Many electronic devices, such as organic light emitting devices (OLEDs), charged coupled devices (CCDs), and thinfilm transistors (TFTs), organic Thin-Film Transistors (TFT), solar cells, comprise active components which are susceptible to degradation from exposure to moisture, oxygen and other gases found in the atmosphere. In order to ensure the longevity of these devices, some form of hermetic packaging is typically used to protect these devices from exposure to degradative substances.
As hermeticity is important for the operation of such devices, hermetic seal testing has become a vital part of the manufacture of these devices. For example, in the semiconductor industry, standard reliability qualification testing includes one or more humidity evaluations in which a sample quantity of devices are subjected to a humid environment and other conditions of elevated temperature, bias, and/or pressure.
A variety of hermeticity tests have been developed to evaluate the hermeticity of encapsulation packages. For example, for detecting fine leaks in integrated circuit (IC) packages, a mixture of helium gas with dry nitrogen, is sealed into the package, and a mass spectrometer is used to detect helium leaking from the package.
While the helium leak test method has been used on many types of semiconductor packages, it suffers from certain problems. Firstly, the helium leak test is generally only applicable for testing packages with cavities, and are not suitable for devices which do not comprise cavities, such as optoelectronic devices, MEMS or any other thin film microelectronic devices. Secondly, helium tends to separate from atmospheric nitrogen present in the device, due to differences in atomic weight, thus migrating to the top of the encapsulation cavity, affecting the thermal conductivity of components in the device as well as the index of refraction of the encapsulation, thereby yielding inaccurate results.
Another method that is currently employed to assess the condition of an electronic component within an encapsulation is direct observation of the optical properties of the electronic component. In OLEDs for example, observation of degradation patterns under the microscope can be carried out to assess the aging of the OLED. However, visual inspection methods are simplistic in nature and can provide only qualitative information about the encapsulated device, but is of limited use for accurate quantitative studies. In a factory line where tests are carried out on large numbers of newly fabricated OLEDs, visual inspection would be highly time consuming as well. Visual inspection is furthermore also not feasible once the OLED is integrated into a display panel.
In the past, some attempts have been made to integrate gas permeation sensors into encapsulation packages. U.S. Pat. No. 3,943,557 describes a semiconductor package with a hermeticity detector integrated into the semiconductor package. The hermeticity detector comprises a set of interdigitated electrodes that are spaced apart by a layer of cobalt oxide. In the absence of moisture, no current passes between the interdigitated electrodes. In the presence of moisture, however, the resistivity of the cobalt oxide reduces significantly, thereby shorting the interdigitated electrodes to trigger an alarm to indicate the ingression of moisture.
German Patent Application Publication No. DE 102 08 767 describes a measuring device for determining the permeability of a material that is used to form an encapsulation. The measuring device comprises a layer of corrosion sensitive material present in the encapsulation for electrical resistance measurements. As the corrosion-sensitive metal is degraded by oxygen/moisture entering the encapsulation, its electrical resistance increases. Moisture within the encapsulation is detected by monitoring the changes in electrical resistance in the corrosion sensitive element.
Despite these developments, limitations in integrated sensors in encapsulated packages still exist. In particular, the problem of sensors not having sufficient sensitivity to detect gas permeation at low levels of 10−4 g/m2/day or better still remains. Continuing efforts are therefore needed to improve their reliability and accuracy.
Accordingly, an object of the present invention is to provide an encapsulated device having at least one gas permeation sensor enclosed within the encapsulation for measuring the permeation of gas into the encapsulation.