KR100557616B1 - Photoresist polymer for resist flow process and photoresist composition containing the same - Google Patents

Abstract

The present invention relates to a method of forming a photoresist resin, a photoresist composition and a contact hole using the same for a resist flow process, wherein the photoresist polymer of Chemical Formula 1 has excellent physical properties as a photoresist as well. When used in combination with a polymer having different physical properties, it shows even better flow characteristics. Therefore, the photoresist composition including the polymer of Formula 1 and a mixed resin thereof may be usefully applied to a resist flow process.

[Formula 1]

Where

R ₁ , R ₂ , R ₃ , R ₄ , R ₅ , R ₆ , a, b and c are as defined in the specification.

Description

Photoresist polymer for resist flow process and photoresist composition containing same {Photoresist polymer for resist flow process and photoresist composition containing the same}

Figure 1a is a graph showing the change in the size of the contact hole with respect to the burning temperature change of the conventional photosensitive agent.

Figure 1b is a cross-sectional photograph showing that the contact hole pattern profile is modified as a result of performing a flow process using a conventional photosensitive agent.

2 is a graph showing the change in the size of the contact hole with respect to the burning temperature change of the photosensitizer of the present invention.

3 to 8 are cross-sectional photographs of contact hole patterns formed in Embodiments 4 to 9 of the present invention.

The present invention relates to a photoresist resin used in a resist flow process, a photoresist composition and a method for forming a contact hole using the same. More specifically, the photoresist polymer of the present invention alone By performing a resist flow process using a photoresist composition comprising a photoresist resin containing a polymer having a different physical property thereto, or in a conventional resist flow process, the flow of the photoresist composition suddenly occurs and the profile of the pattern is bent. The present invention relates to a method for forming a contact hole that can prevent a pattern from being buried due to a loss or overflow.

When forming a contact hole, which is one of semiconductor fine pattern forming processes, a resist flow process is a process technology for forming a fine contact hole having a resolution higher than that of an exposure apparatus.

The resist flow process is a process technology that has been developed in recent years and introduced into a mass production process. The exposure process and development process are performed to form a photosensitive film pattern using a photosensitive agent having a resolution of exposure equipment, and then the glass transition temperature of the photosensitive agent. The above means a process of applying thermal energy to allow the photosensitive agent to be thermally flowed. At this time, the pattern already formed by the supplied thermal energy is thermally flowed in the direction of decreasing the original size to finally obtain a fine pattern required for the integration process.

By introducing such a resist flow process, it is possible to form a fine pattern below the resolution of the exposure equipment as described above, but the biggest disadvantage of this process is the flow of the photoresist at a specific temperature, mainly above the glass transition temperature of the photoresist resin. This suddenly rises and the profile of the pattern can be bent or collapsed, the phenomenon that the pattern is embedded when excessive flow occurs (hereinafter referred to as "over flow") appears. This is because most photosensitizers are very sensitive to the heat applied, resulting in poor temperature control or excessive heat flow due to longer flow times than set values. Referring to FIG. 1A, which shows such a phenomenon, when a resist flow process is performed using a conventional photoresist composition including a single photoresist resin, when a heat is applied in a flow bake process, the photoresist composition gradually flows and reaches a specific temperature. This happens suddenly. As a result, the formed pattern is curved and contracted as shown in FIG. 1B. This phenomenon occurs because the amount of polymer that can be flowed in the upper layer, the central portion and the lower layer in the formed contact hole pattern is different. There is less time flow and the pattern formed after the flow is bent.

In order to solve the above problems, conventionally, a method of increasing the temperature uniformity of a baking oven applying heat or precisely controlling the time maintained in the baking oven is used. It is not a level that solves the overflow problem, and simply adjusting the oven does not improve the shape of the curved pattern that exists for the above reason.

Therefore, the present inventors mixed the photoresist resin having different physical properties with the photoresist composition, and when the photoresist composition is heat-flowed in the baking oven and reaches a specific temperature, the flow is slowed and the degree and direction of the flow can be adjusted. The present invention has been completed by finding that the pattern can be prevented from bending, collapsing or being buried.

An object of the present invention is a photoresist resin used in a resist flow process; A photoresist composition containing the same; A method of forming a photoresist pattern using the same; And it provides a method for forming a contact hole using the formed photoresist pattern.

In order to achieve the above object, the present invention provides a photoresist composition comprising a photoresist polymer having excellent flow characteristics and a resin containing the polymer alone or a mixed resin of the polymer and a photoresist polymer having different physical properties.

Hereinafter, the present invention will be described in detail.

In the present invention, first, a photoresist polymer of Formula 1 is provided. The polymer of Formula 1 is an environmentally stable chemically amplified photoresist (ESCAP) type polymer and includes an acid sensitive protecting group.

[Formula 1]

Where

R ₁ and R ₂ are hydrogen or methyl,

R ₃ , R ₄ and R ₆ are hydrogen or substituted or unsubstituted C ₁ -C ₁₀ straight or branched alkyl,

R ₅ is an acid sensitive protecting group,

The molar ratio of a: b: c is 0.055: 0.03-0.04: 0.005-0.01.

The acid sensitive protecting groups are groups which can be desorbed by acid, see US 5,212,043 (May 18, 1993), WO 97/33198 (September 12, 1997), WO 96/37526 (Nov. 28, 1996), EP 0 794 458 (September 10, 1997) EP 0 789 278 (August 13, 1997) and US 5,750,680 (May 12, 1998) and the like can be used, and t-butyl, tetrahydropyran-2-yl, 2-methyl tetrahydropyran-2-yl, tetrahydrofuran-2-yl, 2-methyl tetrahydrofuran-2-yl, 1-methoxypropyl, 1-methoxy-1-methylethyl, 1-ethoxy Selected from the group consisting of propyl, 1-ethoxy-1-methylethyl, 1-methoxyethyl, 1-ethoxyethyl, t-butoxyethyl, 1-isobutoxyethyl and 2-acetylment-1-yl desirable.

Preferred examples of the photoresist polymer include the following compounds:

Poly {4- [2- (hydroxyphenyl) propyl] phenyl methacrylate / t-butyl acrylate / styrene};

[Formula 1a]

Poly {4- [2- (hydroxyphenyl) propyl] phenyl acrylate / 2-methyl-3,3-dimethyl-but-2-yl (but-2-yl) acrylate / styrene};

[Formula 1b]

Poly {4- [2- (hydroxyphenyl) hexafluoropropyl] phenyl acrylate / t-butyl acrylate / styrene};

[Formula 1c]

The present invention also provides a photoresist composition for resist flow use used to form a fine contact hole.

The photoresist composition for a resist flow of the present invention includes a photoresist resin, a photoacid generator and an organic solvent, and as the photoresist resin, the polymer of Chemical Formula 1 may be used alone, or the acetal polymer of Chemical Formula 2 may be used. Mixed resins can also be used.

[Formula 2]

Where

R ₇ and R ₈ are hydrogen or methyl,

R ₉ , R ₁₀ , R ₁₁ , R ₁₂ and R ₁₃ are hydrogen or substituted or unsubstituted C ₁ -C ₁₀ straight or branched alkyl,

n is an integer selected from 1 to 5,

The mol% ratio of a: b: c: d is 60: 33-35: 5-0-2.

Preferred examples of the polymer of Formula 2 are as follows:

Poly [4-hydroxystyrene / 4- (1-ethoxy) ethoxy styrene / styrene / neopentylglycoldiacrylate];

[Formula 2a]

Poly [4-hydroxystyrene / 4- (1-ethoxy) ethoxy styrene / styrene];

[Formula 2b]

Poly [4-hydroxystyrene / 4- (1-benzyloxy) ethoxy styrene / styrene];

[Formula 2c]

Poly [4-hydroxystyrene / 4- (1-ethoxy) ethoxy styrene / styrene / 2,4-dimethyl-2,4-pentanedioldiacrylate];

[Formula 2d]

When using the mixed resin of the polymer of Formula 1 and Formula 2, the mixing ratio of the polymer of Formula 1 and the polymer of Formula 2 is 1 to 99% by weight: 99 to 1% by weight.

The photoresist composition of the present invention is excellent in resolving power and flow characteristics even when the polymer of Formula 1 is used alone, but when the mixed resin of the polymers of Formula 1 and Formula 2 is used, it shows even better flow characteristics.

The photoresist polymer of Chemical Formula 1 has a relatively low glass transition temperature, excellent adhesion to a silicon wafer, and excellent dissolution properties with respect to a conventional alkaline developer, so that the photoresist composition including the polymer of Chemical Formula 1 alone may be exposed. The contrast between the area and the unexposed areas is excellent and the resolution is excellent.

In addition, unlike the conventional photoresist for KrF, the photoresist polymer of Chemical Formula 1 is an acrylate-based resin, so the glass transition temperature and flow characteristics are very different. Therefore, when used in combination with polyhydroxy styrene-based acetal resin and ESCAP-type resin, the flow of the photoresist composition is improved at a certain temperature or higher when heat is applied in the flow bake process, and thus has a gentle flow sensitivity. (See FIG. 2), a desired process margin can be obtained. In addition, the flow tendency of each polymer is different, so that the deformation of the pattern can be prevented by appropriately adjusting the mixing ratio. The flow characteristics of the polymers are improved when using the mixed resin, because when the two polymers are present in a mixture, new physical properties are completely different from each other. This may be due to the difference in the glass transition temperature (T _g ) of each resin, or may be due to crosslinking between the resins or crosslinking within the resin itself.

On the other hand, the photoacid generator and the organic solvent used in the photoresist composition of the present invention can be used in the conventional photoresist composition.

The photoacid generator may be used as long as it is a compound capable of generating an acid by light, US 5,212,043 (May 18, 1993), WO 97/33198 (September 12, 1997), WO 96/37526 (1996. 11. 28), EP 0 794 458 (September 10, 1997) EP 0 789 278 (August 13, 1997), and US 5,750,680 (May 12, 1998) and the like, and are mainly sulfur-based or onium Salt based compounds are used. The photoacid generator is diphenyl iodo hexafluorophosphate, diphenyl iodo hexafluoro arsenate, diphenyl iodo hexafluoro antimonate, diphenyl paramethoxyphenyl triflate, diphenyl paratoluenyl triflate, diphenyl Paraisobutylphenyl triflate, diphenylpara-t-butylphenyl triflate, triphenylsulfonium hexafluoro phosphate, triphenylsulfonium hexafluoro arsenate, triphenylsulfonium hexafluoro antimonate, triphenylsulfonium One or two or more onium salt or sulfide compounds selected from the group consisting of triflate and dibutylnaphthylsulfonium triflate may be used, and those used in a ratio of 0.01 to 10% by weight based on the photoresist resin used desirable.

In addition, the organic solvent is US 5,212,043 (May 18, 1993), WO 97/33198 (September 12, 1997), WO 96/37526 (November 28, 1996), EP 0 794 458 (September 10, 1997) ) EP 0 789 278 (August 13, 1997) and US 5,750,680 (May 12, 1998) and the like, preferably propylene glycol methyl ether acetate, propylene glycol methyl ether, ethyl lactate, methyl 3-meth It is preferable to use one selected from the group consisting of oxypropionate, ethyl 3-ethoxypropionate and cyclohexanone in a ratio of 100 to 1000% by weight relative to the photoresist resin.

In addition, the present invention provides a method of forming a pattern incorporating a resist flow process using the photoresist composition, specifically

(a) applying a photoresist composition of the present invention above a predetermined etching layer to form a photoresist film,

(b) forming a primary photoresist pattern by a photolithography process,

(c) subjecting the primary photoresist pattern to a flow bake process such that the photoresist is thermally flowed to obtain a secondary photoresist pattern.

Exposure sources that can be used in the photolithography process include KrF (248 nm), ArF (193 nm), VUV (157 nm), EUV (13 nm), E-beam, X-ray or ion beam. In the case of the resin of 1, since the absorbance in the KrF region is low, it is most preferable to use a KrF light source as the exposure source.

In the flow baking process of step (c), thermal energy is applied at approximately 120 to 190 ° C, preferably 140 to 170 ° C.

In addition, the present invention provides a semiconductor device manufactured using the above-described method for forming a contact hole.

Hereinafter, the present invention will be described in detail by examples. However, the examples are only to illustrate the invention and the present invention is not limited by the following examples.

I. Synthesis of Photoresist Polymer

Example 1. Preparation of Poly {4- [2- (hydroxyphenyl) propyl] phenyl methacrylate / t-butyl acrylate / styrene}

4- [2- (hydroxyphenyl) propyl] phenyl methacrylate (0.055M), t-butyl acrylate (0.04M), styrene (0.005M) and AIBN were dissolved in 20 ml of anhydrous tetrahydrofuran. After the polymerization was carried out at 100 ° C. for 8 hours under an Ar (argon) stream (0.1torr or less). After polymerization, the reaction mixture was dropped in petroleum ether to precipitate a polymer, which was filtered and dried to obtain the title polymer of Chemical Formula 1a (61%).

Example 2. of poly {4- [2- (hydroxyphenyl) propyl] phenyl acrylate / 2-methyl-3,3-dimethyl-but-2-yl (but-2-yl) acrylate / styrene} Produce

4- [2- (hydroxyphenyl) propyl] phenyl acrylate (0.055M), 2-methyl-3,3-dimethyl-but-2-yl acrylate (0.04M), styrene (0.005M), AIBN After dissolving in 20 ml of anhydrous tetrahydrofuran, the polymerization was carried out for 8 hours at 100 DEG C under Ar stream (0.1torr or less). After polymerization, the reaction mixture was dropped in petroleum ether to precipitate a polymer, which was filtered and dried to obtain the title polymer of Chemical Formula 1b (60%).

Example 3. Preparation of Poly {4- [2- (hydroxyphenyl) hexafluoropropyl] phenyl acrylate / t-butyl acrylate / styrene}

4- [2- (hydroxyphenyl) hexafluoropropyl] phenyl acrylate (0.055M), t-butyl acrylate (0.03M), styrene (0.01M) and AIBN were dissolved in 20 ml of anhydrous tetrahydrofuran. The polymerization was carried out at 100 DEG C for 8 hours under Ar stream (0.1torr or less). After polymerization, the reaction mixture was dropped in petroleum ether to precipitate a polymer, which was filtered and dried to obtain the title polymer of Chemical Formula 1c (63%).

II. Preparation and Pattern Formation of Photoresist Composition

Example 4.

10 g of the resin synthesized in Example 1 and 0.1 g of triphenylsulfonium triflate were dissolved in 50 g of PGMEA (propylene glycol methyl ether acetate), and then filtered through a 0.2 μm filter to prepare a photoresist composition.

The composition was coated on a wafer and baked at 110 ° C. for 90 seconds. After baking, exposure was performed using a 0.60NA KrF exposure apparatus (Nikon S201). After exposure, the plate was baked at 130 ° C. for 90 seconds again, and developed using a 2.38 wt% TMAH developer to obtain a contact hole pattern of 200 nm. The contact hole pattern was baked at 153 ° C. for 90 seconds to obtain a 100 nm size contact hole pattern as a result of the resist flow process (see FIG. 3).

Example 5.

10 g of the resin synthesized in Example 2 and 0.1 g of triphenylsulfonium triflate were dissolved in 50 g of PGMEA, and then filtered through a 0.2 μm filter to prepare a photoresist composition.

The composition was coated on a wafer and baked at 110 ° C. for 90 seconds. After baking, exposure was performed using a 0.60NA KrF exposure apparatus (Nikon S201). After exposure, the plate was baked at 130 ° C. for 90 seconds again, and developed using a 2.38 wt% TMAH developer to obtain a contact hole pattern of 200 nm. The contact hole pattern was baked at 148 ° C. for 90 seconds and a resist flow process was performed to obtain a contact hole pattern having a size of 90 nm (see FIG. 4).

Example 6.

10 g of the resin synthesized in Example 3 and 0.1 g of triphenylsulfonium triflate were dissolved in 50 g of PGMEA, and then filtered through a 0.2 μm filter to prepare a photoresist composition.

The composition was coated on a wafer and baked at 110 ° C. for 90 seconds. After baking, exposure was performed using a 0.60NA KrF exposure apparatus (Nikon S201). After exposure, the plate was baked at 130 ° C. for 90 seconds again, and developed using a 2.38 wt% TMAH developer to obtain a contact hole pattern of 200 nm. The contact hole pattern was baked at 154 ° C. for 90 seconds to obtain a 80 nm size contact hole pattern as a result of the resist flow process (see FIG. 5).

Example 7.

5 g of the resin synthesized in Example 2 and a poly [4-hydroxystyrene / 4- (1-ethoxy) ethoxy styrene / styrene] resin of the above formula (2b) (a = 60 mol%, b = 35 mol%, c = 5 mol%, d = 0 mol%) 5 g and triphenylsulfonium triflate 0.1 g were dissolved in 50 g of PGMEA, and then filtered through a 0.2 μm filter to prepare a photoresist composition.

The composition was coated on a wafer and baked at 110 ° C. for 90 seconds. After baking, exposure was performed using a 0.60NA KrF exposure apparatus (Nikon S201). After exposure, the plate was baked at 130 ° C. for 90 seconds again, and developed using a 2.38 wt% TMAH developer to obtain a contact hole pattern of 200 nm. The contact hole pattern was baked at 156 ° C. for 90 seconds and a resist flow process was performed to obtain a contact hole pattern having a size of 70 nm (see FIG. 6).

Example 8.

5 g of the resin synthesized in Example 1 and a poly [4-hydroxystyrene / 4- (1-ethoxy) ethoxy styrene / styrene] resin of the above formula (2b) (a = 60 mol%, b = 35 mol%, c = 5 mol%, d = 0 mol%) 5 g and triphenylsulfonium triflate 0.1 g were dissolved in 50 g of PGMEA, and then filtered through a 0.2 μm filter to prepare a photoresist composition.

The composition was coated on a wafer and baked at 110 ° C. for 90 seconds. After baking, exposure was performed using a 0.60NA KrF exposure apparatus (Nikon S201). After exposure, the plate was baked at 130 ° C. for 90 seconds again, and developed using a 2.38 wt% TMAH developer to obtain a contact hole pattern of 200 nm. The contact hole pattern was baked at 156 ° C. for 90 seconds to obtain a 50 nm size contact hole pattern as a result of the resist flow process (see FIG. 7).

Example 9.

5 g of the resin synthesized in Example 3, and poly [4-hydroxy styrene / 4- (1-ethoxy) ethoxy styrene / styrene / 2,4-dimethyl-2,4-pentanediol diacryl Rate] 5 g of a resin (a = 60 mol%, b = 33 mol%, c = 5 mol%, d = 2 mol%) and 0.1 g of triphenylsulfonium triflate were dissolved in 50 g of PGMEA, and then filtered through a 0.2 μm filter to form a photoresist composition. Was prepared.

The composition was coated on a wafer and baked at 110 ° C. for 90 seconds. After baking, exposure was performed using a 0.60NA KrF exposure apparatus (Nikon S201). After exposure, the plate was baked at 130 ° C. for 90 seconds again, and developed using a 2.38 wt% TMAH developer to obtain a contact hole pattern of 200 nm. The contact hole pattern was baked at 157 ° C. for 90 seconds to obtain a 50 nm size contact hole pattern as a result of the resist flow process (see FIG. 8).

As described above, the photoresist polymer of Chemical Formula 1 of the present invention has excellent resolution and flow characteristics, and shows excellent physical properties as a photosensitizer itself (see Examples 3 to 5). It can be seen that when used in admixture with the polymer, it shows even better flow properties (see Examples 6-8).

Therefore, by using the photoresist composition of the present invention, a phenomenon in which the contact hole pattern has a curved profile or is buried by a sudden flow of a photosensitive agent in a resist flow process for forming a fine contact hole pattern below a resolution of an exposure apparatus can be prevented. The contact hole pattern of 100 nm or less can be obtained uniformly.

Claims (18)

A photoresist polymer comprising a polymerized unit represented by the following formula (1). [Formula 1]

Where R ₁ and R ₂ are hydrogen or methyl, R ₃ , R ₄ and R ₆ are hydrogen or substituted or unsubstituted C ₁ -C ₁₀ straight or branched alkyl, R ₅ is an acid sensitive protecting group, The molar ratio of a: b: c is 0.055: 0.03-0.04: 0.005-0.01. The method of claim 1, The acid sensitive protecting groups are t-butyl, tetrahydropyran-2-yl, 2-methyl tetrahydropyran-2-yl, tetrahydrofuran-2-yl, 2-methyl tetrahydrofuran-2-yl, 1- Methoxypropyl, 1-methoxy-1-methylethyl, 1-ethoxypropyl, 1-ethoxy-1-methylethyl, 1-methoxyethyl, 1-ethoxyethyl, t-butoxyethyl, 1- Photoresist polymer, characterized in that it is selected from the group consisting of isobutoxyethyl and 2-acetylment-1-yl. The method of claim 1, The polymerized unit may be selected from the group consisting of poly {4- [2- (hydroxyphenyl) propyl] phenyl methacrylate / t-butyl acrylate / styrene}; Poly {4- [2- (hydroxyphenyl) propyl] phenyl acrylate / 2-methyl-3,3-dimethyl-but-2-yl (but-2-yl) acrylate / styrene}; And poly {4- [2- (hydroxyphenyl) hexafluoropropyl] phenyl acrylate / t-butyl acrylate / styrene}. A photoresist composition comprising the photoresist polymer of claim 1, a photoacid generator and an organic solvent. The method of claim 4, wherein The composition is a photoresist composition, characterized in that it further comprises a polymer comprising a polymer unit of the formula (2). [Formula 2]

Where R ₇ and R ₈ are hydrogen or methyl, R ₉ , R ₁₀ , R ₁₁ , R ₁₂ and R ₁₃ are hydrogen or substituted or unsubstituted C ₁ -C ₁₀ straight or branched alkyl, n is an integer selected from 1 to 5, The mol% ratio of a: b: c: d is 60: 33-35: 5-0-2. The method of claim 5, The polymer of the formula (2) is a poly [4-hydroxy styrene / 4- (1-ethoxy) ethoxy styrene / styrene / neopentyl glycol diacrylate]; Poly [4-hydroxystyrene / 4- (1-ethoxy) ethoxy styrene / styrene]; Poly [4-hydroxystyrene / 4- (1-benzyloxy) ethoxy styrene / styrene]; And poly [4-hydroxystyrene / 4- (1-ethoxy) ethoxy styrene / styrene / 2,4-dimethyl-2,4-pentanedioldiacrylate]. Resist composition. The method of claim 5, The mixing ratio of the polymer of Formula 1 and the polymer of Formula 2 is 1 to 99% by weight: 99 to 1% by weight ratio. The method of claim 4, wherein The photoacid generator is diphenyl iodo hexafluorophosphate, diphenyl iodo hexafluoro arsenate, diphenyl iodo hexafluoro antimonate, diphenyl paramethoxyphenyl triflate, diphenyl paratoluenyl triflate, diphenyl Paraisobutylphenyl triflate, diphenylpara-t-butylphenyl triflate, triphenylsulfonium hexafluoro phosphate, triphenylsulfonium hexafluoro arsenate, triphenylsulfonium hexafluoro antimonate, triphenylsulfonium A photoresist composition comprising one or more selected from the group consisting of triflate and dibutylnaphthylsulfonium triflate. The method of claim 4, wherein The photoresist composition is a photoresist composition, characterized in that used in 0.01 to 10% by weight relative to the photoresist resin. The method of claim 4, wherein The organic solvent is selected from the group consisting of propylene glycol methyl ether acetate, propylene glycol methyl ether, ethyl lactate, methyl 3-methoxypropionate, ethyl 3-ethoxypropionate and cyclohexanone. Resist composition. The method of claim 4, wherein The organic solvent is a photoresist composition, characterized in that used in 100 to 1000% by weight relative to the photoresist resin. (a) forming a primary photoresist pattern on the predetermined etching layer using the photoresist composition of claim 4; (b) performing a resist flow process on the primary photoresist pattern to obtain a secondary photoresist pattern by performing a thermal flow of the photoresist. The method of claim 12, Step (a) is (i) forming a photoresist film by applying the photoresist composition of claim 4 on the etched layer; (ii) forming a primary photoresist pattern by a photolithography process. The method of claim 12, The temperature of said resist flow process is 120-190 degreeC, The formation method of the photoresist pattern characterized by the above-mentioned. The method of claim 13, An exposure source used in the photolithography process is selected from the group consisting of KrF (248 nm), ArF (193 nm), VUV (157 nm), EUV (13 nm), E-beam, X-ray and ion beam. Method of forming a resist pattern. The method of claim 12, And the primary and secondary photoresist patterns are L / S patterns or contact hole patterns. A semiconductor device manufactured by the method of claim 12. A photoresist resin comprising a first polymer comprising a polymerized unit of Formula 1 and a second polymer comprising a polymerized unit of Formula 2. [Formula 1]

Where R ₁ and R ₂ are hydrogen or methyl, R ₃ , R ₄ and R ₆ are hydrogen or substituted or unsubstituted C ₁ -C ₁₀ straight or branched alkyl, R ₅ is an acid sensitive protecting group, The molar ratio of a: b: c is 0.055: 0.03-0.04: 0.005-0.01. [Formula 2]

Where R ₇ and R ₈ are hydrogen or methyl, R ₉ , R ₁₀ , R ₁₁ , R ₁₂ and R ₁₃ are hydrogen or substituted or unsubstituted C ₁ -C ₁₀ straight or branched alkyl, n is an integer selected from 1 to 5, The mol% ratio of a: b: c: d is 60: 33-35: 5-0-2.

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