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.

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.

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.

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.

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.

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

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

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.

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).