Method and apparatus for in-situ and on-line monitoring of a trench formation process

摘要

With a single wafer dry etching equipment, the trench formation process in a silicon wafer is usually monitored by time-based ellipsometry techniques and destructive SEM cross-section analysis to ex-situ determine trench profile and depth. In particular, the thickness of the SiO₂ layer which is redeposited during the trench formation is monitored by ellipsometry. On the contrary, the apparatus (20) of the present invention allows full in-situ and on-line monitoring of the trench formation process. It is essential that the chamber (22) of the etching equipment be provided with side and top quartz windows. Two spectrometers (30A, 30B) are connected to their respective windows (26A, 26B) by two optical fibers (29A, 29B), so that the optic fibers look at the wafer through the plasma (27) at zero and normal angle of incidence with respect to the wafer plane, respectively. Both spectrometers are tuned to look at the same species radiation, e.g. one SiBr band. Signals outputted by the spectrometers viewing the side and top windows are different:side signal Il as a function of time t is representative the band intensity variation during the trench etching;top signal It as a function of time t is representative of both the band intensity variation and the wafer surface reflectivity. Thus, signal It is a mixed signal with band intensity and interferometric components.;According to the monitoring method of the present invention the interferometric component signal Ii is extracted by subtracting the side from the top signal. The interferometric signal Ii is of the quasi-periodic damped sine-shaped type. Then, envelope signals Ja and Jb of said interferometric signal Ii, and finally the amplitude variation signal I of the said envelope signals are generated.;The trench depth D is determined in real time by equation:D = k√$\mathop{\mathrm{image}}$ \overline{\text{I(t=0)}}$$$\mathop{\mathrm{image}}$ \overline{\text{-}}$$$\mathop{\mathrm{image}}$ \overline{\text{I}}$$wherein k is a coefficient depending on the etching system that is used and is determined by a preliminary calibration step.;The thickness of the redeposited SiO₂ layer is computed in real time by the equation:$\mathop{\mathrm{image}}$ wherein λ is the wavelength of the selected species (e.g. SiBr), n is the refractive index of the redeposited SiO₂ layer, and tu is the half-period of the interferometric signal Ii.;D and Th are continuously monitored until the desired final parameters Df and Thf are obtained.;The above method has important applications in the fabrication of deep trenches for DRAM storage capacitors.