Copper thin-film fabrication process

Copper Sulfide—Cadmium Sulfide. This thin-film solar cell was used in early aerospace experiments dating back to 1955. The Cu S band gap is ca 1.2 eV. Various methods of fabricating thin-film solar cells from Cu S/CdS materials exist. The most common method is based on a simple process of serially overcoating a metal substrate, eg, copper (16). The substrate first is coated with zinc which serves as an ohmic contact between the copper and a 30-p.m thick, vapor-deposited layer of polycrystaUine CdS. A layer is then formed on the CdS base by dipping the unit into hot cuprous chloride, followed by heat-treating it in air. A heterojunction then exists between the CdS and Cu S layers. 

In the previous section, it was mentioned that Zhang et al. [51,52] developed a technique to micromachine a thin metal film made out of copper. This material had a number of perforations that followed a predetermined pattern and such a design can be changed fairly easily by simply changing one of the masks used in the fabrication process. Although copper is not an ideal material to be used as a DL in a fuel cell due to the contamination issues, the development of fabrication techniques similar to those mentioned... 

A hybrid ink has been prepared from copper rich CuSe nanoparticles and an indium precursor solution to form CuInSe2 thin films for solar cell applications. Monoethanolamine (MEA) was used as a capping ligand that chelates with the Cu-Se nanoparticles and In precursor. Cu-rich Cu-Se nanoparticles were synthesized and subsequently washed with MEA. Resultantly, a Cu-MEA complex was formed and copper content of the nanoparticles was reduced. CuInSca thin films were deposited at 450 °C using this hybrid ink. PV devices fabricated by using these films demonstrated a power conversion efficiency of 5.04% as compared to an efficiency of 1.04% for normally synthesized copper rich CuSe nanoparticles. This observation confirmed that the Cu-MEA complex had a strong influence on the performance of CuInSc2 based solar cells produced with the hybrid ink process. 

If the subsequent assembly process calls for thermocompression or thermosonic wire bonding, plated copper under the wire-bond sites should be avoided. This can be accomplished by selectively plating the copper. When the adhesion layer is Nichrome or titanium-tungsten, then precision-deposited thin-film resistors can be fabricated. 

In all but the simplest electronic circuits, it is necessary to have a method for fabricating multilayer interconnection structures to enable all the necessary points to be connected. The thick film technology is limited to three layers for all practical purposes because of yield and planarity considerations, and thin-film multilayer circuits are quite expensive to fabricate. The copper technologies can produce only a single layer because of processing limitations. 

TLCPs have many outstanding properties that uniquely qualify them for these high performance multilayer boards (12). Table 7 compares the applicability of LCPs with other state-of-the-art materials for electronic packaging. They can be made into very thin, self-supporting films (<50 J.m) with a controllable CTE. By processing LCP films as described above, circuit substrates can be manufactured with a CTE around 7 ppm/°C and thermal stability over 250°C. TLCPs do not require secondary resins for fabrication into MLBs, they can be thermally bonded to themselves and to copper foil. Control of molecular orientation has been shown to result in a substrate with the desired CTE of 6 to 7 ppm/°C for matching alumina, or 16 ppm/°C for matching copper. 

Flexible circuits. Flexible circuits (flex circuits) are analogous to rigid printed-circuit boards except they are fabricated from a thin flexible dielectric film to which is adhesively bonded a thin copper foil. The copper is then photoetched to form a circuit pattern using normal photolithography processes. A plastic film (coverlay) is then adhesive bonded to the etched... 

Vacuum deposition processes can be used to form freestanding structures by depositing the film on an appropriately shaped mandrel. On the mandrel there is either a parting layer , such as evaporated NaCl, or the surfaces may be non-adhering, such as copper on the oxide on stainless steel. This technique is used to fabricate thin-walled structures and windows. In some cases, the mandrel must be dissolved to release the deposited form. 

---