1-1 Motivation
With the progress of the information technology, the demand and importance of data storage systems rise significantly. Due to the rapid growth of data and the trend of removable media, the high-capacity optical data storage systems are the focus of development. Therefore, the systems evolve from CD to DVD to become a high-capacity and removable storage system. The design of optical pick-up heads plays a key role and has great influence in the performance of the optical data storage system.
Figure 1-1 is the schematic of a conventional optical pick-up head. The optical pick-up head is composed of a laser diode, grating plates, a polarizing beam splitter, a collimator, a quarter wave plate, a 45° mirror, an objective lens, and photodiodes. The working principle of the optical pick-up head is that the laser emitted from the laser diode passes through the grating plates to produce two secondary beams in order to maintain the precise tracking of the light-spot. The beam passes through the polarizing beam splitter and is converted into a parallel beam by the collimator. Then, the beam becomes circularly polarized when passing through the quarter wave plate, and is focused on the surface of the disk (i.e., CD or DVD) by the objective lens after being reflected by the 45° mirror. The reflected beam from the disk becomes orthogonally linearly polarized with regard to the source beam after passing through the quarter wave plate and is reflected by the polarizing beam splitter into the detection photodiodes.
In conventional optical pick-up heads, a number of optical components are needed to detect the optical information signal. As a result of using discrete components, it becomes inevitably thick and large. Compared with the conventional pick-up head, miniaturized devices have the characteristics of shorter response time, less power consumption, and smaller size. In the trend of miniaturization, the Micro-Electro-Mechanical System (MEMS) is an appropriate choice since MEMS is a technology employing the semiconductor fabrication process to minimize electro-mechanical devices. Furthermore, MEMS devices can be integrated with signal processing circuits on the same chip, such as CMOS-MEMS technology, to reduce the noise interference and signal distortion.
Figure 1-2 is a monolithic free-space optical pick-up head developed by M. C.
Wu et al. [1]. The pick-up head contains a semiconductor edge-emitting laser, a beam splitter, three miro-Fresnel lens (including one collimating lens and two focusing lenses), and two 45° reflective mirrors. All optical components are built
Figure 1-1: Conventional optical pick-up head
monolithically on the silicon substrates.
For a full functional micro optical pick-up head, the objective lens and/or the micro-actuator must be included, as shown in Figure 1-3. In this thesis, the micro objective lens in the optical pick-up head is the major topic. For a DVD application, the numerical aperture (N.A.) of the lens is 0.65.
Figure 1-2: SEM micrograph of the free-space integrated optical pick-up head. [1]
Figure 1-3: Diagram of the MEMS-based optical pick-up head.
Silicon Substrate Laser Diode Coupling
and Shaping Grating Beam Splitting
Beam Bending and Steering
High N.A.
Focusing and Actuators Objective Lens
~mm
~mm
1-2 Microlens
There are three types of focusing microlens in the micro-optics, namely the refractive type, the reflective type, and diffractive type, as illustrated in Figure 1-4 [2].
Among them, the refractive and diffractive microlens will be discussed in this section.
In addition, different fabrication processes of the microlens developed in the past are also discussed in this section.
1-2-1 Refractive Microlens
A great variety of fabrication techniques have been applied to the fabrication of refractive optical elements (ROEs). Fabrication of refractive microlenses is often based on some analog physical process. Due to the analog nature of most fabrication techniques used for ROEs, their fabrication is often more difficult compared to the fabrication of diffractive optics. However, even the difficulties in the fabrication of refractive microlenses, the reflow methods have been used to fabricate the refractive microlens successfully.
In the reflow process shown in Figure 1-5, the photoresist is first spin-coated on the silicon substrate to the thickness necessary to produce lenses of given focal length when melted. Then, photolithography is used to define the pattern size of the microlens. Finally, heat is applied to melt the photoresist so that surface tension causes the resist to adopt a hemispherical form, and the microlens is completed.
Figure 1-4: Different types of optical functions. [2]
Although the reflow process is simple, the focal length of the microlens is difficult to control. Figure 1-6 shows an out-of-plane refractive microlens fabricated with the reflow technique and mounted on a surface-micromachined vertical plate [3].
Refractive microlens can also be made from the pre-shaped photoresist by a laser direct write technique [4], the droplets method [5], as shown in Figure 1-7, or the ink-jet process [6]. Furthermore, the refractive microlens pattern can be transferred to the substrate by accurately controlling the substrate-to-photoresist etch selectivity in reactive ion etching (RIE). Figure 1-8 shows a F-number 4.2, 200 μm diameter microlens array and a F-number 0.86, 80 μm diameter linear microlens array in fused silica [7].
Mask Photoresist
Silicon
Heat Melting Expose
Develop
Figure 1-5: Reflow process of the microlens.
Figure 1-6: SEM micrograph of the refractive microlens (diameter: 300 μm). [3]
Figure 1-7: SEM micrographs of a microlens array cured by UV light. [5]
Figure 1-8: SEM micrographs of mirolens arrays fabricated in fused silica. [7]
1-2-2 Diffractive Microlens
In conventional macrooptics, the use of refractive elements dominates, while diffractive elements are only used in spectroscopy mostly. However, diffraction plays a much stronger role in microoptics. Diffractive optics can be viewed as an approach to the fabrication optimized for the lithographic techniques. In ROEs, the light is manipulated by analog phase elements of considerable thickness in relation to the optical wavelength. For micro components, such as aspheric lenses, in particular, the fabrication is impossible with mechanical profiling techniques (due to the small lateral extension) and very challenging with microlithography (due to the large phase depth of the component). The solution to the fabrication problem lies in the periodic nature of the light wave U(x). If a light wave is delayed by one wavelength (corresponding to a phased lag of φ=2π), no difference to the original wave can be found, due to
2
0 0
( , ) ( ) i ( ) i ( , 2 )
U xϕ =A x eϕ = A x eϕ π+ =U x ϕ+ π (1-1) For example, retardation occurs when the wave passes through a dielectric material (e.g., glass or photoresist). The insensitivity of the light wave to phase jumps of 2Nπ (N is an integer) allows one to reduce the thickness of an optical element without changing its effect on a monochromatic wave. In transmission the maximum thickness of the corresponding optical component can be reduced to
max 1
t λ
= n
− (1-2) where n denotes the refractive index of the component material and λ is the wavelength of the incident light, as shown in Figure 1-9. The blazing of continuous phase functions results in laterally periodic elements, and such profiling depths are readily fabricated with lithographic techniques.
(
nλ−1)
Figure 1-9: Blazing of a lens resulting in a reduced thickness.
Diffractive microlenses are very attractive for integration with free-space micro-optical bench (FS-MOB) because:
(a) their focal length can be precisely defined by photolithography;
(b) microlenses with a wide range of N.A. values can be defined;
(c) microlenses with diameters as small as a few tens of micrometers can be made;
(d) their thickness is on the order of an optical wavelength [8].
The thin construction is particularly suitable for the surface micromachining process because the thicknesses of the structure layers are only on the order of 1 μm.
There are two types of diffractive microlenses, one is the continuous kinoform lens, i.e., the Fresnel lens, and the other is the multiple-level binary microlens, as shown in Figure 1-10. Fabrication of binary microlenses on the silicon substrate has already been demonstrated. In the multiple-level microlenses, log n2 masks are needed for the n-level lens. The fabrication steps of a four-level microlens are shown in Figure 1-11 as an example. In order to achieve better approximation of the lens, the number of levels should be as many as possible. The precision of the alignment is the key point in the method when the number of levels increases. Therefore, the complexity and the misalignment are the drawbacks of the process. Figure 1-12 shows the SEM micrograph of a binary micro-Fresnel lens with a diameter of 280 μm, a focal length of 500 μm, and an optical axis which is 254 μm above the silicon surface.
Figure 1-10: Schematic diagrams of (a) a continuous Fresnel zone plate and (b) a multiple-level binary microlens. [8]
Silicon substrate Lens material
Photoresist Lens material
(a) Deposit the lens material, then spin-coat and pattern the photoresist.
(b) Use RIE to etch the lens material for the first step.
(c) Spin-coat and pattern the resist for the second step.
(d) Etch the lens material by RIE.
Figure 1-11: Fabrication process of a four level microlens by the binary method.
(a)
(b)
Figure 1-12: SEM micrograph of the out-of-plane micro-Fresnel lens. [8]
1-2-3 Gray Scale Mask
The gray-scale mask technology uses the sub-micrometer resolution and locally modulates the intensity of the ultra-violet (UV) light through the mask to fabricate the microlens. The technology takes one-mask photolithography for a three-dimensional structure, so this process is simpler than the binary method. There are several methods to fabricate the gray-scale mask which are described as follows.
(1) Half-tone Mask
Half-tone mask, as schematically shown in Figure 1-13, uses different square patterns in the mask to cause different gray levels [9]. The basic concept is that the resolution of the photolithography system must be larger than the minimum size of the pattern on the mask so that an averaged gray scale pattern can be formed on the surface of the photoresist. If the resolution of the photolithography system is smaller than the mask pattern, the pattern on the mask will be completely transferred to the photoresist and no gray scale effect can be achieved. Figure 1-13 is the schematic diagram of a three-level gray-scale mask pattern and the resulting photoresist structure, and the actual resist structure is shown in Figure 1-14.
Figure 1-13: Schematic diagram of a three-level gray-scale mask pattern and the resulting photoresist structure. [9]
Figure 1-14: SEM micrograph of the three gray levels patterned in AZ4620 photoresist resulting from a similar mask pattern in Figure 1-13. [9]
(2) High-Energy Beam-Sensitive Glass (HEBS)
The HEBS glass used for the gray-scale mask consists of a low-expansion zinc borosilicate glass, i.e., a white-crown glass [10]. The base-glass composition consists of silica, metal oxides, nitrates, halides, and photoinhibitors. Typically, TiO2, Nb2O5,
or Y2O3 are used as photoinhibitors to dope the silver-alkali-halide, i.e., (AgX)m(MX)n, complex crystals. These complex crystals are the HEBS materials, and will be changed by high energy beams, such as electron beams. The HEBS glass has different gray-scale levels with different illuminated energy, and the resolution is much higher due to the electron beam size. Figure 1-15 shows a gray-scale HEBS mask pattern, and Figure 1-16 is the SEM micrograph of the actual structure fabricated in a quartz substrate.
Figure 1-15: Optical micrograph of the gray-scale mask. [10]
Figure 1-16: SEM of a diffractive optical element in a quartz substrate. [10]
(3) Laser Direct Writing
The laser direct-writing gray-scale masks also have been applied to fabricate diffractive microlenses. Figure 1-17(a) shows the laser-written sixteen-level microlens pattern before the etching process, and Figure 1-17(b) is a microphotograph of the etched microlens [11].
(a)
(b)
Figure 1-17: (a) A laser direct-writing gray-scale microlens mask and (b) the etched 16-level microlens structure. [11]
(4) Focused Ion Beam (FIB)
Finally, FIB can be used to mill the substrate to fabricate the microlenses.
Compared with other conventional techniques, such as laser direct writing and electron-beam direct lithography, it has the advantage of one-step fabrication without any pattern transfer steps. The substrate material selectivity is not required. This method is investigated in this research and the details of the FIB system will be stated as follows.
1-3 Focused Ion Beam System
A focused ion beam system is a mask-less processing equipment used to make a wide variety of small structures in various materials by utilizing sputtering etching or ion beam induced deposition. Since the 1980s, it has been used in photo-mask repair, modification of the wiring of integrated circuits, processing and observation of cross sections of integrated circuits.
The typical FIB instrument consists of a vacuum chamber, a liquid metal ion source (LIMS), an ion column, a sample stage, detectors, gas inlets, and a computer to control the complete system as shown in Figure 1-18 [12]. The capabilities of the FIB for small probe sputtering are made possible by the LMIS, which can provide a source of ions of ~5 nm in diameter. Several metallic elements or alloy sources can be used in a LMIS, and gallium (Ga) is currently the most commonly used material. Once the Ga ions are extracted from the LMIS, they are accelerated through a potential down the ion column. Figure 1-19 is a schematic diagram of the FIB column. The sample stage typically provide 5-axis movement (X, Y, Z, rotation, and tilt), and all five axis stage motions may be motorized for automatic positioning. Typically, the imaging detector of the FIB system is used to collect secondary electrons for image information. Gas delivery systems can be used in conjunction with the ion beam to
produce site specific deposition of metals, such as Platinum (Pt) or Tungsten (W), or insulators or to provide enhanced etching capabilities. All the devices and systems are controlled by a computer.
FIB instruments may be stand-alone single beam instruments. Alternatively, FIB columns have been incorporated into other analytical instruments such as a scanning electron microscope (SEM), Auger electron spectroscopy, transmission electron microscope (TEM), or secondary ion mass spectrometry. The most common one is a FIB/SEM dual-beam instrument, as is used in this research. The electron beam can be used for imaging without concern of sputtering the sample surface, so very creative ion beam milling and characterization can be obtained. In addition, electron beam deposition of materials can be used to produce very low energy deposition that will not affect the underlying surface of interest as dramatically as ion beam assisted deposition. The column arrangement of the dual-beam FIB systems is shown in Figure 1-20.
Figure 1-18: Schematic diagram of a basic FIB system. [12]
ion column
detector sample
vacuum chamber sample stage gas injection
needles
Figure 1-19: Schematic diagram of the basic FIB column. [12]
Figure 1-20: Schematic diagram of a dual-beam FIB column arrangement. [12]
Electron Column
52° Stage Tilt
Cross-Section Face Ion Column
Viewing with Ion Beam Viewing with Electron Beam
Onscreen Views
Cross-Section Face Cross-Section Face
Not Visible
Completely Visible (Not to Scale) Stage
Suppressor & LMIS Extractor Cap
Beam Acceptance Aperture Lens 1
Beam Defining Aperture Beam Blanking
Deflection Octopole Lens 2
FIB systems have been used for the modification or fabrication of prototype optoelectronic and magnetic devices, integrated circuits, the preparation of specimens for material analysis, and the micromachining of MEMS. In addition, FIB milling can be used to fabricate diffractive microlenses. Figures 1-21(a) and 1-21(b) show diffractive lenses with a designed depth of 1.06 μm fabricated on silicon [13]. Also, some interesting thoughts have been demonstrated by using FIB systems, such as a nano wine glass made by carbon deposition or ion beam milling, namely a “nano milling machine” or a “nano lathe” [14]. The SEM micrograph of the nano wine glass produced with the “nano milling machine” is shown in Figure 1-22.
(a)
Figure 1-21: Diffractive lens on silicon by use of FIB milling. It was characterized by the AFM with a tapping mode. (a) 2D profile in the horizontal direction. (b) SEM micrograph of the fabricated device. [13]
(b)
Figure 1-21: (Continued) Diffractive lens on silicon by use of FIB milling. It was characterized by the AFM with a tapping mode. (a) 2D profile in the horizontal direction. (b) SEM micrograph of the fabricated device. [13]
Figure 1-22: SEM micrograph of the nano wine glass made by FIB milling. [14]
In this thesis, the FIB system is used to mill the silicon nitride films on the silicon substrate to produce microlenses. Since silicon nitride has high absorption in the ultraviolet (UV) wavelength used in photolithography, such milled pattern can
also be used as a gray-scale mask. Moreover, the detailed design concepts and fabrication process of the microlenses and gray-scale masks will be discussed in Chapters 2 and 3.
The specifications and functions of the FIB system in the Semiconductor Research Center (SRC) at National Chiao Tung University (NCTU) are listed below [15]:
z xT Nova NanoLab 200 system of FEI company
z Ion source: Ga+ LMIS
z Acceleration voltage range: 0.5 to 30 kV for electron beam and 5 to 30 kV for ion beam
z Image resolution: 1.5nm for SEM and 7 nm for FIB
z Working distance range: 50 mm
z Data storage
z Process yield engineering
z Etching
z Lithography
z Metal (i.e., Pt and W) and other materials deposition
z Fabrication of micro- and nanostructures
1-4 Objectives and Thesis Overview There are two major objectives in this thesis:
(a) using the FIB system to directly write the microlens patterns on the silicon nitride film to form the lens.
(b) producing microlenses by using the gray-scale mask on the silicon nitride film fabricated by the FIB system.
The fundamental principles and design concepts of the gray-scale mask and
microlenses are described in Chapter 2. The fabrication processes of the lens and mask are discussed in Chapter 3. Then, the profiles of the FIB pattern measured by AFM and the results of the optical experiments of the lens are described and discussed in Chapter 4. Finally, conclusion and future work of the research are discussed in Chapter 5.