the shielding portion in the alternating PSM region may be performed in separate steps.  
         [0052]     In this case, the process of forming the shielding portion  4  in the PSG region, and the process of forming the shielding portion in the alternating PSM region can be easily optimized. For example, the step of forming the resist pattern for forming the shielding portion  4  in the PSG region may be performed using a photo repeater which is a apparatus having a higher alignment precision as compared with an EB exposure apparatus.  
         [0053]     Next, as shown in  FIG. 5D , a resist pattern  12  is formed on the shielding portions  4  and quartz glass substrate  1 . The resist pattern  12  forms the first and second transmitting portions (phase grooved portions) in the PSG and alternating PSM regions.  
         [0054]     Next, as shown in  FIG. 5E , the surface of the quartz glass substrate  1  is etched by an RIE process (dry process) using the resist pattern  12  as a mask. As a result, a plurality of grooved portions (trenches) are formed in the surface of the quartz glass substrate  1 . These grooved portions have substantially vertical side walls.  FIG. 5E  shows seven grooved portions. Thereafter, the resist pattern  12  is removed.  
         [0055]     Among the plurality of grooved portions, the grooved portions in the PSG region form the second transmitting portions  6 . Among the plurality of grooved portions, the grooved portions in the alternating PSM region form the second transmitting portions through a wet process of  FIG. 5G .  
         [0056]     Moreover, portions in which any shielding portion  4  is not formed and any grooved portion is not formed form the first transmitting portions  5  in the PSG region and the alternating PSM region.  
         [0057]     Next, as shown in  FIG. 5F , a resist pattern  13  is formed on the shielding portions  4  and the quartz glass substrate  1 . The resist pattern  13  forms second transmitting portions (phase grooved portions) in the alternating PSM region. The PSG region is masked by the resist pattern  13 . The alternating PSM region excluding the grooved portions is masked by the resist pattern  13 .  
         [0058]     Next, as shown in  FIG. 5G , the surface of the quartz glass substrate  1  is etched by the wet process using the resist pattern  13  as a mask, and the grooved portions in the alternating PSM region are expanded in the lateral and vertical directions. The grooved portions expanded in the lateral and vertical directions form second transmitting portions  6 ′ in the alternating PSM region.  
         [0059]     Thereafter, as shown in  FIG. 5H , the resist pattern  13  is removed, and the exposure mask PM 2  is obtained.  
         [0060]      FIG. 6  shows a sectional view of the exposure mask PM 2  to which concrete dimensional values are attached.  
         [0061]     A depth of the grooved portion of the second transmitting portion  6 ′ is strictly controlled in such a manner that the absolute value of the difference between the phase of the light transmitted through the first transmitting portion  5  in the alternating PSM region and that of the light transmitted through the second transmitting portion  6 ′ is 180°.  
         [0062]     The depth of the grooved portion of the second transmitting portion  6 ′ having a depth corresponding to 180° described above can be formed only by the RIE process without using any wet process. However, the grooved portion of the second transmitting portion  6 ′ is formed using the RIE and wet processes for the following reasons.  
         [0063]     Reaction products generated in the RIE process of  FIG. 5E  adhere to the side walls of the grooved portions. When the reaction products adhere to the side walls of the grooved portions, light transmission intensity of the grooved portion is reduced as compared with a non-grooved opening (surface portion of the quartz glass substrate  1  coated with the resist pattern  12 ). The reduction of the light transmission intensity of the grooved portion causes a dimensional error of the resist pattern formed on the wafer.  
         [0064]     Then, after the RIE process, by performing the wet process, the reaction products adhering to the side walls of the grooved portions is removed and the grooved portion of the second transmitting portion  6 ′ having the depth corresponding to 180° described above is formed.  
         [0065]     A total etching amount in the RIE and wet processes needs to be set to an amount corresponding to 180° described above.  
         [0066]     In this case, distribution of the etching amounts in a depth direction (vertical direction) in the RIE process and in a depth direction (vertical direction) in the wet process is determined in consideration of a side etching amount (etching amount in the lateral direction, required for removing the reaction products) required in the wet process.  
         [0067]     Generally, an etching amount corresponding to 75° is selected in the RIE process of  FIG. 5E , and an etching amount corresponding to 105° is selected in the wet process of  FIG. 5E . In this case, the depth of the grooved portion of the second transmitting portion  6 ′ in the PSG region is a depth corresponding to 75°.  
         [0068]     In the case of W 1 :W 2 :W 3 =n:1:1, the condition that one of plus primary diffracted light and minus primary diffracted light turns to be vanished is represented by the following equation. 
 
360°/( n+ 2)+θ=180°  [Equation 3]
 
         [0069]     When θ=75° is substituted into [Equation 3], n=1.4286. A line width ratio (W 1 :W 2 :W 3 ) of the PSG pattern is designed beforehand at 1.43:1:1.  
         [0070]     That is, the line width ratio of the PSG pattern is selected in such a manner that the etching of the quartz glass substrate  1  for forming the PSG pattern is performed only by the RIE process without using any wet process. Therefore, a test mark having a high dimensional precision is manufactured.  
         [0071]     Furthermore, the method of manufacturing the exposure mask PM 2  of the present embodiment is the same as the conventional method of manufacturing an exposure mask except the pattern layout (shape and dimension of the pattern) on the quartz glass substrate  1 .  
         [0072]     Therefore, according to the present embodiment, it is possible to easily realize the exposure mask including the device pattern and the test mark having the high precision without incurring any increase of the manufacturing cost.  
         [0073]     Moreover, when the device pattern is omitted from the exposure mask, it is possible to easily realize a focus test mark including high precision test mark without incurring any increase of manufacturing cost.  
         [0074]      FIG. 7  shows a relation between n and θ in the case of W 1 :W 2 :W 3 =n:1:1.  
         [0075]     In  FIG. 7 , c denotes a ratio (light intensity ratio) of an intensity of the plus primary diffracted light to that of the minus primary diffracted light. The primary diffracted light having a higher intensity is selected for a denominator of the light intensity ratio.  
         [0076]     A thick solid line shows ε=0. A curve of ε=0 shows a relation between n and θ in a case where one primary diffracted light completely vanishes ε=0.01 (ideal condition).  
         [0077]     On the other hand, two thin solid lines show ε=0.01. A curve of ε=0.01 shows a relation between n and θ in a case where one primary diffracted light has an intensity of about 1% of that of the other diffracted light.  
         [0078]     Moreover, two dotted lines show ε=0.02. A curve of ε=0.02 shows a relation between n and θ in a case where one primary diffracted light has an intensity of about 2% of that of the other diffracted light. In a region held between these two dotted lines, focus measuring is sufficiently correctly performed. This region is represented by [Equation 2] described above.  
         [0079]     Here, Table 1 shows a relation between n and θ (ideal value, effective range) in the case of ε=0.02.  
                                                           TABLE 1                                       θ                    Ideal   Effective           n   value   range                            0.5   36°   19 to 53°           1.0   60°   43 to 77°           1.5   77.143°      60 to 94°           2.0   90°   73 to 107°           2.5   100°    83 to 117°           3.0   108°    91 to 125°                  
 
         [0080]     As θ is closer to the ideal value, needless to say, a focus measurement precision becomes higher. However, when ease of designing of the mask is considered, n is preferably a definite number (round number).  FIGS. 8A  to  8 F show sectional views of PSG patterns corresponding to n=0.5 (θ=36°), 1.0 (θ=60°), 1.5 (θ=77.143°), 2.0 (θ=90°), 2.5 (θ=100°), 3.0 (θ=108°). Especially, the mask of  FIG. 8D  is a mask comprising a structure described as one example in Jpn. Pat. No. 3297423. Masks having realistic θ and n are the masks shown in  FIGS. 8A, 8B , and  8 F.  
         [0081]     The focus test mask PM 1  of the present embodiment will be described further. As described above, the focus test mask PM 1  of the present embodiment comprises the test mark TM and the reference pattern  3 .  
         [0082]     As the reference pattern  3 , mainly there are three types shown in  FIGS. 9A  to  9 C. That is, there are a large isolated pattern  3   a  shown in  FIG. 9A , a diffraction grating pattern  3   b  shown in  FIG. 9B , and an asymmetrical diffraction grating  3   c  whose direction is opposite to that of the PSG pattern  2  as shown in  FIG. 9C .  
         [0083]     Since the large isolated pattern  3   a  has a broad DOF, the measuring having a broad focus range is possible. Since an influence of coma aberration of the diffraction grating pattern  3   b  is equivalent to that of an asymmetrical diffraction grating, the measuring which is not influenced by the coma aberration is possible. Moreover, since a relative shift amount of the opposite directed asymmetrical diffraction grating  3   c  is double, the measuring having twice sensitivity is possible.  
         [0084]     Moreover, to remove the measurement error attributed to the measuring apparatus, as shown in  FIGS. 10A  to  10 C and  11 A to  11 C, it is preferable to use the test mark TM having a structure in which one PSG pattern  2  is put between two reference patterns (two large isolated patterns  3   a , two diffraction grating patterns  3   b , or two asymmetrical diffraction gratings  3   c ). Conversely, even when using a structure in which one reference pattern is put between two PSG patterns, similarly the measurement error attributed to the measuring apparatus can be removed. It is to be noted that in  FIG. 10  and subsequent figures, patterns are not necessarily denoted with reference numerals for simplicity, and the same hatching shows the same portion.  
         [0085]     Furthermore, when astigmatism exists in a projection lens, as shown in  FIG. 12 , by the use of the test mark TM having a structure in which PSG patterns  2  in two directions crossing each other at right angles are disposed in the vicinity of each other, not only the focus but also the astigmatism can be measured.  
         [0086]      FIG. 13  show plan views showing test marks corresponding to automatic measurements by an alignment shift inspection apparatus.  FIG. 14  show sectional views along arrows A-A′ and B-B′ of  FIG. 13D .  
         [0087]     A test mark TM shown in  FIG. 13A  includes two asymmetrical diffraction grating patterns  2  (the first and second asymmetrical diffraction grating patterns), and two reference patterns (the first and second reference patterns)  3   a.    
         [0088]     In a test mark TM shown in  FIG. 13B , reference patterns  3   a  in the test mark TM shown in  FIG. 13A  is replaced with reference patterns  3   b.    
         [0089]     In a test mark TM shown in  FIG. 13C , reference patterns  3   a  in the test mark TM shown in  FIG. 13A  is replaced with reference patterns  3   c.    
         [0090]     A test mark TM shown in  FIG. 13D  comprises a rectangular test mark TM 1 , and a rectangular test mark TM 2  provided in such a manner as to surround the test mark TM 1 . The test marks TM 1 , TM 2  are test marks obtained by changing a linear test mark obtained by vertically extending the test mark of one of  FIGS. 13A  to  13 C into a rectangular shape.  
         [0091]     That is, the test mark TM shown in  FIG. 13D  includes the first and second asymmetrical diffraction grating patterns and the first and second reference patterns, the first asymmetrical diffraction grating pattern and the first reference pattern are disposed in parallel on the first line, and the second asymmetrical diffraction grating pattern and the second reference pattern are disposed in parallel on the second line vertical to the first line.  
         [0092]     In the test marks of  FIGS. 13A  to  13 D, a relation between a focus value and a positional shift is not linear.  
         [0093]     Examples of a test mark which improve the situation include test marks shown in  FIGS. 15A and 15B . Exposure is performed using a test mark TMa shown in  FIG. 15A , and thereafter the exposure is performed using a test mark TMb shown in  FIG. 15B . Measurement patterns formed by these exposures have advantages that a relation between the focus value and the positional shift is substantially linear.  
         [0094]      FIG. 16  is a plan view showing an exposure mask including the test marks TMa, TMb. The test marks TMa, TMb are arranged in a peripheral portion of an exposure region of an exposure mask.  
         [0095]      FIG. 17  is an explanatory view of a double exposure method using the exposure mask of  FIG. 16 . In this method, the double exposure is performed utilizing an overlap region  24  of exposure regions  22 ,  23  adjacent to each other on a wafer  21 .  
         [0096]     That is, two exposure steps for two adjacent exposure regions  22 ,  23  are performed in such a manner that the pattern of the test mark TMa transferred in the exposure region  22  and the pattern of the test mark TMb transferred in the exposure region  23  overlap each other in a predetermined manner in the overlapped region  24 . Accordingly, two exposure steps for performing the double exposure of the test marks do not have to be separately added.  
         [0097]      FIG. 18  shows a plan view of the test mark for measuring a focus based on a profile of reflected light.  
         [0098]     The test mark of  FIG. 18  includes a plurality of asymmetrical diffraction grating patterns  2  and a plurality of reference patterns  3 . The asymmetrical diffraction grating patterns  2  and the reference patterns  3  are arranged in parallel on a line, and shifted in a lateral direction by a certain amount, while the asymmetrical diffraction grating patterns  2  and the reference patterns  3  are alternately arranged in a vertical direction.  
         [0099]     In the configuration, since the patterns alternately shift in a vertical direction by the focus, the profile of the reflected light changes by interference by the shift. A focus value can be measured from the change of the profile. The profile of the reflected light can be acquired, for example, using a device referred to as a scatterometoroy. When the test mark of  FIG. 18  is used, there is an advantage that the double exposure is not required.  
         [0100]     A focus measuring method of the present embodiment is as follows.  
         [0101]     In the focus measuring method of the present embodiment, a projection exposure apparatus is used which projects an image of a mask pattern formed on a photo mask onto a substrate via an optical projection system. The image of the test mark formed on the photo mask (focus test mask) of the present embodiment is projected onto the substrate using the projection exposure apparatus, and a defocus amount on the surface of the substrate is acquired using the image.  
         [0102]     In further detail, at first, a photosensitive agent is applied on the substrate.  
         [0103]     Next, images of the diffraction grating pattern and reference pattern in the test mark in the photo mask are exposed at the same time on the substrate.  
         [0104]     Next, the pattern transferred onto the substrate is developed.  
         [0105]     Next, a relative distance between the images of the diffraction grating pattern and the reference pattern formed on the substrate is measured, and the defocus amount is acquired based on the measured distance.  
         [0106]     A method of manufacturing a semiconductor device of the present embodiment is as follows.  
         [0107]     In the method of manufacturing the semiconductor device of the present embodiment, the projection exposure apparatus which projects the image of the mask pattern formed on the exposure mask onto the substrate via the optical projection system is used. The images of the device pattern and the test mark formed on the photo mask (exposure mask) of the embodiment are projected onto the substrate using the projection exposure apparatus, an actual semiconductor product is manufactured using the images, and the defocus amount of the surface of the substrate is acquired. Various combinations of the phase shift mask PSM of the exposure mask of the embodiment and the test mark TM are considered, and any combination may be used.  
         [0108]     In further detail, at first, a photosensitive agent is applied on the substrate.  
         [0109]     Next, the images of the asymmetrical diffraction grating pattern, reference pattern, and device pattern in the photo mask are exposed on the substrate at the same time.  
         [0110]     Next, the pattern transferred on the substrate is developed.  
         [0111]     Next, the device pattern formed on the substrate is inspected.  
         [0112]     Next, when a defect is detected in the device pattern formed on the substrate in a step of inspecting the device pattern, the relative distance between the images of the diffraction grating pattern and the reference pattern is measured, and the defocus amount is acquired based on the measured distance.  
         [0113]     Next, the device pattern in which the defect is detected, and the defocus amount are recorded in a memory device. A type of the memory device is not especially limited.  
         [0114]     Next, the photo mask related to the device pattern in which the defect is detected is corrected.  
         [0115]     Here, a case where the defocus amount is acquired when the defect is detected has been described, but the defocus amount may be acquired irrespective of presence of detection of the defect.  
         [0116]     By constructing a database of a wafer history including the device pattern and the defocus amount, a cause for an inadvertently generated yield reduction can be easily investigated, and the reduction of the yield can be prevented beforehand.  
         [0117]     Examples of the semiconductor device include a liquid crystal display (LCD) and devices using LCD (e.g., a cellular phone, liquid crystal television, personal computer, PDA) in addition to a DRAM, logic LSI, and system LSI (DRAM embedded LSI).  
         [0118]     As described above, the photo mask in which the line width ratio of the shielding portion, transmitting portion, and phase grooved portion is 2:1:1 and a phase difference between the lights transmitted through the transmitting portion and the phase grooved portion is 90° is a most ideal photo mask for focus measuring. However, even by the use of the photo mask of the embodiment in which the phase difference and the line width ratio satisfy [Equation 2] instead of the above-described ideal photo mask, the high precision focus measuring substantially similar to that by the ideal photo mask is possible. Accordingly, for example, even when the asymmetrical diffraction grating is formed on a usual alternating type phase shift mask, a grooved amount can be set with respect to the phase difference having few loads on the mask manufacturing, and a high precision focus measuring technique equivalent to a conventional technique can be realized even in various exposure masks.  
         [0119]     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.