CVD slides

CHEMICAL VAPOR CHEMICAL VAPOR DEPOSITION (CVD) AND DEPOSITION (CVD) AND

PHYSICAL VAPOR PHYSICAL VAPOR DEPOSITON (PVD) DEPOSITON (PVD)

TECHNIQUESTECHNIQUESHaris MehmoodHaris Mehmood

Dilawaiz FazalDilawaiz Fazal

MS-59 ElectricalMS-59 Electrical

CHEMICAL VAPOR DEPOSITIONCHEMICAL VAPOR DEPOSITION

In CVD, gases are introduced into the In CVD, gases are introduced into the deposition reactor that react and form deposition reactor that react and form the desired film on the substrate.the desired film on the substrate.

Thin Film deposition are done through Thin Film deposition are done through these different CVD schemes.these different CVD schemes.

(APCVD, LPCVD, PECVD, MOCVD)(APCVD, LPCVD, PECVD, MOCVD)

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PHYSICAL VAPOR DEPOSITION PHYSICAL VAPOR DEPOSITION (PVD)(PVD)

PVD is used to deposit thin films by PVD is used to deposit thin films by the condensation of a vaporized form the condensation of a vaporized form of the material onto the surface.of the material onto the surface.

Example include evaporation (MBE) Example include evaporation (MBE) and sputter deposition.and sputter deposition.

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ATMOSPHERIC CHEMICAL ATMOSPHERIC CHEMICAL VAPOR DEPOSITION VAPOR DEPOSITION

(APCVD)(APCVD)

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ATMOSPHERIC PRESSURE CVDATMOSPHERIC PRESSURE CVD

Simplest CVD process at an Simplest CVD process at an atmospheric pressure of 760 torr. atmospheric pressure of 760 torr.

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Wafers are heated by graphite susceptor Wafers are heated by graphite susceptor which in turn is heated by RF coilswhich in turn is heated by RF coils

Wafers are usually placed at some tilt angle Wafers are usually placed at some tilt angle from horizontal to ensure equal gas flowsfrom horizontal to ensure equal gas flows

The films deposited by CVD can be doped The films deposited by CVD can be doped by arsine (AsHby arsine (AsH33), phosphine (PH), phosphine (PH33) and B) and B22HH66

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CHEMICAL REACTIONCHEMICAL REACTION

APCVD is mostly used for epitaxial silicon APCVD is mostly used for epitaxial silicon deposition. deposition.

SiClSiCl44 + 2H + 2H22 → Si + 4HCl → Si + 4HCl

This reaction occurs at the growing surface on the This reaction occurs at the growing surface on the

substrate at the temperature of 1150-1300substrate at the temperature of 1150-1300ooC for 3 -C for 3 -5 minutes5 minutes and at atmospheric pressure (760 torr)and at atmospheric pressure (760 torr)

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DEPOSITION RATEDEPOSITION RATE

Measure of how fast the film can be grown Measure of how fast the film can be grown on the substrate. Figure shows the concent.on the substrate. Figure shows the concent.

and fluxes in the gas-phase boundaryand fluxes in the gas-phase boundary

layer and at the wafer surface.layer and at the wafer surface.

FF11 = h = hGG (C (CGG-C-CSS)) ------- (i)------- (i)

FF22 = k = kSSCCS S ----------------(ii)----------------(ii)

hhG G is the mass transfer coefficient.is the mass transfer coefficient.

kkSS is the chemical surface reaction rate is the chemical surface reaction rate

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The mass transfer coefficient is a diffusion rate constant that The mass transfer coefficient is a diffusion rate constant that relates the mass transfer rate (n), mass transfer area (A), and relates the mass transfer rate (n), mass transfer area (A), and concentration gradient (Δc) as driving force:concentration gradient (Δc) as driving force:

hhG G = n / (A*Δc)= n / (A*Δc)

According to the steady state conditions, the two fluxes must According to the steady state conditions, the two fluxes must

be equal to each other:be equal to each other:

CCSS = C = CGG ( 1 + k( 1 + kSS / h / hGG ) ) -1-1

The deposition rate is given by The deposition rate is given by

v = F/Nv = F/N

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ATMOSPHERIC PRESSURE CVDATMOSPHERIC PRESSURE CVD ADVANTAGESADVANTAGES

-- Low equipment costLow equipment cost

- SiCl- SiCl4 4 is non-toxic, inexpensive and easy to is non-toxic, inexpensive and easy to purifypurify

- High deposition rates- High deposition rates

DISADVANTAGESDISADVANTAGES

- SiCl- SiCl4 4 requires higher temperature than silane requires higher temperature than silane processprocess

- Periodic manual cleaning of injectors are - Periodic manual cleaning of injectors are requiredrequired

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ATMOSPHERIC PRESSURE CVDATMOSPHERIC PRESSURE CVD VLSI APPLICATIONSVLSI APPLICATIONS - Used for making thick oxides- Used for making thick oxides

- Epitaxial is used to improve the performance of - Epitaxial is used to improve the performance of dynamic random access memory devices and dynamic random access memory devices and CMOS ICs as APCVD manufactures good quality CMOS ICs as APCVD manufactures good quality epitaxial silicon layerepitaxial silicon layer

HAZARDS AND SAFETY PRECAUTIONSHAZARDS AND SAFETY PRECAUTIONS The by-product of epitaxial silicon reaction, HCl The by-product of epitaxial silicon reaction, HCl

gas, is very dangerous and toxic. Brief exposure of gas, is very dangerous and toxic. Brief exposure of the gas to the operator can occur during manual the gas to the operator can occur during manual cleaning. Respirators, acid gloves, safety glasses cleaning. Respirators, acid gloves, safety glasses

are to be worn to minimize the risk involved. are to be worn to minimize the risk involved. 1111

LOW PRESSURE CHEMICAL LOW PRESSURE CHEMICAL VAPOR DEPOSITON (LPCVD)VAPOR DEPOSITON (LPCVD)

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LOW PRESSURE CVDLOW PRESSURE CVD

LPCVD reactors opearte at medium vacuum LPCVD reactors opearte at medium vacuum pressure (30-250 Pa).pressure (30-250 Pa).

Wafers stand vertically, perpendicular to the gas Wafers stand vertically, perpendicular to the gas flow.flow.

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CHEMICAL REACTIONSCHEMICAL REACTIONS

LPCVD deposition of Silicon Oxide (LTO)LPCVD deposition of Silicon Oxide (LTO)

SiHSiH44 + O + O22 → SiO → SiO22 + 2H + 2H2 2 (at 430(at 430ooC)C)

LPCVD deposition of Silicon Oxide (HTO)LPCVD deposition of Silicon Oxide (HTO)

SiHSiH22ClCl22 + 2N + 2N22O→ SiOO→ SiO22 + 2HCl + 2N + 2HCl + 2N2 2 (at 900(at 900ooC)C)

LPCVD deposition of Silicon NitrideLPCVD deposition of Silicon Nitride

3SiH3SiH22ClCl22 + 4NH + 4NH33→ Si→ Si33NN44 + 6HCl + 6H + 6HCl + 6H2 2 (at 750(at 750ooC)C)

LPCVD deposition of Polysilicon and MetalsLPCVD deposition of Polysilicon and Metals

SiHSiH44 → Si + 2H → Si + 2H2 2 (at 600(at 600ooC)C)

WFWF6 6 ++ 3H3H22 → W + 6HF → W + 6HF 1414

LOW PRESSURE CVDLOW PRESSURE CVD DEPOSITION RATEDEPOSITION RATE

We have DWe have DGG proportional to proportional to 1/P1/Ptottot

Lowering the pressure of the Lowering the pressure of the gas stream increases the gas stream increases the diffusion and extends the diffusion and extends the higher temperature (1/T). higher temperature (1/T).

Use PV = nRT Law.Use PV = nRT Law. Since hSince hG G is larger at lower is larger at lower

pressure, surface reaction pressure, surface reaction will be carried out at low will be carried out at low

pace. See Figure for pace. See Figure for clarification.clarification. 1515


ADVANTAGESADVANTAGES - Lower chemical reaction temperature as - Lower chemical reaction temperature as

compared to APCVDcompared to APCVD

- Good step coverage and uniformity- Good step coverage and uniformity

- Less dependance on gas flow dynamics- Less dependance on gas flow dynamics

- Highly pure sample obtained- Highly pure sample obtained

DISADVANTAGESDISADVANTAGES - Lower deposition rate than APCVD- Lower deposition rate than APCVD

- Frequent use of toxic and flammable gases such - Frequent use of toxic and flammable gases such as ammonia, silane, fluorine, hydrogen fluoride, as ammonia, silane, fluorine, hydrogen fluoride, HCl, etc.HCl, etc.

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VLSI APPLICATIONSVLSI APPLICATIONS - Gate contacts in MOS and interconnected lines made by - Gate contacts in MOS and interconnected lines made by

LPCVD-assisted PolysiliconLPCVD-assisted Polysilicon

- Thick oxides are used for isolation between metal - Thick oxides are used for isolation between metal interconnects.interconnects.

- Silicon Nitride is used for encapsulation- Silicon Nitride is used for encapsulation

- Oxide/Nitrides are used in FLASH memories- Oxide/Nitrides are used in FLASH memories

- Metals for interconnected lines in CMOS- Metals for interconnected lines in CMOS

HAZARDS AND SAFETY PRECAUTIONSHAZARDS AND SAFETY PRECAUTIONS Ammonia, silane are very dangerous, flammable and toxic. Ammonia, silane are very dangerous, flammable and toxic.

Brief exposure of the gas to the operator can occur during Brief exposure of the gas to the operator can occur during manual cleaning. Personal proctective equipment is worn to manual cleaning. Personal proctective equipment is worn to minimize the risk such as Respirators, acid gloves, safety minimize the risk such as Respirators, acid gloves, safety glasses, armguards, aprons.glasses, armguards, aprons.

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PLASMA ENHANCED PLASMA ENHANCED CHEMICAL VAPOR CHEMICAL VAPOR

DEPOSITION (PECVD)DEPOSITION (PECVD)

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PLASMA ENHANCED CVDPLASMA ENHANCED CVD

PECVD is mostly used to deposit dielectricsPECVD is mostly used to deposit dielectrics The plasma is created by RF power ionizing some of The plasma is created by RF power ionizing some of

the reacting gases between the two electrodes. The the reacting gases between the two electrodes. The energetic species are used to help form the films on energetic species are used to help form the films on top of the wafers.top of the wafers.

Presence of plasma lowers the temperature at which Presence of plasma lowers the temperature at which reaction is possible. reaction is possible. 1919

PLASMA ENHANCED CVDPLASMA ENHANCED CVD

CHEMICAL REACTIONSCHEMICAL REACTIONS

3SiH3SiH44 + 4NH + 4NH33→ Si→ Si33NN44 + 24H + 24H2 2 (below 350(below 350ooC)C)

PECVD is carried out at the frequency ranging from PECVD is carried out at the frequency ranging from 100 kHz to 40 MHz. The process does not require 100 kHz to 40 MHz. The process does not require deep vacuum so the reduced pressure between 50 deep vacuum so the reduced pressure between 50 mTorr tomTorr to

5 Torr is used.5 Torr is used.

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PLASMA ENHANCED CVDPLASMA ENHANCED CVD DEPOSITION RATEDEPOSITION RATE PECVD has lower deposition rate than that of APCVD and PECVD has lower deposition rate than that of APCVD and

LPCVD. LPCVD.

ADNAVTAGESADNAVTAGES - Lower process temperature- Lower process temperature

- Films of good dielectric properties- Films of good dielectric properties

- Good conformal step coverage- Good conformal step coverage

DISADVANTAGESDISADVANTAGES - The highly energetic plasma gases can pose damage to the - The highly energetic plasma gases can pose damage to the

substratesubstrate

- High cost of equipment and toxic byproduct- High cost of equipment and toxic byproduct2121

PLASMA ENHANCED CVDPLASMA ENHANCED CVD VLSI APPLICATIONSVLSI APPLICATIONS - PECVD nitride is suitable for passivation layers- PECVD nitride is suitable for passivation layers - PECVD processed products are used in VLSI-DRAM - PECVD processed products are used in VLSI-DRAM

memory cellsmemory cells - PECVD a-Silicon is used in solar cells- PECVD a-Silicon is used in solar cells - Used for depositing materials like Silicon Oxide, - Used for depositing materials like Silicon Oxide,

Silicon Nitride, Carbon, Amorphous Silicon and Silicon Nitride, Carbon, Amorphous Silicon and polysiliconpolysilicon

HAZARDS AND SAFETY PRECAUTIONSHAZARDS AND SAFETY PRECAUTIONS Silane and SiSilane and Si33NN44 are very dangerous and toxic. Brief are very dangerous and toxic. Brief

exposure of the gas to the operator can occur during exposure of the gas to the operator can occur during manual cleaning. Personal proctective equipment is manual cleaning. Personal proctective equipment is worn to minimize the risk such as Respirators, acid worn to minimize the risk such as Respirators, acid gloves, safety glasses, armguards, aprons.gloves, safety glasses, armguards, aprons.

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MOLECULAR BEAM EPITAXY MOLECULAR BEAM EPITAXY (MBE)(MBE)

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MBEMBE MBE is used to deposit MBE is used to deposit

single crystals.single crystals. MBE creates a molecular MBE creates a molecular

beam of material by beam of material by evaporating it under ultra-evaporating it under ultra-high vacuum (5 e-11 mbar high vacuum (5 e-11 mbar pressure) and is focused on pressure) and is focused on the substrate by effusion the substrate by effusion cellscells

RHEED (reflection high RHEED (reflection high energy electron diffraction) energy electron diffraction) is used to monitor film is used to monitor film growthgrowth

Mass spectrometer collects Mass spectrometer collects stoichiometric datastoichiometric data

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MBEMBE CHEMICAL REACTIONCHEMICAL REACTION To deposit silicon layer, SiTo deposit silicon layer, Si22HH6 6 is evaporated in the is evaporated in the

effusion cells to give silicon effusion cells to give silicon

SiSi22HH66 → 2Si + 3H → 2Si + 3H2 2 (at 600(at 600ooC)C)

DEPOSITION RATEDEPOSITION RATE Slow deposition rates require proportionally better Slow deposition rates require proportionally better

vacuum. In MBE, evaporation is carried out at very low vacuum. In MBE, evaporation is carried out at very low deposition rates. For good epitaxy, we want rate to bedeposition rates. For good epitaxy, we want rate to be

R < (constant) e R < (constant) e –Ed/(k*Tc)–Ed/(k*Tc)

where Ewhere Edd = desorption energy = desorption energy TTcc = epitaxy critical temperature = epitaxy critical temperature

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MBEMBE ADVANTAGESADVANTAGES - Well-controlled process- Well-controlled process

- Grow film with good crystal stucture- Grow film with good crystal stucture

- often use multiple sources to grow alloy films- often use multiple sources to grow alloy films DISADVANTAGESDISADVANTAGES - Low growth rate- Low growth rate

- Sophisticated and expensive equipment- Sophisticated and expensive equipment VLSI APPLICATIONSVLSI APPLICATIONS - Used in high frequency receivers (portable - Used in high frequency receivers (portable

phones)phones)

- Optoelectronics (DVD, Digital Computers)- Optoelectronics (DVD, Digital Computers)

- For producing superconductors (YBa- For producing superconductors (YBa22CuCu33OO7-87-8))

- Used in silicon process technology- Used in silicon process technology 2626

METAL ORGANIC CHEMICAL METAL ORGANIC CHEMICAL VAPOR DEPOSITION VAPOR DEPOSITION

(MOCVD)(MOCVD)

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Compare of epitaxial Compare of epitaxial methodsmethods

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Growth method time features limit

MBE

(Molecular Beam Epitaxy)

1958

1967

Deposit epilayer at ultrahigh vacuum

Hard to grow materials with high vapor pressure

MOCVD

(Metal-Organic Chemical Vapor Deposition)

1968 Use metalorganic compounds as the sources

Some of the sources like AsH3 are very toxic.

2. The MOVCD growth 2. The MOVCD growth systemsystem

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Reactor-1Reactor-1

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MOCVD Growth SystemMOCVD Growth System

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Vacuum andExhaust system

Gas handle system

ComputerControl

Reactor

Gas handling systemGas handling system

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The function of gas handling system is mixing of the gas that will enter the reactor. Timing and composition of the gas entering the reactor will determine the epilayer structure.

Leak-tight of the gas panel is essential, because the oxygen contamination will degrade the growing films’ properties.

Accurate control of flow rate, pressure and temperature can ensure the stable and repeat.

Exhaust systemExhaust system

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Pump and pressure controller

For low pressure growth, we use mechanic pump and pressure controller to control the growth pressure. The pump should be designed to handle large gas load.

Waste gas treatment system and safety concerns

The treatment of exhaust gas is a matter of safety concern.

The MOCVD system for GaAs and InP use toxic materials like AsH3 and PH3. The exhaust gases still contain some not reacted AsH3 and PH3, Normally, the toxic gas need to be removed by using chemical scrubber and extra safety precautions need to be carried out to minimize the risks involved.

3. Metalorganic compound3. Metalorganic compound

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MOCVD is used for preparing compound semiconductors. The vapor pressure of the MO source is an important consideration in MOCVD, since it determines the concentration of source material in the reactor and the deposition rate. Too low of a vapor pressure makes it difficult to transport the source into the deposition zone and to achieve reasonable growth rates. Too high of a vapor pressure may raise safety concerns if the compound is toxic.

Vapor pressure of most Vapor pressure of most common MO compoundscommon MO compounds

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Compound P at 298 K

(torr)

A B Melt point

(oC)

(Al(CH3)3)2 TMAl 14.2 2780 10.48 15

Al(C2H5)3 TEAl 0.041 3625 10.78 -52.5

Ga(CH3)3 TMGa 238 1825 8.50 -15.8

Ga(C2H5)3 TEGa 4.79 2530 9.19 -82.5

In(CH3)3 TMIn 1.75 2830 9.74 88

In(C2H5)3 TEIn 0.31 2815 8.94 -32

Zn(C2H5)2 DEZn 8.53 2190 8.28 -28

Mg(C5H5)2 Cp2Mg 0.05 3556 10.56 175

Log[p(torr)]=B-A/T

ReactionReaction

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6. MOCVD grow GaN and 6. MOCVD grow GaN and related materialsrelated materials

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High temperature

treatment

Bufferlayer

EpilayerGrowth

TMGa

NH3

Temperature

1150oC

550oC

1050oC

Two Step MOCVD Growth procedure

Ga(CH3)3+NH3 GaN+CH4

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Advantages:

• Highly flexible—> can deposit semiconductors, metals, dielectrics.•Used for preparing compound semicondutors

Disadvantages:

•HIGHLY TOXIC! Very expensive source material.•Environmental disposal costs are high

Documents

CVD slides