A software platform currently in beta testing creates a virtual fab to help the IC manufacturing value chain reach environmental targets by quantifying the manufacturing impact through a bottom-up approach with realistic process conditions and validated data from partners as well as imec state-of-the-art fab and equipment.

Early in my career, I worked for a company that invented a step-and-repeat process for shooting circuit images onto wafers. The company also made equipment that deposited photoresist on wafers, sent them to lithography equipment &mdash once called “steppers” but today called “scanners” &mdash and received wafers back for photoresist development. At that time, engineers gave no thought to the resulting energy consumption or gas emissions.

Times have changed. We now see that we’ve been wasteful, and engineers are looking into ways to reduce the environmental impact of electronics manufacturing. That includes semiconductor processes. How can semiconductor process engineers get a grip on and address sustainability in IC manufacturing? Engineers at imec have developed a software platform that models the emissions produced in semiconductor manufacturing; it can be used to prioritize high-emission process areas like lithography and etch for development. EE World spoke with Emily Gallagher, a member of the technical staff at imec about imec.netzero which is currently in development but will be made available to the industry.

To shrink the size of semiconductors and increase circuit density, process complexity increases, driving more steps that cause an increase in carbon emissions. That’s where imec’s Sustainable Semiconductor Technology and Systems program (SSTS) comes in.

Working in imec’s patterning department, Gallagher has recently shifted most of her work to sustainability, which led to investigating and modeling the lithography and etching processes in semiconductor manufacturing. Working with partners, imec has developed the imec.netzero modeling platform, a virtual fab. Through imec.netzero, process engineers can model the inputs and outputs of the processes from a sustainability perspective. Although imec.netzero is currently in use by imec’s SSTS partners. Imec is also testing a more limited version of imec.netzero for public use.

“We recognized that there wasn’t a common language for assessing sustainability early in the IC chip design process,” said Gallagher. “What is the impact of fabricating a semiconductor wafer?”

“The modeling software,” continued Gallagher “will eventually be made public because our mission is to help the semiconductor industry improve its environmental footprint. It starts from the very bottom, looking at the different tools required to make a chip: what processes are needed, which gases are consumed, what utilities are required?”

The software uses tool and process information functions as building blocks that are assembled logically to mimic the process flow of an actual fab building for a given technology. Through those calculations, engineers can create Pareto charts to see which processes are the big drivers in equivalent CO2 emissions. The normalized chart in Figure 1 shows that as logic nodes become more advanced, the environmental impact of those technologies grows.

Gallagher noted that patterning produces about 45% of the overall CO2 equivalent emissions for the N3 logic node, making lithography and etching appropriate areas for emission-reduction efforts.

Gallagher continued by saying that management at large companies such as Microsoft and Apple understand the need to lower emissions, not just their own operations but their entire value chain including the chips they use. “That’s where we help because we can create a quantitative tool for identifying problem areas and defining development projects to help drive down the semiconductor process emissions.”

Not just CO₂
We tend to think of CO₂ as the gas that needs reduction, but many other gases have higher global warming potential than CO2. Fluorine-containing etch gases and hydrogen trap much more heat in the atmosphere than CO₂ for the same mass of gas emitted. How is hydrogen used in lithography? Gallagher explained.

The most advanced scanners used in manufacturing today (Figure 2) are EUV lithography tools that use 13.5 nm light to put the pattern on the wafers. The EUV photons are absorbed by most materials, so the optical mirrors and photomask are reflective. During exposure, the EUV light is scanned across the patterned photomask to project the pattern onto the wafer. This process is repeated at different locations across the wafer to generate multiple copies of the 4X reduced mask across the entire wafer. /

Contamination can be deposited on the reflective surfaces. Any loss of reflectivity is a problem for throughput. Hydrogen plus the EUV light in the scanner creates a hydrogen plasma that etches hydrocarbons to keep the optics clean.

Hydrogen is light and relatively transmissive to EUV light and can be used to create flow patterns that keep particles away from reflective surfaces. That’s important, especially in the region of the EUV source that creates a 13.5 nm light by introducing tin droplets. These droplets are heated until a plasma is formed; the process generates both desired EUV light and undesired Sn debris. To minimize the migration of debris into sensitive regions of the scanner, the flows in the source are especially high.

According to Gallagher, hydrogen has a high global warming potential that’s about 13 times CO₂. Currently, it’s diluted or just burned off, not recycled.

In a press release, Gallagher said that imec recently installed a hydrogen recovery system for EUV lithography in its 300 mm wafer cleanroom, resulting in approximately 70% reuse and recovery. Additionally, imec is increasing its focus on materials and exposure conditions that reduce the EUV dose required. There is an obvious correlation between reducing the dose and reducing emissions per wafer. Engineers at imec have also identified etching directions for improved sustainability with a focus on the overall reduction of conventional gas consumption. Replacing existing high global warming potential gases will be a future area of development.

Measurements
Like any software platform, imec.netzero needs data. That comes from both measurements at imec and partner-provided information. The process can become relatively complex. For example, power and material consumption can differ in process and idle modes. Additionally, process engineers must consider more than just process gas flow. “You need to account for the production of the gas, the volume of process gases for a given type of process, and then for the abatement of the gas afterward. There’s a lot of tool information required to create this emissions calculator.”

“For lithography, emissions are dominated by the electricity required to create the EUV photons. For etch tools, emissions are driven through high global warming potential (GWP) gases like CF4, CHF3, NF3, and SF6.

The global-warming potential of gas is based on a multiplier relative to its warming potential versus CO2. Many of the gases used in etching today, noted Gallagher, have very high global-warming potentials — thousands of times that of CO₂. To assess and improve etch process emissions, one must consider not just the incoming gases but process byproducts and what remains after abatement (the systems that are used to reduce emissions below their respective threshold limit value for release into the atmosphere). “In general,” said Gallagher, “there are a lot of unreacted byproducts that aren’t fully abated, or it’s very environmentally expensive to abate them.”

What can engineers do?
When asked this question, Gallagher replied “Without intervention, the emissions associated with the IC industry are likely to increase by as much as 12%. This is the result of two trends: the increased emissions associated with more advanced technologies that suggest a 2x growth over the ten years from 2020 to 2030 and the increase in the market size if the industry doubles by 2030. Those two effects create an 8% growth, but it could be slower or faster than that. As fabs deploy renewable energy and state-of-the-art greenhouse gas abatement is deployed in every fab, there can be a significant reduction in emissions, but we will still be 2.5x off the 50% reduction target of the Paris Agreement. To achieve further reduction, research across the value chain is needed, and it is needed now.”

Next up at imec: semiconductor packaging.