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Title:Pulsed Sheet Electron Beam Plasma-Assisted CVD of Silicon Films
Author(s):Shaheen, Mohammad A.
Doctoral Committee Chair(s):Ruzic, David N.
Department / Program:Nuclear Engineering
Discipline:Nuclear Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Engineering, Materials Science
Abstract:Pulsed Sheet Electron Beam Plasma-Assisted Chemical Vapor Deposition (PSEB-CVD) is a novel method of thin film deposition which is a variant on the conventional PECVD and an alternative to remote PECVD. PSEB-CVD uses a pulsed electron beam generated plasma, whose dimension is confined to a narrow sheet that passes over the substrate at a controllable height. Variations in plasma pulse width, cathode voltage, sheet beam-to-substrate distance, gas type and pressure can vary the type and energy of the species arriving at the substrate. Specifically, the ratio of $\rm SiH\sb3/SiH\sb2$ flux to the substrate can be increased by a factor of 10 by placing the wafer at least 5 cm from the sheet beam and increased by 3 orders of magnitude by operating the plasma at a 10% duty cycle. The increased $\rm SiH\sb3/SiH\sb2$ flux ratio results in better film quality due to the larger surface mobility of SiH$\sb3$ when compared to SiH$\sb2.$ This improvement, however, is accompanied by a linear decrease in deposition rate, from 25 A/min for the dc case without a sheet beam, to 5 A/m for the 0.5 duty cycle case with the wafer at 5 cm from the substrate. A system based on the PSEB-CVD principles was designed and built to allow the creation of a sheet e-beam at a variable distance from a heated substrate in a 5% silane/He plasma. Also, a plasma-pulsing circuit that can deliver square pulses of widely varying shapes has been built and used to create a pulsed e-beam plasma with varying pulsing conditions. A model of the sheet e-beam plasma kinetics, silane chemistry and surface deposition is used to guide the choice of the experimental parameters so as to effectively select a specific radical for deposition. The pulsed plasma was characterized with Langmuir probe analysis which showed that for the case of a He plasma there was a sharp increase in electron density immediately after the pulse was turned off. For the pulsed silane/He plasma, this effect was not as large, but unlike the He plasma, the floating potential increased for a few ms's after initiating the pulse. The silane/He plasma may have had a strong e-beam component. A recipe was developed for the optimum operating conditions of the PSEB-CVD system based on an analysis of the system operating under a variety of conditions. Growth of Si films in the 100-600 A thickness range was demonstrated as a proof of principle of the PSEB-CVD method. The films were characterized for uniformity, impurity content and crystallinity by a variety of surface analysis techniques including Profilometer, AES, EBSD, SEM, XRD and AFM. The films grown were found to be pure to a detection limit of 0.2%. Diffraction data, as well as grain surface morphology, were used to characterize crystallinity. The films deposited without a sheet beam were found to be amorphous, while the ones grown in a sheet beam were partially polycrystalline (30%). An x-ray diffraction analysis on films deposited in pulsed (0.5 duty cycle) sheet beam (substrate height = 5 cm) indicated the possibility that the films could be preferentially oriented. The films were typically grown at temperatures of 370$\sp\circ$C and 250 mTorr pressure. The Nm uniformity was also greatly improved with the use of the sheet e-beam configuration. The improved crystallinity confirms that deposition quality is improved as a result of beam confinement and plasma pulsing.
Issue Date:1997
Description:307 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1997.
Other Identifier(s):(MiAaPQ)AAI9812834
Date Available in IDEALS:2015-09-28
Date Deposited:1997

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