Challenges with Industrialization of Atomic Layer Deposition of Silicon Nitride

Scaling of next generation logic and memory semiconductor devices requires conformal thin films on high aspect ratio features at low deposition temperatures (<500ºC).  Atomic layer deposition (ALD) is an ideal method for depositing these films and is currently being used in high volume manufacturing (HVM) to deposit high-k dielectric materials (HfO₂, ZrO₂, etc.) as well as the more traditional dielectrics silicon oxide (SiO₂) and silicon nitride (SiN_x).  With the notable exception of silicon nitride, all of these films can be deposited at sufficiently low temperatures to satisfy the integration requirements while maintaining reasonable throughput and cost.  In the HVM environment, silicon nitride ALD has historically been deposited in large furnaces operating at high temperatures, and although both plasma and thermal deposition of silicon nitride have been reported at low temperatures^1-2, translation of these low temperature R&D films into HVM films has been challenging.

In this presentation, we discuss some of the challenges faced with addressing the requirements associated with bringing low temperature ALD SiN_x into HVM, which include achieving high wafer throughput and low per wafer cost, while simultaneously keeping the resistance of the film to multiple downstream etches and cleans, which may include exposure to hydrofluoric acid (HF).  Generic to all ALD processes is the high cost of the precursors relative to traditional chemical vapor deposition (CVD); in the case of SiN_x, this is exacerbated by the relative low “reactivity to cost ratio” of available silicon precursors.  Cost can partially be addressed with chlorosilane precursors, but the chlorine byproducts create challenges with deposition chamber materials and effluent management.  Low reactivity of the silicon precursors poses challenges with respect to process design; the choice of spatial ALD vs. temporal ALD and isobaric vs. non-isobaric deposition sequences affects the final cost and throughput.  Lastly, the use of plasmas in ALD poses challenges for achieving isotropic film properties over the complex topography on today’s semiconductor substrates.

References:

1. H. Goto, K. Shibahara and S. Yokoyama, Applied Physics Letters, 68, 3257 (1996).

2. J. Klaus, A. Ott, A. Dillon and S. George, Surface Science Letters, 418, L14 (1998).