Ultra thin silicon nitride films by plasma ALD using Plasma FlexAL® and OpAL®

Fig 1: Refractive index measured using

in situ spectroscopic ellipsometry as high as 1.95

Qi Fang discusses his successful development of an industry leading silicon nitride ALD process, and describes how to master this notoriously difficult process.

Silicon nitride is a well known material that played an important role in the microelectronics industry for many years; it is CMOS compatible, a good passivation layer, good diffusion barrier and exhibits low leakage. A wide variety of techniques such as PECVD, LPCVD and ICP-CVD have been employed to deposit silicon nitride, but with the current relentless drive towards nano-scale technology a new demand has arisen for ultrathin and highly conformal layers of Si3N4. Atomic Layer Deposition is the ideal technique to meet these demands.

Silicon nitride has long been a challenge by ALD. By their nature ALD precursors tend to be highly moisture sensitive and the preferential reaction is always with oxygen containing species such as background moisture in the chamber. Oxford Instruments’ applications engineers have invested a great deal of time and taken certain steps with the hardware to get from the starting point of SiONx with only 5% nitrogen to SiNx and only 3% oxygen; better than any published literature at this deposition temperature using a metal organic precursor.

The developed SiNx process uses an amino-silane precursor and nitrogen/hydrogen plasma as the two halves of the surface reaction. Comparatively few literature reports exist on SiNx by ALD, but those that do mostly use plasma ALD – from the literature and the author’s own experience it is doubtful that satisfactory film quality can be
achieved by pure thermal ALD methods and especially using nonchlorine based precursors at less than 400ºC deposition temperature.

The refractive index measurement is very sensitive to oxygen contamination; a lower refractive index means more oxygen contamination in the film. Figure 1 shows the refractive index measured using in situ spectroscopic ellipsometry as high as 1.95.

The wet etch rate of the SiNx films is comparable to those of PECVD deposited films at similar temperatures, see figure 2.

Figure 2: Wet etch rates of SiNx are comparable to PECVD films deposited at a similar temperature. A thermal SiO2 wafer was dipped for comparison in the 10:1 BHF solution.

Oxford Instruments is continuing to develop the SiNx process to lower the carbon and oxygen levels even further using alternative precursors which cannot be disclosed for commercial reasons.

For more information on Oxford Instruments ALD tools click here