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1. (Department of Electrical and Computer Engineering and Inter-University Semiconductor Research Center, Seoul National University, Seoul, 08826, Korea)

Poly-Si piezoresistors, diaphragm, barometric pressure sensor, piezoresistive pressure sensor, COMSOL multiphysics

I. INTRODUCTION

Recently, pressure sensors for barometric applications have played an important role in the pressure sensor market[1-6]. These barometric pressure sensors have become an essential part of various applications like weather forecasting, altimeters, automotive industry, industrial measurement and control system[8,9]. Also, barometric pressure sensors that can measure the height in response to the atmospheric pressure are expected to be installed in various smart devices [5,7-12].

Barometric pressure sensors based on Microelectromechanical systems (MEMS) have replaced the conventional barometric pressure sensors because of their small size and better sensor characteristics. Most of reported MEMS pressure sensors are fabricated by forming a cavity under the substrate and sealing it. The volume change of the cavity according to the atmospheric pressure is read as a change of capacitance or a change of piezoresistance. The cavity can be formed by process of etching the back-side substrate or process using the KOH solution[7,9,13]. However, these methods require a relatively large chip area and are expensive. In addition, they are incompatible with Complementary metal-oxide semiconductor (CMOS) technology. In this paper, we fabricate a novel barometric pressure sensor using a process compatible with the silicon CMOS technology. The sensor occupies a small area and can be integrated with circuits. This barometric sensor can read the change in atmospheric pressure as a change in piezoresistance of the Poly-Si electrode and amplifies it with a Wheatstone bridge fabricated on the same chip. Our sensor can detect the pressure changes in the range of 100 ~ 1013 hPa and can be used as barometer or altimeter.

II. FABRICATION

The barometric pressure sensor was fabricated on a 6-inch silicon (Si) wafer using conventional Silicon CMOS technology. Key fabrication steps of the barometric pressure sensor are shown in Fig. 1. 6-inch p-type (100) Si wafer was used as a substrate. A 300 nm-thick silicon dioxide (SiO2) layer was grown by thermal oxidation. Photoresist (PR) patterns for line-shape etching holes with a width of 0.5 μm are formed on the SiO2 layer. Fig. 2(a) is the top Scanning Electron Microscopy (SEM) view taken after the etching holes are formed in the SiO2 layer by etching the exposed SiO2 layer. Next, the upper portion of the Si substrate through the etching holes is isotropically etched using sulfur hexafluoride (SF6) gas, thereby forming a 2 μm depth cavity. There are 3 × 3 μm SiO2 square patterns between the etching holes at 8 μm intervals so that the underlying Si substrate is not fully etched during the isotropic etch. The remaining unetched silicon supports the diaphragm as an anchor to prevent collapse. Fig. 2(b) is the top SEM view taken after isotropic etching through the etching holes. Note the thickness of the remaining SiO2 layer is 0.1 μm after forming the cavity. To close the line-shape etching holes, Plasma Enhanced Tetraethyl Orthosilicate (PE-TEOS) oxide was deposited and this oxide layer was etched-back. Fig. 2(c) is the top SEM view taken after the PE-TEOS oxide layer was etched-back. The diameter of etching holes reduced to about 180 nm. Next, the PE-TEOS oxide was deposited again and etched-back one more time. The etch holes are completely sealed. Fig. 2(d) and Fig. 2(e) show top and cross-sectional SEM views, respectively, after closing the etching holes. The remaining SiO2 layer and the deposited PE-TEOS oxide layer have a thickness of 0.5 μm and become a diaphragm bending up and down with atmospheric pressure. Note the oxide after depositing the PE-TEOS oxide is etched by 0.1 μm. The cavity sealed by the PE-TEOS oxide layer is formed below the 0.5 μm thick diaphragm. If there is a pressure difference between the bottom and top of the diaphragm, the diaphragm will bend up or down. Next, undoped Poly-Si is deposited by LP-CVD (Low Pressure Chemical Vapor Deposition). This Poly-Si is used as the piezoresistors of barometric pressure sensor. Boron ions in a dose range of 3 ~ 5 × 1015 cm-2 are implanted into the Poly-Si. Annealing of implanted boron ions at 1050 ℃ for 5 seconds is followed by Poly-Si patterning using a dry etch process. A SiO2 layer of 10 nm and a Si3N4 layer of 25 nm are formed in order and used as a passivation layer. Then, the exposed passivation layer is etched using patterned PR to create a contact window for connecting the metal layer to the Poly-Si. After removing the PR, a metal (Aluminum) layer is formed and patterned for metal wires and pads.

Fig. 1. Schematic cross-sectional views for key process steps (a) patterning of line-shape etching holes having a width of 0.5μm, (b) isotropic etching using SF6 gas though the etching holes, (c) sealing with PE-TEOS layer, (d) deposition of Poly-Si, (e) patterning of the Poly-Si, (f) deposition of SiO2/ Si3N4 passivation layer, (g) deposition and patterning of the metal layer.

Fig. 2. (a) Top SEM view taken after patterning line-shape etching holes and anchors, (b) top SEM view taken after isotropic etching through the etching holes, (c) top SEM view taken after first PE-TEOS oxide deposition and etch-back, (d) top SEM view taken after filling the etching holes with PE-TEOS oxide, (e) cross-sectional SEM view taken after filling etching holes with the PE-TEOS oxide.

III. EXPERIMENTAL RESULTS AND DISCUSSIONS

To increase the sensitivity of the barometric pressure sensor, air pockets are added around the sensor. In one sensor, all air pockets are connected together to form a cavity. Fig. 3(a) shows the arrangement of air pockets. These air pockets expected to increase the stress on the diaphragm where the piezoresistors are formed. Because anchors of 3 × 3 μm are arranged except for 30 × 50 μm in the center region of the sensor, the diaphragm except the center region does not move to pressure changes. This makes stress due to the pressure difference to be concentrated only in the center region.

Fig. 3. (a) Top view of the fabricated barometric pressure sensor, (b) Magnified top view of barometric pressure sensor, (c) Equivalent circuit of Wheatstone bridge consisting of piezoresistors.

Table 1 compares the size with the previously fabricated pressure sensors. Our pressure sensor has a thin diaphragm compared to other pressure sensors. The thin diaphragm has a relatively high sensitivity and can respond to pressure changes even with a small-sized diaphragm.

Table 1. Size comparisons with other fabricated barometric pressure sensors

 Reference number(s) This Work [6][6] [9][9] [13][13] Diaphragm Size (μm) 100×100 200×200 680×680 500×500 Diaphragm Thickness (μm) 0.5 1.1 3~6 3

The Simulation results of the deflection of the diaphragm using COMSOL Multiphysics is shown in Fig. 4. Fig. 4(a) shows schematic of diaphragm having Poly-Si piezoresistors of 0.35 μm thickness on the 0.5 μm SiO2 layer. The diaphragm has a maximum deflection of 174 nm (Fig. 4(b)), and Fig. 4(c) shows the deflection due to the atmospheric pressure change. The diaphragm has a sensitivity of 0.17 nm/hPa and maximum deflection of the diaphragm is 174 nm at 12 hPa.

Fig. 4. Simulation result of the barometric pressure sensor (a) Schematic of diaphragm and Poly-Si piezoresistors of the sensor, (b) 3D deflection image of the diaphragm under the applied differential pressure, (c) Deflection of the diaphragm versus differential atmospheric pressure of the diaphragm．

The effect of doping concentration of Poly-Si is to control the relative contribution of grain and grain boundary. As the doping concentration increases, the effect of the grain boundary deceases and then the piezoresistive property improves[2]. Poly-Si doped with boron usually has the highest piezoresistive property at 1019 ~ 1020 cm-3[3]. In this paper, 3 × 1015 cm-2 ~ 5 × 1015 cm-2 boron ions are implanted with an energy of 90 keV into the 350 nm-thick Poly-Si, which is expected to give a boron concentration in the range of 8.6 × 1019 cm-3 ~ 1.4 × 1020 cm-3.

Fig. 5(a) shows the resistance change versus pressure curves of Poly-Si piezoresistor at different implantation doses of boron ions. Measurements were performed in the pressure range from 100 hPa to 1013 hPa, and a linear decrease in piezoresistance is observed with increasing atmospheric pressure. As the doping concentration of boron in Poly-Si increases, the piezoresistance change slightly increases.

Fig. 5. (a) Resistance change curve of Poly-Si piezoresistor as a parameter of a boron implantation dose, (b) Output voltage change of the Wheatstone bridge circuit with the pressure as a parameter of boron implantation dose.

In order to increase the output and decrease the temperature drift of barometric pressure sensors, four strain gauges are connected together as a Wheatstone bridge. Each arm of the Wheatstone bridge is made up of Poly-Si resistor. Fig. 3(b) shows the layout where resistors are placed. The two opposing resistors are fixed resistors and the other two opposing resistors are composed of variable piezoresistors. The variable resistors are placed on the diaphragm and the fixed resistors are placed on the silicon substrate. Fig. 3(c) shows the equivalent circuit of Wheatstone bridge consisting of Poly-Si piezoresistors. When a pressure difference occurs, only the variable resistors are deformed. Under constant voltage bias and zero pressure input condition, the output voltage ($V_{out}$) of the bridge is

(1)
$V_{o u t}=\frac{\left(R_{1} \times R_{3}\right)-\left(R_{2} \times R_{4}\right)}{\left(R_{1}+R_{2}\right) \times\left(R_{3}+R_{4}\right)} \times V_{B}$

where $V_{B}$ is bias voltage. $R_{1}$, $R_{2}$, $R_{3}$ and $R_{4}$ are the resistances of the four gauges, respectively. Because $R_{1}$, $R_{2}$, $R_{3}$ and $R_{4}$ are designed to have the same value, $V_{P}$ = 0.

When a pressure difference between two sides of the diaphragm exists, the strain resistance will be changed by the deformation of the diaphragm. Then the piezoresistances of $R_{1}$ and $R_{3}$ will increase, and $R_{2}$ and $R_{4}$ will not change. For their symmetrical locations, $\Delta R_{1}$=$\Delta R_{3}$=$\Delta R$ and $R_{2}$=$R_{4}$=$R$. Consequently, the bridge loses balance if there is a pressure change, and its output voltage is

(2)
$V_{o u t}=\frac{\Delta R}{(2 R+\Delta R)} \times V_{B}$

Fig. 5(b) shows the result of the output voltage change of the Wheatstone bridge circuit with atmospheric pressure changes in the range of 100 hPa to 1013 hPa. The sensitivity of the barometric pressure sensor can be confirmed by the slope of the graph (Fig. 5(b)) because it decreased linearly with increasing atmospheric pressure. As the boron concentration of Poly-Si increases, the sensitivity of the barometric pressure sensor increases. The barometric pressure sensor has a sensitivity of 2.50 μV/hPa at a boron dose of 5 × 1015 cm-2. The measured performance of barometric pressure sensor with the dose of Poly-Si piezoresistors is summarized in Table 2.

Table 2. The performance of barometric pressure sensor according to the dose of Poly-Si piezoresistors

 Poly-Si Dose 3 × 1015 cm-2 4 × 1015 cm-2 5 × 1015 cm-2 Sensitivity 2.01 μV/hPa 2.25 μV/hPa 2.50 μV/hPa

IV. CONCLUSIONS

We have proposed a barometric pressure sensor with a sealed cavity below the diaphragm and piezoresistors on the diaphragm. The process flow for the fabrication of the sensor is compatible with conventional CMOS process only except the formation of the cavity. 0.35 m thick Poly-Si which was doped by boron implantation was used as a piezoresistor to read the deflection of the diaphragm as a resistance change. The piezoresistance change due to the atmospheric pressure change was amplified and shown as a voltage change through the Wheatstone bridge circuit. The change due to atmospheric pressure change was shown as a linear output voltage. The boron implantation dose of 5 × 1015 cm-2 gave the largest sensitivity (2.50 μV/hPa).

ACKNOWLEDGMENTS

This work was supported by the National Research Foundation of Korea (NRF-2016R1A2B3009361), and the Brain Korea 21 Plus Project in 2018.

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Author

Dongkyu Jang

received the B.S. and M. S. degrees in electrical engi-neering from Korea University, Seoul, in 2008 and 2010, respectively.

In 2010, he joined at Samsung Electronics, where he has been working in the area of DRAM integration.

He is currently pursuing the Ph.D. degree with the Department of Electrical and Computer Engineering, Seoul National University, Seoul, South Korea.

He is also with the Inter-University Semiconductor Research Center, SNU.

His current research interests include pressure sensors and gas sensors.

Yoonki Hong

received the B.S. degree in Electrical and Computer Engineering from Seoul National University (SNU), Seoul, Korea in 2013.

He is currently working toward a combined master’s and doctorate program in Department of Electrical and Computer Engineering at SNU.

He is also with the Inter-University Semiconductor Research Center, SNU.

His current research interests include MOSFET-based gas sensor and humidity sensor.

Seongbin Hong

received the B.S. degree in Electrical and Computer Engineering from Seoul National University, Seoul, Korea in 2016.

He is currently working toward a combined master’s and doctorate program in Department of Electrical and Computer Engineering at SNU.

He is also with the Inter-University Semiconductor Research Center, SNU.

His current research interests include FET-based sensor platform design and fabrication.

Jong-Ho Lee

received the B.S. degree from Kyungpook National University, Daegu, Korea, in 1987 and the M.S. and Ph.D. degrees from Seoul National University, Seoul, in 1989 and 1993, respectively, all in Electronic Engineering.

In 1993, he worked on advanced BiCMOS process development at ISRC, Seoul National University as an Engineer.

In 1994, he was with the School of Electrical Engineering, Wonkwang University, Iksan, Chonpuk, Korea.

In 2002, he moved to Kyungpook National University, Daegu, Korea, as a Professor of the School of Electrical Engineering and Computer Science.

Since September 2009, he has been a Professor in the School of Electrical and Computer Engineering, Seoul National University, Seoul, Korea.

From 1994 to 1998, he was with ETRI as an invited member of technical staff, where he worked on deep submicron MOS devices, device isolation.

From August 1998 to July 1999, he was with Massachusetts Institute of Technology, Cambridge, as a postdoctoral fellow, where he was engaged in the research on sub–100 nm double-gate CMOS devices.

He has authored or coauthored more than 216 papers published in refereed journals and over 326 conference papers related to his research and has been granted 85 patents in this area.

He received 18 awards for excellent research papers and research excellence.

He invented bulk FinFET, Saddle FinFET (or bCAT) for DRAM cell, and NAND flash cell string with virtual source/drain, which have been applying for mass production.

His research interests include CMOS technology, nonvolatile memory devices, thin film transistors, sensors, neuromorphic technology, and device characterization and modeling.