Saturday, March 26, 2016

New cyclic azasilanes as volatile and reactive precursors for ALD of SiO2 from Gelest

Here is a very good publication brought to my attention by Henrik Pedersen on Twitter. It is a nice screening exercise of a new class Si precursors for ALD of SiO2 using ozone fom Gelest Inc. SiO2 is one of the most important materials today in the 2nd ALD boom besides silicon nitride. 

As reported earlier here the equipment market for ALD single and multi wafer tools is expected to reach $1.2 billion in the next 2-3 years. One of the reasons behind tremendous growth expectation  is that LPCVD and PECVD just can´t meet the requirements of conformal growth and low thermal budget required by the sub 20 nm Logic and Memory technologies and especially for multi-patterning and also due to the 3D path of 3DNAND. One additional challenge that has to be confronted is to have a stable process that is not affected by surface loading difference due to different chip designs - just imagine the issues with having one specific CVD recipe for each specific litho layer in each specific product in a foundry like TSMC or Globalfoundries.

Here is an excellent publication from Dina Triyoso at Globalfoundries explaining loading effects in the 28nm spacer module PECVD vs ALD SiNx that is free at Research Gate.

The excellent work is form Nicholas Strandwitz research group at the Department of Materials Science and Engineering and Center for Advanced Materials and Nanotechnology, Lehigh University, Bethlehem, USA and has been performed using a the well known workhorse in ALD - the Ultratech/Cambridge Nanotech Savannah S100. This one seems also to be a nice version with a Quartz crystal microbalance (QCM) integrated in the lid from Ultratech.

Cyclic azasilanes as volatile and reactive precursors for atomic layer deposition of silicon dioxide

Ling Ju and Nicholas C. Strandwitz 
J. Mater. Chem. C, 2016, Advance Article, DOI: 10.1039/C5TC03896K

A suite of four volatile aminosilanes, cyclic azasilanes, was used to deposit silicon dioxide (SiO2) films by atomic layer deposition (ALD) over the temperature range 100–300 °C by reaction with O3. The unstable Si–N bonding makes the cyclic azasilanes chemically reactive with hydroxyl surfaces through a ring-opening reaction. Subsequent oxidation with O3 affords silanol groups, which are amenable to further reaction with cyclic azasilanes. The influence of azasilane and O3 exposure times on the growth rate was examined in detail. The growth rates obtained by spectroscopic ellipsometry are 0.6–1.2 Å per cycle for various azasilanes under different ALD conditions, due to side chain structure variation of the precursors. Refractive indices (1.45–1.46) and band gaps (8.5–8.7 eV) are found to be similar to thermal oxide. X-Ray photoelectron spectroscopy (XPS) revealed 3–5 at% C and 0.2–0.4 at% N in the films and an O/Si ratio of ∼1.9 when deposited at 190 °C. The first silane pulse resulted in a surface coverage of ∼1.2 molecules per nm2 as determined by microbalance measurements. The O3 oxidation rate is faster for silanes with Si–OMe groups than those with Si–Me functionalities, and less effective at lower temperatures for some silane precursors. These cyclic azasilanes are promising precursors for ALD SiO2 and surface functionalization, and the variation in the structures provides possibilities to study reaction mechanisms and control surface chemistry.

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