Two-chip Si Pressure Sensor Kiyotaka Osanai,1 Kiyonari Itoh,1 Yukio Matsuki,1 Mikio Hashimoto,2 Shinichi Murashige,2 Takeshi Shiojiri,2 and Shougo Mitani2 We have developed a “Two-chip Si pressure sensor” that consists of two chips. One is a Si pressure sensor chip and the other is an ASIC for signal conditioning of a sensor chip. Comparing with monolithically integrated pressure sensors, this configuration enables higher accuracy, wider pressure range and operating temperature, more flexible operation to changes of supply voltages and output ranges, which are suitable to fulfill diversifying demands from customers. Wide operational temperature ranges from −40 - 125 °C and a total high accuracy of ±1.5% FS (0 - 85 °C) is obtained. 1. Introduction Small and light-weight Si pressure sensors are used in a variety of applications in common living. Especially in the application of electric appliances, medical and automobiles, developing user-friendly “sensing devices” is becoming a big need and with existing electronics and higher functionalized boon, the demand for pressure sensor functions is becoming more sophisticated year by year. Rapidly, higher functionalized boon of cellar phones and digital steal cameras are also making more rooms in the field of new applications. A special fine processing technology called Micro Electro Mechanical Systems (MEMS) is now widely used for accelerometers and Si microphones, pressure sensors also fall in the category of MEMS devices and are one of the earliest devices among commercialized MEMS products. In the case of pressure sensors, it’s almost mandatory to use a special configuration package or chassis with a pressure introducing port to apply a pressure onto the diaphragm, which is working as a sensing portion. Different from standard semiconductor element which is just handling electrical inputs and outputs, when a pressure sensor is in operation the stress is caused in the chip by the pressure applied and for this reason the whole configuration needs to be designed properly, considering the stress caused by not only the assembling process but also the configuration. Si pressure sensor chip has a Wheatstone bridge that consists of four piezo-resistive resistors and generates the DC voltage output in proportion to the 1 Akita branch of Silicon Technology Department of Electron Device Laboratory 2 Silicon Technology Department of Electron Device Laboratory Fujikura Technical Review, 2008 pressure changes. These outputs are small and have characteristic variations such as an offset voltage, a span voltage and temperature characteristics, which makes it difficult to be used in high-accuracy measurements. In most cases, amplification and temperature compensation circuit is added to overcome these issues. The peripheral circuit described above can be realized by using the interfaces such as monolithic integration of these functions onto a sensor chip itself or adding external discrete circuit. Or through trimming, i.e. the optimization of the circuit constants, there are analogue adjustments by thin films’ laser trimming and digital adjustments by Application Specific Integrated Circuit (ASIC). We have been selecting the best way to fit for the product line. One of our products having peripheral circuit around a sensor chip portions are called “One chip integrated pressure sensors” and widely accepted by the market due to their small sizes and user-friendliness. But thinking about the demands of high accuracy, widening pressure ranges and operational temperatures and changing the output swing voltages, “One chip integrated pressure sensors are not necessarily the best solution, and the versatile sensors which are much more flexible to realize the demands listed above are eagerly anticipated. In this paper, we are going to describe high accuracy and easily customized “Two-chip Si pressure sensor”, which we have developed, mainly with its development concept and basic characteristics. 2. Development concept 2.1. Multi chip configuration As explained before, one-chip integration is very effective to decrease its size, but does not fit for pro49 Pressure sensor Coarse offset voltage Fine offset voltage Temperature sensitivity of offset voltage Temperature coefficient of span voltage Gain EEPROM Bridge resistance + + − − + Pressure sensor analog output − Gain circuit Temperature sensor analog output Temperature sensor ASIC Condenser for noise reduction Condenser for low-pass filter Vcc GND Fig. 1. Block diagram of Two-chip Si pressure sensor. ducing many kinds of products by changing pressure sensor portions. The reason is that there must be careful investigations and considerations needed to make a single chip optimized for not only circuitries for signal conditions of sensor, but also the sensor itself. What we’ve done is a multi chip approach, which is to select the pressure sensor chip with the characteristics suitable for the final requirements and to combine it with ASIC chip for signal conditioning. By doing this, we can use the single ASIS chip without modifications to cover from low- to high-pressure range. 2.2. High accuracy consideration 1) It is widely called “trimming,” to adjust sensor characteristics together with temperature characteristics. As explained before, there are two circuit adjustment ways. One is an analogue approach like thin film cutting by laser, the other is a digital approach like storing the trimming data in the internal memory and retrieving them for the purpose of adjusting circuitries after D/A conversion. Comparing these two methods, the latter can be done more accurately because there’s no need for accessing the trimming resisters, and so in situ applying pressures to the sensor and changing environment temperatures, trimming can be easily done. Block diagram of “Two-chip Si pressure sensor” is shown in Fig.1. ASIC is made by complementary metal-oxide semiconductor (CMOS) process and the switched capaci50 S: Switch I: Current V1 f: Frequency V2 C: Capacitance R: Resistance V1 V2 Q=C·(V1-V2) Ieq=Q·f=C·f·(V1−V2) Req=(V1-V2)/Ieq=1/(C·f) Fig. 2. Switched capacitor circuit and equivalent resistance. tor circuit is employed for an amplification circuit. As shown in Fig.2, this circuitry realizes equivalent resistors by the combination of MOS switches and capacitors. In general, a differential amplifier needs the tight ratio of pair resistors rather than the absolute value of themselves. Pair capacitors can be easily made identical when CMOS process is employed., i.e. the ratio of these capacitors is very close to 1. So the switched capacitor circuit is suitable for an instrumentation amplifier because of almost the same value of resistors, which can be obtained through the precise controllability of capacitor values of CMOS process. The parameters we should trim for a sensor are offset voltage coarse/fine adjustments, amplifier gain, temperature sensitivity of offset voltage and temperature coefficient of span voltage. When the adjustment range for 1 bit is small, we can do very fine adjustments. On the other hand, we should have large memory areas for them and increased chip sizes, which also increases chip cost. We’ve optimized these relationships considering the accuracy and cost at the same time. 2.3. Low voltage drive/low power consumption considerations 2) There seems to be a couple of applications like a cellar phone and a PDA, which assumes battery operations. The voltage required was about 2.7 - 3.3 volts and to reduce the total power consumptions for both sensor and ASIC should be minimized. To reduce the total power consumption more intermittent drive of ASIC by external signals can also be proposed. In the case of 5 kohm Wheatstone bridge sensor with our ASIC, the current consumptions are around 2 mA typically when the ASIC is driven by 5 V. The temperature dependencies of the current consumptions are shown in Fig.3. In the case of 3 V drive of the same ASIC with the same sensor, the current consumption becomes as low as around 1.2 mA typically. Also, the intermittent drive makes it possible to comply with a system that requires power management. The voltage dependencies of offset voltages and span voltages are shown in Fig.4 which is very good linearity. 2.4. Multi sensing considerations It’s not unusual to integrate multi devises into one chip, but depending on the combination of the sensors, there are cases that make evaluation of the sensors difficult and make the application less userfriendly. In the case of Two-chip Si pressure sensors, the ASIC is doing temperature compensations by knowing the temperature from the temperature sensor in the ASIC. These temperature sensors can also be used for simple temperature measurements from −40 to 125 °C. 3. Manufacturing process and output characteristics 3.1. Manufacturing process The process flow is shown in Fig.5. Two chips, sensor and ASIC, are placed side by side and connected directly by gold wires. Sensor chip ASIC chip Die bonding current consumption (mA) Wire bonding 3.0 2.5 DC+5 V drive, Sensor=5 kΩImp. Lead trimming & forming 2.0 1.5 Laser marking 1.0 0.5 0.0 −50 0 50 100 temperature (°C) 150 Digital trimming / Measurement Fig. 5. Process flow. Fig. 3. Temperature dependence of current consumption. Gauge pressure type output voltage (V) 5.0 4.0 span voltage Absolute pressure type 3.0 offset voltage 2.0 1.0 10 0.0 4.0 4.5 5.0 5.5 5 6.0 power supply voltage (V) 0 unit: mm Fig. 4. Power supply voltage dependence of sensor offset and span output voltage. Fujikura Technical Review, 2008 Fig. 6. Finished Two-chip Si pressure. 51 0.30 4.90 +1.5 %FS @250 kPa-abs output voltage (V) output voltage (V) @20 kPa-abs 0.25 0.20 0.15 −1.5 %FS 20 40 60 temperature (°C) 80 4.85 4.80 4.75 −1.5 %FS 4.70 0.10 0 +1.5 %FS 0 100 20 40 60 80 100 temperature (°C) Fig. 7. Characteristics of absolute pressure type sensor in 250kPa-abs range. 0.60 4.60 @0 kPa @1 MPa output voltage (V) output voltage (V) 85 °C +1.5 %FS 0.55 0.50 0.45 0.40 85 °C −1.5 %FS 0 20 40 60 80 100 temperature (°C) 120 140 85 °C +1.5 %FS 85 °C −1.5 %FS 4.55 4.50 4.45 4.40 0 20 40 60 80 100 temperature (°C) 120 140 Fig. 8. Characteristics of gauge pressure type sensor in 1MPa range. 3.2. Output characteristics after packaging The absolute together with gauge pressure type sensors with ASIC in a single surface mount packages are shown in Fig.6. The dimensions excluding the pressure ports are 9.0 × 8.0 × H 4.6 mm and 9.8 × 9.5 × H 4.8 mm for the absolute and gauge type respectively. We’ve measured the output voltage characteristics applying 5 V to the sensors. Figure 7 is the sensor characteristics 250 kPa absolute pressure and Figure 8 is for 1 MPa gauge pressure normalized by full scale output of the unit (FS). From these both are having the accuracies within the range of ±1.5 %FS/0 ~ 85 °C and even 1 MPa gauge type maintains the accuracy as high as ±1.5 %FS and up to 120 °C, which can be used in the application that require very high accuracies. 4. Conclusion The main functionalities and basis characteristics of Two-chip Si pressure sensor are summarized in Table 1. By implementing multi chip configuration, we have successfully developed the “Two-chip Si pres- 52 Table 1. Main characteristics of Two-chip Si pressure sensor. Flexible with the changes of pressure ranges and output voltages High accuracy (±1.5 %FS / 0 ~ 85 °C) Wide operating voltages from DC 3 ~ 6V Multi-sensing of the pressure and temperature is possible Wide operating temperature (−40 ~ +125 °C) Wide output voltage range (Vss ~ Vdd) Intermittent drive with the “enable terminal” is possible sure sensor” which fits for high-accuracy measurements and is easily customized according to customer requests. We are planning to broaden out product lines aiming at the medical and automotive markets which need high accuracy. References 1) Takahashi, et al. : Digital Signal Conditioning Technique for Silicon Piezoresistive Pressure Sensor, Fujikura Giho, No.96, pp.54-60, 1999 2) Yomogida, et al. : One-chip Integrated Pressure Sensor of Low Voltage Excitation, Fujikura Giho, No.101, pp.75-80, 2001