Electrical properties of SiNx films with embedded

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Jul 14, 2017 - bridge-type capacitive switches, fabricated with a standard photolitho- graphic process. .... macroscopic power law Kohlrausch behavior.
Microelectronics Reliability 76–77 (2017) 614–618

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Electrical properties of SiNx films with embedded CNTs for MEMS capacitive switches M. Koutsoureli a,⁎, G. Stavrinidis b, D. Birmpiliotis a, G. Konstantinidis b, G. Papaioannou a a b

Physics Department, University of Athens, 15784 Panepistimioupolis, Athens, Greece IESL – FORTH, GR-71110 Heraklion, Greece

a r t i c l e

i n f o

Article history: Received 28 May 2017 Received in revised form 21 June 2017 Accepted 7 July 2017 Available online 14 July 2017 Keywords: MEMS capacitive switches Dielectric charging Reliability Silicon nitride Nanostructured dielectrics

a b s t r a c t The present paper aims to provide a better insight on the electrical properties of silicon nitride (SiNx) dielectric films with embedded carbon nanotubes (CNTs) that can be used in RF MEMS capacitive switches. The effect of the embedded CNTs on the leakage current density and on the discharging processes of the films has been probed with the aid of metal-insulator-metal (MIM) capacitors and it has been found that the presence of CNTs results in an increase of leakage current density as well as to an acceleration of the charge displacement through the bulk of the dielectric films. Finally, the charging and discharging processes have been investigated in MEMS capacitive switches and it has been found that the use of CNTs in SiNx films results in an enhancement of charging processes but it also accelerates the discharging process. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction MEMS capacitive switches possess many benefits over both conventional electronics switches as well as Ohmic-contact ones. These benefits arise from high figure of merit, ultra-low effective on-resistance, low power consumption, large power handling and ultra-high linearity [1,2]. The low loss and power consumption of these switches are excellent for constructing phase shifters and routing networks at X-band and above [3,4]. In spite of these advantages they have not yet reached commercialization and be “component off the shelf” due to reliability problems, the most important of which is still the charging of the dielectric film [1,2]. During the last two decades, a significant effort has been made to study the electrical properties of various dielectric materials in order to control and mitigate the dielectric charging. The majority of these dielectrics are discussed in [5]. Silicon nitride (SiNx) is presently the most commonly used dielectric in MEMS capacitive switches. The electrical properties of silicon rich material (SiNx, x b 1.33) are determined by the material stoichiometry since beyond the percolation threshold, at xc = 1.0, the Si\\Si bonds fail to form continuous percolation paths across the network [6]. For this reason, the Si-rich material has been intensively investigated [7–12] in the view of providing a potential solution to mitigate the dielectric charging. In order to increase the charge draining through the bottom electrode C. Bordas et al. [13] deposited a two layer SiNx, of which the top one included carbon nanotubes (CNTs) of various concentrations. The paper reported a huge ⁎ Corresponding author. E-mail address: [email protected] (M. Koutsoureli).

http://dx.doi.org/10.1016/j.microrel.2017.07.041 0026-2714/© 2017 Elsevier Ltd. All rights reserved.

increase of the dielectric “Figure of Merit” with increasing the CNTs concentration, which was translated into a several orders of magnitude improvement of the MEMS lifetime. The effect of carbon nanotube geometry on tunneling assisted electrical network in nanocomposites has been presented in [14,15]. Finally, the electrical properties of silicon nitride films with gold nanorod arrays and the performance of MEMS capacitive switches with such dielectric films has been in depth analyzed in [16]. In spite of the reported promising results of embedding CNTs into silicon nitride dielectric film of MEMS capacitive switches, there are still issues, such as the film geometry and CNTs density that require further investigation. The present work aims to provide a better understanding on the role of these parameters on the mitigation of dielectric charging. The study has been performed by using MIM (Metal-Insulator-Metal) capacitors and RF MEMS capacitive switches with SiNx dielectric films with embedded CNTs. Finally, a reference SiNx material (without CNTs) has been also fabricated in order to compare the results with the pristine material. 2. Experimental details The dielectric films of utilized devices (MIM capacitors and RF MEMS capacitive switches) have been fabricated with the following steps: First, a solution of CNTs in propanol was deposited on the bottom electrode via spin coating and on top of that, a 100 nm SiNx has been deposited with the plasma enhanced chemical vapor deposition (PECVD) method to embed the CNTs. After that, oxygen plasma was performed with reactive ion etching (RIE) in order to etch uncovered CNTs and a final layer of 100 nm SiNx was grown by PECVD. We mention that the diameter of the CNTs used in the present work is 1 nm, their length is

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2–3 μm and the frequency of the spinner used in the deposition process was 1000 rpm. Apart from these, a reference sample with SiNx has been fabricated by PECVD method in two steps (100 nm each step) without using CNTs solution. The utilized MIM capacitors (Fig. 1) have been fabricated with symmetric metal contacts (Pt/Au) of 1 mm diameter and the dielectric film was 200 nm SiNx with embedded CNTs. SEM images of the utilized films showed us that the dispersed CNTs are rather isolated and they do not seem to form agglomerates (Fig. 2). Considering this and assuming that all CNTs are dispersed on the same horizontal layer, we have made a rough estimation regarding the density of CNTs on the SiNx matrix. Assuming that each CNT covers an area of 2 μm diameter and that the diameter of a circular MIM capacitor is 1 mm, we conclude number that the CNTs density is c.a. 3 CNTs . We mention that the above esti10 μm2 mation is quite rough and further measurements are presently in progress in order to have a more accurate CNTs density value. The switches used in the present work (Fig. 3) are unpackaged bridge-type capacitive switches, fabricated with a standard photolithographic process. The dielectric film was grown on top of a Cr/Au bottom metal electrode. The membrane is an evaporated Cr/Au layer and in the unactuated position, it is suspended about 1 μm above the dielectric. The pull-in voltage of the switches is Vpi ≈ 35 V. The charging procedure in MEMS switches was performed under a bias of 40 V and the up-state capacitance-voltage (C-V) characteristics were monitored after each successive stress step with the aid of a Boonton 72B capacitance meter, with a resolution of 0.2 fF and at 1 MHz, while sweeping the voltage in 50 mV steps. The required bias was applied to the transmission line by a 487 Keithley voltage source– picoampere meter. The duration of each stress step was 2 min while the total stress time was 20 min. After stress, the devices were assessed by obtaining the up-state C-V characteristics for 5.5 h in order to monitor the shift of the bias for minimum capacitance towards the pre-stress level. The bias for up state capacitance minimum was determined by fitting a parabola to the experimental data, assuming a very small capacitance variance. All measurements on MEMS switches have been performed in a cryostat, under vacuum, with prior 2 h annealing at 160 °C, in order to avoid any interference from humidity. Current-voltage (I-V) characteristics have been obtained in MIM capacitors in a vacuum cryostat and at room temperature, for fields up to 2.5 MV/cm with the aid of a Keithley 6487 source-meter/electrometer. The DC bias was applied to the top electrode and the voltage ramp on I-V measurement was performed with a rate of 50 mV/s. Finally, the discharging process through the bulk material has been investigated in MIM capacitors with the aid of a single-point Kelvin Probe system [17], at room temperature and at ambient conditions. The Kelvin Probe is a non-contact, non-destructive vibrating capacitor device used to measure contact potential difference between a conducting specimen and a vibrating probe tip which is placed near the surface of interest. The surface potential of the utilized devices is thus directly measured during discharge, while the device is not in

Fig. 1. Schematic representation of utilized MIM capacitors with SiNx/CNTs films.

615

Fig. 2. Photo of samples with SiNx/CNTs film. White arrows show the embedded CNTs.

contact with the measuring system. A polarization field with intensity of 1 MV/cm has been applied for 5 min at room temperature. The ensuing discharging process has been assessed by measuring the decay of surface potential using a Single Point Kelvin Probe system (KP010) for a time period of about 104 s.

3. Results and discussion The presence of CNTs in the dielectric film introduces conduction paths through percolation [18]. The effect of the embedded CNTs on the electrical properties of the dielectric material will be determined by several parameters such as the CNTs density, length, orientation, shape (straight, helical), etc. [18,19]. In the case of MEMS capacitive switches with CNTs embedded in the upper [13] or lower part of the dielectric film, the impact of the CNTs is expected to be more complex. In such a case, during charging the generated electric field is expected to be inhomogeneous, which will give rise to non-negligible potential fluctuation at the interface of the pristine and CNTs embedded SiNx films and at the free surface of the MEMS dielectric film. This effect is expected to be enhanced by the effect of field emission that may occur at the tip of CNTs. On the other hand, a reverse process is expected to take place during discharge. Taking into account that the assessment of dielectric charging/discharging in MEMS capacitive switches requires the understanding of the charge injection and transport mechanisms, the magnitude of injected charge and the charge kinetics it becomes

Fig. 3. Photo of RF MEMS capacitive switches used in the present work.

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obvious that better understanding of these processes requires the assessment of both MEMS capacitive switches and MIM capacitors. 3.1. Assessment of MIM capacitors Fig. 4 shows the I-V characteristics of MIM capacitors with reference SiNx material and with SiNx material doped with CNTs. It is thus interesting to notice that the leakage current of SiNx/CNTs films increases by almost two orders of magnitude with respect to the reference material. In order to understand this behavior, it is important to bear in mind the following: The length of the incorporated CNTs is larger than the thickness of the first SiNx layer (100 nm), thus the CNTs are expected to be distributed with random orientation in the first SiNx layer giving rise to inhomogeneous electric field across this layer. Taking these into account and neglecting any interfacial phenomena that may be present between the two SiNx layers (due to the two step deposition process of the SiNx material), we expect that when we apply a potential difference across the metal electrodes of the MIM capacitor the applied electric field on the reference material will be homogeneous. On the contrary, the generated electric field across the SiNx film with CNTs is expected to be inhomogeneous and as already mentioned field emission charge injection is expected to take place. Fowler–Nordheim (F–N) theory describes the emission of electrons from a metal due to very high electric field and it takes place through sharp asperity paths, where the electric field is locally enhanced by several orders of magnitude. In terms of measured current (I) versus applied bias (V) the Fowler–Nordheim equation is expressed as [20]:   B I ¼ A∙V 2 ∙ exp − V

ð1Þ

where A is a parameter proportional to effective emitting area and the parameter B is inversely proportional to field enhancement factor (γ) [20]. The presence of field emission process was tested through the F–N plot for applied electric field intensities larger than ~1.5 MV/cm. Fig. 5 shows the characteristic signature plot of the F–N mechanism that describes field emission processes only in the composite dielectric film with embedded CNTs. In order to get more information on this effect in our material, assuming that the work function of the CNTs is Φ = 4.5 eV [21], we calculated a value of a field enhancement factor of γ = (2.2 ± 0.8) × 106. Field emission processes have been intensively investigated in CNTs [22–26] and it has been found that they exhibit low turn-on fields [25,26] and very large enhancement factor (γ), which can be as high as 106 [23]. The value of γ has been found to depend on many parameters. First, the dimensions of the CNTs and more

Fig. 5. FN signature plot for SiNx with embedded CNTs films, in agreement to Eq. 1.

specifically their aspect ratio are directly correlated to their emission properties [22,24,26]. In addition, the distribution of CNTs with respect to the applied electric field plays a significant role on their characteristics [24,26] and any CNTs agglomerate that may have been formed, any structural defects of CNTs that may be present [26] as well as the overlaying SiNx film, are expected to affect the macroscopic manifestation of the observed phenomena. We finally mention that although it is commonly supposed that field emission processes on CNTs take place at the nanotube's end surface it has been also reported that emission from the side wall of a nanotube may also arise [26,27], a phenomenon that may result to an increase of field amplification factor [26]. Regarding the effective emitting area, it can be calculated from the intercept of the F-N signature plot and its value is expected to be quite inaccurate due to large dispersion of our experimental data (Fig. 5). We thus obtained that the effective emitting area in our case is α = (4.3 ± 0.3)×10−20 cm2. The small value of this parameter may be attributed to a possible emission from both nanotube's end surface as well as from their sidewalls [26,27], due to random distribution and shape of CNTs in SiNx matrix. Finally, the absence of field emission above 40 V (2 ΜV/cm) and the saturation of the measured current (Fig. 5) has to be attributed to the current limiting action of the overlaying SiNx film and/or on the effect of adsorbate states on the CNTs [25,26]. Regarding the discharge process through the bulk material, it has been investigated with the aid of KP method in MIM capacitors as well. This experimental method provides information similar to point charging decay assessment, studied with KPFM [28,29]. Moreover, due to the presence of top electrode, any non-uniform charge distribution is screened and the top electrode potential arises from a uniform charge distribution that is practically the same with the average charge calculated from the bias shift of up-state C\\V characteristic minimum [30]. The decay of surface potential (Us) was been found to obey a stretched exponential law of the form: "   # t β U S ðt Þ ¼ U S;0 ∙ exp − τ

Fig. 4. I-V characteristics for reference SiNx material and SiNx with embedded CNTs.

ð2Þ

where US,0 is the surface potential immediately after charging (i.e. at t = 0 s), τ is the characteristic time of the discharging process and β is the stretched exponential factor with 0 b β b 1. As presented on Fig. 6, the incorporation of CNTs results to a decrease of the discharging time to almost one order of magnitude.

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617

the Vmin shift on REF material follows a power law relationship of the form: V min ðt Þ ¼ V 1 ∙t b

Fig. 6. Normalized values for the surface potential decay for the utilized samples.

3.2. Assessment of MEMS capacitive switches The charging process in MEMS capacitive switches is assessed with the shift of C-V minimum of up-state capacitance of MEMS switches [31]. The assessment in utilized MEMS capacitive switches revealed that the charging and discharging behavior showed significant differences between the samples with reference SiNx (REF) and SiNx with embedded CNTs (SiNx/CNTs) materials. The shift of bias at minimum up-state capacitance (Vmin) during charging is presented on Fig. 7 for both REF and SiNx/CNTs materials. The incorporation of CNTs on SiNx enhances the charging process due to field emission into the dielectric films, since larger shift of Vmin has been observed. Moreover, it has been found that Vmin shift on SiNx/CNTs material follows a stretched exponential build-up: "   #! t β τ

V min ðt Þ ¼ V ∞ ∙ 1− exp −

ð3Þ

where V∞ = (6.45 ± 0.14) V is the shift of Vmin at steady state polarization, τ ≈ (40 ± 2)s is the characteristic time of charging process and β ≈ (0.53±0.11) is the stretched exponential factor. Contrary to these,

Fig. 7. The shift of bias at minimum up-state capacitance (Vmin) during charging for both REF and SiNx/CNTs materials. Straight lines correspond to fitting of the experimental data with Eqs. (3) and (4).

ð4Þ

where V1 = (2.57 ± 0.04) and b = (0.130 ± 0.005) are the fitting parameters of the power law form. The above-mentioned differences may be attributed to different polarization mechanisms that appear on the two investigated materials. Taking into consideration the observations from MIM capacitors that showed the dominance of Fowler–Nordheim mechanism for electric fields larger than 1.5 MV/cm on SiNx/CNTs material, we might conclude that this mechanism may be responsible for these differences. Moreover, the presence of CNTs on SiNx films is expected to enhance the applied electric field on the film during charging and to increase the inhomogeneity of the applied field in the dielectric material. In order to obtain a better understanding on the physical interpretation of this relaxation process, we may adopt the model of a distribution and superposition of relaxation times τ in the dielectric material, which under the application of a step voltage leads to the sum of independently relaxing exponential polarization processes and therefore to t−b law [32]. The contribution from the double wall model under the presence of dipoles at the interface between the two SiNx layers cannot be excluded [33]. According to these, the absence of CNTs, which are expected to significantly contribute to charging and discharging thus affecting the interface charge density in the reference material, is expected to lead to a power law behavior [33]. Both mechanisms may lead to time constants τ much larger than the time window of the experiment and therefore to macroscopic power law Kohlrausch behavior. The subsequent discharging process on MEMS switches revealed small differences between REF and SiNx/CNTs materials. The shift of Vmin on both materials (Fig. 8) obeyed a stretched exponential law of the form: "   # t β V min ðt Þ ¼ V 0 ∙ exp − τ

ð5Þ

where V0 is the value of Vmin immediately after charging (i.e. at t = 0 s), τ is the characteristic time of the discharging process and β is the stretched exponential factor with 0 b β b 1. The parameter β is found to be (0.43 ± 0.04) and (0.30 ± 0.02) for REF and SiNx/CNTs materials respectively. The smaller value of β for SiNx/CNTs material indicate a more complex discharging process in this case. Moreover, the characteristic time τ for REF and SiNx/CNTs materials is found to be (1.6 ± 0.6)×104 s and (1.2 ± 0.3)×104 s respectively, which leads to a faster discharging process for the SiNx/CNTs material. The latter result agrees with the obtained results from MIM capacitors, although the differences observed from KP measurements between the two materials were larger. These differences in magnitude of discharging time constant obtained from MIM capacitors and MEMS switches may be attributed to different charging processes as mentioned before. This is because in MEMS switches, during pull-in state, charges are directly injected through the contacting asperities and through field emission in the non-contacting areas [34,35] leading to non-uniform charge distribution at the surface of the dielectric [36]. On the contrary, in MIM capacitors the metal electrodes form a rather “perfect contact” which leads to a much larger surface potential (we mention that at the beginning of the discharge process we have measured a surface potential of about U0 ≈ 35 V with the aid of KP system) and therefore larger electric field responsible for the discharging process. Finally, it is essential to mention that although no results regarding the effect of the embedded CNTs on the RF characteristics of MEMS switches are available, due to the fact that the paper focuses on the electrical properties of the dielectric material, it is expected that the RF performance would be negligibly affected, as long as the conduction mechanism is not close to percolation threshold. Below percolation

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the bulk material towards the bottom electrode has been found to be accelerated when CNTs are embedded into the lower half of SiNx film. In RF MEMS switches, the introduction of CNTs in SiNx films has been found to enhance charging due to Fowler-Nordheim injection. The discharge process of MEMS switches is also found to become faster when CNTs are present in the dielectric films. The simplicity of deposition of SiNx with embedded CNTs films is promising for the fabrication of reliable MEMS capacitive switches. Obviously, the physical processes involved in the charging and discharging mechanisms in bare SiNx and SiNx with embedded CNTs, their dependence on the dielectric film structure and the optimum density of embedded CNTs require further investigation, which is presently in progress. References

Fig. 8. (a) The shift of typical up-state C-V characteristic in a MEMS switch with reference SiNx film during discharge. (b) The shift of bias at minimum up-state capacitance (Vmin) during discharging for both REF and SiNx/CNTs materials. Straight lines correspond to fitting of the experimental data with Eq. (5).

threshold, we expect that the CNTs presence will only contribute to charge collection and increase of conductivity, as reported by Efros et al. in [37]. 4. Conclusions Nanostructured dielectric material for RF MEMS capacitive switches has been fabricated by embedding CNTs into PECVD SiNx. The proposed fabrication process is simple and it takes place in two steps, in order to incorporate CNTs on the lower SiNx layer and thus assist the charge drain towards the bottom metal contact (transmission line) instead of trapping them at the interface between the two SiNx layers. The electrical conduction processes of these films have been probed with the aid of MIM capacitors, by measuring I-V characteristics and by employing a single-point Kelvin Probe system in order to monitor the decay of surface potential during discharge. It has been clearly shown that in films with embedded CNTs the leakage current density is significantly larger and at high electric field intensities (larger than ~1.5 MV/cm) the field emission processes become the dominant charge transport mechanism. Moreover, the discharging process due to charge displacement through

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