Process development for gas sensors employing tungsten trioxide nanostructured layers

On the projected device the intention is to realize the interdigitated electrodes on the top of the active layer. To increase the sensitivity of the sensor the dimension of the gap between the electrodes should be as small as possible. This also reduces the dimension of the sensor and consequently reduces the cost that is proportional to the occupied area. The shape of the first electrode design is not efficient for entire wafers but this design has been performed to exactly fit in the anodized area of a quarter of a 2 inches wafer which the area of the first anodizing tool present in the URV laboratory. The electrodes design has been optimized to increase the probability to obtain a working sensor (with a measurable resistance) in the anodized area.
Further design developments should optimize the area of the whole wafer, but this can only be done when all the processes are controlled with a high precision. The fabrication parameters to evaluate, in order to get electrodes with the minimum gap distance, are:
• the resolution of the laser equipment;
• the dimension of the AZ1505 photoresist;
• the thickness of the metal deposited;
• the lift off process (acetone attack);
In a theoretical way it will be possible to design electrodes with a 1μm gap as the laser and the resist can reach this gap dimension. In fact, this gap distance can be obtained with the Laser equipment of the URV Clean Room but only on the photoresist, so without depositing any metal onto the resist and without the possibility to do the lift off. Hence, the critical parameters are the deposition thicknesses of the metal films and the lift off process. In order to find the minimum gap distance, as the design of the pattern is composed of four interdigitated electrodes, each of this electrodes has been designed with a different gap of 10 μm, 20 μm, 40μm and 80μm (the electrodes dimension is the half of gap dimension).
This design has been performed on resist covered samples with the laser equipment that gives the possibility to have a high design flexibility and avoid fabricating a mask for each design test. For the desired deposition thickness it has been found that the 10 μm, 20 μm and 40μm are not acceptable gap distances as the acetone attack cannot remove the resist and all the electrodes are short circuited.