FIELD OF THE INVENTION
The present invention relates to etching and concerns especially a method for etching as defined in the preamble to claims 1 and 3, an etching fluid as defined in the preamble to claim 16, and use as defined in claim 17 as well as an apparatus for etching as defined in claim 18. Moreover, the invention relates to a plating method as defined in the preamble to claims 19 and 20.
The invention is especially applicable to the manufacture of matrices for optical storage media and in the electronics industry in the manufacture of printed boards and integrated circuits.
In many contexts it is desirable to provide small structures in the surface of a material. A known method for removing material in small dimensions is etching. A common field using etching is the production of electric conductors on printed circuits by removing portions of an electrically conductive layer.
In e.g. the electronics industry there is a need of producing smaller and smaller components, for instance by removing material in very small dimensions by etching. Today it is possible to produce etched structures having a width and depth below 1 μm.
For producing such small structures by etching, it is also desirable to remove material to varying degrees in different directions, i.e. to control the etching effect of an etchant in different directions. Etching with the same etching effect in all directions is generally called isotropic etching, whereas etching with a varying etching effect is called anisotropic etching.
When making small structures by etching, use is today made of different etching methods. These can be divided into dry methods and wet methods .Dry etching methods include, for instance, ion-beam etching which is a mechanically machining method, and plasma etching which is a chemically machining method. Wet etching methods include chemical etching and electrochemical etching.
In mechanical methods,.such as ion-beam etching, a surface which is to be etched is bombarded with high-energy ions. The ions remove atoms from the surface mechanically. Such etching thus is anisotropic.
In chemically acting dry etching methods, such as plasma etching, the ions are guided to a surface of a substance to be etched by means of an electric field. Such etching is carried out mainly by chemical reactions and therefore is not as anisotropic as purely mechanical etching methods. Some degree of mechanical etching can also take place in chemically acting etching.
In plasma etching, an electric field is applied over a gas. The field is strong enough to make the gas be converted and ionised to form a reactive plasma. Reactive ions are passed by the-electric field to a surface to be etched and react therewith in an etching manner.
Dry etching methods are today used in the electronics industry for production of electronic components. Anisotropic etching of structures in small dimensions, 1 μm and less, can be effected.
A serious drawback of the dry etching methods used today is that they are difficult to control since a large number of variables which affect the etching must be kept within strict tolerances. Thus, the technical equipment will be complicated and expensive, The cost of the equipment will also be affected by the size of the workpiece that is to be etched and increases significantly if the equipment is to be dimensioned for the handling of large workpieces.
When producing electronic components, such as integrated circuits and semiconductor components, great requirements are placed on the purity of the components. This requires in dry etching methods, especially in mechanically acting methods, careful cleaning of the etched material, since it has usually been contaminated by residual products. The cleaning operation itself involves an additional step, which besides being time-consuming also requires the use of cleaning agents which in turn have a negative influence on the environment.
To prevent the areas which are not be etched from being affected by the etching, it is common when etching very small structures and necessary in mechanically acting etching methods to mask these areas with a protective layer, called resist. When mechanically affected, for instance by ion bombardment, also the protective layer will be affected during etching. This results in two drawbacks, on the one hand that the protective layer must be very thick so as not to be removed completely in the course of etching and, on the other hand, that the contour of the protective layer towards the surface to be etched becomes uneven owing to the removing effect of the ions, which results in an uneven etched contour.
The substance to be etched is masked also in wet etching methods. Then the substance is immersed in an etching fluid containing an etchant, which, when contacting the substance, is capable of etching.
In chemical etching use is made of an etching fluid containing a solution of an etchant which is capable of etching, by spontaneous chemical reaction, a substance, i.e. the etchant will etch directly when contacting the substance to be etched. The etching occurs isotropically. The etching rate is affected by etching time, temperature and concentration of etchant. The etching fluid usually contains an oxidising agent, for instance BR2, H2O2, HNO3, a completing agent, for instance H2SO4, HF, NaOH, and a solvent, for instance water or methanol. Examples of generally used and recommended compositions of etching solutions for different metals are disclosed in, for instance, “Handbok i metallmikroskopiering” (in English: Handbook of Metallographic Microscopy”), Helfrid Modin and Sten Modin (1977, Meritförlaget, Johanneshov, Sweden). Typical concentrations of etchant for etching small Structures, microstructures, are, for etching of e.g. chromium or copper, 0.8-1.2 M.
Some solvents dissolve a given crystal plane in a substance to be etched more rapidly than other planes, for instance in a semiconductor material, which results in a directionally dependent etching effect, i.e. anisotropic etching.
In electrochemical etching, the etching fluid contains an electrolyte, e.g. a salt solution, which in itself is not capable of etching the substance to be etched by spontaneous chemical reaction, i.e. the etchant does not etch merely by contacting the substance. By applying an electric voltage in the etching fluid between the substance to be etched and an electrode immersed in the etching fluid, an electrolytical process, however, will be begun, in which the substance to be etched is the one pole, usually the anode, and the electrode the other pole. In the electrolytical process, electric current flows in the etching fluid, and ions in the etching fluid react in an etching manner with the substance to be etched.
The etching rate is essentially proportional to the strength of current. The etching will be slightly anisotropic, although not to the same great extent as is possible in dry etching methods. For instance, it is possible to etch in an electrochemical manner structures having a depth-to-width ratio of 1:2.
Several techniques are known for applying voltage in pulses to obtain a good etching effect when using different electrolytes as etching fluid.
In wet etching methods, above all in chemical etching, so-called underetching occurs owing to the isotropic etching properties, i.e. etching off material under the surface that is coated with a protective layer. As a result, it is not possible in purely chemical etching to produce grooves or lines having a greater depth than width. Nor is it possible in electrochemical etching to etch, in case of small dimensions, grooves or lines whose depth exceeds the width. The possibilities of making narrow grooves, e.g. in order to arrange conductors closely together, are thus restricted when using wet etching methods. Furthermore, it is today not possible to produce by wet etching methods even structures, for instance grooves having straight walls, whose width or depth is less than 1 μm.
In wet etching, use is today generally made of fluids which are strongly toxic and harmful to the environment, which in itself is an environmental problem.
Purely chemical etching is also a process which is difficult to control since a plurality of parameters influence the speed of the process.
In electrochemical etching, all surfaces to be etched must be connected to an electric pole during the entire etching procedure. When making printed circuits, this is done by all conductors being interconnected at a connection point during the etching procedure. After completion of the etching, the connection point is removed mechanically in a special production step.
Many experiments have been made, however not quite successful so far, to provide a wet etching method that can be used in the production of small electronic circuits, such as integrated circuits.
OBJECTS OF THE INVENTION
One object of the present invention is to provide a new and enhanced method for etching by eliminating the above-mentioned drawbacks of the prior-art technique.
A special object is to provide an enhanced method for etching small structures, essentially structures which in one or more directions have a dimension less than 50 μm, and above all structures which in one or more directions have a dimension less than 10 μm.
A further object is to provide a method for wet etching, which permits etching of smaller structures than before, especially structures which in one or more directions have a dimension less than 1 μm .
A special object is to provide a method for wet etching, which permits anisotropic etching of small structures.
A particular object is to provide an easily controllable method for etching small structures.
A further object of the invention is to provide an enhanced method for plating.
SUMMARY OF THE INVENTION
According to the invention, these and other objects that will appear from the following description will be achieved by a method, an etching fluid, the use thereof, an apparatus for carrying out the method and a plating method, which are of the types described by way of introduction and which, in addition, have the features defined in the characterising clause of claims 1, 3, 16, 17, 18, 19 and 20, respectively.
The invention is based on the surprising discovery that an etching fluid which has been diluted to have a negligible etching effect, can be used for anisotropic etching while subjected to an electric field.
In one aspect, the invention concerns etching of an electrically conductive substance by means of an etchant, which is present in a dilution which is diluted to such an extent that it is not practically usable for chemical etching. The concentration of the etchant is so low that such reactions between the etchant and the substance to be etched as result in the removal of atoms from the substance to be etched occur sporadically only. By producing an electric field in the etchant solution between an electrode and a surface portion of the substance to be etched, there is formed a local concentration of etchant on the surface portions of the substance to be etched. This results in a significant increase of the etching rate of the etchant, at the same time as the etching direction of the etchant is affected.
It is also possible to regard the invention as a method of transferring the conditions prevailing in dry etching methods to a wet environment in an etching fluid, In this manner, the advantages of dry and wet etching methods have been combined, while eliminating the drawbacks of the respective methods.
The invention relates to etching of an electrically conductive substances Experiments have been made using various metals such as Cu, Ni, Ti, Al and Cr, but the inventive method is expected to work for other conductive materials, such as alloys, and for semiconductors. The electrical conductivity of the substance to be etched should be such as to allow an electric field to form in the dilute solution between the substance to be etched and an electrode.
The crystal structure of the substance to be etched is not critical, and the substance to be etched can thus be monocrystalline as well as polycrystalline.
The etchant should be capable of reacting, in solution, in an etching manner with a surface, intended to be etched, of the substance to be etched. Besides it is assumed that the etchant should be of such a nature as to be kinetically affected by an electric field, thereby permitting a local concentration of the etchant.
An important feature of the invention is that the etchant is present in solution of low concentration. On the basis of the experiments that have been carried out, it seems difficult to achieve an anisotropic etching effect in concentrations of the etchant above 200 mM. However, it has not been possible to determine a lower limit of concentration for a satisfactory function. It is also assumed that the etchant must have sufficient movability in the solution to permit a local concentration of the etchant.
It is assumed that the electric field has two functions, concentrating the etchant locally and accelerating the etching, of which the first-mentioned function is presently assumed to be the most important.
It is supposed that the electric field should be directed to the surface of the substance that is to be etched. To make it possible to locally increase the concentration of the etchant, the extent of the electric field adjacent to the surface that is to be etched should be relatively restricted.
It is preferred that the etchant, at least in concentrated solution, is capable of etching the substance to be etched in the absence of an electric field, i.e. that the etchant is capable of spontaneous chemical etching of the substance to be etched. Even if the new, low concentration of the etchant according to the invention in certain applications can be expected to confer advantages also in connection with electrochemically etching etchants, the results which have been best so far have been obtained in experiments with chemically etching etchants.
In view of what has been said above, the invention may also be regarded as a method for anisotropic etching of a structure in an electrically conductive substance to be etched by means of an etchant which in concentrated solution is usable for isotropic etching of structures in the substance to be etched, said method being characterised by the steps of contacting the substance to be etched with the etchant in a solution which is so diluted that the resulting etching rate implies that the etchant is unusable for said isotropic etching of structures; and subjecting the etchant adjacent to the substance to be etched to an electric field of such a strength that anisotropic etching of the substance to be etched is accomplished at an etching rate which is relevant for producing said structure in the substance to be etched.
The invention is especially directed to the production of small structures in the order of 50 μm and less in respect of etching width as well as etching depth. The invention has been found especially advantageous when producing structures whose width or height is less than 10 μm.
By the solution having an extremely low concentration of etchant and the relevant etching process occurring under the action of an electric field, the etching process obtains an essentially improved controllability and anisotropy compared with prior-art wet methods. This makes it possible to produce and use small etched structures, on the one hand for known constructions and, on the other hand, in new technical fields.
An important property of the invention is that it is possible to etch lines and grooves having a greater depth than width. In experiments, the depth-to-width ratio of an etched groove has been measured to be 3.5:1 when etching a thin copper foil.
The method is inexpensive and requires but relatively simple equipment. Since the etching fluids used have a low concentration of etchant in solution, the etching fluids can be made practically non-poisonous, which results in benefits in the working environment as well as in the exterior environment.
Moreover, the inventive method exhibits low sensitivity to variations in temperature. For example, excellent results have been obtained when etching in the temperature range of 15° C.-30° C. In this range, no considerable temperature influence could be demonstrated, and it is therefore assumed that the desired result can be achieved within a considerably wider range of temperature.
The method does also not exhibit any critical sensitivity to variations in concentration within an effective range of concentration. Experiments have shown that about a concentration value giving good etching results for a certain combination of etchant/etching fluid, it is possible to change the concentration value by a factor two without the etching result being significantly deteriorated.
The inventive method also permits anisotropic etching of small structures without using a protective layer, resist, on surrounding areas of the substance to be etched, since practically no etching occurs in ream that are not subjected to an electric field.
Preferably, the etching fluid in dilute solution is present during etching in such a state that its capability of etching spontaneously, i.e. in the absence of an electric field, is limited to an etching rate of 5 nm/s. If the etchant etches spontaneously at a higher rate, the process will be difficult to control and relatively isotropic, which when using a protective layer results in underetching.
To provide anisotropic etching, the spontaneous etching capability of the etchant is preferably limited to 4 nm/s at most. In experiments, it has been found that further restrictions of the spontaneous etching rate to 3 nm/s and less give still better results, above all a higher degree of anisotropy and the possibility of etching smaller structures. The maximum spontaneous etching rate that can be permitted with maintained controllability of the etching process depends on the composition of the substance to be etched and the size of the structure to be etched.
For instance, experiments have been made with copper as substance to be etched and an ammonium persulphate solution as etching fluid, which has a spontaneous etching rate of about 3 nm/s. It has been possible to measure a depth-to-width ratio of 3:1 in the groove. In experiments at still lower etching rates, the etching process has become still more controllable, and a width-to-depth ratio of 3.5:1 has been measured.
In other experiments in etching chromium, extremely good results have been obtained with etching solutions having a spontaneous etching rate below 0.3 nm/s.
In a preferred embodiment of the etching method, the etchant, which preferably etches isotropically in the absence of an electric field, is caused to etch, by means of the electric field, anisotropically at a higher rate, preferably at at least the double rate, and more preferred at a rate which is ten times higher. Still better results can be expected when increasing the etching rate further, such as 50 times or 100 times. When etching chromium, the etching rate in the desired direction has been increased from below 0.3 nm/s to above 55 nm/s, thus in the order of 200 times, under the action of an electric field.
The preferred concentration of the etchant is 100 mM at most, preferably 20 mm at most, and more preferred 10 mM at most. It may be generally said that the controllability of the etching process, especially when etching small structures, increases with a reduced concentration of the etchant. In some contexts, it has been found advantageous to have concentrations of the etchant below 2 mM, and especially advantageous to have etchant concentrations of 1 mM and less.
The etchant according to the present invention can preferably be defined as an ionic substance capable of reacting in an etching manner with the substance to be etched. The concentrations that are stated in connection with the invention concern the concentration of the etchant which is active according to the invention.
The step of subjecting the etchant to an electric field preferably comprises contacting an electrode with the etchant and applying a voltage between the electrode and the substance to be etched. The distance between the electrode and the etchant is 3 cm at most and preferably 1 cm at most, and more preferred 1 mm at most. The closer to the surface of the substance that is to be etched the electrode is arranged, the higher the etching rate and the better the controllability in the etching process. Various, still shorter distances down to 4 nm have been tested successfully, It may be generally said that when the area round a surface which is to be etched is covered by a protective layer, the demands placed on the design of the electrode and the distance therefrom are not as high as in the case when no protective layer is arranged. For carrying out anisotropic etching of small structures below 50 μm, it is assumed in the latter case that the electrode should be arranged closer than 50 μm to the surface that is to be etched. It is also assumed that the surface area and surface shape of the electrode are important for controlling the extent of the electric field. It is above all important to concentrate the electric field in the area that is to be etched.
Since the inventive method is a wet etching method, the substance to be etched is not contaminated with material that has been removed by etching, like in dry etching methods, but instead the removed material will be collected adjacent to the electrode.
It is preferred that the voltage between the electrode and the substance to be etched is at least 0.5 V, preferably at least 1 V and more preferred at least 1.5 V, and 10 V at most, preferably 5 V at most and more preferred 3 V at most. Good results have been achieved in the range 2 V-2.8 V and particularly good results have been achieved in the range 2.4V-2.6 V. It is difficult to determine a lower limit for the voltage required for etching, and the above-mentioned values are values where a practically usable etching rate is reached. However, it is important that the voltage is not as great as and reversed to the electrochemical potential between the substance to be etched and the electrode, which implies that all etching effect ceases. It may be generally said that the higher voltage applied, the more rapid the etching. At higher voltage levels, the etching is transformed into polishing, in which case no effective etching can be accomplished. A further increase of the voltage results in uncontrolled discharges in the border line between etching solution and substance to be etched, so-called pitting.
It may be concluded from the experiments carried out that the strength of the electric field is important to the process, but that it is difficult to derive an unambiguous connection for this. Experiments rather demonstrate that the voltage level is more critical for producing a good result.
In a preferred embodiment, the electric field is pulsed such that etching occurs during a plurality of first periods. Between the first periods the field is given a reversed direction during a plurality of second periods.
In a special embodiment, there is preferably applied during these second periods, between the electrode and the substance to be etched, a voltage, the size of which corresponds to and the direction of which is reversed to the electrochemical potential between the electrode and the substance to be etched, in the etchant. A reverse voltage thus arises and stops all chemical etching,
In a further special embodiment, there is applied between said first periods a reverse voltage, the size of which is greater than that of the electrochemical potential. In this context, the etching process is reversed and a plating operation is carried out during the second intermediate periods by a certain amount of previously etched-away substance being returned. As a result, the design of the surface structure can be further controlled.
It is assumed that rapidly passing from a first period to a subsequent second period of one of the types described above ensures that residuals from the etching are released from the surface that is to be etched. If these residuals are allowed to cover the surface of the substance that is to be etched, further etching of this surface is prevented, and therefore the etching will be more isotropic.
Preferably, said first periods are as great as the time interval therebetween and amount to 200 ms at most, preferably 100 ms at most, and at least 10 ms, preferably at least 50 ms. It may be generally said that the pulsing, which serves to release residual products, is most important when etching structures having a greater etching depth than etching width, since it is on these occasions that the anisotropic etching effect is most important. The greater the ratio between etching depth and etching width, the shorter should be the periods.
In a special aspect of the invention, it is also possible to coat, by means of a concentrated electric field and a small electrode arranged adjacent to a material to be plated in a strongly diluted plating solution a surface having small structures in a manner corresponding to that in etching according to the invention. It is thus possible on the one hand to carry out plating as a partial step during etching and, on the other hand, to carry out plating of small structures under purely plating-chemical conditions.