The Kelvin Probe Force Microscope is a type of scanning probe instrument that is used to discern the different work functions of a sample. A sharp probe at the end of a cantilever is lowered onto a substrate where electrostatic forces, caused by the difference in work function cause the cantilever to oscillate at the modulated frequency. Using this instrument, high resolution images can be obtained, mapping the surface electronic characteristics. However, developments of this instrument have generally been limited to obtaining higher resolution images as well as reducing noise in the output, limiting the widespread appeal of this expensive instrument. There exist many applications where extremely cheap, low footprint and easy-to-use Kelvin Probe Force Microscopes would be beneficial. In order to cheaply produce this microscope in batch, a post-processed CMOS-MEMS device is utilized. The polysilicon resistors act as a strain gauge such that a conventional optical system will not have to be employed. The ability to use integrated bimorph actuators on chip allow for movement of the cantilever without the employment of large piezoelectric stages with creep effects. Embedded electronics can be fabricated with the CMOS process alongside the MEMS device, allowing full integration of an on board amplifier and read out system. In general, a large table top system can be minimized onto the size of a <1 mm2 area, a microcontroller and a computer. In this work, a Kelvin Probe Force Microscope is designed, fabricated and validated. A MEMS device was designed following similar characteristics of a generic cantilever beam. The stiffness, length, resonant frequency, and other tip characteristics can be mimicked with careful design. The resultant designs were fabricated using a CMOS-MEMS process. In order to obtain a sharper tip with modified characteristics, various methods were employed; such as gallium-aluminum alloy tip formation as well as electroless plating onto the tip of the device. Finally, the resultant device is tested against a sample. It was seen that the MEMS device followed similar characteristics of the conventional microscope itself, validating the equations that define the method. Bimorph actuators were tested to show movement, allowing the integration of the cantilever with the XYZ-stage. Work function changes are observed while scanning different materials. It is shown throughout the course of this thesis, that a true Kelvin Probe Force Microscope can be designed, fabricated and validated using CMOS-MEMS technology.

The Kelvin measurement is a sensitive and label-free method based on work function measurements. Work function, the minimum energy required to extract an electron from a metallic material, can be shifted by ionic charges and dipoles present on the surface. The scanning Kelvin nanoprobe (SKN), a probe-based microscopic imaging device, was used in the detection of work function changes induced by surface-immobilized oligonucleotide / DNA microarrays. The scanning Kelvin nanoprobe was able to study DNA microarrays smaller than 100 µm in size, produced with solution concentrations lower than 10 µmol/L. The limit of detection was estimated to be 15 ng DNA. Better than ± 10% relative variation was achieved for replicate spots. It was observed that higher surface densities of immobilized DNA molecules produced greater work function changes than lower surface densities. Surface saturation with increasing solution concentrations was observed as well. Also, longer strands of DNA produced greater work function changes than shorter strands. Statistical analysis of the results confirmed that non-complementary DNA strands could be differentiated from complementary strands by the Kelvin measurement. Single base mismatches on the complementary DNA strands were also detected by the Kelvin measurement. Different substrate materials were tested in the search for reliable and inexpensive sample slides with satisfactory DNA immobilization efficiency. Materials such as silicon wafers, gold-coated glass slides, gold-coated stainless steel slides, and gold compact discs (CD) were tested. A surface property comparison of gold-coated glass slides and compact discs was made by atomic force microscopy (AFM), and revealed very different microscopic features. The effect of cleaning on gold-coated glass slides was examined by time-of-flight secondary ion mass spectrometry (TOF-SIMS). Technical improvements were made to the SKN equipment progressively. Several revisions to the tip holder design have been employed for better electromagnetic shielding, enhanced robustness and easier tip change. An older signal generator was replaced with a professional PC audio card to provide more stable signal and more convenient on-screen fine tuning, also at a reduced cost. The Labview-based controlling program has also been improved through multiple iterations.

The Kelvin measurement is a sensitive and label-free method based on work function measurements. Work function, the minimum energy required to extract an electron from a metallic material, can be shifted by ionic charges and dipoles present on the surface. The scanning Kelvin nanoprobe (SKN), a probe-based microscopic imaging device, was used in the detection of work function changes induced by surface-immobilized oligonucleotide / DNA microarrays. The scanning Kelvin nanoprobe was able to study DNA microarrays smaller than 100 µm in size, produced with solution concentrations lower than 10 µmol/L. The limit of detection was estimated to be 15 ng DNA. Better than ± 10% relative variation was achieved for replicate spots. It was observed that higher surface densities of immobilized DNA molecules produced greater work function changes than lower surface densities. Surface saturation with increasing solution concentrations was observed as well. Also, longer strands of DNA produced greater work function changes than shorter strands. Statistical analysis of the results confirmed that non-complementary DNA strands could be differentiated from complementary strands by the Kelvin measurement. Single base mismatches on the complementary DNA strands were also detected by the Kelvin measurement. Different substrate materials were tested in the search for reliable and inexpensive sample slides with satisfactory DNA immobilization efficiency. Materials such as silicon wafers, gold-coated glass slides, gold-coated stainless steel slides, and gold compact discs (CD) were tested. A surface property comparison of gold-coated glass slides and compact discs was made by atomic force microscopy (AFM), and revealed very different microscopic features. The effect of cleaning on gold-coated glass slides was examined by time-of-flight secondary ion mass spectrometry (TOF-SIMS). Technical improvements were made to the SKN equipment progressively. Several revisions to the tip holder design have been employed for better electromagnetic shielding, enhanced robustness and easier tip change. An older signal generator was replaced with a professional PC audio card to provide more stable signal and more convenient on-screen fine tuning, also at a reduced cost. The Labview-based controlling program has also been improved through multiple iterations.

This study addresses the development of a sensor to monitor chemical adsorption and charge transfer processes on a surface using a contact potential difference probe (CPD). The current investigation is an outgrowth of ongoing research on non-vibrating CPD probes (nvCPD) which led to the recent development of a novel measurement technique utilizing optical stimulation: optically stimulated CPD (osCPD). Primary outcomes of this thesis are the theoretical modeling, fabrication and demonstration of a functional osCPD sensor. The research also involved significant engineering and experimentation in the design, development, and application of this sensor to oil condition monitoring. This technique measures dielectric and chemical properties of a fluid at the interface between the fluid and a semiconductor substrate. Chopped visible light is used to stimulate the rear surface of a semiconductor substrate, and a CPD probe measures the work function response of the semiconductor on the front surface of the substrate. The work function response is influenced by the nature and quantity of adsorbed species on the top surface, allowing the probe to detect changes in chemical composition at the substrate/fluid interface. An analytical model is developed that relates the osCPD sensor output signal to the chemical and dielectric properties of the oil sample, as well as to the geometry, composition, and control inputs of the silicon substrate and test fixture. In this investigation, the osCPD sensor was used to evaluate dielectric and chemical properties of commercially available engine oil. Oil samples were intentionally degraded through thermal aging (oxidation) and through addition of known contaminants. The osCPD sensor shows good sensitivity to depletion of antioxidants in the oil, as well as to the presence of ferric chloride, an oil-soluble salt typically used to calibrate laboratory test equipment.

This thesis addresses the use of a vibrating Kelvin probe to monitor the change in the front surface potential of a silicon wafer while the rear surface is illuminated with monochromatic, visible light. Two tests were run to verify the change in surface potential. One test increased the intensity of the light and the other increased the wavelength while recording the front surface potential. The change in the surface potential for a range of intensities of incident light was recorded and analyzed. The results show that the change in surface potential increased with increasing intensity. For each wafer, the smallest change in surface potential occurred at the lowest intensity, 3.77 mW. In the same respect, the largest change in surface potential occurred at the highest intensity, 17.8 mW. For all wafers, the change in surface potential ranged from approximately 8 mV at 3.77 mW to approximately 80 mV at 17.8 mW. The change in the surface potential for a range of wavelengths of incident light was also recorded and analyzed. The results showed that the change in surface potential formed a skewed bell curve with increasing wavelength of incident light. For each wafer, the largest change in surface potential occurred at mid-range wavelengths, between 600 nm and 700 nm. The smallest change in surface potential occurred at 450 nm, the shortest wavelength, and 800 nm, the longest wavelength. For all wafers, the change in surface potential ranged from approximately 8 mV at 800 nm to approximately 165 mV at 700 nm. A model based on excess electron diffusion within the silicon wafer was used to predict material properties. After curve fitting the model with experimental results, an excess electron lifetime of ôN = 17 µs and surface recombination rates of sFRONT = sREAR = 18,000cm/s were predicted. These values suggest poor silicon wafer quality relative to commercial silicon devices. Regardless of the quality, the results show that the front surface potential of a silicon wafer is affected by incident light on the rear surface. The quantitative effect of the light is dependent on the properties of the light and the material properties of the silicon wafer.

Many proteins are known to actively interact with biological, as well as inorganic and synthetic surfaces that are widely used in nano- and bio-technology as biosensing platforms and in tissue engineering. Amyloid fibrils are insoluble protein aggregates in beta-sheet conformation that are implicated in at least 20 diseases for which no cure is currently available. The molecular mechanism of fibril formation, as well as the mechanism of fibril clusters interacting with lipid membrane surfaces is currently unknown. The lipid membrane surface has a complex biochemical composition and is also electrostatically non-homogeneous. Currently, the experimental data available for amyloid fibril formation both on lipid and artificial surfaces is limited. The goal of our study is to investigate how the physical properties of the surfaces affect binding of amyloid peptides and affect the fibril formation. We seek to elucidate the effect of electrostatic interactions of amyloid peptides with surfaces using Atomic Force Microscopy (AFM) and Kelvin probe force microscopy (KPFM). We show using KPFM that electrostatic domains readily form within biological systems such as lung surfactant and lipid monolayers. We compared three different implementations of KPFM to demonstrate that frequency modulated (FM-) KPFM provides significant advantages over other modes. We also present a study of Amyloid beta (1-42) fibril formation on model surfaces, which are uniformly charged or possess periodicity of charges and hydrophobic functionality based on thiol self-assembly. Effect of membrane composition, surface charge, and presence of steroids will be discussed.

We have studied the introduction of defects in silicon wafers with low dose channelling ion implantation. (For complete abstract open document)

Orientador: Monica Alonso Cotta / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-11T21:43:56Z (GMT). No. of bitstreams: 1 NarvaezGonzalez_AngelaCarolina_M.pdf: 14145813 bytes, checksum: 31ac8f1ebde240684c9bbe88b9c9b7a7 (MD5) Previous issue date: 2008 / Resumo: As propriedades elétricas de nanofios (InAs, InP, InP-InAs-InP, InAsP) individuais e em junções foram estudadas implementando simultaneamente as técnicas Non Contact Atomic Force Microscopy NC-AFM (para aquisição da topografia) e Amplitude-sensitive Modulated Kelvin Probe Microscopy AM-KPFM (fornece medidas do Potencial de Superfície), permitindo correlacionar as propriedades elétricas com a estrutura da amostra. Em particular, o comportamento do Potencial de Superfície (PS) em função do diâmetro do nanofio foi investigado e utilizado na identificação do material que o compõe. Em uma primeira etapa, a técnica AM-KPFM foi caracterizada, principalmente em termos de resolução para análise de nano-objetos. Nossos resultados evidenciaram um fator de escala presente associado à eletrônica do equipamento, que somente permitiu realizar uma análise qualitativa dos dados adquiridos. Além disso, foi observada uma diminuição no contraste nas medidas elétricas quando o tamanho do objeto analisado diminui. Medidas em nanofios individuais de InP e InAs permitiram estabelecer que há uma queda no PS quando o diâmetro do fio diminui. Este comportamento é o resultado de duas contribuições: a perda no contraste (efeito de tamanho na medida) e o incremento local da função trabalho, que poderíamos associar ao aumento da proporção entre a carga superficial e a carga no interior do fio. Nas junções, há um aumento no PS na região da junção, indicando a formação de uma barreira de energia associada à acumulção de carga. Isto isola as junções do comportamento típico observado em nanofios individuais. Medidas em junções montadas em dispositivos poderiam complementar o estudo deste tipo de configurações. A caracterização do PS em função do diâmetro para os nanofios de InP e InAs permitiu a identificação do material (InAs ou InP) presente ao longo dos fios heteroestruturados de InP-InAs-InP, mostrando também a presença da nanopartícula de ouro usada como catalisador no crescimento. Os contrastes no PS ao longo do fio não se correlacionam diretamente às imagens de Microscopia Eletrônica de Transmissão, sugerindo que a interface elétrica é diferente da metalúrgica. Nos nanofios de InAsP, pelo contrário, os dados obtidos indicam a formação de uma liga ternária / Abstract: The electric properties of InAs, InP, InP-InAs-InP and InAsP nanowires (NWs) -assembled both individually and in junctions - were studied by simultaneous imple-mentation of Non Contact Atomic Force Microscopy NC-AFM (for topography) and Amplitude-sensitive Modulated Kelvin Probe Microscopy AM-KPFM (for Surface Potential distribution), obtaining spatially resolved electrical measurements of the sample structure. In particular, the SP vs NW diameter behavior was investigated and used to identify the material composing the nanowires. In a first approach, AM-KPFM was characterized mainly in terms of resolution for the analysis of the nano-objects. Our results suggest there is a scale factor on our measurements associated to the equipment electronics, that limited our discussion to a qualitative interpretation of the acquired data. Also, a contrast decrease on SP measurements was observed when the size of the object is reduced, comparatively to the tip. The experimental results on individual InAs and InP nanowires showed a SP saturation level (SPsat) below which SP drops with the NW diameter. This behavior came from at least two contributions: a loss of SP contrast due to object/tip size effects and a local increment on work function, that we associate to the larger surface/volume ratio close to the NW tip which makes the material more intrinsic. For NWs on junctions, a larger SP value is correlated to the regions where the junction is formed, possibly due to charge accumulation. Measurements of junctions assembled on devices could complement the study of this kind of structures. The SP vs diameter characterization of InAs and InP nanowires also allowed the identification of the material along the heterostructured InP-InAs-InP nanowire, showing the presence of the Au nanoparticle used to catalyze the growth. The SP image is not directly correlated with HR-TEM images, suggesting that electric and metallurgic interfaces are not the same. For InAsP nanowires, the acquired data indicate the formation of an homogeneous ternary alloy / Mestrado / Física da Matéria Condensada / Mestre em Física

Current magnetic data storage technology is encountering certain fundamental limitations that present roadblocks to its scalability to areal densities of 1 Tbit/in^2 and beyond. Next generation magnetic storage technology is expected to use optical near field techniques to heat the magnetic film locally to write data bits. This requires experimental measurement of thermal conductivity of materials with sub--100 nm resolution. This is essential for the tailoring of the thin film stack to optimize the heat transfer of the process. This can be accomplished with a simple modification to a traditional atomic force microscopy (AFM) system. The modification requires the deposition of a thin metal film on the AFM cantilever thus creating a bimetallic cantilever. The curvature of a bimetallic cantilever is sensitive to temperature. Another modification is the use of a heating laser to raise the temperature of the cantilever so that when it scans across a sample with areas of varying thermal conductivity the bimetallic deformation of the heated cantilever is altered. The resulting system is sensitive to local variations in thermal conductivity with nanoscale resolution. Nanoscale thermal conductivity measurements can then be used to optimize the heat transfer properties of the materials used in a heat assisted magnetic recording system. AFM technology can also play a key role in the development of next generation solid-state chemical sensors. An AFM can be used to measure the workfunction of a material with near atomic resolution thus enabling the study of chemical reactions with high spatial resolution. Since chemical sensors typically use a chemical reaction at their front end to monitor the prescience of a gas, an AFM system can thus be used to understand and optimize the properties of the chemical reaction by monitoring the local workfunction. In this thesis, I explain the use of atomic force microscopy in measuring thermal and chemical properties of materials with applications towards the magnetic storage industry and chemical sensing.

This dissertation details the study of band bending in n- and p-type GaN samples with a Kelvin probe utilizing different illumination geometries, ambients (air, oxygen, vacuum 10-6 mbar), and sample temperatures (77 – 650 K). The Kelvin probe, which is mounted inside an optical cryostat, is used to measure the surface potential. Illumination of the GaN surface with band-to-band light generates electron-hole pairs, which quickly separate in the depletion region due to a strong electric field caused by the near-surface band bending. The charge that is swept to the surface reduces the band bending and generates a surface photovoltage (SPV). Information about the band bending can be obtained by fitting the SPV measurements with a thermionic model based on the emission of charge carriers from bulk to surface and vice versa. The band bending in freestanding n-type GaN templates has been evaluated. The Ga-polar and N-polar surfaces exhibit upward band bending of about 0.74 and 0.57 eV, respectively. The surface treatment also plays a major role in the SPV behavior, where the SPV for mechanical polished surfaces restores faster than predicted by a thermionic model in dark. When measuring the photoluminescence (PL) signal, the PL from mechanically polished surfaces was about 4 orders of magnitude smaller than the PL from chemically mechanically polished surfaces. The PL and SPV behaviors were explained by the presence of a large density of defects near the surface, which quench PL and aid in the restoration of the SPV via electron hopping between defects. Temperature-dependent SPV studies have also been performed on doped n- and p-type GaN samples. In Si-doped n-type GaN, the estimated upward band bending was about 1 eV at temperatures between 295 and 500 K. However, in p-type GaN, the downward band bending appeared to increase with increasing temperature, where the magnitude of band bending increased from 0.8 eV to 2.1 eV as the temperature increased from 295 to 650 K. It appears that heating the p-type GaN samples allows for band bending values larger than 1 eV to fully restore. Pre-heating of samples was of paramount importance to measure the correct value of band bending in p-type GaN. The slope of the dependence of the SPV on excitation intensity at low temperatures was larger than expected; however, once the temperature exceeded 500 K, the slope began to reach values that are in agreement with a thermionic model.