EP1473755B1 - Device and method for controlling a dose of electrons emitted from a microemitter - Google Patents

Description

TECHNICAL AREA

The present invention relates to a device and a method for controlling and controlling a dose of electrons emitted by a micro-transmitter, for example by a microtip.

STATE OF THE PRIOR ART

In the remainder of the description, micro-transmitters of the micropoint type will be considered as nonlimiting examples.

The field of microtips, joined today by that of nanotubes, defines a field of applications both in the field of FED ("Field Emission Display") displays and that of micro-transmitters, in which the requirements in terms of order and control of emitted flows are very severe.

In the case of hot emission (diodes, triodes, cathode ray tubes), the electrons acquire, by thermal agitation, a sufficient energy (called "work output") to reach above the potential barrier, which holds them back. to the nuclei. They then move towards the surface of the material and, if there is an electric field that attracts, they can be extracted from this material. At ordinary temperature, the thermal stirring energy is insufficient for the electrons to leave the material.

In the case of the cold emission, which is based on the principle of a field effect control in a vacuum chamber, a tunnel effect allows the electrons to be extracted from the emitter (cathode) in a vacuum, then to be collected on an anode. Transmitters working in cold emission are considered as voltage-controlled current sources, the flow of electrons emitted obeying the Fowler-Nordheim equations.

This is for example the case of a microtip 10 Tungsten, used in electron emitter. His electrical diagram is represented on the Figure 1A . An electron flow is established between the anode 11 and the cathode 12. A control voltage is applied between the extraction grid 13, called gate, and the cathode 12. Figure 1B presents the behavioral symbol of such a microtip 10 usable with a generic electric simulator ("Spice" type).

The emission regime of such a microtip 10 is characterized by a strong non-linearity of the emission current I _tip as a function of the voltage applied to the extraction grid 13. The tip current I _tip meets the law : $${\mathrm{I}}_{\mathrm{tip}}\mathrm{=}{\mathrm{at}}_{\mathrm{<figure-callout\; id="2"\; label="fn\; V"\; filenames="imgf0004.png"\; state="\{\{state\}\}">fn\; </figure-callout>}}{\mathrm{V}}^{}{\mathrm{}}^{\mathrm{2}}\exp {{}^{\mathrm{-}{\mathrm{b}}_{\mathrm{fn}}}\mathrm{/}}_{{\mathrm{V}}_{\mathrm{gc}}}$$

The coefficients a _fn and b _fn depend on the geometric characteristics of the microtip. Such a current-voltage characteristic is illustrated on the figure 2 . An example of an operating point (I _tip = I _on for V _gate-cathode = V _on ) is represented in this figure. The ideal characteristic is referenced 14.

In reality, such a feature is not reproducible from one microtip to another. Curves 15 shown in dotted lines are thus obtained.

One of the drawbacks of the cold emission is therefore to reveal a certain instability in the current value, which is equivalent to a noise that is generated by fluctuations in the output work inherent in local surface contaminations. These fluctuations vary from one microtip to another and are also variable over time, for the same microtip.

• une commande en courant par un dispositif de régulation de courant : une telle possibilité est utilisée dans les FED (« Field émission display ») via un transistor simple ou « multigate » situé en série dans le circuit de la cathode, comme décrit dans les documents référencés [1] et [2] en fin de description. Le courant émis par chaque micropointe peut théoriquement être programmé. Il est indépendant de la qualité et des caractéristiques de chaque micropointe. D'une micropointe à une autre, ou dans le temps, c'est la tension V_gc qui est modulée. Un des défauts d'une telle commande est de mixer au niveau du circuit de commande et de contrôle du transistor une tension basse (LV) et une tension haute (HV), parce que l'électrode d'extraction doit être portée à quelques dizaines de volts. L'affichage visuel s'accommode de la précision et de la fréquence de fonctionnement limitées de ce type de commande.
• une commande en tension : si l'on n'y prend garde, c'est le courant d'émission qui est modulé, ce qui peut être inacceptable pour certaines applications. Dans la mesure où l'excursion en courant est connue, notamment les extremums, et où la quantité à contrôler est la charge électrique, une telle solution est satisfaisante lorsqu'elle est conjuguée à une fenêtre temporelle d'observation variable, T_nom. $$\mathrm{Q}\mathrm{=}{\mathrm{I}}_{\mathrm{nom}}\mathrm{*}{\mathrm{T}}_{\mathrm{nom}}\mathrm{=}\mathrm{2}{\mathrm{I}}_{\mathrm{nom}}\mathrm{*}\frac{{\mathrm{<figure-callout\; id="2"\; label="T\; nom"\; filenames="imgf0004.png"\; state="\{\{state\}\}">T</figure-callout>}}_{\mathrm{nom}}}{\mathrm{2}}\mathrm{=}\frac{{\mathrm{I}}_{\mathrm{<figure-callout\; id="2"\; label="nom"\; filenames="imgf0004.png"\; state="\{\{state\}\}">nom</figure-callout>}}}{\mathrm{2}}\mathrm{*}\mathrm{2}{\mathrm{T}}_{\mathrm{nom}}$$
  

Two types of microtip commands are possible:

• current control by a current regulation device: such a possibility is used in the FED ("Field emission display") via a single transistor or "multigate" located in series in the cathode circuit, as described in the documents referenced [1] and [2] at the end of the description. The current emitted by each microtip can theoretically be programmed. It is independent of the quality and features of each microtip. From one microtip to another, or in time, the voltage V _gc is modulated. One of the faults of a such control is to mix at the level of the control circuit and control of the transistor a low voltage (LV) and a high voltage (HV), because the extraction electrode must be increased to a few tens of volts. The visual display accommodates the limited accuracy and operating frequency of this type of control.
• a voltage control: if we are not careful, the emission current is modulated, which may be unacceptable for some applications. Insofar as the current excursion is known, especially the extremums, and where the quantity to be controlled is the electric charge, such a solution is satisfactory when it is combined with a variable observation time window, T _nom . $$\mathrm{Q}\mathrm{=}{\mathrm{I}}_{\mathrm{last\; name}}\mathrm{*}{\mathrm{T}}_{\mathrm{last\; name}}\mathrm{=}\mathrm{2}{\mathrm{I}}_{\mathrm{last\; name}}\mathrm{*}\frac{{\mathrm{T}}_{\mathrm{<figure-callout\; id="2"\; label="last\; name"\; filenames="imgf0004.png"\; state="\{\{state\}\}">last\; name</figure-callout>}}}{\mathrm{2}}\mathrm{=}\frac{{\mathrm{I}}_{\mathrm{<figure-callout\; id="2"\; label="last\; name"\; filenames="imgf0004.png"\; state="\{\{state\}\}">last\; name</figure-callout>}}}{\mathrm{2}}\mathrm{*}\mathrm{2}{\mathrm{T}}_{\mathrm{last\; name}}$$
  

The device of the invention is a circuit of this type, which is naturally faster and whose linearity defects noted are corrected, the HV extraction gate control circuits being independent of LV circuits for controlling the electric charge, which simplifies the implementation of the circuit and reduces the sensitivity to noise.

• En boucle ouverte dans le cas d'une calibration séquentielle, puis de la programmation d'un certain nombre de mesures avec une même référence comme illustré sur la figure 3A.
• En boucle fermée dans le cas d'un asservissement du courant en temps réel comme illustré la figure 3B, et comme décrit dans le document référencé [3].

Several solutions are therefore possible to measure the quantity of electrons emitted by a microtip. In some cases, as illustrated in the Figures 3A and 3B it is possible to work in current regulation. The emission during a certain time of a calibrated current (generator 16) makes it possible to delimit an electric charge according to the law Q = It Such a current regulation system comprises a sensing element for detecting the peak current 17, an element of control of the reference current 18 and a current adjusting element 19. This system can operate:

• Open loop in the case of a sequential calibration, then programming a number of measurements with the same reference as shown in the figure 3A .
• In a closed loop in the case of real-time current control as shown in FIG. figure 3B and as described in referenced document [3].

In the illustrated embodiment on the figure 3A , the specifications of the system must allow sequential persistence of the necessary instants to carry out the calibrations. Such an embodiment does not make it possible to correct imperfections of the electron beam whose frequency of recurrence is greater than the refreshing frequency of the calibrations.

In the illustrated embodiment on the figure 3B the stability of the feedback port is essential and must be guaranteed most often at the cost of active compensation of the bandwidth of the looped system, and therefore to the detriment of its performance in speed.

The requirements in terms of speed, stability, noise, and linearity do not allow, in many applications to use such achievements.

A global method for conducting a low electrical charge check consists, by using a few configuration input variables, in defining the desired quantity of charge, interrupting the electron beam when such a desired dose has been reached ( "Dose control"). In this case the quantity of electrical charges is defined a priori. The device allowing such a control must operate on a peak current dynamics, including in particular the fluctuations of the current in time for the same microtip. Such a method theoretically allows a very good linearity. However, the use of real functional modules and the requirement of high frequency operation results in strong nonlinearities of the controlled electric charge as a function of the current regime.

A document of known art, referenced [4] at the end of the description, describes a two-dimensional array of miniature cathodes used as electron beam emitters, which are addressable numerically. This network comprises an internal electronic focusing for each transmitter, a closed-loop electron dose control circuit for controlling each transmitter by precisely controlling the electron flow. Such a circuit of Dose control, connected to a transmitter, provides a dose, delivered during each write cycle, adapted despite transmitter-to-transmitter mismatch, temperature and aging effects. This control circuit makes it possible to terminate the emission at a fixed dose and not at a fixed time. It is an integrated component and connected to the transmitter.

But such a control circuit is a source of non-linearities. It also does not allow, for a linear or two-dimensional array of microtips, to compensate for the dispersion of doses emitted due to the current dispersions inherent in the microtips.

The object of the invention is to compensate for such nonlinearities so as to make the control device linear and usable, and to provide specific solutions for linear or two-dimensional devices.

STATEMENT OF THE INVENTION

• un module senseur qui reçoit le courant fourni par le micro-émetteur ainsi qu'une tension pour ajuster le point de polarisation dudit dispositif,
• un module comparateur qui reçoit le signal de sortie dudit module senseur ainsi qu'une tension de seuil permettant le réglage de la quantité d'électrons à émettre,
• un module logique qui reçoit le signal de sortie du module comparateur, ainsi qu'un signal de démarrage pour initialiser l'émission électronique,et un signal logique pour définir si le micro-émetteur doit ou non émettre,
• un module de commande qui reçoit le signal de sortie dudit module logique qui élabore les tensions nécessaires à l'initialisation et à l'extinction de l'impulsion de courant du micro-émetteur,
• des moyens de variation de la tension de seuil tels que, pendant l'émission d'électrons, la somme S= N_start + N_measure + N_off reste constante, N_start étant le nombre d'électrons du temps d'initialisation de l'impulsion de courant, N_measure étant le nombre d'électrons du temps de mesure de cette impulsion de courant, N_off étant le nombre d'électrons du temps d'extinction de cette impulsion de courant.

The invention relates to a device for controlling and controlling a dose of electrons emitted by a micro-transmitter, for example a microtip, characterized in that it comprises:

• a sensor module which receives the current supplied by the micro-transmitter and a voltage for adjusting the bias point of said device,
• a comparator module which receives the output signal of said sensor module and a threshold voltage for adjusting the quantity of electrons to be emitted,
• a logic module which receives the output signal of the comparator module, as well as a start signal for initializing the electronic transmission, and a logic signal for defining whether or not the micro-transmitter must transmit,
• a control module which receives the output signal of said logic module which generates the voltages necessary for the initialization and extinction of the current pulse of the micro-transmitter,
• means for varying the threshold voltage such that, during the emission of electrons, the sum S = N _start + N _measure + N _off remains constant, N _start being the number of electrons of the initialization time of the current pulse, N _measure being the number of electrons of the measurement time of this current pulse, N _off being the number of electrons of the extinction time of this current pulse.

In a first exemplary embodiment, the device of the invention comprises means for modulating in time the threshold voltage from the initialization signal so as to program a variable dose control over time such that the excess of electrons emitted during the initialization and extinction times is strictly compensated by a decrease over time of the programmed dose.

• un module de détection du courant de micro-émetteur, qui est capable de reproduire exactement le courant de pointe I_tip ou d'introduire un gain sur le courant,
• un module de génération de tension variable qui délivre en sortie une tension de consigne V2 = f (I_tip) .

In a second embodiment, the device of the invention further comprises:

• a micro-transmitter current detection module, which is able to reproduce exactly the tip current I _tip or introduce a gain on the current,
• a variable voltage generation module which outputs a set voltage V2 = f (I _tip ).

• un module senseur, qui reçoit le courant fourni par le micro-émetteur ainsi qu'une tension pour ajuster le point de polarisation,
• un module comparateur qui reçoit le signal de sortie dudit module senseur ainsi qu'une tension de seuil permettant le réglage de la quantité d'électrons à émettre,
• un module logique qui reçoit le signal de sortie du module comparateur, ainsi qu'un signal de démarrage pour initialiser l'émission électronique,et un signal logique pour définir si le micro-émetteur doit ou non émettre,
• un module de commande qui reçoit le signal de sortie dudit module logique qui élabore les tensions nécessaires à l'initialisation et à l'extinction de l'impulsion de courant du micro-émetteur,
• des moyens de variation de la tension de seuil tels que, pendant l'émission d'électrons, la somme S= N_start + N_measure + N_off reste sensiblement constante, N_start étant le nombre d'électrons du temps d'initialisation de l'impulsion de courant, N_measure étant le nombre d'électrons du temps de mesure de cette impulsion de courant, N_off étant le nombre d'électrons du temps d'extinction de cette impulsion de courant

The invention also relates to a linear or matrix device for controlling and controlling electron doses emitted by a set of micro-transmitters, characterized in that it comprises, for each micro-transmitter:

• a sensor module, which receives the current supplied by the micro-transmitter and a voltage to adjust the bias point,
• a comparator module which receives the output signal of said sensor module and a threshold voltage for adjusting the quantity of electrons to be emitted,
• a logic module which receives the output signal of the comparator module, as well as a start signal for initializing the electronic transmission, and a logic signal for defining whether or not the micro-transmitter must transmit,
• a control module which receives the output signal of said logic module which generates the voltages necessary for the initialization and extinction of the current pulse of the micro-transmitter,
• means for varying the threshold voltage such that, during the emission of electrons, the sum S = N _start + N _measure + N _off remains substantially constant, N _start being the number of electrons of the initialization time of the current pulse, N _measure being the number of electrons of the measurement time of this current pulse, N _off being the number of electrons of the time extinguishing this current pulse

• une étape de conversion du courant fourni par le micro-émetteur et d'ajustement du point de polarisation de fonctionnement,
• une étape de comparaison du signal obtenu en sortie de l'étape précédente à une tension de seuil permettant le réglage de la quantité d'électrons à émettre,
• une étape logique, pour initialiser l'émission électronique, et pour définir si le micro-émetteur doit ou non émettre,
• une étape de commande qui élabore les tensions nécessaires à l'initialisation et à l'extinction de l'impulsion de courant du micro-émetteur,

caractérisé en ce qu'il comprend une étape de variation de la tension de seuil telle que, pendant l'émission d'électrons, la somme S= N_start + Nmeasure + Noff reste constante, N_start étant le nombre d'électrons du temps d'initialisation de l'impulsion de courant, N_measure étant le nombre d'électrons du temps de mesure de cette impulsion de courant, N_off étant le nombre d'électrons du temps d'extinction de cette impulsion de courant.The invention also relates to a method for controlling and controlling a dose of electrons emitted by a micro-transmitter comprising

• a step of converting the current supplied by the micro-transmitter and adjusting the operating polarization point,
• a step of comparing the signal obtained at the output of the preceding step with a threshold voltage for adjusting the quantity of electrons to be emitted,
• a logical step, to initialize the electronic transmission, and to define whether the micro-transmitter must emit or not,
• a control step which elaborates the voltages necessary for the initialization and extinction of the current pulse of the micro-transmitter,

characterized in that it comprises a step of varying the threshold voltage such that, during the emission of electrons, the sum S = N _start + Nmeasure + Noff remains constant, N _start being the number of electrons of the time of initialization of the current pulse, N _measure being the number of electrons of the measurement time of this current pulse, N _off being the number of electrons of the extinction time of this current pulse.

• émission électronique par cathode froide,
• commande et contrôle de faibles charges électriques,
• compensation d'erreurs de mesures de charges,
• haute fréquence de fonctionnement,
• solution compatible avec des circuits intégrés spécifiques (ASIC).

Such an invention has a wide field of application:

• electronic emission by cold cathode,
• control and control of small electrical loads,
• compensation of load measurement errors,
• high frequency of operation,
• solution compatible with specific integrated circuits (ASIC).

BRIEF DESCRIPTION OF THE DRAWINGS

• The Figures 1A and 1B illustrate respectively the electrical diagram, and the behavioral symbol of a microtip,
• Fig. 2 illustrates current-voltage characteristics of a microtip.
• the Figures 3A and 3B respectively illustrate a system for regulating the current of a microtip in open loop and in closed loop,
• the figure 4 illustrates a device for controlling and controlling an electron dose emitted by a microtip,
• the figure 5 illustrates the sensor module of the device of the figure 4 ,
• the figure 6 illustrates the comparator module of the device of the figure 4 ,
• the Figures 7A and 7B are chronograms illustrating the operation of the device of the figure 4 ,
• the figure 8 illustrates a materialization of the error on the number of electrons programmed,
• the figure 9 illustrates a compensation of the error on the number of electrons programmed by variable threshold,
• the figure 10 illustrates the decomposition of a current pulse into elementary time,
• the figure 11 illustrates a simplified decomposition compared to that illustrated on the figure 10 ,
• the figure 12 illustrates the distribution of doses during the different elementary times,
• the Figures 13A and 13B illustrate curves giving the number of electrons relative to the peak current respectively without using compensation and using active compensation on the current,
• the figure 14 illustrates an example of time compensation according to the invention,
• the Figures 15 and 16 illustrate a simplified scheme of compensation according to the peak current according to the invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The device for controlling and controlling a dose of electrons emitted by a micro-transmitter, illustrated on the figure 4 , consists of a microtip 10, with an anode 11, a cathode 12, and an extraction grid 13, capable of supplying a current when the voltage of the extraction grid 13 with respect to the cathode 12 becomes greater than the extraction voltage in the vacuum. Spur capacitances 20 and 21 are inherent in the manufacture of such a microtip 10 in microtechnology.

• un module senseur 30 qui réalise une conversion électrons-tension, et qui reçoit le courant Ic fourni par cette micropointe 10 ainsi qu'une tension V1 pour ajuster le point de polarisation dudit dispositif, la sensibilité d'un module
  
  pouvant s'exprimer en volts/électrons,
• un module comparateur 31 qui reçoit le signal de sortie Vse dudit module senseur 30 ainsi qu'une tension de seuil V2 permettant le réglage de la quantité d'électrons à émettre, et qui délivre un signal de détection de charge suffisante Vcom,
• un module logique 32 qui reçoit ce signal Vcom, ainsi qu'un signal de démarrage Start pour initialiser l'émission électronique, et un signal logique data pour définir si la micropointe doit ou non émettre,
• un module de commande 33 qui reçoit le signal de sortie dudit module logique 32 ainsi que des signaux Vg-on et Vg-off, qui élabore les tensions nécessaires à l'initialisation et à l'extinction de l'impulsion de courant de la micropointe (plusieurs dizaines de volts).

This device comprises:

• a sensor module 30 which performs an electron-voltage conversion, and which receives the current Ic supplied by this microtip 10 and a voltage V1 to adjust the polarization point of said device, the sensitivity of a module
  
  can express itself in volts / electrons,
• a comparator module 31 which receives the output signal Vse of said sensor module 30 as well as a threshold voltage V2 for adjusting the quantity of electrons to be emitted, and which delivers a sufficient charge detection signal Vcom,
• a logic module 32 which receives this signal Vcom, as well as a start signal Start to initialize the electronic emission, and a logic signal data to define whether or not the microtip must emit,
• a control module 33 which receives the output signal of said logic module 32 as well as Vg-on and Vg-off signals, which generates the voltages necessary for initialization and extinction of the current pulse of the microtip (several tens of volts).

This device is indeed applicable to an arrangement of several microtips either in the form of a linear arrangement (barette), or in the form of a two-dimensional arrangement (matrix). All combinations of arrangements are also possible. This device can be realized in specific high-voltage technology, and can control the electron doses emitted with high rates.

We will now analyze each of these modules 30, 31, 32 and 33.

Sensor Module 30

The role of this module 30 is to process the basic information available on the microtip 10 and to convert it into a quantity that can be compared to an input quantity, in order to take a decision on the number N of emitted electrons .

This module may advantageously consist of a CTIA amplifier ("capacitive transimpedance amplifier") which performs a current-voltage conversion. The input variable is then the cathode current of the microtip I _c . This amplifier is characterized by its conversion gain R which is expressed in Volt / e ^- . It consists of an amplifier 35, a feedback capacitor (C _fb ) 36, and a resetting device 37. For the output excursion ΔV _s of the sensor module, the following is obtained: $$\mathrm{\Delta }{\mathrm{V}}_{\mathrm{s}}\mathrm{=}\frac{\mathrm{-}{\mathrm{I}}_{\mathrm{vs}}\mathrm{*}{\mathrm{T}}_{\mathrm{int}}}{{\mathrm{VS}}_{\mathrm{fb}}}\mathrm{=}\frac{\mathrm{NOT}\mathrm{*}{\mathrm{q}}_{\mathrm{e}}}{{\mathrm{VS}}_{\mathrm{fb}}}\mathrm{=}\mathrm{NOT}\mathrm{\Re }$$

• Elle n'est pas sensible, en ce qui concerne le signal, aux capacités parasites situées en amont.
• Son gain de conversion peut être fixé de manière précise. Il est défini par la valeur de C_fb. Il peut être, par exemple, de 23µV/e^- pour C_fb = 7fF.
• Le point de polarisation de la cathode est fixé par la variable externe V1

Such a solution is advantageous with respect to a solution achieving a direct integration on the microtip capability for several reasons:

• It is not sensitive, as far as the signal is concerned, to the parasitic capacitors situated upstream.
• Its conversion gain can be set precisely. It is defined by the value of C _fb . It can be, for example, 23μV / e ^- for C _fb = 7fF.
• The polarization point of the cathode is fixed by the external variable V1

Comparator Module 31

• la tension V_se de sortie du module senseur 30,
• la tension de commande V2 qui fixe la valeur du seuil de comparaison.

This module 31 receives on its inputs two analog voltages:

• the output voltage V _se of the sensor module 30,
• the control voltage V2 which sets the value of the comparison threshold.

• Tant que V_se>V2, la sortie logique V_com reste à « 1 ».
• Lorsque V_se=V2, la sortie logique V_com commute et vient se positionner au « 0 » logique.

This module comprises an open-loop amplifier 40 whose output level comprises two states (VDD and VSS) equivalent to two logic states as a function of the input voltages:

• As long as V _is > V2, the logic output V _com remains at "1".
• When V _se = V2, the logic output V _com switches and is positioned at the logic "0".

Logic Module 32

• de maintenir (« latch ») la prise de décision V_com obtenue en sortie du module comparateur 31 jusqu'à l'arrivée d'un signal de remise à zéro.
• de générer des phases non-recouvrantes utiles pour la remise à zéro du module senseur 30 et du module de commande 33.

This module 32 has several functions of sequencing and of generating internal signals. Its role is:

• to maintain ("latch") the decision V _com obtained at the output of the comparator module 31 until the arrival of a reset signal.
• generating non-overlapping phases useful for resetting the sensor module 30 and the control module 33.

This module is initialized by a start signal start at the beginning of the sequence, and obeys the data signal, as illustrated by the following table: Data Action 1 Issue of the microtip 0 No issue of the microtip

Control module 33

This module 33 is responsible for setting the extraction gate voltage necessary for the microtip to transmit the desired current synchronously with the appearance of the start signal. When the dose of electrons emitted has been reached (Vcom decision signal emitted by the comparator module 31). This module 33 cuts the flow by bringing the extraction gate voltage to a level such that the current electronics is diminished by several decades. These ignition and extinction values depend on the transconductance of the microtip and its geometric model. The control voltages can be switched from 20V to approximately 50V, which then requires the use of a specific high voltage technology (HVCMOS). The main function of this module 33 is therefore to perform the translation level [0-3v] to [20v-50v].

Such a control and control device has many limitations, inherent in the principle used. Indeed, the voltage V _is obtained at the output of the sensor module 30 is proportional to the cathode current I _c emitted by the microtip. Considering V1 as the initialization voltage level, the number N _e of electrons emitted by the microtip is such that: $${\mathrm{NOT}}_{\mathrm{e}}\mathrm{=}\frac{{\mathrm{Q}}_{\mathrm{e}}}{\mathrm{q}}\mathrm{=}\frac{\left({\mathrm{V}}_{\mathrm{himself}}\mathrm{-}\mathrm{V}\mathrm{1}\right)}{\mathrm{\Re }}\mathrm{with}\mathrm{\Re }\mathrm{=}\frac{\mathrm{q}}{{\mathrm{VS}}_{\mathrm{fb}}}$$

Qe being the electric charge emitted and q the charge of the electron.

A calibrated load Qc can therefore be programmed by V2 with the relation: $$\mathrm{Nc}\mathrm{=}\frac{{\mathrm{Q}}_{\mathrm{vs}}}{\mathrm{q}}\mathrm{=}\frac{\left(\mathrm{V}2\mathrm{-}\mathrm{V}\mathrm{1}\right)}{\mathrm{\Re }}$$

• un courant nominal Ic_nom serait interrompu au bout d'un temps t_nom.
• un courant 2*Ic_nom serait interrompu au bout d'un temps t_nom/2.
• un courant nominal 0.5*Ic_nom serait interrompu au bout d'un temps 2*t_nom.

Les aires représentées en grisé dans chacun des trois cas sont égales.The value of the comparison threshold V2 sets the programmed electrical load. If all the modules were perfect, the sensor module 30 would immediately transmit a representation V _se of the cathode current I _c , the comparator module 31 would have no delay, and the extraction gate control would instantly activate the establishment or extinction of the electronic flow, according to the chronogram of the Figure 7A . Whatever the level of the electronic current, the charge emitted would be identical and, as shown in FIG. Figure 7B :

• a nominal current Ic _name would be interrupted after a time t _nom .
• a current 2 * Ic _name would be interrupted after a time t _nom / 2.
• a nominal current 0.5 * Ic _name would be interrupted after a time 2 * t _name .

The shaded areas in each of the three cases are equal.

In reality, the overall duration of the current pulse is not linear as a function of the programmed current level. Indeed, because of parasitic capacitances 20 and 21 mentioned above, a switching of several tens of volts of the extraction grid 13 transiently disturbs the input of the sensor module 30 whose polarization must be maintained to avoid any saturation thereof. this. Such saturation would then require a significant time constant for a return to equilibrium and would not allow operation at high frequency. During this time maintaining the polarization of the 30 sensor module at the establishment of the electronic flow, electronic charges are already issued and are recorded in the overall balance of charges, although we can not measure because they depend on the level of current that is not known from the outset. Such a phenomenon is a first source of non-linearities.

Another phenomenon occurs at the extinction of the electron beam, when V _is reached V2. The comparator module 31 has a delay in the decision-making inherent in any electronic module. During this delay, the microtip 10 continues to emit and there is therefore an additional extinguishing charge which is added to the overall balance of the charges emitted. The figure 8 , which represents a materialization of the error on the number N of programmed electrons, illustrates such a phenomenon. If one traces, as a function of time, the number of electrons emitted with respect to the number of electrons programmed, with constant delay, an error is noted on the number of electrons emitted which depends on the level of current. In this figure the curve 45 corresponds to 2 * Iinom, the curve 46 corresponds to Iinom and the curve 47 to Iinom / 2, the curve 48 corresponds to the number of electrons emitted. There is therefore an overshoot on the load transmitted compared to the programmed load, which is a second source of non-linearities.

A first solution for compensating for such nonlinearities uses a comparison threshold that varies as a function of time. Simply send a ramp 50, or a "staircase", on the input V2 of the comparator module 31 as illustrated on the figure 9 .

The object of the invention is to compensate for such non-linearities by proposing other methods of compensation by controlling the cathode current I _c and by feedback on the value of the threshold V 2.

• t1 : temps d'établissement de la tension Vgate + reset CTIA
• t2 : temps de maintien du reset CTIA pour annuler les effets d'injection de charges et les transitoires,
• t3 : temps de mesure,
• t4 : temps de retard de prise de décision du comparateur,
• t5 : temps de retard dû à la coupure de Vgate (logique),
• t6 : délai pour éteindre le flux d'électrons.

By analyzing the profile 55 of the current pulse of the microtip, it is possible to break it up into a series of elementary times t1 to t6:

• t1: voltage setting time Vgate + reset CTIA
• t2: hold time of the reset CTIA to cancel the effects of charge injection and transients,
• t3: measurement time,
• t4: delay of decision making of the comparator,
• t5: delay time due to Vgate cut (logic),
• t6: delay to turn off the electron flow.

• t1+t2=t_start : temps d'initialisation qui s'étend depuis t_début (correspondant au début de l'impulsion) jusqu'à t_{début_contrôle} (correspondant au début effectif du contrôle de dose)
• t3=t_measure : temps de mesure réellement contrôlable qui s'étend depuis t_{début_contrôle} jusqu'à t_{fin_contrôle} (correspondant à la fin du contrôle de la dose)
• t4+t5+t6=t_off : temps d'extinction qui s'étend depuis t_{fin_contrôle} jusqu'à t_fin correspondant à la fin effective de l'émission de dose

Some of these elementary times can be grouped together, leading to the simplified model:

• t1 + t2 = t _start : initialization time that extends from t _start (corresponding to the beginning of the pulse) until t _{start_control} (corresponding to the effective start of the dose control)
• t3 = t _measure : measurement time actually controllable that extends from t _{start_control} until _{t_control end} (corresponding to the end of the dose control)
• t4 + t5 + t6 = t _off : extinguishing time which extends from t _{end_control} until t _end corresponding to the effective end of the dose emission

If we consider that the current reaches its nominal value I _{steady_state} rapidly during the initialization time t _start and that it is maintained during the extinction time t _off , it is therefore at first constant order for the duration of the current pulse. Indeed, at the beginning, the V _gate establishment time is short, and in the end, the logical _gate delay and V _gate delays are largely dominated by the delay of the comparator module 31 in decision making.

Avec $${\mathrm{N}}_{\mathrm{measure}}\mathrm{=}\frac{\left({\mathrm{V}}_{\mathrm{2}}\mathrm{-}{\mathrm{V}}_{\mathrm{1}}\right)}{\mathrm{\Re }}\mathrm{avec}\mathrm{\Re }\mathrm{=}{\mathrm{q}}_{\mathrm{e}}\mathrm{/}\mathrm{Ctia}$$

The total dose emitted in number of electrons can be expressed as: $${\mathrm{NOT}}_{\mathrm{beam}}\mathrm{=}{\mathrm{NOT}}_{\mathrm{merasure}}\mathrm{+}\frac{{\mathrm{I}}_{\mathrm{steady}\mathrm{-}\mathrm{state}}\mathrm{*}\left({\mathrm{t}}_{\mathrm{start}}\mathrm{+}{\mathrm{t}}_{\mathrm{off}}\right)}{{\mathrm{q}}_{\mathrm{e}}}$$

With $${\mathrm{NOT}}_{\mathrm{measure}}\mathrm{=}\frac{\left({\mathrm{V}}_{\mathrm{2}}\mathrm{-}{\mathrm{V}}_{\mathrm{1}}\right)}{\mathrm{\Re }}\mathrm{with}\mathrm{\Re }\mathrm{=}{\mathrm{q}}_{\mathrm{e}}\mathrm{/}\mathrm{CTIA}$$

The predicted electron dose is fixed by N _measure , but an excess dose is actually added because of non-zero initialization and extinction times. The figure 12 illustrates a curve of the number of electrons emitted as a function of the current regime.

In theory, as indicated previously, the number of electrons emitted should remain the same whatever the current I _tip , as illustrated by the horizontal curve 56.

The curves 57 and 58 illustrate the number of electrons emitted respectively during the initialization and extinction times. The sequencing can be such that the times t _start and t _off remain constant whatever the current, that is to say that the electrons emitted during these times t _start and t _off depend only on the current regime (affine function ).

The number of electrons emitted appears on curve 59, which for any value of abscissa X represents the sum of curves 56 + 57 + 58.

The relative numerical indication obtained from these curves shows an error on the number of electrons emitted with respect to the setpoint by a factor of 1.3 to 2.6 in excess. This is not acceptable for the desired emission control accuracy.

The object of the device of the invention is to be able to accurately transmit a programmed number of electrons regardless of the current regime of the microtip and to interrupt the electron beam as soon as this value has been reached. The sum of the electrons emitted during each of the times described above must therefore remain constant, ie the total number of electrons emitted is linear and constant, regardless of the peak current I _tip .

The law of variation of the number of electrons emitted during the initialization and extinction times of the current pulse (affine function) is known. It is therefore possible to intervene on the control of number N _measure of electrons actually measured so that the sum S = N _start + N _measure + N _off remains constant. N _measure decreases as I _tip grows.

To do this, the value of the threshold detection voltage V2 is modified during the electronic exposure. The compensation is carried out on quantities of surplus electrons answering the law: $$\frac{{\mathrm{I}}_{\mathrm{tip}}\mathrm{*}\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}}$$

Two types of compensation are possible: time compensation or compensation depending on the current. The Figures 13A and 13B respectively illustrate the theoretical curves 60 and measured 61 and the theoretical curves 60 and measured 61 'of the relative number of electrons as a function of the peak current I _tip , respectively without compensation and with compensation as a function of the current. Curve 61 'illustrates the improvement that is desired by using such active compensation as a function of current.

We can see on the Figure 13B the stability of the number of electrons emitted as a function of the peak current, although there is an inherent offset to the method used. Indeed the time called t _measure can not be zero because we would not control anything. The minimum time necessary for the proper operation of the compensation must be such that the noise brought back by the sensor module 30 remains low in front of the signal processed by this module (typically N _offset = 400 electrons, ie ΔV _{s_min} = 8mV).

The invention also relates to a linear or array device for controlling and controlling electron doses emitted by a set of micro-transmitters, which comprises, for each micro-transmitter, the different modules 30, 31, 32 and 33 as well as means for variations of the threshold voltage, as described above.

Examples of realization Time compensation

Such compensation is illustrated on the figure 14 . It does not cover all the needs. It is able to compensate for differences between microtips, but not for high frequency fluctuations on the same microtip. However, it can be used when it is certain that the frequency of recurrence of the current fluctuations is lower than the frequency of appearance of the programmed pulses. The threshold voltage V2 is modulated in time from the start initialization signal so as to program a variable dose control over time such that the excess of electrons emitted during the phases t _start and t _off is strictly compensated. by the decrease over time of the programmed dose. $$\mathrm{Scheduled\; dose}\mathrm{=}{\mathrm{NOT}}_{\mathrm{prog}}\mathrm{=}\frac{\left|\mathrm{V}\mathrm{2}\left(\mathrm{t}\right)\mathrm{-}\mathrm{V}\mathrm{1}\right|}{\mathrm{\Re }}\mathrm{with}\mathrm{\Re }\mathrm{=}\frac{{\mathrm{q}}_{\mathrm{e}}}{{\mathrm{VS}}_{\mathrm{tia}}}$$

This temporal variation is controlled by the generator 65.

Active compensation according to the current

When the frequency of the fluctuations of the current is such that it may vary during an elementary exposure time, the previous time correction is no longer sufficient. Indeed, in the balance sheet expression of the number of electrons emitted: $${\mathrm{NOT}}_{\mathrm{e}}\mathrm{-}\mathrm{=}\frac{{\mathrm{I}}_{\mathrm{tip}}\mathrm{*}\mathrm{T}}{{\mathrm{q}}_{\mathrm{e}}}$$

The two variables I _tip and T vary simultaneously during the control. It is therefore no longer possible to control one of the variables while measuring the other. An active correction must be made according to the current.

The figure 15 illustrates a simplified scheme of compensation according to the peak current. A peak current detection module 67 is able to reproduce exactly the peak current or to introduce a gain (X) on this current, for example by means of a current mirror. It is this output current that is measured by the sensor module 30. The input current Itip also serves as a reference for the variable voltage generation module 68 which outputs a set voltage V2 = f (I _tip ). . The decision on the time is always taken by the comparator module 31, but the decision threshold V2 is indexed on the instantaneous value of the emission current. This leads to an optimal compensation.

• Phase d'initialisation $$N_{\mathit{start}}=\frac{I*\left(t_{\mathit{début\_contr}\hat{o}\mathit{le}}-t_{\mathit{début}}\right)}{q}$$
  
• Phase de mesure $$N_{\mathit{measure}}=\frac{I*\left(t_{\mathit{fin\_contr}\hat{o}\mathit{le}}-t_{\mathit{début\_contr}\hat{o}\mathit{le}}\right)}{q}$$
  
• Phase d'extinction $$N_{\mathit{stop}}=\frac{I*\left(t_{\mathit{fin}}-t_{\mathit{fin\_contr}\hat{o}\mathit{le}}\right)}{q}$$
  

More precisely, by taking again the notations of the figure 11 , we can calculate the number of electrons emitted in each of the phases:

• Initialization phase $${\mathrm{NOT}}_{\mathit{start}}=\frac{I*\left(t_{\mathit{début\_contr}\hat{o}\mathit{the}}-t_{\mathit{beginning}}\right)}{q}$$
  
• Measurement phase $${\mathrm{NOT}}_{\mathit{measure}}=\frac{I*\left(t_{\mathit{fin\_contr}\hat{o}\mathit{the}}-t_{\mathit{début\_contr}\hat{o}\mathit{the}}\right)}{q}$$
  
• Extinction phase $${\mathrm{NOT}}_{\mathit{stop}}=\frac{I*\left(t_{\mathit{end}}-t_{\mathit{fin\_contr}\hat{o}\mathit{the}}\right)}{q}$$
  

The number of electrons deposited in excess to compensate by modifying the voltage V2 corresponds to $${\mathrm{NOT}}_{t_{\mathrm{start}}}\mathrm{+}{\mathrm{NOT}}_{\mathrm{stop}}=\frac{I}{q}\left[\left(t_{\mathit{début\_contr}\hat{o}\mathit{the}}-t_{\mathit{beginning}}\right)+\left(t_{\mathit{end}}-t_{\mathit{fin\_contr}\hat{o}\mathit{the}}\right)\right]=\frac{I}{q}\left[t_{\mathit{start}}-t_{\mathit{off}}\right]$$

From where according to ΔV2: $${\mathrm{NOT}}_{\mathrm{start}}\mathrm{+}{\mathrm{NOT}}_{\mathrm{stop}}=\frac{\mathrm{VS}*\Delta V2}{q}\mathrm{either\; \Delta V}2=\mathrm{VS}\left[t_{\mathit{start}}+t_{\mathit{off}}\right]$$

The capacity of the sensor block and the times └ t _start + t _off ┘ being known, the variation of V2 to be programmed is directly proportional to I. The difference of voltage to be programmed with respect to Vref (voltage to be applied to obtain during the phase of measurement of the desired dose if Nstart and Nstop did not exist) can therefore be implemented, for example by means of a resistor R _L making it possible to establish a voltage R _L * I with R _L = (t _start + t _off ) / C. In the particular case where the amplifier CTIA is recharged to a high state, this voltage R _L * I must be added to the voltage Vref to stop, faster than in the ideal case (without Nstart and Nstop), the power of the micropoint and so his show.

Block 68 of the figure 15 can then for example be realized in the manner illustrated on the figure 16 .

The dimensions of the transistors are chosen to fulfill the specified function in a manner known to those skilled in the art.

• de compenser les non-uniformités d'émission de micropointes ou de tout autre dispositif individuellement,
• de réaliser ces différentes fonctions dans un circuit intégré spécifique (ASIC ou « Application Specific Integrated Circuit »).
• de participer, en conséquence, à l'amélioration des rendements de fabrication des micropointes et de leur durée de vie,
• de pouvoir accéder ainsi à des grandes tailles d'émetteurs bidimensionnels sans complexifier le nombre d'interfaces périphériques (auto-traitement du signal in-pixel).

Such an embodiment is advantageous in the sense that it makes it possible to perform all the required functions near or in the electron emission site, which has the advantage of:

• to compensate for the non-uniformity of emission of microtips or any other device individually,
• perform these different functions in a specific integrated circuit (ASIC or "Application Specific Integrated Circuit").
• to participate, therefore, in the improvement of micropoint production yields and their lifetime,
• to thus be able to access large sizes of two-dimensional emitters without complicating the number of peripheral interfaces (self-processing of the in-pixel signal).

REFERENCES

1. [1] "Structure optimization of transistor-based Si field emitter arrays" by T. Matsukawa, K. Koge, S. Kanemaru, H. Tanoue and J. Itoh (TIDW'98, pages 671-674, EDF 2-4 )
2. [2] " Active matrix field-emitter arrays for the next-generation FEDs "by J. Itoh, S. Kanemaru, T. Matsukawa (199, SID )
3. [3] US 6,392,355 B1
4. [4] " Digital electrostatic electrom-beam array lithography "by LR Baylor, DH Lowndes, Simpson ML, Thomas CE, Guillorn MA, Merkulov VI, Whealton JH, Ellis ED, DK Hensley, AV Melechko (J.Vac.Sci.Technol.B20 (6 ), Nov-Dec 2002, pages 2646-2650 )

Claims (8)

1. Device for switching and controlling an electron dose emitted by a micro-emitter comprising:

   - a sensor module (30) that receives the output current from the micro-emitter and a voltage to adjust the polarization point of the said device,

   - a comparator module (31) that receives the output signal from the said sensor module, and a threshold voltage to adjust the quantity of electrons to be emitted,

   - a logical module (32) that receives the output signal from the comparator module (31), and a start signal to initialize the electron remission, and a logical signal to define whether or not the micro-emitter should emit,

   - a control module (33) that receives the output signal from the said logical module that generates the voltages necessary for initialization and extinction of the micro-emitter current pulse,
   characterized in that it comprises;

   - means of varying the threshold voltage such that the sum S = N_start + N_measure + N_off remains substantially constant during the electron remission, where N_start is the number of electrons at the current pulse start time, N_measure is the number of electrons at the measurement time of this current pulse, and N_off is the number of electrons at the extinguishing time of this current pulse.
2. Device according to claim 1, comprising means of modulating the threshold voltage (V2) in time starting from the initialization signal (start) so as to program an electron dose control that is variable in time such that excess electrons emitted during the start (t_start) and extinguishing (t_off) times are strictly compensated by a reduction of the programmed dose in time.
3. Device according to either claim 1 or 2, comprising:

   - a module for detecting the micro-emitter current (67), capable or reproducing the tip current I_tip exactly, or adding a gain on the current,

   - a variable voltage generation module (68) that outputs a set voltage V2 = f(I_tip).
4. Linear or matrix switching and controlling device for electron doses emitted by a set of micro-emitters, comprising the following for each micro-emitter:

   - a sensor module (30) that receives the output current from the micro-emitter and a voltage to adjust the polarization point,

   - a comparator module (31) that receives the output signal from the said sensor module and a threshold voltage to adjust the quantity of electrons to be emitted,

   - a logical module (32) that receives the output signal 1 from the comparator module (31), and a start signal to initialize the electron emission, and a logical signal to define whether or not the micro-emitter should emit,

   - a control module (33) that receives the output signal from the said logical module that generates the voltages necessary for initialization and extinction of the micro-emitter current pulse,
   characterized in that it comprises the following for each micro-emitter :

   - means of varying the threshold voltage such that during the electron emission, the sum S = N_start + N_measure + N_off remains substantially constant, where N_stare is the number of electrons at the current pulse start time, N_measure is the number of electrons at the measurement time of this current pulse, N_off is the number of electrons at the extinguishing time of this current pulse.
5. Device according to any one of the above claims, in which each micro-emitter is a microtip.
6. Process for switching and controlling an electron dose emitted by a micro-emitter comprising:

   - a step to convert the current output by the micro-emitter and to adjust the operating polarization point,

   - a step to compare the signal obtained at the output from the previous step with a threshold voltage for adjustment of the quantity of electron to be emitted,

   - a logical step to initialize the electron emission, and to define whether or not the micro-emitter should emit,

   - a control step that generates the voltages necessary for initialization and for extinction of the micro-emitter current pulse, characterized in that it comprises:

   - a step to vary the threshold voltage (V2) such that during the emission of electrons, the sum S = N_start + M_measure + N_off remains approximately constant, N_start being the number of electrons at the current pulse start time, Measure being the number of electrons at the measurement time of this current pulse, N_off being the number of electrons at the extinguishing time of this current pulse.
7. Process according to claim 6, comprising a step in which the threshold voltage (V2) is modulated with time starting from the initialization signal (start) so as to program an electron dose control that is variable in time such that excess electrons emitted during the start (t_start) and extinguishing (t_off) times are all or partly compensated by a reduction of the programmed dose in time
8. Process according to claim 6, comprising:

   - a step for detecting the tip current, capable of reproducing the tip current I_tip exactly, or adding a gain on the current,

   - a step to generate a variable voltage (68) that output a set voltage V2 = f(I_tip).

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