The breakthrough that has enabled Intel to become the first manufacturer to build a 45nm part is developing High-K Metal Gate transistors. This was necessary to ensure that Moore's law could continue, which states that transistor counts on mainstream CPUs would double every two years.
As Intel's own presentation slide has it, "High-k + metal gate transistors are the biggest advancement in transistor technology since the introduction of polysilicon gate MOS transistors in the late 1960s".
The problem is one of current leakage. Previously up to 65nm technology, Intel has used a silicon dioxide (Si02) gate dielectric inside its transistors. What is a gate dielectric? A dielectric is an insulator, a material that does not transmit electricity, which is necessary in a transistor to be able to turn the current on and off, which is what enables it to function as an electronic switch.
Silicon dioxide has traditionally been used inside processors, but as the manufacturing process hit 65nm, the gate diecletric was a mere five atoms thick, any thinner and it would start causing significant current leakage. This would mean than the transistors would not be able to achieve a fully off state, so there would be little difference between idle state and full power state, making for a very inefficient design. It's also pretty stonking difficult to make a wall less than five atoms thick. (Trust me, I've tried).
To get round these problems, Intel has replaced the silicon dioxide used in the gate dielectric with a thicker hafnium-based high-k material. ‘High-k' refers to a dielectric material with a high dielectric constant,ie. one that insulates consistently and effectively. Intel claims that this reduces the current leakage by more than ten times over the silicon dioxide that was used in the past. This means that you can start increasing the transistor count and keep the processor small and power efficient.
Intel hasn't revealed what materials it has using to make this high-k material, presumably in a bid to delay its competitors in catching up. Just to keep things moving along at a pace, Intel already has very early SRAM 32nm parts up an running, though as already mentioned, we won't actually see a processor based on this until 2009.