运算放大器：静电放电-ESD

Hello,andwelcometothevideofortheTIPrecisionLabdiscussingelectrostaticdischarge,orESD.Inthisvideowe’llexplainhowESDcandamagesemiconductorcomponents.WewillalsogivedetailoninternalESDprotectioncircuitsthatareincludedinsemiconductordevices.Finally,wewillexplainhowESDperformanceischaracterizedsothattherobustnessofthedeviceisunderstood.大家好，欢迎来到TIPrecisionLabs（德州仪器高精度实验室），讨论关于electrostaticdischarge（ESD）静电释放的话题。本节视频将介绍ESD是如何损坏半导体元器件的，并且详细介绍半导体器件中的ESD保护电路的设计。最后，我们将介绍ESD性能指标是如何测定的，以帮助更好地理解器件的ESD性能。Manycommonitemscandevelopastaticelectricalcharge.Insulatorsareespeciallypronetodevelopinglargestaticcharges.Somematerialstendtobecomepositivelychargedandothersbecomenegativelycharged.Whenevertwoitemswithachargeimbalancearebroughtincloseproximity,youcanhaveasuddenflowofelectricitybetweenthetwoobjects,calledanelectrostaticdischarge,ESD.ESDcanshowupasavisiblesparkandoftenhasvoltagelevelsinthethousandsofvolts.Fromasemiconductorperspective,ESDisthemostcommonwaythatdevicesaredamaged.很多常见的物体会积累静电。特别是绝缘体很容易积累大量的静电。一些物体倾向于带正电，另外一些物体倾向于带负电。当两个电荷不同的物体靠到一起的时候，两个物体之间会产生一个瞬时电流，称为ESD（静电释放）。ESD电压经常能达到几千伏，产生可见的电火花。对于半导体器件来说，ESD是最常见的损坏原因。Thetableatthetopofthisslidelistssomematerialsthatgeneratesignificantstaticcharges.Alsonoticethatthestaticchargeisdependentontherelativehumidity.Lowerrelativehumiditygeneratesmorestaticcharge.ThetableatthebottomoftheslideshowsESDdischargevoltagesvs.humanawareness.NoticethatwhenyoucanseeanESD“spark”thevoltageisverylarge,at8000Vormore.上面的这个表格罗列了一些生活中常见动作所产生的静电电压。注意到，静电电压和相对湿度有关。相对湿度低的时候，产生的静电电压更高。下面的表格展示了人体对静电放电的感受程度。注意到，当你看到电火花的时候，静电电压已经很高，在8kV及以上。DifferentdevicetypesaresusceptibletodifferentlevelsofESDvoltage.Somedevices,suchasMOSFETs,aresusceptibleatverylowvoltages.WewillseelaterthatESDprotectioncircuitsareusedinsidemostsemiconductordevicestoenhanceESDrobustness.TheESDrobustnessofaMOSFETcanbeimprovedfromonlytenvoltstothousandsofvoltswithadditionalESDprotectioncircuits.不同类型的器件对ESD的耐压值也不一样。一些器件，比如MOS管，在很低的电压时就会损坏。后面我们将会看到，大部分的芯片中采用的ESD保护电路能够提升器件的ESD性能。在添加了ESD保护电路以后，MOS器件的ESD耐压值可以从10V提升到几千伏。Let’slookmorecloselyatthetypicalwayinwhichsemiconductordevicesaredamagedbyESD.ConsideralargeESDpotential,orvoltage,whichisappliedbetweentheinvertinginputandnegativesupplypinofanopamp.ThisplacesalargevoltagefromthegatetothesourceofoneoftheinputMOSFETs,whichcancausedamagetothedevice.KeepinmindthatthethicknessoftheMOSFETgateoxidemayonlybeafewnanometers,makingitverysusceptibletothiskindofdamage.我们来详细的观察一下，ESD是怎样损坏半导体器件的。假如，当一个幅值很高的静电或者电压加到运放的反相端和负电源端；这就会在输入MOS管的栅极和源极之间产生一个很高的电压，这个电压会击穿栅氧化层，损坏器件。值得注意的是，MOS管栅极氧化层的厚度通常只有几个nm，在这种电压下非常容易损坏。TherearetwomaincategoriesofESDeventsforsemiconductordevices.An“outofcircuit”ESDeventcanoccurtoaloosedevice;thatis,adevicebeforeitissolderedtoaprintedcircuitboard.Outofcircuiteventscanhappenduringmanufacturing,factorytest,ortheassemblyprocess.Ingeneral,factorytestandmanufacturingprocessesaredesignedtominimizeexposuretoESD.An“in-circuit”ESDeventreferstodamagecausedonafullyassembledPCB,orendproduct.Inthiscasetheproductpackaging,productdesign,andthedevice’sownrobustnessdeterminetheESDsusceptibilityoftheproduct.TheESDprotectioncircuitsthatareincludedinthedeviceareintendedtoprotectagainstout-of-circuitevents.针对ESD事件，有两种主要的分类方法。在电路组装前，即芯片未焊接到PCB上之前，ESD会造成器件损坏。这些可能发生在生产，工厂测试和组装的过程中。一般来说，工厂测试和生产工艺在设计的时候，就考虑到最小化ESD的发生。电路组装后的ESD指的是在一个组装好的PCB或者终端产品上，发生的ESD事件。这种情况下，产品的包装，设计以及器件本身的抗ESD性能，决定了产品的ESD特性。器件内部的ESD保护电路是为了防止器件在电路组装前的ESD损坏。TherearesomegeneralprecautionsthatcanbefollowedtominimizeESDdamagetodevicesandassembledproducts.Ingeneral,theseprecautionsuseresistivematerialstodissipatecharge.Forexample,theanti-staticwriststrapshowninthepictureallowsacontrolleddischargeofstaticelectricitytoground.Typicallytheimpedanceofantistaticmaterialsisinthemegaohms.Notethattheanti-staticbag,box,andworksurfacealsocontainresistivematerialsthatareintendedtoslowlyneutralizecharge.我们可以采取一些基本的预防措施来最小化ESD对器件和产品的损坏。一般来说，这些预防措施采用电阻性材料来释放静电。比如，图中所示的抗静电手环能够让人体上的静电，可控地释放到大地上。这些抗静电材料的阻值一般在兆欧级别。值得注意的是，图中所示的抗静电包裝，抗静电盒以及抗静电工作平台，同样都包含了电阻性材料，能够缓慢地释放静电。DuringtheinitialdevelopmentofasemiconductorproducttheESDrobustnessischaracterized.First,astatisticallysignificantsampleofdevicesisfullytestedusingautomatictestequipment.Generally,mostofthedatasheetparametersaremeasuredduringthisautomatedtest.Second,aspecializedESDtestsystemisusedtoapplyasimulatedESDpulsetothedevice.ThespecificcharacteristicsoftheESDpulsearecontrolledbythetesthardware.Forexample,theamplitudeandESDmodelcanbeselected.VariousESDmodelswillbediscussedlater,butinshorttheESDmodelsetsthecapacitance,inductanceandchargeoftheESDpulsetosimulatearealworldESDevent.DuringthetesttheESDpulseisappliedtomanydifferentcombinationsofdevicepinsforasignificantsampleofdevices.Also,differentlevelsofESDpulsesareappliedtothedevice;forexample,1kV,2kV,3kV,andsoon.Finally,thecomprehensivetestisrepeatedonthedevices.TheESDratingofthedeviceisdeterminedbythehighestESDlevelthatalldevicesinthesamplepass.ThisESDratingislistedintheabsolutemaximumratingtableofthedatasheet.在半导体器件最初的生产过程中，芯片的ESD防护性能（参数）就已经确定