a-IGZO TFT is a promising candidate device for an alternative to poly-Si TFTs or a-Si TFTs, because they provide better uniformity in terms of their important device parameters, including the threshold voltage and mobility due to their amorphous phase, and a high mobility (>10 cm2/Vs) is attainable with these devices even in the amorphous phase. Recently, a-IGZO TFTs have been extensively studied by various groups. However, there is little report on interface and bulk effect on device performances and reliability as separately. For investigating the interface and bulk effect, we fabricated two a-IGZO thin film transistors with different channel deposition conditions, RF and DC magnetron sputtering. Specific conditions of RF and DC sputtering are described as follows; magnetron power density of 1.4 W/cm2/2.0 W/cm2 in Ar/O2 gas ratio of (65/35)/(72/28), and the entire gas pressure was 5.0 mTorr and 3.4 mTorr, respectively.In order to characterize the channel quality, C-V curve was measured with various frequencies of (10KHz˜1MHz). RF sputtered channel has higher frequency dependency compared to DC sputtered channel. It means that RF sputtered channel has higher bulk traps in channel compared to DC sputtered channel.
Device performance was characterized through the ID-VG measurement. Electrical parameters of RF and DC sputtered devices are VT=3.5/2.7V, on-off ratio=105/08, SS=2/0.4 V/decade, and uFE= 5/11 cm2/V-s, respectively. It is thus clear that the device performance of DC sputtered device is more superior to RF sputtered device. Therefore, it can be said that the poor device performance of RF device is ascribed to insufficient channel quality, as mentioned in C-V curve.
For reliability study, we measured PBTI and ID-VG hysteresis with normalized gate stress bias and high temperature hot chuck system. Through the unchanged field effect mobility during the stress and relaxation time, and nearly recovered VT and subthreshold slope (SS) after long relaxation time, we were able to know that pre-existed trap was main factor of reliability degradation. Moreover, SS degradation during stress time is more severe in RF device than DC device. It is also proving that RF channel characteristic is worse than DC channel. In high temperature, leakage current increments of RF device were more severe than DC device. This also indicates that bulk traps of RF device in channel region are larger than that of DC device. VT shift of DC sputtered device for PBT stress and hysteresis was higher than that of RF sputtered device. As well known, PBTI is closely related to insulator bulk traps, which shows that channel deposition conditions affect insulator trap characteristics. In summary, device performance of a-IGZO deposited by DC sputtering is better than RF sputtering, which is because DC sputtering improves channel quality of a-IGZO. However, VT shift of DC sputtered device are worse than RF sputtering, which may be related with high magnetron power density.