CVD diamond window development at IAP/GYCOM

Institute of Applied Physics, Nizhny NovgorodGYCOM Ltd., Nizhny Novgorod

A.Litvak, A.L.Vikharev, V.N.Ilin, et al

Presented by G. Denisov

IAP RAS GYCOM

Motivation

Windows based on a CVD diamond are very expensive but today they are the only option for megawatt CW windows. Calculations show that the windows based on the best discs are capable to transmit 2-3 MW power. Diamond windows allow edge cooling of discs by water at room temperature. This makes their use much more comfortable as compared with some windows operating at cryogenic temperatures or using sophisticated coolants.

Last years significant efforts were done by IAP/GYCOM in order to solve the whole scope of problems associated with the use of CVD diamond windows in gyrotrons: growing of discs, their cutting and polishing, and then high-temperature brazing and mounting to a tube.

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IAP RAS GYCOMDiamond disks grown from the gaseous phase in microwave discharge plasma

Industrial reactorCVD technology for growing polycrystalline diamond disks 75 mm in diameter and up to 2 mm thick has been mastered to create output windows for 1 CW gyrotrons. The disks has low loss tangent~ (1−2)·10-5 at the frequency of 170 GHz and are grown at the rate of 1.5-1.6 µm per hour.

Studies of the ways to speed up the growth of diamond disks in the CVD reactor

Application of the pulse-periodic regime of discharge maintenance

growth speed-up by 1.5-3 times

Application of the discharge produced by millimeterwaves

growth speed-up by 7-10 times

MPACVD reactors

1

2

3

4 56

7 8

9 10

4

1

23

5

MSU IAF ASTEX

Operation frequency 2.45 GHzMicrowave power 6 kWGas pressure 50 - 150 Torr

Gas mixture H2 + CH4+O2Deposition area 20 - 50 cm2Growth rate 0.5 - 2 µm / h

Operation frequency 915 MHz Gas mixture H2 + CH4+O2Microwave power 20-60 kW Deposition area 70 - 150 cm2

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Growth rate of diamond films

1014

1013

1015 1016 1017

1014

1015

1016

[H], cm-3

[CH 3

], cm-

3

DCarcjet

CombastionTorch

Hot filamentMicrowave

RFTorch

10 3 µm/h10 2 µm/hG = 10 µm/h1 µm/h

X d ef

= 1 0

2

1

1 0- 2

1 0- 4

Growth rate

Relative defect density

K≈ 2

where concentration of H atomand CH3 radical at the substrateare in cm-3

[D.G. Goodwin, J. Appl. Phys. 74(11) (1993) 6888]

GCH Hcm H

∝⋅ +−[ ][ ]

[ ]3

15 33 10

XGHdef K∝ [ ]

CH H CH H4 3 2+ ↔ +

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Diamond growth in CW 2,45 GHz reactor at IAP

Operation frequency 2.45 GHzMicrowave power 6 kWGas pressure 100 - 150 Torr

Gas mixture H2 + CH4+O2Sample diameter 50 - 75 mmGrowth rate 1-2 µm / h

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Nucleation side Growth side

Non-polished diamond disk of 75 mm in diameter and 1.4 mm thick at center

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Non-polished diamond disk of 75 mm in diameter and 1.65 mm thick at center

1000 1100 1200 1300 1400 1500 1600 1700 1800 1900Raman shift, cm-1

I, a.u.

1000 1100 1200 1300 1400 1500 1600 1700 1800 1900Raman shift, cm-1

I, a.u.

Center

Edge

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Polycrystalline diamond disks were polished by mechanical lapping method using homemade machines

Diamond disks after polishing, diameter – 75 mm, thickness - 1.12 mm and 1.35 mm

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Recently grown discs

100 200 300

tanδ[10-6]5 0 22DB6

54DB1

40050 2 5 0 35 0

5

10

15

20

25

305 0 22DB6

54DB1

40050 2 5 0 35 0

5

10

15

20

25

30

5

10

15

20

25

30

Frequency [GHz]

53DB1

6 3DB1

64DB1

52DB1

IAP RAS 5

150

IAP RAS 6

Comparison with De Beers diamond disks

Growth of atomic hydrogen density [H]

Increase of specific power

Increase of electron density Ne

Generation of plasma by quasi-optical system

Millimeter wave radiation

GCH Hcm H

∝⋅ +−[ ][ ]

[ ]3

15 33 10

Increase of growth rate

Increase of microwave frequency

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1 – upper focusing mirror, 2 – down focusing mirror, 3 – flat mirror, 4 – wave beam splitter, 5 – substrate, 6 - gas discharge plasma, 7 – circular corrugated waveguide, 8 – miter bend, 9 – gyrotron, 10, 11 – mode converter to wave beam

11

22

3

4

1

1

5

6

77

8

9

10

11

View A

A

Gyrotron-based system

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High-speed growing of polycrystalline diamond filmsReactor

based on a 10 kW/30 GHz gyrotron 11

22

3

4

1

1

5

6

77

8

9

10

11

View A

A

0.2 0.4 0.6 0.80

2

4

6

8

G, µm/h

[CH4], %

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Plasma reactor

11

224

3

8 7

8

6

5

Process parameters:Substrate 60 and 75 mm Si Gas mixture Ar + H2 + CH4Microwave power 5 – 8 kW Gas pressure 100 – 400 TorrSubstrate temperature 600 – 900C Deposition time 1-100 hours

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0.2 0.4 0.6 0.80

2

4

6

8

G, µm/h

[CH4], %

Growth rate versus methane concentration at gas pressure 200 Torr

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Diamond disks for gyrotron windows are brazed to copper cuffs with a high temperature alloy in order a tube can pass the process of baking out. High temperature brazing was developed in Gycom company.The window was tested with 170GHz/0.65MW/20sec microwaves1.4kW absorbed power; 2-3 sec temperature stabilization

Brazing IAP RAS GYCOM

GyrotronGyrotronGyrotronGyrotron with a circular Brewster window with a circular Brewster window with a circular Brewster window with a circular Brewster window

2006200620062006 ProofProofProofProof----of principle test of a of principle test of a of principle test of a of principle test of a gyrotrongyrotrongyrotrongyrotron with circular with circular with circular with circular Brewster window (0.1 sec, 0.7Brewster window (0.1 sec, 0.7Brewster window (0.1 sec, 0.7Brewster window (0.1 sec, 0.7----0.9MW, 11 modes in 0.9MW, 11 modes in 0.9MW, 11 modes in 0.9MW, 11 modes in the range of 105the range of 105the range of 105the range of 105----143 GHz)143 GHz)143 GHz)143 GHz)

July 2007July 2007July 2007July 2007 ---- 10 sec test of the tube with high10 sec test of the tube with high10 sec test of the tube with high10 sec test of the tube with high----efficient efficient efficient efficient mode converter and Brewstermode converter and Brewstermode converter and Brewstermode converter and Brewster----angle diamond windowangle diamond windowangle diamond windowangle diamond window

BN ceramic, BN ceramic, BN ceramic, BN ceramic, highhighhighhigh----temperature temperature temperature temperature

brazingbrazingbrazingbrazing

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SummaryWindows based on a CVD diamond are very expensive but today they are the only option for megawatt CW windows. IAP/GYCOM solve the whole scope of problems associated with the use of CVD diamond windows in gyrotrons: growing of discs, their cutting and polishing, and then high-temperature brazing and mounting to a tube. Two setups for growing diamond discs are in operation: the first one bought from Aixtron company, is based on a magnetron and conventional receipts for discs growing, the second one is an experimental gyrotron-based setup operating at high frequency 30 GHz and it aims to increase the disc growth rate. Two more setups are under construction at IAP.IAP/GYCOM discs are brazed at near 800°C temperature to metal constructions. One of the brazed window was successfully tested in the transmission line out of the 170 GHz gyrotron with pulses 0.65 MW/20 sec.Multi-frequency gyrotron with a diamond Brewster–angle window will be tested in July 2007.

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