低溫複晶矽薄膜電晶體的電容模型及其頻率響應

國立交通大學

電子工程學系 電子研究所

碩 士 論 文

低溫複晶矽薄膜電晶體的電容模型

及其頻率響應

Capacitance Model and its Frequency Response

of Low-Temperature Polycrystalline Silicon

Thin Film Transistors

研 究 生：邵而康

指導教授：張俊彥 院士

冉曉雯 博士

低溫複晶矽薄膜電晶體的電容模型及其頻率響應

Capacitance Model and its Frequency Response of

Low-Temperature Polycrystalline Silicon Thin Film Transistors

研 究 生：邵而康 Student：Er-Kang Shaw

指導教授：張俊彥 院士 Advisor : Dr. Chun-Yen Chang

冉曉雯 博士 Dr. Hsiao-Wen Zan

國 立 交 通 大 學

電子工程學系 電子研究所

碩 士 論 文

A Thesis

Submitted to Department of Electronics Engineering and Institute of Electronics

College of Electrical Engineering and Computer Engineering National Chiao Tung University

In Partial Fulfillment of the Requirements For the Degree of

Master in

Electronics Engineering July 2007

Hsinchu, Taiwan, Republic of China

低溫複晶矽薄膜電晶體的電容模型及其頻率響應

研究生： 邵而康 指導教授：張俊彥 院士

冉曉雯 博士

國立交通大學

電子工程學系 電子研究所碩士班

摘要

近年來低溫複晶矽薄膜電晶體已經引起大量的研究，其應用的方向是相當廣 泛的。當低溫複晶矽薄膜電晶體應用為驅動電路時，將操作於閘極交流信號。因 此低溫複晶矽薄膜電晶體對於元件在閘級交流信號下的頻率響應就具有相當大 的重要性。 在本篇論文中，我們使用準分子雷射製作低溫複晶矽薄膜電晶體，利用電容 -電壓量測和阻抗分析儀研究準分子雷射製作的複晶矽薄膜電晶體，其中，調變 在不同準分子雷射能量密度下的複晶矽薄膜品質，我們觀察電容在不同的閘極偏 壓下對頻率的變化，因此，從薄膜本身的特性，建立出以物理意義為根本的電容 模型。 在電容-電壓量測下，我們針對空乏區研究其頻率響應，並且在高頻操作狀

態下觀察到量測的電容值急遽下降。因此，除了考慮到薄膜缺陷造成在不同頻率 下的電容變化，也把從重摻雜的源極跟汲極端往通道中央提供的橫向流動的載 子，所造成的特徵電容和特徵電阻引入；此處的特徵電容跟特徵電阻可由電導-頻率的峰值來計算。此外，利用兩個和缺陷相關的模擬參數，就能模擬出在空乏 區中，各個閘極偏壓下的電容-頻率曲線；而其模擬結果與實驗數據都相當吻合。 模擬結果也說明了在不同閘極偏壓下缺陷狀態的反應時間，以及橫向載子流動所 形成的電阻。當閘極偏壓越靠近元件導通區時，缺陷狀態的反應時間越快，薄膜 的特徵電阻越小。

Capacitance Model and its Frequency Response of

Low-Temperature Polycrystalline Silicon Thin Film Transistors

Student: Er-Kang Shaw Advisor: Dr. Chun-Yen Chang

Dr. Hsiao-Wen Zan

Department of Electronics Engineering and Institute of Electronics

National Chiao Tung University, Hsinchu, Taiwan

Abstract

In recent years, low temperature polycrystalline silicon thin-film transistors

(TFTs) have been investigated extensively for their wide applications. The low

temperature polycrystalline TFTs are operated under gate alternating current signal.

Therefore, the studies of frequency response of low temperature polycrystalline TFTs

under gate alternating signal become very important.

In this thesis, the low temperature polycrystalline silicon TFTs fabricated by the

excimer laser annealing (ELA) are analyzed by the measurements of the

capacitance-voltage (C-V) and impedance analyzer. Here, by adjusting different poly

silicon crystalline film qualities due to different excimer laser energy densities, the

variation of the measured capacitance under different gate biases is observed. Hence,

proposed.

Under the C-V measurement, we focus on the frequency response in the

depletion region, and observe the measured capacitance decreases drastically under

high frequency operation. Consequently, we not only study the capacitance variation

under different frequency caused by the defects existing in the thin film, but also

introduce the characteristics capacitance (Cpoly) and the characteristics resistance

(Rpoly) due to the carrier lateral flow from the heavily doped source and drain to the

channel; Rpoly and Cpoly can be extracted by the peak of conductance-frequency (G/f-f)

curves. Besides, two fitting parameters related to the traps are used to model the C-V

curves under each gate bias in the depletion region. Good agreements are found when

comparing the modeling results and the experimental data.

The proposed capacitance model also illustrates the response time of the trap

states and Rpoly under different gate biases. As gate bias approaches the turn-on region,

Acknowledgement

時間飛逝，兩年的碩士班生涯一眨眼就過去了，在這段期間，首先要感謝我 的指導老師 張俊彥老師和冉曉雯老師。在我的碩士班研究題目上，十分感謝張 俊彥老師給我的建議，和對於這個主題的未來展望，您無時無刻都積極且正面地 鼓勵我去思考、去實作，更讓我了解到實在的努力加上明確的方向才會導向成功 的道路。而冉曉雯老師無論是在生活上，抑或是研究上都給予我許許多多的幫助 和指導；每當我在處理事情的方法上遇到瓶頸的時候，您總是深入淺出的引導 我，告訴我要腳踏實地、循序漸進的去解決所面臨的問題，讓我懂得如何在謎團 中，以銳利的眼光去找出問題的癥結，抽絲剝繭的去尋找答案。在此，要感謝我 敬重的兩位指導老師！謝謝你們的教誨！ 在我的電容模型的實驗上，我要特別感謝明達學長，謝謝你指導我一切研究 上的知識，也提供我實驗上所需要的機台；雖然常去叨擾你，你仍然盡心盡力地 協助我，十分感謝！也要感謝我要感謝實驗室的每一個人：國錫學長、士欽學長、 政偉學長，謝謝你們的指導，在我對於研究上的一些疑惑，和你們討論後總是能 激發我一些靈感，啟發我再去思考的動力。謝謝已經畢業的學長姐：貞儀、章祐、 全生；謝謝你們一年的陪伴和經驗上的傳承，每每和你們討論，才發現我的孤陋 寡聞，再三咀嚼你們所說的話，才能略知一二，進而培養我做研究以及處理事情 應有的態度。此外，同屆的夥伴們：光明、德倫、皇維、廷遠、睿志、文馨、芸 嘉，雖然彼此的研究題目不同，但非常感謝你們大家的陪伴！彼此為了相同的目 標而努力，相互加油打氣的感覺真的很棒！謝謝實驗室的學弟妹們，你們讓我感 到非常溫暖！ 最後，也是最重要的，謝謝我的家人，在人生的道路上，給我力量面對每個 挑戰，度過每個關卡；感謝你們從小到大的支持與照顧！ 邵而康 2007 夏 于台北

Contents

Chinese Abstract I

English Abstract III

Acknowledgment . V

Contents VI

Table Captions VIII

Figure Captions IX

Chapter 1. Introduction

1.1 An overview of poly-Si TFTs technology 1

1.2 RPI CV model of Poly-Si TFT . 2

1.3 Motivation 4

1.4 Thesis Outline 4

Chapter 2. Experimental Procedures 2.1 The device fabrication process . 6

2.2 C-V and C-f measurement 7

Chapter 3. Results and Discussions 3.1 The observation of C-V and C-f curve . 8

3.1.1 C-V curve analysis 8

3.1.2 C-f curve analysis 11

3.2 The proposed equivalent circuit model 12

3.2.2 Methods of device parameter extraction .. 15

3.3 The fitting results and the dependence of parameters on gate bias 16

Chapter 4. Conclusion 18

References 20 Tables

Table Captions

Table.1 The response time of ELA p-type poly-Si TFTs fabricated under different

Figure Captions

Chapter 1.

Fig.1-2-1 Transmission line model proposed by RPI.

Chapter 2.

Fig.2-1-1 The schematic cross sectional view of Poly-Si TFT.(a) is n-type ELA poly-Si

TFT, (b) is p-type ELA poly-Si TFT.

Chapter 3.

Fig.3-1-1 (a) C-V curves of ELA p-type poly-Si TFT. W/L=600µm /6µm. Frequency is

from 10k to 500 kHz. ELA laser energy is 380 mJ/cm2. (b) VG is from -1V to

-2.2V.

Fig.3-1-2 C-V curves of ELA p-type poly-Si TFT. W/L=600µm /6µm. Frequency is

from 10k to 500 kHz. ELA laser energy of (a) is 360 mJ/cm2, of (b) is 340

mJ/cm2.

Fig.3-1-3 SEM images of poly silicon film. ELA laser energy of (a) is 380 mJ/cm2, (b)

is 360 mJ/cm2, (c) is 340 mJ/cm2.

Fig.3-1-4 The dependence of activation energy on gate voltage under three laser

energy densities. These are ELA p-type poly-Si TFTs. W/L=600µm /6µm.

Fig.3-1-5 Trap density extraction by modified Levinson theory under three laser

Fig.3-1-6 C-f curves of ELA p-type poly-Si TFT. W/L=600µm /6µm. ELA laser

energy is 380 mJ/cm2, 360 mJ/cm2, and 340 mJ/cm2.

Fig.3-1-7 C-V curves of ELA p-type poly-Si TFT. W/L=600µm /6µm. Frequency is

from 10k to 10 MHz. ELA laser energy is 380 mJ/cm2.

Fig.3-1-8 C-V curves of ELA p-type poly-Si TFT. W/L=600µm /600µm. Frequency is

from 10k to 10 MHz. ELA laser energy is 380 mJ/cm2.

Fig.3-1-9 C-V curves of ELA n-type poly-Si TFT. W/L=600µm /6µm. LDD=0.75 µm.

Frequency is from 10k to 10 MHz. ELA laser energy is 380 mJ/cm2.

Fig.3-1-10 C-f curves of ELA p-type poly-Si TFT. W/L=600µm /6µm. VG is biased

from -1 V to -3 V. ELA laser energy is 380 mJ/cm2.

Fig.3-1-11 C-f curves of ELA p-type poly-Si TFT. W/L=600µm /600µm. VG is biased

from -2 V to -8 V. ELA laser energy is 380 mJ/cm2.

Fig.3-1-12 C-f curves of ELA n-type poly-Si TFT. W/L=600µm /6µm. LDD=0.75 µm.

VG is biased from -1 V to 1 V. ELA laser energy is 380 mJ/cm2.

Fig.3-2-1 (a) Capacitance equivalent circuit model of poly-Si TFTs for short channel.

(b) Capacitance equivalent circuit model of poly-Si TFTs.

Fig.3-2-2 G/f-f curves of ELA p-type poly-Si TFT. W/L=600µm /6µm. ELA laser

energy is 380 mJ/cm2. VG is from -1V to -3V.

energy is 380 mJ/cm2. VG is from -1V to -8V.

Fig.3-3-1 The curves are calculated using the model. ELA p-type poly-Si TFT.

W/L=600µm /6µm. ELA laser energy is 380 mJ/cm2.

Fig.3-3-2 Extracted Rpoly of ELA p-type poly-Si TFT. W/L=600µm /6µm.

Fig.3-3-3 Extracted Rpoly of ELA p-type poly-Si TFT. W/L=600µm /6µm, and 600µm

/600µm.

Fig.3-3-4 Trap response time of ELA p-type poly-Si TFT. W/L=600µm /6µm.

Fig.3-3-5 The curves are calculated using the model. ELA p-type poly-Si TFT.

W/L=600µm /600µm. ELA laser energy is 380 mJ/cm2.

Fig.3-3-6 Channel trap response time of ELA p-type poly-Si TFT. W/L=600µm

Chapter 1

Introduction

1.1 An overview of poly-Si TFTs technology

In recent years, polycrystalline silicon thin film transistors (poly-Si TFTs) have

drawn much attention because of their widely applications on active matrix liquid

crystal displays (AMLCDs) [1]-[3]. On the other hand, poly-Si TFTs have been applied

to some memory devices such as dynamic random access memories (DRAMs) [4],

static random access memories (SRAMs) [5], electrical programming read only

memories (EPROM) [6], and electrical erasable programming read only memories

(EEPROMs) [7]. Besides, linear image sensors [8], thermal printer heads [9],

photodetector amplifier [10], scanner [11], neutral networks [11] have also adopted

poly-Si TFTs technology. Within these, the application in AMLCDs is the most

primary trend, which leads to a rapid development of poly-Si TFT technology.

Compared to amorphous silicon (a-Si) TFTs, the major attraction of applying

poly-Si TFTs in AMLCDs lies in the greatly improved carrier mobility in poly-Si film

which usually exceeds 100 cm2 / Vs by present mature technology and the capability

of integrating the pixel switching elements, panel array, and peripheral driving circuit

poly-Si TFTs, the dimension of the poly-Si TFTs can be designed smaller than that of

a-Si TFTs to achieve high density and high resolution AMLCDs.

In order to make high performance poly-Si TFTs, low-temperature technology is

required for the commercial flat-panel displays (FPD) on inexpensive glass substrate,

whereas the maximum process temperature is limited to less than 600°C[15]. There

are three major low-temperature a-Si crystallization methods to achieve high

performance poly-Si thin film: solid phase crystallization (SPC), excimer laser

crystallization (ELC), and metal-induced lateral crystallization (MILC). The SPC

method has the advantage of good uniformity and smooth interface, but the

throughput is limited by the long crystallization time. In recent years, ELC has been

used to crystallize the a-Si to poly-Si with lower temperature compared to SPC. In

addition, the ELC method does not require a long time crystallization process, so it is

suitable for mass production with high throughput. However, the roughness and

uniformity of poly-Si film are important issues that may degrade the electrical

characteristics if the laser energy is not accurately controlled.

1.2 RPI CV model of Poly-Si TFT

In RPI AC models, the grain boundary and intra-grain trap states in poly-Si result

frequency. Here, the source and drain terminals were shorted to ground, and a small

signal is applied to the gate of the device. The frequency dispersion phenomenon can

be comprehended by considering the transmission line model that is illustrated in

Fig.1-2-1. A typical analysis of the effective circuit model forms complex impedance

between the gate and source/drain terminals which depends on different frequencies.

For single crystalline silicon transistors, the impedances are very small due to high

mobility. Therefore the capacitance is almost independent of frequency. Besides,

previous research also referred to later flow transmission line model [16]-[17], and

introduced the influence of depletion capacitance (CD). These statements indicated the

variation of measured capacitances with different frequencies primarily arises from a

frequency response associated with the later flow of carriers into and out of the

channel from adjacent source and drain region.

However, the defects existed at the interface between gate oxide and poly-Si film,

the grain boundary defects and the intra-grain defects are other factors to affect the

device performance which are likely to respond to the small ac signal. Because of the

response time of these defects, the measured capacitance is also a function of

frequency in the depletion region. Therefore, the effect of trap capacitance (Ct) should

1.3 Motivation

Most of the previous studies have focused on the analysis of the defect density

within the energy bandgap, the dependence of mobility on different biases, film

qualities, environment temperatures, and the extraction of effective parasitic

resistance by drain current-gate voltage (ID-VG) measurement. In addition, in recent

years, the fabrication technology of poly-Si TFTs have been improved a lot. Due to

high mobility and small dimension of device which could be fabricated on the glass

substrate, these characteristics had attracted the industry to apply poly-Si TFTs to

high frequency peripheral circuit.

Here, we would like to propose an effective circuit model which is based on

physical mechanism and relates to the previous researches to extract some of the

physical parameters such as CD, Ct and Rt by C-V, C-f and G-f measurement. Thus, in

this thesis, we focus on the defects and the characters of the poly-Si films which

respond to small ac signal by C-V, C-f measurement.

1.4 Thesis Outline

This thesis is organized into the following manner.

In chapter 1, a brief overview of poly-Si TFTs for various kinds of applications is

RPI CV model of Poly-Si TFTs and the other background studies are discussed briefly.

Finally, the motivation of this work is expressed.

In chapter 2, detailed fabrication processes of ELA and SPC poly-Si TFTs are

introduced, respectively. The measuring conditions are described concisely.

In chapter 3, the discussion is divided into three parts: First, the phenomena of

the C-V and C-f curves are analyzed. Second, an equivalent circuit model is proposed

to illustrate each effective capacitance and resistance. Moreover, the methods of the

parameters extraction are explained. Finally, the fitting results of the proposed model

are compared with the measured capacitance. Besides, the dependence of these

parameters on gate bias is discussed.

In chapter 4, the conclusions are given including the physical mechanism of the

proposed capacitance model and the explanation of the phenomenon of the frequency

Chapter 2

Experimental Procedures

2.1 The device fabrication process

In this experiment, typical top-gate, coplanar self-aligned poly-Si TFTs are fabricated by ELA method.

The schematic cross sectional view of these devices are shown in Fig.2-1-1, Fig.2-1-1(a) and Fig.2-1-1(b) are n-type with lightly doped drain (LDD) structure and p-type poly-Si TFTs, respectively; the fabrication process is described below.

First, buffer layer was deposited on the glass substrate to prevent the diffusion of

impurities existing in the glass substrate from the silicon layer. Then, undoped 50 nm

thick amorphous-Si layer were deposited on buffer layer. After that, amorphous-Si

films were recrystallized by ELA method under several different laser energies; here,

the laser energy densities are 340 mJ/cm2, 360 mJ/cm2, and 380 mJ/cm2, respectively.

The recrystallized poly-Si films were patterned into the active islands with different

dimensions. Afterward, the gate insulator was deposited; a 50 nm-thick oxide layer

was deposited on the poly-Si film, and then a 20 nm-thick nitride layer was deposited

on the oxide layer. Next, phosphorus ions and boron ions were implanted to form the

n+, n-, and p+ source/drain regions and these dopants were activated by thermal

gate regions as the metal pads.

In our experiment, we compared several C-V and C-f measurement results under

three different ELA laser energies.

2.2 C-V and C-f measurement

In our experiment, HP4156 was applied to measure the current-gate voltage

(ID-VG) in order to check the device performances. Besides, HP4284 was used to

measure C-V under several frequencies which range from 10 kHz to 1 MHz. In

addition, we also measured capacitance-frequency (C-f) by HP4194 in several gate

voltages that are biased in the depletion region, and the measured frequencies were

Chapter 3

Results and Discussions

3.1 The observation of C-V and C-f curve

3.1.1 C-V curve analysis

As we could observe that the measured CG-DS-V curves gradually increase from

turn-off region, depletion region, to turn-on region in Fig.3-1-1(a), and the frequency

dispersion phenomenon appears in the depletion region shown in Fig.3-1-1(b). The

laser energy density of ELA poly-Si TFTs is 380 mJ/cm2 which provides excellent

device performance. Other measured C-V curves of different laser energy densities

are also shown in Fig.3-1-2(a) and Fig.3-1-2(b), which are 360 mJ/cm2 and 340

mJ/cm2, respectively. The frequency dispersion phenomenon in the depletion region

becomes more and more serious; it is because of the degradation of the poly silicon

films under these laser conditions. Fig.3-1-3 is the images of Scanning Electron

Microscope (SEM) which is capable of producing high-resolution images of poly

silicon film surface. We find that the grains of poly silicon films in Fig.3-1-3(a) are

more uniform and larger than that in Fig.3-1-3(b) and Fig.3-1-3(c). The trap response

time listed in Table.1 is extracted while VG bias is in the middle of the depletion

VG is shown in Fig.3-1-4, where the dimension of these poly-Si TFTs are identical;

W/L = 600µm /6µm. The activation energies of these three different device which the

laser energy densities are 340 mJ/cm2, 360 mJ/cm2, and 380 mJ/cm2 are about the

same; 0.65 eV. As device turn on, activation energies decrease to zero. Here, we could

observe that when the laser energy density is 380 mJ/cm2, the activation energy of the

device decreases faster and more steeply than the other ones. In other words, the

poly-Si film recrystallized in this condition which has larger and more uniform grains

results in better film quality and fewer trap states for the carriers to fill compared to

the other device recrystallized under 360 mJ/cm2 and 340 mJ/cm2. Fig.3-1-5 shows the

method of trap density extraction presented by modified Levinson theory [18]; the

gentler the curve is, the fewer trap charges the film contains. Therefore, Levinson

theory also identifies that when the laser energy density is 380 mJ/cm2, poly-Si film

has better quality than that recrystallized by the other two conditions. Fig.3-1-6 shows

the C-f fitting curves based on our proposed capacitance model that will be mentioned

later and the measured capacitance converted from C-V. Here, we suppose the

phenomenon is mainly affected by the defects which are trapped in the bandgap trap

states. Because of the response time of the carriers released from trap states to

conduction are different in each gate bias; meanwhile, the frequency of ac signal is

hardly observe the capacitance caused by the carriers that are capable of following the

small ac signal from10 kHz to several hundred kHz. When frequency increases to 10

MHz as shown in Fig.3-1-7, which is ELA p-type poly-Si TFT with W/L=600µm /6µm.

the measured capacitance reduces dramatically and achieves a specific saturation

value in the turn-on region. It’s because of the carriers provided from source/drain that

are unable to penetrate into the whole intrinsic poly-Si channel which means the

carriers only partially respond to the channel region near the source/drain region and

beneath the gate oxide. Fig.3-1-8 shows the C-V curve of the device with channel

length as 600 µm, the capacitance value degrades when the frequencies are from 100

kHz to 10 MHz. Comparing Fig.3-1-8 to Fig.3-1-7, the capacitance degradation

phenomenon of the device with channel length as 600 µm biased in the turn-on region

becomes severer due to the carriers provided from source/drain merely respond to an

effective small areas which are adjacent to the source/drain region. Fig.3-1-9 shows

C-V curve of n-type poly-Si TFT with W/L=600µm /6µm and LDD structure; the

LDD length is 0.75 µm. The capacitance degradation phenomenon in the turn-on

region is not as severe as p-type one, but still decreases obviously as frequency

increases to 10 MHz. We attribute the variation between n-type and p-type device to

the differences of the mobility between electrons and holes; the effective mass of

capable of following the small ac signal under high frequency operation than holes

such as to penetrate into the middle of the channel region and respond.

3.1.2 C-f curve analysis

The measured C-f curves of ELA p-type poly-Si TFT which dimension is

W/L=600µm /6µm are shown in Fig.3-1-10. Here, the gate biases are within the

depletion region. All the measured capacitances under these gate biases decrease

slightly from 1 kHz to 1 MHz, respectively; then decrease drastically as the frequency

increases over 1 MHz. The frequency of ac signal is too high for the carriers to inject

from the source/drain region to respond to the whole channel. In other words, the

reduced capacitance value is observed because the carriers are gradually incapable of

following the small ac signal over 1 MHz. For that reason, we consider when the

frequency increases around to 15 MHz, there must be an additional effect to reduce

the measured capacitance, and this effect has to be taken into account especially in

high frequency condition.

The same phenomenon also occurs in the device with channel length as 600 µm

shown in Fig.3-1-11, but the capacitance decreases drastically in lower frequency

which ranges roughly from 10 kHz to 100 kHz. We can observe that as frequency

performance under high frequency operation. Fig.3-1-12 shows the C-f curve of

n-type poly-Si TFT with W/L=600µm /6µm and LDD structure; the LDD length is

0.75 µm. Different from p-type poly-Si TFTs, the capacitance degradation

phenomenon should be occurred above 15 MHz because we observe all the

capacitance values biased in the depletion region keep decreasing but do not achieve

small specific constants respectively. Therefore, the capacitance should be degraded

as the frequency extends over 15 MHz, which corresponds to the supposition:

electrons are more capable of following the small ac signal under high frequency

operation than holes. In other words, n-type poly-Si TFTs are more promising in high

frequency operation than p-type ones due to higher carrier mobility.

3.2 The proposed equivalent circuit model

3.2.1 The capacitance equivalent circuit model of the Poly-Si TFT

A capacitance equivalent circuit model is proposed shown in Fig.3-2-1. The

equivalent circuit shown in Fig.3-2-1(a) is adaptable for short channel device; on the

other hand, the equivalent circuit shown in Fig.3-2-1(b) is complete model which also

can be applied for long channel poly-Si TFTs. In the effective device capacitor, a

highly resistive intrinsic poly-Si channel capacitance and resistance have to be

which the area is the product of device width and length; Rpoly and Cpoly are the

characteristics resistance and capacitance of the carrier lateral flow in the channel,

respectively. CD represents the capacitance of depletion. Ct and Rt are the capacitance

and the resistance which result from the trap states in the depletion region and connect

in parallel with CD. Cct and Rct are the capacitance and the resistance which result

from the trap states in the channel. Here, the trap states are caused by interface defects,

grain boundary defects, and intra-grain defects. Besides, the specific response time

constant of charge trapped in the trap state is the multiplication of Rt and Ct, and is a

function of gate bias due to the relative position from trap state level to conduction.

In the turn-off region, Ct is also neglected because carriers can be supplied

neither from the heavily doped source/drain regions nor from the intrinsic poly-Si

bulk. Therefore, there is no frequency response under the strong reverse gate bias. In

addition, Cpoly is expected to be absent due to the intrinsic poly-Si bulk is fully

depleted. Therefore, only a series combination of Ci and CD is regarded as the

capacitance equivalent circuit.

In the turn-on region, because a large number of carriers are injected from the

heavily doped source/drain regions and accumulated at the interface, the amount of

charges that are trapped at all these trap states can be neglected in comparison to the

why only Ci is observed as the frequency swept below several hundred kHz in C-V

and C-f measurements is the carriers form a thin, highly concentrated layer which acts

like a capacitor plate equal in area to the gate. However, in the high frequency region

which is nearly over 1 MHz, a series combination of Ci and Cpoly is expected to

explain the decreasing measured capacitance due to resistance-capacitance (RC) time

constant of poly-Si. In other words, the frequency is too high for the carriers to charge

and discharge, hence, Rpoly and Cpoly are important parameters that should not be

ignored under high frequency operating conditions.

In device depletion region, as we observed the C-f curve in Fig.3-1-5, Cpoly and

Rpoly still affect the drastic degradation of measured capacitance in high frequency

region, on the other hand, Ct and Rt should be considered in the equivalent circuit

model especially when frequency ranges from 1 kHz to 1 MHz because the number of

defect trapped charges is comparable to the number of mobile charges supplied by

source/drain region in the intrinsic poly-Si channel. Hence, Ct and Rt that are in

parallel with CD can describe why the measured capacitance has a subtle decrease

roughly below 1 MHz. Consequently, the effective capacitance of the equivalent

circuit model is given by:

(1) 1 2 1 2 2 2 2 1 2 1 2 / ( ) 1 1 _poly( ) C C C C C C C R C C ω ω ⎡ ⎤ = ⎢ + ⎥ + _⎢⎣ + + _⎥⎦

, where

(2)

(3)

(4)

The capacitance depends strongly on the change of frequency in the turn-on

region and the depletion region; here, we focus on the discussion of the frequency

response in the depletion region, and the equations shown above indicate that

capacitance is a function of frequency under the depletion operating conditions related

to carrier lateral flow and trap response time.

3.2.2 Methods of device parameter extraction

In our studies which focus on the analysis in the depletion region, main

parameters are extracted from C-f and conductance-frequency (G/f-f) curves. In low

frequency region, the value of measured capacitance at 1 kHz is the value of Cp in

series with Ci. Here CP is the summation of CD and Ct affected by frequency.

Therefore, Rt and Ct are the fitting parameters that we determine to describe the

phenomenon of the slightly decreasing capacitance. In high frequency region, we

regard the value of measured capacitance as Cpoly at 15 MHz. Besides, according to

1 i P i P C C C C C = + 2 2 2 1 t P D t t C C C R C ω = + + 2 2 2 2 1 ct poly ct ct C C C R C ω = + +

equation (1), C1 calculated from the effective capacitance of Ci and CP, is larger than

Cpoly at least by one order in the depletion region; hence Cpoly can be neglected, and

Rpoly can be obtained by the peaks of the G/f-f curves shown in Fig.3-2-2 under

different gate biases. Fig.3-2-2 shows the G/f-f curve of p-type poly-Si TFT with

W/L=600µm /6µm biased under depletion region. In addition, the response time

constant of the trap level is the product of Rt and Ct. Fig.3-2-3 shows the G/f-f curve

of p-type poly-Si TFT with W/L=600µm /600µm, Rpoly under each gate bias is still

obtained by the peak of the curve. However, the curve in the device with channel

length as 600 µm is asymmetric, which is considered an envelope formed by two

curves, and caused by the trap states in the channel. Therefore, Cct and Rct are the

fitting parameters that we determine to describe the phenomenon of the lingeringly

decreasing capacitance. Besides, the response time constant of the channel trap level

is the product of Rct and Cct.

3.3 The fitting results and the dependence of parameters on gate bias

Fig.3-3-1 shows the curves fitted by the equivalent model and the measured data

of ELA p-type poly-Si TFT which dimension is W/L=600µm /6µm under each gate

bias. We can see this model for short channel device is very similar to the measured

from the depletion region to the turn-on region, Rpoly decreases swiftly and almost

reaches a specific constant value. The comparison of Rpoly under two different lengths

is shown in Fig.3-3-3, a prominent difference in the depletion region near the turn-off

region can be observed. Furthermore, under the same gate bias, Rpoly of longer

channel length device is larger which means the carrier lateral flow response to the

change of the channel length. The dependence of the trap response time which is the

product of Rt and Ct on gate bias is shown in Fig.3-3-4. We can observe that the trap

response time becomes faster as VG is biased from the depletion region to the turn-on

region. In other words, the trap charges are easier to release from the trap level to

conduction. Fig.3-3-5 shows the curves fitted by the equivalent model and the

measured data of ELA p-type poly-Si TFT which dimension is W/L=600µm /600µm

under each gate bias. This model for long channel device is still similar to the

measured value. However, some deviations occur in smaller gate biases, such as -2.5

V, -3 V. It is because the value of Cp defined at 1 kHz is not correct; the accurate Cp

should be calculated less than 1 kHz as the capacitances reach individually stable

values. Thus, lower measured frequency and less noise are required. Fig.3-3-6 shows

the channel trap response time, similarly, τct is the product of Rt and Ct on gate bias.

We can observe that channel trap response time becomes faster as VG is biased from

Chapter 4

Conclusion

In this thesis, we studied capacitance model and its frequency response of

poly-Si TFTs. We introduced a physically-based equivalent circuit to illustrate the

dependence of capacitance on frequency and gate bias. Here, we focused on the

depletion region due to the drastic frequency response. For device with channel length

as 6 µm, the slight decrease in measured capacitance under low frequency condition

which ranges from 1 kHz to about 1 MHz results from the defects existing in the

poly-Si film. As frequency increases over 1 MHz, the ac signal is too high for the

carriers to inject from the source/drain region to respond in the whole channel.

Therefore, the reduced capacitance can attribute to the influence of the characteristics

resistance (Rpoly) and capacitance (Cpoly) of the carrier lateral flow in the channel area

which are considered connecting in series with an effective capacitance; the effective

capacitance is combined by the insulator capacitance and depletion capacitance in

series connection. The fitting curves calculated using the proposed model explained

the measured capacitance very well.

As a result of the dependence of carrier lateral flow on frequency, Rpoly extracted

high frequency operation.

As the device dimensions scaled down, the influence of the characteristics

resistance and capacitance of the carrier lateral flow in the channel area is not obvious

but still can not be neglected, especially under high frequency operating conditions.

On the contrary, Rpoly is affected by the long channel length because the carriers can

not totally respond to the whole channel area under high frequency operation; thus,

Rpoly has much larger variation compared to the shorter channel one. We can observe

that τt becomes faster as VG is biased from the depletion region to the turn-on region.

In other words, these trap charges are easier to release from the trap level to

conduction.

In conclusion, the effects of defects, device dimension, gate bias and frequency

are considered in the proposed capacitance model. With the proposed equivalent

circuit, we can develop physically-based C-V model for LTPS TFTs to replace current

References

[1] A. G. Lewis, I-W. Wu, T. Y. Huang, A. Chiang, and R. H. Bruce, “Active matrix

liquid crystal display design using low and high temperature processed polysilicon

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[2] Y. Matsueda, M. Ashizawa, S. Aruga, H. Ohshima, and S. Morozumi, “New

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[3] R. G. Stewart, S. N. Lee, A. G. Ipri, D. L. Jose, D. A. furst, S. A. Lipp, and W. R.

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W. Lan, “Characteristics and Three-Dimensional Integration of MOSFETs in

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[8] T. Kaneko, Y. Hosokawa, M. Tadauchi, Y. Kita, and H. Andoh, “400 dpi Integrated

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1086-1039, 1991.

[9] Y. Hayashi, H. Hayashi, M. Negishi, T. Matsushita, “A Thermal Printer Head with

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Solid-State Circuits Conference (ISSCC), pp. 266, 1998.

[10] N. Yamauhchi, Y. Inaba, and M. Okamamura, “An Integrated

Photodector-Amplifier using a-Si p-i-n Photodiodes and Poly-Si Thin Film

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[11] M. G. Clark, “Current status and future prospects of poly-Si devices,” IEE

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Physics, vol. 69, pp. 1703-1706,1991

[13] T. J. King, K. C. Saraswat, “Low-temperature fabrication of poly-Si thin-film

transistors,” IEEE Electron Device Letters., vol. 13, pp. 309-311, 1992

[14] H. Kuriyama, S. Kiyama, S. Noguchi, T. Kuahara, S. Ishida, T. Nohda, K. Sano,

H. Iwata, S. Tsuda, S. Nakano, “High mobility poly-Si TFT by a new excimer laser

annealing method for large area electronics,” IEDM Tech. Dig, vol. 91, pp.

563-566, 1991

[15] S. W. Lee, S. K. Joo, “Low temperature poly-Si thin-film transistor fabricated by

metal-induced lateral crystallization,” IEEE Electron Device Letters., vol. 17, pp.

160-162, 1996

[16] Jong S. Choi, “Frequency-Dependent capacitance-voltage characteristics for

amorphous silicon-based metal-insulator-semiconductor structures” IEEE Electron

Device., vol. 39, 1992

[17] Mark D. Jacunski, “Characterization and modeling of short channel poly silicon

thin film transistors”, RPI Jacunski Dissertation

[18] Horng Nan Chern, Chung Len Lee, Tan Fu Lei “An analytical model for the

above-threshold characteristics of polysilicon thin-fm transistors”, IEEE Electron

1.23E-06

2.92E-06

5.31E-06

τ

_t

( s )

-1.7

1.5

-2.0

V

_G

( V )

380

360

340

mJ/cm

2

1.23E-06

2.92E-06

5.31E-06

τ

_t

( s )

-1.7

1.5

-2.0

V

_G

( V )

380

360

340

mJ/cm

2

The response time of ELA p-type poly-Si TFTs fabricated under different excimer laser energy densities. W/L=600µm /6µm.

gate

Insulator

G

S D

Ci Rch

p

+

p

+

(a)

(b)

Fig.2-1-1 The schematic cross sectional view of Poly-Si TFT.(a) is n-type ELA poly-Si TFT, (b) is p-type ELA poly-Si TFT.

V_G ( V ) -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 C ( F ) 0.0 4.0e-13 8.0e-13 1.2e-12 1.6e-12 2.0e-12 2.4e-12 2.8e-12 3.2e-12 10k 30k 50k 100k 300k 500k V_G ( V ) -5 -4 -3 -2 -1 0 1 C ( F ) 0.0 4.0e-13 8.0e-13 1.2e-12 1.6e-12 2.0e-12 2.4e-12 2.8e-12 3.2e-12 10k 30k 50k 100k 300k 500k

Fig.3-1-1 (a) C-V curves of ELA p-type poly-Si TFT. W/L=600µm /6µm. Frequency

is from 10k to 500 kHz. ELA laser energy is 380 mJ/cm2. (b) VG is from

-1V to -2.2V. Turn-off region Turn-on region Depletion region (a) (b)

V_G ( V ) -5 -4 -3 -2 -1 0 1 C ( F ) 0.0 4.0e-13 8.0e-13 1.2e-12 1.6e-12 2.0e-12 2.4e-12 2.8e-12 3.2e-12 10k 30k 50k 100k 300k 500k V_G ( V ) -5 -4 -3 -2 -1 0 1 C ( F ) 0.0 4.0e-13 8.0e-13 1.2e-12 1.6e-12 2.0e-12 2.4e-12 2.8e-12 3.2e-12 10k 30k 50k 100k 300k 500k

Fig.3-1-2 C-V curves of ELA p-type poly-Si TFT. W/L=600µm /6µm. Frequency is

from 10k to 500 kHz. ELA laser energy of (a) is 360 mJ/cm2, of (b) is 340

mJ/cm2. (a)

Fig.3-1-3 SEM images of poly silicon film. ELA laser energy of (a) is 380 mJ/cm2, (b) is 360 mJ/cm2, (c) is 340 mJ/cm2.

(a)

(b)

V_G ( V ) -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 Ea ( e V ) 0.0 0.2 0.4 0.6 0.8 340 mJ/cm2 360 mJ/cm2 380 mJ/cm2

Fig.3-1-4 The dependence of activation energy on gate voltage under three laser energy densities. These are ELA p-type poly-Si TFTs. W/L=600µm /6µm.

trap density

380mJ/cm

2

: 8.97x10

12

1/cm

2

360mJ/cm

2

: 1.06x10

13

1/cm

2

340mJ/cm

2

: 1.81x10

13

1/cm

2 1/(V_G-V_FB)2 ( V-2 ) 0.00 0.02 0.04 0.06 0.08 0.10 ln[I DS /( V G -V FB )V DS ] -12 -11 -10 -9 -8 -7 380 mJ/cm2 360 mJ/cm2 340 mJ/cm2

Fig.3-1-5 Trap density extraction by modified Levinson theory under three laser energy densities. These are ELA p-type poly-Si TFTs. W/L=600µm /6µm.

Frequency ( Hz ) 104 ₁₀5 C ( F ) 5.0e-13 7.0e-13 9.0e-13 1.1e-12 1.3e-12 1.5e-12 1.7e-12 1.9e-12 380 mJ/cm2 360 mJ/cm2 340 mJ/cm2 model at 380 mJ/cm2 model at 360 mJ/cm2 model at 340 mJ/cm2 VG= -1.5V VG= -2V VG= -1.7V

Fig.3-1-6 C-f curves of ELA p-type poly-Si TFT. W/L=600µm /6µm. ELA laser

V_G ( V ) -5 -4 -3 -2 -1 0 1 C ( F ) 0.0 4.0e-13 8.0e-13 1.2e-12 1.6e-12 2.0e-12 2.4e-12 2.8e-12 3.2e-12 10k 100k 500k 1M 10M

Fig.3-1-7 C-V curves of ELA p-type poly-Si TFT. W/L=600µm /6µm. Frequency is

V_G ( V ) -20 -15 -10 -5 0 C ( F ) 0.0 4.0e-11 8.0e-11 1.2e-10 1.6e-10 2.0e-10 2.4e-10 2.8e-10 10k 100k 500k 1M 10M

Fig.3-1-8 C-V curves of ELA p-type poly-Si TFT. W/L=600µm /600µm. Frequency is

V_G ( V ) -5 -4 -3 -2 -1 0 1 2 3 4 5 C ( F ) 0.0 4.0e-13 8.0e-13 1.2e-12 1.6e-12 2.0e-12 2.4e-12 2.8e-12 3.2e-12 10k 100k 500k 1M 10M

Fig.3-1-9 C-V curves of ELA n-type poly-Si TFT. W/L=600µm /6µm. LDD=0.75 µm.

Frequency ( Hz ) 103 ₁₀4 ₁₀5 ₁₀6 ₁₀7 C ( F ) 0.0 5.0e-13 1.0e-12 1.5e-12 2.0e-12 2.5e-12 3.0e-12 3.5e-12 -3V -2.4V -2.2V -2V -1.8V -1.6V -1.4V -1.2V -1V

Fig.3-1-10 C-f curves of ELA p-type poly-Si TFT. W/L=600µm /6µm. VG is biased

Frequency ( Hz ) 103 ₁₀4 ₁₀5 ₁₀6 ₁₀7 C ( F ) 0.0 4.0e-11 8.0e-11 1.2e-10 1.6e-10 2.0e-10 2.4e-10 2.8e-10 -8V -7V -6V -5V -4V -3.5V -3V -2.5V -2V

Fig.3-1-11 C-f curves of ELA p-type poly-Si TFT. W/L=600µm /600µm. VG is

Frequency ( Hz ) 103 ₁₀4 ₁₀5 ₁₀6 ₁₀7 C ( F ) 0.0 5.0e-13 1.0e-12 1.5e-12 2.0e-12 2.5e-12 3.0e-12 3.5e-12 1V 0.7V 0.5V 0.3V 0V -0.3V -0.5V -0.7V -1V

Fig.3-1-12 C-f curves of ELA n-type poly-Si TFT. W/L=600µm /6µm. LDD=0.75

C

_i

R

_poly

C

_poly

C

_D

R

_t

C

_t

R

_ct

C

_ct

C

_i

C

_D

R

_t

C

_t

C

_poly 1 2

C

poly

1

2

2 R

_poly

2 R

_poly

C

_i

R

_poly

C

_poly

C

_D

R

_t

C

_t

R

_ct

C

_ct

C

_i

R

_poly

C

_poly

C

_D

R

_t

C

_t

C

_D

R

_t

C

_t

R

_ct

C

_ct

C

_i

C

_D

R

_t

C

_t

C

_poly 1 2

C

poly

1

2

2 R

_poly

2 R

_poly

C

_i

C

_D

R

_t

C

_t

C

_i

C

_D

R

_t

C

_t

C

_i

C

_D

R

_t

C

_t

C

_i

C

_D

R

_t

C

_t

C

_i

C

_D

R

_t

C

_t

C

_poly 1 2

C

poly 1 2

C

poly

1

2

C

_poly

1

2

2 R

_poly

2 R

_poly

Fig.3-2-1 (a) Capacitance equivalent circuit model of poly-Si TFTs for short channel. (b) Capacitance equivalent circuit model of poly-Si TFTs.

(a)

Frequency ( Hz ) 103 ₁₀4 ₁₀5 ₁₀6 ₁₀7 G/ f ( S/Hz ) 0 1e-12 2e-12 3e-12 4e-12 5e-12 6e-12 7e-12 8e-12 9e-12 1e-11 -3V -2.4V -2.2V -2V -1.8V -1.6V -1.4V -1.2V -1V

Fig.3-2-2 G/f-f curves of ELA p-type poly-Si TFT. W/L=600µm /6µm. ELA laser

Frequency ( Hz ) 103 ₁₀4 ₁₀5 ₁₀6 ₁₀7 G/f ( S/ Hz ) 0 1e-10 2e-10 3e-10 4e-10 5e-10 6e-10 7e-10 -8V -7V -6V -5V -4V -3.5V -3V -2.5V -2V -1V

Fig.3-2-3 G/f-f curves of ELA p-type poly-Si TFT. W/L=600µm /600µm. ELA laser

Frequency ( Hz ) 103 ₁₀4 ₁₀5 ₁₀6 ₁₀7 C ( F ) 0.0 4.0e-13 8.0e-13 1.2e-12 1.6e-12 2.0e-12 2.4e-12 2.8e-12 3.2e-12 3.6e-12 -3V -2V -1.4V model at -3V model at -2V model at -1.4V

Fig.3-3-1 The curves are calculated using the model. ELA p-type poly-Si TFT. W/L=600µm /6µm. ELA laser energy is 380 mJ/cm2.

V_G ( V ) -8 -7 -6 -5 -4 -3 -2 -1 Rpo ly ( Ω ) 1e+4 1e+5 1e+6 1e+7 W/L=600/6 μm W/L=600/600 μm V_G ( V ) -3.0 -2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 Rpo ly ( Ω ) 1e+4 2e+4 3e+4 4e+4 5e+4 6e+4 7e+4 8e+4 9e+4 1e+5 W/L=600/6 μm

Fig.3-3-2 Extracted Rpoly of ELA p-type poly-Si TFT. W/L=600µm /6µm.

Fig.3-3-3 Extracted Rpoly of ELA p-type poly-Si TFT. W/L=600µm /6µm, and 600µm

V_G ( V ) -3.0 -2.5 -2.0 -1.5 -1.0 τt ( s ) 0 1e-6 2e-6 3e-6 4e-6

Frequency ( Hz ) 103 ₁₀4 ₁₀5 ₁₀6 ₁₀7 C ( F ) 0.0 4.0e-11 8.0e-11 1.2e-10 1.6e-10 2.0e-10 2.4e-10 2.8e-10 -8V -4V -3V -2.5V model at -8V model at -4V model at -3V model at -2.5V

Fig.3-3-5 The curves are calculated using the model. ELA p-type poly-Si TFT. W/L=600µm /600µm. ELA laser energy is 380 mJ/cm2.

V_G ( V ) -8 -7 -6 -5 -4 -3 -2 τct ( s ) 0 2e-6 4e-6 6e-6 8e-6 1e-5

簡 歷

姓名：邵而康

出生日期： 中華民國七十二年四月二十七日 (1983.04.27)

地址：台北市延吉街 30 巷 15 號 2 樓

學歷：

國立交通大學電子物理系學士

(2001.09-2005.06)

國立交通大學 電子研究所碩士班

(2005.09-2007.06)

論文題目：

低溫複晶矽薄膜電晶體的電容模型及其頻率響應

Capacitance Model and its Frequency Response of

Low-Temperature Polycrystalline Silicon Thin Film Transistors