Gas purging anf monitoring apparatus for thin-film chemical vapour deposition

Date

10-1994

Degree

Bachelor of Science in Applied Physics

College

College of Arts and Sciences (CAS)

Adviser/Committee Chair

Demetrio A Yco, Jr.

Abstract

The purging and monitoring of air inside a Chemical-Vapour-deposition (CVD) chamber is vital in the growth of thin-films desired quality. This is because the presence of air particularly of oxygen and water vapors produces reactions that are unwanted in the desired quality of thin-films prepared by CVD. A system for purging and monitoring air inside the chamber is developed. de Gas purging is accomplished by allowing instrument- gill carbon dioxide gas to flow through the chamber at pressures higher than that of air inside the chamber. Due to molecular collisions at higher concentrations and pressures of the purging gas, air is purged out of the chamber. The monitoring of the presence of air inside the chamber is facilitated by the Thermal Conductivity Method. This method utilizes the difference in the thermal conductivities of the two gases mentioned. This is done by utilizing a Wheatstone Bridge with heated resistors. The amount of heat absorbed from these resistors varies for different gases. Thus the resulting resistances of the resistors in the bridge vary to produce an imbalance in the bridge in the form of a measurable EMF. Several configurations for the system are observed. In the first configuration, the dual plastic gas-valve in the inlet of the chamber was not removed. The cross sectional area of the opening of this gas valve differed significantly from the cross sectional area of the rubber hose which produces a difference in the mass flowrates of the gases in the two paths flowing towards the Thermal Conductivity Detector. The difference of the mass flowrates of the gases produced a signal in the detector which was noted. In the second configuration, the dual plastic gas valve was replaced by a single inlet made of glass whose cross sectional area is the same as that of the rubber hoses. The carbon dioxide gas was allowed to flow through different pressures and the time it takes for the bridge to balance was noted. In the third configuration, the chamber was heated to an initial temperature. Temperature control of the chamber is accomplished by placing an insulated heating element made of Kanthalux wire beneath the reactor chamber connected to a 1000 Watt, 110 Volt Variac. The variation in the initial and final temperatures of the chamber after some time and the initial temperature of the chamber and the signal produced by the detector after a certain time is noted. In the fourth configuration, the chamber was also heated to an initial temperature. The time it takes for the chamber to stabilize to a certain temperature and the corresponding signal produced by the detector is noted. The characteristic relationship obtained between the pressure of the carbon dioxide gas and the time it takes for the detector to balance is

t = m(1/p) + b

where m= constant in psi x s

p =pressure of the Carbon Dioxide gas

b =constant in seconds

This empirical equation was obtained for pressure from 4 psi to 12 psi. However it is also holds for pressures greater than 12 psi.

Thus the Gas Purging and Monitoring System for Thin-Film Chemical Vapour Deposition developed is a promising yet inexpensive means of detecting and eliminating air inside the Chemical Vapour Deposition Chamber.

Language

English

Location

UPLB Main Library Special Collections Section (USCS)

Call Number

Thesis

Document Type

Thesis

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