Patent Number: 044343710
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows an electron beam lithography machine 10 having a Gaussian electron beam 12. The electron beam emitted from electron source 14 passes through blanking aperture 16 and then through blanking deflector 18. Beam 12 is focused by lenses 20 and 22 and is demagnified by lens 24 onto target 26. Lens 24 contains the final beam aperture, virtual image 25 of which exits at plane 28 imaged by lens 22 and virtual image 29 at plane 30 imaged by lens 20. The equivalent virtual source is indicated at 31. In order to achieve blanking, the beam 12 must be deflected sufficiently far to miss the aperture hole in the blanking aperture 16. Typical Gaussian beam systems require deflection angles near 2.10.sup.-3 radians. With high-speed Gaussian beams, blanking times are in the range of 1 to 10 nanoseconds. Blanking is accomplished by applying beam deflecting energy to blanking deflector 18. In the blanked position, the equivalent ray path is shown in dashed lines. The dashed ray paths shown below aperture 16 are "virtual" and serve to indicate where the blanked beam would have to go if it were to pass through the final aperture (in plate 24). FIG. 2 is a schematic ray trace diagram of an electron beam machine 32 which provides a shaped beam to the target. Electron source 34 delivers electron beam 36 through lens 38 which focuses the beam toward beam shaper aperture 40. The beam 36 from source 34 is not as finely defined as in the Gaussian system, but the beam shaper aperture cuts off the sides of the beam so that the resultant electron beam below shaped aperture 30 is of selected shape and has well defined edges. Blanking deflector 42 is positioned down the beam path to apply lateral force to the beam to direct it away from the target. Lenses 44 and 46 are sequentially positioned along the beam path toward 48, and the aperture in lens 46 acts as the blanking aperture. The dot-dashed lines below blanking deflector 42 indicate the virtual source 35 and virtual paths 37 of the deflected electron beam. In shaped beam electron beam lithography machines spot exposure times are on the order of 10 to 1,000 nanoseconds, and blanking times must be approximately one order of magnitude shorter than the exposure time so that the spot edge definition is not reduced. Thus, blanking times for shaped beam systems should be in the range of 1 to 100 nanoseconds. This is the time from the beginning of the blanking pulse to the completion of beam swing out of the blanking aperture. The deflection angle in a typical shaped beam electron beam machine is about 1.10.sup.-3 radians. The blanking deflectors 18 and 42 can be electrostatic plates. For a set of electrostatic deflection plates of reasonable size, the deflection angle given above for Gaussian beams and a beam energy of 20 thousand volts, the required driving voltage becomes 20 volts which exceeds the capability of state of the art drivers with nanosecond rise time. Thus, electrostatic deflection is not practical. If blanking is to be achieved by magnetic deflection, and if a single turn coil is considered to minimize self inductance, a coil of reasonable dimensions requires a coil current of approximately one ampere, which exceeds the capability of state of the art amplifiers which have nanosecond rise time. One of the problems of electrostatic deflection is that the electrostatic plates are subject to voltage pulse reflections since they fail to provide a termination which matches the transmission line impedance. The reflected waves travel back and forth between the driver and the plates in tens of nanoseconds so that they can lead to periodic unblanking pulses until the wave energy has become dissipated. In accordance with this invention, the blanking deflectors 18 and 42 are each shaped so that each provides electrostatic as well as magnetic deflection and also serves as a line termination. FIGS. 3 and 4 illustrate the preferred embodient of the blanking apparatus 50. The blanking apparatus 50 is the same in both of these figures, and is the apparatus 18 in FIG. 1 and is the apparatus 42 in FIG. 2. In FIG. 3 apparatus 50 is shown as being connected to the output of one driver amplifier 52 through coaxial cable 54. In FIG. 4, blanking apparatus 50 is shown as being connected to differential twin driver amplifiers 56 and 58 respectively through coaxial cables 60 and 62. As seen in FIGS. 3 and 4, upper plate 64 has a beam opening 66 for passage of the electron beam 67, which is the same as beams 12 and 36. The beam 67 is to be deflected away from the blanking aperture to cut off the beam. Upper plate 64 is preferably normal to the beam path, as is lower plate 68 with its beam opening 70. In FIG. 3, upper plate 64 is connected at its adjacent edge to center conductor 72 of coaxial cable 54, while lower plate 68 is connected at its adjacent edge to outer conductor 74 of the coaxial cable 64. Secured to the outer edge of upper plate 64 and extending substantially parallel to the beam path 67 is outer deflection plate 74. Inner deflection plate 76 is substantially parallel to outer deflection plate 74 and on the opposite side of the beam path. Inner deflection plate 76 is connected to the inner edge of lower plate 68. In addition, resistor 78 interconnects the outer edges upper plate 64 and lower plate 68. As indicated in FIGS. 3 and 4, tab 80 is formed extending to the right of outer deflection plate 74 to hold resistor 78 away from plate 74. Similarly, plate 68 extends to the right of the plane defined by deflection plate 74, for resistor connection. The structure in FIG. 3 is configured to produce both electrostatic and magnetic deflection. A one turn magnetic deflection coil is composed of conductor 72, upper plate 64, resistor 78 with its connections, lower plate 68 and outer conductor 73. This one turn magnetic deflection coil is a loop which is positioned in a plane substantially parallel to the beam path 67. When driver-amplifier 52 drives current around that single loop, a magnetic field is generated which causes deflection of beam 68. The deflection due to this magnetic field is in the direction normal to deflector plates 74 and 76, that is generally left to right in FIGS. 3 and 4. At the same time, the current charges up deflection plates 74 and 76 which act as capacitive deflection plates which deflect the beam 68 in the same direction as the magnetic deflection. Resistor 78 is chosen so that the reactance of the blanking apparatus 10 matches the impedance of coaxial feedline 54, which in turn matches the output impedance of driver-amplifier 52. By combining the effect of capacitance and magnetic deflection, the drive current and voltage levels can be significantly reduced as compared to individual magnetic or capacitive deflection. In addition, unwanted pulse reflections can be avoided. These improvements are achieved by employment of the following concepts: first, reflections are minimized by sending the drive pulses through transmission lines and by using deflection elements which terminate the lines with their own characteristic impedance. The simplest approach to such impedance matching is to employ a coaxial cable 54 as indicated, in combination with the termination, impedance matching resistor 78. The upper limit of blanking frequency is where the reactive impedance of the blanking apparatus 50 becomes comparable to the impedance of resistor 78. With a 50 ohm cable 54, this occurs at about 500 megahertz. The upper frequency limit is thus about twice that of untuned capacitive or magnetic deflection systems. The effects of the electrostatic and magnetic deflection on the beam are approximately equal. In theory, the electrostatic deflection would be about three times the magnetic deflection, but the large unavoidable stray capacitance brings the effect of electrostatic deflection approximately equal to the magnetic deflection. FIG. 4 illustrates twin differential driver-amplifiers 56 and 58 respectively connected through coaxial cables 60 and 62 to the inner corners of plate 68 and 64. By the use of twin differential driver-amplifiers as indicated, either their deflection amplitude or the deflection speed for blanking can be increased by a factor of 2. In summary, the blanking apparatus 50 and its drive permits either a reduction in the drive requirements (per driver-amplifier) by a factor of 4 or an increase in blanking speed by a factor of 4 over conventional approaches. For example, with a pair of ultra-fast, commercially available operational amplifiers, Gaussian beams can be blanked at about 1 nanosecond. Such amplifiers are available from National Semiconductor and are identified as type LH0063. Such amplifiers have outputs in the order of 5 to 10 volts into 50 ohm loads and possess slew rates as high as 6,000 volts per microsecond. Such amplifiers are suitable for this application. This invention has been described in its presently contemplated best mode and it is clear that it is susceptible to numerous modifications, modes and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty. Accordingly, the scope of this invention is defined by the scope of the following claims.