692 IEEE ELECTRON DEVICE LETTERS, VOL. 39, NO. 5, MAY 2018

Fluorination-Enabled Monolithic Integration ofEnhancement- and Depletion-Mode

Indium-Gallium-Zinc Oxide TFTsZhuoqun Feng, Lei Lu , Member, IEEE, Sisi Wang, Jiapeng Li , Zhihe Xia ,

Hoi Sing Kwok, Fellow, IEEE, and Man Wong, Senior Member, IEEE

Abstract— Common metal-oxide thin-film transistors(TFTs) are unipolar (n-type) in nature, operating either in theenhancement-mode (EM) or depletion-mode (DM). A simpletechnology allowing easy monolithic integration of bothEM and DM TFTs is desirable for improved performanceof a metal-oxide circuit. In this letter, the channel regionsof select TFTs in a circuit were fluorinated. These TFTsremained EM when subjected to a heat-treatment in anon-oxidizing atmosphere, while the rest was turned DM.The difference between the threshold voltage of an EMand a DM TFT can be simply and precisely modulatedby controlling the heat-treatment temperature and dura-tion. An inverter monolithically constructed of EM andDM indium-gallium-zinc oxide TFTs is presented as ademonstration.

Index Terms— Indium-gallium-zinc oxide, thin-film tran-sistor, fluorination, inverter, depletion-load, thresholdvoltage, donor-defects.

I. INTRODUCTION

W ITH their relatively low process temperature, highfield-effect mobility, low leakage current and high

transparency [1], thin-film transistors (TFTs) based on metaloxides (MOs) such as indium-gallium-zinc oxide (IGZO)are being deployed in high-end display products. Theirgood electrical performance also enabled the construction ofmore sophisticated electronic circuits, including amplifiers [2],shift registers [3], radio-frequency identification tags [4] andanalog-to-digital converters [5], etc.

While technologies for n-type MO TFTs are relativelymore mature, the lack of an adequate technology for

Manuscript received March 7, 2018; revised March 19, 2018; acceptedMarch 20, 2018. Date of publication March 23, 2018; date of currentversion April 24, 2018. This work was supported in part by the Hong KongGovernment through the Theme-Based Research Project and Technol-ogy Fund under Grant T23-713/11-1 and in part by the Partner StateKey Laboratory on Advanced Displays and Optoelectronics Technologiesunder Grant ITC-PSKL12EG02. The review of this letter was arrangedby Editor S. Hall. (Zhuoqun Feng and Lei Lu contributed equally to thiswork.) (Corresponding author: Lei Lu.)

Z. Feng, S. Wang, J. Li, Z. Xia, and M. Wong are with the Departmentof Electronic and Computer Engineering, The Hong Kong University ofScience and Technology, Hong Kong.

L. Lu and H. S. Kwok are with the Department of Electronic andComputer Engineering, The Hong Kong University of Science and Tech-nology, Hong Kong, and also with the Jockey Club Institute for AdvancedStudy, The Hong Kong University of Science and Technology, Hong Kong(e-mail: eelulei@ust.hk).

Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LED.2018.2818949

p-type MO TFTs hinders the implementation of complemen-tary type circuits. To improve the performance of a unipolarcircuit, one would desire a technology offering both depletion-mode (DM) and enhancement-mode (EM) TFTs by allow-ing easy and robust tuning of their corresponding thresholdvoltage (Vth) [6].

Hitherto several approaches of modulating Vth havebeen attempted, employing different back-channel cappinglayers [7], or using active layers of different materials [8], [9]or thickness [6]. While feasible, these techniques incur asignificant increase in process complexity. The dual-gateTFT approach is not only technologically involved, but alsorequires a more complicated driving scheme [10], [11]; andthe approach of illumination-bias stress [12] of select TFTsdemands an additional time-consuming, post-fabrication stresstreatment.

A technology for monolithically integrating EM and DMIGZO TFTs is presently described, requiring merely theaddition of a simple patterning step to define a photoresistmasking layer for selecting the TFTs into which fluorine (F)is introduced. These “fluorinated” TFTs remain EM aftera subsequent heat-treatment in a non-oxidizing atmosphere,such as nitrogen (N2), while those not fluorinated are turnedDM with their Vth shifting to a negative value. This simpleintegration technology is demonstrated by the characterizationof an inverter monolithically constructed of EM and DM TFTs.

II. EXPERIMENTAL RESULTS

The monolithic integration scheme, applied to an IGZO-based elevated-metal metal-oxide (EMMO) architecture [13],is schematically illustrated in Figure 1.

Subsequent to the definition of the source/drain (S/D) metalelectrodes, a photoresist implantation mask was patterned toexpose select EMMO TFTs. F ions at a dose of 1016 cm−2

were implanted (Fig. 1a) to a depth just above the 20-nmthick IGZO channel regions of these TFTs. The mask was thenremoved and a 2-hr, 400 °C anneal in oxygen (O2) (Fig. 1b)was applied to drive in the implanted F. The distribution of F inthe resulting fluorinated IGZO, dubbed IGZO:F [14], wasinvestigated using secondary ion-mass spectrometry (SIMS)(Fig. 2). Those TFTs masked by the photoresist during theimplantation retained the regular un-fluorinated IGZO in thechannels. The O2-anneal simultaneously oxidized the channel

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FENG et al .: FLUORINATION-ENABLED MONOLITHIC INTEGRATION 693

Fig. 1. Schematic cross-sections of EMMO TFTs subjected to(a) selectively masked F implantation, (b) O2-anneal, and (c) additionalN2-anneal.

Fig. 2. SIMS depth profiles of F in IGZO:F before and after O2-anneal.

Fig. 3. The transfer characteristics of (a) IGZO and (b) IGZO:F TFTssubjected to a 2-hr, 400 °C O2-anneal (�,�) and an additional 10-min,350 °C N2-anneal ( , ).

to maintain its high resistivity [14] and activated the con-ductive S/D regions [13] by forming donor-defects under thegas-impermeable metal electrodes.

Measured at a drain voltage (Vd) of 1 V after the oxi-dizing activation anneal and shown in Fig. 3 are the draincurrent (Id) vs. gate voltage (Vg) transfer characteristics ofregular IGZO and IGZO:F TFTs with equal channel length(L) and width (W ) of 100 µm. Both worked in the EM withsimilar performance metrics (Table I) including Vth (defined

TABLE ICHARACTERISTICS OF THE IGZO AND IGZO:F TFTS ACTIVATED

WITH AN O2-ANNEAL AND FOLLOWED BY AN

ADDITIONAL N2-ANNEAL

Fig. 4. Vth dependence on the duration of 300 °C and 350 °C N2-annealfor both IGZO and IGZO:F TFTs.

as the Vg to obtain an Id of 10 nA), subthreshold slope (SS),on-off current ratio and field-effect mobility (µFE).

The TFTs were subsequently subjected to a 10-min, 350 °Canneal in N2, resulting in parallel shifts of the transfer curvesin the negative Vg direction. The shift of the regular IGZOTFT (Fig. 3a) was sufficiently large to turn Vth negative, thusconverting the TFT from EM to DM; but that of the IGZO:FTFT (Fig. 3b) was small enough to retain a positive Vth, thusalso the EM nature of the TFT. All other characteristics werewell preserved (Table I). Consequently, it is possible to realizecircuits with monolithically integrated EM and DM TFTs withthe addition of a simple mode-determining masking step forselective fluorination.

Displayed in Fig. 4 is the dependence of Vth on the tem-perature and duration of the additional non-oxidizing annealin N2. At 350 °C, the Vth of the IGZO TFT continuouslyshifted negatively with increasing duration, reaching −13.8 Vafter 30 minutes, while the IGZO:F TFT remained EM with apositive Vth. This is a strong indication of the fluorination-enhanced resistance against the generation of annealing-induced donor-defects in IGZO:F. The observed dependenceof Vth on duration can be applied to precisely adjust theVth difference between the EM and DM TFTs. At a lowertemperature of 300 °C, the kinetics of defect generation wassufficiently slow [14] such that the Vth remained largelyunchanged for both types of TFTs.

The diffusion of oxygen-containing species out of the IGZOchannel region through the 300 nm-thick gas-permeable sili-

694 IEEE ELECTRON DEVICE LETTERS, VOL. 39, NO. 5, MAY 2018

Fig. 5. The XPS spectra of O1s with the Gaussian–Lorenz deconvo-lution for (a) un-annealed IGZO, (b) un-annealed IGZO:F, (c) IGZO and(d) IGZO:F subjected to a 30-min, 350 °C N2-anneal.

Fig. 6. Inverter based on IGZO:F EM drive-TFT and IGZO DM load-TFT:(a) schematic circuit diagram; (b) the voltage transfer curves with supplyvoltage VDD varying from 2 to 10 V; (c) the voltage gain at a VDD of 10 V.

con oxide etch-stop layer (Fig. 1) during the non-oxidizinganneal led to the thermally induced generation of donor-defects [15] such as oxygen vacancies [16]. The resultingincrease in channel carrier concentration is responsible forthe negative shift in Vth. The smaller shift of the IGZO:FTFT (Figs. 3 and 4) suggests fluorination improves the sta-bility of IGZO:F against the thermally induced generation ofdefects.

Samples of un-annealed IGZO and IGZO:F and thosesubjected to a 30-min, 350 °C N2-anneal were prepared.Their O1s x-ray photo-electron spectra (XPS) are comparedin Fig. 5. Each spectral peak can be resolved into twoGaussian-Lorentz sub-peaks centered at 531.0 and 531.9 eV,respectively attributed to O2− ions in fully oxidized and O-deficient sites. The area percentage of a sub-peak is takento reflect the population of the corresponding species. Afterthe N2-anneal, the defect-related 531.9-eV sub-peak increasedin IGZO and IGZO:F respectively by ∼11% and ∼5%. Thiscorrelates well with the higher reliability [16] and thermalstability of IGZO:F, plausibly attributed to the replacement

of the weakly bonded O ions with F ions that form strongerbonds with the cations in IGZO:F [17].

As a demonstration, an inverter circuit (Fig. 6a) was con-structed by monolithically integrating IGZO and IGZO:F TFTsrespectively as a DM load and an EM driver. Both TFTsoperated in the EM after the 2-hr, 400 °C O2-anneal, thusrendering a poor load-TFT with a limited pull-up capability.This is reflected in the inadequate characteristics indicatedby the dashed lines in the output voltage (VOUT) vs. inputvoltage (VIN) transfer curves shown in Fig. 6b. After theadditional 10-min, 350 °C N2-anneal, theVth of the IGZO load-TFT was adjusted to −1.4 V. The voltage swing indicatedby the solid lines in Fig. 6b reflects a visible improvement,with the absolute DC small signal gain (AV ≡ −dVOUT/dVIN)increasing to ∼40 at an operating point of ∼1.5V and asupply voltage VDD of 10V (Fig. 6c). This inverter exhibitsa relatively higher performance than that of the previouslyreported DM/EM-based inverters [6], [7], [12].

III. CONCLUSION

Fluorination has been shown to significantly improve theresistance of IGZO against the thermally induced generationof donor-defects in a non-oxidizing atmosphere. A technologyfor monolithic integration of enhancement- and depletion-mode IGZO TFTs is presently described, requiring merely theaddition of a simple patterning step to define a photoresistmasking layer for the fluorination of the TFTs destined tooperate in the enhancement-mode. The difference betweenthe threshold voltages of the two modes of TFTs can besimply and precisely modulated by controlling the temperatureand duration of the non-oxidizing heat-treatment. An invertermonolithically constructed of EM and DM TFTs is presentedas a demonstration, exhibiting improved voltage swing andlarger small-signal gain.

ACKNOWLEDGMENT

The assistance of the staff from both Nanosystem Fabrica-tion Facility and Materials Characterization and PreparationsFacility is gratefully acknowledged.

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