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

Introduction

Silicon nitride (SiN_x) 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 SiN_x 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 SiN_x 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 SiN_x was grown by thermal ALD in 1997, when Morishita³ deposited SiN_x 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 No. 215155) at temperatures ranging from 525–650 °C. While hydrazine has since been replaced by more convenient nitrogen sources, HCDS has remained an important precursor for SiN_x ALD. Later reports⁴ of ALD using HCDS and ammonia demonstrate successful deposition of SiN_x 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 SiN_x is the large precursor exposure (10⁷–10¹⁰ L) required to achieve saturation. Of note, only chlorosilane precursors have been used for thermal ALD of SiN_x, 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 SiN_x 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 SiN_x 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 SiN_x 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 SiN_x 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 SiN_x ALD was tris(dimethylamino) silane (TDMAS, Cat. Nos. 570133, 759562) in 2008.¹² Using a remote ICP nitrogen-forming gas plasma to successfully deposit SiN_x, though with carbon impurities of 5–10%. Provine et al.¹³ improved on these results and were able to grow SiN_x 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 SiN_x deposition. Knoops et al. deposited high quality SiN_x 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 SiN_x, which is attributed to the high film density of the film.

All organosilane precursors using nitrogen plasma, causes the GPC to drop to nearly zero. When NH₃ plasma is used, -NH₂ is a common surface termination. Huang et al.¹⁵ predicted a high activation energy barrier for BTBAS and other amine groups, and this prediction was experimentally confirmed by Provine for TDMAS and NH₃ plasma growth. Density functional theory simulations predicted that BTBAS would only adsorb at undercoordinated nitrogen or silicon sites, which NH₃ plasma cannot provide. Another issue with most organosilanes is the low GPC — typically less than 0.3 Å/cycle. Organosilanes are large molecules that provide only a single silicon atom, so steric hindrance likely plays a role, though the requirement for undercoordinated surface sites probably hinders growth as well. Finally, conformality over HAR structures is limited for organosilanes. Faraz et al. found at best 50% conformality for sidewalls (26 vs 13 nm) when using di(sec-butylamino)silane (DSBAS) and N₂ plasma.¹ These results are common, and are likely due to the soft saturation observed for N₂ plasma exposure. Typically sidewalls are exposed to lower plasma density during the deposition process compared to the top and bottom of the structure.

Heterosilanes

The last group of precursors encompasses all non-organic and non-halide precursors. Silica is simplest of the precursors and has already been discussed. Well-known for use its use in PECVD, SiN_x can be deposited using SiH₄ and N₂ plasma.¹⁶ The disadvantage of this approach is the long (60 s) N₂ plasma time required for saturation. Likewise, S-H contamination is problematic and likely results in low film density and high WER. Trisilylamine (TSA) is another Si precursor that has been investigated for SiN_x deposition. Triyoso et al.¹⁷ demonstrated the growth of SiN_x with TSA and N₂/H₂ plasma at 300 and 400 °C. GPC depended greatly on plasma conditions, and varied from 1.3 to 2.1 Å/cycle and WER in 100:1 HF was approximately 1 nm/min. When comparing PEALD SiN_x with PECVD SiN_x, they found TSA-based PEALD improved transistor performance with higher drive current, higher hole mobility, and improved I_on/ I_off ratio. Jang et al.¹⁸ deposited SiN_x with TSA and NH₃ plasma, though with a lower GPC (0.6 Å/cycle vs. ~1.5 Å/cycle). This has important implications for conformal deposition over HAR structures, since NH₃ plasma offers improved step coverage. Finally, neopentasilane (NPS) and N₂ plasma were used to grow SiN_x between 250 and 300 °C.¹⁹ Film properties were found to be similar to that of TSA, though NPS has a slightly higher GPC (1.2 vs. 1.4 Å/cycle). The WER for SiN_x grown with NPS was strongly plasma dependent, but optimized conditions resulted in a WER of between 2 and 3 nm/min. Both TSA and NPS are interesting precursors due to their high silicon percentage relative to molecular weight, and they offer higher GPC than organosilanes while maintaining a lower WER than typical chlorosilanes.

Summary and Outlook

The need for high quality, conformal SiN_x films grown at low temperature is increasing, and both academia and industry are working to develop advanced processes and precursors. Currently available precursors offer a range of advantages and disadvantages. Chlorosilanes provide enhanced GPC and conformality over HAR structures, but lack wet etch resistance and film density. Organosilanes enable the growth of SiN_x films with extremely low WER, comparable to or lower than those achieved with LPCVD, but are hindered by low GPC and poor conformality. The heterosilanes, which include trisilylamine and neopentasilane, offer both good GPC, which aids in throughput, and low WER. Further investigation is needed to see if these precursors can deliver conformality over HAR structures. While not a silicon precursor, it is worth discussing current work on nitrogen sources. Recent advances in hydrazine delivery technology have enabled the use of ultra-high purity hydrazine in thermal ALD. While toxicity is still a concern, the overall safety of the source has been improved.²⁰ Thermal ALD of TaN and WN with hydrazine as the nitrogen source has been demonstrated. Likewise, low resistivity of TiN deposited at 275–350 °C has been reported. Few reports exist regarding the deposition of SiN_x with hydrazine, but the growth of SiN_x passivation layers using HCDS and hydrazine at 285 °C has been successful.²¹ Chlorine contamination is a problem; these issues can likely be associated with the extremely low deposition temperature since similar issues occur for PEALD below 300 °C. Current work using ultra-high purity hydrazine as a source for SiN_x deposition is promising, and good SiN_x properties have been obtained in depositions occurring between 350 and 400 °C, which should enable the development of low temperature thermal ALD of SiN_x. Together, improvements in nitrogen sources and silicon precursors portend a bright future for atomic layer deposition of SiN_x.