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A wafer charge-up-reducing system of a high-current ion implanter
Nuclear Instruments and Methods North-Holland. Amsterdam
in Physics
Research
A WAFER CHARGE-UP-REDUCING T. HIGUCHI, Sumitomo
M. SAT0
B37/38
SYSTEM
(1989) 605-608
605
OF A HIGH-CURRENT
ION IMPLANTER
and T. TAMAI
Eaton Nova Corporatron, 1501, Imazaike,
Toyo-shi, Ehime PrejI, Japan
A cylindrical target electron shower gun and a charge monitor control system have been developed. The gun has a cylinder closely surrounding the ion beam. The cylinder confines electrons in the ion beam and its vicinity. This makes the gun very effective and the positive/negative charge-up stripe on wafers is minimized, so that positive charge-up can be eliminated without a significant increase of negative charge-up. The charge monitor consists of a small charge pickup chip on the wafer side of the disk and a capacitive sensing system that is on the back side of the disk. This provides a differential signal of the voltage on the charge-up chip. On the chip the voltage is built up depending on the beam condition and the surrounding disk surface condition. The voltage of the chip is very sensitive to changes in the type of wafers and the usage of electron shower, proving the system to be a relative monitor of wafer charge. Combining the monitor and shower gun to make a feedback loop, wafer charge-up becomes controllable. The efficiency of the system is proven by many customer evaluations with their own wafers.
1. Introduction With the increase of integration of VLSI, gate oxide degradation and pattern breakage by charge-up due to high-current ion implantation has become one of the major causes of yield loss. To overcome this problem a new electron shower gun and charge monitor controller have been developed. The gun can reduce positive charge-up without significant increase of negative charge-up. The charge monitor gives signals correlating to the voltage of floating patterns on the wafers. By controlling the electron shower gun by feedback using the charge monitor signal, a very reliable charge-up suppressing system has been developed. In this article the concept of the system and a brief evaluation of results will be presented.
2. Electron shower gun
In a high-current ion implanter wafers are on a disk which is spinning at high speed. The beam is scanning slowly in the radial direction, so each pattern will first get electrons many times and then get net positive charges many times and then again electrons. As a result, on the wafers a negative/positive/negative charge-up stripe pattern will be produced in the radial direction (fig. 1). The oscilloscope traces in fig. 3 show this for one pass through the beam. This will occur when wafers are implanted either with or without an electron shower gun. When a direct flooding type of shower gun is used, electrons flood very widely on the disk so that it is difficult to reduce positive charge-up without increasing negative charge-up. Even when implanting without a shower gun, if the beam hits the grounded metal portion of the disk, many secondary electrons will come out from the disk and get onto the wafers. This may produce fairly large negative charge-up. To reduce pattern damage by charge-up it is necessary to reduce both local
2.1. Concept -
From our long period of investigation on damage of VLSI patterns on wafers it became fairly clear that the voltage on a floating pattern on a wafer changes along with the beam scan and it has both positive and negative peak voltages in each beam scan. The thin insulator film may reach break-down at either voltage peak. The reason why floating patterns have both negative and positive charge-up is that accompanying the ion beam there are electrons that spread widely around the beam. In the beam there are some electrons, but it has a net positive charge, and around the beam there are electrons, giving this area a negative charge. 0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Disk
Electrons
\
’
Fig. 1. Charge-up
on wafers.
V. NEW IMPLANTATION
EQUIPMENT
T. Higuchi er al. /A
606 Secondary
Eleclrons
\
wafer charge-up-reduang
data are shown in fig. 3 which displays the capacitive sensor voltage vs time. By emitting electrons the gun can eliminate positive charge-up without a large increase of negative charge-up as shown in fig. 3 (2). We have done many customer evaluation implantations with 1 Mbit devices and in all cases the results show that there is very little pattern damage (less than 10%) and that this is true for a very wide range of primary electron currents of the shower gun (20-100
tmm DISK LOW Energy
Seconddry
system
Electrons
tram Gun
mA).
3. Charge monitor 3.1. Concept Fig. 2. Concept of the cylindrical target electron showergun positive and negative charge-up peaks. This means we must flood electrons just on the area where the ion beam is striking and eliminate electrons flying toward other areas. The cylindrical target electron shower gun is a solution of this problem. In fig. 2 the concept of the gun is shown. The cylinder at the ground potential surrounding the beam extends very near to the wafers. This confines the electrons within the cylinder and prevents them from spreading widely. The flange at the wafer end of the cylinder is to prevent electrons from flying back to the disk. 2.2. Performance The performance of the gun has been investigated using the charge monitor explained in the next section. It was shown that the new gun can reduce positive and negative charge-up considerably even without emitting electrons. The comparison results of charge monitor
Fig. 3. Reduction of positive charge-up by the electron gun (“As+, 50 keV, 10 mA implantation).
For a long time we have been investigating charge-up monitor methods on wafers during implantation. The first step was a capacitive sensor in front of the wafers on the disk [4]. By that we can see a very clear differential signal, if we apply an artificial voltage on a flat plate imitating the wafers, but if we try to monitor charge-up on the actual patterned wafer during implantation, we cannot get the signal that we expect. After investigating the charge-up phenomena further, it became clear that the result is very natural. Over all, no significant charge-up occurs on the wafer as a whole, but local and time-dependent charging does occur. To measure charge-up as a function of time, we invented a charge monitoring system. A conceptional drawing of the charge monitor is shown in fig. 4. A small conductive charge-up chip is placed between the wafers on the front side of the disk. This chip is isolated from the disk, connected to the disk through a capacitor and guarded by another conductive element which is also insulated from the disk, to eliminate geometrical and secondary electron influences. When the ions or electrons strike this chip, charge is accumulated in the capacitor. The charge-up voltage appears on the chip.
shower Fig. 4. Concept
of the charge monitor.
T. Higuchi et al. / A wafer charge-up-reducing system
Fig. 5. Explanation
of the charge monitor
607
signal.
The voltage is dependent on the beam current and electrons surrounding it. The number of electrons that can be captured on the wafer or monitor chip is dependent on the wafer surface condition, disk condition, usage of electron shower gun, vacuum level and beam condition. The charge-up chip is wired to the capacitive sensing plate located on the back side of the disk. The capacitive sensing plate has the same voltage as the charge-up chip. A small circular plate - capacitive sensor - is installed on the process chamber cover, facing the capacitive sensing plate on the disk. A charge corresponding to the charge-up voltage is induced on the capacitive sensor when the capacitive sensing plate passes over the capacitive sensor.
Fig. 7. Change of the monitor signals with the electron gun (75As’. 50 keV. 10 mA implantation).
shower
voltage of the chip. Negative signals correspond to positive charge-up. The monitor signal changes with radial scan (beam scan). When the chip is in the ion beam spot, a positive peak is observed. On the other hand, just before the beam strikes the chip, the negative peak appears. Monitor signals, strongly relating to the type of wafers on the disk, are shown in fig. 6. It can also be observed that, when we use the new electron shower, the positive peak decreases dramatically without increasing the negative peak (fig. 7).
3.2. Performance 4. Charge monitor controller Typical data of the charge monitor are shown in fig. 5. The first peak of the signal is proportional to the
4.1. System In order to attain optimal yield for implantation using high-current ion implanters, it is preferable to have some method to control wafer charge-up. The charge-up monitor controller was developed by combining the new electron shower gun and the charge monitor mentioned above. 4.2. Function
Fig. 6. Change of the monitor signal by the type of wafers (“As+, 50 keV, 10 mA implantation).
The charge controller has two operating modes: (a) Charge monitor feedback: The filament current of the shower gun is controlled so that the voltage signal from the charge monitor is within the specified level. (b) Disk current feedback: From some experiments it V. NEW IMPLANTATION
EQUIPMENT
T. Higuchi et al. /A
608
wafer charge-up-reducing
was shown that the voltage signal from the charge monitor has a relationship to the current flow down to ground from the disk during implantation. This current signal also has some charge control value, if we control secondary electron current so that the disk current will be in the specified range.
system
The authors wish to express their sincere appreciation to the customers who kindly conducted evaluation implantations to our system and gave us valuable suggestions.
References 5. Summary We have invented a very effective electron shower gun and charge monitoring system. The effectiveness of the system has been proven by many customer evaluations using their own wafers. But we are not yet satisfied with the results and our best effort to get even better results will be continued.
[l] J.M. Hall, H. Glawischnig and W. Holtschmidt, Nucl. Instr. and Meth. B21 (1987) 350. [2] R. Outcault, C. McKenna, T. Robertson and L. Biondo, Nucl. Instr. and Meth. B21 (1987) 354. [3] A.J.T. Holmes, Phys. Rev. Al9 (1979) 389. [4] M. Mack et al., Nucl. Instr. and Meth. B6 (1985) 405.
in Physics
Research
A WAFER CHARGE-UP-REDUCING T. HIGUCHI, Sumitomo
M. SAT0
B37/38
SYSTEM
(1989) 605-608
605
OF A HIGH-CURRENT
ION IMPLANTER
and T. TAMAI
Eaton Nova Corporatron, 1501, Imazaike,
Toyo-shi, Ehime PrejI, Japan
A cylindrical target electron shower gun and a charge monitor control system have been developed. The gun has a cylinder closely surrounding the ion beam. The cylinder confines electrons in the ion beam and its vicinity. This makes the gun very effective and the positive/negative charge-up stripe on wafers is minimized, so that positive charge-up can be eliminated without a significant increase of negative charge-up. The charge monitor consists of a small charge pickup chip on the wafer side of the disk and a capacitive sensing system that is on the back side of the disk. This provides a differential signal of the voltage on the charge-up chip. On the chip the voltage is built up depending on the beam condition and the surrounding disk surface condition. The voltage of the chip is very sensitive to changes in the type of wafers and the usage of electron shower, proving the system to be a relative monitor of wafer charge. Combining the monitor and shower gun to make a feedback loop, wafer charge-up becomes controllable. The efficiency of the system is proven by many customer evaluations with their own wafers.
1. Introduction With the increase of integration of VLSI, gate oxide degradation and pattern breakage by charge-up due to high-current ion implantation has become one of the major causes of yield loss. To overcome this problem a new electron shower gun and charge monitor controller have been developed. The gun can reduce positive charge-up without significant increase of negative charge-up. The charge monitor gives signals correlating to the voltage of floating patterns on the wafers. By controlling the electron shower gun by feedback using the charge monitor signal, a very reliable charge-up suppressing system has been developed. In this article the concept of the system and a brief evaluation of results will be presented.
2. Electron shower gun
In a high-current ion implanter wafers are on a disk which is spinning at high speed. The beam is scanning slowly in the radial direction, so each pattern will first get electrons many times and then get net positive charges many times and then again electrons. As a result, on the wafers a negative/positive/negative charge-up stripe pattern will be produced in the radial direction (fig. 1). The oscilloscope traces in fig. 3 show this for one pass through the beam. This will occur when wafers are implanted either with or without an electron shower gun. When a direct flooding type of shower gun is used, electrons flood very widely on the disk so that it is difficult to reduce positive charge-up without increasing negative charge-up. Even when implanting without a shower gun, if the beam hits the grounded metal portion of the disk, many secondary electrons will come out from the disk and get onto the wafers. This may produce fairly large negative charge-up. To reduce pattern damage by charge-up it is necessary to reduce both local
2.1. Concept -
From our long period of investigation on damage of VLSI patterns on wafers it became fairly clear that the voltage on a floating pattern on a wafer changes along with the beam scan and it has both positive and negative peak voltages in each beam scan. The thin insulator film may reach break-down at either voltage peak. The reason why floating patterns have both negative and positive charge-up is that accompanying the ion beam there are electrons that spread widely around the beam. In the beam there are some electrons, but it has a net positive charge, and around the beam there are electrons, giving this area a negative charge. 0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Disk
Electrons
\
’
Fig. 1. Charge-up
on wafers.
V. NEW IMPLANTATION
EQUIPMENT
T. Higuchi er al. /A
606 Secondary
Eleclrons
\
wafer charge-up-reduang
data are shown in fig. 3 which displays the capacitive sensor voltage vs time. By emitting electrons the gun can eliminate positive charge-up without a large increase of negative charge-up as shown in fig. 3 (2). We have done many customer evaluation implantations with 1 Mbit devices and in all cases the results show that there is very little pattern damage (less than 10%) and that this is true for a very wide range of primary electron currents of the shower gun (20-100
tmm DISK LOW Energy
Seconddry
system
Electrons
tram Gun
mA).
3. Charge monitor 3.1. Concept Fig. 2. Concept of the cylindrical target electron showergun positive and negative charge-up peaks. This means we must flood electrons just on the area where the ion beam is striking and eliminate electrons flying toward other areas. The cylindrical target electron shower gun is a solution of this problem. In fig. 2 the concept of the gun is shown. The cylinder at the ground potential surrounding the beam extends very near to the wafers. This confines the electrons within the cylinder and prevents them from spreading widely. The flange at the wafer end of the cylinder is to prevent electrons from flying back to the disk. 2.2. Performance The performance of the gun has been investigated using the charge monitor explained in the next section. It was shown that the new gun can reduce positive and negative charge-up considerably even without emitting electrons. The comparison results of charge monitor
Fig. 3. Reduction of positive charge-up by the electron gun (“As+, 50 keV, 10 mA implantation).
For a long time we have been investigating charge-up monitor methods on wafers during implantation. The first step was a capacitive sensor in front of the wafers on the disk [4]. By that we can see a very clear differential signal, if we apply an artificial voltage on a flat plate imitating the wafers, but if we try to monitor charge-up on the actual patterned wafer during implantation, we cannot get the signal that we expect. After investigating the charge-up phenomena further, it became clear that the result is very natural. Over all, no significant charge-up occurs on the wafer as a whole, but local and time-dependent charging does occur. To measure charge-up as a function of time, we invented a charge monitoring system. A conceptional drawing of the charge monitor is shown in fig. 4. A small conductive charge-up chip is placed between the wafers on the front side of the disk. This chip is isolated from the disk, connected to the disk through a capacitor and guarded by another conductive element which is also insulated from the disk, to eliminate geometrical and secondary electron influences. When the ions or electrons strike this chip, charge is accumulated in the capacitor. The charge-up voltage appears on the chip.
shower Fig. 4. Concept
of the charge monitor.
T. Higuchi et al. / A wafer charge-up-reducing system
Fig. 5. Explanation
of the charge monitor
607
signal.
The voltage is dependent on the beam current and electrons surrounding it. The number of electrons that can be captured on the wafer or monitor chip is dependent on the wafer surface condition, disk condition, usage of electron shower gun, vacuum level and beam condition. The charge-up chip is wired to the capacitive sensing plate located on the back side of the disk. The capacitive sensing plate has the same voltage as the charge-up chip. A small circular plate - capacitive sensor - is installed on the process chamber cover, facing the capacitive sensing plate on the disk. A charge corresponding to the charge-up voltage is induced on the capacitive sensor when the capacitive sensing plate passes over the capacitive sensor.
Fig. 7. Change of the monitor signals with the electron gun (75As’. 50 keV. 10 mA implantation).
shower
voltage of the chip. Negative signals correspond to positive charge-up. The monitor signal changes with radial scan (beam scan). When the chip is in the ion beam spot, a positive peak is observed. On the other hand, just before the beam strikes the chip, the negative peak appears. Monitor signals, strongly relating to the type of wafers on the disk, are shown in fig. 6. It can also be observed that, when we use the new electron shower, the positive peak decreases dramatically without increasing the negative peak (fig. 7).
3.2. Performance 4. Charge monitor controller Typical data of the charge monitor are shown in fig. 5. The first peak of the signal is proportional to the
4.1. System In order to attain optimal yield for implantation using high-current ion implanters, it is preferable to have some method to control wafer charge-up. The charge-up monitor controller was developed by combining the new electron shower gun and the charge monitor mentioned above. 4.2. Function
Fig. 6. Change of the monitor signal by the type of wafers (“As+, 50 keV, 10 mA implantation).
The charge controller has two operating modes: (a) Charge monitor feedback: The filament current of the shower gun is controlled so that the voltage signal from the charge monitor is within the specified level. (b) Disk current feedback: From some experiments it V. NEW IMPLANTATION
EQUIPMENT
T. Higuchi et al. /A
608
wafer charge-up-reducing
was shown that the voltage signal from the charge monitor has a relationship to the current flow down to ground from the disk during implantation. This current signal also has some charge control value, if we control secondary electron current so that the disk current will be in the specified range.
system
The authors wish to express their sincere appreciation to the customers who kindly conducted evaluation implantations to our system and gave us valuable suggestions.
References 5. Summary We have invented a very effective electron shower gun and charge monitoring system. The effectiveness of the system has been proven by many customer evaluations using their own wafers. But we are not yet satisfied with the results and our best effort to get even better results will be continued.
[l] J.M. Hall, H. Glawischnig and W. Holtschmidt, Nucl. Instr. and Meth. B21 (1987) 350. [2] R. Outcault, C. McKenna, T. Robertson and L. Biondo, Nucl. Instr. and Meth. B21 (1987) 354. [3] A.J.T. Holmes, Phys. Rev. Al9 (1979) 389. [4] M. Mack et al., Nucl. Instr. and Meth. B6 (1985) 405.