L.J. GOLONKA, A. DZIEDZIC, H. ROGUSZCZAK, S. TANKIEWICZ, D. TERECH; Novel technological and constructional solutions of pressure sensors made in LTCC technology, Proc. SPIE, vol. 4516 (2001), Optoelectronic and Electronic Sensors IV, p.10-14
Novel Technological and Constructional Solutions of Pressure Sensors Made in LTCC Technology Leszek J. Golonka, Andrzej Dziedzic, Henryk Roguszczak, Szymon Tankiewicz, Damian Terech Institute of Microsystem Technology, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland Phone/Fax: +48 – 71 – 355 48 22, E- mail:
[email protected] Abstract Paper describes the design, technology aspects and exploitation parameters of thick film p ressure sensors manufactured in LTCC technology. The piezoresistive properties of thick film resistors (DP 2041, 10 k /□) were determined. The pressure sensors were fabricated in LTCC DP 951 tape with thickness of 165 m (before firing). Two types of sensor were designed: - 2D sensor (planar) with the membrane next to the electronic board, - 3D sensor (vertical), where sensor and electronic board were mounted in one three -dimensional LTCC structure. In order to convert an output signal fro m the sensor into linearly proportional, standard (4 20 mA) current signal the electronic circuitry was used. The circuitry was fabricated in SMT technology on LTCC ceramics. Parameters of pressure sensors were determined in the range of differential pressure from 0 down to –100 kPa. Kewords: LTCC, resistor, piezoresistivity, Wheatstone bridge, pressure transducer
1. Introduction The first thick film pressure sensors were described in the 80-t ies [1,2]. The piezoresistive effect in th ick film resistors was utilised in the sensors [3,4]. The sensors were made on an alumina, wh ich was the most common used substrate material in thick film technology. Due to Low Temperature Cofiring Ceramics (LTCC) invention, the new possibilities appeared in the sensor’s design and construction. LTCC ceramics allo ws on integrating both sensor and microelectronics transducer in one structure. The results of our investigation on a new design of pressure sensors are presented in the paper. The achieved sensor’s parameters are similar to the parameters of co mmercially produced sensors (e.g. Entran Dev ices, USA).
2. Technology Two types of sensors were designed: 2D sensor (planar) with the membrane next to the electronic board, and 3D sensor (vertical), where the sensor and electronic board are mounted in one three -dimensional LTCC structure. 2D pressure sensor consisted of 4 green tape tapes. The membrane was made fro m the t wo upper tapes. It was situated next to the electronic circuit (Figure 1). Four thick film p iezo resistors were printed on the membrane. The hole made in two lower tapes determined the memb rane diameter. The vias in two upper tapes (tapes 3 and 4 in the Figure 1) were used for electrical supply connection to Whetstone’s bridge and for sending the bridge output signal to the transducer’s electronic circuit. In the second type of the sensor (3D), the pressure sensor and the transducer were situated vertically, one over another. They formed 3D LTCC structure. The 3D sensor consisted of eight green tapes (Figure 2). The three lowest tapes formed the membrane. The thick film p iezo resis tors were made on the tape 3. They were connected in a Whetstone’s bridge configuration. The central holes made in the tapes 4-6 determined the memb rane diameter. Four vias were made in the tapes 4-8. They were filled with conduction ink. The vias provided electric supply to the bridge input, and signal fro m the bridge output to the electronic transducer on the highest tape (Fig. 2). The 2D and 3D pressure sensors were fabricated in LTCC DP951 tape with thickness of 165 m each (before cofiring process). The green tapes were cofired in typical firing profile reco mmended by Du Pont for DP 951 tape.
8 7
6 5
4
4
3
3
2
2
1
1
Fig. 1. Cross-section of 2D pressure sensor
Fig. 2. Cross-section of 3D LTCC pressure sensor
The electronic circuit o f the trans ducer was based on XTR 105 Burr-Brown element. The piezoresistor’s bridge was supplied fro m constant current source (0.8 mA). Th is kind of supplying determined a stable working conditions. The change of the bridge piezoresistors resistance was transformed to voltage signal. The voltage from the bridge output was sent to the input of the electronic signal conditioning circuit. The circuit converted voltage signal to current signal in 4 20 mA standard.
3. Piezoresistive properties of DP 2041 resistors Piezoresistive properties are characterized by coefficient called Gauge Factor (GF), wh ich is defined as GF = R / R
(1)
where R/ R – relative resistance changes and = l / l – relative change in length of the resistor (strain ). So-called longitudinal (GFL ) and transversal (GFT) gauge factors were calculated for cases where applied stresses are parallel or perpendicular to current flowing through the resistor. Piezoresistive properties were characterized both for surface and buried LTCC resistors. Th e DP 2041 resistor ink with 10 kΩ/□ nominal sheet resistance and PdAg-based DP 6146 conductor were used in fabrication of 0.87 0.87 mm2 test resistors (dimensions after firing process). The well-known method of bent cantilever was used to GF calculation of LTCC resistors – one sample tip was mounted in holder whereas the second tip was bent by micro metric screw. The resistance and deflection of p iezo resistors were measured directly during bending and next the strain was calculated based on knowledge of the beam geometry and deflection at the free beam end. Having the calcu lated strain of the resistor the gauge factor can be easily calculated fro m the above Eq. (1). 1,5x10 1,0x10
-3
DP 2041 ink -3
GFT = 3,2
R/R0
5,0x10 -4 0
-5,0x10 -4 -1,0x10 -3 -1,5x10 -3 -4
-4,0x10
-4
-2,0x10
0
-4
2,0x10
-4
4,0x10
Fig. 3. Typical dependence of relative resistance changes versus strain
R□ [k /□]
GFL
GFT
Surface resistor
7.4
10.0
6.7
Buried resistor
29.6
5.6
3.2
Table 1. Sheet resistance as well as longitudinal and transversal gauge factor for surface and buried DP 2041 resistors Based on the results given in Table 1 it was concluded that: Sheet resistance of buried LTCC resistors is higher than for surface ones (for the same resistor materials and parameters of screen printing process), Longitudinal gauge factor exceeds transversal one both for surface as well as buried resistors, Gauge factor of surface resistors is higher than buried ones.
4. Measurements of pressure actuator exploitation parameters Co mplex measurements of such exploitation parameters as transducer sensitivity in the whole pressure scale range, nonlinearity error, hysteresis error, temperature zero shift – TZS and temperature sensitivity shift – TSS was made both for 2D and 3D LTCC pressure transducers. Their results are presented and discussed below. The exp loitation parameters of basic structure of piezoresistive pressure sensor, this is Wheatstone bridge, were measured in the first stage. This permited to qualify technology used for sensor fabrication, wh ich made possible optimization of manufacturing process and sensor construction. The second stage was c onnected with design and fabrication of full pressure transducer, i.e. with integration of Wheatstone bridge with electronic amp lifying circuit which converts the output signal of the unbalanced bridge into 4 20 mA standard current signal. 4.1. 2D pressure transducer The output signal of the 2D LTCC pressure transducer is shown in Fig. 4. Significant nonlinearity in such transducer response is clearly visib le and nonlinearity error arising fro m this is shown in Fig. 5. 5
Nonlinearity error [% FSO]
20 18 16
I [mA]
14 12 10 8
4 3 2 1 0 -1
6
up down
4 2
-2 -3
0
10
20
30
40
50
60
70
80
90
100
p0-p [kPa]
Fig. 4. Output characteristics of 2D pressure transducer
0
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100
p 0-p [kPa]
Fig. 5. Nonlinearity error of 2D pressure transducer
The nonlinearity error reaches value of 4.5 % FSO (full-scale output) for the largest measured differential pressure. This decreases significantly the measurement accuracy and makes transducer calibration difficult. In this case to precise linearization of the output signal fro m the bridge is necessary by means of specialized linearization circu its or application of microcontroller. Th is leads to complication of transducer construction and increase of fabrication costs. Moreover, the hysteresis error of 2D version is relat ively large ( 0.7 % FSO), which gives larger pressure inaccuracy (Fig. 6).
Hy steresis error [% FSO]
0,1 0,0 -0,1 -0,2 -0,3 -0,4 -0,5 -0,6 -0,7 0
10
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100
po-p [kPa]
Fig. 6. Hysteresis error of 2D pressure transducer versus differential pressure Measurements performed for tested 2D pressure transducer at various operating temperatures have shown its great temperature sensitivity in spite of electronic circuit application. Temperature zero shift reaches to 40 %/ 50°C, whereas temperature sensitivity shift 2,5 %/50°C. The above results condemn 2D (p lanar) version as a useable pressure transducer. Therefore further investigations were concentrated on 3D (vert ical) version, which possessed much better parameters. 4.2. 3D pressure transducer Vertical pressure transducer was characterized in the same manner. Its output signal is visible in Fig. 7. The selfdesigned, high accuracy amplifier based on Burr-Bro wn XTR 105 integrated circuit made possible linear conversion of voltage output from unbalanced Wheatstone bridge onto 4 20 mA standard current signal. Current-voltage transducer restricted sensor nonlinearity to the level 0.5 % FSO (Fig. 8). The largest errors appear in the middle and at the extreme limits of measuring range. Hysteresis was compensated five-times (to the level of 0.14 % FSO – Fig. 9) in co mparison with planar version. 0,5
18
0,4
Nonlinearity error [% FSO]
20
16
I [mA]
14 12 10 8 6
up down
4 0
10
20
30
40
50
60
70
80
90
0,3 0,2 0,1 0,0 -0,1 -0,2 -0,3
100
p0-p [kPa]
Fig. 7. Output characteristics of 3D pressure transducer
0
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40
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60
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100
p0- p [kPa]
Fig. 8. Nonlinearity error of 3D pressure transducer
However connection of electronic circuit together with piezoresistive bridge on the same LTCC structure made worse of selected exp loitation parameters. For exa mp le the increase of temperature zero shift fro m 0.46% to 1.12% FSO was noted. Probably this was caused by temperature sensitivity of fabricated amplifying circu it. But hysteresis error was kept at the same level as for typical co mmercial pressure sensors. On the other hand, our current-voltage transducer permitted for small improvement of temperature sensitivity shift (fro m 0.82 %/50°C to 0.67 %/50°C) in co mparison with unbalanced Wheatstone bridge).
L.J. GOLONKA, A. DZIEDZIC, H. ROGUSZCZAK, S. TANKIEWICZ, D. TERECH; Novel technological and constructional solutions of pressure sensors made in LTCC technology, Proc. SPIE, vol. 4516 (2001), Optoelectronic and Electronic Sensors IV, p.10-14
Hy steresis error [% FSO]
0,08 0,04 0,00 -0,04 -0,08 -0,12 0
10
20
30
40
50
60
70
80
90
100
p0-p [kPa] Fig. 9. Hysteresis error of 3D pressure transducer versus differantial pressure
5. Summary and conclusions Planar (2D) and vertical (3D) LTCC pressure transducers were made and characterized. Electrical p roperties of both versions of pressure sensors are compared in Table 2. 2D transd ucer served only for technology and topology qualification onto explo itation parameters of pressure transducer. But only 3D version permitted both for topology and construction optimization as well as minimizat ion of errors appearing in pressure measuremen ts. This transducer was characterized by appropriate sensitivity in the whole pressure scale, insignificant nonlinearity and hysteresis level and very small shift of the output characteristics at various operating temperatures. All of explo itation paramete rs were fully co mparable with commercially availab le pressure transducers. Therefore further optimization and then mass production of 3D LTCC pressure transducer seems fully possible. Output signal Linearity (% of full scale) Hysteresis (% of full scale) TZS (% FSO/ 50°C) TSS (%/50°C)
Version 2D 4 20 mA 4.5 0.7 40 2.5
Version 3D 4 20 mA 0.5 0.14 1.12 0.67
Table 2. Co mparison of exploitation parameters of 2D and 3D LTCC pressure transducers
References [1] Cattaneo A., Dell’Acqua R., Dell’Orto G., Piro zzi L., Canali C. – A practical utilization of the piezo resistive effect in thick film resistors: a low cost pressure sensor, Proc. IMS (ISHM-USA), 1980, pp. 221-228 [2] Cattaneo A., Dell’Acqua R., Forlan i F., Piro zzi L. – Lo w cost thick-film pressure sensor, SAE Technical Paper Series, No. 800023, SA E Meeting, Detroit, 1980, pp. 49-54 [3] Prudenziat i M., Morten B. – Piezoresistive properties of thick-film resistors – an overview, Hybrid Circuits, No. 10, 1986, pp. 33-37 [4] Handbook of Sensors and Actuators, Vol. 1 – Th ick Film Sensors, M. Prudenziati (ed itor), Elsevier Science B.V., 1994 [5] Kalita W., Potencki J., Slosarčik S. – Integrated pressure/frequency converter in LTCC technology, Proc. 6 th Scientific Conf. “Technologia Elektronowa”, ELTE’97, pp. 597-601 (in Po lish) [6] Bansky J., Kalita W., Potencki J., Slosarčik S. – Integrated converter of pressure/vacuum into frequency based on LTCC technology, Proc. o f 21st Conf. of ISHM Po land, Ustroń, 1997, pp. 73-76 [7] Kalita W., Malita D., Slosarčik S. – Integrated converter of pressure/vacuum into frequency based on LTCC technology, Proc. of 1st Int. Sy mp. on Microel. Techn. and Microsyst., Rzeszów, 1997, pp. 119 -124
