FR2860339A1 - Electron emitter device for electronic display, has current collection unit that is insulated from extraction grid and collects current from micro-emitter, and polarization units, one polarizing grid and another polarizing cathode - Google Patents

Abstract

The device has a current collection unit (40) that is insulated from an extraction grid (26) and collects current transmitted from a micro-emitter (24). Polarization units (34, 23) polarize respectively the extraction grid and a cathode. An anode (36) controls the current emitted by the micro-emitter, based on the measurement of the emitted current.

Description

DEVICE FOR MEASURING THE INTENSITY OF ELECTRON BEAMS EMITTED BY A

MATRIX OF INDIVIDUAL SOURCES

  DESCRIPTION TECHNICAL FIELD AND PRIOR ART

  The invention refers to the field of micro-electron emitters. These micro-sources are used, for example, in the new electronic display techniques or in new applications for direct writing in nanolithography. The associated devices use in particular cold sources such as metal micropoints or silicon or even more recently NTC (carbon nanotubes).

  The operating principle of a micro-source of electrons is shown in FIG.

  The cold emission is based on the principle of extraction by field effect in a vacuum chamber. An Icath current is supplied to the cathode 6. A tunnel effect allows the electrons to be extracted from the emitter (top of the tip 2) in the vacuum, using the Vgext extraction voltage applied to the gate The emitters working in cold emission are considered as voltage-controlled current sources, the flow of emitted electrons obeying the Flower-Nordheim equations.

  The intensity of the electron beams generated is often the seat of instabilities (especially for metal emitters). These instabilities in time can be minimized by adapting the control type of the micro-transmitter, for example with a

D

  controlled device with a current source 10 constant in time (Figure 2) between the transmitter 2 and the cathode 6. In this case, transient operation at high frequency is hardly possible. As regards the spatial inhomogeneities between the emitters, a resistive layer 13 may be introduced at the cathode 6 to standardize the currents emitted by the different emitters.

  These devices can therefore be controlled either by current or by voltage.

  Moreover, in certain areas, and in particular in the field of nanolithography, the positioning on the target (focusing) and the number of electrons emitted must be precisely controlled.

  Several types of focusing are used and can be complementary with the magnetic and / or electrostatic focusing with additional grids, especially available in microtechnologies.

  The control of the number of electrons emitted is generally done by a current control during the emission, or more generally by a direct control of the charges emitted. This control is generally performed on the current emitted by the cold cathode, at the actual level of the emitter 2, benefiting from the fact that these structures are collectively constructed on silicon wafers on which electronic processing can develop.

  The document EP 1249855 illustrates this concept of measurement of current being done in the foot of the cathode, which makes it possible to control in voltage several grids on the path of the electrons (extraction grid and focusing grid).

  The prior art with measurement of the current or the number of electrons emitted by the cathode (in order to control the dose of electrons received by the anode) has limitations: a fraction of this emitted current can be derived by the extraction grid 8, - a significant leakage current can exist between the cathode 6 and the extraction grid 8, the capacitive currents flowing in the cathode 6 during the switching of the extraction grid can also distort this dose control.

  DISCLOSURE OF THE INVENTION The invention aims to solve these problems.

  The invention relates to an electron-emitting device, comprising a substrate, a cathode, electron-emitting means, for example one or more nanotubes or one or more micro-tips, an extraction grid, an anode, means current collection, isolated from the extraction grid and arranged to collect a portion of the current emitted by the emitter means, means for measuring this portion of current emitted, and means for controlling, according to a measurement of this part of the current emitted, polarization means of the electron emitting means, namely polarization means of the extraction grid and / or the cathode.

  According to the invention, the electron extraction function (by the assembly consisting of the extraction grid and the cathode conductor) is separated from the anode current measurement function. Collection means, for example an electrode or a collection grid, are placed in alignment with the anode beam.

  It is therefore possible to measure a known fraction of the emitted current and to deduce the current sent to the anode.

  The current collection means, which can also provide a focusing function, make it possible to collect a fraction of the anode current in order to control the dose of electrons actually sent to this anode. Such a device thus makes it possible to overcome the limitations of the prior art, in particular the current possibly derived by the extraction grid and leakage currents between the cathode and the extraction grid.

  The invention therefore particularly relates to a quadrupole electron emitter device, in which the control of the number of electrons emitted towards the anode is achieved using means, for example a grid, independent of the extraction grid of the electrons. electrons.

  The collection means comprise for example one or more electrodes or collection grids.

  The polarization means of the extraction grid or the cathode can advantageously operate in pulses.

  Advantageously, the substrate is a CMOS substrate, in which case electrical feedthroughs may make it possible to connect the collection means and the extraction grid to the CMOS substrate.

  The current collection means, for the purpose of measuring it, are for example separated from the extraction grid by a layer of dielectric material.

  According to another particular embodiment, the collection means are connected to the conventional transmission device by electrical and mechanical interconnection means such as a micro-ball or a pillar.

  The current measuring means may be located at the level of the substrate of the conventional emission device or at the level of the substrate used for producing the collection means.

  The current measuring means may comprise an amplifier on which a capacitor or resistor is mounted in feedback. A current mirror measurement setup is also possible.

BRIEF DESCRIPTION OF THE FIGURES

  1-3 illustrate prior art devices; FIGS. 4-10C illustrate embodiments or specific aspects of devices and methods according to the invention.

  DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION A first example of an embodiment of the invention is shown in FIG.

  According to this example, an electron-emitting device according to the invention comprises a conventional emission device formed of a base support 15 on which is developed a first structure, or first transmission means 22. structure comprises a cathode 20, a micro-emitter 24 (tip or nanotube) and a first extraction grid 26, the extraction gate-cathode distance being regulated by the thickness of a dielectric 28, which is for example the order of the micrometer. Polarization means 34 and / or 23 make it possible to bias the extraction grid and the cathode respectively and thus to control the current emitted by the micro-transmitter.

  The device of the invention also comprises collection means 40, for example having an electrode or a collection gate, can be positioned above the emission site. They are connected to means 42 for measuring the current.

  These collection means are placed in the path of the emitted electrons in order to take a part and allow the passage of the rest of the emitted electrons to the anode. For this, orifices (or openings) are provided at these collection means. These orifices may be circular, oval or rectangular, they may also have other advantageous geometries. As illustrated in FIGS. 10A and 10B, they may also have the shape of circular sectors 100, 102, 104 or the shape illustrated in FIG. 10C (indented circle).

  Depending on the geometry chosen and the polarization applied, it is possible to determine, according to the classical laws of optics and electromagnetics, the part of the electrons collected and that of the electrons actually transmitted towards the anode. Thus, the measurement of the collected current will give a precise indication of the electrons arriving on the anode (and therefore of the emitted dose).

  With respect to a circular orifice, these cut-out orifices of FIGS. 10A-10C make it possible to collect electrons at several electron beam levels and not only at the edges of the beam, thus making it possible to be less sensitive to the inhomogeneities that may appear on the electron beam. edges.

  These orifices typically have, depending on the application envisaged, diameters of the order of a few tenths of a micron to a few microns for the monolithic devices (FIG. 7), diameters of the order of a few microns to a few tenths of a micron. for hybrid devices (FIGS. 8A, 8B).

  The means 40 are isolated and separated from the grids 26 by means of a dielectric 32 in the embodiment of FIG. 4.

  The adjustment variables of these means 40 are for example the diameter of the hole 41, if it is an electrode seen as a diaphragm, or its polarization, if it is seen as a refocusing electrode. The polarization level of these means will be for example a few volts above the polarization level of the cathode.

  The current collection means 40 are positioned in the transmission axis, the distance relative to the first extraction grid 26 being regulated by the thickness of the dielectric layer 32.

  By placing the current measurement means 42 (ammeter) in the supply circuit of the collection means, it is possible to measure the electron beam, or a quantity proportional to the anode current, and to interact with the current of the micro-transmitter, either via the control of the extraction grid 26 and / or via the control of the cathode 20. An adjustment can be made using means 36 of counter-reaction. These feedback means may for example be composed of a voltage current converter associated with an amplification module and, if necessary, an inverter. They thus make it possible, from the current collected at the level of the collection gate, to establish the voltage to be applied at the level of the cathode and / or the extraction grid. The invention thus makes it possible to use anode current control and regulation means separated from the extraction grid.

  The grids 26 are of metal type. More generally they are conductive (for example Polycrystalline Silicon).

  The emitter tips 24 are conductive, for example silicon or molybdenum.

  The extraction grid 26 has for example a thickness of a few hundred nm to a few micrometers.

  The thickness of the dielectric 28 is typically of a few hundred nm (for example between 0.4 and 0.7 μm).

  The dielectric 32 may have a thickness of between 1 μm or a few micrometers and a few mm, for example 5 mm.

  The thickness of the collecting gate 40 may vary between, for example, 0. 1 pm (integrated embodiment) to a few hundred micrometers (in hybrid embodiment), for example 500 pm.

  The distance between the substrate 20 and the anode 36 is about 1 mm for the intended application.

  It can vary from 10 pm to 10 mm depending on the application.

  A first voltage generator 34 for example establishes a positive ddp between the first extraction grid 26 and the cathode 20 to allow the electrons to escape from the tip in a vacuum. The electron beam is oriented towards the anode 36 with a certain angular opening. To collect the electrons, the anode 36 is for example increased to a few hundred volts positively. The means 40 collect electrons, which the means 42 convert into current measurement, information that the means 36 can use to regulate the extraction of electrons as a function, for example, of a set value of the emitted current.

  The operating frequencies of the source are preferably in the high frequency range, above 1 Mhz.

  The physical realization of the known microwires according to the prior art imposes non-ideal structures, as represented in FIG. 5A. Spurious capacitances between the tip 24 and the gate 40 induce, in particular, large displacement currents at the moment of switching.

  In the case of a control by the extraction grid (FIG. 5A), the cathode potential is maintained at a constant voltage, the potential of the extraction grid is pulsed between a high level and a low level (see FIG. the voltage Vg on the timing diagram of FIG. 5B). The high level corresponds to a period during which the micro-transmitter emits, the low level corresponds to a period during which the micro-transmitter does not emit (see the anode current in Figure 5B).

  According to the invention it is possible, from the current Ig collected at the collection gate (proportional to the anode current in its central part), to act on the potential of the extraction gate to modulate the emission of the micro-transmitter.

  This can be done either by modulating the high level of the voltage Vg, or by changing the duration of transmission by varying the duration of this high level.

  It can be seen in FIG. 5B that, at the time of the commutations of the potential of the extraction grid, significant current peaks transiently at the level of the current of the collection gate. It may therefore be interesting to defer the measurement of the collection current so as to avoid disturbances related to these switches.

  Figures 6A and 6B illustrate a device similar to the device of Figures 5A and 5B but in this case, the control of the micro-transmitter is governed by the cathode. The potential of the extraction grid is therefore constant while the cathode potential is pulsed between a high level and a low level, the latter level corresponding to the emission period of the micro-transmitter.

  According to the invention, it is possible from the collected Ig current to act on the potential of the cathode (Vcathode) to modulate the emission of the microemitter. This can be done by modulating in amplitude or duration the low level of the cathode voltage.

  It can be seen in FIG. 6B that the current collected in this case is less sensitive to switching of the cathode voltage than in the previous case.

  Whatever the embodiment envisaged, it is possible, as illustrated in FIG. 7, to produce the device of the invention comprising a micro-transmitter 24, comprising a cathode, an extraction grid 26 and a collection gate on a substrate. CMOS 60. Electrical bushings 50, 52 are made to connect the extraction gates 26 and the collection gates 40 to the CMOS substrate 60, in which current control and gate control blocks are located. These processing units use a mixed LV / HV technology (low voltage / high voltage), the control and control is done in LVCMOS and the control of the emission in HVCMOS. A collective manufacturing method makes it possible to align the collection grid 40 on the emissive tip 24.

  The collection gate may, as illustrated in FIG. 8A, be attached above a conventional transmission device, an electrical connection and a mechanical support can be made by means of hybridization means, for example a micro-wave. ball 70 or any other means of interconnection (pillar, via, ...). In fact, the grid or collection means are connected by the means 70 to a conductive zone 71 located in the conventional emission device at the extraction grid but isolated in this extraction grid by the insulating zone. 27 (for example SiO2). In this case, there is no more dielectric between the gate 26 and the collection means 40, the hybridization means 70 for maintaining a gap between these elements which ensures, combined with the insulating zone 27, the effect insulation between them.

  In the case illustrated in FIG. 8A, it is a passive collection gate, where the current measurement means 42 and the treatment of the collected current are located in the CMOS substrate 60.

  Hybridization beads 70 are for example composed of fusible alloys of metals. The beads may be circular, oblong or any other form, in the form of buffer or mushroom in particular.

  The height of these hybridization balls 70 makes it possible to control the spacing between the electrode 41 and the substrate which contains the emission means 24. The hybridization beads preferably have micrometric dimensions, these microbeads preferably having a size of between one micrometer and several hundred micrometers.

  Such hybridization means make it possible to maintain a fairly precise spacing distance between the means 40 and the emission gate 26, typically of the order of a few hundred microns, and this, with a precision of the order of a fraction of microns.

  The hybridization techniques also make it possible to control the superposition alignment of the diaphragm apertures 41 with respect to the emitting sources 24, which allows a self-alignment between the electron beam emitting means and the means 40, before proceeding to fix the whole. The fixing is carried out for example by buffering and / or by partial welding of the fusible alloy hybridization beads according to the hybridization techniques, for example described in Electronic Production and Test-Advanced Packaging p32-34 April 1999.

  As illustrated in FIG. 8B, a silicon wafer can be used as a substrate for producing the collection gate. This substrate can then also be used to perform, at the same level as the collection grid, the current measuring means and associated processing. We can talk about active collection grid.

  An advantage of this variant is to increase the available area to achieve the electronic processing blocks and especially to differentiate the low voltage analog portion at the silicon substrate of the gate 40 collection, and the analog portion 34 of high voltage switching at base silicon level 60, thus limiting, inter alia, interference problems between these two parts and also allowing the use of two substrates completely different technologies.

  The example of FIG. 9A illustrates the means for measuring the current of a measurement signal which is amplified by an amplifier 80 on which a capacitor 82 is mounted in counter-reaction. It is then possible to convert the measured current into voltage, which is more easily exploitable with a limited number of components (CTIA). The variation of the output voltage is then expressed by: AVS (G) - Igate * T Cfb where T represents the integration time of the current, or the analysis time. This structure is not very sensitive to rapid changes in the current. The value of the capacitor 82 is for example of the order of 10 fF, which leads to sensitivities of the order of 20 pV / electron.

  The structure illustrated in FIG. 9B, with a counter-reaction resistor 84, makes it possible to represent instantaneous variations of output voltage on instantaneous variations of the input current. The variation of output voltage is expressed in this case.

  Finally, FIG. 9C illustrates an assembly with current mirror measurement: an image of the collection gate current Ig can be used to generate a difference current Iref -Ig, which can be exploited.

  A device according to the invention, whatever the embodiment envisaged, makes it possible to compensate for technological non-uniformities in space or the non-uniformities of the known electron sources.

  The various embodiments described above illustrate a single emitting tip 24, but the invention applies to a set of emitters or emitting points, for example arranged in a matrix, each emitter being provided with means such as the means These means may, depending on the case, be common to a set of transmitters. It is thus possible to make an array of individual sources operating in the manner described above and with the advantages disclosed.

Claims (16)

  An electron emitting device comprising a substrate (15, 60), a cathode (20), electron emitting means (24), an extraction grid (26), an anode (36), means (40) current collection, isolated from the extraction grid and arranged to collect a portion of the current emitted by the emitter means, means (42) for measuring the collected current, and means (36) for controlling , as a function of a measurement of the collected current, the current emitted by the electron emitting means.   2. Device according to claim 1, the electron emitting means comprising at least one microtip or nanotube.   3. Device according to one of claims 1 or 2, the means (40) of current collection comprising at least one electrode or a collection gate.   4. Device according to one of claims 1 to 3, the current control means emitted by the electron emitting means comprising means (34) of polarization in pulses of the grid (26) of extraction.   5. Device according to one of claims 1 to 3, the means for controlling the current emitted by the electron emitting means comprising means (23) for biasing the pulses of the cathode.   6. Device according to one of claims 1 to 5, the substrate being a CMOS substrate (60).   7. Device according to claim 6, electrical bushings for connecting the collection means (40) and the extraction grid (26) to the CMOS substrate (60).   8. Device according to one of claims 1 to 7, the collection means being isolated from the extraction grid (26) by a layer of dielectric material (32).   9. Device according to claim 8, the layer of dielectric material having a thickness of between 0.1 μm and 500 μm.   10. Device according to one of claims 1 to 7, the means (40) of collection being connected by means (70) of electrical and mechanical interconnection such as a micro-ball or a pillar to a conductive zone (71). ).   11. Device according to claim 10, the means (42) for measuring current being located in the substrate.   12. Device according to one of claims 1 to 10, the means (40) for measuring current being formed on a substrate on which are located the collection means.   13. Device according to one of claims 1 to 12, the current measuring means (42) comprising an amplifier (80) on which a capacitor (82) or a resistor (84) is mounted in feedback.   14. Device according to one of claims 1 to 12, the means (42) for measuring current comprising a current mirror measurement mounting.   15. Device according to one of claims 1 to 14, the collection means comprising an orifice (41).   16. Device according to claim 15, the orifice being circular or having circular sectors (100, 102, 104).