MEMS Characterization Platform Solutions for Microsystems Characterization
A new paradigm for MEMS characterization The MEMS Characterization Platform (MCP) is a new concept of laboratory instrumentation for microelectromechanical systems. It enables accurate stationary and dynamic characterization of electrical and mechanical parameters of capacitive MEMS devices with resonance frequencies up to 500 khz, giving you directly fundamental information about the true behavior of your MEMS (the resonance frequency, the quality factor, the rest capacitance, the pull-in and pull-out voltages ). Several other process and device parameters can be extracted indirectly by means of embedded software routines (electrostatic softening, mechanical offsets, process overetch, mechanical nonlinearities,...). Dedicated test procedures can be tailored to the customer needs. And all of this comes in a single instrument! Primarily developed for MEMS inertial sensors (accelerometers and gyroscopes) the MCP can be used for the characterization of a wider range of devices (including MEMS magnetometers, micromirrors, microphones, pressure sensors and several test devices). Analytical capabilities Measurement modes (single ended and differential): - rest capacitance - stationary capacitance variation - dynamic capacitance variation under user-selectable electrostatic stimuli (sinusoidal, square, triangular or arbitrary waveform) - dynamic capacitance variation under synchronized external forces (voltages, accelerations, angular velocity,...) - frequency sweep Direct parameter extraction (no input needed from the user): - capacitive offset (in differential MEMS configurations) - pull-in and pull-out voltages - resonance frequency - quality factor - Bode plots - 3 db bandwidth in over-damped MEMS - electrostatic softening effects - mechanical nonlinearities Indirect measurements (with inputs provided by the user): - mechanical offset (in differential MEMS configurations) - surface charging - process over/under etching - quadrature error measurements in gyroscopes - fracture strength and Young modulus evaluation - fatigue, aging tests or other reliability tests - adhesion forces and surface contact aging 2
MCP working principle The system integrates a unit for the generation of suitable electromechanical stimuli and a section for lock-in based, high-resolution capacitive sensing. The system can be either coupled to probing stations for wafer-level testing, or easily adapted to standard or customized sockets. The MCP comes in a two-differential-channels configuration; this means that you have up to 8 connectors that you can use to plug your MEMS to the MCP: i) 1 rotor connection; ii) 4 stator connectors that can be used either as actuation or sense ports, with fully switchable configurations; iii) 2 auxiliary DC voltage connectors (e.g. for mode tuning or quadrature compensation); iv) 1 ground connection. Configuration examples Differential parallel-plate MEMS accelerometer: one electrode is used as actuator, the other one as sensor. Differential sensing with auxiliary self-test electrodes can be also implemented. Drive mode of a tuning fork gyroscope: external comb-finger electrodes are used as actuators, the internal ones as sensors. A push-pull configuration can also be implemented as the drive signals can be independently configured. Quadrature error can be measured by applying the sense connectors to the sense mode electrodes (not shown), with dedicated routines. 3
Direct measurements examples Capacitance-to-voltage measurement Quasi-stationary C-V curves with automatic calculation of the parabola slope, the capacitance variation and the values of the pull-in and pull-out voltages. Resonance frequency and Q-factor Under-damped MEMS: fitting the time response to a voltage step allows precise measurements of resonance frequency and Q-factor. Frequency sweep mode Over-damped MEMS: automatic routine allows to evaluate the -3dB bandwidth of real single-pole/ two-pole MEMS as well as the Q-factor. 4 Mechanical & electrical nonlinearities evaluation by up/downward frequency sweep.
Indirect measurements examples Squeeze-film damping analysis G. Langfelder, C. Buffa, A. Frangi, A. Tocchio, E. Lasalandra, A. Longoni, Z-axis magnetometers for MEMS inertial measurement units using an industrial process, IEEE Transactions on Industrial Electronics, vol. 60, p. 3983-3990. Quality factor versus number of parallel plates sensing cells in MEMS magnetometers operated at resonance. Thanks to the high versatility of the MCP, you can choose to bias your device as in operating conditions or to measure the free response: this is enabled by the low electrostatic perturbations that the MCP applies to the MEMS during readout. Fatigue propagation monitoring Measurements of the relative elastic stiffness change during fatigue cycles on MEMS specimens. The real time analysis allows capturing damage accumulation generated by subcritical, few-nm-long cracks propagating from natively formed cracks at the MEMS surface. G. Langfelder, S. Dellea, F. Zaraga, D. Cucchi, and M. A. Urquia, The dependence of fatigue in microelectromechanical systems from the environment and the industrial packaging, IEEE Transactions on Industrial Electronics, vol. 59, no. 12, pp. 4938 4948, Dec. 2012. 5
MCP Software and graphical interface The MCP comes with a complete software that allows the user to configure the test bench, to acquire the MEMS response and to elaborate data for parameters calculation, with easy commands in a single interface (see our demos on www.itmems.it). The software includes five main optimized routines: - Calibrate: initial self-calibration against parasitic components; - Rest Capacitance Measurement: accurate absolute measurements of the single-ended or differential capacitance (by automatically switching the MEMS connections); - C-V Curve: execution of back and forth C-V curves, regulation of maximum voltage, resolution, delay between steps, and number of averages; the elaboration of the acquired C-V data to calculate the parabolic fitting coefficients and automatically detect the pull-in and pull-out voltages is also included; - Dynamic Measurement: measurement of the dynamic response of the MEMS under vari ous types of fully configurable predefined or user-defined stimuli; the elaboration of the acquired dynamic data to calculate the spectral response, and in turn the resonance frequency, the quality factor (both for over- and under-damped systems) and the -3dB bandwidth (for over-damped systems). - Frequency Sweep: evaluation of the frequency response of the MEMS through a sinusoi dal voltage signal varying its frequency between a user-configurable range; this routine allows the user to choose both the voltage amplitude and the direction of the sweep (i.e. from low frequencies to high frequencies or viceversa). The MCP software further includes a powerful set of export tools for acquired data, elaborated data, and figures: simple ASCII files, TDMS files (with test-bench data grouped by MEMS and type of measurement), Microsoft Excel spreadsheets, direct export of figures in Matlab or.bmp formats. The software also includes easy configurable automatic test routines for the measurement of large volumes of devices, with pass/fail boundaries. 6
Technical specifications MCP A new concept of laboratory instrumentation for MEMS Interface with MEMS under test: differential MEMS (Stator1 - Rotor - Stator2) or doubly differential MEMS (Actuator1 - Sensor1 - Rotor - Sensor2 - Actuator2) configurations for sensing capacitances up to 10pF and MEMS resonance frequencies up to 500kHz. Switching matrix: the MEMS stators can be software-configured as actuators or sensors and easily exchanged. The system is compatible with suitable switch matrices (not included) for MEMS with more pads to be measured. Rotor and stators selectable biasing: the MCP includes the possibility to bias the rotor and stators with DC voltages, which allows the characterization of MEMS devices in biasing conditions similar to those used in operation. High-frequency signal: up to 1 MHz (MCP-A) or 5 MHz (MCP-G) sine wave with selectable voltage amplitude (10 mv to 700 mv values). Measurement resolution: 0.3 %/Hz 1/2 of the measured capacitance within the range between 200 ff to 10 pf, at 50 mv test signal. Actuation voltages: stationary and dynamic signals up to ± 13V (MCP-A version) or to ±23V (MCP-G version). MEMS connection: BNC connectors to specific sockets, boards or probe stations (other types of connector available upon request). PC connection: Plug & Play USB 2.0 connections to a pre-configured workstation (not included). Power supply: 12 V power adapter from the main power line (100-220 V and 50-60 Hz compatible). Overall dimensions [mm 3 ]: 204x305x80. The MCP comes in different versions depending on voltages and frequencies ranges, to learn more please visit our website www.itmems.it. 7
ITmems S.r.l. Spin-off of Politecnico di Milano Via Francesco D Ovidio 3, 20131 Milano (Italy) www.itmems.it MCP brochure rev 0.3, August 2014