Editorial July 2023 Continuing the Semiconductor Saga of Setting up the Manufacturing in India

Rajiv Parikh

15 Jul 2023

1. In deposition, thin films of conducting, isolating or semiconducting materials are deposited on the wafer to enable the first layer to be printed on it. There are special recipes that are programmed in these tools provided by companies like Applied Materials and KLA Tencor to deposit chemicals and other gases on the silicon wafers, all well defined by the receipt. The tools are of the size of a 300 sq ft room and there are several such tools setup in the fabrication unit.
    
2. The wafer is then covered with a light sensitive coating called the photo-resist. There are two types of resist: positive and negative. The main difference between positive and negative resist is the chemical structure of the material and the way that the resist reacts with light. With positive resist, the areas exposed to ultraviolet light change their structure and are made more soluble – ready for etching and deposition. The opposite is true for negative resist, where areas hit by light polymerize, meaning they become stronger and more difficult to dissolve. Positive resist is most used in semiconductor manufacturing because its higher resolution capability makes it the better choice for the lithography stage. Several companies around the world produce resist for semiconductor manufacturing, such as Fujifilm Electronics Materials, The Dow Chemical Company and JSR Corporation. [info extracted from asml.com]
    
3. Lithography determines how small the NPN and PNP transistors on the chip can be. The number of transistors is based on Moore's Law which states that in an integrated circuit, it doubles about every two years. It is not a law of physics but an observation and projection of a historical trend of chip manufacturing, which results in smaller devices as chip sizes keeps on decreasing every 2 years! During this stage, the chip wafer is inserted into a lithography machine (usually these machines are manufacturers by companies like ASML, where it is exposed to deep ultraviolet (DUV) or extreme ultraviolet (EUV) light. This light has a wavelength anywhere from 365 nm for less complex chip designs to 13.5 nm, which is used to produce some of the finest details of a chip – some of which are thousands of times smaller than a grain of sand. Light is projected onto the wafer through the 'reticle', which holds the blueprint of the pattern to be printed. The system's optics (lenses in a DUV system and mirrors in an EUV system) shrink and focus the pattern onto the resist layer. As explained earlier, when light hits the resist, it causes a chemical change that enables the pattern from the reticle to be replicated onto the resist layer.
    
4. Etching removes the degraded resist later to reveal the intended pattern. The wafer is baked and some of the resist is washed away to reveal a 3D pattern of open channels. Etch processes must precisely and consistently form increasingly conductive features without impacting the overall integrity and stability of the chip structure. Advanced etch technology is enabling chipmakers to use double, quadruple and spacer-based patterning to create the tiny features of the most modern chip designs. As with resist, there are two types of etch: 'wet' and 'dry'. Dry etching uses gases to define the exposed pattern on the wafer. Wet etching uses chemical baths to wash the wafer. Companies such as Lam Research, Oxford Instruments and SEMES develop semiconductor etching systems. As with resist, there are two types of etch: 'wet' and 'dry'. Dry etching uses gases to define the exposed pattern on the wafer. Wet etching uses chemical baths to wash the wafer. Companies such as Lam Research, Oxford Instruments and SEMES develop semiconductor etching systems.
    
5. Ion implementation is the process of bombarding the wafer with positive and negative ions to tune the electrical properties of part of the pattern. Raw silicon – the material the wafer is made of – is not a perfect insulator or a perfect conductor. Silicon’s electrical properties are somewhere in between. Directing electrically charged ions into the silicon crystal allows the flow of electricity to be controlled and transistors – the electronic switches that are the basic building blocks of microchips – to be created. This process is known as ‘ion implantation’.
    
6. Packaging is the next step where a diamond saw is used to slice and dice each individual chip. Cut from a 300-mm wafer, the size most often used in semiconductor manufacturing, these so-called 'dies' differ in size for various chips. Some wafers can contain thousands of chips, while others contain just a few dozens. The chip die is then placed onto a 'substrate'. This is a type of baseboard for the microchip die that uses metal foils to direct the input and output signals of a chip to other parts of a system. And to close the lid, a 'heat spreader' is placed on top. This heat spreader is a small, flat metal protective container holding a cooling solution that ensures the microchip stays cool during operation.