Brett M. Clark

Sources of variability in alpha emissivity measurements at LA and ULA levels, a multicenter study

Alpha emissivity measurements are important in the semiconductor industry for assessing the suitability of materials for use in production processes. A recently published round-robin study that circulated the same samples to several alpha counting centers showed wide center-to-center variations in measured alpha emissivity. A separate analysis of these results hypothesized that much of the variation might arise from differences in sample-to-entrance window separations. XIA recently introduced an ultra low background counter, the UltraLo-1800 (“UltraLo”), that operates in a fundamentally different manner from the proportional counters used at most of the centers in the original study. In particular, by placing the sample within the counting volume, it eliminates the sample-to-entrance window separation issue noted above, and so offers an opportunity to test this hypothesis. In this work we briefly review how the UltraLo operates and describe a new round-robin study conducted entirely on UltraLo instruments using a set of standard samples that included two samples used in the original study. This study shows that, for LA (“Low Alpha” between 2 and 50 α/khr-cm2) sample measurements, the only remaining site-to-site variations were due to counting statistics. Variations in ULA (“Ultra-Low Alpha”<2 α/khr-cm2) sample measurements were reduced three-fold, compared to the earlier study, with the measurements suggesting that residual activity variations now primarily arise from site-to-site differences in the cosmogenic background.

Multicenter comparison of alpha particle measurements and methods typical of semiconductor processing

Alpha counting measurement methods have been widely used in the semiconductor industry for many years to assess the suitability of materials for semiconductor production and packaging applications. Although a number of published articles describe aspects of this counting, a multicenter, comparative trial has not been carried out to assess the methodological accuracy of current methods. This paper reports on experience with a 9 center, international, round-robin style trial using a shared set of samples to quantify variability in alpha emission measurements. Four samples representing low and ultralow alpha materials were counted by each participating lab in a blinded trial. The consensus mean emissivity for low alpha material was estimated as 30.9 khr-1-cm-2 with a range from 20.2 to 45.5, less than half of which can be attributed to counting uncertainty or other known sources of error. A strong correlation for replicate measurements within a lab was also observed supporting the conclusion that there are systematic variations in equipment or calibration among labs. Eleven of 23 measurements of ultralow alpha materials were within 1 standard deviation of the consensus mean and 7 were at or below background. The high level of counting uncertainty for these measurements is thought to be sufficient to mask any systematic variation similar to the low alpha observations. Comparison of the reported values with a standard calculation demonstrates that there are also differences in the interpretation of the values reported for emissivity and error, underscoring the need for careful interpretation of results.

Energy Dependent Efficiency in Low Background Alpha Measurements and Impacts on Accurate Alpha Characterization

A difference in low emissivity alpha particle measurements for the same samples was observed between an Alpha Sciences model 1950 gas proportional counter and the XIA UltraLo-1800 ionization chamber. The energy dependent efficiency was evaluated for each instrument to reconcile the difference, and the implication of instrument design bias on measurement results is presented.