Radiation Damage

Unlike silicon detectors, where the whole volume contributes to the charge collection, only the surface is active in integrated circuits. All the active and passive components are built into a thin layer (approximately $1\,\rm\mu m$), while the silicon bulk is inactive.

There are several effects of radiation to integrated circuits. Most of them are related to ionizing particles similar to silicon detectors. Tab.  summarizes the effects which must be considered in future collider experiments such as CMS.

Table: Important effects of radiation on integrated circuits in the environment of future high energy collider experiments.

Effect	Scope	Persistent
Single Event Upset (SEU)	digital	no
	analog	no
Single Event Latchup (SEL)	digital	yes
Single Event Gate Rupture (SEGR)
Oxide Charging	analog	yes

A digital SEU (single event upset) occurs when enough charge is deposited close to sensitive areas of a flip-flop cell such that the cell state flips. This can be the result of the high local ionization of a recoil atom produced by a nuclear interaction. Such a flipped memory cell disturbs the state machine of the circuit or register settings stored in such flip-flop cells. An SEU is non-destructive and the aftermath can be cleaned by resetting the circuit and possibly reloading the memory registers. It is possible to reduce the impact of SEUs by introduction of ``triple-voting''. This design feature foresees three flip-flop cells in parallel where the state is determined by a majority vote. Thus, a single cell can flip without disturbing the circuit and a single event upset can be reported or even self-repaired.

In analog circuits, an SEU can induce transients which might be misinterpreted as signals. For example, the analog pipeline in the APV front-end amplifier stores the sampled output of the shaper stage in capacitors. Naturally, these elements are susceptive to SEUs, resulting in fake signals.

When the localized charge deposition is strong enough to produce a conductive channel between power levels (e.g. in a CMOS inverter) a high current state is produced which is likely to exceed the chip specifications. Such a SEL (single event latchup) or SEGR (single event gate rupture) can destroy the circuit by overheating. A fast (electronic) fuse can avoid such damage if power is taken away immediately.

Radiation also affects the oxide between gate and channel of a CMOS transistors, which leads to changes in channel noise and transconductance. With proper design, these changes are not critical, especially when the bias currents and voltages of amplifiers can be adjusted. In contrast to silicon detectors, the doping concentration levels of integrated circuits are higher by orders of magnitude, such that the radiation induced change is negligible.

There are specialized radiation hard manufacturing processes which were originally developed for military and space applications. An example of such a process, in which prototypes of the CMS tracker electronics were built, is the DMILL technology by Temic [36]. Fortunately, commercial submicron processes which are very popular for integrated circuits today, are intrinsically radiation tolerant due to their small structures. Together with special radiation tolerant design rules such as guard rings around vulnerable cells or triple-voting, this technology offers an inexpensive alternative to specialized radiation hard processes. The CMS Silicon Strip Tracker electronics will be entirely manufactured in the standard IBM $0.25\,\rm\mu m$ deep submicron CMOS process [37].