In our day-to-day lives, we are surrounded by semiconductors: tiny computer chips that power everything from our mobile phones to our cars. The demand for semiconductors has exploded in recent years and it has become a challenge for manufacturers to produce enough to meet industry needs. This is partly because each semiconductor is made using multiple intricate steps of patterning, deposition, planarization, etching, cleaning, and doping. There can be between 10 and 100 of these steps for etching and cleaning alone, depending on the type of semiconductor being manufactured. Between each step, the semiconductor must be cleaned using powerful, sometimes toxic, chemicals. In addition to being time and resource-intensive, this generates large amounts of materials to be disposed of and requires a special focus on the health of those working with these chemicals every day.

To make semiconductor manufacturing safer and more sustainable, we are developing new green solvent formulations that are safer, more efficient, and better for the environment.

Each semiconductor is made by building up patterns of different materials onto a substrate, which is usually silicon. This is achieved using a process called lithography. First, a sacrificial light-sensitive material, known as a photoresist, is placed onto the surface of the chip. Next, a mask is placed over the chip to only allow light onto certain areas of the chip. When the light hits the photoresist, a reaction occurs that chemically alters the material. The chip is then exposed to a developer, which removes only the material that was exposed to the light. As a result, the pattern from the mask is transferred onto the photoresist on the chip. This pattern is then used to layer on other materials, such as metals and insulators, using a method called etching. The entire process is repeated multiple times until the semiconductor is built. Each time a new material is added to the chip, the photoresist material must be completely dissolved through a process known as “wafer cleaning” before the next layer is added.   

Manufacturers build chips using either a “positive tone” or a "negative tone” process. During the positive tone process, the photoresist material that’s been exposed to the light is dissolved and removed. During the negative tone process, the areas that interact with the light are made insoluble and remain behind. However, the underlying chemistry of the negative tone process is different, and is much harder to remove in the wafer cleaning step. Traditional solvents used for the positive tone process are no longer effective for these negative-tone resists, and longer cleaning processes using more aggressive chemicals are required.

“There are a number of problems that can happen with using negative-tone resist processes in manufacturing,” explains David Rennie, US Technical Service Manager. “First, if you don’t dissolve the photoresist materials fully within each step you end up with leftover particles, which can produce defective chips. It also can clog filters in the cleaning equipment, which are expensive to replace and requires the production line to stop and the whole batch of cleaning solvent — around 10 gallons of chemicals — to be discarded. Undissolved resist will also shorten the effective life of the solvent which increases chemical consumption. Second, if you use more aggressive chemicals, you can damage the metallic components of the chip, rendering them unusable. And third, the number of wafers produced each hour will decrease because the cleaning step in the negative tone process takes longer. Overall, this means fewer usable semi-conductors are produced for each batch of solvent and more chemical waste is generated.”

In addition to the industry’s need for more efficient cleaning solvents, there is a desire by all those working in semiconductor manufacturing to use safer, less toxic chemicals.

“We are motivated both by the need to develop wet cleaning formulations for our customers’ bespoke manufacturing processes and a commitment to move away from the traditional, sometimes toxic, solvents of the past,” says Rennie.

The most problematic chemicals in semiconductor cleaning are N-methylpyrrolidone (NMP) and tetramethylammonium hydroxide (TMAH), as well as the commonly used solvent dimethyl sulfoxide (DMSO). DMSO is itself non-toxic, but it penetrates the skin easily and can potentially carry other toxins from the cleaning process.

“When we set out to design a new formulation for a customer with a challenging negative tone process, we knew all of the functional targets we needed to meet, but we were also able to find a way to formulate the product using less toxic, safer chemicals and without using NMP, TMAH, or DMSO,” says Rennie.

The resulting product was AZ^® Remover 910 [1], which is now being used by customers for a range of different positive- and negative- tone manufacturing processes. “Every customer is using it in a different way,” says Rennie, “but in one company where it has been used for high-volume semiconductor manufacturing for more than a year, they are using 40% less chemicals than they were previously.”

This brings benefits not only in terms of improved efficiency and productivity, but also in a reduced chemical consumption that results in less waste, enabling customers to reduce their environmental footprint.

As semiconductor technology advances and chips continue to shrink in size, the demand for innovative, high performance, and environmentally friendly cleaning solvents is growing.  
“Photoresist materials are being developed to meet new applications for semiconductors all the time, and some of these materials need to withstand higher temperatures or plasma etching processes,” says Rennie. This presents new challenges for manufacturers as they try to improve the materials they use.

To keep up with these challenges, our R&D scientists are using a combination of theoretical chemistry — looking at the chemical properties of the solvents — along with empirical testing of different solvent blends for their cleaning performance. “The theoretical chemistry gives us a starting point, and then the empirical testing is essential for seeing how well the different formulations remove the photoresist, and whether they affect the metal layers of the chips,” says Rennie.

“As the march of technology continues, we need to keep innovating our formulations to keep up with those challenges and provide solutions for the semiconductor industry. The difference is that all our innovation is now being done with less toxic chemicals, finding combinations of green solvents that will work.”

_{[1] https://www.emdgroup.com/en/news/green-solvents-for-chip-production-22-07-2021.html}

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