MEMS Based Capacitive Sensors for Biomedical Applications

MEMS Based Capacitive Sensors for Biomedical Applications

Abstract–This paper presents the Modeling of Silicon based capacitive sensors for biomedical applications. Using the Micro Electro Mechanical Systems (MEMS) technology MEMS sensors are widely used in biomedical applications due to its advantages of miniaturization low power consumption easy to measurement and telemetry.

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 This work demonstrates the design of MEMS based capacitive pressure sensor. We will study the deflection of a fine membrane of silicon (100) of circular form to the perfectly embedded at its edges under uniform and constant pressure. The capacitive response of the sensor obtained is linear in the range of pressure of 0 – 40KPa (0 – 300mmHg) with better sensitivity.

This paper also presents a novel complementary metaloxide-semiconductor(CMOS) micromachined capacitive flow sensor for respiratory monitoring. Airflow induces a pressure change on the suspended sensing plate and causes a capacitance change with respect to the bottom electrode. The microstructure fabricated by post-CMOS metal etch occupies an area of 190 × 190 μm2 and possesses a sensing capacitance of 180 fF. Output waveform of consecutive breaths is successfully measured with an output noise of 14 μV for a measuring bandwidth of 0.5 Hz, which is equivalent to a minimum detectable capacitance change and airflow velocity of 0.13 aF and 0.2 mm/sec, respectively.

I also propose the application of MEMS capacitive pressure sensor for continuous glucose monitoring in diabetic patients. Two electrically charged parallel plates separated by a suitable dielectric medium, situated inside a micro-chamber isolated from the external environment by a felicitous semipermeable membrane constitute the fundamental portion of the device. Determination of glucose concentration is achieved through the measurement of the pressure changes experienced by the capacitive pressure sensor, which is a result of change in viscosity of the test solution as an outcome of competitive binding of glucose.

1. INTRODUCTION

1. Definition of ‘capacitance’

In electrical terms, Capacitance is the ratio of the change in an electric charge in a system to the corresponding change in its electric potential. There are two closely related notions of capacitance: self-capacitance and mutual capacitance. [1]

The capacitance is a function only of the geometry of the design (e.g. area of the plates and the distance between them) and the permittivity of the dielectric material between the plates of the capacitor. For many dielectric materials, the permittivity and thus the capacitance, is independent of the potential difference between the conductors and the total charge on them.

2. Mutual Capacitance

A common form is a parallel-plate capacitor, which consists of two conductive plates insulated from each other, usually sandwiching a dielectric material. In a parallel plate capacitor, capacitance is very nearly proportional to the surface area of the conductor plates and inversely proportional to the separation distance between the plates.

If A is the area of the plates and d is he distance between them, then the capacitance C is given by :

3. Capacitive sensing modes

There are five capacitive sensing modes:

In the first mode the distance d varies, which leads to nonlinear output. In the second mode, an intermediate plate moves relative to two fixed plates, providing a differential measurement. In the third mode, the overlapping area varies, providing more linear output. Fourth mode, the overlapping area varies in a differential way. In the last mode, the dielectric constant varies.

2. OPERATING PRINCIPLE

I have discussed operating principle of three capacitive MEMS sensors: 1) Blood pressure sensor 2) Flow sensor for respiratory monitoring and 3) Continuous Glucose Monitoring

1. Blood pressure and heart rate pressure sensor

For biomedical applications, sensors requirements are small size, very low power consumption, low temperature sensitivity, high dynamic range, and high pressure sensitivity [3]. Pressure sensor MEMS is one of the few devices that have been widely used in biomedical applications. Amongst different types of pressure sensors parallel plate capacitive pressure sensors is one of the most widely studied devices in terms of device physics, performance and reliability [4].

The capacitive pressure sensor uses a pair of parallel plates which forms a capacitor. The upper plate acts as movable plate which is fixed from four sides. When pressure is applied on the upper plate it deforms which changes the distance between two plates of capacitor.

This change in capacitance can be observed to sense the pressure [5]. The pressure range that can be measured by the diaphragm depends on its dimensions (surface area and thickness), geometry, edge conditions, and material. For example, in biomedical applications to measure the blood pressure and heart rate, pressure microsensors are required to operate in the range of 0-40 kPa (0-300 mmHg) [6].

The deflection w of a circular plate with fully clamped perimeter as a function of radius w(r) is given by:

Where R is the radius of the plat 0 ≤ r ≤ R and the deflection at the center of the plate Wo is given by:

In the above equation, D is the flexural rigidity of the plate which is given by:

Where E, h and v are Young’s modulus, diaphragm thickness, and Poisson’s ratio, respectively.

The two-sided edge clamping allows the diaphragm to deflect more in the center, whereas a gradual decrease in deformation can be noted near the clamped edges. This results in a non-uniform change in the distance between the diaphragm and the bottom electrode. Capacitance thus is also not uniform throughout the area, hence integrating capacitance over the 2D distance between the electrodes will give the exact total capacitance. This can be given as:

Where d_o is the distance between plates at zero pressure and D(x,y) is the distance after deflection.

The parameters to be determined for effective deformation analysis are displacements, strains and stresses. The in-plane components are assumed to be uniform through the plate thickness as the diaphragm is of homogeneous material. Hence the dependence on z becomes negligible and all the components become functions of x and y only. The in plane stress tensor can then be given by:

The diaphragm thickness is very small compared to the other dimensions, hence assumption of Kirchhoff’s hypothesis is considered in the analysis of deflection. Further the maximum displacement will be half the thickness of the diaphragm, therefore 2D plane stress analysis of thin plate is best suited. The stress components and the stress matrix can be simplified as:

2. Flow sensor for respiratory monitoring

Pressure induced by momentum change of air flow vertical to the chip is detected by a suspended microstructure which contains the metal-2 layer sandwiched between intermetal dielectric thin films. This symmetrical composition (thickness ∼ 2.64 μm) is intended to minimize the overall structural curl caused by individual thin-film strain gradient.

Capacitive detection of plate motion is achieved through the metal-2 and polysilicon electrodes, which are separated by a 0.64-μm air gap and 2-μm silicon dioxide. The suspended microstructure comprises a square plate of 100 × 100 μm2 and four meandering springs (beam width = 4 μm), occupying a total area of

190 × 190 μm2. Airflow produces a pressure on sensor surface due a momentum change (P = ρv²; ρ is mass density of air and v is the airflow velocity). The pressure is estimated to be 1.2 Pa for a normal breathing flow velocity of 1 m/sec.

3. Continuous glucose monitoring

A remotely applied AC electromagnetic field generates a time-dependent torque on the magnetized permalloy strips, causing the diaphragm to vibrate. A biocompatible glucose-sensitive polymer solution (PAA-ran-PAAPBA) fills the microchamber for affinity glucose binding and detection. As glucose molecules permeate through the CA membrane, binding with the polymer leads to increased viscosity and hence viscous damping on the diaphragm vibration, producing a measureable capacitance change across the electrodes.

3. FABRICATION

I have discussed fabrication processes of all three capacitive MEMS sensors: 1) Blood pressure sensor 2) Flow sensor for respiratory monitoring and 3) Continuous Glucose Monitoring

1. Blood pressure and heart rate pressure sensor

The rectangular structured diaphragm of the capacitive pressure sensor is designed with regular perforations for increased deflection and to reduce electrostatic pull-in. Further, clamping only at the short side edges of the diaphragm enables increased low pressure sensitivity compared to the all edge clamped sensor. When the pressure load is applied on the diaphragm, the distance between the diaphragm and the bottom electrode decreases, this causes the capacitance to increase. The capacitance due to fringe capacitance is made negligible by increasing the length of the diaphragm.

This capacitive MEMS sensor was fabricated based on a surface micromachining technology called PolyMEMS-INAOE®.

As they were designed with the same structure, the aluminum and poly-based sensors were both fabricated with the same mask set and similar steps. The main fabrication steps are the following:

• insulation film deposition over the substrate;
• first (lower) metallic electrode, deposition and patterning;

• deposition of a dielectric film over the lower electrode;
• deposition and patterning of the sacrificial film;
• upper electrode deposition and patterning;
• releasing of the upper electrode;
• cavity sealing by deposition of the polyimide film; and
• Patterning and testing of the electric contacts.

Aluminum based sensors are characterized by a low temperature processing, it allows for a full compatibility with flexible Polyimide substrates; these sensors have been designed with a lower aluminum electrode and an upper composite polyimide/aluminum electrode.

The fabrication process for the aluminum-based sensors is briefly described in the following figure:

The fabrication starts with a 2 inches, n-type moderated-resistivity silicon wafers used as a rigid substrate. The substrates were thermally oxidized for electrical isolation, Figure (a). A 0.5-_m thick aluminum film (Metal 1) was e-beam evaporated and patterned to define the lower electrode, Figure (b). A 0.2-_m thick, low-temperature SiO2 film is Atmospheric Pressure Chemical Vapor deposited, at 400°C, using silane (SiH4) and oxygen as reactive gases. This film defines the dielectric layer over the lower electrode, Figure (c). A sacrificial 0.8-_m-thick polyimide film is deposited; a polymer precursor (PI2610 from HD MicroSystem) is spin-coated at 7500 rpm for 30 s for obtaining the Polyimide layer; the soft bake is carried out at 150°C for 90 s and the final curing is done at 370°C in N2 ambience. The oxide sacrificial layer is etched out through the perforation on top side of the diaphragm, using it as the release holes. The dimension of the release hole being 10 μm ×10 μm is spaced 10 μm apart throughout the entire diaphragm, thus obtaining relatively less deflection at high pressure ranges.  Figure (d). Based on ozone plasma reactive ion etching (RIE), the polyimide film is patterned to define a mesa-like structure for clamping the upper electrode, Figure (e). Then, a second aluminum film (0.5-_m thick) is evaporated over the polyimide structures. The metallic film is patterned to define the upper electrode, the closed and sectioned walls (see windows) and the interconnecting lines, Figure (f). The sacrificial polymer is etched by ozone RIE, where the lateral windows allow for the channels to release the structure, Figure (g). Now the cavity contains a dynamic capacitor whose double dielectric is composed by SiO²/air. Finally, a 1.5-_m-thick polyimide film is spin-coated at 5000 rpm for 30 s; a soft bake is carried out at 150°C for 90 s and the final curing is done at 370°C in a N² ambience, Figure (h).

2. Flow sensor for respiratory monitoring

The sensors were fabricated in a two-polysilicon-four-metal 0.35-μm CMOS process. After completion of the CMOS process, a sacrificial wet etch was performed to remove the stacked metal/via layers for structural release [55]

3. Continuous glucose monitoring

The device consists of a magnetically driven, vibrating

Parylene diaphragm situated inside a microchamber that is further sealed by a cellulose acetate (CA) semi-permeable membrane. The diaphragm is deposited with a gold electrode and further electroplated with permalloy strips. A bottom gold electrode is separated from the vibrating diaphragm by a sealed air chamber.

4. APPLICATION

In addition to the applications listed above, there are several other applications of the sensors which are discussed below:

1. Blood pressure and heart rate pressure sensor

[66]

1. Intracranial Pressure:

Intracranial hypertension is an acute and chronic condition with a variety of causes that include traumatic brain injury, aneurysms, brain tumors, hydrocephalus, stroke, and meningitis.

2. Intraocular Pressure:

The monitoring of intraocular pressure (IOP) is crucial for the diagnosis and monitoring of glaucoma.

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3. Cardiovascular Pressure:

Chronic blood pressure monitoring is critically needed to monitor various conditions of the cardiovascular system (e.g., restenosis, hypertension, and heart failure) as well as to assess the efficacy of surgical interventions as is the case in monitoring repaired aneurysms.

4. Bladder Pressure:

Urinary incontinence is a common issue that is currently diagnosed using a series of tests collectively known as urodynamics.

5. Intra-Abdominal Pressure:

Abnormalities in pressure within the thoracic cavity have multifactorial etiologies (e.g., edema, pneumothorax, sleep apnea).

2. Flow sensor for respiratory monitoring

In artificial ventilation, continuous monitoring of gas flow and volume is essential to deliver precise amount of gas to patient with the aim to reduce risks of iatrogenic diseases, such as barotrauma and volutrauma.

Relevant examples are the use of flow sensors in spirometry for monitoring expiratory and inspiratory gases to estimate some indexes to diagnose asthma and chronic obstructive pulmonary diseases. [45]

3. Continuous glucose monitoring

Continuous Glucose Monitoring (CGM) is a method to track glucose levels throughout the day and night. CGM systems take glucose measurements at regular intervals, 24 hours a day, and translate the readings into dynamic data, generating glucose direction and rate of change reports.

Having this context helps CGM users proactively manage glucose highs and lows, plus gives added insight into impacts that meals, exercise and illness may have on an individual’s glucose levels. CGM can also contribute to better diabetes management by helping to minimize the guesswork that comes with making treatment decisions based solely on a number from a blood glucose meter reading.

 Studies have shown that some CGM systems may help reduce A1C levels and reduce the risk for hypoglycemia, whether users are on insulin injections or pump therapy. [33]

5. CONCLUSION

When a pressure is applied on the sensing electrode, the membrane deforms. This deformation, as expected is maximum at the center and minimal at the periphery. This brings the change in capacitance, which can be translated to the pressure applied.

For flow sensors, a convenient post-CMOS micromachining process is used to fabricate a novel capacitive flow sensor. Highly sensitive detection is demonstrated through monolithic sensor integration to suppress the parasitic effect. It is thus very promising to use the CMOS MEMS approach to implement the complete sensor system-on-chip for long-term respiratory monitoring.

Diabetes mellitus is a chronic condition which is characterized by elevated blood glucose levels [8]. The proposed system could be used to investigate hyperglycemic condition. Unbridled fluctuations of blood glucose levels may lead to micro vascular and macro vascular complications, multiple organ failure and diabetic coma [9]. The panacea for these adversities is incessant measurement of blood glucose level.

6. FUTUTRE WORK

Pressure Sensor:

• The work in progress is the complete characterization of the sensor arrays and their full integration with a telemetric system for monitoring a wide range of pressures.

Flow sensor:

• The upward plate motion can be better detected in the future by placing an additional metal-4 sensing electrode on top of the suspended plate. To prevent water drop or dirt from falling to sensor surface, the top metal layer can be designed to have small blocking holes to pass air flow only.

Glucose Sensor:

• The sensor measures the glucose concentration continuously, like CGM devices, but with the key features of lasting for 14 days and requiring no calibrations, suggesting that improvements in CGM are possible.
• Another research area that requires attention is fault detection. In fact, the availability of a quasi-continuous time BG profile could help in detecting transient and permanent faults of the CGM sensor-insulin pump system. [65]

7. REFERENCES

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[8] Design and analysis of capacitive MEMS viscometric sensor for CGM

by P.N. Prabhakaran and M.Renuga.

[9] A MEMS viscometric sensor for continuous glucose monitoring by

Yongjun Zhao, Siqi Li Et al.

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[66] Lawrence Yu, Brian J. Kim,and Ellis Meng, “Chronically Implanted Pressure Sensors: Challenges and State of the Field”, Published online 2014 Oct 31. doi:  [10.3390/s141120620]

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[33] https://www.dexcom.com/continuous-glucose-monitoring

[65]  Bequette B.W. Fault detection and safety in closed-loop artificial pancreas systems. J. Diabetes Sci. Technol. 2014;8:1204–1214. doi: 10.1177/1932296814543661.