2015 DGON Inertial Sensors and Systems - Physical Logic

Closed-loop MEMS accelerometer: from design to production

B. Grinberg1, A. Feingold1, L. Koenigsberg1, L. Furman1

1

Physical Logic Ltd. Ben Zion Galis 48 74927948 Petah Tikva ISRAEL

Inertial Sensors and Systems 2016 Karlsruhe, Germany

Abstract The paper reports on a navigation grade 30g MEMS accelerometer based on digital ΔΣ modulation and capacitive sensing. The closed-loop accelerometer is a fully integrated system comprising a uniquely designed MEMS device enclosed in a specially built LCC package with a proprietary ASIC. Like Physical Logic’s high-end open-loop MEMS accelerometers, now known as the MAXL-OL-2000 series, the closed-loop design benefits from the in-plane architecture using SOI wafer. Key features of the design are discussed, from the MEMS transducer to system wide considerations of low noise, high linearity, and robust stability of the control design. System level simulation results are presented and compared to the test results from the most recently fabricated MAXL-CL-3030 closed-loop ΔΣ accelerometer with 30g range. Physical Logic has developed a closed-loop MEMS accelerometer with an objective to reach inertial navigation grade performance. Both the high-end open-loop and closed-loop MEMS accelerometers employ a similar transducer design. In a different manner from the commonly used out-of-plane technique for bulk micromachining, an in-plane design using SOI wafer was adopted. The advantages of the approach are full bridge capacitive sensing for parasitic rejection, a highly symmetric mechanical structure for better temperature stability and elimination of the need for vacuum packaging for better reliability. A large proof mass, which is realized in SOI wafer handle layer, contributes to enhanced sensitivity. The closed-loop system architecture operates as a 4 th order ΔΣ modulator used to convert external acceleration into a high frequency single bit digital signal. The design challenges and considerations are described with emphasis on noise, linearity, and stability. The test results of the MAXL-CL-3030 closed-loop ΔΣ accelerometer confirm the navigation grade design. Measurements are presented demonstrating results of 100 dB gain up to 100 Hz. [5]

Figure 2. Open-loop frequency response

A detailed simulation was then used to verify and finalize the set of compensator parameters. This simulation includes a detailed modeling of the noise sources, a bit level digital section including the loop filter model, and a non-linear comparator model. In addition many carefully modeled non-idealities are introduced to test for robustness of the design. In the design of the system, described in this paper, the simulation was performed using Simulink platform of MATLAB. 2.2.

MEMS design

The MEMS device is based on an in-plane bulk micromachining technology. The in-plane concept has several key advantages over the more conventional out-of-plane concept. Most importantly the non-linearity of capacitance sensing based on gap-changing is avoided, it makes possible the implementation of a full bridge sensing topology with a large capacitor for effective parasitic rejection, and a highly symmetric mechanical geometry is attainable in which each of the four sensing capacitors is equally distributed on the area of the MEMS die. The process flow used for the MEMS device in the closed-loop accelerometer is adopted from the open-loop MEMS device of the Physical Logic's MAXL-OL-2000 accelerometers series. A Silicon On Isolator (SOI) wafer is processed using Deep Reactive Ion Etching (DRIE) on both back and front sides to create an in-plane translational displacement proof mass of about 1.2 miligram. Such a massive seismic mass becomes possible due to the [6]

back side etching of the wafer's handle layer (see Figure 3). The large proof mass allows for a more sensitive accelerometer, thereby increasing the SNR. The proof mass is suspended on a set of carefully designed springs to provide a spring-mass system with the desired natural frequency and damping. The comb fingers used for capacitive sensing or electrostatic actuation are formed in the device layer. More details on the in-plane concept and the process flow in which it is realised can be found in previous publications [5, 6].

Figure 3. SOI wafer processing

Although sharing a similar process flow, closed-loop and open-loop MEMS devices differ significantly in their topology. To enable closed-loop operation, the MEMS device includes two sets of comb fingers for electrostatic actuation. The actuator is separated electrically and geometrically from the comb fingers associated with the proof mass displacement sensing. The separation ensures effective electrical crosstalk reduction. In the closed-loop device, special emphasis is made on the symmetry of the metal wiring and stress isolation of the proof mass in order to greatly reduce MEMS induced bias variations. 2.3.

System architecture

The architecture of the sensor is schematically described in Figure 4. The heart of the system, composed of the MEMS device and the ASIC, is enclosed in a specially designed LCC44 ceramic packaging. The package is mounted on a thick PCB board, which includes a crystal to generate a precise clock for the system, a voltage regulation circuitry to provide a highly stabilized reference voltage for the sensor, and a micro controller to govern configuration of various system parameters during start-up.

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Figure 4. Closed-loop MEMS accelerometer system architecture

The ASIC is designed as a mixed-signal system on a chip (see Figure 5.). At the sensing interface, there is an Analog Front End (AFE) circuit comprising a capacitance to voltage converter which translates the proof mass displacement into voltage [2], and a programmable voltage gain amplifier. Following the AFE is a coarse 12-bit ADC that converts the analog output from the AFE block into a digital signal. The coarse ADC allows digital implementation of the compensator or loop filter. The advantages of implementing the compensator in the digital domain were mentioned above in Section 2.1. The MSB output of the digital compensator is converted back to the analog domain by a DAC. In the feedback path there is a high voltage pulser that provides a pulse density modulation signal for electrostatic driving of the MEMS structure. The above mentioned blocks of the ASIC are the main blocks which are directly involved in the closed-loop operation. To achieve the high specifications of the closed-loop accelerometer, there are additionally other important supporting blocks such as power management (PM), a temperature sensor with an analog output, SPI control interface to realize an SPI communication with the external micro controller, and a clock divider block, which divides the precise clock to support the timing for the various ASIC blocks. Components such as the crystal clock generator and the precise voltage reference, although assembled on the PCB board outside the ceramic package, are specified to ensure minimal sensitivity to environmental changes.

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Figure 5. Mixed-signal ASIC

The specifics of the architecture of the closed-loop accelerometer system are designed to ensure stability and minimize sensitivity to the environment. The critical system blocks like the MEMS device and the ASIC are assembled at a minimal distance between them to avoid parasitic effects due to electrical wiring connections. To ensure minimal sensitivity of the bias to temperature changes, the glue is chosen according to tight specifications and a proprietary gluing technique is used to attach the MEMS die to the ceramic package. Finally, the ceramic package is sealed in a perfectly controlled N 2 atmosphere and fine leak tested. 3.

Design agreement

Design agreement and consistency of the production units establish that the production line parameters that determine sensor performance are well understood and under tight control. In this section we show measurement results of the production Physical Logic MAXL-CL3030 units, and compare them to the design simulation. A short description of the production worthy MAXL-CL-3030 units will be followed by an overview of the measurement and simulation results. 3.1.

Physical Logic MAXL-CL-3030

First engineering prototypes of the MAXL-CL-3030 were tested at the end of 2014 and reported in [5]. Since then, Physical Logic has been advancing the sensor and its fabrication line towards a production worthiness level. Significant progress along with impressive

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measurement results have been presented in [6]. In this paper we present results of sensors which are currently in the final stage of qualification. The sensor is a completely self-contained system. In the current configuration the sensor comprises an LCC44 package (see Figure 6) on a PCB board (5.4 X 5.4 cm, 15 gm) with only one side occupied. However, other configurations are possible since the specific functions and geometry of the PCB board are easily custom arranged to enable proper integration of the sensor as part of a system. The sensor uses a single 5V power supply and provides a digital acceleration output along with an analog temperature measurement.

Figure 6. Top view of MAXL-CL-3030 accelerometer

3.2.

Measurement vs. simulation results

The acceleration output of the sensor is a 1-bit digital signal with a high sampling rate, as described in Section 2.1. The 1-bit signal covers the full range of the accelerometer, where the value "1" represents the maximum positive full range acceleration and the value "0" represents the maximum negative full range acceleration. On a system level, this 1-bit signal would naturally be decimated to a required frequency and digital resolution, with decimator output representing a value between "0" and "1" of the original 1-bit signal. Taking into consideration the electrostatic actuation force applied, the sensor’s scale factor, SF, is given by (2) 9.81∙M

SF [bit/g]= 2∙Cx∙V2 ,

(2)

where Cx is the sensitivity of the actuator capacitor, V is the feedback voltage, and M is the mass of the proof mass. Substituting the design values in (2), the scale factor of the system is estimated to be 13 mbit/g. This SF value demonstrates that the sensor supports a sensing range up to 38.5 g, in keeping with the design value. A four point tumble test measurement (see Figure 7) has been performed on a large quantity of MAXL-CL-3030 sensors from different fabrication batches. The average scale factor

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measured is 12.8 mbit/g with a 5% standard deviation, showing good correspondence and consistency to the designed value.

Figure 7. Four-point tumble test measurement result

A reliable system design verification of a closed-loop ΔΣ accelerometer is a PSD analysis of its 1-bit output. The PSD plot result from the complete simulation described in the Section 2.1 is compared to a PSD plot derived from the 1-bit measurement acquired from the MAXLCL-3030 (see Figure 8).

Figure 8. Noise spectrum of 1-bit output. Measurement vs. System Simulation

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There is an excellent matching between the system simulation and a real measurement. The PSD shows the expected 80 dB/Dec noise shaping, confirming the 4 th order ΔΣ modulation. The in-band (up to 400Hz) noise floor is around -135 dB, which using the scale factor, results in ~15 µg/√Hz. Over a typical sample of fifty sensors, an average noise floor of 20 µg/√Hz was measured with a standard deviation of 15%. 4.

Measurement results

This section presents an update on some important results presented in [5, 6] measured from the MAXL-CL-3030. Improvements and consistency of specification measurements are presented. 4.1 Bias and SF stability In [6] we presented results of 25 hour stability measurements based on data acquisition from sensors mounted in a vertical static position. From such an experiment only a combined bias and scale factor stability result can be observed. In the experiment presented in this paper, the data acquisition was performed for a longer period of time (90 hours) and a tumble test was done once each hour to extract the bias, scale factor, and misalignment parameters. Figures 9 and 10 show the bias and scale factor stability results. A simple temperature compensation model was subtracted from the raw data.

Figure 9. Bias stability

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Figure 10. Scale factor stability

Summarizing this experiment we report a less than 20 µg bias stability and a less than 10 ppm scale factor stability during a 90 hour period of time. 4.2 Vibration rectification error In this experiment we calculate sensor’s bias error during 5 g RMS random vibration in 20 Hz - 2 kHz frequency range. Due to the improved loop filter design we have been able to significantly reduce the vibration induced bias previously reported in [5, 6]. Figure 11 shows a typical rectification error plot measured on MAXL-CL-3030. Over a typical sample of fifty sensors we get an average of 6 µg/g2rms Vibration Rectification Error (VRE) with 20% standard deviation.

Figure 11. VRE under 5g random vibration

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4.3 Linearity The improved loop filter design yields much better linearity than was reported in our previous publications. Figure 12 presents a scale factor linearity error as a function of the precise centrifuge acceleration input. The error is calculated as a deviation from the reference linear curve that passes through 0 g and 1 g average output, i.e. the SF calculated from a tumble test. For convenience an upper limit of the error, using a parabolic model is shown (in red). The sensor’s overall error up to 25 g range is bounded by a parabolic upper limit defined by K2=23 µg/g2. The worst case non-linearity error through the 25 g range is less than 0.012%. This test has been performed on a large quantity of accelerometers and the result in Figure 12 is typical.

Figure 12. Linearity error

Table 1 summarizes the results presented above. Closed-loop MEMS accelerometers featuring short term navigation performance has matured to production level and it brings a verity of new possibilities for a great number of applications.

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Table 1. Close loop MEMS accelerometer performance Parameter

5.

Measured value

Sensing range

30 g

Bandwidth

>300 Hz

Noise

20 µg/√Hz, 1σ

Bias stability 90 hours