Background Art It is in many cases desirable to produce small structures in a material surface of a substrate. This can be achieved by electrochemical plating of the substrate.

Alternatively, the substrate can be subjected to electro- chemical etching. In both cases, such electrochemical processing takes place in a cell or container, which defines a chamber containing at least one electrode and an opposite substrate holder. The electrode and the sub- strate holder are connected to a voltage source, which during the electrochemical processing establishes an electrical field between the electrode and a substrate carried by the substrate holder. An electrolyte is re- ceived between the electrode and the substrate. In a plating process, ions pass from the electrolyte and are deposited on the surface of the substrate. In an etching process, parts of the surface of the substrate are dis- solved and pass to the electrolyte in the form of ions.

In one type of plating process, the surface of the substrate can be provided with the desired structure, so that an inverted copy is transferred to the metal layer which is deposited on the substrate acting as a cathode.

The thus structured metal layer can then be released from the substrate and used as a stamper (matrix), for in- stance in injection moulding of various products, such as CD, CD ROM, DVD etc. and also vinyl records, holograms etc., or in so-called nanoimprint lithography for manu-

facturing structures in semiconductor materials etc. Al- ternatively, the structured metal layer can be used as a so-called father by a new metal layer being deposited thereon for making an inverted copy which is used either as a stamper as above or as a so-called mother on which a metal layer is deposited in order to form sons which in turn are used as stampers. This type of plating is disclosed in e. g. US-A-5,843,296 and US-A-5,427,674. In an alternative plating process, an unstructured substrate is arranged as a cathode in the chamber. By suitable masking of the substrate, a desired structure can be formed on the same during plating. According to one more alternative plating process, an even metal layer is depo- sited on an unstructured substrate. This plating process thus results in an unstructured raw matrix of high sur- face smoothness which in a separate subsequent step is structured by a suitable method, such as etching. This type of method is disclosed in W099/63535.

In an etching process, the substrate, which is ar- ranged as an anode in the chamber, can be structured by suitable masking thereof. Alternatively, an electrode surface can be caused to move in a given pattern over the substrate. Different types of electrochemical etch- ing are known from e. g. EP-A-392 738, W098/10121 and EP-A-563 744. Like in plating, the structured substrate can be used directly as a stamper, for instance in injec- tion moulding or in nanoimprint lithography, or be used for making a mother etc.

A plating cell according to US-A-5,427,674 defines a chamber containing a substrate holder and an electrode in the form of a basket of titanium, which carries balls or rods of the metal that is to be deposited on a substrate.

As metal ions are being deposited on the substrate from the electrolyte, new metal ions are emitted from the balls or rods to the electrolyte, so that its metal ion content is essentially constant. The bottom of the basket is plane and provided with a plurality of holes. A sub-

strate holder is arranged opposite the bottom of the bas- ket. During plating, an electrolyte is circulated con- tinuously through the chamber. The electrolyte is pumped vigorously into the space between the electrode and the substrate carried by the substrate holder through holes in the peripheral wall of the chamber, so that turbulence is generated in the electrolyte to eliminate any concen- tration gradients therein. The electrolyte flows up through the electrode and out of the chamber through a central outlet above the electrode. A pressure above at- mospheric is maintained in the chamber during the entire plating process.

The above-mentioned US-A-5,427,674 also discloses an alternative embodiment. In this case the electrode is a so-called dimensionally stable sheet-like anode (DSA) of e. g. platinum-coated titanium. This electrode is essen- tially inert during the plating process. Therefore a com- pensating device is provided for the chamber and designed to add in a dosed manner a metal hydroxide to the elec- trolyte to compensate for the metal ions that are removed from the electrolyte and deposited on the substrate. In this embodiment, a smaller distance between the electrode and the substrate is allowed, which in turn results in higher current density between the electrode and the sub- strate and lower power consumption. In this case the electrolyte is stated to conveniently flow out of the chamber through an electrolyte outlet in the peripheral wall of the chamber.

In both embodiments above, the electrolyte is pumped vigorously through the chamber to generate the desired turbulence. Simultaneously with the turbulence, also a pressure above atmospheric is generated in the chamber, which causes a risk of leakage of electrolyte to the sur- roundings. In view of the risk of leakage, the parts of the cell are therefore in many cases of a strong con- struction and held together by means of a powerful clamping device, for instance a piston-and-cylinder

unit and an abutment, or a plurality of clamping screws.

Another problem of the above apparatus arises when gas forms in the chamber during the plating process, for instance by an inevitable chemical reaction at the elec- trode. Such gas has been found to interfere with the plating process and cause undesired irregularities in the material deposited on the substrate.

In a plating apparatus according to US-A-5,785,826, the substrate is rotated in the chamber during the plat- ing process in order to eliminate any concentration gra- dients in the electrolyte. It is, however, relatively complicated to make an electrically connected substrate rotate in the chamber. Moreover, also this construction will probably have the above problems with gas which pos- sibly interferes with the plating process.

The above problems are also to be found in prior art apparatus for electrochemical etching.

Objects of the Invention An object of the present invention is to wholly or at least partly overcome the above problems according to prior art, and in particular to provide a simplified apparatus and a simplified method for electrochemical processing.

It is also an object in an apparatus and a method for electrochemical processing to minimise the risk of leakage of electrolyte to the surroundings.

Another object is to provide an apparatus and a method for electrochemical processing which result in high surface accuracy of the processed substrate, inde- pendently of the character of the substrate.

It is also an object to minimise the risk of distur- bance in the electrochemical process as gas develops in the chamber.

Summary of the Invention These and other objects that will be evident from the following description are now at least partly achieved by an apparatus and a method according to the

independent claims. Preferred embodiments are defined in the dependent claims.

Owing to the turbulence generator, an optional degree of turbulence can be achieved between the elec- trode member and the substrate carried by the substrate holder, in order to minimise concentration gradients in the electrolyte, above all at the surface of the sub- strate. Thus the turbulence is easily generated mechani- cally by rotation of the stirring portion in the chamber and is thus independent of a possible flow of electrolyte through the chamber. The flow of electrolyte through the chamber can thus be optimised for every situation. For instance, the flow can be kept so low that a constant composition in the electrolyte is exactly guaranteed, whereby the tank volume of electrolyte in the apparatus can be reduced. Moreover, an arbitrary pressure can be established in the chamber during the electrochemical processing, as will be discussed in more detail below.

According to a preferred embodiment, the turbulence generator comprises a drive shaft whose one end is con- nected to the stirring portion arranged in the chamber and comprising at least one blade extending radially from said end. Each blade is designed to screen radially out- wards from said end essentially the same amount of the surface of the electrode member. Each irregularity in the electrical field between the electrode member and the substrate results in a corresponding irregularity of the processed substrate. This embodiment guarantees uniform disturbance in the electrical field between the electrode member and the substrate when the stirring portion is caused to rotate. It is also preferred for each blade to be sheet-like in a plane parallel with the electrode mem- ber, and for each blade in said plane to be defined by two essentially rectilinear edges which extend from the end of the drive shaft at a given angle to one another.

Thus a compact turbulence generator is provided, which without introducing any irregularities into the elec-

trical field allows a desirably small distance between the electrode member and the substrate.

According to one more preferred embodiment, the stirring portion is rotatable about an axis of rotation which coincides with a centre axis of the chamber. Thus a uniform distribution of the turbulence across the surface of the substrate is achieved. Moreover, a stable turbu- lent motion can be established in the chamber, especially if the chamber is rotationally symmetrical about said centre axis. Such a turbulent motion will by centripetal forces automatically urge gas, which possibly forms in the chamber during processing, towards the axis of rota- tion. Preferably, the container has an outlet which is arranged adjacent to the axis of rotation, in an upper portion of the chamber, so that this gas can be withdrawn in a controlled fashion from the chamber. The outlet is preferably connected to the suction device, so that the gas is automatically drawn out together with the electro- lyte. It is also preferred for the electrode member to be arranged above the substrate holder and to have a central hole adjacent to the axis of rotation. Thus, gas drawn towards the axis of rotation can flow through the hole into the upper portion of the chamber and from there out through the outlet.

According to a preferred embodiment, the electrode member is plane, essentially inert during processing and formed with a plurality of perforations distributed across its surface. This allows a compact design with a small distance between the electrode member and the sub- strate, which enables low power consumption and use of small flows of electrolyte. The perforations result in an additionally improved withdrawal of gas forming adjacent to the inert electrode member. The electrode member is preferably a dimensionally stable electrode made of mesh network.

According to another preferred embodiment, an abut- ment is formed to sealingly abut against one side of the

substrate facing away from the substrate holder and with said side define a stabilising chamber, in which at least during processing such a pressure prevails that the sub- strate assumes an essentially plane form, i. e. suitably essentially the same pressure as in the chamber, or a somewhat lower pressure to bring the substrate into plane engagement with a supporting surface of the abutment.

This ensures that the substrate during processing assumes a plane form independently of the subatmospheric pressure in the processing chamber, and thus uniformity of the electrical field between the electrode and the substrate is also guaranteed, even during processing of flexible and/or thin substrates.

According to one more preferred embodiment, a pres- sure control means is adapted to establish a subatmos- pheric pressure in the chamber during the electrochemical processing. Thus, the occurrence of leakage of electro- lyte from the apparatus can be essentially eliminated by the container being given a self-sealing design, i. e. that the subatmospheric pressure in the chamber acts to hold the parts of the container together. Thus, the need for an external locking device which holds the parts of the container together during the electrochemical processing is eliminated. Another advantage is that the container can be opened automatically after completed processing by pressure compensation in the chamber.

According to a preferred embodiment, a suction de- vice is connected to an outlet of the container in order to draw electrolyte through the chamber during generation of the subatmospheric pressure therein. A suitable sub- atmospheric pressure can easily be achieved by adaptation of the suction effect to, for instance, the dimension of the electrolyte inlet of the container. The subatmos- pheric pressure should be such that leakage of electro- lyte is essentially eliminated while at the same time a sufficient throughput of electrolyte is allowed during processing. The throughput of electrolyte should be such

that a substantially constant quality of the electrolyte in the chamber is maintained during processing. The suc- tion effect also permits safe withdrawal of gas which possibly forms in the chamber during processing.

According to one more preferred embodiment, the apparatus comprises at least two parts which in a co- operating and sealing fashion define the chamber during processing. These parts can thus automatically be held together by the subatmospheric pressure in the chamber during processing. A first part preferably comprises a shell-like casing which defines an internal space which extends to a mouth portion and within which the electrode member and the stirring portion of the turbulence genera- tor are arranged. A second part preferably comprises the substrate and cooperates with the mouth portion while closing the space and defining the chamber. Thus, the space can automatically be closed by the second part, i. e. the substrate and any further components, being mounted in the apparatus. In this context, it is also preferred for the substrate holder to be connected to the casing to define the mouth portion and to have an annular receiving surface, which faces away from the electrode member, and for the substrate during closing of the space to be receivable on the receiving surface of the sub- strate holder, said receiving surface preferably being connected to the voltage source. Thus, the substrate serves as the second part cooperating with the casing, and the space in the casing is automatically closed and forms the chamber when the substrate is mounted in the apparatus. Moreover, the apparatus can easily and safely be connected to the voltage source since both the elec- trode member and the substrate holder are incorporated into the first part. If the receiving surface of the sub- strate holder is connected to the voltage source, the substrate is automatically connected to the voltage source during mounting in the apparatus. Preferably, a sealing means is arranged in the receiving surface of the

substrate holder for sealing engagement with the sub- strate during processing. Thus, a self-sealing design of the container is easily achieved, by the subatmospheric pressure in the chamber striving to draw the substrate towards the sealing means in the receiving surface of the substrate holder.

The above discussion of preferred embodiments of the inventive apparatus is to a corresponding degree also applicable to the method of the invention.

Brief Description of the Drawings The invention will now be described in more detail with reference to the accompanying drawings which by way of example schematically illustrate currently preferred embodiments.

Fig. 1 is a plan view from below of a container with an internal processing chamber before mounting of a sub- strate.

Fig. 2 is sectional view of the container along line 2-2 in Fig. 1 after mounting of a substrate.

Fig. 3 is an overall view of a plating apparatus according to the invention.

Fig. 4 is an overall view of a plating apparatus with a container according to an alternative embodiment shown partly in cross-section.

Description of Preferred Embodiments The following description of preferred embodiments is directed at plating of a substrate. A person skilled in the art understands that the description is also applicable to etching of a substrate, by reversing the voltage which drives the electrochemical process.

Figs 1-3 show a first embodiment of a plating appa- ratus according to the invention. The plating apparatus comprises a plating container or cell 1, which is shown in detail in Figs 1 and 2. The cell 1 comprises a shell- like casing 2 which defines an internal space 3. The space 3 is essentially circular-cylindrical and, thus, rotationally symmetrical about a centre axis A. The

space 3 is defined by a plane end wall 4 and a circum- ferential peripheral wall 5. A circumferential opening in the peripheral wall 5 forms an electrolyte inlet 6.

An annular opening in the end wall 4 adjacent to the centre axis A forms an electrolyte outlet 7. A plane electrode plate 8 is arranged adjacent to the end wall 4 and serves as an anode. Connections 9 extend through the casing 2 from the anode 8 and form connecting means 10 on the outside of the casing 2 for connection to a voltage source 11 (Fig. 3). The anode 8 is suitably a dimen- sionally stable anode (DSA), for instance of iridium- oxide-activated titanium, which is formed as a plane fluid-permeable grid.

The cell 1 further comprises a mechanical turbulence generator 12 with a stirring portion which is rotatably arranged outside the anode inside the space 3. The turbu- lence generator 12 has a drive shaft 13 which is arranged through a hole 14 extending along the centre axis A of the casing 2. A packing 15 is arranged to sealingly en- gage the drive shaft 13. The drive shaft 13 extends through a central hole 16 in the anode 8 and defines with the edges of the hole 16 an annular gap essentially aligned with the electrolyte outlet 7. The proximal end of the drive shaft 3 is connected to a drive motor (not shown) which is activatable to rotate the stirring por- tion, which in this case comprises four blades 17 pro- jecting from the distal end of the drive shaft 13. The blades 17 are sheet-like and arranged parallel with the anode 8. Each blade 17 is defined by two essentially rec- tilinear edges 18 which extend from the distal end of the drive shaft 13 at a given angle to one another (Fig. 1).

During rotation, the blades 17 will thus screen, radially outwards from the distal end, essentially the same amount of the surface of the anode 8.

The cell 1 comprises an annular substrate holder 60 which is arranged to define a mouth portion of the space 3. A plane receiving surface 19 of the holder 60 faces

away from the anode 8 and is accessible from the outside of the casing 2. Connections 20 extend through the casing 2 from the holder 60 and form connecting means 21 on the outside of the casing 2 to be connected to the voltage source 11 (Fig. 3). A circumferential packing 22 is ar- ranged in a circumferential groove 23 in the receiving surface 19 and projects therefrom. The substrate holder 60 is arranged to carry a substrate S plane-parallel with the anode 8.

Fig. 2 shows a substrate S which is mounted for plating in the cell 1. As will be described in more detail below, a subatmospheric pressure is established in the cell 1 during the plating process, which is filled with an electrolyte. This subatmospheric pressure draws the substrate S towards the receiving surface 19 of the cathode holder 60 so that the space 3 is sealingly closed and a plating chamber is defined. The substrate S is also brought into electric contact with the cathode holder 60.

In this case the substrate consists, for example, of a metal sheet or a metallised body, such as a glass plate.

During plating, the substrate S is placed on the cathode holder. A subatmospheric pressure is established in the chamber by electrolyte being continuously drawn out through the central outlet 7 and a new electrolyte being allowed to be drawn into the chamber via the pe- ripheral inlet 6. The size of the subatmospheric pressure in the chamber depends on the relationship between the suction effect at the outlet 7 and the pressure drop in the inlet 6 and associated conduits (cf. Fig. 3). In the shown embodiment, a subatmospheric pressure of about 300 kPa has been found to function well. The size of the subatmospheric pressure has been found not to be criti- cal. If the subatmospheric pressure is too small, how- ever, leakage of electrolyte from the cell 1 will arise.

If the subatmospheric pressure is too great, the flow of electrolyte through the chamber will be too small. A cer- tain throughput of electrolyte is in fact desirable since

the supply of new electrolyte ensures that the electro- lyte keeps a constant quality in the chamber, for in- stance with regard to composition, temperature, pH etc.

The plating is initiated by activation of the voltage source 11 (Fig. 3), whereby an electrical field is estab- lished through the electrolyte between the anode 8 and the substrate S, which functions as a cathode. During plating, the blades 17 are rotated continuously in the chamber for mixing of the electrolyte and generation of turbulence, above all on the surface of the substrate S.

After completed plating, the pressure in the chamber is equalised, and the substrate S is released from the holder 60.

The cell 1 described above is included in a plating apparatus, whose other parts will be described below with reference to Fig. 3. The plating apparatus has an elec- trolyte feeding system 30 comprising a first tank 31 which via a feeding conduit 32 communicates with the electrolyte inlet 6 of the cell, and a second tank 33 which via a pump 34, an ejector 35 and a particle filter 36 communicates with the first tank 31. The suction side of the ejector 35 communicates with the electrolyte out- let 7 of the cell 1. The first tank 31 has an overflow 37 which communicates with the second tank 33. The first tank 31 is provided with a heater 38 for heating the electrolyte, a temperature sensor 39 and a pH meter 40.

In the preferred embodiment as described above, use is made of a so-called dimensionally stable anode 8, which is essentially inert relative to the electrolyte.

The metal ions in the electrolyte which are consumed during plating are in this case replaced by means of a compensating device 41 which prepares a water-based slurry containing the corresponding metal ions. A pump 42 is arranged for dosed supply of the slurry to the second tank.

The selection of electrolyte is, of course, con- trolled by which metal is to be deposited on the sub-

strate S. In nickel plating, the electrolyte can, for example, be based on nickel sulphamate and water. In this case, hydrogen ions form at the anode 8 during plating while at the same time nickel ions are precipitated on the substrate S. These hydrogen ions are neutralised by adding an alkaline substance. This takes place by means of the compensating device 41, which prepares the slurry of an alkaline metallic salt, in this case nickel car- bonate, nickel hydroxide, nickel oxide or nickel oxide hydroxide. The anions of the salt react with the hydrogen ions and form, depending on the selection of salt, water or carbon dioxide. Thanks to the metal ions and the alka- line substance being supplied in the form of a salt, the metal ions can in a controlled manner be supplied to the electrolyte based on the pH of the electrolyte measured by the meter 40.

During plating, also gas forms at the inert anode 8, in the above example oxygen. This gas can interfere with the electrical field between the anode 8 and the sub- strate S. The grid-like anode 8, however, allows at least part of the formed gas to move through the many perfora- tions of the grid to the side of the anode 8 facing away from the substrate S, where the gas does not affect the electrical field. Moreover, the blades 17 will during rotation cause a turbulent motion in the electrolyte in the chamber. This turbulent motion generates centripetal forces which draw the gas formed at the anode 8 towards the centre of rotation where the gas in a controlled man- ner is drawn out through the outlet 7. Gas on the side of the anode 8 facing the substrate S can move freely up to the outlet 7 through the gap 16.

The plating apparatus shown in Fig. 3 operates in the following manner. During plating, the pump 34 feeds electrolyte from the second tank 33 through the ejector 35 to the first tank 31. The ejector 35 draws electrolyte from the cell 1 into the first tank 31, and the suction effect of the ejector 35 causes electrolyte to circulate

in a loop between the first tank 31 and the cell 1 while at the same time a subatmospheric pressure is established in the plating chamber of the cell 1. The filter 36 sepa- rates any particles in the electrolyte. A stable electro- lyte level is established in the first tank 31 via the overflow 37. The slurry is fed from the compensating de- vice 41 to the second tank 33 based on the output signal of the pH meter 40. The plating apparatus is conveniently controlled by a main control unit (not shown), such as a computer or PLC.

Fig. 4 shows an alternative embodiment of a plating apparatus. Parts equivalent to those in Figs 1-3 have been given the same reference numerals, and the following description will focus on differences relative to the apparatus described above.

The cell 1 is provided with a form-stabilising abut- ment 50, which comprises a sheet-like body 51 with a cen- tral supporting surface 52, an engaging surface 53 ex- tending round the supporting surface 52 and a circumfer- ential groove 54 in the engaging surface 53. The abutment 50 also comprises a circumferential packing 55 which is mounted in the groove 54 and which projects from the engaging surface 53. Openings 56 are formed in the sup- porting surface 52 and extend through the body 51. The openings 56 are in fluid communication with the ejector 35.

During processing, the abutment 50 is engaged with the substrate S which is carried by the substrate holder 60. The surface 53 sealingly abuts against the substrate S, so that a stabilising chamber is defined between the supporting surface 52 and the substrate S. During proc- essing, the ejector 35 establishes such a pressure in the stabilising chamber that the substrate S is held essen- tially plane, suitably by the substrate S being applied by suction to the supporting surface 52. Thus the pres- sure in the stabilising chamber is suitably essentially the same as or lower than the pressure in the plating

chamber on the opposite side of the substrate S. Ac- cording to an alternative embodiment (not shown), the supporting surface 52 is replaced by a hole which with the substrate defines the stabilising chamber. In this embodiment, a pressure is established in the stabilising chamber which is essentially the same as the pressure in the plating chamber, whereby the substrate S is held essentially plane during processing. The pressures on both sides of the substrate S can possibly be adjusted by means of a variable throttle (not shown) or the like in the conduit between the outlet 7 and the ejector 35 and/or in the conduit between the openings 56 and the ejector 35. The embodiment in Fig. 4 is particularly advantageous when processing flexible and/or thin sub- strates, such as metal foils, since these can be guaran- teed to be held fully plane during processing.

In Fig. 4 it may also be noted that the electrolyte feeding system 30 is somewhat simplified and only com- prises a first tank 31, a pump 34 and an ejector 35.

It will be appreciated that the invention is not restricted to the above detailed description of currently preferred embodiments and can be modified within the scope of the appended claims. For example, a correspond- ing apparatus and a corresponding method can be used for electrochemical etching.