US 20070045842A1 (19) United States (12) Patent Application Publication (10) Pub. No.: US 2007/0045842 A1 Weiser et al. (43) Pub. Date: Mar. 1, 2007 (54) LEAD-CONTAINING SOLDER BUMPS (60) Provisional application No. 60/417.241, filed on Oct. 8, 2002. (76) Inventors: Martin W. Weiser, Liberty Lake, WA (US); Nancy F. Dean, Liberty Lake, Publication Classification WA (US); Brett M. Clark, Spokane, WA (US); Michael J. Bossio, Spokane, (51) Int. Cl. WA (US); Ronald H. Fleming, HOIL 23/248 (2006.01) Spokane, WA (US); James P. Flint, (52) U.S. Cl. ........................... 257/737; 257/734; 257/772 Spokane, WA (US) Correspondence Address: (57) ABSTRACT WELLS ST. JOHN P.S. 601 W. FIRST AVENUE, SUITE 1300 The invention includes solder materials having low concen SPOKANE, WA99201 (US) trations of alpha particle emitters, and includes methods of (21) Appl. No.: 11/592,347 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 (22) Filed: Nov. 2, 2006 containing and lead-free materials through purification of Related U.S. Application Data the materials. The invention also includes methods of esti mating the fractionation of a low concentration of one or (62) Division of application No. 10/670,319, filed on Sep. more alpha particle emitters during purification of a mate 26, 2003. rial. y=6E-05x1-OOO2 ge SERIES 1 - LINEAR (SERIES 1) TOTAL IMPURITIES IN PPM Patent Application Publication Mar. 1, 2007 Sheet 1 of 3 US 2007/0045842 A1 50 44 44% 14 Arc az7 - A AAGAAAAG A&Z Patent Application Publication Mar. 1, 2007 Sheet 2 of 3 US 2007/0045842 A1 21.)*–||–)((INITSERIES)4=7FF77()„No. ??>1620-ajU2,E-,J •sikºsI 200H/2 WO/SIN000 VHa7v InOH/2 WO/SIN000 WH-7v Patent Application Publication Mar. 1, 2007 Sheet 3 of 3 US 2007/0045842 A1 —(VGN/7SEINES)(? —(VGN/7SJ12775)(1 E-L’OE-TJºSººs! õEZ?T2,972 01OZ09:0709 |nOH/2 WO/SIN000 VHgTv US 2007/0045842 A1 Mar. 1, 2007 LEAD-CONTAINING SOLDER BUMPS semiconductor devices because the alpha particles can induce so-called soft errors. The errors are referred to being RELATED PATENT DATA “soft” in that the errors are not permanent. However, the errors will typically cause at least one round of incorrect 0001. This application is related to U.S. Provisional calculations. Application Ser. No. 60/417.241, which was filed on Oct. 8, 2002. 0009. There are numerous sources for alpha particles, including reactions caused by cosmic rays. However, the TECHNICAL FIELD Source which is frequently most problematic for semicon ductor device packages is solder utilized for forming various 0002 The invention pertains to semiconductor packages: interconnections relative to semiconductor dies. For lead-containing solders and anodes; and methods of remov instance, the wafer-bump technique is becoming relatively ing alpha-emitters from materials. common for forming high density interconnects to semicon ductor dies. The bumps are portions of solder formed over BACKGROUND OF THE INVENTION electrical nodes associated with a semiconductor die pack 0003 Solders are commonly utilized in semiconductor age. If the solder utilized in the bumps has alpha particle device packaging. If the Solders contain alpha particle emit emitting components, the alpha particles are frequently ting isotopes (referred to herein as alpha particle emitters), emitted close to integrated circuitry associated with the emitted alpha particles can cause damage to packaged semiconductor die. semiconductor devices. Accordingly, it is desired to reduce 0010 Occasionally, the solder formed over the electrical the concentration of alpha particle emitters within the sol nodes is in the form of large pillars. Such pillars are ders. frequently referred to as columns. For purposes of interpret 0004. An exemplary prior art semiconductor package is ing this disclosure, the term “bump' is to be understood to shown in FIG. 1 as a package 50, with the exemplary encompass various forms of solder formed over electrical package representing a flip-chip construction. The package nodes, including the forms commonly referred to as col comprises a semiconductor component 12 (Such as, for S. example, an integrated circuit chip). The package also 0011 A typical component of many solders is lead. comprises a board 14 utilized to support the semiconductor However, one of the lead isotopes (specifically 'Pb) has a component 12. A plurality of contact pads 38 (only some of decay chain that leads to alpha particles. Further, various which are labeled) are joined to chip 12, and a plurality of common contaminants of lead can emit alpha particles, contact pads 40 (only some of which are labeled) are joined including, for example, isotopes of uranium, thorium, to board 14. Solder balls or bumps 39 (only some of which radium and polonium. are labeled) are provided between pads 38 and 40 to form electrical interconnects between pads 38 and 40. The elec 0012. The alpha particle emitters present in lead can be trical connection utilizing the solder balls or bumps 39 with present in the ore from which the lead is initially refined. pads 38 and 40 can incorporate so-called wafer bump Alpha particle emitters can be alternatively, or additionally, technology. introduced during processing and/or use of the lead. For instance, phosphoric acid and some antistatic systems con 0005 Suitable encapsulant 44 can be provided over the tain alpha particle emitters; some abrasives and cleaning chip 12 and substrate 14 as shown. Additionally, and/or agents can introduce alpha particle emitters into lead; and alternatively, thermal transfer devices (not shown) Such as Smelting of commercial lead can introduce uranium, thorium heat sinks and heat spreaders can be provided over the chip and other alpha particle emitters into the lead from gangue 12. rock. 0006 Contact pads 30 (only some of which are labeled) 0013 The amount of alpha particle emitters present in are on an underside of the board 14 (i.e., on a side of board lead is typically determined by an alpha flux measurement, 14 in opposing relation relative to the side proximate chip with results stated in terms of alpha particle counts per unit 12). Contact pads 30 typically comprise stacks of copper, area per hour (cts/cm/hr). It is possible to commercially nickel and gold. Solder balls 32 (only some of which are obtain lead having an alpha flux of from 0.002 to 0.02 labeled) are provided on the contact pads and utilized to cts/cm/hr, but it is very difficult to obtain a material with a form electrical interconnections between the contact pads 30 lower alpha flux. However, the semiconductor industry is and other circuitry (not shown) external of the chip package. requesting materials with significantly lower alpha flux, The contact pads 40 can be connected with pads 30 through including for example, materials having an alpha flux of less circuit traces (not shown) extending through board 14. than 0.0001 cts/cm/hr. 0007. The shown package 50 has solder proximate chip 0014) Among the difficulties associated with reducing the 12 from at least balls 39, and possibly through wafer bumps concentration of alpha flux emitters in a material to associated with pads 38 and/or pads 40. There can be other extremely low levels is a difficulty in measuring the con applications of solder within package 50 which are not centration of the emitters at flux levels below 0.002 cts/cm/ specifically shown. For instance, a solder paste can be hr. Unless the concentration can be measured, it is difficult provided between chip 12 and various thermal transfer to monitor a purification process to determine if alpha devices. particle emitters are being removed. For instance, it can be 0008. The solders utilized in package 50 can be prob difficult to determine at any given stage of the purification lematic, as discussed above, in that the Solders can contain process if alpha particle emitters are fractionating with a alpha particle emitters. Alpha particles are problematic for material or away from the material. US 2007/0045842 A1 Mar. 1, 2007 0.015 Although the discussion above focuses on remov 0.001 cts/cm/hr, preferably less than 0.0005 cts/cm/hr, ing alpha particle emitters from lead-containing solders, it more preferably less than 0.0002 cts/cm/hr, and even more should be understood that alpha particle emitters are also preferably less than 0.0001 cts/cm/hr. problematic in other materials. For instance, one of the methods utilized to reduce the concentration of alpha par BRIEF DESCRIPTION OF THE DRAWINGS ticle emitters in solder has been to create so-called lead-free solders. Such solders contain little, if any, lead, which is 0022 Preferred embodiments of the invention are desirable from an environmental perspective. However, the described below with reference to the following accompa solders can still have an undesirable amount of alpha particle nying drawings. emitters present therein. Exemplary lead free solders are Sn: 0023 FIG. 1 is a diagrammatic cross-sectional side view 3.5% Ag: Sn: 4% Ag: 0.5% Cu; and Bi: 2-13% Ag, where the percentages are by weight. of a prior art semiconductor package construction. 0024 FIG. 2 is a graph of alpha counts/cm/hour versus 0016 One of the methods which has been utilized for total impurities for a material purified in accordance with an reducing the number of alpha particle emitters in lead aspect of the present invention. containing solders is to start with lead materials which have very few emitters therein. Presently there are three sources 0025 FIG. 3 is a graph of alpha counts/cm/hour versus of such materials.