Plasma is the fourth state of matter and in the last three years, it has turned out to be valuable for surface modification and deposition of a variety of materials. Plasma is employed to make surfaces ready for die attach, wire bonding, and mold/encapsulation in IC packaging applications.
Furthermore, plasma-enhanced surface activation and contamination removal processes increase the reliability and yield, as well as improve the production of advanced technology products. The surfaces in several optoelectronic devices are prepared using plasma-enhanced contamination removal prior to eutectic die attach and wire bonding.
In this article, some examples of plasma surface modification in the optoelectronic as well as the microelectronic industries are discussed.
Substrate materials and adhesives used for attachment do not usually have the needed physical or chemical properties to allow good adhesion and necessitate surface modification.1
During plasma surface modification, an interaction takes place between the plasma-generated excited species and a solid interface. The plasma process results in a chemical and/or physical modification of the surface’s first few molecular layers without affecting the properties of the bulk. Typical materials used in the microelectronics and optoelectronics industries are:
- Metals such as copper, aluminum, silver, gold, tungsten, nickel, and palladium
The effectiveness of the plasma on such complex interfaces is determined by the plasma source gases, the plasma operating parameters, and the configuration of the plasma system.
Surface modification processes can be classified into four types:
- Contamination removal
- Surface activation
A particular process is chosen based on the physical and chemical composition of the material to be processed, and the necessary resultant process. Another factor that must be taken into consideration is the plasma process and the subsequent processing steps. Usually, surface modification is sensitive to environmental exposure and time, where the surface is susceptible to lose its plasma-induced physical and chemical properties.
Figure 1 illustrates an automated in-line plasma system, which has gained popularity because of the consistency it offers. Using these systems, it would be possible to execute surface modification processes individually, right before the subsequent step in the assembly process.
Figure 1. This automated, in-line plasma tool is designed for surface modification processes. The upstream and downstream transfer mechanisms and the compact high-density plasma chamber are contained within a single enclosure.
The image below represents the single-strip, compact plasma chamber, which can process BGA-type substrates with argon plasma. The characteristic photon emission of the argon plasma can be visibly observed through the chamber window.
This single-strip, compact plasma chamber processes BGA-type substrates with argon plasma.
The table below illustrates the process applications.
|Process Applications for Plasma Surface Enhancements
|Plasma Source Gas
||Surface Modification Processes
||Advanced Technology Application
Substrate Polymer–Metal Adhesion
Oxidation Process (Organic Removal)
Mold and Encapsulant Adhesion
||Mold and Encapsulant Adhesion
Reduction Process (Metal Oxide Removal)
Eutectic Die Attach
|Carbon Tetrafluoride (CF4) and
or Sulfur Hexafluoride (SF6) and
||Polymer Etch–Fiber Stripping
Thin Film Etch–Oxides, Nitrides
Surface contamination removal involves the removal of micron-level contamination using the physical and/or chemical energy of the plasma. This process employs ablation, in which the surface is hit with positive ions. The ablation process can eliminate contamination from the surface and can make the surface rough on an atomic scale, which is demonstrated by atomic force microscopy.2
The chemical process is widely employed to eliminate oxidation and residual materials, typically below a few microns, for example, organic films. The chemical process uses reduction or oxidation chemistry through the gas-phase radicals.
Insufficient solder reflow in eutectic die attach leads to poor wire-bond pull strength and voiding, which in turn cause specific contamination problems in microelectronic and optoelectronic package reliability. Wire-bond pad contamination could be a result of previous processing steps, for example, die attach epoxy bleed or environmental exposure (i.e., bond pad metal oxidation).
It is possible to prepare bond pads through a physical, chemical, or combined physical-chemical process using argon and oxygen source gases. Oxygen-based plasma will make use of the oxygen radicals to chemically react with the epoxy, thereby producing volatile gas-phase by-products that can be pumped from the vacuum chamber.
The effectiveness of oxygen-based plasma to eliminate die bond epoxy bleed has been established widely.3 In case oxidation is the main problem, the bond pad surfaces can be prepared by using a physical process. It has been demonstrated that an argon plasma treatment of PBGA strips can improve the wire-bond pull strength by up to 24.3%.4
Metal oxidation can act as a physical barrier for wire bonding as well as solder reflow. It is possible to reduce metal oxides by using a combined physical and chemical process with argon and hydrogen source gases. For example, copper oxide is reduced to copper in hydrogen plasma when hydrogen radicals react with the metal oxide.
CuO + 2H• → Cu + H2O
Ablation will make the surface rough even when a contamination source is absent, thereby offering a larger surface area for wire bonding. This leads to enhanced wire-bond uniformity from one bond to another.4
Plasma surface activation uses gases like nitrogen, hydrogen, ammonia, and oxygen. Upon exposure to the plasma, these gases will dissociate and react with the surface, forming different chemical functional groups on the surface. The chemical activity of the surface is changed by these functional groups. The chemical bonds between the new functional groups and the bulk material are strong, where the new functional groups have the ability to further bond with adhesives, resulting in better adhesion.
The functional groups also maximize the surface area available for the adhesive, thus dispersing the load across a wider area and yielding better adhesive strength. The surface type and the selection of gases determine the functional group that will be replaced on the surface.4
In microelectronic applications, plasma surface activation before die attach provides better heat transfer, enhanced contact, and negligible voiding.
In semiconductor applications, the mold/encapsulant material serves to offer adequate adhesion to a variety of package components, chemical resistance, mechanical strength, good corrosion resistance, matched CTE to the materials it interfaces with, high thermal conductivity, and high moisture resistance in the temperature range used.
The delamination along the interfaces is the main reliability problem for plastic-encapsulated microcircuits; therefore, the potential to form exceptional bonding with package components and to remain bonded is very essential.
It has been demonstrated that plasma treatment can improve the bond strength at the plastic encapsulant, gold-plated copper leadframe interface through an enhanced chemical compatibility with the molding compound.2 Initial analyses have also revealed that nickel surfaces treated with water-based plasma can improve the adhesion of the mold compound to the nickel surface.
Plasma etch is characterized by the chemical reactivity of the discharge. Source gases employed in the etching process dissociate within the plasma, forming a combination of highly reactive species. The major advantage of chemical plasma is its chemical selectivity.
The process chemistry can be optimized so that a single material can be selectively etched when other materials are present. For example, the dissociation of oxygen and carbon tetrafluoride (CF4) in right concentrations produces highly reactive fluoro, oxy, and oxyfluoro radicals that rapidly break carbon-carbon bonds within a number of materials.
Volatile by-products are formed by the reaction at the solid interface, and these by-products are ultimately pumped from the vacuum system. Plasma etch has a range of applications that are unique to semiconductor and optoelectronic processing, such as thin-film etch, photoresist removal, and polymer etch.
In optoelectronic manufacturing, plasma etch is employed to form stripped fibers through the restricted removal of the urethane acrylate buffer coating.
Traditional optic fibers comprise of a cylindrical core enclosed by a cladding material, and the cladding is covered by a buffer material. The core is the light-carrying element, and the total internal reflection in the fiber is supported by the cladding.
The buffer must be stripped for a range of applications, such as amplifier seeding, hermetic sealing, fiber Bragg gratings, fiber arrays, and pigtailing of laser diodes. For example, fiber Bragg gratings are widely used to develop devices for dense wavelength division multiplexing (DWDM).
Figure 2 illustrates a fiber with the buffer material removed and the core and glass cladding revealed. When the fiber buffer is removed, it is necessary for the urethane acrylate polymer to be completely removed and the innate strength of the glass core to be appropriately maintained. Reducing the plasma etch of the glass core requires the buffer removal process to be strongly controlled.
Figure 2. Illustration displays a fiber with the buffer material removed, and the glass cladding and core exposed.
During plasma-induced cross-linking, inert gases such as argon or helium are used to eliminate some of the atomic species from the surface, thereby producing reactive surface radicals. These radicals subsequently react within the surface forming chemical bonds, which result in a cross-linked surface. This technique is used on polymeric substrates, such as those used for PBGA packages.
Argon plasma is effective in sputtering several nanometers of material from the sample surface, thus making the surface rough on the nanometer scale. The resultant cross-linking improves the bonding of metal layers to the plasma-treated polymer laminate.
Surface modification processes performed using gas phase plasma technology are widely employed in the microelectronic and optoelectronic industries. Usually, adhesives and substrate materials do not have the needed physical or chemical properties to allow good adhesion and necessitate surface modification.
At the time of plasma surface modification, interaction takes place between a solid interface and the plasma-generated excited species. The plasma process boosts a physical and/or chemical modification of the first few molecular layers of the surface without affecting the properties of the bulk.
As discussed earlier, surface modification can be categorized into four groups (contamination removal, etch, surface activation, and cross-linking), with the selection of a specific process governed by the resultant process requirement.
- F.D. Egitto and L.J. Matienzo, “Plasma Modification of Polymer Surfaces for Adhesion Improvement,” IBM Journal of Research and Development, July 1994, p. 423.
- S. Yi, J. Kim et al., “Bonding Strengths at Plastic Encapsulant-Gold-Plated Copper Leadframe Interface,” Microelectronics Reliability, January 2000, p. 1212.
- M.White, “The Removal of Die Bond/Epoxy Bleed Material by Oxygen Plasma,” Proceedings 32nd IEEE Electronic Components Conference, 1982, p. 262.
- L. Wood, C. Fairfield et al., “Plasma Cleaning of Chip Scale Packages for Improvement of Wire Bond Strength,” Chip Scale Package Seminar, December 2000.
- E. Finson, S. Kaplan et al., “Plasma Treatment of Webs and Films,” Society of Vacuum Coaters, 38th Annual Technical Conference Proceedings, 1995.
This information has been sourced, reviewed and adapted from materials provided by Nordson MARCH.
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