這是集成電路物理設計的第一個系列【physical cell】的第二篇文章,本文主要講TAP Cell相關知識: 1,什么是TAP Cell? TAP Cell只有兩個連接關系:nWell連接VDD,pSub連接VSS。 TAP Cell沒有 input pin和output pin,內(nèi)部沒有任何邏輯功能。 早期的standard cell內(nèi)部包含tap cell (taped standard cell),新工藝的standard cell內(nèi)部一般不包含tap cell,而是使用單獨的tap cell (tapless standard cell)。
2,為什么要加入TAP Cell?
3,如何Insert TAP Cell?
實際上并不需要每個standard cell的nWell/pSub都需要連接VDD/VSS,只需要Row上隔一段距離連接就可以。 tap cell在擺放好macro之后,在擺放standard cell之前放置。 兩個tap cell之間的擺放距離需要根據(jù)DRC rule來設置。 FC/ICC2 cmd: >create_tap_cells -lib_cell */TAPCELL -distance $distance -minn_horizontal_periphery_spacing 1 -pattern stagger -no_abutment -no_abutment_horizontal_spacing 1 -no_abutment_corner_spacing 3 -skip_fixed_cells.Innovus cmd:
>set_well_tap_mode -rule $welltap_D -bottom_tap_cells $bottom tap_cell -top_tap_cell $top_tap_cells -cell $tap_cells>set_well_tap_mode -insert_cells {{TAPCELL1 rule $distance1} {TAPCELL2 rule $distance2}} >addWellTap -cell $tap_cells -cellInterval [expr 4*$welltap_D] -checkerBoard -incr *
4,什么是Latch Up?
Latch Up是CMOS電路的一種寄生現(xiàn)象,當Latch Up發(fā)生時,會形成一條從VDD到GND的低電阻通路,這會產(chǎn)生大電流,對芯片造成不可逆的損傷。 在CMOS電路中,兩個寄生的BJT結(jié)構(PNPN)會形成Latch Up結(jié)構。 PNPN寄生結(jié)構一般是不會處于開啟狀態(tài),但當其達到觸發(fā)條件時,會瞬態(tài)開啟并形成一個從VDD到VSS的低電阻通路,且該狀態(tài)發(fā)生后便不再需要觸發(fā)條件也可以自己維持觸發(fā)狀態(tài)。
下圖是常見的CMOS電路中PNPN結(jié)構寄生產(chǎn)生的Latch Up結(jié)構,為避免觸發(fā)該Latch Up 結(jié)構,需要將寄生的兩個PN二極管反偏防止其導通,即p-substrate連接GND,nWell連接VSS(這也是tap cell的作用)。
5,如何預防Latch Up的發(fā)生? Guard rings
provides a better way to collect the minority carriers.Guard ring consist of a P+ ring on Psubstrate and N+ ring on Nwell all around the nMOS and pMOS.These rings contains as many as contacts as per design rules.N+ rings connected to VDD and P+ ring connected to GND.There rings will collect the minority carriers and avoid the development of potential difference between body and source which activate the BJTsWell isolatioon by trench Isolate the NMOS and PMOS using oxide trench and Buried oxide
Epitaxial layer This is called P on P+. provides a low impedance path for minority carriers.
Retrograde well doping In retrograde well doping, peak concentration is in deep inside the nWell, no in surface.
Combination of epitaxial layer and retrograde well doping ESD circuit SOI technology No Latch Up Issue. Less parasitic capacitance, leads to less leakage current. Self heating problem.
6,參考資料
https://www./channel/UCVWaC1gXZfHNqwdl6jovsjQ
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