The semiconductor industry relies on aggressive process chemicals to manufacturer wafers with microscopic precision. Highly corrosive gases and potent solvents are used to selectively etch semiconductor materials. Distribution systems or components in contact with these chemicals must have the cleanest, corrosion resistant surface their design will allow. The specification responsible for establishing this surface quality is SEMI F19.
SEMI F19, “Specification for the Surface Condition of the Wetted Surfaces of Stainless Steel Components,” is the next specification introduced in our blog series. The specification was first published in 1994, and at the time of this writing version 0815 was approved in 2015.
Semi F19 is widely used in the semiconductor industry and has also been adopted outside of semiconductor use as an effective benchmark for passivation or electropolish surface quality for austenitic stainless steels. 316L stainless steel surfaces processed to the highest grade set in SEMI F19 will have an excellent level of corrosion resistance due to their high level of chromium enrichment at the surface, and their minimal surface roughness provides the most cleanable surface possible should contamination occur.
Terminology Introduced in SEMI F19
The first three sections of SEMI F19 cover the specification’s Purpose, Scope, and Referenced Standards. Section four is a list of acronyms and definitions. Many of the definitions are for surface defects common to stainless steel plus electropolishing process artefacts. In this specification, defects include cracks, inclusions, blisters, dents, pits, stringers, and scratches on the metal surface. Some common electropolish-related terms defined in F19, also included in ASTM B912, are frostiness and orange peel. Both artefacts result from inconsistent temperature or current density during the electropolish operation.
Requirements and Table 1
Following a section on ordering information, SEMI F19 presents the Requirements Section. SEMI F19 has three quality grades: General Purpose (GP), High Purity (HP), and Ultra-High Purity (UHP). Each grade has different requirements for: Surface Roughness, Surface Defects, Surface Contamination, and Surface Chemistry, which are detailed in Table 1 of the specification.
SEMI F19 references SEMI F37, “Method for Determination of Surface Roughness Parameters for Gas Distribution System Components,” on testing requirements to measure the roughness values Ra and Ry. GP, HP, and UHP all require certain Ra average, Ra maximum, and Ry maximum reading to be considered acceptable.
Did you know that Passivation post Electropolishing provides increased corrosion resistance?
Sure, electropolishing does result in a passive stainless steel surface. However, we have found that employing Astro Pak’s Ultra Pass® passivation treatment after electropolishing can provide incremental corrosion resistance, achieving higher chrome to iron ratios, resulting in a higher quality passive layer.
This requirement sets a limit on the number of defects (as defined in the Terminology Section) permissible in a batch of acquired Scanning Electron Microscope (SEM) images. SEMI F73, “Test Method for Scanning Electron Microscopy (SEM) Evaluation of Wetted Surface Condition of Stainless Steel Components,” is the specification governing the acquisition settings and the required number of micrographs. GP has no surface defect requirement; HP limits an average of 30 defects per photo with no count exceeding 50, while UHP’s limits are 10 average and 20 maximum.
SEMI F19 prohibits the presence of all elements apart from: iron, chromium, nickel, molybdenum, manganese, silicon, and carbon present at the surface when analyzed by Energy Dispersive Spectroscopy (EDS). This requirement applies to both HP and UHP grades; GP does not have a contamination requirement.
The surface chemistry requirements of SEMI F19 make it a popular benchmark for passivation efficacy. GP grade components must be passivated per ASTM A967, further no surface chemical analysis is required. HP and UHP grades must be analyzed, preferably using X-Ray Photoelectron Spectroscopy (XPS) but Auger Electron Spectroscopy (AES) is allowed, for the total chromium to iron ratio (Cr/Fe), the chromium oxide to iron oxide ratio (CrOX/FeOX), and the oxide layer thickness. HP must have both ratios higher than 1.0 while UHP requires Cr/Fe greater than 1.5 and CrOX/FeOX greater than 2.0. SEMI’s specification for conducting XPS work is the document SEMI F60 while AES work is covered in SEMI F72. The two higher grades also have a limitation on the presence of carbon, sulfur, phosphorous, nitrogen, and silicon at the immediate surface.
Along with these chromium enrichment ratios, the passive layer must be at least 15 Angstroms thick for HP and UHP grades. Oxide thickness is determined from the depth profile provided by both AES and XPS testing. The passive layer thickness is defined as the depth at which the oxygen concentration is half of its maximum value. Figure 1 below from SEMI F60 is a theoretical depth profile detailing how the oxide layer thickness, and other layers commonly found in passivated stainless steel are measured.
Corrosion Resistance in SEMI F19
A fifth requirement is mentioned in Table 1 of SEMI F19, Corrosion Resistance. SEMI F19 does not set any requirement on corrosion resistance per its three grades, it mentions the exact resistance shall be agreed between the end user and supplier. The specification calls out ASTM G150 and SEMI F77, both electrochemical tests to determine the Critical Pitting Temperature, as the corrosion resistance tests to be used to set a resistance level.
Did you know Nitric Acid is an etchant?
Nitric acid removes free iron, but, if incorrectly prepared/diluted, can also etch the base metal due to its indiscriminate nature. Alternatively, Astro Pak’s Ultra Pass® citric acid blend selectively targets free iron, rendering it chemically non-reactive, leaving the base metal intact. It also achieves a passive layer, leading to a more corrosion resistant surface, making it the preferred choice for passivation.
SEMI F19 provides an excellent set of requirements for ensuring that your system or components perform as intended. All three grades defined in the specification result in a clean, corrosion resistant process surface. Among these, the UHP grade is one of the best industry-accepted quality requirements currently available for 316L stainless steel. Astro Pak has proven processes to meet these surface conditions through our electropolishing division and our proprietary, citric acid based passivation blend, Ultra Pass®. Contact Astro Pak today to find out how we can partner with you to achieve your ultra high purity process goals.
Appendix: Commentary on Chromium Enrichment
The most important entries with respect to passivation in the Terminology Section of SEMI F19 are Cr/Fe and CrOX/FeOX. Cr/Fe is the total ratio of chromium to iron in the passive layer. This definition is taken from SEMI F60, SEMI’s specification on conducting XPS analysis on the passive layer of stainless steel samples. Cr/Fe does not consider the oxidation state of the metals analyzed, it includes both metallic and oxidized forms of each metal and can be influenced by the presence of any free iron on the surface or metallic iron and chromium if the x-ray beam probes into the base metal.
CrOX/FeOX on the other hand only considers oxidized chromium and oxidized iron. These compounds are what provide the corrosion resistance of the passive layer, with chromium oxide being the more important contributor. XPS is accepted in the industry as the more accurate technique for determining passive layer concentration while AES is preferred for determining passive layer depth. Figure 2 is a section of a spectrum acquired by XPS, showing the different characteristic peaks for oxidized iron and metallic iron. The area under the curve of each distinct peak relates to the concentration of each species present. In the case of Figure 2, about two thirds of the total iron measured in this sample is oxidized.