Saturday, August 27, 2016

Entegris launch Fluorine-Free Tungsten (FFW) for 3D NAND

At the resent "4th Annual Yield Breakfast Forum: Yield Enhancement Challenges in Today’s Memory IC Production" Jim O’Neill from Entegris presented "Fluorine-Free Tungsten (FFW) for 3D NAND" (SEMICON West, JULY 14, 2016).

Many of you who have been working on the ASM Pulsar ALD reactor are familiar with the Entegris (originally ATMI) ProEvap(TM) solid source evaporator vessel that can deliver a high dose of low vapor pressure metal chlorides e.g. ZrCl4 and HfCl4 as well as MO-precursors like La(THD)3. This evaporation technology was one of the key technologies that enabled the ALD success story for the introduction of HfO2 in 45 and 32/28 nm CMOS High-k / Metal Gate transistors.


FFW is developed targeting 3D NAND gate contacts (avoids Fluorine attack of node dielectric). To achieve good film properties precursor purification is critical and a Solid delivery vessel enables consistent precursor delivery (Information & Figures from Entegris, used with permission)
 
For Tungsten metallization in CMOS, DRAM and 3DNAND tungsten hexafluoride is typically used, either in a CVD or an advanced pulsed CVD process. However, there is an ever ongoing issue with fluorine diffusion into the active device layers causing reliability issues. Until today this issue has typically been prevented by a TiN diffusion barrier, but scaling down the geometrical constraints has now forced reducing the TiN barrier thickness to a point that it can no longer hinder fluorine diffusion into the device and therefore there is a need to drastically reduce fluorine in the tungsten contacts. For this the industry has developed different solutions, including:

1) Metal organic tungsten precursors without fluorine. However, all these molecules have until today failed to reproduce the low contact resistance from a WF6 process.

2) Reducing agent treatments during deposition, which is believed to be the trick in the WF6-basded Lam Research New ALTUS(R) Max E Series for Low-fluorine, Low-stress, and Low-resistivity ALD Tungsten process (Link). Lam Research reports that this process reduce fluorine content by two orders of magnitude.

Besides the high purity precursor and solid source deliver vessel high temperature in-line gas filters are needed to deliver a particle free precursor flow. Later Entegris will also add a flow monitoring system that is currently under development - Advanced IR Sensor monitoring precursor flux (SSFM) (Information & Figures from Entegris, used with permission).

However, there is another solution and that is to move to a different metal halide precursor, namely Tungsten Hexachloride. The issue until now has been to deliver a high constant gas flow over long time allowing high volume production. That´s why it is very interesting to read about the new larger ProEvap(TM) system, which as reported is able to handle up to 6kg of solid precursor (previously  < 1kg).  


The new ProEvap(TM) vessel is also interesting in an additional way since it can be provided with an internal corrosion protective coating by CVD, ALD or PVD. (Information & Figures from Entegris, used with permission).

A special thank you to Ed Korczynski (Sr. Technical Editor, SST/SemiMD) for giving me the insights to this technology and Entegris for releasing the information and figure.