How to make a microchip?

The microchip is a powerful tool to bring affordable devices to everyone around the world. Typically there are over 350 process steps to make a microchip. However we will cover, only the basics here.

Microchips are made of large numbers of small circuit elements known as transistors. A microchip can contain from a few hundreds to a billion transistors. In order to fit all these circuits on a microchip they must be extremely small. Historically circuits have been measured in microns, or 1/1000th of a millimeter. Approximately 70 times smaller than the width of a human hair. Today’s advanced technologies produce circuits so small they are measured in nanometers or 1/1000th of 1 micron. That is thousands of times smaller than the width of a human hair.

Transistors and Digital circuits operate similar to a light switch. They turn the current on and off. This on-off switching is how information is managed and controls how information is stored in a microchip. Electronic circuits consist of two parts; one is the switch, that starts and stops the flow of electrical current, a device that starts or stops the flow of electric current by making or breaking the circuit. Another is the wire- the circuit pathway that connects the switches to form the circuit. Materials through which electricity flows with relative ease are called conductors. 

Copper and other metals are examples of conductive materials. In contrast, a material that does not allow electrons to flow through it is called an insulator. For example, glass. Semi conducting materials are unique because they can be both an insulator and a conductor. Hence the term semi-conductor.

A transistor is formed with semiconducting areas that allow the flow of electricity and insulating areas that don’t allow the flow of electricity. Transistors are the fundamental building blocks of a microchip and are built on a silicon base. Silicon is one of the most common elements on Earth and its unique electrical properties have made it the most commonly used semiconductor. To manufacture the silicon wafer, sand and other materials are processed at high temperatures to purify the silicon. A seed-crystal or single crystal is brought into contact with molten silicon.

The seed is slowly rotated and pulled from the crucible producing a large silicon cylinder called an ingot. When completed these cylinders can be 5 feet long and weigh hundreds of pounds.  The ingot is then sliced and polished into thin discs called wafers, approximately the thickness of a credit card, which then become the foundation of a microchip.

The diameter of wafers has grown over time, starting at less than 2" in 1970 to 300 mms or the equivalent of 12 inches today. The larger the wafer, the more microchips that can be made at one time. Typically 200 to several thousand microchips can be made on a single 300 mm wafer. There are several process steps that are used repeatedly in the creation of a microchip.

Three of the most important steps are Film deposition, where a thin layer of film is deposited on the wafer, Photo lithography where a pattern of the microchip is created when a layer of photo resist is exposed to ultraviolet light, and film removal where the exposed areas of the wafers are etched away while the protected areas are not, leaving a pattern on the wafer. These processes are repeated many times, building layer upon layer of the microchip.

Film deposition consists of creating thin uniform layers of the material on the wafer surface. There are three primary techniques for film deposition - Chemical Vapor Deposition (CVD), Physical Vapor deposition (PVD) and Electrochemical Plating ECP.

CVD is a chemical process and can be used to deposit both insulating and conducting films. During CVD gases containing the elements or the material to be deposited are introduced into a chamber, where a chemical reaction occurs to form a solid thin film on the wafer.

On certain microchips such as microprocessors, the first layer formed uses a CVD process called epitaxial deposition or Epi. Epi involves growing a layer of single crystal silicon directly on a wafer creating better electrical properties, which results in enhanced performance for advanced microchips.

A thin layer of insulating material is deposited on the epi forming the foundation of the transistor.

The next process is photolithography, a process where a mask, the pattern of the microchip's circuitry is positioned in a precise location above the wafer. This process, also known as patterning is repeated several times during the microchip making process. In photolithography, the wafer is first coated with a layer of a light sensitive material called photo resist. The mask is then placed above the wafer and ultra violet light is flashed through it exposing the photo resist in select areas, creating a pattern.

Each layer of the microchip uses a mask with a different pattern.
 To ensure that all layers are aligned with each other and that every microchip is the same, the size and pattern of the shapes on the wafer must be exact.

To achieve this, a high precision Optical Lithography tool steps through every microchip on the wafer, exposing the pattern, one microchip at a time. The photo resist in the exposed areas is then removed, revealing the wafer surface beneath. The material from the exposed surface is chemically removed, leaving a 3 dimensional pattern on the wafer. This process known as etch, transfers the desired pattern from the photo resist to the wafer, the pattern of a circuit. Once the etch step is completed, the remaining photo resist is removed.

The next step in creating the transistor is ion implantation. Extremely small amounts of impurities called dopants are accelerated to a high velocity, so they can penetrate or implant the wafer surface, changing the electrical properties of the silicon. The wafer then goes through a high temperature anneal or Rapid Thermo Processing or RTP. RTP activates the dopants changing the semiconducting nature of the silicon and repairs the silicon structure that was damaged during the implantation step.

During this process, the wafer is exposed to a highly controlled thermal cycle that heats the wafer from room temperature to 1100 degrees Celsius within seconds.

There are many implantation and annealing steps involved in the creation of the transistor. Once the transistors are made they are connected with many levels of metal wiring, called interconnects, which are separated from each other using insulators. The final combination of transistors and interconnects forms the structure known as an integrated circuit. Today's most advanced logic devices use copper interconnects. Aluminum interconnects are most commonly used in memory chips. There are several common methods required to create these interconnect layers. CVD, PVD and ECP.

To make these interconnects, an insulating layer of oxide is deposited between the transistors and the first layer of interconnects to prevent shorting out the circuits. The transistors are connected to the first layer of interconnects through openings in the insulating layer called contacts. These contacts are different from all the other layers of the interconnect, because they touch the transistors directly. Once the contacts are completed, the deposition, photolithography and etching steps are repeated in the successive interconnect layers. One method to deposit metal layers on the wafer is through Physical Vapor Deposition ( PVD ). In a high vacuum environment, a gas is accelerated towards a metal target.

This target material is the source of the metal atoms that will make up the conducting film on the wafer. Metals commonly used are aluminum, copper and titanium. The accelerated gas is directed at the target and physically knocks off tiny amounts of the target material, depositing a thin conductive film on the wafer.

The second method for metal deposition is atomic layer deposition or ALD. ALD is a new emerging type of CVD, capable of forming extremely thin layers. In ALD, chemicals are introduced sequentially into a low-pressure chamber. The resulting chemical reaction forms an ultra thin layer of material. Each such deposition cycle deposits a thickness equivalent to single atomic layer. By repeating the cycle the desired thickness is achieved.  Ultra thin films can thus be deposited with precise control.

A third method is electrochemical plating or ECP. In ECP the wafer is immersed in a solution containing dissolved copper. A Voltage applied between the solution and the wafer pulls the dissolved copper to the wafer, depositing the copper film. This plating process continues until the desired thickness of copper is achieved. The ECP deposition step leaves the surface uneven, making it difficult to pattern the wafer for the following interconnect layer. To smooth the entire surface of the wafer, a process called chemical mechanical planarization, or CMP, is used. The next photolithography step now has a flat surface on which to create a pattern for the subsequent layer of the microchip.

CMP is both a chemical and a mechanical process. During CMP, the circuit side is held against a rotating pad while an abrasive liquid chemical called slurry is added. After the deposition of the metal is completed and the surface is smoothed out with the CMP step, another layer of insulating material is deposited between every interconnect layer to prevent shorting out the metal lines on the microchip. These process steps are repeated until all the interconnects have been created and the microchip's structure is complete.

Today's most advanced microchips can have up to ten layers of interconnects. Because a single processed wafer can have a value of tens of thousands of dollars, it becomes increasingly important to verify the integrity of each process step, and if errors are found they must be immediately corrected.

Various metrology and inspection steps are used to monitor wafer-manufacturing processes throughout the fabrication sequence. Metrology and Inspection technologies include:-
1) Defect Inspection
2) Defect Review Scanning Electron Microscopy (DR-SEM)
3) Critical Dimension Measurement Scanning Electron Microscopy (CD - SEM)

Defect Inspection locates defects on the patterned wafers.
DR-SEMs use electrons to image and then automatically classify defects on the wafer, such as particles, scratches or residues enabling engineers to determine the source of the defect.
CD-SEMs measure the "critical dimension" (CD) of the sub-micron-sized circuits in a microchip, assuring the accuracy of the manufacturing process.

CD measurements are typically performed after the photolithography and etch steps. All of these processing has to be done in extremely clean manufacturing facilities called fabrication plants or (fabs), because a single particle of dust can destroy a microchip. The air in the fabs is two million times cleaner than the air we breathe in our homes. To keep the air in the fab clean portions of the building are sectioned into areas called clean rooms and special clothing commonly called the bunny suits are worn by the engineers who work inside. Bunny suits are made up of a special material that stops hair and skin particles from getting into the air. In some ultra clean clean rooms engineers must wear breathing apparatus to clean the air they exhale.

Once all the processing steps are completed, the wafer is cut into individual microchips, which are then tested, packaged and assembled. Microchips are then shipped around the globe and are incorporated into the devices we use everyday. When you pick up the phone, turn on the computer, start your car or watch television, it is the microchip that enables these devices, which shapes today's modern society.

With the broadest product line in the industry, Applied Materials makes the equipment used by chipmakers to create smaller and more powerful microchips. These include CVD, EPI, ETCH, IMPLANT, RTP, PVD, ALD, ECP, CMP and Inspection Systems. Virtually every new microchip in the world is made using an Applied Material system.

Article contributed by Applied Materials