vlsi technology 2

7/27/2019 VLSI technology 2

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Lithography


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Lithography is the process of transferring patterns of geometric

shapes in a mask to a thin layer of radiation-sensitive materials

(called resist) covering the surface of a semiconductor wafer


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Resolution is defined to be the minimum feature

dimension that can be transferred with high fidelity to

a resist film on a semiconductor wafer

Registration is a measure of how accurately patterns

on successive masks can be aligned or overlaid with

respect to previously defined patterns on the samewafer

Throughput is the number of wafers that can beexposed per hour for a given mask level and is thus a

measure of the efficiency of the lithographic process


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An IC fabrication facility requires a clean room, particularly in

lithography areas. Airborne particles settling on semiconductor

wafers and lithographic masks behave as opaque patterns that can besubsequently transferred to the wafers.

Particle 1 may

result in the

formation of a

pinhole in theunderlying layer

Particle 2 may

cause constriction

of current flow inthe metal runner

Particle 3 may lead to a short circuit between the two

conducting regions and render the circuit useless


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Clean RoomIn a clean room, the total

number of dust particles

per unit volume must be

tightly controlled along

with other parameterssuch as temperature,

humidity, pressure, and so

on.

A class X clean room is

usually defined to be one

that has a dust count of

Xparticles (diameters of

0.5 m or larger) per

cubic foot.

For modern lithographic processes, a class 10 or better clean room is required


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Optical Lithography Shadow Printing

Contact printing yields very highresolution (~ 1 m), but suffers from

major drawback caused by dust

particles or silicon specks

accidentally embedded into themask.

Proximity printing is not as prone toparticle damage. However, the small gap

between the mask and wafer (typically 10

m to 50 m) introduces optical diffraction

at the feature edges on the photomasks andthe resolution is degraded.


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Shadow Printing

The minimum linewidth that can be printed, lm

, in shadow

printing is roughly given by

lm = (g)1/2

where is the wavelength of the exposure radiation and g is

the gap between the mask and the wafer and includes thethickness of the resist. For typical values of (~ 0.4 m) and g

(~ 50 m), lm

is on the order of 4.5 m. The above equation

imparts that the minimum linewidth can be improved byreducing the wavelength (that is, going to deep UV spectral

region) or the gap g.


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Projection Printing

In order to circumvent

problems associated with

shadow printing, projectionprinting exposure tools have

been developed to project an

image of the mask patternsonto a resist-coated wafer

many centimeters away from

the mask. The small imagearea is scanned or stepped

over the wafer to cover the

entire surface.


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The resolution of a projection

system is given by

lm

= /NA

where lis the wavelength of the

exposure radiation and NA isthe numerical aperture given by

NA = sin n

where denotes the refraction index of the imaging medium ( = 1 in air) and is

the half angle of the cone of light converging to a point image at the wafer.

n

The depth of focus, z, can be expressed as

z = lm/ 2 tan lm/2 sin = / [2 (NA)2]nResolution can be enhanced by reducing . This explains the trend towards

shorter wavelength in optical lithography.


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Mask Defects

The number of mask defects has a

profound effect on the final IC yield,

which is defined as the ratio of good

chips per wafer to the total number ofchips per wafer. As a first-order

approximation,

Y exp{-DA}where Yis the yield, D is the average

number of "fatal" defects per unit

area, and A is the area of an IC chip.

If D remains the same for all masklevels, N, then

Y exp{-NDA}


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Phase

ShiftMask

The phase-shifting layer that covers adjacent apertures reverses the sign of the

electric field. Although the absolute intensity at the mask is unchanged, the

electric field of these images at the wafer, shown by the dotted line, can be

canceled (right figure). Consequently, images that are projected close to one

another can be separated completely. A 180o phase change occurs when a

transparent layer of thickness d=

/ 2 (n 1), where n is the refraction index and is the wavelength, covers one aperture as shown in the figure.


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A photoresist is a radiation-sensitive compound. For positive resists, the

exposed region becomes more soluble and thus more readily removed in

the developing process. The net result is that the patterns formed in the

positive resist are the same as those on the mask. For negative resists,the exposed regions become less soluble, and the patterns engraved are

the reverse of the mask patterns

A positive photoresist consists of three constituents: photosensitive

compound, base resin, and organic solvent. Prior to exposure, the

photosensitive compound is insoluble in the developer solution. After

irradiation, the photosensitive compound in the exposed pattern areasabsorbs energy, changes its chemical structure, and transforms into a

more soluble species. Upon developing, the exposed areas are expunged

Photoresists


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Negative photoresists are polymers combined with aphotosensitive compound. Following exposure, the

photosensitive compound absorbs the radiation energy and

converts it into chemical energy to initiate a chain reaction,

thereby causing cross-linking of the polymer molecules.

The cross-linked polymer has a higher molecular weight

and becomes insoluble in the developer solution. After

development, the unexposed portions are removed

One major drawback of a negative photoresist is that the

resist absorbs developer solvent and swells, thus limitingthe resolution of a negative photoresist


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The left figure depicts a typical

exposure response curve for a positive

resist. The resist has a finite solubility

in the developer solution even prior to

exposure. At a threshold energy, ET,

the resist becomes completely soluble.

ET

therefore corresponds to the

sensitivity of the photoresist. Another

parameter, , is the contrast ratio and

is given by:1

1ln

= E

ET

A larger implies a more rapid

dissolution of the resist with an

incremental increase of exposure

energy and results in a sharper image.

The edges of the resist image are

generally blurred due to diffraction.

The right figure shows the response

curve for a negative photoresist. Thesensitivity of a negative photoresist is

defined as the energy required to retain

50% of the original resist film thickness

in the exposed region.


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Pattern TransferThe wafer is placed in a clean room

that typically is illuminated with

yellow light as photoresists are not

sensitive to wavelengths greater than

0.5 m. The wafer is held on a

vacuum spindle, and approximately 1

cm3 of liquid resist is applied to the

center of the wafer. The wafer is than

spun for about 30 seconds. The

thickness of the resulting resist film,

lR

, is directly proportional to its

viscosity as well as the percent solid

content indigenous to the resist, and

varies inversely with the spin speed.

For spin speeds in the range of 1000

to 10000 rpm, film thicknesses on the

order of 0.5 to 1 m can be obtained.


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The wafer is then given a pre-exposure bake (80oC to 100oC) to

remove solvent and improve adhesion. The wafer is aligned withrespect to the mask in an optical lithographic system prior to

exposure to UV or deep UV light. For a positive photoresist, the

exposed portions are dissolved in the developer solution. The wafer is

then rinsed, dried, and then put in an ambient that etches the exposedinsulating layer but does not attack the resist. Finally, the resist is

stripped, leaving behind an insulator image (or pattern) that is the

same as the opaque image on the mask. For a negative photoresist,the exposed area becomes insoluble, and the final insulator pattern is

the reverse of the opaque image on the mask.

The insulator image can be employed as a mask for subsequentprocessing. For instance, ion implantation can be performed to dope

the exposed regions selectively.


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Lift-Off Technique

This method works if the film thickness is smaller than that of the

photoresist


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Lithographic Techniques

El t lith h ff hi h l ti b f th ll l th f


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Generation of micron and submicron

resist geometries

Highly automated and preciselycontrolled operation

Greater depth of focus

Direct patterning without a mask

Electron lithography offers high resolution because of the small wavelength of

electrons (< 0.1 nm for 10-50 keV electrons). The resolution is not limited by

diffraction, but by electron scattering in the resist and the various aberrations of the

electron optics. Electron lithography has the following advantages:

The biggest disadvantage of electron

lithography is its low throughput

(approximately 5 wafers / hour at less than

0.1 m resolution). Therefore, electronlithography is primarily used in the

production of photomasks and in situations

that require small number of custom

circuits.


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Electron Resists For a positive electron resist, the

polymer - electron interactioncauses chain scission, that is,

broken chemical bonds. The

irradiated areas can be dissolved

in a developer solution that attackslow molecular - weight material.

Common positive electron resists

are poly (methyl methacrylate),

abbreviated PMMA, and poly(butene-1 sulfone), abbreviated

PBS. Positive electron resists

typically have resolution of 0.1m

or better.When electrons impact a negative electron resist, polymer linking is induced.

Poly (glycidyl methacrylate-co-ethyl-acrylate), abbreviated COP, is a common

negative electron resist. Like a negative photoresist, COP swells during

developing, and resolution is limited to about 1m.


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X-Ray LithographyX-ray lithography employs a shadow

printing method similar to optical

proximity printing. The x-ray

wavelength (0.4 to 5 nm) is much

shorter than that of UV light (200 to

400 nm). Hence, diffraction effects are

reduced and higher resolution can be

attained. For instance, for an x-ray

wavelength of 0.5 nm and a gap of 40

m, lm

is equal to 0.2 m. X-ray

lithography has a higher throughput

when compared to e-beam lithography

because parallel exposure can be

adopted. However, on account of the

finite size of the x-ray source and the

finite mask-to-wafer gap, a penumbral

effect results which degrades the

resolution at the edge of a feature.


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The penumbral blur, , on the edge of the resist image is given by

= ag/L

where a is the diameter of the x-ray source, gis the gap spacing, and L is the

distance from the source to the x-ray mask. Ifa = 3 mm, g= 40 m, and L =

50 cm, is on the order of 0.2 m.

An additional geometric effect is the lateral magnification error due to the

finite mask-to-wafer gap and the non-vertical incidence of the x-ray beam.

The projected images of the mask are shifted laterally by an amount d,

called runout given by

d= rg/L

where rdenotes the radial distance from the center of the wafer. For a 125-mm wafer, the runout error can be as large as 5 m forg= 40 m and L =

50 cm. It is compensated for during the mask making process.


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Ion Lithography

Ion lithography may achieve higher resolution than optical, x-ray, or electron

lithographic techniques because ions undergo no diffraction and scatter much less

than electrons. In addition, resists are more sensitive to ions than to electrons andthere is the possibility of a resistless wafer process. However, ion beams are larger

than electron beams. Due to the slow throughput, the most important application

of ion lithography is the repair of masks for optical or x-ray lithography.

The figure depicts the

computer trajectory of 50

H+

ions implanted at 60keV. The spread of the ion

beam at a depth of 0.4 m

is only 0.1 m.

O ti l lith h i th i t


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Conventional optical

lithography is considered

difficult for a design rule ofmuch less than 0.1 m due to

its resolution limit. For deep

sub-micrometer structures,

the two remaining optionsare electron beam direct

writing or x-ray lithography.

Perfect x-ray masks are

difficult to make and thethroughput of electron

lithography is slow (the

throughput varies as the

reciprocal of the square ofthe minimum feature length).

For mass production, the cost

and footprint (required floor

area) of the machine mustalso be minimized.

Optical lithography is the main stream

technology and some commercial resists can

resolve down to 0.1 m or lower


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Etching


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Wet Chemical Etching

(1) Transportation of

reactants to the reactingsurface (e.g. by diffusion)

(2) Chemical reactions atthe surface

(3) Transportation of the

products from the surface

(e.g. by diffusion)


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Silicon Wet EtchingFor semiconductor materials, wet chemical etching usually

proceeds by oxidation, accompanied by dissolution of the oxide.

For silicon, the most commonly used etchants are mixtures of

nitric acid (HNO3) and hydrofluoric acid (HF) in water or aceticacid (CH3COOH). The reaction is initiated by promoting

silicon from its initial oxidation state to a higher oxidation state:

Si + 2h+ Si2+


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The holes (h+) are produced by the autocatalytic process:

HNO3 + HNO2 2NO2- + 2h+ + H2O2NO2

- + 2H+ 2HNO2

Si2+ combines with OH- (formed by dissociation of H2O) toform Si(OH)2 which subsequently liberates H2 to form SiO2:

Si(OH)2 SiO

2+ H

2

SiO2 then dissolves in HF:

SiO2

+ 6HF H2

SiF6

+ H2

O

The overall reaction can be written as:

Si + HNO3 + 6HF H2SiF6 + HNO2 + H2O + H2


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Although water can be used as a diluent, acetic acid is

preferred as the dissociation of nitric acid can be retarded inorder to yield a higher concentration of the undissociated

species

At very high HF and low HNO3 concentrations, the etching

rate is controlled by HNO3, because there is an excess

amount of HF to dissolve any SiO2 formed At low HF and high HNO3 concentrations, the etch rate is

controlled by the ability of HF to remove SiO2 as it is being

formed. The etching mechanism is isotropic, that is, not

sensitive to crystallographic orientation


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Insulating / Metal Film Etching

Etching of insulating and metal films is usually

performed with chemicals that dissolve thesematerials in bulk form and involves their

conversion into soluble salts and complexes

Film materials will etch more rapidly than their

bulk counterparts

The etching rates are higher for films that possesspoor microstructures or built-in stress, are non-

stoichiometric, or have been irradiated


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Isotropic / Anisotropic Etching

Wet etching is usually isotropic,undercutting the layer underneath

the mask and resulting in a loss of

resolution in the etched pattern If patterns are required with

resolution much smaller than the

film thickness, anisotropic etching(i.e., 1 > A

f> 0) must be used:

Af

= 1 l/hf

= 1 - vl/v

v

where Af is the degree ofanisotropy, and v

land v

vare the

lateral and vertical etch rates,

respectively. For isotropic etching,

vl= v

vandA

f= 0


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Plasma

A plasma is a fully or partially ionized gas composed

of ions, electrons, and neutrals. The plasma most

useful to ULSI processing is a weakly ionized plasma

called glow discharge containing a significant density

of neutral particles (>90% in most etchers). Althoughplasma is neutral in a macroscopic sense, it behaves

quite differently from a molecular gas, because it

consists of charged particles that can be influenced by

applied electric and magnetic fields

A plasma is produced when an


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p p

electric field is applied across two

electrodes between which a gas is

confined at low pressure, causing

the gas to break down and become

ionized.

Simple DC (direct current) power

can be used to generate plasma,

but insulating materials require

AC (alternate current) power to

reduce charging.

In plasma etching, an RF (radio frequency) field is usually used to

generate the gas discharge. One reason for doing so is that the electrodesdo not have to be made of a conducting material. Besides, electrons can

gain sufficient energy during oscillation to cause more ionization by

electron neutral atom collisions. As a result, the plasma can be

generated at pressures lower than 10-3 Torr.

The free electrons released by photo ionization or field emission from a


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The free electrons released by photo-ionization or field emission from a

negatively biased electrode create the plasma. The free electrons gain

kinetic energy from the applied electric field, and in the course of theirtravel through the gas, they collide with gas molecules and lose energy.

These inelastic collisions serve to further ionize or excite neutral species

in the plasma via the following reaction examples:

e- + AB A- + B+ + e- (Dissociative attachment)e- + AB A + B + e- (Dissociation)e- + A A+ + 2e- (Ionization)

Some of these collisions cause the gas molecules to be ionized and create

more electrons to sustain the plasma. Therefore, when the applied

voltage is larger than the breakdown potential, the plasma is formedthroughout the reaction chamber. Some of these inelastic collisions can

also raise neutrals and ions to excited electronic states that later decay by

photoemission, thereby causing the characteristic plasma glow.


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Interaction of plasmas with surfaces is often

divided into two components: physical andchemical

A physical interaction refers to the surfacebombardment of energetic ions accelerated acrossthe plasma sheath, and the loss of kinetic energy by

the impinging ions causes ejection of particlesfrom the sample surface

Chemical reactions are standard electronic bondingprocesses that result in the formation ordissociation of chemical species on the surface

Plasma Etching Mechanism


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Plasma Etching Mechanism

Sputtering Systems


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Sputtering Systems (Top) Sputtering - etching system: Inertgas ions accelerated by the applied

electric field sputter off the surface.There is essentially no sputtering of the

sidewalls to attain a high degree of

anisotropy, but the selectivity is poor.

In reactive ion etching (RIE), the noble

gas plasma is replaced by a molecular

gas plasma similar to that in plasma

etching. Under appropriate conditions,both RIE and plasma etching can give

high selectivity and a high degree of

anisotropy

(Bottom) Parallel - plate plasma etching system: The plasma is confined between

the two closely spaced electrodes. Molecular gases containing one or more

halogen atoms are fed through the gas ring.

Ion-Assisted The etching rate can be enhanced by


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Reaction

g y

ion-assisted chemical reactions.

This figure depicts the etch rate as a

function of flow rate of XeF2

molecules with and without 1 keV

Ne+ bombardment. The lateral etchrate depends only on the ability of

XeF2

molecules to etch silicon in the

absence of energetic ions impacting

the surface, whereas the vertical etch

rate is a synergistic effect due to both

Ne+ bombardment and XeF2

molecules.

The degree of anisotropy can

generally be enhanced by increasing

the energy of the ions

When a gas is mixed with one or


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When a gas is mixed with one or

more additive gases, both the etch

rate and selectivity can be altered.

In this example, the etching rate of

SiO2

is approximately constant for

addition of up to 40% hydrogen,

while the etch rate for silicon drops

monotonically and is almost zero at

40% H2.

Selectivity (ratio of the etch rate of

silicon dioxide to silicon) exceeding

45:1 can be achieved with CF4 - H2reactive ion etching. This process is

thus useful when etching a SiO2

layer that covers a polysilicon gate.


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Lift Off versus Direct Etching


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Lift-Off versus Direct Etching

Disadvantages of the

lift-off technique:

(1) Rounded feature

profile

(2) Temperature

limitations

(> 200oC to 300oC)


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Polysilicon and Dielectric Film

Deposition


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Thin Films in Microelectronics

Polycrystalline silicon or polysilicon

Doped or undoped silicon dioxide

Stoichiometric or plasma-deposited silicon

nitride

Polysilicon


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Polysilicon

Gate electrode material in MOS devices

Conducting materials for multilevel

metallization

Contact materials for devices with shallow

junctions

Polysilicon can be undoped or doped with elements such as As,

P, or B to reduce the resistivity. The dopant can beincorporated in-situ during deposition, or later by diffusion or

ion implantation. Polysilicon consisting of several percent

oxygen is a semi-insulating material for circuit passivation.


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Dielectric Thin Films

Insulation between conducting layers

Diffusion and ion implantation masks

Diffusion sources (doped oxide)

Capping doped films to prevent dopant loss

Gettering impurities

Passivation to protect devices fromimpurities, moisture, and scratches

Ph h d d ili di id l f d t


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Phosphorus-doped silicon dioxide, commonly referred to

as P-glass or phosphosilicate glass (PSG), is especiallyuseful as a passivation layer because it inhibits the

diffusion of impurities (such as Na), and it softens and

flows at 950o

C to 1100o

C to create a smooth topographythat is beneficial for depositing metals

Borophosphosilicate glass (BPSG), formed by

incorporating both boron and phosphorus into the glass,flows at even lower temperatures between 850oC and

950oC

The smaller phosphorus content in BPSG reduces theseverity of aluminum corrosion in the presence of

moisture


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Silicon nitride is a barrier to sodium diffusion, isnearly impervious to moisture, and has a low

oxidation rate

The local oxidation of silicon (LOCOS) processalso uses silicon nitride as a mask

The patterned silicon nitride will prevent the

underlying silicon from oxidation but leave the

exposed silicon to be oxidized

Silicon nitride is also used as the dielectric forDRAM MOS capacitors when it combines with

silicon dioxide

Physical Vapor Deposition (PVD)


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Physical Vapor Deposition (PVD)

Evaporation: A vapor is first generated by evaporating a

source material in a vacuum chamber and then transported

from the source to the substrate and condensed to a solid filmon the substrate surface.

Sputtering: The process involves the ejection of surfaceatoms from an electrode surface by momentum transfer from

the bombarding ions to the electrode surface atoms. The

generated vapor of electrode material is then deposited on thesubstrate. Sputtering processes, unlike evaporation, are very

well controlled and generally applicable to all materials such

as metals, insulators, semiconductors, and alloys.

Typical Sputtering System


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Typical Sputtering System

Chemical Vapor Deposition (CVD)


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Chemical Vapor Deposition (CVD)

Atmospheric-pressure chemical vapor deposition (APCVD) Low-pressure chemical vapor deposition (LPCVD)

Plasma-enhanced chemical vapor deposition (PECVD)

Since the steps in a CVD process are sequential the one that


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Since the steps in a CVD process are sequential, the one that

occurs at the slowest rate will determine the deposition rateand the rate-determining steps can be grouped into gas-phaseand surface processes

Gas-phase processes dictate the rate at which gases impinge on

the surface and since such transport processes occur by gas-phase diffusion proportional to the diffusivity of the gas andthe concentration gradient across the boundary layer, they are

only weakly influenced by the deposition temperature The surface reaction rate is greatly affected by the depositiontemperature. At low temperature, the surface reaction rate isreduced so much that the arrival rate of reactants can exceed

the rate at which they are consumed by the surface reactionprocess. Under such conditions, the deposition rate is surface-reaction-rate-limited, and at high temperature, it is usuallymass-transport-limited.

Arrhenius Equation


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q

R = A exp {-qEa/kT}where R is the deposition rate, A is the

frequency factor, q is the electronic

charge, Ea

is the activation energy, k is

the Boltzmann's constant, and T is the

absolute temperature

The activation energy calculated from the

slope of the straight-line plots is roughly1.7 eV.

At high temperature, the reaction becomes faster than the rate at which unreacted

silane arrives at the surface. The reaction in this temperature regime is mass-transport limited, as exemplified by the high temperature (or small 1/T) data. The

linear portion of the lines show the surface-reaction limited conditions, that is, the

rate of reaction is slower than the rate of reactant arrival.

Characteristics and Applications of CVD Processes


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Process Advantages Disadvantages Applications

APCVD Simple reactor, Poor step coverage, Doped/undoped

(low T) fast deposition, particle contamination, low T oxides

low temperature low throughput

LPCVD Excellent purity High temperature, Doped/undoped

& uniformity, low deposition rate high T oxides,

conformal step silicon nitride,

coverage, large polysilicon,wafer capacity, tungsten,

high throughput WSi2

PECVD Low temperature, Chemical (e.g. H2

) Passivation

fast deposition, and particle (nitride), low T

good step contamination insulators over

coverage metals

Simple Commercial Hot-Wall, Low-


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S p e Co e c a ot Wa , ow

Pressure Reactor for High WaferCapacity Deposition of Polysilicon

Horizontal Tube


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Horizontal Tube

APCVD Reactor

Gas Injection-TypeContinuous-Processing

APCVD Reactor

Plenum-Type

Continuous-ProcessingAPCVD Reactor


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Parallel Plate

PECVD Reactor

Single Wafer

PECVD Reactor

Typical Commercial PECVD System


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Common Gases Used in CVD


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Flammable

Exposure ToxicLimits in Air Limit

Gas Formula Hazards (vol %) (ppm)

Ammonia NH3 Toxic, corrosive 16-25 25Argon Ar Inert -- --

Arsine AsH3 Toxic -- 0.05

Diborane B2H6 Toxic, flammable 1-98 0.1

Dichlorosilane SiH2Cl2 Toxic, flammable 4-99 5Hydrogen H2 Flammable 4-74 --

Hydrogen chloride HCl Toxic, corrosive -- --

Nitrogen N2 Inert -- --

Nitrogen Oxide N2O Oxidizer -- --

Oxygen O2 Oxidizer -- --

Phosphine PH3 Toxic, flammable Pyrophoric 0.3

Silane SiH4 Toxic, flammable Pyrophoric 0.5

Polysilicon Deposition


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Polysilicon is deposited by pyrolyzing silane between 575oC and 650oC in alow-pressure reaction:

SiH4 (g) Si (s) + 2H2 (gas)

Either pure silane or 20 to 30% silane in nitrogen is bled into the LPCVD

system at a pressure of 0.2 to 1.0 torr. The properties of the LPCVD

polysilicon films are determined by the deposition pressure, silane

concentration, deposition temperature, and dopant content.

Polysilicon can be doped by adding phosphine, arsine, or diborane to the

reactants (in-situ doping). Adding diborane causes a large increase in the

deposition rate because diborane forms borane radicals, BH3, that catalyzegas-phase reactions and increase the deposition rate. In contrast, adding

phosphine or arsine causes a rapid reduction in the deposition rate, because

phosphine or arsine is strongly adsorbed on the silicon substrate surface

thereby inhibiting the dissociative chemisorption of SiH4.

The dopant concentration in diffused polysilicon often exceeds the

solid solubility limit, with the excess dopant atoms segregated at the


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grain boundaries.

The high resistivity

observed for lightly

implanted polysilicon is

caused by carrier traps

at the grain boundaries.

Once these traps aresaturated with dopants,

the resistivity decreases

rapidly and approachesthat for implanted

single-crystal silicon.

Silicon Dioxide Deposition


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Silicon dioxide films can be deposited at lower than 500oC

by reacting silane, dopant (phosphorus in this example), and

oxygen under reduced pressure or atmospheric pressure

SiH4 (g) + O2 (g) SiO2 (s) + 2H2 (g)

4PH3 (g) + 5O2 (g) 2P2O5 (s) + 6H2 (g)

The process can be conducted in an APCVD or LPCVD

chamber. The main advantage of silane-oxygen reactions is

the low deposition temperature allowing films to be depositedover aluminum metallization. The primary disadvantages are

poor step coverage and high particle contamination caused by

loosely adhering deposits on the reactor walls.

Silicon dioxide can be deposited at 650oC to 750oC in an


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p

LPCVD reactor by pyrolyzing tetraethoxysilane,Si(OC2H5)4. This compound, abbreviated TEOS, is

vaporized from a liquid source

Si(OC2H5)4 (l) SiO2 (s) + by-products (g)

The by-products are organic and organosilicon

compounds. LPCVD TEOS is often used to deposit the

spacers beside the polysilicon gates. The process offers

good uniformity and step coverage, but the high

temperature limits its application on aluminuminterconnects.

Sili b d it d b LPCVD t b t 900oC b


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Silicon can be deposited by LPCVD at about 900oC by

reacting dichlorosilane with nitrous oxide

SiCl2H2 (g) + 2N2O (g) SiO2 (s) + 2N2 (g) + 2HCl (g)

This deposition technique provides excellent uniformity,

and like LPCVD TEOS, it is employed to depositinsulating layers over polysilicon. However, this oxide is

frequently contaminated with small amounts of chlorine

that may react with polysilicon causing film cracking.

Plasma-assisted CVD requires the control and optimization of

h d i f d d l i ddi i


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the RF power density, frequency, and duty cycle in addition to

the conditions similar to those of an LPCVD process such as gas

composition, flow rate, deposition temperature, and pressure.

Like LPCVD at low temperature, the PECVD process is

surface-reaction-limited, and adequate substrate temperaturecontrol is necessary to ensure film thickness uniformity.

By reacting silane and oxygen or nitrous oxide in plasma,silicon dioxide films can be formed by the following reactions:

SiH4 (g) + O2 (g) SiO2 (s) + 2H2 (g)

SiH4 (g) + 4N2O (g) SiO2 (s) + 4N2 (g) + 2H2O (g)

Step CoverageA uniform or conformal step coverage depicted in (a)

results when reactants or reactive intermediates

adsorb and then migrate promptly along the surface


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before reacting.When the reactants adsorb and react without

significant surface migration, the deposition rate is

proportional to the arrival angle of the gas molecules.

If the mean free path of the gas is much larger thanthe dimensions of the step (b), the arrival angle and

consequently the film thickness vary. The film is thin

along the vertical walls and may have a crack at the

bottom of the step caused by self-shadowing.

When there is minimal surface mobility and the mean

free path is short, the arrival angle at the top of the

step is 270o

, thus giving a thicker deposit. The arrivalangle at the bottom of the step is only 90o, and so the

film is thin (c). The thick cusp at the top of the step

and the thin crevice at the bottom combine to give a

re-entrant shape that is particularly difficult to coverwith metal.

Reflow


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Doped oxides used as diffusion sources contain 5 to 15

weight % of the dopant

Doped oxides for passivation or interlevel insulation

contain 2 to 8 wt. % phosphorus to prevent the diffusion of

ionic impurities to the device

Phosphosilicate glass (PSG) used for the reflow process

contains 6 to 8 wt. % phosphorus

Reflow

O id ith l h h t ti ill t


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Oxides with lower phosphorus concentrations will not

soften and flow, but higher phosphorus concentrations can

give rise to deleterious effects for phosphorus can react

with atmospheric moisture to form phosphoric acid which

can consequently corrode the aluminum metallization

Addition of boron to PSG further reduces the reflow

temperature without exacerbating this corrosion problem

Borophosphosilicate glass (BPSG) typically contains 4 to 6

wt. % P and 1 to 4 wt. % B

SEM photographs (3200x) showing

surfaces of 4.6 wt. % P-glass annealed


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g

for different times in steam at 1100oC

0 minute 20 minutes

40 minutes 60 minutes

SEM cross sections (10,000x) of samples with

different weight percent of phosphorus


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different weight percent of phosphorus

annealed in steam at 1100oC for 20 minutes

0.0 wt. % P 2.2 wt. % P

4.6 wt. % P 7.2 wt. % P

Planarization The step coverage ofdeposited oxides can be


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p

improved by planarization oretch-back techniques.

Since the organic resistmaterial has a low viscosity,

reflow occurs during

application or the subsequent

bake. The sample is then

plasma etched to remove all

the organic coating and part of

the PSG, as long as theetching conditions are selected

to remove the organic material

and PSG at equal rates.

Silicon Nitride Deposition


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(1) Stoichiometric silicon nitride (Si3N4) can be deposited at

700oC to 800oC at atmospheric pressure:

3SiH4 (g) + 4NH3 (g) Si3N4 (s) + 12H2 (g)

(2) Using LPCVD, silicon nitride can be produced by reactingdichlorosilane and ammonia at temperature between 700oC and

800oC:

3SiCl2H2 (g) + 4NH3 (g) Si3N4 (s) + 6H2 (g)The reduced-pressure technique has the advantage of yielding

good uniformity and higher wafer throughput.

Silicon Nitride Plasma Deposition


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Hydrogenated silicon nitride films can be deposited by reactingsilane and ammonia or nitrogen in plasma at reduced temperature:

SiH4 (g) + NH3 (g) SiN : H (s) + 3H2 (g)

SiH4 (g) + N2 (g) 2SiN : H (s) + 3H2 (g)

Plasma-assisted deposition is conducted at low temperature by

reacting the gases in a glow discharge. Two plasma depositedmaterials, plasma deposited silicon nitride (SiNH) and plasma

deposited silicon dioxide, are useful in VLSI. On account of the

low deposition temperature, 300oC to 350oC, plasma nitride can be

deposited over the final device metallization. Plasma-deposited

films contain large amounts of hydrogen (10 to 35 atomic %)

bonded to silicon as Si-H, to nitrogen as N-H, and to oxygen as Si-

OH and H2O.

Properties of Silicon Nitride Films


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Deposition LPCVD Plasma

Temperature (o

C) 700-800 250-350Composition Si

3N

4(H) SiN

xH

y

Si/N ratio 0.75 0.8-1.2

Atom % H 4-8 20-25

Refractive index 2.01 1.8-2.5Density (g/cm3) 2.9-3.1 2.4-2.8

Dielectric constant 6-7 6-9

Resistivity (ohm-cm) 1016 106-1015

Dielectric strength (106V/cm) 10 5Energy gap (eV) 5 4-5

Stress (109 dyne/cm2) 10T 2C-5T

Rapid Thermal Chemical Vapor

Deposition (RTCVD) System


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Deposition (RTCVD) System

RTCVD requires higher

temperature but shorter time

to deposit the same materialcompared to LPCVD.

Temperature is the switchthat turns the RTP

deposition process on or off,

avoiding the long ramp-up

and ramp-down times

required in conventional

methods.

To be commercially viable, RTCVD systems


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y , y

must be able to process a single wafer in 1 to 2minutes, giving a corresponding throughput of 30to 60 wafers per hour

For relatively thick deposited films of 100 to 200nm, this requires deposition rates greater than 100nm/min

In comparison, the typical deposition rate inconventional batch LPCVD processes is about 10nm/min

The RTCVD of SiO2 by pyrolysis of TEOS is believed to


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occur by the reaction:

Si(OC2H5)4 SiO2 (s) + 2H2O + 4C2H4

Below 800oC, the deposition rate is controlled by surface

reaction processes with activation energy of 3.3 eV. This

large sensitivity to temperature thus demands tight

temperature control for thin oxide deposition at lowtemperature. Above 800oC, the deposition rate of SiO2approaches 100 nm/min with a lower activation energy,

which meets the throughput and deposition controlrequirements of the RTCVD system. The high operating

temperature makes it infeasible for back end steps in

multilevel-metallization technologies that use aluminium.

Thin oxide applications include in-situ deposition of MOS gate

structures where Si surface cleaning, gate oxide formation, and

polysilicon gate electrode deposition would all occur in a low-


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polysilicon gate electrode deposition would all occur in a low

pressure, multi-chamber cluster tool. RTCVD yields high-quality

films through the use of ultraclean gases in a chamber with a highly

regulated ambient, and examples of these reactions are:

SiH4 + O2 SiO2 + 2H2

SiH4 + 2N2O SiO2 + 2N2 + 2H2O

3SiH2Cl2 + 10NH3 Si3N4 + 6NH4Cl + 6H2

3SiH4 + 4NH3 Si3N4 + 12H2

SiH4 Si + 2H2

vlsi technology 2

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