engineering

Strained silicon

Strained silicon is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Strained silicon rather than just read about it. In short: Strained silicon is a layer of silicon in which the silicon atoms are stretched beyond their normal interatomic distance. This can be accomplished by putting the layer of silicon over a substrate of silicon–germanium (SiGe).

Key takeaways

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

Reference excerpt

Strained silicon is a layer of silicon in which the silicon atoms are stretched beyond their normal interatomic distance. This can be accomplished by putting the layer of silicon over a substrate of silicon–germanium (SiGe). As the atoms in the silicon layer align with the atoms of the underlying silicon germanium layer (which are arranged a little further apart, with respect to those of a bulk silicon crystal), the links between the silicon atoms become stretched, thereby leading to strained silicon. Moving these silicon atoms further apart reduces the atomic forces that interfere with the movement of electrons through the transistors and thus improved mobility, resulting in better chip performance and lower energy consumption. These electrons can move 70% faster allowing strained silicon transistors to switch 35% faster. More recent advances include deposition of strained silicon using metalorganic vapor-phase epitaxy (MOVPE) with metalorganics as starting sources, e.g. silicon sources (silane and dichlorosilane) and germanium sources (germane, germanium tetrachloride, and isobutylgermane). More recent methods of inducing strain include doping the source and drain with lattice mismatched atoms such as germanium and carbon. Germanium doping of up to 20% in the P-channel MOSFET source and drain causes uniaxial compressive strain in the channel, increasing hole mobility. Carbon doping as low as 0.25% in the N-channel MOSFET source and drain causes uniaxial tensile strain in the channel, increasing electron mobility. Covering the NMOS transistor with a highly stressed silicon nitride layer is another way to create uniaxial tensile strain. As opposed to wafer-level methods of inducing strain on the channel layer prior to MOSFET fabrication, the aforementioned methods use strain induced during the MOSFET fabrication itself to alter the carrier mobility in the transistor channel.

History The idea of using germanium to strain silicon for the purpose of improving field-effect transistors appears to go back at least as far as 1991. In 2000, an MIT report investigated theoretical and experimental hole mobility in SiGe heterostructure-based PMOS devices. In 2003, IBM was reported to be among primary proponents of the technology. In 2002, Intel had featured strained silicon technology in its 90nm x86 Pentium microprocessors series in early 2000. In 2005, Intel was sued by AmberWave company for alleged patent infringement related to strained silicon technology.

See also Strain engineering Hall effect Piezo effect

References

Further reading Development of New Germanium Precursors for SiGe Epitaxy; Presentation at 210th ECS Meeting (SiGe Symposium), Cancun, Mexico, October 29, 2006. Shenai, Deo V.; Dicarlo, Ronald L.; Power, Michael B.; Amamchyan, Artashes; Goyette, Randall J.; Woelk, Egbert (2007). "Safer alternative liquid germanium precursors for relaxed graded SiGe layers and strained silicon by MOVPE". Journal of Crystal Growth. 298: 172–175. Bibcode:2007JCrGr.298..172S. doi:10.1016/j.jcrysgro.2006.10.194.

Illustrations

Strained silicon: Strained silicon — Strained silicon

Worked examples

Example 1 — a first encounter with Strained silicon

Start with the simplest possible case. Write down what Strained silicon claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, 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 Strained silicon 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 Strained silicon 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 Strained silicon

In research: Strained silicon appears in engineering 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 Strained silicon 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: Strained silicon is common in secondary-school and first-year university syllabi. It links to neighbouring topics Germanium, Group IV semiconductors, Semiconductor material types, so understanding it makes those chapters shorter.
In everyday life: Look for Strained silicon 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.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Strained silicon in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Strained silicon 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 Strained silicon out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Strained silicon in simple terms?

Why does Strained silicon matter?

Because it connects several engineering 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 Strained silicon?

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 Strained silicon.