GB1573207A - Surface treating agent adapted of intermediate products ofa semiconductor device - Google Patents

Description

(54) SURFACE-TREATING AGENT ADAPTED FOR INTERMEDIATE PRODUCTS OF A SEMICONDUCTOR DEVICE (71) We, TOKYO SHIBAURA ELECTRIC COMPANY LIMITED, a Japanese corporation, of 72 Horikawa-cho, Saiwai-ku, Kawasaki-shi, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a surface-treating agent for use in the treatment of surfaces of intermediate products obtained during the manufacture of a semiconductor device, for example the washing of the surface of intermediate semiconductor products obtained in the respective steps of manufacturing a semiconductor device, the etching of a metal layer used as wiring and the removal of those portions of a positive-working photoresist film which are and are not exposed to light.

Semiconductor devices such as transistors, diodes, integrated circuits, rectifying elements, thyristors, etc. are at present mostly based on silicon. The semiconductor device is generally prepared through various steps of growing required layers in gas phase, forming oxide films and depositing metal layers all on a semiconductor substrate, diffusing an impurity in the semiconductor substrate and fitting electrodes thereto. Since the properties of the semiconductor device are prominently affected by an impurity or contaminant, it is ncessary to wash the surface of an intermediate product obtained in each preceding step in order to remove as much as possible undesired organic and inorganic contaminants, before each succeeding step is conducted.

Depending on the kind of chemicals used as a surface-treating agent and surface-treating temperature, the above-mentioned washing may be effected by various processes.

Basically, the washing comprises the steps of degreasing organic contaminants such as lapping oil, dissolving off the unnecessary metalic contaminants by acid solution and drying the surface of the respective intermediate products by centrifugal dehydration or blowing high purity nitrogen gas on the surface.

As discussed in our copending application No. 35164/76 (Serial No. 1573206), various cleansing and surface treating agents have been proposed for treating surfaces of intermediate products obtained in the manufacture of semiconductor devices but these are not always wholly satisfactory.

In our copending Application No. 35164/76 (Serial No. 1573206) we describe and claim a method of treating (as therein defined) a surface or surface portion (as therein defined) of an intermediate product obtained in the manufacture of a semiconductor device or cleansing a surface of apparatus used in the manufacture of such an intermediate product which comprises contacting the said surface or surface portion with an aqueous solution containing at least one trialkyl(hydroxyalkyl)-ammonium hydroxide of the formula:

in which Rl, R2 and R3 are each alkyl groups containing from 1 to 4 carbon atoms, and R4 is an alkylene group containing from 2 to 4 carbon atoms, in an amount of from 0.01 to 20% by weight, based on the weight of water and trialkyl(hydroxyalkyl) ammonium hydroxide.

The term "surface" or "surface portion", as used in Application No. 35164/76 (Serial No.

1573206), is defined to mean the surface portion of intermediate products obtained in the respective steps of manufacturing a semi-conductor device. Namely, the term "surface portion" denotes the surface of a wafer just as sliced off from an ingot, a wafer in which an impurity has been diffused, an oxide film produced on the wafer, a photoresist mounted on the surface of the wafer and a metal layer deposited on the surface of the wafer. The term "treat" or "treating" is defined to represent the process of washing the above-mentioned various surface portions, removing the photoresist selectively or wholly, and etching metal layers on such surfaces.

The surface treating agent used in the method described in Application No. 35164/76 (Serial No. 1573206) comprises an aqueous solution containing one or more of trialkyl(hydroxyalkyl)ammonium hydroxide (hereinafter abbreviated as "THAH") of the formula:

As mentioned above, R1, R2 and R3 are alkyl groups each having 1 to 4 carbon atoms, and R4 is an alkylene group having 2 to 4 carbon atoms.

THAH compound contained in the surface-treating agent preferably includes trimethyl (2-hydroxyethyl)-ammonium hydroxide (also referred to "choline"), triethyl (2hydroxyethyl)ammonium hydroxide, tripropyl (2-hydroxyethyl) ammonium hydroxide, tributyl (2-hydroxyethyl) ammonium hydroxide, dimethylethyl (2-hydroxyethyl) amonium hydroxide, trimethyl (3-hydroxypropyl) ammonium hydroxide, trimethyl (3-hydroxybutyl) ammonium hydroxide, and trimethyl (4-hydroxybutyl) ammonium hydroxide. Most preferred among these compounds are choline, triethyl (2-hydroxyethyl) ammonium hydroxide, and trimethyl (3-hydroxypropyl) ammonium hydroxide.

THAH can be easily prepared by reacting trialkylamine whose alkyl groups respectively have 1 to 4 carbon atoms with epoxide having 2 to 4 carbon atoms in the presence of water.

THAH itself is not only free from most objectionable contaminants in the manufacture of a semiconductor device such as alkali metals, for example, sodium and heavy metals like copper, silver and gold, but also is not mixed with these contaminants during its manufacture. Therefore, application of THAH in treating the previously defined surface portion does not give rise to the so-called contrary contamination. Further, THAH readily soluble in water can be easily washed off with water. Even where THAH happens to remain attached to the surface portion, heating at about 100"C decomposes THAH into harmless glycol and trialkylamine, offering great advantage.

Where the surface portion of intermediate products obtained in the respective steps of manufacturing a semiconductor device is washed by the surface-treating agent, the product has only to be immersed 5 to 20 minutes in said agent kept at 70 to 900C as more preferably subjected to ultrasonic cleaning. A THAH solution used in this case contains 0.01% to 20% by weight, more preferably 0.1 to 5% by weight, especially 1 to 3% by weight of THAH.

THAH is a very alkaline material. As previously mentioned, an alkaline solution, for example, that containing sodium hydroxide is known to remove organic materials. for example, animal and vegetable oils. The higher the basicity of the alkaline solution, the more effective, said removal. The degree of dissociation pKb of THAH substantially stands at zero measured in water, proving that THAH has a considerably high basicity comparable to sodium hydroxide, and is very effective to remove organic materials deposited on the surface of intermediate products of a semiconductor device. An aqueous THAH solution has a pH value of 12.84 when 1% by weight of THAH is contained and a pH value of 12.20 when THAH content is 0.1% by weight, whereas a 0.5% aqueous solution of sodium hydroxide indicates a pH value of 12.7. This fact shows that even a smaller amount of THAH than sodium hydroxide has a greater removing power.

The THAH solution is very effective to dissolve alkali metals such as sodium, aluminium, amorphous silicon powders, and crystalline silicon powder which are brought into contact with the surface of intermediate products of a semiconductor device, and moreover forms a water-soluble complex with heavy metals such as gold, silver and copper. When contacted by the aqueous THAH solution, the silicon and silicon oxide portions of the surface of a silicon wafer are rendered prominently hydrophilic, and conveniently washed off with water. For instance, where the silicon portion of said surface is treated with dilute hydrofluoric acid, the contact angle of water particles indicates 60 to 700. In contrast where the silicon portion is treated with the aqueous THAH solution, the contact angle of water particles shows 7 , rendering the silicon portion very hydrophilic where the silicon oxide portion of the surface of a silicon wafer is treated with hydrofluoric acid, water particles show a contact angle of 7". When the silicon oxide portion is treated with THAH, water particles present the same contact angle of 7 , making said silicon oxide portion very hydrophilic.

The aqueous THAH solution etches a silicon oxide film thermally produced on a silicon wafer and a silicon oxide film formed by gas phase growth on said wafer at a far smaller rate than an aqueous solution of sodium hydroxide having about the same basicity. Table A below shows a comparison between the etching rate of an aqueous choline solution and that of an aqueous sodium hydroxide solution.

TABLE A Treated surface portion Etching rate 0.5% aqueous 1% aqueous choline solution NaOH solution Thermally produced silicon oxide film (1) 2.6A/min 13A/min Silicon oxide film produced 6.9A/min S6kmin by gas phase growth (2) Note: (1) Steam oxidation at 1200"C (2) Reaction between SiH4 and 2 at 500"C As mentioned above, the aqueous THAH solution etches a silicon oxide film having a relatively great thickness at a small rate, but easily removes an extremely thin silicon oxide film produced, for example, while a silicon wafer is allowed to stand. Such a very thin silicon oxide film produced by atmospheric oxygen generally has a thickness of 20 to 30 .

While hydrofluoric acid decreases the thickness of such thin silicon oxixde film to about 5A, 0.5% aqueous THAH solution of this invention kept at 90"C decreases the thickness of said silicon oxide film to the same extent only in one minute.

The aqueous THAH solution shows the same degree of dependency as a mixed solution of ethylenediamine and pyrocatechol on the orientation of a particular plane of a silicon crystal. Namely, a silicon crystal having a < 111 > plane resists etching by the aqueous THAH solution, whereas a silicon crystal having a < 100 > plane is readily etched by said solution. Figure 3 of the drawings accompanying Application No. 35164/76 (Serial No.

1573206) shows how much an N type silicon wafer having a specific resistance of 30 Q cm and a crystal plane of < 100 > is etched by the different concentration of an aqueous choline solution at 90"C. The etching rate does not change much by the conductivity type and specific resistance of a silicon wafer.

It has been found that etching of the crystal plane of < 100 > can be suppressed by adding to 100 parts by weight of an aqueous THAH solution 0.003 to 30 parts by weight, preferably 0.06 to 3 parts, or more preferably 0.06 to 1.5 parts by weight of H202, without changing the effect of THAH. For illustration, determination was made of the solubility of a mirror-finished silicon wafer having a specific resistance of 1 Q cm and a crystal plane of < 100 > by immersing said wafer 20 minutes at 90"C in five samples of 0.5% aqueous choline solution, that is, four samples, 100 parts by weight of which were mixed with 0.3, 0.03, 0.003 and 0.0003 part by weight of H20,, and the remaining sample consisting of said choline solution itself. It was found that the silicon wafer was etched at the rate of 0.001 microns/min. 0.002 microns/min. 0.140 microns/min. 0.340 microns/min. and 0.301 microns/min by the above-mentioned five samples of 0.5% aqueous choline solution. The above experiments prove that application of H202 in an amount of not less than 0.003 parts appreciably suppressed the etching of the silicon wafer having a < 100 > plane by the choline solution.

Accordingly the present invention provides a surface treating agent for treating intermediate products obtained in the manufacture of semiconductor devices which comprises an aqueous solution containing at least one trialkyl(hydroxyalkyl)ammonium hydroxide of the formula:

(in which R1, R2, R3 and R4 have the meanings defined above) in an amount of from 0.01 to 20% by weight, based on the weight of water and trialkyl(hydroxyalkyl)ammonium hydroxixde, and from 0.001 to 30% by weight, based on the weight of water and trialkyl(hydroxyalkyl) ammonium hydroxide, of hydrogen peroxide.

The present inventors have further found that the treating agent more effectively washes particularly heavy metals by mixing it with a water-soluble complexing agent capable of forming a water-soluble complex with metal in an amont of from 0.01 to 10% by weight, based of the weight of water and THAH. The complexing agent typically includes sodium cyanide, potassium cyanide, ammonium cyanide, sodium ethylenediaminetetraacetate, potassium ethylenediamidetetraacetate, triethanolamine, ethylenediamine, ethylenediaminetetraacetic acid and cuproin, and mixtures thereof.

A smaller amount of the complexing agent than 0.01% by weight is not effective, while a larger amount of the complexing agent than 10 % by weight causes sodium or potassium contained in the complexing agent to exert the previously mentioned harmful effect.

The surface-treating agent of the invention solution may additionally contain 0.001 to 5% by weight, preferably 0.01 to 2% by weight, based on the weight of water and THAH, of a nonionic surfactant. The surfactant, like H2O2, has been found to suppress the etching of a < 100 > plane of a silicon crystal and to decrease the surface tension of an aqueous THAH solution, causing the aqueous THAH solution more wettable to a surface to be treated.

Preferred examples of nonionic surfactant are aliphatic acid esters of polyhydric alcohols, and polyoxyethylene type represented by the formulae: R - O(CH2CH2O)mH;

and R" - COO(CH2CH2O)nH where R and R" are alkyl groups each having 12 to 18 carbon atoms, R' is an alkyl group having 8 or 9 carbon atoms, m is 8 to 15, and n is 5 to 30.

As described above, the surface-treating agent of this invention has the advantages that organic and inorganic contaminants deposited on the surface of intermediate products of a semiconductor device are effectively removed by a very simple process. After treated with the surface-treating agent, the surface of the intermediate products has only to be washed with distilled water by the customary process, followed by drying. It is unnecessary to provide facilities for disposing of gases and liquids derived from organic solvents or for treating various acids as has been the case with application of the prior art surface-treating agent. Moreover. salts obtained by neutralizing THAH are nontoxic, and trialkylamine produced upon decomposition of THAH is only slightly odorous, substantially eliminating the necessity of using any deodorizing device.

The surface-treating agent of this invention is also useful for removal of a positiveworking photoresist film applied in photoetching. Namely, an aqueous solution containing 1 to 10% by weight, preferably 1 to 7% by weight, more preferably 2 to 5% by weight of THAH is used to remove (develop) those portions of the photoresist which are exposed to light, and an aqueous solution containing more than 10 up to 20% by weight, is applied in etching those portions of the photoresist which are not exposed to a light. An aqueous solution of 1 to 10% by weight of THAH effectively removes a positive-working photoresist film. An aqueous solution containing a smaller amount of THAH than 1% by weight results in a low resolving power, whereas an aqueous solution containing a larger amount of THAH than 10% by weight unnecessarily dissolves those portions of the positive-working photoresist film which are not exposed to a light.

What is more convenient is that an aqueous solution containing 0.01 to 20% by weight of THAH is adapted to etch a wiring metal layer, particularly an aluminium layer formed on a semiconductor wafer. Selective etching of a metal layer is generally carried out by photoetching through the steps of vapour-depositing a metal layer on the surface of a semiconductor wafer, coating a photoresist film on said metal layer, etching or developing the photoresist film into a prescribed pattern and selectively etching the metal layer through the photoresist thus etched. The metal layer generally has a thickness of about 1 to 3 microns and is etched in two steps, initially at a relatively quick rate and later at a slow rate when the metal layer is etched to a thickness of about 1000he in order to avoid excess etching.

When etched by the surface-treating agent of the invention, a metal layer is preferably immersed several minutes in an aqueous solution containing 5 to 10% by weight of THAH at 60 to 70"C, and then the aqueous solution is cooled to room temperature to perform the latter step of etching. An aqueous solution containing 0.01 to 5% by weight of THAH has a relatively slow etching rate and should advisably be used in the latter step of etching.

According to this invention, the surface of intermediate products obtained in the respective steps of manufacturing a semiconductor device can be satisfactorily treated, as described above, with an aqueous solution containing at least one of the THAH compounds and hydrogen peroxide. Namely, cleaning of organic and inorganic contaminants and selective etching of a metal layer can be effected by an aqueous solution containing 0.01 to 20% by weight of THAH; selective etching or development of those portions of a positive-working photoresist which are exposed to a light can be carried out by an aqueous solution containing 1 to 10% by weight of THAH; and removal of those portions of the positive-working photoresist which are not exposed to a light is attained by an aqueous solution containing more than 10 up to 20% by weight of THAH. It is possible to mix an aqueous THAH/H202 solution with any of or any combination of 1 to 10% by weight of a complexing agent and 0.001 to 20% by weight of a nonionic surfactant.

The surface-treating agent of this invention formed of an aqueous THAH solution which dissolves crystalline silicon powders and amorphous silicon powders is also applicable in cleaning the surface of the members of a quartz boat, crucible, reaction cylinder, gas phase growth device and the like. Silicon powders, crystalline or amorphous, can be completely washed off without the possibility of the above-mentioned members being unnecessarily etched simply by immersing the members about 20 minutes in a hot aqueous solution containing 0.01 to less than 0.1% by weight of THAH, about 10 minutes in a hot aqueous solution containing 0.1 to less than 1% by weight of THAH; or about 10 minutes in an aqueous solution containing 1 to 20% by weight of THAH and maintained at room temperature.

The above description mainly refers to silicon, but germanium, gallium phosphide and the like semiconductors can also benefit from a surface-treating agent according to the invention.

This invention will be more fully understood from the following examples. All parts and percentages in the examples are by weight, unless otherwise indicated.

Example 1 A sample N type silicon substrate for epitaxial growth doped with high concentration of antimony and having a crystal plane of < 111 > was mechanically mirror finished by the customary process. After allowed to stand 24 hours in the open air, the sample was dipped at 90"C, in a cleaning solution prepared by adding 0.06 parts of H202 to 100 parts of a 1% aqueous choline solution, subjected 10 minutes to ultrasonic cleaning, washed 10 minutes with running distilled water obtained by straining deionized water through a filter of 0.3 micron mesh, and later dried by a high speed rotary drier. An N type epitaxially grown layer having a specific resistance of 5 to 6 Q cm was formed on the sample silicon substrate with a thickness of 10 microns. Lamination defects occurred in the number of zero or two at most per square centimeter. An impurity presented a good profile in the epitaxially grown layer.

Example 2 A sample mirror-finished silicon wafer having a specific resistance of 3 to 5 Q cm and a crystal plane of < 100 > was dipped in a cleaning solution comprising 0.5% aqueous choline solution to which 1 part of H202 were added based on 100 parts of the choline solution. The sample was subjected 10 minutes to ultrasonic cleaning, washed with running distilled water obtained by filtering deionized water, and dried by a rotary drier. A silicon oxide film was thermally formed on the surface of the sample silicon wafer with a thickness of 1000 . The silicon oxide layer was removed by dilute hydrofluoric acid. The sample was dipped in a fresh batch of the above-mentioned choline solution containing H2O2, and subjected 10 minutes to ultrasonic cleaning at 70"C. Later, the sample was again washed 10 minutes with distilled water and dried by a rotary drier.

A silicon oxide film was thermally deposited at 10000C on the surface of the sample silicon wafer thus treated with a thickness of 1500 . The sample now provided with the silicon oxide film was fitted with an aluminium electrode to provide an MOS diode. This diode had flat band voltage of 1.4V, and, when subjected to a bias temperature test at 300"C under bias voltages of +20V and -20V, indicated a shift of only -0.1V at both +BT and -BT forms of the test, that is, when positive and negative bias voltages were impressed, respectively. The surface-treating agent of this invention was found to remove impurities tending to increase the surface energy level of the MOS diode or those like sodium which had a movable nature.

Example 3 This example was conducted in substantially the same manner as in Example 2, excepting that the surface-treating agent formed of the aqueous choline solution containing H202 was replaced by a surface-treating agent prepared by mixing a 0.5% aqueous choline solution with 0.09 parts of H202 and 5 parts of triethanolamine based on 100 parts of the 0.5% choline solution. This example gave the same results as in Example 2.

Example 4 This example was carried out in substantially the same manner as in Example 1, excepting that the cleaning solution was replaced by a 1% aqueous choline solution to which 0.06 parts of H202 and 1 part of NH4CN were added based on 100 parts of the 1% choline solution and than an N type silicon semiconductor substrate had a crystal plane of < 100 > .

Example 5 A sample N type silicon wafer having a specific resistance of 200 to 300 Q cm and lapped with abrasive material having an average particle size of 16 microns was washed with the cleaning agent used in Example 2 by dipping the sample in the solution at 60"C and then subjecting the sample to ultrasonic cleaning. The sample was again treated with a fresh batch of the cleaning solution and further washed 10 minutes with running distilled water obtained by straining deionized water through a filter of 0.3 micron mesh and later quickly dried by a high speed rotary drier. Thereafter, gallium was diffused in the sample by the sealed tube method to form a PN junction. When measured by the ordinary method, the minority carrier stored in the sample silicon wafer had life of 60 to 85 microseconds.

By way of comparison, a sample silicon wafer was washed with a NH4OH-H202 solution and a HC -H202 solution respectively in place of the cleaning solution containing choline in the same manner as described above. Gallium was diffused on the silicon wafer thus treated to provide a PN junction. However, the minority carrier stored in the sample silicon wafer had a life of 30 to 45 microseconds.

The above-mentioned experiments prove that the surface-treating agent of this invention fully cleaned metal contaminants deposited on the surface of a lapped silicon wafer which is usually difficult to clean.

WHAT WE CLAIM IS: 1. A surface treating agent for treating intermediate products obtained in the manufacture of semiconductor devices which comprises an aqueous solution containing at least one trialkyl (hydroxyalkyl) ammonium hydroxide of the formula:

in which R', R2 and R3 are each alkyl groups having from 1 to 4 carbon atoms and R4 is an alkylene group having from 2 to 4 carbon atoms, in an amount of from 0.01 to 20% by weight, based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide, and from 0.001 to 30% by weight, based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide of hydrogen peroxide.

2. A surface treating agent as claimed in claim 1 containing from 0.1 to 5% by weight, based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide, of trialkyl (hydroxyalkyl) ammonium hydroxide.

3. A surface treating agent as claimed in claim 2 containing from 1 to 3% by weight, based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide, of trialkyl (hydroxyalkyl) ammonium hydroxide.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (19)

**WARNING** start of CLMS field may overlap end of DESC **. minutes to ultrasonic cleaning at 70"C. Later, the sample was again washed 10 minutes with distilled water and dried by a rotary drier. A silicon oxide film was thermally deposited at 10000C on the surface of the sample silicon wafer thus treated with a thickness of 1500 . The sample now provided with the silicon oxide film was fitted with an aluminium electrode to provide an MOS diode. This diode had flat band voltage of 1.4V, and, when subjected to a bias temperature test at 300"C under bias voltages of +20V and -20V, indicated a shift of only -0.1V at both +BT and -BT forms of the test, that is, when positive and negative bias voltages were impressed, respectively. The surface-treating agent of this invention was found to remove impurities tending to increase the surface energy level of the MOS diode or those like sodium which had a movable nature. Example 3 This example was conducted in substantially the same manner as in Example 2, excepting that the surface-treating agent formed of the aqueous choline solution containing H202 was replaced by a surface-treating agent prepared by mixing a 0.5% aqueous choline solution with 0.09 parts of H202 and 5 parts of triethanolamine based on 100 parts of the 0.5% choline solution. This example gave the same results as in Example 2. Example 4 This example was carried out in substantially the same manner as in Example 1, excepting that the cleaning solution was replaced by a 1% aqueous choline solution to which 0.06 parts of H202 and 1 part of NH4CN were added based on 100 parts of the 1% choline solution and than an N type silicon semiconductor substrate had a crystal plane of < 100 > . Example 5 A sample N type silicon wafer having a specific resistance of 200 to 300 Q cm and lapped with abrasive material having an average particle size of 16 microns was washed with the cleaning agent used in Example 2 by dipping the sample in the solution at 60"C and then subjecting the sample to ultrasonic cleaning. The sample was again treated with a fresh batch of the cleaning solution and further washed 10 minutes with running distilled water obtained by straining deionized water through a filter of 0.3 micron mesh and later quickly dried by a high speed rotary drier. Thereafter, gallium was diffused in the sample by the sealed tube method to form a PN junction. When measured by the ordinary method, the minority carrier stored in the sample silicon wafer had life of 60 to 85 microseconds. By way of comparison, a sample silicon wafer was washed with a NH4OH-H202 solution and a HC -H202 solution respectively in place of the cleaning solution containing choline in the same manner as described above. Gallium was diffused on the silicon wafer thus treated to provide a PN junction. However, the minority carrier stored in the sample silicon wafer had a life of 30 to 45 microseconds. The above-mentioned experiments prove that the surface-treating agent of this invention fully cleaned metal contaminants deposited on the surface of a lapped silicon wafer which is usually difficult to clean. WHAT WE CLAIM IS:

1. A surface treating agent for treating intermediate products obtained in the manufacture of semiconductor devices which comprises an aqueous solution containing at least one trialkyl (hydroxyalkyl) ammonium hydroxide of the formula:

in which R', R2 and R3 are each alkyl groups having from 1 to 4 carbon atoms and R4 is an alkylene group having from 2 to 4 carbon atoms, in an amount of from 0.01 to 20% by weight, based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide, and from 0.001 to 30% by weight, based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide of hydrogen peroxide.

2. A surface treating agent as claimed in claim 1 containing from 0.1 to 5% by weight, based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide, of trialkyl (hydroxyalkyl) ammonium hydroxide.

3. A surface treating agent as claimed in claim 2 containing from 1 to 3% by weight, based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide, of trialkyl (hydroxyalkyl) ammonium hydroxide.

4. A surface treating agent as claimed in claim 1 containing from 1 to 10% by weight,

based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide, of trialkyl (hydroxyalkyl) ammonium hydroxide.

5. A surface treating agent as claimed in claim 1 containing from more than 10 to 20% by weight, based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide, of trialkyl (hydroxyalkyl) ammonium hydroxide.

6. A surface treating agent as claimed in claim 1 containing from 5 to 20% by weight, based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide, of trialkyl (hydroxyalkyl) ammonium hydroxide.

7. A surface treating agent as claimed in claim 1 containing from 5 to 10% by weight, based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide, of trialkyl (hydroxyalkyl) ammonium hydroxide.

8. A surface treating agent as claimed in any one of the preceding claims containing from 0.06 to 3% by weight, based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide, of hydrogen peroxide.

9. A surface treating agent as claimed in claim 8 containing from 0.06 to 1.5% by weight, based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide, of hydrogen peroxide.

10. A surface treating agent as claimed in any one of the preceding claims in which the trialkyl (hydroxyalkyl) ammonium hydroxide is one or more of trimethyl (2-hydroxyethyl) ammonium hydroxide, triethyl (2-hydroxyethyl) ammonium hydroxide, tripropyl (2hydroxyethyl) ammonium hydroxide, tributyl (2-hydroxyethyl) ammonium hydroxide, dimethylethyl (2-hydroxyethyl) ammonium hydroxide, trimethyl (3-hydroxypropyl) ammonium hydroxide, trimethyl (3-hydroxybutyl) ammonium hydroxide and trimethyl (4hydroxybutyl) ammonium hydroxide.

11. A surface treating agent as claimed in claim 10 in which the trialkyl (hydroxyalkyl) ammonium hydroxide is one or more of trimethyl (2-hydroxyethyl) ammonium hydroxide, triethyl (2-hydroxyethyl) ammonium hydroxide and trimethyl (3-hydroxypropyl) ammonium hydroxide.

12. A surface treating agent as claimed in any one of the preceding claims containing from 0.01 to 10% by weight, based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide, of at least one complexing agent capable of forming a water-soluble complex with metals.

13. A surface treating agent as claimed in claim 12 in which the complexing agent is one or more of sodium cyanide, potassium cyanide, ammonium cyanide, ethylene diamine tetraacetic acid, sodium ethylene diamine tetraacetate, potassium ethylene diamine tetraacetate, triethanolamine and ethylene diamine.

14. A surface treating agent as claimed in claim 12 or claim 13 containing from 0.1 to 3% by weight, based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide, of complexing agent.

15. A surface treating agent as claimed in any one of the preceding claims also containing from 0.001 to 5% by weight, based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide, of at least one nonionic surfactant.

16. A surface treating agent as claimed in claim 15 containing from 0.01 to 2% by weight, based on the weight of water and trialkyl (hydroxyalkyl) ammonium hydroxide, of nonionic surfactant.

17. A surface treating agent as claimed in claim 15 or claim 16 in which the nonionic surface active agent is a polyoxyethylene surfactant of the formula: R - O(CH2CH2O)mH;

or R" - COO(CH2CH2O)nH in which R and R" are each alkyl groups having from 12 to 18 carbon atoms, R' is an alkyl group having 8 or 9 carbon atoms, m is 8 to 15 and n is 5 to 30.

18. A surface treating agent as claimed in claim 17 in which the nonionic surfactant is a polyoxyethylene nonylphenol ether.

19. A surface treating agent as claimed in claim 1 substantially as hereinbefore described with reference to the Examples.