Comparison Of Wet And Dry Oxidation Of Silicon Dioxide

Published Date: 02 Nov 2017

A Anusha, Chithra Parameswaran ,Revathi P and *V.Velmurugan

School of Electronics Engineering, V.I.T University, Vellore, Tamil Nadu, India.

[email protected]

Abstract

Silicon dioxide plays a major role in the present technology. The basic idea is to grow SiO2 layer on Silicon. The oxide is grown on intrinsic silicon substrate by thermal oxidation i.e., wet oxidation and dry oxidation. The thickness is compared for both the oxidation processes. The time of an oxidation cycle is maintained constant and temperature is varied. The oxide thickness is monitored for different number of cycles. The capacitance per unit length is also calculated for the oxide growth in above. As reported experimentally, the wet process gave thicker oxide than dry process. The capacitance per unit length showed a similar result.

Introduction

The surface of silicon substrate or wafer oxidizes to form Silicon dioxide which acts as high quality electrical insulator and during the impurity deposition acts as barrier material. These two properties of Silicon dioxide are the primary factors that leads Silicon to play a dominant role in semiconductor industry. The oxidation of Silicon in steam and dry ambient are compared by using the TCAD tool. Previous work report comparison of the prime models of oxide growth, the Deal Grove and the Massoud[1]. In this paper we are concentrating on Massoud model which governs the oxide growth for thickness less than 30nm. The variation in oxide thickness with oxidation time has been reported in literatures at vast. In this paper we are comparing the oxide thickness with number of cycles using TCAD tool Sprocess. Sprocess facilitates simulation of all the available fabrication processes in the semiconductor domain. Here, we emphasise on oxidation of intrinsic Silicon in dry and wet ambient.

Theory

The SiO2 function layer on a chip is multipurpose. This semiconductor material has the ability to achieve all the properties because of its native oxide layer. Silicon has an easily formed protective oxide, as for other materials we have to depend on the deposited insulators for surface protection. The advantages of using SiO2 is : 1)It acts a diffusion barrier for B,P,As . 2) SiO2 is a good insulator. 3) SiO2 can be etched with HF which leaves Si unaffected. Silicon dioxide (silica) layer is formed on the surface of a silicon substrate by thermal oxidation at higher temperatures. The oxidation of Silicon has been investigated in the temperature range of 800-1200ËšC .The studies involved the use of oxidation atmospheres likes dry oxidation and wet oxidation. The goal of the oxidation is to have a high quality oxide layer on a Silicon Substrate. The main idea is to observe the thickness by both the oxidation process. The thickness of the oxide mainly depends on the temperature and the time.

Wet Oxidation:

It is a form of hydrothermal reaction. The oxidation is done by involving the dissolved components in H2O with oxygen as a oxidizer. When air is used, it is referred as Wet Air Oxidation. In order to avoid excess evaporation of water, the system is maintained under pressure. Generally Wet oxidation grows slowly when compared to dry oxidation but this has become an advantage. The reason is , higher growth rate is the oxidant solubility limit for SiO2, which is much higher for wet oxidation than dry oxidation. Thus wet oxidation is applied for thick oxides in insulation and passivation layers. The chemical reaction is:

Si + 2 H2O → SiO2 + 2H2

Due to its water content, the wet oxide films exhibit a lower dielectric strength and more porosity to impurity penetration than dry oxides. When the electrical and chemical properties of the film are not critical then this wet oxidation is preferred. Mainly wet oxidation is to grow thick oxides such as masking oxides, blanket field oxide etc.

Dry Oxidation:

Dry oxidation has lower growth rate than wet oxidation, although the oxide film quality is better. Therefore thin oxides such as screen oxide, pad oxide and especially the gate oxide uses the dry oxidation processes. Compared with others, this has best material characteristics and quality. The oxidation rate is low and so the oxide thickness can be controlled accurately. The chemical reaction for Silicon and Oxygen is:

Si + O2 → SiO2.

The oxide growth is explained by two methods .They are Deal Grove model and Massoud model.

Deal grove model can describes the growth of oxide with thickness above 30nm only. As the device geometry and size shrinks down, the limitations of this model becomes evident. Massoud model has been suggested as an extension of the Deal grove model which addresses and improves the description in the thin oxide growth regime. This is an analytical model based on parallel oxidation mechanisms. Here the growth rate expression is modified by adding a term that exponentially decays with the oxide thickness. These are inbuilt capabilities of Sprocess and the layer thickness is extracted using the data extraction functions available within it.

The first term gives a linear-parabolic behaviour to the growth where $ B$ and   are the parabolic and linear rate constants, respectively, as defined by Deal and Grove, but their values in the Massoud model are completely different. In Arrhenius-expression the rate constants can be written in the form.

B=CB exp(

Results and Discussion

The oxide thickness is measured for number of cycles temperatures varying from 800ËšC to 1100ËšC using the TCAD Sprocess simulator. The oxide thickness comparisons were made for wet and dry oxidation for the above temperatures. The graphs for oxide thickness Vs number of cycles as obtained are shown below.

Figure 1: tox vs. Number of Cycles for Wet Oxidation

Figure 2: tox vs. Number of Cycles for Dry Oxidation

These graphs were based on the Massoud model. The various temperatures for both oxidations are compared and it reveals a higher growth rate for wet oxidation than dry oxidation, which is in accordance with the experimental reports.

Figure 3: tox vs. Number of cycles for wet and dry oxidations at 800ËšC

Figure 3 shows the plots for oxide thickness Vs number of cycles for wet and dry oxidation at 800ËšC The oxide growth is linear for both the oxidations and wet oxidation has a faster growth rate than dry oxidation.

Figure 4: tox Vs. Number of cycles for both wet and dry oxidations at 900ËšC

Figure 4 shows the plot for oxide thickness Vs number of cycles for wet and dry oxidations at 900ËšC. Wet oxidation grows linearly upto 600 cycles and then in a parabolic manner.

Figure 5: tox vs. Number of cycles for wet and dry oxidation for 1000ËšC

Figure 5 shows the plot for oxide thickness Vs Number of cycles for wet and dry oxidations at 1000ËšC. Wet process gives a steeper curve with higher growth rate per cycle than dry process.

Figure 6: tox Vs. Number of cycles for wet and dry oxidation for 1100ËšC

Figure 6 shows the plot for oxide thickness Vs Number of cycles for both wet and dry oxidations at 1100ËšC .It has been observed that for both wet oxidation and dry oxidation it follows the linear path.

The Capacitance per unit length during the growth process is shown below.

Figure 7 Capacitance per unit length Vs Number of cycles for dry oxidation

Figure8 Capacitance per unit length Vs Number of cycles for wet oxidation

Conclusion

The oxide growth is simulated for dry and wet processes. The thickness and capacitance per unit length of the dielectric is observed with number of cycles and plots for the same are obtained. From the above graphs we infer that wet oxidation has higher growth rate than dry oxidation and oxide film quality is better for wet oxide film.

For wet oxidation in the above plot it can be seen that the increase in temperature from 900 to 1000c leads to approximately double the oxidation rate for every 100c increase in temperature, where as for dry oxidation the oxidation rate is faster than its counterpart for the same temperature values.