SCIS Seals & Valves Group develops seals testing standard

By Paul Trio

SCIS is a SEMI Technology Community that tackles critical component defectivity for the semiconductor manufacturing industry. The organization develops test methods for measuring defects in these critical components. Originally, this SEMI community was looking at challenges surrounding sub-10nm process nodes, but our constituents – Integrated Device Manufacturers (IDMs), capital equipment OEMs, and (sub)component suppliers – felt that the immediate need was for standards that would apply to process nodes that are already being used for volume semiconductor device manufacturing.

IDMs need ways to tell their supply chain how defects attributable to these critical components factor into the overall process-node defect budgets and wafer-contamination limits. Chipmakers and IDMs needed to start with a baseline: How problematic are existing critical components in the overall fab systems and how do these contaminants contribute to defects and how do they affect overall process yields?

These questions must be answered for every component in the fab’s process line including the drums that hold the fab chemistries, fluid delivery systems, and components used in the wafer-processing chamber. All of these critical fab-line components come into contact with each manufactured wafer, in one way or another, and each is a suspect with respect to contamination, defects, and yield problems. SCIS develops test methods for these fab-line critical components testing that are used to identify the defects caused by these components and for establishing baselines.

SCIS has seven working groups dealing with various critical components. Each is developing various test methods for many critical fab-line components. There are many facets with respect to testing each of these critical components.

Take something as simple as a seal, such as an FFKM (perfluoroelastomer, made from polymers) seal. These seals are ubiquitous in fab lines. In harsher environments, such as inside of a processing chamber, these seals are exposed to high temperatures and harsh chemistries. Different FFKM seals will have different characteristics such as thermal resistivity and chemical resistance, depending on customer specifications, and can also vary from one manufacturer to another. In addition, these characteristics can change depending on environmental conditions – or just the passage of time.

SCIS looks at defect traits from the perspective of each component in the fab line and decides which of the components’ parameters contribute most to process defects. Initially, the SCIS Seals & Valves Group collected a list of seal-related issues or parameters. The working group then cross-checked these parameters against different manufacturing processes used in the fab including ALD (atomic layer deposition) and CVD (chemical vapor deposition). Some processes are harder on seals than others. Then the working group prioritized these various parameters according to their contribution to the overall process defect budget. IDMs provided important input during these steps because they work with these seals on a daily basis. At this point, the SCIS working group had a prioritized list of parameters, vetted by various stakeholders in the semiconductor manufacturing industry. The group then set to develop standardized measurement methods for these critical parameters.

Based on this work, the SCIS Seals & Valves Group has already published two documents. The first is a standard that specifies methods for testing seal-induced impurities such as ashing (analysis of metals content of the ash) and TOC (total organic content).

The second document published by the Seals & Valves Group is a guide that documents BKMs (best known methods) for handling seals – from the moment they’re cured in an oven to packaging, shipping, handling in a fab, and installation – a to reduce contamination problems during use. For example, some seals are sensitive to light. Some polymer seals degrade when they come into contact with IPA (isopropyl alcohol), which is often used for prepping. A degraded seal can emit contamination particles during processing, which will cause yields to fall. (This latter bit of information came directly from a major IDM, which demonstrates the invaluable role that users of these components can play in the development of testing standards.)

The Seals & Valves Group’s current work focuses on developing a standard for measuring seal leak rates. This standard will define test methods for evaluating a seal’s ability to maintain pressure under vacuum. Although there are well-established standard for testing seal CSR (compressive stress relaxation) in the aerospace industry, there’s no such standard for the semiconductor industry. So originally, the Seals & Valves Group tried to tackle that challenge by developing a similar standard for SEMI’s constituents. However, a more practical and immediate parametric challenge turned out to be seal leakage rates.

Installed seals are exposed to high temperatures and harsh chemistries in the semiconductor fabrication process. The Seals & Valves Group decided to develop a test method that would determine how well seals perform over time with respect to leakage rates as the seals are exposed to cyclic harsh conditions. The goal is to simulate the working conditions for these seals, as closely as possible and in a repeatable manner.

There are, of course, some challenges associated with this work. For example, IDMs and equipment OEMs don’t want to reveal their exact process conditions as they are proprietary. So the Seals & Valves Group took a step back and focused on developing a test method based solely on exposure to elevated temperatures.

Development of this thermal test requires the design of a standardized test jig to help ensure consistent, repeatable tests, shown in Figure 1.

The seal under test, shown in red in Figure 1, sits at the center of the jig. A second seal, shown in green, is used to seal the actual test environment. Two thermocouples in the jig’s top and bottom monitor of the temperature inside of the jig. There are gas and purge lines for controlling the ambient pressures on either side of the seal under test.

Figure 2 illustrates how the jig is connected to the gas sources.

The seals leak test is based on a helium leak test. Helium is one of the smallest atoms so it will leak through just about any small gap and, with time, permeate through the material as well. In addition, helium is inert, and testing for helium using a mass spectrometer is a well-established technique for leak testing. Helium leak testing can be one thousand to one million times more sensitive than using mechanical, pressure-decay test techniques. The jig’s nitrogen lines serve to purge the test chambers of helium between leak tests.

Developing just a test jig is not sufficient. The Seals & Valves Group also developed a test sequence for using the jig. There were no existing standard, so the group needed to use its knowledge of the seals’ composition and operating conditions to develop certain test parameters. For example, the group elected to use 200°C as the maximum temperature for the high-temperature portion of the test because FFKM seals start to degrade at 250°C.

At this point, the Seals & Valves Group has gone through several iterations of a proposed test sequence. There was some initial reluctance to provide detailed inputs, but after a few iterations of the proposed method (and an understanding that this would become an industry standard to hold suppliers accountable), inputs have become more forthcoming.

This is an excellent example that demonstrates why it’s so important for SCIS working groups to get chipmakers, IDMs, component vendors, and even feedstock materials vendors to participate in these standardization efforts. Standards are far more useful if they’re based on real-world conditions.

Currently, the SCIS Seals & Valves Group is working towards finalizing the seals-leak test sequence. The jig has been designed in AutoCAD and a prototype will soon be manufactured. Although the test and jig have been developed with significant industry participation, the validity of the test has yet to be determined. The validity will be verified though Alpha testing before the jig design and test method are incorporated into a standard.

However, SEMI is not a test house. It’s a facilitator. The testing will therefore be performed by a neutral third party capable of carrying out the test under fab-like conditions. SEMI’s role is to work with different testing entities such as SUNY Polytechnic Institute in Utica, New York or IMEC in Belgium.

SEMI will solicit bids for this work through its SCIS Executive Advisory Committee, which consists of C-level executives from device makers, semiconductor capital equipment OEMs, and major critical component suppliers. This project has leveraged many of the relationships that SEMI has developed over the years and has broken new ground in standards making for SCIS and for SEMI.

For those looking to learn more about SCIS or engage in ongoing efforts, please contact Paul Trio, senior manager of Strategic Initiatives at SEMI, at ptrio@semi.org.