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!

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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.

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(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.

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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.

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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 !

SiO2 Patterning

In IC fabrication it is necessary to deposit/grow and etch insulating layers of Silicon Dioxide. This presents a few problems for standard photolithographic patterning because SiO2 is hydrophilic which can cause photoresist adhesion issues and also the HF etchant attacks most photoresists. These issues combine to leave you with poor pattern definition and often complete photoresist lifting during etch.

The steps I have found to mitigate these issues are (in order): dehydration bake, HMDS vapor prime, thick resist coating, hard bake, and buffered oxide etch.

First, SiO2 is thermally grown on a test wafer using a water vapor source on a nearby hotplate to fill the furnace with steam during oxidation. The first step to ensure good resist adhesion is a dehydration bake which creates a hydrophobic wafer surface. This does not need to be done if the wafer recently came out of the furnace but if it has been in storage, than a bake of up to 700C may be necessary to restore the dehydrated surface. The next step is HMDS vapor priming: 

Here, the wafer is heated to around 200C in the presence of Hexamethyldisilazane (HMDS) vapor forming a surface monolayer on the wafer that further increases resist adhesion. HMDS can also be spin coated but this often yields a far too thick layer and can lead to incomplete photoresist development.

The final steps before etch are to spin the resist and to hard bake it. Naturally, a thicker resist film allows for a longer etch time. For maximum chemical stability, the hard bake should be conducted for extended periods of time close to the resist softening point which is usually around 145C. This can make the photoresist difficult to remove, so an ultrasonic acetone bath may be necessary unless you have proper stripping chemicals.

Instead of a standard HF etch, a buffered oxide etch of NH4F (Ammonium Fluoride) in HF can be used to control the etch rate and photoresist lifting. I use approximately 2-3g of 100% NH4F per 50mL of HF and etch time for 6000Å SiO2 is 20min at 20C.

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Etch failures

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Etch trials

Mark IV Maskless Submicron Photolithography Stepper

Details and photos of my newest maskless photolithography stepper have been posted here: Info Page

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Automated DLP submicron stepper for 2″ (50mm) wafers with LabView control, computer alignment, and wafer vacuum chuck. Based on an old Nikon microscope with custom optics and in-situ UV-VIS spectroscopy for illumination process control. Diffraction-limited resolution is <250nm with a 365nm light source.

SEM image
SEM image

Lift-Off Metal Patterning

Lift-off is a technique that allows you to patten a metal layer without any etchant chemicals. Photoresist is spun on, exposed, and developed then the metal is sputtered or evaporated on top of the resist. The photoresist is then striped and any metal on top of it is peeled off. This leaves metal in only the areas in which the resist was not present after developing.

Typically, negative photoresist is preferred for lift-off for a number of reasons but I did not want to change my existing process so I attempted it with positive AZ4210 resist yielding decent results.

Photoresist can be removed in acetone or developer solution (assuming it is exposed by ambient light during processing) but a lower vapor-pressure solvent is preferred. An ultrasonic bath can also improve lift-off.

If lift-off is difficult, a thicker resist film or thinner metal coating can help. Also, substrate heating during deposition leads to resist softening and side wall coating, making lift-off impossible. I used a Peltier cooler in my chamber to prevent this.

Silicon Wafer Laser Scribing and Cutting

Using a 50W Ytterbium fiber laser to scribe and cut Silicon wafers. Laser wavelength is 1062nm and since Silicon has poor transmission at this low IR wavelength, enough energy is absorbed to make laser marks.

Scribe marks are made at higher speed and must follow perpendicular or parallel to a flat of the <100> wafer so that the scribe lies along a crystal lattice line. When cutting all the way through the wafer, higher power is used and arbitrary shapes that do not follow the crystal lines are possible.

Lithography Projector Mod – Color Wheel Removal

When modifying a projector for photolithography, one thing to note is the UV transmission quality of the optics. Often, the stock color wheel in most DLP projects attenuates that UV significantly on the clear and blue sections. Thus, it is beneficial to remove it however the projector has RPM feedback (from motor back-EMF and/or photodiode) so simply removing it will result in the projector not starting up. A simple RC relaxation circuit can be built to emulate this signal (often 60 or 120hz) so that the projector will function properly.

The circuit shown below is approximate. The 2.4uF capacitor and 3.3KΩ resistor set the frequency of oscillation.

The completed circuit is probed and verified using a Siglent SDS 2304X oscilloscope, using channels 1 and 2 which display the oscillator output and capacitor charging waveforms, respectively, in the last photo.

In the end, the projector is “fooled” into operating with monochrome light output without a color wheel.

IC Design Tools: From Verilog to Masks

There are a number of open source layout and design tools for ICs however here I will just focus on Magic VLSI. Below are the steps to take a design from Verilog through synthesis and layout to the physical mask that is used for fabrication, via the Qflow digital synthesis flow.

The Verilog digital design for this example is a UART interface from www.asic-world.com and is synthesized/routed with the standard SCMOS rules, however a custom .tech file can be specified containing process details and design rules for DRC.

Once Qflow executes successfully, the design can be viewed in Magic after loading the appropriate cells and a GDS file is generated. This is opened in OwlVision GDSII Viewer or similar which can be used to generate the individual mask image files for each layer (active, poly, metal, etc.)

Using the same tools mentioned above, I also designed a simple PMOS chip to test my process. It is scalable to any reasonable size and contains 2 differential amplifier circuits (seen on right and middle) and a number of diodes/resistors and other test features on the left.

The design requires 4 masks for fabrication: active/diffusion, gate oxide, contact, and metal.

Fiducials should be added for subsequent layer alignment. Note: Grateful Dead bears are necessary for the circuit to function correctly.

Silicon Drift Detector Energy Dispersive Spectroscopy (SDD EDS/EDX) Setup

Instillation and setup of a Silicon Drift Detector EDS system. A SDD is a modern detector for Energy-dispersive X-ray spectroscopy that mounts onto an Electron Microscope and allows the user to analyze the composition of a sample. When electrons from the SEM hit the sample, it emits secondary electrons (along with many other particles and energies) which are used by the SEM to produce the image you see on screen and X-rays which are detected by the SDD to produce EDS information. The X-rays have a specific energy which is characteristic of the material and equal to the energy difference between excited and ground state electron orbitals in the atom.

The SDD (30mm2 in my case) quantizes the energies of impinging X-rays and amplifies them to be read by a computer. As compared to traditional Si(Li) detectors, SDDs are faster, have higher resolution, and do not require Liquid Nitrogen cooling.

The detector is mounted to the chamber at a 35° angle which makes it most convenient for use at a WD of 25mm. When using the SEM at lower working distances, the SDD probe is retracted out of the chamber as to avoid collision with the sample stage. The detector is chilled to -30°C with Peltier modules.

The detector has a built in preamplifier. The output is fed to a pulse processor containing an FPGA which connects to a laptop via ethernet.With the SDD, EDS spectra can be acquired in less than a few minutes at high beam currents.

(Click on image to enlarge)

Thanks to Pulsetor LLC, Rick Mott, Jeff Thompson, David Bono, and John Guerard for their extreme generosity and help with the detector.