Pulsed Sheet Electron Beam Plasma-Assisted CVD of Silicon Films

Shaheen, Mohammad A.

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Description

Title

Pulsed Sheet Electron Beam Plasma-Assisted CVD of Silicon Films

Author(s)

Shaheen, Mohammad A.

Issue Date

1997

Doctoral Committee Chair(s)

Ruzic, David N.

Department of Study

Nuclear Engineering

Discipline

Nuclear Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Engineering, Materials Science

Language

eng

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.

Graduation Semester

1997

Type of Resource

text

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