Chocolate characterisation by using Laser Diffraction Particle Size AnalyzerChocolate is produced by processing cocoa powder along with milk and fats. The most important factor which interest costumer to consume chocolates is the flavor strength that chocolate created which is indicated by the taste of chocolate. Chocolate can be either bitter or sweet depending on the amount of cocoa used in the production. The art of consuming chocolates also relies on the chocolates texture, whether the chocolate is rough or smooth on the surface and the hardness of chocolate. The hardness of chocolate is governed by combination of the crystallized lipid phase and the solid dispersed phase. Not only affected by ratio of lipid and solid phase, the hardness of chocolate can vary depends on the packaging along with its storing condition (Windhab,2006). The other factors influenced chocolate consumption is chocolates physical appearance and how the environment affects it such as how the chocolate melts in the tongue due to the heat caused by human body. When aimed for smoother chocolate, milk fat is added into the mixture to soften cocoa butter to produce lighter soft chocolate (Rowat, 2011). According to Powis, 2012, interactions between raw materials combine with the correct manufacturing process would effect on the properties and structure of chocolate produced. Focusing on the physical properties of chocolate by relating back to its processing operations, the most important parameter that needs to be considered in order to produce chocolates as desired is the particle size distribution of chocolate. Particle size distribution becomes a crucial factor because it is closely related to how the produced chocolate will turn out to be especially regarding to its rheological behaviour and mechanical properties (Afoakwa, 2008). Throughout the process of manufacturing chocolate products, particle size distribution must be carefully controlled and maintain constantly since a little difference would lead to significant change to the chocolate produced and would effect on the oral behaviour when the chocolate is being consumed. Furthermore, particle size distribution in chocolate is also used to identify chocolates final structure and its final taste, whether the chocolate will be smooth, creamy or rigid and also the level of bitter and sweetness of chocolate product itself (Do, 2007). Do, 2007 also stated that if the chocolate particles are large meaning that small particle size distribution occurs and would result on a less creamy structure when compared to particles with larger particle size distribution. Chocolate would taste bitter due to small particle size distribution and would result on a more dense structured chocolate (Afoakwa, 2008).

Fig 1. Factors influenced oral processing for chocolate (Gaikwad, 2012)There are many ways to determine the particle size distribution and particle size of chocolates, for example through the process of screening. Screening is commonly used to determine particle size distribution on particles which has size larger than approximately about 40-60 m. Unfortunately, this process become inaccurate when the particle is smaller than the required range (Gaikwad, 2012). For small particle, common methods used to characterize particle size distribution is done by gravimetric registration of settling, observation of the change of transparency of suspension, microscopic particle counters with digital marking, electrical resistance changes and light scattering in laser beam (Mohos, 2010). Some of the methods relating to calculation of particle size which is used in the industry will be explain in this report. Chocolate has small particle size, sizing in approximately 18-35 m (Afoakwa, 2008), therefore determining particle size distribution of chocolate cannot be done by screening method. The most common equipment used in the chocolate manufacturing industry to determine particle size distribution of chocolate is laser diffraction (Malvern Instruments, 2010). Due to its simplicity and reasonable pricing, laser diffraction is a favourable measurement tool to study and examine particle size distribution (Mohos, 2010). Nowadays, modern laser diffraction tools are automated and standardized to allow a more accurate calculation to be resulted. The use of laser diffraction as a measurement tool also offers a less time consuming analytical process when compared to other particle size distribution analytical methods (Afoakwa, 2008).How does Laser Diffraction works? Laser diffraction which is invented based on the theory of light scattering, is now widely used in industry and believed to be the most effective measurement technique in calculation of particle size (Kippax, 2005). Light patterns used to characterise the particle size are then processed into mathematical analysis which is then further processed to produce an accurate picture of particle size distribution of the object. To achieve that goal, laser diffraction is commonly equipped with a computer and microscope to determine the type and shape of observant particle.

Fig 2. How laser diffraction worksAll the equipment required for laser diffraction and how the light scatter on the object and results an image projected to the multi-element detector is shown in Fig2. Chocolate sample is prepared under certain concentration dissolved in a suitable liquid. Fat-containing materials are dissolved in the liquid while particles are suspended. Most commonly used solvent is tricholoroethane, but due to the danger it causes to food products, the use of it has been replaced with sunflower oil or isopropyl alcohol (Malvern Instrument,2010). Chocolate sample is then placed in the middle of parallel beam in where a laser of monochromatic light source is shot at. Once it hit the sample, monochromatic light produces different scattering angles which are then followed through multi-element detector in where it is recorded for analysis. Smaller particles diffract light at a wider angles and different intensities than larger particles. From the detector, numerical values are produced and calculated using optical model and mathematical equation to produce ratio of total volume over number of particles resulting volumetric particle size distribution (Pain, 2005).Fraunhofer diffraction theory and Mie scattering theoryThe signal of measurement from multi element detector is translated into particles size through mathematical calculation based on theory known as Fraunhofer and Mie theory. The correct equation and formulae are needed be applied in order to lessen error calculation especially for the small particles.Mie theory is a more complex theory which is applied to determine particle size distribution while Fraunhofer diffraction is a simpler form of Mie scattering theory. However, fraunhofer diffraction can only be used to measure particles bigger than the incidence wave length (Pain, 2005).According to Fraunhofer diffraction theory, when a spherical particle of diameter (d) is within measurement area, its light intensity distribution of any angle is:

In the equation:f the focal length of receiving lensthe wavelength of incidence lightJ1 first order Bessel function =

X : scattering angle min : 1.22 /DBased on the calculation of light intensity from equation 1, it resulted a graphic drawn in fig 3. From the graphic, it would also represent the distribution of particle sizes of material being observed since light intensity pattern has correspondence relationship with particle size (Interference and Diffraction).

Fig 3. Fraunhofer diffraction patern for a circular aperture (Interference and Diffraction)When smaller particles are required to be measure, Mie theory is used as its based of calculation. However, the use of Mie theory requires knowledge of parameter such as refractive index and absorption value. The refractive index can be calculated by using refractometer measurement or it can also be calculated using empirical methods (Malvern, 2008).

Fig 4. Appearance of particles and the absorption value (Malvern, 2008)While the absorption value can be evaluated by estimating particles physical appearance under a microscope comparing it to fig 4. Based on calculation of calcium carbonate with refractive index of 1.57 and the absorption of 1, 0.1, 0.01 , particle size distribution from those values is plotted in fig 3 (Malvern, 2011).

Fig 5. Particle size distribution resulted from laser diffraction calculation (Malvern, 2011)Advantages and Disadvantages of Laser DiffractionDespite all its advantages stated above, the used of laser diffraction in the industry to analyze particle size distribution addresses few disadvantages within the chocolate manufacturing industry. A list of benefits and disadvantages is stated in table 1.

Table 1. Advantage and Disadvantage of Laser Diffraction Application (Kippax, 2005) AdvantageDisadvantage

Applied in a wide range of industrial application.Low resolving power. Narrow, the difference in size between particles must be at least 15%

This technique is very flexible and able to measure a wide range of particle size from micron to milimeters.

The final result depends on accuracy of refractive index and light absorption and also information about particle shape.

A fast measuring time is needed It cannot be used to measure mix of particles with different optical properties.

Because it requires less-time, measurement can be done over several times providing more data to be averaged therefore resulting a more accurate measurement.Often, problems are caused by particles which strongly absorbs light because it may not produce stable scattering signal.

Ease of verification. Laser diffraction requires no calibration and can be verified easily.

Other techniques that are used to characterise chocolateThere are other similar methods that can be used to analyse particle size distribution of chocolate. One closely related to laser diffraction process is dynamic light scattering (DLS). Similar to laser diffraction, DLS also uses light although the use of light in dynamic light scattering is used to reflect the Brownian motion of particles. Brownian motion is measure in the function of time. Dynamic light scattering is rarely used in characterising chocolate because it produces low resolution (CPS Instrument Europe). Due to chocolate particles which are similar in size, it would be harder to differ from one another.Coulter counter is another method commonly found in industry and mostly prefered as a method in analysing particle size distribution. Volume of particle is measured by allowing number of suspended particles to pass through an orifice which is immersed by an electrode. Electrical resistance measurement technique is used as based calculation of coulter counter process. Although coulter counter produces high resolution result and can be applied to calculate in a wide range of diameter ranging from 0.5-400 m, but this method is not suitable in chocolate characterisation due to high density of chocolate paste. Also coulter counter process needs calibration and produces error when used to measure low particle concentration (Graham, 2003).Not only particle size distribution, rheology of chocolate also plays an important role in determining the quality in taste and physical behaviour of chocolate product. Rheology of chocolate is usually based on yield stress and viscosity parameters. Conventionally, the International Confectionery Association (ICA) recommended the use of rotational viscometer with concentric cylinders for rheological measurement (Afoakwa et al, 2009). The use of rotational rheometer allows controlling the applied shear stress and shearing strain in order to create a flow curve. The viscosity of chocolate is determine by measuring the resistance of rotating cylinder encounters in a chocolate at a specific temperature since viscosity and yield value are temperature dependant. There are three types of rotational rheometer used in the industry for measuring rheological of chocolate; coaxial cylinders, parallel plate and cone and plate rheometers.Nowadays, the method of calculating shear viscosity and yield stress of chocolate using Magnetic Resonance Imaging visometric is widely used in chocolate manufacturer to calculate rheological properties for quality assurance of chocolate product (Wichchukit et al, 2005). Magnetic Resonance Imaging viscometric (MRI) is believed to be a non-destructive technique and a rapid process since shear rate and shear stress value can be calculated from a single flow rate measurement. Other advantage of MRI viscometric is that it is an in-line instruments allowing real time controlling of the process (Goloshevsky, A.G et al. 2005). When compared to other conventional viscometer and rotational viscometer, MRI viscometric is favourable because it provides a velocity profile image that can be used to study slip phenomena (Wichchukit, 2004).The effect of microstructure to the texture and physical appearance of chocolate productsParticle microstructure has significant impact on the rheology of chocolate. The determination of rheological properties of chocolate is important in manufacturing processes for obtaining high quality products with well-defined texture. The texture properties of chocolate are identified by the hardness it is in the mouth, meltabilitily, smoothness and stickiness (El-Kalyoubi,2011). Factors such as fat content, moisture content, emulsifiers, temperature and particle size distribution contributes to the rheological properties which effect texture of chocolate product (Albrecht et al, 2011). Viscosity is one of the rheological properties which is easily influenced by the external factors but can easily calculate and controlled as well. Some studies have found that chocolates of high viscosity have a pasty mount feel which last longer in the mouth than the one with lower viscosity but too high of viscosity would result a very hard solid-like chocolate (El-Kalyoubi, 2011). According to Windhab, to produce a smooth and creamy chocolate, chocolate must be processed to have a viscosity of 1.5-3.5Pa.s at shear rate of 20/s.Plastic viscosity is used to determine pumping characteristics, filling of rough surface, coating and sensory character of chocolate mass (Goncalves, 2010). Viscosity is closely related to particle size distribution, composition and processing strategy. The larger the particle would result a lower viscosity as well as lower yield value (Albrecht et al, 2011). Both viscosity and yield value are temperature dependant, but an increase in temperature would result viscosity to decrease and yield value to rise (Anton Paar).

Fig 6. Relationship between viscosity and shear strees in production of chocolateViscosity is parallelly connected with yield value based on the Casson parameters (Kutschmann,2000). Once the yield value is passed, the chocolate will behave like solid. As shear stress increases, the structure effects get further developed resulting decreasing in viscosity until a shear-stress equilibrium structure is reached. As drawn in Fig.5 at 1, the viscosity reaches its minimum value min approximately 0.5 2 Pa.s and the chocolate structure is completed. When being further processed, agglomerate structure would occur resulting the chocolate to collapse.

Microstructural characterisation of coffee bean using X-Ray MicrotomographyThe most important parameters during roasting in the coffee processing is the microstructural appearance of beans (Schenker, et al, 2000). By studying the change of coffee beans structure, the taste of coffee produced can be modified. Not only type of coffee beans used as raw materials, previous studies has shown that quality of coffee can also be improved by modifying temperature in which the bean is processed (Clarke and Vitzthum, 2001). Roasting is an essential process which need to be done in order to physically treat beans by heating it at temperature up to 250C (Illy and Viani, 1995). Roasting also affects mechanical properties of the coffee beans. The process of roasting is known to produce beans which are characterized to have crispy texture and apparantly more fragile in structure (Pittia et al, 2001). The crisp resembles low water content in the beans which cause a decrease on its density and is often used as parameter in the presence of microspores (Schenker et al, 2000).Pittia et al, 2001 believed that heat reactions determine changes in chemical, chemical-physical and structural properties of coffee beans which affect the characteristic, taste and texture of coffee. Phenomena of heat transfer along with degassing in the process of roasting combined together with gas adsorption capacity are factors that mainly govern by coffee beans structural properties in focus to the change of pore structure. There are several techniques which can be applied to characterize coffee structure such as light microscopy, scanning and cryo scanning electron microscopy (Huang et al, 2002). The most favorable innovation which is commonly applied in industry is the application of 3D imaging analyzes, the X-ray microtomographs. By using these mechanical tools, it would be easier and more accurate to analyze beans microstructural characteristics such as porosity, thickeness and the morphology of beans.In the application of microtomography, there are two devices mostly found in the industry which are synchroton radiaton X-ray microtomographs and dekstop X-ray microtomographs (Goel et al, 2001). Comparing to those two types of microtomographs, synchrotons is a more conventional method applied due to its limitation on the intensity. Synchrotons also uses polychromatic fan beams resulting on low resolution images therefore it would be hard to investigate in greater detail (Goel et al, 2001, Huang et al, 2002).Desktop microtomograph is constantly developed in recent years and produces synchrotons X-ray microtomograph. Synchrotons X-ray microtomograph on the other hand, can produce a monochromatic parallel beam which results a higher intensity, leading to a higher resolution images that are suitable for characterisation of detailed structured material (Samuelsen et al, 2001). According to Samuelsen et al, 2001, this method is widely used in clinical diagnosis especially related to medical application, material science, geology and biology but recently it also has been widely used in the study of food microstructure.

How does X-Ray Microtomography works? Comparing to other characterization technique, X-ray microtomography can be considered as a new method invented in the late 1870 (Sasov, 1987). This technique enables visualisation of samples structure in three dimensional and it is believed to be a non-destructive method which means during preparation of the sample, it does not need to undergo series of processes which might disturb samples structure (Sasov et al, 1998). X-ray microtomography is functioned based on how the X-ray beams interact with the sample provided. X-ray light will be weaken as it passes through a sample. The decrease on the light intensity depends on the density and atomic number of the sample (in this case the coffee bean) and the use of X-ray energy. A reconstruction of the sample can be drawn by using images of the sample projected from different angles. Supported by mathematical calculation, X-ray microtomography creates cross sectional images of the internal structure of the object. When these different consecutive slices are reconstructed a 3D visualization can be obtained with high resolution.

Fig 7. Schematic of the x-ray microtomography device (Davis et al, 2002)

As it can be seen from fig 6 a beam of monochromatic X-ray illuminates a sample placed in front of a phosphor plate. A phosphor plate, in this case, is function to convers light to optical radiation with high resolution (Davis et al, 2002). The intensity of X-ray light is recorded by a digital, electro-optic detector while the projections of the sample are recorded by a CMOS (complementary metal-oxide semiconductor) flat panel detector with an integated scintilator. These projections are used to reconstruct three dimensional (3D) models of the observed sample. Then two dimensional (2D) slices can be cut through the 3D models in different directions. According to (Dalen, 2000), this method offers possibilities to attain resolution until micrometer scale depending on the size of the sample being studied. X-ray microtomography is often used to characterize highly porous material and composite materials due to its ability to provide detail information about the density profile of the sample, pore texture (pore density and pore size distribution), defect and dimensional measurements (Proudhon, 2010).

(a) (b)Fig 8. (a). Grey scale tomography of coffee bean (b). 3D reconstruction of coffee beans using microtomography(Frisullo, 2012)

Advantages and Disadvantages of X-Ray MicrotomographyThe main advantage of tomography is its three dimensions result produced in high resolution whereas other methods only able to construct structures of sample onto a two dimensional image. Three dimensional structures becomes very useful allowing for better observation when studying human tissue (Barbetta, 2012). Microtomograph is excellent for bone imaging with good resolution which can be up to 6m when combined with contrast agent. It is also decent in terms of image acquisition times, which can be in the range of minutes for small animals ( Koo, 2006). One of the major drawbacks of microtomograph is the radiation dosage placed on test animals. Although this is generally not lethal, the radiation is high enough to affect the immune system and other biological pathways which may ultimately change experimental outcomes (Boone,2004). Also, radiation may affect tumor size in cancer model as it mimics radiotherapy and thus extra control groups might be needed to account for this potential variable. In addition, the contrast resolution of microtomography is quite poor and thus it is unsuitable for distinguishing between similar tissue type such as normal vs diseased tissues (Mizutani and Suzuki, 2010).X-ray microtomography is believed to be an efficient technique and prove helpful for identifying organic gem materials, corals and ivory, however it cannot be used to identify sample with metal layers (Karampelas, 2010). In addition, microtomography instrumentation is still very costly and yet it still requires technique to scientifically trained staff for analysis and interpretation. Other techniques that are used to characterise coffeeIn application, the roasting process must be situated in an optimum condition paying particular attention to the degree of roasting in order to produce coffee product as desired Illy and Viani, 1995). Degree of roasting is an important parameter in coffee roasting in order to determine the flavour of coffee product based on the coffee flavour profile as seen in fig 8. The differences in texture, porosity and taste of coffee are likely to be resulted due to different temperature which would lead to the change in degree of roasting.

Fig 9. Flavor by roast degreeDegree of roasting can be determined by visual observation and analytical methods. Visual observation includes judging the coffee bean through its colour change during the process. Although its a less accurate mechanism, but this method is commonly done in rural area. Another way to identify degree of roasting is by listening to the cracking sound resulted during roasting. There are two cracks that would likely to occur during roasting, at about 200-202 C which identifies the beginning of light roast and another crack occurs at 224-226 C (Wang, 2012).Analytical methods is believed to be a more accurate way in quantifying the degree of roast. This method includes the use of gas chromatography - mass spectrometer to measure ratio of free amino acids (Nehring & Maier, 1992) and alkylpyrazines (Hashim&Chaveron, 1995) and GC-MS with stable isotopes as internal standard (SIDA) for calculating the chlorogenic acid content inside the roasted coffee beans (Illy & Viani, 1995). Gas chromatography with mass spectrometer is a reliable analytical instrument in where seperation, identification and quantification of components can occur continously at the same time (Hajslova and Cajka, 2007). Gas chromatography is functioned to seperate compounds into various components and the mass spectrometer does the analysis of the components.

Fig 10. Gas chromatography-mass spectrometerAccording to Sparkman, et al (2011), the process starts by mixture of molecules is injected to the GC. The sample is carried by helium, heated until it become gases. Chemical is then separated in the column based on their volatility. After being separated, molecules are then blasted with electrons which causes them to break up into ions. As ions continue through the mass-spectrometer, they travel through an electromagnetic field that filters the ions based on mass. The filter continously scans through the range of masses as the stream of ions come from the ion source. The number of ions that passes through is measured by using a detector to create a mass spectrum and information is then sent to computer to be analyzed.Meanwhile, despite all its advantages gas chromatography-mass spectrometer has several disadvantages which is the presence of disturbance in the mass spectrum (Gohlke and McLafferty, 1993). This is occured due to unproper separation of gas chromatography which results impurities to enter the mass spectrometer. This problem can be encounter by using GC-MS with stable isotopes as internal standard (SIDA) although it will cost slightly more expensive to purchase the isotopes required (Pickard, et al, 2013).Other physical methods such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), cyro-scanning electron microscopy which have been widely used to determine characteristic of roasted coffee beans especially regarding to its porosity and changes of structure (Massini et al., 1990). The drawback of those methods is that it requires complicated sample preparation. Before being analysed, the sample needs to be freezed or dehydrated hence it would affect on the change of structure in certain types of product (Frisullo, 2012) (a) (b)Fig 11. (a). TEM micrograph of a cell wall in a coffee bean(b). Cryo-SEM micrograph of coffee bean tissue (Schenker, 2000)

The effect of microstructure to the texture and physical appearance of coffee productsThe unique characteristic of food product are defined by many aspects and mostly observed by the product appearances. Coffee for example, physical characteristics of coffee product is closely related to how coffee bean is being treated. Size, shape and colour are important parameter which has been commonly used to grade dry beans (Illy and Viani, 1995). However, coffee beans have a porous, spongy texture therefore they are vulnurable of fungi. Infected beans would result an off-flavour, strong smell and decrease in quality if the bean is too moist (Sivetz, 1991). According to Dutra, et al. 2001, the taste and texture of coffee product is likely to have developed during the roasting steps. During roasting, beans is treated through series of processes where chemical and physical changes is likely to occur (Dutra, et al, 2001). Temperature and heat transfer factor becomes important in determining the characteristic of coffee beans and it must be maintain in a constant condition to prevent beans from burning. Commonly, time-temperature profile graphs is used to condition coffee bean roasting process in order to develope a unique flavour desired. (Buffo and Cardelli-Freire, 2004).Beans which are infected by environmental condition (such as immature beans, split in beans, beans damaged by insect) also plays important role in determining the flavour of roasted beans. Non damaged beans are heavier in weight comparing to faulty beans and believed to havea a higher water activity (Mazzafera, 1994). Due to higher water content, non faulty beans takes a longer time for the first crack to occurs.Coffee are commonly categorized as light, medium or dark roast based on the heat process. On different level, coffee beans tends to result specific flavor and different appearances as stated in table 2. A sour and underdeveloped flavour is commonly found during light roast level which is indicated by the first crack. Further heat treatment results a first crack is known as medium level. During this level, sweated process occurs producing a bright brown coloured coffee with well-balanced taste. When roasting continuous, dark roast level occurs resulting low acidity and a shiny coffee beans (Lyman et al, 2003).Table 2. Roast level and their flavors NotesSurfaceFlavour

LightDuring this stage, a pop sound is heard or known as the first crack. It occurs due to the rise of temperature in coffee bean.DryHigh acidity is achieved during this process and the weigh of bean is decreasing due to water being vaporized.

MediumFollowing the first crack, the roast level reaches its medium phase.DryCaramelisation of sugar occured and the level of acidity is decreasing. It produce a larger bean in shape.

DarkIn this phase, another pop sound is released and the bean become very oily. This phase is called the second crack.ShinyThe flavour is more bittersweet and it smells like roasted bean.

Microstructural characterisation of thin films using Spectroscopic EllipsometryMeasurement of thin film has been a challenge for past several years but the problem has been solved with the innovation of light analysis technique known as spectroscopic ellipsometer. Spectroscopic ellipsometer is a tool which enables measuring optical constant (extinction coefficient and refractive index) of film structures as well as calculating film thickness of thin film (Horiba). Spectroscopic ellipsometer is generally used to measure materials such as semiconductor thin films, dielectric gates, dielectric films like YSZ, characterisation of photoresist in UV, optical coating thermal barrier coating with YSZ thin films for turbo reactor blades, self-assembled monolayer, chemistry-polymer thin films, optical components like mirrors, coating and LCD displays (Jasco). Due to its sensitivity to very thin layers, this analytical method is commonly applied in measuring very thin layers ranging from 1 A to 1micron (Laughlin, 2005). Ellipsometer is designed with two different polarization measurements allowing twice much information therefore a more accurate analysis can be produced (Hiroyuki, 2007). Ellipsometer can also be set at variety of incidence angles resulting an increase amount of data which will be used for analysis.

Fig 12. Measurement principle of ellipsometryHow does Ellipsometry works? Ellipsometry works by lighting sample with a parallel beam of monochromatic light being set to a variable angle of incidence. The change of polarisation after the light is reflected from the surface of sample is analysed using polarisation-sensitive analyser (Pascu, 2009). From the change of polarisation, measurement of optical parameters from the sample is carefully conducted. On the other hand, variable angle of spectroscopic ellipsometry is used to determine optical constants which are refractive index and extinction coefficient through the reflection of light beam from the surface of the sample (Rothen, 1974). Refractive index (n) represents the propagation speed of the wave through the sample and direction of propagation while extinction coefficient (k) relates to how much energy of the wave is absorbed in a material (Den Boer, 1995). Refractive index and extinction coefficient is then modelled through computer software in order to calculate thickness of single layer. By processing data from many points on the sample, it is then able to generate two dimensional and three dimensional sample surface.There are three most common used ellipsometer applied in the industry for characterisation of thin films (Tompkins, 2005). Those elipsometer differs in term of polarizer system and analyser, also the source of beam from the light is coming. The basic design of ellipsometer are nulling ellipsometer, rotating analyser and photo elastic modulator ellipsometer. Comparing those three ellipsometer designs, the most simplest design used is the nulling ellipsometer (Nulling Ellipsometry). Although the disadvantages when this method is being applied are the optical component is set to a specific wavelength and it cannot be modified. Also due to the measurement which occurs in real time, a minor mistake would affect the end of calculation resulted.

Fig 13. Nulling ellipsometerRotating analyser ellipsometer is the oldest most-commercially used equipment in the industry. Similar to other types of ellipsometer, this tool uses linearly polarized incident light reflected from the sample to analyse the thickness of thin film. The only difference is that the polarisation analyser is a rotating analyser, which is used as a detector to measure output intensity as a function of polarization angle (Chen, 1987). Rotating analyser ellipsometer is favourable because it is a less time consuming process and relatively easy to control being compared to nulling ellipsometer and photo elastic modulator ellipsometer.

Fig 14. Rotating analyzer ellipsometerPhoto elastic modulator is a new innovation of ellipsometer developed. Due to the ability of not requiring any moving parts to get the data, this ellipsometer is known as solid state ellipsometer. Photo elastic modulator works as programmed by computer and easier to control electronically by using a remote. PEM produces light beam, polarized at 50 kHz and it is well-equipped with static analyser allowing the light to reflected from the sample surface into detector and generated signal from it (Hinds Instruments).

Fig 15. Photo elastic modulator ellipsometer

Calculation of Film Thickness by using Spectroscopic Ellipsometer

Fig 16. Ambient film substrate system (Pedersen, 2004)Thickness of the film can be calculated when measurement are done in three system as drawn in fig. 15 keeping in mind that the refractive indexes in all media are known. When the light from ellipsometer hits the surface at ambient, o is created. As it continues its way down to film and substrate, it created the angle of reflection of 1 and 2. Based on method of calculation in ellipsometry written by Pedersen, (2004), the angle of reflections are then used to calculate reflection coefficients using equation of 2.a and 2.bP = . (2.a)P = .. (2.b)Equation 2.a and 2.b is then inserted in complex reflection ratio equation given in 2.c to find the value of P (Azzam & Bashara, 1977, p.288). Once P is found, further calculation is done to determine the value of refractive index (X) using equation 2.d. Refractive index is related to the film thickness stated in equation 2.eP = P . (2.c)P = . (2.d)Where A = ; B = ; C = ; D = ; E = ; F =

ln (X) = - j.4 .. (2.e)d = If a set of ellipsometric parameters are measured at a given angle of incidence and a given wavelength with the thickness of film is the only unknown, thus the thickness of a sample can be calculated.Advantages and Disadvantages of Spectroscopic EllipsometryThe use of ellipsometry is believed to be a non destructive technique because it requires uncomplicated sample preparation. This method is also favorable due to less time consuming process and ease in operation (Collins, 1990). Other advantages of spectroscopic ellipsometry is that it is very sensitive, can be applied in both measuring single and multi layer film and also has the capability to measure ultra thin films as thin as 1 Amstrong (Morton, 2002). Direct measurement of optical constant (extinction coefficient and refractive index) can be achieved in a more accurate manner compared to other analytical methods. Despite all the advantages, spectroscopic ellipsometry has some drawbacks as well such as significant change in temperature during the process would cause an error to the result. This process also requires variety of voltages to be applied for different wave length Pascu, 2009. Technical difficulties are also found when characterizing thin films which is thinner than 10 nm since it would be hard to differentiate between layer and substrate and also on thick films since there is interference oscillation which is difficult to resolve . Spectroscopic ellipsometry is a very sensitive device that thin film being observerd must be uniform in thickness to get a more accurate measurement and reducing error in the result. Another factors that needs to be carefully watched is the roughness of the film. To achieve accurate calculation, thin films must be smooth to prevent light from depolarized (Den Boer, 1995).Other techniques that are used to characterise thin filmsThere are several techniques which can be applied to characterise thin films especially in measuring the thickness of film. One of the most commonly used and cheap in application is spectroscopic reflectance. This method has been widely used to measure the thickness, roughness and optical constant of thin films. Film thickness of 10A-100A can be measured by spectral reflectance by varying the wavelength of reflectance (Filmetrics,2005). However, when films are too thin, too numerous or to complicated to be measured, this method would result inaccuracy and error to the system. In the industry, spectral reflectance is often used to measure SiO2 on silicon (Hlubina, 2009).Other method used as thin film measurement techniques is X-ray reflectivity (XRR). This method is believed not only able to determine thin film thickness but also other film parameters such as density, surface, interface roughness and can be used to analyze the layer structure of film. According to Yasaka, 2010, X-ray reflectivity measurements were performed using X-ray diffractometer. A rotating anode Cu was used along with the X-ray beam was monochromatized using monochromator. The incident and reflected beams were collimated and the reflection intensity was measured by a scintillation counter. Thickness and roughness determination is needed to be done in high accuracy therefore it is essential to precisely align a sample position to the X-ray beam (Inaba, 2008). The sample is mounted on a vertical sample stage and allignment is done automatically via computer.The effect of microstructure to the texture and physical appearance of thin filmsWhen thin films are analyzed, the most important factor that needs to be considered particularly related to the microstructure of the film itself because a slight change in the formation of microstructure often result a significant change to the thin films properties. The effect of change in microstructure would affects mechanical properties of thin film such as hardness and plasticity properties (Bull, 2012). Experiment conducted by Bull focuses on the affect of grain size to the hardness of thin films. His investigation resulted that smaller grain size of film leads to a harder film. The hardness of film increases in the experiment as the grain size decreases but the hardness of thin film remains constant when large grain size is being observed. Furthermore, the hardness of thin films plays an important role on determining its plasticity properties. Yong (2005) observed the affect of microstructural changes of thin films towards films plasticity properties. Plasticity properties of film occurs due to annealing process. Prior to annealing process, the stress strain curves shows changes from elastic to plastic behaviour. This transition can affect to diversity of stress between grains. Due to heat treatment dring annealing process, it results a decrease in the thickness of the films (Naceur, 2012). Thicker films is believed to have a more step by step progress from elastic to plastic behavior when compared to films which are thinner in height because thicker films have a wider grain size distribution (Yong, 2005). When comparing films wih similar thickness, a more significant transition is seen in all films due to annealing. As the thickness of film decreases, the yield stress increases a little but it soon drops significantly after the process of annealing. After the process of heating, when the temperature of film is cooled down, thermal transfer occur in the film and substrate. Heat transfer resulted development of tensile stress to occur in grains hence a full plastic flow is resulted. Thus immediately after cooling, all grains are roughly at their yield stress. The same yield stress is caused by stress relaxation and elastic anisotropy of the grain. Macroscopically, this results in a sharp transition of the stress-strain curve from the elastic regime due to plastic regime.

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