Chapter 1: Introduction
1.3 Organization of this Thesis
Besides this chapter, this thesis is divided into the following chapters.
In Chap. 2, we describe the process flow of fabricating the n-chanel poly-silicon
TFTs. Also we present the measurement setup.
In Chap. 3, we show the response of poly-silicon TFT gas sensors to the
environments with various pressure and/or various gas compositions. The change of
electrical characteristics to different environments and the effect of structural
parameters are recorded and discussed. Results of nano-wire gas sensors are also
presented and addressed.
Finally, we conclude this thesis in Chap. 4 by summarizing the experimental results
and major findings.
Chapter 2:
Device Fabrication and Measurement Setup
2.1 Fabrication of Planar Poly-Si TFTs
The poly-silicon TFT gas sensors were fabricated on 6-inch p-type (100) Si wafers
with resistivity of 15~25Ω-cm and the thickness of 655~695μm. Note that the
wafers will serve as the back-gate of the completed devices. A wet oxide layer with
thickness of 500 Å serving as gate dielectric first grew on poly-silicon wafer. And
then a poly-Si layer with thickness of 1000 Å was deposited on the wet oxide. The
poly-Si was heavily doped with P+ at energy of 15 keV and dose of 5x1015 /cm2.
Afterwards, source/drain (S/D) photoresist patterns were formed on the poly-Si layer
by a standard lithography step, followed by a plasma etch step to form S/D (Figure
2-1). Second poly-Si layer was subsequently deposited on the wafer to serve as the
channel layer. In the fabrication, the poly-Si layers were deposited twice to form the
S/D and channel separately, so the thickness of the channel can be modulated flexibly.
In this regard, 4 different channel thicknesses, namely, 70 Å, 300 Å, 500 Å, 1000 Å,
were explored in this work. Channel pattern was then formed with another standard
lithographic and etch steps (Figure 2-2).
2.2 Nano-wire Gas Sensor Device Structures and Fabrication [16]
The nano-wire field effect transistors (NWFETs) were also fabricated on 6-inch
p-type wafers capped with a nitride layer. Figure 2-3(a) and (b) show the top (layout)
and cross-sectional views (along the A – B direction in Figure 2-3(a)), respectively, of
the device, dubbed as device A.
As shown in Figure 2-3, the Si wafers were first capped with a 100 nm-thick nitride
layer. Then, a 100nm-thick oxide layer was deposited by LPCVD. After deposition of
the oxide layer, standard photolithographic and etch steps were employed to form the
oxide dummy structures. Subsequently, a 100 nm-thick amorphous-Si layer was
deposited and then annealed at 600°C for 24 hours in N2 ambient to transform it into
poly-Si. Afterwards, the source/drain (S/D) was doped with phosphorus ion
implantation with a dose of 5E15 cm-2. After the generation of S/D photoresist
patterns with a lithographic step, a reactive plasma etching step was performed to
form the S/D regions. Using the anisotropic sidewall-spacer etching process, poly-Si
NW channels were formed at the sidewalls of the oxide dummy structures during the
S/D etching step. By precisely controlling the etching time, the cross-sectional
dimensions of poly-Si NW channels could be scaled down to sub-100 nm scale.
Subsequently, all devices were then covered with a 200-nm-thick TEOS oxide
passivation layer. Finally, the poly-Si NW was exposed by a 2-step dry/wet etching
process. Note that in the etching steps the wet etching rate of TEOS oxide should be
carefully adjusted so that an oxide layer would remain on source/drain regions while
the channels were exposed. This would prevent the conduction from source to drain
via the test solution.
2.3 Gas Sensing Measurements
The electrical characteristics of devices were measured in different environmental
conditions (pressure and composition), including vacuum (2x10-1 torr), nitrogen,
ammonia, steam, mixture of ammonia and nitrogen, mixture of nitrogen and steam,
and so on. For this purpose, a measurement system schematically shown in Figure 2-4
was constructed. In the system a closed chamber is used to house the test devices. A
pump connected to the chamber is used to pump out the air in the chamber and the
pressure as low as 2x10-1 torr can be achieved. After pumping, various gases (nitrogen,
ammonia, and steam) can be injected into the chamber to vary the chamber gas
composition. Note that nitrogen and ammonia are stored in the steel cylinders, while
the steam (vaporized H2O) is stored in sampling bags. Flow of the gases is controlled
by valves. The electrical characteristics are measured by Keithley4200 with setup
measurement software, Interactive Characterization Software (ICS). This
measurement system is owned by Laboratory of Enzyme and Protein Engineering,
Department of Biological Science and Technology, National Chiao Tung University.
In this study, Id-Vg and Id-time measurements were constantly performed. For the
Id-Vg measurements, Vd was fixed at 0.5 V while Vg was kept at low values in order
not to cause stability and reliability issues on the test devices. For the Id-time
measurements, first we probed the basic electrical characteristics of the device and
chose an appropriate gate voltage in the subthreshold region to ensure obvious change
if the varied environment indeed affects the device. In the process the drain voltage
was also kept at 0.5 volt.
Some major parameters of the transistors, including threshold voltage (Vth) and
subthreshold swing (SS), are defined below [17].
The threshold voltage:
Vth = VFB – (Qd+qD)Cox + 2ΦF (2-1).
In the expression, Qd (Coul/cm2) is the effective charge stored in the depletion region
of channel when MOSEFT is turned on, Cox is the capacitance of gate oxide per unit
area, D is the doping concentration of the channel, and VFB is the flat-band voltage
with the following relation:
VFB =ΦMS – Qi/Cox (2-2),
ΦMS =ΦM -ΦS, (2-2a),
where ΦM is the work function of gate material ,ΦS is the work function of the
channel material, and Qi is the charge in oxide that includes mobile ions, trapped
charge, fixed charge, and charge in interface of oxide and silicon. ΦF is the silicon
bulk potential:
ΦF = ln(p/ni) x (kT /q) = ln(ni/n) x (kT/q),
where kT/q is thermal energy, k is Boltzmann constant, T is absolute temperature, q is
the electron charge, p is the concentration of holes at thermal equilibrium, n is the
concentration of electrons at thermal equilibrium, and ni is the intrinsic concentration
of electrons and holes.
The on-current in the linear region (Vg > Vth, Vg-Vth > Vd ) can be expressed as
Id = μCox x (W/L) x [(Vg - Vth) x Vd – Vd2/2] (2-3),
where μ is the carrier mobility, W is the channel width, and L is the channel length.
The on-current in the saturation region (Vg > Vth, Vg-Vth < Vd ) can be expressed
as :
Id = μCox x (W/L) x (Vg - Vth)2 x 1/2 (2-4).
Definition of subthreshold swing (SS) is :
(2-5).
These electrical parameters can be obtained from the Id-Vg curves at Vd of 0.5V.
Chapter 3 Electrical Characteristics of Back-Gate TFT Gas Sensor
3.1 Basic Electrical Characteristics of Devices with Different Channel Structure under Atmosphere
The electrical characteristics of NWFET and back-gated planar TFTs with channel
thickness: 70Å, 300Å, 500Å and 1000Å are shown in Figure 3-1. The measurements
were done under atmosphere and at room temperature. For the planar devices the
drain current increases with increasing channel thickness in general. The device with
70Å channel thickness exhibits the lowest on and off current. This phenomenon is
attributed to the high resistance of the ultra-thin channel. However, the device with
70Å thickness channel also depicts slightly better subthreshold swing (SS) over the
remaining planar devices. This implies that device with thinner channel has better
gate controllability.
By comparing the transfer characteristics of NWTFT with those of the planar
devices, as shown in Figure 3-1, clearly the NWFET exhibits much improved
performance in terms of higher on-current, larger on/off current ratio, and steeper SS.
Part of the improvement can be attributed to nanowire’s high surface/volume ratio.
However, some hydrogen species are suspected to have been contained in the poly-Si.
These H species come from the underlying SiN (see Figure 2-3) which was deposited
with H-related reaction gas (e.g., SiH4 and NH3), thus abundant of hydrogen atoms
are incorporated in the SiN film [18]. Portion of the H species diffuse into the poly-Si
nanowire and passivate the defects existing therein in the subsequent process steps,
thus the device shows improved performance.
We can speculate reasonably that there is a conducting path from the source to
drain near the film surface (Figure 3-2) in addition to the nominal conduction channel
manipulated by the back gate near the oxide/film interface. The surface conduction
path is significantly affected by the environment as there is no dielectric passivation,
and less affected by the back gate unless the channel film is sufficiently thin. Below
are the results of a series of experiments carried out to examine the above postulation.
3.2 The Effects of Measurement Environment on Poly-Si TFTs
3.2.1 Device Characteristics in Vacuum and in Nitrogen AmbientTypical 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.
3.3 Effects of Ammonia
In this experiment, the device was first tested in normal ambient. Then the
device was put into the chamber and the chamber was pumped down to 0.2 Torr.
After the second measurement performed in vacuum, the nitrogen was injected into
the chamber to increase the pressure to 1 atm, and then the third measurement was
performed. Afterwards, 1.5 ppm ammonia (NH3) was injected into the chamber.
Finally, the fourth measurement was performed.
The results are shown in Figures 3-12(a) to (d) for devices with different channel
thickness. Also the results of Id-time measurement performed on a TFT with 300 Å
channel thickness are shown in Figure 3-13. Basically the results shown in Figures
3-12(a)~(d) are the same as those shown in Figures 3-3(a)~(d) at the first three stages
measured in normal air, vacuum, and N2 ambient. As the ammonia is introduced, no
obvious effect on the Id-Vg curve is observed for all the test devices regardless of the
channel thickness. Figure 3-13 also shows similar trend. The observation implies the
1.5 ppm ammonia does not result in significant passivation effect as the moisture does
to recover the device characteristics.
In [21] it was mentioned that ammonia (NH3) might dissociatively absorb at 300
K to form SiH and SiNH2 on the surface of Si, as shown in Figure 3-14. Though slow
thermal decomposition of NH2 to N and H2 occurs even at 320 K, most Si-NH2 is
stable up to 630 K [22]. However, owing to the trace amount of NH3 presenting in the
ambient, these H-related species cannot exhibit significant passivation effect on the
device characteristics.
3.4 Effects of Moisture and Moisture/Ammonia Mixture
Here we first examined the response of device characteristics to the injection of
moisture and ammonia. In this test the former three stages of measurements were the
same as those described in former section. Then the humidity was injected into the
chamber until the moisture content in the chamber reached 42.6 % and the fourth
measurement was performed. Finally, 1.5 ppm ammonia was injected into the
chamber before the fifth measurement was performed.
The results are shown in Figures 3-15(a)~(d) for devices with different channel
thickness. Since the effects of vacuum and N2 ambient have been addressed in former
sections, here we just discuss the effect of moisture and moisture/NH3 mixture. As
can be seen in the figures, the input of moisture can indeed improve the device
characteristics. In [23], it was mentioned that in a H2O molecule the oxygen end tends
to bond with a Si-Si dimmer on Si surface, whereas the hydrogen ends tend to attach
to dangling bonds within the neighboring row of dimmers. The bindings, Si-H and
Si-OH, can passivate the defects in poly-Si (Figure 3-16). However, as mentioned
above, such passivation is limited to the region closest to the surface of poly-Si,
therefore the characteristics cannot fully recover.
One interesting phenomenon observed in Figures 3-15(a)~(d) is the dramatic
increase in the drain current as the ammonia is mixed with the steam in the
environment. In [24], the extent of the hydrolytic reaction in wet ammonia: H2O +
NH3 -> NH4+ + OH-, was mentioned (Figure 3-17). Formation of NH4+ can get more
OH- dissociated from H2O and thus the passivation effect can be greatly enhanced as
compared with ammonia or moisture alone.
However, some remarks on the effect should be given. First, the dependence
between the extent of drain current enhancement and the channel thickness is not the
same as that illustrated in Figures 3-3(a)~(d). From Figure 3-15(d), the characteristics
measured under moisture/ammonia mixture have higher off-current than the original
curve. Moreover, in the figures the SS for the condition is larger than the original
transfer curves. It is obvious that most of the conduction current pass through near the
top surface of channel (Figure 3-2) and thus weaken the controllability of back-gate.
From this observation, we conclude that the passivation mechanism of
moisture/ammonia mixture mainly occurs in the top surface region of the poly-Si
channels.
Next we investigate the interaction of ammonia with the moisture in the normal
air and the resultant impact on device performance. In the testing of Figure 3-18, we
made first device measurement under atmosphere. After closing the test chamber we
injected ammonia into the chamber directly without vacuum process and then
performed the second measurement. Finally we opened the chamber and made the
third measurement after 5 min the ammonia dispersed away from the chamber (5
minutes later from opening the chamber).
In Figures 3-18(a)~(d), as NH3 is injected, not only the Vth but also the SS
reduces dramatically, unlike the situation shown in Figures 3-15(a)~(d). As mentioned
in Sec. 3.1, the fresh devices contain an amount of H-related species. It is speculated
that those H-related species could accelerate the penetration of ammonia and thus the
passivation is not restricted to the regions near the surface. In other words, the region
close to the oxide/channel interface is also passivated by the ammonia even when the
film is thick. From the above results and discussion, we infer that the moisture in the
air reacts with ammonia and results in the enhancement of defect passivation. For the
air reacts with ammonia and results in the enhancement of defect passivation. For the