Control of Photo-Assisted Etching of Si in Chlorine Containing Plasmas



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Plasma etching is critical for the fabrication of very large-scale integration (VLSI) microelectronic devices. In-plasma photo-assisted etching (PAE) with ion energy below the ion-assisted etching (IAE) was discovered in our laboratory several years ago. PAE may be advantageous or detrimental, depending on the situation. Thus, it is desirable to be able to control PAE. This thesis reports fundamental studies of PAE suppression using oxygen additions to the plasma and PAE enhancement by extra VUV flux irradiating the Si substrate. O2 addition to a baseline plasma was used to control PAE of p-type Si (100) and poly-Si in a high density, Faraday-shielded, Ar/Cl2 (225/25 sccm) inductively coupled plasma. The etching rate of samples etched under PAE condition was not affected by 0.1 or 0.25 sccm O2 addition to the base case Ar/Cl2 plasma, while etching stopped with more than 0.5 sccm O2 addition. Ion assisted etching, was not affected by up to 2 sccm of oxygen addition, but decreased rapidly when more than 5 sccm O2 was added to the plasma. Thus, there was a window of O2 additions that completely suppressed PAE but allowed IAE to proceed unobstructed. Vacuum transferred X-ray photoelectron spectroscopy (XPS) showed that PAE-treated samples had a thicker surface oxide and lower chlorine concentration (SiCl). In contrast, the surface of IAE samples contained Si dangling bonds and higher silicon chlorides (SiCl2 and SiCl3). The PAE rate of p-type Si was enhanced by extra vacuum ultraviolet (VUV) photon flux. Etching was carried out in the Ar/Cl2 ICP used above (main ICP). Additional photon flux was provided by a compact Ar/He (2.5/47.5 sccm) plasma source (auxiliary ICP), in tandem with the main ICP. The auxiliary ICP was also used as a standalone plasma source to determine (using VUV spectroscopy) the composition of the Ar/He gas mixture that maximized the 106.7 nm line (1s4) of the resonant state of argon. Most PAE-treated samples had a rough surface under the scanning electron microscope (SEM). A significant etching enhancement of 51% was found at a pressure of 15 mTorr, where the PAE rate with only the 60 W main ICP ON was 0.069 μm/min, increasing to 0.104 μm/min when the 200 W auxiliary ICP was also turned ON. XPS survey results showed that PAE with only the main ICP ON resulted in the same [Cl]/[Si] surface concentration ratio of about 0.15, when both ICPs were ON, suggesting that the chemical composition of the silicon surface was not affected by the additional VUV photon flux. High resolution XPS indicated that the extra VUV photon flux from the auxiliary ICP may induce surface conversion of lower chlorides to higher chlorides. Surprisingly large PAE yields 356 and 135 Si/photon were found at neutral-to-photon flux ratios of 19 × 104 and 0.9 × 104, respectively.



Plasma science and technology, plasma etching processes, surface reactions and plasma-surface interactions, photo-assisted etching, vacuum ultraviolet, XPS, SEM, laser interferometry


Portions of this document appear in: Du, Linfeng, Emilia W. Hirsch, Demetre J. Economou, and Vincent M. Donnelly. "Effects of O2 addition on in-plasma photo-assisted etching of Si with chlorine." Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 38, no. 5 (2020): 053003.