At the 2018 Bay Area Maker Faire, our project Uncovering the Silicon showed off a number of simple and complex integrated circuits (with rather large feature size) under the microscope. We had a great time helping visitors look at the features and get a glimpse of what’s inside those black box integrated circuit packages. To take this to the next level for this year’s Maker Faire, we decided to try and close the loop; to take one simple integrated circuit and elucidate its workings well enough that visitors to our booth will be able to see every single component of the circuit, understand their function, and relate it to the macroscopic behavior of the chip. For this, we picked what turns out to be a rather obscure chip: the Fairchild μL914, which is a dual 2-input NOR gate. This chip belongs to the resistor–transistor logic (RTL) family.
Here’s what the chip looks like. It’s in a funny old “glob-top” can package with eight leads.
Here’s the pinout; there are two NOR gates in the chip, plus power and ground.
Ken Shirriff built a circuit with the chip to demonstrate its functionality. When we push either of the two buttons for one of the gates, that LED will turn off.
Here’s the schematic diagram, adapted from the original datasheet. If you look at the left side, if either of those inputs goes high, the transistor pulls the output low.
John McMaster decapped a few of the chips and sent us a die photo. He made a video about the process — no small feat. We’ll be bringing one of these bare chips and a microscope (equipped with both eyepieces and a camera) to Maker Faire.
For the macroscopic scale, we approached visualizing this circuit from a couple of angles: the physical structure of the chip, and the electronic structure of the circuit.
Eric Schlaepfer used the die photo to model the structure of the chip in CAD.
Simultaneously, Ken designed a printed circuit board version for use with discrete components that maintained the same structure as the IC.
Working from Eric’s CAD model, we built a single NPN transistor model from layers of colored acrylic. If you lift it up, and look through the transparent middle layers, you can tell that the emitter (red) is embedded into the top of the base (yellow) and does not go all the down way through it. (Transistors like these are planar: The emitter is above the base, and the base is above the collector.)
The top layer of this little model has labels for the collector, emitter and base. It is removable so that the layers of the model can be more easily inspected.
The model of the chip die includes a transparent cover representing the oxide layer, and that supports the metal layer with the wire bond pads on the edges.
One of the reasons that this particular chip is educational to look at is that there are a few unused components on the die. There are two unused transistors: one of them is unconnected, and the other is shorted. There are also several unused resistors (resistors are the dogbone shapes). The unconnected and unused components are easier to see, and provide a visual example that is useful for understanding what the connected components look like under the metal layer.
It is also fun to imagine what other circuits could have been made with different connections.
We glued most of the layers together, but left the top two layers removable so that it is easier to see the internal structure when the top is removed.
(Aside: we left out most of the epitaxial pocket material, because even though we used transparent acrylic to represent it, the layers of the components are much more visible without it present.)
There are cutouts in the oxide layer where the metal layer connects to the circuitry below.
One of the most noticeable things you see when you look at this type of IC under the microscope is the bond wires. We’ve used silver glitter hot glue sticks to represent them.
The glob of melted glue represents where the wire is bonded to the pad.
When you look straight down on the model with its glitter bond wires, it looks very similar to what you’ll see in the microscope.
To round things out for our acrylic model, we made a physical legend to make it easier to identify all of the parts of the model.
Once Ken got his PCBs back from our friends at OSHPark, he built it up with the same example circuit.
The PCBs turned out beautifully, and it’s great to see the familiar discrete packages on the enlarged circuit. Ken has published the PCB design on Github.
We hope to see you at Maker Faire this weekend!
Bonus: Ken laid out some hypothetical alternate metal layers to use the same die to create different chips.
The book, published by No Starch Press, turned out beautifully. It has good pictures, clear drawings, and bright colors.
It brings a few of our classic projects onto the printed page, including LED-lit Sea Urchins, Electric Origami, the Dark Detecting LED, and Edge-lit Cards. Thank you, John, for letting us be a part of this!
For this year’s Bay Area Maker Faire we are excited to be collaborating with Eric Schlaepfer and Ken Shirriff. We’ll be bringing decapped chips like the MOS 6502, the 555 timer and 741 op-amp along with microscopes to let visitors see what’s inside of famous and interesting integrated circuits. We’ll also be bringing large scale reference models, including the MOnSter 6502.
Maker Faire is May 18-20 at the San Mateo Expo Center. If you’re looking for us at Maker Faire, our exhibitor number is 65553 and our project name is Uncovering the Silicon. We look forward to seeing lots of you there!
We’ve written about the Silicon Valley Electronics Flea Market many times before. Make that many, many, many, many, many, many, many, many times before. It’s a great source of inspiration, beautiful objects and interesting conversations.
We’re writing about it now because it has moved locations! The April 14th flea market will be at the parking lot of the Sunnyvale Fry’s. We’ll hope to see you all there this weekend!
On Monday, February 19, we’ll be celebrating Presidents Day at The Tech Museum in San Jose.
Spend your Presidents Day with us! We’re bringing you even more hands-on science fun than usual. You’ll build straw rockets and design colorful climbing robots. We’re also teaming up with Kickstarter to give you a sneak peek at some new tech.
The hours are 10:00 a.m. – 5:00 p.m. and we’ll be bringing the MOnSter 6502 and demonstrating how microprocessors work with our giant version of the classic MOS 6502.
Last year while attending FIRST robotics competitions with the Firebots, I had the privilege of serving as a judge at both the Central Valley Regional and the Sacramento Regional. Judging gives an opportunity to get to know the folks involved in the competition, whether they’re students, mentors, or other volunteers like you. I’ve judged and volunteered at a few events now, and one of the great things to see is the way that the community builds and nurtures itself.
One of the students I met in past years, Callie, had graduated from her team, but keeps coming back as a volunteer. Callie was refereeing at both events, and shines brightly as a role model. Literally. She built an LED tiara and programmed it to light in the event colors of red, white, and blue.
She’s a student at UC Davis, and is a truly wonderful role model for the high school students at the events. While you don’t necessarily need an LED tiara to shine as a role model, Adafruit does have a tutorial so that you can make one, too.