The invention relates to a beam blanker for interrupting a beam of charged particles, the beam blanker showing an axis along which charged particles propagate before entering the beam blanker, the beam blanker comprising means for generating an electric field perpendicular to said axis, the electric field for deflecting the charged particles, and an aperture in a diaphragm, the aperture transmitting the beam when the beam is not interrupted and the diaphragm stopping the beam when the beam is interrupted.
Such a beam blanker is known from U.S. Pat. No. 4,445,041.
Beam blanking is used in for example charged particle instruments, such as Scanning Electron Microscopes, Transmission Electron Microscopes, Focused Ion Beam machines, and the like.
A well-known application is the use of a beam blanker for modulating the beam current in an Electron Beam Lithography System. In such a system a beam of energetic electrons is focused and scanned over a substrate, such as a semiconductor wafer covered with a resist layer. During this scanning the beam is blanked/unblanked, as a result of which part of the resist layer is irradiated by the electron beam and part of the resist layer is not exposed to electrons. Due to the irradiation the properties of the resist are locally changed, and further treatment of the resist results in the forming of a pattern on the wafer. Further processing, such as etching, evaporation of metals, implantation of materials, etc, may then be performed.
The known beam blanker is intended to blank/unblank a beam of electrons as used in such an Electron Beam Lithography System. It comprises a deflector formed by two electrodes and downstream of said deflector a diaphragm with an aperture. The diaphragm intercepts the beam of electrons when the beam is deflected by a deflector, and the aperture transmits the beam when the beam is not or hardly deflected. For a fast blank/unblank speed the diaphragm is positioned at a cross-over position of the beam. The small spot size of the beam at the cross-over enables an abrupt change in current for a small change in deflection, and thus a fast blank/unblank speed for a given dV/dt of the deflection signal. Further downstream of the deflector and the diaphragm a second deflector is placed so that the combined deflection of the two deflectors results in a pivot point at the cross-over. As a result of the pivot point coinciding with the cross-over no change of the position of the spot is observed downstream of the beam blanker when blanking the beam. Due to the limited speed of electrons in a beam the deflectors should be excited with a small time delay. This is achieved by exciting the deflectors with the same signal, but the signal of the second deflector is delayed by adding a delay line.
The voltage needed to blank the beam is typically 6 volts at a beam energy of approximately 20 keV.
According to the patent disclosure the known beam blanker can be used to frequencies of around 300 MHz.
Another application of a beam blanker is for generating a pulsed beam in a Transmission Electron Microscope (TEM). In a TEM a sample is irradiated by a beam of energetic electrons, and electrons transmitted through the sample are used to gather information about the sample. Normally the beam is used to study a sample that does not change in time, or only very little.
Lately there is a demand for studying effects that are time dependent, such as decay effects after probing the sample with a pulse of light, thereby pumping it to an excited state. By irradiating the sample with a train of electron pulses while also illuminating the sample with a time-synchronized train of light pulses from a pico- or femto-second laser, resulting in a train of light pulses with a duration similar to or shorter than the electron pulse, the decay effects can be studied. By introducing a variable phase delay between the two trains of pulses and making recordings for different phase delays, a complete dependency of the decay effects can be recorded. This resulted in the 1999 Nobel Prize for chemistry to professor Zewail for his work on femtochemistry, that is: the study of chemistry in the femtosecond scale by observing molecules with ultra-fast lasers.
It is noted that Zewail did not use a beam blanker to generate a pulsed electron beam, but a pulsed electron source, as described in US patent application No. US2005/0253069. The application describes that an electron source (a heated LaB6 crystal) is pulsed by photo-emission of using a pulsed laser. A problem of a pulsed electron source of the type used by Zewail is that it shows a brightness that is much lower than the brightness of the well-known Schottky emitters routinely used in TEMs.
As known to the person skilled in the art the amplitude of the signal for driving a beam blanker is proportional to the energy of the beam blanked, while the period of the signal (and thus the dV/dt) is proportional with the frequency and/or rise time of the beam. At increasing frequencies the signal for driving the beam blanker must thus have a larger dV/dt. This is difficult to achieve. This is aggravated when the blanker is used for blanking a beam with higher energy, as this implies (using the same dimensions) a blanking signal with increased amplitude and increased dV/dt. It is noted that the 300 keV beam energy typically used in a TEM is more than 10 times higher than the 20 keV used in the beam blanker of the known patent.
There is a need for a high frequency beam blanker operating at a lower power and/or with a higher sensitivity.
The invention intends to provide such a beam blanker.
To that end the beam blanker according to the invention is characterized in that the electric field is generated by a resonant structure with a resonant frequency f, the resonant structure equipped to generate an electric field that sweeps the beam over the aperture, as a result of which the beam is transmitted through the aperture twice per period of the frequency f.
By making the deflection means part of a resonant structure, the amplitude of the deflector is amplified by a factor Q, in which Q is the quality factor of the resonant structure.
Such a resonant structure may include an LC network. In a preferred embodiment the deflector means consist of two electrodes that are part of a capacitor of an LC network.
Alternatively the deflector means are part of a wave guide, the wave guide coupled to a RF oscillator. The waveguide may be open or closed at one end. Small apertures in the waveguide allow the beam to enter and to leave the wave guide. The electric field in the waveguide deflects the beam.
In a preferred embodiment the resonant structure comprises a resonant transmission line (shielded or non-shielded). As is the case for a wave guide, the transmission line may be open ended or closed, as long as the beam passes near a voltage maximum of the transmission line.
It is noted that, as known to the person skilled in the art of RF electronics, when driving the resonant structure by a generator, impedance matching may be done by, for example, stub tuning with one or more opened or closed stubs. At lower frequencies for example tapped inductors and/or capacitors may be used, or for example a helical resonator that may or may not be combined with a capacitor.
It is noted that a beam blanker is described by K. Ura et al., “Picosecond Pulse Stroboscopic Scanning Electron Microscope”, J. Electron Microsc., Vol. 27, No. 4, (1978), p. 247-252. This beam blanker uses a resonant cavity as shown in its FIG. 2a, and a buncher as shown in its FIG. 2b. The document mentions that the cavities are designed using Fujisawa's theory, see K. Fujisawa, “General Treatment of Klystron Resonant Cavities”, IRE Trans. on Microwave Theory and Techniques (October 1958), pages 344-358. It is noted that the theory of Fujisawa only relates to (klystron) cavities with rotational symmetry, and thus the cavities of Hosokawa are rotational symmetric as well. Such cavities do not generate an electric field perpendicular to the rotational axis, as is the case in this invention.
In a preferred embodiment the beam blanker comprises a resonant transmission line and a grounded conductor, and the electric field is generated between the resonant transmission line and the grounded conductor.
The resonant transmission line may be open or closed at the end, and a voltage maximum (a voltage node) occurs at the position where the beam passes the transmission line.
The electric field may be synchronized to, or derived from, a driving signal. The driving signal may be an electric signal, or it may be an optical signal for triggering a photoreceptor, such as a PIN diode or a phototransistor, in an electric circuit.
Synchronizing the signal with an optical signal is especially attractive when synchronizing the signal to an optical probe signal that probes the sample in, for example, a TEM.
Synchronizing the signal with an optical signal is also very attractive when the beam blanker is situated in a high voltage area, such as the gun area of a TEM.
The deflection of the beam depends on the energy of the charged particles. In charged particle instruments the particles are often generated in a gun area and accelerated to their final energy The energy in the gun area is typically lower than 10 keV, while in a Transmission Electron Microscope the final energy with which they impact on a sample is typically between 80 and 400 keV, although higher and lower energies are known to be used. By placing the beam blanker in the gun area the deflection voltage (and thus the power needed to drive the beam blanker) is less than when the beam blanker is operated at the final energy. Preferably the driving of such a signal at high voltage (the gun voltage) is done using previously mentioned triggering by an optical signal, in which the optical signal bridges the gap between ground to high voltage via a fiber.
The resonant frequency f may be derived from a driving signal by injection locking, phase locking or frequency locking the resonant frequency to a, in most cases sub-harmonic, driving signal. Also frequency multiplication may be used to form a high frequency signal from a lower frequency signal.
Instead of driving the resonant structure with a driving signal, the resonant structure may comprise a negative impedance element, such as a Gunn diode or an IMPATT diode. In this way no external driving signal is necessary, as the negative impedance element will make the structure oscillate. It is noted that circuits are known in which the frequency of such a circuit can be tuned in a variety of ways, including mechanical means, electronic means (phase shifters) or injection locking.
For some uses it is necessary to tune frequency and/or phase of the resonant frequency f. This may be achieved by, for example, mechanical tuning means (e.g. a tuning screw), by electronic means (e.g. a phase shifter or a varicap), or in another way (such as the magnetic tuning used in RF isolators/circulators.
It is noted that the latter (the RF isolator/circulator) may also ease the demands on impedance matching to the resonant structure as little energy is reflected back into the circuitry generating the driving signal.
Instead of using a transmission line in the resonant circuit, also a cavity resonator may be used to generate the electric field. The cavity resonator may take the form of a TM or TE waveguide with two holes through which the beam enters and leaves the cavity resonator or waveguide.
Preferably the beam blanker is equipped with an aperture in the form of a slit or a hole with a dimension in the direction in which the beam is deflected of less than 100 μm, preferably less than 10 μm, most preferably less than 1 μm.
In a particle-optical instrument such as a Scanning Electron Microscope (SEM) or a Transmission Electron Microscope (TEM), or an instrument equipped with a SEM and/or a TEM column, the electron source is often a Schottky emitter. The Schottky emitter typically has an emitting diameter of approximately 20 nm, and a cross-over of less than 20 nm is formed by the condenser optics. An aperture of 100 μm, preferably less than 10 μm, and most preferably less than 1 μm can thus be well used to transmit the beam. It is noted that a smaller aperture implies that the beam is blanked at a lower electric field (a smaller deflection), but that too small an aperture implies problematic alignment. However, an alignment of less than 1 μm is well achievable.
The beam blanker is for use in a particle-optical instrument such as a Scanning Electron Microscope (SEM), a Transmission Electron Microscope (TEM) or a Focused Ion Beam (FIB) instrument, or an instrument equipped with a SEM, FIB and/or a TEM column.
Preferably the apparatus is equipped with a laser, such as a nano- or femtosecond laser, producing a train of light pulses for probing the sample. When synchronizing the laser and the beam blanker, time dependent studies on ultra-short (femto-second) scale or longer can be performed.
The synchronization can be achieved by e.g. triggering the beam blanker by a laser pulse. Electrons then irradiate the sample shortly after the triggering took place. A variable phase shift in the beam blankers circuitry can then be used to probe the sample at different delay times.
For longer delay times two beam blankers can be used, one of the beam blankers selecting some of the pulses transmitted by the other. Preferably the two blankers show positional overlap, or even share components, such as the aperture.
As is clear to the person skilled in the art the signal of the beam blankers should be such that the beam passes through both blankers when required. This can be achieved by driving the beam blankers with the same frequency, but varying the phase of one (or both) signals with a phase shifting element. It is also possible to excite one beam blanker with a first frequency and the other with a frequency that has a harmonic or sub-harmonic relation to the first frequency. In that case a zero-crossing of both frequencies, and thus a passing of the beam through both beam blankers, occurs on a regular basis.