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Fabrication of capacitive micromechanical ultrasonic transducers by low-temperature process
Sensors and Actuators A 99 (2002) 85–91
Fabrication of capacitive micromechanical ultrasonic transducers by low-temperature process D. Memmia, V. Fogliettia, E. Ciancia,*, G. Calianob, M. Pappalardob a
b
Istituto di Elettronica dello Stato Solido IESS-CNR, Via Cineto Romano 42, 00156 Rome, Italy Dip. Ingegneria Elettronica, Universita` di Roma Tre, Via della Vasca Navale 84, 00146 Rome, Italy
Abstract This work is focused on the development of an innovative process for the realization of capacitive micromachined ultrasonic transducers (cMUTs) using surface micromachining on silicon with the possibility of integrating front-end electronic. We describe the processing steps and the materials used to obtain an entirely low-temperature (<510 8C) fabrication process. The structural SiOx layer and the SiNx membrane layer are deposited, respectively, by thermal evaporation and by plasma-enhanced chemical vapor deposition (PECVD). These techniques are compatible with the introduction of polyimide as the sacrificial layer. The 100% etch selectivity of the polyimide with respect to SiOx and SiNx allows the fabrication of membranes with a wide variety of shapes and air-gap dimensions. We have optimized thermal annealing treatment to control stress of the PECVD silicon nitride film to obtain optimal mechanical properties of membranes. Transducers have been characterized by electrical impedance analysis, showing resonance frequencies ranging from 4 to 6 MHz. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Polyimide; Silicon nitride; Capacitive transducer; Surface micromachining; Low-temperature process; Thermal annealing
1. Introduction Ultrasounds are used for many applications like nondestructive evaluations, position detectors, flow metering, and are extensively used for medical imaging diagnostic. Recently, capacitive micromachined ultrasonic transducers (cMUTs) have been developed as an attractive alternative to piezoelectric ones. A capacitive ultrasonic transducer consists of many electrostatic cells connected in parallel that essentially work as parallel plate capacitors (like condenser microphones), with a fixed electrode and a free-standing membrane supporting the second electrode. The technological approach that we present in this paper is based on surface micromachining techniques that allow to fabricate the free-standing membranes in a structural layer, of silicon nitride, deposited over a silicon substrate. The great improvement with respect to previous designs [1] is the low temperature of the process. This permits the use of a wide range of materials, like polymers, and of processing steps. The use of plasma-enhanced chemical vapor deposition (PECVD) allows to deposit silicon nitride structural layers at low temperature (400 8C) with characteristics controlled by * Corresponding author. Tel.: þ39-6-415221; fax: þ39-6-41522220. E-mail address: [email protected] (E. Cianci).
process parameters; the membrane stress (from compressive to tensile) is controlled by a post-deposition anneal at 510 8C which is the highest temperature used to fabricate the device. The deposition of silicon monoxide for the cavities walls, called rails, is performed by evaporation and it is a low temperature process step. Two features of the process make it robust and reliable: pre-patterning and use of polyimide as sacrificial layer. For pre-patterning we mean the structuring of polyimide into islands that will define the cavities of final electrostatic cells, before the deposition of the membrane film. The perfect etching selectivity of polyimide against the structural layers, along with pre-patterning, guarantees a precise control of geometry and size of active transducer regions.
2. Process technology The fabrication process flow, depicted in Fig. 1, is as follows: (a) Spinning of polyimide as sacrificial layer; (b) Pre-patterning of polyimide layer; (c–d) Definition of rails by lift-off technique using evaporated silicon monoxide; (e) Deposition of silicon nitride layer for membrane fabrication;
0924-4247/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 9 0 3 - 7
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D. Memmi et al. / Sensors and Actuators A 99 (2002) 85–91
Fig. 1. Schematic representation of CMUT fabrication process steps.
Fig. 2. FTIR absorption spectra of silicon nitride films (a) before and (b) after annealing.
D. Memmi et al. / Sensors and Actuators A 99 (2002) 85–91
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Fig. 3. A membrane before (left) and after (right) annealing process.
(f) Opening of etchant holes; (g) Removal of sacrificial layer; (h) Sealing of etchant holes by deposition of silicon monoxide; (i) Metallization; (j) Deposition of silicon nitride layer for sealing and protection. The devices have been realized on 3 in. silicon wafers, p-type doped with low resistivity (0.1 O cm) because they act as the bottom electrode of the transducer. We have fabricated transducers, designed to work at about 5 MHz, with electrostatic cells whose characteristic dimensions are: 0.5 mm membrane thickness, 0.5 mm vacuum gap,
i.e. the distance of separation between membrane and bottom electrode and 40 mm membrane diameter. 2.1. Definition of transducer active regions The transducer active regions are the cavities upon which membranes are suspended and their control is obtained by pre-patterning of the sacrificial layer. Besides pre-patterning, etching selectivity of sacrificial layer against structural layer is important to avoid damaging of membranes and cavity walls. These considerations led to the choice of polyimide as sacrificial layer enabled by the low temperature (<510 8C) of the process steps [2]. We used a film of Olin Probimide1
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D. Memmi et al. / Sensors and Actuators A 99 (2002) 85–91
112A polyimide [2] spun coated onto the substrate (Fig. 1a) and cured at 410 8C for 30 min and we obtained 1 mm thick polyimide, thinned to 0.5 mm by reactive ion etching. By pre-patterning, circular islands are defined in polyimide layer, to be the final cavities (Fig. 1b); subsequently, the cavities walls, called rails, are defined by lift-off technique using silicon monoxide. SiO is evaporated at low temperature on the pre-patterned polyimide layer, still resist-masked by the first lithographic step (Fig. 1c), and excess material is removed by dissolving the resist (Fig. 1d). SiO is widely used in many applications as coating material due to the ease of evaporation at low temperature, [3] but its use as structural material for rails is new. It was evaporated from a powder source using a resistively heated molybdenum source, at a pressure of 107 mbar with an evaporation rate ˚ /s and with the substrate at room temperature. The of 5 A deposition rate is kept small to avoid formation of large grains (2 mm). The stress of vacuum-deposited SiO film is slightly tensile. The thickness of the SiO film should be the same with the polyimide layer, in order to obtain a planar surface close to the theoretical design.
(<500 8C) of films with a wide range of different mechanical properties [6]. The properties required for the nitride film as membranes are: moderate tensile stress (0.1–0.2 GPa), high Young’s modulus (>200 GPa) and high density (>3100 kg/m3). Inevitably, during deposition some N atoms are bonded to silicon linked with residual hydrogen atoms that gives the unwanted compressive stress.
2.2. Deposition of silicon nitride membranes
2.2.1. Power, pressure and gas flows of deposition A low-power and high-pressure deposition determines many bonds Si–H and Si–Si in addition to Si–N bonds. Much amorphous silicon remains trapped inside the film. This makes the film compact and compressive stressed. A high-power and low-pressure film deposition gives many Si–N–Si bonds, and N–H bonds that make the film fragile with low compressive stress. We can control levels of Si–H and N–H bonds inside of nitride film by changing gas flow composition. When silane flow is low (5 sccm, 1% of N2), we have many N–H bonds, while Si–H bonds are not detectable. In this case the film is fragile and has a high etch rate, but has moderate compressive stress. The opposite happens in case of an abundant silane flow (30 sccm, 6% of N2). A good choice is 4% [5].
The cMUT membranes deposed by PECVD technique are composed of silicon nitride. As opposed to LPCVD [4] film deposition, PECVD allows low-temperature deposition
2.2.2. Temperature of deposition It has been seen that varying the temperature of the substrate from 250 8C (which is the minimum temperature
Fig. 4. SEM micrograph of a silicon monoxide ‘‘pillar’’ evaporated into an etchant hole.
D. Memmi et al. / Sensors and Actuators A 99 (2002) 85–91
for deposition of good film) to 400 8C (the maximum temperature allowed by our reactor) yields a film with decreasing compressive stress. If film deposition at a yet higher temperature had been possible, a film with tensile stress could have been obtained directly. These variations in stress are due to differences in film density connected to hydrogen desorption. The hydrogen loss results in the formation of new chemical bonds which lead to contraction of the film (thus the tensile stress). Annealing following the deposition has revealed itself a necessary procedure for film stress control. It is the only way by which compressive stress is decreased so
89
as to obtain tensile stress. This modification of the film is non-reversible. 2.3. Mechanical tension of the silicon nitride films and its control by annealing process We turned the parameters favoring a low compressive stress are: RF power 10 W, temperature 400 8C, pressure 0.7 mbar. In order to obtain a membrane with a higher Young’s modulus the silicon content in the silicon nitride film has been increased, but this caused an increased compressive stress.
Fig. 5. A portion of the final transducer investigated by (a) SEM and (b) optical microscope.
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D. Memmi et al. / Sensors and Actuators A 99 (2002) 85–91
Fig. 6. (a) Transducer real and imaginary impedance in air; (b) transmission in air at 5.2 MHz.
We obtain nitride film with tensile stress by annealing process at 510 8C that brings to hydrogen desorption and a more dense film. The decrease in Si–H bonds (approx. 2100 cm1) is clearly seen in the absorption spectrum of Fig. 2. The images in Fig. 3 show two AFM three-dimensional reconstruction of membranes before and after annealing process (10 h at 510 8C). 2.4. Membrane release To make the membranes free-standing we have to remove the sacrificial layer. The release of membranes is performed by wet etching of polyimide (Fig. 1g) in a solution of sulphuric acid and hydrogen peroxide, thereby giving an etching selectivity against silicon nitride membranes and SiO rails of 100%. After sacrificial etching, we perform a thermal annealing at T ¼ 510 8C to reduce the
hydrogen inside nitride film and to achieve the tensile stress required. 2.5. Sealing of cavities and metallization Sealing of etchant holes is necessary for immersion uses. Therefore, as a first step in the sealing process, a SiO evaporation is performed (Fig. 1h) with a vertical deposition in order to fill the vias without perturbing the cavity geometry in proximity of the holes (Fig. 4), unlike CVD deposition, that is conformal and causes topological variations of the cavity. As a second step, we depose another nitride film to seal hermetically the cavity (Fig. 1j). Metallization is the last ˚ aluminum is sputtered on both process step (Fig. 1i): 1000 A sides of the wafer and an ohmic contact on the heavily doped substrate is achieved by thermal diffusion of aluminum at 450 8C for 120 min. Fig. 5 shows a portion of the final transducer.
D. Memmi et al. / Sensors and Actuators A 99 (2002) 85–91
3. Impedance characterization Fabricated cMUTs have a 3 mm2 area, consist of 1512 cells of 40 mm diameter, with 0.5 mm thick silicon nitride membranes, and 0.5 mm thick evacuated cavities, connected in parallel. Electrical impedance analysis (Fig. 6a) shows a peak resonance frequency at 5.2 MHz. We also performed a transmitter–receiver measurement in air between two transducers and Fig. 6b shows the signal detected by the receiver.
4. Conclusion This paper reports a low-temperature surface micromachining process which enables the fabrication of cMUT with high reliability. Low-temperature steps, as the deposition of the silicon nitride structural layer and of silicon monoxide as cavities walls, make possible the use of polyimide as sacrificial material providing perfect etching selectivity with respect to the silicon nitride membrane. The technological solutions described above give flexibility of design for different applications.
91
The fabricated CMUTs have shown resonance frequencies between 4 and 6 MHz, and worked as transmitter– receiver in air.
References [1] X. Jin, I. Ladabaum, F.L. Degertekin, S. Calmes, B.T. Khuri-Yakub, Fabrication and characterization of surface micromachined capacitive ultrasonic immersion transducers, IEEE J. Microelectromech. Syst. 8 (1999) 100–114. [2] Technical Specification Sheet of Probimide1 112 A Olin Microelectronic Materials. [3] H. Shibayama, S. Hasuo, T. Yamaoka, Formation of low defect density SiOx film for Josephson integrated circuits, Appl. Phys. Lett. 47 (1985) 429–430. [4] A. Stoffel, A. Kovacs, W. Kronast, B. Muller, LPCVD against PECVD for micromechanical applications, J. Micromech. Microeng. 6 (1996) 1–13. [5] W.A.P. Claasen, W.G.Y.N. Valkenburg, M.F.C. Willemsen, W.M. van de Wijgert, Influence of deposition temperature, gas pressure, gas phase composition, and RF frequency on composition and mechanical stress of plasma silicon nitride layers, J. Electrochem. Soc. 132 (1985) 893–898. [6] B.F. Hanyaloglu, E.S. Aydil, Low-temperature plasma deposition of silicon nitride from silane and nitrogen plasmas, J. Vac. Sci. Technol. A 16 (1998) 2794–2802.
Fabrication of capacitive micromechanical ultrasonic transducers by low-temperature process D. Memmia, V. Fogliettia, E. Ciancia,*, G. Calianob, M. Pappalardob a
b
Istituto di Elettronica dello Stato Solido IESS-CNR, Via Cineto Romano 42, 00156 Rome, Italy Dip. Ingegneria Elettronica, Universita` di Roma Tre, Via della Vasca Navale 84, 00146 Rome, Italy
Abstract This work is focused on the development of an innovative process for the realization of capacitive micromachined ultrasonic transducers (cMUTs) using surface micromachining on silicon with the possibility of integrating front-end electronic. We describe the processing steps and the materials used to obtain an entirely low-temperature (<510 8C) fabrication process. The structural SiOx layer and the SiNx membrane layer are deposited, respectively, by thermal evaporation and by plasma-enhanced chemical vapor deposition (PECVD). These techniques are compatible with the introduction of polyimide as the sacrificial layer. The 100% etch selectivity of the polyimide with respect to SiOx and SiNx allows the fabrication of membranes with a wide variety of shapes and air-gap dimensions. We have optimized thermal annealing treatment to control stress of the PECVD silicon nitride film to obtain optimal mechanical properties of membranes. Transducers have been characterized by electrical impedance analysis, showing resonance frequencies ranging from 4 to 6 MHz. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Polyimide; Silicon nitride; Capacitive transducer; Surface micromachining; Low-temperature process; Thermal annealing
1. Introduction Ultrasounds are used for many applications like nondestructive evaluations, position detectors, flow metering, and are extensively used for medical imaging diagnostic. Recently, capacitive micromachined ultrasonic transducers (cMUTs) have been developed as an attractive alternative to piezoelectric ones. A capacitive ultrasonic transducer consists of many electrostatic cells connected in parallel that essentially work as parallel plate capacitors (like condenser microphones), with a fixed electrode and a free-standing membrane supporting the second electrode. The technological approach that we present in this paper is based on surface micromachining techniques that allow to fabricate the free-standing membranes in a structural layer, of silicon nitride, deposited over a silicon substrate. The great improvement with respect to previous designs [1] is the low temperature of the process. This permits the use of a wide range of materials, like polymers, and of processing steps. The use of plasma-enhanced chemical vapor deposition (PECVD) allows to deposit silicon nitride structural layers at low temperature (400 8C) with characteristics controlled by * Corresponding author. Tel.: þ39-6-415221; fax: þ39-6-41522220. E-mail address: [email protected] (E. Cianci).
process parameters; the membrane stress (from compressive to tensile) is controlled by a post-deposition anneal at 510 8C which is the highest temperature used to fabricate the device. The deposition of silicon monoxide for the cavities walls, called rails, is performed by evaporation and it is a low temperature process step. Two features of the process make it robust and reliable: pre-patterning and use of polyimide as sacrificial layer. For pre-patterning we mean the structuring of polyimide into islands that will define the cavities of final electrostatic cells, before the deposition of the membrane film. The perfect etching selectivity of polyimide against the structural layers, along with pre-patterning, guarantees a precise control of geometry and size of active transducer regions.
2. Process technology The fabrication process flow, depicted in Fig. 1, is as follows: (a) Spinning of polyimide as sacrificial layer; (b) Pre-patterning of polyimide layer; (c–d) Definition of rails by lift-off technique using evaporated silicon monoxide; (e) Deposition of silicon nitride layer for membrane fabrication;
0924-4247/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 9 0 3 - 7
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D. Memmi et al. / Sensors and Actuators A 99 (2002) 85–91
Fig. 1. Schematic representation of CMUT fabrication process steps.
Fig. 2. FTIR absorption spectra of silicon nitride films (a) before and (b) after annealing.
D. Memmi et al. / Sensors and Actuators A 99 (2002) 85–91
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Fig. 3. A membrane before (left) and after (right) annealing process.
(f) Opening of etchant holes; (g) Removal of sacrificial layer; (h) Sealing of etchant holes by deposition of silicon monoxide; (i) Metallization; (j) Deposition of silicon nitride layer for sealing and protection. The devices have been realized on 3 in. silicon wafers, p-type doped with low resistivity (0.1 O cm) because they act as the bottom electrode of the transducer. We have fabricated transducers, designed to work at about 5 MHz, with electrostatic cells whose characteristic dimensions are: 0.5 mm membrane thickness, 0.5 mm vacuum gap,
i.e. the distance of separation between membrane and bottom electrode and 40 mm membrane diameter. 2.1. Definition of transducer active regions The transducer active regions are the cavities upon which membranes are suspended and their control is obtained by pre-patterning of the sacrificial layer. Besides pre-patterning, etching selectivity of sacrificial layer against structural layer is important to avoid damaging of membranes and cavity walls. These considerations led to the choice of polyimide as sacrificial layer enabled by the low temperature (<510 8C) of the process steps [2]. We used a film of Olin Probimide1
88
D. Memmi et al. / Sensors and Actuators A 99 (2002) 85–91
112A polyimide [2] spun coated onto the substrate (Fig. 1a) and cured at 410 8C for 30 min and we obtained 1 mm thick polyimide, thinned to 0.5 mm by reactive ion etching. By pre-patterning, circular islands are defined in polyimide layer, to be the final cavities (Fig. 1b); subsequently, the cavities walls, called rails, are defined by lift-off technique using silicon monoxide. SiO is evaporated at low temperature on the pre-patterned polyimide layer, still resist-masked by the first lithographic step (Fig. 1c), and excess material is removed by dissolving the resist (Fig. 1d). SiO is widely used in many applications as coating material due to the ease of evaporation at low temperature, [3] but its use as structural material for rails is new. It was evaporated from a powder source using a resistively heated molybdenum source, at a pressure of 107 mbar with an evaporation rate ˚ /s and with the substrate at room temperature. The of 5 A deposition rate is kept small to avoid formation of large grains (2 mm). The stress of vacuum-deposited SiO film is slightly tensile. The thickness of the SiO film should be the same with the polyimide layer, in order to obtain a planar surface close to the theoretical design.
(<500 8C) of films with a wide range of different mechanical properties [6]. The properties required for the nitride film as membranes are: moderate tensile stress (0.1–0.2 GPa), high Young’s modulus (>200 GPa) and high density (>3100 kg/m3). Inevitably, during deposition some N atoms are bonded to silicon linked with residual hydrogen atoms that gives the unwanted compressive stress.
2.2. Deposition of silicon nitride membranes
2.2.1. Power, pressure and gas flows of deposition A low-power and high-pressure deposition determines many bonds Si–H and Si–Si in addition to Si–N bonds. Much amorphous silicon remains trapped inside the film. This makes the film compact and compressive stressed. A high-power and low-pressure film deposition gives many Si–N–Si bonds, and N–H bonds that make the film fragile with low compressive stress. We can control levels of Si–H and N–H bonds inside of nitride film by changing gas flow composition. When silane flow is low (5 sccm, 1% of N2), we have many N–H bonds, while Si–H bonds are not detectable. In this case the film is fragile and has a high etch rate, but has moderate compressive stress. The opposite happens in case of an abundant silane flow (30 sccm, 6% of N2). A good choice is 4% [5].
The cMUT membranes deposed by PECVD technique are composed of silicon nitride. As opposed to LPCVD [4] film deposition, PECVD allows low-temperature deposition
2.2.2. Temperature of deposition It has been seen that varying the temperature of the substrate from 250 8C (which is the minimum temperature
Fig. 4. SEM micrograph of a silicon monoxide ‘‘pillar’’ evaporated into an etchant hole.
D. Memmi et al. / Sensors and Actuators A 99 (2002) 85–91
for deposition of good film) to 400 8C (the maximum temperature allowed by our reactor) yields a film with decreasing compressive stress. If film deposition at a yet higher temperature had been possible, a film with tensile stress could have been obtained directly. These variations in stress are due to differences in film density connected to hydrogen desorption. The hydrogen loss results in the formation of new chemical bonds which lead to contraction of the film (thus the tensile stress). Annealing following the deposition has revealed itself a necessary procedure for film stress control. It is the only way by which compressive stress is decreased so
89
as to obtain tensile stress. This modification of the film is non-reversible. 2.3. Mechanical tension of the silicon nitride films and its control by annealing process We turned the parameters favoring a low compressive stress are: RF power 10 W, temperature 400 8C, pressure 0.7 mbar. In order to obtain a membrane with a higher Young’s modulus the silicon content in the silicon nitride film has been increased, but this caused an increased compressive stress.
Fig. 5. A portion of the final transducer investigated by (a) SEM and (b) optical microscope.
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D. Memmi et al. / Sensors and Actuators A 99 (2002) 85–91
Fig. 6. (a) Transducer real and imaginary impedance in air; (b) transmission in air at 5.2 MHz.
We obtain nitride film with tensile stress by annealing process at 510 8C that brings to hydrogen desorption and a more dense film. The decrease in Si–H bonds (approx. 2100 cm1) is clearly seen in the absorption spectrum of Fig. 2. The images in Fig. 3 show two AFM three-dimensional reconstruction of membranes before and after annealing process (10 h at 510 8C). 2.4. Membrane release To make the membranes free-standing we have to remove the sacrificial layer. The release of membranes is performed by wet etching of polyimide (Fig. 1g) in a solution of sulphuric acid and hydrogen peroxide, thereby giving an etching selectivity against silicon nitride membranes and SiO rails of 100%. After sacrificial etching, we perform a thermal annealing at T ¼ 510 8C to reduce the
hydrogen inside nitride film and to achieve the tensile stress required. 2.5. Sealing of cavities and metallization Sealing of etchant holes is necessary for immersion uses. Therefore, as a first step in the sealing process, a SiO evaporation is performed (Fig. 1h) with a vertical deposition in order to fill the vias without perturbing the cavity geometry in proximity of the holes (Fig. 4), unlike CVD deposition, that is conformal and causes topological variations of the cavity. As a second step, we depose another nitride film to seal hermetically the cavity (Fig. 1j). Metallization is the last ˚ aluminum is sputtered on both process step (Fig. 1i): 1000 A sides of the wafer and an ohmic contact on the heavily doped substrate is achieved by thermal diffusion of aluminum at 450 8C for 120 min. Fig. 5 shows a portion of the final transducer.
D. Memmi et al. / Sensors and Actuators A 99 (2002) 85–91
3. Impedance characterization Fabricated cMUTs have a 3 mm2 area, consist of 1512 cells of 40 mm diameter, with 0.5 mm thick silicon nitride membranes, and 0.5 mm thick evacuated cavities, connected in parallel. Electrical impedance analysis (Fig. 6a) shows a peak resonance frequency at 5.2 MHz. We also performed a transmitter–receiver measurement in air between two transducers and Fig. 6b shows the signal detected by the receiver.
4. Conclusion This paper reports a low-temperature surface micromachining process which enables the fabrication of cMUT with high reliability. Low-temperature steps, as the deposition of the silicon nitride structural layer and of silicon monoxide as cavities walls, make possible the use of polyimide as sacrificial material providing perfect etching selectivity with respect to the silicon nitride membrane. The technological solutions described above give flexibility of design for different applications.
91
The fabricated CMUTs have shown resonance frequencies between 4 and 6 MHz, and worked as transmitter– receiver in air.
References [1] X. Jin, I. Ladabaum, F.L. Degertekin, S. Calmes, B.T. Khuri-Yakub, Fabrication and characterization of surface micromachined capacitive ultrasonic immersion transducers, IEEE J. Microelectromech. Syst. 8 (1999) 100–114. [2] Technical Specification Sheet of Probimide1 112 A Olin Microelectronic Materials. [3] H. Shibayama, S. Hasuo, T. Yamaoka, Formation of low defect density SiOx film for Josephson integrated circuits, Appl. Phys. Lett. 47 (1985) 429–430. [4] A. Stoffel, A. Kovacs, W. Kronast, B. Muller, LPCVD against PECVD for micromechanical applications, J. Micromech. Microeng. 6 (1996) 1–13. [5] W.A.P. Claasen, W.G.Y.N. Valkenburg, M.F.C. Willemsen, W.M. van de Wijgert, Influence of deposition temperature, gas pressure, gas phase composition, and RF frequency on composition and mechanical stress of plasma silicon nitride layers, J. Electrochem. Soc. 132 (1985) 893–898. [6] B.F. Hanyaloglu, E.S. Aydil, Low-temperature plasma deposition of silicon nitride from silane and nitrogen plasmas, J. Vac. Sci. Technol. A 16 (1998) 2794–2802.