First IC :)

Preface

I am very excited to announce the details of my first integrated circuit and share the journey that this project has taken me on over the past year. I hope that my success will inspire others and help start a revolution in home chip fabrication. When I set out on this project I had no idea of what I had gotten myself into, but in the end I learned more than I ever thought I would about physics, chemistry, optics, electronics, and so many other fields. Furthermore, my efforts have only been matched with the most positive feedback and support from the world; I owe a sincere thanks to everyone who has helped me, given me advice, and inspired me on this project. Especially my amazing parents, who not only support and encourage me in any way they can but also give me a space to work in and put up with the electricity costs… Thank you!

_D7V8343

Without further ado, I present the first home(garage)made lithographically-fabricated integrated circuit – the “Z1” PMOS dual differential amplifier chip. I say “lithographically-fabricated” because Jeri Ellsworth made the first transistors and logic gates (meticulously hand wired with conductive epoxy) and showed the world that this is possible. Inspired by her work, I have demonstrated ICs made by a scalable, industry-standard, photolithographic process. Needless to say, this is the logical step-up from my previous replication of Jeri’s FET fabrication work. 

Design

I designed the Z1 amplifier looking for a simple chip to test and tweak my process. Layout was done in Magic VLSI for a 4 mask PMOS process (active/doped area, gate oxide, contact window, and top metal.) PMOS has advantages over NMOS as far as mobile ionic contamination that lends it to being fabricated in a garage. The masks are designed in 16:9 aspect ratio for easy projection.

The feature (gate) size is approximately 175μm although there are test features as small as 2μm on the chip. Each amplifier section (center and right) contain 3 transistors (2 for long-tailed differential pair and one as current source/load resistor) which means a total of 6 FETs on the IC. The left portion of the IC contains resistors, capacitors, diodes, and other test features used to characterize the fabrication process. Each node of the differential pairs is broken out to a separate pin on the lead frame so it can be analyzed and external biasing can be added as necessary.

Fabrication

There are 66 individual fabrication steps to make this chip and it takes approximately 12 hours for a full run. The process yield can be as high as 80% for these large features, but is largely dependent on my coffee intake that day. I have also made Youtube videos covering semiconductor fabrication theory and discrete MOSFET fabrication.

50mm (2″) Si wafers are scored into 5.08 x 3.175mm dies (~16mm^2 area) with a Epilog fiber laser. This die size is chosen to fit into a Kyocera 24pin DIP carrier.

Native oxide is stripped off the wafer with a quick dilute HF dip and then they are extensively cleaned in Piranha solution (H2SO4:H2O2), RCA 1 (H2O:NH3:H2O2), RCA 2 (H2O:HCL:H2O2), followed by another dilute HF dip.

The field oxide is thermally grown in a water vapor ambient (wet oxidation) to a thickness of 5000-8000Å.

The oxidized wafer is ready for patterning of the active/doped (P-type) area. Positive photoresist (AZ MiR 701 for SiO2 patterning and AZ 4210 for Al layer) is spun on at around 3000rpm yielding a film of about 1.5μm for the AZ MiR 701 or 3.5μm for the AZ 4210 which is soft baked at 90C on a hotplate.

Lithography process details

The active area mask is exposed with my Mark IV maskless photolithography stepper at 365nm UV and the pattern is developed in TMAH or KOH solution depending on the resist.

Resist pattern
Resist pattern

The resist pattern is then hard baked and a number of other tricks are used to ensure good resist adhesion and chemical stability during the following HF etch step which transfers that pattern to the oxide layer and opens windows to the bare silicon surface for doping. These regions later become the source/drain of the FETs.

Doping is then carried out by either solid or liquid source. The solid source is a Boron Nitride disk that is placed in proximity (<2mm) from the wafer in the tube furnace. Alternatively, spin-on liquid sources can be prepared from Phosphoric or Boric acid in water or solvents and doping is carried out in a standard pre-deposition/HF dip/drive-in/deglaze process.

The above mentioned patterning steps are then repeated twice for the gate oxide layer and then the contact layer. The gate oxide must be much thinner (<~750Å) than the field oxide, so the regions between the source/drain are etched away and a thinner oxide is grown there. Then, since the whole wafer has been oxidized during the doping step, contact windows must be etched for the metal layer to make connection with source/drain doped regions.

na_1

 

(click to enlarge)

Now, all the transistors are formed and are ready to be interconnected and broken out to the lead frame. A blanket layer of Aluminum (400-500nm) is sputtered or thermally evaporated onto the wafer. An alternative would be to use the lift-off process in which the photoresist is patterned first and then metal is deposited.

The metal layer is then patterned with photolithography and etched in hot Phosphoric acid to yield the completed IC. The final steps before testing are visual inspection and high temperature annealing of the Aluminum to create ohmic connections.

IMG_3212

The finished chip is now ready for packaging and testing.

I don’t have a wire bonder (accepting donations!) so my testing right now is limited to manually probing the wafer with sharp tweezers or using a flip-chip board (difficult to align) to connect it to a curve tracer. The differential amplifier is also tested empirically in-circuit to verify operation.

_D7V8375

FET Ids/Vds curve from discrete NMOS device

Of course, these curves are far from ideal (some of which is due to extra contact resistance/other factors like that) but I would expect things to get better with proper wire bonding. This may also account for some of the die to die variation. This page will be updated with new IV, FET, and a differential amplifier characteristic curves soon.

Update 9/3/18: I got a wire bonder! It will definitely take some more practice before I can bond to a chip, but results will be posted. This will also allow for more extensive testing. I just moved out to college so progress will hopefully be made on school breaks.

Thanks for following my work and feel free to contact me with your thoughts at sam@zeloof.xyz !

48 thoughts on “First IC :)”

  1. Wowsers. My final year uni project was making solar cells so I can appreciate all the fab steps you had to do to make devices, and to do it all by yourself and outside a “lab-enviroment” is just wow/10 man.

    How did you manage to get all the equient to do this?

  2. Fascinating! It would be ultra-awesome that this project could be merged with this other: https://dberard.com/home-built-stm/, where he achieves an atom-level resolution scanning electron microscope where a tungsten needle driven by a piezo-actuator scans surfaces and gets a read-out via tunnel effect. You should be neighbour of his garage. 🙂

  3. This is absolutely incredible work. Thank you for taking the extra time of documenting every step as you go and then sharing it with the world.

  4. This is incredible! DIY IC fabrication is something I had always dreamed about, but the dangers of any kind of vapor deposition process kept me away (as well as the cost of anything approaching proper safety equipment). Well, let’s be honest, the equipment in general is cost-prohibitive unless you want to rely on decommissioned surplus (which is still expensive).
    Do you have any comment on the PPE and/or environmental protection considerations you may have (or may not have) undertaken for this project? If that is covered elsewhere in your blog, I apologize for looking over it…

    1. Looks like he’s doing all wet chemistry and vapor/sputter metals (no CVD) so the worst he’s got to deal with is HF and piranha solution but that’s nothing beyond high-school chem lab levels of dangerous.

      If he moves on to trying self-aligned gates then the safety/PPE gets to be tricky due to CVD polysilicon which takes silane or trichlorosilane which are super toxic and pyrophoric and all that fun.

  5. Do you have a Patreon account? I cannot support you in any other meaningful way, but I would be very happy if you continued your work. I want to see some decent open CPUs in ~20 years.

  6. Congratulations on this very important milestone! This is a worthy project indeed.

    If you do more complicated circuits, you might consider using closed MOS transistors. In this case it would have saved you one mask (you don’t need “active”) and a few process steps. The university I studied at used the NMOS version of it and you can see the CMOS version created at RCA: http://www.visual6502.org/wiki/index.php?title=RCA_1802E#Simple_logic_gates

    Some additional advantages are that you get very nice length/width transistor ratios in a roughly square area and transistors in series get the right sizes for best speed since outer rectangles must be larger than inner ones.

    The main problems are that even minimal transistors are a bit large and that all circuits end up connecting to ground (though you can get around that by adding an extra transistor that is always off).

  7. Awesome project, the Amp Hour episode was great, too.
    Are you aware of this project: https://code.google.com/archive/p/homecmos/ ? It isn’t active anymore, but maybe you can find some useful stuff in the archive.

    Would you mind sharing your Magic VLSI design files on GitHub or something for others to play around with? That would be great!

  8. Congratulation ! I can’t tell more. It’s just…
    I would like to know every steps in detail and documentation on how you did it.
    Just in case one day I have a time, money and a garage at the same time…

Leave a Reply to James Thomas Cancel reply

Your email address will not be published. Required fields are marked *