Method for producing low molecular weight cellulose derivative, and method for producing etched inorganic material

Abstract

The present invention provides a method for producing a low molecular weight cellulose derivative from a starting material cellulose derivative, while suppressing desorption of a substituent.　A method for producing a low molecular weight cellulose derivative according to the present disclosure includes subjecting a cellulose derivative (C) which has a constituent unit represented by formula (1) and has a weight average molecular weight of W1, in which W1 is 5 × 104 or more, to an extraction treatment using a polar protic solvent so as to obtain a low molecular weight cellulose derivative (c) which has the constituent unit represented by formula (1) and has a weight average molecular weight of W2, in which W2 is 1 / 2 or less of W1.

Description

Method for producing low molecular weight cellulose derivatives, method for producing etched inorganic materials

[0001] The present disclosure relates to a method for producing a low-molecular-weight cellulose derivative, a silicon nitride film protective agent containing the low-molecular-weight cellulose derivative obtained by the method, and a method for producing a silicon substrate having a surface layer containing a silicon oxide film and a silicon nitride film, in which the silicon oxide film is selectively etched, using the low-molecular-weight cellulose derivative obtained by the method. This application claims priority from Japanese Patent Application No. 2023-177763, filed on October 13, 2023, the contents of which are incorporated herein by reference.

[0002] Cellulose, a major component of biomass, is used in a variety of applications, including as a binder, thickener, coating agent, and raw material for containers and packaging films. To be used in these applications, cellulose must be readily soluble in water. However, because cellulose contains a large number of hydroxyl groups in its molecule, it readily undergoes dehydration reactions to form crosslinked structures, making it difficult to dissolve in water.

[0003] Therefore, a method of substituting the abundant hydroxyl groups of cellulose with ether bonds or the like to impart water solubility to cellulose is known. However, it is difficult to completely proceed with the substitution reaction, and the unsubstituted hydroxyl groups tend to form crosslinked structures, resulting in water-insoluble portions. Furthermore, when a cellulose derivative containing unsubstituted hydroxyl groups is used as a coating agent, it is difficult to form a uniform coating film, and thick film portions and defective portions tend to occur.

[0004] As a method for preventing a cellulose derivative containing unsubstituted hydroxyl groups from generating water-insoluble portions, Patent Document 1 discloses a method for depolymerizing the cellulose derivative with a strong acid such as hydrochloric acid, a strong alkali such as sodium hydroxide, or a cellulolytic enzyme such as cellulase.

[0005] Japanese Patent Application Laid-Open No. 2005-239845

[0006] However, when depolymerization is performed using a strong acid, a strong alkali, or a cellulolytic enzyme, the substituents such as ether bonds of the cellulose derivative are easily eliminated, increasing the degree of unsubstituted hydroxyl groups, which in turn makes it easier to form crosslinked structures. Therefore, it has been found that the cellulose derivative obtained by the method of Patent Document 1 still tends to produce water-insoluble portions.

[0007] Therefore, an object of the present disclosure is to provide a method for producing a low-molecular-weight cellulose derivative from a raw cellulose derivative while suppressing the elimination of substituents. Another object of the present disclosure is to provide a method for producing a low-molecular-weight cellulose derivative from a raw cellulose derivative, the low-molecular-weight cellulose derivative having a reduced degree of crosslinking and an increased degree of substitution. Another object of the present disclosure is to provide a method for producing a silicon substrate having a surface layer containing a silicon oxide film and a silicon nitride film, in which the silicon oxide film is selectively etched, using the low-molecular-weight cellulose derivative obtained by the above method. Another object of the present disclosure is to provide a silicon nitride film protective agent containing a low-molecular-weight cellulose derivative having a low degree of crosslinking and a high degree of substitution.

[0008] As a result of extensive research conducted by the present inventors to solve the above-mentioned problems, they have found that there is a difference in solubility in a protic polar solvent between a cellulose derivative with a low degree of crosslinking and a high degree of substitution and a cellulose derivative with a high degree of crosslinking and a low degree of substitution, and that by performing an extraction treatment using a protic polar solvent, a low-molecular-weight cellulose derivative with a low degree of crosslinking and a high degree of substitution can be selectively obtained in the extract.

[0009] Furthermore, it was found that when an aqueous solution of the low-molecular-weight cellulose derivative with a low degree of crosslinking and a high degree of substitution obtained in this way is applied to the surface layer of an inorganic material having a surface layer containing an oxide film and a nitride film, a coating (particularly a coating of uniform thickness without thick portions or missing portions) can be formed only on the surface of the nitride film, thereby selectively imparting etching resistance to the nitride film. It was also found that treating an inorganic material having a surface layer containing an oxide film and a nitride film with an aqueous solution of the cellulose derivative, followed by etching using a wet etching solution that selectively etches the oxide film, can produce a silicon substrate in which the silicon oxide film is selectively and accurately etched relative to the silicon nitride film. The present disclosure has been completed based on these findings.

[0010] That is, the present disclosure provides a compound represented by the following formula (1): (wherein three R's are the same or different and represent a hydrogen atom, a hydrocarbon group, or a group in which two or more hydrocarbon groups are bonded via a linking group; provided that at least one of the three R's is a hydrocarbon group or a group in which two or more hydrocarbon groups are bonded via a linking group; the hydrocarbon group may have a hydroxyl group or a carboxyl group), and the weight average molecular weight is W1, where W1 is 5×10 4 The present invention provides a method for producing a low-molecular-weight cellulose derivative, which comprises subjecting the above-described cellulose derivative (C) to an extraction treatment using a protic polar solvent to obtain a low-molecular-weight cellulose derivative (c) having a constitutional unit represented by the above formula (1) and having a weight-average molecular weight of W2, where W2 is 1 / 2 or less of W1.

[0011] The present disclosure also provides a method for producing the low-molecular-weight cellulose derivative, wherein the low-molecular-weight cellulose derivative (c) has a degree of molar substitution of 2.5 or more.

[0012] The present disclosure also provides a method for producing the low-molecular-weight cellulose derivative (c), wherein the crosslinking degree of the low-molecular-weight cellulose derivative is 0.1 or less.

[0013] The present disclosure also provides the method for producing the low-molecular-weight cellulose derivative, wherein the Log P value of the protic polar solvent is −0.4 or more and 0.6 or less.

[0014] The present disclosure also provides a method for producing the low-molecular-weight cellulose derivative, wherein the cellulose derivative (C) is hydroxyethyl cellulose.

[0015] The present disclosure also provides a method for producing a silicon substrate having a surface layer containing a silicon oxide film and a silicon nitride film, in which the silicon oxide film has been selectively etched, through the following steps: Step 1: producing a low-molecular-weight cellulose derivative (c) by the production method, and dissolving the produced low-molecular-weight cellulose derivative (c) in water to obtain an aqueous cellulose solution; Step 2: applying the obtained aqueous cellulose solution to a silicon substrate having a surface layer containing a silicon oxide film and a silicon nitride film to obtain a silicon substrate with a protective film; and Step 3: etching the silicon substrate with the protective film using an etching solution that selectively etches the silicon oxide film.

[0016] The present disclosure also provides a compound represented by the following formula (1): (wherein three R's are the same or different and represent a hydrogen atom, a hydrocarbon group, or a group in which two or more hydrocarbon groups are bonded via a linking group; provided that at least one of the three R's is a hydrocarbon group or a group in which two or more hydrocarbon groups are bonded via a linking group; the hydrocarbon group may have a hydroxyl group or a carboxyl group), and the weight average molecular weight is 10 x 10 4 Hereinafter, a silicon nitride film protective agent is provided, which comprises a cellulose derivative having a molar substitution degree of 2.5 or more and a crosslinking degree of 0.1 or less.

[0017] According to the method for producing a low-molecular-weight cellulose derivative of the present disclosure, a cellulose derivative having a low molecular weight, a low degree of crosslinking, and a high degree of substitution can be obtained by a simple method.

[0018] The low-molecular-weight cellulose derivative with a low degree of crosslinking and a high degree of substitution thus obtained can be suitably used for the purpose of selectively forming an etching-resistant coating on a nitride film. Furthermore, by using the cellulose derivative to selectively form an etching-resistant coating on a nitride film of an inorganic material having a surface layer containing an oxide film and a nitride film, and then performing an etching process using a wet etching solution that selectively etches the oxide film, a silicon substrate can be produced in which the silicon oxide film is selectively and precisely etched relative to the silicon nitride film. Furthermore, by using the silicon substrate obtained by the above method, a semiconductor element having a high-precision wiring pattern can be formed, thereby realizing a high-performance semiconductor device.

[0019] [Method for producing low-molecular-weight cellulose derivatives] The method for producing low-molecular-weight cellulose derivatives of the present disclosure is a method in which a cellulose derivative (C) having a weight-average molecular weight W1 is subjected to an extraction treatment using a protic polar solvent to obtain a low-molecular-weight cellulose derivative (c) that has the same structural units as the cellulose derivative (C) and has a weight-average molecular weight reduced to ½ or less of the weight-average molecular weight W1 of the cellulose derivative (C).

[0020] (Raw cellulose derivative (C)) The cellulose derivative (C) is a cellulose derivative that serves as a raw material for the low-molecular-weight cellulose derivative (c). In this specification, it may be referred to as "raw cellulose derivative (C)".

[0021] The starting cellulose derivative (C) is a compound having a structural unit represented by the following formula (1). (In the formula, three Rs may be the same or different and represent a hydrogen atom, a hydrocarbon group, or a group in which two or more hydrocarbon groups are bonded via a linking group. However, of the three Rs, at least one is a hydrocarbon group or a group in which two or more hydrocarbon groups are bonded via a linking group. The hydrocarbon group may have a hydroxyl group or a carboxyl group.)

[0022] The hydrocarbon group is preferably an aliphatic hydrocarbon group, particularly preferably a saturated aliphatic hydrocarbon group, and most preferably an alkyl group. The hydrocarbon group has, for example, 1 to 5 carbon atoms, preferably 1 to 3 carbon atoms, and particularly preferably 1 or 2 carbon atoms.

[0023] Examples of the alkyl group include linear or branched alkyl groups having 1 to 5 carbon atoms, such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a s-butyl group, a t-butyl group, and a pentyl group. Of these, alkyl groups having 1 to 3 carbon atoms are preferred, and alkyl groups having 1 to 2 carbon atoms are particularly preferred.

[0024] The linking group is a divalent group having one or more atoms, such as an ether bond (—O—).

[0025] The group in which two or more hydrocarbon groups are bonded via a linking group is preferably a group represented by the following formula (r): The bond extending from the left end in formula (r) is bonded to the oxygen atom constituting the OR group in formula (1). 1 -(O-R 2 ) n -X (r)

[0026] R in the above formula 1 , R 2 are the same or different and represent a hydrocarbon group. The hydrocarbon group is preferably an aliphatic hydrocarbon group, particularly preferably a saturated aliphatic hydrocarbon group (i.e., an alkylene group). The hydrocarbon group has, for example, 1 to 5 carbon atoms, preferably 1 to 3 carbon atoms, and particularly preferably 1 to 2 carbon atoms.

[0027] The alkylene group is preferably an alkylene group having 1 to 3 carbon atoms, such as a methylene group, a dimethylene group, a methylmethylene group, a 1-methylethylene group, a 1,1-dimethylmethylene group, or a trimethylene group, and more preferably an alkylene group having 1 to 2 carbon atoms.

[0028] In the above formula, X represents a hydroxyl group or a carboxyl group.

[0029] In the above formula, n is (O-R 2) groups, and is, for example, a number greater than 0. The upper limit of n is, for example, 10, preferably 5, and particularly preferably 3. The lower limit of n is preferably 1, and particularly preferably 1.5.

[0030] From the viewpoint of obtaining a low-molecular-weight cellulose derivative (c) having excellent solubility in water, the starting cellulose derivative (C) is a cellulose derivative having a structural unit represented by the formula (1), in which at least one of the three Rs in the formula is a hydrocarbon group having a hydroxyl group (for example, a hydroxyalkyl group, preferably a hydroxy C 1-5 The cellulose derivatives having a hydroxyl group (alkyl group) are preferred.

[0031] Specific examples of the starting cellulose derivative (C) include methyl cellulose, hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), hydroxyethyl methyl cellulose (HEMC), hydroxypropyl methyl cellulose (HPMC), carboxymethyl cellulose (CMC), etc. From the viewpoint of obtaining a low-molecular-weight cellulose derivative (c) having excellent solubility in water, at least one selected from HEC, HEMC, and CMC is preferred, HEC and / or HEMC are particularly preferred, and HEC is most preferred.

[0032] The starting cellulose derivative (C) may form a salt, that is, the hydroxyl group or carboxyl group of the starting cellulose derivative (C) may form a salt.

[0033] Examples of the salts include monovalent metal salts such as alkali metal salts (lithium salts, sodium salts, potassium salts, rubidium salts, cesium salts, etc.), divalent metal salts such as alkaline earth metal salts (calcium salts, magnesium salts, etc.), quaternary ammonium salts, amine salts, substituted amine salts, and double salts thereof. As the salts, alkali metal salts such as sodium salts and quaternary ammonium salts are preferred.

[0034] For example, when the hydroxyl group or carboxyl group of the starting cellulose derivative (C) forms a sodium salt, the hydroxyl group (-OH) becomes -ONa, and the carboxyl group (-COOH) becomes -COONa.

[0035] The degree of etherification (Ds1) of the starting cellulose derivative (C) can be selected, for example, from the range of 0.1 to 3.0. From the viewpoint of obtaining a low-molecular-weight cellulose derivative (c) having excellent solubility in water, the lower limit of the degree of etherification of the starting cellulose derivative (C) is preferably 1.0, particularly preferably 1.5, most preferably 1.8, and particularly preferably 2.0. In the present disclosure, the degree of etherification of the cellulose derivative refers to the average value of the degrees of etherification (degree of substitution) of the hydroxyl groups at the 2-, 3-, and 6-positions of the constituent unit represented by formula (1).

[0036] The degree of polymerization of the constituent unit represented by the above formula (1) in the starting cellulose derivative (C) is, for example, 5 to 500. From the viewpoint of obtaining a low-molecular-weight cellulose derivative (c) having excellent solubility in water, the upper limit of the degree of polymerization is preferably 300, particularly preferably 150. The lower limit of the degree of polymerization is preferably 30.

[0037] The weight average molecular weight (W1) of the raw cellulose derivative (C) is 5 × 10 4 From the viewpoint of obtaining a low-molecular-weight cellulose derivative (c) having excellent solubility in water, the weight-average molecular weight (W1) is 100×10 4 Preferably, 50 x 10 4 Less than 30 × 10 is more preferable. 4 The following is particularly preferred: 25 x 10 4 The following are most preferred:

[0038] The viscosity of a 2 wt % aqueous solution of the starting cellulose derivative (C) at 25°C (B-type viscometer, 30 rpm) is, for example, 500 mPa s or less. From the viewpoint of obtaining a low-molecular-weight cellulose derivative (c) having excellent solubility in water, the viscosity is more preferably 400 mPa s or less, particularly preferably 300 mPa s or less, and most preferably 200 mPa s or less. The lower limit of the viscosity is, for example, 30 mPa s.

[0039] The degree of crosslinking (Dc1) of the starting cellulose derivative (C) is, for example, 0.05 to 0.5. From the viewpoint of obtaining a low-molecular-weight cellulose derivative (c) having excellent solubility in water, the upper limit of the degree of crosslinking is preferably 0.3, and particularly preferably 0.2.

[0040] The degree of crosslinking of the cellulose derivative can be determined by a method in which the cellulose derivative is heat-treated with 0.1 M hydrochloric acid to decompose the crosslinked structure, the resulting decomposition product is mixed with 2,4-dinitrophenylhydrazine to react, a purified reaction product is obtained by extraction, and the resulting reaction product is subjected to HPLC-MS / MS analysis for quantification.

[0041] (Protic Polar Solvent) The protic polar solvent is a solvent that selectively dissolves a low-molecular-weight cellulose derivative having a low degree of crosslinking and a high degree of substitution contained in the raw cellulose derivative (C) (preferably, the low-molecular-weight cellulose derivative (c) described below).

[0042] SP value of protic polar solvent (cal / cm 3 ) 1 / 2 The SP value (value at 25°C calculated by the Fedors method) is, for example, 8.0 to 15.0. From the viewpoint of obtaining a low-molecular-weight cellulose derivative (c) having excellent solubility in water, the upper limit of the SP value is preferably 14.0, particularly preferably 12.0, and most preferably 11.0. The lower limit of the SP value is preferably 9.0, particularly preferably 9.5.

[0043] The dispersion force δD (cal / cm) of the Hansen solubility parameter of a protic polar solvent at 25°C 3 ) 1 / 2 is, for example, 15.0 to 16.5. From the viewpoint of obtaining a low-molecular-weight cellulose derivative (c) having excellent solubility in water, the upper limit of the dispersing power δD is preferably 16.0, particularly preferably 15.9, and most preferably 15.7. The lower limit of the dispersing power δD is preferably 15.4.

[0044] The dipole-dipole force δP (cal / cm) of the Hansen solubility parameter of a protic polar solvent 3 ) 1 / 2is, for example, 6.0 to 11.0. From the viewpoint of obtaining a low-molecular-weight cellulose derivative (c) having excellent solubility in water, the upper limit of the dipole-dipole force δP is preferably 10.0, particularly preferably 8.0, and most preferably 7.0.

[0045] The hydrogen bonding strength δH (cal / cm) of the Hansen solubility parameter of a protic polar solvent 3 ) 1 / 2 In order to obtain a low-molecular-weight cellulose derivative (c) having excellent solubility in water, the upper limit of the hydrogen bonding strength δH is preferably 20.0, particularly preferably 18.0, most preferably 15.0, and particularly preferably 12.0.

[0046] The dispersion force δD, dipole-dipole force δP, and hydrogen bonding force δH of a protic polar solvent may be literature values ​​or may be calculated using computer software (e.g., Hansen Solubility Parameters in Practice 5th Edition 5.0.13 (HSPiP)).

[0047] The protic polar solvent preferably has a LogP value (octanol / water partition coefficient) of -0.4 or more and 0.6 or less, from the viewpoint of obtaining a low-molecular-weight cellulose derivative (c) having excellent solubility in water. The upper limit of LogP is more preferably 0.3, particularly preferably 0.2, most preferably 0.1, and particularly preferably 0.05. The lower limit of LogP is more preferably -0.2, particularly preferably -0.15. The LogP value of the protic polar solvent can be calculated using ChemDraw.

[0048] The melting point of the protic polar solvent under normal pressure is, for example, -100 to -80°C.

[0049] The boiling point of the protic polar solvent under normal pressure is, for example, 50 to 150°C, preferably 70 to 140°C.

[0050] Specific examples of protic polar solvents include C 1, C 2, C 3, C 4, C 5, C 6, C 7, C 8, C 9, C 10, C 11, C 12, C 13, C 14, C 15, C 16, C 17, C 18, C 19, C 20, C 21, C 22, C 23, C 24, C 25, C 36, C 37, C 48, C 59, C 60, C 71, C 72, C 73, C 74, C 75, C 76, C 77, C 78, C 79, C 80, C 81, C 82, C 83, C 84, C 85, C 86, C 87, C 88, C 89, C 90, C 91 1-5Aliphatic alcohols; C such as propylene glycol monomethyl ether (PGME) 2-4 Alkylene glycol mono C 1-5 These may be used alone or in combination of two or more.

[0051] (Extraction Treatment) The extraction treatment can be carried out by using a protic polar solvent as an extractant and bringing the starting cellulose derivative (C) into contact with the extractant.

[0052] As the extracting solution, the above-mentioned protic polar solvent may be mixed with a solvent other than the above-mentioned protic polar solvent, but the proportion of the above-mentioned protic polar solvent in the total amount of the extracting solution is preferably 60% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, particularly preferably 90% by weight or more, most preferably 95% by weight or more, and especially preferably 99% by weight or more.

[0053] The extraction temperature is, for example, 25 to 100° C. The extraction time is not particularly limited, but is, for example, 0.5 to 5 hours.

[0054] An example of a method for contacting the starting cellulose derivative (C) with the extract solution is immersing the starting cellulose derivative (C) in the extract solution. After the extraction treatment, the extract solution in which the starting cellulose derivative (C) has been immersed is subjected to filtration or the like to collect a filtrate, which can then be subjected to a separation means such as concentration, distillation, crystallization, reprecipitation, recrystallization, or column chromatography to obtain a low-molecular-weight cellulose derivative (c).

[0055] The yield of the low-molecular-weight cellulose derivative (c) based on the starting cellulose derivative (C) is, for example, 1 to 10% by weight, preferably 1.5 to 5% by weight.

[0056] (Low-molecular-weight cellulose derivative (c)) The low-molecular-weight cellulose derivative (c) obtained by the above method is a component contained in the raw cellulose derivative (C) that dissolves in the above protic polar solvent, and has the same constitutional unit represented by the above formula (1) as the cellulose derivative (C).

[0057] The weight average molecular weight (W2) of the low molecular weight cellulose derivative (c) is not more than half the weight average molecular weight W1 of the starting cellulose derivative (C).

[0058] The weight average molecular weight (W2) is 1 / 2W1 or less (i.e., 0.5W1 or less), preferably 0.4W1 or less, particularly preferably 0.35W1 or less, and most preferably 0.3W1 or less. The lower limit of W2 may be, for example, 0.05W1 or 0.1W1.

[0059] That is, W2 / W1 is 0.5 or less, preferably 0.4 or less, particularly preferably 0.35 or less, and most preferably 0.3 or less. The lower limit of W2 / W1 may be, for example, 0.05 or 0.1.

[0060] The weight average molecular weight (W) of the low molecular weight cellulose derivative (c) is, for example, 1×10 4 ~10 x 10 4 and preferably 1 × 10 4 ~7 x 10 4 is.

[0061] The viscosity of a 5 wt % aqueous solution of the low-molecular-weight cellulose derivative (c) at 25°C (B-type viscometer, 30 rpm) is, for example, 200 mPa s or less, preferably 180 mPa s or less, more preferably 150 mPa s or less, even more preferably 100 mPa s or less, particularly preferably 80 mPa s or less, most preferably 70 mPa s or less, and especially preferably 50 mPa s or less. The lower limit of the viscosity is, for example, 1 mPa s.

[0062] The degree of etherification (Ds2) of the low-molecular-weight cellulose derivative (c) is preferably higher than the degree of etherification (Ds1) of the starting cellulose derivative (C) in terms of excellent solubility in water. That is, Ds2 / Ds1 is preferably greater than 1, more preferably 1.1 or greater, particularly preferably 1.2 or greater, most preferably 1.25 or greater, and particularly preferably 1.3 or greater.

[0063] The degree of etherification (Ds2) of the low-molecular-weight cellulose derivative (c) is, for example, 2.5 or more, preferably 2.6 or more, particularly preferably 2.7 or more, and most preferably 2.8 or more, with the upper limit of the degree of etherification (Ds2) being 3.0.

[0064] The degree of crosslinking (Dc2) of the low-molecular-weight cellulose derivative (c) is preferably lower than the degree of crosslinking (Dc1) of the starting cellulose derivative (C) in terms of excellent solubility in water. That is, Dc2 / Dc1 is preferably less than 1, more preferably 0.5 or less, particularly preferably 0.1 or less, and most preferably 0.05 or less. The lower limit of Dc2 / Dc1 is 0.

[0065] The degree of crosslinking (Dc2) of the low-molecular-weight cellulose derivative (c) is, for example, 0.1 or less. In terms of excellent solubility in water, the degree of crosslinking (Dc2) is preferably 0.05 or less, more preferably 0.03 or less, and particularly preferably 0.01 or less. The lower limit of Dc2 is 0.

[0066] The low-molecular-weight cellulose derivative (c) obtained by the method for producing a low-molecular-weight cellulose derivative according to the present disclosure has a low degree of crosslinking and a high degree of substitution, as described above, and therefore has excellent water solubility, for example, 5% by weight or more at 20°C.

[0067] As described above, the low-molecular-weight cellulose derivative (c) has excellent solubility in water and does not leave any insoluble portions. Therefore, for example, if an aqueous solution of the low-molecular-weight cellulose derivative (c) is used as a coating agent, a uniform coating film can be formed without thick or missing portions.

[0068] Furthermore, the low-molecular-weight cellulose derivative (c) has the property of easily adhering to silicon nitride films, and therefore can be suitably used as a silicon nitride film protective agent.

[0069] Furthermore, the low-molecular-weight cellulose derivative (c) easily adheres to silicon nitride films but does not easily adhere to silicon oxide films, and even if it does adhere, it easily peels off. When an aqueous solution of the low-molecular-weight cellulose derivative (c) is applied to the surface layer of an inorganic material having a surface layer containing an oxide film and a nitride film, a coating of uniform thickness can be formed only on the surface of the nitride film, and etching resistance can be selectively imparted to the nitride film.

[0070] By treating a silicon nitride film with an aqueous solution of low molecular weight cellulose derivative (c), the etching rate of the silicon nitride film can be reduced compared to when the treatment is not carried out.The etching rate reduction rate of the silicon nitride film due to the aqueous cellulose solution (i.e., the rate at which the etching rate is reduced by the low molecular weight cellulose derivative (c)) depends on the etching treatment conditions, but when carried out according to the method described in Examples, it is, for example, 50% or more, preferably 60% or more, particularly preferably 70% or more, and most preferably 80% or more.The upper limit of the etching rate reduction rate is, for example, 90%.

[0071] On the other hand, there is almost no difference in the etching rate of the silicon oxide film between when the silicon oxide film is treated with an aqueous solution of the low-molecular-weight cellulose derivative (c) and when the treatment is not performed, and the reduction in the etching rate of the silicon oxide film due to the aqueous cellulose solution depends on the etching treatment conditions, but is, for example, 2% or less when performed by the method described in the Examples. The lower limit of the reduction in the etching rate is 0%.

[0072] Because the low-molecular-weight cellulose derivative (c) has the above-mentioned properties, it can be suitably used as an etching selectivity imparting agent that improves the etching selectivity ratio [silicon oxide film / silicon nitride film].

[0073] [Manufacturing Method for Obtaining Etched Silicon Substrate] The manufacturing method for obtaining an etched silicon substrate of the present disclosure is a method for obtaining a silicon substrate having a surface layer containing a silicon oxide film and a silicon nitride film, in which the silicon oxide film of the silicon substrate has been selectively etched, through the following steps: Step 1: Producing a low-molecular-weight cellulose derivative (c) by the manufacturing method described above, and dissolving the produced low-molecular-weight cellulose derivative (c) in water to obtain an aqueous cellulose solution; Step 2: Applying the obtained aqueous cellulose solution to a silicon substrate having a surface layer containing a silicon oxide film and a silicon nitride film to obtain a silicon substrate with a protective film; and Step 3: Etching the silicon substrate with the protective film using an etching solution that selectively etches the silicon oxide film.

[0074] The manufacturing method of the present disclosure may include other steps in addition to the steps described above, and may include, for example, the following step 4 following steps 1, 2, and 3. Step 4: The silicon substrate after the etching process is cleaned with a cleaning agent to remove the protective film.

[0075] (Step 1) Step 1 is a step of producing a low-molecular-weight cellulose derivative (c) by the above-mentioned production method, and dissolving the produced low-molecular-weight cellulose derivative (c) in water to obtain an aqueous cellulose solution.

[0076] The cellulose aqueous solution can be prepared by mixing the low-molecular-weight cellulose derivative (c), a dispersion medium (containing at least water), and, if necessary, other components.

[0077] The concentration of the low-molecular-weight cellulose derivative (c) in the aqueous cellulose solution is, for example, 0.001 to 3 wt %. The lower limit of the concentration is preferably 0.01 wt %, more preferably 0.05 wt %, particularly preferably 0.1 wt %, most preferably 0.25 wt %, and especially preferably 0.3 wt %, from the viewpoint of obtaining the effect of improving etching selectivity. The upper limit of the concentration is preferably 2 wt %, particularly preferably 1.5 wt %, most preferably 1.0 wt %, and especially preferably 0.8 wt %, from the viewpoint of suppressing a decrease in the yield of silicon substrates due to contamination by substances derived from the low-molecular-weight cellulose derivative (c).

[0078] The aqueous cellulose solution may contain one or more water-soluble polymer compounds other than the low-molecular-weight cellulose derivative (c) (e.g., polyvinyl alcohol, polyvinylpyrrolidone, polyethylene glycol, polyglycerin, etc.), but the proportion of the low-molecular-weight cellulose derivative (c) in the total amount (100% by weight) of water-soluble polymer compounds contained in the aqueous cellulose solution is, for example, 50% by weight or more, preferably 60% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, particularly preferably 95% by weight or more, and most preferably 99% by weight or more.

[0079] The aqueous cellulose solution contains at least the low-molecular-weight cellulose derivative (c) as a non-volatile component, and may also contain one or more other components (e.g., polymer compounds including the other water-soluble polymer compounds, dispersants, surfactants, etc.). The proportion of the low-molecular-weight cellulose derivative (c) in the total amount of the non-volatile components is, for example, 50% by weight or more, preferably 60% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, particularly preferably 95% by weight or more, and most preferably 99% by weight or more.

[0080] The non-volatile components of the cellulose aqueous solution are components other than the dispersion medium such as water and organic solvents, and are, for example, components remaining after heating the cellulose aqueous solution at 100° C. for 1 hour under normal pressure.

[0081] (Step 2) Step 2 is a step of applying the above-mentioned cellulose aqueous solution to a silicon substrate having a surface layer including a silicon oxide (SiO2) film and a silicon nitride (Si3N4) film to obtain a silicon substrate with a protective film.

[0082] Methods for forming a protective film by applying an aqueous cellulose solution to a surface layer of a silicon substrate having a surface layer containing a silicon oxide film and a silicon nitride film include the following methods: [1] A method of immersing a silicon substrate having a surface layer containing a silicon oxide film and a silicon nitride film in an aqueous cellulose solution (i.e., a dipping method); [2] A method of applying an aqueous cellulose solution onto a surface layer of a silicon substrate having a surface layer containing a silicon oxide film and a silicon nitride film (i.e., a coating method).

[0083] The thickness of the protective film is, for example, 1 to 200 nm. In particular, from the viewpoint of highly suppressing etching of the silicon nitride film and improving the etching selectivity ratio [silicon oxide film / silicon nitride film], the thickness of the protective film is preferably 5 to 100 nm, and particularly preferably 5 to 30 nm.

[0084] (Step 3) Step 3 is a step of subjecting the silicon substrate with the protective film obtained through step 2 to an etching treatment (i.e., wet etching treatment) using an etching solution that selectively etches the silicon oxide film.

[0085] The etching solution is preferably a composition containing at least one acidic substance selected from hydrofluoric acid (HF), ammonium fluoride (NHF), ammonium hydrogen fluoride, BHF (a mixture of HF and NHF; buffered hydrofluoric acid), DHF (a mixture of HF and HO), and the like.

[0086] The etching method for the silicon substrate with the protective film is not particularly limited, and examples thereof include the following methods: [3] A method of immersing the silicon substrate with the protective film in the etching solution (i.e., a dipping method); [4] A method of applying the etching solution onto the surface layer of the silicon substrate with the protective film (i.e., a coating method).

[0087] (Step 4) Step 4 is a step of cleaning the silicon substrate after etching with a cleaning agent to remove the protective film.

[0088] In many cases, the protective film covering the silicon nitride film remains even after the etching process. In such cases, it is preferable to remove the remaining protective film by washing with a detergent.

[0089] The cleaning agent preferably contains at least water (for example, deionized water).

[0090] The detergent may contain other components in addition to water, such as a base or an acid. Since a base or an acid has the effect of dissolving the low-molecular-weight cellulose derivative (c), adding these to the detergent can enhance the detergency of the detergent.

[0091] Examples of the base include inorganic bases such as sodium hydroxide and potassium hydroxide, organic bases such as tetramethylammonium hydroxide and choline, ammonia, etc. These may be used alone or in combination of two or more.

[0092] Examples of the acid include sulfuric acid, hydrochloric acid, nitric acid, carbonic acid, acetic acid, phosphoric acid, aqua regia, dilute hydrofluoric acid, sulfuric acid / hydrogen peroxide, hydrochloric acid / hydrogen peroxide, etc. These can be used alone or in combination of two or more.

[0093] The cleaning agent may contain, in addition to water, an oxidizing agent that oxidizes the surface of the silicon substrate. If the cleaning agent contains an oxidizing agent, the silicon nitride film to which the remaining protective film is attached is oxidized by the oxidizing agent, and at least a portion of the surface becomes a silicon oxide film. Once the silicon nitride film becomes a silicon oxide film, the low-molecular-weight cellulose derivative (c) has the property of being difficult to adhere to the silicon oxide film and, even if it does adhere, is easily peeled off, making it easy to remove.

[0094] Examples of the oxidizing agent include peroxides. Examples of the peroxides include organic peracids such as performic acid, peracetic acid, perpropionic acid, m-chloroperbenzoic acid, trifluoroperacetic acid, and perbenzoic acid; and inorganic peroxides such as hydrogen peroxide, ammonium persulfate, and sodium perborate. These can be used alone or in combination of two or more.

[0095] The cleaning agent may contain water, a base or an acid, and the oxidizing agent, and may be, for example, an aqueous solution containing ammonia hydrogen peroxide mixture obtained by mixing ammonia and hydrogen peroxide solution, sulfuric acid hydrogen peroxide mixture obtained by mixing sulfuric acid and hydrogen peroxide solution, or hydrochloric acid hydrogen peroxide mixture obtained by mixing hydrochloric acid and hydrogen peroxide solution.

[0096] That is, the cleaning agent may be an aqueous solution containing at least one selected from ammonia hydrogen peroxide, sulfuric acid hydrogen peroxide, and hydrochloric acid hydrogen peroxide.

[0097] By using the cleaning agent, the remaining protective film can be easily removed, and the etched surface layer can be prevented from being damaged when the protective film is removed.

[0098] According to the method of the present disclosure, the etching selectivity ratio [silicon oxide film / silicon nitride film] is significantly increased compared to when etching is performed by a similar method except that a protective film is not provided, and the increase rate is, for example, 2 times or more, preferably 3 times or more, more preferably 4 times or more, and particularly preferably 5 times or more.

[0099] According to the method of the present disclosure, the silicon oxide film of the surface layer containing the silicon oxide film and the silicon nitride film is etched very selectively, thereby making it possible to obtain a silicon substrate having a highly accurate wiring pattern.

[0100] The silicon substrate thus obtained can then be subjected to an electrode formation process, a dicing process, etc., to manufacture a semiconductor element (e.g., a semiconductor chip).Furthermore, the obtained semiconductor element can be subjected to a conventionally known semiconductor device manufacturing process, such as a wire bonding process, a molding process, etc., to manufacture a semiconductor device.

[0101] The above-described configurations and combinations thereof of the present disclosure are merely examples, and additions, omissions, substitutions, and modifications of the configurations are possible as appropriate without departing from the spirit of the present disclosure. Furthermore, each aspect disclosed in this specification can be combined with any other feature disclosed in this specification. Furthermore, the present disclosure is not limited by the embodiments.

[0102] The present disclosure will be explained in more detail below using examples, but the present disclosure is not limited to these examples.

[0103] Example 1: PGME (50 mL) and SE400 (5 g) were added to a flask and stirred at 50°C for 3 hours. The contents of the flask were then filtered using PTFE filter paper with a pore size of 3 μm to obtain a filtrate. The obtained filtrate was treated with an ion exchange resin to remove salts and obtain a purified filtrate. The purified filtrate was reprecipitated with 10 times the weight of PGMEA to obtain a slurry. The obtained slurry was subjected to a filtration process, and the resulting residue was dried to obtain low molecular weight hydroxyethyl cellulose (HEC(s); 0.075 g, yield 1.5%). The viscosity of a 5 wt% aqueous solution of the obtained low molecular weight hydroxyethyl cellulose at 25°C (B-type viscometer, 30 rpm) was 20 mPa·s.

[0104] (Measurement of Weight-Average Molecular Weight) The weight-average molecular weight of the obtained low-molecular-weight hydroxyethyl cellulose was measured by size exclusion chromatography.

[0105] (Measurement of Crosslinking Degree) 0.1 M hydrochloric acid (100 mL) was added to the obtained low-molecular-weight hydroxylethyl cellulose (1 g) and the mixture was allowed to react at 60°C for 1 hour. After the reaction was completed, 150 mL of 0.01 M 2,4-dinitrophenylhydrazine (DNPH) solution (pH approximately 2.5) was added to the solution, and the mixture was allowed to react at approximately 40°C for approximately 1 hour to synthesize aldehyde compound-2,4-DNPH. Methylene chloride was then added to the reaction mixture in a 1:1 volume ratio to extract aldehyde compound-2,4-DNPH into the organic layer. The organic layer was then separated, and the methylene chloride was evaporated to obtain aldehyde compound-2,4-DNPH as a dry solid. This was then placed in an HPLC-MS / MS analyzer to measure the mass spectrum, and the crosslinking degree was calculated.

[0106] (Measurement of molar substitution) The molar substitution of the obtained low-molecular-weight hydroxyethyl cellulose was measured by 13 Measurement was performed by C-NMR.

[0107] Examples 2 to 8 and Comparative Examples 1 to 3 The same procedures as in Example 1 were carried out except that the type of raw cellulose derivative and the extraction conditions were changed as shown in the table below. As a result, in Comparative Examples 1 to 3, the raw cellulose derivative SE400 was completely insoluble in the solvent, and the low-molecular-weight cellulose derivative could not be extracted.

[0108] Comparative Example 4 SE400 (5 g) and pure water (100 mL) were added to a flask and stirred at 95°C for 30 minutes. A strongly acidic cation exchange resin (1 g) was added thereto, and the mixture was stirred at 95°C for 3 hours to carry out depolymerization. The contents of the flask were filtered using cellulose filter paper with a pore size of 3 µm to remove the strongly acidic ion exchange resin, thereby obtaining an aqueous solution of low molecular weight hydroxyethyl cellulose.

[0109] Comparative Example 5 SE400 (0.1 g), pure water (10 mL), and cellulase (0.03 g) were placed in a flask and stirred at 40° C. for 2 hours for enzyme treatment to obtain an aqueous solution of low-molecular-weight hydroxyethyl cellulose.

[0110]

[0111] (Raw Cellulose Derivatives) SE400: Hydroxyethyl cellulose (etherification degree 2.1, crosslinking degree 0.099, weight average molecular weight 20 × 10 4 , viscosity of a 2 wt% aqueous solution at 25°C and 30 rpm of 80-130 mPa·s, manufactured by Daicel Miraize Co., Ltd. SE400 (uncrosslinked): hydroxyethyl cellulose (etherification degree 2.1, crosslinking degree 0, weight average molecular weight 20 × 10 4 ), a 2 wt% aqueous solution having a viscosity of 80-130 mPa·s at 25°C and 30 rpm, manufactured by Daicel Miraize Co., Ltd. SE400*1: SE400 subjected to a depolymerization treatment using a strongly acidic cation exchange resin by the method described in Comparative Example 4. SE400*2: SE400 subjected to a depolymerization treatment using cellulase by the method described in Comparative Example 5. HECald: hydroxyethyl cellulose (etherification degree 2.3, crosslinking degree 0.327, weight average molecular weight 9×10 4 , viscosity of a 5 wt% aqueous solution at 25°C and 30 rpm: 75-150 mPa·s), manufactured by Sigma-Aldrich Corporation; HECwako: hydroxyethyl cellulose (etherification degree 2.3, crosslinking degree 0.25, weight average molecular weight 9.3×10 4 , viscosity of a 5 wt% aqueous solution at 25°C and 30 rpm: 75-150 mPa·s), manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.

[0112] (solvent)

[0113] The solubility in water of the low molecular weight cellulose derivative obtained in Example 1 and the performance of the coating film obtained using the low molecular weight cellulose derivative were evaluated by the following methods.

[0114] <Evaluation of Solubility in Water> The solubility in water of the low molecular weight cellulose derivative obtained in Example 1 was evaluated by the following method. The low molecular weight cellulose derivative (0.5 g) was mixed with pure water (10 mL) and stirred in a ball mill at 50°C for 30 minutes, and then visually inspected to see if any undissolved low molecular weight cellulose derivative remained. The results are shown in Table 3 below.

[0115] <Coating Performance Evaluation> A coating was formed using the low-molecular-weight cellulose derivative obtained in Example 1 using the following method, and the film thickness uniformity and etching resistance were evaluated. (Preparation of Aqueous Cellulose Solution) The low-molecular-weight cellulose derivative (0.3 g) and pure water (100 mL) were mixed and stirred using a ball mill at 50°C for 30 minutes to obtain a cellulose aqueous solution. (Coating Preparation A) The obtained cellulose aqueous solution was spin-coated onto a Si3N4 / silicon substrate (a silicon substrate with a 300 nm-thick Si3N4 film formed by PE-CVD, product name "PE-SiN", manufactured by Advantec Co., Ltd.) using a spin coater (manufactured by Mikasa Co., Ltd.) under conditions of 500 rpm x 5 seconds, 1000 rpm x 5 seconds, and 1500 rpm x 30 seconds. The substrate was then heated on a hot plate at 150°C for 90 seconds to obtain a coated Si3N4 / silicon substrate. The resulting coating was visually inspected for color unevenness. The coating film had a uniform thickness and was free from color unevenness. The results are shown in Table 3 below.

[0116] (Evaluation of Coating Removability) When the Si3N4 / silicon substrate with the coating obtained in Coating Preparation A was immersed in a 2.38 wt % aqueous solution of tetrahydroammonium hydroxide (pH 13.3) at 25°C for 1 minute, the coating was easily peeled off.

[0117] (Etching Treatment A) The Si3N4 / silicon substrate with the coating obtained in Coating Preparation A was immersed in 50 mL of buffered hydrofluoric acid (trade name "BHF110", manufactured by Daikin Industries, Ltd.) and allowed to stand for 5 minutes. The silicon nitride film thickness before and after etching was measured using a spectroscopic ellipsometer (M-2000, manufactured by J.A. woolom), the etching rate (nm / min) of the silicon nitride film was calculated, and the etching resistance was evaluated according to the following criteria. The results are shown in Table 3 below. <Evaluation criteria for etching resistance> Good: The etching rate of the silicon nitride film was less than 4.0 nm / min. Fair: The etching rate of the silicon nitride film was 4.0 nm / min or more but less than 6.0 nm / min. Unfair: The etching rate of the silicon nitride film was 6.0 nm / min or more.

[0118] (Coating Preparation B) The same procedure as in Coating Preparation A was carried out, except that a SiO2 / silicon substrate (a silicon substrate with a 1.5 μm-thick SiO2 film formed by thermal oxidation, product name "Th-SiO2", manufactured by Advantec Co., Ltd.) was used instead of the Si3N4 / silicon substrate. However, no coating formation was observed.

[0119] (Etching Treatment B) The etching rate (nm / min) of the silicon oxide film was calculated in the same manner as in Etching Treatment A, except that the SiO2 / silicon substrate treated with Coating Preparation B was used instead of the Si3N4 / silicon substrate with the coating.

[0120] The etching selectivity ratio [silicon oxide film / silicon nitride film] was calculated from the results of etching treatment A and etching treatment B. The results are shown in Table 4 below.

[0121] The water solubility of the low-molecular-weight cellulose derivatives obtained in Examples 2 to 8 and Comparative Examples 4 and 5, and the performance of the coating films obtained using the low-molecular-weight cellulose derivatives were evaluated in the same manner as in Example 1. The results are shown in Table 3 below.

[0122] Tables 1 to 3 show that the method for producing low-molecular-weight cellulose derivatives disclosed herein can produce low-molecular-weight cellulose derivatives with low crosslinking and high degrees of substitution. Furthermore, the low-molecular-weight cellulose derivatives obtained in this manner have excellent solubility in water, and when an aqueous solution is applied to the surface of a silicon nitride membrane, a uniform coating can be formed. This coating imparts excellent etching resistance to the silicon nitride membrane. On the other hand, conventional methods for lowering the molecular weight of raw cellulose derivatives by depolymerizing them using strong acids or enzymes often result in low-molecular-weight cellulose derivatives with increased crosslinking or decreased degrees of substitution, making it difficult to selectively obtain cellulose derivatives with low crosslinking and high degrees of substitution. Therefore, it is clear that the low-molecular-weight cellulose derivatives obtained by depolymerization do not produce a uniform coating when an aqueous solution is applied to the surface of a silicon nitride membrane. Furthermore, in the method of the present disclosure, since extraction is performed using a protic polar solvent, a low-molecular-weight cellulose derivative with a low degree of crosslinking and a high degree of substitution can be obtained. However, when an aprotic polar solvent is used instead of a protic polar solvent, it is found that the low-molecular-weight cellulose derivative with a low degree of crosslinking and a high degree of substitution cannot be obtained because it is difficult to dissolve the low-molecular-weight cellulose derivative in an aprotic polar solvent.

[0123] Comparative Examples 6 to 11 Etching rates were measured and etching selectivity ratios were calculated in the same manner as in Example 1, except that the water-soluble polymer compounds shown in the table below were used instead of the low-molecular-weight hydroxyethyl cellulose [HEC(s)] obtained in Example 1. The results are summarized in Table 4 below.

[0124] (Water-soluble polymer compounds) HEC(s): low-molecular-weight hydroxyethyl cellulose obtained in Example 1 PGL: polyglycerin (trade name "PGL XPW", weight-average molecular weight 3000, manufactured by Daicel Corporation) PEG1000: polyethylene glycol (trade name "PEG#1000", weight-average molecular weight 1000, manufactured by NOF Corporation) PEG4000: polyethylene glycol (trade name "PEG#4000", weight-average molecular weight 4000, manufactured by NOF Corporation) PVA: polyvinyl alcohol (trade name "GOHSENOL KL-05", manufactured by Nippon Synthetic Chemical Industry Co., Ltd.) PVP: polyvinylpyrrolidone (trade name "Polyvinylpyrrolidone K-30", weight-average molecular weight 40000, manufactured by Nippon Shokubai Co., Ltd.)

[0125] From the examples in Table 4 above, it can be seen that when a cellulose aqueous solution containing a low-molecular-weight cellulose derivative (c) is applied to a silicon substrate having a surface layer containing a silicon oxide film and a silicon nitride film, the etching amount of the silicon nitride film can be significantly reduced without suppressing the etching amount of the silicon oxide film, thereby significantly improving the etching selectivity ratio [silicon oxide film / silicon nitride film]. On the other hand, from the comparative examples, it can be seen that even if a polymer compound has the same water-solubility as the low-molecular-weight cellulose derivative (c), compounds other than cellulose derivatives do not have the effect of improving the etching selectivity ratio [silicon oxide film / silicon nitride film].

[0126] In summary, the configuration of the present disclosure and its variations are described below. [1] Formula (1) below (wherein three R's are the same or different and represent a hydrogen atom, a hydrocarbon group, or a group in which two or more hydrocarbon groups are bonded via a linking group; provided that at least one of the three R's is a hydrocarbon group or a group in which two or more hydrocarbon groups are bonded via a linking group; the hydrocarbon group may have a hydroxyl group or a carboxyl group), and the weight average molecular weight is W1, where W1 is 5×10 4A method for producing a low-molecular-weight cellulose derivative, comprising subjecting cellulose derivative (C) described above to extraction treatment using a protic polar solvent to obtain a low-molecular-weight cellulose derivative (c) having a constitutional unit represented by formula (1) above and having a weight-average molecular weight of W2, where W2 is 1 / 2 or less of W1. [2] A method for producing a low-molecular-weight cellulose derivative according to [1], wherein the low-molecular-weight cellulose derivative (c) has a degree of molar substitution of 2.5 or more. [3] A method for producing a low-molecular-weight cellulose derivative according to [1] or [2], wherein the low-molecular-weight cellulose derivative (c) has a degree of crosslinking of 0.1 or less. [4] W2 is 1 x 10 4 ~10 x 10 4 [5] The method for producing a low-molecular-weight cellulose derivative according to any one of [1] to [4], wherein W2 / W1 is 0.05 to 0.5. [6] The method for producing a low-molecular-weight cellulose derivative according to any one of [1] to [5], wherein the Log P value of the protic polar solvent is -0.4 or more and 0.6 or less. [7] The SP value of the protic polar solvent at 25°C calculated by the Fedors method is 8.0 to 15.0 (cal / cm 3 ) 1 / 2 [8] The method for producing a low-molecular-weight cellulose derivative according to any one of [1] to [6], wherein the dispersive force δD of the Hansen solubility parameter of the protic polar solvent at 25°C is 15.0 to 16.5 (cal / cm 3 ) 1 / 2 and the dipole-dipole force δP is 6.0 to 11.0 (cal / cm 3 ) 1 / 2 and the hydrogen bonding strength δH is 10.0 to 23.0 (cal / cm 3 ) 1 / 2 [9] The method for producing a low-molecular-weight cellulose derivative according to any one of [1] to [7], wherein the protic polar solvent is C 1-5 Fatty alcohol and / or C 2-4 Alkylene glycol mono C 1-5

[10] The method for producing a low-molecular-weight cellulose derivative according to any one of [1] to [8], wherein the protic polar solvent is a C ether. 2-4 Alkylene glycol mono C1-5

[11] A method for producing a low-molecular-weight cellulose derivative according to any one of [1] to [8], wherein the cellulose derivative (C) is an ether.

[11] A method for producing a low-molecular-weight cellulose derivative according to any one of [1] to

[10] , wherein the cellulose derivative (C) is at least one compound selected from the group consisting of methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, and carboxymethyl cellulose.

[12] A method for producing a low-molecular-weight cellulose derivative according to any one of [1] to

[10] , wherein the cellulose derivative (C) is at least one compound selected from the group consisting of hydroxyethyl cellulose, hydroxyethyl methyl cellulose, and carboxymethyl cellulose.

[13] A method for producing a low-molecular-weight cellulose derivative according to any one of [1] to

[10] , wherein the cellulose derivative (C) is hydroxyethyl cellulose.

[14] A method for producing a low-molecular-weight cellulose derivative according to any one of [1] to

[13] , wherein the cellulose derivative (C) has a degree of etherification of 1.0 to 3.0.

[15] A method for producing a low-molecular-weight cellulose derivative according to any one of [1] to

[14] , wherein the degree of polymerization of the constitutional unit represented by formula (1) in the cellulose derivative (C) is 5 to 500.

[16] A method for producing a low-molecular-weight cellulose derivative according to any one of [1] to

[15] , wherein the viscosity of a 2 wt% aqueous solution of the cellulose derivative (C) at 25°C and 30 rpm, as measured using a Brookfield viscometer, is 500 mPa s or less.

[17] A method for producing a low-molecular-weight cellulose derivative according to any one of [1] to

[16] , wherein the degree of crosslinking of the cellulose derivative (C) is 0.05 to 0.5.

[18] A method for producing a silicon substrate, wherein the silicon oxide film of a silicon substrate having a surface layer including a silicon oxide film and a silicon nitride film is selectively etched, by carrying out the following steps:Step 1: A low-molecular-weight cellulose derivative (c) is produced by the method for producing a low-molecular-weight cellulose derivative according to any one of [1] to

[17] , and the produced low-molecular-weight cellulose derivative (c) is dissolved in water to obtain an aqueous cellulose solution. Step 2: The obtained aqueous cellulose solution is applied to a silicon substrate having a surface layer including a silicon oxide film and a silicon nitride film to obtain a silicon substrate with a protective film. Step 3: The silicon substrate with the protective film is etched using an etching solution that selectively etches the silicon oxide film.

[19] The following formula (1) is expressed as follows: (wherein three R's are the same or different and represent a hydrogen atom, a hydrocarbon group, or a group in which two or more hydrocarbon groups are bonded via a linking group; provided that at least one of the three R's is a hydrocarbon group or a group in which two or more hydrocarbon groups are bonded via a linking group; the hydrocarbon group may have a hydroxyl group or a carboxyl group), and the weight average molecular weight is 10 x 10 4

[20] A silicon nitride film protective agent comprising a cellulose derivative having a molar substitution of 2.5 or more and a crosslinking degree of 0.1 or less, represented by the following formula (1): (wherein three R's are the same or different and represent a hydrogen atom, a hydrocarbon group, or a group in which two or more hydrocarbon groups are bonded via a linking group; provided that at least one of the three R's is a hydrocarbon group or a group in which two or more hydrocarbon groups are bonded via a linking group; the hydrocarbon group may have a hydroxyl group or a carboxyl group), and the weight average molecular weight is 10 x 10 4 Hereinafter, the present invention relates to the use of a composition containing a cellulose derivative having a molar substitution degree of 2.5 or more and a crosslinking degree of 0.1 or less as a silicon nitride film protective agent.

[0127] According to the method for producing a low-molecular-weight cellulose derivative disclosed herein, a low-molecular-weight cellulose derivative with a low degree of crosslinking and a high degree of substitution can be obtained by a simple method. The low-molecular-weight cellulose derivative with a low degree of crosslinking and a high degree of substitution thus obtained can be suitably used for selectively forming an etching-resistant coating on a nitride film. Furthermore, by using the cellulose derivative to selectively form an etching-resistant coating on a nitride film of an inorganic material having a surface layer containing an oxide film and a nitride film, and then performing an etching treatment using a wet etching solution that selectively etches the oxide film, a semiconductor element with a high-precision wiring pattern can be formed, thereby realizing a high-performance semiconductor device.

Claims

1. The following formula (1) (wherein, three R's may be the same or different and each represent a hydrogen atom, a hydrocarbon group, or a group in which two or more hydrocarbon groups are bonded via a linking group. However, of the three R's, at least one is a hydrocarbon group or a group in which two or more hydrocarbon groups are bonded via a linking group. The hydrocarbon group may have a hydroxyl group or a carboxyl group), and the weight average molecular weight is W1, where W1 is 5×10 4 A method for producing a low molecular weight cellulose derivative, comprising subjecting the above cellulose derivative (C) to an extraction treatment using a protic polar solvent to obtain a low molecular weight cellulose derivative (c) having a constitutional unit represented by the above formula (1) and having a weight average molecular weight of W2, wherein W2 is 1 / 2 or less of W1.

2. The method for producing a low molecular weight cellulose derivative according to claim 1, wherein the molar substitution of the low molecular weight cellulose derivative (c) is 2.5 or more.

3. A method for producing a low molecular weight cellulose derivative according to claim 1 or 2, wherein the crosslinking degree of the low molecular weight cellulose derivative (c) is 0.1 or less.

4. The method for producing a low molecular weight cellulose derivative according to claim 1 or 2, wherein the Log P value of the protic polar solvent is from -0.4 to 0.

6.

5. A method for producing a low molecular weight cellulose derivative according to claim 1 or 2, wherein the cellulose derivative (C) is hydroxyethyl cellulose.

6. A manufacturing method for obtaining a silicon substrate having a surface layer including a silicon oxide film and a silicon nitride film, in which the silicon oxide film is selectively etched, through the steps of: Step 1: A low molecular weight cellulose derivative (c) is produced by the production method according to claim 1 or 2, and the produced low molecular weight cellulose derivative (c) is dissolved in water to obtain an aqueous cellulose solution. Step 2: The obtained aqueous cellulose solution is applied to a silicon substrate having a surface layer including a silicon oxide film and a silicon nitride film to obtain a silicon substrate with a protective film. Step 3: The silicon substrate with the protective film is etched using an etching solution that selectively etches the silicon oxide film.

7. The following formula (1) (wherein, three R's may be the same or different and each represent a hydrogen atom, a hydrocarbon group, or a group in which two or more hydrocarbon groups are bonded via a linking group. However, of the three R's, at least one is a hydrocarbon group or a group in which two or more hydrocarbon groups are bonded via a linking group. The hydrocarbon group may have a hydroxyl group or a carboxyl group), and the weight average molecular weight is 10×10 4 The silicon nitride film protective agent comprises a cellulose derivative having a molar substitution degree of 2.5 or more and a crosslinking degree of 0.1 or less.