The production of Photolithography masks, reticles, and other precision optical components free of defects and surface contaminants is crucial in the Lithography (“Litho”) and metrology functional areas of microelectronics manufacturing.
The requirement for real-time AMC monitoring is widely known in industries that produce and employ optical components for actinic or inspection applications.
This article will discuss the advantages and uses of Ion Mobility Spectrometry (IMS) using Particle Measuring Systems' AirSentry® II (ASII) Real-time AMC Analyzer in the Metrology, Litho, and Photo Optical lens production industries.
Image Credit: Particle Measuring Systems
The International Roadmap for Devices and Systems (IRDS 2020 Edition) prioritizes eliminating feature defects resulting from patterned images transmitted to the wafer in the Photolithography Functional Area (Litho FA) using advanced Lithography methods.
This has ramifications not only for the Litho FA's developer and scanner operations but also for the use of high-precision optical components, such as those utilized in the production of 193 nm and 266 nm lasers.
Section 1 of this article summarizes the IRDS 2020 Edition’s relevant focus areas for AMC control in the Litho functional area as the semiconductor sector moves forward into the next decade.
Implications include defect-free production of high-precision optical lens surfaces for actinic and inspection procedures of the Litho Functional Area Metrology applications, where high-precision lenses are critical.
Section 2 describes defects created by the interaction of acidic and basic AMC, which generates optical haze on lenses.
This is a mutually concerning issue for the Litho Functional Area (FA) and the Optical component’s production processes, as perfect transmission or reflection is needed during visual inspection metrology tool assembly and usage.
Extreme Ultra Violet (EUV) Litho tools will become increasingly applicable in HVM systems in the next decade.
EUV techniques employ reflective optical components (reticles/masks and mirrors) to impart patterned images into the photoresist chemistry in a vacuum environment. These components, like DUV (and previous generations of Litho FAs), must be protected from particle and AMC contaminants during manufacturing and until final OEM tool integration.
In addition, Section 3 covers well-documented defects specific to the Photolithography FA and undesirable stochastic defects connected with the entire actinic process. One cause of these defects is the Photoresist material’s reaction chemistry.
This article also presents a real-world instance of AMC detection's importance in producing precision optical components and sub-systems.
In this case, the ASII Mobile Cart (with ASII Total Amines and ASII Total Acids analyzers) is used in an optical lens manufacturing process to detect, remove, and control inappropriate levels of airborne ammonia AMC within the production plant.
According to the vision of challenges outlined in the IRDS 2020 edition, real-time AMC detection will remain crucial in enabling process technology development.1
As described above, real-time AMC detection with IMS is the most widely used and benchmarked example in high-volume semiconductor fabrication fab environments, and the AirSentry II has the greatest global footprint of such devices.
Section 1: IRDS Edition 2020 Advanced DUV and EUV Lithography Challenges
According to the 2020 IRDS, the Litho FA must decrease imperfections connected with two general modalities that cause functional defects in the integrated circuit pattern on the silicon wafer.2 The modalities are as follows:
- The distortion of patterns emitted radiated by optical surfaces (masks, mirrors, reticles, lenses, and other optical components) during the exposure stage in advanced Litho processes.
- Stochastic defects are unpredictable time-dependent fluctuations in light exposure and resist chemistry during the actinic exposure stage. More precisely, stochastic defects result from random fluctuations in the number of photons in a discrete exposure of a small area, as well as random placement, response, and dissolution of different molecular components that make up photoresist.
The first of these defects is mostly caused by the existence of AMC in the Litho FA, which can produce surface contamination on masks, reticles, and other precision optical components.
These defects, however, can also be caused by irregularities in the optical components (lenses, waveplates, mirrors, polarizers, prisms, and so on) that have been exposed to AMC during production (Figure 1).
Figure 1. Lens contamination likely due to AMC during lens manufacturing. Image provided by Excelitas Technologies Corp
When integrated into the connected Litho scanner toolset, components having concealed anomalies can emit distorted light patterns.
The EUV process tool employs reflective optical components to project the pattern directly onto the wafer in a vacuum environment. It must reduce wavefront distortion and, when necessary, not limit the ability to correctly focus the beam at the wafer surface.3 (Figure 2)
Figure 2. EUV Litho tool: Optical layout of the engineering test stand5 (M1-M4 main chamber, C1-C3 condensing optics). Image Credit: Particle Measuring Systems
If these precision optical components contain embedded anomalies, the image quality on the wafer will eventually be reduced or limited.
AMC and its products can deposit on optical surfaces during production, packing, or transportation, causing embedded reflective optical component defects (EUV), and transmissive optical component defects (DUV and prior generation).
AMC-related surface contamination and malfunctioning optical components with incorporated AMC-related defects are important concerns in both the Litho FA and Litho Tool OEM assembly, as well as the inspection tool Metrology FA and Metrology tool OEM assembly.
OEMs of Litho and Metrology tools demand the production and delivery of near-perfect light transmission performance from their respective optical component and sub-system supply chains. Slower resist formulation can have been shown to decrease some stochastic defects.2
However, the time-dependent defects linked with AMC exposure (in DUV and preceding generations) would rise proportionally as the exposing process and pre-expose queue periods increase.
In addition, defects are predicted to grow if AMCs exist during the extended exposure durations required to perform more complex DUV, DUV-double, and even DUV-quadruple patterning schemes, as required by IRDS 2020.
Overall, the successful application of advanced Litho methods has resulted in the roadmap’s key issues no longer being resolution-dependent on light wavelength.
The key issues are the transmission of unmodified light patterns passing through the different optical components and subsystems within a Litho tool and the optimal performance of the photoresist material to reduce overlay, critical dimension (CD), and Line Edge Roughness (LER) defects.
As we enter the next decade of Lithography innovation, as outlined in the IRDS, the same concern for optical components in the Litho FA should be extended to the adjacent metrology functional areas using high power and density (focus) DUV lasers.
The production plants where precision optical components and sub-systems (such as DUV lasers) are produced and incorporated into both Litho-scanner and metrology visual-inspection tools will require more stringent AMC standards than conventional Litho FAs.
Section 2: AMC-Caused Haze Formation on Optical Surfaces within Lithography, Metrology, and Component (Sub-System) Manufacturing
One primary goal in minimizing defects in these processes is to prevent haze from forming on mirrors, masks, prisms, lenses, reticles, and other optical components.
This is often the result of acid-base AMC presence in Litho and Metrology FAs and Tool OEMs.
Pattern defects in the Litho FA caused by the AMC-induced formation of salt crystals creating an optical haze on the optical component surfaces are well-documented. They are of major concern wherever these highly designed, precision surfaces are produced and utilized for optimal light transmission.
Optical Haze is caused by the photochemical interaction of airborne acidic and basic AMC (see reaction in Figure 4). Ammonia (NH3) and various primary and secondary amines comprise most basic AMC. Figure 3 illustrates typical reactive acids and bases.
Figure 3. Most Common reactive AMCs present in cleanroom detectable by the ASII *Total Amines, and **Total Acids Analyzer. Image Credit: Particle Measuring Systems
Once created on the optical surfaces, salt haze degrades the image, which is meant to travel through the optics without aberrations in wavelength or emission power during the actinic processes within the Litho scanning tracks.
The resulting image deterioration generates pattern defects on the wafer, contributing to dimensional and overlay defects. One such instance of salt creation is when airborne ammonia and sulfur dioxide (SO2) photochemically combine to produce ammonium sulfate salts.4 (Figure 4)
5
Figure 4. Formation of ammonium sulfate from reactant AMC SO2 and NH3. Image Credit: Particle Measuring Systems
Figure 5. Salt crystals on ArF Lithography reticle photo5. Image Credit: Particle Measuring Systems
Even more worrying is that UV light accelerates the production of salts and enhances optical haze on all surfaces, as shown in Figures 4 and 5.
In the coming years, the creation of these salts will be increasingly prevalent since EUV methods of double and quadruple exposure, as well as slower resist response times, will increase salt and haze production.
Although pellicles are available to assist in keeping masks clean in DUV and prior generations of Litho methods, their transmission is limited, resulting in drastically reduced exposure tool throughput.2
As of 2020, the 7 nm Lithography FA utilizes DUV and immersion patterning. As the industry progresses to 5 and 3 nm logic nodes, EUV will be employed for the lowest pitches, with immersion quadruple patterning for some levels.2
More exposures increase the likelihood of Litho-related defects, particularly those induced by AMC anomalies in precision transmissive and reflective optical components.
As a result, real-time AMC monitoring and control with sub-ppb sensitivity remains a fundamental need in the Litho FA and will be expanded to the optical component and sub-system production sector to improve defect excursions emanating from Litho.
The AirSentry II (ASII) Point of Use Analyzer detects AMC Acids and Amines with high sensitivity, 24/7 coverage, and remarkable temporal resolution, offering AMC concentrations with detection limits as low as 70 ppt (Figure 6).
Figure 6. Lower Detection Limits (LDL). Image Credit: Particle Measuring Systems. Image Credit: Particle Measuring Systems
ASII Note: Figure 6 depicts the lower detection limits for available form factors of the Particle Measuring Systems AirSentry II product line:
- Point of Use (POU) Analyzer: Offers 24x7x365 coverage at crucial single sample sites.
- The AMC Multipoint Manifold System enables automated sampling over 16 or 30 sample channels with up to 75 m of sample tubing length, resulting in 2 - 5 sample trends per day in regions of interest.
- The fully integrated AMC Mobile Cart allows for real-time sampling on a mobile platform and maps concentration gradients across the fab to find sources and readily define new areas by a single user
Section 3: Defects caused by AMC in the Photolithography Functional Area
Specific Defects Caused by Resist Neutralization and Other Stochastic Defects
Many pattern and overlay defects that emerge in the Litho FA actinic processes are classified as stochastic defects. Stochastic defects are changes in light exposure that occur randomly over time and resist chemistry.
This section will detail some of the stochastic defects that can occur when there is acid-catalyzed, chemically amplified resist (CAR) material, which is specific to the DUV Photolithography FA, as a result of photoresist material degradation in the highly controlled pre-Bake, Coat, post-Exposure-Bake (PEB), and develop processes within the Litho Track.
These defects can manifest as missing contact holes, bridging between lines, line openings, or merged contact holes. Recent research has demonstrated that they are more prevalent than a simple extrapolation of Critical Dimension variation based on normal distribution would suggest.
Stochastic defects presently restrict the usable resolution of EUV tools, although they are caused by AMC reactions on the exposed wafer within the EUV tool’s vacuum environment.
The EUV region still requires overall environmental and transport-path AMC control. The acid-catalyzed photoresist is an essential component of DUV’s actinic processes in lithography.
In the case of “positive patterning” lithography, the resist is composed of solvents that are placed onto the wafer substrate and subsequently transformed into water-soluble acids by UV light, which is transmitted via an actinic lens during the exposure process in the exposure tool.
These polymerized, water-soluble resist regions are dissolved in the aqueous development solution during the subsequent development stage following the exposure.
In the instance of "negative-pattering", the exposure tool’s UV light pattern is the image-negative of the positive-patterning image, resulting in the desired resist structures.
When basic AMCs (the most common of which is ammonia) come into contact with the acid catalyst meant to react with the surface resist, UV light exposure slows and, therefore, becomes considerably less effective than when used in the highly precise UV light pattern.
Unwanted and uncontrolled AMC resist-catalyst reactions below ~1 ppb can lead to various pattern and overlay faults. Bridging between lines, missing contact holes, line openings, and merged contact holes are some instances of defects associated with resist degradation.
One of the earliest pattern defects shown to be directly produced by the presence of reactive amines interacting with resist, known as T-Topping (Figure 7), was initially explored and documented in 1997, using IMS as the best choice for real-time in-situ AMC monitoring.1
Figure 7. T-Topping resulting from increasing level of Amines AMC presence from top to bottom. Image Credit: Particle Measuring Systems
For these reasons, most Photolithography Functional Areas often establish upper control limits of ~1 ppbv Total Amines, highlighting the obvious requirement for sub ppbv or ppt detection limits.
Moving into the 2020s, the focus has been placed on implementing even longer exposure and scanning in the DUV Litho processes to handle the double and quadruple exposure required by IRDS.
As a result, a new focus is placed on preventing the emergence of stochastic defects when utilizing the specified slower-reacting stabilized resist.
Over the last few decades, substantial research and implementation efforts have been spent in developing more stable resist and resist additives that can sustain longer queue times in both pre-and post-exposure and baking to prevent the loss of photo-acid due to surface evaporation.3
The need for greater control over harmful environmental contamination is obvious. This includes improved detection and real-time reaction to unwanted AMC concentration levels, the most common of which are the amines class of AMCs.
Real-time AMC monitoring with the ASII’s accuracy, precision, and ease-of-use performance characteristics is becoming increasingly important as new EUV processes are built and maintained in the present decade.
Section 4: AMC Control in Precision Optical Lens Manufacturing
Manufacturing DUV—and EUV-compatible optics and optomechanical assemblies necessitates a level of environmental control that exceeds that of standard particulate-based cleanrooms.
Depending on the UV laser’s wavelength and fluence (energy per area), various levels of molecular contamination control are necessary to avoid laser damage to the optical components.
Table 1 depicts the maximum recommended amounts of organic and inorganic AMC in air for producing medium and high fluence DUV optics. Note that EUV Litho regions employ wavelengths considerably below 100 nm in EUV tools with the primary exposure stage held at vacuum.
The reflective optics in this area of the stage must be devoid of embedded AMC-related defects that may emerge during optics production. Optical materials used in advanced Litho areas must meet the control limitations of 193 nm FAs.
Table 1. Organic and Inorganic molecular contamination limits in optical component and sub-system manufacturing and Litho FA4. Source: Particle Measuring Systems
| λ |
Fluence Level |
Maximum Organic AMC Levels in Air |
Maximum Inorganic AMC Levels in Air |
| 266 |
Medium |
10 ng/L |
Acids: < 10 ppb, Ammonia + Total Amines: 1-2 ppb |
| 266/193 |
High |
5 ng/L |
Acids: < 1-2 ppb, Ammonia Total Amines: 1 ppb |
| *< 100 nm |
Very High |
Meet above, at minimum |
Meet above, at minimum during component and sub-system manufacture |
Higher AMC levels can result in higher light absorption and scattering, laser-induced degradation that causes optical haze, and lower overall performance and lifespan.
The effect of AMC on optical components is often not visible before prolonged exposure to a UV laser or outgassing testing, thus monitoring and controlling AMC in the production environment is crucial to prevent this failure scenario.
From the perspective of optical component and sub-system manufacturing, it must be noted that there is currently no practical method for detecting the presence of AMC adsorbents on the surface of optical components before shipment to Metrology or Litho tool OEMs.
As a result, the defect may emerge after the component has already been installed. Thus, the defect is passed on to the consumer (OEM) rather than intercepted and remedied by the supplier during production.
From this standpoint, real-time AMC monitoring in the optical component and subsystem production plant becomes a leading indication of the effective quality and dependability of the components and subsystems.
Section 5: AirSentry II Application in Precision Optical Component and SubSystem Manufacturing
Optics manufacturers have modified manufacturing methods and cleaning and coating processes to fulfill the stringent material and cleanliness criteria for DUV and EUV qualification.
Excelitas Technologies manufactures optical components and sub-systems for EUV and DUV tool processes. It is a technical leader in producing high-performance, market-driven photonic solutions to address the optronic, lighting, detection, and optical technology requirements.3
Excelitas has recently significantly improved its optics manufacturing functional area, where components and sub-systems are manufactured and integrated into optical metrology equipment using DUV light. Excelitas is spending heavily on specialty equipment to control AMC contamination. This includes:
- AMC flow benches for cleaning, inspecting, and packaging optics
- Specially designed N2 purge cabinets for storing tools and optics between manufacturing stages
- Chemical getters for N2 lines that feed coating chambers and blow-off guns
The Excelitas Approach
Excelitas implemented continuous AMC monitoring of total acids and total amines via the PMS AirSentry II Mobile Cart to monitor and qualify the DUV systems and processes, tested optical surfaces at different stages of production for surface particle contamination, and instituted regular PMs to test room and purge cabinet air for VOCs.
To begin, baseline AMC levels in the clean spaces of concern were determined using the standard Define, Measure, Analyze, Improve, Control (DMAIC) technique.
The Particle Measuring Systems AMC Mobile Cart was used to perform a full mapping of total acids and amines AMC, including an area devoted to DUV part cleaning and storage.
A controlled positive-pressure flow bench equipped with AMC filtration (called the “AMC flow bench”) was used as a pilot-scale proof-of-concept to test the hypotheses that the selected AMC filters and airflow scheme were enough to control and sustain AMC levels within the bench flow and in varying proximities of the bench relative to the room’s baseline AMC concentrations.
To assure data integrity, the Particle Measuring Systems Applications Engineering team provided direct support for Mobile Cart setup, calibration, and operation and offered assistance with data analysis and interpretation.
To sample in DUV regions, the standard mobile cart sample inlet port was linked to an adapter containing roughly 3 meters of 1/4-inch OD PFA tubing that could be easily oriented to better sample local environments.
Figure 8 depicts a time-chart graph showing AMC levels throughout the week, whereas Figure 9 presents the plan view arrangement of the sampled locations.
Figure 8. Amine and acid AMC levels in a controlled (AMC Flow Bench) vs uncontrolled (Transfer Zone) environment over 7 days. Image Credit: Particle Measuring Systems
Figure 9. The layout of the area tested. Image Credit: Particle Measuring Systems
During the first half of the week, tubing was installed within the “AMC Flow Bench”. Tubing was relocated 1.5 feet beyond the flow bench to the "Transfer Zone" later in the week, where exposed components are frequently shifted from the bench to an N2 purge cabinet.
AMC sample mapping in the transition area was used to identify whether more AMC filtration (and maybe air handling system reconfiguration) will be required during the DMAIC’s Improve and Control phase.
Measurements depict that amines and acids within the AMC bench consistently remain below 1 ppb over several days, indicating appropriate conditions for producing high fluence DUV optics. Outside the bench, amines and acid levels increase by about 3 ppb.
While levels outside the AMC bench are adequate for handling a medium fluence optic, producing high fluence optics will necessitate a higher level of AMC control in the room, such as molecular filtration to supplement the particulate (HEPA) filtration already in place and protecting optics from AMC whenever they are transported outside the flow bench.
The AMC Mobile Cart mapping exercise adequately supports the need for AMC filtration, as well as an isolated air handling scheme if it is to be utilized as intended to produce or clean components and subsystems for integration into the optical DUV OEM tool.
Excelitas has deployed two AirSentry II Total Amines and Total Acids Point of Use Analyzers for long-term AMC control and sustainability. The 24/7 real-time POU analyzer sampling method will be deployed as part of the complete AMC control strategy to ensure Litho and Metrology Tools OEMs get high-performance lenses.
Summary
The requirement for AMC control has been well understood from the early days of layering integrated circuits onto bare silicon.
As we go into the next phase, and maybe the final years of Moore’s law, the new methods required in Lithography will necessitate even higher levels of AMC control, not only in Litho but also in other functional areas, and OEM suppliers will be obliged to provide it.
High yields and throughput translate into bottom-line profitability in semiconductor production and improve the viability and survivability of metrology tools and optics OEMs.
These requirements necessitate adopting real-time, high-sensitivity AMC monitoring that provides remarkable temporal resolution, easy-to-interpret findings, and low maintenance. The Particle Measuring Systems AirSentry II AMC Analyzer range of products offers the value and performance required to meet these specifications.
References
- Dean KR, Miller DA, Carpio RA, Petersen JS, and Rich GK. Effects of airborne molecular contamination on DUV photoresists. SEMATECH. 2706 Montopolis Drive, Austin, Texas, USA, 78741-6499.
- IRDS 2020. International Roadmap For Devices and Systems 2020 Edition—Lithography.
- Hollenshead, Jeromy, and Klebanoff, Leonard. Modeling radiation-induced carbon contamination of extreme ultraviolet optics. Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures. 2006; 24(1):64. doi:10.1116/1.2140005.
- Kambara H, Favre, A, Davenet M, and Rodier D. Airborne molecular contamination detection method for photomasks and ultra purging decontamination. Photomask and Next-Generation Lithography Mask Technology XVI, 73791G. Proc. SPIE 7379. 2009 May 11. https://doi.org/10.1117/12.824293.
- Eschbach, et al. ArF Lithography reticle crystal growth contributing factors. Intel. 2004.
This information has been sourced, reviewed and adapted from materials provided by Particle Measuring Systems.
For more information on this source, please visit Particle Measuring Systems.