图像信息原理教学课件电荷耦合器件v2.ppt

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1、电荷耦合器件,(Charge Coupled Device,CCD),固体成像器件,电荷耦合器件CCD(Charge Coupled Device,CCD),互补金属氧化物半导体 CMOS图像传感器（Complementary Metal Oxide Semiconductor,CMOS）,电荷耦合器件CCD,线阵CCD,面阵CCD,表面沟道CCD(SCCD),电荷包存储在半导体与绝缘体之间的界面，并沿界面转移。,体沟道CCD(BCCD),电荷包存储在离半导体表面一定深度的体内，并在半导体内沿一定方向转移,CCD类型,1.电荷耦合器件的工作原理,1.1、CCD工作原理,CCD,光信息,电脉冲,

2、脉冲只反映一个光敏元的受光情况,脉冲幅度的高低反映该光敏元受光照的强弱,输出脉冲的顺序可以反映一个光敏元的位置,完成图像传感,特点：以电荷作为信号,基本功能：电荷的存贮和转移,CCD基本工作原理,信号电荷的产生,信号电荷的存贮,信号电荷的传输,信号电荷的检测,1.1.1 电荷存贮,CCD 是由规则排列的金属氧化物半导体（Metal Oxide Semiconductor,MOS）电容阵列组成。,Metal,Oxide,Semiconductor,如果有光入射到半导体硅片上，在光子的作用下，半导体硅产生电子空穴对，由此产生的光电子被表面的势阱所吸收。而空穴被电场排斥出耗尽区。,当在金属电极上加正

3、电压VG时，在电场的作用下，电极下型区域里的多数载流子空穴被排斥、驱赶，形成了一个耗尽区。,而对于少数载流子电子，电场则吸引它到电极下的耗尽区。耗尽区对于带负电的电子来讲是一个势能很低的区域称为“势阱”。,势阱积累电子的容量取决于势阱的“深度”，而表面势的大小近似与栅压VG成正比。,势阱内吸收的光电子数量与入射光势阱附近的光强成正比。这样一个MOS结构单元就称光敏单元或一个象素；而将一个势阱所吸收集的若干个光生电荷称为一个电荷包。,通常在半导体硅片上制有成千上万个相互独立的MOS光敏单元，如果在金属电极上加上正电压，则在半导体硅片上就形成成千上万的个相互独立的势阱。如果此时照射在这些光敏单元上

4、是一副明暗起伏的图像，那么这些光敏元就会产生出一幅与光照强度相对应的光电荷图像，因而得到影像信号。,1.1.2.电荷耦合,CCD器件每一单元（每一像素）称为一位，有256位、1024位、2160位等线阵CCD。CCD一位中含的MOS电容个数即为CCD的相数，通常有二相、三相、四相等几种结构，它们施加的时钟脉冲也分为二相、三相、四相。二相脉冲的两路脉冲相位相差1800；三相及四相脉冲的相位差分别为1200、900。当这种时序脉冲加到CCD驱动电路上循环时，将实现信号电荷的定向转移及耦合。,图所示TCD1206的相邻两像元，每一位含MOS电容2个,取表面势增加的方向向下，工作过程如图所示：,at=

5、t1时，1电极处于高电平，而2电极处于低电平。由于1电极上栅压大于开启电压，故在1下形成势阱，假设此时光敏二极管接收光照，它每一位（每一像元）的电荷都从对应的1电极下放入势阱。,b t=t2时，1电极上栅压小于2电极上栅压，故1电极下势阱变浅，势阱变深，电荷更多流向2电极下。（由于势阱的不对称性，“左浅右深”，电荷只能朝右转移,c t=t3时，2电极处于高电平，而1电极处于低电平，故电荷聚集到2电极下，实现了电荷从1电极下到2电极下的转移。,d 同理可知，t=t4时，电荷包从上一位的1电极下转移到下一位的1电极下。因此，时钟脉冲经过一个周期，电荷包在CCD上移动一位。,1.1.3 电荷注入,光

6、注入方式 当光照射到CCD硅片上时，在栅极附近的体内产生电子空穴对，其多数载流子被栅极电压排开，少数载流子则被其收集到势阱中形成信号电荷。,在CCD中，电荷注入分为两类：光注入和电注入。,N为入射光的光子流速率；A为光敏单元的受光面积，t为光注入时间,1.1.4 电荷检测,CCD输出结构的作用是将CCD中信号电荷变为电流或电压输出，以检测信号电荷的大小。,浮置扩散放大器属于电压输出方式，目前采用较多。其基本结构和工作原理如下：,如图所示，给出了CCD的电压输出电路。,放大管T1,复位管T2,输出二极管T3,此电荷积分器随T2管的开与关，处于选通和关闭状态，称为选通电荷积分器,放大管T1是源跟随

7、器,复位管T2工作在开关状态,输出二极管T3始终处于强反偏状态,A点的等效电容C由T3管的结电容加上T1管的栅电容构成，它构成一个电荷积分器,如图所示为电压输出工作波形图。,CCD电压输出工作原理为：在每个时钟脉冲周期内，随着时钟脉冲1或2的下降过程，就有一个电荷包从CCD转移到输出二极管T3的N区，即转移到电荷积分器上，引起A点电位变化为：,由于MOS管T1的电压增益为,式中gm为电导，RL为负载电阻，故T1管源极输出电压变化为：,对Vout进行读出，然后T2管栅极RG在复位脉冲R的作用下导通，将电荷包Q通过T2管的沟道抽走，使A点电位重新置在VRD值，为下一次Vout读出作准备。,因为是N

8、沟道，信息电荷为电子，故加负号,当R结束，T2管关闭后，由于T1管处于A点的VRD电位的强反偏状态，此积分器无放电回路，所以A点电位一直维持在VRD值，直到下一个时钟脉冲信号电荷到来为止。,CCD Analogy,A common analogy for the operation of a CCD is as follows:An number of buckets(Pixels)are distributed across a field(Focal Plane of a telescope)in a square array.The buckets are placed on top o

9、f a series of parallel conveyor belts and collect rain fall(Photons)across the field.The conveyor belts are initially stationary,while the rain lowly fills the buckets(During the course of the exposure).Once the rain stops(The camera shutter closes)the conveyor belts start turning and transfer the b

10、uckets of rain,one by one,to a measuring cylinder(Electronic Amplifier)at the corner of the field(at the corner of the CCD)The animation in the following slides demonstrates how the conveyor belts work.,RAIN(PHOTONS),BUCKETS(PIXELS),VERTICALCONVEYORBELTS(CCD COLUMNS),HORIZONTALCONVEYOR BELT(SERIAL R

11、EGISTER),MEASURING CYLINDER(OUTPUT AMPLIFIER),CCD Analogy,Exposure finished,buckets now contain samples of rain.,Conveyor belt starts turning and transfers buckets.Rain collected on the vertical conveyoris tipped into buckets on the horizontal conveyor.,Vertical conveyor stops.Horizontal conveyor st

12、arts up and tips each bucket in turn intothe measuring cylinder.,After each bucket has been measured,the measuring cylinderis emptied,ready for the next bucket load.,A new set of empty buckets is set up on the horizontal conveyor and the process is repeated.,Eventually all the buckets have been meas

13、ured,the CCD has been read out.,Structure of a CCD 1.,The image area of the CCD is positioned at the focal plane of the telescope.An image then builds up that consists of a pattern of electric charge.At the end of the exposure this pattern is then transferred,pixel at a time,by way of the serial reg

14、ister to the on-chip amplifier.Electrical connections are made to the outside world via a series of bond pads and thin gold wires positioned around the chip periphery.,Connection pins Gold bond wires Bond pads Silicon chip,Metal,ceramic or plastic package,Image area,Serial register,On-chip amplifier

15、,Structure of a CCD 2.,CCDs are are manufactured on silicon wafers using the same photo-lithographic techniques used to manufacture computer chips.Scientific CCDs are very big,only a few can be fitted onto a wafer.This is one reason that they are so costly.The photo below shows a silicon wafer with

16、three large CCDs and assorted smaller devices.A CCD has been produced by Philips that fills an entire 6 inch wafer!It is the worlds largest integrated circuit.,Don Groom LBNL,Structure of a CCD 3.,One pixel,Channel stops to define the columns of the image,Transparenthorizontal electrodesto define th

17、e pixels vertically.Also used to transfer the charge during readout,Plan View,Cross section,The diagram shows a small section(a few pixels)of the image area of a CCD.This pattern is reapeated.,ElectrodeInsulating oxiden-type siliconp-type silicon,Every third electrode is connected together.Bus wires

18、 running down the edge of the chip make the connection.The channel stops are formed from high concentrations of Boron in the silicon.,Structure of a CCD 4.,On-chip amplifierat end of the serial register,Cross section ofserial register,Image Area,Serial Register,Once again every third electrode is in

19、 the serial register connected together.,Below the image area(the area containing the horizontal electrodes)is the Serial register.This also consists of a group of small surface electrodes.There are three electrodes for every column of the image area,Structure of a CCD 5.,The serial register is bent

20、 double to move the output amplifier away from the edgeof the chip.This useful if the CCD is to be used as part of a mosaic.The arrows indicate how charge is transferred through the device.,Edge of Silicon,160mm,Image Area,Serial Register,Read Out Amplifier,Bus wires,Photomicrograph of a corner of a

21、n EEV CCD.,Structure of a CCD 6.,OD,OS,RD,SW,Output Node,Substrate,Output Transistor,Reset Transistor,SummingWell,20mm,Output Drain(OD),Output Source(OS),Gate of Output Transistor,Output Node,Reset Drain(RD),Summing Well(SW),Last few electrodes in Serial Register,Serial Register Electrodes,Photomicr

22、ograph of the on-chip amplifier of a Tektronix CCD and its circuit diagram.,Electric Field in a CCD 1.,The n-type layer contains an excess of electrons that diffuse into the p-layer.The p-layer contains an excess of holes that diffuse into the n-layer.This structure is identical to that of a diode j

23、unction.The diffusion creates a charge imbalance and induces an internal electric field.The electric potential reaches a maximum just inside the n-layer,and it is here that any photo-generated electrons will collect.All science CCDs have this junction structure,known as a Buried Channel.It has the a

24、dvantage of keeping the photo-electrons confined away from the surface of the CCD where they could become trapped.It also reduces the amount of thermally generated noise(dark current).,Electric potential,Potential along this line shownin graph above.,Electric potential,Cross section through the thic

25、kness of the CCD,Electric Field in a CCD 2.,During integration of the image,one of the electrodes in each pixel is held at a positive potential.This further increases the potential in the silicon below that electrode and it is here that the photoelectrons are accumulated.The neighboring electrodes,w

26、ith their lower potentials,act as potential barriers that definethe vertical boundaries of the pixel.The horizontal boundaries are defined by the channel stops.,np,Electric potential,Region of maximum potential,pixel boundary,Charge packet,SiO2 Insulating layer,Electrode Structure,pixel boundary,inc

27、omingphotons,Charge Collection in a CCD.,Photons entering the CCD create electron-hole pairs.The electrons are then attracted towards the most positive potential in the device where they create charge packets.Each packet corresponds to one pixel,Charge Transfer in a CCD 1.,In the following few slide

28、s,the implementation of the conveyor belts as actual electronicstructures is explained.The charge is moved along these conveyor belts by modulating the voltages on the electrodespositioned on the surface of the CCD.In the following illustrations,electrodes colour coded redare held at a positive pote

29、ntial,those coloured black are held at a negative potential.,+5V0V-5V,+5V0V-5V,+5V0V-5V,Time-slice shown in diagram,Charge Transfer in a CCD 2.,+5V0V-5V,+5V0V-5V,+5V0V-5V,Charge Transfer in a CCD 3.,+5V0V-5V,+5V0V-5V,+5V0V-5V,Charge Transfer in a CCD 4.,+5V0V-5V,+5V0V-5V,+5V0V-5V,Charge Transfer in

30、a CCD 5.,+5V0V-5V,+5V0V-5V,+5V0V-5V,Charge Transfer in a CCD 6.,+5V0V-5V,+5V0V-5V,+5V0V-5V,Charge Transfer in a CCD 7.,Charge packet from subsequent pixel entersfrom left as first pixel exits to the right.,+5V0V-5V,+5V0V-5V,+5V0V-5V,Charge Transfer in a CCD 8.,On-Chip Amplifier 1.,OD,OS,RD,SW,Output

31、 Node,Output Transistor,Reset Transistor,SummingWell,+5V0V-5V,+10V0V,SW,-end of serial register,Vout,Vout,The on-chip amplifier measures each charge packet as it pops out the end of the serial register.,The measurement process begins with a resetof the reset node.This removes the charge remaining fr

32、om the previous pixel.The resetnode is in fact a tiny capacitance(0.1pF),RD and OD are held at constant voltages,(The graphs above show the signal waveforms),On-Chip Amplifier 2.,OD,OS,RD,SW,Output Node,Output Transistor,Reset Transistor,SummingWell,+5V0V-5V,+10V0V,SW,-end of serial register,Vout,Vo

33、ut,The charge is then transferred onto the Summing Well.Vout is now at the Reference level,There is now a wait of up to a few tens of microseconds while external circuitry measuresthis reference level.,On-Chip Amplifier 3.,OD,OS,RD,SW,Output Node,Output Transistor,Reset Transistor,SummingWell,+5V0V-

34、5V,+10V0V,SW,-end of serial register,Vout,Vout,This action is known as the charge dumpThe voltage step in Vout is as much as several mV for each electron contained in the charge packet.,The charge is then transferred onto the output node.Vout now steps down to the Signal level,On-Chip Amplifier 4.,O

35、D,OS,RD,SW,Output Node,Output Transistor,Reset Transistor,SummingWell,+5V0V-5V,+10V0V,SW,-end of serial register,Vout,Vout,Vout is now sampled by external circuitry for up to a few tens of microseconds.,The sample level-reference level will be proportional to the size of the input charge packet.,2、电

36、荷耦合器件的特性参数,2.1 电荷转移效率和转移损失率,N个电极转移后所剩余的电量为,2.2 驱动频率,驱动频率的下限,在信号电荷的转移过程中，为了避免由于热激发少数载流子而对注入信号电荷的干扰，注入电荷从一个电极转移到另一个电极所用时间t必须小于少数载流子的平均寿命，对于二相来讲，周期为T,载流子的平均寿命与器件的工作温度有关，工作温度越高，热激发的少数载流子平均寿命越短，驱动频率的下限越高,驱动频率的上限,驱动频率升高时，驱动脉冲驱使电荷从一个电极转移到另一个电极的时间t应大于从一个电极转移到另一个电极的固有时间，才能保证电荷的完全转移，否则信号电荷跟不上驱动脉冲的变化，将会使转移效率大大

37、下降。,电荷转移的快慢与载流子的迁移率、电极长度、衬底杂质的浓度和温度等因素有关。,3 电荷耦合摄像器件,3.1、CCD摄像原理,CCD摄像器件可分为线列和面阵两大类。以线列CCD为例说明摄像原理。,CCD线列图象器件由光敏区、转移栅、CCD移位寄存器、电荷注入、信号读出电路等几个部分组成。如图所示是一个N个光敏元的线列CCD,移位转移栅(遮光),光敏单元,转移栅,入射光,CCD摄像过程可归纳为如图所示的五个环节,工作波形如图所示。,2）转移 就是将N个光信号电荷包并行转移到所对应的各位CCD中，t处于高电平。,3）传输 N个信号电荷在二相脉冲1、2驱动下依次沿CCD串行输出。,4）计数 计数

38、器用来记录驱动周期的个数。通常计数器预置值定为N+m,m为过驱动次数。,1）积分 在有效积分时间里，光栅P处于高电平，每个光敏元下形成势阱，光生电子被积累到势阱中，形成一个电信号“图象”。,各个环节分述如下：,3.2、几种常用的CCD驱动方法,CCD驱动时序产生方法多种多样，常用的方法有：,这种方法用数字门电路及时序电路搭成CCD驱动时序电路。一般由振荡器、单稳态触发器、计数器等组成。可用标准逻辑器件搭成或可编程逻辑器件制成。特点是驱动频率高，但逻辑设计比较复杂。,a 直接数字电路驱动方法,b.可编程逻辑器件CPLD,驱动频率和积分时间可以调节,c 单片机驱动方法,单片机产生CCD驱动时序的方

39、法，主要依靠程序编制，直接由单片机I/O口输出驱动时序信号。时序信号是由程序指令间的延时产生。这种方法的特点是调节时序灵活方便、编程简单，但通常具有驱动频率低的缺点。如果使用指令周期很短单片机（高速单片机），则可以克服这一缺点。,在EPROM中事先存放驱动CCD的所有时序信号数据，并由计数电路产生EPROM的地址使之输出相应的驱动时序。这种方法结构简明，与单片机驱动方法相似。,d EPROM驱动方法,e 专用IC驱动方法,利用专用集成电路产生CCD驱动时序，集成度高、功能强、使用方便。在大批量生产中，驱动摄像机等视频领域首选此法，但在工业测量中又显得灵活性不好。可用可编程逻辑器件法代替“专用I

40、C驱动方法”。,3.3、CCD图象传感器的应用,CCD的七个应用领域,).小型化黑白、彩色TV摄像机(面阵CCD应用最广泛的领域。),2).传真通讯系统,用10242048像元的线阵CCD作传真机，可在不到一秒钟内完成A4开稿件的扫描。,3).光学字符识别,代替人眼，把字符变成电信号，进行数字化，然后用计算机识别。,4).广播TV,5).工业检测与自动控制,这是CCD应用量很大的一个领域，统称机器视觉应用。,a.在钢铁、木材、纺织、粮食、医药、机械等领域作零件尺寸的动态检测，产品质量、包装、形状识别、表面缺陷或粗糙度检测。b.在自动控制方面，主要作计算机获取被控信息的手段。c.还可作机器人视觉

41、传感器。,6).可用于各种标本分析（如血细胞分析仪），眼球运动检测，X射线摄像，胃镜、肠镜摄像等。,a.天文摄像观测b.从卫星遥感地面 如：美国用5个2048位CCD拼接成10240位长取代125mm宽侦察胶卷，作地球卫星传感器。c.航空遥感、卫星侦察 如：1985年欧洲空间局首次在SPOT卫星上使用大型线阵CCD扫描，地面分辨率提高到10m。还在军事上应用：微光夜视、导弹制导、目标跟踪、军用图象通信等。,7).天文观测,、微小尺寸的检测（10500um）,用衍射的方法对细丝、狭缝、微小位移、微小孔等进行测量。,根据夫琅和费衍射公式:,可以得到：,例：尺寸测量,d细丝直径；K衍射暗纹级次K=1

42、,2,3；激光波长；,L被测细丝到CCD光敏面的距离；,被测细丝到第K级暗纹的连线，与光线主轴的夹角；,xk第K级暗纹到光轴的距离。,3,当很小时（即L足够大时）Sintg=xk/L,代入,得到：,S暗纹宽度，S=XK/K是相等的，则测细丝直径d转化为用CCD测S,误差分析,由于激光波长误差很小可忽略不计，则,例：He-Ne激光632.8nm,L=1000mm0.5mm,d=500m,当CCD像元选用131m,测量误差：,丝越细，测量精度越高（d越小S越大），甚至可达到d=10-2m.,S的测量方法,图象传感器(IS)输出的视频信号经放大器A放大，再经峰值保持电路PH和采样保持电路S/H处理，变成箱形波，送到A/D转换器进行逐位A/D转换，最后读入计算机内进行数据处理。判断并确定两暗纹之间的像元数ns，则暗纹宽度S=nsp(p为图象传感器的像元中心距)，代入可以得到.,激光测微装置电路框图,.小尺寸测量,小尺寸的检测是指待测物体可与光电器件尺寸相比拟的场合。,式中:f_透镜焦距 a物距 b 像距 放大倍率 n 像元数 p像元间距,解释:由成像公式,作业：画出线阵CCD测量微小物体的原理图，并分析。,