Physical Properties of Liquid Precursors

In striving to adhere to Moore's Law, the semiconductor industry is designing and using new metal-organic compounds. Precursor compounds deliver a metal atom to the surface of a hot silicon wafer in many film deposition processes; the physical property most important for efficient delivery is vapor pressure because it controls the behavior of vapor delivery devices. Therefore, this project’s first goal is to produce accurate vapor pressure data for relevant liquid precursors from room temperature to as high as 200 °C. Such data will improve the modeling of chemical vapor deposition processes and the design and use of bubblers and other devices that deliver precursor vapors. Other goals are to characterize the thermal stability of precursors and to devise a method for measuring vapor pressure that is suitably accurate yet more convenient than existing methods.

Precursors for chemical vapor deposition are frequently liquid compounds that, until recently, were either rare or nonexistent. Their vapors are delivered to the process chamber either by direct injection (flash evaporation) or by bubbling a carrier gas through the liquid held in a "bubbler". Anecdotal information from companies that sell precursors or the associated process equipment had suggested an industrial need for improved property data of these specialty chemicals, especially vapor pressure. That need was verified at an industry workshop.

The project goals are to (1) assess the quality of existing vapor pressure data of compounds such as Hf [(CH₃)₂N]₄ (acronym TDMAH), (2) fill gaps in the existing data by developing apparatus capable of accurate vapor pressure measurements, (3) provide the resulting accurate data to industry, and (4) investigate alternate methods to measure vapor pressure that are suitably accurate yet more convenient than existing methods.

To measure vapor pressure, the laboratory apparatus uses pressure gauges that operate at the same temperature as the sample. This direct method was chosen over indirect methods that rely on, for example, thermogravimetric analysis or nuclear magnetic resonance. Challenges include achieving appropriate control of the sample temperature, handling hazardous samples, and avoiding systematic errors due to the presence of decomposition products, dissolved gases, and other impurities. To measure thermal stability, a gas chromatograph/mass spectrometer is incorporated into the apparatus.

• Measured vapor pressure of water and obtained agreement with literature.
• Characterized temperature and time dependence of pressure.
• Designed and assembled apparatus to handle hazardous effluents.