JPH05241321A - Optical mask and method for correcting this mask - Google Patents

Abstract

PURPOSE:To provide the optical mask which enables correction with high accuracy in correspondence to an unshaped defect shape and the method for correcting this defect in the correction of the defect of a phase shift mask. CONSTITUTION:An etching stopper layer 3 is provided between a phase shifter 4 and a light shielding pattern 2 or a protective film having resistance in a washing stage included in a mask production stage is formed on the upper layer of the etching stopper layer 3 or the defect part is processed by a convergent ion beam. Either system of stopping the processing by the etching stopper layer 3 by the combination use of a reactive gas for the convergent ion beam or selectively removing the etching stopper layer 3 after processing with the convergent ion beam is selected at this time. As a result, the dissolution of the etching stopper layer 3 by the washing stage included in the mask production stage is averted and the stopper layer 3 can be made to remain on the lower layer of the phase shifter 4 and, therefore, the finally processed base which is flat even to the unshaped defect shape is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical mask for semiconductor manufacturing, and more particularly to an optical mask for a phase shift mask having a phase shifter having a predetermined pattern on a transparent substrate and a defect repairing method therefor.

[0002]

2. Description of the Related Art In a conventional photomask, a light-shielding pattern such as chromium is formed on a transparent substrate. However, with the miniaturization of semiconductor patterns, in order to meet the demand for improvement of pattern resolution in the lithography process, the basic structure shown in FIG. 6 in which a transparent pattern for imparting a phase difference to exposure light is formed between two adjacent light-shielding pattern gaps. The phase shift masks that it has are being put to practical use.

When a mask has a defect, the defect is transferred to all the semiconductor chips exposed by the mask, so that the defect of the mask must be completely removed. The defects of the phase shift mask to be removed include the omission of the conventional light shielding pattern,
Alternatively, at the same time as the generation of the extra light-shielding portion, the shifter portion comes off and the unnecessary shifter portion occurs. Of these, other than the omission of the shifter portion, it is possible to correct it by using the conventional local sputter etching with a focused ion beam and local chemical vapor deposition. However, in order to correct the omission of the shifter part, Journal of Applied Physics, Vol. 60, No. 11 (1991), p. 1076, "High Resolution Optical Lithography Technology Using Phase Shift Method".
It is necessary to adopt a method in which a sub-shifter is provided below a normal shifter as described in (4). Further, a method using a focused ion beam is promising for fine local repair, and is applied to the defect repair of the conventional optical mask. JP-A-58-5
Japanese Patent No. 6332 describes the basic concept of defect correction of the optical mask.

[0004]

The correction method reported in the above article is performed in the following steps. As the structure of the phase shift mask, a silicon nitride film having a film thickness for reversing the phase of the exposure light by 180 ° is formed as the sub-shifter 12 on the quartz substrate 1. Form the light-shielding pattern 2 with chrome on it, and also 180 ° in the predetermined gap between those patterns.
The shifter pattern 4 shown in FIG. 7A is formed of silicon oxide whose phase is inverted. The shifter 4 has defects such as shifter omissions 5 and shifter residuals 6 during the formation process. In order to correct this, in this method, first, the resist 13 is applied to the entire surface of the mask, and the defective portions 5 and 6 are spot-exposed. This is developed to form a resist pattern shown in FIG. This is subjected to anisotropic dry etching to remove the silicon oxide shifter 4 as shown in FIG. 7C. In the dry etching here, the etching is stopped on the surface of the sub-shifter 12 by using an etching gas which has selectivity between silicon oxide and silicon nitride. Further, this time, conversely, anisotropic dry etching is performed using an etching gas that gives a selection ratio such that the processing rate of silicon nitride is higher than that of silicon oxide, and the sub-shifter 12 formed of silicon nitride of the lower layer is removed. To do. Since the substrate 1 under the silicon nitride is quartz, that is, silicon oxide, etching stops at the upper surface of the substrate 1. After the above process is completed, the resist 13 is removed and the correction is completed as shown in FIG. Before the defect was corrected, the phase of the exposure light passing through the shifter 4 had an intermediate value between 0 ° and 180 ° at the defect portions 5 and 6, and a shadow was formed during the exposure, but the rest of the shifter 6 is the subshifter 1
2 is removed from above and the shadow caused by this is eliminated. Also,
The shifter omission 5 that occurred in the overlapping portion of the sub-shifter 12 and the shifter 4 has a phase difference of 0 ° because both of the sub-shifter 12 and the shifter 4 are removed. On the other hand, the phase difference is 360 ° at the portion where the sub shifter 12 and the shifter 4 overlap, the phase difference in these regions is substantially eliminated, and the shadow due to the shifter omission 5 disappears. As described above, the shifter 4 can be modified by this method. However, the problem of this method is that the repair process is complicated and there is a risk that a new defect is generated by the repair, and it is difficult to accurately perform the exposure positioning by the spot exposure with a complicated pattern. ..

In order to simplify the correction process and improve the correction accuracy, FIB (Focused Ion Be)
am) is suitable. The defect correction by FIB7 is the process shown in FIG. 6 shifters left in this process
Is removed by sputter etching using FIB7. Although the shifter missing portion 5 is also removed by sputter etching by the FIB 7, the phase shift of this portion is 0 ° as it is, and there is a phase inversion portion between the unshifted shifter portion 4 and the phase shift 180 °. Therefore, there is a shadow at the time of exposure. Therefore, the shifter portion 4 removed by the FIB 7 substantially eliminates the phase difference between the region where the shifter 4 is present and the region where the shifter residue 6 is corrected by digging the substrate 1 by 180 ° phase shift, and a correction method that does not cause a shadow. Has been adopted. However, in this method, when the shifter remainder 6 having an irregular shape and a different height depending on the position is corrected, the unevenness reflecting the shape of the shifter 4 is processed on the substrate 1, so that the phase is disturbed in that portion. , And it is difficult to make a complete correction.

[0006] The next conceivable correction method as a method for solving the processing problem by FIB is a method using FIBAE (FIB Assist Etching). In this method, the FIBAE stopper layer 3 is inserted between the shifter 4 and the chrome pattern 2 to shift the shifter 4
The unevenness of is absorbed here. For example, the substrate 1 is made of quartz, the shifter layer 2 is made of SOG (= spin on glass), and FIBA is used.
When the reactive gas 9 used for E is XeF ₂ (xenon fluoride), Al ₂ O ₃ and Mg are used as the stopper layer 3.
If it is F ₂ or CeF _3, etc., the etching rate is slower than that of the shifter layer 4, and as shown in FIG. 9, processing is almost stopped by the stopper layer 3, and the unevenness of the shifter 4 is thinner than that of the shifter 4 at this stage. It can be changed to a small unevenness in the stop layer 3. After that, the reactive gas 9 is stopped, and the process is switched to the FIB process which is the sputtering process with almost no process selectivity.
As a result, the substrate 1 is dug, and the phase difference between the portion where the shifter omission 5 has been corrected and the portion where the shifter 4 is present is matched,
Eliminates shadows during exposure due to shifter omissions 5. According to this method, since the processed bottom surface can be flattened, defect correction with high depth accuracy can be performed regardless of the shape of the shifter defect. Further, since the FIB 7 capable of focusing to 0.1 μm or less is used, the plane accuracy of repair can also satisfy the defect repair requirement. However, the problem with this method is the cleaning resistance of the stopper layer 3. Generally, in manufacturing a phase shift mask, a cleaning step is always performed after patterning as shown in FIG. In this cleaning, strong cleaning such as ozone sulfuric acid cleaning is performed in order to completely remove the contamination of the mask. In this cleaning, for example, Al ₂ which is a general coating material is used.
O ₃ is not resistant to this cleaning, and A under the chrome pattern 2 is not removed in the cleaning process after forming the chrome pattern 2.
All except l ₂ O ₃ dissolve. Therefore, since the stopper layer 3 does not exist below the transparent portion of the shifter 4 that needs to be corrected, the above-described correction using FIBAE cannot be performed. However, as a characteristic of the material, if a material that transmits exposure light, has high selectivity in FIBAE, can withstand cleaning, and can be formed into a practical film is selected as the material of the stopper layer 3, Hiring is possible.

An object of the present invention is to provide an optical mask and a method for repairing the same, which can repair defects in a phase shift mask with high accuracy regardless of the defect shape.

[0008]

The above-mentioned object is to be provided under a phase shifter of a phase shift mask for inverting the phase of exposure light,
This is achieved by providing a phase shifter and a transparent film having different etching resistance in the FIBAE or mask cleaning step, and absorbing the unevenness of the shifter defect at the time of defect repair by this transparent film. At this time, the resistance to the mask cleaning is ensured by forming the transparent film after forming the chromium pattern or by forming the transparent film in several layers.

[0009]

When the irregular shifter defect is repaired, the stopper layer provided under the shifter has a slow etching rate due to the chemical interaction between the reactive gas and FIB at the time of FIBAE. However, only the amount of physical sputter etching is reflected on the stopper layer, and the unevenness is flattened. Alternatively, in the case of processing by physical sputter etching using only FIB, the stop layer functions as a layer that holds the unevenness of the shifter defect therein, and the stop layer having no cleaning resistance is dissolved in the cleaning step to remove the defective portion. Flatten. By these actions, the unevenness of the shifter defect can be removed from the mask.

[0010]

Embodiments of the present invention will be described below with reference to the drawings.

First Embodiment: FIG. 1 shows a mask structure of a first embodiment of the present invention and a process for correcting a shifter 4. Here, quartz is used for the substrate 1, chromium is used for the light shielding film 2, and SOG is used for the phase shifter 4. Now, as the stopper layer 3, Al ₂ O ₃ , which is generally used as a coating material for optical lenses, is adopted. With this material, when XeF ₂ is used as the reactive gas 9 in FIBAE, the processing speed ratio, that is, the processing selection ratio is S
It has been confirmed that OG / Al ₂ O ₃ is 40 or more.

Conventionally, in the manufacturing process of the mask itself, the FIBAE stopper layer 3 was formed under the chromium pattern 2. However, as shown in the flow of the mask manufacturing process in FIG. 10, a cleaning process is required after forming the chromium pattern 2. Then, the permeated portion of the stopper layer 3 is exposed to a cleaning liquid (for example, ozone sulfuric acid) before the SOG film formation, and in the case of Al ₂ O ₃ , the stopper film 3 is dissolved at this stage because it has no resistance to ozone sulfuric acid. I will end up. This makes it impossible to repair defects by FIBAE. Therefore, after forming the chrome pattern 2, cleaning it, performing defect inspection and defect repair,
A film of l ₂ O ₃ is formed. This is the state shown in FIG. A SOG film is formed thereon and patterned to form a phase shifter 4. Al ₂ O ₃ exposed in the next cleaning step is dissolved with ozone sulfuric acid. However, Al under the shifter 4
₂ O ₃ remains because SOG of the shifter material has resistance to ozone sulfuric acid and functions as a mask. This becomes the stopper layer 3. The state at this stage is shown in FIG. In the subsequent defect inspection process, the shifter remaining 6 or
When the shifter omission 5 is detected, these position coordinates are sent to the defect repairing apparatus, and the process goes to the defect repairing step shown in FIG. In this embodiment, the first stage of defect correction is FIBAE.
Apply. When FIB 7 is irradiated with XeF ₂ acting as the reactive gas 9, SOG is Al _{2 as} described above.
Since it is processed 40 times faster than O ₃ , Al ₂ O ₃ also serves as a stopper against irregular-shaped shifter defects. Now, the shifter 4 is set to a film thickness that reverses the phase of the exposure light by 180 °. When the material of the shifter 4 is SOG, this film thickness is 415 nm, and the required specification of the correction for performing the correction of the phase shifter 4 within 10% of the phase difference 180 ° is the processing depth accuracy. The position of the bottom surface must be within ± 40 nm including the flatness. Considering the actual defect correction, the maximum height of the shifter defect is 415n.
It is rare that the shifter film thickness of m is exceeded. Further, when the shifter defect is repaired, the phase shift amount in the region having the shifter 4 is made uniform, so that the film thickness 41 around the defect portion 5 must be ensured.
Processing is performed by including a 5 nm shifter portion in the irradiation area of FIB7. Therefore, the thickness of the stopper layer 3 may be set so as not to penetrate the stopper layer 3 during the processing in the same ion irradiation region as the shifter 4 of 415 nm and the portion where the shifter 4 is almost removed. That is, since the processing speed is 40 times, if the thickness of the stopper layer 3 is 20 nm with a margin, it is 415 nm as shown in FIG.
When the normal shifter 4 of the above is processed up to the stopper layer 3, the stopper layer 3 of the portion where the shifter 4 is almost removed
The dug depth is about 10 nm (up to 415 nm / 40), and stops in the middle of the stopper layer 3. Therefore, 4
The unevenness of 15 nm can be flattened to the unevenness of about 10 nm. This satisfies the processing specification ± 40 nm. The next correction step is the FIB processing step shown in FIG. Now, the substrate 1 is made of quartz, and the quartz is FIBAE using XeF _2.
Then, the stopper layer 3 is processed about 30 times faster than Al ₂ O ₃ . Therefore, in this step, the reactive gas is not allowed to act so that the unevenness of the substrate processed portion is not enlarged by FIBAE when penetrating the stopper layer 3. At this stage, slight irregularities remain on the stopper layer. However, in the physical sputtering process, the sputter rate, which represents the number of target atoms ejected per incident ion, becomes large as the incident angle deviates from the vertical, so that even slight remaining irregularities cause the phase difference of 180 °. Further flattened during the digging, and shadows during exposure can be completely removed.
The shifter missing defect 5 can be corrected by the above method.
Regarding the shifter residual defect 6, the FIBAE of FIG.
It can be removed by scanning the FIB 7 according to the defect shape in the process.

Second Embodiment: FIG. 3 shows a mask structure and a shifter 4 repairing process of the second embodiment. The mask of this embodiment basically has the same structure as that of the first embodiment, and the only difference is the film thickness of the stopper layer 3. Therefore, in the case of the present embodiment, the stopper layer 3 rather functions as a layer which absorbs irregularities of defects and dissolves. Similar to the first embodiment, the material of each layer is quartz for the substrate, chromium for the light shielding film 2, SOG for the shifter 4, and Al _{2 for the} stopper layer 3.
Let O ₃ . The difference in the mask structure from the first embodiment is the film thickness of the stopper layer 3.

After forming the chrome pattern 2, the stopper layer 3 is formed to obtain the state shown in FIG. This is SOG
Are stacked to form a film, and patterning is performed to form a shifter 4. The SOG film thickness at this time is set to a film thickness that shifts the phase of the exposure light by 180 ° together with the lower layer Al ₂ O ₃ . Next, the cleaning process is started. Since Al ₂ O ₃ is adopted as the stopper layer 3 now, the stopper layer 3 exposed at this stage is
Melts into the state shown in FIG. After that, when a defect inspection of the shifter 4 is performed, a shifter missing defect 5 and a shifter residual defect 6 as shown in FIG. 3B are detected. These are sent to the next defect correction process. FIBAE is not used in the defect correction in this embodiment, and only FIB processing is used.
As shown in FIG. 3C, when processing is performed only with FIB7, unlike FIBAE, there is almost no difference in processing speed between SOG and Al ₂ O ₃ , so most of the unevenness of the shifter defect is preserved in the stopper layer 3. It In this embodiment, in order to remove the unevenness, the mask on which the processing up to the stopper layer 3 has been completed is washed once at this stage. By this washing, FIG.
Although the stopper layer 3 including the unevenness of the shifter defect as shown in (d) is dissolved, the substrate 1 is not dissolved, so that the portion having the shifter defect is flattened. Although this cleaning step is wet etching in the present embodiment, the same result can be obtained even if this cleaning step is replaced with a dry etching step. Further, the shifter residual defect portion 6 in this step is also completely dissolved and removed from the mask because the shifter portion 4 is removed by FIB processing in the previous step. Then again mask FI
Returning to the B processing step, here, the quartz substrate 1 of the shifter missing defect portion 5 is dug only by the amount by which the phase of the exposure light is shifted by 180 °, and the normal shifter portion 4 by which the phase is shifted by 180 ° between SOG and Al ₂ O _3. The phase is adjusted to and the correction is completed. Since only the FIB processing is used in the correction process in this embodiment, the correction device can be simplified.

Third Embodiment: In the first and second embodiments, the stopper layer 3 is formed after the formation of the chromium pattern 2 and its cleaning, and the shifter layer 4 is used as a mask so that the stopper below the shifter 4 is formed. Layer 3 was left. In the third embodiment, FIG.
As shown in, a method of forming a protective film 11 having resistance to cleaning on the stopper layer 3 is adopted.

In this embodiment, Al ₂ O ₃ is first formed as a stopper layer 3 on the quartz substrate 1. Next, an ITO (indium tin oxide) film is formed as the protective film 11 for cleaning. Chromium for the light-shielding film 2 is formed thereon and patterned. After that, a cleaning process is performed, but since the ITO film on the stopper layer 3 has resistance to cleaning, it functions as the protective layer 11 and the stopper layer 3 is not dissolved by cleaning. After that, after inspecting and correcting the defects of the chromium pattern 2, SOG is formed, patterned, and washed again. Here again, the stopper layer 3 is protected by the protective film 11 and remains without being dissolved. This is the state shown in FIG. When the mask is inspected for defects, shifter missing defects 5 and shifter remaining defects 6 are detected. These defects can be corrected in the same manner as in the first embodiment, and in the first step, the defective portions 5 and 6 are processed by FIBAE using XeF ₂ . At this time, the processing speed of the protective film 11 under the shifter 4 is equal to that of the shifter 4, but the stopper layer 3 has a lower processing speed than that of the shifter 4 as described in the first embodiment. It is flattened by the stopper layer 3. FI without using reactive gas 9 as the second stage
In the B processing, the quartz substrate 1 is dug by a phase shift of 180 ° to align the phase with the normal shifter portion 4, and the defect correction is completed.

Fourth Embodiment: FIG. 5 shows a mask structure and a shifter 4 repairing process of the fourth embodiment. Here, the material of each layer is the same as that of the third embodiment. However, in this embodiment, a thickness that shifts the phase of the exposure light by 180 ° is adopted as the thickness of the stopper layer 3.

The mask consists of Al ₂ O ₃ and IT on the quartz substrate 1.
After forming O and a film, chromium is formed and patterned.
Similar to the third embodiment, the stopper layer 3 is removed in the next cleaning step.
Is protected by the ITO film 11 and does not dissolve. Further SOG
Is formed, patterned, and washed. After all, the stopper layer 3 does not dissolve. Here, the film thickness of SOG is set to a thickness that shifts the phase of exposure light by 180 °. This mask defect inspection is performed to correct the shifter missing defect 5 and the shifter remaining defect 6 as shown in FIG. In this embodiment, only FIB processing is used. In FIB processing, the processing speed is almost the same for all layers. Therefore, FIBA
As in the case of using E, the unevenness of the shifter defect is not flattened in the stopper layer 3, and when the FIB processing is performed, the unevenness of the shifter defect remains in the stopper layer 3 and the processed bottom surface is uneven. Is. This is the state shown in FIG. In this embodiment, the stopper layer 3 having the unevenness is dissolved by passing through a washing process thereafter. Then,
As shown in FIG. 5C, the wet etching progresses to the top of the quartz substrate 1, and the etching stops there, so that a flat surface is obtained. At this stage, the phase shift in each part is 360 ° in the normal shifter portion 4, 0 ° in the portion where the shifter missing defect 5 is corrected, and 18 in the portion where the shifter 4 does not exist.
0 ° and 0 ° at the portion where the shifter residual defect 6 is corrected. Therefore, there is substantially no phase difference in the shifter missing portion 5, and the defect can be repaired. However, in the shifter remaining portion 6, since the phase difference between the normal portion and the correction portion is 180 °, a shadow is generated at the time of exposure at the phase change portion. Therefore, in order to make the phase difference in this portion uniform, as shown in FIG. 5D, the portion of the substrate 1 in which the shifter residual defect 6 is corrected is phase 18
Dig only 0 ° using FIB processing. This substantially eliminates the phase difference even in this portion, and the defect correction is completed. In this embodiment, the thickness of the stopper layer 3 is set to 180 degrees.
Although it is set as a degree of shift, if this is set for a phase of 360 degrees,
The digging of the substrate 1 is not performed in the shifter remaining defect portion 6 but in the shifter missing defect portion 5. As a result, the same result as above can be obtained. Further, in the present embodiment, the stopper layer 3 was dissolved in the cleaning after the first stage of correction, but if this is replaced with a dry etching process, good processing without undercut of the layer etched by wet etching is possible. Becomes In this embodiment as well, as in the second embodiment, only the FIB processing is used in the correction process, so that the correction device can be simplified.

Here, in the third and fourth embodiments, the stopper layer 3 and the protective layer 11 are formed under the light shielding pattern 2, but this is the same as in the first and second embodiments. Even if a film is formed on the pattern 2, the same effect can be obtained.

In the above embodiment, the stopper layer 3 is made of Al ₂
O ₃ was adopted, but this can be adopted as long as it is a material satisfying that a sufficient transmittance for exposure light is obtained and that the processing speed in FIBAE is slower than that of the shifter material. Other than Al ₂ O ₃ , Mg
Examples thereof include metal fluorides such as F ₂ and CeF ₃ . Further, the function of the protective film 11 is to obtain a sufficient transmittance for exposure light, not dissolve in the cleaning process in the mask manufacturing process, and have resistance to etching during shifter patterning. Therefore, a material other than the ITO film used in the above embodiments can be used as long as the material satisfies the above conditions.

[0021]

According to the present invention, the shifter defect of the phase shift mask can be corrected with high reproducibility and high accuracy regardless of the shape, so that a practical yield of defect correction can be secured.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a phase shifter correction method according to a first embodiment of this invention.

FIG. 2 is an explanatory diagram of a processing progress state when correcting a phase shifter.

FIG. 3 is an explanatory diagram of a phase shifter correction method according to a second embodiment of the present invention.

FIG. 4 is an explanatory diagram of a phase shifter correction method according to a third embodiment of the present invention.

FIG. 5 is an explanatory diagram of a phase shifter correction method according to a fourth embodiment of the present invention.

FIG. 6 is a sectional view of a basic phase shift mask.

FIG. 7 is an explanatory diagram of a first conventional technique for correcting a phase shift defect.

FIG. 8 is an explanatory diagram of a second conventional technique for correcting a phase shift defect.

FIG. 9 is an explanatory diagram of a third conventional technique for correcting a phase shift defect.

FIG. 10 is a flowchart of a phase shift mask manufacturing process.

[Explanation of symbols]

1 ... Substrate (quartz), 2 ... Light-shielding pattern (chrome), 3 ... Stopper layer (Al ₂ O ₃ ), 4 ... Phase shifter (SOG), 5 ... Shifter missing defect, 6 ... Shifter remaining defect, 7 ... Focused ion beam, 8 ... nozzle, 9 ... reactive gas (XeF _2), 10 ... ion beam scanning range, 11 ... protective layer (ITO), 12 ... sub-shifters _{(Si 3 N 4), 13} ... resist.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor, Fumika Ito, 292 Yoshida-cho, Totsuka-ku, Yokohama, Kanagawa, Ltd., Production Engineering Research Laboratory, Hitachi, Ltd. No. 1 stock company Hitachi Ltd. Musashi factory

Claims (13)

[Claims] 1. A projection optical system mask comprising a transparent substrate on which a light-shielding pattern is formed, and a phase shifter for changing the phase of the exposure light is provided in a specific opening, and the exposure light is provided between the phase shifter and the light-shielding pattern. An optical mask provided with an etching stopper film that transmits light. 2. The optical mask according to claim 1, wherein, in the etching stopper layer, the layer has resistance to etching of a phase shifter. 3. A projection optical system mask comprising a transparent substrate on which a light-shielding pattern is formed, and a phase shifter for changing the phase of exposure light is provided in a specific opening, the layer being below the phase shifter in a phase shift mask manufacturing process. An optical mask comprising: a protective film that is resistant to cleaning and that transmits exposure light; and an etching stopper film that transmits exposure light. 4. The optical mask according to claim 3, wherein in the etching stopper layer, the layer has resistance to etching of a phase shifter. 5. The optical mask according to claim 3, wherein the protective film and the etching stopper layer are provided under a light shielding pattern. 6. The optical mask according to claim 3, wherein the protective film and the etching stopper layer are provided on an upper layer of a light shielding pattern. 7. Al ₂ as the etching stopper layer
The optical mask according to claim 1 or ₃ , wherein a metal oxide such as O ₃ or a metal fluoride such as MgF ₂ or CeF ₃ is used. 8. The optical mask according to claim 3, wherein a transparent film such as an ITO film is used as the protective film. 9. A defect correction method for a projection optical system mask, comprising a light-shielding pattern formed on a transparent substrate, and a phase shifter for changing the phase of exposure light provided in a specific opening, wherein the transparent film is provided below the shifter. Modification of the optical mask characterized by including a step of processing up to and a step of precisely processing the substrate of the mask by a substrate thickness that gives a phase shift amount equal to or an integer multiple of the phase shift amount by the phase shifter. Method. 10. The method of repairing an optical mask according to claim 9, wherein in the step of processing the transparent film, the phase shifter is processed with respect to the transparent film with good selectivity. 11. The method for repairing an optical mask according to claim 9, wherein a focused ion beam is used as the processing method. 12. The method of repairing an optical mask according to claim 10, wherein a focused ion beam and a reactive gas are used in combination as the method of processing with good selectivity. 13. A defect correction method for a projection optical system mask, comprising a light-shielding pattern formed on a transparent substrate, and a phase shifter for changing the phase of exposure light provided in a specific opening. Processing step, selectively processing the transparent film, and processing the mask substrate accurately by a substrate thickness giving a phase shift amount equal to or an integer multiple of the phase shift amount by the phase shifter. A method of repairing an optical mask, comprising the steps of: