Libor Rufer

On-Chip Pseudorandom MEMS Testing

This paper presents a Built-In-Self-Test (BIST) implementation of pseudo-random testing for MEMS. The technique is based on Impulse Response (IR) evaluation using pseudo-random Maximum–Length Sequences (MLS). The MLS approach is capable of providing vastly superior dynamic range in comparison to the straightforward technique using an impulse excitation and is thus an optimal solution for measurements in noisy environments and for low-power test signals. The use of a pseudo-random sequence makes the practical on-chip implementation very efficient in terms of the extra hardware required for on-chip testing. We will demonstrate the use of this technique for an on-chip fast and accurate broadband determination of MEMS behaviour, in particular for the characterisation of cantilever MEMS structures, determining their mechanical and thermal behaviour using just electrical tests.

Pseudorandom Functional BIST for Linear and Nonlinear MEMS

Pseudorandom test techniques are widely used for measuring the impulse response (IR) for linear devices and Volterra kernels for nonlinear devices, especially in the acoustics domain. This paper studies the application of pseudorandom functional test techniques to linear and nonlinear MEMS built-in-self-test (BIST). We will first present the classical pseudorandom BIST technique for linear time invariant (LTI) systems which is based on the evaluation of the IR of the device under test (DUT) stimulated by a maximal length sequence (MLS). Then we will introduce a new type of pseudorandom stimuli called the inverse-repeat sequence (IRS) that proves better immunity to noise and distortion than MLS. Next, we will illustrate the application of these techniques for weakly nonlinear, purely nonlinear and strongly nonlinear devices

Built-in-self-test techniques for MEMS

As predicted by technology roadmaps, embedded micro-electro-mechanical-systems (MEMS) is yet another step in the continuous search for higher levels of integration and miniaturization. MEMS are analog components and the test paradigm is similar to the case of analog and mixed-signal circuits. However, given the fact that they work with signals other than electrical, the test of these embedded parts poses new challenges. In this paper, we will review some recent works in this field and we will present a complete approach to MEMS built-in-self-test (BIST) based on pseudorandom testing.

Mems built-in-self-test using MLS

This paper presents a Built-In-Self-Test (BIST) implementation of pseudo-random testing for Micro Electro-Mechanical Systems (MEMS). The technique is based on Impulse Response (IR) evaluation using Maximum-Length Sequences (MLS). We will demonstrate the use of this technique and move forward to find the signature that is defined as the necessary samples of the impulse response needed to carry out an efficient test. We will use Monte-Carlo simulations to find the set of all fault-free devices under test (DUT). This set defines the impulse response space and the signature space. A DUT will be judged fault-free according to its signature being inside or outside the boundaries of the signature space. Finally, the test quality will be evaluated as function of the probabilities of false acceptance and false rejection, yield and percentage of test escapes. According to these test metrics, the design parameters (length of the MLS and the precision of the analogue to digital converter ADC) will be derived.

On-chip testing of MEMS using pseudo-random test sequences

This paper presents a method for an on-chip fast and accurate broadband determination of MEMS behaviour. This technique is based on impulse response (IR) evaluation using pseudo-random Maximum-Length Sequences (MLS). The MLS approach is capable of providing vastly superior dynamic range in comparison to the straightforward technique using an impulse excitation and is thus an optimal solution for measurements in noisy environments and for low-power test signals. The use of a pseudo-random sequence for testing makes the practical on-chip implementation very efficient in terms of the extra hardware required for on-chip testing. We will exemplify this technique for the case of MEMS structures such as cantilevers and bridges, by determining their mechanical and thermal behaviour using just electrical tests.

A CMOS Compatible Ultrasonic Transducer Fabricated With Deep Reactive Ion Etching

This paper describes design, fabrication, and test of an integrated micromachined ultrasound transducer (MUT). This MUT can work as an emitter and a receiver of ultrasonic signals in air and is intended to be used in applications requiring both acoustic signal generation and sensing. Among these applications there can be detection or distance measurements of different objects where the nature either of these objects or of their surroundings does not allow the use of light-based methods. The device has been fabricated in a 0.8 mum CMOS process combined with deep reactive ion etching, integrating in the same chip a suspended membrane (plate) and the associated interface electronics. The plate is thermally actuated at its resonance frequency (40 kHz) during emission. During reception, a piezoresistive bridge placed on the membrane is used for monitoring its deflections. The main advantage of the design using an actuator and a sensor integrated in one multilayer structure is its simplicity. Comparing with capacitive transducers where two-electrode structure is used, considerations in terms of mechanical stability and membrane damping as well as the fabrication process are not critical. The integrated solution allows the use of an amplifier at the output of the piezoresistive Wheatstone bridge which results in the sensitivity of 35 mV/Pa; the maximum acoustic pressure generated by the transducer at 10 mm is 5 mPa

GaAs and GaN based SAW chemical sensors: acoustic part design and technology

In this paper, we present the design considerations and the technology process of SAW (surface acoustic wave) chemical sensors based either on GaAs or GaN structures. These sensors can be used for identifying environmental contaminants and chemical or biological agents in large applications scale; in this study, we aimed at the measurement of low concentrations of gaseous mercury. We describe the design of the acoustic part of the sensor including the structure for the generation and reception of the surface acoustic wave and the chemoselective coating made of gold. We show the technology process that achieves the device operating at the frequency of 250 MHz. Finally we present some preliminary results obtained from the device

On-chip testing of embedded transducers

System-on-Chip (SoC) technologies are evolving towards the integration of highly heterogeneous devices, including hardware of a different nature, such as digital, analogue and mixed-signal, together with software components. Embedding transducers, as predicted by technology roadmaps, is yet another step in this continuous search for higher levels of integration and miniaturisation. Embedded transducers fabricated with Silicon/CMOS compatible technologies may have a lower performance than transducers fabricated with fully dedicated technologies. However, they offer the Industry the possibility of providing low cost applications for very large market niches, while still keeping the required transducer sensitivity. This is the case, for example, for accelerometers or CMOS imagery. Test technology for SoC devices is rapidly maturing. Yet many difficulties still remain, in particular for addressing the test of analogue and mixed-signal parts. Embedded transducers can be seen as analogue components. But given the fact that they work with signals other than electrical, the test of these parts is even harder to study. In this paper, we will present our work in the field of MEMS testing and its evolution towards transducer on-chip testing.

Behavioral modeling and simulation of a MEMS-based ultrasonic pulse-echo system

This paper describes the design, modeling and simulation of an acoustic microsystem in a pulse-echo ultrasonic application. The microsystem, used as emitter and receiver of ultrasonic signals, consists of a bulk micromachined suspended membrane. During emission, the membrane is placed in an oscillator loop and is thermally actuated at its resonance frequency (approximately equals 40 kHz). This frequency is slightly dependent on the membrane average temperature. The electronic interface circuit monitors this temperature. The membrane oscillations generate an ultrasonic signal (pulse) that propagates in the air and interacts with a solid body. As a result of this interaction, the ultrasonic signal is reflected on the solid surface and is received by the microsystem (echo). During reception, a piezoresistive bridge placed on the membrane is used for monitoring the membrane deflections. The resonance frequency of the membrane is tuned to the emitted frequency by keeping the membrane at the same temperature, achieving then maximum sensitivity. This paper presents in detail the behavioral modeling and simulation of the complete system. Some MEMS parts and the acoustic waves propagation are modeled using an Analogue Hardware Description Language (Verilog-A). The associated electronics are implemented in CMOS and the overall system is simulated with the SpectreHDL simulator in the CADENCE environment.

Electromagnetic modeling of an integrated micromachined inductive microphone

This paper presents a detailed electromagnetic modeling for a new structure of a monolithic CMOS micromachined inductive microphone. This study is of capital importance for the optimization of this new microphone structure based on the variation of mutual inductance between an external fixed inductor and an internal suspended inductor. A prototype fabrication of this microphone has been done in a CMOS compatible process using a 0.6 µm technology. The modeling results show that the use of an AC current in the primary fixed inductor gives a much larger induced voltage in the secondary inductor comparing to the case when a DC current is used. The induced voltage evaluated, in the μV range using a DC current solution, is multiplied by a factor of 3 to 6 when using the AC alternative current. We find a value of 1 nA for the current induced in the secondary inductor, which indicates that the Laplace force generated will have a negligible effect on the membrane displacement. We have proposed the use of the symmetric dual-layer spiral inductor structure that can increase the induced emf value by a factor of 4.