LOW ALPHA EMISSIVE SOLDER BUMPS

Abstract: An improved solder bump composition and method advantageously employs a thin low-alpha layer (310) of lead (Pb) deposited in close proximity to alpha particle sensitive devices, while ordinary (i.e., low cost) Pb is used for the bulk (302) of the solder bump (320). This approach allows for reduced overall cost while still providing protection from alpha-particle induced soft errors. The low-alpha layer reduces the flux of alpha particle into devices in two ways. First, the low-alpha layer is itself essentially Pb<210> free and therefore alpha particle emissions from the low-alpha layer are negligable. Second, the low alpha layer is substantially opaque to alpha particles emitted by the ordinary Pb which include Pb<210>. As a result, sensitive circuits on a semiconductor chip employing the improved solder bump are shielded from alpha particle emissions of the low-cost Pb<210>-containing portion of a solder bump.

Description: LOW ALPHA EMISSIVE SOLDER BUMPS Technical Field The present invention relates to semiconductor packagmg technology, and more particularly to flip chip and related chip scale semiconductor packaging technology Background Art As electronic devices become smaller and more dense, greater demands are placed on the ability to establish efficient, reliable interconnections from a semiconductor chip to a substrate There are at least three known methods for interconnecting chips to substrates Those three methods are face-up wire bonding, faceup tape-automated bonding, and the flip chip method Of these three methods, the flip chip method has often been chosen as a preferred method for semiconductor packaging, for it allows for the mterconnection of high- density devices havmg a large number of input and output paths Specifically, the flip chip method is preferred because it provides short conductive leads from the chip to the substrate, a small device footprint, low mductance, high frequency capabilities, and good noise control A flip chip is a semiconductor chip that is mounted onto a substrate with the surface of the chip facing the substrate Although several materials may be used to form an mterconnection between the flip chip and the substrate, solder is one of the more commonly employed materials In the solder interconnect process termed controlled-coUapse chip connection (C4), a solder bump is deposited on a wettable conductive terminal on the semiconductor chip The semiconductor chip is then aligned with the substrate so that the solder bumps is directly over a solder wettable terminal of the substrate The solder bump is then tacked to the substrate and reflowed, creating an electrical and mechanical connection from the chip to the substrate as well as a path for heat dissipation The C4 process typically uses lead-based alloy as solder For example, lead-tm alloys having a composition of three to five weight percent of tm have been commonly used Although the use of such lead- based alloys allows for desirable processing conditions and reliable interconnect structures, the composition of the solder indirectly causes errors in devices on the semiconductor chip which are sensitive to radiation Most lead which is used to form C4 solder contains a Pb 210 isotope which is a decay product of uranium Pb 210 is a radiogenic isotope which has a half-life of twenty-two years and which eventually emits alpha particles with an energy of approximately 5 5 million electron volts (5 5 MeV) The emission of such alpha particles from the lead solder bump leads to errors in radiation-sensitive devices present on a semiconductor chip bonded to the solder bump Incident ionizing radiation of an alpha particle that impinges upon a semiconductor chip produces a trajectory of electron-hole pairs within the semiconductor chip The charge resulting from those electron-hole pairs, m turn, may collect in a potential well of a device on the semiconductor chip For example, charge may collect in an empty storage capacitor of a memory device If the amount of charge so collected exceeds a critical upset charge value, the storage device registeis as being full rather than empty Hence, an alpha particle may cause a smgle nonrecurring read error on a smgle bit of a memory array In other words, an alpha particle emitted from a solder bump is a potential source of soft errors It is apparent from the precedmg discussion that eliminating all alpha particle emitting isotopes m lead such as Pb 210 would correspondingly lead to a reduction m radiation-induced soft errors in active devices Although perhaps the most direct solution to the problem, completely eliminating radioactive isotopes m solder bumps has proven to be both difficult and very expensive For instance, it is extremely difficult and costly to separate Pb 210 from bulk lead Lead isotope separation necessitates the use of large, expensive instrumentation such as an accelerator Furthermore, because of its limited supply, it is very costly to use lead which naturally has a low abundance of the Pb 210 isotope In fact, such "low-alpha" lead typically costs between fifty and sixty times the cost of lead normally used to form solder bumps Correspondmgly, a need exists for a relatively inexpensive technique to reduce alpha particle emissions of lead-containing solder bumps Besides addressing the problem of soft errors by focusing on the solder composition itself, it is known that soft errors may be reduced by using error correction code Error correction code reduces soft errors by providing a mechanism which both detects and corrects soft errors Even if a storage capacitor is filled by charge resulting from an alpha particle, the error correction code will detect the error and return the device it to its original (and correct) empty state Although error correction code has exhibited a degree of usefulness in reducing soft errors, room for improvement remains In particular, error correction code is costly in that it creates memory redundancies and requires extra logic to implement Correspondmgly, a need exists for reducmg soft errors without havmg to sacπfice memory or havmg to implement extra logic circuitry onto a semiconductor chip In view of at least the foregomg, it is apparent that an economical, improved solder bump which produces reliable, efficient interconnects while reducing or eliminating soft errors caused by alpha particle emissions is desired DISCLOSURE OF INVENTION Accordmgly, an improved solder bump composition and method advantageously employs a thin low- alpha layer of lead (Pb) deposited in close proximity to alpha particle sensitive devices, while ordinary (l e , low cost) Pb is used for the bulk of the solder bump This approach allows for reduced overall cost while still providmg protection from alpha-particle mduced soft errors The low-alpha layer reduces the flux of alpha particle mto devices m two ways First, the low-alpha layer is itself essentially Pb 210 free and therefore alpha particle emissions from the low-alpha layer are negligible Second, the low-alpha layer is substantially opaque to alpha particles emitted by the ordinary Pb that mcludes Pb 210 As a result, sensitive circuits on a semiconductor chip employing the improved solder bump are shielded from alpha particle emissions of the low-cost Pb 21 "-containing portion of a solder bump In one embodiment in accordance with the present mvention, a solder bump formed on an integrated circuit chip having an alpha particle (α-particle) sensitive device or circuit defined thereon includes a first portion and a low-alpha lead layer The first portion includes at least one α-particle emissive constituent The low-alpha lead layer is formed m contact with the mtegrated circuit chip and is positioned between the first portion and the α-particle sensitive device or circuit In some variations, the first portion mcludes a lead contammg solder and an α-particle emissive constituent is Pb 210 In some variations, the low-alpha lead layer is substantially free (relative to the first portion) of α-particle emissive constituents and radioactive decay precursors thereof In some vanations, the low-alpha lead exhibits an α-particle emissive characteristic approximately 100 to approximately 1000 times smaller than that of the first portion In some vanations, the low-alpha lead exhibits an α-particle emissive characteristic of less than about 0 1 - 0 001 α-particles per cm 2 per hour In some variations, the first portion mcludes SnPb or InPb solder In some variations, the low-alpha lead layer mcludes an additional constituent to raise the melting point thereof to above the reflow temperature for the first portion In some variations, the reflow temperature of a SnPb or InPb solder is at least approximately 25°C below the meltmg pomt of even undoped low-alpha lead In another embodiment m accordance with the present mvention, a method for reducmg -particle radiation incident on an α-particle sensitive device or circuit of an mtegrated circuit chip from a lead- contammg solder bump mcludes formmg a low-alpha lead layer on the mtegrated circuit chip and formmg lead-containing solder thereon to define solder bump precursors The solder bump precursors have the low- alpha lead layer disposed between the lead-contammg solder and the α-particle sensitive devices or circuits The method further mcludes reflowing the solder bump precursors under conditions selected to reflow the lead-contammg solder substantially without mixing of the lead contammg solder mto the low-alpha lead layer In some variations, the reflowing is performed at a temperature below the melting point of the low- alpha lead layer but above the reflow temperature of the lead-contammg solder In some variations, the formmg of the low-alpha lead layer mcludes doping low-alpha lead with a constituent selected to raise the meltmg point of the low-alpha lead layer to at least approximately 25°C above the reflow temperature of the lead-containing solder In some variations, the formmg of the low-alpha lead layer mcludes forming multiple sublayers thereof Boundaries between the sublayers substantially limit intrusion of the lead-contammg solder mto the low-alpha lead layer during the reflowing In yet another embodiment in accordance with the present invention, an apparatus includes an mtegrated circuit chip havmg an α-particle sensitive device defined thereon and including a lead-based solder bump for connecting to the mtegrated circuit chip The lead-based solder bump mcludes a low-alpha lead layer formed on a surface of the integrated circuit chip and a lead-contammg solder portion formed on the low-alpha lead layer The low-alpha lead layer is substantially free of α-particle emissive constituents and substantially opaque to -particle emissions from the lead-contammg solder portion In some variations, the apparatus further mcludes a substrate package, a chip carrier, or a printed circuit board, the lead-based solder bump electrically connecting the integrated circuit chip thereto In some vanations, the lead-based solder bump is formed directly over the α-particle sensitive device In some vanations, the α-particle sensitive device is associated with an SRAM memory cell or a DRAM memory cell In some variations, the mtegrated circuit chip includes a microprocessor, a memory, and/or a cache BRIEF DESCRIPTION OF THE DRAWINGS The present mvention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled m the art by referencing the accompanying drawmgs Figure 1A is a side view of a flip-chip packagmg configuration with solder bump interconnections and Figure IB is an above perspective view of such a configuration pnor to mterconnection Figures 2A, 2B, and 2C are a time series of side views of a solder bump after forming of a precursor Pb/Sn bump, after reflow to form a PbSn solder bump, and after attachment to a packagmg substrate Figures 3A and 3B are a time series of side views of solder bump formation in accordance with an embodiment of the present mvention Figure 3 A depicts a low-alpha lead (low α Pb) layer positioned to shield α-particle sensitive devices and/or circuits of a semiconductor chip from α-particles from normal (α emitter bearing) lead Figure 3B depicts a resultant PbSn solder bump after reflow wherem the low α lead layer thereof remains substantially mtact in accordance with various embodiments of the present invention Figures 4A and 4B depict an embodiment of a solder bump wherem the low-α lead layer is formed of multiple sublayers thereof such that the boundaries between the sublayers contribute to the prevention of mixing of normal (α emitter bearmg) lead mto the low α lead layer Figure 5 depicts a chip mcludmg α-particle sensitive devices and/or circuits electrically and mechanically connected to a substrate usmg an unproved solder bump configuration and method includmg a low α lead layer formed and mamtamed m accordance with an embodiment of the present mvention Figure 6A depicts a low-α lead layer positioned to shield α-particle sensitive devices and/or circuits of a semiconductor chip from α-particles from a solder alloy of α emitter bearing lead formed thereon Figure 6A depicts a resultant solder bump after reflow wherem the low-α lead layer thereof remains substantially mtact due to its higher meltmg point than the solder alloy The use of the same reference symbols m different drawmgs mdicates similar or identical items MODE(S FOR CARRYING OUT THE INVENTION Figures 1A and IB depict a flip chip interconnection of chip 110 to substrate 105 usmg solder bumps 101 Solder bumps 101 are formed on chip 110, then positioned and reflowed to form mechanical and electrical connections with matchmg wettable substrate lands 106 on substrate 105 In this way electrical connections can be provided for circuits on chip 110 to the outside world Flip chip technology was developed during the 1960's by IBM and the current state of the art for flip chip and related chip scale semiconductor packagmg technologies is described m a jomt industry standard document, J-STD-012 entitled "Implementation of Flip Chip and Chip Scale Technology" and dated January 19, 1996 Those of skill in the art will appreciate the techniques, materials, compositions, and methods of fabncation for such interconnect technology as described m the joint mdustry standard J-STD-012 (January 1996) is incorporated herem by reference in its entirety Figures 2A, 2B, and 2C depict solder bumps such as bump 101 (Figures 1 A, IB) at various stages of an interconnect and packagmg process In particular, Figure 2A depicts a solder bump precursor 201 which mcludes a body of lead (Pb) and a cap layer of tm (Sn) The bump precursor 201 is formed on a silicon substrate 210 havmg a passivation layer 203 and a metal layer 211 In the embodiment of Figure 2 A, the metal layer mcludes chromium, copper, and gold In general, such a metal layer 211 establishes a pad limiting metal area and is of a solder wettable terminal metallurgy that defines the size and area of a soldered connection, limits the flow of the solder bump, and provides adhesion and contact to chip wrnng Suitable pad limiting metal configurations are known in the art See, for example, J-STD-012 at page 20 The solder bump precursor (201) may be formed by any suitable process including, e g , evaporation, electrodeposition, electroplating, screen printing, etc Although Figure 2A depicts a solder bump precursor for formation of a PbSn solder bump, other solder bump compositions are possible with corresponding modifications to the precursor Common solder bump compositions mclude 90 PbSn, 97 PbSn, 63 SnPb, 50 InPb, and 95 PbSn Whatever the constituents of the solder bump precursor and method of formation, a subsequent non-oxidizing reflow process yields a rounded solder bump For example, solder bump precursor 201 is heated such that the lead and tm components thereof melt to form solder bump 20 IB (Figure 2B) Substrate 205, which mcludes metallization bonding pad 211, is then heated to reflow solder bump 201B and substrate 205 is placed to bond corresponding metallization pads 211 to metal features 212 of silicon substrate 210 The resulting solder bump 20 IC forms the electrical and mechanical bridge between the chip and the next level assembly, in this case, substrate 205 Although not shown m Figures 2A, 2B, and 2C, a variety of techniques can be used to ensure that solder bumps such as solder bump 201c form connections between corresponding metal pads of the substrate and skip to the substrate 205 and the chip 210 without collapse of the solder bump One of these is the controlled collapse chip connection (C4) method in which a solder jomt collecting a flip chip in which the solder bump connects the flip chip and substrate where the chip connection surface tension forces of the liquid solder supports the weight of the chip and controls the collapse height of the jomt Alternative or complementary techniques mclude the use of mechanical standoffs such as a copper ball standoff in the solder jomt or a glass dam As described above, lead in solder bump material (e g , 201, 201B, and 201C), can often contain alpha particle emitters For example, low cost lead often contains traces of thorium and various decay products such as polonium as well as the radiogenic isotope, Pb 210 Figures 3A and 3B, Figures 6A and 6B, and Figures 4A and 4B depict vanous unproved solder bump configurations in which a low-alpha lead layer 310 is used m close proximity to chip 330 that may mclude circuits sensitive to alpha particle emissions Each of the exemplary configurations is now descnbed In a first exemplary configuration, a low-alpha lead layer 310 is employed As shown m Figure 3 A, the low-alpha lead layer 310 is formed on the surface of chip 330 and lead 302 (e g , low cost lead that may contain alpha particle emitters) is formed thereupon As before, a cap layer of tm 301 is formed on top of the lead In this way, a solder bump precursor 320 is provided Solder bump precursor 320 is heated above the meltmg pomts of lead 302 and tm 301 to reflow the metals such that the two alloy to form solder bump 303 as shown m Figure 3B Note that although t 301 and lead 302 alloy to form solder bump 303, low-alpha lead layer 310 remams substantially unmixed with the resultant solder bump 303 As a result, low-alpha lead layer 310 forms an α-particle barner between the solder bump which mcludes alpha emitters and sensitive circuits of chip 330 Several techniques for mamtammg low-alpha lead layer 310 substantially unmixed with the resultant solder bump 303 are described m greater detail below As noted m J-STD-012 (January 1996) at 22-23, 26 soft errors m circuits and devices, e g , memory cells of DRAM and SRAM chips, are a substantial problem In fact, the jomt standard document states that solder must be kept m an appropriate distance from such alpha particle sensitive circuitry For example, a separation of 150 μm is required between (4 bumps (97 PbSn)) and sensitive circuitry on polyrmide passivated devices Although measures such as thicker passivations or metal shields can mitigate the effects of proximity, the use of a low-alpha lead layer m accordance with the present invention has a manufacturing advantage smce change in current process technology is required Using a low-alpha lead layer as described herem, typical solder bumps contammg normal lead (e g , low-cost alpha emitter bearing lead) can be formed directly on typical SRAM or DRAM cells fabricated in 0 35 μm (or smaller) process In general, lead will be considered to be low-alpha lead based on a relative measure of α-particle eπussivity as compared to bulk, low-cost lead and based on the sensitivity of a particular device technology to α-particle upsets In other words, "low" is relative to emissive characteristics of low-cost bulk lead suitable for C4 or other solder bump processes and relative to device requirements An optimal "lowness" for α- particle emissivity is a function of cost and sensitivity of a particular device technology to α-particle upsets For example, given a particular level of device sensitivity, an extremely low emissivity low-alpha lead (e g , less than 0 001 α-particles per cm 2 per hour) may be desireable However, m other situations, a low-alpha lead with an α-particle emissivity of less than 0 1 α-particles per cm 2 per hour may be suitable Because a low-alpha lead layer also acts as a barrier to α-particles from ordinary lead-bearmg solder, thickness of the low alpha lead layer is also a factor Dependmg on market cost, a thinner low-alpha lead layer of extremely low emissivity lead may be preferrable to a thicker low-alpha lead layer of moderately low emissivity lead, or vice versa Appropnate α-particle emissivity characteristics and layer thicknesses will vary for differing device technologies, levels of mtegration, proximities to α-particle sensitive devices, emissivites of bulk, low-cost lead, and lead market conditions However, for 0 35 μm technology used in microprocessor fabrication with solder bumps directly over cache memory cells, low-alpha lead layers of less than 1 μm exhibitmg emissivities of less than approximately 0 05 α-particles per cm 2 per hour are suitable To maintam low-alpha lead layer 310 substantially unmixed with the normal lead of solder bump 303, several techniques are suitable For example, low-alpha lead of layer 310 can be formed with additional constituents so as to raise the meltmg pomt of low-alpha lead layer 310 to above the temperature required to melt tm 301 and lead 302 In this way, low-alpha lead layer 310 remams m solid form even as solder bump 303 is formed Such additional constituent materials need only raise the melting pomt by approximately 30° Suitable additional constituent elements mclude Au (@1-10% by weight), • Ba (@l-10% by weight), Ca (@l-10% by weight), Cu (@l-10% by weight), Mg (@1-10% by weight), Hg (@1-10% by weight), and • Rare earth elements (@l-5% by weight), as well as combinations thereof Of these, constituent element concentrations of Hg (@4-5% by weight), Mg (@2-3% by weight) and Au (@3-4% by weight) are generally preferred Constituent element selections resulting in greater or lesser raismg of meltmg point for low-alpha lead layer 310 may be employed dependmg on the precision of temperature control available in a particular manufacturing process Another technique for ensuring that a low-alpha lead layer remains substantially unmixed with the alpha emitter contammg material mvolves lowermg the meltmg point of the alpha emitter contammg material compared to that of the low-alpha lead Figures 6A and 6B depict an exemplary embodiment m which a PbSn solder alloy 602 is formed on low-alpha lead layer 610 In particular, a low-alpha lead layer 610 is deposited by evaporation (or other method such as electroplating) to a thickness of approximately 1 μm The remainder of solder bump precursor 620 is deposited at a desired Pb/Sn ratio For example, in an evaporative process, a sequence of boats contammg metal targets can be used where the first boat (or boats) contam(s) low-alpha lead and the remamder contain the desired PbSn mixture When the evaporation cycle is complete, (I e , after one complete pass), a layer approximately 1 μm thick of low-alpha lead is formed beneath the PbSn alloy The fractional percentage of Sn is selected to lower the melting pomt of the PbSn alloy below that of the low-alpha lead For example, a 10% Sn fraction lowers the meltmg pomt of the PbSn allow to approximately 300°C as compared with 327°C for low-alpha lead layer 610 In this way, a convenient reflow temperature can be selected to ensure that low-alpha lead layer 610 does not melt and remains substantially unmixed with the PbSn alloy contammg α-particle emitters Therefore, the integrity of low-alpha lead layer 610 is maintained while PbSn solder alloy 602 is reflowed, providing an α-particle shield to sensitive devices on chip 630 (e g , SRAM or DRAM cells) A similar configuration and process is envisioned for Pbln alloys where substantially pure low-alpha lead deposited as an initial layer to a thickness of approximately 1 μm and a Pbln bump is deposited thereupon The fraction of In is chosen so that the meltmg pomt of the Pbln mixture is lower than that of the low-alpha lead As before, this technique provides an undisturbed layer of low-alpha lead to act as a shield to α-particles emitted by constituents of the low-cost lead included m the Pbln mixture Another technique for ensuring that low-alpha lead layer 310 remams substantially unmixed with α- particle emitter bearing lead, even as solder bump 303 is formed, is shown m Figures 4A and 4B Multiple successive component layers of the low-alpha lead layer 310 are formed In the illustrative embodiment of Figures 4A and 4B, five 0 2 μm layers of low-alpha lead 411 are formed by successive layer formmg process steps, e g , evaporation, sputtermg electrodeposition, electroplatmg, screen prmtmg, etc In this way, the aggregate low-alpha lead layer 410 mcludes a boundary layer 412 between the top layer of low-alpha lead and the overlayer of normal lead 402 as well as multiple mteπor boundaries between low-alpha lead layers 410 These boundaries between layers advantageously limit the mixing of bearing lead 402 with at least the lower portion of low-alpha lead layer 410, even when reflow temperatures are sufficient to melt lead 402, tm 401, and low-alpha lead 410 Mixing is prevented by the presence of the mterlayer boundaries (e g , 412 and 413) which act as getteπng sites thereby preventing the uniform flow of heat Figure 4B shows the resultant solder bump 403 after reflow Note that the individual layers of low- alpha lead 410 are preserved Therefore, after reflow, a solder bump m accordance with the present mvention mcludes a low-alpha layer 410 that is essentially free of alpha particle emitting isotopes and that acts as an α- particle barrier layer to α-particle emissions from solder bump 403 (which contains normal, α-particle emitting, lead) In this way, sensitive circuits such as memory cells are protected from α-particle emissions without requiring large amounts of expensive low-alpha lead, error correcting code techniques, or other material overlayers such as indium or gold Referring back to Figures 3A and 3B, typical scale for the solder bump precursor is as follows Lead layer 302 is approximately 60 to 70 μm m thickness, whereas low-alpha lead layer 310 is approximately 1 μm m thickness Although a variety of relative dimensions are possible, thinner low-alpha lead layers are generally advantageous so as to reduce the amount and expense of low-alpha lead consumed On the other hand, thicker layers provide more effective barriers to α-particle emissions and greater protection from the transient errors that can result In practice, a 1 μm layer of low-alpha lead has been found to be effective, although thinner and thicker layers (e g , layers varying from about 0 5 μm to about 5 μm) may also be suitable Such low-alpha lead layers can be formed by evaporation, by sputtering, by electroplatmg, or by any other suitable method Figure 5 depicts a C4 bump interconnection (solder bump 503) between chip 330, 420 and substrate 550, with a low-alpha lead layer 310, 410 interposed between the α-particle emitter bearmg lead of solder bump 503 and a sensitive device region 501 of chip 330, 420 The low-alpha lead layer is formed and maintained by any suitable technique For example, the low-alpha lead layer may be formed and mamtamed through reflow as described above with reference to Figures 3A and 3B (e g , by doping the low-alpha lead layer to raise its meltmg pomt), as descnbed above with reference to Figures 6A and 6B (e g , by formmg a reduced meltmg pomt alloy or mixture on the low-alpha lead layer), or as descnbed above with reference to Figures 4A and 4B (e g , by exploiting getteπng sites a sublayer boundanes) In any case, mixing of normal alpha emitter bearing lead 302, 602, 402 mto the low-alpha lead layer 310, 610, 410 is substantially limited or prevented Illustratively, α-particle sensitive device region 501 may include DRAM or SRAM cells, e g , of a cache memory, for a microprocessor formed on chip 330, 630, 420 In this way, solder bump 503 can be formed m close proximity to sensitive device region 501 without exposmg sensitive device region 501 to soft- error mducmg, solder bump generated α-particle emissions As a result, such sensitive device regions and solder bump interconnections can be formed at higher densities and with fewer α-particle emission related positional constraints It is expected that the advantages of solder bumps formed m accordance with the present mvention will mcrease as feature sizes decrease on chips such as 330, 630, 420 Such decreased feature sizes, for example m sub 0 25 μm advanced microprocessors, increase the vulnerability of devices and circuits, (particularly memory cells of caches) to α-particle mduced soft enors In addition, such higher density circuits mcrease demands for mterconnect density, and hence, for surface area of chip 330, 630, 420 usable for chip scale mterconnect formation While the mvention has been descnbed with reference to various embodiments, it will be understood that these embodiments are illustrative and that the scope of the invention is not limited to them Many vanations, modifications, additions, and improvements of the embodiments described are possible For example, although solder compositions of lead and tin have been described, other compositions bearing α particle emitters may also benefit from a low α lead layer For example, an InSb solder embodiment is envisioned Furthermore, although embodiments have been described (1) in which the meltmg point for a low-alpha lead portion of a solder bump is elevated with respect to an alpha-emitter bearing lead portion thereof, (2) m which the meltmg point for the alpha-emitter bearing lead portion is reduced with respect to the low-alpha lead portion of a solder bump thereof, and (3) in which structural aspects of a low-alpha lead layer are employed to limit mixing of the low-alpha lead layer with alpha-emitter bearmg lead, other embodiments may combme aspects of the described to achieve similar results For example, a low-alpha lead layer may be doped to mcrease its meltmg temperature while a Pb-based alloy may be formed thereon with alloy constituents selected to reduce the alloy meltmg temperature Although embodiments of the present invention have been described pnmanly m the context of a semiconductor device havmg α-particle sensitive memory cells, e g , a DRAM, SRAM or microprocessor mcludmg on-chip cache or mam memory, other α-particle sensitive microelectronics applications are also suitable These and other variations, modifications, additions, and improvements may fall withm the scope of the invention as defined m the claims which follow

Claims: WHAT IS CLAIMED IS: 1 A solder bump formed on an integrated circuit chip having an alpha particle (α-particle) sensitive device or circuit defined thereon, said solder bump compnsmg a first portion mcludmg at least one α-particle emissive constituent, a low-alpha lead layer formed m contact with said mtegrated circuit chip, said low-alpha lead layer positioned between said first portion and said α-particle sensitive device or circuit 2 A solder bump as recited m claim 1 , wherem said first portion includes a lead containing solder and said at least one α-particle emissive constituent mcludes Pb 210 3 A solder bump as recited in claim 1, wherem said low-alpha lead layer is substantially free of α-particle emissive isotopes, and wherem said low-alpha lead layer is substantially opaque to α-particle emissions of said first portion such that solder bump related α-particle mduced soft enors m said α-particle sensitive device or circuit are substantially eliminated 4 A solder bump as recited m claim 1, wherem said low-alpha lead layer mcludes an additional constiment to raise the meltmg pomt of said low-alpha lead layer to above a reflow temperature for said first portion 5 A solder bump as recited m claim 1, wherein said first portion mcludes a lead contammg solder, and wherem said low-alpha lead layer mcludes plural separately formed sublayers thereof, boundaries between said sublayers substantially inhibiting intrusion of said α-particle emissive radioisotope from said lead contammg solder mto said low-alpha lead layer during reflow 6 A solder bump as recited m claim 5, wherem at least one of said plural separately formed sublayers mcludes an additional constituent to raise the meltmg pomt of said sublayer to above a reflow temperature for said lead contammg 7 The solder bump as recited in claim 1, reflowed to form an electrical and mechanical connection to a substrate m a packaged mtegrated circuit 8 The solder bump as recited m claim 7, wherem said packaged mtegrated circuit mcludes a microprocessor with an on-chip cache, and wherem said α-particle sensitive device or circuit includes a memory cell of said an on-chip cache 9 A solder bump as recited in claim 1, wherein said α-particle sensitive device or circuit includes one of an SRAM memory cell and a DRAM memory cell 10 A method for reducmg α-particle radiation incident on an α-particle sensitive device or circuit of an mtegrated circuit chip from a lead-contammg solder bump, said method compnsmg formmg a low-alpha lead layer on said mtegrated cncuit chip and forming lead-contammg solder thereon to define solder bump precursors havmg said low-alpha lead layer disposed between said lead-contammg solder and said α-particle sensitive devices or circuits, reflowing said solder bump precursors under conditions selected to reflow said lead-containmg solder substantially without mixing of said lead contammg solder mto said low-alpha lead layer 11 A method as recited in claim 10, wherem said reflowing is performed at a temperature below the meltmg point of said low-alpha lead layer but above the reflow temperature of said lead-containmg solder 12 A method as recited m claim 10, wherein said formmg said low-alpha lead layer includes dopmg low-alpha lead with a constituent selected to raise the meltmg pomt of said low-alpha lead layer to at least approximately 25°C above the reflow temperature of said lead-containmg solder 13 A method as recited in claim 10, wherem said formmg said low-alpha lead layer includes forming plural sublayers thereof, and wherem boundaries between said sublayers substantially limit intrusion of said lead-contammg solder into said low-alpha lead layer during said reflowing 14 An apparatus compnsmg an integrated circuit chip havmg an α-particle sensitive device defmed thereon, a lead-based solder bump for connecting to said mtegrated circuit chip, said lead-based solder bump mcludmg a low-alpha lead layer formed on a surface of said integrated circuit chip and a lead-containmg solder portion formed on said low-alpha lead layer, said low-alpha lead layer bemg substantially free of α-particle emissive constituents and substantially opaque to α-particle emissions from said lead-contammg solder portion 15 An apparatus as in claim 14, further comprising one of a substrate package, a chip earner, and a printed circuit board, said lead-based solder bump electrically connecting said one of a substrate package, a chip earner, and a prmted circuit board to said integrated circuit chip 16 An apparatus as in claim 14, wherein said lead-based solder bump is formed directly over said α- particle sensitive device 17 A solder bump precursor compnsmg a first lead-based conductive layer formed on a semiconductor chip havmg a radiation sensitive device defined m closed proximity to said solder bump precursor, said first lead-based conductive layer being substantially free of radioactive isotopes and bemg substantially opaque to radiation, and a second lead-based conductive layer formed on the first lead-based conductive layer, wherem compositions of said first and said second lead-based conductive layers are selected to allow reflow of said solder bump precursor at a reflow temperature below a meltmg point of said first lead-based conductive layer 18 A solder bump precursor as m claim 17, wherein said first lead-based conductive layer is substantially free from Pb 210 and the second conductive material mcludes Pb 210 19 A solder bump cormectmg a semiconductor chip to a substrate, chip earner or circuit board, said semiconductor chip mcludmg a device susceptible to a radiation-induced soft error m close proximity to said solder bump, said solder bump compnsmg plural thin lead layers, each less than about 0 5 - 02 μm m thickness, said plural thin lead layers electrically coupled to a circuit of said semiconductor chip, said plural thm lead layers bemg substantially free of α-particle emissive isotopes and bemg substantially opaque to α- particles, each thin lead layer bemg separated from an adjacent thin lead layer by a layer interface, a lead-bearmg solder portion electncally coupled between said plural thm lead layers and said substrate, said lead-bearmg solder portion comprising the substantial proportion of said solder bump, and a transition region extendmg mto less than all of said thm lead layers, said transition region compnsmg a mixture of material from said lead-bearing solder portion and from said thin lead layers 20 A solder bump as recited in claim 19, further compnsmg a minority constituent withm at least one of said plural thm lead layers, said minority constituent selected to mcrease the meltmg temperature of said at least one thm lead layer relative to a reflow temperature of said lead-bearmg solder portion

Inventors: HOSSAIN TIM Z, MAILE BERND, MILLER ROY MARK, TIFFIN DON A, HOSSAIN, TIM, Z., MAILE, BERND, MILLER, ROY, M., TIFFIN, DON, A.
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES, INC.
Application year: 1999
Main classification: H01L21
IPC codes: B23K35, H01L21, H01L23, H05K3