Abstract
Vacuum ultraviolet-absorption spectroscopy (AS) and emission spectroscopy (ES) from delocalized probe plasma are implemented in the downstream chamber of a soft-etch industrial plasma reactor. A capacitively coupled plasma plasma, running in the upper compartment in He/NF3/NH3/H2 mixtures at about 1 Torr, produces reactive species which flow through a shower head into a downstream chamber, where they can etch different μ-electronic materials: Si, SiO2, SiN, etc. The ES reveals the presence of F atoms, while the dissociation rates of NF3 and NH3 are deduced from the AS, as well as the density of HF molecules, produced by chemical chain-reactions between dissociation products of NF3, NH3, and H2. The variations of HF density as a function of the NH3 flow rate suggest the possible formation of NH4F molecules in the plasma.
Highlights
Until recently, they have drawn relatively little scientific interest compared to other plasma sources because they inevitably lead to isotropic etching, which restrict their use to specific applications
The chemistry of downstream plasmas used for isotropic etching of Si alloys is investigated by VUV absorption spectroscopy and optical emission spectroscopy (OES) both in NF3/H2 and NF3/NH3 chemistries
VUVAS indicates the formation of large amounts of HF molecules which are expected to play a role in oxide etching
Summary
Downstream etchers known as remote plasma sources (RPS) were introduced in the early 1970s for resist stripping applications.[1,2] until recently, they have drawn relatively little scientific interest compared to other plasma sources because they inevitably lead to isotropic etching, which restrict their use to specific applications. H addition to NF3 in RPS (which are studied in this paper) leads to new etching possibilities Such processes were introduced[10] to deoxidize silicon surfaces, i.e., etch SiO2 selectively to Si prior to epitaxy, or to clean shallow trench isolation (STI) trenches. The “Smart Etch” process is using energetic H2 or He ions generated in a capacitively coupled plasma (CCP) plasma for the first step, modifying several nm of the SiN material by ion implantation (the use of light ions prevents SiN sputtering),[18,19] Initially, the modified SiN* layer was removed selectively to pristine SiN by a wet HF6 or by gaseous HF.[20]. By directly measuring the densities of NH3, NF3, and HF, as well as the variation of F density as a function of the operating condition, we could get insights into the complex chemistry of these RPS plasmas
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