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Graphene Membranes for Hall Sensors and Microphones Integrated with CMOS-Compatible Processes Sebastian Wittmann, Christoph Glacer, Stefan Wagner, Stephan Pindl, and Max C. Lemme ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00998 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 23, 2019

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ACS Applied Nano Materials

Graphene Membranes for Hall Sensors and Microphones Integrated with CMOS-Compatible Processes Sebastian Wittmann1,2*, Christoph Glacer3, Stefan Wagner4, Stephan Pindl1, Max C. Lemme2,4* 1Infineon Technologies AG, Wernerwerkstrasse 2, 93049 Regensburg, Germany 2RWTH Aachen University, Chair of Electronic Devices, Otto-Blumenthal-Straße 2, 52074 Aachen, Germany 3Infineon Technologies AG, Am Campeon 1-15, 85579 Neubiberg, Germany 4AMO GmbH, Otto-Blumenthal-Straße 25, 52074 Aachen, Germany *Authors to whom correspondence should be addressed: Max C. Lemme - [email protected] Abstract Graphene is a promising candidate for future electronic devices because of its outstanding electronic and mechanical properties. The high charge carrier mobility in graphene, particularly in substrate-free suspended form, suggests applications as Hall effect sensors. In addition, graphene membranes are highly desirable as pressure sensors or microphones. Here, suitable integration processes for freestanding graphene devices with standard CMOS processes are demonstrated. We propose a process flow for graphene membrane-based Hall sensors and microphones that is CMOS back end of the line compatible. The Hall sensors show mobilities up to 11,900 cm2·V-1·s-1, which are higher than in germanium and GaAs based Hall sensors. Graphene-based microphones are resonance free for frequencies up to 700 kHz, i.e. in the acoustic wave region, which is a unique advantage over conventional microelectromechanical (MEMS) microphones.

KEYWORDS: graphene; membrane; Hall sensor; microphone; CMOS integration

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Introduction Graphene is a two-dimensional material with excellent electronic, thermal and mechanical properties, which makes it a promising candidate for future applications in micro- and nanoelectronics, but also for nanoelectromechanical systems1–7. Recent work has focused on the application of graphene as a Hall sensor element because of its high charge carrier mobility8–14. It is well understood that the interaction of the graphene layer with the substrate limits the charge carrier mobility because of doping effects and surface roughness12,15–19. It is therefore advantageous to use freestanding graphene membranes, which eliminates the influence of the substrate. Graphene membranes have also been used as piezoresistive and capacitive pressure sensors3,20 and condenser microphones with graphene as a diaphragm21–23. For the latter, one can expect first resonances in the megahertz range due to graphene’s ultimate thinness24, with potential for several hundred GHz resonance frequencies for membranes with nanoscale diameters25. Thus the bandwidth of graphene-based microphones reaches from infrasound across auditory sound, ultrasound and to hypersonic sound for membrane diameters down to 5 nm. This property opens up a wide application space for graphene microphones. In this work we propose a generally CMOS compatible integration process for Hall sensors and microphones with graphene membranes as active elements. The device structures used in this work are manufactured with CMOS compatible processes, which are currently used in the industrial production of silicon based microphones26. This allows performance benchmarking of graphene-based Hall sensors and microphones in an industrially relevant context. The focus on freestanding graphene is driven by electronic properties, in particular charge carrier mobility, which is improved by the reduction of substrate interactions. As a consequence, high charge carrier mobility was achieved, which leads to high sensitivity Hall sensors. The electrical ACS Paragon Plus Environment

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performance of the Hall sensors was determined by Hall effect measurements in a vander-Pauw geometry. The performance of graphene-based microphones was analyzed using classical microphone measurement techniques like pull-in voltage, frequency response and a new method for sensitivity calculation using Raman spectroscopy. The predicted bandwidth to the hypersonic range is evaluated with graphene-based microphones.

Experimental Hall sensors and microphone test structures were fabricated using standard CMOS processes (Figure 1) to include electrical contacts, cavities and counter electrodes (Figure 2 a), b)). Different designs of the test structures were included to determine the degree of integrability and scalability. The diameters of the cavities were varied for both structures and the number and arrangement of the holes in the counter electrode was altered in the case of the microphones. Commercial graphene sheets were obtained (Graphenea) and transferred using a wet transfer technique27. Before transfer, the test chips were cleaned using oxygen plasma for 120 s at a power of 50 W and an oxygen flow of 20 sccm in a Plasma Electronics MyPlas system. After graphene transfer, the samples were dried in atmospheric conditions for 6 h and treated with acetone steam to remove the PMMA resist for approximately 12 h (graphene side of the test chip facing down to the acetone surface). The electrical characterization of the graphene-based Hall sensor comprised of voltage and current sensitivity and Hall measurements. These were performed in a vacuum chamber with four electrical manipulators with tungsten needles, connected to an Agilent 4156B semiconductor parameter analyzer. A custom-built copper coil below the substrate induces a magnetic field that can be varied by regulating the current flow through the coil. A maximum magnetic field of about 80 mT can be obtained at a ACS Paragon Plus Environment


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current of 15 A. A commercial Infineon TLE4997E2XALA1 Hall-effect sensor powered by a Kethley2000 power supply was used as reference. Charge carrier density and mobility measurements were performed at a current of IC = 1 mA and a voltage of VC = 1 V while the magnetic field was swept. Voltage sensitivity SV, current sensitivity SI , mobility μ, charge carrier density n and magnetic offset Boff were calculated according to equations 1 through 4 (with VH as the Hall voltage and Voff as the Hall voltage without magnetic field) SV =

1 ∂VH =μ VC ∂B

| |

(1)

1 ∂VH IC ∂B

| |

(2)

IC

(3)

SI =

n=

e·|∂VH ∂B|

Boff =

Voff

(4)

SV

Capacitance and frequency response measurements were performed to characterize the graphene-based microphones with precision HP LCR-Meters 4284A and 4192A. Capacitance measurements were carried out with an AC voltage of 10 mV at a frequency of 100 kHz while the bias voltage was swept up to 2 V with a resolution of 0.01 V. The frequency response was measured with an AC voltage of 100 mV while the frequency was swept from 5 Hz up to 700 kHz with a resolution of 100 Hz. The capacitance and the phase angle were extracted. Raman spectroscopy was carried out in a Horiba Raman system with a laser excitation wavelength of 532 nm (2.33 eV). Using a 100x objective with a long working distance focusing lens with 0.21 mm and a numerical aperture of 0.90, we obtained a spot size of ~1.05 µm in x and y direction. For detection, we use a single-mode optical fiber and ACS Paragon Plus Environment

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a spectrometer with a grating of 1,800 lines/mm. All measurements were performed with linear laser polarization, a 100x objective, an integration time of 1 s with an accumulation number of 2 (this is a specific setting of the tool) and a power density of 5.77 mW/µm2. The positions of the G and the 2D mode were extracted with a Lorentzian fit function. The G and the 2D mode depend on mechanical in-plane stress and carrier density and we used the peak positions to calculate the surface tension and the microphone sensitivity (see supporting information).

Results and Discussion The cross section of a microphone structure is shown in Figure 2 c. The perforated counter electrode of the microphone consists of n-doped poly-silicon for electrical contacts and is passivated with silicon nitride on top and silicon dioxide below. The stabilization ridges consist also of n-doped poly-silicon and are covered with silicon dioxide. On top of the structures, gold rings provide electrical contacts to the graphene membranes. No counter electrodes are required for the Hall sensors and hence the structure consists only of gold contacts on the top of the substrate. After graphene transfer onto the substrate, suspended graphene devices were identified with an Olympus Laser scanning Microscope OLS4000 with a diode laser with a wavelength of 405 nm. The height profile of the measurement provides information whether the graphene is suspended or in contact with the perforated back plate, i.e. defective. Height profiles of a graphene-based Hall sensor and a microphone are shown in Figure 3 a and b. In the case of the microphone, the graphene is suspended if the height signal over the cavities in the counter electrode has the same intensity as the contact ring on top. After identifying suspended graphene, the sheets were structured using a pulsed ytterbium fiber laser with a wavelength of 1064 nm (Han’s Laser Technology Industry ACS Paragon Plus Environment


Group). Graphene cutting was achieved at 40 kHz for 10 ns exposure at a power of 500 mW and a total pulse duration of 600 ns with undefined lower emission. The structured devices are shown in Figure 4. The graphene-based Hall sensors have been structured to isolate the graphene layer above the cavity from the rest of the layer on the test structure. A slight misalignment of the laser resulted in the graphene to be in contact with part of the substrate. Hence the extracted charge carrier mobility is a sum of the graphene suspended above the cavity and on the substrate. In the case of the microphone, the graphene on the counter electrodes was removed with the laser to ensure that the graphene membrane is not shortened with the back plate. The etching also minimized parasitic capacitances. In this case, residual graphene on the substrate in contact with the graphene membrane does not influence the electromechanical properties of the graphene microphones. Figure 4 also shows that the MEMS carrier structure was partly melted by the laser. This is due to the fact that the carrier itself is also a membrane and therefore has limited heat dissipation. The device performance was still high, although this may have a detrimental effect on the graphene.

Hall sensors The graphene Hall sensors were characterized for carrier mobility and carrier concentration (Figure 5), as well as current and voltage sensitivity. Figure 6 a) shows the Hall voltage dependence on applied magnetic field for two sensors with different cavity areas that are covered with graphene (78.5 µm2 for sample 1 and 314 µm2 for sample 2). Table 1 summarizes the device parameters extracted from the measurements. There was some misalignment of the laser during the structuring process that resulted in partial parasitic coverage of the substrate with graphene. As a result, the measured Hall voltage is a function of the mean value of the charge carrier densities in graphene over the cavity and on the substrate (VH = 〈n〉 ―1 28). In addition, ACS Paragon Plus Environment

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the fact that the contacts are not at the edges of the active graphene region influences the Hall voltage, which is known to be lower for contacts inside the active region. The measured Hall mobility is therefore lower than the intrinsic mobility29. In both sensors, the active area is identical and defined as the sum of the areas of the suspended graphene (Acavity) and the graphene on substrate (Asubstrate). As a result, the ratio of Acavity/ Asubstrate is greater for sample 2 (Acavity/ Asubstrate ≈ 6.17·10-3) than for sample 1 (Acavity/ Asubstrate ≈ 1.54·10-3). The Hall mobility in sample 2 was measured to be µ2 = 11,900 cm2/Vs, compared to µ1 = 6,240 cm2/Vs in sample 1. Even though the influence of the carrier substrate on the Hall measurement cannot be quantified, the data indicates that the charge carrier mobility increases with an increasing proportion of graphene suspended over the cavity, due to reduced scattering from the substrate surface. In addition, the extracted charge carrier concentrations is lower in sample 2 (n2 = -5.33·1011 cm-2) than in sample 1 (n1 = 1012 cm-2). This confirms our argument, as substrates are known to induce image charges in graphene, i.e. to increase the charge carrier concentration compared to suspended graphene. The current and voltage sensitivities of a Hall sensor describe its electronic response to a magnetic field, and the magnetic offset of the sensor. The samples show voltage and current sensitivities of 0.62 to 1.19 V/V·T and 624 to 1190 V/A·T, respectively. These values outperform commercial silicon based CMOS Hall sensors using Hall effect (0.07 V/V·T and V/A·T for 𝑆𝑉 and 𝑆𝐼)30, which is a result of the higher charge carrier mobility in the suspended graphene. Also the magnetic offset of the samples are comparable to CMOS Hall sensors, which have offsets in the range of 10 mT for van-der-Pauw structures31. The misalignment from the laser structuring and in-plane stress of the graphene membrane are factors for this offset, which can be also seen in CMOS Hall devices32–34 (Table 1).



Microphones The graphene membrane microphones were characterized regarding their pull-in voltage and frequency response. Pull-in measurements on sample 1 (Figure 6 b)) show that the graphene membrane is not in contact with the counter electrode up to a pullin voltage of 1.78 V. At this point, the graphene membrane is pulled far enough towards the electrode to make contact, hence the distance d becomes zero with increasing bias voltage 𝑉𝑏𝑖𝑎𝑠 (𝑑 ∝ 𝑐𝑜𝑛𝑠𝑡. ― 𝑉2𝑏𝑖𝑎𝑠) and the capacitance value increases drastically (𝐶 ∝ 1 (1 ― 𝑉2 )). This correlation results from the equalization of the restoring force 𝐹 ∝ 𝑑 𝑏𝑖𝑎𝑠 𝑅 of the graphene membrane and the electrical force 𝐹𝐸 ∝ 𝑈2. We observed a smaller capacitance step at a bias voltage of Vbias = 1.5 V, which may be attributed to additional slack in the graphene through sliding35. The design and the data suggest an operating voltage of less than 1.8 V which is ideally suited for use in mobile phones with their standard supply voltage of 2 V. Frequency response measurements on sample 2 (Figure 6 b)) show no resonances in the range from 5 Hz up to 700 kHz and therefore no natural frequencies in the audible range. Optical images before and after this measurement show similar height profiles which confirms that the graphene membrane is not in contact with the counter electrode. In contrast to classical MEMS capacitive microphones, which have a typical bandwidth of 2 Hz to 50 kHz36–65, graphene-based condenser microphones may thus also be operated in the lower ultrasonic range without the occurrence of resonances. In addition to these classical characterization methods for microphones, scanning Raman spectroscopy was carried out to estimate the mechanical stress in the graphene membranes. This allows conclusions about the sensitivity of the microphone. The Raman measurements were done on sample 2 and the results are shown in Figure 7. The elongation and the surface tension were estimated with the Raman model described in the supporting information. The data showed that the Raman signal was ACS Paragon Plus Environment

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influenced by the counter electrode, which is why the edges of the perforation holes also become visible as circles (Figure 7). In order to ensure that the influence of the counter electrode does not affect the estimations, only the measured data from the center of the graphene membrane (in-between the edges of the perforation holes in Figure 7) were used to estimate the sensitivity. The average microphone sensitivity from sample 1 was estimated with equations 1 to 13 from the supporting information with the Raman data from the center of the graphene membranes to be 1.051 mV/Pa. This is lower than commercial silicon based capacitive microphones that show sensitivities of 2.4-10 mV/Pa66. However, commercial silicon-based microphones are typically 700 to 1,100 µm in diameter, compared to 40 µm in the membranes investigated in this work67. This gives graphene membrane microphones a clear advantage when it comes to scalability.

Conclusions We demonstrated a fabrication process for Hall sensors and microphones with graphene as an active membrane element in a CMOS compatible device environment for industrial MEMS applications. While in principle compatible with industrial manufacturing68, a very low yield was observed that can be attributed to manual graphene transfer. The graphene membrane Hall sensors showed higher sensitivities than typical CMOS Hall sensors. Since the Hall sensitivity is limited by the charge carrier mobility, it can be increased with higher graphene quality. The magnetic offset of the sensors is severely limited by the alignment of the graphene laser etching process, yet still comparable to silicon-based Hall sensors. The graphene microphone showed pull-in voltages below 2 V, which generally enables the use in mobile phones. The microphones further showed no resonances in infrasound, auditory sound and in the lower ultrasonic range, indicating a wide range of potential applications. Even ACS Paragon Plus Environment


though the graphene-based sensitivities are lower by a factor of 2 to 10 compared to commercial MEMS microphones, they achieve this at a 17.5 to 27.5 times lower diaphragm diameter compared to classical silicon-based microphones, which gives them a large potential cost and size advantage.


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Associated Content Supporting Information Dependencies of charge carrier concentration and mechanical in-plane stress in graphene with Raman G and 2D mode, model for calculation of membrane sensitivity from Raman spectra

Acknowledgements This work was financially supported by the German Ministry of Education and Research (BMBF) under the project GIMMIK (contract no. 03XP0210F) and by the European Commission under the project Graphene Flagship (contract no. 785219).



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Figures

Figure 1: Schematic process sequence for the fabrication of the microphone and Hall structures.


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Figure 2: Optical images of a Hall sensor (a), a microphone (b) and cross-sectional scanning electron micrograph (SEM) of a microphone structure, cut by Focused Ion Beam (FIB) (c). The identifiers V1 07 or V3 09 indicate different designs for Hall sensors and microphones. c) The counter electrode consists of ntype poly-silicon with silicon nitride on top and silicon dioxide below. Gold electrodes are placed on top of the structure for electrical contact to the graphene membrane. The entire structure is covered with a platinum layer from the FIB-Cut. Inset in a) and b): schematic cross-section of the respective devices.



Figure 3: Extracted height profiles of the laser scanning microscopy measurement from graphene covered Hall sensor (a) and microphone (b) structures. The bright green color indicates the top of the structure and darker colors (dark green and blue) indicate subjacent height information.

Figure 4: Optical images of the Hall sensor (a) and microphone (b) after laser structuring. Due to misalignment of the laser, the graphene membrane is also in contact with the substrate. Therefore, the measured Hall voltage corresponds to the mean value of graphene over the cavity and on the substrate 28. In the microphone structure, the graphene layer over the contacts to the counter electrode was ablated with the laser to avoid a short circuit between the individual structures and the graphene membrane. Also, along the contacts to the graphene membrane, the passive and active areas were separated to reduce parasitic capacitance.


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Figure 5: Measurement configuration for charge carrier density (a) and charge carrier mobility (b) extraction from Hall measurement.



Figure 6: a) Hall effect and b) microphone measurements of suspended graphene devices. a) Sample 1 (left) shows a lower mobility and therefore lower sensitivity compared to sample 2 (right). The reason is a higher cavity area in sample 2 (314 µm2) than in sample 1 (78.5 µm2). A higher doping concentration in sample 1 is an indicator for a higher proportion of substrate doping. b) Pull-in and frequency response measurements of graphene-based microphones. Sample 1 (left) shows a pull-in voltage of Vbias = 1.78 V. At about 1.50 V, a small capacitance increase is observed, which we attribute to slip in of the graphene membrane. Sample 2 (right) shows no phase angle alteration ∆𝚯 of 90° is, which means that no natural resonances occur in the measured frequency range of 5 Hz up to 700 kHz.


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Figure 7: Extracted elongation and surface tension from Raman measurements. The elongation and surface tension values were calculated using the Raman model described in the methods section. The Raman signal was strongly influenced by the counter electrode, therefore the edges of the perforation holes became visible as circles.



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Tables Table 1: Extracted Hall sensor parameter from the Hall effect measurements for both samples.

Parameter

Sample 1

Sample 2

𝝁 [cm2/V·s]

6,240

11,900

𝒏 [1011 cm-2]

10

-5.33

𝑺𝑽 [V/V·T]

0.62

1.19

𝑺𝑰 [V/A·T]

624

1,190

𝑩𝒐𝒇𝒇 [mT]

16

48


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