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EXPERIMENTAL APPROACH AND CHARACTERIZATION TECHNIQUES The LTCVD SiNx process was developed in an 8" wafer, stainless-steel, warm wall module of a four-chamber cluster tool. The reactants used were Si14 and ammonia (NH 3). An MKS 1153 solid source delivery system was employed to precisely and reproducibly control the delivery of the Si precursor. In order to systematically approach the development and optimization of low temperature CVD SiN process, a DOE study was implemented. In the DOE study, the Minitab Statistical software was used to plan and execute the deposition experiments and develop a process window. The DOE approach used a response surface model (RSM) to map the entire process parameter space, thus yielding complete and highly useful results [10]. The DOE approach explored the effects on the three key process parameters, namely, ammonia gas flow rate, substrate temperature, and process pressure on the film properties. The SiI 4 precursor flow rate was kept constant at 12 sccm during processing. Table I summarizes the range of deposition parameters investigated and optimized experimental process parameters, that in terms of highest deposition rate at the optimum substrate temperature of 370 TC. Table 1. DOE process window investigated for low temperature CVD SiN films. Process Parameters

Substrate temperature (°C) Pressure (Torr)

"* "* "• "*

"*

Range Investigated

Best parameters (In terms of highest growth rate of - 5.4 nm/min)

300 - 440

370

1- 3

3

NH 3 flow rate (sccm)

300 - 900

600

Si14 flow rate (sccm)

12

12

Si source temperature (°C)

160

160

The resulting DOE-produced films were analyzed as follows: Film thickness and refractive index (n) were measured by ellipsometry using a Tencor UV 1080 spectroreflectometer at X = 600 nm. Compositional analysis and density measurements were performed by Rutherford Backscattering (RBS) at a primary He+ energy of 2.0 MeV. The hydrogen profile in the films was also investigated by Nuclear Reaction Analysis (NRA) 2 using the nuclear reaction 'H(15N, CXq)1 C. The film breakdown electric field (Ebd) and dielectric constant were measured through, respectively, leakage current-voltage (I-V) and high frequency (1 MHz) capacitance voltage (C-V) in a series of Metal-Nitride-Semiconductor (MNS) diodes (Al/SiNg/n-Si) for SiNx thicknesses ranging from 1080 - 2000 A. Aluminum dots of 760 ptm diameter and of 5000 A thickness were evaporated by e-beam through a mask onto the SiN films. As ohmic contact of the Si substrates, an aluminum layer of 5000 A thickness was also evaporated on the substrate side of the SiNg samples. Stress of SiNx films was measured on 4" Si wafers at room temperature by determining the bending of the wafer before and after film deposition. The equipment used was the Tencor FLX 2320 system.

110

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Chemical bonding configurations in the films were investigated by FTIR in the range of 400 - 4000 cm-.

RESULTS AND DISCUSSION Figure 1 shows a typical RBS spectrum of 2000 A - thick SiNx film deposited on Si substrate at temperature of 370 `