WU Zu-mou, BAI Jin-chao, DING Xiang-qian, LIU Ming-xuan, LI Xiao-long, WANG Xun, LIU Hai-peng, SONG Yong-zhi, CHEN Wei-tao

(Beijing BOE Display Technology Co., Ltd., Beijing 100176, China)

Abstract: The mechanism of defects on Cu process along with the development of new product is studied in this paper, and improvement methods are proposed. The results show that the high flow of O2 used in post ashing corrodes the Cu in PVX1 (The First passivation Layer) etch, and makes the Cu hole blackened. After adding the H2 treatment step, CuO can be reduced to Cu, and the hole appears metallic white color during CD (Critical Dimension) measurement. During 1ITO (The First Indium Tin Oxides Layer) etch process, high temperature annealing results in the occurrence of serious oxidation of exposed SD Cu. Skipping anneal or changing mask design can avoid the defect. In the VIA etch step, high content of O2 oxidizes the Cu, resulting in the break of conduction between gate and SD through the hole which leads to the occurrence of vertical line Mura. Reducing the content of O2 of VIA etch can solve the problem.

Key words: array process; Cu corrosion; Cu oxidation; black hole defect; anneal; vertical line Mura

1 Introduction

Driven by the high refresh rate and high resolution of display products, many display plants use low resistivity Cu instead of Al in the gate or SD layer, even in both layers at the same time, so as to avoid the risk of resistor-capacitance delay[1-3]. ORG (Organic) film material have many advantages such as low dielectric constant and high flatness which make it widely used in TFT-LCD[4]. when converting Al to Cu, not only the DEP (Deposition) process changes, but also mask, etch and strip processes have to change correspondingly. So the process becomes more complicated when ORG is added. In this paper, according to a HADS (High Advanced Dimension Switch) mode product with organic film development process, the defects caused by the use of Cu materials are studied and improvement methods are also given to solve the technological challenges.

2 Array Process and Defects

Fig.1 Schematic diagram of organic film product structure

As shown in Fig. 1, a typical array process of 6 mask product is described. The gate layer is firstly fabricated. Then active and SD film are deposited successively and HTM (Half Tone Mask) is used for once exposure which needs twice wet etch, twice dry etch and once ashing processes, thus presenting TFT channel structure. Finally, ORG/1ITO/VIA/2ITO (The Second ITO Layer) layers are made. The thickness of Cu in gate and SD layers are both 300 nm.

Fig.2 Defect of pixel hole in AA zone

Fig. 2 is taken on ORG FICD (Final Inspection Critical Dimension) in AA (Active Area) zone. It can be seen that the pixel hole is very black. This color is abnormal because the inside of the hole is copper and it should be as bright white as the SD line under the same vision.

As shown in Fig. 3, GOA (Gate on Array) hole is taken by optical microscope after the array substrate is fabricated. Black stain and abnormal color can be observed in the left SD hole while the gate hole on the right is clean.

Fig.3 Defect of SD hole in GOA zone

Fig.4 Vertical line Mura

From Fig. 4, we can see that vertical black line appears in monochromatic green picture when the module is lighted. The incidence of this defect is about 20%, and its distribution on array glass (2.2 m×2.5 m) is irregular.

3 Results and Discussions

3.1 Black Hole Defect

The three main steps of the ORG layer are PVX1 DEP, ORG mask and PVX1 etch (also called ORG etch). The difference from other layers is that the organic film here acts as PR (Photoresist), but remains in the array structure without being removed. ORG FICD actually reflects the form after PVX1 etch. It is well known that CuO is black. So the blackening of the hole indicates that the Cu surface inside has been oxidized. In order to clarify the step of Cu oxidation, the pictures obtained before PVX1 deposition and after ORG mask, respectively. Obviously, it can be seen that the position of the base of the hole in Fig. 5 (a) is the same as the SD line. In Fig. 5 (b), all positions except the hole are covered with a layer of PVX1 and ORG, both of which are transparent substances. Therefore, the color of all the locations where the metal is present is the same in DICD (Development Inspection Critical Dimension). Fig. 2 clearly shows that the holes are blackened, illustrating that the defect occurs in PVX1 etch.

Fig.5 Image of TFT in AA. (a)FICD before PVX1 deposition; (b) DICD after ORG mask.

Fig.6 Diagram of CuO protective film

PVX1 etch uses dry etching to remove about 100 nm of SiNx. Actually this process is divided into two steps. In the first step, SF6and O2with a gas flow ratio of 1∶2 are used to react with SiNx. As shown in the equation (1), the role of O2is to promote the dissociation of F*,from SF6so as to increase the etching rate. In the second step, the gas flow ratio of SF6and O2is changed to 1∶30. Undercuts of ORG and PVX (as shown in the Fig. 6) are avoided through the reaction between O2and organic film. Unfortunately, during this process, the high content of O2will react with the exposed Cu, resulting in a darker color at the hole shown in the equation (2):

SiNx+ 4F*→ SiF4↑ + N2，

(1)

Cu + O2→ CuO，

(2)

CuO + H2→ Cu + H2O ↑.

(3)

In order to improve this defect, the usual method is to add a gas treatment process. H2is a reducing gas commonly used in factories. After PVX1 etch is completed, array glass is placed in H2plasma (55 500 mL/min) atmosphere for 40 s and then CD is tested. By contrast, it can be seen from Fig. 7 that the hole appears bright white again just like the metal line. Furthermore, the yield of array glass after H2treatment increased by 36%. All these show that H2plasma is effective for reducing CuO and the treatment can improve the contact resistance.

Fig.7 Image of TFT in AA.(a)FICD after PVX1 etch; (b) FICD after H2 treatment.

3.2 GOA Hole Defect

In order to observe the abnormality of GOA hole, the cross-sectional picture of this area was obtained by SEM (Scanning Electron Microscope) in Fig.8. There is a thick layer of corrosive substance between 2ITO and SD, presumed that SD Cu was reacted. Fig. 9 and Fig. 10 is the picture before 1ITO DEP and after 1ITO etch respectively. According to the pictures, after making 1ITO layer, the surface of SD Cu in GOA area is tightly covered by a layer of corrosion products. While gate Cu remains unchanged because it is covered by GI (Gate Insulator) layer. It can be determined by comparison that the steps of abnormal occurrence is 1ITO layer, which includes 1ITO DEP, 1ITO mask and 1ITO etch. Experience tells us that 1ITO DEP and mask will not cause the deterioration of Cu thin film. Therefore 1ITO etch is the most doubtful step.

1ITO etch includes wet etch and anneal which can convert ITO into a polycrystalline state with low electrical resistance and high transmission rate, so the sample is validated by these two processes separately after 1ITO mask[5]. The results show that no abnormality is found in the samples after wet etch. Samples placed in oven at 230 ℃ for 30 min are shown in Fig. 11(b). There exists a layer of corrosives similar to Fig. 8. This surely serves to illustrate most clearly that the exposed SD Cu will be oxidized by high temperature heating in CDA (Clean dry Air) in anneal process. Thus resulting in the open of GOA circuit.

Fig.8 Image of section of the GOA SD hole

Fig.9 Image of Cu before 1ITO DEP; (a) SD Cu; (b) Gate Cu.

Fig.10 Image of Cu after 1ITO etch. (a) SD Cu; (b) Gate Cu.

Fig.11 Image of SD Cu in active area. (a) After 1ITO wet etch; (b) After 1ITO anneal.

Attention should be paid to the high temperature environment of Cu when ITO layer is made after SD layer in array process sequence. To solve this problem, we can skip anneal step in 1ITO etch and wait for 2ITO anneal as compensation. In 2ITO anneal, 1ITO or 2ITO covers all the exposed SD Cu, so the oxidation phenomenon caused by anneal will not happen again. On the other hand, when changing design of 1ITO mask to make all exposed SD Cu covered with PR, the remained 1ITO can protect Cu from oxidation.

3.3 Vertical Line Mura

Usually, the abnormal hole will affect the input of electrical signals, resulting in poor quality of picture[6]. In the case of vertical line Mura, gate uses Cu and SD uses Al. The hole in GOA can make the electrical conduction between peripheral circuit and data line of the display area. At the beginning, laser is used to break down the Cu hole to make the electrical conduction there as shown in Fig.12(a). The results show that the breakdown of Cu through gate layer makes the Cu/2ITO lap and Mura disappear. While the crossover hole on SD Al is broken down, the defect remains unchanged. Therefore, it can be judged that the hole on the surface of gate layer is abnormal.

Fig.12 (a) Breakdown of hole by laser in peripheral circuit; (b) Schematic diagram of the section of the gate hole.

Then, the abnormal gate Cu holes are tested by focused ion beam microscopy (FIB). As shown in Fig.13, there is no abnormal 2ITO on the surface of the hole and 2ITO lap at the climbing position with right slope angle. So 2ITO layer is conducting normally. More importantly, the slope angle of the hole is also gentle with no undercut exist. Therefore, it is suspec-ted that the surface corrosion of Cu leads to circuit breakage.

Fig.13 FIB test result of hole. (a) Top view; (b) Cross section of abnormal Cu.

Fig.14 Electrical waveform test of data line

In failure analysis, electrical waveform testing is often used to find defects. Compared with the OK sample from Fig. 14, the NG (Not Good) sample has a delay of 0.679 μs at low voltage and 0.39 μs at high voltage in the rise region of waveform. When the fall part of the waveform is compared with the OK sample, the delay of NG sample is 0.141 μs at low voltage and 0.137 μs at high voltage. In Z-inversion display mode, the date line of monochrome picture fluctuates in each frame[7]. Poor contact of data line through the gate holes can cause electric waveform delay in each frame and affect the brightness of the picture. So the vertical line Mura appears in this situation.

VIA etch is dry etch performed using plasma formed of SF6and O2. When the O2content of VIA etch is changed from 62% to 41%, the incidence of vertical line Mura can be reduced to 0%. It can be seen that when the oxygen content in gas of VIA etch is too high, the oxidation reaction of Cu causes the abnormal contact between 2ITO and Cu at the connection hole. The oxidation leads to the increase of data line RC delay and eventually the occurrence of vertical line Mura.

4 Conclusion

The oxidation and corrosion of Cu always run through the process of switching from Al material to Cu. Once the Cu thin film is made, every process step in the following must be considered to avoid its reaction with solid, liquid, gas and plasma. In PVX1 etch, adding H2treatment can reduce CuO to Cu and make the hole appear normal color in CD test. If the 1ITO layer is made after Cu layer, the effect of anneal should be considered. Cu thin film can be protected from oxidation by changing mask design or skipping anneal. In VIA etch, reducing O2content of gas can reduce Cu oxidation, thus avoiding the increase of contact resistance. With the diversification of products, the oxidation and corrosion of Cu may appear in a variety of forms. We firmly believe that these defects can be excellently solved by careful researcher.