Memory chips have been called the crude oil of the twenty-first century. They are used in a wide variety of electronic applications from children's toys to sophisticated communication satellites. The current generation of memory chip (64 Mb) is capable of storing 3,355 pages of text on a piece of about the size of a dime.
What is a Semiconductor?
A number of elements are classified as semiconductors including silicon, zinc, and germanium. These elements have the ability to conduct electrical current, and they can be regulated in the amount of their conductivity. Silicon is the most widely used semiconductor material because it is easily obtained.
Silicon is basically extracted from sand. It has been used for centuries to make cast iron, bricks, and pottery. In ultra-pure form, the controlled addition of minute amounts of certain impurities (called dopants) alters the atomic structure of the silicon. The silicon can then be made to act as a conductor or a nonconductor, depending upon the polarity of an electrical charge applied to it. Hence, the generic term semiconductor.
Early Developments
Semiconductor materials were studied in laboratories as early as 1830. The first materials studied were a group of elements and compounds that were usually poor conductors if heated. Shining light on some of them would generate an electrical current that could pass through them in one direction only.
By 1874, electricity was being used not only to carry power, but to carry information. The telegraph, telephone, and later the radio were the earliest devices in an industry that would eventually be called electronics.
Radio receivers required a device called a rectifier to detect signals. Ferdinand Braun used the rectifying properties of the galena crystal, a semiconductor material composed of lead sulfide, to create the cat's whisker diode for this purpose. Thus was born the first semiconductor device.
The Integrated Circuit
Until 1959, all electronic components were discrete: that is, they performed only one function, and many of them had to be wired together to create a functional circuit. Although a great number of identical discrete transistors could be fabricated on a single wafer, they then had to be cut up and individually packaged in tiny cans. Packaging each component and hand wiring the components into circuits was extremely inefficient. The military sought more efficient methods of making circuits.
New technologies emerged and integrated circuits were soon developed with various components (transistors, resistors, and capacitors) formed on the same chip, but interconnection of the various components still required tedious hand wiring.
In 1959, Jean Hoerni and Robert Noyce developed a new process called planar technology at Fairchild Semiconductor which enabled them to diffuse various layers onto the surface of a silicon wafer to make a transistor, leaving a layer of protective oxide on the junctions. This process allowed metal interconnections to be evaporated onto the flat transistor surface and replaced the hand wiring. The new process used silicon instead of germanium, and made commercial production of ICs possible.
The initial resistance to the new IC technology gave way to enormous popularity. By the end of the 1960s, nearly 90% of all the components manufactured were integrated circuits.
Semiconductor Manufacturing: Fabrication
Semiconductor memory chips are manufactured in cleanroom environments because the circuitry is so small even tiny bits of dust can damage it. Micron 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.
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.
DIFFUSION
A layer of material such as oxide or polysilicon is grown from or deposited onto the wafer. The first material deposited helps create the first layer of the semiconductor "skyscraper."
COAT-BAKE
The photo resist, a light sensitive protective layer, is applied. The liquid photo resist is then baked to form a hardened layer that is light sensitive but resistant to chemical attack. This hardened layer acts much like the film in a camera and is used to transfer circuit images to the wafer.
ALIGN
A reticle with the circuit pattern for a given level is aligned over the wafer. Ultraviolet light shines through the clear portions of the reticle exposing the pattern onto the photosensitive resist.
DEVELOP
The photo resist is chemically treated in a develop process that selectively removes the exposed regions of resist and leaves the unexposed regions containing the pattern information on the reticle.
DRY ETCH
The wafers are placed in a vacuum chamber, and a mixture of gases are pumped in and excited by electricity. This plasma eats away the material not protected by the remaining resist. When the unprotected material has been removed, the remaining material begins the pattern of the circuitry.
WET ETCH & CLEAN
The remaining resist is removed in wet etch to reveal the patterned oxide layer. Then the wafer is cleaned. The process is repeated up to 18 times to create the various layers necessary for each part's circuitry.
In this sterile 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.
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.
In photolithography, 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 hardens and becomes impervious to etchants when exposed to ultraviolet light. This chemical change allows the subsequent developer solution to remove the unexposed photoresist while leaving the hardened, exposed photoresist on the wafer.
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.
The hardened photoresist is then removed (cleaned) with another chemical.
Electrical characteristics of selected areas are changed by implanting energized ions (dopants) into areas not protected by resist or other layers. The dopants come to rest below the wafer's surface, creating the positive and negative areas on the wafer which encourage or discourage the flow of electrical current throughout the die. 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.
Masking Steps
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.
LAYERS OF SILICON, OXIDE, METAL, AND SILICON NITRIDE
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.
While still on the wafer, every integrated circuit is tested and functional and nonfunctional chips are identified and mapped into a computer data file.
Probe performs the first of several tests each component must pass before being shipped to a customer. To test the die, a probe card is positioned over each wafer. Probe needles on the card touch corresponding bond pads on the die. The needles act as transmitters, electronically sending information to and receiving information from the die's memory.
A computer map of the wafer is generated showing the results for each die, and a database of information is created.
Parametric tests are also performed at Probe to define variations in the fabrication process and to check the reliability of the parts.
The tests performed in Probe save packaging and test costs, help increase yields by allowing for repair of some of the non-functional die, and generate important data for design and product engineers and yield analysis and enhancement.
After the wafers in a lot are tested, they are sent to Assembly. In some cases, unpackaged die and wafers are sorted and prepared for external customers who package the die according to their own specification.
In Assembly, the wafers are cut, and the die are removed, packaged, and prepared for final testing and shipping.
Backgrind
Using grindstone wheels, the thickness of the wafer is reduced to provide both a clean, uniform surface and a specified product thickness. Backgrind is capable of grinding each wafer down to approximately 0.3 millimeters—as thin as an average piece of paper.
Wafer Mount / Saw
This process attaches a wafer and a film frame to ultraviolet-sensitive adhesive tape. This tape holds the wafer and die in place during subsequent processing.
A diamond-edged saw blade, approximately the thickness of a human hair, cuts the wafer into individual die. The blade spins at 45,000 revolutions per minute and cuts at a speed of 8.9 centimeters (3.5 inches) per second. During the cutting process, water sprays on both the blade and the wafer to keep the temperature low and remove the debris. After the wafer is cut, a final high-pressure rinse washes it.
Die Attach
Using the wafer map created in Probe, the good die are identified, removed from the wafer, and placed with adhesive on a leadframe or "interposer" at a speed of up to 4,000 die per hour. To remove the die from the tape, needles push up from beneath the tape as a vacuum tip lifts the die from the top. The unqualified die are left on the adhesive as illustrated below.
The good die are then adhered to the interposer--or lead frame--and batches of interposers are cured in an oven to set the adhesive/epoxy.
Wire Bond
Thin gold wire—99.9999% pure and thinner than a human hair—is attached to the die and the interposer. This wire provides the communication path (circuitry connection) between the die and the computer. Ultrasonic gold-ball bonding, a technique that combines ultrasonic energy, heat, and force, is capable of interconnecting the die bond pads to the interposer/leadframe bond pads.
Encapsulation
During Encapsulation, the die and a small portion of the interposer/leadframe are covered with a hard plastic compound to protect the die. The equipment encapsulates the die by moving the interposers into a mold area, using force to inject heated compound into the mold cavities, and curing the compound. The mold is opened, and the interposers are pressed out and cleaned
Lead Finish
Product must now go through either an Electroplating or Solder-ball Attach process. In electroplating, the exposed metal on a leadframe is covered with a conductive metal coating. While submerged in a tin and lead solution, leadframes are charged to attract the tin and lead ions. This results in a uniform plated coating that increases conductivity, keeps the leads from rusting, and provides a clean, even surface.
In solder-ball attach, solder balls and flux are placed on gold-plated pads located on the substrate. When heat is applied to the part, the solder balls adhere to the pads. Leads or solder balls provide the final interconnect between the component and the board application in the end-use product.
Trim and Form
In Trim & Form, leadframes are loaded into trim-and-form machines where the leadfingers are formed step by step until finally the chips are severed from the frames. An opens-and-shorts test is performed on each device, and the devices are sorted into good or reject trays or tubes.
The various positions and shapes of the leads and the package size and shape depend on the final application and the customer's packaging requirements.
Semiconductor Manufacturing: Test/Burn-In
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 test.
There chips are subjected to varying voltages and temperatures to determine their long-term quality and reliability and to eliminate weak chips.
Simulating actual usage, Micron's proprietary AMBYX™ ovens run monitored performance tests on every chip. This helps to guarantee the reliability of the parts customers receive and reduces overall test time and production costs.
Marking/Scanning
Each chip is marked with identifying information. Chips pass under a laser, which etches the product type, date, and speed onto the surface of the package. Each chip is then scanned to ensure that the package and leads meet industry standards. Optics or lasers examine the chips to identify defects.
Finished Goods
Parts are prepared for shipment according to customer needs. For example, customers who put the chips on boards or modules prefer them placed in equidistant pockets on a tape and spooled on a reel for automated product assembly. Other customers want parts packaged in antistatic tubes or trays.
The product is prepared for courier pickup and shipped to computer, peripheral, telecommunications, and transportation customers throughout the world.
In the last 30 years semiconductors have become virtually indispensable in many aspects of daily life. Even people who do not own or use a computer are likely to use semiconductor memory in one way or another.
Many of the fantastic capabilities of our modern world are possible thanks to the semiconductor memory chip.
Access Time
Time interval between the instant that a piece of information is sent to the memory device and the instant it returns.
Alignment
The correct positioning of a mask or reticle relative to a wafer.
Ambient
Room temperature.
Binary
Numbering system using two as a base and requiring only two symbols: 0 and 1.
Bit
(Memory Bit) Short for 'Binary Digit.' The smallest piece of data (a '1' or '0') that a computer recognizes. Combinations of 1s and 0s are used to represent characters and numbers.
Burn-in
The process of exercising an integrated circuit at elevated voltage and temperature. This process accelerates failure normally seen as "infant mortality" in a chip. (Those chips that would fail early during actual usage will fail during this process. Those that pass this test have a life expectancy much greater than that required for normal usage.)
Byte
A number of binary bits, usually eight, that represent one numeric or alphabetic character.
Capacitance
The property of a circuit element that permits it to store an electrical charge.
Capacitor
A discrete device that stores an electrical charge on two conductors separated by a dielectric.
Cell
A tiny area within the memory array that actually stores the bit in the form of an electrical charge.
Cleanroom
The super clean environment in which semiconductors are manufactured. The lower the rating, the cleaner the facility. These rooms typically have hundreds of thousands of particles less per cubic foot than the normal environment.
CMOS
Complementary Metal Oxide Semiconductor. A MOS device containing both N-channel and P-channel MOS active elements. One of two basic processes (MOS and Bipolar) used to fabricate integrated circuits.
CPU
Central Processing Unit. The computer module in charge of retrieving, decoding, and executing instructions.
Design Rules
A set of rules establishing minimum dimensions of a transistor and minimum spacing between adjacent components.
Die
A single rectangular piece of semiconductor material onto which specific electrical circuits have been fabricated; refers to a semiconductor which has not yet been packaged.
Dielectric
A material that conducts no current when it has voltage applied to it. Two dielectrics used in semiconductor processing are silicon dioxide and silicon nitride.
Diffusion
The standard procedure for doping silicon by heating wafers in a furnace from 400 to 1,150 degrees C in an atmosphere of dopant atoms.
Digital
Indicates the representation of data by a series of bits or discrete values, such as 0s and 1s.
Doping
The introduction of an impurity into a semiconductor to modify its electrical properties.
DRAM
Dynamic Random Access Memory. A type of memory component. 'Dynamic' means the device's memory cells need to be periodically recharged. Information stored in the memory cells, as a positive or negative charge, is accessed randomly.
Etch
Removal of specific material (such as portions of a given layer) through a chemical reaction.
Flash
A nonvolatile programmable semiconductor memory product. Flash devices retain the contents of their memory when the power is turned off.
Flat Pack
A flat, rectangular IC package type with the necessary leads projecting from the sides of the package.
Geometries
(Device) sizes within a device referring to the layout of components and interconnects on the U.
IC
Integrated Circuit. A tiny complex of electronic components and their connections produced on a slice of material such as silicon. Commonly referred to as a U or chip.
Ion Implant
The process of introducing selected impurities into a semiconductor via high-voltage ion bombardment to achieve desired electrical properties in selected regions.
KGD
(Known Good Die) Fully tested chips that are ready for bonding into multi-chip modules.
Lithography
The transfer of a pattern or image from one medium to another, as from a mask mask to a wafer.
Logic
The circuits used to control operation of IC devices.
Mask
A chrome and glass pattern for a layer of the wafer used in the photolithography process.
Megabit
One million binary pieces (bits) of information.
Micron
A unit of measure equivalent to one-millionth of a meter, synonymous with micrometer.
Mil
One-thousandth of an inch, equal to 25.4 microns.
Ministack
A three dimensional capacitor structure built between two layers of polysilicon. The three dimensional structure uses vertical and horizontal surfaces to increase total capacitor surface area facilitating large capacitance in a small area.
PC Board
Printed circuit board. The board(s) used in a computer system onto which semiconductor components are connected.
Photolithography
The process used to transfer a pattern or image from the masks to a wafer. The process uses a photosensitive emulsion and light.
Photoresist
A material that prevents etching or plating of the area it covers
PLCC
Plastic leaded chip carrier. A type of semiconductor package.
Planar
A simple flat capacitor built between silicon and polysilicon layers.
Resistor
A semiconductor device that provides resistance to the flow of electricity.
Reticle
A piece of glass with a chrome pattern for several die, used in the photolithography process.
Semiconductor
A solid crystalline substance whose electrical conductivity falls between that of a conductor and an insulator.
Shrink
Reduction in die (chip) size.
Silicon
A nonmetallic element used in the semiconductor industry as a substrate for multiple layers of material, built to form electrical circuits. Silicon is grown from a crystal to form a cylinder shaped 'log.' Slicing the logs into sections 1/40 of an inch thick creates bare wafers.
SIMM
Single In-line Memory Module. A high-density DRAM package alternative consisting of several plastic leaded chip carriers (PLCC) connected to a single printed circuit board (PC board). SIMMs provide an upgrade vehicle for future generations of DRAMs without having to redesign the PC board.
SRAM
Static Random Access Memory. An integrated circuit similar to a DRAM which requires no constant refreshing or recharging. It retains stored information as long as power is applied to the computer, hastening information retrieval process time.
Subassemblies
Two or more individually replaceable items integrated to form a system.
Surface Mount
A PC board assembly technique for high density manufacturing using TSOP, PLCC, and SOJ packages.
Transistor
A semiconductor device that uses a stream of charge carriers to produce active electronic effects.
Wafer
A thin disk (or slice) of silicon on which many separate chips can be fabricated and then cut into individual die.
Yield
Number of acceptable units (die) produced on each wafer compared to the maximum possible
