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Thin film membrane based on a-SiGe: B and MEMS technology for application in cochlear implants
Journal of Non-Crystalline Solids 358 (2012) 2331–2335
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Thin film membrane based on a-SiGe: B and MEMS technology for application in cochlear implants Aurelio Heredia a, c,⁎, Roberto Ambrosio b, Mario Moreno d, Carlos Zuñiga d, Abimael Jiménez b, Karim Monfil b, Javier de la Hidalga d a
Electronics, Department, UPAEP, 21 Sur 1103, Puebla, Mexico Electrical and Computing Department-IIT, UACJ, Av. del Charro 450N, 32310, Cd. Juarez, Mexico BioEngineering and Chemical, Department, UPAEP, 21 Sur 1103, Puebla, Mexico d Electronics Department, INAOE, Luis Enrique Erro No. 1, Santa Maria Tonantzintla, Puebla, Mexico b c
a r t i c l e
i n f o
Article history: Received 9 September 2011 Received in revised form 23 November 2011 Available online 24 January 2012 Keywords: PECVD; Amorphous silicon germanium; Cochlear implant; Pressure sensor; MEMS
a b s t r a c t In this work is presented the fabrication of a thin film membrane as a bio-transducer for aural assistance detection, therefore it will operate at low pressure. The resonant membrane was deposited by PECVD technique at low temperature of deposition T = 270 °C, using SiH4, GeH4, and Boron gases. The membrane was suspended on a micromachined crystalline silicon frame obtained by wet chemical etching. The a-SiGe:B film presented a resistivity of 2.46 × 103 (Ω-cm), resistance of 20.8 kΩ. Using these experimental data we succeeded in designing a simple structure for sensing low pressure variations. The output voltage of the sensor was measured for a range of pressure from 0 to 3000 Pa and at bias voltage of 10 V. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Micro Electro Mechanical Systems (MEMS) technology being to mature and new applications are becoming more attractive for research and development, such as automotive, consumer products and medical applications [1]. MEMS fabrication techniques include bulk and surface micromachining. Using these techniques in conjunction with the deposition of high quality thin films materials many sensor technologies have been developed. The use of thin films obtained at low temperature of deposition (T b 300 C) open the possibility to use different type of substrates such as glass, metal, plastic and biocompatible materials. In addition, the use of low temperature allows the integration of MEMS with electronic circuits, either CMOS (Complementary MOSFET) or TFT (Thin Film Transistors) for flexible electronics [2,3]. Also, different works using thin film in MEMS have been reported, for example: IR bolometers sensors have been developed through surface micromachining, using a-SiGe:H as a sensing layer in a bridge configuration supported by a SiNx thin film, all the materials were deposited by PECVD at low temperature of deposition [4]. In Ref. [5], electrostatic microresonators based in thin film silicon MEMS have been reported, the devices were fabricated on glass substrates at temperatures ≤110 °C using amorphous to microcrystalline silicon. Sarro
⁎ Corresponding author. Tel.: + 52 222 2299400x7143; fax: + 52 222 2299425. E-mail address: [email protected] (A. Heredia). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.12.101
et al in [6] have reported the use of SiC thin films by PECVD for MEMS applications in harsh environment, combining with CMOS technology at low temperature of post-processing. As we mentioned in previous paragraph, another interesting area of MEMS is the developing of medical devices. The use of these devices in medicine has been researched in the field of exploration, diagnostic and surgery [1]. A cochlear implant is an artificial hearing device, which transmits sound directly to the auditory nerve through electrical stimulation of the cochlea, bypassing the ear canal, eardrum, and middle ear bones. Today, piezomaterials are the best choice for direct detection of sound at frequencies between 20 Hz and 20 kHz for auditory applications [7,8]. One of the key issues for achieving low-cost detectors is their monolithic integration and compatibility with CMOS technology. For this purpose, the most widely used detector approach is to implement micro-cochlear implant using micromachined structures on CMOS process. The piezoresistive properties of crystalline silicon have been well studied in the fabrication of strain gauges and other electromechanical transducers [9]. Some works have reported the piezoresistive pressure sensors with diaphragms and piezoresistors made of 6H-SiC, while, others studies have showed sensors with 3 C-SiC piezoresistors on Si or SiO2/Si diaphragms [10]. The piezoresistance effects have been observed in amorphous silicon films (aSi:H,F), this material reported a gauge factor (GF) equal to 33 for ptype films and − 20 in the n-type films [11]. The piezoresistive behavior of n-type and p-type doped μc-Si:H films deposited by radiofrequency (RF) plasma-enhanced CVD on Corning glass substrates at
2332
A. Heredia et al. / Journal of Non-Crystalline Solids 358 (2012) 2331–2335
200 °C has been reported in ref [12], with values of GF between 16 and 23 and − 25 to −40 for p-type and for n-type films respectively. In ref [13], the piezoresistive property of n-type and p-type of μc-Si:H deposited by radio-frequency plasma enhanced chemical vapor deposition (RF) and by Hot Wire on plastic substrates at 100 °C has been demonstrated values of GF for p-type films between 25 and 32 and for n-type films between − 40 and − 10. These examples show the use of silicon thin film technology at low temperature of deposition, which enables the possibility for large area applications in novel substrates. In this context, few works have investigated the potential of amorphous silicon, its alloys for pressure sensors and its applications in medical devices. This is the main reason for investigate this kind of applications taking into account the advantages of low temperature of deposition of the films and integration with silicon technology or another alternative substrates. On the other hand, a drawback for the use of amorphous silicon in the fabrication of devices is its high resistivity. In order to overcome this problem, is necessary to perform the doping of the films. Doping of a-Si:H films can be performed during the deposition process by adding the dopant precursor. Currently at UPAEP/INAOE, we are developing thin film materials based on amorphous silicon germanium alloys (a-SiGe: B), thus the resistivity of the material, can be easily controlled by the doping level. The films used in this work particularly present a piezoresistive effect that can be controlled through variations on doping; therefore, the resistance is affected under pressure conditions. Furthermore the resistance changes on films, the deposition of this material is fully compatible with the CMOS technology. This option opens the possibility to integrate the device with the read-out circuitry; this will lead to a cochlear implant with an improvement in the cost compared with others made of different materials and technology [14]. As mentioned before the device is intended for low sound detection; therefore it will operate at low pressure. In this work the resonant membrane is achieved by depositing the thin film on a silicon nitride diaphragm, which is suspended on a micromachined crystalline silicon frame as is shown in Fig. 1. Here, we explore the use of piezoresistive membrane sensors for acoustic vibration detection at micro-scale. The principle operation and the method of microfabrication for a simple design of MEMS piezoresistive sensor are described. Finally the fabrication of a-SiGe: B sensor directly on Si substrates and the characterization of the sensor under pressure conditions are also demonstrated.
An electrical resistor may change its resistance when it experiences a strain and deformation. This effect provides an easy and direct energy/signal transduction mechanism between the mechanical and the electrical domains. Today, it is used in the MEMS field for a wide variety of sensing applications, including accelerometers and pressure sensors [9]. The resistance value of an element with the length l and the crosssectional area A is given by
2. Principle of operation
G ¼ ΔR=εR
The piezoresistive effect describes the changes of resistivity of material when it is subjected to external stress or material deformation.
where ε is the applied strain. The resistance is measured along its longitudinal axis. The total resistance change is the contribution of changes under longitudinal and transversal stress components, which can be expressed as
R ¼ ρl=A
ð1Þ
The resistance value is determined by the resistivity (ρ) of the material and the dimensions. Consequently, exist different ways to change the resistance; one of this is applying strain. Thus, dimensions, including the length and cross section, will change with strain. Transverse strains may be developed in response to longitudinal loading. If the length of a resistor is increased, the cross section will decrease under finite Poisson's ratios, as is shown in Fig. 2. In general, semiconductors materials exhibit a change in resistivity with strain. For a semiconductor, this change in resistivity with strain can be very large. Resistivity is a direct measure of charge carrier density [15]. The resistivity of a semiconductor material is given by ρ ¼ 1=qNμ
ð2Þ
Where q is the electron charge, N is the number of charge carriers and μ the mobility. The effect of applied stress is to change the number and the mobility of charge carriers within a material, which result in a large change in resistivity. The electron charge and numbers of charge carriers can be controlled during the manufacturing process by changing the amount and type of the material. Regarding to a macroscopic description of the behavior of a piezoresistor under a normal strain. The change in resistance is linearly related to the applied strain, according to [13] ΔR=R ¼ GΔl=l
ð3Þ
The proportional constant G in the above equation is called the gauge factor of a piezoresistor. Rearrange the terms in this equation is possible to have an expression for G
ΔR=R ¼ ðΔR=RÞlongitudinal þ ðΔR=RÞtransverse ¼ Glongitudinal Slongitudinal þ Gtransverse Stransverse
ð4Þ
ð5Þ
In a strain sensor, this effect is commonly measured using Wheatstone bridge circuit. Fig. 3 shows the configuration in which only a single piezoresistor is implemented on the bridge circuit. A voltage is applied to the bridge and changes in resistance of the strain sensor cause changes in the output voltage of the bridge; in this case, the resistance of the variable resistor (Rs) is represented as Rs ¼ R þ ΔR
ð6Þ
Whereas the nominal resistance values of other three resistors are denoted as R. the output voltage is linearly proportional to the input voltage Fig. 1. Configuration and structure for the sensor.
Vout ¼ ð−ΔR=ð2R þ ΔRÞÞVin
ð7Þ
A. Heredia et al. / Journal of Non-Crystalline Solids 358 (2012) 2331–2335
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Fig. 2. Schematic of the device structure under pressure; and how its dimensions change under longitudinal stress [9].
3. Experimental The piezoresistive sensor consists of a membrane floating structure of two layers, as is shown in Fig. 4. The device was fabricated in (100) silicon wafers n-type and we used the MEMS micromachining process. The first layer is formed by a 150 nm thick SiN obtained by (LPCVD). The deposition parameters of SiN films, were as follows: deposition temperature 700 °C, 547 mTorr using SiH4 and NH3. An AMP 3300 PECVD system from Applied Materials was used for the deposition of the a-SiGeB:H amorphous films. The temperature of deposition was set to 270 °C. The PECVD system is a planar reactor with a plate separation of 5 cm and 65 cm of diameter. The RF power is supplied to the cathode (top electrode) through a matching network, and operates in the frequency range of 8 to 110 kHz. 100% SiH4 and 1% B2H6/H2 mixture were used as the gas sources. In all the cases the pressure was maintained at 0.6 Torr, the RF power density was 0.09 W/cm 2 and the RF frequency was set to 110 kHz. The deposition time was set to 35 min, and this resulted in film thicknesses in the 250–300 nm range. The flow ratio Xg = [B2H6]/[SiH4] + [B2H6], was varied from 0 to 0.12 to obtain different
Fig. 3. Wheatstone bridge circuit configuration for Vout sensing.
boron concentration in the films. Electrical characterization was conducted using a Keithley 2400 meter-source and the electrometer 6517A. The output voltage of the sensor under applied pressure is the unbalanced Vout of the Wheatstone bridge, for this type of measurement the device was put in a chamber at atmospheric pressure; an another pressure sensor was used as calibration device, and then the pressure was varied from 0 to 3000 Pa. 3.1. Fabrication of a-SiGe:B piezoresistive Sensor In a general form, the fabrication process of the device is described as follows: a) n-type (100) Si wafers were cleaned and thermally oxidized with a 1.5um of thickness of SiO2; b) a lithography step was performed to define the membrane; c) the next step was done to etch the oxide, the membrane was formed by anisotropic wet etching on silicon from the backside to open almost all the cavity to the front surface, this was done using KOH solution at 85 °C; d) then a silicon nitride film was deposited with the parameters of deposition mentioned in the previous paragraph; e) the next step was the deposition
Fig. 4. A SEM image for the fabricated piezoresistive sensor, the dimensions are 50 x 50 μm2.
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A. Heredia et al. / Journal of Non-Crystalline Solids 358 (2012) 2331–2335
Fig. 5. Current–voltage characteristics for the a-Si-B:H films with a varying of boron content, Xg. Fig. 6. Activation energy (Ea) and resistivity (ρ) of the B-doped a-Si:Ge films with different boron content.
of the best obtained conductivity of a-SiGeB film by PECVD on the front side of the wafer. On the a-SiGeB film was applied a lithography step to define the area of the sensor, the dimensions were 50 × 50 μm 2; f) the film was etched by RIE process using SF6; g) finally an aluminum layer of 0.3 μm was evaporated and patterned. A SEM image of the final device is shown in Fig. 4.
pressure, the resistance increases when the pressure is increased as is shown in Fig. 7. The ΔR = R-R0 is a resistance change, where R0 is zero stress resistance; Fig. 8 shows the variations of ΔR as a function of applied pressure.
4. Results
5. Discussion
In order to determine the material with the best electrical characteristic for the sensor; a patterned aluminum electrodes were placed onto the deposited films. The selected film was used to measure the resistance-pressure characteristics. Fig. 5, shows the measured I-V behavior for different Xg values. A fairly ohmic behavior is clearly seen for all the samples. Also, the resistance of the samples changes with the concentration of boron. It is important to point out that it was not necessary a thermal treatment for alloying the Al to the Bdoped a-Si:Ge in order to get an ohmic contact. Avoiding the alloying step is a potential advantage, because the system AI/P-doped with aSi:Ge when is submitted to thermal treatments at temperatures as low as 250 °C may induce micro-crystallization; and as a consequence, the electrical and optical properties of the film could be changed [14,15]. The carrier transport in amorphous materials (a-Si:H), is a thermally activated process and the conductivity can be expressed as
Intrinsic a-SiGe:H has lower resistivity than a-Si: H or nc-Si: H and moreover depending on the Ge content the resistivity could change in several orders of magnitude as is reported in [16,17]. It means that aSiGe: H is a more versatile material than a-Si: H or nc-Si: H and its properties (resistivity and activation energy) can be tailored in a wide range, according to a specific application. That versatility is also valid for boron doped films (a-SiGe: H,B). It is clear from Eq. (8) that high conductivity corresponds to low activation energy, high resistivity is undesirable for the reading electronics connected to the sensor; thus the circuitry sets an upper limit to the piezoresistive sensor. Regarding to the piezoelectric effect, the films were measured at room temperature, taking into account that there not exist variations in temperature that could be changing the resistance of the film. The value of gauge factor is small when the material device is compared with others based on μc-Si or a-Si: H [12,13], but the behavior of a
σ ¼ σ 0 expð−Ea =kTÞ
ð8Þ
where Ea is the activation energy, k is Boltzman constant and T is the temperature. The resistance and activation energy may be estimated by means of Eq. (8), and measuring the conductivity of the films as a function of temperature. Table 1 summarizes the obtained activation energies for the deposited films and the data are plotted in Fig. 6 including the resistivity. The piezoresistive effect was characterized using the Wheatstone configuration (Fig. 3) at bias voltage of 10 V, and by varying the
Table 1 Measured activation energy Ea (eV) and resistivity (ρ) of the B doped a-SiGe films as a function of boron content. Xg
Ea, (eV)
ρ (Ω-cm), (@T = 300 K)
0 0.06 0.091 0.12
0.75 0.48 0.43 0.46
6.45 × 104 2.46 × 103 8.64 × 103 1.14 × 104
Fig. 7. The resistance measured in the sensor as a function of pressure.
A. Heredia et al. / Journal of Non-Crystalline Solids 358 (2012) 2331–2335
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low deposition temperature to accomplish the compatibility with the silicon technology. The prototype a-SiGeB sensor could be used as an innovative cochlear implant with a simple sensor design. The electrical characterization has shown that a-SiGe: B films are very suitable to integrate this kind of sensor with its read out electronic circuits due to low resistivity. Also, the device presented a good sensitivity compared with other piezoresistive strain sensors. Acknowledgements We would like to acknowledge the support from the investigation department of UPAEP university and to the Master student Eugenio Urrutia, for their assistance as well as for their suggestions during the structural characterization. References Fig. 8. The resistance change (ΔR/R) in the sensor as a function of pressure.
p-type material is presented in the film; however further studies are necessaries in order to determine the mechanical properties of aSiGe: B films. The sensitivity S in a pressure sensor can be defined as the voltage-changing rate per unit pressure, therefore the sensor shows a decrement in its resistance under pressure, which results in a sensitivity of 0.03 mV Pa − 1. The sensitivity S is expressed as a function of diaphragm geometry and the material properties in a separation form. However, as a first time the sensitivity in this work is not too small compared with strain sensor based on crystalline silicon. Furthermore, the films are highly conductive and are suitable for the use in pressure sensor, with the advantage that the a-SiGeB films can be deposited in any substrate at low temperature of deposition. 6. Conclusion A thin film membrane sensor based on silicon germanium boron was fabricated and characterized for potential applications under pressure variations. The fabrication process has been developed at
[1] K.J. Rebello, Proc. IEEE 92 (1) (2004) 43–55. [2] J.P. Conde, J. Gaspar, V. Chu, Thin Solid Films 427 (2003) 181–186. [3] K.H. Cherenack, A.Z. Kattamis, B. Hekmatshoar, J.C. Sturm, S. Wagner, IEEE Electron Device Lett. 28 (11) (2007) 1004–1006. [4] M. Moreno, A. Kosarev, A. Torres, R. Ambrosio, J. Non-Cryst. Solids 354 (2008) 2598–2602. [5] S.B. Patil, V. Chu, J.P. Conde, Procedia Chem. 1 (2009) 1063–1066. [6] V. Rajaraman, L.S. Pakula, H. Yang, P.J. French, P.M. Sarro, Int. J. Adv. Eng. Sci. Appl. Math. 2 (1) (2011). [7] G. Kino, Acoustics Waves, Devices, Imaging and Analog Signal Processing, Prentice Hall, 1987. [8] A. Arnau, Piezoelectric transducers and applications, second ed. Springer-Verlag, Berlin, 2008. [9] Ch Liu, Foundations of MEMS, Chapter 6, Prentice Hall, USA, 2005. [10] C.H. Wu, S. Stefanescu, H.I. Kuo, C.A. Zorman, M. Mehregany, The 11th International Conference on Solid-State Sensors and Actuators, Munich, Transducers'01, 2001. [11] S. Kodato, S. Nishida, M. Konagai, K. Takahashi, J. Non-Cryst. Solids 59 & 60 (1983) 1207–1210. [12] S. Nishida, M. Konagai, K. Takahashi, Jpn. J. Appl. Phys. 25 (1986) 17–21. [13] P. Alpuim, V. Chu, J.P. Conde, IEEE Sens. J. 2 (4) (2002) 336–341. [14] N. Mukherjee, R.D. Roseman, J.P. Willgin, J. Biomed. Mater. Res. (Appl. Biomat.) 53 (2000) 181–187. [15] S.M. Sze, Physics of Semiconductor Devices, John Wiley and Sons, USA, 1981. [16] A. Heredia-J, A. Torres-J, J. De la Hidalga-W, A. Jaramillo-N, J. Sanchez-M, C. Zuñiga-I, A. Perez, M. Basurto-P, Mat. Res. Soc. Proc. 796 (2004) V2.4.1–V2.4.6. [17] A. Torres, M. Moreno, A. Kosarev, A. Heredia, J. Non-Cryst. Solids 354 (2008) 2556–2560.
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Thin film membrane based on a-SiGe: B and MEMS technology for application in cochlear implants Aurelio Heredia a, c,⁎, Roberto Ambrosio b, Mario Moreno d, Carlos Zuñiga d, Abimael Jiménez b, Karim Monfil b, Javier de la Hidalga d a
Electronics, Department, UPAEP, 21 Sur 1103, Puebla, Mexico Electrical and Computing Department-IIT, UACJ, Av. del Charro 450N, 32310, Cd. Juarez, Mexico BioEngineering and Chemical, Department, UPAEP, 21 Sur 1103, Puebla, Mexico d Electronics Department, INAOE, Luis Enrique Erro No. 1, Santa Maria Tonantzintla, Puebla, Mexico b c
a r t i c l e
i n f o
Article history: Received 9 September 2011 Received in revised form 23 November 2011 Available online 24 January 2012 Keywords: PECVD; Amorphous silicon germanium; Cochlear implant; Pressure sensor; MEMS
a b s t r a c t In this work is presented the fabrication of a thin film membrane as a bio-transducer for aural assistance detection, therefore it will operate at low pressure. The resonant membrane was deposited by PECVD technique at low temperature of deposition T = 270 °C, using SiH4, GeH4, and Boron gases. The membrane was suspended on a micromachined crystalline silicon frame obtained by wet chemical etching. The a-SiGe:B film presented a resistivity of 2.46 × 103 (Ω-cm), resistance of 20.8 kΩ. Using these experimental data we succeeded in designing a simple structure for sensing low pressure variations. The output voltage of the sensor was measured for a range of pressure from 0 to 3000 Pa and at bias voltage of 10 V. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Micro Electro Mechanical Systems (MEMS) technology being to mature and new applications are becoming more attractive for research and development, such as automotive, consumer products and medical applications [1]. MEMS fabrication techniques include bulk and surface micromachining. Using these techniques in conjunction with the deposition of high quality thin films materials many sensor technologies have been developed. The use of thin films obtained at low temperature of deposition (T b 300 C) open the possibility to use different type of substrates such as glass, metal, plastic and biocompatible materials. In addition, the use of low temperature allows the integration of MEMS with electronic circuits, either CMOS (Complementary MOSFET) or TFT (Thin Film Transistors) for flexible electronics [2,3]. Also, different works using thin film in MEMS have been reported, for example: IR bolometers sensors have been developed through surface micromachining, using a-SiGe:H as a sensing layer in a bridge configuration supported by a SiNx thin film, all the materials were deposited by PECVD at low temperature of deposition [4]. In Ref. [5], electrostatic microresonators based in thin film silicon MEMS have been reported, the devices were fabricated on glass substrates at temperatures ≤110 °C using amorphous to microcrystalline silicon. Sarro
⁎ Corresponding author. Tel.: + 52 222 2299400x7143; fax: + 52 222 2299425. E-mail address: [email protected] (A. Heredia). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.12.101
et al in [6] have reported the use of SiC thin films by PECVD for MEMS applications in harsh environment, combining with CMOS technology at low temperature of post-processing. As we mentioned in previous paragraph, another interesting area of MEMS is the developing of medical devices. The use of these devices in medicine has been researched in the field of exploration, diagnostic and surgery [1]. A cochlear implant is an artificial hearing device, which transmits sound directly to the auditory nerve through electrical stimulation of the cochlea, bypassing the ear canal, eardrum, and middle ear bones. Today, piezomaterials are the best choice for direct detection of sound at frequencies between 20 Hz and 20 kHz for auditory applications [7,8]. One of the key issues for achieving low-cost detectors is their monolithic integration and compatibility with CMOS technology. For this purpose, the most widely used detector approach is to implement micro-cochlear implant using micromachined structures on CMOS process. The piezoresistive properties of crystalline silicon have been well studied in the fabrication of strain gauges and other electromechanical transducers [9]. Some works have reported the piezoresistive pressure sensors with diaphragms and piezoresistors made of 6H-SiC, while, others studies have showed sensors with 3 C-SiC piezoresistors on Si or SiO2/Si diaphragms [10]. The piezoresistance effects have been observed in amorphous silicon films (aSi:H,F), this material reported a gauge factor (GF) equal to 33 for ptype films and − 20 in the n-type films [11]. The piezoresistive behavior of n-type and p-type doped μc-Si:H films deposited by radiofrequency (RF) plasma-enhanced CVD on Corning glass substrates at
2332
A. Heredia et al. / Journal of Non-Crystalline Solids 358 (2012) 2331–2335
200 °C has been reported in ref [12], with values of GF between 16 and 23 and − 25 to −40 for p-type and for n-type films respectively. In ref [13], the piezoresistive property of n-type and p-type of μc-Si:H deposited by radio-frequency plasma enhanced chemical vapor deposition (RF) and by Hot Wire on plastic substrates at 100 °C has been demonstrated values of GF for p-type films between 25 and 32 and for n-type films between − 40 and − 10. These examples show the use of silicon thin film technology at low temperature of deposition, which enables the possibility for large area applications in novel substrates. In this context, few works have investigated the potential of amorphous silicon, its alloys for pressure sensors and its applications in medical devices. This is the main reason for investigate this kind of applications taking into account the advantages of low temperature of deposition of the films and integration with silicon technology or another alternative substrates. On the other hand, a drawback for the use of amorphous silicon in the fabrication of devices is its high resistivity. In order to overcome this problem, is necessary to perform the doping of the films. Doping of a-Si:H films can be performed during the deposition process by adding the dopant precursor. Currently at UPAEP/INAOE, we are developing thin film materials based on amorphous silicon germanium alloys (a-SiGe: B), thus the resistivity of the material, can be easily controlled by the doping level. The films used in this work particularly present a piezoresistive effect that can be controlled through variations on doping; therefore, the resistance is affected under pressure conditions. Furthermore the resistance changes on films, the deposition of this material is fully compatible with the CMOS technology. This option opens the possibility to integrate the device with the read-out circuitry; this will lead to a cochlear implant with an improvement in the cost compared with others made of different materials and technology [14]. As mentioned before the device is intended for low sound detection; therefore it will operate at low pressure. In this work the resonant membrane is achieved by depositing the thin film on a silicon nitride diaphragm, which is suspended on a micromachined crystalline silicon frame as is shown in Fig. 1. Here, we explore the use of piezoresistive membrane sensors for acoustic vibration detection at micro-scale. The principle operation and the method of microfabrication for a simple design of MEMS piezoresistive sensor are described. Finally the fabrication of a-SiGe: B sensor directly on Si substrates and the characterization of the sensor under pressure conditions are also demonstrated.
An electrical resistor may change its resistance when it experiences a strain and deformation. This effect provides an easy and direct energy/signal transduction mechanism between the mechanical and the electrical domains. Today, it is used in the MEMS field for a wide variety of sensing applications, including accelerometers and pressure sensors [9]. The resistance value of an element with the length l and the crosssectional area A is given by
2. Principle of operation
G ¼ ΔR=εR
The piezoresistive effect describes the changes of resistivity of material when it is subjected to external stress or material deformation.
where ε is the applied strain. The resistance is measured along its longitudinal axis. The total resistance change is the contribution of changes under longitudinal and transversal stress components, which can be expressed as
R ¼ ρl=A
ð1Þ
The resistance value is determined by the resistivity (ρ) of the material and the dimensions. Consequently, exist different ways to change the resistance; one of this is applying strain. Thus, dimensions, including the length and cross section, will change with strain. Transverse strains may be developed in response to longitudinal loading. If the length of a resistor is increased, the cross section will decrease under finite Poisson's ratios, as is shown in Fig. 2. In general, semiconductors materials exhibit a change in resistivity with strain. For a semiconductor, this change in resistivity with strain can be very large. Resistivity is a direct measure of charge carrier density [15]. The resistivity of a semiconductor material is given by ρ ¼ 1=qNμ
ð2Þ
Where q is the electron charge, N is the number of charge carriers and μ the mobility. The effect of applied stress is to change the number and the mobility of charge carriers within a material, which result in a large change in resistivity. The electron charge and numbers of charge carriers can be controlled during the manufacturing process by changing the amount and type of the material. Regarding to a macroscopic description of the behavior of a piezoresistor under a normal strain. The change in resistance is linearly related to the applied strain, according to [13] ΔR=R ¼ GΔl=l
ð3Þ
The proportional constant G in the above equation is called the gauge factor of a piezoresistor. Rearrange the terms in this equation is possible to have an expression for G
ΔR=R ¼ ðΔR=RÞlongitudinal þ ðΔR=RÞtransverse ¼ Glongitudinal Slongitudinal þ Gtransverse Stransverse
ð4Þ
ð5Þ
In a strain sensor, this effect is commonly measured using Wheatstone bridge circuit. Fig. 3 shows the configuration in which only a single piezoresistor is implemented on the bridge circuit. A voltage is applied to the bridge and changes in resistance of the strain sensor cause changes in the output voltage of the bridge; in this case, the resistance of the variable resistor (Rs) is represented as Rs ¼ R þ ΔR
ð6Þ
Whereas the nominal resistance values of other three resistors are denoted as R. the output voltage is linearly proportional to the input voltage Fig. 1. Configuration and structure for the sensor.
Vout ¼ ð−ΔR=ð2R þ ΔRÞÞVin
ð7Þ
A. Heredia et al. / Journal of Non-Crystalline Solids 358 (2012) 2331–2335
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Fig. 2. Schematic of the device structure under pressure; and how its dimensions change under longitudinal stress [9].
3. Experimental The piezoresistive sensor consists of a membrane floating structure of two layers, as is shown in Fig. 4. The device was fabricated in (100) silicon wafers n-type and we used the MEMS micromachining process. The first layer is formed by a 150 nm thick SiN obtained by (LPCVD). The deposition parameters of SiN films, were as follows: deposition temperature 700 °C, 547 mTorr using SiH4 and NH3. An AMP 3300 PECVD system from Applied Materials was used for the deposition of the a-SiGeB:H amorphous films. The temperature of deposition was set to 270 °C. The PECVD system is a planar reactor with a plate separation of 5 cm and 65 cm of diameter. The RF power is supplied to the cathode (top electrode) through a matching network, and operates in the frequency range of 8 to 110 kHz. 100% SiH4 and 1% B2H6/H2 mixture were used as the gas sources. In all the cases the pressure was maintained at 0.6 Torr, the RF power density was 0.09 W/cm 2 and the RF frequency was set to 110 kHz. The deposition time was set to 35 min, and this resulted in film thicknesses in the 250–300 nm range. The flow ratio Xg = [B2H6]/[SiH4] + [B2H6], was varied from 0 to 0.12 to obtain different
Fig. 3. Wheatstone bridge circuit configuration for Vout sensing.
boron concentration in the films. Electrical characterization was conducted using a Keithley 2400 meter-source and the electrometer 6517A. The output voltage of the sensor under applied pressure is the unbalanced Vout of the Wheatstone bridge, for this type of measurement the device was put in a chamber at atmospheric pressure; an another pressure sensor was used as calibration device, and then the pressure was varied from 0 to 3000 Pa. 3.1. Fabrication of a-SiGe:B piezoresistive Sensor In a general form, the fabrication process of the device is described as follows: a) n-type (100) Si wafers were cleaned and thermally oxidized with a 1.5um of thickness of SiO2; b) a lithography step was performed to define the membrane; c) the next step was done to etch the oxide, the membrane was formed by anisotropic wet etching on silicon from the backside to open almost all the cavity to the front surface, this was done using KOH solution at 85 °C; d) then a silicon nitride film was deposited with the parameters of deposition mentioned in the previous paragraph; e) the next step was the deposition
Fig. 4. A SEM image for the fabricated piezoresistive sensor, the dimensions are 50 x 50 μm2.
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Fig. 5. Current–voltage characteristics for the a-Si-B:H films with a varying of boron content, Xg. Fig. 6. Activation energy (Ea) and resistivity (ρ) of the B-doped a-Si:Ge films with different boron content.
of the best obtained conductivity of a-SiGeB film by PECVD on the front side of the wafer. On the a-SiGeB film was applied a lithography step to define the area of the sensor, the dimensions were 50 × 50 μm 2; f) the film was etched by RIE process using SF6; g) finally an aluminum layer of 0.3 μm was evaporated and patterned. A SEM image of the final device is shown in Fig. 4.
pressure, the resistance increases when the pressure is increased as is shown in Fig. 7. The ΔR = R-R0 is a resistance change, where R0 is zero stress resistance; Fig. 8 shows the variations of ΔR as a function of applied pressure.
4. Results
5. Discussion
In order to determine the material with the best electrical characteristic for the sensor; a patterned aluminum electrodes were placed onto the deposited films. The selected film was used to measure the resistance-pressure characteristics. Fig. 5, shows the measured I-V behavior for different Xg values. A fairly ohmic behavior is clearly seen for all the samples. Also, the resistance of the samples changes with the concentration of boron. It is important to point out that it was not necessary a thermal treatment for alloying the Al to the Bdoped a-Si:Ge in order to get an ohmic contact. Avoiding the alloying step is a potential advantage, because the system AI/P-doped with aSi:Ge when is submitted to thermal treatments at temperatures as low as 250 °C may induce micro-crystallization; and as a consequence, the electrical and optical properties of the film could be changed [14,15]. The carrier transport in amorphous materials (a-Si:H), is a thermally activated process and the conductivity can be expressed as
Intrinsic a-SiGe:H has lower resistivity than a-Si: H or nc-Si: H and moreover depending on the Ge content the resistivity could change in several orders of magnitude as is reported in [16,17]. It means that aSiGe: H is a more versatile material than a-Si: H or nc-Si: H and its properties (resistivity and activation energy) can be tailored in a wide range, according to a specific application. That versatility is also valid for boron doped films (a-SiGe: H,B). It is clear from Eq. (8) that high conductivity corresponds to low activation energy, high resistivity is undesirable for the reading electronics connected to the sensor; thus the circuitry sets an upper limit to the piezoresistive sensor. Regarding to the piezoelectric effect, the films were measured at room temperature, taking into account that there not exist variations in temperature that could be changing the resistance of the film. The value of gauge factor is small when the material device is compared with others based on μc-Si or a-Si: H [12,13], but the behavior of a
σ ¼ σ 0 expð−Ea =kTÞ
ð8Þ
where Ea is the activation energy, k is Boltzman constant and T is the temperature. The resistance and activation energy may be estimated by means of Eq. (8), and measuring the conductivity of the films as a function of temperature. Table 1 summarizes the obtained activation energies for the deposited films and the data are plotted in Fig. 6 including the resistivity. The piezoresistive effect was characterized using the Wheatstone configuration (Fig. 3) at bias voltage of 10 V, and by varying the
Table 1 Measured activation energy Ea (eV) and resistivity (ρ) of the B doped a-SiGe films as a function of boron content. Xg
Ea, (eV)
ρ (Ω-cm), (@T = 300 K)
0 0.06 0.091 0.12
0.75 0.48 0.43 0.46
6.45 × 104 2.46 × 103 8.64 × 103 1.14 × 104
Fig. 7. The resistance measured in the sensor as a function of pressure.
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low deposition temperature to accomplish the compatibility with the silicon technology. The prototype a-SiGeB sensor could be used as an innovative cochlear implant with a simple sensor design. The electrical characterization has shown that a-SiGe: B films are very suitable to integrate this kind of sensor with its read out electronic circuits due to low resistivity. Also, the device presented a good sensitivity compared with other piezoresistive strain sensors. Acknowledgements We would like to acknowledge the support from the investigation department of UPAEP university and to the Master student Eugenio Urrutia, for their assistance as well as for their suggestions during the structural characterization. References Fig. 8. The resistance change (ΔR/R) in the sensor as a function of pressure.
p-type material is presented in the film; however further studies are necessaries in order to determine the mechanical properties of aSiGe: B films. The sensitivity S in a pressure sensor can be defined as the voltage-changing rate per unit pressure, therefore the sensor shows a decrement in its resistance under pressure, which results in a sensitivity of 0.03 mV Pa − 1. The sensitivity S is expressed as a function of diaphragm geometry and the material properties in a separation form. However, as a first time the sensitivity in this work is not too small compared with strain sensor based on crystalline silicon. Furthermore, the films are highly conductive and are suitable for the use in pressure sensor, with the advantage that the a-SiGeB films can be deposited in any substrate at low temperature of deposition. 6. Conclusion A thin film membrane sensor based on silicon germanium boron was fabricated and characterized for potential applications under pressure variations. The fabrication process has been developed at
[1] K.J. Rebello, Proc. IEEE 92 (1) (2004) 43–55. [2] J.P. Conde, J. Gaspar, V. Chu, Thin Solid Films 427 (2003) 181–186. [3] K.H. Cherenack, A.Z. Kattamis, B. Hekmatshoar, J.C. Sturm, S. Wagner, IEEE Electron Device Lett. 28 (11) (2007) 1004–1006. [4] M. Moreno, A. Kosarev, A. Torres, R. Ambrosio, J. Non-Cryst. Solids 354 (2008) 2598–2602. [5] S.B. Patil, V. Chu, J.P. Conde, Procedia Chem. 1 (2009) 1063–1066. [6] V. Rajaraman, L.S. Pakula, H. Yang, P.J. French, P.M. Sarro, Int. J. Adv. Eng. Sci. Appl. Math. 2 (1) (2011). [7] G. Kino, Acoustics Waves, Devices, Imaging and Analog Signal Processing, Prentice Hall, 1987. [8] A. Arnau, Piezoelectric transducers and applications, second ed. Springer-Verlag, Berlin, 2008. [9] Ch Liu, Foundations of MEMS, Chapter 6, Prentice Hall, USA, 2005. [10] C.H. Wu, S. Stefanescu, H.I. Kuo, C.A. Zorman, M. Mehregany, The 11th International Conference on Solid-State Sensors and Actuators, Munich, Transducers'01, 2001. [11] S. Kodato, S. Nishida, M. Konagai, K. Takahashi, J. Non-Cryst. Solids 59 & 60 (1983) 1207–1210. [12] S. Nishida, M. Konagai, K. Takahashi, Jpn. J. Appl. Phys. 25 (1986) 17–21. [13] P. Alpuim, V. Chu, J.P. Conde, IEEE Sens. J. 2 (4) (2002) 336–341. [14] N. Mukherjee, R.D. Roseman, J.P. Willgin, J. Biomed. Mater. Res. (Appl. Biomat.) 53 (2000) 181–187. [15] S.M. Sze, Physics of Semiconductor Devices, John Wiley and Sons, USA, 1981. [16] A. Heredia-J, A. Torres-J, J. De la Hidalga-W, A. Jaramillo-N, J. Sanchez-M, C. Zuñiga-I, A. Perez, M. Basurto-P, Mat. Res. Soc. Proc. 796 (2004) V2.4.1–V2.4.6. [17] A. Torres, M. Moreno, A. Kosarev, A. Heredia, J. Non-Cryst. Solids 354 (2008) 2556–2560.