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DEPOSITION OF FLUORINATED SILICON NITRIDE USING CHEMICAL VAPOR DEPOSITION TECHNIQUE IBRAHIM KHAN, H.A. NASEEM, Department of Electrical Fayetteville, AR 72701

PLASMA ENHANCED

S.S. ANG, AND W.D. BROWN Engineering, University

of

Arkansas,

ABSTRACT High hydrogen content of ammonia/silane deposited silicon nitride films has been linked to instability in electronic devices such as MOSFET's MNOSFET's and TFT's. Fluorine has been proposed to help reduce the hydrogen content of these films. In this work, NF 3 was used as a fluorinating agent. Optical and electrical properties were studied as a function of NF 3 flow rate. A maximum optical gap of 5.3 eV was obtained with NF 3 as compared to 3.2 eV without. The dielectric strength improved from 4 to 10 MV/cm with the addition of NF 3. FTIR data show that the Si-N peak shifts to higher wave numbers and the Si-H peak drops below the detection limit with the addition of NF 3. Also, the broad Si-N peak at 850 cm-' is composed of many individual peaks which become discernable when a stainless steel screen is placed in front of the substrate. As the NF3 concentration is increased, a new peak at 1030-1060 cm-1 appears. This may be associated with a N-F vibration (1030 cm-n). INTRODUCTION Silicon nitride is widely used as the final encapsulating layer on processed integrated circuits (IC's) because it is impervious to most chemical contaminants. Conventional PECVD nitride is deposited using SiH4 and NH 3 as the source gases. These films contain 30-40 atomic percent (% at.) hydrogen. Recent studies [1,2] have clearly implicated H from the PECVD nitride encapsulant as the source of degradation of MOSFET's in IC's. A reduction in hydrogen content of the nitride encapsulant is important to the reliability of modern IC's. The hydrogen content of the nitride films can be reduced to less than 15% at. if a SiH4 /N 2 gas mixture is used. However, the deposition rate is very low due to the higher dissociation energy of N2 as compared to NH3. Another approach to reducing hydrogen content is to introduce F into the plasma. Fluorine is an efficient scavenger of hydrogen. Recently, there have been several reports on fluorinated silicon nitride films which have low hydrogen, low trap densities, high resistivity, high thermal stability, and better immunity to hot-electron induced degradation. [3,4,5] In this paper we summarize results of our study on nitride deposition using pure silane and nitrogen. Introducing a grounded screen above the substrate is shown to improve the film quality. The effect of increasing NF3 flow on various film properties is also presented. EXPERIMENTAL PROCEDURES 10

Silicon nitride films were deposited on cleaned (111) n-Si (1fn-cm) substrates for electrical measurements, on high Mat. Res. Soc. Symp. Proc. Vol. 219. 01991 Materials Research Society

758

resistivity double side polished (100) p-Si for FTIR spectroscopy and ellipsometry, and on quartz plates (T20 SUPRASIL-2 from Heraeus Amercil Inc.) for UV-VIS spectroscopy. Substrate temperature was kept at 300 0 C