Direct Write Metallizations for Ag and Al
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Abstract We have employed inks containing nanometer-sized particles of Ag and Al (nano-Ag and nano-Al, respectively) as precursor inks for the formation of contacts to n- and p-type Si, respectively. The particles as formed by the electroexplosion process were dispersed in toluene, applied to Si and annealed above the respective eutectic temperatures. In the case of nano-Ag, this directly yields an ohmic contact. However, the nano-Al was found to be coated with an oxide layer that impairs the formation of an ohmic contact. A chelating chemical etch involving treatment with hexafluoroacetylacetone was developed to remove this oxide coat. This treated nano-Al produced a good ohmic contact. Smooth, pure Ag films have also been deposited by spray printing organometallic inks prepared from Ag(hfa)(SEt 2) and Ag(hfa)(COD). These films are deposited in one step onto heated glass and Si substrates at one atmosphere pressure. The films show resistivities of -2 pQ.cm. These inks appear to be amenable to ink-jet printing of Ag lines and as a low temperature glue for the Ag nanoparticles for thicker metallizations.
Introduction There is an increasing drive to replace current metallization approaches for microelectronics, photovoltaics, toys and a variety of low resolution packaging applications with lower cost simpler approaches. For example in photovoltaics, metallizations involve the deposition of metal films and lines onto the junction part of the device to extract current. At the present, metallizations are carried out by high vacuum approaches (sputtering or evaporation) or screen printing using micron-sized particle inks. Both approaches are mask-based technologies. The ability to directly write conducting metal layers and grids using non-vacuum, spray and inkjet deposition techniques would greatly simplify the metallization process, lowering both the capital and material costs. Ink-jet printing can also give narrower grid lines than screen printing, decreasing shading losses and p therefore increasing cell efficiency[ 1-3]. Figure 1 shows a pictorial representation of a emf Sii Wafer WSi solar cell. Typically it consists of a thin crystalline p-type wafer with a diffused front nn type layer to make a homojunction. Carriers are collected at the front and back contacts after they are generated in the wafer and then Ag grid front contact separated by the junction field. Contact is normally made to the p-type and n-type sides Figure 1. Pictorial representation of a Si solar cell. by alloying Al and Ag metals with the Si to form p+ and n+ contacts, respectively.[4-7] The Al layer and Ag lines act both as contacts to the Si and as electrodes for the cell, with sunlight passing between the Ag grid lines. To allow for direct write metallizations, we are investigating the use of nanoparticlebased inks. In the simplest case, an ink could be formulated using only nanoparticles, however 59 Mat. Res. Soc. Symp. Proc. Vol. 624 © 2000 Materials Research Society
in some cases, to get good particle-particle and particle-substrate int
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