Characterization of Amorphous Silicon by Secondary Ion Mass Spectrometry
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Characterization of Amorphous Silicon by Secondary Ion Mass Spectrometry Yupu Li, Shaw Wang, Xue-Feng Lin, and Luncun Wei Charles Evans & Associates, 810 Kifer Road, Sunnyvale, CA 94086, U.S.A. ABSTRACT Based on various implanted standards, we have used SIMS (Secondary Ion Mass Spectrometry) to characterize amorphous Si thin films with high hydrogen content. SIMS and HFS (Hydrogen Forward Scattering) showed good agreement on the measured total H doses for H-implanted Si samples and the aSi thin films. For the H-implanted Si samples, in the dose range of 2e15 atoms/cm2 to 2e17 atoms/cm2 (corresponding to peak H concentration from 0.32 at.% to 32 at.%), SIMS results showed that the calibration curve is a straight line. In other words, no correction for SIMS quantification is needed when moving from low to high hydrogen content samples. Analysis of P (Phosphorus) in a-Si thin films requires the use of high mass resolution magnetic sector SIMS to separate P and a mass interference from (30Si+H). Using a magnetic sector SIMS instrument, P-doped a-Si thin layers (~ 50nm thick) were analyzed using 3keV O2 beam with oxygen leak for better depth resolution and improved detection limits. For the analysis of C, N, O in a-Si thin films (again approximately 50nm thick) the profiling energy typically needs to be lowered down to 3keV and the material needs to be sputtered at a high rate in order to reach real background levels. In this work, a-Si thin films were also analyzed using a 3keV Cs+ primary ion beam with (Cs2M)+ (M= C, N, O) detection for good depth resolution and detection limits. INTRODUCTION Impurities and dopant profiles in amorphous silicon are important aspects for a-Si(H) solar cell materials [1] and a-Si TFT (Thin Film Transistors) for flat panel display applications [2]. To material engineers, a new challenge is to make more controllable thinner a-Si films for subsequent manufacture into devices. One way to reduce the cost of solar electricity is to reduce the thickness of the films that form a solar cell [1]. It is well known that hydrogen content is key to the optimized function of a-Si(H) solar cells and a-Si TFT devices for flat panel displays [1,2]. For example, it has been reported that by switching a-Si TFT to poly-Si TFT, the mobility can increase from 200 cm2/Vsec (for poly Si). The higher mobility can result in smaller thin film transistors, higher image resolution, lower power consumption and better performance. The production of low hydrogen content aSi is essential for laser-induced crystallization of a-Si for poly-Si TFT applications. Because of these new manufacturing demands there are also new characterization challenges for SIMS and other material characterization techniques. In this paper we report some examples of SIMS analyses of a-Si thin films (down to approximately 50nm thick), using both Cs+ and O2+ primary ion beams generated by Cameca magnetic sector SIMS and PHI quadrupole SIMS instruments. The various analytical modes used and typical results obtained are highlighted in the s
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