Indium Corporation Tech Paper

Challenges in Supply of Ultralow Alpha Emitting Solder Materials

Authored by: Andy C. Mackie, PhD, MSC and Olivier Lauzeral, iRoc Technologies.

The names of high-energy subatomic present in the active device layers of a particles such as alpha, beta, gamma, semiconductor chip. DR. ANDY C. MACKIE x-rays and cosmic rays will be well An alpha particle is an ionized heavy known to every high-school student. particle emitted spontaneously from the Table 1 shows some of the most nucleus of a specific isotope of a high common types of particles and a atomic weight (usually >200 Daltons) description of them. 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 Beta Electron or positron which, by decaying, give rise to alpha Gamma, X-rays Electromagnetic particles. The potential energy of the Andy C. Mackie, PhD, MSc, Senior (photon) particle depends on the nucleus from Product Manager, Semiconductor and Advanced Assembly Materials for Thermal Neutron Uncharged nuclear which it is emitted. Figure 2 shows Indium Corporation, has over 20 years particle the decay series (only alpha emissions experience in wafer fabrication and semiconductor packaging materials Cosmic rays Mixture of mostly shown) for the isotope Thorium 232. and manufacturing. beta, with alpha, As can be readily seen, the highest Dr. Mackie is an industry expert in gamma and other energies correlate with greater nuclear physical chemistry, rheology and types materials science. He is a member instability (shortest half-lives). of the ITRS, AEC, iNEMI and SMTA, Table 1. Most common subatomic species. and is current chair of the editorial advisory board for Chip Scale Review In aerospace electronics, any of these magazine. particles may be encountered and email: [email protected] each presents a unique challenge Full biography: www.indium.com for applications, such as satellite biographies telecommunications. Even at sea level, much of the semiconductor packaging 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 R1

FromFrom One One Engineer Engineer To To Another Another® ® 1 Indium Corporation Tech Paper

+ electronics due to the use of high-Pb solders (high surface P N + P N + P N + areas) as die-attach materials and thinner wafers. P N “Protective” layer - Materials Affected Interconnect layer + - + - Active device layer + - It is no surprise that suppliers of materials for advanced + - - + + - + - electronics and semiconductor assembly are constantly ++ + - Silicon + - being asked to meet increasingly stringent requirements for + ++ performance criteria and also for adherence to standards. Most

readers from a surface mount technology (SMT) background Figure 3. Effect of “Protection” on the depth of the Bragg Peak. will be familiar with RoHS and similar global requirements The energy (in mega electron Volts, MeV) dictates the velocity, for low-lead (Pb) in solders. They may also be familiar with and hence the penetration depth, of the alpha particle which the requirement for low halogen levels, originally driven by may be traveling at a significant fraction of the speed of light. environmental needs, and now also driven in semiconductor packaging by a requirement for increased compatibility Thermal neutrons are also a source of concern in semiconductor with underfill materials (flip-chip) and reducing damage to applications where boron[10] is used, as this boron isotope bondpads (power die-attach applications). The need for low- can capture neutrons and fission to produce alpha particles, a alpha emitting materials now adds to the list of requirements gamma ray and a lithium ion[2]. from customers. False Protection For flip-chip and similar applications (the focus of this study), the primary low alpha and ultralow alpha materials of interest Protection against these high-energy particles is often are those immediately adjacent to the chip surface. With the counter-intuitive. As mentioned, the kinetic energy of the move from solder bumps to copper pillar for sub-100micron alpha particle is (from ½ m.v2 ) a measure of the velocity. As chip-attach designs, these materials include: the particle interacts with the lattice structure, it slows down, creating pairs of holes and electrons (+ and -), until finally it • Organics: absorbs two electrons and forms a helium atom. It therefore – Capillary underfills, wafer level underfills (polymer- seems to make sense to put a “protective” layer in the way to collar), photoresists, ABF, solder mask, substrate epoxy, protect the active semiconductor layer from the accumulation fluxes, molding compounds of charge: unfortunately this (Figure 3) may act to bring closer • Inorganics: to the doped-ion wells of the active layer where the slowing – Dielectrics, fiber filler in substrate, “filler” in capillary alpha particle generates more electron/hole pairs as it slows underfills down. Like a firework going off in a final flare of glory, more • Metals: pairs are generated as the alpha particle’s interactional cross- – Solder, UBM, copper traces section grows and finally slows to a stop (red dot in Figure 3) in a phenomenon known as the Bragg Peak[3]. Although materials sent for testing to iROC have occasionally been found to be “hot” (high alpha emissivity), the usual Over the last several years, JEDEC [4] members have led assumption is that organic materials are relatively easy to efforts to determine the extent of sensitivity of semiconductor obtain free of uranium and thorium isotopes. devices (specifically memory devices – DRAM or Flash memory) to these types of particles, and also neutrons, using Solder is therefore the key focus-point for spontaneous both ambient-level and accelerated particle conditions[5,6]. energetic particle emissions. Experiments have shown that Less well-known is the ability of alpha-particles and other even low levels of lead (Pb) in a SAC (tin-silver-copper) 2 high-energy phenomena to cause “latch-up” where a P-N- alloy can emit levels of alpha as high as 0.3cph/cm , or over P-N junction (effectively two transistors with a common well) 2 orders of magnitude higher than the increasingly accepted 2 are locked into a conductive mode that persists even after the “ultralow alpha level” (ULA) of 0.002cph/cm . signal is removed. This type of phenomenon is becoming a real Note that emerging copper-pillar applications, such as cause for concern in high-temperature power semiconductor Tessera’s microPILR™, also use solder as a means of attaching

From One Engineer To Another® 2 Indium Corporation Tech Paper

the copper pillar to the land on the substrate[7], and will need material is spread to be carefully selected and specified for ultralow alpha into a thin film emissions. in the tray. A thin Measurement of Alpha Emissions plastic (usually 0.002” thick Clear definitions of test methods [8] are forthcoming from Mylar) film is then JEDEC, although some companies (both materials suppliers stretched over and users) have set their own specifications for what is meant the tray with care by terms such as “low alpha” and “ultralow alpha” (ULA). taken to minimize The work by JEDEC and associated industry groups seems air entrapment Figure 4. Flux specimen ready for study. 2 to indicate that a level of 0.002cph/cm is considered an between the acceptable level and should be designated ULA. material in the tray and the plastic film (Figure 4).

Equipment is becoming available that allows an investigator The metal pan is then sealed into the alpha-counter, and a to determine the energy of a particle, and therefore more flow of P10 gas passes over the surface of the plastic film and accurately “fingerprint” the source of the particle. However, in out into the counting chamber, which contains P10 gas. It is current, state of the art analytical equipment, the accelerating a combination of 90% argon and 10% methane, called the voltage used to detect alpha particles is balanced to ensure on “counting gas”, and is used because it ionizes to CH4+ and the one hand that low-energy alpha particles are detected, but Ar+ very easily (low ionization energy). also to ensure that other particles are not detected, or are at least minimized. This usually means that the sensitivity of the Sensitivity of Alpha Emission detector is reduced, usually by around 10%. This has to be accounted for in the final calculation of the particle flux. Measurement Methodology The test method’s sensitivity depends greatly on the extent of the If the signals detected were known to originate from a single measured background radiation. The latest gas flow proportional known source present only in the material under test, then counters on the market have ultra-low background (as low it should be possible to eliminate any need for background as 2cph) thanks to a constant improvement in materials and measurements. However, the reality is that for such a sensitive manufacturing processes. However, the best-case background test method, “false positives” are a frequent occurrence. Such signal is equivalent to 2cph/1000cm2 or 0.002cph/cm2; the false positives may be classified as either, from: same level as the highest allowable ULA emission level. The • Interfering high energy sub-atomic particles best-case signal/noise ratio for ULA materials is therefore less – Cosmic rays than 1:1, which severely limits the sensitivity of the analytical – Stray neutrons methodology and necessitates long periods of study to ensure precision. Even then, precision to four decimal places (such • The measurement system itself as 0.0020cph/cm2) is impossible to guarantee. Note that the – RF interference forthcoming JEDEC document discusses the statistical analysis – Voltage spikes in the supply voltage of the data, and best analytical practices in great depth. • Naturally-occurring radon gas It is also important to reliably quantify the background level • Cross-contamination from other sources, such as dust or of radiation both before and after every single measurement to previous studies ensure that cross contamination, background radiation shifts, Specimen Preparation and analyzer sensitivity drift are accounted for accurately. Accurate measurement of alpha emissions from particles relies on a large surface area of the material under study. Typically, Manufacture of Materials a large metal pan of around 1,000cm2 forms the bottom of Each time an ingredient destined for use in a ULA material a hermetically-sealed unit, with a cleanable plastic tray that comes into contact with any other material, there is the fits inside the pan to hold the material that is under test. The potential for contamination. Alpha-emitting materials range

From One Engineer To Another® 3 Indium Corporation Tech Paper

suppliers where the guarantee is that the materials are in “secular equilibrium.” That is, the alpha emission is from an isotope at the bottom of the decay series that is not increasing in concentration as a result of a faster-decaying parent isotope. 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/ Figure 5. Alpha emission reading versus time. research/mdl/se/SE-Testing-Results-1.pdf. from ambient radon gas (Rn), which is a natural breakdown 3 http://en.wikipedia.org/wiki/Bragg_peak. product of specific uranium and thorium isotopes, to dust with 4 Joint Electronic Device Development Council http://www. jedec.org. adsorbed radon, and other materials that can contaminate. Each time a ULA-material (especially metal in a molten 5 JESD89A - Measurement and Reporting of Alpha Particle and Terrestrial Cosmic Ray Induced Soft Errors in Semiconductor state) is exposed to other materials, the chance of diffusion, Devices. and hence contamination, increases. Experience at Indium 6 JESD89-2A - Test Method for Alpha Source Accelerated Soft Corporation has shown that all potential sources of alpha- Error Rates. emitting isotopes need to be accounted for and eliminated, 7 Solberg “3D Substrate Innovation for Very Fine Pitch Flip- either by prudent choice or materials construction of Chip Applications” IWLPC, Santa Clara 2010. manufacturing equipment, or (in the case of ambient argon) 8 JESD221 - Alpha Radiation Measurement in Electronic by purging. Materials.

Because it is a non-persistent gas, radon is not considered a First published in Chip Scale Review, legitimate source of concern for ULA materials usage, and Nov./Dec. 2010. test labs in both Asia and in the US do not control for radon in the environment during sample preparation, although it naturally purges from the system as the P10 gas flows over the Mylar film covering the test specimen (Figure 5).

Key gating alpha-emissions test points therefore need to be set up for consistent supply of low-alpha material. For a manufacturer of solder pastes, there are four separate critical points: 1. Incoming raw materials (flux ingredients and solders) 2. Solder powder 3. Flux 4. Final solder paste

It is considered that if each of the separate ingredients (1 to 3) is below the desired limit, then that is a good indicator, but not a definitive one, that the material is suitable for use. Another critical factor is the purchase of materials from

From One Engineer To Another® 4