单晶片材料去除和表面生成的研究

Abstract: A new method of ultrasonic chemical mechanical polishing (CMP) combined withultrasonic lapping is introduced to improve the machining performance of carbide silicon (SiC)To fulfill the method, an ultrasonic assisted machining apparatus is designed and manufacturedomparative experiments with and without ultrasonic assisted vibration are conducted. Accordingto the experimental results, the material removal rate (MRR) and surface generation are investigatedThe results show that both ultrasonic lapping and ultrasonic CMP can decrease the two-body abrasionand reduce the peak-to-valley (PV) value of surface roughness, the effect of ultrasonic in lappingcan contribute to the higher MRR and better surface quality for the following CMP The ultrasonicassisted vibration in CMP can promote the chemical reaction, increase the MRR and improve thsurface quality. The combined ultrasonic CMP with ultrasonic lapping achieved the highest MRR of1.057 um/h and lowest PV value of 0.474 um. Therefore this sequent ultrasonic assisted processingmethod can be used to improve the material removal rate and surface roughness for the single crystalSiC wafer.

1.Introduction

Being one of the third-generation (wide bandgap) semiconductor materials, carbide silicon(SiC) has attracted much attention both in academic research and industrial application, because ithas remarkable physical and chemical properties such as high thermal conductivity, high hardnesschemical resistance, temperature resistance, transparency to light wave, etc. (1]. Thus, SiC has becomeone of the most promising materials in optoelectronics and power electronics, especially when thedevice is used in a high temperature, high frequency, high power, and radiation resistant environmentHowever, its mechanical and chemical properties make the surface planarization much more difficultthan silicon and sapphire, which are extensively used as substrate materials, as its Mohs hardnessis close to diamond and the chemical inertness is too strong to react in all known aqueous etchingsolutions at room temperature except in hot Hydrofluoric Acid (HF) solution or phosphoric acidabove 200 °C (2]. It is well known that chemical mechanical polishing (CMP) is still the most efficienttechnology to finish the hard-brittle wafer, which can achieve the ultra-smooth surface with minimadamage (3]. In addition, CMP of SiC substrate using colloidal silica slurry was first reported in1997 by Zhou et al. (4 . This research reported that high temperature and a pH higher than 10 were indispensable conditions to remove material. But the material removal rate (MRR) was still very low, typically less than 100 nm/h.

On considering improving the material removal rate (MRR) of SiC during CMP, the reportediterature mainly focuses on CMP slurry (5-101. On the one hand, CARE (catalyst-referred etching) (11using different oxidants and catalysts was studied to replace the high temperature. Among the oxidantshydrogen peroxide (H,O) was the most widely used to increase the chemical corroding of SiC (79In addition, Fe nanoparticles or hydroxylion was the most widely used catalyst for H, 0, (8,101, but theMRR was still not large. For example, the MRR was 60 nm/h when only HO, was added into theslurry. The MRR was 100 nm /h when both HO, and hydroxyl ion were added into the slurry, and itwas not more than 200 nm/h when the slurry contained HO, and Fe nanoparticles. On the othernand, the addition of a high-hardness abrasive combined with an oxidizer to the slurry is anothelmethod to increase MRR.Hevdemann et al, [51 increased the MRR to about 900 nm /h by addition of anano-diamond and HO,. Jeong et al. (6] increased the MRR to about 500 nm/h also by addition of anano-diamond, But the cost of a nano-diamond abrasive is expensive. and the diamond particle easiloroduces mechanical scratches. From the view of processing, ECMP (electro-chemical mechanicapolishing) [12], PAP (plasma assisted polishing) [13), and PCMP (photocatalysis-assisted CMP) (14have been reported to increase the MRR of single-crystal SiC. Electro-chemical mechanical polishing issuitable for the conductor and needs an electrical system with a cathode in the slurry. Plasma assistedpolishing needs high temperatures, large amounts of power, and complicated equipment, while theMRR is still not large. Photocatalysis-assisted CMP needs a photocatalyst, ultraviolet light, an electroncapturer, and an acid environment, and the MRR is still not large enough.

In contrast, ultrasonic assisted CMP has been reported to be a cost-efficient method to improvethe processing performance. For example, Tsai et al. (15] combined the ultrasonic CMP with ultrasonicdressing of a diamond disk to increase the MRR for copper substrate with an improved surfaceroughness and the torque force was dramatically reduced. The research provided a feasible method toimprove the polishing performance and dressing efficiency simultaneously. Lu et al. (16,17] increasedthe MRR, lowered the surface roughness, and improved the surface flatness for a sapphire substrateby virtue of ultrasonic flexural vibration assisted CMP with a self-designed flexural vibrating plateLi et al. (18,19] made experimental studies on ultrasonic and megasonic vibration assisted CMP forsilicon wafers and the results implied a better effect than CMP without ultrasonic. Liu et al. (201 alsohad a study on the elliptical vibration-aided CMP of monocrystalline silicon, where a mathematicamodel of MRR and the experimental verification were performed. And a good match between themathematical model and experimental results was achieved.

However, ultrasonic assisted CMP of single crystal SiC has been hardly reported. Additionallyultrasonic assisted abrasive machining can decrease the machining forces thereby reducing surfacedamage (21-23]. Thus, this paper introduces the ultrasonic CMP combined with ultrasonic lappingto process the single crystal SiC wafer based on a self-devised and manufactured ultrasonic unitIn addition, the specimens are the Si-face SiC, which are considered to be more useful for epitaxiafilm growth, but more difficult to be removed compared with C-face (9. A series of combinationalexperiments with and without ultrasonic were conducted to investigate the influence of ultrasonicon the MRR and surface generation of SiC, where the surface roughness may not reach the levereported in the references. Firstly, the effect of ultrasonic on the chemical corrosion of Si-face SiCduring CMP is investigated; secondly, the effect of ultrasonic on the CMP performance of Si-face SiC isinvestigated; finally, the surface status is observed by the optical microscope and AFM, the chemicacorroding is measured by XPS, and the MRR is calculated according to the mass change measured belectronic balance.