Doping Silicon with Lasers

Today we're doping silicon wafers with a phosphoric acid and a fiber laser.

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==== Details ====
Impurity dopants are traditionally introduced into a wafer via thermal diffusion, or ion implant. There is an alternate method using lasers to drive the dopant into the wafer, a sort of targeted thermal diffusion. This technique is used in the solar panel industry to form back contacts and selective emitters.

The laser selectively vaporizes the phosphoric acid and melts the silicon, allowing phosphorus to diffuse into the molten region. After the laser pulse ends, the silicon resolidifies and the impurity is trapped inside.

==== Analysis Equipment ====
Phenom XL SEM from Thermo
Gwyddion for AFM post-processing
Blender for 3D AFM rendering

==== Timeline ====
0:00 Intro
0:39 I have no idea what I'm doing
1:09 Doping silicon
2:21 Thermal diffusion
3:19 Ion implantation
4:36 Laser doping
10:06 Resistivity test
10:40 Hot probe test
12:20 Diode test?
15:57 SEM Archive
17:06 Next steps
17:26 New hat!

[SamZeloof]

thanks for sharing! I loved seeing the process and SEM pics. This is def promising for home chip fab. If the resistance goes down then the doping is working! For a while all my PN junction curves looked like that too, fixed it by using Al with a few % Cu mixed in for both P and N contacts and annealing at 450c. Pure aluminum tends to alloy with the silicon and forms spikes that go deep into the wafer and shorts your junction to the undoped silicon below. Not sure if this is a problem with Ti/Al. Also, your sputtered metal electrodes overhang the doped regions. Is there thick oxide on the non-overlapped regions? If the doped regions are 1x1mm, maybe try sputtering metal only on the center 0.5x0.5mm of each to be safe

[BBoman]

Ion Implant process engineer here. Your description of my process was spot on! I'm impressed how you succinctly and thoroughly explained what I do in just over a minute and might have to steal your explanation when people ask me what I do!

You also mention that ion implant will probably never be feasible in a home fab setup, and that's probably true under one condition... if you are concerned with purity of the energies and species being implanted. If you are OK just implanting everything in your source gas, it may be feasible for someone to setup a "shotgun" implanter, which is basically a plasma chamber fed by some dopant-containing gas over the wafer with an acceleration grid in between that sends all the positive ions into the wafer, dopant included. This was actually the first type of implanter ever used in industry. One issue with this design may be uniformity, not sure how this was solved in industrial applications. Another issue is that you still have to anneal the silicon to "activate" the dopant... speaking of, have you considered sending your laser doped wafers through an anneal furnace to "wash out" some of the nonuniformities?

[LanceThumping]

This makes me think that there should be a sub-section of each scientific field (similar to this one at least) that focuses exclusively on the minimization/simplification of all techniques, tools, etc. needed to do certain experiments or create certain devices.
It would be interesting to see a scientific group that actually worked to figure out the minimal setup needed to create usable chips and to shrink those needs.

[kenjinks]

If you input an AC wave into the wafer while using x/y deflection on the scope, you can apply the the AC signal to the x probe, while reading the output with the y probe. You will get a hysteresis curve telling the function of the pn junction at that AC frequency. The freq to use will depend on the capacitance of the junction.

[xTJ]

I love seeing work-in-progress content! It's really cool to watch projects evolve

[jfieqj]

This channel is amazing and deserves a lot more followers. If a video starts with "Hey folks, short video today talking about doping silicon with phosphoric acid and lasers" it's guaranteed to be good.

[BillDemos]

5:08 I am in tears. What a wonderful image.
Edit: Just saw that you were trying to make a diode. Well, the contacts are of paramount importance. Who on earth did tell you to use Al as a p contact???!!! Generally, you need high Workfunction (Φ) if you need an ohmic contact to a p-type semiconductor. You can think of it like this: the n-type semiconductor has a high fermi level and generally likes to give electrons, so you match it with a metal that likes to give electrons, such as: Cesium, Na, K, Aluminium. On the other hand, you match a p-type semiconductor with a metal that likes to retain its electrons, such as: Platinum, Selenium, Copper, Gold. Platinum and Selenium are kings with a work-function Φm being 6.35eV!!!!

[ytcomments]

Multi-layer contacts can be done using, for example, one or more of Mo/W/Ti/Cr as a Shottky or Ohmic contact, depending on your application and diffusion layer doping. Consider making a four probe resistance measurement setup (linear or square) for example with tiny pogo pins. You could also make the opposite side of the wafer back side electrode and measure your diode structures (e.g. pnn or even pin with more pure silicon) through the wafer.

[zagmongor]

As someone who binge watches youtube videos on whatever subject I'm interested in that day, I very much appreciate your meticulously planned and edited videos. As I am usually searching for explanations of certain phenomenon, or in the case of many of your videos, diy versions of expensive processes or simply exposure to those processes I wouldn't normally get to see, I personally don't care so much that you put out videos on a strict schedule. I watch your videos because of how detailed you get and seeing what results you can achieve (and issues you had to resolve) once you've dialed everything in. Needless to say it's still fun to see everything you get to play with.

[Streetsy]

The fact I can follow what you’re saying for the most part, shows how good you are at these educational videos. Thanks for sharing. 😄

[VincentGroenewold]

Oh yeah, work-in-progress is always awesome to me! I mean, with a fully perfected video, while very nice, it does feel a bit like magic. Seeing work-in-progress and sometimes simply failures, makes it real, makes me feel like maybe I can do it as well. So I'm all for it! :) I'll become a Patreon as well in the new year as I finally will have a bit more to spend again. :)

[drewc]

The algorithm brought me here because the intricacies of all this is all way over my head but the general gist of it absolutely fascinating. Thanks.

[takoslvt]

Whoever does the lighting/set design is a genius. It's absolutely beautiful.

[Antine]

I always find that it's amazing that you can make garage-semiconductor

[troy]

Thank you for taking the time to inform me on such an understandable manner i had not heard of any of this despite my interest in electronics and diy hobbies

[izzieb]

This is dope!

No, really - it's doping silicon. Will you be making a video about transistors with this technique?

[ARVash]

Wow! I bet this kind of thing would really help for small scale retro chip production.

[ESniperspreeE]

In college when we built mosefts, diodes, and capacitors we purged the chamber with nitrogen at the end of annealing. I think the final aneal had some hydrogen to get rid of dangling bonds. Been years since I have done it tho. I look forward to your content and my own lab once I can afford to set up my own lab.

[auxchar]

This is really interesting. I wouldn't have thought something like this could remotely work in a home shop at all. I do love that you're talking about it even though it's not quite there yet, I really hope that some other more knowledgeable people help you figure it out. I like these sorts of "work in progress" videos.

[jjones]

Bro. Keep up the good work. See something like this in a home lab gives me mad hope for the future.