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Gas Plasma—A Dry Process for Cleaning & Surface Treatment Gas GasGas Plasma—APlasma—A DryDry ProcessProcess ForFor CleaningCleaning && SurfaceSurface TreatmentTreatment By Lou Rigali Everyone is familiar with some atomic and molecular species in form of gas plasma or glow dis- excited energy states. Many of these charge. The neon bulb is an ex- species can react chemically or ample of a low-temperature gas physically under relatively mild plasma. The sun is a high-tempera- conditions. For instance, paper that ture gas plasma. Lightning is a will “burn” or oxidize rapidly at about high-energy electrical discharge 800 °C will undergo the same reaction that produces a glow or plasma. at about 30–60 °C. While all similar, these forms of The types and nature of the species plasma differ in many important formed will depend on the gases used. details. An explanation can be as The most important active agent is extensive as a doctoral thesis, atomic oxygen in an oxygen plasma. involving chemistry and physics, This is a free radical, and will react but even a simplified description with all organic material to form CO2 can provide meaningful informa- and H2O. When argon is used as a tion. gas, an argon ion can be accelerated in a field and has enough energy to Plasma is a state of matter—a so- break carbon-to-carbon bonds, or to called fourth state, along with gases, displace by sputtering other elements liquids and solids. An ion is a gas on a surface. Whichever gases are used, atom that has become charged by the reaction is at the surface and mate- Fig. 1 losing or gaining an electron. When a rial is removed on the molecular level at significant, but low, fraction of the rates of Ångstroms per sec or min. gas is ionized, the plasma formed is The first commercial application of Semiconductor Applications not in thermal equilibrium, and the gas plasma was for ashing animal Wafer Fabrication temperature of the gas is only about tissue. The technique was used to There are many terms used to de- 30–50 degrees above ambient, while remove the organic matrix and leave scribe the plasma process. It is often the electrons are much hotter. A neon the inorganic residue for subsequent difficult to define these terms, light is an example of this kind of analysis. In this way, the analyst because the same term can apply to plasma. The sun is an example of a could detect trace amounts of ele- more than one process—and vice high-temperature plasma, where the ments in plant leaves, human hair and versa. In the semiconductor industry, gas is all ionized and the gases and animal tissue. Even now, plasma- the terms ashing, stripping and electrons are both at a very high ashing techniques are used to screen etching can all mean the removal of temperature. for the presence of asbestos in lung photoresist from a wafer. Removing Typically, a low-temperature tissue. What began as an analytical metal and oxides is usually referred to plasma is formed under vacuum technique for the chemist, soon grew as an etching process. Thin organic conditions ranging from 100–1000 into a billion-dollar business for and inorganic films, such as nitrides, mTorr, although there are a number of making semiconductor devices. can be deposited on wafers (and other publications that describe an atmo- Plasma, however, is not only used surfaces). Most applications involve spheric process (not a corona or for the fabrication of wafers. It is used making significant changes in the dielectric discharge).1 The energy to for a variety of applications in surface, extending several microns dissociate the gas can be either DC different industries. There is merit in into the surface. voltage or radio frequency from reviewing the various applications in several thousand Hz, up to microwave order to obtain a better understanding Packaging & Assembly frequencies of 10 GHz. It can be of the process and its benefits. Unlike the wafer fab, most of the argued that the chemistry of the Terminology for a specific technology applications in this section involve the plasma is the same, independent of may differ significantly from one first few hundred Ångstroms of the the power source; however, each industry to another, even though the surface. The topography of the method of dissociation has its merits processes may be similar. What a surface is not drastically changed, and and disadvantages. wafer fabrication engineer refers to as most applications involve improving In addition to ions, a plasma resist stripping, an engineer in adhesion. Adhesion to a surface is contains free radicals, which are another field will call cleaning. usually good if the surface has high 10 PLATING & SURFACE FINISHING water drop measure- bond, as shown by the testing and ments are much measurement done in the study by easier to perform, it Herard (Fig. 4).6 is good to know that the measured ESC C/ Marking Cu ratios correlate The replacement of organic solvent- extremely well with based inks with aqueous-based inks water drop contact does not provide the same consistency angles measured on of adhesion. Even heat or hydrogen the same surface (see flame treatment produces inconsistent Fig. 3). adhesion. Again, plasma-treated Fig. 2 Bonding pads on surfaces always provide a better the substrate are also surface energy. Terms such as surface. Some situations will show subject to various and inconsistent cleaning, ablation, treatment or better results than others because of levels of contamination. One source roughening generally describe the the nature of the ink, the encapsulant of contamination on the die surface is sample process. and the marking process. Plasma fluorine ion, most likely left as a A convenient way to measure treatment ensures uniformity of the residue on the wafer or die during the surface energy is by measuring the process, and decreases the variability fab process.4 While the presence of contact angle of a deionized water of the results. Printing or marking on this contamination may not show wire droplet (see Fig. 1). When the contact many different types of surfaces is bond degradation immediately, there angle is small, the surface energy is also improved, especially with the use is evidence showing correlation of the large and the unactivated surfaces are of aqueous-based inks. presence of this element and bond called hydrophobic. Good adhesion failure resulting from the migration of Fluxless Soldering can be obtained when the contact fluorine, causing embrittlement of the angle is less than 10 degrees. A patented process by MCNC, using wire.5 Removal of this contaminant plasma, has been shown to give good Die Bonding may require the use of argon bom- welding results with typical lead and bardment. This is an important Plasma cleaning/treatment of sub- gold solder formulations, without the application of plasma (i.e., the strates will always improve the use of flux, and therefore without the removal of trace amounts of impuri- 7 adhesion of the epoxy and provide a requirement of a flux removal step. ties, such as inorganic ions, by the use better bond between the die and the of argon). Non-semiconductor Applications substrate, as shown in Fig 2. The Removal of Vacuum Grease better bond provides better heat Encapsulation From Machined Copper & dissipation. Studies have also shown In this process, we are asking the Stainless Steel Parts that there is less delamination at the molding compound to provide, in Manufacture of very sophisticated die with those samples that have been addition to all of the other important medical diagnostic equipment that plasma treated.2 required properties, good adhesion to operates under vacuum conditions Wire Bonding a number of different surfaces. requires that the machined metal sub- Electron spectroscopy for chemical Depending on the type of package, the assemblies that go into the product analysis (ESCA) has shown that the molding compound must adhere to the must be absolutely clean from con- presence of carbon on a surface will substrate material, solder mask, die tamination, in particular, but not lim- limit the quality of the wire bond.3 and the metal bond pads. There are ited to carbon. In addition, many of The relative level of carbon contami- many applications that involve the these same machined parts must be nation on a copper surface, for bonding of one material to another— replaced periodically. The wet clean- example, can be determined from the plastic to plastic, metal to plastic, etc. ing process that was used included the ratio of the area of the carbon (C) and In each of these bonding operations, use of acetone and water. The disad- copper (Cu) peaks in ESCA. Because plasma improves the quality of the vantages were that there was always Fig. 3 Fig. 4 12 PLATING & SURFACE FINISHING some residue of the solvent that com- Cracking,” 1993 Electronic Components & ITAP 1996. promised the vacuum; a company Technology Conf., Orlando, FL, June 1993. 8. Fisher, J. ITRI Reports on Micro-Vias, directive required that, wherever 4. Goodman, J., et al. “Fluoride Contamination Printed Circuit Fabrication, pp. 58–60. possible, the use of all organic sol- from Fluoropolymers in Semiconductor Manufacture,” Solid State Technology, July vents be eliminated. About the Author 1990, pp. 65–68. Lou Rigali is a graduate of Northeast- A specific cleaning process was 5. Gore, S. “Degradation of Thick Film Gold ern University in Boston, worked as a developed using O2/CF4 plasma in a Bondability Following Argon Plasma batch plasma system (March PX- Cleaning,” ISHM Proc., 1992, pp. 737– chemist for several years and then 2400). The test samples were con- 742. entered into product marketing with taminated by taking a finger smear of 6.
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