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What are balanced magnetron sputtering and unbalanced magnetron sputtering?什麽是平衡磁控濺射和非平衡磁控濺射?

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What are balanced magnetron sputtering and unbalanced magnetron sputtering?什麽是平衡磁控濺射和非平衡磁控濺射?

發布日期:2019-12-13 作者: 點擊:

什麽是平衡磁控濺射和非平衡磁控濺射?

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    平衡磁控濺射即傳統的磁控濺射,是在陰極靶材背後放置芯部與外環磁場強度相等或相近的永磁體或電磁線圈,在靶材表麵形成與電場方向垂直的磁場。沉積室充入一定量的工作氣體,通常為Ar,在高壓作用下Ar 原了電離成為Ar+離子和電子,產生輝光放電,Ar+ 離子經電場加速轟擊靶材,濺射出靶材原子、離子和二次電子等。


    電子在相互垂直的電磁場的作用下,以擺線方式運動,被束縛在靶材表麵,延長了其在等離子體中的運動軌跡,增加其參與氣體分子碰撞和電離的過程,電離出更多的離子,提高了氣體的離化率,在較低的氣體壓力下也可維持放電,因而磁控濺射既降低濺射過程中的氣體壓力,也同時提高了濺射的效率和沉積速率。


    但平衡磁控濺射也有不足之處,例如:由於磁場作用,輝光放電產生的電子和濺射出的二次電子被平行磁場緊緊地約束在靶麵附近,等離子體區被強烈地束縛在靶麵大約60 mm 的區域,隨著離開靶麵距離的增大,等離子濃度迅速降低,這時隻能把工件安放在磁控靶表麵50~100 mm的範圍內,以增強離子轟擊的效果。這樣短的有效鍍膜區限製了待鍍工件的幾何尺寸,不適於較大的工件或裝爐量,製約了磁控濺射技術的應用。且在平衡磁控濺射時,飛出的靶材粒子能量較低,膜基結合強度較差,低能量的沉積原子在基體表麵遷移率低,易生成多孔粗糙的柱狀結構薄膜。提高被鍍工件的溫度固然可以改善膜層的結構和性能,但是在很多的情況下,工件材料本身不能承受所需的高溫。



    非平衡磁控濺射的出現部分克服了以上缺點,將陰極靶麵的等離子體引到濺射靶前200~300 mm 的範圍內,使基體沉浸在等離子體中,如圖所示。這樣,一方麵,濺射出來的原子和粒子沉積在基體表麵形成薄膜,另一方麵,等離子體以一定的能量轟擊基體,起到離子束輔助沉積的作用,大大的改善了膜層的質量,非平衡磁控濺射係統有兩種結構,一種是其芯部磁場強度比外環高,磁力線沒有閉合,被引向真空室壁,基體表麵的等離子體密度低,因此該方式很少被采用。另一種是外環磁場強度高於芯部磁場強度,磁力線沒有完全形成閉合回路,部分外環的磁力線延伸到基體表麵,使得部分二次電子能夠沿著磁力線逃逸出靶材表麵區域,同時再與中性粒子發生碰撞電離,等離子體不再被完全限製在靶材表麵區域,而是能夠到達基體表麵,進一步增加鍍膜區域的離子濃度,使襯底離子流密度提高,通常可達5 mA/cm2 以上。這樣濺射源同時又是轟擊基體表麵的離子源,基體離子束流密度與靶材電流密度成正比,靶材電流密度提高,沉積速率提高,同時基體離子束流密度提高,對沉積膜層表麵起到一定的轟擊作用。


    非平衡磁控濺射離子轟擊在鍍膜前可以起到清洗工件的氧化層和其他雜質,活化工件表麵的作用,同時在工件表麵上形成偽擴散層,有助於提高膜層與工件表麵之間的結合力。在鍍膜過程中,載能的帶電粒子轟擊作用可達到膜層的改性目的。比如,離子轟擊傾向於從膜層上剝離結合較鬆散的和凸出部位的粒子,切斷膜層結晶態或凝聚態的優勢生長,從而生更致密,結合力更強,更均勻的膜層,並可以較低的溫度下鍍出性能優良的鍍層。


    非平衡磁控濺射技術的運用,使平衡磁控濺射遇到的沉積致密、成分複雜薄膜的問題得以解決,然而單獨的非平衡磁控靶在複雜基體上較難沉積出均勻的薄膜,而且在電子飛向基體的過程中,隨著磁場強度的減弱,一部分電子吸附到真空室壁上,導致電子和離子的濃度下降。對此研究人員開發出多靶非平衡磁控濺射係統,以彌補單靶非平衡磁控濺射的不足。多靶非平衡磁控濺射係統根據磁場的分布方式可以分為相鄰磁極相反的閉合磁場非平衡磁控濺射和相鄰磁極相同的鏡像磁場非平衡磁控濺射,如圖為雙靶閉合磁場和雙靶鏡像磁場。


    比較閉合磁場非平衡靶對和鏡像靶對的磁場分布情況,可以看出在靶材表麵附近磁場差別不大,內外磁極之間橫向磁場對電子的約束形成一個電離度很高的等離子體陰極區,在此區域內的正離子對靶麵的強烈濺射刻蝕,濺射出大量靶材粒子飛向基體表麵。在內部和外環磁極的位置,特別是較強的外環磁極處,以縱向磁場為主,成為二次電子逃離靶麵的主要通道,進而成為向鍍膜區域輸送帶電粒子的主要通道。再比較閉合磁場和鏡像磁場在鍍膜區域內磁場分布,差別就大了,對於鏡像靶對,由於兩個靶磁場的相互排斥,縱向磁場都被迫向鍍膜區外(真空室壁)彎曲,電子被引導到真空室壁上流失,總體上降低了電子進而離子的數量。由於鏡像磁場方式不能有效地束縛電子,因而等離子體的濺射效率未有得到提高。而閉合磁場非平衡靶對在鍍膜區域的縱向磁場是閉合的。隻要磁場強度足夠,電子就隻能在鍍膜區域和兩個靶之間運動,避免了電子的損失,從而增加了鍍膜區域的離子濃度,大幅度提高了濺射效率。

                                                             

                        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.


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