Formation of Inorganic Thin Films by Laser Ablative Cluster Emission

研究表明，激光照射无机物表面，如含铜氧化物超导体，会导致离子、原子和团簇的发射。这一过程取决于激光的影响。利用这种激光烧蚀工艺，在氧气氛下在衬底上形成无机薄膜。在1989年，我们已经证明了Bi_2Sr_2CaCu_2O_8和Bi_2Sr_2CuO_6表面发出团簇，这些相是通过这种团簇形成过程形成的。我们已经开发了这个过程，将目标分解成更小的部分，即离子和原子。这些离子和原子积聚在底物上。在Bi_2Sr_2CuO_6烧蚀过程中，观察到Bi、Sr和Cu的原子形态。Bi_2Sr_2CuO_6薄膜的形成是在各种氧化气体中进行的。结果表明，NO_2>N_2O>O_2对氧化过程的影响依次为：在NO_2气氛下，即使是10^<-4> torr的低压也能有效地使薄膜结晶。在N_2O气氛中，Bi_2Sr_2CuO_6相的结晶温度比在O_2气氛中低50 ~ 100℃。这可以用激光照射形成的O自由基的强氧化能力来解释。在混合的N_2O和O_2气体中，氧气被充分地吸收到薄膜中。仅在O_2气氛下，杂质CuO被分离到表面，形成粗糙的表面。薄膜形成过程中激光的照射对低温薄膜的形成是有效的。即使是10mJ/cm^2量级的弱光也足够有效。因此，激光烧蚀法是制备无机薄膜的有效方法，同时选择较低温度下NO_2和N_2O等气氛是制备无机薄膜的重要条件。

It has been revealed that the laser irradiation of the surface of inorganic substances, such as copper containing oxide superconductor, leads to the emission of ions, atoms and clusters. This process is dependent on the laser influence. By utilizing this laser ablation process, inorganic thin films are formed on substrates in the oxygen atmosphere. In 1989, we have demonstrated that clusters are emitted from the Bi_2Sr_2CaCu_2O_8 and Bi_2Sr_2CuO_6 surface, and these phases are formed using this cluster formation process. We have developed this process to decompose the targets into smaller pieces, that is ions and atoms. These ions and atoms are accumulated on the substrates. In the case of Bi_2Sr_2CuO_6 ablation, atomic species of Bi, Sr and Cu are observed. The thin film formation of Bi_2Sr_2CuO_6 is carried out in the various oxidation gases. As a result, it has been revealed that NO_2>N_2O>O_2 are effective in this order for the oxidation process. In the NO_2 atmosphere, even the low pressure of 10^<-4> torr is effective to crystallize the thin films. In the N_2O atmosphere, the Bi_2Sr_2CuO_6 phase is crystallized at 50 to 100C lower temperature than in the O_2 gas. This is explained by the strong oxidation power of O radical formed by the laser irradiation. In the mixed N_2O and O_2 gas, the oxygen is incorporated enough in the films. In the O_2 atmosphere alone, the impurity CuO is segregated an the surface to make a rough surface. The irradiation of laser light during the thin film formation is effective for the low temperature thin film formation. Even the low intensity light of the order of 10mJ/cm^2 is effective enough. Thus, the laser ablation method is effective for the formation of inorganic thin films and also it is important to choose the atmosphere, such as NO_2 and N_2O, to form thin films at lower temperature.