1. Basic Concepts in Reliability Design - nl3prc

[2] Designing for Reliability3.1 SubstratesMaterials used for semiconductor substrates include Si single-crystal, Si epitaxial, GaAs and GaP.Advances in crystal processing technology have made the manufacture of dislocation-free Si singlecrystalpossible. Currently, attention is being paid to defects occurring during the oxidation anddiffusion processes, distortion and micro-cracks which occur during the crystal-forming process, andstacking faults known as OSFs (oxidation-induced stacking faults). These types of crystal defect, whendense, increase the reverse current in the PN junction, degrading the retention time in DRAM andcausing a leakage current in SRAM. In bipolar devices these defects degrade the breakdown voltage 1)characteristics and reduce h FE . Countermeasures include oxygen density control in the crystal,optimization of gettering and other processes, and cleaning.If the resistance in the Si substrate deviates from the desired value, yields and reliability are affected.This is due to a piezo-resistance effect caused by physical stress during resin encapsulation.Countermeasures include encapsulating the device in low-stress resin or incorporating sufficientmargin in the design to allow for resistance changes.3.1.1 PN JunctionWith improved pattern designs and product process designs, characteristic faults due to degradationin the PN junction have become negligible. However, recent continuing progress in microscopicprocesses has increased the possibility of errors due to external stress factors.One such factor is electrical overstress (EOS). This is caused by an excessive current flow between thepower supply and GND when parasitic PNP and NPN transistors turn on due to the nature of thedevice’s structure. This is especially common in CMOS devices and is referred to as the “latch-up”phenomenon. It induces open failures in Al metal interconnections and bonding wires.Countermeasures include lowering the h FE of parasitic transistors and optimizing the pattern design.However, precautions must also be taken regarding the operating conditions. (See Chapter 5.)Another example of external stress is electrostatic discharge (ESD). When ESD applies a reverse biasto the PN junction, heat due to power dissipation melts the Si, damaging the junction. On-chipprotection is normally provided by inserting resistors and protective diodes to disperse the voltage andcurrent.3.1.2 Alloy SpikesThe primary metallization material for semiconductor devices is Al, since it has a low electricalresistance compared to other materials and since it adheres well to insulating materials such assilicon oxide film. However, high-temperature treatment can cause silicon in the substrate to flow intothe Al film and damage the junction in the area of the contact. This is referred to as an alloy spike. Itlowers the breakdown voltage and causes shorts, especially in shallow junctions.Countermeasures include adding silicon to the Al, and forming barrier metal between the Al and thesilicon.2-15