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Magnetron sputtering (磁控溅射台))

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The sputtering process involves complex scattering process and multiple energy transfer processes溅射过程中涉及到复杂的散射过程和多种能量传递过程

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The sputtering process involves complex scattering process and multiple energy transfer processes溅射过程中涉及到复杂的散射过程和多种能量传递过程

发布日期:2018-01-04 作者:www.cycas.com 点击:

直流溅射法要求靶材能够将从离子轰击过程中得到的正电荷传递给与其紧密接触的阴极,从而该方法只能溅射导体材料,不适于绝缘材料,因为轰击绝缘靶材时表面的离子电荷无法中和,这将导致靶面电位升高,外加电压几乎都加在靶上,两极间的离子加速与电离的机会将变小,甚至不能电离,导致不能连续放电甚至放电停止,溅射停止。故对于绝缘靶材或导电性很差的非金属靶材,须用射频溅射法(RF)。


溅射过程中涉及到复杂的散射过程和多种能量传递过程:首先,入射粒子与靶材原子发生弹性碰撞,入射粒子的一部分动能会传给靶材原子,某些靶材原子的动能超过由其周围存在的其它原子所形成的势垒(对于金属是5-10eV),从而从晶格点阵中被碰撞出来,产生离位原子,并进一步和附近的原子依次反复碰撞,产生碰撞级联。当这种碰撞级联到达靶材表面时,如果靠近靶材表面的原子的动能大于表面结合能(对于金属是1-6eV),这些原子就会从靶材表面脱离从而进入真空。


溅射镀膜就是在真空中利用荷能粒子轰击靶表面,使被轰击出的粒子沉积在基片上的技术。通常,利用低压惰性气体辉光放电来产生入射离子。阴极靶由镀膜材料制成,基片作为阳极,真空室中通入0.1-10Pa的氩气或其它惰性气体,在阴极(靶)1-3KV直流负高压或13.56MHz的射频电压作用下产生辉光放电。电离出的氩离子轰击靶表面,使得靶原子溅出并沉积在基片上,形成薄膜。溅射方法很多,主要有二级溅射、三级或四级溅射、磁控溅射、对靶溅射、射频溅射、偏压溅射、非对称交流射频溅射、离子束溅射以及反应溅射等。

磁控溅射台

由于被溅射原子是与具有数十电子伏特能量的正离子交换动能后飞溅出来的,因而溅射出来的原子能量高,有利于提高沉积时原子的扩散能力,提高沉积组织的致密程度,使制出的薄膜与基片具有强的附着力。


溅射时,气体被电离之后,气体离子在电场作用下飞向接阴极的靶材,电子则飞向接地的壁腔和基片。这样在低电压和低气压下,产生的离子数目少,靶材溅射效率低;而在高电压和高气压下,尽管可以产生较多的离子,但飞向基片的电子携带的能量高,容易使基片发热甚至发生二次溅射,影响制膜质量。另外,靶材原子在飞向基片的过程中与气体分子的碰撞几率也大为增加,因而被散射到整个腔体,既会造成靶材浪费,又会在制备多层膜时造成各层的污染。


为了解决阴极溅射的缺陷,人们在20世纪70年代开发出了直流磁控溅射技术,它有效地克服了阴极溅射速率低和电子使基片温度升高的弱点,因而获得了迅速发展和广泛应用。


其原理是:磁控溅射台在磁控溅射中,由于运动电子在磁场中受到洛仑兹力,它们的运动轨迹会发生弯曲甚至产生螺旋运动,其运动路径变长,因而增加了与工作气体分子碰撞的次数,使等离子体密度增大,从而磁控溅射速率得到很大的提高,而且可以在较低的溅射电压和气压下工作,降低薄膜污染的倾向;另一方面也提高了入射到衬底表面的原子的能量,因而可以在很大程度上改善薄膜的质量。同时,经过多次碰撞而丧失能量的电子到达阳极时,已变成低能电子,从而不会使基片过热。因此磁控溅射法具有"高速"、"低温"的优点。该方法的缺点是不能制备绝缘体膜,而且磁控电极中采用的不均匀磁场会使靶材产生显著的不均匀刻蚀,导致靶材利用率低,一般仅为20%-30%。

        The direct current sputtering method requires that the target can transfer the positive charge from the ion bombardment process to the cathode in close contact with it, so this method can only sputtering the conductor material, which is not suitable for the insulating material, because the ion charge on the surface of the insulating target can not be neutralized when bombarding, which will lead to the potential rise of the target surface, and the applied voltage is almost added to the target, and the ion acceleration and ionization between the two poles The chance will be smaller, even can not ionize, resulting in not continuous discharge or even discharge stop, sputtering stop. Therefore, RF sputtering should be used for insulating target or non-metallic target with poor conductivity. 

      The sputtering process involves complex scattering process and many kinds of energy transfer processes. Firstly, the incident particles collide with the target atoms elastically, and part of the kinetic energy of the incident particles will be transmitted to the target atoms. The kinetic energy of some target atoms exceeds the potential barrier (5-10ev for metals) formed by other atoms around them, so they are collided out of the lattice lattice and generate dislocations Atoms, and further and adjacent atoms in turn repeatedly collide, resulting in a collision cascade. When the collision cascade reaches the target surface, if the kinetic energy of the atoms near the target surface is greater than the surface binding energy (1-6ev for metals), these atoms will separate from the target surface and enter the vacuum. 

    Sputtering coating is a technology that bombards the target surface with charged particles in vacuum to deposit the bombarded particles on the substrate. Usually, the incident ions are produced by glow discharge of low pressure inert gas. The cathode target is made of coating material, the substrate is used as anode, argon or other inert gas of 0.1-10Pa is introduced into the vacuum chamber, and glow discharge is produced under the action of 1-3KV DC negative high voltage or 13.56MHz RF voltage of the cathode (target). The ionized argon ions bombard the target surface, making the target atoms splash and deposit on the substrate, forming a thin film. There are many sputtering methods, including secondary sputtering, tertiary or quaternary sputtering, magnetron sputtering, target sputtering, RF sputtering, bias sputtering, asymmetric AC RF sputtering, ion beam sputtering and reactive sputtering. 

   Because the sputtered atoms are splashed out after exchanging kinetic energy with positive ions with tens of electron volts energy, the sputtered atoms have high energy, which is conducive to improving the diffusion ability of atoms during deposition, improving the density of deposited structure, and making the film and substrate have strong adhesion. 

   During sputtering, after the gas is ionized, the gas ions fly to the cathode target under the action of electric field, and the electrons fly to the grounded wall cavity and substrate. In this way, under low voltage and low pressure, the number of ions produced is small, and the sputtering efficiency of the target is low; while under high voltage and high pressure, although more ions can be produced, the high energy carried by the electrons flying to the substrate is easy to heat the substrate or even cause secondary sputtering, which affects the film quality. In addition, the collision probability between the target atoms and the gas molecules increases greatly in the process of flying to the substrate, so it will be scattered to the whole cavity, which will not only cause the target waste, but also cause the pollution of each layer in the preparation of multilayer. 

   In order to solve the defects of cathode sputtering, DC magnetron sputtering technology was developed in 1970s. It effectively overcomes the weaknesses of low cathode sputtering rate and high substrate temperature caused by electrons, so it has been rapidly developed and widely used. 

  The principle is as follows: in magnetron sputtering stage, due to the Lorentz force on the moving electrons in the magnetic field, their motion path will be bent or even spiral, and their motion path will become longer, so the number of collisions with the working gas molecules will be increased, the plasma density will be increased, and the magnetron sputtering rate will be greatly improved On the other hand, it also improves the energy of the atoms incident on the substrate surface, so it can improve the quality of the film to a great extent. At the same time, the electrons that lost energy after many collisions have become low energy electrons when they reach the anode, so that the substrate will not overheat. Therefore, magnetron sputtering has the advantages of "high speed" and "low temperature". The disadvantage of this method is that the insulator film can not be prepared, and the inhomogeneous magnetic field used in the magnetic control electrode will cause significant inhomogeneous etching of the target, resulting in low utilization rate of the target, generally only 20% - 30%.

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