U.S. patent application number 11/192013 was filed with the patent office on 2005-12-22 for method of forming a lead-free bump and a plating apparatus therefor.
Invention is credited to Kiumi, Rei, Kuriyama, Fumio, Saito, Nobutoshi, Shimoyama, Masashi, Yokota, Hiroshi.
Application Number | 20050279640 11/192013 |
Document ID | / |
Family ID | 32677413 |
Filed Date | 2005-12-22 |
United States Patent
Application |
20050279640 |
Kind Code |
A1 |
Shimoyama, Masashi ; et
al. |
December 22, 2005 |
Method of forming a lead-free bump and a plating apparatus
therefor
Abstract
The present invention relates to a lead-free bump with
suppressed formation of voids, obtained by reflowing a plated film
of Sn--Ag solder alloy having an adjusted Ag content, and a method
of forming the lead-free bump. The lead-free bump of the present
invention is obtained by forming an Sn--Ag alloy film having a
lower Ag content than that of an Sn--Ag eutectic composition by
plating and reflowing the plated alloy film.
Inventors: |
Shimoyama, Masashi;
(Fujisawa-shi, JP) ; Yokota, Hiroshi;
(Fujisawa-shi, JP) ; Kiumi, Rei; (Tokyo, JP)
; Kuriyama, Fumio; (Tokyo, JP) ; Saito,
Nobutoshi; (Tokyo, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
32677413 |
Appl. No.: |
11/192013 |
Filed: |
July 29, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11192013 |
Jul 29, 2005 |
|
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10743757 |
Dec 24, 2003 |
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Current U.S.
Class: |
205/101 ;
204/198; 205/252; 257/E21.175; 257/E21.508; 257/E23.021;
257/E23.069 |
Current CPC
Class: |
H01L 2224/05573
20130101; H01L 2924/01019 20130101; H01L 2924/014 20130101; H01L
2224/05568 20130101; H01L 2924/01023 20130101; H01L 23/49816
20130101; H01L 2924/01092 20130101; H01L 21/2885 20130101; H01L
24/12 20130101; H01L 2924/01004 20130101; H01L 2224/11462 20130101;
H01L 2924/01082 20130101; H01L 24/05 20130101; C25D 5/505 20130101;
H01L 2924/01005 20130101; H01L 2924/01047 20130101; H01L 2924/14
20130101; H01L 2924/01033 20130101; H01L 2924/01322 20130101; H01L
2924/01029 20130101; H01L 2924/01074 20130101; C25D 21/14 20130101;
H01L 2924/01018 20130101; H01L 2924/01022 20130101; H01L 2924/01076
20130101; H01L 2224/056 20130101; H01L 2924/01027 20130101; H01L
2224/13099 20130101; H01L 2924/01006 20130101; H01L 2924/01039
20130101; H01L 24/11 20130101; H01L 2924/01078 20130101; C25D 3/60
20130101; H05K 3/3463 20130101; H01L 21/4853 20130101; H01L
2924/01015 20130101; H05K 3/3436 20130101; H01L 2924/01013
20130101; H01L 2224/056 20130101; H01L 2924/00014 20130101 |
Class at
Publication: |
205/101 ;
205/252; 204/198 |
International
Class: |
C25D 003/60; C25D
017/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 26, 2002 |
JP |
2002-378010 |
Claims
What is claimed is:
1. A method of forming a lead-free bump comprising: carrying out
Sn--Ag alloy plating on a portion on which a bump is formed while
controlling the composition of a plating bath and electrodeposition
conditions so that a plated Sn--Ag alloy film having a lower Ag
content than that of the Sn--Ag eutectic composition is formed; and
then reflowing the plated alloy film.
2. The method of forming a lead-free bump according to claim 1,
wherein the Ag content in the plated Sn--Ag alloy film is 1.6 to
2.6% by mass.
3. The method of forming a lead-free bump according to claim 2,
wherein the maximum temperature of the reflowing the plated alloy
film is not higher than 240.degree. C.
4. The method of forming a lead-free bump according to claim 1,
wherein the control of the composition of the plating bath and the
electrodeposition conditions is carried out by changing the
electrodeposition conditions while keeping the ratio of
concentration of Ag ion to Sn ion in the plating bath constant.
5. The method of forming a lead-free bump according to claim 1,
wherein the control of the composition of the plating bath and the
electrodeposition conditions is carried out by changing the
concentration ratio of Ag ion to Sn ion in the plating bath while
keeping the electrodeposition conditions constant.
6. A plating apparatus for forming a lead-free bump, comprising: a
plating vessel for containing a plating solution having Ag ions and
Sn ions; an anode; a holder for holding a workpiece and feeding
electricity to the workpiece; an electrodeposition power source for
feeding electricity to the anode and to the workpiece held by the
holder; a replenishment mechanism for replenishing the plating
solution with Ag ions and Sn ions; an analyzer for monitoring Ag
ions and Sn ions; and a control mechanism for controlling, on a
basis of analytical information from the analyzer, an Ag content in
a plated Sn--Ag alloy film formed on a surface of the workpiece at
a value lower than an Ag content of an Sn--Ag eutectic
composition.
7. The plating apparatus for forming a lead-free bump according to
claim 6, wherein the Ag content in the plated Sn--Ag alloy film is
controlled within the range of 1.6 to 2.6% by mass.
8. The plating apparatus for forming a lead-free bump according to
claim 6, wherein the Ag content in the plated Sn--Ag alloy film is
controlled by adjustment of concentrations of Ag ions and Sn ions
in the plating solution and/or change of electrodeposition
conditions.
9. The plating apparatus for forming a lead-free bump according to
claim 6, wherein the anode, the holder and the plating vessel are
made of materials whose amount of emission of .alpha.-rays is low
so that an amount of .alpha.-rays emitted from a surface of the
plated Sn--Ag alloy film is made not higher than 0.02
cph/cm.sup.2.
10. The plating apparatus for forming a lead-free bump according to
claim 6, wherein the anode comprises an insoluble anode.
11. The plating apparatus for forming a lead-free bump according to
claim 6, wherein the anode comprises a soluble anode.
12. The plating apparatus for forming a lead-free bump according to
claim 11, wherein the anode comprises an Sn anode.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a lead-free bump and a
method of forming the same, and more particularly to a lead-free
bump with suppressed formation of voids, obtained by reflowing a
plated film of Sn--Ag solder alloy having an adjusted Ag content,
and a method of forming the lead-free bump, and also to a plating
apparatus for forming such a lead-free bump.
[0003] 2. Description of the Related Art
[0004] In surface mounting technology of semiconductor devices or
the like, it is very important to carry out soldering with high
reliability. Although an eutectic solder containing lead
(Sn:Pb=63:37) has heretofore been used widely in soldering, in the
light of environmental contamination and because of the problem of
.alpha.-rays generation from lead, development of lead-free
soldering is under way.
[0005] For example, lead-free soldering by means of printing or
electroplating is being studied. With printing, however, there is a
limit in its approach to fine pitches through the use of a metal
mask. Electroplating is therefore becoming mainstream, for example,
for the formation of wafer bumps.
[0006] In the case of forming wafer bumps by electroplating, a
heating operation (reflowing) is usually carried out to make plated
films into the form of balls. The reflow temperature is preferably
as low as possible in order to avoid thermal damage to other parts
that exist in the substrate. From this viewpoint, many developments
of solder alloys have been directed to making the composition of an
alloy closest possible to the eutectic composition of the alloy in
order to make use of the eutectic point.
[0007] However, the formation of bumps by electroplating has the
problem that upon reflowing of bumps, voids can be formed in the
bumps. The formation of voids is particularly marked with bumps of
an Sn--Ag alloy, lowering the reliability of the bumps.
SUMMARY OF THE INVENTION
[0008] There is, therefore, a need for the development of a means
to form a lead-free Sn--Ag bump by electroplating without formation
of voids upon reflowing, and it is an object of the present
invention to provide such means.
[0009] As a result of studies to obtain a lead-free bump without
formation of voids, it was discovered by the present inventors that
when forming a bump by Sn--Ag solder alloy plating, the Ag content
in the plated film has a great influence on the formation of voids.
In particular, voids can be formed in a lead-free Sn--Ag bump upon
reflowing when the Ag content of the bump is approximately equal to
or higher than the Ag content of the Sn--Ag eutectic composition.
As a result of further study, it has now been found that in order
to securely prevent the formation of voids in a bump of Sn--Ag
solder alloy, it is necessary to form the bump with a plated alloy
film having a lower Ag content than that of the Sn--Ag eutectic
composition (weight ratio Sn:Ag=96.5:3.5/Ag content, 3.5% by
mass).
[0010] It has also been found that contrary to the expectation that
a decrease in the Ag content of a plated alloy film from the Ag
content of the Sn--Ag eutectic composition will incur a rise in the
reflow temperature, the melting point of the alloy film does not
increase significantly with a decrease in the Ag content, that is,
it is not necessary to significantly raise the reflow
temperature.
[0011] The present invention has been accomplished based on the
above findings. Thus, the present invention provides a lead-free
bump obtained by forming an Sn--Ag alloy film having a lower Ag
content than that of an Sn--Ag eutectic composition by plating and
reflowing the plated alloy film.
[0012] The present invention also provide a method of forming a
lead-free bump comprising: carrying out Sn--Ag alloy plating on a
portion on which a bump is formed while controlling the composition
of a plating bath and electrodeposition conditions so that a plated
Sn--Ag alloy film having a lower Ag content than that of the Sn--Ag
eutectic composition is formed; and then reflowing the plated alloy
film.
[0013] The present invention also provides a plating apparatus for
forming a lead-free bump, comprising: a plating vessel for
containing a plating solution having Ag ions and Sn ions; an anode;
a holder for holding a workpiece and feeding electricity to the
workpiece; an electrodeposition power source for feeding
electricity to the anode and to the workpiece held by the holder; a
replenishment mechanism for replenishing the plating solution with
Ag ions and Sn ions; an analyzer for monitoring Ag ions and Sn
ions; and a control mechanism for controlling, on a basis of
analytical information from the analyzer, an Ag content in a plated
Sn--Ag alloy film formed on a surface of the workpiece at a value
lower than an Ag content of an Sn--Ag eutectic composition.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a diagram showing the relationship between the
concentration ratio of Ag ion to Sn ion in an alloy plating
solution and the Ag content in the plated film;
[0015] FIG. 2 is a diagram showing the relationship between the
current density in plating and the Ag content in the plated film,
as observed when the plating is carried out by continuously
applying a direct current;
[0016] FIG. 3 is a diagram showing a difference in the Ag content
in a plated film between a plated film obtained by plating carried
out by continuously applying a direct current (continuous direct
current plating) and a plated film obtained by plating carried out
by intermittently applying a direct current (intermittent
plating);
[0017] FIG. 4 is a diagram showing a plating apparatus according to
an embodiment of the present invention;
[0018] FIG. 5 is a diagram showing an infrared oven for use in
reflowing;
[0019] FIG. 6A is a diagram showing the sampling portions of sample
before reflowing which are used for quantitative analysis of Ag in
Example 1, and FIG. 6B is a diagram showing the sample portions of
samples after reflowing which are used for quantitative analysis of
Ag in Example 1;
[0020] FIG. 7A is an SEM photograph of a bump before reflowing
obtained in Example 1, FIG. 7B is an SEM photograph of the bump
shown in FIG. 7A but after reflowing at 225.degree. C., FIG. 7C is
an SEM photograph of the bump shown in FIG. 7A but after reflowing
at 230.degree. C., and FIG. 7D is an SEM photograph of the bump
shown in FIG. 7A but after reflowing at 238.degree. C.;
[0021] FIG. 8 is an SEM photograph of the cross-section of a bump
formed by subjecting a plated Sn--Ag alloy film having an Ag
content of 2.6% by mass to reflowing at 238.degree. C.; and
[0022] FIG. 9 is an SEM photograph of the cross-section of a bump
formed by subjecting a plated Sn--Ag alloy film having an Ag
content of 3.4% by mass to reflowing at 238.degree. C.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] The lead-free bump of the present invention can be obtained
by depositing a plated Sn--Ag alloy film by Sn--Ag alloy plating
(hereinafter referred to simply as "alloy plating") carried out
under such controlled electrodeposition conditions that the plated
Sn--Ag alloy film has an Ag content which is lower than the Ag
content of the Sn--Ag eutectic composition (i.e. 3.5 wt %), and
then reflowing the plated Sn--Ag alloy film.
[0024] From the viewpoint of preventing the formation of voids, it
may be sufficient merely to control the Ag content in the plated
alloy film such that it is lower than the above-described upper
limit. With the Ag content ranging from 2.6 to 3.5% by mass,
however, voids could be formed in some cases. Thus, in order to
completely avoid voids, the Ag content in the plated alloy film is
preferably made not higher than 2.6% by mass.
[0025] Further, it is desirable from a practical viewpoint that the
reflow temperature be not so high (for example, the maximum reflow
temperature of not higher than 240.degree. C.). For this purpose,
it is preferred that the lower limit of the Ag content in the
plated alloy film be made 1.6% by mass. Thus, in order to provide a
practically desirable bump, the Ag content in the plated alloy film
is preferably made within the range of 1.6 to 2.6% by mass.
[0026] Thus, according to the present invention, it is necessary to
carry out plating while controlling the Ag content in the plated
alloy film at a value lower than 3.5% by mass, preferably from 1.6
to 2.6% by mass.
[0027] Further, with respect to the lead-free bump obtained by
reflowing the plated alloy film as described above, the level of
.alpha.-rays emitted from the surface of the plated alloy film is
preferably not higher than 0.02 cph/cm.sup.2.
[0028] Lead has a plurality of isotopes, including natural
radioactive elements. The isotopes of lead are intermediate
products or final products in uranium or thorium decay series and
emit .alpha.-rays in their decay processes. A-rays can act on
semiconductor devices of a semiconductor integrated circuit and
cause soft errors. Sn and other elements also contain such natural
radioactive elements though in a slight amount. Thus, a lead-free
Sn--Ag bump also emits .alpha.-rays, and it is important to
suppress the emission of .alpha.-rays at a low level. By
suppressing the emission of .alpha.-rays from the surface of the
lead-free bump at a level of not higher than 0.02 cph/cm.sup.2,
soft errors in semiconductor integrated circuit devices due to the
influence of .alpha.-rays can be prevented.
[0029] In general, the composition of deposited components in alloy
plating is determined by the concentrations of the components in a
plating solution and the electrodeposition conditions. Also in
alloy plating according to the present invention, the Ag content in
a plated alloy film can be made within the above-described range by
adjusting the concentration ratio of Ag ion to Sn ion in an alloy
plating solution, and controlling the electrodeposition conditions.
In particular, the Ag content in a plated alloy film can be
controlled by (a) changing the electrodeposition conditions while
keeping the concentration ratio of Ag ion to Sn ion in a plating
bath constant, or (b) by changing the concentration ratio of Ag ion
to Sn ion in a plating bath while keeping the electrodeposition
conditions constant.
[0030] Though the alloy plating solution generally contains,
besides the ions of the metals forming an alloy, a complexing agent
for stabilizing metal ions, a brightening agent for making the
surface of the plated film beautiful and/or other additive(s), the
Ag content in the plated alloy film is primarily determined by the
concentration ratio of Ag ion to Sn ion in the alloy plating bath.
Thus, a plated alloy film having a controlled Ag content can be
obtained by finding a preferred range of the concentration ratio of
Ag ion to Sn ion though experiments, and carrying out plating while
keeping the concentration ratio within the preferred range. In
fact, it has been confirmed that when carrying out alloy plating
under fixed electrodeposition conditions, the Ag content in a
plated alloy film is proportional to the concentration ratio of Ag
ion to Sn ion in a plating solution, as schematically shown in FIG.
1.
[0031] Accordingly, a plated alloy film having a controlled Ag
content can be obtained by immersing a workpiece in an alloy
plating solution with a predetermined concentration ratio of Ag ion
to Sn ion, and carrying out plating under constant
electrodeposition conditions. By reflowing the plated alloy film, a
bump without voids can be obtained.
[0032] The following is an example of an alloy plating solution
usable in the present invention;
[0033] Composition:
[0034] Sn ion (Sn.sup.2+) 10-100 g/L (preferably 35-50 g/L)
[0035] Ag ion (Ag.sup.+): 0.3-8 g/L (preferably 0.6-4 g/L)
[0036] Methanesulfonic acid: 100 g/L
[0037] It is known that, in alloy plating, the composition of
deposited components varies depending on the electrodeposition
conditions. Also in alloy plating according to the present
invention, the Ag content in a plated alloy film can be changed by
changing the electrodeposition conditions.
[0038] Alloy plating according to the present invention may be
carried out in various manners using various types of electric
currents, including direct current plating carried out by
continuously applying a direct current, and an intermittent plating
carried out by intermittently applying a direct current with
periodical rest periods.
[0039] In the case of direct current plating in which a direct
current is applied continuously during plating, the Ag content in
the plated alloy film decreases with an increase in the current
density, as schematically shown in FIG. 2. Preferable current
conditions may be determined experimentally, and plating may be
carried out while keeping the determined conditions. A preferred
current density in direct current plating is about 10 to 100
mA/cm.sup.2.
[0040] In the case of intermittent plating in which a direct
current is applied intermittently with periodical rest periods, as
schematically shown in FIG. 3, the plated alloy film has an Ag
content which is different from the Ag content of a plated alloy
film as obtained by applying the same direct current continuously.
Also with intermittent plating, preferable conditions, such as an
applied voltage, the proportion of rest time, etc. may be
determined experimentally, and plating may be carried out while
keeping the determined conditions. A preferred current density
during application of electric current is about 10-200 mA/cm.sup.2,
and a preferred rest time (zero current) is {fraction (1/10)}-1 of
application time.
[0041] Though each applied voltage in the above two types of
plating varies depending upon conditions such as the intensity of
current, the underlying material, the thickness of plating, the
plating solution, the anode used, etc., it is preferably about 1 to
5 V.
[0042] There is no particular limitation on an apparatus for
carrying out the above-described alloy plating, and a common dip
type plating apparatus may be employed. For a practical operation,
however, it is desirable to use an apparatus which takes account of
a jig structure configured to the mechanical conditions of a
workpiece, a stirring mechanism (paddle structure) for supplying
metal ions uniformly and rapidly to the entire surface of a
workpiece such as a wafer, the shape and size of a mask for
equalizing the electric field distribution, a plating solution
circulation system for removing foreign matter, preventing a change
in the quality of a plating solution and supplying metal ions
uniformly and rapidly to the entire surface of a workpiece,
etc.
[0043] Further, as described above, it is necessary to carry out
alloy plating while adjusting the concentrations of Ag ions and Sn
ions in a plating solution, and controlling the electrodeposition
conditions. It is therefore preferred to use a plating apparatus
which is provided with a replenishment mechanism for replenishing
an alloy plating solution with Ag ions and Sn ions, an analyzer for
monitoring Ag ions and Sn ions, and a control mechanical which,
based on analytical information from the analyzer, adjusts the
concentrations of Ag ions and Sn ions in the alloy plating solution
and/or controls the electrodeposition conditions. FIG. 4 shows an
example of such a plating apparatus.
[0044] In FIG. 4, reference numeral 1 denotes a plating apparatus,
2 denotes a plating vessel, 3 denotes an anode, 4 denotes a holder,
5 denotes a workpiece, 6 denotes an electrodeposition power source,
7 denotes conductive wire, 8a through 8c denote a replenishment
mechanism, 9 denotes an analyzer, 10 denotes a control mechanism,
11 denotes an auto-sampler, 12a through 12c denote feed pumps, 13
denotes a shut-off valve, and 14 denotes a discharge outlet.
[0045] The analyzer 9 periodically or continuously analyzes and
monitors a change in the concentrations of Ag ions and Sn ions, as
an index for control of plating, which is due to consumption or
loss of the ions during operation of the plating apparatus. An
atomic absorption spectrometry device, for example, may be used as
the analyzer 9.
[0046] The control mechanism 10, which comprises, for example, a
computer for control, determines optimum replenishment amounts of
Ag ions (solution), Sn ions (solution), etc. based on analytical
information from the analyzer 9, and actuates the feed bumps 12a
through 12c, which are connected to the replenishment mechanisms 8a
through 8c, so as to add Ag ions (solution) and Sn ions (solution)
to a plating solution.
[0047] The replenishment mechanisms 8a through 8c, besides the
portion for replenishment of Ag ion solution and Sn ion solution,
may additionally comprise a portion for replenishing water for
adjustment of the composition of plating solution, or an
additive.
[0048] The anode 3, the holder 4 and the plating vessel 2 are each
made of a material whose emission of .alpha.-rays is low so that
the amount of .alpha.-rays emitted from the surface of a plated
Sn--Ag alloy film formed by the plating apparatus 1 is made not
higher than 0.02 cph/cm.sup.2. Thus, the amount of natural
radioactive elements taken in a bump of the plated alloy film
formed by plating is made low, so that the amount of .alpha.-rays
emitted from the bump can be made at such a low level. This
effectively suppresses soft errors in semiconductor integrated
circuit devices due to the influence of .alpha.-rays.
[0049] The anode 3 may either be an insoluble anode or a soluble
anode. It is possible with an insoluble anode to continue using it
without a change. When an Sn soluble anode is used as the anode 3,
Sn ions can be supplied from the anode to the plating solution
during plating operation. This facilitates control of the plating
solution and reduces operation for replenishment of metal ions.
[0050] The control mechanism 10 desirably controls the plating
system under optimum electrodeposition conditions (the
above-described current density and voltage application method) for
a particular composition of plating solution, and should make at
least one of control of the electrodeposition conditions and
control of the concentrations of Ag ions and Sn ions by
replenishment of the ions.
[0051] The actual deposition behavior of plating is influenced not
only by the above-described concentration ratio of Ag ion to Sn ion
in an alloy plating solution and the electrodeposition conditions,
but also by many other factors. For example, the Ag content in a
plated alloy film can vary depending on the type of the additive(s)
added in the plating solution. In most cases, however, the specific
ingredients and their amounts of an additive are undisclosed as the
additive manufacturer's know-how.
[0052] For forming bumps according to the present inventions,
therefore, it is necessary to conduct experiments in advance with
varying current densities in electroplating, voltage application
methods and the concentration ratios of Ag ions to Sn ion in an
alloy plating solution, and measure the contents of Ag in the
various bumps formed. Based on the results of measurement, the
optimum conditions for plating can be determined.
[0053] By carrying out plating under the optimum conditions thus
determined, it becomes possible to stabilize the composition of the
plating solution and stably form a bump having a desired Ag
content.
[0054] The plated alloy film thus formed is then subjected to
reflowing to form a bump. The reflowing is carried out by heating
the plated alloy film in an inert gas atmosphere (e.g. nitrogen or
argon atmosphere) using, for example, the apparatus (infrared oven)
shown in FIG. 5. In FIG. 5, reference numeral 20 denotes an
infrared oven, 21 denotes a chamber, 22 denotes a stage, 23 denotes
a silica glass window, 24 denotes an infrared lamp, and 25 denotes
a workpiece.
[0055] The reflowing in this apparatus is carried out, for example,
by setting the workpiece 25, which has undergone alloy plating, in
the chamber 21, allowing nitrogen gas to flow into the chamber 21
at a rate of about 8 to 30 L/min to adequately carry out gas
replacement, and then heating the workpiece 25 through the silica
glass window 23 by the infrared lamp 24.
[0056] The reflow temperature is important for the formation of
bumps according to the present invention. A bump may be formed on a
printed circuit board, etc. Common electronic components are said
to be heat-resistant to a temperature of about 240.degree. C. The
maximum temperature in the step of reflowing a plated alloy film
formed by alloy plating should therefore be not higher than
240.degree. C. Further, the melting point of an Sn--Ag solder is
generally 221.degree. C., and it is generally said that the minimum
reflowable temperature is the melting point +10.degree. C., and
that the reflow temperature must be maintained for 15 to 45
seconds. Taking such requirements into consideration, the
temperature conditions upon reflowing may be exemplified by:
231.degree. C. for 30 seconds with the maximum temperature of
238.degree. C.
[0057] The above-described lead-free bump of the present invention
can be utilized, for example, as a ball-shaped bump on wiring pad
in a mounting substrate.
[0058] In the formation of ball-shaped bumps, metal bond pads are
first formed, and then a resist is applied on the substrate, with
locations for bumps being left, to form a resist pattern. Next,
plated Sn--Ag alloy films having a controlled Ag content are formed
in the above-described manner. Thereafter, the resist is peeled
off, and the alloy films are subjected to reflowing at a
predetermined reflow temperature.
[0059] Any one of the formation of the metal bond pads, the
formation of the resist pattern and the removal of the resist may
be carried out by common methods in the art.
[0060] Further, the lead-free bump of the present invention can be
used to form wiring pads on a variety of semiconductor substrates.
In particular, a lead-free bump can be formed on a semiconductor
substrate of a semiconductor device by the following steps (I) to
(IV):
[0061] (I) Step of forming wiring pads on a semiconductor substrate
of a semiconductor device
[0062] (II) Step of forming a barrier metal on the wiring pads
formed
[0063] (III) Step of forming an Sn--Ag plating on the barrier
metal
[0064] (IV) Step of reflowing the Sn--Ag plating
[0065] The semiconductor device used in step (I) includes an
integrated circuit (IC) and the like. A known barrier metal may be
used as the barrier metal formed on the wiring pads in step
(II).
[0066] The following examples are provided to illustrate the
present invention in greater detail and are not to be construed to
be limiting the invention in any manner.
EXAMPLE 1
[0067] (1) Preparation of Sn--Ag Bump:
[0068] A resist was applied to a thickness of 120 .mu.m on a wafer
in such a manner that a number of holes having an opening size of
100 .mu.m are formed, thereby preparing a sample. The plating area
of the sample was 149.63 cm.sup.2. Plating of the sample was
carried out by the following steps under the following
conditions.
[0069] (Plating Steps)
[0070] Degassing (10 min).fwdarw.Pre-cleaning with 10% sulfuric
acid (1 min).fwdarw.Copper plating.fwdarw.Water-cleaning.fwdarw.Ni
plating Water-cleaning.fwdarw.Sn--Ag alloy plating
[0071] (Plating Conditions)
[0072] (a) Cu Plating
[0073] Plating bath composition:
1 Cu.sup.2+ 220 g/L H.sub.2SO.sub.4 200 g/L HCl 5 mL/L Additive 5
mL/L
[0074] Plating temperature: 25.degree. C.
[0075] Stirring: mechanical stirring (paddle stirring speed 10
m/min)
[0076] Circulation of plating solution: flow rate 2.5 L/min
[0077] Electrode: copper anode, interpolar distance about 7.5 mm,
anode mask .o slashed. 250 mm
[0078] Cathode current density (total current): 5 A/dm.sup.2 (7.48
A)
[0079] Plating thickness: 2 .mu.m
[0080] (b) Ni Plating
[0081] Plating bath composition:
2 Ni(NH.sub.2SO.sub.4).4H.sub.2O 450 g/L H.sub.3BO.sub.3 30 g/L
NiCl.sub.2.6H.sub.2O 10 g/L Additive 2 mL/L
[0082] Plating temperature: 50.degree. C.
[0083] Stirring: mechanical stirring (paddle stirring speed 10
m/min)
[0084] Circulation of plating solution: flow rate 2.5 L/min
[0085] Electrode: nickel anode, interpolar distance about 75 mm,
anode mask .o slashed. 250 mm
[0086] Cathode current density (total current): 3 A/dm.sup.2 (4.49
A)
[0087] Plating thickness: 3 .mu.m
[0088] (c) Sn--Ag Plating
[0089] plating bath composition:
3 Sn.sup.2+ 40 g/L Ag.sup.+ 1.5 g/L methanesulfonic acid 100 g/L
Additive 10 g/L
[0090] [A 2:2:1 (weight ratio) mixture of polyoxyethylene
surfactant, thiourea and cathechol]
[0091] Plating temperature: 25.degree. C.
[0092] Stirring: mechanical stirring (paddle stirring speed 10
m/min)
[0093] Circulation of plating solution: flow rate 2.5 L/min
[0094] Electrode: titanium anode, interpolar distance about 7.5 mm,
anode mask .o slashed. 250 mm
[0095] Cathode current density (total current): 10 A/dm.sup.2
(14.9A), direct current plating
[0096] Plating thickness: 140 .mu.m
[0097] (2) Reflowing
[0098] After the plating described in (1) above, the resist was
removed to expose the plated portions. The plated portions were
reflowed by using an infrared oven as shown in FIG. 5. Temperature
control of the infrared oven was carried out by placing a 2-inch
silicon wafer with a thermocouple embedded in the outermost layer
at the center (temperature measuring wafer made by SensArray
Corporation) on the stage of the infrared oven. The sample to be
reflowed was placed near the thermocouple of the silicon wafer.
After carrying out pre-heating at 150 to 170.degree. C. for 90
seconds, the sample was heated to a reflow temperature in 30
seconds. The reflow temperature was from the minimum reflowable
temperature 231.degree. C. to the maximum temperature 238.degree.
C. After maintaining the reflow temperature for 30 seconds, the
sample was cooled.
[0099] The interior of the infrared oven had been replaced with
nitrogen gas, and the heating was carried out while flowing
nitrogen gas at a rate of 8 L/min. The infrared oven was employed
because of its capability of rapid heating and rapid cooling.
[0100] (3) Composition Analysis of Bump
[0101] The elemental composition of an Sn--Ag alloy bump was
estimated in the following manner. The bump was embedded in a
resin, and the bump was cut to expose a cut surface. After
polishing the cut surface, elementary mapping was performed by EPMA
(Electron Probe Microanalysis). Further, quantitative analysis of
Ag was carried out in three sectional micro-areas (l, c, r) as
shown in FIGS. 6A and 6B, each having an area of about 10
.mu.m.times.10 .mu.m, and the average of the measured valves was
determined as the Ag content of the bump.
[0102] A rough measurement of elemental composition is possible,
without the necessity of cutting a sample to expose a cut surface,
by using a .mu. fluorescent X-ray analyzer. Further, it is also
possible to dissolve the bumps in an acid and analyze the
compositional distribution in the wafer by ICP-MS (Inductively
Coupled Plasma Mass Spectrometer).
[0103] (4) Observation of the Shape of Bump
[0104] After the Sn--Ag alloy plating and the subsequent removal of
the resist, the plated portions and the bumps after reflowing at
various temperatures were observed under an SEM. FIG. 7A shows an
SEM photograph of a bump before reflowing, FIG. 7B shows an SEM
photograph of a bump after reflowing at 225.degree. C., FIG. 7C
shows an SEM photograph of a bump after reflowing at 230.degree.
C., and FIG. 7D shows an SEM photograph of a bump after reflowing
at 238.degree. C.
[0105] (5) Observation of Voids
[0106] A cut surface of a bump after reflowing at 238.degree. C.,
was observed under an SEM. The observation was carried out after
embedding the wafer in a resin, cutting the bump and polishing the
cut surface. As a result, as shown in FIG. 8, no void was observed
in a plated alloy film (bump) according to the present invention,
having an Ag content of 2.6% by mass. In contrast, voids were
formed in a bump of plated alloy film having an Ag content of 3.4%
by mass, as shown in FIG. 9.
EXAMPLE 2
[0107] Alloy plating was carried out with various proportions of Ag
to the total metal in an alloy plating solution, various current
densities upon plating and various current application methods, and
the respective plated alloy films were subjected to reflowing at
238.degree. C. For the bump thus obtained, measurement of the Ag
content and observation of the shape of bump and the presence of
voids were carried out in the same manner as in Example 1. The
results are shown in table 1.
4TABLE 1 Plating Plating conditions Voltage Bump Void Ball solution
Current density application Ag content present (x) formation Ag/Sn
(%) (A/dm.sup.2) method (wt %) absent (.smallcircle.) at
238.degree. C. 4.3 3 DC 5.2 x .smallcircle. 4.4 3 DC 6.4 x
.smallcircle. 4.4 20 CHOP 4.0 x .smallcircle. 4.3 3 DC 7.7 x
.smallcircle. 4.3 20 CHOP 5.1 x .smallcircle. 1.3 3 DC 1.8
.smallcircle. .smallcircle. 1.3 8 DC 0.9 .smallcircle. x 1.3 20
CHOP 1.4 .smallcircle. x 2.2 3 DC 3.4 x .smallcircle. 2.2 20 CHOP
2.1 .smallcircle. .smallcircle. 4.1 3 DC 4.9 x .smallcircle. 3.1 20
CHOP 2.9 .smallcircle. .smallcircle. 3.1 10 CHOP 3.6 x
.smallcircle. 3.1 3 DC 5.0 x .smallcircle. 3.2 3 DC 5.8 x
.smallcircle. 3.2 20 CHOP 2.7 x .smallcircle. 2.3 3 DC 3.5 x
.smallcircle. 2.3 20 CHOP 2.6 .smallcircle. .smallcircle. Note: DC
denotes direct current plating, and CHOP denotes intermittent
plating
[0108] As apparent from the results shown in Table 1, voids are not
formed in a bump (plated alloy film) when the Ag content is 2.9% or
lower. Especially when the Ag content is from 1.8 to 2.6%, the
plated alloy film can be transformed into a ball at the maximum
reflow temperature 238.degree. C. without formation of voids,
providing a highly practical lead-free bump.
[0109] Lead-free bumps according to the present invention are free
of voids, and are of a desirable ball shape that can be formed at a
relatively low reflow temperature. Further, those bumps do not
contain lead, and thus do not cause malfunction of an integrated
circuit due to the emission of .alpha.-rays nor environmental
contamination.
[0110] Lead-free bumps according to the present invention can
therefore be widely used in surface mounting technology (SMT) of
semiconductor devices, etc. and enable a reliable soldering despite
their no inclusion of lead.
* * * * *