SEMICONDUCTOR CELLS FOR ANALYZING MATERIALS

CLAIM OF PRIORITY

Benefit of Priority is hereby claimed to U.S. Patent Application No. 11/683,623, filed March 8, 2007 and entitled "SEMICONDUCTOR UV ABSORPTIVE PHOTOMETRIC CELLS", which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

Various embodiments described herein relate to apparatus, systems and methods associated with semiconductor devices for analyzing materials using their UV absorption properties.

BACKGROUND

There is presently a need to rapidly analyze large numbers of compounds, particularly biomolecules. For example, the completion of sequencing of the human genome has provided very numerous targets to analyze and investigate. Advances in combinatorial chemistry allows generation of very large numbers of target compounds to analyze.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a plan perspective view of an embodiment of a UV absorption array.

Fig. 2 is a plan perspective view of an embodiment of a UV absorption array.

Fig. 3 is a top view of a single semiconductor data storage cell for use in devices of Figs. 1 and 2.

Fig. 4 is a side sectional view taken along lines A-A in Fig. 3.

Fig. 5 is an electrical schematic for the data storage cell of Fig. 3.

Fig. 6 is a plot of threshold voltage for a single semiconductor data storage cell as shown in Fig. 3 versus optical absorption.

DESCRIPTION

In some embodiments, optical photometric analysis may provide a high throughput means to analyze large numbers of compounds, such as biomolecules. Integrated circuit technology may allow one to rapidly detect properties of large numbers of biomolecules in arrays.

An EPROM is a type of non- volatile semiconductor memory chip normally operated as a digital device. Non-volatile means that the chip retains information when power is removed. One class of EPROM devices is erasable with erasing performed by directing ultraviolet (UV) light through a quartz or boron silicate glass window toward a floating gate of the EPROM device. Data in the form of electronic charge is stored on a floating gate electrode that is programmed by manipulation of transistor electrodes to charge the device for one digital state and erased with strong UV light, typically in the range of 225 nm - 275 nm, for the second digital state.

We have found that the same UV wavelengths that are effective for photometric analysis of biomolecules and the like are also effective for electron transfer in floating gate devices, i.e. removing charge by incrementally erasing the devices. In some embodiments herein, this has led to fabricating a UV photometric sensor with higher sensitivity than hereto possible because the device is very sensitive to a narrow UV wavelength band and other wavelengths are self-rejected. In some embodiments, the UV photometric sensor is a type of charge storage device, preferably a floating gate EPROM transistor having a variable threshold voltage that can act in place of a photometer.

In some embodiments, a UV photometric sensor array may be used to analyze an array of biomolecule target sites on a carrier. The sites have biomolecule target materials that are responsive to UV light for determining a biomolecule property. The target sites, supported on a carrier, are transmissive of UV light that is partly attenuated by interaction with the biomolecules. Receiving the attenuated light is an array of non- volatile, individually addressable cells of charge storage devices, such as EPROM transistors, that start out fully charged. Thus, the UV photometric sensor array includes the biomolecule sites, the non-volatile addressable cells separated from the UV photometric sensor array by a UV window with a UV light source used to direct light to the array of biomolecule sites. Upon impingement of UV light, the

biomolecule sites attenuate the UV light in proportion to a known characteristic property of the biomolecules. The attenuated light is transmitted through the window, which is transparent in the UV spectral region, to the non-volatile cells of charge storage devices. The charge storage cells are incrementally discharged or erased in proportion to the amount of UV light absorbed. An analog output amplifier is configured to read a relative change in threshold voltage in each charge storage cell that results from the incremental discharge of the cell, with an output indicating the extent of absorbed UV light. In some embodiments, the UV light transmissive window that separates the biomolecule sites from the addressable charged cells is in a sandwich configuration in which the biomolecule site array, the window and the UV sensor array are in close contact. The addressable aspect of the charged cells allows individual cells to be selected and read for a charge value which is translated to a determination of the biomolecule property.

Because EPROM floating gate transistor devices can be integrated in a chip or wafer, it is possible in some embodiments to combine microarrays of biomolecule sites with a corresponding array of EPROM devices in an overlay relation. Because EPROM floating gate devices have also been constructed with X-Y address circuitry, one can read selected UV photometric sensors individually.

In some embodiments, UV light is directed from a UV specific source through the microarray of molecules for detection of phenomena that would have UV absorption as an indicator. As mentioned, in some embodiments the charge storage devices are of the EPROM type, namely having a UV transmissive window receiving UV light that has been attenuated to some extent by interaction with biomolecules in a well or an array. UV transmissive windows are those that transmit UV light in a passband relatively well compared to other wavelengths that are suppressed or rejected. Here the passband is approximately 225 run - 275 nm. The UV light source may be integrated into the UV photometric sensor array or may be external and separated from the UV photometric sensor array. In an EPROM, the UV light causes incremental erasing of the floating gate, i.e. the transfer of holes from the floating gate to a substrate in a non- volatile or permanent manner in proportion to the amount of incident UV light. A charge storage capacitor would work similarly. The

electrical charge state of the floating gate influences the conduction ability of an associated transistor by a property known as the threshold voltage, Vj, that can be read by an analog output amplifier, such as a difference amplifier. It has been found that the change in threshold voltage can be almost linearly related to UV attenuation. Because both the biomolecules and the EPROM are sensitive to UV light centered at 260 run, the measurements are more sensitive and selective than if made with broadband detectors, such as CCD elements or photodiodes or other photodetectors. The EPROM window acts as a filter, suppressing light of wavelengths not of interest.

Some embodiments of the EPROM device feature a central poly floating gate that is surrounded laterally by a control gate that has a subsurface electrode that is shared with an adjacent control transistor so that EPROM devices can be individually addressed. A layer of quartz, borosilicate glass or other appropriate material provides a UV transmissive window separating the EPROM array from the biomolecule array, but the EPROM array, the window and the biomolecule array are sandwiched together in close proximity. Light is directed through the biomolecule array, through the UV window and onto the EPROM array where changes in threshold voltage of each selected cell can be measured in a predetermined sequence. The change in threshold voltage of each selected EPROM is a photometric measurement of the incremental charge removed from the EPROM by attenuated UV light and a direct measure of light removed in a corresponding biomolecule site, thereby indicating in an analog output amplifier a biomolecule property by translation from a prior calibration.

With reference to Fig. 1 the UV photometric sensor array 10 is seen to have two major components. The first is an array 11 of variable threshold devices set up as transistors of the EPROM type that are mounted in an integrated circuit chip 13. The device array 11 is thus an X-Y array of EPROM transistors as in a semiconductor memory, except not operated as digital devices, but as analog devices. Such an array contains addressing circuitry, power supply circuitry and data reading or sensing circuitry. The addressing circuitry has the capability of individually writing or reading a selected cell. A floating gate in each EPROM serves as a charge storage element for a photometric measuring system. Each EPROM transistor is charged by placing electronic charge, i.e. electrons or holes, on the floating gate. Impingement of UV light on the floating

gate partly erases or removes charge from the floating gate by creation of a discharge path in silicon dioxide. UV light reaches the floating gate through a UV light transparent window 15 covering the floating gate, facing upwardly toward UV beam 25. The impingement of light coming through window 15 onto the floating gate can be the measure of absorption of light through an optical cell through which the UV light passes. The state of charge of the floating gate, i.e. the extent of discharge of the floating gate, is read by electronic circuitry, described below, with data emerging through chip pins 17 to outside data logging equipment.

In some embodiments, window 15 can be quartz or may be a borosilicate glass, preferably having the highest possible UV transmission in the range of 225-275 nm. One such suitable glass is described in U.S. Pat. No. 5,547,904 but many other suitable window materials are known. In some embodiments the biomolecule carrier serves as window 15, if made of appropriate material. In these embodiments the carrier is the UV window as well as being a biomolecule support.

Above the window 15 is a carrier 23 which may be an opaque material having an array of biomolecule sites 21 that are UV light transmissive. The biomolecule sites may be fluidic wells for holding small volumes of liquid target material or solid receptor sites for binding target molecules. In some embodiments, there may be a one-to-one correspondence between biomolecule sites 21 and EPROM transistors in device array 11 of variable threshold transistors. The biomolecule sites have the property of attenuating UV light when target material is stimulated or intercepted by UV light. For example, optical absorbance at 260 nm is routinely used to measure concentration of nucleic acids in solution. This example illustrates a standard measurement of a spectrophoto-meter.

The present discovery allows a number of types of assays. For example, quantification of nucleic acids may occur in some embodiments, by measuring light absorbed at 260 nm. Because single stranded DNA and double stranded DNA have different absorption characteristics at 260 nm, this allows a simplified assay of DNA hybridization. This allows determination of complementary strands of DNA without use of any additional dye or label, as the double stranded DNA is self reporting. In another type of assay, the purity of DNA

could be analyzed. Pure double stranded DNA has (at 50 μg/ml) an absorbance at 260 run of 1.0 and that the ratio of absorbance at 260 and 280 ran of the sample will be greater than 1.8. If the sample has an A260/A280 of less than 1.8, the sample is probably contaminated with protein. In some embodiments, the samples could be placed in microplate wells and the wells of a microplate could be sequentially illuminated with 260 ran and 280 run illumination and absorbance could be sequentially illuminated at each wavelength. Between illuminations the sensor (once read and date logged) could be reset.

Larger targets could also be used in some embodiments. These include cellular assays, beads or other particulate targets. For example, certain pathological cells are characterized by UV absorption changes. A tissue section on a slide could be illuminated with UV light as a whole and pathological cells responsive to UV light localized and enumerated in a single analysis event. The slide must be registered or indexed to the UV sensor array such that X-Y positions on the slide correspond to known UV detector locations. The device could also be used for screening materials for UV absorbance/reflection properties for possible use as skin protection, window coatings or other uses. These are just a few illustrations of the uses of the present devices. Some embodiments of the present invention allow for nearly simultaneous measurements at many thousands of sites where biomolecular or other targets reside at this same wavelength.

In operation, an IC chip with an array of EPROM transistors would be all programmed or fully charged with electrons on the floating gates. This is done with electrical circuitry by injection of charge from a substrate doped region to the floating gate. The window 15 is blocked with an opaque screen to prevent accidental erasing by ambient light or else the window 15 is kept dark. The chip is overlaid with the array of biomolecule target sites in registration with the EPROMs using robotic electromechanical techniques. The opaque screen is removed and biomolecule target sites are illuminated with UV light which is attenuated or absorbed by the biomolecules in relation to a measured parameter. In this illustration, the sites are sufficiently separated so that illumination through one site affects only one EPROM transistor. Each EPROM transistor is incrementally erased, i.e. charge removed from the floating gate in proportion to the UV impinging on the floating gate. This is the amount of light from the UV

source less the amount of light attenuated by the target material of the biomolecule sample and less the light lost in the window. The latter quantity is the same for all EPROMs and can be ignored. In some embodiments, impinging light is pulsed at a known rate so that the amount of illumination for samples can be quantified and reproduced. The charge that is removed may be either holes or electrons depending on whether an NMOS or PMOS EPROM design is used. In a PMOS transistor, holes are the majority carriers and so holes will supply the maximum charge to the floating gate. For either NMOS or PMOS transistors, the role of UV light is the same, i.e. to incrementally remove charge from the floating gate. UV light creates electron-hole pairs in silicon dioxide, the insulating material between the floating gate and the substrate, thereby providing a discharge path for the charged floating gate. Although the silicon dioxide beneath the floating gate might, in some embodiments, be shielded from UV, there are sufficient electron-hole pairs created near the edges of the floating gate to provide a discharge path. By targeting a floating gate with a beam, as in Fig. 2, the beam spot is larger than the floating gate area so that surrounding silicon dioxide is illuminated. The incrementally removed charge changes the threshold voltage of the EPROM transistor and the change in threshold voltage is read by an analog output amplifier.

With reference to Fig. 2, an array of variable threshold EPROM transistors 31 is fabricated as a semiconductor memory on wafer 33, together with ancillary circuitry, including address, read and write circuits. In some embodiments, the memory is arranged in a grid of X-Y memory cells, with most of the cells having substantially the same size. In some embodiments, the memory array is covered with a thin sheet of quartz or borosilicate glass or other material, perhaps 0.1 to 1 mm thick as a UV window with the highest transmission in the 225-275 run range, acting as a UV window 35. Atop the window 35 is a carrier 43 having an array 41 of biomolecule sites. The sites may be micro-wells in a glass plate holding liquid biomolecule target samples. The wells are transparent but surrounding glass is made opaque by masking. In some embodiments, the substrate, wall bottom, or other biomolecule support has similar UV transmission as the window, providing minimal interference with the UV measurements. Alternatively, in some embodiments the biomolecule sites may be solid receptor sites where target biomolecule material is held in place,

using probes or other receptors. In soime embodiments, the sites have approximately the same dimension as a beam spot of an illuminating UV laser beam 45. The beam is steered to a desired X-Y location by scan mirrors 47 and 49 that are controlled by stepper motors and servos to go to a desired target location. A pulsed UV laser 53 directs beam 45 through one or more lenses 51 toward the desired biomolecule site of the array 41 on carrier 43. In passing through the target material at the biomolecule site, UV light is attenuated in the same manner as described above, with the attenuated light being detected in a cell of the UV photometric sensor array. Once a reading is made in one UV photometric sensor cell, the beam is stepped using the scan mirrors 47 and 49 to the next biomolecule site and the process is repeated, hi some embodiments, before receiving attenuated light, a corresponding UV photometric sensor cell is fully charged because the attenuated light causes discharge of the UV photometric sensor cell in proportion to the amount received in accordance with a known relation that is established by calibration.

With reference to Fig. 3, a two-transistor UV photometric sensor cell is built on a semiconductor substrate 61, typically a silicon wafer, hi some embodiments, the cell is part of an X-Y array of cells arranged in an X-Y pattern like a semiconductor memory, or can be a stand alone device for disposable applications. A stand alone device may be a single cell. Ancillary memory circuits, such as address and read-write circuitry, are also included. Some arrays may be packaged as chips, shown in Fig. 1, or used in wafer form as show n in Fig. 2. hi either situation the UV photometric sensor cell exists below a UV transmissive window, not shown, hi some embodiments, the window may be masked to prevent crosstalk from a neighboring cell in a manner so that only a rectangular sensing area 70, having a boundary indicated by a dashed line, is exposed to attenuated UV light. The central shaded zone is floating gate 63, a conductive polysilicon layer separated from the substrate 61 by an insulative layer, typically oxide. The floating gate 63 belongs to an EPROM transistor having subsurface source-drain electrodes and controlled by a control transistor. Floating gate 63 has a rectangular aperture 65 that accommodates a metal contact 67 that extends from above the floating gate 63 to substrate 61 where it makes contact with a subsurface electrode. The sensing area 70 is an integral and unitary part of floating gate 63, except that portions of floating gate 63 on sides

of aperture 65 are masked off. Floating gate 63 is separated from the edge of the isolation region to prevent charge leakage between the floating gate and the isolation region.

Surrounding floating gate 63 is a channel 80 and a polysilicon control gate 73 belonging to a control transistor. The control gate has a loop shape except for a panhandle region 74 that extends beyond the active area of the cell where a metal contact 75 extends from above and contacts the control gate 73. The height of contact 75 is similar to contact 67.

Surrounding control gate 73 are a plurality of peripheral electrode contacts 81, 82, 83-93 disposed in a spaced relation relative to control gate 73 from a level similar to contacts 67 and 75. A boundary 76 defines the extent of the active area of the cell. Shallow trench isolation around boundary 76 electrically isolates cell 60 from neighboring cells.

With reference to Fig. 4, the floating gate portions 63a and 63b are on either side of a central P+ implant region in N- well 59 in P-type wafer substrate 69, the implant serving as a non-shared subsurface electrode 72, having the overlying metal contact 67. The floating gate portions 63a and 63b are over a channel of the floating gate transistor between electrode 72 and source-drain implant regions 66 and 68 that form an opposite subsurface electrode that establish the channel directly below floating gate portions 63 a and 63b and in the substrate.

Edges of the cell are established by shallow trench isolation regions 55 and 57 that extend in a rectangle defining the active region shown by boundary 76 in Fig. 3. Returning to Fig. 4, the subsurface implant regions 62 and 64 are in the outer periphery of the active region and are among a number of similar implants in the substrate beneath the peripheral electrode contacts 81, 82, 83-93. The source-drain implant regions 66, 68 are part of a group of implants that similarly extend in a loop between the floating gate 63 and the control gate 73 such that a source-drain electrode, such as an electrode established by implant region 66 is opposite a peripheral electrode, such as an electrode associated with implant region 62, to define a channel for the control gate, such as control gate region 73b. The same channel is established on the opposite side by an electrode established by implant region 64 defining a channel under control gate region 73a that terminates at an electrode established by implant region 68.

Thus, in some embodiments, a surrounded transistor is seen, having control gate 73 and subsurface electrodes on all sides of gate plus the floating gate 63 with subsurface electrodes on all four sides of the floating gate. The electrode contacts 67, 81 and 88, as well as contact 75 of Fig. 3, allow proper bias to be applied to the transistor for various operations such as fully charging the floating gate transistor and reading the charge on the floating gate.

With reference to Fig. 5, in some embodiments, the two PMOS transistor UV photometric sensor cell 60 having control transistor 101 has external contact 75 associated with control gate 73. Floating gate transistor 103 has a common source-drain electrode 102, corresponding to electrodes associated with implant regions 66, 68 in Fig. 4. Electrical ground is a peripheral electrode contact 81, corresponding to electrodes 81-93 in Fig. 3. The N- well is represented by a common potential at node 159, the N-well potential. The floating gate 63 has no contacts. An electrode 104 of the floating gate transistor 103 is connected to contact 67 where the programming potential Vp is applied. A voltage regulating device 106 assures that a desired voltage is not exceeded during charging of the floating gate. After the cell is charged, the programming potential is removed and the floating gate threshold voltage is read at node 107 when the control transistor 101 is selected by an appropriate read voltage applied at contact 75. The voltage on node 107 is transmitted on line 105 to difference amplifier 109, an analog output amplifier, that compares a reference voltage V _R from node 108 to the measured threshold voltage V _T . Difference amplifier 109 directly measures the changing threshold voltage, Vj, as erasing incremental occurs. When this voltage is plotted, a graph similar to Fig. 6 is observed. As described above, Vj changes in proportion to UV light absorbed by the UV photometric sensor cell.

With reference to Fig. 6, the vertical axis 121 shows the threshold voltage Vj, while the horizontal axis 123 shows light absorbance, an arbitrary scale of attenuated light from a UV source. The curve 125 is almost a linear relationship of plotted points showing how the threshold voltage of an EPROM cell varies with absorption of UV light in a calibrated cell. As more light is absorbed in the cell the threshold voltage of the EPROM decreases as charge is driven from the floating gate. In some embodiments, in an array of cells, each cell is individually charged, illuminated with UV light, whether by a laser or a

broad area source, and then read after discharge by light absorption. The output data is read and recorded and then the next cell is addressed. Each cell can be processed in an interval of about a hundred milliseconds, more or less.

While this patent application has described embodiments of a semiconductor UV photometric sensor for use in analyzing biomolecules sensitive to UV light, other applications exist for the semiconductor UV photometric sensor. For example, in space applications, CMOS semiconductor integrated circuits can malfunction if exposed to UV radiation. The UV photometric sensor of the present invention could be used to shut down CMOS electronics upon detection of UV light that causes a predetermined shift in threshold voltage of the sensor. The window that transmits UV light to the photometric sensor may be used to attenuate UV light so that the characteristic curve of Fig. 6 is in the desired range for providing electronic safety based upon prior calibrations.