How to Protect the Integrity of Wetted Surfaces

The semiconductor industry relies on aggressive process chemicals to manufacture wafers with microscopic precision. To selectively etch semiconductor materials, highly corrosive gases and potent solvents are utilized.

Components or distribution systems that come into contact with these chemicals must possess the cleanest and most corrosion-resistant surface their design allows. Establishing this surface quality is the role of SEMI F19.  

SEMI F19, “Specification for the Surface Condition of the Wetted Surfaces of Stainless Steel Components,” is introduced in this article. Initially published in 1994, the currently approved version, as of the time of writing, is 0815, which was approved in 2015.

SEMI F19 is widely adopted in the semiconductor industry and has also found application beyond semiconductors as a reliable benchmark for passivation or electropolish surface quality of austenitic stainless steels.

316L stainless steel surfaces treated to meet the highest grade specified in SEMI F19 exhibit excellent corrosion resistance due to their high chromium enrichment on the surface. Their minimal surface roughness provides an easily cleanable surface in case of contamination.

Image Credits: Shutterstock / IM Imagery

Terminology Introduced in SEMI F19

The first three sections of SEMI F19 address the specification's Purpose, Scope, and Referenced Standards. Section four comprises a list of acronyms and definitions, including definitions for surface defects commonly found in stainless steel and electropolishing process artifacts.

Defects outlined in this specification encompass cracks, inclusions, blisters, dents, pits, stringers, and scratches on the metal surface. SEMI F19 also defines specific electropolish-related terms, such as frostiness and orange peel, which are also included in ASTM B912.

These artifacts result from an inconsistent temperature or current density during electropolishing.

Requirements and Table 1

Following a section on ordering information, SEMI F19 presents the Requirements Section, which outlines three quality grades: General Purpose (GP), High Purity (HP), and Ultra-High Purity (UHP). Each grade has specific requirements for Surface Roughness, Surface Defects, Surface Contamination, and Surface Chemistry, which are elaborated in Table 1 of the specification.

Surface Roughness

SEMI F19 refers to SEMI F37, "Method for Determination of Surface Roughness Parameters for Gas Distribution System Components," for testing requirements related to measuring roughness values Ra and Ry. To be deemed acceptable, GP, HP, and UHP each have specific criteria for average Ra, maximum Ra, and maximum Ry readings.

Surface Defects

This requirement limits the allowable number of defects (as defined in the Terminology Section) in a batch of acquired Scanning Electron Microscope (SEM) images.  

The specification governing the acquisition settings and required number of micrographs is SEMI F73, "Test Method for Scanning Electron Microscopy (SEM) Evaluation of Wetted Surface Condition of Stainless Steel Components."  

While GP has no surface defect requirement, HP restricts the average to 30 defects per photo with no count exceeding 50, and UHP sets limits at an average of 10 and a maximum of 20 defects.

Surface Contamination

According to SEMI F19, the presence of all elements, except iron, chromium, nickel, molybdenum, manganese, silicon, and carbon analyzed through Energy Dispersive Spectroscopy (EDS), is prohibited. This requirement applies to both HP and UHP grades, while GP has no contamination requirement.

Surface Chemistry

The surface chemistry requirements outlined in SEMI F19 have made it a widely recognized benchmark for assessing passivation efficacy. GP grade components must undergo passivation as per ASTM A967, without requiring surface chemical analysis.

HP and UHP grades must be analyzed, preferably using X-Ray Photoelectron Spectroscopy (XPS). However, Auger Electron Spectroscopy (AES) is also acceptable, to determine the total chromium to iron ratio (Cr/Fe), chromium oxide to iron oxide ratio (CrOX/FeOX), and oxide layer thickness.

HP must have ratios greater than 1.0, while UHP requires Cr/Fe to be greater than 1.5 and CrOX/FeOX to be greater than 2.0. The guidelines for conducting XPS work are provided in SEMI F60, while AES work is covered in SEMI F72.  

The higher-grade components impose limitations on the presence of carbon, sulfur, phosphorus, nitrogen, and silicon at the immediate surface. HP and UHP grades also require a passive layer thickness of at least 15 Angstroms. The oxide thickness is determined from the depth profile obtained through AES and XPS testing.

The passive layer thickness is the depth at which the oxygen concentration is half its maximum value. Figure 1 in SEMI F60 depicts a theoretical depth profile illustrating how the oxide layer thickness and other commonly found layers in passivated stainless steel are measured.

Sketch of a depth profile of passivated stainless steel reproduced from SEMI F60.

Figure 1. Sketch of a depth profile of passivated stainless steel reproduced from SEMI F60. Image Credit: Astro Pak Corporation

Corrosion Resistance in SEMI F19

Table 1 of SEMI F19 mentions a fifth requirement: Corrosion Resistance. Unlike its three grades, SEMI F19 does not establish any specific need for corrosion resistance; instead, it states that the exact resistance level should be agreed upon between the end user and the supplier.

The specification references ASTM G150 and SEMI F77 as the designated electrochemical tests to determine the Critical Pitting Temperature and establish a level of corrosion resistance.

Bottom Line

SEMI F19 presents a comprehensive set of requirements to ensure your system's or components' optimal performance. All three grades defined in the specification result in a pristine surface that is corrosion resistant.  

The UHP grade stands out as one of the industry's top-quality requirements for 316 L stainless steel. Astro Pak possesses proven processes to achieve these desired surface conditions through its electropolishing division and its proprietary blend of citric acid-based passivation, known as Ultra Pass®.

Appendix: Commentary on Chromium Enrichment

Within the Terminology Section of SEMI F19, the most significant entries related to passivation are Cr/Fe and CrOX/FeOX. Cr/Fe represents the overall ratio of chromium to iron in the passive layer.

This definition is derived from SEMI F60, SEMI's specification governing XPS analysis of the passive layer in stainless steel samples.  

Cr/Fe does not account for the oxidation state of the metals analyzed; it encompasses both metallic and oxidized forms of each metal. Additionally, it can be influenced by the presence of free iron on the surface or metallic iron and chromium if the X-Ray beam probes into the base metal.

On the other hand, CrOX/FeOX solely considers oxidized chromium and oxidized iron. These compounds are the primary contributors to the corrosion resistance of the passive layer, with chromium oxide being the more crucial element.  

In the industry, XPS is widely accepted as the more accurate technique for determining the concentration of the passive layer, while AES is preferred for measuring its depth. Figure 2 displays a spectrum section obtained via XPS, showcasing distinctive oxidized and metallic iron peaks.  

The area under each peak reflects the concentration of the corresponding species. In the case of Figure 2, approximately two-thirds of the total iron measured in this sample is oxidized.

Iron-specific section of a spectrum acquired by XPS. Blue peaks are oxidized iron, black peaks are metallic iron.

Figure 2. Iron-specific section of a spectrum acquired by XPS. Blue peaks are oxidized iron, black peaks are metallic iron. Image Credit: Astro Pak Corporation

This information has been sourced, reviewed and adapted from materials provided by Astro Pak Corporation.

For more information on this source, please visit Astro Pak Corporation.


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