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Sains Malaysiana 38(4)(2009): 553557

Statistical Desin of Ultra-Thin SiO2

for Nanodevices(Reka bentuk Statistik SiO

2Ultranipis untuk Nanoperanti)

U. HASHIM*, M. F. A. ABDUL FATAH, I. AHMAD & B. Y. MAJLIS

ABSTRACT

A study was performed on a series of ultra thin SiO2

lms in order to determine the factors affecting the oxide growth

and also the effect of temperature to the lm surface roughness. The samples of ultra thin SiO2

were prepared through a

dry oxidation method using a high temperature furnace. There are three levels of temperature used, that is 900, 950 and

1000C and the samples were grown at 0.333 litre/min, 0.667 liter/min and 1 liter/min oxygen ow rate and different

oxidation times of 1, 2 and 3 minutes. The thickness was determined using an ellipsometer and the micro morphology

of the oxide surface was obtained by using an atomic force microscope (AFM). The thickness of the oxide ranged from 1

to 5 nm. All the data has been interpreted using Taguchis method to analyze the most affecting factors in producing an

ultra thin silicon dioxide. The optimum parameters are 900C, 0.333 litre/min and at 1 minute time. The most inuentialparameter is temperature. The temperature also affects the surface roughness. The AFMresult of 950C withRMSvalue of

0.1088 nm is better than the 900C oxide with RMSvalue 0.4553 nm. This shows that oxides need to be grown at a higher

temperature to provide better surface roughness which is also important in ultra thin gate oxide characteristics.

Keywords:Atomic force microscopy (AFM);CMOS; gate dielectrics; silicon dioxide; Taguchis method; ultra-thin gateoxide

ABSTRAK

Kajian telah dijalankan ke atas beberapa siri lapisan SiO2ultranipis untuk menentukan faktor-faktor yang mempengaruhi

pertumbuhan oksida dan juga kesan suhu terhadap kekasaran permukaan lapisan. Sampel SiO2

lampau nipis telah

disediakan melalui kaedah pengoksidaan kering menggunakan relau bersuhu tinggi. Terdapat tiga peringkat suhu yang

digunakan iaitu 900, 950 dan 1000C dan sampel telah ditumbuhkan dalam 0.333 liter/min, 0.667 liter/min dan 1 liter/

min dan perbezaan masa pengoksidaan, 1, 2 dan aliran oksigen pada kadar 3 minit. Pencirian ketebalan dilakukan

dengan menggunakan elipsometer dan mikromorfologi bagi permukaan oksida diperolehi menggunakan mikroskop daya

atom (AFM). Ketebalan oksida yang diperolehi adalah dalam julat 1 hingga 5 nm. Semua data yang diperolehi dianalisis

menggunakan kaedah Taguchi untuk menganalisis faktor-faktor yang paling mempengaruhi penghasilan SiO2ultranipis.

Parameter yang paling optimum ialah 900C, 0.333 liter/min pada masa 1 minit. Faktor yang paling mempengaruhi

prosess ini ialah suhu tetapi suhu juga mempengaruhi kekasaran permukaan. KeputusanAFMpada 950C dengan nilai

RMS0.1088 nm adalah yang paling baik berbanding pada 900C dengan nilai RMS0.4553 nm. Kajian ini membuktikan

bahawa pertumbuhan oksida perlu dilakukan pada suhu tinggi untuk menghasilkan kekasaran permukaan yang lebih

baik yang juga amat penting bagi ciri get oksida ultranipis.

Kata kunci:CMOS; get oksida ultranipis; get dielektrik; kaedah Taguchi; mikroskop daya atom (AFM); silikon dioksida

INTRODUCTION

Ultra-thin ate oxide is a thin layer of oxide (usually silicon

dioxide) forms insulatin layer between the control ate

and the conductin channel of the transistors, which turns

the current ow on and off. The gate oxide layer, acts as

an insulator, protectin the channel from the ate electrode

and preventin a short circuit.

As circuits are made denser, all of the dimensions

of the transistors are reduced correspondinly; these also

means reducin the thickness of the oxide. However

reducin the thickness is not an easy solution because there

are physical and practical limits on how thin an oxide lmcan be made. Fiure 1 shows the historical trend in oxide

thickness for hih-performance loic applications over the

past decade.

There are few methods of producin ultra-thin ate

oxide but silicon dioxide is usually thermally rown and not

deposited by CVD (chemical vapour deposition). Thermal

oxide has hih interity than most CVD oxide lm and so far

has demonstrated hih uniformities, less defects and hih

dielectric strength than deposited oxide thin lm. Thermal

oxide is normally rown in a diffusion furnace at a hih

temperature usin either wet or dry rowth method. Dry

oxide rowth rate is much slower than wet, for this reason

dry oxidation are primarily used for thin oxide where hihuniformity and hih dielectric strenth are needed.

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The thickness of the oxide must be less than 3 nm to be

considered as an ultra thin lm. Any different in thickness

even if 1 nanometre would ive a different characteristic.

Therefore uniformity of the oxide is a must for achievin

a good thin lm oxide. The thickness must be sufciently

uniform across the wafer, wafer to wafer and from run to

run.

In the desin of the experiments, Tauchis method

is used. From the practical point of view, the oal of

Taguchis method in this experiment is to nd and examine

the interactions between factors. The equilibrium between

levels of different factors, robust tolerance desin, and

costs is based on two main concepts proposed by Tauchi:

quality loss function and sinal/noise ratio. Accordin to

Tauchis quality enineerin philosophy and methodoloy,

there are three important steps in desinin a product or

process: system desin, parameter desin and tolerance

desin. The aim of system desin is to create a product that

indeed has the properties intended for it at the plannin

stae. This involves the development of a prototype, choice

of materials, parts, components, assembly system and

manufacturing processes, so that the product fullls the

specied conditions and tolerances at the lowest costs.

DETAILS OF EXPERIMENT

Details of the experimental setup are summarized as

follows: The thermal rowth of silicon dioxide layers

on silicon wafers are rown by dry oxidation

method usin a hih-temperature furnace. The silicon

dioxide layers was rown from 1 to 3 minutes with

temperatures at 900C, 950C and 1000C and with

various oxygen ow rates. The oxygen ow rates used

are 0.333, 0.667 and 1 litre/minute. The parameters are

arrane in standard L9 orthoonal array, which meansfor any pair of columns, all combinations of factor

level occurs at an equal number of times. Details of the

parameters are shown in Table 1.

The pre-oxidation cleanin sequence consisted of

H2O

2-based solutions of NH

4OH and HCl with appropriate

DI water rinses, followed by a dip in dilute HF and a nal

DI water rinse. This cleanin sequence has been shown

to yield a hydroen-terminated silicon surface (Marras et

al. 2004). The wafers were then dried usin nitroen and

immediately loaded in the oxidation furnace with nitroen

owing. Upon reaching the target oxidation temperature,

the furnace ambient was then switched to dry oxyen.

TABLE 1. Experimental parameters

Experiment Temperature Time Oxygen Flowrate

No (C) (min) (Liter/min)

1 900 1 0.33

2 900 2 0.67

3 900 3 1

4 950 1 0.67

5 950 2 1

6 950 3 0.33

7 1000 1 18 1000 2 0.33

9 1000 3 0.67

FIgURE 1. Historical trends in thickness of SiO2

uses as ate insulator in

CMOS loic vs. year of publication (Blasco 2001; Stathis 2002)

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chanes that affect the thickness of the silicon dioxide

and the results indicates that temperature are affectin

the most.

The best parameter shown to row the thinnest

oxide are at 900C, 1 minute and with 0.333 L/min. In

order to create an ultra-thin silicon dioxide to be usedas a ate oxide, the thickness is not the only factor to

be considered. The uniformity, the surface rouhness

and also the intensity of oxide on the surface are also

important for achieving a good thin lm oxide. Because

of this, it was decided that ultra thin ate oxide should be

rown between 900C to 950C, within 2 minutes time

and with 0.667 litre/min oxygen ow rate. The effect

of this temperature to the surface rouhness has been

obtained in an experiment and it is presented in Fiure 4

and Fiure 5.

Shown in Fiure 4 and 5 are micro morpholoies of

the silicon dioxide reions. Here it is shown that surfacerouhness of the oxides for the second sample is better than

the rst sample. This can be proven by the result ofRMS

for both samples. The root mean square (RMS) for the rst

sample is 0.4553 nm compare to the second sample that is

0.1088 nm. But both samples are maintained at very ood

levels, especially when the RMS measurement is less then

1 nm.

All the thickness data obtained are interpreted

usin Tauchis method to achieve the best parameter of

acquirin the thickness below 3 nm.

To obtain the micro morpholoy of the oxide, the

atomic force microscopy (AFM) was used and also subjected

to the imae enhancin technique to improve the imae.

Two samples have been rown usin dry oxidation method.

The parameters of the sample are 900C, 2 minutes, 1 litre/

min oxygen ow rate and 950C, 2 minutes, 1 litre/min

oxygen ow rate.

RESULT AND DISCUSSION

The thickness of each sample has been obtained at ve

different points using an elipsometer. Using this ve

thickness data, S/N ratio for each sample was calculated

usin the formula iven:

S/N Ratio = (1)

where n = 5 (the number of times the thickness is measured)

andx is the thickness value. The S/N value (Table 2) for

each factor can be measured usin formula (Wu 2002)

iven:

Factors = (2)

whereya,y

band y

cis the value of S/N in the experiment in

which that the factors are involved. The results for each

factor are shown in Fiure 2.

The raph presented in Fiure 3 is the S/N valueof each factor that is affectin the oxide thickness. The

raphical representation is also convenient for drawin

qualitative inferences and choosin the optimum level.

The optimum level of factors can be achieved by choosin

the hihest S/N ratio for each factor; in this way the oxide

thickness below 3 nm is achieved. From the raph in

Fiure 3, the slope of each factor shows the amount of

TABLE 2. The S/N ratio for each sample

Experiment No S/N Ratio Each Sample

1 -2.159

2 -5.758

3 -7.463

4 -8.755

5 -10.213

6 -6.951

7 -8.499

8 -8.568

9 -15.181

FIgURE 2. graphical values of control factors and their levels

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CONCLUSION

The statistical desin of the experiment was used to

evaluate three process variables that are temperature, time

and the oxidation ow rate. The analysis of the graphical

value in Fiure 3 indicated that the temperature is the

most signicant factor in oxide growth. The atomic forcemicroscopy result of 950C with RMS value of 0.1088 nm

is better than the 900C oxide with RMS value 0.4553 nm.

AFM shows that oxidation at hiher temperature is effective

to reduce the surface rouhness. This proves that oxides

need to be rown in a hiher temperature to provide better

surface rouhness which is also important for an ultra thin

ate oxide characteristic. However, because the increase in

temperature also increase the thickness, it is believed that the

statistical desin of experiment can be employed to control

and optimize the effect of all factors in oxidation process.

This will make the realization of achieving at ultra thin

silicon dioxide surface comes true as it is becomin even

more challenin as circuits are made denser and all of the

dimensions were reduced correspondinly.

REFERENCES

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SiO2

on Si. Nanotechnology 12: 110-112.

Hattori, T., Nohira, H. & Takahashi, K. 1999. Initial rowth

steps of ultrathin ate oxides. Microelectronic Engineering

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Marras, A., Munari, I.D., Vescovi, D. & Ciampolini, P. 2004.Performance evaluation of ultra thin ate oxide CMOS

circuits. Solid-State Electronics 48: 551-559.

Michel Houssa. 2004.High-K Gate Dielectrics . United Kindom:

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Mur, P., Semeria, M.N., Olivier, M., Papon, A.M., Ch. Leroux,

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Rios, R. & Arora, N.D. 1994. Determination of Ultra Thin gate

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FIgURE 4. Imaes ofAFM scan and the surface roughness line prole of sample grown n 1 litre/min

of oxygen ow rate for 2 minutes at 900C with the thickness of 1.96 nm

FIgURE 5. Imaes ofAFM scan and the surface roughness line prole of samples grown in 1 litre/min

of oxygen ow rate for 2 minutes at 950C with the thickness of 2.85 nm

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Ryu Hasunuma, Junichi Okamoto, Norio Tokuta, Kikuo

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on Si(111)

Characterized by Conductive Atomic Force Microscopy,

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Yamabe. 2005. Morpholoical Chane in Surface and

Interface durin Ultra thin SiO2 Film growth, TheElectrochemical Society Interface 747.

Stathis, J.H. 2002.Reliability limits for the ate insulator in CMOS

technoloy, IBM Journal of Research and Development:

Scaling CMOS to the limits 46(2/3): 256-286.

Thakur, R.P.S. 1993. Ultrathin gate and Capacitor Dielectric

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U. Hashim*

Institute of Nano Electronic Enineerin (INEE)

Universiti Malaysia Perlis

01000 Kanar, Perlis

Malaysia

M.F.A. Abdul Fatah, I. Ahmad & B.Y. MajlisDepartment of Electrical, Electronics and System Enineerin

Faculty of Enineerin

Universiti Kebansaan Malaysia

43600 Bani, Selanor D.E.

Malaysia

*Correspondin author; email: uda@unimap.edu.my

Received: 2 May 2008

Accepted: 4 December 2008