CHEMICAL VAPOR DEPOSITION REACTORS


Chemical vapor deposition (CVD) reactors are used in applications that involve the deposition of a layer or layers of a substance onto a surface. The figure below is a 3000x magnification of a cubic diamond coated tool.



(Copyright Crystallume Engineered Diamond Products,
Santa Clara, CA)

ATMOSPHERIC PRESSURE CVD


Atmospheric Pressure Chemical Vapor Deposition reactors are the most general type of CVD reactors. An APCVD conveyor system is shown below.



(Copyright SierraTherm Production Furnaces, Inc., Watsonville, CA)

GENERAL INFORMATION

In atmospheric pressure chemical vapor deposition a layer of material several microns (10-6 m) thick is deposited on a receiving surface. The picture below shows a reaction chamber during the reaction used in semiconductor wafer processing. The uniformity of the deposited layer is crucial to the performance of microelectronic devices.



(NASA/copyright nasaimages.org.)

EQUIPMENT DESIGN

The animation below shows a horizontal cold-wall APCVD reactor used in semiconductor wafer production. The wafers are heated by heating the graphite susceptor through induction via copper coils connected to a radio frequency (RF) source. The gas containing the material to be deposited flows over the wafers and the reaction takes place, forming a micron-thick product layer. The susceptor is tilted so as to reduce downstream depletion.



This animation below shows the operation of a vertical-flow cold wall APCVD reactor. The wafers are heated by an RF coil beneath the graphite holder. Instead of having the gas flow horizontally over the wafers, as in the previous design, the gas flows from below and is sent above the wafers, then falls onto the reaction site. The rotating holder results in a much more uniform deposited layer than if it were stationary.



The true advantage to the vertical gas flow design is the continuous supply of fresh reactant gas, which all but eliminates the problem of downstream depletion. Due to its wafer load capability, this design is often used in laboratories or small fabrication lines.


The diagram below shows the flow of wafers and process gas within a horizontal conveyor.



(Copyright Dow Corning Corporation, Midland, MI)

The number of objects, or load, that a reactor can handle adds another element to reactor design. The two reactors shown below are used in semiconductor wafer production. The picture on the left shows a common vertical barrel design, in which the wafers are placed on ledges within the barrel. In the picture to the right wafers are stacked inside a quartz boat within a horizontal reactor.


(Copyright CVD Equipment Corporation, Ronkonkoma, NY)

USAGE EXAMPLES

APCVD reactors are commonly used in the microelectronics industry, where they are used to apply thin layers of semiconductor materials, such as silicon (Si) and gallium arsenide (GaAs). The picture below shows Si wafer-based solar panels that convert sunlight into electricity.



(Copyright CVD Equipment Corporation, Ronkonkoma, NY)

APCVD reactors are used for surface coating in applications including reinforcement, abrasives, thermal insulation, anti-corrosives, electrical shielding and optical reflection. The tools shown below have been coated with CVD diamond coating technologies.



(Copyright Crystallume Engineered Diamond Products, Santa Clara, CA)

Below on the left is a coating system that applies energy saving films to large glass panels. These glass panels are called low-emissivity (Low-E) windows and reduce the infrared radiation from the inside window pane to the outside window pane, reducing energy loss by as much as 30%50%. Installed Low-E windows are shown on the right.


(Pictures copyright CVD Equipment Corporation, Ronkonkoma, NY)

ADVANTAGES

DISADVANTAGES

  • Relatively low operating cost since no vacuum needed
  • Uniformity of deposited layer compromised at higher temperatures and pressures
  • Gas flow dynamics hard to control at high pressures

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LOW PRESSURE CVD


GENERAL INFORMATION

Low Pressure Chemical Vapor Deposition reactors are similar to APCVD reactors, but operate under a vacuum. The LPCVD system shown below is used for the growth of graphene, oxides, nitrides, polysilicon, silicon, and other coating materials.



(Copyright CVD Equipment Corporation, Ronkonkoma, NY)

EQUIPMENT DESIGN

Uniformity of the deposited layer is a main concern when designing a CVD reactor. The lower operating pressure found in LPCVD systems results in better uniformity along the surface, as well as better step coverage. In good step coverage as the surface elevation changes the deposited layer maintains consistent thickness. In poor step coverage, on the other hand, the thickness is smaller at the points of elevation change. Improper step coverage results in poor protection of the tool surface and defects in microelectronic devices.



The LPCVD reactor shown below can hold many wafers and can be configured to load wafers vertically or horizontally. The typical operating pressure for an LPCVD reactor is 0.1 to 2 torr.



(Copyright Dow Corning Corporation, Midland, MI)

USAGE EXAMPLES

LPCVD reactors are used in the same industries as APCVD reactors. The pictures shown below are used in the machinery industry. The picture on the right shows a multi-wafer cluster reactor used in the microelectronics industry.


(Copyright Diamond Tool Coating, North Tonawanda, NY)
(Copyright Process Specialties, Inc.,
Tracy, CA)

ADVANTAGES

DISADVANTAGES

  • Lower reaction temperatures than APCVD reactors
  • Good step coverage and uniformity
  • Less dependence on gas flow dynamics
  • More expensive than APCVD reactors
  • Downstream depletion can occur in horizontal designs

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PLASMA-ENHANCED CVD


GENERAL INFORMATION/EQUIPMENT DESIGN

Plasma-Enhanced Chemical Vapor Deposition (PECVD) reactors use a radio frequency (RF) power to create a plasma field in the deposition gas. The reaction chamber is in a vacuum and at temperatures 200°C lower than other CVD reactors.



(Copyright Dow Corning Corporation, Midland, MI)


(Copyright Oxford Instruments, Concord, MA)

USAGE EXAMPLES

PECVD reactors are used in the microelectronics industry for applications requiring precise tolerances.



(Copyright Oxford Instruments, Concord, MA)

ADVANTAGES

DISADVANTAGES

  • Combination of vacuum pressures and lower temperature produces better uniformity in the deposited layer.
  • Reactor can be used in other microelectronic production process steps.
  • Many more process variables to be controlled compared to other CVD reactors.
  • Cost of operation is increased with increased number of components.

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ATOMIC LAYER DEPOSITION


GENERAL INFORMATION

Atomic layer deposition (ALD) is a variant of CVD used for depositing thin films one atomic layer at a time. The films produced using ALD technology are highly uniform and the process can be thermal or plasma-enhanced. The ALD system shown below can support both thermal and plasma deposition.



(Copyright Cambridge NanoTech Inc., Cambridge, MA)

EQUIPMENT DESIGN

High uniformity in ALD is due to the precursor adsorbing onto the substrate surface a monolayer at a time. Ideally the films are formed a monolayer at a time, but in practice, because of steric hindrance of the precursor molecules, only a fraction of a monolayer is formed during one reaction cycle. The number of deposition cycles determines the film thickness, making it easy to control.


The ALD process alternates between pulsing of reactant gas, which is referred to as precursor gas, and purging of unreacted gas and reaction by-products. The steps are illustrated in the diagram and described below.



(Copyright Nanexa AB, Uppsala, Sweden)

  1. Pulse of precursor is exposed to the surface of the substrate.
  2. Purge of excess unreacted precursor using inert gas.
  3. Pulse of a second precursor followed by a surface reaction.
  4. Purge of gaseous reaction by-products.

Shown below is a diagram of an ALD process chamber. The precursor enters at one end of the chamber where the molecules adsorb onto the surface substrate that is surrounded by a heater. The operating temperature needs to be selected so the reaction happens in a self-limiting manner. If the temperature is set too high or too low, the precursor can decompose or not be reactive enough.



(Copyright Dow Corning Corporation, Midland, MI)

Process chambers are shown in the pictures below. The ALD system pictured on the right has multiple chambers, which allow separate and independent reactions to occur simultaneously.


(Copyright CVD Equipment Corporation, Ronkonkoma, NY)

USAGE EXAMPLES

Atomic layer deposition can be used for a variety of thin film deposition types including oxides such as Al3O4, SiO2, and ZnO, nitrides such as WN, Zr3N4 and TiN and metals such as Pt, W, and Fe. This process can be used for protective coating, microelectronics, optoelectronics and optics.



(Copyright Cambridge NanoTech Inc., Cambridge, MA)

ADVANTAGES

DISADVANTAGES

  • Accurate thickness control over a wide thickness range.
  • Large area uniformity.
  • Operates at low temperatures.
  • No gas-phase reactions
  • Low deposition rates.
  • Sometimes challenging precursor chemistry
  • Sometimes low crystallinity due to low temperatures.

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ACKNOWLEDGEMENTS


Cambridge NanoTech Inc., Cambridge, MA
Crystallume Engineered Diamond Products, Santa Clara, CA
CVD Equipment Corporation, Ronkonkoma, NY
Diamond Tool Coating, North Tonawanda, NY
KEMI Silicon, Inc., San Antonio, TX
Dow Corning Corporation, Midland, MI
Nanexa AB, Uppsala, Sweden
NASA
Oxford Instruments, Concord, MA
Process Specialties, Inc., Tracy, CA
SierraTherm Production Furnaces, Inc., Watsonville, CA


REFERENCES


"Energy Savers: Low-Emissivity Window Glazing or Glass." U.S. Department of Energy. 09      Feb. 2011. Web. 03 Apr. 2011.<http://www.energysavers.gov/your_home      /windows_doors_skylights/index.cfm/mytopic=13430>.
Fogler, Scott H. Elements of Chemical Reaction Engineering. 3rd ed. Englewood Cliffs, NJ:      Prentice-Hall, 1998. Print.
Putkonen, Matti. "Organometallic Precursors for Atomic Layer Deposition." 2005. Precursor      Chemistry of Advanced Materials: CVD, ALD and Nanoparticles. Ed. Roland A. Fischer.      Berlin: Springer, 2005. 125-45. Print.
Van Zant, Peter. Microchip Fabrication. 3rd ed. New York: McGraw-Hill, 1997. Print.

DEVELOPERS


Michael Edison
Joseph Palazzolo
Matthew Robertson
Kelsey Kaplan


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