Silicon Nitride Atomic Layer Deposition: A Brief Review of Precursor Chemistry

Antonio T. Lucero and Jiyoung Kim*, Material Matters, 2018, 13.2

Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, TX 75080, USA
*Email: jiyoung.kim@utdallas.edu

 

Introduction

Silicon nitride (SiNx) is a critical material for semiconductor devices, increasingly used in high-performance logic and memory. Modern, scaled devices require robust SiN films deposited at low temperature (<400 °C) for use as gate sidewall spacers and in self-aligned quadruple patterning.1 Traditional SiNx deposition techniques, including chemical vapor deposition (CVD) and plasma-enhanced chemical vapor deposition (PECVD), are now giving way to atomic layer deposition (ALD). ALD allows more control over the thickness of deposition, work at relatively low temperatures, and conforms over high-aspect ratio structures.2 ALD can be divided into two classes, thermal ALD and plasma-enhanced ALD (PEALD). Both methods have some advantages for SiNx deposition. Thermal ALD allows for conformal deposition over high aspect ratio (HAR) structures (>5000:1), while PEALD can be used at much lower temperatures with lower HAR conformality. Advances in precursor chemistry and nitrogen sources have enabled the tailoring of material properties like the wet etch rate and growth rate to meet research and industry requirements. There are currently three main silicon precursor classes: chlorosilanes, organosilanes, and heterosilanes. Chlorosilanes are silicon precursors where Si-Cl bonding is predominant. Organosilanes are silicon precursors containing organic ligands, although currently this class is limited to aminosilanes in practice. The last group, heterosilanes, includes all other precursors.

Chlorosilanes

Chlorosilanes are an historically important class of silicon precursors that helped to build the semiconductor industry by enabling the production of ultra-high purity silicon. This class includes any silicon precursor containing at least one chlorine-silicon bond. The first SiNx was grown by thermal ALD in 1997, when Morishita3 deposited SiNx using hexachlorodisilane (HCDS, Si2Cl6 , Cat. No. 205184) and hydrazine (N2H4, Cat. No. 215155) at temperatures ranging from 525–650 °C. While hydrazine has since been replaced by more convenient nitrogen sources, HCDS has remained an important precursor for SiNx ALD. Later reports4 of ALD using HCDS and ammonia demonstrate successful deposition of SiNx at temperatures as low as 515–557 °C. In addition, tetrachlorosilane,5 dichlorosilane (DCS),6 and octachlorotrisilane7 have all been used successfully. Since the deposition temperature is fairly high, the physical properties such as density and wet etch rate (WER) in hydrofluoric acid are good. Growth per cycle (GPC) varies, but is typically greater than 1 Å/cycle. A disadvantage of chlorosilanes for thermal ALD of SiNx is the large precursor exposure (107–1010 L) required to achieve saturation. Of note, only chlorosilane precursors have been used for thermal ALD of SiNx, since they are the only precursors stable enough for use above 400 °C, the temperature at which ammonia is activated. With the exception of hydrazine, which will be discussed in a later section, no other nitrogen sources are available for thermal ALD. This limitation has prevented the widespread industry adoption of thermal ALD for SiNx growth.

In order to grow films at low (<400 °C) temperatures, research has focused on the use of plasma to aid deposition.8 Microwave plasma, inductively coupled plasma (ICP), and capacitively coupled plasma (CCP) are most commonly used in combination with sources of reactive nitrogen including ammonia, nitrogen, or nitrogen forming gas (N2-H2). DCS9 and HCDS10 have been used extensively with ammonia in PEALD of SiNx at temperatures from 300–400 °C. Temperatures less than 300 °C can lead to excess chlorine contamination due to the formation of NH4Cl. Ovanesyan et al.10 reported conformal deposition of SiNx on HAR structures using HCDS and NH3 plasma at 400 °C, with hydrogen in the form of -NH and chlorine (<1%) as the primary impurities. Conformal deposition when using NH3 plasma is a strong advantage of chlorosilane precursors. Unfortunately, the WER for SiNx deposited with chlorosilane precursors is reported to be high, and film density is low due to hydrogen incorporation. Recently, a new chlorosilane precursor, pentachlorodisilane (PCDS) has been reported11 that while similar to HCDS, results in a 20% higher GPC (0.78 vs. 1.02 Å/cycle) with similar or better physical properties. Substituting a chlorine atom with a hydrogen appears to lower the steric hindrance of the PCDS molecule and increase its polarity, leading to a precursor with higher reactivity. In addition, a precursor exposure of only 4 × 104 L, or 4–5 orders of magnitude lower than the exposure for thermal ALD processes and 1–2 orders lower than that of other PEALD provides results with a similar GPC. The unique hollow cathode plasma source used for films grown with both HCDS and PCDS results in exceptionally low oxygen contamination in these films.

Organosilanes

The first organosilane used for SiNx ALD was tris(dimethylamino) silane (TDMAS, Cat. Nos. 570133, 759562) in 2008.12 Using a remote ICP nitrogen-forming gas plasma to successfully deposit SiNx, though with carbon impurities of 5–10%. Provine et al.13 improved on these results and were able to grow SiNx with high film density (2.4 g/cm3) and low WER (3 nm/min in 100:1 HF) at a temperature of 350 °C. Performing a hydrogen plasma post-anneal reduced WER to less than 1 nm/min.

Bis(tert-butylamino)silane (BTBAS) is another aminosilane frequently used for SiNx deposition. Knoops et al. deposited high quality SiNx with BTBAS and N2 plasma.14 Film density was very high at 2.8 g/cm3, and film wet etch rate was 0.2 nm/ min for growth at 400 °C. Carbon contamination was less than 2%, but that increased to approximately 10% for films grown at 200 °C. Film properties were similar to those obtained from low-pressure chemical vapor deposition (LPCVD) grown SiNx, which is attributed to the high film density of the film.

All organosilane precursors using nitrogen plasma, causes 500

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