How Plasma-Enhanced Surface Modification …

1 Ubstrate materials and the adhe-sives employed for attachmentoften do not possess the needed physicalor chemical properties to allow for goodadhesion, and require Surface Surface Modification involvesthe interaction of the plasma-generatedexcited species with a solid interface. Theplasma process results in a physical and/or chemical Modification of the first fewmolecular layers of the Surface , whilemaintaining the properties of the materials employed in themicroelectronics and optoelectronicsindustries include ceramics, glass, polymersand metals such as gold, copper, alu-minum, nickel, palladium, tungsten, andsilver. The effectiveness of the plasma onthese complex interfaces is determined bythe plasma source gases, the configurationof the plasma system and the plasmaoperating Modification processes can beclassified into four categories: Contamination Removal Surface Activation Etch Cross LinkingSelection of a specific process is deter-mined by the physical and chemical com-position of the material to be processed,as well as by the ensuing process plasma process and the subsequentprocessing steps must also be Modification is often sensitiveto time and environmental exposure, wherethe Surface may lose its plasma-inducedphysical and chemical in-line plasma systems(Figure 1), which allow the Surface mod-ification process to be performed indi-vidually immediately prior to the nextstep in the assembly process have gainedpopularity due to the consistency they lead photo displays the single strip,compact plasma chamber, which processesBGA-type substrates with argon characteristic photon emission of theargon plasma can be seen through thechamber window.

2 (Process applicationsare shown in the table.) Contamination RemovalSurface contamination removal involvesthe use of the plasma s physical and/orTechnical ForumHow Plasma-Enhanced Surface ModificationImproves the Production of Microelectronicsand OptoelectronicsABSTRACTOver the past 30 years, plasma, the fourth state of matter, has become a very useful method for Surface Modification and deposition of variousmaterials. In IC packaging applications, plasma is employed to prepare surfaces for die attach, wirebonding and mold/encapsulation. Moreover, Plasma-Enhanced contamination removal and Surface activation processes improve the reliability and yield, and alsoenhance the manufacture of advanced technology products. In many optoelectronic devices, Plasma-Enhanced contamination removalis used to prepare surfaces prior to eutectic die attach and wirebonding. This article examines examples of plasma Surface modificationin both the microelectronic and optoelectronic Dr.

3 James D. Getty, March Plasma Systems, Concord, single strip, compact plasma chamber processes BGA-type substrates with argon plasma. Reprinted from the January-February issue of Chip Scale Review. Used by permission; all rights reserved. 2002 Chip Scale Forumchemical energy to remove process employs ablation, wherethe positive ions bombard the ablation process can dislodge con-tamination from the Surface , and canroughen the Surface on an atomic scale,as revealed by atomic force chemical process is widely employedto remove residual materials, typicallyless than a few microns, such as organicfilms, and oxidation. The chemical processemploys either reduction or oxidationchemistry via the gas-phase contamination issues in micro-electronic and optoelectronic packagereliability are poor wirebond pullstrength and voiding, due to insufficientsolder reflow in eutectic die pad contamination can be aby-product from previous processingsteps such as die attach epoxy bleed orenvironmental exposure ( , bond padmetal oxidation).

4 A physical, chemical or combinedphysical-chemical process using argon andoxygen source gases can be employed toprepare the bond pads. An oxygen-basedplasma will take advantage of the oxygenradicals to chemically react with the epoxy,producing volatile gas-phase by-productsthat can be pumped from the success of an oxygen plasma forremoval of die bond epoxy bleed hasbeen widely cases where oxidation is of concern,a physical process can be employed toprepare the bond pad surfaces. An argonplasma treatment of PBGA strips has beenshown to improve the wirebond pullstrength by as much as oxidation can act as a physicalbarrier for both wirebonding and solderreflow. A combined physical and chemi-cal process using argon and hydrogen canreduce the metal oxides. For example,reduction of copper oxide to copper isachieved in a hydrogen plasma via thereaction of the hydrogen radicals withthe metal + 2H Cu + H2 OEven in the absence of a contaminationsource, ablation will roughen the surfaceand provide a larger Surface area for wire-bonding, resulting in improved wire-bond uniformity bond to ActivationPlasma Surface activation employs gases,such as oxygen, nitrogen, hydrogen, andammonia, which, when exposed to theplasma, will dissociate and react with thesurface, creating different chemical func-tional groups on the different functional groups modifythe chemical activity of the Surface .

5 Thenew functional groups have strongchemical bonds with the bulk material andhave the capability to further bond withadhesives to promote better functional groups also increase thesurface area available for the adhesiveand thus will distribute the load over alarger area resulting in improved adhesivestrength. Gas selection and Surface typedetermine the functional group that willbe substituted on the microelectronic applications, plasmasurface activation prior to die attachprovides better contact, improved heattransfer and minimal purpose of the mold/encapsulantmaterial for semiconductor applicationsis to provide adequate mechanical strength,adhesion to various package components,good corrosion and chemical resistance,matched CTE to the materials it interfaceswith, high thermal conductivity and highmoisture resistance in the temperaturerange ModificationAdvanced TechnologySource GasProcessesApplicationArgon (Ar)Contamination Removal AblationWirebondDie AttachCross LinkingSubstrate Polymer Metal AdhesionOxygen (O2)Contamination Removal ChemicalWirebondOxidation Process (Organic Removal) Die AttachSurface ActivationMold and Encapsulant AdhesionEtchPhotoresist RemovalNitrogen (N2) Surface ActivationMold and Encapsulant AdhesionHydrogen (H2)

6 Contamination Removal ChemicalWirebondReduction Process (Metal OxideEutectic Die AttachRemoval)Carbon TetrafluorideEtchPolymer Etch Fiber Stripping(CF4) and Oxygen (O2)Photoresist Removalor Sulfur HexafluorideThin Film Etch Oxides, Nitrides(SF6) and Oxygen (O2)Process Applications for Plasma Surface EnhancementsFigure 1. This automated, in-line plasma tool isdesigned for Surface Modification upstream and downstream transfer mech-anisms and the compact high-density plasmachamber are contained within a single from the January-February issue of Chip Scale Review. Used by permission; all rights reserved. 2002 Chip Scale ability to form good adhesionwith package components and to remainbonded is of paramount importance,since delamination along the interfaces isa major reliability issue for plastic-encap-sulated treatment has been demon-strated to improve the bond strength atthe plastic encapsulant, gold-platedcopper leadframe interface via anenhanced chemical compatibility withthe molding studieshave also indicated that plasma treat-ment of nickel surfaces with water-basedplasma improvesthe adhesion of themold compound to the nickel etch is characterized by the chem-ical reactivity of the discharge.

7 The etchingprocess utilizes source gases that dissociatewithin the plasma, creating a mixture ofhighly reactive species. The advantage ofthis chemical plasma is its process chemistry can be optimizedso that one material can be selectivelyetched in the presence of other example, the dissociation of carbontetrafluoride (CF4) and oxygen in theappropriate concentrations produceshighly reactive oxy, oxyfluoro, and fluororadicals that rapidly break carbon-carbonbonds within numerous reaction at the solid interfaceproduces volatile by-products, which arepumped from the vacuum system. Plasmaetch has many applications specific tosemiconductor and optoelectronic pro-cessing, including photoresist removal, thinfilm etch, and polymer optoelectronic manufacturing,plasma etch has been employed to producestripped fibers through the controlledremoval of the urethane acrylate optic fibers are com-posed of a cylindrical core covered by acladding material and a buffer materialthat encases the cladding.

8 The core is thelight-carrying element, and the claddingpromotes the total internal reflection inthe the buffer is required forvarious applications, including hermeticsealing, pigtailing of laser diodes, fiberarrays, fiber Bragg gratings, and Bragg gratings, for example, arewidely used in the fabrication of devicesfor dense wavelength division multiplex-ing (DWDM).Figure 2 displays a fiber with the buffermaterial removed and the glass claddingand core exposed. A critical requirementin the removal of the fiber buffer is tocompletely remove the urethane acrylatepolymer while maintaining the intrinsicTechnical ForumFigure 2. Illustration displays a fiber with thebuffer material removed, and the glass claddingand core exposed. Gas phase plasma is an electricallyneutral mixture of electrons, ions, rad-icals, photons, recombination productsand neutrals created by the applicationof energy, such as radio frequency(RF), to a source gas contained withina vacuum chamber.

9 (The figure sum-marizes the active species that arepresent in an oxygen plasma.) Freeelectrons initiate the process; exposureof the free electrons to the externalenergy source allows the electrons togain sufficient kinetic energy, so that acollision with another atom or moleculewill result in the formation of ions andradicals. The reactive radical speciesare capable of chemical work wherethe ionized atom and molecularspecies are capable of physical workthrough sputtering. Photon emission within a plasma is aresult of the excited neutrals, ions and freeradicals formed in the plasma losingtheir excess energy. The wavelength ofthe emission is sufficient to breakchemical bonds, and can be usefulwhen treating polymeric non-equilibrium plasma haselectron energies in excess of10,000 C, critical for sustaining theplasma through atomic and molecularionization and dissociation processes,it maintains a gas stream temperaturethat is less than 100 C.

10 The low processtemperature is important to productsthat are temperature sensitive. JGThe Fourth State of MatterThis illustration summarizes the active species that are present in an oxygen plasma. PLASMAFree Radical (highly reactive)OzoneNegative IonsUltraviolet Light PhotonPositive IonsElectronReprinted from the January-February issue of Chip Scale Review. Used by permission; all rights reserved. 2002 Chip Scale of the glass core. Therefore, tightcontrol of the buffer removal process isrequired to minimize the plasma etch ofthe glass LinkingPlasma-induced cross linking employs inertgases such as argon or helium to removesome atomic species from the Surface ,and generates reactive Surface radicals react within the surfaceforming chemical bonds, which resultsin cross-linked Surface . This approach isemployed on polymeric substrates, suchas those used for PBGA plasma effectively sputters nano-meters of material from the sample Surface ,literally roughening the Surface on the nano-meter scale.