oxidation process in ic fabrication

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Oxidation Process in IC Fabrication

To know the basics click on the following links

TAKE A LOOK : IC FABRICATION TECHNIQUES

TAKE A LOOK : SILICON SUBSTRATE PREPARATION

TAKE A LOOK : CHEMICAL VAPOUR DEPOSITION (CVD)

Utility of Thermal Oxidation

The function of a layer of silicon dioxide (SiO2) on a chip ismultipurpose. SiO2 plays an important role in IC technologybecause no other semiconductor material has a native oxide whichis able to achieve all the properties of SiO2. The role of SiO2 in ICfabrication is as below :

It acts as a diffusion mask permitting selective diffusions into siliconwafer through the window etched into oxide.

It is used for surface passivation which is nothing but creatingprotective SiO2 layer on the wafer surface. It protects the junctionfrom moisture and other atmospheric contaminants.

It serves as an insulator on the water surface. Its high relative

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dielectric constant, which enables metal line to pass over theactive silicon regions.

SiO2 acts as the active gate electrode in MOS device structure.

It is used to isolate one device from another.

It provides electrical isolation of multilevel metallization used inVLSI.

It is fortunate that silicon has an easily formed protective oxide, forotherwise we should have to depend upon deposited insulators forsurface protection. Since SiO2 produces a stable layer, this has heldback germanium IC technology.

Growth and Properties of Oxide Layers on Silicon

Silicon dioxide (silica) layer is formed on the surface of a siliconwafer by thermal oxidation at high temperatures in a stream ofoxygen.

Si+02 = SiO2 (solid)

The oxidation furnace used for this reaction is similar to the diffusionfurnace. The thickness of the oxide layer depends on thetemperature of the furnace, the length of time that the wafers are init, and the flow rate of oxygen. The rate of oxidation can besignificantly increased by adding water vapour to the oxygen supplyto the oxidizing furnace.


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Si + 2H2O = SiO2 + 2H2

The time and temperature required to produce a particular layerthickness arc obtained from empirically determined design curves,of the type shown in the figures given below corresponding to dry-oxygen atmosphere and also corresponding to steam atmosphere.

Growth and Properties of Oxide Layers on Silicon

In the past, steam was obtained by boiling ultra-high-purity waterand passing it into the high-temperature furnace containing thesilicon wafers; however, present day technologies generally usehydrogen and oxygen which are ignited in the furnace tube to formthe ultra high-purify water vapour.

The process of silicon oxidation takes place many times during thefabrication of an IC. Once silicon has been oxidized the furthergrowth of oxide is controlled by the thickness of the initial or existingoxide layer.


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Growth Rate of Silicon Oxide Layer

The initial growth of the oxide is limited by the rate at which thechemical reaction takes place. After the first 100 to 300 A of oxidehas been produced, the growth rate of the oxide layer will be limitedprincipally by the rate of diffusion of the oxidant (02 or H20) throughthe oxide layer, as shown in the figures given below.

The rate of diffusion of O2 or H2O through the oxide layer will beinversely proportional to the thickness of the layer, so that we willhave that

dx/dt = C/x

where x is the oxide thickness and C is a constant of proportionality.Rearranging this equation gives

xdx = Cdt

Integrating this equation both sides yields, x2/2 = Ct

Solving for the oxide thickness x gives, x = 2Ct

We see that after an initial reaction-rate limited linear growth phasethe oxide growth will become diffusion-rate limited with the oxidethickness increasing as the square root of the growth time. This isalso shown in the figure below.


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The rate of oxide growth using H2O as the oxidant will be about fourtimes faster than the rate obtained with O2. This is due to the factthat the H2O molecule is about one-half the size of the O2 molecule,so that the rate of diffusion of H2O through the SiO2 layer will bemuch greater than the O2 diffusion rate.

Oxide Charges

The interlace between silicon and silicon dioxide contains atransition region. Various charges are associated with the oxidisedsilicon, some of which are related to the transition region. A chargeat the interface can induce a charge of the opposite polarity in theunderlying silicon, thereby affecting the ideal characteristics of theMOS device. This results in both yield and reliability problems. Thefigure below shows general types of charges.


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Oxide Charges

Interface-trapped charges

These charges at Si-SiO2 are thought to result from several sourcesincluding structural defects related to the oxidation process, metallicimpurities, or bond breaking processes. The density of thesecharges is usually expressed in terms of unit area and energy in thesilicon band gap.

Fixed oxide charge

This charge (usually positive) is located in the oxide withinapproximately 30 A of the Si SiO2 interface. Fixed oxide chargecannot be charged or discharged. From a processing point of view,fixed oxide charge is determined by both temperature and ambientconditions.

Mobile ionic charge

This is attributed to alkali ions such as sodium, potassium, andlithium in the oxides as well as to negative ions and heavy metals.The alkali ions are mobile even at room temperature when electricfields are present.

Oxide trapped charge

This charge may be positive or negative, due to holes or electronstrapped in the bulk of the oxide. This charge, associated with defectsin the Si02, may result from ionizing radiation, avalanche injection.


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Effect of Impurities on the Oxidation Rate

The following impurities affect the oxidation rate

Water1.

Sodium2.

Group III and V elements3.

Halogen4.

In addition damage to the silicon also affects oxidation rate. As wetoxidation occurs at a substantially greater rate than dry oxygen, anyunintentional moisture accelerates the dry oxidation. Highconcentrations of sodium influence the oxidation rate by changingthe bond structure in the oxide, thereby enhancing the diffusion andconcentration of the oxygen molecules in the oxide.

During thermal oxidation process, an interface is formed, whichseparates the silicon from silicon dioxide. As oxidation proceeds, thisinterface advances into the silicon. A doping impurity, which isinitially present in the silicon, will redistribute at the interface until itschemical potential is the same on each side of the interface. Thisredistribution may result in an abrupt change in impurityconcentration across the interface. The ratio of the equilibriumconcentration of the impurity, that is, dopant in silicon to that in SiO2at the interface is called the equilibrium segregation coefficient. Theredistribution of the dopants at the interface influences the oxidationbehaviour. If the dopant segregates into the oxide and remains there


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(such as Boron, in an oxidizing ambient), the bond structure in thesilica weakens. This weakened structure permits an increasedincorporation and diffusivity of the oxidizing species through theoxide thus enhancing the oxidation rate. Impurities that segregateinto the oxide but then diffuse rapidly through it (such as aluminium,gallium, and indium) have no effect on the oxidation kinetics.Phosphorus impurity shows opposite effect to that of boron, that is,impurity segregation occurs in silicon rather than Si02. The same istrue for As and Sb dopants.

Halogen (such as chlorine) impurities are intentionally introducedinto the oxidation ambient to improve both the oxide and theunderlying silicon properties. Oxide improvement occurs becausethere is a reduction in sodium ion contamination, increase in oxidebreakdown strength, and a reduction in interface trap density. Trapsarc energy levels in the forbidden energy gap which are associatedwith defects in the silicon.

Growth and Properties of Thin Oxides

MOS VLSI technology requires silicon dioxide thickness in the 50 to500 A range in a repeatable manner. This section is devoted to thegrowth and properties of such thin oxide. This oxide must exhibitgood electrical properties and provide long-term reliability. As anexample, the dielectric material for MOS devices can be thin thermaloxide. This dielectric is an active component of the storage capacitorin dynamic RAMs, and its thickness determines the amount ofcharge that can be stored.

The growth of thin oxide must be slow enough to obtain uniformity


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and reproducibility. Various growth techniques for thin oxide are dryoxidation, dry oxidation with HCl, sequential oxidations usingdifferent temperatures and ambients, wet oxidation, reducedpressure techniques, and high pressure/low temperature oxidation.High pressure oxidation is discussed later. The oxidation rate will, ofcourse, be lower at lower temperatures and at reduced pressures.Ultra-thin oxide (

There is a benefit of increase in the oxidation rate if the thermaloxidation is carried out at pressures that are much aboveatmospheric pressure. The rate of diffusion of the oxidant moleculesthrough an oxide layer is proportional to the ambient pressure. Forexample, at a pressure of 10 atm the diffusion rate will be increasedby a factor of 10 and the corresponding oxidation time can bereduced by nearly the same factor. Alternatively, the oxidation can bedone for the same length of time, but the temperature required willbe substantially lower.

Thus, one principal benefit of high-pressure oxidation processing islower-temperature processing. The lower processing temperaturereduces the formation of crystalline defects and produces less effecton previous diffusions and other processes. The shorter oxidationtime is also advantageous in increasing the system throughput. Themajor limitation of this process is the high initial cost of the system.

Oxide Masking

The oxide layer is used to mask an underlying silicon surfaceagainst a diffusion (or ion implantation) process. The oxide layer ispatterned by the phtolithographic process to produce regions wherethere are opening or windows where the oxide has been removalto expose the underlying silicon. Then these exposed silicon regionsare subjected to the diffusion (or implantation) of dopants, whereasthe unexposed silicon regions will be protected. The pattern ofdopant that will be deposited into the silicon will thus be a replicationof the pattern of opening in the oxide layer. The replication is a keyfactor in the production of tiny electronic components.


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The thickness of oxide needed for diffusion masking is a function ofthe type of diffusant and the diffusion time and temperatureconditions. In particular, an oxide thickness of some 5000 A will hevufftcieni to mask against almost all diffusions. This oxide thicknesswill also be sufficient to block almost alt but the highest-energy ionimplantation.

Oxide Passivation

The other function of Si02 in IC fabrication is the surfacepassivation. This is nothing but creating protective Si02 layer on thewafer surface. The figure below shows a cross-sectional view of ap-n junction produced by diffusion through an oxide window. Thereare lateral diffusion effects, that is, the diffusion not only proceeds inthe downward direction, but also sideways as well, since diffusion isan isotropic process. The distance from the edge of the oxidewindow to the junction in the lateral direction underneath die oxide isindicated as yj.

Diffusion Masking


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oxidation process in ic fabrication

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