Electrical response of polycrystalline silicon thin film transistor on

steel foil under mechanical strain

Po-Chin Kuo, Abbas Jamshidi-Roudbari and Miltiadis K. Hatalis

Electrical and Computer Engineering, Lehigh University, Bethlehem, PA, 18015

ABSTRACT

This work investigates the effects of mechanical strain on electrical characteristics of

polycrystalline thin film transistors (poly-Si TFTs). Poly-Si TFTs were fabricated on steel foil

substrate and characterized under the strain ranging from -1.2% to 1.1% induced by bending.

The electron mobility increased under tensile and decreased under compressive strain while that

of the hole exhibited an opposite trend. For p-channel TFTs the normalized threshold voltage and

subthreshold slope were a function of strain. In both n- and p-channel TFTs the off current

decreased under tensile while it increased under compressive strain. The observed mobility and

off current trends in poly-Si TFTs under strain are similar to those reported in single crystalline

silicon devices.

INTRODUCTION

Mechanically flexible electronic systems have the potential to realize novel applications and

thus have recently attracted a lot of research interest. Most of the reported flexible electronics are

based on amorphous silicon thin film transistor (a-Si:H TFT) technology because this technology

is deemed more readily compatible with flexible polymer substrates. However, the low carrier

mobility in amorphous silicon and the poor reliability of amorphous silicon TFTs may limit the

applications of this technology. Polycrystalline silicon thin-film transistor (poly-Si TFT)

technology on steel foils has been successfully demonstrated as a platform for high performance

flexible electronics [1] because of the high electron and hole mobility that enables CMOS circuit

architectures and the compatibility of this technology with high processing temperatures. High

resolution flexible displays have also been realized by this technology [2].

One of the most important features of any flexible electronic system is its ability to be

deformed. However, any form of deformation induces mechanical strain to the electronic thin

film devices and circuits fabricated on the flexible substrate. Therefore it is necessary to

understand the effects of mechanical strain on the electrical characteristics of thin film devices.

There are extensive studies over a-Si:H TFTs [3, 4] and also organic TFTs [5] on flexible

substrate under mechanical strain. The relevant work on poly-Si TFTs was lacking thus in our

earlier effort we investigated the effect of tensile strain on mobility of poly-Si TFTs [6]. To make

the work more complete in this paper we report the mobilities and characteristics of both n-

channel and p-channel poly-Si TFTs under both uniaxial tensile and compressive strain induced

by bending.

EXPERIMENT

We fabricated arrays of poly-Si TFTs on a 100 µm thick, type 304, 150 mm in diameter

stainless steel substrate. The polished steel wafers had a surface roughness (Ra) of only 10 nm.

The metal foil wafers were coated with a 3 µm thick PECVD SiO

passivation layer on both

sides in order to electrically isolate the steel substrate. The active semiconductor layer was 50 nm

of a PECVD deposited amorphous silicon film which was laser crystallized by using the

sequential lateral solidification (SLS) method [7]. After patterning the silicon active layer, a 50

nm thick PECVD SiO

layer and a 200 nm thick LPCVD amorphous silicon layer were

deposited as the gate dielectric and the gate electrode layers respectively. The gate electrode

layer was then doped n+ by ion implantation of phosphorus, patterned, and dry etched. The

source and drain regions of n-channel TFTs were then implanted with phosphorous, while those

of p-channel TFTs were implanted with boron. After implantation, an activation anneal at 650 °C

was performed, and then the source, drain and gate regions were silicided using nickel. After

deposition of a PECVD SiO

passivation layer, a 300 nm thick aluminum and a 50 nm thick

nickel was deposited and patterned using lift-off. Finally, a post metallization anneal at 350 °C

followed by a plasma hydrogenation anneal at 300 °C were performed. A cross-sectional view of

the indented poly-Si TFT structure is shown in figure 1. Initial characteristics of TFTs having a

channel width of 16µm and channel length of 8 µm are shown in Table I. Samples being 20 mm

by 35 mm were then cut from the steel wafers for performing the bending experiments.

We applied tensile as well as compressive strain on poly-Si TFTs by bending the sample

outward and inward to fit the outer and inner surfaces of cylindrical tubes with various radii. To

be accurate for strain calculation, the sample was well constrained to conform to the surface of

the cylinder. The strain ε, was then calculated from the relationship ε = z/R, where R is the

bending radius and z is the distance from the neutral plane to the active layer of the TFT. In our

samples the neutral plane is approximately located in the middle of the stainless steel foil

substrate because the TFT layers are thin (~1µm) compared to the steel foil substrate (~100µm)

and the Young’s modulus of stainless steel is larger than that of the thin film materials.

The equivalent uniaxial strain resulting from the bending was in the range from -1.2% to

1.1% and this strain was parallel to the TFT channel direction in this experiment. Figure 2 shows

the configuration of the experimental setup. Transfer characteristics were measured, at a drain to

source voltage of Vds= 0.1 V for n-channel and Vds= -0.1 V for p-channel TFTs, at each

bending radius from the same TFTs. Field effect mobility, threshold voltage and subthreshold

slope were then extracted from these measurements.

Table I. Poly-Si TFTs characteristics (W/L=16µm/8µm)

Figure 1. Poly-Si TFT structure Figure 2. Characterization of poly-Si TFT under

on stainless steel foil tensile (left) and compressive (right) strain

Type

Mobility

(cm

/Vs)

(V)

Subthreshold slope

(dec/V)

n-channel 310 -0.03 4.9

p-channel 105.8 -2.75 3.8

RESULT AND DISCUSSION

Figure 3 shows the transfer characteristics of n-channel and p-channel poly-Si TFTs for

various tensile and compressive strains. As it can be seen by comparing figure 3(a) and 3(b), the

on current increases while the off current decreases in the n-channel TFTs, whereas both on and

off currents decrease in the p-channel TFTs as the applied tensile strain increases. However, all

these trends are reversed under compressive strain as shown in figure 3(c) and 3(d).

The observed off current trends are similar to those reported for the single crystalline silicon

MOSFET devices and which were attributed to the change of silicon band gap under strain [8, 9]

– the energy bandgap increases under uniaxial tensile and decreases under uniaxial compressive

strain. In poly-Si TFTs the thermionic emission is the dominant mechanism for the off leakage

current [10] at low drain bias. The change of silicon bandgap would affect the barrier height

which carriers in grain boundary traps would have to overcome in order to contribute to TFT off

current. As a result for both n- and p-channel TFTs the off current decreases under tensile and

increases under compressive strain.

0.0E+00

5.0E-06

1.0E-05

1.5E-05

2.0E-05

2.5E-05

3.0E-05

1.0E-13

1.0E-12

1.0E-11

1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

-5 0 5 10

(A)

(V)

Strain= 0.95%

Strain= 0.21%

Strain= 0%

(a)

0.0E+00

1.0E-06

2.0E-06

3.0E-06

4.0E-06

5.0E-06

6.0E-06

7.0E-06

1.0E-12

1.0E-11

1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

-10 -5 0 5

(A)

(V)

Strain= 0.95%

Strain= 0.21%

Strain= 0%

(b)

0.0E+00

5.0E-

1.0E-

1.5E-

2.0E-

1.0E-12

1.0E-11

1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

-5 0 5 10

(A)

(V)

Strain= 0%

Strain=- 0.56%

Strain=- 0.95%

(c)

0.0E+00

1.0E-06

2.0E-06

3.0E-06

4.0E-06

5.0E-06

6.0E-06

7.0E-06

8.0E-06

1.0E-11

1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

-10 -5 0 5

(A)

(V)

Strain= 0%

Strain=- 0.56%

Strain=- 0.95%

(d)

Figure 3. Transfer characteristics of poly-Si TFT with (a) n-channel (b) p-channel under tensile

strain and (c) n-channel (d) p-channel under compressive strain

Figure 4(a) and 4(b) show the normalized carrier mobility for n-channel and p-channel poly-

Si TFTs respectively as a function of applied strain. As it can be seen under tensile strain the

electron mobility increases by 20% while that of holes decreases by 10% as the strain increases

from 0% to 0.5%; a further increase in the magnitude of applied strain results in a saturation in

the change of both electron and hole effective mobilities. Under compressive strain the effective

electron mobility decreases by 50% while that of holes increases by 20% monotonously as the

strain increases from 0% to -1.2%. The effect of strain on mobility is more pronounced for n-

channel poly-Si TFTs especially under compression.

In single crystalline silicon MOS devices, the electron and hole mobilities under uniaxial

strain have been extensively studied [8, 11-15] because of the significance of strained silicon

technology to improve performance of nanoscale silicon MOS devices. It was noted that strain-

induced band splitting affects carrier repopulation and suppresses the scattering of carriers.

Furthermore the carrier transport is also affected by a change in the carrier effective mass which

is induced by strain. As a result the mobility of electrons is enhanced under uniaxial tensile

strain while reduced under uniaxial compressive strain parallel to the channel; however, these

trends tend to be reversed for holes. The electron mobility saturation at higher level of tensile

strain is due to the saturation of band splitting effects [13]. Since the poly-Si thin film material,

particularly when it is prepared by the SLS method, is composed by large crystalline silicon

grains, we can infer that the change of electron and hole mobilities under strain in our

experiment is related to the aforementioned effects present in single crystalline silicon devices.

0.4

0.6

0.8

1.2

1.4

1.6

-1.2 -0.8 -0.4 0.0 0.4 0.8 1.2

µ/µ

Strain (%)

n_W16L8

n_W32L8

n_W64L8

(a)

0.4

0.6

0.8

1.2

1.4

1.6

-1.2 -0.8 -0.4 0.0 0.4 0.8 1.2

µ/µ

Strain (%)

p_W16L8

p_W32L8

p_W64L8

(b)

Figure 4. Normalized mobility as a function of strain for (a) n-channel and (b) p-channel poly-Si

TFT

As shown in figure 5(a), in p-channel TFTs the normalized threshold voltage decreases

under tensile and almost remains unchanged under compressive strain; however, for n-channel

TFTs the effect of strain on the change of threshold voltage is not obvious. Figure 5(b) shows the

normalized subthreshold slope of p-channel TFTs which increases under tensile while decreases

under compressive strain. It is interesting to note that this phenomenon is more pronounced in

smaller channel length devices. No obvious trend was found for that of n-channel TFTs under

strain. All the tested poly-Si TFTs remained functional under the applied strain that ranged from

-1.2% to 1.1%.

0.7

0.8

0.9

1.1

1.2

1.3

-1.0 -0.5 0.0 0.5 1.0

th (V)

Strain (%)

p_W16L4

p_W16L8

p_W32L8

(a)

0.6

0.8

1.2

1.4

-1.0 -0.5 0.0 0.5 1.0

S/S

Strain (%)

p_W16L4

p_W16L8

p_W32L8

(b)

Figure 5. Normalized (a) threshold voltage and (b) subthreshold slope of p-channel poly-Si TFT

as a function of applied strain

CONCLUSIONS

We fabricated n-channel and p-channel poly-Si TFTs on stainless steel foil and investigated

the effects of uniaxial strain on the electrical characteristics of these devices. It was observed that

the effective electron mobility increased under tensile and decreased under compressive strain

while that of holes behaved in an opposite way. The effect of strain on mobility is more

pronounced in n-channel poly-Si TFTs especially under compression. For p-channel poly-Si

TFTs the normalized threshold voltage decreased under tensile strain but remained unchanged

under compressive strain, and the normalized subthreshold slope increased under tensile strain

but decreased under compressive strain. No obvious trends were observed for the threshold

voltage and the subthreshold slope of n-channel poly-Si TFTs either under tensile or compressive

strain. The off current for both n- and p-channel poly-Si TFTs decreased under tensile while

increased under compressive strain. The change in the poly-Si TFT device characteristics caused

by the mechanical strain should be taken into account during the design of flexible electronic

circuits when based on this technology. It is important to note that all TFTs remained functional

under the entire range of applied strain from -1.2% to 1.1% with no obvious physical damage

observed. This shows that the poly-Si TFT technology on stainless steel foil is suitable for

flexible electronics application.

ACKNOWLEDGMENT

The authors would like to acknowledge the financial support of this work from ARL, NASA,

The FlexTech Alliance and the Sherman Fairchild Foundation. The authors also wish to thank

Dr. Tolis Voutsas from Sharp Laboratories of America for his assistance with the SLS

crystallization process.

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