Dual-channel microcantilever heaters for volatile organic compound detection and mixture analysis

We report on novel microcantilever heater sensors with separate AlGaN/GaN heterostructure based heater and sensor channels to perform advanced volatile organic compound (VOC) detection and mixture analysis. Operating without any surface functionalization or treatment, these microcantilevers utilize the strong surface polarization of AlGaN, as well as the unique heater and sensor channel geometries, to perform selective detection of analytes based on their latent heat of evaporation and molecular dipole moment over a wide concentration range with sub-ppm detection limit. The dual-channel microcantilevers have demonstrated much superior sensing behavior compared to the single-channel ones, with the capability to not only identify individual VOCs with much higher specificity, but also uniquely detect them in a generic multi-component mixture of VOCs. In addition, utilizing two different dual channel configurations and sensing modalities, we have been able to quantitatively determine individual analyte concentration in a VOC mixture. An algorithm for complete mixture analysis, with unique identification of components and accurate determination of their concentration, has been presented based on simultaneous operation of an array of these microcantilever heaters in multiple sensing modalities.


Dual-channel microcantilever heaters for volatile organic compound detection and mixture analysis
Ifat Jahangir,* and Goutam Koley

(I) Electrical characterization of the sensor channels
The current-voltage (I-V) characteristics of the sensor channels of the MDC-MH and SDC-MH are shown in Figure S1(a) where strong non-linearity is observed. The currents are observed to decrease after ~5 V, which also happened for the heater channel. Figure S1(b) shows how power and resistance changes for the sensor channel of either device as the voltage bias varies. Here we see the resistance to increase by almost 275% for SDC-MH and 380% for MDC-MH as the bias changes from 0 to 14 V, with maximum power dissipation below 3 mW.

(II) Sensitivity of MDC-MH and SDC-MH
We have defined two types of sensitivity parameters in this work, the first one is threshold voltage sensitivity (S v ) and the second one is threshold current sensitivity (S i ). Their general definitions are (from Equation (1) and Equation (4) While S v is an intrinsic property of the sensor, S i is a property that is both device-specific and analyte-specific. In Table S1, we have listed the S v for various devices used in this work. For SDC-MH, the definition of S v is not valid in secondary-heating mode due to the non-linear nature of the response. Also we have shown the S v for two different prototypes for each type of dual channel microcantilever heater to demonstrate the consistency in the observations. Table S2 shows the S i for several analytes from different devices in both self-heating and secondary heating mode.

(III) Comparison between the first and the second derivatives of ΔI/I 0
When multiple VOCs are sensed by the MDC-MH, TMH and SDC-MH (only self-heating mode), the response itself does not give any clear indication of more than one analytes being present in the environment. Therefore, we take the first and second derivatives with respect to heater channel bias to detect small abrupt changes on the responses. In Figure S2, subplots (a), (b), (e) and (f) are reproduced from Figure 6 of the main article, while (c) and (d) represent the first derivatives of (a) and (b). It is obvious that the first derivatives do not provide any distinct indication of multiple VOCs being detected; unlike the second derivatives, which show sharp peaks at the V th of each analyte.

(IV) Threshold Voltage and FWHM for different analytes and concentrations
We have observed different V th for the same analyte using different devices due to the differences in S v . We are showing the V th for both modes of the MDC-MH and the selfheating mode for the SDC-MH and the TMH in Table S3.
In Table S4

(V) Shift in V th in presence of multiple analytes
It has been observed that for self-and secondary heating modes, the V th does not shift noticeably regardless of the number of VOCs present in the system. This makes this technology an ideal solution for analyzing components of a mixture of unknown VOCs of various concentrations. In Table S5, we present the V th for 100 ppm of ethanol obtained from various devices, with or without other analytes being present and detected simultaneously.

(VI) Fabrication details for the microcantilever heaters:
We used deep anisotropic silicon etch technique to achieve suspended cantilever structure, where silicon was the sacrificial layer. All the fabrication steps were carried out in the Institute of Electronics and Nanotechnology (IEN) facility at Georgia Institute of Technology, Atlanta, GA. We started our process with 1.8 cm by 1.8 cm diced pieces of 6 inch AlGaN/GaN HEMT Epi wafer grown on Silicon (111) substrate, purchased from NTT Advanced Technology Corporation, Japan. The wafer had 2 nm iGaN and 15 nm Al 0.25 Ga 0.75 N on 1 µm iGaN, with 300 nm buffer layer separating the GaN layer from 750 µm Si substrate.
At first, using a SiO 2 hard mask deposited by plasma enhanced chemical vapor deposition (PECVD) technique, we used an inductively coupled plasma (ICP) etcher with Cl 2 /BCl 3 to isolate the cantilever mesa, leaving AlGaN layer intact only on the cantilever. Then ICP etching of GaN was performed to make an outline of the cantilever while AlGaN mesa was protected by 1 µm PECVD SiO 2 which was later etched away by HF wet etching. Then 20/100/45/55 nm Ti/Al/Ti/Au metal stack was deposited using e-beam metal evaporator on the cantilevers at the bases, followed by annealing at 800˚C in presence of N 2 for about 60 s to make good ohmic contact. Then the metal contact pads were formed using another stage of lithography and deposition of Ti/Au contacts.
After that the Si at the bottom of the pocket was etched from the backside of the sample. The back pocket outline was defined by a patterned SiO 2 mask layer aligned with the top pocket and then "Bosch process" was used to etch Si using ICP etcher. [2]- [4] Finally the square chips were attached to a dual-in-line (DIP) chip carrier package and all contact pads wire bonded