Precision Micromachining of CVD Diamond Films

1. Introduction

diamond films have been processed by various types of pulsed lasers for this purpose [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] . Nanosecond pulsed, excimer lasers have been applied for surface The excellence in mechanical and electrical properties modifications of CVD diamond films [2] [3] [4] [5] [6] [7] [8] . In these makes diamond films uniquely qualified for applications investigations, the excimer lasers have shown their capato microelectromechanical system (MEMS ), optics, bilities in polishing, etching, and patterning of the electronic packaging and other micro components. To diamond films. Reduced surface roughness, improved fully utilize diamond’s excellent properties in the applicaoptical transmittance, and patterned structures were tions, micron or submicron micromachining of diamond reported from the applications. However, the processes films is required. The laser ablation technique has been have also exhibited some damage, such as transformed extensively recognized to be a unique method for microsurface layers, and spoiled spot edges. These drawbacks machining and designing of micro components [1] . CVD have been obstacles to the application of the nanosecond pulsed excimer lasers to high resolution micromachining of CVD diamond. Recently, femtosecond pulsed UV lasers have been introduced to overcome the drawbacks of nanosecond pulsed lasers [9] [10] [11] [12] . By utilizing the the process showed some improvements, as compared has an average grain size and thickness of 2 mm and to the nanosecond pulsed laser process. Even though 7 mm, respectively. The diamond film was subjected to the ultrashort laser ablation showed improvements on three different processing environments, such as atmosurface finish and purity of the irradiated diamond films, spheric condition, vacuum condition and gas-stream it was not successful in completely removing the impercondition. The interaction, between the CVD diamonds fections, such as rippled surfaces, and blurred interfaces.

and excimer laser pulses, showed drastic changes In the ablation processes by the lasers, plasma formaamong the different processing environments. Other tion is inevitable due to the characteristics of short or than the different gas processing environments, the ultrashort pulsed laser irradiation. Even though the same experimental conditions were applied to the dialaser-material interaction is limited to the nanosecondmond film. The energy fluence, repetition rate and femtosecond range, the plasma extends far longer than number of pulses were fixed at 10 J/cm2, 1 Hz, and 20 the laser pulse duration. In studies of ultrafast imaging times, respectively. A circular beam spot (145 mm diamof nanosecond pulsed laser ablation [13] [14] [15] [16] , it was eter) was illuminated on the film surface by a pin-hole visualized that the formation and expansion of plasma,

(1 mm diameter) mask in the excimer laser imaging induced by ablative material ejection, extended up to system. The temperature, around the irradiated spot, tens of microseconds. The propagation of shock waves is assumed to have an exponential decay type of radial was also observed due to the expansion of the highdistribution, dispersed due to the formation of the temperature plasma. In this respect, the formation and plasma [17] . During the short dwelling (25 ns) of the expansion of the plasma give rise to almost all of the excimer laser pulse on the film surface, atomic or damage, such as surface phase transformation, plasma molecular bonds of the material are disintegrated due material ejection and re-deposition, and spoiled interto the transmission of high-energy fluence from the faces. The plasma generated thermal damage especially laser pulse. After the short interaction, disintegrated has been a major obstruction to micromachining of the atoms and molecules are released from the surface, diamond films in micron and submicron scale. To impleforming plasma due to their high-energy states [13ment the laser ablation technique to the precision micro -15] . The surface of the irradiated spot is heated by the machining of diamond films, technical and fundamental high-temperature plasma, which results in thermal reacapproaches are required to circumvent the deleterious tions, and possibly oxidation. The thermal expansion plasma effects. The aim of this investigation is to develop of the high-temperature plasma induces propagation experimental techniques that compensate for the deleteof stress waves, resulting the ejection of materials and rious plasma-material interactions. Emphases of this deposition of debris around the irradiated spot [17] . research are to achieve maximum dissipation of the high-temperature plasma and proper quenching of the target surface.

2.1. Atmospheric Condition

Fig. 1. Experimental setup and SEM micrographs for the excimer laser irradiation in atmospheric condition: (a) the irradiated spot and surroundings; (b) the irradiated spot; (b1) magnified view of area 1 from (b); and (b2) magnified view of area 2 from (b).

SEM micrographs in Fig. 1 are taken from the 2. Excimer laser micromachining of diamond films irradiation under stationary air in ambient atmosphere. In Fig. 1a , the surrounding of the irradiated spot shows A CVD diamond film was subjected to this preliminary investigation. The polycrystalline diamond film a distinctive plasma affected zone (PAZ ), which has somewhat symmetrical, ring-shaped patterns with The damage on the diamond film appears somewhat different from the inside to the outside of the oval spikes. The areas of PAZ indicated by 1 and 2 in Fig. 1b are magnified in Fig. 1b1 and 1b2, respectively. pattern. In Fig. 2b2 , the area, outside of the oval pattern, show that layers of the diamond crystals are damaged In Fig. 1b1 , the area near the edge of the spot has suffered from heavy deposition of debris and partial by the expansion of the high-temperature plasma. The dispersion of the plasma is faster in the low-pressurized damage of diamond crystals. Energy dispersive X-ray ( EDX ) analysis indicated that the pulsed irradiation medium [13, 16 ] . Even though damage is found in the wake of expanding plasma, the irradiated spot shows eradicated the diamond film, and revealed the silicon substrate, which created the ruffled crater. When the uniform reduction of surface roughness. distance increases from the irradiation spot, the deposition is gradually reduced, as shown in Fig. 1b2 . The 2.3. Gas-stream condition debris and deposition of the ejected materials are observed at distances up to 10 times of the spot In the gas-stream condition, a nozzle was installed diameter. The PAZ is caused by combination of deposto blow a 99.9% argon gas over the surface of the ition of ablated debris and damage upon the thermal diamond thin film, upon the laser pulse irradiation. expansion of high-temperature plasma.