For those of us in the semiconductor industry, the periodic table is more than just a poster on a science classroom wall: it’s a valuable resource that we use to create new logic and memory devices. Recognizing its importance, we join others around the world in celebrating the 150th anniversary of the periodic table.
The United Nations General Assembly and UNESCO, the United Nations Educational, Scientific and Cultural Organization, declared 2019 the International Year of the Periodic Table of Chemical Elements, celebrating the table’s development by Dmitri Mendeleev in 1869. Their proclamation describes the table as “one of the most significant achievements in science, capturing the essence of not only chemistry, but also physics, medicine, earth sciences and biology.”
Chemistry made great strides in 17th and 18th centuries, but by the 19th it had lost some of its momentum. It lacked a unifying theory. A chemist might use various procedures to combine or decompose substances into something new, but there was little understanding of the underlying principles. And though an element was conceived to be a fundamental, irreducible component, there was no understanding of what made it so, or how many elements there might be. Mendeleev’s genius was to see a pattern in the jumble. When he arranged the known elements by increasing atomic weight, he saw that their chemical properties repeated periodically. When he lined them up in a table, starting new rows to align elements with similar properties in the same column, he created the first periodic table.
About 50 years after Mendeleev’s table, in 1913, Niels Bohr proposed a model of the atom. His model described that electrons can only be located in prescribed orbits, or shells, around the nucleus. It is the number of electrons in the outermost shell, called the valence electrons, that largely determine an element’s chemical properties. The periodic columns defined by Mendeleev turn out to be the exact same columns defined by the number of valence electrons.
Valence electrons determine the electrical conductivity of an element; as a result, an element may be classified as a metal, a nonmetal, or a metalloid. Metals represent the largest group; these elements are typically good conductors of heat and electricity. In contrast, nonmetals, also called insulators or dielectrics, are poor conductors of heat and electricity. Metalloids, or semiconductors, as they are often known, are the smallest group – just seven elements. Lying between metals and non-metals, they usually conduct heat and electricity, though not as well as metals. Silicon, the fundamental material at the heart of chips, is the most well-known metalloid. Elements from neighboring columns (such as boron, phosphorus, and arsenic) are common dopants – key to making semiconductors conduct an electric current.
The number of elements used in semiconductor manufacturing has grown dramatically as the technology has evolved. The first semiconductor devices were made of germanium, but germanium was quickly supplanted by silicon and high-volume production of integrated circuits really took off. (Germanium later rose again in popularity in SiGe IC devices.) In the 1990s aluminum, used in early integrated circuits to carry electrical signals around the chip, was replaced by copper, for its higher conductivity. At about the same time, refractory metals (such as tungsten and tantalum) were added, and reactive gases (such as bromine and hydrogen) were introduced for plasma etching. The 2000s saw great activity, especially in the transition metals; nearly every element in the lanthanide series has been evaluated. During that decade, new dielectrics containing exotic components like hafnium and strontium were introduced. Today, more elements continue to be under evaluation, as part of new compounds or for new applications. Ultrathin MgO is the tunneling barrier used in magnetic RAM (MRAM) devices. Both sulfur and selenium are alloyed with metals such as tungsten, molybdenum, and titanium in transition metal dichalcogenides which are a new family of 2D nanosheets that could replace Si as the channel in future transistors. No doubt, more elements will be adopted as scientists search the entire table for new ways to improve manufacturing processes and device performance.