Chapter 4 MICROSYSTEM FABRICATION PROCESSES

Transcription

1 Chapter 4 MICROSYSTEM FABRICATION PROCESSES 4.1 Overview In past few years, advancements in fabrication technologies and state of the art measurement instrumentation led to establishment of micro electromechanical system (MEMS). The one main criterion of MEMS is that there are at least some elements having some sort of mechanical functionality whether or not these elements can move. Micro-scale structures, devices, and systems frequently have the specific applications such as high frequency filters, resonators, electromagnetic field and stress sensors, etc. Micro-scale devices and systems will allow one to access the atomic scale phenomenon, where quantum effects are prominent. The dimensions of MEMS and their components ranges from 100 nm to the centimetre range [133]. MEMS are touching every aspect of our lives as compared to semiconductor technology, information technology, or cellular and molecular biology. These systems have played key roles in many important areas e.g. transportation, communication, automated manufacturing, environmental monitoring, health care, defence systems, and a wide range of consumer products. MEMS are inherently small which gave them attractive characteristics such as reduced size, weight, and power dissipation and improved speed and precision compared to their macroscopic counterparts. Recent fabrication technologies enable the definition of small geometries with dimension control, flexibility in designing, better interfacing with microelectronics, repeatability, reliability, high yield, and low cost are the key parameters to develop MEMS. Besides few special etching, bonding, and assembly techniques, integrated circuit (IC) fabrication technology meets all the criteria required for MEMS fabrication. Conventional machining techniques are not sufficient for MEMS fabrication, but microfabrication provides a powerful tool for batch processing and miniaturization of electromechanical devices and systems. Depending upon the fabrication technique used for fabrication, MEMS devices exhibit a length or width ranging from micrometers to several hundreds of micrometers with a thickness from sub micrometer up to tens of micrometers. This field has expanded to a great extent in recent years with rapid technology advances [134, 135]. 71

2 4.2 Introduction to fabrication technologies The roots of most of the microfabrication techniques are embedded in standard fabrication methods developed for the semiconductor industry. A clear understanding of these techniques is essential before taking up the fabrication of MEMS. Thin film deposition, etching and VLSI micro fabrication disciplines are among them Lithography Lithography is the technique used to transfer a designed pattern onto a substrate. This pattern is subsequently used to etch an underlying thin film for various purposes. Lithography using an ultra violet (UV) light source is most widely used lithography technique in microelectronic fabrication. Electron-beam (e-beam) and X-ray lithography are two other alternatives that have attracted considerable attention in the MEMS and nanofabrication areas [136]. i. Photolithography: In microfabrication techniques photolithography is the most successful technology. Since its invention in 1959, it has been used in semiconductor industry. All the essential components of ICs are made by this technology. The photolithographic techniques used for fabricating microstructures are based on a projection of UV rays through the mask designed using MEMS designing software onto a thin film of photoresist that is spin-coated on a wafer through a high numerical aperture lens system. Multilayered structures can be achieved with the combination of accurate alignment of a successive set of photomasks and exposure of these successive patterns. Photolithography has matured rapidly by continuous improvements in the ability to resolve ever-smaller features. Figure 4.1 shows the photolithography and pattern transfer involving a set of process steps. Photolithographic process has its own specific requirements, but there is a basic common flow of process that are common to most procedures. ii. X-ray lithography: It is one of the forms of shadow printing process in which patterns designed over the mask are transferred into a third dimension in a resist (PMMA) material. This is a chemical process to dissolve volume of material damaged by the X-rays. Parameters like beam exposure, precision of designed structure on mask, purity, and processing of the resist material controls the quality remaining structure. The final quality of the MEMS structure depends upon the precision of electroforming and micro-molding processes. X-ray lithography process is classified as hybrid technology [137]. 72

3 Fig. 4.1 Process flow of basic photolithography followed by pattern transfer [136]. iii. It s bridging the semiconductor and classical manufacturing technology. The ability of X-ray lithography for creating a wide variety of shapes from different materials makes this method similar to classical machining, with the added benefit of high aspect ratios and absolute tolerances that are possible using lithography and other high-precision mold fabrication techniques. E-Beam Lithography (EBL): It is a specialized technique for creating the extremely fine patterns with dimensions to the range in nanometer, required by the modern electronics industry for ICs. It consists of scanning a beam of electrons across a surface covered with a resist film sensitive to those electrons, thus depositing energy in the desired pattern in the resist film. In this technique beam of electron formed and scanned across the surface. The main attributes of this technology are capability of high resolution to atomic level, flexibility to work with a variety of materials, it is slow as compared to optical lithography, and it is very expensive technique [138-73

4 139]. Figure 2.3 shows a block diagram of a typical EBL tool. The working principle of EBL is similar to conventional photolithography. Fig. 4.2 X-ray lithography schematic [138]. The patterns over the spin coated resist are created by the exposure of high energy electron. The substrate usually requires conducting to prevent later electron charging. Then a focused beam of electron is scanned across the sample for exposure. Photolithography is a parallel process of exposure of light but EBL is serial process of exposure where a small beam width is allowed at one time. Developing of the corresponding exposed region will make the resist soluble (positive resist). The resulting patterned resist layer can serve as a mask for later depositing or etching. Lift off process removes the resist mask and excess material on top of it to obtain the final desired applicable devices Thin film deposition Thin film deposition techniques are used extensively in micro/nanofabrication technologies. The deposited thin films have different properties than those of their corresponding bulk forms. The techniques utilized to deposit these materials have a great impact on their final properties e.g. the inter stress (compressive or tensile) in a film is strongly process dependent. Excessive stress may crack or detach the film from the substrate and therefore must be minimized, although it may also be useful for certain applications. In some 74

5 deposition method an intermediate layer may be needed to improve adhesion [140]. Figure 4.4 shows the classification of the deposition techniques. H.V power supply Electron gun Lens power supplies Blanking amplifier Pattern generator Column D/A converters deflection amplifiers Registration unit Laser interferometer stage controller Airlock Stage Chamber Final lens electron detector Computer Vacuum system Vibration isolation table Pattern data storage Fig. 4.3 Schematic of EBL system [140]. i. Evaporation technique: This is an oldest techniques used for depositing thin films but still widely used in the laboratory and industries for depositing metal and metal alloys. Steps taking place in thermal or vacuum evaporation: (i) vapor is generated by boiling or subliming a source material, (ii) the vapor is transported from the source to the substrate, and (iii) the vapor is condensed to a solid film [141]. Evaporants cover an extraordinary range of varying chemical reactivity and vapor pressures. ii. Glow-Discharge Technologies: This technique represents a high source of processes used to deposit and etch thin films. Various methods developed from this technique for thin film deposition are shown in Fig Sputtering and other ion assisted techniques are essential for the fabrication of semiconductor devices and hardware resistant coatings. This method is defined as a partially ionized low pressure gas in a 75

6 quasi neutral state sustained by the presence of energetic electrons. This plasma is created due the mass difference between the electron and the ions, which leads to the more rapid energy transfer to the electron than ions. These high energy electrons have high probability of causing ionization and excitation event when colliding with heavier particles. The generation phenomenon of the particles and their interactions with surface and deposition of thin films are the most important reasons why glow discharge plasmas have become of such importance in material science [142]. Thin Film Deposition Gas Phase Chemical Technique Liquid Phase Chemical Technique Glow Discharge Technique Evaporation Technique Fig. 4.4 Fabrication domains of thin film technology. iii. Gas-Phase Chemical Processes: Thermal oxidation and chemical vapor deposition (CVD) are the two methods used for film formation by purely chemical processes in gas or vapor phases. In CVD process of film deposition vapor of material reacts chemically near or on a substrate surface to form a solid product. The main advantage of CVD is the synthesis of simple and complex compounds with relative ease at low temperature. Chemical as well as physical properties of the thin film can be tailored by controlling the reaction chemistry and deposition conditions [143]. Also parameters such as temperature, pressure, input concentrations, gas flow rates, reactor geometry, and operating principle determine the deposition rate and the properties of the film deposit. iv. Liquid-Phase Chemical Formation: Electrochemical processes are used to grow inorganic thin films from liquid phase by chemical reactions. These processes include reduction plating, electroless plating, conversion coating, and displacement 76

7 deposition. Mechanical technique is another process categorized under liquid phase method. Also liquid phase epitaxy is still being used for growing a number of single-crystal [144, 145]. The choice of deposition technique for thin films is application dependent. Various parameters of thin films are considered before employing a specific technique of deposition. Multiple layers of thin film of different material can be deposited for a large variety of applications using various deposition systems. But contaminations issue are accounted before passing sample from one to another Etching process Etching is another fundamental fabrication step which is very important in VLSI technology. In VLSI and micro/nano fabrication various conducting and dielectric thin films deposited using mask must be removed at some point or another. In addition to thin film etching in micro/nano fabrication sometimes substrate also need to be removed in order to create various MEMS structures. Selectivity and directionality are the important parameters in etching process [146]. In case of isotropic etching, the etchant attacks the material in all directions at the same rate. On the other hand in case of anisotropic etching, the dissolution rate depends on specific directions. Semicircular profile under the mask is obtained in isotropic etching where as in case of anisotropic straight side walls and non circular profile are obtained shown in Fig PPR PPR PPR PPR Substrate Substrate (a) Fig. 4.5 Profile for isotropic (a) anisotropic (b) Etch through a photoresist mask [146]. (b) Etching can be divided into two categories wet and dry etching. i. Wet Etching: This is a superior technique as compared to dry method. Wet etching is insensitively used in micro/nano fabrication, in spite of less frequent application in VLSI fabrication processes. Various combinations of acids and base solutions are used to etch metals. Also there are commercially available etchant formulations 77

8 ii. for aluminum, chromium, and gold. In microfabrication anisotropic and isotropic wet etching of crystalline (silicon and gallium arsenide) and non-crystalline (glass) substrates are very important topics. The beginning of micromachining and MEMS discipline started with the possibility of anisotropic wet etching of silicon. Since 1950s isotropic etching of silicon using HF/HNO 3 /CH 3 COOH are being carried out to thin down the silicon wafer. In bulk micromachining silicon anisotropic wet etching is an important technique. Potassium hydroxide (KOH), ethylene diamine pyrochatechol (EDP), and tetramethyl ammonium hydroxide (TMAH) are three important silicon etchants in this category. These etchant dissolve the silicon along preferred crystallographic direction [147]. Dry Etching: Primarily these are plasma based techniques having several advantages as compared to other methods. This method allows smaller lines to be patterned and high aspect ratio in vertical structures. But the selectivity of dry etching techniques is lower than the wet etching. Different mechanisms are applied to obtain directionality in three basic dry etching techniques namely high pressure plasma etching, reactive ion etching (RIE), and ion milling [148] Micromachining techniques MEMS fabrication techniques are commonly used to build various microdevices (microsensors and microactuator). Micromachining technologies are recently developed for the fabrication of MEMS devices. A greater interest is focused on the achievement of 3D sculptured surfaces and high aspect ratios with complex fine shapes. The dimensional spectrum of the microstructures that can be fabricated using these techniques ranges down to few micron. A wide variety of techniques which are capable of creating micro/nano structures with various degrees of quality and speed are developed. In this section various forms of micromaching techniques are discussed. i. Bulk Micromachining: It is one the oldest and mature MEMS fabrication technology. Also commercially most successful technique used to manufacture sensors and actuator. The basic idea behind bulk micromachining is the selective removal of substrate, which allows creating various micromechanical components such as beams, plates, and membrane that can be used to fabricate a variety of sensors and actuators [149]. Etching (wet and dry) and substrate bonding are the important techniques used in bulk micromachining. Figure 4.6 shows the structure obtained using bulk micromachining with back etching of substrate. 78

9 Substrate Substrate Etchant Fig. 4.6 Wet chemical etching [147]. ii. Surface Micromachining: It is an important MEMS microfabrication technique used to create movable microstructures on top of substrate. These structures are created by deposition of thin film over sacrificial layer which is subsequently etched to obtain movable micromechanical structures. Fabrication of small size structures are the main advantage of this technique [150]. It is relatively easy to integrate the surface micromachined structures with on-chip electronics for increased functionality. Sacrificial Layer Substrate Substrate Substrate Fig. 4.7 Surface micromachining process [150]. Thin film deposition technique had improved in 1980s which revive the interest in surface micromachining technology. In same decade polysilicon surface micromachining was introduced, which opened the door to the fabrication of a 79

10 variety of microsensors. The schematic of surface micromachining process is shown in Fig Sacrificial layer is grown and patterned over the substrate. The structural material is anchored to the opening created in the sacrificial layer. Finally the sacrificial layer is removed, resulting in patterned microstructure. iii. Laser Micromachining: This technique uses light radiation with high energy density as a patterning tool and appears as efficient system for micromachining a wide range of materials without any mechanical or chemical interaction shown in Fig Short wavelength lasers such as excimer and Nd:YAG diode are adopted for micromachining. Excimer lasers have coherent characteristics and short wavelength, which allows the beam to be exposed on the substrate surface with mask project method [151]. Fig. 4.8 Laser micromachining process [152]. Large area machining for complex planar shapes can be obtained by moving mask as well as the work piece. Polymer micromachining is possible with technique to realise microholes, trenches, grooves, and patterns on the substrate with micro precision. Nd:YAG diode lasers are short pulse, high repetition rate, diffraction limited beam quality lasers, so harmonic generation are used to obtain double or triple wavelength laser near IR, visible or UV region. These lasers are used in micromachining of different materials. 80

11 iv. Micro Electro Discharge Machining: In this technique material is removed using electrical discharge between conductive tool and material in micro electro discharge machining [153]. Electro thermal erosion creates small carters which are copied in the material with a non contact system. This technique can cut materials metals, semiconductor, and conductive ceramics. Micro electro discharge machining is used to make micro drill with micro milling approach with simple shaped tools. v. Micro Ultrasonic Machining: In this technology ultrasonic vibration ranging from 30 to 40 khz frequency are used by micro tools to create accurate holes in hard and brittle materials like silicon, borosilicate glass, quartz, and ceramics. Micro ultrasonic machining is a combination of wire electro-discharge grinding (WEDG) and on machine masking method [134]. 4.3 Characterization techniques Nano scale materials demonstrate unique properties as compared to their bulk counterparts. Electronics and optical properties of the nanomaterial, alters their chemical activities and mechanical/structural stabilities. These properties make nanomaterial attractive for sensing purpose. But the nano scale device requires highly sophisticated tools for characterization processes. A large number of analytical tools are developed for the characterization of nanomaterial, nano devices, nano systems, etc. In the coming sections some of the most common characterization techniques are presented Optical microscopy Microstructure of the materials is examined by the researchers and engineers using this technique. In late 1880s the use of light microscope was started to examine the microstructures. It is widely used by the metallurgists to examine metallic materials. Also this technique is used to examine ceramics and polymers. An optical microscope comprises of the following main components: illumination system, objective lens, eyepiece, photomicrographic system, and specimen stage. Optical microscope shown in Fig. 4.9 can be categorized based on the illumination method used (transmitted or reflected light). Reflected light method is commonly used optical microscope where as transmitted light method is used to examine transparent or semi-transparent materials [ ]. On the basis of visible light generation system attached with microscope to analyze the specimen, illumination system can be divided into three categories low-voltage tungsten filament bulbs, tungsten halogen bulbs, and gas discharge tubes based systems. Wavelength 81

12 spectrum ranging from 300 to 1500 nm is provided by tungsten bulb. In colour photography, light temperature is important factor. Tungsten halogen also provides continuous spectrum of light. The light is brighter than ordinary tungsten bulbs. But it requires a heat filter in the light path and good ventilation. Gas discharge tubes are filled with mercury (Hg) or xenon (Xe) vapor which gives them high brightness. Hg gas discharge tubes are commonly used, which gives the arc of discontinuous spectrum but Xe has continuous spectrum. These tubes also require cooling system. The reflected light from the objects enables to analyze the object by optical microscopy. The light wave changes in either amplitude or phase but our eyes can perceive difference in amplitude and wavelength of light not the phase difference. Bright field and dark field imaging are commonly used mode of object examination, which utilizes the difference in the wave amplitude. A special optical arrangement needed to convert the wave phase difference into amplitude difference in phase contrast, polarized light and Nomarski contrast mode. Fig. 4.9 Morden optical microscope [155] X-ray diffraction Crystal structure of material is determined by X-ray diffraction, which is a far most effective method. This method is used to identify chemical compounds from their crystalline structure, but not their composition of chemical elements. Different compounds or phase having same composition can be identified using this method. Three different methods of diffraction are X-ray diffraction, electron diffraction, and neutron diffraction. In 82

13 1912 X-ray diffraction by crystals was discovered making it an extensively studied and used technique for material characterization. X-ray diffraction method can be classified under two categories spectroscopic and photographic. The spectroscopic technique is also known as powder diffractometry. Photographic techniques are not used excessively in modern laboratories are used to determine the unknown crystal structures [158, 159]. X-Ray Diffraction (XRD) is a useful characterization technique used to analyze material crystal structure. The wavelength of X-rays is very small of the order of angstrom; radiation at this wavelength is small enough to hit the multiple planes in the crystal structure [160]. Scattering of x-rays occurs due to interference from the constituent atoms in the crystal. It is the scattering of these waves from the different planes of the atomic structure that gives us a diffraction pattern. Since diffraction pattern is related to x-ray interaction with the planes of the crystal structure, if the planes of the crystal are oriented in a different direction it will yield a different diffraction pattern. These patterns are directly correlated to unique crystal structure [161]. They can give variety of information about the material like crystallite thickness, the interplanar spacing of the structure, orientation of the crystal or crystals in the film and residual stress in the film. This information is provided by the peak position and shape. The interplanar spacing of the crystal is calculated using Bragg s law shown in Fig [160, 161]: 2dsin n (4.1) where d is the distance between lattice plane, θ is the scattering angle, n is the order of diffraction peak, and λ is the wavelength of the x-rays. Fig Bragg s Law [161]. 83

14 In this tool polycrystalline specimen is examined using an X-ray beam of single wavelength. Incident angle of X-ray beam is continuously changed and spectrum of diffraction intensity versus the angle between the incident and diffracted beam is recorded. This technique enables to identify the crystal structure and quality of material by analyzing and comparing them with the spectrum of database containing over 60,000 diffraction spectra of known crystalline substances. Fig Schematic of X-ray diffractometer tool [159]. Figure 4.11 shows the geometrical arrangement of X-ray source, specimen, and detector. The generated X-ray passes through special slits which collimate the X-ray beam. Soller slits are commonly used in the diffractometer. A divergent X-ray beam passing through the slits strikes the specimen. X-rays are diffracted by the specimen and form a convergent beam at receiving slits before they enter a detector. The diffracted beam needs to pass through a monochromatic filter before received by a detector. The filter s arrangement suppress wavelength other than Kα radiations. Commercially available diffractometer uses Bragg Brentano arrangement, in which incident beam is fixed but the sample stage rotates around the axis perpendicular to the plane of incident angle. The technique of thin film X- ray diffractometry uses a special optical arrangement for detecting the crystal structure of thin films and coatings on a substrate. Thin film X-ray diffractometry requires a parallel incident beam, not a divergent beam as in regular diffractometry. A monochromator is placed in the optical path between the X-ray tube and the specimen. The small glancing angle of the incident beam ensures that sufficient diffraction signals come from a thin film. 84

15 4.3.3 Transmission electron microscopy The transmission electron microscope (TEM) is a scientific instrument that uses electrons instead of light to scrutinize objects at very fine resolutions shown in Fig Electron microscopes generate images of material microstructures with much higher magnification. The wavelength of electrons in these microscopes is about 10,000 times shorter than that of visible light. The resolution of electron microscope is of the order of few nanometers. Such high resolution makes electron microscopes extremely useful for revealing ultrafine details of material microstructure. Fig Schematic of TEM tool [162]. High energy electron beam is generated by electron gun for illumination purpose in TEM. Energy determines the wavelength of electrons and wavelength largely determines resolution of the microscope, the acceleration voltage determines the resolution to a large extent. In order to achieve high resolution TEM operates under an acceleration voltage of greater than 100 kv. Disadvantages associated with high voltage electron microscopy are high cost and risk of specimen damage during microscopy [162, 163]. 85

16 The information regarding crystal structure, crystal quality, grain size, and crystal orientation of the specimen axis is provided by TEM. To obtain amplitude contrasted image, an objective diaphragm is inserted in the back focal plane to select the transmitted beam. The crystalline parts in Bragg orientation appear dark and the amorphous are not Bragg oriented parts appear bright. This imaging mode is called bright field mode BF shown in Fig. 4.13(a). If the diffraction is constituted by many diffracting phases, each of them can be differentiated by selecting one of its diffracted beams with the objective diaphragm. This mode is called dark field mode DF shown in Fig. 4.13(b). The BF and DF modes are used for imaging materials to nanometer scale. The selected area diaphragm is used to select only one part of the imaged sample for example a particle or a precipitate. This mode is called selected area diffraction SAED shown in Fig. 4.13(c). The spherical aberrations of the objective lens limit the area of the selected object to few hundred nanometers. Nevertheless, it is possible to obtain diffraction patterns of a smaller object by focusing the electron beam with the projector lenses to obtain a small spot size on the object surface (2-10nm). The spots of SAED become disks whose radii depend on the condenser diaphragm. This is called micro diffraction. SAED and micro diffraction patterns of a crystal permit to obtain the symmetry of its lattice and calculate its inter-planar distances (with the Bragg law). Fig (a)bright Field (b) Dark Field (c) Diffraction Mode for imaging of TEM [163] Scanning Electron Microscopy The scanning electron microscope (SEM) uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. These signals provide information regarding morphology, chemical composition, and crystalline structure and orientation of materials making up the sample [164]. Data is collected over a selected surface of the sample and 2-dimensional image is generated. Conventional SEM technique 86

17 in scanning mode can produce image of structure surface ranging from 1cm to few micron. SEM provides valuable information regarding the structural arrangement, spatial distribution, wire density, and geometrical features of the specimen. Essential components of all SEMs includes Electron Source ( Gun ), Electron Lenses, Sample Stage, Detectors for all signals of interest, Display/Data output devices. SEMs always have at least one detector (usually a secondary electron detector), and most have additional detectors shown in Fig Fig Block diagram of SEM tool [164]. The specific capabilities of a particular instrument are critically dependent on which detectors it accommodates. Accelerated electrons in an SEM carry significant amounts of kinetic energy, and this energy is dissipated as a variety of signals. The incident electrons are decelerated in the solid sample. These signals include secondary electrons, backscattered electrons, diffracted backscattered electrons, photons, visible light, and heat. Secondary electrons and backscattered electrons are commonly used for imaging samples, secondary electrons are most valuable for showing morphology and topography on samples and back scattered electrons are most valuable for illustrating contrasts in composition in multiphase samples. X-ray is produced by inelastic collisions of the incident electrons with electrons in discrete orbital of atoms in the sample. As the excited electrons return to lower energy states, they yield X-rays that are of a fixed wavelength. SEM analysis is considered 87

18 to be non-destructive because X-rays generated by electron interactions do not lead to volume loss of the sample and thus it is possible to analyze the same materials repeatedly X-ray photoelectron spectroscopy In this electron spectroscopy technique electrons emitted from a solid for elemental analysis. The characteristic electrons (Auger or photoelectron) exhibit energy levels, revealing the nature of elements in specimens being examined. Characteristic electrons easily escape from the uppermost atomic layers of the specimen as their energies are relatively low while the X-ray can escape from a much greater depth, which makes this technique suitable for surface chemical analysis. Physically origin of Auger electrons and photoelectrons are different, but the electrons carry similar information regarding the chemical composition of the material surface [165, 166]. X-ray photo spectroscopy (XPS) system includes electron gun, X-ray gun, and analyzer for electron energy. Electrons ejected from upper atomic layer of specimen when the atom absorbs an X-ray photon are known as X-ray photoelectron. Figure 4.15 shows the illustration of the photoelectron emission from an atom when it is excited by X-ray photons. Incident X-ray photon with sufficient energy ( h ) knocks out an inner shell electrons in the K-shell electron ejected from the surface as a photoelectron with kinetic energy E K. Binding energy can be calculated based on the following relationship. E h E (4.1) B K Fig Electron emission in XPS [165]. where is the parameter representing the energy required for an electron to escape from a material s surface, h is Planck s constant, and is the frequency. E depends on both the sample material and the spectrometer. The characteristics value of the binding energies of B 88

19 the atomic electrons can be used to identify elements. X-ray photoelectron spectroscopy (XPS) identifies chemical elements from the binding energy spectra of X-ray photoelectrons. Fig XPS tool layout [166]. XPS measures the electronic states in a solid by irradiating a specimen with monochromatic x-ray radiation and analyzing the emitted photoelectrons. Figure 4.16 shows the schematics of XPS system. It consists of an ultra-high vacuum chamber equipped with an X-ray source, usually MgK and/or AlK (both can be monochromatic), a lens system that collects the ejected electrons, an energy analyser, a detector, and a suitable system for displaying signal intensity as a function of the kinetic or binding energy [167, 168] Atomic force microscopy Atomic Force Microscopy (AFM) technique is developed for surface characterization of sample. AFM is based on inter atomic force which is a non destructive technique as compared to SEM and TEM. In 1981 AFM was developed and capable of mapping three dimensional surfaces [169]. The first practical demonstration of vibrating cantilever in AFM was made in by Wickarmsinghe with an optical interferometer to measure the amplitude of a cantilever vibration [170]. AFM is used in obtaining images with atomic 10 resolution of 10 m or one tenth of nanometer. The probe of AFM is less than 50 nm in diameter and the area scanned by the probe are less than 100 m. Magnification of the 89

20 AFM may be between 100 X and 100,000,000 X in the horizontal and vertical axis. Figure 4.17 illustrates the block diagram of an atomic force microscope. The force on the needle of microscope is nanoscopic and the sample surface is measured with a force sensor. The output of the sensor is sent to a feedback controller which drives a Z motion generator. In order to maintain fixed distance between the probe and surface a feedback controller is used which utilizes the force sensor output. X-Y motion generators then move the probe over the surface in the X and Y axis. The motion of the probe is monitored and used to create an image of the surface. Fig Block diagram of AFM tool [170] Electrical characterization Electrical transport properties are important for micro/nano device characterization, electronic device applications, and the investigation of unusual transport phenomena arising from one-dimensional quantum effects [171]. i. Four probe systems are commonly used for measurement of electrical properties such as electrical conductivity and resistivity of thin films. The four-point probe is used to test the structure throughout the fabrication process to ensure and verify the condition of the device between the various processing steps. Active doping concentration and mobility of the semiconductor can be quantify using resistivity measurement. A four point probe uses four conducting electrodes that are set up to separately carry current and sense voltage. Figure 4.18 shows the most common layout in which four pins lined up linearly with the outer electrode injecting current in to the film and the two inner electrodes measuring the resulting electrical potential. In order to remove the resistance between the metal electrodes and the 90

21 underlying material in the resistivity measurement, injecting current and measuring voltage are done using separate electrode [173]. The four-point collinear probe technique of resistivity measurement involves bringing four, equally spaced, electrical conducting pins in contact with the material of unknown resistance. The array is placed in the centre of the material. An in-line four-point probe is used to determine the specimen sheet resistance at each desired measurement location. The number and positioning of measurement locations is determined by end-use need, or by the parties to the test in the case of referee measurements. At each location, a direct current is passed into the specimen, using two of the probes, as specified, and the potential difference is measured using the other two probes. Fig Schematic of Four-point probes measurement [172]. ii. Capacitance-Voltage measurement: A capacitance-voltage (C-V) curve is generated by plotting the dynamic capacitance as a function of the dynamic voltage drop across the device [173]. C-V analysis is analogous to modulation spectroscopy in which the first, second, and sometimes even third derivative of the raw data are obtained in order to remove background and enhance subtle features in the measured data. 91