Cleanroom packaging films: Beyond clean
02/01/2001
by Stephen W. Van Someren
Although he may not have realized it at the time, the wise sage Francis Bacon was quite possibly the first person to recognize the importance of a higher standard of clean when he coined the phrase, "Cleanliness is next to Godliness," nearly 400 years ago.
And while Bacon may not necessarily have had cleanroom packaging in mind when he spoke those well-known words, he wasn't far off the target when it comes to the significance of unyielding devotion to contamination control.
Bags labeled "Cleanroom prepared," "Packaged in a Class 100 cleanroom" or "Cleanroom admissible" do not achieve the real needs of most manufacturers in today's swiftly changing world. Cleanroom packaging users must invest the energy to craft specifications and processes that will insure the integrity, quality and consistency of performance that the product demands, and that the customer expects. Only by taking steps to advance the science can the consistent quality of a packaged product be guaranteed.
The advancement of product definitions and standards is being led by the users of cleanroom packaging in silicone wafer and hard-disk media manufacturing and in the vacuum deposition equipment industry. These users have a huge stake in high quality and absolute consistency with no room for "wild card" contamination from the packaging film being used. The products being packed by this class of user have the possibility of long-term contact with the packaging film; as a result, all contamination issues are amplified by the exposure time.
Other industrial segments that are becoming more concerned with these same issues are also driven by the need to preserve the quality and consistency of their products, which include optics and lasers, critical filtration, wafer mask, high-purity metal targets and any low ionic content product, such as PVA scrubbers.
Killer contamination
"Killer contamination" does not refer simply to particles any longer. Previously, cleanroom packaging was evaluated by how clean it tested or—in the case of the labeling statements noted above—by feeling comfortable because the product was packed in a so-called "cleanroom." In the past decade, the challenges for total contamination control have evolved as quickly in cleanroom packaging as they have in the new ISO Global Cleanroom Standards.
 A clean packaged wafer boat. |
In his article "Saving the most complex for last" (CleanRooms, Dec. 2000, p. 50), Richard A. Matthews discusses ISO's recognition of the issue and the need for controlling outgassing, surface contamination transfer and airborne molecular contamination in the cleanroom. Quite simply, cleanroom packaging acts as a "mobile cleanroom" surrounding that critical product you have spent so much time engineering and producing. That mobile cleanroom—a bag—has the job of protecting your product from the very dirty outside world and, just as importantly, not causing degradation of the enclosed product.
Clean packaging film manufacturers have been investigating these issues as they have a direct impact on the packaged product, due not only to intimate contact with the packaging, but—in some cases—long-term contact as well. In studies performed by research scientists at Applied Materials, it was discovered that visible black contamination on wipers used to clean components prior to installation was residue from the use of anti-static polyethylene packaging.
As a result, in 1997 a vigorous investigation was launched to understand the problem, with the result being an all-encompassing specification for all clean components. This is just one action being taken by users.
What was impressive about the Applied Materials specification was the effort that went into fully understanding the exact protection needs of each component, and the subsequent tailoring of the solution to the specific need. This is important for two reasons: the first being not to under-protect a product now or in its future development; the second being packaging cost control.
 A lab technician inserts solvent sample from test bag into liquid laser counter to establish cleanliness level. |
Low particle count may not be the only performance parameter to define in your packaging specification. Without question, particle contamination is the first priority in cleanroom packaging. Most users have come to understand that they can expect and demand cleaner films now than they could in the past. Due to this expectation, many critical users' application specifications are calling for packaging films to qualify at a particle level tested down to 0.5 micron instead of the more common NASA specifications, such as JSCM 5322, that at their most critical levels only test down to 5 microns. This is a strong example of how a packaging user has identified the "killer contamination" particle size and taken the steps to control it.
Additionally, a new question about particles is being asked: "What is the make up of the particles?" With the understanding that chemical contamination is a component of the particles being found in testing, we are now looking at ways to identify its composition. This information is critical for some medical device packaging and challenges involving chemical purity.
All flexible cleanroom supplies (wipers, gloves, garments and packaging films) have physical limits to how clean they can be produced. Moreover, the flexible materials break down as they are used, releasing fibers or discharging small amounts of latex or vinyl from abrasion. Packaging films can also be abraded by the packaged product, releasing sub-micron particles. Standards and qualifications of all such products must consider these limitations, while the proper use of vacuum packaging procedures will greatly reduce the opportunity for abrasion from the film and reduce any internal abrasion between products.
The new requirements for high-performance cleanroom packaging now include demands for low outgassing, low non-volatile residue (NVR) and low ionic contamination, and the user's specifications should address any performance parameters that can be determined to be a risk. This can be a formidable challenge for any engineer, but the return in yield and quality enhancement produces an immediate payback.
In the Applied Materials scenario mentioned earlier, it was a common practice to wipe critical components prior to their installation in process tools. Steve Lin and Sarah Graves of Applied Materials, who reported their findings in the October 1998 issue of Micro, found that "This labor-intensive practice results in a significant productivity loss, and tool decontamination has become a critical quality issue, which can only worsen if no effective resolution is implemented soon."
The issuing of a cleanroom packaging specification has resolved this type of situation and reduced the waste of costly production time in the cleanroom. Most packaging challenges are not as complicated as that experienced by Applied Materials, but the cost of improving your packaging in any similar situation is easily offset by yield improvement and reducing secondary cleaning.
 Test sample being flushed with solvent for liquid laser particle tester. |
Non-volatile residue in packaging films is easy to analyze and should be considered in all packaging qualification processes. This test looks for dissolved hydrocarbon residue in the solvent used for particle testing; by weighing the resulting residue, limits can be established for your packaging challenge. Secondary and more sensitive investigations can be done using ash or alpha tests to identify the level of free hydrocarbons as a user compares films to meet important needs. All ethylene-based packaging films will have some level of free hydrocarbon; the chief concern is choosing a material that minimizes damage to your product.
Low outgassing is an important factor in all cleanroom disciplines, and it is even more important to your product once it's in a package. If you are concerned about the outgassing of structural components in your cleanroom—and you should be—consider your package as a minienvironment, traveling to the other side of the world for processing. The opportunity to transfer chemical contamination to the surface of the packed product from outgassing during long-term contact with a large surface area like the inside of a bag is obvious. The challenge to the packaging manufacturer is to engineer films that minimize the components of the film itself that contribute to outgassing.
Mobile ion impurity levels can also have damaging results to high-performance products. The issue of purity has become extremely critical in any application when the component being packaged comes into direct contact with a process substrate. Any ionic contamination that transfers to the component will corrupt the process substrate. In each situation, specifications will need to balance setting maximum damage thresholds against realistic expectations for the packaging film. The science of testing for ions has advanced as quickly as the evolution of test equipment has made it possible. We have seen the measurement of ionic contamination advance from parts per million to parts per trillion, and now to 1012 molecules/cm2.
Developing focused specifications and requirements for your packaging films is critical to the long-term protection of your product. The first step is to understand the critical issues that can degrade the quality and consistency of your product. It is important that all aspects are considered and rated as to level of impact. Remember as you choose packaging films that most of your products will be double bagged. The performance parameters of the two bags can be mixed to add the needed protection the product requires. We have so far only addressed cleanliness parameters, but your specifications will also include the needs for physical protection.
Testing methods are the way we place definable values on particles, NVR, outgassing and ionic contamination limits in your specification. It is important that the test methods and expectations are clearly expressed for your qualification of a packaging film. In addition, the testing expectations for purchasing specifications need to be defined. As you define and request specific testing, remember that this will add costs to your purchasing process and the cost from your vendors.
The cold economic facts of life must also be considered in your development of a specification. In most cases, the more performance you demand, the more it costs. It is important that you do not over-specify or you will be buying more protection than you need. A properly developed specification will protect the product's cleanliness and integrity, and will be paid for by the resultant yield improvement and customer satisfaction.
Steve Van Someren is president of Cleanfilm (Islandia, NY), a manufacturer of cleanroom packaging products for the aerospace and semiconductor industries.
Don't miss Van Someren's conference presentation at CleanRooms East '01, March 12-14 in Boston. Visit www.cleanrooms.com for registration information or call (603) 891-9267.
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Testing methods
The following methods are suggestions for usable, repeatable and well-documented tests. As much as possible use existing standards to control costs. In special applications, try working with your vendor to develop tests that solve the issue.
Particle testing must be liquid solvent based to insure all particles are released from the film's surface. Liquid laser counting systems are the most uniformly accepted and consistent way to determine particle counts. Additionally, completely defined microscope counting systems can be used based on NASA specifications (KSC-C-123H). Most qualified vendors use particle levels as defined in NASA specifications.
Non-volatile residue testing methods are called out in ASTM D2109-71 and NASA KSC-C-123H. NVR is a simple test that can be performed with lab equipment available in most facilities. It is critical with this and all other tests that the cleanliness of the lab area is of a sufficient level to protect the samples from outside contamination. Purity of test solvents must be adhered to insure consistency and accuracy of the results.
ASTM E 595-93 is a most useful and readily available analytical technique for outgassing evaluation. Three measurements are reported: the total mass loss (TML); the mass of collected volatile condensable materials (CVCM); and the weight change of a previously outgassed specimen caused by water vapor recovery (WVR). The evaluation of the results must be compared to the findings of other packaging films and balanced with the needs of your product.
Ionic chromatography is tested by FTIR methods, using solvents to extract contamination on the inside of a bag. Due to the variety of chromatography equipment available, it is harder to call out any specific process. The results of your chosen method will help guide a decision to films that reduce the opportunity for contamination.—SWV