Monolithic-integrated piezoresistive MEMS accelerometer pressure sensor with glass-silicon-glass sandwich structure

A monolithic composite MEMS sensor with sandwich structure is designed, simulated, fabricated and characterized. It consists of a rectangular diaphragm piezoresistive pressure sensor and a double-cantilever-mass piezoresistive accelerometer. The professional MEMS software, Intellisuite 8.5, is used to simulate the performances of the composite sensor. The composite sensor is fabricated on a (100)—silicon wafer by using MEMS bulk-micromachining and anodic bonding technology. One-step wet anisotropic etching process on the backside of the wafer can form the main backside shape of the composite sensor including the mass of accelerometer and the pressure sensing diaphragm. The glass-silicon-glass sandwich structure is formed with α-Si (amorphous silicon)-glass anodic bonding on the top surface of the wafer and Si-glass anodic bonding at the bottom. The fabricated composite sensor is measured, resulting in 33.0 μV/V kPa sensitivity for the 450 kPa-ranged pressure sensor, as well as, 11.2 μV/V g sensitivity for the 125 g-ranged accelerometer. The die size of the composite sensor chip is 2.5 mm × 2.5 mm × 1.4 mm. The measured results show that the composite sensor is appropriate for the application fields such as automobile, aerospace and environment monitoring.


Introduction
With the development of silicon micromachining technology, silicon pressure sensors (Crescini et al. 2003;Suja et al. 1 3 2 Design 2.1 Sensor structure and configuration Figure 1 shows the monolithic composite sensor structure before anodic bonding process which consists of an accelerometer and a pressure sensor. The accelerometer has double-cantilever-mass structure and the pressure sensor has rectangular sensing diaphragm structure. After one-mask wet anisotropic etching, the accelerometer's rectangular diaphragm with a mass and the pressure sensor's rectangular diaphragm are shaped simultaneously. Then, deep reactive ion etching (DRIE) is used to release the accelerometer's diaphragm and form the suspended Fig. 1 The monolithic composite sensor integrated with a piezoresistive accelerometer and a pressure sensor before bonding process: a top view; b cross section schematic diagram; c bottom view double-cantilever-mass structure. The thickness of accelerometer's diaphragm is designed to be 15 μm, the same as that of the pressure sensor's diaphragm, and the mass of accelerometer is designed to be greatest after wet etching to increase the sensitivity of accelerometer. Boron-doped silicon areas on the substrate work as piezoresistors for both accelerometer and pressure sensor. A layer of thermalgrowing SiO 2 and a layer of LPCVD low-stress SiN x are deposited and patterned on the substrate as passivation layers. Around bonding areas, passivation layers are etched as trenches which expose silicon. A layer of 1 μm-thick LPCVD α-Si is deposited and patterned subsequently to connect the silicon substrate by the exposed silicon in trenches, which can help to protect piezoresistors from p-n junction break-down during the α-Si-glass anodic bonding process on the top surface. To ensure the bonding strength and airtightness, instead of Al wires, heavily boron-doped silicon areas are buried beneath the SiO2/SiNx passivation layers to electrically connect Al wires with Al pads. The glass-silicon-glass sandwich structure (Roylance and Angell 1979;Zhang et al. 2013) is formed with α-Si-glass anodic bonding on the top surface and Si-glass anodic bonding at the bottom, as is shown in Fig. 2.

Design of pressure sensor
Dimensional parameters of the pressure-sensing diaphragm and piezoresistors are shown in Fig. 3. The pressure-sensing diaphragm (Santosh Pant 2014, 2016;Bae et al. 2004;Kanda and Yasukawa 1997) is designed to be 950 μm × 450 μm × 15 μm, which features high mechanical linearity under 450-kPa ranged pressures. Four borondoped silicon piezoresistors (R1, R2, R3, R4) are parallelly arranged on edges and the center of the diaphragm along <110> direction to form a Wheatstone full-bridge. The piezoresistors are boron-doped to be about 1 × 1017 cm -3 to reduce the temperature effects on sensitivity. The dimensional parameters of the piezoresistors are designed to be 260 μm (length) × 20 μm (width) × 3 μm (boron-doping depth).

Design of accelerometer
Dimensional parameters of accelerometer's double-cantilever-mass structure are shown in Fig. 4. Two folded boron-doped silicon piezoresistors (R′ 1, R′ 2) are parallelly arranged on the centers of two cantilevers along <110> direction, respectively, and another two (R′ 3, R′ 4) are parallelly arranged on substrate along another direction. Four piezoresistors form a Wheatstone half-bridge with reference resistors R′ 3 and R′ 4. The dimensional parameters of the piezoresistors are designed to be 100 μm (length) × 10 μm (width) × 3 μm (boron-doping depth). The thickness of accelerometer's diaphragm and cantilevers is designed to be 15 μm, the same as that of the pressure  sensor's diaphragm. Two cantilevers are designed to be 80 μm × 60 μm with the distance of 60 μm. The dimensional parameters of the diaphragm are 785 μm × 260 μm, and the distance between the center of etched mass and the edge of the substrate is 770 μm.
In order to increase sensitivity, the mass of the accelerometer should be etched to be greatest. The mass is formed by one-mask anisotropic wet etching process and the convex corner compensation etching technique (Fan and Zhang 2006;Offereins 1990;Schroder et al. 2001) is used to design the greatest mass. Because the thickness of silicon substrate is 380 μm and the thickness of accelerometer's diaphragm is 15 μm, the depth of wet etching is calculated to be 365 μm. The mass etching mask design is illustrated in Fig. 5. The dimensional parameters of mask frame are 1550 μm (length) × 960 μm (width) to ensure the diaphragm area of 865 μm (length) × 260 μm (width) after 365 μm depth wet etching process. The center of the big 440 μm side-length square is 1080 μm away from the edge, and eight small 200 μm side-length squares are around four corners of the big square for wet etching compansation. All parameters designed for squares can guarantee the greatest mass after 365 μm depth wet etching process.

Simulations
The commercial professional MEMS software, Intellisuite 8.5, is used to simulate the performances of the monolithic composite sensor. Intellisuite 8.5 has IntelliEtch module which can demonstrate the etched silicon substrate structure during silicon anisotropic wet etching process, 3D Builder module which can create and mesh a three-dimensional object and TEM module which can perform thermoelectro-mechanical analysis for a multi-conductor electromechanical device in multiple physical domains. Fig. 6a, the model of the pressure sensor, which includes a rectangular pressure diaphragm, piezoresistors and wires, is built and meshed by using 3D Builder and then exported to TEM. The material properties of silicon, aluminum and boron-doped silicon (piezoresistor) are listed in Table 1. Figure 6a results show the stress distribution on the pressure diaphragm under 450 kPa pressure applied on the top surface. After the stress distribution extracted to electromechanical coupling module,

Simulations for accelerometer
In this design, the mass of the accelerometer is formed using silicon anisotropic wet etching process from the backside of the wafer. To maximize the mass size and get a deeper understanding of etching process, silicon anisotropic wet  Fig. 7. IntelliEtch module is based on an atomic representation of the anisotropic wet etching dynamic process of silicon substrate, allowing the use of a wide range of Kinetic Monte Carli (KMC) and Cellular Automata (CA) time-evolution (Zhu and Liu 2000;Yan et al. 2008). SiO 2 and SiN x layers patterned as Fig. 5 are applied on the silicon wafer bottom surface (100) as the etching mask. The anisotropic etch rate of aqueous KOH (45 %, 85 °C) has been calibrated using wagon wheel method (Gosálvez et al. 2011) and is input to the database in software. Figure 8 shows that shapes of accelerometer's mass are formed at different etching depths. Simulation verifies that the etching mask as Fig. 5 can form the greatest mass which is separated to diaphragm edges to be an island.
The accelerometer with double-cantilever-mass structure is modeled according to IntelliEtch simulation results and subsequently electromechanical coupling simulations are conducted using TEM, as is shown in Fig. 8. Under a 5 V power supply and a 125 g acceleration input, the output of Wheatstone bridge is 2.5042-2.4959 V = 8.3 mV. The sensitivity of accelerometer is calculated to be 8.3 mV/ (5 V × 125 g) = 13.3 μV/V g.
Vibration modal analysis of accelerometer is also conducted using TEM, which can predict whether the chip is able to work well in a certain noise circumstance. The first vibration mode of the accelerometer is 8.54 kHz, as is shown in Fig. 9.

Fabrication
The composite sensor is fabricated with bulk-micromachining process and packaged using anodic wafer bonding  Fig. 10, fabrication processes start from n-type (100) double-side polished silicon wafer with a resistivity of 5-10 Ω cm and the thickness of 380 μm. The main steps are as follows: 1. Boron is implanted into the substrate and patterned to form piezoresistors of both accelerometer and pressure sensor. Subsequently heavy boron is implanted to form ohmic contact areas in piezoresistors and connecting wires between Al wires and Al pads. b electric potential of four piezoresistors on the Wheatstone bridge under a 5 V power supply 2. A 0.45 μm-thick thermal-growing SiO 2 layer and a 0.1 μm-thick LPCVD low-stress (150 MPa) SiN x layer are sequentially deposited on Si wafer and patterned to expose the silicon wafer in trenches. These two layers not only work as passivation and insulating layers for the piezoresistors, but also are used as the hard mask for silicon DRIE. 3. A 1 μm-thick LPCVD α-Si layer is deposited on the SiO 2 /SiN x passivation layers and patterned to expose the accelerometer and pressure sensor's working region and the outputpads region. 4. Wire holes are opened on ohmic contact areas by reactive ion etching (RIE). A 1 μm-thick aluminum layer is then magnetron sputtered and patterned for Wheatstone-bridge piezoresistive interconnection and seven output pads.
5. The wafer is soaked into aqueous KOH (45 %, 85 °C) with the clamp protection of the front side of wafer for about 11 h to complete the bottom-release by anisotropic etching. After one-step anisotropic etching, both the accelerometer mass and the pressure sensor diaphragm are formed. 6. The SiO 2 /SiN x layers are drily etched using RIE to expose bare silicon at the bottom surface. The holedrilled Pyrex7740 glass is bonded to the bottom silicon using anodic bonding with parameters of temperature 350 °C, voltage 800 V, current 20 mA, pressure 1600 N and time 20 min. After bottom bonding, the pressure sensor's sensing diaphragm is exposed to external detected pressure environment by the drilled hole in glass.

Fig. 9
Accelerometer vibration mode simulation using TEM: a simulation results of the natural frequency from 1st to 5th vibration modes; b the first vibration mode of the accelerometer 7. The double-cantilever-mass structure is released by DRIE, the Pyrex7740 glass is bonded to the α-Si layer at the top surface using anodic bonding with parameters of temperature 400 °C, voltage 1000 V, current 20 mA, pressure 1600 N and time 20 min. After top bonding, the accelerometer and the pressure sensor are sealed in one cavity at the top surface. 8. The top glass is diced to have the electric pads exposed.
Subsequently, the wafer is saw-diced from the bottom side to form the monolithic composite sensor dies.
The micrographs in Fig. 1a, c show the formed device chip before bonding from the front and backside, respectively. Figure 11 shows the device chip after bonding and dicing. The sensor die is as small as 2.5 mm × 2.5 mm × 1.4 mm. Fig. 12, the performances of the pressure sensor in the monolithic composite sensor are measured in a temperature-controlled environmental chamber where the temperature can be adjusted from room temperature to 300 °C and pressure can be adjusted from 0 to 1000 kPa .

Shown in
Test results are demonstrated in Fig. 13. Figure 13 shows the output voltage versus the pressure of the pressure sensor at 25 °C under a 5 V DC supply. Within the range of 0-450 kPa, by using fitting-line method the sensitivity of the pressure sensor is calculated to be 33.0 μV/V kPa.

Fig. 10
Fabrication processes of the monolithic composite sensor, a Boron is implanted into the substrate and patterned to form piezoresistors. b A 0.45 μm-thick thermal-growing SiO2 layer and a 0.1 μm-thick LPCVD low-stress (150 MPa) SiNx layer are deposited and patterned. c A 1 μm-thick LPCVD α-Si layer is deposited on the SiO2/SiNx passivation layers and patterned. d Wire holes are opened on ohmic contact areas by reactive ion etching (RIE). e The wafer is soaked into aqueous KOH to complete the bottom-release by aniso-tropic etching. f The SiO2/SiNx layers are drily etched using RIE to expose bare silicon at bottom. The Pyrex7740 glass is bonded to the bottom using anodic bonding (g). The double-cantilever-mass structure is released by DRIE, the Pyrex7740 glass is bonded to the α-Si layer at the top surface (h). The top glass is diced to have the electric pads exposed. Subsequently, the wafer is saw-diced from the bottom side to form sensor dies

Accelerometer
Shown in Fig. 14, a precise centrifuge turntable and a centrifuge control system are used for the measurement of the accelerometer. To ensure the dynamic balance, two accelerometers are fixed at two edges of the centrifuge turntable. The rotation-generated centrifugal accelerations are adjustable by the centrifuge control system which is controlled by a computer. A DC 5 V power is supplied to the Wheatstone bridge of the accelerometer. The output signal of the accelerometer is directly input a digital multimeter for voltage readout without any amplification.
The output voltage versus the acceleration under a 5 V supply is shown in Fig. 15. Within the measured range of 0-135 g, by using fitting-line method the sensitivity of accelerometer is calculated to be about 11.2 μV/V g. The zero point output of the accelerometer is measured to be 3.81 mV under a 5 V supply.

Conclusion
The monolithic composite sensor integrated pressure sensor and accelerometer has been designed and fabricated with compatible bulk-micromachining process and glasssilicon-glass three-layer anodic bonding technique. Onemask anisotropic wet etching from the wafer backside is processed to simultaneously form the pressure sensing diaphragm and the mass of the accelerometer. The composite sensor is packaged by Pyrex7740 glass anodic bonding on both sides of the wafer. On the top side of the wafer, a 1 μm-thick LPCVD α-Si layer is deposited to ensure the electrical conduction for anodic bonding and protect the piezoresistors from p-n junction break-down during anodic bonding process. After saw-dicing, this sandwich monolithic composite sensor come to a size of 2.5 mm × 2.5 mm × 1.4 mm. Testing results show that the pressure sensor has 33.0 μV/V kPa sensitivity, and the accelerometer has 11.2 μV/V·g sensitivity. Compared with some piezoresistive accelerometers in literatures or on the market, such as the Infineon SP30T-TPMS products have 8.95 μV/V kPa and 7.74 μV/V g sensitivity, our devised accelerometer exhibits a much increased performance.
Performances of the monolithic composite sensor well meet the requirements of its application fields.