Plasma Science and Technology, Vol.14, No.10, Oct. 2012

Dry Etching Characteristics of AmorphousIndium-Gallium-Zinc-Oxide Thin Films

ZHENG Yanbin (郑艳彬)1, LI Guang (李光)1, WANG Wenlong (王文龙)1,LI Xiuchang (李秀昌)1, JIANG Zhigang (姜志刚)2

1School of Pharmaceutical Sciences, Changchun University of Chinese Medicine,Changchun 130117, China

2The State Key Laboratory of Superhard Materials, Jilin University, Changchun 130112,

China

Abstract Amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) back-

plane technology is the best candidate for flat panel displays (FPDs). In this paper, a-IGZO TFT

structures are described. The effects of etch parameters (rf power, dc-bias voltage and gas pres-

sure) on the etch rate and etch profile are discussed. Three kinds of gas mixtures are compared

in the dry etching process of a-IGZO thin films. Lastly, three problems are pointed out that need

to be addressed in the dry etching process of a-IGZO TFTs.

Keywords: IGZO, TFT, dry etch, plasma

PACS: 52.77.Bn

DOI: 10.1088/1009-0630/14/10/11

1 Introduction

The backplane is the core part of liquid crystal dis-plays (LCDs) and active-matrix organic light-emittingdiode displays (AM-OLEDs). At present, amorphoussilicon (a-Si), low-temperature polysilicon (LTPS) anda-IGZO TFTs backplanes almost dominate the wholeFPD market. a-Si TFTs have low field-effect mobility(∼1 cm2/Vs) [1∼3]. They are insufficient to drive large-size and high-resolution AM-OLEDs. LTPS TFTs havehigh field-effect mobility (∼100 cm2/Vs) [4∼6], but theyare not uniform because of the existence of crystal grainboundaries. Moreover, they require more mask processsteps; at least 7 masks are needed to produce a back-plane and that increases the manufacturing cost [7].LTPS is formed by a-Si using metal-induced lateralcrystallization (MILC) [8] or excimer laser annealing(ELA) [9∼11] technology. Due to the introduction ofmetal ions in the process of MILC, LTPS TFTs have ahigh leakage current [12]. ELA is a mature technologyused for small-size panels, but it is not suitable for large-area glass because of the limited laser beam length andlaser beam instability. a-IGZO TFTs have high mobil-ity (∼10 cm2/Vs) [1.13∼15] and uniformity because oftheir amorphous characteristic. There is no doubt thea-IGZO TFT backplane will be the best candidate forthe next generation of displays. Many studies on thepreparation of a-IGZO have been reported [16∼18], butonly few reports on the etching of a-IGZO. The wetetching rate of the a-IGZO channel is quite fast, even

with a weak acid solution [19]. The dry etching methodis a more controllable method in the TFT manufac-turing process. Table 1 presents a comparison of dryetching and wet etching. Generally, the wet etchingprocess results in an isotropic profile. Therefore, thedry etching method is suitable for a fine-pattern de-sign. Compared to the wet etching process, dry etchinghas little impact on the environment, so it is a moreideal etching method. In this work, the effects of etchparameters, such as the coil rf power, dc-bias voltageand gas pressure, were investigated in the process ofdry etching. In addition, gas mixtures containing CH4,Cl-based etch gas and F-based etch gas were comparedin the etching of a-IGZO. Last, three problems encoun-tered in the dry etching process of a-IGZO were brieflyexamined.

2 The structure of a-IGZO TFT

There are four basic TFT structures, includingbottom-gate staggered, top-gate staggered, bottom-gate coplanar and top-gate coplanar [20]. The firsttype [21,22] is the most commonly used structure. Fig. 1displays the schematic diagram of a bottom-gate stag-gered a-IGZO TFT. As the active layer of a TFT, thepreparation of a-IGZO film and post-annealing treat-ment have important effects on the performance ofTFT, such as the threshold voltage (Vth), on-off cur-rent ratio (Ion/Ioff) and subthreshold swing (SS) [1,7].

Plasma Science and Technology, Vol.14, No.10, Oct. 2012

Table 1. Comparison of dry etching and wet etching

Items Dry etching Wet etching

Etch rate Acceptable, controllable High, hard to exactly control

Profile control Good Poor

Selectivity Acceptable, controllable High, hard to exactly control

Uniformity Good Poor

Repeatability Good Poor

Multi layer etch Possible Difficult

Critical dimension control Capable of defining small Inadequate for defining

feature size (< 100 nm) feature size < 3 µm

Chemical usage Small Large

Equipment cost High Low

Radiation damage Can not be ignored None

Fig.1 The schematic diagram of a bottom-gate staggered

IGZO TFT

3 Effects of etch parameters onetching a-IGZO thin films

The effects of etching are determined by the com-bined action of the coil rf power, dc-bias voltage andgas pressure. We investigated the effect of each param-eter on the dry etching of a-IGZO.

3.1 Coil rf power and dc-bias voltage

Fig. 2 (a) presents the etch rate of a-IGZO thin filmsetched at different rf powers. The etch rate linearlyincreased with the power increasing. This might beattributed to the increase of Ar ions and reactive radi-cals caused by the increase in the plasma density at ahigh coil rf power [23]. So the physical sputtering andthe rate of chemical reactions on the etched surface in-creased. The power densities are usually in the rangeof 0.1∼1 W/cm2. In the case of changing the dc-biasvoltage, the same tendency was observed, as shown inFig. 2 (b). This was attributed to the increased bom-bardment energy of Ar ions at a high dc-bias voltage.

A high coil rf power or a high dc-bias voltage re-sulted in a high degree of anisotropy. The etch profilebecame vertical as the coil rf power or dc-bias voltageincreased. LEE and CHUNG [24,25] had verified this inthe dry etching of IZO thin films with different etchgases, as shown in Fig. 3 and Fig. 4. Similar variationsmay occur during the dry etching of a-IGZO thin films.

Fig.2 Etch rate of IGZO thin films etched at different rf

powers and dc-bias voltages [23]

Fig.3 Etch profile of IZO thin films at various coil rf pow-

ers: (a) 500 W, (b) 700 W, (c) 800 W [24]

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ZHENG Yanbin et al.: Dry Etching Characteristics of Amorphous Indium-Gallium-Zinc-Oxide Thin Films

Fig.4 Etch profile of IZO thin films at various dc-bias volt-

ages: (a) 100 V, (b) 200 V, (c) 250 V [24]

3.2 Gas pressure

With an increasing gas pressure, there is an increasein the amounts of active radicals and ions but a decreasein their mean free paths [24]. These two effects coun-teract each other. We are not sure which effect plays amore vital role in a certain range of the gas pressure. Sothe effects of the gas pressure on the etch rate and etchprofile are uncertain. The etch rate may increase [26],decrease [23,24,26] or remain unchanged [27] with the gaspressure increasing. The gas pressure is usually in therange of 5∼200 mTorr.

4 The dry etching characteristicof a-IGZO with different gasmixtures

Since it was discovered by Professor HideoHOSONO [28] in 2004, there have been only a few re-ports on the dry etching of a-IGZO films up to now.In contrast, many reports on the dry etching of IZOfilms are available. Because of the similarity of the twomaterials, we will explain the dry etching characteristicof a-IGZO films with different gas mixtures by analogywith that of IZO films in the following section.

4.1 Gas mixtures containing CH4

If the etching gas contains CH4, a-IGZO may havea high etch rate because CH3 radicals can react withIn, Ga, or Zn to form volatile products In(CH3)x,Ga(CH3)y and Zn(CH3)z. KHANNA [29] found thesurface morphology of IZO film etched in CH4/Ar/H2

exhibited particle-like features resulting from the pref-erential desorption of In-, and O-containing volatile re-action products, probably InCH3, and CO2. A Zn-richsurface layer was produced after etching in CH4/Ar/H2

because of different formation rates of volatile prod-ucts. A similar phenomenon may appear when etch-ing a-IGZO films. The etch rate may increase with araised ratio of CH4/(CH4+Ar) at first because of theformation of volatile products In(CH3)x, Ga(CH3)y andZn(CH3)z. However, a further increase of CH4 in theCH4/Ar mixtures may decrease the a-IGZO etch ratedue to the increased formation of α-C:H polymer on thea-IGZO surface with high CH3 radicals and insufficiention bombardment. The dry etching of a-IGZO may ex-hibit an ion-assisted etch mechanism that is similar toZnO [30,31].

4.2 Cl-based etch gas

Cl-based etch gas, such as Cl2 [32,33] and BCl3 [34],can be used to etch a-IGZO films. However, the etchrate may be not so fast as using gas mixtures contain-ing CH4 because the surface reaction products ZnCl2,InCl3, GaCl3 are non-volatile compounds in the rangeof pressure (mTorr range) and temperature (∼ 25◦C)used in most of the dry etching experiments [29]. Thea-IGZO etch rate may increase with an increased Cl2 orBCl3 concentration due to the increased chemical reac-tion of the a-IGZO films with Cl radicals in the plasmawith the assistance of Ar ion sputtering. The dry etch-ing process of a-IGZO films obeys a reactive ion etchmechanism that includes ion sputtering and a chemicalreaction.

4.3 F-based etch gas

The a-IGZO films could also be etched with F-basedetch gas, such as CF4

[32,34]. The etch rate is slowerthan that of Cl-based etch gas because the surface re-action product F-based compounds have a high melt-ing point and boiling point (as is illustrated in Table2). However, the etch rate could be tailored by con-trolling the addition of oxygen [35]. The etch rate maydecrease by increasing F-based etch gas because it iseasy to form fluorocarbon polymer film on the a-IGZOfilm with a large amount of F-based etch gas that stopsfurther etching. Ar ion sputtering with chemical assis-tance is the main etch mechanism.

Table 2. Boiling and melting temperatures of the IGZOetching byproducts [34]

Parameters Byproducts

GaCl3 GaF3 ZnCl2 ZnF2 InCl3

Boiling temp. (◦C) 201.2 1000 756 1500 800

Melting temp. (◦C) 77.8 800 275 872 586

5 Conclusion

As the active layer of a TFT, the thickness of a-IGZO film ranges from 30 nm to 80 nm [36]. Cl- andF-based etch gas are more appropriate for dry etchingof a-IGZO films. However, three problems should stillbe addressed in the process of IGZO TFT manufacture.

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Plasma Science and Technology, Vol.14, No.10, Oct. 2012

a. The selectivity of an a-IGZO layer and insulatorlayer should be as large as possible (>6) to ensure asafe insulator layer.

b. In order to ensure good ohmic contact betweenthe a-IGZO layer and S/D electrode, the etch profile ofthe a-IGZO layer should not be more than 75◦.

c. If the dry etching method is adopted to pat-tern the S/D electrodes in the fabrication of bottom-gate staggered a-IGZO TFTs, the selectivity of theS/D metal and a-IGZO should be considered. SHINet al. [33] found that the selectivity of Mo (S/D metal)and a-IGZO could be up to 16.76 and the a-IGZO etchrate was 23.3 nm/min when adopting Cl2/Ar (flow rate:45 sccm/5 sccm) for etching. These results can meetfabrication requirements. There is still an issue whichshould not be ignored. The a-IGZO semiconductorlayer may change into a conductor layer because of ionbombardment in the process of patterning S/D elec-trodes. The electrical properties may deteriorate (suchas the decrease of Ion/Ioff). Fortunately, PARK etal. [37] solved this problem. They applied N2O plasmatreatment on the back surface of the a-IGZO channellayer and got excellent electrical characteristics. Thereason is that oxygen that dissociates from N2O en-ables a conductive a-GIZO back surface to recover.

Due to its excellent performance, many flat paneldisplays, including electronic papers (e-papers), organiclight-emitting-diode displays (OLEDs) and liquid crys-tal displays (LCDs) have adopted the IGZO TFT back-plane [38]. But research on a-IGZO TFT backplaneshas just started in China. Compared with Samsung,LG and Sharp, we still have a long way to go. So weshould increase financial support for related researchand try to achieve great breakthroughs in key technolo-gies of IGZO TFT backplane manufacturing, such asthe preparation, dry etching of a-IGZO films and thedesign of the IGZO TFT structures.

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(Manuscript received 3 May 2011)(Manuscript accepted 28 December 2011)E-mail address of ZHENG Yanbin: ccucm2010@163.com

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