Electrostatic discharge in semicon fabrication: Causes and solutions

By K. Beekmann, Marketing Manager Semiconductor, Precision Polymer Engineering

In the modern electronics marketplace, the demand for mobile computing technology is outstripping that of traditional personal computers and laptops. As technological evolution continues to force down the size and weight of front end interfaces and with an increased requirement in performance output, the challenges of electronic chip and component production are increasing.

At fabrication (fab) level, this dynamic has led the industry to focus on innovation to reduce costs and streamline processes. Perhaps the most significant step-change here has been the shift from 200mm to 300mm wafer processing, allowing fab owners to produce more chips per process step, increasing production output. In addition to increasing wafer size, device dimensions have been significantly reduced, now with 28nm and 20 nm technology nodes reaching mass production.

In recent years the industry has moved towards a greater reliance on automation, lower staffing costs and reducing the risk of contamination posed by human presence and manual handling within the cleanroom. Automated robotic equipment in semiconductor fabs now delivers the many thousands of process steps it takes to produce a finished product wafer. This means there are now many more pick-and-place operations, moving the wafer from the front opening unified pod (FOUP) into the process system or analytical tool. The scale of repetitive mechanical handling and movement throughout the manufacturing process is a significant factor in creating the potential for device damage in semiconductor production; through charging and subsequent Electrostatic Discharge (ESD) or electrostatic attraction of particles.

What is ESD

ESD is the transfer of electrostatic charge between two or more surfaces or bodies at different electrostatic potential. A common route cause of ESD is triboelectric charging, which occurs when two materials in a process such as handling equipment and wafers, repeatedly come into contact and separate. A charge accumulates, eventually creating a discharge, the resultant energy manifesting itself in the form of heat. This is a serious concern for fab owners and a primary cause of damage to sensitive electronic components.

In semiconductor manufacturing, it is possible for nano-scale devices to be significantly damaged by ESD that is imperceptible to the human body. It is suspected that ESD events occur hundreds of times a day below the human sensitivity threshold of 3,000 volts. Even low levels of ESD can have a huge impact on sensitive electrical devices, impacting yield, quality and reliability resulting in a huge cost to manufacturers.

In order to reduce the risk of ESD, system manufacturers have a responsibility to monitor the environment and use appropriate materials and equipment. Grounding and controlling charge leakage paths is a necessity in order to minimize ESD or electrostatic charging.

The impact of electrostatic discharge events

As semiconductor technology has progressed, the industry has seen reduced voltage tolerances and a lower capacity for heat dissipation, leading to new problems and vulnerability allied to electrostatic 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 (1).

When electrostatically induced charge flows rapidly through an integrated circuit in an ESD event,  sufficient heat can be generated to breakdown the gate structure, cause spiking in contacts, junction breakdown, and  burn the interconnects. As devices continue to reduce in size, the  impact of ESD is increased. Thinner gates mean lower breakdown voltage and shallower junctions lead to a higher current density during such an ESD event. Furthermore, device size reduction also decreases the distance between I/O pads, reducing the area available for ESD protection.

Damage from ESD on semiconductor devices can be immediate and catastrophic. ESD events can also lead to partially damaged devices which may still conform to specification however; once built into an electronic system, cause reliability issues or premature failure (2) and can be blamed for millions of dollars of product failures every year.

 

Dissipative elastomers

Elastomers are commonly used as contact or support materials for substrates being processed through production equipment. Their main function is to avoid substrate slippage during fast robotic handler extension and rotation movements.  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, which in turn can lead to induced charging of other conducting materials present on the substrate thereby increasing the likelihood of a subsequent ESD event.

Dissipating elastomeric materials have electrical resistance values in a specific range between those of insulators and conductors. This allows for a controlled release of energy (electrical charge) via a low resistance path to earth. 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 such materials with additional electrostatic dissipative properties, however, often proves a challenge.

The novel elastomer materials developed by Precision Polymer Engineering (PPE) have been characterized to be electrostatically dissipative thereby avoiding electrostatic charging of the material and allowing the slow, safe release of any built up charge minimizing the likelihood of an ESD event. Furthermore, the dissipative properties of fluoroelastomer and perfluoroelastomer polymers have been achieved whilst avoiding the use of metallic based fillers such as silver, nickel or copper which often leads to undesirable mobile ion contamination of sensitive devices. 

Conclusion

The build up of static in semiconductor manufacturing is unavoidable. The industry must therefore work to control and reduce static throughout the various manufacturing stages, working to avoid static charging or dissipate charge and reduce the risks of thermal damage associated with the occurrence of an ESD event. The use of dissipating elastomers has clear benefits; in that wafers are protected from charge build through each manufacturing process step where robotic handlers are employed. By implementing this kind of risk averse procedure; yield, quality and output are improved, lowering costs, resulting in greater profitability. 

For more information about how dissipative elastomers can help you, contact the author at [email protected]

October 6, 2014

 

References:

(1)    “Fundamentals of Electrostatic Discharge” ESD Association

(2)    “Understanding ESD and EOS Failures in Semiconductor Devices” S. Agarwal, Cypress Semiconductor, Electronic Design Feb 2014

For further press information please contact: Ben Johnson, The Scott Partnership, 1 Whiteside, Station Road, Holmes Chapel, Cheshire, United Kingdom Tel: + 44 (0)1477 539539 Fax: +44 (0)1477 539540  E-mail: [email protected] 

 

About Precision Polymer Engineering (PPE)

Precision Polymer Engineering (PPE) designs, develops and manufactures high performance molded rubber seals (elastomer seals), rubber gaskets and rubber components for industries around the world.

PPE develops novel elastomer materials to meet the most demanding sealing applications including extreme temperatures and chemically aggressive environments.  Our seal design experts can work with you to select the best materials and optimize customized sealing solutions for your needs. In addition we can mold rubber seals and rubber gaskets in sizes to suit any sealing application and manufacture them in lead times as fast as 48 hours.

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