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Author Topic: Why gold is used in CPU or RAM ???  (Read 1915 times)

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trankhanh_cr

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Why gold is used in CPU or RAM ???
« on: January 05, 2017, 06:34:52 PM »

Althought character electric conductivity of Ag > Au....
Thanks..
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AWK

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Re: Why gold is used in CPU or RAM ???
« Reply #1 on: January 05, 2017, 08:34:27 PM »

Take into account also other properties
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Arkcon

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Re: Why gold is used in CPU or RAM ???
« Reply #2 on: January 05, 2017, 11:26:04 PM »

Determine the actual amount used, and compare costs, they might be trivial.
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trankhanh_cr

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Re: Why gold is used in CPU or RAM ???
« Reply #3 on: January 06, 2017, 04:46:57 AM »

I don't think because cost.. I think about using Au the best in method function of CPU or RAM....etc.... But i don't know a reason.
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AWK

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Re: Why gold is used in CPU or RAM ???
« Reply #4 on: January 06, 2017, 04:56:02 AM »

Take into account also other properties, eg chemical reactivity.
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trankhanh_cr

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Re: Why gold is used in CPU or RAM ???
« Reply #5 on: January 06, 2017, 07:07:50 AM »

" It depends on the application. For example, high power broadband VHF (television) vacuum tube transmitters used silver plated contacts and resonate cavities. Advise from the manufacture was not to worry about the silver oxidation, that was not a problem at those frequencies. (Source: I was a transmitter engineer who serviced and maintained a variety of 5KW to 50KW VHF television transmitters.)
/u/crocodile_in_denial is spot on, "the emag voodoo can make conductor selection very important"
Electro-magnetic fields at VHF, UHF in high power applications are something like magic. "

https://www.reddit.com/r/askscience/comments/3onixa/if_silver_is_the_most_conductive_element_why_dont/
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trankhanh_cr

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Re: Why gold is used in CPU or RAM ???
« Reply #6 on: January 06, 2017, 07:09:45 AM »

So, problem in here is not cost, oxidate.. but i don't reason..
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Re: Why gold is used in CPU or RAM ???
« Reply #7 on: January 06, 2017, 08:04:39 AM »

Ag easily reacts with sulfur and its compounds.
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Enthalpy

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Re: Why gold is used in CPU or RAM ???
« Reply #8 on: January 07, 2017, 12:55:43 PM »

Aluminium was used long ago, then gold, but Cpu switched to copper during the Pentium III "coppermine" era. Presently, they seem to use more exotic and varied schemes of metallization, possibly varying among the layers. Have a look at that one
https://www.researchgate.net/publication/241159766_Metallization_of_sub-30_nm_interconnects_Comparison_of_different_linerseed_combinations

Whatever the present choice, it can't and doesn't result from trivial parameters like conductivity or cost. There are so many constraints on the production process that designers are happy to find a workable combination. A few requirements (they depend on the metal layer!):
- The metal must be patterned, usually by some exotic etching method, without destroying the photoresist that defines the metal's shape nor the layers below. The etched walls must be vertical.
- It must make good contacts with some layers below (aided by 2-3 additional thinner metal layers usually) and adhere on the varied layers below.
- It may serve as an implantation mask in some processes, so the transistors' source and drain get doped but not the channel under the gate.
- Not destroy nor pollute the gate insulator, which is very few atoms thick. Not pollute the semiconductor below the gate insulator.
- It may have to fill deep narrow trenches properly.
- At least the pad layers must permit ball bonding and still resist some corrosion after that. Not trivial with metal so thin.

And so on an so forth. Conductivity is obviously very important, but the rest is go-nogo. Expect metals much more exotic than Au and Ag! Ru, Ta, W and basically anyone found to fit the task.
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trankhanh_cr

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Re: Why gold is used in CPU or RAM ???
« Reply #9 on: January 19, 2017, 05:37:22 AM »

I received a idea by my teacher that Au has not thermal effect. So i fount f(T) = R (T: temperature; R: resistor) but i don't see...
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Enthalpy

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Re: Why gold is used in CPU or RAM ???
« Reply #10 on: January 21, 2017, 02:14:48 PM »

I don't quite get why putting more time in an reason for gold. As it looks, gold isn't used any more, for 15 years. You could first check that. Your teacher may be outdated.

The temperature stability of the resistivity is of very minor importance. There are dozens of constraints on metals that make a process possible or not, designers are happy to find one metal that fits, so temp drift does NOT determine this choice, definitely.

You won't grasp the kind of constraints on metal choice by general chemistry or electricity data. It's more like: how quickly does it diffuse through silicon and through niobium oxide, does it create deep energy levels in silicon and the insulators, does it cover the lower layers uniformly if it's 10nm thick, does it make eutectics with silicon or react with the insulators, can it be etched away with only gaseous products...

When we switched from Al to Au >30 years ago it was mainly to increase the current density in the wires without causing electromigration, the process that transports metal atoms under high current density. But because Au spoils Si so brutally, we had to put before a thin layer of W or Pt that stopped Au diffusing. And because W and Pt make no good ohmic contacts with Si, we had to put first a thin layer of Ti.

That's just to suggest you that the choices are difficult, so difficult that one single metal was impossible, and that no single parameter among the ones you cite had any role.

That was three decades ago, about at the time of the Intel 286. Everything has completely changed many times since, with Cu replacing Au during the PIII era 1.5 decades ago, later with the high-K insulators that probably demanded other metals, and meanwhile Intel has gone through some 15 process generations.
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