Etching Characteristics of Quartz Crystal Wafers Using Argon-Based Atmospheric Pressure CF4 Plasma Stabilized by Ethanol Addition
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Atmospheric-pressure plasma chemical vaporization machining (AP-PCVM) using helium as a carrier gas to achieve uniform thickness of quartz crystal wafers has been practically applied to the mass production of high-performance crystal units. However, because of the high cost and limited supply of helium, the proposed use of argon instead of helium has garnered interest. Argon plasma at atmospheric pressure tends to become unstable, but it has been reported that the addition of a small amount of ethanol makes it possible to generate a stable and uniformly spread glow discharge atmospheric-pressure argon plasma. In this study, an AP-PCVM experiment was conducted on quartz crystal wafers using an argon-ethanol mixture gas as the carrier gas and CF4 and O2 as process gases. A uniform, stable glow discharge plasma was generated by applying the ethanol-containing process gas, and satisfactory removal characteristics for correcting the thickness deviation of a quartz crystal wafer were obtained.
KeywordsAtmospheric-pressure plasma chemical vaporization machining (AP-PCVM) Quartz crystal wafer Ethanol-added argon-based CF4 plasma Glow discharge plasma
Quartz crystal units have good frequency–temperature characteristics at around room temperature. In addition, the crystal units are very stable both physically and chemically, so the change in frequency with aging is small. Because of these excellent properties, quartz crystal units have been widely used in communication devices such as mobile phones and in consumer devices such as digital cameras and computers in order to provide accurate reference signals [1, 2, 3]. To increase the communication speed, the resonance frequency of the quartz crystal unit should be increased by reducing the thickness of the quartz crystal wafer. In the recent commercial production of quartz resonators, a wafer process was intensively developed to improve the productivity of resonators. To achieve improved productivity by reducing the processing time for frequency adjustment, a uniform thickness is essential for the quartz crystal wafer. However, commercially available quartz crystal wafers, which are formed by conventional mechanical fabrication processes such as cutting with a wire saw, lapping, ion beam figuring, and polishing, typically have a thickness deviation of ± 0.1%. This value is about 100-fold larger than the tolerance required for a commercial product. Furthermore, owing to the poor parallelism and the existence of subsurface damage, many spurious peaks, which deteriorate the resonance characteristics, are observed in a resonance curve . As a highly efficient and damage-free thickness correction technique, atmospheric-pressure plasma chemical vaporization machining (AP-PCVM) has been applied to improve the thickness uniformity of quartz crystal wafers .
AP-PCVM is an ultraprecise figuring technique that uses fluorine radicals generated by atmospheric-pressure plasma to change the surface atoms of a substrate into volatile reaction products to form the desired shape. Since AP-PCVM is a noncontact chemical figuring technique that does not apply a mechanical load to substrates, the breakage of thin brittle materials is prevented and no subsurface damage (SSD) layer is formed during the chemical removal process [6, 7]. Moreover, as AP-PCVM is an atmospheric-pressure process, no vacuum chamber is required. Until now, the most widely used methods for plasma generation at atmospheric pressure have mainly included inductively coupled plasma (ICP) [8, 9], capacitively coupled plasma (CCP) , and microwave plasma (plasma jet) [6, 10]. Since ICP and microwave plasma produce high temperatures exceeding 200 °C, the possibility of both crystal twinning and breaking of the quartz crystal wafer increases. Therefore, CCP is used in AP-PCVM to process the crystal wafers in this study. In our previous research, the thickness uniformity of a commercially available quartz crystal wafer was decreased from 250 to 50 nm by a single correction process without the formation of SSD . KYOCERA has developed the world’s smallest crystal unit (1.0 × 0.8 mm) for smartphones, wearables, and other electronic devices by AP-PCVM. However, helium gas has been used as the carrier gas in AP-PCVM until now. The helium gas used in industry is mostly produced from natural gas. Problems such as the depletion of natural resources and high cost are of wide concern. To solve these problems, researchers have proposed the use of argon gas instead of helium gas as the carrier gas in the generation of atmospheric-pressure plasma, as argon gas can be industrially produced by the fractional distillation of liquid air. Although this will reduce operational cost, the breakdown voltage for argon is much higher than that for helium . As is well known, the higher breakdown voltage may cause the rapid multiplication of electrons after breakdown, leading to the formation of filamentary arc streamers . As reported by Sun et al. , the addition of ethanol to argon has been proven to be very useful for generating an atmospheric-pressure glow discharge plasma. In our study, we used argon with a small amount of ethanol instead of helium as the carrier gas in AP-PCVM. Experiments were conducted to investigate the etching characteristics of quartz crystal wafers by AP-PCVM using ethanol-added argon-based atmospheric-pressure CF4 plasma.
2 Experimental Setup
3 Results and Discussion
3.1 Role of Oxygen in AP-PCVM
3.2 Comparison Between He and Ar-Based Plasmas
3.3 Relationship Between Scan Speed and Removal Volume
As the high breakdown voltage for argon easily leads to the formation of filamentary arc streamers, which can break a quartz crystal wafer, argon has not until now been used as carrier gas in AP-PCVM to correct the thickness distribution of a quartz crystal wafer. In this paper, the etching characteristics of a quartz crystal wafer obtained by AP-PCVM using ethanol-added argon-based atmospheric-pressure CF4 plasma were investigated. The addition of a small fraction of ethanol to argon can generate atmospheric-pressure plasma with a stable glow discharge state instead of arc streamers. To avoid carbon deposition on quartz crystal wafers during etching, O2 was added to the process gas. An O2 flow rate of 4 sccm is considered suitable because a high etching rate can be obtained without carbon deposition. Using argon instead of helium as the carrier gas in AP-PCVM will effectively solve problems such as the depletion of natural resources and high cost.
This study was supported by JST A-STEP Grant Number JPMJTS1623, Japan. This study was also supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number JP19J20167.
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