Single page Print

Intel's Pentium 4 Prescott processor

Plus Northwood's last ride

INTEL IS LAUNCHING A BEVY of new chips today, including new speed grades of the current Pentium 4 and Pentium 4 Extreme Edition processors clocked at 3.4GHz. The biggest news, though, is the new processor core, code-named Prescott. Prescott isn't just a die shrink, though it is that. Prescott is also a major reworking of the Pentium 4's microarchitecture—major enough that I'm surprised Intel didn't opt to call this processor the Pentium 5.

Prescott clock speeds will initially range from 2.8GHz to 3.4GHz. To keep Prescott-based P4s distinct from older "Northwood" cores, Intel is tacking an "E" on to the product names, so they'll be called the Pentium 4 2.8E or 3.2E. The product mix gets most confusing at 2.8GHz, where one could buy four different Pentium 4s: the 2.8GHz (a Northwood core with a 533MHz front-side bus), the 2.8C (Northwood again, but with an 800MHz bus), the 2.8A (Prescott with a 533MHz bus), or the 2.8E (Prescott with 800MHz bus). Clear as mud?

We tested, well, lots of chips against Prescott and the new P4 3.4GHz processors

Anatomy of a die shrink

Anatomy of Intel's 90nm process tech (Source: Intel)

Let's start with a look at the gory details of the die shrink. With Prescott, Intel is moving the Pentium 4 from a 130nm (or 0.13 micron) fabrication process to a 90nm process. As always, such a transition brings immediate benefits in the form of smaller die sizes and, usually, higher potential clock speeds. The conversion to 90nm is far from trivial, though, and Intel has enhanced its manufacturing process in a number of ways in order to facilitate the change.

One of the most notable changes is the use of a strained silicon substrate. When stretched slightly, the lattice structure of silicon atoms spreads out and opens up, allowing for freer flow of electrons. This lower resistance, in turn, allows for smaller gate lengths and faster transistors. Intel claims here that its new process only adds two percent to manufacturing costs, which is remarkable given the use of strained silicon.

Intel's 90nm process replaces the fluorine-doped silicon oxide dielectric film used previously with an even lower capacitance carbon-doped oxide film. This process also employs a layer of nickel silicide, essentially as caps on the transistors, to lower resistance versus the cobalt silicide used in Intel's 130nm process. The result of these changes is gate lengths as small as 50nm. SRAM cells are down from 2 square microns to 1.15.

Not only is the 90nm process smaller, but Intel is also manufacturing Prescott using seven layers of copper interconnects, instead of the six used at 130nm. All told, the changes shrink the Pentium 4's die size to 122 mm2, from 145 mm2 for Northwood—this despite the fact Prescott's transistor count is 125 million, over twice Northwood's 55 million transistors.

The Prescott die (Source: Intel)