Application note:

LD process microscope 10x/0.25 and  thin film organic semiconductor technology

FUNSOM, Soochow University, Prof. Lifeng Chi and Dr. Wenchong Wang

The research group of Prof. Chi and Dr. Wenchong Wang at the University of Suzhou is developing methods that can be used to structure thin organic layers using photolithography. Photolithography is a well-established process in the inorganic semiconductor industry for producing small structures for microchips. With organic materials, the process can’t be applied directly, since these are not stable enough against the radiation power and organic solvents required thereby.

New methods to produce OLEDs, OFETs and organic memory devices

The trick is based on pre-patterning a thin nucleation layer photolithographically on the substrate. The organic material is evaporated from a crucible and deposited onto the pre-structured substrate in vacuum. When all parameters, such as substrate temperature and molecular beam flux, are properly optimized, the organic material grows only on the pre-patterned layer and has the desired morphological microstructure (e.g. crystallinity). Diffusion on the substrate and self-assembly of the molecules play a decisive role here. The method basically allows a simple and lithographical way of producing organic semiconductor devices over large area. It is also possible to deposit several organic layers on top of each other.

The pre-structured substrate is vapor-deposited with organic molecules (Phys. Rev. Letter 2007)

Process chamber with integrated high-resolution optics

To observe the process of deposition in-situ on the pre-structured substrate, a special vacuum chamber was designed. The high-resolution process microscope of Technologie Manufaktur and up to three inverted evaporators for targeted evaporation of organic materials are mounted on top of the chamber (see image below). A heated substrate holder with an xyz-stage is integrated.
The process microscope has a spatial resolution of up to 1 μm at a working distance of 100mm. The long working distance allows the lens to be mounted outside the chamber. The substrate is observed through a vacuum window of the chamber. In addition, various spatially resolved spectroscopic methods, e.g. Photoluminescence or Raman spectroscopy can be realized.

Vacuum process chamber with evaporators and process microscope

The process microscope

To have a long working distance and an optical correction over the very large wavelength range of 200 nm – 2000 nm (DUV-UV-VIS-IR), the lens of the process microscope is designed as a combination of mirrors and lenses. The design is very much like that of a classic Schwarzschild lens, but also includes (refractive) lenses to compensate chromatic errors that arise through the vacuum window. This process microscope is the only „long distance“ microscope on the market for mounting on a standard vacuum flange (DN63) and observation through a vacuum window (registered for patent approval).
The primary mirror has a diameter of 140mm and is of highest surface quality (<λ / 20). The Secondary Mirror is a so-called Mangin Mirror: This optical element functions as a lens with its front side and as a mirror with its rear side. The combination of Mangin Mirror and field lens corrects the chromatic errors produced by the vacuum window. Due to the good chromatic correction, a wide variety of spectroscopic methods – including imaging – can be used in the wavelength range from 200 nm to 2000 nm.
The process microscope has a numerical aperture of 0.25 and resolution of up to 1 μm.

Optical design of the Process Microscope

In-situ imaging and spectroscopy

With the developed process chamber, it is possible to observe the evaporation process in real time with high resolution and to investigate morphological evolution of the growing organic layers.

In the next step, the setup will be extended to allow photoluminescence spectroscopy on small areas of the sample. Thus, evolution of the optical properties of the layer system can then be monitored locally during the depostion. For this purpose, a second image is generated via a beam splitter in which the spectrum is detected with a positionable optical fiber.

Organic material deposited on a patterned metal layer

Application fields in future markets

Thin layers of different organic materials may act as electronic and optoelectronic components such as LEDs and field effect transistors by means of an appropriate structuring. With the method described above, it will be possible to produce highly integrated circuits based on organic materials. The technology would also be suitable to produce micro-displays with very high pixel density. Applications can be found in the areas of virtual / mixed-reality, camera viewfinders and head-up displays.

Example of a microdisplay © Photo Fraunhofer FEP, photographer: Anna Schroll

Publications:

  1. O. Buller & H. Wang & W. Wang & L.F. Chi & A. Heuer (2018) Boundary-induced nucleation control: a theoretical perspective. Phys. Chem. Chem. Phys. 20, 3752-3760
  2. Zhu, Juan & Fontein, Florian & Wang, Hong & Zhong, Qigang & Li, Chenglong & Li, Jianping & Wang, Bo & Liao, Liang-Sheng & Wang, Morgan & Huang, Lizhen & Harald, Fuchs & Wang, Wenchong & Chi, Lifeng. (2017). Molecular Materials and Devices: Micro Organic Light-emitting Diodes Fabricated through Area-selective Growth. Mater. Chem. Front.. 10.1039/C7QM00383H.
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