AC Photovoltaic Inspection of P-N Junctions having High Leakage Current
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AC PHOTOVOLTAIC INSPECTION OF P-N JUNCTIONS HAVING HIGH LEAKAGE CURRENT N. HONMA, H. SHIMIZU, C. MUNAKATA AND M. OGASAWARA* Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185, Japan * Device Development Center, Hitachi Ltd., Oume, Tokyo 198, Japan ABSTRACT A focused photon beam chopped at 2 kHz scans p-n junctions in a p-type Si wafer and ac photovoltages are capacitively measured in order to inspect homogeneities of the junctions. It is found that the ac photovoltages are high not only in the junction areas but also in the field oxide regions around the junctions when the junctions are leaky. This indicates that dense positively charged traps exist at the interface between the heavily boron implanted Si substrate and the field oxide layer around the high leakage junction, and that the traps cause the increase in both the junction leakage current and the ac photovoltage. INTRODUCTION Junctions are one of the fundamental structures in semiconductor devices, such as MOS memory or solid state image devices. With shrinking geometries and with small charge packet sizes, it is becoming increasingly important to reduce junction leakage in such devices. Junction leakage should be inspected in an early stage of device manufacturing in order to obtain a high yield. A technique for imaging the distribution of ac photovoltages induced with a chopped photon beam (PB) has been developed [1]. The ac photovoltage (PV) is the product of junction impedance and photocurrent (PC). When leakage current in a junction is large, junction impedance may possibly be reduced. Therefore, the induced PV is expected to decrease. However, measured PV is actually large when junction leakage is high. Experimental results indicate the importance of the peripheral region of the high leakage junction. Here we report that the dense interface traps around a high leakage junction is the cause of the unexpectedly large ac PV as well as the high leakage current. EXPERIMENTAL (1)
Sample preparation
Many junctions were formed on a p-type (100) Si wafer with a diameter of 127 mm and resistivity of 0. 1 Q m . Then n-type! surface layers were formed by As ion implantation at a dosage of 2 2 X 1018 m- . A channel isolation was formed by oxi(dation of 500 nm thick Si02 followed by B ion implantation. The ,junietiorn depth was 0.2 A m and the junction size was lmmX 2mm. (2)
Experimental apparatus
A scanning photon microscope (SPM) the ac PV distribution in the Si wafer.
121 was used to measure The SPM hias two eal;tihxle
Mat. Res. Soc. Symp. Proc. Vol. 163. ,g199oMaterials Research Society
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ray tubes (CRTs), one provides a blue PB (wavelength X . = 450 nm) and another provides an infrared PB. The maximum viewing size was 150 mm and the PB diameter on a sample wafer was 0.3 mm. The chopping frequency (f) of the PB was 2 kHz. When high resolution was required, a He-Ne laser with wavelength of 633 nm was used as the PB source. The laser beam was modulated at 2 kHz with an acousto-optic modulator. The modulated beam was scanned in the x-y dire
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