Silicide

Key takeaways

Silicide belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Silicide to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Silicide from memory before moving on to harder problems.

Reference excerpt

A silicide is a type of chemical compound that combines silicon and a usually more electropositive element. Silicon is more electropositive than carbon. In terms of their physical properties, silicides are structurally closer to borides than to carbides. Because of size differences however silicides are not isostructural with borides and carbides. Bonds in silicides range from conductive metal-like structures to covalent or ionic. Silicides of all non-transition metals have been described except beryllium. Silicides are used in interconnects.

Structure Silicon atoms in silicides can have many possible organizations:

Isolated silicon atoms: electrically conductive (or semiconductive) CrSi, MnSi, FeSi, CoSi, Cu5Si, (V,Cr,Mn)3Si, Fe3Si, Mn3Si, Mg2(Si,Ge,Sn,Pb), (Ca,Ru,Ce,Rh,Ir,Ni)2Si Si2 pairs: U3Si2, hafnium and thorium silicides Si4 tetrahedra: KSi, RbSi, CsSi Sin chains: USi, (Ti, Zr, Hf, Th, Ce, Pu)Si, CaSi, SrSi, YSi Planar hexagonal graphite-like Si layers: β-USi2, silicides of other lanthanoids and actinoids Corrugated hexagonal Si layers: CaSi2 Open three-dimensional Si skeletons: SrSi2, ThSi2, α-USi2

Preparation and reactivity Most silicides are produced by direct combination of the elements. However, the process is extremely exothermic and control of the reaction poor. In rare cases, a metal oxide can undergo silicobaric reduction, in which pure silicon deoxygenates the metal to a silicon oxide, which is then removed by a vacuum. Alternatively, molten aluminum or copper are adequate solvents for the alloying. A silicide prepared by a self-aligned process is called a salicide. This is a process in which silicide contacts are formed only in those areas in which deposited metal (which after annealing becomes a metal component of the silicide) is in direct contact with silicon, hence, the process is self-aligned. It is commonly implemented in MOS/CMOS processes for ohmic contacts of the source, drain, and poly-Si gate.

Alkali and alkaline earth metals Group 1 and 2 silicides e.g. Na2Si and Ca2Si react with water, yielding hydrogen and/or silanes. At Consumer Electronics Show (CES) 2012 a safe and eco-friendly 1kWh or 3kWh capacity mobile phone charger with sodium silicide that runs on water has introduced for 'people who spend time away from the electricity grid'. Any type of water can be used, including salt water and it can even run on puddle water providing it isn't thickened with mud or any other sediment. Magnesium silicide reacts with hydrochloric acid to give silane:

Mg2Si + 4 HCl → SiH4 + 2 MgCl2 Group 1 silicides are even more reactive. For example, sodium silicide, Na2Si, reacts rapidly with water to yield sodium silicate, Na2SiO3, and hydrogen gas. Rubidium silicide is pyrophoric, igniting in contact with air.

Transition metals and other elements The transition metal silicides are usually inert to aqueous solutions. At red heat, they react with potassium hydroxide, fluorine, and chlorine. Mercury, thallium, bismuth, and lead are immiscible with liquid silicon.

Applications Silicide thin films have applications in microelectronics due to their high electrical conductivity, thermal stability, corrosion resistance, and compatibility with photolithographic wafer processes. For example silicides formed over layers of polysilicon, called polycides, are commonly used as an interconnect material in integrated circuits for their high conductivity. Silicides formed through the salicide process also see use as a low work function metal in ohmic and Schottky contacts. High work function metals are often not ideal for use in metal–semiconductor junctions directly due to fermi–level pinning where the Schottky barrier potential of the junction becomes locked around 0.7–0.8V. For this reason low forward-voltage Schottky diodes and ohmic interconnects between a semiconductor and a metal often utilize a thin layer of silicide at the metal–semiconductor interface.

List (incomplete) Nickel silicide, NiSi Sodium silicide, NaSi Magnesium silicide, Mg2Si Platinum silicide, PtSi (platinum is actually more electronegative than silicon) Titanium silicide, TiSi2 Tungsten silicide, WSi2 Molybdenum disilicide, MoSi2 Neptunium silicide, NpSi2

Illustrations

Worked examples

Example 1 — a first encounter with Silicide

Start with the simplest possible case. Write down what Silicide claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Silicide before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Silicide ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Silicide

In research: Silicide appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Silicide in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Silicide is common in secondary-school and first-year university syllabi. It links to neighbouring topics Anions, Silicides, so understanding it makes those chapters shorter.
In everyday life: Look for Silicide outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Silicide in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Silicide means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Silicide out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Silicide in simple terms?

A silicide is a type of chemical compound that combines silicon and a usually more electropositive element. Silicon is more electropositive than carbon.

Why does Silicide matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Silicide?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Silicide.