integrated-circuit
In a list of ICs, along with the familiar package names such as QFN32, LQFP48, etc., I've seen a few ICs to be listed as DIE for the package size. I've never seen that description before as an IC package size, and Wikipedia does not list it either.
What can it be?
I assume it's some kind of chip-scale package, but it does not reveal the silicon size or any other properties, like number of pins, etc.
Whereas most packages are used by several manufacturers, \$\mu\$MAX seems to be a package solely used by Maxim. It's an 8 pin SMT package, about as wide as an SO-8, but just 3mm long instead of the 5mm of an SO-8. This is achieved by using a 0.65mm pitch instead of 1.27mm.
Other manufacturers also go to ever smaller packages, but most often choose for leadless packages like DFN (Dual Flat No Leads).
Intersil has this list of packages.
NXP has this list.
Unfortunately my experience is that recommended footprints for a certain package aren't always consistent between manufacturers.
Flip chip is about a single die. On the circuitry side solder balls are placed and the thing is mounted upside down on the PCB, hence the name.
Package on package (\$PoP\$) goes one step further: you have a flip chip, with another BGA on top of that, where the lower die occupies the place between the top die's solder balls.
The top die is not a flip chip; note the bonding wires. The bottom die is a flip chip, though the soldered balls aren't visible; the die should have been 1/10mm or so above its substrate.
Like many \$PoP\$ illustrations, this one shows a stacked die at the top, but that's not a requirement to talk about \$PoP\$.
A typical \$PoP\$ application would be to have a microcontroller below and a Flash or RAM device on top, since they will have to be tightly connected anyway.
Best Answer
Danger Will Robinson!
It refers to a "raw die" -- meaning the chip is not packaged. You will get a piece of exposed silicon (possibly encapsulated or partially so, but typically not).
If you are asking this question, then I'm pretty sure it is not what you want. ;-)
If you want an example...
Consider the Max3967A from Maxim Semiconductor.
If you want to buy the conventional packaged version the part number is MAX3967AETG+, but if you just want the raw microchip inside (no package) you want part number MAX3967AE/D.
In the catalog the "package" for the "/D" version will be "DIE" -- meaning no package.
From page 12 of the datasheet:
You can see they dimension the die in the drawing for you. You will need access to a wire bonding machine in order to use a raw die (among other things).
In this microscope image, you can see two wires bonded (attached) to the package in the center.
And in this photo of a thick-film hybrid circuit (taken with a little less magnification) you can see the wires bonded directly to the various die as well as the package forming the connections between the frame and the outside world:
Why you can buy raw dies: