Sensor cell

Details Sensor cell Sensor cell

2001-12-20

2003-12-02

Jackson, Jerome (Department: 2815)

Active solid-state devices (e.g., transistors, solid-state diode

Responsive to non-electrical signal

C257S071000

Reexamination Certificate

active

06657269

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to sensor cells and to sensors which incorporate such sensor cells.
2. Description of Related Art
Chemical sensors incorporating arrays of sensor cells including semiconductor transistors are known. Such sensors have typically used a silicon wafer as the substrate material. However, silicon is a relatively expensive material. Furthermore for certain types of sensors, such as biosensors, disposability of the sensor after use is an especially important issue as the biosensor can only be used once before disposal. When silicon is used as the substrate material, disposing of the used biosensors becomes more problematical.
Additionally, the difficulties associated with fabricating transistor arrays on silicon substrates are known to increase significantly with increase in the size of the array. Hence, with silicon substrates the tendency is for a high density of devices for any given size of array. For biosensors, this high packing density can be problematical because for many applications the active parts of the microelectronic chip incorporating the array must operate in a wet environment.
Many forms of chemical sensors, such as biosensors, have been proposed. One type of multi-biosensor comprises a pH sensor in the form of an array of four Ion Sensitive Field Effect Transistors (ISFET's) in Urination with four Metal Oxide Silicon Field Effect Transistors (MOSPET's) acting as source follower circuits. However, in order to provide sufficient isolation between the ISFET's, the proposed array was relatively bully in size. Furthermore, an IFSET is a form of transistor and considerable problems arise in isolating such devices from a solution being tested. To alleviate the problems of isolation, the ISFET's and MOSFET's have been proposed to be fabricated on a silicon layer in the form of a number of discrete sites supported on a sapphire substrate. Sapphire was used as the substrate material because of its excellent electrical isolation properties. A protectional membrane was then formed over the gate surfaces of the ISFET's, followed by membranes respectively sensitive to the compounds to be tested. The individual sensors so produced functioned as pH sensors and could be used to detect urea, glucose and potassium. However, as mentioned above, the sensor array was of relatively large size, measuring approximately 2 mm in width and 6 mm in length for a four sensor array. Furthermore, sapphire substrates can only be used to fabricate arrays to a certain size and it is well known that the concerns rating to the fabrication of arrays using silicon increase significantly with increase of array size. Additionally, the silicon and, in particular, the sapphire substrate materials are relatively expensive and therefore chemical sensors of the above type are extremely costly to fabricate. This cost aspect is particularly burdensome when considering that many types of sensors can only be used once before disposal. Moreover, these materials are not readily disposable, giving rise to significant environmental concerns regarding disposal after use.
More recently, sub-micron CMOS technology has been proposed for use as a biosensor array for DNA analysis. This technology has enabled an array of up to about 1000 sensor cells to be fabricated on a substrate having a size in the order of a few millimeters square. However, as the CMOS devices are fabricated on a silicon substrate, the proposed array has a high packing density. To isolate the active CMOS devices from the wet operating environment, a specific integrated reaction test chamber is provided in the form of a cavity arranged between two superimposed and hermetically sealed primed circuits. The DNA material to be analysed is separated into its two strands by heating and, using a biochemical process, the stands are labelled with a fluorescent molecule. An analyte containing the DNA strands is then placed in contact with the chip. If a DNA strand has a sequence matching that of a target arranged on an electrode of the sensor, hybridisation occurs which results in a physical localisation of the DNA sample onto the appropriate electrode of the chip. The chip is then rinsed and the sensor is read with a CCD camera. As the DNA strands have been labelled with a fluorescent molecule, relative brightness on the electrodes of the device indicates where bonding has occurred. Key issues in the applicability of such devices are recognised as materials compatibility, manufacturing and packaging in order to reliably deliver a wet-chip concept and these can be compromised by the requirement to achieve a high packaging density on the silicon substrate material. Also, as will be apparent from the above description, such biosensors are relatively expensive to manufacture.
Thin film transistors (TFT's) are relatively inexpensive to manufacture as relatively cheap non-silicon substrates such as soda glass or plastic can be used. The use of a plastics substrate can provide additional benefits as it is a relatively disposable material. Furthermore, TFT's can be readily fabricated as large area arrays and such technology has already found widespread application in industry, such as for example, in the manufacture of active matrix liquid crystal display devices. The manufacturing processes are therefore well proven and a high yield of operable devices can reliably be obtained at relatively low costs, especially in comparison to silicon substrate devices. These advantages are further enhanced when considering that arrays larger than those available from silicon substrates can also be reliably fabricated. The use of silicon wafer substrates for such large area arrays is considered to be extremely problematical as it becomes increasingly difficult and expensive to fabricate the arrays in view of the substrate material itself and the semiconductor fabrication techniques which must necessarily be employed.
There are also drawbacks associated with the performance of such devices when used to sense certain substances. MOSFET's typically comprise a relatively thin layer of silicon dioxide (SiO
2
) supported on a doped silicon substrate. The SiO
2
layer has inherent capacitance which is inversely proportional to the thickness of the layer. If the SiO
2
layer is fabricated to a typical thickness of about 100 nm, there is significant loss of capacitive signal from the device which is due to the inherent capacitance of the SiO
2
layer. If the SiO
2
layer is fabricated as a very thin layer to improve signal output, the devices become very unstable in use. These design conflicts can be alleviated if the sensing electrode is made very small. However, the sensing electrode must be fabricated to a practical size as it is used to receive the substance being identified. The MOSFET gate area must therefore be mad relatively large but this gives rise to the basic fabrication concern regarding the use of silicon transistors for chemical sensors in that the provision of relatively large gate areas significantly reduces the packing density of the transistors which can be accommodated on the finite size silicon substrates, which in turn reduces the number of sensor cells that can be accommodated in the sensor array.
SUMMARY OF THE INVENTION
For chemical or biosensors in particular, the ability of TFT's to be readily fabricated as large area arrays at relatively low cost presents significant advantages in comparison to the conventionally used silicon devices as the need to achieve a very high packing density is not a dominant factor in device design. Hence, the area associated with each sensor cell of an array which receives the sample to be identified can, if necessary, be displaced from the active semiconductor components, alleviating the isolation concerns which exist with the current silicon substrate devices. Furthermore, the sensing areas for receiving a sample to be identified, which may be in the form of electrodes for a DNA sensor, can be made relatively

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