A History: Chemical Delivery
As we look for faster devices, improved medical testing devices, better energy storage capacities, or more energy efficient products in our everyday life, researchers are working on the next breakthrough product that does just that. Once researchers develop a new product, they partner with an industrial partner to scale up production of that product to bring the product to market. Thin films are layers, ranging from a few angstroms to several microns in thickness, of material that are deposited onto the surface of another material. Thin films can be deposited through Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD), Epitaxy, or Growth Mode processes. These films can be used for a variety of applications and purposes such as: reflective coatings, insulating materials in semiconductor devices, bio sensors, hard coatings on tools, and drug delivery applications to name a few.
A specialized type of chemical vapor deposition is Atomic Layer Deposition or Epitaxy. Atomic Layer Epitaxy and Molecular Layering were developed back in the 1960’s in Finland and the Soviet Union respectively. The details about Molecular Layering were kept secret in the Soviet Union. The initial purpose of the Atomic Layer Epitaxy process was to grow a zinc sulfide thin film layer for electroluminescent display panels. This process involved the reaction of zinc chloride, a solid, and hydrogen sulfide, a gas. The resulting reaction produced a display that had a sharper image than the previous deposition process.
The semiconductor industry was looking for ways to grow silicon dioxide films, without destroying or damaging previously built-up layers. One of the first materials used was tetraethyl orthosilicate (TEOS). TEOS bubbler systems were developed to be placed close to CVD tools in the fab. Other early liquid sources in the semiconductor industry include WF6, POCl3, and trans LC. As semiconductor devices became more sophisticated, new materials were used to grow the films and crystal structures needed for the devices.
Chemical Vapor Deposition (CVD) is an important process in the semiconductor industry. Whether it is silicon based, III-V based, or II-VI based, they all use CVD processes, which can be in the form of plasma enhanced, atomic layer, or metal organic. Atomic layer deposition and metal organic deposition processes use similar materials to the same precursor materials. Precursors can be delivered to the process chamber via direct injection, aerosol, or with a carrier gas. It will be dependent on the volatility of the precursor as to which method is used.
Direct Liquid Injection Chemical Vapor Deposition (DLICVD) uses liquid precursors that are injected directly into a vaporization chamber prior to being injected into the process chamber of the CVD tool. This process uses a push gas to push the liquid through a liquid mass flow controller to the vaporization chamber. The line from the vaporization chamber to the chamber must be heat traced. The process is similar to fuel being delivered to a car’s engine via fuel injectors. The liquid precursors can also be a solid material that is dissolved in solution. This method is preferred when growing films with high growth rates.
Aerosol Assisted Chemical Vapor Deposition (AACVD) uses precursors that are liquids and gases that are aerosolized and delivered to the processing chamber. These aerosols are generated by using ultrasonic sound waves. This process is suitable for non-volatile precursors. AACVD process is used to produce liquid crystal displays, LED lights, and transparent conducting oxides.
The most common precursor delivery method uses a carrier gas, such as hydrogen or nitrogen, that is bubbled through a solid or liquid source and delivered to the process chamber. The precursors are loaded into bubblers and placed inside of the tool itself. The carrier gas is bubbled through the bubbler, picking up the precursor material and delivering it to the chamber. Bubblers are placed in water baths so that the temperature of the bubbler can be controlled to give better control of the precursor flow rate into the process chamber. The use of mass flow controllers (MFC’s) can control the flow of the carrier gas to the bubbler.
The industry is pushing to have larger precursor sources placed near the process tools. However, codes are limiting the amount of materials that can be placed in the manufacturing space based upon the hazard classifications of the material. Some precursors are highly flammable, pyrophoric and/or highly reactive, which limits them to small allowable quantities in the manufacturing area. The industry is asking suppliers to develop bulk liquid push systems that can keep the downstream bubblers filled with precursor material. The bulk liquid precursor sources can be placed in areas of the building designed to store large quantities of highly hazardous materials and can dispense these materials as well.