The "Antenna Rules" deal with process induced gate oxide damage caused when exposed polysilicon and metal structures, connected to a thin oxide transistor, collect charge from the processing environment (e.g., reactive ion etch) and develop potentials sufficiently large to cause Fowler Nordheim current to flow through the thin oxide. Given the known process charge fluence, a figure of exposed conductor area to transistor gate area ratio is determined which guarantees Time Dependent Dielectric Breakdown (TDDB) reliability requirements for the fabricator. Failure to consider antenna rules in a design may lead to either reduced performance in transistors exposed to process induced damage, or may lead to total failure if the antenna rules are seriously violated.

The polysilicon rules require that the area of the polysilicon over field oxide divided by the area of the transistor gate (thin oxide area) must be less than Np (where Np is a limit that depends on the process and on design targets). For example, a 200 µm long by 1 µm wide polysilicon wire connected to two channel regions of 2 µm X 0.6 µm and 1 µm X 0.6 µm has an antenna ratio of 111. Usually the polysilicon rules are fairly conservative, so the antenna ratio in this example may be in violation of some fabricator's polysilicon antenna rule.

A similar calculation applies to metal wires connected to transistor gates. In this case, you must first obey the Np:1 rule for any polysilicon. Metal antenna rules can be a little more variable depending upon how conservative a fabricator is in dealing with this damage mechanism. The most conservative approach recognizes that process induced damage is a cumulative effect. In this case, you calculate the total area of the poly + metal1 + metal2 + metal3 + ... and divide by the area of the transistor channels connected to this structure. This ratio must be less than Nm.

A less conservative metal antenna rule defines ratios for each metal level (Nm1, Nm2, Nm3, etc.) and recognize that lower levels of metal are protected by inter-level dielectric during the etch process for the metal above. Other metal rules recognize that photoresist covers metal lines during reactive ion etch, so the antenna rule only needs to consider the area of the exposed metal edges. This results in rules defined for area ratio calculations as follows: Ratio = 2[(L_metal+W_metal)*t_metal]/(W_channel*L_channel), where, W_metal, L_metal, and t_metal are the width, length and thickness of the metal line connected to a total channel area defined by W_channel by L_channel.

If metal1 is connected to an active area junction and to a poly structure connected to a transistor channel, then the Nm:1 rule is relaxed. However, the poly Np:1 ratio rule still applies. The reason for this exception is that charge induced current is safely shunted through the junction to the substrate and, therefore, does not cause gate oxide damage. The junction area ratio (A_total/A_junction) must not be more than N_max:1. Note: A_total is the sum of poly and metal area connected to a thin oxide region.

An additional absolute area rule is also imposed for additional safety margin. The total area of exposed conductor that is electrically connected to a gate channel area limited to A_max µm². Other constraints may apply, but these constraints are specific to a particular fabricator.

There are layout techniques to help deal with antenna ratio rules. For example, if a design uses a large array of clocked devices connected to a single clock source via a metal1 clock distribution structure then a "cut and link" method can be used to moderate the antenna rule effects. In this method, the metal1 distribution structure is divided up into pieces of metal1 connected to gate structures such that the antenna rule is obeyed. Short links from metal1 to metal2 then back to metal1 connect the clock distribution structure in a way that it prevents the total area of the clock distribution structure from being connected to gate poly structures during metal1 etch. If the metal2 links are the minimum links necessary to make the connection, then the current induced in the metal2 area is very small (Nm2 << metal2 rule).