The Effects of Measurement Environment on Poly-Si TFTs

Typical characteristics of the planar TFTs with various film thickness measured

under different test environments are shown in Figures 3-3(a)~(d). In these

measurements, the device was first tested in normal ambient which had a moisture

level of 42%, and then in the closed test chamber (see the description in Chap. 2)

which was pumped down to 0.2 Torr. After the second measurement performed in

vacuum, the vacuum chamber was injected with high purity N₂ (99.999%) to increase

the pressure to around 1 atm, and then the third measurement was performed. Finally

the chamber was open to expose the device to the air again. We then repeated the

measurements to check if the transfer characteristics of the device recovered to the

original characteristics.

In the figures we can find that there is a big change in the Id-Vg curves as the test

environment is switched from atmosphere to vacuum. When measured in vacuum, the

drain current became low and insensitive to the variation of gate voltage. As N₂ was

injected into the chamber, the Id-Vg curve remained the same as that measured in

vacuum.

When the device was put back to the normal air, the I-V characteristics would

recover, but the extent depended strongly on the film thickness. As can be seen in

Figure 3-3(a), the TFT with 70Å-thick channel almost return to the original

characteristics while the other devices with thicker channel do not t recover

completely (Figures 3-3(b) ~3-3(d)).

Note that the high purity N₂ is very dry and contains less moisture, unlike the

normal air which has a relative humidity of 42 %. Thus we postulate that the above

difference in I-V characteristics is due to the existence of moisture and the interaction

between moisture and the poly-Si films. Note the moisture contains H-related species

(such as H or OH) which can help passivate the defects contained in the poly-Si [19].

The schematic illustration about the passivation of defects in the grain boundaries

with H from the moisture in the ambient is shown in Figure 3-4 (a). In the figure and

following discussion, for simplicity, we assume H is the main species responsible for

the passivation of the defects. Note that the fresh devices should contain specific

amount of H throughout the film since the last step in device fabrication was a

de-ionized water rinse after the removal of photoresist with H₂SO₄/H₂O₂. According

to the water passivation effect [14], some H species should remain in the films even

when the device was dried. At vacuum, the amount of active defects in the poly-Si

increases because the vacuum tends to drive out the H from the grain boundaries and

destruct the bonding, such as Si-H, leaving the defects unpassivated (see Figure 3-4).

The amount of dangling bonds increases massively and affects the electrical

characteristics considerably. As the high-purity N₂ is injected into the chamber such

characteristics are retained due to the dry ambient. After returning to the air, the Id-Vg

curve is restored due to the diffusion of H from the moisture contained in the normal

air. However, the concentration of H decreases with increasing depth due to the

diffusion process. The device with70 Å-thick channel restores soon due to its shallow

channel (Figure 3-5(a)). However, for devices with thicker channel it needs longer

time to recover completely. The gated channel (bottom path shown in Figure 3-2) thus

retains a high amount of un-passivated defects, as shown in Figure 3-5(b). This

explains why the characteristics cannot recover completely in Figures 3-3(b)~(d) as

the test environment returns to the normal air.

3.2.2 Id-time Measurements

In this Id-time measurement, the setup of measurement environment is the same

as that described in last sub-section. The measurement scheme is stated in Sec. 2.3,

and the Vg is set at 0.5V. In Figure 3-6, the Id of a TFT with 300Å channel thickness

initially increased as measured under normal atmosphere. To check this phenomenon

we compared the transfer curves of the device before and 60 sec after the above

Id-time measurement. The results are shown in Figure 3-7. We can see that the

threshold voltage becomes smaller while the SS remains unchanged after the Id-time

measurement. To confirm and figure out the mechanism, we made a series of

measurements performed on one another device with the results are shown in Figure

3-8. First, we measured the transfer curve of the device with 300Å channel thickness

under normal air (curve (1) in Figure 3-8). And then we made the Id-Time

measurement (Vg = 3.7V > 0V ) (curve (1) in Figure 3-9) on the device for 2100 sec,

and then the transfer curve was measured again (curve (2) in Figure 3-8). As the trend

shown in Figure 3-7, the threshold voltage of the curve (2) in Figure 3-8 shifts

leftward. And then we made Id-Vg measurement with gate voltage sweeping from 0

to -5V, as shown in Figure 3-10. After that we measured the curve 3) in Figure 3-8

and found the threshold voltage shifts slightly rightward with respect to the curve (2)

shown in the same figure. Before measuring the curve (4) in Figure 3-8, we made

another Id-Time measurement with Vg=-3.7V (< 0V) (curve (2) in Figure 3-9). The

curve (4) shifts leftward and becomes close to the original curve (1) in Figure 3-8.

From the above experimental results we postulate the instability in the transfer

curves of the test devices as measured in the normal ambient is due to the action of

mobile ions, like sodium or potassium ions, presenting in the oxide. This is a well

known issue for old MOSFET technology [20]. Since there is no passivation dielectric

covering the devices, these contamination species are likely to appear in the test

samples. As a positive gate voltage is applied for a sufficient long time, these ions

tend to accumulate near the oxide/channel interface and the threshold voltage is thus

decreased (see Figure 3-11(a)). In opposite situation as the gate bias is negative, these

ions would be attracted toward and accumulate near the oxide/gate interface, leading

to an increase in threshold voltage.

Now let’s return to discuss the results shown in Figure 3-6. While the air was

pumped out to vacuum the text chamber, the Id dropped drastically which is

consistent with the results shown in Figure 3-3(b). After the measurement

environment turned back to atmosphere, the current increased again. The air has

obvious passivation to the device.