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ANTENNA EFFECT

Plasma charging damage refers to the unintended high-field

stressing of the gate-oxide in MOSFET during plasma processing.

The available charges are the net charges collected from the

plasma by the exposed conductor with connection to the gate or

substrate. Both electrons and positive ions from the plasma are

impinging on the exposed conductor during processing.

Depending on the charge balance condition, the electron flux

might not equal the ion flux, a net positive or negative charge

collection rate exists. The collected net charges are channeled to

the gate as shown in fig. 1 where it is neutralized by the current

tunneling across the gate-oxide.

Clearly, the size of the conductor exposed to the plasma plays a

role in determining the magnitude of the net charge collection

rate and therefore the tunneling current by fowler nordheim

tunneling. This is the so called antenna effect. The area ratio of

the conductor to the oxide under the gate is the antenna ratio.

Higher tunneling current means higher damage.

It occurs during the manufacturing process and renders a die

useless.During metallization (when metal wires are laid across

devices), some wires connected to the polysilicon gates of

transistors can be left floating (unconnected) until the upper

metal layers are deposited.

A long floating interconnect (without proper shielding layer of

oxide) can act as a temporary capacitor, collecting charges during

fabrication steps, such as plasma etching. If the energy built up

on the floating node is suddenly discharged ,the logic gate might

suffer permanent damage due to transistor gate oxide

breakdown.

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If the connection to silicon does not exist, charges and may build

up on the inter-connect to the point at which rapid discharge does

take place and permanent physical damage results, e.g., to

MOSFET gate oxides. This destructive phenomenon is known as

the 'antenna effect'.

'Antenna ratio' is defined as the ratio between the physical areaof the conductors making up the antenna to the total gate oxidearea to which the antenna is electrically connected. A higher ratioimplies a greater propensity to fail due to the antenna effect. Thiscan result either from a relatively larger area to collect charge ora reduced gate oxide area on which the charge is concentrated.

Design Solution to reduce Antenna Effects:

Most important methods are jumper insertion and diode insertion to removeantenna violation.

A jumper is a forced layer change from one metal layer to another, and then back tothe same layer. Jumper insertion breaks up a long wire so that the wire connected tothe gate input is shorter and less capable of collecting charge, as shown in Figure.

The advantage of jumper insertion is that it is fully controlled by the routing tool.The disadvantage is that it can potentially contribute to routing congestionproblems in upper metal layers

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In most of the tools, jumper insertion is performed automatically during the routing.

After detailed routing, you can fix antenna violations manually by inserting jumpersby using commands corresponding to the tool you are using.

Diode Insertion:

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As shownin Figure, diode insertion near a logic gate input pin on a net provides a dischargepath to the substrate so that built-up charges cannot damage the transistor gate.

Unfortunately, diode insertion increases cell area and slows timing due to theincrease of logic gate input load. Moreover, diode insertion is not feasible in regionswith very high placement utilization.

In most of the tools, diode insertion is performed automatically when you use therouting command. You can manually insert diodes using the corresponding toolscommands. There are two points in the design flow where you can insert diodes tofix antenna violations.

Design Rules for Some Current Technologies TSMC 0.18um

Metal antenna ratio is not cumulative.

Maximum drawn ratio of field poly perimeter area to the active poly gate areaconnected directly to it 200.

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