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ABSTRACT:
Micro Electro Mechanical Systems (MEMS) in the field of biomedical industry has given rise to Bio MEMS. These encompasses Biosensors, Bio instruments and tools, Bio testing and analysis etc. Bio MEMS present a great challenge to engineers to design and manufacture of these types of sensors and instruments. Biosensor serves as the interface between living and electronic systems. These systems and the sensors should not affect the behavior of the living systems. Thin – film microelectronic technology offers special advantages for manufacturing of Biosensors over the traditional manufacturing methods. These can be used in minimizing the deleterious interactions through the use of small size and mass, Bio compatible material, and physical characteristics that are closely related to living tissues. Not only Bio MEMS the finger print sensor also discussed in this paper.
This paper also describes the concept and sensing principle of our fingerprint sensor, which we named as a MEMS finger sensor .Next, the analytical model of the MEMS structures described. Then, a fabrication process to stack the MEMS structures directly on the sensing circuits is presented. In the process, a sealing technique, STP (spin coating film transfer and hot-pressing) is applied. Finally, we discuss the fabrication results and evaluate the sensor’s ability to obtain fingerprint images regardless of whether the finger is dry or wet.
MEMS
Micro Electronic – Mechanical Systems (MEMS) is the integration of mechanical Elements, sensors, actuators and electronics on a common silicon substrate through Micro fabrication technology.
The electronics are fabricated using Integrated Circuit (IC) process sequences.
(Ex: CMOS, BIPOLAR, or BICMOS processes)
Introduction to MEMS in Biosensor:
Micro – Electro – Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators and electronics of micro fabrication technology. “MEMS technology is currently enjoying a moment of formidable expansion in synergy with the heath sciences, giving rise to the notion of MEMS for biomedical applications – Bio MEMS.” “The Bio MEMS industry is less conservative, and it’s a little bit dangerous because we know less what works and what doesn’t.”
Bio sensors can be manufactured using MEMS technology because it is possible to fabricate sensors and even micro sensors that have the potential for mass production. This can be achieved through the application of thin film technology.
BIOSENSORS
The term biosensor has been applied to devices either: (1) used to monitor living systems, (2) Incorporating biotic elements.
The consensus, however, is that the term should be reserved for use in the context of a sensor incorporating a biological element such as an enzyme, antibody, nucleic acid, microorganism or cell. For the purposes of this tutorial, a biosensor will be defined as:
Analytical devices incorporating biological material or a bromidic intimately associated with or integrated within a physicochemical transducer or transducing Microsystems, which may be optical, electrochemical, thermometric, piezoelectric or magnetic. The usual aim of a biosensor is to produce either discrete or continuous digital electronic. Signals which are proportional to a single analyses or a related group of analyses.
Biosensors are analytical devices which use biological interactions to provide either qualitative or quantitative results.
Biosensors be used:
Biosensors are finding use in increasingly broader ranger of application. The following list describes some of the current applications.
• Clinical diagnosis and biomedicine
• Industrial effluent control
• Pollution control and monitoring
• Mining, industrial and toxic gases
• Military applications.
• Farm, garden and veterinary analysis.
• Process control: fermentation control.
• Food and drink production and analysis.
• Microbiology: bacterial and viral analysis.
• Pharmaceutical and drug analysis.
Fabrication of Biosensors Using Thin Film Technology:
fabricating patterned thin films is to first deposit a of the film that is not desired by chemical etching through Thin – film consists of layers of metals – insulators or, in some cases, semiconductor materials that are usually deposited upon insulating surfaces such as alumina, glass, or polyimide. Thin – films are deposited using vacuum evaporation or sputtering. Thin film technique are relatively expensive because of lower capital equipment expenditures and unit production costs, it has limitation that the quality of the film deposited is lower. Patterned films are highly reproducible using the thin film process. For these reasons, most bio – sensors applications require thin film technology or at least a hybrid between thin and thick film manufacturing techniques. The standard method of a photo mask.
The standard steps found in the fabricating process are:
• Surface preparations clean and dry vapour surface.
• Dehydration bake – dehydrate the water to aid resist adhesion.
• Photo resist apply – coating of the wafer with resist either by spinning or spraying, typically desire a uniform coat.
• Alignment – align pattern on mask to features on wafers.
• Exposure – projection of mask image on resist causing selective chemical property change.
• Hard bake – additional evaporation of the remaining solvent from the resist.
• Inspection – inspect surface for alignment and defects.
• Etch – top layer of the wafer is removed thro the opening in resist layer.
• Photo resists removal – remove photo resist layer from wafer.
• Final inspection – surface inspection for etch regularities and other problems.
• Soft bake – partial evaporation of some of the solvent in the resist may result in a significant loss of mass of resist. Make resist viscous.
• Develop – selective removal of resist after exposure. Usually a wet process.
Applications of Film Technology to Biomedical Sensors
Thin film technology can be applied to the design and fabrication of chemical and physical sensors for biomedical applications.
• Bi potential electrodes.
• Electrolytic conductivity sensor.
• Amperometric Oxygen sensor
• Ion selective Membrane Sensor
• Abdominal Respiration Sensor
• Temperature Sensors
• Capacitive Force Sensors.
Bio sensors Benefits:
Specificity:
Like other bioanalytical methods biosensors use a biologically derived compound as the sensing element. The advantage of biological sensing elements is their remarkable ability to distinguish between the analyses of interest and similar substances. With biosensors, it is possible to measure specific analyses with great accuracy.
Speed:
One characteristic of biosensors that distinguishes them from other bioanalytical methods is that the analyses tracers or catalytic products can be directly and instantaneously measured. There is no need to wait for results from lengthy procedures carries out in centralized laboratories.
Simplicity:
The uniqueness of a biosensor is that the receptor and transducer are integrated into one single sensor. This combination enables the measurement of using regents. For example, the glucose concentration in a blood sample can be measured directly by a biosensor by
simply dipping the sensor in the sample. This is in contrast to the conventional assay in which many steps are used and each step may require a regent to treat the sample.
Continuous Monitoring Capability:
Another advantage that biosensors have over bioanalytical assays is that they can regenerate and reuse the immobilized biological recognition element. For enzyme – based biosensors, an immobilized enzyme can be used for repeated assays this feature allows these devices to be sued for continuous or multiple assays. By contrast, immunoassays, including enzyme linked immunosorbent assay (ELISA), are typical based on irreversible binding and are thus used only once and discarded.
Sensor Construction
A biosensor consists of two primary components: a bio receptor and a transducer.
Overview:
Biosensors consist of bio – recognition systems, typically enzymes or binding proteins, such as antibodies, immobilized onto the surface of physic – chemical transducers. The term immunosensor is often used to describe biosensors which use antibodies as their bio recognition system.
Biorecognition, systems can also include nucleic acids, bacteria and single cell organisms and even whole tissues of higher organisms. Specific interactions between the target analyses and the complementary biorecognition layer produces a physicochemical change which is detected and may be measured by the transducer. The transducer can take many forms depending upon the parameters being measured – electrochemical – optical, mass and thermal changes are the most common.
Bio receptors:
The bio receptor is a bio molecule that recognizes the target analyses. Bio receptors are typically enzymes or binding proteins, such as antibodies, immobilized on to the surface of a physicochemical transducer. Specific interactions between the targets analyze and recognition sites within the bio receptor produces a physicochemical change which is detected and may be measured by the transducer. The following chart summarizes possible bio receptors that can be utilized in a biosensor have to be capable of measuring this signal