Analysis of the Passivation Layer

The high density is a basic property for material as passivation. A growth of film can be divided into three steps, as shown in Fig. 4-1. First, the resolution limit step, the resolution of film growth depends on the interaction between film molecules and surface molecules of substrate. Second, the diffusion limit aggregation step, where molecules diffuse on a surface and stick to any existing island without further diffusion. Third, the coarsening step, the film islands coarsen until the grains touch each other. The top view images of the SiO_x and SiN_x film are characterized by SEM, as shown in Fig. 4-2. Both of the two films are confirmed to complete the three steps of the deposition films. The morphology of the SiO_x and SiN_x film is also observed by AFM, as shown in Fig. 4-3. The surfaces roughness of the SiOx and SiNx

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film is of less than 1 nm. The flat passivation films are suitable for following top electrode process to assemble a back plane of the TFT-LCD.

(a) (b) (c)

Fig. 4-1 Growth steps of film: (a) the nucleation, (b) the diffusion, and (c) the coarsening step, respectively. [31]

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(a) SiOx

(b) SiNx

Fig. 4-2 SEM image of (a) the SiO_x film and (b) the SiN_x film deposited by sputtering, respectively.

SiO

SiN

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(a) SiOx

(b) SiNx

Fig. 4-3 AFM morphology of (a) the SiO_x film and (b) the SiN_x film deposited by sputtering, respectively.

SiN

SiO

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4.2.2 Cross-section Observation

According to the mechanism of gas permeation, the rate of gas permeation is related to the dissolubility of the surface, the rate of diffusion in the film, the thickness of the film, and the pressure difference in the two sides of film. In this experiment, the thickness of the film and the pressure difference in the two sides of film are the same. The rate of diffusion in the film is related to the density of the material. The SEM photos of the cross section of the SiOx

and SiN_x film, as shown in Fig. 4-4, can be directly used to investigate the density of the film.

Compared with the cross section photos of the SiOx and SiNx film with the same thickness, the SiO_x film clearly has more cavities. The result indicates the SiO_x film has lower density than the SiNx film. Therefore, the SiNx film has a better structure as passivation to isolate the a-IGZO film from oxygen and moisture in the ambience.

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(a) SiOx

(b) SiNx

Fig. 4-4 The cross section image of (a) the SiO_x film and (b) the SiN_x film deposited by sputtering, respectively.

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4.2.3 Moisture Content

With the cross section photo of the SiOx and SiNx films, the SiNx films has fewer cavities to be a more suitable passivation layer. To quantize the density of the SiO_x and SiN_x films, the moisture content of the three different samples were measured. The three samples are the a-IGZO film on the substrate, the a-IGZO film with SiO_x as passivation, and the a-IGZO film with SiNx as passivation, respectively. The moisture content in the three samples, as shown in Fig.4-5, shows that the a-IGZO film easy to absorb moisture and increase the interaction with moisture. Nevertheless, the a-IGZO film with the SiOx or SiNx film as passivation can reduce the moisture content. The a-IGZO film with SiN_x has the lowest value of moisture content, confirmed that the SiNx film has a higher density in structure to prevent the a-IGZO film from interacting with oxygen and moisture in the ambience.

IGZO SiOx / IGZO SiNx / IGZO

0 800 1600

m o is tu re c o n te n t (p p m )

film

Fig. 4-5 The moisture content of the three samples included the a-IGZO film, the a-IGZO film with SiOx as passivation, and the a-IGZO film with SiNx as passivation measured by Karl Fischer moisture titrator, respectively.

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4.2.4 Contact Angle Measurement

The dissolubility of the surface and the rate of diffusion in the film are the important issues for materials as passivation. The rate of diffusion in the film was studied by the cross section microphotograph and moisture content, respectively. The contact angle measurement reveals that the hydrophobic ability of the SiO_x and SiN_x films is related to the dissolubility of the surface. The hydrophobic property of the material can decrease the rate of dissolubility on the interface. The photographs of de-ion water (DI water) drop on the SiO_x and SiN_x films are shown in Fig. 4-6, respectively. The contact angle of the SiOx and SiNx films is summarized in Table 4-1. The sequence of contact angle is SiN_x ＞ SiO_x. The higher contact angle indicates a more hydrophobic property and signifies a lower dissolubility occur on the surface.

As a result, the SiN_x films are a more suitable to reduce the rate of gas permeation.

4.2.5 Transmittance

The optical energy band gap of the a-IGZO is about 3 eV that may cause the electrical instability for a-IGZO TFTs under near-ultraviolet illumination. Consequently, precaution is necessary to block the a-IGZO film from interacting with the photo energy over 3 eV. The transmittance of the SiOx and SiNx film were measured by ultraviolet-visible spectroscopy, as shown in Fig. 4-7. The SiO_x film can almost be transmitted from 200 nm to 800 nm.

Nevertheless, the SiNx film has low transmittance under 80 % when the wavelength is lower than 350 nm. According to the definition of transmittance (T) and absorbance (A):

T

Alog₁₀ , low transmittance represents high absorbance. Accordingly, the SiNx film can prevent the back channel of a-IGZO from light illumination when the wavelength is under 350 nm.

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(a) (b)

Fig. 4-6 Contact angle for DI water drop on the (a) SiO_x and (b) SiN_x film deposited by sputtering, respectively.

Table 4-1 Contact angle of the SiO_x and the SiN_x film deposited by sputtering

SiOx SiNx

CA-left(°) 35.45 52.62

CA-right(°) 35.83 52.56

200 250 300 350 400 450 500 550 600 650 700 750 800 0

20 40 60 80 100

Transmittance(%)

Wavelength (nm)

SiNx

SiOx

Fig. 4-7 The transmittance of the SiOx and SiNx film deposited by sputtering.

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