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yousoo
Joined: 19 Mar 2007
Posts: 5
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 09 Apr 2007 8:15 Why do gate use in poly? |
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I`m studying [IC Layout Basics] p.134 - Wiring with poly
I have one question.
Why do gate use in poly? not metal?
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fredflinstone
Joined: 20 May 2005
Posts: 75
Helped: 17
Location: Bangalore, India
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| 09 Apr 2007 10:07 Why do gate use in poly? |
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hi,
the gate of MOSFET use poly instead of METAL so that the MOS structure is self aligned. To explain what is self aligned structure assume that we use metal for making gate. For this we need to first do the source/drain diffusion and then the metal layer. Now on doing so the METAL gate may not be perfectly aligned to the source and drain diffusions and in worse case it can so happen that the metal gate may not be overlapping with either source/drain due to misalignment. This degrades the performance of MOS and I think it may not work as a MOS either. So inorder to workaround this problem what ppl used to do was to increase the gate length thereby ensuring that the gate will have overlap with source and drain diffusions. Now this method had an issue that the gate/source overlap capacitance was huge here affecting the design. Also very small channel lengths was not possible here.
If we use poly then what we do is we will first grow poly gates on the silicon oxide and on top of that we will do diffusion . This ensures that the poly is perfectly alligned with the source and drain diffusion and also shoter channel lengths with lesser gate/source overlap capacitance is made possible. For a more detailed explanation refer Razavi ...
hope this helps.
fred
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leohart
Joined: 10 Nov 2006
Posts: 229
Helped: 12
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| 10 Apr 2007 2:35 Why do gate use in poly? |
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| if u use metal as gate,u can't do selfaligned s,d implant,coz if u doing so,the s,d drivein will melt the metal gate(al gate),so u have to do the s,d implant and drivein(anneal) first then gate deposition.
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fredflinstone
Joined: 20 May 2005
Posts: 75
Helped: 17
Location: Bangalore, India
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| 10 Apr 2007 6:22 Why do gate use in poly? |
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hi leohart,
Thanks for the explanation for why we cant use metal for self aligning. I was not sure about this.
rgds
fred
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yousoo
Joined: 19 Mar 2007
Posts: 5
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| 10 Apr 2007 7:14 Re: Why do gate use in poly? |
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| thank you, thank you all
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acbalbason
Joined: 08 Apr 2007
Posts: 117
Helped: 10
Location: Chung Li, Taiwan
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| 10 Apr 2007 17:20 Re: Why do gate use in poly? |
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hi!
polysilicon has a much higher melting point compared to metals and that will alllow annealing at higher temperature before metal deposition.
- al
Added after 3 minutes:
hi
and... annealing temperatures are usually higher than the metal meltign point so you can't anneal a micro circuit after metal has been deposited. Ü
- al
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Julian18
Joined: 09 Apr 2007
Posts: 128
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| 12 Apr 2007 6:56 Re: Why do gate use in poly? |
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| fredflinstone wrote: |
For a more detailed explanation refer Razavi ...
fred |
refer to which book? or paper?
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fredflinstone
Joined: 20 May 2005
Posts: 75
Helped: 17
Location: Bangalore, India
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| 12 Apr 2007 7:41 Why do gate use in poly? |
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hi Julian,
I was saying to refer Razavi book page no 613 for info about self aligned MOS strcuture. Sorry for creating confusion.
cheers
fred
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siva497
Joined: 16 Apr 2007
Posts: 4
Helped: 1
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| 16 Apr 2007 8:17 Re: Why do gate use in poly? |
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| does any one have pdf of razavi?
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strennor
Joined: 09 Dec 2003
Posts: 66
Helped: 13
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| 16 Apr 2007 9:12 Why do gate use in poly? |
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i think it is available somewhere on internet, maybe this forum.
try search it
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