Electrostatic deflection system with impedance matching for high positioning accuracy

An apparatus and method for deflecting electron beams with high precision and high throughput. At least one electrode of a deflecting capacitor is connected to a signal source via a coaxial cable. A termination resistor is further connected to the coaxial cable and the electrode at the joint of the coaxial cable and the electrode. The termination resistor has a resistance matched to the impedance of the coaxial cable and the electrode has an impedance matched to half of the impedance of the coaxial. The deflecting capacitors of the present invention have a minimized loss of precision due to eddy current. The spacing of electrodes in the deflecting capacitors is reduced by a factor of approximately two compared to the state-the-art system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to an electrostatic deflection system used in electron beam systems.

2. Description of the Related Art

An electron beam is a group of electrons that have approximately the same kinetic energy and move in approximately the same direction. Electron beam technologies are used in many fields, such as cathode ray tubes (CRT), lithography, scanning electron microscopes, and welding. Electron beam systems, such as scanning electron microscopes, vector and raster beam lithography systems, usually have an electron beam column, in which at least one deflection system may be used to deflect electron beams, for example, to ensure the beams strike a target specimen at a precise location.

Electron beams are generally deflected by a magnetic or an electric field. An electrostatic deflection system is a system that uses an electric field to deflect the electron beams. Because an electric field is generally faster than an magnetic field in deflecting an electron beam, electrostatic deflection systems are usually used to implement fast deflection and to achieve high throughput in the electron beam systems. The miminal configuration for an electrostatic deflection system consists a capacitor that forms an electric field between two electrodes. The electron beam is deflected as it passes through this electric. Deflection signals are supplied to the capacitor in the form of an electric signal via a transmission line, for example, a high frequency 50 Ohm coaxial cable. The electric signal results in a voltage difference across the distance between two opposing electrodes of the capacitor. The deflection system is generally designed to minimize reflections of the electric signal due to impedance mismatch in the transmission line by selecting components to optimize impedance matching.

In the state-of-the-art deflection systems, impedance matching is done by making the capacitor a part of the transmission line, which means that the capacitor has the same impedance as the transmission line and the signal flows over the capacitor.FIG. 2illustrates a schematic view of a state-of-the-art deflection system200. Electrodes202and212are disposed opposing to one another and form a capacitor210. The capacitor210is configured to deflect an electron beam201that passes through between the electrodes202and212. A signal source203is adapted to supply a deflection signal to the electrode202via a drive coaxial cable204. Particularly, the signal source203is connected to the electrode202via a core channel204aof the drive coaxial cable204and an outer channel204bof the drive coaxial cable204is grounded. The electrode202is further connected to the ground via a return coaxial cable205and a termination resistor206. As used herein, termination resistor generally refers to any component having a desired impedance and need not necessarily be a resistor. The return coaxial cable205and the termination resistor206are connected in series. Similarly, the electrode212is connected to a signal source213via a drive coaxial cable214and connected to the ground via a return coaxial cable215and a termination resistor216.

Usually, the drive coaxial cables204,214and the return coaxial cables205and215are 50 Ohm coaxial cables, and the termination resistors206and216are equivalent to 50 Ohm resistors. The impedance between each electrode202or212and the ground is 50 Ohm respectively. Therefore, each electrode202or212acts as a part of transmission lines formed by the coaxial cables204-205and214-215respectively. The impedance of virtual coaxial cables204-202-205and214-212-215matches that of the termination resistors206and216respectively. The signal source203outputs a deflection signal that is inverted to a deflection signal from the signal source213such that the resultant voltage across the capacitor210to deflect the electron beam101is twice the amplitude of each of the signal sources203and213.

During operation, the signal source203applies a voltage via drive coaxial cable204. A current passes along the drive coaxial cable204, the electrode202and the return coaxial cable205, and then flows to the termination resistor206. At point207, the impedance of the virtual coaxial cables204-202-205matches the impedance of the termination resistor206. Thus, reflection of wavefronts is minimized. Similarly, the signal source213sends a voltage (inverted of the voltage from signal source203) down the drive coaxial cable214. A current passes a long the drive coaxial cable204, the electrode212, the return coaxial cable215and the termination resistor216. As discussed above, the impedance between each electrode202/212and the ground is 50 Ohm. Since the electrodes202and212are connected to a pair of inverted voltages, the voltage at the middle points between the electrodes202and212equals to the ground. Thus, the capacitor210may be considered as two capacitors (202-ground, and ground-212) in series each has an impedance of 50 Ohm. Therefore, the impedance of the capacitor210is 100 Ohm, or twice of that of the coaxial cable used. Because the impedance of the capacitor is given, among others, by the distance between the two electrodes of the capacitor, the impedance of the capacitor210can be adjusted by distance D1between the electrodes202and212.

FIG. 3illustrates another state-of-the-art deflection system200A. For ease of understanding, identical or similar features are identified by the same numerals inFIGS. 2 and 3. The deflection system200A is similar to the deflection system200inFIG. 2. However, only one signal source203is used in the deflection system200A. The electrode212is grounded. Therefore, the electrodes202and212form a capacitor210A having an impedance matched to that of the coaxial cables204and205. The distance between the electrodes202and212is D2, which is half of D1inFIG. 2if the same kind of coaxial cables are used in both systems.

However, the state-of-the-art electrostatic deflection systems discussed above have several disadvantages. First, because there is a current flows over the electrode into the termination resistor, eddy currents are induced in the neighboring metal areas when the current in the electrode changes with the deflection voltage. The induced eddy-current in turn creates a transient magnetic field which affects the beam deflection. The induced transient magnetic field drives the beam deflection angle causing the deflection system to lose precision, especially when the dwell time of the electron beam is larger than the eddy current transients. The induced magnetic field may also affect the electron beam with a long time constant so that the beam does not settle to a target position for a relatively long time, especially when the time constant of the eddy current is much larger than the dwell time of the beam at one point. Second, two connectors are required for each electrode increasing complexity of the system. Third, a rather large spacing is required between the capacitor electrodes in order to match the impedance of a high frequency cable, which has a standard impedance of 50 Ohm.

Since fast rising, high precision and simplicity are desirable in electron beam systems, a need exists in the art for a method and system for improving electron beam deflection.

SUMMARY OF THE INVENTION

The present invention relates to an apparatus and method for deflecting electron beams with high precision and high throughput.

One embodiment provides an apparatus for deflecting an electron beam. The apparatus comprises first and second opposing electrodes, a first signal source adapted to provide a first deflection signal, a first coaxial cable having a first impedance, wherein a first end of the first coaxial cable is electrically coupled to the first signal source and a second end of the first coaxial cable is electrically coupled to the first electrode, and a first termination component having an impedance matched to the first impedance, wherein a first end of the first termination component is electrically coupled to the second end of the first coaxial cable and a second end of the termination component is electrically coupled to a ground, wherein the first electrode having an impedance relative to the ground matched to approximately half of the first impedance.

Another embodiment provides an apparatus for deflecting an electron beam. The apparatus comprises at least one of a first circuit, each of the first circuit comprising a signal source, a termination component, and a drive cable connecting the signal source and the termination, wherein the drive cable has a first impedance, the termination component is ground and has a resistance matched to the first impedance, and at least one pair of opposing electrodes adapted to deflect the electron beam, wherein one electrode in the at least one pair of electrodes is connected to a respective one of the at least one first circuit such that the electrode is connected to the signal source via the drive cable.

Yet another embodiment of provides a method for providing an electric signal to a deflection system having multiple electrodes. The method comprises providing a first closed circuit, wherein a signal source, a coaxial cable and a termination component are connected in series, connecting one of the electrodes to the first closed circuit at one point, wherein the one point is a joint of the coaxial cable and the termination component, wherein the electrode connected to the first closed circuit and the ground having an impedance matched to a parallel circuit of the coaxial cable and the termination component, and providing a first electric signal using the signal source in the first closed circuit.

DETAILED DESCRIPTION

Embodiments of the present invention may be utilized to improve electron beam deflection. By connecting a single point of an electrode of a capacitor used to deflect the electron beam to a coaxial drive cable and one end of a termination resistor, current flow across the electrode may be prevented. As a result, the inducement of eddy currents and associated transient magnetic fields may be avoided, thereby leading to more precisely controlled deflection. Further, according to some embodiments, lower total capacitor impedance may be utilized, allowing correspondingly shorter distances between electrodes.

While embodiments of the present invention will be described with reference to an electron beam deflection system, those skilled in the art will recognize that the concepts described herein may be applied to control deflection of a variety of different types of charged particle beams used in a variety of different applications. Further, while 50 Ohm transmission cables are described, those skilled in the art will recognize that transmission cables of other impedance values may be accommodated with corresponding changes in capacitor design (electrode size and spacing) and termination resistors.

An Exemplary Beam Column

Electron beam systems generally have an electron beam column configured to shape electron beams and to move shaped electron beams to desired locations on a target.FIG. 1is a schematic view of an electron beam column100in connection with one or more embodiments of the present invention. The electron beam column100includes an electron beam source105adapted to generate an electron beam101, illumination optics110, a blanking deflector115, a blanking aperture120, an upper aperture125, transfer optics130, a shape deflector135, a lower aperture140, a vector deflector145and delivery optics150.

The electron beam source105may be a thermal field emission source, thermal emission source or field emission source. The illumination optics110are configured to assist the electron beam source105to illuminate the upper aperture125, while the transfer optics130are configured to project the electron beam101through the upper aperture125onto the lower aperture140. The delivery optics150are configured to project the deflected electron beam101to a target155. The blanking aperture120is configured to prevent electron beams from reaching the upper aperture125. The upper aperture125and the lower aperture140are configured to form the shape of the electron beam101.

There are three sets of deflectors in the electron beam column100, the blanking deflector115, the shape deflector135and the vector deflector145. The blanking deflector115is configured to deflect the electron beams101in the onto the blanking aperture120, e.g. along line102, so that the electron beam101is prevented from reaching the target155. The shape deflector135and the vector deflector145are configured to shape and move the electron beam101in response to the signals generated by a flash generator. More specifically, the shape deflector135is configured to move the electron beam101such that the overlap of the image or shadow of the upper aperture125with the lower aperture140can be modified. The electron beam101that passes through the lower aperture140has the shape of the overlap of the image of the upper aperture125with the lower aperture140. In this manner, the shape deflector135is configured to shape the electron beam101. The vector deflector145is configured to move the shaped electron beam101to the desired location on the target155. The movement and shaping of the electron beam101is provided in more details in paragraphs 0025–0034 of U.S. patent application Ser. No. 10/996,020, filed in Nov. 22, 2004, entitled “Method for Elimination Low Frequency Error Sources to Critical Dimension Uniformity in Shaped Beam Writing Systems”, which is incorporated by reference.

Exemplary Beam Deflectors

Generally, a blanking deflector has one pair of deflecting electrodes. A shape deflector and a vector deflector may have at least two pairs of deflecting electrodes.FIGS. 4 and 5illustrate embodiments of a pair of deflecting electrodes which can be used in a blanking deflector, a shape deflector or a vector deflector of an electron beam column. While the deflector electrodes are schematically illustrated as flat panels (plates), other shapes, such as curved arcs of a circle, are also contemplated.

Referring first toFIG. 4, a schematic view of a deflection system300, in accordance with one embodiment of the present invention is shown. Electrodes302and312are disposed opposing to one another and form a capacitor310configured to deflect an electron beam301that passes through between the electrodes302and312. A signal source303adapted to supply a deflection signal is connected to a termination resistor306via a drive coaxial cable304. The termination resistor306is further connected to the ground. The electrode302is connected to both of the termination resistor306and the drive coaxial cable304at point307where the termination resistor306connects the drive coaxial cable304.

In one embodiment, a T-piece may be used at point307with the termination resistor306, the drive coaxial cable304and the electrode302each connecting to one leg of the T-piece. Similarly, a signal source313is adapted to supply a deflection signal to the electrode312in the same manner via a termination resistor316and a drive coaxial cable314. The termination resistors306and316is configured to match the impedance of the drive coaxial cable304and314respectively. The impedance of the electrodes302and312are matched to half of the impedance of the coaxial cables304and314respectively.

As a result, there will only be attenuation resulting from the reflection of forward moving waves. Reflected signals at points307and317see two parallel lines each having the impedance of the drive coaxial cables304and314, which corresponds to the impedance between the electrodes302and312. Because the termination resistors306and316are outside the electrodes302and312, no current flows through the electrodes302and312in static mode and only a small current flows through the electrodes302and312during voltage switching. Therefore, only very small eddy currents are created in neighboring materials during switching compared to the state-of-the-art deflection systems.

Usually, the drive coaxial cables304are314are 50 Ohm coaxial cables, and the termination resistors306and316are equivalent to 50 Ohm resistors. The impedance between each electrode302or312and the ground is 25 Ohm respectively. The signal source303outputs a deflection signal that is inverted to a deflection signal from the signal source313such that the driven force produced by the capacitor310to deflect the electron beam301may double the capacity of each signal sources303and313. The voltage at the middle points between the electrodes302and312equals to the ground. Thus, the capacitor310may be considered as two capacitors (302-ground, and ground-312) in series, each having an impedance of 25 Ohm. Therefore, the impedance of the capacitor310is 50 Ohm, or the same as the coaxial cable used and half of the capacitor210inFIG. 2. Thus, distance D3between the electrodes302and312is half of the distance D1ofFIG. 2if other parameters are kept the same.

FIG. 5is a schematic view of a deflection system300A, in accordance with another embodiment of the present invention. For ease of understanding, identical or similar features are identified by the same numerals inFIGS. 4 and 5. The deflection system300A is similar to the deflection system300inFIG. 4. However, only one signal source303is used in the deflection system300A. The electrode312is grounded. Therefore, the electrodes302and312form a capacitor310A having an impedance matched to that of parallel circuit of the drive coaxial cable304and the termination resistor306. The distance between the electrodes302and312is D4, which is half of D3ofFIG. 4if the same kind of coaxial cables are used in both systems.

FIG. 6is a schematic view of a deflection system300B, in accordance with another embodiment of the present invention. For ease of understanding, identical or similar features are identified by the same numerals inFIGS. 4 and 6. In the deflection system300B, a return coaxial cable308is used to connect the termination resistor306and the point307. The return coaxial cable308having the same impedance as the drive coaxial cable304so that impedance is still matched at the point307and between the coaxial cable308and the termination resistor306which is configured to matched to impedance of the drive coaxial cable304. By using the return coaxial cable308, the termination resistor306may be positioned in a convenient location away from the electrode302where power can be easily removed to a heat sink. Similarly, a return coaxial cable318having the same impedance of the drive coaxial cable314is used in connecting the termination resistor316and the electrode312.

FIG. 7is a schematic view of a deflection system300C, in accordance with another embodiment of the present invention. For ease of understanding, identical or similar features are identified by the same numerals inFIGS. 5 and 7. In the deflection system300C, a return coaxial cable308having the same impedance of the drive coaxial cable304is used to connect the termination resistor306and the electrode302.

Therefore, the electron deflection system of the present invention overcomes the disadvantages of the state-of-the-art deflection systems discussed above. The effect of induced transient magnetic fields is minimized because no current flows through the electrodes of the capacitor, thus, improving the precision and shortening the settling time. The system is simplified for only one connector is required for each electrode. The spacing between the capacitor electrodes is reduced by a factor of approximately two compared to the state-of-the-art systems.

CONCLUSION

By connecting a single point of an electrode of a capacitor used to deflect the electron beam to a coaxial drive cable and one end of a termination resistor, current flow across the electrode may be prevented, thereby avoiding the inducement of eddy currents and associated transient magnetic fields. As a result, more precisely controlled beam deflection may be achieved. Further, by reducing the effective impedance seen by reflected signals, lower total capacitor impedance may be utilized, allowing correspondingly shorter distances between electrodes, which may simplify deflector design and manufacture.