SEMI Publishes New PV Standards
SEMI Publishes New PV Standard
The demand for quality silicon has been rising as the demand for solar power and photovoltaic cells to produce power has soared in recent years. A major problem with sourcing this material has been the lack of a standardized test method for detecting elemental impurities in photovoltaic silicon feedstock, as the purity level can have an impact on solar cell efficiency as well as the productivity of some processes used to transform the PV Si feedstock into solar cells. To answer that need, SEMI has released PV1-0309: “Test Method for Measuring Trace Elements in Photovoltaic-Grade Silicon by High-Mass Resolution Glow Discharge Mass Spectrometry” in the March 2009 publication cycle.
This Standard was developed by the International PV Analytical Test Methods Task Force (TF), led by Richard Hockett of Evans Analytical Group. The International PV Analytical Test Methods TF was formed in fall of 2007, and over the course of one full year and three Task Force meetings was able to develop industry consensus and finish work on this first Standard. Hockett described the need for the Standard, stating “Although the total annual volume of silicon feedstock that goes into silicon solar cells has now surpassed the total annual volume of polysilicon used in the silicon semiconductor industry, SEMI PV1 is the first standard test method that specifically addresses the evaluation of some types of PV Silicon feedstock. By standardizing this test method, the PV industry now has an international “ruler” that can be used for commerce.”
Manufacturers of crystalline and micro-crystalline silicon used in solar cells purchase or produce their own silicon feedstock material. This silicon is often called Photovoltaic Silicon (PV Si) Feedstock or Solar Grade (SoG) Silicon and can take many physical forms, including, for example, granules, powders, polysilicon chunks, wafers, reclaimed silicon, and top and tail cuts from silicon boules. The purity level of PV Si Feedstock can affect solar cell efficiency and the productivity of some processes used to transform the PV Si Feedstock into solar cells.
SEMI has many test methods that can be used to determine the purity level of PV Si Feedstock; these depend upon its physical form and the elements of interest: SEMI MF397-02 for measuring net resistivity, SEMI MF1389-00 for measuring dopants by photoluminescence, SEMI MF1724-01 for measuring surface metal contamination on granules, chunks or powders by acid extraction followed by atomic absorption spectroscopy, SEMI MF1188-1105 for measuring interstitial oxygen by Fourier Transform Infrared Spectroscopy, SEMI MF1391-0704 for measuring substitutional carbon by Fourier Transform Infrared Spectroscopy, and SEMI MF1630-0704 for measuring dopants by low temperature Fourier Transform Infrared Spectroscopy. SEMI MF28-0707 can be used to indirectly measure the purity level using photoconductive decay.
However, the existing SEMI test methods do not provide for the direct measurement of a broad range of elemental impurities, and to date there had been no formal consensus on the meaning of the term “Photovoltaic Grade.” SEMI PV1-0309 uses the term “Photovoltaic-Grade” to describe the silicon feedstock to be analyzed by Glow Discharge Mass Spectrometry (GDMS) for impurities. GDMS directly measures the element concentrations below 6N levels (or at ppbwt level) for all elements (except the atmospherics C, O, N, H and noble gases) including all the dopants and metals, regardless of the state of the element (e.g., substitutional, interstitial, or located in a defect such as a grain boundary or the surface). The GDMS measurement can be made on PV Si Feedstock regardless of its physical form, whether granules, powders, polysilicon chunks, wafers, reclaimed silicon, and top and tail cuts from silicon boules. The ability of GDMS to detect impurities in the sub-6N level may be especially useful with the advent of silicon solar cell processing that getters impurities from the bulk so that PV feedstock with purity in the 6N region is acceptable.
While SEMI has been developing and publishing Standards for PV since 1981, PV1-0309 is the first entry into SEMI’s new Photovoltaics volume. SEMI’s PV-related Standards have traditionally been included in our Materials volume, but given the industry demand for these standards and the level of current standards development activity in the PV area, it was time to create a new volume focused on PV. The new PV Standard, PV1-0309, will contribute to improved yield and ensure compatibility of processes worldwide.” From this point on, all new PV Standards published by SEMI will be included in the PV volume and carry the “PV” designation.
Initially formed in 2007, the charter of the SEMI Photovoltaic Standards Technical Committee is to “Explore, evaluate, discuss, and create consensus-based standard measurement methods, specifications, guidelines, and practices that, through voluntary compliance, will promote mutual understanding and improved communication between users and suppliers of photovoltaic manufacturing equipment, materials and services to enhance the manufacturing efficiency and capability so as to reduce manufacturing cost of the photovoltaic (PV) industry.”
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