Problem 3 (Due Wednesday, September 15)

    Chemical Vapor Deposition (CVD) is a method for growing thin film coatings. This process is extensively used by the microelectronics industry to fabricate integrated circuits, such as microprocessors. The films, typically a micron or less in thickness, are grown on silicon wafers, which are several hundred millimeters in diameter and are a few millimeters in thickness.

    Photographs of one of the CVD reactor in my laboratory is shown below. Some of the reactor parts have been labeled. In the process, molecules enter a vacuum chamber in gaseous form. The pressure of the chamber is controlled by using a vacuum pump and a valve. Subsequent chemical reactions, driven either by thermal or electrical energy, produce film deposition on the silicon wafer substrate. The temperature of the substrate is maintained at 25 °C by backside water cooling.

    My research group is investigating how the choice of reactant molecule determines the composition and properties of the deposited films. One of the molecules under study is D₄. The following information was available from the catalog from which the D₄ was purchased:

Melting point: 17 °C Boiling point: 175 °C Vapor Pressure @ 23 °C : 1 torr Hvap: 10.9 kcal/mol Molecular Weight: 296.61

a) What phase or phases of D₄ exist at equilibrium under ambient laboratory conditions of 1 bar and 25 °C?

b) If the gas inside the reactor is pure D₄, what is the maximum operating pressure which avoids condensation of liquid D₄ on to the 23 °C substrate?

c) In order for reactant to flow into the CVD chamber, heating tape is used to elevate the temperature of the crucible which contains a supply of the pure reactant. The higher temperature increases the vapor pressure in the crucible and the resultant pressure difference allows the reactant to flow into the CVD chamber. If a vapor pressure of 10 torr is required, to what temperature must the crucible be heated?

d) Sketch a P-T diagram and show the location of the three conditions defined in parts a, b, and c of this problem.

e) A related molecule, D₃, is also under investigation. What phase or phases of D₃ exist at equilibrium under ambient conditions.

Melting point: 65 °C Boiling point: 134 °C Vapor Pressure @ 70 °C : 70 torr Hvap: 9.5 kcal/mol Molecular Weight: 222.46