Hochratenprozesse für organische Halbleiterbauelemente (English shop)

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This work investigated the influence of the deposition process on the properties of an organic semiconductor device. The main focus was on the deposition of the top electrode. This constitutes the most energy-intensive process step in the fabrication of an organic light-emitting diode (OLED) and can lead to degradation effects in the device, as demonstrated here.
Both transparent contacts of sputtered, aluminium-doped zinc oxide (AZO) and opaque electrodes of physically-thermally deposited aluminium were investigated. The aim of the investigations was to develop processes with the highest possible deposition rates whose deposition does not adversely affect the underlying layers.

The sputtering process can subject the devices to UV and thermal radiation. In addition, particles with high kinetic energy are emitted from the target. These effects increase the diffusivity of the sputtered particles on the substrate surface, but can also adversely affect the chemical and morphological properties of the organic semiconductor layers. In contrast to indium tin oxide, the AZO used here requires that a minimum degree of surface diffusion on the substrate be permitted in order to produce electrically conductive layers.
In order to prevent any effect of the sputtering process on the underlying organic layers, the degradation mechanisms were investigated separately and a graded process regime was developed. This began with the deposition of a poorly conductive layer on the organic semiconductors that could, however, be deposited gently. In the course of layer growth, the process parameters were continuously changed towards higher-energy particles, resulting in more conductive layer structures. The already deposited parts of the graded layer thereby acted as a barrier between the organic molecules and the increasingly conductive regions of the top electrode. This enabled the fabrication of the world’s first OLED with an AZO top contact deposited from a planar magnetron cathode.

In the field of opaque devices, aluminium is predominantly used as the top electrode, deposited by means of physical thermal vapour deposition (PVD). This process generates a great deal of thermal radiation, which can adversely affect the electro-optical efficiency of the OLEDs.
In order to investigate this process, the influence of thermal energy on the morphological and electrical properties of thin organic layers was first quantified. Taking these boundary conditions into account, it was then possible to implement optimisation measures in terms of apparatus and process engineering that almost completely prevent the deposition parameters from affecting the device.

These investigations showed that a higher deposition rate not only increases throughput, but also reduces the overall thermal load on the devices. These results led to the development of a new PVD system with a very high deposition rate. It is based on the flash sublimation method and reduces the coating time for a 100 nm thick aluminium layer from 4 minutes to about 10 seconds.
As a result of the changes in the PVD process, the formation of microcrystals in the deposited aluminium could be demonstrated. The effects of this crystallisation on the morphological and electrical properties of the aluminium layer were investigated, as was the influence of the high-rate process on the electro-optical properties of the OLED. By means of suitable process control, it was possible to completely prevent any effect of the high-rate deposition of the top contact on the function of the OLED.
Furthermore, the high-rate deposition method could be transferred to all organic and inorganic layers of an OLED. In this way, the field of application of flash sublimation could for the first time be extended beyond the material class of pure metals. The brief, intense supply of thermal energy can thereby affect the chemical structure of the molecules themselves. For the electron transport material Alq3, for example, an isomerisation of the molecule depending on the process parameters was demonstrated.
In this method, the onset and duration of deposition during the process depend on the applied temperature gradient and the specific phase transition temperature of the material to be deposited. If the crucible is filled with different materials at the same time, separate layers can be produced at low temperature gradients. If the gradient is increased, layers with gradual intermixing are formed, whose concentration ratio results from the offset of the deposition periods. Using the example of Alq3 and DCM, it could also be shown that at high temperature gradients a single, homogeneously intermixed or doped layer is formed.
Taking the specific phase transition temperatures into account, the method developed here allows the deposition of an OLED consisting of a-NPD, Alq3, LiF and Al in a single fabrication step. The total fabrication time of the semiconductor device is thereby reduced from several hours to about two minutes.