U.S. patent application number 16/129831 was filed with the patent office on 2019-01-24 for method for depositing a copper seed layer onto a barrier layer and copper plating bath.
The applicant listed for this patent is Atotech Deutschland GmbH. Invention is credited to Matthias DAMMASCH, Marco HARYONO, Sengul KARASAHIN, Hans-Jurgen SCHREIER.
Application Number | 20190024239 16/129831 |
Document ID | / |
Family ID | 49231340 |
Filed Date | 2019-01-24 |
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United States Patent
Application |
20190024239 |
Kind Code |
A1 |
DAMMASCH; Matthias ; et
al. |
January 24, 2019 |
METHOD FOR DEPOSITING A COPPER SEED LAYER ONTO A BARRIER LAYER AND
COPPER PLATING BATH
Abstract
The present invention relates to an aqueous electroless copper
plating bath, including a source for Cu(II) ions, glyoxylic acid as
the reducing agent for Cu(II) ions, at least one complexing agent
for Cu(II) ions and at least one source for hydroxide ions selected
from the group consisting of RbOH, CsOH and mixtures thereof,
wherein the electroless copper plating bath is substantially free
of Na+, K+ and tetraalkylammonium ions.
Inventors: |
DAMMASCH; Matthias; (Berlin,
DE) ; HARYONO; Marco; (Berlin, DE) ;
KARASAHIN; Sengul; (Berlin, DE) ; SCHREIER;
Hans-Jurgen; (Velten, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Atotech Deutschland GmbH |
Berlin |
|
DE |
|
|
Family ID: |
49231340 |
Appl. No.: |
16/129831 |
Filed: |
September 13, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14906370 |
Jan 20, 2016 |
10077498 |
|
|
PCT/EP2014/069410 |
Sep 11, 2014 |
|
|
|
16129831 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25D 7/12 20130101; C23C
18/1683 20130101; C23C 18/40 20130101; C25D 5/00 20130101; C23C
18/1653 20130101; C25D 3/38 20130101; C23C 18/1633 20130101; H01L
21/76873 20130101; H01L 21/76898 20130101; H01L 21/288
20130101 |
International
Class: |
C23C 18/40 20060101
C23C018/40; C25D 5/00 20060101 C25D005/00; H01L 21/768 20060101
H01L021/768; H01L 21/288 20060101 H01L021/288; C25D 7/12 20060101
C25D007/12; C23C 18/16 20060101 C23C018/16; C25D 3/38 20060101
C25D003/38 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 25, 2013 |
EP |
13185862.3 |
Claims
1. An aqueous electroless copper plating bath, comprising a. a
source for Cu(II) ions, b. glyoxylic acid as the reducing agent for
Cu(II) ions, c. at least one complexing agent for Cu(II) ions and
d. at least one source for hydroxide ions selected from the group
consisting of RbOH, CsOH and mixtures thereof, wherein said
electroless copper plating bath is substantially free of Na.sup.+,
K.sup.+ and tetraalkylammonium ions.
2. The aqueous electroless copper plating bath of claim 1 with a pH
ranging between 10 and 14.
3. The aqueous electroless copper plating bath of claim 1, wherein
the source for Cu(II) ions is selected from copper sulfate, copper
chloride, copper nitrate, copper acetate, copper methane sulfonate,
copper hydroxide, copper formate and hydrates thereof.
4. The aqueous electroless copper plating bath of claim 1, wherein
the Cu(II) ions have a concentration ranging from 0.05 g/l to 20
g/l.
5. The aqueous electroless copper plating bath of claim 1, wherein
the glyoxylic acid has a concentration ranging from 0.05 g/l to 20
g/l.
6. The aqueous electroless copper plating bath of claim 1, wherein
the glyoxylic acid has a concentration ranging from 0.1 g/l to 10
g/l.
7. The aqueous electroless copper plating bath of claim 1, wherein
the glyoxylic acid has a concentration ranging from 0.2 g/l to 6
g/l.
8. The aqueous electroless copper plating bath of claim 1, wherein
the at least one complexing agent for Cu(II) ions is selected from
the group consisting of carboxylic acids, hydroxycarboxylic acids,
aminocarboxylic acids, alkanolamines, polyols and mixtures
thereof.
9. The aqueous electroless copper plating bath of claim 1, wherein
the at least one complexing agent for Cu(II) ions is selected from
the group consisting of hydroxycarboxylic acids, alkanolamines, and
mixtures thereof.
10. The aqueous electroless copper plating bath of claim 1, wherein
the at least one complexing agent for Cu(II) ions is at least one
polyamino disuccinic acid derivative.
11. The aqueous electroless copper plating bath of claim 1, wherein
the at least one complexing agent for Cu(II) ions is a combination
of i) at least one polyamino disuccinic acid derivative and ii) one
or more compounds selected from the group consisting of
ethylenediamine tetraacetic acid,
N'-(2-Hydroxyethyl)-ethylenediamine-N,N,N'-triacetic acid, and
N,N,N',N'-Tetrakis-(2-hydroxypropyl)-ethylenediamine.
12. The aqueous electroless copper plating bath of claim 1, wherein
the at least one complexing agent for Cu(II) ions is a combination
of i) i) N,N,N',N'-tetrakis-(2-hydroxypropyl)ethylenediamine and
ii) one or more from ethylenediamine tetraacetic acid and
N'-(2-Hydroxyethyl)-ethylenediamine-N,N,N'-triacetic acid.
13. The aqueous electroless copper plating bath of claim 1, wherein
the at least one source for hydroxide ions is CsOH.
14. The aqueous electroless copper plating bath of claim 1,
comprising a wetting-agent selected from the group comprising
cationic wetting agents, anionic wetting agents, non-ionic wetting
agents, amphoteric wetting agents and mixtures thereof.
15. The aqueous electroless copper plating bath of claim 1,
comprising a wetting agent selected from the group consisting of
polyethyleneglycol, polypropyleneglycol,
polyethyleneglycol-polypropyleneglycol co-polymers, alcohol
alkoxylates, alkoxylated fatty acid and mixtures thereof.
16. The aqueous electroless copper plating bath of claim 1, wherein
the concentration of the wetting agent in the aqueous electroless
copper plating bath ranges from 0.1 to 100 g/l.
17. The aqueous electroless copper plating bath of claim 1,
comprising at least one stabilizing agent.
18. The aqueous electroless copper plating bath of claim 17,
wherein the at least one stabilizing agent is selected from the
group comprising mercaptobenzothiazole, thiourea, cyanide and/or
ferrocyanide, cobaltocyanide salts, polyethyleneglycol derivatives,
2,2'-bipyridyl, methyl butynol, and propionitrile.
19. The aqueous electroless copper plating bath of claim 17,
wherein the at least one stabilizing agent has a concentration
ranging from 0.1 mg/l to 1000 mg/l.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a division of and claims priority
to U.S. application Ser. No. 14/906,370, filed 20 Jan. 2016, now
U.S. Pat. No. 10,077,498, which in turn is a U.S. National Stage
Application based on and claiming benefit and priority under 35
U.S.C. .sctn. 371 of International Application No.
PCT/EP2014/069410, filed 11 Sep. 2014, which in turn claims benefit
of and priority to European Application No. 13185862.3 filed 25
Sep. 2013, the entirety of both of which is hereby incorporated
herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a method for depositing a
copper seed layer onto a barrier layer which serves as a plating
base for successive electroplating. The method according to the
present invention is particularly suitable for dual damascene
plating in the manufacture of microchips and the like.
BACKGROUND OF THE INVENTION
[0003] Electroplating of a metal, particularly copper
electroplating, requires an electrically conductive seed layer in
case of substrate surfaces made of typical barrier materials such
as ruthenium and cobalt. Such a seed layer can in principle consist
of any kind of electrically conductive material. However, copper
seed layers are preferred because of the high intrinsic electrical
conductivity of copper.
[0004] Said seed layer has to fulfil several requirements such as a
sufficient adhesion to the underlying barrier layer and to the
metal deposited on top of said seed layer by electroplating.
Furthermore, the seed layer should have a homogeneous (narrow)
thickness distribution and a smooth outer surface. Such
requirements are of particular importance in the manufacture of
microchips and the like, where recessed structures which need to be
coated with such a copper seed layer can have dimensions as low as
in the nanometer range.
[0005] Methods for plating a copper seed layer onto a barrier layer
made of ruthenium are disclosed in U.S. Pat. No. 7,998,859 B2 and
U.S. Pat. No. 7,470,617 B2. Both methods utilize an aqueous
solution comprising a reducing agent to remove undesired surface
oxides from the ruthenium barrier layer prior to electroless copper
plating. Both methods further utilize standard electroless copper
plating bath composition comprising either NaOH of KOH as the sole
source for hydroxide ions. The roughness of the outer surface of
the copper seed layer in both cases is too high (comparative
Examples 1 to 4).
[0006] Accordingly, there is a need to provide a thin copper layer
deposited by electroless plating onto a barrier layer which results
in a copper seed layer having an outer surface with an improved
smoothness.
Objective of the Present Invention
[0007] It is the objective of the present invention to provide a
method for depositing a copper seed layer for successive
electroplating onto a barrier layer which has a homogeneous
thickness distribution and a smooth outer surface.
SUMMARY OF THE INVENTION
[0008] This objective is solved by a method for providing a copper
seed layer on top of a barrier layer comprising, in this order, the
steps of [0009] (i) providing a substrate comprising at least on a
portion of the outer surface a barrier layer, [0010] (ii)
contacting said substrate with an aqueous electroless copper
plating bath which comprises [0011] a. a water-soluble source for
Cu(II) ions, [0012] b. a reducing agent for Cu(II) ions, [0013] c.
at least one complexing agent for Cu(II) ions and [0014] d. at
least one source for hydroxide ions selected from the group
consisting of RbOH, CsOH and mixtures thereof.
[0015] The copper seed layer on top of a barrier layer obtained by
the method according to the present invention provides a sufficient
adhesion to the underlying barrier layer and the metal layer
electroplated onto said copper seed layer. Furthermore, the copper
seed layer has a homogeneous thickness distribution and the
required smooth surface. No activation of the barrier layer with a
noble metal activator is required prior to depositing the copper
seed layer by electroless plating thereon.
BRIEF DESCRIPTION OF THE FIGURES
[0016] FIG. 1 shows a scanning electron micrograph of a copper seed
layer obtained from an electroless plating bath containing NaOH as
the sole source for hydroxide ions and glyoxylic acid as the
reducing agent on a ruthenium barrier layer (Example 1
(comparative)).
[0017] FIG. 2 shows a scanning atomic force microscope (AFM) image
of a copper seed layer obtained from an electroless plating bath
containing NaOH as the sole source for hydroxide ions and glyoxylic
acid as the reducing agent on a ruthenium barrier layer (Example 1
(comparative)).
[0018] FIG. 3 shows a scanning electron micrograph of a copper seed
layer obtained from an electroless plating bath containing NaOH as
the sole source for hydroxide ions and formaldehyde as the reducing
agent on a ruthenium barrier layer (Example 2 (comparative)).
[0019] FIG. 4 shows a scanning atomic force microscope (AFM) image
of a copper seed layer obtained from an electroless plating bath
containing NaOH as the sole source for hydroxide ions and
formaldehyde as the reducing agent on a ruthenium barrier layer
(Example 2 (comparative)).
[0020] FIG. 5 shows a scanning electron micrograph of a copper seed
layer obtained from an electroless plating bath containing NaOH as
the sole source for hydroxide ions and glyoxylic acid as the
reducing agent on a cobalt barrier layer (Example 3
(comparative)).
[0021] FIG. 6 shows a scanning electron micrograph of a copper seed
layer obtained from an electroless plating bath containing KOH as
the sole source for hydroxide ions and formaldehyde as the reducing
agent on a ruthenium barrier layer (Example 4 (comparative)).
[0022] FIG. 7 shows a scanning atomic force microscope (AFM) image
of a copper seed layer obtained from an electroless plating bath
containing KOH as the sole source for hydroxide ions and
formaldehyde as the reducing agent on a ruthenium barrier layer
(Example 4 (comparative)).
[0023] FIG. 8 shows a scanning electron micrograph of a copper seed
layer obtained from an electroless plating bath containing CsOH as
the sole source for hydroxide ions and glyoxylic acid as the
reducing agent on a ruthenium barrier layer (Example 5
(invention)).
[0024] FIG. 9 shows a scanning atomic force microscope (AFM) image
of a copper seed layer obtained from an electroless plating bath
containing CsOH as the sole source for hydroxide ions and glyoxylic
acid as the reducing agent on a ruthenium barrier layer (Example 5
(invention)).
[0025] FIG. 10 shows a scanning electron micrograph of a copper
seed layer obtained from an electroless plating bath containing
CsOH as the sole source for hydroxide ions and formaldehyde as the
reducing agent on a ruthenium barrier layer (Example 6
(invention)).
[0026] FIG. 11 shows a scanning atomic force microscope (AFM) image
of a copper seed layer obtained from an electroless plating bath
containing CsOH as the sole source for hydroxide ions and
formaldehyde as the reducing agent on a ruthenium barrier layer
(Example 6 (invention)).
[0027] FIG. 12 shows a scanning electron micrograph of a copper
seed layer obtained from an electroless plating bath containing
CsOH as the sole source for hydroxide ions and glyoxylic acid as
the reducing agent on a cobalt barrier layer (Example 7
(invention)).
[0028] FIG. 13 shows a scanning atomic force microscope (AFM) image
of a copper seed layer obtained from an electroless plating bath
containing tetramethylammonium hydroxide as the sole source for
hydroxide ions and glyoxylic acid as the reducing agent on a
ruthenium barrier layer (Example 8 (comparative)).
[0029] FIG. 14 shows a scanning atomic force microscope (AFM) image
of a copper seed layer obtained from an electroless plating bath
containing tetrabutylammonium hydroxide as the sole source for
hydroxide ions and glyoxylic acid as the reducing agent on a
ruthenium barrier layer (Example 9 (comparative)).
DETAILED DESCRIPTION OF THE INVENTION
[0030] The substrate material suitable for the method for providing
a copper seed layer on top of a barrier layer according to the
present invention is preferably selected from silicon, a
low-.kappa.-dielectric material on a silicon substrate and glass.
Electronic devices made of such materials comprise microchips,
glass interposers and the like. Recessed structures in such
substrate materials need to be coated or filled with a metal which
is usually deposited by electroplating. The most relevant metal is
electroplated copper.
[0031] One particular application of the method for providing a
copper seed layer on top of a barrier layer according to the
present invention is electroplating a metal into very fine recessed
structures etched into a silicon substrate, a
low-.kappa.-dielectric material on a silicon substrate. Typical
low-.kappa.-dielectric materials are known in the art and comprise
fluorine-doped silicon dioxide, carbon-doped silicon dioxide,
porous silicon dioxide, porous carbon-doped silicon dioxide,
polyimide, polynorbornenes, benzocyclobutane, PTFE and spin-on
silicone based polymeric dielectrics such as
hydrogen-silsesquioxane and methyl-silsesquioxane.
[0032] Such very fine recessed structures can be blind vias (only
having one opening), through vias (having an opening on both sides)
as well as trenches (line patterns and the like). Such recessed
structures usually have geometric dimensions ranging from e.g. 14
nm to several hundred .mu.m or even mm.
[0033] This particular application can be divided in [0034] 1.
dual-damascene applications wherein the recessed structures are
formed in silicon and/or a low-.kappa.-dielectric material and
having an opening size in the nanometer or low micrometer range.
The copper seed layer may have a thickness in the range of 1 to 20
nm and [0035] 2. filling of through silicon vias (TSVs) which are
blind vias in silicon and/or a low-.kappa.-dielectric material on a
silicon substrate. The opening of TSVs is in the micrometer range
and the copper seed layer may have a thickness up to several
hundred nanometers or even one or more micrometer.
[0036] Another particular application of copper seed layers on top
of a barrier layer is in the manufacture of displays such as liquid
crystal based displays wherein a glass substrate is coated with one
or more metallic barrier layers. Electrical circuitry may be formed
by depositing a copper seed layer having a smooth outer surface
onto the one or more barrier layers followed by electroplating of a
metal such as copper thereon.
[0037] All of these applications require a homogeneous thickness
distribution of the copper seed layer and a smooth outer
surface.
[0038] Undesired diffusion and related processes of said
electroplated metal into the substrate material or vice versa must
be suppressed in many cases in order to obtain and/or maintain the
desired properties of the final electronic device. Accordingly, the
electroplated metal is separated by means of one or more barrier
layers deposited by either gas phase methods such as chemical
vapour deposition (CVD), physical vapour deposition (PVD) and
atomic layer deposition (ALD) and plasma-enhanced methods of the
aforementioned, or wet chemical processes such as electroless
plating. Only the most outer barrier layer in case of a stack of
two or more individual barrier layer materials will be in contact
with the electroless copper plating bath. The single barrier layer
material or most outer barrier layer material in case of stacks of
two or more individual barrier layers is selected from cobalt
nickel, ruthenium, tungsten, molybdenum, tantalum, titanium,
iridium, rhodium and alloys of the aforementioned.
[0039] Said "alloys of the aforementioned" comprises nitrides such
as tungsten nitride, tantalum nitride and cobalt nitrides such as
Co.sub.4N as well as phosphides and borides such as cobalt- and
nickel-phosphorous or -boron alloys, ternary cobalt- and
nickel-phosphorous alloys and ternary cobalt- and nickel-boron
alloys (e.g. Ni--Mo--P, Ni--W--P, Co--Mo--P, Co--W--P,
Co--W--B).
[0040] The thickness of said barrier layer or the outermost barrier
layer in case of a stack of several barrier layers usually has a
thickness in the range of 1 to 20 nm.
[0041] Removal of Undesired Surface Oxides from the Barrier
Layer:
[0042] In case the single barrier layer material or outermost
barrier layer material in case of stacks of individual barrier
layers is a ruthenium layer, surface oxides present on top of the
outer surface of said barrier layer materials may need to be
removed prior to depositing the copper seed layer by electroless
plating because they may prevent a full coverage of the copper seed
layer on the whole outer surface of said barrier layer.
[0043] Said surface oxides can be sufficiently removed prior to
step (ii) by treatment of a ruthenium surface with e.g. a
N.sub.2/H.sub.2 plasma and/or by treatment of the ruthenium surface
with a reducing agent provided in a solvent.
[0044] Wet-Chemical Removal of Undesired Surface Oxides from the
Barrier Layer:
[0045] Suitable reducing agents in a solvent for a sufficient
removal of said surface oxides are selected from the group
comprising glyoxylic acid, formaldehyde, hypophosphite, hydrazine,
dimethylamino borane, trimethylamine borane, N-methylmorpholino
borane and sodium borohydride. The concentration of said reducing
agent in the solvent preferably ranges from 0.001 to 10 mol/l, more
preferably from 0.005 to 5 mol/l and most preferably from 0.01 to 2
mol/l.
[0046] The solvent is preferably selected from the group comprising
water, alcohols such as propanol, glycol ethers such as
diethyleneglycol and mixtures thereof.
[0047] Mixtures of water and one or more of the aforementioned
organic solvents preferably comprise 50 wt.-% water and more
preferably less than 50 wt.-% water. The stability of the reducing
agent is improved when utilizing said water-(organic)solvent
instead of pure water or water-(organic)solvent mixtures containing
more than 50 wt.-% water.
[0048] The substrate having a ruthenium barrier layer is preferably
contacted with said reducing agent in a solvent for 30 s to 20 min
and more preferably for 1 to 10 min. The temperature of the
reducing agent-solvent mixture during said process step is
preferably held in a range from 20 to 90.degree. C. and more
preferably 50 to 90.degree. C.
[0049] Said reducing agent-solvent mixture preferably comprises a
wetting agent such as a cationic wetting agent, an anionic wetting
agent, a non-ionic wetting agent, an amphoteric wetting agent and
mixtures thereof. Such types of wetting agents are for example
disclosed in U.S. Pat. No. 7,998,859 B2.
[0050] The concentration of the wetting agent in the reducing
agent-solvent mixture preferably ranges from 0.1 to 100 g/l, more
preferably 0.25 to 25 g/l and most preferably 0.5 to 15 g/l.
[0051] Another suitable wet-chemical method for a sufficient
removal of undesired surface oxides on the outer surface of a
ruthenium barrier layer is the treatment of the ruthenium barrier
layer with nascent hydrogen which is generated in situ in a solvent
by applying a cathodic current to the substrate which is contacted
with said solvent. In this embodiment, nascent hydrogen serves as
the reducing agent provided in a solvent.
[0052] The voltage applied to the substrate preferably ranges from
0.5 to 20 V, more preferably 1 to 15 V and most preferably from 2
to 10 V and is held for 1 to 360 s, more preferably for 10 to 180 s
and most preferably for 20 to 120 s. The solvent is preferably
selected from the group comprising water, alcohols such as
propanol, glycol ethers such as diethyleneglycol, and mixtures
thereof.
[0053] The solvent containing in situ formed nascent hydrogen is
held during this process step at a temperature in the range of 10
to 80.degree. C., more preferably of 15 to 60.degree. C. and most
preferably of 20 to 40.degree. C.
[0054] The solvent preferably comprises a wetting agent selected
from cationic wetting agents, anionic wetting agents, non-ionic
wetting agents, amphoteric wetting agent, and mixtures thereof.
More preferably, the solvent comprises a non-ionic wetting agent or
mixtures of two or more non-ionic wetting agents. Most preferably,
the solvent comprises a non-ionic wetting agent selected from the
group comprising polyethyleneglycol, polypropyleneglycol,
polyethyleneglycol-polypropyleneglycol co-polymers, alcohol
alkoxylates such as branched secondary alcohol ethoxylates and
ethoxylated phenol or naphthol, alkoxylated fatty acid such as
ethoxylated fatty acids and mixtures thereof. The desired removal
of surface oxides with the nascent hydrogen as the reducing agent
is particularly supported by said non-ionic wetting agents.
[0055] The concentration of the wetting agent in the solvent or of
all wetting agents together in case more than one wetting agent is
utilized preferably ranges from 0.5 to 100 g/l, more preferably
from 1 to 25 g/l and most preferably from 5 to 15 g/l.
[0056] In one embodiment of the present invention two or more of
said means for reducing surface oxides on top of a ruthenium
barrier layer are combined. A rinsing step with a rinsing liquid
e.g. with water may separate said means for reducing surface oxides
prior to step (ii). The rinsing liquid may further comprise a
non-ionic wetting agent preferably selected from the group
comprising polyethyleneglycol, polypropyleneglycol,
polyethyleneglycol-polypropyleneglycol co-polymers, alcohol
alkoxylates such as branched secondary alcohol ethoxylates and
ethoxylated phenol or naphthol, alkoxylated fatty acid such as
ethoxylated fatty acids and mixtures thereof.
[0057] The concentration of the optional non-ionic wetting agent in
the rinsing liquid or of all wetting agents together in case more
than one non-ionic wetting agent is utilized preferably ranges from
0.5 to 100 g/l, more preferably from 1 to 25 g/l and most
preferably from 5 to 15 g/l.
[0058] The substrate may be rinsed for example with water prior to
step (ii) of the method according to the present invention.
[0059] Barrier layer materials which are not suitable for
depositing a copper layer by electroless (autocatalytic) plating
thereon require a separate activation prior to step (ii) of the
method according to the present invention. Said activation is known
in the art and for example used in the manufacture of printed
circuit boards and the like. Accordingly, barrier layer materials
which are not suitable for depositing a copper layer by electroless
(autocatalytic) plating thereon can for example be activated by
depositing a noble metal such as palladium by immersion plating
onto said barrier layer material which is then suited for
depositing copper by electroless (autocatalytic) plating
thereon.
[0060] Electroless Plating of the Copper Seed Layer onto the
Barrier Layer:
[0061] One particular drawback of such barrier layer(s) is their
electrical conductivity which is usually much lower than e.g. the
electrical conductivity of copper. Accordingly, a copper seed layer
having an increased electrical conductivity in comparison with
barrier layer materials is required for successive
electroplating.
[0062] The thickness of said copper seed layer preferably ranges
from 50 nm to 2 .mu.m for applications such as filling of through
silicon vias and manufacture of displays. In case of dual damascene
applications, the thickness of the copper seed layer preferably
ranges from 1 to 20 nm.
[0063] The copper seed layer is deposited by electroless plating
onto the barrier layer (or the outermost barrier layer in case of a
stack of two or more individual barrier layers) in step (ii) of the
method according to the present invention.
[0064] "Electroless plating" means "autocatalytic plating". Hence,
the copper plating bath utilized in step (ii) comprises a reducing
agent for Cu(II) ions.
[0065] The electroless copper plating bath utilized in the method
according to the present invention is preferably an aqueous plating
bath, i.e., the solvent is water.
[0066] The water-soluble source for Cu(II) ions is preferably
selected from the group comprising copper sulfate, copper chloride,
copper nitrate, copper acetate, copper methane sulfonate, copper
hydroxide, copper formate and hydrates thereof. The concentration
of Cu(II) ions in the electroless plating bath preferably ranges
from 0.05 to 20 g/l, more preferably from 0.1 to 10 g/l and most
preferably from 0.2 to 6 g/l.
[0067] The electroless plating bath further contains a reducing
agent selected from the group comprising formaldehyde, glyoxylic
acid, glucose, sucrose, cellulose, sorbitol, mannitol,
gluconolactone, hypophosphite, boranes, hydrazine, formic acid,
ascorbic acid and mixtures thereof. Most preferably, the reducing
agent is glyoxylic acid. The concentration of the reducing agent
preferably ranges from 0.1 to 100 g/l, more preferably from 0.5 to
50 g/l and most preferably from 1 to 20 g/l.
[0068] The electroless plating bath further comprises at least one
complexing agent for Cu(II) ions which is preferably selected from
the group comprising carboxylic acids, hydroxycarboxylic acids,
aminocarboxylic acids, alkanolamines, polyols and mixtures
thereof.
[0069] Particularly preferred combinations of a reducing agent and
at least one complexing agent for Cu(II) ions are:
[0070] 1. glyoxylic acid as reducing agent and at least one
polyamino disuccinic acid derivative as the complexing agent for
Cu(II) ions which is disclosed in WO 2013/050332 A2;
[0071] 2. glyoxylic acid as reducing agent and a combination of i)
at least one polyamino disuccinic acid derivative and ii) one or
more compounds selected from the group consisting of
ethylenediamine tetraacetic acid,
N'-(2-Hydroxyethyl)-ethylenediamine-N,N,N'-triacetic and
N,N,N',N'-Tetrakis-(2-hydroxypropyl)-ethylenediamine as complexing
agents for Cu(II) ions which is disclosed in EP 13161330.9; and
[0072] 3. glyoxylic acid as reducing agent and a combination of i)
N,N,N',N'-tetrakis-(2-hydroxypropyl)ethylenediamine and ii) one or
more from ethylenediamine tetraacetic acid, and
N'-(2-Hydroxyethyl)-ethylenediamine-N,N,N'-triacetic acid as
complexing agents for Cu(II) ions which is disclosed in EP
13161418.
[0073] The electroless plating bath further comprises a source for
hydroxide ions which is selected from the group consisting of RbOH,
CsOH and mixtures thereof. The concentration of hydroxide ions is
adjusted to obtain a pH-value for the electroless copper plating
bath which preferably ranges between 10 and 14 and more preferably
between 11 and 13.5.
[0074] The electroless plating bath according to the present
invention and utilized in the method for providing a copper seed
layer on top of a barrier layer is most preferably substantially
free of Na.sup.+ and K.sup.+ ions. "Substantially free of" is
defined herein as not intentionally or purposefully added to the
electroless plating bath.
[0075] It is possible that there is some small quantity of Na.sup.+
and/or K.sup.+ impurity present, but this is neither sought nor
desired.
[0076] The electroless plating bath according to the present
invention and utilized in the method for providing a copper seed
layer on top of a barrier layer is also most preferably
substantially free of tetraalkylammonium ions such as
tetramethylammonium ions.
[0077] In another embodiment of the present invention, said
electroless plating bath comprises Na.sup.+ and/or K.sup.+ ions in
addition to the at least one source for hydroxide ions selected
from the group consisting of RbOH, CsOH and mixtures thereof.
Accordingly, hydroxide ions may be provided in this embodiment of
the present invention in the form of at least 50 mol-% hydroxide
ions provided by sources selected from the group consisting of
RbOH, CsOH and mixtures thereof, and less than 50 mol-% hydroxide
ions are provided by adding NaOH and/or KOH to said electroless
plating bath. Hydroxide ions may also be provided completely in
this embodiment of the present invention by a source for hydroxide
ions selected from the group consisting of RbOH, CsOH and mixtures
thereof, and Na.sup.+ and/or K.sup.+ ions are added to said
electroless plating bath in form of e.g. the sodium and/or
potassium salt of a carboxylic acid, hydroxycarboxylic acid and/or
aminocarboxylic acid as the complexing agent for Cu(II) ions.
[0078] The electroless plating bath utilized in step (ii) of the
method according to the present invention is preferably free of
Na.sup.+ and K.sup.+ ions which are utilized as counter ions in the
source of hydroxide ions in commercially used electroless plating
baths for deposition of copper.
[0079] The roughness of the outer surface of the copper seed layer
is too high when using NaOH or KOH as the sole source of hydroxide
ions (Examples 1 to 4). The roughness of the outer surface of the
copper seed layer is also too high when using tetramethylammonium
hydroxide or tetrabutylammonium hydroxide as the sole source of
hydroxide ions (Examples 8 and 9)
[0080] The electroless plating bath optionally further comprises at
least one stabilizing agent which suppresses undesired reduction of
copper in the plating bath itself. The at least one stabilizing
agent is preferably selected from the group comprising
mercaptobenzothiazole, thiourea, cyanide and/or ferrocyanide,
cobaltocyanide salts, polyethyleneglycol derivatives,
2,2'-bipyridyl, methyl butynol and propionitrile.
[0081] The concentration of the at least one stabilizing agent
preferably ranges from 0.1 to 1000 mg/l, more preferably from 0.5
to 500 mg/l and most preferably from 1 to 250 mg/l.
[0082] In a preferred embodiment of the present invention, the
electroless plating bath further comprises a wetting agent selected
from the group comprising cationic wetting agents, anionic wetting
agents, non-ionic wetting agents, amphoteric wetting agents and
mixtures thereof.
[0083] Most preferably, the electroless plating bath comprises one
or more non-ionic wetting agents selected from the group comprising
polyethyleneglycol, polypropyleneglycol,
polyethyleneglycol-polypropyleneglycol co-polymers, alcohol
alkoxylates such as branched secondary alcohol ethoxylates and
ethoxylated phenol or naphthol, alkoxylated fatty acid such as
ethoxylated fatty acids and mixtures thereof. Particularly said
non-ionic wetting agents reduce the plating rate during electroless
copper plating onto the barrier layer. Thereby, the thickness
distribution of the copper seed layer on top of the barrier layer
is improved, i.e. more homogeneous.
[0084] The concentration of the optional wetting agent (or in case
of a mixture of two or more wetting agents the concentration of all
wetting agents together) preferably ranges from 0.1 to 100 g/l,
more preferably from 0.25 to 25 g/l and most preferably from 0.5 to
15 g/l.
[0085] The substrate can be for example contacted with the
electroless plating bath by dipping the substrate into the
electroless plating bath, by spraying the electroless plating bath
onto the substrate and by dispensing the electroless copper plating
bath onto the substrate. A particularly suitable plating tool which
can be used to carry out the method according to the present
invention is disclosed in US 2012/0213914 A1.
[0086] The electroless plating bath is preferably held at a
temperature in the range of 15 to 80.degree. C., more preferably 20
to 60.degree. C. and most preferably 25 to 45.degree. C. during
step (ii). The substrate is contacted with the electroless plating
bath during step (ii) for 5 s to 30 min, depending on the desired
thickness of the copper seed layer to be deposited and on the
plating rate obtained by the particular plating bath composition
and plating parameters.
[0087] In one embodiment of the present invention, an inert gas is
directed through the electroless plating bath during step (ii)
which increases the plating rate. The inert gas is preferably
selected from nitrogen, argon and mixtures thereof.
[0088] The life time of the electroless plating bath for depositing
the copper seed layer is improved by directing oxygen gas through
the electrolyte before step (ii) and/or between plating steps (ii)
in production.
[0089] Preferably, oxygen gas is directed through the electrolyte
before step (ii) and/or between plating steps (ii) in production,
and/or an inert gas is directed through the electroless plating
bath during step (ii).
[0090] The copper seed layer deposited in step (ii) has a
homogeneous thickness distribution even at a thickness of e.g. 10
nm or even less. Hence, the barrier layer is completely covered by
the copper seed layer even if the copper seed layer only has a
thickness of e.g. 10 nm or even less.
[0091] Furthermore, the outer surface of the copper seed layer is
smooth as desired. The smoothness expressed in S.sub.Q values and
obtained for electroless copper plating at 35.degree. C. for 22 min
with glyoxylic acid as the reducing agent preferably ranges from
0.1 to 4 nm, more preferably 0.2 to 3.9 nm and most preferably from
0.5 to 3.8 nm.
[0092] The smoothness expressed in S.sub.Q values obtained for
electroless copper plating at 35.degree. C. for 22 min with
formaldehyde as the reducing agent preferably ranges from 0.1 to 30
nm, more preferably from 1 to 28 nm and most preferably from 5 to
26 nm.
[0093] The "S.sub.Q value" is defined herein as the root mean
square height of the surface.
[0094] Said S.sub.Q value can be obtained by scanning the outer
surface of the copper seed layer with a scanning atomic force
microscope. Such methods and their application to determine the
smoothness of a metal surface and the respective S.sub.Q value are
known in the art.
[0095] The smoothness of copper seed layers obtained from an
electroless copper plating bath comprising formaldehyde as the
reducing agent is less good than compared to electroless copper
plating baths comprising glyoxylic acid as the reducing agent. The
technical effect of the at least one source for hydroxide ions
selected from the group consisting of RbOH, CsOH and mixtures
thereof is also observed with formaldehyde as the reducing
agent.
[0096] The substrate is then suited for depositing a metal onto the
copper seed layer by electroplating.
[0097] The method for providing a copper seed layer on top of a
barrier layer is also suitable for filling recessed structures such
as dual damascene structures completely with copper instead of only
providing a seed layer for successive electroplating of a metal
such as copper thereon. A recessed structure filled with a copper
deposit is obtained from this embodiment of the present
invention.
[0098] The metal which is deposited by electroplating onto the
copper seed layer can be in principle any metal or metal alloy
which can be electroplated onto a copper seed layer. Most
preferably, the metal which is deposited by electroplating onto the
copper seed layer is copper. Suitable bath compositions and plating
conditions for depositing a metal such as copper or a metal alloy
onto the copper seed layer are known in the art.
EXAMPLES
[0099] The invention will now be further illustrated by reference
to the following non-limiting examples.
[0100] General Procedures
[0101] Substrate Materials:
[0102] The substrate material in case of Examples 1 to 2, 4 to 6, 8
and 9 was silicon in form of wafers having a ruthenium barrier
layer attached to the wafer surface whereon copper was deposited by
electroless plating after reduction of surface oxides and
activation with a noble metal catalyst.
[0103] The substrate material in case of Examples 3 and 7 was
silicon in form of wafers having a cobalt barrier layer attached to
the wafer surface whereon copper was deposited by electroless
plating without reduction of surface oxides and activation with a
noble metal catalyst.
[0104] Reduction of Surface Oxides on Ruthenium as the Barrier
Layer Material:
[0105] The wet-chemical pre-treatment method consisted of dipping
the substrate comprising a ruthenium barrier layer into a solution
consisting of 2 g/l of dimethylamino borane as the reducing agent
in diethylene glycol as the solvent (T=65.degree. C., t=2 min).
[0106] Electroless Copper Plating Baths:
[0107] Two different electroless copper plating bath types were
used. The first type of copper plating bath comprised glyoxylic
acid as the reducing agent and an alkanolamine as complexing agent.
The second type of copper plating bath comprised formaldehyde as
the reducing agent and tartrate as complexing agent.
[0108] Different sources of hydroxide ions were added throughout
the examples in order to obtain a pH value of 13 in the copper
plating baths of both types.
[0109] The plating baths were held during copper deposition at
35.degree. C. and the plating time was 22 min.
[0110] Investigation of the Surface Smoothness of the Copper Seed
Layers:
[0111] The smoothness of the outer surface of the copper seed
layers was determined with a scanning atomic force microscope
(Digital Instruments, NanoScope equipped with a PointProbe.RTM.
from Nanosensors with a tip radius of less than 7 nm), scan size:
1.times.1 .mu.m (glyoxylic acid-based plating baths) and 5.times.5
.mu.m (formaldehyde-based plating baths), scan in tapping mode.
[0112] So values were obtained by these measurements and are
provided with the respective example below.
Example 1 (Comparative)
[0113] Copper was deposited onto a silicon wafer comprising a
ruthenium barrier layer after reduction of the surface oxides. The
sole source of hydroxide ions in the copper plating bath was NaOH
and the reducing agent was glyoxylic acid.
[0114] A scanning electron micrograph of the copper seed layer
outer surface is shown in FIG. 1.
[0115] The S.sub.Q value obtained was 11.71 nm from the scanning
atomic force microscope scan shown in FIG. 2.
Example 2 (Comparative)
[0116] Copper was deposited onto a silicon wafer comprising a
ruthenium barrier layer after reduction of the surface oxides. The
sole source of hydroxide ions in the copper plating bath was NaOH
and the reducing agent was formaldehyde.
[0117] A scanning electron micrograph of the copper seed layer
outer surface is shown in FIG. 3.
[0118] The S.sub.Q value obtained was 31.4 nm from the scanning
atomic force microscope scan shown in FIG. 4.
Example 3 (Comparative)
[0119] Copper was deposited onto a silicon wafer comprising a
cobalt barrier layer after reduction of the surface oxides. The
sole source of hydroxide ions in the copper plating bath was NaOH
and the reducing agent was glyoxylic acid.
[0120] A scanning electron micrograph of the copper seed layer
outer surface is shown in FIG. 5.
Example 4 (Comparative)
[0121] Copper was deposited onto a silicon wafer comprising a
ruthenium barrier layer after reduction of the surface oxides. The
sole source of hydroxide ions in the copper plating bath was KOH
and the reducing agent was formaldehyde.
[0122] A scanning electron micrograph of the copper seed layer
outer surface is shown in FIG. 6.
[0123] The S.sub.Q value obtained was 33.5 nm from the scanning
atomic force microscope scan shown in FIG. 7.
Example 5 (Invention)
[0124] Copper was deposited onto a silicon wafer comprising a
ruthenium barrier layer after reduction of the surface oxides. The
sole source of hydroxide ions in the copper plating bath was CsOH
and the reducing agent was glyoxylic acid.
[0125] A scanning electron micrograph of the copper seed layer
outer surface is shown in FIG. 8.
[0126] The S.sub.Q value obtained was 3.67 nm from the scanning
atomic force microscope scan shown in FIG. 9.
Example 6 (Invention)
[0127] Copper was deposited onto a silicon wafer comprising a
ruthenium barrier layer after reduction of the surface oxides. The
sole source of hydroxide ions in the copper plating bath was CsOH
and the reducing agent was formaldehyde.
[0128] A scanning electron micrograph of the copper seed layer
outer surface is shown in FIG. 10.
[0129] The S.sub.Q value obtained was 24.0 nm from the scanning
atomic force microscope scan shown in FIG. 11.
Example 7 (Invention)
[0130] Copper was deposited onto a silicon wafer comprising a
cobalt barrier. The sole source of hydroxide ions in the copper
plating bath was CsOH and the reducing agent was glyoxylic
acid.
[0131] A scanning electron micrograph of the copper seed layer
outer surface is shown in FIG. 12.
Example 8 (Comparative)
[0132] Copper was deposited onto a silicon wafer comprising a
ruthenium barrier layer after reduction of the surface oxides. The
sole source of hydroxide ions in the copper plating bath was
tetramethylammonium hydroxide and the reducing agent was glyoxylic
acid (example in accordance with US 2004/0152303 A1).
[0133] The S.sub.Q value obtained was 9.3 nm from the scanning
atomic force microscope scan shown in FIG. 13.
Example 9 (Comparative)
[0134] Copper was deposited onto a silicon wafer comprising a
ruthenium barrier layer after reduction of the surface oxides. The
sole source of hydroxide ions in the copper plating bath was
tetrabutylammonium hydroxide and the reducing agent was glyoxylic
acid.
[0135] The S.sub.Q value obtained was 20.07 nm from the scanning
atomic force microscope scan shown in FIG. 14.
* * * * *