28 Enose

Dr. Ayushi Paliwal and Dr. Monika Tomar

1. Introduction

With advancement in technology, number of pollutant gases has been increased significantly. As we all are aware of gases from factories and vehicles contains very harmful gases such as CO, CO2, NO2, SO2, methane etc, which leads to various health problems to the mankind. In view of this continuous monitoring of different gases at various places (especially at resident areas) is essential. To monitor the multiple harmful gases on a single platform, E-nose devices can be fabricated using MEMS (Microelectronic mechanical structures) technique. As the name suggest E-nose is some kind of electronic nose sensing the concentration of various gases similar to the functioning of our nose which keeps on sensing the different gases and keeps on identifying the type of gas. Metal oxide based gas sensors are the most popular sensors when talked in terms of ease of miniaturization and mass production. Metal oxide gas sensors consist of a micro-heater, interdigitated electrodes and a metal oxide film. MEMS technique is a technique in which micron size mechanical structures can be fabricated using the photolithography and micromachining technique. As we have already learned about photolithography and micromachining in our previous modules. In this module we will learn how to make E-nose using photolithography and micromachining.

2. What is E-Nose?

E nose is array of sensors with different sensing layers deposited over it. On interaction with gas each sensor will give different response helping in identifying the type of gas. Different kinds of sensors can be used as gas sensor, such as Surface acoustic wave based, optical, conductometric etc. The two main components of an E-Nose are the “sensing systems” and the “pattern recognition system”. The sensing system has an array of different sensing elements (e.g. gas sensors), where each element is designed for a particular odour (gas) thereby responding by various magnitudes to individual odour (gas) or to a combination. Each odour (gas) vapour presented to the sensor array produces a signature or unique pattern characteristic (finger print or smell print) of the vapour. By presenting different chemicals to the sensor array, a database of signatures is built up, which is used to train the pattern recognition system. The training process is to configure the recognition system to produce unique classifications of each odour (gas) so that unknown odour (gas) could subsequently be classified and identified. Metal oxides (MO) are one of the most commonly used E-Nose sensing technologies. These sensors are already discussed in detail in the previous chapters. These sensors are generally operated at higher temperatures and therefore have high power consumption (around 800mW for Taguchi ceramic sensor). Therefore extensive research is continuing towards the fabrication of smaller, and more energy efficient systems, such as thin film MO sensors or micro-fabricated silicon planar MO sensors. In metal oxide gas sensors, microheaters are used as micro hotplate which provides constant and uniform temperature to the gas sensing film. Sensitivity, response and recovery time of the gas sensor depends on the performance of micro heater. Various parameters such as microheater geometry, size and material are considered to make a low power and thermally stable microheater. However, one needs to optimize the miniaturization along with sensitivity as small area for sensing may lead to saturation at higher concentrations. Different geometries namely s-type, meander shape, fan type, double spiral, etc. have been explored for microheater element considering temperature uniformity and power consumption. Various materials such platinum, poly-Si, nichrome, tungsten, gold, etc. have been explored for microheater application. However, platinum is a clear winner generally, due its stability at high temperature values and chemical stability. Physical verification of the temperature and temperature uniformity is carried out by infrared imaging. Power consumption of the microheater can also be optimized using the pulse mode of operation.

In the present module we are taking example of MO conductometric E-Nose. Schematic of e-nose is shown in figure 1. It can be seen in the figure 1 that there is array of sensor with different sensing layer, each sensing layer is represented by different colour. It may also be noted from the figure 1 that each sensors contains four contact pads, two corresponding to heater and two for electrodes. Heater is incorporated with each sensor in order to provide necessary operating temperature for sensor functioning.

3. MEMS (Microelectronic mechanical structures):

Microelectronic mechanical structures are the electronic circuits fabricated on very thin membranes. Being fabricated on membranes MEMS devices offers various advantages over conventional devices such as small size, high sensitivity and low power consumption. The schematic for the MEMS devices is shown in figure 2. Silicon bulk micromachining is used in order to fabricate MEMS devices. For the bulk micromachining of Silicon wet as well as dry etching technique can be followed each have their advantage and disadvantage as already discussed in the etching module. As can be seen from figure 2 cavity is fabricated on the back side of the device using bulk micromachining of Silicon and Inter digital electrodes have been patterned on the front side on Silicon dioxide membrane in order to obtain MEMS device.

4. Fabrication of E-Nose:

Following steps are used to fabricate E-nose and is shown in figure 3.

• Cleaning: First step towards the fabrication of micro-electronic devices is wafer cleaning. The silicon needs to be cleaned thoroughly following the standard process of cleaning including RCA1, RCA2 and piranha cleaning. The sequence of cleaning Si wafers was first introduced by Ms. Werner Kern, an employee of Radio Corporation of America (RCA), in 1965 and is therefore often referred to as the RCA process. This chemical sequence does not chemically react and attack the Si, in fact removes contamination that resides on the wafer surface. Cleaning of Si wafers is carried out using the following process:
• Pirana cleaning: Si wafers are first cleaned in pirana solution which is a mixture of Sulfuric Acid and Hydrogen Peroxide in 1:1 ratio. Organic and inorganic contaminations are removed from the wafer. Cleaning for 2 to 10 minutes is recommended. Subsequently, wafers are rinsed in De-ionised (DI) water and dried with jet of dry N2 gas.
• RCA cleaning: Ionic and metal contamination present on the surface of Si wafer is removed using RCA solution. RCA solution consists of a mixture of DI water: HCl: H2O5 in 5:3:3 ratio and the solution is heated at 70 oC. The cleaning process is carried out for 20 minutes. Finally wafers are rinsed in DI water and dried with jet of dry N2 gas.
• Oxide Removal: To remove the native oxide from the Si surface, wafers are dipped in a solution of HF:DI water (1:20 ratio) for 5 to 10 second. Finally wafers are rinsed in DI water and dried with jet of dry N2 gas. HF is extremely dangerous and must be handled with great care.
• Thermal Oxidation: A sacrificial layer of SiO2 is needed on the surface of Si wafer for the selective area etching after opening of window. The SiO2 can be grown on the surface using thermal oxidation. Therefore, oxidation of Si wafer is the most important process. Three zone thermal oxidation process is used to grow a thick film of SiO2 over the Si wafer. Thickness of the SiO2 thin film strongly depends on the oxidation time and temperature. The thermal oxidation is carried out at 1100 oC ± 10 oC in 100% O2 ambient. The wet oxidation of Si was performed for 10 hours by passing water vapours with O2 gas. Dry oxidation is carried out initially for 30 minutes followed by 10 hours wet oxidation, and again dry oxidation for 30 minutes. The presence of residual water molecules from the wafer surface was removed during dry oxidation. It may be noted that SiO2 film is growing on both sides of Si wafer.
• Electrodes and heater Patterning: Photolithography needs to be performed for electrodes patterning. The photolithography process includes spin coating of photo-resist, baking, alignment, and exposure followed by developing of the pattern. The exposed photo-resist will be removed from the selective area.Metal will be deposited for electrode over the patterned wafer.After deposition lift off of undesirable photoresist needs to be done in order to get electrodes pattern.
• Sensing Layer Deposition: To fabricate E-Nose different sensing layer needs to be deposited in order to detect multiple gases. Since we are taking the example of array of four sensors, four different sensing layers needs to be deposited. For each sensing layer windows of photoresist are required to be opened followed by sensing layer deposition. This process is followed four times for four different layers and each time a different sensing layer is deposited.
• Cavity fabrication: The wafers were carried out for the photolithography for forming the cavity and bottom electrode. The photolithography process includes spin coating of photo-resist, baking, alignment, and exposure followed by developing of the pattern. The exposed photo-resist will be removed from the selective area of the wafer followed by the etching of silicon dioxide using buffer HF or dry etching and followed by etching of silicon using wet or dry etching.

Figure 3: Fabrication steps for E-Nose

4. Dicing and Packaging:

Since there are large number of E-nose are fabricated on Silicon wafer as shown in figure 4. Separation was performed using dicing and they were get wire bonded to be used in electronic kit.

5. Working and Applications:

All four sensors can be simultaneous exposed to given concentrations of the different gases. The response of each sensor for a particular gas can be plotted on a single graph giving some pattern. Radar plots or polar plots [Johnson and Wichern (1982)] are convenient way to display multivariate dimensional data into the two dimensional data. The axes for each feature radiate from the origin at equal angles and the magnitudes of the features joined by straight lines. A typical radar plot of the response data obtained from an array of four sensors is shown in Fig. 5. Since each gas is behaving differently with these sensing layer, each gas will leads to some pattern. The pattern can be identified in order to identify the gas. Also, the amplitude or area under the curve for each pattern will helps in identifying the concentration of the particular gas.

6. Summary

What is E-nose?

• Technique of fabricating the MEMS based Enose
• Fabrication steps of Enose
• Dicing and packaging of complete packaged Enose
• Working and applications of Enose