Connect with us

What eats N channel silicon?

Discussion in 'Electronic Design' started by [email protected], Apr 22, 2009.

Scroll to continue with content
  1. Guest

    I may need to thin or etch the back side of a c5 process CMOS die.
    What is fluid and eats the silicon?

    Steve Roberts
  2. Guest

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

    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.
  4. Confirming popular expectation, this has now become JT's answer to all
  5. krw

    krw Guest

    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.
  6. Bob Pownall

    Bob Pownall Guest

    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
  7. Guest

    Try standard lapping or sand blasting.

  8. 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

  9. 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.

  10. Martin Brown

    Martin Brown Guest

    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.

    Martin Brown
  11. Guest

    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.

  12. Guest

    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

    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.

  13. E

    E Guest

    Don't forget

  14. Eeyore

    Eeyore Guest

    HF is indeed used in semiconductor processing, most likely as a chemical
    polish in combination with other rather agressive acids and H2O2. At least
    that was the pece of kit I was refurbing. There was an emergeny exit and a
    weight operated 'waterfall' in the event of getting any on you.
    Fortunately the tanks were empty.

    I can't imagine using it on a processed die though.

  15. Eeyore

    Eeyore Guest

    Just a small spot ? Could it withstand Electro Discharge Machining ?

  16. Tim Williams

    Tim Williams Guest

    It doesn't. Silicon is fairly well inert to anything but hydrofluoric
    and its variations. Reason being, fluoride make SiF6(2-) ions,
    keeping it in solution; chroride doesn't, so even if it reacted, you'd
    just be making a bunch of silica goo, most of which would remain stuck
    firmly to the silicon surface. Which actually is exactly what
    happens, and why it doesn't eat it.
    Ammonia is chlorinated, leading to noxious chloramine compounds
    (NH2Cl, NHCl2, NCl3). Trichloramine (nitrogen trichloride) is almost
    famously explosive, being in the same class as the more famous
    nitrogen triiodide.

    Adding acid to a hypochlorite (bleach is equal parts NaOCl, NaCl and a
    little bit of remaining NaOH, hinting at its synthesis) releases
    chlorine gas. The reaction looks something like, 2NaOCl + 2HCl =
    2NaCl + Cl2(g) + H2O. Same thing works with bleaching lime (calcium
    hypochlorite, which unlike sodium hypochlorite, is a stable solid,
    often sold for pool chlorine). Actually, all the pool chlorine
    products will release Cl2 when HCl is added (including H2O2).

    In the lab, chlorine is often generated by dripping sulfuric or
    hydrochloric acid on calcium hypochlorite or TCCA
    (trichloroisocyanuric acid, also used for pools), a fairly
    controllable reaction.

  17. Rich Grise

    Rich Grise Guest

  18. Rich Grise

    Rich Grise Guest

    Why? Chlorine is chlorine, after all.

  19. Rich Grise

    Rich Grise Guest

    Huh? It makes a cloud of ammonium chloride particles, aka Sal Ammoniac.
    Since when is NH4CL toxic?

Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day