WO1999030346A1

WO1999030346A1 - Particle-optical apparatus provided with an acceleration sensor for the compensation of specimen vibrations

Info

Publication number: WO1999030346A1
Application number: PCT/IB1998/001642
Authority: WO
Inventors: Alexander Henstra; Albert Visscher; Kars Zeger Troost; Hendrik Gezinus Tappel
Original assignee: Philips Electron Optics B.V.
Priority date: 1997-12-11
Filing date: 1998-10-19
Publication date: 1999-06-17

Abstract

Vibrations of the specimen in a SEM may lead to a serious reduction of the resolution that can be achieved by means of the SEM. In order to mitigate this problem, it is known to determine the displacements caused by the vibrations of the specimen relative to the electron beam by means of a speed sensor. In the SEM the output signal of this sensor is used to compensate the vibrations of the specimen relative to the electron beam. The compensation signal is added to the control signals for the scan coils in such a manner that the electron beam moves together with the vibrations of the specimen. In a known SEM the speed of the specimen is integrated once so that the output signal of the integrator represents the displacement of the specimen. Generally speaking, this type of vibration compensation is adequate for comparatively low frequencies. According to the invention an acceleration sensor is used for higher frequencies. A higher sensitivity can thus be achieved. If compensation is made only for the natural frequency of the specimen holder, it suffices to reverse the sign of the sensor signal.

Description

Particle-optical apparatus provided with an acceleration sensor for the compensation of specimen vibrations

The invention relates to a particle-optical apparatus which includes:

* a particle source for producing a beam of electrically charged particles which travel along an optical axis of the apparatus in order to irradiate an object in the apparatus by means of the particle beam, * control means for controlling a scanning motion of the particle beam in order to scan the object to be irradiated by means of a particle beam,

* compensation means for compensating vibrations of the object to be irradiated in the apparatus relative to the beam of electrically charged particles, which means include: at least one vibration sensor for determining the relative vibrations, which sensor includes a signal output which is coupled to .an input of the control means in order to supply a compensation signal whereby the vibrations are compensated.

A particle-optical apparatus of this kind is known from United States patent No. 4,948,971. Generally speaking, particle-optical apparatus, such as electron microscopes or electron lithography apparatus, are arranged to irradiate an object to be studied or treated by means of a beam of electrically charged particles (usually an electron beam) which is produced by means of a particle source such as a thermal electron source or an electron source of the field emission type. The aim of the irradiation of the object may be to image these objects to be studied (specimens in electron microscopes) or to form very- small structures on the object, for example for microelectronics (electron lithography apparatus). In both cases a focusing lens is required for the focusing of the electron beam. This lens is used to form an electron beam focus whereby the object to be irradiated is scanned. During focusing the emissive surface of the electron source, or a part thereof, is imaged, generally at a strongly reduced scale, on the object to be irradiated: a specimen to be examined (in the Scanning Electron Microscope or SEM) or on an object on which the relevant microstructure is to be provided (in lithography apparatus). The image of the electron source (the "spot") is formed by means of an imaging lens system. The focusing lens is in that case formed by the objective lens of the spot-forming lens system.

Scanning in the SEM is performed by moving the spot of the electron beam across the specimen in mutually perpendicular directions (the x direction and the y direction) by means of scan coils which are arranged in or near the lens. Scanning in lithography apparatus is performed by moving the spot of the electron beam, again by means of scan coils, across the object to be scanned in conformity with a predetermined pattern (usually calculated by a computer). In both cases control means are provided for controlling the scanning motion. In the SEM these means are formed by signal generators which apply a sawtooth signal to the scan coils for the x direction as well as for the y direction, so that the specimen is always scanned along parallel lines. The scan coils in lithography apparatus are controlled by amplifiers which are controlled by memories which receive information from said computer.

The apparatus described in the cited US patent is a SEM which includes compensation means for compensating vibrations of the specimen relative to the electron beam. The displacements caused by the vibrations of the specimen relative to the electron beam are determined by means of a vibration sensor which is constructed as a speed sensor. The output signal thereof is integrated once so that the output signal of the integrator represents the displacement of the specimen relative to the electron beam. The integrated signal is added to the control signals for the scan coils in such a manner that the electron beam moves together with the vibrations of the specimen, so that the vibrations of the specimen relative to the electron beam are compensated. Generally speaking, this type of vibration compensation offers acceptable results for comparatively low frequencies. For higher frequencies it may occur that the sensitivity of a speed sensor is inadequate in given circumstances. It is an object of the invention to provide an alternative method of compensating the vibrations of the specimen relative to the electron beam. To this end, the particle-optical apparatus according to the invention is characterized in that the vibration sensor is constructed as an acceleration sensor for determining the acceleration of the object to be irradiated in the apparatus relative to the beam of electrically charged particles, and that the signal output of the vibration sensor is coupled to the input of the control means in such a manner that the compensation signal is applied to the input of the control means in phase opposition relative to the vibrations to be compensated.

The invention is based on the recognition of the fact that circumstances may occur in which the use of a vibration sensor in the form of an acceleration sensor is to be preferred in a SEM, because such an acceleration sensor can be constructed to be smaller (depending on the sensitivity .and the frequency range in which it operates) or because a given minimum sensitivity is desired for a given (comparatively high) frequency range. Because the sensitivity of acceleration sensors to high frequencies is generally greater th-an that of speed sensors, the invention offers a solution in those situations.

In a preferred embodiment of the invention a bandpass filter is arranged between the output of the vibration sensor and the input of the control means. This embodiment offers the possibility of tuning the acceleration sensor to a given, narrow frequency range; this range can be selected to be so narrow that, for example only the natural frequency of the specimen holder is transmitted, so that the system reacts only to this most important source of vibrations. Other frequencies can be compensated, if desired, in a known manner which is particularly suitable for the relevant frequencies. An additional advantage of this embodiment of the invention resides in the fact that the relative acceleration must be integrated twice so as to obtain the relative displacement; however, because the vibration compensation is restricted to one frequency only (and a narrow range around that frequency), integrating twice has the same effect as reversal of the sign of the signal to be integrated. The use of an integrator circuit can then be dispensed with while the interchanging of the poles of the output of the vibration sensor suffices.

A further embodiment of the invention is provided with a specimen holder for supporting the object to be irradiated, the acceleration sensor being provided on the specimen holder. In this embodiment it is achieved that the acceleration sensor reacts only to vibrations which must indeed be compensated, i.e. the vibrations of the specimen holder. Other vibrations are thus ignored to a high degree.

Another embodiment of the invention is provided with two acceleration sensors which are arranged to determine the acceleration in mutually different directions, each acceleration sensor being provided with an associated bandpass filter. In this embodiment it is achieved that all vibration directions occurring in the plane of the specimen holder (generally known as the x-y plane in this technique) are compensated. This is because a vibration of arbitrary direction can always be decomposed into two vibrations, each of which constitutes the sensitive direction of one of the sensors. The two different directions may notably be two mutually perpendicular directions, or the different directions may be chosen so as to extend parallel to the directions of two different natural vibrations, resulting in a maximum sensitivity for these vibration directions.

The invention will be described in detail hereinafter with reference to the Figures in which corresponding reference numerals denote corresponding elements. Therein: Fig. 1 shows diagrammatically a particle-optical instrument in which the invention can be used.

Fig. 2 is a diagrammatic representation in the form of a block diagram of the signal processing in the compensation means for compensating vibrations of the specimen relative to the electron beam.

Fig. 1 shows a particle-optical instrument in the form of a part of a column 2 of a scanning electron microscope (SEM). As is customary, an electron source (not shown in the Figure) in this instrument produces a beam of electrons which travels along the optical axis 4 of the instrument. This electron beam can pass through one or more electromagnetic lenses, such as a condensor lens 6, after which it reaches the lens 8. This lens, being a so-called monopole lens, forms part of a magnetic circuit which is furthermore formed by the wall 10 of the specimen chamber Y2. The lens 8, however, may also be constructed as a conventional slit lens, in which case the magnetic circuit is situated completely within the lens and hence the wall 10 of the specimen chamber \2 does not form part of the magnetic circuit. The lens 8 is used to form an electron beam focus whereby the specimen 14 is scanned. Such scanning takes place by moving the electron beam across the specimen in the x direction as well as in the y direction by means of scan coils 16 provided in the lens 8. The specimen 14 is arranged on a specimen holder 20. Secondary electrons which travel back in the direction of the lens 8 are released from the specimen. These secondary electrons are detected by a detector 24 which is provided in the bore of this lens. A control unit 26 is connected to the detector in order to activate the detector and to convert the current of detected electrons into a signal which can be used to form an image of the specimen, for example by means of a cathode ray tube. Even though the detector is arranged in the bore of the monopole lens 8 in this Figure, it is alternatively possible to arrange the detector in the space between the specimen 14 and the monopole lens 8.

On the specimen holder 20 there is mounted an acceleration sensor 30 which outputs a signal which is representative of the magnitude of the acceleration of the specimen table. For this purpose, use can be made of a commercially available acceleration sensor, for example an acceleration sensor as marketed by Brϋel & Kjaer DK-2850, Naerum DK, type 4378 accelerometer with amplifier type 2646. The sensor 30 reproduces the absolute magnitude of the acceleration of the specimen table, i.e. the acceleration relative to the specimen chamber 12 is not determined. Generally speaking, this is not objectionable, because at the frequencies of relevance the amplitudes of the vibrations of the specimen chamber 12 are much lower than those of the specimen holder 20. However, if the vibrations of the specimen chamber 12 are not negligibly small, for example the wall 10 of the specimen chamber 12 can also be provided with an acceleration sensor, the signals of the two sensors then being subtracted from one another. The output signal of the acceleration sensor 30 is applied to control means for controlling the scanning motion of the particle beam for scanning the object to be irradiated. The control means, to be described in detail with reference to Fig. 2, are symbolically represented in Fig. 1 by an amplifier 38 which is connected to the scan coils 16. The output signal of the acceleration sensor 30 is applied to the input 34 of the amplifier 32, the other input 36 of which receives a sawtooth signal for performing the scanning motion.

It is to be noted that even though Fig. 1 shows only one acceleration sensor, several acceleration sensors can be provided on the specimen table 20. Each of these sensors can then be used for a specific vibration direction; the same holds, if desired, for acceleration sensors provided on the wall 10 of the specimen chamber 12. Fig. 2 shows a block diagram of the signal processing in the compensation means for compensating vibrations of the specimen 14 relative to the electron beam. It is assumed that there are provided a first acceleration sensor 30 which is mounted on the specimen holder 20, and a second acceleration sensor 38 which is mounted on the chamber wall 10. In order to derive the relative acceleration of the specimen holder with respect to the specimen chamber, so with respect to the electron beam, the output signal of the acceleration sensor 38 must be subtracted from the output signal of the acceleration sensor 30; this operation takes place in the subtractor circuit 42. The output signal of the subtractor circuit 42 is applied, via the input 34, to an amplifier 42 which forms part of the control means 32 mentioned with reference to Fig. 1. However, it is alternatively possible for the sensors 30 .and/or 32 to be provided with their own amplifier; in that case the amplifier 42 can be omitted. After .amplification by the amplifier 42, the difference signal of the two sensors 30 and 38 is applied to a bandpass filter 44. The passband of this bandpass filter 44 is adjusted to the resonant frequency of the specimen holder 20 or, if there are several resonant frequencies, to the most important resonant frequency (i.e. the frequency having the highest amplitude). After selection of this resonant frequency by the bandpass filter 44, the signal is applied to the inverting input of a differential amplifier 46. This differential amplifier may be constructed in a customary manner as a feedback operational amplifier. The scan signal for the scanning motion of the electron beam is applied to the non-inverting input of the differential amplifier 46. The described composition of the scan signal and the compensation signal utilizes the fact that the relative acceleration must be integrated twice in order to obtain the relative displacement of the specimen holder with respect to the specimen chamber (and hence with respect to the electron beam). However, if the vibration compensation is limited to one frequency only (and to a narrow range around this frequency), integrating twice has the same effect as reversal of the sign of the signal to be integrated. This property is used by applying the compensation signal (i.e. the difference signal appearing at the output of the bandpass filter 44) to the inverting input of the differential amplifier, so that its sign is reversed, before adding it to the scan signal. After the signal thus composed has been given the correct value, if desired, by amplification or attenuation by the amplifier 46, the resultant signal is applied to the scan coils 16 which impart a displacement to the electron beam which is equal to the relative displacement of the specimen holder with respect to the beam and has the same direction as this relative displacement.

Generally speaking, the scan signal will be a sawtooth signal as represented by the symbol in the block 40. However, this is not necessarily so. It is also possible to use other waveforms for scanning the specimen. In that case separate deflection coils must be provided for the vibration compensation, i.e. one set for each direction of vibration compensation. However, it is alternatively feasible to decompose the (non- sawtooth) scan signal first into a signal for one vibration direction and a signal for the other vibration direction, after which the compensation signal is supplied for each of the vibration directions; subsequently, the two decomposed and corrected signals must be composed again.

Claims

CLAIMS:

1. A particle-optical apparatus which includes:

* a particle source for producing a beam of electrically charged particles which travel along an optical axis (4) of the apparatus in order to irradiate an object (14) in the apparatus by means of the particle beam, * control means for controlling a scanning motion of the particle beam in order to scan the object to be irradiated by means of the particle beam,

* compensation means for compensating vibrations of the object (14) to be irradiated in the apparatus relative to the beam of electrically charged particles, which means include: - at least one vibration sensor for determining the relative vibrations, which sensor includes a signal output which is coupled to an input of the control means in order to supply a compensation signal whereby the vibrations are compensated, characterized in that * the vibration sensor is constructed as an acceleration sensor for determining the acceleration of the object (14) to be irradiated in the apparatus relative to the beam of electrically charged particles, and

* that the signal output of the vibration sensor is coupled to the input of the control means in such a manner that the compensation signal is applied to the input of the control means in phase opposition relative to the vibrations to be compensated.

2. A particle-optical apparatus as claimed in Claim 1, in which a bandpass filter is arranged between the output of the vibration sensor and the input of the control means.

3. A particle-optical apparatus as claimed in Claim 1 or 2, which is provided with a specimen holder for supporting the object to be irradiated, the acceleration sensor being provided on the specimen holder.

4. A particle-optical apparatus as claimed in one of the Claims 1 to 3 which is provided with two acceleration sensors which are arranged to determine the acceleration in mutually different directions, each acceleration sensor being provided with an associated bandpass filter.