SiC单晶强等离子体超声微细切割机理及参数控制

In the conventional wire saw cutting of SiC monocrystal, the wire contacts the part, leading to possible wafer damage and deformation. This proposal seeks to investigate a non-contact micro-machining method which combines intensive plasma and ultrasonic vibration to process SiC monocrystal. The wire saw and SiC monocrystal form the cathode and anode, respectively. When the electron flux in the machining micro zone becomes close to the maximum abnormal glow zone of gas discharge by adjusting the electric field with the microsecond pulse current width, the intensive negative oxygen ions have a chemical reaction with the active SiC monocrystal surface in the micro zone and produce silicon dioxide and CO. The CO escapes from the micro-machining zone and the silicon dioxide is removed by the electrolyte motion and wire saw ultrasonic vibration, creating excellent surface integrity and high material remove rate of SiC monocrystal. The proposed investigations include three topics. The first topic will formulate how intensive plasma interacts with SiC monocrystal and the relationship between the plasma intensity and process parameters such as pulse current peak, current on and off interval and inter-electrode gap. The second topic concerns the investigation of the removal and transportation behavior of the silicon dioxide by electrolyte motion and wire saw axial motion with transverse ultrasonic vibration. The third topic seeks to model the dynamic inter-electrode gap and process parameters, such as pulse current peak, current on and off interval and wire saw feed rate, and develop the dynamic inter-electrode gap observer to predict the inter-electrode gap distance and control this distance during the process by adjusting the wire feed rate. This proposal provides a new non-contact micro-machining method for hard and brittle materials, and establishes the theoretical basis and control strategy for this class of manufacturing technology such as Electro-Chemical Machining, Electro-Discharge Machining (EDM) and Wire EDM (WEDM).

针对超硬脆SiC单晶在传统的金刚石线锯接触式切割过程中，存在晶片翘曲、变形和易碎等问题，提出一种强等离子体与超声复合的SiC单晶非接触式微细切割方法。其实质是将线锯与SiC单晶分别作为阴阳极，调控脉冲电场强度，使加工微区电子通量向气体双峰曲线异常辉光区最大值逼近，产生高强度的负氧离子与SiC单晶表面的活性微区发生氧化反应生成二氧化硅和CO，CO逸出释放，二氧化硅在电解液和线锯的超声作用下去除，达到SiC单晶良好的表面质量和高的材料去除率。研究强等离子体对SiC单晶的作用机理，探索等离子体强度与电流参数及极间间距的相关性规律；揭示SiC单晶氧化物在电解液、横向超声和轴向运动的线锯作用下的剥离和去除行为；建立面向电参数与机械参数的动态极间间距的数学模型，设计动态极间间距观测器以预估间距。为非良导电类硬脆材料提出一套非接触式微细切割晶片的方法，并为该类制造技术提供系统的理论基础和控制策略。

项目将等离子体放电加工和固结磨粒金刚石线锯相结合进行半导体单晶材料SiC和Si的晶片加工研究。分析了放电过程中极间电场的建立、极间等离子体放电通道的形成。引入阴极场致电子发射理论，提出了单晶Si放电临界参量—电导率的界定方法，并建立了放电系统中电流密度与电导率的物理模型，确定单晶Si的临界电导率和材料的可加工性。.提出了等离子体放电加工的极间阻抗模型，分析了极间未被击穿时极间电容容值的极限值。通过分析，将极间绝缘工作液介质被击穿后，极间等效为纯电阻模型。在此基础上，建立了基于等效电阻的极间间距物理模型，确定等离子体放电通道的等效电阻率，解释了等离子通道的等效电阻率随极间间距增大而幂律下降的原因。采用概率法对极间间距模型进行验证，结果表明，建立的极间间距模型可以准确预测实际的极间间距。.采用系统辨识方法对等离子体放电系统模型进行了阶次辨识和参数预估，建立了极间间距与电机进给位置的二阶差分模型，设计了最小方差自校正控制器。仿真结果表明：最小方差自校正控制器可以使极间间距稳定跟踪各种参考信号（直线、方波、正弦波等）。在脆性半导体材料等离子体放电和固结磨粒金刚石线锯加工的控制台上的实验结果表明：在实际加工控制过程中，极间间距能够稳定可靠的跟踪期望间距，加工效率提高50%。.采用提出的等离子体与固结磨粒金刚石线锯切割相结合的复合方法切割半导体的实验结果表明，复合加工后工件表面粗糙度相对于电火花切割降低了71%，切缝宽度为电火花切割切缝宽度的94%；相比金刚石固结磨粒线锯切割，复合方法能减少工件的表面划痕和线锯丝的磨损，并且能够抑制碳元素在工件表面的残留，碳元素的残留减少26%。同时复合加工方法能够去除电火花切割工件后表面的凹坑和再凝固物质，提高工件表面质量。