Experimental study on characteristic of ultrathin sapphire wafer polishing with Layer Stacked Clamping (LSC) method

doi:10.21203/rs.3.rs-2250836/v1

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Experimental study on characteristic of ultrathin sapphire wafer polishing with Layer Stacked Clamping (LSC) method

https://doi.org/10.21203/rs.3.rs-2250836/v1

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published 19 Jun, 2023

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Ultrathin sapphire wafer is of great significance in the semiconductor field. In order to explore the effective clamping method of ultrathin sapphire wafer in double-side polishing, this paper studied the characteristic of Layer stacked clamping (LSC) method on polishing ultrathin sapphire wafer with double-side polishing machine. A self-made friction force test platform was built for learning the friction force between sapphire wafer and baseplate with different baseplate (stainless steel, cast iron, aluminum alloy) and different baseplate surface roughness (R_a 3.6 nm, 68.2 nm, 210.1 nm, 517.9 nm). Single factor polishing experiments were carried out on baseplate with different flatness (PV value 5.3 µm, 9.8 µm, 19.9 µm, 29.7 µm) and different thicknesses (0.082 mm, 0.104 mm, 0.119 mm). The double-side polishing experiments were carried out to compare the polishing performance on the ultrathin sapphire polishing between LSC method and traditional paraffin bonding method. The results show that the friction force of stainless steel and iron increase under the adsorption of droplets. Stainless steel performs higher friction force and is more suitable for making the baseplate. The inner fringe of limiter was cut off by the edge of the sapphire wafer and a slope was thereby formed. According to polishing results, LSC method has higher processing efficiency per unit time. The surface roughness, flatness and material removal rate are better than paraffin bonding. Finally, a smooth surface with surface roughness (R_a) 1.3 nm and flatness (PV) 0.988 µm was obtained with LSC method.

Layer Stacked Clamping

Sapphire Wafer

Polishing

Double-sides Polishing

From the crystal rod to the substrate, sapphire substrates must undergo various procedures, including slicing, double-sided grinding, rough polishing, and fine polishing [1]. After CMP polishing, the sapphire substrate will be transferred to an external extension for epitaxy. The sapphire substrate has a thickness of approximately 430 µm. Due to sapphire's poor thermal conductivity, an increase in temperature in the LED's active region will have a catastrophic effect on the light output characteristics and service life of the sapphire substrate [2]. To improve the thermal performance of the sapphire material, the sapphire substrate is back-thinned to less than 100 m following the electrode preparation procedure [3, 4]. Sapphire substrate back thinning is accomplished mostly through grinding and lapping procedures, and following thinning, ultra-precise polishing is necessary to obtain a surface of high quality with minimal damage [5].

Double-side lapping and polishing is a common ultra-precision processing technique that has been widely applied to the ultra-precision processing of various flat components, such as sapphire substrates, quartz wafers, silicon wafers [6]. Kasai[7] utilized a two-plane machining strategy to analyze CMP machining of rigid disks. They constructed a model of the trajectory and relative motion velocity of single abrasive grains for double-side polishing, and combined it with a material removal model that optimized the uniformity of material removal from the upper and lower surfaces. Kim [8] employed diamond cemented grinding discs mixed with Al₂O₃ abrasive to machine sapphire using a dual plane machining process. They discovered that the combined action of diamond and Al₂O₃ caused sapphire material removal. Observing the experimental results of sapphire, Wang [9] analyze the relative movement between workpiece and abrasive grit, and establish a mathematical model on the basis of the double-sided planetary grinding machine. Li [10, 11] conducted orthogonal experiments on double-sided CMP of sapphire wafers, studied the effects of different processing parameters on material removal rate, surface roughness, and depth of SSD, and adopted the optimization method of orthogonal experimental results based on weight matrix, and obtained the effect of each factor on The influence degree of the index value of the orthogonal experiment was compared, the processing results of sapphire wafers under different processing methods were compared, and the material removal equation based on experience and theory was established. Wang [12] examined the effects of grinding pressure, grinding wheel speed, and grinding wheel grit on the surface precision and machining efficiency, approximately obtaining 10 m/min in material removal rate.

With the increasing in the LED business, greater demanding has been placed on the quality of sapphire wafers. For ultra-thin sapphire wafer preparation, double-side polishing technique is the chosen polishing method due to its excellent flatness and parallelism, as well as its high polishing efficiency [13]. However, the majority of commercially available double-side polishing equipment is of the planetary wheel type, which suffered a clamping issue such as insufficient strength and rigidity of the planetary wheel cage, which leads to the runaway and fragmentation of wafer and severely reduces processing efficiency [14]. Current methods for clamping sapphire wafers include vacuum adsorption [15], paraffin bonding [16], planetary wheel clamping [17], and wax-free adsorption pad [18]. Vacuum adsorption uses negative pressure formed between the workpiece and the porous ceramic to adsorb the ultra-thin flat workpiece to the fixture. However, the negative pressure will cause local deformation in the ultra-thin workpiece surface, and the surface flatness of the ultra-thin workpiece is unsatisfying after processing [19–21]. The widely used clamping method for thinning substrate is paraffin bonding [22]. However, in order to take down the wafers, wafers holder must be heated for melting wax after processing and then wafers must be cleaned clearly for a long time, which is not conducive to enhance processing efficiency. As the thickness of flat workpieces is typically tiny, the requirements in material of wand wheel are stringent. At present, only the blue steel wand wheel can handle ultra-thin flat workpieces, and the cost was high. The wax-free adsorption pad can cause a partial vacuum with the porous structure within the polyurethane pad [23]. Then, combining with the sealing effect of liquid droplets, the workpiece can be adsorbed on the wax-free pad, but the employed polishing pad was too soft to grip ultra-thin wafer. Therefore, there is still no viable solution for solving clamping problem of the ultra-thin sapphire wafer. In addition, the investigation on processing sapphire wafer is now focused on the process optimization [24–26], the discussion on the clamping issue was rarely carried out. To further improve the polishing efficiency of ultra-thin sapphire wafers, we propose a novel clamping method for ultra-thin wafers based on the layer stacked clamping (LSC) [27]. The adhesion mechanism and the numerical model of adhesion force were studied, and the validity of the adhesion model was verified through experiments.

To further discuss the characteristics of this method, this paper will be presented from the aspects of friction force with different baseplate materials, the effect of baseplate thickness and flatness, and the polishing results with the LSC method and traditional clamping method.

2.1 Materials

c < 0001 > Sapphire substrates were used in the experiments (2 inch×0.17 in thickness mm). As shown in Fig. 1, 5 points on the work surface were selected for valuing the surface roughness Ra (mean value of each point). P1 was in the center, and other points were selected evenly at a circle with a radius of 20 mm. A white light interferometer (Taylor Hobson CCI HD) was employed for measuring surface roughness R_a. Before measuring, substrates were cleaned by an ultrasonic cleaner for 30 min with dehydrated alcohol. After cleaning, dried gas with high pressure was used to dry the substrate. Then, substrates were placed on the workbench of the white light interferometer. The device can automatically scan the surface topography and deal with the surface profile with software based on the ISO 25178 standard.

2.2 Processing method

The processing method is depicted in Figure. 1. The schematic of the layer stacked clamping (LSC) method is illustrated in Figure. 2(a). Two ring-shaped limiters are attached to both sides of the baseplate. An amount of water was dropped onto a surface of the baseplate, and thereby, the water film was formed after the workpiece was placed inside the limiters. Since the two contacted surfaces were not extremely smooth, van der Waals forces and capillary forces [27, 28] between the baseplate and workpiece were obtained, which induced a great normal adhesion force of workpiece to baseplate. Because the workpiece was also radially limited by the limiter, the movement of the workpiece can be totally restrained. After the two pieces of workpieces were mounted onto the baseplate, the "workpiece–baseplate–workpiece" layer stacked clamping (LSC) method was formed. The double-side plate processing method was used to polish the sapphire workpiece employing the LSC method, as shown in Figure. 2(b). The LSC system was held by the planetary holder, which can be driven by the gear system composite by the out gear and sun gear. The lower plate was rotated by a motor. The upper plate was placed onto the upper side of the LSC system, and the polishing pressure was loaded onto the upper plate. Under the relative velocity of the workpieces to the upper plate and lower plate, the material can be removed efficiently.

2.3 Experimental conditions

According to a previous study of double-sided polishing experiments, a better polishing result can be achieved on the workpiece surface when the speed ratio of outer gear and sun gear i_o=0, the speed ratio of sun gear and lower plate i_p=1.7, speed ratio of upper plate is i_u=-1.2, eccentric distance of slot e_s=25 mm and center distance between base plate and slot e_b=0.5 mm was selected. In this case, the revolution speed of the sun gear is 20 rpm. The revolution speeds of the lower plate and upper plate were 34 rpm and − 24 rpm, respectively.

Polishing slurry with pH 12 was used in the experiments, which mainly consisted of deionized water, abrasive particles SiO₂ (average particle size of 80 nm, 5 wt.%) and an acid-base regulator (NaHCO₃). Nanopoly-100 (Figure. 3) A single-sided polisher was used to polish the substrate to obtain the desired initial surface roughness and flatness. A precision balance (accuracy 0.0001 g) was used to weigh the mass, and the material removal rate (MRR) was calculated by using Eq. (1):

MRR = Δm/Sρt (1)

where Δm is the mass loss before and after polishing, S is the workpiece processing area (S = πD²/4, D is the workpiece diameter), ρ is the material density, and t is the processing time.

The experiments were divided into 4 rounds.

In the first round, the performance of the basis plate materials in the clamping workpiece was studied. Before the tests, the base plates made of different materials (aluminum alloy, cast iron, and 304 stainless steels) were polished to achieve identical initial conditions, including surface roughness and flatness. The initial surface roughness and flatness of sapphire were R_a = 5 nm and PV = 3.4 µm, respectively. The initial flatness of the baseplate was PV = 10 µm. The friction forces between the workpiece and several base plates with and without droplets, namely, the dry and wetness conditions, were measured by the device in Figure. 4. The device consisted of a 3-direction force sensor, an X-direction motion platform, and a fixing bolt. The baseplate was linked to the fixing bolt, which was used to restrain the movement of the baseplate during measurement. The workpiece was fastened to a 3-direction force sensor that was mounted to the X-direction motion platform, as shown in Figure. 4(a). The test procedure was as follows: After dropping a droplet (DI water) onto the sapphire surface, the baseplate was placed over the droplet, and then the LSC was obtained. A weight was used to produce a normal pressure on the workpiece, and then the workpiece and force sensor moved with a constant velocity (0.1 mm/s) in the X-direction. According to the measuring principle, the device was constructed as displayed in Figure. 4(b).

The effect of limiter thickness, ranging from 0.08 mm to 0.14 mm with a 0.02 mm increment, on the polishing performance was learned in the second round. The impact of the base plate flatness on the polishing results was investigated in the third round. Finally, a comparison experiment of the LCS method to the traditional holding method (Paraffin adhesion) was conducted.

3.1 Friction force of different baseplates

Table 1

Experimental parameters
Parameters	Values
External pressure (N)	0.8, 2.8, 4.8, 6.8, 8.8
Contact status	Dry, Wetness (one droplet)
Substrate material	Aluminum alloy, Cast iron, Stainless steel
Surface roughness (nm)	Stainless steel	3.6, 68.2, 210.1, 517.9
	Aluminum alloy	60.2
	Cast iron	63.8

The experimental conditions are displayed in Table. 1. The self-weight of the base plate was 0.8 N, and the extra pressure ranges from 2 N to 8 N in 2 N increments, resulting in total external pressure changes from 2.8 N to 8.8 N. The friction forces of different materials under dry and wet conditions are shown in the figures. 5 and 6. From Figures. 5 (a) and (b), it can be seen that the largest friction force was found between stainless steel and sapphire under dry conditions, and the smallest friction force was observed with the aluminum alloy. This is because the shear strength of stainless steel and cast iron is much greater than that of aluminum alloy, and the shear force is thereby much greater than that of aluminum alloy when tangential displacement occurs. Furthermore, the shear strength of stainless steel was also higher than that of cast iron. Hence, stainless steel performs better in the experiments. From the viewpoint of adhesion theory, the adhesion force was formed between two surfaces due to the contact and deformation of the micro prominent body. The adhesion force is related to the shear strength of the material. By comparing Figs. 5 and 6, it can be seen that the friction force of stainless steel and cast iron was increased under wetness conditions compared with dry conditions, especially at 2.8, 4.8, and 6.8 N external load. This is because the wetness condition increases the adsorption force between the two contact surfaces, and at the same time, the yield limit was also enhanced by the micro bulges, which were lubricated by water during the lateral sliding process, resulting in the enhancement in adhesion force. In this case, the friction force, which was highly determined by the adsorption force and adhesion force, was highly increased.

Figures 7 and 8 show the friction force between the stainless steel and workpiece under dry and wetness conditions. The initial surface roughness ranges from 3.6 nm to 517.9 nm. According to the figures, at R_a = 3.6 nm, the friction force is lowest because a smoother surface cannot form an effective adhesion and furrow effect, which results in a decrease in the friction coefficient between the two surfaces. Under wetness conditions, the friction force increases when the surface roughness R_a increases from 3.6 nm to 210.1 nm and then decreases when the surface roughness R_a reaches 517.9 nm. This is because the additional adhesion force caused by water was offsite to the water lubrication effect in a smoother state, which caused a relatively small alteration in the friction coefficient. For the worst surface roughness, water lubrication contributed mostly to the friction force, and the adhesion force caused by water was lower, which led to a lower friction force. A large friction force should be considered for the baseplate material. According to the results, stainless steel with a surface roughness R_a = 68.2 nm was suitable.

3.2 Limiter thickness

Stainless steel and glass fiber plates are chosen as the baseplate and limiter material. The processing thickness of ultrathin sapphire is significantly influenced by the thickness of the limiter. Hence, the effect of the limiter thickness on the polishing characteristic is further studied. To learn the available thickness of the limiter, extra pressure was applied to the limiter with different thicknesses. The maximum withstanding load at the given thicknesses was investigated. The limiter thickness was gradually reduced in each experiment, as shown in Table 2. Nanopoli-100 was used in this experiment. The sapphire (2 inch) thickness and flatness were 0.17 mm and 3.76 µm, respectively. The initial flatness and weight of the baseplate were PV = 10 µm and 2.1 kg. Each test time was 1 hour. The flow rate of the polishing fluid was 25 ml/min. After polishing, the limiter may be broken due to the overlarge squeezing load induced by the workpiece.

Table 2

Experimental parameters
Parameters	Values
Limiter thickness (mm)	0.082, 0.104, 0.119
External pressure	0.7–10.5 Kg (0.7 increment)

The experimental results of extreme withstanding pressures with different thicknesses are shown in Fig. 9. The figure shows that the reliability of the limiter increases as the thickness increases. The typical failure type of the limiter is depicted in Figure. 10. A slope was observed at the fringe area in the inner ring of the limiter, which was destroyed by the edge of sapphire. When the friction force applied on the workpiece drives the workpiece to change state from the clamping state to the upwarping state and then to squeeze the fringe area, the interference area between the limiter and the sapphire occurs. Once the external shear strength of the limiter increased sharply and exceeded the extreme withstanding capacity of the limiter material, the limiter was broken instantly. In this case, the workpiece fails to be limited any longer.

3.3 Effect of baseplate flatness on sapphire processing

The flatness (PV and RMS value) of the sapphire wafer with different baseplate flatness values during 3 hours of processing is shown in Figure. 11. The limiter thickness was 0.104 mm. It can be observed from the graph that the PV and RMS values of the sapphire wafer grow as the baseplate flatness increases, but the increasing trend was far slower than that of the baseplate. When the workpiece is absorbed by the baseplate, the surface contour of the baseplate is replicated in the workpiece surface. Simultaneously, the workpiece rotates within the fixture, which further promotes the workpiece to replicate the baseplate surface. A typical example is shown in Figure. 11(e). Initially, an extrusion structure was obtained in the middle of the baseplate. During polishing, the interaction force between the extrusion structure and the polishing pad is increased, resulting in a higher material removal rate than other areas. After polishing, a convex shape in the middle portion of the workpiece was generated. To assure processing quality, the baseplate flatness must be no less than the expected workpiece flatness.

3.4 Polishing comparison

Based on the above experimental results, a double-side polishing machine with a layer stacked clamping method was constructed, as shown in Figure. 12. The comparison between the LSC method and traditional paraffin holding method was conducted to further demonstrate the reliability of the laminated clamping method. In this experiment, twelve pieces of sapphire wafers were separated into two groups and attached to the baseplate with laminated clamping and paraffin adsorption. During clamping, a 50 µl droplet was placed on the baseplate surface to form a liquid film for the absorbing workpiece. The weight was then placed on the surface of the workpiece and allowed to rest for two minutes to allow the droplet to spread and form a liquid film. In the traditional holding method with paraffin, the baseplate was heated to melt paraffin on the baseplate surface, and then the workpiece was placed upon the melted paraffin. After that, the paraffin was cooled until it turned solid again. When polishing was complete, the baseplate was heated again to melt the paraffin to remove the workpiece. The baseplate height was adjusted so that the initial height of the two groups of experiments was kept constant. The limiter thickness was 0.102 mm, and the baseplate flatness was 1.085 µm. A workpiece (α-Al₂O₃ C type) 2 inches in diameter and 0.17 mm in thickness was used in the experiments.

The surface roughness variation with polishing time is displayed in Figure. 13. The final surface morphology with LSC and the paraffin holding method is shown in Figure. 14. The variation trend was close to each other, but the surface roughness values with LSC were superior to those with paraffin bonding. Moreover, the final uniformity of surface roughness was CV = 0.1148 with paraffin bonding and CV = 0.0329 with LSC. Figure. 15 shows the variation in the material removal rate with polishing time under the two clamping methods. It is obvious that a higher material removal rate was obtained with the LSC method. The total view of the workpiece after polishing under various clamping methods is shown in Figure. 16. The final flatness was PV = 0.988 µm and PV = 1.159 µm. Thus, the LSC was still superior to the paraffin holding method by considering MRR and flatness. Additionally, the total time for bonding and wafer pickup exceeded 15 minutes with the traditional holding method, and the clean step for removing paraffin cost approximately 10 minutes; the total time was approximately 25 minutes. However, by the LSC method, the clamping operation can be accomplished within 10 seconds, and the clean time was almost 5 minutes. Therefore, the LSC method has higher efficiency in practice.

Layer stacked clamping (LSC) method was proposed to improve the polishing efficiency on ultrathin sapphire wafer by providing a water film between workpiece and baseplate. In order to explore the effective clamping method of ultrathin sapphire wafer in double-side polishing, this paper studied the characteristic of Layer stacked clamping (LSC) method on polishing ultrathin sapphire wafer.

1) Three materials were selected for making baseplate in friction force test. The friction force of stainless steel is the highest, followed by cast iron, and aluminum alloy is the lowest. Stainless steel material is more suitable for making the baseplate. Under the adsorption of droplets, the friction force of stainless steel and cast-iron increased, indicating that water film adsorption has a positive effect on the friction force under certain conditions.

2) The failure of the limiter is related to its thickness. A slope cut by the sapphire edge was found on the inner fringe of baseplate, which results in the failure of the limiter. The flatness of baseplate can not only copy its original error to workpiece surface but also induce higher form error on the workpiece surface.

3) Through the comparison polishing experiment between the paraffin bonding and the LSC method, it was found that the difference between the surface roughness and flatness with the LSC method is better than the paraffin bonding method. Furthermore, the coefficient of variation of surface roughness obtained by the LSC method is better than that obtained by the paraffin bonding method. Compared with the paraffin bonding method, the LSC method has better processing efficiency per unit time for realizing the high-precision double-side ultrathin sapphire polishing.

Acknowledgments This research was financially supported by Basic Research Project of Wenzhou City, China (grant No. G20210002 and G20210001), Natural Science Foundation of Zhejiang Province, China (grant No. LQ22E050008), National Natural Science Foundation of China (Grant No. U20A20293, 52275467, 51905485). A special thanks goes to Ultra-precision Machining Centre of Zhejiang University of Technology.

Authors’ contributions The corresponding author Ming Feng has guided the paper writing and contributed to data discussion and article revision. Zhixiang Chen was responsible for writing, developing the experimental designs and measurements, taking part in ensuring the experimental environment and preparing workpieces and analyzing experimental results. Shunkai Han and Hongyu Chen assisted in data processing and paper revision. Xianglei Zhang was responsible for improving the test jigs.

Funding Basic Research Project of Wenzhou City, China (grant No. G20210002 and G20210001), Natural Science Foundation of Zhejiang Province, China (grant No. LQ22E050008), National Natural Science Foundation of China (Grant No. U20A20293, 52275467, 51905485)

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Conflict of interest The authors declare that they have no conflict of interest.

Ethical approval Not applicable

Consent to participate Not applicable

Consent to publish Not applicable

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Experimental study on characteristic of ultrathin sapphire wafer polishing with Layer Stacked Clamping (LSC) method

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Abstract

Figures

1. Introduction

2. Materials And Methods

2.1 Materials

2.2 Processing method

2.3 Experimental conditions

3. Results And Discussion

3.1 Friction force of different baseplates

3.2 Limiter thickness

3.3 Effect of baseplate flatness on sapphire processing

3.4 Polishing comparison

4. Conclusions

Declarations

References

Status:

Journal Publication

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