Indium Corporation Tech Paper

Challenges in Supply of Ultralow Alpha Emitting Solder Materials

Authored by: Andy C. Mackie, PhD, MSc and Olivier Lauzeral (iROC)

The names of high-energy subatomic present in the active device layers of a particles such as alpha, beta, gamma, semiconductor chip. Andy C. Mackie x-rays and cosmic rays will be well known to every high-school student. An alpha particle is an ionized heavy Table 1 shows some of the most particle emitted spontaneously from the common types of particles and a nucleus of a specific isotope of a high description of them. atomic weight (usually >200 Daltons) atom. Of particular interest and Particle Name Description concern to semiconductor fabricators Alpha Helium (He4) are the elements uranium (U) and nucleus (2 protons thorium (Th), both of which decay to + 2 neutrons) give stable isotopes of lead (Pb), but Andy C. Mackie, PhD, MSc, Global Beta Electron or positron which, by decaying, give rise to alpha Product Manager, Semiconductor and particles. The potential energy of the Advanced Assembly Materials, Indium Gamma, X-rays Electromagnetic Corporation, has over 20 years experience (photon) particle depends on the nucleus from in wafer fabrication and semiconductor Thermal Neutron Uncharged nuclear which it is emitted. Figure 2 shows packaging materials and manufacturing. particle the decay series (only alpha emissions Mackie is an industry expert in physical Cosmic rays Mixture of mostly shown) for the isotope Thorium 232. chemistry, rheology and materials beta, with alpha, As can be readily seen, the highest science. He is a member of the ITRS, gamma and other energies correlate with greater nuclear JEDEC, SEMI, iNEMI and SMTA, and is on the editorial advisory board for Chip Scale types instability (shortest half-lives). Review magazine. Table 1. Most common subatomic species. email: [email protected] Full Biography: indium.com/corporate/ In aerospace electronics, any of these bio/andy_mackie.php particles may be encountered and A QR (quick response) code contains each presents a unique challenge encoded data. for applications, such as satellite When scanned with a smart phone’s telecommunications. Even at sea level, camera (via a QR reader application), it will much of the semiconductor packaging take you to a specific URL or text message. experts are aware of the increasing • Download need for controlled alpha-emissions article in materials that are immediately • Share with adjacent to the chip surface. This need a friend is driven by the shrink in size of the

active device (characterized by the indium.us/E018 “equivalent DRAM gate length”1) Figure 2. Decay series for thorium. Form no. 98692 R0

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+ mount technology (SMT) background P N + P N will be familiar with RoHS and + P N + P N similar global requirements for low- “Protective” layer lead (Pb) in solders. They may also - be familiar with the requirement for Interconnect layer + - + - low halogen levels, originally driven Active device layer + - + - - by environmental needs, and now also + + - driven in semiconductor packaging by a + - ++ + - requirement for increased compatibility Silicon + - + with underfill materials (flip-chip) and ++ reducing damage to bondpads (power die-attach applications). The need Figure 3. Effect of “Protection” on the depth of the Bragg Peak. for low-alpha emitting materials now The energy (in mega electron Volts, interactional cross-section grows and adds to the list of requirements from MeV ) d ict ates t he velocit y, a nd hence t he finally slows to a stop (red dot in Figure customers. penetration depth, of the alpha particle 3) in a phenomenon known as the Bragg which may be traveling at a significant Peak3. For flip-chip and similar applications fraction of the speed of light. (the focus of this study), the primary low Over the last several years, JEDEC4 alpha and ultralow alpha materials of Thermal neutrons are also a source of members have led efforts to determine interest are those immediately adjacent concern in semiconductor applications t he extent of sensit iv it y of sem iconductor to the chip surface. With the move from where boron10 is used, as this boron devices (specifically memory devices – solder bumps to copper pillar for sub- isotope can capture neutrons and fission DRAM or Flash memory) to these types 100micron chip-attach designs, these to produce alpha particles, a gamma ray of particles, and also neutrons, using both materials include: and a lithium ion2. ambient-level and accelerated particle conditions5 6. Less well-known is the • Organics: False Protection ability of alpha-particles and other high- – Capillary underfills, wafer level energy phenomena to cause “latch-up” underfills (polymer-collar), Protection against these high-energy where a P-N-P-N junction (effectively photoresists, ABF, solder mask, particles is often counter-intuitive. As two transistors with a common well) substrate epoxy, fluxes, molding mentioned, the kinetic energy of the are locked into a conductive mode compounds alpha particle is (from ½ m.v2 ) a measu re that persists even after the signal is of the velocity. As the particle interacts • Inorganics: removed. This type of phenomenon is with the lattice structure, it slows down, – Dielectrics, fiber filler in substrate, becoming a real cause for concern in creating pairs of holes and electrons “filler” in capillary underfills high-temperature power semiconductor (+ and -), until finally it absorbs two electronics due to the use of high-Pb electrons and forms a helium atom. It • Metals: solders (high surface areas) as die-attach therefore seems to make sense to put a – Solder, UBM, copper traces materials and thinner wafers. “protective” layer in the way to protect Although materials sent for testing to the active semiconductor layer from the Materials Affected iROC have occasionally been found accumulation of charge: unfortunately to be “hot” (high alpha emissivity), this (Figure 3) may act to bring closer It is no surprise that suppliers of the usual assumption is that organic to the doped-ion wells of the active materials for advanced electronics and materials are relatively easy to obtain layer where the slowing alpha particle semiconductor assembly are constantly free of uranium and thorium isotopes. generates more electron/hole pairs as being asked to meet increasingly it slows down. Like a firework going stringent requirements for performance Solder is therefore the key focus-point off in a final flare of glory, more pairs criteria and also for adherence to for spontaneous energetic particle are generated as the alpha particle’s standards. Most readers from a surface emissions. Experiments have shown

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that even low levels of lead (Pb) in a present only in the material under test, The metal pan is then sealed into the SAC (tin/silver/copper) alloy can emit then it should be possible to eliminate alpha-counter, and a flow of P10 gas levels of alpha as high as 0.3cph/cm2, any need for background measurements. passes over the surface of the plastic film or over 2 orders of magnitude higher However, the reality is that for such a and out into the counting chamber, which than the increasingly accepted “ultralow sensitive test method, “false positives” contains P10 gas. It is a combination of alpha level” (ULA) of 0.002cph/cm2. are a frequent occurrence. Such false 90% argon and 10% methane, called positives may be classified as either, the “counting gas”, and is used because Note that emerging copper-pillar from: it ionizes to CH4+ and Ar+ very easily applications, such as Tessera’s (low ionization energy). microPILR™, also use solder as a means • Interfering high energy sub-atomic particles of attaching the copper pillar to the Sensitivity of Alpha land on the substrate7, and will need to – Cosmic rays be carefully selected and specified for – Stray neutrons Emission Measurement ultralow alpha emissions. • The measurement system itself Methodology Measurement of – RF interference The test method’s sensitivity depends Alpha Emissions – Voltage spikes in the supply voltage greatly on the extent of the measured background radiation. The latest gas flow 8 • Naturally-occurring radon gas Clear definitions of test methods are proportional counters on the market have forthcoming from JEDEC, although • Cross-contamination from other ultra-low background (as low as 2cph) some companies (both materials sources, such as dust or previous thanks to a constant improvement in suppliers and users) have set their own studies materials and manufacturing processes. specifications for what is meant by Specimen Preparation However, the best-case background terms such as “low alpha” and “ultralow signal is equivalent to 2cph/1000cm2 Accurate measurement of alpha alpha” (ULA). The work by JEDEC or 0.002cph/cm2; the same level as the emissions from particles relies on a and associated industry groups seems highest allowable ULA emission level. large surface area of the material under to indicate that a level of 0.002cph/cm2 The best-case signal/noise ratio for ULA study. Typically, a large metal pan of is considered an acceptable level and materials is therefore less than 1:1, which around 1,000cm2 forms the bottom should be designated ULA. severely limits the sensitivity of the of a hermetically-sealed unit, with a analytical methodology and necessitates Equipment is becoming available that cleanable plastic tray that fits inside the long periods of study to ensure precision. allows an investigator to determine the pan to hold the material that is under test. Even then, precision to four decimal energy of a particle, and therefore more The material is spread into a thin film in places (such as 0.0020cph/cm2) is accurately “fingerprint” the source of the tray. A thin plastic (usually 0.002” impossible to guarantee. Note that the the particle. However, in current, state thick Mylar) film is then stretched over forthcoming JEDEC document discusses of the art analytical equipment, the the tray with care taken to minimize air the statistical analysis of the data, and accelerating voltage used to detect alpha entrapment between the material in the best analytical practices in great depth. particles is balanced to ensure on the one tray and the plastic film (Figure 4). hand that low-energy alpha particles are It is also important to reliably quantify detected, but also to ensure that other the background level of radiation particles are not detected, or are at least both before and after every single minimized. This usually means that the measurement to ensure that cross sensitivity of the detector is reduced, contamination, background radiation usually by around 10%. This has to be shifts, and analyzer sensitivity drift are accounted for in the final calculation of accounted for accurately. the particle flux. If the signals detected were known to Manufacture of Materials originate from a single known source Figure 4. Flux specimen ready for study. Each time an ingredient destined for

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A material in secular equilibrium, therefore, has maximum particle emission at time zero and will then only decrease over time. References 1 Described in the International Technology Roadmap for Semiconductors http://www.itrs.net/. 2 Vijaykrishnan et al. “Testing neutron- induced soft errors in semiconductor memories” http://www.cse.psu. edu/research/mdl/se/SE-Testing- Results-1.pdf . 3 http://en.wikipedia.org/wiki/Bragg_ peak 4 Joint Electronic Device Development Council http://www.jedec.org. Figure 5. Alpha emission reading versus time. 5 JESD89A – Measurement and use in a ULA material comes into purges from the system as the P10 gas Reporting of Alpha Particle and contact with any other material, there flows over the Mylar film covering the Terrestrial Cosmic Ray Induced Soft is the potential for contamination. test specimen (Figure 5). Errors in Semiconductor Devices. Alpha-emitting materials range from 6 JESD89-2A – Test Method for Alpha ambient radon gas (Rn), which is a Key gating alpha-emissions test points Source Accelerated Soft Error Rates. natural breakdown product of specific therefore need to be set up for consistent 7 Solberg “3D Substrate Innovation uranium and thorium isotopes, to supply of low-alpha material. For a for Very Fine Pitch Flip-Chip manufacturer of solder pastes, there Applications” IWLPC, Santa Clara dust with adsorbed radon, and other 2010. materials that can contaminate. Each are four separate critical points: 8 JESD221 – Alpha Radiation time a ULA-material (especially metal 1. Incoming raw materials Measurement in Electronic Materials. in a molten state) is exposed to other (flux ingredients and solders) materials, the chance of diffusion, First published in Chip Scale and hence contamination, increases. 2. Solder powder Review, Nov./Dec. 2010. Experience at Indium Corporation 3. Flux has shown that all potential sources 4. Final solder paste of alpha-emitting isotopes need to be accounted for and eliminated, It is considered that if each of the either by prudent choice or materials separate ingredients (1. to 3.) is below construction of manufacturing the desired limit, then that is a good equipment, or (in the case of ambient indicator, but not a definitive one, argon) by purging. that the material is suitable for use. Another critical factor is the purchase Because it is a non-persistent gas, of materials from suppliers where the radon is not considered a legitimate guarantee is that the materials are in source of concern for ULA materials “secular equilibrium.” That is, the usage, and test labs in both Asia and alpha emission is from an isotope at in the US do not control for radon the bottom of the decay series that is not in the environment during sample increasing in concentration as a result preparation, although it naturally of a faster-decaying parent isotope.

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