Fabrication of IR microlens using a fiber laser

In two subsequent stages, various types of silicon microlenses were fabricated. A fiber laser of 1.06 µm was employed to ablate micro dips at the silicon surface and then a mixed acid solution was used to etch the wafer ultrasonically. The experimental data demonstrate spherical, cylindrical IR microlenses and IR microlens array formation with an optimum numerical aperture of 2.39 minimum microlens height of 21 μm, diameter of 32 μm and focal length of 6.7 μm with maximum resolution of 0.395 μm and magnification of (44X) was achieved. The minimum experimentally achieved microlens’ roughness was 45 nm. Theoretical calculations were conducted to estimate the temperature at the silicon surface during the ablation stage. A temperature of approximately 3590 K is established at the center of the laser-silicon in the ablation stage. Due to the controllable laser micromachining process, optoelectronics and biological imaging are viable applications for IR microlens arrays.


Introduction
The significance of miniaturization is growing in a growing array of modern technologies (1).Micromachining, a technology widely employed across various industries, entails the precise elimination of material at high rates, achieving a resolution within the range of a few microns (2).
A microlens refers to a tiny optical lens, typically characterized by its size being less than 1 mm.The use of those lenses to concentrate and guide light is a common practice in optical instruments such as cameras, projectors and sensors (3).
Microlenses can be fabricated from plastic, glass and silicon, among other things (4).Silicon is often used to make IR microlenses because it is evident in the electromagnetic spectrum's infrared part (5).
Silicon microlenses are used in many applications, such as imaging system, micro optics, optical sensors, microscopy, spectroscopy and optical communications (6).By controlling the processing parameters of these processes, the optical properties of the microlens can be tailored to specific applications (7).
These microlenses can be fabricated using techniques such as lasers and etching processing.The fabrication process in laser microprocessing involves creating precise patterns on a silicon substrate to shape the lens structure (8).Moreover, laser-based methodologies employed in the fabrication of microlenses exhibit exceptional precision and adaptability, rendering them a widely favored choice for a diverse range of applications in photonics, microfabrication and optics (9).
On the other hand, the wet etching process entails the targeted elimination of material from a silicon wafer's surface through a chemical solution.Wet etching is a process characterized by isotropy; wherein material removal occurs uniformly in all directions (10).During the wet etching process, it is observed that the laser-produced region exhibits greater etch rate selectivity compared to the unirradiated regions (11).Additionally, the ablation craters tend to expand gradually.Selecting the etchant solution and process parameters is crucial for attaining the intended etch profile and precision (12).
In this work, different shapes of IR microlens and IR microlens array are created using laser ablation and chemical wet etching techniques.Theoretical calculations were conducted to optimize the fabrication process by estimating the heat generated at the silicon surface and the ablation depth in the first stage.

The experimental methodology
A fiber laser was employed for micromachining a silicon substrate and producing various types of microlenses.The production process involves two subsequent stages: laser ablation and chemical etching.A fiber laser of 1.06 µm wavelength and 130 ns pulse duration, 30 KHz frequency and 1-30 W output power was used for the ablation process.Prior to the machining process, the silicon wafer was washed with ethanol and deionized water in an ultrasonic bath and then dried in a clean environment.
Four acids were mixed together for the reshaping process.The volume ratio of the acid mixture was 3:12:1:10 represented by HF (40 W%), HNO 3 (69 W%), H 2 SO 4 (98 W%) and HAC (99 W%).These four acids were thoroughly mixed and the silicon wafer was fully immersed in the acid mixture using ultrasonic etching to increase the regularity of the etching procedure.
The shape and dimensions of the fabricated silicon microlenses were measured using an optical microscope, OLYMPUS BX60M, supported with a high-resolution digital camera.
Finally, a 980 nm diode laser with a spot size of 3 mm and a power of 3 mW was directed on the silicon microlens to investigate the optical characteristics of silicon microlens.The resulting image was then projected onto the high-resolution CCD camera, BC106 THOR-LABS, which was situated behind the silicon microlens.

Results and discussion
The fabricated silicon microlenses were subjected to rigorous theoretical calculations and experimental analysis.

The interaction area
Many shapes of silicon microlenses were fabricated in this work: spherical, cylindrical, spherical MLA and cylindrical MLA. Figure 1(a, c and e) shows the first fabrication stage carried out by the fiber laser.While, the second stage was the etching process with etching time of 40 min is shown in Figure 1(b, d and f).Spherical microlens array(10 × 10) was fabricated with diameters of 74 and 98 µm before and after etching, respectively, as shown in Figure 1(a and b).Moreover, a cylindrical microlens array was fabricated (3 lines) with a diameter of 65 and 102 µm before and after etching, respectively, as shown in Figure 1(c  and d).Moreover, the side view of the silicon cylindrical microlens array with ML depth 80 and 98 µm for the first and second stages, respectively, are shown in Figure 1(e and f).
Figure 1 shows that the etching process of cylindrical microlens was greater than that of spherical microlens due to the cylindrical microlens having a larger surface area.

The experimental calculations
The height and diameter of the microlens were calculated by an optical microscope and analyzed by the ImageJ software.The schematic diagram of fabricated micro holes for the  ablation process (stage 1) was shown in Figure 2(a).The micro holes have been expanded into spherical microlenses during the etching process (stage 2), as illustrated in Figure 2(b), where (d) stands for the diameter and (h) for the height of the microlenses.The focal length, numerical aperture, resolution and magnification for microlens were calculated by following relations (13): where f is the focal length, F # is the F number, NA is the numerical aperture of lens, R is the microlens resolution, M is the microlens magnification, d is the diameter of lens, h is the height of lens, n is the refractive index of lens and λ is the laser wavelength.The properties of the fabricated spherical microlens are given in Table 1.It is found that when the laser power is increased, the height, diameter, focal length, resolution increases too, but the numerical aperture and magnification decrease.The optimum value of the numerical aperture is 2.391 when the laser power is 5 W.Moreover, the properties of the fabricated cylindrical microlenses are given in Table 2.It is found that when the laser power increases, the height, diameter, focal length and resolution increase too, but the numerical aperture and magnification decrease.The optimum value of the numerical aperture is 2.392 when the laser power is 5 W. From Tables 1 and 2, it is found that the diameter and focal length of cylindrical microlens were less than spherical microlens at the same laser parameter due to the anisotropic etching effect in spherical microlens being greater than that for cylindrical microlens.These tables clearly show the best cylindrical and spherical microlens with the greatest resolution and magnification power when using the lowest fiber laser power of 5 W.

The optical properties of silicon microlens
A laser diode of 980 nm was directed to the silicon microlens, while a CCD camera was placed behind the silicon microlens.The laser beam enters the microlens and is transmitted to the CCD camera as shown in Figure 3(a).
The laser beam profiles of various microlens shapes are displayed in Figure 3(b, c and d).The laser beam profile focused by spherical microlens is displayed in Figure 3(b).Figure 3(c) shows the beam profile focused by cylindrical microlens.Meanwhile, the beam profile of the spherical microlens array is depicted in Figure 3(d).Figure 3(b-d) reveals that various laser profiles can be generated by different types of microlenses that can be utilized for excellent beam shaping suitable for many applications.

The theoretical calculations
Mathcad 15 software was utilized to estimate the silicon surface temperature.When the laser irradiates the material, the surface will absorb some of the laser's power, leading to heating the material's surface.Silicon exhibited melting and vaporization temperatures of 1687 and 3538 K, respectively (14).When the laser beam is incident upon the silicon surface, the intense power density of the laser induces a rapid increase in the surface temperature of the silicon.The surface's temperature can be determined by applying a specific mathematical relationship (15): where α is the absorption coefficient, P D is the fiber laser power density, A is the absorptivity, p is the density and c p is the specific heat.Table 3 displays the increase in silicon surface  temperature as a function of fiber laser power.With 21 W of fiber laser power, the surface temperature could reach a temperature of 3970 K.
A model for the laser and silicon wafer interaction was constructed using the COM-SOL software, as depicted in Figure 4.The model simulates the thermal distribution on the surface and subsurface regions during the initial ablation stage.A fiber laser power of 19 W was selected to achieve a laser ablation temperature of 3590 K, as indicated in Table 3. Figure 4(a) illustrates the heat distribution on the interaction area of the silicon   surface during the ablation process.Figure 4(b) illustrates the silicon's ablation depth and the temperature distribution beneath the surface.Figure 5 depicts the temperature variations of silicon throughout the ablation procedures conducted using a fiber laser.Figure 5(a) shows that the surface temperature increased with radiation time.Moreover, the surface temperature of silicon rises to approximately 4600 K within a processing time of 1 s during the ablation process when a fiber laser power of 19 W is utilized, as depicted in Figure 5(a).Figure 5(b) shows the heat-affected zone during the processing, which also increases with the radiation time.Theoretical calculations for the first processing stages were performed using the COMSOL software, as presented in Table 4.
Table 4 summarizes the laser microprocessing of silicon wafers to fabricate silicon microlenses, as conducted through COMSOL software.During the process of laser ablation, it is observed that the temperature of the silicon surface reaches a value of 4600 K after 1 s, with a HAZ width of 0.57 mm.The ablation depth is also measured to be 230 µm, while the ablation width was 110 µm.It has been observed that an increase in the irradiation time leads to an increase in surface temperature, ablation depth, ablation width and the HAZ.Moreover, fiber lasers can maintain the necessary temperature (3750 K) for the ablation for extended periods, starting at 0.5 s, to get HAZ, ablation depth and a diameter of 0.5, 5 and 40 μm, respectively.

Figure 1 .
Figure 1.The optical microscope image (10X magnification) of the interaction area of silicon: (a) spherical MLA after stage 1, (b) spherical MLA after stage 2, (c) cylindrical MLA after stage 1, (d) cylindrical microlens after stage 2, (e) side view of initial shape of cylindrical MLA and (f) side view of the final shape of cylindrical microlens.

Figure 2 .
Figure 2. Schematic diagram of fabrication of silicon microlens by fiber laser and modification with etching.(a) After laser ablation and (b) after etching.

Figure 4 .
Figure 4.The theoretical model of silicon substrate irradiated by fiber laser (a) 3D model of the ablation process and (b) the 2D model for the ablation depth and width.

Figure 5 .
Figure 5.The thermal distribution of the silicon irradiated by fiber laser: (a) silicon surface temperature for the ablation process and (b) the HAZ during the processing.

Table 1 .
Specifications of the silicon spherical microlens.

Table 2 .
Specifications of the silicon cylindrical microlens.

Table 3 .
The silicon surface temperature as a function of the fiber laser power.

Table 4 .
The maximum silicon surface temperature as a function of irradiation time for microlens production.