Scaling Theory in VLSI Design

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Questions and Answers

How does the scaling factor (S) for full scaling typically range historically?

Between 1.5 and 2.0

Between 1.0 and 1.2

Between 0.8 and 1.0

Between 1.2 and 1.5 (correct)

What is the effect of scaling on the chip area for a circuit (A) according to the scaling theory?

Chip area remains unchanged

Chip area doubles with each scaling

Chip area triples with each scaling

Chip area scales by $1/S^2$ (correct)

What happens to the static power in the MOSFET when the scaling factor increases?

Static power decreases by 1/S

Static power remains constant

Static power increases

Static power decreases by S^2 (correct)

Which of the following describes the primary goal of constant-field scaling?

To maintain unchanged electric field Signup and view all the answers

Which parameter is not scaled in constant-field scaling?

Depth of Doping (xj) Signup and view all the answers

What physical size reduction is associated with the subsequent process in full scaling?

30% Signup and view all the answers

By what factor do power supplies and thresholds reduce in full scaling?

30% Signup and view all the answers

What is the effect of the scaling process on the overall power consumption of a MOSFET?

Power consumption reduces to S^2 Signup and view all the answers

What does Power Density measure?

The power consumed per unit area Signup and view all the answers

How does power scale in relation to area during scaling?

Power scales by 1/S^2 and area by 1/S^2 Signup and view all the answers

In Constant Voltage Scaling, what happens to the voltage levels?

They remain un-scaled Signup and view all the answers

What is the result of scaling when using Constant Voltage Scaling?

Power consumption increases because IDS also increases Signup and view all the answers

What can be a drawback of Constant Voltage Scaling?

It leads to unwanted increases in MOSFET characteristics Signup and view all the answers

What aspect of device characteristics does Constant-Field scaling address?

Power density remaining constant Signup and view all the answers

Why is power density considered an important quantity?

It directly affects the reliability of devices Signup and view all the answers

In the equation P = IDS * VDS, how does the instantaneous power behave in Constant-Voltage Scaling?

It scales linearly with the dimensional factor S Signup and view all the answers

What is one primary reason for scaling in VLSI designs?

Improve performance by allowing more complex systems Signup and view all the answers

According to the scaling predictions, how often does the transistor count double?

Every 2-3 years Signup and view all the answers

What was the transistor count of the Pentium 4 Processor in 2001?

42 Million transistors Signup and view all the answers

What is one effect of scaling down the size of transistors?

Reduced cost per transistor Signup and view all the answers

What is the primary goal of the lightly doped drain (LDD)?

Minimize electric field strength Signup and view all the answers

Which of the following was the first integrated circuit introduced?

First Planar Integrated Circuit Signup and view all the answers

Which of these years marks the introduction of the first single chip microprocessor?

1968 Signup and view all the answers

Which of the following is NOT a kind of limit in device scaling?

Biology Signup and view all the answers

What is one proposed solution to address poor electrostatics in MOSFETs?

Double Gate Signup and view all the answers

What major milestone did Intel achieve in 2006?

Ship the first billion transistor microprocessor Signup and view all the answers

What does scaling help to achieve regarding system size?

Reduce the size of the system Signup and view all the answers

What issue arises when the gate oxide thickness falls below 1.5-2.0 nm?

Tunneling through gate oxide Signup and view all the answers

What is a consequence of generating electron-hole pairs in the drain depletion region?

Device reliability issues Signup and view all the answers

Which solution aims to minimize the effects of gate leakage?

Metal Gate Signup and view all the answers

What is the intended effect of using high mobility channels in MOSFETs?

Enhance drive current Signup and view all the answers

Which problem is associated with poor channel transport in MOSFETs?

Decreased Ion Signup and view all the answers

What is the main consequence of constant voltage scaling on power density?

Power density increases significantly due to area scaling. Signup and view all the answers

Which scaling method tends to be a hybrid approach in practical scenarios?

Mixed Scaling Signup and view all the answers

How does constant voltage scaling impact the power consumption as devices' dimensions shrink?

It worsens performance with increased power consumption. Signup and view all the answers

What is the formula used for calculating power density during constant voltage scaling?

$PDensity = S^3 \cdot P$ Signup and view all the answers

Which characteristic typically follows a similar scaling trend to the voltage in a hybrid scaling approach?

Cox' Signup and view all the answers

Why is increased power density considered a negative aspect of constant voltage scaling?

It generates excess heat in a small area. Signup and view all the answers

What happens to the power consumption ($Power'$) under constant voltage scaling?

It increases directly with $S$. Signup and view all the answers

What main aspect of a device is directly impacted by scaling when using constant voltage?

Power Density Signup and view all the answers

What is the primary effect of using thinner gate dielectrics?

Increased power consumption Signup and view all the answers

What is the proposed solution to mitigate the issues with thinner gate dielectrics?

Increasing the dielectric constant Signup and view all the answers

Which material is provided as an example of a high-k dielectric?

HfO2 Signup and view all the answers

What does the equation $I_d = \frac{1}{2} W \mu C_{ox} (V_{GS} - V_T)^2 / L$ describe?

The drain current in a MOSFET Signup and view all the answers

What impact does increasing the dielectric constant of the gate oxide have on leakage current?

Decreases leakage current Signup and view all the answers

What does 'effective thickness' refer to in the context of gate dielectrics?

Equivalent SiO2 thickness Signup and view all the answers

What is the relationship between the strain in silicon and mobility?

Strain enhances mobility Signup and view all the answers

In the context of the equation $C_{ox} = \frac{\epsilon_{ox}}{t_{ox}}$, which variable represents the dielectric constant?

\epsilon_{ox} Signup and view all the answers

Study Notes

Scaling Theory

Scaling involves moving very large-scale integrated (VLSI) designs to new fabrication methods, aiming to shrink circuitry size.
Examples include a 1961 integrated circuit with two transistors, a 2001 Pentium 4 processor containing 42 million transistors, and a 2006 Itanium 2 dual processor with 1.7 billion transistors.

Why Scale?

Improved performance: More complex systems are achievable.
Increased transistor density: Reduces cost per transistor and system size.
Reduced power: Smaller transistors require less supply voltage.

Scaling Predictions

Gordon Moore predicted exponential growth in the number of transistors per integrated circuit (IC).
He predicted a doubling of transistors every 2-3 years.
In 1975, he predicted more than 65,000 transistors in an IC.

Timeline of Major Events

1947: First transistor (Bell Labs).
1958: First integrated circuit (Noyce/Fairchild & Kilby/Texas Instruments).
1968: Noyce and Moore formed Intel.
1971: Intel introduced the 4004.
2006: Intel shipped its first billion-transistor processor.

How Much Can We Shrink?

Chip area (A) scales proportionally to 1/S², or (1/S)(1/S).

Full Scaling (Constant-Field)

The principle involves scaling both horizontal and vertical device dimensions by the same factor (s > 1) while keeping the electric field unchanged.
Scaling process elements by 30% is commonly used.
Power supplies and voltage thresholds should be adjusted by 30% for full scaling.
A scaling factor (S) between 1.2 and 1.5 has been historically common.

Scaling Effect on Device Characteristics

Power: Static power (P) in MOSFETs is defined as P = I_DS V_DS. Both I_DS and V_DS scale with 1/S in full scaling, resulting in P scaling with 1/S².
Power Density: Power density is power per area. It scales as 1/S² and remains constant, which can lead to heat issues as chips get larger with scaling.

Constant Voltage Scaling

This scaling method only changes device dimensions and not voltages.
Power scales as S, which is not desirable.
Power density scales as S³, aggravating heat generation and thus limiting performance and scalability.

Scaling Choices

Full scaling is favorable but sometimes impractical.
Constant-voltage scaling can worsen performance.
A hybrid approach combining both methodologies is often used.

Limits of Scaling

Thermodynamics: Doping concentration in source and drain
Physics: Tunneling through gate oxide
Statistics: Statistical fluctuations in body doping
Economics: Factory cost/ cost-effectiveness

About Gate Oxide Scaling

Gate oxide thickness decreases in proportion to channel length to maintain control over the channel.
For channel lengths below 100nm, the oxide thickness becomes thinner, limiting the number of atomic layers and reaching fundamental limits. Typical structures show that oxide thickness for 90nm Intel devices was ~1.2nm.

Tunneling through Gate Oxide

Oxide thickness limits occur as current density (I_gate) is proportional to 1/T_ox³, and below 1.5-2.0 nm, SiO2 or Si-O-N is no longer suitable.

New Materials and Structures for Advanced MOSFETs

Problems with using current CMOS methods:
- poor electrostatics
- poor channel transport
- S/D parasitic resistance
- gate leakage increased
- gate depletion increased end-of-transistor effect
Solutions:
- Double Gate and FinFET
- Advanced materials such as high dielectric constant (high-k) dielectrics
- lightly doped drains (LDD)

Gate Leakage

Conventional gate dielectrics (SiO₂) experience increased leakage with scaling.
Alternative gate oxides, like HfO₂, are needed since the thickness of SiO₂ approaches a theoretical limit with further scaling.

High Mobility Channel:Strained Silicon

Strained silicon enhances carrier mobility due to changes in the energy structure.

Strained Silicon

Growing strained silicon layers over relaxed SiGe layers (which has a different crystal structure and lattice constant) can be used to control the lattice structure and thus carrier mobility.

Typical device structure

Strained Si is one way to boost the mobility and thus current in silicon devices.
The structure involves strained silicon over relaxed SiGe, to increase device scalability while reducing gate leakage and end-of-transistor and other critical effects.

Mobility Enhancements

Strained Si increases NMOS and PMOS Current by ~ 1.5x, but only ~1.15x for PMOS.
Methods like this are important to scale further.

Double-Gate MOSFET Structures

Different architectures, such as vertical and planar, exist.
Designs like finFETs and double-gate architectures provide more control and scalability.

Advanced MOSFET Structures

Configurations like FinFETs and double-gate MOSFETs are examples of advanced structures.
SOI (Silicon on Insulator) structures

Multigate MOSFET

Advanced multi-gate MOSFET architectures improve voltage and current control.

Short Channel Effects

Adverse effects like threshold voltage reduction and increased leakage current are due to short channel lengths
Types: Vt roll-off (Reverse Short Channel Effect, RSCE), Drain-Induced Barrier Lowering (DIBL), Narrow-Width Effect (Inverse-Narrow-Width Effect), Channel Length Modulation, Punchthrough Effect, Breakdown, Velocity Saturation and Overshoot.

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Description

Explore the principles of scaling in Very Large Scale Integration (VLSI) designs. This quiz covers the importance of scaling, key predictions by Gordon Moore, and significant milestones in the development of integrated circuits. Test your knowledge on how scaling affects performance, transistor density, and power consumption.