high density plasma flood system for wafer charge neutralisation

8/13/2019 High Density Plasma Flood System for Wafer Charge Neutralisation

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High Density Plasma Flood System for W afer Charge NeutralisationH i r o w Ito , Hiroshi Asechia, Yasuhiko Matsunaga , Masahiko Niwayamab,Ken$ Yonedab, Michael Vella , Mike Reilly and Walt Hacker

Applied Materials Japan, Narita, Chiba 286-0825, Japank L S I Process Technology Development Center, Matsushita Electronics Corp.

Electro-Graph, Carlsbad,CA 92009, USAhiro-i to@amat orn

Abstract Plasma Flood System, a low energy electron genera tor, has been widely used as an effective tool to neutralise wafercharging induced by ion implantation. Although it has been successful in achieving the full device yield under high curren t ionimplantation, furt her advancem ent in device design imposed a need to minimise the wafer charging down to a few volts due to th euse of thin ga te oxide o f less than lOnm thickness.High Density Plasma Flood System (HD PFS) was thus developed for the Applied Materials xR series Ion Implanters to maintainthe maximum throughput with high current processes without compromising on device yield. HD PFS is a high efficiency chargeneutraliser that supplies very low energy -dew lectrons at high emission(>3OOmA). The system has a unique configuration ofmagnetic circuit and ar c discharge profilethatenables th e efiective ran spo rt of electrons from the plasma source to th e waferwhile reducing thepower consumption by one order of magnitude. This paper discusses he s tructu re and the performance of theHD PFS in terms of electron transp ort efficiency and energy distribution. Typical operation window is also shown by using theyield of MOS apacitor devices at different gate oxide thickness (3.5,5 and 10nm). Si months of filament life has beendemonstrated.

I . INTRODUCTIONCharge neutralisation of wafers during ion implantation hasbeen one o f the m ost imp ortant tasks for controlling implantprocesses [1-31. Me thod s to achieve the neutralisation havealso gone through a series of evolution due to a need toreduce the energy of compensatory charges, i.e. electrons, asgate oxid es become thinner [4].Dense plasma source technique, such as Plasma FloodSystem (PFS) of Applied Materials [4], is now widely used omaintain good yield for thin gate oxide devices. Although it

has been succ essfil in achieving the task there is still a needto improve the neutralisation performance in order tomaximise the process throughput with increased beamcurrent and very thin gate oxide (thickness less than lOnm).This means that th e elec tron energy must be reduced and theelectron flux must be increased.High Density Plasma Floo d System HD PFS) has thus beendeveloped fo r the Applied Materialsx series Ion Im plantersin order to satisfy the new requirement. HD PFS is animproved version of the existing PFS which generates lowenergy electrons by using a unique acceYdecel method [4].The configuration of the source head has been largelychanged, which resulted in very compact hardware and hightransport efficiency of electrons. Combined with theredesigned Guide Tube , th e system offers the comp leteneutralisation of positive charging du e to ion beam, while therisk of negative charg ing is minimised. OGtirnisation of ar cefficiency also ma de th e life time of filament exceptionallylong (over 6 months). The same design concept is adopted toa PFS for AMAT P190 00,920 0 and 9500 systems [SI.This paper describes the design and the performance of theHD PFS in terms of charging characterisation and th e testdevice results using CHARM [ ] nd MOS capacitors.

. HD PFS Design and OperationA DesignThe schematic view of the HD PFS is shown n Fig.1 Thesource head was largely changed fiom the previous PFSmodel [4] in its shape and mounting. This means theimprovement in arc efficiency and electron transport byoptimising the mag netic field p rofile and the plasma coupling.The system uses a proven acceYdecel operation to generatelow energy electrons by grounding he filament and the GuideTube that transports the electrons with a aid of plasmaconfinement by magnetic cusp field and negative biaspotential [4].The compact design (4Omm in w idth) allowed the system tobe fitted in very small room where the charge neutralisation isrequired. Con trol electronics and softwar e are basicallyidentical to those of the existing PFS.It should also be noted th at th e line of sight to the filament iscompletely eliminated, therefore, tungsten contamination onwafer surface is almost undetectable.Due to the major improvement in electron transportefficiency, typical arc current required to maintain highenough electron emission is now reduced by one order ofmagnitude. Arc current of as low as 0.5A is normallysufficient to perform 2 mA high dose implant. Low arcoperation enabled the filament to last for more than 6months.B OperationExcept for the use of lower arc current, the method tooperate the HDPFS is the same as hat o f the previous PFS.Primary parameter to control the electron emission is arc

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current. Net flux of electrons to the w afer can be con trolledby arc current an do r Guide Tube potential.The knction of the HD PFS c n be well depicted byrecording a few monitoring parameters as described below.view

- a-lxrFig.1 Schematicview of he HD PFS. Plasma in the arc chamber iscoupled onto the beam plasma inside the Guide Tube near the wafer1) Emission Current : Emission currents at 3 0V of arc are

plotted against arc curren t in Fig.2 at tu be bias voltage o f OV.Argon gas was used as a source feed material. Emissioncurrent of HD PFS (1SOmA at 1 A of arc) is higher thanPrevious PFS (10mA at 1A of arc) by one order ofmagnitude . This allowsHDPFS to use very low arc operation(arc current below 1A).2) Wheel Current : Wheel currents are plotted in Fig.3.With HD PFS, positive reading on wheel current disappearsat 0.5A of arc current. On the other hand, previous PFShardly made the wheel current negative with high currentimplantation, indicating that the positive charging alwaysremains.3) C harge Voltage on capacitively coupled charge sensor :

Charge voltages on SOOOA sheet oxide wafer are plotted inFig.4. HD PFS controlled the voltage below OV at low arccurrent (0.5A). P rev io us P FS wa s n ot able to e l i t epositive charging ( > l o w even at maximum arc current (6A),indicating a risk of positive charging dam age.

3 LOEM3f LOEM22

l.OE+OO I0 1 2 3 4 5 6

Arc Currena [A]

Fig.2 Emission Current in acceYdece1mode s ploned against arc current atOV oftube.with As'/SOkeV/ZOmA.4) O peration window : It has been know n experimentallythat the operation window to give a g ood device yield tends

to be between 5 and -15V on Charge Sensor reading. HD

PFS showed the good operation window between 0.5A t o 3Ao f arc current for the high current beam, whereas the previousPFS showed a narrow window towards the maximum arccurrent.

Arc Current [A]Fig.3. Wheel Curren t in acceUdecelmode s plottedaga inst arc curre nt at OVoftubewith As'/SOkeV/2OmA

+ HDP F S Max201510

- Voltage- 5 VoltageHDP F S Min0

Present PFSMax VoltagePreSentPFSMin Voltage

-20rc Current [A]

Fig.4. ChargeVoltage in acceVdecel mode s plottedagainst arc current atOV of ubewth As'/SOkeV/2OmAC Tungsten EmissionfromPFSLevel of tungsten on wafer surface in conjunction with PFSoperation was evaluated with SIMS by usingAs'/SOkeV/20mA/SElS cm-' Implant.Measured level of tungsten fiom the HD PFS was about5E9cm-' which is roughly the same as the detectio n limit ofthe measurement. Tungsten emission was proven to be

minimised due to the low arc current (i.e. low filamentcurrent) operation and the complete elimination of the line ofsight to filament.El. PERFORMANCE on TESTDEVICES

Charge neutralisation performance of HD PFS wasevaluated by using a series of charge sensitive MOScapacitors with different gate oxide thickness and EEPROM .A ntennaMOS capacitorsYield results of antenna MOS capacitors with thin gateoxide (less than lOnm) have demonstrated that the HD PFS

c n effectively neutralise wafer charging with a largeoperation window in beam current, arc current and tube biasvoltage. All th e advanced antenna MOS capacitors for thesetests w ere supplied by M atsushita Electronics Corp.The yield of a test matrix of the MOS capacitors with 1 E6antennas and the gate oxide thickness with Snm on a p-substrate is plotted in Fig.6. The implant wasAs+/2OkeV/lOmA/SElScm-'. The yield was lOO??with OVof tube bias and at ar c currents of 0.5, 1 and 2A. The yield

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was also 100% with IA of arc current and at tube bias of 0,-10 and -2OV.

I--203B* Tube. ias

Voltage [VI

Arc Current [A]Fig.6. Yield o MOS capacitors (1E6 antenna ratio and 5nm gate oxide) isplotted aga inst arc current and tube. bias voltage. The implant wasAs*/20keV/IOmA/5E15an

In comparison, the same implant with the Previous PFSgave a yield of less than 60% for 1E6 antennas with 4A of arcwhen the tube bias was -lOV and -2OV.A result oft he HD PFS and Previous PFS is plotted in Fig.7for the MOS capacitors at antenna ratios of 3.2E4, 1E5,3.2E5 and 1E6 with 5nm gate oxide on n-substrate. Ingeneral, antenna MOS capacitor fabricated on n-su bstrate ismore sensitive for negative charging damage than the o ne onp-substrate. The test was done with no beam at a typicaloperation to see the negative charging breakdown due to highenergy electrons. HD PFS at 1A of arc and the previous PFSat 4A of arc. Yield of HD PFS was 100%for all devices. Onthe other hand, the yield of the previous PFS dro pped to 20%for the 1E6 antenna. The previous PFS caused negativecharging breakdown under the n o beam condition. This is anevidence that high energy electrons can reach the w afers withthe previous PFS when the tube bias is used. From theseresults, it is clear that the net electron energy of HD PFS issignificantlyreduced. Cusp magnets on the guide tube appearto provide the energy filter effect which prevents highenergy secondary electrons fiom reaching the wafer surface.

-Antenna ra t ioFig.7. Yield of the HD PFS and Soleno id PFS with no beam are ploIted orantenna ratiosof 3.2E4, lE5,3.2 ES and 1E6 at Snm gate oxide.

Fig.8-1,2,3 and 4 show the breakdown frequency of theantenna MOS capacitors at different gate oxide thickness(IOnm,7.5~1,nm and 3 . 5 ~ 1 ) .Data indicate a dependenceof the damage on electron energy and the two major deviceparameters i.e. antenna ratio and oxide thickness [7, 81.Change in electron energy means a difference in flux ratiobetween primary electrons fiom th e arc chamber (low energyelectrons) and secondary electrons produced along the

I beamline (higher en e re electrons).

Fig.8-1 and 2 show that HD PFS can hardly cause anydamage for the devices of 7 . 5 ~ 1ate oxide or thicker unlessthe electron energy is deliberately increased. s the gateoxide becomes thinner, the breakdown frequency increased asshown in Fig.8-3 and 4 However, even with the extremelycharge sens itive devices such as 3.5nm gate oxide and 1E6antenna ratio, HD PFS can still achieve perfect yield bycontrolling the electron energy to the lower en d.

Antenna R atioFig 8-1 Yield \ersus Electron energy versus Antenna Ratio with lOnm pateoxide MOS capacitors The implant was As /20keV/l W S F 5 cm

Antenna RatioFig.8 -2. Yield versus Electron energy versus Antenna Ratio with 7.5 nm gate

oxide MOS capacitors.The mplant was As /20 kcV /IW SE 15 cm.

Antenna Ratio

Fig.8-3. Yield versus Electron energy versus Antenna Ratio with 5nm gateoxideMOS capacitors.The implant was As/20keV/10mA/5E15 cmThe yield of MOS capacitors on p-substrate at a typicaloperating condition of HD PFS, 1A of arc and OV of guidetube, are plotted in Fig.9 for different antenna ratio and gateoxide thickness. The implant condition wasAs/ZOkeV/1OmA/5El5 cm -2. T he yield was lOO?h on all

cases (antenna ratio at iE5 and iE6, gate thiciiess at i ijnm,480



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5nm and 3.5nm). T his is a clear indication that HD PFS has alarge enough window t o cover from the present technology(devices of lOnm gate) t o the future requirement (devices of3.5nm gate) [ IO]

Fig.8-4. Yield versus Electron energy versus Antenna Ratio with 3Snm gateoside M OS capacitors. The implant was AP/20keV/lOmA/5El5crn. .

Gate oxide thlcknessml

Fig.9. The yield ofMOS apacitors at a hpical operating condition of HDPFS. The mplant was As'/2OkeV/lOmA/5E 15 cm.2.HD FS was operatedat 1A of arc and OV ofguide tube with 1.4 sccm argon.B PROMHD PFS dem onstrated a precise Vt control with remarkableThe results are shown inniformity across the wafer.Figs.5-land 2.

Fig.5-1 Vt shfi on EEPROM . Implant condition : As'/40keV/12mA/4E15cm-'. HDPFS 0.5N -lOV . A verage Vt shift 0.OOV. Sigm a 0.01V . I91

N CONCLUSIONHD PFS has been developed for Applied Materials ionimplanters in order to achieve an ideal charging control for0.15um technology and beyond. The system offers an ample

supply (1 O m A at 1A or arc) of low energy electrons (

high density plasma flood system for wafer charge neutralisation

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