Study of a Si3N4 passivation layer in Si/SiO2-based piezoresistive pressure sensors

Miniaturized pressure sensors are of great interest in many industrial applications such as thermodynamics, biosensors, soil and fluid mechanics as they yield high sensitivity, self sensing abilities and a compact design. Silicon plays a significant part in the production of such sensors, offering dependable micromachining capabilities and delivering excellent sensitivity and accuracy.[1] Silicon-based piezoresistive pressure sensors are typically composed of doped silicon piezoresistances mounted in a Wheatstone bridge configuration which measures the pressure due to a change in the electrical resistances. As piezoresistive pressure sensors are electromechanical devices, enhancing their overall sensitivity can be achieved by optimizing the electrical properties of the piezoresistors in conjunction with the mechanical properties of the membrane. This work has two main purposes. The first one is to characterize the Si3N4/SIO2 interface with Si by means of MOS capacitor structures to quantify the potential passivation effects related to the Si3N4 dielectric layer. This dielectric stack is of particular interest in the context of sensing applications as the residual stress of both material are self-compensating. In this context three stacks are considered : SiO2/Si as a reference stack, Si3N4/Si and Si3N4/SiO2/Si, each of which have an aluminium backgate electrode whose post-deposition annealing temperature is investigated. C-V measurements with the mercury probe are performed in order to extract the electrical figures of merit such as the total dielectric stack capacitance 𝐶𝑖𝑛𝑠, the flatband voltage 𝑉𝑓𝑏, the fixed dielectric stack charge 𝑄𝑓 and the interface traps density 𝐷𝑖𝑡. The second purpose of this work is to propose a stack for piezoresistive pressure sensor designed with a Si3N4 passivation layer based on the design from a previous project conducted at UCLouvain with piezoresistors mounted in a Wheatstone bridge configuration. Then to fully characterize the fabricated sensors with mechanical measurements of the membrane deflection with pressure, electrical measurements of the I-V characteristics of the piezoresistors and electromechanical measurements of the full bridge with pressure. Sensors with a balanced bridge with low voltage offset drifts and good normalized sensitivity [ppm/Pa/mm^2] are targeted. The characteristics of the studied MOS stacks which are the total insulator capacitance 𝐶𝑖𝑛𝑠., the flatband voltage 𝑉𝑓𝑏 and the fixed charge in the insulator layer 𝑄𝑓 are extracted from C-V measurements. The increase in aluminium annealing temperature showed to reduce the fixed charge density but its limit to 432°C, to prevent silicon nitride delamination and aluminium melting, restrained the improvement of fixed charge density. With regard to the interface trap density 𝐷𝑖𝑡 estimated using both the conductance and high-low frequency method, their estimated values were questioned as no correction was applied to the measured conductance and capacitance. The deflection with pressure of five n-type sensors showed good agreement with theoretical curves from membrane deflection theory and allowed to estimate a resulting stress on small and large membranes ranging from 50MPa to 70MPa tensile. The balance of the bridge was assessed for three sensors through I-V measurements and the piezoresistances values ranging from 7.7kPa to 21.018kPa were extracted from the linear current range. The current and bridge output voltages variation with current were measured with a positive pressure difference applied on the backgate of the membrane. The pressure ranged from 0kPa to 20kPa with 5kPa increments for 0.5x0.5mm^2 membranes and from 0kPa to 2kPa for 2x2mm^2 ones with 0.5kPa increments. Both the current and output voltages variation with pressure displayed a linear relation to pressure for positive pressure differences. Finally the maximal sensitivity obtained for a n-type sensor was computed and it reached 2.096ppm/Pa/mm^2 for an orthogonal 0.5x0.5mm^2 membrane.