What do potato chips bags and semiconductor wafers have in common? Yes, they both involve chips of some kind, but their creation also utilizes chemical vapor deposition. Chemical vapor deposition is the process by which a gaseous mixture is exposed to a substrate to form a layer of a specific material.
CVD Process in Semiconductors
While this is a relatively general definition, it describes a fundamental manufacturing technology for the world in which we live. Nearly all semiconductor manufacturing processes utilize chemical vapor deposition, meaning most modern electronics would not exist without this technology.
There are varying methods for chemical vapor deposition (CVD) that rely on differing chemistries, substrate materials, temperatures, pressures, and durations. However, nearly all CVD processes utilize two main procedures:
- Decomposition reaction of a gaseous compound
- Combination reaction of one or more of those elemental parts on a substrate material.
For the decomposition and combination reactions to occur correctly, the manufacturer must increase the temperature and/or reduce the pressure. This addition of energy allows the bonds to break and reform, similar to how water evaporates in hot or low-pressure air.
Chemical Vapor Deposition Applications: Science of Making Rock Candy
To give context to the fundamentals of this reaction outside of the realm of semiconductors, let's consider how confectioners create rock candy. Yes, rock candy, the ultimate childhood indulgence of pure sugar crystals on a wooden stick.
- First, sugar (our monomer) heats and dissolves into water to form a supersaturated solution mixture of water and decomposed sugar (i.e., carbon, hydrogen, oxygen).
- Next, the wood (our substrate) is exposed to the mixture to act as the interface of the upcoming heterogeneous reaction.
- As the solution cools, the wood substrate acts as a catalyst location for the sugar monomers to begin reforming a sugar polymer in a combination reaction.
- As the heterogeneous reaction continues, the polymers continue to grow until the solution reaches a desired equilibrium with the sugar and water, thus forming the large sugar crystals we know and love.
Although a simple example, chemical vapor deposition in the semiconductor world works on the same principle:
- Compounds break down into monomers.
- A substrate acts as a catalytic surface for a heterogeneous combination reaction of polymers.
- A crystalline structure of nearly pure polymer chains forms.
Chemical Vapor Deposition Techniques
There are a large variety of chemical vapor deposition methodologies, but you'll commonly find three techniques in modern manufacturing of devices such as CMOS and MEMS ICs, RF Switches, microphones, and opto-acoustic modulators:
1. Atmospheric Pressure CVD (APCVD)
As the name suggests, APCVD occurs at atmospheric pressure. APCVD is often used for graphene synthesis. To achieve the decomposition and combination reactions in APCVD, manufacturers must use extremely high temperatures. The process requires temperatures from 1000-1300C, making APCVD thermally expensive.
2. Low-pressure CVD (LPCVD)
LPCVD processes often utilize a vacuum and a furnace tube to decrease the pressure of the process gases, allowing them to react at lower temperatures than APCVD. Common LPCVD pressures can range from 10-1000Pa. This kinetically controlled process is a higher-volume, lower-cost process when compared to PECVD and is capable of processing over 100 wafers per run at 600-900C.
3. Plasma Enhanced CVD (PECVD)
Utilized for processes with an even more stringent thermal budget, PECVD processes can typically process a small batch of wafers while operating between 100-400C. In PECVD, cold plasmas are introduced to the deposition chamber to increase the deposition particle's electron temperatures. By simply changing the pressure, cold plasmas can use the energy of these electrons to dissociate the reactive gas and form the solid film of the substrate, allowing for much lower reaction temperatures.
CVD in Semiconductor Manufacturing
Wafer surfaces contain thin coatings of CVD-created polymer chains. One of the most common applications of CVD in modern manufacturing is creating integrated circuits. CVD is also utilized in solar cell manufacturing, where manufacturers grow silicon on monocrystalline silicon substrates, which forms a silicon layer 15-50 microns thick.
Manufacturers also use chemical vapor deposition to grow 3C- and 6H- silicon carbide (SiC) on silicon wafer substrates. Various techniques exist to grow silicon carbide, and some methods even introduce n-type and p-type dopants into the monocrystalline SiC films. Although we can create silicon carbide crystals in other ways, chemical vapor deposition has made it much more cost-effective to develop relatively thick, impurity-free SiC crystals for use in electronics manufacturing.
Without chemical vapor deposition technology, the semiconductor industry (and many other industries) would be significantly less productive. CVD allows technology to create incredibly small, powerful, and useful devices across nearly all industries. Whether we utilize CVD to deposit aluminum onto a potato chip bag's polypropylene layers or we use it to create the next generation of artificial intelligence, this process has a significant impact on our modern way of life.