U.S. patent application number 11/291679 was filed with the patent office on 2006-08-10 for membrane-limited selective electroplating of a conductive surface.
Invention is credited to Stephen Mazur.
Application Number | 20060175202 11/291679 |
Document ID | / |
Family ID | 36216955 |
Filed Date | 2006-08-10 |
United States Patent
Application |
20060175202 |
Kind Code |
A1 |
Mazur; Stephen |
August 10, 2006 |
Membrane-limited selective electroplating of a conductive
surface
Abstract
This invention relates to processes and apparati for selectively
electroplating a metal layer or layers into recessed topographic
features on a conductive surface. The processes and apparati of the
invention are useful for fabricating metal circuit patterns, for
example for creating copper interconnects between integrated
circuit elements embedded in a thin layer of dielectric material on
the surface of a semiconductor wafer.
Inventors: |
Mazur; Stephen; (Wilmington,
DE) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1128
4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
36216955 |
Appl. No.: |
11/291679 |
Filed: |
November 30, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60631583 |
Nov 30, 2004 |
|
|
|
Current U.S.
Class: |
205/149 ;
204/252 |
Current CPC
Class: |
C25D 5/34 20130101; H01L
21/76877 20130101; C25D 5/02 20130101; H01L 21/2885 20130101; H05K
3/423 20130101; H05K 2201/09563 20130101; C25D 17/001 20130101;
H05K 2203/0557 20130101; C25D 17/008 20130101; C25D 7/123
20130101 |
Class at
Publication: |
205/149 ;
204/252 |
International
Class: |
C25D 7/00 20060101
C25D007/00; C25B 9/00 20060101 C25B009/00 |
Claims
1. A process of electroplating metal onto a conductive surface,
wherein the conductive surface comprises plateaus and trenches, the
method comprising: (a) contacting the conductive surface with an
electroplating solution comprising platable metal ions; (b)
providing an ion-conducting membrane comprising a first surface and
an opposing second surface, wherein the membrane is substantially
impermeable to the platable metal ions in the electroplating
solution; (c) providing an anolyte composition which contacts an
anode and the first surface of the membrane; (d) positioning the
second surface of the membrane in close proximity to, or in
sensible contact with, the conductive surface; and (e) applying a
voltage between the anode and the conductive surface to
electroplate at least a portion of the metal ions in the
electroplating solution onto the conductive surface to form metal
layers on the plateaus and in the trenches, wherein the thickness
of metal electroplated in the trenches is greater than the
thickness of the metal layer electroplated on the plateaus.
2. The process of claim 1, wherein the anolyte composition
comprises water, an aqueous solution, a low-conductivity fluid, a
conductive solution, a conductive fluid, a conductive slurry, or a
conductive gel.
3. The process of claim 1, wherein substantially all of the
platable metal ions in the electroplating solution are cations, or
complexes having a positive net charge, and the membrane is an
anion-selective ion-conducting membrane.
4. The process of claim 1, wherein substantially all of the
platable metal ions in the electroplating solution are anions, or
complexes having a negative net charge, and the membrane is a
cation-selective ion-conducting membrane.
5. The process of claim 1, wherein the membrane comprises a
polymeric ionomer functionalized with acid groups having a pKa less
than 5.
6. The process of claim 5, wherein the polymeric ionomer is a
perfluorosulfonic acid/PTFE copolymer.
7. The process of claim 1, wherein the conductive surface and the
membrane are moved relative to each other in such a way that the
area of contact of the membrane moves across the conductive
surface.
8. The process of claim 1, wherein the platable metal ions comprise
a metal selected from silver, nickel, cobalt, tin, aluminum,
copper, lead, tantalum, titanium, iron, chromium, vanadium,
manganese, zinc, zirconium, niobium, molybdenum, ruthenium,
rhodium, hafnium, tungsten, rhenium, osmium, iridium, and
combinations thereof.
9. The process of claim 8, wherein the platable metal ions comprise
silver, nickel, cobalt, tin, copper, or aluminum.
10. The process of claim 1, wherein the voltage is applied in such
a way as to generate a constant current.
11. The process of claim 1, wherein the voltage is varied with time
between selected voltage values.
12. The process of claim 1, wherein the trenches have lateral
dimensions in the range of from about 0.01 micron to about 100
microns.
13. An apparatus for electroplating metal onto a conductive
surface, the conductive surface comprising plateaus and trenches,
the apparatus comprising: (a) a fluid source providing the
conductive surface with an electroplating solution comprising
platable metal ions; (b) a charge-selective ion-conducting membrane
comprising a first surface and an opposing second surface, wherein
the membrane is substantially impermeable to the platable metal
ions in the electroplating solution, and is adapted for the second
surface to be placed in close proximity to or in sensible contact
with the conductive surface; (c) an anode in electrical contact
with the first surface of the membrane; and (d) a power source
capable applying a voltage between the anode and the conductive
surface to generate a flow of electrical current in an amount
sufficient to electroplate at least a portion of the metal ions in
the electroplating solution onto the conductive surface.
14. The electroplating apparatus of claim 13, wherein the anode is
in sensible contact with the first surface of the charge-selective
ion-conducting membrane.
15. The electroplating apparatus of claim 13, wherein the anode
comprises a porous electrochemically inactive material with at
least one surface that is flat and smooth.
16. The electroplating apparatus of claim 13, further comprising a
porous non-conducting spacer that is disposed between the membrane
and the anode, wherein the porous non-conducting spacer comprises a
material selected from open-cell polymeric foams, open-cell
polymeric gels, woven fabrics, non-woven fabrics, paper, felt, and
porous ceramics.
17. The electroplating apparatus of claim 13, further comprising an
electrically insulating mask covering a portion of the first or
second surface of the membrane, wherein the electrically insulating
mask comprises a polyolefin or a halogenated polyolefin.
Description
FIELD OF THE INVENTION
[0001] This invention relates to processes and apparati for
selectively electroplating a metal layer or layers into recessed
topographic features on a conductive surface. The processes and
apparati of the invention are useful for fabricating metal circuit
patterns, for example for creating copper interconnects between
integrated circuit elements embedded in a thin layer of dielectric
material on the surface of a semiconductor wafer.
BACKGROUND
[0002] In the damascene process for fabricating integrated
circuits, the electrical interconnections are created as patterns
of lines and holes etched through a dielectric layer on the surface
of the wafer. Such patterns are then filled with metallic copper,
and electroplating is commonly used. An ideal deposition process
would completely fill the recesses in the dielectric layer with
copper to a level that is flush with the surrounding plateau
surfaces and not deposit any copper on the plateau surfaces.
[0003] Although conventional electroplating technology can provide
control over thickness and uniformity of the plated layer, no
practical method has been disclosed that selectively deposits a
metal layer into the holes and trenches or the recessed areas in
the dielectric layer and simultaneously precludes depositing a
metallic layer of comparable thickness on top of the plateaus
separating the circuit features. More often, under conventional
electroplating conditions, a thick layer of copper is deposited on
the plateaus and must be removed by a highly specialized polishing
process to planarize the surface, and smooth it to within an
extremely fine tolerance, and simultaneously avoid loss of, or
damage to, the circuit features.
SUMMARY OF THE INVENTION
[0004] One aspect of the present invention is a process of
electroplating metal onto a conductive surface, wherein the
conductive surface comprises plateaus and trenches, the method
comprising: [0005] (a) contacting the conductive surface with an
electroplating solution comprising platable metal ions; [0006] (b)
providing an ion-conducting membrane comprising a first surface and
an opposing second surface, wherein the membrane is substantially
impermeable to the platable metal ions in the electroplating
solution; [0007] (c) providing an anolyte composition which
contacts an anode and the first surface of the membrane; [0008] (d)
positioning the second surface of the membrane in close proximity
to, or in sensible contact with, the conductive surface; and [0009]
(e) applying a voltage between the anode and the conductive surface
to electroplate at least a portion of the metal ions in the
electroplating solution onto the conductive surface to form metal
layers on the plateaus and in the trenches, wherein the thickness
of metal electroplated in the trenches is greater than the
thickness of the metal layer electroplated on the plateaus.
[0010] Another aspect of the present invention is an apparatus for
electroplating metal onto a conductive surface, the conductive
surface comprising plateaus and trenches, the apparatus comprising:
[0011] (a) a fluid source providing the conductive surface with an
electroplating solution comprising platable metal ions; [0012] (b)
a charge-selective ion-conducting membrane comprising a first
surface and an opposing second surface, wherein the membrane is
substantially impermeable to the platable metal ions in the
electroplating solution, and is adapted for the second surface to
be placed in close proximity to or in sensible contact with the
conductive surface; [0013] (c) an anode in electrical contact with
the first surface of the membrane; and [0014] (d) a power source
capable applying a voltage between the anode and the conductive
surface to generate a flow of electrical current in an amount
sufficient to electroplate at least a portion of the metal ions in
the electroplating solution onto the conductive surface.
[0015] These and other aspects of the present invention will be
apparent to those skilled in the art in view of the present
disclosure and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The description of the invention is aided by use of the
following figures, which are not intended to be drawn to scale:
[0017] FIG. 1A shows a cross-section of damascene wafer showing
copper circuit features 9 embedded in dielectric layer 10;
[0018] FIG. 1B shows a cross-section showing deposition of copper
layer 17 onto sub-mircon topographic features under conventional
transport-limited electroplating conditions;
[0019] FIG. 2A shows a schematic illustration of membrane-limited
electroplating into a topographic recess employing an
anion-conducting membrane with an acidic copper sulfate plating
solution;
[0020] FIG. 2B shows a schematic illustration of membrane-limited
electroplating into a topographic recess employing a
cation-conducting membrane with a basic cyanocuprate plating
solution;
[0021] FIG. 3 shows a schematic cross-section of a membrane-limited
electroplating apparatus employing hydrostatic pressure to seal the
membrane 13 against the substrate;
[0022] FIG. 4 shows a schematic cross-section of a membrane-limited
electroplating apparatus employing mechanical force from a porous
anode 18 with smooth flat surface to seal the membrane 13 against
the substrate;
[0023] FIG. 5 shows a schematic cross-section of a membrane-limited
electroplating apparatus employing mechanical force from a porous
spacer 19 with smooth flat surface to seal the membrane 13 against
the substrate; and
[0024] FIG. 6 shows a schematic cross-section of a membrane-limited
electroplating apparatus employing a low conductivity fluid 20 as
the anolyte and mechanical force from a porous anode 18 with smooth
flat surface to seal the membrane 13 against the substrate.
DETAILED DESCRIPTION
[0025] One embodiment of this invention is an apparatus for
electroplating metal onto a conductive surface, the conductive
surface comprising plateaus and trenches, the apparatus comprising:
[0026] (a) a fluid source providing the conductive surface with an
electroplating solution comprising platable metal ions; [0027] (b)
a charge-selective ion-conducting membrane comprising a first
surface and an opposing second surface, wherein the membrane is
substantially impermeable to the platable metal ions in the
electroplating solution, and is adapted for the second surface to
be placed in close proximity to or in sensible contact with the
conductive surface; [0028] (c) an anode in electrical contact with
the first surface of the membrane; and [0029] (d) a power source
capable applying a voltage between the anode and the conductive
surface to generate a flow of electrical current in an amount
sufficient to electroplate at least a portion of the metal ions in
the electroplating solution onto the conductive surface.
[0030] Unless otherwise stated, the following terms when used
herein have the meanings set forth below.
[0031] By "trenches" is meant recessed features on the substrate or
conductive surface. In the present specification, "trenches,"
"recessed features," "holes," "recessed trenches," "topographic
recesses," and "vias," may be used alternatively, in conjunction,
selectively, or interchangeably. Unless otherwise specified, usage
of any of these terms includes every type of recessed feature that
is not a plateau and the meaning is construed to comprehensively
include all types of features.
[0032] By "plateau," is meant the generally flat area of the
substrate or conductive surface that is at the level of the top of
the trenches and/or vias.
[0033] Unless otherwise specified, a "conductive" fluid or solution
has conductivity greater than about 5 mS/cm, preferably equal to or
greater than about 30 mS/cm, more preferably equal to or greater
than about 100 mS/cm.
[0034] A "conductive surface" has a sheet resistance no greater
than about 10 milli-ohms per square.
[0035] A "low-conductivity" fluid or solution has conductivity
below about 1000 .mu.S/cm. "Non-platable metal ions" are known to
those skilled in the art and include, for example, Na and K.
[0036] The electroplating solution may contain, in addition to the
platable metal ions, other electrolytes, surfactants, and/or other
additives well known in the art and variously designated as
"brighteners", "levelers", or "accelerators".
[0037] Any apparatus or device suitable for supplying
electroplating solution to an area between a membrane and a
substrate is useful, including, for example, baths and sprayers.
The electroplating solution can be supplied at any pressure, and
the supply can be intermittent or continuous. The electroplating
solution can be of one composition, or can change composition
during the plating process.
[0038] To drive the electroplating process, a source of DC
electrical power is connected between the conductive surface (which
functions as the cathode) and the anode. The source of DC power can
be steady or can advantageously provide pulses and/or variable DC
power. A typical damascene wafer comprises a barrier layer that may
not provide sufficient electrical connection to a power source and
thus cannot by itself serve as a cathode. For this reason a seed
layer of metal, for example copper, covers the barrier layer to
provide the electrical connection to the power source.
[0039] The anode is an electrically conductive material such as a
metal or alloy (e.g., stainless steel, platinum, palladium or the
dimensionally stable anodes commonly used in the chlor-alkali
process) or carbon. Electrical contact between the anode and the
first surface of the membrane can beneficially be through an
anolyte contacting the anode and the first surface of the membrane,
wherein preferably the anolyte is a conductive solution, fluid, or
composition.
[0040] The anolyte also acts as a source and/or sink for ions
passing through the membrane. The anolyte solution may comprise
water, a polar organic solvent, or a combination of such solvents
and beneficially also includes solutes such as acids, bases or
salts. Higher conductivity anolyte compositions, for example having
a conductivity of at least 20 mS/cm, are generally preferred, as
they can reduce voltage loss of current passing from the anode and
through the anolyte. The anolyte can contain one or more
non-plating metal ions, e.g., Na, K, or such, For use with
anion-conducting membranes, the anolyte should not contain any
readily reducible negatively charged anions. For use with
cation-conducting membranes, the anolyte should not contain any
readily reducible positively charged cations.
[0041] The membrane-limited selective electroplating process can be
used for deposition of a wide variety of platable metals and metal
alloys. Suitable metals include silver, nickel, cobalt, tin,
aluminum, copper, lead, tantalum, titanium, iron, chromium,
vanadium, manganese, zinc, zirconium, niobium, molybdenum,
ruthenium, rhodium, hafnium, tungsten, rhenium, osmium, iridium,
and combinations thereof. Preferred metals include silver, nickel,
cobalt, tin, aluminum, copper, lead and combinations thereof. The
method is particularly suitable for electroplating copper and/or
copper-containing alloys on damascene wafers.
[0042] Membrane-limited selective electroplating may employ
conventional electroplating solutions comprising salts of metal
ions or complex metal ions and other ingredients, for example acids
or bases, buffers, surfactants and/or other additives known in the
electroplating art. Any or all adjuvants known for use in
electroplating solutions can be used in the processes herein. The
platable metal in the electroplating solution either has a positive
charge or a negative charge. Common commercial aqueous
electroplating solutions fall into two general categories,
depending upon whether the dissolved metal ions are positively
charged cations or negatively charged anions. Membrane-limited
selective electroplating may use either type of plating solution
depending on the type of membrane that is employed. It is desirable
that substantially all (meaning at least 80%) of the platable metal
ions in electroplating solutions for use with an anion-conducting
membrane are present in the form of positively charged cations. On
the other hand, substantially all of platable metal ions in
electroplating solutions for use with a cation-conducting membrane
are desirably in the form of negatively charged anions.
[0043] The electroplating solution for copper plating is commonly
either an acid solution containing, for example CuSO.sub.4 in
aqueous H.sub.2SO.sub.4, or a solution containing basic cyanide or
other nitrogen-containing-ligand, for example CuCN and NaCN in
aqueous NaOH or Na.sub.2CO.sub.3. In the former example, the
platable copper species is the hydrated cupric cation
Cu(H.sub.2O).sub.n.sup.+2, whereas in the latter example the
platable copper species is the complex cyanocuprate anion
Cu(CN).sub.3.sup.-2.
[0044] The ion-conducting membrane serves two functions. The first
function of the ion-conducting membrane is to displace plating
solution from the plateau areas of the conductive surface while
trapping plating solution within the recessed areas. The second
function of the membrane is to serve as a gate that allows certain
ions to carry electrical current through the membrane, but
specifically prevents electrochemically active metal ions from
contacting or plating onto the plateau areas.
[0045] Suitable charge-selective ion-conducting membranes include
film-forming ionic polymers that are stable under the conditions of
the electroplating process. Ionic polymer membranes useful in
electro-coating, electro-dialysis, the chloralkali process and
fuel-cells may also be useful in the electroplating process herein.
The ion-conducting membrane can be of any thickness, but
advantageously the membrane thickness is greater than the width of
trenches to be filled with electroplated metal. In practice, the
membrane thickness is typically at least 2 times the largest width
of trenches to be filled with electroplated metal. Exemplary
thickness of the membrane is in the range of from about 40 microns
to about 500 microns, or alternatively about 3 to about 120 mils.
The reason for having appreciable thickness is that the thickness
will resist bending and, coupled with the stiffness of the
membrane, should be sufficient so that the membrane does not
conform to the topography of the conductive surface. The
distribution of charged moieties in the pores need not be uniform,
and the membrane can comprise one or more separate membranes
laminated one to another.
[0046] Similarly, the membrane is advantageously sufficiently stiff
and incompressible so that the active portion of the membrane does
not conform to the topography of the conductive surface. The
stiffness and compressibility of the membrane can vary with degree
of saturation as well as the ion content in the membrane, but
generally, most commercially available Nafion.RTM. and Flemion.RTM.
cation-exchange membranes, or Fumatech FAP and PCA60 anion-exchange
membranes have the requisite stiffness and incompressibility when
the saturation level is near 100%.
[0047] Suitable ion-conducting membranes are substantially
impervious (impermeable) to the platable metal ions in the
electroplating solution. By substantially impermeable to the metal
ions in the electroplating solution we mean firstly, that for
cation-exchange membranes, the transference number for cations is
at least 0.9, and secondly, that at least 80% of the platable metal
ions are anions. Similarly for anion-exchange membranes, the
transference number for anions is at least 0.9, and at least 80% of
the platable metal ions are cations. Under conditions where
electroplating requires transfer of a cation, a cation-conducting
membrane is used. An exemplary cation-conducting membrane comprises
a polymeric ionomer functionalized with at least one type of acidic
moiety. Cation-selective ion-conducting membranes (also called
cation-exchange membranes), generally comprise organic polymer
films with acidic functional groups (e.g., --CO.sub.2H or
--SO.sub.3H), bound covalently to the polymeric backbone. In one
embodiment, the cation-conducting membranes are formed from
polymeric ionomers functionalized with strong acid groups that have
a pKa of less than about 3. Sulfonic acid groups are preferred
strong acid groups. Preferred polymeric ionomers are copolymers of
fluorinated and/or perfluorinated olefins and monomers containing
strong acid groups. An exemplary cation-selective membrane may have
1.0 to 4.0 milli-equivalents of strong acid groups per cubic
centimeter of membrane. Suitable membranes include
polytetrafluorethylene polymer-based membranes, perfluorocarboxylic
acid/PTFE copolymers, polymeric ionomers functionalized with both
sulfonic acid groups and carboxylic acid groups, and
perfluorosulfonic acid/ polytetrafluorethylene copolymer membranes.
Other acid moieties can be attached to the membrane as an
alternative to, or in addition to, the carboxylic acid moieties
and/or the sulfonic acid moieties, including for example a
sulfanilamide moiety, a phosphonate moiety, a sulfonyl moiety, or
any combination thereof, wherein the acidic moieties can
independently be substituted with, for example, a C.sub.1 to
C.sub.4 alkyl group.
[0048] Commercially available cation-conducting membranes useful in
the processes of this invention include Flemion.RTM.
perfluorocarboxylate ionomer membranes (Asahi Glass Co., Ltd,
Yokahama, Japan) and/or Nafion.RTM. perfluorosulfonate ionomer
membranes (E.l. du Pont de Nemours, Inc., Wilmington, Del.), which
are composed of fluorocarbon chains bearing highly acidic
carboxylic and sulfonic acid groups, respectively. On exposure to
water, the acid groups of Nafion.RTM. ionize, leaving fixed
sulfonate anions and mobile hydrated protons. The protons may be
readily exchanged with various metal cations. Nafion.RTM. is
particularly well-suited for use in membrane-limited selective
electroplating due to its strong common-ion exclusion, high
conductivity, strong acidity, chemical stability and robust
mechanical properties.
[0049] In one embodiment the membrane is layered, and comprises a
fluoropolymer membrane comprising at least two integrally laminated
layers including a first layer made of a perfluorocarbon polymer
having carboxylic acid groups as its ion exchange groups, and a
second layer comprising perfluorocarbon polymer having sulfonic
acid groups as its ion exchange groups. Alternatively, the layers
can be separated by a fluid layer. A suitable membrane can be a
single layer having both sulfonic and carboxylic groups made, for
example, by the copolymerization of a carboxylic acid type monomer
with a sulfonic acid type monomer, or by the copolymerization of a
carboxylic acid type monomer with a sulfonic acid type monomer, or
by impregnating a sulfonic acid type fluoropolymer membrane with a
carboxylic acid type monomer, followed by polymerization. Suitable
membranes include those formed from a blend comprising a sulfonic
acid group-containing polymer and a carboxylic acid
group-containing polymer, which is laminated on a sulfonic acid
group membrane, as described in U.S. Pat. No. 4,176,215, and herein
incorporated by reference.
[0050] Under conditions where electroplating requires transfer of
an anion, an anion-conducting membrane is used. An anion-conducting
membrane comprises a polymeric ionomer functionalized with at least
one type of basic moiety, for example quaternary ammonium groups.
Tertiary or lower amino groups are also suitable functional groups.
Anion-selective ion-conducting membranes (also called
anion-exchange membranes) generally comprise organic polymer films
with positively charged covalently bound functional groups such as
ammonium ions --NH.sub.3.sup.+, NH.sub.2R.sup.+, --NHR.sub.2.sup.+,
or --NR.sub.3.sup.+, or basic salts such as --NRH.sub.2OH,
NR.sub.2HOH, or NR.sub.3OH, where R is an organic radical. When
saturated with water, these functional groups hydrate and
dissociate. The resulting cations --NH.sub.3.sup.+,
--NRH.sub.2.sup.+, --NR.sub.2H.sup.+, and NR.sub.3.sup.+ remain
confined within the membrane while the hydroxide ions --OH, are
free to diffuse, migrate and exchange with other anions in adjacent
solutions. An exemplary anion-conducting membrane may have 5 to 200
microequivalents of basic moieties per cm.sup.2 of membrane area.
Examples of anion-selective ion-conducting membranes (anion
conducting membranes) include the PC amine-functionalized epoxide
polymers (PCA--Polymerchemie Altmeier GmbH, Heusweiler, Germany).
Strongly basic styrenic anion-conductive membranes can be formed
from a cross-linked poly-styrene-divinylbenzene that is
chloromethylated using a Lewis acid and further functionalized by
addition of a tertiary amine. Methods for making anion-conducting
membranes can be adapted from methods to make anion-exchange
membranes described in U.S. Pat. No. 6,646,083, which is
incorporated by reference herein.
[0051] Also provided are processes for electroplating metal onto
conductive surfaces.
[0052] According to one embodiment there is provided a process for
electroplating metal onto a conductive surface, wherein the
conductive surface comprises plateaus and trenches, the method
comprising: [0053] (a) contacting the conductive surface with an
electroplating solution comprising platable metal ions; [0054] (b)
providing an ion-conducting membrane comprising a first surface and
an opposing second surface, wherein the membrane is substantially
impermeable to the platable metal ions in the electroplating
solution; [0055] (c) providing an anolyte composition which
contacts an anode and the first surface of the membrane; [0056] (d)
positioning the second surface of the membrane in close proximity
to, or in sensible contact with, the conductive surface; and [0057]
(e) applying a voltage between the anode and the conductive surface
to electroplate at least a portion of the metal ions in the
electroplating solution onto the conductive surface to form metal
layers on the plateaus and in the trenches, wherein the thickness
of metal electroplated in the trenches is greater than the
thickness of the metal layer electroplated on the plateaus.
[0058] Electroplating occurs when a portion of the external surface
of the ion-conducting membrane is brought into sensible contact
with a portion of the conductive surface that is covered by the
electroplating solution, with the substrate held at a voltage more
negative than the open circuit voltage. The term "sensibly contact"
means there may be a thin layer of fluid disposed between the
membrane and the wetted plateaus; preferably there is none. Some
solution remains trapped within the recessed features. The trapped
electroplating solution within the recesses serves as a source of
metal ions in an electroplating step. The continuous or
intermittent exchange of the plating solution within the recessed
features with fresh plating solution is usually achieved by moving
the membrane with respect to the conductive surface, where the
exposed portions of the surface not contacting the membrane are
subject to a rinse of fresh electroplating solution. A membrane
moving along a surface may not displace all the electroplating
solution from the plateaus, and controlling the velocity of the
membrane and the pressure exerted on the membrane can influence the
thickness of any layer of electroplating solution disposed between
the plateaus and the membrane.
[0059] The conductive surface typically comprises a metal such as
copper, but other metals may be used as long as they provides
suitable electrical contact to the power source and adhesion to the
plated copper.
[0060] The distance between the membrane and the plateaus desirably
provides an electroplating solution layer between the plateaus and
the membrane that is less than twice the depth of the trenches
preferably much less than twice the depth of the trenches. The
"depth of the trenches" is the difference in height between the
floor of trenches and vias to be filled and the top of the
surrounding plateaus. For example, if the membrane contacts and
electroplates material onto a damascene wafer where the depth of
the trench to be filled is about 1 micron, then the average height
of the electroplating solution disposed between a membrane and a
nearby plateau is less than 0.5 microns, and may preferably be less
than 10 nanometers. When the membrane is being moved relative to
the conductive surface, this layer may provide lubrication between
the membrane and the conductive surface.
[0061] In a process of the current invention, the pressure exerted
by the membrane on the conductive surface is sufficient to reduce
the thickness of the layer of electroplating solution disposed
between the top of plateaus and the membrane to the desired
thickness. For example, the pressure can range from about 0.03 to
about 30 psi.
[0062] The velocity of the membrane relative to the conductive
surface may range from about 0 to about 200 cm/sec or more, but is
typically from about 1 cm/sec to about 30 cm/sec.
[0063] Then the membrane is held in sensible contact with the
conductive surface and a suitable voltage is applied between the
anode and cathode under these conditions, metal ions of the
electroplating solution are reduced to elemental metal and are
deposited into the trenches of the conductive surface. Unlike
conventional electroplating, a disproportionately large portion of
the electrical current flows through small volumes of the
electroplating solution trapped within the topographic recesses
(trenches) of the surface. The entrapment of the electroplating
solution is achieved by holding the conductive surface in intimate
contact with a first surface of a charge-selective ion-conducting
membrane. The conductive surface can be held stationary and the
membrane moved, or the membrane can be held stationary and the
surface moved, or both the surface and the membrane can be in
motion. The relative motion may be parallel or perpendicular to the
conductive surface or some combination of the two.
[0064] The rate of depletion of platable metal ions in the
electroplating solution is typically not constant, and the rate of
plating from a trapped volume of electroplating solution can slow
over time as the concentration of platable ions in the
electroplating solution is depleted. In some embodiments, the
amount of time for which a given portion of electroplating solution
is trapped without being refreshed is at least sufficient to lower
the average concentration of platable metal ions by at least 30% in
the layer of fluid disposed between the membrane and the recessed
areas. As the recessed areas become filled with copper, they retain
progressively less electroplating solution, so that metal ion
depletion occurs more rapidly. There is no advantage in allowing
the concentration to fall to less than 90% of its original value.
The amount of time to electroplate before replenishing or replacing
the electroplating solution disposed in a trench can in some cases
beneficially be changed as an endpoint is approached.
[0065] If the membrane is moving relative to the surface, then
fresh electroplating solution is supplied to the area between the
membrane and the surface. Since electroplating solution is
typically disposed on the conductive surface prior to the membrane
passing over that surface, it is important to prevent current
flowing to the plateau areas outside the area where the membrane
sensibly contacts the surface. Once the membrane contacts the
surface, then electroplating solution is displaced from the
plateaus, and the electroplating process can beneficially proceed.
In one embodiment of the invention, in order to avoid plating metal
onto the plateau areas the electrical circuit is temporarily opened
or the voltage is set to the open-circuit voltage during
disengagement, movement, and re-engagement of the membrane.
[0066] In other embodiments, such as those represented in FIGS.
3-6, different areas of the surface may be systematically engaged
and disengaged from the membrane by continuously moving the
membrane across the conductive surface. In that way fresh plating
solution is continuously provided to the recessed areas without
need to interrupt the current. Moreover, since at any given time
the current flows only to a localized area of the surface, which
contacts the membrane, the uniformity of deposition in recesses
over the entire surface can be systematically optimized by
regulating the integrated residence times in localized areas. The
rate or velocity at which the membrane moves across the surface
determines the rate at which fresh plating solution is supplied to
the recessed areas: the greater this velocity, the greater the rate
of supply.
[0067] During the electroplating process, there can be a
substantial flux of solvent across the membrane and the surface
during the electroplating process, and an accompanying substantial
change in concentrations of constituents in the electroplating
solution. In addition, the composition of the anolyte changes as
various reagents are either consumed or generated by anodic
reactions and other reagents may accumulate or be lost through the
membrane. In order to maintain uniform processing conditions, it is
advantageous to maintain substantially stable compositions for the
anolyte and electroplating solutions. Therefore, it may be
advantageous to remove and replace used solutions from the anode
compartment and from the conductive surface, or to otherwise affect
the composition to maintain stable concentrations in the anolyte
and electroplating solution.
[0068] As metal is deposited into the recessed areas, the recessed
volumes become progressively filled with metal and retain
progressively less plating solution when pressed against the
membrane surface. Consequently, the metal ions in the recess
volumes become more rapidly depleted. Near the end of the process,
when the recessed volumes are nearly filled with metal and approach
the level of the plateau areas, the rate of deposition will
eventually decrease to a negligible value. Accordingly, the process
of the invention is self-limiting in the sense that the plating
process automatically slows as the recessed volumes have been
filled with metal to a level approaching or comparable with the
plateau areas. The corresponding decrease in plating current may be
used as a diagnostic indication of the process end-point. Near the
end of the process, it may be advantageous to intentionally create
a small gap of less than 1 micron between the membrane and the
plateaus, which will result in plating a small quantity of copper
or other metal on plateaus, to insure the trenches are completely
filled and to make sure there is sufficient electroplating solution
between the bottom of trenches and vias to result in an minimum
rate of metal deposition. Increasing velocity, lowering the
hydrostatic pressure, or a combination thereof, may be used to
increase the thickness of the layer of electroplating solution
disposed on plateaus as the endpoint of the polishing is
neared.
[0069] FIG. 2A illustrates one embodiment of a process of the
present invention wherein an anion-conducting membrane is employed
in conjunction with an acidic CuSO.sub.4 plating solution. The
surface of the substrate 10 (the conductive surface) is initially
covered by the plating solution 12, but a first surface of the
membrane 13 is then pressed against the substrate surface so as to
displace the plating solution from the plateau surfaces while
leaving small volumes of electroplating solution entrapped in the
recesses or the trenches 12. The membrane may be held or pressed
against the surface of the substrate by hydrostatic pressure of the
anolyte solution 16. When a suitable potential difference is
applied between the anode 15 and the substrate surface 11,
dissolved Cu.sup.+2 ions within the recessed cavity or trench 9 are
reduced to copper metal (Cu.sup.0) which plates onto the recessed
surface, while SO.sub.4.sup.-2 and HSO.sub.4.sup.- ions carry the
current by migrating across the membrane 13 to the anolyte solution
16 surrounding the anode 15. If the anode is electrochemically
inert and the anolyte contains primarily water containing little or
no easily oxidized solutes, then the principle anodic reaction will
be oxidation of water to O.sub.2 and H.sup.+. To the extent that
the membrane 13 is impermeable to cations, and to the extent the
membrane displaces plating solution from the plateaus, there may,
and typically is, water or other solvent permeation through the
membrane. Little or no Cu.sup.+2 ions may diffuse or migrate from
electroplating solution disposed in a recess to the plateau
surfaces, and little or no Cu.sup.0 will plate onto those surfaces.
The net result of these processes is that as Cu.sup.0 plates onto
the recessed surfaces 17 of the trench 9, CuSO.sub.4 is removed
from the solution in the recessed cavity and H.sub.2SO.sub.4
accumulates in the anolyte solution.
[0070] FIG. 2B illustrates one embodiment of the invention wherein
a cation-conducting membrane is employed in conjunction with a
basic plating solution comprised of CuCN and advantageously other
salts such as NaCN in aqueous NaOH. The surface of the substrate is
initially covered by the plating solution, and a first surface of
the membrane is then pressed against the substrate surface so as to
displace the plating solution from the plateau surfaces while
leaving plating solution 12 trapped in the recesses. Hydrostatic
pressure can advantageously be applied to the anolyte solution 16
contacting the second, opposing, surface of the membrane 13 in
order to urge the first surface of the membrane 13 against the
plateau areas of the substrate. The anolyte solution 16 may
comprise water, a polar organic solvent, or a combination of such
solvents and may include solutes such as bases or salts, but need
not contain any electrochemically active metal ions. The anode 15
is an electrically conductive material such as a metal or carbon.
Depending upon the composition of the anolyte 16 and the anode 15,
the anodic reaction may comprise oxidation of the anode 15 to yield
soluble oxidation products (a sacrificial anode), or may comprise
oxidation of some component of the anolyte solutions 16. If the
anode is electrochemically inert and the anolyte contains primarily
de-ionized water containing little or no easily oxidized solutes,
then the principle anodic reaction will be oxidation of OH.sup.- to
O.sub.2.
[0071] It shall be understood that the examples illustrated in
FIGS. 2A and 2B and described in the previous paragraphs are only
representative examples. Many different types of apparatus, plating
solution, anolytes and electrode reactions can be utilized in
membrane-limited electroplating, as will be apparent to those
skilled in the art.
[0072] The apparatus for membrane-limited electroplating is
advantageously designed so that limited or no electrolytic current
can flow to plateau areas of the substrate not sealed by the
membrane. FIGS. 3-5 illustrate various methods to restrict the flow
of current to areas of the substrate sensibly contacted by the
membrane.
[0073] FIG. 3 illustrates in cross-section an apparatus for
membrane-limited electroplating. In this apparatus the seal between
plateau areas on the substrate 10 (the conductive surface) and the
membrane 13 is maintained by hydrostatic pressure applied to the
anolyte solution 16 on the upper (second) surface of the membrane
13. However, not all areas of the substrate 10 are sealed by the
membrane 13. It is therefore advantageous to prevent electrolytic
current from flowing to the unsealed areas in order to prevent
deposition of metal onto plateaus exposed to plating solution in
those areas by providing an electrically insulating barrier mask 14
disposed on or over the membrane to cover those areas of the
membrane which do not contact the substrate. The mask 14 may
comprise a thin, flexible polymeric film bonded, laminated or
sealed against either the first or second surface of the membrane
13. If the anolyte is water-based or inorganic acid-based, then the
mask may comprise any water-immiscible solvent, oil, or grease
disposed on the membrane that reduces the electrical conductivity
of the membrane by at least factor of 2. The masking may be
disposed on the exterior of the membrane, as shown, or
alternatively, may be disposed on or against the opposite,
interior, side of the membrane. In one embodiment the
ion-conducting membrane is cast on an impermeable web or membrane
having openings that define the active area of the membrane.
Examples of materials suitable for construction of the mask 14
include, but are not limited to, polyolefins and halogenated
polyolefins.
[0074] Another embodiment of the invention is illustrated in FIG.
4. In this apparatus the seal between plateaus areas on the
substrate and the second surface of the membrane 13 is maintained
by mechanical force between the anode 18 and the upper (first)
surface of the membrane 13. For this purpose the anode 18 must be a
porous structure in order to maintain a layer of anolyte solution
or composition 16 adjacent to the anode/membrane interface.
Alternatively, the anode may be an electrochemically inactive,
conductive material such as carbon or a noble metal with at least
one smooth, flat surface. The anode 18 is immersed in a second
fluid (the anolyte 16) and is separated from the plating solution
12 by the membrane 13. In addition, the lower surface of the anode
18 must be sufficiently smooth and flat to maintain a tight seal
between the lower (second) surface of the membrane 13 and the
plateau surfaces on the substrate 10. Examples of materials
suitable for construction of the porous anode 18 include, but are
not limited to, porous sintered metals, porous carbon or carbon
fiber felt or paper. In one embodiment of the invention, the anode
18 comprises a porous, electrochemically inactive, conductive
material such as carbon or a noble metal with at least one smooth,
flat surface. As in FIG. 3, an electrically insulating barrier mask
14 is employed in this embodiment to prevent electric current from
flowing to areas of the substrate not contacting the membrane.
[0075] FIG. 5 illustrates an embodiment in which the membrane is
pressed against the surface of the substrate by mechanical force
applied by a porous non-conducting spacer or support 19 situated
between the membrane and the anode 15. For this purpose the lower
surface of the porous support 19 is smooth and flat and
sufficiently compliant to maintain a uniform pressure between the
lower (first) surface of the membrane and the substrate plateaus.
The porous support 19 must contain pores or channels filled with
anolyte solution or composition in order to maintain a
substantially uniform distribution of electrolytic current between
the anode and the recessed areas of the substrate. Examples of
materials suitable for construction of the porous support 19
include but are not limited to, open-cell polymeric foams or gels,
woven or non-woven fabrics, papers, felts, or porous ceramics.
[0076] In yet another embodiment of the invention (not shown),
similar to that shown in FIG. 5, the seal between plateaus areas on
the substrate and the membrane is maintained by mechanical force
between a thin, porous, compliant metal sheet anode, and the upper
(first) surface of the membrane. This force is applied via a
porous, elastic structure. Such structures may comprise open-cell
foams, honeycomb structures, woven or nonwoven papers or cloths.
Materials suitable for construction of the porous backing material
may include, but are not limited to, silicone elastomers and
fluoropolymer elastomers. The anode must be porous in order to
maintain anolyte solution or composition at the interface between
the membrane and the anode.
[0077] FIG. 6 illustrates an embodiment in which the anolyte
solution has been replaced by a low-conductivity fluid, for example
de-ionized water (DIW). The bottom side of the membrane 13 makes
intimate contact with the substrate 10 only in areas opposite the
anode 18, whereas in surrounding areas where the membrane 13 is
disengaged from the substrate 10 a gap exists between the anode 18
and the disengaged upper side of the membrane 13. Because of the
low conductivity of de-ionized water 16, any current passing
through this gap will be subject to an ohmic resistance
proportional to the width of the gap. The voltage applied between
the anode 18 and the substrate 10 is maintained just large enough
to provide a desired current, for example a current density between
10 and 200 mA/cm.sup.2, to recessed areas (trenches and vias) 9
sealed by the membrane 13 where no gap is present between the anode
18 and the first (upper) surface of the membrane 13. Under these
conditions the ohmic resistance due to a small gap, for example 0.1
mm, beyond the edge of the anode 18 will be sufficient to reduce
the voltage difference and the current density to a negligible
value so that little or no electroplating occurs on areas beyond
the edges of the anode 18. To the extent that the low-conductivity
fluid prevents electrolytic current flowing to areas of the
substrate not contacting the membrane, this embodiment need not
require an electrically insulating barrier mask.
[0078] The area of contact between the membrane 13 and the
substrate 10 may be continuously moved over the surface of the
substrate 10 in such way that the area under the anode 18 always
remains in contact. In this manner, fresh plating solution can be
continuously replaced in the recessed features 9 and plating
current can be maintained without interruption until the desired
amount of metal has been deposited.
[0079] In the embodiment represented in FIG. 6 the low-conductivity
anolyte surrounding the anode will gradually become contaminated by
ions, especially when using an anion-conducting membrane in
conjunction with an acidic plating solution. Therefore, in order to
prevent the conductivity of the low-conductivity fluid from
increasing to a point where current can flow beyond the edges of
the anode, the low-conductivity fluid anolyte advantageously is
continuously replaced.
[0080] Embodiments of this invention are not limited to a single
area of contact between the membrane and the substrate. An
apparatus of this invention can comprise a multiplicity of contact
areas involving a single large membrane, a multiplicity of
membranes and/or a multiplicity of insulating masks, and may
further comprise a multiplicity of anodes and a multiplicity of
anolyte solutions. Such embodiments can provide advantages for
increasing the productivity of the process and/or improving the
macroscopic uniformity of the process.
[0081] Although the present invention is described with reference
to certain preferred embodiments, it is apparent that modification
and variations thereof may be made by those skilled in the art
without departing from the spirit and scope of this invention as
defined by the appended claims. In particular, it will be clear to
those skilled in the art that the present invention may be embodied
in other specific forms, structures, arrangements, proportions, and
with other elements, materials, and components, without departing
from the spirit or essential characteristics thereof. One skilled
in the art will appreciate that the invention may be used with many
modifications of materials, methods, and components otherwise used
in the practice of the invention, which are particularly adapted to
specific substrates and operative requirements without departing
from the principles of the present invention. The presently
disclosed embodiments are therefore to be considered in all
respects as illustrative and not restrictive, the scope of the
invention being indicated by the appended claims, and not limited
to the foregoing description.
* * * * *