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Saturday, July 19, 2008

Antenna Effects

Antenna Effects:

Modern wafer processing uses ‘Plasma etch’ (or ‘dry etch’). Plasma is an ionized/reactive gas used to etch. It allows super control of pattern (shaper edges / less undercut) and also allows several chemical reactions that are not possible in traditional (wet) etch. Apart from this, several unwanted things happen just because of several plasma processing steps. One of them is the charging damage.
Plasma charging damage refers to the unintended high-field stressing of the gate-oxide in MOSFET during plasma processing. The stress voltage that develops across the gate and substrate of a MOSFET during plasma processing basically comes from three sources.

  • Non uniform distribution of plasma potential across the wafer.
  • Charging filtering (shading) due to microscopic topography on the wafer.
  • AC effects due to the nature of RF discharge that sustain the plasma.
The stress voltages due to AC effects are quite small in most cases and cannot cause damage by themselves. They do, however, add to the magnitude of stress voltages developed by either non-uniform plasma potential or topographic filtering of charge or the sum of both.

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. This is the so called “antenna effect”. The area ratio of the conductor to the oxide under the gate is the antenna ratio. The antenna ratio, in a rough sense, is a current multiplier that amplifies the tunneling current density across the gate-oxide. For a given antenna ratio, a larger tunneling current is supported when the plasma density is higher. Higher tunneling current means higher damage.
This antenna effect can be understood in a different way also. 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.
In other word the 'antenna' is an inter-connect, i.e., a conductor like polysilicon or metal, that is not electrically connected to silicon, i.e., not 'grounded', during the processing steps of the wafer. The connection to silicon would normally provide an electrical path to bleed-off any accumulated charges. 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'.

  • Charge builds up (DC) on the metal wires (antenna) during the application of the plasma etch
  • Because the gate of a MOSFET is like a capacitor
  • If the potential on the gate reaches a certain value it will ‘break down’
  • ‘Punch through’ occurs
  • The gate is damaged irreparably
The 'antenna ratio' of an inter-connect is used to predict if the antenna effect will occur. 'Antenna ratio' is defined as the ratio between the physical area of the conductors making up the antenna to the total gate oxide area to which the antenna is electrically connected. A higher ratio implies a greater propensity to fail due to the antenna effect. This can result either from a relatively larger area to collect charge or a reduced gate oxide area on which the charge is concentrated.

Charge Build up is affected by:
  • Diffusion path
    • There is an NP diode to substrate at the drain/source of any output pin
    • During plasma-etch this diode is reverse biased and at high temp
    • This causes the diode to behave like a resistor
  • Gate Area
    • Larger gate_area == larger gate ‘capacitor’
    • At fixed ‘charge’, voltage potential reduces as cap size increases
    • Reducing the voltage prevents ‘punch through’
  • Diffusion Area
    • Bigger diffusion == Smaller resistor
    • Smaller R allows more current to pass
  • Wire length
    • Longer wires act as antennas to ‘pick up’ more charge
The conditions that lead to antenna formation depend on the technology used to fabricate the chip and must be determined empirically for each process. Once they have been identified, they can be used to define a set of antenna rules, similar to conventional DRC rules, that can be coded. Factors these rules need to consider is whether the antenna should be based on the "top" area of the metal or on its "sidewall" area.
  • In an aluminum-based process, charge accumulation occurs during the ETCH step. The top of the metal is protected by a resist during this step, so the antenna rules for this process should be based on the metal sidewall area.
  • In copper-base technologies, charge accumulation occurs during CMP (Chemical-Mechanical Polishing). In this process, the sides of the metal are protected, so the antenna rules need to be based on the metal's top surface area.
A number of techniques can be utilized to minimize the antenna effect. For example, the occurrences of antennas can be predicted and their ratios calculated using design verification and layout software known as 'design rule check' ('DRC') programs. Then by adjusting the physical layout of the inter-connects, the antenna ratios can be reduced to an acceptable level. In addition, processing steps utilizing plasma can be optimized to reduce the build-up of charges on any antennas that do exist on devices.
To avoid antenna problems, you must design all net topologies so that no gate is vulnerable to a large amount of floating charge. Antenna rules are commonly expressed as a ratio of wire area over gate area (Aw/Ag) for each metal and cut (via) layer. This ratio indirectly states how much floating charge a transistor gate can handle by specifying how much wire can be connected to the input of the logic gate before antenna problems occur.

Design Solution to reduce Antenna Effects:
  • Router options
    • Break signal wires and route to upper metal layers by jumper insertion
    • All metal being etched is not connected to a gate until the last metal layer is etched.
  • Dummy transistors
    • Addition of extra gates will reduce the capacitance ratio.
    • PFETs more susceptible than NFETs
    • Problem of reverse Antenna Effects.
  • Embedded Protection Diode
    • Connect reverse biased diodes to the gate of transistor (during normal circuit operation, the diode does not affect functionality).
  • Diode insertion after placement and route
    • Connect diodes only to those layers with antenna violations.
    • One diode can be used to protect all input ports that are connected to the same output ports.

Most important methods are jumper insertion and diode insertion to remove antenna violation. We are discussing these two methods here in detail.
Jumper techniques are the most effective method of avoiding antenna-effect problems. Diode insertion can repair the remaining antenna problems. However, it is costly in terms of cell area size and it complicates the netlist verification process.
Jumper Insertion:
A jumper is a forced layer change from one metal layer to another, and then back to the same layer. Jumper insertion breaks up a long wire so that the wire connected to the 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 congestion problems in upper metal layers. There are also situations for which there are no valid jumper solutions.
Figure : Jumper Insertion Breaks Up a Long Wire

In most of the tools, jumper insertion is performed automatically during the routing. After detailed routing, you can fix antenna violations manually by inserting jumpers by using commands corresponding to the tool you are using. When you execute those commands, tool detects and fixes antenna violations using jumpers and a tailored ripup and reroute strategy.
The Importance of Jumper Location in Repairing Antenna Violations
Figure shows two nets with the same separation between the input and output pins, but slightly different jumper locations. The first one has an antenna violation, and the second one does not.
Figure: Impact of Jumper Location

The difference is that the first net has a long metal1 connection to the input pin. The wire area as detected by the input pin of the first net is significant, and, therefore, the antenna ratio is exceeded.
This example shows that antenna violations can be avoided through the use of jumpers (also known as “bridges”). A jumper directs the net to a higher metal layer before descending again. In the process of metallization, the pin is connected to a small amount of wire area, except on the highest layer, avoiding any antenna problem below that layer.
Unfortunately, the use of jumpers might only defer the antenna problem to the highest metal layer of the jumper, where antenna violations might still occur because all geometries of the net are physically connected to each other. For this reason, it is important that the output pin have some ability to solve antenna violations.
Diode Insertion
Figure : Diode Inserted Near a Logic Gate Input Pin

As shown in Figure, diode insertion near a logic gate input pin on a net provides a discharge path to the substrate so that built-up charges cannot damage the transistor gate.
Unfortunately, diode insertion increases cell area and slows timing due to the increase of logic gate input load. Moreover, diode insertion is not feasible in regions with very high placement utilization.
In most of the tools, diode insertion is performed automatically when you use the routing command. You can manually insert diodes using the corresponding tool’s commands. There are two points in the design flow where you can insert diodes to fix antenna violations.
  • Inserting Diodes Before Detailed Placement
Normally, the diode is added only to the pins that need it. The antenna checker is called for each pin in question to decide first, if the pin has antenna violations and second, if a jumper has failed in the area of the pin because the area is blocked and a large enough hole does not exist.
  • Inserting Diodes After Detailed Placement
After detailed routing, the antenna violations can still exist for various reasons. For example, there can be too much congestion to insert a jumper or the diffusion strengths of the output pins are too weak. In these cases, diode insertion is a viable choice.
The semiconductor manufacturer generally provides the gate area or size, and the antenna checker calculates the appropriate wire area using the wire (charge) accumulation method specified by the manufacturer.
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 area connected directly to it 200.
  • When the protection diode is not used, the maximum ratio of each metal (for M1 to M5) perimeter area to the active Poly gate area 400.
  • Antenna Ratio = 2[(L+W1)*t]/W2*l
L: floating metal length connected to gate
W1: floating metal width connected to gate
t: metal thickness
W2: connected transistor channel width
l: connected transistor channel length
Design Rules for Some Current Technologies –IBM 0.13um
  • Antenna ratio is non-cumulative.
  • Poly antennae larger than 100 are NOT ALLOWED
  • Floating gate devices with metal antennae larger than 150 are NOT ALLOWED
  • Every N-well is required to be tied down by a N+ diode
Antenna Rules:
In most cases, antenna rules are in the form of:
(antenna-area) / (gate-area) < (max-antenna-ratio)
  • Gate-area
    • Boolean AND of the ‘poly’ and the ‘diffusion’ layers
    • Recognized as gate area of the transistors by essentially all foundries
  • Antenna-area
    • Amount of metal area attached to the input pin
    • Calculation method varies for different processes
  • Max-antenna-ratio
    • Represents max allowed ratio of antenna area to gate area
    • Calculation method varies for different processes
  • There are 2 ways to calculate antenna area:
    • Side-Wall Area = (W + L) * 2 * Thickness
    • Polygon Area = W * L
Calibre antenna rules
M6_DIO = NET AREA SD >= 0.16
{ @ (M6 area / gate area + ACCUMULATE ) > (600 in OD2, 5500 not in OD2) (without effective diode)
@ (M6 area / gate area)+ ACCUMULATE > Ratio (with effective diode)
!!AREA(M6) * !!AREA(GATE) *
- (!AREA(M6)+!AREA(GATE)) * LargeNumber
Magma antenna rules:
rule antenna ratio metal_rule $l -area_type area -accumulation_type path \
-ratios {{{{0 600 0} {0.16e-12 43072.96 456e12}} {METAL6}}
Synopsys antenna rules:
define_antenna_rule -mode \
-diode_mode \
-metal_ratio \
define_antenna_layer_rule -mode \
-layer \
-ratio \
-diode_ratio <{v0 v1 v2 v3 [v4]}>
During the IC manufacturing process, the metal layer is exposed to conditions that lead to the build-up of an electrostatic charge. The amount of charge that builds up depends on a number of factors; the most important from an antenna standpoint is how much metal is exposed. As more metal is exposed, the maximum charge that accumulates on the net that the metal is part of also increases. The substrate remains at ground since it is connected to the fabrication device. As a result a voltage gradient develops across the gate oxide. When this gradient becomes large enough, it is relieved via an explosive discharge (i.e. "lightning"). The problem is more significant at smaller technologies because the damage resulting from the discharge is more likely to extend across the entire length of the gate.
Antenna rule checking is different for every process technology because the method for expressing antenna ratio is not standardized.
Antenna repair is accomplished by inserting a reverse-bias diode on the violating net as close to the gates being protected as practical. During normal chip operation, the reverse bias prevents electrons from flowing from the net through the diode and into the chip's substrate. During fabrication, however, the charge on the net can build to the point where the voltage drop across the diode exceeds its break-down voltage. This voltage is greater than the normal operating voltage, but less than the voltage at which an electrostatic discharge at the gate can be expected. When this happens, the diode allows electrons to flow from the net to the substrate and thus limits how much charge can accumulate on the net. The process is non-destructive, and it's possible that the net could discharge through the diode several times during the fabrication process.
The other way to repair is to "break up" the antenna by shifting briefly to a different metal. When this metal layer is fabricated, the long piece on one side is no longer electrically connected to the gate and does not contribute to any antenna effects. When it is eventually connected through the higher-level metal "bridge," it is no longer exposed to the charge accumulation and again does not contribute to an antenna violation.
Causes of antenna defects:
  • Electrostatic charge collection on wires while the metallization is being deposited. (This is usually referred as ‘charge-collecting antenna problem’ or simply antenna problem)
Most important ways to repair the antenna violation:
  • Using jumpers to break up long wires connected to gates (Jumpers are a short metal segment inserted onto a long route of another metal layer).
  • Using diodes to provide a discharge path to the substrate by contact to a diffusion area.
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