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Voltage measurements using the time-of-flight electron spectrometer
Microelectronic Elsevier
Engineering
133
24 (1994) 133-138
VOLTAGE MEASUREMENTS SPECTROMETER
USING THE TNE-OF-FLIGHT
ELECTRON
A.R. Dinnis University of Edinburgh, Electrical Engineering Department, The King’s Buildings, Edinburgh EH9 3% Scotland. U.K. ABSTRACT Ihe trme-ofjllght (TO/*) electron .spectrometer measures the trme tuken,for u short pulse of secondury electrons to travel ,fiom the .speclmen surfuce to u ,f&st mrcrochunnel plute detector pluced some wuy up the electron-opticul column. The shupe of’ the resultmg pulse 1s directly rekuted to the complete energy spectrum of‘ the emitted secondury electrons. Results ure presented jtir un experrmentd set-up bused on u stun&d scunnrng electron microscope column with un “single-pole” find lens which allows u udd~tronul reiutively lurge unobstructed workmg drstunce above the .speclmen. Methods ,for the ruprd e.xtructlon of yuuntltutlve voltuge dutu ure described.
Advantages of the TOF spectrometer There are significant signal-to-noise advantages in using an appropriate multi-channel spectrometer compared with the retarding-field spectrometer which is commonly used in electron-beam IC testing instruments [I], [2]. If the full range of low-energy secondaries is detected, this advantage can be up to a factor of 30 times, but is still significant even when only the higher-energy secondaries are used. However, this is not the only advantage; the fact that the whole spectrum of electron energies is available in every pulse is also important. Each pulse contains all the information required to find the potential of the point of emission, provided there are sufficient electrons in the pulse. Thus, with suitable processing electronics it will be possible to produce rapid measurements of potentials without the need for “linearising” feedback as is used in retarding-field spectrometers [3]. This brings, among other advantages, the possibility of simpler systems for quantitative multistroboscopic sampling and for the rapid production of quantitative voltage contour maps. Experimental arrangements As described previously [4J, in order to extract the secondary electrons and ensure that the time-of -flight is constant irrespective of the angle of emission, it is necessary to place the specimen at or above the peak of the field distribution in a magnetic immersion lens. In the experimental set-up (Figure 1) this is a “single-pole” lens [5] placed below the normal objective lens of a Cambridge instruments S-l 00 scanning electron microscope (SEM) and with its axis aligned as closely as possible with that of the column. The primary beam is pulsed by a set of chopping plates, placed in this
0167-9317/94/$07.00
0 1994 - Elsevicr Science B.V. All rights reserved.
134
A.R. Dinnis I Vollap
by T’OF
ELECTRON
GUN
L-l
-L-r’
ANODE PLATES
i2ik?ENSER
CHANNEL-PLATE ASSEMBLY
’
:
’ :
I
DRIFT TUBE
I
’ I I ’ 0: ;i I :
STANDARD
1 :
LENS
‘: II
1,’
5, m
SPECIMEN ADDITIONAL SINGLE-POLE LENS
Figure 1 I SEM column with additional single-pole lens and microchannel plate detector
A.R. Dinnis I Voltage by TOF
instance immediately below the gun anode. Secondary electrons leaving the specimen will then spiral back through the fields of the additional lens and the standard lens and are collected at a point some distance above these lenses by a microchannel plate detector having a fast response. A drift-tube liner is inserted through the lens bore so that the energy and hence the time-of-flight can be controlled by varying the potential of the tube appropriately. The electron detector is housed in a short column section inserted directly below the second condenser lens and consists of two microchannel plates in cascade followed by a wide-band transimpedance amplifier. The bandwidth of this arrangement is restricted by the input impedance of the amplifier and the capacitance presented to its input terminals and in the present case is about 70 MHz. The collection efficiency of the system is restricted by the use of the standard narrow-bore SEM final lens, but is still sufficient to give a useful s&ral. It should also be realised that the magnetic field along the axis deviates considerably from the theoretical optimum and that a customdesigned column would give a significant improvement in performance. The scanned area is small because of the immersion lens and it will be necessary to use a variableaxis lens if large areas are to be scanned.
BSE peak
~___~____,____+____,____+___i-_____~__-~----~_ I , I , ! 0 I I I , : , /
20 ns per division Figure 2. Time-of-flight signal with wave applied to the specimen.
a 3V square
Voltage measurements
Figure 2 shows a typical TOF waveform recorded on a 100 MHz bandwidth analogue oscilloscope. The drift-tube was held at + I OV and a 3V square-wave applied to a bond-pad on the specimen, the two levels of the square-wave giving rise to the double trace in the figure. The left-most peak is due to the fast ( 1 keV) backscattered electrons which inevitably travel back up the column but can be separated because of the clear time interval before the secondaries arrive and also give a reliable datum for measurement of time delays of the secondaries. The shapes of the secondary electron peaks do not correspond precisely to the theoretical predictions, due to the non-
135
136
A.R. Dinnis I Voltage by 7‘OF
(a>
mm
I cb
(b) Figure 3. Method for using 2-channel system to provide quantitative voltage measurement. (a) Timing intervals Ca and Cb are chosen appropriately for the range of specimen voltages being measured. (b) Outline of circuit for measurement system.
A.R. Dinnis I Voltage by TOF
137
optimum lens fields and the restricted bandwidth of the overall system, but there is a clear voltage-contrast signal. In order to produce a low-noise picture on the analogue oscilloscope, a peak probe current of approximately 100 nA and a pulse length of about 10 ns were used. A rapid means of extracting the true voltage of the specimen from the TOF signal is desirable when testing integrated circuits in an industrial environment. The method we have hitherto used involved the recording of the TOF signal on a digitising oscilloscope, followed by off-line processing on a personal computer. While this process could be speeded up considerably, it could not give a result quickly enough, for example, to give a real-time video voltage-contrast map. A simple 2-channel system, outlined in Figure 3 has therefore been devised to give rapid results. The TOF signal is divided into two channels by the switching signals Ca and Cb, which are synchronised to the specimen signal (or to the BSE peak). As the specimen voltage changes, the relative signals in the two channels change in a predictable way; a given voltage will always give rise to the same ratio between Ra and Rb. A potential calculated from the ratio of these quantities will therefore not be affected by moderate changes in beam current or topography etc. The present system converts the repetitive pulse signals to a low-frequency signal so that the signal-processing can be carried out by analogue operational-amplifier circuts or by simple ND converters and subsequent digital processing, to give an output which is simply the specimen voltage. The system is not yet complete, but a typical output measured on one channel of the system is shown in Figure 4. A low-frequency triangular wave is applied to the specimen (Vs) and a similar triangular wave (Ra) is produced after the detectoritilter stage in channel A. This is what would be expected from a consideration of Figure 3(a), as the secondary electron peak varies in the region between the signals labelled V2 and V 1, and is similar to what would be expected from a retarding-field spectrometer.
1.0 v per division
p-i
,
____
j
i____f___l____~____i____~----~___~.
I
/
:
)
/
:
I
:
Figure 4. Output signal, Ra, for 1V triangular wave, Vs, applied to specimen. The low-pass signal detector shown in Figure 3b is included in order to simplify the circuitry required to establish the technique. Work is also proceeding on a fast
138
A.R. Dinnis I Voltage by 1‘OF
analogue system which takes as input the fast pulses Ra and Rb and outputs a video bandwidth signal which can be used in waveform reconstruction or image display. Consideration is also being given to the relative advantages of increasing the number of channels from the minimum value of?. However, it seems likely that if the ultimate precision is required, the method employing the digitising oscilloscope and software processing is the logical option. Conclusions It has been demonstrated that the TOF spectrometer can be used to make voltage measurements on a wide range of specimens, including those which require a large free working distance above the specimen. It can thus be used where mechanical probes are needed to connect with a specimen. or where additional devices such as scanning probe microscopes are to be used.
With suitable processing electronics. the TOF spectrometer rapid quantitative voltage measurements.
has the potential
to give
References 1. Khursheed A: “Multi-channel vs. conventional retarding field spectrometers for voltage contrast”, Microelectronic Engineering, 16, 43-50 (1992) 2. Dubbeldam L and Kruit P: “First experimental results of an e-beam tester with dispersive secondary electron energy analyzer”, Microelectronic Engineering, 7, 231-234 (1987) 3. Gopinath A and Sanger CC: “A technique for the linearization of voltage contrast in the scanning electron microscope”, J. Phys. E: Scientrfic instruments, 4, 334-336 (1971) 4 Dennis AR, Khursheed A and Smart PD: “The time-of-flight voltage contrast spectrometer first results”, Microelectronic Engineering, 16, 35-42 (1992) 5 Garth SCJ. Nixon WC and Spicer DF: “Magnetic field extraction of secondary electrons for accurate Integrated circuit voltage measurement”, J. Vat. Sci. Technol. B, 4, 217-220 (1986)
Engineering
133
24 (1994) 133-138
VOLTAGE MEASUREMENTS SPECTROMETER
USING THE TNE-OF-FLIGHT
ELECTRON
A.R. Dinnis University of Edinburgh, Electrical Engineering Department, The King’s Buildings, Edinburgh EH9 3% Scotland. U.K. ABSTRACT Ihe trme-ofjllght (TO/*) electron .spectrometer measures the trme tuken,for u short pulse of secondury electrons to travel ,fiom the .speclmen surfuce to u ,f&st mrcrochunnel plute detector pluced some wuy up the electron-opticul column. The shupe of’ the resultmg pulse 1s directly rekuted to the complete energy spectrum of‘ the emitted secondury electrons. Results ure presented jtir un experrmentd set-up bused on u stun&d scunnrng electron microscope column with un “single-pole” find lens which allows u udd~tronul reiutively lurge unobstructed workmg drstunce above the .speclmen. Methods ,for the ruprd e.xtructlon of yuuntltutlve voltuge dutu ure described.
Advantages of the TOF spectrometer There are significant signal-to-noise advantages in using an appropriate multi-channel spectrometer compared with the retarding-field spectrometer which is commonly used in electron-beam IC testing instruments [I], [2]. If the full range of low-energy secondaries is detected, this advantage can be up to a factor of 30 times, but is still significant even when only the higher-energy secondaries are used. However, this is not the only advantage; the fact that the whole spectrum of electron energies is available in every pulse is also important. Each pulse contains all the information required to find the potential of the point of emission, provided there are sufficient electrons in the pulse. Thus, with suitable processing electronics it will be possible to produce rapid measurements of potentials without the need for “linearising” feedback as is used in retarding-field spectrometers [3]. This brings, among other advantages, the possibility of simpler systems for quantitative multistroboscopic sampling and for the rapid production of quantitative voltage contour maps. Experimental arrangements As described previously [4J, in order to extract the secondary electrons and ensure that the time-of -flight is constant irrespective of the angle of emission, it is necessary to place the specimen at or above the peak of the field distribution in a magnetic immersion lens. In the experimental set-up (Figure 1) this is a “single-pole” lens [5] placed below the normal objective lens of a Cambridge instruments S-l 00 scanning electron microscope (SEM) and with its axis aligned as closely as possible with that of the column. The primary beam is pulsed by a set of chopping plates, placed in this
0167-9317/94/$07.00
0 1994 - Elsevicr Science B.V. All rights reserved.
134
A.R. Dinnis I Vollap
by T’OF
ELECTRON
GUN
L-l
-L-r’
ANODE PLATES
i2ik?ENSER
CHANNEL-PLATE ASSEMBLY
’
:
’ :
I
DRIFT TUBE
I
’ I I ’ 0: ;i I :
STANDARD
1 :
LENS
‘: II
1,’
5, m
SPECIMEN ADDITIONAL SINGLE-POLE LENS
Figure 1 I SEM column with additional single-pole lens and microchannel plate detector
A.R. Dinnis I Voltage by TOF
instance immediately below the gun anode. Secondary electrons leaving the specimen will then spiral back through the fields of the additional lens and the standard lens and are collected at a point some distance above these lenses by a microchannel plate detector having a fast response. A drift-tube liner is inserted through the lens bore so that the energy and hence the time-of-flight can be controlled by varying the potential of the tube appropriately. The electron detector is housed in a short column section inserted directly below the second condenser lens and consists of two microchannel plates in cascade followed by a wide-band transimpedance amplifier. The bandwidth of this arrangement is restricted by the input impedance of the amplifier and the capacitance presented to its input terminals and in the present case is about 70 MHz. The collection efficiency of the system is restricted by the use of the standard narrow-bore SEM final lens, but is still sufficient to give a useful s&ral. It should also be realised that the magnetic field along the axis deviates considerably from the theoretical optimum and that a customdesigned column would give a significant improvement in performance. The scanned area is small because of the immersion lens and it will be necessary to use a variableaxis lens if large areas are to be scanned.
BSE peak
~___~____,____+____,____+___i-_____~__-~----~_ I , I , ! 0 I I I , : , /
20 ns per division Figure 2. Time-of-flight signal with wave applied to the specimen.
a 3V square
Voltage measurements
Figure 2 shows a typical TOF waveform recorded on a 100 MHz bandwidth analogue oscilloscope. The drift-tube was held at + I OV and a 3V square-wave applied to a bond-pad on the specimen, the two levels of the square-wave giving rise to the double trace in the figure. The left-most peak is due to the fast ( 1 keV) backscattered electrons which inevitably travel back up the column but can be separated because of the clear time interval before the secondaries arrive and also give a reliable datum for measurement of time delays of the secondaries. The shapes of the secondary electron peaks do not correspond precisely to the theoretical predictions, due to the non-
135
136
A.R. Dinnis I Voltage by 7‘OF
(a>
mm
I cb
(b) Figure 3. Method for using 2-channel system to provide quantitative voltage measurement. (a) Timing intervals Ca and Cb are chosen appropriately for the range of specimen voltages being measured. (b) Outline of circuit for measurement system.
A.R. Dinnis I Voltage by TOF
137
optimum lens fields and the restricted bandwidth of the overall system, but there is a clear voltage-contrast signal. In order to produce a low-noise picture on the analogue oscilloscope, a peak probe current of approximately 100 nA and a pulse length of about 10 ns were used. A rapid means of extracting the true voltage of the specimen from the TOF signal is desirable when testing integrated circuits in an industrial environment. The method we have hitherto used involved the recording of the TOF signal on a digitising oscilloscope, followed by off-line processing on a personal computer. While this process could be speeded up considerably, it could not give a result quickly enough, for example, to give a real-time video voltage-contrast map. A simple 2-channel system, outlined in Figure 3 has therefore been devised to give rapid results. The TOF signal is divided into two channels by the switching signals Ca and Cb, which are synchronised to the specimen signal (or to the BSE peak). As the specimen voltage changes, the relative signals in the two channels change in a predictable way; a given voltage will always give rise to the same ratio between Ra and Rb. A potential calculated from the ratio of these quantities will therefore not be affected by moderate changes in beam current or topography etc. The present system converts the repetitive pulse signals to a low-frequency signal so that the signal-processing can be carried out by analogue operational-amplifier circuts or by simple ND converters and subsequent digital processing, to give an output which is simply the specimen voltage. The system is not yet complete, but a typical output measured on one channel of the system is shown in Figure 4. A low-frequency triangular wave is applied to the specimen (Vs) and a similar triangular wave (Ra) is produced after the detectoritilter stage in channel A. This is what would be expected from a consideration of Figure 3(a), as the secondary electron peak varies in the region between the signals labelled V2 and V 1, and is similar to what would be expected from a retarding-field spectrometer.
1.0 v per division
p-i
,
____
j
i____f___l____~____i____~----~___~.
I
/
:
)
/
:
I
:
Figure 4. Output signal, Ra, for 1V triangular wave, Vs, applied to specimen. The low-pass signal detector shown in Figure 3b is included in order to simplify the circuitry required to establish the technique. Work is also proceeding on a fast
138
A.R. Dinnis I Voltage by 1‘OF
analogue system which takes as input the fast pulses Ra and Rb and outputs a video bandwidth signal which can be used in waveform reconstruction or image display. Consideration is also being given to the relative advantages of increasing the number of channels from the minimum value of?. However, it seems likely that if the ultimate precision is required, the method employing the digitising oscilloscope and software processing is the logical option. Conclusions It has been demonstrated that the TOF spectrometer can be used to make voltage measurements on a wide range of specimens, including those which require a large free working distance above the specimen. It can thus be used where mechanical probes are needed to connect with a specimen. or where additional devices such as scanning probe microscopes are to be used.
With suitable processing electronics. the TOF spectrometer rapid quantitative voltage measurements.
has the potential
to give
References 1. Khursheed A: “Multi-channel vs. conventional retarding field spectrometers for voltage contrast”, Microelectronic Engineering, 16, 43-50 (1992) 2. Dubbeldam L and Kruit P: “First experimental results of an e-beam tester with dispersive secondary electron energy analyzer”, Microelectronic Engineering, 7, 231-234 (1987) 3. Gopinath A and Sanger CC: “A technique for the linearization of voltage contrast in the scanning electron microscope”, J. Phys. E: Scientrfic instruments, 4, 334-336 (1971) 4 Dennis AR, Khursheed A and Smart PD: “The time-of-flight voltage contrast spectrometer first results”, Microelectronic Engineering, 16, 35-42 (1992) 5 Garth SCJ. Nixon WC and Spicer DF: “Magnetic field extraction of secondary electrons for accurate Integrated circuit voltage measurement”, J. Vat. Sci. Technol. B, 4, 217-220 (1986)