Lifetime Mapping Technique for Ultrathin Silicon-on-Insulator Wafers

Abstract

Highly sensitive lifetime mapping equipment using the UV laser excited differential microwave photoconductivity decay technique was developed to observe the defect and contamination distribution in ultrathin silicon-on-insulator ͑SOI͒ layers. The technique was successively applied to the evaluation of state-of-the-art SOI wafers. Through theoretical analysis, it was quantitatively confirmed that the lifetime is shortened as the SOI film becomes thin, due to an increase in interface recombination. The large improvement in sensitivity attained by the differential detection enabled us to evaluate nonuniformities in the active layer quality by overcoming the difficulty of detection normally experienced by the decrease in the thickness of the SOI layer. Using light illumination with an extremely short penetration depth and the barrier effect of the buried oxide layer, it was found that the generated carriers were confined to the active layer, and lifetime data reflecting the quality of the SOI layer could be obtained. In the lifetime map for an SOI layer with a thickness of 52 nm, numerous islandlike spots with long lifetime, which were believed to be caused by HF defects or thin silicon areas, were detected.

In-line Process Monitoring for Amorphous Oxide Semiconductor TFT Fabrication using Microwave-detected Photoconductivity Decay Technique

“…Figure 1 shows a typical µ-PCD decay curve of the a-IGZO thin film. The decay curve can be divided into three components; the peak value, and the two kinds of decay including the fast and the slow decay constants, which is different from the decay curves obtained on LTPS thin films [10]. According to our previous study [3], we assumed that the density of the carriers participating in the microwave conductivity obeys the following equation after photoexcitation:…”

“…The µ-PCD method is a non-destructive, contactless technique that enables characterization of the capture, recombination, and relaxation of laser-excited carriers by observing the temporal response of the microwave reflectivity [10]. The apparatus of µ-PCD system is shown in Fig.…”

Thin Film Polycrystalline Silicon Solar Cells

“…HPWVA was carried out at various annealing temperatures to improve the minority carrier lifetime. Quality of the poly-Si films was characterized by a differential microwave photo-conductivity decay (µ-PCD) system (Kobelco Research Institute, Inc., LTA-1510EP) [18]. This µ-PCD system can coincidentally detect microwave reflections from two areas with and without excess carriers generated by pulse laser illumination, and decay curves with high signal/noise ratio can be obtained from the difference of the two signals, even in the case of thin films.…”

New equations to calculate carrier recombination lifetime of silicon epitaxial layer, based on open circuit voltage decay method

“…2,3) The microwave-photoconductive decay (μ-PCD) method, in which the carrier recombination process is observed as the conductivity decay of the semiconductor after carrier injection by laser irradiation, is widely employed as an evaluation technique for the recombination lifetime of silicon wafers owing to its reliability and capability of high-resolution two-dimensional map evaluation of the whole wafer. 4,5) However, μ-PCD has difficulty in measuring the recombination lifetime of the silicon epitaxial layer on a highly doped substrate, such as a p− silicon epitaxial layer on a p+ silicon wafer or an n− silicon epitaxial layer on an n+ silicon wafer. Since the sheet conductance of the epitaxial layer after carrier injection is smaller than that of the highly doped p+ or n+ substrate, the total decay of sheet conductance from the epitaxial wafer is too small to measure.…”

“…Since the sheet conductance of the epitaxial layer after carrier injection is smaller than that of the highly doped p+ or n+ substrate, the total decay of sheet conductance from the epitaxial wafer is too small to measure. 6) Although differential μ-PCD has also been proposed, 5) this method cannot accurately measure the recombination lifetime of the epitaxial layer. 7) The open circuit voltage decay (OCVD) method, 8) in which the recombination process in a diode is measured as a voltage decay across the diode after carrier injection by forward current, can measure the recombination lifetime in the i-layer of PiN diodes.…”