FR2751128A1 - Metal cleaning especially for cleaning optical disk manufacturing die - Google Patents

Abstract

A metal surface cleaning process involves treatment with a microwave-generated chemically pure plasma. Preferably, the treatment is repeated 1-10 (especially 5) times and comprises a first stage employing an oxidising gas plasma (preferably an oxygen/argon, CF4, CF4/argon or especially oxygen plasma) and a second stage employing a deoxidising gas plasma (preferably an argon, argon/ammonia or especially ammonia plasma). The metal surface preferably consists of nickel, cadmium, tantalum or a mixture of these metals, especially nickel.

Description

METHOD FOR CLEANING METAL SURFACES
BY PLASTIC
DESCRIPTION
Technical area
The present invention relates to a method for cleaning metal surfaces, in particular matrices used in the manufacture of optical discs such as CDs, CD-ROMs and DVDs, by purely chemical plasma.

 This method is applicable to the cleaning of metal surfaces in general, and more particularly to metal surfaces having functional micron cavities, for example to the surfaces of matrices used for the manufacture of optical discs, requiring an effective, selective and nonabrasive cleaning.

 In all cases, the problem lies in the difficulty of removing dirt on a metal surface which has functional micron cavities, that is to say a fine and precise topography, without damaging this topography.

 In the case of an optical disk array, the cleaning can be intended on the one hand for the metal surface of the new array, at its exit from the manufacturing unit, on the other hand for the maintenance cleaning of the surface of the. matrix throughout its use for printing optical discs.

 Indeed, the manufacture of an array of optical discs comprises the manufacture of a parent element by electrolytic deposition of a metal, for example nickel, on a glass support comprising a topography of resin covered with a thin layer of 'silver. The father element thus produced makes it possible, by electrolytic deposition, to manufacture the matrix of optical disks.

 For printing optical discs, this matrix is fixed in an injection press into which polycarbonate is introduced under pressure.

A plastic disc is obtained on which the imprints of the micron cavities of the matrix are reproduced (this disc is then covered with a reflective layer of aluminum to produce an optical disc).

 During the manufacture of the parent element of the matrix and of the matrix, the surfaces of the parent element of the matrix and of the matrix are soiled in particular by resins, photoresists, stains of water, acetone, electrolytic bath, e-tc .... . and by traces of manipulation.

 It is therefore necessary to clean these metal surfaces at the end of the manufacturing unit in order to optimize, from their first use, the printing of the matrix and of the optical discs.

 During the use of the matrix for printing optical discs, polycarbonate accumulates in the micron cavities of the matrix, making printing of optical discs less precise, and thus greatly limiting the life of the matrix.

 Maintenance cleaning of the matrix surface is therefore also necessary throughout the life of the matrix, in order to remove traces of polycarbonate and traces of handling.

Prior art
The common technique used for industrial cleaning of metal surfaces is a chemical wet treatment. This technique involves the use of concentrated acid baths and rinsing solutions.

 However, this type of cleaning of metal surfaces has many disadvantages, in particular pollution and cost, for a quality of cleaning which is not always satisfactory. Indeed, the concentrated acid baths used for cleaning surfaces must then be reprocessed, resulting in a non-negligible cost; in addition, the rinsing solutions are seldom purified and are rejected in the nature posing a real problem of pollution.

 In addition, this chemical wet technique can be abrasive for certain metallic surfaces and does not always make it possible to restore used surfaces with functional micron cavities to the quality of their new condition.

 The inventors are interested in the techniques of plasma cleaning of surfaces usually used in silicon technology.

 The principle of these techniques is either to bombard the surface to be cleaned with reactive compounds or ions obtained by electric discharges, or to put this surface in contact with activated chemical species in a plasma obtained generally by a wave source. In the first case, it is a plasma cleaning by ion bombardment, in the second case, a purely chemical plasma cleaning.

 The quality of cleaning is measured by a factor of dissociation of the stains from the surface to be cleaned, that is to say by the ratio of stains detached from the surface, by cleaning, to the total stains on the surface before cleaning.

 Concerning plasma cleaning by ion bombardment, the plasma is generated by a discharge of an electrical power of low frequency, about 50 KHz, or of medium frequency, about 13.5 MHz, in a reactor in which a partial pressure of a gas is kept low by pumping. The gas used is fluorine, oxygen or chlorine and the partial pressure of this gas in the reactor is from 0.135 Pa to 135 Pa.

 The plasma thus generated induces self-polarization of the substrate to be cleaned. The ions formed are of high energy (from 100 to 1000 eV) and when they reach the surface of the substrate, they are accelerated, and they bombard this surface by tearing off more or less material, depending on whether it has bonds more or less strong chemicals, giving rise to more or less volatile compounds. The dissociation factor of this type of plasma is low and of the order of 10-6.

 To this physical action is superimposed the chemical reaction action of the reactive ions or compounds such as fluorine, oxygen or chlorine on the surface.

The volatile reaction products are removed by a pumping system.

The document Le Vide, Les Couches Thins,
Supplement No. 256-March-April 1991 Polymers Degradation Under Plasma Exposure: Influence of Physical
Properties, pp. 265-267 describes an example of plasma cleaning of phenolic resins on silicon surfaces by ion bombardment.

 This type of cleaning is not completely selective with respect to the surfaces to be cleaned and in particular at the level of the topographies that these surfaces may include, because it accelerates the effect of bombardment on the topographic reliefs. This ion bombardment is a fairly violent physico-chemical abrasion.

 In addition, in the case of certain metallic surfaces, in particular nickel, cadmium, tantalum surfaces, etc., the compounds formed are not very volatile and are redeposited on the walls of the reactor, thus creating a poisoning effect of the enclosure being manifested by a slowing down of the efficiency of the process.

 Concerning purely chemical plasma cleaning, it consists in bringing a surface to be cleaned into contact with activated chemical species in a plasma obtained by a wave source in the enclosure of a reactor.

 Thus, application WO 95/07152 describes a surface treatment in which the surface to be treated is brought into contact with a reactive gas and subjected simultaneously to the action of ultraviolet radiation or of a laser beam. The oxidizing reagent can be oxygen, chlorine or fluorine.

The document The Void, Thin Films
Supplement No. 256-March-April 1991, p. 256-258 relating to the role of neutral species during polyolefin treatments in spatial post-discharge of a microwave oxygen discharge, is a study intended to determine the optimal conditions for the treatment of polyolefin surfaces in order to improve wettability and adhesion.

 Application FR-A-2,534,040 describes a process in which a plasma is used in the manufacture of integrated circuits. This plasma is intended to eliminate a layer of resin constituting a protective mask in the manufacture of these circuits. The gas mixture used is a mixture of oxygen and a rare gas such as argon.

 In silicon technology, microwave plasma cleaning is frequently used. The plasma is created by a microwave source and has a very strong dissociation of the reactive gas species with very low energy ions of the order of a few electrons volt. The substrate is placed outside the plasma source, so it is not polarized by the source. In the absence of this polarization, the ions or the activated species of the atoms of the gas introduced into the enclosure of a reactor are only chemically active on the silicon surface to be cleaned.

Statement of the invention
The present invention specifically relates to a method of cleaning a metal surface which makes it possible to solve the problems mentioned above.

 This method of cleaning a metal surface is characterized in that it comprises at least one sequence of treatment with at least one purely chemical plasma, generated by means of microwave waves.

The treatment sequence consists for example of a first step using an oxidizing gaseous plasma followed by a second step using a deoxidizing gaseous plasma.

 The plasma is generated by means of microwave waves produced by an excitation device. It is advantageously generated by waves generally having a frequency in the range from 0.3 GHz to 13 GHz, and an excitation device having a power of approximately 100 to approximately 2000 W is preferably used.

 Generally, to carry out this treatment, the operation is carried out using gas plasma under pressures which can range from 0.135 Pa to 13.5 Pa and at a gas flow rate which can range from 1 cm 3 / s to 200 cm 3 / s.

 The first step, using an oxidizing gas plasma, generally lasts from 1 s to 600 s, and the second step, using a deoxidizing gas plasma, generally lasts from 1 s to 600 s.

The oxidizing gas plasma is for example a plasma comprising oxygen, hereinafter called oxygen plasma, a plasma of a mixture of oxygen and argon, a plasma comprising CF4, hereinafter called plasma
CF4, a plasma of a mixture of CF4 and argon, a plasma comprising SF6, or a plasma of a mixture of at least two of the abovementioned gases. Preferably, an oxygen plasma is used.

 The deoxidizing gas plasma is for example a plasma comprising ammonia, a plasma comprising argon, or a mixture of ammonia and argon. Preferably, a mixture of ammonia and argon is used, hereinafter called ammonia plasma.

 When the oxidizing gas plasma is an oxygen plasma, the oxidizing gas plasma has a flow rate of approximately 1 cm3 / s to 200 cm3 / s, preferably a flow rate of 3.5 cm3 / s, and a pressure of approximately 0.135 Pa to about 13.5 Pa, preferably 10 Pa. The proportion of oxygen in an oxygen plasma is generally from about 5% to 100% by volume, preferably from 5% to 50% by volume. The plasma being generated by means of microwave waves produced by an excitation device having a power of 100 W to 2000 W, preferably 400 W. The duration of this oxidation stage with oxygen plasma is approximately 1 s to 600 seconds, preferably 30 seconds.

When the oxidizing gas plasma is a plasma of
CF4, the gaseous plasma of CF4 has a flow rate of approximately 1 cm3 / s to 200 cm3 / s, preferably 10 cm3 / s, and a pressure of approximately 0.135 Pa to 13.5 Pa, preferably 10 Pa The proportion of CF4 in the plasma is approximately 10% to 50% by volume, preferably from 10% to 30% by volume, the plasma being generated by means of microwave waves produced by an excitation device having a power from about 100 W to 2000 W, preferably 600 W. The duration of this CF4 plasma step is from about 1 second to 600 seconds, preferably from 30 seconds.

 The oxidation of soils by the gas plasma of the first step is erased by the deoxidation by the gas plasma of the second step, this avoids a strong degradation of the surface to be cleaned while erasing the defects due to pollution by products of combustion or fat.

 When the deoxidizing gaseous plasma is an ammonia plasma, the proportion of ammonia in the plasma is generally from 10% to 100% by volume, preferably from 2% to 50% by volume.

 In this second step, when the deoxidizing gas plasma is an ammonia plasma, the operation is carried out at a gas flow rate of approximately 1 cm 3 / s to 50 cm 3 / s, preferably 2 cm 3 / s, at a pressure of approximately 0.135 Pa at 13.5 Pa, preferably 10 Pa, the gas plasma is generated by means of microwave waves produced by an excitation device having a power set at about 100 W at 2000 W, preferably set at 300 W, and the step has a duration of about 1 s to 600 s, preferably 30 s.

 The treatment sequence is reproduced from 1 to 10 times, preferably 5 times.

The metallic surface is a surface which comprises, for example, a metal chosen from nickel, cadmium, tantalum, etc., or a mixture of these metals, preferably the surface is a nickel surface.

 The metal surface is a surface which can comprise a fine topography, for example micron cavities, having a functional importance, as in the case of matrices intended for the printing of optical discs, and parent elements of these matrices.

The method according to the invention makes it possible to clean metal surfaces such as CD matrices,
CD-ROM and DVD as well as the father elements of these matrices. It can effectively remove stains from resin, photoresists, acetone, polycarbonate, water stains and handling, and even give new cleaned surfaces a higher quality than new uncleaned surfaces.

 The result of the cleaning according to the invention results in a lower error rate when printing the optical discs from the matrices, and the printing of the matrices from their parent element, and by a duration of greater life of matrices and their father element. This process is also less polluting and more economical than the processes usually used for these metal surfaces.

Examples of devices making it possible to implement the method according to the invention are described in the documents Le Vide, Les Couches Thins
Supplement No. 25- March-April 1991, the request
US-A-4,804,431 and application FR-A-2,534,040.

Other characteristics and advantages of the invention will appear better on reading the description which follows, given of course by way of nonlimiting illustration with reference to the appended figure which represents a device for implementing the method of invention. The device shown in this figure is that of the application
FR-A-2 534 040.

Example of application of the method according to the invention to the cleaning of an optical disk array
In the figure, it can be seen that the device used in this example of plasma cleaning applied to an optical disk array comprises an enclosure 1 which can be brought to practically vacuum by a pumping system 3 provided with a valve 2. In the enclosure, is arranged a support 5 adapted to receive the matrix to be cleaned 7. At the upper part of the enclosure, a cylindrical cell 9 made of quartz in which a plasma can be generated, opens into the enclosure by a flared part 10 situated opposite of the matrix and also made of quartz. The opening of this flared part 10 has dimensions greater than those of the matrix to be treated.

 The cell 9 can be supplied with oxygen, ammonia, CF4, SF6 and argon or by a mixture of these, by a pipe 11 connected to reservoirs 13 and 15 of these gases by means of valves. flow adjustment 17 and 19.

 The device also comprises an excitation device for exciting by periodic waves in the microwave domain the column of gas present in the cylindrical cell 9 thus generating a plasma. This excitation structure comprises a waveguide 21 of rectangular section whose walls associated with the long side of the section have a circular opening allowing the passage of the cylindrical cell 9 through the waveguide.

 Sealing means are provided at the crossing of the cell 9 and of the part 10 in the enclosure 1.

 The dimensions of the cross section of the waveguide are chosen according to the frequencies used to generate the gaseous plasma in the cell 9. For example, for the use of frequencies of 2.45 GHz, the guide wave has a cross section of 4.3 cm in height and 8.6 cm in width.

 At one of its ends, the waveguide comprises a transition element 23 enabling the guide 21 to be supplied by a high frequency generator 25.

 At the other end of the waveguide 21, is installed an adapter stud 27 secured to a rod 29 making it possible to slide the piston over a stroke of length L. This length L is preferably of the order of magnitude of the wavelength ig of the fundamental mode of the waveguide 21. The length of the waveguide is generally of the order of 10 to 15 times the radius of the cylindrical cell 9, this in order to avoid reflections inside the guide, which would affect the homogeneity of the plasma created in cell 9.

 The piston 27 has an adaptation role allowing maximum absorption of the high frequency energy supplied by the generator to the gas column of the cell.

 The distance separating the plasma from the matrix and that between the arm of the flared part are optimized to obtain good homogeneity of the plasma and of the reaction. The opening of this flared part has dimensions greater than those of the matrix to be cleaned.

 The microwave source is of the surface wave type (not limiting), a second RF source can be coupled to the first. The power used varies from 0 to 600 W. The cleaning time is adjustable as well as the powers.

 The diameter of the vacuum chamber is 25 cm, and 5 cm in height. The gas arrives from above, the flows are controlled by mass flowmeters.

 The matrix to be cleaned comprises a metallic nickel surface, a few tenths of a millimeter thick, deposited by electrolysis, and has a fine and precise topography formed by oblong cuvettes of dimensions varying in width between 1.6 to 0.6 μm and between 1 and 10 µm in length.

This matrix advantageously presented defects or alterations, detected by a dynamic test whose specifications are recorded in 1 Orange
Book for CDs. These faults cause errors when printing optical discs which may not be correctable or correctable.

 Non-correctable faults are named E32.

The correctable faults are named Ell, E21,
E31, E14, E22. The alteration of the matrix is also reflected through a block error rate (BLER), reflectivity or an I3R parameter, I3 being the signal of information of three successive bits identical and R the reflectivity.

 In this example, in the treatment sequences, we use an oxidizing oxygen plasma and a deoxidizing ammonia plasma.

 The oxidizing plasma is an oxygen plasma at a gas flow rate of 3.5 cm / s, and at a pressure of 10 Pa, the proportion of oxygen in the plasma is 5 to 50% by volume, the power of the source. of microwave waves is 400 W-and the duration of this first stage is 30 s.

 The deoxidizing gaseous plasma is an ammonia plasma at a flow rate of 2 cm3 / s, and at a pressure of 10 Pa, the proportion of ammonia in the plasma is from 2 to 50% by volume, the power of the source of microwave waves of 300 W and the duration of this second stage is 30 s.

 The total duration of a sequence is 1 min. This sequence was reproduced 5 times, for a total duration of 5 min.

The cleaning process according to the invention made it possible to practically eliminate the Ell defects without changing
I3R, resulting in a lower BLER rate.

Claims (16)

 1. A method of cleaning a metal surface, characterized in that it comprises at least one treatment sequence with at least one purely chemical plasma generated by means of microwave waves.  2. The method of claim 1, wherein the treatment sequence consists of a first step using an oxidizing gas plasma followed by a second step using a deoxidizing gas plasma.  3. Method according to claim 2, in which the oxidizing gas plasma is chosen from an oxygen plasma, a plasma of an oxygen / argon mixture, a CF4 plasma, a plasma of a CF4 / argon mixture.  4. Method according to any one of claims 2 to 3, wherein the deoxidizing gaseous plasma is chosen from an ammonia plasma, an argon plasma or a plasma of an ammonia and argon mixture.  5. The method of claim 2, wherein the oxidizing gas plasma is an oxygen plasma and the deoxidizing gas plasma is an ammonia plasma.  6. The method of claim 5, wherein in the first step of the sequence, the oxidizing gaseous plasma has a flow rate of 1 to 200 cm3 / s and a pressure of 0.135 to 13.5 Pa, the proportion of oxygen in the plasma is from 5 to 100% by volume, the plasma being generated by means of microwave waves produced by an excitation device having a power of 100 to 2000 W, the first step of said method having a duration of 1 to 600 s.  7. The method of claim 5, wherein in the first step of the sequence, the gaseous oxygen plasma has a flow rate of 3.5 cm3 / s, and a pressure of 10 Pa, the proportion of oxygen in the plasma is from 5 to 50% by volume, the plasma being generated by means of microwave waves produced by an excitation device having a power of 400 W, the first step of said process having a duration of 30 s.  8. The method of claim 3, wherein in the first step of the sequence, the gaseous plasma of CF4 has a flow rate of 1 to 200 cm3 / s and a pressure of 0.135 to 13.5 Pa, the proportion of oxygen in the plasma is 10 to 50% by volume, the plasma being generated by means of microwave waves produced by an excitation device having a power of 100 to 2000 W, the first step of said process having a duration of 10 to 600 s  9. The method of claim 8, wherein in the first step of the sequence, the gas plasma of CF4 has a flow rate of 10 cm3 / s and a pressure of 10 Pa, the proportion of CF4 in the plasma is 10 to 30 % by volume, the plasma being generated by means of microwave waves produced by an excitation device having a power of 600 W, the first step of said method having a duration of 30 s.  10. Method according to any one of claims 5 to 9, in which in the second step of the sequence, the gaseous ammonia plasma has a flow rate of 1 to 50 cm3 / s, and a pressure of 0.135 to 13.5 Pa, the proportion of ammonia in the plasma is 10 to 100% by volume, the plasma being generated by means of microwave waves produced by an excitation device having a power of 100 to 2000 W, the second step of said process having a duration of 1 to 600 s.  11. The method of claim 10, wherein in the second step of the sequence, the gaseous ammonia plasma has a flow rate of 2 cm3 / s, and a pressure of 10 Pa, the proportion of ammonia in the plasma is 2 at 50% by volume, the plasma being generated by means of microwave waves produced by an excitation device having a power of 300 W, the second step of said method having a duration of 30 s.  12. Method according to any one of claims 6 to 11, in which the sequence is reproduced from 1 to 10 times.  13. The method of claim 12, wherein the sequence is reproduced 5 times.  14. The method of claim 13, wherein the metal surface comprises a metal selected from nickel, cadmium, tantalum or a mixture of these metals.  15. The method of claim 14, wherein the metal surface is nickel.  16. The method of claim 15, wherein the metal surface is an optical disk array, or a parent element of an optical disk array.