As scaling progressed, the conventional SiO₂ gate dielectric reached its limits. Making the SiO₂ film thinner strengthens the gate field, but beyond a certain point the following problems arise:

● Tunneling current (leakage current) increases sharply

● Power consumption rises

● Transistor reliability deteriorates

The solution introduced to overcome these problems was the High-k (high-dielectric-constant material) + Metal Gate structure = HKMG (High-k Metal Gate).

It can be called the revolutionary technology that made the scaling generations of 45nm → 32nm → 22nm possible.

(1) The Limits of Conventional SiO₂

SiO₂ was an excellent gate dielectric, but once its thickness falls below about 1nm, electrons begin to pass straight through it because of quantum tunneling.

→ Gate leakage current increases, causing defects and heat generation.

 

(2) The Adoption of High-k Materials (such as HfO₂)

Because High-k materials have a high dielectric constant, they keep the physical film thick while behaving electrically like a thin film.

In other words, they offer the following advantages:

● Leakage current can be reduced dramatically

● Devices can be scaled down while maintaining electrical performance

 

HfO₂ (hafnium oxide) in particular has become the industry standard, thanks to its suitably high dielectric constant and good compatibility with silicon.

Once High-k materials were adopted, the conventional polysilicon gate ran into the following problems:

● Polysilicon introduces impurities into the High-k layer, degrading performance

● Gate resistance increases

● Adjusting the work function becomes difficult

 

For these reasons, metal gates were introduced.

Advantages of metal gates:

● The work function can be chosen freely, making it easier to optimize nMOS and pMOS separately

● Lower resistance enables higher speed

● Good compatibility with High-k materials

 

Today, the common approach is to adjust the work function by stacking multiple metals, centered on TiN (titanium nitride).

HKMG was a game changer for scaling, delivering the following benefits.

(1) A Dramatic Reduction in Leakage Current

Because High-k allows a thicker physical film, leakage is reduced to 1/10 to 1/100 of that of conventional SiO₂.

→ A major contribution to lower power consumption.

 

(2) Improved Transistor Switching Characteristics

Metal gates have lower resistance than polysilicon. As a result:

● Gate RC delay is reduced

● Switching speed improves

● They are advantageous for high-frequency applications

 

(3) Continued Scaling (Extending Moore’s Law)

Without HKMG, scaling beyond 45nm would not have been possible. HKMG can be considered the foundational technology for scaling all the way to today’s FinFET and GAA era.

HKMG faces the following challenges:

● Maintaining interface quality between High-k and Si

● Tuning the work function of the metal gate

● Optimizing thermal processing (annealing)

● Forming high-compression, low-stress films

At advanced nodes, the Gate Last approach has become mainstream: performance is secured by moving metal gate formation to a later step in the process.

At leading-edge nodes, HKMG continues to evolve in the following directions:

● Multilayer high-k dielectric stacks (e.g., HfO₂ + Al₂O₃)

● Technologies to reduce random telegraph noise (RTN)

● Alloying for fine-tuning the work function

● Design optimization for GAA

● Gate structures capable of handling high currents for AI chips

Combined with the GAA structure, HKMG has become a core technology supporting efforts to break through the limits of scaling.

● High-k materials were introduced to overcome the limits of SiO₂

● They dramatically reduce leakage current

● Metal gates optimize electrical characteristics and the work function

● Without HKMG, the performance of the FinFET era would not have been achievable

● HKMG remains an essential structure in the GAA generation and will continue to evolve

1.Explain why High-k materials became necessary.

2.Why do problems arise when a polysilicon gate is combined with a High-k material?

3.What are the advantages of a metal gate?

 

Column Supervisor: Koji Kakumoto (President, OTIS Co., Ltd.) After studying languages abroad and working in planning and development at a trading company, he joined OTIS Co., Ltd. in 2011. Working mainly in the Corporate Planning Department while also overseeing the manufacturing and engineering divisions, he has served as President since 2018, driving business growth and strengthening the organization.

 

This article is a general technical explanation intended for educational purposes and does not refer to any specific company, product, or technology.