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Miniature traveling wave deflection for electron beam analog to digital conversion
Microelectronic Engineering 84 (2007) 806–809 www.elsevier.com/locate/mee
Miniature traveling wave deflection for electron beam analog to digital conversion Rafael Aldana *, Fabian Pease Stanford University, Stanford, CA 94305, United States Available online 30 January 2007
Abstract Analog to digital conversion using miniature electron optical structures has been previously reported and shown to offer an extraordinary combination of speed and resolution (6 bits at 100 GHz sampling). This is best achieved using multiple beams. One disadvantage of that proposed system was the high-voltage required (50 V). We can reduce this value by using traveling wave deflection. But the classical square meander configuration is dispersive for all beams except the on axis one. We describe a triangular meander deflection electrode with geometry determined by the speed ratio between the beam and the slowed wave; the deflector is non-dispersive for all parallel beams, even if they are misaligned with the plates main axis. This structure reduces the required voltage to 10 V and simulations show it works at least up to 15 GHz for a single beam. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Electron beam deflection; Microfabrication; Analog-digital conversion; Microstrip
1. Introduction Electron beam based analog to digital conversion (Fig. 1) benefits greatly from miniaturization of the deflection system [1], and from the use of parallel beams in order to reduce the requirements in the readout electronics needs. Miniaturization of the parallel-plate deflector lowers the transit time, thus enhancing frequency response, without sacrificing sensitivity as the deflection depends on the aspect ratio of the plates not on the size. The use of a traveling wave deflector [2] reduces the power consumption since it requires a lower (<50 V) analog voltage for the same deflection, further improves the deflection sensitivity without compromising speed, and offers the possibility of influencing parallel beams with the same deflection plate. However, miniaturization of the traveling wave deflector has challenges including feasibility of microfabrication and use with multiple beams. Different non-miniaturized traveling-wave structures (Fig. 2) have been described, each with its advantages *
Corresponding author. E-mail address: [email protected] (R. Aldana).
0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.041
and disadvantages [3]. Some of these 3D structures, such as the D helix [4] (Fig. 2c and e), would have better performance at small scales, but are more difficult to miniaturize using planar MEMS fabrication techniques; we have concentrated in the 2D planar structures. 2. Travelling wave structures The most used slow wave structure is the classical ‘castellated’ meander deflector (Fig. 3a). This structure is non-dispersive only for the beam along its center (path 1). For a sinewave deflecting voltage, V(t), an electron along path 1 will receive a periodic series of equal deflecting impulses irrespective of the frequency of V(t) because the transit time of the electron between impulses matches the delay of V(t) along the meander. However an electron along path 2, offset by x1, will still receive a periodic series of impulses but the corresponding adjacent delays of V(t) along the meander will be unequal. Thus in general the electron will receive a periodic series of pairs of unequal impulses and their sum will depend on the frequency of V(t) and on x1. A similarly unacceptable situation holds for the inclined path (#3). In earlier applications this prob-
R. Aldana, F. Pease / Microelectronic Engineering 84 (2007) 806–809
807
Dectector arays Digital output Deflection plate 0V
+Vanalog
Lz
Lx
Electron lens
-Vanalog
V Lz tan(α ) = x Vz Lx
electron bunches Sub-ps laser pulses
Photocathode
Fig. 1. ADC architecture: sub-ps mode-locked laser pulses strike a photocathode to generate electron bunches. Each electron bunch is deflected by the analog voltage and strikes one of the arrays of detectors generating a digital value. By using multiple beams and staggering the light pulses (as shown) we can increase the conversion speed.
lem was less significant because only a single beam was used and the structures were larger (>1 mm): the line sections transversal to the beam could be considered to have a constant potential, with delay lines connecting them together and providing most of the slowing. Miniaturization makes these delay lines comparable in length to the
transversal sections, so the delay caused in this sections is no longer negligible. The problem can be overcome using the ‘sawtooth’ triangular meander structure (Fig. 3b). Now the transit time s of the electron between 2 consecutive deflectors depends on the offset x1 such that s can match the corre-
Fig. 2. Different Traveling wave structures (from [3]).
808
R. Aldana, F. Pease / Microelectronic Engineering 84 (2007) 806–809
Fig. 3. Voltage impulses for input sinewave of arbitrary frequency seen by an electron along a traveling wave deflector, depending on its transverse position and inclination to the optical axis. The total deflection is the sum of the impulses. In the square type, the deflection is dependent on frequency for all beams except the on-axis beam (1a) as it is the only one that receives equal impulses; off-axis parallel beams (2a) receive unequal impulses from even and odd plates, and inclined beams (3a) see a modulated voltage. Using triangular type (b) all beams receive equal impulses independent of frequency.
sponding delay of V(t). Correct matching is achieved when sin (h/2) = Velectron/vV(t) independent of x1. For an inclined beam (path 3) the outcome is also satisfactory. This structure has all the same benefits of the meander one in terms of sensitivity and transit time. Two versions of the proposed triangular meander are being fabricated, both consisting of a zigzagging gold microstrip on a glass substrate with angle h of 27° in order to match the 5 kV electron beam speed (0.14 c) with the calculated propagation velocity of the wave in the microstrip (0.6 c). The first version (Fig. 4a) is a standard microstrip with trace width of 164 lm, patterned on top of a 100 lm glass substrate with a ground plane on the back side of the substrate; the beam stands between the trace and a
microstrip ground
top electrode
beam
0.5mm
glass microstrip ground
Fig. 4. Experimental sawtooth structure geometry: 10 fold, 1 lm trace thickness, 163 lm trace width, 100 lm glass substrate.
top electrode located 800 lm from the substrate. The second version (Fig. 4b) is an inverted microstrip of 100 lm width patterned on top of a 500 lm substrate with the top electrode acting as a ground plane located 850 lm apart.
Fig. 5. Simulation results: transmission parameter S12 (and 3 dB frequency cut-off) for the experimental structures (Fig. 4) both meander and sawtooth. A simple microstrip of equivalent length has similar simulated performance which means the imperfections (corners) cause negligible reflections.
R. Aldana, F. Pease / Microelectronic Engineering 84 (2007) 806–809
809
SMA vacuum feedthroughs electron bunches
SMA connectors
V analog
Lz = 1mm
50 Ohm term
GND
Lx = 400 μm
Phosphor or MSM detectors
b
f (Hz)
Feedthrough
Plates
Fig. 6. (a) Experimental deflecting stage: three-axis movable simple parallel plates of 1 mm width and 0.4 mm gap; fed through 6 GHz capable SMA feedthroughs. (b) Network analyzer experimental frequency response of vacuum feedthrough and deflector.
Further additions of the structure include: (a) Trimming or mitering of the corners to reduce reflections due to steep angles and uncompensated corners. (b) Patterning the ground plane and the substrate to minimize crosslink between lines and mitigate the forward z-axis wave guiding mode. (c) Hanging plates, replacing most microstrip dielectric with air, lowering the dielectric losses and maximizing the electric field affecting the beam. (d) Using a coplanar waveguide structure to facilitate UHF vacuum feeding. 3. Simulations High-frequency simulations using finite elements techniques (HFSSÓ) indicate slightly lower frequency cut-off for triangular structure than that of the square one (Fig. 5). When the simulations were performed on a structure 10 times longer than the proposed one (trace length around 300 mm), the difference was exacerbated due to the reflections caused by the steep angles in the acute corners (26°) which is less important in smaller scales. The modifications of the sawtooth structure were able to reduce the difference (through the mitering) and even overcome it, giving a 2 times frequency cut-off improvement when a ‘hanging’ double cantilever structure with patterned ground plane is used, due to lower dielectric losses in air than in the glass. Finally simulations were done to minimize the reflections in the ‘hanging’ structure caused by the discontinuity in the dielectric, from air to glass; similarly to the experimental results of [5], to compensate the step discontinuity the trace
width has to be changed not in the glass/air edge but a distance from it. Through the simulation we found this distance to be 3 times the trace width in air, in the air side. 4. Experiments The experimental system of Fig. 6 was built to validate the simulations. It consists of a thermionic electron gun and a SMA-fed 3D stage that holds the deflection plates. Engineering high-frequency feedthroughs proved challenging. The current feedthroughs (ISI 9252004 double ended high-frequency SMA UHV feedthrough) have been tested to achieve 6 GHz transmission response, and a set of simpler parallel (Fig. 6b) plates show a 5 GHz frequency response. Experiments are in progress to extend the validation to the triangular traveling wave structure. Acknowledgements The authors wish to acknowledge DARPA and the Office of Naval Research through Department of the Navy Grant Number N66001-04-1-8910, and La Caixa foundation for funding. References [1] R.F.W. Pease, K. Ioakeimidi, R. Aldana, R. Leheny, J. Vacuum Sci. Technol. B 21 (2003). [2] A.V. Haeff, Device for the method of controlling high frequency currents, US Patent no. 2064469, 1936. [3] Jack E. Day, Adv. E. E P 10 (1958). [4] S.T. Smith, R.V. Talbot, C.H. Smith, Proc. I.R.E 40 (1952). [5] A. Kraus, Rohde und Schwarz Mitteilungen 8 (December) (1956).
Miniature traveling wave deflection for electron beam analog to digital conversion Rafael Aldana *, Fabian Pease Stanford University, Stanford, CA 94305, United States Available online 30 January 2007
Abstract Analog to digital conversion using miniature electron optical structures has been previously reported and shown to offer an extraordinary combination of speed and resolution (6 bits at 100 GHz sampling). This is best achieved using multiple beams. One disadvantage of that proposed system was the high-voltage required (50 V). We can reduce this value by using traveling wave deflection. But the classical square meander configuration is dispersive for all beams except the on axis one. We describe a triangular meander deflection electrode with geometry determined by the speed ratio between the beam and the slowed wave; the deflector is non-dispersive for all parallel beams, even if they are misaligned with the plates main axis. This structure reduces the required voltage to 10 V and simulations show it works at least up to 15 GHz for a single beam. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Electron beam deflection; Microfabrication; Analog-digital conversion; Microstrip
1. Introduction Electron beam based analog to digital conversion (Fig. 1) benefits greatly from miniaturization of the deflection system [1], and from the use of parallel beams in order to reduce the requirements in the readout electronics needs. Miniaturization of the parallel-plate deflector lowers the transit time, thus enhancing frequency response, without sacrificing sensitivity as the deflection depends on the aspect ratio of the plates not on the size. The use of a traveling wave deflector [2] reduces the power consumption since it requires a lower (<50 V) analog voltage for the same deflection, further improves the deflection sensitivity without compromising speed, and offers the possibility of influencing parallel beams with the same deflection plate. However, miniaturization of the traveling wave deflector has challenges including feasibility of microfabrication and use with multiple beams. Different non-miniaturized traveling-wave structures (Fig. 2) have been described, each with its advantages *
Corresponding author. E-mail address: [email protected] (R. Aldana).
0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.041
and disadvantages [3]. Some of these 3D structures, such as the D helix [4] (Fig. 2c and e), would have better performance at small scales, but are more difficult to miniaturize using planar MEMS fabrication techniques; we have concentrated in the 2D planar structures. 2. Travelling wave structures The most used slow wave structure is the classical ‘castellated’ meander deflector (Fig. 3a). This structure is non-dispersive only for the beam along its center (path 1). For a sinewave deflecting voltage, V(t), an electron along path 1 will receive a periodic series of equal deflecting impulses irrespective of the frequency of V(t) because the transit time of the electron between impulses matches the delay of V(t) along the meander. However an electron along path 2, offset by x1, will still receive a periodic series of impulses but the corresponding adjacent delays of V(t) along the meander will be unequal. Thus in general the electron will receive a periodic series of pairs of unequal impulses and their sum will depend on the frequency of V(t) and on x1. A similarly unacceptable situation holds for the inclined path (#3). In earlier applications this prob-
R. Aldana, F. Pease / Microelectronic Engineering 84 (2007) 806–809
807
Dectector arays Digital output Deflection plate 0V
+Vanalog
Lz
Lx
Electron lens
-Vanalog
V Lz tan(α ) = x Vz Lx
electron bunches Sub-ps laser pulses
Photocathode
Fig. 1. ADC architecture: sub-ps mode-locked laser pulses strike a photocathode to generate electron bunches. Each electron bunch is deflected by the analog voltage and strikes one of the arrays of detectors generating a digital value. By using multiple beams and staggering the light pulses (as shown) we can increase the conversion speed.
lem was less significant because only a single beam was used and the structures were larger (>1 mm): the line sections transversal to the beam could be considered to have a constant potential, with delay lines connecting them together and providing most of the slowing. Miniaturization makes these delay lines comparable in length to the
transversal sections, so the delay caused in this sections is no longer negligible. The problem can be overcome using the ‘sawtooth’ triangular meander structure (Fig. 3b). Now the transit time s of the electron between 2 consecutive deflectors depends on the offset x1 such that s can match the corre-
Fig. 2. Different Traveling wave structures (from [3]).
808
R. Aldana, F. Pease / Microelectronic Engineering 84 (2007) 806–809
Fig. 3. Voltage impulses for input sinewave of arbitrary frequency seen by an electron along a traveling wave deflector, depending on its transverse position and inclination to the optical axis. The total deflection is the sum of the impulses. In the square type, the deflection is dependent on frequency for all beams except the on-axis beam (1a) as it is the only one that receives equal impulses; off-axis parallel beams (2a) receive unequal impulses from even and odd plates, and inclined beams (3a) see a modulated voltage. Using triangular type (b) all beams receive equal impulses independent of frequency.
sponding delay of V(t). Correct matching is achieved when sin (h/2) = Velectron/vV(t) independent of x1. For an inclined beam (path 3) the outcome is also satisfactory. This structure has all the same benefits of the meander one in terms of sensitivity and transit time. Two versions of the proposed triangular meander are being fabricated, both consisting of a zigzagging gold microstrip on a glass substrate with angle h of 27° in order to match the 5 kV electron beam speed (0.14 c) with the calculated propagation velocity of the wave in the microstrip (0.6 c). The first version (Fig. 4a) is a standard microstrip with trace width of 164 lm, patterned on top of a 100 lm glass substrate with a ground plane on the back side of the substrate; the beam stands between the trace and a
microstrip ground
top electrode
beam
0.5mm
glass microstrip ground
Fig. 4. Experimental sawtooth structure geometry: 10 fold, 1 lm trace thickness, 163 lm trace width, 100 lm glass substrate.
top electrode located 800 lm from the substrate. The second version (Fig. 4b) is an inverted microstrip of 100 lm width patterned on top of a 500 lm substrate with the top electrode acting as a ground plane located 850 lm apart.
Fig. 5. Simulation results: transmission parameter S12 (and 3 dB frequency cut-off) for the experimental structures (Fig. 4) both meander and sawtooth. A simple microstrip of equivalent length has similar simulated performance which means the imperfections (corners) cause negligible reflections.
R. Aldana, F. Pease / Microelectronic Engineering 84 (2007) 806–809
809
SMA vacuum feedthroughs electron bunches
SMA connectors
V analog
Lz = 1mm
50 Ohm term
GND
Lx = 400 μm
Phosphor or MSM detectors
b
f (Hz)
Feedthrough
Plates
Fig. 6. (a) Experimental deflecting stage: three-axis movable simple parallel plates of 1 mm width and 0.4 mm gap; fed through 6 GHz capable SMA feedthroughs. (b) Network analyzer experimental frequency response of vacuum feedthrough and deflector.
Further additions of the structure include: (a) Trimming or mitering of the corners to reduce reflections due to steep angles and uncompensated corners. (b) Patterning the ground plane and the substrate to minimize crosslink between lines and mitigate the forward z-axis wave guiding mode. (c) Hanging plates, replacing most microstrip dielectric with air, lowering the dielectric losses and maximizing the electric field affecting the beam. (d) Using a coplanar waveguide structure to facilitate UHF vacuum feeding. 3. Simulations High-frequency simulations using finite elements techniques (HFSSÓ) indicate slightly lower frequency cut-off for triangular structure than that of the square one (Fig. 5). When the simulations were performed on a structure 10 times longer than the proposed one (trace length around 300 mm), the difference was exacerbated due to the reflections caused by the steep angles in the acute corners (26°) which is less important in smaller scales. The modifications of the sawtooth structure were able to reduce the difference (through the mitering) and even overcome it, giving a 2 times frequency cut-off improvement when a ‘hanging’ double cantilever structure with patterned ground plane is used, due to lower dielectric losses in air than in the glass. Finally simulations were done to minimize the reflections in the ‘hanging’ structure caused by the discontinuity in the dielectric, from air to glass; similarly to the experimental results of [5], to compensate the step discontinuity the trace
width has to be changed not in the glass/air edge but a distance from it. Through the simulation we found this distance to be 3 times the trace width in air, in the air side. 4. Experiments The experimental system of Fig. 6 was built to validate the simulations. It consists of a thermionic electron gun and a SMA-fed 3D stage that holds the deflection plates. Engineering high-frequency feedthroughs proved challenging. The current feedthroughs (ISI 9252004 double ended high-frequency SMA UHV feedthrough) have been tested to achieve 6 GHz transmission response, and a set of simpler parallel (Fig. 6b) plates show a 5 GHz frequency response. Experiments are in progress to extend the validation to the triangular traveling wave structure. Acknowledgements The authors wish to acknowledge DARPA and the Office of Naval Research through Department of the Navy Grant Number N66001-04-1-8910, and La Caixa foundation for funding. References [1] R.F.W. Pease, K. Ioakeimidi, R. Aldana, R. Leheny, J. Vacuum Sci. Technol. B 21 (2003). [2] A.V. Haeff, Device for the method of controlling high frequency currents, US Patent no. 2064469, 1936. [3] Jack E. Day, Adv. E. E P 10 (1958). [4] S.T. Smith, R.V. Talbot, C.H. Smith, Proc. I.R.E 40 (1952). [5] A. Kraus, Rohde und Schwarz Mitteilungen 8 (December) (1956).