21 Thermal Oxidation

Dr. Ayushi Paliwal and Dr. Monika Tomar

1. Introduction

As we have studied in past chapters, in order to achieve weak signal amplification Field Effect Transistors (FETs) have been predominantly used over the last few decades. A field effect transistor has a very simple geometry. A channel, through which the current flows in this semiconductor based device, is sandwiched between a source and a drain electrodes. Another electrode called the gate electrode is provided, as can be seen in Figure 1.1. This gate electrode controls the functioning of the Filed Effect Transistor. By applying certain amount of voltage to the gate electrode the effective electrical diameter of the channel semiconducting portion of a FET can be controlled. These FETs serve multipurpose applications today from acting as an oscillator, to amplify the weakest of the signals such as the wireless signals. This is because even the smallest of variations in the value of the applied gate voltage can lead to a significantly large amount of variation in the value of the channel current. Due to their multiple applications in real world a lot of time and research has been carried out in the development of better and more useful incarnations of Field Effect Transistors. Metal Oxide Field Effect Transistors (MOSFET) is one such incarnation which has revolutionised the applicative aspect of Filed Effect Transistors. MOSFET is a four terminal device where just as FET we have a gate electrode, source electrode and drain electrode, but we also have body contact or a substrate contact which in general is coupled with the source electrode. Either the p-type or the n-type semiconducting material can be used as the semiconducting material for the channel of MOSFET. As can be seen in Figure 1.2 the most important aspect of MOSFET (also the reason for the name) is that the gate electrode has an oxide layer over it. This oxide layer exists between the channel and the gate electrode. This oxide layer insulates the metal electrode from the channel semiconducting material, which was also the reasoning behind naming these devices Insulated Gate Field Effect Transistors in the early phases of discovery. You will study more about MOSFET in the subsequent chapters but here we would like to focus on one very important aspect of Metal Oxide Field Effect Transistors and that is the “oxide”. Since their introduction MOSFET has become predominant technological force in the area of Field Effect Transistors and that is the easy availability of high quality thick oxide layer. As silicon can easily form an oxide, this silicon oxide layer can be fabricated by many processes. These include Plasma Enhanced Chemical Vapour Deposition (PECVD), Electrochemical Anodization, and Thermal Oxidation etc. Since, PECVD and electrochemical anodization require costly chemicals and toxic gases, thus thermal oxidation is mostly preferred. Also, comtamination may occur in PECVD and inferior electrical properties of oxides are obtained as compared to thermally grown oxides. Thus, Thermal Oxidation is the most reliable and also the most popular technique for deposition of silicon dioxide and we will be focusing on the same during the course of this module.

Figure 1.2 – Metal Oxide Filed Effect Transistor (MOSFET)

Following thermal oxidation techniques are used for the growth of the oxide layer

• Rapid Thermal Processer (RTP)
• Diffusion furnace (horizontal or vertical)

RTP can be used to reduce the thermal redistribution of impurities at high temperature. For small devices this is an important consideration and as a result most engineers make use of low temperature processes. However, in Rapid thermal oxidation, the thick oxide films cannot be produced as it may generate stress in the film. Thus, for higher thickness, Diffusion furnace process is preferred.

2. Thermal Oxidation

As discussed in the section above, one of the most common technique for fabrication of micro thin film of oxide (which is mostly Silicon Oxide0 on the surface of Silicon is thermal oxidation. This process takes place inside a diffusion furnace in between a temperature range of 800˚C to 1500˚C. The oxide layer grown from thermal oxidation process specifically on silicon is not only useful as a passivation layer (protecting layer) or as an electrically insulating layer but also acts as a hard mask for nitride etching or as a sacrificial layer for surface micromachining or for drive-in diffusion process. Either a Rapid Thermal Processer (RTP) or a diffusion furnace (horizontal or vertical) is used for the growth of the oxide layer which can also be called as a thermal oxide layer. For procuring a thin dry layer of thermal oxide or for devices with low heating capacity Rapid Thermal Processer technique is used.

Figure 1.3 shows the schematic of three zone oxidation furnace that can be used for making SiO2 thin film over Si wafer. The growth of the oxide film is governed by the diffusion of oxygen into the substrate (Si wafer), which means the film growth is actually downwards into the substrate. As the thickness of the oxidized layer increases, the diffusion of oxygen to the substrate becomes slower leading to a parabolic relationship between film thickness and oxidation time as show in Figure 1.4.The temperature is raised to 800° C – 1100° C in three different zones to speed up the process. The center zone is kept at maximum temperature and sides zones are kept at slightly lower temperatures. This process is naturally limited to materials that can be oxidized, and it can only form films that are oxides of that material.The gas is purged into the thermal furnace from one end. This furnace is made up of quartz and hence can resist such high temperature easily. The furnace is surrounded by resistance heaters which produce the heating effect and are coupled with thermocouples to monitor this heating. This furnace is supported by ceramic supports having comb like structure. The gas filled in from one side is vented out from the other side through the end cap which is provided with a vent hole.Thermal oxidation of Si is accomplished by placing the silicon wafers vertically in an open ended quartz tube. The quartz tube is placed in a resistance heated furnace at 900 to 1100 oC. The process of thermal oxidation is divided into two categories: (i) dry oxidation and (ii) wet oxidation.

2.1 Dry and Wet Oxidation

The two process of thermal oxidation for the production of oxide (SiO2) thin film can be described as:

Dry Oxidation

• In this process oxygen gas is exposed to the silicon wafer at approximately 1100˚C (or above). The salient feature here is that this oxygen is dry, that is, oxygen is provided directly without any contamination of water. A very high quality oxide is produced using dry thermal oxidation process which is described in Figure 1.5 (a).

• For devices such as MOSFET normally the dry oxidation method is used to procure the oxide thin film.

• The chemical reaction for this process is very simple and can be described as, Si(Solid) + O2 (Gas) → SiO2(Solid)

• One major disadvantage of dry thermal oxidation method is that growth speed is very slow.

Wet Oxidation

• This process involves the burning of hydrogen and oxygen in there purest form at a temperature as high as 1100˚C. This reaction leads to production of a water vapour rich atmosphere in the oxidation furnace.
• This vapour than acts on the surface of the silicon substrate, which is in form of a thin wafer, producing the silicon oxide layer as can be seen in Figure 1.5 (b).
• The chemical reaction for this process is very similar to the one in dry thermal oxidation,Si(Solid) + 2H2O(gas) → SiO2(Solid) + 2H2(gas)
• Though wet oxidation method gives a comparatively faster growth rate but the disadvantage is that the quality of thermal “wet” oxide produced is not so good as obtained from dry oxidation process.
• This technique is mostly used to produce oxides that can be use as a masking layer or as sacrificial layer for the process of surface micromachining.

The growth rate in dry oxidation is very slow as compared to wet oxidation. However, the electrical properties of SiO2 films grown by dry oxidation are comparatively better than that grown using wet oxidation method. For making thick oxide films, wet oxidation is preferred over dry oxidation, but it introduces more structural defects and higher density of interface state. Thermal oxidation of silicon results in the formation of relatively dense trap free films, which can be used to protect Si wafer during its various high temperature cycles and processing steps. SiO2 is very well known insulator so it is typically used to form electrical insulation or as passivation layer that is used for other device fabrication purposes later in a process sequence.

Other than these two methods another thermal oxidation process is to use oxygen rich in chlorine content to oxidise the thin silicon wafer. This method is popularly termed as Cl Oxidation (Chlorine Oxidation) and the chlorine rich oxygen is produce by flowing oxygen through any of the chlorine containing compounds such as hydrochloric acid (HCl), Tri-cholor ethylene (TCE), Trichloroacetic acid (TCA) etc. Also Sodium (Na) ions are introduced in order to improve the quality of the thermal silicon oxide film. This leads to the reduction in mobile charges and also decreases the amount of crystal defects in the silicon oxide film, hence giving us a high quality pure side layer. Figure 1.6 describes the Chlorine Oxidation process.

2.2 Model for Thermal Oxidation of Silicon Wafer

Deal- Grove Model

The Deal-Grove model is used to understand the formation of silicon oxide thin film over the surface of silicon wafer. The model describes the oxidation reaction occurring at the interface between oxide layer and the substrate (Si) instead of reaction between oxide and ambient gas. The most prominent of model is described below in figure 1.7. When the oxidising species which will be oxygen gas molecules in the case of dry thermal oxidation and water vapour in case of wet thermal oxidation interact with the surface of silicon wafer, it leads to the concentration of the above mentioned species to reach C0 molecules/cm3. The value of C0 is approximated to be 5.2 x 1016 molecules/cm3 for oxygen molecules in dry thermal oxidation,when the temperature of furnace is maintained at 1100˚ C and the pressure applied is 1 atm. The value of C0 at same parameters for wet oxidation process for water molecules is approximately 3 x 1019 molecules/cm3. The Deal- Grove model can be divided into three parts for better understanding:

(i) Transportation of oxidizing species from bulk of the oxidizing gas to outer surface of oxide layer.

(ii) Transportation of oxidizing species across the oxide film towards silicon.

(iii) Reaction of oxidizing species at the interface with silicon wafer and form a new layer of SiO2.

Now once the silicon oxide layer is formed on Si wafer, the oxidising species (oxygen or water vapour) start diffusing through the oxide layer. This diffusion leads to the concentration of oxidising species at the surface of Si surface which is denoted as Cs. The flux value for diffusion can be expressed as,

Where, D is the diffusion coefficient of oxidizing species and x is the thickness of the silicon oxide layer.Now, the value of flux at the surface of the silicon wafer is,

The reaction of the oxidizing agents with silicon forms silicon dioxide. Let C1 be the number of molecules of the oxidizing species in a unit volume of the oxide. There are 2.2 × 1022 silicon dioxide molecules/cm3 in the oxide, and we add one oxygen molecule (O2) to each silicon dioxide molecule, whereas we add two water molecules (H2O) to each silicon oxide molecule. Therefore, C1 for oxidation in dry oxygen is 2.2 × 1022cm−3, and for oxidation in water vapor it is twice this number (4.4 × 1022cm−3). Thus, the growth of the oxide layer thickness is given by:

It may be noted from equation (4) that the thickness of oxide layer (x) varies linearly with oxidation time (t) for small t. Whereas the said variation is parabolic with t for large value of t as per equation (5).Equation (3) is generally written as:

2.3 Oxidation Rate Factors

As we described in the last section the rate of thermal oxidation initially varies linearly with oxidation time (equation 4) and then eventually takes a parabolic route (equation 5). Other than oxidation time there are many other factors which affect the rate of thermal oxidation growth of silicon. Few of them are:

• Orientation of the Si crystal

As we are well aware that silicon being a crystal has various crystal orientations. The growth of oxide thin film varies according to the orientation of the crystal plane.

• Oxidation Methods

Different oxidation techniques lead to different ambient environment for oxidation. As mentioned earlier, dry oxygen process lead to a slow growth rate for oxide layer, while using hydrogen in the wet oxidation technique or chlorine along with oxygen lead to a fast growth rate for oxide layer.

• Concentration of Dopant

Doping of Si substrate is recommended to enhance oxidation rate. The heavy doping of substrate weakens the strength of oxide bonds making it easy for the gas to penetrate through the oxide layer to reach the substrate surface. So heavy doping of substrate enhances and accelerates the rate of thermal oxidation of oxide film.

• Pressure

Pressure of ambient gas plays a major role in determining the rate of oxidation. If an ultra thin layer of oxidised film is require, then the oxidation process is carried out at very low pressure. Also oxidation temperature gets reduced significantly at very high pressure. As mentioned earlier, after certain period of time the oxidation rate becomes parabolic and this parabolic rate constant directly depends on the partial pressure of the oxygen used.

3. Why Silicon Oxide?

Another important thing to understand here is the reason behind fabricating silicon oxide layer. We have already established that silicon can be oxidised easily. A shallow layer consisting of native oxides is formed easily on the surface of silicon even at room temperature, hence making silicon a good candidate for oxidation material. Silicon oxide plays a significant role in MOS technology. It is also strongly resistant to most of the chemical which are generally used for the processing of silicon wafer. It acts as a barrier for impurities and dopants, safeguarding the purity of silicon wafer. This highlights the role of SiO2 as a masking layer. Silicon oxide is stable at high temperatures, has a wide band gap, very high dielectric strength and insulation abilities. This coupled with the easy fabrication of SiO2 layer, that has resulted in making silicon oxide the most desirable candidate for a passivation and protective layer.

3.1 Structure of Silicon Oxide

Silicon oxide is in general grown as amorphous. It has a 3-D tetrahedral network. As is described in Figure 1.8 one silicon atom is surrounded by four oxygen molecules. Though amorphous in nature SiO2 has short range order. Oxygen atom in silicon oxide is shared in a tetrahedral, a few times known as bridging bonds and sometimes the oxygen atoms have non-bridging bonds.The non-bridging bonds lead to fused silica state while bridging bonds form quartz. Also hydrogen atoms can exist as impurities in the SiO2 structure, especially for wet thermal oxidation. Also sodium and other dopants can also exist as impurities. The distance between Si and O molecule is 0.162 nm while Si-Si bond distance is 0.31 nm.

3.2 Properties of Silicon Oxide

As discussed earlier the silicon oxide layer acts as a passivation layer and protective layer. Silicon oxide has many attractive properties, such as,

• Excellent Electrical Insulator

Silicon oxide has an energy gap of 9 eV. The resistivity of SiO2 is 1 x 1020 ohm-cm.

• High Breakdown Electric Field

Electric field breakdown can rise as high as 10 MV/cm.

• Stable and Reproducible Si/SiO2
• Conformal oxide growth on exposed Si surface.
• SiO2 is a good diffusion mask for common dopants

On comparing diffusion rate in silicon and silicon oxide we get, DSiO2<<DSi.

• Very good etching selectivity between Si and SiO2.

Some important properties of silicon oxide has been described in table 1.1.

Table 1.1 – Properties of silicon oxide.

4. Summary 

• In this module we have established the importance of oxide layer in many devices such as MOSFET.
• Though there are many ways in which we can achieve this oxide layer, but the one preferred by most is thermal oxidation method.
• Silicon wafer is cleaned and kept in a thermal furnace which can go up to temperature as high as 1500°c.
• This oxidation furnace is than subjected to either oxygen (dry thermal oxidation) or water molecules (wet thermal oxidation). The molecules of oxygen or water react with the silicon surface forming a thin oxide layer gradually.
• This oxide layer acts as an insulating material and can also be used as a protective layer. It is used for surface micro matching acting as a sacrificial layer.
• Depending on the requirement for quality of oxide layer we can choose either from dry or wet thermal oxidation.
• The configuration and applications of silicon oxide layer were further discussed in this module.

you can view video on Thermal Oxidation