U.S. patent application number 09/795124 was filed with the patent office on 2001-09-27 for method for anisotropic etching of structures in conducting materials.
This patent application is currently assigned to Obducat AB. Invention is credited to Heidari, Babak, Olsson, Lennart.
Application Number | 20010023829 09/795124 |
Document ID | / |
Family ID | 26662742 |
Filed Date | 2001-09-27 |
United States Patent
Application |
20010023829 |
Kind Code |
A1 |
Olsson, Lennart ; et
al. |
September 27, 2001 |
Method for anisotropic etching of structures in conducting
materials
Abstract
A method for plating an electrically conductive substance, which
includes the steps of contacting the electrically conductive
substance with a plating agent in dilute solution, in which the
plating agent is present in a concentration of 200 mM at most, and
subjecting the plating agent adjacent to the electrically conducive
substance to an electric field.
Inventors: |
Olsson, Lennart; (Malmo,
SE) ; Heidari, Babak; (Lund, SE) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT &
DUNNER LLP
1300 I STREET, NW
WASHINGTON
DC
20005
US
|
Assignee: |
Obducat AB
|
Family ID: |
26662742 |
Appl. No.: |
09/795124 |
Filed: |
March 1, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09795124 |
Mar 1, 2001 |
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09262740 |
Mar 5, 1999 |
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6245213 |
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09262740 |
Mar 5, 1999 |
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PCT/SE97/01480 |
Sep 5, 1997 |
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60025138 |
Sep 6, 1996 |
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Current U.S.
Class: |
205/103 ;
204/242; 205/105; 205/107; 205/640; 216/100; 216/102; 216/105;
216/99 |
Current CPC
Class: |
C23F 1/02 20130101; H05K
3/07 20130101; C25F 3/02 20130101 |
Class at
Publication: |
205/103 ;
205/640; 216/99; 216/100; 216/102; 216/105; 205/107; 205/105;
204/242 |
International
Class: |
C23F 001/00; C25F
003/00; C25F 003/04; C25F 003/12; C25D 005/18; C25D 017/00; C25F
007/00; B23H 003/00; B23H 005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 6, 1996 |
SE |
9603260-2 |
Claims
1. 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, characterised
by the steps of contacting the substance to be etched with the
etchant in a solution which is so diluted that the etchant is
unusable for 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 said structure in
the substance to be etched is accomplished.
2. The method for etching as claimed in claim 1, wherein the
etchant is present in a concentration of 200 mM at most.
3. The method as claimed in claim 1 or 2, wherein the etchant, in
concentrated solution, is capable of etching the substance in the
absence of an electric field.
4. The method for etching as claimed in any one of claims 1-3,
wherein the capability of the etchant, in dilute solution, of
etching the substance to be etched in the absence of an electric
field is 5 nm/s at most, preferably 3 nm/s at most.
5. The method for etching as claimed in any one of the preceding
claims, wherein the strength of the electric field is such that the
etchant, in dilute solution, is given an increased etching rate
which is preferably doubled, and more preferred is at least ten
times greater than in the absence of the electric field.
6. The method as claimed in any one of the preceding claims,
wherein the etchant in dilute solution is present in a
concentration of 100 mM at most, preferably in a concentration of
50 mM at most, and more preferred in a concentration below 10
mM.
7. The method as claimed in any one of the preceding claims,
wherein the etchant is an ionic substance having the capability of
reacting in an etching manner with the substance to be etched.
8. The method as claimed in any one of the preceding claims,
wherein the step of subjecting the etchant to an electric field
comprises contacting an electrode with the etchant and applying a
voltage between the electrode and the substance to be etched.
9. The method as claimed in claim 8, wherein the electrode is
arranged at a distance from the surface to be etched, the distance
being 3 cm at most and preferably 1 cm at most, and more preferred
1 mm at most.
10. The method as claimed in claim 8 or 9, wherein the applied
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.
11. The method as claimed in any one of claims 8-10, wherein the
electrode has a tapering portion directed to the electrically
conductive substance and arranged at a distance of 10 nm at most
from the substance to be etched.
12. The method for etching as claimed in any one of the preceding
claims, wherein etching is carried out during a plurality of first
periods, between which the electric field changes.
13. The method for etching as claimed in claim 12, wherein the
electric field, between said first periods, is of reversed
direction during second periods.
14. The method as claimed in claim 13, wherein plating is
accomplished during said second periods, substance to be etched
being returned.
15. The method for etching as claimed in any one of claims 12-14,
wherein between said first periods, measurement of the etched depth
is carried out during third periods, in which no electric field
affects the etchant.
16. The method for etching as claimed in any one of claims 12-15,
wherein 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.
17. An etching fluid, characterised in that it comprises an etchant
in dilute solution, in which the etchant is present in a
concentration of 200 mM at most, preferably 100 mM at most and more
preferred 20 mM at most.
18. Use of an etching fluid as claimed in claim 17 for producing
structures which are 50 .mu.m or less.
19. An apparatus for carrying out the method as claimed in any one
of claims 1-16.
20. A method for plating an electrically conductive substance,
characterised by the steps of contacting the electrically
conductive substance with a plating agent in dilute solution, in
which the plating agent is present in a concentration of 200 mM at
most, and subjecting the plating agent adjacent to the electrically
conductive substance to an electric field.
21. A method for plating a structure on an electrically conductive
substance to be plated by means of a plating agent which in
concentrated solution is usable for isotropic plating of structures
on the substance to be plated, characterised by the steps of
contacting the substance to be plated with the plating agent in a
solution which is so diluted that the plating agent becomes
unusable for isotropic plating of structures, and subjecting the
plating agent adjacent to the substance to be plated to an electric
field of such a strength that an anisotropic plating of the
substance to be plated is accomplished at a plating rate which is
relevant for producing said structure on the substance to be
plated.
22. The method as claimed in any one of claim 20 or 21, wherein
plating occurs during a plurality of second periods, between which
etching occurs during a plurality of first periods.
Description
FIELD OF THE INVENTION
[0001] 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.
[0002] 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.
BACKGROUND ART
[0003] 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.
[0004] 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
.mu.m.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] Dry etching methods are today used in the electronics
industry for production of electronic components. Anisotropic
etching of structures in small dimensions, 1 .mu.m and less, can be
effected.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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 BR.sub.2,
H.sub.2O.sub.2, HNO.sub.3, a completing agent, for instance
H.sub.2SO.sub.4, 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,
Meritforlaget, 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] Several techniques are known for applying voltage in pulses
to obtain a good etching effect when using different electrolytes
as etching fluid.
[0020] 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 .mu.m.
[0021] 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.
[0022] Purely chemical etching is also a process which is difficult
to control since a plurality of parameters influence the speed of
the process.
[0023] 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.
[0024] 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
[0025] 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.
[0026] 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 .mu.m, and above all
structures which in one or more directions have a dimension less
than 10 .mu.m.
[0027] 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 .mu.m .
[0028] A special object is to provide a method for wet etching,
which permits anisotropic etching of small structures.
[0029] A particular object is to provide an easily controllable
method for etching small structures.
[0030] A further object of the invention is to provide an enhanced
method for plating.
SUMMARY OF THE INVENTION
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] The invention is especially directed to the production of
small structures in the order of 50 .mu.m 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 .mu.m.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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.degree.
C.-30.degree. 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.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] 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.
[0052] 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.
[0053] 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.
[0054] 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.
[0055] 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.
[0056] 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.
[0057] 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 .mu.m, it is assumed in the latter case that the electrode
should be arranged closer than 50 .mu.m 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.
[0058] 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.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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,
[0063] 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.
[0064] 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.
[0065] 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.
[0066] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0067] The invention will now be described in more detail with
reference to the drawings, which for the purpose of exemplification
illustrate embodiments of the invention and in which:
[0068] FIG. 1 is a schematic view of an arrangement of an apparatus
for carrying out the inventive method;
[0069] FIG. 2 is a schematic cross-sectional view of an etched
groove; and
[0070] FIG. 3 is a schematic cross-sectional view of an embodiment
of the invention having a pointed electrode.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
[0071] An embodiment of the invention will now be described in more
detail with reference to FIG. 1. A substance 6 to be etched of an
electrically conductive material is immersed in a vessel 2
containing an etching fluid 4. The etching fluid 4 contains an
etchant in dilute solution. Moreover, an electrode 8 is immersed in
the etching fluid 4. The electrode 8 is arranged at a distance from
a surface 7 to be etched of the substance 6 to be etched. A control
unit 12, which comprises a voltage source, is connected between the
electrode 8 and the substance 6 to be etched.
[0072] The control unit 12 preferably is a device which is capable
of controlling current between and voltage across the substance 6
to be etched and the electrode 8.
[0073] Such controlling can be carried out by, for instance, a
computer program. The control unit 12 should be capable of letting
current flow in both directions. Besides, it should be possible,
when there is no current flow between the electrode and the
substance to be etched, to read the voltage that arises through the
electrochemical potential difference between the substance 6 to be
etched and the electrode 8. The latter is intended for reading the
etching depth/topography of the substance to be etched through said
potential difference.
[0074] The etching fluid 4 is chemically etching and the etchant is
capable of etching the substance 6. The etchant is present in the
etching fluid 4 in such a dilute solution that the etching fluid is
not usable for spontaneous chemical etching. The etching fluid can
be prepared, for instance, by diluting 20 times, 100 times or more
a commercially used etching fluid, which in normal use, i.e. in
concentrated form, is usable for chemical etching of the substance
to be etched. The etchant thus reacts in a chemically etching
manner with the substance to be etched. The etchant concentration
in the dilute solution allows merely sporadic etching activities
which result in the removal of atoms from the substance to be
etched.
[0075] A voltage, typically in the order of 1 V-4 V, is applied
between the electrode and the substance to be etched. Thus, an
electric field forms in the etching fluid 4 between the electrode 8
and the surface 7 to be etched of the substance 6.
[0076] The electric field produces a local etchant concentration on
the surface 7 to be etched and increases the tendency of the
etchant to react with the substance to be etched, and therefore the
etchant will, under the action of the electric field, etch the
surface 7 to be etched at a considerably increased etching
rate,
[0077] The substance 6 to be etched is masked adjacent to the
surface 7 to be etched with a protective layer, a so-called resist
layer, with which the etchant does not react. It is common to use a
photoresist layer, which is formed by a per se known, photographic
exposing and developing procedure.
[0078] Two different embodiments will be described below with
reference to an arrangement of the type described above.
Example 1
[0079] In this experiment, chromium was etched with an etchant in
9.11 mM solution, and the experiment comprised the following
steps:
[0080] A board of nickel was coated with a 100 nm-thick layer of
chromium by electrochemical plating. The chromium surface was
coated with a protective layer in the form of a photoresist layer,
whereupon a pattern (lines and points having a width of 0.5 .mu.m)
was exposed and developed for etching of uncovered surfaces of the
substance to be etched.
[0081] The etching fluid was prepared by mixing the etchant
Ce(NH.sub.4).sub.2(NO.sub.3).sub.6 in a quantity of 2 g in 1 ml
CH.sub.3COOH and 500 ml deionised water (18 M.OMEGA. water) and was
filtered in a 0.2 .mu.m filter. The chemical etching rate of the
solution then was <0.3 nm/s.
[0082] A flat electrode of TiN used as cathode and the Ni-Cr-board
used as anode were immersed in the etching fluid together with a
third electrode (chemically stable) which was used as reference
electrode for voltage measurement. The cathode and the anode were
applied in plane-parallel configuration and at a distance of 1 mm
from each other.
[0083] A device for generating current in the fluid between cathode
and anode was connected.
[0084] The potential of the anode was set at +0.7 V and the
potential of the cathode was set at 0 V, which resulted in a total
etching stop, whereupon a pulse having the length 100 ms and the
amplitude +2.5 V was applied to the cathode. The temperature was
constantly kept at 25.degree. C.
[0085] When carrying out the experiment, the chromium layer was
completely etched through after two pulses (thus, after less than
200 ms effective etching time). Then the cathode voltage was again
set at 0 V in order to avoid underetching. No underetching could be
measured.
[0086] This experiment is one in a series of etching experiments in
etching chromium. In the accompanying Table I, some test parameters
are stated. Etching fluids having an etchant concentration of up to
45 mM have been tested with excellent results. The best results,
i.e, perpendicular and even edges with no noticeable underetching,
were obtained with concentrations below 25 mM . Etching while using
constant direct current, 50 ms pulse and 100 ms pulse, gave similar
results. The experiment described above resulted in the lowest
etchant concentration and the best etching result.
Example 2
[0087] In this experiment, a 5 .mu.m-thick copper layer on a
commercial laminate was etched with an 87.72 mM etching fluid. The
experiment comprised the following steps:
[0088] A pattern (5 .mu.m to 25 .mu.m thick lines) for etching of
uncovered surfaces of the substance to be etched was produced in a
manner corresponding to the above described experiment by means of
a photoresist layer on the laminate.
[0089] The etching fluid was prepared by mixing 2 g ammonium
persulphate with 100 ml deionised water. The chemical etching rate
of the solution then was <3 nm/s.
[0090] A flat electrode of TiN, which was used as cathode, and the
copper laminate, which was used as anode, were immersed in the
etching fluid together with a third electrode (chemically stable),
which was used reference electrode for voltage measurement. The
cathode and the anode were applied in plane-parallel configuration
and at a distance of 2 mm from each other.
[0091] A device for generating current in the fluid between cathode
and anode was connected.
[0092] The potential of the anode was set at -0.3 V and the
potential of the cathode was set at 0 V, which resulted in a total
etching stop, whereupon a pulse having the length 50 ms and the
amplitude +3.5 V was applied to the cathode.
[0093] A slight flow of etching fluid was pulsed across the copper
surface in a manner synchronised with the lower value of the
cathode pulse.
[0094] The copper layer was completely etched through after 180
pulses (thus, after less than 9 s effective etching time). Then the
cathode voltage was again set at 0 V in order to avoid
underetching. The total underetching was measured to be 1 .mu.m
maximum.
[0095] A corresponding experiment in etching of copper has also
been carried out with a corresponding etching fluid at 22 mM
concentration of etchant. This resulted in no appearance of
underetching. The test parameters are stated in the enclosed Table
II.
[0096] In the experiment according to Example 2, the potential of
the anode and cathode has been varied such that the potential of
the anode, after carrying out some etching, was set at a lower
value than -0.3 V during periods when the potential of the cathode
was 0. In this context, plating-chemical conditions will form, in
which the liquid with etched-away substance acts as a plating agent
and etched-away substance is returned to the open copper surfaces.
By the thus-achieved plating during periods between periods when
etching is carried out, the surface structure of the etched
surfaces is changed.
[0097] The following conclusions have been drawn from the
experiments described above:
[0098] a) Etching fluids etching chemically at a rate of up to 3
nm/s and probably also higher and/or having a concentration of
etchant of up to 90 mM and probably also higher are extremely
usable for accomplishing anisotropic etching of structures in the
order of 5 .mu.m.
[0099] b) The smaller structures to be etched, the lower chemical
etching rate of the etching fluid and the lower concentration of
the etchant are required for anisotropic etching.
[0100] It has not yet been possible to unambiguously determine what
laws determine the etching according to the inventive method.
Without binding the invention to a specific theory, it is assumed
that the invention functions as follows.
[0101] The etching fluid has such a low concentration of the
etchant that the reactive ions get in contact with the substance to
be etched sporadically only and cause isotropic etching reactions
on the surface of the substance to be etched, By applying a voltage
in the etching fluid between an electrode and a limited surface of
the substance to be etched, an electric field and a local
concentration of active ions are produced. The electric field also
accelerates active ions so as to give them a speed towards the
substance to be etched.
[0102] The etching fluid has, in its capacity as ion solution,
electrical conductivity, and therefore a current flows between the
electrode and the substance to be etched when voltage is applied
therebetween. Under the action of the current, the capacity of the
active ions of reacting in an etching manner with the substance to
be etched increases by the current accelerating the etching
reactions between the active ions and the substance to be
etched.
[0103] The anisotropic properties of the etching method are assumed
to be caused mainly by the electric field initially giving the
active ions a speed in the direction of the field. When etching,
for instance, a groove which has already been etched to a certain
depth, i.e. such that the groove already has a bottom and two
walls, anisotropic etching requires that the active ions which are
passed into the groove be mainly caused to etch the bottom of the
groove. If the active ions should follow the field lines of the
electric field, a great part of the ions would be caused to etch
the walls of the groove. Since this is not the case in the
inventive method, it is assumed that the inertia of masses of the
active ions in the groove prevails over the effect of the electric
field.
[0104] By using very low concentrations the process will be
controllable. Too high concentrations result in relevant etching
taking place also in the absence of an electric field. Besides, it
is not possible with initially high concentrations to achieve
satisfactory anisotropy during etching, since the electric field
produces an uncontrollable concentration of active ions.
[0105] Reference is now made to FIGS. 2a-d. An inconvenience when
etching is that residual products, which form as the active ions
react in an etching manner with the substance to be etched, form a
blocking layer after some time, thereby preventing further active
ions from contacting the substance to be etched. FIG. 2a
illustrates how ions in the etching fluid are pulled in different
directions under the action of the electric field. In FIG. 2b,
active ions 14 have reached the substance 6 to be etched and have
begun the etching and the forming of residual products 16. FIG. 2c
illustrates how a blocking layer of residual products 16 has been
formed. The forming of such a blocking layer also prevents
anisotropic etching. The inconvenience is particularly pronounced
in anisotropic etching of a bottom surface in a groove. This
inconvenience is obviated by changing the direction of the electric
field, such that the active ions 14 are pulled away from the
substance 6 to be etched in the direction of the electrode 8. This
is illustrated in FIG. 2d. As a result, the blocking layer is
broken and the residual products 16 can be spread in the etching
fluid. Even if the blocking layer is broken at regular intervals by
changing the direction of the electric field, the removal of
residual products 16 is assumed to be the limiting factor for
anisotropy when etching small grooves having a greater depth than
width. For maintaining anisotropy as far as possible when etching
deep structures, it is preferred to let the electric field change
direction by pulsating the voltage. The deeper the structure, the
shorter etching pulse time is required.
[0106] By changing the voltage in pulses of different wave shape,
different etching geometries will form. In the special embodiment
of the invention when plating is carried out between periods of
etching, particular possibilities of additional process control are
achieved.
[0107] Only small currents are required to obtain a good etching
result with an electrically conductive etching fluid. Thanks to
this, all surfaces that are to be etched need not be connected to a
common electric pole during the entire etching procedure. In the
presence of a surface, which is to be etched and is connected to
the electric pole, of the substance to be etched, the etching fluid
has in fact the capacity of conducting small currents to another
surface to be etched of the substance and thus connecting this
other surface to be etched to the electric pole.
[0108] This property is usable, for instance, when producing narrow
grooves in an electrically conductive substance to be etched, which
is present in a thin layer on an insulating material. The entire
layer to be etched is, in the initial etching phase, electrically
connected to the same electric pole. When the groove has been
etched throughout along a line between the edges of the groove, the
electric connection between the edges of the groove is interrupted.
In the etching method according to the invention, small currents
will continue to be conducted between the surfaces of the substance
to be etched, for instance between the two sides of the groove and
possibly also a remaining intermediate portion of the substance to
be etched, and therefore the etching procedure continues until one
chooses to stop it, when the etching is completed. Therefore no
interconnection of a plurality of separate remaining portions is
necessary.
[0109] Special embodiments of the inventive method will now be
described.
[0110] When etching a substance consisting of two or more
superimposed layers of conductive material having different
electrochemical potentials, it is possible to easily determine the
etched depth. This is accomplished by determining, in the absence
of an applied voltage, the galvanic voltage between the substance
to be etched and a reference electrode. When the substance to be
etched comprises two different materials which are, to a different
extent, in electrically conductive contact with the etching fluid,
different galvanic voltages will form in the fluid.
[0111] In a special embodiment of the invention, shown in FIG. 3,
the electrode 8 is formed with a tapering portion 18 directed to
the electrically conductive substance 6. The electrode has a tip
which is insulated with an electrically insulating layer except at
its outer end 20. By using such a pointed electrode 6, it is
possible to etch very small structures anisotropically without the
presence of a layer protecting against etching. In experiments with
such a pointed electrode at a distance of 4 nm from the substance
to be etched, it has been possible to etch grooves having a width
of 35 nm without using a layer protecting against etching.
[0112] By means of the invention and various embodiments thereof, a
great number advantages are achieved. For instance, it may be
mentioned that the method is extremely gentle, on the one hand
since the etching solution is strongly diluted and, on the other
hand, since only low voltages are used. This results in the layer
protecting against etching not being subjected to any considerable
action during etching. It is therefore possible to use a very thin
layer protecting against etching, down to molecular thickness, of
an insulating substance. Moreover, it is possible to maintain even
edges of the protective layer adjacent to a surface to be etched
during etching, and thus to etch even contours, also in very small
structures, down to nanometer level. Up to now, this has not been
possible to achieve by using a prior-art etching method.
[0113] The inventive method is also more rapid than plasma etching,
and etching equipment for high-accuracy etching of large workpieces
conforms to etching equipment for etching of small workpieces, and
therefore high-accuracy etching of large workpieces has now been
rendered possible at a reasonable cost.
1TABLE I Conc. Ce(NH.sub.4).sub.2(NO.sub.3).sub.6 CH.sub.3COOH
H.sub.2O Voltage Temp. Total time [mMLAR] No. [g] [ml] [ml] [v]
[.degree. C.] [110 nm] active 16- 20 3.5 1000 2 V 25 36.21 17 25 5
1000 DC/(50 ms pulse) 2 V 25 45.39 18* 5 5 500 DC .ltoreq.2 V 25
.ltoreq.2 s 18.07 19* 5 1 400 DC/(50 ms pulse) 2 V 25 .ltoreq.2 s
22.75 20* 5 1 1000 DC/(100 ms pulse) 2 V 25 .ltoreq.2 s 9.11 * =
For optimum results. Easy to control the edge shape (chemical
etching <3 .ANG./s) - = Some underetching
[0114]
2TABLE II Ammonium persulphate H.sub.2O Conc. No. [g] [ml] Note
[mMLAR] 3 2 100 (2 v, 60 ms pulse), (10-15 s), 5-10 .mu.m Cu 87.72
4 0.5 100 (2 v, 150 ms pulse), (10 s), 5 .mu.m Cu 21.93
* * * * *