Characterization of Defects Created in Silicon Due to Etching in Low-Pressure Plasmas Containing Fluorine and Oxygen
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EXPERIMENTAL In this study we used phosphorous-doped Czochralski (CZ) - grown silicon wafers with (100) orientation having a room temperature resistivity of - 30 0 cm. The silicon wafers were positioned on the water-cooled (- 200 C) quartz electrode of a diode reactor to which 13.56 MHz rf power of 200 W was applied. Mixtures of oxygen either with SF6 (SF6/02) or with CF4 (CF4/02) where used as etching gases, the fraction of oxygen in the plasma was varied from 0% to 60%. RIE was performed at a plasma pressure of 25 mTorr for 1 min. Prior to plasma exposure the samples were etched for 30 s in dilute HF. The PL measurements were performed at a temperature of 2 K using the 514 nm line of an Ar+-ion laser as an excitation source. PL was dispersed with a SPEX 1-m double-grating monochromator and detected by a liquid-nitrogen-cooled North Coast Ge detector. The PL signal was recorded with a conventional lock-in technique in phase with the frequency of a mechanical chopper. The spectra were not corrected for the response of the detection system. Gold Schottky diodes were prepared for the DLTS measurements. The DLTS data were retrieved using a standard experimental set-up with temperature variation in the range 80 - 340 K. PL CHARACTERIZATION OF RIE-INDUCED DEFECTS Comparison of CF4 and SF6 RIE Fig. I shows typical PL spectra of n-type Si wafers subjected to CF4 and SF6 RIE. All spectra contain phosphorous (P) bound exciton (BE) lines related to the no-phonon (NP) and transverse optical (TO) phonon-assisted transitions. The relatively broad band detected at the low energy side of pTO line corresponds to electron-hole droplet (EHD) recombination. After the RIE etching two excitonic lines at 0.7893 and 0.9695 eV, known as C- and G-lines [15], appear in the spectrum. In addition, the broad PL band in the 0.70 - 1.00 eV spectral region is detected after the SF6 RIE. The appearance of C- and G- lines after both SF6 and CF4 RIE indicates radiation damage of the near-surface region. Both PL lines are usually observed after the irradiation by high energetic particles [15]. The formation mechanism of C- and G- centers is well established and includes the radiation-induced displacement of a carbon atom to form either a close pair of interstitial oxygen and carbon atoms (C-centers) or carbon substitutional - Si interstitial - carbon substitutional complexes (G-centers). The formation of these defects after CF4 RIE is explained
U,,
W
CF4 RI
0.70
0.80
0.90
1.00
1.10
1.20
PHOTON ENERGY (eV) Fig. 1. PL spectra at 2K after RIE etching using SF6 and CF4 as a plasma. The dip around 0.90 eV is due to water vapour absorption. 600
either as a result of bombardment-induced displacement of residual carbon atoms [11] or as a result of carbon implantation from the etching plasma [10]. For SF6 RIE, which does not contain carbon atoms in the etching gas, ion bombardment should be considered as a dominant mechanism of C- and G- centre formation in agreement with Ref. 14. The post-RIE damage is commonly assumed to decrease with increasing
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