Plasma Deposition of Silicon Nitride

This paper reports on the preparation and characterization of thin films of silicon nitride deposited on heated silicon substrates from Si(CH, ),-NHS-H, mixtures activated at room temperature by an a-c. discharge at low frequency. The films were deposited at 800°C. Deposition rate as well as refractive index were measured and several parameters were systematically varied , including deposition time (1-3 h), total gas flow rate, chamber pressure (20-45 kPa), peak driving voltage (8-12 kV) and frequency (400-600Hz). The composition of the films was studied using transmission electron microscopy (106V), Auger spectroscopy and infrared spectroscopy. In plasma CVD reactors, the substrates are generally immersed in the discharge and resistively heated l Using such a device, the discharge is produced in the gaseous reactant mixture which is maintained at low pressure (5-200Pa) under an r.f. plasma. Thus "silicon nitride'' films (actually Six N,H, ) are deposited at temperatures between 20 and 500°C from SiH,-NH3-Ar miktures 2 at temperatures between 300 and 600°C from SiH4-NH3-N, mixtures /3/ and recently at temperatures between 25 and 200°C from the reactant mixture Si(CH,),-NH, /4/. Moreover, similar silicon nitride films have been deposited at room temperature using low frequency (50 Hz) plasma CVD with SiH4-N, mixtures at about 160 Pa total pressure /5/. Finally, as a matter of comparison, Si3N, can be deposited on graphite subtrates by low pressure (520-1320 Pa) pyrolysis of Si(CH,), -NHS-H, mixtures in a hot wall reactor (1000-1200°C) /6/. Silicon nitride deposits made below a temperature of 500°C contain non-negligible quantities of hydrogen. On the other hand, fabrication of deposits above 1000°C can lead to undesirable reactions with the substrate. Here, we describe a technique which enables the fabrication of hydrogen-free layers of silicon nitride at a temperature between 500 and 1000 "C. 2-EXPERIMENTAL The film deposition equipment is shown schematically in Figure 1. The vertical reactor is a silica tube 620 mm high and 40 mm in diameter. The lower part of the reactor is the electronic activation zone where an a.c. discharge is produced in the gaseous reactant mixture. The high voltage discharge electrode is constituted by a Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1991250 JOURNAL DE PHYSIQUE IV THERMAL ACTIVATION ZONE ELECTRONIC ACTIVATION Z O N E Fig. 1.Film deposition apparatus. (1) gas inlet; (2) water jacket; (3) high voltage electrode ; (4) gaseous gap (5) silica tube ; (6; inductive coil ; (7) susceptor ; (8) molybdenum wire. stainless steel cylindrical inner grid, the radius of which is very close ( about 1 mm ) to the radius of the outer tube, acting as grounded electrode. The square mesh aperture of the grid (112 pm) has been chosen to be about twice the Debye length /7/ in such a plasma. The outer tube is cooled directly with water from the laboratory supply. Electrically, the cold plasma zone presents a capacitive load to the power supply due to both the gaseous gap and the dielectric barrier present (Figure 2). D i e l e c t r i c Q u a r t z tube Gaseous gap 4 I-@ VOLTAGE HIGH Fig.2. -Equivalent circuit: 1/C, = l/ C, + l/ C, The power and control circuit has been described previously /8/. The power W dissipated in the gaseous gap can be calculated from the Manley expression /9/ : W= 4f C, V. (Vm -C, V, / C, C, ) where f is the frequency (Hz), C, and C,, the capacitances (F) respectively of the dielectric barrier and of the plasma reactor, V,, the peak driving voltage (V) and V, , the peak across the gaseous gap (V) The upper part of the reactor is the thermal activation zone-The graphite susceptor of the silicon substrate (6x6mm) is sustained by a molybdenum wire within the central axis of the tube and heated by an inductive coil. The temperature of the susceptor is controlled and measured by an optical pyrometer. Before any experiment, the reactor was evacuated to a pressure of about 10-I Pa. A mixture of tetramethyl silane (TMS), ammonia (NH, ) and hydrogen (H,) was used for depositing thin films. Whisle the gaseous mixture was flowing into the reactor, the plasma was initiated until the suceptor temperature is adjusted. Such a general film deposition equipment characterized mainly by successive use of electronic and thermal activations has been patented by our research team /10/ . So only preliminary results will be presented in this paper in order to judge the ability of this novel technique to produce good quality thin films. A Rudolph Research automatic ellipsometer (A=63280 nm) was used to measure film thickness and index of refraction. The composition of the films was studied using AUGER electron spectroscopy (RIBER SIA 200), infrared spectroscopy (PERKIN-ELMER 1760 X) and electron energy loss spectroscopy (EELS) on the 1 MV electron microscope from the CEMES CNRS, located in TOULOUSE. 3-RESULTS AND DISCUSSION The experimental data show the influence of the following parameters on the deposition process: deposition time, total gas flow rate Q, reactor total pressure P, frequency and peak driving voltage. The substrate temperature was kept constant (800°C).