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高性能的热释电红外传感器
该传感器测量物理量的传感作为热量再将温度信号转换为电信号的传感器称为热。这一类型的热传感器是热释电红外传感器。热传感器不受欢迎主要是因为他们缓慢的设备，和他们相比，光子探测器的灵敏度低。一个主要的优势，热探测器光子探测器，它们可以在室温下操作。这是一本研究的主要动机。本文的目的是设计一个非制冷红外热释电探测器，探测灵敏度高。高性能的热导率传感器降低到基板或热沉了。热释电被定义为与相应的温度变化的偏振态的变化。钛酸铅钙（PCT）是一种铁电钙钛矿材料。它具有很高的热释电系数，高介电常数，如果存放在一个适当的比例可以在6 / W x106v热导传感器和使用该设计的基板之间的范围内的一个非常高的热电响应率被发现是9.51x10-9 W / K为低，小于辐射热3.69x10-7 W / K电导检测器提出了这两种减振器的设计。设计的效率和红外传感器的顶表面的方向被发现两侧各有55°视野。用于制造设备的各种制备方法进行了详细讨论，最好的方法是在其他类型的比较提到。是详细讨论了减振器和检测机构的应用。
电磁辐射具有电气和磁性元件。电磁波是由运动的带电粒子产生。这些波被称为“电磁辐射”（EM）因为他们散发的能量以光子的形式。他们必须穿越空间的能力，通过空气和其它材料。电磁辐射是按波的无线电波，微波，红外线，可见光，紫外线辐射的波长为几种类型，X射线和伽马射线。光子是最基本的“单位”的所有形式的电磁辐射。电磁辐射具有能量和动量，可以赋予物质与它互动。
那是在绝对温度0 K的任何物体发出的红外辐射被认为。对测试有许多来源的红外辐射如红外灯泡，硅碳棒，电加热器，水银灯等目的，红外探测器一般大致可分为两类：光子探测器和热探测器。光子探测器的灵敏度是由半导体材料的能带隙的用来感测温度的变化确定。这些探测器需要外部冷却机制，保持高效运行的特定的温度的装置，大大增加了成本和复杂性的最终的红外传感器或影像系统。相比之下，热红外探测器通常在接近室温的温度通常不需要低温冷却。这使得红外热成像仪成本更低，各种军事和民用领域有吸引力。
辐射温度计是非接触式温度传感器测量从现场对被调查对象接受热电磁辐射量温度。辐射温度计有点和阵列器件。阵列式辐射温度计是用来绘制温度分布在给定的区域。由此产生的图像可以被看作是一个二维温度图下面积的检验。他们主要用在金属，半导体，塑料等制造过程，辐射温度计使自动化和反馈控制，为产品的热图像进行分析，对任何故障
和裂纹是从成型炉产品若发现故障，它将被从输送机。这些设备被用于消防人员在火提高自己的知名度。这种辐射温度计的室温附近是使用碲镉汞作为传感器材料的讨论。
热成像和人体检测使用辐射温度计测量在一个相对大面积多点的温度和显示热像图。在红外光谱的9-14米区域的热成像摄像机辐射检测。作为一个对象的对象增加，相应增加辐射温度。这种类型的热成像是用于军事和安全应用。最近，机场工作人员都在使用热成像检测猪流感疑似病例的世界。脑肿瘤的热分布和周围的大脑皮层可以用红外摄像机目前的技术映射。红外热像仪是用在许多行业在检查结构的保温结构的热泄漏。
三个主要因素影响红外探测器的选择，噪声特性，探测灵敏度和光谱响应。这些特点主要是由吸收机制，影响探测器材料，红外窗口，和斩波频率。热探测器相比，光子探测器具有宽的光谱响应，所以当选择一个光子探测器宜选择检测器具有很好的响应接近频带被使用。根据探测器的时间响应的应用也被认为是一个特定的红外探测器选择的重要因素。除了上述因素，鲁棒性强，制造成本，使用方便，包装也起辅助作用的红外探测器的选择。一个典型的探测器在室温工作会降低检测系统的成本作为一个整体，因为它避免了低温冷却系统的使用。
在光子探测器，光子（电磁波的基本部分）是由半导体材料吸收，从而产生自由电荷载体。这些自由电荷载体测量作为一个电流在二极管。这是工作机制的光子型红外探测。红外光子探测器的第一代大多用于扫描系统，第二代光子红外系统作为盯着系统，和IR系统的第三代集中在改善热分辨率，帧速率，和多色的能力。一个用于构建一个红外光子探测系统最重要的材料碲镉汞。碲镉汞是用来在大规模红外焦平面阵列的构建。（J. Bajaj，罗克韦尔科学中心，1999）制备混合红外焦平面阵列具有光谱范围从1µm碲镉汞已被视为检测红外辐射的材料之一，从1958。一种碲化汞镉的主要缺点是它的汞具有较高的蒸气压的制造。PbSnTe是材料的并行开发的HgCdTe的继任者但由于大量的硅和高介电常数使用热膨胀系数降低了。
测辐射热计工作在对应的电阻变化的温度变化的测量原理。电阻传感器放置在吸热材料，在一个孤立的表面。金属薄膜和热敏电阻（根据温度变化的电阻变化）测辐射热计已在使用。半导体测辐射热计的外部冷却的被用在他们的高灵敏度和探测空间应用。最近已经尝试用新材料的开发具有较大的温度系数的测辐射热计。一个很有前途的材料是半导体掺镧钛酸锶钡。液态氦冷却的辐射热测量仪仍然采用远红外光谱和天文的非制冷探测器的性能是不够的。构建这种类型的测辐射热计的最佳材料的选择是非常重要的。
在最近的过去锗测辐射热计的详细研究。这些测辐射热计的性能提高由于温度低于
4K。最简单的方法是泵氦降低操作温度。这样的操作温度1K和2K之间可以得到的，但（Drew和西弗斯，1969）描述了一个在约0.3K操作氦低温恒温器。微机械加工技术是采用从基板获得传感器的隔离。偏置电流是需要校准的设备和设置的电阻变化的测量参考点。高电阻温度系数（TCR）和一个小的1 / f噪声是理想的材料性能的一个完美的测辐射热计10。同时，它必须能够集成温度传感材料与信号读出电路（如CMOS晶片）的具有成本效益的方式。今天，最常见的测辐射热计温度传感材料是钒合金的氧化层，金属测辐射热计的多孔硅（Si）和陶瓷热。
相当数量的工作已经由帕特利1971热释电探测器的发展。人们已经注意到，在大多数情况下，从过去的几十年里，硫酸三甘氨酸（TGS）或其衍生物的热释电探测的最合适的材料。其他热释电材料如铌酸锶钡（SBN）（玻璃和艾布拉姆斯1971），硫酸锂和锆钛酸铅的家庭成员（马勒等人1972，双论等人1972）可能更适合某些特定的应用。材料性能的最佳选择了帕特利讨论（1970）。一个有效的热释电传感器材料的最相关的材料性能的热释电系数，介电常数和热容量。在一个理想的材料热释电系数要大，其介电常数小，热容量小的元件。TGS不具有已知的最大的热释电系数在室温下。更高的价值在BST和一些掺杂锆钛酸铅陶瓷的发现，但更大的热释电系数的材料具有更大的介电常数和损耗大的因素。在最佳的热释电探测器的主要噪声源是具有介电损耗相关的约翰逊噪声，其次是放大器的噪声。一个小的TGS检测器的能力可能会下降得如此之低，它可以成为小于放大器的输入容量。由这些材料制成的探测器比TGS更强大，虽然这种优势被夸大了，他们正在生产满足不需要最高性能的应用程序的简单廉价的探测器的要求。在TGS探测器性能的改进已经在TGS的质量改进带来的相关的11高输入阻抗低噪声放大器的性能。这些结果表明，热释电探测器非制冷红外探测器中最好的。利用热释电探测器这样的景点，它在室温下操作，具有非常高的电压响应率和探测率。 在最近的时代，Hybrid FPA的取代点探测器。随着MEMS制造技术，可以生产具有很好的减振器设计非制冷热释电红外探测器。的大小的装置减少已达到饱和点和三维堆叠的设备已经开始出现。读出热释电探测器中电子已经开始被集成在传感器使用焊料球。焊料球的高度提供了必要的隔离和传感器之间的基板，即使这些方法仍在研究；他们证明具有很高的灵敏度和响应红外探测器可能会在不久的将来，在微观尺度的制造。
在吸收器吸收器设计的蜘蛛网开始研究J. J块等人关于新型毫米波天文测辐射热计的设计。他们提出了一个蜘蛛网吸收塔的设计，取得了较好的吸收。后来F B liewiett等人。1999提出了红外和亚毫米蛛网测辐射热计低Tc超导转变边缘温度计的制备表征。此
后，许多涉及吸收器设计的形状像一个网状的有损线的设计已经在考虑为他们有很高的吸收率。m.j.m.e de倪维尔等人提出了一种氮化硅网络结构高温超导测辐射热计。这种装置在5.4x1010厘米√赫兹/ W许多这些结构顺序有很高的探测用红外检测测辐射热计。这方面可作为检测热释电传感器结合这些吸收剂可以在很高的探测率的使用。d.m.dennison l.n.hadley和提出了一种新的季度波吸收器的设计在其两部学报，1947。本设计共分三层。在上面的金属膜的阻抗与自由空间相匹配，使入射辐射的吸收和发射50% 50%。第二层是一介电层，和第三层是全反射层，反射透射辐射。辐射相消干涉的中间层产生超过95%易吸收。这样的一个减震器的设计是本设计的提出。这些减振器的设计，如果实施，可能会导致非常高的吸收。当这些吸收剂结合高效的热释电传感器，其结果是在室温下具有很高的灵敏度和探测率。
热电性可以定义为自发极化的温度依赖性的某些固体可以是单晶或多晶材料。如果材料的温度是由一个小数量的提高，材料的极化变化，电流可以从晶体的某些面或磁盘的某些面之间测量。
为了将热释电材料，材料的每个基本单元必须有一个电偶极子。如果偶极子在整个材料的排列在这样一种方式，自我消除不发生然后材料会表现出电极化称为自发极化。如果材料是保持在一个恒定的温度，内部的自发极化的表面电荷的积累所掩盖，如果材料经受温度变化，偶极子的强度变化，导致表面电荷重新分布。这种效应可以通过连接一个电表之间的导电电极放置在适当的表面物质的测定。这种效应被称为热释电效应。
一个简单的热释电探测器由热释电元件与金属电极的相对面。一般铁电材料是最适合于热释电探测器。他们有不同的偏振方向不同的领域众多，这样的净效应是零。通常极化了东方这些域相互平行。即使在完全极化检测器，没有可观察到的电压被发现，因为它的内部极化的表面电荷积累通过各种泄漏路径的两个表面之间的平衡。因此，热释电探测器只能用于检测调制信号。当探测器是由一个热源加热，电磁辐射，这种情况下，的量是由温度变化和材料的热释电系数确定的偏振变化。因此，电荷在电容测量的两个金属电极形成有直接的联系，由出现在探测器的热通量引起的极化。
还有许多其他的噪声参数的检测器集的检测限。最大的热释电效应的一类材料称为铁电体的观察。一个探测器的用处可通常在最小入射功率评估。这可以表现为两者的响应和在检测器和放大器的电子产生的噪声函数。与热释电探测器相关的简单电路，使得它的混合设计一个合适的人选。混合设计是结合CMOS和MEMS工艺制作下读取传感器电子。
在某些应用中，热释电探测器容易微音。微音是由环境因素造成的设备中的机械振动
产生的电输出。这种类型的噪声的噪声很容易成为主导形式。这种类型的噪声的主要原因是热释电材料的压电性。颤噪效应是由安装探测器的接触点上或更少的刚性结构。
热释电探测器工作在两种操作模式是电流模式和电压模式。在电压模式，热电元件的热电电流电荷的电容器，以及由此产生的电压由源极跟随器电路的测量。常用的调制频率在1-10赫兹之间的电压模式的探测器在热、电1 / f特性的时间常数，典型信号的几个MV。在电流模式的热释电流由一个电流电压转换器转换（基本上与反馈元件，运放也被称为跨阻放大器TIA）。电流型探测器正常工作的热性能和电气时间常数之间，在从1Hz到1千赫的频率，典型的信号100毫伏以上。该探测器的频率响应定义的热性能和电气时间常数，得到的信号是至关重要的。热时间常数是对环境的热电元件的热耦合的方法是在两种操作模式，有效的。电气时间常数被定义为电压模式的热释电材料的电容和栅极电阻的产品只能在小范围内变化。在电流模式下，它被定义为产品的反馈电阻和反馈电容。另外的热释电信号电流模式实现的增益高，可以通过改变反馈电阻很容易调整，而在电压模式的增益只有0.8左右。所以在当前模式的频率响应和探测器的信号电压可以设计更独立，其结果可能运行在高达1 kHz的频率，导致在一个非常短的响应时间。
在这一章中的定义和在热释电探测器热释电物理与相应方程的讨论。的工作原理和热平衡方程进行了详细的讨论。研究发现，钙钛矿材料优于其他材料，因为它具有更好的控制材料的性能。各种材料被视为该项目的传感器材料。材料如陶瓷，钽酸锂，和铌酸锶钡可作为传感器材料。但是PCT钙改性钛酸铅为从钙钛矿型铁电家庭传感器材料。选择这种材料作为传感器材料的原因将在下一章中讨论。电压响应率，探测灵敏度和噪声参数与相应的方程的讨论。
该红外热释电传感器达到一个非常高的电压响应率和探测率DRV，采用有效的减振器设计的结合非常低的热导率的衬底材料。吸收塔的设计是一个热传感器的一个重要组成部分。有很多传统上已被用于吸收剂。热最大吸收可通过吸收层沉积在探测器。有吸收层具有很高的效率，两个主要的要求是，A减震器必须能够在大的波长范围工作和B它应该有一个低的热质量。有传统上已被用于设计的三个主要类型。
附件2：外文原文
HIGH DETECTIVITY PYROELECTRIC INFRARED SENSOR The sensors which measure physical quantities by sensing them as thermal quantities and then converting the thermal signals into electrical signals are called thermal sensors. One such type of thermal sensor is a pyroelectric infrared sensor. Thermal sensors are not popular mainly because they are slow devices, and they have low sensitivity when compared to photon detectors. One major advantage of thermal detectors over photon detectors is that they can operate at room temperature. This is one of the main motivations for this research. The aim of the thesis is to design an uncooled, high detectivity pyroelectric infrared detector. The high detectivity is obtained by reducing the thermal conductivity from the sensor to the substrate or the heat sink. Pyroelectricity is defined as the change in polarization with corresponding change in temperature. Lead calcium titanate (PCT) is a material which is a ferroelectric perovskite. It has a very high pyroelectric co-efficient, high dielectric constant and if deposited in a proper ratio could yield a very high pyroelectric response in the range of 6 x106V/W. The thermal conductance between the sensor and the substrate using the proposed design is found to be as low as 9.51x10-9 W/K, less than the radiative thermal conductance 3.69x10-7 W/K. Two kinds of absorber designs are proposed with this detector. The efficiency of the design and the directivity of the top surface of the infrared sensor are found to have 55° field of view on both sides. Various fabrication methods for fabricating the device have been discussed in detail and the best methods have been mentioned in comparison over the other types. The development of the absorber and its application in the detection mechanism is discussed in detail.
Electromagnetic radiation has electrical and magnetic components. Electromagnetic waves are produced by the motion of electrically charged particles. These waves are also called
A photon is the basic
Any object that is above the absolute temperature 0 K is considered to emit infrared radiation. For the purpose of testing there are many sources of infrared radiation like the infrared bulb, globar, electric heater, mercury lamp, etc. Infrared detectors in general can be broadly classified into two groups: photon detectors and thermal detectors. The sensitivity of photon detectors is determined by the energy band gap of the semiconductor material utilized to sense
the temperature change. These detectors require an external cooling mechanism to maintain the devices at a particular temperature for efficient operation, which significantly increases the cost and complexity of the final IR sensor or imagery systems. In contrast, thermal IR detectors often operate near room temperature and usually do not require cryogenic cooling. This makes thermal IR imagers less costly and, thus attractive for various military and civilian applications.
Radiation Thermometers are non-contact temperature sensors that measure temperature from the amount of thermal electromagnetic radiation received from a spot on the object under investigation. Radiation thermometers are available as point and array devices. The array type radiation thermometers are used to plot temperature distributions in the given area. The resulting image can be viewed as a 2-D temperature map of the area under inspection. They are used mainly in manufacturing process of metals, semiconductors, plastics, etc. Radiation thermometers enable automation and feedback control, as the thermal image of the product is analyzed for any faults and cracks as it comes out of the molding furnace and if the product found faulty it will be rejected from the conveyer. These devices are being used by fire fighters to improve their visibility during fire. One such radiation thermometer for near room temperature is discussed in using HgCdTe as the sensor material.
Thermal imaging and human body detection uses radiation thermometers to measure temperature at many points on a relatively large area and display a thermogram. Thermal imaging cameras detect radiation in the 9-14 m region of the infrared spectrum. As the temperature of an object increases the radiation emitted by that object increases correspondingly. This type of thermography is particularly used for military and security applications. Recently, airport personnel all around the world used thermography to detect suspected swine flu cases. The thermal profile of brain tumors and the surrounding cerebral cortex can be mapped with current technology using Infrared cameras . Thermography is used in many industries to inspect the thermal insulation in structures and the leakage of heat from a structure.
The three main factors that influence the choice of an IR detector are the noise characteristics, detectivity, and the spectral response. These main characteristics are influenced by the absorption mechanisms, detector material, IR window, and the chopping frequency. Thermal detectors have a wide spectral response when compared to photon detectors, and so when choosing a photon detector it is advised to choose a detector which has a good response as close to the spectral band being used. Depending on the application of the detector the response time is also considered as an important factor for selection of a particular IR detector. Other than the above mentioned factors, robustness, cost of fabrication, ease of use, and packaging also play a secondary role in the selection of an IR detector. Typically a detector which works in room temperature will reduce the cost of the detection system as a whole as it avoids the use of
cryogenic cooling system.
In photon detectors, the photons (basic part of the electromagnetic wave) are absorbed by the semiconductor material, which generates free charge carriers. These free charge carriers are measured as a current across the diode. This is the working mechanism of the photon based Infrared detection. The first generation of infrared photon detectors were mostly used as scanning systems, the second generation photon infrared systems was used as staring systems, and the third generation of IR systems concentrates on improving the thermal resolution, frame rates, and multicolor capability . One of the most important materials used to construct an IR photon detection system is HgCdTe. Mercury cadmium telluride is used to construct IRFPA’s on a large scale. ( J.Bajaj, Rockwell science center, 1999) have fabricated a hybrid IRFPA which has a spectral region from 1-16µm. HgCdTe has been considered as one of the materials for Infrared radiation detection from 1958 . One of the major disadvantages of Mercury cadmium telluride is its fabrication as Hg has a high vapor pressure. PbSnTe was a material that was developed in parallel with HgCdTe as a successor but due to a large coefficient of thermal expansion with silicon and high dielectric constant its use was reduced.
A bolometer works on the principle of measurement of change in resistance corresponding to the change in temperature. The resistance sensor is placed below a heat absorbing material and on an isolated surface. Both metal film and thermister (change in resistance based on change in temperature) bolometers have been in use. Semiconductor bolometers which are externally cooled are being used in space applications for their high sensitivity and detectivity. Recently attempts have been made to develop bolometers by using newer materials with larger temperature coefficients. A promising material is the semiconducting lanthanum-doped barium strontium titanate . Liquid helium cooled bolometers are still employed in far infrared spectroscopy and astronomy where the performance of the uncooled detectors is inadequate. The selection of the optimum material for constructing this type of bolometer is very critical.
A detailed study of the Ge bolometer was done in the recent past. The performance of these bolometers improves since the temperature is reduced below 4K. The simplest procedure is to pump the helium to reduce the operating temperature. In this way an operating temperature between 1K and 2K can be obtained, but (Drew and Sievers, 1969 ) have described a Helium cryostat operating at about 0.3K. Micromachining techniques are employed to obtain the isolation of the sensor from the substrate. A bias current is needed to calibrate the device and to set a reference point for measurement of the change in resistance. A high temperature coefficient of resistance (TCR) and a small 1/f noise are desirable material 10 properties for a perfect bolometer. At the same time, it must be possible to integrate the temperature sensing material together with signal read-out electronics (e.g. a CMOS wafer) in a cost efficient way. Today, the
most common bolometer temperature sensing materials are vanadium alloy oxide layer , metal bolometers on porous silicon (Si) and ceramic bolometers .
Considerable amount of work has been made in the development of pyroelectric detectors by Putley 1971, Hadni 1971. It has been noted that for most purposes, from the past decades triglycine sulphate (TGS) or its derivatives are the most suitable materials for pyroelectric detection. Other pyroelectric materials such as strontium barium niobate (SBN) (Glass and Abrams 1971), lithium sulphate and members of the lead zirconate titanate family (Mahler et al 1972, Yamaka et al 1972 ) may be more suitable for some specific applications. The optimum choice of material properties has been discussed in detail by Putley (1970) . The most relevant material properties for an efficient pyroelectric sensor material are the pyroelectric coefficient, the dielectric constant and the thermal capacity. In a desirable material the pyroelectric coefficient should be large, both components of the dielectric constant small and the thermal capacity small. TGS does not possess the largest known pyroelectric coefficient at room temperature. Higher values being found in BST and in some doped lead zirconate titanate ceramics, but the materials with larger pyroelectric coefficients have much larger dielectric constants with large loss factors. In the best pyroelectric detectors the dominant source of noise is the Johnson noise associated with the dielectric loss, followed by the amplifier noise. The capacity of a small TGS detector could fall so low that it could become less than the amplifier input capacity. Detectors made from these materials are somewhat more robust than TGS, although this advantage has been exaggerated, and they are being produced to meet requirements for simple cheap detectors for applications not requiring the highest performance. The improvement in the performance of TGS detectors has been brought about by improvements in the quality of TGS and of the performance of the associated 11 high input impedance low noise amplifier. These results indicate the pyroelectric detector amongst the best uncooled thermal detectors. The attractions of using a pyroelectric detector in this way are that it operates at room temperature and it has a very high voltage responsivity and detectivity.
In recent times, hybrid FPA’s have replaced point detectors. With the advancement in MEMS fabrication technologies, it is possible to produce uncooled pyroelectric infrared detectors with a very good absorber design. The reductions of the size devices have stated reaching saturation point and 3-D stacked devices have started emerging. The read out electronics in pyroelectric detectors have started to be integrated under the sensor using a solder ball. The height of the solder ball gives the necessary isolation between the sensor and the substrate Even though these methods are still under research; they prove that IR detectors with
very high sensitivity and responsivity could be manufactured in micro scale in the near future.
Spider-web absorber designs in absorbers started with the research by J.J block et al about a novel bolometer design for mm-wave astrophysics. They had proposed a spiderweb absorber design that yielded better absorption. Later F.B liewiett et al. 1999 proposed the fabrication and characterization of infrared and sub-mm spiderweb bolometers with low Tc superconducting transition edge thermometers. Thereafter a lot of designs involving absorber designs shaped like a mesh of lossy wires have been under consideration as they have very high absorptivity. M.J.M.E de Nivelle et al proposed a high Tc bolometer with silicon nitride web structure. This device had a very high detectivity in the order of 5.4x1010 cm √Hz/W. a number of these structures were used with bolometers for infrared detection. This aspect can be used to inspect the use of pyroelectric sensors combined with these absorbers which could yield in very high detectivity. L.N.Hadley and D.M.Dennison proposed a novel quarter wavelength absorber design in their two part journal , in 1947. The design consists of three layers. The metal film on top is impedance matched to free space, so that 50% of the incident radiation is absorbed and 50% is transmitted. The second layer is a dielectric layer, and the third layer is a total reflection layer which reflects back the transmitted radiation. The radiation destructively interferes in the middle layer yielding more than 95% absorption easily. One such absorber design is proposed in this design. These absorber designs, if implemented, could result in very high absorption. When these absorbers are combined with highly effective pyroelectric sensors, the result is very high sensitivity and detectivity at room temperatures.
Pyroelectricity can be defined as the temperature dependence of the spontaneous polarization of certain solids which may be either single crystals or poly crystalline materials. If the temperature of the material is raised by a small amount, the electrical polarization of the material is changed and a current can be measured from certain faces of the crystal or between certain faces of the disk.
In order for a material to be pyroelectric, each fundamental unit of the material must have an electric dipole. If the dipoles throughout the material are aligned in such a way that the self-cancellation does not occur then the material will exhibit an electrical polarization called spontaneous polarization . If the material is maintained in a constant temperature, the internal spontaneous polarization is masked by the accumulation of charges on the external surface, if
the material undergoes a change in temperature (chopper ON and OFF as shown in the diagram), the strength of the dipoles changes, causing the surface charges to redistribute themselves. This effect can be measured by connecting an ammeter between conductive electrodes placed on the appropriate surfaces of the substance. This effect is called pyroelectric effect.
A simple pyroelectric detector consists of a pyroelectric element with metal electrodes on its opposite faces. Generally ferroelectric materials are best suited for pyroelectric detectors . They have large number of different domains with different direction of polarization, such that the net effect is zero. Usually poling is done to orient these domains parallel to each other. Even across a perfectly poled detector, no observable voltage is found, because its internal polarization is balanced by a surface charge which accumulates via various leakage paths between the two faces. For this reason the pyroelectric detector can only be used to detect a modulating signal. When the detector is heated by a source, electromagnetic radiation is this case, the polarization changes by an amount that is determined by the temperature change and pyroelectric coefficient of the material. Thus the charge measured across the capacitor formed by the two metal electrodes is directly linked to the polarization caused by the heat flux that occurs in the detector.
There are many other noise parameters that set the detection limit for the detector. The largest pyroelectric effects are observed in a class of materials called ferroelectrics. The usefulness of a detector can be usually assessed in terms of the minimum detectable incident power. This can be represented as a function of both the responsivity and the noise generated in the detector and its amplifier electronics. The simple circuitry associated with the pyroelectric detector makes it a suitable candidate for hybrid designs. Hybrid designs are the combination of a CMOS and MEMS process to fabricate the read out electronics under the sensor.
In certain applications pyroelectric detectors are prone to microphony. Microphony is the electrical output produced by the mechanical vibration in the device due to environmental factors. This type of noise becomes easily dominant form of noise. The main cause of this type of noise is the piezoelectric nature of pyroelectric materials. Microphony effects are reduced by mounting the detector on point contacts or a less rigid structure.
The pyroelectric detector operates in two modes of operation they are the current mode and the voltage mode. In the voltage mode, the pyroelectric current charges the pyroelectric element capacitor, and the resulting voltage is measured by a source follower circuit. At common modulation frequencies between 1-10 Hz voltage mode detectors operate beyond the thermal and electric time constant in 1/f behavior, typical signals are a few mV. In current mode the pyroelectric current is transformed by a current-voltage-converter (basically on OpAmp with feedback components, also called a transimpedance-amplifier TIA). Current mode detectors
normally operate between the thermal and the electrical time constant, at frequencies from 1Hz up to 1 kHz, with typical signals about 100 mV or more. For the detector performance the frequency response defined by thermal and electrical time constant and the resulting signal is of key importance. The thermal time constant is a measure of the thermal coupling of the pyroelectric element to the environment is effective in both operation modes. The electrical time constant in voltage mode is defined as the product of the pyroelectric material capacitance and the gate resistor and can be changed only in a small range. In current mode it is defined as the product of feedback resistor and feedback capacitance. Additionally the achievable gain of the pyroelectrical signal in current mode is much higher and can be adjusted easily by changing the feedback resistor, while in voltage mode the gain is only around 0.8. Therefore in current mode the frequency response and signal voltage of the detector can be designed much more individually, which results in possible operation at high frequencies up to 1 kHz resulting in a very short response time .
In this chapter the definition of pyroelectricity and the physics behind pyroelectric detectors were discussed with corresponding equations. The operating principle and the heat balance equation were discussed in detail. It is found that perovskite materials are superior to other materials since it has better control over the properties of the materials. Various materials were considered as the sensor material for this project. Materials like PZT, Lithium tantalate, and Strontium barium niobate can be used as the sensor material. However PCT Calcium Modified Lead titanate was chosen as the sensor material from the perovskite ferroelectric family. The reasons for choosing this material as the sensor material will be discussed in the next chapter. The voltage responsivity, detectivity and the noise parameters were discussed with corresponding equations.
The proposed Infrared pyroelectric MEMS sensor achieves a really high voltage responsivity RV and detectivity D, by using an efficient absorber design in combination with very low thermal conductivity to the substrate material. The absorber design is an important part of a thermal sensor. There are a lot of absorbers that have been used traditionally. Maximum absorption of heat can be obtained by having an absorbing layer deposited on the detector. There are two main requirements for the absorbing layer to have very high efficiency, they are, a) the absorber must be able to work at large range of wavelengths and b) it should have a low thermal mass. There are three main types of designs that have been used traditionally.
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高性能的热释电红外传感器
该传感器测量物理量的传感作为热量再将温度信号转换为电信号的传感器称为热。这一类型的热传感器是热释电红外传感器。热传感器不受欢迎主要是因为他们缓慢的设备，和他们相比，光子探测器的灵敏度低。一个主要的优势，热探测器光子探测器，它们可以在室温下操作。这是一本研究的主要动机。本文的目的是设计一个非制冷红外热释电探测器，探测灵敏度高。高性能的热导率传感器降低到基板或热沉了。热释电被定义为与相应的温度变化的偏振态的变化。钛酸铅钙（PCT）是一种铁电钙钛矿材料。它具有很高的热释电系数，高介电常数，如果存放在一个适当的比例可以在6 / W x106v热导传感器和使用该设计的基板之间的范围内的一个非常高的热电响应率被发现是9.51x10-9 W / K为低，小于辐射热3.69x10-7 W / K电导检测器提出了这两种减振器的设计。设计的效率和红外传感器的顶表面的方向被发现两侧各有55°视野。用于制造设备的各种制备方法进行了详细讨论，最好的方法是在其他类型的比较提到。是详细讨论了减振器和检测机构的应用。
电磁辐射具有电气和磁性元件。电磁波是由运动的带电粒子产生。这些波被称为“电磁辐射”（EM）因为他们散发的能量以光子的形式。他们必须穿越空间的能力，通过空气和其它材料。电磁辐射是按波的无线电波，微波，红外线，可见光，紫外线辐射的波长为几种类型，X射线和伽马射线。光子是最基本的“单位”的所有形式的电磁辐射。电磁辐射具有能量和动量，可以赋予物质与它互动。
那是在绝对温度0 K的任何物体发出的红外辐射被认为。对测试有许多来源的红外辐射如红外灯泡，硅碳棒，电加热器，水银灯等目的，红外探测器一般大致可分为两类：光子探测器和热探测器。光子探测器的灵敏度是由半导体材料的能带隙的用来感测温度的变化确定。这些探测器需要外部冷却机制，保持高效运行的特定的温度的装置，大大增加了成本和复杂性的最终的红外传感器或影像系统。相比之下，热红外探测器通常在接近室温的温度通常不需要低温冷却。这使得红外热成像仪成本更低，各种军事和民用领域有吸引力。
辐射温度计是非接触式温度传感器测量从现场对被调查对象接受热电磁辐射量温度。辐射温度计有点和阵列器件。阵列式辐射温度计是用来绘制温度分布在给定的区域。由此产生的图像可以被看作是一个二维温度图下面积的检验。他们主要用在金属，半导体，塑料等制造过程，辐射温度计使自动化和反馈控制，为产品的热图像进行分析，对任何故障
和裂纹是从成型炉产品若发现故障，它将被从输送机。这些设备被用于消防人员在火提高自己的知名度。这种辐射温度计的室温附近是使用碲镉汞作为传感器材料的讨论。
热成像和人体检测使用辐射温度计测量在一个相对大面积多点的温度和显示热像图。在红外光谱的9-14米区域的热成像摄像机辐射检测。作为一个对象的对象增加，相应增加辐射温度。这种类型的热成像是用于军事和安全应用。最近，机场工作人员都在使用热成像检测猪流感疑似病例的世界。脑肿瘤的热分布和周围的大脑皮层可以用红外摄像机目前的技术映射。红外热像仪是用在许多行业在检查结构的保温结构的热泄漏。
三个主要因素影响红外探测器的选择，噪声特性，探测灵敏度和光谱响应。这些特点主要是由吸收机制，影响探测器材料，红外窗口，和斩波频率。热探测器相比，光子探测器具有宽的光谱响应，所以当选择一个光子探测器宜选择检测器具有很好的响应接近频带被使用。根据探测器的时间响应的应用也被认为是一个特定的红外探测器选择的重要因素。除了上述因素，鲁棒性强，制造成本，使用方便，包装也起辅助作用的红外探测器的选择。一个典型的探测器在室温工作会降低检测系统的成本作为一个整体，因为它避免了低温冷却系统的使用。
在光子探测器，光子（电磁波的基本部分）是由半导体材料吸收，从而产生自由电荷载体。这些自由电荷载体测量作为一个电流在二极管。这是工作机制的光子型红外探测。红外光子探测器的第一代大多用于扫描系统，第二代光子红外系统作为盯着系统，和IR系统的第三代集中在改善热分辨率，帧速率，和多色的能力。一个用于构建一个红外光子探测系统最重要的材料碲镉汞。碲镉汞是用来在大规模红外焦平面阵列的构建。（J. Bajaj，罗克韦尔科学中心，1999）制备混合红外焦平面阵列具有光谱范围从1µm碲镉汞已被视为检测红外辐射的材料之一，从1958。一种碲化汞镉的主要缺点是它的汞具有较高的蒸气压的制造。PbSnTe是材料的并行开发的HgCdTe的继任者但由于大量的硅和高介电常数使用热膨胀系数降低了。
测辐射热计工作在对应的电阻变化的温度变化的测量原理。电阻传感器放置在吸热材料，在一个孤立的表面。金属薄膜和热敏电阻（根据温度变化的电阻变化）测辐射热计已在使用。半导体测辐射热计的外部冷却的被用在他们的高灵敏度和探测空间应用。最近已经尝试用新材料的开发具有较大的温度系数的测辐射热计。一个很有前途的材料是半导体掺镧钛酸锶钡。液态氦冷却的辐射热测量仪仍然采用远红外光谱和天文的非制冷探测器的性能是不够的。构建这种类型的测辐射热计的最佳材料的选择是非常重要的。
在最近的过去锗测辐射热计的详细研究。这些测辐射热计的性能提高由于温度低于
4K。最简单的方法是泵氦降低操作温度。这样的操作温度1K和2K之间可以得到的，但（Drew和西弗斯，1969）描述了一个在约0.3K操作氦低温恒温器。微机械加工技术是采用从基板获得传感器的隔离。偏置电流是需要校准的设备和设置的电阻变化的测量参考点。高电阻温度系数（TCR）和一个小的1 / f噪声是理想的材料性能的一个完美的测辐射热计10。同时，它必须能够集成温度传感材料与信号读出电路（如CMOS晶片）的具有成本效益的方式。今天，最常见的测辐射热计温度传感材料是钒合金的氧化层，金属测辐射热计的多孔硅（Si）和陶瓷热。
相当数量的工作已经由帕特利1971热释电探测器的发展。人们已经注意到，在大多数情况下，从过去的几十年里，硫酸三甘氨酸（TGS）或其衍生物的热释电探测的最合适的材料。其他热释电材料如铌酸锶钡（SBN）（玻璃和艾布拉姆斯1971），硫酸锂和锆钛酸铅的家庭成员（马勒等人1972，双论等人1972）可能更适合某些特定的应用。材料性能的最佳选择了帕特利讨论（1970）。一个有效的热释电传感器材料的最相关的材料性能的热释电系数，介电常数和热容量。在一个理想的材料热释电系数要大，其介电常数小，热容量小的元件。TGS不具有已知的最大的热释电系数在室温下。更高的价值在BST和一些掺杂锆钛酸铅陶瓷的发现，但更大的热释电系数的材料具有更大的介电常数和损耗大的因素。在最佳的热释电探测器的主要噪声源是具有介电损耗相关的约翰逊噪声，其次是放大器的噪声。一个小的TGS检测器的能力可能会下降得如此之低，它可以成为小于放大器的输入容量。由这些材料制成的探测器比TGS更强大，虽然这种优势被夸大了，他们正在生产满足不需要最高性能的应用程序的简单廉价的探测器的要求。在TGS探测器性能的改进已经在TGS的质量改进带来的相关的11高输入阻抗低噪声放大器的性能。这些结果表明，热释电探测器非制冷红外探测器中最好的。利用热释电探测器这样的景点，它在室温下操作，具有非常高的电压响应率和探测率。 在最近的时代，Hybrid FPA的取代点探测器。随着MEMS制造技术，可以生产具有很好的减振器设计非制冷热释电红外探测器。的大小的装置减少已达到饱和点和三维堆叠的设备已经开始出现。读出热释电探测器中电子已经开始被集成在传感器使用焊料球。焊料球的高度提供了必要的隔离和传感器之间的基板，即使这些方法仍在研究；他们证明具有很高的灵敏度和响应红外探测器可能会在不久的将来，在微观尺度的制造。
在吸收器吸收器设计的蜘蛛网开始研究J. J块等人关于新型毫米波天文测辐射热计的设计。他们提出了一个蜘蛛网吸收塔的设计，取得了较好的吸收。后来F B liewiett等人。1999提出了红外和亚毫米蛛网测辐射热计低Tc超导转变边缘温度计的制备表征。此
后，许多涉及吸收器设计的形状像一个网状的有损线的设计已经在考虑为他们有很高的吸收率。m.j.m.e de倪维尔等人提出了一种氮化硅网络结构高温超导测辐射热计。这种装置在5.4x1010厘米√赫兹/ W许多这些结构顺序有很高的探测用红外检测测辐射热计。这方面可作为检测热释电传感器结合这些吸收剂可以在很高的探测率的使用。d.m.dennison l.n.hadley和提出了一种新的季度波吸收器的设计在其两部学报，1947。本设计共分三层。在上面的金属膜的阻抗与自由空间相匹配，使入射辐射的吸收和发射50% 50%。第二层是一介电层，和第三层是全反射层，反射透射辐射。辐射相消干涉的中间层产生超过95%易吸收。这样的一个减震器的设计是本设计的提出。这些减振器的设计，如果实施，可能会导致非常高的吸收。当这些吸收剂结合高效的热释电传感器，其结果是在室温下具有很高的灵敏度和探测率。
热电性可以定义为自发极化的温度依赖性的某些固体可以是单晶或多晶材料。如果材料的温度是由一个小数量的提高，材料的极化变化，电流可以从晶体的某些面或磁盘的某些面之间测量。
为了将热释电材料，材料的每个基本单元必须有一个电偶极子。如果偶极子在整个材料的排列在这样一种方式，自我消除不发生然后材料会表现出电极化称为自发极化。如果材料是保持在一个恒定的温度，内部的自发极化的表面电荷的积累所掩盖，如果材料经受温度变化，偶极子的强度变化，导致表面电荷重新分布。这种效应可以通过连接一个电表之间的导电电极放置在适当的表面物质的测定。这种效应被称为热释电效应。
一个简单的热释电探测器由热释电元件与金属电极的相对面。一般铁电材料是最适合于热释电探测器。他们有不同的偏振方向不同的领域众多，这样的净效应是零。通常极化了东方这些域相互平行。即使在完全极化检测器，没有可观察到的电压被发现，因为它的内部极化的表面电荷积累通过各种泄漏路径的两个表面之间的平衡。因此，热释电探测器只能用于检测调制信号。当探测器是由一个热源加热，电磁辐射，这种情况下，的量是由温度变化和材料的热释电系数确定的偏振变化。因此，电荷在电容测量的两个金属电极形成有直接的联系，由出现在探测器的热通量引起的极化。
还有许多其他的噪声参数的检测器集的检测限。最大的热释电效应的一类材料称为铁电体的观察。一个探测器的用处可通常在最小入射功率评估。这可以表现为两者的响应和在检测器和放大器的电子产生的噪声函数。与热释电探测器相关的简单电路，使得它的混合设计一个合适的人选。混合设计是结合CMOS和MEMS工艺制作下读取传感器电子。
在某些应用中，热释电探测器容易微音。微音是由环境因素造成的设备中的机械振动
产生的电输出。这种类型的噪声的噪声很容易成为主导形式。这种类型的噪声的主要原因是热释电材料的压电性。颤噪效应是由安装探测器的接触点上或更少的刚性结构。
热释电探测器工作在两种操作模式是电流模式和电压模式。在电压模式，热电元件的热电电流电荷的电容器，以及由此产生的电压由源极跟随器电路的测量。常用的调制频率在1-10赫兹之间的电压模式的探测器在热、电1 / f特性的时间常数，典型信号的几个MV。在电流模式的热释电流由一个电流电压转换器转换（基本上与反馈元件，运放也被称为跨阻放大器TIA）。电流型探测器正常工作的热性能和电气时间常数之间，在从1Hz到1千赫的频率，典型的信号100毫伏以上。该探测器的频率响应定义的热性能和电气时间常数，得到的信号是至关重要的。热时间常数是对环境的热电元件的热耦合的方法是在两种操作模式，有效的。电气时间常数被定义为电压模式的热释电材料的电容和栅极电阻的产品只能在小范围内变化。在电流模式下，它被定义为产品的反馈电阻和反馈电容。另外的热释电信号电流模式实现的增益高，可以通过改变反馈电阻很容易调整，而在电压模式的增益只有0.8左右。所以在当前模式的频率响应和探测器的信号电压可以设计更独立，其结果可能运行在高达1 kHz的频率，导致在一个非常短的响应时间。
在这一章中的定义和在热释电探测器热释电物理与相应方程的讨论。的工作原理和热平衡方程进行了详细的讨论。研究发现，钙钛矿材料优于其他材料，因为它具有更好的控制材料的性能。各种材料被视为该项目的传感器材料。材料如陶瓷，钽酸锂，和铌酸锶钡可作为传感器材料。但是PCT钙改性钛酸铅为从钙钛矿型铁电家庭传感器材料。选择这种材料作为传感器材料的原因将在下一章中讨论。电压响应率，探测灵敏度和噪声参数与相应的方程的讨论。
该红外热释电传感器达到一个非常高的电压响应率和探测率DRV，采用有效的减振器设计的结合非常低的热导率的衬底材料。吸收塔的设计是一个热传感器的一个重要组成部分。有很多传统上已被用于吸收剂。热最大吸收可通过吸收层沉积在探测器。有吸收层具有很高的效率，两个主要的要求是，A减震器必须能够在大的波长范围工作和B它应该有一个低的热质量。有传统上已被用于设计的三个主要类型。
附件2：外文原文
HIGH DETECTIVITY PYROELECTRIC INFRARED SENSOR The sensors which measure physical quantities by sensing them as thermal quantities and then converting the thermal signals into electrical signals are called thermal sensors. One such type of thermal sensor is a pyroelectric infrared sensor. Thermal sensors are not popular mainly because they are slow devices, and they have low sensitivity when compared to photon detectors. One major advantage of thermal detectors over photon detectors is that they can operate at room temperature. This is one of the main motivations for this research. The aim of the thesis is to design an uncooled, high detectivity pyroelectric infrared detector. The high detectivity is obtained by reducing the thermal conductivity from the sensor to the substrate or the heat sink. Pyroelectricity is defined as the change in polarization with corresponding change in temperature. Lead calcium titanate (PCT) is a material which is a ferroelectric perovskite. It has a very high pyroelectric co-efficient, high dielectric constant and if deposited in a proper ratio could yield a very high pyroelectric response in the range of 6 x106V/W. The thermal conductance between the sensor and the substrate using the proposed design is found to be as low as 9.51x10-9 W/K, less than the radiative thermal conductance 3.69x10-7 W/K. Two kinds of absorber designs are proposed with this detector. The efficiency of the design and the directivity of the top surface of the infrared sensor are found to have 55° field of view on both sides. Various fabrication methods for fabricating the device have been discussed in detail and the best methods have been mentioned in comparison over the other types. The development of the absorber and its application in the detection mechanism is discussed in detail.
Electromagnetic radiation has electrical and magnetic components. Electromagnetic waves are produced by the motion of electrically charged particles. These waves are also called
A photon is the basic
Any object that is above the absolute temperature 0 K is considered to emit infrared radiation. For the purpose of testing there are many sources of infrared radiation like the infrared bulb, globar, electric heater, mercury lamp, etc. Infrared detectors in general can be broadly classified into two groups: photon detectors and thermal detectors. The sensitivity of photon detectors is determined by the energy band gap of the semiconductor material utilized to sense
the temperature change. These detectors require an external cooling mechanism to maintain the devices at a particular temperature for efficient operation, which significantly increases the cost and complexity of the final IR sensor or imagery systems. In contrast, thermal IR detectors often operate near room temperature and usually do not require cryogenic cooling. This makes thermal IR imagers less costly and, thus attractive for various military and civilian applications.
Radiation Thermometers are non-contact temperature sensors that measure temperature from the amount of thermal electromagnetic radiation received from a spot on the object under investigation. Radiation thermometers are available as point and array devices. The array type radiation thermometers are used to plot temperature distributions in the given area. The resulting image can be viewed as a 2-D temperature map of the area under inspection. They are used mainly in manufacturing process of metals, semiconductors, plastics, etc. Radiation thermometers enable automation and feedback control, as the thermal image of the product is analyzed for any faults and cracks as it comes out of the molding furnace and if the product found faulty it will be rejected from the conveyer. These devices are being used by fire fighters to improve their visibility during fire. One such radiation thermometer for near room temperature is discussed in using HgCdTe as the sensor material.
Thermal imaging and human body detection uses radiation thermometers to measure temperature at many points on a relatively large area and display a thermogram. Thermal imaging cameras detect radiation in the 9-14 m region of the infrared spectrum. As the temperature of an object increases the radiation emitted by that object increases correspondingly. This type of thermography is particularly used for military and security applications. Recently, airport personnel all around the world used thermography to detect suspected swine flu cases. The thermal profile of brain tumors and the surrounding cerebral cortex can be mapped with current technology using Infrared cameras . Thermography is used in many industries to inspect the thermal insulation in structures and the leakage of heat from a structure.
The three main factors that influence the choice of an IR detector are the noise characteristics, detectivity, and the spectral response. These main characteristics are influenced by the absorption mechanisms, detector material, IR window, and the chopping frequency. Thermal detectors have a wide spectral response when compared to photon detectors, and so when choosing a photon detector it is advised to choose a detector which has a good response as close to the spectral band being used. Depending on the application of the detector the response time is also considered as an important factor for selection of a particular IR detector. Other than the above mentioned factors, robustness, cost of fabrication, ease of use, and packaging also play a secondary role in the selection of an IR detector. Typically a detector which works in room temperature will reduce the cost of the detection system as a whole as it avoids the use of
cryogenic cooling system.
In photon detectors, the photons (basic part of the electromagnetic wave) are absorbed by the semiconductor material, which generates free charge carriers. These free charge carriers are measured as a current across the diode. This is the working mechanism of the photon based Infrared detection. The first generation of infrared photon detectors were mostly used as scanning systems, the second generation photon infrared systems was used as staring systems, and the third generation of IR systems concentrates on improving the thermal resolution, frame rates, and multicolor capability . One of the most important materials used to construct an IR photon detection system is HgCdTe. Mercury cadmium telluride is used to construct IRFPA’s on a large scale. ( J.Bajaj, Rockwell science center, 1999) have fabricated a hybrid IRFPA which has a spectral region from 1-16µm. HgCdTe has been considered as one of the materials for Infrared radiation detection from 1958 . One of the major disadvantages of Mercury cadmium telluride is its fabrication as Hg has a high vapor pressure. PbSnTe was a material that was developed in parallel with HgCdTe as a successor but due to a large coefficient of thermal expansion with silicon and high dielectric constant its use was reduced.
A bolometer works on the principle of measurement of change in resistance corresponding to the change in temperature. The resistance sensor is placed below a heat absorbing material and on an isolated surface. Both metal film and thermister (change in resistance based on change in temperature) bolometers have been in use. Semiconductor bolometers which are externally cooled are being used in space applications for their high sensitivity and detectivity. Recently attempts have been made to develop bolometers by using newer materials with larger temperature coefficients. A promising material is the semiconducting lanthanum-doped barium strontium titanate . Liquid helium cooled bolometers are still employed in far infrared spectroscopy and astronomy where the performance of the uncooled detectors is inadequate. The selection of the optimum material for constructing this type of bolometer is very critical.
A detailed study of the Ge bolometer was done in the recent past. The performance of these bolometers improves since the temperature is reduced below 4K. The simplest procedure is to pump the helium to reduce the operating temperature. In this way an operating temperature between 1K and 2K can be obtained, but (Drew and Sievers, 1969 ) have described a Helium cryostat operating at about 0.3K. Micromachining techniques are employed to obtain the isolation of the sensor from the substrate. A bias current is needed to calibrate the device and to set a reference point for measurement of the change in resistance. A high temperature coefficient of resistance (TCR) and a small 1/f noise are desirable material 10 properties for a perfect bolometer. At the same time, it must be possible to integrate the temperature sensing material together with signal read-out electronics (e.g. a CMOS wafer) in a cost efficient way. Today, the
most common bolometer temperature sensing materials are vanadium alloy oxide layer , metal bolometers on porous silicon (Si) and ceramic bolometers .
Considerable amount of work has been made in the development of pyroelectric detectors by Putley 1971, Hadni 1971. It has been noted that for most purposes, from the past decades triglycine sulphate (TGS) or its derivatives are the most suitable materials for pyroelectric detection. Other pyroelectric materials such as strontium barium niobate (SBN) (Glass and Abrams 1971), lithium sulphate and members of the lead zirconate titanate family (Mahler et al 1972, Yamaka et al 1972 ) may be more suitable for some specific applications. The optimum choice of material properties has been discussed in detail by Putley (1970) . The most relevant material properties for an efficient pyroelectric sensor material are the pyroelectric coefficient, the dielectric constant and the thermal capacity. In a desirable material the pyroelectric coefficient should be large, both components of the dielectric constant small and the thermal capacity small. TGS does not possess the largest known pyroelectric coefficient at room temperature. Higher values being found in BST and in some doped lead zirconate titanate ceramics, but the materials with larger pyroelectric coefficients have much larger dielectric constants with large loss factors. In the best pyroelectric detectors the dominant source of noise is the Johnson noise associated with the dielectric loss, followed by the amplifier noise. The capacity of a small TGS detector could fall so low that it could become less than the amplifier input capacity. Detectors made from these materials are somewhat more robust than TGS, although this advantage has been exaggerated, and they are being produced to meet requirements for simple cheap detectors for applications not requiring the highest performance. The improvement in the performance of TGS detectors has been brought about by improvements in the quality of TGS and of the performance of the associated 11 high input impedance low noise amplifier. These results indicate the pyroelectric detector amongst the best uncooled thermal detectors. The attractions of using a pyroelectric detector in this way are that it operates at room temperature and it has a very high voltage responsivity and detectivity.
In recent times, hybrid FPA’s have replaced point detectors. With the advancement in MEMS fabrication technologies, it is possible to produce uncooled pyroelectric infrared detectors with a very good absorber design. The reductions of the size devices have stated reaching saturation point and 3-D stacked devices have started emerging. The read out electronics in pyroelectric detectors have started to be integrated under the sensor using a solder ball. The height of the solder ball gives the necessary isolation between the sensor and the substrate Even though these methods are still under research; they prove that IR detectors with
very high sensitivity and responsivity could be manufactured in micro scale in the near future.
Spider-web absorber designs in absorbers started with the research by J.J block et al about a novel bolometer design for mm-wave astrophysics. They had proposed a spiderweb absorber design that yielded better absorption. Later F.B liewiett et al. 1999 proposed the fabrication and characterization of infrared and sub-mm spiderweb bolometers with low Tc superconducting transition edge thermometers. Thereafter a lot of designs involving absorber designs shaped like a mesh of lossy wires have been under consideration as they have very high absorptivity. M.J.M.E de Nivelle et al proposed a high Tc bolometer with silicon nitride web structure. This device had a very high detectivity in the order of 5.4x1010 cm √Hz/W. a number of these structures were used with bolometers for infrared detection. This aspect can be used to inspect the use of pyroelectric sensors combined with these absorbers which could yield in very high detectivity. L.N.Hadley and D.M.Dennison proposed a novel quarter wavelength absorber design in their two part journal , in 1947. The design consists of three layers. The metal film on top is impedance matched to free space, so that 50% of the incident radiation is absorbed and 50% is transmitted. The second layer is a dielectric layer, and the third layer is a total reflection layer which reflects back the transmitted radiation. The radiation destructively interferes in the middle layer yielding more than 95% absorption easily. One such absorber design is proposed in this design. These absorber designs, if implemented, could result in very high absorption. When these absorbers are combined with highly effective pyroelectric sensors, the result is very high sensitivity and detectivity at room temperatures.
Pyroelectricity can be defined as the temperature dependence of the spontaneous polarization of certain solids which may be either single crystals or poly crystalline materials. If the temperature of the material is raised by a small amount, the electrical polarization of the material is changed and a current can be measured from certain faces of the crystal or between certain faces of the disk.
In order for a material to be pyroelectric, each fundamental unit of the material must have an electric dipole. If the dipoles throughout the material are aligned in such a way that the self-cancellation does not occur then the material will exhibit an electrical polarization called spontaneous polarization . If the material is maintained in a constant temperature, the internal spontaneous polarization is masked by the accumulation of charges on the external surface, if
the material undergoes a change in temperature (chopper ON and OFF as shown in the diagram), the strength of the dipoles changes, causing the surface charges to redistribute themselves. This effect can be measured by connecting an ammeter between conductive electrodes placed on the appropriate surfaces of the substance. This effect is called pyroelectric effect.
A simple pyroelectric detector consists of a pyroelectric element with metal electrodes on its opposite faces. Generally ferroelectric materials are best suited for pyroelectric detectors . They have large number of different domains with different direction of polarization, such that the net effect is zero. Usually poling is done to orient these domains parallel to each other. Even across a perfectly poled detector, no observable voltage is found, because its internal polarization is balanced by a surface charge which accumulates via various leakage paths between the two faces. For this reason the pyroelectric detector can only be used to detect a modulating signal. When the detector is heated by a source, electromagnetic radiation is this case, the polarization changes by an amount that is determined by the temperature change and pyroelectric coefficient of the material. Thus the charge measured across the capacitor formed by the two metal electrodes is directly linked to the polarization caused by the heat flux that occurs in the detector.
There are many other noise parameters that set the detection limit for the detector. The largest pyroelectric effects are observed in a class of materials called ferroelectrics. The usefulness of a detector can be usually assessed in terms of the minimum detectable incident power. This can be represented as a function of both the responsivity and the noise generated in the detector and its amplifier electronics. The simple circuitry associated with the pyroelectric detector makes it a suitable candidate for hybrid designs. Hybrid designs are the combination of a CMOS and MEMS process to fabricate the read out electronics under the sensor.
In certain applications pyroelectric detectors are prone to microphony. Microphony is the electrical output produced by the mechanical vibration in the device due to environmental factors. This type of noise becomes easily dominant form of noise. The main cause of this type of noise is the piezoelectric nature of pyroelectric materials. Microphony effects are reduced by mounting the detector on point contacts or a less rigid structure.
The pyroelectric detector operates in two modes of operation they are the current mode and the voltage mode. In the voltage mode, the pyroelectric current charges the pyroelectric element capacitor, and the resulting voltage is measured by a source follower circuit. At common modulation frequencies between 1-10 Hz voltage mode detectors operate beyond the thermal and electric time constant in 1/f behavior, typical signals are a few mV. In current mode the pyroelectric current is transformed by a current-voltage-converter (basically on OpAmp with feedback components, also called a transimpedance-amplifier TIA). Current mode detectors
normally operate between the thermal and the electrical time constant, at frequencies from 1Hz up to 1 kHz, with typical signals about 100 mV or more. For the detector performance the frequency response defined by thermal and electrical time constant and the resulting signal is of key importance. The thermal time constant is a measure of the thermal coupling of the pyroelectric element to the environment is effective in both operation modes. The electrical time constant in voltage mode is defined as the product of the pyroelectric material capacitance and the gate resistor and can be changed only in a small range. In current mode it is defined as the product of feedback resistor and feedback capacitance. Additionally the achievable gain of the pyroelectrical signal in current mode is much higher and can be adjusted easily by changing the feedback resistor, while in voltage mode the gain is only around 0.8. Therefore in current mode the frequency response and signal voltage of the detector can be designed much more individually, which results in possible operation at high frequencies up to 1 kHz resulting in a very short response time .
In this chapter the definition of pyroelectricity and the physics behind pyroelectric detectors were discussed with corresponding equations. The operating principle and the heat balance equation were discussed in detail. It is found that perovskite materials are superior to other materials since it has better control over the properties of the materials. Various materials were considered as the sensor material for this project. Materials like PZT, Lithium tantalate, and Strontium barium niobate can be used as the sensor material. However PCT Calcium Modified Lead titanate was chosen as the sensor material from the perovskite ferroelectric family. The reasons for choosing this material as the sensor material will be discussed in the next chapter. The voltage responsivity, detectivity and the noise parameters were discussed with corresponding equations.
The proposed Infrared pyroelectric MEMS sensor achieves a really high voltage responsivity RV and detectivity D, by using an efficient absorber design in combination with very low thermal conductivity to the substrate material. The absorber design is an important part of a thermal sensor. There are a lot of absorbers that have been used traditionally. Maximum absorption of heat can be obtained by having an absorbing layer deposited on the detector. There are two main requirements for the absorbing layer to have very high efficiency, they are, a) the absorber must be able to work at large range of wavelengths and b) it should have a low thermal mass. There are three main types of designs that have been used traditionally.