Article pubs.acs.org/est
Environmental Life Cycle Assessment of a Carbon Nanotube-Enabled Semiconductor Device Lindsay J. Dahlben,†,‡ Matthew J. Eckelman,§ Ali Hakimian,† Sivasubramanian Somu,† and Jacqueline A. Isaacs*,† †
Department of Mechanical and Industrial Engineering and Center for High-rate Nanomanufacturing, Northeastern University, 360 Huntington Avenue, Boston, Massachusetts 02115, United States ‡ Raytheon Company, Integrated Defense Systems, 350 Lowell Street, Andover, Massachusetts, 01810 § Department of Civil and Environmental Engineering, Northeastern University, 360 Huntington Avenue, Boston, Massachusetts 02115, United States ABSTRACT: Carbon nanotubes (CNTs) demonstrate great promise in a variety of electronic applications due to their unique mechanical, thermal, and electrical properties. Although commercialization of CNTenabled products is increasing, there remains a significant lack of information regarding the health effects and environmental impacts of CNTs as well as how the addition of CNTs may affect the environmental profile of products. Given these uncertainties, it is useful to consider the life cycle environmental impacts of a CNTenabled product to discover and potentially prevent adverse effects through improved design. This study evaluates the potential application of CNT switches to current cellular phone flash memory. Life cycle assessment (LCA) methodology is used to track the environmental impacts of a developmental nonvolatile bistable electromechanical CNT switch through its fabrication, expected use, and end-of-life. Results are reported for environmental impact categories including airborne inorganics, land use, and fossil fuels, with the largest contributions from gold refining processes and electricity generation. First-order predictions made for the use and end-of-life (EOL) stages indicate that the CNT switch could provide potential improvements to reduce environmental burden during use, although CNT release could occur through existing EOL processes.
■
INTRODUCTION Carbon nanotubes (CNTs) are nanoscale cylindrical allotropes of carbon that are becoming increasingly utilized in electronic device research, development, and production. A particularly promising set of applications exists in the semiconductor industry as components for memory storage devices, interconnects, and transistors.1 At the same time, however, the literature indicates considerable variability and uncertainty regarding the health impacts, reactivity, ecological effects, and environmental fate and transport of CNTs.2−5 In order to consider the widespread technological potential of CNTs in the context of their potential risks, the environmental implications of CNT-enabled products should be evaluated during development to identify and control potential risks that may occur throughout the entire life cycle of the product.6−8 There are two main categories of CNTs: single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs). SWCNT geometries reflect that of a single graphene sheet seamlessly wrapped into a tubular shape, whereas MWCNTs consist of an array of these nanotubes that are concentrically arranged. CNTs are highly desirable for use in a variety of different applications and industries due to their unique properties, including high tensile strength, a high © XXXX American Chemical Society
Young’s modulus, good thermal conductivity, and, depending on the chirality, semiconducting or metallic conductivity.9 Mechanically, the strength, stiffness, and flexibility of nanotubes are significantly higher than those of conventional carbon fibers.10 SWCNTs in particular also have a very high current densitythe current density for metallic SWCNTs is approximately 1,000 times greater than for copper.11 Since the initiation of the National Nanotechnology Initiative (NNI) in 2001, the NNI has invested over $18 billion in research and development (R&D) in the field of nanotechnology and has a proposed budget of nearly $1.8 billion for the 2013 fiscal year.12 According to Nanowerks,13 there are more than 100 companies globally that are fabricating CNTs, and this is expected to double within the next five years. CNTs currently account for a 28% market share of overall nanomaterials demand. The global CNT industry had a total production value of $668 M in 2010, with $631 M attributed to MWCNTs and $37 M attributed to SWCNTs. Total Received: December 31, 2012 Revised: April 26, 2013 Accepted: May 28, 2013
A
dx.doi.org/10.1021/es305325y | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Article
may very well alter the product’s environmental impact and require different handling during use and end-of-life (EOL) management. In this work, an original environmental assessment of a high order, low concentration CNT product is presented. The specific CNT product evaluated is a developmental semiconductor device that utilizes a SWCNT to form a nonvolatile bistable electromechanical CNT switch (Figure 1). (More
production is forecast to grow to $1.1 billion by 2016 at an annual growth rate of >10%.13 This growth is expected to be driven by the electronics and data storage sector as nanotube technology, scalability, repeatability, reliability, and supply quality improves. While SWCNTs are more expensive to produce than MWCNTs, they are more likely to be used for advanced electronic products that require higher nanotube order and alignment. The electronics industry is expected to utilize CNTs for many purposes, but applications including sensors and memory devices will require their use at a high order and low concentration. CNT synthesis is typically done using one of the following four methods: arc discharge, laser ablation, chemical vapor deposition (CVD), and high-pressure carbon monoxide (HiPco) processes. Following CNT synthesis, semiconductor device fabrication is a series of manufacturing processes used to create many nanoenabled devices and generally follows a set of common processes. Fabrication is done on high-purity crystalline silicon waferstypically 200 and 300 mm in diameter for industrial manufacture. Manufacture of semiconductor devices consists of layering, patterning, and functionalizing the wafer’s surface to create chips. Although there are variations in the recipes and sequencing, the unit operations are reasonably generic: wafer cleaning, furnace operations, lithography, etch, metallization, chemical mechanical polishing (CMP), diffusion, and ion implantation. The amount and frequency of processing to create a given device depends on the function and complexity desired and can easily take hundreds of steps. The semiconductor manufacturing industry is both energyand water- intensive.14,15 Wafer fabrication itself consumes a vast amount of energy due to the use of processing equipment and cleanroom environments, including air flow recirculation systems, process cooling water systems, and pumps and compressors for ultrapure water and gases.15 The industry also makes use of numerous chemicals (many of them highpurity), as reagents or processing aids. Although the exact types and amounts of chemicals differ greatly between the process recipes, fabrication facility, and target semiconductor device, there are some generalities that can be made. For example, nitrogen is by the far the most widely used gas in wafer fabrication, with other gases including oxygen and argon. Wafer cleaning and wet etch processes use a variety of cleaning agents including sulfuric acid, hydrogen peroxide, hydrochloric acid, hydrofluoric acid, ammonium hydroxide, and phosphoric acid and sometimes perfluoro-compounds such as SF6 and CHF3. Further, wafer fabrication utilizes a substantial amount of high purity water, requiring a limit of only a few parts per billion of impurities. A number of previous environmental assessments have been performed for CNT-enabled products, including construction materials, polymer composites, textiles, and electronic applications such as lithium-ion batteries and field emission displays.16−19 In the first three categories, the CNTs are dispersed within the host material to form a reinforced composite. Such products can be referred to as having a low order, high concentration of CNTs, due to the natural, random dispersion of CNTs within the host material and the large amount of CNTs used in the product, respectively. Generally, CNT-enabled devices that require high order and low concentration are more likely to be in the R&D phase, whereas several low order, high concentration products are already on the market. These variations in CNT order and concentration
Figure 1. Isometric and cross-section schematic of CNT switch. The SWCNT spans across gold electrodes and with applied voltage, it deflects to complete the circuit.
information on the principles of operation and functionality can be found in the patent filing.20) The present study provides a detailed life cycle inventory for the production of this nanoenabled semiconductor device and quantifies the environmental impacts of synthesis and the devices life cycle stages of fabrication to expected use to EOL management to provide an overall, first-order prediction of the product’s environmental footprint and the relative contribution of different materials and processes. Thus, this work provides an evaluation of the environmental implications of using CNTs in electronic components relative to other semiconductor manufacturing processes and offers insights for sustainable development of CNT-enabled technologies.
■
METHODOLOGY Environmental evaluation of the CNT switch is carried out using a process-based life cycle assessment (LCA) methodology with four distinct stages to generate a comprehensive overview of the product’s total environmental effect: goal and scope definition, inventory (LCI), impact assessment (LCIA), and interpretation.21 This is a “cradle-to-gate” LCA that includes upstream inputs such as raw materials extraction and processing of the input materials (metals, high-purity chemicals, the Si wafer, and the CNTs themselves) and energy as well as the inputs and emissions associated with device fabrication. The study scope is extended in later sections to include environmental estimations during the expected use and end-of-life stages. Minor processes such as assembly of the non-CNT electronic components, transport of the device to end-users, and refining of recovered materials were not assessed. Figure 2 illustrates the system boundaries of this study. Fabrication of the CNT switch uses a total of 15 process steps and employs wafer cleaning, furnace, lithography, etching, and metallization semiconductor operations as well as an additional CNT assembly operation to integrate the CNTs onto devices. Results are reported for a projected full-scale manufacturing environment (a scaled-up version of CNT B
dx.doi.org/10.1021/es305325y | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Article
Figure 2. Scope of this life cycle assessment.
fabrication process step is a unit operation with material and energy inputs and related emissions that is performed on the wafer to provide a specific function. CNT LCI data are utilized from the HiPco SWCNT production process developed in previous work.24 The input data, which include nanotube synthesis, purification, inspection, and packaging, are included as an input to CNT switch fabrication. SimaPro 7.3 was utilized as the primary modeling platform for access to inventory databases, libraries, and impact assessment methods.25 LCI data were modeled using matching entries from the ecoinvent 2.2 database, while the US EPA’s TRACI2 impact assessment method was used to translate resource use and emissions into indicators of environmental impact, including greenhouse gas emissions, acidifying and eutrophying emissions, ozone depletion, smog formation, ecotoxicity, and human health damages from cancer, respiratory effects, and other diseases.26
switch fabrication as it is currently performed in the laboratory), which makes estimates for realistic, commercialscale wafer processing capacities, effective process yields, cycle times, and production volumes. Table 1 sequentially lists the CNT switch fabrication process steps. In general, the steps are typical wafer fabrication Table 1. CNT Switch Fabrication Process Sequence step #
unit operation
1
wafer clean
2 3 4 5 6 7
furnace metallization lithography lithography lithography lithography
8 9 10 11 12
wafer clean etch etch wafer clean metallization
13 14 15
lithography wafer clean CNT assembly
function particulate removal film deposition film deposition resist coat resist bake film patterning resist development rinse and dry film patterning resist removal rinse and dry film deposition film patterning rinse and dry CNT deposition
recipe standard prediffusion clean 500 nm SiO2 wet oxidation 20 nm W sputter 150 nm PMMA spin coat oven bake optical switch mask alignment MIBK/IPA solution
■
RESULTS AND DISCUSSION Fabrication Processes and Inventory. An aggregated inventory of categorized inputs per wafer is shown in Table 2. The standard prediffusion clean (Step 1) uses the greatest amount of input material, in particular deionized water at 2.6 L/wfr. Because this step is a series of rinses and washes, all of the inputs become emissions, resulting in the greatest contributor to mixed wastewater to treatment, though pollutant loads are minimal. Over the entire fabrication process, water is the largest input by mass at 6.8 kg/wfr, followed by acids/ etchants and bases and gases, both at 0.2 kg/wfr. The mass dominance of water in the life cycle inventory is in line with previous studies of semiconductor devices.14 Results state total fabrication energy use at 13.1 kWh/wfr. Total primary energy use is dominated by the metallization operations at 9.5 kWh/wfr, because of the electricity required to run the e-beam evaporator and sputter deposition system. Etch operations are the second greatest contributors to energy use at 3.0 kWh/wfr due to the plasma etch system. These unit operations have a higher use of energy per wafer in part because of their low wafer processing capacity. Because of the small quantities of CNTs employed in the device, energy use associated with CNT production and incorporation, while substantial on a per unit mass basis, is relatively small. On a life cycle basis, energy use dominates emissions to air, as a result of fossil fuel combustion. Direct processing emissions into air are mostly benign N2 from the wafer cleaning steps. Impact Assessment. LCIA results are broken down by fabrication step (Figure 3A) and by material and energy inputs (Figure 3B). The life cycle impact assessment reveals that, for
cascade rinse and dry SF6, CHF3 plasma etch HF wet etch cascade rinse and dry 2 nm Cr, 50 nm Au e-beam evaporation lift-off cascade rinse and dry dielectrophoresis
processes with the exception of the final step, CNT assembly. In this step, the SWCNT is deposited onto the switch template using dielectrophoresis. A small droplet of CNT-suspended solution is placed onto the switch substrate, and when a nonuniform electric field is applied, the CNTs align across the trenches as desired. A thorough and detailed description of all 15 fabrication process steps can be found in Dahlben.22 LCI data were organized in a process-based manner derived from a similar inventory module developed by Murphy et al.23 In convention with previous studies, the functional unit was taken to be all devices manufactured on a single wafer. Fabrication is performed on 3-in. Si wafers (surface area = 45 cm2, mass = 4 g), with 84 switches manufactured on one wafer. Process parameters, material and energy inputs and associated emissions were gathered through observation of laboratory practices, including documenting the types and amounts of input materials, investigating waste treatment of spent materials, recording equipment used, and estimating energy consumption in the fabrication process. Associated with each C
dx.doi.org/10.1021/es305325y | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Article
Table 2. Energy and Categorized Input Materials for Fabrication of One Wafer fabrication inputs
relevant processes (step #s)
g/wfr
acids/etchants and bases HCl HF H2SO4 H2O2 NH4OH gases Ar CHF3 N2 O2 SF6 photoresists PMMA metals Au Cr W solvents acetone IPA MIBK CNTs
214 14.3 16.0 112 69.8 1.8 205 0.56 0.47 203 0.87 0.06 2.6 2.6 1.5 1.4 0.02 0.01 111 8.1 86.9 16.1 5.7 × 10−16
deionized water (L) energy (kWh)
6.8 13.1
wafer clean (1) wafer clean (1); resist removal (10) wafer clean (1) wafer clean (1); film patterning (13) film patterning (13) film patterning (9) film patterning (9) wafer clean (1, 8, 11, 14); wet oxidation (2); optical mask alignment (6); wet oxidation (2) film patterning (9) PMMA resist coat (4) metallization - film deposition (12) metallization - film deposition (12) metallization - film deposition (3) film patterning (13) resist development (7); film patterning (13) resist development (7) CNT deposition (15) wafer clean (1, 8, 11, 14); wet oxidation (2); resist removal (10); CNT deposition (15) all except wet lithography (7,10,13)
Figure 3. Life cycle impact assessment results across multiple environmental impact categories by (A) fabrication step and (B) material and energy input.
nearly every category, two upstream processes are responsible for the great majority of impacts, namely electricity generation and the mining and refining of gold. Step 12, the film deposition of Cr and Au using e-beam evaporation, is also one of the most energy-intensive unit operations and thus makes the greatest contributions to all impact categories. Generally, the next most impactful operation is Step 9, the plasma etch, which is also relatively energy intensive (3 kWh/wfr). For ozone depletion, the use of methyl fluoride (CH3F) in plasma etching is an important contributor to impacts, while the use of sulfur hexafluoride (SF6) also contributes to greenhouse gas emissions. Following tungsten sputter deposition, the remaining 12 process steps cumulatively contribute
