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A novel method is proposed to fabricate micro Diffractive Optical Elements (DOE) using micro cutting tools shaped with focused ion beam (FIB) milling. Micro tools with nanometric cutting edges and complicated shapes are fabricated by controlling the tool facet’s orientation relative to the FIB. The tool edge radius of less than 30 nm is achieved for the nano removal of the work materials. Semi-circular micro tools and DOE-shaped micro tools are developed to fabricate micro-DOE and sinusoidal modulation templates. Experiments show that the proposed method can be a high efficient way in fabricating micro-DOE with nanoscale surface finishes.
©2010 Optical Society of America
1. Introduction
Micro optical elements (MOE) are key components for building compact optoelectronic systems. Micro Diffractive Optical Elements (MDOE) have shown many unique advantages, such as small volume and weight, good optical quality and large apertures. With the trend of device miniaturization, there is increasing demand for MDOE for applications in solar condenser lenses, infrared sensors, laser beam shaping, and high-performance optical imaging systems.
There are several methods for fabricating micro optical elements, such as, Binary Optics Fabrication method [1], Direct Writing method using a laser or a focused ion beam [2,3], Single-point Diamond Turning (SPDT) technology [4], etc. The lithography method needs multi-step processes with high cost, while the beam direct writing method shows lower fabrication efficiency. SPDT is considered as one of the most appropriate processing methods for various types of Micro Optical Elements. It can precisely control the fabrication process with a high machining accuracy, and it is much suitable for processing complex micro diffractive optical elements.
However, with the optical elements miniaturization, the development of the micro tools with non-traditional shape and sharp edges used in SPDT is becoming a very important topic in the MOE fabrication. Due to the size and configuration limitations of traditional cutting tools in SPDT, some areas with a high aspect ratio in the MDOE cannot be machined, as shown in Fig. 1(a) . This will cause the shadowing effect, which will directly degrade the optical properties of the device and reduce its diffraction efficiency [5]. While this problem can be overcome by using a micro hemi-spherical tool, as shown in Fig. 1(b), the sharp cutting in the MOE demands non-traditional shape micro machining tool fabrication, which is a key topic in the micro fabrication research.
Fig. 1 Shadowing effect comparison in MDOE fabrication with spherical and hemi-spherical tools. (a) Traditional shape tool, (b) Micro hemi-spherical tool.
Micro tool processing methods mainly include precision mechanical grinding [6], micro electro discharge machining (µEDM) [7], etc. In recent years, focused ion beam (FIB) milling has also been applied in the micro tool’s fabrication [8,9]. It can achieve high-precision material removal and geometry accuracy using high energy focused ion beam bombardment.
In this paper, micro diffractive optical elements were fabricated by using micro tools with non-traditional geometry, fabricated with a focused ion beam. The key fabrication techniques for the development of the complex shape micro tools and MDOE were investigated in detail.
2 Experimental setups
The experiments for micro tools fabrication were performed under a FIB/SEM dual beam system equipped with a high resolution rotational equipment. The FIB system uses a focused Gallium ion beam working under an accelerating voltage ranging from 5 to 30 kV, and a probe current ranging from 1 pA to 20 nA. The translation stage can be tilted within 15°~60°. Tool rotation is controlled by a rotational axis that can rotate unlimitedly with a minimum step size of 10−7 rad per pulse. Different tool faces can be milled by accurately adjusting their relative positions to the FIB by the rotation and sample tilt control. Tool blanks with end diameters of around 30~100 μm were used here by a precision lapping method.
3 Results and discussions
3.1 Micro tool fabrication
During the process of the micro tool fabrication by the FIB milling, the FIB milling sequence and tool position relative to ion beam are critical factors in determining micro tool characteristics, such as cutting edges, rake face, and relief angle. The FIB would produce sharp edges on the side of facets furthest from the ion source, while the facet edge closest to the ion source is rounded because the part of the Gaussian beam intensity would extend outside the defined pattern boundary, as shown in Fig. 2 .
Therefore, in order to produce sharp cutting edges, tool position with respect to the ion beam and the fabrication sequences are crucial.
Based on the sharp edge generation analysis above, only three steps were proposed here to fabricate micro tool with sharp cutting edges by the FIB method. Figure 3 illustrates the fabrication procedure for an arc-shaped micro tool. The dark parts in the figure represent the removal regions by the FIB milling. Firstly, a smooth rake face is created on one side of the tool by the FIB milling in Fig. 3(a). Secondly, the tool is rotated clockwise with its rake face away from the ion source. The two side faces of the micro tool are then created by FIB milling, respectively. Thirdly, a desired cross-sectional shape is milled by the FIB bitmap patterning method [3], as shown in Fig. 3(c). In the bitmap patterning method, the FIB milling time at a location can be accurately controlled by the color value of the bitmap predefined. By controlling the sample stage tilt angle and the tool rotation, a proper relief angle can be achieved by adjusting the FIB incidence angle with respect to the surface normal. Finally, an arc micro tool with sharp cutting edges is obtained, as shown in Fig. 3(d). The SEM image of the arc micro tool fabricated is shown in Fig. 4 .
Fig. 4 SEM images of the FIB fabricated arc-shaped micro tool. (a) Spherical micro tool, (b) hemi-spherical micro tool.
In the FIB milling process, a large ion beam current can be selected first to mill the micro tool’s outline configuration to increase the process’s efficiency. Then a small ion beam current can be chosen to create the final smooth tool face. The ion beam current used in the area of A and B in Fig. 4(a) is 1 nA and 20 nA, respectively. In addition, precisely controlling the beams’ overlapping is also very important to obtain a smooth micro tool face, which can be accurately controlled in the bitmap patterning method. The major advantages of the FIB based micro tool fabrication method include the better geometry control of micro tools with sharp edge without the introduction of any machining stress comparing with ordinary precision grinding method.
The FIB-shaped micro tools’ edge radius has been measured by Atomic Force Microscope (AFM), as shown in Fig. 5 . The scanning probe is sharpened by FIB milling before the tool’s edge radius measurement. By considering the AFM probe’s broadening effect from the AFM results, the developed micro tools’ edge radius is can be approximately 25nm.
Fig. 5 Results of the micro tool edge radius measured by AFM. (a) Sharpening AFM tip, (b) Edge result of micro tool.
Comparing with the ordinary SEM cross-section observation method [7,8], the AFM measurement method can show the three-dimensional morphology of the Micro tool edges without tool destruction.
3.2 Sinusoidal modulation template developed
A sinusoidal modulation template has been machined by an arc-shaped micro tool fabricated using single-point diamond turning. The arc-shaped micro tool is made from a single crystal diamond, as shown in Fig. 4(a). The micro tool’s nose radius is 10 μm, with 0 ° rake angle and 12.4 ° relief angle.
For the micro optical elements’ fabrication using the micro tool, the micro-tool’s processing path, which is also called the Numerical Control (NC) path, should be defined [10]. First, the three-dimensional data model of Sine surface array is built. Second, the two-dimensional track of tool path is designed according to the micro tool’s specific parameters. Then, project the tool path two-dimensional track to the three-dimensional Sine surface model, and the intersections between them would form the NC tool path. Finally, the required surface structure would be realized after the micro tool follows the tool path in ultra precision machining.
The machined sinusoidal modulation template is measured by a Veeco NT9300 interferometer, as shown in Fig. 6 . From the figure we can see that the structure surface is close to the ideal of the sine graph, the cycle and amplitude of sine curve are close to the defined values. The single-crystal diamond micro-cutting tools prepared by FIB were used for the precise machining of sinusoidal surface array.
Fig. 6 The sinusoidal surface machined by micro tool. (a)A sinusoidal template by WLI, (b) Cross-section of the sinusoidal results.
3.3 Micro Fresnel optical components fabrication
The working principle of a Fresnel lens is to divide the conventional lenses into several regions with equal spacing and remove the central part of each region while maintaining the required surface curvature, as shown in Fig. 7 .
The Micro Fresnel optical element designed here has a diameter of 15 mm, a curvature radius of 30 mm, and the interval between the adjacent rings of 30 μm. The largest depth is calculated to be 7.678 µm, which provides an important reference for tool design and machining.
According to the geometric relationship, the incomplete machine parameters can be calculated. We define the width of the incomplete turning area in a ring as x, the sharp angle for the MDOE is θ, as shown in Fig. 1. Thus the relationship between the width of incomplete turning part using the traditional shape tool xt and hemispherical shape tool xh with the corresponding tool nose radius R should be:
And theθ can be approximately calculated from:Where, d is the maximum cutting depth of the Fresnel components; l is the distance between the neighboring rings.According to the formulas (1)~(3), decreasing the tool radius can reduce the incomplete machine area. The hemi-spherical micro tool with R = 7.73μm nose radius was fabricated by the FIB, as shown in Fig. 8 . The micro Fresnel mirror designed with parameters d = 7.678 µm, l = 30µm, and it can be calculated from the equations above that the width of incomplete turning area xt and xh is 9.65µm and 1.92µm, respectively. Moreover, the volume for the incomplete turning with the hemispherical shape tool would greatly reduced comparing with the traditional shape tool. Therefore, the shadowing effect influence for MDOE can be greatly reduced by choosing the hemispherical shape micro tool.
The hemi-spherical micro tool was used to fabricate micro Fresnel optical component by the ultra-precision machining. Then through the micro-feed in x-axis and z-axis, the micro Fresnel profile is machined step by step. The machining parameters are as follows: spindle speed is 1900 rev/min, feed rate is 1mm/min, and maximum depth of cut on z-axis is 1μm /loop. The micro Fresnel lens was machined on a 6061 aluminum workpiece. The machined Fresnel optical lens is measured by a Veeco NT9300 interferometer, as shown in Fig. 9(a) (b).
Fig. 9 Data of micro Fresnel diffractive optical component. (a) Micro Fresnel optical component, (b) Morphology of the micro Fresnel lens, (c) Diffraction rings.
The micro Fresnel optical component’s optical property has been tested using a He-Ne laser with a 632.8nm wavelength. After passing through the extender lens and the collimated lens, the He-Ne laser irradiated the micro Fresnel optical component surface. The diffraction rings were formed, whose order of diffraction can be up to 22, as shown in Fig. 9(c). It is shown that the micro Fresnel optical lens fabricated by the method shows high resolution and high diffractive efficiency.
Finally, a novel concept is proposed here to fabricate MDOE by using a DOE-shaped micro tool. By transferring the profile of the object MDOE, the micro tool with DOE-shaped cross section is developed by the FIB bitmap patterning method, as shown in Fig. 10(a) . The MDOE is machined on a 6061 aluminum by the DOE-shaped micro tool, as shown in Fig. 10(b). The spindle speed is 1500 rev/min, and the maximum depth of cut is 1.2μm. This method could be useful to fabricate micro DOE array efficiently.
Fig. 10 MDOE fabricated by a DOE-shaped micro tool by FIB. (a) DOE-shaped micro tool by FIB, (b) MDOE machined by the DOE-shaped micro tool.
4 Conclusions
A novel method is proposed to fabricate Micro Diffractive Optical Elements (MDOE) using micro cutting tools shaped with focused ion beam (FIB) milling. Micro tools with about 25nm cutting edges and complicated shapes can be fabricated by the FIB method. MDOE and sinusoidal modulation templates are fabricated by the developed semi-circular micro tools and DOE-shaped micro tools. Experiments show that the proposed method can be a high efficient way in fabricating MDOE with a nanometric surface finish.
Acknowledgements
The research work is supported by ‘111’ project by the State Administration of Foreign Experts Affairs and the Ministry of Education of China (B07014), National High Technology Research and Development Program of China (863 Program-No. 2009AA044305&2009AA044204), National Natural Science Foundation of China (No. 50905126, 90923038&50935001), and Natural Science Foundation of Tianjin. The authors would like to thank Mr. S. Wu, Mr. Y. Sa and Mr. G. S. Zeng for assistance in the micro tool edge radius measurement and micro DOE’s optical experiments.
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