Solvent annealing process for forming a thin semiconductor...

Details Solvent annealing process for forming a thin semiconductor... Solvent annealing process for forming a thin semiconductor...

2000-08-31

2001-11-06

Bowers, Charles (Department: 2813)

Semiconductor device manufacturing: process

Having organic semiconductive component

Reexamination Certificate

active

06312971

ABSTRACT:

TECHNICAL FIELD
The invention relates generally to manufacturing of semiconductor devices and more particularly to the manufacture of patterned organic layers in organic material-based electronic devices.
BACKGROUND OF THE INVENTION
Thin films of organic semiconductor material can be used in the construction of thin-film transistors (TFT). Performance of organic TFT's (OTFT), characterized by parameters such as the field effect mobility and threshold voltage, depends in part upon the molecular structure of the semiconductor film. Factors such as interfacial structure, the degree of molecular order and crystalline orientation of the thin film affect film properties.
Ordering of the semiconductor depends in turn on how the thin film is deposited. It is generally believed that increasing the amount of molecular order—by increasing crystal size, reducing the density of crystalline defects, or improving short-range molecular order-permits charge carriers, i.e. electrons or holes, to more efficiently move between molecules. This can increase the field effect mobility.
Advantageous molecular order that gives high field effect mobility can be achieved using some relatively expensive deposition techniques. In contrast, deposition techniques that enable inexpensive production or production of films of a desired uniformity and thickness can produce films that exhibit a relatively small field-effect mobility.
For example, a solvent cast film that is permitted to slowly dry often exhibits a relatively high field effect mobility when incorporated into an OTFT. Unfortunately, some deposition techniques that are more amenable to manufacturing do not readily permit slow evaporation of solvent. For example, though spin coating can yield relatively uniform thin films, the solvent usually leaves the film relatively quickly, generally leading to a low degree of crystal order. Field-effect mobility, for example, can be a factor of about 10 to 100 smaller than for cast films. Other manufacturing processes such as screen printing or various thin-film coating methods may yield desirable film morphology, but not desirable molecular order.
What are needed are lower cost manufacturing methods that produce relatively high quality organic semiconductor films.
SUMMARY OF THE INVENTION
In one aspect, the invention relates to the manufacture of displays that utilize arrays of OTFT'S. The invention is of particular use in the production of displays with an electrophoretic display medium and mitigates the problem of poor quality organic layers obtained by lower cost manufacturing processes. The invention permits manufacturing of higher quality organic semiconductor layers while still utilizing lower cost organic film deposition methods.
In one aspect, after deposition of the organic semiconductor film, the film is annealed under the influence of a vapor derived from a solvent. The solvent is chosen for its ability to assist molecular rearrangement after diffusing into the organic semiconductor film. Annealing conditions are selected to provide molecular rearrangement that improves electrical properties while avoiding conditions that damage the film. Annealing conditions in some embodiments are determined by systematically varying the solvent temperature, the organic semiconductor film temperature, and the annealing time until an appropriate level of improvement in electrical properties is obtained.
In one embodiment, a substrate is formed by depositing an organic semiconductor film via a lower cost method such as printing or spin coating on a support substrate. A portion of a solvent is vaporized to bring the vapor into contact with the film. The chemical potential of the vapor molecules is controlled to provide an interaction with the organic semiconductor film to alter the molecular arrangement of the film. Some embodiments further entail placing the substrate on a first temperature controlled stage and placing the solvent on a second temperature controlled stage. The chemical potential of the vapor is adjusted by controlling the temperature of the solvent. Appropriate annealing conditions are obtained by adjusting the temperature of the solvent, the substrate, and the anneal time. This process can assist manufacturing of lower cost displays that utilize arrays of organic thin-film transistors.


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