The wire bonding process has to be enhanced for high device reliability and lower manufacturing costs, thereby guaranteeing good yields and bond strengths. Poor bond strengths and low yields are usually caused by the selection of materials in modern packaging or upstream contamination sources. Gas plasma technology can be employed to clean pads prior to wire bonding to boost yields and bond strengths.
Gas plasma is a robust, efficient resource, and when used appropriately, it can significantly improve the yield, manufacturability, and reliability of modern semiconductor packages. Plasma is used to enhance the consistency and pull strength of wire bonds; increase fillet height, fillet uniformity, and underfill adhesion for flip chip devices; and alter surfaces for enhanced adhesion in mold and encapsulation processes.
Several factors influence the effectiveness of a plasma process, including the selection of process parameters, chemistry, part placement, time, power, and electrode configuration. For a particular packaging application, process chemistry and electrode configuration are the key factors.
Correct implementation of plasma in packaging such complex devices needs insights about the device to be packaged, including the pre-processing steps and any sensitivity, its materials of construction, as well as plasma technology.
Thin Film Metallization
In the existing advanced substrate technologies, low-cost substrates are produced using very thin gold plating usually on palladium or nickel metallization. The thickness of gold is extremely thin, typically less than 50 nm.
This gold thickness is a challenge for the plasma system when seeking to use plasma for bond pad contamination removal because of epoxy bleed-out caused while the die attach step is taking place. The presence of epoxy can result in poor bonding yields and wire bond pull strengths. The challenge is to properly remove the organic resin bleed with plasma without damaging or getting rid of the thin film gold required for the wire bonding step.
It is possible to use two plasma modes to treat the substrate prior to wire bonding: direct downstream or plasma. In the direct plasma mode, an energy source is used to ionize and dissociate a source gas that produces gas plasma composed of physically and chemically active components.
Samples to be plasma treated are placed straightaway in the gas discharge, on or near the system’s electrode plates with full exposure to the working species of the plasma (for instance, byproducts, ions, and free radicals). The kind of working species to which the substrate is exposed is a function of the source gas selected.
If argon was used as a source gas, for instance, the argon ions produced by plasma would have an impact on the surface of the substrate and remove the organic residue via a sputtering mechanism. In the example discussed below, a quad flat no-lead (QFN) package with 25 nm of gold on palladium was tested for wire bond upgrading with and without argon direct plasma.
The die was attached with conductive epoxy, oven cured, direct plasma treated, and wire bonded with 25 μm wire. A statistically valid set of samples created a mean pull strength of 10.00 g with a CpK of 2.07 with plasma, compared to a mean pull strength of 3.89 with a CpK of 0.03 without plasma.
This example shows that it is possible to use direct plasma to significantly improve wire bond pull strengths under tightly regulated process settings.
Oxygen can also be selected as the source gas. Here, the active species created in the plasma include oxygen ions, oxygen radicals, and byproducts like ozone. The plasma-produced oxygen radicals oxidize the organic resin, generating water and gas phase carbon dioxide with a little assistance from the oxygen ions.
Downstream ion-free plasma (IFP) is a substitute to direct plasma. IFP is a pure chemical plasma, free from ions and photons accountable for the physical component. The IFP process involves the production of active species upstream of the sample processing area, which is then followed by diffusion of the active species through a gas baffle assembly.
The gas baffle eliminates the photons, ions, and electrons, thereby allowing the substrate to be exposed only to the radicals as well as byproducts created in the upstream plasma. The downstream plasma mode can be used every time the die or substrate is sensitive to the exposure of ions or photons produced in the direct plasma.
An example when thinking about the use of direct plasma versus downstream plasma is the use of thin metallization while processing substrates. In a single plasma cycle, all of the gold on the substrate bond pads can probably be eliminated, having a significant impact on the wire bond pull strengths.
In the subsequent example, identical QFN packages with wire bond pads made up of 25 nm of gold on palladium were die attached using conductive adhesive, oven cured, direct plasma treated using argon source gas under differing power and time settings, and wire bonded using 25 μm wire.
Table 1 illustrates the significance of tightly regulating the plasma process to ensure that all of the organic resin bleed is eliminated without sputtering the thin film gold on the bond pad. The pull strength data (Table 1) were collected with a constant plasma power, argon source gas, and pressure by altering the plasma process time.
||Mean Pull Strength (grams)
Although the “Under Treated” sample showed a little improvement over the “No Plasma” sample, in comparison with the “Optimized” settings, it was evident that the process did not entirely remove the epoxy bleed. An additional set of experiments was carried out to show the importance of tightly regulating the plasma process. The “Over Treated” sample produced poor pull strengths as the thin film gold bond pad material was eliminated.
IFP plasma can be used in situations where the semiconductor device equipment or the substrate metallization is sensitive to direct plasma exposure. A thin-film gold QFN package was die attached using conductive adhesive, cured thermally, plasma treated under ion-free and direct plasma conditions, and wire bonded using 25 μm wire.
In ion-free plasma, the QFN package is exposed only to the chemically active oxygen radicals, limiting the effect of gold sputtering. Figure 1 illustrates pull strengths for these QFN packages under no plasma, direct oxygen plasma, and IFP oxygen plasma.
In both plasma circumstances, the wire bond pull strength and CpK are significantly enhanced when compared to the no-plasma condition. In contrast, the direct plasma situation is a little better, specifying that removal of the organic resin bleed is not the only mechanism for improved bond pull strength.
Additional studies are being carried out to better comprehend the above observation.
Additive Substrate Technology
There are three main types of metallization methods in the production of substrates: subtractive, additive, and semi-additive. The traditional methods involve using subtractive metallization, including the application of a blank metal followed by photolithography and metal etch of the metal to develop the substrate traces.
In additive plating, the metal traces are directly constructed on the substrate. Additive plating is often used as it offers advantages for small geometries necessary in high-density substrates. There are two typical sources of contamination with additive plating: nickel diffusion from the plating that can have an impact on wire bonding pull strength and yield, and organic impurity from the substrate manufacturing process.
A correctly configured plasma system can efficiently treat these contamination sources and improve wire bond yields.
The efficiency of plasma for boosting wire bond pull strength under settings of oxygen-based plasma, argon-based plasma, and no plasma was examined by using an additive-plated substrate. The results are given in Table 2, demonstrating that both plasma processes significantly improve the pull strengths while preserving high CpK values.
However, from the pull strength data, it cannot be understood whether the pull strength enhancement is due to the reduction of nickel on the bond pad surface or the elimination of organic contamination.
||Average Pull Strength (grams)
In order to gain more insights into plasma-enhanced pull strength enhancement, X-ray photoelectron spectroscopy (XPS) was employed to evaluate the performance of the two diverse plasma processes for the elimination of the nickel and organic contamination. Relative concentrations of carbon, nickel, and gold were measured on the substrate bond pads. Table 3 provides the results.
From the no-plasma condition data, it can be observed that the gold bond pad is contaminated with organic contamination as revealed by the high carbon and nickel content. While the oxygen-based process is efficient for eliminating organic contamination via a chemical mechanism, it does not successfully treat the nickel. An argon sputtering process will be very efficient in removing the nickel contamination.
|Oxygen Based Plasma
|Argon Based Plasma
By examining the oxygen-based data in detail, it can be said that plenty of the organic contamination confining the wire bond pull strength is successfully eliminated and the remaining organic contamination is adventitious carbon as observed both from the increase in pull strengths illustrated in Table 2 and the relative increase in the concentration of gold shown in Table 3.
Furthermore, the relative increase in the nickel content for the oxygen-based plasma is perhaps because of the exposure of the bond pad nickel contamination lying under the organic and the effective reduction of the top layer organic contamination. But it can be observed that the relatively small quantity of nickel does not seem to be the key factor in the pull strength enhancement when the no-plasma setting in Table 2 is compared with both of the plasma conditions.
A sputtering mechanism is used by the argon-based plasma to remove the nickel as well as the organic. The data in Table 3 reveal the reduction both in the carbon and nickel levels. The small improvement in the pull strengths for the argon process as illustrated in Table 2 is possibly because of the decrease in the nickel quantity on the bond pad.
When looking for the type of plasma chemistry required for the additive plated substrates, users have to balance throughput requirements with the chemistry. Usually, the chemically based processes, like the oxygen plasma, will provide shorter cycle times when compared to those guided just by sputtering processes. In both cases, the plasma process enables these substrates to be wire bonded.
Advanced packaging technologies continue to stimulate the advancement of material innovations to match the requirements of additional functionality in smaller packages. Plasma processing using these advanced materials is often necessary for wire bonding applications to allow suitable pull strengths and improved bonding yields.
Material sensitivity considerations and attention to the contamination sources are crucial while setting up the plasma system. When plasma is perfectly configured, it turns out to be an empowering technology for the development of advanced package reliability and yields.
This information has been sourced, reviewed and adapted from materials provided by Nordson MARCH.
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