1 Introduction

Sapphire, the hexagonal closely packed single crystal of aluminum oxide (α-Al2O3), is used in manufacturing cutting tools and magnetic tape heads in the electronic industry, etc., because of their good strength, high hardness, high-temperature stability and excellent wear resistance [1]. Consequently, machining this material is quite difficult and expensive. Polishing of sapphire corroded in molten KOH under the conditions of 300 °C. Ultra-short laser have several advantages for fabricating microstructures on hard materials [2, 3]. Femtosecond laser is a promising tool for microstructure fabrication in such a hard material, due to the short laser pulse time and the extremely high irradiance as compared with other ultra-short lasers [4, 5]. Hence, the method of femtosecond lasers combined with chemical etching is promising for the micro-/nanoscale fabrication of many types of materials. More recently, micro-machining for surface and photo-sensitivity structuring by femtosecond lasers has attracted much interest. They fabricated microfluidic channels [6], microlens array [7], surface structures for integrated optical [8–12], microfluidic devices [13–15] and so on.

In 2008 Dirk and Jens discovered microchannels and self-assembled nanostructures in the volume of sapphire by combining femtosecond laser irradiation and chemical etching with aqueous solution of HF acid [16]. Vilar et al. [17] produced sapphire surface morphology and phase transformations by femtosecond laser radiation. Studies showed self-ordered periodic structures with an average spatial periodicity of ~300 nm and the interaction were more disruptive and extensive fracture, exfoliation and ejection of ablation debris occurred. These pioneering works have demonstrated the possibility of the process, but the mechanism responsible for the increased etching rate and the chemical etching properties has yet to be understood.

In this paper, a mechanism is presented that leads to modified sapphire which has faster etching rates based on Raman spectrum and XRD spectrum. We propose a fabrication method based on femtosecond laser irradiation and chemical etching for fabricating microstructures of arbitrary size and shape on sapphire substrates. A detailed analysis of the etching process is reported, which gives an insight into a better understanding of modified microstructure etching properties.

2 Experimental setup

The schematic of experimental setup is shown in Fig. 1. The sapphire samples used in our experiment were round piece with a thickness of 1 mm and a diameter of 25 mm, which were prepared by optical polishing the surfaces.

Fig. 1
figure 1

Schematic sketch of experimental setup

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The femtosecond laser (Spectra-Physics, Inc) with central wavelength of 800 nm, pulse width of 120 fs and repetition rate of 1 kHz is utilized for the micromachining. The laser pulse energy is attenuated through a neutral density attenuator to less than 8 mW before the 20× objective lens (NA = 0.25). A 0.5-mm-thick BBO crystal (θ = 29.5°) is used to change the frequency of the laser before the objective. To write adjacent lines, samples are mounted on a three-axis translation stage (resolution Δx = 100 nm; maximum speed v max = 2 mm/s). The propagation direction of the laser is along x axis as shown in Fig. 1, and the used scanning speed ranges from 0.1 to 0.6 mm/s.

After irradiation, the corrosion took place in a room-temperature bath of 40% KOH solution for typically from 20 to 80 min. Microstructures are formed in LSM images. After etching, specimens are rinsed in de-ionized water and cleaned in an ultrasonic bath. The irradiation generally produced groove structures approximately 1–2 μm depth and 5–7 μm width as shown in Fig. 2. Figure 2a shows microstructures without corrosion, and Fig. 2b shows microstructures etched for 80 min. Figure 2 shows the burr of rough microstructures surface removed by etching. Chemical etching improves the surface roughness and expands the dimensions of microstructures clearly.

Fig. 2
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Microstructures without corrosion compared with microstructures etched for 80 min

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3 Experimental results

Al2O3 has more than 10 types of phases as far as known. Alpha alumina (α-Al2O3) is water-free Al2O3, known as corundum, which has high-temperature stability and excellent wear resistance [18]. Besides, gamma alumina (γ-Al2O3) is also water-free Al2O3, belongs to cubic crystal and is a porous and unstable material with high dispersion [11]. Hence, γ-Al2O3 is easier to be etched than α-Al2O3. Figure 3 shows typical Raman spectrums of the non-modified and modified areas. Raman spectrum of specimens in the pristine condition and excitation wavelength is 785 nm. The non-modified spectrum has five bands located at 375.9, 413.5, 428.7, 642.3 and 747.2 cm−1, which is in good agreement with the spectrum of the natural sapphire crystal α-Al2O3 [19]. The band at 416.9 cm−1 is O–Al–O antisymmetrical stretch vibration and the band at 643.3 cm−1 is O–Al–O stretching vibration in the modified part spectrum. The two Raman bands have shifts that may belong to impurities or other defects in the crystal. Laser irradiation destroys the structure of sapphire.

Fig. 3
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Raman spectrums of the non-modified and modified area in synthetic sapphire

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γ-Al2O3 does not show any bands in Raman spectrum [20]. The modified area (γ-Al2O3) is investigated by X-ray diffraction (XRD) and displayed in Fig. 5. Diffraction peaks with 2θ of 37.98, 39.66, 45.79 and 67.16 cm−1 were observed and assigned to γ-Al2O3. The non-modified area (α-Al2O3) is investigated by X-ray diffraction (XRD) and displayed in Fig. 4. Diffraction peaks with 2θ of 25.58, 35.15, 37.78, 43.36 cm−1 and 57.50 were observed and assigned to α-Al2O3. Sapphire crystal is a simple colorless oxide crystal, whose chemical constituent is Al2O3 and crystalline form is α-Al2O3. α-Al2O3, α-Al2O3 is the most stable alumina phase because of its trigonal system. Sapphire samples change from colorless transparent to white non-transparent by femtosecond laser irradiation. Figures 4 and 5 show that the observed results indicate a conversion from α-Al2O3 into γ-Al2O3 due to modified synthetic sapphire by femtosecond laser irradiation.

Fig. 4
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XRD spectrum of the non-modified area in sapphire

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Fig. 5
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XRD spectrum of the modified area in sapphire

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Through Fig. 3 Raman spectrum and Fig. 5 XRD spectrum, we propose a mainly responsible mechanism for the increase in etching rate: crystal form transformed one type of sapphire to another polymorph in the laser track. Sapphire crystal is trigonal system α-Al2O3, and Al and O atoms are arranged symmetrically rule and hard to be destroyed by aqueous solution of KOH alkali. The Al–O bond was changed when femtosecond laser focuses on sapphire internal and produces a certain pressure. At the same time, sapphire (α-Al2O3) of six-party close packing structure transforms into γ-Al2O3 of random cubic close packing structure. Therefore, the transformation makes a sharp drop in samples’ chemical stability and leads to the corrosion rate increase extremely. In order to better understand the impacts of machining parameters on corrosion properties, an experiment is investigated. The experiment studied the influence of laser power, scanning velocities and etching time on dimensions of microstructures.

Figures 6 and 7 show samples fabricated by laser pulse with power of 3, 4, 5, 6, 7 and 8 mW, respectively, at speed of 0.3 mm/s. Then, sapphire samples are etched for 20, 40, 60, 80 and 100 min. Figure 6 shows the corrosion rate increased with the increase in laser scanning speeds. In addition, the corrosion rate peaks at 20 min and then becomes slow. Therefore, the corrosion rate is in a fast-slow pattern. We suspect a reason for the corrosion rate pattern. Femtosecond laser irradiates on sapphire, which forms two parts, laser ablation area and modified area. The laser ablation area is fine micro- and nanostructures on the surface of the grooves. The modified area is not removed, but the sapphire changed into γ-Al2O3 by laser. At the beginning 20 min of etching, the aqueous solution of KOH alkali reacts with completely modified area that is the major components of γ-Al2O3. As the reaction carried through, KOH reacts with the not completely modified area that consists of γ-Al2O3 and α-Al2O3. The maximum etch width ranges from 0.5 to 2.5 μm and depth ranges from 1 to 2.5 μm. Therefore, modified dimensions do not exceed 2.5 μm in a certain range of pulse energy. Modified dimensions do not increase significantly with enhancing the pulse energy. After pulse energy increased to 5 mW, modified dimensions almost have no change. Sapphire modified area (α-Al2O3 changed into γ-Al2O3 by laser) dimension is in a limited range of about 2.5 μm.

Fig. 6
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Relationship between microstructure width and laser power

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Fig. 7
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Relationship between microstructure depth and laser power

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Figures 8 and 9 show samples fabricated by laser pulse with scanning speed of 0.2, 0.3, 0.4, 0.5 and 0.6 mm/s, respectively, at 3 mW. Then, sapphire samples are etched by KOH solution. The corrosion rate increased with the increase in laser scanning speed. Additionally, it shown the width etching rate increases with the increase in laser scanning speed but the depth etching rate descends with the increase in laser scanning speed. In addition, energy attenuation caused by generated opaque γ-Al2O3 hinders laser machining and conversion progress. Hence, machined microstructures are limited in a dimension range and modified dimensions do not exceed 2.5 μm in a certain range of scanning speed and power.

Fig. 8
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Relationship between microstructure width and laser scanning speed

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Fig. 9
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Relationship between microstructure depth and laser scanning speed

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4 Conclusions and summary

An effective method for the corrosion of sapphire by femtosecond laser ablation has been proposed in this study. This method reduces the stringent requirements of sapphire corrosion and has a high corrosion speed at room temperature. When femtosecond laser focuses on sapphire internal and produces a certain pressure, sapphire (α-Al2O3) of six-party close packing structure transforms into γ-Al2O3 of random cubic close packing structure. It reveals that femtosecond laser modification greatly improved the corrosion rate of sapphire. The impacts of three machining parameters on corrosion properties of modified sapphire were investigated. Microstructure dimension increased with the increase in power and the decline of speed. But in a certain range of scanning speed and pulse energy, dimension of sapphire removed by KOH solution is about 2 μm for 20 min. Hence, by choosing appropriate laser power, scanning speed and etching time, high-quality arbitrary microstructures fabricated by femtosecond laser combined with chemical etching fabricate in sapphire. It would be a potential application in photonic crystal-based integrated optical elements or microfluidic devices in life science, biology or chemistry.