A polishing process for nonlinear optical crystal flats based on an annular polyurethane pad
Highlights
► Annulus Chemical Mechanical Polishing process is developed based on the conventional CMP process for polishing crystals. ► A septum & holder system suitable for the crystal is employed to replace the carrier & driving spindle. ► A turning system and ring conditioner are developed for planarization of the base plate and the pad respectivley.
Introduction
The nonlinear optical (NLO) crystals are mainly used for frequency conversion of lasers. Many excellent NLO crystals have been developed since 1980s, such as Beta-Barium Borate (beta-BaB2O4, BBO), Lithium Triborate (LiB3O5, LBO), and Yttrium Calcium Oxoborate (YCOB) [1], [2], [3]. The realization of their excellent properties depends, to a great extent, on the ability to fabricate the high quality surfaces [4]. Considering the scarceness and preciousness of crystal materials as well as their anisotropic properties, processes for fabrication of these fragile NLO crystals should be cautiously proposed. However, few works on this topic have been published in the literature [4], [5]. The difficulties for fabricating this kind of optics lie in two major aspects: one is the strict requirement of ultra-precision on the surface figure while the other is to avoid surface defects as well as reducing subsurface damage (SSD).
A LBO crystal flat, to be employed in our Nd:YAG laser, has a size of 50 mm × 50 mm × 7 mm. The properties of this LBO crystal are shown in Table 1. The surface figure and the roughness of the two 50 mm × 50 mm working surfaces are required to be lower than λ/8 (Peak-Valley value, P-V, λ = 632.8 nm) and 1 nm (Root-Mean-Square value, RMS) respectively. Little defect on the surface is permitted as well as subsurface damage and residual stress.
Although Magnetorheological Finishing (MRF) process can be an alternative for ultra-precision optics among the advanced polishing processes published in the literature, the Hydroxy-Fe in magneto-rheological fluid would contaminate the surface layer of the workpiece [6], [7]. For the computer controlled small-tool polishing process that has been extensively used for large optics, convergence of the full-aperture P-V value is seriously deteriorated for small optics due to the instability of the material removal function on the optic edge areas [8], [9], [10], [11]. In addition, the small tool employed in the process induces local stress on the contact area [12], which may damage the isotropic crystals. The ion beam polishing, another process emerging in recent years, shows enormous superiority in the determination and precision. However, the heated ions will cause thermal deformation or even destruction of the elements [13], [14], [15].
In the semiconductor industry, CMP is the most common polishing process for planarization of wafers [16], [17], [18]. During CMP processes, a wafer is rotated about its axis while being pressed face-down by a carrier and a carrier film against a rotating polishing pad covered with colloidal silica slurry of specific chemical properties [19]. In order to achieve the ultra-precision optical surface of this fragile and anisotropic LBO flat, a new Annulus Chemical Mechanical Polishing (ACMP) process was developed on the basis of the conventional CMP process. The carrier and driving spindle in the conventional process were replaced by a septum and holder system suitable for the workpiece. The process is characterized by an annular polyurethane pad which has three major merits: the effective, fast dressing and topography-correction of polyurethane pads, defect-free surface and low subsurface damage of the workpiece.
The detailed description of the process is presented in the next sections as well as a discussion on the achievement of final polishing quality. Following that, trial experiments of polishing the LBO flat are conducted with the results shown afterwards.
Section snippets
Technological process
ACMP process is characterized by an annular polyurethane pad fixed on the metal base plate and a polytetrafluoroethylene (ptfe) septum on the exterior of the workpiece. The ptfe septum is used to pre-press the pad on the workpiece edge. The workpiece and the septum are held in the cavity of the circular holder, which is also manufactured by ptfe. With the driving wheels on the periphery of the holder, rotating speed of the holder can be accurately controlled. The polyurethane pad is kept flat
Experimental and results
A GR-35 polyurethane pad manufactured by Universal Photonics Inc. utilizing polyurethane-based foam is used as the polishing pad. This pad has a Shore Hardness of approximately 37 and a uniform thickness of 1.27 mm. It also contains a distribution of asperities and pores. Grooves are periodically machined into the pad surface to readily transport the polishing slurry into the polish site. The groove pattern of GR-35 polyurethane pad is a regular intersecting orthogonal channel pattern (x–y
Conclusions
A practical process (ACMP) for polishing ultra-precision crystal flats has been proposed and demonstrated by the authors. The annulus pattern of the pad employed in ACMP is viable to its topography control. By incorporating the tuning system for planarization of the base plate and the diamond conditioner ring for planarization of the pad, the topography of the annular polyurethane pad could be figured to be extremely flat. The ACMP process adopts a ptfe septum and a holder for holding the flats
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