CURRNET STATUS OF FIELD AIDED LATERAL CRYSTALLIZATION
Duck-Kyun Choi, Hyun-Chul Kim, Tae-Seok Han
Division of Materials Science and Engineering, Hanyang University, 17 Haengdang-Dong, Seongdong-Ku, Seoul 133-791, South Korea
Young-Bae Kim
Department of Physics, North Carolina State University, 2700 Stinson Drive, Box 8202, Raleigh, North Carolina 27695, U.S.A.
As a low temperature crystallization process, the field-aided lateral crystallization (FALC) technique possesses outstanding advantages such as directional crystallization, high crystallization rate and inherent gettering effect owing to the effect of electric field and can lead on to high performance transistors. It turned out that the FALC behaviors resulted from the competition between electromigration effect and the magnitude of potential gradient. In order to demonstrate the feasibility of FALC process, an array of FALC processed poly-Si TFT was fabricated. The transistor characteristics were evaluated and compared to those fabricated without an electric field.
INTRODUCTION
Low temperature polycrystalline silicon (poly-Si) has received a great deal of attention for the application in either high resolution active matrix-liquid crystal displays (AM-LCDs) or active matrix-organic light emitting diode (AM-OLED) because of its superior carrier mobility than amorphous silicon thin films.1 For the display application, the issues of poly-Si thin film are large grain size, uniformity in large area, fast crystallization and reasonably low process temperature etc. Although there are various methods to crystallize amorphous silicon (a-Si) thin film at low temperature, basically, they can be classified in two groups. One is laser assisted process and the other is thermal process. The laser assisted processes are currently leading technologies in spite of some drawbacks such as non-uniformity in poly-Si and high process cost. The metal induced lateral crystallization (MILC) is the most widely known thermal method.2 In this process, using small amount of Ni as a crystallization catalyst, one can reduce the crystallization temperature significantly than that with solid phase crystallization (SPC).3 However, MILC process also has some problems in requirements for transistor application. The crystallization rate is very low and it is only dependent on the process temperature. Above all, the major problem of MILC is the undesirable metal residue in channel area
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after the completion of crystallization which will act as a leakage path in poly-Si TFT. Therefore, we modified this process by applying an electric field during crystallization.
Over the past decade, we have investigated the crystallization behaviors of field aided lateral crystallization (FALC). In this report, we firstly like to introduce the crystallization behaviors of FALC process in terms of process temperature and electric field intensity. Then we will propose the crystallization mechanism of FALC process4. For the device application of FALC process, array of n-channel thin film transistor (n-TFT) was fabricated and evaluated on 2 inch square glass substrate. Important device characteristics such as field effect mobility, threshold voltage, on/off current ration will be discussed compare to that of MILC transistors.
EXPERIMENTAL
Amorphous silicon films with a thickness of 50 nm were deposited on Corning 1737 glass substrates at 400℃ by low pressure chemical vapor deposition (LPCVD) using disilane (Si2H6) as a source gas. In order to investigate the crystallization behavior T-shape patterns (120 ㎛ in width) were generated by using a photolithography process. Before Ni deposition, native oxide that might have been formed on the a-Si film was removed by dilute HF. Then, 2~5nm-thick Ni was deposited using a DC magnetron sputter at room temperature. Finally, Ni was selectively deposited on outside of T-shape patterns using lift off process.
A DC power supply was used to apply an electric field to a patterned substrate through electrodes formed on opposite sides of the substrate during the thermal annealing in a tube furnace. The heat treatment was performed for 5 hours in the temperature range of 400℃ ~ 500℃ in 25℃ intervals in N2 ambient to prevent oxidation of the metal catalyst or the amorphous silicon films.
Arrays of transistor patterns of W/L from 10/10um to 100/100um were defined on a-Si film by photolithography process. Active layers were formed by reactive ion etching (RIE) using SF6 and O2 mixed gases. Using via-hole photolithography, Ni layer of 2nm thickness was deposited only on source and drain regions, which was followed by phosphorus doping using same mask pattern. All the sources and drains of the transistors were connected separately to 100nm-thick Mo electrodes. The crystallization processes was performed in N2 ambient for 5 hours at 550oC with applying the electrical field intensity of 200V/cm. After crystallization, gate oxide and gate electrode were formed on channel region. In the panel, some of the pixel transistors were not connected to the electrodes so as to compare the characteristics of transistors processed by FALC and MILC.
After the crystallization process, the velocity of crystallization was calculated with measuring the crystallization distance with an optical microscope. The grain size and grain shape were analyzed by scanning electron microscope. A degree of crystallization was calculated by deconvolution between a-Si peak and poly-Si peak from the original
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Raman spectrum. The electrical characteristics were measured using HP4140B pA meter/DC voltage source.
RESULTS AND DISCUSSION 1. Crystallization behaviors of FALC
The crystallization morphologies in the T-shaped patterns for both the MILC and the FALC specimens at various crystallization temperatures were examined by using an optical microscope. As seen in the case of MILC in Fig. 1(a), the crystallization proceeded from the edge of the pattern with a uniform speed of 0.86 ㎛/h at the crystallization temperature of 500�. This conformal crystallization behavior results from the random nature of thermal diffusion and is a typical behavior in the MILC process.
Fig. 1. Optical microscope images of crystallized patterns showing the crystallization velocity
after annealing for 5 hrs at (a) 500� by using MILC and at (b) 400�, (c) 450�, and (d) 500� by using FALC with an applied voltage of 180 V/cm.
On the other hand, directional crystallization from the negatively biased side toward
the positively biased side occurred in the FALC-processed specimen due to the effect of the electric field. The crystallization velocities were measured to be 2㎛/h, 12㎛/h, and 60㎛/h when the FALC temperatures were 400�, 450�, and 500�, respectively. For the 450� sample in Fig. 1(c) where the crystallization boundary is bowed, the average of the crystallization distance was selected in calculating the crystallization velocity. Surprisingly, the crystallization velocity of the FALC sample at 400� was even faster than that of the MILC sample at 500�. At the same crystallization temperature of 500�,
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it becomes almost two orders of magnitude larger than the value for the MILC-processed one.
It is a well known fact that the Ni catalyst and the amorphous Si form NiSi2, and that this silicide phase mediates the crystallization of amorphous silicon at low temperature. Hayzeldon and others reported that the crystallization velocity of amorphous silicon is determined by the diffusivity of Ni atoms in the silicide phase, and is thus driven by the chemical activity difference.5 Therefore, the crystallization proceeds uniformly from the Ni coated area to the pattern in the metal-induced lateral crystallization. On the other hand, in FALC process, the crystallization velocity increases monotonically as the applied voltage increases to a point and then decreases beyond the critical voltage (Fig. 2).6
30 60 90 120 150 180 210
20
40
60
80
100
120
Cry
stal
lizat
ion
velo
city
(um
/hou
r)
Electric filed (V/cm) Fig. 2. Crystallization velocity as a function of electric field intensity at 450oC.
Microstructures of poly-Si are observed by scanning electron microscope and the
results are shown in Fig. 3. As shown in Fig. 3(a), FALC processed poly-Si has elongated grains up to 2um parallel to the direction of electric field. On contrast, MILC processed poly-Si resulted in smaller grain size than that of FALC and does not show directionality in grains. In addition, it is not uneasy to recognize that the degree of crystallization in poly-Si region by FALC process is much higher than that by MILC process. These results indicate that one can control the channel microstructure by adapting FALC process. Since the degradation of carrier mobility in channel is directly associated with the grain boundary scattering, it is expected that the field effect mobility can be enhanced in FALC processed poly- Si TFTs.
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Fig. 3. FE-SEM images showing the microstructures of poly-Si areas crystallized by (a) FALC process having electric field of 200V/cm and (b) MILC process under the zero electric field
2. Crystallization mechanism of FALC
Since the field-aided lateral crystallization (FALC) process is induced by electric
potential gradient as well as the chemical potential gradient, the crystallization behavior is closely related to the electric field distribution, depending on the field intensity, pattern size, shape, etc. The enhanced crystallization velocity from the applied electric field may be interpreted as a Joule heating effect. Measured temperatures on the specimen surface and the calculated temperature increases from the current flow during the crystallization process, however, reveal negligible Joule heating effect, few degree increments in temperature.
Fig. 4. Schematic drawing of the potential energy distribution of positively charged Ni ion in amorphous silicon
Under the influence of an electric field, the driving force of the Ni atoms in the silicide
phase can be schematically presented as Fig. 4. The electric field intensity determines the slope of the periodical potential curve. The atomic electron affinity of Ni and Si is 1.155
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eV and 1.385 eV, respectively.7 Therefore, it is reasonable to assume that the charge state of the Ni atom in the silicide phase is positive. Under this assumption, the Ni atoms would migrate to the negative electrode. However, the results of the experiment show the opposite behavior, implying that a stronger driving force against the potential gradient must exist. The force, which pushes the atom against the electrostatic field, is an electron wind effect caused by the electromigration. Despite the light mass of an electron, when a significant amount of electrons are present (or when the current density is high enough), the momentum exchange induces the atoms to migrate along the electron flow direction. For example, a positively charged Ni atom at the saddle point of the free energy diagram shown in Fig. 4 diffuses toward the higher potential side with the aid of the electron wind. Therefore, the apparent charge of a Ni atom could appear as negative. This is called an effective charge and is presented in Eq. (1).
φ∇⎟⎠⎞
⎜⎝⎛−∇−= *
iii
iii eZkT
CDCDJ (1)
Here, the Ji, Di, ∇Ci, k, T, e, Zi
*, and ∇φ represent flux, diffusivity, concentration gradient, the Boltzman constant, absolute temperature, electron charge, effective charge, and electric potential gradient, respectively. Two terms are associated with Zi
*: electron wind term (Zi
*wind, negative value) and drift term (Zi
*drift, positive value).
In order to prove the proposed theory, verification must be presented. The question becomes, “is there enough current flow inside the amorphous silicon pattern to cause such a phenomenon?” However, answering the question is not a simple matter because of the temperature dependence on the electrical resistivity. At room temperature, the metal silicide crystallization mediator, NiSi2 has a higher resistivity than pure Ni, and the resistivity of NiSi2 increases as the temperature increases. Meanwhile, the resistivity of amorphous silicon decreases as the temperature increases because of the exponential increase in the intrinsic carrier concentration. Hence, the resistivities of the Ni coated and Ni free areas at the crystallization temperature are quite comparable. The result of the experiment, measuring the current level variation during the heat treatment of simple test patterns, is presented in Fig. 5. As indicated in the figure, the resistivity of the Ni-free amorphous silicon was reduced by 4 orders of magnitude at 510oC. The current density under the bias of 100 V is in the range of 100 A/cm2. Ni-deposited amorphous silicon, on the other hand, increased in resistivity during the ramping due to the formation of NiSi2. The resistivity then fell off along the amorphous silicon curve due to the intrinsic carrier increase of the amorphous silicon in the Ni coated film. The resistivity further decreased during crystallization because the polycrystalline silicon formed by the metal induced crystallization (MIC) has a lower resistivity than that of the amorphous silicon. The remaining fraction of crystalline silicon in the channel region increased in size as crystallization proceeded. Such decrease in resistivity was not observed in pure amorphous silicon film.
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Fig. 5. Resistivity of amorphous silicon and Ni-deposited amorphous silicon as a function of
annealing time.
3. Devices characteristics
An optical microscope image of the common electrodes and active areas with W/L of 10/100um, 50/100um and 100/100um are shown in Fig. 6(a). Left side of each active area is connected to Mo common electrode, and right side is connected to another Mo common electrode. With common electrodes, we could apply very uniform electrical field strength of 200V/cm to each isolated transistors in array module. Fig. 6(a) shows images after crystallization and the details are presented schematically in Fig. 6(b) for the clarity. Fig. 6(c) shows the optical microscope image of a channel with W/L of 100/100um after crystallization by FALC process for 5 hours. The channel region is fully crystallized under the electric field of 200V/cm. In this image, we cannot find the marker in the channel region which appears always in the case of MILC processed due to the remnant metal impurity. This result implies that the channel area is directionally crystallized in FALC process. From the mechanism of Ni induced crystallization that grain boundary consists of Ni rich phase, the metal residue at the boundary is a major path for the large off-state leakage current in TFTs. Directional crystallization in FALC process can sweep this harmful metal residue out to either source or drain area, so one can overcome this inherent problem in metal induced crystallization. On the other hand, in case of MILC (Fig. 2(d)), the channel area is not fully crystallized in the identical process time to FALC process. This is one of the evidences that the crystallization velocity of FALC is faster than that of MILC, which is another advantage of FALC process.
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Fig. 6. (a) Optical micrograph of TFTs array connected to common electrodes, (b) schematic diagram of TFTs array and common electrode, (c) fully crystallized channel having W/L =
100/100um under the electrical field strength of 200 V/cm, and (d) partially crystallized channel under zero electric field (MILC)
Fig. 7 shows Raman spectrum of high quality poly-Si crystallized by FALC process.
Raman spectrum from poly-Si grown by post annealing of a-Si can be usually divided into two distinct peaks corresponding to an amorphous phase and a crystalline phase. The amorphous phase peak is known to be low intensity and broad peak around 480 cm-1 while the crystalline phase has sharp peak at 520 cm-1. Poly-Si peak is also reported by Tay et al. around 505.5 cm-1.8 The crystalline volume fraction of poly-Si was calculated from the integrated intensities of partial peaks, with Gaussian fits for the amorphous peak (Ia) and Lorentzian fits for the crystalline peaks (Ic). 9 Tsu et al. reported following equation for crystalline volume fraction (Xc). In this equation, γ is the ratio of the backscattering cross-sections between amorphous and crystalline phases10, and the most widely used value is 0.8.
ac
cc II
IX)(γ+
= (2)
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According to equation (2), Raman spectrum shown in Fig. 7 was deconvoluted by three partial peaks (dashed lines), and the crystallization volume fraction turned out to be 0.94. In this figure, TO band associated with amorphous phase around 480 cm-1 almost disappeared because of the high quality of FALC processed poly-Si.
Fig. 7. Raman spectrum of FALC processed poly-Si film. Dashed peaks represent crystalline peak at 520 cm-1, poly-Si peak at 510 cm-1, and a-Si peak at 483 cm-1 by the deconvolution of original
Raman spectrum, respectively
Fig. 8(a) represents the transfer curve of drain current (Ids) vs. gate voltage (Vgs) at the drain voltage (Vds) of 0.1V fabricated by FALC and MILC process, respectively. The important device parameters are listed in Table 1. Both FALC and MILC processed TFTs show large Ion/Ioff ratio over 106. The field effect mobility was calculated from the equation (2) in linear region (Vds=0.1V), and they were about 120 cm2/Vs in FALC TFT and 40 cm2/Vs in MILC TFT, respectively.
dsoxmgs
dsm V
LWC
VIg µ=
∂∂= (3)
Table 1. Device parameters of FALC TFT and MILC TFT
FALC TFT MILC TFT W/L (um) 10/10 10/10
Mobility(cm2/Vs)
120 40
Threshold Voltage
4.9 6.4
Ion/Ioff 106 106 Off-state Current 6.4X10-12 6.7X10-11
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at Vgs = -10V Higher crystalline volume fraction, directional crystallization parallel to the channel
and larger grain size in FALC processed poly-Si are attributed to the high field effect mobility. In addition, off state leakage current of FALC processed TFT is smaller than that of MILC processed TFT at -10V. This stable leakage current behavior at off-state comes from the minimal metal residue in channel area because of the directional crystallization by applying electric field during annealing process in FALC. The threshold voltage was calculated from an Ids
1/2-Vgs curve, and it reveals 4.9V in case of FALC TFT which is about 1.5V lower than that of MILC TFT.
-10 -5 0 5 10 15 201E-13
1E-12
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
FALC MILC
n-channel poly-Si TFTsW/L=10um/10umVd = 0.1V
Dra
in c
urre
nt(A
), I ds
Gate voltage(V), Vgs
(a)
0 5 100.0
5.0x10-7
1.0x10-6
1.5x10-6
2.0x10-6
2.5x10-6
3.0x10-6
3.5x10-6
Dra
in c
urre
nt(A
), I d
Drain voltage(V), Vd
at Vg
0V 2V 4V 6V
W/L = 10um/10um
(b)
Fig. 8. (a) Transfer characteristics of n-channel TFTs at a drain voltage of 0.1V fabricated by
FALC and MILC process. In FALC processed TFTs, electric field of 200V/cm was applied through common electrode during annealing (b) Ids-Vds curves for various gate biases fabricated
by FALC process
CONCLUSION
The FALC process demonstrated superiority to the MILC process in the sense that considerable degree of crystallization can be achieved at a crystallization temperature as low as 400℃. The crystallization velocity of the FALC process is 70 times larger than that of the non-electric field crystallization process, MILC. Such crystallization behaviors are attributed to the electric field and the proposed mechanism suggests that the crystallization velocity and direction are determined by competition between electron wind effect and the magnitude of electric potential gradient.
In FALC processed poly-Si, the crystalline volume fraction calculated from Raman spectrum reached 0.94. The field effect mobility at the drain voltage of 0.1V and off-state leakage current at gate voltage of -10V were superior to those by MILC process and were 120cm2/Vs and 6.4x10-12A, respectively.
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ACKNOWLEDGMENTS
This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the National Research Laboratory (NRL) program (grant number: M10400000272-05J0000-27210)
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