On-site fluorine chamber cleaning for semiconductor thin-film processes: Shorter cycle times, lower greenhouse gas emissions, and lower power requirements

Abstract

Regular removal of thin-film deposits in chemical vapor deposition (CVD) reaction chambers is an important requirement in semiconductor manufacturing technology. Most often plasma etching using fluorine-containing gases like sulfur hexafluoride SF6 and nitrogen trifluoride NF3 is widely applied, because of the efficient reactions of fluorine radicals with the residues and the high volatility of the resulting fluoride products. Concerns have been expressed, however, about the global warming impact of such gases: SF6 and NF3 exhibit global warming potentials of 22,600 and 17,200 times that of CO2 as measured over 100 years [1] [2]. While previous work has shown that NF3 may be preferred to SF6 because of a gas utilization near 100% for small area scale reactors [3], significantly lower gas usage was measured for commercial reactors. Weiss et al. estimate that 16% of the global production of NF3 is released in the atmosphere, with atmospheric concentration rising 11% per year [4]. Alternative chemical cleaning using environmentally friendly processes exist, however. Molecular fluorine has been already used for chamber cleaning in other applications such as semiconductor thermal oxide / nitride, atomic layer deposition (ALD), and plasma enhanced chemical vapor deposition (PeCVD), and LCD display manufacturing. As a non-polar gas, it has no global warming potential [5]. Besides environmental advantages, molecular fluorine offers process and consumption advantages over other cleaning gases. In particular, F2 requires low energy to be dissociated. While 79.5 kJ are sufficient to obtain 1 mole of atomic fluorine from F2, 281 kJ and 329 kJ are necessary from NF3 and SF6 respectively. Thus much more F2 can be activated per unit time, resulting in a faster clean. Another advantage is the gain in mass efficiency. Because the nitrogen atoms from NF3 do not contribute to the cleaning process, molecular fluorine needs 20% less mass to supply the same amount of atomic fluorine radicals as NF3. The absence of the spectator nitrogen atom will be shown to also reduce deleterious temperature effects at the remote plasma source (RPS) as well as the process chamber, resulting in shorter overall clean cycles due to stabilized cleaning temperatures, and significant energy savings. Here we report an effective cleaning process of various process chambers using a variety of deposition techniques and chemistries — PECVD for amorphous silicon; ALD for ultra-low temperature silicon oxide, and thermal diffusion for doped poly-silicon. Fluorine cleaning performances are compared to NF3. Several parameters, such as clean rate, temperature rise at the remote plasma source and chamber during cleaning, and power consumption, are measured. We illustrate with data from various production chambers and explain the underlying thermodynamics and kinetics. Finally, process benefits of faster cleaning times, faster cycle times, temperature stability, and lower material use are demonstrated.