Fish & Chips
Background
In recent time a significant focus has been set on fish behaviour and fish population estimations due to the expected endangerment of certain species of fish mainly as a result of excessive fishing activity. Reliable monitoring of individual fish behaviour and migration in their natural environment is crucial in order to make accurate population estimations and plans for preservation of the different species.
Data Storage Tags (DSTs) are small autonomous measuring systems that can deliver precise information about the surrounding environment of fish. When a fish is caught the DST is sutured to its back and the fish is released. For a period of time the DST measures parameters in the fish surroundings and stores the data in an internal memory. When fishermen re-catch the fish, an award is offered for the return of the DST, and researchers can then retrieve the measured data.
The usefulness of current DSTs is strongly limited by size, price and limited measuring capabilities. The DSTs comprise of a sensor part, interface electronics, a non-volatile memory and an energy source. The aim of our project is to increase measuring capabilities and to shrink the system size while keeping in mind that the design is only a complete success if tags can be produced at a price comparable or preferably lower than that of the DSTs available today.
By making an integrated multi-sensor and applying custom made electronics and packaging concept the total system size can be shrunk to a size mainly limited by the size of the needed energy source.
Photo of a tagged cod ready for release into the ocean. Photo by Stefan Neuenfeldt, DTU-Aqua.
We work on the design and fabrication of multi-sensor chips capable of measuring depth, temperature, salinity and light intensity. From well-established polynomial fits to empirical data of the conductivity, temperature and pressure, the salinity can be calculated. The light intensity day variation can be used to deduct the longitude and latitude. This information can be used to reconstruct fish migration and fish patterns of behaviour.
Facts
To achieve detailed information of fish behaviour it is necessary to measure many different parameters very accurately, with high sensitivity and fast response time. This is possible by micro technology. Further, integrating many sensors in one single chip allow for miniaturising so smaller fish can be tagged. Fast response time can be achieved by exposing the sensors directly to the surrounding media, which challenges the packaging solution.
The sensor chips we have developed consist of 4 integrated sensors; pressure sensor, light sensor, temperature sensor and salinity sensor. In order to measure salinity a direct exposure to the seawater is crucial. Further a faster response time in general is achieved by the direct contact to the surroundings and a more precise pressure measurement can be achieved when the signal does not have to be transferred through a bulky encapsulation.
Photo of a stack of fabricated sensors. The size is 3.9 x 7.3 x 1.2 mm3.
To achieve the direct exposure a potting tube concept has been chosen. Batteries and PCB with electronics and sensor chip is placed in a polystyrene tube which is subsequently filled with a polymer. This seal off all but the sensing part that will stick out in one end of the package.
The sensor chip is fabricated by conventional bulk micromachining. The miniaturized multi sensor consists of two separate chips. One containing pressure sensor and light sensor. The other chip contains the temperature sensor. Each chip further contains a pair of conductivity electrodes. The temperature sensor and interconnects are defined in titanium disilicide. Both chips are covered with silicon nitride as protective coating. The deposited LPCVD nitride is 120 nm thick and serves together with a 170 nm thick silicon dioxide as an optical filter for the light sensor.
The two chips are assembled by anodic bonding to opposing sides of a Pyrex wafer. By utilizing both sides the chip can either be miniaturised or allow for more sensor without increasing the total area of the surface. The bonding also serves as a seal for the reference cavity of the pressure sensor. The complete triple stack multi-sensor is 3.9 × 7.3 × 1.2 mm3.
The chip is flip chip mounted to a PBC substrate and wire bonds are connecting the sensors on the other side the chip. The wires are glob topped. The substrate is placed in a polystyrene tube and potted with an epoxy.
The project is carried out in collaboration with DTU Aqua - National Institute of aquatic resources and DELTA - Danish Electronics, Light and Acoustics.
Image of the chip after epoxy potting.