PARTICLE DETECTOR FOR DETECTING CHARGED PARTICLES

Abstract

The invention relates to a particle detector, comprising: a measuring electrode for measuring charged particles, a detection device for detecting the charged particles measured by the measuring electrode, and an evaluation device for determining the number of charged particles detected by the detection device. The detection device has a charge amplifier for converting a charge signal generated by the charged particles into a voltage signal and an amplifier device for amplifying the voltage signal.

Claims

1. A particle detector comprising: a measuring electrode for measuring charged particles, a detection device for detecting the charged particles measured by the measuring electrode, and an evaluation device for determining the number of charged particles detected by the detection device, characterized in that the detection device has a charge amplifier for converting a charge signal generated by the charged particles into a voltage signal and an amplifier device for amplifying the voltage signal.

2. The particle detector according to claim 1, wherein the evaluation device is designed to evaluate the voltage signal in a respective counting window around a measuring time of the charged particles at the measuring electrode in order to determine the number of charged particles detected.

3. The particle detector according to claim 2, wherein the detection device has a trigger device for defining the respective counting window.

4. The particle detector according to claim 3, wherein the counting window defined by the trigger device is shifted in time to later times preferably by dT.sub.i/2 such that the counting window is around the respective measuring time T.sub.i.

5. The particle detector according to claim 2, wherein the evaluation device is designed to filter the voltage signal in the respective counting window in order to increase the signal-to-noise ratio.

6. The particle detector according to any of claim 2, wherein the evaluation device is designed to determine a voltage difference in the voltage signal before the measuring time and after the measuring time in order to determine the number of charged particles detected in a respective counting window.

7. The particle detector according to claim 6, wherein the evaluation device is designed to determine the voltage signal after the measuring time at a sampling time within the respective counting interval in order to determine the voltage difference, for which the following applies: 3/f.sub.0<t.sub.s,i<4.5/f.sub.0, wherein f.sub.0 denotes a resonance frequency of the charge amplifier.

8. The particle detector according to claim 1, wherein the charge amplifier is a low-noise charge amplifier which has a signal-to-noise ratio greater than 10 dB in a predetermined frequency interval.

9. The particle detector according to claim 1, wherein the charge amplifier has a phase margin of at least 45°, preferably of at least 60°.

10. The particle detector according to claim 1, wherein the amplifier device has an amplification factor which is adjustable by means of the evaluation device as a function of the voltage signal.

11. The particle detector according to claim 10, wherein the amplification factor is adjustable over at least four decades, preferably over at least five decades.

12. The particle detector according to claim 1, wherein the measuring electrode is designed as a Faraday cup.

13. The particle detector according to claim 1, further comprising: an extraction device for extracting the charged particles.

14. The particle detector according to claim 13, further comprising: a particle guide device for guiding the charged particles from the extraction device to the measuring electrode.

15. The particle detector according to claim 13, wherein the extraction device and/or the particle guide device are designed to filter the charged particles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Exemplary embodiments are shown in the schematic drawings and are explained in the following description. In the drawings:

[0045] FIG. 1 is a schematic representation of a particle detector for detecting charged particles, comprising a measuring electrode, a detection device and an evaluation device,

[0046] FIG. 2 is a schematic representation of a voltage signal at the output of an amplifier device of the detection device and a charge signal of a charged particle with an elementary charge, which is measured by the measuring electrode,

[0047] FIG. 3 is a schematic representation of the voltage signal from FIG. 2 in which a plurality of particles impinges on the measuring electrode at different measuring times,

[0048] FIG. 4 is a schematic representation analogous to FIG. 3, in which a trigger signal and a plurality of counting windows for determining a respective number of charged particles impinging on the measuring electrode are shown,

[0049] FIG. 5 is a schematic representation of the voltage signal in one of the counting windows of FIG. 4, in which a voltage difference is determined in order to determine the number of charged particles,

[0050] FIG. 6 shows schematic representations of an ideal voltage signal at the output of a charge amplifier of the detector device with different phase margins,

[0051] FIG. 7 is a schematic representation of the signal-to-noise ratio of the charge amplifier in a frequency interval of interest,

[0052] FIG. 8 is a schematic representation of the voltage signal at the output of the amplifier device with different amplification factors, and

[0053] FIG. 9 is a schematic representation analogous to FIG. 8, in which the course of the voltage signal is shown over a longer period of time.

DETAILED DESCRIPTION

[0054] In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

[0055] FIG. 1 shows schematically the structure of a particle detector 1 for detecting charged particles 2, which in the example shown are ions. The charged particles 2 emanate from a particle source 3 which is arranged outside of the particle detector 1, for example in a chamber not shown in the drawing. The charged particles 2 enter the particle detector 1 from the environment via an aperture opening of an extraction device 4. A particle guide device 5 in the form of ion optics is used to conduct or guide the charged particles 2 from the extraction device 4 to a measuring electrode, which in the example shown is designed as a Faraday cup 6. The charged particles 2 propagate from the particle source 3 along a straight trajectory to the Faraday cup 6. The charged particles 2 entering at an input aperture of the Faraday cup 6 can be measured by the Faraday cup 6 with practically no backscattering.

[0056] Both the extraction device 4 and the particle guide device 5 can serve to filter the charged particles 2 so that only charged particles 2 with certain (known) mass-to-charge ratios can enter the Faraday cup 6. For the filtering, the extraction device 4 can, for example, have a Nielsen grid with metal grid structures. The particle guide device 6 can enable the charged particles 2 to be filtered, for example by a Fourier-based filtering method. In the event that the path between the extraction device 4 and the measuring electrode in the form of the Faraday cup 6 is small compared to the mean free path of the charged particles 2, the provision of the particle guide device 5 may be dispensed with.

[0057] The particle detector 1 also has a detection device 7 for detecting the charged particles 2 detected by the measuring electrode in the form of the Faraday cup 6 and an evaluation device 8 for determining the number of charged particles 2 detected by the detection device 7 (detection electronic unit).

[0058] In the example shown in FIG. 1, the detection device 7 is constructed as an analog circuit and should be arranged as close as possible to the Faraday cup 6.

[0059] The evaluation device 8 is used for digital processing of analog signals that are provided at the outputs of the detection device 7. For the digital processing in the evaluation device 8 (evaluation electronic unit), the analog signals are converted into digital signals with the aid of A/D converters (not shown). The evaluation device 8 can be connected to other digital devices, for example to a measuring computer or the like, with the aid of a digital interface indicated by a double arrow.

[0060] The detection device 7 of FIG. 1 has a charge amplifier 9 for converting a charge signal Q(t) generated by the charged particles 2 into a voltage signal U(t). An amplifier device 10 connected downstream of the charge amplifier 9 is used to generate an amplified voltage signal U(t) at the output of the amplifier device 10 (denoted by U(t) in FIG. 1).

[0061] FIG. 2 shows the current I.sub.ion(t) when an elementary charge (a charged particle 2 with an elementary charge) impinges on the Faraday cup 6. The charge signal Q(t) forms the area under the current curve I.sub.ion(t) shown in FIG. 2. FIG. 2 also shows the amplified voltage signal U(t) at the output of the amplifier arrangement 10, which results from the charged particle 2 impinging with the elementary charge.

[0062] FIG. 3 shows a representation analogous to FIG. 2 of the repeated impinging of charged particles 2 with an elementary charge on the Faraday cup 6. In the example shown in FIG. 3, the charged particles 2 impinge on the Faraday cup 6 with a constant period of 16 microseconds. As can be seen in FIG. 3, the voltage signal U(t) decreases after the impinging of a respective charged particle 2 because a discharge takes place via the electronic unit of the detection device 7.

[0063] In order to evaluate the voltage signal U(t) to determine the number (or, if the charge of the particles 2 is known, equivalent) of the total charge Q.sub.N of the charged particles 2 detected, it is advantageous if the voltage signal U(t) to minimize the noise is only evaluated in a respective counting window dT.sub.i (i=1, 2, . . . ) around a measuring or collision time T.sub.i of a respective charged particle 2 (or of several charged particles 2) at the Faraday cup 6. The collision time T.sub.i typically forms the middle of the time interval of the respective counting window dT.sub.i, i.e. the counting window dT.sub.i extends from T.sub.i−dT.sub.i/2 to T.sub.i+dT.sub.i/2.

[0064] The definition of the counting window dT.sub.i can in principle take place in the evaluation device 8 by a suitable evaluation of the digitized voltage signal U(t).

[0065] In the example shown in FIG. 1, the particle detector 1 has a trigger device 11, more precisely a trigger circuit, for defining a respective counting window dT.sub.i. Alternatively, the trigger device might be integrated and part of the evaluation device 8, determine a trigger signal from an analog or digital voltage signal. In the example shown, the trigger device 11 comprises a bandpass filter 12 for filtering the frequencies of the voltage signal U(t) relevant for triggering as well as a combined amplifier and trigger electronic unit 13, which in the example shown has a threshold switch in the form of a Schmitt trigger. The (binary) trigger signal T(t) present at the output of the trigger device 11 is also shown in FIG. 4. The switchover between the two binary states of the trigger signal T(t) occurs when the voltage signal U(t) exceeds or falls below a threshold value. The trigger signal T(t) thus also enables the collision times T.sub.i of the charged particles 2 on the Faraday cup 6 to be counted.

[0066] As can also be seen from FIG. 4, the trigger signal generated during triggering or the corresponding counting window dT′.sub.i determined by the trigger device 11 is in each case before the measuring time T.sub.i or, more specifically, the measuring time T.sub.i forms the end of the counting window dT′.sub.i. To determine the number Q.sub.N of charged particles 2, the counting window dT′.sub.i is therefore shifted in time to later times by the evaluation device such that the counting window (dT.sub.i) is around the respective measuring time T.sub.i. Preferably, the counting window dT.sub.i is shifted by dT.sub.i/2, so that the measuring time T.sub.i is in the middle of the counting window dT.sub.i.

[0067] The counting windows dT.sub.i shown in FIG. 4 are selected to be comparatively long in order to clarify the representation; the duration of a respective counting window dT.sub.i is typically significantly shorter than shown in FIG. 4.

[0068] The voltage signal U(t) at the output of the amplifier device 10 is typically very noisy, particularly in the case of weak collisions (i.e. when a small quantity of particles 2 collide) at the Faraday cup 6. The noise is caused by the electronic unit, for example by the charge amplifier 9, as described in more detail below.

[0069] FIG. 5 shows the (noisy) voltage signal U(t) during a counting window dT.sub.i, an ideal voltage signal U.sub.id(t) and a filtered voltage signal U.sub.f(t) that was generated by a sliding averaging of the voltage signal U(t) in order to increase the signal-to-noise ratio. Instead of sliding averaging, other noise-reducing algorithms can also be carried out by the evaluation device 8. For example, the typical shape of the voltage signal U(t) for a particular particle collision can be determined as a convolution term in order to improve the signal-to-noise ratio with the aid of known algorithms, for example wavelet or Fourier-based algorithms, for example by at least 10 dB or 20 dB.

[0070] To determine the number Q.sub.1 of charged particles 2 impinging on the Faraday cup 6 in the respective counting window dT.sub.i, a voltage difference U.sub.i in the voltage signal U(t), more precisely the filtered voltage signal U.sub.f(t), is determined before the measuring time T.sub.i and after the measuring time T.sub.i, as shown in FIG. 5. The voltage difference U.sub.i is determined on the basis of a baseline in the form of a voltage level that the filtered voltage signal U.sub.f(t) has at the start of the counting window dT.sub.i.

[0071] To determine the voltage difference U.sub.i, the voltage signal U(t) in the example shown in FIG. 5 is evaluated after the measuring time T.sub.i at a sampling time t.sub.s,i within the respective counting window dT.sub.i around the last particle collision time, for which the following applies: 3/f.sub.0<t.sub.s,i<4.5/f.sub.0, wherein f.sub.0 denotes a passage resonance frequency of the charge amplifier 9.

[0072] A sampling time t.sub.s,i which lies in the time interval specified above has proven to be advantageous for determining the number Q.sub.i of charged particles 2 in the respective time window dT.sub.i, as will be explained below with reference to FIG. 6.

[0073] FIG. 6 shows the voltage signal X.sub.s(t) of an idealized (standardized) voltage jump U.sub.0, wherein the following applies to the associated voltage signal X.sub.s(t):

[00003]$X_s\left(t\right)=1-e^{-\alpha .Math.{\omega }_{o}t}.Math.\left[\cos \left(\sqrt{1-{\alpha }^2}{\omega }_{o}t\right)+\frac{\alpha }{\sqrt{1-{\alpha }^2}}\sin \left(\sqrt{1-{\alpha }^2}{\omega }_{o}t\right)\right]$

[0074] wherein α forms a measurement for the phase margin of the charge amplifier 9 (with 0<α<1) and wherein ω.sub.0 denotes the passage circular resonance frequency of the charge amplifier 9 (ω.sub.0=2 TT f.sub.0, wherein f.sub.0 denotes the passage resonance frequency of the charge amplifier 9. The passage resonance frequency f.sub.0 can for example be on the order of MHz, for example: f.sub.0=10 MHz. In FIG. 6, the ideal voltage signal X.sub.s(t) is shown as an example for four values of the phase margin α=[0.900, 0.625, 0.425, 0.280]. The sampling time t.sub.s,i should on the one hand be in the steady state of the charge amplifier 9, and on the other hand not be too long after the measuring time T.sub.i. For the optimal sampling time t.sub.s,i, the interval already specified above has proven to be advantageous: 1<t.sub.s,i/1.5, with T.sub.0≈3/f.sub.0.

[0075] In order to avoid that the steady state of the charge amplifier 9 is reached at a comparatively late point in time at which further particles may impinge on the Faraday cup 6, it has proven to be advantageous if the charge amplifier 9 has a phase margin of at least 45°, preferably at least 60°, as is the case in the example shown in FIG. 5. It has also proven to be advantageous if the charge amplifier 9 is a low-noise charge amplifier 9, which has a signal-to-noise ratio of at least 10 dB in a predetermined frequency interval of interest between a lower frequency f.sub.1 and an upper frequency f.sub.2.

[0076] The signal-to-noise ratio is defined as follows:

[00004]$\mathrm{SNR}=\frac{\Delta u_Q}{u_N}=\frac{\Delta u_Q}{\sqrt{{\int }_{f_1}^{f_2}{e}_N^2.\mathrm{df}}}$

[0077] wherein Δ.sub.uQ denotes the useful voltage signal (in V) and e.sub.N.sup.2 denotes the noise component (in V/Hz) caused by the electronic unit of the charge amplifier 9. The lower frequency f.sub.1 of the frequency range of interest is typically on the order of kHz, for example from approx. 20-100 kHz; the upper frequency f.sub.2 of the frequency range of interest is typically on the order of MHz, for example 15 MHz or above.

[0078] FIG. 7 shows typical noise behavior of the low-noise charge amplifier 9, or its frequency-dependent noise density e.sub.N within a frequency range between f.sub.1/f.sub.0 and f.sub.2/f.sub.0 standardized to the resonance frequency f.sub.0 of the charge amplifier 9, wherein f.sub.0 denotes the passage resonance frequency of the charge amplifier 9.

[0079] To determine the number of impinging particles 2 or the number of (elementary) charges Q.sub.N proportional to this number, the sum of the charges Q.sub.i collected in the respective counting windows dT.sub.i is formed, which is proportional to the voltage difference calculated in the manner described above.

[0080] With a charge-to-voltage conversion factor CF of the charge amplifier 9, which can, for example, be on the order of approx. 100 nV/As, the following results for the total charge Q.sub.N in N time windows dT.sub.i (1=1, . . . , N):

[00005]$Q_N=\underset{i=1}{\overset{N}{.Math.}}{Q}_i=\underset{i=1}{\overset{N}{.Math.}}\frac{U_i}{{10}^{\mathrm{Nx}}.\mathrm{CF}}$

[0081] The factor 10.sup.Nx forms the amplification factor of the amplifier device 10.

[0082] Since the voltage signal U(t) at the output of the amplifier device 10 increases steadily in the case of closely successive collisions of charged particles 2, but with a steady increase in the signal level the incline is lower, the base level also increases more slowly with increasing time or with an increasing number of collisions—in the worst case almost logarithmically. However, the voltage signal U(t) at the output of the amplifier device 10 always returns to the initial potential in the long term. It is therefore advantageous if the amplification factor 10.sup.Nx of the amplification device 10 is adjustable.

[0083] In the amplifier device 10 shown in FIG. 1, the amplification factor 10.sup.Nx is adjustable in stages over five decades (Nx=1, . . . 5), i.e. an amplification of 10.sup.1 to 10.sup.5 can be generated with the amplifier arrangement 10. For this purpose, the amplifier device 10 has, for example, five amplifier stages connected in series which are not shown in FIG. 1 for reasons of clarity. Each amplifier stage has an amplification factor of 10.sup.1 and can be switched on or off individually by the evaluation device 8. It is understood that the amplifier device 10 can have more or fewer amplifier stages and that the amplification factor of a respective amplifier stage does not necessarily have to be one decade (10.sup.1).

[0084] As a function of the signal height (level) of the voltage signal U(t), the evaluation device 8 defines an optimal amplification factor 10.sup.Nx.opt of the amplifier device 10, at which the voltage signal U(t) at the output of the amplifier device 10 is not overdriven. For this purpose, it is advantageous if the evaluation device 8 can read out a respective output of each of the five amplifier stages of the amplifier device 10. This is also advantageous so that the evaluation device 8 can carry out an automatic offset adjustment of the amplifier stages of the amplifier device 10 when a respective measurement is started. If necessary, the amplifier device 10 resets individual amplifier stages from overdrive to readiness for measurement.

[0085] FIG. 8 shows that the optimal, non-overdriving amplification factor is 10.sup.4 both in the case that the number of charged particles 2 is elementary charges and in the case that there are approx. 10.sup.5 elementary charges (i.e. N.sub.x,opt=4). FIG. 9 shows that the respective voltage signals U(t) at the output of the amplifier device 10 decrease again after an initial increase with an increasing number of collisions. With the amplifier device 10 with five amplifier stages, a dynamic range of the particle detector 1 of approximately five decades can thus be achieved. The amplifier device 10 therefore practically takes over the function of a secondary electron multiplier (SEM).

[0086] In summary, a particle detector 1 can be implemented in the manner described above, which on the one hand is robust and has a high stability thanks to the use of the Faraday cup 6 or a measuring electrode and on the other hand covers a high dynamic range of up to 5-6 decades. As an alternative to using a Faraday cup 6, another measuring electrode can also be used in order to measure the charged particles 2. For example, the charged particles 2 can be measured in a non-destructive manner, in that induction charges are measured using the measuring electrode 6.

[0087] Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above.

[0088] Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are described as example forms of implementing the claims.