Dual Gate Etching: The Doping Effect

The etch rate and profiles for n- and p-doped as well as intrinsic poly-Si can differ from each other quite significantly depending on the plasma conditions and chemistry. Lee et al. explain these etch rate differences by Coulombic forces between the dopants and the halogen atom or molecule at the surface. Coulomb attraction between uncompensated donors (As or P) and chemisorbed halogens enhances etch rates in n-type silicon. Coulomb repulsion between uncompensated acceptors (B) and chemisorbed halogens inhibits etch rates in p-type silicon. This effect is the strongest in chlorine (slide 1).

Slide 2 shows the etch rates for n-doped and undoped poly-Si as well as the doped / undoped "selectivities" for HBr, Cl2, NF3 and CF4/O2 plasmas. All other reactor parameters such as pressure and power settings were kept constant. The experiment supports the finding that chlorine shows the strongest doping effect and the the chlorine in typical gate etch gas mixtures is the culprit for etch rate differences and profile distortions. Pure HBr exhibits the weakest doping effect followed by CF4/O2 and NF3.

Zhang et al. reported similar results for n-, p- and intrinsic poly-Si for CF4, HBr/Cl2 ans HBr/O2. Their findings show that HBr/O2 has the strongest doping effect for n-doped poly-Si. The effect for p-doped poly-Si is overall much smaller and similar for all three chemistries (slide 3).

Advanced gate structures are frequently pre-doped, i.e. the n- or p-dopants are introduced before the gate structures are etched. The doping levels rise as the devices are being scaled and can reach concentrations that lead to severe profile distortions. It was fund that the addition of CF4 to a traditional HBr / Cl2 / O2 poly-Si gate mainetch can reduce the doping effect. This is an additional benefit of the selfclean silicon etch chemistry. Slide 4 illustrates the effect of CF4 addition for various CF4 gas flows.

Slide 5 shows the doped/undoped selectivity as a function of pressure for 3 different poly-Si main etch processes: 1. HBr/Cl2/O2; 2. HBr/Cl2/O2/CF4 and 3. HBr/Cl2/O2/NF3. The etch rate difference increase at with higher pressure and have a maximum at around 20 mTorr (smallest Vdc). As expected, the doping effect for the Cf4 added process is small. Interestingly no effect was observed for NF3 addition. This points to carbon inhibitors rather than fluorine effect as mechanism for lowered doping sensitivity.

More plasma etch applications …