national formosa university institute of electro-optical and material science 太陽能工程 -...

National Formosa University

Institute of Electro-Optical and Material Science

太陽能工程 -太陽能電池篇

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Introduction for PECVDIntroduction for PECVDPlasma Enhanced Chemical Vapor Plasma Enhanced Chemical Vapor

DepositionDeposition




Vacuum level：

• According to the different behavior of the size of the gas pressure and gas movement, vacuum divided into five levels：

Pa torr

Rough vacuum

Airflow morphology ： Viscous flow

1atm~102 Pa 760~1 torr

Medium vacuum

Airflow morphology ： Transition flow

102~10-1 Pa 1~10-3torr

High vacuum

Airflow morphology ： Molecular flow

10-1~10-5 Pa 10-3~10-7 torr

Ultra-high vacuum

Airflow morphology ： Single-molecule movement

10-5~10-8 Pa 10-7~10-10 torr

Ultra-high vacuum

Airflow morphology ： Single-molecule movement

less 10-8 Pa less 10-10 torr

Under standard atmospheric pressurePressure =1 atm =760 mmHg =760 torr = 1.013 x 105 Pa (?kg/cm2) =1013 mbar =14.7 psi =7.6 x 105 micron

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Mean free path• Gas molecules in the movement, each molecule before the collision to

other molecules, the average walking distance is called the gas mean free path, usually represented by the symbol 「 λ 」 , unit is cm. at room temperature 20 ° C （ The higher the temperature, molecular motion faster）

Pressure Mean free path

1 x 10-3 torr 5cm

1 x 10-4 torr 50cm

1 x 10-5 torr 5m

1 x 10-6 torr 50m

1 x 10-7 torr 500m

1 x 10-8 torr 5km

1 x 10-9 torr 50km

1 x 10-10 torr 500km

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Vacuum pump operating rangeRough vacuum Medium vacuum High vacuum Ultra-high vacuum

Pressure (mbar)




Classification of the vacuum gauge( 一 ) Divided according to the measuring method:：1. Direct ： The size of the direct measurement of the gas molecules acting on the wall or

membrane of the sensing element force, reading is usually nothing to do with the gas species, a rough vacuum regardless of the often used this way.

2. Indirect ： Characteristics of the measuring gas in a vacuum, such as thermal conductivity, viscosity, etc., to be converted into pressure, high vacuum regardless of this approach is commonly used in reading and gas type.

( 二 ) Divided on the basis of the work principle ：1. Mechanical ： Wall or membrane of the sensing element to the force acting on a size to

determine the size of the pressure.2. Electronic ： Number of different vacuum gas molecules caused by the thermal conductivity,

viscosity, ionizing the measured strength of electronic signals converted into the size of the pressure.

( 三 ) Divided according to the measurement range：1. Rough vacuum: 1 atm ~ 102 Pa, Gauge U-tube 、 Gauges Bourdon 。2. Medium vacuum: 102~10-1 Pa, Gauge thermocouple 、 Gauges capacitance manometer3. High 、 Ultra-high vacuum: 10-1~ 10-10 Pa Gauges cold cathode 、 Gauge ionization




Thin Film Deposition

SputterEvaporation

CVD

(Chemical Vapor Deposition)

PVD

( Physical Vapor Deposition)

Reactive SputterAC SputterDC Sputter

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PVD thin film growth mechanism• First of all, the atoms reach the substrate must have a vertical movement, atoms in

order to “adsorption” on a substrate. these atoms in a chemical reaction to form a thin film substrate. thin film composition atoms in the surface of the substrate diffusion movement, this phenomenon is known as the adatom “Surface Migration”. when the atoms collide with each other combination of the trip atoms, Known as “nucleation”.

• Atoms must reach a certain size, the continuous stable growth. therefore, the small clusters will be inclined to each other polymerization, formation of a larger atoms, to cut overall energy. Atoms continue to grow will form the “island”. gaps between the nuclear island need to fill in order of atomic island junction and the formation of the entire continuous film. unable to bonding of atoms and the substrate, will be taken from the substrate surface detachment, to formation of free atoms, this step is called the atomic “desorption”.

Substrate

Adsorption Chemicalreaction

DesorptionSurface Migration nucleation




Sputter (PVD)

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PVD-Sputtering

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PVD-Sputtering

•The frequency of using magnets to scan the film thickness can be controlled, scanningmore the number of film thickness and the thicker.

•In general, more suitable for thin film deposition conditions: high substrate temperature, low pressure, clean, smooth and non-reaction with the deposited film and the lattice size similar substrate.

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Vacuum evaporation, sputtering and ion deposition three kinds of PVD method of characteristics.

PVD Evaporation

Vacuum

evaporation Sputtering Ion deposition

Ions generated heat kinetic heat

Thin film growth rate Can improve ( < 75 μm/min)

Pure metal other than low

(Cu： 1μm/min)

Can improve( < 25 μm/min)

Particle Atoms, ions Atoms, ions Atoms, ions

Deposition uniformity if no gas mix in , and it will

be worse

good， but the film thickness

uneven

good， but the film thickness

uneven

Deposition of metal YES YES YES

Evaporation alloy YES YES YES

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Vacuum evaporation, sputtering and ion deposition three kinds of PVD method of characteristics.

PVD Evaporation Vacuum

evaporation Sputtering Ion deposition

Evaporation heat-

resistant compounds YES YES YES

Particle energy 0.1~0.5eV 1~100eV 1~100eV

The impact of the inert

gas ion usually not Yes ， or not because

of its shapeYES

Mixing between

the surface and layer usually not YES YES

Heating (external heating)

YES usually not Yes and No

Deposition rate 10-9 m/sec

1.67~1250 0.17~16.7 0.5~833

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Vapor deposition, molecular beam epitaxy and sputtering PVDcharacteristics comparison

Nature of the

method

Deposition rate Large-size control Precise composition control Can be deposited materials

Manufacturing cost

真空蒸鍍(Evaporation)

Slow Poor Less Poor Poor

分子束磊晶成長(MBE)

Slow Poor Excellent Poor Poor

濺鍍 (Sputter) Best Best Best many Excellent

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Thin Film Deposition

CVD

(Chemical Vapor Deposition)

PVD

( Physical Vapor Deposition)

By PressureAPCVD760 torrLPCVD 10-1 torrPECVD

500 mtorr

By Energy Resistance Heat

Re induction Glow Discharge

Photons

By Reactor Type

Hot/Cool wall

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化學氣相沉積五個主要機制：CVD 5 mechanisms• (1) Import the main airstream of the reactants (laminar flow (?))• (2) internal diffusion of reactants• (3) Atomic adsorption• (4) Surface chemical reaction• (5) Resultant outer diffusion and remove

基板At High temperature

(200-500C))

(4)main airstream(1)Reactant

SiH4, H2High concent

(2)internal diffusion Low concent

(3) Adsorption

(5) outer diffusion

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Boundary layer

(6)Chemical reaction




Atmospheric pressure chemical vapor deposition(APCVD)

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The so-called atmospheric pressure chemical vapor deposition method, as the name implies, is the pressure to tap into the atmospheric pressure CVD reactor of a deposition, the deposition rate of this method is very fast (high deposition rate),approximately 600-1000 nm / min. Close to atmospheric pressure(1 atm) the APCVD the operating pressure, the pressure of molecular collisions between the high frequency of homogeneous nucleation "gas phase reactions likely to occur, and easy to produce particles (easily generate particles). APCVD use in industrial applications are concentrated in the larger particle endure the process, such as the protection layer (just for to passivation).




APCVD Reactor Shower head

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Low-pressure chemical vapor deposition(LPCVD)

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Low-pressure chemical vapor deposition during thin film deposition, the gas pressure inside the reactor lowered to below about 100 torr. A chemical vapor deposition reaction. Reaction at low pressure, the film deposited by LPCVD method has better step coverage, but the lower the frequency of collisions between gas molecules makes the LPCVD deposition rate is slow compared to APCVD.The deposition rate is lower.




LPCVD System

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Plasma enhanced chemical vapor deposition(PECVD)

• In the CVD reaction, the decomposition of gas molecules need to have sufficient excitation energy .Plasma enhanced chemical vapor deposition

method, the reaction gas in an electromagnetic field energy, and a variety of chemical reactions in the plasma body quickly, resulting in a short period of time to complete the chemical vapor deposition.

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• Belongs to the non-equilibrium plasma in the PECVD .In the body of such a plasma, the free electrons of the absolute temperature is usually higher than the average gas temperature of 1-2 class times, these high-energy

electron impact gas molecules of the reactants, excitation and ionization, resulting in very lively chemical properties of free radicals group. Addition , the ions hit the substrate surface, resulting in a more lively surface structure, there by speeding up the chemical reaction. In order to reduce the reaction temperature required to reach the lowered energy consumption for heating, in

PECVD share of the weight in the CVD process to gradually become a major thin film deposition tools in Taiwan, especially for IC wafer BEOL metal and dielectric deposition of the plasma membrane.




Belongs to the non-equilibrium plasma in the PECVD. In the body of such a plasma, free electrons of the absolute temperature is usually higher than the average gas temperature 1-2 class times ， Free Electrons have high temperature (energy). They will bomb the reactants and gases. The gases will be ionized. The ionics will be active to react with other ionics. These high-energy electron impact reactant gas molecules, so that excitation and ionization, and chemical properties of very lively radicals. Addition, ion bombardment to the substrate surface, The ionics will also bomb on the substrate surface. Sometime it help deposition, but Sometime it is a damage.More lively surface structure, thereby speeding up the chemical reaction. In order to reduce the reaction temperature required to reach the lowered energy consumption for heating, in PECVD share of the weight in the CVD process to gradually become a major thin film deposition tools in Taiwan, especially for IC wafer BEOL metal and dielectric deposition of the plasma membrane.

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Plasma Enhanced CVD System13.56 MHz/ 60MHz

50-1000W

Shower head

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太陽能工程 -太陽能電池篇Various comparative advantages and disadvantages of CVD

Process Advantage Shortcoming Application

APCVD Simple structure of the reactor

Fast deposition rate

Low temperature process

Step coverage of the poor particle pollution

Low temperature oxide

LPCVD High-purity

Step coverage excellent

Can be deposited on large area chips

High-temperature process

Low deposition rate

High-temperature

oxide

Polysilicon

Tungsten, silicide tungsten

PECVD Low temperature process

High deposition rate

Step coverage

Chemical pollution

Particle pollution

Low-temperature insulator

Passivation layer

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CVD System

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CVD System

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Thin Film - PECVD

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PECVD 在 Solar Cell 上的應用 a-Si:HPECVD way to the deposition of thin films, generally have a high hydrogen content in the case of Hydrogen amount is very a high, because the Plasma in the H atoms with unsaturated bonds, not when the deposition statement of Si and N to form the Si-H and NHkey results. Since H is easily affected by temperature has been released (H will be released at a high temperature), resulting in the instability of the TFT device characteristics.

H in plasma will connected with unsaturated Si and N to become Si-H and N-H

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Hydrogen treat for SiNx layer

After the g-of SiNx do an H2, Plasma, change the surface structure of g-of SiNx, to fill some of the Dangling Bond, in order to avoid the Channel with α-Si Defect trap caused by Cell Mobility lower

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Preparation and Properties of Preparation and Properties of amorphous siliconamorphous silicon

1. Thin film deposition methoda-Si and its alloys is the plasma CVD, the heat of CVD,

reactive sputtering or optical CVD method, vapor-phase synthesis method to prepare thin films.The use of a-Si solar cell, to the plasma of CVD method prepared, it is the first production method described, followed by optical CVD method, Doping.

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Plasma CVD methodPlasma CVD methodAs shown the generated SiHx (x 3) ≦ response (neutral and ionic).These reactions are diffusion to reach 100 to 300.C substrate, on which a variety of reactions (adsorption, detachment, pulled out, insert and surface diffusion process), the formation of a-Si film.

Materials and gases to stimulate dissociation film

Excitation, Decomposition

Transport

Surface reaction

Film

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Commercialization of solar cells using the plasma technology applications

Effect Passivation Antireflection Abosrbers Contacts

Material SiNx:H

SiO2

SiO2, TiO2, Si3N4

a-Si:H, Cu, In, Ga, Se, S,CdTe

Al, Ag, NiV, Mo, SbTe,

ITO, Ti, Pd, ZnO:Al, i-ZnO

Equipment and

technology

PECVD Sputtering PECVD Sputtering

Sputtering PECVD Evaporation Evaporation

Sputtering

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Applied by different generations of solar cells, plasma equipment

Solar cell classification

Applications of plasma devices

Plasma coating membranes

2004 2020

The first generation

Silicon type PECVD 、 Etcher

Antireflection film batch coated

93% 50%

Second-generation

Film-type(Silicon 、II-VI 、 III-V)

PECVD 、 PVD Transparent conductive film, the metal electrode, the silicon thin film

7% 42%

Third-generation

Nano, organic, dye

PVD 、 PECVD SiNx/SiOx

film 、 Transparent conductive filmTiO2/Ta2O5 、Metal electrodes

0% 8%

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Thin film growth

Chip film, the initial covered on the wafer surface, many gas molecules or other child, such as atoms and ions. These particles may be because the

chemical reaction, the solid particles, and then deposited on the wafer surface; or lose part of the kinetic energy through a surface diffusion

campaign, the wafer surface adsorbed (absorbed) were deposited. According to the order of occurrence can be divided into the following five steps (a) crystal growth (b) the grain growth and (c) grain coalescence

(d) seam Road to fill the (e) The deposition film growth.

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thin film deposition steps

(a) crystal growth (b) grain growth and (c) grain coalescence (get together) (d) The seam Road to fill the gap (e) The deposition film growth(1) physical adsorption on the wafer surface adatom(2) adatom back to the gas phase by absorption of the solution

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Thin film structure• island structure： In the deposition, when the atoms or molecules are mutually binding capacity than the substrate

strong island structure, such as metal insulator, graphite and other substrates.• layer structure ： As the atoms and the substrate bonding is stronger than the others. The first layer

of coverage completed with the second layer of bonding will be weak, such as semiconductor thin film of single crystal growth.

• Stranski-Krastanov(S.K.) structure ： Do the aforementioned model of an intermediate process, the current study such behavior is

not fully understood, there may be disturbance to the growth of layered structure due to the binding energy.

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Residual stress• Within the film and the substrate must have the existence of residual stress (thermal

expansion coeff.), residual stress according to their form to distinguish can be divided into two, divided into the two kinds of tensile stress and compressive stress.

• The film tensile stress of the role, then the film - substrate will show a concaveshape• Film by the compressive stress of the role, then the film - substrate will

show aconvex shape

(a) Tensile stress (b) Compressive stress36




VHF-CVD reactor electrode structureIts structure for the strip-type electrodes, However, due to the spacing of theelectrode structure is very narrow, and therefore will produce a standing wave effect, affecting the quality of thin film growth, the left side of the SignalModulator generated waves phase, the elimination of standing wave effect, in order to improve the filmdeposition quality.

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Applications of PECVD in TFPV• a-Si:H film quality related to the experimental parameters:(1) Substrate temperature (2)SiH4/H2 ratio (3)Total gas flow rate

(4)RF power density (5)Electrode to substrate distance (6) growth pressure, ……

• To increase the growth rate of a-Si:H films , some possible problems:

(1)Increase the power density of the plasma power: increase free radical production rate, but will also enhance the pears bombarded by an increase in film stress.

(2)Increase the gas pressure :the homogenous reaction generates more, the powder easily occur, the plasma will easily generate porous film.

(3)Increase viscosity coefficient of free Radicals: cause free radical surface diffusion will decrease to reduce the decline in film quality.

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Applications of PECVD in TFPV

• a-Si ： H thin film deposited by PECVD has lower defect density than by sputter due passivation of dangling bonds by H atoms

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Applications of PECVD in TFPV

• Doped a-Si ： H thin film

• Doped a-Si ： H can be fabricated by mixing PH3 and mixing B2H6 or into SiH4/H2 gas in plasma deposition process.

• Conductivity of a-Si ： H may be varied more than a factor of108

• But doping in a-Si ： H inevitable leads to creation of dangling bonds, higher defects density and shorter diffusion length of carriers.

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Microcrystalline Xicheng film agencies• As raw material, SiH4, SiH3 of SiH4 reaction and very stable, is generally

considered the main precursor for the SiH4 there is a proportion of about1/1000.• Hydrogen atoms from SiH4 and H2, but with SiH4 to produce the binding reaction.• Of SiH4 + H → H2 + SiH3 extinguished, there is no hydrogen dilution

system,the hydrogen atom is almost impossible to reach the substrate surface. To growthe crystal structure must be about 10 times more hydrogen dilution.

Crystalline deposition of microcrystalline silicon thin film changes with thethickness and hydrogen dilution schematic

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grain boundary

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Grain size and shape

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