A Semiautomated Tool for Fabrication of Vacuum based Optoelectronic Devices: Cluster Tool
In today’s era, organic light emitting diodes (OLEDs) are becoming a promising opto-electronic device in the field of display and lighting technology. They have grown with the continuous development of amorphous organic semiconductor. These organic LEDs has got the fame because of the smooth surface of the amorphous organic films which ensures the high stability and uniformity of the devices under high electrical excitation. Due to these qualities OLEDs have move to the wide commercialization. These OLEDs are fabricated on basis of two types of materials: (a) polymer based (b) Small molecule based. The fabrication of these amorphous film-based OLEDs can be roughly categorized into two processes: (a) solution processible (spin coated) (b) vacuum deposited. Due to the limitation of polymer-based OLEDs with display technology(ref2), industries have switched to the small molecules. Though the fabrication of small molecules-based OLED is cost effective, still vacuum based fabrication is preferred. It is observed that the film density, transition temperature and degree of horizontal molecular orientation of small molecules spin coated film is inherently lower than those of corresponding to vacuum deposited films. For the vacuum deposited films, the refractive index and the extinction coefficient in the horizontal direction are much larger than those in the vertical direction which suggests that the molecules are significantly oriented in the horizontal direction. Conversely, the molecular orientation in the spin coated films is random as that in the vacuum deposited film. So, for the small molecule- based OLEDs, vacuum sublimation method is used at the industries. In general, commercial system comes with a single chamber integrated inside the inert gas filled glove boxes for lab-scale research, which has limitations of cross contamination and also takes away a larger portion of an expensive floor space of glove box. Cluster tool design is a solution, however, designing such a multi-chamber tool requires lots of iteration to get an optimal performance. In general, such tools needs to be attached with an inert gas filled glove box via load-lock design, hence larger foot print is required. Effusion cells design in commercial setups require much larger seed amount to start with for vacuum sublimation process. Further, a flexibility in design aspects is limited due to in-general a closed configuration packing. Such closed packing also results in poor debugging the faults if it occurs in system over the time of operation. Indigenous semi-automated design which is unique in its design aspect to provide an economical solution for lab-scale research on ambient sensitive optoelectronic semiconductor materials for multi-stack device. Further modification is possible based on addition of layer or scaling up the size over which thin-films need to be deposited. Customized thickness monitor sensors with line of site with respect to each effusion cell to have a controlled rate of deposition. Special design of effusion cells further reduces the cost of in general expensive organic semiconductor compounds in test runs. Samples can be transferred in inert conditions without even being attached to inert gas filled glove box with cluster tool, hence no larger foot print and allows an option to keep the two tools separately at different locations for space optimization. Salient features/Advantages/Novelty of the proposed technology. This invention deals with development of a customized tool to fabricate a multilayer thin-film optoelectronic device. Materials utilized for these thin-films are sensitive to ambient and hence special arrangement is needed to ensure until one finish the complete device fabrication, sample does not get exposed to ambient. There are chances of cross contamination and hence there is a dedicated thin-film deposition chamber for each layer of the device stack. Each chamber has multiple effusion cells to the vacuum evaporation/sublimation of material simultaneously with a controlled deposition rate from 0.1 A/s to 2 A/s. After each deposition specimen gets transferred from one deposition chamber to another one via load-lock conditions without bringing the chamber in refill state. Whole of this cluster unit can be attached to an inert gas filled glove box or a special arrangement of inert atmosphere can be used at opening door of the chamber to ensure that samples remain in inert atmosphere.
Department of Physics