MEMS技术第六讲噪声与器件的性能.ppt

MEMS和微系统设计 课程的基本目的 掌握MEMS设计的基本过程掌握mems机电及其耦合分析的基本理论掌握机电结构微器件的性能分析方法了解热 流体的基本理论了解MEMS的发展前沿知识培养对MEMS的兴趣 课程内容 MEMS概述及MEMS设计的概述工艺简要回顾系统设计 工艺设计及版图设计主要的机械 电子元件及其设计基础多域耦合设计 以机电耦合为例子器件性能的估计简单的其他域的元件及其简要设计要点设计实例 本章提纲 噪声处理的基本知识和类型器件的分辨率系统的分辨率 Noise Noise RandomfluctuationofagivenparameterI t Inaddition anoisewaveformhasazeroaveragevalue Wecan thandlenoiseatinstantaneoustimesButwecanhandlesomeoftheaveragedeffectsofrandomfluctuationsbygivingnoiseapowerspectraldensityrepresentationThus representnoisebyitsmean squarevalue NoiseSpectralDensity Wecanplotthespectraldensityofthismean squarevalue CircuitNoiseCalculations Deterministic Random HandlingNoiseDeterministically Candothisfornoiseinatinybandwidth e g 1Hz Canapproximatethisbyasimusoidalvoltagegenerator especiallyforsmallB say1Hz Thisisactuallytheprinciplebywhichoscillatorswork oscillatorsarejustnoisegoingthroughatinybandwidthfilter Why Neithertheamplitudenorthephaseofasignalcanchangeappreciablywithinatimeperiod1 B SystematicNoisecalculationprocedure GeneralCircuitwithSeveralNoiseSource Assumenoisesourcesareuncorrelated1 Forreplacew adeterministicsourceofvalue SystematicNoiseCalculationProcedure 2 Calculate treatingitlikeadeterministicsignal 3 Determine4 Repeatforeachnoisesource 5 Addnoisepower meansquarevalues TotalRMSvalue 噪声源 干扰噪声 理论可消除 隔离和共地 机械噪声 thermalnoise browniannoise 电噪声thermalnoise johnsonnoise shotnoise 散粒噪声 1 fnoise Mechanical Brownian Noise ImpingingmoleculesgiverisetoaBrowniannoiseforce NoiseforceappliedtoM k bsystemresultsinrandomBrownianmotionwithafrequency depenndentpowerspectrum Implications f f1 ElectronicNoiseSources Thermalnoiseinresistorsgeneratedbyrandommotionofelectronsorholeswhitespectraldensity upto10THz spectraldensity Example R 1k T 300K ThermalNoise cont Thermalnoisecurrent findNortonequivalent Example R 1k T 300K BW 1MHz ResistorNoiseinMEMS InterconnectresistanceRinttocapacitivepositionsensors routinginpolySi0canleadtoahighresistances duetorelativelyhighsheetresistanceofthislayer inertialMEMSoftenhavecompliantsuspensions largenumberofsquaresinpolySi1andsignificantcontributiontoRint Flicker 1 f Noise NoisemechanismrequireaaDCcurrent incontrasttothermalnoiseOriginof1 fnoiseinMOSFETs surfacestatesNearDC thenoisecurrentdiverges MOSFETNoisesources Whenbiasedinsaturation VDS VDS sat thenoisecanberepresentedbyaninputnoisevoltageandaninputnoisecurrent Originof1 fnoiseinMOSFETs surfacestates channelresistanceinsaturation inversedependenceongatearea largetransistorshavelower1 fnoise neglectingDCgatecurrentandit sshotnoise MOSFETNoisesource cont EquivalentMOSFETsmall signalmodelwithinput referrednoisesourcesCrossoverfrequencybetweenthermalandflickernoisecanrangefrom1kHzto1MHz SNRandDR Defined Signal to noiseratio SNRNote1 PnoiseiscalculatedoveralimitedbandwidthNote2 Vn rmsistakenastheminimumdetectablesignal intheabsenceofspecialcodingorsignalprocessingDynamicrange DR SignalandNoisewaveforms Sinusoidwithamplitudenormalizedto1 Gaussiannoisewithrmslevelnormalizedto1 notethatpeak peaknoiselevelisoccasionallyashighas6 本章提纲 噪声处理的基本知识和类型器件的分辨率系统的分辨率 对简单模型加速度计噪声的估计 Where Vout n Vs n Ve1 n A1 Ve2 n A2Vout n是电路最终输出的噪声 Vs n是sensor noise Ve1 n Ve2 n是一级放大器和二级放大器的电噪声 A1和A2是一级和二级放大器的放大倍数 按照等效噪声的功率谱计算办法 这些信号将成平方关系 这也解释了为什么放大器一般采用二级放大 而且第一级一般采用低噪声低倍数放大的缘故 上式还可以看出传感器的灵敏度是可以放大的 但分辨率和信躁比是不可以放大的 例子 Resolution noise equivalentoutput sensitivity unitisthesameasinput amplifywillnotbeusefulfortheresolution butforsensitivity X A1 Ynoise equitvalent A2 Yele equitvalent Y Xisinput Yisoutput A1issensitivityofsensor A2iselectricalamplifier Ynoise equitvalentisnoiseequitavalentY Yele equitvalentiselectronic noiseequitavalent thewholesensitivityofsystemisY X A1 A2 resolutionofthesystemis A2 Ynoise equitvalent Ynoise equitvalent Y X 由于原子的热运动 1 2KT的随机能量噪声在任意地方任何静止时刻都是存在的 可以用能量的观点来进行静态估计 由于电子的热运动形成了电流 电流随机存在 均值为零 动态随机存在 功率随温度可变 从频率域看来 其功率在频率上保持4KT 只对电阻有效 类似的 对应于机械系统 4KT只对应阻尼有效 对于二阶机械系统 4KT df c 此处 df是带宽 c是阻尼 不是阻尼比什么的 即F c v 就是系统的扰动功率 即1 2m v v t 也可写作F v 此处 F是扰动力 v是速度 其大小可以用F c算出来 相当于二阶电子系统的V V R F相当于R 11233 Areviewtolastlesson usingpiezoresistorasexampleCapacitorsensor sameStonebridgeasusedinpiezoresistor inputaVininputpositivecurveandanegativecurvevoltage V V aexampleisgiventocaculatesensitivity noise resolutionanasys withconsiderationwithamplifycircuit noisecamefromthedistributecapacitancewhichinfluencecapacitormuchmorethanpiezoresistor112334 capacitorsensor acalculationtohowf andresolutionofaaccerameteraredesignedduetonoise 1kHzfrequencyrequirementandnoiseexistingaretranferedintoaresolutionof0 1mg distributedcapacitorandamplifiercircuitnoise andhowtogetridofthosenoises Example GyroMDSCalculation 根据下图 画出压阻加速度计的噪声模型 途中的Cp需要改成R和Cp的并联 计算方式可参考陀螺的计算过程 ThegyrosensepresentsalargeeffectivesourceimpedanceCurrentsaretheimprtantvariable voltagesare opend outMustcomparei0withthetotalcurrentnoiseieqTOTgoingintotheamplifiercircuit B ThesimplePressuresnesorInthesimplepressuresnesor themovingmassisdirectlyexposedtotheincidentacousticpressureandthedisplacementofthismassismeasured SuchasensorisshownschematicallyinFig 3 ForanarbitraryforceF thedisplacementresponseZis IEEETRANSACTIONSONELECTRONDEVICES VOL 40 NO 5 MAY1993Mechanical ThermalNoiseinMicromachinedAcousticandVibrationSensors Forsignal theforceF psS whereSistheareaofthetransducerfaceandpsisthespectraldensityofthesignalpressure fornoise theforce sothesignal to noiseratiois 例子 压力 电容的例子 以差压 电压 电源供电来检查电容的变化 需要后接放大电路 放大电路的输入电容Cp对被测电容的影响较大 Cp通常比参考电容C0要大的多 而且传感器的输入电容还包括 引线电容 10pf m 解决问题的办法包括 第一个放大器是1 1的放大器 放大器的输出反馈到放大器输入电容的一端 输入电容的另一端接放大器的输入 这样输入电容处于V 0的状态 本章提纲 噪声处理的基本知识和类型器件的分辨率系统的分辨率 IEEESENSORSJOURNAL VOL 5 NO 3 JUNE2005Mechanical ThermalNoiseinMEMSGyroscopes VibratoryGyroscopes GeneratemomentumbyvibratingstructuresAgain conservationofmomentumleadstomechanismsformeasuringrotationrateandorentationExample vibratingmassinarotatingframe BasicVibratoryGyroscopeOperation PrincipleofOperationDrive SenseReponsespectra TuningForkGyroscope MEMS BasedTuningForkGyroscope In planedriveandsensemodespickupz axisrotationsMode matchingformaxmumoutputsensitivityFrom Zaman Ayazi etal MEMS 06 MEMS BasedTuningForkGyroscope DriveAxisEquivalentCircuit GeneratesdrivedisplacementvelocityxdtowhichtheCoriolisforceisproportional ToSenseAmplifer forsynchronization Drive to SenseTransferFunction GyroRedoutEquivalentCircuit forasingletine GyroSenseElementOutputCircuit GyroSenseElementOutputCircuit Easiesttoanalyzeifallnoisesourcearesummedatcommonnode GyroRedoutEquivalentCircuit fo