Transmited light micrograph of a break in the chip of a synchronous dynamic random-access memory (SDRAM) module (capacity 128 MB, PC-100 serie). Random-access technology allows the CPU to read and write data very quickly. The chip was fabricated by photolithography, using several overlapping metalization layers (we can see in the break) and vertical contacts to interconnect them in a mono-crystal silicon wafer, which serves as the substrate. Organized into several million memory cells (a capacitor and a transistor per unit), each one may store a binary digit, or bit. To preserve the information, a periodic refresh of written data is required. Nevertheless, the chip can reach very high densities of storage and is therefore generally used as the main memory in personal computers. It is by far the most widely used RAM. Field of view size: 148 x 222 痠 (0.006 x 0.009 in)