Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 9: Photoresist

 

We’ve just got off a quick overview on organic chemistry. Now we’re getting back to photoresist. The point of photoresist, if you’ll recall, is to take a pattern so you can print stuff on your wafer. To do that it has to be a chemical that responds to ultraviolet light. And I mean more “responds to” than get a mild sunburn; it’s got to chemically change so you can transfer the pattern of light into a pattern of stuff.

It’s a polymer made of benzene rings. Someone’s showing off.

The word “Photoresist” covers a great deal of variation, but the nature of the job it has to do requires certain commonalities. For starters, rather than all one substance, it’s a mix of three different things. You’ve got a photoactive compound, naturally. You’ve also got a resin, for stability. And then you’ve also got a solvent, for instability. The solvent keeps your resist liquid so you can apply it evenly. The resin keeps it solid once it’s on, so that it doesn’t move around as you’re working with it. That picture up there is of a resin. A thing called meta-cresol novolac. Can’t tell you why.

What’s it do? What you’ve got there is the basis of a polymer. It’s a benzene ring, connected to another benzene ring by a carbon atom on the bottom. It’s got an oxygen group of the top left carbon in the benzene ring and a methyl group off the right. What you’re looking at is a chain of two monomers; add another fifty to two hundred of those rings with their groups and you’re in business. Your resin molecules are big enough to get tangled with each other and massive enough that they’re hard to dissolve.

The solvent is there to keep your resist liquid, so you can apply an even coat. Here’s how you do that; you vacuum your wafer down against a chuck. You put liquid photoresist on top of it. Then you spin it around at something like 3000 RPM (and accelerate up to that quickly for best results). The slower you spin it the thicker your final resist is going to be. Most of the resist will fly off the sides, leaving a uniform layer coating the wafer. Less than 1% of your starting resist stays on the wafer. Seems like a waste, honestly.

After you have resist on your wafer you bake it to drive most of the solvent out. (100 C for 60 seconds, perhaps.) You needed it as a liquid to get it on the wafer, now you need it as a solid so that your pattern sticks. And speaking of patterns, this is the photosensitive compound; the thing that reacts to light:

Serve up one of these trays at your next party for organic chemists. It’ll be a smash.

This is Diazoquinone, or DQN for short. It’s an older form of resist; not sensitive enough for the really small feature sized modern applications. The principles apply though. Alright, I’m gonna be straight with you; it’s the only one pictured in my textbook. And hey, the molecule is complex enough, isn’t it? Let’s unpack this thing.

Looking at it like a checkmark. Starting from the top left you’ve got a benzene ring, then there are a couple carbon atoms in between (one atom with methyl groups off on either side). Then another benzene ring. Following? After the benzene ring, there’s a Silicon cracker with an oxygen on either side. That silicon atom is double-bonded to both oxygen slices, and has one more bond on either side to continue the molecule. That’s six valence electrons; you’ve gone past the 3s and 3p orbitals and gotten into the 2p shell as well. That can work with the higher atomic numbered elements; they have more electrons and more physical volume.

We’ve got one more benzene ring at the bottom of the cookie sheet, and a cyclohexane ring. Cyclo meaning circle and hex meaning there’s six of them. (Couldn’t we call benzene cyclohexane then? You could say 1,3,5 cyclohexene, but benzene covers the thing well enough.) This ring isn’t a benzene ring; the bonds are wrong. The oxygen atom requires two bonds, so you don’t get the same resonance structure. There’s also a pair of Nitrogen atoms, in the form of squeaky white cheese curds. Have I mentioned recently how much I love Wisconsin?

Those nitrogen atoms are the weak point on this molecule. If this was a boss fight in a video game those nitrogen atoms would be blinking red. Let’s hit one with a photon and see what happens.

That’s a hickory-smoked string cheese photon. The best kind of photon, in my experience.

Nitrogen normally wants to form three bonds (Why three? Good question. Nitrogen has five electrons in its outer shell; it makes three bonds, gets a time share on three electrons and has one more pair floating around unbonded.) Okay, count the bonds there. One atom is fine; it’s got three bonds between it, the other nitrogen atom and the rest of the molecule. And the other? Well, it’s just sort of hanging out. Makes an unstable situation. Here we see the photon has broken off the nitrogen into its own separate N2 molecule. The nitrogen is happy now, but the energy levels aren’t stable on the main molecule. Let’s see what happens next:

The molecule has undergone Wolff Rearrangement, just like I do every full moon.

The ring has only five carbon atoms in it now; one has jutted up underneath the oxygen. Better, but not perfect. If you add water though…

If this molecule were a video game boss this would be it’s dreaded final form. As a molecule it’s still it’s final form, it’s just not dreaded.

That COOH group (there’s an H there; ran out of frame on my camera and had devoured too much of the molecule to reassemble it once I discovered the error). Right. That group is a common one in organic chemistry; it’s a carboxylic acid group. What’s that last word? Acid. What’s that mean? It means this compound will now dissolve in the presence of a base.

You develop your wafers by spraying them with a solution of Sodium Hydroxide (NaOH) or Potassium Hydroxide (KOH). The water in your solution causes that last transformation, and the base rips your Diazoquinone out of the mix. You’re left with just the resin in the exposed areas, and that’s not strong enough on its own to avoid being washed away. Final result: neat little lines all the way through your photoresist just waiting for you to do something with them.

That’s a positive photoresist. What about a negative photoresist? Instead of producing an acid for you to wash away, the ultraviolet light causes your negative photoresist to cross-link. That is, your polymers stick one to another and become even harder to wash away. The downside is that you have to use harsher chemicals to develop and strip off negative photoresists. The upside is that you don’t necessarily have to strip the remaining photoresist away. Example

You’ll note the carbon atoms are represented by Colby jack cheese rather than the straight Colby as before. Found a picture of this molecule late enough that I had already disposed of my previous materials.

That’s a big one. It’s a member of a class generally called polyimide. That whole thing? That’s one monomer. You get long chains of this stuff. That molecule acts as both your resin and your photoactive compound (or rather acted; it’s in its final form there.)

In a polyimide process, you’d start with your polyimide in solution with NMP. That stuff will kill you. But I digress. When you expose your wafers, the ultraviolet light causes your polyimide chains to link together, tangling themselves like so many earbud cables. Makes it very difficult for those sections to dissolve. You develop it with more NMP; the stuff that wasn’t exposed falls back into solution and leaves clear areas on your parts. Run that stuff through a cure cycle to drive off any remaining solvent and to finish the cross-linking, and you’re done. You end up with a durable and flexible plastic layer on your parts. I’ll have more to say about polyimide processing anon.

While there’s more that can and ought to be said about the resist coating process, I’m going to move on to exposing. Join us next week for “The Mercury Light Quick-Tan” or “Check Out My Sweet UV Laser!”

This is part nine of my ongoing series on How to Build a Computer, the “It’ll make a man out of you” way. You may find previous parts under the tag How to Build a Computer. This week’s post has been brought to you by The Altoona Butcher Shop! If you want your photons hot and hickory-smoked, Altoona Meats! We also sell meats that aren’t tuna.

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There are 15 comments.

  1. Percival Thatcher
    Percival Joined in the first year of Ricochet Ricochet Charter Member

    Hank Rhody, Probably Mad: Nitrogen has five atoms in it’s outer shell …

    Five electrons.

    Sorry. It’s like my brain hit a pothole.

    Good post.

     

    • #1
    • July 28, 2018, at 1:05 PM PDT
    • 3 likes
  2. Hank Rhody, Badgeless Bandito Contributor

    Percival (View Comment):

    Hank Rhody, Probably Mad: Nitrogen has five atoms in it’s outer shell …

    Five electrons.

    Sorry. It’s like my brain hit a pothole.

    Good post.

    Gah. Fixed. Also took out the second-guessing.

    • #2
    • July 28, 2018, at 1:18 PM PDT
    • 3 likes
  3. Gary McVey Contributor
    Gary McVey Joined in the first year of Ricochet Ricochet Charter Member

    This is not only an excellent series, it’s the first place I’ve ever found useful advice on keeping nitrogen happy. I mean, with nitrous oxide it keeps us happy. It’s only fair that we return the favor. 

    • #3
    • July 28, 2018, at 1:50 PM PDT
    • 3 likes
  4. Jim McConnell Member
    Jim McConnell Joined in the first year of Ricochet Ricochet Charter Member

    Hank, I tried it, just as you illustrated, but it didn’t work. So I just gave up and ate the components. Am I in trouble?

    • #4
    • July 28, 2018, at 1:51 PM PDT
    • 6 likes
  5. Arahant Member

    Now, I’m hungry. I wonder if the local market carries summer sausage. I could use a bit of oxygen.

    • #5
    • July 28, 2018, at 1:57 PM PDT
    • 3 likes
  6. Matt Balzer, Imperialist Claw Member

    Jim McConnell (View Comment):

    Hank, I tried it, just as you illustrated, but it didn’t work. So I just gave up and ate the components. Am I in trouble?

    It depends. If you ran out of components and are still hungry then probably. 

    • #6
    • July 28, 2018, at 4:09 PM PDT
    • 2 likes
  7. Hank Rhody, Badgeless Bandito Contributor

    Matt Balzer (View Comment):

    Jim McConnell (View Comment):

    Hank, I tried it, just as you illustrated, but it didn’t work. So I just gave up and ate the components. Am I in trouble?

    It depends. If you ran out of components and are still hungry then probably.

    You, ah, you might also want to invest in a stool softener.

    • #7
    • July 28, 2018, at 4:37 PM PDT
    • 3 likes
  8. Arahant Member

    Hank Rhody, Probably Mad (View Comment):
    You, ah, you might also want to invest in a stool softener.

    Or prunes, figs, dates, and whole grains, if you can eat them.

    • #8
    • July 28, 2018, at 5:02 PM PDT
    • 2 likes
  9. John H. Member
    John H. Joined in the first year of Ricochet Ricochet Charter Member

    This is Diazoquinone, or DQN for short. It’s an older form of resist; not sensitive enough for the really small feature sized modern applications.

    Back in, what was it, the 20th century, I’d read C&E News, and when it wasn’t doing dutiful calisthenics for or rather against global warming, it would talk about stuff like this. My ever-likely-to-be-faulty memory was that the way to make things smaller on a wafer was to use ever shorter wavelengths of light – which demanded totally different compounds to absorb that light. You just can’t put a different functional group on diazoquinone.

    You may have already covered this. I have been meaning to read these posts more closely – they are exactly what Ricochet needs. A few weeks ago my thermostat failed and whoever took the call said it might be a battery. Noooo; the thermostat doesn’t have a battery. Anyway, as I ever-so-likely-faultily recall, bistable multivibrators retain their state even after power-down. Just as Ricochet didn’t ever say anything about Tuktoyaktuk until this week, so it may not ever have said anything about bistable multivibrators…until you came along! I must check.

    • #9
    • July 28, 2018, at 6:19 PM PDT
    • 5 likes
  10. Hank Rhody, Badgeless Bandito Contributor

    John H. (View Comment):
    My ever-likely-to-be-faulty memory was that the way to make things smaller on a wafer was to use ever shorter wavelengths of light – which demanded totally different compounds to absorb that light. You just can’t put a different functional group on diazoquinone.

    Yeah; I’ve been wondering about that one myself. Haven’t yet resolved the question. Your mercury lamp has emission lines at 430 nm down to something like 230 nm (don’t have my references in front of me.)

    How exactly do you write a pattern at 20 nm when your wavelength doesn’t get smaller than 230 nm?

    Suppose I should figure that one out before I have to write a post on it.

    • #10
    • July 28, 2018, at 9:04 PM PDT
    • 3 likes
  11. RightAngles Member

    The best thing about these posts is that I’ve learned I can make a computer out of food! I can’t wait to get started! Thanks, Hank!

    • #11
    • July 30, 2018, at 11:11 AM PDT
    • 7 likes
  12. Judge Mental Member

    RightAngles (View Comment):

    The best thing about these posts is that I’ve learned I can make a computer out of food! I can’t wait to get started! Thanks, Hank!

    Yesterday, I started my car with an ice cream sandwich.

    • #12
    • July 30, 2018, at 11:17 AM PDT
    • 7 likes
  13. RightAngles Member

    Judge Mental (View Comment):

    RightAngles (View Comment):

    The best thing about these posts is that I’ve learned I can make a computer out of food! I can’t wait to get started! Thanks, Hank!

    Yesterday, I started my car with an ice cream sandwich.

    See?

    • #13
    • July 30, 2018, at 11:29 AM PDT
    • 4 likes
  14. Matt Balzer, Imperialist Claw Member

    RightAngles (View Comment):

    Judge Mental (View Comment):

    RightAngles (View Comment):

    The best thing about these posts is that I’ve learned I can make a computer out of food! I can’t wait to get started! Thanks, Hank!

    Yesterday, I started my car with an ice cream sandwich.

    See?

    It sounds good until he has to admit he ate his keys.

    • #14
    • July 30, 2018, at 11:42 AM PDT
    • 6 likes
  15. Arahant Member

    Oh, you kids!

    • #15
    • July 30, 2018, at 11:42 AM PDT
    • 3 likes