Mask plate and preparation method thereof

Abstract

The invention discloses a mask plate and a preparation method thereof. The preparation method comprises the following steps: cleaning and drying a silicon wafer; etching a male mold pattern which is complementary with the mask plate in shape on the silicon wafer; a Ti / Cu composite film is deposited on the front face of the wafer male mold, then invar alloy is electroformed, and annealing is conducted after electroforming is completed; carrying out chemical polishing until the wafer male mold pattern is completely exposed; a hard material film layer is deposited on the face, with the invar alloy, of the wafer male die; coating photoresist on the back surface and the front surface of the wafer respectively, covering the back surface of the wafer with a mask plate for exposure and development, and then soaking the wafer in an alkali solution containing hydroxyl ions until the silicon wafer in an area not protected by the photoresist disappears; removing the photoresist; performing dry etching to remove the hard material film layer corresponding to the pixel hole area; the Ti / Cu composite film is removed through etching, and the mask plate is obtained after cleaning and drying; according to the mask plate, the stress deformation of the evaporation pixel opening holes and the evaporation alignment holes is small, the cleaning resistance is high, and the shape of the opening pattern is accurate.

Description

Technical Field

[0001] The invention belongs to the technical field of mask plates, and in particular relates to a mask plate and a preparation method thereof. Background Art

[0002] With the development of high PPI OLEDos devices, it is inevitable that OLEDos with direct RGB patterned structure will replace OLEDos with white light + color filter structure. Therefore, the development of high PPI fine mask has become the main research direction for many OLED panel and upstream and downstream manufacturers. The main solutions currently include: fine mask of Invar material by calendering process, fine mask of Invar material by electroforming process, fine mask of Invar and polymer composite material, fine mask of silicon-based silicon nitride, nickel plating on the surface of silicon-based silicon nitride mask, etc.

[0003] Among them, the silicon-based silicon nitride mask FSM (Fine Si Mask) has been used in the production of OLEDos samples with more than 3000PPI, and has achieved great success. However, due to the high brittleness of silicon nitride, it is very difficult to use and clean, and the cost is too high, making it difficult to promote. Therefore, some manufacturers are still actively developing fine masks made of Invar material to obtain a reusable low-cost alternative. The most important solution is the silicon-based electroforming Invar mask.

[0004] Since it is difficult to eliminate residual stress in the invar alloy film in the silicon-based electroformed Invar mask, the pixel openings and alignment holes of the film will be deformed, which seriously affects the pixel opening accuracy and alignment accuracy of the mask. For silicon-based silicon nitride masks, although the silicon nitride film has a high hardness and good shape retention, and the pixel openings and alignment holes will not be deformed, the mask is very easy to be damaged by external forces during storage, use and cleaning due to the hardness and brittleness of the silicon nitride material. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a mask plate and a preparation method thereof, wherein a wafer positive mold is first etched on a silicon wafer, and then a layer of Ti / Cu composite film is deposited in the wafer positive mold. Then, after electroforming invar alloy and polishing, a layer of hard material film is further deposited on the surface of the invar alloy, and then the silicon wafer in the pixel area and the hard material film layer at the pixel opening are sequentially etched away. The hard material film layer is not easily deformed, and the pixel holes and alignment holes of the invar film are prevented from being deformed during the evaporation mask plate process, thereby improving the opening accuracy of the pixel opening and alignment holes.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing a mask plate, the method comprising the following steps:

[0008] (1) Cleaning and drying the silicon wafer;

[0009] (2) etching a positive mold pattern complementary to the shape of the mask plate on the silicon wafer to obtain a wafer positive mold;

[0010] (3) depositing a Ti / Cu composite film on the front side of the wafer positive mold, and then electroforming an Invar alloy until the Invar alloy completely covers the wafer positive mold pattern, and annealing is performed after the electroforming is completed;

[0011] (4) chemical polishing is performed until the wafer positive mold pattern is completely exposed;

[0012] (5) depositing a hard material film layer on the side of the wafer positive mold with Invar alloy, and cleaning and drying it;

[0013] (6) coating photoresist on the back and front sides of the wafer respectively, using a mask plate to cover the back side of the wafer for exposure and development, exposing the areas corresponding to the vapor-deposited pixel holes and the areas corresponding to the vapor-deposited alignment holes of the mask plate, and placing the areas other than these under photoresist protection, and then immersing the wafer in an alkaline solution containing hydroxide ions until the silicon wafer in the areas not protected by the photoresist disappears;

[0014] (7) removing photoresist;

[0015] (8) The mask plate is placed with the side containing the hard material film layer facing downward in a dry etching machine for dry etching to remove the hard material film layer corresponding to the pixel hole area. After cleaning and drying, the mask plate is obtained.

[0016] In step (3), the thickness of the deposited Ti / Cu composite film is 6 to 10% of the depth of the positive mold pattern.

[0017] In step (3), the Invar alloy is Invar 36.

[0018] In step (3), the annealing conditions are: the annealing atmosphere is a mixture of nitrogen and hydrogen, and the annealing is carried out at 300-800° C. for 1.5-2.5 hours.

[0019] In step (5), the hard material film layer is one of a silicon nitride layer, a diamond layer, a silicon oxide layer, a chromium layer, a carbon fiber layer, and a low expansion glass layer. The compressive strength of these film layers is higher than 700 MPa, and the linear thermal expansion coefficient at 20°C is lower than 10×10 -6 / ℃, if the thermal expansion coefficient of the superhard material is too high, it will cause the pixel openings of the mask to deviate too much due to thermal expansion during the evaporation process, and there will also be a risk of separation or even damage of the invar and the superhard material.

[0020] In step (6), the alkali solution containing hydroxide ions is a potassium hydroxide solution with a mass concentration of 15 to 25%.

[0021] In step (8), the dry etching conditions are: using SF 6 and O 2 Mixed gas, etch for 0.5-1min at 180-220W and 35-45℃.

[0022] The present invention also provides a mask plate prepared according to the preparation method, wherein the mask plate comprises a mask base, and a first mask layer and a second mask layer sequentially arranged on the mask base; pixel through holes are arranged in both the first mask layer and the second mask layer, and the positions and sizes of the pixel through holes of the two are the same;

[0023] The first mask layer is an Invar alloy layer; the second mask layer is a hard material film layer; and the mask base is a silicon wafer.

[0024] The thicknesses of the first mask layer and the second mask layer are 1-10 μm and 0.05-1 μm respectively.

[0025] In the preparation method of the mask provided by the present invention, a layer of Ti / Cu composite film is first deposited in the wafer positive mold, and then after electroforming the invar alloy and polishing, a layer of hard material film is continuously deposited on the surface of the invar alloy. The hard material film is not easily deformed, so as to prevent the pixel holes and alignment holes of the invar film from deforming during the evaporation of the mask, thereby improving the opening accuracy of the pixel openings and alignment holes. The Ti / Cu composite film layer can serve as a barrier layer when the hard material layer is etched, and as the etching of the hard material layer proceeds, the Ti / Cu composite film layer will gradually become thinner or even disappear, thereby achieving the effect of removing the Ti / Cu composite film layer and reducing the thermal deformation of the mask, and reducing the residual stress deformation of the invar film by adding the hard material film layer, thereby achieving the effect of killing two birds with one stone. Compared with the prior art, the present invention has the following beneficial effects:

[0026] The preparation method of the mask provided by the present invention is simple, and the shape of the obtained mask opening pattern is accurate. The present invention uses the composite film of Invar alloy / hard material as the working film layer of the evaporation mask, so that the working film layer of the mask combines the advantages of the flexibility of the Invar alloy material and the shape retention of the hard material, and the stress deformation of the evaporation pixel opening and the evaporation alignment hole is small, and the cleaning resistance is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 A top view of the mask provided by the present invention, 1-silicon wafer, 2-pixel opening, 3-evaporation alignment hole, 4-Invar alloy layer, 5-hard material film layer;

[0028] Figure 2 A side view of the mask provided by the present invention;

[0029] Figure 3 A schematic diagram of the manufacturing process flow of the mask plate provided by the present invention. DETAILED DESCRIPTION

[0030] The present invention is described in detail below with reference to the embodiments.

[0031] A method for preparing a mask plate comprises the following steps:

[0032] (1) Standard cleaning of silicon wafers (RCA)

[0033] Use single-sided polished wafers, the crystal orientation of the polished surface is <100> Direction, the wafer diameter is 150 or 200 mm, and the thickness is 315 μm. The wafer is placed in a wet cleaning machine for cleaning. The first cleaning (SC-1) is performed with deionized water (H 2 O), a mixture of 30% wt hydrogen peroxide solution and ammonium hydroxide heated at 75°C, with a weight ratio of 15:3:1, for 10 minutes. After the wafer is removed, it is rinsed in a cascade of deionized water for 5 minutes. Then the wafer is placed in H 2 The oxide on the wafer is removed by 1 minute in a hydrofluoric acid bath with a volume ratio of O:HF of 50:1. After the wafer is removed, it is cascade rinsed again in deionized water for 5 minutes. Next is the second cleaning (SC-2), using a 75°C heated mixture of deionized water, 30%wt hydrogen peroxide solution and 37%wt hydrochloric acid solution, with a weight ratio of 15:3:1, for 10 minutes. After the wafer is removed, it is cascade rinsed with deionized water for 5 minutes, and then immediately placed in a spin rinse dryer.

[0034] (2) Silicon wafer etching

[0035] First, use a spin coater to spin-coat HMDSHDMS on the front side of the wafer and bake it at 150°C for 1 min. Then, spin-coat a 1.5 μm thick i-line positive photoresist layer on the front side and bake it at 150°C for 1 min. Then, use a photomask to irradiate the wafer with 150 mJ / cm 2 The photoresist was patterned and exposed with a dose of . After exposure, a post-exposure bake was performed at 105°C for 1 min, and then developed with Microposit MF-321 developer for 5 min. Silicon was etched using a Drytek 482Quad Etcher under the following conditions: the etching gas was SF 6and Ar, the volume ratio of the two is 1:1, the etching power is 600W, the temperature is 40℃, the time is 4min, and the etching depth is 3.3μm. The resist is removed in a solvent stripping bath, the solvent stripping bath is Baker PRS-2000resistStripper, and the stripping conditions are: soaking at 90℃ for 10min. In this way, a single crystal silicon positive mold wafer with patterned protrusions is obtained.

[0036] (3) Electroforming Invar and annealing

[0037] The wafer positive mold was transferred to the PVD equipment, and a Ti / Cu composite film consisting of a 10nm Ti layer and a 200nm Cu layer was deposited on the front of the silicon wafer by sputtering. Subsequently, the film was transferred to the electroforming system for deposition of Invar film. The electroforming conditions were: current density 1A / dm 2 , 35℃, pH=2.5-3, filter diameter=0.1μm, anode is nickel particles and iron foil, electroforming for 15-20min, depositing Invar 36 alloy with a thickness of 3.5μm. After the electroforming is completed, the silicon-based Invar casting is taken out and cleaned with a wafer cleaner for 10min of methyl pyrrolidone, isopropanol and carbon dioxide / water two-fluid circulation. After drying with nitrogen, the thickness of the electroformed layer is measured by a focused ion beam microscope (FIB), and the stoichiometric ratio of the electroformed layer is measured by the energy spectrometer accessory of the FIB equipment. The thickness measurement needs to be measured at 25 points in the non-display area of ​​the mask, and the thickness of the Invar layer is required to be 3.5μm and the uniformity is <5%. The optimal composition of Invar36 is Ni (36%) / Fe (63%) / Co (0.1-1%). After the casting is cleaned and dried again, it is placed in a tubular furnace and N is passed through. 2 / H 2 The mixed gas was annealed at 600℃ for 2.0h and then cooled to room temperature, so that the crystal structure of electroformed Invar changed from body-centered cubic to face-centered cubic, and the linear thermal expansion coefficient changed from 10×10 -6 / ℃ reduced to <3×10 -6 / ℃.

[0038] (4) Chemical Mechanical Polishing

[0039] The annealed casting with known Invar layer thickness was polished using chemical mechanical polishing equipment, ultimately retaining a 3μm thick Invar layer. At this point, 0.3μm of over-etching had been performed, removing the seed layer and a portion of silicon in the raised portion, completely exposing the single crystal silicon positive mold pattern.

[0040] (5) Silicon nitride deposition

[0041] The wafer was transferred to the PECVD equipment and a 1μm thick SiNx layer was deposited on the invar side.

[0042] (6) Silicon wafer etching

[0043] The wafer with silicon nitride deposited on the surface was placed in a wafer cleaning machine for 10 min of methyl pyrrolidone, isopropanol and carbon dioxide / water two-fluid cycle cleaning. After drying with nitrogen, a 3 μm thick silicon dioxide was spin-coated on the back of the wafer. B3, cover the exposure and develop through the photolithography mask to expose the area corresponding to the vapor-deposited pixel hole of the mask and the area corresponding to the vapor-deposited alignment hole. These other areas are placed under the protection of photoresist to prevent etching by alkaline solution. Then spin-coat the entire silicon nitride surface on the front side of the wafer with photoresist to prevent the etching solution from corroding the silicon nitride during the back silicon etching process. In order to etch the wafer, a 77°C bath containing a 20% mass concentration of potassium hydroxide solution is used. KOH usually etches silicon at a rate of 1μm per minute, so a 315μm wafer takes about 5 hours. After the wafer is etched through, it is rinsed in a deionized water cascade for 10 minutes.

[0044] (7) Removal of photoresist

[0045] After rinsing, remove with Piranha Etching Solution B3, rinse again with deionized water for 10 min. Subsequently, dip the wafer into isopropyl alcohol and slowly pull it out, allowing it to dry as it leaves the solution to prevent drying marks and debris.

[0046] (8) SiNx etching

[0047] The front side (silicon nitride side) of the mask after the backplane silicon is removed and cleaned is placed in a dry etching machine with the face downwards, and the SiNx on the front side of the mask is etched. The etching conditions are: the etching gas is SF 6 and O 2 According to the mixed gas composed of 1:1 volume ratio, the etching power is 200W, the temperature is 40℃, and the time is 0.5-1min. The area without Ti / Cu / Invar alloy shielding will be etched away, and the area with Ti / Cu / invar alloy shielding will be retained. During this silicon nitride etching process, the Ti / Cu composite film at the bottom of invar will be reduced to a certain thickness. After the mask is taken out of the dry etching machine, it is cleaned again with methyl pyrrolidone, isopropyl alcohol and carbon dioxide / water two-fluid circulation for 10 minutes in a wafer cleaning machine, and then dried with nitrogen.

[0048] The mask plate structure obtained in this embodiment is as follows: the mask plate includes a mask base, and a first mask layer and a second mask layer sequentially arranged on the mask base; pixel through holes are arranged in the first mask layer and the second mask layer, and the positions and sizes of the pixel through holes of the two are the same; the first mask layer is an invar alloy layer; the second mask layer is a silicon nitride layer; and the mask base is a silicon wafer. In this structure, due to the complementary mechanical properties of invar alloy and silicon nitride, the stress deformation of the mask plate vapor deposition pixel opening and vapor deposition alignment hole is small, and the cleaning resistance is high.

[0049] The detailed description of a mask plate and a method for preparing the same with reference to the above embodiments is illustrative rather than restrictive, and several embodiments may be listed according to the limited scope. Therefore, changes and modifications without departing from the overall concept of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for preparing a mask plate, characterized in that: The preparation method comprises the following steps: (1) Cleaning and drying the silicon wafer; (2) etching a positive mold pattern complementary to the shape of the mask plate on the silicon wafer to obtain a wafer positive mold; (3) depositing a Ti / Cu composite film on the front side of the wafer positive mold, and then electroforming an Invar alloy until the Invar alloy completely covers the wafer positive mold pattern, and annealing is performed after the electroforming is completed; (4) chemical polishing is performed until the wafer positive mold pattern is completely exposed; (5) depositing a hard material film layer on the side of the wafer positive mold with Invar alloy, and cleaning and drying it; (6) coating photoresist on the back and front sides of the wafer respectively, using a mask plate to cover the back side of the wafer for exposure and development, exposing the areas corresponding to the vapor-deposited pixel holes and the areas corresponding to the vapor-deposited alignment holes of the mask plate, and placing the areas other than these under photoresist protection, and then immersing the wafer in an alkaline solution containing hydroxide ions until the silicon wafer in the areas not protected by the photoresist disappears; (7) removing photoresist; (8) The mask plate is placed with the side containing the hard material film layer facing downward in a dry etching machine for dry etching to remove the hard material film layer corresponding to the pixel hole area. After cleaning and drying, the mask plate is obtained.

2. The preparation method according to claim 1, characterized in that: In step (3), the thickness of the deposited Ti / Cu composite film is 6 to 10% of the depth of the positive mold pattern.

3. The preparation method according to claim 1, characterized in that: In step (3), the Invar alloy is Invar36.

4. The preparation method according to claim 1, characterized in that: In step (3), the annealing conditions are: the annealing atmosphere is a mixture of nitrogen and hydrogen, and the annealing is carried out at 300-800°C for 1.5-2.5h.

5. The preparation method according to claim 1, characterized in that: In step (5), the hard material film layer is one of a silicon nitride layer, a diamond layer, a silicon oxide layer, a chromium layer, a carbon fiber layer, and a low expansion glass layer.

6. The preparation method according to claim 1, characterized in that: In step (6), the alkali solution containing hydroxide ions is a potassium hydroxide solution with a mass concentration of 15 to 25%.

7. The preparation method according to claim 1, characterized in that: In step (8), the dry etching conditions are: using a mixed gas of SF6 and O2, etching at 180-220W and 35-45°C for 0.5-1min.

8. The mask plate prepared by the preparation method according to any one of claims 1 to 7.

9. The mask plate according to claim 8, characterized in that: The mask plate comprises a mask base, and a first mask layer and a second mask layer sequentially arranged on the mask base; pixel through holes are arranged in both the first mask layer and the second mask layer, and the positions and sizes of the pixel through holes of the two are the same; The first mask layer is an Invar alloy layer; the second mask layer is a hard material film layer; and the mask base is a silicon wafer.

10. The mask plate according to claim 9, characterized in that: The thicknesses of the first mask layer and the second mask layer are 1-10 μm and 0.05-1 μm respectively.

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