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Review of nanoparticles in ultrapure water: definitions and current metrologies for detection and control

Maria Pia Herrling1*, Philippe Rychen1

1Ovivo Switzerland AG, Innovation Department, Hauptstrasse 192, Aesch, 4147, Switzerland*Mariapia.herrling@ovivowater.com

Ultrapure water (UPW) is one of the main materials for electronics fabrication and therefore, it needs to be monitored for critical parameters such as nanoparticles (NP). The state-of-the-art online measurement techniques are challenged by particles at the killer particle sizes smaller than 10 nm. Due to the uncertainties in NP detection, the identification of NP sources and sinks in UPW system is limited nowadays. This review article aims to give an overview on the current developments and perspectives in metrologies for detection and control. The following topics will be discussed: transferability of general definition of NP to UPW, state-of-the-art particle analytics, sources and sinks of NP in UPW systems as well as dominant particle interactions responsible for NP contamination.

Keywords: nanoparticles, monitoring, analytical methods, semiconductors, removal, ultrapure water, particles

INTRODUCTION

The global market for electronic components is booming and is expected to reach US$191.8 billion by the year 2022 [1]. Ultrapure water (UPW) forms a crucial part of the supply chain in the electronics industry. For the fabrication of electronic devices, quality control of UPW for impurities which will cause wafer defects is a major challenge. These defect causing impurities may include (nano)-particles, metals, ionic species, organic compounds, bacteria, and others [2, 3]. Nanoparticles (NP) become especially of interest due to the decreasing feature size of devices, which thus implies a shrinking killer defect size for NP at 1.5 to 10 nm as recently discussed at the UPW Micro Conference 2017 [4]. According to the ITRS 2015 (YE report), the number of particles in UPW should be less than 1000 #/L (particles per litres) (1 #/mL (particles per millilitres)) at point of entry (POE) and less than 100 #/L (0.1 #/mL) at EUV mask production (POE) for particles larger than the critical particle size [5]. The critical particle size is based on half-pitch DRAM. To meet this and future recommendations, interdisciplinary research

in the field of NP in UPW – especially regarding analytics and removal strategies – is needed [6, 7]. In this context, the main challenges proposed by the community are:

• The gap in methodology for accurately monitoring NP in UPW at low concentrations and small particle sizes (< 10 nm).

• The gap in methodology in control, where a deeper understanding in NP behaviour and removal strategies during UPW generation is required.

These two challenges are strongly connected, because without appropriate measurement techniques, the evaluation for NP removal is limited. Beyond that, there is a need for impact studies to identify specific impacts of NP on the wafer yield and efficient rinsing protocols [5]. However, this will not be the focus of this article. The purpose of this review article is to present an overview on the current developments in metrologies for detection and control of NP in UPW. Therefore, the following topics

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will be addressed: transferability of general definition of NP to UPW, state-of-the-art particle analytics and identification of sources and sinks for NP in UPW systems addressed by particle interaction scenarios.

DEFINITIONS OF NP: ARE THEY APPLICABLE TO UPW?

For a better understanding of NP contamination in UPW in terms of particle chemistry and physico-chemical behaviour and for regulatory purposes, a common definition of NP would be directive. Therefore, the term NP should be defined first by existing standards.

There are several accepted definitions for NP available (e.g. by International Organization for Standardization (ISO), Organization for Economic Co-operation and Development (OECD), Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR) and others), which mainly utilize size and geometric boundaries for a measurable definition. We will focus on the definition given by DIN ISO/TS 80004-1(2015) as it is one of the most utilized definitions. According to ISO, the nano-scale is the length scale between 1 nm and 100 nm. The lower limit is set to 1 nm to exclude single or small groups of molecules from the definition. A nanoparticle belongs to the group of nano-objects which is a “…discrete piece of material with one, two or three external dimensions in the nanoscale…”. Nanoparticles do have all three dimensions in the

nano-scale. If the largest and shortest dimension of the axis of a nanoparticle differ strongly, the term nano-fiber or nano-platelets should be used [8]. Due to their small size, NP do have different physico-chemical properties compared to the same material in bigger size, e.g. regarding reactivity or adsorption. More information about the theory of NP can be found elsewhere [9, 10].

Before the term nanoparticle has been introduced, the terms colloid or colloidal suspensions were present and were often used as synonyms for NP. According to IUPAC (International Union of Pure and Applied Chemistry), colloids are defined as objects having one dimension between 1 and 1000 nm and, therefore, the definitions are overlapping [11]. The definition includes any kind of material, such as organic and inorganic core materials.

To demonstrate the state of the research, the number of publications per year are shown in Figure 1. The data includes all publications available on Web of Science across all disciplines with the target keyword found in the title and/or topic (abstract, title, keywords). This means that also irrelevant publications might be included into the counts. Research on colloids started in the 1930s and the number of publications/year increased constantly. In comparison, research on NP began later, but the number of publications/year rose quickly up to 19,000 in 2016. This can be explained by the numerous and diverse applications of nanotechnologies nowadays. The application

Figure 1: Number of publications published between 1943 and 2016.

The curves represent the hits by keywords “nanoparticles”, “colloidal”, “wastewater”, “ultrapure water” and “electronics” in the title or topic literature search in Web of Science (source: https://login.webofknowledge.com, 2017, Clarivate Analytics (access date: 09/29/2017)). Between 1950 and 1980 only 3 points per year where taken (figure created by M.P. Herrling, 2017).

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of NP in electronic industry rose parallel to the overall increase of publications for the keyword “nanoparticles” only. With the extensive production of nanomaterials, their analysis, impact and control in, for example, water streams received more and more attention in research. In other fields, such as nanoparticles in UPW, fewer articles were published on Web of Science. Reasons might be that related publications might have published in other journals or platforms and might be found by other keywords such as “killer particles” or “critical sized particles” instead of nanoparticles. The research about nanoparticles in UPW for electronic industry started around the 2000s [12] and has a great potential presently.

Now, the question arises whether the presented definitions can be transferred to NP in UPW. A harmonization of terminology for NP might be helpful, especially in order to promote common understanding within the UPW community. Furthermore, a species definition of NP in UPW might

help for recommendations for the quality of UPW. In this context it might be discussed:

• Should geometric boundaries for NP in UPW be defined, especially approaching killer defect sizes smaller than 1 nm? Should the lower size limit of a NP be established at the size of a single water molecule (molecule size of H2O is ~0.3 nm) because UPW aims for highest purity or should the definition cover the range up to colloidal size (1 nm – 1000 nm)?

• Should high molecular weight nm-sized organic/inorganic clusters in UPW [6, 13] be considered as NP and officially be included into the definition? Should the definition of a NP in UPW depend their chemical composition and/or on specific defects caused on a wafer?

• Should other “particulate” compounds, such as NVR be considered as NP as they are captured by the same measurement technique e.g. CPC.

Measurement principle Examples for devices Parameters Online Batch

liquid (optical) particle counter LPC

UDI 20, PMS PS, PC X

Nanocount 30+, Lighthouse PS, PC X

KL-30AX, Rion PS, PC X

condensation particle counter system

LNS Liquid NP sizer system 9310, Kanomax FMT

PSD, PC X

CPC Scanning threshold particle counter 9010, Kanomax FMT

PS, PC X

acoustic particle counter APC Particle scout 20, Uncopiers PS, PC X

laser induced breakdown detection

LIBD Magellan NP trace analyzer, Cordouan Technologies

PS, PC X X

dynamic light scattering DLSZetasizer Nano ZS, Malvern PSD, zeta potential X

Nano partica SZ-100, Horiba PSD, zeta potential X

NP tracking analysis NTA ViewSizer 3000, Manta Instruments

PSD, PC, visualization X

Benchtop scanning electron microscopy

SEM Phenom ProX scanning electron microscope

PSD, PC, morphology, shape, etc.

X

Table A: Selection of state-of-the-art particle measurement devices. No claim for completeness.

Abbreviations: particle size (PS), particle size distribution (> 10 channels) (PSD), particle counts (PC). Data was extracted from device specification sheets. The data has been extracted from the specification sheets provided by the manufacturers (table created by M.P. Herrling, 2017).

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• Should bacteria which are seen as larger particles, which can be separated by filters [6, 14], be included in the definition? Should a NP be any matter that can be filtered out or particulate matter that is deposited on a wafer after drying?

Regarding the chemistry of NP, a definition of their physical state (e.g. soft and hard materials) would be directive. The physical state of particulate matter depends upon the surrounding water chemical conditions and temperature. Furthermore, a clear distinction between dissolved and particulate substances in UPW might help to differentiate NP. In other sectors, such as health sector, it has been claimed that more metrics should be taken into account for the definition, such as physico-chemical properties (e.g. agglomeration, density etc.) and thresholds for associated metrics [15].

METROLOGIES FOR NP DETECTION

There are various methods available for particle analysis regarding their chemical composition, particle size, particle surface, shape etc. However, those techniques can only partly meet the demands for UPW monitoring: on-line measurement and delivery of particle counts with high measurement precision. For UPW in electronics manufacturing facilities, the particle size (-distribution) (PS, PSD) and particle number concentration/ particle count (PC) are of main interest. In the following, an overview on state-of-the-art techniques will be given in order to point out potential deployments in metrologies for detection. The selected devices represent only small share of devices, which are commercially available nowadays. Therefore, there is no claim for completeness. Table A shows examples for devices categorized by following measurement principles:

• conventionally used in UPW systems: liquid optical particle counters (LPC),

• systems involving condensation particle counters (LNS, CPC),

• new online monitoring device acoustic particle counter (APC),

• recently laser induced breakdown detection (LIBD),

• examples for methods for NP characterization in batch mode: dynamic light scattering (DLS) and NP tracking analysis based on recording Brownian motion (NTA),

• imaging techniques as e.g. (benchtop) scanning electron microscopy (SEM).

For further information, please refer to SEMI F63, where an overview on existing UPW particle monitoring methods is given [16]. Please note that LPC and NTA are based in light scattering phenomena, however for better understanding different abbreviations have been chosen. Non-volatile residuals (NVR) are not considered here.

The methods listed in Table A have been compared regarding their performance in particle size range (Figure 2). Geometric boundaries for the target area (yellow highlighted) were set according the ISO definition of NP (1 – 100 nm) [8] and the killer particle size according IRDS 2017 (1.5 – 10 nm) [4]. Bars in Figure 2 marked with an asterisk (*) refer to threshold sizes and larger particles will also be considered. Figure 2 shows that great efforts have been put into the development of methods for NP characterization.

Commercially available LPC enable the detection down to 20 nm. LPC are widely used for NP monitoring in UPW [2] and serve as reference for comparison studies, as e.g. for LPC and LNS [17]. A few other methods cover particles smaller than 20 nm, such as online measurement systems using CPC. Besides PC and PS analysis, high resolution PSD can be provided by LNS. APC is a new method [18], which has been proved for UPW monitoring in comparative study with LPC [19]. APC covers particles down to 20 nm. Literature mentions the potential to detect NP down to < 10 nm in near future [18].

Another new approach is offered by LIBD, which has recently been applied for the first time in UPW [20]. In comparison to the previously mentioned online measurement devices, DLS is not commonly used for NP detection in UPW. Its limitations are the batch measurement mode and its inability to provide particle number concentrations. However, DLS enables the detection of particles down to less than 1 nm. Newer devices based on light scattering, such as NTA offer the possibility for PSD, PC and the particle visualization [21].

Another batch analysis method, which has been introduced for UPW application is SEM [22, 23]. Depending on the specifications of SEM and sampling procedure, small particles (< 10 nm) can be detected. Furthermore, elemental analysis is possible [2, 13]. Here, a benchtop device has been selected due to compact design for potential application onside.

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In conclusion, research on new developments in online particle analytics is ongoing. Commercially available devices are able to detect particles down to 10 nm and less, however, for the time being, the target particle size of 1 – 10 nm can only be covered by LNS and state-of-the-art techniques using batch measurement mode.

OCCURRENCE OF NP IN UPW

In UPW systems for both outlet and point-of-use (POU) measurements, a broad range of NP concentrations have been reported in the last years. In the following sections, some examples will be given. Using LPC concentrations of 1.53 to 157 #/mL >20 nm have been found in differently equipped UPW systems [24]. With SEM, concentrations of

12 #/mL >10 nm were reported [25]. Appling the LIBD method, much higher NP concentrations of 2.9•104 #/mL >10 nm and 8•103 #/mL >20 nm were measured [20]. Much higher concentrations have also been found using LNS (1•107 to 1•108 #/mL >5 nm) [17].

This large difference in particle counts between SEM, LIBD, LPC and LNS in 4-5 orders of magnitude leave big uncertainties as to the reliability of the measurement principles and techniques. Looking at the high variability of the results, analytical techniques with advanced detection efficiency at high sensitivity are needed. A critical role is played by the minimum NP sample concentration to achieve statistically significant results. This minimum required concentration depends on numerous parameters, such as the specific technical details of the device,

Figure 2: Particle size range covered by different methods for NP characterization given by the manufacturer.

The maximal particle size detectable with SEM was estimated according to sample size and view field. The yellow highlighted area in the graph represents the NP size range of special interest for UPW monitoring. Bars marked with asterisk (*) refer to threshold sizes. The data has been extracted from the specification sheets provided by the manufacturers (figure created by M.P. Herrling, 2017).

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sample volume, sample flow, sampling time, level of background noise, statistical analysis approach, and other factors. Figure 3 presents the relationship between the (minimal) particle size and the (minimal) particle concentration specifications for the given methods in the particle size range of 1 – 100 nm. The yellow highlighted area is defined according to the recommendations of the ITRS 2015 (1 particle/mL) [5]. A similar analysis has been presented before [26, 27], however specific data from manufacturers was not included. The presented data was extracted from data sheets by the manufacturers. The required particle concentration is set either by the typical measurement range or particle thresholds. For LPC, the particle concentration is assumed to be equal to the zero counts. The data for SEM represents device specific estimations.

Figure 3 shows that the recommendations cannot be met by currently available methods. This highlights that the ongoing research on method development is essential to better understand the “real” occurrence of NP in UPW [3]. On the one hand, low particle numbers might be tackled by concentrating NP by reverse osmosis, for example. On the other hand, improvement of sensitivity of analytical components as e.g. LPC sensitivity are currently under evaluation [26]. All presented measurement techniques do have certain advantages and drawbacks. The combination of different methods could give a more complete picture of NP in UPW, e.g. by correlating with sum parameters (TOC etc.). Comparative studies for the different NP detection methods are recommend for a better assessment of NP [17].

Figure 3: Correlation minimal specifications in terms of particle concentration and particle size.

The yellow highlighted area represents the NP size range and concentration of special interest for UPW monitoring. For the LPC, the minimal NP concentration needed is assumed to be equal to the zero counts. The data has been extracted from the specification sheets provided by the manufacturers (figure created by M.P. Herrling, 2017). Some data are not provided.

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SOURCES AND SINKS FOR NP IN UPW SYSTEMS

To effectively set sampling points and later to control NP occurrence, the sources of NP in UPW systems should be investigated in more detail. Here, four possible sources for NP along the UPW system from pre-treatment to polishing are discussed. The sources of particles in UPW have been summarized in SEMI-F75 [2]. Those sources are addressed by scenarios for dominant particle interactions, an example of which is shown in Figure 4.

• Raw Water: NP can originate in the raw water used as feed. Larger NP might be degraded and could be transported through the system. It is assumed that those NP form the minority of the NP present in UPW.

• System components: NP can originate from physical-chemical erosion of UPW system components, such as piping components, valves, tanks, linings, flowmeters etc. [2, 28]. The NP release depends upon hydraulic conditions and material properties.

• Process steps: NP can originate from UPW system process steps such as e.g. ion exchange resins beds as described in SEMI-C93 [29]. Abrasion of the resin material might result in the release of NP during resin changing and hydraulic transport is possible [30, 31]. In this context, interactions of NP (e.g. accumulation, sorption, desorption) with surfaces (e.g. membranes, resins, system components) influence the fate of NP in UPW, see Figure 4.

• Neo-formation: NP can originate from neo-formation processes, either where changing pH conditions (ion exchange resins) or energy transfer (UV reactors) in micro-environments occurs [17, 32]. One commonly known example is the formation of colloidal silica [2].

This leads to the assumption that complex particle interactions are involved in the contamination of UPW with NP, especially when considering that NP are unstable particulate matter. Indications for their origin can be given by their chemical composition. Using SEM in combination with elemental analysis (EDX), a great variety of elements have been found: iron,

Figure 4: Possible NP interactions influencing their fate in UPW systems with examples for scenarios.

The figure is a schematic representation, dimensions are out of scale. Ions are examples for cations and anions present in aqueous solution (figure created by Herrling, 2017).

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calcium, carbon, oxygen, sulphur, zinc, magnesium, chrome and others, but in most cases silica [13, 22, 25, 33]. This indicates that NP might be organic as well as inorganic and various sources are possible (1-4 listed above). To control NP in UPW, there are mainly two strategies:

• One strategy to control NP in UPW is to restrict their sources by replacing system components or process improvements. One proposed process improvement might be to remove H2O2 from UPW [34] to lower the damage to the ion exchange resin, or increase the resin purity [30]. But further research is needed to confirm this.

• Another strategy is to find sinks for NP in UPW processes. A potential sink for NP are filters, especially ultrafiltration modules.

It is assumed that the filtration performance at the killer particles size is not sufficient. The filtration capability is <100% for NP <15 nm [13]. The particle rejection by filtration depends among other factors on: particle nature (e.g. shape), particle concentration, transmembrane pressure,

media pressure [35]. Therefore, the development of new tighter filter media is ongoing [22] and recommended [3]. The direct comparison of various membrane systems, such as ultrafiltration and cartridge filters [35, 36] is difficult.

Table B summarizes recent particle rejection test for filter media with different pore sizes and particle challenges. Studies from other disciplines (drinking water, surface water) were included, these use larger particle sizes, but achieved comparable NP removal as in UPW. In UPW, the reported removal was >96% for injected particle challenges, except the particle monitoring of Murayama et al. (2016). The analytical method of choice (LNS, SEM, mass spectrometry (MS) etc.) seems to have a minor influence on the removal results. Besides the conventionally used filter tests, new test procedures and poly-dispersed particle challenges are recommended to give more detailed insights [37, 38]. The procedures for rating of filters regarding their capacity to remove NP are described in SEMI C79 [39]. NP removal from UPW by other process steps, such as membrane distillation, has also been investigated [40].

Filter Pore size

Water matrix NP type

Initial NP size (nm)

Analysis Feed conc. Removal Reference

ceramic flat-sheet membranes

20 nmsynthetic fresh water

Ag-NP 83.6 MS 1 mg/L 98.6 ± 4.68[41] †

TiO2 NP 33.7 1 mg/L 95.6 ± 0.99

hydrophilic regenerated cellulose membranes

10 kDa water SiO2 NP 28 turbidity 1.44 g/L >99.6% [42] †

100 kDa

water SiO2 NP 28 turbidity 1.44 g/L >99.6% [42] †

ultrafiltration modules

10 kDa UPW SiO2 NP 12 LNS 2E10, 2E9, 4E9 p/mL

>96% [43]

microfiltration cartridge

n.m. UPW SiO2 NP 12 LNS 1E8, 2E8 p/mL

>90% [43]

ultrafiltration modules

10 nm UPW no injection

> 10 nm*

SEM no injection >89% [44]

ultrafiltration modules 4 kDa UPW SiO2 NP 12

LPC, STPC, SEM

0 – 1E9 p/mL >99% [23, 30]

Table B: Selected NP rejection studies in water treatment systems.

NP sizes marked with asterisk (*) indicate the size of particles being captured by the measurement technique. The pore size is listed as given in the references and cannot directly be compared. Studies highlighted with cross (†) were conducted in laboratory scale. N.m. means not mentioned. The data has been extracted from cited references (table created by M.P. Herrling, 2017).

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In conclusion, the majority of the NP can be rejected, however, the requirements for UPW might not be fulfilled. Based on the uncertain concentration levels of NP in UPW systems, their removal is assessed by individual particle challenges. The advances in particle detection will allow to better identify NP sinks and control their fate (e.g. by final and POU filtration) in UPW systems, as also stated by IRDS 2016 [3].

CONCLUSIONS

This review article allows the following conclusions to be drawn:

• Chemical and hydraulic conditions in UPW are very distinct from other liquids and even water for other applications and therefore, it should be discussed if the term NP might be defined more precisely for UPW systems. A harmonization of terminology might be helpful in terms of process sensitivity and to meet future quality recommendations as the sector of nanotechnology in microelectronics is expanding.

• State-of-the-art online measurement techniques cannot fully cover the killer particle size of 1.5 – 10 nm at target particle concentrations. Further research is needed to improve the sensitivity of the analytical devices and develop new on-line particle counting methods [2], fully independent from NP composition.

• The NP concentration measured in UPW systems (below 102 up to 108 particles per mL) might depend on the metrology of detection applied. The combination of different methods and more comparative studies are recommended for a better assessment of NP in UPW.

• A deep understanding of NP interactions in UPW is an advantage to control their sources and sinks and improve UPW quality. Mechanisms such as NP degradation, neo-formation, NP release and surface interactions might influence the fate of NP in UPW systems. In conjunction, filtration steps represent sinks for NP, however, might not be a sufficient controlled barrier, so far.

AUTHOR CONTRIBUTIONS STATEMENT

M.P.H conceived and conducted the literature research, M.P.H and P.R. analysed the results and wrote the article. All authors have reviewed the manuscript.

ACKNOWLEDGEMENTS

The authors would like to thank the manufacturers of measurement devices for the constructive and informative discussions, which have strongly contributed to this review article.

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