Elemental Analysis, Inc.

Ultra-Trace Bulk Analysis of Polysilicon by Instrumental Neutron Activation Analysis

The rapid growth in solar cell production has fueled demand for high-purity silicon. About ninety percent of the current solar cell market is based on solar cells using silicon and the majority of the raw material for the process is derived from polycrystalline silicon. The demand for polysilicon from the solar industry is growing at up to 40% annually and it is anticipated that the use of polysilicon for solar cells will be three to four times that of the semiconductor industry in about ten years. The projected global demands for polysilicon are such that the two largest polysilicon producers have announced plans to increase their production capacities to over 58,000 metric tons in the next three years and several other companies, with no prior polysilicon experience, are constructing facilities to enter the market.1

Although the purity requirements of silicon for solar cells (five to seven 9s) are lower than that for semiconductors (nine 9s), the power conversion efficiency of solar cells is largely dependent on impurity levels in the silicon raw materials. Measurement of element concentrations in the polysilicon raw material and the process wafers is, therefore, essential as new polysilicon production technologies are developed to lower solar cell production costs and for the maintenance of quality control during the manufacturing of solar cells. The quality control of the polysilicon is especially important as new producers of this high purity material enter the market. Since it was first applied in 1960 for the analysis of tantalum, instrumental neutron activation analysis (INAA) continues to be one of the most sensitive and accurate techniques for meeting industries need for the trace element analysis of high-purity silicon. In keeping with the industry expansion, the demand for INAA of high-purity silicon has more than doubled over the last three years. This paper presents a brief overview of INAA of high-purity silicon.

The idea of using neutrons as an analytical probe for elemental analysis was first proposed and demonstrated by Von Hevesy and Levi for the analysis of trace quantities of rare earths in geological materials in 1936. Since then, the excellent sensitivity, selectivity and precision of INAA have made it one of the most versatile and widely employed elemental analysis techniques. Because most materials are “transparent” to both the probe (neutrons) and the signal (gamma rays), there are few matrix effects associated with the analysis and standardization of the measurement is simple and straightforward. Moreover, because little, if any, sample manipulation is required, INAA is a highly sensitive technique that can be applied to bulk samples and is relatively free of reagent and laboratory contamination.

In INAA, stable nuclei in the sample undergo neutron induced nuclear reactions when the sample is exposed to a flux of neutrons. The most common neutron reaction is neutron capture by a stable nucleus (AZ) that produces a radioactive nucleus (A+1Z). The “neutron rich” radioactive nucleus then decays, with a unique half-life, by the emission of a beta particle. In the vast majority of cases, gamma-rays are also emitted in the beta decay process and a high-resolution gamma-ray spectrometer is used to detect these “delayed” gamma rays from the artificially induced radioactivity in the sample for both qualitative and quantitative analysis. A schematic illustration of the neutron capture INAA process is given in Figure 1. The energies of the delayed gamma rays are used to determine which elements are present in the sample, and the number of gamma rays of a specific energy is used to determine the amount of an element in the sample. For example, when a sample that contains iron is irradiated, a fraction of the 58Fe atoms in the sample will capture a thermal (or low energy) neutron and become 59Fe. The 59Fe atoms are radioactive and have a half-life of 44.5 days. When the 59Fe atoms beta decay to 59Co, a 1099 keV gamma ray is emitted 56 % of the time. The amount of iron in the original sample can be determined by measuring the number of 1099 keV gamma rays emitted from the sample in a given time interval after the sample has been exposed to a flux of neutrons. A description of the procedures used to quantify an analyte in INAA is beyond the scope of this article. The physical principles of the analysis are so well understood that neutron activation analysis is one of the primary techniques used by the National Institute of Standards and Technology to certify the concentration of elements in standard reference materials.

Advantages of INAA Return to Top

The major advantage of INAA is that it provides accurate results for large, bulk samples (tens of grams) without having to dissolve or digest the sample. Moreover, by employing an appropriate surface etch procedure, it is possible to ensure that the trace elements observed in the INAA measurement are coming from the bulk material and are not a result of surface contamination at the production facility or in the analytical lab; critical information when evaluating bulk material from a new production technology or new production facility. Total reflection x-ray fluorescence analysis, secondary ion mass spectrometry, and vapor phase decomposition inductively coupled plasma mass spectrometry are the techniques routinely used in house by the semiconductor industry to ensure the purity of the semiconductor and solar grade silicon. INAA is an excellent complement to these surface analysis techniques in that it can provide similar sensitivities on large, bulk silicon samples. An example of the use of INAA to examine impurities in silicon produced by the silane and metallurgical routes can be found in the recent paper by Holt et al.

Disadvantages of INAA Return to Top

As with all analytical techniques, there are drawbacks to using INAA. One major disadvantage is that the technique requires access to a high-flux neutron source to obtain the sensitivities listed in Table 1. As a result, the technique cannot be performed “in house” by industry. A second disadvantage of INAA is the time required for the analysis. Given the continuous production schedules on which the semiconductor industry operates, an analytical protocol that takes four to five weeks can be inconvenient. (If information is needed on only a few elements with shorter half-lives, it is possible to reduce the analysis time.) The third major disadvantage of INAA is that it cannot provide information on some of the light elements, particularly B, C, O and Al, that are monitored to ensure optimum performance of semiconductor devices.

Conclusion Return to Top

INAA has had a long and successful history of application in the semiconductor silicon industry for analysis of bulk samples and it is an excellent tool to assist industry in quality control as demand for polysilicon continues to grow.