U.S. patent application number 11/592396 was filed with the patent office on 2007-03-01 for lead-containing anodes.
Invention is credited to Michael J. Bossio, Brett M. Clark, Nancy F. Dean, Ronald H. Fleming, James P. Flint, Martin W. Weiser.
Application Number | 20070045838 11/592396 |
Document ID | / |
Family ID | 32093989 |
Filed Date | 2007-03-01 |
United States Patent
Application |
20070045838 |
Kind Code |
A1 |
Weiser; Martin W. ; et
al. |
March 1, 2007 |
Lead-containing anodes
Abstract
The invention includes solder materials having low
concentrations of alpha particle emitters, and includes methods of
purification of materials to reduce a concentration of alpha
particle emitters within the materials. The invention includes
methods of reducing alpha particle flux in various lead-containing
and lead-free materials through purification of the materials. The
invention also includes methods of estimating the fractionation of
a low concentration of one or more alpha particle emitters during
purification of a material.
Inventors: |
Weiser; Martin W.; (Liberty
Lake, WA) ; Dean; Nancy F.; (Liberty Lake, WA)
; Clark; Brett M.; (Spokane, WA) ; Bossio; Michael
J.; (Spokane, WA) ; Fleming; Ronald H.;
(Spokane, WA) ; Flint; James P.; (Spokane,
WA) |
Correspondence
Address: |
WELLS ST. JOHN P.S.
601 W. FIRST AVENUE, SUITE 1300
SPOKANE
WA
99201
US
|
Family ID: |
32093989 |
Appl. No.: |
11/592396 |
Filed: |
November 2, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10670319 |
Sep 26, 2003 |
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11592396 |
Nov 2, 2006 |
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60417241 |
Oct 8, 2002 |
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Current U.S.
Class: |
257/734 ;
257/E23.021; 257/E23.069 |
Current CPC
Class: |
C22B 13/06 20130101;
H01L 2924/01015 20130101; H01L 2924/01006 20130101; H01L 23/3128
20130101; H01L 24/10 20130101; B23K 35/025 20130101; H05K 3/3485
20200801; H01L 23/49816 20130101; H01L 2924/00014 20130101; C25C
1/18 20130101; H01L 2924/01029 20130101; H01L 2224/05573 20130101;
H01L 2924/01012 20130101; H01L 2924/01049 20130101; H01L 2924/01082
20130101; H01L 2924/01084 20130101; H01L 2224/05568 20130101; H01L
2924/01079 20130101; B23K 35/26 20130101; H05K 3/3463 20130101;
H01L 2924/01033 20130101; H01L 2924/01005 20130101; H01L 2224/16225
20130101; H01L 2924/01092 20130101; H01L 2224/131 20130101; H01L
2224/13099 20130101; H01L 2924/01047 20130101; H01L 2924/01088
20130101; H01L 2924/014 20130101; H01L 2924/14 20130101; B23K
35/268 20130101; H01L 24/13 20130101; H01L 2224/13 20130101; H01L
2924/12042 20130101; H01L 2224/13 20130101; H01L 2924/00 20130101;
H01L 2924/12042 20130101; H01L 2924/00 20130101; H01L 2924/00014
20130101; H01L 2224/05599 20130101; H01L 2224/131 20130101; H01L
2924/014 20130101 |
Class at
Publication: |
257/734 |
International
Class: |
H01L 23/48 20060101
H01L023/48 |
Claims
1-9. (canceled)
10. A lead-containing anode having an alpha flux of less than 0.001
cts/cm.sup.2/hr, the lead-containing anode comprising at least
about 50 weight % lead.
11. The lead-containing anode of claim 10 having an alpha flux of
less than 0.0005 cts/cm.sup.2/hr.
12. The lead-containing anode of claim 10 having an alpha flux of
less than 0.0002 cts/cm.sup.2/hr.
13. The lead-containing anode of claim 10 having an alpha flux of
less than 0.0001 cts/cm.sup.2/hr.
14-29. (canceled)
Description
RELATED PATENT DATA
[0001] This application is related to U.S. Provisional Application
Ser. No. 60/417,241, which was filed on Oct. 8, 2002.
TECHNICAL FIELD
[0002] The invention pertains to semiconductor packages;
lead-containing solders and anodes; and methods of removing
alpha-emitters from materials.
BACKGROUND OF THE INVENTION
[0003] Solders are commonly utilized in semiconductor device
packaging. If the solders contain alpha particle emitting isotopes
(referred to herein as alpha particle emitters), emitted alpha
particles can cause damage to packaged semiconductor devices.
Accordingly, it is desired to reduce the concentration of alpha
particle emitters within the solders.
[0004] An exemplary prior art semiconductor package is shown in
FIG. 1 as a package 50, with the exemplary package representing a
flip-chip construction. The package comprises a semiconductor
component 12 (such as, for example, an integrated circuit chip).
The package also comprises a board 14 utilized to support the
semiconductor component 12. A plurality of contact pads 38 (only
some of which are labeled) are joined to chip 12, and a plurality
of contact pads 40 (only some of which are labeled) are joined to
board 14. Solder balls or bumps 39 (only some of which are labeled)
are provided between pads 38 and 40 to form electrical
interconnects between pads 38 and 40. The electrical connection
utilizing the solder balls or bumps 39 with pads 38 and 40 can
incorporate so-called wafer bump technology.
[0005] Suitable encapsulant 44 can be provided over the chip 12 and
substrate 14 as shown. Additionally, and/or alternatively, thermal
transfer devices (not shown) such as heat sinks and heat spreaders
can be provided over the chip 12.
[0006] Contact pads 30 (only some of which are labeled) are on an
underside of the board 14 (i.e., on a side of board 14 in opposing
relation relative to the side proximate chip 12). Contact pads 30
typically comprise stacks of copper, nickel and gold. Solder balls
32 (only some of which are labeled) are provided on the contact
pads and utilized to form electrical interconnections between the
contact pads 30 and other circuitry (not shown) external of the
chip package. The contact pads 40 can be connected with pads 30
through circuit traces (not shown) extending through board 14.
[0007] The shown package 50 has solder proximate chip 12 from at
least balls 39, and possibly through wafer bumps associated with
pads 38 and/or pads 40. There can be other applications of solder
within package 50 which are not specifically shown. For instance, a
solder paste can be provided between chip 12 and various thermal
transfer devices.
[0008] The solders utilized in package 50 can be problematic, as
discussed above, in that the solders can contain alpha particle
emitters. Alpha particles are problematic for semiconductor devices
because the alpha particles can induce so-called soft errors. The
errors are referred to being "soft" in that the errors are not
permanent. However, the errors will typically cause at least one
round of incorrect calculations.
[0009] There are numerous sources for alpha particles, including
reactions caused by cosmic rays. However, the source which is
frequently most problematic for semiconductor device packages is
solder utilized for forming various interconnections relative to
semiconductor dies. For instance, the wafer-bump technique is
becoming relatively common for forming high density interconnects
to semiconductor dies. The bumps are portions of solder formed over
electrical nodes associated with a semiconductor die package. If
the solder utilized in the bumps has alpha particle emitting
components, the alpha particles are frequently emitted close to
integrated circuitry associated with the semiconductor die.
[0010] Occasionally, the solder formed over the electrical nodes is
in the form of large pillars. Such pillars are frequently referred
to as columns. For purposes of interpreting this disclosure, the
term "bump" is to be understood to encompass various forms of
solder formed over electrical nodes, including the forms commonly
referred to as columns.
[0011] A typical component of many solders is lead. However, one of
the lead isotopes (specifically .sup.210Pb) has a decay chain that
leads to alpha particles. Further, various common contaminants of
lead can emit alpha particles, including, for example, isotopes of
uranium, thorium, radium and polonium.
[0012] The alpha particle emitters present in lead can be present
in the ore from which the lead is initially refined. Alpha particle
emitters can be alternatively, or additionally, introduced during
processing and/or use of the lead. For instance, phosphoric acid
and some antistatic systems contain alpha particle emitters; some
abrasives and cleaning agents can introduce alpha particle emitters
into lead; and smelting of commercial lead can introduce uranium,
thorium and other alpha particle emitters into the lead from gangue
rock.
[0013] The amount of alpha particle emitters present in lead is
typically determined by an alpha flux measurement, with results
stated in terms of alpha particle counts per unit area per hour
(cts/cm.sup.2/hr). It is possible to commercially obtain lead
having an alpha flux of from 0.002 to 0.02 cts/cm.sup.2/hr, but it
is very difficult to obtain a material with a lower alpha flux.
However, the semiconductor industry is requesting materials with
significantly lower alpha flux, including for example, materials
having an alpha flux of less than 0.0001 cts/cm.sup.2/hr.
[0014] Among the difficulties associated with reducing the
concentration of alpha flux emitters in a material to extremely low
levels is a difficulty in measuring the concentration of the
emitters at flux levels below 0.002 cts/cm.sup.2/hr. Unless the
concentration can be measured, it is difficult to monitor a
purification process to determine if alpha particle emitters are
being removed. For instance, it can be difficult to determine at
any given stage of the purification process if alpha particle
emitters are fractionating with a material or away from the
material.
[0015] Although the discussion above focuses on removing alpha
particle emitters from lead-containing solders, it should be
understood that alpha particle emitters are also problematic in
other materials. For instance, one of the methods utilized to
reduce the concentration of alpha particle emitters in solder has
been to create so-called lead-free solders. Such solders contain
little, if any, lead, which is desirable from an environmental
perspective. However, the solders can still have an undesirable
amount of alpha particle emitters present therein. Exemplary lead
free solders are Sn:3.5% Ag; Sn:4% Ag:0.5% Cu; and Bi:2-13% Ag,
where the percentages are by weight.
[0016] One of the methods which has been utilized for reducing the
number of alpha particle emitters in lead-containing solders is to
start with lead materials which have very few emitters therein.
Presently there are three sources of such materials. The sources
are (1) very old lead where the .sup.210Pb has substantially all
decayed; (2) some specific PbS ore bodies which have very little
.sup.210Pb therein, and which have been carefully refined; and (3)
lead which has been subjected to laser isotope separation to remove
the .sup.210Pb from the lead. Various problems exist with all of
the sources. For instance, the first source utilizes very old Pb,
and such is often poorly refined and therefore contains various
radionuclides as contaminants. The second source typically does not
have a low enough alpha particle emitter concentration to meet the
ultimately desired requirements of the semiconductor industry. The
third source is very energy intensive to form, and therefore is not
commercially feasible.
SUMMARY OF THE INVENTION
[0017] In one aspect, the invention includes a method of refining a
material. An initial composition of the material is provided. The
initial composition has an alpha flux of greater than or equal to
0.002 cts/cm.sup.2/hr. The material is purified to form a second
composition of the material. The second composition has an alpha
flux of less than 0.001 cts/cm.sup.2/hr, preferably less than
0.0005 cts/cm.sup.2/hr, more preferably less than 0.0002
cts/cm.sup.2/hr, and even more preferably less than 0.0001
cts/cm.sup.2/hr. The purification can comprise, for example,
electro-refining and/or chemical refining.
[0018] In yet another aspect, the invention includes a
semiconductor structure comprising a solder having an alpha flux of
less than 0.001 cts/cm.sup.2/hr, preferably less than 0.0005
cts/cm.sup.2/hr, more preferably less than 0.0002 cts/cm.sup.2/hr,
and even more preferably less than 0.0001 cts/cm.sup.2/hr.
[0019] In yet another aspect, the invention includes a
lead-containing anode having an alpha flux of less than 0.001
cts/cm.sup.2/hr, preferably less than 0.0005 cts/cm2/hr, more
preferably less than 0.0002 cts/cm.sup.2/hr, and even more
preferably less than 0.0001 cts/cm.sup.2/hr.
[0020] In yet another aspect, the invention includes a
lead-containing solder bump having an alpha flux of less than 0.001
cts/cm.sup.2/hr, preferably less than 0.0005 cts/cm.sup.2/hr, more
preferably less than 0.0002 cts/cm.sup.2/hr, and even more
preferably less than 0.0001 cts/cm.sup.2/hr. The bump can, in
particular aspects, be in the form of a column.
[0021] In yet another aspect, the invention includes a
lead-containing solder paste having an alpha flux of less than
0.001 cts/cm.sup.2/hr, preferably less than 0.0005 cts/cm.sup.2/hr,
more preferably less than 0.0002 cts/cm.sup.2/hr, and even more
preferably less than 0.0001 cts/cm.sup.2/hr.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] Preferred embodiments of the invention are described below
with reference to the following accompanying drawings.
[0023] FIG. 1 is a diagrammatic cross-sectional side view of a
prior art semiconductor package construction.
[0024] FIG. 2 is a graph of alpha counts/cm.sup.2/hour versus total
impurities for a material purified in accordance with an aspect of
the present invention.
[0025] FIG. 3 is a graph of alpha counts/cm.sup.2/hour versus
copper concentration for a material purified in accordance with an
aspect of the present invention.
[0026] FIG. 4 is a graph of alpha counts/cm.sup.2 hour versus total
impurities for a material purified in accordance with an aspect of
the present invention.
[0027] FIG. 5 is a graph of alpha counts/cm.sup.2/hour versus
silver concentration for a material purified in accordance with an
aspect of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] This disclosure of the invention is submitted in furtherance
of the constitutional purposes of the U.S. Patent Laws "to promote
the progress of science and useful arts" (Article 1, Section
8).
[0029] One aspect of the invention is a recognition that even
though a lead isotope (.sup.210Pb) is the prominent alpha emitter
of lead containing high concentrations of alpha emitters, such is
not the case with lead containing low or very low concentrations of
alpha emitters. For purposes of interpreting this disclosure and
the claims that follow, lead containing low concentrations of alpha
emitters is lead having an alpha flux of about 0.02
cts/cm.sup.2/hr, and lead containing very low concentrations of
alpha emitters is lead having an alpha flux of about 0.002
cts/cm.sup.2/hr.
[0030] It appears that uranium isotopes, thorium isotopes, and
perhaps other non-lead isotopes are the primary alpha emitters of
lead containing low or very low concentrations of alpha emitters.
This conclusion is based on the observation that alpha flux versus
time for lead containing low and very low concentrations of alpha
particle emitters does not follow the secular equilibrium curve
predicted from .sup.210Pb decay. The recognition that the primary
alpha emitters of lead are non-lead isotopes leads to improved
methods for reducing the alpha emitter level of lead initially
containing low and very low concentrations of alpha particle
emitters. Specifically, the alpha emitter level can be reduced with
purification methods that remove the minor amounts of non-lead
contaminants from the lead. An exemplary method developed in
accordance with aspects of the present invention is
electro-refining utilizing a bath containing nitric acid:water
(with the nitric acid concentration being from about 2% to about
50%, by volume)
[0031] The recognition that non-lead impurities are a primary alpha
emitters for lead containing low or very low concentrations of
alpha emitters is also supported by an observation that alpha flux
of such lead can scale with impurity content of the lead.
[0032] Although the invention is generally described herein with
reference to removing alpha particle emitting materials from lead,
it is to be understood that the invention can also be utilized for
purifying materials other than lead (for example metals, such as
tin, silver, copper, indium, bismuth etc. that are frequently
incorporated into lead-free solders). The removal of alpha
particles can be particularly beneficial for metals utilized in
semiconductor manufacture; such as, for example, metals
incorporated into wafer bump solders.
[0033] One aspect of the invention is a method of indirectly
tracking a low concentration of at least one alpha particle emitter
during purification of an elemental material. The alpha particle
emitter is a contaminant, rather than an isotope of the material.
In particular aspects the material can be lead and the alpha
particle emitters are isotopes of thorium and uranium. One or more
contaminants are identified which are present in the elemental
material at a greater concentration than the alpha particle
emitters, and which fractionate similarly to the emitters during
purification. The identified contaminants are preferably substances
which can be easily tracked during the purification. The
fractionation of the contaminants refers to distribution of the
contaminants between a portion remaining with the elemental
material which is to be purified during the purification process,
and a portion separating from the elemental material. Preferably,
the fractionation will be such that substantially entirely all of
the contaminants separate from the elemental material during the
purification process.
[0034] The fractionation of the contaminants is tracked during the
purification, and from such tracked fractionation is extrapolated
the fractionation of one or more alpha particle emitters.
Accordingly, the fractionation of the alpha particle emitter is
inferred (i.e., indirectly determined), rather than being directly
measured. Such can avoid problems associated with attempting to
measure an alpha flux for a material having a very low
concentration of alpha particle emitters. A low or very low
concentration of alpha particle emitters generates an alpha flux
which is typically not significantly different from background, and
which is accordingly typically very difficult to measure with a
high degree of confidence. Since the alpha particle emitters are
contaminants rather than isotopes of the elemental material being
purified, the alpha particle emitters will fractionate with other
contaminants if the purification utilizes methodology which very
specifically fractionates the elemental material from contaminating
substances. Exemplary methodology having appropriate specificity is
electro-refining. Chemical refining can also be used, either in
addition to or alternatively to the electro-refining.
[0035] The elemental material which is to be purified can, as
discussed above, be lead, and can initially comprise at least 99.99
weight % lead prior to the purification. Accordingly, the elemental
material which is to be purified can be considered to consist
essentially of, or consist of, lead. In other aspects, the
elemental material which is to be purified can consist essentially
of, or consist of Ag, Sn, Cu, Bi and In. In specific aspects, the
material which is to be purified can be ultimately used in a
lead-free solder.
[0036] The method of purification can be any suitable method,
including, for example, chemical refining and/or electro-refining.
In an exemplary method of purifying lead, electro-refining is
utilized with a bath comprising water and one or more of nitric
acid, methane sulfonic acid, fluosilicate and fluoborate. In
particular aspects of the invention it is found that
electro-refining of lead in a bath comprising or consisting
essentially of nitric acid and water (with the nitric acid being
present to a concentration of from about 2% to about 50%, by
volume) can be particularly effective for reducing the alpha flux
of lead that initially contains low or very low concentrations of
alpha emitters. Exemplary conditions for the electro-refining of
the lead in a nitric acid bath include a bath temperature of from
about 70.degree. F. (21.degree. C.) to about 100.degree. F.
(38.degree. C.), a treatment time of from about several hours to
about several days, and a treatment current density of from about
1A/ft.sup.2 to about 70 A/ft.sup.2.
[0037] Several graphs are provided with this disclosure (labeled as
FIGS. 2-5), and such graphs evidence that the alpha flux of a
material can scale with the total amount of impurities in the
material. Accordingly, the fractionation of alpha emitting
particles during a purification process can be extrapolated from
the fractionation of other impurities during the purification
process. Purification of 99.99% pure silver to 99.999% pure silver
resulted in a decrease of alpha activity from 0.0162.+-.0.0017
cts/cm.sup.2/hr to 0.0062.+-.0.0007 cts/cm.sup.2/hr. Purification
of 99.99% pure tin to 99.999% pure tin resulted in a decrease of
alpha activity from 0.0066.+-.0.0005 to 0.0007.+-.0.0008.
TABLE-US-00001 TABLE 1 Trace Element Components of Silver
Concentration Concentration Element (ppm) in 99.99% Ag (ppm) in
99.999% Ag Au 7 <0.5 Cu 5 <0.1 Bi 10 <0.1 Fe 0.7
<0.1
[0038] TABLE-US-00002 TABLE 2 Trace Element Components of Tin
Concentration Concentration Element (ppm) in 99.99% Sn (ppm) in
99.999% Ag Ag 1 0.1 Cu 1 0.1 Mg 0.3 0.1 Si 0.3 0.1 Pb 10
<0.2
[0039] As stated above, the methodologies of the present invention
can be utilized for purifying materials associated with lead-free
solders, as well as for purifying materials associated with
lead-containing solders. The utilization of the methodologies for
reducing alpha emitter concentrations in lead-free solders can have
numerous advantages. For instance, it is frequently assumed that
lead-free solders will have little or no alpha emitters therein.
Such assumption is a poor assumption, and is based on the mistaken
belief that .sup.210Pb is the primary alpha emitter in solders.
Ideally there would be no alpha emitters present in a lead-free
material, or at least the level of alpha emission would be below
the very low threshold discussed above (i.e., having an alpha flux
below about 0.002 cts/cm.sup.2/hr). Such is not available in
lead-free solders conventionally utilized in semiconductor
packaging, but can be achieved in lead-free solders prepared
utilizing methodologies of the present invention. Specifically,
methodologies of the present invention can substantially remove
alpha emitters (such as isotopes of thorium and uranium) from
lead-free solder materials. The removal of the alpha emitting
contaminants can most easily be verified by measuring the alpha
flux of the sample since they are normally present below the
detection limits of most analytical methods such as glow discharge
mass spectrometry (GDMS). However analytical methods like GDMS can
be used to track the concentration of non alpha emitting impurities
that are present in higher concentration than the alpha emitters
and are removed at a similar rate as the alpha emitters during
purification.
[0040] In compliance with the statute, the invention has been
described in language more or less specific as to structural and
methodical features. It is to be understood, however, that the
invention is not limited to the specific features shown and
described, since the means herein disclosed comprise preferred
forms of putting the invention into effect. The invention is,
therefore, claimed in any of its forms or modifications within the
proper scope of the appended claims appropriately interpreted in
accordance with the doctrine of equivalents.
* * * * *