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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'.

Simplification:
  • 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
A.R.4_A.R.6.M6
{ @ (M6 area / gate area + ACCUMULATE ) > (600 in OD2, 5500 not in OD2) (without effective diode)
@ (M6 area / gate area)+ ACCUMULATE > Ratio (with effective diode)
NET AREA RATIO M6 M6_DIO HV_GATE GATE > 0 ACCUMULATE ACC_M5
[
!!AREA(M6) * !!AREA(GATE) *
(!AREA(M6_DIO)*(!!AREA(HV_GATE)*(AREA(M6)/AREA(GATE)-600) +
!AREA(HV_GATE)*(AREA(M6)/AREA(GATE)-5500)) +
!!AREA(M6_DIO)*(AREA(M6)/AREA(GATE)-AREA(M6_DIO)*456-43000))
- (!AREA(M6)+!AREA(GATE)) * LargeNumber
]
}
ACC_M6 = NET AREA RATIO M6 GATE >= 0 ACCUMULATE ACC_M5
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 \
-cut_ratio
define_antenna_layer_rule -mode \
-layer \
-ratio \
-diode_ratio <{v0 v1 v2 v3 [v4]}>
Summary:
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.

63 comments:

  1. I seldom leave comments on blog, but I have been to this post which was recommended by my friend, lots of valuable details, thanks again.

    ReplyDelete
  2. Hi,
    Nice to here that you like this Post. Please keep visiting my blog and let me know if you need anyother specific detail.

    ReplyDelete
    Replies
    1. If you can write something on verilog than it will be great

      Delete
  3. seems you have worked in all eda companies.. gr8

    ReplyDelete
  4. great work , cheeeers , certainly good for the freshers

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  5. Amazing knowledge...well written to...

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  6. Thanks a lot for sharing . I couldn't have understood the concept better from any other source.

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  7. thanks for the detailed explanation...:-)

    ReplyDelete
  8. Really, Very good and detailed explanation.
    Good no. of ways has been mentioned to overcome the ANTENNA effect.
    Thanks

    ReplyDelete
  9. Excellent analysis and great presentation.

    ReplyDelete
  10. Kudos to you for giving such detailed and graphical explanation of this topic. I understood many concepts related to Antenna problem because of this.

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  11. thank you so much,i would like to know what will happen for hanged metal1 it will not find any discharging path na after metal2 fabrication it may aid the charge on metal1(hanged)and metal2 and hence may damage the gate oxide na??can you pls clarify more on it..Thanx in advance.

    ReplyDelete
    Replies
    1. You are right. So if the metal1 has too much charge that it can damage the gate oxide, it will do that but if that's not the case then it will wait for metal2 and then metal3 and so on.
      So the point is the moment accumulated charge is sufficient to destroy thr gate oxide, it will do.

      Delete
  12. can u giv me a lnk for documentation,ppt and abstract for this topic..i would lik to giv a tech seminar on this...wil it be good ?

    ReplyDelete
    Replies
    1. hi Unknow,

      Its very difficult for me to give you any ppt right now. Because I don't have anything handy currently. Apart of my blog, may be wikkipedia website can help you.

      http://en.wikipedia.org/wiki/Antenna_effect

      Sorry man - That I am unable to help you regarding this right now.

      Delete
  13. can u giv me a lnk for documentation,ppt and abstract for this topic..i would lik to giv a tech seminar on this...wil it be good ?

    ReplyDelete
  14. its very usefull for me thank you ........ can u plz explain me wht propotion use in anteena effect.........

    ReplyDelete
  15. its very usefull to understand anteena concept Good work, any other blogs.

    ReplyDelete
  16. Hi
    This article is really very nice and it helps me to understand very clearly.
    I have a QUESTION : In some articles i have red, when a tool puts a jog on metal then it takes a higher or one above metal. But not lower or one below metal. why? Would you please explain me.

    Thanks
    AD

    ReplyDelete
    Replies
    1. no u can not go to lower level metal c because whn the gate s connected by M2 and ur getting antenna error.
      if u go for jumper of m1 near the gate, M1 s fabricated first and then M2 so v r getting error from M2 , M1 and M2 are connected and that may harm our gate.
      If u go for jumper of m3 near the gate, M2 s fabricated first as a two piece near the gate and after the jumper.
      den we fabricate m3 so charges in m2 s removed before m3 fabrication, so it will not harm the gate.

      Delete
  17. Hi,
    Thanks a lot.
    good explanation.

    ReplyDelete
  18. Hi, if we are connecting the antenna metal to higher metal,the top metal still contributes to antenna effect right?

    ReplyDelete
    Replies
    1. See,there will be a small chance that top metal will not contribute to the antenna.This is because top metal length used as jumper is small.Unless top metal, which is used in the same net(of lower metal ,suffering antenna) and used in different location ,add up to cross antenna ratio for that metal layer(top metal),it will not not happen.

      Delete
  19. Hello Expert

    I have one more doubt regarding antenna effect
    if one of the metal is connected to drain and other end connected to
    gate (ex inverter connected to another inverter ) is there a need for
    antenna protection ?? because one end is connected to diffusion which
    by default has a parasitic reverse bias diode (bw diffusion and
    substrate) so i feel it is protected by default am i right ?

    ReplyDelete
    Replies
    1. Kishan,
      You have to go for Antenna protection in this case also. See in layman language - why there is a problem. Whenever (even for a instant) if there exist enough charge on the gate (which comes from the wire) which can damage it - we should go for the Antenna protection. Now the other end can be connected to anywhere - It doesn't matter. It all matter how long the wire is -- if wire is too long and after that its connected to ground - even in that case there may be a problem of Antenna.

      Actually these problems comes during the fabrication- so these have no major relationship with the logical connection and whatever you are talking .. is the logical connection.

      I hope you got my point. Still if these is any doubt - feel free to post here.

      Delete
    2. This comment has been removed by the author.

      Delete
    3. Thanks a lot expert but first step in fab is substrate, diffusion, poly and metal so a reverse bias diode is already formed there which can breakdown at high reverse voltage and form a low impedance path to substrate so dont the charges leak away during the process?

      Thanks and regards

      Delete
    4. yes.. we dont need diode or jumper in dis case. It will go through the diffusion. we need diode only when the gate s connected by M1(lower layer) and drain s connected by M2(higher layer) and antenna error s from M1(lower layer) in dis case only v need diode or jumper.

      Delete
    5. Hi Kishan,

      Suppose the net is routed through M1-M5 layers and it's having antenna violation on M3.
      So while M3 is manufactured there's excess charge on it. But M4 and M5 aren't manufactured yet. So the piece of net on M3 which is having excess charge, isn't connected to the driver's output pin (diffusion) yet. Hence there's no path for the charge to go into the diffusion.

      By the time M4 and M5 are manufactured, the charge on M3 is already grounded (I don't know exactly how though), and there's no antenna violation anymore.

      Correct me if I am wrong somewhere.

      Thanks,
      Sagar

      Delete
  20. hi,

    may i know in a design with seven layers is there any chance of getting antenna violations at seventh layer..
    say if exists then how to overcome with that violations
    as there is no room to insert diode.. what could be the other chance of removing it.. please let me know..

    ReplyDelete
    Replies
    1. there are chances -- any where and any layer -- that's the reason you can see that the foundry have rules for all the layers...
      Now for such violations always prefer jumpers.

      "Using jumpers to break up long wires connected to gates".

      Its a mostly recommended methodology - that for M1 or Max M2 - Use Diode and for upper layer use Jumpers.

      Delete
  21. Hello,

    I have a question about the antenna rules....i hav observed that the topmost layer will have the antenna ratio very less [0.1]...can you pls tell me why is that ? All other layers will have antenna ratios as 200,300 etc...But why only Top most layer has less ratio ??

    ReplyDelete
  22. Really helpful thank you for sharing it !!!

    ReplyDelete
  23. very much helpful, well explained, cleared most of my doubts on antenna effect

    ReplyDelete
  24. This comment has been removed by the author.

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  25. Superb xlanation...thank u

    ReplyDelete
  26. Hi experts could u plz explain detaildly regording antenna dide working. .
    Thanks in advance.
    Regards,
    Siddharth.

    ReplyDelete
  27. Really good explanation to understand the Antenna Issue, I have one question, I have antenna Violation on Top Metal Layer and my design is very congested so I can not put the Antenna Diode. Usually for Jumper you move to higher metal layer and then come down to lower metal Layer. In this scenario can Jump to Lower metal Layer and then again going back to Top metal layer will fix the Antenna Violation or we need to try something else.

    ReplyDelete
    Replies
    1. no u can not go to lower level metal and then to top level.. c because whn gate s connected to m3 that s done first after the m4(consider top layer) is done. so both are connected and error is coming from m4(top layer). so u hav to connect a diode or try to reduce the length of top layer.

      Delete
  28. Nice explanation. No need refer anything else for understanding Antenna effect. Nice work !!!!

    ReplyDelete
  29. I think you guys did an excellent job explaining the antenna effect. I will be looking forward to more information
    as a layout technician. This will continue to help me emensely. Great Work!! and Thank You

    ReplyDelete
  30. This comment has been removed by the author.

    ReplyDelete
  31. Great explanation......:)
    This data much better than Art of Analog Layout - Alan Hestings

    ReplyDelete
  32. Thanks alot...:)

    ReplyDelete
  33. Could you expalin how to choose diode specifications or layout area for Antenna effect?

    ReplyDelete
  34. Can you please explain the detailed operation of antenna diode ???

    ReplyDelete
    Replies
    1. Yes, i too need some explanations regarding the operation of antenna diode.

      Delete
  35. can you please tell me why higher metal layers doesnot accumulate charge ?

    ReplyDelete
  36. I just get artictle about tunnel diode, if you need to get more information about this electronic device check this.

    ReplyDelete
  37. Hi

    A great article to understand antenna effect. Thanks for the detailed explanantion.
    I have one query regarding use of antenna diode. What type of diodes are prefarable(p+/n+ diode)? and How are we connecting it?

    Thanks

    ReplyDelete
    Replies
    1. Hi,

      We prefer NMOS devices as the diodes. Source and Drain connected to antenna affected signal, Gtae and substarte are connected to GND.

      Delete
  38. Please explain the antenna diode functioning in detail?

    ReplyDelete
  39. cheers.. keep-up the good work!!!!

    ReplyDelete
  40. I have one doubt on diode insertion in reverse bias for antenna effect. why we have to connect diode in reverse bias only. pls explain

    ReplyDelete
  41. Can you please explain the three sources of the stress voltage mentioned ?

    ReplyDelete
  42. Hii,

    while calculating the maximum antenna ratio in the formula, i have seen a parameter called "Diode Protection" (DP) value, when diodes are used for preventing antenna violations. it would be very helpful if you could explain that. i have read somewhere, it is a ratio that can range from 0 to 1e6. can you please tell me whats that ratio is and how the value is determined for preventing violations.

    Thank you very much for this post.. very useful

    ReplyDelete
  43. Hi, nice explanation for Antenna effect given. But i coundt get the Operation of the Antenna Diode, in this scenario. Is is a shottkey diode/tunnel diode ? or a normal diode, which will be damaged if it conducts in reverse bias.

    ReplyDelete
  44. Hi,
    nice to read this, got idea what is antenna effect, causes and remedies but i didn't get the functionality of inserting diode i mean how it is operate. can u please explain in detail operation of inserting diode

    ReplyDelete

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