EP4526914A1

EP4526914A1 - Method and apparatus for controlling an electron beam with a phase modulator driven by analog multiplexing

Info

Publication number: EP4526914A1
Application number: EP23725737.3A
Authority: EP
Inventors: Vincenzo Grillo; Alberto RONCAGLIA; Enzo ROTUNNO; Luca BELSITO
Original assignee: Consiglio Nazionale delle Richerche CNR
Current assignee: Consiglio Nazionale delle Richerche CNR
Priority date: 2022-05-17
Filing date: 2023-05-16
Publication date: 2025-03-26
Also published as: WO2023222733A1; IT202200010184A1

Abstract

A method for controlling an electron beam with a phase modulator (20), the phase modulator comprising input contacts (22) configured to receive input signals, and an array of MEMS devices (23) for controlling an electron beam (EB), each MEMS device comprising a MEMS electrodes (24) operable to control the electron beam based on the input signals, wherein the method comprises sending to an individual input contact (22) a respective multiplexed input signal formed by a plurality of superimposed AC signals at different frequencies, demultiplexing the multiplexed input signal to a plurality of frequency channels (25) respectively associated to the MEMS electrodes (24), and rectifying and DC-converting the demultiplexed input signal in such a way that each MEMS electrode (24) is biased with a DC signal corresponding to the amplitude of one of the AC signals.

Description

Method and apparatus for controlling an electron beam with a phase modulator driven by analog multiplexing

The present invention relates in general to the field of electron optics based on the use of Micro Electro-Mechanical Systems (MEMS) devices.

There is currently an intense competition in the field of electron microscopy towards the manufacturing of MEMS devices that are designed to be used inside electron microscopes. The advantages of such devices are two-fold: 1) they can produce the same effect of regular electronic lenses with small values of the applied voltages; 2) unusual designs for electron optics can be possible.

Towards this direction, two main approaches have been reported so far in the scientific literature: 1) on one side, a matrix of individual Einzel lenses that plays the role of an electron phase modulator, 2) on the other side, a variety of devices of specific designs of the electrodes to produce peculiar phase plates for electrons. Among the latter, considering the most recent examples, mention is made to the Orbital Angular Momentum (0AM) sorter, a phase plate able to generate spiral electron beam wavefronts, devices for generation of caustics for computational “ghost imaging”.

In each of these devices the availability of a large number of electrodes is a fundamental issue to add flexibility and control to the phase plate operation. Increasing the number of controllable electrodes requires as well an increase of the number of external contacts of the device holder, which is however limited to the available space allowed by the holder. If the electrical connection with the MEMS devices is implemented by simple pressure contacting tips, between 8 and 16 contacts can be achieved (16 with large efforts). Using wire bonding, a larger number of contacts can be handled precisely, but in this way the interchangeability of the MEMS devices of the holder is much more difficult and time-consuming.

An aim of the invention is to provide a way to increase the number of controllable electrodes without correspondingly increasing the number of external contacts of the holder. In view of the above, there is proposed a method for controlling an electron beam with a phase modulator, said phase modulator comprising a plurality of input contacts configured to receive input signals, and an array of MEMS devices for controlling an electron beam, each MEMS device comprising a plurality of MEMS electrodes operable to control the electron beam based on said input signals, wherein each input contact is connected to a respective MEMS device of the array of MEMS devices, wherein the method comprises sending to an individual input contact of the plurality of input contacts a respective multiplexed input signal formed by a plurality of superimposed alternating current - AC signals at different frequencies, demultiplexing the multiplexed input signal to a plurality of frequency channels respectively associated to the MEMS electrodes of the MEMS device connected to the individual input contact, and rectifying and DC-converting the demultiplexed input signal in such a way that each MEMS electrode is biased with a direct current - DC signal corresponding to the amplitude of one of the AC signals.

A further object of the invention is an apparatus for controlling an electron beam, including a phase modulator comprising a plurality of input contacts configured to receive input signals, and an array of MEMS devices for controlling an electron beam, each MEMS device comprising a plurality of MEMS electrodes operable to control the electron beam based on said input signals, wherein each input contact is connected to a respective MEMS device of the array of MEMS devices, waveform generating means connected to the input contact and operable to send to an individual input contact of the plurality of input contacts a respective multiplexed input signal formed by a plurality of superimposed alternating current - AC signals at different frequencies, demultiplexing means integrated on the phase modulator and configured to demultiplex the multiplexed input signal to a plurality of frequency channels respectively associated to the MEMS electrodes of the MEMS device connected to the individual input contact, and rectifying and DC-converting means integrated on the phase modulator and configured to rectify and DC-convert the demultiplexed input signal in such a way that each MEMS electrode is biased with a direct current - DC signal corresponding to the amplitude of one of the AC signals.

The invention using a multiplexing technique allows to increase the number of controllable electrodes keeping the number of external contacts of the device holder constant. For instance, with a conventional device holder having 8 contacts, 8-M electrodes can be controlled, where M is the number of electrodes of the individual MEMS device. Therefore, even if the electrical connection with the MEMS devices is implemented by simple pressure contacting tips, a relatively large number of electrodes can be controlled. Using wire bonding, an even larger number of contacts can be handled precisely and a consequently larger number of electrodes can be controlled, to the detriment, however, of the interchangeability of the phase modulator.

Further characteristics and advantages of the invention will be presented in the following detailed description, which refers to the attached drawings, provided purely by way of nonlimiting example, in which:

Figure 1 is a schematic representation of an electron microscope provided with an apparatus according to the invention;

Figure 2 is an enlarged representation of a device holder and a chip of the apparatus of Figure 1, wherein the chip integrates a phase modulator;

Figure 3 is a partial plan view of the chip showing the phase modulator;

Figure 4 is a scheme showing the basic principles of the signal multiplexing technique utilized in the invention;

Figure 5 is a scheme showing the architecture and operation of the present invention.

Figure 1 shows an electron microscope 10 having a per se known architecture. In the drawing, EB designates an electron beam used in the microscope, 11 designates an anode, 12 designates condenser lenses, 13 designates a sample holder, 14 designates an objective lens and 15 designates projecting lenses. The architecture of the electron microscope is not essential to the present invention.

A phase modulator 20 is interposed between the anode 11 and the first of the condenser lenses 12. The phase modulator 20 is a tunable phase plate operable to shape the electron beam wavefront by controlling the phase of the electron beam EB. While the phase modulator is disclosed herein as used in an electron microscope, it is intended that the invention can be applied wherever there is a need to control the phase of an electron beam.

With reference to Figures 2 and 3, the phase modulator 20 comprises a chip 21 on which the components of the phase modulator 20 are integrated. The phase modulator 20 comprises a plurality of input contacts 22 configured to receive (voltage or current) input signals from an external control unit CU (see Figure 1), and an array of Micro Electro-Mechanical Systems (MEMS) devices 23 for controlling the electron beam EB. In general, these MEMS devices 23 can be electrostatic elements or actuators such as, for example, Einzel lenses or needle electrodes used for example in spiral phase plates and 0AM sorters. Each MEMS device 23 comprises a plurality of MEMS electrodes 24 operable to control the electron beam EB based on the input signals received by the input contacts 22. In the example shown in Figure 3, the MEMS electrodes 24 are configured as needles arranged around the electron beam axis z; in this example, each input contact 22 of the phase modulator chip 21 controls four electrodes/needles 24 of a respective MEMS device 23. For instance, the four needles circled by a dashed line in Figure 3 are controlled by the input contact on the top left position of the array of input contacts 22 of Figure 2. According to other embodiments, the MEMS electrodes can be configured, for instance, as electrodes of Einzel lenses.

Each MEMS device 23 is connected (more precisely, the MEMS electrodes 24 of each individual MEMS device 23 are connected) to a respective one of the input contacts 22.

The phase modulator 20 is coupled to a conventional device holder 30 configured to support the phase modulator 20. The device holder 20 carries a plurality of external contacts 31 intended to be connected to the input contacts 22 of the phase modulator 20, for example by pressure contacting tips or wire bonding. An electrical wiring 32 contained in the device holder is connected to the external contacts 31 and is configure to connect the phase modulator to the control unit CU.

A signal multiplexing technique is used to control the MEMS electrodes 24 with a limited number of input contacts 22. The basic principle of this signal multiplexing technique is illustrated in Figure 4.

By superimposing two AC signals at different frequencies (for instance coo and 3coo, given by the light gray and dark grey traces in Figure 4 top, respectively), a non- sinusoidal signal represented by the trace in the top-center plot is obtained. The amplitude spectrum A(co) of such a signal is constituted by two lines located at the frequencies of the fundamental components coo and 3 coo. If the composite signal is generated in parallel on two electrodes as shown in Figure 4 bottom, its different harmonic components can be selected by using passband filters centred on these two frequencies, in order to obtain a different component on each electrode.

By exploiting this property, AC signals at different frequencies can be multiplexed from one input contact 22 to several output electrodes 24 by mixing the signals within a single, non- sinusoidal waveform generated on the input contact 22 and using appropriate passband filters Hi(co) and H2(co) to select the individual harmonic components on each output electrodes. Using the input electrodes 22 for the AC multiplexing, a multiplication of the programmable signals that can be used in the MEMS devices 23 of the phase plate 20 could be achieved.

In the scheme of Figure 5, the multiplexing of the signals from the input contacts 22 of the chip 21 to the MEMS electrodes 24 utilized in the phase plate device is represented. On each of the N input contacts 22 of the chip 21, a waveform generator (WGi, ..., WGN in the figures) integrated into an external board 40 associated to the control unit CU is used to produce a composite waveform constituted by the superposition of harmonic components with different frequencies and amplitudes. Each input contact 22 (INi, ..., INN in the figure) is split into M channels 25 on which the signal is processed by M non-overlapping passband filters 26 (Fi, F2, ..., FM in the figure), centred on the frequencies of the harmonic components. Each set of M channels and related filters Fi, F2, ..., FM is associated to an individual MEMS device 23 of the phase modulator 20. In this way, after filtering, the sine signals mixed in the input waveform are separated on each multiplexing channel 25. Each of these signals can then be rectified and DC-converted by a simple circuit constituted by a diode 27 in series with a capacitor 28, as shown in the figure, obtaining a DC voltage whose value will be close to the peak voltage of the filtered harmonic wave. Such voltage can be used as a bias for an individual MEMS electrode 24 (OUTi, ... , OUTNM) of the programmable phase plate integrated in the chip 21.

Using this technique, an M-fold multiplication of the N input contacts of the chip 21 could be obtained, where M is expected to depend on the number of non-overlapping filters that can be obtained in the target operation frequency range of the circuit. Since the DC voltages obtained after rectification and conversion of the harmonic components of the signal will closely match the amplitudes of the harmonic components, any voltage could in principle be obtained on the MEMS electrodes 24 of the phase plate, provided that the waveform generator 40 used is able to mix sinusoidal signals with arbitrary amplitude on the input contacts 22.

The passband filters 26 can be for example passive RC or RLC filters, electromechanical filters implemented with a quartz or a MEMS resonator with or without signal amplification after the filter. All these filters 26 are integrated on the chip 21 in a suitable way for the space constraints of the device holder 30.

The scheme of Figure 5 summarizes the operation of the external board 40 and of the MEMS devices 23. The external board 40 provides N output channels. On each one, a waveform constituted by M tuneable sinusoidal waveforms with frequency e.g. in the range 1 Hz-1 MHz plus one DC channel and tunable amplitude can be produced by the system. The chip 21 is powered through the USB connection of the control unit UB and also through an additional external power supply.

The above described scheme suffers potentially of some limitation because of 1) the cross talk between channels since the linear passive filters can only produce limited reduction of the unwanted frequencies, 2) only positive (or negative) value of the bias are possible, 3) the overall sum of all M bias on the M channels is limited by the breakdown of insulations so that it is not possible to have all biases to the nominal maximum at the same time.

A possible way to increase the multiplexing capacity is using filter with narrower passband, such as resonant RLC or micromechanical filters, to decrease the crosstalk and accommodate more channels in the IkHz-lMHz range.

A partial compensation of cross talk is also possible by diagonalizing the in/out matrix for each channel. Since negative values cannot be used it is necessary to add a baseline of bias so that all values are referred to such base line. In the example shown in Figure 2, such a baseline of bias is introduced through two of the input contacts 22.

It is possible to further increase the number of addressed phase elements. In facts there are cases in which what matters is only the phase in the proximity of the electrodes and not its long range field. This is the case of Einzel lenses array. In this case the phase is described by the ambiguity of a factor 2K. The formula is

<p = mod(CVd, 2TT) where d is the electrode thickness, C is a coupling constant and V is the applied voltage and mod(x, y) is the function that calculates the remainder of the division of the two arguments.

An appropriate manufacture of the MEMS devices allows to drive two electrodes of thickness d- and d₂ with the same voltage but still obtain two different phases within a certain discretisation limit that is roughly gauged imagining that n = (CV_maxd, 2TT) steps in the phase difference are possible. So, the phase discretisation is at best

In many cases a precision of 0.02K can be reached. When used alone (i.e. without the multiplexing architecture shown in Figure 5), the above described arrangement could allow to control 2-N MEMS electrodes with N input contacts. However, if this arrangement is applied to the architecture of Figure 5, this results in having each frequency channel 25 connected to two different MEMS electrodes 24. Therefore, the number of MEMS controllable through the input contacts 22 is further increased by 2 times.

Claims

1. A method for controlling an electron beam with a phase modulator (20), said phase modulator comprising a plurality of input contacts (22) configured to receive input signals, and an array of Micro Electro-Mechanical Systems - MEMS devices (23) for controlling an electron beam (EB), each MEMS device comprising a plurality of MEMS electrodes (24) operable to control the electron beam based on said input signals, wherein each input contact

(22) is connected to a respective MEMS device of the array of MEMS devices (23), wherein the method comprises sending to an individual input contact (22) of the plurality of input contacts a respective multiplexed input signal formed by a plurality of superimposed alternating current - AC signals at different frequencies, demultiplexing the multiplexed input signal to a plurality of frequency channels (25) respectively associated to the MEMS electrodes (24) of the MEMS device (23) connected to the individual input contact (22), and rectifying and DC-converting the demultiplexed input signal in such a way that each MEMS electrode (24) is biased with a direct current - DC signal corresponding to the amplitude of one of the AC signals.

2. Apparatus for controlling an electron beam, including a phase modulator (20) comprising a plurality of input contacts (22) configured to receive input signals, and an array of Micro Electro-Mechanical Systems - MEMS devices

(23) for controlling an electron beam (EB), each MEMS device comprising a plurality of MEMS electrodes (24) operable to control the electron beam based on said input signals, wherein each input contact (22) is connected to a respective MEMS device of the array of MEMS devices (23), waveform generating means (40) connected to the input contacts (22) and operable to send to an individual input contact of the plurality of input contacts a respective multiplexed input signal formed by a plurality of superimposed alternating current - AC signals at different frequencies, demultiplexing means (26) integrated on the phase modulator (20) and configured to demultiplex the multiplexed input signal to a plurality of frequency channels (25) respectively associated to the MEMS electrodes (24) of the MEMS device (23) connected to the individual input contact (22), and rectifying and DC-converting means (27, 28) integrated on the phase modulator (20) and configured to rectify and DC-convert the demultiplexed input signal in such a way that each MEMS electrode (24) is biased with a direct current - DC signal corresponding to the amplitude of one of the AC signals.

3. Apparatus according to claim 2, wherein said frequency channels (25) are connected to the same individual input contact (22), and wherein said demultiplexing means comprise a plurality of passband filters respectively associated to the frequency channels (25), each passband filter having a frequency band tuned to the frequency of one of the AC signals.

4. Apparatus according to claim 2 or 3, wherein said MEMS devices are Einzel lenses, wherein said DC signal is a voltage signal, and wherein each frequency channel (25) is connected to two different MEMS electrodes (24) of different thicknesses so that the two MEMS electrodes can be driven with the same voltage signal to impart different phases to the electron beam.

5. An electron microscope comprising an apparatus according to any of claims 2 to 4.