Electron Beam Processing of Semiconductors

Introduction

In the semiconductor industry, there are certain application areas where switching speed and related performance criteria are critically important. A good example of this would be discrete devices, such as power diodes. The ability to increase the switching speed of a device while minimizing backward current leakage is especially important in applications involving high power and high frequency operation.

Generally, power silicon devices operating against frequencies below 400 hertz (microsecond ranges) do not need any special enhancement in terms of switching speed and carrier lifetime control. When these devices are to be incorporated into integrated circuits or other power control systems above 400 hertz, secondary processes (such as electron beam irradiation, or gold doping) are typically used to increase switching speed by shortening carrier lifetime. There exists a well-documented industry need for power semiconductor and transistor devices that can operate in the megahertz range, so the practical range of enhancements required spans several orders of magnitude.  

Basic Principles

Semiconductor wafers are frequently irradiated using high voltage accelerated electrons (electron beams, or b-radiation) produced by commercially available linear accelerators. Accelerator voltages higher than 5 million electron volts (MeV) are preferred, since they provide more uniform penetration through the thickness of the wafer or semiconductor device. Efficiency (power utilization) also increases with higher voltage.

The underlying purpose is to generate specific types and sizes of defects within the crystalline matrix controlling concentration and distribution. Depending on the semiconductor device that will be built on the modified substrate, these defects are critical to performance related to specific applications. Fuochi and colleagues demonstrated in 1995 that the proportional concentration of complex defects produced by irradiating between beam energies of 6.6 MeV and 12 MeV is ten times greater than simple defects, when compared to irradiation at 1 MeV.

Irradiation doses required to achieve the desired effects vary according to exact product nature and performance specifications, but are generally in the range of 0.3 to 20 kGy. Dose is typically delivered in one or two passes. Semiconductors are usually packaged for beaming in static-free envelopes or polystyrene flat carriers, both of which are suitable for processing using both horizontal and vertical beams  

Established Products

Electron beam processing has been employed as an alternative to gold doping in a number of products, a sampling of which is illustrated in the table below. Applications for these families of devices include: DC motor controls, anti-lock brake systems, electronic lamp ballasts, general-purpose inverters, uninterruptable power supplies, high-voltage specialty power supplies, surge protectors, isolators, etc.

Key Advantages of Electron Beam Processing Technology

The main advantage of electron beam processing is that it can be used to enhance switching speeds in semiconductor devices without proportionate increases in backward leakage of current. This is possible because unlike gold doping, for example, the irradiation process does not involve adding hetero-atoms to the silicon matrix. It is these hetero-atoms that while increasing switching speed, also produce higher, and often unacceptable, backward current leakage levels. Another major advantage of electron beam semiconductor enhancement technology is that the end product is re-workable. An off-spec irradiation-treated silicon wafer can be returned to its initial (pre-irradiation) state by annealing with heat. No such recovery is possible with gold doping, as the hetero-atom inclusions are permanent. A gold-doped off-spec wafer thus becomes expensive scrap. Finally, whereas high temperature processes, such as gold doping, can only be performed early in the manufacturing process, in many cases prior to electrical measurements that might provide more accurate estimates of the optimum doping levels needed, electron beam processing can take place at almost any step of the manufacturing process, up to and including the irradiation of individual, packaged or mounted chips.  

Conclusion

Electron beam processing has several advantages over gold doping. Similar switching speeds can be achieved with either technology, but irradiation has advantages that are increasingly important for scaled high frequency devices. It is possible to attain higher overall yield levels with irradiation when the ability to rework or retune carrier lifetime and switching speed is considered.

High voltage electron irradiation is a preferred processing technique since it allows manufacturers to take full advantage of the uniformity of defect distribution within the enhanced device as compared to diffusion-controlled thermal techniques.

Appendix - Performance Uniformity in Thyristors
(Data Normalized)

¹ Fuochi, P.G., Martelli, A., Gombia, E., Mosca, R., Fasce, F., Pasqualetti, M., Portesine, M., Zambelli, M., Icardi, M., Fimiani, S., Electron Irradiation of Power Semiconductor Devices in Italy, NTIS Report Number PB95-26586INZ (1995).

² Fuochi, P.G., Martelli, A., Gombia, E., Mosca, R., Fasce, F., Pasqualetti, M., Portesine, M., Zambelli, M., Passerini, B., Current status and prospects of electron irradiation of power semiconductor devices in Italy, Proceedings of the Seventh International Meeting of Radiation Processors (1989).