I was studying the Pentium processor's die and I noticed...

I was studying the Pentium processor's die and I noticed strange connections that I didn't understand. These turned out to be "antenna diodes", special diodes that protect the chip from damage during manufacturing. Let's take a look... 1/6

A closeup of the Pentium die showing transistors and metal wiring. Arrows indicate two places where the wiring is connected to the silicon substrate.

Like 23 November at 20:30 | Open on oldbytes.space

13 comments

Ken Shirriff

The problem is that the chip manufacturing process uses plasma to etch away unwanted metal to produce wiring. However, long wires on the chip can pick up electrical charge from the plasma. This charge can damage the chip's transistors. In particular, each transistor has a thin layer of "gate oxide", just a few hundred atoms thick, that can easily be damaged. 2/6

This diagram shows the structure of a MOSFET transistor. It has three connections: source, gate, and drain. The source and drain are embedded in the silicon. However, the gate is separated from the silicon by an extremely thin layer of insulating oxide.

23 November at 20:34 | Open on oldbytes.space

Ken Shirriff

The solution is to add extra diodes to the circuitry, special devices that will drain away the dangerous charge.
These diodes are relatively rare, only needed when long wires on the chip are connected to a transistor gate on one end and the other end isn't connected yet. The long wire acts (metaphorically) as an antenna, so the protective diode is called an antenna diode. 3/6

A closeup of the silicon surface showing some transistors consisting of doped silicon regions and polysilicon lines. This photo has PMOS transistors at the top and NMOS transistors at the bottom. Several square regions of doped silicon form antenna diodes. I removed the metal layers of the chip for this photo.

23 November at 20:40 | Open on oldbytes.space

Ken Shirriff

This closeup photo shows the three layers of wiring on the Pentium and the transistors underneath. The L-shaped hook towards the lower left is a connection to an antenna diode. 4/6

A closeup of the Pentium die showing wiring and circuitry. The top metal lines run vertically with a golden color. Underneath, the middle metal lines run horizontally. At the silicon surface, the bottom metal wiring connects the transistors to form logic gates. The doped silicon regions and polysilicon wires are visible as grayish lines on the gray silicon background. Towards the lower left, an L-shaped metal wire is connected to an antenna diode, a dark gray square.

23 November at 20:44 | Open on oldbytes.space

Ken Shirriff

Here's a photo of the Pentium die with main functional blocks labeled. In this photo, the three layers of metal wiring almost completely hide the underlying silicon. 5/6

A die photo of the Pentium processor. It consists of a complex pattern of golden and reddish rectangular regions. I've labeled the main functional blocks: the 8K code cache and 8K data cache on the left. The main integer execution unit is in the middle with the floating point unit to the right. Instruction fetching and decoding is at the top.

23 November at 20:53 | Open on oldbytes.space

Ken Shirriff

For more on antenna diodes in the Pentium, see my latest blog post:
https://www.righto.com/2024/11/antenna-diodes-in-pentium-processor.html

23 November at 20:53 | Open on oldbytes.space

Tuckers Nuts Resist😈!

@kenshirriff
🥥 Fascinating stuff, Ken.
This has the feel of archeology at a microscopic level. The photos have the feel of pre-Columbian art in Mexico/South America. 🥥

23 November at 22:45 | Open on mstdn.social

nils

@kenshirriff How is it that old saying goes? Something along the lines of "There are two kinds of electronics engineers, those that build antennas on purpose and those that build them on accident."
Very interesting stuff!