Soft errors alpha particles

A barrier coating may also be required for protection against alpha particles. High-density devices can suffer soft errors when alpha particles emitted from trace quantities of thorium or uranium in packaging materials strike the active surface. Thus, the surface must be protected against this possibility. Polyimides are again recommended for such applications. 

The demand for high purity metals has tremendously increased due to needs in the information technology and telecommunications industries. High purity Al is used in various microelectronic devices and semiconductor detectors are made of hyperpure Ge and Si, etc. Impurity levels depend upon both the raw material and the purification method and may influence the function of the device. For example, in semiconductor materials, alpha particle emission creates damage causing the so-called soft error effect. For this reason, U and Th levels are strictly limited in these applications. 

Alpha particle A product given off by the decay of radioactive material (usually emitted by traces of radioactivity in IC ceramic packaging materials). This particle has a positive charge equal to twice that of an electron and is emitted at a very high velocity. Alpha particles can cause temporary memory upsets in DRAMs, which is known as soft error rate. 

There is an increasing amount of li t sensitive opto-devioes that need UV-VIS protection. Alpha particle radiation is caused by a very low level of uranium and cosmic radiation present as background radiation in the device package and the atmosphere, respectively, and which could generate a temporary soft error in operating dynamic random access memory (DRAM), such as the 64 K, 256 K, 1 and 4 Megabits DRAM devices. This type of alpha particle radiation has become a major concern, especially in high density memory devices. 

May, T.C. and Woods, M.H., Alpha-particle-induced soft errors in dynamic... 

The same coating is also used to protect static RAM chips from alpha particle radiation, which can cause soft errors, and it reduces the probable failure rate by at least one order of magnitude. Calculations indicated that a 0-5 mm layer would stop 8 MeV alpha particles, and that the coating was fully compatible in all respects. The authors reported that the resultant soft error rate of 0-1% per 1000 h established in service was the lowest level of activity observed for any packaging material presently in use. 

Low alpha particle emission ingredients may need to be pure with respect to actinide elements, to avoid alpha particle emissions that can cause soft errors in dynamic random access memories of 64K capacity and upwards. 

Low alpha particle emission materials are available enabling the encapsulation of 64 and 256K dynamic RAMs without the fear of soft error generation due to alpha particle emissions from the moulding compound. 

With the increase in packing density of dynamic RAMs, soft errors caused by alpha particles are becoming an increasingly important cause of device malfunctioning. (For further information on soft errors see Section 10.3.) Alpha particles are emitted from uranium and thorium isotopes residual in most packaging materials and thin films of polyimides deposited upon a chip surface have been successfully evaluated as alpha particle barriers. Polyimides can be made pure enough to contain no detectable amounts of uranium or thorium and a 40 xm thick film may reduce the soft error generation rate by up to 1000 times. A typical example of a commercially available polyimide alpha particle barrier is DuPont s Pyralin PIH 61454. 

There are multiple sources for the radiation that causes soft errors. They fit within the broad categories of cosmic rays, (boron) neutron capture, alpha (a) particles from materials in microelectronic packages, and electrical noise or electromagnetic interference (EMI) [67]. The... 

Soft errors in microelectronic packages are a difficult, troublesome problem to address. Eliminating them is virtually impossible. Soft error rate effects can be reduced in a number of ways, including the use of shielding, software correction methods (although this reduces performance), alpha particle-tolerant structures (e.g., SOI), and utilization of low-alpha-emitting materials. Let us now focus on the reduction of alpha particle-induced SER from traditional lead-based solders. 

FIG. 23 Schematic depicting the sequence of events in which alpha particles cause soft errors in memory devices, (a) A device node prior to an event, (b) Silicon is ionized along an alpha particle trajectory creating electron-hole pairs, (c) Presence of an oxide layer below the active device significantly reduces the collection of charge. (From Ref. 67.)... 

Based on the discussions in prior sections, the soft error concern associated with traditional lead-based solders is quite clear, but what is less clear is why lead-free solders can potentially pose a SER concern. Because the overriding SER concern with lead-based solders is alpha particle emission by Pb °, then it may be reasoned that solders which do not contain Pb do not emit alpha particles. Understandably, there are many in the industry making that assumption. Although lead-free solders may be the best path to very low alpha particle-emitting solders for SER-sen-sitive products, they can also be sources of radioactivity unless proper processing steps are taken. 

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