MEMS sensitive structure sand and dust friction contamination test and electrical variation characteristics

The impact of dust pollution on the sensitive components of Micro-Electro-Mechanical Systems (MEMS) has been a hot topic in current reliability research. However, previous studies have rarely focused on the fundamental analysis of the influence of dust pollutants on the sensitive components of MEMS, and these research results lack universality. This paper aims to present experimental results on the electrical characteristic changes of MEMS thermal sensitive resistors based on dust friction, providing a more detailed description and explanation. Through dust friction experiments, we investigated the effects of dust particle size and relative humidity on the electrical characteristics of MEMS devices. We chose the resistance value output as the electrical characteristic parameter, based on the performance exhibited during the dust friction experiments. We conducted a detailed analysis of the reasons for the changes in the electrical characteristic parameters. The research results showed that when dust particles settle on the device, the failure mode of the device manifests as resistance drift. The inducing factor is the electrostatic current generated by the friction between dust particles, which enters the sensitive components through the principle of electrostatic dissipation. The measurement principle leads to changes in the resistance value measurement, although the actual resistance value does not change. The magnitude of this electrostatic current is directly proportional to the size of the particles and inversely proportional to the relative humidity. This finding provides theoretical guidance for identifying failure-inducing factors in MEMS failure modes.


1.Introduction
In the field of Microelectromechanical Systems (MEMS) flow sensors [1], gas sensors [2], and optical MEMS, the sensitive structures or components of the devices need to be directly exposed to the working environment, making them susceptible to contamination from dust and other pollutants present in the environment.This contamination can lead to performance degradation of the devices, affecting their operational lifespan and stability.Additionally, these pollutants may cause degradation of the electrical characteristic parameters of the products during operation, posing significant challenges to the reliability of MEMS devices.The existing dust test standards such as IEC 60529-2013, GB/T 4208-2017, GB/T 2423.37-2006, and GJB 150.12A2009 are not applicable for dust pollution tests on MEMS sensitive structures [3] [4].In order to investigate the specific impact of dust particles on the failure modes of MEMS devices and evaluate their reliability, we propose a dust particle friction experiment.This experiment aims to explore the influence of electrostatics [5] generated by the friction between dust particles on the electrical characteristics of MEMS sensors, providing guiding principles for studying the fundamental principles of MEMS failure.
In previous studies, Wei Yujin [6] conducted modeling and simulation experiments on the contamination of the sensitive area of MEMS flow meters using COMSOL software.The analysis focused on the influence of dust with different thicknesses on the sensitive area.Bai Xuejie [7] quantitatively analyzed the effects of particle size, collision velocity, particle type, and contact mode on the surface potential and morphology changes of silicon dioxide.
This article provides a detailed exploration of the influence of the size of dust particles and relative humidity on the electrical characteristics of MEMS devices in sand dust pollution experiments.In the second section, we delve into the principles of dust settling, electrostatic dissipation, and multimeter measurement.The third section comprehensively elucidates the mechanisms by which the size of sand dust particles and relative humidity affect the electrical characteristics of the devices through theoretical derivation and descriptive simulation.Finally, in the fourth section, we present the research results and conduct in-depth discussions, comparing and analyzing them with existing research findings.

Dust falling dust principle
When MEMS thermistor is placed in a contaminated test chamber, dust pollutants gradually accumulate on the sensitive structure.As time goes on, the deposition amount increases, leading to continuous changes in the electrical characteristics of the sensor.In order to evaluate the influence of different particle sizes and relative humidity on the device's electrical characteristics, this study utilized MATLAB software to establish a simulation model.By incorporating a particle deposition model, We are able to simulate the impact of frictional electrostatics caused by dust pollutants on the electrical characteristics of the sensitive structure, and combine the simulation data to perform reliability modeling and assessment.
The dimensionless deposition rate of pollutants is calculated using the particle deposition model proposed by Fan F G et al. [8], as shown in Equation ( 1): ,Other situations His model belongs to the category of empirical models, which have been developed by referencing experimental data, thereby ensuring a remarkable alignment with real-life sedimentation processes.The model encompasses a multitude of parameters, including dimensionless sedimentation velocity, Reynolds number, and Schmidt number.The dimensionless sedimentation velocity serves as a crucial indicator of turbulent friction velocity, frequently employed to evaluate the magnitude of turbulence.Reynolds number, a dimensionless quantity, serves as a discriminant for the flow state of viscous fluids.Conversely, Schmidt number, another widely utilized dimensionless parameter, plays a pivotal role in quantifying the extent of molecular diffusion, exerting a profound influence on the diffusion zone.

Principle of frictional electrostatic dissipation
Firstly, let us assume that under the same humidity conditions, the dissipation coefficient of like charges remains constant.The dissipation equation in two-dimensional scale [9] can be expressed as： In this context, U represents the indicative potential of the sample, while t signifies the dissipation time and D represents the dissipation coefficient.The simple numerical solution within the simulated range is obtained through the discretization function ( ) , n i j u , which corresponds to the continuous equation ( , , ) U x y t with the inclusion of 0 20 ,0 20 , , x um y um x i y t n t         .By employing finite difference approximation, the desired results can be achieved： ( ) ( ) In this context, ( 1)   , n i j u  and ( ) , n i j u respectively represent the surface potential at a point with a step size of 1 n  and n 。 x  and y  represent the positional changes of the charge in the lateral and longitudinal directions during the two-dimensional dissipation process.By using the surface potential at a step size of n and the predetermined dissipation coefficient of D , the surface potential at a step size of 1 n  can be calculated： The initial value of the surface potential of the sample is simulated based on the surface potential distribution data at 0 minutes of dissipative time.In the simulation process, the boundary conditions are set to maintain a constant surface potential value at each boundary point, consistent with the experimentally measured values.To ensure the stability and accuracy of the simulation, it is necessary to ensure that the maximum time interval between each time step in the simulation is smaller than a certain threshold：

Principle of measuring electrical characteristics
In this experiment, we have opted for resistance value output as the electrical characteristic parameter to more precisely evaluate the properties of the tested sample.To accomplish this objective, we require the use of a precise multimeter for resistance measurement.The principle of resistance measurement using a multimeter is based on Ohm's law, in which the internal voltage source of the multimeter is derived from an internal battery.The resistance value of the multimeter comprises several crucial components, including the tested resistance, adjustable resistance (for different range settings of internal resistance), fixed resistance, and zero-adjustment resistance.By measuring the current, we can accurately calculate the resistance value.Specifically, we can employ the following formula for calculation: I = U / (Rg + Rfixed + Rzero + Rtest), where U represents the voltage of the internal battery, Rg is the resistance of the meter head, Rfixed is the fixed resistance connected in series with the meter head, Rzero is the adjustable zero-adjustment resistance, and Rtest is the resistance being measured.To enhance comprehension of the measurement principle, a measurement schematic is provided in Figure 1.When the sand particles descend and come into contact with the thermistor, the sand particles possess initial kinetic energy, causing mutual friction and generating a minute electric current that enters the thermistor.When measuring the resistance using a multimeter, the measured current value in the circuit will be higher than the actual current value： In the provided equation, the variable A I represents the resultant current obtained from the measurement, while the variable F I signifies the static electricity current produced by the friction between individual sand particles.On the other hand, the variable Z I denotes the genuine current traversing through the resistor under examination.It is important to note that the magnitude of the final measured current surpasses the actual current flowing through the resistor.Following the principles of multimeter measurement elucidated above, this leads to lower resistance value being recorded compared to its true value.However, when the sand particles remain undisturbed for a certain duration, the static electricity current gradually diminishes, ultimately approaching zero.Consequently, the resistance measurement obtained during this state reflects the genuine resistance value.

Figure 1 .
Figure 1.Internal schematic diagram of the multimeterWhen the resistance Rx in the circuit is zero, the current reaches its maximum value.By adjusting the resistance R, the angle of deflection of the pointer on the measuring instrument can be made to reach the full-scale value, indicating a current value of I0=E/R in the circuit.As the resistance Rx being measured increases, the current I=E/(R+Rx) decreases gradually, leading to a decrease in the deflection angle of the pointer.Consequently, the resistance value scale on the multimeter dial is marked in the opposite direction.The presence of dust pollution can be observed in Figure2：

Figure 2 .
Figure 2. Schematic diagram of sand and dust pollution