NXP (former Philips Semiconductor) sent a sample of a LPC1102 ARM Cortex-M0 microcontroller in a tiny 16-pin WL-CSP (wafer-level chip-scale-package). This means that the package has the size of the chip die, or the pure die is used directly. The die is only 2.2x2.3mm with a ball pitch of 0.5mm. At the top of the die solder bumps similiar to a BGA package are used to connect the chip to a PCB.
The first image shows the whole chip (tightly fitting in the microscope's view at a magnification of 2x). The 16 balls are easily visible. At the border of the chip all pads are placed. Only a few of them are connected to the balls via the signal redistribution layer. On some unconnected pads there are scratches from the wafer prober needles during final product test.
At a magnification of 5x the connections of the redistribution layer to the chip pads reveal a nice pit.
At the top right corner a chip label shows that it really is a LPC111XL with just some peripherals not connected to the outer world. It is not sure whether NXP uses fully functional dies and just doesn't tell anybody, or if the product test revealed that these unused peripherals don't work. The latter case is clever, because they can sell chips which would otherwise be rejects.
An even close look at a magnification of 20x shows one wire of the redistribution layer and the underlying semiconductor layers. The following images have three different focus settings. Watch the sharp spots.
There are a lot more colorful places on the chip. These complicated and irregular parts usually are analog peripherals. These are hand crafted by design engineers.
At 50x magnification it is really tedious to find a focus plane. The following images are details of the previous two.
The digital parts are implemented with standard cell libraries of logic gates (AND, OR, D-FF, ...). Automatic tools optimize the placement and routing to connect them. The following image shows a detail of logic structures. In the mostly green area many horizontal connections between the cells are visible. Below these (yellowish) vertical connections are visible. These wires completely cover the actual logic cells below them. At some places filler structures are inserted to avoid areas of too inhomogeneous metal distribution. The two vertical lines at the left might be power distribution at the chip.
Many more images with varying magnifications and focus depths are available in the album.