Help purchasing a BRD laser

[alanbrito]

Gotcha. Figured there was something exotic going on.

Sorry for editing so majorly, I should fully think-out my posts before posting, instead of editing so much.

By "regular masks", do you mean it can't be done with contact aligners? Or can't be done with projection as well? (By projection, I mean like a stepper, non-contact, focusing, still using conventional lithography light sources).

And with the laser, are you planning on scanning the laser to form the pattern, or projecting through a mask?

The types of structures that I need to make are hard to make with masks because of the actual type of structures that I need and because of the dimensions that I need to achieve. And yeah, I am scanning it. Glad to see someone understands what I am trying to do.

 

[alanbrito]

I just read the edited post, pullbangdead. I will have to reply more in a minute. Gotta go meet the boss. Thanks for the comments, btw.

 

[pullbangdead]

I just read the edited post, pullbangdead. I will have to reply more in a minute. Gotta go meet the boss. Thanks for the comments, btw.

No problemo.

Definitely a challenge you have here.

Let's see...scanning a 405nm laser across a substrate, spot size=smallest feature size.

Honestly, I disagree with HIMNL. I think alignment of an optical system would be easier than doing the software side. Plus some other problems. For instance, if you are using a thick resist (SU-8 seems to lend itself to pretty thick resists, but I have no idea what thickness you're using), then you're going to have trouble using an objective with such a short focal length/high NA. It's converging/diverging so fast that even if your minimum spot size is nice and tiny and focused in the middle of the film, your spot at the surface of the resist film is going to be bigger. Depending on resist film thickness, you can do some calculations to balance the minimum spot size at the center of your film with the spot size at the surface of the film.

For instance, if the objective of a sled is truly NA=.85, the half-angle divergence is a whopping 58 degrees. So if you have a micron of resist, the spot size at the surface of the resists is going to be 1.6 microns in diameter. Even if you get the beam waist down into the size you want, it's still going to be big at the surface of the resist with such a high NA lens.

Plus with such a short focal length, that becomes a difficult problem to set up a scanning system with such a tiny throw. Setting up an x-y table and a good optical set-up on a table seems an easier way to do it, to me at least. You can easily mount the diode on an optical table, and then use whatever optical components you like.

Lots of stuff to think about.

Any maybe you've already thought of a lot of these things and already worked them out, like maybe you're already using a high-NA lens to focus onto the resist and it's not as big a problem as I thought it might be. No idea, just throwing out the things that come to mind for me.

 

[alanbrito]

Uhm, from your posts, i suppose you're speaking about making very thin elements in photolitography field, like interference masks and similars, and that you are going to use or build some sort of precision plotter (i can be wrong, thinking this, if so, my apologies)

But you have also to considerate that, using a so precise focusing system, you also need a feedback system and an active focusing system, cause any variation of distance between the laser assembly and the plate, change your focus .....

Basically, the same exact problem that the ones that have invented the CD / DVD readers had to manage at the start ..... any oscillation from the disk had to be detected in real time from the reading sensor, and compensated in real-time ..... (for this the lens of the readers/burners is mounted on a mobile system) .....

So, basically, you can adapt an already mounted sled, for this ..... think about this: a sled from a burner is an already assembled focusing / beamshaping unit, including diffraction and separation gratings, prisms, focusing lenses, splitters, and a reading / quadrature photoelement, already aligned and matched (I mean ..... the sled is not just a focusing lens ..... the beam become partially FAC-corrected, diffracted for purify and eliminate interference franges, shaped, prefocused, reflected on the disk, then the reflex from the disk become reflected again through another focusing element collimated with an optical chip that not only read the data, but also detect any defocusing or distance variation, and send signals to the driver for correct it in realtime ..... if you have to build, and overall align, all this alone, it's an almost impossible challenge, where instead in the sled is already done )

So, why don't use it, just making brand-new just the driver / focusing driver / feedback circuit ? ..... not a small or easy work, but muuuuuch more easy than the alignment part

You are so very correct. CD/DVD/BD systems all have autofocusing features to account for variations, and I do not think I would be able to tamper with those electronics or even attempt to make my own. I already have the scanning system figured out, I will mount my sample on an XY motorized stage with roughly 10 nm precision; then make the stage move either manually or by eventually programming something that would allow it to understand vector commands so all I would have to do is input an image. The other problem is the vibrations and irregularities, and that comes into account when working at these small dimensions. The whole setup is on an optical bench, basically a level table with air legs. I can try this a few times and hope for the best for now... The problem with making a whole autofocusing feedback system is that there will be nothing to focus on at the beginning, just a bare substrate covered with an equally bare photoresist. Quite a challenge.

Gotcha. Figured there was something exotic going on.

Sorry for editing so majorly, I should fully think-out my posts before posting, instead of editing so much.

By "regular masks", do you mean it can't be done with contact aligners? Or can't be done with projection as well? (By projection, I mean like a stepper, non-contact, focusing, still using conventional lithography light sources).

And with the laser, are you planning on scanning the laser to form the pattern, or projecting through a mask?

What I meant to do is scanning, the problem with masks is first, the dimensions (commercial masks can be made with a 100 nm spot size, at best, but trying to 1 um or smaller features when the mask looks awkward (because of the 100nm mask spot size) is one of the problems. The other problem is I am making a very periodic arrangement of lines, sort of a diffraction grating in multiple orientations, and the mask people said it might be undoable because they were afraid the mask itself wouldn't hold up because of the complexity in the structure.

No problemo.

Definitely a challenge you have here.

Let's see...scanning a 405nm laser across a substrate, spot size=smallest feature size.

Honestly, I disagree with HIMNL. I think alignment of an optical system would be easier than doing the software side. Plus some other problems. For instance, if you are using a thick resist (SU-8 seems to lend itself to pretty thick resists, but I have no idea what thickness you're using), then you're going to have trouble using an objective with such a short focal length/high NA. It's converging/diverging so fast that even if your minimum spot size is nice and tiny and focused in the middle of the film, your spot at the surface of the resist film is going to be bigger. Depending on resist film thickness, you can do some calculations to balance the minimum spot size at the center of your film with the spot size at the surface of the film.

For instance, if the objective of a sled is truly NA=.85, the half-angle divergence is a whopping 58 degrees. So if you have a micron of resist, the spot size at the surface of the resists is going to be 1.6 microns in diameter. Even if you get the beam waist down into the size you want, it's still going to be big at the surface of the resist with such a high NA lens.

Plus with such a short focal length, that becomes a difficult problem to set up a scanning system with such a tiny throw. Setting up an x-y table and a good optical set-up on a table seems an easier way to do it, to me at least.

Lots of stuff to think about.

You nailed it. I already have the XY problem solved, I have a manual Z stage but it only has 25 um accuracy, so I am considering also having a motorized z stage to hold the laser and to make it move up and down in small increments to run some resolution/focal spot tests. What I have already done is use a 488 nm argon ion laser and moved the Z position of the focusing objective lens (up to 0.75 NA) and I got spot sizes down to 5 um, perhaps. To test for what the best spot size is, I finely tuned it by exposing just simple lines at different Z heights, and determined where the best spot was by looking at which line was thinner. If I had more accuracy in the vertical direction and even possibly an automatic Z stage, it would make things easier, but its doable for now. That's why I want to keep the sled as is and don't even move the laser, since it's already setup to work that way.

Also, you are quite correct about the high NA issue with thicker resists. I am using a not so viscous formulation of SU-8, called SU-8 2000.5, which means I am getting a 0.5 um = 500 nm thick resist at 3500 RPM spin speed, which was verified with a profilometer. The best possible focus might occur when the focal spot is inside the resist. I can always thin the resist down to compensate for this, I have been able to coat 100-200 nm layers without much problem.

Thanks so much for all you guys with this valuable information. What I have decided to do for now is purchase a few PHR-803T sleds in case I burn them, the drivers from modwerx, the driver test load, and maybe for later use an Aixiz module. If all goes well and we need more power (100 mW at this wavelength is more than enough to expose the resist because of the small spot size and increased fluence to the resist), I might even consider building some sort of setup with a 12X diode and burn the living shit out of stuff... Maybe I'll make a hole through the wall. I was reading some posts and people are getting upwards of 700 mW of power out of the 12x BDR-205s.

Now consider getting something like that, a whole setup that wouldn't cost more than $300, versus a commercial 405 nm laser diode system, for example sold by Newport, which retails at like $20K, and only has 80 mW power. Ha. Eat me, Newport.

 

[pullbangdead]

Sounds reasonable.

You mention that your pattern is a periodic arrangement of lines...if that is the case, you may want to take a serious look at holographic lithography. It only works for things that are periodic arrays, but it may be doable for your pattern. Or it may not, I don't know.

If the holographic lithography is doable for your, then it could save you time and money, as well as likely giving you better resolution if done well.

Heck, I don't know anyone who has done it, but you can apparently even do electron beam interference lithography, on the same interference principle as the holographic technique but using electrons. Higher throughput than scanning the beam, very good resolution.

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ETA: Missed the fact before that you got the spotsize down to 5um. I bet you can do better than that with some careful optical design, but if it's good enough for you, then it's good enough for you.

Sucks though that you can't use regular lithography, I'm sure you wish you could use it too. We don't have any trouble getting 2um ridges without any extra effort, just a fairly standard process. Although I suppose you could also get such features with this, even if your spot size is 5um, if you're exposing the areas adjacent to the smallest features. But it seems like you've thought of all this stuff already. I just like interesting problems.

 

[HIMNL9]

@ pullbangdead: sorry, i don't work with SU8, so i don't know what type of resist it is, nor what thickness it can reach in the layering process ..... i just supposed that, if he need some interference gratings, he was using very thin layers ..... ofcourse, i can be wrong in this.

Basically, what i meant is that, being a sled an already aligned system, he can use it "as is" for the mechanical/optical part, rebuilding just the control part ..... but now i realize that there is also another thing, to considerate, and that is the reflectivity index of the photoresist layer .....

I mean, the optic "receiver" IC in the sled, can probably detect the "image" of the focused spot on the resist, and this can be used for a "feedback loop" control system, so when the image become bigger or distorted, the focusing lens can be moved for refocus it ..... but my line of think, ofcourse, can be vanified if the resist don't have enough reflective index ..... if, basically, the "image" of the spot is not intense enough for be correctly detected from the quadrants of the IC ..... (i'm trying to simplify at maximum, sorry if the concept is not too clear)

And, for alanbrito, if you have a perfectly flat and constant assembly, then no, you don't need a feedback for the focusing system, but keep in mind about tolerances ..... like, if you have a perfect plate for hold the item that you're impressing, and if that plate can effectively keep that item perfectly flat, and again, if the plotting system have the X and Y axis that are perfectly parallel to that plate, then is ok, but with the level of precision that you are requiring, also 1/100 of mm or less in bending or not parallelism (or in extreme cases, ambient vibrations or thermal deformation of the structure), can ruin your final work ..... but here, only you know the system that you are building, so only you can decide if this can happen or not

 

[proud2deviate]

It just occurred to me that if you would still like to use the sled in its' entirety, you could simply fix the head in place with epoxy, and move the whole sled to get the focal point where you want it. Targeting it correctly might be a chore, since it would need to be pretty close to your substrate. Maybe rig up a fiber optic camera/microscope to see what's going on?

 

[alanbrito]

It just occurred to me that if you would still like to use the sled in its' entirety, you could simply fix the head in place with epoxy, and move the whole sled to get the focal point where you want it. Targeting it correctly might be a chore, since it would need to be pretty close to your substrate. Maybe rig up a fiber optic camera/microscope to see what's going on?

That's what I am planning on doing, moving the optical head/diode up and down to adjust for focus with the lens position fixed with glue on the actuator. the working distance will probably be around 1 mm, so yeah, it will be difficult but still doable once I have full accurate control on how high I place the laser assembly.

Sounds reasonable.

You mention that your pattern is a periodic arrangement of lines...if that is the case, you may want to take a serious look at holographic lithography. It only works for things that are periodic arrays, but it may be doable for your pattern. Or it may not, I don't know.

If the holographic lithography is doable for your, then it could save you time and money, as well as likely giving you better resolution if done well.

Heck, I don't know anyone who has done it, but you can apparently even do electron beam interference lithography, on the same interference principle as the holographic technique but using electrons. Higher throughput than scanning the beam, very good resolution.

-------------------------------

ETA: Missed the fact before that you got the spotsize down to 5um. I bet you can do better than that with some careful optical design, but if it's good enough for you, then it's good enough for you.

Sucks though that you can't use regular lithography, I'm sure you wish you could use it too. We don't have any trouble getting 2um ridges without any extra effort, just a fairly standard process. Although I suppose you could also get such features with this, even if your spot size is 5um, if you're exposing the areas adjacent to the smallest features. But it seems like you've thought of all this stuff already. I just like interesting problems.

Interference lithography works really well, but for gratings in one direction. Mine will be oriented in multiple directions, so it needs to be scanned. I already did some interference lithography with several lasers and got good results. As for electron beam interference lithography, I have not tried that yet. I guess I could get a source of electrons and then do the same thing, but the optics/electronics for that tend to be more complicated. Lasers are much simpler to work with. Also, I did try some regular electron beam lithography using a system built for that and I got amazing results, but the cost of using that machine to make my samples was way too high and the exposure times as well.

Thanks again guys for your input. Parts are arriving soon, hopefully today, and I will put up my results and probably ask questions on how to set it up.

I got the sled, the drivers from Modwerx and the driver test dummy load. I have a power supply, as well, so all I need now is to put the whole system together once it arrives.