U.S. patent application number 14/777793 was filed with the patent office on 2016-09-22 for electroless copper plating solution.
The applicant listed for this patent is ATOTECH DEUTSCHLAND GMBH. Invention is credited to Birgit BECK, Frank BRUNING, Johannes ETZKORN, Elisa LANGHAMMER, Christian LOWINSKI, Michael MERSCHKY, Jorg SCHULZE.
Application Number | 20160273112 14/777793 |
Document ID | / |
Family ID | 47997197 |
Filed Date | 2016-09-22 |
United States Patent
Application |
20160273112 |
Kind Code |
A1 |
BRUNING; Frank ; et
al. |
September 22, 2016 |
ELECTROLESS COPPER PLATING SOLUTION
Abstract
The invention relates to an electroless aqueous copper plating
solution, comprising a source of copper ions, a reducing agent or a
source of a reducing agent, and a combination comprising i)
N,N,N',N'-Tetrakis (2-hydroxypropyl)ethylenediamine or a salt
thereof, and ii)
N'-(2-Hydroxyethyl)-ethylenediamine-N,N,N'-triacetic acid or a salt
thereof, as complexing agents, as well as to a method for
electroless copper plating utilizing said solution and the use of
the solution for the plating of substrates.
Inventors: |
BRUNING; Frank; (Berlin,
DE) ; BECK; Birgit; (Berlin, DE) ; LANGHAMMER;
Elisa; (Berlin, DE) ; ETZKORN; Johannes;
(Bochum, DE) ; MERSCHKY; Michael; (Berlin, DE)
; SCHULZE; Jorg; (Oranienburg, DE) ; LOWINSKI;
Christian; (Birkenwerder, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ATOTECH DEUTSCHLAND GMBH |
Berlin |
|
DE |
|
|
Family ID: |
47997197 |
Appl. No.: |
14/777793 |
Filed: |
March 25, 2014 |
PCT Filed: |
March 25, 2014 |
PCT NO: |
PCT/EP2014/055979 |
371 Date: |
September 17, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23C 18/405 20130101;
C23C 18/38 20130101; C23C 18/40 20130101 |
International
Class: |
C23C 18/38 20060101
C23C018/38; C23C 18/40 20060101 C23C018/40 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 27, 2013 |
EP |
13161418.2 |
Claims
1. An electroless aqueous copper plating solution, comprising a
source of copper ions, a reducing agent or a source of a reducing
agent, and a combination comprising i) N,N,N',N'-Tetrakis
(2-hydroxypropyl)ethylenediamine or a salt thereof, and ii)
N'-(2-Hydroxyethyl)-ethylenediamine-N,N,N'-triacetic acid or a salt
thereof, as complexing agents.
2. The electroless aqueous copper plating solution according to
claim 1, wherein the combination of complexing agents further
comprises iii) ethylenediamine tetraacetic acid or a salt
thereof.
3. The electroless aqueous copper plating solution according to
claim 1, wherein the ratio of the total molar amount of all
complexing agents to copper ions is in the range of 1:1 to 8:1.
4. The electroless aqueous copper plating solution according to
claim 1, wherein the ratio of the molar amount of complexing agent
i) to the molar amount of complexing agent ii) ranges from 1:0.05
to 1:20.
5. The electroless aqueous copper plating solution according to
claim 2, wherein the ratio of the molar amount of complexing agent
i) to the molar amount of a mixture of complexing agent ii) and
complexing agent iii) ranges from 1:0.05 to 1:20.
6. The electroless aqueous copper plating solution according to
claim 1, wherein the reducing agent is selected from glyoxylic acid
and formaldehyde.
7. A method for electroless copper plating, the method comprising
contacting a substrate with an electroless aqueous copper plating
solution according to claim 1.
8. The method according to claim 7, wherein the substrate is a
substrate made from glass, ceramic or plastics.
9. The method according to claim 7, wherein the substrate is a
glass panel.
10. The method according to claim 7, wherein the substrate has a
surface area of at least 5 m.sup.2.
11. (canceled)
12. The method according to claim 7, wherein on the substrate a
copper layer with a thickness of 0.5 .mu.m to 3 .mu.m is
produced.
13. The method according to claim 7, wherein on the substrate a
copper layer with a roughness, expressed as the root-mean-square
roughness parameter, of 5 to 60 nm is produced.
14. (canceled)
15. (canceled)
16. (canceled)
17. The electroless aqueous copper plating solution according to
claim 2, wherein the ratio of the total molar amount of all
complexing agents to copper ions is in the range of 1:1 to 8:1.
18. The electroless aqueous copper plating solution according to
claim 2, wherein the ratio of the molar amount of complexing agent
i) to the molar amount of complexing agent ii) ranges from 1:0.05
to 1:20.
19. The electroless aqueous copper plating solution according to
claim 3, wherein the ratio of the molar amount of complexing agent
i) to the molar amount of complexing agent ii) ranges from 1:0.05
to 1:20.
20. The electroless aqueous copper plating solution according to
claim 17, wherein the ratio of the molar amount of complexing agent
i) to the molar amount of a mixture of complexing agent ii) and
complexing agent iii) ranges from 1:0.05 to 1:20.
21. The electroless aqueous copper plating solution according to
claim 2, wherein the reducing agent is selected from glyoxylic acid
and formaldehyde.
22. A method for electroless copper plating, the method comprising
contacting a substrate with an electroless aqueous copper plating
solution according to claim 2.
Description
[0001] The present invention relates to an electroless copper
plating solution, a method for electroless copper plating utilizing
said solution and the use of the solution for the plating of
substrates.
[0002] Electroless plating is the controlled autocatalytic
deposition of a continuous film of metal without the assistance of
an external supply of electrons. Non-metallic surfaces may be
pretreated to make them receptive or catalytic for deposition. All
or selected portions of a surface may suitably be pretreated. The
main components of electroless copper baths are the copper salt, a
complexing agent, a reducing agent, and, as optional ingredients,
an alkaline agent, and additives, as for example stabilizers.
Complexing agents are used to chelate the copper to be deposited
and prevent the copper from being precipitated from solution (i.e.
as the hydroxide and the like). Chelating copper renders the copper
available to the reducing agent which converts the copper ions to
metallic form.
[0003] U.S. Pat. No. 4,617,205 discloses a composition for
electroless deposition of copper, comprising copper ions,
glyoxylate as reducing agent, and a complexing agent, for example
EDTA, which is capable of forming a complex with copper that is
stronger than a copper oxalate complex.
[0004] U.S. Pat. No. 7,220,296 teaches an electroless plating bath
comprising a water soluble copper compound, glyoxylic acid and a
complexing agent which may be EDTA.
[0005] US 2002/0064592 discloses an electroless bath comprising a
source of copper ions, glyoxylic acid or formaldehyde as reducing
agent, and EDTA, tartrate or alkanol amine as complexing agent.
[0006] Performance of a copper plating solution is difficult to
predict and strongly depends on its constituents, especially the
complexing agent and the reducing agent, and the molar ratio of its
constituents.
[0007] An object of the invention was to provide with an
electroless copper plating solution with improved performance,
particularly an improved copper deposition rate. A further object
of the present invention was to provide an electroless copper
plating solution for obtaining copper deposits with low
roughness.
[0008] The present invention provides with an electroless copper
plating solution, comprising [0009] a source of copper ions, [0010]
a reducing agent or a source of a reducing agent, and [0011] a
combination comprising [0012] i) N,N,N',N'-Tetrakis
(2-hydroxypropyl)ethylenediamine or a salt thereof, and [0013] ii)
N'-(2-Hydroxyethyl)-ethylenediamine-N,N,N'-triacetic acid or a salt
thereof, as complexing agents.
[0014] The combination of complexing agents may further comprise
[0015] iii) ethylenediamine tetraacetic acid or a salt thereof.
[0016] N,N,N',N'-Tetrakis (2-hydroxypropyl) ethylenediamine is
hereinafter abbreviated as "Quadrol", which is a trademark of BASF
company.
[0017] Ethylenediamine tetraacetic acid is hereinafter also named
as "EDTA".
[0018] N'-(2-Hydroxyethyl)-ethylenediamine-N,N,N'-triacetic acid is
hereinafter also named as "HEDTA".
[0019] In one embodiment the electroless copper plating solution
preferably does not contain cyclohexane diamine tetraacetic acid
(CDTA) in an amount ranging from 0.1 mM to 5.5 M. In another
embodiment the electroless copper plating solution preferably does
not contain CDTA at all.
[0020] One or more of the above mentioned objects are achieved by
the electroless copper plating solution (hereinafter abbreviated as
the "solution") according to claim 1, or by advantageous
embodiments as described in dependent claims and the description.
The copper plating solution of the invention shows an improved
copper deposition rate. At the same time, a low roughness of copper
surfaces can be reached, which is crucial for performance of
certain electronic devices. Due to a higher deposition rate, a
higher thickness of copper layer can be reached at the same process
time.
[0021] The solution according to the invention and the method
according to the invention are preferably used for the coating of
printed circuit boards, chip carriers and semiconductor wafers or
also of any other circuit carriers and interconnect devices. The
solution is used in particular in printed circuit boards and chip
carriers, but also in semiconductor wafers, to plate surfaces,
trenches, blind micro vias, through hole vias (through holes) and
similar structures with copper.
[0022] Particularly, the solution of the invention or the method of
the invention can be used for deposition of copper on surfaces, in
trenches, blind-micro-vias, through-hole-vias, and comparable
structures in printed circuit boards, chips, carriers, wafers and
various other interconnect devices. The term "through hole vias" or
"through holes", as used in the present invention, encompasses all
kinds of through hole vias and includes so-called "through silicon
vias" in silicon wafers.
[0023] Another application wherein the solution can be used with
beneficial effects is metallization of smooth, substrates made from
glass, ceramic or plastics, preferably with a large surface area.
Examples are any kind of displays, as for example any kind of
TFT-displays and liquid crystal displays (LCD). As mentioned above,
a low roughness of copper surfaces can be reached with the solution
of the invention. This effect is advantageous particularly for
display applications since a copper layer can be produced with a
good conductivity.
[0024] The electroless copper plating solution of the invention can
be beneficially used for deposition of copper on a glass substrate,
particularly with large surface area, such as glass panels. Glass
substrates are used, without limitation, for display applications,
as mentioned above. Wet electroless copper deposition, with a
solution as mentioned above, on a glass substrate is beneficial in
comparison to metal sputtering processes that have been used so
far. Benefits that can be reached with wet electroless deposition
in comparison to sputtering techniques are, inter alia, reduced
internal stress and reduced bending of a glass substrate, reduced
equipment maintenance, effective use of metal, reduced material
waste, reduced process temperature.
[0025] Further, the electroless copper plating solution of the
invention can be beneficially used for plating of glass substrates,
particularly glass panels for displays.
[0026] Anyhow, common wet electroless deposition usually produces
rougher metal surfaces than sputtering processes. In the case of
display production this causes poor switching properties,
especially unfavorable prolonged switching times. Thus, for display
production it is necessary to generate metal layers with a
roughness in the range achieved by sputtering processes.
Surprisingly the electroless copper plating solution of the
invention is not only able to generate metal layers at higher
deposition rate, but simultaneously with a low roughness in a range
achieved by sputtering processes.
[0027] Moreover, substrates for display production are activated by
metal seed layers for subsequent deposition of metal layers in
order to build necessary circuitry and switching elements. Thus,
the metal seed layers already display the future pattern of
circuitry and switching elements that comprise small and/or
isolated activated areas as well as a combination of small and
larger activated areas. A high copper deposition rate on glass
substrate is reached with the solution of the invention, especially
on glass substrates that have these small and/or isolated activated
areas. In addition the solution of the invention also is able to
deposit metal layers with uniform thickness simultaneously onto
small and larger activated areas at high deposition rates.
[0028] The solution of the invention is an aqueous solution. The
term "aqueous solution" means that the prevailing liquid medium,
which is the solvent in the solution, is water. Further liquids,
that are miscible with water, as for example alcohols and other
polar organic liquids, may be added.
[0029] The solution of the present invention may be prepared by
dissolving all components in aqueous liquid medium, preferably in
water.
[0030] The solution contains a copper ion source, which may for
example be any water soluble copper salt. Copper may for example,
and without limitation, be added as copper sulphate, copper
chloride, copper nitrate, copper acetate, copper methane sulfonate
((CH.sub.3O.sub.3S).sub.2Cu), copper hydroxide; or hydrates
thereof.
[0031] The reducing agent serves for reducing the copper ions in
order to obtain metallic copper for plating. Reducing agents that
can be employed are for example, and without limitation,
formaldehyde, glyoxylic acid, hypophosphite, hydrazine, and
borohydride. Preferred reducing agents are formaldehyde and
glyoxylic acid.
[0032] The term "source of a reducing agent" means a substance that
is converted to a reducing agent in the solution. The source is for
example a precursor of a reducing agent that converted to the
reducing agent. An example is given below with respect to glyoxylic
acid.
[0033] A particularly preferred reducing agent is glyoxylic acid
because of safety, health and environmental requirements. Even
though formaldehyde is a very important and established reducing
agent of the common electroless copper plating process, it was
classified as a probable human carcinogen. Thus, the electroless
aqueous copper plating solution in one embodiment comprises
glyoxylic acid or a source of glyoxylic acid. In this embodiment,
the solution of the invention does not contain formaldehyde, or, in
other words, the solution is according to this embodiment free of
formaldehyde.
[0034] The term "source of glyoxylic acid" encompasses all
compounds that can be converted to glyoxylic acid in aqueous
solution, such as precursors. A preferred precursor is dichloro
acetic acid. Glyoxylic acid is the reducing agent for the reduction
of copper ions to elementary copper. In the solution, glyoxylic
acid and glyoxylate-ions may be present. As used herein the term
"glyoxylic acid" includes salts thereof. The exact nature of the
species, acid or salt, present will depend on the pH of the
solution. The same consideration applies to other weak acids and
bases.
[0035] In addition to one of the above-mentioned reducing agents,
one or more additional reducing agents may be added, as for example
hypophosphoric acid, glycolic acid or formic acid, or salts of
aforementioned acids. The additional reducing agent is preferably
an agent that acts as reducing agent but cannot be used as the sole
reducing agent (cf. for example the disclosure in U.S. Pat. No.
7,220,296, col. 4, I. 20-43 and 54-62). Therefore, such additional
reducing agent is in this sense also called an "enhancer".
[0036] Electroless copper baths using reducing agents described
above preferably employ a relatively high pH, usually between 11
and 14, preferably between 12.5 and 13.5, and are adjusted
generally by potassium hydroxide (KOH), sodium hydroxide (NaOH),
lithium hydroxide (LiOH), ammonium hydroxide or quarternary
ammonium hydroxide, such as tetramethylammonium hydroxide (TMAH).
Thus, the solution may contain a source of hydroxide ions, as for
example and without limitation one or more of the compounds listed
above. A source of hydroxide is for example added if an alkaline pH
of the solution is desired and if the pH is not already in the
alkaline range by other constituents.
[0037] Preferred is the use of potassium hydroxide. Potassium
hydroxide is of advantage, if glyoxylic acid is used as the
reducing agent, because the solubility of potassium oxalate is
high. Oxalate anions are formed in the solution by the oxidation of
the glyoxylic acid. Thus, potassium hydroxide is especially
preferable for stability of the solution of the present
invention.
[0038] The solution of the present invention further comprises a
mixture of the complexing agent i) Quadrol or a salt thereof with
the complexing agent ii) HEDTA or a salt thereof. The mixture of
the complexing agent i) Quadrol or a salt thereof with the
complexing agent ii) HEDTA or a salt thereof may further comprise
the complexing agent iii) EDTA or a salt thereof. The addition of
Quadrol or a salt thereof to complexing agent ii) or to a mixture
of complexing agent ii) and complexing agent iii) results in an
efficient increase in copper deposition. Before the present
invention was made, it had been observed that increase of metal
deposition rate leads to increased roughness of metal surfaces. In
the present invention a high copper deposition rate and a copper
surface with low roughness are obtained, surprisingly.
[0039] The salts of Quadrol, HEDTA or EDTA may be any suitable,
water soluble salts. The counterions of the salts of Quadrol, HEDTA
or EDTA are preferably selected from alkali metal ions, earth
alkali metal ions and ammonium ions. The counterions of the salts
of Quadrol, HEDTA or EDTA are more preferably selected from lithium
ions, sodium ions, potassium ions, magnesium ions, calcium ions and
ammonium ions.
[0040] The solution of the present invention is free of toxic
co-metals. The solution of the present invention is especially free
of nickel. Nickel forms a more stable complex with the complexing
agents used herein than copper. It therefore reduces copper
complexation and negatively affects or impedes copper deposition.
Moreover, the presence of nickel in the bath would lead to unwanted
nickel deposition, which has to be avoided especially in display
production.
[0041] In one embodiment of a solution of the present invention,
the molar ratio of the complexing agents, related to the total
molar amount of all complexing agents, to copper ions is in the
range of 1:1 to 10:1, preferably 1:1 to 8:1, more preferably 2:1 to
8:1, even more preferably 2:1 to 5:1, still even more preferably
1.5:1 to 4:1, most preferably 2:1 to 4:1. The molar ratio of the
complexing agents, related to the total molar amount of all
complexing agents, to copper ions is defined as the ratio of the
total molar amount of all complexing agents to the molar amount of
copper ions. The total molar amount of all complexing agents is the
sum of the individual molar amounts of all complexing agents. "All
complexing agents" may be a mixture of complexing agent i) and
complexing agent ii) or may be a mixture of complexing agent i),
complexing agent ii) and complexing agent iii). In the examples the
amount of complexing agents is also given as equivalents. One
equivalent is the amount of a complexing agent which completely
complexes a given amount of copper ions. In the case of Quadrol,
EDTA and HEDTA or a salt thereof one equivalent of complexing agent
corresponds to a molar ratio of complexing agent to copper ions of
1:1. In the case of Quadrol, EDTA and HEDTA a molar ratio of 1:1 to
10:1 of complexing agent(s) to copper ions means 1 to 10
equivalents of complexing agent(s) related to copper.
[0042] Less complexing agent leads to instability of the bath or
deposition does not start. More complexing agent in relation to
copper leads to a high density of the bath, which also leads to
reduced lifetime and instability of the bath. Using these ranges
leads to a beneficial combination of high copper deposition rate
and low roughness.
[0043] In another embodiment, the molar ratio of the complexing
agents, related to the total molar amount of all complexing agents,
to copper ions is in the range of 3:1 to 8:1, more preferably 3:1
to 5:1, even more preferably 3:1 to 4:1. Using these ranges leads
to a particularly beneficial combination of high copper deposition
rate and low roughness. A very reproducible performance, a very
reproducible copper deposition, and copper layers with very uniform
thickness can be obtained.
[0044] In one embodiment, the ratio of the molar amount of
complexing agent i) to the molar amount of complexing agent ii)
ranges from 1:0.05 to 1:20, preferably from 1:0.1 to 1:10, more
preferably from 1:1 to 1:5, even more preferably from 1:1 to 1:4,
most preferably from 1:2 to 1:4. The ratio of the molar amount of
complexing agent i) to the molar amount of a mixture of complexing
agent ii) and complexing agent iii) (complexing agent i):
[complexing agent ii)+complexing agent iii)]) ranges from 1:0.05 to
1:20, preferably from 1:0.1 to 1:10, more preferably from 1:1 to
1:5, even more preferably from 1:1 to 1:4, most preferably from 1:2
to 1:4.
[0045] The molar amount of a mixture of complexing agent ii) and
complexing agent iii) ([complexing agent ii)+complexing agent
iii)]) is the sum of the individual molar amounts of complexing
agent ii) and complexing agent iii).
[0046] In another embodiment, the ratio of the molar amount of
complexing agent i) to the molar amount of complexing agent ii)
ranges from 1:0.05 to 1:5, preferably from 1:0.05 to 1:3, more
preferably from 1:0.1 to 1:2. The ratio of the molar amount of
complexing agent i) to the molar amount of a mixture of complexing
agent ii) and complexing agent iii) (complexing agent i):
[complexing agent ii)+complexing agent iii)]) ranges from 1:0.05 to
1:5, preferably from 1:0.05 to 1:3, more preferably from 1:0.1 to
1:2.
[0047] In another embodiment, the ratio of the molar amount of
complexing agent i) to the molar amount of complexing agent ii)
ranges from 1:5 to 1:20, preferably from 1:7 to 1:15, more
preferably from 1:7 to 1:10. The ratio of the molar amount of
complexing agent i) to the molar amount of a mixture of complexing
agent ii) and complexing agent iii) (complexing agent
i):[complexing agent ii)+complexing agent iii)]) ranges from 1:5 to
1:20, preferably from 1:7 to 1:15, more preferably from 1:7 to
1:10.
[0048] Using the ranges outlined above leads to a beneficial
combination of high copper deposition rate and low roughness.
[0049] In one embodiment, the electroless aqueous copper plating
solution comprises, as complexing agents, a combination of [0050]
i) N,N,N',N'-Tetrakis (2-hydroxypropyl)ethylenediamine (Quadrol) or
a salt thereof, and [0051] ii)
N'-(2-Hydroxyethyl)-ethylenediamine-N,N,N'-triacetic acid (HEDTA)
or a salt thereof.
[0052] In a still further embodiment, the electroless aqueous
copper plating solution comprises, as complexing agents, a
combination of [0053] i) N,N,N',N'-Tetrakis
(2-hydroxypropyl)ethylenediamine (Quadrol) or a salt thereof,
[0054] ii) N'-(2-Hydroxyethyl)-ethylenediamine-N,N,N'-triacetic
acid (HEDTA) or a salt thereof, and [0055] iii) ethylenediamine
tetraacetic acid (EDTA) or a salt thereof.
[0056] The solution of the invention in one embodiment contains
following kinds of ingredients in following concentrations:
[0057] Copper ions: 1-5 g/l, corresponding to 0.016-0.079 mol/l,
preferably 2.0-3.0 g/l
[0058] Reducing agent: 0.027-0.270 mol/l, preferably glyoxylic
acid: 2-20 g/l, or formaldehyde: 0.8-8.5 g/l.
[0059] Complexing agents (total amount of all complexing agents):
5-50 g/l, preferably 20-40 g/l, more preferably 20-30 g/l.
[0060] The solution of the present invention may comprise--and does
not necessarily comprise--further components, as for example
stabilizers, surfactants, additives, as rate controlling additives,
grain refining additives, pH buffers, pH adjusters, and enhancers.
Such further components are for example described in following
documents, which are incorporated by reference in their entirety:
U.S. Pat. No. 4,617,205 (particularly disclosure in col. 6, I.
17-col. 7, I. 25), U.S. Pat. No. 7,220,296 (particularly col. 4, I.
63-col. 6, I. 26), US 2008/0223253 (cf. particularly paragraphs
0033 and 0038).
[0061] Stabilizing agents, also referred to as stabilizers, are
compounds that stabilize the electroless plating solution against
unwanted outplating in the bulk solution. The term "outplating"
means unwanted and/or uncontrolled deposition of copper, for
example on the bottom of a reaction vessel or on other surfaces.
Stabilizing function can for example be accomplished by substances
acting as catalyst poison (for example sulfur or other chalcogenide
containing compounds) or by compounds forming copper(I)-complexes,
thus inhibiting the formation of copper(I)oxide.
[0062] The solution of the present invention may comprise one or
more of a stabilizing agent. Suitable stabilizers are, without
limitation, dipyridyls (2,2'-dipyridyl, 4,4'dipyridyl),
phenanthroline, mercapto-benzothiazole, thio-urea or its
derivatives, cyanides like NaCN, KCN, K.sub.4[Fe(CN).sub.6];
Na.sub.2S.sub.2O.sub.3, K.sub.2S.sub.2O.sub.3, thiocyanates,
iodides, ethanolamines, polymers like polyacrylamides,
polyacrylates, polyethylene glycols, or polypropylene glycols and
their co-polymers.
[0063] In another aspect, the present invention relates to a method
for electroless copper plating, the method comprising contacting a
substrate with an electroless copper plating solution as described
above.
[0064] For example, the substrate may be dipped or immersed in the
solution of the invention. In the method a whole surface of a
substrate may be plated with copper, or only selected portions.
[0065] It is preferred that the solution be agitated during use. In
particular, work- and/or solution-agitation may be used.
[0066] The method will be carried out for a sufficient time to
yield a deposit of the thickness required, which in turn will
depend on the particular application.
[0067] One envisaged application of the method is the preparation
of printed circuit boards. The electroless deposition of copper
according to the method of the invention can particularly be used
for the through-plating of holes, surfaces, trenches, blind micro
vias in printed circuit boards. Double sided or multilayer boards
(rigid or flexible) may be plated by means of the present
invention.
[0068] The method of the invention may be useful in providing
electroless copper deposits with a thickness in the range of 0.05
.mu.m to 10 .mu.m, preferably between 0.1 .mu.m-10 .mu.m, 0.1
.mu.m-5 .mu.m, 0.5 .mu.m-3 .mu.m. The thickness of copper layer is
determined with white light interferometry, as described in the
examples.
[0069] The method of the invention produces copper layers on the
substrate with a roughness, expressed as the root-mean-square
roughness parameter, of 5 nm to 60 nm, preferably 5 nm-55 nm and
more preferably 10 nm-45 nm. The obtained roughness is lower by 30%
to 60%, preferably by 40% to 50%, than a method using complexing
agent ii) only or using complexing agent iii) only or using a
mixture of complexing agents ii) and iii) only. In this case the
term "only" means: without addition of Quadrol. The roughness of
the copper layer is determined with white light interferometry, as
described in the examples.
[0070] Substrates that are generally used for printed circuit board
manufacture are most frequently epoxy resins or epoxy glass
composites. But other substances, notably phenolic resins,
polytetrafluoroethylene (PTFE), polyimides, polyphenyleneoxides,
BT(bismaleintriazine)-resins, cyanate esters and polysulphones can
be used.
[0071] Aside from the application of the method in the production
of printed circuit boards, it may be found to be useful in plating
substrates made from glass, ceramic or plastics, as for example
ABS, polycarbonate, polyimide or polyethylene terephthalate.
[0072] In another embodiment of the method, the substrate is a
substrate made from glass, ceramic or plastics, preferably with a
large surface area. A large surface area, means preferably an area
of at least one m.sup.2, preferably at least 3 m.sup.2, more
preferably at least 5 m.sup.2. A large surface area means in
another embodiment preferably an area of 1 m.sup.2 to 9 m.sup.2,
more preferably 3 m.sup.2 to 9 m.sup.2, even more preferably 3
m.sup.2 to 6 m.sup.2, still more preferably 5 m.sup.2 to 6 m.sup.2.
The substrate has preferably a smooth surface. The term smooth
means preferably a roughness (Sq or RMS) of a few nanometers.
Preferably the roughness is 5-30 nm, measured as RMS. Explanations
of the method for roughness measurement and the terms "Sq" and
"RMS" are given in the examples.
[0073] In a special embodiment, the substrate is a glass substrate,
preferably a glass panel. Said glass substrates, especially glass
panels can be used for application in TFT displays, such as liquid
crystal displays. Thus, the glass substrate is particularly such
one that fulfils the specifications as used in display production,
as for example thickness and smoothness. A preferred glass is free
from alkali, such as alkali free boro-silicate-glass.
[0074] Glass substrates may be pretreated before the method of the
invention is carried out, for example with metal seeds, as further
explained below.
[0075] In one embodiment of the method of the present invention,
the method is carried out at a temperature in the range of
20-60.degree. C., preferably 30-55.degree. C. It has been shown in
the present invention that when Quadrol is used as complexing
agent, in combination with another complexing agent, copper
deposition can be done at lower temperatures than in absence of
this component. Even though the temperature is lower, the
deposition rate is higher than with a bath that does not contain
Quadrol.
[0076] The substrate, i.e. the surfaces of the substrate that are
to be plated with copper, particularly non-metallic surfaces, may
be pretreated by means within the skill in the art (as for example
described in U.S. Pat. No. 4,617,205, col 8) to make it/them more
receptive or autocatalytic for copper deposition. All or selected
portions of a surface may be pretreated. A pretreatment is,
however, not necessary in every case and depends on the kind of
substrate and surface. Within the pretreatment, it is possible to
sensitise substrates prior to the deposition of electroless copper
on them. This may be achieved by the adsorption of a catalysing
metal (such as a noble metal, for example palladium) onto the
surface of the substrate.
[0077] A pretreatment process strongly depends on parameters such
as the substrate, the desired application, and the desired
properties of the copper surface.
[0078] An exemplary and non-limiting pretreatment process,
especially for printing circuit board laminates and other suitable
substrates, may comprise one or more of the following steps [0079]
a) optionally cleaning and conditioning the substrate to increase
adsorption. With a cleaner, organics and other residues are
removed. It may also contain additional substances (conditioners)
that prepare the surface for the following activation steps, i.e.
enhance the adsorption of the catalyst and lead to a more uniformly
activated surface, [0080] b) etching, to remove oxides from the
surface of the copper, especially from inner layers in holes. This
may be done by persulphate or peroxide based etching systems,
[0081] c) contacting with a pre-dip solution, such as a
hydrochloric acid solution or sulfuric acid solution, optionally
with an alkali metal salt, such as sodium chloride, also in the
pre-dip solution, [0082] d) contacting with an activator solution,
that contains colloidal or ionic catalysing metal, such as a noble
metal, preferably palladium, causing the surface to become
catalytic. The pre-dip in step c) serves to protect the activator
from drag-in and contaminations, and optionally, particularly if
the activator contains ionic catalysing metal, [0083] e) contacting
with a reducer, wherein the metal ions of an ionic activator are
reduced to elemental metal. [0084] or, if the activator contains
colloidal catalysing metal, [0085] f) contacting with an
accelerator, wherein components of the colloid, for example a
protective colloid, is removed from the catalysing metal.
[0086] In another kind of pretreatment process a permanganate
etching step is employed. The so-called Desmear process is a
multi-stage process, the steps of which are a swelling step, a
permanganate etching step and a reduction step. The sweller used in
the swelling step is made of a mixture of organic solvents. During
this step drill smear and other impurities are removed from the
surfaces of the substrate. A high temperature of 60-80.degree. C.
promotes the infiltration of the sweller which leads to a swelled
surface. Therefore a stronger attack of the subsequently applied
permanganate solution is possible during the permanganate etching
step. Afterwards the reduction solution of the reduction step
removes the manganese dioxides produced during the permanganate
step from the surfaces. The reduction solution contains a reducing
agent and optionally a conditioner.
[0087] The desmear process may be combined with the above described
steps. The desmear process may be performed before step a) of the
above described pretreatment process or the desmear process may be
performed instead of steps a) and b) of the above described
pretreatment process.
[0088] In a pretreatment process which is particularly suitable in
metallization for display applications and in metallization of
glass substrates, a surface is only contacted with a pre-dip
solution and an activator solution and then with the solution of
the invention. Contacting with a cleaning solution and an adhesion
enhancer before the pre-dip step are optional steps that can be
carried out in advance.
[0089] Still another process, which is often used for glass
substrates, may be carried out with following steps before copper
plating: A glass surface that is to be plated exhibits metal seed
layers. The metal seed layers may be brought onto the surface by
sputtering techniques. Exemplary seeds are layers composed of
copper, molybdenum, titanium, or a mixture thereof. Said pretreated
glass surface is contacted with an activator solution that contains
ionic catalysing metal, such as a noble metal, preferably
palladium, causing the surface to become catalytic. The ionic
catalysing metal is reduced onto the surface by the seed metal.
[0090] In this process, addition of a further reducer may be
omitted. This process is especially used in copper plating of glass
substrates for display applications.
[0091] The exemplary pretreatment processes, or single steps
thereof, may be combined to alternative pretreatment processes, if
found necessary.
[0092] In a further aspect, the present invention relates to the
use of the electroless copper plating solution as described above
for the plating of printed circuit boards, wafers, Integrated
circuit substrates, molded interconnect device (MID) components,
displays, such as liquid crystal or plasma displays, particularly
displays for electronic devices or TVs, display components, or
plastic parts, such as plastic parts for functional or decorative
purposes.
DESCRIPTION OF FIGURES
[0093] FIG. 1 Effect of a combination of Quadrol with the further
complexing agent EDTA on copper thickness and roughness in a
plating process
[0094] FIG. 2 Effect of a combination of Quadrol with the further
complexing agent HEDTA on copper thickness and roughness in a
plating process
[0095] The invention is now described in further detail by the
following examples. These examples are set forth to illustrate the
present invention, but should not be construed as limiting the
present invention.
[0096] Method of Roughness Measurement:
[0097] An Optical profilometer/White light interferometer, Model
MIC-520, of ATOS GmbH (Germany) was used to measure the thickness
(Height difference between base plane and plated pattern) and
surface roughness of electrolessly plated copper layers. White
light interferometry is an optical microscopy method known to
persons skilled in the art which projects the target area of a
sample onto a CCD camera. Using interference objectives equipped
with an internal beam splitter, a high-precision reference mirror
is projected onto the CCD camera as well. Due to the overlay of
both images, a spatially resolved interferogram is created which
reflects the height differences between the very flat reference
mirror and the sample of interest. In order to image samples with
large height distribution a vertical scan scheme is used, i.e.
interferograms of the area of interest are imaged as a series
within a range of different sample-objective distances. From these
data a full three dimensional image is compiled. Using this method,
topographic images in the range of 60 .mu.m.times.60 .mu.m to 1.2
mm.times.1.2 mm can be recorded with a vertical resolution in the
range of a few nm.
[0098] The topographic data are used to calculate surface roughness
expressed as the root-mean-square roughness parameter, abbreviated
as Rq or RMS on surface profiles (profile roughness parameter) and
abbreviated as Sq on surface topographies (areal roughness
parameter). The meaning of Rq is identical to the meaning of RMS.
Rq has the meaning as defined in DIN EN ISO 4287 (German and
English version of 1998, Chapter 4.2.2) and Sq has the meaning as
defined in ISO 25178-2 of April 2012 (Chapter 4.1.1).
[0099] In addition the topographic data are used to calculate the
thickness of the plated copper layers as height difference between
the substrate surface (base plane) and the surface of the plated
metal pattern. For calculating topographic images, layer thickness
and surface roughness the Optical profilometer/White light
interferometer, Model MIC-520, of ATOS GmbH (Germany) was equipped
with the computer software Micromap 123, version 4.0, by Micromap
Corporation.
[0100] The mode of measurement was Focus 560 M. The topographic
images were measured with an objective lens with 10 times
magnification and an ocular with 2 times magnification. The
topographic images were recorded in the range of 312
.mu.m.times.312 .mu.m and consist of 480.times.480 points.
EXAMPLE 1
Combination of Quadrol with a Further Complexing Agent
[0101] Substrate: Alkali-free borosilicate glass, thickness 0.7 mm,
sputtered seed layer of copper.
[0102] Pre Treatment:
[0103] 1. alkaline cleaner 40.degree. C./1 min
[0104] 2. rinsing with H.sub.2O
[0105] 3. sulfuric acid pre dip solution, room temperature (RT)/20
sec
[0106] 4. ionic Pd-activator (exchange-reaction between Cu and Pd)
RT/2 min
[0107] 5. rinsing with H.sub.2O
[0108] Electroless copper plating solutions were manufactured. As
complexing agents combinations of Quadrol/EDTA (comparative
example) and Quadrol/HEDTA (inventive example) were employed.
Quadrol was added in amounts of 0 g/l, 2.7 g/l and 5.4 g/l,
respectively. Cu.sup.2+ ions were added as CuSO.sub.4*6H.sub.2O.
The pH of the baths was 13.2 at 21.degree. C.
[0109] Substrates were contacted with the respective plating
solutions as described above at 45.degree. C. for 12 min each. The
samples of deposited Cu layers were analysed according to the
described method in measuring mode "Focus 560 M". The results are
shown in the following tables 1 and 2. FIGS. 1 and 2 show a chart
of the results obtained.
TABLE-US-00001 TABLE 1 Combination of Quadrol/EDTA Sample No. 1 2 3
(comparative) (comparative) (comparative) EDTA 14.0 g/l 14.0 g/l
14.0 g/l EDTA in Equivalents 1.5 1.5 1.5 Quadrol 0 g/l 2.7 g/l 5.4
g/l Quadrol in Equivalents 0 1.1 2.3 Quadrol % vol. 0 10 20
Cu.sup.2+ 2.0 g/l 2.0 g/l 2.0 g/l KOH 8 g/l 8 g/l 8 g/l
Formaldehyde 4.7 g/l 4.7 g/l 4.7 g/l 2,2'-Dipyridyl 1 mg/l 1 mg/l 1
mg/l Temperature [.degree. C.] 45 45 45 Dwell Time [min] 12 12 12
Cu Thickness [.mu.m] 0.63 0.78 0.78 Sq [nm] 71 51 35
TABLE-US-00002 TABLE 2 Combination of Quadrol/HEDTA Sample No. 4 5
6 (comparative) (inventive) (inventive) HEDTA 17.1 g/l 17.1 g/l
17.1 g/l HEDTA in Equivalents 1.9 1.9 1.9 Quadrol 0 g/l 2.7 g/l 5.4
g/l Quadrol in Equivalents 0 1.1 2.3 Quadrol % vol. 0 10 20
Cu.sup.2+ 2.0 g/l 2.0 g/l 2.0 g/l KOH 8 g/l 8 g/l 8 g/l
Formaldehyde 4.7 g/l 4.7 g/l 4.7 g/l 2,2'-Dipyridyl 1 mg/l 1 mg/l 1
mg/l Temperature [.degree. C.] 45 45 45 Dwell Time [min] 12 12 12
Cu Thickness [.mu.m] 0.88 1.06 1.07 Sq [nm] 58 54 40
[0110] "Dwell time" means the time of contacting the substrates
with the electroless copper plating solutions.
[0111] A combination of Quadrol/EDTA (table 1, FIG. 1) or a
combination of Quadrol/HEDTA (table 2, FIG. 2) leads to increased
copper thicknesses in comparison to EDTA alone or
[0112] HEDTA alone, respectively, when the same process time is
chosen. The results show that addition of Quadrol increases the
deposition rate while significantly reducing the roughness of the
deposited copper layers. If Quadrol is added to a solution already
containing HEDTA the roughness in relation to the deposition rate
is lower than if Quadrol is added to a solution already containing
EDTA alone.
EXAMPLE 2
Comparative Example
[0113] Substrates as used in Example 1 were pre-treated as
described in Example 1.
[0114] Electroless copper plating solutions were prepared as
described in Example 1. The copper plating solutions contained a
combination of complexing agents Quadrol and HEDTA in a molar ratio
of 1:20. The total molar amount of complexing agents was varied in
relation to the molar amount of copper ions as shown in Table 3. As
stabilizer a mixture of cyanide and sulphur compounds was
added.
[0115] Two pre-treated substrates (sample A and B) were contacted
with the respective plating solutions as described above at
45.degree. C. for 10 min each. The samples of deposited Cu layers
were analysed as described in Example 1. The results are shown in
Table 3.
TABLE-US-00003 TABLE 3 Variation of ratio of complexing agents to
copper ions. Sample No. 7 8 (comparative) (comparative) HEDTA in
equivalents 0.476 10.48 Quadrol in equivalents 0.024 0.52 Total
equivalents of 0.5 11 complexing agents Cu.sup.2+ 2.5 g/l 2.5 g/l
Cu.sup.2+ in equivalents 1 1 NaOH 8 g/l 8 g/l Formaldehyde 4.5 g/l
4.5 g/l Stabilizer 4 mg/l 4 mg/l Temperature [.degree. C.] 45 45
Dwell Time [min] 10 10 Cu Thickness [.mu.m] Sample A 1.25 1.77
Sample B 1.23 1.95 Mean 1.24 1.86 Sq [nm] Sample A 93 161 Sample B
104 187 Mean 99 174
[0116] Both electroless copper plating solutions deposited copper
with a high deposition rate but the roughness of the copper layers
obtained was too high. In addition, when Quadrol and HEDTA were
used in a molar ratio to copper ions of 0.5:1, the electroless
copper plating solution became instable. When Quadrol and HEDTA
were used in a molar ratio to copper ions of 11:1, the deposited
copper layers showed wild growth and blistering.
* * * * *