Microelectronic Pillintroduction

Microelectronic PillIntroduction

1. INTRODUCTION

We are familiar witha wide range of sensors in the field ofelectronics. They are used widely in the various experiments and research activities too. This microelectronic pill is such a sensor with a number of channels and is called as a multichannel sensor. As the name implies this sensor is a pill. That is it is meant to go inside the body and to study the internal conditions.

Earlier it was when transistor was invented, that radiometry capsules were first put into use. These capsules made use of simple circuits forstudying the gastrointestinal tract. Some of the reasons that prevented their use was their size and their limitation ofnot to transmit through more than a single channel. They had poor reliability and sensitivity. The lifespan of the sensors were also too short. This paved the way for the implementation of single channel telemetry capsules and they were later developed to overcome the demerits of the large size of laboratory type sensors.

The semiconductor technologies also helped in the formation and thus finally the presently seenmicroelectronic pill was developed. These pills are now used for taking remote biomedical measurements in researches and diagnosis. The sensors make use of the micro technology to serve the purpose. The main intention of using the pill is to perform an internal study and recognize or detect the abnormalities and the diseases in the gastrointestinal tract. In this GI (Gastro Intestinal) tract we cannot use the old endoscope as the access is restricted.A number of parameters can be possibly measured by these pills and they include conductivity, pH temperature and the amount of dissolved oxygen in the gastrointestinal tract.

Figure 1: Microelectronic Pill

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Microelectronic PillBlock Diagram

1. BLOCK DIAGRAM

The design of the microelectronic pillis in the form of a capsule. The encasing it has is biocompatible. Inside this are multi- channel (four channels) sensors and a control chip. It also comprises ofa radio transmitter and twosilver oxide cells. The four sensors are mounted on the two silicon chips. In addition to it, there are a control chip, one access channel and aradio transmitter. The four sensors commonly used are a temperature sensor, pH ISFET sensor, a dual electrode conductivity sensor and a three electrode electrochemical oxygen sensor. Among these the temperature sensor, the pH ISFET sensor and the dual electrode conductivity sensor are fabricated on the first chip. The three electrode electrochemical cell oxygen sensor will be on chip 2. The second chip also consists of a NiCr resistance thermometer which is optional.

Figure 2: Block diagram

Microelectronic pill consists of 4 sensors (2) which are mounted on two silicon chips (Chip 1 & 2), a control chip (5), a radio transmitter (STD- type1-7, type2-crystal type-10), silver oxide batteries (8), 1-access channel, 3-capsule, 4- rubber ring, 6-PCB chip carrier.

The microelectronic pill consists of a machined biocompatible (non-cytotoxic), chemically resistant polyether-terketone (PEEK) capsule and a PCB chip carrier acting as a common platform for attachment of sensors, ASIC, transmitter & batteries. The fabricated sensors were each attached by wire bonding to a custom made chip carrier made from a 10pin, 0.5pitch polymide ribbon connector. The connector in turn was connected to an industrial STD, flat cable plug (FCP) socket attached to the PCB carrier chip of the microelectronic pill, to facilitate the rapid replacement off the sensors when required. The PCB chip carrier was made from 2 STD. 1.6 mm-thick fiber glass boards attached back to back epoxy resin which maximized the distance between the 2 sensor chips. The sensor chips are connected to both sides of the PCB by separate FCP sockets, with sensor chip 1 facing the top face, with the sensor chip 2 facing down. Thus, the oxygen sensor on chip 2 had to be connected to the top face by three 200nm copper leads soldered onto the board. The transmitter was integrated in the PCB which also incorporated the power supply rails, the connection points to the sensors, as well as the transmitter & the ASIC & the supporting slots for the capsule in which the carrier is located.

The ASIC was attached with double-sided Cu conducting tape prior to wire bonding to the power supply rails, the sensor inputs & the transmitter (a process which entailed the connection of 64 bonding pads). The unit was powered by 2 STD. 1.55V SR44 Silver oxide (Ag2O) cells with a capacity of 175mAh. The batteries were connected & attached to a custom made 3-pin, 1.27mm pitch plug by electrical epoxy. The connection on the matching socket on the PCB carrier provided a three point power supply to the circuit comprising a negative supply rail (1.55V).

The capsule was machined as two separate screw-fitting compartments. The PCB chip carrier was attached to the front section of the capsule (fig 2). The sensor chips were exposed to the ambient environment through access ports & were sealed by 2 stainless steel clamps incorporating a 0.8 µm thick sheet of Viton fluoroelastometer seal. A 3mm diameter access channel in center of each of the steel clamps (incl. the seal), exposed in sensing regions of the chips. The rear section of the capsule is attached to the front section by a 13mm screw connection incorporating a Viton rubber O-ring. The seals rendered the capsule water proof, as well as making it easy to maintain (e.g. during sensor & battery replacement). The complete prototype was 16*55mm & weighs 13.5g including the batteries.

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Microelectronic PillBasic Components

1. BASIC COMPONENTS

1. Sensors

Figure 3: Sensors

There are basically 4 sensors mounted on two chips- Chip 1 & chip 2. On chip 1 (shown in fig 2 a), c), e)), temperature sensor silicon diode (4), pH ISFET sensor (1) and dual electrode conductivity sensor (3) are fabricated. Chip 2 comprises of three electrode electrochemical cell oxygen sensor (2) and optional Ni Cr resistance thermometer (5).

1. Sensor chip 1

An array consisting of both temperature sensor & pH sensor platforms were cut from the wafer and attached onto 100-µm- thick glass cover slip cured on ahot plate. The plate acts as a temporary carrier to assist handling of the device during level 1 oflithography when the electric connection tracks, electrode bonding pads are defined. Bonding pads provide electrical contact to the external electronic circuit.

Lithography was the first fundamentally new printing technology since the invention of relief printing in the fifteenth century. It is a mechanical Plano graphic process in which the printing and non-printing areas of the plate are all at the same level, as opposed to intaglio and relief processes in which the design is cut into the printing block. Lithography is based on the chemical repellence of oil and water. Designs are drawn or painted with greasy ink or crayons on specially prepared limestone. The stone is moistened with water, which the stone accepts in areas not covered by the crayon. Oily ink, applied with a roller, adheres only to the drawing andis repelled by the wet parts of the stone. Pressing paper against the inked drawing then makes the print.

Lithography was invented by Alois Senefelder in Germany in 1798 and, within twenty years, appeared in England and theUnited States. Almost immediately, attempts were made to print pictures in color. Multiple stones were used; one for each color, and the print went through the press as many times as there were stones. The problem for the printers was keeping the image in register, making sure that the print would be lined up exactly each time it went through the press so that each color would be in the correct position and the overlaying colors would merge correctly.

Early colored lithographs used one or two colors to tint the entire plate and create a water color-like tone to the image. This atmospheric effect was primarily used for landscape or topographical illustrations. For more detailed coloration, artists continued to rely on hand coloring over the lithograph. Once tinted lithographs were well established, it was only a small step to extend the range of color by the use of multiple tint blocks printed in succession. Generally, these early chromolithographs were simple prints with flat areas of color, printed side-by-side.

Increasingly ornate designs and dozens of bright, often gaudy, colorscharacterizedchromolithography in the second half of the nineteenth century. Overprinting and theuse of silver and gold inks widened the range of color and design. Still a relativelyexpensive process, chromolithography was used for large-scale folio works andilluminated gift books that often attempted toreproduce the handwork of manuscripts ofthe Middle Ages. The steam-driven printing press and the wider availability ofinexpensive paper stock lowered production costs and made chromolithography moreaffordable. By the 1880s, the process was widely used for magazines and advertising.At the same time, however, photographic processes were being developed that wouldreplace lithography by the beginning of thetwentieth century.

Chip 1 is divided into two- LHS unit having the temperature sensor silicon diode, while RHS unit comprises the pH ISFET sensor.

DT-670-SD Silicon Diode Features

Figure 4: DT-670-SD

• It measures the body core temperature.
• Also compensates with the temperature induced signal changes in other sensors.
• It also identifies local changes associated with tissue inflammation & ulcers.

ISFET

Figure 5: ISFET

Ion Selective Field Effect Transistor ISFET; this type of electrode contains a transistor coated with a chemically sensitive material to measure pH in solution and moist surfaces. As the potential at the chemically active surface changes with the pH, the current induced through the transistor varies. A temperature diode simultaneously monitors the temperature at the sensing surface. The pH meter to a temperature compensated pH reading correlates the change in current and temperature.

This devicehas an affinity for hydrogen ions, which is the basis for the determination of the pH. The surface of the sensitive area of the sensor contains hydroxyl groups that are bound to an oxide layer. At low pH values hydrogen ions in the sample will bind to these hydroxyl groups resulting in a positively charged surface. In alkaline environments hydrogen ions are abstracted from the hydroxyl groups, leading to a negatively charged surface.

Thus, each pH change has a certain influence on the surface charge. On its turn, this attracts or repulses the electrons flowing between two electrodes in the semiconductor device. The electronics compensates the voltage in order to keep the current between the two electrodes at its set point. In this way this potential change is related to the pH.

Attachment of a polymer membrane on the ISFET introduces the possibility to go beyond the measurement of pH toward other ions. In this plastic layer certain chemicals (ionophores), which can recognize and bind the desired ion, are put in. Now, complex formations of the ionophore and the ion introduce a charge. The potential change is a measure for the ion concentration. Typically, these sensors can be used in a concentration range between app. 10.5 up to 1 mol/l.

1. Sensor chip 2

The Level 1 pattern (electric tracks, bonding pads, and electrodes) was defined in0.9µm UV3 resist by electron beam lithography. A layer of 200nm gold (including an adhesion layer of 15nm titanium and 15nm palladium) was deposited by thermal evaporation. The fabrication process was repeated.

Oxygen sensor detection principle

Most portable or survey instruments used for workplace evaluation of oxygen concentrations make use of "fuel cell" type oxygen sensors. "Fuel cell" oxygen sensors consist of a diffusion barrier, a sensing electrode (cathode) made of a noble metal such as gold or platinum, and a working electrode made of a base metal such as lead or zinc immersed in a basic electrolyte (such as a solution of potassium hydroxide).

Oxygen diffusing into the sensor is reduced to hydroxyl ions at the cathode:

O2 + 2H2O + 4e-OH –

Hydroxyl ions in turn oxidize the lead (or zinc) anode:

2Pb + 4OH 2PbO + 2H2O + 4e –

This yields an overall cell reaction of:

2Pb + O22PbO

Fuel cell oxygen sensors are current generators. The amount of current generated is proportional to the amount of oxygen consumed (Faraday's Law). Oxygen reading instruments simply monitor the current output of the sensor.

An important consideration is that fuel cell oxygen sensors are used up over time. In the cell reaction above, when all available surface area of the lead (Pb) anode has been converted to lead oxide (PbO), electrochemical activity ceases, current output falls to zero, and the sensor must be rebuilt or replaced. Fuel cell sensors are designed to last no more than one to two years. Even when installed in an instrument which is never turned on, oxygen sensors which are exposed to atmosphere which contains oxygen are generating current, and being used up.

Oxygen sensors are also influenced by the temperature of the atmosphere they are being used to measure. The warmer the atmosphere, the faster the electrochemical reaction. For this reason oxygen sensors usually include a temperature compensating load resistor to hold current output steady in the case of fluctuating temperature. (Microprocessor based instrument designs usually provide additional signal correction in software to further improveaccuracy.) Another limiting factor is cold. The freezing temperature of electrolyte mixtures commonly used in oxygen sensors tends to be about 5oF (-20oC). Once the electrolyte has frozen solid, electrical output falls to zero, and readings may no longer be obtained. There are two basic variations on the fuel cell oxygen sensor design. These variations have to do with the mechanism by which oxygen is allowed to diffuse into the sensor. Dalton's Law states that the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the various gases. The partial pressure for oxygen is the fraction of the total pressure due to oxygen. Partial atmospheric pressure oxygen sensors rely on the partial pressure (or pO 2) of oxygen to drive molecules through the diffusion barrier into the sensor. As long as the pO 2 remains constant, current output may be used to indicate oxygen concentration. On the other hand, shifts in barometric pressure, altitude, or other conditions which have an effect on atmospheric pressure will have a strong effect on pO 2 sensor readings. To illustrate the effects of pressure on pO 2 sensors, consider a sensor located at sea level where atmospheric pressure equals 14.7 PSI (pounds per square inch). Now consider that same sensor at an elevation of 10,000 feet. Although at both elevations the air contains 20.9 percent oxygen, at 10,000 feet the atmospheric pressure is only 10.2 PSI! Since there is less force driving oxygen molecules through the diffusion barrier into the sensor, the current output is significantly lower.

"Capillary pore" oxygen sensor designs include a narrow diameter tube through which oxygen diffuses into the sensor. Oxygen is drawn into the sensor by capillary action in much the same way that water or fluid is drawn up into the fibers of a paper towel. While capillary pore sensors are not influenced by changes in pressure, care must be taken that the sensor design includes a moisture barrier in order to prevent the pore from being plugged with water or other fluids.

Figure 6: Capillary pore type oxygen sensor

Effects of contaminants on oxygen sensors

Oxygen sensors may be affected by prolonged exposure to "acid" gases such as carbon dioxide. Most oxygen sensors are not recommended for continuous use in atmospheres which contain more than 25% CO 2.

Substance-specific electrochemical sensors

One of the most useful detection techniques for toxic contaminants is the use of substance-specific electrochemical sensors installed in compact, field portable survey instruments. Substance-specific electrochemical sensors consist of a diffusion barrier which is porous to gas but nonporous to liquid, reservoir of acid electrolyte (usually sulphuric or phosphoric acid), sensing electrode, counter electrode, and (in three electrode designs) a third reference electrode. Gas diffusing into the sensor reacts at the surface of the sensing electrode. The sensing electrode is made to catalyze a specific reaction. Dependent on the sensor and the gas being measured, gas diffusing into the sensor is either oxidized or reduced at the surface of the sensing electrode. This reaction causes the potential of the sensing electrode to rise or fall with respect to the counter electrode. The current generated is proportional to the amount of reactant gas present.

This two electrode detection principle presupposes that the potential of the counter electrode remains constant. In reality, the surface reactions at each electrode causes them to polarize, and significantly limits the concentrations of reactant gas they can be used to measure. In three electrode designs it is the difference between the sensing and reference electrode which is what is actually measured. Since the reference electrode is shielded from any reaction, it maintains a constant potential which provides a true point of comparison. With this arrangement the change in potential of the sensing electrode is due solely to the concentration of the reactant gas.

Figure 7: Three electrode electrochemical sensor