U.S. patent application number 13/829433 was filed with the patent office on 2014-09-18 for method of filling through-holes.
The applicant listed for this patent is ROHM AND HAAS ELECTRONIC MATERIALS LLC. Invention is credited to Leon R. BARSTAD, Nagarajan JAYARAJU, Elie H. NAJJAR.
Application Number | 20140262801 13/829433 |
Document ID | / |
Family ID | 50396860 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140262801 |
Kind Code |
A1 |
JAYARAJU; Nagarajan ; et
al. |
September 18, 2014 |
METHOD OF FILLING THROUGH-HOLES
Abstract
The methods inhibit or reduce dimpling and voids during copper
electroplating of through-holes with flash copper layers in
substrates such as printed circuit boards. An acid solution
containing disulfide compounds is applied to the through-holes of
the substrate followed by filling the through-holes with copper
using an acid copper electroplating bath which includes additives
such as brighteners and levelers.
Inventors: |
JAYARAJU; Nagarajan;
(Framingham, MA) ; NAJJAR; Elie H.; (Norwood,
MA) ; BARSTAD; Leon R.; (Raynham, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ROHM AND HAAS ELECTRONIC MATERIALS LLC |
Marlborough |
MA |
US |
|
|
Family ID: |
50396860 |
Appl. No.: |
13/829433 |
Filed: |
March 14, 2013 |
Current U.S.
Class: |
205/131 |
Current CPC
Class: |
H05K 3/423 20130101;
C25D 5/10 20130101; C25D 5/34 20130101; C25D 3/38 20130101; C23C
18/1653 20130101; C25D 5/02 20130101 |
Class at
Publication: |
205/131 |
International
Class: |
C25D 5/02 20060101
C25D005/02 |
Claims
1. A method comprising: a) providing a substrate with a plurality
of through-holes and a layer of copper flash on a surface of the
substrate and walls of the plurality of through-holes; b) applying
an aqueous acid solution to at least the plurality of
through-holes, the aqueous acid solution comprising one or more
disulfide compounds having a formula: ##STR00005## wherein X is
sodium, potassium or hydrogen, R is independently hydrogen or an
alkyl, n and m are integers of 1 or greater, the one or more
disulfide compounds are in amounts of 50 ppb to 10 ppm; and c)
electroplating at least the through-holes with copper using an acid
copper electroplating bath comprising one or more brighteners and
one or more levelers.
2. The method of claim 1, wherein the disulfide compounds are in
amounts of 50 ppb to 500 ppb.
3. The method of claim 1, wherein X is sodium or hydrogen.
4. The method of claim 1, wherein R is independently hydrogen or
(C.sub.1-C.sub.6)alkyl.
5. The method of claim 1, wherein m and n are independently
integers of 1-6.
6. The method of claim 1, wherein the disulfide compound is
bis(3-sulfopropyl)disulfide.
7. The method of claim 1, wherein the one or more levelers of the
copper electroplating bath are reaction products of one or more
imidazole compounds and one or more epoxy compounds.
Description
FIELD OF THE INVENTION
[0001] The present invention is directed to a method of filling
through-holes having a layer of flash copper which reduces or
inhibits the formation of dimples and voids. More specifically, the
present invention is directed to a method of filling through-holes
having a layer of flash copper which reduces or inhibits the
formation of dimples and voids by applying an aqueous acid
pretreatment solution containing disulfide compounds at low
concentrations to the through-holes with the flash copper layer
followed by filling the through-holes with copper using an acid
copper electroplating bath containing brighteners and levelers.
BACKGROUND OF THE INVENTION
[0002] High density interconnects is an important design in the
fabrication of printed circuit boards with microvias and
through-holes. Miniaturization of these devices relies on a
combination of thinner core materials, reduced line widths and
smaller diameter through-holes and blind vias. The diameters of the
through-holes range from 75 .mu.m to 200 .mu.m. Filling the
through-holes by copper plating has become more and more difficult
with higher aspect ratios. This results in larger voids and deeper
dimples. Another problem with through-hole filling is the way they
tend to fill. Unlike vias which are closed at one end through-holes
pass through a substrate and are open at two ends. Vias fill from
bottom to top. In contrast, when through-holes are being filled
with copper, the copper tends to begin to deposit on the walls at
the center of the through-hole where it plugs at the center forming
"butterfly wings" or two vias. The two vias fill to complete the
deposition of the holes. Accordingly, the copper plating baths used
to fill vias are not typically the same as are used to fill
through-holes. Plating bath levelers and other bath additives are
chosen to enable the right type of fill. If the right combination
of additives is not chosen then the copper plating results in
undesired conformal copper deposition.
[0003] Often the copper fails to completely fill the through-hole
and both ends remain unfilled. An incomplete through-hole fill with
copper deposit in the center with unfilled ends is sometimes
referred to as "dog-boning". The open spaces at the top and bottom
of the holes are referred to as dimples. Entire dimple elimination
during through-hole filling is rare and unpredictable. Dimple depth
is perhaps the most commonly used metric for quantifying
through-hole fill performance. Dimple requirements depend on
through-hole diameter and thickness and it varies from one
manufacturer to another. In addition to dimples, gaps or holes
referred to as voids may form within a copper through-hole fill.
Larger dimples affect further processing of the panel and larger
voids affect device performance. An ideal process completely fills
through-holes with a high degree of planarity, i.e., build up
consistency, without voids to provide optimum reliability and
electrical properties and at as low as possible a surface thickness
for optimum line width and impedance control in an electrical
device.
[0004] Another problem associated with through-hole filling is
filling through-holes with electrolytic copper when the
through-hole walls have a layer of flash copper. Typically
substrates containing through-holes, such as printed circuit
boards, are copper clad with a layer of electroless copper on a
surface and on the walls of the through-holes. Electroless copper
thickness is usually greater than 0.25 .mu.m. Such electroless
copper layers tend to oxidize. Often printed circuit boards are
electrolessly plated with copper and stored for a period of time
prior to further processing. Prolonged periods of exposure to air
as well as general handling of the boards result in relatively
rapid oxidation of the electroless copper layer. To address this
problem the industry electroplates a layer of flash copper 2 .mu.m
to 5 .mu.m thick on the surface of the electroless copper prior to
storage to protect the electroless copper from oxidation. Also, the
thicker flash copper layer allows for removal of any oxide
formation during storage by conventional etching processes whereas
such etching cannot be done on the thinner electroless copper
without the danger of damaging or removing the electroless copper
layer. Unfortunately, electrolytic copper flash adds to the
difficulty of filling through-holes. Dimpling and void formation
frequently occur when workers try to fill through-holes using acid
electrolytic copper plating baths.
[0005] Accordingly, there is a need for a method to improve
through-hole filling in substrates which have a flash copper
layer.
SUMMARY OF THE INVENTION
[0006] Methods include providing a substrate with a plurality of
through-holes and a layer of copper flash on a surface of the
substrate and walls of the plurality of through-holes; applying an
aqueous acid solution to at least the plurality of through-holes,
the aqueous acid solution consisting essentially of one or more
disulfide compounds having a formula:
##STR00001##
wherein X is sodium, potassium or hydrogen, R is independently
hydrogen or an alkyl, n and m are integers of 1 or greater, the one
or more disulfide compounds are in amounts of 50 ppb to 10 ppm; and
electroplating at least the through-holes with copper using an acid
copper electroplating bath comprising one or more brighteners and
one or more levelers.
[0007] The methods reduce or inhibit dimple formation and voids
during through-hole filling. Dimples are typically less than 10
.mu.m deep. The reduced depth of the dimples and void area improves
throwing power, thus provides a substantially uniform copper layer
on the surface of the substrate and good through-hole filling.
DETAILED DESCRIPTION OF THE INVENTION
[0008] As used throughout this specification, the abbreviations
given below have the following meanings, unless the context clearly
indicates otherwise: g=gram; ml=milliliter; L=liter; cm=centimeter;
mm=millimeter; .mu.m=micron; ppm=parts per million; ppb=parts per
billion; .degree. C.=degrees Centigrade; g/L=grams per liter;
A=amperes; dm=decimeters; DI=deionized; wt %=percent by weight;
T.sub.g=glass transition temperature; Void=a space free of copper
within a through-hole otherwise filled with copper metal; aspect
ratio of a through-hole=height of the through-hole/the diameter of
the through-hole; dimple depth=distance from the deepest point of
the dimple to the level of copper plated on the surface of a
substrate; void area of a single
through-hole=0.5A.times.0.5B.times..pi. where A is height of the
void and B is the diameter of the void at its widest point in a
through-hole; through-hole area=height of the
through-hole.times.the diameter of the through-hole; and % void
area=void area/through-hole area.times.100%.
[0009] The terms "printed circuit board" and "printed wiring board"
are used interchangeably throughout this specification. The terms
"plating" and "electroplating" are used interchangeably throughout
this specification. The term "throwing power" means the ability to
plate in low current density areas with the same thickness as in
higher current density areas. All amounts are percent by weight,
unless otherwise noted. All numerical ranges are inclusive and
combinable in any order except where it is logical that such
numerical ranges are constrained to add up to 100%.
[0010] Aqueous acid solutions consist essentially of one or more
disulfide compounds having a formula:
##STR00002##
wherein X is sodium, potassium or hydrogen, preferably X is sodium
or hydrogen; R is independently hydrogen or an alkyl, preferably R
is independently hydrogen or (C.sub.1-C.sub.6)alkyl, more
preferably R is independently hydrogen or (C.sub.1-C.sub.3)alky,
most preferably R is hydrogen; n and m are integers of 1 or
greater, preferably n and m are independently integers of 1 to 3,
more preferably n and m are 2 or 3, most preferably n and m are 3.
Preferably the disulfide compound is bis(3-sulfopropyl)disulfide or
its sodium salt. Preferably the aqueous acid solution consists of
water, one or more inorganic acids and one or more compounds having
formula (I) above. Preferably the aqueous acid solution is free of
any additional components.
[0011] The one or more disulfide compounds are included in the
aqueous acid solution in amounts of 50 ppb to 10 ppm, preferably 50
ppb to 500 ppb, more preferably 100 ppb to 500 ppb. The lower the
concentration included in the acid solution the better because such
disulfide compounds typically form breakdown products which may
hinder uniform copper electroplating and through-hole filling. In
addition, many copper electroplating baths used to fill
through-holes include such disulfide compounds as brighteners or
accelerators. The combination of the breakdown products in the
copper electroplating bath and the acid solution may further
increase the probability of poor through-hole filling. Accordingly,
concentrations of the disulfide compounds in the ppb range are most
preferred.
[0012] Inorganic acids include, but are not limited to sulfuric
acid, hydrochloric acid, nitric acid, hydrofluoric acid or
phosphoric acid. Preferably the inorganic acid is sulfuric acid,
hydrochloric acid or nitric acid, more preferably the acid is
sulfuric acid or hydrochloric acid. Such acids may be included in
the aqueous acid solutions in amounts of 0.5 wt % to 20 wt %,
preferably 5 wt % to 15 wt %, more preferably from 8 wt % to 12 wt
%. Typically the pH is 0 to 1, more typically less than 1.
[0013] The aqueous acid solutions may be applied to cleaned copper
clad substrates with a plurality of through-holes by any suitable
method, such as by immersing or dipping the substrate into the
solution. The solution may be applied to the substrate by spraying
it onto the substrate or by applying the solution with an atomizer
using conventional apparatus. Temperatures may range from room
temperature to 60.degree. C., typically from room temperature to
40.degree. C.
[0014] The substrates are typically plated with a layer of
electroless copper such that the electroless copper is adjacent a
surface of the substrate and the walls of the through-holes. The
electroless copper may have a thickness, typically, from 0.25 .mu.m
to 6 .mu.m, more typically from 0.25 .mu.m to 3 .mu.m. The
electroless copper is plated with a layer of electrolytic flash
copper to protect it from corrosion. The thickness of the
electroplated flash copper adjacent the electroless copper layer
ranges from 0.5 .mu.m to 15 .mu.m, typically from 1 .mu.m to 10
.mu.m, more typically from 1 .mu.m to 5 .mu.m.
[0015] The through-holes of the substrate typically range in
diameter from 75 .mu.m to 200 .mu.m. The through-holes traverse the
width of the substrates and are typically 100 .mu.m to 400
.mu.m.
[0016] Substrates include printed circuit boards which may contain
thermosetting resins, thermoplastic resins and combinations
thereof, including fiber, such as fiberglass, and impregnated
embodiments of the foregoing.
[0017] Thermoplastic resins include, but are not limited to acetal
resins, acrylics, such as methyl acrylate, cellulosic resins, such
as ethyl acetate, cellulose propionate, cellulose acetate butyrate
and cellulose nitrate, polyethers, nylon, polyethylene,
polystyrene, styrene blends, such as acrylonitrile styrene and
copolymers and acrylonitrile-butadiene styrene copolymers,
polycarbonates, polychlorotrifluoroethylene, and vinylpolymers and
copolymers, such as vinyl acetate, vinyl alcohol, vinyl butyral,
vinyl chloride, vinyl chloride-acetate copolymer, vinylidene
chloride and vinyl formal.
[0018] Thermosetting resins include, but are not limited to allyl
phthalate, furane, melamine-formaldehyde, phenol-formaldehyde and
phenol-furfural copolymers, alone or compounded with butadiene
acrylonitrile copolymers or acrylonitrile-butadiene-styrene
copolymers, polyacrylic esters, silicones, urea formaldehydes,
epoxy resins, allyl resins, glyceryl phthalates and polyesters.
[0019] The printed wiring boards may include low or high T.sub.g
resins. Low T.sub.g resins have a T.sub.g below 160.degree. C. and
high T.sub.g resins have a T.sub.g of 160.degree. C. and above.
Typically high T.sub.g resins have a T.sub.g of 160.degree. C. to
280.degree. C. or such as from 170.degree. C. to 240.degree. C.
High T.sub.g polymer resins include, but are not limited to,
polytetrafluoroethylene (PTFE) and polytetrafluoroethylene blends.
Such blends include, for example, PTFE with polypheneylene oxides
and cyanate esters. Other classes of polymer resins which include
resins with a high T.sub.g include, but are not limited to, epoxy
resins, such as difunctional and multifunctional epoxy resins,
bimaleimide/triazine and epoxy resins (BT epoxy),
epoxy/polyphenylene oxide resins, acrylonitrile butadienestyrene,
polycarbonates (PC), polyphenylene oxides (PPO), polypheneylene
ethers (PPE), polyphenylene sulfides (PPS), polysulfones (PS),
polyamides, polyesters such as polyethyleneterephthalate (PET) and
polybutyleneterephthalate (PBT), polyetherketones (PEEK), liquid
crystal polymers, polyurethanes, polyetherimides, epoxies and
composites thereof.
[0020] The dwell time for the solution on the substrate may range
from 0.5 to 5 minutes, preferably from 0.5 minutes to 3 minutes,
more preferably from 0.5 to 2 minutes. The treated substrate is
then electroplated with copper using an acid copper electroplating
bath to fill the through-holes. In addition to one or more sources
of copper ions and one or more acids, the acid copper
electroplating bath also includes at least one or more brighteners
and one or more levelers.
[0021] Sources of copper ions include, but are not limited to water
soluble halides, nitrates, acetates, sulfates and other organic and
inorganic salts of copper. Mixtures of one or more of such copper
salts may be used to provide copper ions. Examples include copper
sulfate, such as copper sulfate pentahydrate, copper chloride,
copper nitrate, copper hydroxide and copper sulfamate. Conventional
amounts of copper salts may be used in the compositions. Copper
salts are included in the bath in amounts of 50 g/l to 350 g/L,
typically 100 g/L to 250 g/L.
[0022] Acids include, but are not limited to sulfuric acid,
hydrochloric acid, hydrofluoric acid, phosphoric acid, nitric acid,
sulfamic acid and alkylsulfonic acids. Such acids are included in
conventional amounts. Typically such acids are included in the acid
copper baths in amounts of 25 g/l to 350 g/L.
[0023] Brighteners include, but are not limited to
3-mercapto-propylsulfonic acid and its sodium salt,
2-mercapto-ethanesulfonic acid and its sodium salt, and
bissulfopropyl disulfide and its sodium salt,
3-(benzthiazoyl-2-thio)-propylsulfonic acid sodium salt,
3-mercaptopropane-1-sulfonic acid sodium salt,
ethylenedithiodipropylsulfonic acid sodium salt,
bis-(p-sulfophenyl)-disulfide disodium salt,
bis-(.omega.-sulfobutyl)-disulfide disodium salt,
bis-(.omega.-sulfohydroxypropyl)-disulfide disodium salt,
bis-(.omega.-sulfopropyl)-disulfide disodium salt,
bis-(.omega.-sulfopropyl)-sulfide disodium salt,
methyl-(.omega.-sulfopropyl)-disulfide sodium salt,
methyl-(.omega.-sulfopropyl)-trisulfide disodium salt,
O-ethyl-dithiocarbonic acid-S-(.omega.-sulfopropyl)-ester,
potassium salt thioglycoli acid, thiophosphoric
acid-O-ethyl-bis-(.omega.-sulfpropyl)-ester disodium salt,
thiophosphoric, acid-tris(.omega.-sulfopropyl)-ester trisodium
salt, N,N-dimethyldithiocarbamic acid (3-sulfopropyl) ester, sodium
salt, (O-ethyldithiocarbonato)-S-(3-sulfopropyl)-ester, potassium
salt, 3-[amino-iminomethyl)-thiol-1-propanesulfonic acid and
3-(2-benzthiazolylthio)-1-propanesulfonic acid, sodium salt.
Preferably the brightener is bissulfopropyl disulfide or its sodium
salt. Typically the brighteners are included in amounts of 1 ppb to
500 ppm, preferably from 50 ppb to 10 ppm.
[0024] Levelers included in the conformal acid copper
electroplating baths are typically reaction products of
heterocyclic aromatic compounds with epoxy compounds. Synthesis of
such compounds is disclosed in the literature such as in U.S. Pat.
No. 8,268,158. Preferably the levelers are reaction products of at
least one imidazole compound of the formula:
##STR00003##
wherein R.sub.1, R.sub.2 and R.sub.3 are independently chosen from
H, (C.sub.1-C.sub.12)alkyl, (C.sub.2-C.sub.12)alkenyl, and aryl and
provided that R.sub.1 and R.sub.2 are not both H. That is, the
reaction products contain at least one imidazole wherein at least
one of R.sub.1 and R.sub.2 is (C.sub.1-C.sub.12)alkyl,
(C.sub.2-C.sub.12)alkenyl, or aryl. Such imidazole compound is
substituted with a (C.sub.1-C.sub.12)alkyl,
(C.sub.2-C.sub.12)alkenyl, or aryl at the 4- and/or 5-position.
Preferably, R.sub.1, R.sub.2 and R.sub.3 are independently chosen
from H, (C.sub.1-C.sub.8)alkyl, (C.sub.2-C.sub.7)alkenyl and aryl,
more preferably H, (C.sub.1-C.sub.6)alkyl, (C.sub.3-C.sub.7)alkenyl
and aryl, and even more preferably H, (C.sub.1-C.sub.4)alkyl,
(C.sub.3-C.sub.6)alkenyl and aryl. The (C.sub.1-C.sub.12)alkyl
groups and the (C.sub.2-C.sub.12)alkenyl groups may each optionally
be substituted with one or more of hydroxyl groups, halogen, and
aryl groups. Preferably, the substituted (C.sub.1-C.sub.12)alkyl
group is an aryl-substituted (C.sub.1-C.sub.12)alkyl group, and
more preferably is (C.sub.1-C.sub.4)alkyl. Exemplary are
(C.sub.1-C.sub.4)alkyl groups include, without limitation, benzyl,
phenethyl, and methylnaphthyl. Alternatively, each of the
(C.sub.1-C.sub.12)alkyl groups and the (C.sub.2-C.sub.12)alkenyl
groups may contain a cyclic alkyl or cyclic alkenyl group,
respectively, fused with an aryl group. As used herein, the term
"aryl" refers to any organic radical derived from an aromatic or
heteroaromatic moiety by the removal of a hydrogen atom.
Preferably, the aryl group contains 6-12 carbon atoms. The aryl
group in the present invention may optionally be substituted with
one or more of (C.sub.1-C.sub.4)alkyl and hydroxyl. Exemplary aryl
groups include, without limitation, phenyl, tolyl, xylyl,
hydroxytolyl, phenolyl, naphthyl, furanyl, and thiophenyl. The aryl
group is preferably phenyl, xylyl or naphthyl. Exemplary
(C.sub.1-C.sub.12)alkyl groups and substituted
(C.sub.1-C.sub.12)alkyl groups include, without limitation, methyl,
ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl,
n-pentyl, 2-pentyl, 3-pentyl, 2-(2-methyl)butyl,
2-(2,3-dimethyl)butyl, 2-(2-methyl)pentyl, neopentyl,
hydroxymethyl, hydroxyethyl, hydroxypropyl, cyclopentyl,
hydroxcyclopentyl, cyclopentylmethyl, cyclopentylethyl, cyclohexyl,
cyclohexylmethyl, hydroxyclohexyl, benzyl, phenethyl,
naphthylmethyl, tetrahydronaphthalenyl and
tetrahydronaphthylmethyl. Exemplary (C.sub.2-C.sub.8)alkenyl groups
include, but are not limited to allyl, styrenyl, cyclopentenyl,
cyclopentylmethyl, cyclopentenylethyl, cyclohexenyl,
cyclohexenylmethyl and indenyl. Preferably, the at least one
imidazole compound is substituted with a (C.sub.1-C.sub.8)alkyl,
(C.sub.3-C.sub.7)alkenyl, or aryl at the 4- or 5-position. More
preferably, the at least one imidazole is substituted with
(C.sub.1-C.sub.6)alkyl, (C.sub.3-C.sub.7)alkenyl, or aryl at the 4-
or 5-position. Still more preferably, at least one imidazole is
substituted at the 4- or 5-position with methyl, ethyl, propyl,
butyl, allyl or aryl. The imidazole compounds are generally
commercially available from a variety of sources, such as
Sigma-Aldrich (St. Louis, Mo.) or may be prepared from literature
methods.
[0025] One or more of the above described imidazole compounds are
reacted with one or more epoxy compounds having formula:
##STR00004##
where Y.sub.1 and Y.sub.2 are independently chosen from hydrogen
and (C.sub.1-C.sub.4)alkyl, R.sub.4 and R.sub.5 are independently
chosen from hydrogen, CH.sub.3 and OH, p=1-6 and q=1-20.
Preferably, Y.sub.1 and Y.sub.2 are both H. When p=2, it is
preferred that each R.sub.4 is H, R.sub.5 is chosen from H and
CH.sub.3, and q=1-10. When p=3, it is preferred that at least one
R.sub.5 is chosen from CH.sub.3 and OH, and q=1. When p=4, it is
preferred that both R.sub.4 and R.sub.5 are H, and q=1. Exemplary
compounds of formula (III) include, but are not limited to:
1,4-butanediol diglycidyl ether, ethylene glycol diglycidyl ether,
di(ethylene glycol) diglycidyl ether, poly(ethylene glycol)
diglycidyl ether compounds, glycerol diglycidyl ether, neopentyl
glycol diglycidyl ether, propylene glycol diglycidyl ether,
di(propylene glycol)diglycidyl ether, and poly(propylene
glycol)diglycidyl ether compounds. Poly(ethylene glycol)diglycidyl
ether compounds of formula III are those compounds where each of
R.sub.4 and R.sub.5=H, p=2, and q=3-20, and preferably q=3-15, more
preferably q=3-12, and still more preferably q=3-10. Exemplary
poly(ethylene glycol)diglycidyl ether compounds include
tri(ethylene glycol)diglycidyl ether, tetra(ethylene
glycol)diglycidyl ether, penta(ethylene glycol)diglycidyl ether,
hexa(ethylene glycol)diglycidyl ether, nona(ethylene
glycol)diglycidyl ether, deca(ethylene glycol)diglycidyl ether, and
dodeca(ethylene glycol)diglycidyl ether. Poly(propylene
glycol)diglycidyl ether compounds of formula III are those
compounds where each of R.sub.4=H and one of R.sub.5=CH.sub.3, p=2,
and q=3-20, and preferably q=3-15, more preferably q=3-12, and
still more preferably q=3-10. Exemplary poly(propylene
glycol)diglycidyl ether compounds include tri(propylene
glycol)diglycidyl ether, tetra(propylene glycol)diglycidyl ether,
penta(propylene glycol)diglycidyl ether, hexa(propylene
glycol)diglycidyl ether, nona(propylene glycol)diglycidyl ether,
deca(propylene glycol)diglycidyl ether, and dodeca(propylene
glycol)diglycidyl ether. Suitable poly(ethylene glycol)diglycidyl
ether compounds and poly(propylene glycol)diglycidyl ether
compounds are those having a number average molecular weight of
from 350 to 10000, and preferably from 380 to 8000.
[0026] Other additives which may be included in the copper
electroplating baths are one or more complexing agents, one or more
sources of chloride ions, stabilizers such as those which adjust
mechanical properties, provide rate control, refine grain structure
and modify deposit stress, buffering agents, suppressors and
carriers. They may be included in the conformal copper
electroplating bath in conventional amounts.
[0027] Through-hole filling is typically done at current densities
of 0.5 A/dm.sup.2 to 5 A/dm.sup.2, preferably from 1 A/dm.sup.2 to
3 A/dm.sup.2. The plating bath temperature may range from room
temperature to 60.degree. C., typically from room temperature to
40.degree. C. Electroplating is done until the through-holes are
filled with minimum copper on the surfaces to make it easier for
post treatment and prepare the substrate for further
processing.
[0028] The methods reduce or inhibit dimple formation and voids
during through-hole filling. The void area as well as the % void
area of through-holes is reduced or eliminated. Dimple formation is
10 .mu.m or less, typically dimple size is less than 10 .mu.m with
no voids in the through-holes which is the preferred industry
standard. The reduced depth of the dimples and voids improves
throwing power, thus provides a substantially uniform copper layer
on the surface of the substrate.
[0029] The following examples are included to further illustrate
the invention but are not intended to limit its scope.
Example 1
Comparative
[0030] Two FR4/glass-epoxy coupons 5 cm wide, 15 cm long and 100
.mu.m thick with a plurality of through-holes were provided by Tech
Circuit. The through-holes had an average diameter of 100 .mu.m.
The coupons contained a layer of electroless copper on one side and
on the walls of the through-holes. The thickness of the copper
layer on each coupon was 0.3 .mu.m. The two coupons were
pre-cleaned using a conventional copper cleaner. One coupon was
placed into a dessicator. The other coupon was then placed in a
plating cell which contained a copper electroplating bath with a
formula as shown in Table 1.
TABLE-US-00001 TABLE 1 COMPONENT AMOUNT Copper sulfate pentahydrate
220 g/L Sulfuric acid 40 g/L Chloride ion from hydrochloric acid 50
ppm Polyethylene glycol 2 g/L
4-phenylimidazole/imidazole/1,4-butandiol 50 mg/L diglycidyl ether
copolymer Bis-sodium sulfopropyl)-disulfide 10 mg/L
[0031] The coupon was connected to a conventional DC rectifier. The
counter electrode in each plating cell was an insoluble. The
plating bath was air agitated during electroplating. The current
density was set at 1 A/dm.sup.2. Copper electroplating was done for
20 minutes at room temperature to deposit a flash copper layer on
the electroless copper layer on the surface and walls of the
through-holes of 5 .mu.m. The coupon with the flash copper was then
placed in the dessicator with the other coupon which included only
the electroless copper layer for storage for the interim prior to
further treatment and electroplating to discourage oxide formation
on the copper.
[0032] Each coupon was removed from the dessicator and cleaned
using a conventional copper cleaner. Each coupon was then placed
into separate plating cells containing the copper electroplating
bath of Table 1. The counter electrode was an insoluble anode.
Copper electroplating was done at a current density of 1.5
A/dm.sup.2 for 82 minutes with continuous air agitation of the bath
at room temperature. After electroplating the coupons were removed
from the plating cells, rinsed with DI water and sectioned for an
analysis of copper layer uniformity and through-hole filling. The
sectioned samples were examined using a conventional optical
microscope. Good plug and fill with an average dimple depth of 4.3
.mu.m and an average void area of 10% was observed on the coupon
which was only electrolessly plated with copper. The coupon having
the flash copper had through-holes without plug and fill or the
through-holes were only partially filled.
Example 2
Comparative
[0033] Three FR4/glass-epoxy coupons 5 cm wide, 15 cm long and 100
.mu.m thick with a plurality of through-holes were provided by Tech
Circuit. The through-holes had an average diameter of 100 .mu.m.
The coupons were processed through electroless copper using
CIRCUPOSIT.TM. 880 Electroless Process plating formulations and
method. The thickness of the electroless copper layer on each
coupon was 0.3 .mu.m. Each coupon was cleaned and electroplated
with a flash copper layer 5 .mu.m thick as described in Example 1
above. Each coupon was then placed in a dessicator during the
interim before further processing to discourage any oxide formation
on the copper.
[0034] Upon removal of the coupons from the dessicator, they were
cleaned using a conventional copper cleaner to remove any possible
oxide layer and have a clean copper surface for plating. Each
coupon was then placed into separate plating cells with a fresh
copper electroplating bath having the formulation in Table 1. The
plating baths were air agitated during electroplating. One coupon
was plated at 1.5 A/dm.sup.2, the second at 2.2 A/dm.sup.2 and the
third was plated at 3 A/dm.sup.2 for 82 minutes, 63 minutes and 41
minutes, respectively. Plating was done at room temperature. After
electroplating the coupons were removed from their plating cells,
rinsed with DI water and allowed to air dry. Each was then
sectioned and examined under an optical microscope for an analysis
of through-hole filling. None of the through-holes in any of the
sections examined were completely filled. Filling was irregular.
Large dimples in excess of 10 .mu.m were observed in the
through-holes which appeared to be more than 50% filled. Most of
the through-holes which had some filling showed extensive
dog-boning in the through-hole centers; however, most of the
through-holes examined were not filled. The variation in the
current density did not appear to affect through-hole filling.
Example 3
[0035] Three FR4/glass-epoxy coupons 5 cm wide, 15 cm long and 100
.mu.m thick with a plurality of through-holes were provided by Tech
Circuit. The through-holes had an average diameter of 100 .mu.m.
The coupons had a layer of electroless copper 0.3 .mu.m thick. Each
coupon was cleaned and electroplated with a flash copper layer 5
.mu.m thick as described in Example 1 above. The coupons were
stored in a dessicator prior to further treatment and plating.
[0036] Upon removal from storage, the flashed coupons were cleaned
to remove any oxide layer and have a clean copper surface for
plating. After cleaning, one coupon was transferred to a plating
bath having the formula of Table 1. The second coupon was first
immersed in an aqueous solution of 5.5 ppm
bis(3-sulfopropyl)disulfide, sodium salt (SPS) and 10 wt % sulfuric
acid for two minutes and then transferred into the copper
electroplating bath. Copper electroplating was done at a current
density of 1.5 A/dm.sup.2 with continuous air agitation of the bath
for 82 minutes. Plating was done at room temperature. After
electroplating the coupons were removed from the plating cells,
rinsed with DI water and sectioned for an analysis of copper layer
uniformity and through-hole filling. The sectioned samples were
examined under an optical microscope. No plug or partially filled
holes were seen on the flash copper coupon which was not treated
with the aqueous acid solution containing SPS. However, superior
through-hole fill with an average dimple depth of 3.6 .mu.m and an
average void area of 6.3% was achieved on the flash coupon which
was immersed in the acid solution containing SPS.
Example 4
[0037] Six FR4/glass-epoxy coupons 5 cm wide, 15 cm long and 100
.mu.m thick with a plurality of through-holes were provided. The
through-holes had an average diameter of 100 .mu.m. The coupons
contained a layer of electroless copper on a surface and on the
walls of the through-holes. The thickness of the copper layer was
0.3 .mu.m. Each coupon was electroplated with a flash copper layer.
Five electroless copper coupons were flashed with copper of
different thickness of 1 .mu.m, 2 .mu.m, 3 .mu.m, 4 .mu.m and 5
.mu.m and the sixth coupon was flashed with 5 .mu.m. The copper
electroplating bath and plating method were as described in Example
1 above. The coupons were all placed in a dessicator in the interim
prior to any further processing.
[0038] Upon removal of the flashed coupons from the dessicator,
each was cleaned to remove any oxide and have a clean copper
surface for plating. Coupons 1-5 were transferred into a plating
bath having the formulation as shown in Table 1 above. The sixth
coupon was first immersed in an aqueous solution of 5.5 ppm
bis(3-sulfopropyl)disulfide, sodium salt (SPS) and 10 wt % sulfuric
acid for two minute and then transferred into the copper
electroplating bath. Copper electroplating was done at a current
density of 1.5 A/dm.sup.2 with continuous air agitation of the bath
for 82 minutes. Plating was done at room temperature. After
electroplating the coupons were removed from the plating cells,
rinsed with DI water and sectioned for an analysis of copper layer
uniformity and through-hole filling. The sectioned samples were
examined under an optical microscope. No plug or partially filled
holes were seen on the flash copper coupons 1-5. The through-holes
of the sixth coupon which was treated in the acid solution with SPS
had an average dimple depth of 3.63 .mu.m and an average void are
of 6.1%. Superior results were achieved with the coupon immersed in
the SPS acid solution prior to through-hole filling.
Example 5
[0039] Eight FR4/glass-epoxy coupons 5 cm wide, 15 cm long and 100
.mu.m thick with a plurality of through-holes were provided. The
through-holes had an average diameter of 100 .mu.m. The coupons had
an electroless copper layer 0.3 .mu.m thick. Each coupon was
cleaned then electroplated with a flash copper layer 5 .mu.m thick
as described in Example 1 above. The coupons were stored in a
dessicator during the interim between flash copper plating and
further processing.
[0040] Upon removal of the coupons from the dessicator, they were
cleaned then each coupon was immersed in separate aqueous solutions
of SPS and 10 wt % sulfuric acid for two minutes. The SPS
concentration of each solution varied as shown in Table 2 below.
After two minutes the coupons were removed from the solutions then
placed in plating cells containing a copper electroplating bath as
described in Table 1 above. The coupons were electroplated with
copper for 82 minutes with air agitation to a surface thickness of
25 .mu.m. The current density was maintained at 1.5 A/dm.sup.2. The
plating was done at room temperature. After copper plating was
completed the coupons were removed from the plating cells, rinsed
with DI water and allowed to air dry at room temperature. Each
coupon was then sectioned to examine the dimple height and voids of
the through-holes. The dimple depth was measured using an optical
microscope. The dimple depth was the distance from the deepest part
of the dimple to the level of the copper layer on the surface of
the coupon as measured in microns. The area of a particular void
was determined using the formula: void
area=0.5A.times.0.5B.times..pi. where A is the height of the void
and B is the diameter of the void at its widest point. The formula
used to determine % void area=void area/hole area.times.100% where
hole area is height of the through-hole without any copper flash
layer.times.the diameter of the through-hole. The results are in
Table 2 below.
TABLE-US-00002 TABLE 2 DIMPLE HEIGHT VOID SAMPLE SPS ppm .mu.m % 1
2 0.3 0 2 2.75 2.7 2.2 3 5.5 3.63 6.1 4 11 1.63 5.4 5 25 11.1 1.8 6
50 34.5 1.4 7 500 60.1 0 8 1000 67.2 0
[0041] Samples 1-4 had acceptable dimple depth below 10 .mu.m.
Although sample 4 had an average dimple depth of 1.63 .mu.m which
was lower than the dimple depth of samples 2-3, the overall results
showed that as the concentration of SPS increased the dimple
increased. Accordingly, as the concentration increased you began to
lose fill performance. At high concentrations such as 500 ppm you
completely lose fill performance. Although none of the
through-holes examined in samples 7 and 8 had any observable voids,
as the concentration of SPS declined from 5.5 ppm in sample 3 to 1
ppm in sample 1, the void area of the through-holes decreased.
Accordingly, as the concentration of the SPS in the acid solution
decreased there was a trend for decreased dimple depth and
reduction in void area in the through-holes.
Example 6
[0042] Two FR4/glass-epoxy coupons 5 cm wide, 15 cm long and 100
.mu.m thick with a plurality of through-holes were provided. The
through-holes had an average diameter of 100 .mu.m. The coupons
included a layer of electroless copper 0.3 .mu.m thick. Each coupon
was cleaned and then electroplated with a flash copper layer 5
.mu.m thick as described in Example 1 above. The coupons were then
placed in a dessicator prior to any further processing.
[0043] Upon removing the coupons from the dessicator, each was
cleaned then immersed in separate aqueous solutions of SPS and 10
wt % sulfuric acid for two minutes. The SPS concentration of each
solution varied as shown in Table 3 below. After two minutes the
coupons were removed from the solutions and then placed in plating
cells containing a copper electroplating bath as described in Table
1. The coupons were electroplated with copper over 82 minutes to a
surface thickness of 25 .mu.m with air agitation. The current
density was maintained at 1.5 A/dm.sup.2. Plating was done at room
temperature. After copper plating was completed the coupons were
removed from the plating cells, rinsed with DI water and allowed to
air dry at room temperature. Each coupon was then sectioned to
examine the dimple depth and void area of the through-holes. The
results are in Table 3 below.
TABLE-US-00003 TABLE 3 DIMPLE HEIGHT SAMPLE SPS ppb .mu.m VOID % 1
100 4.2 0 2 500 0.9 0.1
[0044] Samples 1-2 had dimples well below 10 .mu.m with an average
void area of 0% in sample 1 and only 0.1% in sample 2. Even at very
low concentrations in the ppb range SPS effectively reduced dimple
depth and voids in copper electroplated through-holes.
* * * * *