I always see illustrations like the following one:
In the examples I saw always a single die from a wafer is inserted into an IC package. Is this always like this or can an IC include more than one die?
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Yes, they can include more than one die. This is called an MCM, or Multi-Chip Module.
A common example is eMMC Flash, which includes a bare NAND device coupled with a controller on the same BGA package.
Some high density DRAM and flash devices will use ‘stacked die’, with the upper die made slightly smaller to allow wire-bond to the substrate.
Bigger examples of MCMs include multi-core CPUs like some versions of Intel Xeon and AMD Epyc.
Some FPGAs use stacked die approach to achieve even higher density than is possible with a single die.
More about MCM technology here: https://www.mccoycomponents.com/blog/view/understanding-chiplet-in-one-article
And it's here we learn a shiny-new marketing technobabble word: chiplet. Which is basically a chip, designed to be connected to other chips (excuse me, other chiplets) on the same substrate in Lego-like fashion to make an even bigger chip. Which is still an MCM, thank goodness.
And here I was, up until some minutes ago, thinking chiplets were the leftover broken pieces in the bottom of tortilla chip bowl. You know, those pesky little doodads too small to hold the salsa picante, but still edible if you're desperate.
They can contain multiple dies, the main requirement is that the package contains enough pins and space for the dies,
To my knowledge it is less common now, but was used to mix dies based around different things, generally an analog and a digital die, where combining them would have been more difficult to manufacture, or perhaps it was combining 2 existing product dies in the same package.
These dies where then had some internal bond wires linking the 2 dies.
Your question is possibly accidentally confusing. The IC, integrated circuit is....integrated...The chip/die are the same thing one chunk/chip of the wafer with layers of signals and material to make up the integrated circuit. Very many chips today have multiple ip blocks, if you want to put dram or usb or pcie or ethernet on your chip you are likely going to buy some percentage of that IP from someone else, unless the name on the door is one of a very very short list, you are going to buy the cell library to make your chip which includes and and or gates but also I/O pads and other items. You then put all of that together. If it resembles a system, things that used to be separate chips integrated into the same chip then that is a system on a chip, single chip.
Then your question implied more than one chip on a package, which yes that is becoming increasingly common as answered by others multi chip package. If you want to have two chips (dies) on the same package connected to each other you cant do that edge of die you have to have a pcb essentially so these multi chip packages are essentially a pcb with multiple chips and then all of that combined has pins/legs/balls and usually a lid some material to cover all of it.
The integrated circuit is an integrated...circuit...so it is all integrated into the chip/die, all the ip and glue logic all in one design one chip. Then for various reasons instead of one chip per package separated on an end users pcb with power supplies, connections in the pcb, etc, there can be advantages for everyone to have a multi chip module, smaller footprint on the pcb, fewer signals/power supplies/etc (sometimes) higher speed connections between chips on the module, higher yield vs making a larger chip with the features, can combine chips from different vendors, etc. (the chips on mcms are assumed to be designed for an mcm but some may work standalone).
Another one is package on package, if you look at the original raspberry pi's the processor was (still is of course) a BGA but on the top it had pads so that another bga could be soldered on it. The dram was soldered on top of the broadcom processor and then that was soldered onto the pcb. Designed for phones and other space limited applications and much better/easier control of the external dram interface, much less footprint on the board for a small amount of additional height.
There is nothing magical about ic chips. When you break off or slice the wafer into individual chips it is not as violent but it is like bending a dvd until it snaps or scoring glass and snapping that, the edges take on stress. so you often will see a die is tested on wafer, then they know which ones not to package, but after packaging you need to test it again because of fractures around the edge may have penetrated deep enough to damage the part. Sometimes it is cheaper to dice and package and test rather than test and dice and package and test. But this makes it such that you cant just take two chips and kinda push them together to make a larger thing. You need some solution like a pcb to connect the signals between chips. And you will find that with a number of bigger parts even if it is not a multi-chip-module it is still on a pcb because the ball field is larger than the die and you certainly dont want the die any larger than it has to be for multiple reasons. And not uncommon to then see a metal lid glued on top if that such that you dont see what is going on underneath. Sometimes you dont see the lid and you can see the little pcb...