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January 15, 2013

Next Steps in Low Alpha Material Technology

Next Steps in Low Alpha Material Technology
Brett Clark, Honeywell Electronics Materials
Alpha emissions from IC packaging materials have become a greater concern as device geometries continue to decrease and design complexity increases. The trend towards flip chip and 3D-IC architecture, in particular, has increased the need for reliable low alpha packaging materials. In these designs, packaging features such as wafer-level solder bumps and copper pillar solder caps are located close to the transistors of the device. This increases the transistors' vulnerability to alpha emissions from these features and can lead to higher soft error rates.

Alpha emissions from packaging materials are commonly associated with Pb-based solder. However, the transition to Pb-free solder materials has not eliminated the problem as these materials still contain trace amounts of alpha-emitting isotopes such as polonium-210, or 210Po (a daughter of the lead isotope, 210Pb). Furthermore, the alpha emissions from some Pb-free materials have been shown to increase significantly over time, long after they have been processed. The magnitude of the increase is too large to be attributed to disruptions in the secular equilibrium of the alpha-emitting isotopes. For example, in one experiment, Honeywell observed alpha emissions from samples of tin increase by a factor of 3 over several months. The samples all had relatively low levels of alpha emissions initially.

Technologists at Honeywell have proposed two complementary mechanisms to explain the time-dependency of alpha emissions in these samples: microsegregation and diffusion. A series of experiments were conducted to validate these mechanisms in tin.

Mechanism 1: Microsegregation
Microsegregation is a phenomenon that occurs in certain solid/liquid systems whereby trace elements preferentially segregate in the liquid phase versus the solid phase. Thus, as the material solidifies, from the surface inward, trace elements (including alpha emitters) become concentrated in the liquid phase. To verify that microsegregation occurs during the processing of tin, the distribution of alpha emitters in three 1-mm thick samples of processed tin sheet material was determined. For each sheet, sequential layers of material were etched away, and alpha flux measurements were performed at each new surface. The results (Figure 1) showed that the concentration of alpha emitters in the samples increased linearly with depth, providing evidence of microsegregation.
Next Steps in Low Alpha Material Technology
Figure 1. Alpha emitter distribution in tin sheets

Microsegregation explains why significant levels of alpha emissions were not initially observed in the tin samples. Alpha particles can only penetrate 20 micrometers of tin material, so if most of the alpha emitters were concentrated within the interior of the tin sheets, their emissions would not have been detected.

Mechanism 2: Diffusion
For alpha emissions from the tin samples to have increased over time, the alpha emitters themselves must have diffused to the surface of the tin sheets where they could be detected. Prior literature has reported the diffusion characteristics of 210Po--the isotope primarily responsible for alpha emissions--in lead. An experiment was conducted to determine whether 210Po would behave in a similar manner in tin. In the experiment, a mass of low alpha grade Sn doped with 210Po was divided into two halves. One half (the test sample) was subjected over a period of 4 months to a series of cycles of heating to 200C followed by measurement of alpha flux. The other half (the control sample) was kept at room temperature and its alpha flux was measured at a similar frequency over the same period. The measurements were conducted with a XIA UltraLo 1800 gas ionization chamber -- an instrument that utilizes an active noise rejection algorithm combined with pulse height analysis to enable low background alpha energy spectroscopy.
Next Steps in Low Alpha Material Technology
Figure 2. 210Po-doped tin alpha emission versus time

The results for both test and control samples are displayed in Figure 2. The data indicate that heating the sample accelerated the diffusion of 210Po by a factor of approximately 240 relative to the diffusion rate at room temperature. During each heating cycle, in only a few hours the alpha emissions from the test sample rose to the level of alpha emissions reached by the
control sample after 4 months.
Next Steps in Low Alpha Material Technology
Figure 3. Alpha energy spectrum of tin before and after heating

The alpha energy spectrum of the sample before and after heating (Figure 3) confirmed that the increase in alpha emissions observed in both the test sample as well as the control was due to the diffusion of polonium to the surface of the samples. In these samples, the concentration of 210Po was so small (less than 1 part per trillion) that it could only be determined by measuring the amount of alpha radiation emitted.

The Implications for Packaging Materials
The experiments described above identified two separate mechanisms that explain the increase in alpha emissions over time from processed Pb-free solder materials. Microsegregation, which occurs during the processing of Pb-free solder material, confines most of the alpha emitters to the interior of the material where their emissions cannot be detected. The contaminants subsequently diffuse to the surface of the material, resulting in an observed increase in alpha emissions from the material. Since increases in alpha emissions from packaging materials can lead to higher soft error rates in devices that feature flip chip or 3D-IC architecture, it is important that such materials are screened and latent alpha emitting contaminants are removed.

At Honeywell, technologists have designed a proprietary refining process (patent pending) that selectively removes alpha emitters from tin-solder material. The process is capable of yielding material that does not exceed alpha emission levels of 0.001 counts hr-1/cm-2. At current technology
nodes, materials whose alpha emissions do not exceed 0.002 counts hr-1/cm-2 are considered sufficient. However, as bond pitch widths in ever more compact 3D-IC architecture and packaging continue to shrink, a higher grade of material will be required.

Although all statements and information contained herein are believed to be accurate and reliable, they are presented without guarantee or warranty of any kind, express or implied. Information provided herein does not relieve the user from the responsibility of carrying out its own tests and experiments, and the user assumes all risks and liability for use of the information and results obtained. Statements or suggestions concerning the use of materials and processes are made without representation or warranty that any such use is free of patent infringement and are not recommendations to infringe any patent. The user should not assume that all toxicity data and safety measures are indicated herein or that other measures may not be required.

Brett Clark
Honeywell Electronics Materials