Oxygen-assisted laser cutting and drilling of CVD diamond

v.v. Migulin1, V. G. Ralchenko1 and Y.-J. Baik2

'General Physics Institute, ul. Vavilova 38, 1 1 7942 Moscow, Russia2Korea Institute ofScience and Technology, P. 0. Box 131, Cheongryang, Seoul, Korea

ABSTRACT

A Nd:YAG laser (1.06 jtm wavelength) was used for cutting and drilling of thick free-standingdiamond films produced by microwave plasma chemical vapor deposition. The laser machiningrate was evaluated at different irradiation parameters such as laser power, beam scanning velocityand number of repetitive scans. Carbon oxidation in presence of oxygen flow in thecutting/drilling area is shown to result in smooth and clean sidewalls and top surface aroundgrooves and holes. A beneficial effect of 02 stream on cutting and drilling rate was alsoobserved.

Keywords: diamond film, laser cutting, drilling, oxidation.

1. INTRODUCTION

Chemical-vapor-deposited polycrystalline diamond films is superior material for many applications, heatspreaders, infrared and millimeter wave optics, cutting tools, cantilevers for scanning probe microscopes, sensorsand radiation detectors being only few examples. To be used in a particular device diamond shaping and surfacefinishing are required, such as cutting, drilling, patterning or surface smoothing. Because of extreme hardness ofdiamond its treatment is a difficult task, yet the diamond shaping can be successfully performed with lasers (see[1] for a review). In the present paper we report on cutting and drilling of thick (up to 0.6 mm) CVD diamondfilms with a Nd:YAG laser. Oxygen injection into irradiation zone is found to improve both the rate and qualityof processing.

2. EXPERIMENTAL

High quality diamond films of 200-600 tm thickness were grown on Si substrates in microwave plasmaCVD using CH-H2-O2 gas mixtures as described elsewhere [2]. Then free-standing plates of 57 mm diameterhave been prepared by chemical removal of the substrate. The films displayed a columnar structure with well-faceted surface morphology. Figure 1 shows an example of cross-section of a 0.5 mm thick film in the regionclose to growth side where crystallites with lateral size of 40-50 pm are seen. As the large grains result in highsurface roughness, the laser irradiation was performed only on smooth, fine-grain substrate side to define moreclearly the laser ablated areas. Raman spectra taken at 488 nm excitation wavelength showed a strong narrow(3 .5-4. 1 cm1) diamond peak at 1332 cm1 frequency and a very weak broad peak centered at 1 500 cm' ascribed toamorphous carbon inclusions.

A Nd:YAG laser of up to 14 Watts output power operated at 1.06 pm wavelength, 10 ns pulse duration and10 kHz repetition rate was used in cutting/drilling tests. In spite of low absorption of diamond for wavelengthslonger than 2c225 rim (fundamental absorption band edge), this laser still is suitable for diamond ablation due toeffect of graphitization of diamond surface. Once an opaque graphitized layer is formed by a single pulse or aseries of pulses the ablation rate becomes practically independent on wavelength of incident radiation providedequal energy density and pulse duration used [3]. The samples were placed horizontally on a computer-controlledX-Y table for precise positioning and motion of the workpiece under laser irradiation. The laser beam was

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directed normally (in Z direction) to the film surface being focused with a silica lens of 30 mm focal distance.Irradiation was always performed in ambient atmosphere, also an assistant oxygen jet could be supplied to laser-diamond interaction zone. The 02 stream inclined at 45° with respect to laser beam axis was feed through aninjector placed at vicinity of irradiated zone (Fig. 2). The role of oxygen stream was (i) to remove moreefficiently ablation products and prevent debris re-deposition. and (ii) to enhance diamond etching via oxidation(burning) effect [41.

Fig. 1. SEM picture of cross section of 0.5 mm thickdiamond film in the vicinity of growth surface.Notice columnar grain structure and high roughnessof the faceted surface.

Oxygen

Laser beam

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3. RESULTS3.1. Drilling

Through holes of 100-250 m diameter were drilled using a circular motion of laser beam. To concentratelaser energy as much as possible, the beam was narrowed to a diameter of about 30 tm. the diameter of beamorbit" being adjusted to 100 j.im or 200 tim. Typically the orbital beam velocity was 0.5-1.0 m.mls. Upon

irradiation in air the diamond surface around the holes was heavily contaminated with re-deposited carbonejected from the crater as shown in Fig. 3a. Raman spectra taken at the rim with black debris revealed thepresence of glassy carbon (nanocrystalline graphite). in contrast, when the oxygen jet was supplied to the drilledarea a very clean surface was observed around the hole (Fig. 3b).

An array of 100 and 200 .tm holes shown in Fig. 4 evidences that the reproduction of hole shape and size isgood. Also cross-sections of the holes are seen at the edge of the plate. Those cross-sections were made by lasercutting of one row of holes to visualize their inner structure. The hole sidewalls are rather smooth and nearlyvertical. The through holes were produced by three rotation of laser beam at laser power of 11.5 W. Figure 5shows 100 .tm diameter holes in case when irradiation process has been terminated just after one rotation of thebeam. i.e. prior a through hole is formed. A core remained in the center of the holes still can he seen. If theirradiation would continue the material in the core is believed to be ablated at the very end of the drilling processdue to overheating as a result of reducing thermal contact between the core and surrounded material.

k

i

Beam MotionI//

Fig. 2. Geometry of 02-assisted laser cuttingprocess.

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Fig. 3. Through holes of 200 microns diameter drilled inwith assistant O stream (h). The view is from substrateshape of the hole and clean surface due to oxygen action.

0.5 mm thick diamond film without O injection (a). andside of the film. Laser power is 14 W. Notice improved

I * C

V * * I

V I I I I

Fig. 4. Array of 100 and 200 .tm holes drilled in 0.5nrni thick diamond plate using triple rotation of thelaser beam. Notice cross-sections of through holes atthe plate edge, and one incompletely drilled hole atthe third row.

OO_irn' .,11

-

Fig. 5. Cross-sections of 100 im holes with a corenot completely removed. The drilling process wasinterrupted after one rotation of laser beam. Beamcircular velocity is 0.5 minIs, laser power is 11 .5 W.

Not only the quality of drilled holes improved in case of oxygen jet addition, also beneficial. is the effectof oxygen on drilling rate. The drilling time as function of laser power for 200 jim holes in 0.5 mm thick plateproduced with and without oxygen supply is shown in Fig. 6. The drill time is reduced with oxygen addition.especially at higher power, for instance at Pl4 W the drill time decreases from 35 seconds to 4 seconds.

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80.)60

4O

Fig. 6. Drilling time of 200 jim diameter thoughholes in 0.5 mm thick diamond film as function oflaser power without O injection (open circles) andwith 0- assisted jet (closed circles).

Fig. 7. Trenches fabricated in 450 jim thick diamondfilm by 02-assisted cutting at laser power 10 W.Laser beam scanned repetitively 10 times along eachtrench at different beam velocities (from left to rightgroove): 0.5: 1:2; 5; and 10 mmls.

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3.2. CuttingIn cutting experiments laser parameters were systematically varied to optimize the etching process.

Generally, the groove depth increases monotonicallv but nonlinearly with laser power and number of repetitivescans of the beam, and decreases with beam velocity. Figure 7 shows a SEM picture of series of cuts produced atlaser power P=l0 W using different beam velocities. V. The left groove was made at V0.5 mmls. the next fourgrooves being cut at increasing beam velocities of 1. 2. 5 and 10 mnils. Each cut was produced by 10 passes ofthe beam at a condition that the focus position was fixed at the film surface and did not change during processing.The kerfs have a conical shape looking very similar. The kerf width at entrance (irradiated) side decreases from45 jini to 30 jim when scan velocity increases. No debris within the kerfs or outside are observed owing tooxidation andlor blowing off ablation products by flowing oxygen. It was possible to make more verticalsidewalls by shifting the beam focus position deeper between successive scans.

Figure 8 shows how the groove depth changes with number of scans with and without oxygen jet. The dataare compared for beam velocities of 1.0 nimls and 10 mrnls at laser power of 6.5 W. Upon repetitive scanning thedepth increases noticeably after second scan. but further scanning results only in a slight increment in groovedepth. For instance, the depth is 170 jim after single scan and is 280 jim after 12 scans in the presence of 02 jet.The oxygen feed enhances the cutting rate. especially at low powers, by a factor of up to 3.

Figure 9 illustrates capabilities of figured laser cutting showing a 0.4 mm diamond substrate with 200 jimwide slots cut at on four sides. The slots may be useful for enhanced heat dissipation through the heat spreaderedges. More complex shapes have been also cut with this laser, including a number of artistic designs.

120 0

100 S

• th 02 injectiono -- without 02

0

20

0 —-----6

0

•----s8 10 12 14

Laser power, W

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Laser power P = 6.5 WV.. wth02

250V

200v without 02

-1500Beam velocity100 • 1.0mm/s

-v 10mm/s50

0 2 4 6 8 10 12Nuner of scans

Fig. 8. Groove depth produced at laser power 6.5 W Fig. 9. Diamond film shaped for thermalversus number of scans without (open points) and management application for the case when a heatwith 0. injection (closed points). The scan velocit\ flux to be dissipated through the plate edges. Theis 1 mm/s (squares) and 10 minIs (triangles), plate size is 6x6x0.4 mm. Slots at the edges have

been cut at laser power of 10 W and beam velocit)l0nimls.

4. CONCLUSIONS

CVD diamond wafers of thickness of up to 0.6 mm were cut and drilled with a Nd:YAG laser at differentirradiation conditions. An acceleration of ablation rate and a reduction of carbon re-deposition effects wereobserved when an oxygen jet was directed to laser-diamond interaction zone. This beneficial effect is ascribed tochemical reaction of graphitized diamond on kerfsidewalls and ejected ablation products with oxygen.

5. ACKNOWLEDGEMENTS

This work was supported in a part by Korea Ministry of Information and Communication.

6. REFERENCES

1. V.6. Ralchenko and S.M. Pimenov. 'Laser processing of diamond films". Diamond Films and Technology, 7(1997) 15-40.

2. V.6. Ralchenko, A.A. Smolin, V.1. Konov. K.F. Sergeichev. l.A. Sychov. 1.1. Vlasov. V.V. Migulin. S.V.Voronina and A.V. Khomich, "Large-area diamond deposition by microwave plasma", Diamond and RelatedMaterials, 6 (1997) 417-421.

3.1 .V. Kononenko. V.6. Ralchenko. 1.1. Vlasov. S.V. (Iarnov and VI. Konov, "Ablation of CVD diamond withnanosecond laser pulses of UV-IR range", paper presented at Diamond 1997, Edinburgh, Scotland. 3-8August 1997. paper #15.152 (to he published in Diamond and Related Materials).

4. V.6. Ralchenko. K.G. Korotushenko. A.A. Smolin and EN. Loubnin. "Fine patterning of diamond films bylaser-assisted chemical etching in oxygen", Diamond and Related Materials, 4 (1995) 893-896.

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