Etchants containing filterable surfactant

This invention relates to improved buffered oxide etchant and metal etchant compositions containing soluble surfactant additives. These additives are capable of maintaining reduced surface tensions for increased wetting stability and are useful in integrated circuit manufacture. Specifically, this invention relates to the utilization, in the surfactant composition, of a three-component surfactant blend comprising a combination of a linear perfluorocarboxylic acid, a cyclic alkylamine and an aliphatic alcohol. The three-component surfactant system of the invention is incorporated and finds utility in a known variety of etchants including for example: ammonium fluoride-hydrofluoric acid (HF) blends; ammonium fluoride HF—phosphoric acid (PA) blends; PA—nitric acid—acetic acid blends; PA-nitric acid-acetic acid-fluoboric acid-HF blends; PA—nitric acid—sulfuric acid—HF blends; nitric acid—sulfuric acid—HF blends; nitric acid-HF-acetic acid and nitric acid-sulfuric acid blends. Each of the foregoing compositions is available commercially from the General Chemical, Electronic Chemicals Group, Hollister, Calif.

As integrated circuit component dimensions become smaller, physical wetting by etchant solutions on a substrate becomes more difficult. This is especially important for those buffered etchant solutions used in silicon dioxide etching, which in the absence of surfactants exhibit unacceptably high surface tension.

Due to the increasingly stringent requirements suitable silicon dioxide etchants must possess lower surface tension wherein the surfactant incorporated maintains an acceptable level of surface activity in hydrofluoric acid and ammonium fluoride etch systems.

Prior attempts have been made to produce stable aqueous solutions of surfactant containing ammonium fluoride and hydrofluoric acid etchants. For example, in U.S. Pat. No. 4,517,106 a fluoroalkylsulfonate is used to improve the wetting properties of the acid solutions. In U.S. Pat. No. 4,761,245 a nonionic alkylphenol polyglycidol is used for the same purpose.

In U.S. Pat. No. 4,795,582, it is disclosed that surfactants selected from the group consisting of aliphatic carboxylic acids and salts thereof, aliphatic amines and aliphatic alcohols are added to aqueous mixtures of ammonium fluoride and hydrofluoric acid to improve surface wetting. Among the various surfactant compounds, octylamine is disclosed in this patent as being suitable.

In U.S. Pat. No. 5,496,485 the patentee discloses the utilization of branched aliphatic amine surfactants. The addition of relatively substantial quantities, i.e., amounts in excess of 100 ppm based on the weight of etchants, are functionally required according to that patent.

The present invention provides the means to produce significantly improved surfactants from a practical standpoint of filterability, effectiveness and reduction of the surfactant quantity required.

In accordance with the invention, filtration of the surfactant must be conducted so as to reduce to a minimum the presence of particulates which may adversely affect the end use of microelectronic components processed with compositions containing the surfactant.

In general, in pilot plant runs, integration of the surfactant into the etchant solution is effected by using a rotating mixer with a mixing period of the order of about 3 to 5 minutes. Also, in pilot plant runs, to assure a reasonable period for maximum removal of undesirable particulates from the surfactant containing etchant solution (water being the solvent unless otherwise indicated), a recirculating filtration for a period of the order of at least about 12 hours is employed. The normal flow rate through the centrifugal pump associated with the recirculating filtration system is approximately one gallon per minute.

In commercial runs, i.e., in 100 pound to 3000 pound batches, for example, a rotating mixer of a kind employed in the art is used to blend the surfactant into the etchant solution using a minimum blending period of approximately 3–5 minutes. The blend is then subjected to suitable multipass filtration operation that is continued until the desired minimum level or elimination of undesirable particulates is attained while still retaining a minimum but sufficient quantity of surfactant to provide the desired surface tension reduction in the etchant composition.

It is apparent that the quantity of surfactant retained in the etchant, for practical reasons, is the minimum amount that provides the suitable surface tension reduction. Generally, an effective amount of surfactant retained in the filtered etchant may vary from about 10 ppm to about 100 ppm with about 35 to about 65 ppm (based on the weight of the etchant) being the more optimal quantities.

It is accordingly, an object of the present invention to provide improved silicon dioxide etching solutions having the desirable etching rates and superior wetting compared to prior art etching solutions. By incorporating in the etchants the specific surfactant of the invention, the etchants of the invention retain excellent surface active properties after filtration.

It is a further object of the invention to provide an etching solution, which after a filtration through medium of the order of 0.1 micron, yields a product in which the etchant: retains an effective level of surfactant; is essentially free of metal ions; has a superior rate of etching at etching temperatures used for prior art solutions; provides better wetting characteristics for improved etch uniformity; and does not leave residues or adversely affect photoresist adhesion.

These and other objects of the invention are provided by the addition of a novel multi-component surfactant to aqueous etching solutions of the kind described above.

The criteria for a successful surfactant comprises a chemical additive that provides:

1. Surface tension in etchant is less than or equal to 45 dynes/cm 2 and preferably those in which the surface tension in etchant is less than or equal to 25 dynes/cm 2 2. Filterability: Successful removal of sub-micron particulates through multiple pass filtration to achieve:

10 counts maximum per ml at greater than or equal to 1.0 micron 100 counts maximum per ml at greater than or equal to 0.5 micron

while retaining surfactant in etchant. Encouraged for further testing if sub-micron particulates at level of ≦50 counts/ml at ≧1.0 micron. 3. Trace metallic contribution of surfactant to etchant is ≦10 ppb for all the common metallic elements such as Al, Cr, Fe, Ca, Na, and Ni; as these are critical to the semiconductor manufacturing process. 4. Foaming: Insignificant, the height of foam column disappears in less than 5 minutes, or invisible.

A suitable surfactant should meet the criteria described in 1–4 above. Experiments were carried out in pilot scale (1 lb. to 100 lbs. batches) as well as full scale production of several batches of quantities up to 3000 lbs. in accordance with the present invention. These experiments successfully demonstrated production of suitable qualifying etchants.

The improved etchants, in accordance with the invention, are obtained by adding to tri-component surfactant system solutions of concentrations between about 0.1% and 99% by weight of the compounds selected from those described herein below as a, b, and c. The surfactant content in the etchant comprises a small amount, i.e., on the order of at least about 10 ppm. The tri-component surface active agent comprises (i) a cyclic amine, (ii) a straight chain perfluorocarboxylic acid, and (iii) an aliphatic alcohol.

In accordance with the preferred embodiments of the present invention, an aqueous composition for etching and cleansing semiconductor devices is provided in which the etchant is selected from those that comprise:

(a) ammonium fluoride and hydrofluoric acid; (b) phosphoric acid mixtures with the compounds of (a), or with at least one of the following acids of nitric, acetic, sulfuric, hydrofluoric acid, and fluoroboric acid; and (c) mixtures of hydrofluoric acid at least one acid selected from nitric acid, acetic acid and sulfuric acid.

The novel etchants according to the invention are obtained by adding to said etchant at least about 10 ppm, based on the weight of the etchant of a surfactant comprising a blend of:

(i) an aliphatic cyclic amine of the formula

(CH 2 ) x−1 CHNH 2

wherein x has a value of 5–8;

(ii) a linear perfluorocarboxylic acid of the formula

C n−1 F 2n−1 COOH

wherein n has a value of 4 to 11 and yields a perfluoramide of the formula

C n−1 F 2n−1 CONHCH(CH 2 ) x−1

when reacted with the cyclic amine; and

(iii) an alcohol of the formula

C n H 2+1 OH

wherein n is an integer from 1 to 5.

Although quantities of the above-described tri-component surfactant in amounts substantially in excess of 10 ppm, i.e., even exceeding 100 ppm, may be incorporated, such greater amounts are unnecessary from a practical standpoint. Generally, a small amount of surfactant between about 10 to about 100 ppm and preferably 15 ppm to 90 ppm, based on the weight of the etchant is sufficient. The etchant containing the surfactant of the invention when used in recirculation filtration tanks, produce an etchant in which the filterable surfactant that is retained in the etchant. Use of the surfatant also, advantageously, does not give rise to significant foaming.

The etchant compositions of the present invention are useful for etching or cleaning semiconductor devices and are particularly effective when used for etching oxide formed on the surface of silicon.

The invention will further be illustrated by reference to the following procedure and examples describing the preparation of the surfactants of the invention. In preparing the surfactants, the following reaction is applied:

wherein R is the fluorinated hydrocarbon chain of the perfluorocarboxylic acid, R′ is the cyclic hydrocarbon portion of the cyclic amine.

In the examples, a surfactant was added to a mixture of predetermined amounts of hydrofluoric acid, ammonium fluoride and deionized water to prepare an etchant composition. The prepared composition was kept standing at room temperature and test samples were periodically withdrawn to measure the change of surface tension of the composition.

An illustrative typical reaction of the perfluorocarboxylic acid and the cyclic amine of perfluorooctanoic acid and cycloheptylamine are cited as follows:

Preparation: 10% of surfactant as mixed in 50% by weight isopropyl or 2-propanol (IPA) stock solution is prepared in accordance with the reaction above using perfluorooctanoic acid and cycloheptyl amine. Composition of 100 g of 10% surfactant include:

The following is a typical example of Synthesis and Preparation of the Claimed Surfactants

wherein R is the fluorinated hydrocarbon chain of the perfluorocarboxylic acid, Ŕ is the cyclic hydrocarbon portion of the cyclic amine.

i.e., reaction of perfluorooctanoic acid and cycloheptylamine

An aqueous solution of 10% of surfactant in 50% 2-propanol (by weight)

i.e., refer to the reaction above of perfluorooctanoic acid and cycloheptyl amine. Composition of 100 g of 10% surfactant include: 5 g Acid (approx. 99% purity) 5 g amine (approx. 99% purity) 45 g 2-propanol (approx. 99% purity) 45 g deionized H 2 O

Various runs were conducted to demonstrate that the surfactant(s) met the desired criteria. Generally the experimental runs were carried out in pilot scale (1 lb. to 100 lbs. batches).

General Run

A preliminary testing was conducted prior to filtration to evaluate whether individual components of the claimed surfactant(s), would work individually, need modification, or require combining to give a basic understanding which would lead to synthesis of the surfactant(s) of the invention. Potential surfactant candidates were prepared and evaluated in an aqueous mixture comprised of about 35% by weight of ammonium fluoride, NH 4 F, and about 6.3% by weight of hydrofluoric acid, HF, (defined as BOE 7-1). Dosages were varied and dependant on which surfactant was employed, incremental additions were performed, measurements of surface tension at 25° C. were made until surface tension stabilized or until the surfactant reached a saturation point (limit in solubility). The results are given in Table I below:

Later Experiments and Examples (1–4) were mainly focused on either one of the surfactant compositions H, I, J, K, L, and M (of Table I).