Memory Chip Manufacturing Part 3
All of the circuit elements (transistor, resistor, and capacitor) are constructed during the first few mask operations. The next masking steps connect these circuit elements together.
An insulating layer of glass (called BPSG) is deposited and a contact mask is used to define the contact points or windows of each of the circuit elements. After the contact windows are etched, the entire wafer is covered with a thin layer of aluminum in a sputtering chamber.
The metal mask is used to define the aluminum layer leaving a fine network of thin metal connections or wires.
The entire wafer is then covered with an insulating layer of glass and silicon nitride to protect it from contamination during assembly. This protective coating is called the passivation layer. The final mask and passivation etch removes the passivation material from the terminals, called bonding pads. The bonding pads are used to electrically connect the die to the metal pins of the plastic or ceramic package.
Every integrated circuit is tested. Functional and nonfunctional chips are identified and mapped into a computer data file. A diamond saw then cuts the wafer into individual chips. Nonfunctional chips are discarded and the rest are sent on to be assembled into plastic packages. These individual chips are referred to as die.
Before the die are encapsulated, they are mounted on to lead frames, and thin gold wires connect the bonding pads on the chip to the frames to create the electrical path between the die and lead fingers.
Product samples are taken out of the normal product flow for environmental and reliability assurance testing. These quality assurance tests push chips to their extreme limits of performance to ensure high-quality, reliable die and to assist engineering with product and process improvements.
During Encapsulation, lead frames are placed onto mold plates and heated. Molten plastic material is pressed around each die to form its individual package. The mold is opened, and the lead frames are pressed out and cleaned.
Electroplating is the next process where the encapsulated lead frames are "charged" while submerged in a tin/lead solution. The tin/lead ions are attracted to the electrically charged leads to create a uniform plated deposit which increases the conductivity and provides a clean consistent surface for surface mount applications.
In Trim & Form, lead frames are loaded into trim-and-form machines where the leads are formed step by step until finally the chips are severed from the frames. Individual chips are then put into antistatic tubes for handling and transportation to the test area for final testing.
Each memory chip is tested at various stages in the manufacturing process to see how fast it can store or retrieve information, including the high temperature burn-in in Micron's proprietary AMBYX� ovens which test the circuitry of each chip, ensuring the quality and reliability. This monitored burn-in provides feedback throughout the process, allowing identification and correction of manufacturing problems.
The completed packages are inspected, sealed, and marked with a special ink to indicate product type, date, package code, and speed.