Chapter 4

UEEP2613

Microelectronic Fabrication

Oxidation

Prepared by

Dr. Lim Soo King

24 Jun 2012

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Chapter 4 ..........................................................................................113

Oxidation ..........................................................................................113

4.0 Introduction ............................................................................................ 113

4.1 Chemistry of Silicon Dioxide Formation ............................................. 115

4.2 Application of Silicon Dioxide ............................................................... 116

4.3 Factors Influencing the Growth of Silicon Dioxide ............................ 119

4.4 Analysis of Silicon Dioxide Growth ...................................................... 123

4.5 Quality of Silicon Dioxide ...................................................................... 127

Exercises ........................................................................................................ 128

Bibliography ................................................................................................. 130

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Figure 4.1: Oxidation process silicon ............................................................................... 114 Figure 4.2: Volume expansion of silicon dioxide ............................................................ 114 Figure 4.3: Schematic of a basic oxidation furnace ......................................................... 115 Figure 4.4: Silicon dioxide as diffusion mask .................................................................. 116 Figure 4.5: Screen oxide used in ion implantation ........................................................... 117

Figure 4.6: Pad oxide and barrier oxide in STI process ................................................... 117 Figure 4.7: LOCOS process ............................................................................................. 118 Figure 4.8: Process step showing shallow trench isolation structure ............................... 118 Figure 4.9: Process step of deep filled trench with LOCOS ............................................ 119 Figure 4.10: Silicon oxide thickness with time of growth.................................................. 120

Figure 4.11: Illustration of the rate of silicon dioxide formation between wet and dry

methods .......................................................................................................... 121 Figure 4.12: (a) Depletion effect of p-type silicon and (b) pile-up effect of dopant for n-

type silicon ..................................................................................................... 122 Figure 4.13: The model for describing thermal oxidation .................................................. 123 Figure 4.14: Activation energy and pre-exponentials constant various oxidation processes

for (111) silicon. All C2 values should be divided by 1.68 for (100) silicon . 127 Figure 4.15: The value of B and B/A versus temperature for dry oxygen and stream

oxidation methods .......................................................................................... 127

Figure 4.16: The charges associated with thermal oxide ................................................... 128

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Chapter 4

Oxidation

_____________________________________________

4.0 Introduction

Silicon dioxide SiO2 is a very good dielectric material. Its dielectric constant is

3.9. Silicon dioxide is used mainly for masking where dopant cannot be

diffused, passivation, and insulation. Thus, oxidation of silicon is necessary

throughout the modern integrated circuit fabrication process. Producing high

quality IC’s requires not only an understanding of the basic oxidation

mechanism but ability to form a high quality oxide in a controlled and

repeatable manner is important. In addition to ensure the reliability of the ICs

the electrical properties of the oxide must understand.

Oxidation is one of the most important thermal processes beside diffusion,

chemical vapor deposition CVD etc. It is an adding oxygen process to silicon

wafer to form silicon dioxide on the surface of wafer. Silicon is very reactive

with oxygen. Thus, in nature most silicon exists in the form of silicon dioxide

such as quartz and sand. Silicon dioxide is a dense material that fully covers the

surface of silicon. To continue the oxidation, oxygen molecules have to diffuse

across the oxide layer to reach the silicon atoms underneath and react with

them. When bare silicon is exposed to the atmosphere, it reacts almost

immediately with oxygen or moisture in the air to form a thin layer of silicon

dioxide of about 10 to 20o

A called native oxide. This thickness of silicon dioxide

is sufficient enough to stop the further oxidation of the silicon at room

temperature due to low diffusivity. Figure 4.1 illustrates the oxidation process

of silicon. As the process progress, the silicon-silicon dioxide interface of the

original silicon has been shifted inward, while the oxide is expanded outward.

In the constrained environment, the ratio of expansion is 1:0.45.

One silicon atom is used to form one molecule of silicon dioxide SiO2.

Based on the densities of silicon NSi and silicon dioxide Nox, which are

4.99x1022

cm-3

and 2.27x1022

cm-3

respectively, the recession is 45.5% meaning

every unit thickness of oxide formed required approximately 0.45 unit thickness

of silicon.

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Figure 4.1: Oxidation process silicon

In oxidation process, oxygen is in gas phase and silicon is in solid form.

Therefore, while silicon dioxide is growing, it consumes the substrate silicon

and the film grows into the silicon substrate. As the result, the volume of at the

silicon-silicon dioxide interface is expanded. Figure 4.2 illustrates the expanded

silicon dioxide at the interface.

(a) Unit of silicon dioxide (b) Unconstrained expansion (c) Constrained expansion

Figure 4.2: Volume expansion of silicon dioxide

Semiconductor can be oxidized by various methods that include thermal

oxidation, electrochemical anodization, and plasma enhanced chemical vapor

deposition PECVD. Among the methods, thermal oxidation is the important

method in today modern integrated circuit fabrication. The basic set-up of a

thermal oxidation furnace is shown in Fig, 4.3. It consists of a resistance heated

furnace, a cylindrical fused-quartz tube containing the wafer held vertically in a

slotted boat, and a source of either pure dry oxygen or pure water vapor. The

04 Oxidation

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temperature of oxidation is in the range of 900oC to 1,200

oC with typical gas

flow rate of 1.0 liter per minute.

Figure 4.3: Schematic of a basic oxidation furnace

4.1 Chemistry of Silicon Dioxide Formation

As discussed earlier, oxidation is a process of growing a thin layer of

amorphous silicon dioxide. The process is also termed as chemical vapor

deposition CVD. There are several methods to grow oxide, which are dry and

wet methods. Thermal oxide is grown using oxygen and silicon yields dry

oxide.

Si + O2 SiO2 (4.1)

Silicon dioxide SiO2 can also be grown using hot steam to get wet oxide.

Si + 2H2O SiO2 + 2H2 (4.2)

In terms of quality, dry oxide is better than wet dry due to the rate of growth is

much slower than the corresponding wet oxide growing. If one needs thick

oxide, wet oxide method is preferred due to rate of growth is higher.

Anodic oxide is formed in gaseous or liquid medium by electric field

induced transportation of mobile ion. This method is also termed as low

pressure chemical vapor deposition LPCVD.

Other methods are: low temperature (4000C to 500

0C) chemical vapor

deposition using silane SiH4 and oxygen O2 and high temperature at 1,0000C

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deposition using tetrachlorosilane SiCl4 with carbon dioxide CO2, oxygen O2,

and water H2O.

SiH4 + O2 SiO2 + 2H2 (4.3)

At this point, it is worth to mention that there is another passivation film, which

is silicon nitride Si3N4 film. This film is also used to passivate semiconductor

device because it acts as a barrier to the diffusion of metal ions, particularly

sodium ions. Reactive sputtering of ammonia or nitrogen is the most common

way to deposit this type of film.

3SiH4 + 4NH3 C900~ 0

Si3N4 + 12H2 (4.4)

or

3SiCl4 + 4NH3 C1200~C550 00

Si3N4 + 12HCl (4.5)

4.2 Application of Silicon Dioxide

Oxidation of silicon is one of the basic processes throughout the IC’s process.

There are many applications for silicon dioxide. One of them is as diffusion

mask. Dopant such as phosphorus and boron have lower diffusion rate in silicon

dioxide than in silicon. Therefore, any etching windows on the masking oxide

layer, one can dope silicon substrate at the designated area by dopant diffusion

process as shown in Fig. 4.4.

Figure 4.4: Silicon dioxide as diffusion mask

Screen oxide is commonly used for ion implantation process. It can help to

prevent silicon contamination by blocking the sputtered photoresist. It can also

minimize the channel effect by scattering the incident ions before they enter a

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single crystal silicon substrate. The thickness of screen oxide is about 100 to

200o

A . Figure 4.5 illustrates the screen oxide used in ion implantation.

Figure 4.5: Screen oxide used in ion implantation

Silicon dioxide is also used as the barrier layer to prevent contamination of the

silicon substrate before the trench fill the shallow trench isolation STI. Trench

fill is a dielectric CVD process in which undoped silicate glass USG is

deposited to fill the trench for electrical isolation of neighboring transistor.

Since the CVD process always bring a certain level of contamination, silicon

dioxide acts as the barrier to block contamination. Fig. 4.6 illustrates the pad

oxide and the barrier oxide in shallow trench isolation STI process.

(a) Trench etch

(b) Trench refill

(c) Pad oxide and CMP processes

Figure 4.6: Pad oxide and barrier oxide in STI process

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Localized oxidation of silicon LOCOS has better isolation effect than the

blanket field oxide. LOCOS process uses a thin layer of oxide 200 to 500o

A as

pad layer to buffer the strong tensile stress of the LPCVD nitride. It is illustrated

in Fig. 4.7.

(a) Pad oxidation nitride deposition, and patterning

(b) Oxidation

(c) Pad oxide and nitride strip

Figure 4.7: LOCOS process

Trench isolation either shallow or deep refilled types are used in advanced MOS

and bipolar processes. Shallow trench isolation STI process step is shown in

Fig. 4.8.

(a) Stack and trench etch (b) Pad oxide undercut (c) Liner oxidation

(d) CVD oxide gap filled (e) CMP and HF dip (f) H2PO4 nitride strip

Figure 4.8: Process step showing shallow trench isolation structure

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Deep trench refilled with polysilicon is used to form trench capacitor used in

DRAM memory design. It is also used as isolation in SiGe heterojunction HBT

technology. The process step of deep trench refilled with polysilicon with

combination of LOCOS field oxidation is shown in Fig. 4.9.

(a) Trench etching with SiN mask and oxidation

(b) Poly-Si deposition and etching back

(c) SiN patterning and field oxidation

Figure 4.9: Process step of deep filled trench with LOCOS

The deep trench is formed from reactive ion etching process. This process can

create deep trench of high aspect ratio. The surface of trench is passivated with

a layer of thermally grown oxide. The rest of process steps should be self

explanatory.

4.3 Factors Influencing the Growth of Silicon Dioxide

There are basically five parameters influencing the rate of growth of silicon

dioxide. They are oxidation gas, temperature, crystal orientation, type dopant,

and doping concentration.

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As mentioned early, oxidation is a process of diffusion of oxygen or OH

ion into silicon dioxide to reach the silicon dioxide-silicon interface and react

with silicon to form silicon dioxide. As the thickness of silicon dioxide is

increased, the rate of diffusion becomes slower. In the initial growth process,

the rate of thickness is directly proportional to the time of growth. However,

after certain time, the thickness of oxide is proportional to the square root of

growth time. Figure 4.10 illustrates the thickness of oxide growth with respect

to time of growth.

Figure 4.10: Silicon oxide thickness with time of growth

Wet oxide growth method has higher rate of growth than the dry oxide method.

It is because hydroxide OH ion has higher diffusivity than oxygen molecular in

the silicon dioxide. The illustration is shown in Fig. 4.11. Results show in Fig.

4.10(a) have lower rate of oxidation than results show in Fig. 4.11(b). The

reason is due to diffusivity of the hydroxide ion is higher than dry oxygen. The

results also show higher temperature has higher growth rate due to higher

diffusion.

04 Oxidation

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(a) Silicon dry oxidation

(b) Silicon wet oxidation

Figure 4.11: Illustration of the rate of silicon dioxide formation between wet and dry

methods

The grow rate of silicon dioxide is also dependent on the crystal orientation of

the silicon. Silicon crystal of orientation (111) has higher rate of oxide

formation than the (100) orientation due to the fact that the surface density of

(111) orientation is higher than (100) orientation.

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The rate of silicon dioxide formation is also dependent on the temperature

of the oxidation. The temperature diffusivity follows equation (4.5).

kT/EexpDD ao (4.5)

where Do is the intrinsic diffusivity and Ea is the activation energy.

Oxidation rate is also depending to dopant and doping concentration of

silicon. Generally, heavily doped silicon oxidizes faster than lightly doped

silicon. During oxidation, boron tends to be drawn up to the silicon dioxide and

causes depletion of the boron concentration at silicon-silicon dioxide interface

resulting thinner silicon dioxide than the silicon doped with n-type dopant. n-

type dopants such as phosphorus, arsenic, and antimony have the opposite

effect. The doping concentration at the silicon-silicon dioxide interface is higher

than the original doping concentration due to pile-up effect of dopant at silicon-

silicon dioxide interface. As the result, thicker silicon dioxide will be grown.

Fig. 4.12 shows the pile-up effect of n-type dopant and the depletion effect of

the p-type dopant.

(a) p-type silicon (b) n-type silicon

Figure 4.12: (a) Depletion effect of p-type silicon and (b) pile-up effect of dopant for n-type

silicon

Addition of gas such as hydrochloric acid HCl, which is commonly used in the

gate oxidation process, will suppress mobile ions. This helps to improve the

oxidation rate by approximately 10%. Chlorine ion will bind with dangling

electron of silicon atom at the interface that helps to minimize the space charge.

04 Oxidation

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Thus, it improves the reliability of the IC. The concentration of chlorine cannot

be too high because too much chlorine introduced in silicon dioxide would

affect the stability of oxide. Extra chlorine ion affects voltage bias of the gate.

4.4 Analysis of Silicon Dioxide Growth

The kinetic of silicon oxidation can be described by the model shown in Fig.

4.13. C0 is the surface concentration of oxidant, in which it has unit

molecule/cm3. The magnitude of C

0 is generally at equilibrium and it is

proportional to the partial pressure of the oxidant adjacent to the surface of

oxide. At temperature 1,000oC and pressure of 1atm, value of C0 is 5.2x10

16cm

-3

for dry oxygen and 3.0x1019

cm-3

for water vapor.

Figure 4.13: The model for describing thermal oxidation

The flux F1 of oxidant that diffuses through the silicon dioxide layer, which

resulting the concentration CS at the surface of silicon is equal to

)t(d

)CC(D

dx

dCDF

ox

S01

(4.6)

04 Oxidation

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where D is the diffusion coefficient of the oxidant and dox(t) is the thickness

already present oxide layer. At the surface of silicon, the oxidant reacts

chemically with silicon. With the assumption that the rate of reaction is

proportional to the concentration CS of oxidant at the surface of silicon, the flux

F2 at the surface is equal to

S2 CF (4.7)

where is the surface reaction rate. At steady state, it is F1 = F2 = F. The flux F

is equal to equation (4.8) after substituting CS from equation (4.7) into equation

(4.6).

)/D()t(d

DCF

ox

0

(4.8)

Let C1 be the number of molecules of oxidant in a unit volume of oxide. The

density of silicon dioxide is 2.2x1022

cm-3

. One oxygen molecule is added to

each silicon dioxide, while two water molecules are added to each silicon atom.

Therefore, C1 for oxidation for oxygen is 2.2x1022

cm-3

and for oxidation in

water vapor is 4.4x1022

cm-3

. With this understanding, the growth rate of oxide

layer is given by

)/D()t(d

C/DC

C

F

dt

)t(dd

ox

10

1

ox

(4.9)

The solution of this equation can be solved by setting the condition that at time t

= 0, the oxide thickness is equal to di. i.e. dox(0) = di. The result is the general

equation for oxidation of silicon, which is

)t(C

DC2)t(d

D2)t(d

1

0ox

2

ox

(4.10)

where )DC2/(C/Dd2d 01i

2

i . It represents a time coordinate shift to account

for the initial oxide layer of thickness di. From equation (4.10), the solution for

oxide thickness dox(t) at time t is equal to

1

DC

)t(C21

D)t(d

1

2

0ox (4.11)

For short time equation (4.11) is deduced to

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)t(C

C)t(d

1

0ox

(4.12)

For large value of time, equation (4.12) is deduced to

)t(C

DC2)t(d

1

0ox (4.13)

Equation (4.10) can be written in more compact form, which is shown in

equation (4.14).

)t(BAdd ox

2

ox (4.14)

where

D2

A ,1

0

C

DC2B , and

1

0

C

C

A

B . A and B are coefficients depending on

temperature, crystal orientation, activation energy, and gas mixture. These

coefficients shall be dealt from fitting the graph from experimental data. Letting

dox = di at t = 0 and substituting them into equation (4.14), the parameter ,

which is defined as shift in time coordinate to represent the presence of initial

oxide thickness di or time spent to growth the initial thickness of oxide di, is

equal to

B

Add i

2

i (4.15)

Based on the result shown in Fig. 4.10 or Fig. 4.11, the analysis of oxidation

process shows that the grown oxide thickness dox can be approximated by

solving quadratic equation (4.14), which will yield solution for oxide thickness

dox(t) equation (4.16).

1

)B4/(A

)t(1

2

A)t(d

2ox (4.16)

This quadratic equation has two limit forms of the linear and parabolic growth

whereby they are described by equation (4.17) and (4.19) respectively. The

initial phase of oxide growth is a linear process because the time t is small.

Based on equation (4.16), the oxide thickness dox is deduced to

dox(t) = )t(A

B (4.17)

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This equation is true only if (t+) << B4

A2

, which shall mean a short oxidation

time as discussed earlier.

Based on equation (4.16), oxidation time tl is equal to equation (4.18) if

one assumes that there is no prior grown oxide.

A/B

)t(dt ox

l (4.18)

As time goes on, the process is slow down due to lower diffusivity of oxidation

agent in the already grown silicon dioxide. The thickness dox(t) will follow

equation (4.19) if there is prior grown oxide.

dox(t) = )t(B (4.19)

Equation (4.19) is true only if t >> and t >>B4

A2

, this shall mean that it has

long oxidation time. The time tp of growth for the oxide is

B

)t(dt

2

oxp (4.20)

Based on experimental data, the coefficient B and B/A can be described by

Arrhenius expression which are shown in equation (4.21) and (4.22)

respectively.

)kT/Eexp(CB 11 (4.21)

and

)kT/Eexp(CA/B 22 (4.22)

E1 and E2 are activation energies associated with the physical process that B and

B/A are represented. C1 and C2 are pre-exponential constants. These mentioned

constant values for various types of oxidation processes can be obtained from

Fig. 4.14. Alternatively, the value of B and B/A can be obtained from the graph

shown in Fig. 4.15.

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Ambient B B/A

Dry O2 C1 = 7.72x10

2mhr

-1

E1 = 1.23eV

C2 = 6.23x106mhr

-1

E2 = 2.0eV

Wet O2 C1 = 2.14x10

2mhr

-1

E1 = 0.71eV

C2 = 8.95x107mhr

-1

E2 = 2.05eV

H2O C1 = 3.86x10

2mhr

-1

E1 = 0.78eV

C2 = 1.63x108mhr

-1

E2 = 2.05eV

Figure 4.14: Activation energy and pre-exponentials constant various oxidation processes

for (111) silicon. All C2 values should be divided by 1.68 for (100) silicon

Figure 4.15: The value of B and B/A versus temperature for dry oxygen and stream

oxidation methods

Based on equation (4.18) and (4.20), the total time t to grow silicon oxide of

thickness dox is equal to

B

)t(d

A/B

)t(dt

2

oxox (4.23)

4.5 Quality of Silicon Dioxide

Oxide used for masking is usually grown by wet oxidation. A typical cycle

consists of dry-wet-dry sequence. The growth rate of wet oxide is much higher

04 Oxidation

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than growth of dry oxide. However, the quality of dry oxide is higher due to

denser and has breakdown voltage in region of 5 – 10MV/cm. Owing to this fact,

thin oxide is usually grown using dry oxidation.

MOS devices are affected by charges in the oxide and traps at SiO2-Si

interface. The basic classification of these traps and charges are shown in Fig.

4.16. The interface trapped charges Qit are due to the SiO2-Si interface

properties and dependent on the chemical composition of the interface. These

traps are located at SiO2-Si with energy state located within silicon forbidden

band-gap. The fixed charge Qf is located within approximately 3.0nm of the

SiO2-Si interface. Fixed charge is positive and depends on oxidation and

annealing condition. Oxide trapped charge Qot is associated with defects within

silicon dioxide. This charge is created like X-ray radiation or high energy

bombardment. Mobile ionic charges Qm due to contamination from sodium or

other alkali, are mobile within the oxide under raised temperature and high

electric field operation. In this condition the mobile ions are moving forth and

back through the oxide layer and cause threshold voltage shifts.

Figure 4.16: The charges associated with thermal oxide

Exercises

4.1. The densities of silicon NSi and silicon dioxide Nox are 4.99x1022

cm-3

and

2.27x1022

cm-3

respectively. Prove that each unit thickness of silicon

dioxide formed, it utilizes 0.45 unit thickness of of silicon.

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4.2. Name three factors that influencing the oxide growth.

4.3. What is the purpose of screen oxide?

4.4. What is the purpose of growth LOCOS?

4.5. State a reason why hot stream has higher oxidation rate than dry oxygen.

4.6. State the reason why the linear oxidation of (111) orientation is faster

than the (100) orientation silicon crystal.

4.7. HCl is introduced to improve the reliability of oxide by binding with

dangling electron of silicon during oxidation. State the reason why HF

gas cannot be used.

4.8. Calculate the time taken to grow 2m thick oxide using hot stream at

temperature 1,1000C.

4.9. Find the thickness of the recessed oxide to be etched if a 1.2m thick

LOCOS is to be grown as shown in figure below.

4.10. The full recessed localized oxidation of silicon (LOCOS) is to be grown

for a CMOS integrated circuit that used (100) p-type silicon substrate and

has prior etch of 0.8m. Calculate the time taken to grow the oxide

thickness using wet oxygen at temperature 1,0000C.

4.11. Given that (1.2m)3 of silicon volume is to be grown into silicon dioxide

as shown in figure below. Assuming constrained oxide growth,

calculating the volume of silicon dioxide grown.

04 Oxidation

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4.12. Find the thickness of the recessed silicon to be etched if the oxide

structure shown in the figure is to be grown.

Bibliography

1. JD Pummer, MD Del, and Peter Griffin, “Silicon VLSI Technology”

Fundamentals, Practices, and Modeling”, Prentice Hall, 2000.

2. Hong Xiao, “Introduction to Semiconductor Manufacturing Technology”,

Pearson Prentice Hall, 2001.

3. SM Sze, “VLSI Technology”, second edition, McGraw-Hill, 1988.

4. CY Chang and SM Sze, “ULSI Technology”, McGraw-Hill, 1996.

5. Gary S. May and Costas J. Spanos, “Fundamentals of Semiconductor

Manufacturing and Process Control”, IEEE Wiley-Interscience, 2006.

Index

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A

Activation energy ............................................... 122 Ammonia ........................................................... 116 Anodic oxide ...................................................... 115 Antimony ........................................................... 122 Arsenic ............................................................... 122

B

Barrier oxide ...................................................... 117 Boron ................................................................. 116

C

Carbon dioxide ................................................... 116 Chemical vapor deposition ................ 113, 115, 117 Chlorine ............................................................. 123 CVD ......................... See Chemical vapor deposition

D

Dielectric constant ............................................. 113 Diffusion ............................................................ 116 Diffusivity ........................................................... 122 Dry oxide ............................................................ 115

E

Electrochemical anodization ............................. 114

F

Fixed charge ....................................................... 128

H

HBT technology .................................................. 119 Hydrochloric acid ............................................... 122

I

Interface trapped charges ................................. 128 Intrinsic diffusivity ............................................. 122

L

Localized oxidation of silicon ..................... 118, 119 LOCOS .................. See Localized oxidation of silicon Low pressure chemical vapor deposition .. 115, 118 LPCVD See Low pressure chemical vapor deposition

M

Memory

Dynamic RAM ................................................. 119 Mobile ionic charge............................................ 128

N

Native oxide ....................................................... 113 Nitrogen ............................................................. 116

O

Oxidation ............................................................ 113 Oxide trapped charge ........................................ 128 Oxygen ............................... 113, 114, 115, 120, 124

P

Pad oxide............................................................ 117 Phosphorus ................................................ 116, 122 Plasma enhanced chemical vapor deposition .... 114 Polysilicon .......................................................... 119

R

Reactive ion etching ........................................... 119 Reactive sputtering ............................................ 116

S

Screen oxide ....................................................... 117 Semiconductor

SiGe ................................................................ 119 Shallow trench isolation ............................. 117, 118 SiGe heterojunction ........................................... 119 Silane .................................................................. 115 Silicon ................................................................. 113 Silicon dioxide .................... 113, 114, 115, 117, 120 Silicon nitride ..................................................... 116 Sodium ....................................................... 116, 128 STI ............................... See Shallow trench isolation

T

Tetrachlorosilane ............................................... 116 Thermal oxide .................................................... 115

U

Undoped silicate glass........................................ 117

W

Wet oxide ........................................................... 115