How Are Semiconductors Made?

Semiconductors are amongst the smallest and most detailed technologies that exist. One thumb-sized chip can contain billions of transistors – the miniature units used for conducting and switching electrical current. Not surprisingly, the fabrication process is often long and complex. Inorganic semiconductors specifically require a highly interdisciplinary process involving physics, chemistry, electronics, metallurgy, and more. Organic semiconductors, such as PEDOT:PSS, are an attractive alternative with tunable electronic properties, better flexibility and cheaper manufacturing.

How Silicon Semiconductors Are Made

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Silicon is the fundamental building block of the semiconductor. As a raw material, silicon is the second most abundant material on Earth and has properties that make it an ideal semiconductor. It is extracted from silica – silicon dioxide – found in sand and quartz.

Preparing Silicon

Silicon mining is common in the US, China, and Australia. Mined silicon has impurities, such as iron which is ideal for certain applications like metallurgy. However, the silicon needed for semiconductor manufacture has to be very pure. Semiconductor grade silicon is produced using the Siemens Process to make the material over 99.9% pure.

The pure silicon is shaped into a cylindrical ingot by dipping the material into molten silicon. The monocrystalline structure is doped with boron or phosphorus to alter its electrical properties. The doped silicon is then cut into thin, flat wafers.

Fabricating a Silicon Semiconductor

The fabrication process transforms the wafer into a platform for housing microscopic electrical components. The manufacture of microchips can be categorised into a series of essential steps involving:

1. Deposition
2. Resisting
3. Lithography
4. Etching
5. Doping
6. Packaging

Deposition

To begin, the material is polished to achieve a smooth, reflective finish. This makes it suitable for a patterned layer to be printed on top.

The silicon wafer is first set in a thermal processing system. The material is exposed to oxygen at high temperature to create a layer of silicon dioxide. Next silane and ammonia gases are introduced to create a silicon nitride top layer.

Resisting

A coating device, such as a spin coater, is used to spin the wafer at high speed to uniformly apply a UV-sensitive layer. The coating is called photoresist, or simply ‘resist’, and comprises two kinds:

• Positive resist exposes the wafer to UV light, increasing the solubility in preparation for lithography. This is the most common type.
• Negative resist is stronger and more difficult to dissolve.

Lithography

The wafer is processed in a lithography machine which projects the desired pattern across the wafer multiple times over. Either deep UV or extreme UV light is focused onto the resist layer. Deep UV uses optical lenses and extreme UV uses mirrors. This causes a chemical change to occur in the coating.

Etching

An etching machine is used to engrave the pattern onto the wafer. This etching selectively removes photoresist material from the wafer, using a plasma etch system, to reveal the desired pattern. The wafer is then baked and developed. The process is either ‘wet’ (utilizing chemical baths) or ‘dry’ (utilizing gases).

The stages of lithography and etching are repeated many times so that different patterns are laid on top of one another. This creates layers of transistors connected via copper wiring which enable signals to travel across the chip.

Doping

Positive and negative ions are used to tune the electrical conducting properties of the pattern on the wafer.

Packaging

Diamonds extract an individual chip from the silicon wafer. One chip is typically 300mm. Once removed, chips are positioned onto a substrate with metal foils directing input and output signals to other parts of the system. The chip is then sealed in a protective epoxy resin casement.

Issues with Silicon Manufacturing

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Microchip manufacture is a complicated and time-consuming process. Defects can easily occur, for example during the lithography step in the process. This means a lengthy measuring and testing process to ensure strict standards are met.

As microchips become smaller, the manufacturing steps become more complex. Alongside this, there are less than two dozen companies worldwide that make them in large enough quantities. This is despite the ever-growing demand for chips.

The expansion of semiconductor manufacturing worldwide comes at a cost to the environment. The process involves vast quantities of water, power, and the release of greenhouse gases. The demand for semiconductors has created a rapidly expanding but largely hidden source of carbon emissions in the technological supply chain. According to one study conducted by Greenpeace, semiconductor manufacturing is projected to emit 86 million tonnes of carbon dioxide equivalent by 2030. This equates to more than Portugal’s total emissions for 2021.

Semiconductor manufacturers, such as Samsung and TSMC, have joined RE100 ––a global pledge to achieve net zero by 2050. A limiting factor of the initiative is the scale of semiconductor manufacturing facilities concentrated in the Asia-Pacific region, where there is heavy reliance on fossil fuels and a lack of accessibility to greener sources.

Austrian chipmaking firm, Infineum, has turned to using hydrogen as a means of becoming more environmentally friendly. There is consensus that other companies should look to follow suit on becoming greener. Greenpeace advocate for reducing the target timeline for renewable energy from the current 2050 to 2030 as an attainable goal for large, well-establishes chip manufacturers.

The issues with semiconductor manufacturing in terms of time, efficiency and environmental impact has triggered an increase in research and development. This has involved scoping out new materials and looking for ways to increase precision during manufacturing, for example in the deposition of materials.

Organic Semiconductors

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Organic semiconductors like PEDOT:PSS have attracted wide attention due to their potential applications in flexible and stretchable electronics. They form an integral component of organic field effect transistors. Applications include use in integrated circuits, flexible displays, memory devices, and wearable materials.

Fabrication of Organic Semiconductors

Materials for organic semiconductors can be synthesized from readily available, inexpensive precursors. The organic materials are subject to more flexible and efficient solution processing techniques such as using spin coating, slot die coating, dip coating, which are readily available to most university laboratories. These methods can be easily translated to high throughput printing techniques, such as inkjet printing and roll-to-roll fabrication. Unlike with traditional physical vapor deposition techniques used in the case of silicon, solution processing can take place at room temperature, further reducing energy costs.

The structure and organisation of organic semiconductor films can massively affect their performance and carrier transport. Although small molecules can form high-quality crystal structures, you have to maintain tight control over their molecular alignments. Solution processing is ideal under these circumstances as the process can be carefully controlled. This allows the time and space needed to assemble crystalline structures.

Solution processing is thus widely used in the laboratory to control aggregation making highly ordered or aligned molecular structures. These techniques have been used to successfully prepare highly aligned crystals and nanowires. Conventional solution processing methods can be achieved using a spin coater, slot die coater, or dip coater. All methods “aim to attain better molecular ordering and alignment of organic semiconductors, which is significant for enhancing the electrical performance of the final devices” (Wu, et al., 2022).

Thin Film Alignment

The principles of solution processing techniques for the control of alignment in organic semiconductors has been classified into four categories (Wu et al., 2022):

1. Template/substrate assisted alignment - Focuses on the interface interaction between semiconductors and their substrates.
2. Directional-drying-induced alignment - Focuses on molecular chain alignment caused by drying of the solution in a specific direction.
3. Meniscus guided alignment - Based on alignment behaviour in meniscus (curved surface between air and solution) formed by relative movement of the solution. Important for techniques such as slot-die coating.
4. External force induced alignment - Focuses on alignment resulting from an external force acting on the solution system.

Limitations of Organic Semiconductors

Newer processing methods are not without limitations. The performance of solution-processed organic semiconductor thin films is easily affected by fabrication conditions, including concentrations, temperatures, and solvent properties. These factors make it difficult to control the crystallization process and the lack of precision increases the risk of lattice misorientation.

The Future of Semiconductor Manufacturing

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The techniques for printing organic semiconductors have yet to be scaled up from the laboratory to industrial settings. Directions for future research include:

• How to achieve high orientation and effective molecular stacking with solution processing.
• Finding more environmentally friendly manufacturing processes, including the use of green solvents.
• Gaining a better understanding of the crystallization process.

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