Chemical Vapor Deposition (CVD)

Chemical vapor deposition (CVD) is the most widely used technique for depositing many materials in the semiconductor industry, including a wide range of dielectrics and many metals and metal alloys. In theory, it is quite simple: Two or more materials in a gaseous form are introduced into a reaction chamber where they chemically react with one another to form a new material that is deposited on the wafer surface. A good example is the deposition of silicon nitride (Si₃N₄), formed by the reaction of silane and nitrogen.

In practice, however, the reactions can be quite complicated in that there are a wide variety of deposition parameters that must be considered: the pressure of the chamber, the temperature of the wafer, the flow rate of the gases (how quickly they are introduced into the chamber), the flow path of the gases across the wafer (Figure), the chemical make-up of the gases, the ratio of gases to one another, the role of any reaction byproducts, and whether or not any outside sources of energy are needed to speed up or induce the desired reaction. Adding sources of energy such as plasma energy, of course, introduces a whole new set of variables, such as the ion-to-neutral flux ratio, the ion energy and the rf bias on the wafer.

Then, consider the variables in the deposited film: thickness uniformity across the wafer and over features (the latter known as step coverage), stoichiometry of the film (its chemical composition and distribution), crystal orientation and defect densities, for example. Of course, the deposition rate is also an important factor since that determines the throughput of the reactor — a higher deposition rate often represents a tradeoff with the quality of the film. Since material deposits not only on the wafer but on other parts of the chamber, the frequency and thoroughness of chamber cleaning is also important.

In CVD, one of the critical parameters that determines film uniformity across the wafer is how gases flow across the wafer. Shown are modeled pathlines of SiH4 concentration and tungsten deposition rates on a wafer and pedestal in a Novellus Concept Three ALTUS system. (Source: Novellus Systems, Fluent.)

CVD technologies are often sorted by reactor type and/or pressure, including low-pressure CVD (LPCVD), atmospheric-pressure CVD (APCVD), sub-atmospheric-pressure CVD (SACVD) ultrahigh-vacuum CVD (UHVCVD), plasma-enhanced CVD (PECVD), high-density plasma CVD (HDPCVD) and rapid thermal CVD (RTCVD). Then there's metal-organic CVD (MOCVD), which warrants its own category because of the nature of the sources of metal, which are typically liquids that must be vaporized before they are introduced into the chamber. Just to confuse things, some refer to MOCVD as organo-metallic CVD (OMCVD).

In the past, the most common reactor vessel for LPCVD and APCVD was a simple furnace and even today furnaces are widely used to deposit basic films such as Si₃N₄ and silicon dioxide (silicon in the presence of oxygen will form or "grow" high-quality SiO₂, but that consumes silicon; it's also possible to deposit SiO₂ by reacting silane with oxygen — both can be done in furnaces).

More recently, the push to single-wafer processing has led to a new class of CVD reactors. Most of these use a plasma, in part to speed up the reaction process, but also to provide an "extra knob" that is used to control the quality of the deposited film. PECVD and HDPCVD are particularly interesting in that, by adjusting the power and bias and other parameters, it's possible to get a simultaneous deposition and etching action. By fine-tuning the dep:etch ratio, it's possible to get a very good gap filling process.

An interesting debate for many metals and metal alloys is if they are best deposited by physical vapor deposition (PVD) or by CVD. Although CVD has better step coverage than PVD, films like copper seed layers and tantalum nitride diffusion barriers are today deposited by PVD since there is already a huge installed based of PVD systems and engineers have a high comfort level with PVD. Some suggest that, as step coverage becomes more of an issue (particularly on the sidewalls of vias), CVD will become necessary. A similar type of argument exists when it comes to low-k dielectrics: Are they best deposited by CVD or by a spin-on process?