A new range of dissipative materials based on fluoroelastomer and perfluoroelastomer polymers is designed for wafer processing and wafer handling applications.
By KNUT BEEKMANN, Precision Polymer Engineering, Blackburn, Lancashire, U.K.
All industries are now focusing on creating increasingly innovative products at a greatly reduced cost, providing customers with new cost efficient solutions. As new technologies are being produced, there is a need for an improved supply chain, providing new materials, improving wafer throughputs and lowering defects – all resulting in reduced costs for manufacturers.
When dealing specifically with the electronics industry, there is a particular focus for all major manufacturers on electrostatic charge reduction and elimination of electrostatic discharge (ESD). Electro- static discharge is something we all experience on a regular basis – when walking across a carpeted floor and then touching a door handle, for example, or during a lightning storm. Although in everyday situations it is unlikely that any real lasting damage will be sustained, it is possible for electronic devices to be damaged by ESD events that are imperceptible to the human body. Even relatively low electrostatic voltages can have a huge impact on sensitive electrical devices, impacting yield, quality and reliability. It is suspected that ESD events occur hundreds of times a day. Although these are often not seen, they can have a significant impact on electrical equipment, resulting in a huge cost to manufacturers. In fact, it has been estimated that the cost of static associated damage ranges up to 33% for the electronic industry and between an average of 16 to 22% for component manufacturers [1] – a number which all would like to see dramatically reduced.
The principle of ESD
The initial creation of electrostatic charge requires energy to be transferred to a material which can occur when two materials repeatedly come into contact and separate. This is referred to as triboelectric charging. During this process a chemical bond is formed between the two surfaces as they come into contact, allowing charges to move from one material to the other to equalize their electrochemical potential. This creates a net charge imbalance between the objects. When separated, some of the bonded atoms keep extra electrons, while others give them away, though the imbalance will be partially destroyed by tunnelling or electrical breakdown (usually corona discharge). In addition, some materials may exchange ions of differing mobility, or exchange charged fragments of larger molecules.
When objects at different electrostatic potentials are brought together, the result is a rapid transfer of charge between the objects – the spark or ‘shock’ we commonly recognise as static electricity.
The triboelectric effect is not very predictable, and only broad generalizations can be made. However, materials such as glass or silica, plastics and elastomers, all of which are a fundamental requirement of semiconductor device manufacturing, either as a handling material or as part of the device, can each be a source of electrostatic charge. Once a material has become charged, the electrostatic field can also induce a charge distribution in nearby ungrounded conducting materials.
The level and sign of charge will mostly depend on the types of material, as summarized in TABLE 1.
TABLE 1. Example materials in the triboelectric series.
The impact of electrostatic charging
ESD has been an issue across multiple industries for as long as manufacturing has been taking place. Military forts in the 1400s were using static control procedures and devices trying to prevent inadvertent electrostatic discharge ignition of gunpowder stores. By the 1860s, paper mills throughout the U.S. employed basic grounding, flame ionization techniques, and steam drums to dissipate static electricity from the paper web as it travelled through the drying process [2].
As electronic device technology has progressed, we have seen reduced voltage tolerances and lower capacity for heat dissipation. The age of electronics brought with it new problems allied to electro- static discharge. Today, ESD impacts productivity and product reliability in virtually every aspect of the global electronics environment and emphasis on minimizing electrostatic charging and ESD has become hugely important [2].
When flowing through an integrated circuit, electrostatically induced charge can generate sufficient heat to break down the gate structure, cause spiking in contacts, junction breakdown and burn the interconnects. ESD events can also create a weakness that can lead to reliability issues or premature failure [3]. Damage from ESD on semiconductor devices can be immediate and catastrophic and can be blamed for millions of dollars of product failures a year.
ESD events may also interfere with the control electronics of the process tool. ESD creates electro- magnetic energy transmitted in the form of waves in the radio frequency range, leading to electromagnetic interference (EMI) [4]. Finally, electrostatic charging of materials can also lead to attraction and subsequent adhesion of particles from the ambient or from within the process tools. Electrostatic attraction will not distin- guish between material types and create potential yield reducing damage, as both insulating and ungrounded conducting matter can be equally influenced.
The failures can all have a dramatic impact on manufacturers, especially when we consider the
cost of repairing equipment that has been exposed to ESD – especially if such failures happen once the component has been installed into a system. While a simple piece of electronic technology may cost only $10 to replace and retest on its own, when it fails in the field it could mean a cost of hundreds or thousands of dollars. Recent approximations propose that the cost of repairing an ESD-damaged product increases tenfold at every level between individual component to system [1].
Semiconductor manufacturing
Modern semiconductor production plants have become significantly more automated, especially as wafer sizes have increased to 300mm. Wafers can undergo more than 1000 process steps and multiple robotic wafer handling cycles per process step, providing opportunities for static charges to accumulate and discharge. To a large extent, therefore, static build up is inevitable. In order to counteract this risk, system manufacturers have a responsibility to monitor the environment and use appropriate materials and equipment, as well as ensuring grounding and controlled leakage paths in order to control ESD.
Plastics and elastomers are commonly used to contact or support substrates through production lines. They serve their main purpose in several ways; substrates do not slide as they move and they should potentially be able to withstand raised temperatures without creating adhesion issues. However, these contact materials are primarily insulators. Whenever a substrate is in contact with a handling device and subsequently separates, triboelectric charging will take place. This increases the likelihood of a subsequent ESD event or induced charging of materials.
Rather than using insulators to contact the substrates, these materials should be electrostatically dissipative and have a low resistance path to ground. To ensure this, electrostatically dissipative materials need to have a volume or surface resistance between that of insulating and conducting materials at between 1×104 and 1×1011 ohms [2].
In addition to the properties previously mentioned, materials need to be compatible with semiconductor devices and not contribute to already sensitive levels of contamination. Elasto-meric materials such as ethylene-propylene polymers (EPD/EPDM) can be obtained in dissipative variants; however these invariably contain metallic elements. Dissipative elastomer materials for semiconductor processing or handling should have controlled constituents and avoid common metallic filler.
Dissipative elastomers
Particular properties of fluoroelastomers and perfluoroelastomers, such as chemical resistance, higher temperature compatibility and low levels of contaminants, make them particularly suitable for semiconductor applications. Finding materials with additional electrostatic dissipative properties, however, often proves a challenge.
FIGURE 1. EDX spectra of electrostatically dissipative perfluoroelastomer.
FIGURE 2. EDX spectra of electrostatically dissipative fluoroelastomer.
To meet this need, a completely new range of dissipative materials based on fluoroelastomer and perfluoroelastomer polymers has been specifically designed for wafer processing and wafer handling applications. FIGURES 1 and 2 show an energy dispersive X-ray spectroscopy (EDX) analysis carried out on both polymer types and demonstrates a complete absence of metallic based filler and an entirely organic composition. This is summarized in TABLE 2. Despite avoiding the use of metallic based additives, volume resistance values can be obtained that are well within the dissipative range (FIGURES 3 and 4).
TABLE 2. EDX compositional analysis in weight.
FIGURE 3. Perfluoroelastomer volume resistance.
FIGURE 4. Fluoroelastomer volume resistance.
Conclusion
Technological progress within the semiconductor industry brings with it greater yield sensitivity, along with a common desire to reduce costs. These two factors are also related to defects and hence, how well those defects are controlled throughout an increasingly automated manufacturing process. An important part of reducing defects, both during and after production, includes management of electro-static charging in order to avoid damage from ESD events. Greater use of dissipative materials is an obvious way of minimising charge build-up and reducing ESD events.
However, it is also essential to ensure these materials are compatible with the process environment and the devices themselves. With the correct material and precautions in place, it is possible to avoid damage from static charge, particle contamination through electrostatic attraction and process tool interference through EMI.
References
1. “Guidelines for Static Control Management,” Steven Halperin Eurostat 1990.
2. “Fundamentals of Electrostatic Discharge,” ESD Association
3. “Understanding ESD and EOS Failures in Semiconductor Devices,” S. Agarwal, Cypress Semiconductor, Electronic Design Feb 2014
4. “Preventing Electrostatic Problems in Semiconductor Manufacturing,” A.J. Steinman Compliance Engineering, 2004 Annual reference guide
5. “ESD Protection While Handling LEDs,” C. Lee, D. Ying, C. Wittmann, A. Stich, Osram Application note, December 2013.
KNUT BEEKMANN is the Marketing Manager for Semiconductors, Precision Polymer Engineering, Blackburn, Lancashire, U.K.