What eats N channel silicon?

I don't know about silicon, but hydrofluoric acid comes to mind, it etches glass.

Leftist weenies ?:-)

...Jim Thompson

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| 1962 | I love to cook with wine Sometimes I even put it in the food

Muriatic acid eats through pretty much anything, including someone's concrete garage floor once. But be careful, this stuff is brutal. Can be bought at any pool store, sometimes even Walmart, Home Depot or Lowes.

Warning: Never, never store that in close vicinity to chlorine, bleach or anything like that.

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Regards, Joerg

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Why not contact a materials company and have them do it?

for instance.

A chemical etch is not directional. I can't see any method that would appreciably etch the back of a die that wouldn't mess up the rest of the chip. For instance, you will probably damage the passivation. As you probably know, the standard way to thin a die is backlap, but I've only heard of that being done on a wafer basis, not a die.

Confirming popular expectation, this has now become JT's answer to all questions.

Depends on the chemical. Concentrated KOH is an anisotropic silicon etch, but to make it work you have to get the back surface really clean, i.e. strip off all the incidental oxide and nitride films that are always there, and for that you need RIE or some even nastier chemicals like HF for the oxide and conc phosphoric acid for the nitride. The back end stuff (wiring etc) will not survive these.

Thinning silicon is usually done by bonding the wafer face down on a sheet of glass or another wafer, grinding and polishing the back surface. It's a variation of one of the two main processes for making SOI wafers, namely oxide bonding.

Cheers

Phil Hobbs

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Muriatic acid == Hydrochloric acid == stomach acid. Yes, it etches concrete. Indeed it's used to clean concrete before painting.

Hydrofluoric acid is the really nasty stuff. It'll eat glass.

I didn't think Hydrochloric acid did much to elemental silicon. IIRC, it's used to etch SiO2 from Si when processing chips.

Hydrochloric acid and chlorine bleach (sodium hypochlorite)? How does that reaction work? Ammonia and chlorine bleach is the classic death in a bucket.

IRFNA.

Well, properly chosen resist can give decent protection for a while..

You might try posting on sci.engr.semiconductors. It's not a real active group, but you might get lucky.

Unfortunately, most of my semiconductor experience is with depositing films, rather than etching them.

Someone already mentioned photoresist to protect the front side while you're etching the back. Black wax is another possibility.

Bob Pownall

Try standard lapping or sand blasting.

Herbert

snipped-for-privacy@uakron.edu wrote: : I may need to thin or etch the back side of a c5 process CMOS die. : What is fluid and eats the silicon?

Check K.R.Williams et. al. "Etch rates for micromachining peocessing, part II" in Journal of Microelectromechanical systems, vol.12 no.6 p.761 (2003). You'll want to figure out how to protect the top side of the die, and maybe think about other selectivity questions (eg. will you need selectivity between P vs N doped Si - I guess not), whether the roughness of the etched surface is an issue etc.

The simplified answer is: the "trilogy" isotropic Si etch, consisting of 126 parts of HNO3, 60 parts of H2O and 5 parts of NH4F, applied at -20C temperature.

Regards, Mikko

snipped-for-privacy@uakron.edu wrote: : I may need to thin or etch the back side of a c5 process CMOS die. : What is fluid and eats the silicon?

Another 'fluid' is HNO3:HF:CH3COOH which according to Franssila's "Introduction to microfabrication" can reach as high as 10um/min etch rate for Si. As the stuff contains HF, I'd expect it to eat also the surface oxide. You'd need to protect the sides of the die somehow. I was assuming you know or can figure out the required safety precautions, the possible need for additional cleaning steps (eg. RCA-wash) and so on, and you were merely asking about the etchant recipe?

Like others have noticed in this thread, the standard method is mechanical lapping with a diamond-blade grinder, but I haven't heard of tooling which would allow thinning of diced chips instead of full wafers.

Regards, Mikko

It is worth pointing out to the OP that any silicon etch mixes involving HF or NH4F should not be taken lightly. The HF antidote should be on hand and he should have seen one of the HF safety films first.

This is one of those questions that if you need to ask how to do it you probably should not even be considering attempting it. sci.chem will provide a few gory tales of mishandling errors with HF.

He would be better off sending it to someone who does the task routinely. Back thinning is used on some of the high end scientific imaging CCDs.

Regards, Martin Brown

Actually I have a low budget customer who may soon be my employer if he gets the grant. I want to thin a small spot on the backside of the chip (and its a big easy to handle sensor die) before blasting a tiny via in it with a Q switched yag or Excimer laser. This is a patterned sensor more then a IC. We're using silicon more or less for the fact that the patterning can write the 5 uM electrode structure we need.

I have 7.5 years as a university lab tech with some stuff far worse then HF, so safety concerns are a given. I worked with use Osmium Tetroxide and some other nasties.

BYW, if anybody knows a fab that does sample quantities, that would be interesting too. I know about MOSiS.

I'll probably clean it with H202/Piranha first.

We have been advised to avoid mechanical drilling or scribing due to fragility of the silicon.

So thanks for the replies so far.

Steve

NO joking, I now find myself on a Fed list, probably because I have phoned you Jim. So , as a proud American concerned with maintaining Freedom and saving gas, I guess I am now a PROUD CONSERVATIVE EXTREMIST! '

Actually I had a run in the other day with some nice Feds, concerned about the strange " antennas" seen on my Equinox while driving next to a airport.

Those "antennas" save gas, add about 1-2 MPG at highway speeds, and they are not antennas:

But they are ugly and to the uneducated , make my Equinox look like it has a electronic warfare system.

Steve

Liberals should be sure to store together under the kitchen sink ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

You'll get a toxic gas cloud.

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Regards, Joerg

http://www.analogconsultants.com/

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Don't forget

-ek