This invention relates to etching solutions used in the manufacture of integrated circuits. More particularly this invention relates to etching solutions containing wetting agents to properly wet substrates of components in the manufacture of integrated circuits. As integrated circuit component dimensions decrease, physical wetting by etchant solutions on substrate surfaces becomes more difficult. This is especially important for buffered oxide etchants i.e., ammonium fluoride/hydrofluoric acid solutions used in silicon dioxide etching as these solutions exhibit extremely high surface tension values at typical etching temperatures. Because of the relatively low surface energies of the masking materials used and the photoresist topography, it is difficult to properly wet the substrate, which results in non-uniform etching and poor line reproduction.

While pretreatment ("predipping") of the component into a surfactant solution before placing the substrates in the etching solution is known, more satisfactory results are obtained by the addition of the wetting agent to the etching solution. However, most surfactants commonly used in the industry are insoluble in buffered oxide etchant solutions containing ammonium fluoride and hydrofluoric acid. Further, sufficient amounts of the wetting agent must remain in the etching solution after filtration through filters on the order of 0.2 microns to provide the desired wetting of the substrates during the etching process.

Wetting agents containing fluorochemical compounds have been employed as additives to NH ₄ F/HF etching solutions to improve wetting properties. Examples of fluorochemical compounds include perfluo inated alkyl sulfonates described in U.S. Patent No. 4,517,106, issued May 14, 1985, and fluorinated cycloalkane and cycloalkene sulfonates taught in U.S. Patent No. 4,620,934, issued November 4, 1986 to R. J. Hopkins et al; fluorine-containing carboxylic acids described in U.S. Patent No. 4,582,624, issued April 15, 1986 to N. Enjo et al; and fluorine-containing diamine compounds as described in Japanese Patent Ko ai Publication No. 53,980/1983 published March 30, 1983 by Daikin Kogyo KK. Some of these wetting agents are readily removed during filtration through sub-micron filters; others have low solubility in the etching solutions; and in addition, these wetting agents are expensive to use.

There is a need for wetting agents which are readily soluble in buffered etching solutions containing ammonium fluoride, which retain their solubility and wetting characteristics after repeated filtrations through sub-micron filters and in addition, provide excellent wetting properties at reduced costs. It is an object of the present invention to provide etching solutions for use in integrated circuit manufacture having excellent wetting characteristics which are retained during filtration through sub-micron filters. Another object of the present invention is to provide buffered etching solutions containing ammonium fluoride and hydrogen fluoride having excellent wetting characteristics for components having small and large geometries.

A further object of the present invention is to provide a process for etching silicon dioxide substrates having desirable etching rates.

These and additional objects of the present invention are accomplished in an etching solution comprising an aqueous solution of ammonium fluoride and a wetting amount of a sulfate ester of an alkylphenol polyglycidol ether or an alkyl amine glycidol adduct. The glycidol compounds which are useful as wetting agents include the sulfate esters of alkylphenol polyglycidol ethers having an alkyl substituted phenol adduct with any suitable number of glycidol groups and at least about 0.1 moles of sulfate ester or a water-soluble sulfate salt. Suitable anionic sulfate esters of alkylphenol polyglycidol ethers may be represented by the formula:

wherein R represents an alkyl group having from about 4 to about 12 carbon atoms, x is from about 3 to about 15, M represents H, an alkali metal, an alkaline earth metal, ammonium, or an amine, and y is from 0.2 to about 4. The alkylphenol polyglycidol ether sulfate esters which are useful as wetting agents in the etching solutions of this invention having the above formula include those in which R represents alkyl groups including, for example, butyl, hexyl, octyl, nonyl, and dodecyl. Preferred embodiments include those in which R represents an alkyl group having from about 8 to about 12 carbon atoms and these include octyl, nonyl, and dodecyl groups. The polyglycidol ether groups represented by x

are preferably in the range of from about 6 to about 12. It should be noted that x represents an average number of glycidol units per alkylphenol unit. The sulfate group may be in the free acid ester form in which case M represents H, or a water-soluble salt thereof where M represents an alkali metal, an alkaline earth metal, ammonium, or an amine. Suitable salts include those in which M represents sodium or potassium, ammonium, calcium or magnesium or an amine such as ethanola ine, diethanol- amine, triethanolamine, methylamine, dimethylamine, trimeth la ine, ethylamine, triethylamine, diethylamine, butylamine, propylamine, cyclohexylamine, morpholine, pyridine, octanolamine, octylamine, and the like.

Preferred embodiments of the wetting agent used in the etching compositions of the present invention are those in which R represents an alkyl group having from about 8 to about 12 carbon atoms; M represents an alkali metal such as Na or K, ammonium (NH.) or an amine; and y is from about 1 to about 3. More preferred embodiments for etching compositions to be used in etching small geometries are those free of mobile ions and M is ammonium, and R represents an alkyl group located primarily in the para position. These wetting agents do not incorporate metallic ions which may be present during the etching process.

Anionic alkylphenol polyglycidol ether sulfate esters of the above type can be produced using processes such as those described in U.S. Patent Nos. 2,213,477 and 2,233,281 to produce the alkylphenol polyglycidol ethers followed by the esterification method, for example, described in U.S. Patent Nos. 2,758,977 or 3,725,080.

Alkyl amine glycidol adducts which are useful as wetting agents in the etching solutions of the presen invention include those having a primary or secondary alkyl amine group and any suitable number of glycidol groups. Suitable alkyl amines include homogeneous amine groups as well as mixtures such as those sold commercially as coco amines, soya amines, and tallow amines or mixed fatty amines.

Preferred embodiments of the nonionic alkyl amine glycidol adducts employed in the eching solution o the present invention are those represented by the formulas:

wherein R represents an alkyl group having from about

8 to about 18 carbon atoms and mixtures thereof,

Y represents H, (CH ₂ J—C ^H H—CH ₂ 0)__H, or 20 mixtures thereof; n is from about 1 to about 20; and m + n are from about 2 to about 20; or

wherein R' and R" represent independently selected alkyl groups, the sum total of carbon atoms in R' + R" being from about 8 to about 18; and x is from 1 to about 20. Amine glycidol adducts represented by the above formulas include those of Formula I in which R represents as alkyl groups, for example, octyl, nonyl, decyl, hendecyl, dodecyl or coco or lauryl, tetradecyl.

pentadecyl, hexadecyl, heptadecyl, octadecyl or stearyl or tallow or soya, and mixtures thereof. The adducts are generally a mixture in which Y represents both H and the glycidol group and the amine is therefore a mixture of secondary and tertiary amine mixture. Preferred embodi¬ ments of the amine glycidol adducts represented by Formul

I are those in which Y represents OH

(CH ₂ —CH—CH ₂ 0) _m H and m + N are from about 2 to about 16. Amine glycidol adducts represented by Formula

II include those, for example, in which R' represents butyl, hexyl, octyl, nonyl, decyl or coco, hendecyl, dodecyl or lauryl or coco, tetradecyl, pentadecyl, hexadecyl, or heptadecyl, and R" represents, for example, methyl, ethyl, butyl, hexyl, or octyl. x in Formula II is preferably from about 2 to about 16.

More preferred embodiments of the amine glycidol adducts include those in which R represents an alkyl group having from about 12 to about 16 carbon atoms and mixtures thereof; and m + n are from about 2 to about 10. It is noted that the m, n or x represent an average number of glycidol units per alkyl amine unit. The nonionic amine glycidol adducts used as wetting agents can be prepared, for example, by the method described in U.S. Patent No. 2,089,569, published August 10, 1937 by L. Orthner and C. Heuck.

These liquid wetting agents are employed in any suitable amounts which, in the buffered oxide etching solutions, will enhance the wetting of the substrate to be etched. Suitable wetting amounts include those in the range of from about 5 to about 50,000 parts

per million by weight, preferably from about 25 to abou

30,000, and more preferably from about 50 to about 5,00 parts per million.

The novel etching solutions of the present invention, as buffered oxide etchants, are aqueous solutions of ammonium fluoride having a concentration o between about 15 percent and about 40 percent by weight of NH4.F. Admixed with the ammonium fluoride is an aqueous solution of hydrogen fluoride in amounts which provide the buffered oxide etchant with at least about 3 parts by volume of NH.F to about 1 part by volume of HF and up to about 100 parts by volume of NH.F to about 1 part by volume of HF. In preparing the buffered oxide etching solutions of the present invention, commercially available concentrated aqueous solutions of NH.F

(normally about 40 percent by weight) and HF (normally about 49 percent by weight) may be employed.

Other acids could be used in place of hydroge fluoride in the ammonium fluoride etching solutions of * the present invention including, for example, hydrogen chloride, acetic acid, nitric acid, fluoboric acid, and the like.

The novel etching solution of the present invention may be prepared in any suitable manner. For example, an aqueous solution of ammonium fluoride and th wetting agent may be prepared and the hydrogen fluoride then added; the aqueous solutions of ammonium fluoride and hydrogen fluoride and the nonionic wetting agent may be simultaneously blended or the aqueous mixture of ammonium fluoride and hydrogen fluoride prepared and the wetting agent then blended in.

The novel etching solutions of the present invention all retain their wetting properties after 0.2 micron filtration, even under continuous filtration

conditions. Moreover, the etching solutions after filtering have the property of wetting substrates more effectively and yield more uniform results by etching small geometries (1 to 5 microns) and large geometries (>5 micron) of silicon dioxide in a patterned resist at the same rate without additional deleterious effects. Further, the nonionic wetting agents do not incorporate metallic ions in their structure.

Other additives normally used in buffered oxide etchant solutions may be included in the novel etching solutions of the present invention. For example, polar solvent diluents such as acetic acid, ethylene glycols, glycerol, and lower alkyl alcohols having from 1 to about 4 carbon atoms may be included. The etching solutions are used to etch silicon dioxide coated substrates in the manufacture of integrated circuits employing methods and procedures known in the semiconductor industry.

The novel etching solutions of the present invention provide etchant solutions with reduced surface tensions after microfiltration which improves substrate wetting and yields superior etchant performance resulting in cleaner surfaces and greater uniformity of etched profiles without incorporating foreign metallic ions. The solutions are stable and have desirable shelf lives without phase separations of the wetting agent from the NH.F and HF components.

The following examples illustrate the present invention with no intention of being limited thereby.

EXAMPLE 1

Preparation of the Sulfate Ester (1) of Nonylphenol Polyglycidol (10) Ether*

In a reactor fitted with a thermometer, a magnetic stirrer, and an N ₂ gas inlet, was placed 130 g (.13 mole) nonylphenol polyglycidol (10) ether. The flask was heated to 123°C and 13.0 g (.13 m) sulfamic acid was added. After an initial temperature drop, the temperature quickly rose, reaching a peak of 153°C, then fell back to 125°C. The product mixture was stirred and heated for an additional 40 minutes. After cooling to 55°C, 40 ml ethanol, 36 ml water, and 4 ml 30 percent H ₂ 0 ₂ were added. The temperature was increased to 70°C and heated an additional hour with stirring. An additional 6 ml water was then added, the reaction mixture cooled to ambient temperature, and the product collected. The product, a 1.0 m sulfate ester o nonylphenol polyglycidol (10) weighed 222 g and was obtained as a 65 percent active solution.

(*01in Glycidol Surfactant 10G, a product of Olin Corporation.)

EXAMPLE 2

Preparation of Sulfate (1.5) Ester of Nonylphenol Polyglycidol (10) Ether

In a reactor of the type used in EXAMPLE 1 wa placed 137 g (.14 mole) nonylphenol polyglycidol (10) ether. The flask was heated to 125°C and 6.86 g

sulfamic acid is added. After stirring and allowing the reaction temperature to return to 125°C from the resulting exotherm, an additional 6.86 g of sulfamic aci was added. The procedure was repeated with an additiona 6.86 g sulfamic acid so that a total of 20.6 g (.21 mole) sulfamic acid was reacted. The reaction mixture was stirred at 120° to 125°C for two hours. The temperature was then reduced by cooling to 70°C and 42 ml ethanol, 5 ml H ₂ 0 ₂ and 43 ml water added. After stirring for an additional hour, the product was collected. The product, 1.5 m sulfate ester of nonylphenol polyglycidol (10) was obtained as a 65 percent active solution and weighed 240 g.

EXAMPLES 3-4

Using the procedure of EXAMPLE 1, the sulfate ester (1) of nonylphenol polyglycidol (6) ether and the sulfate ester (3) of nonylphenol polyglycidol (10) ether were prepared.

EXAMPLE 5

A buffered oxide etching aqueous solution was prepared containing 7 parts by volume of 40 percent ammonium fluoride and 1 part by volume of 49 percent hydrogen fluoride. To 800 mis of the etching solution was added 250 parts per million of the nonylphenol polyglycidol sulfate (1) ester containing about 10 glycidol units as prepared in EXAMPLE 1 as the wetting agent. The surface tension of the etching solution was measured with a Du Nouy Ring Tensiometer at 25°C. The solution was continuously filtered through a 0.2 micron

polytetrafluoroethylene filter (Millipore) at a rate of 800 ml/min for 5 to 5-1/2 hours and the surface tension measured again. The results are shown in TABLE I below.

EXAMPLES 6-8

The procedure described in EXAMPLE 5 was followed exactly using the wetting agents prepared in EXAMPLES 2, 3, and 4 respectively. The results are show in TABLE I below.

TABLE I

Surface Tension (dynes/c

Before After Example No. Wetting Agent Filtration Filtratio

5 Nonylphenol poly- 32.4 35.9 glycidol (10) sulfate (1) ester

6 Nonylphenol poly- 34.0 34.0 glycidol (10) sulfate (1.5) ester

7 Nonylphenol poly- 30.0 32.7 glycidol (6) sulfate (1) ester

8 Nonylphenol poly- 35.3 38.7 glycidol (10) sulfate (3) ester

EXAMPLES 5-8 show that the loss in wetting activity after continuous filtration over an extended period of time is negligible.

EXAMPLES 9-12

Four inch silicon wafers were oxidized thermally to about 6000 A (determined by ellipsometry)

coated with positive photoresist to a thickness of about 1.2 microns, and patterned with masks having geometries of about 1.6 to about 2 microns. The etch time was then calculated. One wafer was immersed in the buffered oxidized etching solution of EXAMPLE 5. The etching, at 25°C, was conducted until the 100 percent etch time, as calculated, was attained. The etched pattern was then rinsed in water. The wafer was then inspected with a light microscope with a magnification in the range of 400X to 1000X. This procedure was repeated with the buffered oxide etching solutions of EXAMPLES 6, 7, and 8.

Comparative Examples A, B, and C

The procedure of EXAMPLES 9-12 was repeated exactly with three 4-inch silicon wafers. The patterned wafers were immersed in a buffered oxide etching solution (volume ratio 7:1 of NH ₄ F:HF) which contained no wetting agent for etch times of 100 percent, 105 percent, and 110 percent. The etched wafers were then rinsed in water and inspected using the procedure of EXAMPLES 9-12. The results are given in TABLE II below.

TABLE II

Example % of Etch 2 1.8 1.6 No. Time Micron Micron Micron

9 100 100 100 100

10 100 100 100 100

11 100 100 100 100

12 100 100 100 100

Comparative 100 63 51 42 A

Comparative 105 98 95 89 B

Comparative 110 100 100 98 C

The results in TABLE II illustrate that the etching solutions of the present invention (EXAMPLES

9-12) etch small geometries completely with the desired etch time without requiring overetching which results in the loss of pattern definition.

EXAMPLE 13

Preparation of Hexadecylamine Glycidol ( 5 ) Adduct

Into a reactor fitted with a dropping funnel, a N-, gas inlet, a stirrer, a thermometer, a condenser, and a gas vent, was placed 36 g (0.15 mole) of hexadecylamine. Under a nitrogen atmosphere, glycidol (55.5 g, 0.75 mole) was added dropwise to the hexadecylamine at 150° to 160°C. Upon completion of the addition, the reaction mixture was stirred at 150

to 160°C for an additional 45 minutes. The reaction mixture was cooled and the product removed from the flask. The product, identified as a hexadecylamine adduct with 5 moles of glycidol, weighed 91 grams.

EXAMPLE 14

Using the process of EXAMPLE 13 , the hexadecylamine adduct with 12 moles of glycidol was prepared .

EXAMPLE 15

Preparation of Cocoamine Glycidol (3) Adduct

Into a reactor of the type used in EXAMPLE 13 was placed 92.5 g (0.5 m) of cocoamine*. Under a nitrogen atmosphere, glycidol (111 g, 1.5 m) was added dropwise to the cocoamine at 150° to 170°C. After complete addition of the glycidol and post-reaction,

101.75 g of a product was removed and identified as an adduct of cocoamine and 3 m of glycidol.

(*Armeen C, a product of Akzo Chemie America.)

EXAMPLE 16

To 101.25 g (0.25 m of the 3:1 adduct) of the reaction mixture remaining in the reactor of EXAMPLE 15 was added 37 g (0.5 ) of glycidol dropwise at 160°C. Upon completion of the glycidol addition and post-reaction, this product was cooled to room

temperature. The product weighed 138.2 g and was identified as an adduct of cocoamine with 6 moles of glycidol.

EXAMPLES 17-18

Using the process of EXAMPLES 15 and 16, the cocoamine adducts with 2 moles of glycidol and with 5 moles of glycidol were prepared.

EXAMPLE 19

Using the process of EXAMPLE 13, dodecylamine (Ar een 12D, a product of Akzo Chemie America) was reacted with glycidol to produce the dodecylamine adduct of 6 moles of glycidol.

EXAMPLE 20

A buffered oxide etching aqueous solution was prepared containing 7 parts by volume of 40 percent ammonium fluoride and 1 part by volume of 49 percent hydrogen fluoride. To 800 mis of the etching solution was added 250 parts per million of the hexadecylamine glycidol (5) adduct prepared in EXAMPLE 13. The surface tension of the etching solution was measured with a Du Nouy Ring Tensiometer at 25°C. The solution was continuously filtered through a 0.2 micron polytetrafluoroethylene filter (Millipore) at a rate of 800 mls/min for 5 to 5-1/2 hours and the surface tension measured again. The results are shown in TABLE III below.

EXAMPLES 21-24

The procedure of EXAMPLE 20 was repeated exactly with the substitution for the hexadecylamine glycidol (5) adduct of the following wetting agents.

EXAMPLE 21 Hexadecylamine Glycidol (prepared in

(12) Adduct EXAMPLE 2)

EXAMPLE 22 Cocoamine Glycidol (2) (prepared in Adduct EXAMPLE 5)

EXAMPLE 23 Cocoamine Glycidol (5) (prepared in Adduct EXAMPLE 6)

EXAMPLE 24 Dodecylamine Glycidol (prepared in (6) Adduct EXAMPLE 7)

The results are given in TABLE III below.

TABLE III

Surface Tension (dynes/cm

Before After Example No. Wetting Agent Filtration Filtration

20 Hexadecylamine 39.2 40.7 Glycidol (5) Adduct

21 Hexadecylamine 39.4 44.0 Glycidol (12)

Adduct

22 Cocoamine 34.2 36.5 Glycidol (2)

Adduct

23 Cocoamine 37.5 40.5 Glycidol (5)

Adduct 24 Dodecylamine 38.0 37.8

Glycidol (6) Adduct

TABLE III shows that the etching solutions of EXAMPLES 20-24 of the present invention have only a small loss in wetting activity after continuous filtration over an extended period of time.

EXAMPLES 25-26

Four inch silicon wafers were oxidized thermally to about 6000 A, (determined by ellipsometry) coated with positive photoresist to a thickness of about 1.2 microns, and patterned with masks having geometries of about 1.6, about 1.8 and about 2 microns. The etch time was then calculated. One wafer was immersed in the buffered oxide etching solution of EXAMPLE 13. The etching, at 25°C, was conducted until the 100 percent etch time, as calculated, was attained. The etched pattern was then rinsed in water. The wafer was then inspected with a light microscope with a magnification in the range of 400X to 1000X. This procedure was repeated exactly with the buffered oxide etching solution of EXAMPLE 14. The results are given in TABLE IV below.

Comparative Examples D, E, and F

The procedure of EXAMPLE 25 was repeated exactly with three 4-inch silicon wafers. The patterned wafers were immersed in a buffered oxide etching solution (volume ratio 7:1 of NH.F:HF) which contained no wetting agent for etch times of 100 percent, 105 percent, and 110 percent. The etched wafers were then rinsed in water and inspected using the procedure of EXAMPLE 15. The results are given in TABLE IV below.

TABLE IV

Percent of Completely Etched Geomotries

Example % of Etch 2 1.8 1.6 No. Time Ilicron Micron Micron

25 100 100 100 100 Hexadecylamine Glycidol (5) Adduct

26 100 100 100 100 Dodecylamine Glycidol (6) Adduct

Comparative 100 63 51 42 D

Comparative 105 98 95 89 E

Comparative 110 100 100 . 98 F

EXAMPLES 25-26 illustrate that the etching solutions of the present invention etch small geometries completely within the desired etch time without requiring overetching which results in the loss of pattern definition.