Martin Nese

Guard ring design for high voltage operation of silicon detectors

Abstract The termination of the depletion zone towards the non-depleted part of silicon affects the total device leakage current, the long term stability, the noise level and the radiation hardness of silicon detectors. This paper describes computer simulations and experiments to develop guard ring structures for use in silicon detectors requiring thick depletion layers, high operating voltages and biasing beyond depletion without increase in the leakage current and the noise. Computer simulation of a simplified structure is used to understand the influence from the oxide charges and the substrate doping concentration for a segmented guard structure with several floating diffusion strips. Results from the simulations are compared with measurements on devices. The numerical results are found to be in agreement with experimental data. It is found that segmented guard structures with floating diffusion strips have high breakdown voltages and low leakage currents. The effects of floating metal field plates over the oxide between the floating diffusion strips are studied on two different guard structures by measuring the potential on the diffusion strips and the leakage currents in the guard and active diode. The results show that floating intermediate field plates reduces the influence from oxide charges and stabilises the device against environmental influence.

Silicon-to-silicon anodic bonding with a borosilicate glass layer

Anodic bonding of silicon wafers by sputter deposited glass films, silicon-to-silicon anodic bonding, is presented as a promising sealing method in microengineering. A reliable process for wafer-to-wafer bonding is described and data concerning yield and bonding strength are given. Cathodic bonding is reported in a discussion about the bonding mechanism. Different sputter deposited and annealed Pyrex 7740 layers are evaluated as sealing material. Some advantages of silicon-to-silicon anodic bonding as a mounting method for micromechanical sensors are quantified.

Boron etch-stop in TMAH solutions

Abstract Etch rates of 〈100〉 single-crystal silicon in tetramethyl ammonium hydroxide (TMAH) solutions have been measured as a function of boron doping concentration with the purpose of studying the feasibility of an etch-stop. The boron concentration has been varied up to 2.5 × 1020 cm−3. An etch ratio of 1:40 between the heavily and lightly boron-doped silicon has been obtained. This ratio may depend slightly on the temperature of the etch, but no significant variation with etchant concentration has been observed for TMAH concentrations in the range 23–32 wt.%. Preliminary experiments on the effect of adding pyrazine to the etch solution indicate that pyrazine increases the etch rate slightly and seems to have the effect of reducing surface roughness.

Anodic bonding of silicon to silicon wafers coated with aluminium, silicon oxide, polysilicon or silicon nitride

Abstract Silicon to silicon wafer bonding by use of sputter deposited borosilicate film is a promising mounting method for micromechanical components. This method has been developed to bond 3 inch borosilicate sputter coated silicon wafers to silicon wafers coated either with aluminium, silicon dioxide, polysilicon or silicon nitride. The bondings were performed at temperatures ranging from 300 to 400 °C which enables application of this technique on metallised devices. The bond strengths of the different samples bonded with these methods are all in the region 5–25 MPa. Some samples were exposed to water for 300 h to test the media compatibility, and some samples were thermal shock tested by repeatedly exposing to liquid nitrogen. No significant difference in bond strength has yet been verified statistically for the different sample configurations. We have also observed good correlation between destructive bond strength testing and non-destructive infrared microscope inspection.

Silicon-to-thin film anodic bonding

A process for silicon-to-thin film anodic bonding with polysilicon, silicon oxide, silicon nitride or aluminium as the thin film materials has been developed. Silicon wafers covered with these thin films have been sealed together by anodic bonding using thin sputter-deposited glass layers as sealing material. The bond strengths of the samples have been tested by pull tests. Some samples were exposed to water for 300 h to test the media compatibility. IR microscopy has been shown to be a good method to uncover bonding voids. Bond strength tests of three-inch silicon-to-silicon anodic bonded wafers are shown to be in excellent agreement with the bonding yield expected from IR-microscope inspection.

New method for testing hermeticity of silicon sensor structures

Abstract A method for testing the hermeticity of different wafer-bonding processes used in silicon sensor devices is proposed. The method is based on measuring the gas concentration in a sealed silicon cavity by Fourier-transform infrared spectroscopy (FTIR). The gas concentration and thereby the leakage into the sealed silicon cavity after external pressure exposure is measured by FTIR absorbance. In this work the method has been evaluated by measuring a test silicon cavity filled with a controlled amount of test gas. N 2 O is evaluated as the test gas in this experiment.

A model for the etch-stop location on reverse-biased pn junctions

Abstract This paper reports a model to predict where the silicon anisotropic electrochemical etching terminates on reverse-biased pn junctions. The model explains why the etching process terminates well before the metallurgical junction. The effects of the substrate doping, the type of junction (step or graded), the etching temperature and voltage bias, as well as the technique used (three and four electrodes) are analysed and compared with the experimental data. Some limitations and deviations from this theory are also pointed out.

New Method For Testing Hermeticity Of Silicon Sensor Structures

A method for testing hermeticity of different wafer bonding processes used in silicon sensor devices is proposed. The method is based on measuring the gas concentration in a sealed silicon cavity by Fourier Transform Infrared spectroscopy (FTIR). The gas concentration and thereby the leakage into the sealed silicon cavity after external pressure exposure is measured by FTIR-absorbance. In this work the method was evaluated by measuring a test silicon cavity filled with a controlled amount of test gas. N/sub 2/O as testing gas was evaluated in this experiment.

Batch processing for micromachined devices

Micromachined device technology has emerged during the last three decades. At first it was mainly a technological spin-off from microelectronics/integrated circuit technology. Sensor applications gave the main market pull, batch processing the key to high quality at low cost and silicon micromachining established itself as a unique process technology with distinctive features. Today, these devices have matured into a separate industry sector with their own market and manufacturing infrastructure, also with micromachining of other materials than silicon. They are used in microelectronic systems with widespread applications, ranging from low-cost, high-volume automotive applications to high-cost, low-volume instrumentation applications. The micromachined devices have during these years shown a much slower learning curve than microelectronics in general, making them bottlenecks for performance and cost improvements in their systems. The herald of the rapid development of integrated circuit technology-batch processing-is one of the important keys to ease these bottlenecks. The most important batch processes for micromachined devices are highlighted, and recommendations for future batch processing developments for micromachined devices are given.

A model for the etch stop location on reversed biased pn junctions

This paper reports a model to predict accurately where the silicon electrochemical etching terminates on a reversed biased pn junction. One of the most suitable and successful etch stop techniques is the electrochemical etch stop (ECES), achieved by reverse biasing a pn junction. Although it has been expected, it is clear from published results that the ECES does not terminate at the metallurgical junction. In most applications, accurate control of the obtained membrane thickness is compulsory. Therefore, the understanding of the premature stop of the etching process and the prediction of its exact location is of major importance in the MEMS field.