- Email: [email protected]
Particle generation in ion implanters
MEMa&*, C.C. Angel, ML. Varian
Ion Implant
Systems,
35 Day
Xoad.
Pascucci, D. Prisby Gloucester,
MA OI930,
US.4
Abstract Optical elements at potential represent a serious concern for particle generation in ion implanters. Conditioning of accelerationcolumns is a well known phenomenon in acceleratorsbut lesswell known is the fact that the microdischarging, which accompanies the conditioning process, results in high particle production. All elements at voltage will exhibit microdischarging, but careful design can greatly reduce it and control the associated particle generation. Moreover, microdischarging can be detected, so that the implanter control system tail detect and interlock recipes against microdlscharge generated particulate contamination.
1. Introduction The smaller device geometries now in production necessitateextending particle control to smaller particles and den?,?nritighter control at all particle size levels. While mechaniiai problems continue to contribute particle adders, increasingiy the problems observed are fundamental in nature and associatedwith the ion beam generation and transport processes.Beam ger .~a:-^-dparti&s ir: the anaTyzeiflight tube [l-3; and at the mass slit [I] have already been highlighted as major contributors to ion implanter particulates. The present work has focussed on particle generation by optical elements which are operated at elevated voltages. It hasbeen found that the microdischarging associatedwith such elements can be a copious source of particles. With the emphasison short, compact beamlines to maximize low energy beam transmission [4] and large beam sizesto minimize charging [s] and wafer temperature excursions [6], modern semiconductor implanter design makes the implanted wafer a relatively easy target for the particies erupting for microdischarge arc spots, By careful examination of the microdischarging elements, significant improvements have been developed and incorporated in the EIOOOHPion implanter. 2. Microdischarging and particle generation on high v&age elements The need to condition accel columns is a well known phenomenon in accelerators[s-lo]. The physics of the process is complicated. For new columns microscopic ’ eorrcspondingauthor. 0168-583X/9S/$09.50
ssm
0168-S83X/94300458-7
@ I!295ElsevierScienceE.V.
All
protuberances may be involved [9]. However, columns which are vented to air or those that are cleaned with solvents must also be conditioned, and it has been suggested that in such cases the microdischarging is due to adsorbed water or organics [lo] which initiate the regenerative exchange of positive and negative ions between tke electrodes, resulting in a cascadeand microdischarging. Conditioning is accompanied by microdischarging which is short it! duration, typically a few microsecondsor less, and, with peak currents of tens of amperes, is much less than a full arc down of the column. The resulting localized surface heating smooths protuberances as well as evaporating adsorbed contaminants. Thus the voltage holdover increaseswith time. Because of the dependence of microdischarging on organic contaminants, back sputtered wafer deposits from photoresist aggravate microdischarging. The short beamline lengths and large beam sizes of modern machines may place the column electrodes in close proximity to the wafers. The backsputtering can be considerable and the deposits may become insulating, resulting in undesirable surface electric field gradients. Under these conditions surface discharges may occur and microdischarging may become much more severe, necessitating the cleaning of the electrodes. Beam strike on the accel electrodes, which will greatly increase microdischarging, is to be avoided and frequently dictates an aperture at the entrance to the accel column. Microdischarging dissipates energy, much of it expended at the cathode arc spot [ll]. After time the electrode which behavesas the cathode for the microdischarging will become pitted where the individual discharges have struck, while the electrode which behaves as the anode receives much of the removed material [lo]. This pitting may be microscopic in size or may be visible to the eye. Material is evaporated from this arc spot but the
rightsreserved
Fig. 1. Acceleration
suppression
explosive formation cf the cathode spot also results in the ejection of numerous particles. Depending on the material the particles may be molten (e.g aluminum) or solid
Fig. 2. Machine
computer
output
electrode
micradischarge.
(graphite). As a result of radiation, molten particles may or may not solidify before striking the wafer. Anders et al, have measured the particle flux and size distributions from
with accel suppression
pulse stretching.
Fig. 3. Initial accel electrode set. Gap stress: gap I = 4 kV/mm; gap 2 = 5.4 kV/mm.
such arc spots [I?]. While it is strongly peaked at the smallest particle size measured, 0.1 p,m, the greatest mass transfer occurs for particles in the l-5 Frn range. Microdischarging occurs not only at accel column electrodes but at any electrode at potential within the ion implanter and all are potential sourcesof particles. Significant microdischarging can be expected at field gradients of the order of 1 kV/mm or more. In the case of electrodes, which have no obvious anode or cathode pair, the disc!;arge path may not be obvious. However, the beam can prt~videa conduction path to other elements in the beamline and in such casesthe proximity of the optical element to the beam may be a factor encouraging microdischarging. As in the accel column protection must be provided against incidental beam str:%e :R the ekments and afse against strike by the beam halo. In the present work two different cases of microdischarging were studied in the ElOOO,the acre1 column and the main Faraday electrostatic bias ring. In both cases
Particle
100000
3.1. Detection
of micro8ischarging
Microdischarging typically involves significant current surgesto the electrodes and thus current measurementsare preferred in their detection. Fig. 1 shows a typical microdischarge at the accel suppression electrode measured by a Tektronix AM503 current probe. The duration is about I ~.LP and the peak current, about 15 A. In order to be able to rec;lrd snrchan event using the implanter control computer, which samples only two or three times per second, a fast pulse detection circuit was included within each of the supplies. A one shot circuit to stretch the current pulse to between 400 and 500 ms was used, and
I
I
I
1
I
1 I F I I I I I i -+-
10000~
Accel column
3.
I I I
I
Adders so.2 wn
effective countermeasures were identified, which greatly reduced particle production.
As+20mA60Kv3E15 As+20mAl70Kv3El5
--
implant
Number
Fig. 4. Particle bursting as a result of a sudden increase in column voltage; 150 mm wafers.
Fig. 5. Upgraded ElOO0HP accel electrodeset.Gapstress;gap 1 = 2.2 kV/mm; gap 2 = 22 kV/m.
this stretched pulse was then electrically added to the DC sense signal. The pulse stretcher gives no information about the amplitude or energy of the microdischarge, only that the peak current exceeded a preset threshold, in this case 1 A. The particle production correlates well with the number of microdischargesmeasured in this fashion (see below). The ElOOOHPnow includes the above circuitry for each of the power supplies used ?o bias the --c*:elcolumn. Pig. 2 shows a micodischarge detected in the suppression supply. Of course,these dischargesoccur in electrode pairs but the corresponding electrode is not shown. In AdSon the software detects the discharges and interlocks implant start against excessive discharge rates through a customized configuration file. 3.2.
hitial
accel
electrode
110 kV applied across the column giving a total beam energy of 170 keV. In this figure the beam travels from left to right. The maximum voitage gradient is relatively high, of the order of 4 te 5 kV/mm. Under constant voltage conditions microdischarging is infrequent but, if a sufficient numbers of implants are performed and wafer particles measured, the implants which suffer a microdischarge average > IQ0 particles 2 0.2 pm added per 150 mm wafer more than the imp!ants that did not suffer a microdischarge.When high current implants are performed over a period of time without voltage on the accel column a buildup of backsputtered deposits on the electrodes may occur. ff the voltage is then increased microdischarging will occur. Pig. 4 shows an example of the parti& contaminatiou that lesu!ts. The points plotted are each the average of the added particles to three 150 mm wafers placed equispacedaround the disk. Particle Levelsare quite low until the voltage is elevated when they increase more than 3 orders of magnitude. Re-conditioning is stow; in
se:
The standard accel column electrode set is modeled in Pig. 3 for a 20 mA arsenic beam extracted at 50 kV with
-e
10000
- As+ 3E15 2C -_. - ..^ AstlrtlsrumAJ70Kv~
I
I
I1
20
70
120
220
170 Imhnt
Fig. 6. Particle bursting i? the upgr&d
Number
accel electrode set. I. ADVAI’JCED IMPLANTERS/PROCESS
CONTROL
t\ven~~’ implants tt:c particle levels have recovered only
one order of magniti:udeof the initial rise.
4. I. Bias ring niicrodischargittg
‘Fo overcome the problems depicted in Fig. “1: the design of the accel column was n-examined. The eiectrode set was redesigned for lower electrical stressesand the electrodes configured so that the first and last electrdes increased the shielding of electrodes two and three from beam strike and back-sputtered deposits. Fig. 5 shows the path of the same beam as mcdeled in Fig. 3. The new electrode set, which is standard in the E100OHP,has Iess than half the stresslevels of the original set (2.2 kV/mm). The electrodesare mounted to cones which are identical to those of the original design except that slots to improve pumping through the column have been machined in them. These slots are covered with a heavy gauge aluminum mesh which provides continuity of the electric potential over the cone while giving high vacuum cc>nductance. The particle results of the new cone and electrode set are shown in Fig. 6. The testing methodology is identical to that shown in Fig. 4 except that the low energy exposure periocl is proceeded by a 150 kV test, which performs the initial tube conditioning and quantifies the high energy partic%,performance. The microdischarging particle burst is reduced an order of magnitude over the initial accei set and the subsequentconditioning time is far shorter. One other column de:;‘y:R has been developed and ifsted Wiih the inIesl k+;minimizkg tk park% burs; ihat accompaniesconditioning. While this set reduces the burst shown in Fig. 6 by an order of magnitude and provides even faster conditioning, it is not well suited for decel operation and is being examined for future release with additional beam optics upgrades.
The El000 includes a Faraday in front of the wafer which has been demonstrated to be particularly beneficial in reducing the effects of water charging [$. The system includes a negatively biased suppression electrode at the entrance which generateselectrical stressesof the order of 1 kV/mm. This bias ring is also found to suffer from the effects of microdischarging. The electrode, being closer to the disc, is more prone to backsputtered deposits. Experime~tnllv. microdischar&a “-‘l..-“‘ with larger ” n is more frPnl?rm? beams &here the edges of the beam are closer to the electrode, suggesting a discharge path involving the beam. This is confirmed by ihe pitting that occurs since it is observed to be opposite the beam rather than in the gap between ground and the suppressionring. The pitting that occurs on the bias ring is smaller than on the accel column electrodes, probably because the available stored energy dissipated is much less. Detection of the microdischarging at the bias ring is accomplished in the same manner as in the accel.power supplies aliowing the implanter control system to monitor its occurrence effectively. In the EIOOOHPinterlocking of the recipes is again in place to protect against excessive microdischarging. 4.2. Ptirticle ch ff rging.
generation
by Faraday
bias ring microdis-
The number of particles generated per Faraday bias ring microdischarge is much lower than that due to the accel column, but under normal production there may be more events. Because of the stochastic nature of particle production, carefui statistical analysis of the implant re-
Particle Adders >0.2 Mm
90%Confidence Upper Limit ~. , f .-
0
10
20
30 Number
40 of Bias 2 Dbcharges
50 During Implant
Fig.7. Correlationof particleaddersto Faradaybiaselectrodemicrodischarging.
60
suits oi lots of 25 batchesof clean wafers was required to separate microdischargeproduced particles from those produced from other sources. Each batch consisted of a full disc of I8 wafers and several lots were run to test for repeatability. Snce microdischarges are short they might be expected io affect only a few wafers per implant. Thus the following variables were measured - the average adders for the disk, the sum of the counts for wafers more than three standard deviations from the average (i.e. high flyers) and the average for the disk with the high flyers excluded (truncated average). To determine the statistical significance of the microdisharging the results of the 25 batches were divided into two groups, not necessarily equal in numbers; those with high numbers of microdischarges during the implants and those with low numbers of microdischarges, The siatislica! significance of :he averages were compared using standard techniques [ 131. Clean or new bias rings condiditen rapidly after initial installation and the rate of microdischarging falls typically to 20 per hour or less. The statistical tests showed no correlation between the microdischarging and particulate contamination for the clean bias rings. To make measurements with a coated bias ring an aluminum &:trode was obtained from a prt:ducticn implanter, the entire face of which was heavily coated with a black insulating deposit. Microdischarge pits were in evidence in the corners of Ihe ring. A coated electrode i;uch as this can have as many as several hundred micrcdischargesper hour and in this case a correlation between the microdischarging and the particle contamination could easily be measured, as seen in Fig. 7. The least square fit line indicates that about 10 particles > 0.2 pm are added per wafer L>r each microdischarge.
trade. Howcsci, integmimr of the dischargecurrent puiuos showed tt;ai tlic 3U!?iipF ouikiit filter capacitor in rite ia& ring power supply cmntrihti~d to the discharg- ‘T .1C”I energy on the fitter capacitor bias int~~1?ii7si&1. :crpi ..
4.3. Correction
Microdischargirrg of optical elements at high voltages can be a significant contributor to the overall particle performance of an ion implanter. The microdischarge particle contributors are sporadic and frequently appear as so
of bias ring particle
5. Conclrr§ioss
crwmination
Unlike the accel column it was no: possible to reduce, conveniently, the electrical stressin the Faraday bias elecPartick Adders z-0.2urn
0
2
4
6
8 Number
Fig. 8. &educed
particle
adders
with
10
12
14
of Gas 2 @ischarges micrcdischarging
16 Durbq
energy
18 imipfad
control.
1.ADVANCEDIMPLANTERS/PKOCESS
CONTROL
called high flyers in the particte data. Many different types of optir;al elements can be affected and the solution of the particle problem depends on the element and its location in t1.r beamline. In general, electrical stresses should be minimized, the element must be shielded from deposits and beam strike by the beam halo, and available discharge energies must be minimized.
Acbowledgements
The authors wish to thank Tony Renau for providing the OPTION code with which the modeling was performed.
References [l] ME. Mack, A. Freytsis, M.L. Pascucci, D.J. Prisby and J. Sedgewick, in: Eon Implantation Technology, eds. D.F. Downey, M. Farley, KS. Jones and G. Ryding (North Holland, Amsterdam; I9931 p. 593. [2] M. Jones, F. Sinclair, J. Blake and S. Shields, in: Ion Implantation Technology, eds. D.F. Downey, M. Farley, KS. Jones and G. Ryding (North Holland, Amsterdam, 1993) p. 57”. [3! P. Sferlazzo, D.A. Brown, SE. Beck and J.F. G’Hanlon, in: Ion implantation Technology, eds. D.F. Downey, M. Farley,
KS. Jones and 1993) p. 565.
G.
Ryding tNorth Holland, .4msterdam,
[4] A. Renau, E. Evans and P, Sullivan, Part II of these Proceeoings (IIT’ (North-Holland, Amsterdam, 1995). [5] D.L. Smatlak. ME. Mack and S. Mehta, these Proceedings (IIT’941, Nucl. Instr. and Me&. B 96 (19951 22. 161ME. Mack, in: Handbook of Ion Implantation Technology, ed. J.F. Ziegler (North Holland, Amsterdam, 1992) p, 599. PI D.A. Brown, P. Sferlazzo and J.F. GHanIon, Nucl. Instr. and Meth. B 55 (I9911 348; D.A. Brown, P. Sferlazzo and J.F. O’Hanlon, J. Vat. Sci. Technol. 9 (199112808; D.A. Brown, P. Sfcrlazzo, S.E. Beck and J.F. O’Hanlon, J. Appl. Phys. 71 (1992) 2937. [81 M.R. Shubaly, IEEE Trans. Nucl. Sci. NS-26 (1979) 3065; IEEE Trans. Nucl. Sci. NS-30 (X983) 1399; J.D. Hepburn, M.R. Shubaly and J. I&tin, Inst. Phys. Conf. Ser. 54, chap. 5 (Institute of Physics, London, 1980) p. 158. E91 S. Humpbries, Principles of Ckarged Particle Acceleration (Wiley, New York, 1986). ri01 P. Bolin, F. Tse, W. Bell and M. Mulcahy, Vacuum Insulation, High Voltage Technical Seminar, Boston, September, 1969. fill F. Schwirzke, M.P. Hallal and X.K. Maruyama, IEEE Trans. Plasma Sci. 21 (1993) 410. WI S. Anders, A. Anders, KM. Yu, X.Y.Yao and LG. Brown, IEEE Trans. Plasma Sci. 21 (1993) 440. iI31 M.G. Natrella, Experimental Statistics, National Bureau of Standards, Washington (1966). [I41 P. Lundquist, T. Albertson, K Cahill and M.E. Mack, Part II of these Proceedings (III’941 (North-Holland, Amsterdam, 19951.
Ion Implant
Systems,
35 Day
Xoad.
Pascucci, D. Prisby Gloucester,
MA OI930,
US.4
Abstract Optical elements at potential represent a serious concern for particle generation in ion implanters. Conditioning of accelerationcolumns is a well known phenomenon in acceleratorsbut lesswell known is the fact that the microdischarging, which accompanies the conditioning process, results in high particle production. All elements at voltage will exhibit microdischarging, but careful design can greatly reduce it and control the associated particle generation. Moreover, microdischarging can be detected, so that the implanter control system tail detect and interlock recipes against microdlscharge generated particulate contamination.
1. Introduction The smaller device geometries now in production necessitateextending particle control to smaller particles and den?,?nritighter control at all particle size levels. While mechaniiai problems continue to contribute particle adders, increasingiy the problems observed are fundamental in nature and associatedwith the ion beam generation and transport processes.Beam ger .~a:-^-dparti&s ir: the anaTyzeiflight tube [l-3; and at the mass slit [I] have already been highlighted as major contributors to ion implanter particulates. The present work has focussed on particle generation by optical elements which are operated at elevated voltages. It hasbeen found that the microdischarging associatedwith such elements can be a copious source of particles. With the emphasison short, compact beamlines to maximize low energy beam transmission [4] and large beam sizesto minimize charging [s] and wafer temperature excursions [6], modern semiconductor implanter design makes the implanted wafer a relatively easy target for the particies erupting for microdischarge arc spots, By careful examination of the microdischarging elements, significant improvements have been developed and incorporated in the EIOOOHPion implanter. 2. Microdischarging and particle generation on high v&age elements The need to condition accel columns is a well known phenomenon in accelerators[s-lo]. The physics of the process is complicated. For new columns microscopic ’ eorrcspondingauthor. 0168-583X/9S/$09.50
ssm
0168-S83X/94300458-7
@ I!295ElsevierScienceE.V.
All
protuberances may be involved [9]. However, columns which are vented to air or those that are cleaned with solvents must also be conditioned, and it has been suggested that in such cases the microdischarging is due to adsorbed water or organics [lo] which initiate the regenerative exchange of positive and negative ions between tke electrodes, resulting in a cascadeand microdischarging. Conditioning is accompanied by microdischarging which is short it! duration, typically a few microsecondsor less, and, with peak currents of tens of amperes, is much less than a full arc down of the column. The resulting localized surface heating smooths protuberances as well as evaporating adsorbed contaminants. Thus the voltage holdover increaseswith time. Because of the dependence of microdischarging on organic contaminants, back sputtered wafer deposits from photoresist aggravate microdischarging. The short beamline lengths and large beam sizes of modern machines may place the column electrodes in close proximity to the wafers. The backsputtering can be considerable and the deposits may become insulating, resulting in undesirable surface electric field gradients. Under these conditions surface discharges may occur and microdischarging may become much more severe, necessitating the cleaning of the electrodes. Beam strike on the accel electrodes, which will greatly increase microdischarging, is to be avoided and frequently dictates an aperture at the entrance to the accel column. Microdischarging dissipates energy, much of it expended at the cathode arc spot [ll]. After time the electrode which behavesas the cathode for the microdischarging will become pitted where the individual discharges have struck, while the electrode which behaves as the anode receives much of the removed material [lo]. This pitting may be microscopic in size or may be visible to the eye. Material is evaporated from this arc spot but the
rightsreserved
Fig. 1. Acceleration
suppression
explosive formation cf the cathode spot also results in the ejection of numerous particles. Depending on the material the particles may be molten (e.g aluminum) or solid
Fig. 2. Machine
computer
output
electrode
micradischarge.
(graphite). As a result of radiation, molten particles may or may not solidify before striking the wafer. Anders et al, have measured the particle flux and size distributions from
with accel suppression
pulse stretching.
Fig. 3. Initial accel electrode set. Gap stress: gap I = 4 kV/mm; gap 2 = 5.4 kV/mm.
such arc spots [I?]. While it is strongly peaked at the smallest particle size measured, 0.1 p,m, the greatest mass transfer occurs for particles in the l-5 Frn range. Microdischarging occurs not only at accel column electrodes but at any electrode at potential within the ion implanter and all are potential sourcesof particles. Significant microdischarging can be expected at field gradients of the order of 1 kV/mm or more. In the case of electrodes, which have no obvious anode or cathode pair, the disc!;arge path may not be obvious. However, the beam can prt~videa conduction path to other elements in the beamline and in such casesthe proximity of the optical element to the beam may be a factor encouraging microdischarging. As in the accel column protection must be provided against incidental beam str:%e :R the ekments and afse against strike by the beam halo. In the present work two different cases of microdischarging were studied in the ElOOO,the acre1 column and the main Faraday electrostatic bias ring. In both cases
Particle
100000
3.1. Detection
of micro8ischarging
Microdischarging typically involves significant current surgesto the electrodes and thus current measurementsare preferred in their detection. Fig. 1 shows a typical microdischarge at the accel suppression electrode measured by a Tektronix AM503 current probe. The duration is about I ~.LP and the peak current, about 15 A. In order to be able to rec;lrd snrchan event using the implanter control computer, which samples only two or three times per second, a fast pulse detection circuit was included within each of the supplies. A one shot circuit to stretch the current pulse to between 400 and 500 ms was used, and
I
I
I
1
I
1 I F I I I I I i -+-
10000~
Accel column
3.
I I I
I
Adders so.2 wn
effective countermeasures were identified, which greatly reduced particle production.
As+20mA60Kv3E15 As+20mAl70Kv3El5
--
implant
Number
Fig. 4. Particle bursting as a result of a sudden increase in column voltage; 150 mm wafers.
Fig. 5. Upgraded ElOO0HP accel electrodeset.Gapstress;gap 1 = 2.2 kV/mm; gap 2 = 22 kV/m.
this stretched pulse was then electrically added to the DC sense signal. The pulse stretcher gives no information about the amplitude or energy of the microdischarge, only that the peak current exceeded a preset threshold, in this case 1 A. The particle production correlates well with the number of microdischargesmeasured in this fashion (see below). The ElOOOHPnow includes the above circuitry for each of the power supplies used ?o bias the --c*:elcolumn. Pig. 2 shows a micodischarge detected in the suppression supply. Of course,these dischargesoccur in electrode pairs but the corresponding electrode is not shown. In AdSon the software detects the discharges and interlocks implant start against excessive discharge rates through a customized configuration file. 3.2.
hitial
accel
electrode
110 kV applied across the column giving a total beam energy of 170 keV. In this figure the beam travels from left to right. The maximum voitage gradient is relatively high, of the order of 4 te 5 kV/mm. Under constant voltage conditions microdischarging is infrequent but, if a sufficient numbers of implants are performed and wafer particles measured, the implants which suffer a microdischarge average > IQ0 particles 2 0.2 pm added per 150 mm wafer more than the imp!ants that did not suffer a microdischarge.When high current implants are performed over a period of time without voltage on the accel column a buildup of backsputtered deposits on the electrodes may occur. ff the voltage is then increased microdischarging will occur. Pig. 4 shows an example of the parti& contaminatiou that lesu!ts. The points plotted are each the average of the added particles to three 150 mm wafers placed equispacedaround the disk. Particle Levelsare quite low until the voltage is elevated when they increase more than 3 orders of magnitude. Re-conditioning is stow; in
se:
The standard accel column electrode set is modeled in Pig. 3 for a 20 mA arsenic beam extracted at 50 kV with
-e
10000
- As+ 3E15 2C -_. - ..^ AstlrtlsrumAJ70Kv~
I
I
I1
20
70
120
220
170 Imhnt
Fig. 6. Particle bursting i? the upgr&d
Number
accel electrode set. I. ADVAI’JCED IMPLANTERS/PROCESS
CONTROL
t\ven~~’ implants tt:c particle levels have recovered only
one order of magniti:udeof the initial rise.
4. I. Bias ring niicrodischargittg
‘Fo overcome the problems depicted in Fig. “1: the design of the accel column was n-examined. The eiectrode set was redesigned for lower electrical stressesand the electrodes configured so that the first and last electrdes increased the shielding of electrodes two and three from beam strike and back-sputtered deposits. Fig. 5 shows the path of the same beam as mcdeled in Fig. 3. The new electrode set, which is standard in the E100OHP,has Iess than half the stresslevels of the original set (2.2 kV/mm). The electrodesare mounted to cones which are identical to those of the original design except that slots to improve pumping through the column have been machined in them. These slots are covered with a heavy gauge aluminum mesh which provides continuity of the electric potential over the cone while giving high vacuum cc>nductance. The particle results of the new cone and electrode set are shown in Fig. 6. The testing methodology is identical to that shown in Fig. 4 except that the low energy exposure periocl is proceeded by a 150 kV test, which performs the initial tube conditioning and quantifies the high energy partic%,performance. The microdischarging particle burst is reduced an order of magnitude over the initial accei set and the subsequentconditioning time is far shorter. One other column de:;‘y:R has been developed and ifsted Wiih the inIesl k+;minimizkg tk park% burs; ihat accompaniesconditioning. While this set reduces the burst shown in Fig. 6 by an order of magnitude and provides even faster conditioning, it is not well suited for decel operation and is being examined for future release with additional beam optics upgrades.
The El000 includes a Faraday in front of the wafer which has been demonstrated to be particularly beneficial in reducing the effects of water charging [$. The system includes a negatively biased suppression electrode at the entrance which generateselectrical stressesof the order of 1 kV/mm. This bias ring is also found to suffer from the effects of microdischarging. The electrode, being closer to the disc, is more prone to backsputtered deposits. Experime~tnllv. microdischar&a “-‘l..-“‘ with larger ” n is more frPnl?rm? beams &here the edges of the beam are closer to the electrode, suggesting a discharge path involving the beam. This is confirmed by ihe pitting that occurs since it is observed to be opposite the beam rather than in the gap between ground and the suppressionring. The pitting that occurs on the bias ring is smaller than on the accel column electrodes, probably because the available stored energy dissipated is much less. Detection of the microdischarging at the bias ring is accomplished in the same manner as in the accel.power supplies aliowing the implanter control system to monitor its occurrence effectively. In the EIOOOHPinterlocking of the recipes is again in place to protect against excessive microdischarging. 4.2. Ptirticle ch ff rging.
generation
by Faraday
bias ring microdis-
The number of particles generated per Faraday bias ring microdischarge is much lower than that due to the accel column, but under normal production there may be more events. Because of the stochastic nature of particle production, carefui statistical analysis of the implant re-
Particle Adders >0.2 Mm
90%Confidence Upper Limit ~. , f .-
0
10
20
30 Number
40 of Bias 2 Dbcharges
50 During Implant
Fig.7. Correlationof particleaddersto Faradaybiaselectrodemicrodischarging.
60
suits oi lots of 25 batchesof clean wafers was required to separate microdischargeproduced particles from those produced from other sources. Each batch consisted of a full disc of I8 wafers and several lots were run to test for repeatability. Snce microdischarges are short they might be expected io affect only a few wafers per implant. Thus the following variables were measured - the average adders for the disk, the sum of the counts for wafers more than three standard deviations from the average (i.e. high flyers) and the average for the disk with the high flyers excluded (truncated average). To determine the statistical significance of the microdisharging the results of the 25 batches were divided into two groups, not necessarily equal in numbers; those with high numbers of microdischarges during the implants and those with low numbers of microdischarges, The siatislica! significance of :he averages were compared using standard techniques [ 131. Clean or new bias rings condiditen rapidly after initial installation and the rate of microdischarging falls typically to 20 per hour or less. The statistical tests showed no correlation between the microdischarging and particulate contamination for the clean bias rings. To make measurements with a coated bias ring an aluminum &:trode was obtained from a prt:ducticn implanter, the entire face of which was heavily coated with a black insulating deposit. Microdischarge pits were in evidence in the corners of Ihe ring. A coated electrode i;uch as this can have as many as several hundred micrcdischargesper hour and in this case a correlation between the microdischarging and the particle contamination could easily be measured, as seen in Fig. 7. The least square fit line indicates that about 10 particles > 0.2 pm are added per wafer L>r each microdischarge.
trade. Howcsci, integmimr of the dischargecurrent puiuos showed tt;ai tlic 3U!?iipF ouikiit filter capacitor in rite ia& ring power supply cmntrihti~d to the discharg- ‘T .1C”I energy on the fitter capacitor bias int~~1?ii7si&1. :crpi ..
4.3. Correction
Microdischargirrg of optical elements at high voltages can be a significant contributor to the overall particle performance of an ion implanter. The microdischarge particle contributors are sporadic and frequently appear as so
of bias ring particle
5. Conclrr§ioss
crwmination
Unlike the accel column it was no: possible to reduce, conveniently, the electrical stressin the Faraday bias elecPartick Adders z-0.2urn
0
2
4
6
8 Number
Fig. 8. &educed
particle
adders
with
10
12
14
of Gas 2 @ischarges micrcdischarging
16 Durbq
energy
18 imipfad
control.
1.ADVANCEDIMPLANTERS/PKOCESS
CONTROL
called high flyers in the particte data. Many different types of optir;al elements can be affected and the solution of the particle problem depends on the element and its location in t1.r beamline. In general, electrical stresses should be minimized, the element must be shielded from deposits and beam strike by the beam halo, and available discharge energies must be minimized.
Acbowledgements
The authors wish to thank Tony Renau for providing the OPTION code with which the modeling was performed.
References [l] ME. Mack, A. Freytsis, M.L. Pascucci, D.J. Prisby and J. Sedgewick, in: Eon Implantation Technology, eds. D.F. Downey, M. Farley, KS. Jones and G. Ryding (North Holland, Amsterdam; I9931 p. 593. [2] M. Jones, F. Sinclair, J. Blake and S. Shields, in: Ion Implantation Technology, eds. D.F. Downey, M. Farley, KS. Jones and G. Ryding (North Holland, Amsterdam, 1993) p. 57”. [3! P. Sferlazzo, D.A. Brown, SE. Beck and J.F. G’Hanlon, in: Ion implantation Technology, eds. D.F. Downey, M. Farley,
KS. Jones and 1993) p. 565.
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