There is this chronic issue that haunted us for many years, with the team efforts in finding rootcauses, today we finally able to conclude something out of it:
Issue is High RS VS02.
Test that we plotted to run our hypothesis:
Run 6 t/w with different setup
1. I=0.63mA, IPFG=6A, RS=196.3 (1.42%) rem: clear scan map with 5 lines
2. I=0.63mA, IPFG=0.1A, RS=173.9 (0.42%)
3. I=0.63mA, PFG OFF, RS=174.4 (1.09%) rem: nonuniform oxide, RS should be lower
4. I=6.5mA, IPFG=6A, RS=175.7 (0.59%)
5. I=6.5mA, IPFG=0.1A, RS=172.8 (0.55%)
6. I=6.5mA, PFG OFF, RS=172.9 (0.41%)
Summary:
1. PFG isolation good
2. Small RS difference between high and low beam current (may be expressed as
0.5%, high current - OVERDOPE). Probably, high beam forces higher beamline pressure. It seems, by itself, faraday HAS NO STATIC LEAKAGE. Also, no scale error detected - no problem with dose processor.
3. PFG leakage can be expressed as 15-20 uA/1A PFG current.
4. Because day shift test beam was 2 mA and I PFG = 4A, expected leakage between 3-4%, what gives RS 95-96 Ohm/sq.
5. As mentioned above, in case low beam with high PFG arc current scan map observed. This map had lowest RS (and lowest leakage) when disk was on top and highest RS (and leakage) when disk in bottom position.
When disk travel up and down, because of 7 degrees tilt, space between disk and exit of faraday cage changing by about 0.7 inches (Space highest when disk on the bottom). Hence, leakage INCREASING with increments distance between faraday and disk. Leakage on the bottom is about 95 uA while 63 uA at top disk position. It may happen either exit fence magnet field weak or excessive distance between faraday and disk (or both together).
With assumption model:
1)correct magnet field,
2)linear dependance of leakage from space, and
3)travelling distance 6 inches, might expect extra gap between faraday and disk above 1.5 inches.
If this distance can't be found, weak magnet field should be the reason. (Rem: to eliminate effect of leakage, perhaps, smaller distance required, since magnet field drops not linearly, but as 1/x^3 (in some geometries as 1/x^5, because no magnet charges in nature)
6. Medium current t/w showed acceptable result, because beams difference (with high current test) is about 2-3 times, while PFG arc current - 8 times. Hence for medium current tests situation with leakage better about 3 times. But need to keep in mind, medium current test might be done with beam above 1 mA and with beam 0.5 mA, hence, result may drift signicantly.
7. Leakage getting not important in cases high beam current and low PFG arc current. Let say, Axcellis recommend DO NOT APPLY PFG for all ranges of boron implantations)
8. Leakage getting devastating in case of low current with PFG ON (let say, for 30 uA implant beam)
9. BLIND OR UNREASONABLE INCREASING PFG ARC CURRENT HAS STRONGLY NEGATIVE EFFECT.
Note: All leakage current estimations done on analysis of doping levels
