Part 6, Giving Semiconductors Electrical Properties: The Deposition and Ion Implantation Processes

Semiconductor chips consist of many microscopic layers smaller than a fingernail and as thin as a sheet of paper. Semiconductors are stacked high and solid to form a complex structure similar to a high-rise building. To form this structure, photolithography—which involves coating the top of a monocrystalline silicon (Si; single- crystal silicon, the raw material for semiconductors) wafer with a thin film in stages and drawing circuits, etching—selectively removing unnecessary materials, and cleaning steps are repeated multiple times. After the etching and cleaning processes, a thin film divides, connects, and protects the circuits. Now, we will examine the deposition process for making thin films and the series of processes for giving semiconductors electrical properties.

Deposition Process for Thinly Coating Wafers

The term “thin film” refers to a film that is 1 micrometer or thinner (µm, one-millionth of a meter); this thickness cannot be achieved with simple mechanical processing. Deposition refers to the series of processes for coating a thin film at a desired molecular or atomic level onto a wafer. Because the coating is so thin, precise, and accurate technology is required to uniformly apply the thin film onto a wafer.

▲Structure of a deposited semiconductor

Deposition can largely be divided into two types. The two types are physical vapor deposition (PVD) and chemical vapor deposition (CVD). PVD is used primarily to deposit metal films and is not accompanied by chemical reaction. Meanwhile, CVD involves applying external energy to a vapor of particles formed by a chemical reaction of a gas. The vapor is ejected toward a surface for deposition. This technique can be used to deposit films onto conductors, insulators, and semiconductors alike. CVD is the most widely used method of deposition in current semiconductor processes. CVD can be further divided into thermal CVD, plasma CVD, and photo-induced CVD, depending on the external energy source used. Of these, plasma CVD is the most widely used, for its ability to form films at low temperature, regulate uniformity of film thickness, and process high volumes. The thin film formed through the deposition process has two layers: a metal (conductive) layer which connects the electrical signals among the circuits, and an insulating layer which electrically isolates the internal connection layers or keeps out contaminants.

Ion implantation: Turning the wafer into a semiconductor

A process is still required to give the semiconductor electrical characteristics. A semiconductor has both the characteristics of a conductor and an insulator, and ion implantation is the process that essentially turns a silicon wafer into a semiconductor. Pure silicon is an insulator and does not conduct electricity, but adding impurities impart conductive properties and make it able to conduct current. The impurities are called ions. These ions are turned into fine gaseous particles, then implanted into the front face of the wafer to the desired depth. The impurities used come from Group 15 (phosphorus, P and arsenic, AS) or Group 13 (boron, B) on the periodic table. Implanting a Group 15 element gives an n-type semiconductor, while implanting a Group 13 element gives a p-type semiconductor. The deposition process is crucial, in that how thin and uniform the thin film deposited is makes or breaks semiconductor quality. Future semiconductor circuit structures will come in at thicknesses several million times thinner than a human hair. To give these circuits electrical properties, even more advanced deposition technologies creating thinner and more uniform films will be necessary. Next time we’ll learn about metal interconnects, which is designed to link the elements created through the oxidation, photolithography, etching, and deposition processes and make them function as a circuit.