Page 211 - Semiconductor Manufacturing Handbook

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CHEMICAL VAPOR DEPOSITION

14.12 WAFER PROCESSING

With good design, showerheads can deliver species with 1 percent uniformity. This allows processes
that are mass transfer limited to be deposited uniformly. A system combining such a showerhead
with a uniform platen heater can successfully deposit films over the full range from mass transfer
limited to kinetically limited. This versatility has made this design very popular.

14.3.2 Pumps
CVD processes send large volumes of reactive and potentially corrosive gases to the pumps. Because
of this, capture pumps like cyropumps are not practical. Typically mechanical pumps such as a roots
blower with a rotary vane pump would be used. Concern over pump oil diffusing into the process cham-
ber has led to the replacement of vane pumps with oil free or dry pumps. In applications requiring
process pressures of less than 100 mtorr, a turbo pump is generally used. Pumping technology is dis-
14
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cussed in more detail in books on vacuum systems such as Roth and O’Hanlon, and Lafferty. 16
14.3.3 Wafer Handling
To automate the CVD process tool and reduce the possibility of particle contamination, all wafer
movement in a production tool is done by robot arms. The cassette or front opening unified pod
(FOUP) of wafers is placed in the tool by the operator. From there, the wafers pass through a load
lock in which the pressure is reduced from atmospheric pressure to the process pressure. The robot
then moves the wafer into the CVD process chamber where the film is deposited. Once complete,
the robot arms move the wafer back to the load lock and then to the FOUP.

14.3.4 Gas Delivery System

The flow control components of a CVD system are generally clustered in a single module—the gas
box. All the process gases enter the gas box from the fab facilities typically through 1 / 4 in diameter
electropolished stainless steel tubing. Pneumatic values control the flow of gases. The magnitude of
the flow is regulated by mass flow controllers. The valves and mass flow controllers (MFCs) are con-
trolled by the system computer, which executes the process recipes. These recipes instruct various
valves to open and set the MFCs to the proper set points.
Many CVD processes use precursors that are liquids at room temperature. In order to get this mate-
rial into the process chamber, it must be vaporized, then delivered in a controlled manner. There are
two common methods used for this depending on the vapor pressure of the precursor and its sensitiv-
ity to decomposition. The simplest method is to bubble an inert gas through the liquid, allowing the
bubbles to pick up some of the vapor. This method is easy to implement and inexpensive, but is not
suitable for fragile materials that will decompose after extended time at elevated temperatures.
Alternatively, the liquid can be sprayed onto a heated surface and evaporated there. Then the flow can
be precisely controlled with a liquid flow controller, and the thermal exposure is limited. Often the
gas lines downstream of the evaporator must be heated to prevent the precursor from recondensing.

14.3.5 RF System
For plasma-enhanced CVD systems or those using a plasma clean, an additional set of components
is required to generate the plasma. An RF generator is used to produce the high-frequency energy
needed by the plasma. This is typically at 13.56 MHz, but other frequencies are also used.
Commercial generators are usually designed to send the electrical energy into a 50-Ω load. Since the
load presented by the CVD chamber can vary widely depending on the chamber geometry, gases
used, pressure, and the like, a matching network must be inserted between the generator and the
chamber. Matching networks designed for 13.56 MHz operation are usually adaptive. They will vary
their capacitance or inductance to match changes in the chamber impedance, so the RF generator
always sees a 50-Ω load. If the match is unable to do this, much of the energy will be reflected back
to the generator. Its protective circuitry will then shut it down.

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