[Leapfrog: First Look]
Transistor Recovers From Midlife Crisis With Fundamental Material Changes
Not since Neil Armstrong took his first steps on the moon has the transistor seen such a dramatic change, and that change holds some big promises.
WHAT'S ALL THIS HIGH-K STUFF, ANYHOW? Gate leakage current, source-to-drain leakage current, and increased heat dissipation have plagued the industry as transistors have scaled down in size. In 2004, Intel found that leakage power accounted for 25% of total chip power, and this number continues to increase with each new process shrinkage.
"As more and more transistors are packed onto a single piece of silicon, the industry continues to research current leakage reduction solutions," said Mark Bohr, Intel senior fellow. "Meanwhile our engineers and designers have achieved a remarkable accomplishment that ensures the leadership of Intel products and innovation. Our implementation of novel high-k and metal gate transistors for our 45-nm process technology will help Intel deliver even faster, more energy-efficient multicore products that build upon our successful Intel Core 2 and Xeon family of processors and extend Moore's Law well into the next decade."
Gate leakage occurs when electrons leak across the very dielectric barrier intended to keep them in place. Source-to-drain leakage takes place when the transistor is supposed to be in the "off" state, yet current still flows from source to drain as it would in the "on" state.
The term "high-k" implies that the material maintains a high dielectric constant (with respect to SiO2) that creates a high field effect between the gate and the channel. Also, it features great electronic insulation properties. The "k" refers to the material's ability to hold electric charge.
When evaluating the performance of two dielectric materials (hafnium and SiO2 in this case), the relative physical thickness required to produce the same gate capacitance can be compared, providing an idea of relative "electrical thickness." This is commonly known as the equivalent oxide thickness (EOT). An EOT of 1 nm would result from using a 10-nm thick dielectric featuring a k value of 39. Comparably, the k value of SiO2 is 3.9.
Metal gates also help increase the gate field effect. Together, the HK+MG stackup helps to significantly reduce leakage while increasing the gate capacitance and drive current. For its 45-nm technology, Intel deposits the hafnium-based dielectric using an approach that deposits one atomic layer at a time.
Known as Atomic Layer Deposition (ALD), this process provides exquisite control of the dielectric thickness. Intel claims the overall cost of its 45-nm process is in line with previous-generation process-based manufacturing trends.
The "secret sauce" that makes these HK+MG transistors unique is the combination of the hafnium's thickness, the type of hafnium, and the metals used for the gate electrodes. The combination determines the net benefits gained and dictates the reduction in leakage currents (gate and source to drain), the transistor density increase, and the switching speed increase.
None of the companies have disclosed any part of their respective combinations other than to point out that hafnium is being used to instantiate the high-k part of the equation. The only apparent certainty is Intel's feeling that it has the best "secret sauce," which will at least help keep the company one generation ahead of its closest rival and may even increase that gap.
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