Researchers at Infineon Technologies have developed a technique to solder different types of chip together to form a

The name SOLID is derived from the soldering process used, which is
diffusion soldering (the exact term is solid-liquid interdiffusion). Before
soldering, the upper and lower sides of the "sandwich" chip are coated with
a very thin layer of copper to which a 3micron thick solder is applied. Both
chips are then soldered together at a temperature of 270C and 3bar of
pressure to create a permanent bond. The combined chips are not higher than
"normal" chips because thinned silicon wafers are used. Infineon has reduced
silicon wafers from 120microns to 60microns thick. The outer layer of the
packages is a compound plastic material.
The first prototype to be produced in the new technology is a smartcard
controller. Current smartcard controller products combine two chips on a
single planar surface - one for logic and the other for memory. Therefore
the controller has only a limited memory capacity, typically 32KBytes. A
first prototype of a smartcard controller manufactured by Infineon using the
SOLID process has 160KBytes of non-volatile memory.