M. Smiljanic

Numerical calculation of photodetector response time using Ramo's theorem

The work presents a methodology for numerical calculation of response time of p-i-n photodiodes using the corpuscular approach, i.e. the Ramos theorem. The calculations were performed for the Si p-i-n photodiode homostructure which is now already standard and for a high-speed InP-InGaAs-InP photodetector heterostructure. The results show a good agreement with the results obtained using the usual collective approach, however in our calculation the numerical calculation is significantly simplified, and the physical presentation of the process is extremely simple.

Intelligent industrial measurement instruments with silicon piezoresistive MEMS pressure sensors

In this paper we first describe the latest silicon piezoresistive MEMS pressure sensor developed and manufactured at the Center of Microelectronic Technologies (ICTM-CMT). We introduce the concept of intelligent industrial instrumentation, and present a method that enables simultaneous high-performance pressure and temperature measurement to be realized using the described sensor. Experimental verification of the method was performed using the intelligent industrial instrument platform developed at ICTM-CMT. The obtained results have been discussed. They indicate that it is possible to meet or exceeded the measurement performance of existing industrial-grade equipment by using the described approach. An important advantage of the method is that it can enable the temperature measurement capability to be added to some existing intelligent transmitters without hardware modifications. A new intelligent industrial liquid level transmitter, based on the described method and developed at IHTM-CMT, is also presented.

Simulation and experimental study of maskless convex corner compensation in TMAH water solution

Maskless etching with convex corner compensation in the form of a 〈1 0 0〉 oriented beam is investigated using both experiments and simulations. The maskless convex corner compensation technique is defined as a combination of masked and maskless anisotropic etching of {1 0 0} silicon in 25 wt% TMAH water solution at a temperature of 80 °C. This technique enables fabrication of three-level micromachined silicon structures with compensated convex corners at the bottom of the etched structure. All crystallographic planes that appear during etching are determined and their etch rates are used to calculate the etch rate value in an arbitrary crystallographic direction necessary for simulation by an interpolation procedure. A 3D simulation of the profile evolution of the etched structure during masked and maskless etching of silicon based on the level set method is presented. All crystallographic planes of the etched silicon structures determined in the experiment are recognized in the corresponding simulated etching profiles obtained by the level set method.

Micromachining by maskless wet anisotropic etching {hkl} structures on {100} oriented silicon

The paper presents investigation of micromechanical technique using 3D anisotropically etched Si structures. Ridge-like structure oriented along (110) direction is first etched with mask following maskless etching in 25 wt. % TMAH solution in water at 80°C. The ridge structures investigated by maskless etching are convex prismatic edges included by {100} and {111} planes. Experimental results verify that the cutting planes developed at the ridge corners are {733} planes. From analytical relations applied to experiments, the ratio between the etching rates for the {733} (the fastest etching plane at the edge step) and {100} planes is found.

Boron Redistribution During SOI Wafers Thermal Oxidation

We fabricated silicon-on-insulator (SOI) pressure sensors intended for operation in a wide temperature range. After the thermal oxidation process, the measured sheet resistance of the silicon active layer was higher than expected for the case when the dopant distribution in the active layer is uniform and the layer thickness reduced. The reason was the redistribution of dopant (boron) in the active layer during the high temperature thermal oxidation processes. We developed a model to calculate the boron redistribution in an SOI wafer which was initially uniformly doped. The temperature dependence of hole mobility (and, based on it, the sheet resistance and its temperature dependence) was determined for a calculated impurity profile after oxidation in wet O 2. The experimental temperature dependence of sheet resistance was obtained by measuring the temperature dependence of piezoresistor resistance. The best match between the theoretical and experimental TCR (temperature coefficient of resistance) was achieved by slightly modifying the Aroras model for hole mobility

Glass micromachining with sputtered silicon as a masking layer

In this work we present the not so commonly use of RF sputtered silicon as a masking layer for glass wet etching. The main advantages of this technique are low deposition temperature of silicon layer compared to PECVD and LPCVD processes, simplicity to use and low cost. Si layers were sputtered on 2.5×2.5cm2 Pyrex 7740 substrates. The measured thickness of deposited silicon layer was 2.2μm. Silicon layer was patterned using lift-off technique. Etching was done in undilluted HF (49%) with estimated etch rate of ~8μm/min. Good quality of the glass surface without pinholes and notch defects on the glass etched edges suggests that using sp-Si layer as a masking material for glass etching is feasible.

Microfabrication by mask-maskless wet anisotropic etching for realization of multilevel structures in {100} oriented Si

Step-like structures oriented along 〈110〉 or 〈100〉 directions on {100} oriented Si substrates are first etched with mask following maskless etching in 30 wt. % KOH solution in water at 80°C. The step structures investigated by maskless etching are convex prismatic edges bounded by {100} and {111} planes for 〈110〉 oriented steps or only {100} planes for 〈100〉 oriented steps. Experimental results are used to verify the nature of {hkl} cutting planes that are developed at the convex corners of the steps during maskless anisotropic etching. From analytical relations and experimental results, the ratio between the etching rates for the {hkl} (fast-etching plane at the edge step) and {100} planes is found for both step orientations.

A measurement setup enabling automatic characterization of silicon piezoresistive MEMS pressure sensors

In this paper a measurement setup is presented that enables automatic characterization of silicon piezoresistive MEMS pressure sensors. It is used for determination of the pressure and temperature dependences of sensor electrical parameters. Some of the equipment in the setup, and the software application that controls the process, are developed at the Center of Microelectronic Technologies (CMT). The main objective of the work is to make the sensor characterization experiment as efficient as possible, without sacrificing measurement performance. An example of obtained measurement results is given for 3 sensors fabricated at CMT. The work greatly facilitates small series production of pressure sensors and instruments, as well as research activities in the field of pressure sensors at CMT.

A method enabling simultaneous pressure and temperature measurement using a single piezoresistive MEMS pressure sensor

In this paper we present a high-performance, simple and low-cost method for simultaneous measurement of pressure and temperature using a single piezoresistive MEMS pressure sensor. The proposed measurement method utilizes the parasitic temperature sensitivity of the sensing element for both pressure measurement correction and temperature measurement. A parametric mathematical model of the sensor was established and its parameters were calculated using the obtained characterization data. Based on the model, a real-time sensor correction for both pressure and temperature measurements was implemented in a target measurement system. The proposed method was verified experimentally on a group of typical industrial-grade piezoresistive sensors. The obtained results indicate that the method enables the pressure measurement performance to exceed that of typical digital industrial pressure transmitters, achieving at the same time the temperature measurement performance comparable to industrial-grade platinum resistance temperature sensors. The presented work is directly applicable in industrial instrumentation, where it can add temperature measurement capability to the existing pressure measurement instruments, requiring little or no additional hardware, and without adverse effects on pressure measurement performance.

Prevention of convex corner undercutting in fabrication of silicon microcantilevers by wet anisotropic etching

This paper presents microcantilever fabrication on {100} oriented Si substrate using bulk micromachining technique. Detailed characteristics of convex corner compensation design for 〈100〉 oriented beam for two etching solutions: 25 wt. % TMAH and 30 wt. % KOH water solutions at 80 °C are given.