Intel 8080 Microprocessor

I wanted to decap an iconic Intel 8080 microprocessor. Although (low resolution) die photos can be readily found, I wanted to take a closer look myself. Launched in April 1974 the 8080 was the successor to the 8008.  It was the first Intel CPU on an NMOS process and the first in a 40 pin package.

My one is an 8080A (With max clock speed of 3.125MHz) Intel 8080Afrom week 35 1984

I initially thought the 435 date code meant 1974 week 35 (Which would have been a very early part.) But then realized the ’79 was the design revision. Intel did a design revision in 1979 and were clearly still in production in 1984, ten years after the launch.

The 8080 was designed by Frederico Faggin along with Stan Mazor. One backstory that I find fascinating is that Faggin not only designed this (And the 4004.) He was also the inventor of the Self-Aligned silicon gate process – one of the most important process steps that was used in every CMOS chip for the next 30 years. Even by the 1980’s the idea of a process engineer also designing chips was unthinkable, the two engineering disciplines are just two separate. Faggin left Intel to found Zilog in Nov 1974 (And designed the hugely successful Z80 CPU) and founded and was CEO/Chairman of Synaptics for many years a company making touchpad interfaces who are still around today.

Die Photo

The 8080A was 4.28mm x 4.18mm (17.9mm2) a small area die by modern day examples.Intel 8080A die photoclick on image for high resolution version

Made on a 6μm enhancement transistor NMOS process. The layout (All done manually) has dense packing of both metal and polysilicon layers. Here the polysilicon is being used as routing as the data fans out from the ROM

As well as the main die marking

 

 

 

 

 

 

There are two other die markings “mel” and “i/IL”

The “mel” marking is intringuing.  The ‘el’ are styled the same as the Intel logo, which suggest to me that this is not a designer initial.  

Datasheet and Pin Out

The datasheet was easy to find.  From this I was able to identify the pin out, and some of the key feature blocks

One of the big changes from the 8008 was the 16 bit address register array (Or stack pointer) that enabled the part to address 64kB of memory (Through the 16 Address Array input pads down the right side of the die).  The instruction register has two blocks of ROM each containing 324 bits by my count.  The bi-directional 8 bit data pins are on the top right.

Polysilicon Die Photo

With the dense metal not all features can clearly seen. Whilst 8080 die photos are not hard to find, I have never seen a polysilicon layer die photo of an 8080 anywhere.  So using a paste of Armour etch (HF based) and water to remove the dielectrics, and HCl (Muriatic acid) to etch the Aluminum I created a polysilicon die photo.

I am well pleased with the result!Intel 8080A poly die photoclick on image for high resolution version

Here is a crop from the image where I have marked up the layers to explain what you can seeI may have over-etched the oxides a bit (using homemade pastes and only having one sample it is a bit hit or miss). But you can clearly see all the transistor gates, and the source/drain regions (Active area) and even all the contacts.

This image (Plus metal die photo) would be sufficient to fully reverse engineer the chip with several hours of effort tracing the circuit.  Of course in the 1970’s the 8080 was reverse engineered by dozens of companies (Including Soviet, Polish and Czechoslovakia companies). Many second sources were made by AMD, TI, NEC, Siemens amongst others as cited in the wiki article. Back in 1970 the same microscope and etching was available, but of course there was no digital cameras.  To create a stitched image like this would have required physically taping together many paper photos!

Memory

With the metal removed the layout is much more easy to view.   Here is the a portion of the  16 bit register array at metal, and poly
Only after removing the metal can you really see the rather complex layout

And here is the register area zoomed in, first with metalThen with metal removed, you can clearly see the ROM structure (where there is a contact over active area a pass transistor is created, in empty cells there is field oxide present (No transistor)

Finally there is one structure in the bottom left quadrant that I could not figure out.  It is repeated 8 times, and it looks like there are two polysilicon to active area capacitor. Again first with metaland at poly

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