Achieving surface cleanliness according to ASML’s grade 2 and grade 4 standards

Cleanliness is an essential but often underappreciated requirement for manufacturing and operating high-tech components. Even microscopic contaminants on part surfaces can lead to degraded performance and reduced yields in industries like semiconductor fabrication. Contamination becomes increasingly problematic as industries push towards miniaturization and higher precision.

For example, a particle just 10 nanometers wide can cause failure of leading-edge semiconductor devices with feature sizes below 10 nanometers. Manufacturing processes must achieve exceptional cleanliness levels.

As one of the world’s leading makers of photolithography systems for chip patterning, ASML is a standard-bearer for cleanliness in the semiconductor industry. Their photolithography scanners contain some of the most sophisticated optics ever developed. Components inside ASML tools must meet rigorous cleanliness criteria to function at specified through puts and yields.

This post examines how suppliers can achieve surface cleanliness according to ASML’s demanding grade 2 and grade 4 requirements. First, ASML’s cleanliness grading system, including contaminant limits and inspection methods, will be reviewed. Next, major challenges will be discussed in attaining grade 2 and 4 cleanliness. Approaches for manufacturing, cleaning, handling, and inspection to reach grades 2 and 4 will then be detailed. Key differences between grades 2 and 4 are also outlined.

By elucidating ASML’s cleanliness grades and how to accomplish them, this post aims to advance manufacturing best practices for high-tech industries.

ASML’s Cleanliness Grading System

ASML institutes rigorous cleanliness criteria for components via its grading system to enable the production of hyper-advanced photolithography systems. Understanding these quantitative cleanliness grades and verification techniques is the first step for suppliers in achieving conformance. This section summarizes the essentials of ASML’s grade 2 and grade 4 designations for surface cleanliness.

ASML separates cleanliness into two main grades: 2 and 4. Grade 2 represents an extremely clean level needing stringent contamination control. Applications require the highest precision where even slight contamination causes problems, like optics and certain electronic modules. Grade 4 signifies a generally clean status with modest contaminant limits, suitable for less sensitive but still important uses.

For both grades, surface cleanliness requirements exist for three contaminant categories:

  • Category 1 – Process residues like metal chips/burrs
  • Category 2 – Particulates and fibers
  • Category 3 – Stains from residues, fingerprints, etc.

Grade 2 permits only an exceptionally low level of category 1 and 2 defects – ≤2 total per square decimeter visually or ≤4 under UV-A light inspection. No films or stains (category 3) can be present. Grade 4 allows ≤2 category 1 and 2 contaminants per square decimeter visually, or ≤4 by UV-A, with category 3 stains ≤8 square centimeters per square meter total and ≤3 Square centimeters individually.

Multiple verification techniques like visual and UV-A light inspection against known standards are essential to prove cleanliness specifications are satisfied during manufacturing. ASML provides explicit inspection guidance. Statistical sampling may be appropriate once a process is proven.

Achieving stringent ASML cleanliness grades enables the defect-free performance of sophisticated systems. Suppliers must exercise diligence in upholding standards through strict process and contamination controls. With rapid innovation in high technology, ASML’s specifications will only intensify. Understanding current requirements provides a foundation for the future.

Challenges in Reaching Grade 2 and 4 Standards

Despite detailed specs, realizing grade 2 and 4 cleanliness involves substantial challenges:

Contamination Sources

Contamination sources abound during manufacturing and assembly. Machining generates particulates and metal residue. Joining processes like welding produce fumes, spatter, and flux residue. Even airborne dust in the assembly environment can contaminate surfaces.

Cleaning Difficulties

Complex shapes with deep crevices, holes, and micro-scale surface patterns in precision components complicate cleaning. While ultrasonic baths remove surface contaminants, penetrating intricate geometries is difficult.

Process Control

The stringent quantitative requirements for both grade levels demand excellent process control and operator skill. Grade 2, in particular, allows no margin for error – just a couple of errant particles or micro-spots per square decimeter can fail inspection. Statistics-based process monitoring is imperative.

Suppliers must mitigate contamination sources, develop innovative cleaning methods, and prevent process/human variations to satisfy ASML’s exacting standards.

Achieving Grade 2 Surface Cleanliness

ASML provides comprehensive protocols to achieve grade 2 surface cleanliness during manufacturing, cleaning, inspection, and handling:

Manufacturing Guidelines

Regular cleaning between steps prevents particle and residue buildup. Dedicated equipment and tools produce less particulate. Extrusion and molding processes are optimized for consistency.

Cleaning Procedures

Thorough ultrasonic washing and high-temperature nitrogen drying have proven effective for metal parts:

  1. Ultrasonic alkaline detergent wash
  2. Multiple ultrasonic rinses, the final one with deionized water
  3. Heat drying with filtered nitrogen flow

All steps occur in a cleanroom with operators wearing gloves to prevent recontamination.

For optics, plasma cleaning can further enhance surface cleanliness.

Inspection Methodology

ASML guidelines describe inspection methodology for verifying grade 2 cleanliness—initial visual inspection under bright lighting checks for obvious contamination. Primary quantitative verification uses UV-A illumination and counting of fluorescing spots down to the single digit per square decimeter sensitivity required.

Contamination Control

Stringent protocols govern packaging, transport, and handling to maintain cleanliness. Protective garments, gloves, and cleanrooms help prevent recontamination.

Suppliers can readily satisfy requirements by implementing the comprehensive grade 2 guidelines. The protocols provide a rigorous pathway to achieve cleanliness.

Achieving Grade 4 Surface Cleanliness

The path to accomplishing grade 4 surface cleanliness involves similarities with grade 2, albeit with less stringent requirements:

  • Regular cleaning during manufacturing minimizes particulate and residue buildup
  • Contamination prevention via clean equipment and tooling management is vital
  • For cleaning, ultrasonic washing, and rinsing steps remove contamination
  • Visual and UV-A inspections verify acceptable cleanliness levels
  • Packaging protects cleanliness after processing

Realizing grade 4 cleanliness still requires diligence across manufacturing, cleaning, inspection, and handling to meet specifications. While less stringent than grade 2, discipline in following protocols is vital to avoid cleanliness excursions.

Comparison of Grade 2 and Grade 4

Grade 2 mandates significantly more stringent surface cleanliness than Grade 4:

  • Lower contaminant limits across all particle, fiber, and residue categories
  • More rigorous contamination avoidance in manufacturing
  • Tighter control of cleaning chemistry and processes
  • More sensitive visual and quantitative UV-A inspection techniques
  • Ultra-clean handling, packaging, and transport protocols

The differential cleanliness grades provide a calibration scale for suppliers. The higher precautions and diligence required for grade 2 correspond to the extreme contamination susceptibility of the highest-performance optical, electronic, and precision mechanical systems.


Through quantitative cleanliness grades, targeted guidelines, and rigorous verification protocols, ASML enables suppliers to deliver components with exceptional surface cleanliness tailored to application requirements. Controlling contamination sources, proven cleaning methods, diligent handling procedures, and repeatable inspection techniques allow grade 2 and 4 specifications to be consistently achieved.

ASML’s standards provide clarity amid the ceaseless trend toward more demanding cleanliness needs in high technology. As miniaturization and precision continue intensifying, understanding and realizing ASML’s cleanliness system will become more vital. With rapid innovation, adapting emerging methods to improve cleanliness further will also be key – but ASML’s specifications provide the critical foundation.

Written by Timothy Egger

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