Design and fabrication of a CMOS-compatible MHP gas sensor

Most semiconductor gas sensors operate at temperature of 100 ∼ 500 °C to achieve good sensitivity and fast response character,^1–4 therefore, micro-hotplate is widely used in micro gas sensors due to its outstanding advantages of low power consumption, fast thermal response time and easy to be integrated as sensor array.^5–7 By integrating MHP gas sensor array with IC on one chip for signal processing and temperature compensation, intelligent gas sensors can be achieved, thus, CMOS-compatible MHP gas sensors are of great interest.^6–11 

Heaters, free-standing membrane, gas sensing films and electrodes are essential factors for MHP gas sensors.^6–8,11 Many CMOS-compatible MHP gas sensors use poly-Si resisters as heaters.^10–12 However, during operating at high temperature for a long time, the resistances drift of poly-Si cause long-term stability problems.^13,14

In the modern standard CMOS processes, tungsten, with excellent mechanical strength as well as high temperatures stability, is used as a plug material to form via between different metal layers and the silicon substrate. With nonconventional layout design, MHPs with the tungsten resistor as the heater and thermometer have been developed,^15,16 which is a better choice for CMOS-compatible micro gas sensors. For the free-standing membrane, the dielectric multilayer depends on the CMOS processes,^6,7,11,12 and the release of the membrane is carried out in the post-CMOS processes.^9–12,16,17 Gas sensing materials are usually deposited on the MHPs by RF sputtering^9,12 or drop-coating process^2,18,19 after the release of the membrane. Electrodes are another important factor in sensor design. Since aluminum is the only choice in standard CMOS technique, it has been used as the electrodes of the sensing film.^10,17 But contact problem between the Al electrodes and the sensing films as a result of the oxidation of Al will lead to the invalidation of the gas sensor after long time high temperature operation.¹⁰ 

In this paper, a novel CMOS-compatible MHP gas sensor is designed and fabricated with a standard industrial CMOS technology followed by post-CMOS processes. The design and fabrication of the heater, free-standing membrane, gas sensing film and its electrodes are presented in detail, and the performance characteristics of the device are reported.

The fabrication processes of the MHP gas sensor are shown in Fig. 1. The heater and membrane are fabricated during the CMOS processes (0.5 μm MPW project, China) as shown in Fig. 1(a)–1(c). The membrane of MHP is formed by the SiO₂/Si₃N₄ inter-layer dielectric (ILD) layers. The tungsten plug between the first and the second metal layer is designed as a zigzag resistor heater embedded in the center of the membrane, as shown in Fig. 1(b) and Fig. 2(a). The MHP membrane is a 5 μm thick SiO₂/Si₃N₄ multi-layer membrane with an active heating area of 100 × 100 μm (Fig. 2(a)).

The final step of the CMOS processes is pad etching, and the dielectric layers in the etching windows are also etched during this step. However, the dielectric layers in the etching windows are much thicker than the dielectric layer on the pads, therefore, a residual SiO₂ layer with thickness of about 600 nm remains in the etching windows, as shown in Fig. 1(c). This residual SiO₂ with a light blue color can be observed in the etching windows as shown in Fig. 2(a).

The post-processing steps begin from the releasing of the membrane. A 1.2 μm thick Si₃N₄ was deposited by PECVD (Oxford NGP80) on the back of the chips to protect the back from being etched during the subsequent process of wet etching of silicon. In order to obtain a suspended member, it was necessary to remove the residual SiO₂ in the etching windows over the silicon substrate. A dip of 150 seconds in BOE (Buffered Oxide Etch) was found to effectively remove the oxide layer. And then the MHP membrane was immediately released by bulk silicon etching without attacking the exposed aluminum using a 5 wt.% TMAH (Tetramethyl Ammonium Hydroxide) dual doped with 1.6 wt.% Si powder and 0.5 wt.% (NH₄)₂S₂O₈ at 85 °C,^20,21 as shown in Fig. 1(d) and Fig. 2(b). The SEM photo of the released MHP is shown in Fig. 3, which clearly shows that the MHP is suspended by four legs over the etching cavity.

The oxidation of aluminum electrodes during the annealing of the sensitive material and the long time high temperature operation of the sensor will lead to a poor electrical measurement.^10,17 Hence, a Pt/Ti layer of 200 nm was sputtered on top of the Al electrodes and patterned with lift-off technique to ensure a stable ohmic contact to the sensitive layer, as shown in Fig. 1(e).

The gas sensor chips were packaged and wire-bonded in a standard DIP-16 ceramic package. The deposition of SnO₂ gas sensing film was the last step of the gas senor fabrication process flow. SnO₂ slurry, which was a mixture of SnO₂ nano-powder, surfactant and deionized water, was drop coated on the active area of the MHP. The crystal structure of the SnO₂ nano-powder purchased in CISC 725 was characterized by X-ray diffraction (XRD, D/MAX 2400, Cu Kα radiation with wavelength 1.5418 Å). The result is shown in Fig. 4. All the diffraction peaks can be indexed as tetragonal rutile phase SnO₂ (JCPDS card No. 41-1445). In a sensing film coating procedure, a micropipettor with an internal diameter of 60 μm was installed at a certain angle (30° ∼ 60°) to the horizontal stage of an optical microscope, and the tip of the micropipettor was first moved above the MHP using a manupulator, and then a droplet of the slurry with about hundreds of micrometers in diameter was extruded from the tip and coated on the MHP. The droplet remained a hemisphere shape after it was coated on the MHP (Fig. 2(c)). Finally, in order to evaporate the solvents in the droplet as well as to obtain good electrical contact between the gas sensing electrodes and the sensing film, thermal annealing was carried out using a muffle furnace with low heating rate of 3 °C/min and then holding at 90 °C for an hour. Fig. 2(d) shows the photo of the MHP gas sensor after annealing.

The temperature properties of the tungsten heaters of as-fabricated chips were calibrated¹⁶ in a muffle furnace from ambient temperature to 350 °C with an accuracy of 1 °C. The curves of resistance temperature coefficient calibration are shown in Fig. 5. These curves are approximately linear and the TCR (temperature coefficient of resistance) of the tungsten heaters are about 1.9 ‰/°C, therefore, they also acts as the thermometers of the MHPs.

The thermal efficiency of the MHP was subsequently acquired by electrically heating with a digital source meter (Keithley 2400) and measuring the resistance values. Fig. 6 shows the power consumption versus temperature for the device. The measurement results show that the thermal impedance of the MHP with electrodes and sensitive materials is about 16 °C/mW and a temperature of 300 °C is achieved at the expense of about 19 mW (Fig. 6). Much higher temperatures are attainable by the tungsten heaters,^13,14 but at these temperatures, the aluminum tends to be poor stability due to electro-migration and high temperature oxidation.

The thermal time constant of the MHP was measured by applying a square wave at a frequency of 20 Hz. Fig. 7 shows the rise time from ambient temperature to 300 °C. The suspended membrane is thermally isolated from the rest of the chip. The thermal dissipation of the membranes is quite small, hence, they have very fast thermal response time of 8 ms (10% to 90% rise time) to 300 °C, which enables pulse mode operation, further reducing the power consumption.

The tests of the long-term reliability of the MHPs were carried out by applying a suitable DC heating power on the heater of the MHP to the sensor up to 300 °C in the constant heating mode. The resistances of the heaters were measured during a stability test of 336 hours. Fig. 8 shows the stability of the tungsten heaters of two MHPs in constant heating mode. A poly-silicon heater of a MHP in our previous work²² is also measured for comparison. Obviously, the change of resistance of the tungsten heaters (<0.3%) are far smaller than the poly-silicon heater (<5.3%). As variation of heater resistance leads to the variation of operation temperature, the MHP gas sensor with tungsten heater has much better stability than the one with Poly-Si heater.

This paper describes a novel CMOS-compatible MHP gas sensor, employing tungsten resistive heaters. The devices have been designed and fabricated with a standard CMOS technology followed by post-CMOS processes. The post processing steps have been demonstrated to be fully CMOS-compatible, including the release of the thermal isolated membrane, the deposition of platinum thin film on the top of the aluminum electrodes and the drop-coating of sensitive layer. At room temperature in atmosphere, the devices have low power consumption (19 mW) and fast thermal response time (8 ms) for heating up to 300 °C. The heater of the devices can operate reliably with negligible change in the resistance (<0.3%) at an operation temperature of 300 °C under constant heating mode for 336 h. These works provide the basis for further study on the single-chip integration of gas sensors with integrated circuits.

The authors would like to thank the financial supports from NSFC of China (Grants No. 61274076, 61131004, and 61001054).