CN118707799A - Photomask substrate and preparation method thereof - Google Patents

Abstract

The present invention discloses a photomask substrate and a preparation method thereof, wherein the photomask substrate comprises: a glass substrate, a chrome film layer and a photoresist layer, wherein a silicon dioxide transition layer is provided between the glass substrate and the chrome film layer. The technical solution of the present invention can significantly improve the adhesion of the photomask substrate, effectively reduce defects such as line missing in the manufacturing process, greatly improve product quality and service life, and reduce production costs, especially in the field of high-precision semiconductor applications, and has good operability and repeatability.

Description

Photomask substrate and preparation method thereof

Technical Field

The present invention relates to the field of new materials, in particular to the field of semiconductor material manufacturing, and specifically to a photomask substrate and a preparation method thereof.

Background Art

The application fields of photomask substrate products include optics, TFT, FPT, semiconductors, etc. The photomask substrate is made into a mask after exposure-development-etching-stripping. The mask is composed of a glass substrate and a chrome film layer that makes up the lines. The adhesion between the chrome film layer and the glass substrate determines the number of times the mask can be used or its service life. The stronger the adhesion, the longer the service life of the mask, and the raw material cost and production cost of the mask manufacturer will be reduced.

During the mask manufacturing process, due to the influence of the development or degumming process, defects appear in the final graphic lines (commonly known as white defects in the industry), and the missing lines on the mask cause the mask to be unusable. The missing lines on the mask are related to the adhesion between the chrome film layer and the glass substrate, and the adhesion between the photoresist layer and the chrome film layer. The weaker the adhesion, the more likely it is that the product will be scrapped during the mask manufacturing process, resulting in a lower customer yield, especially in some cases, where the missing lines on the mask graphics account for more than 80% of the defective rate of the mask process.

Specifically, the missing of mask pattern lines is increasingly occurring during the mask manufacturing process, especially in the field of high-precision semiconductor applications. Even very small pattern missing can still cause a large number of products to be scrapped. Line missing during the process is the biggest scrap factor for mask defects, but the current mask substrate manufacturing process still uses processes to handle the cleanliness of the glass substrate and control the temperature, and pays little attention to the microstructure of the chromium film layer. When applied to the field of high-precision semiconductors, such as the application field of semiconductor chips with a precision of 28nm and below, the research and development of the film layer microstructure is an important compensation for the impact of subtle pattern missing.

Currently, the main means to improve adhesion in the industry is to clean the glass substrate to improve the cleanliness between the glass substrates, or to bake and heat the glass substrate to remove residual water vapor on the glass substrate, thereby improving the adhesion between the glass substrate and the chromium film layer.

The Chinese patent document with the publication number CN103235480A and the publication date of August 7, 2013, entitled "New three-layer film structure photomask and preparation method thereof" discloses a three-layer structure of a photomask. In the technical solution disclosed in the patent document, a transition layer is added between the glass substrate and the chromium film, and the transition layer is used to block the precipitation of sodium ions, causing the film layer composition to change.

Based on this, it is expected to obtain a photomask substrate, which can overcome the shortcomings of the prior art by improving the adhesion between the glass substrate and the chrome film layer in the photomask substrate, and this improvement can be simple in process and easy to implement, and the product defect rate and service life can be significantly improved.

Summary of the invention

In view of the above-mentioned deficiencies in the current photomask substrate manufacturing, the present invention provides a photomask substrate and a preparation method thereof to solve the problem of line missing due to insufficient adhesion in the mask plate manufacturing process in the prior art, thereby improving the service life of the mask plate and the product yield.

In order to achieve the above object, the present invention adopts the following technical scheme:

In a first aspect, the present invention provides a photomask substrate, the photomask substrate comprising: a glass substrate, a chrome film layer and a photoresist layer.

Wherein, a silicon dioxide transition layer is provided between the glass substrate and the chrome-plated film layer.

Preferably, the microstructure of the silica transition layer includes first silica particles and second silica particles located between the gaps of the first silica particles, wherein the particle size of the first silica particles is 50-60 nm, and the particle size of the second silica particles is 10-15 nm.

This structure can significantly improve the adhesion between the glass substrate and the chromium film layer. This is because: the interlayer structure in the prior art actually consists of chromium and chromium oxide, while the glass substrate is mainly composed of silicon dioxide. One of the two layers is a glass layer, which is composed of silicon dioxide, and the other layer is grown by coating deposition, which is mainly chromium oxide. The difference between the two layers is large, and the adhesion strength is not good enough.

Unlike the prior art, the technical solution described in the present invention is to first generate a silicon dioxide transition layer on the glass substrate. Since the components of the silicon dioxide transition layer and the glass substrate are basically the same, the bonding force between the two will be relatively high. Moreover, the film layer formed by silicon dioxide is a hard film that can withstand the influence of the internal stress of the glass substrate. Moreover, the silicon dioxide transition layer can also allow _SiO2 to capture oxygen from the dangling bonds on the surface of the glass substrate to form chemical bonds, and make it have a stronger chemical bond force, thereby greatly improving the adhesion performance between the glass substrate and the film layer.

Preferably, the thickness of the silicon dioxide transition layer is 80-150 nm.

Preferably, the chromium film layer comprises chromium particles, chromium oxide particles and chromium oxynitride particles, wherein the particle size of the chromium particles, chromium oxide particles and chromium oxynitride particles is 10-20 nm.

Preferably, the chromium film layer has a five-layer structure, the thickness of the outermost layer is 15-30 nm, and the thickness of the other four layers is 70-90 nm. The outermost layer refers to the layer away from the glass substrate.

In a second aspect, the present invention provides a method for preparing the above-mentioned photomask substrate, which comprises the following steps:

Step S1: treating the glass substrate to remove dirt from the surface;

Step S2: heating the glass substrate;

Step S3: stacking a silicon dioxide transition layer on the glass substrate;

Step S4: stacking a chromium film layer on the silicon dioxide transition layer to obtain a film-plated sheet;

Step S5: baking the coated sheet and air cooling after baking;

Step S6: spin-coating the coated sheet, performing pre-baking treatment, and finally obtaining a photomask substrate.

In the technical solution described in the present invention, the silica transition layer and the chromium film layer are both formed by combining fine microparticles (for example: first silica particles, second silica particles, chromium particles, chromium oxide particles and/or chromium oxynitride particles), and both have certain micropores. In step S3, the particles of the silica transition layer and the glass substrate are sputtered and doped with each other. In the sputtering film forming process of this case (i.e., step S4), the particles of the silica transition layer and the chromium film layer are also sputtered and doped with each other, and finally form a strong bonding layer. By strengthening the bonding force between the silica transition layer and the glass substrate, and the bonding force between the chromium film layer and the silica transition layer, the adhesion between the chromium film layer and the glass substrate is enhanced, and finally the performance of the photomask substrate obtained in this case is significantly improved due to the prior art, and its defective rate is low.

It should be noted that, in the technical solution of the present invention, in order to improve the adhesion between the glass substrate and the chromium film, step S1 is used to remove the surface dirt of the glass substrate so that the glass substrate is in a clean state without any dirt residue. This is because the higher the cleanliness of the glass substrate, the higher the adhesion between the chromium film layer and the glass substrate. The treatment means for the glass substrate can adopt existing technical processes, such as: SPM cleaning, ultrasonic cleaning or brushing.

In order to improve the adhesion between the glass substrate and the chromium film, the glass substrate is heated in step S2, and the heating temperature is preferably 150-200°C. This is because: when the temperature of the glass substrate is higher, the film layer adhesion is stronger, but too high a temperature may also cause the glass substrate to be damaged or even melted. Based on this, the inventor of this case sets the heating temperature of the glass substrate within a suitable temperature range to improve the adhesion between the glass substrate and the chromium film layer in the photomask substrate of the present invention.

Preferably, in step S3, the stacked silicon dioxide transition layer adopts medium frequency magnetron sputtering coating technology, wherein, in the medium frequency magnetron sputtering coating, the power used is 0.5-1KW, the volume ratio of oxygen and argon is 1:1-2:1, and the target material used is a silicon target.

Preferably, in step S4, the stacked chromium film layer is deposited by DC magnetron sputtering, wherein the DC magnetron sputtering deposition operation includes:

Step S41: using a power of 1.5 to 2.5 Kw and a sputtering speed of 110 mm/min to 200 mm/min to form a four-layer structure in the chromium film layer;

Step S42: Use 3.5-5Kw and a sputtering coating speed of 400mm/min-700mm/min to form the outermost layer of the chromium film.

It should be noted that the five-layer structure of the chromium film layer described in the present invention, wherein the first four layers constitute the light-shielding layer of the photomask substrate, and the outermost layer constitutes the anti-reflection layer of the photomask substrate, the microstructure particle size in the light-shielding layer of the photomask substrate is between 10 and 20 nm, which is easier to form a strong adhesion with the silicon dioxide transition layer.

Preferably, in step S41, the ignition atmosphere is argon, and a small amount of oxygen is introduced as a reaction atmosphere, and the gas ratio of argon to oxygen is 15-20:1.

Preferably, in step S42, the ignition atmosphere is nitrogen, with a small amount of argon and oxygen introduced, and the ratio of nitrogen, argon and oxygen is 15-20:1:2.

In some embodiments, the thickness of the photomask substrate is 70-80 nm, and the thickness of the anti-reflection layer of the photomask substrate is 20-30 nm.

Preferably, in step S5, the baking temperature is 80-100° C., the baking time is 10-30 min, and the baking is followed by air cooling for 60-120 min.

Preferably, in step S4, the sputtering target is a chromium target.

It should be noted that in step S5, baking and air cooling are performed to remove the residual stress generated between the chromium film layers. Specifically, after baking, the surface state of the chromium film layer changes, the contact angle increases to 30-50, and the adhesion between the chromium film layer and the photoresist layer increases. Therefore, if the adhesion is not strong enough, the photoresist layer will not be well protected, resulting in abnormal missing lines in the chromium film layer. In this case, the missing can be greatly reduced by further strengthening the adhesion.

Preferably, in step S6, the rotation speed of the spin coating is 1500-3000 rpm, and the coating time is 5-20 s.

In step S3, the chromium film layer is sputtered at low sputtering power and low speed. The particle size of the particles in the single-layer chromium film layer is 10-20nm. Five layers are sputtered, four of which are thicker, with a total thickness of 80nm, and the top layer is thinner, with a thickness of 20nm.

The silicon dioxide transition layer is completed by medium frequency magnetron sputtering coating, using 0.5 to 1KW power, the ratio of oxygen to argon is 1:1 to 2:1, and the target material used is a silicon target.

Compared with the prior art, the technical solution of the present invention has the following advantages and beneficial effects:

The photomask substrate of the present invention improves the performance of the photomask substrate itself by improving the structure between the glass substrate and the chrome film layer, especially improves the adhesion between the two, thereby increasing product quality and service life. In particular, improving the adhesion of the photomask substrate can reduce white defects during the mask plate manufacturing process.

In addition, the photomask substrate described in the present invention improves the adhesion of the photomask substrate, thereby improving the extension of the service life of the mask. Prolonging the use time of the mask can reduce the procurement cost of the mask industry. In addition, the enhancement of adhesion can reduce the probability of product scrapping due to minor defects on the substrate surface during use.

The technical solution of the present invention can significantly improve the adhesion of the photomask substrate, reduce defects such as line missing in the mask manufacturing process, and improve product quality and service life.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

FIG1 is a schematic structural diagram of a photomask substrate according to an embodiment of the present invention;

FIG2 is a schematic structural diagram of a silicon dioxide transition layer of a photomask substrate according to an embodiment of the present invention;

FIG3 is an electron image of a silicon dioxide transition layer of a photomask substrate according to an embodiment of the present invention;

FIG. 4 is a process flow chart of a method for preparing a photomask substrate according to an embodiment of the present invention.

DETAILED DESCRIPTION

The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

FIG. 1 is a schematic structural diagram of a photomask substrate according to an embodiment of the present invention.

As shown in FIG. 1 , in this embodiment, the photomask substrate includes a glass substrate 1 , a chrome film layer 3 , and a photoresist layer 4 , wherein a silicon dioxide transition layer 2 exists between the glass substrate 1 and the chrome film layer 3 .

In addition, further referring to FIG. 1 , it can be seen that the chrome film layer 3 in this embodiment includes a five-layer structure, namely, a photomask substrate light-shielding layer 31 (schematically drawn as one layer in FIG. 1 , but in the actual preparation process, it actually has four layers) and a photomask substrate anti-reflection layer 32 .

In addition, it should be noted that the thickness of the light shielding layer 31 of the photomask substrate is 80 nm, and the thickness of the anti-reflection layer of the photomask substrate is 20 nm.

Of course, in some other implementations, the thickness of the photomask substrate shielding layer 31 is any value within the range of 70 to 90 nm, and the thickness of the photomask substrate anti-reflection layer 32 is any value within the range of 20 to 30 nm.

Furthermore, in the present embodiment, the chromium film layer 3 includes chromium particles, chromium oxide particles, and chromium oxynitride particles, wherein the particle diameters of the chromium particles, chromium oxide particles, and chromium oxynitride particles are 10 to 20 nm.

The structure of the silicon dioxide transition layer 2 can be seen in Figures 2 and 3, wherein Figure 2 is a schematic structural diagram of the silicon dioxide transition layer of the photomask substrate described in the present invention in one embodiment; and Figure 3 is an electron mirror image of the silicon dioxide transition layer of the photomask substrate described in the present invention in one embodiment.

As shown in FIG. 2 , and with reference to FIG. 1 and FIG. 3 when necessary, the thickness of the silica transition layer 2 is 100 nm, wherein the microstructure of the silica transition layer includes first silica particles 21 and second silica particles 22 located between the gaps of the first silica particles 21 , wherein the particle size of the first silica particles 21 is 50 to 60 nm, and the particle size of the second silica particles 22 is 10 to 15 nm.

Of course, in some other embodiments, the thickness of the silicon dioxide transition layer 2 is 80-150 nm.

FIG. 4 is a process flow chart of a method for preparing a photomask substrate according to an embodiment of the present invention.

As shown in FIG. 4 , in this embodiment, the method for preparing a photomask substrate includes the following steps:

Step S1: treating the glass substrate to remove dirt from the surface;

Step S2: heating the glass substrate;

Step S3: stacking a silicon dioxide transition layer on the glass substrate;

Step S4: stacking a chromium film layer on the silicon dioxide transition layer to obtain a film-plated sheet;

Step S5: baking the coated sheet and air cooling after baking;

Step S6: spin-coating the coated sheet, performing pre-baking treatment, and finally obtaining a photomask substrate.

Preparation Example 1

The structure of the photomask substrate obtained in Preparation Example 1 can be seen in FIGS. 1 to 3 , and the specific steps of the preparation method are as follows:

Step S1: Using a tank cleaning machine, the glass substrate is first cleaned with SPM, then with ultrasonic cleaning, and finally with IPA drying to completely remove dirt on the surface of the glass substrate.

Step S2: In the coating machine, the temperature of the glass substrate is heated to 175°C to make the glass substrate reach a suitable temperature and create good conditions for the subsequent coating process; in the coating machine, the glass substrate is plasma bombarded with an energy set to 10ev to further clean and activate the surface of the glass substrate, and then step S3 is executed.

Step S3: Use a silicon target as a sputtering target at the first cathode target position of the coating machine, and adopt medium frequency magnetron sputtering coating. The sputtering power is set to 0.5KW, the oxygen flow rate is 50sccm, and the argon flow rate is 100sccm, and a silicon dioxide film layer with a thickness of 100nm is produced.

Step S4: using a chromium target as a sputtering target at the second cathode target position of the coating machine, and adopting magnetron sputtering coating to obtain a coated sheet;

Among them, in step S4, the process used for the first four layers (i.e., the light shielding layer of the photomask substrate) is that the sputtering power is 1.5KW, the sputtering coating speed is 110mm/min, and the single layer thickness of the light shielding layer of the photomask substrate is 20nm. The composition analysis shows that the main components are chromium and a small amount of chromium oxide. The ignition atmosphere of the light shielding layer of the photomask substrate is argon, and a small amount of oxygen is introduced as the reaction atmosphere. The gas ratio of argon to oxygen is 20:1;

The process used for the last layer (i.e., the anti-reflection layer of the photomask substrate) is as follows: the sputtering power is 4KW, the sputtering thickness is 15nm (i.e., the thickness of the anti-reflection layer of the photomask substrate is 15nm), and the sputtering coating speed is 400mm/min. The composition analysis shows that the main component is chromium oxynitride, and the ignition atmosphere of the anti-reflection layer of the photomask substrate is nitrogen, with a small amount of argon and oxygen, and the ratio of nitrogen, argon and oxygen is 20:1:2;

The coated sheet is cleaned and dried with IPA to clean the coated surface and remove possible impurities, and then step S4 is performed;

Step S5: Place the coated sheet on a hot plate and bake it. The baking temperature is set to 90° C. for 10 min, and then cool it in air for 120 min.

Step S6: Spin-coating the film-coated sheet with the rotation speed set to 2000 rpm and the coating time set to 15 s. After coating the photoresist, pre-baking treatment is performed to obtain a corresponding photomask substrate.

It should be noted that in step S2, the optical parameters of the silicon dioxide transition layer were monitored using a companion film, and the transmittance at 248 nm was 92%. The companion film was sampled, and SEM test was performed after sample preparation to observe the film microstructure, and it was found that the film microstructure of the silicon dioxide transition layer in Preparation Example 1 was composed of first silicon dioxide particles with a size of 50 nm, and the pores were filled with second silicon dioxide particles with a size of 10 to 15 nm.

In addition, in step S4, the companion film is sampled, and then SEM test is performed to observe the microstructure of the film layer. It is found that the chromium film layer in this preparation example is composed of particles with a particle size of 10 to 20 nm.

Furthermore, in step S5, a contact angle test is performed on the cooled coated sheet, and the test result shows that the contact angle is 45°.

Preparation Example 2

The structure of the photomask substrate obtained in Preparation Example 2 can be seen in FIGS. 1 to 3 , and the specific steps of the preparation method are as follows:

Step S1: using a SPIN cleaning machine, the glass substrate is first cleaned by SPM, then ultrasonically cleaned, and dried by nitrogen.

Step S2: In the coating machine, the temperature of the glass substrate is heated to 200°C.

Step S3: The first cathode target position of the coating machine uses a silicon target as a sputtering target material, and adopts medium-frequency magnetron sputtering coating with a sputtering power of 1.2KW, oxygen 150sccm, and argon 150sccm to produce a silicon dioxide film layer with a thickness of 95nm.

Step S4: The second cathode target position of the coating machine uses a chromium target as a sputtering target material and adopts magnetron sputtering coating.

Among them, the process used for the (i.e., the light shielding layer of the photomask substrate) is that the sputtering power is 1.8KW, the sputtering coating speed is 200mm/min, and the single layer thickness of the light shielding layer of the photomask substrate is 17nm. The composition analysis shows that its main components are chromium and a small amount of chromium oxide. The ignition atmosphere of the light shielding layer of the photomask substrate is argon, and a small amount of oxygen is introduced as the reaction atmosphere. The gas ratio of argon to oxygen is 15:1;

The process used for the latter layer (i.e., the anti-reflection layer of the photomask substrate) is a sputtering power of 5KW, a sputtering thickness of 25nm (i.e., the thickness of the anti-reflection layer of the photomask substrate is 25nm), and a sputtering coating speed of 700mm/min. Analysis of its components shows that its main component is chromium oxynitride, and the ignition atmosphere of the anti-reflection layer of the photomask substrate is nitrogen, with a small amount of argon and oxygen introduced, and the ratio of nitrogen, argon and oxygen is 20:1:1.

Step S5: Place the coated sheet on a hot plate and bake at a temperature of 100° C. for 30 minutes, and cool in air for 90 minutes.

Step S6: Spin-coating the film-coated sheet at a rotation speed of 1500 rpm for a coating time of 30 seconds. After coating the photoresist, pre-baking is performed.

It should be noted that in step S2, a companion sheet was used to monitor the optical parameters of the silicon dioxide transition layer, and the transmittance at 248 nm was 94%.

In addition, in step S4, the companion film is sampled, and then SEM test is performed to observe the microstructure of the film layer. It is found that the chromium film layer in this preparation example is composed of particles with a particle size of 10 to 20 nm.

Furthermore, in step S5, a contact angle test is performed on the cooled coated sheet, and the test result shows that the contact angle is 25°.

Verification Example

It should be noted that there are many methods for characterizing the adhesion of mask substrates, and the methods for characterizing adhesion are inconsistent, and the standards for monitoring methods are not unified. There are no national or international standards in the mask substrate industry, and there are usually only a few methods commonly known in the industry, such as: monitoring the tearing of the film layer of the mask substrate coating, monitoring the increase of pinholes, or monitoring the film layer demolding; using cotton swabs + alcohol/acetone wiping and other methods to monitor the demolding of the lines after the mask substrate is patterned by photolithography.

In the technical solution described in the present invention, the adhesion is measured in the following manner:

(1) Use yellow 3M tape to monitor the film tearing of the coated sheet and observe the increase in the number of pinholes under an optical microscope every 200 times.

The final monitoring data is shown in Table 1 below.

Table 1.

Number of tearing Number of pinholes 0 0 200 0 400 0 600 0 800 0 1000 1 1200 1 1400 2 1600 2

It can be seen from Table 1 that the mask substrate obtained by the preparation method of the present invention can be torn off 1600 times with only 0 to 2 additional pinholes, while after 1600 times of producing the mask substrate by conventional methods, the additional pinholes are usually around 10.

(2) The photomask substrate obtained in Preparation Example 1 or Preparation Example 2 was subjected to exposure-development-etching-stripping treatment to prepare a mask. Mark 20 fields of view on the mask under an optical microscope, and mark and record the number and position of line loss. Wipe 20 fields of view with cotton swab + alcohol for 3 minutes, observe 20 fields of view with an optical microscope, and compare the line loss.

After the mask substrate obtained by the preparation method of the present invention was subjected to photolithography for patterning, a cotton swab + alcohol/acetone wiping method was used to monitor the slight missing of lines. The final result showed that no line missing occurred in this case, while line missing was very likely to occur in the conventional method. In some cases, when a single mask was observed in more than 4 fields of view, line missing would be found in 1 to 2 fields of view; between batches of masks, there was a 5% ratio of line missing.

In summary, the technical solution of the present invention enhances adhesion performance, reduces line missing, increases mask life, and reduces costs by stacking a silicon dioxide transition layer between the glass substrate and the chromium film layer, and performs well in the field of high-precision semiconductor applications and improves the yield rate. The baking and air cooling treatment of the film-coated sheet removes residual stress, increases the contact angle, enhances the adhesion between the chromium film layer and the photoresist layer, and reduces line missing anomalies.

The technical solution and method described in the present invention has strong operability and repeatability, and is a practical and efficient solution.

The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with the art within the technical scope disclosed in the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims (10)

1. A photomask substrate, characterized in that the photomask substrate comprises: a glass substrate, a chrome film layer and a photoresist layer, wherein a silicon dioxide transition layer is provided between the glass substrate and the chrome film layer. 2. The photomask substrate according to claim 1 is characterized in that the microstructure of the silica transition layer includes first silica particles and second silica particles located between the gaps of the first silica particles, wherein the particle size of the first silica particles is 50 to 60 nm, and the particle size of the second silica particles is 10 to 15 nm. 3 . The photomask substrate according to claim 1 , wherein the thickness of the silicon dioxide transition layer is 80 to 150 nm. 4. The photomask substrate according to claim 1 or 2, characterized in that the chromium film layer has a five-layer structure, the outermost layer has a thickness of 15 to 30 nm, and the thicknesses of the other four layers are 70 to 90 nm, and the outermost layer refers to the layer away from the glass substrate. 5. A method for preparing the photomask substrate according to any one of claims 1 to 4, characterized in that the method comprises the following steps: Step S1: treating the glass substrate to remove dirt from the surface; Step S2: heating the glass substrate; Step S3: stacking a silicon dioxide transition layer on the glass substrate; Step S4: stacking a chromium film layer on the silicon dioxide transition layer to obtain a film-plated sheet; Step S5: baking the coated sheet and air cooling after baking; Step S6: spin-coating the coated sheet, performing pre-baking treatment, and finally obtaining a photomask substrate. 6 . The preparation method according to claim 5 , characterized in that, in the step S2 , the glass substrate is heated to a temperature of 150-200° C. 7. The preparation method according to claim 5 is characterized in that, in the step S3, the stacked silicon dioxide transition layer adopts medium frequency magnetron sputtering coating technology, wherein, in the medium frequency magnetron sputtering coating, the power used is 0.5 to 1 kW, the volume ratio of oxygen and argon is 1:1 to 2:1, and the target material used is a silicon target. 8. The preparation method according to claim 5, characterized in that in the step S4, the stacked chromium film layer is deposited by DC magnetron sputtering, wherein the DC magnetron sputtering deposition operation comprises: Step S41: using a power of 1.5 to 2.5 Kw and a sputtering speed of 110 mm/min to 200 mm/min to form a four-layer structure in the chromium film layer; Step S42: Use 3.5-5Kw and a sputtering coating speed of 400mm/min-700mm/min to form the outermost layer of the chromium film. 9. The preparation method according to claim 5, characterized in that, in the step S5, the baking temperature is 80-100°C, the baking time is 10-30 minutes, and the baking is followed by air cooling for 60-120 minutes. 10 . The preparation method according to claim 5 , characterized in that, in the step S6 , the rotation speed of the spin coating is 1500 to 3000 rpm, and the coating time is 5 to 20 seconds.