Electronic devices fabricated with integrated circuit (IC) technologies require biases and conduits to external contacts, where such biases are applied or measured and current is conducted into and out of the electronic devices.
A conductor passing over an underlying substrate is isolated from the substrate by an oxide or insulator layer. The voltage of the interconnect line creates an electric field between the conducting interconnect line and the underlying substrate. If this electric field is sufficiently large, an accumulation or inversion layer may be formed. If an inversion layer forms, there is a depletion region separating the induced channel from the underlying semi-conducting substrate. If this junction is reverse biased, the underlying induced channel connects to the drain contact and the depletion region is then reverse biased. The reversed biased depletion region delivers current to the inversion channel. The current is conducted along the channel to the drain contact and adds to the drain current. As the drain voltage changes so does the recovered depletion leakage current contribution. As the drain voltage is increased, the amount of back bias on the induced junction is increased. The depletion related volume and the amount of thermally generated leakage current are also increased. Such leakage current, if blocking means are not provided, adds to the leakage current, creating a net increase in drain current and a drain voltage dependence of the drain current magnitude. The current contribution may be substantial even at zero drain voltage if the substrate channel is reverse biased by a substrate-applied voltage. Leakage current is acquired without a drain voltage applied as long as the drain to substrate voltage biases the induced depletion regions with the thermally generated leakage current.
While one solution is to increase the field oxide thickness, and this is the solution typically used by industry (thick field oxide), this solution is insufficient for the biosensor for several reasons. One reason is the desire to apply large reverse biases to the P drain and channel diode junction (with the underlying N-substrate). The arguments conceptually carry through for N-channel devices as well. P-Channel devices are used by way of example in this document since many biochemicals are negatively charged.
Some improvement may be achieved by making the surface N+ surrounding the sensor, if care is taken not to form a tunneling junction. However, this solution will also fail for sufficient drain to substrate reverse biases and for some biochemical attached charge conditions.
This problem is non-obvious since normal FET structures are isolated and the field oxide is designed not to invert the surface. Such inversion does not occur because a large reverse bias on the drain and channel region is normally not applied. For the biosensor, a large reverse bias is desirable at times because this can decrease the buried channel region and thus increases biosensor sensitivity or is used as a measurement parameter.
A second non-obvious situation arises for any field oxide thickness. If biochemicals attached to the field region of the IC chip, the electric field has the same value inside of the field oxide regardless of the field oxide thickness. For example, a heavy coating of negatively charged biochemicals induce a P-channel on the N-substrate surface and the attendant PN junction depletion region. Thus, by simply creating a sensor condition where the amount of charge attached to the field oxide surfaces surrounding the sensor is great enough to induce a channel (negative charges may induce a P-Channel), the problem of channel and depletion formation with attendant leakage current can occur. This situation is non-obvious since IC chips do not typically have such attached charges on the field oxide, and the field oxide thickness is considered the design feature that blocks inversion of the underlying substrate. Field oxide thickness is ineffective in the attached charge chase where the Gaussian field is strong enough to induce a channel connecting to the source or drain of the sensor.
Needs exist for enhanced biosensor performance and designs.
The present invention may take several different forms to improve biosensor performance.
The first form is an increase in field oxide thickness. The field oxide is increased such that the difference between the substrate voltage and overlying drain and/or source conduit voltage creates an electric field across the field oxide that is lower than that needed to form an inversion layer and a pseudo PN junction.
Another form is to create P+ wells to isolate the sensor from the substrate. An epitaxial N layer is placed on a P substrate. This PN junction provides basic isolation of the sensor from the substrate. However, by placing P+ posts through the N epi layer, the N epi layer is isolated from the remainder of the substrate. A back bias exists between the P region and the portion of the N-well containing the sensor, in this example, a P-Channel device. Here the well is made as small are possible depending on the size of the actual sensor active region.
Another form is to create trench isolation. The sensor is surrounded and abutted with a trench cut deep into the semiconductor substrate. A substrate contact to the underlying N region permits the reverse bias to the back gate of the sensor channel with the attendant sensitivity and sensor parameter measuring features enabled. If there is some inversion of the N surface to become a P-channel, then current cannot flow around the channel between the source S and drain D. Leakage current is reduced substantially by trench isolation, whether caused by interconnects conduits or attached biochemical charge induction. The amount of induced junction leakage is kept negligible by maintaining minimum sensor, channel, source and drain inversionable areas.
Another form is mesa isolation. This form has the same function as the trench isolation but covers a much wider area.
Another form is conductive shield surface protection. A structure includes a sensor region fabricated on a semiconductor substrate and a conduction region protected by insulator materials and added to the remainder of the semiconductor substrate region to protect the surface from undesirable inversion or other phenomena. The conductive layer has a pre-selected bias applied with respect to the back gate bias. The pre-selected bias is chosen to keep the surface of the semiconductor under the interconnect conductors from displaying unwanted surface effects such as inversion, depletion and related unwanted leakage currents adversely contributing to the sensor signal.
An FET can acquire significant thermally generated depletion current contributions to a drain current in spite of using a very large field oxide. Field oxides are used to prevent such occurrences but it is clear that regardless of the field oxide thicknesses, such introduced erroneous drain currents may occur. The bio charge induced channel linked to the drain contact and leakage current generating depletion regions is completely independent of field oxide thickness. The field oxide approach of conventional IC chips is not adequate for certain conditions of the Silicon biosensing platform.
These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the drawings.
The present invention may take several different forms to improve biosensor performance. The first form is an increase in field oxide thickness. The field oxide is increased such that the difference between the substrate voltage and overlying drain and/or source conduit voltage creates an electric field across the field oxide that is lower than that needed to form an inversion layer and a pseudo PN junction.
Another form of improvement is to create P+ wells 13 to isolate the sensor from the substrate 17. An epitaxial N layer 19 is placed on a P substrate 17. A PN junction 21 provides basic isolation of the sensor from the substrate. However, by placing P+ posts 13 through the N epi layer 19, the N epi layer 19 is isolated from the remainder of the substrate. A back bias exists between the P region and the portion of the N-well containing the sensor, in this example, a P-Channel device. Here the well is made as small are possible depending on the size of the actual sensor active region.
Another form of improvement is to create trench isolation. The sensor is surrounded and abutted with a trench 35 cut deep into the semiconductor substrate 17. A substrate contact 31 to the underlying N region permits the reverse bias to the back gate of the sensor channel with the attendant sensitivity and sensor parameter measuring features enabled. If there is some inversion of the N surface to become a P-channel, then current cannot flow around the channel between the source S and drain D. Leakage current is reduced substantially by trench isolation, whether caused by interconnects conduits or attached biochemical charge induction. The amount of induced junction leakage is kept negligible by maintaining minimum sensor, channel, source and drain inversionable areas.
Another form of improvement is mesa isolation. This form has the same function as the trench isolation but covers a much wider area.
These methods may be used individually or in combination.
Another form of improvement is conductive shield surface protection. A structure includes a sensor region fabricated on a semiconductor substrate and a conduction region protected by insulator materials and added to the remainder of the semiconductor substrate region to protect the surface from undesirable inversion or other phenomena. The conductive layer has a pre-selected bias applied with respect to the back gate bias. The pre-selected bias is chosen to keep the surface of the semiconductor under the interconnect conductors from displaying unwanted surface effects such as inversion, depletion and related unwanted leakage currents adversely contributing to the sensor signal.
A partial list of these applications target molecule classes are listed in Table I by way of example. Table I is a partial list of a wide variety of chemicals that can be targeted or sensed with the performance improved biosensors. The term “receptor” is used in the most general sense. A chemical has an affinity for another chemical, a target chemical. Generally, that affinity is selective and specific to the target chemical. Antibodies are one example, nerve receptors another, drug receptors another, oligos are others.
Applications of the Si based biosensor platform are described in general applications regimes, by way of example, by the list provided in Table II.
While the invention has been described with reference to specific embodiments, modifications and variations of the invention may be constructed without departing from the scope of the invention.
This application claims the benefit of U.S. Provisional Application No. 60/568,297, filed May 3, 2004, which is hereby incorporated by reference in its entirety.
Number | Date | Country | |
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60568297 | May 2004 | US |