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alted | 3 years ago

> the benefits are not as great as I think

It's this one.

You're probably thinking of an electrical insulator solely as a material with low conductivity. Vacuum, air, glass (SiO2) (which is the default insulator in chip manufacturing), and many other insulators all have such negligibly small conductivity it doesn't matter here.

But all insulators have a second relevant property: their permittivity (quantified by a number called the material's "dielectric constant". When this is relevant, people often call the insulator a "dielectric"). When an insulator is between two conductors at different voltages, it forms a capacitor. In wiring this is typically undesired because the capacitor takes energy whenever the conductor voltages change.

In fact, the capacitance of the gate insulator in transistors is what causes most heat dissipation in CPUs! (Which, of course, is a big limit to scaling transistor density right now.) Unfortunately, this is fundamental to how transistors work.

Anyway, for wiring you want the capacitance formed by the insulator to be as small as possible, which you do by choosing a material with the smallest dielectric constant. The dielectric constant of SiO2 glass is about 4 times greater than both air and vacuum, which are about equally good.

But keeping a vacuum in a sealed area on a chip is occasionally used for MEMS devices like accelerometers, gyroscopes, and resonators, which would be slowed down mechanically by air pressure.

discuss

order

kurthr|3 years ago

Even those MEMS devices (like sealed poly and Bosch etched bulk resonators) don't use a full vacuum. It's inconvenient and difficult to control. Instead a controlled amount of non-reactive gas is included to give repeatable results (eg damping). Having a known dielectric even close to 1 is fine and atmospheric pressure effects are negligible (eg <0.1%).

In the case of electrical interconnects I'd be surprised, if they weren't sealed with an atmospheric pressure inert gas (N2, CO2, Ar) just to keep moisture out or at least drying agent. Moisture (and dog forbid, condensation) is more likely to cause catastrophic effects than dry air. You could easily go from 1.0005 to 10... and that's ignoring conductivity and dielectric loss.

tarlinian|3 years ago

To be clear, you want to decrease the permittivity that adds capacitance where the accumulation of charge serves no purpose (i.e., in the interconnects) and increase permittivity where we want to have a lot of charge (i.e., on the actual transistor gates so we can produce a large enough electric field to switch the transistor), which is why gate dielectrics use high-k materials like HfO2 and interconnect layers use low-k dielectrics like carbon-doped oxides.)