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1、 EMD Electronics is the electronics business of Merck KGaA,Darmstadt,Germany in the U.S.and Canada.Investing in a Sustainable Semiconductor FutureMaterialsMatterIntroduction Emissions from semiconductor manufacturing are a growing segment of global greenhouse gas(GHG)emissions.There are two reasons
2、behind this trend.First,the demand for semiconductor chips is growing.Our technology appears in everything from mobile phones to automobiles,where the number of chips per vehicle increases every year.Data storage,which relies on the semiconductor industry,is exploding.Second,todays manufacturing pro
3、cesses have more deposition and etch steps than ever.Each step consumes water and electricity and creates GHG emissions.Semiconductor companies large and small talk about achieving climate neutrality by 2030.That sounds like a great goal.But merely achieving that goal wont solve the emissions proble
4、m.How we get there matters.Buying carbon offsets is an easy way out.It isnt the best long-term answer,because many offsets are not as effective as they claim to be.Some may even make the problem worse,defeating the purpose 1.And relying on offsets can make internal actions seem less pressing.It is b
5、est to see offsets as a temporary or last-choice option.Semiconductor industry leaders are,of course,doing more than buying offsets.They are investing in renewable energy,improving the energy efficiency of their processes,and finding ways to reduce waste.These actions are helpful,and we must do more
6、.Despite modest success in reducing emissions per wafer or per revenue,demand for semiconductor chips is growing faster than the improvements can handle.We need more drastic reductions,and that starts by examining the sources.“Reducing PFC emissions from fabs is critical for the next stage of semico
7、nductor industry growth,since fluorocarbons and other gases with high global warming potential comprise a large portion of overall fab emissions.Careful design of etch gas modules will enable fabs to reduce emissions at the source while continuing to drive etch performance.Head of Specialty Gases Re
8、search&Development,MerckDr.James Nehlsen2Emissions sourcesTaking the first stepFor the semiconductor manufacturing supply chain,emissions come from three primary sources.First is the energy and resources needed to produce raw materials.Second is the energy required to transform those materials into
9、products.The third source is the energy used when operating the products(equipment and electronic devices).Taking the iPhone as an example,about 80%of its emissions come from making the phone.The longer the phone stays in use,the better.Individuals can help reduce emissions by waiting to trade in th
10、eir phones for newer models.At the same time,manufacturers can reduce emissions per device and make their products easier to repair.Once companies migrate to powering their fabs and factories with renewable energy,materials are the next-largest category of GHG emissions.Materials-related emissions f
11、all into either Scope 1(direct)or Scope 3.Regarding Scope 3,GHG emission per ton of raw materials varies wildly.Changes to the chemicals and materials used in production make a difference.The more important factor,however,is the Scope 1 emissions.Process gases used during manufacturing consume energ
12、y and can become a direct source of GHG emissions if released into the atmosphere.Device manufacturers can only switch to more sustainable materials if those materials are available at scale and meet their required performance specifications.Therefore,materials suppliers near the beginning of the va
13、lue chain have a responsibility to help their customers,and their customers customers,achieve their sustainability goals.We need continued investment in research and development to qualify new materials for use in a wide range of tools.With thousands of materials-based process steps involved in semi
14、conductor manufacturing,the biggest challenge is knowing where to start.In this white paper,we examine a ubiquitous process,dry etch,to demonstrate how modifying just one of these steps can make a difference.Dry etch is used in a wide range of applications from front end-of-line(FEOL)to back end-of-
15、line(BEOL)for both memory and logic devices.It is a critical process step for many 3D structures with high aspect ratios(HAR).It also uses process gases that contribute significant GHG emissions.Just imagine the positive impact on the environment if we can replace these gases,address the challenges,
16、and adopt a new,more effective approach.3Global warming potentialGHG emissions are expressed as tons of carbon dioxide equivalent(CO2e).Its not surprising that when people consider greenhouse gases,they usually think of CO2 and perhaps methane(CH4).But there are hundreds of greenhouse gases,most of
17、which are more potent than CO2.The effect is not minor.The global warming potential(GWP)of many of the process gases used in fabs is several orders of magnitude greater than that of CO22.As a result,etching and cleaning process-es are responsible for most of the direct GHG emissions from the semicon
18、ductor industry.Data from 2017(see Figure 1)suggested that these steps emit 74%of the total 3.Because the number of etching steps has increased since then,the percentage is probably even higher today.Etching uses high-GWP gases and also generates them as byproducts.Carbon tetrafluoride(CF4)and trifl
19、uoromethane(CHF3)are the most common byproducts.It is important to avoid releasing these high-GWP gases into the atmosphere.Fabs have long been aware of this issue,and many fabs use abatement techniques to contain and destroy some of the problem compounds.Water scrubbers in fabs capture certain comp
20、ounds and neutralize them,but scrubbing does not break down high-GWP fluoro-carbons.Point-of-use(POU)thermal abatement devices reduce fluorocarbon emissions,but they arent a cure.The abatement devices also burn fuel and create liquid or solid waste.In addition,they are only partial-ly effective in r
21、emoving the GHGs.It is better to avoid the gases with the highest GWP in the first place.CF4 and sulfur hexafluoride(SF6)are the most problematic because of their relatively low destruction efficiencies(70%-90%).Even with abatement,10%to 30%of the gases are not destroyed.Figure 1.Role of fluorinated
22、 materials in semi emissions.(Data sources:US EPA GHGRP,Electronics Industry,Oct 2018 and IPCC 5th Assessment Report,2014)4Improving the sustainability of dry etchDry etching is often used in complex designs with HAR features.These structures enable necessary increases in feature density but are als
23、o difficult to etch.The more aggressive the device geometry,the longer the etching time,and the more likely that something will go wrong(see Figure 2a).It is difficult to remove layers from deep trenches without redepositing byproducts at the bottom of the trench or on the sidewalls(see Figure 2b).E
24、tchant performance is critical.Etch rate needs to be as high as possible to keep throughput reasonable when etching deep,narrow features.Selectivity is crucial to ensure accurate pattern transfer.New dielectrics and thinner layers put additional demands on selectivity.Any changes in materials or pro
25、cesses designed to improve sustainability will not succeed if their implementation comes at the expense of performance or yield.Dry etching involves plasma etchants.When etching silicon dioxide(SiO2)dielectrics,carbon-based,fluorinated gases are used.Traditional dry etchants include CF4 and octafluo
26、rocyclobutane(C4F8).These gases are stable,easy to handle,relatively safe,and offer good etch performance.Unfortunately,the stability that makes them excellent etchants also means they are harder to abate.The higher the decomposition temperature,the more energy the abatement process consumes.Figure
27、2a.Typical HAR etch challengesFigure 2b.Surface reactions5The GWP is only one aspect of the emissions potential of a gas.It addresses the input but does not consider byproducts generated during processing.These byproducts may also be greenhouse gases with a different GWP.For example,emissions from a
28、 system using C4F8 may include up to a dozen other chemicals.Changing the input gas changes the output,but not always as expected.Multiple chemical reactions occur inside the dry etch chamber.The details vary depending on the chemistry of the surface and the etching conditions.Gas pressure,plasma po
29、wer,and etchant flow rate affect the results.Studies on CF4 and C4F8 demonstrate how doubling plasma power can cut emissions by 10%to 15%percent(Table 1).Adjusting oxygen levels inside the chamber can improve etch-gas efficiency and reduce total GHG emissions.We need direct emissions measurements to
30、 show what species are emitted and in what quantities during a particular dry etch process.Mass spectrometry is the standard method for making these measurements 4.With this data,we can evaluate abatement methods for each gas species.From a sustainability perspective,replacing conventional dry etcha
31、nts with low-GWP gases is great;however,in practice,it takes work to do it right.Readily available drop-in replacements that meet all the requirements might not exist.Solutions will likely involve a combination of new materials,changes in process conditions,and more efficient abatement.Table 1.A ser
32、ies of runs using either CF4 or C4F8 shows the impact of the impact of inductively coupled plasma power plasma power on the resulting emissions.Process GasHigh-GWP Emitted SpeciesEmissions(kg CO2eq)1200W900W600WCF4CF4.350.359.387C4F8CF4C2F6C4F8.861.880.9856Choosing better process gasesIf we want to
33、migrate away from traditional dry etchants,there are many fluorocarbon molecules to consider.Figure 3 shows the atmospheric lifetime and GWP of a few of these molecules.Fluorocarbon refrigerants like C2H3F3 and C2H2F4 have shorter lifetimes than traditional etchants.These arent ideal,though,because
34、their GWP is still much greater than that of CO2.Although many fluorocarbons have extremely high GWP,not all fluorocarbon molecules are green-house gases.For example,hexafluoro-1,3-butadiene(C4F6)has a GWP of 0.003.Its atmospheric life-time is one day,and it is not considered a greenhouse gas 2.C4F6
35、 has been commercially available since the early 2000s for the etching of silicon dioxide and silicon nitride 5.Simply choosing an input gas with low GWP and confirming output emissions is not enough.Additional factors complicate the gas-selection procedure.Some etchants are,by nature,corrosive and
36、reactive.They need to be to do their job.Others are non-reactive,which is why they are hard to abate.Many etchants are toxic.Even though ventilation systems and protective clothing protect workers from im-mediate hazards,it is best to consider the hazard profile of all fab chemicals.Cost,safety,scal
37、ability,waste generation,and abatement efficiency all come into play when search-ing for replacement etchants.Etch performance is critical.It would be great if one process gas could meet the requirements for all these factors.The reality is more like the examples in Table 2.Engineers should expect t
38、o make trade-offs when selecting the best option.Figure 3.The sustainability challenge:Addressing emissions with chemistry.(Data Source:Inventory of U.S.Greenhouse Gas Emissions and Sinks:1990-2018,Annex 6,US EPA)7The selection processKnowing that trade-offs are necessary,how do we approach selectio
39、n?It starts with a comprehensive search through hundreds of possible molecules.Within the universe of fluorocarbons,subtle structural changes can make significant differences in etch performance and environmental impact.A typical research,screening,testing,and pilot-scale manufacturing development c
40、ycle can take five to 10 years.Add in time to develop a process that works in high-volume manufacturing,and it can take between eight and 15 years from concept to scalable product.That is far too long.2030 is only seven years away.Data science and automation can speed up the screening process.Predic
41、tive models screen out molecules that do not meet minimum requirements.Only molecules that pass the screening move forward.If the GWP index,etch performance,or costs are unacceptable,that molecule is not an option.The next screening step examines expected output emissions.Computational quantum chemi
42、stry can help scientists estimate likely byproducts,reducing the time needed for extensive laboratory measurements.A third level of screening checks for waste production,possible hazards,scalability,and abatement efficiency.Molecules that pass all the theoretical checks head to the lab for synthesis
43、,purification,and testing.Unexpected impurities might cause yield problems.At this stage,there are no easy shortcuts.Rigorous testing is necessary to qualify any new etchant.Table 2.Example of new material selection process.A comprehensive evaluation of etchant material and etch process optimization
44、 will drive to successful development of low-GWP gases.8Ideally,automation in the early research stages can cut years off the total development time.Collaboration with customers can further speed up the process because it allows some steps to happen in parallel.Co-development also reduces the risk.E
45、arly testing at a customer site can make it easier to go from small-scale to high-volume production.The customer can help the supplier identify potential problems upfront and make process changes before attempting to scale production.SummaryMaterials are a major contributor to GHG emissions from the
46、 semiconductor industry.Within the ma-terials category,process gases used in etching and cleaning steps are the largest source.Given that,we need to speed up the process of qualifying new gases with lower GWP.A combination of automated screening,rigorous testing,and customer collaboration can reduce
47、 development time from eight to 15 years down to as low as three years.Decarbonizing the semiconductor industry wont be easy.By tackling the largest problems first and streamlining the process as much as possible,we stand a much higher chance of success.While im-proving dry etch processes alone wont
48、 save society from the problems of climate change,its a good start.We all have a responsibility to do our part.Lets work together to make a difference.“It is amazing where collaboration can take us.Sustainability is no longer the result of individuals;only by working together can we get closer to ou
49、r goals for a sustainable future!Head of Sustainability Electronics,MerckBritta Grundke91 Patrick Greenfield,“Revealed:more than 90%of rainforest carbon offsets by biggest provider are worthless,analysis shows,”The Guardian,18 Jan 2023.https:/ Gunnar Myhre et al,“Anthropogenic and Natural Radiative
50、Forcing.”In:Climate Change 2013:The Physical Science Basis.Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change Stocker,T.F.,D.Qin,G.-K.Plattner,M.Tignor,S.K.Allen,J.Boschung,A.Nauels,Y.Xia,V.Bex and P.M.Midgley(eds.).Cambridge University Pr
51、ess,Cambridge,United Kingdom and New York,NY,USA,2013.3 U.S.EPA Greenhouse Gas Reporting Program(GHGRP),Electronics Manufacturing Sector Profile,2017.2011-2017 Greenhouse Gas Reporting Program Industrial Profile:Electronics Manufacturing Sector(epa.gov).4 International SEMATECH Manufacturing Initiat
52、ive,“Guideline for Environmental Characterization of Semiconductor Process Equipment,”Technology Transfer#60124825A-ENG,December 2006.5 A Nicoletti et al.,“C4F6 1,3 Hexafluorobutadiene-A New Etching Gas:Studies on Material Compatibility,Behavior in Inductively Coupled Plasma and Etch Processes Perfo
53、rmance,”NIST Publications,1 Jun 2003.https:/www.nist.gov/publications/c4f6-13-hexafluorobutadiene-new-etching-gas-studies-material-compatibility-behavior-0References“Were designing a process for large-scale NF3 abatement as a prerequisite to meet our long long-term GHG goals and to drive decarboniza
54、tion in the industry.One pilot project that has been very successful has prevented over 90%of the NF3 from reaching the atmosphere.This project offers enormous promise to reduce emissions of a harmful gas that can remain in the atmosphere for hundreds of years.Secondly,we are working with our custom
55、ers to identify alternative low-GWP gases,and that will help prevent emissions of gases that stay in the atmosphere for thousands of years and will enable our customers to reduce their scope on emissions.VP and Global Head,Semiconductor Materials,MerckAnand Nambiar10In addition to the etchant gases
56、discussed in this paper,Merck is committed to finding alternatives to the harmful per-and polyfluoroalkyl substances(PFAS)used in electronics manufacturing.https:/bit.ly/SustainableMaterialsSolutionsLearn aboutour PFAS Challenge here:EMD Electronics is the electronics business of Merck KGaA,Darmstadt,Germany in the U.S.and CanadaSCAN ME