Influence of Current Injection into a-Sin:H Films on Charge Trapping Defects
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INFLUENCE OF CURRENT INJECTION INTO a-SiN:H FILMS ON CHARGE TRAPPING DEFECTS Tomoki Oku, Kiyoshi Kawabata, Yukio Higaki, Teruhito Matsui, Hirozo Takano and Mutsuyuki Otsubo, Optoelectronic and Microwave Devices R&D Laboratory, Mitsubishi Electric Corporation, 4-1 Mizuhara, Itami, Hyogo 664, Japan
ABSTRACT The degradation mechanism of a-SiN:H films under current injection is investigated. It is shown that the degradation of a-SiN:H films is closely related to the hole trapping into Si-Si, Si-H, and Si 0 defects. It is presumably concluded that the hole trapping centers are the Si-Si defects in the valence band tail. We estimate that the hole trapping cross-section is nearly equal to 10"2Ocm 2 .
INTRODUCTION Hydrogenated amorphous silicon nitride film deposited by plasma-enhanced chemical vapor deposition (CVD) is mainly used as a dielectric for capacitors in GaAs monolithic microwave integrated circuits. To attain highly reliable capacitors, it is important to investigate the mechanism of the time-dependent dielectric breakdown (TDDB) of a-SiN:H films. Many experiments and theories claim that the generation of neutral and/or charged defects plays an important role of TDDB of silicon nitride [1] as well as silicon dioxide [2]. However, the origin of the charge has not yet been clarified from defects and defect reactions on an atomic scale. To characterize the defects and defect reactions, we evaluate the time dependence of injecting current and generated charge under a constant bias voltage since it is sensitively influenced by the defects. Using a-SiN:H films deposited under various conditions, we relate the time dependence of injecting current to the densities of defects. The defects are characterized by Fourier transform infra-red spectrometer (FTIR), Auger electron spectroscopy (AES), and electron spin resonance (ESR).
EXPERIMENT Metal-insulator-semiconductor (MIS) diodes were fabricated on p-type silicon wafers. The evaporated Au/Ti films were used as the upper electrodes. The a-SiN:H films were deposited by varying the NH 3 to SiH 4 gas flow ratio (R = NH 3 /SiH 4 ) at Mat. Res. Soc. Symp. Proc. Vol. 209. 01991 Materials Research Society
488
300°C with a commercial parallel plate CVD system. The thickness (d) and the dielectric constant (Er) of the films were measured by an ellipsometer and a capacitance meter at 1MHz, respectively. The time dependence of injecting current into the a-SiN:H films was measured under a constant minus bias voltage (V) applied to the upper electrodes. The NH 3/SiH 4 ratio, the film thickness, the dielectric constant, and the applied voltage are listed in Table I. The flat-band voltage shifts (A&VFB) of the MIS diodes at R = 10.0 and 20.0 were measured by a capacitance-voltage (C-V) technique at 1MHz after the bias voltage was applied for few tens seconds. Repeating the measurement, we obtained the time dependence of flat-band voltage shifts. Assuming that the charge in the a-SiN:H films is uniformly trapped, the trapped charge (Qt) is given by Qt = - (2yr0/d)&VFB
(1)
where c
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