Thin Films, Big Impact

THIN-FILM DEPOSITION

Silicon nitride, silicon dioxide, Si, and many types of metal thin films are deposited during IC fabrication. Deposited films are usually not single-crystalline.

Three Kinds of Solid

A solid material may be crystalline, polycrystalline, or amorphous. They are illustrated in Fig. 3–16. A crystalline structure has nearly perfect periodic structure as described in Section 1.1. Silicon wafers and epitaxially deposited films (see Section 3.7.3) fall in this category as do high-quality gemstones such as ruby and sapphire (Al2O3 with impurities that produce the characteristic colors) as well as diamond.

Often, materials are polycrystalline, which means the material is made of densely packed crystallites or grains of single crystal. Each grain has a more or less random orientation. The interface between crystallites is called a grain boundary. Each grain may be 10–10,000 nm in size. Metal films and Si films deposited at higher temperatures fall in this category, as do all metal objects that we encounter in daily life. Because each grain contains a large number of atoms, polycrystalline materials have basically the same properties as single crystalline materials. In particular, polycrystalline and crystalline silicon have qualitatively similar electronic properties.

An amorphous material has no atomic or molecular ordering to speak of. It may be thought of as a liquid with its molecules frozen in space. Thermally grown or deposited SiO2, silicon nitride, and Si deposited at low temperature fall in this category. At high temperatures, Si atoms have enough mobility to move and form crystallites on the substrate.

Carrier mobilities are lower in amorphous and polycrystalline Si than in single-crystalline Si. However, transistors of lower performance levels can be made of amorphous or polycrystalline Si, and are widely used in flat-panel computer monitors and other displays. They are called thin-film transistors or TFTs.

Sputtering

Sputtering is performed in a vacuum chamber. The source material, called the sputtering target, and the substrate holding the Si wafer form opposing parallel plates connected to a high-voltage power supply. During deposition, the chamber is

first evacuated of air and then a low-pressure amount of sputtering gas (typically Ar) is admitted into the chamber. Applying an interelectrode voltage ionizes the Ar gas and creates a plasma between the plates. The target is maintained at a negative potential relative to the substrate, and Ar ions are accelerated toward the sputtering target. The impacting Ar ions cause target atoms or molecules to be ejected from the target. The ejected atoms or molecules readily travel to the substrate, where they form the desired thin film. A simplified illustration of the sputtering process is shown in Fig. 3–17. A DC power supply can be used when depositing metals, but an RF supply is necessary when depositing insulating films. Sputtering may be combined with a chemical reaction in reactive sputtering. For example, when Ti is sputtered in a nitrogen-containing plasma, a TiN (titanium nitride) film is deposited on the Si wafer. Sputtering is the chief method of depositing Al and other metals. Sputtering is sometimes called a method of physical vapor deposition (PVD).

Chemical Vapor Deposition (CVD)

While sputtering is a relatively simple and satisfactory way of depositing thin film over flat surfaces, it is directional and cannot deposit uniform films on the vertical walls of holes or steps in the surface topography. This is called a step coverage problem. CVD, on the other hand, deposits a much more conformal film, which covers the vertical and horizontal surfaces with basically no difference in the film thickness.

In CVD, the thin film is formed from gas-phase components. Either a compound decomposes to form the thin film or a reaction between gas components takes place to form it. A schematic of the CVD process is shown in Fig. 3–18. The CVD process is routinely used to deposit films of SiO2, Si3N4 (a dielectric with excellent chemical and electrical stability), and polycrystalline silicon or poly-Si (see the sidebar “Three Kinds of Solid”).

These are some commonly used chemical reactors in the CVD deposition process:

• Poly-Si: SiH4 (silane) → Si + 2H2

• Si3N4: 3SiH2 Cl2 (dichlorosilane) + 4NH3 → Si3N4 + 6HCl + 6H2

• SiO2: SiH4 + O2 → SiO2 + 2H2 High-temperature

• SiO2: SiH2Cl2 + 2H2O → SiO2 + 2HCl + 2H2

A high-temperature oxide (HTO) is particularly conformal because the high deposition temperature promotes particle movement on the surface so that even sidewall coverage is excellent. Commonly used CVD processes include low-pressure chemical vapor deposition or LPCVD, and plasma-enhanced chemical vapor deposition or PECVD processes. Low pressure offers better thickness uniformity and lower gas consumption. A simple LPCVD deposition system is illustrated in Fig. 3–19a. In PECVD, the electrons in the plasma impart energy to the reaction gases, thereby enhancing the reactions and permitting lower deposition temperatures. Figure 3–19b shows the schematic of a PECVD reactor.

Dopant species can be introduced during the CVD deposition of Si. This doping process is called in situ doping and is a method of heavily doping the Si film.

Epitaxy

Epitaxy is a very special type of thin-film deposition technology. Whereas the deposition methods described in the preceding section yield either amorphous or polycrystalline films, epitaxy produces a crystalline layer over a crystalline substrate. The film is an extension of the underlying crystal. In a CVD reactor with special precautions to eliminate any trace of oxide at the substrate surface and at sufficiently high temperature, an arriving atom can move over the surface till it stops at a correct location to perfectly extend the lattice pattern of the substrate crystal. Figure 3–20a illustrates the epitaxy process. Selective epitaxy (Fig. 3–20b) is

a variation of the basic epitaxy technology and has interesting device applications. In selective epitaxy deposition, an etching gas is introduced to simultaneously etch away the material. The net deposition rate is positive, i.e., atoms are deposited, only over the single crystal substrate. There is no net deposition over the oxide mask because the deposition rate over the oxide is lower than the etching rate.

Epitaxy is useful when we want a lightly doped layer of crystal Si over a heavily doped substrate. Also, a different material may be epitaxially deposited over the substrate material as long as the film and the substrate have closely matched lattice constants. Epitaxially grown dissimilar materials are widely used in light-emitting diodes and diode lasers. The interface between two different semiconductors is called a heterojunction. An application example of selective hetero-junction epitaxial growth (of SiGe over Si).

Conclusion

From a few atomic layers to complex material stacks, thin-film deposition is the quiet architect behind every functional layer in modern chips. Whether it’s laying down insulators, gate oxides, or metallic conductors, this process determines not only electrical performance but also scalability, yield, and device reliability.

In an industry where a single nanometer can spell success or failure, the precision and innovation within deposition techniques like CVD, PVD, Sputtering, and epitaxy are nothing short of revolutionary.

The transistor may be the star, but these invisible layers are its stage.

Up Next: We’ve seen how chips are built from the ground up. Next, we’ll explore how billions of transistors are wired, packaged, tested, and qualified to become the processors, memory units, and smart sensors that drive the digital world.