What are balanced magnetron sputtering and unbalanced magnetron sputtering?什么是平衡磁控溅射和非平衡磁控溅射？

什么是平衡磁控溅射和非平衡磁控溅射？

    平衡磁控溅射即传统的磁控溅射，是在阴极靶材背后放置芯部与外环磁场强度相等或相近的永磁体或电磁线圈，在靶材表面形成与电场方向垂直的磁场。沉积室充入一定量的工作气体，通常为Ar，在高压作用下Ar 原了电离成为Ar+离子和电子，产生辉光放电，Ar+ 离子经电场加速轰击靶材，溅射出靶材原子、离子和二次电子等。

    电子在相互垂直的电磁场的作用下，以摆线方式运动，被束缚在靶材表面，延长了其在等离子体中的运动轨迹，增加其参与气体分子碰撞和电离的过程，电离出更多的离子，提高了气体的离化率，在较低的气体压力下也可维持放电，因而磁控溅射既降低溅射过程中的气体压力，也同时提高了溅射的效率和沉积速率。

    但平衡磁控溅射也有不足之处，例如：由于磁场作用，辉光放电产生的电子和溅射出的二次电子被平行磁场紧紧地约束在靶面附近，等离子体区被强烈地束缚在靶面大约60 mm 的区域，随着离开靶面距离的增大，等离子浓度迅速降低，这时只能把工件安放在磁控靶表面50～100 mm的范围内，以增强离子轰击的效果。这样短的有效镀膜区限制了待镀工件的几何尺寸，不适于较大的工件或装炉量，制约了磁控溅射技术的应用。且在平衡磁控溅射时，飞出的靶材粒子能量较低，膜基结合强度较差，低能量的沉积原子在基体表面迁移率低，易生成多孔粗糙的柱状结构薄膜。提高被镀工件的温度固然可以改善膜层的结构和性能，但是在很多的情况下，工件材料本身不能承受所需的高温。

    非平衡磁控溅射的出现部分克服了以上缺点，将阴极靶面的等离子体引到溅射靶前200～300 mm 的范围内，使基体沉浸在等离子体中，如图所示。这样，一方面，溅射出来的原子和粒子沉积在基体表面形成薄膜，另一方面，等离子体以一定的能量轰击基体，起到离子束辅助沉积的作用，大大的改善了膜层的质量，非平衡磁控溅射系统有两种结构，一种是其芯部磁场强度比外环高，磁力线没有闭合，被引向真空室壁，基体表面的等离子体密度低，因此该方式很少被采用。另一种是外环磁场强度高于芯部磁场强度，磁力线没有完全形成闭合回路，部分外环的磁力线延伸到基体表面，使得部分二次电子能够沿着磁力线逃逸出靶材表面区域，同时再与中性粒子发生碰撞电离，等离子体不再被完全限制在靶材表面区域，而是能够到达基体表面，进一步增加镀膜区域的离子浓度，使衬底离子流密度提高，通常可达5 mA/cm2 以上。这样溅射源同时又是轰击基体表面的离子源，基体离子束流密度与靶材电流密度成正比，靶材电流密度提高，沉积速率提高，同时基体离子束流密度提高，对沉积膜层表面起到一定的轰击作用。

    非平衡磁控溅射离子轰击在镀膜前可以起到清洗工件的氧化层和其他杂质，活化工件表面的作用，同时在工件表面上形成伪扩散层，有助于提高膜层与工件表面之间的结合力。在镀膜过程中，载能的带电粒子轰击作用可达到膜层的改性目的。比如，离子轰击倾向于从膜层上剥离结合较松散的和凸出部位的粒子，切断膜层结晶态或凝聚态的优势生长，从而生更致密，结合力更强，更均匀的膜层，并可以较低的温度下镀出性能优良的镀层。

    非平衡磁控溅射技术的运用，使平衡磁控溅射遇到的沉积致密、成分复杂薄膜的问题得以解决，然而单独的非平衡磁控靶在复杂基体上较难沉积出均匀的薄膜，而且在电子飞向基体的过程中，随着磁场强度的减弱，一部分电子吸附到真空室壁上，导致电子和离子的浓度下降。对此研究人员开发出多靶非平衡磁控溅射系统，以弥补单靶非平衡磁控溅射的不足。多靶非平衡磁控溅射系统根据磁场的分布方式可以分为相邻磁极相反的闭合磁场非平衡磁控溅射和相邻磁极相同的镜像磁场非平衡磁控溅射，如图为双靶闭合磁场和双靶镜像磁场。

    比较闭合磁场非平衡靶对和镜像靶对的磁场分布情况，可以看出在靶材表面附近磁场差别不大，内外磁极之间横向磁场对电子的约束形成一个电离度很高的等离子体阴极区，在此区域内的正离子对靶面的强烈溅射刻蚀，溅射出大量靶材粒子飞向基体表面。在内部和外环磁极的位置，特别是较强的外环磁极处，以纵向磁场为主，成为二次电子逃离靶面的主要通道，进而成为向镀膜区域输送带电粒子的主要通道。再比较闭合磁场和镜像磁场在镀膜区域内磁场分布，差别就大了，对于镜像靶对，由于两个靶磁场的相互排斥，纵向磁场都被迫向镀膜区外（真空室壁）弯曲，电子被引导到真空室壁上流失，总体上降低了电子进而离子的数量。由于镜像磁场方式不能有效地束缚电子，因而等离子体的溅射效率未有得到提高。而闭合磁场非平衡靶对在镀膜区域的纵向磁场是闭合的。只要磁场强度足够，电子就只能在镀膜区域和两个靶之间运动，避免了电子的损失，从而增加了镀膜区域的离子浓度，大幅度提高了溅射效率。

                        What are balanced magnetron sputtering and unbalanced magnetron sputtering?

   Balanced magnetron sputtering is the traditional magnetron sputtering, which is to place a permanent magnet or electromagnetic coil with the same or similar magnetic field intensity between the core and the outer ring behind the cathode target, and form a magnetic field perpendicular to the electric field direction on the target surface. The deposition chamber is filled with a certain amount of working gas, usually ar. under the action of high pressure, AR is ionized into Ar + ions and electrons, generating glow discharge. The Ar + ions bombard the target with electric field acceleration, splashing out the target atoms, ions and secondary electrons.

   Under the action of mutually perpendicular electromagnetic field, electrons move in a cycloidal way and are bound to the target surface, which prolongs their motion trajectory in the plasma, increases their participation in the process of gas molecule collision and ionization, ionizes more ions, improves the ionization rate of the gas, and maintains the discharge under a lower gas pressure. Therefore, magnetron sputtering can not only reduce the sputtering process At the same time, the sputtering efficiency and deposition rate are improved.

  For example, due to the effect of magnetic field, the electrons produced by glow discharge and the secondary electrons ejected are tightly confined to the target surface by parallel magnetic field, and the plasma area is strongly bound to the target surface about 60 mm With the increase of the distance from the target surface, the plasma concentration decreases rapidly. At this time, the workpiece can only be placed in the range of 50-100 mm on the surface of the magnetron target to enhance the effect of ion bombardment. Such a short effective coating area limits the geometric size of the workpiece to be plated, which is not suitable for larger workpiece or charging capacity, and restricts the application of magnetron sputtering technology. In the balanced magnetron sputtering, the energy of the target particles is low, the bonding strength of the film base is poor, the mobility of the deposited atoms with low energy on the surface of the substrate is low, and it is easy to form porous and rough columnar structure films. Increasing the temperature of the workpiece can improve the structure and performance of the film, but in many cases, the workpiece material itself can not bear the required high temperature.

  The appearance of unbalanced magnetron sputtering overcomes the above shortcomings. The plasma on the cathode target surface is introduced to the range of 200-300 mm in front of the sputtering target to immerse the substrate in the plasma, as shown in the figure. In this way, on the one hand, the sputtered atoms and particles are deposited on the surface of the substrate to form a thin film; on the other hand, the plasma bombards the substrate with a certain amount of energy, which plays the role of ion beam assisted deposition and greatly improves the quality of the film. The unbalanced magnetron sputtering system has two structures: one is that the core magnetic field strength is higher than the outer ring, the magnetic field line is not closed, and it is led to the true The plasma density on the surface of the substrate is low, so this method is rarely used. The other is that the magnetic field strength of the outer ring is higher than that of the core, and the magnetic field lines do not form a closed circuit completely. Some of the magnetic field lines of the outer ring extend to the surface of the substrate, so that some secondary electrons can escape from the target surface along the magnetic field lines, and collide with the neutral particles at the same time. The plasma is no longer completely limited to the target surface area, but can reach the substrate On the surface, the ion concentration in the coating area is further increased, so that the ion current density of the substrate is increased, usually up to 5 mA / cm2. In this way, the sputtering source is also an ion source that bombards the substrate surface. The current density of the substrate is proportional to the current density of the target, the current density of the target increases, the deposition rate increases, and the current density of the substrate increases, which has a certain impact on the surface of the deposited film.

   Ion bombardment by unbalanced magnetron sputtering can clean the oxide layer and other impurities of the workpiece before coating, activate the surface of the workpiece, and form a pseudo diffusion layer on the surface of the workpiece, which is helpful to improve the binding force between the film layer and the workpiece surface. In the process of coating, the modification of the film can be achieved by the bombardment of charged particles. For example, ion bombardment tends to strip the particles from the loose and protruding parts of the film, cut off the dominant growth of crystalline or condensed state of the film, so as to produce a denser, stronger and more uniform film with stronger binding force, and can plating a good coating at a lower temperature.

   The application of unbalanced magnetron sputtering technology solves the problem of deposition of dense and complex films. However, it is difficult for a single unbalanced magnetron target to deposit uniform films on a complex substrate. In the process of electrons flying to the substrate, with the weakening of magnetic field strength, some electrons are adsorbed on the wall of the vacuum chamber, resulting in the absorption of electrons and ions The concentration decreased. For this reason, researchers have developed a multi-target unbalanced magnetron sputtering system to make up for the shortage of single target unbalanced magnetron sputtering. According to the distribution of magnetic field, multi-target unbalanced magnetron sputtering system can be divided into closed field unbalanced magnetron sputtering with opposite adjacent poles and mirror field unbalanced magnetron sputtering with the same adjacent poles, as shown in the figure, double target closed field and double target mirror field.

   Comparing the magnetic field distribution of the closed magnetic field non-equilibrium target pair and the mirror target pair, it can be seen that there is little difference between the magnetic field near the target surface, and the transverse magnetic field between the internal and external magnetic poles constrains the electrons to form a plasma cathode region with high ionization degree. In this region, the positive ions are strongly sputtered and etched on the target surface, splashing a large number of target particles flying to the substrate surface. In the inner and outer ring magnetic poles, especially in the strong outer ring magnetic pole, the longitudinal magnetic field is the main channel for secondary electrons to escape from the target surface, and then become the main channel for transporting charged particles to the coating area. Comparing the magnetic field distribution of the closed magnetic field and the mirror image magnetic field in the coating area, the difference is great. For the mirror image target pair, due to the mutual repulsion of the two target magnetic fields, the longitudinal magnetic field is forced to bend outside the coating area (the vacuum chamber wall), the electrons are guided to the vacuum chamber wall, and the total number of electrons and ions is reduced. The sputtering efficiency of the plasma has not been improved because the mirror magnetic field can not effectively bind the electrons. However, the longitudinal magnetic field of the non-equilibrium target pair in the coating area is closed. As long as the magnetic field strength is enough, electrons can only move between the coating area and two targets, avoiding the loss of electrons, thus increasing the ion concentration in the coating area and greatly improving the sputtering efficiency.