Contributor Post Created with Sketch. How to Build a Computer 34: Etching

 

We’re moving back from the series on measurement to the whole process of making computer bits out of silicon. Way back, starting with Computers 7, I started a series on patterning; how you can take an idea and draw it small enough that you can apply that pattern to these really tiny circuits. I went over, step by step, each thing you need to do to create the pattern. I skipped entirely the bit where I tell you what, exactly, you do with one of those patterns when you’ve got them. This is the first of a couple of articles that fit, in manufacturing terms, between Computers 15 (Developing), and 16 (Stripping). You develop your pattern on with photoresist, this is how you make it permanent.

We’ll start with etching. Broadly speaking ‘etching’ covers any process where you start with more material and end up with less material. I mean aside from gambling. Let’s say you’ve got your silicon wafer, you want to etch some of that silicon away. To do this we start by burning your wafer. …Okay, perhaps that’s poor phrasing. Put the flamethrower down and I’ll describe what I mean. To protect your silicon wafer from the damage the etching process would do to it we’re going to want to mask it, with a silicon dioxide layer. Heat your wafer up in the presence of oxygen and this happens:

O2 + Si —> SiO2

Simple enough, right? You heat it to a temperature between 600 and 1200 C, over a specific period of time, and you can control exactly how thick of an oxide layer you ‘grow’ this way. (Adding oxygen to the silicon crystal pushes the structure upwards and out since there’s now more material. Hence the ‘growth’ term.) Okay, you’ve got this oxide layer, why exactly did we want it?

There are two basic problems with any kind of etching; you want to etch the things you want to etch away completely, and you want to not etch away any of the things you don’t want gone. Therefore we need to mask off the parts we need to keep. Silicon is pretty tough stuff; you need some real nasty chemistry to eat away at it. We’ll need a mask that will survive that etching process. Some things that will etch silicon won’t touch the oxide layer. We use the oxide layer to protect the stuff that needs to stick around.

Wait. How do we put a pattern into the oxide layer? With photoresist to put a pattern on the oxide, and then it’s own etchant. Okay, let’s skip the whole patterning discussion. Assume we’ve got a pattern in a nice, thick, photoresist layer. Now dunk your wafer in hydrofluoric acid. Nasty stuff. You’ll probably want to buffer that acid with something weaker (ammonium fluoride seems to be popular), and maybe some hydrochloric acid to deal with impurities. With that lovely cocktail, you’ll eat through your oxide in a quick, controlled manner.

SiO2 + 6 HF —> 2H2O + H2SiF6

(But what about the ammonium? That’s there to buffer the solution; slow down the reaction to get a controlled rate of etching. It doesn’t need to personally etch the material to be useful.)

Okay, you’ve eaten through your oxide down to the point where you’re seeing bare silicon. What’s the acid going to do to the silicon? Here’s your vocabulary word for the day: selectivity. “Selectivity” is the way you describe how much an acid will etch one substance over another. If a particular acid will eat through your clothes five times faster than it’ll eat through your skin then it has a high selectivity between the two materials. Also, you should probably wash that off, maybe change your pants.

But back to the silicon, what’s your hydrofluoric acid going to do to it? Not much on its own. That vitalizing cocktail will etch silicon dioxide much more quickly than it will silicon, which means that you can reasonably trust it to etch all the oxide off without altering your silicon. (Generally speaking, high selectivity is useful.) You’ll still want to rinse that off before processing it any further.

Okay, but hydrofluoric acid is pretty nasty stuff. If we can’t rely on that to etch the silicon, what’ll get the job done? There’s this lovely mix of Hydrofluoric and Nitric acid we can use. The nitric acid (HNO3) will leave hydroxide ions in the solution

HNO3 + H2O + HNO2 —> 2HNO2 + 2OH(-) + 2H(+)

The hydroxide ions will react with the surface of the silicon:

Si(4+) + 4OH(-) —> SiO2 + H2

Silicon dioxide is then, in turn, dissolved into the hydrofluoric acid as I described a couple of equations ago. But wait, didn’t we make our protective layer out of silicon dioxide? Hope we made it thick enough that it’ll survive the etching we’re doing to the silicon. Again, selectivity matters but in this case, we can get by with a low selectivity. If you put down twenty nanometers of oxide but you only need to etch ten nanometers of silicon, even if they’re at the same rate you won’t get through the oxide before you’re finished with your etch. Again you’ll want to rinse it pretty quick to stop the etching from going any further.

Of course, all that’s talking in one dimension. The problem with chemistry is that it doesn’t much care which direction it’s etching in; it’s just happy to etch. That can cause all kinds of problems. Here, let me show you, using pancakes:

Rather than acid, I find pancake etching is best done with a surface treatment of maple syrup and then mechanical ablation by means of a fork.

A wafer made of pancakes. I’ve already started the etch. I suppose I could have added a masking layer but I didn’t also feel like frying up some sausages to layer on top. Let’s say each pancake is ten nanometers thick. (You probably need a bit more flour in your batter.) You etch ten nanometers deep to get through the first pancake. But while you’re etching the next pancake you’re also widening the hole; it’s etching towards the sides just as it’s etching downward. It’s undercutting the sausage mask.

You’ll note the hole has gotten wider as well as deeper. Also, there appears to be debris in it; whether that’s unetched material, contamination, or some sort of deposit from your etchant solution you should probably figure that out before making too many more pancakes.

How do we deal with that? Couple things. Perhaps the simplest solution, you can offset your mask. If you expect it to etch five nanometers towards the sides in the time it etches to the depth you need, then maybe you can add five more nanometers of coverage. That way, when you etch under the five nanometers you end up with the size hole you always wanted. Great, right? Works just fine until you need to etch a feature deeper than you need it wide. You end up offsetting your mask so much it piles onto the mask from the other side.

What you need in that case is what’s called an anisotropic etch. That is, it’ll etch one direction much more quickly than any others. How do you do that? Join us fortnight next as we get into the question of dry etching in ‘The Pernicious After-Effects of the Eighteenth Amendment’ or ‘I Like my Silicon Etching like I Like my Martinis.’


This is part 34 of my ongoing series on building a computer, the Punk Rock way. You may find previous parts under the tag How to Build a Computer. This week’s post has been brought to you by all those 90’s revolutionary punk anthems whose words are sounding more and more reasonable to your older, conservative self, even as you reject their motivating philosophies.

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  1. Matt Balzer, Imperialist Claw Member

    Hank Rhody-Badenphipps Esq: ‘I Like my Silicon Etching like I Like my Martinis.’

    Straight gin with a nod toward France in place of vermouth?

    • #1
    • July 13, 2019, at 12:52 PM PDT
    • 3 likes
  2. Gary McVey Contributor
    Gary McVey Joined in the first year of Ricochet Ricochet Charter Member

    There was a longtime cliche comic figure, the pompous 19th century would-be seducer, and his line was always, “Why don’t you come up and see my etchings?” Now I know why. Evidently he was trying to get through their silicon dioxide protective mask. 

    • #2
    • July 13, 2019, at 12:52 PM PDT
    • 7 likes
  3. Hank Rhody, Badgeless Bandito Contributor

    Gary McVey (View Comment):

    There was a longtime cliche comic figure, the pompous 19th century would-be seducer, and his line was always, “Why don’t you come up and see my etchings?” Now I know why. Evidently he was trying to get through their silicon dioxide protective mask.

    A girl wears that much makeup you start to fantasize about chemical means of peeling it off.

    …that sounds more dirty than I was intending, but I don’t think I can come up with any phrasing that doesn’t.

    • #3
    • July 13, 2019, at 12:57 PM PDT
    • 6 likes
  4. Gary McVey Contributor
    Gary McVey Joined in the first year of Ricochet Ricochet Charter Member

    There’s so much humorously presented hard science, chemical and materials engineering here that it’s easy to just go, “Uhhh, information good” in appreciation. Herr Doktor Professor Heinrich Rhody deserves a sharper readership than that. 

    AFAIK–and this shows how little I know–the earliest microprocessors covered in your series could hardly even have been conceptualized without the limited, growing confidence gained by early manipulation of silicon chemistry, which unavoidably introduced the idea of masking and the unexpected consequences that led to excessive scrap rates. 

    • #4
    • July 13, 2019, at 11:49 PM PDT
    • 3 likes
  5. Jack Shepherd Coolidge

    Gary McVey (View Comment):

    There’s so much humorously presented hard science, chemical and materials engineering here that it’s easy to just go, “Uhhh, information good” in appreciation. Herr Doktor Professor Heinrich Rhody deserves a sharper readership than that.

    AFAIK–and this shows how little I know–the earliest microprocessors covered in your series could hardly even have been conceptualized without the limited, growing confidence gained by early manipulation of silicon chemistry, which unavoidably introduced the idea of masking and the unexpected consequences that led to excessive scrap rates.

    It is amazing how much water is required to turn a wafer into a chip. And it’s all due to etching.

    And the fact that water is still pretty cheap.

    The Intel fab in Rio Rancho still uses about as much water as Albuquerque. But we’re the ones shamed into using less.

    HA!

    Agriculture and Manufacturing use all the water.

    Not residential.

    (Sorry to derail, Hank. Nice work, as always.)

    • #5
    • July 14, 2019, at 2:33 AM PDT
    • 5 likes
  6. Gary McVey Contributor
    Gary McVey Joined in the first year of Ricochet Ricochet Charter Member

    Doesn’t sound like derailment to me, D, but an important side issue to etching. It’s now possible to do it cleaner than ever before. Is that just theory, or has the industry cleaned up some of its act? 

    • #6
    • July 14, 2019, at 2:40 AM PDT
    • 1 like
  7. Jack Shepherd Coolidge

    Gary McVey (View Comment):

    Doesn’t sound like derailment to me, D, but an important side issue to etching. It’s now possible to do it cleaner than ever before. Is that just theory, or has the industry cleaned up some of its act?

    I sincerely doubt it. When Intel and AMD build these fabs, investing a billion or more dollars into the community, they negotiate cheap as chips water deals as part of the bargain. Any savings they make they pocket. And the “cleaner” way of doing things is rarely cheaper for them

    • #7
    • July 14, 2019, at 2:48 AM PDT
    • 2 likes
  8. Gary McVey Contributor
    Gary McVey Joined in the first year of Ricochet Ricochet Charter Member

    dnewlander (View Comment):

    Gary McVey (View Comment):

    Doesn’t sound like derailment to me, D, but an important side issue to etching. It’s now possible to do it cleaner than ever before. Is that just theory, or has the industry cleaned up some of its act?

    I sincerely doubt it. When Intel and AMD build these fabs, investing a billion or more dollars into the community, they negotiate cheap as chips water deals as part of the bargain. Any savings they make they pocket. And the “cleaner” way of doing things is rarely cheaper for them

    It’s like another “Chinatown”. The skullduggery behind little-known government-industry deals to parcel out water rights.

    Who knows, maybe the age of Epstein is ripe for a “Chinatown” remake. 

    • #8
    • July 14, 2019, at 2:54 AM PDT
    • 4 likes
  9. Jack Shepherd Coolidge

    dnewlander (View Comment):

    Gary McVey (View Comment):

    Doesn’t sound like derailment to me, D, but an important side issue to etching. It’s now possible to do it cleaner than ever before. Is that just theory, or has the industry cleaned up some of its act?

    I sincerely doubt it. When Intel and AMD build these fabs, investing a billion or more dollars into the community, they negotiate cheap as chips water deals as part of the bargain. Any savings they make they pocket. And the “cleaner” way of doing things is rarely cheaper for them

    As an example, Facebook is in the midst of building a data center in Los Lunas, about 20 miles south of me.

    They got the tiny (well, not so much anymore) village of Los Lunas to basically turn over all of their water rights on the Rio Grande to cool the place. In exchange, LL got a $5 billion facility that’s going to be staffed by about 50 people.

    I think they’re stupid, but other minds may vary.

    OH! And they expected our electric company to build a new transmission line from a “solar plant” to their data center for free. Our public resource commission said, “No”. And they’ve appealed it. Twice.

    WTF should I pay for Facebook’s power line?

    • #9
    • July 14, 2019, at 3:02 AM PDT
    • 6 likes
  10. Jack Shepherd Coolidge

    Gary McVey (View Comment):

    dnewlander (View Comment):

    Gary McVey (View Comment):

    Doesn’t sound like derailment to me, D, but an important side issue to etching. It’s now possible to do it cleaner than ever before. Is that just theory, or has the industry cleaned up some of its act?

    I sincerely doubt it. When Intel and AMD build these fabs, investing a billion or more dollars into the community, they negotiate cheap as chips water deals as part of the bargain. Any savings they make they pocket. And the “cleaner” way of doing things is rarely cheaper for them

    It’s like another “Chinatown”. The skullduggery behind little-known government-industry deals to parcel out water rights.

    Who knows, maybe the age of Epstein is ripe for a “Chinatown” remake.

    I haven’t actually seen Chinatown, but I know enough about it to understand what you’re saying.

    Chinatown remake is much more relevant than a Star 80 remakes, which I believe is happening.

    • #10
    • July 14, 2019, at 3:06 AM PDT
    • 3 likes
  11. Gary McVey Contributor
    Gary McVey Joined in the first year of Ricochet Ricochet Charter Member

    Gary McVey (View Comment):

    dnewlander (View Comment):

    Gary McVey (View Comment):

    Doesn’t sound like derailment to me, D, but an important side issue to etching. It’s now possible to do it cleaner than ever before. Is that just theory, or has the industry cleaned up some of its act?

    I sincerely doubt it. When Intel and AMD build these fabs, investing a billion or more dollars into the community, they negotiate cheap as chips water deals as part of the bargain. Any savings they make they pocket. And the “cleaner” way of doing things is rarely cheaper for them

    It’s like another “Chinatown”. The skullduggery behind little-known government-industry deals to parcel out water rights.

    Who knows, maybe the age of Epstein is ripe for a “Chinatown” remake.

    The original “Chinatown” is a cynical mystery involving agricultural water rights, one of the most ancient of human property fights. The sequel, “The Two Jakes”, is essentially about the next phase of Los Angeles’ God-given advantages, oil, which was practically everywhere in the basin, with more drilling than is remembered or almost more than can be imagined now.

    Skip a step that can be filled in later. The next phase of vast mega-industry development has software and hardware components, both of them basically in the Bay Area, but more and more a lot of it down here on Silicon Beach. From there, the pampered deciders survey maps of the southwest and examine resources.

    • #11
    • July 14, 2019, at 3:30 AM PDT
    • 3 likes
  12. Reese Member
    Reese Joined in the first year of Ricochet Ricochet Charter Member

    OT, but

    …a Star 80 remake, which I believe is happening.

    Noooo! Mariel Hemingway was perfect in it. And the Schneider character was also portrayed perfectly– perfectly creepy. 

    • #12
    • July 14, 2019, at 11:50 AM PDT
    • 4 likes
  13. Jack Shepherd Coolidge

    Reese (View Comment):

    OT, but

    …a Star 80 remake, which I believe is happening.

    Noooo! Mariel Hemingway was perfect in it. And the Schneider character was also portrayed perfectly– perfectly creepy.

    She got implants for that role. That’s dedication to your craft.

    Or vanity. Or Hollywood. I don’t know.

    • #13
    • July 14, 2019, at 12:31 PM PDT
    • 3 likes
  14. Percival Thatcher
    Percival Joined in the first year of Ricochet Ricochet Charter Member

    dnewlander (View Comment):

    Reese (View Comment):

    OT, but

    …a Star 80 remake, which I believe is happening.

    Noooo! Mariel Hemingway was perfect in it. And the Schneider character was also portrayed perfectly– perfectly creepy.

    She got implants for that role. That’s dedication to your craft.

    Or vanity. Or Hollywood. I don’t know.

    All of the above, but particularly the last two.

    • #14
    • July 14, 2019, at 12:34 PM PDT
    • 1 like
  15. Gary McVey Contributor
    Gary McVey Joined in the first year of Ricochet Ricochet Charter Member

    Now, back to hydroflouric acid and oxidation that results in crystal formation…

    • #15
    • July 14, 2019, at 12:40 PM PDT
    • 2 likes
  16. Jack Shepherd Coolidge

    Gary McVey (View Comment):

    Now, back to hydroflouric acid and oxidation that results in crystal formation…

    What, you’d prefer discussing silicon, instead of appreciating silicone?

    • #16
    • July 14, 2019, at 12:42 PM PDT
    • 4 likes
  17. Judge Mental Member

    I feel I have to point out that if you’re etching your lines in stacks of pancakes, this computer is going to be the size of Texas.

    • #17
    • July 14, 2019, at 12:44 PM PDT
    • 6 likes
  18. Matt Balzer, Imperialist Claw Member

    Judge Mental (View Comment):

    I feel I have to point out that if you’re etching your lines in stacks of pancakes, this computer is going to be the size of Texas.

    I’m not seeing the problem there, other than you’d have to put it in Alaska.

    • #18
    • July 14, 2019, at 12:46 PM PDT
    • 4 likes
  19. Jack Shepherd Coolidge

    Matt Balzer, Imperialist Claw (View Comment):

    Judge Mental (View Comment):

    I feel I have to point out that if you’re etching your lines in stacks of pancakes, this computer is going to be the size of Texas.

    I’m not seeing the problem there, other than you’d have to put it in Alaska.

    Crazy people:

    “Sony, because Americans’ hands are too big.”

    • #19
    • July 14, 2019, at 12:53 PM PDT
    • 3 likes
  20. Matt Balzer, Imperialist Claw Member

    dnewlander (View Comment):

    Matt Balzer, Imperialist Claw (View Comment):

    Judge Mental (View Comment):

    I feel I have to point out that if you’re etching your lines in stacks of pancakes, this computer is going to be the size of Texas.

    I’m not seeing the problem there, other than you’d have to put it in Alaska.

    Crazy people:

    “Sony, because Americans’ hands are too big.”

    It’s not our fault people in other countries have small hands.

    • #20
    • July 14, 2019, at 12:58 PM PDT
    • 1 like
  21. Judge Mental Member

    dnewlander (View Comment):

    Matt Balzer, Imperialist Claw (View Comment):

    Judge Mental (View Comment):

    I feel I have to point out that if you’re etching your lines in stacks of pancakes, this computer is going to be the size of Texas.

    I’m not seeing the problem there, other than you’d have to put it in Alaska.

    Crazy people:

    “Sony, because Americans’ hands are too big.”

    You’re killin’ me Smalls.

    https://www.youtube.com/watch?v=96iJsdGkl44

    • #21
    • July 14, 2019, at 1:02 PM PDT
    • 1 like
  22. OldDanRhody's speakeasy Member

    I worked maintenance for a time in a facility that manufactured circuit boards: layers of copper alternated with layers of insulating material. Each copper layer had a different circuit etched into it, and the completed assembly could have as many as eighty layers thickness. Many chemical baths – mostly nasty, gold and silver plating processes, heated hydraulic presses under high vacuum, lots of water usage. The head of the maintenance department wouldn’t enter the various rooms where the chemistry was being applied to the product…

    Mrs. OD rebelled at my working hours (12-hour shifts, weekend nights) so I had a plausible excuse for seeking employment elsewhere before very long.

    • #22
    • July 15, 2019, at 8:36 AM PDT
    • 4 likes
  23. Randy Weivoda Moderator
    Randy Weivoda Joined in the first year of Ricochet Ricochet Charter Member

    Hank Rhody-Badenphipps Esq: What you need in that case is what’s called an anisotropic etch. That is, it’ll etch one direction much more quickly than any others. How do you do that? Join us fortnight next as we get into the question of dry etching in ‘The Pernicious After-Effects of the Eighteenth Amendment’ or ‘I Like my Silicon Etching like I Like my Martinis.’

    You still know how to write a cliffhanger, Hank.

    • #23
    • July 15, 2019, at 10:31 AM PDT
    • 3 likes
  24. Hank Rhody, Badgeless Bandito Contributor

    Randy Weivoda (View Comment):

    Hank Rhody-Badenphipps Esq: What you need in that case is what’s called an anisotropic etch. That is, it’ll etch one direction much more quickly than any others. How do you do that? Join us fortnight next as we get into the question of dry etching in ‘The Pernicious After-Effects of the Eighteenth Amendment’ or ‘I Like my Silicon Etching like I Like my Martinis.’

    You still know how to write a cliffhanger, Hank.

    They say explaining a joke ruins the humor, but there really isn’t much humor in it to begin with, so..

    “as we get into the question of dry etching in…”

    • #24
    • July 15, 2019, at 12:08 PM PDT
    • 3 likes
  25. SkipSul Coolidge
    SkipSul Joined in the first year of Ricochet Ricochet Charter Member

    I swear you’re writing these solely as various meals inspire you.

    • #25
    • August 5, 2019, at 7:52 AM PDT
    • 3 likes

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