Process Tutorial

What are chips made of?
Semiconductors, MEMS, and now Nanodevices, are fabricated on thin slices of silicon called wafers . Once all processing is completed the wafer is cut into small pieces commonly referred to as chips . In many cases these chips are used to control the functions of a computer, hence the term " Computer Chips ". On a 100mm wafer there may be 1000 chips of a particular size, but on a 200mm wafer you could fit as many as 4000 chips of the same size. Since there is only a slight difference in the cost of processing the larger wafers as opposed to the smaller ones, it may be more cost effective for companies to fabricate their devices on the larger size wafers. When each chip can be sold for as much as $150.00 this can make a significant difference in profit.

What are Thin Films?
Thin films are layers of either conductive or non-conductive films that are added to the top surface of the wafer. These films may be used to form interconnects between devices or insulators between interconnect layers. The process step that produces the thin film is referred to as deposition . A variety of materials can be deposited on a wafer. Three main categories of thin films materials are conductors, insulators, and semiconductors. Several different techniques are employed for deposition, but the process can generally be thought of as a uniform layering of material on the surface of the wafer. Two common techniques are Plasma Enhanced Chemical Vapor Deposition (PECVD), which uses electrical energy to speed up a reaction between reactive gases while under vacuum, and Low Pressure Chemical Vapor Deposition (LPCVD) which uses heat to speed up a reaction between reactive gases while under vacuum. The result of this chemical reaction is a material or "film" which is deposited onto the wafer surface. Oxide is often deposited using the PECVD method when high temperature thermal oxidation is not possible.

What is Thermal Oxidation?
Thermal Oxidation is the process of growing a film called Silicon Dioxide, more commonly referred to as Oxide. Thermal oxidation of silicon is easily achieved by heating a silicon wafer to temperatures typically in the range of 900C to 1200C in either a steam or oxygen environment, after which a chemical reaction will take place. In the simplest terms Oxide is "Silicon Rust" on the surface of a silicon substrate. The fact that silicon can be oxidized is one reason it is so widely used in chip manufacturing.

How Oxide is used?
Oxide is primarily used as an insulator between electrical pathways in miniature electronic devices, or what you might think of as a computer chip. Just like the electrical system in your car, the electrical pathways on a chip must follow a certain configuration. If a wire is connected in the wrong place this can create a short. The electrical system in a computer chip works exactly the same way except instead of the wires being protected by plastic coverings they are insulated by silicon dioxide. That's what is meant when we refer to Silicon Dioxide as an insulator.

Metal Films
Various types of elemental metal (e.g. aluminum, gold, copper, nickel, titanium), metal alloys, and other metal compounds are used to from interconnection layers within a device. The layers of metal added during the fabrication process create electrical connections and allow current to flow throughout the device. You can think of this as the electrical system of a computer chip. Metals are deposited by a variety of techniques, the most common being thermal evaporation and sputtering. After the metal has been deposited onto the entire wafer, it may be patterned into interconnects using the Photolithography and Etch techniques described below.

What is Silicon Nitride?
Silicon nitride is a thin film that can be deposited onto a silicon wafer during device manufacturing using either of the two CVD methods described above. Silicon Nitride has many useful properties. One reason this material is so widely used is its ability to withstand exposure to a number of chemicals used during the device fabrication process. Consequently, silicon nitride is often used as a protected layer during chemical etching. This unique material can also be very flexible without cracking or tearing. Silicon Nitride is often used in MEMS devices as a membrane or "diaphragm" in miniature pressure sensors. These types of pressure sensors are being used in today in the automotive industry in ABS brake systems, fuel systems, and emission control systems. Silicon Nitride is also used for its optical properties. This material can be used in optical devices that direct light into fiber, which have revolutionized the telecommunications industry. This use of fiber optics in communications has enabled our communications network to become faster and more reliable.

Photolithography
Many steps in the device fabrication process are intended to impact only certain areas of the wafer. To define the desired areas on the wafer (and block the remaining areas) a series of masking layers are used. This allows us to transfer shapes that are created using CAD software onto the surface of the wafer with sub-micron resolution. The process of photolithography involves the use of a material called photoresist to generate a specific pattern on the surface of the wafer. Photolithography is useful because it can transfer the pattern to the wafer surface very quickly. Photoresist is a light-sensitive material which can be processed into a specific pattern after being exposed to light energy in the shape of the desired pattern. The process is somewhat similar to photography where light energy forms a pattern on the film. Once the photoresist has been developed, only certain areas of the wafer are covered with photoresist. The subsequent process steps will only affect the areas there is no photoresist present.

Etch
Once the photolithography process has been completed, the areas that remain unprotected by photoresist are removed in a process called etching. By etching the wafer you are creating a three dimensional representation of the design that was transposed onto the wafer during the photolithography process. The etching is usually achieved by either placing wafers into a chemical bath (Wet Etching) or by activating certain reactive gases with electrical energy while the wafers are placed in a chamber that is under vacuum (Plasma Etching).

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