scaling of pfc abatement using plasma burn boxes*0 20 40 60 80 100 0 200 400 600 800 1000 cf 4...

SCALING OF PFC ABATEMENT USING PLASMABURN BOXES*

Xudong “Peter” Xu and Mark J. KushnerUniversity of Illinois

Department of Electrical and Computer EngineeringUrbana, IL 61801

November 1998

*Work supported by National Science Foundation andSemiconductor Research Corp.

UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS

AGENDA

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• Introduction • Geometries of Etching Reactor and Burn Box • Plasma Chemistry for Ar/PFCs/O 2

• Consumption and Generation of PFCs in Etching Reactor

• Comparison of Predicted C 2F6 Abatement with Experiments

• Plasma Remediation of PFCs in Burn Box

• Concluding Remarks

GWP (100-year ITH)Lifetime (years)

CF4

C2F6

C3F8

CHF3

SF6

NF3

CO2

5000 10,000 15,000 25,000 50,000

6,50050,000

9,20010,000

2,6007,000

11,700250

23,9003,200

13,100740

2001


PERFLUOROCOMPOUNDs (PFCs)

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• Perfluorocompounds (PFCs) are widely used as process gases in microelectronics fabrication for etching and chamber cleaning.

• PFCs are absorbers of infrared radiation having long atmospheric lifetimes and thus have high global warming potential.

*

* J. V. Gompel and T. Walling, Semiconductor International, Semp., 1997, p95


INVESTIGATION OF PLASMA GENERATION ANDREMEDIATION OF PFCS

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• Since finding suitable substitutes for PFCs is unlikely, remediation of the effluent of plasma etching reactors must be done.

• Destruction of C 2F6 by plasmas has been experimentally demonstrated in

low pressure devices.

• Computer modeling is a useful tool for investigate the physical processes occurring in plasma remediation of PFCs.

• 2-d HPEM simulations have been performed to investigate- Consumption and generation of PFCs in ICP etching reactors - Abatement of PFCs in a plasma burn box

GEOMETRIES OF ETCHING REACTOR AND PLASMABURN BOX


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• The test system consists of a plasma etching chamber and a downstream plasma burn box.

• The plasma etching chamber is a 13.56 MHz ICP reactor using Ar/C 2F6 at 10 mTorr. A 20 cm diameter wafer sits on the substrate

• The burn box, which is down stream of the turbopump, is also an ICP reactor with O 2 injection at the higher pressure of 150 mTorr.

Burn Box

Plasma EtchingChamber

Wafer

Coils

O2

Coils

Showerhead Nozzle (Ar /C2F6)

Turbopump


OXIDATION CHEMISTRY IN BURN BOX

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O + CF3 → COF2 + F O + CF2 → COF + F

O + CF2 → CO + F + F O + CF → CO + F

O + COF → CO2 + F

O(1D) + CF 3 → COF2 + F O(1D) + CF 2 → COF + F

O(1D) + CF 2 → CO + F + F O(1D) + CF → CO + F

O(1D) + COF → CO2 + F

O+ + CF4 → CF3+ + FO O+ + C2F6 → CF3+ + CF3 + O

O+ + C2F6 → C2F5+ + FO

COF + CF2 → CF3 + CO COF + CF2 → COF2 + CF

COF + CF3 → CF4 + CO COF + CF3 → COF2 + CF2

COF + COF → COF2 + CO


EXPERIMENTAL ABATEMENT OF C2F6 IN BURN BOX

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• Sawin and Vitale * investigated the abatement of C 2F6 in an ICP burn box using an internal coil.

• C 2F6: 200 sccm

• O 2: 200 sccm

• Pressure: 500 mTorr

C2F6/O2 =50/50

* 51st Annual Gaseous Electronics Conference

D = 10 cm

0

20

40

60

80

100

0 200 400 600 800 1000

C F4(cal.)

C2F

6(cal.)

C F4(exp.)

C2F

6(exp.)


SIMULATION RESULTS FOR EXPERIMENTAL CONDITIONS

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• C 2F6: 200 sccm• O 2: 200 sccm• Pressure: 500 mTorr

Percent of C 2F6:remaining andpercent conversion of CF 4

Power (w)

• The decomposition of C 2F6 is mainly caused by electron impact.

C 2F6 + e → CF3+ + CF3 + e + e

C 2F6 + e → CF3 + CF3 + e

C 2F6 + e → CF3- + CF3 0.16

C2F6 + e → CF5 + F - 0.84

• The generation of CF 4 is via radical

recombination. CF 3 + F → CF4

• The formation of CF 4 depends on the

gas temperature [k=1.78•10 -8•(T/298) -7.73

•exp(-4271/T) cm 3•s -1] and the availability

of F atoms to recombine with CF 3.

• The branching ratio for C 2F6 + e → negative ions significantly changes the formation of CF 4.


SIMULATION RESULTS FOR ABATEMENT EXPERIMENT

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• Plasma is generated only near the coils for 500 mTorr pressure due to the finite skin depth.

• Lower level of C 2F6 decomposition is due, in part, to pass through of gases in center.

• C 2F6: 200 sccm• O 2: 200 sccm• Pressure: 500 mTorr• Power: 500 Watts

4.8

0.02

E (10 11cm-3)

0 50

Radius (cm)

26Source-C2F6 (10 17cm-3)

0.0

-5.2

0 515

26

Radius (cm)

Log scale

PLASMA PARAMETER IN ETCHING REACTOR


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• A “standard” ICP reactor having a 4-turn coil is used as a baseline.• Power: 650 W • Gas Mixture: Ar/C 2F6 =60/40• Gas flow: 200 sccm • Pressure: 10 mTorr

11.30.0

Radius (cm) 0 15

0

15

[Eθ] (V/cm)

150 Radius (cm)

1.460.00

E (10 11 cm-3)

1.50.0

15

[P] (W/cm 3)

0Radius (cm)


FEEDSTOCK NEUTRAL DISSOCIATION

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C2F6(10 13 cm-3)

15

0150

Radius (cm)

CF3 (10 13 cm-3) CF2 (10 13 cm-3)

150 Radius (cm)

• C 2F6 can be dissociated mainly to CF 3, and CF3 can be continuously dissociated to generate CF 2. Due to subsequent reassociation, the densities of C 2F6 increase near pump port.

C2F6 + e → CF3 + CF3 C2F6 + Ar * → CF3 + CF3 + Ar

CF3 + e → CF2 + F + e CF3 + Ar * → CF2 + F + Ar

CF3 + CF3 +M → C2F6 + M

150 Radius (cm)

4.60

0.00

6.03

0.00

6.49

0.00


PFC GENERATION IN ETCHING CHAMBER

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• Some PFCs are generated due to radical recombination.

CF3 + F → CF4 CF3 + F 2 → CF4 + F

CF2 + CF3 → C2F5 CF2 + CF2 → C2F4

150 Radius (cm)

C2F4 (10 12 cm-3) C2F5 (10 12 cm-3)

150 Radius (cm)

CF4(10 13 cm-3)

15

0150

Radius (cm)

2.45

0.00

2.22

0.00

8.98

0.00


C2F6 CONSUMPTION IN ETCHING REACTOR

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Power (w)Gas Flow Rate

(SCCM)

600500

400

50

80

60

70

100150

200250 300

Input C2F6 = 40%

C2F6 (%)

2030

4050

60Gas Flow Rate

(SCCM)

100150

200250

300

60

70

POWER =500 W

Power (w)

600500

400C2F6 (%)20

3040

50 60

Gas Flow Rate = 200 SCCM

50

60

70

• A design of experiment has been performed to determine the functional relationships for C 2F6 consumption.

• The consumption increases nearly linearly with a decrease in C 2F6 percentage and gas flow rate, and with an increase of power deposition.


PLASMA PARAMETERS IN BURN BOX

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• A “standard” ICP burn box having a 2-turn coil with radius 4.25 cm and height 26.5 cm is used as a baseline.

• Power: 650 W • Effluent: 227.7 sccm• O 2 injection: 90 sccm

• Pressure: 150 mTorr

21.1

0.12

Power ( W cm -3) Electric field ( V cm -1)

11.9

0.08

1.60

0.76

0.01

E density ( 10 12 cm-1) E Temperature ( ev )

3.59

1.93

0.02

9.98 5.64


EFFLUENT CmFn REMOVAL IN PLASMA BURN BOX

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1.94

0.01

2.66

0.02

1.53

0.72

0.01

7.65

3.62

0.05

0.92 1.26

• CF 2, CF3, C2F4 and C2F5 from the etching reactor effluent are almost completely removed in the burn box.

CF2 (10 14cm-3) CF3 (10 14cm-3) C2F4 (10 13cm-3) C2F5 (10 13cm-3)

99.95%Abatement

99.99%Abatement

99.97%Abatement

99.9996%Abatement


C2F6 ABATEMENT AND CF4 GENERATION

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1.27

0.01

6.15

3.10

0.04

0.64

• In the burn box, C 2F6 in the etching reactor effluent is abated by 75%, but a significant amount of CF 4 can be generated (triple CF 4 in the effluent).

C2F6 (10 15cm-3) CF4 (10 14cm-3)

C2F6 ABATEMENT AND CF4 GENERATION vs POWER, O2• C 2F6 abatement increases with increasing power and O 2 fraction.

• More dissociation of C 2F6 → CF3, O2 → O • Subsequent reactions of O + CF 3• CF 4 generation increases with increasing power and decreasing O 2. CF4 is generated by the reaction of CF 3 + F. In O atom rich environments, CF 3 reacts to produce COF 2.


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Relative Percent(to input C 2F6 amount)

150

100

Power (w)600

1000

400

O2 (sccm)80050

40

60

80

Power (w)600

800

400

40

60

O2 (SCCM) 150

100

50

1000

20

80

OXIDATION PRODUCT GENERATION vs POWER, O2


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• CO is preferentially formed at high power and high O 2 since CF2 leads to CO. The sources of CF 2 and O are large at higher discharge power.

• CO 2 and COF2 are preferentially formed at high O 2, however subsequent dissociation of CO 2 and COF2 reduces this sensitivity.

Relative Percent (to input C 2F6 amount)

Power (w)600

800

400 O2 (%)

150

50100

0

50

100

1000

O2 (sccm)

150

50

100

Power (w)

1000800

600400

10

20

30

Power (w) O2 (SCCM)

150

100

50

80

40

60

1000800

600400

100


CONCLUSION

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• PFC consumption and generation in an ICP plasma etching reactor, and PFC abatement in an ICP burn box have been discussed.

• C 2F6 consumption for Ar/C 2F6 gas mixtures in the etching reactor is

proportional to ICP power deposition, and inversely proportional to C 2F6

mole fraction and total gas flow rate.

• PFCs from the plasma etching effluent can be abated in the ICP burn box at high power and high O 2. With low O 2, C2F6 can be decomposed,

however CF 4 is generated. The major oxidation products are CO 2, CO and

COF2.

• Low decomposition of C 2F6 in can result from the finite electromagnetic

skin depth resulting in “outside” power deposition and “pass through” of gases in the center.

scaling of pfc abatement using plasma burn boxes*0 20 40 60 80 100 0 200 400 600 800 1000 cf 4...

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