Carla Frege, Felipe Lopez-Hilfiker, Ben Bensaoula, Liang Zhu
The Need for Improved AMC Detection
For semiconductor manufacturers, the accurate measurement of airborne molecular contaminants (AMCs) is critical for establishing effective process controls. As node technologies continue to advance, reducing in size and increasing in complexity, the effects of foreign molecular contaminants become increasingly detrimental to wafer production yields. Growing concerns related to material supply chains are forcing manufacturers of both small and large node technologies alike to pursue improvements in the monitoring of AMCs to implement effective preventive maintenance schedules that reduce downtime and ensure optimum wafer yields.
Along with the advancement of production processes and machines, there are emerging demands for higher performance analytical instruments that report AMC concentrations. To ensure stringent control over the macro and micro manufacturing environment, contaminants at parts-per-trillion volume (pptv) concentrations need to be inventoried. Ultra-fast instrument response is required to increase the sample throughput and provide alarm at early-stage AMC incidents.
Semiconductor manufacturing processes are subject to micro contamination attributed to the ambient air quality comprised of both external (make-up air) and internal (e.g., tools, chemicals, workers, material coatings) sources . Cross contamination between ambient, process, and storage environments increases AMC complexity.
Although some sources can be identified, many are still difficult to understand and their impact on semiconductor fabrication remains largely unknown. The availability of comprehensive, real-time measurements at with pptV sensitivity has been limited to a few solutions that are inadequate in their speed, sensitivity, and compound coverage.
Here we review critical contaminant categories monitored with the Semicon AMC Monitor.
Airborne Molecular Contaminants
Airborne molecular contaminant is a generic description of a substance in the form of a gas or aerosol that compromises the manufacturing process. In 2007 the International Roadmap for Devices and Systems (IRDS) introduced definitions for AMC categories: molecular acids, molecular bases, refractories, dopants, and condensables. Ten years later in their 2017 edition it was recognized that this approach was not enough as more contaminants were discovered that did not fall into any of these categories.
Current technologies used for AMC detection are limited by either by poor duty cycle and/or compound coverage. While chromatographic techniques are used less frequently, they persist despite their 30-minute temporal resolution and high cost of maintenance. Other online methods such as cavity ring-down spectroscopy (CRDS) measure compounds based on the extinction of their specific wavelength, which limits the performance to measure only one, or a couple of compounds, and reporting every few minutes. Thus, most available and deployed technologies are limited to measuring only one AMC category.
Semicon AMC Monitors offer a state-of-the-art solution to the inadequacies of traditional systems. With one instrument, semiconductor manufacturers can measure several AMC categories simultaneously, in real time, with single digit pptV detection limits. Full mass spectra acquisition improves retrospective analysis when new compounds of interest emerge. These characteristics make Semicon AMC Monitors ideal for monitoring the most complex and demanding environments, ensuring precision and robust coverage, applicable to a variety of fab processes.
Molecular Acids (MAs)
The influence acids have on wafer yield is problematic; particularly, acids. They corrode metallic lines and pads, contribute foreign deposition on wafer surfaces, haze lithography masks, and deteriorate HEPA filters. The degradation of filters can lead to insufficient sanitization, the generation of unwanted boron species, and cause other effects that are detrimental to manufacturing processes, production equipment, and final products. The full effects of trace acid concentrations remain uncertain since monitoring solutions offering real-time, comprehensive measurement of inorganic and organic acids with single digit pptv sensitivity are not widely implemented or available. Conventional methods for acid detection listed in the International Technology Roadmap for Semiconductors (ITRS) 2017 report provide limited sensitivity (>100 pptv), while <5 pptV presence of total inorganic acids are required for most fabs practicing 5 nm processes or smaller. With single digit pptV detection limits, Semicon AMC Monitors provide precision acid monitoring, greatly outperforming currently deployed techniques.
Molecular Bases (MBs)
Among AMC compounds, ammonia, amines, and amides compose a unique species category. The properties of these compounds make them undergo rapid acid-base reactions, forming small salt particles that cause unwanted material deposition. Ammonia exhibits a strong affinity for metal surfaces such as copper, which leads to persistent contamination risk. Although ammonia remains a dominant base, various amines and amides can also be observed in fab processes, causing the deterioration of lithographic performance, and uneven etching. Reporting individual bases is required to understand their full impact on manufacturing processes and to design the most efficient abatement strategy. Measurements with Semicon AMC Monitors demonstrate that bases generally persist on FOUP surfaces longer than acids. Reactions between acids and bases, at extremely low concentrations, leads to the formation of new particles, a process known as nucleation. For this reason, the simultaneous monitoring of both acids and bases is critical. Semicon AMC Monitors register trace levels of molecular acids and bases in seconds, enabling manufacturers to elect the most efficient protocols for optimum FOUP cleaning.
Molecular Condensables (MCs) and Volatile Organic Compounds (VOCs)
These categories include plasticizers, phosphates, antioxidants, siloxanes, and VOCs. The presence of VOCs contributes to salt formation on wafer surfaces, also known as hazing. The hydrolysis of acetic acid derived from methyl ether acetate (PGMEA) has been known to cause hazing. On the other hand, it has been demonstrated that refractive organosilicons alter the light scattering properties of optical lenses when they deposit during lithographic processes . Semicon AMC Monitors accurately detect these compounds with excellent time response and sensitivity.
AMC Detection Use Cases
Strategic placement of monitors in the fab ensures that environmental conditions are observed to improve control and minimize failures caused by contamination. The sensitivity and ultra-fast time response of Semicon AMC Monitors improves control for high-throughput processes. Semicon AMC Monitors favor a diversity of applications, including AMC control at the process and tool level, air chemical conditioning, FOUP monitoring, material research and development, forensic failure analysis and remediation, leak detection, exhaust monitoring, and quality control.
Effective cleanroom design aims to maximize production yields for environmentally sensitive materials (microelectronics) and processes (wafer fabrication) . Cleanrooms are classified based on the number and size of particles allowed per volume of air. Measuring airborne particle precursors is critical to meet stringent standards and to ensure satisfactory yield.
AMCs originate from a diverse range of sources, including people, ventilation systems, and material off gassing. Even in the most pristine cleanroom environment, many compounds are still detected with Semicon AMC Monitors. Measurements in an ISO 6 cleanroom are shown in Figure 3. Here, compounds of different molecular categories were directly sampled, revealing a wide variety of compounds, including inorganic acids, VOCs, and fluorinated compounds (with several PFAS).
One of the most common outgassing sources in the fab environment are the front opening unified pods; referred to as FOUPs. FOUPs are used to transfer wafers from one process to the next to reduce contamination risk. Studies have shown that cross contamination can occur when outgassed AMCs from contaminated wafers remain in a FOUP which then contaminates the next batch of clean wafers . Wafer transport by FOUP between the many areas of a fab presents the greatest potential for cross contamination.
plications for both acid and base FOUP outgassing were performed with a Semicon AMC Monitor. In both instances, trace levels of AMCs were observed after more than 10 hours of continuous flushing, demonstrating the long residence time of some compounds and the importance of their traceability.
The results from this simulation demonstrate fast and sensitive FOUP screening using Semicon AMC Monitors. Integration of this solution in FOUP-by-FOUP or batch wafer processing provides precise detection of trace contaminants, improving yield and reducing queue times.
Semicon AMC Monitors are innovative and robust real-time analyzers. They are a critical solution for any fab pursuing optimal performance. With industry-leading sensitivity and minimal operational overhead, they provide comprehensive detection for a complete range of molecular categories.