The Time is Now: Sustainable Semiconductor Manufacturing

Historically, volatile perfluorocarbon (PFC) compounds have played an important role in semiconductor fabrication as a source of reactive fluorine atoms used to remove material in etching and chamber cleaning processes. They are stable and tend to have long lifetimes in the atmosphere, making them potent greenhouse gases with high warming potential.

In 1999, quite early in the history of global warming awareness, semiconductor manufacturers committed to reduce PFC emissions by at least 10% below baselines for each region over the next 10 years. By 2010 they had achieved a reduction of 32%, far surpassing the original goal. At that time, they recommitted to further reductions, targeting a normalized emission rate(NER – kilograms of CO2 e per cm2 of silicon) 30% lower than the 2010 baseline.

By 2020 they had achieved a 22.9% decrease, this despite increasingly complex products with more layers, and advanced etch processes that use new gases. The WSC is now working to establish a new PFC reduction 10-year goal that will estimate emissions using the latest 2019 methodologies from the IPCC. (FIGURE 1). The PFC story provides an excellent example of the challenges encountered in the search for solutions to complex environmental problems.

The success in reducing PFC emissions had two major components: the switch to non-PFC NF3 for many chamber cleaning applications and the adoption of abatement technologies that could destroy unconsumed PFC gases that exited the process chamber. PFCs can be destroyed by a plasma or fuel-based burner. Plasma has the advantage of not adding carbon emissions from the burning fuel. However, in practice the choice of technology is often driven by the relative cost and availability of electrical power versus fuel gas.

Even when electrical power is available and cost-effective, the source of that power, i.e., coal or renewable, must also be considered in the net-zero carbon Scope 2 calculation. The solution is further complicated by the design of the burner. Although NF3 itself contains no carbon, burning it in an open flame of hydrocarbon fuel can create PFCs.

The problem originates in the wide variations of temperature across an open flame, and the solution lies in a specially designed burner, known as an inward fired combustor, which maintains a more uniform temperature in the critical regions and largely separates the carbon-containing fuel from the process gases.

Semiconductor manufacturers have long been sensitive to the large power demands of their manufacturing processes, if not from an environmental perspective, then from the point of view of cost. Most estimates put the share of energy used by process equipment at just under half of the total used by a Fab. Of that, about half is used by the pumps used to maintain the vacuum conditions required for many processes to operate.

Pump manufacturers have continuously improved the energy efficiency of their products since the early days of the industry. New mechanisms, higher shaft speeds, power inverter technology and new materials have all contributed. Much of the low hanging fruit was picked long ago, but some areas do remain where further improvements are possible.

One of the most promising is the implementation of idle mode operation, sometimes called green mode, in which the pump is put into a low power state when the process it serves is idle. The greatest challenge is the close coordination required between process equipment in the fab and pumps in the SubFab.

Technologies already exist to support that coordination and some of the resistance to its implementation must be attributed to the reluctance of operators to change any aspect of a high-yield high-volume manufacturing operation. Green mode operations could provide near-term reductions in GHG emissions without requiring fundamental changes to ongoing production.

Unfortunately, other processes coming online now at advanced nodes have the potential to greatly increase power consumption, among them is EUV lithography which uses approximately 10X as much power as conventional 193nm immersion lithography. There are offsetting factors, such as a reduction in the number of processing steps. Still, an IMEC analysis [4] estimated a 3.46 increase in power consumption and a 2.5X increase in GHG emissions per wafer in going from the 28nm node to the 2nm node.

The first manufacturer to implement EUV lithography in high-volume production, a major foundry, saw its normalized energy consumption (KWH per 8-inch equivalent wafer mask layer) increase more than 25%, to 12.5KWH in 2019 after hovering several years just below 10KWH.

While the trend in semiconductor manufacturing is clearly one of increasing power consumption (FIGURE 3), the solution is equally clear. Energy consumption must switch to renewable sources.

It is a solution not lost on major manufacturers. In 2020, the same foundry signed the world’s largest renewable energy purchase agreement, a 20-year deal buying all the energy from a 920-megawatt offshore wind farm being built nearby, and committed to using 100% renewable energy by 2050. [6]. The largest US IDM has committed to 100% renewable energy by 2030. Other major players have made similar commitments, though, as always, the devil is in the details.

The IPCC’s evidence-based conclusions are unequivocal: humans are causing the environment to warm. Its predictions are dire, and the costs of inaction, human and economic, far exceed the costs of mitigation. Although the semiconductor industry is not today among the largest emitters of greenhouse gases, our emissions are significant and growing rapidly.

Measures exist that can provide short-term reductions in GHG emissions from the semiconductor manufacturing process, including well designed PFC abatement and green-mode vacuum pump operation. By far the most critical change is a shift to renewable energy sources across the industry and throughout the supply chain. With the return of the United States to the Paris Agreement, the conference of parties once again includes all the major economic powers. We must demand coordinated leadership and financial commitments from our national leaders and continue to work at all levels, local to global, to reduce GHG emissions.

DR. CZERNIAK is Environmental Solutions Business Development Manager at Edwards Vacuum, where over a long career, he has previously served in a number of leadership positions. He is also a visiting industrial professor of chemistry at Bristol University and a semiconductor industry representative to the IPCC, in which capacity he participated as a reviewer of the recent assessment report (AR6).