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Deflection signal generator for electron beam machines
Microelectronics and Reliability
Pergamon Press 1970. Vol. 9, pp. 345-347.
Printed in Great Britain
D E F L E C T I O N S I G N A L GENERATOR FOR E L E C T R O N BEAM M A C H I N E S N. M. LINDSAY, A. R. D I N N I S a n d W. E. J. FARVIS
Electrical Engineering Department, University of Edinburgh A b s t r a c t - - A brief review of the available techniques for reducing electron beam aberrations is
presented. The paper then describes a D-to-A system capable of resolving 1 part in 100,000 in the analog output. This system is being developed to control the spot deflection in a high resolution electron beam machine. INTRODUCTION
IT is expected that the role of the electron beam machine (EBM) will become increasingly important as more sophisticated electron beam systems and machining processes are developed. Whereas sub-micron circuit detail is beyond the scope of conventional photolithographic methods, the electron beam lithography process tl-3~ will be useful down to at least 500 A and, as the beam is easily deflected, a mask is not necessary and very complex patterns can be traced out. A standard method for deflecting the beam across the work piece employs two digital to analog (D-to-A) current converters supplying coils which deflect the electron beam along mutually perpendicular axes on the work piece (the X and Y axes). This method is the obvious choice for highly automated systems, as the process can be controlled by an on-line computer. Often the work piece is mounted on a mechanically moving coordinate table so as to supplement the amount of electron beam deflection that is available, since the usable range of deflection is usually only 10004000 spot diameters. Deflection beyond this range causes serious defocusing with a loss of beam resolution and power density at the work piece. CORRECTION OF DEFLECTION ABERRATIONS
T h e electron optical system is shown in Fig. 1. As the beam is deflected through a small angle 0, the spot diameter increases by an amount proportional to 0~0~, where cc is a measure of the convergence or angular aperture of the beam. Distortion
Field emitter gun
Deflection coils
0~-~ ~
./,~
~;;.,,~,.,/ ,
/Best focus
//
Work piece
FIG. 1. Sketch of electron optical system.
of the pattern due to deflection non-linearity occurs but is easily corrected by suitably processing the input signals. For high resolution work the maximum deflection for a given number of spot diameters is small and, if the beam aperture e is also small, the defocussing is greatly reduced. By employing a high brightness electron source, practical beam currents may be transmitted through a very small aperture. It is thus a great advantage to use, in the EBM system, field emitter cathodes or Schottky
345
346
N. M. L I N D S A Y , A. R. D I N N I S and W. E. J. F A R V I S
cathodes made from crystal oriented material, as these give much higher brightness (higher electron emission density and coherence) than conventional cathodes.14, 5) The resolution is further improved by dynamically focussing the beam so that the best focus lies on the surface of the work piece, (6) and this involves reducing the field of the final lens by an amount proportional to the square of the deflection. This is particularly effective if a favourable distribution is found for the deflection fields, so as to reduce astigmatism or over-focussing in the plane of the deflectionJ 7~ Such refinements allow a much greater range of beam deflection than is normal. A limit due to beam dispersion is reached at about V/AV, where V and A V are respectively the accelerating potential and the voltage spread of electrons in the beam. Thus with a system operating a field emitter source, where A V is approximately 0.1 V, and with a beam potential of 10 kV, one could expect a maximum deflection range of 100,000 spot diameters. In such a system a high precision moving co-ordinate table would be~unnecessary. PROTOTYPE DEFLECTION SIGNAL GENERATOR
The need arises for D-to-A equipment with very high dynamic range to generate the deflection currents, and also provision of an analog signal which closely approximates the square of the deflection, to be used for focus correction. These requirements are embodied in the circuit of Fig. 2. T h e circuit is used in conjunction with a primary D-to-A voltage source, in this case a ladder network with a 10 V reference supply and controlled by a 12-bit reversible binary counter. The ladder output is accurate to better than 1 part in 4000. A pulse from the final flip-flop of the binary counter triggers a reversible ring counter which selects one of four output stages. The output current, which drives the deflection coil, is the sum of currents through the precision resistors RA1 to RA¢, summed at terminal A, and the correction signal is derived from the currents through RBI to RB4,summed at terminal B. A and B are maintained near earth potential by the operational amplifiers A 1 and .42. When the first stage is switched in, S 1 conducts, S 2 to S 7 are open, and the currents through RA1 and RB1 are modulated by the ladder output via amplifier A a and transistor
•+15v
~'-
To terminal B channel Y
P = ; M' J '; R6 o-----
-
1
0
;' ;2=i:
I%
S
R.
.
.
.
.
.
REZ,
.
.
.
e,4~ R~ R,~
.
.
-15
v
-10v
l Input to ring counter
Fie. 2. Deflection signal generator (Channel X).
T 2. The biasing of T 2 to T 5 is such that only Te conducts at this stage. When the ring counter switches to the second stage (T 7 conducting), S 2 clamps RA1 and RB1 across the ladder reference voltage and the feedback to A 3 is now switched by S a to RA2 and RB2. T3 conducts, and T 4 and T 5 are still biased off. R 7 is chosen to allow T 2to be near saturation at switchover, and so the current through S 2 is minimized, similarly for the remaining two stages. The resistors RB1to RB4are chosen so that the current at terminal B increases in four linear segments approximating to the square of the current through A. T h e differential amplifiers have offset voltages of less than 1 mV which is less than a 1-bit increment in the ladder output, so the output accurately expands the input dynamic range by a factor of four. The linearity of this output is, however, limited by the tolerance of the precision resistors (0.1 per cent). Mercury reed switches, actuated by low impedance coils, were chosen in order to simplify the design and construction of the prototype and ensure
DEFLECTION
S I G N A L G E N E R A T O R F O R E L E C T R O N BEAM M A C H I N E S
accuracy. Their main drawbacks are their bulk and slow response time, but these hardly matter in the present application. T h e slow response gives rise to transients in the output of up to 2 msec duration, so the EBM is programmed to cut off the beam for this period at each switchover. By using M O S T s to switch in the feedback and transistors switching to the reference voltage, the transients can be reduced and a much higher input rate allowed. Additional reed svdtches (not shown in Fig. 2) are used to reverse the deflection coil connections, thus allowing deflection over four quadrants. DISCUSSION
Figure 3 shows the response from the prototype, which is in fact a single channel, eight-stage version. By increasing the number of stages still further, a dynamic range of up to 100,000 is feasible. However, the long-term repeatability, determined mainly by drift in the resistors and reference supply, would not normally approach this figure. It should be noted that for EBM applications, a very large dynamic range and adequate short-term repeatability are important features, but a high degree of linearity in the output is not necessary. We envisage using such a system to control machining over a 5 m m square with resolution of about 0.1 ~m, using electron lithography tech-
347
niques. During machining time the total drift should not be appreciable, and by using a clean ultra-high vacuum and avoiding ferromagnetic materials in the EBM, inaccuracies due to stray charges and magnetic hysteresis will be eliminated. A full assessment of the system is, however, not yet possible, and must await completion of the vacuum system and a high-performance version of the D-to-A generator.
Acknowledgements--The authors wish to thank the
Science Research Council who have sponsored this work. Patent rights rest with the National Research Development Corporation, who are co-operating with the development of this system.
REFERENCES
1. H. N. G. KINC, Microelectronics 1, 28 (1967). 2. T. H. P. CHANGand W. C. NIXON, I E E E 9th Ann. Symp. Electron. Ion, Laser Beam Technol., Berkeley, California (1967). 3. Y. TARUI, S. DENDA, H. BABA,S. MIYAUCHIand K. TANAKA,Microelectron. ~ Reliab. 8, 101 (1969). 4. T. E. EVERHART,J. Appl. Phys. 38, 4944 (1967). 5. A. V. CREWE, M. ISAACSONand D. JOHNSON, Rev. Scient. Instrum. 40, 241 (1969). 6. K. SCHLESINGERand R. A. WAGNER, I E E E T r a m Electron Dev. ED- 12, 478 (1965). 7. C. C. T. WANG, I E E E Trans. Electron Dev. ED-15,
603 (1968).
Pergamon Press 1970. Vol. 9, pp. 345-347.
Printed in Great Britain
D E F L E C T I O N S I G N A L GENERATOR FOR E L E C T R O N BEAM M A C H I N E S N. M. LINDSAY, A. R. D I N N I S a n d W. E. J. FARVIS
Electrical Engineering Department, University of Edinburgh A b s t r a c t - - A brief review of the available techniques for reducing electron beam aberrations is
presented. The paper then describes a D-to-A system capable of resolving 1 part in 100,000 in the analog output. This system is being developed to control the spot deflection in a high resolution electron beam machine. INTRODUCTION
IT is expected that the role of the electron beam machine (EBM) will become increasingly important as more sophisticated electron beam systems and machining processes are developed. Whereas sub-micron circuit detail is beyond the scope of conventional photolithographic methods, the electron beam lithography process tl-3~ will be useful down to at least 500 A and, as the beam is easily deflected, a mask is not necessary and very complex patterns can be traced out. A standard method for deflecting the beam across the work piece employs two digital to analog (D-to-A) current converters supplying coils which deflect the electron beam along mutually perpendicular axes on the work piece (the X and Y axes). This method is the obvious choice for highly automated systems, as the process can be controlled by an on-line computer. Often the work piece is mounted on a mechanically moving coordinate table so as to supplement the amount of electron beam deflection that is available, since the usable range of deflection is usually only 10004000 spot diameters. Deflection beyond this range causes serious defocusing with a loss of beam resolution and power density at the work piece. CORRECTION OF DEFLECTION ABERRATIONS
T h e electron optical system is shown in Fig. 1. As the beam is deflected through a small angle 0, the spot diameter increases by an amount proportional to 0~0~, where cc is a measure of the convergence or angular aperture of the beam. Distortion
Field emitter gun
Deflection coils
0~-~ ~
./,~
~;;.,,~,.,/ ,
/Best focus
//
Work piece
FIG. 1. Sketch of electron optical system.
of the pattern due to deflection non-linearity occurs but is easily corrected by suitably processing the input signals. For high resolution work the maximum deflection for a given number of spot diameters is small and, if the beam aperture e is also small, the defocussing is greatly reduced. By employing a high brightness electron source, practical beam currents may be transmitted through a very small aperture. It is thus a great advantage to use, in the EBM system, field emitter cathodes or Schottky
345
346
N. M. L I N D S A Y , A. R. D I N N I S and W. E. J. F A R V I S
cathodes made from crystal oriented material, as these give much higher brightness (higher electron emission density and coherence) than conventional cathodes.14, 5) The resolution is further improved by dynamically focussing the beam so that the best focus lies on the surface of the work piece, (6) and this involves reducing the field of the final lens by an amount proportional to the square of the deflection. This is particularly effective if a favourable distribution is found for the deflection fields, so as to reduce astigmatism or over-focussing in the plane of the deflectionJ 7~ Such refinements allow a much greater range of beam deflection than is normal. A limit due to beam dispersion is reached at about V/AV, where V and A V are respectively the accelerating potential and the voltage spread of electrons in the beam. Thus with a system operating a field emitter source, where A V is approximately 0.1 V, and with a beam potential of 10 kV, one could expect a maximum deflection range of 100,000 spot diameters. In such a system a high precision moving co-ordinate table would be~unnecessary. PROTOTYPE DEFLECTION SIGNAL GENERATOR
The need arises for D-to-A equipment with very high dynamic range to generate the deflection currents, and also provision of an analog signal which closely approximates the square of the deflection, to be used for focus correction. These requirements are embodied in the circuit of Fig. 2. T h e circuit is used in conjunction with a primary D-to-A voltage source, in this case a ladder network with a 10 V reference supply and controlled by a 12-bit reversible binary counter. The ladder output is accurate to better than 1 part in 4000. A pulse from the final flip-flop of the binary counter triggers a reversible ring counter which selects one of four output stages. The output current, which drives the deflection coil, is the sum of currents through the precision resistors RA1 to RA¢, summed at terminal A, and the correction signal is derived from the currents through RBI to RB4,summed at terminal B. A and B are maintained near earth potential by the operational amplifiers A 1 and .42. When the first stage is switched in, S 1 conducts, S 2 to S 7 are open, and the currents through RA1 and RB1 are modulated by the ladder output via amplifier A a and transistor
•+15v
~'-
To terminal B channel Y
P = ; M' J '; R6 o-----
-
1
0
;' ;2=i:
I%
S
R.
.
.
.
.
.
REZ,
.
.
.
e,4~ R~ R,~
.
.
-15
v
-10v
l Input to ring counter
Fie. 2. Deflection signal generator (Channel X).
T 2. The biasing of T 2 to T 5 is such that only Te conducts at this stage. When the ring counter switches to the second stage (T 7 conducting), S 2 clamps RA1 and RB1 across the ladder reference voltage and the feedback to A 3 is now switched by S a to RA2 and RB2. T3 conducts, and T 4 and T 5 are still biased off. R 7 is chosen to allow T 2to be near saturation at switchover, and so the current through S 2 is minimized, similarly for the remaining two stages. The resistors RB1to RB4are chosen so that the current at terminal B increases in four linear segments approximating to the square of the current through A. T h e differential amplifiers have offset voltages of less than 1 mV which is less than a 1-bit increment in the ladder output, so the output accurately expands the input dynamic range by a factor of four. The linearity of this output is, however, limited by the tolerance of the precision resistors (0.1 per cent). Mercury reed switches, actuated by low impedance coils, were chosen in order to simplify the design and construction of the prototype and ensure
DEFLECTION
S I G N A L G E N E R A T O R F O R E L E C T R O N BEAM M A C H I N E S
accuracy. Their main drawbacks are their bulk and slow response time, but these hardly matter in the present application. T h e slow response gives rise to transients in the output of up to 2 msec duration, so the EBM is programmed to cut off the beam for this period at each switchover. By using M O S T s to switch in the feedback and transistors switching to the reference voltage, the transients can be reduced and a much higher input rate allowed. Additional reed svdtches (not shown in Fig. 2) are used to reverse the deflection coil connections, thus allowing deflection over four quadrants. DISCUSSION
Figure 3 shows the response from the prototype, which is in fact a single channel, eight-stage version. By increasing the number of stages still further, a dynamic range of up to 100,000 is feasible. However, the long-term repeatability, determined mainly by drift in the resistors and reference supply, would not normally approach this figure. It should be noted that for EBM applications, a very large dynamic range and adequate short-term repeatability are important features, but a high degree of linearity in the output is not necessary. We envisage using such a system to control machining over a 5 m m square with resolution of about 0.1 ~m, using electron lithography tech-
347
niques. During machining time the total drift should not be appreciable, and by using a clean ultra-high vacuum and avoiding ferromagnetic materials in the EBM, inaccuracies due to stray charges and magnetic hysteresis will be eliminated. A full assessment of the system is, however, not yet possible, and must await completion of the vacuum system and a high-performance version of the D-to-A generator.
Acknowledgements--The authors wish to thank the
Science Research Council who have sponsored this work. Patent rights rest with the National Research Development Corporation, who are co-operating with the development of this system.
REFERENCES
1. H. N. G. KINC, Microelectronics 1, 28 (1967). 2. T. H. P. CHANGand W. C. NIXON, I E E E 9th Ann. Symp. Electron. Ion, Laser Beam Technol., Berkeley, California (1967). 3. Y. TARUI, S. DENDA, H. BABA,S. MIYAUCHIand K. TANAKA,Microelectron. ~ Reliab. 8, 101 (1969). 4. T. E. EVERHART,J. Appl. Phys. 38, 4944 (1967). 5. A. V. CREWE, M. ISAACSONand D. JOHNSON, Rev. Scient. Instrum. 40, 241 (1969). 6. K. SCHLESINGERand R. A. WAGNER, I E E E T r a m Electron Dev. ED- 12, 478 (1965). 7. C. C. T. WANG, I E E E Trans. Electron Dev. ED-15,
603 (1968).