A Novel Polishing Process with Rigid-Flexible Composite Structure Plate and Its Performance in Polishing Sapphire Wafer

A novel exible polishing process has been developed for sapphire wafer by using a polishing plate with rigid-exible composite structure to satisfy the demands of excellent surface shape accuracy and high surface topography quality simultaneously. This new polishing plate was fabricated by alternately casting and curing the ring structure of soft and hard unsaturated resins. It is found that the overall stiffness of the polishing plate is improved due to the “hard support frame” of rigid-exible polishing plate, as well as the ability of removal selectivity of the polishing plate is strengthened. The topography quality and shape accuracy of sapphire wafer polished by presented novel polishing process have been compared with those polished by conventional exible polishing, respectively. Both experiment and simulation results are shown that the surface roughness and topographical variations of sapphire wafer polished by the novel rigid-exible composite structure polishing plate have been greatly improved. Comparing with the conventional exible polishing, the surface shape accuracy of the sapphire wafer polished by the presented novel polishing process can be improved by 54.1%.


Introduction
As a typical engineering ceramic material, single crystal sapphire consisting of α-Al 2 O 3 has been widely used in many applications such as optics, electronics, and temperature sensing etc., and is the most common wafer used in light emitting diodes (LEDs) by virtue of its excellent mechanical and optical properties such as great hardness, good thermal stability, chemical inertness, and good light transmission [1][2][3][4]. The surface quality of processed sapphire wafer plays a critical role in these applications, as well as in shape accuracy. Generally, the processed surface of sapphire wafer is required to be smooth and at without sub-surface damage to ensure the performance requirements in practice [5][6][7][8]. However, as a typical hard-brittle and di cult-to-process material, it is a great challenge for sapphire to satisfy those demands.
Currently, the abrasion processing of sapphire wafer mainly includes slicing, lapping and polishing [9].
The main planarization machining to realize the precision requirement for sapphire wafer is abrasive machining, especially by using abrasive particles with ne grain size [10,11]. A high concentration of slurry that contains relatively large abrasive particles of high hardness, such as diamond and alumina, has been widely used with a metal-resin platen in the conventional mechanical polishing [12,13].
However, hard abrasives during mechanical polishing lead to poor roughness and heavy scratch on sapphire surface, resulting in a subprime pre-machined surface for next processing step. On the other hand, uneven removal of surface materials can be found due to the uneven dispersion and uncontrollable trajectory of abrasive particles, which lead to the decline of surface shape accuracy. Therefore, it is extremely di cult to obtain ne surface and high surface shape accuracy simultaneously through traditional free abrasive polishing.
In recent years, various reform technologies have been applied to improve the mechanical polishing quality of sapphire wafer, such as xed abrasive machining [14,15], mixed abrasive machining [16], ultrasonic vibration-assisted machining [17,18], hydrodynamic machining [19], catalyst-assisted machining [20,21], composite abrasive machining [22][23][24][25]. Although these methods exhibit machining performance superior to the conventional method, some problems associated with the inherent properties of free and xed abrasive machining still exist. For example, the random distribution and uncontrollable trajectory of the abrasive particles in free abrasive polishing result in the uneven distributed topography and the severe bad of surface shape accuracy. Compared to free abrasive process, xed abrasive polishing exhibits a signi cantly worse surface quality, especially a large number of scratches and pits [26].
As a novel super-precision machining process, exible polishing has been widely used for machining hard and brittle materials as well as various kinds of optical materials, such as SiC, sapphire, and GaN.
Flexible polishing is to use the soft binder with yield effect to restrict the movement of abrasive particles within a certain range under certain constraints to form a soft polishing plate. Under a certain polishing pressure, the yielding effect from abrasive polishing plate caused by soft binder is used to make the large-size abrasive particles yield, which enables large-size abrasive particles to achieve relatively high surface quality instead of smaller ones. Therefore, uneven dispersion and uncontrollable trajectory problems of ultra ne abrasives can be effectively avoided. Flexible polishing has been reported as an ideal approach to satisfy the processing demands of scratch-free and nano-scale roughness in wafer surfaces [27]. Yuan et al. [28] prepared a new soft abrasive polishing plate for monocrystalline silicon polishing. Based on the "trap effect" of the polishing plate during processing, the scratches caused by large abrasive particles on the machined wafer can be effectively avoided. Xu et al. [29][30][31] used sol-gel technology to disperse ultra ne abrasive evenly in sodium alginate solution, and then solidi ed with Ca 2+ solution to form semi-xed exible polishing plate. This technology was proposed for the mechanical polishing of single crystal sapphire, single crystal silicon carbide and other photoelectric wafer materials to achieve nano-scale surface roughness. Bai et al. [32] used the constrained abrasive particle effect of magnetorheological polishing to achieve ultra-smooth and low damage polishing of hard and brittle materials, signi cantly reducing or even eliminating the damage caused by large-size abrasive particles to the machined surface. The above research results show that compared with the traditional free abrasive and xed abrasive polishing, using the exible polishing plate with hard abrasive to process the wafer material can obtain a smoother and less sub surface damage surface. However, in the process of wafer polishing, it is not enough to only obtain better surface quality, but also need higher surface shape accuracy. Due to the surface of exible polishing plate is relatively soft, it will lead to certain deformation of polishing plate under the action of polishing pressure, which is di cult to ensure the surface shape accuracy of wafer processing. Therefore, it is essential to develop a new method to meet the perfect surface and high surface shape accuracy of sapphire wafer polishing at the same time.
In this study, we introduce a novel exible polishing by using rigid-exible composite structure polishing plate (RCSPP), and investigated its polishing performance. In the RCSPP, the rigid structural material without any abrasive is used as the "support frame", which can reduce the deformation of the polishing plate and ensure the surface shape accuracy in the polishing process. Meanwhile, the soft structural material is used as the "processing layer" to realize the exible polishing of the wafer and obtain better wafer surface quality. A conventional exible polishing is performed for comparative study. The surface topography and shape accuracy of sapphire wafer after the exible polishing with different plates are performed, and the MRR of exible polishing process is measured. It will be shown that using this polishing process instead of conventional exible polishing, smoother surface and high surface shape accuracy can be obtained at the same time. Furthermore, through nite element analysis, the effect of rigid structure on surface shape accuracy is discussed in detail.

Manufacturing procedures of polishing plates
The manufacturing procedures of RCSPP via casting technique mainly includes seven procedures, as shown in Fig. 1: (a) promote agent and curing agent are successively added into the hard unsaturated resin solution and stirred evenly by thermostatic mixer; (b) the hard unsaturated resin solution was poured on the annular groove, which is pasted on the glass plate; (c) wait for the hard unsaturated resin solution to cure for 4 hours; (d) only the acrylic plates are removed; (e) promote agent, alumina abrasives (the average particle size is 40 µm) and curing agent are successively added into the soft unsaturated resin solution in and stirred uniformly; (f) wait for the soft unsaturated resin solution to cure for 4 hours to form RCSPP; (g) take out the RCSPP with the turnover formwork. Moreover, the conventional exible polishing plate is made from soft unsaturated resin by mixing, screeding and curing processes, as shown in Fig. 1(e) and (h).

Polishing tests
Commercial lapping-machined sapphire wafer-oriented (0 0 0 1) plane with two-inch diameter was used, and its original surface roughness Ra was about 1 µm. Polishing experiments were conducted on a rotary-type polishing machine (AUTOPOL-1200S, Kejing, China) with conventional exible polishing plate and RCSPP, respectively. Deionized water was applied as the coolant. The schematic diagram of the sapphire wafer polishing process is shown in Fig. 2.
The acting force between the wafer carrier and the polishing plate was 5 kgf. The workpiece/plate rotation speed was 60/120 rpm, and the processing time was 120 min. After processing, the wafers were cleaned immediately with ethanol and deionized water under sonication. The weight of the wafers before and after polishing was measured by a precision electronic balance with 0.01 mg precision (GE0505, Yoke, China) to calculate the MRR according to the following formula [33]: The surface pro le and roughness of processed sapphire wafers were evaluated using a contact roughness instrument (MarSurf XR20, Mahr, Germany). For each machined wafer, nine testing points roughness Ra that evenly dispersed on the wafer surface were conducted at different locations, as illustrated in Fig. 3, and the average roughness Ra was calculated. The standard deviation (SD) was calculated by Eq. (2) using nine testing points thickness changes (the same testing points as mentioned above) measured by a digital micrometer (ACE-G3121, Rapid measurement, China), which was used to evaluate the surface shape accuracy. The SD can be expressed as: Moreover, the compression tests of rigid and exible materials were carried out on a universal testing machine of mechanics of materials (2382, Instron, USA) to obtain the mechanical properties parameters of materials used for simulation.

Effect Of Rigid Structure On Surface Shape Accuracy
In order to study the effect of rigid structure of rigid-exible composite plate on surface shape accuracy, stress and displacement between different polishing layers and sapphire wafer surface were performed by nite element analysis. Abaqus nite element program (Version 6.14, Dassault Systemes Simulia Corp., Providence, R.I.) is used to analyze the material removal process during conventional exible and RCSPP polishing from a microscopic point of view.
On the whole, due to the surface of sapphire wafer is uneven and continuous [34], assuming the local area of sapphire wafer surface is a unidirectional curved surface. The local area of contact interface between polishing plate and workpiece is taken as the geometrical and mesh model as shown in Fig. 4. The sapphire wafer and polishing plate are established by solid element type for the authenticity of the geometrical model. In order to improve the calculate accuracy, hexahedron mesh is used on sapphire wafer and polishing plate. The number of divisiory unit of sapphire wafer is 2700, the number of rigid area and exible area of RCSPP's divisiory unit are 4500 respectively (Fig. 4a), the number of the conventional exible polishing plate's divisiory unit is 9000 (Fig. 4b). The mechanical property parameters of sapphire wafer, rigid material and exible material are listed in Table 1. In terms of boundary conditions, sapphire wafer is loaded in the Y direction and constrained by three rotational degrees of freedom and two displacement degrees of freedom of X and Z axis and bottom of the polishing plate is completely xed. The simulation results of local stress and displacement at the processing interface between sapphire wafer surface and different polishing layers at different positions are shown in Fig. 5. For the conventional exible polishing, as shown in Fig. 5(a), when the sapphire wafer interacts with the conventional exible polishing plate, the exible structural materials containing abrasives will undergo elastic deformation under the action of polishing pressure, resulting in the processing area adhering to the whole non-plane of the sapphire wafer, thus reducing the surface shape accuracy of the sapphire wafer. Meanwhile, following ve cases of material removal process during RCSPP polishing are addressed: 1) When the local highest point of wafer surface contacts with the rigid structural materials of RCSPP at the position shown in Fig. 5(b), it can be seen that under the action of polishing pressure, the stress and displacement change area almost only occurs in the rigid structural materials without any abrasive, while at this time, the exible structural material with hard abrasives has no contact with the wafer surface.
Therefore, the polishing plate only supports the sapphire wafer at this position without any material removal.
2) When the local highest point of wafer surface contacts with the rigid structural materials of RCSPP at the position shown in Fig. 5(c), the stress area is similar to that in Fig. 5(b), and the exible structural material of the polishing plate begins to deform slightly due to the small deformation of the rigid material, and the material is rarely removed near the local highest point.
3) When the local highest point of the sapphire wafer contacts the intersection of rigid and exible structural materials of RCSPP, as shown in Fig. 5(d), the local highest point of sapphire wafer contacts with the soft and hard areas simultaneously. Due to the different hardness of two unsaturated resins, the exible structural material is relatively less stressed under the support of the rigid structural material, and its exibility makes the displacement change greatly. Meanwhile, under the support of the rigid structural material without any abrasive, the exible structural material containing abrasives being to slightly remove the wafer material near the local highest point. 4) When the local highest point of wafer surface contacts with the exible structural materials of RCSPP at the position shown in Fig. 5(e), the stress diagram shows that when the local highest point of sapphire wafer reaches the limit of the soft region, the stress of the rigid structural materials is the largest at the boundary line, which supports the sapphire wafer. Locally machining surface is divided into processing area and non-processing area by the position of the support, in which the upper location of the support is a non-processing area, and the lower location of the support is a processing area in contact with the soft area with abrasives. 5) When the local highest point of wafer surface contacts with the exible structural materials of RCSPP at the position shown in Fig. 5(f), under the support action of rigid structure, the local highest point of the sapphire wafer move to the location with maximum displacement in exible structural materials, and the contact arc length between wafer surface and exible structural materials reaches the maximum, as well as the material remove.
Based on the above analysis, the existing of rigid structure in RCSPP makes the exible structural materials with abrasives obtain the ability of removal selectivity, which means that the material in the convex area of the wafer surface will be removed preferentially. On the contrary, the material in the concave area will be removed as the convex area is planarized. Based on this characteristic, the novel exible polishing with RCSPP can effectively improve the surface shape accuracy in the polishing of sapphire wafer.

Comparison of planarization behavior
In order to compare the planarization behavior of different exible polishing processes, the wafers were polished using conventional and RCSPP polishing after lapping process. Two variations of surface roughness Ra of sapphire wafer after two kinds of exible polishing are shown in Fig. 6. Under the yielding effect of exible material, it is observed that the wafer surface roughness Ra decreases from about 1 µm to 0.7 µm after four polishing intervals in conventional exible polishing. Meanwhile, the surface roughness of sapphire wafer processed by RCSPP polishing is similar to that of the conventional exible polishing under the same machining parameters, and the surface roughness of sapphire wafer is reduced by nearly 0.3 µm. Figure 7 shows the surface pro le curves of sapphire wafers before and after polishing. As shown in Figure 7(a), lapping-machined sapphire wafer surface exhibits a large number of deep scratches and irregularity on the surface of sapphire wafer. These scratches could also cause an obvious increase of roughness. In terms of the above results, both of the two exible polishing approaches could effectively smooth the surface as presented in Figure 7(b) and (c). While the great smoother surface acquired from the exible polishing by using conventional and RCSPP polishing, the processed wafers have a higher de nition than original surface, and the vast majority of scratches are removed as shown in Figure 7(b) and (c). It can also be shown that the value of PV (peak-to-valley) is > 9 nm before polishing, and <5 nm after polishing. Both kinds of exible polishing can signi cantly reduce the scratch depth of sapphire wafer. These results show that the surface roughness polished by rigid-exible composite structure polishing is consistent with that obtained by conventional exible polishing, indicating that the rigid structural in novel exible polishing will not reduce the planarization ability of the polishing plate.

Comparison of surface shape accuracy
Two kinds of exible polishing are used to compare the surface shape accuracy of polished sapphire wafer, as shown in Fig. 8. It can be seen that with the extension of polishing time, the surface shape accuracy of sapphire wafer processed by novel exible polishing with RCSPP has been signi cantly improved. Comparing with the conventional exible polishing, the surface shape accuracy of the sapphire wafer polished by the presented novel polishing process can be improved by 54.1%. Due to the existence of "hard support frame" of rigid structural materials in the novel polishing process, which cause the overall stiffness of the polishing plate is improved as well as obtained the ability of removal selectivity, the surface atness error of the polishing plate in the polishing process is dramatically reduced.
Therefore, the use of RCSPP can signi cantly improve the surface shape accuracy of processed wafer. Figure 9 shows the MRR of sapphire wafers processed by conventional exible and RCSPP polishing. It can be seen that the MRR was affected by the rigid structure in RCSPP. As presented in Fig. 8, the material removal rate of sapphire wafer polished by RCSPP polishing is 1.2 nm/min lower than that of the conventional exible polishing. It can be explained by that the presence of rigid structural materials without any abrasive, the effective number of abrasive particles in contact with sapphire wafer surface is reduced. These results indicate that the material removal of sapphire wafer is affected by the rigid structure, but its quantitative relationship needs further investigation.

Conclusions
In this study, a novel exible polishing process has been presented with RCSPP to meet the demands of excellent surface accuracy and high surface topography quality simultaneously for sapphire wafer. The conventional exible and RCSPP polishing have been performed for comparative study. Meanwhile, combined with nite element analysis, the effects of rigid structure on surface shape accuracy have been discussed in detail. The following conclusions can be drawn as: 1. The rigid structure can greatly improve the surface shape accuracy of sapphire wafer in proposed novel exible polishing process. Comparing with the conventional exible polishing, the surface shape accuracy of the sapphire wafer polished by the presented novel polishing process can be improved by 54.1%. Meanwhile, it is found that the surface of sapphire wafer polished by novel exible polishing has the same surface planarization ability, surface roughness and topographical variation as the conventional exible method, but the MRR will be reduced accordingly. 2. Due to the existence of rigid structural in RCSPP, on the one hand, the overall stiffness of polishing plate is improved and the deformation of the polishing plate in the polishing process is reduced; On the other hand, it enables the exible abrasive materials to obtain the ability of removal selectivity.

Figure 1
Schematics of the manufacturing procedures of conventional exible polishing plate and RCSPP.

Figure 2
Schematic diagram of the rotary-type polishing experimental setup.

Figure 3
Testing points of surface roughness and thickness change.

Figure 4
Geometrical and mesh model of the local area of contact interface between polishing plate and workpiece.

Figure 5
Stress and displacement at the local action area between different polishing plates and sapphire wafer surface.

Figure 6
The surface roughness Ra of sapphire wafer after two kinds of exible polishing. Surface shape accuracy of sapphire wafers processed by conventional exible and RCSPP polishing.