The Material Properties of Silicon Nitride Formed by Low Energy Ion Implantation

Abstract

This paper reports on silicon nitride formation by low energy implantation of nitrogen or ammonia into silicon. Extensive material investigation on nitrogen-implanted films using various analytical methods, including ellipsometry, chemical etching, transmission electron microscopy (TEM), x-ray photoelectron microscopy (XPS), infrared transmission spectroscopy (IR), and Rutherford backscattering spectroscopy (RBS) is discussed. A new technique to derive the film thickness, density, refractive index, and dielectric constant is developed. This method employs a combination of ellipsometry, capacitance measurement, and nitrogen areal density obtained by RBS. A major finding indicates that the implanted nitride films have a low density compared to CVD nitride. Other material properties are also summarized.Thermal nitridation and nitridation of oxide have been shown to have certain advantages over thermal oxidation (1-3). The thin dielectric obtained is very uniform and exhibits high breakdown strength. In addition, the nitrided layer has a higher dielectric constant than oxide and is impervious to impurity diffusion. The film is also less susceptible to process-induced gate failures (3). With the scaling of the gate dielectric in VLSI, thin nitride is a potential candidate to be investigated.We have shown that thin nitride can be formed by using low energy implantation of N2 in an ion milling machine (4). Since the ion milling machine can deliver a high current density over a large area, the through-put of this process is comparable with single-wafer processing. The implantation of N2 is accompanied by surface sputtering. The sputter-implantation process produces a very uniform layer. Rutherford backseattering spectroscopy (RBS) indicates that the total nitrogen incorporated into the Si surface is about 3.2 -3.5 • 1018 cm -2 at 1.7 keV. The as-implanted layer has a N/Si ratio of 1. A damaged layer about 3 -4 nm thick at the interface extending into the Si substrate is also present. After thermal treatment, the ratio approaches the stoichiometric nitride value of 1.3. The damage to the Si substrate is reduced but not totally removed after annealing at 900~ for 1/2h. A damaged layer about 0.8 nm thick remains. Transmission infrared spectroscopy shows the existence of N-Si bonding in both the as-implanted and the annealed samples. However, the transmission minimum shifts from about 850 to 800 cm -I after heat-treatment. ExperimentsThe substrates used in the studies were (100)-oriented silicon wafers. The native oxide is removed before loading into an ion milling machine (Veeco Microetch System). The system is first pumped down to below 2 • 10 -6 torr and then the implantation is carried out with nitrogen or ammonia at a pressure of 8 • 10 -5 torr. The total dose of the implant is always greater than 0.3 C/cm ~. The samples are then annealed in dry nitrogen or oxidized in wet oxygen. Results Ellipsometry measurements.--Ellipsometry is oftenused to obtain the thickness and the refractive index of a dielectric layer independ...

Oxidation Studies of Crystalline CVD Silicon Nitride

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Oxynitride films formed by low energy NO+ implantation into silicon

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The Effect of Nitrogen Concentration at Oxynitride Gate Insulators Formed by 28N2 + Implantation into Silicon with Additional Conventional or Rapid Thermal Oxidation

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