Enhanced defect generation in gate oxides of P-channel MOS transistors in the presence of water

Massengill and other RER faculty members for introducing me to concepts in microelectronics and radiation effects through their courses and out-of-class discussions. Low-frequency noise generated in electronic devices is a key problem in analog circuits and systems, as it places a limit on the lower limit of the amplitude range of signals that can be detected and processed in circuits. In particular, there has been a long-standing debate as to whether the 1/f noise is dominated by number fluctuation noise due to gate oxide traps or bulk mobility fluctuations.

The background noise spectrum is mainly due to the random thermal motion of the charge carriers in the channel, preamplifier noise, and the 60 Hz line frequency. As we saw above, low-frequency noise in MOS devices is due to the interaction of charge carriers in the channel with traps in the oxide, so as radiation creates more traps, the low-frequency noise properties will also change. He suggested that during the first stage, the holes created by the radiation release hydrogen ions in the SiO2 mass.

In most MOS devices, the interfacial traps above the mid-gap Si energy are acceptor-like, while those in the lower half of the bandgap are donor-like. In this work, we studied the effects of exposure to moisture at elevated temperatures on radiation-induced charge accumulation in the oxides of MOS transistors using current-voltage measurements and low-frequency noise measurements, as will be discussed in the following sections of this thesis. The MOS transistors tested were manufactured in the 1980s using Sandia 4/3-μm technology [31] and came from two different production lots, G1916A and G1928A.

In this technology, the radiation-induced charge build-up in the gate oxides normally dominates the radiation response.

Figure 1. A typical noise waveform is illustrated [1].

Current-Voltage Measurements

All device terminals are connected to four SMUs of an Agilent 4156B semiconductor parameter analyzer. To derive the threshold voltage, VT, for MOSFETs, the drain bias is held constant at ±0.1 V (depending on whether the transistor is n-channel or p-channel), while the gate bias is swept below threshold in reverse by small steps. The threshold voltage can be derived by drawing a tangent to the linear portion of the ID-VG curve using the parameter analyzer.

The discontinuity that this tangent makes on the VG axis is then rectified by VD/2 to obtain the threshold voltage. After deriving the preradiation threshold voltage values for the control and moisture-exposed transistors, the MOSFETs were characterized by 1/f noise measurements. Some of them were then irradiated using a 10-keV ARACOR Model 4100 X-ray irradiator at a dose rate of about 31 krad(SiO2)/min to study how the oxide traps and interface traps that were created change the noise characteristics with low frequency. and current-voltage characteristics of MOS transistors.

The shifts in threshold voltages due to oxide traps and interface traps can be determined using charge-separation techniques as described below. Here, Cox is the oxide capacitance per unit area and ΔNot is the areal density of oxide traps, projected onto the Si/SiO2 interface. The center-gap method requires the extrapolation of the subthreshold region of the I-V curve to the center-gap current.

Here, φs is the band bending at the surface, given by φs = (kT/q)ln(Na/ni) at midgap, Na is the channel doping, ni is the internal carrier concentration, LD is the Debye length, β = q /kT and Cm = μeff (W/2L), where μeff is the effective mobility. Using these values, the midgap current is determined and the I-V curve is extrapolated to where it intersects the midgap current to obtain the VMG.

Low Frequency Noise Experiments

For a measurement bandwidth of 1 kHz, the high-pass filter of the preamplifier is set to approximately 1 kHz. There are larger voltage shifts due to interface trapped charge, ΔVit and oxide trapped charge, ΔVot. This is in sharp contrast to the n-channel transistors, where both the HAST-exposed and the non-HAST n-channel transistors showed comparable voltage shifts.

If the values of the power spectral density at f = 10 Hz from each trace of Figure 12 as a function of Vd2, according to the number fluctuation model, the power spectral density of drain-voltage fluctuations associated with capture and release of charge carriers is expected to vary as. Therefore, it is expected that the slope of a log-log plot of SVd vs.Vd2, as in Figure 13(b), will be close to unity. b) Best linear fit of the data shown in (a). The "Pre" and the "Post" values of the normalized noise level K-value for both the control and the HAST-exposed transistors before and after irradiation are shown below.

This relatively small change (less than a factor of 2 change in noise magnitude with HAST exposure) is typical of results observed for other nMOS transistors from all four groups. Similar increases in noise were observed for pMOS transistors from all studied groups exposed to HAST, both before and after irradiation. Water molecules first diffuse into the field oxide (FOX) (in the shaded region) and then diffuse into the gate oxide laterally (as shown) from the FOX regions, as shown schematically in Figure 18. This is a generic representation of a MOS transistor surrounded by a field oxide, and not the actual diagram of the devices used in this study for which a similar diagram is not available.).

Trapping the holes in O vacancies leads to an increase in the oxide captured charge. Phosphorus forms a stronger complex with H2O in SiO2 because the H in the resulting -P-OH bonds forms hydrogen bonds with the O of the Si-O ring network. This hydrogen bond is not visible in the B-OH complex due to the different polarities of B-O and P-O bonds, as shown in Figure 20.

We found that Sandia 4/3 µm pMOS transistors are much more affected by moisture exposure than nMOS transistors. The magnitudes of n-channel MOS gate oxide voltage shifts for transistors exposed to moisture at elevated temperatures and then irradiated with 10 keV X-rays are consistent with previous work. The increased vulnerability of pMOS transistors can be partially attributed to the absence of phosphorus in the field oxide regions of pMOS transistors, which allows for greater moisture diffusion and defect generation in these gate oxides than in nMOS transistor oxides.

More detailed studies need to be done to identify the specific microscopic origin of the defects causing the noise. This is an example of the code written in C to control Stanford Research Systems. Svensson, “The defect structure of the Si-SiO2 interface, a model based on trivalent silicon and its hydrogen bonds,” in The Physics Of SiO2 and its Interfaces, S.

Chen, “The effects of hydrogen in hermetically sealed packages on the total dose and dose rate response of bipolar linear circuits,” IEEE Trans.