[HN Gopher] Antenna Diodes in the Pentium Processor
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Antenna Diodes in the Pentium Processor
Author : chmaynard
Score : 57 points
Date : 2024-11-23 20:35 UTC (2 hours ago)
(HTM) web link (www.righto.com)
(TXT) w3m dump (www.righto.com)
| kens wrote:
| Author here: I know this is a very obscure topic, but hopefully
| it will be interesting to some. Let me know if there are any
| questions...
| jiggawatts wrote:
| Absolutely fascinating!
|
| It's precisely these orthogonal, secondary concerns that make
| every industry more difficult than people on the outside might
| think.
|
| Articles like yours shed light on these challenges.
|
| I'm reminded of a recent project working on a (small!) data
| warehouse where for the first time in my career I had to not
| only be concerned with theoretical performance of queries, such
| as the presence or absences of indexes, but orthogonal concerns
| such as the time taken to rewrite terabytes of data on disk
| during night ETL jobs... combining with the "change rate" of
| the source data.
|
| Your article is a similar concern that only specialists in the
| in the industry are even aware of: it's not enough to logically
| route connections -- a challenging optimisation all by itself
| -- but there are these competing physical optimisation issues
| as well that need to be simultaneously optimised!
| iakov wrote:
| Your pictures give insight into such a small world, individual
| freaking transistors on a CPU chip! Reading textbooks and wiki
| is one thing, but seeing silicon spliced up and photographed up
| close is another. Very interesting read, and very well
| presented too, thank you.
| rietta wrote:
| Are the antenna diodes only there to reduce damage during
| manufacture or is there also impact runtime in an electromagnetic
| noisy environment?
| kens wrote:
| The antenna diodes are only relevant during manufacturing, when
| a metal line is connected on one end but not the other. ESD
| diodes on the other hand protect inputs against electrostatic
| discharge when the chip is in use.
| neuroelectron wrote:
| I thought they were there to allow Van Eck phreaking of the
| processor state
| Germont wrote:
| There is a tiny amount of extra capacitance on the net because
| of the reverse-biased junction of the antenna diode, but that's
| it. These diodes do get taken into account in when determining
| timing though.
| pclmulqdq wrote:
| Fun fact about "antennas" in chip manufacturing: They have
| nothing to do with actual antennas. Charge can build up on long
| wires during manufacturing because the chemicals involved are not
| neutral and have some interactions with exposed wires. That
| charge needs to go somewhere to protect the rest of the circuits.
| There's nothing RF about this.
|
| Later technologies (28 nm and below) have extensive design rules
| around prevention of "antenna" effects.
| anonymous_user9 wrote:
| I think that's incorrect. The article and the Wikipedia page on
| the antenna effect say antenna effects are caused by plasma
| etching, which uses RF to create the plasma.
| kens wrote:
| It's a bit confusing. The plasma is created by RF, but the RF
| doesn't cause the antenna effect (nor do "chemicals"). The
| charged ions and electrons in the plasma are what cause the
| charge buildup. The wire acts as an antenna in a metaphorical
| sense, not a literal sense, as I mentioned in Footnote 3.
| Germont wrote:
| Now go look at the need for antenna diodes in SOI technology:)
| with the substrate no longer the safe haven, a lot more oxides
| can be exposed to large differential voltages during
| manufacturing.
| lizknope wrote:
| I was following this discussion on the /r/chipdesign subreddit
| when Ken posted there a few days ago. Nice to see that he gave
| credit and linked to that thread.
|
| I'm a physical design engineer that uses software from Cadence
| and Synopsys to do chip layout of blocks with billions of
| standard cells. Our flow automatically puts antenna diodes in for
| all block input pins. Then the tools are usually good enough to
| breakup internal nets with layer jumping to avoid antennas.
|
| Some of the charge also comes from the CMP process. Modern chips
| have about 20 layers of metal but there are lots of other via
| layers in between those and then all the base layers with the
| actual transistors. You want the wafer to be flat before building
| the next layer.
|
| https://en.wikipedia.org/wiki/Chemical-mechanical_polishing
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