Plasma, the fourth state of matter, has become a useful method for surface modification and deposition of various materials over the past three years. Plasma is used to prepare surfaces for wirebonding, die attach, and mold/encapsulation in IC packaging applications.
In addition, plasma-enhanced surface activation and contamination removal processes improve the yield and reliability, and enhance manufacturing of sophisticated technology products. Before eutectic die attach and wirebonding, plasma-enhanced contamination removal is employed to prepare surfaces in many optoelectronic devices.
This article discusses some examples of plasma surface modification in both the optoelectronic and microelectronic industries.
Adhesives and substrate materials used for attachment often lack the required chemical or physical properties to enable good adhesion, and require surface modification.1
During plasma surface modification, an interaction occurs between a solid interface and the plasma-generated excited species. The plasma process leads to a physical and/or chemical modification of the first few molecular layers of the surface without compromising the bulk’s properties. Standard materials used in the optoelectronics and microelectronics industries include:
- Metals such as aluminum, copper, gold, silver, nickel, tungsten, and palladium
The plasma operating parameters, the plasma source gases, and the configuration of the plasma system determine the effectiveness of the plasma on these complex interfaces.
Surface modification processes can be divided into four categories:
- Contamination Removal
- Surface Activation
- Cross Linking
Selection of a specific process depends on the chemical and physical composition of the material to be processed, and the ensuing process required. Another factor that needs to be considered is the plasma process and the following processing steps. Often, surface modification is sensitive to environmental exposure and time, where the surface is likely to lose its plasma-induced chemical and physical properties.
Figure 1 shows an automated in-line plasma system, which have become popular due to the consistency they provide. With these systems, surface modification processes can be carried out individually, immediately before the next 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 lead photo shows the compact, single strip plasma chamber, which is capable of processing BGA-type substrates with argon plasma. Through this chamber window, the characteristic photon emission of the argon plasma can be seen clearly.
This single strip, compact plasma chamber processes BGA-type substrates with argon plasma
The following table shows 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
During surface contamination removal, the physical and/or chemical energy of the plasma is used to remove micron-level contamination. This process uses ablation, where the surface is bombarded by positive ions. The ablation process can dislodge contamination from the surface, and can roughen the surface on an atomic scale, which is revealed by atomic force microscopy.2
The chemical process is extensively used to remove residual materials, usually less than a few microns, such as organic films, and oxidation. The chemical process uses oxidation or reduction chemistry via the gas-phase radicals.
Due to inadequate solder reflow in eutectic die attach, poor wirebond pull strength and voiding cause specific contamination issues in optoelectronic and microelectronic package reliability. Wirebond pad contamination could be a by-product from earlier processing steps such as environmental exposure (i.e., bond pad metal oxidation) or die attach epoxy bleed.
Bond pads can be prepared with a chemical, physical, or combined physical-chemical process using oxygen and argon source gases. An oxygen-based plasma will take advantage of the oxygen radicals to chemically react with the epoxy, creating volatile gas- phase by-products that can be pumped from the vacuum chamber.
The effectiveness of an oxygen-based plasma to remove die bond epoxy bleed has been demonstrated extensively. 3 If oxidation is a major concern, a physical process can be used to prepare the bond pad surfaces. It has been shown that an argon plasma treatment of PBGA strips can enhance the wirebond pull strength by up to 24.3%.4
Metal oxidation can serve as a physical barrier for both solder reflow and wirebonding. Metal oxides can be reduced by a combined chemical and physical process using hydrogen and argon source gases. For instance, reducing copper oxide to copper is achieved in a hydrogen plasma when hydrogen radicals react with the metal oxide.
CuO + 2H• → Cu + H2O
Ablation will roughen the surface even in the absence of a contamination source, providing a larger surface area for wirebonding. This results in improved wirebond uniformity bond to bond.4
Gases such as oxygen, hydrogen, nitrogen, and ammonia are used by plasma surface activation. When exposed to the plasma, these gases will dissociate and react with the surface, producing varying chemical functional groups on the surface. The surface’s chemical activity is modified by these functional groups. The new functional groups have strong chemical bonds with the bulk material and are capable of further bonding with adhesives, leading to better adhesion.
The surface area available for the adhesive is also increased by the functional groups, distributing the load across a larger area and resulting in better adhesive strength. The functional group, which will be replaced on the surface4, is determined by the selection of gases and the type of the surface.
In microelectronic applications, plasma surface activation before die attach offers improved heat transfer, better contact, and minimal voiding.
In semiconductor applications, the purpose of the mold/encapsulant material is to provide sufficient adhesion to different package components, mechanical strength, chemical resistance, good corrosion, matched CTE to the materials it interfaces with, high moisture resistance in the temperature range used, and high thermal conductivity.
As delamination along the interfaces is a major reliability issue for plastic-encapsulated microcircuits, the ability to form excellent adhesion with package components and to remain bonded is very important.
It has been shown that plasma treatment can enhance the bond strength at the plastic encapsulant, gold-plated copper leadframe interface through an improved chemical compatibility with the molding compound.2 Inital studies have also shown that nickel surfaces treated with water-based plasma can enhance the adhesion of the mold compound to the nickel surface.
Plasma etch is chracterized by the discharge’s chemical reactivity. Source gases used by the etching process dissociate inside the plasma, producing a mixture of highly reactive species. Chemical selectivity is a major benefit of chemical plasma.
It is possible to optimize the process chemistry, so that a single material can be selectively etched in the presence of other materials. For instance, the dissociation of oxygen and carbon tetrafluoride (CF4) in appropriate concentrations creates highly reactive fluoro, oxy, and oxyfluoro radicals that quickly break carbon-carbon bonds inside various materials.
Volatile by-products are produced by the reaction at the solid interfacel, and these by-products are eventually pumped from the vacuum system. Plasma etch has various applications that are specific to optoelectronic and semiconductor processing, such as polymer etch, thin film etch, and photoresist removal.
In optoelectronic manufacturing, plasma etch is used to create stripped fibers via the controlled removal of the urethane acrylate buffer coating.
Conventional optic fibers contain a cylindrical core covered by a cladding material, and the cladding is encased by a buffer material. The light-carrying element is the core, and the total internal reflection in the fiber is promoted by cladding.
The buffer needs to be stripped for various applications, such as hermetic sealing, amplifier seeding, fiber arrays, fiber Bragg gratings, and pigtailing of laser diodes. For instance, fiber Bragg gratings are extensively employed to fabricate devices for dense wavelength division multiplexing (DWDM).
Figure 2 displays a fiber with the buffer material removed and the core and glass cladding exposed. While removing the fiber buffer, the urethane acrylate polymer should be fully removed and the inherent strength of the glass core should be properly maintained. In order to minimize the plasma etch of the glass core, the buffer removal process should be tightly controlled.
Figure 2. Illustration displays a fiber with the buffer material removed, and the glass cladding and core exposed.
Inert gases such as helium or argon are used by plasma-induced cross linking to remove some atomic species from the surface, creating reactive surface radicals. These radicals then react inside the surface forming chemical bonds, which lead to cross-linked surface. This method is used on polymeric substrates, like those employed for PBGA packages.
Argon plasma efficiently sputters nanometers of material from the surface of the sample, roughening the surface on the nanometer scale. The resulting cross linking enhances the adhesion of metal layers to the plasma-treated polymer laminate.
Surface modification processes using gas phase plasma technology are extensively used in both the optoelectronic and microelectronic industries. Often, adhesives and substrate materials lack the required chemical or physical properties to enable good adhesion, and require surface modification.
During plasma surface modification, interaction occurs between a solid interface and the plasma-generated excited species. The plasma process promotes a physical and/or chemical modification of the first few molecular layers of the surface while maintaining the bulk’s properties.
As mentioned before, surface modification can be classified into four categories (contamination removal, etch, surface activation, and cross linking), with the selection of a specific process established by the subsequent process requirement.
1. 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.
2. S. Yi, J. Kim et al., “Bonding Strengths at Plastic Encapsulant-Gold-Plated Copper Leadframe Interface,” Microelectronics Reliability, January 2000, p. 1212.
3. M.White,“The Removal of Die Bond/Epoxy Bleed Material by Oxygen Plasma,”Proceedings 32nd IEEE Electronic Components Conference, 1982, p. 262.
4. L. Wood, C. Fairfield et al., “Plasma Cleaning of Chip Scale Packages for Improvement of Wire Bond Strength,” Chip Scale Package Seminar, December 2000.
5. 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.
For more information on this source, please visit Nordson MARCH.