Chemical-mechanically polishing large areafree-standing CVD diamond films

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(1. Hebei Plasma Diamond Technology Co., Ltd., Shijiazhuang 050000, China) (2. Hebei Institute of Laser, Shijiazhuang 050000, China)

Abstract Chemical mechanical polishing is used to polish large area free-standing CVD diamond films with diameters of 66 mm. The influence of polishing plates (iron plate, asphalt plate and soft cushion) and oxidizing agents (K2S2O8 and K2FeO4) on polishing results are investigated. Profilometer (tip radius 5 μm), optical microscope and Raman spectroscopy are used to evaluate the polishing effects of the CVD diamond films. Results show that the material removal rate of iron plate is the highest, while the surface polished by soft cushion is the most uniform with roughness of 2 nm. It is also found that the most effective oxidizing agent is K2FeO4. In conclusion, the optimum polishing conditions are soft cushion and K2FeO4.

Key words polishing plate; oxidizing agent; material removal rate; surface roughness

With the outstanding mechanical strength, thermal conductivity, chemical stability and optical transmittance, CVD diamond film has attracted more and more attention in recent years[1]. However, the unpolished CVD diamond films can not meet the industrial requirement due to a large quantity of defects, especially in the optical fields. Therefore, many polishing methods have been proposed, such as UV laser polishing, mechanical polishing, tribo-chemical polishing, thermochemical polishing, and chemical-assisted mechanical polishing[2]. The films with a diameter of 66 mm usually can reach nm-level surface roughness by conventional mechanical polishing in some suitable conditions. But it usually results in hot deformation and crack propagation of films to some extent. And the other polishing methods either need some special or costly apparatus or require high temperatures[3-8]. It is important to polish at normal conditions and room temperature while achieving ultraprecise machining of diamond with low scathing results. The chemical mechanical polishing will be the most potential polishing technique.

Up to now, there are limited investigations about chemical-mechanically polishing CVD diamond films. THORNTON, MALSHE and WANG et al. used molten salts such as potassium nitrate and sodium nitrate or their mixtures to polish CVD diamond films[9-11]. The material removal rate is very high but it also has some disadvantages such as high polishing temperature, serious corrosion, more contaminants on the surface and poor surface roughness. CHENG et al. polished CVD diamond films by chemical-assisted mechanical method with six different slurries[12]. The optimum polishing conditions is using K2S2O8for coarse polishing followed by KMnO4for fine polishing. The lowest roughness of 10 nm was obtained after 5 hours. But the film used in their experiment is just 20 μm in thickness and the polishing area (225 mm2) is small. YUAN et al. put forward a kind of locally heated chemical mechanical polishing method[13-14], through which the roughness of a small area (100 mm2) free-standing CVD diamond film could reach 0.187 nm after 8 hours polishing.

Although these experimental results are corking, they still need more efforts to apply the technique to actual industrial production. Firstly, the sizes of industrially produced films are usually tens or hundreds of times larger than those of experimental samples. It will lead to a great difficulty in forcing the films with equal load. Furthermore, the produced films are usually too fragile to carry tremendous weight. Secondly, using hard polishing plates could not get a well-distributed surface but only locally good roughness.

In this paper, an experiment is designed based on large area free-standing CVD diamond films whose diameters are 66 mm. Three polishing plates, namely iron plate, asphalt plate and soft cushion, are selected and two frequently-used strong oxidants, namely potassium persulfate (K2S2O8) and potassium ferrate (K2FeO4) are chosen. A proper amount of dilute sulfuric acid is added to the two polishing slurries to promote oxidation reaction. The optimum polishing condition is obtained experimentally.

1 Experimental procedure

The diameter of CVD diamond films used in the experiment is 66 mm and the thickness is about 600 μm. Before polishing, every sample is pretreated with coarse grinding and fine grinding so that their surface roughnesses are all below 100 nm.

Three polishing plates, namely iron plate, asphalt plate and soft cushion are compared to check which works best. During the process, keep the same rotational speed 90 r/min and same load 5.5 kg. The saturated K2FeO4solution and diamond powder (maximum size of 0.2 μm) mixed with dilute sulfuric acid are separately dripped into the polishing plates, because K2FeO4is easily decomposed into Fe(OH)3under acidic conditions. The roughness is measured every 3 hours until it does not change five times in a row. For each diamond film, measure four points along its diameter and the minimum one is taken as the roughness. Take the time when the roughness is no longer changing as polishing time.

Two frequently-used strong oxidizing agents, namely K2S2O8and K2FeO4, are chosen to get the optimum polishing slurry. Both of them are dripped into the polishing process separately from the diamond powder (maximum size of 0.2 μm) mixed with dilute sulfuric acid. Every agent is prepared on saturated liquid state at room temperature. The roughness is measured every 3 hours until it does not change five times in a row. Also, measure four points along the sample diameter, but the average one is taken as the roughness value. After polishing time for 45 hours, the roughness of the two polished surface are compared by evaluating their effect using profilometer (tip radius 5 μm), optical microscope and Raman spectroscopy.

2 Results and discussion

Firstly, material removal rates of iron plate, asphalt plate and soft cushion are obtained, which are 1.428 μm/h, 0.674 μm/h and 0.074 μm/h respectively. The surface roughness before and after polishing of different polishing plates at rotational speed 90 r/min and same load 5.5 kg are shown in Fig. 1. Surface roughness varying with polishing time of different polishing plates are shown in Fig. 2.

Fig. 1 Surface roughness varying with different polishing plates

It can be found that the roughness of films polished by iron plate is almost invariant after 24 hours, which means that it needs 24 hours to achieve the minimum roughness using iron plate. And they are 42 hours and 45 hours respectively for asphalt plate and soft cushion to reach minimum roughness. The material removal rates of iron plate and asphalt plate are both very high, but only a very small area can achieve the minimum roughness, which means that the polishing results are not uniform. The removal rate of soft cushion is relatively low, but the surface roughness can reach 2 nm and the polished surface is more uniform. This is mainly because, soft cushion have closer contact with the diamond surface and better water absorption.

Fig. 2 Surface roughness varying with polishing time of different polishing plates

It is concluded that soft cushion gets the well-distributed surface. Therefore, it is chosen as the polishing plate in the following experiment.

Surface roughness varying with polishing time of different oxidants are shown in Fig. 3. It is indicated that the surface roughness of CVD diamond films decreases rapidly and then slowly during polishing process.

Fig. 3 Surface roughness varying with polishing time of different oxidants

Fig. 4 shows the optical micrographs of diamond films. Fig. 4a is the micrograph of the as-grown polycrystalline diamond film, which is very rough and has many surface bulges. Because the gap between bulges and pits is too big, the images of bulges are not clear. The surface becomes smooth and surface bulges disappear after finely grinding as shown in Fig. 4b, whose surface roughness decreases to below 100 nm. Fig. 4c and Fig. 4d are the micrographs of the CVD diamond films polished by K2FeO4and K2S2O8respectively. The polished surfaces become smooth with little pits, though the existence of pits between crystals is an inherent phenomenon in the CVD polycrystalline diamond films. Comparing the two polished surfaces, the one in Fig. 4c is smoother than that in Fig. 4d. And surface roughness measured reaches 2 nm using oxidant K2FeO4after 45 hours, but 15 nm using oxidant K2S2O8after the same polishing time. It is concluded that K2FeO4has stronger oxidizing ability than K2S2O8does and that diamond surface polished by K2FeO4is better than that by K2S2O8at the same polishing time.

(a) As-grown diamond film(b) Finely ground surface(c) Film polished by K2FeO4(d) Films polished by K2S2O8Fig. 4 Optical micrographs of diamond films

The photo of film polished by K2FeO4is shown in Fig. 5, whose surface is very smooth like a mirror, and the reflections of a coin and ceilings can be easily seen.

Fig. 5 Photo of film polished by K2FeO4

Raman spectroscopy can distinguish different forms of carbon. As shown in the Fig. 6, CVD diamond films are characterized by Raman spectra. Each of these curves has a strong Raman peak at 1332 cm-1, and there is no obvious strong peak anywhere else. It indicates that the diamond films prepared by the DC arc plasma jet have high quality, and a large number of non-diamond phases have not been found after oxidation.

Fig. 6 Raman spectra of CVD diamond films at different states

As for the broad peak around 1420 cm-1appeared in Raman spectra, ZHU et al. have proved that it is not an intrinsic Raman peak[15]. It will vanish whether increasing or decreasing the excitation wavelength. The peak value of untreated diamond film is the minimum and the film after grinding is the second. The highest two curves are almost same. The peak value of the untreated samples is very small, it means that there are a lot of coarse and cluttered grains on the surface of diamond film produced by the DC arc plasma jet. The experimental results show that chemical mechanical polishing does not introduce more non-diamond impurities into diamond films.

3 Conclusions

The experimental results show that the large area free-standing CVD diamond film polished by soft cushion is the most uniform and the surface roughness can reach 2 nm. And the most effective oxidizing agent is K2FeO4.