How Micron Manufactures Memory

Micron manufactures memory - Here’s how!
Memory chips are integrated circuits with various components (transistors, resistors, and capacitors) formed on the same chip. These integrated circuits begin as silicon, which is basically extracted from sand. Turning silicon into memory chips is an exacting, meticulous procedure involving engineers, metallurgists, chemists and physicists.
Memory is produced in a very large facility called a fab, which contains many cleanroom environments. Semiconductor memory chips are manufactured in cleanroom environments because the circuitry is so small that even tiny bits of dust can damage it. Micron’s Boise facility covers over 1.8 million square feet and has class 1 and class 10 cleanrooms. In a class 1 cleanroom, there is no more than 1 particle of dust in a cubic foot of air. In comparison, a clean, modern hospital has about 10,000 dust particles per cubic foot of air. The air inside a cleanroom is filtered and recirculated continuously, and employees wear special clothing such as dust-free gowns, caps, and masks to help keep the air particle-free. This special clothing is commonly referred to as a bunny suit.

The first step from silicon to integrated circuit is the creation of a pure, single-crystal cylinder, or ingot, of silicon six to eight inches in diameter. These cylinders are sliced into thin, highly polished wafers less than one-fortieth of an inch thick. Micron uses six- and twelve-inch wafers in its fabrication processes. The circuit elements (transistors, resistors, and capacitors) are built in layers onto the silicon wafer.

Most chip designs are developed with the help of computer systems or computer-aided design (CAD) systems. Circuits are developed, tested by simulation, and perfected on computer systems before they are actually built. When the design is complete, glass photomasks are made—one mask for each layer of the circuit. These glass photomasks are used in a process called photolithography.

In the sterile cleanroom environment, the wafers are exposed to a multiple-step photolithography process that is repeated once for each mask required by the circuit. Each mask defines different parts of a transistor, capacitor, resistor, or connector composing the complete integrated circuit and defines the circuitry pattern for each layer on which the device is fabricated.

Memory Chip Manufacturing Part 2
At the beginning of the production process, the bare silicon wafer is covered with a thin glass layer followed by a nitride layer. The glass layer is formed by exposing the silicon wafer to oxygen at temperatures of 900 degrees C or higher for an hour or more, depending on how thick a layer is required. Glass (silicon dioxide) is formed in the silicon material by exposing it to oxygen. At high temperatures, this chemical reaction (called oxidation) occurs at a much faster rate.
Next, the wafer is uniformly coated with a thick light-sensitive liquid called photoresist. Portions of the wafer are selected for exposure by carefully aligning a mask between an ultraviolet light source and the wafer. In the transparent areas of the mask, light passes through and exposes the photoresist.

Photoresist undergoes a chemical change when exposed to ultraviolet light. This chemical change allows the subsequent developer solution to remove the exposed photoresist while leaving the unexposed photoresist on the wafer. Wafers are exposed to a multiple-step photolithography process that is repeated once for each mask required by the circuit.

The wafer is subjected to an etch process (either wet acid or plasma dry gas etch) to remove that portion of the nitride layer that is not protected by the hardened photoresist. This leaves a nitride pattern on the wafer in the exact design of the mask. Hundreds of memory chips can be etched onto each wafer. The hardened photoresist is then removed (cleaned) with another chemical.

Dopants are frequently introduced as part of the layer formation in high temperature diffusion operations or with ion implanters. These dopants tailor the silicon's conductive characteristics making it either negative (n-type) or positive (p-type). These basic steps are repeated for additional layers of polysilicon, glass, and aluminum.

The finished wafer is an intricate sandwich of n-type and p-type silicon and insulating layers of glass and silicon nitride.

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.

The PCB is a critical part of the memory module. It enables your computer to access the memory. For this reason, Micron engineers place significant effort on correctly designing the PCB. Each design is tested by simulation and undergoes multiple design improvements prior to release for production.

PCBs are built in arrays, or sheets, made up of several identical boards. After assembly, the array will be separated into individual modules, similar to how a chocolate bar can be broken into small squares. By varying the total number of PCBs in each array based on size, Micron maximizes the number of modules made from a given amount of raw materials. The Micron Design Engineering group also interacts frequently with system manufacturers' engineers to optimize the design process and improve manufacturability of the customer's modules.

When the module design is perfected and the PCBs produced, memory module assembly begins! Assembly entails an intricate soldering procedure that attaches memory chips to the PCB.

Throughout the entire module assembly process, Micron takes great precaution to eliminate electrostatic discharge (ESD), or what most of us refer to as static electricity. ESD damage is a leading cause of device failure. That same "shock" you feel after shuffling your feet across carpet then touching something can completely destroy a memory chip. In fact, a person passing within 12 inches of an unprotected chip can cause damage. Micron team members wear protective clothing and use anti-static equipment during the assembly process. This ensures that any electrical charges on people or equipment will not transfer to the memory modules. Additionally, after every manufacturing step, the product is checked and verified and in-line Statistical Process Control (SPC) data is gathered. These checks provide immediate feedback to ensure continuous improvement.

Next, the assembled chips and boards pass through an oven. The heat melts, or reflows, the solder into a liquid stage. When the solder cools, it solidifies, leaving a permanent bond between chips and board. The surface tension of the molten solder prevents the chips from misaligning during this process.

Once the chips are attached, the array is separated into individual modules. Micron team members visually inspect each module. Many modules also undergo additional inspections using automated X-Ray equipment to ensure no unreliable solder joints exist. All Micron Memory Modules meet IPC-A-610 acceptance criteria - the industry standard recognized worldwide.

Micron then tests and tags the modules. We use custom equipment to automatically test performance and functionality. This eliminates any possibility of an operator mistakenly placing a failed module in a passing location. Certain modules are programmed with an identifying "Dog Tag" that your PC will recognize and read.

Finally, the modules are sampled through one last Quality Inspection. They are placed into ESD safe plastic trays or bags and are ready for delivery. Finished product is shipped to the customer. Micron is the memory supplier to the top PC manufacturers in the world.

Micron Quality
Much of the success of Micron and its subsidiaries is due to their long-standing commitment to quality. For example, Micron Technology, Inc., uses a unique intelligent burn-in and test system called AMBYX as part of its stringent quality assurance and testing process. The AMBYX system tests chips under varying temperatures, voltages, and operational conditions, providing reliability data on every device made. Our memory manufacturing processes are ISO 9001 certified - the most comprehensive level of certification, in the internationally recognized ISO family of specifications for quality assurance management systems. In February 1997, Micron Technology, Inc. was one of the first companies in the United States to attain certification under the new ISO 14001 Environmental Management Systems Standard. ISO 14001 are the emerging voluntary international environmental management standards, which ensure that organizations have effective environmental systems.

Who else manufacturers memory?
There are only a handful of "true" manufacturers of memory - that is, companies who fabricate the memory chips. These manufacturers sell their chips mostly to major computer manufacturers for use in their systems. In the memory upgrade market, however, there are a number of vendors who claim to be memory manufacturers, but the truth is, these vendors buy the memory chips from a manufacturer like Micron, then, merely assemble the modules. Other vendors in the upgrade market simply buy the modules from a manufacturer, repackage them, and sell them under their brand name. We believe that Micron is the first true memory manufacturer to ship its products directly to consumers.

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