1. What type of material is a memory chip made out of?
its a chip of silicon with special impurities that are layered to make circuits. Look up intergrated circuits, memory chips are basically make out of the same stuff all ICs, processors, logic chips etc etc. are made of.
2. What functions does the aluminum plate on a RIMM serve?
heat dissipation is my guess, I dont have any of that type memory but its the fashion now to press metal plates against the memory chips (heat spreaders) to help remove heat from the chip and thus allow it to operate at higher frequencies without overheating.
to be specific on the first, typical memory is DRAM.. dynamic random access memory. made with CMOS, one transistor and one capacitor per bit.. as opposed to SRAM that goes into processor cache, which runs a lot faster, but is 6 transistors per bit.