5.8 Sspd_chapter 3_section 3.4.schottky diode and section 3.5.

3.4.schottky diode.

Metal Aluminum is the commonly used interconnection path in Integrated Circuit Technology because it provides a good conducting path and it makes ohmic contacts on the contact pads of the solid state devices being integrated. Bulk ‘Al’ has a resistivity of 2.7×10 ^-6 Ω-cm but in thin film form its resistivity is higher by a factor of 20 hence it high current densities as high as ~ 10 ⁵ A/cm ² which is required for high speed switching networks it leads to serious reliability problems of a phenomena known as electromigration which is a major cause of the breakdown of VLSI and ULSI integrated circuits.

At nodes (length of the channel) below 90nm, the pitch (the lateral width of the interconnect) of interconnection is 45nm or less. At this level of integration, to build complex and dense circuits the multilevel metallization is routinely employed. As much as 10 multi layers are used. The metal thickness ranges from a fraction of μm to several μm. The thinner interconnects route signals while thicker layers serve as power bus. Electrical connection between two adjacent levels of interconnects are made through a VIA. To reduce the contact resistance at a contact pad a Silicide such as NiSi ₂ is added. In electromigration (also known as the electron wind effect) metal ions are forced to move downstream due to the high electron current densities, along the the grain structure. This results into voids and hillocks in the film at curves and forks.The thinning due to voids leads to excessive heating and break in the conducting path.

Electromigration is prevented by controlling the grain structure along the microcrystallites that form the metal lines. Larger grains have less surface area and therefore resist electromigration. Some alloying metals, such as copper, accumulate in the grain boundaries, locking the atoms thetre into place and thus preventing electromigration. Copper also prevents hillocking of Al films and thus prevents non-unifporm thermal effects. It is thus increasingly common to add Cu to Al in small quantities to form interconnects. Since 1998 IBM has introduced Cu interconnects leading to a much higher performing ICs with no problem of electromigration even at interconnect pitch lower than 90nm. Copper has replaced Aluminum as the interconnect material in nodes below 90nm. Cu has excellent electromigration reliability and 40% lower resistance than Al. It may be deposited by electroplating or by CVD. Copper patterns are etched by damascene process. This wil be given in supplementary sheets.

But ‘Al’ is itself P-Type material as is evident from Figure 3.26 and Table 3.4.1. Hence its deposition on P-Type Semi-conductor will give an ohmic contact but its deposition on N-Type Semiconductor gives rise to rectifying contact and such an interface is a PN Junction diode which in reality is Metal-Semiconductor Diode with very fast switching properties. This Metal Semiconductor was studied by Schottky in great details and hence it is called Schottky Diode.