1. Introduction

Extensive research has been carried out with femtosecond pulsed laser for sub-micron machining. It has been proved that by controlling the fluence of the laser spot such that only a small part of the intensity peak has enough energy to induce material breakdown, it is possible to breakthrough the limitation of wavelength, producing sub-spot-size features^[1,2]. This unique advantage of femtosecond optical pulse makes it one of the most attractive research interests in the field of sub-micron machining. It is predicted that with femtosecond pulse the machining resolution of laser can be reduced dramatically, down to 20nm^[3].

Though femtosecond pulsed laser ablation presents excellent opportunity for sub-spot size machining, the quality of the ablated features can be affected by many factors. In this letter, we intend to describe the effect of spatial defects of the laser beam induced by grating pairs integrated in the laser system. The defects found in the sub-micron features are not evident when the feature size is big. Hence, a careful study and subsequent elimination of these errors is required for ablating sub-micron features with high edge acuity, shape symmetry and spatial resolution.

For this investigation, a chirped pulse amplification (CPA) Ti:sapphire system comprising of a pulse stretcher, a regenerative amplifier pumped by a Nd:YFL laser and a compressor is used. This produces pulses of 150fs duration at a repetition rate of 1kHz with a fundamental wavelength of 800nm. The amplified pulse coming out from the compressor module enters through a second harmonic generator. The final machining beam is of 400nm in wavelength. Transmitting the beam through the second harmonic crystal also stretches the pulse duration, resulting in final pulse duration of 300fs.

To smoothen the Gaussian profile of the laser spot and to remove scattered light, a spatial filter is placed after the laser source. A lens of 19mm focal length focuses the 6mm beam into a tightly focused spot of 2.5 µm diameter (measured from experimental results) on to the specimen. The specimen, a finely sputtered platinum film of 1000Å onto a quartz substrate of 3.0mm thickness, is mounted on a three-axis translation stage. The specimen is scanned under the laser spot so that a group of pits can be machined. The pulse energy is controlled at very low fluency region so that only a small portion of the spot peak is above the threshold of the target material, which enables sub-spot size features^[4]. The SEM image of a 320nm pit ablated by the system with pulse energy of 15nJ, is shown in figure 1. It can be seen from the figure that there are two small pits of shallower depth symmetrically located at each side of the machined pit. An in-depth study of the optical configuration of the laser machining system reveals the reason for these symmetrically located pits that accompany the machining spot.

 

Fig. 1. Pits created on 1000Å platinum film. (pulse energy of 15nJ, 100 pulses, wavelength of 400nm, pulsewidth of 300fs)

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The fundamental pulses emitting from oscillator, whose energy in the range of few nano-joules, do not have high enough power to cause breakdown in the target material. The energy of the optical pulse is amplified by Chirped Pulse Amplification. It consists of three processes viz., pulse stretching, pulse amplification and pulse recompression. Pulse stretching elongates the duration of the pulse from femtosecond region to picosecond region so that the peak power of the pulse can be brought down to facilitate amplification. Recompression is the reverse process of pulse stretching, shortening the duration of amplified pulse as close to the fundamental pulse as possible. For both stretching and recompression, two-pass grating-pair is the most commonly used configuration. Since gratings are involved, the errors raised by grating deficiencies will have significant effect on the created structures.

Consider a diffraction grating consisting of a pattern of grooves whose nominal spacing is ‘d’. Scattered light is defined as light leaving the surface of a diffraction grating that does not follow the grating equation for the nominal groove spacing, mλ=d (sinθ_i+sinθ_r) where m denotes the order number of the diffracted beam. This is analogous to the concept of scattered light for a mirror, which is the light leaving the mirror surface that does not follow the law of reflection. For gratings produced by conventional mechanical ruling engineering, light can be scattered by a grating due to surface irregularities, irregularities in the position of the grooves, and irregularities in the depth of the grooves. Surface irregularities refer to those tiny scratches, pinholes and roughness on the scale of the incident wavelength due to metal or photoresist coating. These irregularities will introduce spatial frequencies in the groove pattern other than that of the groove spacing d, giving rise to constructive interference of the diffracted light at angles that do not follow the grating equation mentioned above.

An ideal grating is one of infinite size with perfectly uniformed groove spacing and uniformly illuminated with collimated light. It is designed to diffract only in the directions dictated by the grating equation. In practice, the finite size of a real grating will generate small secondary maxima between the orders due to groove spacing imperfections. With classically ruled gratings it is long taken for granted that, given the extreme sensitivity of gratings to periodic errors, no grating will fail to generate multiplicity of the faint secondary images accompanying each principal maximum of the ideal grating. The faint secondary images are called ghosts because of the fact that they are the exact low intensity replicas of the principal images. According the nature of the groove error, the ghosts can be classified into two types as Rowland ghosts and Lyman ghosts ^[5,6]. Both Rowland and Lyman ghosts follow the grating equation, but for spatial frequencies other than 1/d, which is the frequency of grating. Rowland ghosts can be traced to the inevitable defects in the lead screw of ruling engine and its mounting, which have much longer periodicity compared to the groove spacing, whereas Lyman ghosts are caused by short periodicity (on the order of the groove spacing). The inverse relationship between periodicity and the corresponding diffraction angle leads Lyman ghosts to be very far away from the principal maximum. But the Rowland ghosts occur closely spaced and symmetric about either side of the principal maximum and the intensity can be as high as a disastrous 1%.

2. Experimental results and discussion

To better understand the concept of Rowland ghosts, experiments were conducted with a grating of 100 lines/mm, illuminated by a red laser of 623.8nm. When the grating spacing is high, Rowland ghosts locate outside the laser spot, therefore, can be detected by a beam profiler. Figure 2 gives an intensity profile of the Rowland ghosts symmetrically located at either side of the first diffraction order. But in the case of ultrashort pulsed laser machining, the gratings used in the setup are usually above 1200grooves/mm. Hence the distance between the ghosts in the ablated feature is as small as 1µm as shown in Figure 1. This distance is much smaller than the tightly focused laser spot size, which is 2.5µm in diameter, which makes it impossible to be blocked by using a spatial filter.

At high fluency region where energy of full spot is above the ablation threshold, the created structure has a size equal to or bigger than the spot size as shown in figure 3. From the figure it can be seen that the feature size is much bigger compared to the one in figure 1. Due to the big size of the produced structure, the disturbance of the Rowland ghost is negligible. While at low fluency region, the pulse energy is just enough to start an ablation and only the peak portion of the spot can create material damage, where the Rowland ghosts become obvious since they are very close to the intensity peak. From this discussion it can be understood that the effect of Rowland ghosts become prominent at the low fluency region where sub-micron machining becomes a reality.

 

Fig. 2. Rowland ghost measured from first order diffraction of a grating of 100µm-space, illuminated by 632.8nm red laser. Spot diameter of 260µm.

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Fig. 3. Pits created on 1000Å platinum film. (pulse energy of 20nJ, 100 pulses, wavelength of 400nm, pulsewidth of 300fs)

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Since sub-micron machining is the requirement, it is imperative that this error needs to be eliminated. To eliminate this error, experiments were conducted at a lower fluency region with pulse energy of 13nJ and the results of three pits with a feature size of 250nm is shown in figure 4. It can be seen from the figure that the Rowland ghost does not appear. The reason is that, at lower fluency region, the Rowland ghosts disappear due to their lower intensity in comparison to their principal maximum. One of the advantages of femtosecond pulse ablation is that the threshold value can be precisely defined. Therefore, even very small energy variation inside the machine spot will make difference. The centre portion of the spot can still create damage, producing pits, While the two ghosts located cannot induce damage, even though their energy is slightly lower.

3. Conclusion

In this work, capability of ultrafast lasers to produce sub-micron holes has been demonstrated. While analysing the holes, formation of ghosts have been experienced which is not prominent while machining features in the micron scale. It has been concluded that these ghosts are raised by the deficiencies of grating spacing. When sub-spot-size micromachining is concerned, even a minimal ghost effect will create defect in the produced structure. To address this need some of the methods to eliminate the ghosts are discussed. It has been concluded that ghosts can be eliminated by improving the quality of the gratings produced by ruling engine or be replaced by Holographic gratings which are free from spacing error. Alternatively, from the discussion it can also be concluded that ghosts are eliminated while machining materials such as quartz, platinum, fused silica etc., which have high threshold fluence. Thus, for sub-micron-machining, either the use of high fluence materials are the use holographic gratings for pulse stretching and pulse compression shall eliminate the presence of ghosts.

 

Figure 4. Pits created on 1000Å platinum film. (pulse energy of 13nJ, 100 pulses, wavelength of 400nm, pulsewidth of 300fs)

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