Device Failures due to Wafer Crystal Defects

Crystallographic defects that are present in silicon wafers do affect the electrical performance of devices built onto these wafers. Important device properties that are impacted by defects within the silicon crystal include: 1) minority carrier lifetimes; 2) leakage currents in p-n junctions; 3) collector-emitter leakage currents in bipolar transistors; 4) MOS gate oxide quality; and 5) MOS threshold voltages. Extreme cases of degradation due to the presence of these defects can result in device failures.

'Minority carrier lifetime' refers to the average time interval between the generation and recombination of a minority carrier (electrons in p-type silicon and holes in n-type silicon) in a semiconductor crystal. Recombination, which involves a free electron and a hole combining to annihilate each other, is a process by which carriers return to their inactive state, e.g., the electron falls from the conduction band back to the valence band where it can no longer conduct. The minority carrier lifetime affects the performance of the device in many ways, although generally (but not exclusively) a longer minority carrier lifetime is desired.

The presence of defects and electrically neutral impurities in the crystal decreases the minority carrier lifetime, since these defects and impurities tend to form recombination centers within the crystal that 'trap' active carriers. These recombination centers are actually additional localized intermediate energy levels within the semiconductor bandgap, which is the separation between the lowest conduction band energy level and the highest valence band energy level. Imperfections that introduce such intermediate energy levels include point defects, clusters of point defects, and dislocations.

Reverse-biased p-n junctions are supposed to be non-conducting, and must therefore exhibit as little leakage current as possible. Excessive leakage currents across p-n junctions can have various detrimental effects: the device's power dissipation may increase, DRAM storage stability may degrade, device voltage/current characteristics may shift, etc.

Two common defects that can result in increased p-n junction current leakage are transition metal precipitates and dislocations. Dislocations that are decorated by electrically active metal impurities can appear across the junction and cause it to exhibit excessive current leakage. Dislocations and the strain caused by accumulating precipitates in the p-n junction can also form carrier generation-recombination centers that may lead to higher current leakage.

Another common device problem that is attributed to crystal defects is increased 'collector-emitter' leakage currents in bipolar transistors. The interesting aspect of this issue is the fact that the collector-base and base-emitter leakage currents may remain at minimal levels, even while there is a marked increase in collector-emitter leakage. Thus, in such a degradation, a certain amount of current flows from collector to emitter even if there's no current flowing into the base.

Excessive collector-emitter leakages have been attributed to several types of crystal defects. For instance, a dislocation that passes through the transistor from the collector to the emitter, if decorated with metal impurities, can allow appreciable current to flow between the collector and emitter even if the base current is zero. Diffusion may also be enhanced along such a dislocation during emitter formation, contributing to the collector-emitter leakage current. Dislocations that increase collector-emitter current leakage by way of enhanced diffusion are known as 'collector pipes.' Collector pipes in bipolar transistors, which result in significant collector-emitter leakage currents even in the absence of a base current, have been subjected to numerous studies using electron-beam-induced current (EBIC) techniques.

The gate oxides of MOS devices must exhibit minimal oxide leakage current and high oxide breakdown voltage. Stacking faults in the silicon substrate, usually caused by metallic contamination during oxidation, have been shown by studies to increase gate oxide current leakage and lower the gate oxide's breakdown voltage. Higher gate oxide defect density, which also reduces the oxide's breakdown voltage, has also been correlated with the presence of oxygen precipitates at the wafer surface.

The threshold voltages of MOS transistors on a wafer depend on the resistivity of the wafer substrate. Keeping these threshold voltages within the required specifications is important to proper device operation. Wafer-to-wafer variability in resistivity can be caused by temperature fluctuations when the silicon ingot was produced, or by alterations in the carrier concentrations resulting from the presence of oxygen precipitates.

Primary Reference: Wolf and Tauber, Silicon Processing for the VLSI Era Vol. 1, Lattice Press

LINKS: Crystal Defects; Silicon Wafers; Crystal Growing; Die Failures

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