Thick-film copper metallization

Thick-film copper metallization is typically selected for the low cost of the conductor inks in comparison to gold inks. Other fhick-film conductor inks, such as silver and palladium silver, are also used when low material cost is... 

For substrates metallized with thick-film copper that will see subsequent soldering, manufacturers typically plate them with nickel and optionally with a top layer of gold. Some manufacturers will solder dip their copper thick-film subsfrafes after nickel plating. The resulting metallization has toe same solderability and wire bondability characteristics as DEC and plated copper. 

As in the case of anodic processes, current oscillations occur in the potential region where there is a transition between the metal active and passive state and vice versa. The systems exhibiting oscillations are related to electrodissolution of iron and less to that of other metals such as cobalt, stainless steel, copper, and silver (see, e.g., [113-116] and references therein). In this case, the periodic process involves formation and dissolution of comparatively thick films of metal oxides, hydroxides, or other insoluble compounds. 

Adhesion of copper films to PMDA/ODA polyimide was determined by peel tests conducted on samples that were prepared by vapor-depositing a thin layer of copper onto the polyimide and then building the thickness of the metal layer to about 18 p,m by electrodeposition of copper. Results of the adhesion measurements correlated well with substrate pretreatment. When the substrate... 

In our spectroscopic study, where no potential was applied to the metal, albumin did not appear to exhibit a corrosive effect on the thin metal films. If metallic copper or nickel is ionized or solubilized and removed from the surface, the metallic layer will decrease in thickness. Metals absorb strongly in the mid-infrared and thus, a decrease in the thickness of this absorbing layer would result in an increase in the penetration depth of the evanescent... 

The 64k, 80 pm x 80pm sized tilting mirrors are built on the top of a CMOS-based control ASIC. In order to reduce the topography of the underlying metallization/passivation structures, a 2.5pm-thick PECVD oxide film is first deposited on the ASIC. An ILD oxide CMP step based on Klebosol 30N50 colloidal silica slurry is used for planarization. In order to connect the ASIC with the deflection electrodes above (see Fig. 14.10), vias have to be etched into the planarized dielectric film. Then, a copper metal stack including a TaN barrier has to be deposited and a two-step Cu damascene CMP process has to be performed. As this process is equivalent to Cu damascene in microelectronics fabrication, standard Cu CMP slurries can be used. 

The electropolymerization method of functional monomers has proved successful in many cases, in particular to incorporate various ligands and their metallic complexes, like salen [66,245], porphyrin [246], diphosphine [247], pyridine [71,80,248,249], crown ether [250,251], metallofullerene [252], tetraazacyclotetradecane [253], or Prussian blue type [254] in a conjugated organic material like PPy, PTh, or PANE Thick films are likely to be obtained provided that the polymer is redox active at the deposition potential due to this, Zotti et al. demonstrated that 5,5 -bis(3,4-(ethylenedioxy)thien-2-yl)-2,2 -bipyridine could be electropolymerized when complexed by iron and ruthenium, but not in the case of complexation by nickel or copper [71]. 

The electrodeposition process, which is typically restricted to electrically conductive materials and is carried out in a liquid solution of ions (electrolyte), is well suited to make films of metals such as copper, gold, and nickel. The films can be made in any thickness firom <0.1 to >100 pm. Other materials including metal oxides can be deposited as well. There are basically two technologies for plating electroplating and electroless plating... 

The method of deposition is what differentiates the hybrid circuit from other packaging technologies and may be one of two types thick film or thin film. Other methods of metallizing a ceramic substrate, such as direct bond copper, active metal brazing, and plated copper, may also be considered to be in the hybrid family, but do not have a means for directly fabricating resistors and are not considered here. Semiconductor technology provides the active components, such as integrated circuits, transistors, and diodes. The passive components, such as resistors, capacitors, and inductors, may also be fabricated by thick- or thin-film methods or may be added as separate components. 

Metal-plated articles are common in our society. Jewelry and tableware are often plated with silver. Gold is plated onto jewelry and electrical contacts. Copper is plated onto many objects for decorative purposes (Figure 21-5). Automobiles formerly had steel bumpers plated with thin films of chromium. A chrome bumper required approximately 3 seconds of electroplating to produce a smooth, shiny surface only 0.0002 mm thick. When the metal atoms are deposited too rapidly, they are not able to form extended lattices. Rapid plating of metal results in rough, grainy, black surfaces. Slower plating produces smooth surfaces. Tin cans are steel cans plated electrolytically with tin these are sometimes replaced by cans plated in a fraction of a second with an extremely thin chromium film. 

To obtain finer lines and smaller vias, one can use photoimageable thick-film process for dielectric and conductors and diffusion patterning. The photoimageable thick-film process involves the use of a photoactive paste printed on a substrate and exposed through artwork or a mask to define circuif characteristics, lines, and vias. The materials are developed in an aqueous process and then fired using the conventional thick-film fechnique. Copper, silver, and gold metallizations are used, and layer coxmts of up to 10 circuit layers are possible. 

For many power systems, hundreds of amperes of current may be flowing through a substrate in a relatively small area. To prevent severe losses in the conductors, the metallization must be very thick and low in resistivity. One approach to this problem is direct bond copper (DBC), which was developed by General Electric in the mid-1970s. Unlike thick-film or thin-film conductors, DBC can be purchased with metal thicknesses up to 0.65 mm (25 mil). Combined with the low resistivity of copper (0.12 mD/n) and a high thermal conductivity substrate such as AIN, this approach creates nearly the ideal substrate for this type of application. 

Ceramic technology offers a wide choice of conductor metallizations, and various conductor technologies are used with ceramics. Screen-printed and photo-defined, thick-film, thin-film, electroplating [3], electroplating over thick film, and direct bond copper (DBC) [4] are tfie most prevalent metallizations. 

It is common to refer to thick-film metallizations as "gold," or "silver," or "copper" — the conducting metal component in the paste. It is important to keep in mind that the typical conductors for ceramics are compositions of glasses, ceramic powders, and conducting metal particles. As a result, the conductivity of typical gold conductors is 30-50% that of bulk copper and that of typical silver conductors, 70-90% that of bulk copper. The conductivity of plated thick-film and DBC approach that of bulk copper. Table 2.1 summarizes the properties of typical conductors for ceramic application. 

Thick-film ink manufacturers do not publish thermal conductivity data for their conductor and via-fill inks. Work by Harshbarger [16] and Krum [17] has shown that the thermal conductivity of thick-film conductor inks is approximately 20% of that of pure metals. A first-order approximation of the thermal conductivity of thick-film conductor inks is to multiply the percentage of the published value of electrical conductivity (the reciprocal of resistivity) of the ink and the thermal conductivity of the pure metal in the ink. For copper thick film, as described in the above example, the electrical... 

The portion of the heat path of interest here is the thermal resistance of the metallization, Q,neiaiibation, which is equal to the sum of the thermal resistances in the vertical direction of the plated copper (Gj) and the thick-film adhesion layer (62). 

Copper can be electroplated onto the substrate in two techniques. In the first process, as shown in Figure 8.25a, an adhesion or seed layer is deposited on the ceramic. This adhesion layer can be thin film, either sputtered or evaporated, thick film, or refractory metallization. A sputtered or evaporated gold layer is deposited on top of the seed layer for thin-film metallization. Copper is then electroplated to the required thickness. This is followed by electroplated nickel and an optional gold electroplate. Figure 8.26 shows the buildup of plated copper metallization. 

For those packages and substrates where the pattern is defined first, either with thick-film or refractory metallization, electrolytic copper plating requires that the traces be electrically connected. To accomplish this requires the use of temporary shorting bars. 

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