Sensitive field effect device and manufacturing method thereof

Abstract

The present invention concerns a sensitive field effect device (100) comprising a semiconductor channel (110), a source electrode (120) connected to said semiconductor channel (110), a drain electrode (130) connected to said semiconductor channel (110), such that said semiconductor channel (110) is interposed between said source electrode (120) and said drain electrode (130), a gate electrode (140) and a dielectric layer (150) interposed between said gate electrode (140) and said semiconductor channel (110), characterized in that said semiconductor channel (110) is a layer and is made of an amorphous oxide and in that said sensor means (170, 171, 172, 173, 174, 175, 175) are configured to change the voltage between said gate electrode (140) and said source electrode (120) upon a sensing event capable of changing their electrical state. The present invention also concerns a sensor and a method for manufacturing said field effect device (100).

Claims

1. A sensitive field effect device comprising; a substrate; and a plurality of ionization sensitive field effect devices, arranged on said substrate, wherein each of said ionization sensitive field effect devices comprises: a semiconductor channel comprising a layer made of an amorphous oxide, a source electrode connected to said semiconductor channel, a drain electrode connected to said semiconductor channel, such that said semiconductor channel is interposed between said source electrode and said drain electrode, a gate electrode, wherein a gate capacity (C.sub.G) of the gate electrode is a function of a gate-source voltage (V.sub.GS) at different frequencies, a dielectric layer interposed between said gate electrode and said semiconductor channel, the dielectric layer arranged to be exposed to an ionizing radiation, the dielectric layer comprising at least one main layer made of at least one material including at least one cation element with an atomic number selected so as to increase an absorption of said incident ionizing radiation to be sensed, wherein said at least one material of said dielectric layer has a high atomic number Z, said atomic number Z being higher than 36 (Z>36), at least one RFID transmitter including an RFID chip connected to the source electrode and the drain electrode of a related field effect device, wherein the RFID transmitter is configured to be remotely connectable to a RFID receiver (RFID-R); wherein said RFID transmitter is configured to change state from a first logic state of high-impedance to a second logic state of low-impedance and is further configured to transmit the state change to said RFID receiver (RFID-R), and wherein the gate electrode is connected to a first terminal of a photodiode and the source electrode is connected to a second terminal of the photodiode and, responsive to receiving light, the photodiode conducts and causes a potential between the gate electrode and the source electrode to vary the RFID transmitter to transmit the state change.

2. The device according to claim 1, wherein said amorphous oxide is a high mobility amorphous oxide type and is selected from a group consisting of: Indium Gallium Zinc Oxide (IGZO) and/or Indium Hafnium Zinc Oxide (IHZO) and/or Zinc Tin Oxide (ZTO) and/or Gallium Zinc Tin Oxide (GZTO).

3. The device according to claim 1, wherein said at least one main layer of said dielectric layer is made of Yttrium Oxide (Y.sub.2O.sub.3) and/or Zirconium Oxide (ZrO.sub.2) and/or Hafnium Oxide (HfO.sub.2) and/or Tantalum Pentoxide (Ta.sub.2O.sub.5) and/or Bismuth Oxide (Bi.sub.2O.sub.3).

4. The device according to claim 1, wherein said dielectric layer is in electrical contact with said source electrode and said drain electrode.

5. The device according to claim 1, wherein said dielectric layer includes a thickness greater than or equal 150 nm.

6. The device according to claim 1, wherein said dielectric layer is a multilayer, comprising one or more insulating layers with an atomic number lower than the atomic number of said main layer.

7. The device according to claim 6, wherein said one or more insulating layers of said dielectric layer are made of silicon-dioxide (SiO.sub.2) and/or aluminum-oxide (Al.sub.2O.sub.3).

8. The device according to claim 6, wherein said dielectric layer comprises the combination of said insulating layer, with lower atomic number atoms, and said main layer repeated from two to ten times, and a top layer of the said insulating layer with lower atomic number atoms.

9. The device according to claim 1, wherein the device comprises at least one substrate, on which said gate electrode and said dielectric layer are deposited.

10. The device according to claim 9, wherein said at least one substrate is flexible.

11. The device according to claim 9, wherein said at least one substrate is made of polyethylene naphthalate.

12. The device according to claim 1, wherein said gate, source and drain electrodes are made of conductive materials, including at least one of: molybdenum (Mo), copper (Cu), aluminum (Al), gold (Au) and/or silver (Ag), conducting oxides, Gallium-doped Zinc Oxide (GZO) or Indium Zinc Oxide (IZO) and/or conducting polymers.

13. The device according claim 1, wherein said field effect devices are arranged as an array or as a matrix.

14. The device according to claim 1, wherein said RFID transmitter comprises an RFID chip and an antenna.

15. The device according to claim 1, wherein the device further comprises a plurality of RFID transmitters, each one being operatively connected to a related field effect device.

16. The sensitive field effect device of claim 1, wherein the photodiode is connected in parallel to a capacitor, and wherein the gate electrode is connected to a first terminal of the capacitor and the source electrode is connected to a second terminal of the capacitor.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The present invention will now be described, for illustrative but not limitative purposes, according to its preferred embodiments, with particular reference to the figures of the enclosed drawings, wherein:

(2) FIG. 1 shows a perspective scheme of the architecture of a single ionization sensitive field effect device according to the present invention;

(3) FIG. 2 shows arrays of ionization sensitive field effect device according to FIG. 1 arranged on a flexible substrate;

(4) FIGS. 3a, 3b and 3c show a simplified model for explaining the ionization damage effect and the related carrier accumulation in the ionization sensitive field effect device of FIG. 2;

(5) FIG. 4 shows a plot with transfer characteristics before and after exposure of X-ray radiation;

(6) FIGS. 5a and 5b shows plots of ionization sensitive field effect device threshold voltage Vt before and after exposure for 1 second to X-ray;

(7) FIGS. 6a and 6b shows the dynamics of the ionization sensitive field effect device threshold voltage recovery after X-ray exposure;

(8) FIG. 7 shows a general electrical connection of the field effect device according to the invention with an RFID transmitter;

(9) FIG. 8 shows a schematic diagram of the electrical connection of the field effect device according to the invention with an RFID transmitter;

(10) FIG. 9 shows a characteristic curve of the drain-source impedance Z as a function of the gate-source voltage, together with the RFID logic state function;

(11) FIGS. 10a and 10b show the transfer characteristics of the field effect device according to the invention and its frequency response;

(12) FIG. 11 shows a circuit diagram of the field effect device according to the present invention adapted to detect ionizing radiations, connected to a RFID transmitter;

(13) FIG. 12 shows a characteristic curve of the drain-source impedance Z as a function of the gate-source voltage, together with the RFID logic state function referred to FIG. 11;

(14) FIG. 13 shows a circuit diagram of the field affect device according to the invention adapted to detect light radiation;

(15) FIG. 14 shows the circuit diagram of FIG. 13 where the field affect device according to the invention is connected to a RFID transmitter;

(16) FIG. 15 shows a characteristic curve of the drain-source impedance Z of the field affect device according to the invention is a function of time, together with the RFID logic state;

(17) FIGS. 16a and 16b show the operation of a circuit diagram of the field affect device according to the invention adapted to detect mechanical stresses;

(18) FIGS. 17a and 17b show a circuit diagram of the field affect device according to the invention adapted to detect temperature variations; and

(19) FIGS. 18a and 18b show a circuit diagram of the field affect device according to the invention adapted to detect chemical substances.

(20) In the various figures, similar parts will be indicated by the same reference numbers.

DETAILED DESCRIPTION

(21) Referring to FIG. 1, a first embodiment of a field effect device 100 according to the invention is shown, wherein details of the device architecture are also specified, for detecting electromagnetic radiations and particularly ionizing radiations, such as X-rays and the like.

(22) The device 100 mainly comprises source, drain and gate electrodes, indicated with reference numbers 120, 130 and 140 respectively, a semiconductor channel layer 110, arranged on a dielectric layer 150, and a substrate 160.

(23) Said source electrode 120, said drain electrode 130 and said gate electrode 140 are made of a conductive material, such as molybdenum (Mo), copper (Cu), aluminum (Al), gold (Au) or silver (Ag), or even conducting oxides, such as Gallium Zinc Oxide, Indium Zinc Oxide.

(24) Said dielectric layer 150 has a thickness greater than 150 nm, so as to prevent the tunneling effects and the relevant leakage currents. This allows also reducing the overall leakage current effects possibly due to other phenomena.

(25) The semiconductor channel layer 110 is arranged between, and in electric contact with said source electrode 120 and said drain electrode 130. Said semiconductor channel layer 110 is made of a high mobility amorphous oxide. In particular, said semiconductor channel layer 110 can be made of Indium Gallium Zinc Oxide (IGZO), Indium Hafnium Zinc Oxide (IHZO), Zinc Tin Oxide (ZTO) or Gallium Zinc Tin Oxide (GZTO).

(26) The high mobility amorphous oxides used for said semiconductor channel layer 110 are fabricated by photolithography. They feature high bandwidth, low parasitic capacitance and minimal gate current, as it can be appreciated from FIG. 2 and are thus compliant to low-voltage operation, particularly with RFID circuits.

(27) Said field effect device 100 comprises also sensor means, so that, upon sensing and event to be detected, the impedance, commonly referred to with the letter Z, of the semiconductor channel layer 110 is varied by over an order of magnitude, thus allowing, for example, the connection of a RFID transmitter, as better explained below.

(28) In the present embodiment, in addition to the above, the semiconductor channel layer 110 is exposed to the radiation to be detected.

(29) The sensing means provided in the field effect device 100 according to the present embodiment comprises multilayer dielectric oxide structure dielectric layer 150, comprising a main layer, combined with one or more further layers with lower atomic number atoms.

(30) Said main layer is a high-Z layer, this meaning that the atomic number is higher than 36. In particular, said main layer is made of at least one of the following materials: Yttrium Oxide (Y.sub.2O.sub.3); Zirconium Oxide (ZrO.sub.2); Hafnium Oxide (HfO.sub.2); Tantalum Pentoxide (Ta.sub.2O.sub.5); Bismuth Oxide (Bi.sub.2O.sub.3).

(31) Said one or more further layers with lower atomic number atoms is an insulating layer that can be for example SiO.sub.2 or Al.sub.2O.sub.3.

(32) The bottom layer of the multicomponent dielectric is said insulating layer with lower atomic number atoms, followed by the main layer. This structure is then repeated two to ten times, followed by a top layer of the said insulating layer with lower atomic number atoms.

(33) The dielectric layer 150 is deposited on said flexible substrates 160 such as Polyethylene Naphthalate, to achieve flexible sensitive field effect device 100. More specifically, in FIG. 2, arrays patterned on polymeric/plastic substrates are shown, comprising a plurality of ionization sensitive field effect devices 100.

(34) In addition to the above, it is seen that the gate electrode 140 is arranged between said flexible substrates 160 and said dielectric layer 150.

(35) The operation of the device 100, based on the accumulation of ionization charges in the multilayer dielectric oxide structure 150 due to X-ray radiation as described above is as follows.

(36) In FIGS. 3a-3c, basic processes how the device operates are schematically depicted.

(37) The formation of ionization charge is initiated by the absorption of a high-energy photon or photoelectron 200 (see FIG. 3a), giving the generation of an electron hole pair 310 in the dielectric layer 150.

(38) The electrons 311 (see FIG. 3b) are collected in a fast process, namely faster than respective holes collection process, at the gate electrode 140.

(39) The remaining hole charges 312 move, as said, slowly in a thermally activated hopping process to the interface with the semiconductor channel 110 (see FIG. 3c).

(40) Thus, as a consequence of X-ray exposure, a larger drain current and a shift in threshold voltage to negative values result.

(41) FIG. 4 shows as an example ionization sensitive field effect device 100 transfer curves, measured in saturation (drain electrode 130 voltage VD=20 Volts) before and after exposure to a total dose of 60 mGy (in the air; Gy is Gray unit measure) emitted by a molybdenum (Mo)-tube as a X-ray source, operated at a dose rate of 60 mGy/s.

(42) The transfer characteristics show a shift of 0,326 V to more negative gate potentials, after X-ray exposure (reference to FIG. 3a state). No other transistor parameter, such as sub-threshold slope or mobility is influenced by the exposure.

(43) The shift is a consequence of the positive carriers accumulation at the interfaces between the dielectric layer 150 and the semiconductor channel 110 (see FIG. 3c). Positive carriers accumulation influences the voltage between said source electrodes 120 and said gate electrode 130 at fixed current flux. By measuring the variation of the gate potentials, it is possible to infer the amount of X-ray dose to which the device 100 has been exposed to.

(44) Sensitivity of the device 100 to ionization depends crucially on the composition and thickness of the high-Z dielectric multilayer 150.

(45) Low X-ray attenuation length (λ<60 μm) in this layer is achieved due to the high atomic number (e.g. using Tantalum Oxide), whereas an oxide layer (e.g. SiO.sub.2) serves to maintain good isolating and interfacial properties. Increasing the thickness of multilayer dielectric oxide structure 150 allows to further improve the X-ray response, as demonstrated in FIG. 5a. This figure shows how, by increasing the thickness of the dielectric layer 150 from 114 nm to 381 nm, the threshold shift increases by a factor of 6 as well.

(46) Furthermore the ionization sensitive field effect device 100 according to the invention, allows to perform quantitative dosimetry by monitoring variations in threshold, as it offers a linear dependency between exposure dose and threshold shift for total doses remaining below 100 mGy.

(47) This finding is shown in FIG. 5b, which shows the variation of threshold voltage caused by exposure to X-ray 200 of varying dose-rates for a period of 1 sec. The inset shows the resulting shift in threshold voltage as a function of dose. The linear fit results in a sensitivity of 5.5 mV/mGy.

(48) The device 100 operates as an integrating dosimeter and maintains the threshold voltage after the X-ray exposure event.

(49) FIGS. 6a and 6b displays a recovery measurement, in which the threshold voltage was monitored for 22 hours after an exposure to a dose of 21 Gy. The curve follows a stretched exponential with a characteristic time-scale of a few hours.

(50) From these data, it is possible to calculate the time in which readout of the device 100 has to be carried out, to keep the error due to recovery drift below 10%. Due to the stretched exponential behavior, this time interval depends on the detected dose with longer retention times being observed at lower doses. From these data, defining a maximum readout time, which is plotted in FIG. 6b as a function of the radiation dose, is possible.

(51) The maximum readout time interval is defined here as the time after an X-ray exposure event in which the drift in threshold voltage remains below 10%.

(52) In the range where the device 100 shows a linear response, the maximum readout time exceeds 300 sec. This long retention time makes even the sequential readout of a large detector array feasible.

(53) The manufacturing of an ionization sensitive field effect device 100 comprises the following main steps.

(54) The gate electrode 140 is deposited on the flexible substrates 160 using materials processing via physics routes such as sputtering or chemical routes, such as solution based processes, such as screen printing, sol gel, among others. Patterning of gate electrodes 140 is performed with photolithography and etching processes or lift-off.

(55) The dielectric layer 150 can be deposited using different techniques such as RF-sputtering, from two separate targets containing SiO.sub.2 and Ta.sub.2O.sub.5 or via solution-based processes, containing the elements above mentioned. Both processes can be run in parallel. For the sputtering case, a mechanical shutter was used, so as to create the multilayer structure of the dielectric layer 150, by alternating depositions of SiO.sub.2 only and co-depositions of SiO.sub.2 and Ta.sub.2O.sub.5. For solution route method, the thickness was determined by the number of deep steps (e.g. sol gel deep coating), or by the amount of material spread over the surface (e.g. screen printing). Patterning of dielectric layer 150 is performed with photolithography and etching processes or lift-off.

(56) The subsequent step is to deposit a high mobility amorphous oxide semiconductor channel 110 on the dielectric layer 150, using materials processing via physics routes, such as sputtering or chemical routes, such as solution based processes (e.g. screen printing, sol gel, among others). Patterning of oxide semiconductor channel 110 is performed with photolithography and etching processes or lift-off.

(57) Finally, said source electrode 120 and said drain electrode 130 are deposited using materials processing via physics routes such as sputtering or chemical routes, such as solution based processes, such as screen printing, sol gel, and the like, after a photolithography process. Said source electrode 120 and said drain electrode 130 are patterned using lift-off.

(58) Field effect device 100 can be also connected to a RFID transmitter. In particular, referring to FIG. 7, general electrical connection of the field effect device 100 according to the invention is seen, with an RFID transmitter 400, properly integrated together with said device 100.

(59) Said RFID transmitter 400 comprises a RFID chip 401, connected to the source electrode 120 and the drain electrode 130, namely it is parallel connected to the semiconductor channel layer 110, and an antenna 402 is in its turn connected to said RFID chip 400. In the figure, typical levels of impedance Z determining the logic states are also shown.

(60) In addition, FIG. 8 shows the connection between the field effect device 100 with a RFID transmitter 400, whose RFID chip 401 is connected between the source 120 and the drain 130 of the field effect device 100, where the RFID transmitter 400 is remotely connected to a RFID receiver RFID-R, through the antenna 402.

(61) Upon a sensing event, the potential V.sub.GS between the gate terminal 140 and the source terminal 120 is subject to a variation. Then the semiconductor channel layer 110 impedance Z varies according to a characteristic curve like the one plotted in FIG. 9.

(62) The RFID transmitter state is then switched from high-impedance (0 logic state) to low-impedance (1 logic state), as shown in said FIG. 9.

(63) As it can be seen, upon the sensing event the semiconductor channel layer 110, the impedance Z decreases considerably, allowing the change of the logic state, if measured in voltage, between the source 120 and the drain 130 electrodes of the field effect device 100.

(64) FIGS. 10a and 10b show the transfer characteristics of a field effect device 100 and its frequency response. In particular, in said FIGS. 10a and 10b there are respectively plotted the drain currents and the gate currents as a function of gate-source voltage, at different drain voltages, and the gate capacity (in picoFarad) C.sub.G as a function of the gate-source voltage V.sub.GS, at different frequencies.

(65) Upon a sensing event, the voltage at the gate electrode 140 remains constant, thus leaving unaltered the impedance Z probed by the RFID chip 401. In this way, the occurrence of the sensing event is stored in a passive memory state, as better explained below, where some applications of the field effect device 100 are examined, particularly directed toward the application of the RFID devices, for which, as it is well known, high sensitive devices are required to improve the detection sensitivity.

(66) Indeed, in many applications it is necessary to guarantee that environmental conditions for example in a working place, in a storage room, in a technical engine (motor, computer) or in a parcel remain within a certain tolerance interval to avoid a possible health risk or damage.

(67) For the applications described below the following parameters are relevant: sufficiently high on/off ratio that matches the impedance requirements of typical RFID chip 401; steep sub-threshold slope, which determines the width of the transition region, in which the sensor state is not well defined; low parasitic capacitance to realize a high impedance state also at high operation frequencies; a low leakage current (<10 nA/cm.sup.2), to realize stable charge storage; and a high cut-off frequency (>30 kHz) to realize a low-impedance state at RFID chip 401 frequencies.

(68) From the polarization standpoints, it is apparent that the device 100 is operated close to threshold, therefore even small voltage variations normally between the gate electrode 140 and the source electrode 120 lead to large variations in semiconductor channel 110 impedance that are directly detected by the RFID sensor 400 terminals.

(69) FIG. 11 shows the field effect device 100, according to the first embodiment, where a RFID transmitter 400 is connected between the source electrode 120 and the drain electrode 130, then parallel with the semiconductor channel layer 110. The source electrode 120 is also connected with the gate electrode 140. An ionization radiation beam, X-rays in the figure, beams on the semiconductor channel layer 110, causing the logic state switch, as shown in FIG. 12, due to the impedance increase of the semiconductor channel layer 110.

(70) This logic switch configuration is read by RFID transmitter 400, which transmits an RFID receiver RFID-R, by the related antenna 402.

(71) Referring to FIG. 13-15, a second embodiment of the field effect device 100 is shown, where the sensing means comprises a photodiode 170 and a capacitor 171, both parallel connected to said gate electrode 140 and said source electrode 120.

(72) When a light beam, having a wavelength within the visible band or in the nearby of the same (e.g. UV or IR bands), reaches said photodiode 170, the latter conduces, so that the potential between the gate electrode 140 and the source electrode 120 varies. FIG. 15 shows the variation of the impedance of the semiconductor channel layer 110 and the consequent RFID chip 401 logic state switch as a function of time.

(73) FIGS. 16a and 16b show a third embodiment of the invention, wherein the sensing means of said field effect device 100 according to the invention comprises a capacity 171, connected between said gate electrode 140 and said source electrode 120, and a rectifier diode 172, series connected with a piezoelectric sensor 173, connected to said gate electrode 140 and to said source electrode 120.

(74) In case of a mechanical stress is applied to the piezoelectric sensor 173, the gate-source potential changes, causing a variation of the impedance of the semiconductor channel layer 110, thus allowing the logic switching off the field effect device 100.

(75) Referring now to FIGS. 17a and 17b, a fourth embodiment of the invention is shown, in which sensing means comprises a capacitor 171 and a pyroelectric sensor 174, capable of sensing the environment temperature. The capacitor 171 and said pyroelectric sensor 174 are series connected.

(76) Said sensing means are connected between the gate electrode 140 and the source electrode 120, as the other embodiments above described.

(77) The operation of this embodiment is the analogous of the others. Variation of the gate-source potential V.sub.GS causes the variation of the impedance Z of the semiconductor channel layer 110.

(78) Referring to FIGS. 18a and 18b, a fourth embodiment of the invention is shown, wherein the sensor means now comprises a capacitor 171 and a chemical sensor 175. The connection of said chemical sensor 175 and said capacitor 171 is the same the third embodiment described above.

(79) Also the electrical operation is the same of the third embodiment shown in FIGS. 17a and 17b, with the apparent difference that in that case it is a chemical substance detection, as sensing event, that changes the electrical state of the sensor and then the gate-source voltage, so as to change the impedance between said source electrode 120 and said drain electrode 130.

(80) As it can be seen, in all the embodiments described above, a RFID chip 401 can be connected between said source electrode 120 and said drain electrode 130, so as to detect the variation of the potential between said electrodes, in transmitting the states change, namely the detection of ionization radiation, light, mechanical stress, variation of temperature and detection of chemical substances to a RFID receiver RFID-R.

(81) It is seen that even the above mentioned passive components, as the capacitor 171, as well as the different sensors, can be integrated in a single device.

(82) In addition, the several above mentioned sensing means can be combined each other.

(83) An advantage of the field effect device according to the invention is that of enabling to create low-cost RFID sensors capable of monitoring if certain environmental conditions (ionizing radiation, light, temperature, chemicals, mechanical forces) go beyond a defined threshold. Once conditions are beyond threshold, the RFID sensor state is changed and the event can be traced at a later time using a wireless RFID readout systems.

(84) A further advantage of the device according to the invention is that of introducing innovative nanostructured materials allowed to combine unique electronic sensor performance, with the possibility to deposit sensors in microstructured arrays on large-areas and on flexible plastic substrates.

(85) The present invention has been described for illustrative but not limitative purposes, according to its preferred embodiments, but it is to be understood that modifications and/or changes can be introduced by those skilled in the art without departing from the relevant scope as defined in the enclosed claims.