Sapphire Devices Research Articles

Effect of amino acid complexing agents on chemical mechanical polishing performance and action mechanism of C-plane sapphire: Combining experiments and theoretical calculations

Chemical mechanical polishing (CMP) of sapphire faces greater challenges as the performance and processing requirements of sapphire devices continue to improve. Amino acids are often used as complexing agents on CMP because of their high water solubility, environmental friendliness and strong complexing ability. In this study, a combination of experimental techniques and theoretical calculations were used to investigate the effects of glycine (Gly), serine (Ser) and lysine (Lys) as complexing agents on the CMP properties of C-plane sapphire. The existence forms of three amino acid molecules under alkaline conditions were analyzed, and their possible binding modes to sapphire were discovered. Experimentally, it can be found that all three amino acids can improve the performance of C-plane sapphire CMP in different degrees. The measure results of X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) show that the three amino acids can be complexed with the Al(OH)4‒ ions produced by the sapphire to form soluble substance, thus speeding up the removal of sapphire. By calculation, it can be shown that the process of complexation is that the –NH2 and –OH groups in amino acids form covalent bonds with Al atom in Al(OH)4‒. Finally, the complexation capacity of the three amino acids is in the following order: Gly < Ser < Lys. Therefore, this study can confirm that amino acids as complexing agent have a good complexing effect on sapphire CMP, and the addition of –NH2 and –OH groups can help to enhance the complexing ability of the complexing agent, which has a certain good guiding significant for the selection of sapphire CMP complexing agent.

Effect of hydroxy carboxylates as complexing agent on improving chemical mechanical polishing performance of M-plane sapphire and action mechanism analysis

As sapphire device performance continues to improve, greater challenges are posed to the chemical mechanical polishing (CMP) of sapphire, with its high degree of hardness and brittleness. M-plane sapphire substrates are not widely used because they are more difficult to process, despite having higher luminous efficiency than C-plane substrates. In this study, the effect of three hydroxyl carboxylates, namely potassium tartrate (PT), potassium citrate (Cit) and sodium gluconate (Gluc), as complexing agents on the CMP of M-plane sapphire was investigated to obtain a high material removal rate (MRR) and low root mean square surface roughness (Sq). First, the chemical reactivities of the three complexing agents were predicted with Material Studio (MS) software. The predicted results showed that the complexing ability of the three complexing agents was greatest for Gluc, followed by Cit, with PT having the least complexing ability. Experimental results confirmed that Gluc was the optimal complexing agent for the M-plane sapphire CMP. The mechanism of action during CMP was revealed by X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (FTIR). The results showed that the Al(OH)4‒ ions produced by the sapphire were complexed by Gluc to form the soluble complex Al(OH)4‒/Gluc‒. At the same time, a solid phase reaction also occurred between the M-plane sapphire, SiO2, and water during CMP, and Al2Si2O7∙2H2O was generated. After polishing with the optimized slurry, the M-plane MRR was improved to 5.358 μm/h, a 50% improvement compared with the reference slurry, and the Sq decreased from 0.345 nm to 0.172 nm. These findings provide important guidance for the development of high-performance sapphire devices.

Modeling a broadband detector for an experiment that measures the speed of gravity over short distances

In order to investigate the behavior of gravitational signals while travelling through a medium an experiment was designed, aimed at measuring the speed of these signals over short distances. The experiment contains 2 sapphire devices that behave as a detector, which are suspended in vacuum and cooled down to 4.2 K. The amplitude of the detecting device is measured by an ultralow, phase-noise microwave signal that uses resonance in the whispering gallery modes. Since sapphire has a quite high mechanical Q, the detection band is expected to be small, thus reducing the detection sensitivity. A new shape for the detecting device is presented in this work, yielding a detection band of several hundred Hertz. With the aid of a Finite Element Program the normal mode frequencies of the detector are determined.

Modelling an Experiment to Measure the Speed of Gravity In Short Distances

An experiment to measure the speed of gravitational signals in short distances has been developed with the goal to study its behaviour as traveling through a medium different from air. The experiment is composed of three sapphire devices suspended in vacuum and cooled down to 4.2 Kelvin. The amplitudes of the central device (detector) is monitored by an ultralow phase noise microwave signal using resonance in the whispering gallery modes. The other two sapphire devices are excited by piezoelectric crystals, which make the two devices vibrate at the same frequency and phase. Between the two vibrating devices and the detector, a different medium will be placed, and then the speed is measured and compared with the case where the medium is pure air. The modelling of the experiment is made assuming the detector as a spring-mass system. The results show that the detection is achievable.

Comparing the performance of a femto fiber-based laser and a Ti:sapphire used for multiphoton microscopy applications.

Ti:sapphire laser systems are the more extended excitation sources in multiphoton (MP) microscopy. Although tunable, the cost, size, and lack of portability often limit their use in some research fields. Femtosecond fiber-based lasers represent an attractive alternative since they are portable, compact, and affordable. Most MP applications using these devices employ wavelengths beyond 1000 nm. This work evaluates the performance of a mode-locked fiber-based laser emitting at 780 nm (within the spectral region often used with Ti:sapphire devices) for use in MP imaging microscopy. MP images acquired with this laser system have been compared with those obtained with a "regular" solid-state source. Results herein show that the images recorded with both laser sources were similar, independently of the depth location of the imaged plane. The structural information contained in the images hardly differed. Moreover, the images of deeper layers improved by means of adaptive optics were also similar. These ultrafast laser sources are expected to enhance the impact of MP microscopy in basic research, as well as in biomedical environments.

Indirect determination of hematology reference intervals in adult patients on Beckman Coulter UniCell DxH 800 and Abbott CELL-DYN Sapphire devices.

Background Conventional establishment of reference intervals for hematological analytes is challenging due to the need to recruit healthy persons. Indirect methods address this by deriving reference intervals from clinical laboratory databases which contain large datasets of both physiological and pathological test results. Methods We used the "Reference Limit Estimator" (RLE) to establish reference intervals for common hematology analytes in adults aged 18-60 years. One hundred and ninety-five samples from 44,519 patients, measured on two different devices in a tertiary care center were analyzed. We examined the influence of patient cohorts with an increasing proportion of abnormal test results, compared sample selection strategies, explored inter-device differences, and analyzed the stability of reference intervals in simulated datasets with varying overlap of pathological and physiological test results. Results Reference intervals for hemoglobin, hematocrit, red cell count and platelet count remained stable, even if large numbers of pathological samples were included. Reference intervals for red cell indices, red cell distribution width and leukocyte count were sufficiently stable, if patient cohorts with the highest fraction of pathological samples were excluded. In simulated datasets, estimated reference limits shifted, if the pathological dataset contributed more than 15%-20% of total samples and approximated the physiological distribution. Advanced sample selection techniques did not improve the algorithm's performance. Inter-device differences were small except for red cell distribution width. Conclusions The RLE is well-suited to create reference intervals from clinical laboratory databases even in the challenging setting of a adult tertiary care center. The procedure can be used as a complement for reference interval determination where conventional approaches are limited.

AlN/GaN/Sapphire heterostructure for high-temperature packageless acoustic wave devices

Surface acoustic waves (SAW) technology is very promising to achieve wireless sensors able to operate in high temperature environments up to possibly 1000 °C. However, there is currently a bottleneck related to the packaging of such sensors. The current high-temperature packaging solutions can withstand 600 °C at most. This limitation could be overtaken by the development of packageless devices, based on the waveguiding layer acoustic waves (WLAW) principle. In such devices, the acoustic wave is confined inside an inner layer and is then isolated from undesired surface perturbations like dust deposition.In this paper, we investigate the performance of an AlN/IDT/GaN/Sapphire WLAW device used as a temperature sensor able to operate up to 500 °C. After validating a room-temperature GaN material constant set with basic SAW measurements performed on IDT/GaN/Sapphire structure, the AlN/IDT/GaN/Sapphire device is simulated to determine the optimal relative thicknesses of AlN and GaN films in order to obtain a good wave confinement. Based on these calculations, an experimental WLAW device is performed and electrically characterized. The full wave confinement is experimentally confirmed by the lamination of an acoustic absorber on top of the device: no change in the scattering parameters was observed. The WLAW device is then electrically characterized between the ambient temperature and 500 °C. A temperature coefficient of frequency (TCF) value of −34.6 ppm/°C is obtained, demonstrating the potential of the WLAW AlN/IDT/GaN/Sapphire structure as a packageless temperature sensor. Finally, the theoretical TCF of the AlN/IDT/GaN/Sapphire structure was numerically calculated by changing the material constants of AlN, GaN and Sapphire according to the temperature coefficients available in the literature. The theoretical and experimental data were found in good accordance.

(Invited) Epitaxy of GaN on Si (111) for Power Electronics, RF and LEDs

The semiconductor industry has a highly developed infrastructure for silicon processing with a high level of automation, state of the art facilities and the ability to deal with large volumes which lead to lower fabrication costs. GaN on silicon can take advantage of this maturity of process, especially with the use of 200mm substrates giving access to large CMOS compatible fabs. This requires specific growth tools, such as that shown in figure 1, and a typical GaN on silicon wafer is shown in figure 2. Combining the industrialization techniques of silicon with the outstanding properties of a wideband gap material such as GaN can result in high performance devices and new functionality for a wide variety of applications in power conversion, radio-frequency electronics and opto-electronics. Power electronics is currently a very active topic, with multiple academic and industrial players working on GaN based materials, in particular due to the availability of HEMT structures as shown in figure 3 that can be integrated on silicon substrates. Such structures have high carrier mobility and the capability to withstand high critical electrical fields, which makes them extremely promising for power conversion applications at lower cost compared to bulk silicon carbide devices for example. Moreover, with aggressive device scaling compared to silicon power transistors, such devices allow higher frequency operation which in combination with high temperature operation, ensure more compact and greener energy converters at more competitive prices. Beyond traditional power conversion, radio frequency applications are also a field where GaN is of great interest. Although the market is today mainly covered by GaN on silicon carbide layers to address high performance demands such as military radar, high volume markets are now pushing for products at more competitive prices. In particular GaN on silicon is a candidate of increasing interest for the next generation of 5G networks in development. Finally, GaN on silicon has been proposed for many years as a solution to reduce LED fabrication costs. However the move into production has not occurred on a large scale, in particular due to the ever decreasing costs of GaN on sapphire devices and the difficulties of growing the very high quality GaN required for LEDs on silicon substrates. However, new applications such as micro-displays, with pixels shown in figure 4, are once again making GaN on silicon attractive for LEDs production, as large wafers and low defectivity of CMOS compatible processing mean that pixel yield and therefore device yield can be maintained, which is much harder to achieve for traditional GaN on sapphire production. This talk will describe the basics of growth of GaN on silicon, and the techniques of strain engineering required to produce flat wafers at room temperature. We will also look at the different in-situ characterizations available and discuss the different information that we can extract from these to ensure easier development of layers and better control of production. We will then examine the specific challenges of GaN on silicon growth, and the difficulties associated with the integration of these layers into CMOS compatible fabs. In particular, we will look at the different forms of defectivity found during GaN on silicon growth, with an example shown in figure 5. These defects must be avoided, to ensure high performance and high yield, to ensure CMOS compatibility and also to avoid an impact on the fragility of wafers, which is a serious issue for GaN on silicon wafers. We will also present some of the current solutions being implemented by LETI in order to counteract these difficulties and move towards truly CMOS compatible layers. An example of a fully processed wafer is shown in figure 6. Finally we will examine the specific challenges for GaN on silicon for each of the above applications, in terms of layer performance and integration difficulties, before looking ahead to future concepts for GaN on silicon layers, and further integration of these layers including hybrid LED micro-displays, and integrated circuits for power electronics. Figure 1

Impact of Trench Dimensions on the Device Performance of GaN Vertical Trench MOSFETs

In this letter, we have examined the impact of trench dimensions on the breakdown voltage and ON-resistance of trench MOSFETs fabricated on sapphire and bulk GaN substrates. Contrary to simulation studies, the breakdown voltage decreased with an increase in trench dimensions in devices fabricated on sapphire substrates. However, such breakdown voltage dependence with trench dimensions was not observed in devices fabricated on bulk GaN substrates of the same area. The observed trend on GaN on sapphire devices was associated with the equivalently reduced number of dislocations per device area. These results give an insight into how dislocations could affect breakdown voltage in power MOSFETs.

Over 1000 V/30 mA operation GaN-on-Si MOSFETs fabricated on Si substrates

We demonstrated the operation of GaN-on-Si metal–oxide–semiconductor field effect transistors (MOSFETs) for power electronics components. The interface states at SiO2/GaN were successfully improved by annealing at 800°C for 30min in N2 ambient. The interface state density was less than 1×1011cm–2eV−1 at Ec−0.4eV. The n+ contact layers as the source and drain regions as well as the reduced surface field (RESURF) zone were formed using a Si ion implantation technique with the activation annealing at 1200°C for 10s in rapid thermal annealing (RTA). As a result, we achieved an over 1000V and 30mA operation on GaN-on-Si MOSFETs. The threshold voltage was +2.6V. It was found that the breakdown voltage depended upon the RESURF length and nitride based epi-layer thickness. In addition, we discussed the comparison of each performance of GaN-on-Si with -sapphire devices.

Low cost miniature thermoelectric generator

A low-cost thermoelectric generator realised using integrated circuit technology is described. The thermopile comprises polycrystalline silicon thermoelements on a quartz substrate and employs a design previously reported by the authors for a silicon on sapphire device. An approximate 10 fold decrease in the substrate thermal conductivity is achieved. Results of output measurements are presented and compared with those of previous work highlighting the advantages of the new approach.

Assessment of a new device for laser angioplasty

Experimental work has shown that a transparent laser device delivering pulsed energy to an artery results in a smaller area of surrounding damage than does an opaque device with a continuous wave laser. The combination of a transparent ball-tipped device with a pulsed Nd-YAG laser has been investigated. The system delivers pulses of 100 microseconds at a rate of 10 Hz and average energy of 0.5 J per pulse with an energy loss of 5-10% between the output at the laser rail and the fibre tip. The dose/response was measured and showed that on normal aorta under saline the device produces craters with a depth of 5 microns/J and 1.5 mm radius. There is a 100% increase in dose response with diseased aorta and a 50% increase when exposure is carried out under blood. The effect of a varying angle of incidence upon the arterial wall has been measured. Angulation of the device at 10 degrees from the perpendicular reduces the crater depth to 50%, as compared with a 50% reduction at 60 degrees using a bare fibre. As estimated with a thermal camera in air, the device heats up to a maximum of 50 degrees C during a 50 J exposure, compared to 110 degrees C after 5 J for the sapphire device. Artificial circulation experiments were carried out using diseased femoral vessels occluded by a ligature. The new system recanalised 100% of occlusion in straight vessels, and 40% of occlusions in curved vessels at a radius of 2.5 cm.(ABSTRACT TRUNCATED AT 250 WORDS)