METHOD IN AND APPARATUS FOR ETCHING OR PLATING OF SUBSTRATES.

Field of the Invention The present invention relates generally to pro- cessing of substrates, e. g. by plating or etching.

The invention relates specifically to cleaning of a substrate after completed processing.

The present invention is especially usable in electrochemical plating and etching. Therefore the description of background art, as well as objects of the invention and embodiments thereof, will be given with reference to such electrochemical processing, in particular plating. However, the invention is also applicable to other processing of a surface of a sub- strate, for instance in chemical etching or in dissolv- ing of surface layers, such as resist layers after expo- sure, by means of a solvent or the like.

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

Alternatively, the substrate can be subjected to elec- trochemical etching. In both cases, such electrochemi- cal 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 received 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 instance 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.

Alternatively, 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 arranged 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.

In a conventional plating chamber, such as described in e. g. US-A-5,427,674, the anode is formed as a basket of titanium, which carries balls or rods of the metal to be deposited on the substrate. At the same rate as metal ions are deposited on the substrate from the electrolyte, new metal ions are released from the balls or rods to the electrolyte so that its metal ion content is essentially constant. In an alternative embodiment, which is also mentioned in the above US-A-5,427,674, a dimensional stable, plate-shaped anode (DSA) of e. g. platinum-coated titanium is used instead. The plating chamber is provided with a compensating device which is adapted to supply in a dosed manner to the electrolyte a metal hydroxide so as to compensate for the metal ions which are removed from the electrolyte and deposited on the substrate. In this embodiment, a smaller distance between anode and cathode is allowed, which in turn allows higher current density between anode and cathode and also lower power consump- tion.

When the plating process in the chamber is complet- ed, the substrate and the metal deposited thereon are cleaned from electrolyte by means of a flushing fluid, usually clean ion-free water. This cleaning takes place by the chamber being opened and the substrate transfer- red to a separate flushing unit. During cleaning, the flushing fluid will be mixed with electrolyte, and the resulting waste fluid mixed with metal ions will be a residual product that must be taken care of. In many cases, the transfer of the substrate to the flushing unit occurs manually, which means that the operator is exposed to substances in the electrolyte which are injurious to health, such as nickel. In manual handling, there is also a certain risk of contamination of the electrolyte that is present in the plating chamber. An automated cleaning of the plated substrate requires expensive equipment, such as a computer-controlled robot unit, for transfer-

ring the substrate from the plating chamber to the flush- ing unit.

The above problems are also to be found in prior-art apparatus for chemical or electrochemical etching, and dissolution of a surface layer, for instance an exposed resist layer, on a substrate.

Object of the Invention It is an object of the present invention to wholly or at least partly overcome the above problems according to prior-art technique, and in particular to provide a method in processing a substrate as well as a processing apparatus which minimises the emission of environmentally dangerous residual products.

It is also an object to reduce the operator's expo- sure to substances which are injurious to health during cleaning of the processed substrate.

One more object is to minimise the risk of contami- nation of the processing fluid, i. e. the electrolyte, the etchant or the solvent, during cleaning of a substrate processed with this processing fluid.

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

Preferred embodiments are defined in the dependent claims.

An advantage of the inventive method is that the substrate is flushed with a flushing fluid in situ in the processing chamber. Thus, the working operations for cleaning the processed substrate are significantly simplified. The cleaning can easily be automated without necessitating expensive auxiliary equipment. Moreover, the cleaning takes place in an essentially closed envi- ronment inside the chamber, which means that the envi- ronment's exposure to substances dangerous to health is minimised. Thanks to the fact that the flushing fluid after passing the chamber is cleaned by means of a clean-

ing device and recirculated to the flushing fluid tank, an essentially closed process with a minimum of emissions of environmentally dangerous residual products, such as metal ions, is made possible.

According to a preferred embodiment, any flushing fluid admixed to the processing fluid is separated from the processing fluid, after which the thus separated flushing fluid via the cleaning device is recirculated to the flushing fluid tank. This measure makes it pos- sible to continuously maintain a given quality of the processing fluid.

According to another preferred embodiment, the pro- cessing fluid is fed through the chamber during process- ing, preferably by processing fluid during processing being circulated between a first processing fluid tank and the chamber, while at the same time processing fluid is circulated between the first processing fluid tank and a second processing fluid tank. Thus, processing fluid of a desirable quality can always be supplied to the chamber from the first tank. For instance, in electrochemical processing, the ion concentration of the processing fluid can be adjusted in the second tank, from which the pro- cessing fluid can be supplied via a filter to the first tank, so that a clean processing fluid of optimal concen- tration is available therein. Moreover the processing fluid can be kept at a given level in the first tank, for instance by means of an overflow connected to the second tank, in order to eliminate the arising of variations in the flow of processing fluid through the chamber. More- over, flushing fluid admixed to the processing fluid can be separated from the processing fluid in the second tank. This can easily be carried out by a gas flow being supplied above the processing fluid in the second tank and by this gas flow being received and cooled in a con- denser to form a condensate containing the admixed flush- ing fluid.

According to another preferred embodiment, especial- ly intended for electrochemical plating, the ion concen- tration of the processing fluid, i. e. the electrolyte, in the second tank is adjusted by a dosed supply of alkaline anions and metal ions which are identical with metal ions present in the electrolyte. This embodiment is particu- larly preferred when using a dimensionally stable anode in the chamber. Thus, a constant metal ion content can easily be maintained in the electrolyte. Preferably, a filter is arranged between the second tank and the first tank, so that undissolved residues of the added ions can be prevented from reaching the first tank and from this the chamber. Preferably, the anions and metal ions can be supplied as a slurry of an alkaline metallic salt. The supply of the alkaline metallic salt is controlled pre- ferably based on the pH of the electrolyte received in the first or second tank. Consequently, simple and accu- rate control is allowed. During the plating process, hydrogen ions in fact form at the anode in proportion to the amount of deposited metal ions on the substrate, which means that the pH also indicates the metal ion content of the electrolyte. It is preferred for the alkaline metallic salt to contain anions in the form of carbonate ions, hydroxide ions, oxide ions or oxide hydroxide ions since these are capable of neutralising the formed hydrogen ions without giving rise to undesired residual products in the electrolyte.

According to another preferred embodiment, the flushing fluid is cleaned after passing the chamber in the cleaning device by separating ions dissolved in the flushing fluid, and said ions are recirculated to the processing fluid, preferably to the second tank. This permits an essentially fully closed process with minimum amounts of residual products. The recirculation of the ions can take place manually or automatically.

According to another preferred embodiment, the flushing fluid fed through the chamber is directed during

a first period of time to the first or the second tank and during a subsequent second period of time via the cleaning device to the flushing fluid tank. Thus, the amount of processing fluid that is admixed to the flush- ing fluid can be minimised. This is advantageous since as a rule it is easier to remove flushing liquid, typically water, from the processing fluid than processing fluid from the flushing fluid.

According to a preferred embodiment, the flushing fluid fed through the chamber is transferred to a secon- dary tank, and the flushing fluid is supplied from the secondary tank to the flushing fluid tank via the clean- ing device, which collects the ions dissolved in the flushing fluid. By contaminated flushing fluid being sup- plied to the secondary tank, flushing fluid of a desir- able quality is always available in the flushing fluid tank. Moreover the flushing fluid can be kept at a given level in the flushing fluid tank, for instance by means of an overflow connected to the secondary tank, in order to minimise the risk of production troubles during the flushing of the substrate.

It is also preferred for a subatmospheric pressure to be generated in the chamber during processing. The risk of processing fluid leaking from the chamber is thus easily eliminated. Furthermore the sealing of the chamber can be simplified since the subatmospheric pressure auto- matically strives to keep the parts of the chamber toge- ther.

According to a preferred embodiment, the processing fluid circulating between the first and second tanks passes through an ejector, which during generation of a subatmospheric pressure in the chamber draws electrolytes from this and thus causes the processing fluid to circu- late between the first tank and the chamber. Consequently the number of pumps or the like can be minimised since one pump can perform the circulation of processing fluid both between the first and second tanks and between the

first tank and the chamber. Furthermore a subatmosphe- ric pressure in the chamber can easily be achieved, for instance by suitable adaption of the suction effect of the ejector to the dimension of the processing fluid inlet of the chamber.

According to one more preferred embodiment, the cir- culation of processing fluid between the first and second tanks is interrupted after completed processing, whereby also the circulation of processing fluid between the first tank and the chamber is interrupted, after which flushing fluid is supplied from the flushing fluid tank to the chamber so as to generate a pressure above atmos- pheric in the chamber, which is thus opened and allows the flushing fluid to flow down into a collecting vessel.

This method is simple and robust since the chamber is automatically opened when being filled with flushing fluid. Conveniently the chamber is opened only to such an extent that the flushing fluid can flow out, in order to minimise the environment's exposure to substances injurious to health.

The above discussion of preferred embodiments of the inventive method is correspondingly applicable also to the apparatus according to the invention.

Brief Description of the Drawings The drawings will now be described in more detail with reference to the accompanying drawings which by way of example illustrate a currently preferred embodiment relating to plating.

Fig. 1 is a schematic view of an embodiment of a plating apparatus according to the invention.

Fig. 2 shows in more detail a plating apparatus corresponding to the embodiment in Fig. 1.

Description of a Preferred Embodiment The preferred embodiment of an inventive plating apparatus will in the following first be described gene- rally with reference to Fig. 1. The plating apparatus comprises a plating chamber 1 with an anode 2 and an

opposite cathode holder (not shown), which carries the substrate S which is to be plated. A voltage source (not shown) is connected to the anode 2 and the cathode holder in order to establish during the plating process an elec- trical field between the anode 2 and the substrate S, which acts as a cathode. An electrolyte feeding system 10 communicates with the chamber 1 to feed an electrolyte through the chamber 1 during plating. A flushing water system 20 also communicates with the chamber 1 in order to feed flushing water through the chamber 1 after com- pleted plating, for washing of the plated substrate S.

Valves 30,31,32 are arranged to selectively control the feeding of electrolyte and flushing water to and from the chamber 1. The flushing water system 20 comprises a cleaning device 20'which is designed to separate ions from incoming flushing water. A conduit 33 extends from the cleaning device 20'to the electrolyte feeding system 10 for transferring separated ions to the same. The elec- trolyte feeding system 10 contains a concentrating device 10'which is designed to separate water from the electro- lyte. A conduit 34 extends from the concentrating device 10'to the flushing water system 20 for transferring separated fluid to the same.

The apparatus shown in Fig. 1 generally operates in the following fashion. During plating, the valves 30,32 are closed and the valve 31 open, and electrolyte is cir- culated continuously between the electrolyte feeding sys- tem 10 and the chamber 1. The system continuously condi- tions the electrolyte, inter alia with regard to its metal ion content, pH and temperature. After completed plating, the plated substrate S is flushed in situ in the chamber 1 by the valve 30 being opened and flushing water being pumped into the chamber 1. A non-return valve 35 prevents flushing water from flowing into the electrolyte feeding system 10. During a first period of time, the flushing water is allowed to press electrolyte out of the chamber 1 and into the system 10. When almost all

the electrolyte has been pressed out, the valve 31 is closed and the valve 32 is opened, so that flushing water and the remaining electrolyte are allowed to circulate between the flushing water system 20 and the chamber 1.

The plated substrate S is flushed clean from electrolyte.

The cleaning device 20'of the system continuously cleans the flushing water received from the chamber 1. In the meantime the concentrating device 10'operates to sepa- rate any flushing water admixed to the electrolyte and transfer this to the cleaning device 20'of the system 20. Subsequently the apparatus is ready to perform a new plating process. The plating apparatus is suitably con- trolled by a main control unit (not shown).

The construction and function of the apparatus in Fig. 1 will now be described in more detail with refe- rence to Fig. 2.

The electrolyte feeding system 10 comprises a first tank 11, which via a feeding duct 12 communicates with the chamber 1, and a second tank 13, which via a pump 14, an ejector 15 and a particle filter 16 communicates with the first tank 11. The suction side of the ejector 15 communicates with an electrolyte outlet of the chamber 1.

The first tank 11 has an overflow 17 which communicates with the second tank 13. The concentrating device 10'of the system 10 comprises an air fan 11'which communicates with the upper part of the second tank 13, and a cooling tower 12'extending from the upper part of the tank 13.

The first tank 11 is provided with a heater 18A for heat- ing the electrolyte, a temperature sensor 18B and a pH meter 18C.

In the preferred embodiment, the anode 2 (Fig. 1) in the chamber 1 is a dimensionally stable anode which is essentially inert relative to the electrolyte. The metal ions in the electrolyte which are consumed during plating are replaced in this case by means of a compensating device 40 which prepares a water-based slurry containing

corresponding metal ions. A pump 41 is designed for dosed supply of the slurry to the second tank 13.

The choice of electrolyte is of course controlled by which metal is to be deposited on the substrate S (Fig. 1). In nickel plating, the electrolyte can be based on e. g. nickel sulphamate and water. In this case hydro- gen ions are formed at the anode 2 during plating while at the same time nickel ions are precipitated on the sub- strate S. These hydrogen ions are neutralised by supply- ing an alkaline substance. This takes place by means of the compensating device 40, which prepares the slurry of an alkaline metallic salt, in this case nickel carbonate, nickel hydroxide, nickel oxide or nickel oxide hydroxide.

The anions of the salt react with the hydrogen ions and form, depending on the salt chosen, water or carbon dioxide. Owing to the metal ions and the alkaline sub- stance being supplied in the form of a salt, metal ions can be supplied in a controlled fashion to the electro- lyte based on the pH of the electrolyte measured by the meter 18C.

The flushing water system 20 comprises a clean water tank 21, which via a feeding pump 22 and the valve 30 communicates with the feeding duct 12 and the chamber 1, and a secondary tank 23, which via a pump 24 and the cleaning device 20'communicates with the clean water tank 21. The cleaning device 20'is an ion filter, for instance a filter for reverse osmosis. The cleaning device 20'is designed to separate ions from the flushing water and has an ion outlet which via the duct 33 is con- nected to the compensating device 40 for transferring ion concentrate to this. The cleaning water tank 21 has an overflow 25 which communicates with the secondary tank 23. The secondary tank 23 communicates with the cooling tower 12'via the duct 34 for receiving condensed water from the same.

The plating chamber 1 has a pressure-controlled outlet (not shown) which is closed at subatmospheric

pressure in the chamber 1 and is open at pressure above atmospheric in the same. For instance, the plating cham- ber 1 may consist of two parts which are held together at internal subatmospheric pressure and which automatically are separated at internal pressure above atmospheric. A collecting vessel 50 is arranged under the plating cham- ber 1 for receiving electrolyte and flushing water from the pressure-controlled outlet, as will be described in more detail below. The collecting vessel 50 communicates via the respective valves 31,32 with the first tank 11 and the secondary tank 23.

The plating apparatus shown in Fig. 2 operates as follows. During plating, the valves 30,31,32 are clos- ed. The pump 14 feeds electrolyte from the second tank 13 through the ejector 15 to the first tank 11. The ejec- tor 15 draws electrolyte from the chamber 1 into the first tank 11, and the suction effect of the ejector 15 causes electrolyte to circulate in a loop between the first tank 11 and the chamber 1. This loop is designed in such manner that a subatmospheric pressure is established in the chamber 1. The filter 16 separates any particles in the electrolyte. Via the overflow 17, a stable elec- trolyte level is established in the first tank 11. The compensating device 40 feeds the slurry to the second tank 13 based on the output signal of the pH meter 18C.

The concentrating device 10'separates continuously or intermittently water from the electrolyte in the second tank 13 and transfers this water to the secondary tank 23.

After the plating has been completed, the plated substrate S is flushed in situ in the chamber 1 by the pump 14 being stopped, the valve 30 being opened and the feeding pump 22 being started to pump flushing water into the chamber 1. The non-return valve 35 prevents the flushing water from flowing into the first tank 11. The water flowing into the chamber 1 increases the pressure in the chamber 1 so that its pressure-controlled outlet

(not shown) is automatically opened and the electrolyte is pressed out into the collecting vessel 50. At the same time the valve 31 is caused to open so that the electro- lyte is allowed to flow down into the first tank 11. When almost all the electrolyte has been pressed out of the chamber 1, the valve 31 is closed and the valve 32 is opened, so that flushing water and the remaining electro- lyte are allowed to flow down into the secondary tank 23.

The chamber 1 is now passed by flushing water, and elec- trolyte is flushed away from the plated substrate S. In the meantime the pump 24 is started, and contaminated flushing water is fed from the secondary tank 23 through the filter 20'to the clean water tank 21. A stable water level is maintained in the clean water tank 21 by means of the overflow 25. During the flushing process, the fan 11'is activated to feed a flow of air above the electro- lyte in the second tank 13. This flow of air entrains water vapour from the electrolyte and flows up in the cooling tower 12', in which the water vapour is made to condense. The formed condensate is transferred to the secondary tank 23, from which it is pumped via the filter 20'to the clean water tank 21 which always contains essentially ion-free flushing water.

Thus, according to the invention a method and an apparatus are provided which in a cost-efficient and simple manner minimise the emission of residual products which are harmful to the environment. Moreover, the ope- rator's exposure to substances dangerous to health when cleaning the plated substrate is reduced. Also the risk of contamination of the electrolyte when cleaning a plat- ed substrate is minimised.

It should be noted that the invention is not restricted to the above detailed description of the currently preferred embodiment, and can be modified within the scope of the appended claims. The apparatus described above can to a large extent also be used for electrochemical etching of a substrate. The basic prin-

ciples of the invention can also be applied to chemical etching of a substrate, and to dissolving a surface layer on a substrate, for instance after exposing a resist layer, for masking the substrate before a subsequent etching or plating step.

It may also be noted that in some applications there may be reason to operate with a pressure above atmosphe- ric in the processing chamber.

In some applications, the chamber need not be conti- nuously passed by a processing fluid during processing.

For example, it may in some kind of electrochemical etch- ing be sufficient to supply processing fluid to the cham- ber before the actual processing starts.